TiO₂Stability Landscape:  Polymorphism, Surface Energy, and Bound Water Energetics

Abstract

The energetics of pure-phase rutile nanorods and spherical anatase nanoparticles have been studied by high-temperature drop solution calorimetry in 3Na 2 O‚4MoO 3 solvent at 975 K and water adsorption calorimetry in a wide range of particle sizes (surface area) from 6 to 40 nm (5-270 m 2 /g). The calorimetric surface enthalpies for rutile and anatase, calculated as 2.22 ( 0.07 and 0.74 ( 0.04 J/m 2 , respectively, are in general agreement with Ranade et al. 's results (Proc. Natl. Acad. Sci. U.S.A., 2002, 99, 6476), although their numerical values are somewhat different because of impurities and unaccounted bound water in the previous work. This study supports the energy crossovers previously proposed for the TiO 2 polymorphs. The energetics of water adsorption were measured using a commercial Calvet microcalorimeter coupled with a gas dosing system. This permitted the calculation of differential and integral enthalpies of water adsorption that characterize how tightly water binds to rutile and anatase surfaces and the calculation of adsorption entropies, which reflect the surface mobility of adsorbed water. The integral enthalpy of tightly bound water (relative to liquid H 2 O standard state) is -18 kJ/mol for anatase and -40 kJ/mol for rutile. As seen previously for Al 2 O 3 , the TiO 2 polymorphs with higher surface energy bind water more tightly. The calculated entropy changes for the adsorption of water on TiO 2 are more negative than the entropy changes for the condensation of gaseous water to hexagonal ice. This finding suggests highly restricted mobility of molecules adsorbed at initial stages of adsorption (low coverage) and, possibly, dissociative adsorption on both rutile and anatase surfaces. However, nanoparticles contain both tightly bound water and loosely bound water. The latter is characterized by energetics of bulk water. The stabilizing water contribution to the overall energy of the system makes the hydrated nanophase samples more stable. The recommended transformation enthalpy for bulk anatase to bulk rutile is -1.7 ( 0.9 kJ/mol.

Hubbard-Ucorrected Hamiltonians for non-self-consistent random-phase approximation total-energy calculations: A study of ZnS,TiO2, and NiO

“…[76]. Two recent experiments [76,77] found similar enthalpy differences of 0.027 and 0.017 eV/f.u. The measurement of this quantity is a significant experimental challenge, requiring careful control of impurity concentration and synthesis conditions [76].…”

Hydrothermal synthesis of anatase nanoleaves and size dependence of anatase–rutile transformation upon heating

“…The data obtained by Navrotsky et al [11,12] , Fig. 6, give a guideline for understanding this process where two opportunities may be foreseen.…”

“…Curves are numbered according to Table 1 Int Nano Lett (2016) 6:111-117 113 structure of the samples studied would be preserved. On the other hand, both anatase and brookite are metastable against the bulk rutile, and the smaller the particle, the more heat is evolved upon the anatase-bulk rutile transformation [12]. In this case, one may anticipate that should aggregation occur upon heating, rutile would be formed as the most thermodynamically stable phase, and the greater the S sp .…”

“…Preparation of materials often includes their thermal pretreatment, and this requires the knowledge regarding their phase state. Precise calorimetric measurements by Navrotsky and co-workers have shown [11,12] that depending on the size, the thermodynamic stability of the TiO 2 polymorphs at ambient temperatures is significantly different. For the smallest particles, the anatase structure is the most stable.…”

Determinação das energias de superfície do SnO2 puro e dopado

“…1 [24]; 57,4 kJ/mol [25]). Este resultado, no entanto, é consistente com outros dados reportados para óxidos [5,26]. Nota-se ainda no gráfico que a curva tem um decaimento rápido nesta energia, mostrando alguns "degraus" nas energias.…”