Grain Boundary Facilitates Photocatalytic Reaction in Rutile TiO2 Despite Fast Charge Recombination: A Time-Domain ab Initio Analysis

Resolving the Puzzle of Charge Carrier Lifetime in ZnO by Revisiting the Role of Oxygen Vacancy

Zinc oxide (ZnO) is a wide bandgap prototypical n-type semiconductor due to the presence of intrinsic oxygen vacancies (VO). The VO can readily transfer to the most energetically favorable +2 charged VO (VO2+) by losing two electrons mediated by the metastable VO1+ defect. Nevertheless, the influence of charged VO on the charge dynamics in ZnO and the underlying mechanisms remain elusive. By performing nonadiabatic molecular dynamics simulations of the charge trapping and recombination processes, we show that both VO1+ and VO2+ slow down the nonradiative electron-hole recombination via assisted defect states and, thus, extending charge carrier lifetime compared to pristine ZnO. Our study contributes to identifying the different recombination pathways that take place in VO1+ and VO2+ of n-type ZnO systems, providing useful guidance for designing high-performance ZnO-based devices.

Elucidating the role of adsorption during artificial photosynthesis: H2O and CO2 adsorption isotherms over TiO2 reveal thermal effects under UV illumination

Adsorption measurements of CO₂ and H₂O over TiO₂ surfaces in dark and under illumination were carried out to reveal the ensuing bottlenecks of the initial steps of the artificial photosynthesis reaction. When the adsorption isotherms of both CO₂ and H₂O were measured under illumination, the results were comparable to isotherms measured at higher temperatures in dark. This evidence is interpreted as the presence of hot spots, due to charge carrier recombination reactions. Differential heat of adsorption measurements revealed that H₂O adsorption on TiO₂ is stronger, and with a higher coverage than that of CO₂. Dissociation of water is an energetically uphill reaction, and the local hot spots due to charge carrier recombination in indirect bandgap semiconductors can enhance the reaction probability. At higher temperatures, higher reaction probabilities are expected and estimated by a thermodynamic analysis for water splitting reaction. The potential role of these hot spots during natural and artificial photosynthetic reactions is discussed.

CO Adsorbate Promotes Polaron Photoactivity on the Reduced Rutile TiO2(110) Surface

Polarons play a major role in determining the chemical properties of transition-metal oxides. Recent experiments show that adsorbates can attract inner polarons to surface sites. These findings require an atomistic understanding of the adsorbate influence on polaron dynamics and lifetime. We consider reduced rutile TiO₂(110) with an oxygen vacancy as a prototypical surface and a CO molecule as a classic probe and perform ab initio adiabatic molecular dynamics, time-domain density functional theory, and nonadiabatic molecular dynamics simulations. The simulations show that subsurface polarons have little influence on CO adsorption and CO can desorb easily. On the contrary, surface polarons strongly enhance CO adsorption. At the same time, the adsorbed CO attracts polarons to the surface, allowing them to participate in catalytic processes with CO. The CO interaction with polarons changes their orbital origin, suppresses polaron hopping, and stabilizes them at surface sites. Partial delocalization of polarons onto CO decouples them from free holes, decreasing the nonadiabatic coupling and shortening the quantum coherence time, thereby reducing charge recombination. The calculations demonstrate that CO prefers to adsorb at the next-nearest-neighbor five-coordinated Ti³⁺ surface electron polaron sites. The reported results provide a fundamental understanding of the influence of electron polarons on the initial stage of reactant adsorption and the effect of the adsorbate-polaron interaction on the polaron dynamics and lifetime. The study demonstrates how charge and polaron properties can be controlled by adsorbed species, allowing one to design high-performance transition-metal oxide catalysts.

Water Splitting with a Single-Atom Cu/TiO2 Photocatalyst: Atomistic Origin of High Efficiency and Proposed Enhancement by Spin Selection

Anatase TiO2 is an intensely investigated photocatalytic material due to its abundance and chemical stability. However, it suffers from weak light harvesting and low photocatalytic efficiency. Experiments show that light absorption and photocatalytic properties can be enhanced simultaneously by TiO2 doping with well-dispersed Cu atoms, forming a single-atom catalyst (Cu/TiO2) that can be used for solar water splitting and other applications. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrate that Cu/TiO2 is inactive before light irradiation due to rapid electron-hole recombination via both shallow and deep traps. Surprisingly, the shallow trap is more detrimental to the Cu/TiO2 performance than the deep trap because it couples better to free carriers. After light irradiation, leading to electron transfer and Cu/TiO2 protonation, the shallow trap is eliminated, and a local distortion around the Cu atom stabilizes the deep trap state on the Cu d-orbital, decoupling it from free charges and giving rise to high photocatalytic hydrogen generation activity. We further demonstrate that the photocatalytic performance of Cu/TiO2 can be enhanced by spin selection, achievable experimentally via optical intersite spin transfer or chiral semiconductor coating. Both H adsorption and spin selection enhance charge carrier lifetimes by an order of magnitude. The spin selection mechanism does not require formation of the H species, which necessitates concurrent sources of electrons and protons and which is intrinsically unstable because water splitting involves frequent proton shuffling. Our results rationalize the experimental observations at the atomistic level, provide mechanistic insights into operation of single atom photocatalysis, and demonstrate that spin selection can be used to develop advanced and efficient systems for solar energy conversion.

Controlling Charge Carrier Trapping and Recombination in BiVO4 with the Oxygen Vacancy Oxidation State

The lack of an in-depth understanding of the intrinsic oxygen vacancy (O_V) defect properties in the photoanode BiVO₄ limits the further improvement of its photoelectrochemical water splitting performance. To address this issue, nonadiabatic molecular dynamics simulations are performed to study the impact of O_V on charge carrier lifetimes in BiVO₄. The simulations show that a neutral O_V gives rise to local structural distortions due to the formation of V-O-V bonds, forcing the electrons trapped on the nearer of the two V atoms to form two deep polaron-like V⁴⁺ hole traps. These localized midgap states greatly accelerate nonradiative electron-hole recombination compared to that of pristine BiVO₄, reaching a time scale of several nanoseconds in good agreement with experiments. The ionized O_V state restores the bandgap to its value in pristine BiVO₄ and restores the charge carrier lifetimes due to the fast loss of coherence time. Our study reveals the mechanism of the detrimental role of O_V in BiVO₄ and provides valuable insights for improving the performance of the BiVO₄ photoanode by ionizing the oxygen vacancy.

Crystal Symmetry and Static Electron Correlation Greatly Accelerate Nonradiative Dynamics in Lead Halide Perovskites

Using a recently developed many-body nonadiabatic molecular dynamics (NA-MD) framework for large condensed matter systems, we study the phonon-driven nonradiative relaxation of excess electronic excitation energy in cubic and tetragonal phases of the lead halide perovskite CsPbI₃. We find that the many-body treatment of the electronic excited states significantly changes the structure of the excited states' coupling, promotes a stronger nonadiabatic coupling of states, and ultimately accelerates the relaxation dynamics relative to the single-particle description of excited states. The acceleration of the nonadiabatic dynamics correlates with the degree of configurational mixing, which is controlled by the crystal symmetry. The higher-symmetry cubic phase of CsPbI₃ exhibits stronger configuration mixing than does the tetragonal phase and subsequently yields faster nonradiative dynamics. Overall, using a many-body treatment of excited states and accounting for decoherence dynamics are important for closing the gap between the computationally derived and experimentally measured nonradiative excitation energy relaxation rates.

Rapid Decoherence Induced by Light Expansion Suppresses Charge Recombination in Mixed Cation Perovskites: Time-Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we have investigated the effect of light-induced lattice expansion on the nonradiative electron-hole recombination in the mixed-cation perovskite FA_0.75MA_0.25PbI₃. We demonstrate that charge carrier lifetime extends by a factor of 1.5 within 1% lattice expansion; the bandgap grows only by 0.04 eV; the electron-phonon coupling increases by 13%; and the decoherence time shortens by 37%. The small bandgap change has negligible influence on recombination times. Lattice expansion enhances atomic fluctuations that lead to the enhancement of electron-phonon coupling and acceleration of decoherence. By creating several high-frequency phonon modes, the lattice expansion shortens the decoherence time further. As a result, rapid decoherence beats an enhanced electron-phonon coupling, playing the dominant role in suppressing the nonradiative electron-hole recombination. The light-induced lattice expansion or strain effects provide a rational route to improve the perovskite photovoltaic and photoelectronic device performance.

Anomalous Temperature-Dependent Charge Recombination in CH3NH3PbI3 Perovskite: Key Roles of Charge Localization and Thermal Effect

Optimizing metal halide perovskite solar cells necessitates understanding of nonradiative electron-hole recombination because it comprises a dominant route for charge and energy losses. In principle, the electron-hole recombination rate increases as temperature grows due to enhanced electron-phonon coupling. Experiments defy this expectation in MAPbI₃ (MA = CH₃NH₃⁺). By performing nonadiabatic (NA) molecular dynamics analyses combined with time-domain density functional theory simulations, we demonstrate that nonradiative electron-hole recombination in MAPbI₃ at high temperature occurs more slowly than that at low temperature. First and most important, increasing temperature enhances thermal disorder and leads to significant distortion of the inorganic Pb-I framework, giving rise to electron and hole wave functions locating spatial separation and reducing NA coupling by a factor of 28% in comparison with low temperature. Second, rising temperature enhances the thermal fluctuations of both the inorganic and organic components that accelerate decoherence process by a factor of 12%. Both factors, particularly the small NA coupling, contribute to suppressing electron-hole recombination at high temperature. The simulations show excellent agreement with experiments and emphasize how the charge localization driven by thermal effects impacts electron-hole recombination in perovskites and advances our understanding of the unusual charge dynamics.

Unravelling the Effects of Pressure-Induced Suppressed Electron-Hole Recombination in CsPbBr3 Perovskite: Time-Domain ab Initio Analysis

Using nonadiabatic (NA) molecular dynamics simulations, we demonstrate pressure-dependent electron-hole recombination in all-inorganic CsPbBr₃ perovskite. In particular, electron-hole recombination under 1 atm takes place in several hundred picoseconds, agreeing well with experiments. An increase of pressure causes PbBr₆ octahedron distortion, including contraction of both Pb-Br-Pb angles and Pb-Br bond lengths, leading to a decrease in decoherence time and NA coupling and thus slowing electron-hole recombination. When the pressure reaches a critical pressure of 1.20 GPa, a phase transition occurs in which the charge carrier lifetime is longest and extends to several nanoseconds. When the pressure is increased over the threshold, the shrinkage of Pb-Br bond length is inhibited and the contraction of Pb-Br-Pb angles primarily induced the PbBr₆ octahedron distortion. Such a situation gives rise to a mild NA coupling and decoherence time, restoring the recombination time to over half of a nanosecond. Our study uncovers the mechanisms for the pressure-suppressed charge recombination and provides an advanced route toward further development of photovoltaic performance of perovskite materials.

Symmetry Breaking at MAPbI3 Perovskite Grain Boundaries Suppresses Charge Recombination: Time-Domain ab Initio Analysis

The influence of grain boundaries (GBs) on charge carrier lifetimes in methylammonium lead triiodide perovskite (MAPbI₃) remains unclear. Some experiments suggest that GBs promote rapid nonradiative decay and deteriorate device performance, while other measurements indicate that charge recombination happens primarily in non-GB regions and that GBs facilitate charge separation and collection. By combining time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we demonstrate that charge separation and localization happening at MAPbI₃ GBs due to symmetry breaking suppresses charge recombination. Even though GBs lower the MAPbI₃ bandgap and charge localization enhances interactions with phonons, electron-hole separation decreases the NA coupling, and the excited state lifetime remains virtually unchanged compared to the pristine perovskite. Our study rationalizes how GBs can have a positive influence on perovskite optoelectronic properties and advances fundamental understanding of charge carrier dynamics in these fascinating materials.

Interface structure prediction via CALYPSO method

The atomistic structures of solid-solid interfaces are of fundamental interests for understanding physical properties of interfacial materials. However, determination of interface structures faces a substantial challenge, both experimentally and theoretically. Here, we propose an efficient method for predicting interface structures via the generalization of our in-house developed CALYPSO method for structure prediction. We devised a lattice match toolkit that allows us to automatically search for the optimal lattice-matched superlattice for construction of the interface structures. In addition, bonding constraints (e.g., constraints on interatomic distances and coordination numbers of atoms) are imposed to generate better starting interface structures by taking advantages of the known bonding environment derived from the stable bulk phases. The interface structures evolve by following interfacially confined swarm intelligence algorithm, which is known to be efficient for exploration of potential energy surface. The method was validated by correctly predicting a number of known interface structures with only given information of two parent solids. The application of the developed method leads to prediction of two unknown grain boundary (GB) structures (r-GB and p-GB) of rutile TiO₂ Σ5(2 1 0) under an O reducing atmosphere that contained Ti³⁺ as the result of O defects. Further calculations revealed that the intrinsic band gap of p-GB is reduced to 0.7 eV owing to substantial broadening of the Ti-3d interfacial levels from Ti³⁺ centers. Our results demonstrated that introduction of grain boundaries is an effective strategy to engineer the electronic properties and thus enhance the visible-light photoactivity of TiO₂.