V-doped activated Ru/Ti2.5V0.5C2 dual-active center accelerate hydrogen production from ammonia borane

Designing and constructing the active center of Ru-based catalysts is the key to efficient hydrolysis of ammonia borane (NH3BH3, AB) for hydrogen production. Herein, V-doped Ru/Ti2.5V0.5C2 dual-active center catalysts were synthesized, showing excellent catalytic ability for AB hydrolysis. The corresponding turnover frequency value was 1072 min-1 at 298 K, and the hydrolysis rate rB of AB was 235 × 103 mL·min-1·gRu-1. X-ray photoelectron spectroscopy results indicated that the interaction between V-doped Ti3C2 and catalytic metal Ru transfers electrons from Ti to Ru, resulting in electron-rich Ru species. According to density functional theory calculations, the activation energy and reaction dissociation energy of the reactants AB and H2O on V-doped catalysts were lower than those of Ru/Ti3C2, thus optimizing the catalytic kinetics of AB hydrolysis. The modification strategy of V-doped Ti3C2 provides a new pathway for the development of high-performance catalysts for AB hydrolysis.

Vacancy Engineering in 2D Transition Metal Chalcogenide Photocatalyst: Structure Modulation, Function and Synergy Application

Transition metal chalcogenides (TMCs) are widely used in photocatalytic fields such as hydrogen evolution, nitrogen fixation, and pollutant degradation due to their suitable bandgaps, tunable electronic and optical properties, and strong reducing ability. The unique 2D malleability structure provides a pre-designed platform for customizable structures. The introduction of vacancy engineering makes up for the shortcomings of photocorrosion and limited light response and provides the greatest support for TMCs in terms of kinetics and thermodynamics in photocatalysis. This work reviews the effect of vacancy engineering on photocatalytic performance based on 2D semiconductor TMCs. The characteristics of vacancy introduction strategies are summarized, and the development of photocatalysis of vacancy engineering TMCs materials in energy conversion, degradation, and biological applications is reviewed. The contribution of vacancies in the optical range and charge transfer kinetics is also discussed from the perspective of structure manipulation. Vacancy engineering not only controls and optimizes the structure of the TMCs, but also improves the optical properties, charge transfer, and surface properties. The synergies between TMCs vacancy engineering and atomic doping, other vacancies, and heterojunction composite techniques are discussed in detail, followed by a summary of current trends and potential for expansion.

Quantum confinement in chalcogenides 2D nanostructures from first principles

We investigated the impact of quantum confinement on the band gap of chalcogenides 2D nanostructures by means of density functional theory. We studied six different systems: MoS₂, WS₂, SnS₂, GaS, InSe, and HfS₂and we simulated nanosheets of increasing thickness, ranging from ultrathin films to ∼10-13 nm thick slabs, a size where the properties converge to the bulk. In some cases, the convergence of the band gap with slab thickness is rather slow, and sizeable deviations from the bulk value are still present with few nm-thick sheets. The results of the simulations were compared with the available experimental data, finding a quantitative agreement. The impact of quantum confinement can be rationalized in terms of effective masses of electrons and holes and system's size. These results show the possibility of reliably describing quantum confinement effects on systems for which experimental data are not available.

Rational Design of Semiconductor Heterojunctions for Photocatalysis

Electronic structure calculations provide a useful complement to experimental characterization tools in the atomic-scale design of semiconductor heterojunctions for photocatalysis. The band alignment of the heterojunction is of fundamental importance to achieve an efficient charge carrier separation, so as to reduce electron/hole recombination and improve photoactivity. The accurate prediction of the offsets of valence and conduction bands in the constituent units is thus of key importance but poses several methodological and practical problems. In this Minireview we address some of these problems by considering selected examples of binary and ternary semiconductor heterojunctions and how these are determined at the level of density functional theory (DFT). The atomically precise description of the interface, the consequent charge polarization, the role of quantum confinement, the possibility to use facet engineering to determine a specific band alignment, are among the effects discussed, with particular attention to pros and cons of each one of these aspects. This analysis shows the increasingly important role of accurate electronic structure calculations to drive the design and the preparation of new interfaces with desired properties.

Band offset in semiconductor heterojunctions

Semiconductor heterojunctions are widely applied in solid-state device applications, including semiconductor lasers, solar cells, and transistors. In photocatalysis they are of interest due to their capability to hinder charge carriers' recombination. A key role in the performance of heterojunctions is that of the alignment of the band edges of the two units composing the junction. In this work, we compare the performances of three widely applied approaches for the simulation of semiconductors heterostructures, based on density functional theory calculations with hybrid functionals. We benchmark the band offsets of ten semiconductors heterostructures for which experimental values are available: AlP/GaP, AlP/Si, AlAs/GaAs, AlAs/Ge, GaAs/Ge, GaP/Si, ZnSe/Ge, ZnSe/AlAs, ZnSe/GaAs, and TiO₂/SrTiO₃. The methods considered are (i) the alternating slabs junction (ASJ), (ii) the surface terminated junction (STJ), and (iii) the independent units (IU) approach. Moreover, two different ways to determine a common reference have been considered, (i) the plane averaged electrostatic potential, and (ii) the energy of the core levels. Advantages, drawbacks and overall performances of each method are discussed. The results suggest that the accuracy in the estimation of the band offsets is ∼0.2 eV when the ASJ method is applied. The STJ approach provides a similar accuracy, while the neglection of any interface effect, as in the IU method, provides only a qualitative estimate of the band offset and can result in significant deviations from the experiment.

First-Principles Study of Na Intercalation and Diffusion Mechanisms at 2D MoS2/Graphene Interfaces

Na-ion batteries (NIBs) are emerging as promising energy storage devices for large-scale applications. Great research efforts are devoted to design new effective NIB electrode materials, especially for the anode side. A hybrid 2D heterojunction with graphene and MoS2 has been recently proposed for this purpose: while MoS2 has shown good reversible capacity as a NIB anode, graphene is expected to improve conductivity and resistance to mechanical stress upon cycling. The most relevant processes for the anode are the intercalation and diffusion of the large Na ion, whose complex mechanisms are determined by the structural and electronic features of the MoS2/graphene interface. Understanding these processes and mechanisms is crucial for developing new nanoscale anodes for NIBs with high performances. To this end, here we report a state-of-the-art DFT study to address (a) the structural and electronic properties of heterointerfaces between the MoS2 monolayers and graphene, (b) the most convenient insertion sites for Na, and (c) the possible diffusion paths along the interface and the corresponding energy barrier heights. We considered two MoS2 polymorphs: 1T and 3R. Our results show that 1T-MoS2 interacts more strongly with graphene than 3R-MoS2. In both cases, the best Na host site is found at the MoS2 side of the interface, and the band structure reveals a proper n-type character of the graphene moiety, which is responsible for electronic conduction. Minimum-energy paths for Na diffusion show very low barrier heights for the 3R-MoS2/graphene interface (<0.25 eV) and much higher values for its 1T counterpart (∼0.7 eV). Analysis of structural features along the diffusion transition states allows us to identify the strong coordination of Na with the exposed S atoms as the main feature hindering an effective diffusion in the 1T case. These results provide new hints on the physicochemical details of Na intercalation and diffusion mechanisms at complex 2D heterointerfaces and will help further development of advanced electrode materials for efficient NIBs.