Water splitting with polyoxometalate-treated photoanodes: enhancing performance through sensitizer design

Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO(2), polyoxometalate (POM) water oxidation catalyst [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)](10–) (1), and both known photosensitizer [Ru(bpy)(2)(H(4)dpbpy)](2+) (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)(2)(H(2)dpbpy)](H(2)2). Both triads, containing catalyst 1, and catalyst-free dyads, produce O(2) with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H(2)2 absorbs light more strongly than P2, and increases O(2) quantum yields by up to 270%. TiO(2)-2 based photoelectrodes are also more stable to desorption of active species than TiO(2)–P2: losses of catalyst 1 are halved when pH > TiO(2) point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O(2) are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO(2), and is also consistent with the 4(th)1-to-dye electron transfer limiting performance rather than catalyst TOF(max). Transient absorption reveals that TiO(2)–2–1 has similar 1(st) electron transfer dynamics to TiO(2)–P2–1, with rapid (ps timescale) formation of long-lived TiO(2)(e(–))–2–1(h(+)) charge separated states, and demonstrates that metallation of the crown ether groups (Na(+)/Mg(2+)) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO(2)-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO(2) can oxidize water without a catalyst.