Plasmonic Nanostructures Induced Photochemistry on Surfaces

Micha Asscher, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel
Gil Toker, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel
Liat Zilberberg, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel
Serge Mitlin, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel

The search for direct evidence of plasmon enhanced photochemistry and catalysis at the vicinity of metal nanostructures is accelerating in recent years due to its potential importance in solar energy related applications1,2. Such enhancement has been predicted theoretically more than 3 decades ago, during the early reports on Surface Enhanced Raman Spectroscopy (SERS) but has not yet been demonstrated under well controlled UHV environment. Extreme enhancement of photo-desorption cross section at nanometer size sharp edges and tips of silicon has been demonstrated in our group3. In this report we present the first attempt to address this issue under well-defined ultra-high vacuum conditions, on top of clean (non-colloidal) silver nanoparticles at the size range of 10-120nm. Ethyl chloride molecules adsorbed at 45K on silver clusters decorated SiO2/Si(100) substrate were used as our probe molecule for photo-reactivity and selectivity studies. Experimental parameters included gradual increase of the silver particles size and total coverage, excitation laser wavelength and in particular varying the distance of the probe molecule from the silver particle's surface via Xe spacer layers (see figure 1 below). Pump-probe experiments employing two laser wavelengths were used to dissociate the molecule (355nm) and to excite the TiO2 film modified surface plasmon resonance of the silver particles (532nm). This experiment has revealed that no observable enhancement of photo-reactivity could be detected when the two nsec laser pulses overlap in time. The overall list of observations has led us to conclude that the photo-induced mechanism is governed by hot-electrons excitation. Following the excited electrons energy, affected by work function modifications, with respect to the ethyl chloride affinity level confirm our proposed mechanism.

Another hybrid system composed of silver nano-particles embedded within vacuum prepared TiO2 thin film (20nm thick) has been tested for its photo-catalytic activity. This composite film has been studied while irradiating an aqueous solution of methylene blue (MB) dye molecule at 355nm, revealing an enhanced photo-degradation of the MB molecules by the hybrid Ag-TO2 system compared to the pure titania substrate. An indirect hot electron mediated MB degradation mechanism via the titania conduction band will be discussed.

Figure 1: (a) Continuous Ag layer (b) Separated 120nm Ag particles (c) Excitation wavelength effect on EC photochemistry yield as a function of Xe spacer layer thickness.

1. N. J. HALAS ET. AL., JACS, 136, 64 (2014) ; 2. P. CHRISTOPHER ET. AL., ACS CATALYSIS, 4, 116 (2014)
3. G. Toker and M. Asscher, Phys. Rev. Lett., 107, 167402 (2011).