Ab Initio Self-Trapped Excitons

We propose a new formalism and an effective computational framework to study self-trapped excitons (STEs) in insulators and semiconductors from first principles. Using the many-body Bethe-Salpeter equation in combination with perturbation theory, we are able to obtain the mode- and momentum-resolved exciton-phonon coupling matrix element in a perturbative scheme and explicitly solve the real space localization of the electron (hole), as well as the lattice distortion. Further, this method allows us to compute the STE potential energy surface and evaluate the STE formation energy and Stokes shift. We demonstrate our approach using two-dimensional magnetic semiconductor chromium trihalides and a wide-gap insulator BeO, the latter of which features dark excitons, and make predictions of their Stokes shift and coherent phonon generation which we hope will spark future experiments such as photoluminescence and transient absorption studies.

Thermal-Stabilized Protonated TiO2 for Heat-Accelerated Photoelectrochemical Water Splitting

Enhancing the charge separation efficiency is a big challenge that limits the energy conversion efficiency of photoelectrochemical (PEC) water splitting. Surface states generated by protonation of TiO2 are the efficient charge separation passageways to massively accept or transfer the photogenerated electrons. However, a challenge is to avoid the deprotonation of a protonated TiO2 photoelectrode at the operation temperature. Here, we found that the terminal hydroxyl group (OHT) as surface states on the TiO2 surface generated via electrochemical protonation of TiO2 at 90 °C [90-TiO2-x-(OH)x] is thermally stable. As a result, the thermally enhanced photocurrent of the 90-TiO2-x-(OH)x electrode reached 1.05 mA cm-2 under 80 °C, and the stability was maintained up to 10 h with a slight photocurrent decrease of 3%. The thermally stable surface states as charge separation paths provide an effective method to couple the heat field with the PEC process via thermal-stimulating hopping of polarons.

An embedded cluster CASPT2 study of the Ce:YVO4 spectrum

Multiconfigurational theory, in combination with the embedded cluster approach, is a precise and ab initio approach to describe the electronic structure of solids. In this work, the spectrum of a Ce(III) dopant in YVO4 has been studied by complete active space perturbation theory of the second order (CASPT2), with the host material represented as a set of ab initio model potentials and point-charges. We assess the sensitivity of the spectrum to the size of both the embedded cluster size as well as the size of the electronic basis set. A comparison of our best computational model with experimental results shows that the embedding approach is robust and can accurately model the spectrum of low-concentration dopants in complex host materials.

Oxygen-deficient TiO2-x interlayer enabling Li-rich Mn-based layered oxide cathodes with enhanced reversible capacity and cyclability

The unique anion redox mechanism of Li-rich Mn-based layered oxide (LMLO) cathodes endows them with a higher specific capacity compared with conventional cathodes. However, the irreversible anion redox reactions can cause structural degradation and sluggish electrochemical kinetics in the cathode, resulting in a poor electrochemical performance in the batteries. Thus, to address these issues, a single-sided conductive oxygen-deficient TiO2-x interlayer was applied on a commercial Celgard separator as a coating layer towards the LMLO cathode. After coating TiO2-x, the initial coulombic efficiency (ICE) of the cathode increased from 92.1% to 95.8%, the capacity retention improved from 84.2% to 91.7% after 100 cycles, and the rate performance of the cathode was significantly enhanced from 91.3 mA h g-1 to 203.9 mA h g-1 at 5C. Operando differential electrochemical mass spectroscopy (DEMS) showed that the coating layer could restrain the release of oxygen in the battery, especially from the initial formation process. The X-ray photoelectron spectroscopy (XPS) results demonstrated that the favorable oxygen absorption by the TiO2-x interlayer benefitted the suppression of side reactions and cathode structural evolution and favored the formation of a uniform cathode-electrolyte interphase on the LMLO cathode. This work provides an alternative path to address the issue of oxygen release in LMLO cathodes.

Models of Polaron Transport in Inorganic and Hybrid Organic-Inorganic Titanium Oxides

Polarons are a type of localized excess charge in materials and often form in transition metal oxides. The large effective mass and confined nature of polarons make them of fundamental interest for photochemical and electrochemical reactions. The most studied polaronic system is rutile TiO₂ where electron addition results in small polaron formation through the reduction of Ti(IV) d⁰ to Ti(III) d¹ centers. Using this model system, we perform a systematic analysis of the potential energy surface based on semiclassical Marcus theory parametrized from the first-principles potential energy landscape. We show that F-doped TiO₂ only binds polaron weakly with effective dielectric screening after the second nearest neighbor. To tailor the polaron transport, we compare TiO₂ to two metal-organic frameworks (MOFs): MIL-125 and ACM-1. The choice of MOF ligands and connectivity of the TiO₆ octahedra largely vary the shape of the diabatic potential energy surface and the polaron mobility. Our models are applicable to other polaronic materials.

Tuning a small electron polaron in FePO4 by P-site or O-site doping based on DFT+U and KMC simulation

Due to the existence of a small polaron, the intrinsic electronic conductivity of olivine-structured LiFePO₄ is quite low, limiting its performance as a cathode material for lithium-ion batteries (LIBs). Previous studies have mainly focused on improving intrinsic conductivity through Fe-site doping while P-site or O-site doping has rarely been reported. Herein, we studied the formation and dynamics of the small electron polaron in FeP_1-αX_αO₄ and FePO_4-βZ_β by employing the density functional theory with the on-site Hubbard correction terms (DFT+U) and Kinetic Monte Carlo (KMC) simulation, where X and Z indicate the doping elements (X = S, Se, As, Si, V; Z = S, F, Cl), and α and β indicate the light doping at the P position (α = 0.0625) and O position (β = 0.015625), respectively. We confirmed the small electron polaron formation in pristine FePO₄ and its doped systems, and the polaron hopping rates for all systems were calculated according to the Marcus-Emin-Holstein-Austin-Mott (MEHAM) theory. We found that the hopping process is adiabatic for most cases with the defects breaking the original symmetry. Based on the KMC simulation results, we found that the doping of S at the P site changes the polaron's motion mode, which is expected to increase the mobility and intrinsic electronic conductivity. This study attempts to provide theoretical guidance to improve the electronic conductivity of LiFePO₄-like cathode materials with better rate performance.

Three-in-one oxygen-deficient titanium dioxide in a pomegranate-inspired design for improved lithium storage

Defects engineering has played an ever-increasing important role in electrochemistry, especially secondary lithium batteries. TiO₂ is regarded as a promising anode due to its attractive cycling stability, low volume strain and great abundance, while challenges of intrinsic poor electrical and ionic conductivity remain to be addressed. Herein, we report a three-in-one oxygen vacancy (V_O)-involved pomegranate design for TiO_2-x/C composite anode, which provides highly improved electrical conduction, shortened Li⁺ pathway and promoted Li⁺ redox. N-doped mesoporous carbon acts as a robust scaffold to support the whole structure, electron highway and efficient reductant to generate V_O on TiO₂ nanoparticles during crystallization. Theoretical calculations reveal the crucial role of surface V_O on TiO₂ in Li electrochemistry. Resultantly, the optimal TiO_2-x/C anode achieves significantly enhanced cycling performance (203 mAh/g retained after 2000 cycles at 1 A/g). Postmortem analysis reveals the robustness of V_O and reasonable structure stability upon cycles for improved battery performance.

Detection and Correction of Delocalization Errors for Electron and Hole Polarons Using Density-Corrected DFT

Modeling polaron defects is an important aspect of computational materials science, but the description of unpaired spins in density functional theory (DFT) often suffers from delocalization error. To diagnose and correct the overdelocalization of spin defects, we report an implementation of density-corrected (DC-)DFT and its analytic energy gradient. In DC-DFT, an exchange-correlation functional is evaluated using a Hartree-Fock density, thus incorporating electron correlation while avoiding self-interaction error. Results for an electron polaron in models of titania and a hole polaron in Al-doped silica demonstrate that geometry optimization with semilocal functionals drives significant structural distortion, including the elongation of several bonds, such that subsequent single-point calculations with hybrid functionals fail to afford a localized defect even in cases where geometry optimization with the hybrid functional does localize the polaron. This has significant implications for traditional workflows in computational materials science, where semilocal functionals are often used for structure relaxation. DC-DFT calculations provide a mechanism to detect situations where delocalization error is likely to affect the results.

Diagrammatic quantum Monte Carlo toward the calculation of transport properties in disordered semiconductors

A new diagrammatic quantum Monte Carlo approach is proposed to deal with the imaginary time propagator involving both dynamic disorder (i.e., electron–phonon interactions) and static disorder of local or nonlocal nature in a unified and numerically exact way. The establishment of the whole framework relies on a general reciprocal-space expression and a generalized Wick’s theorem for the static disorder. Since the numerical cost is independent of the system size, various physical quantities, such as the thermally averaged coherence, Matsubara one-particle Green’s function, and current autocorrelation function, can be efficiently evaluated in the thermodynamic limit (infinite in the system size). The validity and performance of the proposed approach are systematically examined in a broad parameter regime. This approach, combined with proper numerical analytic continuation methods and first-principles calculations, is expected to be a versatile tool toward the calculation of various transport properties, such as mobilities in realistic semiconductors involving multiple electronic energy bands, high-frequency optical and low-frequency acoustic phonons, different forms of dynamic and static disorders, and anisotropy.

Modeling polarons in density functional theory: lessons learned from TiO2

Density functional theory (DFT) is nowadays one of the most broadly used and successful techniques to study the properties of polarons and their effects in materials. Here, we systematically analyze the aspects of the theoretical calculations that are crucial to obtain reliable predictions in agreement with the experimental observations. We focus on rutile TiO₂, a prototypical polaronic compound, and compare the formation of polarons on the (110) surface and subsurface atomic layers. As expected, the parameterUused to correct the electronic correlation in the DFT +Uformalism affects the resulting charge localization, local structural distortions and electronic properties of polarons. Moreover, the polaron localization can be driven to different sites by strain: due to different local environments, surface and subsurface polarons show different responses to the applied strain, with impact on the relative energy stability. An accurate description of the properties of polarons is key to understand their impact on complex phenomena and applications: as an example, we show the effects of lattice strain on the interaction between polarons and CO adsorbates.

Effects of oxygen vacancies on the photoexcited carrier lifetime in rutile TiO2

The photoexcited carrier lifetime in semiconductors plays a crucial role in solar energy conversion processes. The defects or impurities in semiconductors are usually proposed to introduce electron-hole (e-h) recombination centers and consequently reduce the photoexcited carrier lifetime. In this report, we investigate the effects of oxygen vacancies (O_V) on the carrier lifetime in rutile TiO₂, which has important applications in photocatalysis and photovoltaics. It is found that an O_V introduces two excess electrons which form two defect states in the band gap. The lower state is localized on one Ti atom and behaves as a small polaron, and the higher one is a hybrid state contributed by three Ti atoms around the O_V. Both the polaron and hybrid states exhibit strong electron-phonon (e-ph) coupling and their charge distributions become more and more delocalized when the temperature increases from 100 to 700 K. Such strong e-ph coupling and charge delocalization enhance the nonadibatic coupling between the electronic states along the hole relaxation path, where the defect states behave as intermediate states, leading to a distinct acceleration of e-h recombination. Our study provides valuable insights to understand the role of defects on photoexcited carrier lifetime in semiconductors.

TiO2 Polarons in the Time Domain: Implications for Photocatalysis

Exploiting the availability of solar energy to produce valuable chemicals is imperative in our quest for a sustainable energy cycle. TiO₂ has emerged as an efficient photocatalyst, and as such its photochemistry has been studied extensively. It is well-known that polaronic defect states impact the activity of this chemistry. As such, understanding the fundamental excitation mechanisms deserves the attention of the scientific community. However, isolating the contribution of polarons to these processes has required increasingly creative experimental techniques and expensive theory. In this Perspective, we discuss recent advances in this field, with a particular focus on two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT), and discuss the implications for photocatalysis.

A complete ab initio thermodynamic and kinetic catalogue of the defect chemistry of hematite α-Fe2O3, its cation diffusion, and sample donor dopants

This paper studies comprehensively the defect chemistry of and cation diffusion in α-Fe₂O₃. Defect formation energies and migration barriers are calculated using density functional theory with a theoretically calibrated Hubbard U correction. The established model shows a good agreement with experimental off-stoichiometry and cation diffusivities available in the literature. At any temperature, and are the predominant ionic defects in hematite at the two extremes of oxygen partial pressure (pO₂) range, reducing and oxidizing, respectively. Between these two extremes, an intrinsic electronic regime exists where small polaronic electrons and holes are the dominant charge carriers. The calculated migration barriers show that Fe ions favor the diffusion along the 〈111〉 direction in the primitive cell through an interstitial crowdion-like mechanism. Our model suggests that cation diffusion in hematite is mainly controlled by the migration of , while may contribute to cation diffusion at extremely low pO₂. Our analysis in the presence of two sample donor dopants Ti and Sn indicates that high temperature annealing at T > 1100 K is needed to prepare n-type hematite at ambient pO₂, consistently with prior experimental findings. Alternatively, annealing at lower temperatures requires much lower pO₂ to avoid compensating the donors with Fe vacancies. A synergistic comparison of our theoretical model and the experimental results on Ti-doped hematite led us to propose that free electrons and small polarons coexist and both contribute to n-type conductivity. Our validated model of defective hematite is a foundation to study hematite in applications such as corrosion and water splitting.

Hole Polaron Migration in Bulk Phases of TiO2 Using Hybrid Density Functional Theory

Understanding charge-carrier transport in semiconductors is vital to the improvement of material performance for various applications in optoelectronics and photochemistry. Here, we use hybrid density functional theory to model small hole polaron transport in the anatase, brookite, and TiO₂-B phases of titanium dioxide and determine the rates of site-to-site hopping as well as thermal ionization into the valance band and retrapping. We find that the hole polaron mobility increases in the order TiO₂-B < anatase < brookite and there are distinct differences in the character of hole polaron migration in each phase. As well as having fundamental interest, these results have implications for applications of TiO₂ in photocatalysis and photoelectrochemistry, which we discuss.

Experimental and theoretical investigation of the enhancement of the photo-oxidation of Hg0 by CeO2-modified morphology-controlled anatase TiO2

This study aims to investigate the coeffects of predominantly exposed anatase TiO₂{001} and {101} and CeO₂ loading on the photo-oxidation of Hg⁰ to relieve the adverse effects caused by higher temperatures of 50-250 °C. The effect of loading CeO₂ on the photocatalytic activity of morphology-controlled TiO₂ was not only investigated using DFT with U correction but also experimentally analyzed by characterizing the electrochemical properties and the formation of free radicals. The theoretical calculation showed that CeO₂ loading on TiO₂{101} was more stable than that on TiO₂{001}. Accordingly, a larger portion of CeO₂ was observed to anchor to the (101) plane than to the (001) plane. CeO₂ loading is more beneficial for increasing the distribution of photo-induced electrons and holes on the surface of 7%CeTi than on the surface of TiO₂ and increases the energy difference between the conduction band edge of 7%CeTi and the standard redox potential of O₂/·O₂^-. Correspondingly, the photocatalytic removal efficiencies (PREs) of Hg⁰ by 7%CeTi were significantly enhanced compared with those of pristine TiO₂. The effect of CeO₂ was highly morphologically dependent on the photocatalytic activity. This study provides valuable insight into surface engineering strategies for morphology-controlled photocatalysts for air pollution control technology.

Dynamics of Photoexcited Small Polarons in Transition-Metal Oxides

The dynamics of photoexcited polarons in transition-metal oxides (TMOs), including their formation, migration, and quenching, plays an important role in photocatalysis and photovoltaics. Taking rutile TiO₂ as a prototypical system, we use ab initio nonadiabatic molecular dynamics simulation to investigate the dynamics of small polarons induced by photoexcitation at different temperatures. The photoexcited electron is trapped by the distortion of the surrounding lattice and forms a small polaron within tens of femtoseconds. Polaron migration among Ti atoms is strongly correlated with quenching through an electron-hole (e-h) recombination process. At low temperature, the polaron is localized on a single Ti atom and polaron quenching occurs within several nanoseconds. At increased temperature, as under solar cell operating conditions, thermal phonon excitation stimulates the hopping and delocalization of polarons, which induces fast polaron quenching through the e-h recombination within 200 ps. Our study proves that e-h recombination centers can be formed by photoexcited polarons, which provides new insights to understand the efficiency bottleneck of photocatalysis and photovoltaics in TMOs.

Water-Hydrogen-Polaron Coupling at Anatase TiO2(101) Surfaces: A Hybrid Density Functional Theory Study

Defects and water generally coexist on the surfaces of reducible metal oxides for heterogeneous photocatalysis in aqueous environments, which makes quantification and understanding of their coupling essential for development of practical solutions. Here we explore and quantify the coupling between water (H₂O)- and hydrogen (H)-induced electron-polarons on the TiO₂ anatase (101) surface by means of first-principles simulations. Without H₂O, the hydrogen-induced electron-polaron localizes preferentially around the energetically favored subsurface H site. Its hopping barrier to neighboring sites in the subsurface is about 0.29 eV. Conversely, following H₂O adsorption, surface trapping of the electron-polaron becomes energetically favored, and the diffusion barrier from subsurface to surface decreases by 0.15 eV. H₂O adsorption is shown to be effective in decreasing the proton diffusion energy barrier within the same layer by reducing the polaron-proton coupling and promoting diffusion toward the subsurface in line with a recent experimental observation on water-dispersed anatase TiO₂ nanoparticles.

In Situ Tuning of Defects and Phase Transition in Titanium Dioxide by Lithiothermic Reduction

Defects engineering of oxides plays a vital role in tuning their physicochemical and electronic properties and thereby determining their potential applications. However, the safe and controllable production of effective defects in the oxides is still challenging. Here, we report a facile one-pot solid lithiothermic reduction approach to generate graded oxygen defects in TiO₂ nanoparticles. Various levels of lithium reduction are systematically studied, and meanwhile, a distinct phase transition from anatase TiO₂ to cubic Li_xTiO₂ is observed with the increasing lithium ratio. The structure and evolution of surface defects and bulk phase transition are investigated in detail. Afterward, we demonstrate their applications in carbon dioxide photoreduction and photothermal imaging. The slightly reduced TiO₂ with effective oxygen defects affords a highly broadened solar spectrum absorption and yields significantly enhanced visible photocatalytic activity in CO₂ conversion, which is further revealed by theoretical calculations. The highly reduced TiO₂ with obvious phase transition shows enhanced solar absorption and achieves high photo-thermal-conversion efficacy, showing huge potential in photo-thermal-related applications. The lithiothermic reduction is a general and effective approach to produce defects and induce phase transition in oxides, which can be used in multiple applications.

A comparative first-principles investigation on the defect chemistry of TiO2 anatase

Understanding native point defects is fundamental in order to comprehend the properties of TiO₂ anatase in technological applications. The previous first-principles reports of defect-relevant quantities, such as formation energies and charge transition levels, are, however, scattered over a wide range. We perform a comparative study employing different approaches based on semilocal with Hubbard correction (DFT+U) and screened hybrid functionals in order to investigate the dependence defect properties on the employed computational method. While the defects in TiO₂ anatase, as in most transition-metal oxides, generally induce the localization of electrons or holes on atomic sites, we notice that, provided an alignment of the valence bands has been performed, the calculated defect formation energies and transition levels using semilocal functionals are in a fair agreement with those obtained using hybrid functionals. A similar conclusion can be reached for the thermochemistry of the Ti-O system and the limit values of the elemental chemical potentials. We interpret this as a cancellation of error between the self-interaction error and the overbinding of the O₂ molecule in semilocal functionals. Inclusion of a U term in the electron Hamiltonian offers a convenient way for obtaining more precise geometric and electronic configurations of the defective systems.

Oxidation-State Constrained Density Functional Theory for the Study of Electron-Transfer Reactions

We propose a new constrained density functional theory (CDFT) approach which directly controls the oxidation state of the target atoms. In this new approach called oxidation-state constrained density functional theory (OS-CDFT), the eigenvalues of the occupation matrix obtained from projecting the Kohn-Sham wave functions onto the valence orbitals are constrained to obtain the desired oxidation states. This approach is particularly useful to study electron transfer problems in transition metal-containing systems due to the multivalent nature of the transition metal ions. The calculation of the forces on the ions and of the coupling constant was implemented under the OS-CDFT scheme to allow efficient and accurate study of electron transfer reactions. We demonstrated the application of this method in the study of different electron transfer reactions including the aqueous ferrous-ferric self-exchange reaction, polaron hopping in the TiO₂ anatase and bismuth vanadate, and photoexcited electron transfer in the sapphire.

Electronic Properties of Realistic Anatase TiO2 Nanoparticles from G0W0 Calculations on a Gaussian and Plane Waves Scheme

The electronic properties of realistic (TiO₂)_n nanoparticles (NPs) with cuboctahedral and bipyramidal morphologies are investigated within the many-body perturbation theory (MBPT) G₀W₀ approximation using PBE and hybrid PBEx (12.5% Fock contribution) functionals as starting points. The use of a Gaussian and plane waves (GPW) scheme reduces the usual O⁴ computational time required in this type of calculation close to O³ and thus allows considering explicitly NPs with n up to 165. The analysis of the Kohn-Sham energy orbitals and quasiparticle (QP) energies shows that the optical energy gap (O_gap), the electronic energy gap (E_gap), and the exciton binding energy (ΔE_ex) values decrease with increasing TiO₂ NP size, in agreement with previous work. However, while bipyramidal NPs appear to reach the scalable regime already for n = 84, cuboctahedral NPs reach this regime only above n = 151. Relevant correlations are found and reported that will allow one to predict these electronic properties at the G₀W₀ level in even much larger NPs where these calculations are unaffordable. The present work provides a feasible and practical way to approach the electronic properties of rather large TiO₂ NPs and thus constitutes a further step in the study of realistic nanoparticles of semiconducting oxides.

Polarons from First Principles, without Supercells

We develop a formalism and a computational method to study polarons in insulators and semiconductors from first principles. Unlike in standard calculations requiring large supercells, we solve a secular equation involving phonons and electron-phonon matrix elements from density-functional perturbation theory, in a spirit similar to the Bethe-Salpeter equation for excitons. We show that our approach describes seamlessly large and small polarons, and we illustrate its capability by calculating wave functions, formation energies, and spectral decomposition of polarons in LiF and Li_{2}O_{2}.

Investigating Polaron Formation in Anatase and Brookite TiO2 by Density Functional Theory with Hybrid-Functional and DFT + U Methods

Anatase and brookite are robust materials with enhanced photocatalytic properties. In this study, we used density functional theory (DFT) with a hybrid functional and the Hubbard on-site potential methods to determine electron- and hole-polaron geometries for anatase and brookite and their energetics. Localized electron and hole polarons were predicted not to form in anatase using DFT with hybrid functionals. In contrast, brookite formed both electron and hole polarons. The brookite electron-polaronic solution exhibits coexisting localized and delocalized states, with hole polarons mainly dispersed on two-coordinated oxygen ions. Hubbard on-site potential testing over the wide 4.0-10 eV range revealed that brookite polarons are formed at U = 6 eV, while anatase polarons are formed at U = 8 eV. The brookite electron polaron was always localized on a single titanium ion under the Hubbard model, whereas the hole polaron was dispersed over four oxygen atoms, consistent with the hybrid DFT studies. The anatase electron polarons were dispersed at lower on-site potentials but were more localized at higher potentials. Both methods predict that brookite has a higher driving force for the formation of polarons than anatase.

Assessment of Density Functional Theory in Predicting Interaction Energies between Water and Polycyclic Aromatic Hydrocarbons: from Water on Benzene to Water on Graphene

The interactions of water with polycyclic aromatic hydrocarbons, from benzene to graphene, are investigated using various exchange-correlation functionals selected across the hierarchy of density functional theory (DFT) approximations. The accuracy of the different functionals is assessed through comparisons with random phase approximation (RPA) and coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] calculations. Diffusion Monte Carlo (DMC) data reported in the literature are also used for comparison. Relatively large variations are found in interaction energies predicted by different DFT models, with GGA functionals underestimating the interaction strength for configurations with the water oxygen pointing toward the aromatic molecules. The meta-GGA B97M-rV and range-separated hybrid, meta-GGA ωB97M-V functionals provide nearly quantitative agreement with CCSD(T) values for the water-benzene, water-coronene, and water-circumcoronene dimers, while RPA and DMC predict interaction energies that differ by up to ∼1 kcal/mol and ∼0.4 kcal/mol from the corresponding CCSD(T) values, respectively. Similar trends among GGA, meta-GGA, and hybrid functionals are observed for larger polycyclic aromatic hydrocarbons. By performing absolutely localized molecular orbital energy decomposition analyses (ALMO-EDA), it is found that, independently of the number of carbon atoms and exchange-correlation functional, the dominant contributions to the interaction energies between water and polycyclic aromatic hydrocarbon molecules are the electrostatic and dispersion terms while polarization and charge transfer effects are negligibly small. Calculations carried out with GGA and meta-GGA functionals indicate that, as the number of carbon atoms increases, the interaction energies slowly converge to the corresponding values obtained for an infinite graphene sheet.

Effective dielectric function of laser-pumped anatase nanoparticles: influence of free carriers trapping and depletion of valence band

Anatase nanoparticles were examined using the z-scan technique with simultaneous measurements of the intensity of the scattered laser light. In the course of the z-scan experiments, the average intensity of probing laser pulses varied between 1.0·10⁶ W/cm² and 1.08·10¹¹ W/cm² at the wavelength equal to 355 nm and between 1.0·10⁷ W/cm² and 1.0·10¹² W/cm² at 532 nm. The pulse duration was equal to 10 ns in all cases. A method for recovery of an effective dielectric function of nanoparticles is suggested. It was found that, in the case of the interband transition, the recovered dielectric functions for a short duration of the laser pulse sequences can be fitted by parametric dependencies corresponding to the Lorentz model. A kinetic model describing the changes in the population of mobile carriers is considered. It was found that the efficiency of the charge recombination is considerably less than the efficiency of the trapping. The dwell time of the mobile charge carriers before being captured was estimated as ≈13 ms.

Intrinsic charge trapping in amorphous oxide films: status and challenges

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states stemming from the disorder of amorphous metal oxide networks. We start from presenting the results for amorphous (a) HfO₂, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy measurements and theoretical calculations using density functional theory shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO₂. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modeling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO₂ and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO₂, a-Al₂O₃, a-TiO₂. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO₂ and a-SiO₂ weakens Hf(Si)-O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O^2- ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions.

Tuning defects in oxides at room temperature by lithium reduction

Defects can greatly influence the properties of oxide materials; however, facile defect engineering of oxides at room temperature remains challenging. The generation of defects in oxides is difficult to control by conventional chemical reduction methods that usually require high temperatures and are time consuming. Here, we develop a facile room-temperature lithium reduction strategy to implant defects into a series of oxide nanoparticles including titanium dioxide (TiO₂), zinc oxide (ZnO), tin dioxide (SnO₂), and cerium dioxide (CeO₂). Our lithium reduction strategy shows advantages including all-room-temperature processing, controllability, time efficiency, versatility and scalability. As a potential application, the photocatalytic hydrogen evolution performance of defective TiO₂ is examined. The hydrogen evolution rate increases up to 41.8 mmol g⁻¹ h⁻¹ under one solar light irradiation, which is ~3 times higher than that of the pristine nanoparticles. The strategy of tuning defect oxides used in this work may be beneficial for many other related applications.

Defective oxides are attractive for energy conversion and storage applications, but it remains challenging to implant defects in oxides under mild conditions. Here, the authors develop a versatile lithium reduction strategy to engineer the defects of oxides at room temperature leading to enhanced photocatalytic properties.

Oxygen induced enhancement of NIR emission in brookite TiO2 powders: comparison with rutile and anatase TiO2 powders

Brookite TiO₂ attracts considerable attention in photocatalysis owing to its superior performance in several photocatalytic reactions. In this work, we investigated the behavior of charge carriers in brookite, rutile, and anatase TiO₂ by using photoluminescence (PL) and transient absorption (TA) spectroscopies. PL measurements revealed that brookite TiO₂ exhibits a visible and a NIR emission at ∼520 nm and ∼860 nm, respectively. Addition of methanol vapor quenched both the visible and NIR emissions by the hole-consuming reaction of methanol. However, exposure to O₂ shows curious behaviors: the visible emission was quenched but the NIR emission was enhanced. These results can be accounted for by the enhancement of upward band bending resulting in the effective separation of electrons and holes into the bulk and the surface, respectively. Furthermore, the shallowly trapped electrons, which are responsible for visible PL, are consumed by O₂; hence, the visible emission is quenched. However, in the case of NIR emission, the deeply trapped electrons are responsible and they are mainly located at the surface defects. The O₂ adsorption promotes the hole accumulation at the surface and then assists the recombination of these deeply trapped electrons, resulting in the enhancement of the NIR emission. We also found that the lifetime of NIR emission (τ₁ = 43 ± 0 ns and τ₂ = 589 ± 1 ns) was much longer than that of visible emission (τ₁ = 15 ± 0 ns and τ₂ = 23 ± 0 ns), since the mobility of these deeply trapped electrons to encounter with holes is lower than that of the shallowly trapped electrons. However, even for this slow NIR emission, the actual lifetime of the deeply trapped electrons estimated by TA (1.5 ± 0.0 μs and 17 ± 0 μs) was one or two orders of magnitude longer, confirming that non-radiative recombination is dominant and it is much slower than radiative recombination: TAS and PL provide detailed information on the radiative and non-radiative recombination processes. The PL of anatase and rutile TiO₂ powders was also measured and the difference from brookite TiO₂ was discussed.

Weak Donor-Acceptor Interaction and Interface Polarization Define Photoexcitation Dynamics in the MoS2/TiO2 Composite: Time-Domain Ab Initio Simulation

To realize the full potential of transition metal dichalcogenides interfaced with bulk semiconductors for solar energy applications, fast photoinduced charge separation, and slow electron-hole recombination are needed. Using a combination of time-domain density functional theory with nonadiabatic molecular dynamics, we demonstrate that the key features of the electron transfer (ET), energy relaxation and electron-hole recombination in a MoS₂-TiO₂ system are governed by the weak van der Waals interfacial interaction and interface polarization. Electric fields formed at the interface allow charge separation to happen already during the photoexcitation process. Those electrons that still reside inside MoS₂, transfer into TiO₂ slowly and by the nonadiabatic mechanism, due to weak donor-acceptor coupling. The ET time depends on excitation energy, because the TiO₂ state density grows with energy, increasing the nonadiabatic transfer rate, and because MoS₂ sulfur atoms start to contribute to the photoexcited state at higher energies, increasing the coupling. The ET is slower than electron-phonon energy relaxation because the donor-acceptor coupling is weak, rationalizing the experimentally observed injection of primarily hot electrons. The weak van der Waals MoS₂-TiO₂ interaction ensures a long-lived charge separated state and a short electron-hole coherence time. The injection is promoted primarily by phonons within the 200-800 cm^-1 range. Higher frequency modes are particularly important for the electron-hole recombinations, because they are able to accept large amounts of electronic energy. The predicted time scales for the forward and backward ET, and energy relaxation can be measured by time-resolved spectroscopies. The reported simulations generate a detailed time-domain atomistic description of the complex interplay of the charge and energy transfer processes at the MoS₂/TiO₂ interface that are of fundamental importance to photovoltaic and photocatalytic applications. The results suggest that even though the photogenerated charge-separated state is long-lived, the slower charge separation, compared to the electron-phonon energy relaxation, can present problems in practical applications.

Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts

In this mini-review, we discuss what insight computational modelling can provide into the working of photocatalysts for solar fuel synthesis and how calculations can be used to screen for new promising materials for photocatalytic water splitting and carbon dioxide reduction. We will extensively discuss the different relevant (material) properties and the computational approaches (DFT, TD-DFT, GW/BSE) available to model them. We illustrate this with examples from the literature, focussing on polymeric and nanoparticle photocatalysts. We finish with a perspective on the outstanding conceptual and computational challenges.

Continuum and atomistic description of excess electrons in TiO2

The modelling of an excess electron in a semiconductor in a prototypical dye sensitised solar cell is carried out using two complementary approaches: atomistic simulation of the TiO2 nanoparticle surface is complemented by a dielectric continuum model of the solvent-semiconductor interface. The two methods are employed to characterise the bound (excitonic) states formed by the interaction of the electron in the semiconductor with a positive charge opposite the interface. Density-functional theory (DFT) calculations show that the excess electron in TiO2 in the presence of a counterion is not fully localised but extends laterally over a large region, larger than system sizes accessible to DFT calculations. The numerical description of the excess electron at the semiconductor-electrolyte interface based on the continuum model shows that the exciton is also delocalised over a large area: the exciton radius can have values from tens to hundreds of Ångströms, depending on the nature of the semiconductor (characterised by the dielectric constant and the electron effective mass in our model).

Electronic structures of bare and terephthalic acid adsorbed TiO2(110)-(1 × 2) reconstructed surfaces: origin and reactivity of the band gap states

Ti₂O₃-row contributed band gap states are sensitive to TPA adsorption, resulting in the redistribution of Ti 3d states at the interface.