The description of the band gap of halide perovskites at the level of density functional theory (DFT) has been subject of several studies but still presents significant problems and deviations from experimental values. Various approaches have been proposed, including the use of system-specific hybrid functionals with a variable amount of exact exchange or the explicit inclusion of spin–orbit coupling (SOC) effects. In this work, we present a pragmatic recipe to compute the band gap of halide perovskites with a minimum average error. The recipe is tested on a set of 36 halide perovskites of the type ABX₃ [A = Cs, methyl-ammonium (MA), and formamidinium (FA); B = Ge, Sn, and Pb; and X = Cl, Br, and I] for which experimental estimates of the band gap have been reported in the literature. Upon assessment of the accuracy of commonly used DFT functionals and the analysis of their performances based on error and statistical analysis, we suggest a strategy to compute band gaps in halide perovskites with a single functional. This is based on the use of the hybrid HSE06 functional where SOC is included exclusively for Pb-containing compounds. The results are rationalized in terms of the materials’ chemical nature and are corroborated by the prediction of their expected efficiencies in solar cells. The calculated efficiencies from band gaps obtained with the proposed approach closely follow the experimental trend, demonstrating the importance of adopting a reliable but material-independent computational strategy to screen new halide perovskite materials for solar energy conversion.

The surface functionalization of TiO₂-based materials with alkylsilanes is attractive in several cutting-edge applications, such as photovoltaics, sensors, and nanocarriers for the controlled release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES) is able to self-assemble to form monolayers on TiO₂ surfaces, but its adsorption geometry and solar-induced photodegradation pathways are not well understood. We here employ advanced experimental (XPS, NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools to investigate the preferential interaction mode of APTES on anatase TiO₂. We demonstrate that monomeric APTES chemisorption should proceed through covalent Si–O–Ti bonds. Although dimerization of the silane through Si–O–Si bonds is possible, further polymerization on the surface is scarcely probable. Terminal amino groups are expected to be partially involved in strong charge-assisted hydrogen bonds with surface hydroxyl groups of TiO₂, resulting in a reduced propensity to react with other species. Solar-induced mineralization proceeds through preferential cleavage of the alkyl groups, leading to the rapid loss of the terminal NH₂ moieties, whereas the Si-bearing head of APTES undergoes slower oxidation and remains bound to the surface. The suitability of employing the silane as a linker with other chemical species is discussed in the context of controlled degradation of APTES monolayers for drug release and surface patterning.

The hydrogen evolution reaction (HER) has a key role in electrochemical water splitting. Recently a lot of attention has been dedicated to HER from single atom catalysts (SACs). The activity of SACs in HER is usually rationalized or predicted using the original model proposed by Nørskov where the free energy of a H atom adsorbed on an extended metal surface M (formation of an MH intermediate) is used to explain the trends in the exchange current for HER. However, SACs differ substantially from metal surfaces and can be considered analogues of coordination compounds. In coordination chemistry, at variance with metal surfaces, stable dihydride or dihydrogen complexes (HMH) can form. We show that the same can occur on SACs and that the formation of stable HMH intermediates, in addition to the MH one, may change the kinetics of the process. Extending the original kinetic model to the case of two intermediates (MH and HMH), one obtains a three-dimensional volcano plot for the HER on SACs. DFT numerical simulations on 55 models demonstrate that the new kinetic model may lead to completely different conclusions about the activity of SACs in HER. The results are validated against selected experimental cases. The work provides an example of the important analogies between the chemistry of SACs and that of coordination compounds.

One-pot three-component regioselective azide-alkyne cycloadditions are central reactions for synthesizing pharmaceuticals and fine chemicals and are also applied for in vivo metabolic labeling biotechnology. Homogeneous catalysts based on copper species coordinated with ancillary ligands are regularly used to perform this reaction, offering superior catalytic activity and selectivity compared to conventional heterogeneous counterparts based on supported copper nanoparticles. However, the challenge of catalyst recovery limits the use of these homogeneous compounds in many large-scale applications. In this work, we report the high catalytic performance of a family of Cu-based single-atom catalysts for triazole synthesis, with an emphasis on the fundamental understanding of the structure and function of the catalyst. The catalysts were prepared via tricyanomethanide polymerization to create a joint electronic structure where the mesoporous graphitic carbon nitride carrier acts as a ligand for the atomically dispersed copper species. The material properties and the precise metal location/coordination (i.e., deposited in the heptazine pore of carbon nitride, substituted in the framework of carbon nitride, hosted in a vacancy, or entrapped in sandwich-like arrangement) were characterized through a battery of spectroscopic and theoretical methods. The catalysts were employed in the synthesis of 1,2,3-triazoles employing azide-alkyne click reaction under base-free conditions. The single-atom Cu catalysts demonstrated improved activity and selectivity compared to the homogeneous reference catalyst. Density functional theory calculations corroborated the results and showed that the reaction proceeds through a barrier given by the activation of the acetylenic moiety on Cu₁. The activity of this step was primarily affected by the coordination of the metal with the support. Therefore, understanding the metal coordination in single-atom catalysts is critical to further optimizing single-atom catalysts and greening synthetic chemistry.