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.

Reproduction of the band gaps of semiconductors and insulators represents a well-known problem for standard DFT approaches based on semilocal functionals. The problem can be partly solved using hybrid functionals, in which a given portion of exact exchange is mixed with the DFT exchange. Recently, a new class of dielectric-dependent functionals has been introduced in which the amount of exact exchange is derived from the static dielectric function of a given compound. In this study we considered in a systematic way on an equal footing a set of 24 nonmagnetic three-dimensional (3D) bulk metal oxides and 24 quasi-two-dimensional (quasi-2D) semiconductors (oxides, hydroxides, chlorides, oxyhalides, nitrides, and transition metal dichalcogenides) and computed the corresponding Kohn−Sham band gaps with three global hybrid functionals and four range-separated hybrid functionals. These in turn were divided into standard (PBE0, B3LYP, HSE06, SC-BLYP) and dielectric-dependent (DD-B3LYP, DD-SC-BLYP, DD-CAM-B3LYP) functionals. We also performed a statistical analysis of the DFT data set along with structural parameters of these 2D and 3D materials. The surprising result is that overall there is no real improvement with the use of dielectric-dependent functionals compared to PBE0, HSE06, and B3LYP. Short-range DD-SC-BLYP gives a minor improvement in the band gaps for bulk metal oxides compared with standard SC-BLYP, but the mean absolute error is still 0.12 eV higher than with B3LYP. The use of dielectric-dependent standard or short-range functionals such as DD-B3LYP or DD-HSE06 worsens the situation. However, the dielectric-dependent version of the long-range-separated functional implemented with the Coulomb attenuating method (CAM), DD-CAM-B3LYP, leads to a clear improvement for band gaps of quasi-2D materials. On the basis of this analysis, the conclusion is that the use of a standard hybrid functional such as B3LYP or HSE06 is recommended for nonmagnetic bulk 3D metal oxides. On the other hand, the treatment of layered materials such as MoO 3 or V 2 O 5 benefits from the use of dielectricdependent range-separated functionals.

In view of their ability to absorb visible light and their high surface catalytic activity, metal sulfides are rapidly emerging as promising candidates for CO 2 photoreduction, scoring over the traditional oxide-based systems. However, their low conversion efficiencies due to serious radiative recombination issues and poor stability restrict their real-life applicability. Enhancing their performance by coupling them with other semiconductor-based photocatalysts or precious noble metals as cocatalysts makes the process cost intensive. Herein, we report the single-phase ternary sulfide Cu 3 SnS 4 (CTS) as a robust visible-light photocatalyst for selective photoreduction of CO 2 to CH 4 . It showed a remarkable 80% selectivity for CH 4 evolution with the rate of 14 μmol/g/h, without addition of any cocatalyst or scavenger. The mechanistic pathway for catalytic activity is elucidated by first principle calculations and in situ ATR, which imply a formaldehyde pathway of hydrocarbon production. The Cu−Sn termination of the surface is shown to be the key factor for competent CO 2 absorption and activation as confirmed from our X-ray spectroscopy measurements and first principle calculations. This study provides a foundation and insights for the rational design of sulfide-based photocatalysts to produce renewable fuel.

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With the aim of manipulating the mechanical properties of the recently discussed two-dimensional material MXene, we investigate the effect of alloying. We consider substitutional doping of B and V at Ti and C sites of Ti 2 C. Calculations of quantities such as in-plane stiffness, Young's modulus, and critical strain through rigorous first-principles technique establish that B doping is highly effective in improving the elastic properties. Oxygen passivation of B-doped Ti 2 C in addition to improved elastic properties also exhibits reasonably high critical strains making them ideally suited for applications in flexible devices. Our study further reveals the presence of strong spin-phonon coupling in unpassivated Ti 2 C compounds which influences the mechanical behavior. The damage of Ti 2 C in its magnetic ground state of A-type antiferromagnetic structure is found to occur at much higher strain than that of the nonmagnetic Ti 2 C.