Structural Changes in Palladium Nanofilms during Thermal Oxidation
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ctural Changes in Palladium Nanofilms during Thermal Oxidation A. M. Samoylova, *, S. A. Ivkova, D. I. Pelipenkoa, M. K. Sharova, V. O. Tsyganovaa, B. L. Agapovb, E. A. Tutovc, and P. Badicad a
Voronezh State University, Voronezh, 394018 Russia Research Institute of Electronic Engineering, Voronezh, 394033 Russia c Voronezh State Technical University, Voronezh, 394026 Russia d National Institute of Materials Physics, Bucharest-Magurele, 077125 Romania *e-mail: [email protected] b
Received May 3, 2019; revised May 12, 2020; accepted May 18, 2020
Abstract—Nanocrystalline PdO films have been characterized by X-ray diffraction, scanning electron microscopy, and electron probe microanalysis. The results demonstrate that thermal oxidation in an O2 atmosphere causes ~35-nm-thick nanocrystalline Pd films on SiO2/Si(100) substrates to undergo a sequence of phase transformations resulting in PdO formation, followed by PdO decomposition into metallic Pd at T > 1120 K. In the range 670–970 K, the a and c tetragonal cell parameters of the nanocrystalline PdO films increase monotonically with increasing temperature. The present and previously reported data have been used to construct a model for the unit cell in the crystal structure of palladium(II) oxide. Based on the quasichemical approach, we propose a model that accounts for the observed increase in the tetragonal cell parameters and the p-type conductivity of the nanocrystalline PdO films in terms of the formation of excess interstitial oxygen atoms. Keywords: palladium, palladium(II) oxide, nanostructure, crystal structure, thermal oxidation, gas sensors DOI: 10.1134/S0020168520100131
INTRODUCTION Gas sensors, including portable personal instruments, allow one to prevent technological and domestic accidents with explosive gases and are necessary for safety systems in a variety of industrial processes employing toxic and flammable volatile substances [1–3]. Metal oxide semiconductors have been used in various gas sensors for half a century [1, 4–6]. The functional properties of p-type metal oxide semiconductors, such as Cr2O3, Cu2O, PdO, and others, have so far been studied only fragmentarily, even though these materials have great potential for use in gas sensors [5, 6]. The gas sensing properties of nanostructures based on palladium(II) oxide, which has p-type conductivity [7, 8], have attracted researchers’ attention relatively recently. Experimental data demonstrate that sensors based on PdO nanostructures with various morphological architectures offer high sensitivity, high sensor response stability, a short recovery time, and good sensor signal reproducibility for hydrogen, carbon monoxide, organic vapor, nitrogen(IV) oxide, and ozone detection in atmospheric air [7–13].
It is known that the phase diagram of the palladium–oxygen system has not yet been studied in sufficient detail, the PdO nonstoichiometry region has not been determined, and there are contradictory views in the literature as to the nature of the point defects responsible for the p-type conducti
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