Research Article Cite This: ACS Catal. 2018, 8, 5952−5962
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Tailoring Crystallographic Orientations to Substantially Enhance Charge Separation Efficiency in Anisotropic BiVO4 Photoanodes Jaesun Song,† Min Ji Seo,† Tae Hyung Lee,‡ Yong-Ryun Jo,† Jongmin Lee,† Taemin Ludvic Kim,‡ So-Young Kim,† Seung-Mo Kim,† Sang Yun Jeong,† Hyunji An,† Seungkyu Kim,† Byoung Hun Lee,† Donghwa Lee,§ Ho Won Jang,‡ Bong-Joong Kim,† and Sanghan Lee*,†
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†
School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea ‡ Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea § Department of Materials Science and Engineering, and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea S Supporting Information *
ABSTRACT: In photoelectrochemical (PEC) water splitting, BiVO4 is considered the most promising photoanode material among metal oxide semiconductors because of its relatively narrow optical bandgap and suitable band structure for water oxidation. Nevertheless, until now, the solar-to-hydrogen conversion efficiency of BiVO4 has shown significant limitations for commercialization because of its poor charge transport. Various strategies, including the formation of a heterojunction and doping of electron donors, have been implemented to enhance the charge transport efficiency; however, fundamental approaches are required for further enhancement. In this regard, we report the fundamental approach for BiVO4 thin film photoanodes by fabricating epitaxial oxide thin films with different crystallographic orientations for PEC water splitting. The crystalline anisotropy generally reveals distinct physical phenomena along different crystallographic orientations. In the same vein, in terms of the anisotropic properties of BiVO4, the electrical conductivity of BiVO4 is greater along the ab-plane than along the c-axis. Consequently, as the crystallographic orientation of the BiVO4 thin film changes from (001) to (010), the charge transport properties in the epitaxial BiVO4 thin film are significantly enhanced. Thus, at 1.23 VRHE, the photocurrent density of the epitaxial BiVO4 (010) thin film (2.29 mA cm−2) is much higher than that of the epitaxial BiVO4 (001) thin film (0.74 mA cm−2) because of significant enhancement in charge transport properties even for undoped BiVO4. These results strongly suggest that the growth of epitaxial BiVO4 thin films with specific crystallographic orientations has great potential to considerably improve the charge transport efficiency of photoanodes for solar water splitting. KEYWORDS: BiVO4, photoelectrochemical, orientation, epitaxial, hole diffusion length
1. INTRODUCTION Photoelectrochemical (PEC) water splitting has attracted a lot of interest as an ideal method for environmentally friendly hydrogen production because of its potential to harvest sustainable and clean energy by forming hydrogen and oxygen using solar energy.1−4 Metal oxide semiconductors, such as TiO2,5 α-Fe2O3,6 and WO3,7 have been examined as promising photoanode materials (which generate the oxygen evolution reaction (OER) in aqueous solutions by PEC reaction) because of their natural stability under highly oxidative conditions. Monoclinic BiVO4 is the most promising photoanode material for water oxidation because it has a relatively narrow optical band gap of ∼2.4 eV, which can effectively absorb a wide range of the visible light spectrum. Furthermore, it has a © 2018 American Chemical Society
suitable band structure for water oxidation with a relatively negative conduction band edge position (∼0 V vs the reversible hydrogen electrode (RHE, VRHE)).8−10 Therefore, the theoretical maximum photocurrent (Jmax) for BiVO4 is high (7.5 mA cm−2) under AM 1.5G illumination.9 Nevertheless, until now, the solar-to-hydrogen conversion efficiency of BiVO4 significantly limits its commercialization because of its especially poor charge transport properties. Therefore, various efforts have been made to enhance the charge transport properties based on eq 111−14 Received: March 5, 2018 Revised: May 15, 2018 Published: May 22, 2018 5952
DOI: 10.1021/acscatal.8b00877 ACS Catal. 2018, 8, 5952−5962
Research Article
ACS Catalysis JPEC = Jabs × ηsep × ηtran
the b- and c-axes is observed for monoclinic BiVO4 by using density functional theory (DFT) calculations (Figure S1). In terms of the crystalline anisotropy properties of BiVO4 in particular, it should be especially noted that the electrical conductivity of BiVO4 is greater along the ab-plane than along the c-axis.45,46 Herein, considering the hopping transport mechanism of BiVO4,47 the electrical conductivity(σ) can be described with eq 2
(1)
where JPEC is the practical PEC water oxidation photocurrent density of the photoanode material, Jabs is the photocurrent density (assuming an absorbed photon conversion efficiency of 100% and calculated as Jmax × light harvesting efficiency (LHE)), ηsep is the charge separation efficiency, and ηtran is the surface charge transfer efficiency. Among these factors, ηtran has been greatly improved by coating the BiVO4 surface with an OER catalyst.12,15 For example, when an OER catalyst, such as cobalt phosphate, is coated on the surface of BiVO4, ηtran (1.23 VRHE) approaches 100%. Therefore, this study focuses on increasing the value of ηsep to achieve high-performance BiVO4 photoanodes. However, it is difficult to increase the value of ηsep; many charge carriers recombine because the hole diffusion length is shorter than the light absorption length.12,16−18 Although various strategies, such as forming heterojunctions,15,17−23 doping electron donors,12,16,24−28 and introducing ferroelectric materials,29,30 have been widely implemented to enhance the value of ηsep and overcome the challenging limitations, new approaches are required for further enhancement. In this trend, recent studies on the growth of epitaxial BiVO4 thin film photoanodes were performed to understand its fundamental properties for PEC water splitting.31−35 The establishment of epitaxial metal oxide thin film photoanodes could maximize the potential of the oxide thin film photoelectrodes and seek further breakthrough development by exploring its fundamental properties as the cornerstone. For example, epitaxial BiVO4 thin film photoanodes enable an indepth understanding of the fundamental PEC properties of the photoanodes themselves by minimizing any other factors, such as impurities, structural defects, and grain boundaries, in the thin film photoanodes. On this basis, the new development and tailoring of functionalities in complex oxide thin films, such as the effects of a doping profile or biaxial strain and the manipulation of its band structure, could provide new routes for significantly improving its photoefficiency.35 For that reason, several studies have achieved simultaneous growth of an epitaxial BiVO4 thin film and identified its photocatalytic performance. Van et al. synthesized epitaxial monoclinic BiVO4 (a = 5.1956 Å, b = 5.0935 Å, c = 11.6972 Å, β = 90.387°) thin films on a cubic yttria-stabilized zirconia (YSZ, a = 5.145 Å) substrate by pulsed laser deposition (PLD) and identified its photocathodic properties.33 Subsequently, self-assembled BiVO4−WO3 heterostructured thin films were grown epitaxially on a YSZ (001) substrate to explore the charge interaction of oxide heterostructures.31 The growth of epitaxial BiVO4 thin films on a SrTiO3 (STO, a = 3.905 Å) substrate was also achieved through the effect of the inserted WO3 template layer, which could reduce the lattice mismatch between BiVO4 and STO from an average of ∼8 to ∼3%.32 However, all previous studies related to the growth of epitaxial BiVO4 thin films have focused on those grown along the c-axis.31−37 In other words, although many studies have been conducted on the growth of epitaxial BiVO4 thin films, the growth of epitaxial BiVO4 thin films with different crystallographic orientations has not been examined. This crystalline anisotropy can generally reveal distinct physical phenomena along different crystallographic orientations, such as crystal facet engineering,38−40 anisotropic electronic structure,41,42 different ferroelectric switching,43,44 and so forth. As an example, highly anisotropic electronic band structure along
⎛ −E ⎞ σ(T) = σ0(T )exp⎜ σ ⎟ ⎝ kBT ⎠
(2)
where Eσ is the conductivity activation energy, and the conductivity activation energy is significantly related to the generation of carriers. In other words, the enhanced electrical conductivity of BiVO4 is directly related to the increase in the hopping sites by lowering the conductivity activation energy. These anisotropy properties of BiVO4 can be attributed to the differences in the crystalline structure of BiVO4, including the arrangement of V ions.46 Considering the crystalline structural origin of the anisotropic transport in BiVO4, the charge carrier transport along the c-axis is only generated by nearest-neighbor hopping, whereas the charge carrier transport along the abplane is generated by nearest-neighbor hopping including nextnearest-neighbor hopping, which causes enhanced hopping in the ab-plane. In this regard, we propose a new approach to improve the charge transport efficiency of BiVO4 by investigating its fundamental properties by fabricating epitaxial oxide thin films with different crystallographic orientations for PEC water splitting. The c-axis-oriented epitaxial BiVO4 (001) thin films were grown on YSZ (001) substrates, whereas the b-axisoriented epitaxial BiVO4 (010) thin films were grown on STO (001) substrates, as shown in Figure 1. Both b- and c-axis-
Figure 1. Schematic of epitaxial BiVO4 thin films grown (a) in the caxis orientation on a YSZ (001) substrate and (b) in the b-axis orientation on an STO (001) substrate.
oriented BiVO4 thin films exhibit good lattice matching (lattice mismatch
