Electroless patterned assembly of metal nanoparticles on hydrogen-terminated silicon surfaces for applications in photoelectrocatalysis.

International audienceThe deposition of gold and platinum nanoparticles (NPs) on hydrogen-terminated Si(100) (Si(100)-H) surfaces has been performed by galvanic displacement using fluoride-free sub-millimolar metallic salt solutions. The scanning electron microscopy (SEM) images showed the formation of oblate hemispherical NPs, with an average diameter of ca. 40 nm and an average height of 20 ± 10 and 10 ± 5 nm for Au and Pt, respectively. Furthermore, the calculated number density was (6.0 ± 0.8) × 10(9) Au NPs cm(-2) and (6.6 ± 1.3) × 10(9) Pt NPs cm(-2) with a larger size distribution measured for Au NPs. The Au 4f and Pt 4f X-ray photoelectron spectra of the metallized surfaces were characterized by a principal component corresponding to either the metallic gold or platinum. However, two other components located at higher binding energies were also visible and ascribed to gold or platinum silicides. Using this fluoride-free deposition process and a "reagentless" UV photolithography technique, we have also demonstrated that it was possible to prepare metallic NP micropatterns. Following this approach, single metal (Au) and two metals (Au and Pt) patterns have been produced and characterized by energy-dispersive X-ray spectroscopy (EDS) which revealed the presence of the expected metal(s). Such metallic NP micropatterned surfaces were used as photocathodes for H(2) evolution from water as a proof-of-concept experiment. These electrodes exhibited much higher electrocatalytic performance than that of nonmetallized Si(100)-H, both in the absence of light and under illumination. The overpotential for hydrogen evolution was significantly decreased by ca. 450 mV with respect to Si(100)-H (measured for a current density of 0.1 mA cm(-2)) under identical illumination conditions

Crossover in structure and dynamics of a primary alcohol induced by hydrogen-bonds dilution

International audiencePrimary alcohols show a prominent Debye process in the dielectric relaxation located at a timescale longer than the main structural relaxation. Böhmer and co-workers studied dilution effects of the hydrogen bonding network of n-butanol (BuOH) with n-bromobutane (BuBr) to better understand the origin of this process. Interestingly, this work has evidenced a crossover in Debye relaxation time (τ D ) for a critical concentration in BuBr x c = 0.5. By using molecular dynamics simulations and NMR experiments we propose here to explore further dilution effects on the dipolar and translational dynamics. Moreover, we discuss the relation between structural and dynamical properties in the context of a detailed study of the microstructure and the H-bond network. The overall results are consistent with the existence of a topological change in the liquid structure occurring at about x c = 0.5 from a hypernetted percolating network to independent nanodomains of n-butanol molecules embedded in the n-bromobutane phase