TiO 2-Based Nanomaterials : Design , Synthesis , and Applications

1National Engineering Laboratory of Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou 215123, China 2School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 3Kazuo Inamori School of Engineering, Alfred University, Alfred, NY 14802, USA 4School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 5State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

photocatalytic degradation of ethylene pollutant. Compared to commercial P25, the photocatalytic results demonstrated that the TiO 2 powder with or without N-doping was a higher efficient photocatalyst. Moreover, the author deeply studied and discussed the photocatalytic mechanism under visiblelight irradiation. M. Fujita et al. in "Preparation of Oleyl Phosphate-Modified TiO 2 /Poly(methyl methacrylate) Hybrid Thin Films for Investigation of Their Optical Properties" employed oleylphosphate modified TiO 2 nanoparticles to prepare polymethylmethacrylate-(PMMA-) based hybrid materials via the ex situ route. The hydrophobic modification of TiO 2 nanoparticle surfaces through stable Ti-O-P bonds was verified to effectively suppress the aggregation of TiO 2 nanoparticle in polymer matrices to achieve high refractive index. The composite thin film exhibited the highest refractive index ( = 1.86) with 20 vol% content of TiO 2 and still kept excellent optical transparency even with a TiO 2 content up to 70 mass%. We believe that the strategy with the ex situ route for preparation of TiO 2 /polymer hybrids after surface modification enables us to control the refractive indices easier than the in situ route and would have huge impact in optical films related to TiO 2 nanoparticles.
W. Guan et al. in "Preparation and Photocatalytic Performance of Nano-TiO 2 Codoped with Iron III and Lanthanum III" synthesized metal (Fe 3+ , La 3+ ) doping nanoscale titanium dioxide (nano-TiO 2 ) via sol-gel method to improve its photocatalytic activity and utilization of visible light. The modified sol-gel method was verified to be an effective technique for codoping the TiO 2 lattice with Fe 3+ and La 3+ and restricted the growth of doped TiO 2 crystal. Furthermore, the catalytic mechanism which was revealed for metal doping of nano-TiO 2 was proposed. Codoping of nano-TiO 2 with the tombarthite metal mixture had a synergistic effect on the photodegradation reaction of methyl orange. The codoped nano-TiO 2 exhibited superior photocatalytic activity compared to the sum of the single-doped nano-TiO 2 samples. This work provided a potentially attractive and effective approach for TiO 2 photocatalysis to resolve the environmental problem.
X. H. Dai et al. in "Attenuating Immune Response of Macrophage by Enhancing Hydrophilicity of Ti Surface" constructed Ti samples with high contrast of surfaces hydrophilicity. Experimental results showed that highly hydrophilic Ti surface (Ti-H 2 O 2 ) yielded good biocompatibility and less multinucleated cells formation in vitro. The secretion of TNF-and IL-10 quantified by ELISA revealed that more hydrophilic Ti surface leads to lower activation status of macrophages. Moreover, the NF-B assay revealed that NF-B/TNF-might be the possible mechanism underlying behind surface hydrophilicity modulating immune response. All these results suggested that hydrophilic Ti surface might be more favorable in attenuating macrophage immune response via NF-B signaling, which may provide new insight in surface-designing of novel implant devices.

Acknowledgments
The editors thank the authors for their efforts and time spent for each manuscript. The lead editor thanks all editors for the time spent in reviewing, assigning reviews, and commenting on submitted manuscripts. The editors hope that this special issue will be useful to investigators in functional TiO 2 -based materials.

Introduction
Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds with two or more fused aromatic rings. PAHs are produced mainly by the incomplete combustion of fossil fuels. They are widespread contaminants found in natural bodies of water [1,2]. Many PAHs have a variety of mutagenic and carcinogenic effects in organisms. It is reported that low molecular weight PAHs (LMW PAHs, compounds containing three or less fused benzene rings) are more susceptible to biodegradation, while high molecular weight PAHs (HMW PAHs, compounds containing four or more fused benzene rings), which are highly mutagenic and carcinogenic, are more recalcitrant [3,4]. Therefore, it is difficult to eliminate PAHs through traditional biological water treatment [5].
An advanced oxidation technique called TiO 2 -photocatalyzed degradation has attracted attention as a method of eliminating a variety of organic compounds because the process can achieve effective mineralization under mild temperature and pressure conditions [6,7]. Some biorefractory substances, such as norfloxacin [8], dyes [9][10][11], and 4-chlorocatechol [12], have been reported to degrade successfully using this technique. Some PAHs were also reported to be photocatalytically degraded successfully with the presence of TiO 2 in aqueous environment. Wen et al. [13] examined the photocatalytic degradation of pyrene (PYR) that preadsorbed onto TiO 2 surface before UV irradiation. They found that the pH of the dispersion and the ratio of PYR/TiO 2 : water had little effect on the photooxidation rate of PYR, while the extent of surface coverage and the addition of Fe 3+ affected it greatly. Lin and Valsaraj [14] 2 Journal of Nanomaterials studied the degradation of PYR and phenanthrene (PHE) in a dilute water stream by an annular photocatalytic reactor with TiO 2 immobilized on a quartz tube. They found that both PAHs degrade to CO 2 and H 2 O in the presence of quinone. Dass et al. [15] found that acenaphthene, anthracene (AN), fluorine (FL), and naphthalene (NP) undergo efficient photocatalytic oxidation in aqueous suspensions of TiO 2 upon irradiation with a 500 W super-high pressure mercury lamp as well as sunlight. Lair et al. [16] studied the degradation of NP in water by photocatalysis in UV-irradiated TiO 2 suspensions. They found that the optimum TiO 2 was 2.5 g L −1 . They also observed that CO 3 2− strongly inhibited NP adsorption and removal, while pH had insignificant effect upon the NP removal, and temperature slightly accelerated the NP degradation. Through the identification of the main intermediates of NP photodegradation, they proposed that NP degraded by oxygenated radicals and by direct oxidation in the holes formed. Woo et al. [17] investigated the effects of acetone on the photocatalytic degradation efficiency and pathways of NP, acenaphthylene (ACN), PHE, AN, and benzo[a]anthracene in synthetic water. They observed that PAHs photolysis generated toxic intermediate products, especially in the case of ACN and PHE. However, all PAHs complete detoxification was achieved in a 24 h of UV/TiO 2 system by 100 mg L −1 of catalyst. They also found that acetone enhanced PAHs degradation, but 16% acetone addition significantly altered the degradation pathway of NP and ACN. Vela et al. [18] investigated the removal of a mixture of six PAHs (BaP, benzo[b]fluoranthene, benzo[ghi]perylene, benzo[k]fluoranthene, fluoranthene, and InP) from groundwater by ZnO and TiO 2 photocatalysis in tandem with Na 2 S 2 O 8 as oxidant under natural sunlight. The photocatalytic experiments showed that the addition of photocatalyst strongly improved the elimination of PAHs in comparison with photolytic tests. The time required for 90% degradation in the condition of the study was in the ranges 7-15 min and 18-76 min for ZnO and TiO 2 systems, respectively.
Although the TiO 2 -photocatalyzed degradation technology has been extensively investigated in pollutant treatment in the last two decades, its practical application is limited because of the disadvantages of TiO 2 , such as its low surface area, low adsorbability for pollutants, fast recombination of photogenerated electrons-hole pair, the maximum absorption in the ultraviolet light region, and the difficulty of separation from treated water [19,20]. Numerous approaches, such as composite semiconductor, noble metal loading, ion doping, nanotube formation, and sensitization, have been used to modify TiO 2 to overcome the above disadvantages [21][22][23][24]. SiO 2 is one of the most popular coupled materials in composite semiconductors. Mixed TiO 2 -SiO 2 has been reported to be three times more photoactive than TiO 2 alone [25]. The addition of SiO 2 particles not only alters the size and shape of the TiO 2 particles, but also increases the thermal stability and adsorbability of TiO 2 particles [26,27]. Platinum (Pt) is one of the most common noble metals loading on TiO 2 . Ishibai et al. [28] reported their Pt-TiO 2 possessed high photocatalytic activity under visible light irradiation, as well as under UV light irradiation.
They suggested that the surface complex formation associated with Ti-O-Pt chemisorption dominated the visible light reactivity. Devipriya et al. [29] prepared Pt-TiO 2 and immobilized it on ceramic tiles. They found that the catalyst was effective for the solar photocatalytic removal of chemical and bacterial pollutants from water. The optimum loading of Pt on TiO 2 was found to be 0.5%. Ahmed et al. [30] reported that their Pt-TiO 2 samples are more active than the corresponding bare TiO 2 for both methanol oxidation and dehydrogenation processes. The particle size of Pt-TiO 2 was decreased with the increasing of platinum loading. Some researchers attributed the enhanced photocatalytic activity to the increased light absorption and the retardation of the photogenerated electron-hole recombination [30,31]. As the Fermi level of Pt is lower than that of TiO 2 , photoexcited electrons can be transferred from conduction band to Pt particles deposited on the surface of TiO 2 , while photogenerated valence band holes remain on the TiO 2 . These activities greatly reduce the possibility of electron-hole recombination, resulting in stronger photocatalytic reactions [31].
Our previous works proved that a porous photocatalyst Pt/TiO 2 -SiO 2 could efficiently decompose PYR, a four-ring PAH [34,35]. It was also interesting to know the efficiency of Pt/TiO 2 -SiO 2 for decomposing other PAHs. However, the PAH group of organic compounds is very large (over 100 toxic compounds). It would consume much time and money to identify the degradability of all PAHs in Pt/TiO 2 -SiO 2 suspension. To solve this problem, quantitative structure activity relationships (QSAR) modeling, which correlates and predicts transport and transformation process data of organic pollution from their structural descriptors, may be used to study photocatalysis mechanisms and generate predicted photocatalysis process data efficiently [36,37].
It is greatly important to develop QSAR models in which quantum chemical descriptors are used. Quantum chemical descriptors clearly describe defined molecular properties. They can easily be obtained by computation. Lu et al. [38] studied QSAR of phenols and anilines for predicting the toxicity of these compounds to algae. Hu and Aizawa [39] studied the QSAR for the estrogen receptor binding affinity of phenolic chemicals. de Lima Ribeiro and Ferreira [40] studied the QSAR of 67 PAHs in order to predict the phototoxicity of these compounds. Chen et al. [41][42][43][44] studied the QSAR in order to predict the photolysis of PAHs and dibenzo-pdioxin. In the above studies, the quantum chemical descriptors of energy of the highest occupied molecular orbital ( homo ), the energy of the lowest unoccupied molecular orbital ( lumo ), and the GAP, the difference between lumo and homo (GAP = lumo − homo ), were proven to be significant for PAH photochemical QSAR studies [43,45,46]. lumo and homo can serve as measures of the molecular capacity to donate or to accept an electron pair, respectively. The GAP expresses the necessary energy to excite an electron from HOMO to LUMO [40]. These descriptors can be obtained by semiempirical molecular orbital algorithms [40,47].
The present study aimed to further understand the function of Pt/TiO 2 -SiO 2 and to investigate the photocatalytic degradation character and kinetics of PAHs. Six PAHs were tested in Pt/TiO 2 -SiO 2 suspension. The PAHs included three  LMW PAHs, NP, FL, and PHE, and three HMW PAHs, PYR, benzo[a]pyrene (BaP), and dibenzo[a,h]anthracene (DahA). The selected PAHs are frequently detected in natural bodies of water [1,2,48]. The HMW PAHs are all acutely carcinogenic PAHs. The physicochemical properties of the selected PAHs are listed in Table 1. The degradation kinetics for photocatalytically degradable PAHs were also studied. lumo , homo , and GAP were adopted as the molecular descriptors for NP, FL, PHE, PYR, BaP, and DahA. They have been calculated in a previous research [40]. Based upon the results, the primary correlation between the photocatalytic degradability of PAHs over Pt/TiO 2 -SiO 2 in water and their quantitative molecular structure was studied. Based upon the analysis of correlation, the photocatalytic degradability of 67 PAHs was predicted, and the predictions were partly verified by experiments of photocatalytic degradation of anthracene (AN) and Indeno[1.2.3.cd]pyrene (InP).

Materials and Reagents.
All reagents were of analytical quality, and all solvents were of HPLC grade. All PAHs were purchased from Kanto Chemical Co., Inc., (Japan). The Pt/TiO 2 -SiO 2 used in the present study was synthesized by the method presented in previous work [35] under a compaction pressure of 1.83 × 10 5 kPa, Pt coating ratio of 0.4 wt%, TiO 2 : SiO 2 of 1 : 1, and calcination temperature of 973 K. The scanning electron microscopy (SEM) images ( Figure 1) show the surface roughness and morphology of Pt/TiO 2 -SiO 2 . The X-ray diffraction (XRD) and the energydispersive analysis of the X-ray (EDAX) analysis could give some interesting information of the structure of Pt/TiO 2 -SiO 2 . The XRD spectrum ( Figure 2) indicates that TiO 2 in Pt/TiO 2 -SiO 2 is in anatase form, and EDAX spectrum (

Photocatalytic Degradation of PAHs.
The reaction solutions were prepared by introducing a proper volume of stock solution (50 mg L −1 ) into a water/methanol (99/1) solvent to achieve a required final concentration. Water was purified by Milli-Q Plus system (Millipore). Methanol (1%) was added to minimize the adsorption of PAHs to the glassware walls. The reaction mixture was 100 mL of initial concentration of PAHs at 5 × 10 −8 M and 0.3 g L −1 of Pt/TiO 2 -SiO 2 in a glass beaker, continuously mixed with a magnetic stirrer. A black-light lamp (27 w) emitting monochromatic radiation at 368 nm was placed above the reaction solution. The distance between the lamp and the surface of the solution was 5 cm. Before irradiation, the solution was stirred in the dark for 30 min to allow the system to reach adsorption equilibrium.
A sample approximately 2 mL was taken at the designed time interval during irradiation. The same volume of ethanol was added to the sample to prevent the loss of PAHs on wall of glassware, and then it was centrifuged and filtered through a 0.45 m cellulose filter to remove all solid particles. The treated sample was analyzed by high-performance liquid chromatography (HPLC; Hitachi L-7300 HPLC) with a fluorescence detector Hitachi L-7485. The chromatography equipment was equipped with a C-18-reversed phase separation column and the mobile phase was a 90/10 volumetric ratio mixture of methanol/water. The detecting wavelengths for NA, FL, PHE, PYR, BaP, DahA, AN, and InP were 216/308, 210/310, 248/365, 245/390, 297/430, 286/430, 297/390, and 300/500 nm (excitation/emission nm), respectively.

Photocatalytic Degradation Kinetics of PAHs.
Pseudofirst-order kinetics (1) is normally assumed for PAH photolysis [12,49]: Therefore, the rate constant and half-life time of PAHs degradation were calculated using the following equations: where 0 and are PAHs' concentration at times zero and , respectively, is the rate constant, and 1/2 is the half-life time.

Data Set of Quantitative Molecular Structure of PAHs.
The present work studied 67 nonsubstituted PAHs containing 2-7 rings with 5 and 6 carbon atoms ( Figure 4). The electronic descriptors of lumo , homo , and the GAP for PAHs obtained using the AM1 algorithm reported by de Lima Ribeiro and Ferreira [40] were selected in this study.

Photocatalytic Degradation of PAHs in Pt/TiO 2 -SiO 2
Suspension. The photolysis and photocatalysis of the selected PAHs are shown in Figure 5. It was observed that Pt/TiO 2 -SiO 2 played a very different role in photocatalysis of LMW PAHs and HMW PAHs. The presence of Pt/TiO 2 -SiO 2 increased the degradation rates of HMW PAHs efficiently, while the presence of Pt/TiO 2 -SiO 2 inhibited the degradation rates of LMW PAHs. Similar results were obtained in previous research, which confirms that the presence of TiO 2 resulted in a slower photooxidation rate of FL [50]. This difference is considered to be related to the PAHs' molecular structure, which determines the reactivity of a PAH.   On the contrary, the rate constant of photocatalysis for PYR, BaP, and DahA of 0.0155, 0.0452, and 0.0192, respectively, is all higher than the rate constant of 0.0043, 0.0398, and 0.0112, respectively. The correlation coefficients indicate that both the photolysis and photocatalysis of PAHs fit the pseudo-first-order equation very well, except FL, for which the 2 for photolysis and photocatalysis are 0.7332 and 0.6774, respectively. This might be attributed to the special molecular structure of FL, which is the only PAH with a 5-carbon ring structure and which has the largest GAP value of 8.5021 eV among the tested PAHs (see Section 3.3), resulting in a more difficult photoinduced reaction for FL. The results indicate that the photolytic and photocatalytic degradation mechanisms of FL are different from that of other tested PAHs.

Kinetics of Photocatalytic
3.3. lumo , homo , and GAP of PAHs. The lumo , homo , and GAP of NP, FL, PHE, PYR, BaP, and DahA obtained by AM1 algorithm in de Lima Ribeiro and Ferreira research [40] are summarized in Table 3.
It is observed that the values of homo of HMW PAHs are all higher than those of LMW PAHs. On the contrary, the GAP values of HMW PAHs molecular are all lower than those of LMW PAHs. The GAP values of the PAHs for which degradation could be accelerated by Pt/TiO 2 -SiO 2 are all smaller than or equal to 7.4529 eV; on the other hand, the GAP values of PAHs for which degradation was inhibited by Pt/TiO 2 -SiO 2 are all larger than or equal to 8.2086 eV.
These results indicate that more photoenergy is required for triggering the degradation of LMW PAHs than that of HMW PAHs. It can be understood that GAP values can serve as a measure of the excitability of the molecule: the smaller the GAP of a PAH, the easier it will be excited.

Prediction of the Photocatalytic Degradability of 67 PAHs.
The photochemical properties of PAHs undoubtedly depend Journal of Nanomaterials 7 As mentioned above, the maximum GAP of DahA of tested PAHs for which degradation could be accelerated by Pt/TiO 2 -SiO 2 was 7.4529 eV, and the minimum GAP of PHE of tested PAHs for which degradation was accelerated by Pt/TiO 2 -SiO 2 was 8.2086 eV. Therefore, one can deduce that when the GAP of a PAH is less than or equal to 7.4529 eV, this PAH can be degraded in Pt/TiO 2 -SiO 2 -UV system. On the other hand, when the GAP of a PAH is larger than or equal to 8.2086 eV, this PAH cannot be degraded in Pt/TiO 2 -SiO 2 -UV system. When the GAP of a PAH is between 7.4529 eV and 8.2086 eV, the degradation potential of this PAH is uncertain in the experimental condition of our study.
Following the above approach, the photocatalytic degradability of 67 PAHs was predicted and is listed in Table 4. The results show that 46 PAHs are potentially photocatalytically degradable. All of these 46 PAHs, except AN (anthracene, labelled no. 1), were all HMW PAHs. There were four PAHs for which no photocatalytic degradability was predicted. All four of these PAHs were LMW PAHs (labelled nos. 2, 14, 18, and 58). For these LMW PAHs, biological treatment can be a very good complement [51,52]. The photocatalytic degradability of another seventeen PAHs was determined to be uncertain, most of which were 4∼6-ring PAHs.

Verification of Prediction.
To verify the above prediction, the photolysis and photocatalysis of AN (number 1, 3-ring PAH) and InP (number 62, 6-ring PAH), whose GAPs are 7.2795 eV and 6.8528 eV, respectively, were examined in Pt/TiO 2 -SiO 2 -UV system. We selected AN and InP because they are also PAHs commonly found in water [18,53], and AN is the only LMW PAH which is assumed to be photocatalytically degradable. If the prediction was right, the degradation of these two PAHs should be accelerated in the Pt/TiO 2 -SiO 2 -UV system. It is shown clearly in Figure 6 that the degradation rate of AN and InP is indeed improved with the presence of Pt/TiO 2 -SiO 2 . The photocatalytic degradation of AN and InP in the Pt/TiO 2 -SiO 2 -UV system can also be described by the first-order kinetic model. The rate constant, half-life time, and the total removal efficiency of photocatalysis for AN are 0.0766 min −1 , 9.05 min, and 100%, respectively, while those for InP were 0.0079 min −1 , 87.74 min, and 63.11%, respectively. Therefore, the prediction is verified in a way.
For further study, the determination of photocatalytic degradability for the remaining PAHs can be identified by setting a new boundary value of GAP, which can be obtained by appropriate testing of the uncertain PAHs. The prediction is significant for realistic water treatment. By knowing the value of GAP, the photocatalytic degradability of some pollutants in the photocatalyst suspension might be known without doing any experimentation. The GAP value can be obtained easily from previous research or by computation. Therefore, large expenditures of time and money required for the determination can be avoided.

Conclusions
The present study was undertaken to probe the correlation between the photocatalytic degradability of PAHs

Introduction
Recently, the nanomaterial, such as nanoiron, titanium dioxide photocatalysts (TiO 2 ), has applied for groundwater remediation, degradation of hazardous materials, and environment cleanup [1][2][3]. However, the wide band gap of TiO 2 for anatase crystal structure requires UV light and occupied less than 10% energy in solar spectrum for the excitation of electron-hole pairs [4], which restricts the application of TiO 2 because of low photo quantum efficiency. Various metals such as Ag have been doped into the TiO 2 lattice to shift its absorption from UV to visible light, and doping improved their photocatalytic activities [5]. In addition, literatures also fabricated the nanotube array or hybrid structures of element-doped-TiO 2 to enhance the photocatalytic activity [6,7]. However, none of them gave satisfactory results due to their thermally instability and increased carrier trapping [8]. Relatively, the doping of anion on TiO 2 such as nitrogen and carbon element doped and codoped has considerable effect on increasing its photocatalytic activity [9][10][11][12][13][14], in which, the nitrogen seems to be the most effective dopant due to its similar size to oxygen and metastable defect complex, as well as small ionization energy [15], and N-doped TiO 2 (N-TiO 2 ) has an advantage on photocatalytic activity [16,17]. Hence, literatures focused on discussing the structure of N-TiO 2 using the X-ray photoelectron spectroscopy (XPS), electron spin resonance, or Xray absorption near edge structure. Some reports claim that the band gap of the N-TiO 2 is reduced due to a rigid valence band shift upon doping [18], others attributed the observed absorption of visible light by N-TiO 2 to the excitation of electrons from localized N-impurity states in the band-gap [8,19]. In general, the change in banding structure of N-TiO 2 was attributed to the N interstitial doping, N substitution, and O vacancy [20][21][22]. However, till now the structure of N-TiO 2 is still under debate. The preparatory methods and conditions such as nitrogen precursor and sintering temperature may considerably affect the nitrogen state in the doped TiO 2 [19,23]. But, only few systematic discussions have been found regarding the dominated factor ratio of N to Ti and calcination temperature for the activity of N-TiO 2 , simultaneously.
In other words, it is necessary to understand the chemical fine-tuning and further optimization of the visible-light photocatalytic activity of N-TiO 2 . The goal of this study is (1) to develop a simple method for the synthesis of N-TiO 2 nanomaterials using facie method at various conditions of nitrogen precursor amounts and composite calcination temperatures, (2) characterization of N-TiO 2 composite materials, (3) carried out the photocatalytic activity of as prepared N-TiO 2 under visible light.
Ethylene is a plant hormone that controls many plant responses, and its higher concentration can affect the food and horticultural product industry [24], safe handling, and the parent compound of important environmental contaminants, and the photocatalytic activity for the degradation was quite different from that of volatile chlorinated organic compounds such as trichloroethylene [25]. Therefore, batch experiments were conducted for the removal of ethylene to evaluate the N-TiO 2 photoactivity under visible light irradiation and have also been compared with that of commercial photocatalysts Degussa P25. Materials characterizations have been performed through specific surface area analysis, X-ray diffraction (XRD), scanning electron microscope (SEM), electron Spectroscopy for Chemical Analysis (ESCA), and diffuse reflectance ultraviolet-visible absorption spectra (DRS UV-vis).

Chemicals and Synthesis of N-TiO 2
Composite. All the chemicals were used as procured without further purification. Commercial photocatalyst P25 was purchased from Degussa. Ammonium hydroxide (NH 4 OH, 33 wt%), titanium tetraisopropoxide (TTIP), and absolute ethanol (99.8%) were procured from J. T. Baker. Synthesis procedure of N-TiO 2 composite was modified as reported in literatures [16,26], and ammonium hydroxide was chosen as a nitrogen precursor. Different amounts of ammonium hydroxide (0-200 mL) and DI water (200-0 mL) were mixed with absolute ethanol (200 mL) in a flask, and then a 400 mL ammonia solution (concentration 0-8.4 M) was obtained. The 10 ml TTIP was added dropwise into the ammonia solution and stirred vigorously for 4 h at 4 ∘ C in a water bath. After that the hydrolysis and condensation reaction was kept on stirring for 24 h at room temperature, and then the sol-gel solutions stood to 24 h in aged conditions. The ensuing colloid was then centrifuged and washed thoroughly with distilled water to remove the residual reactants. The final precipitate was dried at 105 ∘ C overnight in air and calcined at various temperatures for 5 h to obtain the final N-TiO 2 composite. The synthesized N-TiO 2 composites were labeled as N T , where and represent the amount (mL) of ammonium hydroxide added during the synthesis procedure and calcinations temperature (unit is 100 ∘ C per scale), respectively. For example, the N 150 T 5 catalyst indicates the addition of 150 mL ammonium hydroxide during the sol-gel procedure of amorphous N-TiO 2 catalyst, followed by its calcination at 500 ∘ C temperature. Pure TiO 2 catalyst was named as N 00 T y ( means "zero"), and there was not any addition of ammonium hydroxide during synthesis procedure, such that N 0 T 3 refers to the pure TiO 2 catalyst which was calcined at 300 ∘ C temperature.

Characterization of Composite.
The composite was characterized by means of XRD, DRS UV-vis, XPS, and specific surface area. XPS were recorded on an ESCA spectrometer (ULVAC-PHI, PHI 5000, Japan) operated at 150 W and were used to investigate the surface properties in the valence band of the composite. The shift of the binding energy due to relative surface charging was corrected using the C 1s as an internal standard. The optical absorption responses of composite were obtained from the pressed disk composite using UV-vis (Hitachi, U-3900H, Japan) equipped with an integrated sphere, and BaSO 4 was used as a reference. Specific surface areas of samples were obtained by the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption apparatus (Micromeritics ASAP 2020, USA). Samples were degassed at 150 ∘ C for 2 h prior to the measurements. The crystal phases of the composites were analyzed by XRD (PANalytical X'Pert Pro MRD, USA) with Cu-Ka radiation and diffraction angle range of 2 which is 20 ∘ to 80 ∘ . The crystal size was determined according to Scherrer's equation and fraction of anatase phase in the mixture was calculated by formula proposed by Spurr [27].

Photocatalytic Activity of N-TiO 2 .
During this study, removal ethylene was studied in batch experiment modes to evaluate the photocatalytic activity of N-TiO 2 samples as well as commercial photocatalyst P25. All the powders (0.5 g) were coated onto the flask bottle (catalyst film was kept at 1 mg/cm 2 ) and caped with sleeve stoppers. The constant ethylene was injected into the flask to get initial ethylene concentration (about 140 or 85 ppmv in different test); a 150 L aliquot regularly was withdrawn and injected into gas chromatograph equipment equipped with flame ionization detector (PerkinElmer Clarus 500, USA) to analyze the ethylene concentration. The visible illumination was provided by six three-colored fluorescent lamps (March T5-8W/865), located above the flasks equipped with a filter to cut the UV light region. The visible irradiance 610 nm was determined by detector (international light SED005) and about 0.62 mW/cm 2 at the bottom of flask.

Result and Discussions
3.1. BET, XRD, and SEM Analysis. The characteristics of composites including surface area, anatase phases, and crystallite size are summarized in Table 1 and Figure 1, in which, the  Journal of Nanomaterials crystallite size and phase content of sample is calculated using the Scherrer equation from XRD peak position and full width half maximum [27]. Comparing Figure 1(a) with Figure 1(b), the unobvious change in the BET surface area and crystalline size of samples were observed after the addition of N precursor amounts. However, the clear changes in composites at different calcination temperatures depicted that the dominant factor for surface area and crystalline size of catalysts are calcination temperature, not the addition of N amounts. The pore size and pore volume of composite relative to surface area decreased with the increase of the calcination temperature, and this result is attributed to the pores collapsed and bigger crystallites aggregation of catalyst [28]. The XRD patterns of N-TiO 2 samples in different conditions are shown in Figure 2. The diffraction peaks marked with letters "A" and "R" on the figure corresponds to the anatase and rutile phase, respectively. In Figure 2(a), the major peak of 2 theta at about 25.5 ∘ corresponding to crystal plane (101) of anatase, which became thinner and the relative intensity was increased with calcination temperature. Such stronger crystalline flair of catalyst would enhance its photoactivity. The XRD patterns of N-TiO 2 synthesized with different nitrogen precursors presented in Figure 2(b) have also shown the similar results. It was also depicted that the nitrogen precursor amount did not significantly affect the N-TiO 2 crystal phase and crystal size.
The N-TiO 2 catalyst (N 150 T 3 ) shows great effect on the photoactivity and also prevents the conversion of N-doped amount into nitrogen gas during calcination process. This sample synthesized with 150 mL N precursor was tested to investigate the effects of calcination temperature and the result is presented in Figure 2(c).
In Figures 2(a) and 2(c), the anatase phase on pure and N-doped TiO 2 sample started to appear at 200 ∘ C and 300 ∘ C calcination temperatures, respectively. The phase transformation from anatase to rutile was observed after the 600 ∘ C and 700 ∘ C calcination temperatures. This result can be implied that the nitrogen-doped catalyst retarded the phase transformation [10]. Such higher thermal stability could elevate the anatase crystallinity and promoted the photo-induced charge separation and transportation [29]. The interesting brookite phase was only observed in pure TiO 2 (see Figure 2(a)) and disappeared as the beginning of rutile phase, and further the inhibited form of the brookite phase in the N-TiO 2 was implied ( Figure 2(c)), since the addition of nitrogen in N-TiO 2 catalyst. Figures 2(b) and 2(c) showed that the N-doped TiO 2 samples had typical peaks of polycrystalline anatase structure and rutile without any detectable dopant related peaks, such as TiN, which was possibly due to the movement of dopant ions either into interstitial positions or substitutional sites of the TiO 2 structure or ion concentration, was detected to be too low [30].
The morphology and particle size of the N-TiO 2 (N 150 T 5 ) and pure TiO 2 (N 00 T 5 ) samples can be obtained from an examination of the SEM image ( Figure 3). Single particle in agglomerates in both of them exhibited uniform spherical shape; the size was in range from 20 to 25 nm and a narrow size distribution appeared. Both of these sizes were slightly smaller than the crystallite sizes determined from the XRD in Figure 1(b). The results of similar size and morphology also indicated that the N precursor did not have significant effect on the particle size and shape of TiO 2 catalyst.

DRS UV-Vis and ESCA.
In this study, the N-TiO 2 powders sintered below 600 ∘ C, all exhibited a pale yellow color which suggests its ability to absorb light in the visible region [31]. However, Qiu and Burda infer that the obtained UV-vis spectra that localized defect states may also play a role in providing visible light activity [32]. Optical absorption spectra of the N-TiO 2 and Degussa P25 samples are shown in Figure 4, and the absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic band gap absorption of TiO 2 . A significant shift of the absorption edge to a lower energy in the visible-light region was observed for N-doped TiO 2 . This result indicates that band-gap narrowing has successfully been achieved by the doping of N into the TiO 2 lattice, presumably due to the modification of the band structure Journal of Nanomaterials  Band gap (eV)  Figure 4: The UV-vis adoption pattern of N-TiO 2 (a) prepared with various ammonium hydroxide amount added and (b) calcined at different temperature. [33]. The absorption spectra of the N-doped TiO 2 samples show a stronger absorption than undoped TiO 2 as well as P25 in the UV-vis light region and a red shift in the band gap transition except N 50 T 3 sample. The red shift is ascribed possibly to the oxygen vacancy [34] or the fact that N doping can narrow the band gap of the TiO 2 [35,36]. Generally, the photocatalytic activity is proportional to the photo numbers absorbed by photocatalyst per second and the efficiency of the band gap transition [28], the N doping expand the wavelength response range of TiO 2 into the visible region and also increased the number of photo-generated electrons and holes to enhance the photocatalytic performance of N-TiO 2 . The enhanced activity in N-TiO 2 catalysts also was demonstrated in Figure 6. Figure 4(a) revealed that the absorbance increases with increasing added nitrogen precursor amounts of N-TiO 2 synthesized condition. Unfortunately, such absorbance is not proportional to its photoactivity. Further, the UV-vis absorbance spectra of appropriate samples with different calcined temperature are shown in Figure 4(b). The absorbance of all N-TiO 2 samples in the visible region is higher than that of P25 and may be ascribed to the fact that the narrowing of band gap as well as localized defect resulted from N doping and calcination. Higher than 700 ∘ C calcination, the N-TiO 2 powders show a significant red shift in the band gap transition due to the formation of rutile phase and growth of TiO 2 crystallite. However, interestingly, amorphous N 150 T 2 powders also showed a greatest red shift. As the calcination temperature increased from 300 ∘ C to 600 ∘ C, the intensive absorption of N 150 T 4 , N 150 T 3 , and N 150 T 5 were observed in the visible region between 400 and 500 nm, which is the typical absorption feature of nitrogen doped TiO 2 [10], whereas it was not observed on N 150 T 6 sample even if N 150 T 6 appeared better photoactivity than N 150 T 3 . However, the aforementioned result could not possibly provide a definite relation between absorbance and photoactivity of as-prepared sample, which is due to that the absorption features in the visible range originated from color centers through doping processes or posttreatments rather than by narrowing of the band gap [32,37]. Literature also reports the intensive absorption of ion doped TiO 2 that can be assigned to oxygen vacancies produced by thermal treatment, which form localization levels within the band gap [34]. Moreover, the origin of visible light absorption originated from N and O vacancies suggested that the N dopants can only affect the absorption below 500 nm wavelength, while the O vacancies are responsible for the induced absorption at wavelengths above 500 nm [38].
ESCA analyses of N 1s and Ti 2p were performed on pure TiO 2 , P25, and N-TiO 2 catalysts and results are shown in Figure 5. In Figure 5(a), the binding energy (BE) peaks corresponding to N 1s core-levels for N-TiO 2 samples prepared with various amounts of N precursor and calcined at 300 ∘ C are observed one major peak at 398-402 eV. However, sometime the binding energy observed at 400 eV is ascribed to N atoms which are incorporated into the TiO 2 lattice as N 2 molecules [33]. Saha and Tompkins investigated the N 1s ESCA spectra during the oxidation of TiN and assigned the peaks as atomic -N (396 eV) and molecularly chemisorbed -N2 (400 eV and 402 eV) [31]. Those different N 1s spectral features are still debated and dependent on different preparation methods and conditions [35]. Further, the Ti 2p 1/2 and Ti 2p 3/2 spin-orbital splitting photoelectrons for anatase TiO 2 were located at binding energies of about 464.0 and 458.2 eV, respectively, which were assigned to the presence of typical Ti 4+ . Same Ti 2p peaks are located at same binding energy and distribution ( Figure 5(b)), because the N precursor amount was not a dominating factor in the electronic structure of N-TiO 2 , although the different activity of N-TiO 2 synthesized in N precursor amount is shown in Figure 6(a).
The content of nitrogen were maintained in all the composites smaller than 1% and found decreased with increasing calcination temperature as shown in Figure 5(c), which may be due to the replacement of N in the matrix by O and    Journal of Nanomaterials transform to nitride result in lower N concentrations [32]. Literature reports that the N dopant was adsorbed NO or bond to oxygen vacancy site in interstitial formation (Ti-O-N), especially at such low N doping concentrations [39]. The N 1s peaks also shifted towards higher binding energy with increasing calcination temperature level and appeared as different distribution. However, high sintering temperature can also cause the loss of dopants from N-TiO 2 catalyst. Therefore, an appropriate sintering temperature would be more important to the activity of N-doped TiO 2 [32]. At the same time, in Figure 5(d), the Ti 2p peaks undergone a shift towards higher binding energy with increasing calcination temperatures. This indicates the lowering of electronic density around the central Ti ion by introducing more O next to Ti [32]. The above mentioned binding energy shift pointed out that the calcination temperature may be a dominating factor in the electronic structure of N-TiO 2 , and this is found consistent with XRD and BET analyses, and thus the calcination temperature would be key factor for the characteristics of N-TiO 2 based on synthesized method of this study. Figure 6 shows the photocatalytic performances for ethylene removal over the various N-TiO 2 and pure TiO 2 in batch reactions under the visible light irradiation. The rate of photocatalytic oxidation has been described by a pseudo-firstorder equation, as presented in (1) [24] and the rate constant values are given in Table 1. Consider

Photoactivity of N-TiO 2 under Visible Light Illumination.
where 0 is the initial concentration of ethylene, is the ethylene at time , and is the apparent pseudo first photocatalytic oxidation rate constant. Examination of (1) suggests that the apparent pseudo first photocatalytic oxidation rate constant can be determined experimentally from a plot of ln( 0 / ) versus time, which has a slope of − . All the photocatalytic oxidation rate constants ( ) of catalysts are summarized in Table 1. Figure 6(a) shows the effect of N-TiO 2 synthesized at different nitrogen precursor with 300 ∘ C calcination temperature, and the order of the reaction rates (Table 1) Then, the N-TiO 2 catalyst was synthesized at 150 mL nitrogen precursor and sintered from 300 ∘ C to 800 ∘ C temperatures to investigate the effects of sintering temperature on N-TiO 2 photoactivity. In Figure 6(b), the activity of N 00 T 5 (Pure TiO 2 sintered at 500 ∘ C temperature) was found better than commercial TiO 2 P25 under visible light irradiation due to the transfer of electron and hole between two phases (mix phase of anatase with few fraction of brookite) which is ascribed to the oxygen vacancy resulted from the calcination at 500 ∘ C [40].
Moreover, the optimized N 150 T 5 photocatalyst has shown better ethylene degradation than undoped N 00 T 5 and others. Based on the experimental results, the nitrogen doping would improve the photoactivity of undoped TiO 2 under visible light irradiation. Both N 150 T 7 and N 150 T 8 showed less photoactivity due to the existence of rutile phases and the tiny BET surface area as shown in Figure 1(b). However, the largest specific surface area of N 150 T 3 should have theoretically better photoactivity for the removal of ethylene due to the abundance of active sites. More effectiveness of N 150 T 5 was also observed for visible region of solar illumination and there are some other facts that may affect photoactivity of TiO 2 under visible illumination such as O vacancy resulted from calcination and N impurity acted as light sensitization [41,42]. We have found that not only is the N-TiO 2 sample oxygen-deficient but also it involves a small amount of Ndoped. N doping in TiO 2 was in interstitial formation (Ti-O-N) and that function retarded the reoxidation of oxygen deficient TiO 2 , which is essentially indispensable for visible light sensitization [36].

Conclusions
N-TiO 2 nanocatalysts can be developed directly by so-gel method under the conditions: 150 mL ammonium hydroxide precursor added and 500 ∘ C calcination temperature. The ethylene removal efficiency was dependent on the optimal contents of nitrogen precursor, surface area, and oxygen vacancy and good crystallization resulted from the adapted calcination temperatures [32,43]. The N-TiO 2 powders showed a stronger absorption in the visible light region from 400 to 500 nm wavelength, which was possibly due to narrow band gap by mixing of N 2p states with O 2p states on the top of the valence band or a creation of N-induced mid-gap level. Other more intensive absorption in the visible light region longer than 500 nm wavelength is ascribed to oxygen vacancy resulted from the calcination and N doping. The N-doping wet-method has an advantage on photocatalytic activity as compared to metal ion doping (thermally instability). In addition, the doped TiO 2 enhanced the photoactivity may be further investigated in depth [32], such as use of first principle calculations, origin of the enhanced visible light absorption in N-doped anatase TiO 2 [44], or approaches to combine TiO 2 with another mineral such as schorl. [3] Y.-T. Lin

Introduction
Supply of safe and aesthetically pleasing water, for human consumption, is essential for all treatment facilities. A common and recurrent problem in drinking water is the formation of earthy-musty taste and odor [1]. Taste and odor episodes in drinking water are the cause for most consumer complaints [2]. There is growing concern about off-flavour in Rawal Lake Filtration Plant, which intensifies in summer months [3][4][5]. Sensory analysis detected earthy and musty off-flavours in raw, as well as in treated supplies from this facility. This study focused on Geosmin (C 12 H 22 O) and 2-MIB (C 11 H 20 O) as surrogate for earthy and musty offflavours. Earthy and musty off-flavours are produced as secondary metabolite by two groups of aquatic microorganism: (i) anabaena for Geosmin and (ii) Phormidium spp. for 2-MIB [6]. Both compounds are semivolatile, saturated tertiary alcohols that can be detected at extremely low concentrations of 6-20 ng/L [7]. The main problem with the presence of Geosmin/2-MIB is that conventional water treatment processes including coagulation, flocculation, and filtration are inadequate for their removal [8]. Oxidants including KMnO 4 , ozonation, and UV/H 2 O 2 have been tested for removal of odorous compounds [9,10] but due to the complex tertiary structures of Geosmin and 2-MIB, these are resistant toward oxidation and most aquatic oxidants [11]. Activated carbon in combination with sand media in filtration plants is considered as one of the best available practice for the removal of off-flavours from water, but at relatively higher amount of pollutant levels, its adsorption efficiency drops [12][13][14]. Literature review reveals that nanoparticles have some advantages in using adsorption processes to solve many environmental issues due to their unique surface and structural properties [15]. A number of studies have been reported on photo catalysis of Geosmin and 2-MIB [16]; however, to the best of our knowledge no previous study has been reported on adsorption of Geosmin and 2-MIB using

Material Systems.
The GC standard of Geosmin and 2-MIB were purchased from Sigma-Aldrich (Germany) at a concentration of 2 mg/L and 10 mg/L in methanol, respectively. Stock solutions were prepared in 10 mL methanol. General purpose reagent titanium (IV) oxide (Riedel-De Haen) was used as a source of titania nanoparticles; ferrous chloride and platinum chloride were purchased from Merck, Germany. Other chemicals including GC grade methanol, dichloromethane (DCM), and wood based granular and powdered activated carbon (Norit C1A) were procured from Acros, Organics, USA.

Synthesis of Nanoparticles.
Liquid impregnation method was used to prepare metal doped titania nanoparticles [19] as described in Table 1.

Scanning Electron Microscopy of Adsorbents. SEM images
were measured using JEOL JSM 6460 scanning electron microscope. Microscope operated at an acceleration voltage of 5, 10, and 15 kV and filament current of 60 mA. SEM was performed to find the surface morphology, topography, microstructure, and composition of adsorbents. The porous texture characterization of the samples was obtained by physical adsorption of gases. From N 2 adsorption data at −196 ∘ C (Autosorb-6B apparatus from Quantachrome [20]) the specific BET surface area ( BET ) was determined by applying the Brunauer-Emmett-Teller (BET) equation.

Analytical Method.
Geosmin and 2-MIB were analysed using solid phase microextraction coupled with gas chromatograph-flame ionized detector (SPME-GC-FID). One gram of sodium chloride was placed in a 15 mL vial containing 3 mL of the aqueous sample. The vial was sealed with Teflon-lined septum. The samples were heated to 65 ∘ C, and the SPME fiber (divinylbenzene-carboxenpolydimethylsiloxane SPME fiber) was inserted into heads space for 15 min equilibrium adsorption period. The fiber was then withdrawn from the sample and injected into the injection port of Shimadzu 2010 series Gas Chromatograph coupled with FID. The column used was fused silica capillary column with a length of 30 m, inner diameter of 0.32 mm, and wall thickness of 0.5 m. The column temperature was set at 60 ∘ C for 1 min and then raised to 160 ∘ C @ 15 ∘ C/min, held at 160 ∘ C for 1 min, then raised to 200 ∘ C @ 10 ∘ C/min, again held at 200 ∘ C for 1 min, and finally temperature was raised to 260 ∘ C and held constant for 1 min. Total program time was 16.67 min. Detector temperature was 280 ∘ C while injector was set at 300 ∘ C. Split ratio was zero. Nitrogen was the makeup gas and Helium was used as carrier gas. Total flow rate was 6.6 mL/min. The extracts were stable and could be stored for up to two months with no apparent loss of analytes and concentrations as low as 1 ng/L could be detected.

Synthetic Solutions Preparation.
Geosmin and 2-MIB solutions used in this study were prepared in the laboratory by dissolving stock solutions in ultrapure water. This study was aimed at testing adsorbents for their ability to adsorb earthymusty odorants for the concentrations which are present in lake water. For this reason, sampling was performed from Rawal Lake Filtration Plant (RLFP) in the month of July, when taste and odor episode in treated water was at their maximum levels. RLFP draws raw water from Rawal Lake, Pakistan. Its treatment processes include coagulation, flocculation, sedimentation, sand filtration, and disinfection, which were inadequate to remove taste and odor from water. Average concentration of Geosmin and 2-MIB in treated water samples was 690 ng/L and 678 ng/L, respectively; hence, synthetic solution with 700 ng/L of odorants was prepared for batch adsorption and column adsorption experiments. All reactors were wrapped with aluminium foil during nanoparticle experiments, to restrict photo catalytic activity and to ensure adsorption as the only mechanism of odorant removal.  best stirring rate (between 25 and 200 rpm), with optimised adsorbent doses and contact time.

Removal Efficiency Calculation.
To determine the removal efficiency of selected adsorbents, 100 mL of test solution with initial off-flavours concentration of 700 ng/L was taken in 250 mL conical flask at room temperature. 125 mg of the adsorbents (GAC, Fe, and Pt doped nanoparticles) were added in each test solution and stirred at orbital shaker at 100 rpm for one hour. All the samples were stored in dark brown vials, till final concentration analysis. Samples were subjected to filtration through prewashed 0.45 m filters, to remove any solid adsorbents residue, prior to the gas chromatographic analysis.
Amount of Geosmin and 2-MIB adsorbed (ng/g) was calculated using [21] where is the initial analyte concentration, is the residual equilibrium analyte concentrations (ng/L), is the volume of the solution (mL), and is the mass of adsorbent (mg) used in test solution. For kinetic studies, 500 mg of adsorbent doses was introduced into 1000 mL solutions with initial analytes concentration of 700 ng/L and placed at an orbital shaker at a rotation speed of 100 rpm. Samples were taken at predetermined intervals up to 120 min. The kinetic analysis was carried out at room temperature (25 ± 1 ∘ C).

Pilot Scale Column Experiments.
These experiments were designed to investigate efficiency of granular activated carbon and metal doped titania nanoparticles in fixed bed columns.
Column design was based on study by Tang et al. [19]. Pilot setup consisted of three glass columns each having internal diameter 1.5 cm and empty bed contact time was 10.5 min. Media depth for all columns was 12 cm. Column one is comprised of granular activated carbon having 0.56 mm mean particle size. Other two columns were filled by glass beads coated with Fe and Pt doped titania nanoparticles. Each column had glass wool at their base, as the supporting media. Flowrate through these columns was controlled using a peristaltic pump attached to the three-way manifold. 700 ng/L Geosmin and 2-MIB solutions were passed through these columns at 2 mL/min. Figure 1 shows schematic of the pilot column setup. were dipped in 10% hydrogen fluoride solution and covered for 24 hours. Next, glass beads were stirred with metal doped titania nanoparticles aqueous solution, at 250 rpm for half an hour. After oven drying at 110 ∘ C for 20 min these coated beads were shifted to furnace at 400 ∘ C. Finally these were washed with distilled water, dried, and stored in dark.

Breakthrough Curves.
Removal efficiency of the adsorption columns was represented by "break through curves" drawn from results, which showed that the concentration ratios ( / ) were a function of throughput volumes. Keeping in view the very low concentration of the pollutants, the breakthrough time and exhaustion time were set as / = 0.02 and / = 0.95, respectively. Samples for residual Geosmin and 2-MIB analysis were drawn after 10 bed volumes.

Column Desorption Study.
In this study, after exhaustion of column, desorption study was carried out by pouring 50 mL of 0.1 M NaOH solution as an eluent, which was known to be efficient for column recovery [22]. Moreover, the adsorbent column was washed with distilled water before the next adsorption-desorption cycles. The adsorption-desorption cycles were repeated trice using the optimized condition to check the sustainability of the column for repeated use.

Characterisation of Adsorbents.
For the direct examination of particle size and surface morphology of the samples SEM was used. All the particles were spheroid or oblate spheroid loosed and macrospores can be clearly seen in the SEM micrographs. Scans of nanoparticles at ×30,000 resolution are represented in Figure 2  Journal of Nanomaterials   indicating the high surface area for adsorption. BET surface areas for three adsorbents tested were 567 m 2 /g, 423 m 2 /g, and 274 m 2 /g for Pt-TiO 2 , Fe-TiO 2 , and activated carbon, respectively.

Adsorption Conditions
Optimization. Adsorption condition optimization results are demonstrated in Figure 3. The batch adsorption experiments were carried out by using various amounts adsorbent types.
Similar removal trends were observed for both Geosmin and 2-MIB removal, with increasing adsorbent doses. It was observed that, even on increasing the amount of adsorbent doses in the solution, the percentage removal of Geosmin and 2-MIB increased gradually and % removal plateaued at 125 mg/L for metal doped nanoparticles while 160 mg/L was found to be optimized GAC dosage. Adsorption capacities of the three adsorbent types at different contact times, when all other variables are constant, were determined. The maximum adsorption capacity of Geosmin and 2-MIB was obtained within contact time of 60 min, with optimized adsorbents dosage for all adsorbents studied. After optimizing PAC dosage and contact time final experiments were performed to explore optimum mixing rate. Results illustrate that stirring rate had remarkable effects on rate of adsorption as with the increase in speed contact between adsorbent and adsorbate increases. Maximum adsorption of Geosmin and 2-MIB was observed at mixing rate of 100 rpm using optimized adsorbent dozes, within a contact time of 60 min.

Removal Efficiency of Adsorbents.
Removal efficiencies of the selected adsorbents are given in Table 2. Results indicate that removal efficiencies of metal doped titania particles were much higher, as compared to Granular activated carbon. Overall, Pt doped titania nanoparticles showed the best removal efficiency towards Geosmin and 2-MIB.

Adsorption Isotherms.
Removal efficiencies of adsorbents were tested through isotherm studies. The linear form of the isotherm studies helped in better understanding of the adsorption phenomena. Freundlich and Langmuir equation explained adsorption of Geosmin and 2-MIB on different adsorbent concentrations, as discussed in the next sections.
Freundlich equation was used in this study as given below [23] Log = log + 1 log , where = equilibrium loading of Geosmin and 2-MIB on the GAC (ng/mg), = equilibrium concentration of Geosmin and 2-MIB in the water (ng/L), = adsorption capacity at unit concentration (ng/mg), 1/ = heterogeneity of adsorbent, and 1/ and were obtained from slope and intercept of linear plot between log and log .
From data stated in Table 3, it is clear that the Freundlich model is statistically valid for all adsorbents analyzed. To compare different adsorbents, usually the absorbent with the higher amount of adsorbate adsorbed, at the desired effluent concentration, is preferred for the particular application. A number of carbon physical and chemical properties affect adsorption toward a particular adsorbate. For Geosmin and 2-MIB, the hydrophobic compounds, the amount of surface area associated with the adsorbate is critically important. The Freundlich constants ( and ) given in Table 3 for each adsorbent and the resulting curves are shown in Figure 4  Mathematical form of Langmuir isotherm is given below where = equilibrium loading of Geosmin and 2-MIB on the GAC (ng/mg), = equilibrium concentration in the water (ng/L), and = maximum adsorption capacity (ng/mg). Values of and were obtained directly from intercept and slope of linear plot between / and . Langmuir constants are described in Table 4.
Langmuir isotherms curves for Geosmin and 2-MIB are shown in Figure 5. Values for adsorption coefficient and monolayer capacity are higher for Pt doped 8 Journal of Nanomaterials   The shape of the isotherm can be used to examine the favourability of the adsorbent. It was done by another dimensionless quantity, , given by the following equation: where is the dimensionless separation factor, is the initial Geosmin and 2-MIB concentrations, and is the Langmuir constant.
values ranged between 0.69 and 0.57 for Geosmin and 0.71-0.56 for 2-MIB. It showed favourable isotherms for both off-flavours. Values for both isotherm constants are higher in case of Geosmin which clearly suggest preferred adsorption of Geosmin on all adsorbents, compared to 2-MIB. Cook at al., 2004, also reported that Geosmin showed better adsorption than 2-MIB on activated carbon and this was attributed to the lower molecular weight and higher solubility of Geosmin [25].

Kinetic Isotherms .
A pseudo-second-order model (5) was used to describe the kinetics of adsorption [26]: where is the adsorption capacity (mg/g) at any time interval, is the adsorption capacity (mg/g) at the equilibrium, and (g mg −1 min −1 ) is the pseudo-second-order rate constant.
Journal of Nanomaterials   Integrating (5) with the following boundary conditions: = 0 to and = 0 to , gives Equation (6) can be linearized as follows: Thus, the parameters and can be obtained from the intercept and slope of the plot of ( / ) against .
The kinetic data obtained from batch adsorption tests was analyzed by using the pseudo-second-order model. Figure 6 shows the linearized plots of the results. The kinetics of Geosmin and 2-MIB adsorption were studied in batch adsorption tests, with three different adsorbents. Results clearly indicate that the removal efficiency is a function of contact time; moreover, maximal and values were obtained with Pt doped nanoparticles. All pseudo-secondorder parameters are listed in Table 5. The data demonstrate a good compliance with the pseudo-second-order equation, as the regression coefficients for the linear plots were higher than 0.98 for all the adsorbents tested.

Column Experiments Results
. Column efficiencies were determined in terms of breakthrough and column exhaustion times. At the beginning, removal through columns was very efficient as the adsorbents were fresh with all of their adsorption sites available. With the passage of time, some of the adsorption sites got exhausted and effluent concentrations started rising. After all the adsorption sites were exhausted, the inlet and the outlet concentrations became nearly the same. Column 1 having granular activated carbon was not found very effective either for Geosmin or 2-MIB. It attained early breakthrough and exhaustion points. Figure 7 illustrates breakthrough curves, breakthrough points, and exhaustion points for Geosmin and 2-MIB, respectively.
Overall comparison of adsorbents indicated that Ptdoped titania nanoparticles were most efficient for Geosmin and 2-MIB removal. Geosmin removal was a bit higher than 2-MIB in this case as well.

Column Desorption.
For a viable sorption process, columns desorption and regeneration are important. The exhausted 12 cm bed volume columns (for all the three types of columns) were successfully regenerated using 0.1 M of NaOH solution. The results ( Figure 8) show that, on average, the columns could be used again by losing only 8% of their adsorption ability in successive cycle.

Geosmin and 2-MIB Removal Using RLFP Treated Water.
The above-stated experiments were performed using synthetic solution of Geosmin and 2-MIB. Finally, experiments were conducted with the RLFP treated water samples. Average concentration of Geosmin and 2-MIB in composite water samples was 690 ng/L and 678 ng/L, respectively. Water quality data for water from RLFP is given in Table 6.
A comparison of taste and odor removal efficiency using adsorption and Column tests is presented in Figure 9. According to the results better removal was achieved with synthetic water as compared to the sample of treated water from RLFP. This difference can be attributed to organic matter and several interfering species in the treated water samples ( Table 6). High removal efficiencies of 97-99% for Geosmin and 96-98% for 2-MIB from both synthetic and treated water samples confirm effectiveness of metal doped nanoparticles.

Conclusions
The present experimental results suggest that metal doped titania nanoparticles demonstrate significant adsorption potential for the accelerated removal for earthy-musty odor producing compounds in the drinking water. Study shows that metal doping increases BET surface area, thus enhancing efficiency of titania to adsorb Geosmin and 2-MIB. The concentrations examined in this study are typically the levels that cause odor problems in lake water. Whilst activated carbon is still useful for off-flavours removal, metal doped titania nanoparticles are far more effective. Fe-TiO 2 showed 95% and 93% Geosmin and 2-MIB removal. Pt-TiO 2 was found to be most efficient; it removed 98% of Geosmin and 97% of 2-MIB as compared to 82% and 73% Geosmin and 2-MIB removal by most widely used granular activated carbon. Smaller size, extremely large surface area (567 m 2 /g), and more active adsorption site make Pt-TiO 2 as superior adsorbent. Further work is also being undertaken to refine the process for implementation in commercial applications.

Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.

Introduction
Transparent materials with high refractive indices have been extensively required for numerous applications, including optical devices, lenses, antireflective coatings, and waveguides [1][2][3]. In these applications, synthetic polymers have been employed as optical materials exhibiting excellent formability and tunable chemical and physical properties. However, refractive indices of common polymers are limited to the range from 1.3 to 1.7 [4]. Recently, functional polymer-based hybrids incorporating inorganic fillers with high refractive indices have attracted increasing attention for their ability to improve the optical properties of conventional polymers [5]. TiO 2 is an attractive inorganic component for preparation of hybrid materials because of such characteristics as its high refractive index ( = 2.5-2.7), nonadsorption of visible light, nontoxicity, and low cost [6]. One important issue in the preparation of transparent TiO 2 /polymer hybrids is the suppression of Rayleigh scattering at the interfaces between inorganic components and polymer matrices. In general, the diameters of inorganic fillers should be smaller than 40 nm, which is one-tenth the minimum wavelength of visible light, to suppress Rayleigh scattering [7]. Thus, TiO 2 nanoparticles (NPs) of desirable size, below 40 nm, as high refractive inorganic fillers, should be employed in the preparation of transparent polymer-based hybrids. In addition, homogeneous dispersion of TiO 2 NPs in a polymer matrix is also required to maintain the transparency after formation of the hybrids. TiO 2 NPs easily aggregate in hydrophobic polymer matrices because their surfaces are covered with hydrophilic hydroxyl groups [8]. Surface modification of TiO 2 NPs with organic groups is consequently an important technique for improving their affinity with polymer matrices [7] to suppress their aggregation.
Carboxylic acids [9,10], silane coupling reagents [11,12], and phosphorus coupling reagents [13,14] have been generally used for surface modification of various metallic oxides including TiO 2 because of their high reactivity to the surfaces. In the case of inorganic fillers in polymer-based hybrids for optical applications, monolayered stable surface modification is strongly desired to maintain their uniform and stable dispersion in the polymer matrix. For modification with carboxylic acids, the drawback is instability of Ti-O-C bonds with respect to hydrolysis. It is well-known that silane coupling reagents undergo inter-and intramolecular condensation (namely, homocondensation) in the presence of a trace of water to form multilayers comprising silane coupling molecule moieties on their surfaces. Phosphorus coupling reagents react with the TiO 2 surfaces easily to form Ti-O-P bonds, on the other hand, which are stable with respect to hydrolysis, and no homocondensation of phosphorous coupling reagents proceeds under mild conditions [15]. Among phosphorus coupling reagents, phosphoric acid esters are attractive, since they can be easily prepared from commercially available phosphoric acids and alcohols [16]. Another advantage is the flexibility of their structures, leading to facile introduction of desirable functional groups, which play an important role in their compatibility with both polymer matrices and solvents [17][18][19][20][21].
Poly(methyl methacrylate) (PMMA) is a typical optical polymer exhibiting high transparency and excellent formability. Since the refractive index of PMMA is relatively low for optical applications ( = 1.49) [22], incorporation of TiO 2 NPs in PMMA matrices has been investigated extensively for preparation of PMMA-based hybrid materials exhibiting high refractive indices [10,[23][24][25][26][27][28][29][30][31][32][33][34]. Yuwono et al. prepared transparent TiO 2 /PMMA hybrid films via an in situ sol-gel route [24]. The refractive index of the hybrid film increased to 1.780 with incorporation of 60 mass% TiO 2 into PMMA. Lee and Chen also investigated the in situ sol-gel route for preparation of hybrids using titanium(IV) -butoxide and PMMA-based polymers bearing trimethoxysilyl residues [25]. The refractive indices of the hybrid films increased linearly from 1.508 to 1.867 with TiO 2 contents in the range from 2.9 to 70.7 mass% TiO 2 . In both studies, the in situ sol-gel route was used to generate anatase TiO 2 NPs ( = 2.5) in the polymer matrices. In the in situ method, it is difficult to control the precise amount of TiO 2 content and the dispersion of NPs in PMMA. An ex situ route in which inorganic fillers are incorporated into polymer matrices after surface modification, on the other hand, enables easier adjustment of the refractive indices of TiO 2 /PMMA hybrids than the in situ route. Thus, the ex situ route enables us to achieve excellent dispersion of well-defined rutile TiO 2 NPs ( = 2.7) into polymer matrices.
In this study, we report the preparation of PMMA-based hybrid thin films by incorporating TiO 2 NPs modified with oleyl phosphate (OP) via the ex situ route. It has been demonstrating that an oleyl group is an attractive group for surface modification of TiO 2 NPs, though stability is not sufficient for modifications with oleic acid and oleylamine [35] with respect to hydrolysis. The surfaces of the TiO 2 NPs were modified with OP, and a stable dispersion of the resulting modified TiO 2 NPs in organic solvent was achieved [36]. It should be noted that stability of resulting Ti-O-P bonds is important when TiO 2 NPs are used as nanofillers via the ex situ route. In this study, an aqueous dispersion of rutile TiO 2 was used as a starting material and was reacted with OP to achieve surface modification. The resultant OP-modified TiO 2 NPs were dispersed in a PMMA matrix to prepare TiO 2 /PMMA hybrid thin films, and their optical properties were investigated as a function of the TiO 2 content.

Materials.
A 15 mass% aqueous TiO 2 dispersion and OP (monoester and diester mixture) were kindly supplied by Sakai Chemical Co., Ltd. (Osaka, Japan). All reagents were used as received without further purification. Poly(methyl methacrylate) (PMMA) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan).

2.2.
Analyses. Infrared (IR) spectra were recorded on a JASCO FT-IR-460 Plus spectrometer using the KBr disk technique. X-ray diffraction (XRD) patterns were obtained with a Rigaku RINT-2500 diffractometer (monochromated CuK radiation). Solid-state 13 C and 31 P nuclear magnetic resonance (NMR) spectroscopy was performed using a JEOL CMX-400 spectrometer at 99.55 and 160.25 MHz, respectively. Solid-state 13 C NMR spectra were obtained with cross-polarization (CP) and magic angle spinning (MAS) techniques (pulse delay 5 s, contact time 1.5 s, spinning rate 8 kHz). Solid-state 31 P NMR spectra were obtained with a MAS technique only (pulse delay 20 s, spinning rate 8 kHz). Thermogravimetric analysis (TG) was performed with a PERKIN ELMER TGA7 thermobalance in the temperature range from 30 to 800 ∘ C at a heating rate of 10 ∘ C/min under an air flow. A transmission electron microscopy (TEM) image was obtained with a JEOL JEM-1011 microscope operating at 100 kV accelerating voltage. The distribution of particle sizes was measured by dynamic light scattering (DLS) technique using a NIKKISO Nanotrac Wave-EX150 at a wavelength of 780 nm. In obtaining TEM image of TiO 2 /PMMA hybrids, the sample was prepared on a Si wafer and an ultrathin section of the film was cut by a focused ion beam instrument (SEIKO EG&G SMI 2050). Ultraviolet-visible (UV-Vis) spectra were recorded on a JASCO V-630 spectrometer in the transmittance mode. The refractive indices were determined using an Otsuka Electronics FE-3000 refractive film thickness monitor.  TiO 2 NPs. OP moiety was covalently bound to the surface of TiO 2 NPs using an aqueous dispersion diluted with 15 mL of methanol, based on a previous study [36]. Briefly, 0.49 mL of OP dissolved in 20 mL of methanol was added to the TiO 2 dispersion and stirred Journal of Nanomaterials 3 for 18 h at room temperature. The precipitate was separated by centrifugation at 3500 rpm for 10 min and redispersed in 5 mL of toluene. The precipitate was washed twice with 15 mL of ethanol by the dispersion-precipitation process using super-centrifugation at 13500 rpm for 5 min to remove unreacted OP. The precipitate was then dispersed in 10 mL of toluene, following evaporation of the solution in vacuo. Finally, the remaining OP-modified TiO 2 NPs (OP TiO 2 ) were redispersed in 20 mL of toluene.

Preparation of TiO 2 /PMMA Hybrid Thin Films.
Prescribed amounts of the OP TiO 2 /toluene dispersion and toluene were added to a vial containing 200 mg of PMMA. In all the PMMA-based hybrid samples (0-20 vol% TiO 2 content), the total dispersion volume was 10 mL. After stirring for 5 h at 40 ∘ C, a dispersion of OP TiO 2 and PMMA was obtained. Hybrid thin films were prepared by spin-coating of the dispersion on glass substrates at 2000 rpm for 20 s and subsequent heating at 100 ∘ C for 1 h.

Characterization of TiO 2 NPs.
The FTIR spectra of OP TiO 2 , OP, and bare TiO 2 NPs are shown in Figure 1. In the spectrum of OP TiO 2 , absorption bands assignable to oleyl groups are present: stretching vibrations of -HC=CHappear at 3001 cm −1 , ] (CH 3 ) at 2954 cm −1 , ] (CH 3 ) at 2871 cm −1 , ] (CH 2 ) at 2921 cm −1 , ] (CH 2 ) at 2851 cm −1 , and (CH 2 ) at 1458 cm −1 . These bands are also present in the spectrum of OP [39]. Notable differences among these IR spectra are observed in the P-O stretching region. Although a ](P=O) absorption band at 1151 cm −1 is clearly observed in the spectrum of OP, the corresponding band disappears in the spectrum of OP TiO 2 . P-O stretching absorption bands, which are not observed in the spectrum of TiO 2 NPs, are present at 1029 and 1008 cm −1 in the spectrum of OP TiO 2 . These observations suggest that the OP moiety is bound to the TiO 2 surface [13].
The solid-state 13 C CP/MAS NMR spectrum of OP TiO 2 is shown in Figure 2. The carbon signals of free OP in the liquid-state 13 C NMR spectrum (Figure 2(a) The solid-state 31 P MAS NMR spectrum of OP TiO 2 is shown in Figure 3. In the liquid-state 31 P NMR spectrum of free OP (Figure 3(a)), the signals of free mono-and diesters are observed at 4.3 and 3.1 ppm, respectively. In the OP TiO 2 , the signals are observed at 4.5, 2.1, and −1.3 ppm after the modification reaction. Guerrero et al. reported that the NMR signal of organophosphorous compounds shifted upfield after dehydration condensation between Ti-OH and P-OH [13]. In addition, the phosphorous signals shifted downfield with the coordination of the phosphoryl oxygen with Lewis acid sites because of an increase in the electrophilicity of phosphorous atoms. The disappearance of the ](P=O) absorption band from FTIR suggests that the phosphoryl oxygen in OP may react preferentially with the Lewis acid sites of the TiO 2 surfaces. Thus, the signal at −1.3 ppm is assignable to the tri-and bidentate environments of mono-and dioleyl phosphate moieties, respectively. The signal at 2.1 ppm is likely to be due to the bidentate environment of a monooleyl phosphate moiety produced by a reaction involving P=O and Lewis acid sites. The formation of monodentate binding by a reaction between Lewis acid sites on TiO 2 NPs and In the TEM image of OP TiO 2 ( Figure S2), TiO 2 NPs can be observed as a large number of dark spots in the range of 4-20 nm. Almost all of the TiO 2 NPs are individually divided by border lines with a few nm thickness, and they appear to correspond to OP layers on the surface. Considering the length of the oleyl groups (=2.3 nm) [40], OP moieties could be present as monolayers on the surfaces of the TiO 2 NPs. The particle size distribution is determined by DLS using dispersion of OP TiO 2 in toluene ( Figure S3 in Supplementary Material). The size distribution of the OP TiO 2 NPs is shown to be relatively narrow with the monodispersion, and the median size is 16.1 nm. In contrast, bare TiO 2 NPs exhibit a 4.9 nm median size for the starting aqueous dispersion. Since individual particle shapes of OP TiO 2 are clearly observed in the TEM images, aggregation comprising a limited number of TiO 2 NPs could be modified with OP. Thus, it is demonstrated that OP-modified TiO 2 NPs are successfully prepared as nanofillers for optical applications because their size can be controlled at below 40 nm to suppress Rayleigh scattering effectively.

Optical Properties of OP TiO 2 /PMMA Hybrid Thin
Films. OP TiO 2 /PMMA hybrid thin films are transparent even though they contain 20 vol% TiO 2 NPs ( Figure S4 in  for suppression of Rayleigh scattering therefore leads to high transparency of the OP TiO 2 /PMMA hybrid thin films. The thicknesses of the OP TiO 2 /PMMA hybrid thin films are shown in Figure 6. The film thickness increases in the range of 150-260 nm with increases in TiO 2 volume fraction. In general, the thickness of spin-coated polymer film depends mainly on the viscosity and density of a polymer solution at a certain spin speed [41]. TiO 2 NPs have a greater density than PMMA, and PMMA is diluted with a low viscosity OP TiO 2 dispersion. The effect of density seems to dominate and the addition of the OP TiO 2 dispersion to the PMMA solution results in the formation of a thicker film than neat PMMA. The refractive indices of the OP TiO 2 /PMMA hybrid thin films at 633 nm are plotted against the TiO 2 volume fraction, as shown in Figure 7. The refractive index of the neat PMMA film is 1.49, and the indices of OP TiO 2 /PMMA hybrid thin films increase linearly with increases in the TiO 2 volume fraction, suggesting that the refractive indices of hybrid thin films can be controlled by adjusting the TiO 2 volume fraction. At 20 vol% TiO 2 content, the refractive index rises to 1.86. This value is comparable with those of flint glass ( = 1.4-2.0) [42] and sapphire crystal ( = 1.77) [43], suggesting the potential usefulness of the OP TiO 2 /PMMA hybrids in the present study for various optical applications.
In a previous report on TiO 2 /PMMA hybrids, Lee and Chen reported the preparation of anatase TiO 2 /PMMA hybrid films using titanium butoxide and a PMMA derivative bearing trialkoxysilane in the side chains via the in situ sol-gel route [25]. The refractive indices of the hybrid films increased linearly between 1.508 and 1.867 for 2.9-70.7 mass% TiO 2 content. In the present study, rutile TiO 2 NPs, which have a higher refractive index than that of anatase, were dispersed in PMMA via the ex situ route. The refractive indices of the hybrid thin films in the present study are slightly higher than those in the previous report using anatase in the TiO 2 content range between 0.9 and 21.6 vol%. In this study, OP used as a surface modifier has a relatively low refractive index ( = 1. 45-1.48), and surface modification with OP should decrease the volume fraction of TiO 2 NPs compared with the hybrid film comprising only PMMA and TiO 2 in the previous report. Thus, the refractive indices of hybrid thin films using OP TiO 2 were similar to those in the previous report in spite of the dispersion of rutile TiO 2 NPs.

Conclusions
OP was covalently bound to the surfaces of TiO 2 NPs to prepare TiO 2 /PMMA hybrid thin films with high refractive indices. After the surface modification of TiO 2 NPs, IR and the solid-state 31 P MAS NMR results revealed the formation of Ti-O-P bonds, and the oleyl groups on the surfaces played an important role in suppressing the aggregation of TiO 2 NPs in toluene. The particle sizes of the OP-modified NPs were smaller than 40 nm and were homogeneously dispersed in PMMA. The OP TiO 2 /PMMA hybrid thin films exhibited excellent optical transparency, and their transmittance values were larger than 89.0% at 633 nm. The refractive indices of the hybrid thin films increased with increases in the volume fraction of TiO 2 NPs, and their refractive index reached 1.86 at 20 vol% TiO 2 content. The present results demonstrate that the surface modification of TiO 2 NPs is an excellent method for the preparation of TiO 2 fillers applicable to the ex situ method for preparing various TiO 2 /polymer hybrids.

Introduction
Because of excellent mechanical and corrosion resistance properties, titanium-(Ti-) based materials have been widely used in making dental and orthopedic implants [1,2]. However, immune responses mediated by inflammatory cells have been found to significantly impact the biocompatibility and function of implanted medical devices or prostheses and can ultimately precipitate their clinical failure [3,4]. The paradigm for developing an ideal implant material has been shifted using from inert to immunomodulatory materials, emphasizing the importance of immune cells in the evaluation of a candidate material [5]. Macrophages, the major effector cells in the immune response to implants, have been reported to determine the long-term immune response and outcome of an inflammatory reaction [6,7]. Modulation of adhesion and activation of macrophages with anti-inflammatory drugs can reduce immune responses [8]. However, in the clinic the long-term systematic use of drugs can compromise host defenses, leading to impaired immune responses and an increased potential for infection [9]. To avoid this problem, appropriate surface modifications of implants have been considered as an alternative approach to inhibit or attenuate inflammation reaction temporarily for achieving high success rates [10][11][12].
Recently, several types of surface modification methods have been introduced to slow macrophage-mediated immune responses triggered by Ti-based implants, including coatings, sand blasting, and acid etching [13][14][15]. However, delamination of the coating was found to sometimes occur during implant placement or in load bearing situations, which was probably caused by poor stability of the coating bound to the substrate [13]. Additionally, normal bone mineralization can be inhibited by the release of constituents, such as Al 2 O 3 or Ta 2 O 5 , from the material when they were modified by sand blasting [14]. The etched implant surface has been shown to be capable of promoting osseointegration because of hydrophilicity [15]. Furthermore, surface hydrophilicity of Ti-based implants has been established to modulate osteogenic cell adhesion, proliferation, and differentiation [16]. Therefore, it can be hypothesized that incorporating surface hydrophilicity might be a promising alternative for regulating macrophage behavior. Nevertheless, little attention has been paid to the immunoregulatory effect of surface hydrophilicity of titanium on the macrophage behavior and data about this topic has been needed.
Herein, the purpose of this work is to investigate the effects of the hydrophilicity of Ti surface on macrophage behavior in vitro. Two kinds of Ti surfaces with different hydrophilicity including polished Ti surface (Ti-polished) and etched Ti surface (Ti-H 2 O 2 ) were used in present study. The morphology, roughness, and wettability of the Ti surfaces were firstly characterized. The morphology of macrophage RAW264.7 cells on the Ti surfaces was observed by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Cell proliferation was assessed by CCK-8 assay. The activation status of macrophages was quantified by measuring TNF-and IL-10 secretion by ELISA assay. Subsequently, the underlying mechanism of modulation of surface hydrophilicity on the immune response was explored by both immunofluorescence and western blotting analysis.

Samples Preparation.
Ti disks with 20 mm in diameter and 2 mm thick were used to prepare two kinds of different Ti surfaces including hydrophilic (Ti-H 2 O 2 ) and polished (Tipolished). Ti-polished surfaces were prepared by polishing with sand paper from 400 to 1500 grit. Ti-H 2 O 2 surfaces were obtained via immersing the Ti disks in 10% H 2 O 2 solution for 24 h.

Sample
Characterization. The surfaces were imaged by scanning electron microscope (SEM, Hitachi S-4800, Tokyo, Japan) at a 15 kV accelerating voltage. Atomic force microscopy (AFM, SPA-300HV, Seiko, Tokyo, Japan) was performed to measure the surface roughness parameter Ra over a 10 × 10 m area. X-ray photoelectron spectroscopy (XPS, Thermo V6 Escalab 250, West Sussex, UK) was carried out to analyze element composition of these Ti surfaces. Surface water contact angles of Ti materials were measured using a drop shape analysis system (JY-82A, China).

Scanning Electron Microscopy (SEM).
Cells were incubated on Ti disks for 1, 3, and 7 days. After incubation, cells were fixed with 2.5% glutaraldehyde and incubated with 0.18 mol/L sucrose solution. Samples were dehydrated through a graded ethanol series (30-100%) and air-dried.

Confocal Laser Scanning Microscopy (CLSM).
Cells were cultured on Ti disks for 7 days. Cells were then fixed with 4.0% paraformaldehyde, treated with Alexa Fluor 546phalloidin (50 g/mL) for 1 h at room temperature, and then stained with DAPI for 10 min according to the manufacturer's directions. Images of the stained cells were acquired using CLSM (Zeiss, LSM 780, Oberkochen, Germany).

Cell Proliferation Assay.
To assess macrophage activity on Ti disks, cell counting kit-8 (CCK-8) was performed on days 1, 3, 5, and 7. According to the manufacturer's directions, CCK-8 solution was added to each well of the plate at a ratio of 1 : 10 and then was incubated for 2 h in an incubator. After incubation, absorbance was measured using a microplate reader (Bio-Rad, Hercules, CA, USA).

Cell Cytokine Secretion Examination.
To analyze cytokine secretion, cells were cocultured with Ti disks and supernatants were collected at days 1, 3, 5, and 7. The levels of TNFand IL-10 in the supernatants were quantified using an ELISA kit (R&D Systems, Minneapolis, MN, USA) according to the manufacturer's directions.

Assessment of NF-B.
In accordance with the manufacturer's directions, cells grown on Ti disks for 3 days were fixed, blocked, and incubated with anti-NF-B p65 primary antibody at 4 ∘ C overnight. Then samples were treated with an appropriate Cy3-conjugated secondary antibody for 1 h and stained with DAPI for 5 min at room temperature. A confocal microscopy (Zeiss, LSM 780, Oberkochen, Germany) was used to observe NF-B activation.

Statistical
Analysis. Data were analyzed using an independent Student's -test and are presented as means ± standard deviation (S.D.). < 0.05 was considered to indicate a statistically significant difference.

Results and Discussion
Given the importance of macrophages in bone dynamics, some studies have investigated interactions between bone substitute biomaterials (e.g., bioceramics, polymers, or titanium) and macrophages [11,17,18]. However, in the field of implant materials research, the immunoregulatory effect of surface hydrophilicity of titanium on the macrophage Journal of Nanomaterials behavior and its underlying mechanism still remain largely unknown. To devise a strategy for Ti surface design for clinical regenerative therapies, additional data regarding the effects of surface hydrophilicity on immune responses and the underlying mechanisms involved will be required. The results in present study demonstrated that a more hydrophilic Ti surface might more favorably attenuate macrophage immune responses via NF-B-TNF-signaling.
The SEM observation clearly indicated that Ti-H 2 O 2 possessed a nanostructure with a uniformly distributed nanoparticle compared to Ti-polished surface (Figure 1(a)). As shown in Table 1, AFM images showed that the two kinds of Ti surfaces shared similar roughness that Ra of Ti-polished and Ti-H 2 O 2 were 23.25 ± 5.89 nm and 87.13 ± 23.72 nm. While Ti-H 2 O 2 surface (35.21 ± 3.08 ∘ ) was found to be more hydrophilic than the Ti-polished surface (64.46 ± 2.384 ∘ ). The quantified element composition given by XPS indicated a little increment in the O1s peak for the hydrophilic Ti-H 2 O 2 surface compared to hydrophobic Ti-polished surface, supported by atomic ratios of oxygen ( Figure 2 and Table 2). The results were consistent with previous report that O 2 − and -OH group would be generated on surface when treated with H 2 O 2 solution [19]. Thus, the two kinds of Ti surface models used in our work presented significantly different surface hydrophilicity and similar roughness.
The proliferation rate of cells on the Ti surfaces partially reflects biocompatibility of the materials, which is essential for implant survival. Mouse macrophage RAW264.7 cells viability on different Ti surfaces was measured by CCK-8 assay for 1, 3, 5, or 7 days (Figure 3). Overall, relative high hydrophilic Ti-H 2 O 2 surface resulted in enhanced cell proliferation. Although cell viabilities on different surfaces  were similar after 1 day of culture, significantly higher cell viabilities were found on hydrophilic Ti-H 2 O 2 surfaces compared to hydrophobic Ti-polished surfaces after culture for 3 days. Additionally, this trend continued to be maintained  at days 5 and 7. These data demonstrate that hydrophilic Ti-H 2 O 2 surfaces are more biocompatible than hydrophobic Tipolished surfaces. The excellent biocompatibility of Ti-H 2 O 2 surfaces for macrophage may be benefit for wound healing. Previous studies also indicated that improved wound healing was associated with enhanced macrophage recruitment [20]. The influence of materials on macrophage morphology played an important role in the assessment of materials immunoregulatory property. Cell shape changes have been reported to be related to different functional states of cells, such as proliferation, nuclear organization, and differentiation [21][22][23]. Additionally, cell shape was found to modulate the phenotypic polarization of macrophages. In this study, we clearly observed different initial cell adhesion behaviors of macrophages on two types of Ti surfaces after 1 day of culture (Figure 4(a)). More filopodia were observed on hydrophobic Ti-polished surfaces than on those of hydrophilic Ti-H 2 O 2 surfaces. Filopodia are specialized macrophage adhesion structures that can form in the early stages of cell adhesion and consist of punctuate F-actin on plasma membrane extensions [24], which facilitate attachment and migration [25,26]. It has been proposed that a round shape might indicate a resting nonactivated macrophage, whereas the presence of multiple filopodia represents an activated macrophage [26]. Therefore, the difference in filopodia numbers clearly suggested that macrophages would be more prone to become activated on the hydrophobic Ti-H 2 O 2 surface than on the hydrophilic Ti-polished.
After 7 days of culture, SEM observation showed that several suspected FBGCs formed by cell fusion appeared on Ti-polished surfaces, whereas only elongated cells with numerous cellular projections were present on Ti-H 2 O 2 surfaces (Figure 4(a)). Furthermore, immunofluorescence staining of cytoskeleton organization and CLSM analysis of nuclei confirmed that more FBGCs formed on hydrophobic Ti-polished surfaces than on hydrophilic Ti-H 2 O 2 surfaces (Figure 4(b)). This phenomenon was consistent with previous work of Collier et al. who reported that hydrophilic surfaces exert a robust inhibitory effect on FBGCs formation [25]. The formation of FBGCs in response to implanted biomaterials or medical devices, also known as a foreign body reaction, has been implicated in the degradation and cracking of implanted material and in initiating the formation of fibrous capsules that surround the implanted biomaterials [27,28]. FBGCs with multiple nuclei might release profibrotic factors, including transforming growth factor beta (TGF-) and platelet-derived growth factor (PDGF), which could in turn activate fibroblasts and endothelial cells to facilitate the subsequent fibrotic encapsulation and inflammatory response [21,29]. Thus, increased FBGCs formation on hydrophobic Tipolished surfaces might imply a more serious inflammatory response of macrophages, and hydrophilic Ti-H 2 O 2 surfaces might be more beneficial for inhibiting inflammation.
Notably, the two types of Ti surfaces also yielded macrophages with significantly different cell morphologies. More macrophages with spindles appeared on Ti-H 2 O 2 surface, whereas almost all macrophages exhibited a round cell appearance with multiple filopodia on Ti-polished surfaces (Figure 4(b)). These results suggested that Ti-H 2 O 2 surfaces might promote the polarization of macrophages towards a prohealing macrophage phenotype, implying that they can exert anti-inflammatory activities and improve wound healing responses. Our results were also consistent with previous report that prohealing cells exhibited a more elongated shape compared to proinflammatory cells [30]. By contrast, the round cell morphology on Ti-polished surfaces might be a phenotypic marker of proinflammatory macrophages, leading to fibrotic responses and inflammation around any materials.
It has been well established that the immune system is closely related to skeletal development and that they share many common cytokines [31]. Those cytokines can mediate macrophage-dependent inflammatory cascades that are triggered by innate immune activation and act to influence implant integration and osteogenesis [32][33][34][35]. Proinflammatory cytokines can enhance osteoclast differentiation and resorbing activity and inhibit osteoblast activity and bone formation, whereas anti-inflammatory cytokines exert opposite effects [33][34][35]. TNF-, a key proinflammatory regulator that is mainly released by stimulated macrophages, contributes to fibrosis via TNF receptor 2, which then acts through extracellular signal-regulated kinase 1/2 (ERK1/2) to stimulate cell proliferation and inhibit collagen degradation [33]. TNF-was also found to activate the ERK-specific mitogen-activated protein kinase (MAPK) pathway, leading to increased TGF-1 production, which can facilitate fibrosis Journal of Nanomaterials [36]. The ELISA quantitative analysis showed that TNFsecretion was significantly increased on hydrophobic Tipolished surfaces compared to hydrophilic Ti-H 2 O 2 surfaces at all time points (Figure 5(a)). Production of the proinflammatory cytokine TNF-peaked on both Ti surfaces after 1 day of culture and then decreased with the time, suggesting that a robust inflammatory response occurred at an early stage of culture. IL-10 is a typical anti-inflammatory cytokine that plays a critical role in suppressing proinflammatory cytokines [35]. In contrast to the pattern of proinflammatory cytokine secretion, production of the anti-inflammatory cytokine IL-10 increased in a time-dependent manner on both Ti surfaces ( Figure 5(b)). These results indicated that the surface wettability of Ti implants has a profound effect on the immune response of macrophages. Our results are also in line with previous studies. Lee et al. reported that hydrophilic titanium elicited fewer inflammation reactions [37]. Tsang et al. demonstrated that hydrophilic carbon nanofibers (CNFs) caused a weaker macrophage-dependent inflammatory response than hydrophobic CNFs [38]. Taken together, hydrophilic Ti-H 2 O 2 surfaces may potentially be better osteogenesis-promoting materials, due to the ability to inhibit the production of proinflammatory cytokines and promote anti-inflammatory molecule secretion.
The confirmation that macrophage behavior can be affected by different hydrophilic Ti surfaces prompted us to further explore the possible mechanism underlying the hydrophilicity-dependent inflammatory response to implanted materials. Activation of the NF-B pathway, a key intracellular regulator of inflammatory signaling, has been known to promote the secretion of proinflammatory cytokines, including TNF-and IL-1 [30,31]. In our work, the significantly different levels of TNF-production on these two Ti surfaces implied that the NF-B pathway might be critical in the hydrophilicity-dependent immune response. The NF-B family consists of five members-p50, p52, p65 (RelA), c-Rel, and RelB-that share an N-terminal Rel homology domain (RHD) [32]. Most of these proteins can form different NF-B homodimers and heterodimers. Among the heterodimers, p50/p65 heterodimers are considered to be key mediators of gene expression [33]. Therefore, p65 was chosen in present research as the mark for NF-B expression. It can be seen from immunofluorescence images and western blotting results in our work that more NF-B p65 was present in macrophages cultured on Tipolished surfaces than in those cultured on Ti-H 2 O 2 surfaces ( Figure 6). Combined with inflammatory cytokines secretion results, we can conclude that NF-B signaling may be involved in the regulation effects of hydrophilic Ti surfaces on the acute phase inflammation response of macrophage. Conclusively, these findings could provide an approach to optimize the Ti implant materials for desired bone integration and orthopedic applications.

Conclusions
In this study, Ti surface hydrophilicity-dependent immune response and underlying mechanism were explored. The results showed that more hydrophilic Ti surface (Ti-H 2 O 2 ) yielded enhanced cell adhesion, increased proliferation, and less multinucleated cells formation. The hydrophilic Ti-H 2 O 2 surfaces that exhibited the inhibition in the production of pro-inflammatory cytokines and the promotion in the secretion of anti-inflammatory molecules may be more beneficial to serve as potentially osteogenesis-promoting materials. Moreover, it was revealed that hydrophilic Ti surface might be more favorable in attenuating macrophage immune response via NF-B signaling. These results may provide new insight in surface-designing of novel implant devices.

Introduction
Global environmental pollution and energy shortages are becoming increasingly serious problems [1,2]. The control of environmental pollution has become a major and urgent topic of concern. In 1972, Fujishima and Honda published the first article in Nature declaring that the semiconductor titanium dioxide crystal electrode has the ability to photocatalytically split water to produce hydrogen [3,4]. This discovery signaled the beginning of heterogeneous photocatalysis research in the area of semiconductors. Photocatalysis technology, as a representative of green chemistry, is widely applied in many areas, such as wastewater treatment, air purification, and solar energy transfer and storage [5][6][7]. Nanoscale titanium dioxide (nano-TiO 2 ) has become a preferred material for these applications due to its high catalytic activity and stable chemical properties and because it is cheap and nontoxic [8][9][10]. However, there are some disadvantages of using nano-TiO 2 , such as the high recombination rate of photoproduced electron-hole pairs, low quantum efficiency, and poor photocatalytic performance [9][10][11][12]. Nano-TiO 2 can only use the ultraviolet portion of the solar spectrum range (only 3-5% of the total range) due to a wide band gap (3.2 eV), which leads to low effective utilization of sunlight [13][14][15]. Researchers have used a variety of methods to modify nano-TiO 2 , including noble metal modification, compound semiconductor, dye sensitization, metal ion doping, and others [16][17][18][19]. With these modification methods, the recombination rate of photogenerated electron-hole pairs of nano-TiO 2 photocatalyst is decreased, and the photocatalysis efficiency and range of visible light that generates a response are increased [20][21][22][23]. Our research group found that modification by codoping with two elements can increase the visible light photocatalytic activity of nano-TiO 2 .
The main aim of the present study was to increase the visible light photocatalytic activity of nano-TiO 2 . The originality and significance of this study are described as follows.
(3) The effects of various parameters, such as the roasting temperature, roasting time, catalyst dosage, initial pH value, doping amount, and doping type on the photocatalytic activity were investigated. In conclusion, a new mechanism for metal doping of nano-TiO 2 was proposed.

Preparation of Nano-TiO 2
Photocatalysts. The preparation process for nano-TiO 2 photocatalysts is shown in Figure 1. First, 100 mL absolute ethanol and 5 mL glacial acetic acid were added to a 250 mL beaker. After magnetic stirring for 30 min, the pH value of the solution was adjusted to 2 using nitric acid. The obtained mixture was designated solution A. Then, 15 mL ethanol and different doses (doping percentage is described as mole ratio) of modifier (Fe 3+ and/or La 3+ ) were added to a separate beaker to obtain solution B. Solution B was then added to solution A to obtain solution C. Then, 20 mL tetrabutyl titanate was added to the mixed solution C, followed by the addition of 5 mL distilled water. This solution was then stirred for 4 h. Sol TiO 2 was obtained after sealing the container for 2 days. The sol TiO 2 gradually formed nano-TiO 2 upon drying, grinding, and heat treatment.

Characterization of Photocatalysts.
The phase of the asprepared samples was analyzed using a Shimazu XRD-6000 X-ray diffractometer with a copper target (Cu K , = 0.15406 nm), a voltage of 40.0 KV, and a current of 30.0 mA.
Surface charge analysis was conducted using a British Kratos XPSAM800 multifunctional surface analysis electron spectrometer, with an Al target (1486.6 ev) X-ray gun operating under 12 kv × 15 ma power. The analysis chamber background vacuum was 2 × 10 −7 Pa, adopting FAT working style. The spectrometer was operated with Cu2P3/2 (932.67 ev), Ag3d5 (368.30 ev), and Au4f7/2 (84.00 ev) prototype correction, and data were corrected using carbon pollution Cls (284.8 ev).
The compounds produced in photocatalyzed reactions were identified using UV-Vis absorption spectroscopic analysis. This study used a Shimadzu UV-Vis 2550 spectrophotometer (integrating sphere method) for fixed UV-Vis spectroscopy. A fixed amount of photocatalyst powder was placed in a quartz ware, using standard BaSO 4 powder as a reference. Spectra were collected with a sweep rate of 1800 nm/min and a scanning range of 300-650 nm.
The molecular structures of reaction produces were analyzed by fluorescence spectroscopy (FS). This study used solid form testing with a fixed fluorescence intensity at a certain incident light wavelength (275 nm), and the results were combined with the experimentally determined photocatalytic activity to analyze the fluorescent light characteristics of the photocatalysts.

Target Compounds.
This study used methyl orange (chemical name, dimethylamino azo benzene sulfonic acid sodium), which is produced by nitriding aminobenzene sulfonic acid via N, N-dimethylaniline coupling, as the target compound for assessing photocatalyst activity. The molecular formula of methyl orange is C 14 H 14 O 3 N 3 SNa. Its molecular weight is 327.34 Da, and its molecular structural contains a benzene group and N and S heteroatoms as shown in Scheme 1 [24]. Its molecular structure has certain representativeness. Remediation of methyl orange is a widespread problem, because this material is widely used as an industrial dye and is harmful to the aquatic environment. Methyl orange shows obvious absorption of visible light, as its absorbance and concentration have a linear relationship within a certain range, according to the Lambert-Beer law. In the continual degradation process of methyl orange, the maximum absorption wavelength has been at near 465 nm, almost without deviation. The UV-Vis absorption spectra of methyl orange solution undergoing degradation with TiO 2 photocatalyst is shown in Figure 1.

Photocatalytic Reaction Experiment.
In a typical photocatalytic experiment, with a 30-W UV lamp and 35-W xenon lamp as light sources, 100 mL methyl orange solution (10 mg/L) and photocatalyst were added to five 300 mL beakers to form separate mixed suspensions. Prior to illumination, these mixed suspensions were stirred using a magnetic stirrer for 30 min. Then, the mixed suspensions were illuminated (the distance from the liquid level to the UV lamp was 10 cm) for 180 min. The mixtures obtained after illumination were separated by centrifugation for 20 min.

Photocatalytic Activity Experiment.
The photocatalytic activity of the as-prepared TiO 2 samples was evaluated according to the decolorization rate of methyl orange solution. First, 100 mL methyl orange solution and a specified amount of TiO 2 photocatalyst were placed in a homemade photocatalytic reaction container. After 30 min of magnetic stirring, the reaction mixture was illuminated using the UV lamp and xenon lamp. By measuring the absorbance of the solution at the maximum absorption wavelength of methyl orange ( max = 465 nm), the decoloring rate can be calculated as follows: where is the decoloring rate, 0 is the initial absorbance before illumination, and is the absorbance after illumination time, . Figure 2 shows the XRD spectra for the different types of nano-TiO 2 photocatalyst treated at 500 ∘ C for 2 h. Compared to the standard X-ray spectrum of TiO 2 , it can be seen that the nano-TiO 2 powders and Fe 3+ /La 3+ co-doped nano-TiO 2 powders were anatase phase. As shown in Figure 2, the peak shapes of (101), (004), and (200) crystal plane diffraction was sharp, indicating that anatase phase had completely developed. Due to the small amounts of iron and lanthanide used for doping, no metal oxide diffraction peaks were observed corresponding to Fe 3+ or La 3+ . According to previous reports, in the La 3+doped nano-TiO 2 , La 3+ ions on the surface of nano-TiO 2 are oxidized and form a single layer of lanthanide oxide, which is difficult to detect by XRD. According to the solid physical band theory, in the nano-TiO 2 crystal, the ionic radius of Ti 4+ is 0.074 nm, and the ionic radius of Fe 3+ is 0.069 nm. Thus, Fe 3+ can easily spread into the nano-TiO 2 lattice and replace Ti 4+ in the nano-TiO 2 lattice. The ionic sizes of Fe 3+ and Ti 4+ ions are different, leading to nano-scale TiO 2 crystal lattice deformation. Upon La 3+ doping onto nano-TiO 2 , La 3+ replaces the lattice Ti 4+ . The ionic radius of La 3+ is 0.115 nm, which is larger than that of Ti 4+ . Thus, the substitution of La 3+ for Ti 4+ will cause distortion and inflation of the nano-TiO 2 crystal lattice, which will improve the photocatalytic activity of the material.

Phase Distribution, Particle Size, and Lattice Distortion of the Prepared Nano-TiO 2 Photocatalysts.
The size of the nanoscale grain obtained using this formula, that is, the first particle size of oriented crystal growth, cannot reflect particle agglomeration. The calculation results are shown in Table 1.
The calculation results show that the average particle size of Fe 3+ /La 3+ codoped nano-TiO 2 was lower than that of pure TiO 2 . The particle size of 0.01% Fe 3+ /0.6% La 3+ -doped TiO 2 was the smallest among those tested (6.1 nm). According to the results of photocatalytic degradation of methyl orange solution, the photocatalytic activity of 0.01% Fe 3+ /0.6% La 3+doped TiO 2 was the best among the photocatalysts tested. The average grain size of tombarthite-doped TiO 2 was smaller 10    than that of pure TiO 2 , indicating that the mixture of tombarthite ions inhibited the growth of the nanocrystalline phase. The average grain size of nano-TiO 2 codoped with tombarthite ions and transition metal ions was smaller, indicating that doping improved this inhibition. Doping with metal ions will affect the phase transition temperature, grain size, and other parameters and cause lattice distortion. Fe 3+ partly replaced lattice Ti 4+ , inevitably causing oxygen defects, and the existence of oxygen vacancies is thought to promote grain growth of the rutile phase. Therefore, Fe 3+ doping has a beneficial effect on the transformation of nano-TiO 2 from anatase to rutile type. Doping with La 3+ can inhibit the transformation of TiO 2 from anatase to rutile and thereby increase the content of the highly photocatalytic anatase phase, causing the grain size of nano-TiO 2 to decrease and thus the quantization effect to increase. Therefore, the synergistic effect of codoping with Fe 3+ and La 3+ makes the photocatalyst activity higher than the sum of the activities with single ion doping. According to Figure 2, upon doping with a small amount of Fe 3+ and La 3+ , the diffraction peaks of nano-TiO 2 shift towards the low angle direction, indicating that the diffraction peaks of Fe 3+ /La 3+ codoped nano-TiO 2 catalyst are wider than those of pure TiO 2 . Compared with pure nano-TiO 2 , the particle size of doped nano-TiO 2 was reduced. This is because a certain amount Fe 3+ and La 3+ penetrates the nano-TiO 2 crystal lattice, restricting the transfer and rearrangement of Ti and  O ions, inhibiting the growth of nano-TiO 2 crystals and decreasing the particle size. ). This is due to doping with Fe 3+ and La 3+ . In the Fe 3+ /La 3+ codoped TiO 2 , these two peaks shift 0.5 eV toward the higher energy direction, indicating that the effective positive charge of Ti was increased. Upon doping with elemental Fe and La, on the surface or in the lattice of nano-TiO 2 , electronic redistribution occurs and leads to a decrease in the Ti outer electron density, a reduction in the shielding effect, and an increase in the electron binding energy. These effects are beneficial for increasing photocatalytic activity. The binding energy difference between catalyst Ti2p and O1s is 71.3 eV, which indicates that Ti in the three prepared catalysts is in the tetravalent form (TiO 2 ). According to Figures 3(a)-3(c), the peaks at 529.6-529.8 eV in the O1s high-resolution XPS patterns are mostly related to Ti, and the surface hydroxyl or oxygen in oxide defects is the key. Hydroxyl groups on the surface of the catalyst are considered to be an important factor affecting photocatalytic activity. A hydroxyl group on the nano-TiO 2 catalyst surface can capture light and generate an ⋅OH free radical, which has strong oxidation ability. The ⋅OH free radical is the main strong oxidizer in the photocatalytic reaction. Therefore, as the hydroxyl content on the surface of nano-TiO 2 catalyst increases, the surface becomes more conducive to the generation of ⋅OH free radicals and the quantization efficiency is further improved, thereby effectively improving the catalytic activity of the nano-TiO 2 catalyst. According to Figures 3(a) and 8(b), in the Fe2p highresolution XPS patterns for 0.01% Fe 3+ doped nano-TiO 2 and 0.01% Fe 3+ /0.6% La 3+ doped nano-TiO 2 , Fe2p peaks appear at 710.68 eV and 710.98 eV. This is trivalent iron, indicating that iron doped on TiO 2 is in the form of Fe 2 O 3 . In addition, as shown in Figure 3(c), no Fe2p peak appears in the TiO 2 XPS spectrum, indicating that elemental Fe exists only in crystalline Fe 3+ /La 3+ doped nano-TiO 2 . Elemental Fe in the three prepared catalysts is in the tetravalent form.

Elemental Analysis of the Prepared Nano-TiO
According to Figures 3(a) and 3(c), in the La3d highresolution XPS patterns of 0.6% La 3+ doped nano-TiO 2 and 0.01% Fe 3+ /0.6% La 3+ doped nano-TiO 2 , La3d peaks appear at 835.75 eV and 836.20 eV. In Figure 3(d), no La3d peaks appear in the TiO 2 XPS spectrum at these positions, confirming that elemental La was present only in La 3+ doped nano-TiO 2 and Fe 3+ /La 3+ codoped nano-TiO 2 powders. The difference in the binding energies of La 3+ doped nano-TiO 2 and Fe 3+ /La 3+ codoped nano-TiO 2 is 0.5 eV. This suggests that La 3+ doping changed the electronic distribution on the nano-TiO 2 surface or lattice, thus improving photocatalytic performance. In the La3d spectrum, two peaks appear for La3d 3/2 and La3d 5/2 . According to previous reports, La exists in the form of La 2 O 3 [25,26]. Thus, La 3+ ions did not enter into the lattice of TiO 2 . This is because the ionic radius of La 3+ ions is bigger than that of Ti 4+ ions, and thus, La 3+ ions cannot enter into the lattice of TiO 2 . Figure 4 shows the fluorescence spectra of TiO 2 and the three prepared nano-TiO 2 photocatalysts. According to Figure 4(a)(A-D), the fluorescence spectra for Fe 3+ /La 3+ codoped nano-TiO 2 and pure TiO 2 have a fluorescence peak at 417 nm. The intensity of this peak for Fe 3+ /La 3+ codoped nano-TiO 2 is lower than that in the spectra for nano-TiO 2 doped with either metal or pure TiO 2 . Combined with the experimental results for methyl orange solution decolorization, this decrease in fluorescence intensity indicates a reduced recombination rate of photo-produced electronhole pairs, and thus, an increased photocatalytic activity. The above results show that the improvement in visible light catalytic activity is due to the reduction of the light carrier recombination rate by doping. Figure 4(b) shows the fluorescence spectra of nano-TiO 2 doped with different amounts of La 3+ . Figure 4(c) shows the fluorescence spectra of nano-TiO 2 doped with different amounts of Fe 3+ . Figure 4(d) shows the fluorescence spectra of nano-TiO 2 doped with different amounts of Fe 3+ and La 3+ . According to these spectra, TiO 2 shows a strong peak at 417 nm, and the position of this peak is not affected by doping of the nano-TiO 2 with any amount of Fe 3+ and/or La 3+ . The intensity of this peak is weaker in the spectra for Fe 3+ /La 3+ codoped nano-TiO 2 . La and Fe exist in the form of La 2 O 3 and Fe 2 O 3 , respectively, and these metal oxides can function as agents to capture photo-produced electrons. After capture of a photo-produced electron, it is difficult for the electron to recombine with a hole. Together with the results of the methyl orange decolorization experiments, these results showing that the fluorescence intensity of codoped samples is smaller indicate that their photocatalytic activity is better. Codoping with Fe 3+ and La 3+ reduces the recombination rate of photo-produced electron-hole pair and improves the quantum efficiency, thus leading to improvement in the photocatalytic efficiency. Figure 5(a) shows UV-Vis absorption spectra of nano-TiO 2 doped with different amounts of Fe 3+ . Relative to the absorption spectra of pure TiO 2 , the absorption band edge of Fe 3+ doped nano-TiO 2 shows an obvious red-shift. The obviously enhanced absorption strength in the visible area is beneficial for improving the utilization of sunlight and the photocatalytic efficiency. The main reason for this improvement is that the radius of Fe 3+ (0.064 nm) is similar to that of Ti 4+ (0.068 nm), and thus, Fe 3+ can replace some Ti 4+ in the lattice and create lattice defects. Impurity level formed in nano-TiO 2 band gap, and the energy of semiconductor optical electronic transiting to guide reduced, smaller energy photoproduction electronic can also transit, so the spectrum redshift, light response range extended. Figure 5(b) shows the UV-Vis absorption spectra of nano-TiO 2 doped with different amounts of La 3+ . The absorption sideband of La 3+ doped nano-TiO 2 moved towards the longer wavelengths, and the absorption rate of light increased. This is because the radius of La 3+ is 0.115 nm, which is larger than that of Ti 4+ (0.068 nm). Thus, La 3+ has difficultly entering the Journal of Nanomaterials lattice of nano-TiO 2 . When the quantity of doped La was very small, La covered the surface of nano-TiO 2 mainly in its oxide form, hindering grain growth. This led to the particles of La-doped nano-TiO 2 being smaller than those of nondoped nano-TiO 2 . The tombarthite elements covering the surface of nano-TiO 2 particles can absorb light over a wide range and transfer the energy to the nano-TiO 2 , thus improving the photocatalytic reactivity. Figure 6 shows the UV-Vis absorption spectra for singleand codoped nano-TiO 2 photocatalysts. The absorption bands for these nano-TiO 2 catalysts shifted to the visible light region at varying degrees. For catalyst doped with La 3+ and Fe 3+ individually, the absorption edge moved 35 nm and 41 nm toward the visible light region, respectively. That for nano-TiO 2 photocatalyst codoped with 0.01% Fe 3+ and 0.6% La 3+ moved 49 nm toward the visible light region. Therefore, the absorption sideband of Fe/La codoped nano-TiO 2 red-shifted more than that for the Fe 3+ or La 3+ singledoped nano-TiO 2 , and the absorption of visible light by the codoped catalyst is stronger than that by the singledoped catalysts. This indicated that the codoping with both elements has a synergistic effect. The main reasons are as follows: the 3d orbital of Fe 3+ is above the valence band of nano-TiO 2 . Electrons on the 3d orbital can absorb 415 nm visible light and transit it to nano-TiO 2 to create Fe 4+ , and thus, Fe 3+ acts as an electron trap. The vacant 5d orbital of La 3+ serves as a good electron transfer orbital. This orbital can be used to transfer the photo-produced electrons in the TiO 2 photocatalytic reaction, and thus, La 3+ also acts as an electron trap. Therefore, codoping with Fe 3+ and La 3+ inhibited recombination of photo-produced electrons and holes, and thereby improved the quantum efficiency of photoproduction.

Influence of Doping Amount on
the Photocatalytic Activity of Nano-TiO 2 3.5.1. Influence of Doping with Fe 3+ or La 3+ on the Photocatalytic Activity of Nano-TiO 2 . Figure 7(a) demonstrates the influence of Fe 3+ doping concentration on the photocatalytic activity of nano-TiO 2 . According to Figure 7(a), the optimum doping amount of Fe 3+ is 0.01%, which gives methyl orange decolorization rates of 93.5% with 3 h of UV illumination (versus 56.88% for pure TiO 2 ) and 29.8% with 5 h of visible light illumination (versus 4.2% for pure TiO 2 ). Doping with Fe 3+ causes the nano-TiO 2 to not only be able to capture electrons, but also to capture holes and the carrier is easily released. Thus, doping with Fe 3+ can increase the photocatalytic activity of the nano-TiO 2 catalyst. Doping with a small amount of Fe 3+ can reduce the recombination rate of electrons and holes and enhance the photocatalytic activity of nano-TiO 2 in the visible region, by improving the visible light utilization efficiency. At a low doping concentration, Fe 3+ can play a dual role as an electron and a hole trap, and thereby improve the photocatalytic activity of the catalyst. At a high doping concentration, Fe 3+ can reduce the quantum efficiency of photo-produced electrons and holes, leading to a decrease in the photocatalytic activity of the catalyst. This also can explain the influence of the doping amount on the photocatalytic activity via the process of capturing electrons and holes crossing the barrier. The recombination rate depends on the distance, , of separation between the electron and hole [27]: where composite is the recombination rate constant, 0 is the capture carrier hydrogen-like wave equation, and is the distance of separation between the electron and hole. According to the formula above, when the doping concentration is less than the optimum value, the semiconductor does not have enough traps to catch carriers. When the doping concentration is larger than the optimum value, due to the reduction in the average distance between the electrons and traps, the recombination rate grows exponentially as the doping concentration is increased. Thus, use of the optimum doping amount of transition metal ions is critical. Figure 7(b) shows the influence of La 3+ doping concentration on the photocatalytic activity of nano-TiO 2 . As shown in Figure 7(b), doping with tombarthite element La 3+ improved the photocatalytic activity of nano-TiO 2 . The methyl orange degradation rate under visible light illumination is greatly improved by La doping. The methyl orange decolorization rate increased as the doping amount of La 3+ increased. The highest photocatalytic activity was observed for a doping concentration of La 3+ of 0.6%. The methyl orange decolorization rate was 88.1% with 3 h of UV irradiation (pure TiO 2 ) and 27.4% with 5 h of visible light irradiation (versus 4.2% for pure TiO 2 ). With greater doping amounts, the photocatalytic activity did not continue to increase, but instead decreased. Doping with La ions increased catalytic activity, because tombarthite elements can produce electron configuration, polycrystalline type, and thermal stability. Doping with the appropriate amount of a tombarthite element has a positive role in improving the crystal type and photocatalytic properties of nano-TiO 2 . Because the La 3+ radius is 0.106 nm, which is different from that of Ti 4+ (0.068 nm), doping with La ions caused an increase in oxygen vacancy and defects on the Journal of Nanomaterials surface of the nano-TiO 2 , effectively inhibiting the nano-TiO 2 photo-production of electron-hole pairs, thereby improving the photocatalytic activity. However, too much tombarthite element also may cause a free electron transfer center to become a free electron recombination center and increase the photo-production of electron-hole pairs, thus reducing the photocatalytic activity. The f orbital of tombarthite elements can have a coordination effect with the degradation substrate, and doping with a certain amount of La ions can effectively separate the nano-TiO 2 photo-produced electrons and holes, generating many active groups with strong oxidizing ability involved in the photocatalytic oxidation reduction reaction, thereby improving the photocatalytic activity of the catalyst. However, when the doping amount exceeds a certain concentration, too much tombarthite metal ion deposition on the surface of nano-TiO 2 hinders electron and hole transfer from the surface of the catalyst. Thus, tombarthite metal ions on the surface of the nano-TiO 2 become charge carrier recombination centers, resulting in a decrease in catalytic activity.
3.6. The Influence of Fe 3+ /La 3+ Codoping on Nano-TiO 2 Photocatalytic Activity. Figure 8 shows the influence of Fe 3+ /La 3+ codoping on the photocatalytic activity of nano-TiO 2 under UV illumination, and Figure 9(a) shows the effect of Fe 3+ /La 3+ codoping on the photocatalytic activity of nano-TiO 2 under visible light illumination. According to the results shown in these figures, the catalytic activity of codoped nano-TiO 2 is higher than that of catalyst doped with either Fe 3+ or La 3+ . The 0.01% Fe 3+ and 0.6% La 3+ codoped nano-TiO 2 possessed the highest photocatalytic activity. After 3 h of UV irradiation, the decolorization rate of methyl orange for 0.01% Fe 3+ and 0.6% La 3+ codoped nano-TiO 2 was 99.8%. After 5 h of visible light irradiation, the decolorization rate of methyl orange for 0.01% Fe 3+ and 0.6% La 3+ codoped nano-TiO 2 was 40.7%. Both of these rates are greatly improved over those achieved by pure TiO 2 . Doping with transition metal Fe 3+ ions alone did not hinder the modification of tombarthite ions, but worked together with tombarthite La 3+ ions to further improve the activity of the photocatalyst. The experimental results show that there are optimum doping amounts for both Fe 3+ and La 3+ . A high concentration of doping ions can reduce the photocatalytic activity. Under the conditions of high concentrations, neither Fe 3+ nor La 3+ can effectively penetrate the crystal lattice of nano-TiO 2 , and therefore, these ions gather on the surface of crystals. An excessive of doping ions can catch large numbers of electrons and holes, reduce the quantum efficiency, and reduce the activity of catalysts. For low doping concentration, an increase in the doping ion concentration can improve the optical carrier separation effect. Therefore, because the thickness of the space between electrons and the surface of nano-TiO 2 decreases with an increasing amount of doping tombarthite element, when the optimum concentration of doping metal is reached, the distance between the electrons and the surface is equal to the penetration depth of incident light into the solid and photoproduction of electrons and holes is achieved by optimal light irradiation, benefiting the photocatalytic reaction. The combined effects of Fe 3+ and La 3+ upon codoping of nano-TiO 2 photocatalyst promoted the optimum separation of photoproduced electrons and Journal of Nanomaterials holes and thus improved the photocatalytic activity of the photocatalyst.

Kinetics of the Photocatalytic Activity of Codoped Nano-
For a heterogeneous photocatalytic system, such as the nano-TiO 2 photocatalytic system, the reaction rate of photocatalytic oxidation can be described by the Langmuir-Hinshelwood dynamics equation as follows [28]: where is the concentration of reactant, is the activity constant, and is the adsorption equilibrium constant of the reaction. Integration of (3) gives When the concentration is small, (4) can be transformed into where is the apparent rate constant and is a constant. The kinetics for the degradation of methyl orange by the different prepared nano-TiO 2 photocatalysts were investigated in the present study. The relationship between ln( 0 / ) ( 0 is the initial concentration and is the concentration at time ) and photocatalysis time is shown in Figure 9(b). The fitting of the data for the photocatalytic degradation of methyl orange with a first-order kinetic curve is shown in Table 2.
According to Table 2, the value for the fitted straight line is far less than 0.01, indicating that ln( 0 / ) and are significantly linearly correlated. As shown in Figure 9, under visible light irradiation, the degradation of methyl orange by different doped nano-TiO 2 catalysts is well described by first-order reaction kinetics. The high correlation coefficients indicate that this model can be used to describe this photodegradation reaction.

Conclusions
In the present study, nano-TiO 2 powder photocatalyst was prepared and modified via a sol-gel method by doping with either Fe 3+ or La 3+ individually or codoping with both Fe 3+ and La 3+ . Codoping of nano-TiO 2 photocatalysts with both Fe 3+ and La 3+ resulted in better catalytic performance than that achieved by doping with either Fe 3+ or La 3+ , as well as better inhibition of nanocrystal growth and better refinement of grain size. Doping with tombarthite ions can effectively inhibit the shift of nano-TiO 2 from anatase to rutile. La 3+ doping changed the nano-TiO 2 surface or lattice electron distribution. The sol-gel method can be used to effectively dope the lattice of nano-TiO 2 with Fe 3+ and La 3+ . Compared with catalyst doped with only Fe 3+ or La 3+ , the light absorption intensity of Fe 3+ /La 3+ codoped nano-TiO 2 photocatalyst was stronger. This is because the absorption band edge redshifted obviously, and the spectral response range was extended into the visible light region, increasing the utilization of visible light. Fe 3+ /La 3+ codoped nano-TiO 2 photocatalyst showed superior photocatalytic performance compared to the single-doped samples. Because nano-TiO 2 codoped with two elements can achieve higher catalytic activity under visible light, this approach increases the potential utility of nano-TiO 2 photocatalyst materials in important environmental purification processes.