DOI 10.1007/s11595-009-4533-7
Effect of pH Values on Photocatalytic Properties of Bi2WO6 Synthesized by Hydrothermal Method
GAO Chunmei, WANG Zhiyu*, YU Zhongping, YE Bo, LIU Bo,
FAN Xianping, QIAN Guodong
（State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China）
Abstract: Highly crystalline orthorhombic Bi2WO6 powders were hydrothermally synthe-
sized from aqueous solutions of Na2WO4·2H2O and Bi (NO3)3·5H2O over a wide range of pH. The
effect of pH on morphologies, sizes and properties of the Bi2WO6 crystals was investigated. The band
gaps of the as-prepared Bi2WO6 were determined from the onset of the absorption edge of UV-vis
diffuse reflectance spectra. The methyl orange photodegradation was employed as a probe reaction to
test the photocatalytic activity of the as-prepared samples under visible light irradiation. The photo-
catalytic activities of methyl orange degradation under visible light irradiation are strongly dependent
on the pH used in the synthesis. The highest efficiency is observed at pH＝7.
Key words: Bi2WO6; hydrothermal synthesis; photocatalytic; visible light irradiation
1 Introduction
Generally, two approaches can be exploited to de-velop the visible-light-irradiation photocatalysts: modi-fication of TiO2 and development of a novel material. The former has been largely investigated by doping dif-ferent ions[1-3] to extend absorption wavelength range to visible region. Recently, many investigations have been undertaken on the latter strategy. A great number of novel, undoped, single-phase, multiple oxide semicon-ductor photocatalysts have been developed, such as BiVO4, CaBi2O4, CaIn2O4 and so on. They all show certain absorption ability under visible-light irradiation.
Bi2WO6 is the simplest members of the aurivillius family with the layer structure. Up to date, some meth-ods have been exploited to prepare Bi2WO6 crystals, such as solid-state method[4], ultrasonic-assisted method [5] and hydrothermal processes[6-9]. As well known, differ-ent material preparation methods may have some im-portant different effects on material microstructure and properties. Local chemical heterogeneity, large sized particles and low surface area led by high-temperature sintering for a very long time, which may result in mul-tiphase powders and are not favorable for the photo-catalytic performance, were the deadly shortcomings of the solid-state method. Recently, hydrothermal route among other environmental friendly aqueous processes have received much attention. Shi et al[10] firstly synthe-sized Bi2WO6 through hydrothermal processes and Yu et al[11] firstly reported Bi2WO6 powders that have visi-ble-light photocatalytic activity at room temperature were prepared by a hydrothermal process. Hydrothermal synthesis offers many advantages over conventional solid-state methods, such as mild synthesis conditions, high dispersivity, high degree of crystallinity, high purity, small particle size and narrow particles size distribution and so on. Moreover, the hydrothermal conditions, such as the ratio of raw materials, synthesis temperature and time, pH values, among others, are easily controlled. Through hydrothermal method, hierarchical uniform microspheres[12], nanoplates[13], flower-like superstruc-ture[14]etc of Bi2WO6 were attained by controlling the experiment conditions. Yao et al[9] also employed mi-crowave during the hydrothermal synthesis of Bi2WO6 nanocrystals.
Bi2WO6 was successfully synthesized by hydro-thermal processes in a wide range of pH. The effects of pH on morphologies and particle sizes were investigated. Photocatalytic activity of Bi2WO6 was evaluated by degradation of methyl orange under visible light irradia-tion.
2 Experimental
2.1 Preparation
The following procedures were adopted for the preparation of Bi2WO6 samples. All reagents used were of analytic grade without further purification. The pre-cursors for hydrothermal synthesis of Bi2WO6 were
(Received: May 6, 2008; Accepted: Sep.10, 2008)
GAO Chunmei(高高高): E-mail: fly girl fly@https://www.doczj.com/doc/ea7163662.html,
?王王王
Corresponding author: WANG Zhiyu():Prof.;E-mail: wangzhiyu@https://www.doczj.com/doc/ea7163662.html,
Funded by the PCSIRT, the National Natural Science Foundation of China (Nos. 50532030 and 50625206) and the Zhejiang Provincial Natural Science Foundation of China (No. Z4080021)
prepared by coprecipitation from inorganic metal salts. The raw materials were corresponding Na2WO4·2H2O and Bi (NO3)3·5H2O in 1∶2 molar ratio. An amount of Bi (NO3)3·5H2O was firstly dissolved into 10.0 mL of 2.0 M HNO3 solution, while an amount of Na2WO4·2H2O was added to 10.0 mL of 1.0 M NaOH solution. Mixing the two solutions, the beaker was magnetically stirred at ambient temperature for 30 min to complete the precipitate reaction. After that, the pH was adjusted to a specific value using 1.0 M NaOH and HNO3 solution. Then the resulting precursor solution was transferred into a 50 mL Teflon-lined autoclave and filled with distilled water up to 80% of the total volume. Subsequently, the autoclave was maintained at 473 K for 20 h and then air cooled to room temperature naturally. Finally, the resulting light yellow products were col-lected and repeatedly washed with distilled water and absolute ethanol several times, and then dried at 373 K. Meanwhile Bi2WO6 was also prepared by traditional solid-state reaction according to a procedure reported previously[4] for comparison.
2.2 Characterization
The X-ray diffraction (XRD) patterns, which were obtained on a C-98 diffractometer with Cu Kα radia-tion at a scan rate of 0.1°/s in the 2θ range 20-70°, were used to determine the identity of any phase present. The morphology and microstructure of Bi2WO6 crystals were observed by field emission scanning electron mi-croscopy (FE-SEM, JEOL JSM-6700F) and transmis-sion electron microscopy (TEM, JEOL JEM-200CX). The UV-visible diffuse reflectance spectra (DRS) of the photocatalyst were measured by a UV-visible spectro-photometer (UV4100, Hitachi Company) using BaSO4 as a reflectance standard and were converted from UV-visible absorption spectra according to the Kubelka-Munk equation.
2.3 Photocatalysis
Photocatalytic activity of the as-prepared samples was evaluated by the decomposition of methyl orange under visible light irradiation (λ>400 nm). A 420 W halogen HQI-BT 400D lamp (OSRAM, made by SIEMENS Company in Germany) was used as the light source. Aqueous solution of methyl orange 10 mL, 20 mg/L and 20 mg of photocatalyst were placed in a beaker. Prior to irradiation, the suspension was ultrasonically sonicated for 5 min and then magnetically stirred under dark conditions for 30 min to establish adsorp-tion-desorption equilibrium. The suspension was then irradiated using the above light source. At regular inter-vals, samples were extracted and centrifuged to remove photocatalyst particles from the methyl orange aqueous solution for analysis. The concentration of methyl or-ange was determined by measuring the absorbance value at approximately 464 nm using a UV4100 spectrometer (Hitachi Company). 3 Results and Discussion
3.1 Hydrothe rmal synthe sis of ortho-
rhombic Bi2WO6
The crystal structure and average crystallite size of the as-prepared samples were investigated by XRD and the results are shown in Fig.1. No diffraction peaks were observed for the starting precipitate precursors obtained by inorganic precipitate reaction before hydrothermal treatment. Therefore, it can be concluded that the hydrothermal treatment is essential for the formation of crystalline Bi2WO6. No peaks of any other phases or impurities are detected when pH>5. These diffraction peaks of the samples can be indexed with the ortho-rhombic Bi2WO6 (JCPDS Card: 73-1126). However, most of the peaks emerge but a little one mark as (■) belong to Bi2O3 phase.
when pH＝4, it can be clearly seen that the as-prepared samples at low pH shows weak crystalliza-tion. With increasing pH, the diffraction peak intensity obviously increases and the width of the peak becomes gradually narrower. With the improvement of crystalli-zation, the crystallites grow larger as estimated accord-ing to the famous Scherrer equation, the surface areas of the samples decrease.
3.2 Morphologies of Bi2WO6 samples
The Morphologies and microstructures of the as-prepared samples were characterized by TEM and FE-SEM. The results are presented in Fig.2. Obviously,
the morphologies and dimensions of the samples are
strongly dependent on the pH value. The shape of Bi2WO6 (pH＝4) (Fig.2(d)) is completely different from others. Large compact particles (hundreds of nm in size) of irregular shape organized in larger aggregates are visible and the size is inhomogeneous. With the increase of pH value, the Bi2WO6 crystallites become sheet-shaped. The Bi2WO6 (pH＝7) (Fig.2(b)) display mainly regular thin sheet-shaped morphology and the Fig.1 XRD patterns of Bi2WO6 samples prepared at
various pH values
smallest size of ca 100 nm(Fig.2(f)). As the pH goes up to 8, besides the sheet-shaped crystals can be seen (Fig.2(a)),that there is a certain quantity of small irregular articles adhering to the plates. Though the Bi 2WO 6 pre-pared by solid state reaction(SSR) (Fig.2e) presents also sheet-shaped, it has bigger size than the sample prepared by hydrothermal process.
The EDS analysis (the percent content of the elements
is wt%) reveals that the elemental composition of the
nanosheets is bismuth, tungsten and oxygen with a molar ratio of Bi ∶W = 0.303∶
0.153, which is very close to the
Bi
∶
W molar ratio (2∶1) in orthorhombic Bi 2WO 6. The result is in well accordance with that of XRD patterns.
3.3 Photoabsorption properties
Light absorption of the material and the migration of
the light-induced electrons and holes of the samples were
measured by UV-vis diffuse reflectance spectroscopy
(DRS). The absorption spectra were transformed from
the DRS according to the Kubelka–Munk equation. The as-prepared samples present almost the same absorption edge around 450 nm. Fig.3 shows a typical diffuse reflec-tion spectrum of good-quality Bi 2WO
6 (pH
＝7) and Bi 2WO 6 (pH ＝4). The diffuse reflectance spectra of the
Bi 2WO 6 powders prepared at pH=4 have a tail at the ab-sorption edge. It suggests the formation of surface states
and impurity levels, which can also be confirmed by the XRD patterns(Fig.1). The band gap of Bi 2WO 6 was esti-mated to be 2.75 eV from the onset of the absorption edge, which is similar to the value in the Ref.[6,15] and bigger than Zou’s result (2.69 eV). Compared to the SSR sample, the absorption of Bi 2WO 6 nanoplates appears to blue-shift obviously. This can be attributed to the nanosized effect.
3.4 Photocatalytic activity
Photocatalytic activity of the Bi 2WO 6 powders was evaluated by the photodegradation of methyl orange in 20 mg/L aqueous solution. The irradiation time depend-ent absorption spectra of methyl orange during photode-gradation are presented in Fig.4. Sunlight irradiation over
aqueous MO/Bi 2WO 6 dispersions leads to a decrease in
absorption intensity and a blue-shift of the wavelength.
With the increase of irradiation time, the characteristic absorption band at 464 nm for methyl orange shrinks gradually, and the absorbance of the band at 464 nm decreases from 1.495 to 0.09; meanwhile, the decolora-tion of solution is observed. The absorption band at 464 nm is associated with the azo bond (-N ＝N-), ie , the chro-mophoric group of the methyl orange dye. The absorption band representing the -N ＝N- bond disappeared, re-vealing that the methyl orange azo dye has been degraded
with the photocatalysis of the Bi 2WO 6 powders.
The remaining ratio, defined as (C /C 0)×100%, can be used to evaluate the extent of the degradation of the
methyl orange. As shown in Fig.5, the photocatalytic
activity of Bi 2WO 6 with different pH values varying from
4 to 8 has significant difference. As a result of surface adsorption, methyl orange also has a tiny decrease in the contrastive experimentation using P2
5 as photocatalyst. The concentration of methyl orange decreased negligi-bly over the time span of these experiments in the ab-
Fig.2 SEM images of Bi 2WO 6 powders prepared at (a) pH ＝8; (b) pH ＝7;
(c) pH ＝6; (d) pH ＝4; (e) SSR and (f) TEM image (pH ＝7)
Fig.3 UV-vis diffuse absorption spectra of Bi 2WO 6 samples Fig.4 Absorption spectral changes that occur during the photo-catalytic degradation of aqueous solution of 20 mg/L methyl orange solution with catalyst loading of 2 g/L Bi 2WO 6
sence of photocatalysts excluding P25. A blank experi-ment in the absence of irradiation but with the prepared catalysts demonstrates that no significant change in the concentration was found. The experiment results appear that the degree of decolorization of dye solution increases with increasing pH values, and reaches the highest value at pH ＝7 and then decreases. The photocatalytic activi-ties of most samples were higher than that of the sample prepared by traditional solid state reaction.
Crystallinity and the surface area of the photocata-lyst are two important factors influencing the photo-catalytic activity. The higher is the crystalline quality, the smaller is the amount of defects. The defects operate as trapping and recombination centers between photogen-erated electrons and holes, resulting in a decrease in the photocatalytic activity. Therefore, a high degree of crystallinity is required for photocatalysts. At the same time, the smaller the particle size is, the bigger the surface area is, and the more the active sites are, which can lead to higher photocatalytic activity. From the XRD (Fig.1) and SEM (Fig.2) results, with the increase of the pH value, the crystallinity of Bi 2WO 6 increased, and the contents of the impurities decreased, which benefit to the photocatalytic properties. However, the particle size increased too, which lead to lower specific surface area and lower absorptive capacity of MO on the surface of Bi 2WO 6 particles, and then the lower photocatalytic ac-tivity. The Bi 2WO 6 (pH ＝7) has highest crystallinity and smallest particle size, so the photocatalytic activity reaches the higher value at pH ＝7.
4 Conclusion Bi 2WO 6 were synthesized via mild hydrothermal treatment. The band gap of Bi 2WO 6 crystals was esti-mated to be about 2.7
5 eV from the onset of UV-Vis absorption spectra of the photocatalyst. The experiment and results indicate that the photocatalytic activity of Bi 2WO
6 increases with increasing pH values, reaches the
highest value at pH ＝7 and then decreases, due to the
samples’ better crystallization and smaller particle size.
References
[1] U Shahed, M Khan, M Al-Shahry. Efficient Photochemical
Water Splitting by a Chemical Modified n -TiO 2[J]. Science , 2002, 297: 2243-2245
[2] S Sakthivel, H Kisch. Daylight Photocatalysis by Car-bon-modified Titanium Dioxide[J]. Angew. Chem. Int. Edit., 2003, 42(40): 4908-4911
[3] T Umebayashi, T Yamaki, H Itoh. Band Gap Narrowing of
Titanium Dioxide by Sulfur Doping[J]. Appl. Phys. Lett., 2002, 81(3): 454-456
[4] Junwang Tanga, Zhigang Zoub, Jinhua Yea. Photocatalytic
Decomposition of Organic Contaminants by Bi 2WO 6 under Visible Light Irradiation[J]. Catal. Lett., 2004, 92(2): 53-56 [5] Lin Zhou, Wenzhong Wang, Lisha Zhang. Ultrasonic-assisted
Synthesis of Visible-light-induced Bi 2MO 6 (M =W, Mo) Photocatalysts[J]. J. Mol. Catal. A: Chem., 2007, 268: 195-200 [6] Chuan Zhang, Yongfa Zhu. Synthesis of Square Bi 2WO 6
Nanoplates as High-Activity Visible-Light-Driven Photo-catalysts[J]. Chem. Mater ., 2005, 17: 3537-3545
[7] Hongbo Fu, Liwu Zhang, Wenqing Yao, Yongfa Zhu. Photo-catalytic Properties of Nanosized Bi 2WO 6 Catalysts Synthe-sized via a Hydrothermal Process[J]. Applied Catalysis B: Environmental., 2006, 66: 100-110
[8] Jiaguo Yua, Jianfeng Xiong, Bei Cheng, Ying Yub, Jianbo
Wang. Hydrothermal Preparation and Visible-light Photo-catalytic[J]. Journal of Solid State Chemistry, 2005,178: 1968-1972
[9] Shushan Yao, JiyongWei, BaibiaoHuang, et al . Morphology
Modulated Growth of Bismuth Tungsten Oxide Nanocrys-tals[J]. Journal of Solid State Chemistry, 2009, 82: 236-239 [10] Yanhui Shi, Shouhua Feng, Changsheng Cao. Hydrothermal
Synthesis and Characterization of Bi 2MoO 6 and Bi 2WO 6[J]. Materials Letters, 2000, 44: 215-18
[11] Jiaguo Yu, Jianfeng Xiong, Bei Cheng, Ying Yu, Jianbo
Wang. Hydrothermal Preparation and Visible-light Photo-catalytic Activity of Bi 2WO 6 Powders[J]. Journal of Solid State Chemistry, 2005, 178: 1968-1972
[12] Yuanyuan Li, Jinping Liu, Xintang Huang, and Guangyun Li.
Hydrothermal Synthesis of Bi 2WO 6 Uniform Hierarchical Microspheres[J]. Crystal Growth & Design , 2007, 7: 1350-1355 [13] Chuan Zhang, and Yongfa Zhu. Synthesis of Square Bi 2WO 6
Nanoplates as High-Activity Visible-Light-Driven Photo-catalysts[J]. Chem. Mater., 2005, 17: 3537-3545 [14] Lisha Zhang, Wenzhong Wang, Zhigang Chen, Lin Zhou, Haolan Xua and Wei Zhu. Fabrication of Flower-like Bi 2WO 6 Superstructures as High Performance Visible-light Driven Photocatalysts[J]. J. Mater. Chem ., 2007, 17:
2526-2532
[15] Shicheng Zhang, Chuan Zhang, Yi Man, Yongfa Zhu. Visi-ble-light-driven Photocatalyst of Bi 2WO 6 Nanoparticles Prepared via Amorphous Complex Precursor and Photo-catalytic Properties[J]. J. Solid State Chem., 2006, 179: 62-69
Fig.5 Photocatalytic degradation of methyl orange (20 mg/L) with various photocatalysts under visible light (λ>400 nm) as function of irradiation time: (a) P25, (b) pH ＝4,
(c) pH ＝5, (d) SSR, (e) pH ＝6, (f) pH ＝8, (g) pH ＝
7