Elsevier

Thin Solid Films

Volume 544, 1 October 2013, Pages 452-456
Thin Solid Films

Electric field assisted aerosol assisted chemical vapour deposition of nanostructured metal oxide thin films

https://doi.org/10.1016/j.tsf.2013.02.082Get rights and content

Highlights

  • Electric field assisted chemical vapour deposition method

  • Ability to create high surface area film architectures

  • Can produce enhanced sensor response

  • Good control over film properties

Abstract

Nanostructured thin films of tungsten, vanadium and titanium oxides were deposited on gas sensor substrates from the electric field assisted chemical vapour deposition reaction of tungsten hexaphenoxide, vanadyl acetylacetonate and titanium tetraisopropoxide respectively. The electric fields were generated by applying a potential difference between the inter-digitated electrodes of the gas sensor substrates during the deposition. The deposited films were characterised using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The application of an electric field, encouraged the formation of interesting and unusual nanostructured morphologies, with a change in scale length and island packing. It was also noted that crystallographic orientation of the films could be controlled as a function of electric field type and strength. The gas sensor properties of the films were also examined; it was found that a two to three fold enhancement in the gas response could be observed from sensors with enhanced morphologies compared to control sensors grown without application of an electric field.

Introduction

Metal oxide thin films have been subjected to enormous research activities in recent years due to their wealth of interesting properties and applications in a diverse range of areas from chromogenic materials [1], [2], catalyst materials [3], energy materials [4], [5] and sensors [6], [7]. It is highly desirable to be able to produce materials with high surface areas and tailored nano-structures in order to maximise the material performance and efficiency in these increasingly important applications.

Chemical vapour deposition is a useful method for depositing thin films of metal oxides and has been shown to be capable of producing a range of material architectures [8]. The effect of an electric field in chemical vapour deposition reactions of any kind is largely unknown; Williams et al. noted that whilst trying to follow WO3 film growth from an aerosol assisted chemical vapour deposition (AACVD) using impedance measurements, the bias of the measuring circuit led to a change in film microstructure [9]. We have conducted a number of studies on the effect of electric fields in the chemical vapour deposition reactions to produce various metal oxides [10], [11], [12], [13]. In this paper we describe the use of electric field assisted chemical vapour deposition to produce thin films of vanadium, titanium and tungsten oxides and their application as metal oxide gas sensors.

Section snippets

Film synthesis

The deposition of tungsten oxide was carried out on alumina gas sensor substrates. The substrates were 3 mm × 3 mm chips with inter-digitated gold electrodes with a gap of 45 μm between the electrodes. 50.8 μm platinum wires (Alfa Aesar), were spot welded to the platinum heater track on the bottom of the substrates and the electrodes on the top, using a Macgregor DC601 parallel gap resistance welder. The chips were then further welded onto individual sensor housings. A dielectric glaze (4913G, ESL

Results

Scanning electron microscopy images of the deposited metal oxide films are shown in Fig. 2. The vanadium dioxide films prepared from the AACVD route (Fig. 2C.) had an island growth morphology composed of 100 nm particles that have agglomerated into larger islands, a microstructure typical of AACVD reactions [16]. It is clear from the SEM of films prepared with an applied electric field that nanorod structures are developing secondary nucleation sites and film growth is progressing in a pseudo

Discussion

Previous investigations [10], [11], [12], [13] have shown that the application of an electric field during a CVD reaction influences the crystallographic orientation, microstructure and subsequent properties of the deposited materials. Indeed it is well known that sensor microstructure plays an important role in determining the gas response of a gas sensing material [24]. We have suggested previously that a variety of phenomena may be occurring when an electric field is applied to CVD growth

Conclusion

The use of electric fields in the aerosol assisted chemical vapour deposition reactions of titanium isopropoxide, tungsten hexaphenoxide in toluene or vanadyl acetylacetonate in ethanol on gas sensor substrates led to the production of titanium, tungsten or vanadium oxide thin films. Changing the strength of the electric field led to large changes in morphology, thickness, implied growth rate and preferential orientation of the films. The gas sensor properties of the films were examined by

Acknowledgments

RB thanks the Royal Society for a Dorothy Hodgkin Fellowship and the EPSRC for financial support (grant number EP/H005803/1). Mr. Kevin Reeves is thanked for his invaluable assistance with electron microscopy.

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