Harvesting solar energy for artificial photosynthesis is an emerging area in alternative energy research. In the present article, we have investigated the photocatalytic properties of single-layer group IV–VI monochalcogenides, MXs (M = Ge, Si, Sn and X = S, Se) based on first-principles electronic structure calculations. Our dispersion corrected DFT calculations show that these materials have moderate cohesive energies (<120 meV/atom), which are indicative of favorable isolation of MX monolayers by mechanical, sonicated, or liquid-phase exfoliation. The calculated band gaps using hybrid density functional method (HSE06) reveal that all of the MXs show larger band gaps than the minimum energy required for the water splitting reaction (1.23 eV). Considering band edge alignments, all the MXs other than SiS have an acceptable alignment of conduction band minima but not the valence band maxima. We have evaluated the overpotentials for both oxygen and hydrogen evolution reactions. Interestingly, considering contribution from overpotentials, we have tuned the band alignments by varying the pH of the medium. At a basic pH, GeS and SiSe exhibit excellent photocatalytic properties whereas for SiS, an acidic pH is required. Additionally, the optical absorption spectrum shows excellent absorption in the visible region indicating efficient harvesting of solar radiation. They are substantially stable even in aqueous environment indicating their robust stability at ambient electrochemical conditions.

Black phosphorus (BP), despite possessing a favorable direct band gap, suffers from structural instability at ambient conditions that limits its utility for lithium ion batteries (LIB). In this Letter, we have proposed h-BN as an effective capping agent for black-phosphorene (Pn) for application as an anode material in both LIBs and sodium ion batteries (SIBs). The binding energy of Li/Na in the h-BN/black-Pn heterostructure is greatly enhanced (2.81 eV/2.55 eV) vis-a-vis pristine Pn (1.80 eV/1.59 eV) along with reduction in the barrier for movement of Li/Na within the layers. Significantly, lithiation/sodiation of these heterostructures does not alter the packing patterns due to insignificant volume changes (∼1.5−2.0%). The theoretical specific capacities for h-BN/black-Pn is 607 and 445 mA h g −1 for LIB and SIB, respectively, which are larger than those for existing commercial anode materials. Clearly, the high capacity, low open-circuit voltage, small volume change, and high mobility of Li/Na within the layers make h-BNcapped black-Pn an excellent anode material in LIBs/SIBs. The heterostructure exhibits an interesting semiconductor → metal electronic phase transition upon lithiation/sodiation. C hemical energy is the most appropriate form of energy storage in terms of energy density. Among the various available energy storage technologies, lithium ion batteries (LIBs) and sodium ion batteries (SIBs) have become prime candidates in next-generation energy storage devices. Due to their high energy density, enhanced rate capabilities, and good cycle life, LIBs are already in use for anode materials.

Choice of suitable electrode material is a fundamental step in Li-ion battery (LIB) to achieve enhanced performance. In the present study we have explored the feasibility of phosphorene analogs, i.e. group IV monochalcogenides (SiS, SiSe, GeS, GeSe,SnS and SnSe) monolayers to serve as anode material in LIB by density functional theory(DFT). Our exploratory study indicates lithium binds efficiently to these monolayers of which Li@SiS and Li@SiSe show appreciable stability which are comparable to phosphorene. Zero point energy corrected minimum energy pathway (MEP) for Li diffusion demonstrates high anisotropy for both SiS and SiSe with a low diffusion barrier of ~0.15eV along the zigzag direction.Inclusion of corrections due to quantum effects like the zero point energy (ZPE) and quantum mechanical tunneling (QMT) increase the diffusion rates by 6-10 % at room temperature and become increasingly significant as temperature is reduced (40-55 % increment at T=100K). The calculated theoretical capacity for SiS and SiSe are 445.7 mAhg -1 and 250.44 mAhg -1 respectively which are well above existing commercially available used anode materials. Both SiS and SiSe preserve their structural integrity upon lithiation justifying their role as host material for lithium. A semiconductor → metallic transition is observed upon full lithiation for both. All these exceptional properties including low diffusion barrier, moderate to high specific capacity, low open circuit voltage (OCV), small volume change and good electrical conductivity, suggest that monolayer SiS and SiSe could serve as a promising electrode material in LIB.

Phosphorene (Pn) is stabilized as a layered material like graphite, yet it possess a natural direct band gap (Eg = 2.0 eV). Interestingly, unlike graphene, Pn exhibits a much richer phase diagram which includes distorted forms like the stapler-clip (black Pn, α form) and chairlike (blue Pn, β form) structures. The existence of these phases is attributed to pseudo-Jahn-Teller (PJT) instability of planar hexagonal P6(6-) rings. In both cases, the condition for vibronic instability of the planar P6(6-) rings is satisfied. Doping with electron donors like tetrathiafulvalene and tetraamino-tetrathiafulvalene and electron acceptors like tetracyanoquinodimethane and tetracyanoethylene convert blue Pn into N-type and black Pn into efficient P-type semiconductors, respectively. Interestingly, pristine blue Pn, an indirect gap semiconductor, gets converted into a direct gap semiconductor on electron or hole doping. Because of comparatively smaller undulation in blue Pn (with respect to black Pn), the van der Waals interactions between the dopants and blue Pn is stronger. PJT distortions for two-dimensional phosphorus provides a unified understanding of structural features and chemical reactivity in its different phases.

Phosphorene, the monolayer form of black phosphorus, is the most recent addition to graphene-like van der Waals two-dimensional (2D) systems. Due to its several interesting properties, namely its tunable direct band gap, high carrier mobility, and unique in-plane anisotropy, it has emerged as a promising candidate for electronic and optoelectronic devices. Phosphorene (Pn) reveals a much richer phase diagram than graphene, and it comprises the two forms namely the stapler-clip like (black Pn, α form) and chairlike (blue Pn, β form) structures. Regardless of its favorable properties, black Pn suffers from instability in oxygen and water, which limits its successful applications in electronic devices. In this Perspective, the cause of structural diversity of Pn, which leads to different properties of both black and blue Pn, is discussed. We provide possible solutions for protecting phosphorene from chemical degradation and its applications in the field of energy storage namely for Li and Na ion batteries.