Synthesis and characterization of boron doped bismuth oxide thin film by spray pyrolysis for a fuel cell application

Abstract

Bismuth oxide (Bi2O3) thin films and Boron (B) doped Bismuth oxide thin films with different B concentrations (1.0 at. % ~ 7.0 at. %) have been prepared by spray pyrolysis method on a glass substrate at 350 0C substrate temperature. The surface morphology, structural, optical and electrical properties of the as-deposited films has been studied in details. The energy dispersive X-ray (EDX) data of the Bi2O3 and B doped Bi2O3 thin films were taken. From EDX data, atomic weight % of Bismuth and Oxygen in the Bi2O3 films is found to be 72.21 at.% and 27.79 at.%, respectively. After B doping, the atomic weight % of boron is increasing on the other hand the atomic percentage of bismuth and oxygen is decreasing. The Field emission scanning electron microscopy (FESEM) micrographs of the Bi2O3 and B doped Bi2O3 films have been taken at 350 0C.The FESEM micrographs of as-deposited films show uniform surface and deposition covers the substrate well. Multigonal or semicircle shaped particles have been observed on the whole substrate of the as-deposited thin films. With Boron concentration the grain size is increasing and after 5 at.% B doped, the surface of the deposited thin films turns into the glassy form. The crystal structure and the crystallite size of the as-deposited Bi2O3 and B doped Bi2O3 thin films were studied by X-ray diffraction (XRD). From the XRD data it is clear that the Bi2O3 and B doped Bi2O3 thin films are polycrystalline in nature with mixed phases of monoclinic and tetragonal structures. For pure Bi2O3 the crystallite size calculated from (108) plane is found 22.72 nm and for B doping the crystallite size decreases up to 3.0 at.% and then increases. The dislocation density was found for pure Bi2O3 is 1.93´10-3 line/nm2 and after B doping the dislocation density increases upto 3.0 at.% and then decreases. The micro strain of the deposited films shows the same nature of dislocation density. The values of inter planer spacing of the thin films are found in good agreement with those adopted in JCPDS card no. 27-1773, 27-0049. The optical studies were carried out by UV-Vis spectroscopy in the range of 200 to 1100 nm. Various optical constants such as absorption coefficient, extinction coefficient, optical band gap, refractive index and dielectric constant of the films have been measured. The transmittance for pure Bi2O3 is found 39% and after 1.0 at.% B doping, the transmittance drastically decreases but above 1.0 at.% B doping, the transmittance is increased in the visible range (400 - 700) nm. The band gap of the deposited films varies from 3.99 to 3.70 eV. The extinction coefficient and refractive index of the deposited films are increasing with increasing wavelength in the visible range (400 - 700) nm. The dielectric loss of the pure and B doped films shows that the dielectric loss is increasing with increasing wavelength in the visible range to near infrared region (400- 1100) nm. The electrical studies were carried out using four-probe method. The electrical study of the deposited films confirms that the temperature dependent resistivity of the deposited films is increasing with B doping concentration. The resistivity was found to be of the order of 103-m. The activation energy of the deposited films at high temperature (378 - 448) 0K gradually decreases with B doping concentration from 0.2923 eV to 0.2466 eV and at low temperature region the activation energy varies from 0.1655 eV to 0.0447 eV. Form the surface morphology study, we found less porous films and from the electrical study we found that the conductivity of the deposited thin films are decreasing with B doping concentration. This study reveals that B doped Bi2O3 thin film could be used in fuel cell applications and other optoelectronic devices more effectively.