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Organic quinones conjugated with a conductive support like graphene via a covalent and/or non-covalent approach are emerging as low-cost and sustainable alternatives to conventional pseudocapacitive materials because of their fast and reversible redox kinetics. Herein, for the first time, reduced graphene oxide (rGO) networks functionalized with the eco-friendly dopamine (DA) moiety in a combined covalent and non-covalent manner have been explored by a facile hydrothermal synthesis method as high-performance pseudocapacitive materials. Further, a unique in situ electrochemical polymerization approach has been undertaken in an attempt to boost the overall storage capacity as well as cycling stability. Electrochemical tuning of the active material can result in an enhancement in the specific capacitance via dual improvement in faradaic and capacitive current as compared to the initial xerogel material, highlighting the significance of the in situ process. The electrode material shows a highest specific capacitance (CSP) of 348 F g−1 at a current density of 0.5 A g−1 in 1 M H2SO4 while retaining 60% of its initial capacitance even at a very high current density of 200 A g−1. A metal-free all-solid-state symmetric supercapacitor was developed with the in situ electropolymerized active material and the as-fabricated device exhibits an excellent CSP of 218 F g−1 at 0.325 A g−1 in 1 M PVA/H2SO4 gel electrolyte with a maximum energy density of 30.3 W h kg−1 and power density up to 13 kW kg−1. Most importantly, the material exhibited an extraordinary stability of 53 000 charge–discharge cycles while retaining 94% of its initial capacitance, while the device also shows an excellent capacitance retention of 92% even after 10 000 continuous cycles. Simultaneously, the DFT study was amended to understand the covalent and non-covalent interaction of the redox species, its charge storage mechanism and charge density distribution as well as to calculate the density of states.

The facile synthesis of a highly durable, low-cost and robust electrocatalyst for hydrogen generation from water is vital to address the existing environmental issues, as well as to provide an environmentally-friendly clean and green energy supply. Electrochemical deposition of single-phase nickel phosphide on galvanostatically deposited copper foam (Cuf@Ni5P4) core–shell nanostructure offers an innovation in the structural design of a new platform for novel electrocatalysts. The Cuf@Ni5P4 provides a superior three-dimensional conductive channel for ion transport during the catalytic process. The catalyst exhibits an excellent electrocatalytic activity towards the hydrogen evolution reaction (HER) in acidic media. The superhydrophilic and aerophobic properties of the porous electrode help the H2 gas bubbles to quickly leave the surface. Interestingly, it requires a very low overpotential of 90 mV for HER at a current density of 10 mA cm−2. The very small Tafel slope of 49 mV dec−1 and the very high exchange current density (∼0.76 mA cm−2) originate from the large electrochemically active surface area and the fast mass and electron transfer efficiency of the Cuf@Ni5P4 catalyst. A theoretical study was performed to investigate the mechanism underlying the HER activity in Cu-supported Ni5P4 at an atomic scale. Density functional theory (DFT) calculations suggest a very high negative Gibbs free energy change (ΔGH*) in Ni5P4 (0001)/Cu (111) upon hydrogen adsorption, which is actually responsible for the excellent HER activity of the catalyst. Furthermore, it shows remarkable durability for hydrogen generation under low (10 mA cm−2) and high current densities (160 mA cm−2) for >84 h with ∼96% retention of the overpotential, establishing a low-cost and efficient catalyst for sustainable, future energy generation strategies.

Piezoelectric nanomaterials have been emerging as flagship materials for harvesting nanoelectromechanical energy. Pristine, semiconducting 1T-MX2 (M = Zr and Hf; X = S, Se, and Te) monolayers are intrinsically centrosymmetric, and hence non-piezoelectric. This inversion symmetry is broken in their Janus monolayer (non-centrosymmetric) structures, leading to the emergence of a high degree of piezoelectricity in them. This brings along a new dimension in nanoscale piezoelectricity, as the origin of this piezoelectricity is predominantly ionic in nature, in contrast to the 1H-MoS2 monolayer, where it is of electronic character. DFT calculations reveal the piezoelectric coefficient (d22 = 4.68–14.58 pm V−1) in these Janus monolayers to be much higher than that in single layer 1H-MoS2 (d11 = 2.99 pm V−1). 9% uniaxial tensile strain applied along the arm-chair direction is found to raise d22 in HfSSe Janus monolayers to 123.04 pm V−1, which reaches the level of piezoelectric coefficients in the state-of-the-art perovskites. The major contribution of the ionic component to the piezoelectric coefficient is attributable to the predominance of ionic character in the interatomic bonds in these monolayers, which arises from the decoupled band edges, i.e., no hybridization between the band edge states (chalcogen-p and metal-d). Contrarily, 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers with coupled band edges are held together mainly by covalent bonds, resulting in the dominance of electronic contribution to piezoelectricity. The nature of band edges causes a lower deformation potential for electrons in 1T Hf and Zr based dichalcogenide monolayers and their Janus structures with respect to 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers. This induces a much higher electron mobility in the former than in 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers. The carrier mobility calculated using Lang et al.'s formalism [Phys. Rev. B, 2016, 94, 235306] agrees well with the experimentally measured electron mobility. Our predictive findings underscore the imminent need to synthesize these 1T-MX2 semiconducting Janus structures to induce a high level of piezoelectricity together with robust electron mobility.

2D semiconducting transition-metal-dichalcogenides (TMDCs) have drawn a surge of research interests in ultra-thin nanoelectronics and optoelectronics, owing to their moderate charge carrier mobility. Due to a large variation in reported data on charge carrier mobilities of these free-standing monolayers, a more accurate determination of carrier mobilities is a must. This article highlights room temperature carrier mobilities calculated in group-VI B TMDC (i.e., MX2 (M = Mo, W; X = S, Se, Te)) monolayers via a systematic comparison of reliable models used within the framework of density functional theory. Additionally, piezoelectric effects on carrier mobility have been incorporated. The robust formulation by Lang et al. [Phys. Rev. B, 2016, 94, 235306] gives the most accurate estimation, where the carrier (i.e. electron or hole) mobility and the ratio of hole to electron mobility reach closest to the experimentally measured value. The higher mobility of holes with respect to electrons is attributed to the smaller deformation potential of the holes, while smaller carrier effective masses and larger elastic moduli cause larger carrier mobility in WX2 relative to MoX2. Carrier mobility is found to drop from S to Te and also from W to Mo in MX2 on account of the decrease in the elastic moduli, which is consistent with the increase in the macroscopic static dielectric constant. Erroneously high electron mobility (>1000 cm2 V−1 s−1), which is two orders of magnitude higher than the experimental ones, has been disputably reported earlier for MoTe2 and WS2 monolayers. It owes its origin to the ultrasmall deformation potential found therein. High (∼1.4–2.3) hole to electron mobility ratio in MX2 monolayers is found to occur in our work as MoSe2 > WS2 > MoS2 > WSe2 > WTe2 > MoTe2. This large carrier mobility ratio will facilitate an efficient separation of electron–hole pairs, which is particularly useful in optoelectronic applications, e.g., photovoltaics, photocatalysis and artificial photosynthesis. In all terms including optical absorbance, a WS2 monolayer is found to be most suitable for photophysical processes. Using the Nernst formulation, water splitting and CO2 photoreduction on WS2 are found to be favorable over a wide range of pH.

A nanohybrid material containing carbon-supported molybdenum carbide and nitride nanoparticles of size ranging from 8 to 12 nm exhibits excellent HER catalytic activity. This molybdenum based catalyst (MoCat) is designed as a highly efficient, low-cost (precious-metal-free), highly stable electrocatalyst for water electrolysis in acidic media, and synthesized using a simple methodology. These nanoparticles (β-Mo2C and γ-Mo2N) were produced in situ using a metal precursor and C/N source via a controlled solid state reaction. An overpotential of 96 mV for driving a current density of 10 mA cm−2 was measured for the MoCat catalyst, which is very close to that of commercially available Pt/C catalysts (61 mV).