Template Engineering of CuBi₂O₄ Single‐Crystal Thin Film Photocathodes

Abstract

photoconversion of semiconducting photoelectrodes in photo-electrochemical (PEC) water-splitting, it is necessary to synthesize high-quality single-crystal thin films. [1-7] High-crystalline oxide thin films provide an ideal platform to enhance photoconversion efficiency using various strategies such as ferroelectric oxide domain engineering and interfacial engineering. [3-7] For example, the interlayer insertion of atomically sharp polar LaAlO 3 lowered the charge transfer resistance between the WO 3 photoanode and the Nb:SrTiO 3 substrate. [6] Furthermore, crystal facet engineering, which controls the preferred crystallographic orientation, has become an emerging approach to enhance the photocurrent or photostability in oxidebased photoelectrodes such as TiO 2 , BiVO 4 , and Cu 2 O thin films. [8-12] Therefore, the synthesis of single-crystal oxide thin film photoelectrodes is significantly important for the development of photoelectrodes toward efficient PEC water-splitting. While the investigation of single-crystal thin film photoanodes such as TiO 2 and BiVO 4 are relatively focused, [13-15] there are few studies for single-crystal thin film photocathodes such as copper-based oxides. One of the major challenges facing To develop strategies for efficient photo-electrochemical water-splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single-crystal thin films. However, it is challenging to synthesize high-quality single-crystal thin films from copperbased oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi 2 O 4 (CBO) single-crystal thin film photocathode is achieved using a NiO template layer grown on singlecrystal SrTiO 3 (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain-matching epitaxy, and forms a type-II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single-crystal thin film photocathode demonstrate −0.4 and −0.7 mA cm −2 at even 0 V RHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H 2 O 2 as an electron scavenger, respectively. The successful synthesis of high-quality CBO single-crystal thin film would be a cornerstone for the in-depth understanding of the fundamental properties of CBO toward efficient photo-electrochemical water-splitting.

Overcoming Phase‐Purity Challenges in Complex Metal Oxide Photoelectrodes: A Case Study of CuBi2O4

“…We find that the bandgaps of all films are similar to that of the single crystal, 1.90 ± 0.05 eV, consistent with previous reports. [32,81] The extracted bandgap of the α/β-Bi 2 O 3 film is 2.85 ± 0.05 eV, which is in agreement with reported values of both αand α/β films. [69,82] In the FH-treated film, an additional peak is seen at higher photon energies of ≈2.75-3.0 eV.…”

“…However, they agree with recently reported photocurrent densities of CuBi 2 O 4 with similar thicknesses deposited by PLD. [81] We attribute the modest photocurrent densities mainly to their low specific surface area and absence of strategies to improve the photocurrent, such as doping (i.e., with Ag), [33] using selective contacts, and creating an internal gradient of copper vacancies, that were reported for these films. [34,51,54] The photocurrent densities of both photoelectrodes were similar, −0.38 mA cm −2 at 0.6 V versus RHE (back illumination), with onset potentials at ≈+1.2 V versus RHE.…”

Facile Aqueous Solution‐Gel Route toward Thin Film CuBi2O4 Photocathodes for Solar Hydrogen Production

“…The p‐type semiconducting CuBi 2 O 4 has been identified as a promising metal oxide photocathode with a suitable band gap of 1.5–1.9 eV and an aptly aligned conduction band edge for the hydrogen evolution reaction (HER). [ 20–28 ] An internal photovoltage of ≈1 V is plausible from CuBi 2 O 4 photocathodes. A PEC stack is envisaged by coupling CuBi 2 O 4 with suitable photoanode or providing external bias of ≈0.2–0.3 V. [ 29,30 ] The preparation of p‐type CuBi 2 O 4 has been well‐reported in literature through various methods such as electrodeposition, pulsed laser deposition (PLD), drop‐casting, RF‐magnetic sputtering, spin‐coating, hydrothermal growth, etc.…”

“…A PEC stack is envisaged by coupling CuBi 2 O 4 with suitable photoanode or providing external bias of ≈0.2-0.3 V. [29,30] The preparation of p-type CuBi 2 O 4 has been well-reported in literature through various methods such as electrodeposition, pulsed laser deposition (PLD), drop-casting, RF-magnetic sputtering, spin-coating, hydrothermal growth, etc. [20][21][22][23][24][25][26][27][31][32][33] Solution-based approaches are ideal for the scaleup of deposition processes. However, the facile, scalable, and green aqueous deposition of thin-film CuBi 2 O 4 is hampered by the solubility of bismuth in water.…”

Enhanced charge collection and surface activity of a CuBi2O4 photocathode via crystal facet engineering

“…[17][18][19] Analogous to other transition metal oxide semiconductors, CBO exhibits polaron transport, which can signicantly reduce charge mobility and thus limit the diffusion length of photocarriers. 20,21 As a result, the photocarriers only present a diffusion path of 45 nm, and a very low percentage of charge carriers can participate in the surface reaction. 10,22 On the other hand, the sluggish hydrogen evolution reaction (HER) kinetics also cause charge accumulation at the surface/subsurface region, further inducing charge recombination.…”