We investigated the photoelectrochemical kinetics of (ligand stabilized) Q-CdS particles adsorbed on a gold electrode by Intensity Modulated Photocurrent Spectroscopy. With this method, we hoped to unravel the mechanism of photocurrent generation in the gold/Q-CdS system [1]. Suspensions of 4-nm size quantised CdS particles were prepared by a standard procedure. The CdS particles were adsorbed in a monolayer on a bare gold electrode an on a gold electrode with a hexanedithiol SAM. The morphology of Q-CdS monolayers was investigated with STM (in KFA Julich, under guidance of Dr. Meissner and Dr. Hiesgen). Tunneling spectroscopy was used to probe the electronic properties of individual particles; the results suggest that there is a weak electronic coupling between the gold and the CdS particles, hence that the gold/CdS junction must be considered as a tunnel junction.
The photocurrent-potential curves of a gold/Q-CdS electrode were measured in O.5 M KCl and in alkaline solutions containing tartrate as a hole scavenger. IMPS measurements were performed with UV light from an Ar-laser, the light intensity being harmonically modulated with an acousto-optic modulator. The opto-electric transfer function measured in the onset region of the photocurrent was reminescent of recombination occuring via two electron tunneling processes between gold and Q-CdS:

Q(e,h_t) ---> Q⁺(h_t) ---> Q

Here, Q(e,h_t) stands for the CdS particle with an electron in the LUMO (conduction band), and a hole trapped on a S^2- surface ion. The electron tunnels from the conduction band to empty states above the Fermi-level in the gold. Consecutively, an electron from the Au tunnels to an empty surface state (surface trapped hole). The characteristic frequency of the IMPS response was about 2 π x 200 s^-1. According to our model, the characteristic frequency corresponds to k_{CB  Au}, the tunnel rate per Q(e,h_t) particle. Photoinduced electron tunneling at such a low rate again indicate the existence of a long lived excited state in Q-CdS. It can be concluded that Q(e,h_t) is identical to the long-lived dark excitonic state observed by time resolved light absorption spectroscopy [2,3]. Indeed, tunneling at a rate of about 2 π x 200 s^-1 from Q(e,h_t) to gold can compete with the decay of Q(e,h_t) due to electron capture by S^-a (i.e. surface trapped hole) which has a life time of about 50 x 10^-3 s.
The IMPS response in alkaline aqueous electrolyte shows two semicircles in the {Re=pos., Im=neg.} quadrant, reminescent of charge separation by two electron tunneling processes:

Q(e,h_t) ---> Q⁺(h_t) ---> Q

From the characteristic frequencies, it followed that electron tunneling in this solution was slower than in 0.5 M KCl. This might be due to the fact that in alkaline electrolytes, Q-CdS particles are covered with a CdO or Cd(OH)₂ layer, increasing the average tunnel distance.

[1]	S. Ogawa, F.-R. F. Fan, and A. J. Bard, J. Phys.Chem. 99 (1995) 11182.
[2]	W. J. Albery, P. N. Bartlett and J. D. Porter, J. Electrochem. Soc. 131 (1984) 2892.
[3]	W. J. Albery, G. T. Brown, J. R. Darwent, and E. Saievar-Iranizad, J. Chem. Soc. Faraday Trans. 1, 81 (1985) 1999.

[BACK] [MAIN] [FURTHER]