In this paper, we present two approaches to synthesize nanostructured metal oxide semiconductors in a form of multi-layer thin films later assembled as a conductometric gas-sensors. The first approach produces a combination of thin solid film of tungsten trioxide (WO3) with nanoclusters of cupric oxide (CuO) prepared by a magnetron-based gas aggregation cluster source (GAS). The second method is a two-step reactive magnetron sputtering forming a nanostructured copper tungstate (CuWO4) on-top of a WO3 film. Both methods lead to synthesis of nanosized hetero-junctions. These greatly improve the sensorial response to hydrogen in comparison with a WO3 thin film alone.
High-power impulse magnetron sputtering of a Ta target in precisely controlled Ar+O2+N2 gas mixtures was used to prepare amorphous N-rich tantalum oxynitride (Ta–O–N) films with a finely varied elemental composition. Postdeposition annealing of the films at 900°C for 5 min in vacuum led to their crystallization without any significant change in the elemental composition. The authors show that this approach allows preparation of a Ta–O–N film with a dominant Ta2N2O phase of the bixbyite structure. As far as the authors know, this phase has been neither experimentally nor theoretically reported yet. The film exhibits semiconducting properties characterized by two electrical (indirect or selection-rule forbidden) bandgaps of about 0.2 and 1.0 eV and one optical (direct and selection-rule allowed) bandgap of 2.0 eV (suitable for visible-light absorption up to 620 nm). This observation is in good agreement with the carried out ab initio calculations and the experimental data obtained by soft and hard X-ray photoelectron spectroscopy. Furthermore, the optical bandgap is appropriately positioned with respect to the redox potentials for water splitting, which makes this material an interesting candidate for this application.
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