Letter pubs.acs.org/JPCL
Transforming Anodized WO3 Films into Visible-Light-Active Bi2WO6 Photoelectrodes by Hydrothermal Treatment Charlene Ng, Akihide Iwase, Yun Hau Ng, and Rose Amal* ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *
ABSTRACT: We directly transformed anodized tungsten oxide film (WO3·2H2O) into bismuth tungstate (Bi2WO6) by substituting the intercalated water molecules with [Bi2O2]2+ in a hydrothermal treatment. The resultant Bi2WO6 was readily used as an electrode to produce anodic photocurrent in H2 evolution on the Pt counter electrode observed under visible light irradiation.
SECTION: Energy Conversion and Storage
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uch research has been directed toward photoelectrochemical (PEC) water splitting using semiconductors owing to its potential to cleanly generate inexhaustible hydrogen (H2) fuel sources from water and solar energy.1 Although various photoelectrodes for water splitting have been reported, the number of visible-light-responsive oxide photoelectrodes is still limited, for example,WO3,2,3 Fe2O3,4 and BiVO4.5 Hence, it is of great value to develop visible-light-active photocatalysts that can efficiently exploit the visible light in solar energy conversion reactions. Bismuth tungstate (Bi2WO6) has been demonstrated by several authors as a photocatalyst for the evolution of O2 from water and the degradation of organic compounds under visible light irradiation.6−10 Generally, Bi2WO6 thin films are synthesized by directly coating Bi2WO6 particles on an electrode substrate using the spin-coating technique,11 dipcoating method,12 or electrostatic self-assembly deposition method.13 However, these existing coating techniques often result in unstable films with relatively lower activities due to the poor attachment of particles onto substrates such as FTO and ITO glass.14 Recently, Zhu et al. developed a hard-templatedirected synthesis method to prepare porous Bi2WO6 thin films, while Zhang et al. demonstrated the synthesis of a Bi2WO6 photonic crystal film for visible-light-driven activities.15,16 There are only limited studies relating to the direct synthesis of Bi2WO6 films without having particulate Bi2WO6 as an intermediate. The discovery of alternatives in the development of direct film synthesis of this ternary metal oxide is therefore desirable. We have previously reported the synthesis of flower-like WO3·2H2O with layered structured thin films via the anodization technique (Scheme S1, Supporting Information).17 © 2012 American Chemical Society
Although anodization is useful for the preparation of metal oxide films, simple oxides are usually produced. On the other hand, the synthesis of Bi2WO6 particles from a layered tungstate precursor by the hydrothermal method has also been widely reported.18 This motivates us to investigate the direct synthesis of complex oxide, Bi2WO6 films by the anodization of tungsten foil and subsequent hydrothermal treatment. Here, we show for the first time the feasibility of transforming a simple oxide of layered as-anodized WO3·2H2O films into a ternary oxide electrode of Bi2WO6 without the need for particulate Bi2WO6 as an intermediate. Insight into the process of transformation from tungsten foil to Bi2WO6 via anodized tungstate was also presented. Additionally, PEC water splitting was demonstrated by employing Bi2WO6 as a photoanode. Figure 1 summarizes the materials derived from different synthetic conditions involving the anodization of tungsten metal and subsequent hydrothermal treatment in achieving Bi2WO6. Flower-structured WO3·2H2O (sample A) was obtained under unstirred conditions, while porous WO3 films (sample B) were achieved under stirred condition introduced during the anodization process. Flower-structured WO3 (sample C) was attained after WO3·2H2O (sample A) experienced the loss of water molecules through calcination at 400 °C for 4 h.17 Figure 2A and B depicts the SEM image of the flowerstructured as-anodized WO3·2H2O film, while Figure 2C and D Received: February 12, 2012 Accepted: March 14, 2012 Published: March 14, 2012 913
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Figure 1. Flowchart summarizing the materials derived by various synthetic conditions involving the anodization of tungsten metal and subsequent hydrothermal treatment.
along the (001) plane, which is parallel to the intrinsic layer structure of Bi2WO6.19,20 X-ray diffraction (XRD) patterns of the samples before and after the hydrothermal treatment are revealed in Figure 3A. Calcination at 400 °C for 4 h was performed on the film after the hydrothermal treatment. XRD confirmed the presence of monoclinic WO3·2H2O in the as-anodized film, while the prominent (131) and (060) peaks that arise after the hydrothermal treatment can be well assigned to orthorhombic Bi2WO6. Orthorhombic Bi2WO6 was obtained following hydrothermal treatment prior to the annealing process; calcination was subsequently performed to improve the crystallinity of the film. These results confirmed the successful formation of Bi 2 WO 6 by employing the as-anodized WO3·2H2O film as the tungsten precursor. For comparison, nonlayered structure tungsten precursors prepared by anodization were also employed: amorphous porous WO3 and monoclinic flower-structured WO3 (samples B and C in Figure 1, respectively). The absence of orthorhombic Bi2WO 6 formation (Figure S1, Supporting Information) on these films signifies the importance of a layered structure in the formation of Bi2WO6 during hydrothermal treatment. Given that orthorhombic Bi2WO6 is composed of alternating [Bi2O2]2+ and WO6 octahedral layers, it is postulated that the layered structure of WO3·2H2O favors the formation of orthorhombic Bi2WO6 through a substitution process of the H2O molecules with the [Bi2O2]2+ layers during hydrothermal, as depicted in Figure 3B.21,22 In Figure 4A, a glancing angle XRD (GAXRD) technique that manipulates the angle of incident X-rays enables thin-film analysis at different depths; a small glancing angle analyzes the top layer of the film, while a larger glancing angle investigates into the bulk of the film. At a glancing angle of 0.3°, the XRD pattern shows minute peaks corresponding to WO3. Subsequently, the peak intensity of WO3 increases as the glancing angle increases from 0.3 to 1.0°, where the XRD analyzes deeper within the film. These results indicate that the WO3 is positioned under the Bi2WO6 layer, which lies between the tungsten substrate and the top Bi2WO6 film. We have confirmed the existence of this thin layer of nonlayered amorphous WO3 originally positioned underneath of the flower-structured WO3 in our previous study,17 indicating
Figure 2. SEM images of the as-anodized WO3·2H2O film (A,B) and Bi2WO6 following hydrothermal treatment (C,D) at different magnifications. The circled area in (C) indicates the pointed edges of the square/rectangular platelets observed after the hydrothermal treatment.
illustrates the morphology of the Bi2WO6 film after hydrothermal treatment. An assembly of intertwined platelets radiating in all directions that lead to the formation of a flower-like morphology can be observed in Figure 2A and B. In contrast, the film that underwent the hydrothermal process composed of square/rectangular platelets with a thickness of 10−15 nm and the flower structures seemed to be unwrapped (Figure 2D) following the hydrothermal treatment. The unwrapped flower structures may possibly be caused by the pressurized system during the hydrothermal process and/or the substitution process of interstitial water that is discussed later. The pointed edges of the square/rectangular platelets that are apparent in the circled area of Figure 2C are also evidently absent in the as-anodized film (Figure 2A). The formation of these square/rectangular platelets at pH 7 without any surfactants can be attributed to the high anisotropic growth 914
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Figure 3. (A) XRD patterns of the films before and after hydrothermal reaction. (B) The schematic structure of transformation of anodized WO3·2H2O into Bi2WO6.
Figure 4. (A) GAXRD patterns of the Bi2WO6 film at 0.3, 0.5, and 1.0° glancing angle. (B) XPS elemental depth profile of the Bi2WO6 film. The inset is the SEM image of the side profile of the Bi2WO6 film. (C) The SEM-EDX mapping analysis of the elements (Bi, W, and O) to illustrate the homogeneity of the Bi2WO6 layer formed after hydrothermal treatment.
thin film during the transformation of layered WO3·2H2O to Bi2WO6 via a substitution mechanism. The presence of the thin porous WO3 is observed toward the end of region 2, where the Bi content reaches zero atomic percent. In region 3, the tungsten content reaches 100%, indicating that the XPS analysis has reached the tungsten substrate. Interestingly, the inset in Figure 4B shows that the thickness of the film is approximately 1.2 μm, which is consistent with the film before hydrothermal treatment, as illustrated in Figure S2 (Supporting Information). Hence, the analogous thickness and morphology after transformation further supported the involvement of a substitution process in the formation of Bi2WO6. The XRD and XPS analyses are also further confirmed by the SEM-EDX mapping (Figure 4C) on the resulting film. The SEM-EDX mapping scan is performed on the side profile of the resulting film to observe the change in the concentration of Bi, O, and W
that the amorphous WO3 was produced by anodization, not by hydrothermal treatment. These results are further supported by the XPS elemental depth profile of the Bi2WO6 film (Figure 4B). The etching time and atomic ratio do not correspond to the actual respective thickness and stoichiometric ratio due to preferential sputtering on the uneven surface that arises during the etching process. Region 1 reflects a Bi rich layer at the surface that has maximum exposure to the Bi precursors during hydrothermal treatment. Subsequently, the Bi content decreases with depth and remains constant, indicating the formation of pure Bi2WO6 in this region. In region 2, a mixture of Bi2WO6 and WO3 exists, with the amount of Bi decreasing with depth while the W content increases with depth. This can be explained by the limited diffusion of Bi precursors or [Bi2O2]2+ into the bulk material that generates a gradient of Bi concentration throughout the 915
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Figure 5. (A) Current−potential curves of WO3 and Bi2WO6 films under dark and light condition (scan rate of 0.02 V/s in 0.1 M H2SO4 aq. versus Ag/AgCl, λ ≥ 420 nm). The horizontal dotted lines indicate the position of zero current. (B) UV−vis absorbance spectra and wavelength-dependent photocurrent of the Bi2WO6 film (electrolyte, 0.1 M H2SO4; potential, 1.0 V versus Ag/AgCl; light source, 300 W Xe lamp with cutoff filters).
nm, corresponding to a band gap energy of 2.8 eV, which illustrates its ability to respond under visible light irradiation. The onset wavelength of photocurrent also corresponds well with the absorption edge of the UV−vis spectra, confirming that the generation of photocurrent originates from the excitation of the band gap of Bi2WO6. Figure 6A shows the current generation and the rate of hydrogen (H2) gas evolution during a 4 h PEC water splitting reaction with a Bi2WO6 electrode as the photoanode. The
elements with depth. The Bi atoms can be observed throughout the depth of the film, with the greatest intensity near the surface of the film and decreasing with depth. This confirms that the transformation to Bi2WO6 is not solely confined to the surface of the film. On the other hand, the intensity of W atoms increases with depth, indicating the presence of WO 3 positioned under the Bi2WO6 layer. The current−potential study of the Bi2WO6 and WO3 film under dark and light illumination in 0.1 M H2SO4 is illustrated in Figure 5A. The WO3 film was prepared by a similar anodization method and subsequently calcined at 400 °C for 4 h to achieve a monoclinic structure (sample C in Figure 1). No significant change in the morphology is observed after calcination. The onset potential is the minimum potential or voltage required for continuous anodic photocurrent to be observed and is illustrated by the interception of the curves under dark and light conditions. As observed in Figure 5A, the onset potential values were observed at −0.60 V for Bi2WO6 and −0.23 V for WO3 during the sweeping toward the positive potential direction. The presence of Bi2WO6 shifted the onset potential of the electrode by 0.37 V to the negative direction, which is of practical importance for the PEC water splitting reactions. Because both Bi2WO6 and WO3 are n-type semiconductors, the onset potential represents the flat band potential and also the potential of the conduction band. As mentioned earlier, Bi2WO6 is composed of a layered structure with the perovskite-like slab of WO6 and [Bi2O2]2+ layers. In general, a metal oxide with a layered structure usually leads to a wider band energy when compared to its bulk counterpart. Factors contributing to the wider band include the thickness and interaction of the perovskite layers that cause the excited energy state to be more localized.23 Therefore, the conduction band consisting of W 5d orbitals in the layered Bi2WO6 is more negative than that of the bulk WO3. The reason for Bi2WO6 having a similar band gap than WO3, regardless of the higher conduction band, is the presence of Bi. On the basis of density functional theory calculations, the valence band of Bi2WO6 consists of both Bi 6s and O 2p orbitals, which result in a new hybridized valence band with energy higher than that of the O 2p orbital in WO3.21,24,25 The optical behavior of the Bi2WO6 film investigated by the UV−vis absorbance spectra and wavelength-dependent photocurrent measurements is illustrated in Figure 5B. The absorption edge is positioned at 440
Figure 6. (A) Anodic photocurrent of Bi2WO6 and the rate of H2 evolution (Δ) for PEC water splitting using a Bi2WO6 electrode under visible light irradiation. Applied potential, 1.0 V versus Pt; electrolyte, 0.1 M H2SO4 solution; light source, 300 W Xe lamp with a cutoff filter (λ ≥ 420 nm). (B) Schematic of the two possible electron pathways for the Bi2WO6 and WO3 heterojunction electrode and the Bi2WO6 electrode. 916
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layer and subsequent shielding from the visible light illumination by the Bi2WO6 layer. In summary, we have successfully developed a simple direct film route that involves hydrothermal treatment on anodized hydrated tungstate film. The Bi2WO6 generated an anodic photocurrent and produced H2 from the counter Pt electrode under visible light irradiation. The similar morphology and film thickness attained after the transformation to Bi2WO6 verified the proposed substitution process of the H2O molecules in the anodized film with the [Bi2O2]2+ layers during hydrothermal treatment. The importance of a layered structure in the formation of Bi2WO6 through a substitution process during hydrothermal treatment is confirmed by the absence of orthorhombic Bi2WO6 formation in porous amorphous WO3 and flower-structured monoclinic WO3. This understanding certainly provides a new platform for the development of complex ternary metal oxide photoelectrodes.
negligible dark current measured under the absence of irradiation confirms the generation of excited carriers under visible light irradiation. Furthermore, no H2 gas was detected during the dark measurement. When an applied voltage of 1.0 V versus Pt (two-electrode system) is applied, H2 gas evolution is detected under visible light irradiation. The anodic photocurrent is observed to be approximately 6.5 μA,26 while the H2 gas evolution rate remains approximately 0.1 μmol h−1 within 4 h of visible light illumination. The theoretical rate of H2 gas evolution calculated from the photocurrent measured matched with the actual rate of H2 gas evolution and reflects the close 100% faradaic efficiency achieved during reaction. Furthermore, the performed PEC water splitting reaction is an uphill energy conversion process as it proceeds by applying an external applied voltage smaller than 1.23 V. Bi2WO6 is also recognized as an oxygen-evolving photocatalyst and has been reported by Kudo et al. to exhibit O2 evolution activity under visible light irradiation.6,20 The absence of O2 detection could be explained by the high solubility of O2 in water (1.4 μmol/ mL under 1 atm at 20 °C)27 and the photoadsorption of O2 molecules onto the surface of the Bi2WO6 electrode.28,29 The limited release of the O2 gas molecules from the electrolyte and photoanode leads to a low production rate that is beyond the detection limit of the gas chromatograph. The possibility of photocorrosion by the excited holes is also eliminated by comparing the XRD spectra of the samples that are being utilized before and after the PEC measurements (Figure S3, Supporting Information). On the basis of the onset potential and optical band gap energies of the materials, we propose two possible electron pathways experienced by the photoexcited electrons, as depicted in Figure 6B. The first pathway involves the direct injection of the photogenerated electrons to the collecting substrate (W metal) from the synthesized Bi2WO6 layer. An alternative pathway arises due to the presence of WO3 that is located in between Bi2WO6 and the tungsten substrate, which may be beneficial for charge separation caused by the difference in the position of the conduction and valence bands of the two semiconductors. Similar heterojunction films such as WO3/ BiVO4,30,31 WO3/SrNb2O6,32 and Fe2O3/WO333 have shown enhanced performance as compared to their individual components. It was discussed that the improved properties of these heterojunction films is mainly ascribed to the improved charge separation efficiency induced by the band energy alignment present in the heterojunction. This results in an improvement on the overall photocurrent conversion efficiency. Under visible light illumination, excited electrons are generated in the conduction band of Bi2WO6. As the conduction and valence bands of Bi2WO6 are more negative than the corresponding bands of WO3, this condition favors the injection of photogenerated electrons from the conduction band of Bi2WO6 to that of the WO3, which are located nearer to the tungsten substrate, as verified by GAXRD and XPS depth profile results. Subsequently, the electrons will be transported through an external circuit to the Pt counter electrode where H2 evolves. In contrast, the holes generated by Bi2WO6 can migrate to the electrode/electrolyte surface to produce O2 from the oxidation of H2O. It is important to note that despite the presence of this WO3 layer, the electrode mainly exhibits the behavior of Bi2WO6 because the WO3 layer exists in a small amount and is located underneath of the Bi2WO6 layer. The photocurrent generated from the WO3 layer can also be considered negligible due to the low thickness exhibited by this
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ASSOCIATED CONTENT
S Supporting Information *
(1) Experimental details and (2) supporting figures. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by the ARC Centre of Excellence for Functional Nanomaterials. The authors would also like to acknowledge the UNSW Mark Wainwright Analytical Centre, and particularly thank Dr. Yu Wang and Dr. Bill Gong for their generous help in the XRD and XPS analysis.
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REFERENCES
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