Titanyum dioksit katkılı niobyum pentoksit filmlerin optik, yapısal ve elektrokromik özellikleri

Optical, structural and electrochromic properties of titanium dioxide mixed niobium pentoxide films

Özet

Elektrokromik camlar, uygulanan bir gerilim ile optik özelliklerini değiştirebildikleri için son yıllarda yoğun ilgi konusu olmuşlardır. Bu camlara uygulanan gerilimin ters yönde çevrilmesi ile camların optik özelliklerinin tekrar eski durumuna geri dönmesi, elektrokromik camları teknolojik açıdan çok önemli bir konuma getirmektedir. Elektrokromik camlar; arabalarda (tavan camlarında, yan camlarda, ön camda ve dikiz aynalarında), binalardaki pencerelerde ve ekran uygulamalarında kullanılmaktadırlar. Bu camlar gerek mimari açıdan görsel bir güzellik sağlamakta, gerekse ısı ve ışık kontrolü yaptıkları için enerji tasarrufu sağlamaktadırlar. Niobyum pentoksit, elektrokromik açıdan oldukça verimli bir malzemedir. Birçok elektrokromik malzeme kristal halde iyi derecede elektrokromik özellik göstermezken niobyum pentoksit bu özelliği göstermektedir. Ayrıca, niobyum pentoksit amorf halde iken kahverengi, kristal halde iken mavi renklenme gösterir. Bu özelliği ile diğer elektrokromik malzemelerden ayrılır. Niobyum pentoksit, kendisine katkılanan malzemeye göre de farklı renkte elektrokromik renklenme gösterebilmektedir. Bu çalışmada saf niobyum pentoksitin yanısıra hacimce %20, %40, %60 ve %80 titanyum dioksit katkılı niobyum pentoksit filmler de hazırlanmıştır. Elde edilen filmlerin optik, yapısal ve elektrokromik özellikleri incelenmiştir. Bu filmlerin bazıları ısıl işleme tabi tutulmuş ve ısıl işlemin filmlere etkisi incelenmiştir. Titanyum dioksit katkı miktarının artması ile filmlerin yük tutabilme kapasitesinin arttığı gözlenmiştir.

Abstract

Electrochromism is the phenomenon displayed by some chemical species of reversibly changing color when a voltage is applied. As the color change is persistent and energy need only be applied to effect a change, electrochromic materials are used to control the amount of light and heat allowed to pass through windows, and has also been applied in the automobile industry to automatically tint rear-view mirrors in various lighting conditions. Electrochromic windows (or "smart windows") are windows that can be darkened or lightened electronically. A small voltage applied to the windows will cause them to darken; reversing the voltage causes them to lighten. This capability allows for the automatic control of the amount of light and heat that passes through the windows, thereby presenting an opportunity for the windows to be used as energy-saving devices. Electrochromic properties of niobium pentoxide were firstly reported in 1980. After the first study of electrochromic properties of sol-gel deposited niobium pentoxide in 1991 it became an extensively studied electrochromic material. Niobium pentoxide coated electrochromic films show both bronze and pale blue coloration if coated films are amorphous or crystalline, respectively. Niobium chloride (NbCh) and niobium ethoxide (Nb(OC2Hs)5) are the most used precursors to obtain niobium pentoxide sol. Sol-gel dip coating and spin coating methods are the most used methods to prepare niobium pentoxide sol. Not only sol-gel method but also reactive DC magnetron sputtering, indirect reactive sputtering, thermal oxidation, pulsed laser and chemical vapor deposition methods were frequently used for obtaining niobium pentoxide thin films. There are only a few studies concerning doped niobium pentoxide thin films. Some of these studies can be summarized as Sn, Zr, Ti, Mo, Li and titanium dioxide (TİO2). Although there are many studies concerning electrochemical properties of niobium pentoxide only a few of studies were interested in optical properties. Structural studies on niobium pentoxide showed that it is amorphous up to 450°C and starts crystallization above this temperature. At least 12 crystal structures of niobium pentoxide have been identified and the most often phases have been labeled as TT, T, M, B and H. Low temperature (~500°C), medium temperature (~800°C) and high temperature (~ 1000° C) forms of niobium pentoxide are called TT (or T), M (or B) and H phases, respectively (Ko and Weissman, 1990). Phase transformation in titanium dioxide has been widely studied for optical and electronic applications because they have a high refractive index, a high dielectric constant, high photocatalytic activity, and good physical and chemical stability (Oh et. ah, 2003). The high refractive index and low absorption coefficient of titanium dioxide make it suitable for optical coating in silicon solar cell and optical thin film device. Titanium dioxide films have also attracted attention for use in the fabricating capacitors of microelectronic devices due to their high dielectric constants. There are also some studies concerning electrochromic properties of titanium dioxide. In this study we investigated and compared the Structural, optical and electrochemical properties of titanium dioxide mixed niobium pentoxide films deposited by sol-gel dip coating method at 134 mm/min dipping rate. The films have high transmittance values of between 0.74 and 0.88. A steady increase was observed in the thickness of the films with increasing titanium dioxide concentration for both as deposited and heat treated films. Refractive indices of the films at 550 nm wavelength lie between 1.80 and 1.91. All as deposited films have band gap values about 3.35 ± 0.02 eV whereas those of heat treated films are about 3.18 ± 0.02 e V. Height profile analysis of three dimensional atomic force microscope pictures shows that surface smoothness of the films decreases with increasing titanium dioxide concentration. It was found that charge density values of the films increase with increasing titanium dioxide doping concentration for as deposited films. Charge density values of as deposited films are 5.5, 7.7, 8.8 and 13.3 mC/cm2for 0%, 5%, 10% and 15% titanium dioxide mixed niobium pentoxide films, respectively. On the other hand, charge density values of heat treated films did not show important variation with increasing titanium dioxide concentration.