Theoretical Approaches to Excited-State-Related Phenomena in Oxide Surfaces

Modeling Heterogeneous Catalysis Using Quantum Computers: An Academic and Industry Perspective

Heterogeneous catalysis plays a critical role in many industrial processes, including the production of fuels, chemicals, and pharmaceuticals, and research to improve current catalytic processes is important to make the chemical industry more sustainable. Despite its importance, the challenge of identifying optimal catalysts with the required activity and selectivity persists, demanding a detailed understanding of the complex interactions between catalysts and reactants at various length and time scales. Density functional theory (DFT) has been the workhorse in modeling heterogeneous catalysis for more than three decades. While DFT has been instrumental, this review explores the application of quantum computing algorithms in modeling heterogeneous catalysis, which could bring a paradigm shift in our approach to understanding catalytic interfaces. Bridging academic and industrial perspectives by focusing on emerging materials, such as multicomponent alloys, single-atom catalysts, and magnetic catalysts, we delve into the limitations of DFT in capturing strong correlation effects and spin-related phenomena. The review also presents important algorithms and their applications relevant to heterogeneous catalysis modeling to showcase advancements in the field. Additionally, the review explores embedding strategies where quantum computing algorithms handle strongly correlated regions, while traditional quantum chemistry algorithms address the remainder, thereby offering a promising approach for large-scale heterogeneous catalysis modeling. Looking forward, ongoing investments by academia and industry reflect a growing enthusiasm for quantum computing's potential in heterogeneous catalysis research. The review concludes by envisioning a future where quantum computing algorithms seamlessly integrate into research workflows, propelling us into a new era of computational chemistry and thereby reshaping the landscape of modeling heterogeneous catalysis.

Atomic and Electronic Structures of Co-Doped In2O3 from Experiment and Theory

The synthesis and properties of stoichiometric, reduced, and Co-doped In2O3 are described in the light of several experimental techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet (UV)-visible spectroscopy, porosimetry, and density functional theory (DFT) methods on appropriate models. DFT-based calculations provide an accurate prediction of the atomic and electronic structure of these systems. The computed lattice parameter is linearly correlated with the experimental result in the Co concentration ranging from 1.0 to 5.0%. For higher Co concentrations, the theoretical-experimental analysis of the results indicates that the dopant is likely to be preferentially present at surface sites. The analysis of the electronic structure supports the experimental assignment of Co2+ for the doped material. Experiments and theory find that the presence of Co has a limited effect on the material band gap.

Theoretical insights into dopamine photochemistry adsorbed on graphene-type nanostructures

The equilibrium geometry structures and light absorption properties of the dopamine (DA) and dopamine-o-quinone (DAQ) adsorbed on the graphene surface have been investigated using the ground state and linear-response time-dependent density functional theories. Two types of graphene systems were considered, a rectangular form of hexagonal lattice with optimized C-C bond length as the model system for graphene nanoparticles (GrNP) and a similar system but with fixed C-C bond length (1.42 Å) as the model system for graphene 2D sheet (GrS). The analysis of the vertical excitations showed that three types of electronic transitions are possible, namely, localized on graphene, localized on the DA or DAQ, and charge transfer (CT). In the case of the graphene-DA complex, the charge transfer excitations were characterized by the molecule-to-surface (MSCT) character, whereas the graphene-DAQ was characterized by the reverse, i.e. surface-to-molecule (SMCT). The difference between the two cases is given by the presence of an energetically low-lying unoccupied orbital (LUMO+1) that allows charge transfer from the surface to the molecule in the case of DAQ. However, it was also shown that the fingerprints of excited electronic states associated with the adsorbed molecules cannot be seen in the spectrum, as they are mostly suppressed by the characteristic spectral shape of graphene.

Activating two-dimensional semiconductors for photocatalysis: a cross-dimensional strategy

The emerging two-dimensional (2D) semiconductors substantially extend materials bases for versatile applications such as semiconductor photocatalysis demanding semiconductive matrices and large surface areas. The dimensionality, while endowing 2D semiconductors the unique properties to host photocatalytic functionality of pollutant removal and hydrogen evolution, hurdles the activation paths to form heterogenous photocatalysts where the photochemical processes are normally superior over these on the mono-compositional counterparts. In this perspective, we present a cross-dimensional strategy to employ thenD (n= 0-2) clusters or nanomaterials as activation partners to boost the photocatalytic activities of the 2D semiconductors. The formation principles of heterogenous photocatalysts are illustrated specifically for the 2D matrices, followed by selection criteria of them among the vast 2D database. The computer investigations are illustrated in the density functional theory route and machine learning benefitted from the vast samples in the 2D library. Synthetic realizations and characterizations of the 2D heterogenous systems are introduced with an emphasis on chemical methods and advanced techniques to understand materials and mechanistic studies. The perspective outlooks cross-dimensional activation strategies of the 2D materials for other applications such as CO2removal, and materials matrices in other dimensions which may inspire incoming research within these fields.

Density Matrix Embedding Using Multiconfiguration Pair-Density Functional Theory

We present a quantum embedding method for ground and excited states of extended systems that uses multiconfiguration pair-density functional theory (MC-PDFT) with densities provided by periodic density matrix embedding theory (pDMET). We compute local excitations in oxygen mono- and divacancies on a magnesium oxide (100) surface and find absolute deviations within 0.05 eV between pDMET using the MC-PDFT, denoted as pDME-PDFT, and the more expensive, nonembedded MC-PDFT approach. We further use pDME-PDFT to calculate local excitations in larger supercells for the monovacancy defect, for which the use of nonembedded MC-PDFT is prohibitively costly.

Toward a Rigorous Theoretical Description of Photocatalysis Using Realistic Models

This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

Magnetic nature and hyperfine interactions of transition metal atoms adsorbed on ultrathin insulating films: a challenge for DFT

The magnetic ground state and the hyperfine coupling parameters of some first-row transition metal (TM) atoms (Ti, Cr, Mn, Fe, Co, and Ni) adsorbed on ultrathin insulating oxide films are studied by means of DFT calculations. The results obtained using GGA, screened hybrid, and GGA+U functionals are compared for TMs adsorbed on free-standing MgO(100). Then, the case of adsorption on MgO mono- and bilayers supported on Ag(100) is studied using GGA+U. Along with the problematic aspects inherent to the calculation of hyperfine coupling constants, a critical dependence on the magnetic state and electron configuration of the TM is reported, which implies a real challenge for the state-of-the-art DFT methods. In the cases where all functionals considered provide a coherent magnetic and electron configuration, however, the calculated hyperfine parameters do not depend significantly on the choice of the functional. In this respect, the role of the metal support in the hyperfine coupling constants is highly system-dependent and becomes crucial in all cases where the support modifies the oxidation state of the adatom, induces a change in the bonding site or simply induces a rearrangement of the orbital energy diagram. This has important implications for the modelling of single TM atoms deposited on insulating ultrathin films supported on metals for application in quantum technologies or as memory devices.

Evaluation of molecular photophysical and photochemical properties using linear response time-dependent density functional theory with classical embedding: Successes and challenges

Time-dependent density functional theory (TDDFT) based approaches have been developed in recent years to model the excited-state properties and transition processes of the molecules in the gas-phase and in a condensed medium, such as in a solution and protein microenvironment or near semiconductor and metal surfaces. In the latter case, usually, classical embedding models have been adopted to account for the molecular environmental effects, leading to the multi-scale approaches of TDDFT/polarizable continuum model (PCM) and TDDFT/molecular mechanics (MM), where a molecular system of interest is designated as the quantum mechanical region and treated with TDDFT, while the environment is usually described using either a PCM or (non-polarizable or polarizable) MM force fields. In this Perspective, we briefly review these TDDFT-related multi-scale models with a specific emphasis on the implementation of analytical energy derivatives, such as the energy gradient and Hessian, the nonadiabatic coupling, the spin-orbit coupling, and the transition dipole moment as well as their nuclear derivatives for various radiative and radiativeless transition processes among electronic states. Three variations of the TDDFT method, the Tamm-Dancoff approximation to TDDFT, spin-flip DFT, and spin-adiabatic TDDFT, are discussed. Moreover, using a model system (pyridine-Ag20 complex), we emphasize that caution is needed to properly account for system-environment interactions within the TDDFT/MM models. Specifically, one should appropriately damp the electrostatic embedding potential from MM atoms and carefully tune the van der Waals interaction potential between the system and the environment. We also highlight the lack of proper treatment of charge transfer between the quantum mechanics and MM regions as well as the need for accelerated TDDFT modelings and interpretability, which calls for new method developments.

Robust ΔSCF calculations with direct energy functional minimization methods and STEP for molecules and materials

The direct energy functional minimization method using the orbital transformation (OT) scheme in the program package CP2K has been employed for Δ self-consistent field (ΔSCF) calculations. The OT method for non-uniform molecular orbitals occupations allows us to apply the ΔSCF method for various kinds of molecules and periodic systems. Vertical excitation energies of heteroaromatic molecules and condensed phase systems, such as solvated ethylene and solvated uracil obeying periodic boundary conditions, are reported using the ΔSCF method. In addition, a Re-phosphate molecule attached to the surface of anatase (TiO₂) has been investigated. Additionally, we have implemented a recently proposed state-targeted energy projection ΔSCF algorithm [K. Carter-Fenk and J. M. Herbert, J. Chem. Theory Comput. 16(8), 5067-5082 (2020)] for diagonalization based SCF in CP2K. It is found that the OT scheme provides a smooth and robust SCF convergence for all investigated excitation energies and (non-)periodic systems.

Surface modeling of photocatalytic materials for water splitting

The photocatalyst surface is central to photocatalytic reactions. However, it has been a challenge to explicitly understand both the surface configuration and the structure-dependent photocatalytic properties at the atomic level. First-principles density functional theory (DFT) calculations provide a versatile method that makes up for the lack of experimental surface studies. In DFT calculations, the initial surface model greatly affects the accuracy of the calculation results. Consequently, establishing a more realistic and more reliable material surface models is undoubtedly the first step and the most important link in theoretical calculations. The aim of this Perspective is to provide a general understanding of the methods for the surface modeling of photocatalytic materials in recent years. We begin with a discussion of the basic theories applied in photocatalytic surface research, followed by an explanation of the importance of surface modeling in photocatalysis. We then elaborate on the advantages and disadvantages of the basic surface model and briefly describe the latest surface modeling methods. Finally, we evaluate the rationality of current surface modeling methods. We summarize this Perspective by prospecting the developing directions of photocatalytic surface research in the future. It is believed that a reasonable surface model should be verified by both experimental characterization and theoretical computation with negative feedback.

Excited States of Crystalline Point Defects with Multireference Density Matrix Embedding Theory

Accurate and affordable methods to characterize the electronic structure of solids are important for targeted materials design. Embedding-based methods provide an appealing balance in the trade-off between cost and accuracy─particularly when studying localized phenomena. Here, we use the density matrix embedding theory (DMET) algorithm to study the electronic excitations in solid-state defects with a restricted open-shell Hartree-Fock (ROHF) bath and multireference impurity solvers, specifically, complete active space self-consistent field (CASSCF) and n-electron valence state second-order perturbation theory (NEVPT2). We apply the method to investigate the electronic excitations in an oxygen vacancy (OV) on a MgO(100) surface and find absolute deviations within 0.05 eV between DMET using the CASSCF/NEVPT2 solver, denoted as CAS-DMET/NEVPT2-DMET, and the nonembedded CASSCF/NEVPT2 approach. Next, we establish the practicality of DMET by extending it to larger supercells for the OV defect and a neutral silicon vacancy in diamond where the use of nonembedded CASSCF/NEVPT2 is extremely expensive.

A novel synthetic carbon and oxygen doped stalactite-like g-C3N4 for broad-spectrum-driven indometacin degradation

Achieving efficient solar utilization is a primary goal in the field of photocatalytic degradation of PPCPs. For this study, a broad-spectrum carbon and oxygen doped, porous g-C₃N₄ (COCN) was synthesized via a simple co-pyrolysis of dicyandiamide and methylamine hydroiodide (CH₅N·HI). The 0.3COCN demonstrated an excellent photocatalytic degradation of indometacin (IDM), which was 5.9 times higher than bulk g-C₃N₄. The enhanced photocatalytic activity could be ascribed to the broad-spectrum utilization of solar light and improved charge separation efficiency. Reactive species (RSs) scavenging experiments have shown that O₂^·- and ¹O₂ were the dominant active species. Further, the 0.3COCN exhibits excellent yield of hydroxyl radicals which was confirmed by electron spin resonance (ESR) spectra. Meanwhile, the degradation pathways of IDM were proposed according the HRAM LC-MS/MS and total organic carbon (TOC). This research provided a new strategy for a broad-spectrum photocatalyst, and a promising strategy for environmental remediation.

High dielectric ternary oxides from crystal structure prediction and high-throughput screening

The development of new high dielectric materials is essential for advancement in modern electronics. Oxides are generally regarded as the most promising class of high dielectric materials for industrial applications as they possess both high dielectric constants and large band gaps. Most previous researches on high dielectrics were limited to already known materials. In this study, we conducted an extensive search for high dielectrics over a set of ternary oxides by combining crystal structure prediction and density functional perturbation theory calculations. From this search, we adopted multiple stage screening to identify 441 new low-energy high dielectric materials. Among these materials, 33 were identified as potential high dielectrics favorable for modern device applications. Our research has opened an avenue to explore novel high dielectric materials by combining crystal structure prediction and high throughput screening.

Prediction of Absorption Spectrum Shifts in Dyes Adsorbed on Titania

Dye adsorption on metal-oxide films often results in small to substantial absorption shifts relative to the solution phase, with undesirable consequences for the performance of dye-sensitized solar cells and optical sensors. While density functional theory is frequently used to model such behaviour, it is too time-consuming for rapid assessment. In this paper, we explore the use of supervised machine learning to predict whether dye adsorption on titania is likely to induce a change in its absorption characteristics. The physicochemical features of each dye were encoded as a numeric vector whose elements are the counts of molecular fragments and topological indices. Various classification models were subsequently trained to predict the type of absorption shift i.e. blue, red or unchanged (|Δλ| ≤ 10 nm). The models were able to predict the nature of the shift with a good likelihood (~80%) of success when applied to unseen data.

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.

Braiding kinetics and spectroscopy in photo-catalysis: the spectro-kinetic approach

The combination of kinetic and spectroscopic tools has become a key scientific methodology for the understanding of catalytic behavior but its application in photocatalysis has inherent difficulties due to the nature of the energy source of the reaction. This review article provides an overview of its use by, first, presenting mechanistically derived kinetic formulations and spectroscopic data handling methods including intrinsic expressions for light and, second, highlighting representative examples of application. To do it we consider universal catalytic systems, particularly (although not exclusively) titania-based materials, and the most frequent hole and/or electron triggered reaction schemes. This review also provides a general framework to pave the way for the future progress of the spectro-kinetic approach in the photocatalysis area.

Effect of doping and chemical ordering on the optoelectronic properties of complex oxides: Fe2O3-V2O3 solid solutions and hetero-structures

The electronic and optical properties of α-(Fe_1-xV_x)₂O₃ at low (x = 0.04) and high (x = 0.5) doping levels are investigated using a combination of periodic and embedded cluster approaches, and time-dependent density functional theory. At low V concentrations the onset of the optical absorption is ∼0.5 eV (i.e., nearly 1.6 eV lower than that in pure α-Fe₂O₃) and corresponds to the electron transitions from V 3d to Fe 3d* orbitals. At high V concentrations, optical absorption energies and intensities are sensitive to specific arrangements of Fe and V atoms and their spin configuration that determine Fe-V hybridization. The onset of the lowest inter-vanadium absorption band in the case of Fe₂O₃/V₂O₃ hetero-structures is as low as ∼0.3 eV and the corresponding peak is at ∼0.7 eV. In contrast, in the case of solid solutions this peak has lower intensity and is shifted to higher energy (∼1.2 eV). Analysis of the orbital character of electronic excitation suggests that Fe₂O₃/V₂O₃ hetero-structures absorb light much more effectively than random alloys, thus promoting efficient photo-induced carrier generation. These predictions can be tested in α-(Fe_1-xV_x)₂O₃ thin films synthesized with well-controlled spatial distribution of Fe and V species.

Performance of a modified hybrid functional in the simultaneous description of stoichiometric and reduced TiO2polymorphs

A modified hybrid functional is proposed to properly describe the electronic structure and physical properties for stoichiometric and reduced TiO₂polymorphs.

First-principles study of relative stability of rutile and anatase TiO2 using the random phase approximation

By considering high-order correlations using the random phase approximation, rutile is correctly predicted to be more stable than anatase.

Adsorption properties of trifluoroacetic acid on anatase (101) and (001) surfaces: a density functional theory study

Trifluoroacetic acid adsorbs preferentially on the TiO₂(001) surface working as a growth control agent.

Perspectives on heterogeneous photochemistry

Heterogeneous photochemistry has a potentially important role in production of energy, in environmental remediation and in sustainable production of chemicals. Photochemical efficiency depends on both materials properties and the desired chemical reaction that is promoted through creation of an excited state. A detailed understanding of the interplay between materials properties and reactivity requires a molecular-scale approach that determines the elementary steps in the overall process. This personal account summarizes the role of defects in determining the photochemical and thermal reactions on rutile titania, a model for semiconductor metal oxide photocatalysts that defects, e.g., Ti interstitials present in the subsurface region, and O adatoms on the surface, have a substantial impact on the efficiency for photochemical conversion through modification of molecular binding and also through likely modification of charge carrier dynamics. Design of materials must include engineering of the optical and electronic properties of the semiconductor photocatalyst, and understanding of the key photochemical steps involved in specific processes to ensure proper alignment of their electronic states with the band structure of the material. Thus, fundamental surface science studies and development of time-dependent theoretical methods that map out the reaction mechanism for photochemical processes on materials with controlled composition and structure are critical.

Electronic structure of low-dimensional 4d(5) oxides: interplay of ligand distortions, overall lattice anisotropy, and spin-orbit interactions

The electronic structure of the low-dimensional 4d(5) oxides Sr2RhO4 and Ca3CoRhO6 is herein investigated by embedded-cluster quantum chemistry calculations. A negative tetragonal-like t2g splitting is computed in Sr2RhO4 and a negative trigonal-like splitting is predicted for Ca3CoRhO6, in spite of having positive tetragonal distortions in the former material and cubic oxygen octahedra in the latter. Our findings bring to the foreground the role of longer-range crystalline anisotropy in generating noncubic potentials that compete with local distortions of the ligand cage, an issue not addressed in standard textbooks on crystal-field theory. We also show that sizable t2g(5)-t2g(4)eg(1) couplings via spin-orbit interactions produce in Sr2RhO4 ⟨Z⟩ = ⟨Σ(i)l(i)·s(i)⟩ ground-state expectation values significantly larger than 1, quite similar to theoretical and experimental data for 5d(5) spin-orbit-driven oxides such as Sr2IrO4. On the other hand, in Ca3CoRhO6, the ⟨Z⟩ values are lower because of larger t2g-eg splittings. Future X-ray magnetic circular dichroism experiments on these 4d oxides will constitute a direct test for the ⟨Z⟩ values that we predict here, the importance of many-body t2g-eg couplings mediated by spin-orbit interactions, and the role of low-symmetry fields associated with the extended surroundings.

Understanding the reactivity of metallic nanoparticles: beyond the extended surface model for catalysis

Metallic nanoparticles (NPs) constitute a new class of chemical objects which are used in different fields as diverse as plasmonics, optics, catalysis, or biochemistry.

Electronic and chemical properties of a surface-terminated screw dislocation in MgO

Dislocations represent an important and ubiquitous class of topological defect found at the surfaces of metal oxide materials. They are thought to influence processes as diverse as crystal growth, corrosion, charge trapping, luminescence, molecular adsorption, and catalytic activity; however, their electronic and chemical properties remain poorly understood. Here, through a detailed first-principles investigation into the properties of a surface-terminated screw dislocation in MgO we provide atomistic insight into these issues. We show that surface dislocations can exhibit intriguing electron trapping properties which are important for understanding the chemical and electronic characteristics of oxide surfaces. The results presented in this article taken together with recent experimental reports show that surface dislocations can be equally as important as more commonly considered surface defects, such as steps, kinks, and vacancies, but are now just beginning to be understood.

Self-Ordered Titanium Dioxide Nanotube Arrays: Anodic Synthesis and Their Photo/Electro-Catalytic Applications

Metal oxide nanotubes have become a widely investigated material, more specifically, self-organized titania nanotube arrays synthesized by electrochemical anodization. As a highly investigated material with a wide gamut of applications, the majority of published literature focuses on the solar-based applications of this material. The scope of this review summarizes some of the recent advances made using metal oxide nanotube arrays formed via anodization in solar-based applications. A general methodology for theoretical modeling of titania surfaces in solar applications is also presented.