To solve resource, energy, and environmental issues, development of sustainable clean energy system is strongly required. In recent years, hydrogen has been paid much attention to as a clean energy. ...Solar hydrogen production by water splitting using a photocatalyst as artificial photosynthesis is a promising method to solve these issues. Efficient utilization of visible light comprised of solar light is essential for practical use. Three strategies, i.e., doping, control of valence band, and formation of solid solution are often utilized as the useful methods to develop visible light responsive photocatalysts. This mini-review introduces the recent work on visible-light-driven photocatalysts developed by substitution with metal cations of those strategies.
Mn-, Ru-, Rh-, and Ir-doped SrTiO3 possessed intense absorption bands in the visible light region due to excitation from the discontinuous levels formed by the dopants to the conduction band of the ...SrTiO3 host. Mn- and Ru-doped SrTiO3 showed photocatalytic activities for O2 evolution from an aqueous silver nitrate solution while Ru-, Rh-, and Ir-doped SrTiO3 loaded with Pt cocatalysts produced H2 from an aqueous methanol solution under visible light irradiation (λ > 440 nm). The Rh(1%)-doped SrTiO3 photocatalyst loaded with a Pt cocatalyst (0.1 wt %) gave 5.2% of the quantum yield at 420 nm for the H2 evolution reaction.
Photocatalytic H2 evolution over aqueous TiO2 suspension, with methanol as holes scavenger, is systematically studied as a function of anatase and rutile phase compositions. The highly crystalline, ...flame-synthesized TiO2 nanoparticles (22−36 m2 g−1) were designed to contain 4−95 mol % anatase, with the remaining being rutile. Although the amount of photocurrent generated under applied potential bias increases with increasing anatase content, a different trend was observed during photocatalytic H2 evolution in suspension form; that is, without potential bias. Here, synergistic effects in terms of H2 evolution were observed for a wide range of anatase contents, from 13 to 79 mol %. At the optimal 39 mol % anatase, the photocatalytic activity was enhanced by more than a factor of 2 with respect to the anatase- and rutile-rich phases. The synergistic effect in these mixed anatase−rutile phases was thought to originate from the efficient charge separation across phase junctions. No synergistic effect was observed for the physically mixed anatase and rutile particles due to insufficient physical contact. Here, we also identify the formation of highly reducing hydroxymethyl radicals during the simultaneous oxidation of methanol, which efficiently inject additional electrons into the TiO2 conduction band, that is, current-doubling, for heterogeneous (instead of homogeneous) H2 evolution.
CuGaS2, (AgInS2)x-(ZnS)2-2x, Ag2ZnGeS4, Ni- or Pb-doped ZnS, (ZnS)0.9-(CuCl)0.1, and ZnGa0.5In1.5S4 showed activities for CO2 reduction to form CO and/or HCOOH in an aqueous solution containing K2SO3 ...and Na2S as electron donors under visible light irradiation. Among them, CuGaS2 and Ni-doped ZnS photocatalysts showed relatively high activities for CO and HCOOH formation, respectively. CuGaS2 was applied in a powdered Z-scheme system combining with reduced graphene oxide (RGO)-incorporated TiO2 as an O2-evolving photocatalyst. The powdered Z-scheme system produced CO from CO2 in addition to H2 and O2 due to water splitting. Oxygen evolution with an almost stoichiometric amount indicates that water was consumed as an electron donor in the Z-schematic CO2 reduction. Thus, we successfully demonstrated CO2 reduction of artificial photosynthesis using a simple Z-scheme system in which two kinds of photocatalyst powders (CuGaS2 and an RGO-TiO2 composite) were only dispersed in water under 1 atm of CO2.
Sodium niobates with nanowire morphology (NaNbO3-NW) were synthesized in a large scale by use of a niobium oxooxalate complex as the starting material. This NaNbO3-NW showed definitely enhanced ...photocatalytic activity for H2 or O2 evolution in the presence of sacrificial reagents and an overall water splitting under UV-light irradiation, as compared with a bulky counterpart (NaNbO3-B). This is the first example that an overall water splitting into H2 and O2 proceeded on the semiconductor nanowire photocatalyst.
Ir cocatalyst-loaded SrTiO3:Ir has arisen as a promising photocatalyst for H2 evolution with a response to the whole range of visible light up to 800 nm. The key factor for the high activity lies in ...H2-reduction after loading of an Ir cocatalyst to obtain metallic and adhesive Ir-cocatalysts.
We investigated an effect of Ag(i)-substitution at the Cu sites in CuGaS2 with a chalcopyrite structure on physicochemical, photocatalytic and photoelectrochemical properties. Cu1-xAgxGaS2 (x = ...0-0.8) solid solutions possessed 2.34-2.35 eV of band gaps. Ru-loaded Cu1-xAgxGaS2 (x = 0-1.0) powders showed photocatalytic activity for hydrogen evolution from an aqueous solution containing SO32- and S2- as electron donors under simulated sunlight irradiation. The photocatalytic activity of CuGaS2 (x = 0) was improved, when excess Ga2S3 was added in a starting material for the preparation to give a single phase of a chalcopyrite structure. Cu1-xAgxGaS2 (x = 0.1-1.0) showed higher activity than the CuGaS2 (x = 0), when prepared with the optimum excess Ga2S3. Cu1-xAgxGaS2 (x = 0-0.8) electrodes gave cathodic photocurrents under visible light irradiation, indicating that they possessed p-type characters. Cu0.8Ag0.2GaS2 gave the largest cathodic photocurrent and possessed the most positive onset potential of the cathodic photocurrent among the Cu1-xAgxGaS2 (x = 0-0.8) electrodes. Loading a Ru cocatalyst increased the cathodic photocurrent. Solar water splitting proceeded without an external bias using a photoelectrochemical cell consisting of the Ru-loaded Cu0.8Ag0.2GaS2 photocathode and a CoO-loaded BiVO4 photoanode. Thus, the photocatalytic and photoelectrochemical performances of CuGaS2 were improved by Ag(i)-substitution at the Cu sites.
A visible-light-driven Z-scheme photocatalytic CO2 reduction reaction (CO2RR) to produce CO was demonstrated using an aqueous particulate dispersion containing two bare semiconductors, ...(CuGa)0.3Zn1.4S2 for CO2RR and BiVO4 for water oxidation. The semiconductors were mixed with a water-soluble cobalt tris(dimethylbipyridine) complex. The CO selectivity was 98% (against H2), and the rate of CO generation was 1–2 orders of magnitude higher than those of previously-reported aqueous suspension photocatalytic systems. O2 was continuously evolved, and isotope tracer analyses confirmed that CO2 was the carbon source for CO. Experimental studies and calculations suggest that the Co complex acts dual-functionally in synergy with (CuGa)0.3Zn1.4S2 and BiVO4: it behaves as an efficient ionic electron mediator, and also acts as a new active CO2RR cocatalyst after a structural change by accepting photoexcited electrons from (CuGa)0.3Zn1.4S2. This simple method, operating in a self-optimizing manner in solution, has great potential to help achieve sustainable, highly active artificial photosynthetic systems.
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•Co complexes were designed as redox electron mediators for visible-light photocatalytic liquid-phase Z-scheme CO2 reduction.•Activity of aqueous suspension of two semiconductor particulates was highly dependent on redox potential of the Co complex.•Use of Co-tris(dimethyl-bipyridine) complex resulted in the Z-schematic CO2-to-CO conversion selectivity of 98%.•The CO generation rate was improved by one to two orders of magnitude over conventional aqueous suspension Z-scheme systems.•The Co complex acted dual-functionally as an electron mediator and as a cocatalyst for CO2 reduction in the aqueous solution.
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