Graphene-Assisted Controlled Growth of Highly Aligned ZnO Nanorods and Nanoribbons: Growth Mechanism and Photoluminescence Properties

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

Hydrogen Evolution Reaction Activity of Graphene–MoS₂ van der Waals Heterostructures

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 MoS₂/graphene (MoS₂/GR) van der Waals (vdW) vertical heterostructures is studied. Density functional theory (DFT) based calculations show that the presence of MoS₂ induces p-type doping in GR, which facilitates hydrogen adsorption in the GR side compared to the MoS₂ side with Δ<i>G</i>_H closer to 0 eV in the MoS₂/GR bilayer vertical stacks. The activity maximizes in graphene with monolayer MoS₂ and reduces further for bilayer and multilayers of MoS₂. The PEC HER performance is studied in various electrodes, namely, single-layer graphene, single- and few-layered MoS₂, and their two different types of vertical heterojunctions having different stacking sequences. The graphene on top of MoS₂ 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

Covalently Connected Carbon Nanotubes as Electrocatalysts for Hydrogen Evolution Reaction through Band Engineering

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