دانلود رایگان مقاله لاتین انرژی سطحی بر ثبات آناتاز از سایت الزویر
عنوان فارسی مقاله:
اثرات انرژی سطحی بر ثبات آناتاز و روتایل نانوبلورها: نمودار پیش بینی برای دوپ پنتا اکسید نیوبیم دی اکسید تیتانیوم
عنوان انگلیسی مقاله:
Surface energy effects on the stability of anatase and rutile nanocrystals: A predictive diagram for Nb2O5-doped-TiO2
سال انتشار : 2017
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مقدمه انگلیسی مقاله:
1. Introduction
Titanium dioxide presents three main polymorphs: anatase, rutile and brookite. All three polymorphs can be readily synthesized in the laboratory and typically, the metastable anatase and brookite will transform into the thermodynamically stable rutile upon calcination at temperatures exceeding 600 ◦C [1]. Some studies have been performed on the thermodynamic viewpoint to better understand the stability of the TiO2 polymorphs [2–7]. Even though rutile is the most stable form of TiO2 and anatase is known as a metastable phase, at the nanoscale, anatase becomes more stable than rutile [2,8]. This is related to the competition between surface and bulk energies, leading to a crossover in thermodynamic stability when particle sizes are decreased. Anatase thus shows a range of particle sizes in which it is thermodynamically stable as long as coarsening is avoided [2]. Several dopants have been studied targeting the control of TiO2 stability at the nanoscale and its properties. Among them, niobium improves TiO2 performance for oxygen sensing [9–11], solar cells applications [12,13], and photocatalysis [14–16]. When dopingTiO2 with Nb2O5, since the ionic radius of Nb5+ (0.64 Å) is only slightly larger than Ti4+ (0.605 Å), Nb5+ forms solid solution in the lattice of TiO2. One Ti cationic vacancy is then created per every four Nb ions to compensate the excess charge, or a stoichiometric reduction of Ti4+ to Ti3+ can take place to accommodate it [17]. Atanacio et al. [18] have showed that niobium has a potential to segregate on the surface of TiO2 under oxidizing conditions. The segregation tendency is enhanced by the presence of titanium vacancies, which are formed at the TiO2/O2 interface and trapped at the surface. The use of dopants prone to segregation has been cited as a tool to decrease interface energies in oxides [19–24] and observed to follow Eq. (1): s = 0 + sHseg (1) where s is the surface energy of the doped material; 0 represents the surface energy of undoped material; s is the solute excess at the surface; and Hseg is the enthalpy of segregation. From Eq. (1), one can draw the conclusion that the surface energies decrease with an increase in the surface excess of the dopant, a concept well established for liquids by Gibbs’ adsorption isotherm. If a dopant is changing the surface energy of an oxide with intrinsic polymorphism, the stability of particular phases can be remarkably affected when the surface area is enlarged (smaller grain sizes). As demonstrated for Y and Ca doped zirconia [25,26], a http new phasediagramcomplementing the conventionaltemperaturecomposition diagram must be introduced, with the grain size as a new key variable to account for the surface energetic contribution. Quantitative prediction of the effect of the dopant on the surface energy is therefore of interest to enable phase stability design. Many methods have been proposed in the literature to measure the surface energy of oxides [19]. Among them, water adsorption microcalorimetry has recently demonstrated potential to accurately assess surface energies of doped systems [27] and was recently explored in TiO2 based systems [28]. The principle of this technique is the relationship between the heat of adsorption of water molecules on the surface of the oxide and the surface energy itself. An adsorption theory previously described in details enables calculating the surface energy of the anhydrous surface state or at a given hydration condition [27,29]. In this work, surfaces energies of 0 to 2 mol% Nb2O5-doped TiO2 were measured using water adsorption microcalorimetry [29]. The data was combined with oxide melt drop solution enthalpy to deliver reliable thermodynamic information on the bulk energetics of the system and the results used to build a new, predictive nanophase diagram for anatase-rutile in the studied compositional range
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کلمات کلیدی:
Modeling the Morphology and Phase Stability of TiO2 Nanocrystals in ... pubs.acs.org/doi/pdf/10.1021/ct0499635 by AS Barnard - 2005 - Cited by 163 - Related articles tension of low index hydrated stoichiometric surfaces of anatase and rutile, which are presented ... affect the crystallinity and phase transition temperature. In. [PDF]Particle size effects on transformation kinetics and phase stability in ... www.minsocam.org/msa/AmMin/toc/Articles_Free/1997/Gribb_p717-728_97.pdf by AMYA GRIBB - 1997 - Cited by 640 - Related articles estimates are used to show that a 15% greater surface energy for rutile causes the total free energy of rutile to ... GRIBB AND BANFIELD: NANOCRYSTALLINE TiO2 membranes for gas ... The effect of average crystallite size on anatase to ru-. Handbook of Soil Sciences: Resource Management and Environmental ... https://books.google.com/books?isbn=1439803072 Pan Ming Huang, Yuncong Li, Malcolm E. Sumner - 2011 - Science Resource Management and Environmental Impacts, Second Edition Pan ... Dislocations are created when nanocrystalline materials grow by oriented ... 4.2.1.2 Stability The stability of NPs is a function of their surface energy, and ... For example, the surface energies of small rutile particles are higher than those for anatase ... [PDF]Structure-Reactivity Relationships of Anatase and Rutile TiO2 ... citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.428.6408&rep=rep1... by L Österlund - 2010 - Cited by 7 - Related articles defined anatase and rutile TiO2 nanocrystals correlations between the ... surface energy of anatase, while the {110} crystal faces of rutile have much lower surface ... fraction of {001} faces by using fluorine-terminated surfaces to change the relative stability of .... Field-induced polarization effects and state of aggregation may. Surface energy effects on the stability of anatase and rutile nanocrystals isi-dl.com/item/76958 Surface energy effects on the stability of anatase and rutile nanocrystals: A predictive diagram for Nb2O5-doped-TiO2. sciencedirect.com ...
