Photoelectrochemistry on Ru-2,2′-bipyridine-phosphonate-derivatized TiO2 with the l3-/I- and quinone/hydroquinone relays.: Design of photoelectrochemical synthesis cells

Photocurrent measurements have been made on nanocrystalline TiO2 surfaces derivatized by adsorption of a catalyst precursor, [Ru(tpy)(bpy(PO3H2)2)(OH2)](2+), or chromophore, [Ru(bPY)2(bpy(PO3H2)(2))](2+) (tpy is 2,2':6',2-terpyridine, bpy is 2,2'-bipyridine, and bpy(PO3H2)(2) is 2,2'-bipyridyl-4,4'-diphosphonic acid), and on surfaces containing both complexes. This is an extension of earlier work on an adsorbed assembly containing both catalyst and chromophore. The experiments were carried out with the I-3(-/I-) or quinone/hydroquinone (Q/H(2)Q) relays in propylene carbonate, propylene carbonate-water mixtures, and acetonitrile-water mixtures. Electrochemical measurements show that oxidation of surface-bound Ru-III-OH23+ to Ru-IV=O2+ is catalyzed by the bpy complex. Addition of aqueous 0.1 M HCIO4 greatly decreases photocurrent efficiencies for adsorbed [Ru(tpy)(bpy(PO3H2)(2))(OH2)(12+) with the I-3(-/I-) relay, but efficiencies are enhanced for the Q/H(2)Q relay in both propylene carbonate-HCIO4 and acetonitrileHCIO(4) mixtures. The dependence of the incident photon-to-current efficiency (IPCE) on added H(2)Q in 95% propylene carbonate and 5% 0.1 M HCIO4 is complex and can be interpreted as changing from rate-limiting diffusion to the film at low H(2)Q to rate-limiting diffusion within the film at high H(2)Q. There is no evidence for photoelectrochernical cooperativity on mixed surfaces containing both complexes with the IPCE response reflecting the relative surface compositions of the two complexes. These results provide insight into the possible design of photoelectrochemical synthesis cells for the oxidation of organic substrates.