- Email: [email protected]
Preparation and Photoelectrochemical Properties of Bi2MoO6 Films
ACTA PHYSICO-CHIMICA SINICA Volume 23, Issue 11, November 2007 Online English edition of the Chinese language journal ARTICLE
Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(11): 1671−1676.
Preparation and Photoelectrochemical Properties of Bi2MoO6 Films Yi Man,
Ruilong Zong,
Yongfa Zhu*
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
Abstract:
Bi2MoO6 films on ITO glass substrates were prepared from amorphous complex precursor by dip-coating technique.
The relationships between conditions of preparation, structures, morphologies, and photoelectrochemical properties of Bi2MoO6 films were investigated by using scanning electron microscopy (SEM), X-ray diffraction (XRD), laser Raman spectroscopy (LRS), diffuse reflectance spectroscopy (DRS), photocurrent-action curves, and incident photon-to-current conversion efficiency (IPCE). Bi2MoO6 films calcined at 500 for 1 h were the γ-Bi2MoO6 phase, and Bi2MoO6 nanoparticles grew along (131) plane. The thickness of the films obtained was about 69 nm. The size of Bi2MoO6 nanoparticles increased with rising calcination temperature and extension of calcination time; in addition, γ-Bi2MoO6 changed into β-Bi2MoO6 and γ′-Bi2MoO6 at 525 . Bi2MoO6 films had visible-light response, and detectable photocurrent was generated under visible-light (λ>400 nm) irradiation. The IPCE of the optimized Bi2MoO6 films was 2.14% at 400 nm. The photocurrent density and IPCE could be controlled by modifying the surface structure of Bi2MoO6 films, which could be achieved by changing the preparation conditions. Key Words:
Bi2MoO6 films; Amorphous complex precursor; Photoelectrochemistry; Visible light
At present, environmental pollution and energy crisis have become critical problems to be resolved for the sustainable development of human society. Semiconductor photocatalysis has hence gained more attention due to its potential application in environmental purification[1−3], solar energy conversion[4,5], and H2 production by splitting water[6]. TiO2 is an excellent photocatalyst because of its high activity, low cost, and good stability. In addition to TiO2, other nanosized mixed oxides, such as tantalates[7,8], vanadates[9,10], and tungstates[2,3,11−15], have been reported for UV and visible-light photocatalytic activities by many researchers. However, their visible-light absorption is still little, which limits their solarlight utilization. Therefore, it is necessary to develop novel visible-light photocatalysts that have high visible-light activity and good stability. Bismuth molybdates have three crystalline structures: α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6[16]. γ-Bi2MoO6 is an Aurivillius-type polymorphic compound. It is composed of 2+ octahedral MoO2+ 4 layers and five-coordinated Bi2O2 , and Bi
atom layers are sandwiched between WO6 (MoO6) octahedral layers[17]. Recently, Kudo[18,19] found that Bi2MoO6 showed excellent visible-light photocatalytic activity for the production of O2 from AgNO3 solution. Bi2MoO6 films could also have potential application in novel solar cells, but the photoelectric conversion of the film under visible light has not been reported until now. In this work, Bi2MoO6 films on ITOconductive glass have been prepared by the direct pyrolysis of amorphous complex precursor. The structure and the photoelectrochemical properties of the films have been investigated in detail.
1 1.1
Experimental Preparation of Bi2MoO6 films
Bi2MoO6 films were prepared by using the dip-coating method from amorphous complex precursor. Typically, 0.02 mol diethylenetriaminepentaacetic acid (H5DTPA) and 7.5 mL concentrated ammonia solution (about 7.0 mol·L−1) were
Recieved: June 27, 2007; Revised: August 26, 2007. ∗ Corresponding author. Email: [email protected]; Tel/Fax: +8610-62787601. The project was supported by the National Natural Science Foundation of China (20433010, 20571047). Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
added into 100 mL hot distilled water under magnetic stirring. After dissolution, 0.01 mol Bi(NO3)3 and 0.714 mmol (NH4)6Mo7O24·6H2O powders were added. The mixture was stirred and heated at about 80 to promote the dissolution and reaction, until it became a colorless transparent solution. Then, the solution was vaporized slowly in an oven at 80 . After that, the solution became a piece of transparent glass-like material and then it was grinded to a powder sample. The powder dissolved in distilled water to form the 30% (w, mass fraction) precursor solution. Before dip-coating, the ITO glasses were first scrubbed with detergent and washed thoroughly using distilled water. After treatment with a mixture of 30% H2O2, concentrated ammonia solution, and distilled water (volume ratio is 1:1:6) for 10 min, they were immersed in ultrasonic baths of ethanol for about 15 min. Finally, each ITO plate was exposed to an 11 W mercury lamp in an enclosed housing for UV irradiation. The Bi2MoO6 films were prepared as follows: the cleaned ITO glass was vertically dipped in precursor solution for 3 min, and raised by 3 cm·min−1. After drying in an oven at 80 , the films were calcined at various temperatures in air for different times. The above steps were repeated three times, and temperature-series samples were obtained. 1.2
Sample characterization
The morphology of Bi2MoO6 films was detected by using a KYKY-2800 scanning electron microscopy (SEM). The crystal structure of the films was investigated by using X-ray diffraction (XRD) (Rigaku D/MAX-RB diffractometer) with Cu Kα radiation (tube voltage 40 kV, tube current 50 mA). Raman spectra were recorded on a RM 2000 microscopic confocal Raman spectrometer (Renishaw Company, England) with an excitation wavelength of 514 nm laser at 0.5 mW. UV-visible diffuse reflectance spectra (DRS) were recorded in the range of 200−700 nm using BaSO4 as a reference sample in Hitachi U-3010 UV-visible spectrometer. Auger electron spectroscopy (AES) was obtained using a PHI-700 scanning auger microscopy system, and the sputtering rate was approximately 7 nm·min−1 for a thermal oxidized SiO2 thin film. The lamp-house was a 500 W Xe lamp (Beijing Trusttech Company, China). The average light intensity (λ>400 nm) was about 190 mW·cm−2. The SAP301 grating monochromator (Beijing Zhuoli Hanguang Instrument Company, China) was used for producing monochromatic light. Photoelectrochemical measurements were performed by home-made three electrode quartz cells. Bi2MoO6 film on ITO, Pt electrode on ITO, and Hg/Hg2Cl2/sat.KCl were used as working electrode, counter electrode, and reference electrode, respectively. The electrolyte was made of propylene carbonate solution containing 0.05 mol·L−1 I2 and 0.5 mol·L−1 LiI. The irradiation area was about 2 cm2. The incident photon-toelectron conversion efficiency (IPCE) was measured by a
home-made two-electrode Bi2MoO6 device. Bi2MoO6 film on ITO and Pt electrode on ITO were used as working electrode and counter electrode, respectively. Propylene carbonate solution containing 0.05 mol·L−1 I2 and 0.5 mol·L−1 LiI was the electrolyte, and the corresponding irradiation area was about 0.126 cm2.
2 2.1
Results and discussion Morphology and phase structure of Bi2MoO6 films
XRD patterns of Bi2MoO6 films calcined at different temperatures for 1 h are shown in Fig.1. No diffraction peaks were observed on ITO glasses. Based on JCPDS 21-0102, γ-Bi2MoO6 phase could be obtained after calcination at 475, 500, and 525 . The intensity of the diffraction peaks did not strengthen with the increase in the calcination temperature. The sample calcined at 500 showed the strongest γ-Bi2MoO6 peaks, indicating that well-crystallized γ-Bi2MoO6 films could be formed at 500 . In addition to γ-Bi2MoO6 phase, the β-Bi2Mo2O9 and γ′-Bi2MoO6 phases also appeared for the samples calcined at 525 , suggesting that γ-Bi2MoO6 crystalline phases changed at higher temperature. At the bottom of Fig.1, a standard XRD pattern of γ-Bi2MoO6 is provided. The standard intensity of the (131) peak was about 5 times that of the (200) or (002) peak, which could be expressed as I(131)/I(200)=5. However, the value of I(131)/I(200) was almost 11 in the calcined series samples. It implied that the crystal had special anisotropic growth along the (131) plane. The morphologies of ITO and Bi2MoO6 films calcined at different temperatures for 1 h are shown in Fig.2. The surface of Bi2MoO6 films calcined at 425 was smooth, and the particles were small. The sample calcined at 500 was also smooth, and the particles became bigger. When the sample was calcined at 525 , there were many large stick-like particles above the films. The SEM images of ITO, precursor, and Bi2MoO6 films calcined at 500 for different times are shown in Fig.3. There were many bubbles on the precursor films, and particles
Fig.1
XRD patterns of Bi2MoO6 films calcined at different temperatures for 1 h
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
Fig.2 SEM images of ITO (a) and Bi2MoO6 films calcined at different temperatures for 1 h (b−d) (a) ITO, (b) 475 , (c) 500 , (d) 525
Fig.3
SEM images of ITO (a), precursor (b), and Bi2MoO6 films calcined at 500 for different times (c−f) (a) ITO, (b) precursor, (c) 0.5 h, (d) 1 h, (e) 2 h, (f) 4 h
appeared only after calcination. The films calcined within 1 h were smooth, and there were a few large particles on the films calcined for 2 h. When the calcination time was prolonged to 4 h, more large particles appeared. Thus, it could be concluded that the Bi2MoO6 particles grew larger as the calcination time prolonged and the calcination temperature increased. Raman spectrum of Bi2MoO6 film calcined at 500 for 1 h is shown in Fig.4. There was a very strong band at 797 cm−l along with two shoulder bands at 853 and 718 cm−1, corresponding to Mo–O stretching modes of the distorted MoO6 octahedra[20]. In addition, bands below 400 cm−1 at 356, 283, 193, and 137 cm−l also corresponded to Mo–O band. From AES surface spectrum of Bi2MoO6 films, Bi, Mo, and O auger
kinetic energy peaks could be seen clearly, but there was no InMNN and SnMNN auger kinetic energy peak at 404 eV and 430 eV, respectively. It implied that the Bi2MoO6 films were compact enough to cover the entire ITO surface. AES depth-profile spectra of Bi2MoO6 films calcined at 500 for 1 h are shown in Fig.5. The atomic ratio of Bi, Mo, and O was almost the same all along the depth, which was about 2:1:6, close to the stoichiometric proportion of the atoms in Bi2MoO6. The thickness of the film was only about 69 nm, which was deduced based on the crossing point of Bi and In atoms. In summary, Bi2MoO6 films could cover the ITO surface after calcination, and the atom ratio was according to the stoichiometric proportion of Bi2MoO6. Bi2MoO6 nanoparticles
Fig.5 Fig.4 Raman spectrum of Bi2MoO6 films calcined at 500 for 1 h
AES depth-profile spectra of the Bi2MoO6 films calcined at 500 for 1 h
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
had special anisotropic growth along the (131) plane. As the calcination time prolonged and the calcination temperature increased, the particles became larger. Bi2MoO6 crystalline phase changed at 525 in our experimental condition. 2.2
Photoelectrochemical properties of Bi2MoO6 films
Fig.6 shows the DRS spectra of both Bi2MoO6 films calcined at 500 for 1 h and Bi2MoO6 nanosheet powder. Bi2MoO6 films had visible-light absorption, and the steep shape of the spectrum indicated that the visible-light absorption came from the band-gap transition[21]. The color of Bi2MoO6 films was yellow, in accordance with the extension of its absorption edge to 480 nm. The band-gap energy estimated from the (Ahν)0.5 (A: absorbance) versus photon-energy plot was 2.61 eV. Compared with Bi2MoO6 nanosheets, the blue shift of Bi2MoO6 films could be observed clearly because the crystalline sizes of the powder and film were far larger than 5 nm. Therefore, the blue shift could not have resulted from the quantum-size effect but can be ascribed to their different degrees of crystallization[15]. The open-circuit voltages of Bi2MoO6 films prepared under different calcination conditions were measured. The opencircuit voltages of Bi2MoO6 films calcined at 475, 500, and 525 for 1 h were 0.275, 0.283, and 0.233 V, respectively; whereas the open-circuit voltages of Bi2MoO6 films calcined at 500 for 0.5, 1, 2, and 4 h were 0.304, 0.283, 0.268, and 0.135 V, respectively. The photoinduced electrons and holes diffused to electrode/solution interface, and this diffusion generated the open-circuit voltage[22]. Hence, the value of open-circuit voltage reflected the number of photoinduced electrons and holes[23]. The open-circuit voltages of Bi2MoO6 films were minimized with the increasing in calcination time and temperature. The most possible reason was the enlargement of the particles with longer calcination time and higher calcination temperature, which diminished the formation and separation of the photoinduced electrons and holes. Photocurrent-generation curves of Bi2MoO6 films prepared under different calcination conditions are shown in Fig.7. Bi2MoO6 films could generate photocurrent under visible-light (λ>400
Fig.7 Photocurrent generation from Bi2MoO6 films under the visible-light irradiation (a) series of Bi2MoO6 films calcined at different temperatures for 1 h; (b) series of Bi2MoO6 films calcined at 500 for different times
nm) irradiation. Photocurrent-generation curves of Bi2MoO6 films calcined at different temperatures for 1 h are shown in Fig.7a. The photocurrent intensity did not exhibit linear relationship with the calcination temperature, and Bi2MoO6 films calcined at 500 showed the strongest photocurrent intensity. This phenomenon was because of the different structures of Bi2MoO6 films calcined at different temperatures. Photocurrent-generation curves of Bi2MoO6 films calcined at 500 for different times are shown in Fig.7b. The photocurrent intensity also did not exhibit linear relationship with the calcination time. The samples calcined for 1 h showed the strongest photocurrent intensity, whereas those calcined for 1.5 and 0.5 h showed the second-strongest intensity, and the samples calcined for 4 h showed the weakest response. In addition, the value of visible-light response was almost the same for several repeated experiments, indicating that the Bi2MoO6 films had excellent light stability. The relationships between visible-light response and preparation conditions will be discussed in the next part. IPCE action curves of Bi2MoO6 films calcined at different temperatures for 1 h under visible-light (λ>400 nm) irradiation are shown in Fig.8. The value of IPCE was calculated using the following expression: IPCE =
Fig.6
DRS of Bi2MoO6 films calcined at 500 for 1 h and Bi2MoO6 powder
12.5 j × 100% λI
where j is the photocurrent intensity (µA·cm−2) and λ is wavelength, I is the incident light intensity (W·m−2). As shown in Fig.8, the conversion efficiency was related to the wavelength of light irradiation, the trends of which
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
defects and controlling the size of particles.
3
Conclusions
Bi2MoO6 films on ITO glass substrates have been prepared from amorphous complex precursor by dip-coating technique, and the thickness of the obtained films was less than 100 nm. Bi2MoO6 films had visible-light response, and detectable photocurrent was generated under visible-light (λ>400 nm) irradiation. The IPCE of Bi2MoO6 films calcined at 500 for 1 h could reach 2.14% at 400 nm. Fig.8 IPCE action curves of Bi2MoO6 films calcined at different temperatures for 1 h
matched the absorption spectra of Bi2MoO6 films. The conversion efficiency reduced with the increase in wavelength. The possible reason was that the photo energy reduced as the wavelength increased, so photoinduced electrons and holes easily recombined, resulting in the diminishing of efficiency. The IPCE of Bi2MoO6 films calcined at 500 for 1 h could reach 2.14% at 400 nm. Compared with Fig.7a and Fig.8, in temperature series samples the changes in visible-light response were in accordance with those of IPCE. The Bi2MoO6 films calcined at different temperatures displayed different visible-light responses, which could be investigated from the formation and recombination of photoinduced electrons and holes. Bi2MoO6 films calcined at low temperature (475 ) existed many defects due to poor crystallization, and these defects could act as electron-hole recombination centers. Even if the number of photoinduced electrons and holes was large after visible-light excitation, the films displayed weak photocurrent and low IPCE because the electron-hole recombination at the interface of Bi2MoO6 films and electrolyte had a dominant effect. The particles of Bi2MoO6 films calcined at high temperature (525 ) were large, and other Bi2MoO6 crystalline phases existed; hence, the number of photoinduced electrons and holes was small, which resulted in the low visible-light response. The visible-light response and IPCE yielded the highest values at 500 because Bi2MoO6 crystalline phase could be well formed at this temperature. Similarly, the visible-light responses of Bi2MoO6 films calcined for different times could also be explained based on the formation and recombination of photoinduced electrons and holes. Bi2MoO6 films calcined for short time (0.5 h) were not well crystallized; the electron-hole recombination at the interface had a dominant effect, so the visible-light response was low. The particles of Bi2MoO6 films calcined for long time (4 h) were large, so the number of photoinduced electrons and holes was small, and the visible-light response was weak. In conclusion, the visible-light response and IPCE of Bi2MoO6 films were affected by their structure. High visible-light response and IPCE could be obtained by reducing the
References 1 Kutsuma, S.; Toma, M.; Takeuchi, K.; Ibusuki, T. Environ. Sci. Technol., 1999, 33: 1071 2 Fu, H. B.; Pan, C. S.; Yao, W. Q.; Zhu Y. F. J. Phys. Chem. B, 2005, 109: 22432 3 Zhang, S. C.; Zhang, C.; Man, Y.; Zhu Y. F. J. Solid State Chem., 2006, 179: 62 4 Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev., 1995, 95: 69 5 Zhang, S. C.; Yao, W. Q.; Zhu, Y. F.; Shi, L. Y. Acta Phys. -Chim. Sin., 2007, 23(1): 111 6 Kudo, A.; Hijii, S. Chem. Lett., 1999, 10: 1103 7 He, Y.; Zhu, Y. F.; Wu, N. Z. J. Solid State Chem., 2004, 177: 3868 8 Xu, T. G.; Zhao, X.; Zhu, Y. F. J. Phys. Chem. B, 2006, 110: 25825 9 Zhang, S. C.; Zhang, C.; Yang, H. P.; Zhu, Y. F. J. Solid State Chem., 2006, 179: 873 10 Zhang, L. W.; Fu, H. B.; Zhang, C.; Zhu, Y. F. J. Solid State Chem., 2006, 179: 804 11 Fu, H. B.; Zhang, L. W.; Yao, W. Q.; Zhu, Y. F. Appl. Catal. B: Environ., 2006, 66: 100 12 Zhang, C.; Zhu, Y. F. Chem. Mater., 2005, 17: 3537 13 Zhao, X.; Xu, T. G.; Yao, W. Q.; Zhang, C.; Zhu, Y. F. Appl. Catal. B: Environ., 2007, 72: 92 14 Huang, G. L.; Zhang, C.; Zhu, Y. F. J. Alloy. Compd., 2007, 432: 269 15 Zhao, X.; Yao, W. Q.; Wu, Y.; Zhang, S. C.; Yang, H. P.; Zhu, Y. F. J. Solid State Chem., 2006, 179: 2562 16 Keulks, G. W.; Krenzke, L. D.; Notermann, T. M. Adv. Catal., 1978, 27: 183 17 Buttrey, D. J.; Vogt, T.; White, B. D. J. Solid State Chem., 2000, 155: 206 18 Yu, J.; Kudo, A. Chem. Lett., 2005, 34: 1528 19 Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B, 2006, 110: 17790 20 Hardcastle, F. D.; Wachs, I. E. J. Phys. Chem., 1991, 95: 10763 21 Kudo, A.; Tsuji, I.; Kato, H. Chem. Commun., 2002, 17: 1958 22 Wei, P. H.; Yao, F. Y.; Wang, Y. J. J. Shandong Normal Univ. (Nat. Sci. Ed.), 2000, 15(2): 151 23 Chen, J. T.; Li, X. J.; Yang, Y.; Wang, L. Y.; He, M. X. Chinese Journal of Catalysis, 2004, 25(5): 397
Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(11): 1671−1676.
Preparation and Photoelectrochemical Properties of Bi2MoO6 Films Yi Man,
Ruilong Zong,
Yongfa Zhu*
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
Abstract:
Bi2MoO6 films on ITO glass substrates were prepared from amorphous complex precursor by dip-coating technique.
The relationships between conditions of preparation, structures, morphologies, and photoelectrochemical properties of Bi2MoO6 films were investigated by using scanning electron microscopy (SEM), X-ray diffraction (XRD), laser Raman spectroscopy (LRS), diffuse reflectance spectroscopy (DRS), photocurrent-action curves, and incident photon-to-current conversion efficiency (IPCE). Bi2MoO6 films calcined at 500 for 1 h were the γ-Bi2MoO6 phase, and Bi2MoO6 nanoparticles grew along (131) plane. The thickness of the films obtained was about 69 nm. The size of Bi2MoO6 nanoparticles increased with rising calcination temperature and extension of calcination time; in addition, γ-Bi2MoO6 changed into β-Bi2MoO6 and γ′-Bi2MoO6 at 525 . Bi2MoO6 films had visible-light response, and detectable photocurrent was generated under visible-light (λ>400 nm) irradiation. The IPCE of the optimized Bi2MoO6 films was 2.14% at 400 nm. The photocurrent density and IPCE could be controlled by modifying the surface structure of Bi2MoO6 films, which could be achieved by changing the preparation conditions. Key Words:
Bi2MoO6 films; Amorphous complex precursor; Photoelectrochemistry; Visible light
At present, environmental pollution and energy crisis have become critical problems to be resolved for the sustainable development of human society. Semiconductor photocatalysis has hence gained more attention due to its potential application in environmental purification[1−3], solar energy conversion[4,5], and H2 production by splitting water[6]. TiO2 is an excellent photocatalyst because of its high activity, low cost, and good stability. In addition to TiO2, other nanosized mixed oxides, such as tantalates[7,8], vanadates[9,10], and tungstates[2,3,11−15], have been reported for UV and visible-light photocatalytic activities by many researchers. However, their visible-light absorption is still little, which limits their solarlight utilization. Therefore, it is necessary to develop novel visible-light photocatalysts that have high visible-light activity and good stability. Bismuth molybdates have three crystalline structures: α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6[16]. γ-Bi2MoO6 is an Aurivillius-type polymorphic compound. It is composed of 2+ octahedral MoO2+ 4 layers and five-coordinated Bi2O2 , and Bi
atom layers are sandwiched between WO6 (MoO6) octahedral layers[17]. Recently, Kudo[18,19] found that Bi2MoO6 showed excellent visible-light photocatalytic activity for the production of O2 from AgNO3 solution. Bi2MoO6 films could also have potential application in novel solar cells, but the photoelectric conversion of the film under visible light has not been reported until now. In this work, Bi2MoO6 films on ITOconductive glass have been prepared by the direct pyrolysis of amorphous complex precursor. The structure and the photoelectrochemical properties of the films have been investigated in detail.
1 1.1
Experimental Preparation of Bi2MoO6 films
Bi2MoO6 films were prepared by using the dip-coating method from amorphous complex precursor. Typically, 0.02 mol diethylenetriaminepentaacetic acid (H5DTPA) and 7.5 mL concentrated ammonia solution (about 7.0 mol·L−1) were
Recieved: June 27, 2007; Revised: August 26, 2007. ∗ Corresponding author. Email: [email protected]; Tel/Fax: +8610-62787601. The project was supported by the National Natural Science Foundation of China (20433010, 20571047). Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
added into 100 mL hot distilled water under magnetic stirring. After dissolution, 0.01 mol Bi(NO3)3 and 0.714 mmol (NH4)6Mo7O24·6H2O powders were added. The mixture was stirred and heated at about 80 to promote the dissolution and reaction, until it became a colorless transparent solution. Then, the solution was vaporized slowly in an oven at 80 . After that, the solution became a piece of transparent glass-like material and then it was grinded to a powder sample. The powder dissolved in distilled water to form the 30% (w, mass fraction) precursor solution. Before dip-coating, the ITO glasses were first scrubbed with detergent and washed thoroughly using distilled water. After treatment with a mixture of 30% H2O2, concentrated ammonia solution, and distilled water (volume ratio is 1:1:6) for 10 min, they were immersed in ultrasonic baths of ethanol for about 15 min. Finally, each ITO plate was exposed to an 11 W mercury lamp in an enclosed housing for UV irradiation. The Bi2MoO6 films were prepared as follows: the cleaned ITO glass was vertically dipped in precursor solution for 3 min, and raised by 3 cm·min−1. After drying in an oven at 80 , the films were calcined at various temperatures in air for different times. The above steps were repeated three times, and temperature-series samples were obtained. 1.2
Sample characterization
The morphology of Bi2MoO6 films was detected by using a KYKY-2800 scanning electron microscopy (SEM). The crystal structure of the films was investigated by using X-ray diffraction (XRD) (Rigaku D/MAX-RB diffractometer) with Cu Kα radiation (tube voltage 40 kV, tube current 50 mA). Raman spectra were recorded on a RM 2000 microscopic confocal Raman spectrometer (Renishaw Company, England) with an excitation wavelength of 514 nm laser at 0.5 mW. UV-visible diffuse reflectance spectra (DRS) were recorded in the range of 200−700 nm using BaSO4 as a reference sample in Hitachi U-3010 UV-visible spectrometer. Auger electron spectroscopy (AES) was obtained using a PHI-700 scanning auger microscopy system, and the sputtering rate was approximately 7 nm·min−1 for a thermal oxidized SiO2 thin film. The lamp-house was a 500 W Xe lamp (Beijing Trusttech Company, China). The average light intensity (λ>400 nm) was about 190 mW·cm−2. The SAP301 grating monochromator (Beijing Zhuoli Hanguang Instrument Company, China) was used for producing monochromatic light. Photoelectrochemical measurements were performed by home-made three electrode quartz cells. Bi2MoO6 film on ITO, Pt electrode on ITO, and Hg/Hg2Cl2/sat.KCl were used as working electrode, counter electrode, and reference electrode, respectively. The electrolyte was made of propylene carbonate solution containing 0.05 mol·L−1 I2 and 0.5 mol·L−1 LiI. The irradiation area was about 2 cm2. The incident photon-toelectron conversion efficiency (IPCE) was measured by a
home-made two-electrode Bi2MoO6 device. Bi2MoO6 film on ITO and Pt electrode on ITO were used as working electrode and counter electrode, respectively. Propylene carbonate solution containing 0.05 mol·L−1 I2 and 0.5 mol·L−1 LiI was the electrolyte, and the corresponding irradiation area was about 0.126 cm2.
2 2.1
Results and discussion Morphology and phase structure of Bi2MoO6 films
XRD patterns of Bi2MoO6 films calcined at different temperatures for 1 h are shown in Fig.1. No diffraction peaks were observed on ITO glasses. Based on JCPDS 21-0102, γ-Bi2MoO6 phase could be obtained after calcination at 475, 500, and 525 . The intensity of the diffraction peaks did not strengthen with the increase in the calcination temperature. The sample calcined at 500 showed the strongest γ-Bi2MoO6 peaks, indicating that well-crystallized γ-Bi2MoO6 films could be formed at 500 . In addition to γ-Bi2MoO6 phase, the β-Bi2Mo2O9 and γ′-Bi2MoO6 phases also appeared for the samples calcined at 525 , suggesting that γ-Bi2MoO6 crystalline phases changed at higher temperature. At the bottom of Fig.1, a standard XRD pattern of γ-Bi2MoO6 is provided. The standard intensity of the (131) peak was about 5 times that of the (200) or (002) peak, which could be expressed as I(131)/I(200)=5. However, the value of I(131)/I(200) was almost 11 in the calcined series samples. It implied that the crystal had special anisotropic growth along the (131) plane. The morphologies of ITO and Bi2MoO6 films calcined at different temperatures for 1 h are shown in Fig.2. The surface of Bi2MoO6 films calcined at 425 was smooth, and the particles were small. The sample calcined at 500 was also smooth, and the particles became bigger. When the sample was calcined at 525 , there were many large stick-like particles above the films. The SEM images of ITO, precursor, and Bi2MoO6 films calcined at 500 for different times are shown in Fig.3. There were many bubbles on the precursor films, and particles
Fig.1
XRD patterns of Bi2MoO6 films calcined at different temperatures for 1 h
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
Fig.2 SEM images of ITO (a) and Bi2MoO6 films calcined at different temperatures for 1 h (b−d) (a) ITO, (b) 475 , (c) 500 , (d) 525
Fig.3
SEM images of ITO (a), precursor (b), and Bi2MoO6 films calcined at 500 for different times (c−f) (a) ITO, (b) precursor, (c) 0.5 h, (d) 1 h, (e) 2 h, (f) 4 h
appeared only after calcination. The films calcined within 1 h were smooth, and there were a few large particles on the films calcined for 2 h. When the calcination time was prolonged to 4 h, more large particles appeared. Thus, it could be concluded that the Bi2MoO6 particles grew larger as the calcination time prolonged and the calcination temperature increased. Raman spectrum of Bi2MoO6 film calcined at 500 for 1 h is shown in Fig.4. There was a very strong band at 797 cm−l along with two shoulder bands at 853 and 718 cm−1, corresponding to Mo–O stretching modes of the distorted MoO6 octahedra[20]. In addition, bands below 400 cm−1 at 356, 283, 193, and 137 cm−l also corresponded to Mo–O band. From AES surface spectrum of Bi2MoO6 films, Bi, Mo, and O auger
kinetic energy peaks could be seen clearly, but there was no InMNN and SnMNN auger kinetic energy peak at 404 eV and 430 eV, respectively. It implied that the Bi2MoO6 films were compact enough to cover the entire ITO surface. AES depth-profile spectra of Bi2MoO6 films calcined at 500 for 1 h are shown in Fig.5. The atomic ratio of Bi, Mo, and O was almost the same all along the depth, which was about 2:1:6, close to the stoichiometric proportion of the atoms in Bi2MoO6. The thickness of the film was only about 69 nm, which was deduced based on the crossing point of Bi and In atoms. In summary, Bi2MoO6 films could cover the ITO surface after calcination, and the atom ratio was according to the stoichiometric proportion of Bi2MoO6. Bi2MoO6 nanoparticles
Fig.5 Fig.4 Raman spectrum of Bi2MoO6 films calcined at 500 for 1 h
AES depth-profile spectra of the Bi2MoO6 films calcined at 500 for 1 h
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
had special anisotropic growth along the (131) plane. As the calcination time prolonged and the calcination temperature increased, the particles became larger. Bi2MoO6 crystalline phase changed at 525 in our experimental condition. 2.2
Photoelectrochemical properties of Bi2MoO6 films
Fig.6 shows the DRS spectra of both Bi2MoO6 films calcined at 500 for 1 h and Bi2MoO6 nanosheet powder. Bi2MoO6 films had visible-light absorption, and the steep shape of the spectrum indicated that the visible-light absorption came from the band-gap transition[21]. The color of Bi2MoO6 films was yellow, in accordance with the extension of its absorption edge to 480 nm. The band-gap energy estimated from the (Ahν)0.5 (A: absorbance) versus photon-energy plot was 2.61 eV. Compared with Bi2MoO6 nanosheets, the blue shift of Bi2MoO6 films could be observed clearly because the crystalline sizes of the powder and film were far larger than 5 nm. Therefore, the blue shift could not have resulted from the quantum-size effect but can be ascribed to their different degrees of crystallization[15]. The open-circuit voltages of Bi2MoO6 films prepared under different calcination conditions were measured. The opencircuit voltages of Bi2MoO6 films calcined at 475, 500, and 525 for 1 h were 0.275, 0.283, and 0.233 V, respectively; whereas the open-circuit voltages of Bi2MoO6 films calcined at 500 for 0.5, 1, 2, and 4 h were 0.304, 0.283, 0.268, and 0.135 V, respectively. The photoinduced electrons and holes diffused to electrode/solution interface, and this diffusion generated the open-circuit voltage[22]. Hence, the value of open-circuit voltage reflected the number of photoinduced electrons and holes[23]. The open-circuit voltages of Bi2MoO6 films were minimized with the increasing in calcination time and temperature. The most possible reason was the enlargement of the particles with longer calcination time and higher calcination temperature, which diminished the formation and separation of the photoinduced electrons and holes. Photocurrent-generation curves of Bi2MoO6 films prepared under different calcination conditions are shown in Fig.7. Bi2MoO6 films could generate photocurrent under visible-light (λ>400
Fig.7 Photocurrent generation from Bi2MoO6 films under the visible-light irradiation (a) series of Bi2MoO6 films calcined at different temperatures for 1 h; (b) series of Bi2MoO6 films calcined at 500 for different times
nm) irradiation. Photocurrent-generation curves of Bi2MoO6 films calcined at different temperatures for 1 h are shown in Fig.7a. The photocurrent intensity did not exhibit linear relationship with the calcination temperature, and Bi2MoO6 films calcined at 500 showed the strongest photocurrent intensity. This phenomenon was because of the different structures of Bi2MoO6 films calcined at different temperatures. Photocurrent-generation curves of Bi2MoO6 films calcined at 500 for different times are shown in Fig.7b. The photocurrent intensity also did not exhibit linear relationship with the calcination time. The samples calcined for 1 h showed the strongest photocurrent intensity, whereas those calcined for 1.5 and 0.5 h showed the second-strongest intensity, and the samples calcined for 4 h showed the weakest response. In addition, the value of visible-light response was almost the same for several repeated experiments, indicating that the Bi2MoO6 films had excellent light stability. The relationships between visible-light response and preparation conditions will be discussed in the next part. IPCE action curves of Bi2MoO6 films calcined at different temperatures for 1 h under visible-light (λ>400 nm) irradiation are shown in Fig.8. The value of IPCE was calculated using the following expression: IPCE =
Fig.6
DRS of Bi2MoO6 films calcined at 500 for 1 h and Bi2MoO6 powder
12.5 j × 100% λI
where j is the photocurrent intensity (µA·cm−2) and λ is wavelength, I is the incident light intensity (W·m−2). As shown in Fig.8, the conversion efficiency was related to the wavelength of light irradiation, the trends of which
Yi Man et al. / Acta Physico-Chimica Sinica, 2007, 23(11): 1671−1676
defects and controlling the size of particles.
3
Conclusions
Bi2MoO6 films on ITO glass substrates have been prepared from amorphous complex precursor by dip-coating technique, and the thickness of the obtained films was less than 100 nm. Bi2MoO6 films had visible-light response, and detectable photocurrent was generated under visible-light (λ>400 nm) irradiation. The IPCE of Bi2MoO6 films calcined at 500 for 1 h could reach 2.14% at 400 nm. Fig.8 IPCE action curves of Bi2MoO6 films calcined at different temperatures for 1 h
matched the absorption spectra of Bi2MoO6 films. The conversion efficiency reduced with the increase in wavelength. The possible reason was that the photo energy reduced as the wavelength increased, so photoinduced electrons and holes easily recombined, resulting in the diminishing of efficiency. The IPCE of Bi2MoO6 films calcined at 500 for 1 h could reach 2.14% at 400 nm. Compared with Fig.7a and Fig.8, in temperature series samples the changes in visible-light response were in accordance with those of IPCE. The Bi2MoO6 films calcined at different temperatures displayed different visible-light responses, which could be investigated from the formation and recombination of photoinduced electrons and holes. Bi2MoO6 films calcined at low temperature (475 ) existed many defects due to poor crystallization, and these defects could act as electron-hole recombination centers. Even if the number of photoinduced electrons and holes was large after visible-light excitation, the films displayed weak photocurrent and low IPCE because the electron-hole recombination at the interface of Bi2MoO6 films and electrolyte had a dominant effect. The particles of Bi2MoO6 films calcined at high temperature (525 ) were large, and other Bi2MoO6 crystalline phases existed; hence, the number of photoinduced electrons and holes was small, which resulted in the low visible-light response. The visible-light response and IPCE yielded the highest values at 500 because Bi2MoO6 crystalline phase could be well formed at this temperature. Similarly, the visible-light responses of Bi2MoO6 films calcined for different times could also be explained based on the formation and recombination of photoinduced electrons and holes. Bi2MoO6 films calcined for short time (0.5 h) were not well crystallized; the electron-hole recombination at the interface had a dominant effect, so the visible-light response was low. The particles of Bi2MoO6 films calcined for long time (4 h) were large, so the number of photoinduced electrons and holes was small, and the visible-light response was weak. In conclusion, the visible-light response and IPCE of Bi2MoO6 films were affected by their structure. High visible-light response and IPCE could be obtained by reducing the
References 1 Kutsuma, S.; Toma, M.; Takeuchi, K.; Ibusuki, T. Environ. Sci. Technol., 1999, 33: 1071 2 Fu, H. B.; Pan, C. S.; Yao, W. Q.; Zhu Y. F. J. Phys. Chem. B, 2005, 109: 22432 3 Zhang, S. C.; Zhang, C.; Man, Y.; Zhu Y. F. J. Solid State Chem., 2006, 179: 62 4 Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev., 1995, 95: 69 5 Zhang, S. C.; Yao, W. Q.; Zhu, Y. F.; Shi, L. Y. Acta Phys. -Chim. Sin., 2007, 23(1): 111 6 Kudo, A.; Hijii, S. Chem. Lett., 1999, 10: 1103 7 He, Y.; Zhu, Y. F.; Wu, N. Z. J. Solid State Chem., 2004, 177: 3868 8 Xu, T. G.; Zhao, X.; Zhu, Y. F. J. Phys. Chem. B, 2006, 110: 25825 9 Zhang, S. C.; Zhang, C.; Yang, H. P.; Zhu, Y. F. J. Solid State Chem., 2006, 179: 873 10 Zhang, L. W.; Fu, H. B.; Zhang, C.; Zhu, Y. F. J. Solid State Chem., 2006, 179: 804 11 Fu, H. B.; Zhang, L. W.; Yao, W. Q.; Zhu, Y. F. Appl. Catal. B: Environ., 2006, 66: 100 12 Zhang, C.; Zhu, Y. F. Chem. Mater., 2005, 17: 3537 13 Zhao, X.; Xu, T. G.; Yao, W. Q.; Zhang, C.; Zhu, Y. F. Appl. Catal. B: Environ., 2007, 72: 92 14 Huang, G. L.; Zhang, C.; Zhu, Y. F. J. Alloy. Compd., 2007, 432: 269 15 Zhao, X.; Yao, W. Q.; Wu, Y.; Zhang, S. C.; Yang, H. P.; Zhu, Y. F. J. Solid State Chem., 2006, 179: 2562 16 Keulks, G. W.; Krenzke, L. D.; Notermann, T. M. Adv. Catal., 1978, 27: 183 17 Buttrey, D. J.; Vogt, T.; White, B. D. J. Solid State Chem., 2000, 155: 206 18 Yu, J.; Kudo, A. Chem. Lett., 2005, 34: 1528 19 Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B, 2006, 110: 17790 20 Hardcastle, F. D.; Wachs, I. E. J. Phys. Chem., 1991, 95: 10763 21 Kudo, A.; Tsuji, I.; Kato, H. Chem. Commun., 2002, 17: 1958 22 Wei, P. H.; Yao, F. Y.; Wang, Y. J. J. Shandong Normal Univ. (Nat. Sci. Ed.), 2000, 15(2): 151 23 Chen, J. T.; Li, X. J.; Yang, Y.; Wang, L. Y.; He, M. X. Chinese Journal of Catalysis, 2004, 25(5): 397