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Reducing Graphene Oxide on a Visible-Light BiVO4 Photocatalyst for an Enhanced Photoelectrochemical Water Splitting Yun Hau Ng,†,§ Akihide Iwase,†,§ Akihiko Kudo,‡ and Rose Amal*,† †
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ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia, and ‡Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
ABSTRACT Bismuth vanadate (BiVO4) is incorporated with reduced graphene oxide (RGO) using a facile single-step photocatalytic reaction to improve its photoresponse in visible light. Remarkable 10-fold enhancement in photoelectrochemical water splitting reaction is observed on BiVO4-RGO composite compared with pure BiVO4 under visible illumination. This improvement is attributed to the longer electron lifetime of excited BiVO4 as the electrons are injected to RGO instantly at the site of generation, leading to a minimized charge recombination. Improved contact between BiVO4 particles with transparent conducting electrode using RGO scaffold also contributes to this photoresponse enhancement. SECTION Energy Conversion and Storage
BiVO4 from nanowire to nanopyramid.17 We have also demonstrated photoelectrochemical water splitting using BiVO4 electrodes to be influenced by the contact between photocatalysts and FTO probed by BiVO4 with various sizes.18 As the basis for thin-film photoanode in photoelectrochemical process, in general, pure BiVO4 falls short on one main principal barrier, namely, the high charge-carrier's recombination rate. Transport of photogenerated electrons across the particle network randomly exposes them to many grain boundaries and recombination centers prior to their collection at the electrode surface. This phenomenon results in only modest photocurrent generated in the pure BiVO4 film. In searching for new tools to enhance photoactivity of semiconductors, the graphene-based nanocomposite system has stood out as recent studies have shown its usefulness in electronics, catalysis, and photovoltaic devices.19-21 Owing to the abundance of delocalized electrons from the conjugated sp2-bonded carbon network, graphitic carbon enhances the transport of electrons photogenerated in semiconductor particles, leading to an increase in the photoconversion efficiency of the system. Graphene-based metal or semiconductor nanocomposites are generally synthesized using graphene oxide (GO) as the precursor, followed by its chemical reduction to reduced graphene oxide (RGO). Various methods have been reported, and the use of toxic reducing agent hydrazine or high temperature is sometimes unavoidable.22,23 Kamat et al. recently discovered an alternative facile photocatalytic
T
he demand for clean energy technology has triggered research in nanostructured semiconductor films for solar cells, water splitting, and environmental remediation application.1-6 Of particular interest are titanium dioxide (TiO2) particulate films synthesized using various methods including the hydrothermal and anodization methods.7-9 To date, TiO2 serves as the benchmark material for many photoconversion processes. Despite initial success in achieving considerable efficiency in converting light energy into useful chemicals and electricity, TiO2 activation limited to UV light only has substantially hampered its relevance in a genuine solar light energy conversion application. From the viewpoint of solar energy utilization, the development of photocatalysts capable of photoinduced charge separation upon excitation in the visible spectral region is emerging as the important research direction in this field.10-12 Bismuth vanadate (BiVO4) has been an ideal visible lightdriven semiconductor with narrow band gap energy of 2.4 eV (λ2.4 eV) in a standard three-compartment cell, BiVO4 and BiVO4-RGO films undergo charge separation; then, the electrons flow through the circuit, thus generating photocurrent. Magnitude of the anodic photocurrent, generated by the films, characterizes the electrons collection efficiency at the FTO collecting electrode. Figure 3a shows the voltage-current functions of BiVO4 and BiVO4-RGO during repeating ON-OFF illumination cycles. Both samples exhibit prompt and reproducible photocurrent, whereas a controlled experiment using only RGO film does not generate any photoresponse. Using a dropcast method, BiVO4 thin film generates a low photocurrent density (∼8 μA cm-2), which is consistent with those observed in other laboratories.16,18 Other works showed that pressuring the thin film at 200 kg cm-2 or directly growing BiVO4 on conducting electrode can improve the adhesion of BiVO4 to the conducting electrode, thus improving the photocurrent generation (ranging from tens of microamperes to millamperes per square centimeter).14,16 Overall, photocurrent generation of the BiVO4-RGO photoelectrode surpasses those generated by pure BiVO4. Magnitude of the saturated photocurrent density generated in BiVO4-RGO (∼70 μA cm-2) is nearly one order larger than that of pure BiVO4. Interestingly, a UV-excited TiO2 (Degussa P25) photoelectrode with the same surface density prepared under similar experimental conditions produces less photocurrent density (∼50 μA cm-2) than the visible light-triggered BiVO4RGO. This significant improvement of BiVO4 in generating a competent visible light response is of great importance in designing efficient visible light photoelectrochemical solar cells and photocatalysts. In a separate experiment of water splitting reaction, H2 and O2 evolution measurements were conducted in a two-electrode system using 0.1 M Na2SO4 electrolyte solution, and an external bias voltage of 0.8 V between Pt counter electrode and BiVO4 or BiVO4-RGO photoanode was applied. Photoexcited electrons flow through an external circuit to the Pt wire to reduce H2O to H2, whereas photogenerated holes (hþ) are consumed for water oxidation on the photoanode. A negligible amount of gas, beyond the limitation of quantification, was observed on pure BiVO4, whereas a steady evolution of H2 and O2 was quantified on BiVO4-RGO at the
basis of the identical peak positions and intensities for BiVO4 components in all samples, the introduction of GO to the synthesis and the successive photocatalytic reduction of GO do not alter the crystallinity and phases of BiVO4. This is a crucial factor when identifying the effect of RGO in its photoelectrochemical properties. Figure 2a shows the morphology of the synthesized BiVO4 and BiVO4-RGO composites under a field-emission SEM microscope. Pure BiVO4 synthesized using the solid-liquid phase reaction of bismuth nitrate and vanadium oxide under acidic conditions possesses large particle sizes ranging from 150 to 500 nm. The particles seize smooth surfaces and appear to aggregate. When GO was introduced to the system, interaction between its oxygen-functional groups, especially the carboxylic species, and the hydroxyl groups of oxide materials led to the dispersion and adhesion of BiVO4 particles on GO sheets.37 This intimate interaction enables the electron transfer from BiVO4 to GO during the photoexcitation process. Careful examination of the SEM image reveals the BiVO4 being comprehensively integrated within the matrix of RGO; that is, the particles are seen deposited on top of as well as underneath the slightly transparent RGO. (See Figure SI2 of the Supporting Information for pure RGO SEM image.) This nanostructure enables a multichannel environment to facilitate the efficient charge interaction within the composite. We obtained uniform thin films of BiVO4 and BiVO4-RGO by dropcasting 0.5 g/L of particles suspended in ethanol onto FTO transparent electrodes. Figure 2b shows photographs of the pure BiVO4 and BiVO4-RGO photoelectrodes. Using suspension with concentration higher than 0.5 g/L caused the formation of uneven films with significant surface roughness. These homogeneous thin film photoelectrodes are robust in both acidic and basic media, even under stirring and purging conditions. The microscopic feature of the dropcasted nanocomposite thin films (Figure SI3 of the Supporting Information) exhibited essentially identical morphological characteristics as its powdery dispersed counterpart: the RGO flakes are seen well-decorated with BiVO4 particles. Most importantly, surface contact between the RGO flake and FTO is established because this is crucial in shuttling photogenerated electrons from excited BiVO4 to the external circuit, which is further discussed below.
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Figure 4. Normalized plots of the photocurrent-time dependence for both BiVO4 and BiVO4-RGO electrodes. Inset represents a typical photocurrent transient response curve at a constant potential.
photoconversion efficiency indicates the improved charge collection efficiency in the presence of RGO. Inset in Figure 4 shows the typical photocurrent transient profile of the BiVO4 and BiVO4-RGO electrodes at a constant potential. Transient of photocurrent has been employed to study the charge recombination behavior of a semiconductor electrode,38 and it can be normalized by defining the parameter as ð1Þ D ¼ ðI ðtÞ - I ðstÞ Þ=ðI ðinÞ - I ðstÞ Þ
Figure 3. (a) Visible light voltage-photocurrent functions of BiVO4, BiVO4-RGO, and TiO2 (under UV irradiation). (b) IPCE and diffuse reflectance spectra of BiVO4 and BiVO4-RGO.
rate of 0.75 and 0.21 μmol h-1, respectively. The successful splitting of water using an external bias at 0.8 V in this system, lower than the theoretical value of 1.23 V, illustrates that the uphill reaction occurs through light energy conversion. These results are evidence of the constructive effect of the RGO in promoting electron shuttling and suppressing charge recombination to realize improved utilization of charge carriers for the overall water splitting reaction. The slight deviation from the stoichiometric ratio of hydrogen to oxygen generated is possibly due to the consumption of small amounts of hþ by the RGO. Further investigations in the stability of RGO in the presence of various holes scavengers are now in progress. The incident photon-to-current-conversion efficiency (IPCE) of BiVO4-RGO and their diffuse reflectance spectra is recorded in Figure 3b. Photocurrent action spectra were recorded at 0.75 V versus Ag/AgCl with a monochromatic excitation source. IPCE action spectra determine the amount of photogenerated electrons collected at the contact per photon irradiated on the photoelectrochemical cell surface and are calculated by normalizing the photocurrent values with incident light energy and intensity. Both samples demonstrate a photocurrent onset at 520 nm corresponding to the band gap of BiVO4. The onset measured in IPCE fits well with the diffuse reflectance spectra, indicating that the photocurrent generation occurs upon band gap photoexcitation of BiVO4. This would exclude the possibility of the excited RGO playing the role of the electron source in generating photocurrent. In the absence of RGO, a trivial IPCE of BiVO4 at 0.3% (at 400 nm) was observed, whereas the IPCE response shows a remarkable enhancement to 4.2% when RGO is incorporated. One order of magnitude enhancement in
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where I(t) is the photocurrent at a time t, I(in) is the initial photocurrent at t = 0, and I(st) is the steady-state photocurrent. Tafalla et al. defined the transient time constant, τ, as the time at which ln D = -1.39 Figure 4 shows the plots of ln D versus time for photocurrent transient responses of BiVO4 and BiVO4-RGO photoelectrodes biased at 0.75 V. For BiVO4, τ = 2.8 s, compared with BiVO4-RGO, τ=7.6 s, and it is a general observation that τ is larger in BiVO4-RGO in the entire range of applied bias, which reflects the slower recombination process in BiVO4RGO photoelectrode. The decay time (in the range of seconds) is comparable to that of the colloidal TiO2 electrodes reported by Hagfeldt et al.38 Although the recombination mechanisms cannot be concluded from these measurements, it elucidates that the significant enhancement in IPCE and water splitting reaction on BiVO4-RGO film are owing to its slower charge recombination rate. In general, photoactivity of BiVO4 is limited by its rapid charge recombination upon excitation, thus limiting the charge carriers' contribution to the overall photocurrent generation and photocatalytic reaction. Charge recombination is promoted by the existence of many grain boundaries among semiconductor particles as well as poor contact with the flat FTO surface due to its 3D shape, as indicated by the SEM images in Figure SI1a of the Supporting Information. Photogenerated electrons have to diffuse through countless recombination centers prior to reaching the FTO electrode, resulting in the loss of the vast majority of these charge carriers. By providing a low-resistant electrons pathway, the presence of RGO facilitates efficient electron transfer from excited BiVO4 particles to the FTO electrode,
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SUPPORTING INFORMATION AVAILABLE Experimental details, SEM images of pure RGO and photoelectrodes, relationship between photocurrent transient and recombination process, V 2p and Bi 4f XPS spectra, and Raman spectroscopy analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: r.amal@ unsw.edu.au.
Author Contributions: §
These authors contributed equally.
ACKNOWLEDGMENT We thank the Australian Research Council for its financial support. Y.H.N. thanks Prof. Prashant V. Kamat from the University of Notre Dame for assistance and advice in establishing the experimental procedures for BiVO4-graphene photocatalytic synthesis. Dr. Bill Gong from Solid State & Elemental Analysis Unit of UNSW is gratefully acknowledged for his help in XPS measurement and analysis.
Figure 5. Electron transport in a photoelectrochemical cell based on BiVO4 and reduced graphene oxide.
REFERENCES whereas the RGO itself does not directly participate in the charge generation. Combining the physical and electrochemical characterizations allows the mechanism of improved electrons transport in the BiVO4-RGO film to be postulated (Figure 5). The primary step in the process is the photoinduced charge separation within BiVO4 particles, followed by the transfer of electrons to RGO sheets and then to the collecting electrodes. It is reasonable to consider that the electron transfer from BiVO4 to RGO proceeds readily, given the flatband potential of -0.30 and ∼0 V versus NHE for BiVO4 and graphene at pH 7, respectively.16 Previous studies have also shown that RGO serves excellently as electron acceptor and mediator.40 In addition, RGO sheets also provide better media for the dispersion of BiVO4 particles and improve contact with flat FTO surface. As intimate surface contact being established between RGO and FTO during the film fabrication (Figure SI1b of the Supporting Information), electrons injected from BiVO4 are quickly transported to the collecting electrode through the presence of the large pool of delocalized electrons from its π-π graphitic carbon network. The results presented in this study highlight the constructive effect of incorporating BiVO4 with RGO in the photoelectrochemical water splitting reaction. We have demonstrated the feasibility of using BiVO4 visible light photocatalyst to reduce GO photocatalytically. Usefulness of RGO in promoting charge collection and charge transport by accepting photogenerated electrons from BiVO4 and shuttling them to the collecting electrode, yielding an enhancement in IPCE as great as one-order of magnitude, has been proven. The BiVO4-RGO system also generates higher photocurrent density in visible light than that generated by the TiO2 system in UV light. Moreover, significant H2 and O2 evolution was seen in the BiVO4-RGO photoelectrochemical water splitting cell, whereas negligible gas evolution was observed in the pure BiVO4 cell, showing the great advancement of the synthesized BiVO4-RGO composite for water splitting.
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