Nanostructured copper oxides as ethanol vapour sensors

Abstract

We report on ethanol (C₂H₅OH) vapour sensing devices based on nanostructured cupric oxide (CuO) and cuprous oxide (Cu₂O) thin films, which are deposited using RF sputtering at relatively low temperature and power conditions: at 120 °C, single stoichiometry CuO and Cu₂O films are deposited using the sputtering power of 200 and 250 W, respectively. At such sputtering conditions CuO films exhibit smaller nanocrystallite base dimensions (∼30 nm), in comparison to Cu₂O films (∼85 nm), which significantly enhance surface area to volume ratio. Both nanostructured CuO and Cu₂O gas sensors are able to detect ethanol vapour as low as several ppm and at relatively low operating temperatures of 180 and 260 °C, respectively. The sensors showed high sensitivity and repeatability, as well as fast response and recovery towards ethanol vapour.

Introduction

In the past two decades, tremendous efforts have taken place to enhance the performance of gas sensors based on semiconducting metal oxide films via incorporating nanomaterial into their structure [1], [2], [3], [4], [5], [6], [7], [8]. The nanostructured metal oxide films are favourable due to their unique properties such as large surface area to volume ratio, presence of size effect (Debye length), and incorporation of various well-controlled dopants at different stoichiometry. These nanostructures are used with the aim of obtaining improved gas sensitivity and a rapid response [5], [9], [10].

In the family of metal oxide semiconductors, copper oxides (Cu_xO – cuprous oxide (Cu₂O) and cupric oxide (CuO)) are amongst the most promising materials for gas sensing due to their high absorption coefficients, favourable electronic structures and earth-abundance [11], [12]. Copper oxides are intrinsic p-type semiconductors with relatively narrow energy band gaps of ∼1.2 eV (CuO) and ∼2.1 eV (Cu₂O) that exhibit a variety of interesting properties, which can be fully exploited in gas and vapour sensors, including ethanol vapour. Ethanol vapour sensors are extensively used in the fields of wine quality monitoring, breath analysis and food industries [13], [14].

To date, the majority of work in the field of metal oxide gas and vapour sensors has been devoted to n-type semiconductors such as ZnO, SnO₂, In₂O₃, WO₃, V₂O₅ and TiO₂ [15], [16], whilst the number of reports on the sensing properties of p-type metal oxide semiconductors, such as nanostructured Cu_xO, are significantly lower [9], [17]. This may be due to the higher carrier mobilities of n-type metal oxides (e.g. SnO₂ ∼160 cm² V⁻¹ s and ZnO ∼200 cm² V⁻¹ s) in comparison to p-type metal oxides (e.g. Cu₂O ∼10 cm² V⁻¹ s) [18]. However, p-type metal oxides such as Cu_xO still have great merit for gas sensing applications. They are remarkable catalytic materials that can operate at relatively low temperatures in comparison to n-type metal oxides. Additionally their low bandgaps (less than 2 eV) help in designing and implementing visible-light optoelectronically tunable semiconducting sensors. While the large bandgap of many n-type semiconducting metal oxides can only tuned in the UV range, which is not practical for many safe sensing applications.

Nanostructures of Cu_xO for ethanol vapour sensing can be prepared by several techniques such as thermal oxidation [19], [20], [21], hydrothermal [22], [23], [24], [25], chemical vapour deposition [9], DC reactive magnetron sputtering [26], [27] and solvothermal techniques [28], [29]. However, amongst these techniques, RF magnetron sputtering has several advantages such as high control over the deposition parameters (e.g. thickness and uniformity). Additionally, a high degree of film engineering and tuning (crystallinity, morphology of the grains and their stoichiometry) is possible [30], [31], [32], [33]. To the best of our knowledge, to-date there have been no reports on nanostructured Cu_xO films for ethanol vapour sensing formed using the RF magnetron sputtering approach. Our goal in this work is to demonstrate that RF magnetron sputtering can be applied as a deposition process for the fabrication of Cu_xO ethanol vapour sensors, and to provide an explanation of their mode of operation.

In this work, we synthesize and characterize the RF sputtered Cu_xO films and investigate their ethanol vapour sensing performance. We show that relatively low RF powers and substrate temperatures can be used for depositing single stoichiometry Cu₂O and CuO with the aim of producing nanostructures with well-engineered morphological sizes. The resulting CuO and Cu₂O films were then characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photospectroscopy (XPS), conductive atomic force microscopy (c-AFM) and Raman spectroscopy to assess their conditions. For gas sensing characterization, the CuO and Cu₂O films were sputtered onto quartz substrates with pre-patterned gold interdigitated transducers (IDTs) and tested towards ethanol vapour of concentrations in the range 12.5–500 part per millions (ppm).