Preparation-method-dependent morphological, band structural, microstructural, and photocatalytic properties of noble metal-GaNbO sub(4) nanocomposites

• Published in: RSC advances 2013-08, Vol.3 (37), p.16817-16828
• Main Authors: Devi, Suman, Kelkar, Sarika, Kashid, Vikas, Salunke, Hemant G, Gupta, Narendra M
• Format: Article
• Language: English
• Subjects: Conduction band; Gold; Microstructure; Nanocomposites; Photocatalysis; Photocatalysts; Platinum; Semiconductors
• ISSN: 2046-2069
• DOI: 10.1039/c3ra43477j

We report the distinct physicochemical and photophysical properties of gallium niobate photocatalysts (bandgap: similar to 3.1 eV), prepared by a solid-state (SS) reaction and sol-gel (SG) method and dispersed with a noble metal ( similar to 0.5% of Pt, Au, or RuO sub(x)) cocatalyst. SG-GaNbO sub(4) comprised smaller size particles ( similar to 20-50 nm) and a larger surface area ( similar to 160 m super(2) g super(-1)) compared to SS-GaNbO sub(4) (particle size similar to 30-150 nm, surface area similar to 27 m super(2) g super(-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-GaNbO sub(4) 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-GaNbO sub(4) 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 GaNbO sub(4), as confirmed by ab initio electronic structure calculation. These photocatalysts, however, showed good activity for semiconductor-mediated photo-dissociation of aqueous methanol to produce H sub(2); a cocatalyst-dependent activity trend, Pt > RuO sub(x) > Au, was observed. Doping of S at similar to 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.