High Purity Anatase TiO₂ Nanocrystals:  Near Room-Temperature Synthesis, Grain Growth Kinetics, and Surface Hydration Chemistry

Abstract

High purity, spherical anatase nanocrystals were prepared by a modified sol-gel method. Mixing of anhydrous TiCl(4) with ethanol at about 0 degrees C yielded a yellowish sol that was transformed into phase-pure anatase of 7.7 nm in size after baking at 87 degrees C for 3 days. This synthesis route eliminates the presence of fine seeds of the nanoscale brookite phase that frequently occurs in low-temperature formation reactions and also significantly retards the phase transformation to rutile at high temperatures. Heating the as-is 7.7 nm anatase for 2 h at temperatures up to 600 degrees C leads to an increase in grain size of the anatase nanoparticles to 32 nm. By varying the calcination time from 2 to 48 h at 300 degrees C, the particle size could be controlled between 12 and 15.3 nm. The grain growth kinetics of anatase nanoparticles was found to follow the equation, D(2) - D(0)(2) = k(0)t(m)e((-)(E)(a)/(RT)) with a time exponent m = 0.286(+/-9) and an activation energy of E(a) = 32 +/- 2 kJ x mol(-)(1). Thermogravimetric analysis in combination with infrared and X-ray photoemission spectroscopies has shown the anatase nanocrystals at different sizes to be composed of an interior anatase lattice with surfaces that are hydrogen-bonded to a wide set of energetically nonequivalent groups. With a decrease in particle size, the anatase lattice volume contracts, while the surface hydration increases. The removal of the surface hydration layers causes coarsening of the nanoparticles.

Review of the anatase to rutile phase transformation

“…The kinetics of these processes typically are considered in terms of temperature and time. In terms of the former, pure bulk anatase is considered widely to begin to transform irreversibly to rutile in air at *600°C [6,66,99]; however, the reported transition temperatures vary in the range 400-1200°C [42,66,[108][109][110][111][112] owing to the use of different methods of determining the transition temperatures, raw materials, and processing methods. The anatase to rutile transformation is not instantaneous; it is timedependent because it is reconstructive [2,109,113].…”

“…Excluding studies which used titania of unusually small particle size or long soak times, and studies which did not use onset temperatures as the defined transition These data indicate that there remains some uncertainty about the energetics of the phase transformation Appearance of detectable rutile peak by XRD [99] 2007 (900) Sol-gel synthesised powder Reported value: apex of broad DTA peak (data not shown) [158] a Data in parenthesis do not correspond to the onset of the transformation temperatures, it can be seen that the reported transition onset temperatures, as determined by XRD, appear to converge around 600°C. Although it is difficult to ascertain the intrinsic behaviour of titania, this is accepted to be the region of the onset temperature of the anatase to rutile transformation in bulk pure anatase in air [2,7,123,154].…”

“…When titanium alkoxides undergo hydrolysis, the result is agglomorated titanium hydroxide groups which are often in the form of white amorphous precipitates [46,95,204]. Such solutions also can be synthesised using TiCl 4 [96,99,103,104,202] which can result in crystalline phases of high purity. The use of a low pH limits condensation reactions [197], imposes a repulsive charge and limits the size of these precipitates resulting in the presence of free [Ti(OH) x (OH 2 ) 6-x ] (4-x)?…”

Thickness Effect on Fluctuation of Electron States in Thin Film and Implications for Lattice Constant Change Due to Size Reduction

“…An interesting phenomenon has been observed in anatase TiO 2 . In some cases, contraction of the lattice parameter when reducing the particle size has been observed [39][40][41][42][43], which is similar to the behavior of metals. In other cases the antithetical phenomenon was investigated, where expansion of the lattice parameter was observed when reducing the material size [44].…”

Toxicity Associated with the Photo Catalytic and Photo Stable Forms of Titanium Dioxide Nanoparticles Used in Sunscreen lotion

“…The main toxicity of these nanoparticles was shown to be due to surface impurities, 87 which often occur as a result of the synthesis process. The use of titanium alkoxides as synthesis precursors, [88][89][90][91] as well as the use of acetic acid, 89 alcohol, 92 oils, 93 and other organic solvents 88,91,92 as reaction media are examples of such sources. Post-synthetic stabilization of nanoparticles through the use of surfactants also introduces extraneous material which may interfere with the accurate observation of the toxicity phenomenon.…”