Gold Nanoclusters Deposited on SiO2 via Water as Buffer Layer: CO-IRAS and TPD Characterization

CO adsorption properties on gold clusters were studied by infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). Two growth procedures that differ in the final gold clusters morphology were compared. In the first, the clusters were prepared by direct deposition (DD) of gold atoms on SiO2/Si(100) substrates. The second growth mode is based on initial evaporation of gold atoms on top of amorphous solid water as a buffer layer at 100 K that separates the small gold seed clusters from the substrate. Subsequent annealing to 300 K desorbs the water molecules, resulting in nanocluster growth and their (cold) deposition on the substrate in a buffer layer assisted growth (BLAG) mechanism. It is demonstrated here for the first time that one can independently control cluster size and density by repeating the BLAG procedure as many times as needed. BLAG clusters are more 3D in nature, have larger height to diameter ratio, yet their interaction with CO is very similar to DD clusters. This is reflected by the practically identical CO stretch observed on both clusters at 2106 ( 2 cm-1. The CO stretch frequency was found BLAG clusters size (2-10 nm) independent. CO molecules that are most strongly bound at the perimeter of the gold clusters do not contribute to the IRAS signal. TPD measurements have shown that CO interaction with BLAG clusters is somewhat weaker than with the DD clusters, indicated by lower and cluster size dependent peak desorption temperature (170-190 K for BLAG vs 230-240 K for DD clusters). Smaller clusters lead to higher COdesorption temperature. The area under the CO-TPD peak, linearly increases with the number of multiple BLAG cycles. The high-temperature tail of the TPD peaks above 200 K has been correlated with cluster perimeter gold atoms that seem to interact with the underlaying SiO2 substrate.