Effect of thermal annealing on the phase evolution of silver tungstate in Ag/WO₃ films

Highlights

• Ag₂WO₃ is prepared in a facile way by annealing multi-layers of Ag and WO₃ films.

• The variation of phase fraction of Ag and Ag₂WO₃ is studied with temperature.

• The optimum temperature for the formation of silver tungstate is obtained as 400 °C.

• The photoluminescence spectra show an emission peak at a wavelength of 441 nm.

Abstract

Silver/tungsten oxide multi-layer films are deposited over quartz substrates by RF magnetron sputtering technique and the films are annealed at temperatures 200, 400 and 600 °C. The effect of thermal annealing on the phase evolution of silver tungstate phase in Ag/WO₃ films is studied extensively using techniques like X-ray diffraction, micro-Raman analysis, atomic force microscopy and photoluminescence studies. The XRD pattern of the as-deposited film shows only the peaks of cubic phase of silver. The film annealed at 200 °C shows the presence of XRD peaks corresponding to orthorhombic phase of Ag₂WO₄ and peaks corresponding to cubic phase of silver with reduced intensity. It is found that, as annealing temperature increases, the volume fraction of Ag decreases and that of Ag₂WO₄ phase increases and becomes highest at a temperature of 400 °C. When the temperature increases beyond 400 °C, the volume fraction of Ag₂WO₄ decreases, due to its decomposition into silver and oxygen deficient phase Ag₂W₄O₁₃. The micro-Raman spectra of the annealed films show the characteristic bands of tungstate phase which is in agreement with XRD analysis. The surface morphology of the films studied by atomic force microscopy reveals that the particle size and r.m.s roughness are highest for the sample annealed at 400 °C. In the photoluminescence study, the films with silver tungstate phase show an emission peak in blue region centered around the wavelength 441 nm (excitation wavelength 256 nm).

Introduction

The distinct properties of semiconductor materials give rise to promising applications in solar cells [1], light-emitting diodes [2], smart windows [3], gas sensing [4] and other optical applications [5]. Metal tungstates are important semiconductors which have been studied for many years because of their potential applications in photoluminescence [6], catalysis [7], optics [8], humidity sensors [9], [10], dosimetry [11], magnetic applications [12], etc. Of several metal tungstates, silver based materials exhibit high photo-catalytic properties under visible light for the degradation of methyl orange dye and Escherichia coli bacteria destruction [13], [14]. Hu et al. [13] obtained irregular particles of Ag₂WO₄ powders by microwave assisted synthesis at different pHs with photo-catalytic activity for the degradation of different organic dyes under ultraviolet and visible light. Wang et al. [14] prepared Ag₂WO₄ powders with good antimicrobial action by the supersonic assisted homogeneous precipitation method. Liu et al. [15] used chemical solution method for the synthesis of Ag₂WO₄/AgCl nanorods for plasmonic photo-catalyst applications. In addition to these applications, Ag₂WO₄ is getting attraction since it can be used as solid lubricant materials at relatively higher temperatures [16]. Therefore achieving precise control over the synthesis and properties of metal tungstates has been a hot research subject. This paper presents a facile way to prepare Ag₂WO₄ thin films by thermal annealing multi-layers of Ag and WO₃ thin films prepared by RF magnetron sputtering technique and the films thus obtained are characterized using techniques like X-ray diffraction, atomic force microscopy, micro-Raman analysis and photoluminescence spectroscopy. The importance of the present technique is that the multistage sintering of precursor powders and prolonged heat treatment for obtaining metal tungstates used in other methods can be avoided. The effect of thermal annealing in the phase evolution of silver tungstate in Ag/WO₃ film is studied extensively.