Visible Light Photocatalysis via CdS / TiO 2 Nanocomposite Materials

Nanostructured colloidal semiconductors with heterogeneous photocatalytic behavior have drawn considerable attention over the past few years. This is due to their large surface area, high redox potential of the photogenerated charge carriers, and selective reduction/oxidation of different classes of organic compounds. In the present paper, we have carried out a systematic synthesis of nanostructured CdS-TiO2 via reverse micelle process. The structural and microstructural characterizations of the as-prepared CdSTiO2 nanocomposites are determined using XRD and SEM-EDS techniques. The visible light assisted photocatalytic performance is monitored by means of degradation of phenol in water suspension.


INTRODUCTION
Anatase TiO 2 is the most widely used semiconductor photocatalyst for an effective decomposition of organic compounds in air and water under UV light irradiation, with wavelength shorter than 387 nm [1,2].The major problem is that only about 4% of the solar spectrum falls in this UV range.The efficient use of sunlight becomes an appealing challenge for developing photocatalysts [3][4][5].One approach for achieving this objective is to sensitize TiO 2 by using a narrow band gap semiconductor with a higher conduction band than that of TiO 2 .CdS with band gap energy of 2.5 eV is considered to be one of the many sensitizers used for large band gap semiconductors because of the ideal position of its conduction and valence band edges.CdS alone, however, shows negligible photocatalytic activity because of its instability and rapid electron-hole pair (EHP) recombination rates.Studies have proven that with the appropriate particle interaction, CdS-TiO 2 nanocomposites can efficiently decompose organics such as phenol and methylene blue under visible light irradiation less than 495 nm [6][7][8].
The reaction as shown in Figure 1 occurs when a CdS particle is excited by a photon with a wavelength less than 495 nm.An EHP is formed, and subsequently the photogenerated electron is quickly transferred to the conduction band of a coupled TiO 2 particle that has a conduction band edge more positive than the CdS particle (∼ 0.5 eV).The photogenerated hole in the quantum-sized CdS particle can theoretically migrate to the surface and participate in the oxidation of adsorbed organics.The electrons that are transferred to the conduction band of TiO 2 have no holes to recombine with and therefore participate in reduction reactions according to the conduction band energy level of TiO 2 .In order to establish a quantum size effect, one must have to synthesize extremely small particles of the order of the excitonic diameter and to stabilize them against further growth.Reverse micelle process provides a controlled environment to achieve this goal.
In the present study, we have carried out the successful formulation of CdS-TiO 2 nanocomposite via reverse micelle process and studied their structural, microstructural, UV-Vis spectral characteristics and visible light photocatalytic behavior.

Materials and method
The chemicals used for the synthesis of CdS-TiO 2 nanocomposite are of the purest quality and are used as received: Journal of Nanomaterials In the next step, adding 1 M Cd(ClO 4 ) 2 .× H 2 O to this reverse micelle to form microemulsion "A."Similarly, the microemulsion "B" is formed by adding 0.1 M Na 2 S to a separate reverse micelle solution.Mixing the microemulsions A and B with vigorous stirring at room temperature thus produces CdS yellowish microemulsion "C."Another microemulsion "D" is formed by adding 1 M Ti(OPr) 4 /2propanol in separate reverse micelle and stirred continuously for 1 hour.The water to alkoxide ratio (h) have been varied and optimized for the TiO 2 precipitation, ( The microemulsions of "C" and "D" are finally mixed and vigorously stirred to form CdS-Xwt.% TiO 2 (x= 5, 10, 50).The CdS-TiO 2 precipitate obtained is then washed couple of times with DI water, ethanol, and acetone to remove the surfactant.The amorphous nanocomposite of CdS-TiO 2 is calcined at 500 • C under the flow of N 2 for 3 hours.

Structural and microstructural characterization
The structural characterization for the phase identification and particle size analysis has been carried out using Philips X'pert pro powder X-ray diffractometer with CuKα radiation (λ = 1.54060Å).The incident and diffraction slit width used for all the experiments are 1 • and 2 • , respectively, and the incident beam mask used corresponds to 10 mm.The XRD profile analysis and particle sizes have been calculated using PANalytical X'pert Highscore software version 1.0 e.The microstructural characterizations such as surface morphology and chemical composition were performed using scanning electron microscope SEM (Hitachi S-800) both in imaging and EDS modes.Genesis software has been used to analyze the microstructures and elemental composition of the CdS-TiO 2 nanocomposites.

BET surface area measurements
Catalysts and photocatalysts are often characterized by their interaction with gases.At low temperatures, nonreactive gases (nitrogen, argon, krypton, etc.) are physisorbed by the surface.Through gas physisorption the total surface area of the sample can be calculated by the BET method.The Chem-BET 3000 from Quantachrome Instruments has been employed to determine the surface area and a pore size distribution of the pure TiO 2 , pure CdS, and CdS-TiO 2 nanocomposites.A known amount of sample (∼ 100 mg) was placed in a glass tube and the sample was outgassed at 300 • C for 5 hours.Multipoint BET method using nitrogen as the adsorbate gas, the isotherm has been measured at 77 K.

UV-Vis transmittance spectroscopy
The transmittance measurement is performed by UV-Vis spectrometer.The powder samples of pure TiO 2 and CdS-TiO 2 nanocomposite are made into slurry by dissolving it in (Acetylacetone + H 2 O + Triton X-100) solution.Thus obtained slurry was spin coated at 4000 rpm on to the glass slide for UV-Vis spectrum studies.The homogeneous films obtained from spin coating samples were then subjected to UV-Vis spectroscopy measurements in transmittance and absorption modes using an Ocean Optics USB2000 fiber optic spectrometer.

Photocatalytic reactor testing
Photocatalytic experiments using visible light and a combination of UV-A and visible light were performed using a single lamp and an annular reflector to provide irradiation to all sides of the reaction vessel as shown in Figure 2. The immersion well was used as a means of concentrating the aqueous solution to the perimeter of the reaction vessel to optically optimize the system.A 1000 W metal halide lamp was experimentally chosen, since it adequately replicated both the pattern and intensity peaks of the solar spectrum more accurately than other visible sources such as high-pressure sodium and fluorescent lamps.
The metal halide spectrum, as shown in Figures 3(a) and 3(b), was found to contain sufficient UV-A radiation in the range of 350 nm-400 nm to serve as both a UV-A and visible light source.For photocatalytic experiments using visible light region, a longpass UV filter provided by Edmund Optics was used to cut off the wavelengths shorter than 400 nm, at a cost of approximately 8% attenuation at wavelengths greater than 400 nm.The system was cooled from 110 • C to 70 • C using ventilation provided by three fans inputting and removing air from the surface of the lamp and inside the reactor.

Phenol degradation by UV-Vis spectroscopy measurement
Photocatalytic phenol degradation experiments were conducted using the following procedure.A solution of deionized water, phenol, and the catalyst under test were mixed  using magnetic stirring, and air was injected to scavenge photogenerated electrons and also to help suspend the catalyst.The phenol concentration in all experiments was 40 ppm which was selected as a value large enough to accurately measure using absorption measurements and small enough to degrade within hours when exposed to an efficient photocatalyst.Air was dispersed in the aqueous solution through a 12inch rod sparger and was used in place of pure O 2 to simulate real-world experimental conditions.Periodic 1.5 mL samples were retrieved from the reaction vessel in desired time increments and placed in Eppendorf microcentrifuge tubes for analysis of the photocatalytic degradation of phenol.The nanoparticle catalysts were separated from the phenol and water mixtures by centrifuging the Eppendorf tubes for 10 minutes at 4000 rpm and 3220 rcf using an Eppendorf 5810 R centrifuge.UV-Vis spectroscopy using an Ocean Optics USB2000 fiber optic spectrometer was used to analyze the degradation of phenol with respect to time by monitoring the intensity of the absorption peak of phenol located at 268.63 nm.Phenol degradation was determined with respect to its initial concentration, and therefore all degradation plots are normalized based on C/C 0 values which are interpreted as the concentration at the measured time divided by the initial concentration.

Structural characterization
Figure 4 represents the X-ray diffraction patterns of (a) pure CdS nanoparticle (b) pure TiO 2 anatase and (c) CdS-50 wt.% TiO 2 nanocomposite prepared via reverse micelle process.No impurity peaks from the precursor ingredients such as cadmium perchlorate hydrate and sodium sulfide are observed for the pure TiO 2 and CdS as shown in Figures 4(a) and 4(b).However, for the CdS-50 wt.% TiO 2 nanocomposite, impurity peaks due to sodium perchlorate (NaClO 4 ) (see Figure 4(c)) are observed at the angle of orientations 22.4098 • and 30.21111 • .This may be due to the starting precursor reactions, which needs further filtration and purification.Also from these X-ray patterns, it is easily discernable that there exist two different grain sizes of the nanocomposite mixture due to CdS nanoparticle and TiO 2 anatase.We have calculated the average crystallite sizes from the X-ray diffraction patterns (Figure 4) according to Scherrer's equation and given in Table 1.The nanocomposite CdS-50 wt.% TiO 2 exhibits the larger crystallites when compared to either pure TiO 2 or pure CdS.This is due to high temperature (500 • C) calcination involved after the composite formation in reverse micelle process.

Microstructural and chemical characterization
The microstructural characterization of the CdS-50 wt.% TiO 2 nanocomposite is carried out via scanning electron microscopy.Figures 5(a) and 5(b) show the SEM image and EDS spectrum of the CdS coupled TiO 2 nanocomposite structure.Reverse micelle process of preparing semiconductor nanocomposite yields homogeneous particles, thus improves the interparticle electron transfer from high-lying conduction band semiconductor with a small band gap (e.g., CdS ∼ 2.5 eV) to low-lying conduction band semiconductor with a large band gap (e.g., TiO 2 ∼ 3.2 eV).The EDS profile shows correct stoichiometry of CdS to TiO 2 in the nanocomposite structure.The atomic ratio of Cd and Ti in the nanocomposite structure was 3 at.%: 9 at.%. obtained from the SEM-EDS spectrum as shown in Figure 5(b).
The growth of intergrain boundaries between the CdS and TiO 2 nanostructures and the particle size varies with the experimental parameters such as water-to-surfactant ratio (w 0 ), water-to-alkoxide ratio (h), pH concentration of obtained solution, ratio of CdS to TiO 2 precursors, and calcination temperatures.The optimized parameters for the reverse micelle nanocomposite CdS-50 wt.% TiO 2 structure are given in Table 2.The standards for determining the optimizing conditions given in Table 2 are explained below.
The molar ratio of H 2 O to surfactant AOT (w 0 ) was varied as 2, 4, 6, 8, and 10.The reverse micelle and gelation formation rates are highly controlled by varying this ratio.Rapid gelation with larger crystallites occurred during the higher molar ratio of water to surfactant, (e.g., w 0 = 8, 10) whereas slow gelations are due to the lower value of w 0 (e.g., 2, 4).An optimum value of [AOT]/[H 2 O] = 6 was found and used for the further experiments.Similarly, the ratio of [H 2 O]/[Ti-alkoxide] was optimized to obtain appropriate concentration of TiO 2 .The acidic/basic nature of the reverse micelle CdS-TiO 2 is easily controlled by decreasing or increasing the pH value.The rapid precipitation effect was monitored and controlled by suitably adjusting the pH to 9. Thus obtained CdS-TiO 2 nanocomposite was then calcined at different temperatures under flowing N 2 atmosphere.Increasing the calcination temperature above 500 • C, the appearance of rutile TiO 2 structure is inevitable and thus lowers the photocatalytic effect of CdS-TiO 2 nanocomposites.Calcination temperature below 500 • C yields poor crystalline structure of CdS and TiO 2 .Hence the optimum calcination temperature was found to be 500 • C for rest of the experiments.

UV-Vis spectral response
The UV-Vis transmittance spectral response for the pure TiO 2 and CdS-50 wt.% TiO 2 nanocomposite is shown in Figure 6.The onset of transmittance starts below 350 nm and a sharp transition occurred at around 380 nm for the pure TiO 2 anatase structure.Whereas for the case of CdS-50 wt.% TiO 2 nanocomposite prepared by reverse micelle process, the spectral curve extends to the visible light region with the onset value corresponding to 490 nm.The absorption edge of TiO 2 in CdS-TiO 2 nanocomposites observed above 400 nm indicates a significant "blue shift" of the bandgap energy compared to pristine TiO 2 .The quantum-sized effect of CdS in the nanocomposite structure seems plausible explanation for this "blue shift" effect [20].However, in the present study, the quantum CdS is not observed due to the coalescent or agglomeration of nanoparticles in the reverse micelle process.This may suggest that the nanocomposite coupled semiconductor (CdS-50 wt.% TiO 2 ) is expected to increase photogenerated charge carriers under the irradiation of visible light.It is noteworthy to mention that the absorption spectrum due to TiO 2 is not present in the CdS-TiO 2 nanocomposite films due to the different film thickness between TiO 2 and CdS nanoparticles.Besides, the concentration of TiO 2 also affects the optical absorption of CdS-TiO 2 nanocomposites in the UV range.

Photodegradation of phenol in water suspension
Photoactivity of the as-prepared CdS-Xwt.%TiO 2 nanocomposite materials is determined by the degradation of phenol in water suspension under visible light irradiation.Photocatalytic experiments using purely visible light, a longpass UV filter provided by Edmund Optics was used to cut off the wavelengths below 400 nm.Phenol is chosen as a  test degradant for its photoactivity behavior.The primary reason is that phenol is an ideal degradant for visible light photocatalysis measurements because it absorbs only UV-C radiation with an absorption peak between 265 nm and 270 nm.This implies that it will not be susceptible to photolysis when irradiated by UV-A and visible light.Phenol is also a test degradant that exhibits application, since it is a common contaminant of industrial effluents.Photocatalytic oxidation of phenolic compounds in industry has been suggested to replace other processes such as activated carbon absorption, chemical oxidation, biological treatment, wet oxidation, and ozonolysis [21].A third reason phenol is used is that the photooxidation of phenolic compounds illuminated TiO 2 surfaces has been widely studied and the reaction kinetics are found to be first order following the well established Langmuir-Hinshelwood equation [22,23].The normalized phenolic concentrations (C/C 0 ) at regular intervals for the CdS-Xwt.%TiO 2 nanocomposite have been plotted in Figure 7.There is no direct correlation of rate of degradation and the CdS content was observed in the CdS-TiO 2 nanocomposite structure.However, the photodegradation of phenol concentration up to 40% is achieved in the CdS-50 wt.% TiO 2 nanocomposite material.Great efforts are presently being carried out to tailor the nanocomposite system by optimizing  the experimental conditions and solve the corrosion behavior of CdS for the effective photoactivity under visible light irradiations.We will also attempt to synthesize these systems by mechanochemical process employing high energy planetary milling and the results will be forthcoming.

CONCLUSIONS
We have systematically carried out the synthesis of pure TiO 2 , pure CdS and CdS coupled TiO 2 nanocomposite by employing reverse micelle process.The X-ray phase analysis of these systems shows evidence for the formation of heterojunction CdS-Xwt.%TiO 2 nanocomposite structure with average crystallite sizes ranging from 18 nm to 60 nm.The microstructural investigation of the nanocomposite envisages the correct stoichiometry of CdS to TiO 2 and homogeneous surface morphology.The surface-coated CdS and TiO 2 anatase nanocomposite extends the absorption band edge in the visible region as observed from the UV-Vis measurements.The as-synthesized nanocomposite materials exhibit better photoactivity under visible light irradiation than that of pure TiO 2 .

Figure 3 :
Figure 3: (a) UV-Vis metal halide spectrum compared to solar irradiation and (b) effects of UV-cutoff filter on metal halide irradiation pattern.