JOURNAL OF CHEMISTRY Materials Formation of uniform size anatase nanocrystals from bis(ammonium lactato)titanium dihydroxide by thermohydrolysis Hermann Mo Èckel,* Michael Giersig and Frank Willig Hahn-Meitner-Institut Berlin GmbH, Abt. CD, Glienicker Str. 100, D 14109 Berlin, Germany. E-mail: moeckel@hmi.de Received 5th July 1999, Accepted 13th September 1999 Bis(ammonium lactato)titanium dihydroxide decomposes in neutral aqueous solution at temperatures above 100 ³C to produce anatase nanocrystals of preferentially oblate habitus, and with a very narrow size distribution. There is almost no dependence of size on precursor concentration, and a time dependence is observed only in the very beginning of crystal growth. The crystal size is mainly determined by the reaction temperature, increasing from ca. 2 nm at 120 ³C to about 20 nm at 300 ³C. We assume the temperature dependent formation of a layer of byproduct ammonium lactate at the crystallite surface where it interferes with growth reactions. After removal of this layer, continued heating yields larger crystallites in typical anatase shapes, indicating that Ostwald ripening has now become effective. Introduction Nanocrystalline TiO 2 can be prepared in various ways. One of the most common is hydrolysis and polycondensation of titanium tetraalkoxides (e.g. isopropoxide) or TiCl 4 . 1±10 We have found bis(ammonium lactato)titanium dihydroxide (ALT) to be an interesting and quite convenient starting material in TiO 2 crystallite synthesis. Recently, ALT has also been mentioned as a new base material in preparing catalysts, 11 in making electrodes for medical treatment and diagnosis, 12 and in generating UV-protective ®lms on various surfaces. 13 While titanium alkoxides hydrolyse rapidly, ALT is stable at ambient temperature in neutral solution. Aqueous NaOH decomposes ALT to yield TiO 2 , NH 3 and sodium lactate. High pH hydrolysis of ALT is slower and more easily controlled than titanium alkoxide decomposition. We will report on this reaction elsewhere. 14 Acid decomposition of ALT has recently been described by Baskaran et al. 13 Thermohydrolysis of ALT appeared particularly interesting since preliminary experiments yielded good, almost mono- disperse nanocrystals. Nanocrystalline TiO 2 having well de®ned size and narrow size distribution would be desirable for making thin ®lms. The present work was carried out in order to investigate the possible in¯uence of various experi- mental parameters (concentration of precursor ALT and of byproduct ammonium lactate, temperature and time exposed to high temperature) on crystal growth and size distribution. The products were analyzed using transmission electron microscopy (TEM). Experimental ALT is available (Aldrich) as a 50% (w/w) aqueous solution (ALT50). It was used as received or diluted as necessary with de-ionized water. [ALT, M~294.12; ALT50, d~1.222 kg L 21 , C~2.08 mol L 21 ]. Ammonium lactate (HAL) was obtained (Aldrich) as a 20% aqueous solution [HAL, M~107.11, d~1.054 kg L 21 ]. Thermal decomposition experiments up to 300 ³C were performed in sealed glass ampoules, or, in a few cases, in a 15 mL titanium autoclave at higher temperature. A sealed ampoule was placed in an empty HPLC column, 125620 mm SS (pressure tube). The tube was ®lled with water and pressurized by an HPLC pump (Knauer 64), using a discarded HPLC column (25064 mm, 5 mm packing) or a suf®cient length of 0.1 mm I.D. capillary as a restrictor. The pressure was adjusted via the ¯ow rate to a value above the vapor pressure of water at the desired temperature. The pressure tube was suspended vertically in an adjustable temperature oven, and heated to 200 ³C within 15 min. During heating and cooling periods, strong pressure changes occur due to the thermal expansion behaviour of water. To avoid ampoule cracking, these changes were compensated for by manually adjusting the ¯ow rate. Ampoules from high temperature (y250±300 ³C) reactions have to be opened cautiously because the internal pressure might be very high due to secondary reactions. HPLC tests were performed on a 25064 mm ODS column (Nucleosil 10 mm grain, 10 nm pores) with MeOH±H 2 O (80 : 20) eluent and UV detection at 254 nm. In some experiments, organic components were removed from the product. An aqueous solution of ALT (0.415 mol L 21 ) was heated to 260 ³C for 24 h. The product was centrifuged and the liquid decanted. The precipitate was washed using ultrasound and centrifugation, ®rst with diluted NH 3 and methanol in water, then with water. For yield determination, samples were ultrasonically treated for maximum homogeneity. Aliquots of 50 mL were dried at 100 ³C, then baked at 300 ³C for 19 h and, after weighing, at 400 ³C for another 19 h. There was no further weight decrease after the 300 ³C treatment, indicating that all organic material had been removed. The TiO 2 samples were examined using a Philips CM12 (accelerating voltage 120 kV) electron microscope equipped with a super twin lens and EDAX analyzer. High resolution images were taken using a slow scan CCD camera under Scherze defocus conditions. 15 For the HRTEM studies the sample was adsorbed on electron microscopic grids coated with a very thin (50 A Ê ) amorphous carbon ®lm. Results General Under mild reaction conditions (0.153 g ALT mL 21 , 200 ³C for 3 days) there was no indication of new compounds being formed except for TiO 2 and probably ammonium lactate. In particular, no new absorptions were seen in the UV-VIS spectrum, except for some transmission decrease due to J. Mater. Chem., 1999, 9, 3051±3056 3051 This Journal is # The Royal Society of Chemistry 1999 Published on 01 January 1999. Downloaded on 12/10/2015 09:10:48. View Article Online / Journal Homepage / Table of Contents for this issue