Morphological and structural characterization of WO3 and Cr–WO3 thin films synthesized by sol–gel process

doi:10.1016/j.tsf.2009.12.019

Thin Solid Films

Volume 518, Issue 16, 1 June 2010, Pages 4512-4514

https://doi.org/10.1016/j.tsf.2009.12.019 Get rights and content

Abstract

WO₃ thin films were prepared by spin-coating methanol solutions of a tungsten chloromethoxide, and easily modified with Cr by the addition of Cr 2-ethylhexanoate. The films were heat-treated up to 700 °C, and characterized by X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy and Electron Energy Loss spectroscopy. The film morphology was rough and porous, not depending on Cr presence, while their structure was constituted by packed spheroidal or elongated dense structures, giving rise to the peculiar film surface morphology. Cr was distributed in the film structure without phase separations, up to as high as 5% Cr atomic concentration.

Introduction

Tungsten trioxide (WO₃) is a promising material for gas-sensing applications [1], [2], [3], and has been prepared by a variety of physical and wet chemical techniques [4]. Among the latter, the sol–gel method has been widely applied for the simplicity and the easy doping of the final material. The latter feature is of particular interest for WO₃, since it has been shown that the addition of chromium remarkably increased the ammonia sensing performances of WO₃-based thick film sensors [5]. For fully exploiting the potential of sensors integrated in silicon-technology based devices, it was then of interest to develop the thin film technology of such materials. In this paper we report the successful sol–gel synthesis of pure and Cr-doped WO₃ thin films, and the related morphological and structural characterization, which are of particular interest for gas-sensing materials. We used a convenient route that was already exploited in the past for the preparation of WO₃ thin films [7], [8], [9]. Starting from WCl₆, a chloromethoxide was prepared by reaction with methanol. The simultaneous presence of chloro and alkoxo ligands stabilized the precursor molecules, and the addition of acetylacetone and of methanol further improved the solution stability: no precipitation occurred for more than one month. Moreover, film deposition by spin-coating could be achieved without a strict control of the atmospheric moisture. Finally, the chromium precursor was chosen in order to avoid the formation of precipitates on the film surface after the spin-coating step: this phenomenon may easily occur when reaching high additive concentrations with inorganic salts as precursors. The careful design of the sol and additive chemistry allowed deposition of uniform thin films, which were subsequently heat-treated for achieving crystallization and elimination of any organic residual. It will be shown that easy deposition of WO₃ thin films can be achieved, with a rough and porous structure on a microscopic scale, and that Cr incorporation is possible with a homogeneous distribution of Cr up to very high concentrations.

Section snippets

Experimental

All the chemicals were provided by Sigma-Aldrich in analytical grade or, as concerns WCl₆, with a 99.9% purity. For preparing the starting WO₃ precursor sols, a solution of acetylacetone (acacH; acacH:W molar ratio was 3) in 10 ml of methanol was slowly added to 1 g of WCl₆ in a glove-box purified from moisture and oxygen. The blue color of the prepared sols was due to the formation of W(V) chloroalkoxide complex, previously described as WCl₃(OCH₃)₂ [6]. The solution was further diluted with 10 ml

Results and discussion

The film evolution with the heat-treatment temperature was monitored by XRD and the results are shown in Fig. 1. For the various film compositions, some common trends are observed: the films are amorphous up to 300 °C, while for higher heating temperatures the film crystallization is evident. The observed peaks can belong to both the WO₃ δ (triclinic) and γ (monoclinic) crystallographic phases. The XRD patterns are not able of distinguishing between the two phases, whose coexistence is very

Conclusions

WO₃ thin films can be conveniently prepared by spin-coating methanol solutions of a tungsten chloromethoxide, without a stringent control of the deposition environment. The addition of chromium can be carried without phase separation of chromium oxides up to a large Cr concentration, by using a metalorganic Cr precursor easily dissolved in the starting WO₃ sol. The rough and porous structure of the thin films is a consequence of the high hydrolytic reactivity of the precursor, so the film is

Acknowledgements

The XRD unit of the Serveis Científico-Tècnics of the University of Barcelona is gratefully acknowledged for its cooperation. This work was supported by the European Union in the frame of the NANOS4 project (Grant NMP4-CT-2003-001528).

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Cited by (4)

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The crystallization of WO3 films is, in general, very complex and for our samples starts, although with a slow kinetic, at 500 °C and is almost completed after annealing at 600 °C for 1 h. This transition is often found at lower temperatures (350–550 °C), when the samples are grown by other deposition techniques [17,32,54], or by sol–gel methods [23,55,56], or finally by sol–gel employing a templating agent [42]. In these previous studies the presence of hydrous phases [23,42], not found in this work, has been also reported.
The electronic, morphological and structural properties of WO₃ thin films, synthesized via a sol–gel route and deposited on ITO/glass substrates by spin-coating, were analyzed as a function of annealing temperature (100–700 °C range) by Scanning Electron Microscopy, Atomic Force Microscopy, micro-Raman spectroscopy, X-ray Diffraction and Photoelectron Spectroscopy. We have found evidence of two competing processes when the film is annealed at high temperatures (600–700 °C): a structural phase transition from amorphous to crystalline WO₃ and a temperature-activated diffusion of sodium ions, from the substrate into the WO₃ film, which induces the formation of sodium tungstate. The surface of the films was found to be oxygen deficient after deposition but reverted to fully oxidized WO₃ after high temperature annealing in air. The annealing also induced a restructuring of the films with formation of nano-crystalline aggregates. The influence of film thickness on these processes was also investigated.
Suppression of the NO<inf>2</inf> interference by chromium addition in WO<inf>3</inf>-based ammonia sensors. Investigation of the structural properties and of the related sensing pathways
2013, Sensors and Actuators, B: Chemical
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Finally, the highlighted species are used for hypothesizing specific ammonia adsorption pathways which do not produce NOx, whose parasitic response is then hampered. A detailed investigation of the synthesis process was carried out in [4–6]. Briefly, WCl6 (1 g) was reacted with a solution of acetylacetone (molar ratio to W was 3) in 10 ml of methanol.
WO₃ and Cr–WO₃ powders were prepared by sol–gel process, with Cr:W atomic concentration ranging from 2% to 8.8%. WCl₆ was used as W precursor and reacted with methanol in presence of acetyacetone as stabilizer. The required amount of Cr 2-ethylhexanoate was then added to the resulting solution, which was subsequently dried. The resulting powder was heat-treated at temperatures ranging from 200 to 700 °C. With increasing the Cr concentration, the samples heat-treated at 500 °C contained an increasing amount of the additional phase Cr₂WO₆, as evidenced by X-ray diffraction and Transmission Electron Microscopy. The sensing tests toward ammonia gas, from 50 to 500 ppm showed that, up to 5% concentration, Cr addition lowered the best operating temperatures and/or enhanced the response with respect to pure WO₃, then the response remarkably decayed. DFT modeling showed that Cr(III) incorporation was more favorable in interstitial position while Cr (VI) was more favored in substitutional configuration. In this case, highly acidic Cr sites are present, enhancing the adsorption of ammonia species and reactions not involving NO₂ as by-product. Beyond 5% Cr concentration, Cr₂WO₆ formation extracts Cr from the WO₃ structure, so decreasing the ammonia response. Moreover, the appearance of Cr₂WO₆ seems to deactivate the surface adsorption properties of the material.
Visible-light-induced superhydrophilicity of crystallized WO<inf>3</inf> thin films fabricated by using a newly isolated W<sup>6+</sup> complex salt of citric acid
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¹: Current Address: Electrochemical Processes Unit, IMDEA Energy, c/Tulipan, s/n, E-28933 Mostoles, Madrid, Spain.

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Morphological and structural characterization of WO3 and Cr–WO3 thin films synthesized by sol–gel process

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Sens. Actuators B

Sens. Actuators B

J. Mater. Chem.

J. Appl. Physi.

Chem. Mater.

J. Electrochem. Soc.

Morphological and structural characterization of WO₃ and Cr–WO₃ thin films synthesized by sol–gel process