Synthesis , Characterization , and Photocatalytic Activity of TiO 2 Microspheres Functionalized with Porphyrin

In order to utilize visible light more efficiently in the photocatalytic reaction, TiO2 microspheres sensitized by 5-(4allyloxy)phenyl-10,15,20-tri(4-methylphenyl)porphyrin (APTMPP) were prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and UV-vis diffuse reflectance spectroscopy, and so forth, The characterization results indicated that APTMPP-MPS-TiO2 was composed of the anatase crystal phase. The morphology of the composite materials was spheriform with size of 0.3–0.7 μm and the porphyrin was chemisorbed on the surface of TiO2 through a Si–O–Ti bond. The photooxidation of α-terpinene was employed as the model reaction to evaluate the photocatalytic activity of APTMPP-MPS-TiO2 microspheres under visible light. The results indicated that the photodegradation of α-terpinene was significantly enhanced in the presence of the APTMPP-MPS-TiO2 compared with the nonmodified TiO2 under visible light.


Introduction
The application of semiconductors in heterogeneous photocatalysis to eliminate various pollutants in aqueous systems has gained significant attention in the last decade [1][2][3].TiO 2 is considered a preferred and potential semiconductor photocatalytic material for applications requiring antimicrobial and sterilizing characteristics because some of its forms have reasonable photoactivity.Besides, it has many advantages such as inexpensiveness, easy production, (photo)chemical and biological stability, and innocuity to the environment and to human beings.It has been used for water remediation being effective for the photodegradation of many harmful organic pollutant to innocuous inorganic species [4][5][6][7].However, some drawbacks limit its application especially in the large-scale industry.One important disadvantage of pure titania is characterized by low quantum efficiency because of high bandgap energy (∼3.2 eV) and high rate of e − and h + recombination.The light absorption region of anatase-typed TiO 2 particles does not fit with the solar spectrum because the solar energy above 3.0 eV (λ ≤ 410 nm) only makes up less than 5% of whole sunlight.Therefore, the development of low-bandgap photoactive material, the so-called visible light photocatalyst, is strongly urged for solving environmental problems.To overcome this problem, different approaches have been proposed in the literature in which the response of the semiconductor was extended toward the visible region.Narrowing the bandgap is an effective way to enhance the photocatalytic performance of titania, and this can be done with metals and nonmetals ion doping [8][9][10], ion implantation, and photosensitization [11][12][13][14][15].Among them, dye sensitization is considered to be an efficient method to modify the photoresponse properties of TiO 2 particles [16][17][18].The dyes used are erythrosine B, rose bengal, porphyrin, and so forth.Similarly to other photosensitizers, porphyrins are recognized to be the most promising sensitizers.They not only can be involved in the reaction mechanism by transferring electrons to the conduction band of TiO 2 or to adsorbed O 2 , but also they can cooperate with TiO 2 participating in photooxidation reactions by means of a direct activation of O 2 [19][20][21][22][23][24].Recently, the photocatalytic activity of TiO 2 powders International Journal of Photoenergy impregnated with copper porphyrins used as sensitizers for the decomposition of 4-nitrophenol has been investigated [25][26][27][28].But Some of them suffer from a reversible binding and the support is broken during the washing of the solvent under working conditions.A useful means to avoid these limitations is to establish the stable covalent binding between porphyrins and TiO 2 samples.
We recently reported efficient photooxidations using heterogeneous photosensitizers prepared by the covalent immobilization of metal-free porphyrins on silica microspheres [29].To further study the photosensitization of metal-free porphyrins, one novel metal-free porphyrin derivatives, that is, 5-(4-allyloxy)phenyl-10,15,20tri(4-chlorophenyl)porphyrin was synthesized and characterized.The catalytic activity of metal-free porphyrin complex covalent binding to TiO 2 is studied upon irradiation the sample with visible light.The oxidation of α-terpinene has been chosen as a model reaction.The purpose of this paper is to study the structural relationship between porphyrin complex and TiO 2 in the prepared composite by sol-gel processing.Furthermore, the stability of photocatalysts was also investigated and discussed.

Physical Measurements.
The scanning electron microscopy (SEM) was performed with a JSM-6330F Field Emission Scanning Electron Microscope.The XRD curves were got from Rigaku D/max 2550 VB/PC (Japan).The surface property of samples was characterized by X-ray photoelectron spectroscopy (XPS) on a PerkinElmer PHI 1600 ESCA system.The specific surface areas of the samples were measured using nitrogen adsorption method at 77 K and the Brunauer-Emmett-Teller (BET) analysis using a Flow Sorb 2300 apparatus (Micromeritics International).The pore size distributions were calculated with the DFT Plus software (Micromeritics), applying the Barrett-Joyner-Halenda (BJH) model considering cylindrical geometry of the pores.Thermal analysis was performed with a Netzsch TG-209 Thermogravimetric Analyzer.UV-vis and IR spectra were recorded on a Shimadzu UV-3150 spectrophotometer and an EQUINOX 55 Fourier transformation infrared spectrometer, respectively.Diffuse reflectance spectra in the 200-800 nm were obtained by using a Shimadzu UV-3150 UV-vis spectrophotometer. 1 H NMR spectra were recorded on a 300 MHz Bruker-AMX spectrophotometer with tetramethylsilane (TMS) as internal reference.Mass spectrometry analysis was performed on a Thermo LCQDECA-XP spectrometer.An iodine tungsten lamp (200W, Shanghai), ITL, was used as a light source.A distance of about 5 cm between the lamp and reactor was maintained.

Synthesis of APTMPP-MPS-TiO 2 . TiO 2 microspheres
were prepared by sol-gel processes [33].10 mL of Ti(OBu) 4 is dissolved in 10 mL of anhydrous C 2 H 5 OH (99.7%) to produce Ti(OBu) 4 -C 2 H 5 OH solution.Meanwhile, 5 mL of water and 3 mL ammonia were added to another 10 mL of anhydrous C 2 H 5 OH in turn to form an ethanol-NH 3water solution.After the two resulting solutions are stirred for 30 min, respectively, the Ti(OBu) 4 -C 2 H 5 OH solution is slowly added dropwise to the ethanol-NH 3 -water solution under vigorously stirring to carry out a hydrolysis.Thus, a semitransparent sol is gained after continuously stirring for 1 h.Then, 2 mL ammonia was added into the mixture (pH 11) and the solution was stirred vigorously another 12 h at room temperature.After completion of the reaction, the resulting precipitate was collected by centrifugation, washed repeatedly with alcohol, and then dried in a vacuum and white powder was obtained.To complete the surface modification with APTMPP-MPS, 40 mg APTMPP-MPS was dissolved in 30 mL of CHCl 3 and 1 g finely TiO 2 powder was transferred into a weighing bottle.Subsequently, After ultrasonication for 2 min and stirring for 20 min vigorously, the mixture bottle was kept at 80 • C in the water bath to vaporize the liquid under stirring.The product was washed with CHCl 3 in a Soxhlet extractor overnight to remove unbound APTMPP.Finally, the solids were dried at 110 • C for 12 h.Unbound APTDCPP was quantified through spectrophotometric measurement at 518 nm, by using a calibration curve obtained by using a suitably diluted solution.Thus, the amounts of bound porphyrins were determined by difference between the two measurements.APTMPP in APTMPP-MPS-TiO 2 was 0.0202 mmol/g obtained by this method.

Approach of Calculation.
The degradation rate of αterpinene in the reaction process could be calculated by the following formula: where A t is the absorbance of α-terpinene measured at 265 nm at time t, and A 0 is the initial absorbance prior to reaction.The residual concentrations of α-terpinene could be calculated by the following formula: where C t is residual concentration of α-terpinene at time t and C 0 is the initial concentration of α-terpinene.by the condensation reaction of APTMPP-MPS and TiO 2 microspheres.

Preparation and Characterization of APTMPP-MPS-TiO
The morphology of TiO 2 and APTMPP-MPS-TiO 2 was obtained by scanning electron micrograph as shown in Figure 1.The micrograph shows irregularly shaped spherical particles with loose and discrete structure.Distribution of particle size of TiO 2 and APTMPP-MPS-TiO 2 over a broad range was observed with the average size of 0.7 μm.No change has happened in the diameter of TiO 2 after APTMPP bonding on it.
The UV-vis DRS spectra of APTMPP-MPS-TiO 2 and pure TiO 2 are shown in Figure 2. It is worth noting that no shift of the bandgap edge of TiO 2 can be observed in loaded sample.APTMPP-MPS-TiO 2 exhibits typical porphyrin absorption features with a strong Soret band at 419 nm and moderate Q bands at 519, 552, 595, and 650 nm, the strong band appears at 419 nm arising from the transition of a 1u (π)-e g (π), and the less intense bands in the 500-650 nm region corresponding to the a 2u (π)-e g (π) transition attributed to porphyrin [36] while there is no absorption above 400 nm for pure TiO 2 .It can be found that APTMPP-MPS-TiO 2 composites exhibit a broader absorption range than pure TiO 2 .
Figure 3 shows the FT-IR spectra of APTMPP-MPS-TiO 2 .The peak around 1630 cm −1 is due to the bending vibration of the O-H bond of chemisorbed water, and the peak around 3400 cm −1 is due to the stretching mode of the O-H bond of free water.The IR band at 400-850 cm −1 and 1400 cm −1 corresponds to the Ti-O-Ti stretching vibration mode in crystal TiO 2 .Unlike the pure TiO 2 [37], the modified TiO 2 samples have a new broad IR band at about 928 cm −1 corresponding to the vibrations of Ti-O-Si bonds [38].The existence of Ti-O-Si bond verifies the formation of chemical bonding between titania and organic component.That at around 1000 cm −1 is assigned to symmetric and asymmetric Si-O-Si stretching vibrations in siloxane network.Some new bands between 1300 and 1400 cm −1 attributed to the pyrrole C=N stretching also indicate that the presence of porphyrins in the hybrid materials.Figure 4 shows the TG and DTG for APTMPP-MPS-TiO 2 sample.From the TG-DTG analysis of the APTMPP-MPS-TiO 2 , the weight of particles sharply decreases up to 450 • C and slowly decreases from 450 to 800 • C. TG-DTG analysis of the sample also shows the endothermic peak at 100 • C and two exothermic peaks at 290 and 450 • C. It is thought that the peak at 100 • C is due to free adsorbed water, and the peak at 290 • C is due to the decomposition of the porphyrin and residual hydroxy group.In addition, the peak at 450 • C corresponds to the crystallization of the amorphous phase into the anatase phase.Above 650 • C, it can be assumed that the product completely transforms into the anatase phase because there is no change in particle weight.
From the X-ray diffraction patterns of the samples shown in Figure 5, we can see only that the anatase crystalline phase was present in both samples.The major phase of the photocatalyst is pure anatase without brookite.No rutile peaks were observed for all titania.The peaks appearing at 2θ = 25.3 • , 38.7 • , 47.6 • , and 54.8 • are attributed to anatase phase.It was confirmed that Ti(OC 4 H 9 ) 4 was oxidized into TiO 2 , and the formed TiO 2 was mostly anatase by the calcination in the air.In addition, immobilization of APTMPP-MPS on TiO 2 has little effects on the interlayer spacing.This demonstrates that the porphyrins grafted on TiO 2 surface does not crystallize.
Figure 6 shows the nitrogen adsorption-desorption curves measured by BET method and the pore-size distribution calculated from desorption isotherms of APTMPP-MPS-TiO 2 prepared by sol-gel method.As seen, the isotherm of the modified sample shows a combination of type I [39].At low relative pressure, the isotherm exhibits high adsorption, indicating that as-prepared samples contain micropores (type I).While at high relative pressure from 0.6 to 1.0, the curve exhibits a little biger adsorption, indicating the bigger crystallites aggregation form bigger pores.The BET surface area and specific pore volume of the composite are 266.3m 2 g −1 and 0.23 cm 3 g −1 .Figure 7 shows the pore size distribution curve of the corresponding products by the BJH method.It can be seen that the diameter of pore ranged from 1.8 to 4.0 nm, and the largest pore diameter was 2.1 nm.In order to analyze the chemical composition of the APTMPP-MPS-TiO 2 , we performed X-ray photoelectron spectroscopy (XPS) analyses.Figure 8 shows XPS survey spectrum for the surface of APTMPP-MPS-TiO 2 particles.The XPS spectrum reveals characteristic peaks from Ti, C, Si, S, N, and oxygen.Parameters of the XPS peak for Ti(2P 3/2 ) and O(1S) are listed in Table 1.The peaks of APTMPP-MPS-TiO 2 are shifted to higher binding energies by 0.28 and 0.62 eV for Ti(2P 3/2 ) and O(1S) than TiO 2 [40], respectively.The increase of binding energy for Ti(2P 3/2 ) and O(1S) while APTMPP-MPS adsorbed on TiO 2 indicates chemical bonding at the interface of the silica coating layer and titania particle surface.Figure 9 shows the XPS spectra of O(1S) peak for APTMPP-MPS-TiO 2 .The shape of a wide and asymmetric peak of XPS O(1S) spectra indicated that there would be more than one chemical state according to the binding energy.It includes crystal lattice oxygen (O Ti−O or O Si−O ) and chemisorbed oxygen (O O−H ).The main contribution is attributed to Ti-O in TiO 2 , and the other two kinds of oxygen contributions can be ascribed to the -OH in Ti-OH and the Si-O in SiO 2 , respectively [41,42].Usually, hydroxyl groups measured by XPS are ascribed to the chemisorbed water.As seen, the hydroxyl content increased in the sample.This is probably due to the fact that the films easily adsorb water vapor in air, leading to the formation of hydroxyl on the films.This also corresponds to the results of previous TG and FTIR.

α-Terpinene
Photooxidation.The photocatalytic activities for the degradation of α-terpinene under visible light irradiation using prepared photocatalysts were tested.αterpinene degradation occurred promptly in the visible irradiation under the adding of the APTMPP-MPS-TiO 2 as confirmed by the UV-vis spectra changes of APTMPP-MPS-TiO 2 (Figure 10).The absorption spectrum of α-terpinene in ethanol was characterized by the band of 265 nm in   the UV-vis region.As observed in Figure 10, the intensity of the 265 nm absorption band decreased rapidly under visible light irradiation, indicating the degradation of αterpinene in the presence of APTMPP-MPS-TiO 2 microspheres.For comparison, blank experiments, in which the photooxidation experiment of α-terpinene was performed in dark in the presence of APTMPP-MPS-TiO 2 or irradiated with visible light with TiO 2 , were done and no obvious catalytic results were observed.It believes that both visible light and APTMPP-MPS-TiO 2 were indispensable to the photooxidation of α-terpinene.
A tentative photocatalytic mechanism of APTMPP-MPS-TiO 2 was deduced and shown in Scheme 2. When the visible light irradiates on the surface of APTMPP-MPS-TiO 2 , the porphyrin molecules adsorbed on the surface of TiO

Effect of Catalyst Amount on the Photocatalytic Activity.
The photocatalytic activities are also influenced by the catalyst amount, as shown in Figure 11.It reveals that APTMPP-MPS-TiO 2 hybrids have higher photocatalytic activity than that of only TiO 2 particles and the photooxidation efficiency increases with an increase in APTMPP-MPS-TiO 2 concentration up to 2.0 mg/mL and then remains almost constant above a certain level.This has been explained as the concentration of the APTMPP-MPS-TiO 2 increased; the number of photons absorbed and the number of α-terpinene molecules adsorbed increased with respect to an increase in the number of APTMPP-MPS-TiO 2 molecules.The density of the molecule in the area of illumination also increases and thus the rate gets enhanced.After a certain level, the dye molecules available are not sufficient for adsorption by the increased number of catalyst molecule.Hence, the additional catalyst powder is not involved in the photocatalytic activity and the rate does not increase with increase in the amount of catalyst beyond certain limit.Replotting Figure 11(a) in the ln(C 0 /C t )−t coordinates (Figure 11(b)).It is found that α-terpinene's photooxidation follows the pseudo-firstorder kinetic in the lower initial APTMPP-MPS-TiO 2 amount (0.5-1.5 mg/mL).However, when APTMPP-MPS-TiO 2 amount was greater than 1.5 mg/mL, the photocatalytic oxidation would deviate from the pseudo-first-order kinetic with respect to APTMPP-MPS-TiO 2 amount.

Stability and Reusability of the Catalysts.
For the assessment of catalyst stability and reusability, APTMPP-MPS-TiO 2 sample was used in the degradation of different solutions of α-terpinene in four consecutive experiments.Catalyst was recovered using centrifuge during sampling and after each experiment.The UV-vis and IR spectra as well as the morphology of the recovered hybrid material did not show any substantial change compared to fresh material.All reaction solutions analyzed after catalytic reactions did not show the characteristic soret band of porphyrins, which indicated that no APTMPP lost from TiO 2 .Using the recovered APTMPP-MPS-TiO 2 as catalyst to catalyze α-terpinene photooxidation in the same conditions, they retain their high catalytic activity after being recycled four times.For example, the photooxidation percentages of αterpinene for four consecutive experiments are as follows: 87, 84, 80, 79, and 81% in the same conditions (1 mg/mL APTMPP-MPS-TiO 2 , 3.7 × 10 −4 M α-terpinene, 60 min).
These results indicate that APTMPP-MPS-TiO 2 has highly chemical stability and is recoverable and reusable.

Conclusions
In this work, novel APTMPP-MPS-TiO 2 hybrids with high photocatalytic activity were synthesized by immobilizing metal-free porphyrin on titania sol particles.The metalfree porphyrin can extend the light absorption of TiO 2 into visible region and make α-terpinene be effectively photooxidated under visible light.The degradation reaction process of α-terpinene completely obeys the first-order law.Furthermore, the photocataysts are stable, harmless, and they can be reused with high efficiency.