Plasma-chemical synthesis of copper oxide nanoparticles in a low-pressure arc discharge А.V. Uschakov a, b , I.V. Karpov a, b , А.А. Lepeshev a, b, * , M.I. Petrov a, c a Krasnoyarsk Scientific Center of the Siberian Branch of the Russian Academy of Science, 660036, Krasnoyarsk, Russia b Siberian Federal University, 660041, Krasnoyarsk, Russia c Kirensky Institute of Physics, Russian Academy of Sciences, 660036, Krasnoyarsk, Russia article info Article history: Received 30 May 2016 Received in revised form 15 August 2016 Accepted 16 August 2016 Available online 18 August 2016 Keywords: Copper oxide nanoparticles Low pressure arc discharge Plasma-chemical synthesis Paschen curve Similarity parameter abstract The influence of a pressure of gas mixture (10 vol% O 2 þ 90% N 2 ) on an average size of copper oxide nanoparticles, produced in the plasma of low pressure arc discharge, has been studied as a basic process variable. A correlation between the dependence of average particle size on gas mixture pressure and the dependence of discharge gap voltage on product of interelectrode distance by a gas mixture pressure, has been found. The estimation was carried out by means of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). A mathematical model of the cathode region, which shows the applicability of the similarity theory to the low pressure arc discharge, has been represented. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Copper oxide with nanosized particles and films are useful materials in a variety of applications such as photovoltaic devices [1,2], electrochromic devices [3], thin film transistors [4], and chemical sensors [5], owing to their moderate band gap, low cost production, and high optical transparency. Copper oxide nano- particles are commonly synthesized by wet chemical processes [6e8]. However, it is not easy to ensure the homogeneity and crystallinity of the nanoparticles when fabricated through such methods. This is because wet chemical processes involve low temperatures. In contrast to wet chemical processes, thermal plasma-based ones involve the evaporation of the constituent metals at temperatures higher than 10,000 K, followed by the rapid condensation of the gas phases [9e13]. Thermal plasma-based processes have other advantages over wet chemical processes. One is that, using these processes, it is possible to prevent the end products from being contaminated by impurities; this is not the case with wet chemical processes [14,15]. So far, a number of thermal plasma based processes have been developed. Among them, the vacuum arc plasma evaporation (VAPE) method is commonly used for the deposition of thin films [16] and the fabrication of nanoparticles [17e22]. We recently reported that particle phases with different stoichiometries, including Cu2O and CuO, can be produced by varying the deposition conditions, such as oxygen partial pressure, discharge power, processing pressure, and substrate temperature [23]. According to the previous research, the properties of powders produced in the plasma arc discharge of low pressure, depend mainly on the gas mixture pressure in a plasma-chemical reactor [24]. Dispersion of produced powders depends mainly on two processes: evaporation and ionization of a liquid metal on a cath- ode surface, and condensation form a plasma-vapor phase. In a sufficiently high vacuum of ~10 1 Pa powder consists of particles of two types: spherical particles with size of 0.5e3 mm, wherein fine condensate dropped, and spherical particles with size of 0.5e10 mm, which represent agglomerates of smaller particles with size about 0.1 mm [24,25]. The percentage of nanopowders in this case is negligible. The experimental results clearly show the ad- vantages of powders obtained at different pressures of the gas mixture. Change of the pressure in the plasma-chemical reactor does not lead to change of the particle size distribution function. This shows predominantly thermal nature of arc powders synthe- sis, where the particles are formed due to vapor condensation. The purpose of this work is to define the mechanism influencing the pressure of gas mixture in a plasma-chemical reactor on an average * Corresponding author. Krasnoyarsk Scientific Center of the Siberian Branch of the Russian Academy of Science, 660036, Krasnoyarsk, Russia. E-mail address: sfu-unesco@mail.ru (А.А. Lepeshev). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum http://dx.doi.org/10.1016/j.vacuum.2016.08.007 0042-207X/© 2016 Elsevier Ltd. All rights reserved. Vacuum 133 (2016) 25e30