Frank E. Osterloh

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]

Nanoscale strontium titanate photocatalysts for overall water splitting.

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.

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.

Quantum confinement controls photocatalysis: a free energy analysis for photocatalytic proton reduction at CdSe nanocrystals.

The ability to adjust the mechanical, optical, magnetic, electric, and chemical properties of materials via the quantum confinement effect is well-understood. Here, we provide the first quantitative analysis of quantum-size-controlled photocatalytic H2 evolution at the semiconductor-solution interface. Specifically, it is found that the hydrogen evolution rate from illuminated suspended CdSe quantum dots in aqueous sodium sulfite solution depends on nanocrystal size. Photoelectrochemical measurements on CdSe nanocrystal films reveal that the observed reactivity is controlled by the free energy change of the system, as determined by the proton reduction potential and the quasi-Fermi energy of the dots. The corresponding free energy change can be fitted to the photocatalytic activity using a modified Butler-Volmer equation for reaction kinetics. These findings establish a quantitative experimental basis for quantum-confinement-controlled proton reduction with semiconductor nanocrystals. Electrochemical data further indicate that proton reduction occurs at cadmium sites on the dots, and that charge separation in these nanocrystals is controlled by surface effects, not by space charge layers.

Particle suspension reactors and materials for solar-driven water splitting

Reactors based on particle suspensions for the capture, conversion, storage, and use of solar energy as H2 are projected to be cost-competitive with fossil fuels. In light of this, this review paper summarizes state-of-the-art particle light absorbers and cocatalysts as suspensions (photocatalysts) that demonstrate visible-light-driven water splitting on the laboratory scale. Also presented are reactor descriptions, theoretical considerations particular to particle suspension reactors, and efficiency and performance characterization metrics. Opportunities for targeted research, analysis, and development of reactor designs are highlighted.

Structure defects in g-C3N4 limit visible light driven hydrogen evolution and photovoltage

Graphitic carbon nitride (g-C3N4) is a promising visible-light-responsive photocatalyst for hydrogen generation from water. As we show here, the photocatalytic activity of g-C3N4 is limited by structure defects generated during the calcination process. Specifically we find that the photocatalytic hydrogen production rate from aqueous methanol is inversely related to the calcination temperature (520–640 °C). The highest activity of 0.301 mmol h−1 g−1 is observed for the sample prepared at the lowest processing temperature. Surface photovoltage (SPV) spectroscopy shows that the maximum photovoltage is reduced (from 1.29 V to 0.62 V) as the processing temperature is increased, in accordance with higher defect concentrations and faster electron–hole recombination. The defects also produce additional optical absorption in the visible spectra and cause a red shifted, weakened photoluminescence (PL). Based on the sub-gap signal in the SPV and PL spectra, defect energy levels are +0.97 V and −0.38 V (vs. NHE) in the band gap of the material. According to Fourier transform infrared (FTIR) spectra, the defects are due to amino/imino groups in the g-C3N4 lattice.

Extrinsic magnetoresistance in magnetite nanoparticles

Magnetite (Fe3O4) nanoparticles, 8 to 9 nm in size, have been synthesized using an aqueous precipitation technique. X-ray diffraction and chemical titration confirm a single cubic spinel phase with expected stoichiometry. Superparamagnetic behavior has been observed in pressed pellets of the nanoparticles above 200 K. Spin-dependent tunneling through adjacent particles has led to a negative magnetoresistance, −8.6% at 200 K and −4.5% at 300 K in a 70 kOe field. This is caused by the field-induced alignment of the nanoparticle magnetization directions.

Limiting factors for photochemical charge separation in BiVO4/Co3O4, a highly active photocatalyst for water oxidation in sunlight

Chemical modification of BiVO4 nanoparticles (Scheelite, EG = 2.62 eV) with chemically deposited Co3O4 nanoparticles improves the photocatalytic water oxidation activity by a factor of 17 to 11 mmol g−1 h−1 under visible light (AQE 10% at 435 nm) and to 1.24 mmol g−1 h−1 under sunlight from aqueous 0.02 M NaIO4. This activity ranks among the highest among known visible light driven water oxidation photocatalysts. Based on systematic electrochemical, photoelectrochemical, and surface photovoltage measurements, the high photocatalytic activity can be attributed to the electrocatalytic properties of the Co3O4 cocatalyst and to the formation of a heterojunction at the BiVO4–Co3O4 interface.

Photochemical Charge Separation in Nanocrystal Photocatalyst Films: Insights from Surface Photovoltage Spectroscopy

Photochemical charge generation, separation, and transport at nanocrystal interfaces are central to photoelectrochemical water splitting, a pathway to hydrogen from solar energy. Here, we use surface photovoltage spectroscopy to probe these processes in nanocrystal films of HCa2Nb3O10, a proven photocatalyst. Charge injection from the nanoparticles into the gold support can be observed, as well as oxidation and reduction of methanol and oxygen adsorbates on the nanosheet films. The measured photovoltage depends on the illumination intensity and substrate material, and it varies with illumination time and with film thickness. The proposed model predicts that the photovoltage is limited by the built-in potential of the nanosheet-metal junction, that is, the difference of Fermi energies in the two materials. The ability to measure and understand these light-induced charge separation processes in easy-to-fabricate films will promote the development of nanocrystal applications in photoelectrochemical cells, photovoltaics, and photocatalysts.