Application of TitaniumDioxide-Graphene Composite Material for Photocatalytic Degradation of Alkylphenols

Titanium dioxide-graphene (TiO2-G) composite was used for the photodegradation of alkylphenols in wastewater samples. e TiO2-G composites were prepared via sonochemical and calcination methods. e synthesized composite was characterized by Xray diffraction (XRD), infrared spectroscopy (IR), scanning electronmicroscopy (SEM), transmission electronmicroscopy (TEM), energy dispersive X-ray analysis (EDX), and �uorescence spectroscopy. e photocatalytic efficiency was evaluated by studying the degradation pro�les of alkylphenols using gas chromatography-�ame ionization detector (GC-FID). It was found that the synthesized TiO2-G composites exhibit enhanced photocatalytic efficiencies as compared to pristine TiO2.e presence of graphene not only provides a large surface area support for the TiO2 photocatalyst, but also stabilizes charge separation by trapping electrons transferred from TiO2, thereby hindering charge transfer and enhancing its photocatalytic efficiency.


Introduction
Alkylphenol ethoxylates (APEs) are a class of nonionic surfactants that have been used extensively as detergents, emulsi�ers, and dispersing agents [1].It has been reported that during the biodegradation of APEs in conventional wastewater treatment plants (WWTPs), alkylphenols (APs) like octylphenol (OP) and nonylphenol (NP) are produced as persistent metabolites which possess higher toxicity and estrogenic activity and have greater tendency towards bioaccumulation [2].With widespread use of these APs and poor efficiency of biodegradation, it is difficult to remove completely from wastewater treatment plants.NP is listed as a priority pollutant in the Water Framework Directive (WFD) [3], and studies have shown that trace levels of OP and NP can exert estrogenic effects on aquatic organisms and mammals [4].e concentrations of APs detected in wastewater effluents were in the range of 0.01-15 gL −1 [1] while the proposed environmental quality standard (EQS) of the WFD for nonylphenol and octylphenol in surface waters is 0.3 gL −1 and 0.1 gL −1 , respectively [5].
Advanced oxidation process (AOP) involving UV/TiO 2 is a promising remediation technique for these phenolic endocrine-disrupting compounds since there is usually UV disinfection step in wastewater treatment plants and it is cheap.Titania is the most common semiconductor photocatalyst used due to its strong oxidizing power, nontoxicity, and long-term photostability [6].In addition, studies have shown that complete mineralization of alkylphenols to CO 2 and H 2 O can be achieved via the use of TiO 2 [7].However, it suffers from the major drawback of low photocatalytic efficiency due to the high rate of electron/hole pair recombination [8].us the development of new materials for modifying TiO 2 is needed to increase the photocatalytic activity for its practical applications in WWTPs.
When the TiO 2 photocatalyst is irradiated with energy equal to or greater than the bandgap energy (3.2 eV), the electrons move to the conduction band (CB) to generate positive holes in the valence band (VB) [10].e positive holes can react with adsorbed H 2 O to form hydroxyl radicals while the electrons react with O 2 to form superoxide radicals.Being highly reactive, the OH and O 2 radicals can oxidize the pollutants in solution or react with adsorbed pollutants.Otherwise, these electron and hole pairs can recombine.e high rate of electron and hole pair recombination thus restricts the efficiency of TiO 2 photocatalysis.Many efforts are focused on enhancing the photoactivity of TiO 2 , and some of these include increasing the adsorption abilities of the photocatalyst surface by adding a coadsorbent [7] and modi�cation of TiO 2 with metal or semiconductor to improve the separation between free carriers [11].
Graphene is a �at monolayer of carbon atoms tightly packed in a two-dimensional honeycomb lattice, which has received tremendous attention over the years due to its extraordinary electronic, thermal, and mechanical properties [12].It is a 0 ev bandgap semiconductor [13] with high mobility of charge carriers [14], high speci�c area (∼2600 m 2 g −1 ) [15], and a high adsorption capacity.Given the properties of graphene, the presence of graphene can enhance the photocatalytic efficiency of TiO 2 [16][17][18][19].Since cost is an important consideration for practical applications in WWTPs, one added advantage of using TiO 2 -graphene composites as photocatalyst is that bulk production of graphene sheets can be produced from the inexpensive and abundant graphite at low costs [9].
In this study, TiO 2 -graphene composite was synthesized and characterized.e photocatalytic efficiencies for the degradation of 4-n-heptylphenol, 4-n-octylphenol, and 4nonylphenol of the synthesized composites were compared with TiO 2 using GC-FID.e degradation pro�les of these APs were studied because they are the reported byproducts of the widely used commercial formulations of APEs.

Experimental
2.1.Chemicals and Reagents.e acid-functionalized graphite oxide was obtained from Associate Professor Loh Kian Ping's research group at the National University of Singapore and was synthesized via Hummer's method [20].Tetrabutyl orthotitanate (purity ≥ 97%) was purchased from Fluka (Buchs, Switzerland), and hydrazine (35 weight % solution in water) was obtained from Sigma-Aldrich (Milwaukee, WI, USA).Technical ethanol was obtained from Riverbank Chemicals Pte Ltd (Singapore).e commercial photocatalyst, TiO 2 P25 (Degussa AG, Germany) with primary particle diameter of 21 nm and speci�c surface area of 50 ± 15 m 2 g −1 , was used for comparison purposes.e following chemical standards (purity ≥ 98%) were obtained from Wako Chemicals (Tokyo, Japan): 4-n-heptylphenol, 4-n-octylphenol, and 4-nonylphenol (mixture of NP isomers).e derivatization agent bis(trimethylsilyl)tri�uoroacetamide (BSTFA) (purity > 98%) was obtained from Merck (Darmstadt, Germany).Ultrapure water was prepared on a Milli-Q water system (Milford, MA, USA).For the HPLC-grade organic solvents, acetonitrile was obtained from Lab-Scan Analytical Sciences (Bangkok, ailand), and hexane was obtained from Fischer Scienti�c (Loughborough, UK).Stock standard solutions of the individual standards were prepared in acetonitrile at 1000 mgL −1 and stored at 4 ∘ C. Aqueous solutions (200 mL) containing 5 mg/L mixtures of the analytes were prepared using ultrapure water.

Materials.
Photodegradation was initiated via the use of a 15 W germicidal mercury lamp with a working wavelength of 254 nm from Ster-L-Ray, Atlantic Ultraviolet Corp. (Hauppauge, NY, USA).A jacketed Pyrex glass column (310 mm height and 70 mm inner diameter) was bought from UFO Labglass (S) Pte Ltd and was used as the photoreactor.
2.3.Synthesis of TiO 2 -Graphene Photocatalyst.e graphene sheets were combined with TiO 2 nanoparticles through sonochemical and calcination methods [21].Firstly, graphite oxide was sonicated in technical ethanol (20 mL) for 15 minutes to disperse them well and form graphene oxide solution (GO).Tetrabutyl orthotitanate (TBOT) was used as the titanium precursor, and it was added to the GO solution.Next, the mixture was sonicated for 30 minutes to improve the interaction between these chemicals.Milli-Q water (80 mL) was added, and the sol sample formed by hydrolysis was treated with ultrasonic irradiation in an ultrasonic cleaning bath (Branson 1210-USA).e sol sample was then aged at 25 ∘ C for 20 hours to further hydrolyze TBOT and form monodispersed TiO 2 nanoparticles.e samples were dried in a 100 ∘ C oven.In order to reduce the GO to graphene (G), hydrazine solution (5 mL) was added to the dried sample and it was stirred for 24 hours.e reduced sample, TiO 2 -graphene (TiO 2 -G), was then washed with ultrapure water and dried in a dessicator.Finally, in order to convert the amorphous TiO 2 to crystalline TiO 2 which possesses higher photocatalytic activity, the mixtures were calcined under nitrogen atmosphere at 400 ∘ C for 1 hour.TiO 2 -G composites with different mass ratios of titanium and carbon were synthesized (1 : 0.1 and 1 : 0.2).e preparation process of TiO 2 was similar to that of TiO 2 -G, except that the addition of GO and reduction with hydrazine steps were skipped.

Characterization Techniques.
Powder X-ray diffraction (XRD) was used for phase identi�cation and to estimate the crystallite size of the anatase nanoparticles.e XRD spectra were obtained at room temperature with a Siemens X-ray diffractometer (Model: B5005) using Cu K radiation (  1.5406 � Å), and data were collected from 2 = 20 ∘ to 80 ∘ at a step size of 0.020 ∘ /s.FTIR spectra of TiO 2 , GO, and TiO 2graphene were obtained via a Varian 3100 FT-IR spectrophotometer using KBr pellets.Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) of the synthesized composite were performed via a JEOL JEM 3010 electron microscope to observe the surface morphology of the TiO 2 -G composite.EDX analysis was recorded on a JEOL JED-2300 scanning electron microscope to identify the elemental composition.e charge recombinations of TiO 2 -G and TiO 2 were compared by obtaining the �uorescence emission spectra.Fluorescence spectroscopy analysis was carried out at room temperature via a Perkin Elmer Luminescence spectrometer (LS 55) equipped with a powder holder accessory, and the excitation wavelength was set at 300 nm.

2.�. Study o� Photode�radation
Pro�les.e photocatalytic efficiencies of the synthesized composites were evaluated by studying the degradation pro�les of alkylphenols in aqueous media under Ultraviolet C (   nm) irradiation.A batch reactor, which was housed in a black box, was used for the photocatalytic experiments, and the schematic diagram is shown in Figure 1.e solution was continuously stirred in order to obtain a homogeneous solution.Cooling water was circulated in the outer jacket in order to keep the reaction temperature constant.In each photodegradation test, 20 mg of photocatalyst was dispersed in 200 mL solution containing 5 ppm mixture, and the amount of suspended photocatalyst was kept at 0.1 g/L.To allow the adsorption-desorption equilibrium of alkylphenols on the photocatalyst, the suspension was stirred for 5 minutes under dark conditions.Prior to turning on the UV lamp, the concentration of the solution was determined, which was considered as the initial concentration ( 0 ).

Analytical Procedure.
In order to monitor the variation of analyte concentrations with time and obtain the degradation pro�les, sample aliquots were withdrawn from the photoreactor at 45-minute intervals (over a period of 3 hours) and �ltered through a 0.2 m syringe �lter to remove the photocatalyst particles before analyses.Liquidliquid extraction with previously optimized conditions [1] was used for study the degradation pro�le� brie�y, a 5 mL of sample was adjusted to pH 2 using 1 M HCl in a 10 mL centrifuge tube and extracted twice with 200 L of dichloromethane.To remove trace amounts of water, anhydrous sodium sulphate was added to the organic layer and 100 L of bis(trimethylsilyl)tri�uoroacetamide (BSTFA) was added.e mixture was kept in a 60 ∘ C water bath for 20 min to complete the derivatization.From this, 2 L was injected to a GC-FID for analysis.

GC-FID Analysis. GC analyses were performed via an
Agilent 7890A GC-FID system with a 7683B autosampler.e GC was equipped with a DB-5 fused silica capillary column, 30 m × 0.25 mm i.d.× 0.25 m �lm thickness (J�� Scienti�c, Folsom, CA, USA).Helium was used as the carrier gas at a �ow rate of 1 mLmin −1 . 2 L of the derivatized sample was injected into GC-FID under the splitless mode.e injection temperature was set at 300 ∘ C and the temperature programming was as follows: 60 ∘ C (held for 2 min), 20 ∘ C/min to 180 ∘ C, and 5 ∘ Cmin −1 to 220 ∘ C (held for 8 minutes).

Results and Discussion
3.1.XRD Characterization.Although there are three crystalline forms of TiO  (anatase, brookite, and rutile), the anatase form of titania is reported to give the best combination of photoactivity and photostability [22].e XRD spectra of the synthesized TiO  and TiO  -graphene are shown in  63.08 ∘ are attributed to the anatase phase [23].e absence of the peaks at 2 = 27.5 ∘ and 30.8 ∘ indicates that the rutile and brookite phases are not present.In addition, it is observed that the peak width broadened slightly with the introduction of graphene.It can be seen from Table 1 that the average crystallite size of TiO  -G is smaller than that of pristine TiO  .
It is thus inferred that due to the strong interactions between TiO  and graphene during the hydrolysis of the sol samples, the TiO  crystalline particles were unable to grow larger [18].
3.2.FTIR Spectroscopy.FTIR spectroscopy was used to characterize the interaction between graphene and TiO  nanoparticles.e FTIR spectrum of graphene oxide (Figure 3(a)) shows bands which are attributed to the epoxide ( C−O−C at 1250 cm −1 ), carboxylic acid ( CO at 1720 cm −1 ), and hydroxyl groups ( C−OH at 1365 cm −1 ) [24].Conversely, it can be seen from Figure 3(c) that most of these oxygen functionalities had been removed in the synthesized TiO graphene composite.A new band at ∼1580 cm −1 which may be attributed to the skeletal vibrations of the graphene sheets [25] and a strong band at ∼500-700 cm −1 , which is due to the Ti-O-Ti vibration [26] con�rms the presence of both graphene and TiO 2 in the synthesized composite.e IR spectrum of TiO 2 shows a broad band at ∼3400 cm −1 and the band at ∼1625 cm −1 originates from the surface-adsorbed water, and this indicates the presence of −OH groups on the surface of titania [27].Comparatively, the band at ∼2400-3400 cm −1 of TiO 2 -graphene is broader, thus indicating the presence of hydrogen bonding in the synthesized composite.Since hydrazine reduction introduces nitrogen groups onto graphene, the OH groups present on the surface of titania could form hydrogen bonds with the NH groups present on the graphene sheets to form the composite.
In addition, a new band at ∼1210 cm −1 is observed in the spectrum obtained for TiO 2 -G, and this can be assigned to Ti-O-C vibrations [28].Since hydrazine does not cause a full reduction of graphene oxide [20], the Ti-O-C bond may be formed when OH groups present in TiO 2 react with residual OH groups present on the graphene sheets in removing water.

Characterization of TiO 2 -G by Transmission Electron
Microscopy.From the TEM images, it can be seen that the graphene sheets are coated with TiO 2 nanoparticles.Figure 4(c) illustrates the homogeneous dispersion of TiO 2 nanoparticles on the graphene sheet, and it can be speculated that under ultrasonic irradiation, the graphene sheets interact with TiO 2 through strong chemisorption and physisorption [29].In addition, the graphene sheets appear transparent and are folded over one edge [24].us, this indicates that single graphene sheets have been isolated.

Energy Dispersive X-Ray Elemental Microanalysis. e
EDX spectrum shows the presence of Ti, C, and O atoms in the synthesized TiO 2 -graphene composite (Figure 5).It can be seen from Table 2 that the atom percentage of O is slightly more than twice that of Ti. is indicated that some oxygen functionalities remained on the graphene sheets aer hydrazine reduction, which is in agreement with the IR spectrum obtained.

Fluorescence Emission Spectroscopy.
Since photocatalytic activity is a function of lifetime and trapping of electron and hole pairs, �uorescence emission spectra are a useful characterization technique to investigate the efficiency of charge carrier trapping and to understand the fate of e−/h+ pairs in semiconductor particles like TiO 2 [21].Recombination of electron and hole pairs can emit energy in the form of �uorescence [30], and as seen from Figure 6, the �uorescence intensity of TiO 2 -G is lower than that of TiO 2 .is indicates that the recombination of electron and hole pairs is suppressed in the presence of graphene, which possibly indicates an increase in photocatalytic efficiency.
In addition, the �uorescence emission spectra displayed 3 main peaks at 380 nm, 420 nm, and 484 nm for both TiO 2 and TiO 2 -G, which are attributed to the self-trapped excitons localized on TiO 6 octahedra and oxygen vacancies [31].us, this indicates that the presence of graphene does not alter the mechanism of TiO 2 photocatalysis.

Method Validation.
Due to the polarity of APs, derivatization of the analytes is required prior to GC-FID analyses [32].Silylation using bis(trimethylsilyl)tri�uoroacetamide (BSTFA) is a rapid and commonly used derivatization technique in which the active hydrogen of the hydroxyl groups is converted into trimethylsilyl (TMS) group [33].ermally stable and highly volatile derivatives are obtained, and this results in improved gas chromatographic parameters such as accuracy, reproducibility, sensitivity, and resolution [34].Liquid-liquid extraction was employed to monitor the degradation rates since it is a simple, fast, and reliable technique for monitoring the disappearance of the parent compound [35].In order to access the practicality and suitability of this proposed LLE method, the optimized extraction conditions were used to determine the extraction method's repeatability, linearity, limits of detection (LOD), and limits of quanti�cation (LOQ).e results are summarized in Table 3. Repeatability was evaluated by triplicate analysis at the various analyte concentrations within the linear range of the extraction method.Satisfactory repeatability of relative standard deviations (RSDs) 9 to 12% was obtained.e linearity of this extraction method was evaluated at �ve different concentrations, ranging from 5 to 1000 g/L.e limit of detection of the analytes was determined (between 0.4 and 0.8 g/L) based on S/N ratio is 3, while the limit of quanti�cation (LOQ) was calculated (between 1.3 and 2.6 g/L) based on the de�nition of   .

Comparison of Photocatalytic Efficiency of the Synthe-
sized TiO 2 -G, TiO 2 , and Commercial Photocatalyst P25 on Alkylphenols in Wastewater Samples.Prior photocatalysis, wastewater samples were collected from local drainages and extracted using optimized LLE method.No target compounds were detected in the wastewater samples.erefore, experiments were designed with spiked (100 g/L of alkylphenols) wastewater samples.e degradation pro�les of alkylphenols are presented in Figure 7. e concentrations of photocatalyst used for each experiment is 0.1 g/L.A low dosage of photocatalyst loading was chosen for the comparison of photocatalytic efficiency since it would be more cost effective for practical applications.In order to con�rm that the source of decrease in analyte response was due to photodegradation rather than sample loss from extraction, a control experiment was carried out in which the sample solution was continuously stirred over a period of 3 hours and the variation in concentration was monitored.It can be seen that the percentage of the original analyte le remained unchanged.us photodegradation and not sample loss was the main source of declining peak areas which was observed in the subsequent experiments.
In addition, to monitor the effects due to direct photolysis only, the sample solution was irradiated with UV light.It was observed that the concentrations decreased slightly with UV irradiation, with more than 90% of the original solution still remaining at the end of 3 hours.
When the photocatalysts were added, the peak areas of the analytes declined signi�cantly, thus indicating the efficacy of advanced oxidation processes for the removal of these phenolic endocrine disrupting compounds.Figures 8(a 4. It was observed that TiO 2 -graphene composite with Ti : C ratio of 1 : 0.1 exhibited enhanced photocatalytic efficiencies for the photodegradation of the analytes over that of pristine TiO 2 , which was synthesized in the same way.As a comparison, the photocatalytic efficiencies were compared to that of the commercial TiO 2 photocatalyst, which has a phase composition of 80% anatase and 20% rutile.e synthesized TiO 2 -graphene (1 : 0.1) also displayed higher photocatalytic efficiency as compared to that of the commercial TiO 2 photocatalyst P25 for the degradation of 4-nonylphenol and 4-n-octylphenol.On the other hand, the TiO 2 -graphene photocatalyst with a higher carbon loading (Ti : C ratio of 1 : 0.2) had the highest photocatalytic efficiency for the photodegradation of 4-n-heptylphenol.e degradation rate of 4-nonylphenol was the lowest since technical nonylphenol is a mixture of isomers which contain both straight-and branched-chain NPs [36].

Performance of TiO 2 -G Composite on Photocatalytic
Efficiency.From the �uorescence emission spectra obtained, it can be seen that the presence of graphene can suppress the recombination of electron and hole pairs of TiO 2 .Under UV irradiation, electrons are excited from the valence band (VB) to the conduction band (CB) of the anatase, thus creating a hole in the valence band.In the absence of graphene, most of these charges recombine and only a small fraction participate in photocatalytic reactions (<1%), thus resulting in low photoefficiency.Given the semiconducting and unique electronic properties of graphene, electrons from the anatase particles can be transferred to the graphene sheets, thus allowing charge separation, stabilization, and hindered recombination [37].Consequently, the holes on the anatase particles are longer-lived and thus the TiO 2 -G composite has higher photocatalytic efficiency.
In addition, the large speci�c surface area of graphene sheets can also account for the enhanced photocatalytic efficiency of TiO 2 -G.Graphene sheets can adsorb the alkylphenol molecules in aqueous solutions and concentrate them on the surface of titania [38].It was noted that the APs at time = 0 min are largely decreased from the prepared initial concentration due to strong adsorption on the photocatalyst surface [32].Given the highly hydrophobic nature of the long-chained alkylphenols, they tend to adsorb on the highly hydrophobic graphene sheets.As a result, the dispersion of TiO 2 on graphene sheets and adsorption of the alkylphenol molecules created many active sites for photocatalytic degradation and thus enhanced the photocatalytic efficiency.e photocatalytic efficiency of the TiO 2 -G composite would increase with increase of the graphene percentage at optimum experimental conditions.e TiO 2 -G composite with Ti : C mass ratio of 1 : 0.1 displayed highest photocatalytic efficiency for the degradation of 4-nonylphenol and 4-n-octylphenol.One reason for the reduced photocatalytic efficiency with increase of graphene content is that higher composition of graphene sheets can shield the UV light from absorption by TiO 2 nanoparticles [18].us, with reduced UV adsorption by the TiO 2 particles, the composite would have reduced photocatalytic efficiency.Further studies are required to investigate this mechanism.From the experimental results obtained, higher photocatalytic efficiencies can be achieved with a graphene composition which is only 10% that of TiO 2 present for 4-n-nonylphenol and 4-octylphenol whereas a higher graphene loading (20% of TiO 2 present) in the synthesized composite was favorable for photodegradation of 4-heptylphenol.us, the presence of a small percentage of graphene can have synergetic effects on the photocatalytic efficiency of TiO 2 .

Conclusion
In this study, TiO 2 -graphene composite has been successfully synthesized and characterized.Compared to that of pristine TiO 2 , TiO 2 -Graphene exhibited enhanced photocatalytic efficiency of alkylphenols.By varying the composition of graphene in the photocatalyst, the photocatalytic efficiency was also varied.e photocatalytic efficiency of the photocatalyst exhibited better photocatalytic efficiency than the commercial TiO 2 photocatalyst, P25 with optimal loading of graphene.is might be due to the following reasons: (i) large surface area of graphene allowed it to act as a coadsorbent for the alkylphenol molecules and it resulted in more active sites for photocatalytic reactions to occur and (ii) graphene could inhibit the recombination of electron and hole pairs of TiO 2 , thus enhancing charge transfer and allowing the oxidation of adsorbed molecules.

F 5 :F 6 :a
(a) EDX area analysis and (b) SEM micrograph for the TiO 2 -graphene composite.Fluorescence emission spectra of (a) titanium dioxide and (b) titanium dioxide-graphene.T 1: Structural data for the synthesized composites based on XRD spectra.Determined by the use of the Debye-Scherrer equation [9] using the full width at half maximum (FHWM) for the 101 anatase XRD peak.T 2: EDX elemental microanalysis (atom %) of TiO 2 Titanium dioxide-graphene (1 : 0.1) 40.22 42.48 17.30 ) and 8(b) show the chromatogram aer the dark adsorption period (  ) and the chromatogram aer the addition of TiO 2 -G photocatalyst with UV irradiation for 3 hours, respectively.e percentage of individual analytes that remained aer 3 hours of UV irradiation with the different photocatalysts is shown in Table