Light absorption across the bandgap in semiconductors is exploited in many important applications such as photovoltaics, light emitting diodes and photocatalytic conversion. Metals differ from semiconductors in that there is no energy gap separating occupied and unoccupied levels; however, it is still possible to excite electrons between bands. This is evidenced by materials with metallic properties that are also strongly coloured. An important question is whether such coloured metals could be used in light harvesting or similar applications. The high conductivity of a metal would preclude sufficient electric field being available to separate photocarriers; however, the high carrier mobility in a metal might also facilitate kinetic charge separation. Here we clearly demonstrate for the first time the use of a red metallic oxide, Sr(1-x)NbO(3) as an effective photocatalyst. The material has been used under visible light to photocatalyse the oxidation of methylene blue and both the oxidation and reduction of water assisted by appropriate sacrificial elements.

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Mesoporous monocrystalline rutile TiO 2 has been fabricated at low temperature using mesoporous silicas SBA-15 and KIT-6 as hard templates. The key step of the synthetic process was introducing titanium nitrate complex into the template pores and allowing it to dry, dehydrate, decompose, and finally, form TiO 2 crystals in the pores. It was found that the reaction temperature and concentration of HNO 3 in the used precursor had great effects to the crystallization of TiO 2 . Removal of the silica templates after the TiO 2 crystallization has been investigated. Crystallization of TiO 2 in cage-containing mesoporous silicas, FDU-12 and SBA-16 was not successful, further confirming the previous speculation about strong interaction between the crystals and the wall of silica cages. The porous titanium oxide specimens were characterized by using various techniques, including XRD, HRTEM, and nitrogen adsorption/desorption. Proton conductivity and Li-ion insertion property of the samples were also examined. The highest conductivity, 8 × 10 -3 S cm -1 , was obtained at 50 °C under 100% RH and 1 Li + could be accommodated per TiO 2 unit (335 mA h/g) for the first discharge.

The low range of A-site deficiency in perovskite structures with Ni cations was verified by neutron powder diffraction, transmission electron microscopy, and thermogravimetric analysis. A thermodynamic approach has been utilized, for the first time, to predict the extent of A-site deficiencies within the perovskite structure, introducing simple prediction criteria that could be adopted for designing advanced materials.

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Simply mixing a Cu­(II) salt and 1,2-ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catalysis, depending on pH. In phosphate buffer at pH 12, the Cu­(II) en complex formed in solution is decomposed to give a phosphate-incorporated CuO/Cu­(OH)2 film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm2 was obtained at an overpotential of 540 mV, a significant enhancement compared to other Cu-based surface catalysts. The results of electrolysis studies suggest that the solution en complex decomposes by en oxidation to glyoxal, following Cu­(II) oxidation to Cu­(III). At pH 8, the catalysis shifts from heterogeneous to homogeneous with a single-site mechanism for Cu­(II)/en complexes in solution. A further decrease in pH to 7 leads to electrode passivation via the formation of a Cu­(II) phosphate film during electrolyses. As the pH is decreased, en, with pK b ≈ 6.7, becomes less coordinating and the precipitation of the Cu­(II) film inhibits water oxidation. The Cu­(II)-based reactivity toward water oxidation is shared by Cu­(II) complexation to the analogous 1,3-propylenediamine (pn) ligand over a wide pH range.

Ferroelectric materials hold great promise in the field of photocatalytic water splitting due to their spontaneous polarization that sets up an inherent internal field for the spatial separation of photogenerated charges. The ferroelectric polarization, however, is generally accompanied by some intrinsic defects, particularly oxygen vacancies, whose impact upon photocatalysis is far from being fully understood and modulated. Here, we have studied the role of oxygen vacancies over the photocatalytic behavior of single-domain PbTiO3 through a combination of theoretical and experimental viewpoints. Our results indicate that the oxygen vacancies in the negatively polarized facet (001) are active sites for water oxidation into O2, while the defect-free sites prefer H2O2 as the oxidation product. The apparent quantum yield at 435 nm for photocatalytic overall water splitting with PbTiO3/Rh/Cr2O3 is determined to be 0.025%, which is remarkable for single undoped metal oxide-based photocatalysts. Furthermore, the strong correlation among oxygen vacancies, polarization strength, and photocatalytic activity is properly reflected by charge separation conditions in the single-domain PbTiO3. This work clarifies the crucial role of oxygen vacancies during photocatalytic reactions of PbTiO3, which provides a useful guide to the design of efficient ferroelectric photocatalysts and their water redox reaction pathways.

Tantalum nitride (Ta3 N5 ) highlights an intriguing paradigm for converting solar energy into chemical fuels. However, its photocatalytic properties are strongly governed by various intrinsic/extrinsic defects. In this work, we successfully prepared a series of Mg-doped mesoporous Ta3 N5 using a simple method. The photocatalytic and photoelectrochemical properties were investigated from the viewpoint of how defects such as accumulation of oxygen and nitrogen vacancies contribute to the catalytic activity. Our findings suggest that Mg doping is accompanied by an accumulation of oxygen species and a simultaneous elimination of nitrogen vacancies in Ta3 N5 . These oxygen species in Ta3 N5 induce delocalized shallow donor states near the conduction band minimum and are responsible for high electron mobility. The superior photocatalytic activity of Mg-doped Ta3 N5 can then be understood by the improved electron-hole separation as well as the lack of nitrogen vacancies, which often serve as charge-recombination centers.