Modeling the Photochromism of S‑Doped Sodalites Using DFT, TD-DFT, and SAC-CI Methods

S-doped sodalite minerals of the Na₈Al₆Si₆O₂₄(Cl,S)₂ formula, also known as hackmanites, are computationally investigated for the first time, in order to understand their photochromic properties. With combined periodic boundary conditions and embedded cluster-type approaches, this paper brings a theoretical overview of the photochromism mechanism, also called tenebrescence in geology. Time-dependent density functional theory (TD-DFT) calculations of sodalite systems containing electrons trapped in Cl vacancies showed an absorption spectrum and a simulated color in agreement with experiment. This modeling highlights the huge effect of the F center’s environment such as the direct contribution of the β cage on the trapped electron and a strong vibronic coupling of the absorption spectrum. TD-DFT and post-Hartree–Fock (SAC-CI) calculations were also operated on S₂^2–-containing systems in order to determine the exact mechanism of coloration and discoloration, supporting that the key step is a direct through-space charge transfer between the S₂^2– ion and a Cl vacancy. The geometry modification induced by this charge transfer leads to a large electronic reorganization stabilizing the F center, thus explaining the high stability of the colored state of the mineral

Semiconductors Used in Photovoltaic and Photocatalytic Devices: Assessing Fundamental Properties from DFT

The photovoltaic and photocatalytic systems generally use at least one semiconductor in their architecture which role is to absorb the light or to transport the charge carriers. Despite the large variety of working principles encountered in these systems, they share some fundamental steps such as light absorption, exciton dissociation, and charge carrier diffusion. These phenomena are governed by fundamental properties of the semiconductor like the bandgap, the dielectric constant, the charge carrier effective masses, and the exciton binding energy. The ability of density functional theory to compute all of these properties is evaluated. From the particularly good results obtained with the HSE06 functional, it can be concluded that DFT is a reliable tool for the evaluation and prediction of these key properties which open nice perpectives for <i>in silico</i> design of improved semiconductors for solar energy application. In the light of these calculations, some experimental observations on the difference of efficiencies between semiconductors like TiO₂ anatase and rutile or ZnO are interpreted

Diffusion Kinetics of Gold and Copper Atoms on Pristine and Reduced Rutile TiO₂ (110) Surfaces

Statistical mechanics and transition-state theory have been used to investigate the diffusion kinetics of gold and copper atoms on pristine and various reduced surfaces of rutile TiO₂ (110). A DFT+<i>U</i> approach has been employed to calculate potential energy maps and to evaluate the required diffusion activation barriers. The role of the support reducibility has been examined on the adsorption properties (optimal structures, energetics, and spin polarization) and diffusion kinetics, especially for the reduced support presenting a single subsurface oxygen vacancy. This approach has allowed us to demonstrate key discrepancies between Au and Cu atoms and to sketch out a comparative scenario for the early-stage nucleation of Au and Cu nanoparticles on the various surface states of TiO₂ (110)

First-Principles Modeling of Dye-Sensitized Solar Cells: Challenges and Perspectives

Since dye-sensitized solar cells (DSSCs) appeared as a promising inexpensive alternative to the traditional silicon-based solar cells, DSSCs have attracted a considerable amount of experimental and theoretical interest. In contrast with silicon-based solar cells, DSSCs use different components for the light-harvesting and transport functions, which allow researchers to fine-tune each material and, under ideal conditions, to optimize their overall performance in assembled devices. Because of the variety of elementary components present in these cells and their multiple possible combinations, this task presents experimental challenges. The photoconversion efficiencies obtained up to this point are still low, despite the significant experimental efforts spent in their optimization.The development of a low-cost and efficient computational protocol that could qualitatively (or even quantitatively) identify the promising semiconductors, dyes, and electrolytes, as well as their assembly, could save substantial experimental time and resources. In this Account, we describe our computational approach that allows us to understand and predict the different elementary mechanisms involved in DSSC working principles. We use this computational framework to propose an in silico route for the ab initio design of these materials. Our approach relies on a unique density functional theory (DFT) based model, which allows for an accurate and balanced treatment of electronic and spectroscopic properties in different phases (such as gas, solution, or interfaces) and avoids or minimizes spurious computational effects.Using this tool, we reproduced and predicted the properties of the isolated components of the DSSC assemblies. We accessed the microscopic measurable characteristics of the cells such as the short circuit current (<i>J</i>_sc) or the open circuit voltage (<i>V</i>_oc), which define the overall photoconversion efficiency of the cell. The absence of empirical or material-related parameters in our approach should allow for its wide application to the optimization of existing devices or the design of new ones

Through-Space Charge Transfer in Rod-Like Molecules: Lessons from Theory

Time-dependent density functional theory calculations are performed within a range-separated hybrid framework to quantify the efficiency of through-space charge transfer (CT) in organic rod-like push–pull compounds. Our model allows us to quantify the CT distance, the amount of transferred electron, as well as the spread of the charges. The impact of several kinds of variations has been investigated: (1) the nature and length of the π-conjugated bridge; (2) the strength of the terminal groups; (3) the presence of a central groups; and (4) the use of a polar environment. In α,ω-nitro-dimethylamino chains, we found that the charge transfer is maximized when four to six conjugated rings are separating the donor and the acceptor. The maximum CT distance is ca. 5 Å for these chains but can be improved by 1–2 Å in polar environments. Adding a stronger electron-donating group does not systematically induce an enhancement of the CT if a strong electron-accepting moiety is used, the latter tending to extract the electron from the conjugated chains rather from the donor moiety. There is indeed a fine equilibrium to respect to improve CT. This investigation is a further step toward the rational optimization of charge transfer properties

Modeling Dye-Sensitized Solar Cells: From Theory to Experiment

Density functional theory (DFT) and time-dependent DFT are useful computational approaches frequently used in the dye-sensitized solar cell (DSSC) community in order to analyze experimental results and to clarify the elementary processes involved in the working principles of these devices. Indeed, despite these significant contributions, these methods can provide insights that go well beyond a purely descriptive aim, especially when suitable computational approaches and methodologies for interpreting and validating the computational outcomes are developed. In the present contribution, the possibility of using recently developed computational approaches to design and interpret the macroscopic behavior of DSSCs is exemplified by the study of the performances of three new TiO₂-based DSSCs making use of organic dyes, all belonging to the expanded pyridinium family

Enhanced Kinetics of Hole Transfer and Electrocatalysis during Photocatalytic Oxygen Evolution by Cocatalyst Tuning

Understanding photophysical and electrocatalytic processes during photocatalysis in a powder suspension system is crucial for developing efficient solar energy conversion systems. We report a substantial enhancement by a factor of 3 in photocatalytic efficiency for the oxygen evolution reaction (OER) by adding trace amounts (∼0.05 wt %) of noble metals (Rh and Ru) to a 2 wt % cobalt oxide modified Ta₃N₅ photocatalyst particulate. The optimized system exhibited high quantum efficiencies (QEs) of up to 28 and 8.4% at 500 and 600 nm in 0.1 M Na₂S₂O₈ at pH 14. By isolation of the electrochemical components to generate doped cobalt oxide electrodes, the electrocatalytic activity of cobalt oxide on doping with Ru or Rh was improved in comparison with cobalt oxide, as evidenced by the onset shift for electrochemical OER. Density functional theory (DFT) calculations show that the effect of a second metal addition is to perturb the electronic structure and redox properties in such a way that both hole transfer kinetics and electrocatalytic rates improve. Time-resolved terahertz spectroscopy (TRTS) measurement provides evidence of long-lived electron populations (>1 ns; with mobilities μ_e ≈ 0.1–3 cm² V^–1 s^–1), which are not perturbed by the addition of CoO_<i>x</i>-related phases. Furthermore, we find that Ta₃N₅ phases alone suffer ultrafast hole trapping (within 10 ps); the CoO_<i>x</i> and M/CoO_<i>x</i> decorations most likely induce a kinetic competition between hole transfer toward the CoO_<i>x</i>-related phases and trapping in the Ta₃N₅ phase, which is consistent with the improved OER rates. The present work not only provides a novel way to improve electrocatalytic and photocatalytic performance but also gives additional tools and insight into understand the characteristics of photocatalysts that can be used in a suspension system