82 | December 2021 www.facs.website none-negligible role in the catalytic enhancement.24-27 These studies provided various experimental and theoretical evidences supporting our conclusion that the Pt(II)-catalyzed H2 evolution is often accelerated via formation of a hydridodiplatinum(II,III) intermediate: 2Pt(II) + H+ + e- —> Pt(II)-Pt(III)-H. The stabilization of this intermediate has been interpreted in terms of the enhanced basicity of the electron pair in the filled Pt(II) dz2 orbital because of destabilization at the s*(dz2-dz2) antibonding orbital by the stack of two square-planar Pt(II) units. The high HER activity initially found for the amidate-bridged Pt(II)2 dimers may be similarly explained. As noted earlier,17 the hydride formation accompanies the formal oxidation at the metal center (i.e., Pt(II)2 Pt(2.5+)2 + e-) and therefore the strongly donating property of ligands together with the filled-filled dz2-dz2 interaction greatly contributes to the thermodynamic stability of the Pt(II)-Pt(III)-H intermediate. Several other groups have so far reported on the results consistent with our conclusion.28 On the other hand, we also attempted to develop the dyads and triads constructed by the covalent linkage of components selected from WRC, PS, Acceptor, and Donor.29-32 The first successful model was a PS-WRC dyad given by the amide coupling of [Ru(bpy)2(5amino-phen)]2+ (phen = 1,10-phenanthroline) and PtCl2(dcbpy) (dcbpy = 4,4’-dicarboxy-bpy) (Figure 2),24 which was turned out to be the first example of a photo-hydrogen-evolving molecular device promoting the water reduction to H2 in the presence of Donor (EDTA) without any additional components. Based on the photocatalysis experiments combined with the in-situ dynamic light scattering (DLS) measurements, the lack of colloidal platinum formation was clearly evidenced for many of such molecular devices developed in our group.25,30,31,33 Developing PECs with a dark cathode rather than Tandem PECs As mentioned above, the two-phase gas evolution technique adopted in our molecular-based photoelectrochemical cells (PECs) has a great advantage (Figure 3c). The original concept was developed by Fujishima and Honda in 1972 (Figure 3a).11 In their report, a TiO2 electrode, corresponding to an n-type semiconductor (SC), was irradiated by visible light to evolve O2 and H2 at the photoanode (TiO2) and the dark cathode (Pt), respectively, without applying external bias with the two compartments separated by a glass frit (Figure 3a). They also pointed out that the photoirradiation at the cathode by replacing Pt by a p-type SC should result in more efficient water splitting. By following this concept, the molecular-based PECs with both electrodes subjected for light illumination are intensively studied for overall water splitting in recent years.34-37 Typically, such PECs consist of a photoanode modified with PS and WOC and a photocathode similarly modified with PS and WRC (Fig. 3b). These PECs are thus classified as Tandem PECs, where TiO2, SnO2, WO3 or BiVO4 (n-type SC) is adopted in the photoanode, while NiO, p-Si or p-GaP (p-type SC) is used in the photocathode. The word “tandem” denotes that two photosensitizers are connected in a tandem fashion in order to successively transfer a single electron based on twice of one-photon pumping. It means that two photons are required for the overall one-electron transfer. Theoretically, at least 8 photons must be absorbed to split water: 2H2O + 8hn 2H2 + O2, leading to the value of 50% in the highest attainable quantum efficiency for the overall photoreaction with this Tandem PECs. Some researchers misleadingly define that “Tandem” is equivalent to the “Z-scheme in oxygenic photosynthesis by green plants (PS-II and PS-I drive water oxidation and NADPH production, respectively)”. However, this is not the case because NADPH is not the only photoproduct. More importantly, the electron transport chain connecting the PS-II and PS-I generates the proton gradient energy across the Thylakoid membrane which is used to mechanically drive the ATP synthase. To keep the value of 100% regarded as the standard theoretical maximum for the quantum efficiency of photoreactions, our Figure 2. Schematic representation of multi-component system for photochemical HER, and molecular structures of the Pt(II)2 dimers (WRC) together with the dinuclear Ru-Pt photocatalyst (PS-WRC dyad).
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