Unravelling the mysteries of solar steam nanobubbles
Recently it was reported that illuminating a dispersion of gold nanoparticles in water with sun or laser light results in the formation of a layer of vapor (or ‘nanobubble’) around the particles. This nanobubble exhibits temperatures well above 100°C and pressures up to ~15 atm.  While partners of the University of Twente and TNO Eindhoven are exploring the fundamentals of nanobubble formation by a number of techniques, my project focuses on trying to use the vapor layer as nanoreactor for Aqueous Phase Reforming (APR), circumventing the need for externally applied high temperature and pressure and instead only using sunlight.
APR involves the conversion of biomass-derived alcohols into H2 and CO2.  However, APR is not catalyzed by gold, but by platinum and/or nickel. Therefore, my project will focus on synthesizing gold nanoparticles (NPs) between 5 and 100 nm coated with a thin oxide layer (1-5 nm) which will both prevent the gold NPs from aggregation and function as a support for the catalyst. An advantage of this approach is the possibility to use the abovementioned particles for SHINERS, Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy , a technique closely related to Surface-Enhanced Raman Spectroscopy (SERS). Consequently, the effect of gold NP shape and size and the nature/thickness of the oxide layer on the formation of the nanobubble, the catalysis and the enhanced Raman signal by the presence of hotspots will be investigated.
Figure 1: left: Shell-Isolated Nanoparticle, SHIN, of a gold particle coated with a thin oxide layer (silica, titania, zirconia, niobia) on which the Pt or Ni catalyst is deposited; right: APR reactor loaded with SHINs in aqueous solution and a biomass-derived substrate. Laser light is guided into the reactor to induce heating, catalytic reactions and/or vibrational measurements by SHINERS or ATR-IR.