Sulolit Pradhan

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.

Photoluminescence and conductivity studies of anthracene-functionalized ruthenium nanoparticles

Carbene-stabilized ruthenium nanoparticles were functionalized with anthryl moieties by olefin metathesis reactions with 9-vinylanthracene, at a surface concentration of about 19.7%, as estimated by (1)H NMR spectroscopic measurements. Because of the conjugated metal-ligand interfacial bonding interactions, UV-vis measurements of the resulting nanoparticles showed a new broad absorption band centered at 612 nm, in addition to the peaks observed with monomeric vinylanthracene. FTIR measurements depicted apparent red-shifts of the aromatic vibrational stretches as compared to those of the monomeric vinylanthracene, suggestive of decreasing bonding order of the aromatic moieties as a result of extended conjugation between the particle-bound anthracene groups. Photoluminescence measurements confirmed the notion that effective intraparticle charge delocalization occurred by virtue of the conjugated metal-ligand interfacial bonding interactions, with apparent red-shifts of the excitation peaks and blue-shifts of the emission features, as compared to those of the monomeric vinylanthracene. The diminishment of the Stokes shift was, at least in part, attributed to the different chemical environments surrounding the anthryl moieties on the nanoparticle surface. Electronic conductivity measurements showed that because of the conjugated Ru[double bond, length as m-dash]C π bonds, the activation energy for interparticle charge transport was about one order of magnitude lower than that observed with particles passivated by alkanethiolates. Additionally, whereas the original carbene-stabilized nanoparticles exhibited a semiconductor-metal transition within the temperature range of 100 to 320 K, anthracene-functionalized nanoparticles displayed apparent semiconducting behaviors with the ensemble conductivity increasing monotonically with temperature, most likely due to the disordering within the nanoparticle solids that arose from the different structures of the carbene ligands and anthryl moieties. These studies indicate that anthracene functionalization may be exploited as an effective route towards the manipulation of nanoparticle optoelectronic properties.

Single electron transfer in thermally annealed nanoparticle dropcast thick films

A very simple and effective procedure based on thermal annealing was reported in inducing discrete charge transfer in nanoparticle solid films. The particle ensembles were prepared by dropcasting a particle solution onto an interdigitated array electrode. The as-prepared particle films exhibited only linear featureless current-potential profiles in conductivity measurements, whereas after thermal annealing, well-defined staircase features of single electron transfer started to emerge at temperatures higher than 300 K. This was accounted for by the combined consequence of structural rearrangements of nanoparticle cores within the organic protecting matrix and thermal activation of interparticle charge transport.