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

Abstract

A new benign aqueous route toward bismuth‐containing photoelectrodes is proposed to eliminate the need for harmful organic solvents and/or acids. A CuBi2O4 photocathode is prepared by stabilizing the metal ions through complexation in pH neutral aqueous solutions. Merits of the proposed approach are elemental homogeneity (with unique doping possibilities) and ease of fabrication (e.g., high scalability). The prepared aqueous CuBi2O4 precursor forms a nearly phase‐pure kusachiite crystalline phase free of organics residuals and capable of water reduction due to its sufficiently negatively positioned conduction band at −0.4 V versus RHE. Deposition on fluorine doped tin oxide coated glass (FTO/glass) substrates and thermal treatment leads to uniform but granular films of CuBi2O4 with excellent control over stoichiometry and thickness, owing to the facile and non‐destructive synthesis conditions. Ultimately, the optimized CuBi2O4 photocathodes produce AM1.5G photocurrent densities of up to −1.02 mA cm−2 at 0.4 V versus RHE with H2O2 as an electron scavenger, competing with bare CuBi2O4 prepared through less benign non‐aqueous organic synthesis routes.

Photoelectrochemical properties of p-type CuBi2O4 prepared by spray pyrolysis of carbon-free precursor aqueous solution combined with post-annealing treatment

“…34,35 Among them, the CuBi 2 O 4 (CBO) material can be recognized as one of the promising candidates for the photoelectrode material for the PEC HER involving CO 2 reduction. 36–48 CBO has a tetragonal crystal structure and consists of stacks of square planar Cu( ii )O 4 groups linked to distorted trigonal Bi( iii )O 6 polyhedral groups. 34 The bandgap ( E g ) energy of the CBO material has been reported to be 1.5–1.8 eV (adsorption edge wavelength up to 830 nm), 37,38,42,44 which leads to a theoretical STH energy conversion efficiency limit of ∼24% at an external quantum efficiency (EQE) of 100%.…”

“…, F-doped tin oxide coated glass (FTO) and Au) in air. 35–39 CBO precursor compounds have been prepared on the substrates by deposition of the precursor solution using wet processes such as electrodeposition, 36–39,49,52 drop-casting methods, 36,44,45,48,50 spin-coating methods, 46,47 and spray pyrolysis methods. 40–43,51,53 Concerning the wet process to prepare CBO-based photocathodes, most of the precursor solution is based on non-aqueous organic solvents ( e.g.…”

“…, polyethylene glycol (PEG) and triethyl orthoformate (TEOF)) to dissolve and stabilize the Cu and Bi complex in the solvent. 38,41,46,47 The role of the organic additives in the precursor solution is to mitigate the hydrolyzed precipitation of Bi subnitrate species as time advances in the precursor solution. 38,41,46 Very recently, Joos et al have established an impressive synthesis route using citrate-ligand-based aqueous-sol–gel methods to fabricate the CBO photoelectrode prepared by a combination of spin-coating and thermal annealing in air.…”

“…38,41,46,47 The role of the organic additives in the precursor solution is to mitigate the hydrolyzed precipitation of Bi subnitrate species as time advances in the precursor solution. 38,41,46 Very recently, Joos et al have established an impressive synthesis route using citrate-ligand-based aqueous-sol–gel methods to fabricate the CBO photoelectrode prepared by a combination of spin-coating and thermal annealing in air. 46 However, CO 2 emissions associated with the combustion and decomposition of carbon species present in the precursor compounds (ligands of the metal complex) and the solvents in which they are dissolved are considered issues.…”

“…38,41,46 Very recently, Joos et al have established an impressive synthesis route using citrate-ligand-based aqueous-sol–gel methods to fabricate the CBO photoelectrode prepared by a combination of spin-coating and thermal annealing in air. 46 However, CO 2 emissions associated with the combustion and decomposition of carbon species present in the precursor compounds (ligands of the metal complex) and the solvents in which they are dissolved are considered issues. To overcome this, it is preferable to establish a method for the preparation of precursor compounds via a carbon-free wet process, leading to a large-scale process with a low environmental impact.…”