Erwin M. Sabio

Artificial inorganic leafs for efficient photochemical hydrogen production inspired by natural photosynthesis.

Adv. Mater. 2010, 22, 951–956 2010 WILEY-VCH Verlag Gm Using sunlight to split water molecules and produce hydrogen fuel is one of the most promising tactics for controlling our carbon-based energy ‘‘habit’’. Of the various possible methods, nature provides a blueprint for converting solar energy in the form of chemical fuels. A natural leaf is a synergy of elaborated structures and functional components in order to produce a highly complex machinery for photosynthesis in which light harvesting, photoinduced charge separation, and catalysis modules combine to capture solar energy and split water into oxygen and ‘‘hydrogen’’ (in the form of reducing equivalents) efficiently. Thus, the design of efficient, cost-effective artificial systems by the coupling of leaflike hierarchical structures and analogous functional modules under the guidance of the key steps of natural photosynthesis—capture of sunlight photons, electron–hole separation with long lifetimes, and energy transduction into hydrogen—would be a major advance in the development of materials for energy conversion. Here, we present a general strategy to assemble man-made catalysts (Pt/N-doped TiO2) into leaf-shaped hierarchical structures, named artificial inorganic leaf (AIL), for efficient harvesting of light energy and photochemical hydrogen production. This concept may broaden the horizon for the design of artificial photosynthetic systems based on biological paradigms and provides a working prototype to exploit solar energy for sustainable energy resources. Many research efforts have been carried out to develop artificial photosynthetic systems by constructing a variety of analogous molecular systems consisting of electron donors and acceptors to mimic light-driven charge separation, which occur in photosynthetic reaction centers, or by assembling semiconductor photocatalysts into various nanostructures. Though significant progress has been achieved, most research only focused on the functional imitation of photosynthesis and neglected the structural effect. Actually, the whole structure of natural leaves strongly favors light harvesting: the focusing of light by the lenslike epidermal cells, the multiple scattering and absorbance of light within the veins’ porous architectures, the light propagation in the columnar cells in palisade parenchyma acting as light guides, the enhanced effective light pathlength and light scattering by the less regularly arranged spongy mesophyll cells, and the efficient light-harvesting and fast charge separation in the high surface area three-dimensional constructions of interconnected nanolayered thylakoid cylindrical stacks (granum) in chloroplast. Meanwhile, the photosynthetic pigments in chloroplast successfully perform electron transfer and energy transduction. Thus, in order to mimic a photosynthetic system, it may be necessary for an artificial system to both have similar hierarchical structures for efficient light-harvesting and charge-separation-analogous functional modules, which could i) absorb incident photons, generating excited states, ii) transfer this excitation energy to a donor/ acceptor interface, where photochemical charge separation takes place. Meanwhile, such a system should be able to transfer charge away from the interface in order to limit the rate of wasteful recombination reactions, and iii) it should couple the photochemically generated charges to appropriate catalysts for the production of hydrogen. So our approach for artificial photosynthesis is to construct an artificial leaf by copying the complex architecture of leaves, replace the natural photosynthetic pigments with man-made catalysts, and realize efficient light-harvesting and photochemical hydrogen production. We first demonstrate this new concept with N-doped TiO2, a widely used visible-light-responsive photocatalyst for hydrogen production. Recently, there has been a strong interest in doping TiO2 with anions such as N, S, B, P, C, and halogens. [22–25]

Photocatalytic Water Oxidation with Nonsensitized IrO2 Nanocrystals under Visible and UV Light

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.

Photocatalytic water oxidation with suspended alpha-Fe2O3 particles-effects of nanoscaling

Alpha-Fe2O3 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 Fe2O3 nanocrystals for photocatalytic water oxidation. Three morphologies of hematite were compared, including bulk-type-α-Fe2O3 (Bulk-Fe2O3, 120 nm), ultrasonicated Bulk-Fe2O3 (Sonic-Fe2O3, 44 nm), and synthetic Fe2O3 (Nano-Fe2O3, 5.4 nm) obtained by hydrolysis of FeCl3·6H2O. According to X-ray diffraction, all phases were presented in the alpha structure type, with Nano-Fe2O3 also containing traces of β-FeOOH. UV/Vis diffuse reflectance revealed an absorption edge near 600 nm (EG = 2.06 eV) for all materials. Cyclic voltammetry gave the water oxidation overpotentials (versusNHE at pH = 7, at 1.0 mA cm−2) as η = +0.43 V for Nano-Fe2O3, η = +0.63 V for Sonic-Fe2O3, and η = +0.72 V for Bulk-Fe2O3. Under UV and visible irradiation from a 300 W Xe-arc lamp, all three materials (5.6 mg) evolved O2 from water with 20.0 mM aqueous AgNO3 as sacrificial electron acceptor. The highest rates were obtained under UV/Vis (>250 nm) irradiation with 250 μmol h−1 g−1 for Bulk-Fe2O3, 381 μmol h−1 g−1 for Sonic-Fe2O3 and 1072 μmol h−1 g−1 for Nano-Fe2O3. Turnover numbers (TON = moles O2/moles Fe2O3) were above unity for Nano-Fe2O3 (1.13) and Sonic-Fe2O3 (1.10) but not for Bulk-Fe2O3 (0.49), showing that the nanoscale morphology was beneficial for catalytic activity.

Improved niobate nanoscroll photocatalysts for partial water splitting.

Layered K(4)Nb(6)O(17) is a known UV-light-driven photocatalyst for overall water splitting, with a band gap of 3.5 eV. Following ion exchange and exfoliation with tetrabutylammonium hydroxide, the layered material separates into nanosheets that coil into 1.0±0.5 μm long and 10±5 nm wide nanoscrolls to reduce their surface energy. Pt and IrO(x) (x=1.5-2) nanoparticles were photochemically deposited onto the surface of the nanoscrolls to produce two- and three-component photocatalysts. Under UV irradiation, the nanostructures produced H(2) from pure water and aqueous methanol, with turnover numbers ranging from 2.3 and 18.5 over a 5 h period. The activity of the catalysts for H(2) evolution can be directly correlated with the varying overpotentials for water reduction (210-325 mV). From water, no oxygen is evolved. Instead, the formation of surface-bound peroxides in a 1:1 stoichiometry with H(2) is observed. Slow photochemical oxygen evolution can be achieved with the sacrificial electron acceptor AgNO(3), and under an electrochemical bias. The electrochemical water oxidation overpotentials are ca. 600 mV across the series of scrolls. From the photo onset potential the conduction band edge for the unmodified scrolls is estimated as -0.75 V at pH 7. Deposition of a co-catalyst is found to depress this value by 58 mV (IrO(x)), 148 mV (Pt/IrO(x)), and 242 mV (Pt). However, because water oxidation remains rate-limiting, this does not affect the overall performance of the catalysts.

Charge separation in a niobate nanosheet photocatalyst studied with photochemical labeling.

Photolabeling was employed to probe charge separation and the distribution of redox-active sites on the surface of nanosheets derived from the layered photocatalysts KCa(2)Nb(3)O(10). Electron microscopy reveals 1-50 nm particles of silver, gold, iridium oxide, and manganese dioxide particles and small atomically sized clusters of platinum and IrO(x) on the nanosheet surfaces and along the edges. The sizes, shapes, and particle densities vary with the deposition conditions, i.e., the precursor concentration and the presence of sacrificial agents. Overall, the study shows that photogenerated electrons and holes are accessible throughout the nanosheets, without evidence for spatial charge separation across the sheet.

Metallic LiMo3Se3 nanowire film sensors for electrical detection of metal ions in water.

LiMo 3Se 3 nanowire film sensors were fabricated by drop-coating a 0.05% (mass) aqueous nanowire solution onto microfabricated indium tin oxide electrode pairs. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM), the films are made of a dense network of 3-7 nm thick nanowire bundles. Immersion of the films in 1.0 M aqueous solutions of group 1 or 2 element halides or of Zn(II), Mn(II), Fe(II), or Co(II) chlorides results in an increase of the electrical resistance of the films. The resistance change is always positive and reaches up to 9% of the base resistance of the films. It occurs over the course of 30-240 s, and it is reversible for monovalent ions and partially reversible for divalent ions. The signal depends on the concentration of the electrolyte and on the size and charge of the metal cation. Anions do not play a significant role, presumably, because they are repelled by the negatively charged nanowire strands. The magnitude of the electrical response and its sign suggest that it is due to analyte-induced scattering of conduction electrons in the nanowires. An ion-induced field effect can be excluded based on gated conductance measurements of the nanowire films.

Photocatalytic water splitting with nano-K4Nb6O17

As many water splitting photocatalysts only evolve hydrogen under irradiation due to complications with high energy water oxidation steps, a three component nano-catalyst was designed by combining sites for water reduction and oxidation with solar-charge-supplying semiconductor. The semiconductor framework was derived from K4Nb6O17, a known UV-light photocatalyst with a band gap of 3.5 eV. Following ion exchange and exfoliation with terabutyalammonium hydroxide, the layered material separates into nanosheets that coil into 1 ± 0.5 um long and 10 ± 5 nm wide nanoscrolls to redue surface energy. PT (reduction sites) and IrOx (oxidation sites) were photochecmically deposited on the surface of the nanoscrolls to produce two-and three-component nanostructures. Upon irradiation with UV-light, H2 was evolved from aqueous methanol and pure water, substoichiometric O2 from aqueous AgNo3. The band structures of each catalyst and reason for lack of O2 evolution from pure water was evaluated with cyclic voltammetry and photoelectrochemistry.