Photocatalytic Degradation of 4-Chlorophenol by CuMoO 4-Doped TiO 2 Nanoparticles Synthesized by Chemical Route

The photocatalytic degradation of 4-chlorophenol (4-CP) in aqueous solution was studied using CuMoO4doped TiO2 nanoparticles under Visible light radiation. The photocatalysts were synthesized by chemical route from TiO2 with different concentration of CuMoO4 (CuxMoxTi1 − xO6; where, x values ranged from 0.05 to 0.5). The prepared nanoparticles are characterized by XRD, BET surface area, TEM, UV-vis diffuse reflectance spectra, Raman spectroscopy, XPS and EDAX spectroscopy were used to investigate the nanoparticles structure, size distribution, and qualitative elemental analysis of the composition. The CuxMoxTi1 − xO6 (x = 0.05) showed high activity for degradation of 4-CP under visible light. The surface area of the catalyst was found to be 101 m/g. The photodegradation process was optimized by using CuxMoxTi1 − xO6 (x = 0.05) catalyst at a concentration level of 1 g/l. A maximum photocatalytic efficiency of 96.9% was reached at pH = 9 after irradiation for 3 hours. Parameters affecting the photocatalytic process such as catalyst loading, concentration of the catalyst and the dopant concentration, solution pH, and concentration of 4-CP have been investigated.


Introduction
The photocatalytic degradation of different toxic compounds such as organic or inorganic pollutants, eliminated through photochemical reaction by using TiO 2 photocatalysts, has been widely studied [1][2][3][4][5][6][7][8].It is an attractive technique for the complete destruction of undesirable contaminants in both liquid and gaseous phase by using artificial light of solar illumination [9].
There are so many techniques used for the complete annihilation of undesirable contaminants in both liquid and gaseous phase by using artificial light and nano photocatalyst TiO 2 [10][11][12].However, there are two serious limitations, which were found in the conventional TiO 2 catalyst system that limits its practical applications.First, setting velocity of aggregated TiO 2 (average diameter of 0.2 µm) is very slow, thus requiring a long retention time in the clarifier.Second, as the quantity of TiO 2 is increased in order to increase the photocatalytic rate, the high turbidity created by the high TiO 2 concentration can decrease the depth of UV penetration.This effect can drastically lower the rate of photocatalytic reaction on a unit TiO 2 weight basis.Therefore the applications for metal ions have been used for doping TiO 2 [13][14][15][16] to increase the photocatalytic property by influencing generation and recombination of the charge carriers under light.Barkat et al and Woo et al reported that photodegradation of 2-chlorophenol by Co-doped TiO 2 and 4-chlorophenol by Ni 2+ -doped TiO 2 were photoactive under UV light but they did not investigate the degradation of 2/4-chlorophenol with P25 TiO 2 [17][18].The effects of copper (II) ions have been studied on the photodegradation of the insecticide monocrophos [19], photocatalytic degradation of sucrose [20], acetic acid [21], phenol [22], and methyl orange [23].But there is no such example of copper molybdenum doped TiO 2 photocatalysts.A successful application of CuMoO 4 -doped TiO 2 is the easy degradation of 4-chlorophenol in aqueous medium in presence of UV light.It is of interest to know how the photocatalytic degradation induced by TiO 2 doped metal ions will affect the treatment of water.The dopant ions or oxides can also modify the band gap or act as change separators of the photoinduced electronhole pair thus enhancing the photocatalytic activity [24].
In this paper, we report the preparation of copper molybdenum doped TiO 2 nano photocatalyst by chemical solution decomposition methods.The photocatalytic activities of the synthesized copper molybdenum doped TiO 2 nanocatalysts were compared with P 25 titania by examining the photodegradation of 4-CP as a model photocatalytic reaction under visible light.The Cu x Mo x Ti 1 − x O 6 (x = 0.05) (CMT1) shows better photocatalytic activity compared to P25 TiO 2 and the other compositions of copper molybdate doped titanium dioxide Cu photocatalyst for photodegradation of 4-CP.

Synthesis of Nanosized Anatase
CuMoO 4 -Doped TiO 2 The total synthesis was carried out in two steps by chemical solution decomposition method (CSD).In the first step the stock of Cu(NO 3 ) 2 •6H 2 O (Aldrich, 99.99%), (NH 4 ) 2 MoO 4 (Aldrich, 99.99%) and titanium tartarate solutions were prepared.The titanium tartarate solution was prepared by the following procedure.TiO 2 powder (Aldrich, 99.99%) is dissolved in 40% HF solution in a 500 ml teflon beaker kept on an water bath for ~24 h.During warming on the water bath, the solution was shaken occasionally.The clear fluoro complex of titanium was then precipitated with 25% NH 4 OH solution.The precipitate was filtered and thoroughly washed with 5% aqueous solution of NH 4 OH to make the precipitate fluoride free.Then the hydroxide precipitate of titanium is dissolved in tartaric acid (Aldrich, 99.99%) solution.The strength of the Ti 4+ in titanium tartarate solution was estimated by gravimetric method.
In the second step, the equivalent amount of copper nitrate, ammonium molybdate, and titanium tartarate solution were taken in a beaker as per chemical composition.The complexing agent TEA (triethanolamine) (MERCK, Mumbai, India) (where molecular ratio of metal ion:TEA = 1:3) was added to the homogeneous solution of constituents maintaining pH at 6 -7 by nitric acid (65%) and ammonia.The mixed solution was dried at 200˚C, resulting in a black carbonaceous light porous mass which was calcinated at three different temperatures namely 500˚C, 600˚C and 700˚C for 2 h at a heating rate of 5˚C/min for different chemical compositions of Cu x- Mo x Ti 1 − x O 6 (x = 0.05, 0.1, 0.5) nano powders.Complete synthesis procedure is presented below in the Flowchart 1 diagram.

Photocatalytic Activity
The photocatalytic activities of the newly prepared nanosized Cu x Mo x Ti 1 − x O 6 (x = 0.05, 0.1, 0.5) powders were characterized using photodegradation of 4-chlorophenol to carbon dioxide and water in aerated aqueous solution as a model photoreaction.The photocatalytic reactions were carried out by slow stirring the mixture using a magnetic stirrer with simultaneous irradiation by visible light source using a 300-W Xe lamp with a cut off filter (λ > 420 nm).The reactions were performed by adding nano powder of each photocatalyst (0.1 g) and the concentration of 4-CP is 50 ppm, into each set of a 100 ml of different solution of 4-CP.However, the efficiency of photodegradation of 4-CP is maximum at 10 ppm in the presence of prepared catalyst.The system was thoroughly repeated by several cycles of evacuation.
A small volume (1ml) of reactant liquid was siphoned out at regular interval of time for analysis.It was then centrifuged at 1500 rpm for 15 min, filtered through a 0.2 µm-millipore filter to remove the suspended catalyst particles and analyzed for the residual concentration of 4-CP by high performance liquid chromatograph (HPLC).The efficiency of the decolorization process at pH = 9 is measured by the following Equation ( 1), as a function of time.

 
Here C 0 and C are the initial and remaining 4-CP concentrations in the solution, respectively.

Characterization
The crystallinity of the prepared nano powders was checked by powder X-ray diffraction (XRD) with a Rigaku Model Dmax 2000 diffractometer using CuKa radiation (λ = 1.54056Å) at 50 kV and 150 mA by scanning at 2˚ 2θ min −1 .Scherer's equation was applied using the (101) peak to determine the pseudo-average particle size of nanosized anatase CuMoO 4 doped TiO 2 to reveal the effects of the preparation parameters on the crystal growth: D = Kλ/(βcosθ), where K was taken as 0.9, and β is the full width of the diffraction line at half of the maximum intensity.
The energy dispersive X-ray spectroscopy (EDX) (JEOL JMS-5800) was used to study the qualitative elemental analysis and element localization on samples being analyzed.BET surface area measurements were carried out using a (BECKMAN COULTER SA3100) on nitrogen adsorption desorption isotherm at 77K.
The morphology and the size of TiO 2 nanocrystallites ere investigated by high resolution transmission elec-w tron microscopy (HRTEM) with a JEOL-2010F at 200 kV.The particle size distribution of TiO 2 nanocrystallites was determined by directly measuring the particle sizes on the TEM images.The average particle size of each sample was determined by using the size distribution data based on a weighted-averages method.
The catalytic activity of the prepared nanoparticles was measured in a batch photoreactor containing appropriate solutions of 4-CP with visible light irradiation of 300-W Xe lamp.High performance liquid chromatography (HPLC) was used for analyzing the concentration of 4-CP in solution at different time intervals during the photodegradation experiment.
Raman spectrum was obtained by using a Perkin Elmer Spectrum GX Raman instrument.The UV-vis diffuse reflectance spectra of the prepared powders were obtained by a UV-vis spectrophotometer (UV-1601 Shimadzu) at room temperature

Transmission Electron Microscopy (TEM) Study 3.1. XRD Analysis
The XRD pattern of copper molybdenum doped titanium Bright field TEM (Model JEOL-2010F) micrograph of  However, the average crystallite size of CMT determined by the peak broadening method was found to be about 12 -13 nm obtained from XRD analysis (shown in Table 1).
The corresponding selected area electron diffraction pattern of the same sample (CMT1) showed distinct rings, characteristic of a single crystalline nanoparticle as shown in Figure 4 (inset).

Specific Surface Area (BET) Analysis
The BET of different compositions of CMT1, CMT2, CMT3, P25 TiO 2 , CM and CT calcined at 550˚C temperatures is listed in Table 1, which is measured by dini-T.K. GHORAI trogen adsorption-desorption isotherm in BECKMAN COULTER SA3100.It is noted that BET decreases as dopant concentration of metal ions increases at a particular composition.The sample CMT1 having high specific surface area, which was about 101 ± 5 m 2 /g, provided good photocatalytic properties among all the photocatalysts against 4-CP.Hence the large surface area enhanced photocatalytic activity through efficient adsorption of the reactant on the catalyst surface.

Raman Spectroscopy
Raman analysis of CuMoO 4 -doped TiO 2 alloy may allow us to rationalize these results.The Raman spectra of prepared nanoparticles calcined at 550˚C with varying mol% of CuMoO 4 in TiO 2 is shown in Figure 5.The analysis of Raman bands suggests that all active materials having bands at 337 cm -1 for CuO 2 [25] in-plane bond-bending mode and 971 cm -1 for Mo [26] correspond to Mo = O bond stretching modes.Except the above two bands, all the mentioned bands matched with characteristic bands of titania.shows the binding energy of O (1s) at 533.4 eV and Ti (2p) at 454.9 eV (2p 3/2 ) and 461.3 eV (2p 1/2 ) corresponding to Oxygen and Ti metal.

UV-VIS Diffuse Reflectance Spectrum
The UV-vis diffuse reflectance spectrum and band gap energy of all the compositions are shown in Figure 7 and Table 1 respectively.From Figure 7 and Table 1 we may conclude that the UV-vis diffuse reflectance spectrum of CuMoO4 doped TiO2 and pure TiO2, gave distinct band gap absorption edges at 409 nm, 401 nm, 397 nm and 387 nm for doped CMT1, CMT2, CMT3 and pure TiO2 and corresponding band gap energies are 3.03, 3.09, 3.12 and 3.20 eV respectively.At lowest concentration of CuMoO 4 , the absorption edge shift is maximum hence the corresponding calculated band gap energy is minimum.This is explained considering that when the amount of dopants is small, the metals ions are well incorporated into the lattice withstanding the evolvement of local strains.On the other hand, when the dopants are in excess, CuMoO 4 cannot enter the TiO 2 lattice but cover on the surface of TiO 2 in MO 3 form, and leads to the formation of heterogeneity junction.So, Cu x Mo x Ti 1 − x O 6 (x = 0.05) photocatalysts has lower band gap energy (3.03 eV) highest within the temperature range in which the experiments were carried out photocatalytic activity compared to other dopant concentrations and P25 TiO 2 .

Photocatalytic Activity of the Prepared Samples
The evaluation of the efficiency of photodegradation of 4-CP as a function of different experimental parameters is demonstrated in Figures 8-11.To study the effect of the catalyst on the 4-CP photodegradation rates, samples are annealed at different calcinations temperatures.The activities strongly depended on the calcination temperature of the catalysts.Figure 8 summarizes the results of these experiments.The highest degradation of 4-CP was achieved with samples that were annealed at 550˚C.However, increase in the calcinations temperatures of the catalysts decrease the photocatalytic activity due to increase of particle size and decrease of the specific surface area.The crystalline nature of the anatase structure is primarily responsible for the photocatalytic activity of the nanoparticles.Particles with anatase structure are known to have a better photocatalytic activity [27].Moreover, the small particle size of CMT1 (about ~10 nm) provides a large surface area where the catalytic reactions could occur and the photoreactivity is enhanced.The effect of the dopant concentration on the photo- catalytic activity of different compositions of copper molybdate doped titanium dioxide photocatalyst, on photodegradation of 4-CP has been presented in Figure 9.
From Figure 9, it could be noted that the dopant concentration in TiO 2 has a great impact upon its photocatalytic activity to decolorize 4-CP solution at pH = 9.The photodegradation efficiency of 4-CP decreased with increasing CuMoO 4 concentration, reaching a maximum value of 96.9% with sample containing 0.05 mol% Cu-MoO 4 that was annealed at 550˚C.The photodegradation efficiency of 4-CP using all the photocatalyst are presented in Table 1.The anatase CMT1 nanocrystallites with regular crystal surfaces should have less surface defects, giving highly efficient photocatalysis by suppression of electron-hole pair recombination through redox cycle between Cu(II) and Cu(I).Cu(II) ions work as electrons scavengers which may react with the superoxide species and prevent the holes-electrons (h + /e -) recombination and consequently increase the efficiency of the photo-oxidation.The possible reaction is shown below: Defect sites are identified as Ti 3+ on the TiO 2 surface due to adsorption and photoactivation of oxygen thus, increasing the photocatlaytic efficiency.Electron can be also excited from defect energy levels Ti 3+ , to the TiO 2 conduction band and photodegradation occurs.mainly a photolysis process.The degradation of 4-CP is increased by adding CuMoO 4 doped TiO 2 with CuMoO 4 concentration of 0.05 mol%.The degradation reached a maximum value of 96.9 with catalyst dosage of 1 g/L and the optimize concentration of 4-CP is 50 ppm.However, a further increase in the catalyst dosage slightly decreased the degradation efficiency.The photodecomposition rates of pollutants are influenced by the active site and the photo-absorption of the catalyst used.Adequate loading of the catalyst increases the generation rate of electron/hole pairs for enhancing the degradation of pollutants.However, addition of a high dose of the semiconductor decreases the light penetration by the photocatalyst suspension [28] and reduces the degradation rate.A Langmuir-Hinshelwood type [29] of relationship can be used to describe the effect of 4-CP concentration on its degradation.
The pH of the solution has a strong effect on the photodegradation process, as shown in Figure 11.Degradation efficiency of 4-CP has not been found to be significant at low pH values but increased rapidly with increase of the pH, attaining a maximum value of 96.9% for pH of 9. Further increase in pH of 4-CP decreases the photodegradation efficiency, because the protons are potential determining ions for TiO 2 , and the surface charge development is affected by the pH [30].
Upon hydration, surface hydroxyl groups (TiOH) are formed on TiO 2 .These surface hydroxyl groups can undergo proton association or dissociation reactions, thereby bringing about surface charge which is pH-dependent and photodegradation occurs.
However, the dopant concentrations of the prepared catalyst above an optimal value result in the formation of  recombination centers which traps the charges for a very long time thereby reducing the photodegradation performance.

Conclusions
In this study, nanophotocatalyst CuMoO4 (5 mole%) doped TiO2 synthesized by CSD method is more photoactive than all other compositions of copper molybdate doped TiO2 and P25 TiO 2 due to high surface area (101 m 2 /g), lower band-gap (3.03 eV) and photochemical degradation on 4-CP through redox cycle between Cu(II) and Cu(I).The typical composition of the prepared Cu-MoO4 doped TiO2 was CuxMoxTi1 − xO6 with the value of x ranging from 0.05 to 0.5.The photocatalytic activity strongly depends on CuMoO4 doping concentration.The photodegradation process was optimized by using Cux MoxTi1 − xO6 (x = 0.05) catalyst at a concentration level of 1g/L.A maximum photocatalytic efficiency of 96.9% was reached at pH = 9 after irradiation for 3 hours.The light absorption measurements confirmed that the presence of 5 mol% CuMoO4 doped TiO2 structure caused significant absorption shift into the visible region compared to the pure TiO2 powder.

Flowchart 1 .
Synthesis of different composites of nanosized copper molybdenum doped titanium dioxide Cu x Mo x Ti 1-x O 6 (x = 0.05, 0.1, 0.5) photocatalysts.dioxide, Cu x Mo x Ti 1 − x O 6 [when x = 0.05 (CMT1), 0.1 (CMT2) & 0.5 (CMT3)], copper doped TiO 2 [Cu-TiO 2 (CT)] and CuMoO 4 (CM) are shown in Figure 1 at 550˚C.It can be seen that the peaks at 2θ of 25.26˚, 38.16˚, 48.17˚, 54.03˚, 55.12˚, and 64.69˚ are assigned to (101), (004), (200), (105), (211), and (204) respectively (JCPDS data File No. 84 1285) lattice planes of TiO 2 , which are attributed to the signals of anatase phase.No additional peaks were found to be present, which could be assigned to the CuMoO 4 anorthic phase that indicated that the resulting nano powder was alloy of copper molybdate with titanium dioxide.Rutile phase was not observed for all specimens using different Ti precursors.The XRD patterns of Cu x Mo x Ti 1 − x O 6 (x = 0.05) recorded at room temperature after annealing the samples at various temperatures, indicated no change of crystallographic characteristics shown in Figure 2. Furthermore, the EDAX spectroscopy measurements (Figure 3) show a molar ratio of CuMoO 4 : TiO 2 equal to about 0.05:0.95.

Figure 2 .Figure 4 .
Figure 2. XRD of Cu x Mo x Ti 1 − x O 6 (x = 0.05) recorded at room temperature after annealing the samples at various temperatures.

Figure 6 FigureFigure 5 .
Figure 6 shows the results of XPS spectra of Cu x Mo x Ti 1−x O 6 (x = 0.05).CuMoO 4 doped TiO 2 where the concentration of CuMoO 4 is 0.05 mole, the Cu 2 O/CuO and MoO 3 are shown in Figure 6 to identify the copper and molybdenum state on the surface of TiO 2 .The Cu (2p)-binding energies of Cu 2 O/CuO were found to be 932.8 and 953.4 eV, respectively and corresponding Mo (2p)-binding energy is 233.0 eV.According to the position and the shape of the peaks, the copper on the surface of TiO 2 may exist in multiple-oxidation states.Oxygen and Ti show surface characteristic photoelectron peaks.Figure

Figure 7 .
Figure 7.The UV-visible diffuse reflectance spectra of M-Ti samples with the highest dopant-atom content.

Figure 8 .
Figure 8. Photocatalytic effect of the CMT1 crystal structure on the 4-CP photodegradation at different calcinations temperatures.

Figure 10 Figure 9 .
Figure10shows the effect of the CuMoO 4 -doped TiO 2 dosage on the 4-CP degradation.It can be seen that in the absence of the catalyst about 12% of 4-CP was removed at pH 9 after 3 h of irradiation of UV.This is