The influence of oxidation temperature on structural, optical and electrical properties of thermally oxidized bismuth oxide films

Abstract

Monoclinic bismuth oxide (Bi₂O₃) films have been prepared by thermal oxidation of vacuum evaporated bismuth thin films onto the glass substrates. In order to obtain the single phase Bi₂O₃, the oxidation temperature was varied in the range of 423–573 K by an interval of 50 K. The as-deposited bismuth and oxidized Bi₂O₃ films were characterized for their structural, surface morphological, optical and electrical properties by means of X-ray diffraction, scanning electron microscopy (SEM), optical absorption and electrical resistivity measurements, respectively. The X-ray analyses revealed the formation of polycrystalline mixed phases of Bi₂O₃ (monoclinic, α-Bi₂O₃ and tetragonal, β-Bi₂O₃) at oxidation temperatures up to 523 K, while at an oxidation temperature of 573 K, a single-phase monoclinic α-Bi₂O₃ was formed. From SEM images, it was observed that of as-deposited Bi films consisted of the well-defined isolated crystals of different shapes while after thermal oxidation the smaller dispersed grains were found to be merged to form bigger grains. The changes in the optical properties of Bi₂O₃ films obtained by thermal oxidation at various temperatures were studied from optical absorption spectra. The electrical resistivity measurement depicted semiconducting nature of Bi₂O₃ with high electrical resistivity at room temperature.

Introduction

Bismuth oxide (Bi₂O₃) has attracted a great amount of attention and become one of the important functional materials due to its peculiar properties like energy gap, refractive index, dielectric permitivity, oxygen mobility as well as remarkable photoconductivity and photoluminescence [1], [2], [3]. Because of good dielectric characteristics, it has applications such as coatings, Schottky barrier solar cell, metal/insulator/semiconductor (MIS) capacitors, etc. [1], [4], [5]. Because of its high photoconductivity, it is widely used in third-order nonlinear optical glasses [6], optoelectronics, optical coatings, electrochromic materials [7], as well as in transparent and superconductor ceramic glass manufacturing [8], [9], [10], [11]. The Bi₂O₃ system exhibits high oxide ionic conductivity and have been proposed as good electrolyte materials for application, such as solid oxide fuel cell (SOFC) and oxygen sensor [12]. Furthermore, owing to their high oxygen-ion conductivity characteristics which provide oxygen for the hydrogen abstraction, the Bi₂O₃ also finds extensive application as a catalyst for industrial selective oxidation reactions, especially for propylene selective oxidation and ammoxidation to acrolein and acrylonitrile, respectively [13]. Recently, nanocrystalline Bi₂O₃ with large surface area, electrochemical stability and pseudocapacitive behavior has found significant contribution to supercapacitor technology [14]. The Bi₂O₃ plays a significant role within the class of semiconducting oxide materials due to a variety of physical properties which intimately depends on its numerous polymorphs [15]. It is well known that, Bi₂O₃ has four main crystalline phases with two non-stoichiometric phases, denoted by α-, β-, γ-, δ-Bi₂O₃ and non-stoichiometric phases are Bi₂O_2.33, Bi₂O_2.75 [16]. Depending on preparation technique, the electrical conductivity of bismuth oxide may change by five orders of magnitude, while its band gap may change from 2 to 3.96 eV [15]. Thus the preparation of nanocrystalline Bi₂O₃ films is creditable.

Previously many researchers have endeavored to obtain the Bi₂O₃ films by thermal treatment to bismuth (Bi) films. For example, the group of Leontie et al. [2], [3], [17], [18] reported that the mixed phases of Bi₂O₃ like β-Bi₂O₃, α-Bi₂O₃, BiO, etc. could be obtained by thermal oxidation of vacuum evaporated Bi films at different oxidation temperatures, heating and cooling rates. The polycrystalline and multiphase (α-Bi₂O₃ and β-Bi₂O₃) films were obtained by Ismail [19] by rapid thermal oxidation with aid of halogen lamp at 773 K for 45 s in static air condition. Huang et al. [20] prepared Bi₂O₃ thin films by the oxidation process of the electrodeposited metallic Bi films and depending on the deposition potential of Bi films; they obtained pure α-Bi₂O₃ and β-Bi₂O₃.

In the present work, single-phase monoclinic Bi₂O₃ thin films have been obtained by thermal oxidation of vacuum evaporated Bi films onto the glass substrate. The Bi films were oxidized at different temperatures keeping the fixed oxidation time, heating and cooling rate. The films were characterized using XRD, scanning electron microscope, optical absorption and electrical resistivity measurement techniques.