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

Abstract

fuel can be used on-site and on-demand but can also be stored and transported for off-site use. [5] However, practical PEC applications put stringent demands on photoabsorber materials in terms of efficiency, cost, and stability. Significant trade-offs have to be made, and this has thus far impeded the commercialization of PEC technology. [3,4] High solar to hydrogen conversion efficiencies approaching 20% have been achieved with photoelectrodes based on high-quality III-V semiconductors, such as GaInP 2 and GaAs. [6] However, their cost is likely to be prohibitive, and many of them suffer from instability under PEC operating conditions. The primary materials criteria are suitable bandgap energy to absorb a large fraction of solar photons with sufficient energies to enable water splitting, good electrical conductivity to enable photogenerated charge carrier extraction, favorable energy-band positions to enable carrier injection, and long-term stability in an aqueous environment. [4] Additionally, the material should be abundant and inexpensive in order to make PEC technology competitive with the chemical production of hydrogen from coal or natural gas. Almost all possible elemental and binary semiconductors have been investigated as photoelectrodes for water splitting, but none fulfill all the requirements. Therefore, the search will have to be expanded to ternary or even more complex materials. Metal-oxides offer many unique advantages as photoabsorber materials for PEC water splitting. [7-9] They have a variety of The widespread application of solar-water-splitting for energy conversion depends on the progress of photoelectrodes that uphold stringent criteria from photoabsorber materials. After investigating almost all possible elemental and binary semiconductors, the search must be expanded to complex materials. Yet, high structural control of these materials will become more challenging with an increasing number of elements. Complex metal-oxides offer unique advantages as photoabsorbers. However, practical fabrication conditions when using glass-based transparent conductive-substrates with low thermal-stability impedes the use of common synthesis routes of high-quality metal-oxide thin-film photoelectrodes. Nevertheless, rapid thermal processing (RTP) enables heating at higher temperatures than the thermal stabilities of the substrates, circumventing this bottleneck. Reported here is an approach to overcome phasepurity challenges in complex metal-oxides, showing the importance of attaining a single-phase multinary compound by exploring large growth parameter spaces, achieved by employing a combinatorial approach to study CuBi 2 O 4 , a prime candidate photoabsorber. Pure CuBi 2 O 4 photoelectrodes are synthesized after studying the relationship between the crystal-structures, synthesis conditions, RTP, and properties over a range of thicknesses. Single-phase photoelectrodes exhibit higher fill-factors, photoconversion efficiencies, longer carrier lifetimes, and increased stability than nonpure photoelectrodes...

Enhancing the Photon Absorption and Charge Carrier Dynamics of BaSnO3 Photoanodes via Intrinsic and Extrinsic Defects

“…Interestingly, after H 2 S annealing, the mobility increases by ∼25 times, with a maximum of 0.025 cm 2 V –1 s –1 . We briefly note that the mobility values obtained for the PLD BaSnO 3 and H 2 S–BaSnO 3 samples are comparable to the TRMC-determined mobilities of other metal oxide photo-electrode materials, such as α-SnWO 4 , BiVO 4 , CuBi 2 O 4 , and TiO 2 . ,− …”

Understanding the Role of Oxygen and Hydrogen Defects in Modulating the Optoelectronic Properties of P-Type Metal Oxide Semiconductors

“…Our results are in agreement with those of Gottesman et al who observed that decreasing the thermal process duration of CBO resulted in improved performance. 46 In summary, we found that oxygen vacancies and hydrogen impurities are natural defects in CBO films, and their prevalence can be controlled by thermal treatments in a controlled atmosphere. These defects have significant deleterious impacts on the PEC performance.…”

“…Therefore, the ideal process conditions for the as-grown films evaluated within this investigation should involve asingle annealing in O 2 . Our results are in agreement with those of Gottesman et al who observed that decreasing the thermal process duration of CBO resulted in improved performance …”

Progress in Ternary Metal Oxides as Photocathodes for Water Splitting Cells: Optimization Strategies

“…Some AB 2 O 4 compounds not containing iron have also attracted considerable interest. For instance, CuBi 2 O 4 has also been widely investigated, especially in the last years, [90][91][92][93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108] since Arai et al discovered its potential use as a new visible-lightresponsive p-type semiconductor in 2007. [90] It crystallizes in a kusachiite-type structure consisting of CuO 4 units staggered along the c-axis and separated by Bi 3þ ions, each of which is linked to six O atoms (Figure 5a).…”