Determining a suitable noble-metal-free catalyst for hydrogen evolution reaction (HER) by photoelectrocatalytic (PEC) water splitting is an enduring challenge. Here, the molecular origin of number of layers and stacking sequence-dependent PEC HER performance of MoS 2 /graphene (MoS 2 /GR) van der Waals (vdW) vertical heterostructures is studied. Density functional theory (DFT) based calculations show that the presence of MoS 2 induces p-type doping in GR, which facilitates hydrogen adsorption in the GR side compared to the MoS 2 side with ΔG H closer to 0 eV in the MoS 2 /GR bilayer vertical stacks. The activity maximizes in graphene with monolayer MoS 2 and reduces further for bilayer and multilayers of MoS 2 . The PEC HER performance is studied in various electrodes, namely, single-layer graphene, single-and few-layered MoS 2 , and their two different types of vertical heterojunctions having different stacking sequences. The graphene on top of MoS 2 sequence showed the highest photoresponse with large reaction current density and lowest charge-transfer resistance toward HER, in aggrement with the DFT calculations. These findings establish the role of stacking sequence in the electrochemistry of atomic layers, leading to the design of new electrocatalysts by combinatorial stacking of a minimal number of layers.

Controlled assembly of mesoscopic structures can bring interesting phenomena because of their interfaces. Here, carbon nanotubes (CNTs) are cross-coupled via a C–C bonding through Suzuki reaction resulting in three-dimensional (3D) CNT sponges, and these 3D CNTs are studied for their efficacy toward the electrocatalytic hydrogen evolution reaction (HER) in acidic mediumone of the promising methods for the production of a renewable energy source, hydrogen. Both single and multiwall CNTs (SWCNTs and MWCNTs) are studied for the development of 3DSWCNTs and 3DMWCNTs, and these 3D CNTs are found to be HER active with small reaction onset potentials and low charge-transfer resistances unlike their uncoupled counterparts. First-principle density functional calculations show that the combination of electron acceptor and donor bonded to the CNT network can provide a unique band structure modulation in the system facilitating the HER reaction. This study can provide possibilities for band engineering of CNTs via functionalization and cross-coupling reactions.

Single molecule detection using graphene can be brought by tuning the interactions via specific dopants. Electrostatic interaction between the most electronegative element fluorine (F) and hydrogen (H) is one of the strong interactions in hydrogen bonding, and here we report the selective binding of ammonia/ammonium with F in fluorographene (FG) resulting to a change in the impedance of the system. Very low limit of detection value of ~0.44 pM with linearity over wide range of concentrations (1 pM–0.1 μM) is achieved using the FG based impedance sensor, andthisscreen printed FG sensor works in both ionized (ammonium) and un-ionized ammonia sensing platforms. The interaction energies of FG and NH3/NH4+ are evaluated using density functional theory calculations and the interactions are mapped. Here FGs with two different amounts of fluorinecontents −~5 atomic% (C39H16F2) and ~24 atomic% (C39H16F12) - are theoretically and experimentally studied for selective, high sensitive and ultra-low level detection of ammonia. Fast responding, high sensitive, large area patternable FG based sensor platform demonstrated here can open new avenues for the development of point-of-care devices and clinical sensors.

Although graphene technology has reached technology readiness level 9 and hydrogen fuel has been identified as a viable futuristic energy resource, pristine atomic layers such as graphene are found to be inactive towards the hydrogen evolution reaction (HER). Enhancing the intrinsic catalytic activity of a material and increasing its number of active sites by nanostructuring are two strategies in novel catalyst development. Here, electrocatalytically inert graphene (G) and hexagonal boron nitride (hBN) are made active for the HER by forming van der Waals (vdW) heterostructures via vertical stacking. The HER studies are conducted using defect free shear exfoliated graphite and hBN modified glassy carbon electrodes via layer by layer sequential stacking. The G/hBN stacking pattern (AA, AB, and AB') and stacking sequence (G/hBN or hBN/G) have been found to play important roles in the HER activity. Enhancement in the intrinsic activity of graphene by the formation of G/hBN vdW stacks has been further confirmed with thermally reduced graphene oxide and hBN based structures. Tunability in the HER performance of the G/hBN vdW stack is also confirmed via a three-dimensional rGO/hBN electrode. HER active sites in the G/hBN vdW structures are then mapped using density functional theory calculations, and an atomistic interpretation has been identified.

New layered solids by the combinatorial stacking of different atomic layers are emanating as novel candidates for energy efficient devices. Here, sequentially stacked single layer graphene-molybdenum disulfide (MoS) van der Waals (vdW) solids are demonstrated for their efficacy in the catalysis of hydrogen evolution reaction (HER), and importance of their stacking order in tuning the photo-electrocatalytic (PEC) efficiency is unraveled. Single layer graphene and a few layered MoS stacked vdW solids based transparent flexible electrodes were prepared, and a particular stacking sequence where top-graphene: bottom-MoS/polydimethylsiloxane (PDMS) geometry (MSGR) exhibited the lowest onset and over potentials and a very high exchange current density (j ∼ 245 ± 1 μA cm) in acidic HER in comparison to the individual layers and other stacked configuration (MoS on top of graphene on PDMS, GRMS). The HER studies under dark and white light illuminations were conducted to explore the PEC responses of the devices. The augmented HER performance of MSGR is further confirmed from the charge transfer resistance measurements using electrochemical impedance spectroscopy. Role of graphene plasmonics and MoS to graphene electron transfer were studied, and this study unravels the importance of a new factor, stacking order of vdW layers, while designing novel devices from the layered solids.