Probing Nanoparticles using Electrochemistry Coupled with Spectroscopy

Probing Nanoparticles using Electrochemistry Coupled with Spectroscopy 

This originates from the changes in Ef-LDOS at the nanocrystalline metal surface and at the adsorbate, induced by electrochemical potential control [142]. A layer model analysis is used to describe the Pt NMR spectrum of nanoscopic materials [144]. It is also possible to correlate the electronegativity of the adsorbates with the Knight shift associated with the Pt nanoparticles [138]. The orientation of adsorbates on metallic substrates under potential control conditions has also been explored [122, 131]. Tong and co-workers have recently demonstrated the use of EC-NMR to investigate the electronic environment of the core of MPCs [145]. 

Other spectroscopic techniques that have been used with electrochemistry to probe nanoparticles include electronic and vibrational spectroscopies. The spec-troelectrochemistry of nanosized silver particles based on their interaction with planar electrodes has been studied recently [146] using optically transparent thin layer electrodes (OTTLE). Colloidal silver shows a surface plasmon resonance absorption at 400 nm corresponding to 0.15 V vs. Ag/AgCl. This value blue shifts to 392 nm when an Au mesh electrode in the presence of Ag colloid is polarized to —0.6 V (figure 20.12). The absorption spectrum is reported to be quite reproducible and reversible. This indicates that the electron transfer occurs between the colloidal particles and a macroelectrode and vice versa. The kinetics of electron transfer is followed by monitoring the absorbance as a function of time. The use of an OTTLE cell ensures that the absorbance is due to all the particles in the cell between the cell walls and the electrode. The distance over which the silver particles will diffuse has been calculated to be 80 pm in 150 s, using a diffusion coef- 

Weaver and co-workers [150] carried out potential-dependent infrared spectroscopy to characterize adsorption of CO on nanosized platinum particles. Large particles of diameter 4 nm and above show C-0 stretching frequencies similar to those of platinum macroelectrodes. Small particles of diameter 2-4 nm, on the other hand, show a red shift in C-0 frequency, approaching that of platinum carbonyl clusters. The authors ascribe this observation to the changes in the platinum surface coordination number, consistent with pseudo-spherical packing-density considerations. A time-resolved IR absorption technique has also been used to monitor electrocatalytic reactions using platinum nanoparticles [151]. 

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Probing Nanoparticles using Electrochemistry Coupled with Spectroscopy I 665... 

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