Nanoscale Effects on Thermodynamics and Phase Equilibria in Oxide Systems

Navrotsky, Alexandra

doi:10.1002/cphc.201100129

Cited by 176 publications

(201 citation statements)

References 53 publications

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“…2 nm, would stabilize the cubic phase) [35,36]. Obviously, this particle-size driven structural transition is not a ZrO 2 exclusive situation [37]. It has been frequently concluded that microparticulated samples of these oxides adopt the thermodynamically stable structural variety, whereas those shown by nanoparticulated ones are metastable [38].…”

Section: Resultsmentioning

confidence: 98%

Large scale synthesis of nanostructured zirconia-based compounds from freeze-dried precursors

Gómez¹,

Villanueva²,

Vie³

et al. 2013

Journal of Solid State Chemistry

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a b s t r a c tNanocrystalline zirconia powders have been obtained at the multigram scale by thermal decomposition of precursors resulting from the freeze-drying of aqueous acetic solutions. This technique has equally made possible to synthesize a variety of nanostructured yttria or scandia doped zirconia compositions. SEM images, as well as the analysis of the XRD patterns, show the nanoparticulated character of those solids obtained at low temperature, with typical particle size in the 10-15 nm range when prepared at 673 K. The presence of the monoclinic, the tetragonal or both phases depends on the temperature of the thermal treatment, the doping concentration and the nature of the dopant. In addition, Rietveld refinement of the XRD profiles of selected samples allows detecting the coexistence of the tetragonal and the cubic phases for high doping concentration and high thermal treatment temperatures. Raman experiments suggest the presence of both phases also at relatively low treatment temperatures.

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Section: Resultsmentioning

confidence: 98%

Large scale synthesis of nanostructured zirconia-based compounds from freeze-dried precursors

Gómez¹,

Villanueva²,

Vie³

et al. 2013

Journal of Solid State Chemistry

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“…The low SE of the layered structure CaMnO materials (significantly smaller than for Mn 3 O 4 , Mn 2 O 3 , or MnO 2 ) suggests that water is not strongly bound to the surface sites, because, for many oxides, the enthalpy of H 2 O chemisorption becomes more exothermic with increasing SE (28,(34)(35)(36). In comparing TiO 2 and SnO 2 (rutile structure), Ma et al (35) have argued that the better gas-sensing ability of the latter stems from a lower surface energy, lower water coverage on the surface, and weaker binding of surface water, enabling energetically easier access of other gas molecules to the SnO 2 surface.…”

Section: Discussionmentioning

confidence: 97%

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

Birkner

Nayeri²,

Pashaei³

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Previous measurements show that calcium manganese oxide nanoparticles are better water oxidation catalysts than binary manganese oxides (Mn 3 O 4 , Mn 2 O 3 , and MnO 2 ). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodynamic stability and a high surface area, their low surface energy suggests relatively loose binding of H 2 O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidation enthalpy independent of the Mn 3+ / Mn 4+ ratio and much smaller in magnitude than the Mn 2 O 3 -MnO 2 couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidation states.nanomaterials | thermochemistry R esearch on calcium manganese oxides (CaMnOs) has been inspired by the water-oxidation centers in photosystem II (1-5), which is a manganese-calcium cluster Mn 4 CaO 5 (H 2 O) 4 supported by a protein environment. Because the capability for efficient water oxidation is unique to photosystem II among all biological photosystems (6, 7), these CaMnO materials that mimic the elemental composition, manganese oxidation state, and particle size of the photosynthetic water oxidation center are of special interest (8)(9)(10)(11)(12). Although a number of other metal compounds function as water-oxidizing catalysts (13), many contain rare and expensive metals like iridium and ruthenium; the advantage of manganese oxides (Mn-oxides) is that they are earth-abundant, inexpensive, and environmentally friendly (1)(2)(3)(4)(5)14).CaMnO phases have short-range order structure and lamellar morphology (12), which are hallmarks of phyllomanganates (classically known as hydrous Mn-oxides), and they possess a layered structure (15)(16)(17)(18)(19)(20). In this family of structures, manganese octahedra (and vacancies) form the layers, and charge-balancing cations (i.e., alkali and alkaline earth ions, protons) and water occupy the interlayer space.Our previous calorimetric studies of various oxide nanoparticles, including those of manganese, iron, and cobalt (21), show that different polymorphs and phases with different oxidation states have significantly different surface energies. For particle sizes below 100 nm, these differences affect the free energies of phase transitions and of oxidation-reduction reactions, shifting the latter by as much as several log(fO 2 ) units at a given temperature. This thermodynamic effect on redox equilibria is significant when one considers electrochemical potentials for water splitting or other catalytic reactions involving nanoparticles (14). In a chargecompensated layered structure, the Mn(III)/Mn(IV) equilibrium will be different from that of binary Mn-oxide phase assemblages, namely, Mn 3 O 4 (hausmannite), Mn 2 O 3 (bixbyite), and MnO 2 (pyrolusite). Thus, particle size, morphology, crystal structure, and mixed valence in the more complex ...

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“…Although the relative stability of the polymorphs depends on bulk properties such as the enthalpy of formation and molar volume, the contribution of surface free energy often results in a dependence of stability on crystal size (58,59). This dependence can even invert the sequence of polymorph stability relative to that observed for the bulk phases (58).…”

Section: The Influence Of Surface Energy On Pathwaysmentioning

confidence: 99%

“…Cryo-TEM observations showing coaligned arrays of particles that appear separated by a solvent layer underline the importance of the solvent in mediating attachment (62). Because the solvation energy of a surface generally becomes more exothermic with increasing surface energy (59), the dynamics of CPA should also be affected by surface energies. In particular, high-energy surfaces with loosely held solvent may be more reactive toward other species in the solution, including other particles.…”

Section: The Influence Of Surface Energy On Pathwaysmentioning

confidence: 99%

Crystallization by particle attachment in synthetic, biogenic, and geologic environments

Yoreo

Gilbert

Sommerdijk

et al. 2015

Science

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Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

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Nanoscale Effects on Thermodynamics and Phase Equilibria in Oxide Systems

Cited by 176 publications

References 53 publications

Large scale synthesis of nanostructured zirconia-based compounds from freeze-dried precursors

Large scale synthesis of nanostructured zirconia-based compounds from freeze-dried precursors

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

Crystallization by particle attachment in synthetic, biogenic, and geologic environments

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