Highly Improved Quantum Efficiencies for Thin Film BiVO4

ARTICLE pubs.acs.org/JPCC

Highly Improved Quantum Efficiencies for Thin Film BiVO4 Photoanodes Yongqi Liang,† Toshiki Tsubota,‡ Lennard P. A. Mooij,† and Roel van de Krol*,† †

Materials for Energy Conversion and Storage (MECS), Department of Chemical Engineering, Delft University of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands ‡ Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Fukuoka, Japan ABSTRACT: BiVO4 has received much recent interest as a promising photocatalyst for oxygen evolution from water, but little is known about the factors that limit its performance as a photoanode. In this article, we report on highly efficient and reproducible BiVO4 photoanodes prepared by a new spray pyrolysis recipe. For undoped films deposited on a transparent conducting substrate (F-doped SnO2, FTO), electron transport and charge collection at the back-contact were found to limit the photoresponse. Electron transport could be greatly enhanced by donor-doping with 1% W, while the charge collection problem has been solved by introducing a thin (∼10 nm) interfacial layer of SnO2 in between the FTO and the BiVO4. This layer presumably acts as a hole mirror that prevents recombination via FTO-related defect states at the FTO/BiVO4 interface. By addressing these two issues, the external quantum efficiency (IPCE) of spray-deposited BiVO4 films was improved by a factor of ∼7, leading to an unprecedented IPCE of 46% (at 450 nm) at 1.63 VRHE for 1% W-doped BiVO4 films deposited on FTO/SnO2.

’ INTRODUCTION Ever since the introduction of BiVO4 (bandgap ∼2.4 eV) as a photocatalyst for O2 evolution,1 various structures of BiVO4 have been prepared to improve its visible light response for solar water splitting. Powders made up from nanoparticles and thin films are the two most common forms adopted for the photocatalytic reaction. While research on powders offers the advantage of easy screening of the photocatalyst (BiVO4 with a quantum yield of 9% at 450 nm has been reported),2 the unavoidable mixing of O2 and H2 in a single volume is an important practical disadvantage. Apart from the obvious safety concerns, one also has to pay a significant energy penalty for separating these gases. This can be avoided by using thin films on conductive substrates as photoelectrodes. In such a system, oxygen and hydrogen are produced at different electrodes, and separation is trivial. Moreover, the compatibility with external bias or a tandem cell configuration allows a large degree of control over the charge transport processes.3 Until now, BiVO4 films prepared by drop casting,4 sol
gel processing,5 and polymer-assisted deposition6 typically show very low quantum efficiencies (500 nm) with many voids between the grains. The bottom layer has a significantly smaller grain size (450 nm is due to the weak light absorption at the band edge that is common for semiconductors with an indirect bandgap. The drop of the IPCE below 360 nm is consistent, although somewhat more pronounced, with the decrease in absorption observed in Figure 3. The much stronger decrease in IPCE for back-side illuminated BiVO4 films at wavelengths below 360 nm, shown in Figure 4b, is due to the absorption by the FTO glass substrate. In order to design an effective strategy for improving the photoresponse, elucidation of the rate-limiting step is of great importance for photoelectrodes. Since electron
hole pairs in BiVO4 are mostly generated within the penetration length of the light (∼100 nm), the electrons need to travel across the entire thickness of the film (∼200 nm) to the BiVO4/FTO interface for collection when illumination is from the front. In contrast, the holes need to travel across the entire thickness to the BiVO4/ H2O interface for collection when illumination is from the back. This is illustrated in Figure 4c,d. As shown in Figure 4b, the IPCE for back-side illumination is larger than that for front-side illumination. Since all other parameters are the same, this indicates that electron transport is slower than hole transport in these BiVO4 films. This is consistent with the picture that emerges when comparing the electronic structure and the crystal structure of the material. Electronic structure calculations show that the conduction band of BiVO4 consists mainly of V-3d orbitals.15,16 However, the crystal structure reveals that the VO4 tetrahedra are not interconnected with each other, i.e., they do not share corners or edges.16 This means the (nearly) free electrons in BiVO4 have to hop between VO4 tetrahedra, which explains the poor electron transport properties. At first sight, the layered structure of smaller and larger particles in the film (Figure 1) could also provide a reasonable

explanation for the larger back-illumination photocurrents. The smaller particles have a larger specific surface area, which is expected to improve the water oxidation kinetics. However, at the low light intensities used in Figure 4b (∼1 μW/cm2), slow water oxidation kinetics are very unlikely to limit the overall performance of the photoanode. We verified this by control experiments using a different spray nozzle, which yielded films with a homogeneous particle size (i.e., no layered structure). Back-side illumination of these films again gave higher photocurrents than that of front-side illumination, with a ratio similar to that observed in Figure 4b. This confirms that the difference in front- and back-side illumination photocurrents is indeed due to poor electron transport properties and not to the morphology of the film. We now turn our attention to the collection of the photogenerated electrons that arrive at the FTO/BiVO4 interface. To investigate whether this step limits the overall performance, the band alignment between BiVO4 and FTO was modified by inserting an undoped SnO2 layer. Figure 5a shows the change of the IPCE spectrum when a SnO2 interfacial layer (∼10 nm thick) is present in between FTO and BiVO4. While the shape of the IPCE spectrum remains unchanged, the IPCE at 450 nm increases from 7% to 26% after the SnO2 interfacial layer is introduced. Because the SnO2 layer has no obvious influence on the structure of the BiVO4 films (XRD and Raman spectra do not show any changes), the efficiency increase is attributed to passivation of the interface between FTO and BiVO4 by the SnO2 layer. To explain this, we first discuss the band alignment of the unmodified FTO/BiVO4 interface. From the work of Long et al. a work function of ∼4.7 eV can be derived for BiVO4,4 while values of 4.4
4.9 eV have been reported for FTO.17 This implies that BiVO4 forms an ohmic or perhaps a mild Schottky barrier contact with FTO, as indicated in Figure 5c. As SnO2 is expected to have a somewhat larger work function than FTO due to its lower carrier concentration, the contact to BiVO4 will become less ohmic in nature when the SnO2 layer is inserted. This would result in a decrease in the IPCE, which is in contrast with the increase observed here. 17596

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The Journal of Physical Chemistry C

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Figure 5. (a) Comparison of the IPCE spectra of BiVO4 on FTO with and without an ∼10 nm SnO2 interfacial layer. (b) Comparison of the IPCE spectra of undoped BiVO4 and 1% W doped BiVO4 on FTO, both with the SnO2 interfacial layer. The IPCE spectra are measured under 1.63 VRHE bias and back-side illumination. The schematic diagrams c
e illustrate the recombination at the defect state present at the FTO/BiVO4 interface and the hole mirror effect of the SnO2 layer, respectively.

Instead, we believe that the observed efficiency improvement is related to the presence of a defect state acting as a recombination center at the FTO surface. This is explained by the diagrams shown in Figure 5c
e. In the absence of an SnO2 interfacial layer (Figure 5c), the photogenerated electrons will be trapped at this defect state (DS*), and a negative charge accumulates at the FTO/BiVO4 interface. Additional electrons will be repelled by this negative charge and remain in the bulk. This quickly reduces the band bending in BiVO4, which allows the photogenerated holes to reach the FTO/BiVO4 interface and recombine with the electrons trapped in the defect state (Figure 5d). The defect state then basically acts as a channel for Shockley
Read
Hall recombination at the interface. The insertion of the SnO2 interfacial layer prevents the photogenerated holes from reaching the defect state; as indicated in Figure 5e, the holes are reflected back into the BiVO4 when reaching the SnO2/BiVO4 interface. This results in less recombination and a higher collection efficiency. We emphasize that this so-called hole mirror effect by itself, i.e., without the presence of a recombination center at the interface, cannot explain the efficiency increase, as has been suggested before.8,18 This is because in the absence of the electron trap DS*, the holes would only be able to reach the FTO/BiVO4 interface, and experience the benefit of the hole mirror, at the high illumination intensities normally required for flattening the bands. We already observe the beneficial effect of the SnO2 hole mirror at light intensities of a few μW/cm2. This is far below the intensity normally required to flatten the bands in the absence of electron trapping/accumulation at the interface. Interestingly, the beneficial effect of a SnO2 interfacial layer has also been observed for R-Fe2O3 (hematite) photoanodes in combination with FTO19,20 but not for hematite in combination

with other substrates such as n-type Si.21 This suggests that these interfacial recombination centers are a particular property of the FTO substrate. While the chemical nature of this defect is not clear, an associated defect consisting of an in-plane surface oxygen vacancy adjacent to a bridging 4-fold coordinated Sn2+ species is a likely candidate, as this is known to introduce deep states in the bandgap of SnO2.22,23 As mentioned above, electron transport is found to be slower than hole transport in BiVO4. To improve the electron transport, the concentration of the electrons inside BiVO4 can be tuned with n-type dopants. Normally, mobile electrons are introduced by the presence of oxygen vacancies (VO3 3 ) in BiVO4 according to eq 1. BiVO4 1 OXO sf V O3 3 þ O2 ðgÞ þ 2e0 2

ð1Þ

Since the VO3 3 concentration cannot be easily controlled during spray pyrolysis, we have added W to improve the electron transport in BiVO4. As illustrated in the electronic compensation reaction below, tungsten can increase the free electron concentration by acting as a donor-type dopant: 2BiVO4

2WO3 þ Bi2 O3 sf 2BiXBi þ 2W V3 þ 8OXO þ

1 O2 ðgÞ þ 2e0 2

ð2Þ Here, we also added a small excess of Bi2O3 in order to keep the ratio of A- and B-type cations the same, thereby avoiding the energetically unfavorable formation of highly charged Bi vacancies.13 The IPCE spectrum of 1.0% W-doped BiVO4, together with the IPCE spectrum of an undoped BiVO4 photoelectrode, is shown in Figure 5b. A systematic increase of the IPCE is 17597

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The Journal of Physical Chemistry C observed along the entire absorption spectrum of W-doped BiVO4, and the IPCE reaches 46% at 450 nm when biased at 1.63 VRHE, with a maximum of 52% at 370 nm. The relatively high potential of 1.63 VRHE was chosen to allow direct comparison with the results of Sayama et al., who first reported on the photoelectrochemical performance of thin film BiVO4 photoanodes, and to the best of our knowledge are still the only ones to show IPCE values larger than 20% in the absence of sacrificial agents.7 It should be emphasized that all metal oxide photoanodes require a bias voltage, for which usually the (somewhat arbitrary) value of 1.23 VRHE is taken. At this potential, the IPCE of our BiVO4 is ∼36% at 450 nm (based on the data in Figure 4a), which is still more than 1.5 times higher than any previously reported values under the same conditions. Further improvements may be possible by optimizing the W concentration using, e.g., combinatorial methods, as demonstrated in a recent study on W- and Mo-doped BiVO4.24 Care should be taken to avoid concentrations that are too high, as this might lead to enhanced recombination25 or even phase segregation.

’ CONCLUSIONS The spray deposition process shows excellent reproducibility, with IPCE values consistently above 40% between 350 and 450 nm at a bias of 1.63 VRHE and under back-side illumination. Up to now, the highest efficiency that we have achieved under these conditions is 46% for 1% W-doped BiVO4 films on FTO coated with a thin SnO2 interfacial layer and without any sacrificial agents present in the electrolyte. These high efficiencies have been obtained by addressing two key issues: (i) improving the inherently slow electron transport in BiVO4 by donor-doping with tungsten and (ii) incorporating a hole mirror in the form of a very thin layer of SnO2 underneath the BiVO4 to avoid recombination at defect states at the FTO/BiVO4 interface. Since this recombination appears to be a particular feature of the FTO layer, we believe that the strategy of using a thin SnO2 layer as a hole mirror can also benefit other photoanode materials, as was already demonstrated for R-Fe2O3.19 The integration of the IPCE of our best BiVO4 sample over the standard AM1.5 solar spectrum26 gives a photocurrent density of 1.4 mA/cm2 when biased at 1.23 VRHE. This would correspond to a Solar-To-Hydrogen (STH) power conversion efficiency of 1.7%, provided that the bias voltage is generated by a photovoltaic cell that is positioned behind the BiVO4 photoanode in a tandem configuration. Although there is still a long way to go in order to achieve the theoretical maximum value of 7.6 mA/cm2 (assuming all incident photons with an energy exceeding the bandgap contribute to the photocurrent, resulting in a STH efficiency of 9.3%), these results clearly show that monoclinic scheelite BiVO4 is a promising photoanode material for photoelectrochemical water splitting.

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’ AUTHOR INFORMATION Corresponding Author

*Fax: +31-(0)15-2787421. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is financially supported under the Sustainable Hydrogen program (project nr. 053.61.009) by NWO-ACTS. We thank Steven Kleijn for helpful discussions. T.T. thanks the Kyushu Institute of Technology for supporting his visit to TUDelft. 17598

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