Debraj Ghosh

Janus Nanostructures Based on Au−TiO2 Heterodimers and Their Photocatalytic Activity in the Oxidation of Methanol

Au-TiO2 snowman-like heterodimer nanoparticles were prepared by a surface sol-gel process based on gold Janus nanoparticles whose surface-protecting monolayers consisted of a hemisphere of hydrophobic 1-hexanethiolates and the other of hydrophilic 2-(2-mercaptoethoxy)ethanol. Transmission electron microscopic measurements showed that the resulting TiO2 nanoparticles (diameter 6 nm) exhibited well-defined lattice fringes that were consistent with the (101) diffraction planes of anatase TiO2. The heterodimer nanoparticles displayed apparent photoluminescence that was ascribed to electronic transitions involving trap states of TiO2 particles, and the photocatalytic activity was manifested by the oxidative conversion of methanol into formaldehyde, which was detected quantitatively by the Nash method. The enhanced photocatalytic performance, as compared to that of the TiO2 nanoparticles alone, was ascribed to the charge separation of photogenerated electrons and holes at the Au-TiO2 interface that was facilitated by the close proximity of the gold nanoparticles. These results suggested that (i) there were at least two possible pathways for photogenerated electrons at the TiO2 conduction band, decay to the trap states and transfer to the gold nanoparticles, and (ii) energy/electron transfer from the trap states to gold nanoparticles was less efficient. In essence, this study showed that the snowman-like heterodimers might be exploited as a homogeneous photocatalytic system for the preparation of functional molecules and materials.

Palladium nanoparticles passivated by metal-carbon covalent linkages

Stable palladium nanoparticles were prepared by the passivation of palladium–carbon covalent linkages by the reduction of diazonium derivatives. The resulting particles were characterized by TEM, UV-vis, FTIR, and NMR measurements. Interestingly, in electrochemical studies of the electronic conductivity of the particle solid ensembles, Pd–biphenyl particles exhibited metallic temperature dependence within the temperature range of 80 to 320 K; whereas the conductivity of Pd–decylphenyl particles showed a transition from semiconductor to metal at 180 K, manifested by their distinctly different temperature dependence. Control experiments with alkanethiolate-protected palladium nanoparticles in the same temperature range exhibited only semiconductor-like conductivity which increased with increasing temperature. The discrepancy was interpreted using Motts model for metal–insulator transition and ascribed to the strong Pd–C interactions and low contact resistance, which facilitated the interparticle charge transfer.

A double substrate “sandwich” structure for fiber surface enhanced Raman scattering detection

A double substrate “sandwiching” structure has been designed and tested for molecular detection using surface enhanced Raman scattering (SERS). With silver (Ag) nanoparticles as SERS substrates and rhodamine 6G (R6G) as a test molecule, the results show that the “sandwich” configuration exhibits significantly higher SERS enhancement compared to just one of the substrates or a simple sum of the signals from the two separate substrates. The improved SERS sensitivity is attributed to a stronger electromagnetic field enhancement by the double substrate sandwich structure.

Fiber surface enhanced Raman scattering (SERS) sensors based on a double substrate “sandwich” structure

A new configuration was designed and tested based on ldquosandwichingrdquo target analyte molecules between two metal nanostructure substrates using surface enhanced Raman scattering (SERS), which exhibits significantly higher SERS enhancement compared to just one substrate.