Spatially Controlled CO2 Conversion Kinetics in Natural Leaves for Motion Generation.

Living systems that can spontaneously exhibit directional motion belong to diverse classes such as bacteria, sperm and plankton. They have fascinated scientists in recent years to design completely artificial or biohybrid mobile objects. Natural ingredients, like parts of plants, have been used to elaborate miniaturized dynamic objects, which can move when they are combined with other, non-natural, building blocks. Herein, we report that the precise structural tailoring of natural plant leaves allows generating a spatially predefined and confined release of oxygen gas, due to the conversion of carbon dioxide. This constitutes the driving force for generating motion, which is solely due to the respiration of leaves by photosynthesis. The rate of gas evolution can be fine-tuned by changing the light intensity and the leaf size, allowing ultimately to control the motility of objects with dimensions ranging from the millimeter to the micrometer scale.

Light and electric field induced unusual large-scale charge separation in hybrid semiconductor objects.

Separation of electric charges is the most crucial phenomenon in natural photosynthesis, and is also extremely important for many artificial energy conversion systems based on semiconductors. The usual roadblock in this context is the fast recombination of electrons and holes. Here we demonstrate that the synergy of light and electric fields allows separating very efficiently electric charges over an unusually large distance in TiO2. The generated internal electric field can also be used to shuttle electrons simultaneously to the two opposite sides of a hybrid TiO2-polyaniline object. This counterintuitive behavior is based on the combination of the principles of bipolar electrochemistry and semi-conductor physics.

Evaluation of local oxygen flux produced by photoelectrochemical hydroxide oxidation by scanning electrochemical microscopy.

Several in-situ electrochemical approaches have been developed for performing a localized photoelectrochemical investigation of the photoanode. One of the techniques is scanning electrochemical microscopy (SECM), which probes local heterogeneous reaction kinetics and fluxes of generated species. In traditional SECM analysis of photocatalysts, evaluation of the influence of radiation on the rate of studied reaction requires an additional dark background experiment. Here, using SECM and an inverted optical microscope, we demonstrate the determination of O2 flux caused by light-driven photoelectrocatalytic water splitting. Photocatalytic signal and dark background are recorded in a single SECM image. We used an indium tin oxide electrode modified with hematite (α-Fe2O3) by electrodeposition as a model sample. The light-driven flux of oxygen is calculated by analysis of SECM image recorded in substrate generation/tip collection mode. In photoelectrochemistry, the qualitative and quantitative knowledge of oxygen evolution will open new doors for understanding the local effects of dopants and hole scavengers in a straightforward and conventional manner.

Growth of sillenite Bi12FeO20 single crystals: structural, thermal, optical, photocatalytic features and first principle calculations.

Ideal sillenite type Bi12FeO20 (BFO) micron sized single crystals have been successfully grown via inexpensive hydrothermal method. The refined single crystal X-ray diffraction data reveals cubic Bi12FeO20 structure with single crystal parameters. Occurrence of rare Fe4+ state is identified via X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The lattice parameter (a) and corresponding molar volume (Vm) of Bi12FeO20 have been measured in the temperature range of 30-700 °C by the X-ray diffraction method. The thermal expansion coefficient (α) 3.93 × 10-5 K-1 was calculated from the measured values of the parameters. Electronic structure and density of states are investigated by first principle calculations. Photoelectrochemical measurements on single crystals with bandgap of 2 eV reveal significant photo response. The photoactivity of as grown crystals were further investigated by degrading organic effluents such as Methylene blue (MB) and Congo red (CR) under natural sunlight. BFO showed photodegradation efficiency about 74.23% and 32.10% for degrading MB and CR respectively. Interesting morphology and microstructure of pointed spearhead like BFO crystals provide a new insight in designing and synthesizing multifunctional single crystals.

Electrocatalyst on insulating support?: Hollow silica spheres loaded with Pt nanoparticles for methanol oxidation.

Electrocatalytic oxidation of methanol on silica hollow spheres, loaded with platinum nanoparticles (Pt-SiO2-HS), is reported. The functionalized hollow silica spheres were prepared by the surfactant (lauryl ester of tyrosine) template-assisted synthesis. These spheres were loaded with platinum nanoparticles by γ-radiolysis. Energy-dispersive X-ray analysis (EDAX) and X-ray photoelectron spectroscopy (XPS) analyses confirmed presence of Si and Pt in the composite. High-resolution transmission electron microscopy showed the formation of uniformly deposited Pt nanoparticles over the hollow spheres with a predominant Pt(111) lattice plane on the surface. In spite of the poor conducting nature of the silica support, the oxidation potential and current density per unit mass for methanol oxidation were noted to be ca. 0.72 V vs NHE and 270 mA mg(-1), respectively, which are among the best values reported in its class. The composite did not show any sign of a degradation even after repeated use. In fact, the anodic current was found to increase under constant polarization, which is attributed to a facile reaction between adsorbed CO with a surface hydroxyl group present on the silica support. These results are in favor of Pt-SiO2-HS as a promising electrocatalyst material in the direct methanol fuel cell (DMFC) applications.

M-Au/TiO2 (M = Ag, Pd, and Pt) nanophotocatalyst for overall solar water splitting: role of interfaces.

M-Au/TiO2 (M = Ag, Pd, Pt) composites were prepared through a facile one-pot photodeposition synthesis and evaluated for solar water splitting (SWS) with and without a sacrificial agent. The M-Au combination exhibits a dominant role in augmenting the H2 generation activity by forming a bi-metallic system. Degussa P25 was used as a TiO2 substrate to photodeposit Au followed by Au + M (M = Ag/Pd/Pt). The SWS activity of the M-Au/TiO2 was determined through photocatalytic H2 production in the presence of methanol as a sacrificial agent under one sun conditions with an AM1.5 filter. The highest H2 yield was observed for Pt0.5-Au1/TiO2 and was around 1.3 ± 0.07 mmol h(-1) g(-1), with an apparent quantum yield (AQY) of 6.4%. Pt0.5-Au1/TiO2 also demonstrated the same activity for 25 cycles of five hours each for 125 h. Critically, the same Pt0.5-Au1/TiO2 catalyst was active in overall SWS (OSWS) without any sacrificial agent, with an AQY = 0.8%. The amount of Au and/or Pt was varied to obtain the optimum composition and it was found that the Pt0.5-Au1/TiO2 composition exhibits the best activity. Detailed characterization by physico-chemical, spectral and microscopy measurements was carried out to obtain an in-depth understanding of the origin of the photocatalytic activity of Pt0.5-Au1/TiO2. These in-depth studies show that gold interacts predominantly with oxygen vacancies present on titania surfaces, and Pt preferentially interacts with gold for an effective electron-hole pair separation at Pt-Au interfaces and electron storage in metal particles. The Pt in Pt0.5-Au1/TiO2 is electronically and catalytically different from the Pt in Pt/TiO2 and it is predicted that the former suppresses the oxygen reduction reaction.