Tuning the energy band-gap of crystalline gallium oxide to enhance photocatalytic water splitting: mixed-phase junctions

Abstract

Rational design and fabrication of mixed-phase oxide junctions is an attractive strategy in the photocatalytic applications. A new tuneable α-β mixed-phase Ga2O3 has recently been discovered to have the high activity in the photocatalytic water splitting. Here we perform a first-principles study to reveal the nature of efficient separation of photogenerated carriers achieved by the mixed-phase Ga2O3. It is found that the tensile strain and lattice misfitting at the interface junctions significantly tune their energy bands. As the interior angles between two components change, the characteristics of valence band-edge states can be significantly different. Through the analysis on the bonding strength of the bonds near the interfaces, and the comparison of calculated and experimentallyobserved carrier migration directions, we suggest a favorable junction for the efficient separation of photogenerated carriers which has a type-II band alignment with 0.35 eV higher valance band of α−Ga2O3 than that of β-Ga2O3, and the conduction band offset of only 0.07 eV. It seems that the electron migration across the phase boundary from the α− to β−Ga2O3 mainly follows the adiabatic electron-transfer mechanism due to the strong orbital coupling between the conduction bands of two phase materials.

Band-to-band transitions, selection rules, effective mass, and excitonic contributions in monoclinic β−Ga2O3

“…Computational results consistently predict a very small anisotropy, but span a relatively wide range of values: from (0.12-0.13)m e (GGA-DFT) [66], through (0.23-0.24)m e (local density approximation DFT) [72], to 0.39m e [76]. At the hybrid HF-DFT level the reported values are more consistent: (0.26-0.27)m e [10], (0.27-0.28)m e [27,71] with the HSE functional, and 0.34m e for the B3LYP functional [73].…”

Anisotropy, phonon modes, and free charge carrier parameters in monoclinicβ-gallium oxide single crystals

“…[32,33]). Earlier calculations using various approaches obtained 0.28 m e [102,103], 0.34m e [25], and 0.390m e [7]. Calculations that did not use a hybrid functional approach led to smaller values of (0.23 .…”

“…Few reports exist on theoretical prediction and experimental determination of static and high-frequency dielectric constants and their anisotropy [22][23][24][25][26][27][28]. Calculations predict effective mass parameters [7,8,[29][30][31], and few experiments were reported [32,33]. Theoretical descriptions of Brillouin zone center phonon modes are reported [28], and phonon band structures and density of states allowed prediction of thermal transport properties.…”

“…The highly anisotropic monoclinic β-gallia crystal structure (β phase) is the most stable crystal structure among the five phases (α,β,γ ,ε, and δ) of Ga 2 O 3 [5,6]. Mixed phase α-β Ga 2 O 3 oxide junctions were recently discovered for high-activity photocatalytic water splitting [7]. Current research focusses on the development of single-crystalline group-III sesquioxide semiconductors with low defect densities for potential use as active materials in electronic and optoelectronic devices [8].…”

Electrophoretic Deposition of Gallium with High Deposition Rate

“…Gallium and its compounds can find many applications in microelectronics [1], solar cells [2,3], solar water splitting [4,5], microwave circuitry [6], optoelectronics [7], flexible neuroprobes [8], self-repairing electrodes [9], and micro switches [10]. Considerable efforts have already been made to OPEN ACCESS deposit gallium thin films, which typically involve electroplating with highly toxic gallium salts.…”