A New Photocatalyst CdWO4 Prepared with a Hydrothermal Method

J. Phys. Chem. C 2008, 112, 17351–17356

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A New Photocatalyst CdWO4 Prepared with a Hydrothermal Method Dong Ye, Danzhen Li,* Wenjuan Zhang, Meng Sun, Yin Hu, Yongfan Zhang, and Xianzhi Fu* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou UniVersity, Fuzhou, 350002, People’s Republic of China ReceiVed: July 5, 2008; ReVised Manuscript ReceiVed: September 5, 2008

A novel photocatalyst CdWO4 was synthesized via a hydrothermal process, and its high photocatalytic activity was revealed. The influence of preparation conditions on crystal structure, optical properties, and photocatalytic activity of CdWO4 catalyst was investigated. The results showed that the sample was irregular short rods with a monoclinic wolframite structure. In photocatalytic performance, CdWO4 showed a good ability toward the photodegradation of methyl orange (MO) and Rhodamine B (RhB). The specimen prepared at pH ) 4 through annealing process had the best activity in photodegradation of MO in aqueous solution under UV light irradiation. But there were no obvious differences in activity performance when annealing temperature was below 873 K, indicating that the temperature had little influence on photocatalytic activity of CdWO4. Introduction Photocatalytic degradation of organic compounds for the purpose of purifying wastewater from industries and households has attracted much attention in recent years.1-3 However, the research of various oxide semiconductor photocatalysts were mainly focused on Ti,4-6 Nb,7 Ta,2,8,9 and In,10 etc. Thus, exploitation of a new photocatalyst system is compulsory. Recently, it has been reported that, for compounds consisting of sixth group element, some polytungstates were homogeneous photocatalysts for H2 evolution from an aqueous solution containing an electron donor.11 However, there were only a few reports about W-based heterogeneous photocatalysts, Na2W4O13, Bi2W2O9, (NaBi)0.5WO4, AgInW2O8, Bi2WO6.12-16 These Wbased photocatalysts can produce not only H2 but also O2 from an aqueous solution containing sacrificial reagents. Except those, it was also reported that Bi2WO6 and ZnWO4 were good photocatalysts for photodegradation of organic compounds under UV light irradiation.17-20 These results suggest the possibilities that some W-based photocatalysts with the activity for environmental applications may be developed. Cadmium tungstate (CdWO4) with a monoclinic wolframite structure is one of the families of metal tungstates. It has aroused much interest because of its low radiation damage, high average refractive index, and high X-ray absorption coefficient.21 CdWO4 has been used as a popular X-ray scintillator22 with such exceptional advantages as high efficiency, high chemical stability, high stopping power, and short decay time and has also a promising application as an advanced medical X-ray detector in computerized tomography.23 However, the investigation on the photocatalytic activity of CdWO4 has never been reported before. It is, therefore, of great interest to examine the photocatalytic properties of pure CdWO4 material. It is well-known that nanometer-sized inorganic lowdimensional systems exhibit a wide range of optical and electric properties24 that rely sensitively on both size and morphology.25,26 The fabrication of nanocrystallite, such as nanoparticles, nanorods, nanofibers, and nanotubes, has attracted intense attention recently because of their promising applications in various fields * To whom correspondence should be addressed. Phone/Fax: (+86)59183779256. E-mail: (D. L.) [email protected], (X. F.) [email protected].

of technology with respect to their collective optical, magnetic, and electronic properties.27-30 The preparation of CdWO4 with different morphologies has been reported elsewhere.31-33 As we known, hydrothermal method offers many advantages over other conventional methods, such as mild synthesis conditions, high degree of crystallinity, high purity, and narrow particle size distribution of product. So it is a very popular synthesis method to synthesize photocatalysts. CdWO4 was prepared by a simple hydrothermal method according to literature.31 In this work, the photocatalytic activities of the asprepared samples for the MO and RhB photodegradation were first investigated. To further understand the mechanism of the CdWO4 photocatalysis, the effects of preparation conditions and annealing temperature on the photocatalytic activity of CdWO4 were discussed. 2. Experimental Section 2.1. Synthesis. All chemicals used were analytic grade reagents without further purification. Appropriate amounts of Na2WO4 · 2H2O and (CH3COO)2Cd · 2H2O were put into a 100mL stainless steel autoclave, and then 75 mL of distilled water was added with strongly magnetic stirring at room temperature. The pH value of the mixed solution from 3 to 10 was adjusted dropwise with CH3COOH, HCl, and NaOH (0.5 M). The autoclave was sealed and maintained at the temperature of 433 K for 12 h and then cooled to room temperature naturally. After the above hydrothermal treatment, the white precipitate was centrifuged and washed with distilled water and absolute ethanol for several times to remove any impurities, and then dried at 333 K. Finally, the products were sintered at different temperatures for 2 h. ZnWO4 was synthesized through a hydrothermal method, which was the same as CdWO4. The analytic grade reagents of Na2WO4 · 2H2O and (CH3COO)2Zn · 2H2O were used as precursors without further purification. According to literature,34 Ti(OC3H7)4 was used as precursor to prepare TiO2 colloidal solution. To get the TiO2 photocatalyst, the approximate process can be shown as follows: A mixture of 1.3 mL of HNO3, 180 mL of H2O, and 15 mL of Ti(OC3H7)4 was peptized, dialyzed, and concentrated at room temperature to form a highly dispersed TiO2 colloidal solution. Then the

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17352 J. Phys. Chem. C, Vol. 112, No. 44, 2008 colloidal solution was dried at 333 K. After that, the solid of TiO2 was annealing at 673 K for 2 h. 2.2. Characterizations. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The transmission electron microscopy (TEM) image was measured by JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. The powders were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. A suspension in ethanol was sonicated, and a drop was dripped on the support film. The Brunauer-Emmett-Teller (BET) surface area was measured with an ASAP2020 M (Micromeritics Instrument Corp.). UV-vis diffuse reflectance spectra (DRS) were recorded on a Varian Cary 500 Scan UV-vis-NIR spectrophotometer with BaSO4 as the background and in the range of 200-800 nm. The photoluminescence (PL) excitation and emission spectra were taken on a FL/FS 900 time-resolved fluorescence spectrometer. 2.3. Photocatalytic Experiments. Photocatalytic reactions were performed in a quartz tube with 4.6 cm inner diameter and 17 cm length. Four 4-W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5) were used as illuminating source. 150 mg of powder photocatalysts was suspended in 150 mL of MO aqueous solution (10 ppm) and stirred overnight before irradiation to ensure that adsorption/ desorption equilibrium had been reached. A 3 mL aliquot was taken at 20 min intervals during the experiment and centrifuged (TDL-5-A) to remove the powders. The filtrates were analyzed on a Varian UV-vis spectrophotometer (model: Cary-50). The percentage of degradation is reported as C/C0. C is the maximum peak of the absorption spectra of MO for each irradiated time interval at wavelength 464 nm. C0 is the absorption of the starting concentration when adsorption/desorption equilibrium was achieved. To test its photocatalytic lifetime, the as-prepared CdWO4 was recycled and reused five times in the decomposition of MO under the same conditions. After each photocatalytic reaction, the aqueous solution was centrifuged to recycle the CdWO4 powders that were then dried at 333 K for another test. As well as MO, the photodegradation toward RhB (150 mL, 1.0 × 10-5 mol L-1) for catalysts were also investigated. 3. Results and Discussion 3.1. Formation of CdWO4 Crystalline. Figure 1 shows the XRD patterns of the as-synthesized CdWO4. XRD patterns can be indexed to a pure monoclinic phase of well-crystallized CdWO4 with a wolframite structure, well consistent with the reported data (JCPDS 14-676). Distinctive peaks at 23.28, 29.00, 29.55, 30.51, 35.37, 35.68, and 47.65° match well with the (110), (-111), (111), (020), (002), (200), and (220) crystal planes of CdWO4, respectively. The XRD patterns for CdWO4 prepared at different initial pH values are shown in Figure 1a. The results reveal that, with the increase of pH value, the intensity of the diffraction peaks decrease, especially in the (110) and (020) crystal planes. The results suggested that the initial pH value is important for the crystallization of as-prepared CdWO4. To investigate the influence of annealing temperature on the crystallization of CdWO4, Figure 1b shows the XRD for samples annealed at different temperatures; it can be illuminated that there are no obvious changes on crystallization and grain diameter when annealing temperatures are below 873 K. Table 1 shows the BET surface area for samples annealing at different temperatures. Just as shown, there are no obvious

Ye et al.

Figure 1. Figure 1. XRD patterns of CdWO4: (a) CdWO4 prepared at different initial pH values and annealing at 873 K for 2 h; (b) CdWO4 annealing under different temperature and all the samples prepared at initial pH ) 4.

TABLE 1: Influence of Annealing Temperature (for 2 h) on BET Surface Area for Sample Prepared at pH ) 4 annealing temperature (K) 2

-1

BET surface areas (m g )

precursor 14.95

573

673

773

873

13.98 13.52 13.14 12.79

differences in surface area when the annealing temperatures are below 873 K. The result is consistent with the photocatalytic activity performance. 3.2. Morphologies of CdWO4 Crystalline. The morphologies of the as-synthesized CdWO4 are demonstrated in the TEM images shown in Figure 2. As shown in Figure 2a, the as-synthesized CdWO4 is an irregular short rod with pointed ends, and some particles with pointed ends are also observed. It implies that these particles are intermediates of the rods and further form the short rods through prolonging them along the (100) direction. A representative HRTEM image showing clear lattice fringes is shown in Figure 2b. The interlayer spacing of 0.308 nm corresponds to the (-111) plane of CdWO4. 3.3. Electronic Structure Calculation. In this work, highly crystalline CdWO4 is identified with monoclinic symmetry, a ) 50.26 nm, b ) 50.78 nm, and c ) 58.67 nm; R ) 90.0°, β ) 90.0°, and γ ) 91.47°. To study the electronic structure of CdWO4, the density functional theory (DFT) calculations have been performed. Figure 3 shows the energy band dispersion and density of states (DOS). The valence band maximum (VBM) was located at the B point, while the conduction band minimum (CBM) was at the Y point, giving an indirect gap semiconductor nature of CdWO4. These results can be obtained as follows: an occupied band on the low-energy side consists of the O 2s orbitals mainly. The occupied band in the middle consists of the hybrid orbital of O 2p, Cd 3d, and W 5d. The occupied

Photocatalyst CdWO4

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Figure 2. TEM images of CdWO4 (sample prepared at pH ) 4 and annealing at 873 K for 2 h): (a) image at low magnification. (b) HRTEM image of CdWO4.

Figure 3. DFT calculations for CdWO4: (a) energy band dispersion; (b) density of states.

band on the high-energy side, corresponding to the broad valence band, consists of only the O 2p orbitals. The lowest unoccupied band mainly comprises the W 5d orbitals with a small contribution of the O 2p and Cd 4s, corresponding to conduction band. As for the unoccupied energy levels, the bottom of the conduction band is formed by the W 5d orbitals, but there are also small bands composed of Cd 4p and O 2p. The partial DOS for CdWO4 is also shown in Figure 4. The Fermi level is set to zero on the abscissa. The highest-occupied molecular orbital (HOMO) shows the p orbital lobes on the O atom, indicating that the orbital is purely composed of the O 2p orbitals. The lowest-unoccupied molecular orbital (LUMO) is mainly formed by the W 5d orbital, but the O 2p is included to some degree. This mixing is rather common for metal oxide semiconductors including the d10 elements.35-37 Thus, the highest-occupied and lowest-unoccupied molecular orbital levels are composed of the O 2p orbitals and the W 5d orbitals, respectively. The band gap of CdWO4 is estimated to be 3.1 eV (the band gap calculated by DFT is smaller than that obtained experimentally, which is frequently pointed out as a common feature of DFT calculations).38 The band structure indicates that charge transfer upon photoexcitation occurs from the O 2p orbitals to the empty W 5d orbitals. 3.4. Optical Properties of CdWO4 Crystal. The UV-vis diffuse reflectance spectra of CdWO4 are shown in Figure 5.

As shown, the ability of absorbing toward UV light for CdWO4 is enhanced with the increase of annealing temperature. It may be owing to the improvement of crystallization through annealing treatment. Another result can be indicated that there are two transition bands at 220 and 270 nm, which can be identified in the tungstate group. For an indirect gap semiconductor, it is well-known that the relation between absorption coefficient and band gap energy can be described by the formula: (F(R)hν)1/2 ) A(hν - Eg), where hν and Eg are photon energy and optical band gap energy, respectively, and A is the characteristic constant of semiconductors. From the equation, (F(R)hν)1/2 has a linear relation with hν. By extrapolation of the linear relation to (F(R)hν)1/2 ) 0, Eg of sample can be got. According to the electronic structure calculation, the CdWO4 is an indirect gap semiconductor. The sample presents absorption edge around 313nm. The band gap Eg of 3.9 eV can be estimated. Room temperature PL property of the as-prepared CdWO4 was also investigated. As shown in Figure 6 the sample exhibits a blue-green emission peak at 487 nm in the PL spectrum, with the excitation wavelength 290 nm. It is proposed that WO6 octahedral structure in CdWO4 are the luminescent centers and a charge-transfer transition between the O 2p orbitals and the empty d orbitals of the Central W ion is responsible for this luminescence.39

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Figure 6. PL spectrum (with the excitation wavelength 290 nm) of CdWO4 (pH ) 4 and 873 K for 2 h). The inset is the excitation spectrum of CdWO4 (with the emission wavelength 487 nm).

Figure 4. The partial density of states for CdWO4.

Figure 7. (a) Temporal changes of MO concentration as monitored by the UV-vis absorption spectra at 464 nm on different CdWO4; (b) temporal absorption spectral patterns of MO during the photodegradation process on sample (pH ) 4 and 873 K for 2 h). Figure 5. Diffuse reflectance spectra of the CdWO4 (pH ) 4) at different annealing temperatures for 2 h and optical band gap energy Eg of CdWO4 (inset).

3.5. Photocatalytic Activities of CdWO4 Catalysts. Temporal changes in the concentration of MO monitored by examining the variations in the maximal absorption in UV-vis spectra at 464 nm for different samples and the temporal evolution of the spectral changes taking place during the photodegradation of MO for samples are shown in parts a and b of Figure 7, respectively. The results show that the sample prepared at pH ) 4 has the highest photocatlytic activity, and after 100 min of irradiation, the conversion of MO can be nearly

up to 100%. The blank experiment with catalyst absence was also performed. It can be found that MO is almost not decomposed under the same irradiation time, indicating that presence of CdWO4 photocatalyst plays an important role in the photodegradation of MO. It is well-known that the preparation condition has relation with crystallization and purity for the sample. Liao et al.31 have reported that the initial pH value was an important factor for the synthesis of CdWO4. It implied that when the pH value is below or above some a value, some impurities would be brought in. As shown in Figure 1a, when pH > 4, the intensity of diffraction peak decreases with the increase of pH value, especially in the (110) and (020) crystal planes. Therefore, the

Photocatalyst CdWO4

Figure 8. Temporal changes of MO concentration on CdWO4 prepared at initial pH ) 4 and annealing at different temperatures.

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Figure 10. Photocatalytic degradation of RhB (1 × 10-5 mol L-1) by the as-prepared sample (pH ) 4 and 873 K for 2 h, 0.15 g).

Figure 9. The lifetime for photodegradation of MO on sample (pH ) 4 and 873 K for 2 h).

differences in their photocatalytic activity can be attributed to the impurities which can promote the recombination of photogenerated holes with photogenerated electrons, and on the other hand, the decrease of crystallization may be another reason. The influence of annealing temperature on the photocatalytic activity is shown in Figure 8. It can be found that when annealing temperatures are below 873 K, the influence of annealing temperature on photocatalytic activity can be ignored. It well matches with results of XRD and BET. However, another result is indicated that annealing treatment is good for photocatalytic activity. Maybe it can be explained that some impurities absorbed on the surface of sample are eliminated through annealing treatment. As a result, the chance for the contact of MO with sample can be enhanced. Besides, it may be owing to the enhanced absorbance toward UVlight through annealing treatment. The stability of a photocatalyst is important for its application. Herein, the stability of CdWO4 was investigated. The lifetime for CdWO4 is shown in Figure 9. The results reveal that the sample does not exhibit obvious loss of activity. To investigate the photoactivity of CdWO4 toward other organic compounds, RhB was employed, too. As shown in Figure 10, RhB can be degraded almost completely after 40 min irradiation with UV light (254 nm). According to ref 40, the activity for other W-based photocatalysts can be indicated as follows: Bi2WO6 presented the photocatalytic activity in the wide spectral scope, including UV and visible light irradiation. ZnWO4 only displayed relatively high photoactivity under UV ligtht. However, PbWO4 showed poor photoactivity under any light irradiation. So, it is interesting to contrast CdWO4 with ZnWO4 which almost has the same absorbance spectral scope as CdWO4. As shown in Figure 11a, in comparison with CdWO4,

Figure 11. (a) Photocatalytic photodegradation of MO (10 ppm) and RhB (1 × 10-5 mol L-1) in the ZnWO4 and CdWO4 suspensions. (b) Temporal changes of MO concentration for CdWO4 prepared at initial pH ) 4 and TiO2.

the ZnWO4 prepared at 433 K for 12 h showed a lower activity in degradation MO, and there was not an obvious difference in the photoactivity toward RhB for the two catalysts. Therefore, the order about photocatalytic activity of W-based photocatalysts may be shown as Bi2WO6 > ZnWO4 ∼ CdWO4 < PbWO4. As shown in Figure 11b, TiO2 showed a higher velocity in degradation MO compared with CdWO4. But, it also can be pointed out that, toward the same initial concentration of MO, the time used to degrade it completely was almost the same for the two photocatalysts. Except that, toward CdWO4, as shown in Figure 9 it can still keep a well activity after a long time irradiation even when the wavelength of UV light was 254 nm. So CdWO4 can be a good candidate for potential photocatalyst. 3.6. Mechanism for Photodegradation. Terephthalic acid photoluminescence probing technique (TA-PL) has been widely used in the detection of hydroxyl radicals.41 2-Hydroxyl-terephthalic

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Ye et al. photoactivity toward RhB too. The mechanism for the photodegradation performance needs to be researched further. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20537010, 20677010, and 20873023), an “863” Project From the MOST of China (2006AA03Z340), National Basic Research Program of China (973 Program, 2007CB613306), and the Natural Science Foundation of Fujian, China (2003F004, 2005HZ1007). References and Notes

Figure 12. (a) The temporal changes in fluorescence intensity for different conditions under UV light (254 nm) irradiation. (b) The changes of hydroxyl radicals for CdWO4 (pH ) 4 and 873 K for 2 h).

acid, which is generated when terephthalic acid captures the hydroxyl radicals, performs a strong fluorescence characteristic, so the hydroxyl radicals can be detected indirectly by monitoring the fluorescence intensity changes of CdWO4/TA solution. Figure 12a shows the temporal changes in fluorescence intensity at 426 nm under UV light (254 nm) irradiation for different conditions, and the fluorescence intensity increases steadily with the irradiation time. As shown, the conditions for without illumination and without catalyst were also performed. In fact, the experiment in dark was done for 1 h. The result can be attained that the intensity for hydroxyl radicals will be hold in initial value with the absence of illumination. Compared with blank experiment, it is indicated that more hydroxyl radicals would be generated with catalyst. The changes of hydroxyl radicals for CdWO4 are shown in Figure 12b. It can be concluded that hydroxyl radicals are indeed generated on CdWO4 under UV irradiation from Figure 12, and the rate of hydroxyl radical generation for different conditions is in accordance with the performances of MO degradation, suggesting hydroxyl radicals may be the main reactive species. 4. Conclusions CdWO4 with irregular short rods was prepared in this article. A high photocatalytic activity for the MO degradation was exhibited. The sample prepared with initial pH ) 4 and through annealing process showed the highest photocatalytic activity toward MO due to good crystallization and well pure phase. Both the annealing process and initial pH values of the mixed solution had influences on photocatalytic activity of CdWO4, but the latter played a more important role. Except that the as-prepared sample showed good

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