Tantalum Pentoxide: From Crystal Structures to Applications in Water Splitting

Abstract

The environment and energy crisis due to carbon dioxide emissions from burning fossil fuels have increased the urgency to explore sustainable energy, while hydrogen with high energy density and environmental friendliness represents a promising solution. Currently, hydrogen production from renewable-energy-driven water splitting via electrocatalytic, photocatalytic, and photoelectrochemical processes are actively investigated, where the design of stable and active earth-abundant catalysts is one of the most important scientific questions. Among the numerous metal oxide catalysts, tantalum pentoxide (Ta 2 O 5 ) has recently received great attention due to its suitable band gap and high stability as an anode for water splitting. In this Review, we start with a summary of diverse Ta 2 O 5 phases with different crystalline structures. We also summarize the experimental methods for the synthesis of different Ta 2 O 5 phases, followed by discussions of the electronic structures of each phase from theoretical calculations. Then, we review the recent applications of Ta 2 O 5 as the anode catalyst for the oxygen evolution reaction (OER) and the cathode catalyst for hydrogen evolution reaction (HER). Finally, we discuss the challenges and opportunities of using Ta 2 O 5 -based materials as viable catalysts for water splitting, with an emphasis on the crucial role of the synergistic effort from both theory and experiment.

Phase comparison and equation of state for Ta2O5

“…Our calculated E-V curves (figure 6) agree with previous studies [10,19,31] in finding that λ is the most energetically favorable of the plausible phases, followed by L SR . Phase B and several variations of γ have been calculated with even lower E [19,22], as well as good matches to the experimentally observed band structure [23,38].…”

“…The other plausible phases (whether or not β R is included) have a combined parameter range of K 0 = 138.03 ± 3.68 GPa and K ′ 0 = 1.82 ± 0.45. These can be viewed as the most likely room-temperature EoS parameters for Ta 2 O 5 in the absence of definitive evidence to support one structure over the others, even if the analyzed material is a mix of phases, as may be the case for certain synthesis methods [10,59]. Assuming the Ta 2 O 5 phase is L SR , we find it to be more compressible than determined by Stavrou et al (K 0 = 199 GPa, K ′ 0 = 0.1) [18].…”

“…Table 1 lists the names, space groups, and published ambient unit cell volumes (V 0 ) of the phases considered in this study. This list is based on similar compilations in the literature [10,19,21,39]. The supplementary material provides a list of other proposed phases.…”

“…The literature on Ta 2 O 5 crystallography dates back 70 years and includes more than a dozen crystalline phases, as well as many studies that use the same name for different structures. Several previous publications provide overviews and comparisons of Ta 2 O 5 structure proposals [10,12,19,25], which we briefly summarize here. Early studies on Ta 2 O 5 recognized two forms that crystallized at varying T [26][27][28].…”

“…Unusually for such a widely used material, the crystal structure of Ta 2 O 5 at ambient conditions remains controversial, with numerous candidate structures appearing in the literature and disagreement between studies that attempt to adjudicate between them. Synthesized bulk Ta 2 O 5 is often amorphous [9,10], while crystals tend to have high concentrations of defects that impede structural determinations [11]. Resolving the ambient crystal structure is important because predictions of various properties, including the band structure [12,13] and mechanical strength [7,14] depend on it.…”