ORIGINAL PAPER The effect of heat treatment in the reducing atmosphere on the physical properties of TiO 2 thin films prepared by sol–gel method Maryam Sadat Ghorashi • Azarmidokht Hosseinnia • Fereydoun Alikhani Hessari • Yadolah Ganjkhanlou Received: 23 January 2013 / Accepted: 11 May 2013 Ó Springer Science+Business Media New York 2013 Abstract Multilayered nanostructured TiO 2 thin films were prepared by sol–gel and dipping deposition on quartz substrate followed by thermal treatment under reducing atmosphere (20 %H 2 –80 %Ar). Heat treatment at progres- sively higher temperatures caused structural, morphological, and optical changes, which were investigated by X-ray dif- fraction (XRD), atomic force microscopy, scanning electron microscopy, and UV–Vis spectroscopy. The conductivities of the thin films were also measured by 4-point probe method. The XRD results showed that the calcined TiO 2 thin films consist of single anatase phase which was completely transformed into rutile phase after heat treatment at 1,000 °C. The grains of films grew by intra-agglomerate densification after heat treatment at higher temperatures. The root mean square roughness of the samples was found to be in the range of 0.58–3.36 nm. The partially reduced TiO 2 samples have red-shifted transmittance bands due to new energy band formed by oxygen vacancies. The electrical conductivity of the films was also enhanced after heat treatment in reducing atmosphere. Keywords TiO 2  Non-stoichiometry  Reducing atmosphere  Electro-optics 1 Introduction TiO 2 materials are promising compounds for variety of applications including photo-electrochemical water-splitting, degradation of environmental pollutants [1–3], solar cells [4], gas sensors [5], and optoelectronics [6] devices. Unfortu- nately as a result of its wide band gap (3.2 eV for the anatase phase), TiO 2 absorbs only a small fraction (about 5 %) of the solar energy. This is especially a matter of concern for the generation of hydrogen from water to reach commercial efficiencies. Accordingly, there has been many efforts to increase the photoactivity of TiO 2 under visible light region by ion implantation, doping with metals, electron beam and plasma treatment [7–11]. In order to enhance the overall efficiency of the photo-electrochemical devices, the con- comitant increasing of the quantum efficiency and the elec- trical conductivity of TiO 2 is an important issue for study [12]. Oxygen deficiency in TiO 2 is reported to yield enhanced photocatalytic activity under a visible light environment and improve the electrical conductivity [13–15]. TiO 2 films have been prepared by various techniques such as sol–gel [16–18], evaporation [19], pulsed laser deposition [3], reactive sputtering [20], spray pyrolysis [21], and chemical vapor deposition [22]. Among the above mentioned methods, sol–gel is one of the most uti- lized techniques as a consequence of its low processing cost, simplicity, excellent homogeneity and possibility to tailor the film properties. TiO 2 thin films with desired physical properties can be achieved by varying film processing conditions. There are many reports about the effect of annealing or calcination temperature on the TiO 2 film properties [23–25]. However, in none of them the heat treatment procedure was applied in the reducing atmosphere with the intention to produce a deviation from the stoichiometry. The formation of oxygen vacancies and Ti 3? is probable in TiO 2-x . However, the predominant defect is not definitely known yet [26, 27]. In this study, TiO 2 films were prepared by sol–gel route and exposed to thermal treatment in the reducing atmosphere. A M. S. Ghorashi  A. Hosseinnia (&)  F. A. Hessari  Y. Ganjkhanlou Materials and Energy Research Center, P.O. BOX 14155-4777, Tehran, Iran e-mail: a-hosseinnia@merc.ac.ir 123 J Sol-Gel Sci Technol DOI 10.1007/s10971-013-3069-7