Thermally-induced microstructural evolution in nanoparticle-based CuO, WO$_3$ and CuO-WO$_3$ thin films for hydrogen gas sensing

TL;DR

This paper explores how annealing affects the microstructure of nanoparticle CuO, WO$_3$, and their composites, revealing phase changes and stability differences, and evaluates their hydrogen sensing performance.

Contribution

It provides new insights into the thermal stability, phase evolution, and gas sensing properties of nanoparticle-based CuO, WO$_3$, and CuO-WO$_3$ thin films.

Findings

CuO films show increased crystallinity and particle growth with temperature.

WO$_3$ and CuO-WO$_3$ films are more thermally stable.

At 400°C, CuO-WO$_3$ films form a novel $ extgamma$-CuWO$_4$ phase.

Abstract

This study systematically investigates the microstructural evolution of nanoparticle-based CuO, WO$_3$, and composite 'CuO-WO$_3$' thin films induced by their post-deposition annealing. The films were reactively deposited using a magnetron-based gas aggregation technique, with the composite films consisting of alternating monolayers of CuO and WO$_3$ nanoparticles. After deposition, the films were annealed in synthetic air at temperatures ranging from 200 to 400$^\circ$C and characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Annealing of the CuO films led to the most pronounced changes associated with a gradual enhancement of crystallinity accompanied by significant particle growth with increasing annealing temperature, while the WO$_3$ and CuO-WO$_3$ films were more thermally stable to crystallization and particle growth. Notably, at 400$^\circ$C, the CuO--WO$_3$ films crystallized into a novel $\gamma$-CuWO$_4$ phase. The annealed films were further evaluated for their gas-sensing performance upon H$_2$ exposure and the obtained results were analyzed in relation to film properties and the microstructural evolution induced by annealing.