Characterization and Photoelectrochemistry of Layer-by-Layer Self Assembled Films of CdS Quantum Dots in Polyelectrolyte
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Characterization and Photoelectrochemistry of Layer-by-Layer Self Assembled Films of CdS Quantum Dots in Polyelectrolyte Matrix Lara. I. Halaoui Department of Chemistry, American University of Beirut, Beirut, 110236 Lebanon Email: [email protected] ABSTRACT Photoelectrochemical studies of polyacrylate-capped CdS nanoparticles embedded in a polyelectrolyte matrix on electrode surfaces are presented. Multilayered films of polyacrylatecapped Q-CdS are assembled layer-by-layer on solid surfaces modified with a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). The characterization of the layer-bylayer growth by means of UV-visible absorption, photoluminescence, FT-IR spectroscopy and ellipsometry measurements revealed the incorporation of the nanoparticles in the matrix and the reproducible surface-charge reversal taking place. In this paper, we present initial photocurrentvoltage studies of films thus formed, where the feasibility of charge conduction through the polyelectrolyte matrix and the dependence of photocurrent magnitude on the number of bilayers are demonstrated. In addition, the dependence of photocurrent direction on the electrode potential, and the observation of anodic photocurrent spikes attributed to surface states in the presence of dissolved oxygen are reported. INTRODUCTION Understanding the processes of charge separation upon light absorption, charge recombination, hopping at interfaces, and transfer to solution species in semiconductor nanoparticles and films of nanoparticles is crucial to gain a fundamental knowledge about this size regime, and for the development of a future nanotechnology. Achieving this goal necessitates devising controlled means for the assembly of semiconductor nanoparticles as films on conducting surfaces, and investigating the properties of such composite systems. In the quantum size regime, semiconductor nanoparticles exhibit a quantized eigenspectrum and an increase in the energy gap relative to the band gap of the bulk solid [1,2]. The synthesis, physical, and optical properties of many semiconductor nanoparticles, including CdS quantum dots (Q-CdS) have been extensively investigated [3]. There are also many reports regarding the photoelectrochemical properties of nanoparticles in solution or assembled on electrode surfaces [4-12]. In this paper, we present initial studies on light-induced charge transfer at multilayers of polyacrylate-capped Q-CdS assembled in poly(diallyldimethylammonium chloride) (PDADMA) by means of electrostatic attraction between the negatively charged capping agent and the cationic polyelectrolyte. This layering process follows the work of Iler for the layering of colloidal particles [13], and has been employed to incorporate various materials in polyelectrolytes [6, 14-16]. By investigating the photoelectrochemistry of films of Q-CdS/PDADMA, we report (1) the feasibility of electrical communication between the CdS nanoparticles and the electrode surface via charge hopping through the polyelectrolyte matrix, (2) the measurement of bot
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