Nanostructured Materials Based on Thin Films and Nanoclusters for Hydrogen Gas Sensing ^†

Abstract

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.

1. Introduction

Nanostructured metal-oxide semiconductors (MOS) have been investigated for their sensorial activity for more than fifty years. A tremendously large variety of synthesis methods was developed [1]. Usually, these methods are wet-techniques or CVD ones, all providing a cheap and easy way to prepare a sufficiency of MOS. However, sometimes the tuning of the nanostructural properties or using the recipes for a wider spectrum of materials is complicated or even not possible [2].

In this work we present two magnetron-based techniques which enabled us to synthesize nanostructured composite materials later utilized as a conductometric hydrogen gas sensor. Materials exhibit enhanced sensitivity towards H₂ thanks to the nano-structure.

Figure 1. SEM micrographs of investigated structures. (a) CuO clusters on the WO₃ film; (b) CuO cluster covered with WO₃ film; (c) CuWO₄ nanostructures on WO₃ thin film.

Figure 1. SEM micrographs of investigated structures. (a) CuO clusters on the WO₃ film; (b) CuO cluster covered with WO₃ film; (c) CuWO₄ nanostructures on WO₃ thin film.

2. Materials and Methods

Thin films of WO₃ were deposited on SiO₂ substrates using a reactive dc magnetron sputtering (60 W) from a metallic target. A mixture of argon and oxygen at a partial pressure ratio of 1:3 was used as a working gas. The substrate was heated to 400 °C.

Clusters of CuO were deposited from the copper target in the gas aggregation gas cluster source using a clean Ar as a working gas. The detailed description of the vacuum chamber and further parameters can be found in Ref [3].

CuWO₄ nanostructures were formed by reactive deposition from a copper metallic target using a reactive rf sputtering (230 W) in a mixture of argon and oxygen at a partial pressure ratio of 1:4. The substrate was heated to 400 °C. For more details see Ref. [4].

3. Results and Discussion

The SEM topographies of referred materials can be found in Figure 1. The relative response towards hydrogen of composite was increased in comparison with WO₃ thin film alone as can be seen in Figure 2. Only the proper amount of CuO or CuWO₄ leads to enhancement. The response of WO₃ decreases to low concentration or to high amount of added material (not shown).

The explanation is based on the formation of nanosized heterojunctions which reduces the conductive channel in thin film. For CuO clusters and WO₃ they are p-n type, and in case of CuWO₄ they are hetero n-n. Further details can be found in papers [3,4].

4. Conclusions

We were able to enhance the sensorial response of WO₃ thin film towards hydrogen by synthetizing CuO nanoclusters or CuWO₄ nanostructures. The deposition techniques are advantageous for their integration with microcircuit devices since they do not require any wet steps and the materials were used “as-deposited” without any need of annealing or sintering.