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.

We demonstrate graphene-assisted controlled fabrication of various ZnO 1D nanostructures on the SiO2/graphene substrate at a low temperature (540 °C) and elucidate the growth mechanism. Monolayer and a few layer graphene prepared by chemical vapor deposition (CVD) and subsequently coated with a thin Au layer followed by rapid thermal annealing is shown to result in highly aligned wurtzite ZnO nanorods (NRs) with clear hexagonal facets. On the other hand, direct growth on CVD graphene without a Au catalyst layer resulted in a randomly oriented growth of dense ZnO nanoribbons (NRBs). The role of in-plane defects and preferential clustering of Au atoms on the defect sites of graphene on the growth of highly aligned ZnO NRs/nanowires (NWs) on graphene was established from micro-Raman and high-resolution transmission electron microscopy analyses. Further, we demonstrate strong UV and visible photoluminescence (PL) from the as-grown and post-growth annealed ZnO NRs, NWs, and NRBs, and the origin of the PL emission is correlated well with the X-ray photoelectron spectroscopy analysis. Our results hint toward an epitaxial growth of aligned ZnO NRs on graphene by a vapor-liquid-solid mechanism and establish the importance of defect engineering in graphene for controlled fabrication of graphene-semiconductor NW hybrids with improved optoelectronic functionalities.

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.

We demonstrate efficient physical functionalization of single layer graphene with Au nanoparticles mediated by in-plane defects in graphene grown by a chemical vapor deposition technique. The effect of an ultrathin Au layer on the single layer, bi layer, and few layer graphene with intrinsic defects was studied by resonance Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). We observed a striking enhancement in the intensity of sharp D and D′ bands after sputter deposition of an ultrathin Au layer on graphene. In contrast, G and 2D bands show a lower enhancement in intensity and a change in line width due to the charge transfer from Au to the graphene and strong interaction between the Au and graphene layers, respectively. X-ray photoelectron spectroscopy (XPS) analysis shows a 40% decrease in integrated intensity ratio of sp 3 and sp 2 bands in C-1s spectra after Au functionalization indicating bonding of Au atoms preferentially at the defect sites in graphene. This was further substantiated by HRTEM imaging and position dependent Raman spectral line shape analysis. The calculations of interdefect distance and areal defect density from the Raman analysis on the graphene−Au hybrid are in close agreement with the HRTEM analysis. Further, Raman spectral line shape dependence of Au functionalization on the number of layers in graphene reveals that Au functionalized single layer graphene behaves like a pristine bi layer graphene due to strong interaction between Au and the graphene layer. These results open up possibilities for efficient physical functionalization of graphene with foreign atoms through defect engineering for novel applications of graphene in catalysis, biosensors, and optoelectronic and photonic devices.

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.