NiO x (0 < x < 1) modified SrTiO 3 (STO) is one of the best studied photocatalysts for overall water splitting under UV light. The established mechanism for this and many other NiO x containing catalysts assumes water oxidation to occur at the early transition metal oxide and water reduction at NiO x . Here we show that NiO x -STO is more likely a three component Ni-STO-NiO catalyst, in which STO absorbs the light, Ni reduces protons, and NiO oxidizes water. This interpretation is based on systematic H 2 /O 2 evolution tests of appropriately varied catalyst compositions using oxidized, chemically and photochemically added nickel and NiO nanoparticle cocatalysts. Surface photovoltage (SPV) measurements reveal that Ni(0) serves as an electron trap (site for water reduction) and that NiO serves as a hole trap (site for water oxidation). Electrochemical measurements show that the overpotential for water oxidation correlates with the NiO content, whereas the water reduction overpotential depends on the Ni content. Photodeposition experiments with NiCl 2 and H 2 PtCl 6 on NiO-STO show that electrons are available on the STO surface, not on the NiO particles. Based on photoelectrochemistry, both NiO and Ni particles suppress the Fermi level in STO, but the effect of this shift on catalytic activity is not clear. Overall, the results suggest a revised role of NiO in NiO x -STO and in many other nickel-containing water splitting systems, including NiO x -La : KTaO 3 , and many layered perovskites.

Rutile IrO(2) is known as being among the best electrocatalysts for water oxidation. Here we report on the unexpected photocatalytic water oxidation activity of 1.98 nm ± 0.11 nm succinic acid-stabilized IrO(2) nanocrystals. From aqueous persulfate and silver nitrate solution the nonsensitized particles evolve oxygen with initial rates up to 0.96 μmol min(-1), and with a quantum efficiency of at least 0.19% (measured at 530 nm). The catalytic process is driven by visible excitations from the Ir-d(t(2g)) to the Ir-d(e(g)) band (1.5-2.75 eV) and by ultraviolet excitations from the O-p band to the Ir-d(e(g)) (>3.0 eV) band. The formation of the photogenerated charge carriers can be directly observed with surface photovoltage spectroscopy. The results shed new light on the role of IrO(2) in dye- and semiconductor-sensitized water splitting systems.

Alpha-Fe 2 O 3 is cheap and abundant, and has a visible light indirect (phonon assisted) band gap of 2.06 eV (600 nm) due to a d-d transition, and a direct band gap at 3.3 eV (375 nm), associated with the ligand to metal charge transfer process. Here we describe results on using freely dispersed Fe 2 O 3 nanocrystals for photocatalytic water oxidation. Three morphologies of hematite were compared, including bulk-type-a-Fe 2 O 3 (Bulk-Fe 2 O 3 , 120 nm), ultrasonicated Bulk-Fe 2 O 3 (Sonic-Fe 2 O 3 , 44 nm), and synthetic Fe 2 O 3 (Nano-Fe 2 O 3 , 5.4 nm) obtained by hydrolysis of FeCl 3 $6H 2 O. According to X-ray diffraction, all phases were presented in the alpha structure type, with Nano-Fe 2 O 3 also containing traces of b-FeOOH. UV/Vis diffuse reflectance revealed an absorption edge near 600 nm (E G ¼ 2.06 eV) for all materials. Cyclic voltammetry gave the water oxidation overpotentials (versus NHE at pH ¼ 7, at 1.0 mA cm À2 ) as h ¼ +0.43 V for Nano-Fe 2 O 3 , h ¼ +0.63 V for Sonic-Fe 2 O 3 , and h ¼ +0.72 V for Bulk-Fe 2 O 3 . Under UV and visible irradiation from a 300 W Xe-arc lamp, all three materials (5.6 mg) evolved O 2 from water with 20.0 mM aqueous AgNO 3 as sacrificial electron acceptor. The highest rates were obtained under UV/Vis (>250 nm) irradiation with 250 mmol h À1 g À1 for Bulk-Fe 2 O 3 , 381 mmol h À1 g À1 for Sonic-Fe 2 O 3 and 1072 mmol h À1 g À1 for Nano-Fe 2 O 3 . Turnover numbers (TON ¼ moles O 2 / moles Fe 2 O 3 ) were above unity for Nano-Fe 2 O 3 (1.13) and Sonic-Fe 2 O 3 (1.10) but not for Bulk-Fe 2 O 3 (0.49), showing that the nanoscale morphology was beneficial for catalytic activity.

SrTiO(3) (STO) is a large band gap (3.2 eV) semiconductor that catalyzes the overall water splitting reaction under UV light irradiation in the presence of a NiO cocatalyst. As we show here, the reactivity persists in nanoscale particles of the material, although the process is less effective at the nanoscale. To reach these conclusions, Bulk STO, 30 ± 5 nm STO, and 6.5 ± 1 nm STO were synthesized by three different methods, their crystal structures verified with XRD and their morphology observed with HRTEM before and after NiO deposition. In connection with NiO, all samples split water into stoichiometric mixtures of H(2) and O(2), but the activity is decreasing from 28 μmol H(2) g(-1) h(-1) (bulk STO), to 19.4 μmol H(2) g(-1) h(-1) (30 nm STO), and 3.0 μmol H(2) g(-1) h(-1) (6.5 nm STO). The reasons for this decrease are an increase of the water oxidation overpotential for the smaller particles and reduced light absorption due to a quantum size effect. Overall, these findings establish the first nanoscale titanate photocatalyst for overall water splitting.

The photocatalytic hydrogen production of CdSe nanocrystals (1.75-4.81 nm) in the presence of aqueous sodium sulphite depends exponentially on the bandgap of the particles, confirming that the material's activity is controlled by the degree of quantum confinement.