230 research outputs found
Ab Initio Study of Phase Stability in Doped TiO2
Ab-initio density functional theory (DFT) calculations of the relative
stability of anatase and rutile polymorphs of TiO2 were carried using
all-electron atomic orbitals methods with local density approximation (LDA).
The rutile phase exhibited a moderate margin of stability of ~ 3 meV relative
to the anatase phase in pristine material. From computational analysis of the
formation energies of Si, Al, Fe and F dopants of various charge states across
different Fermi level energies in anatase and in rutile, it was found that the
cationic dopants are most stable in Ti substitutional lattice positions while
formation energy is minimised for F- doping in interstitial positions. All
dopants were found to considerably stabilise anatase relative to the rutile
phase, suggesting the anatase to rutile phase transformation is inhibited in
such systems with the dopants ranked F>Si>Fe>Al in order of anatase
stabilisation strength. Al and Fe dopants were found to act as shallow
acceptors with charge compensation achieved through the formation of mobile
carriers rather than the formation of anion vacancies
Freeze Casting: From LowāDimensional Building Blocks to Aligned Porous StructuresāA Review of Novel Materials, Methods, and Applications
Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of wellācontrolled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freezeācasting processes are described and the relationships between processing and structure are examined. Recent progress in freezeācasting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored microā and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freezeācast geometriesābeads, fibers, films, complex macrostructures, and nacreāmimetic compositesāare presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freezeācasting techniques and lowādimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.TU Berlin, Open-Access-Mittel - 202
Theoretical insights into the hydrophobicity of low index CeO2 surfaces
The hydrophobicity of CeO2 surfaces is examined here. Since wettability
measurements are extremely sensitive to experimental conditions, we propose a
general approach to obtain contact angles between water and ceria surfaces of
specified orientations based on density functional calculations. In particular,
we analysed the low index surfaces of this oxide to establish their
interactions with water. According to our calculations, the CeO2 (111) surface
was the most hydrophobic with a contact angle of {\Theta} = 112.53{\deg}
followed by (100) with {\Theta} = 93.91{\deg}. The CeO2 (110) surface was, on
the other hand, mildly hydrophilic with {\Theta} = 64.09{\deg}. By combining
our calculations with an atomistic thermodynamic approach, we found that the O
terminated (100) surface was unstable unless fully covered by molecularly
adsorbed water. We also identified a strong attractive interaction between the
hydrogen atoms in water molecules and surface oxygen, which gives rise to the
hydrophilic behaviour of (110) surfaces. Interestingly, the adsorption of water
molecules on the lower-energy (111) surface stabilises oxygen vacancies, which
are expected to enhance the catalytic activity of this plane. The findings here
shed light on the origin of the intrinsic wettability of rare earth oxides in
general and CeO2 surfaces in particular and also explain why CeO2 (100) surface
properties are so critically dependant on applied synthesis methods
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