Mixing Thermodynamics and Photocatalytic Properties of GaP–ZnS solid solutions

Abstract

Preparation of solid solutions represents an effective means to improve the photocatalytic properties of semiconductor-based materials. Nevertheless, the effects of site-occupancy disorder on the mixing stability and electronic properties of the resulting compounds are difficult to predict and consequently many experimental trials may be required before achieving enhanced photocatalytic activity. Here, we employ first-principles methods based on density functional theory to estimate the mixing free energy and the structural and electronic properties of (GaP)x(ZnS)1−x solid solutions, a representative semiconductor-based optoelectronic material. Our method relies on a multi-configurational supercell approach that takes into account the configurational and vibrational contributions to the free energy. Phase competition among the zinc-blende and wurtzite polymorphs is also considered. We demonstrate overall excellent agreement with the available experimental data: (1) zinc-blende emerges as the energetically most favorable phase, (2) the solid solution energy band gap lies within the 2-3 eV range for all compositions, and (3) the energy band gap of the solid solution is direct for compositions x ≤ 75%. We find that at ambient conditions most (GaP)x(ZnS)1−x solid solutions are slightly unstable against decomposition into GaP-and ZnS-rich regions. Nevertheless, compositions x ≈ 25, 50, and 75% render robust metastable states that owing to their favorable energy band gaps and band levels relative to vacuum are promising hydrogen evolution photocatalysts for water splitting under visible light. The employed theoretical approach provides valuable insights into the physicochemical properties of potential solid-solution photocatalysts and offers useful guides for their experimental realization.

Strain engineering of oxide thin films for photocatalytic applications

“…For photocatalytic water-splitting purposes, the electronic band structure of unstrained CeO 2 (111) presents two important limitations. First, the corresponding E g is too large for absorption of visible light, and second, the position of the VB (CB) edge is too far below (too close to) the water oxidation potential of −5.6 eV (the proton reduction potential of −4.4 eV) [2,3]. Interestingly, biaxial strain can be used to partly overcome both of those limitations.…”

“…For instance, the band gap (E g ) of photocatalytic materials must be below ∼ 3 eV in order to absorb solar radiation within the visible spectral range. At the same time, a semiconductor able to catalyze the synthesis of H 2 in water or the reduction of CO 2 into CH 4 should possess a conduction-band edge higher in energy than the corresponding redox potential (−4.4 and −4.6 eV, respectively, relative to the vacuum level) and the valence-band edge lower than the H 2 O oxidation potential (−5.6 eV) [2,3]. Furthermore, active photocatalysis requires efficient separation of the photogenerated charge carriers (low exciton binding energy) and their rapid transportation to the reaction sites (long electronhole recombination time) [4].…”

Giant Thermal Enhancement of the Electric Polarization in FerrimagneticBiFe1−xCoxO3Menéndez1, Cazorla2 2020Phys. Rev. Lett.Self Cite150120View full textAdd to dashboardCite

“…3(d); we assume that at low temperatures the magnetic and configurational entropies are small and very similar for the two phases (i.e., the fM i g, fE i g, and configurational degeneracy spectra calculated for T and R are much alike, Fig. 2 [48]), thus they hardly have any influence on ΔG.…”

“…2). At equilibrium conditions, and considering only configurational effects, each configuration contributes to the total magnetization of the crystal as [48,49]…”

“…3 [40]). The corresponding P-T phase boundary, determined with the condition [48]. The magnetic moment, total energy, and relative degeneracy of each representative configuration are indicated.…”