Chromium and Ruthenium-Doped Zinc Oxide Thin Films for Propane Sensing Applications

Heberto Gómez-Pozos,José González-Vidal,Dwight Acosta,Luis Castañeda,María De La Luz Olvera,Maximino Avendaño-Alejo,Jorge Rodríguez-Baez,Arturo Maldonado,Gonzalo Torres

doi:10.3390/s130303432

Abstract

Chromium and ruthenium-doped zinc oxide (ZnO:Cr) and (ZnO:Ru) thin solid films were deposited on soda-lime glass substrates by the sol-gel dip-coating method. A 0.6 M solution of zinc acetate dihydrate dissolved in 2-methoxyethanol and monoethanolamine was used as basic solution. Chromium (III) acetylacetonate and Ruthenium (III) trichloride were used as doping sources. The Ru incorporation and its distribution profile into the films were proved by the SIMS technique. The morphology and structure of the films were studied by SEM microscopy and X-ray diffraction measurements, respectively. The SEM images show porous surfaces covered by small grains with different grain size, depending on the doping element, and the immersions number into the doping solutions. The sensing properties of ZnO:Cr and ZnO:Ru films in a propane (C3H8) atmosphere, as a function of the immersions number in the doping solution, have been studied in the present work. The highest sensitivity values were obtained for films doped from five immersions, 5.8 and 900, for ZnO:Cr and ZnO:Ru films, respectively. In order to evidence the catalytic effect of the chromium (Cr) and ruthenium (Ru), the sensing characteristics of undoped ZnO films are reported as well.

Highlights

Zinc oxide is one of the most important multifunctional semiconductor oxides because of its physical properties, such as resistivity control over the range 10−3–105 Ωcm, high transparency in the visible range, chemical and thermal stability at room temperature, a direct wide bandgap, around3.37 eV, and a large exciton binding energy of 60 meV [1]
The surface profile or the rms roughness of the films was measured, and values between 10–20 nm were estimated with an accuracy of 10%
Deposition of all samples were reached by six immersions in a 0.6 M starting solution, prepared from zinc acetate, and zinc oxide (ZnO) films were doped by an additional process consisting in one, three or five immersions of the ZnO films into a solution containing Cr or Ru

Summary

Introduction

3.37 eV, and a large exciton binding energy of 60 meV [1] These characteristics make ZnO thin films very attractive for different applications, such as, solar cell transparent contacts [2], surface acoustic wave systems [3], liquid crystal displays [4], gas sensors [5,6], and other optoelectronic devices [7]. Undoped semiconductor oxides are catalytically active, a dopant element (catalyst) is often added to improve their sentitivity and selectivity In this respect, different elements have been tested as catalysts in ZnO films, and it has been widely documented that an adequate selection of the catalyst, depending on the detecting gases, leads to an improvement of the sensing properties [14,15,16,17,18,19,20]. ZnO samples can be processed by different deposition techniques, such as, thermal evaporation [21], sputtering [22], chemical vapor deposition [23], chemical spray [24], and sol- gel technique [25]

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