Facile Preparation of Efficient WO 3 Photocatalysts Based on Surface Modification

Tungsten trioxide (WO 3 ) was surface modified with Cu(II) nanoclusters and titanium dioxide (TiO 2 ) nanopowders by using a simple impregnation method followed by a physical combining method. The obtained nanocomposites were studied by scanning electron microscope, X-ray photoelectron spectroscopy spectra, UV-visible light spectra, and photoluminescence, respectively. Although the photocatalytic activity ofWO 3 was negligible under visible light irradiation, the visible light photocatalytic activity of WO 3 was drastically enhanced by surfacemodification of Cu(II) nanoclusters and TiO 2 nanopowders.The enhanced photocatalytic activity is due to the efficient charge separation by TiO 2 and Cu(II) nanoclusters functioning as cocatalysts on the surface. Thus, this simple strategy provides a facile route to prepare efficient visible-light-active photocatalysts for practical application.


Introduction
As one of the most important transition metal oxides, tungsten trioxide (WO 3 ) has attracted considerable attention due to its promising physical and chemical properties [1,2].Considering its small band gap, stable physicochemical properties and resilience to photocorrosion effects, WO 3 has been widely considered as a feasible candidate for visible-light photocatalysts [1][2][3].However, several fundamental issues have to be addressed before they are economically available for large scale industrial applications.For example, pure WO 3 is usually not efficient photocatalysts because of the high electron-hole recombination rate and the difficulty in the reduction of oxygen, due to the negative position of its conduction band (CB) [4].Thus, many efforts have been made to improve the activity of WO 3 , such as morphology control, doping, nanostructure construction, and surface modification [4].One of the most promising ways to accomplish this goal is to design heterogeneous catalysts [5].So far, various heterogeneous WO 3 based heterogeneous structures, such as WO 3 /SiO 2 , WO 3 /TiO 2 , WO 3 /NiO, and Pt/TiO 2 -WO 3 , have been designed toward good catalytic performance [5][6][7][8][9][10][11][12].Titanium dioxide (TiO 2 ) has attracted much attention as a suitable semiconductor to construct heterogeneous structures with WO 3 , due to its low cost, nontoxicity, and suitable band structure [5,[13][14][15][16].The valence band (VB) and CB potentials of TiO 2 are more cathodic than those of WO 3 [16].The coupling of TiO 2 and WO 3 can lead to photogenerated electron and hole transfer from one semiconductor particle to another; those are electrons transfer from the CB of TiO 2 down to the CB of WO 3 and holes transfer from the VB of WO 3 to that of TiO 2 [17].This process suppresses the recombination of photogenerated carriers and leads to improved photocatalytic efficiency of the system [17].To further increase the photocatalytic performance of the system, metal or metal oxide particles, such as Au, Pt, and CuO, were introduced into the system to promote the reduction reaction of electrons with oxygen molecules, leading to efficient consumption of electrons [10][11][12].For example, copper ions modified WO 3 /TiO 2 nanocomposites, prepared by Hosogi and Kuroda, exhibited efficient photocatalytic activities [12].However, there are still many problems in the practical application of these reported catalysts, including the complexity of the preparation procedures, the requirement for expensive raw materials, and the difficulties for large scale production [5][6][7][8][9][10][11][12].For instance, sol-gel method and urea as raw materials were needed in the preparation of copper ions modified WO 3 /TiO 2 nanocomposites [12].Thus, efforts aimed at improving the photocatalytic performance and the preparation process of WO 3 are still needed.
In the present work, we reported efficient Cu(II) nanoclusters modified WO 3 /TiO 2 nanocomposites (Cu(II)-WO 3 /TiO 2 ) through a facile preparation process.In this process, Cu(II) nanoclusters were deposited on WO 3 using a simple impregnation method and TiO 2 nanopowders were introduced into the Cu(II) nanoclusters modified WO 3 (Cu(II)-WO 3 ) by a physical combination method.The introduced Cu(II) nanoclusters and TiO 2 nanopowders functioned as cocatalysts on the surface of WO 3 .Thus, the obtained Cu(II)-WO 3 /TiO 2 products exhibited an enhanced visible light photocatalytic activity.

Experimental
2.1.Materials.Commercial tungsten (VI) oxide (Sigma-Aldrich; for monoclinic WO 3 , particle size is ∼100 nm) was used as the initial WO 3 .CuCl 2 ⋅2H 2 O (Sigma-Aldrich) was used as the source of Cu(II) nanoclusters.Degussa (Evonik) P25 TiO 2 nanopowders (particle size ∼25 nm) was used as the raw material of TiO 2 .All of these commercial materials were used as received, without further purification.Distilled water was applied in the experimental process.

Preparation of the Composites.
Cu(II) nanoclusters were grafted on the surface of WO 3 by using an impregnation method, as reported previously [18,19].CuCl 2 ⋅2H 2 O was used as the Cu(II) nanoclusters source to prepare Cu(II)-WO 3 .1 g WO 3 powder with 0.1% weight fraction of Cu to WO 3 was dispersed in 10 mL distilled water.0.1% weight fraction of Cu to WO 3 has demonstrated the optimized amount of Cu(II) nanoclusters for Cu(II)-WO 3 systems [18].The suspension was heated at 90 ∘ C and stirred for 1 h in a vial reactor to hydrolyze the CuCl 2 source and generate Cu(II) nanoclusters on the surface of WO 3 .Then, the suspension was filtered twice with a membrane filter (0.025 m, Millipore) and washed with sufficient amounts of distilled water.The resulting residue was dried at 110 ∘ C for 24 h and subsequently grounded into fine powder using an agate mortar and pestle.
The mixing of Cu(II)-WO 3 with TiO 2 was performed using a physical mixing method.Typically, 1 g Cu(II)-WO 3 powder and 1% weight ratio TiO 2 were mixed in an agate mortar and grounded into fine powder using a pestle for 1 h.

Sample Characterizations.
Scanning electron microscope (SEM) images were taken using a field-emission SEM (FE-SEM, Hitachi S-4800).Photoluminescence PL spectra were obtained by using a Hitachi F-4500 fluorophotometer with an excited wavelength of  = 325 nm at room temperature.UV-visible reflectance spectra were obtained by the diffuse reflection method using a spectrometer (UV-2550, Shimadzu).Surface compositions were studied by X-ray photoelectron spectroscopy (XPS; model 5600, Perkin-Elmer).The binding energy data were calibrated with reference to the C 1s signal at 284.5 eV.

Catalytic Activity
Testing.Photocatalytic activity of the WO 3 samples was evaluated in terms of the decolorization of methylene blue (MB) dye under visible irradiation.20 mg sample was dispersed into 100 mL of 10 mg/L MB solution and stirred in the dark for 1 h to reach a complete adsorptiondesorption equilibrium.Then the solution was irradiated with ∼20 mW/cm 2 visible light (>420 nm, with a light filter L42 (Asahi Techno-Glass)) under continuous stirring.With a given irradiation time interval, some specimens (5 mL) were taken from the dispersion and were centrifuged (4000 rpm).The clear upper solution was subjected to UV-Vis spectrophotometer (UV-2550, Shimadzu).The concentration of MB was determined from the absorbance at the wavelength of 665 nm.

Results
Figure 1 shows the SEM images of the obtained samples.It can be seen that the bare WO 3 samples contained many particles.These particles have a clear surface and a size of ∼100 nm.After Cu(II) nanoclusters grafting, the particle morphology still remained, indicating the grafting of Cu(II) nanoclusters did not affect its morphology (Figure 1(b)).After the modification with TiO 2 nanopowders, some small particles could be observed in TiO 2 mixed Cu(II)-WO 3 (Cu(II)-WO 3 /TiO 2 ) samples (Figure 1(c)).These small particles have a size around several tens of nanometers, coinciding with the size of TiO 2 nanopowders.In the paper, except specially noted, the weight ratios of Cu and TiO 2 to WO 3 were set to 0.1% and 1%, respectively, In order to determine the surface composition and chemical states of the surface elements, XPS spectra were recorded, as shown in Figure 2. In the W 4f and O 1s core-level spectra of the samples (Figures 2(a) and 2(b)), no obvious differences could be seen in the chemical states of elements W and O, demonstrating that neither the surface grafting of Cu(II) nanoclusters nor physical mixing of TiO 2 powders affected the bonding structure between tungsten and oxygen.In the Cu 2p core-level spectra (Figure 2(c)), Cu signals were clearly observed in Cu(II)-grafted samples, such as Cu(II)-WO 3 and Cu(II)-WO 3 /TiO 2 , confirming that Cu(II) was successfully grafted on the surface of WO 3 , while, in the Ti 2p core-level spectra (Figure 2(d)), Ti signal was only observed in Cu(II)-WO 3 /TiO 2 composites, indicating the TiO 2 was well mixed with WO 3 powders.
Figure 3 shows the UV-Vis of the samples.It clearly shows that WO 3 has a good visible light absorption property, indicating it is a potential visible light photocatalyst.The absorption edge of WO 3 is located at ∼460 nm, which corresponds to the interband transition of WO 3 [18].This interband absorption indicates a band gap of ∼2.7 eV, which coincides with the reported values of 2.7 eV for WO 3 [20,21].After Cu(II) nanoclusters grafting, an additional light absorption at the range of ∼700-800 nm was clearly superimposed on the light absorption of WO 3 , as shown in the inset of Figure 3.This additional light absorption can be attributed to the d-d transition of Cu(II) [18].After further modification with TiO 2 powders, the interband transition of WO 3 was not changed, due to the small amount of TiO 2 powders and their large band gap [13][14][15].Notably, the additional visible light absorption caused by the d-d transition of Cu(II) can still be observed in the mixed nanocomposites, proving the existence of Cu(II) nanoclusters (inset of Figure 3).The grafting of Cu(II) nanoclusters to the surface switched its photocatalytic activity.It can be seen that MB dye was almost degraded by Cu(II)-WO 3 with 2 h of visible light irradiation.Interestingly, after further modification with TiO 2 nanopowders, Cu(II)-WO 3 /TiO 2 exhibited an enhanced photocatalytic activity compared with that of Cu(II)-WO 3 .MB dye was completely degraded by Cu(II)-WO 3 /TiO 2 nanocomposites with 1.5 h of visible light irradiation, revealing the high photocatalytic activity of the Cu(II)-WO 3 /TiO 2 nanocomposites.Figure 4(c) shows the pseudo-first-order kinetic rate for the photochemical degradation of MB by Cu(II)-TiO 2 samples.The pseudo-first-order kinetic rate was calculated according to the equation of ln(  /) = , where /  is the normalized MB concentration,  is the reaction time, and  is the pseudo-first-rate constant.It can been seen that the Cu(II)-WO 3 /TiO 2 samples presented the highest reaction rate.The reaction rate was sharply decreased when bare WO 3 was used.The result was consistent with the MB decomposition curves in Figure 4(b).Figure 4(d) shows the cycling measurements of MB decomposition over Cu(II)-WO 3 /TiO 2 .Similar  values were obtained after 5-cycle measurements, suggesting a good stability for the photocatalytic application of Cu(II)-WO 3 /TiO 2 .
We also investigated the influences of experimental parameters on the photocatalytic performances of Cu(II)-WO 3 /TiO 2 samples.Figure 5 shows the photocatalytic performances of Cu(II)-WO 3 /TiO 2 samples with different ratios of TiO 2 .It can be seen that the activity of the samples was increased with the ratio of TiO 2 to WO 3 in the beginning.After the highest activity was achieved at the ratio of 1%, the photocatalytic activity was decreased again with the increase of ratio.These results revealed that the Cu(II)-WO 3 /TiO 2 samples obtained with 1% TiO 2 have the optimum amount of TiO 2 for hole separation and reaction.It has been reported that the amount of TiO 2 to mix with WO 3 was important for the photocatalytic reaction [22,23].Thus, the Cu(II)-WO 3 /TiO 2 samples with a TiO 2 ratio of 1% exhibited the highest photocatalytic performance.

Discussions
Figure 6 shows the energy levels of TiO 2 and WO 3 [18].TiO 2 , as one of the most efficient photocatalysts, has a high potential CB and a deep VB.Thus, electrons in its CB have sufficient reduction power for oxygen reaction with single electron and holes in its VB have large oxidation power for organic compounds decomposition, respectively.Consequently, TiO 2 has a very high efficiency for photocatalytic reactions under UV light irradiation.However, TiO 2 can only be activated under UV light irradiation, owing to its large band gap.WO 3 is sensitive to visible light because of its proper band gap, 2.7 eV [20,21].Notably, both the CB and VB positions of WO 3 are more positive than those of TiO 2 .As a result, photogenerated electrons can be transferred from the CB of TiO 2 to the CB of WO 3 and photogenerated holes can be transferred from the VB of WO 3 to that of TiO 2 [17].Moreover, if photons do not have enough energy to excite TiO 2 but have enough energy to excite WO 3 , hole in the VB of WO 3 is still possibly transferred to the VB of TiO 2 [24].This process suppresses the recombination of photogenerated carriers and indicates that TiO 2 can act as hole cocatalyst [12,17,24].On the other hand, the CB potential of WO 3 is lower than the potential for reduction reaction of oxygen molecules, leading to the insufficient consumption of electrons in CB.When Cu(II) nanoclusters were modified on the surface of WO 3 , the photogenerated electrons in the CB of WO 3 can be transferred to the Cu(II) nanoclusters.The transferred electrons can be consumed by multielectron reduction reactions with oxygen molecules in the Cu(II) nanoclusters [18,19].In other words, Cu(II) nanoclusters function as efficient electron cocatalysts [12,18,19].Consequently, the activity of Cu(II)-WO 3 can be further enhanced by combining with TiO 2 .
Figure 7 shows the PL spectra of bare WO 3 , Cu(II)-WO 3 , and Cu(II)-WO 3 /TiO 2 , respectively.The main emission peak for WO 3 is centered at about 460 nm, which is approximately equal to the band gap energy of WO 3 [20,25]  that bare WO 3 exhibited the highest PL intensity among these samples, indicating the highest recombination rate of electrons and holes [26].After the Cu(II) nanoclusters were grafted, the intensity of the PL emission decreases, which can be attributed to the decrease of the efficient electron trapping and consumption on Cu(II) nanoclusters [18,19].The emission intensity of the Cu(II)-WO 3 /TiO 2 was lower than that of bare WO 3 and Cu(II)-WO 3 , which indicated that the recombination rate of photogenerated charge carriers was the lowest in the Cu(II)-WO 3 /TiO 2 .The PL results confirmed the importance of the modification of Cu(II) nanoclusters and TiO 2 nanopowders for hindering the recombination of electrons and holes.Thus, efficient visible light photocatalytic activity can be achieved in Cu(II) and TiO 2 modified WO 3 .

Conclusions
Efficient WO 3 photocatalysts were prepared by being simply surface modified with Cu(II) nanoclusters and TiO 2 nanopowders.In this prepared system, Cu(II) nanoclusters and TiO 2 nanopowders were deposited on the surface of WO 3 using a simple impregnation method and a physical combination method, respectively.Cu(II) nanoclusters and TiO 2 nanopowders functioned as efficient cocatalysts on the surface of WO 3 , which acted as photocatalyst.Thus, efficient charge separations and reactions can be achieved in this Cu(II)-WO 3 /TiO 2 system, resulting in efficient visible light photocatalytic reaction for organic compounds decomposition.The simple strategy opens an avenue for designing efficient visible-light-active photocatalysts for practical application.

Figure 4
Figure 4  represents the variation of MB concentration by photocatalytic reaction with the samples under visible light (>420 nm) irradiation.Typical evolution of MB concentration during photocatalytic reaction on Cu(II)-WO 3 /TiO 2 is presented in Figure4(a).Under light irradiation, the characteristic MB absorption peak decreased sharply and almost no color was observed after 90 minutes of irradiation, indicating that MB was completely degraded by Cu(II)-WO 3 /TiO 2 .Comparative studies among bare WO 3 , Cu(II)-WO 3 , and Cu(II)-WO 3 /TiO 2 show that bare WO 3 has a negligible activity under visible light irradiation (Figure4(b)).The grafting of Cu(II) nanoclusters to the surface switched its photocatalytic activity.It can be seen that MB dye was almost degraded by Cu(II)-WO 3 with 2 h of visible light irradiation.Interestingly, after further modification with TiO 2 nanopowders, Cu(II)-WO 3 /TiO 2 exhibited an enhanced photocatalytic activity compared with that of Cu(II)-WO 3 .MB dye was completely degraded by Cu(II)-WO 3 /TiO 2 nanocomposites with 1.5 h of visible light irradiation, revealing the high photocatalytic activity of the Cu(II)-WO 3 /TiO 2 nanocomposites.Figure4(c) shows the pseudo-first-order kinetic rate for the photochemical degradation of MB by Cu(II)-TiO 2 samples.The pseudo-first-order kinetic rate was calculated according to the equation of ln(  /) = , where /  is the normalized MB concentration,  is the reaction time, and  is the pseudo-first-rate constant.It can been seen that the Cu(II)-WO 3 /TiO 2 samples presented the highest reaction rate.The reaction rate was sharply decreased when bare WO 3 was used.The result was consistent with the MB decomposition curves in Figure4(b).Figure4(d)shows the cycling measurements of MB decomposition over Cu(II)-WO 3 /TiO 2 .Similar  values were obtained after 5-cycle measurements, suggesting a good stability for the photocatalytic application of Cu(II)-WO 3 /TiO 2 .We also investigated the influences of experimental parameters on the photocatalytic performances of Cu(II)-WO 3 /TiO 2 samples.Figure5shows the photocatalytic performances of Cu(II)-WO 3 /TiO 2 samples with different ratios of TiO 2 .It can be seen that the activity of the samples was increased with the ratio of TiO 2 to WO 3 in the beginning.After the highest activity was achieved at the ratio of 1%, the photocatalytic activity was decreased again with the increase of ratio.These results revealed that the Cu(II)-WO 3 /TiO 2 samples obtained with 1% TiO 2 have the optimum amount of TiO 2 for hole separation and reaction.It has been reported that the amount of TiO 2 to mix with WO 3 was important for the photocatalytic reaction[22,23].Thus, the Cu(II)-WO 3 /TiO 2 samples with a TiO 2 ratio of 1% exhibited the highest photocatalytic performance.