Direct synthesis of anatase TiO 2 aerogel resistant to high temperature under supercritical ethanol Yong Kong a,b , Xiao-Dong Shen a,b,n , Sheng Cui a,b a College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China b State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China article info Article history: Received 25 August 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: TiO 2 aerogel Sol–gel Supercritical ethanol drying Crystal structure Nanocrystal abstract TiO 2 aerogel (TA) was directly prepared by supercritical ethanol drying of TiO 2 gel which was synthesized by a simple one-step sol–gel process. High temperature of the supercritical ethanol drying led to the decomposition of most of the organic groups in the TiO 2 gel and formation of anatase TiO 2 . Thermogravimetric (TG) revealed that the resulting TA was anatase TiO 2 with a small amount of organic groups. X-ray diffraction (XRD), scanning electron microscope (SEM) and N 2 isothermal adsorption/ desorption tests indicated that the crystalline structure (crystalline size and phase) and pore structure of TA were very stable after thermal treatment below 700 1C. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Aerogels are unique porous materials with a distinctive micro- structure consisting of pores and particles in the nanometer size range, therefore, they possess excellent properties such as high surface area and porosity, large pore volume, low density and small particle size [1–3]. Among the aerogels that have been developed, TiO 2 aerogel (TA) is considered as a promising material with reference to the green world objectives. They are nontoxic and inexpensive, present a notable UV light absorption, photo- corrosion resistance, oxidative properties and can be used as catalysts, catalyst carrier, and solar cells [4–9]. Generally, aerogels are prepared by the sol–gel technique followed by supercritical drying [1–3]. Aerogels obtained from this process are amorphous which have a large number of organic groups. When the thermal treatment is performed to crystallize these amorphous aerogels, it may result in the disappearance of some unique properties of aerogels, such as high surface area, large pore volume and nanoscale particle [10,11]. Furthermore, the crystalline nature will be changed during heat treatment. For example, the photoactivity of TiO 2 catalysts is influenced by the physical properties of TiO 2 , such as crystal structure, particle size, surface area and porosity [12–15]. Therefore, we developed a direct synthesis of anatase TA by drying the gel in high-temperature supercritical ethanol. 2. Experimental Tetrabutylortotitanate (TBOT), nitric acid (HNO 3 , mass fraction of 65% in water), deionized water (W) and anhydrous ethanol (EtOH) were used as raw materials without further purification. TA was synthesized by a one-step sol–gel process followed by the supercritical ethanol drying. The as-prepared TA was treated in a muffle furnace for 3 h in air with a heating rate of 20 1C/min. The samples treated at 400, 500, 600, 700, and 800 1C were denoted as TA-400, TA-500, TA-600, TA-700, and TA-800, respectively. 3. Results and discussion In this work, the formation of anatase TA went through super- critical drying as well as the crystalline phase transformation in the supercritical ethanol apparatus. The high pressure and the ethanol in the vessel can avoid damaging TiO 2 gel by high temperature before the system reaches to supercritical condition. Under supercritical condition, the supercritical ethanol can avoid the agglomeration between nanoparticles [16]. Fig. 1 shows the Thermogravimetric (TG) curve of the as- prepared TA. A mass loss below 225 1C is due to the adsorbed impurities and moisture. A mass loss above 225 1C is due to decomposition of the organic groups in TA, such as –OH, CH 2 and CH 3 . It should be noted that the mass loss of TA is very low, approximately 4.4%. It is reasonable that the high-temperature supercritical condition leads to the decomposition of most of the organic groups in the TiO 2 gel. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.004 n Corresponding author at: Nanjing University of Technology, College of Materials Science and Engineering, Nanjing 210009, China. Tel.: þ86 25 83587235; fax: þ86 25 83221690. E-mail address: xdshen@njut.edu.cn (X.-D. Shen). Materials Letters 117 (2014) 192–194