Effect of polaronic charge transfer on band alignment at the Cu/TiO2 interface

We present a first principles investigation into the electronic properties of an extended interface between rutile TiO2 and Cu. We show that owing to the highly polarizable nature of TiO2, the interface is unstable to the spontaneous formation of small electron polarons at the interface. The resulting dipole leads to an increase in the conduction band offset by 0.4 eV and the presence of a band of occupied states related to Ti d states 1.4 eV below the Fermi energy. This effect should be expected more generally at interfaces between highly polarizable oxides and metals but is missed by standard first principles approaches. Given the ubiquitous nature of such interfaces, this previously overlooked effect may have important implications for diverse applications across science and technology

Origin of differences in the excess volume of copper and nickel grain boundaries

The excess volume associated with grain boundaries is one of the primary factors driving defect segregation and diffusion which controls the electronic, mechanical and chemical properties of many polycrystalline materials. Experimental measurements of the grain boundary excess volume of fcc metals Cu and Ni have shown a difference of over 40%. The difference in lattice constant between Cu and Ni is only 3%, therefore this substantial difference is currently lacking explanation. In this article we employ a high throughput computational approach to determine the atomic structure, formation energy and excess volume of a large number of tilt grain boundaries in Cu and Ni. By considering 400 distinct grain boundary orientations we confirm that theoretically there is a systematic difference between the excess volumes in the two materials and we provide atomistic insight into the origin of the effect

Facet-dependent electron trapping in TiO2 nanocrystals

The trapping of electrons at surfaces of nanocrystalline titanium dioxide can be decisive in controlling performance for diverse applications in photocatalysis, energy storage, and solar energy generation. Here, we employ first-principles calculations to elucidate the factors which influence electron trapping for all low index surfaces of rutile TiO2. We show that different surface orientations exhibit markedly different electron affinities: some preferring to trap electrons with others repelling electrons. We demonstrate that local variations in trapping energy are linked to variations in electrostatic potential and ion coordination providing atomistic insight into this effect. The equilibrium nanocrystal morphology exposes both electron-trapping and electron-repelling facets and therefore is predicted to possess highly anisotropic electron-trapping properties. We discuss how knowledge of surface-specific trapping properties can be utilized to design a number of nanocrystal morphologies which may offer improved performance for applications. (Figure Presented)

First-Principles Investigation of Titanium Nanoparticle Oxidation

We perform first-principles calculations to investigate the initial stages of titanium nanoparticle oxidation. We determine the most stable structure of a 181-atom decahedral nanoparticle with various oxygen coverages ranging from a single atom to full oxidation of the surface. Linear O_ad–Ti–O_ad bonding configurations on the nanoparticle surface are found to be most stable for low oxygen coverage. The degree of lattice expansion is observed to gradually increase with increasing oxygen content up to 8.2% for full oxidation of the surface. To investigate likely mechanisms for subsequent subsurface oxidation, we calculate energy barriers for many inequivalent oxygen diffusion pathways. We find that the most favorable pathways involve penetration of oxygen into subsurface octahedral sites in the center of facets where the strain is largest. The results provide atomistic insight into the oxidation behavior of Ti nanoparticles and highlight the important role played by adsorption induced strain

Electronic Properties of {111} Twin Boundaries in a Mixed-Ion Lead Halide Perovskite Solar Absorber

We present first-principles theoretical predictions of the electronic properties of {111} twin boundaries in pure formamidinium lead iodide (FAPI) as well as a mixed-ion lead halide perovskite containing formamidinium, Cs, I, and Br. We find that the {111} twin boundary is extremely stable in pure FAPI but introduces no electron or hole trapping states and presents relatively small barriers (<100 meV) to transport of electrons and holes, suggesting that they are relatively benign for solar cell performance. However, in the mixed-ion perovskite, twin boundaries serve as a nucleation site for formation of an I- and Cs-rich secondary phase. The reduced band gap in this segregated phase leads to hole trapping and is likely to enhance electron–hole recombination and lead to reduced open-circuit voltage in solar cell devices. These results highlight the role of twin defects as nucleation sites for secondary phases, which can be extremely detrimental to solar cell performance

Passivating Grain Boundaries in Polycrystalline CdTe

Using first-principles density functional calculations, we investigate the structure and properties of three different grain boundaries (GBs) in the solar absorber material CdTe. Among the low ∑ value symmetric tilt GBs ∑3 (111), ∑3 (112), and ∑5 (310), we confirm that the ∑3 (111) is the most stable one but is relatively benign for carrier transport as it does not introduce any new states into the gap. The ∑3 (112) and ∑5 (310) GBs, however, are detrimental due to gap states induced by Te–Te and Cd–Cd dangling bonds. We systematically investigate the segregation of O, Se, Cl, Na, and Cu to the GBs and associated electronic properties. Our results show that co-doping with Cl and Na is predicted to be a viable approach passivating all gap states induced by dangling bonds in CdTe

Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2

Particle/particle interfaces play a crucial role in the functionality and performance of nanocrystalline materials such as mesoporous metal oxide electrodes. Defects at these interfaces are known to impede charge separation via slow-down of transport and increase of charge recombination, but can be passivated via electrochemical doping (i.e., incorporation of electron/proton pairs), leading to transient but large enhancement of photoelectrode performance. Although this process is technologically very relevant, it is still poorly understood. Here we report on the electrochemical characterization and the theoretical modeling of electron traps in nanocrystalline rutile TiO2 films. Significant changes in the electrochemical response of porous films consisting of a random network of TiO2 particles are observed upon the electrochemical accumulation of electron/proton pairs. The reversible shift of a capacitive peak in the voltammetric profile of the electrode is assigned to an energetic modification of trap states at particle/particle interfaces. This hypothesis is supported by first-principles theoretical calculations on a TiO2 grain boundary, providing a simple model for particle/particle interfaces. In particular, it is shown how protons readily segregate to the grain boundary (being up to 0.6 eV more stable than in the TiO2 bulk), modifying its structure and electron-trapping properties. The presence of hydrogen at the grain boundary increases the average depth of traps while at the same time reducing their number compared to the undoped situation. This provides an explanation for the transient enhancement of the photoelectrocatalytic activity toward methanol photooxidation which is observed following electrochemical hydrogen doping of rutile TiO2 nanoparticle electrodes

Small-polaron mediated recombination in titanium dioxide from first principles

Nonradiative recombination leads to losses in efficiency in optoelectronic devices such as photovoltaic cells and light-emitting diodes. Charges trapped at point defects or self-trapped as a small polaron may act as recombination centers. Using various phases of titanium dioxide as an example, we provide first-principles predictions that small hole polarons in the bulk of the crystal would exhibit significant rates of recombination with electrons in the conduction band. However, small hole polarons trapped at a model grain boundary are predicted to have much higher nonradiative recombination rates, which can be attributed to softer phonon modes in the vicinity of the boundary as well as greater electron-phonon coupling. These findings have ramifications in materials other than titanium dioxide, and we propose strategies to reduce the degree of recombination that would occur at grain boundaries