Capturing Metastable Oxide Semiconductors for Applications in Solar Energy Conversion

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Many small bandgap semiconductors have been discovered or predicted to exist beyond the edges of stability, that is, accessible only as metastable solids that are thermodynamically unstable. In many cases, these metastable semiconductors have been revealed to have technologically promising properties for solar energy conversion, such as in photocatalysis or in photovoltaics. This Account presents a review of research results selected from my group and others in recent years on these semiconductors. Notably, these include the chemical systems of mixed-metal oxides (i.e., M′MO x ; M = Ti(IV), Nb(V), or Ta(V) cation; M′ = Ag(I), Cu(I), Sn(II), Pb(II), or Bi(III) cation), which have diverse structure types and compositions. High photocatalytic activities have been found for the light-driven reduction or oxidation of water as p-or n-type photoelectrodes, respectively, or as suspended powders in aqueous solutions. These have exhibited new combinations of favorable semiconductor properties, such as deep visible-light absorption, near-optimal band edge energies, defect tolerance, and functional carrier mobilities and charge separation. As described herein, this set of properties is inextricably linked to their metastable nature, that is, the crystalline structures and compositions needed for these characteristics lead naturally to thermodynamic instabilities. This Account focuses on current research efforts that have begun unlocking the potential of these semiconductors via new recent advances in (1) synthetic approaches that enable their preparation and (2) the understanding of structure−property relationships discovered at the precipices of stability that lead to the improved semiconductor properties. For example, low-temperature reactions have been developed to facilitate greater kinetic control, such as with the use of molten salts, and have been a key factor in preparing many of these semiconductors. As a result, a plethora of promising new mixed-metal oxide systems have been uncovered that exhibit band gaps spanning the range of photon energies from ∼1.3 to >3.0 eV. Especially relevant for visible-light applications are the Cu(I)-and Sn(II)-containing semiconductors. For example, n-type Sn(II)-titanates and p-type Cu(I)-niobates can be synthesized by flux methods and exhibit some of the smallest known visible-light band gaps that also maintain suitable conduction and valence band edges for driving the water-splitting half reactions. Kinetic stabilization of these metastable semiconductors against thermally driven phase segregation is increased with the formation of solid solutions for both the M and M′ cation sites, leading to effective strategies to more finely tune their band gaps, band edge energies, and photoelectrochemical properties. Many unique and useful relationships are emerging between the synthesis and structures of metastable semiconductors and their physical properties, leading to more efficient solar energy conversion.

Renaissance of Topotactic Ion‐Exchange for Functional Solids with Close Packed Structures

“…For example, this typically occurs between the layers or through the pore openings that are found in layered or porous crystalline structures [3–5] . Additionally, thermodynamically unstable compounds, i. e., metastable compounds, can frequently be synthesized via maintenance of the underlying crystalline structure during the ion‐exchange reaction [6–8] . Thus, low‐temperature ion‐exchange reactions provide a valuable kinetic handle for the formation of new, and frequently metastable, crystalline solids.…”

Cs,Rh-codoped WO3with a core–shell structure responsive up to 600 nm as an O2-evolving photocatalyst forZ-schematic water splitting

“…Therefore, various visible-light-driven photocatalysts have been developed for water splitting systems. [1][2][3][4][5][6][7][8][9] Z-schematic water splitting systems are useful to utilize photocatalysts with narrow band gaps (BGs). Powder-based Zschematic water splitting systems are constructed with a H 2evolving photocatalyst, an O 2 -evolving photocatalyst, and a redox electron mediator, as shown in Fig.…”

Circumventing Thermodynamics to Synthesize Highly Metastable Tin(II) Perovskites: Nano Eggshells of SnHfO3

“…These phases constitute an elusive and dynamic frontier due to the synthetic challenges of kinetic stabilization, which is lacking in conventional solid-state synthetic approaches at high temperatures. 3,6,7 Several low-temperature 'Chimie Doucie' techniques have been effectively employed in the crystallization of thermodynamically unstable oxides. 8 Topotactic ion-exchange mediated by a molten salt flux has recently been demonstrated as an effective method for the synthesis of highly metastable oxides.…”