Preparation-method-dependent morphological, band structural, microstructural, and photocatalytic properties of noble metal–GaNbO4 nanocomposites

Abstract

We report the distinct physicochemical and photophysical properties of gallium niobate photocatalysts (bandgap: ∼3.1 eV), prepared by a solid-state (SS) reaction and sol–gel (SG) method and dispersed with a noble metal (∼0.5% of Pt, Au, or RuOx) cocatalyst. SG–GaNbO4 comprised smaller size particles (∼20–50 nm) and a larger surface area (∼160 m2 g−1) compared to SS–GaNbO4 (particle size ∼30–150 nm, surface area ∼27 m2 g−1). XRD patterns revealed a preparation-dependent variation in the relative intensity of prominent reflections. In TEM examination, SG samples exhibited small-range grain boundaries and heterogeneous metal/substrate interfacial contacts, while SS–GaNbO4 had long-range ordering. Laser-Raman and thermoluminescence investigations revealed that lattice distortion, defect-induced inter-bandgap charge trapping states, and the local environment around the metal/semiconductor interfaces may also depend on the preparation method. Metal–GaNbO4 nanocomposites showed no activity for the dissociation of pure water under UV (>250 nm) irradiation, despite the favourable conduction and valence band potentials. This was attributed to the sharp Ga and Nb d-levels in the narrow conduction band of GaNbO4, as confirmed by ab initio electronic structure calculation. These photocatalysts, however, showed good activity for semiconductor-mediated photo-dissociation of aqueous methanol to produce H2; a cocatalyst-dependent activity trend, Pt > RuOx > Au, was observed. Doping of S at ∼5% of the oxygen sites led to decreased photoactivity, ascribed to the presence of localized S 3p states just above the O 2p valence level. In conclusion, besides band characteristics, certain morphological and microstructural properties play a crucial role in the photoactivity of the metal/oxide nanocomposites.