The behavior of excess electrons in undoped and defect free bulk anatase and rutile TiO2 has been investigated by state-of-the-art electronic structure methods including hybrid density functional ...theory (DFT) and the random phase approximation (RPA). Consistent with experiment, charge trapping and polaron formation is observed in both anatase and rutile. The difference in the anisotropic shape of the polarons is characterized, confirming for anatase the large polaron picture. For anatase, where polaron formation energies are small, charge trapping is observed also with standard hybrid functionals, provided the simulation cell is sufficiently large (864 atoms) to accommodate the lattice relaxation. Even though hybrid orbitals are required as a starting point for RPA in this system, the obtained polaron formation energies are relatively insensitive to the amount of Hartree-Fock exchange employed. The difference in trapping energy between rutile and anatase can be obtained accurately with both hybrid functionals and RPA. Computed activation energies for polaron hopping and delocalization clearly show that anatase and rutile might have different charge transport mechanisms. In rutile, only hopping is likely, whereas in anatase hopping and delocalization are competing. Delocalization will result in conduction-band-like and thus enhanced transport. Anisotropic conduction, in agreement with experimental data, is observed, and results from the tendency to delocalize in the 001 direction in rutile and the (001) plane in anatase. For future work, our calculations serve as a benchmark and suggest RPA on top on hybrid orbitals (PBE0 with 30% Hartree-Fock exchange), as a suitable method to study the rich chemistry and physics of TiO2.
Hydrogen spillover is the surface migration of activated hydrogen atoms from a metal catalyst particle, on which they are generated, onto the catalyst support. The phenomenon has been much studied ...and its occurrence on reducible supports such as titanium oxide is established, yet questions remain about whether hydrogen spillover can take place on nonreducible supports such as aluminium oxide. Here we use the enhanced precision of top-down nanofabrication to prepare controlled and precisely tunable model systems that allow us to quantify the efficiency and spatial extent of hydrogen spillover on both reducible and nonreducible supports. We place multiple pairs of iron oxide and platinum nanoparticles on titanium oxide and aluminium oxide supports, varying the distance between the pairs from zero to 45 nanometres with a precision of one nanometre. We then observe the extent of the reduction of the iron oxide particles by hydrogen atoms generated on the platinum using single-particle in situ X-ray absorption spectromicroscopy applied simultaneously to all particle pairs. The data, in conjunction with density functional theory calculations, reveal fast hydrogen spillover on titanium oxide that reduces remote iron oxide nanoparticles via coupled proton-electron transfer. In contrast, spillover on aluminium oxide is mediated by three-coordinated aluminium centres that also interact with water and that give rise to hydrogen mobility competing with hydrogen desorption; this results in hydrogen spillover about ten orders of magnitude slower than on titanium oxide and restricted to very short distances from the platinum particle. We anticipate that these observations will improve our understanding of hydrogen storage and catalytic reactions involving hydrogen, and that our approach to creating and probing model catalyst systems will provide opportunities for studying the origin of synergistic effects in supported catalysts that combine multiple functionalities.
Acetic acid is one of the simplest molecules containing a carboxylic moiety, a common anchoring groups used to functionalize TiO2-based devices. The behavior of acetic acid in proximity of the ...anatase (101) surface has been investigated by means of first-principles density functional theory (DFT) calculations, including explicit liquid solvent in the simulations. A novel acetic acid binding mode, characterized by proton insertion below the first layer of oxide atoms, has been found employing a sufficiently thick anatase slab model. Hybrid DFT calculations show that the subsurface proton insertion leads to a trap-state for excess electrons, favoring localization below the surface edge. Proton deintercalation represents the largest barrier for acid mobility and desorption. However, if the proton is adsorbed on top of the surface, the acid molecule can partially detach from the surface and easily move toward a thermodynamically more stable state. A series of consecutive changes in the adsorption mode can lead to long-range diffusion of the molecule along the 010 direction of the surface, with a barrier of only E act = 20.3 kJ/mol. Similarly, the free energy barrier to completely detach an acetic acid molecule from the surface into the solvent has been computed to be E act = 51.0 kJ/mol, if the proton is adsorbed on top of the anatase slab. Where significant, a comparison between the “explicit liquid” environment and the more often employed “solvent monolayer” environment has been carried out, highlighting the importance of solvent interactions.
Lithium intercalation across the anatase (101) surface is investigated with hybrid DFT. It is demonstrated that the upper surface layers of the oxide are geometrically less constrained, allowing ...stabilization of both excess electrons (by 5 × 10–2 eV) and intercalated Li+ cations (by 1.1 × 10–1) eV compared to bulk anatase. Li+ ions tend to segregate in the first subsurface layers, forming rows along the 010 crystal direction. Also, excess electrons localize preferentially subsurface, as confirmed by these hybrid and additional random phase approximation (RPA) calculations. Furthermore, subsurface Li+ ions act as pinning centers for adsorbates, stabilizing them by roughly 1.1 × 10–1 eV, with some variations depending on the adsorption geometry and on the vicinity of the cations. The cation incorporation mechanism in the oxide is also affected by the presence of the interface: The two step Li+ insertion process has an activation energy of 1.43 eV, more than double the activation energy for cation migration in bulk systems. A monolayer of acetonitrile solvent on top of the slab facilitates the process, lowering the barrier to 7.6 × 10–1 eV. The interaction between Li+ ions and electrons is also stronger at the (101) interface and can be quantified in 9 × 10–2 eV, more than double that of its bulk value. The combined effect of adsorbate pinning and enhanced electron localization constitute a reservoir of shallow trap states in the proximity of the adsorbed donor molecules, likely to facilitate charge-transfer mechanisms. In contrast, the magnitude of the Li+/electron interaction, even if enhanced at the interface, is still low enough to allow subsequent charge migration toward other acceptor centers in the oxide, likely with a mechanism spanning different time scales.
The behavior of excess electrons in undoped and defect free bulk anatase and rutile TiO
2
has been investigated by state-of-the-art electronic structure methods including hybrid density functional ...theory (DFT) and the random phase approximation (RPA). Consistent with experiment, charge trapping and polaron formation is observed in both anatase and rutile. The difference in the anisotropic shape of the polarons is characterized, confirming for anatase the large polaron picture. For anatase, where polaron formation energies are small, charge trapping is observed also with standard hybrid functionals, provided the simulation cell is sufficiently large (864 atoms) to accommodate the lattice relaxation. Even though hybrid orbitals are required as a starting point for RPA in this system, the obtained polaron formation energies are relatively insensitive to the amount of Hartree-Fock exchange employed. The difference in trapping energy between rutile and anatase can be obtained accurately with both hybrid functionals and RPA. Computed activation energies for polaron hopping and delocalization clearly show that anatase and rutile might have different charge transport mechanisms. In rutile, only hopping is likely, whereas in anatase hopping and delocalization are competing. Delocalization will result in conduction-band-like and thus enhanced transport. Anisotropic conduction, in agreement with experimental data, is observed, and results from the tendency to delocalize in the 001 direction in rutile and the (001) plane in anatase. For future work, our calculations serve as a benchmark and suggest RPA on top on hybrid orbitals (PBE0 with 30% Hartree-Fock exchange), as a suitable method to study the rich chemistry and physics of TiO
2
.
Hybrid DFT and RPA calculations predict that polarons form in both anatase and rutile, but that the transport mechanism might be different.
An extended theoretical investigation of the electronic and interface properties of titania and alumina with and without supported platinum nanoparticles is presented and compared to recent ...experimental data with the aim to understand the mechanism of hydrogen activation, adsorption, and spillover. Thirteen-atom platinum particles on titania adopt a distinct different structure than on alumina, which results in distinct hydrogen coverages. Upon hydrogen adsorption, titania is reduced with creation of Ti(III) electronic trap states, strongly interacting with the surface adsorbed protons. The combined Ti(III)/proton migration rate is slower than the one of single surface protons, and it is not influenced by the presence of coadsorbates, such as water molecules. Hydrogen is instead heterolytically split on defect sites on alumina with the formation of a surface proton and a hydride moiety, bound to the particularly reactive surface tricoordinated aluminum site. The electronic structure of alumina is only marginally altered, without formation of defect states. The mobility of the hydride moiety is limited, in particular in the presence of coadsorbed water molecules, that compete for the adsorption sites. Modeling of hydrogen spillover from a platinum cluster to the metal oxide demonstrates that the spillover rate depends on the hydrogen partial pressure and on the thickness of the oxide acceptor layer. Kinetic Monte Carlo results confirm that hydrogen spreading on titania leads to a homogeneous coverage, while on alumina hydrogen can only be found up to a few nm from the platinum cluster because of kinetic competition between diffusion and desorption.
LiFePO
4
is a key material in the field of rechargeable lithium batteries. In this study the transport properties of partially delithiated LiFePO
4
are investigated by molecular dynamics simulations, ...giving particular emphasis to the effect of the presence of Li-Fe anti-site defects on the mechanisms of Li
+
diffusion in the structure, with the aim to deepen the comprehension of the connection between cationic disorder and dimensionality of Li
+
ion transport in this system. The main results indicate that Li diffusion in the system is preferentially anisotropic, with the 0 1 0 direction being the most favourable one. However, Li migration along different directions is possible under specific circumstances, as the Li-Fe anti-site defects effectively act as centres promoting the crossing between adjacent channels running along the 0 1 0 direction.
Li ion diffusion along the 0 1 0 direction in delithiated Li
0.75
FePO
4
from molecular dynamic simulations.
Transition metal lithium orthosilicates are promising cathode materials for lithium-ion batteries. Here we report a combined experimental (in situ X-ray diffraction) and computational (static lattice ...and molecular dynamics) study of the thermal behavior of the Li2Fe0.5Mn0.5SiO4 orthosilicate from room temperature to 950°C. Our X-ray results showed that Pmnb polymorph is the most stable all over the explored temperature range. A significant cation disorder up to 80%, based on the anti-site defect, was found. The defect concentration depends on the synthesis route and temperature, and is completely reversible after the thermal treatments. Moreover, a careful analysis of the impurity phases allowed us to identify Li2SiO3, Fe3O4 and Li3Fe5O8, the last one never reported before. The minimization of defects by opportunely tuning the synthetic parameters would be of great importance in view of potential applications of these materials in lithium batteries.
A combined experimental in situ X-ray diffraction and computational study of the thermal behavior of the Li2Fe0.5Mn0.5SiO4 is reported herein. The anti-site defect does justify the diffraction patterns changes with temperature. Display omitted
► Study of the thermal behavior of Li2Fe0.5Mn0.5SiO4. ► The anti-site defect does justify the diffraction patterns changes with temperature. ► The Pmnb polymorph is stable in the investigated temperaturerange.
Transition metal lithium orthosilicates are promising cathode materials for lithium-ion batteries. Here we report a combined experimental (in situ X-ray diffraction) and computational (static lattice ...and molecular dynamics) study of the thermal behavior of the Li{sub 2}Fe{sub 0.5}Mn{sub 0.5}SiO{sub 4} orthosilicate from room temperature to 950 °C. Our X-ray results showed that Pmnb polymorph is the most stable all over the explored temperature range. A significant cation disorder up to 80%, based on the anti-site defect, was found. The defect concentration depends on the synthesis route and temperature, and is completely reversible after the thermal treatments. Moreover, a careful analysis of the impurity phases allowed us to identify Li{sub 2}SiO{sub 3}, Fe{sub 3}O{sub 4} and Li{sub 3}Fe{sub 5}O{sub 8}, the last one never reported before. The minimization of defects by opportunely tuning the synthetic parameters would be of great importance in view of potential applications of these materials in lithium batteries. - Graphical abstract: A combined experimental in situ X-ray diffraction and computational study of the thermal behavior of the Li{sub 2}Fe{sub 0.5}Mn{sub 0.5}SiO{sub 4} is reported herein. The anti-site defect does justify the diffraction patterns changes with temperature. Highlights: ► Study of the thermal behavior of Li{sub 2}Fe{sub 0.5}Mn{sub 0.5}SiO{sub 4}. ► The anti-site defect does justify the diffraction patterns changes with temperature. ► The Pmnb polymorph is stable in the investigated temperaturerange.
