Esat Pehlivan

Fractal dimensions of niobium oxide films probed by protons and lithium ions

Cyclic voltammetry (CV) and atomic force microscopy (AFM) were used to determine fractal surface dimensions of sputter deposited niobium pentoxide films. Peak currents were determined by CV measurements. Power spectral densities obtained from AFM measurements of the films were used for calculating length scale dependent root mean square roughness. In order to compare the effect of Li and H ion intercalation at the fractal surfaces, LiClO4 based as well as propionic acid electrolytes were used. The CV measurements gave a fractal dimension of 2.36 when the films were intercalated by Li ions and 1.70 when the films were intercalated by protons. AFM measurements showed that the former value corresponds to the fractal surface roughness of the films, while the latter value is close to the dimensionality of the distribution of hillocks on the surface. We conclude that the protons are preferentially intercalated at such sites.

Electrochromic Properties of Pure and Doped Nb2 O5 Thin Films

Niobium pentoxide is one of the promising materials in electrochromic applications. In this study we investigate electrochromic properties of sol-gel dip coated undoped and ZrO2 doped Nb2O5 thin films. Nb2O5 sol was mixed with 5 vol. % and 10 vol. % of ZrO2 sol. Coated samples were subjected to heat treatment at 550°C for crystallization. Electrochromic and optical properties of these films were investigated by using cyclic voltammeter and spectrophotometer. It has been observed from cyclic voltammetry measurements heat treatment causes a deterioration in the charge density of the pure Nb2O5 films. Introduction Some metal oxides such as WO3, Nb2O5, MoO3, Co2O3, CeO2, TiO2 and MnO2 exhibit the ability of changing their optical properties when an external voltage is applied. This property is called electrochromism. These metal oxides are very suitable to produce smart windows in buildings, electronic displays or mirrors [1, 2]. Interest in Nb2O5 has increased in the last decade because of its promising electrochromic properties. Sol-gel is the most used method to obtain electrochromic Nb2O5 films [3-9]. In this work, undoped and ZrO2 doped Nb2O5 thin films were coated using sol-gel dip coating method. These films were also heat treated at 550°C. Electrochromic and optical properties of these films were investigated. Experimental Niobium (V) ethoxide (Nb(OC2H5)5, 99.95%, Aldrich) and zirconium (IV) propoxide (Zr(OC3H7)4, 70 wt. % solution in 1-propanol, Aldrich) were used as precursors for preparing Nb2O5 and ZrO2 sols, respectively. The Nb2O5 sol was doped with ZrO2 sol at the volume ratios of 5 vol. % and 10 vol. %. Microscope slides (Corning 2947) and In2O3:SnO2 (ITO) coated glasses were used as substrates. The substrates were ultrasonically cleaned in deionized water and acetone, respectively, and they were dried in a hot air stream. Dip coating method at 134 mm/min dipping speed was used for coating. The substrates were coated as much as 9 layers to increase the thickness. The films were heated at 100°C for 30 minutes between successive coatings by a microprocessor-controlled furnace (Carbolite, CWF 1100). After finishing the coatings the samples were heated from room temperature to 550°C (approximately 2.5°C/min heating rate) and annealed at 550°C for 15 minutes for crystallization. A computer controlled Parstat 2263 Advanced Electrochemical System (Princeton Applied Science) potentiostat/galvanostat were used for cyclic voltammetry and chronoamperometry measurements. Electrochromic measurements were done by using a 3-electrode cell containing a 0.1 M solution of LiClO4 dissolved in propylene carbonate as the electrolyte. The reference electrode was a silver wire and the counter electrode was a platinum wire. Scanning rate of 50 mV/sec was used for cyclic voltammetry measurements. Time per step of 20 sec was used for chronoamperometric measurements. Optical transmittance values of the films were measured by a Perkin-Elmer Lambda-2 model spectrophotometer over the spectral range from 200 to 1100 nm at normal incidence. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 375-378 doi:10.4028/www.scientific.net/KEM.264-268.375