"Energetics of Nanomaterials" Page: 3 of 14
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03415/2005 TUE 09:23 FAT -07529307 UCD NEAT ORU THERMO
Treat Walker BYi undergrad Cp BYU
Aaron Lewis BYU undergrad Cp Wisconsin Med College
Tyler Meldrum BYU undergrad Cp BY
Thomas Par-y BYU undergrad Cp BYU
TiO, AT THE NANOSCALE
The phases of TiO; (rutile. anatasc, brookite) arc an excellent system To study the interplay of
polymorphism, particle size, and hydration. Our goal has been to study these effects on enthalpy, heat capacity,
entropy, and f&o energy, and to link these properies to a molecular-level understanding of structure and bonding.
We realized at the beginning of this project that obtaining a suite of welt-defined materials with controlled
chemistry, particle size, and water content was a crucial first step of our studies. This has been a larger focus of the
project than initially envisioned but it has paid off well yielding both ftmdancotal understanding and excellent
samples for experiments.
Synthesis and coarsening kinetics
Recognizing the importance of well-characterized samples, we have developed and improved upon several
synthetic methodologies that allow us to produce both phase pure and chemically pure nanoparticles. First, using a
hydrothermal technique we synthesized rutile nanorods with high crystallinity by starting with the hydrolysis of
TiCS and we used a modified sol-eel method to obtain high quality spherical anatase nanoparticles. Details can be
found in our publication [Li 2003].
Secondly, we have produced nanopar:icles free of adsorbed chlorine or organic species. Rutile nanorods
and anatase nanosphcres were achieved in 40 g quantities with extremely low impurities by numerous washings of
the reaction samples using a centrifuge until the pH of the wash solution was approxi-nately 7. The resulting rutile
and anatase nanoparticles were white: analysis by a commercial laboratory found only carbon and chlorine at the
ppm level in both samples.
Thirdly, we characterized the samples extensively using a variety of techniques, including: (i) XRD, which
showed single-phase rutile and anatase nanoparticles, (ii) TEE, which clearly showed that rutile nanoparticles are
rod-shaped and anatase nanoparticles are spherical with uniform size distributions. (iii) Various wet and dry
4hcmica-U analyses, which showed impurities only on the ppm level. (iv) TQ-DSC, which showed that both TiO
nanoparticles dehydrated over a wide temperature range to about 750 0C and quantified the water content of the
samples as a function of particle size (see below). (v) EPR. which showed the absence of Ti' in both rutile and
anatase samples. (vi) Raman, which gave the standard spectra for single phase anatase or rutile but showed
systematic shifts in Raman mode frequencies with increasing particle size (see Section 4.4). (vii) IR, which showed
the presence of hydration layers in TiO2 nanoparticles (the size of the hydration layers become larger with
decreasing particle size, consistent with our TC-DSC measurements). (viii) BET, which gave surface areas
consistent with our XRD and TEM results.
We have also been able to develop irnproved models that describe the grain growth kinetics for TiO2
nanopar-cles, giving us precise control of the particle sizes. Using TiOCII as the starting material (do = 0), we
obtained rtile nanoparticles in a single step with temperature, T.
6 6 and reaction time, t, nas two effective variables to control the
- particle sizes. The rutile particle size (rod diameter), D, and t
5 'Q follow the relationship InD=2.74(4)+0.20(2)linr (see
Figure.1). We also found a linear relationship between In (D) and
-- 3n 9 t= .5(3) -4.1(1) x 0/T Combining both equations, we
3--a E - obtained a generalized model for the grain growth kinetics of
__ rutile nanocrystals under hydrothermal conditions,
0 2 4 6 2' = 6.94 x 10-- - -' where E4= 170.8 k/mcI
The grain growth kinetics for anatase nanoparticles
Figure. Rate cfgrain growth for rutie TiOr synthesized using a so-gel method can be significantly different
nanopw-ices. ABITJ - ofher 199ij from those synthesized using hydrothermal methods or vapor
Work done at B177
ER 15237, "Energetics ofNanomaterials"
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Navrotsky, Professor Alexandra. "Energetics of Nanomaterials", report, January 31, 2005; United States. (digital.library.unt.edu/ark:/67531/metadc778992/m1/3/: accessed September 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.