Pushing the Limits of Metastability in Semiconducting Perovskite Oxides for Visible-Light-Driven Water Oxidation

Abstract

A synthetic route has been discovered to thermodynamically unstable, i.e., metastable, Sn(II)−perovskite oxides that have been highly sought after as lead-free dielectrics and small bandgap semiconductors. A highly facile exchange of Sn(II) is found by using a low melting SnCl 2 /SnF 2 peritectic flux, yielding mixed A-site (Ba 1−x Sn x )ZrO 3 and mixed A-and B-site (Ba 1−x Sn x )(Zr 1−y Ti y )O 3 solid solutions that exhibit a very high metastability, with up to 60% Sn(II) cations and a calculated reaction energy for decomposition of up to −0.3 eV atom −1 . Kinetic stabilization of the higher Sn(II) concentrations is achieved by the high cohesive energy of the perovskite compositions containing Zr(IV) and mixed Zr(IV)/Ti(IV) cations. Significantly red-shifted bandgaps are found with increasing Sn(II) substitution, enabling the optical absorption edge to be broadly tuned from ∼3.90 to ∼1.95 eV. Percolation pathways are calculated to occur for BSZT compositions with >12.5% Sn(II) and >25% Ti(IV) cations. High photocatalytic rates are found for molecular oxygen production for compositions which exceed the percolation thresholds, wherein extended diffusion pathways should "open up" across the structure and the charge carriers become delocalized rather than trapped. These results establish the critical importance of synthetically accessing metastable semiconductors for the discovery of advanced optical and photocatalytic properties.

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

“…These have involved metal oxides in the pyrochlore [28][29][30] and other structure types. [31][32][33][34] The Maggard and Jones groups have pioneered recent studies of Sn(II) exchange into the perovskite structure type, BaMO 3 (M = Ti, Zr, or Hf) with close-packed 'BaO 3 ' layers, as illustrated in Figure 3 and in the reactions below:…”

Capturing Metastable Oxide Semiconductors for Applications in Solar Energy Conversion

“…As described below, crystalline semiconductors containing new combinations of these M/M′ cations have been prepared in my laboratory and others and investigated for their photoelectrochemical properties. − ,− Many representatives within these systems are listed in Table in order of increasing band gaps. Notably, a predominant fraction of ∼80% of these semiconductors has been found to occur as thermodynamically unstable phases, as denoted in the right-hand column.…”

“…In addition, the binary oxides may oxidize in air or decompose in vacuum; for example, both Ag 2 O and SnO disproportionate in vacuum at temperatures of only ∼250–300 °C. Molten salt reactions have been extensively used to synthesize metastable oxides, ,,,− ,− ,− as listed in Table and shown in the SEM images in Figure . In this technique, a salt mixture at a low-melting eutectic composition is used to dissolve and react the metal oxides at relatively low temperatures, as reviewed previously. , The salt mixture can be selected from among, for example, metal halides or metal sulfates such as NaCl/KCl or Na 2 SO 4 /K 2 SO 4 , among many possibilities.…”

“…Thermodynamically stable structures and compositions have been the foundation for establishing structure–property relationships and driving much technological progress. Recent research, however, has revealed growing numbers of crystalline semiconductors that occur near to or “over the edges” of stability, that is, existing as metastable solids that are thermodynamically unstable but that are synthesizable and kinetically trapped. − While compilations of thermodynamic data for solids extend back for many decades, the unknown vastness of metastable space has only begun to be revealed in recent experimental and computational studies. − Thermodynamic instability can result from nonoptimal structures and bonding that, concomitantly, yield physical properties that are both desirable and otherwise challenging to obtain. For example, concepts such as “redox balance” have been key to understanding the stability of oxides under oxidizing or reducing conditions in solid oxide fuel cells .…”

“…In metastable semiconductors, thermodynamic instabilities frequently result from simpler, non-redox processes such as phase segregation. Key properties that can emerge in these types of semiconductors include defect tolerance, , low effective masses of charge carriers, , corrosion resistance, or visible-light band gaps. , Recent results from my group and others have begun to probe the fundamental reasons why these properties are difficult to attain in stable semiconductors for efficiently capturing and utilizing solar energy in photocatalytic reactions.…”