Enhanced photoelectrochemistry in CdS/Au nanoparticle bilayers

Three different configurations of Au-nanoparticle/CdS-nanoparticle arrays are organized on Au/quartz electrodes for enhanced photocurrent generation. In one configuration, Au-nanoparticles are covalently linked to the electrode and the CdS-nanoparticles are covalently linked to the bare Au-nanoparticle assembly. The resulting photocurrent, phi = 7.5%, is ca. 9-fold higher than the photocurrent originating from a CdS-nanoparticle layer that lacks the Au-nanoparticles, phi = 0.8%. The enhanced photocurrent in the Au/CdS nanoparticle array is attributed to effective charge separation of the electron-hole pair by the injection of conduction-band electrons from the CdS- to the Au-nanoparticles. Two other configurations involving electrostatically stabilized bipyridinium-crosslinked Au/CdS or CdS/Au nanoparticle arrays were assembled on the Au/quartz crystal. The photocurrent quantum yields in the two systems are phi = 10% and phi = 5%, respectively. The photocurrents in control systems that include electrostatically bridged Au/CdS or CdS/Au nanoparticles by oligocationic units that lack electron-acceptor units are substantially lower than the values observed in the analogous bipyridinium-bridged systems. The enhanced photocurrents in the bipyridinium-crosslinked systems is attributed to the stepwise electron transfer of conduction-band electrons to the Au-nanoparticles by the bipyridinium relay bridge, a process that stabilizes the electron-hole pair against recombination and leads to effective charge separation.