Pradheep Thiyagarajan

Hierarchical Metal/Semiconductor Nanostructure for Efficient Water Splitting

A hierarchically patterned metal/semiconductor (gold nanoparticles/ZnO nanowires) nanostructure with maximized photon trapping effects is fabricated via interference lithography (IL) for plasmon enhanced photo-electrochemical water splitting in the visible region of light. Compared with unpatterned (plain) gold nanoparticles-coated ZnO NWs (Au NPs/ZnO NWs), the hierarchically patterned Au NPs/ZnO NWs hybrid structures demonstrate higher and wider absorption bands of light leading to increased surface enhanced Raman scattering due to the light trapping effects achieved by the combination of two different nanostructure dimensions; furthermore, pronounced plasmonic enhancement of water splitting is verified in the hierarchically patterned Au NPs/ZnO NWs structures in the visible region. The excellent performance of the hierarchically patterned Au NPs/ZnO NWs indicates that the combination of pre-determined two different dimensions has great potential for application in solar energy conversion, light emitting diodes, as well as SERS substrates and photoelectrodes for water splitting.

Optimization for visible light photocatalytic water splitting: gold-coated and surface-textured TiO2 inverse opal nano-networks

A gold nanoparticle-coated and surface-textured TiO2 inverse opal (Au/st-TIO) structure that provides a dramatic improvement of photoelectrochemical hydrogen generation has been fabricated by nano-patterning of TiO2 precursors on TiO2 inverse opal (TIO) and subsequent deposition of gold NPs. The surface-textured TiO2 inverse opal (st-TIO) maximizes the photon trapping effects triggered by the large dimensions of the structure while maintaining the adequate surface area achieved by the small dimensions of the structure. Au NPs are incorporated to further improve photoconversion efficiency in the visible region via surface plasmon resonance. st-TIO and Au/st-TIO exhibit a maximum photocurrent density of ∼0.58 mA cm(-2) and ∼0.8 mA cm(-2), which is 2.07 and 2.86 times higher than that of bare TIO, respectively, at an applied bias of +0.5 V versus an Ag/AgCl electrode under AM 1.5 G simulated sunlight illumination via a photocatalytic hydrogen generation reaction. The excellent performance of the surface plasmon-enhanced mesoporous st-TIO structure suggests that tailoring the nanostructure to proper dimensions, and thereby obtaining excellent light absorption, can maximize the efficiency of a variety of photoconversion devices.

MoSx supported hematite with enhanced photoelectrochemical performance

By creating a p–n heterojunction of molybdenum sulfide (MoSx)/Ti-doped Fe2O3 (Ti-Fe2O3), we successfully addressed electron–hole transfer problems of hematite and thus achieved the enhanced photoelectrochemical (PEC) performance. MoSx/Ti-Fe2O3 with a thin MoSx layer on the surface of Ti-Fe2O3 fabricated a p–n junction that provided facile charge transfer pathways due to an internal electric field between Ti-Fe2O3 and MoSx, and achieved suppressed charge recombination. The optimized MoSx/Ti-Fe2O3 sample showed a 240% increased photocurrent density (3.03 mA cm−2) over pristine Fe2O3 at RHE 1.50 V. All our data including IPCE, PL, and EIS clearly confirmed the improved PEC performance of MoSx/Ti-Fe2O3 achieved by the formation of a p–n junction with a facile electron–hole transport pathway.

A case study: effect of defects in CVD-grown graphene on graphene enhanced Raman spectroscopy

Graphene-enhanced Raman spectroscopy (GERS) is a technique to increase the Raman scattering of adsorbed probe molecules on graphene. Here we systematically explore the effect of the method used to transfer the CVD-grown graphene onto another substrate on Raman scattering. We have found that graphene transferred using poly methyl methacrylate (PMMA) produces 6 times the Raman scattering signal increase of that produced by graphene transferred using thermal release tape. The reason for this is that PMMA-assisted graphene contains a larger amount of defects such as carboxyl and hydroxyl groups that help the attachment of probe molecules to the graphene surface, leading to improved π–π* interactions and thus easier charge transfer between the probe molecules and graphene. Our results indicate the need for a much closer look at the functional groups of graphene which are different for the two transfer methods.

Thermoelectric properties of nanoporous three-dimensional graphene networks

We propose three dimensional-graphene nanonetworks (3D-GN) with pores in the range of 10 ∼ 20 nm as a potential candidate for thermoelectric materials. The 3D-GN has a low thermal conductivity of 0.90 W/mK @773 K and a maximum electrical conductivity of 6660 S/m @ 773 K. Our results suggest a straightforward way to individually control two interdependent parameters, σ and κ, in the nanoporous graphene structures to ultimately improve the figure of merit value.

Thermal conductivity reduction in three dimensional graphene-based nanofoam

This work investigates the thermoelectric properties of a three dimensional nanofoam of few layer graphene (3D-NFG) decorated with holes having diameter of several tens of nanometer. The nanoporous 3D graphene structures were fabricated by a chemical vapor deposition method to ensure high electrical conductivity required for potential applications as thermoelectric materials. The thermal conductivity of the suspended 3D-NFG samples was measured by an optothermal method and found to be 10.8 W m−1 K−1. The substantially reduced values of thermal conductivity were attributed to the small diameter of the pores and their inhomogeneous distribution. Suppression of heat conduction with preserved electrical conductivity is beneficial for the proposed thermoelectric applications.

Highly Enhanced Raman Scattering on Carbonized Polymer Films

We have discovered a carbonized polymer film to be a reliable and durable carbon-based substrate for carbon enhanced Raman scattering (CERS). Commercially available SU8 was spin coated and carbonized (c-SU8) to yield a film optimized to have a favorable Fermi level position for efficient charge transfer, which results in a significant Raman scattering enhancement under mild measurement conditions. A highly sensitive CERS (detection limit of 10-8 M) that was uniform over a large area was achieved on a patterned c-SU8 film and the Raman signal intensity has remained constant for 2 years. This approach works not only for the CMOS-compatible c-SU8 film but for any carbonized film with the correct composition and Fermi level, as demonstrated with carbonized-PVA (poly(vinyl alcohol)) and carbonized-PVP (polyvinylpyrollidone) films. Our study certainly expands the rather narrow range of Raman-active material platforms to include robust carbon-based films readily obtained from polymer precursors. As it uses broadly applicable and cheap polymers, it could offer great advantages in the development of practical devices for chemical/bio analysis and sensors.