AVS2012 Session SS+EN+OX-ThM: Catalysis and Photocatalysis on Oxides

Hydrogen production through the photoelectrochemical (PEC) water splitting is one of the attractive ways to convert solar energy to storable chemical energy. The availability of powder semiconductor materials through coating methods for preparing photoelectrodes is a one of strong point of PEC cells. Even if the surface is a rough particle layer, the electrolyte solution automatically forms the desirable solid-liquid interface for whole semiconductor surfaces, where photoexcited carriers are separated by electric field.

Oxynitride and oxysulfide materials are promising candidates for photoelectrodes for water splitting. Among them, LaTiO₂N has a proper band structure from the view point of driving solar water splitting. The material shows photocatalytic hydrogen and oxygen evolutions in half reactions using sacrificial reagents, indicating that the material have a proper band structure to drive PEC water splitting. LaTiO₂N absorbs visible light up to 600 nm (E_g = 2.1 eV), so that they can capture more solar energy than oxide photocatalysts, which typically have absorption in the UV region. Photoelectrodes based on the material have been studied extensively, however, the photocurrent was low due to the rack of good preparation method of the electrode.

In the present study, we report a novel fabrication method of photoelectrodes for PEC water splitting using semiconductor powders. This method, which we have termed the particle transfer (PT) method, is shown to be applicable to a variety of semiconductor powders. LaTiO₂N was demonstrated to exceed those prepared by the conventional method of photoelectrode fabrication from powder materials.

Difficulties in achieving control over carrier concentration have impeded progress toward tailoring the electric fields in semiconducting oxide photocatalysts based upon principles of electronic band engineering drawn from classical optoelectronics. The present work demonstrates such principles using the model case of methylene blue photo-oxidation over thin-film anatase TiO₂ grown by atomic layer deposition. The carrier concentration in the polycrystalline semiconductor is controlled over a range of 2.5 orders of magnitude via an unconventional means - film thickness, which indirectly influences the concentration of electrically active donor defects at grain boundaries. Over this range, the reaction rate constant varies by more than a factor of 10, and is well described by a quantitative one-dimensional model for photocurrent. The model suggests that the changes in rate result fundamentally from variations in the width of the space charge layer near the surface. Electrical characterization of the films by capacitance-voltage measurements and ultraviolet photoelectron spectroscopy, together with detailed physical characterization by a variety of techniques, confirm this picture. Prospects for better control of grain boundary donor defects through film synthesis procedures are discussed.

Rutile TiO₂(110) was employed as a model oxide surface to investigate the adsorption behavior of CO₂ by means of scanning tunneling microscopy (STM) and density functional theory (DFT). STM images of partially reduced TiO₂(110) surfaces obtained before and after in-situ dosing of CO₂ molecules at 50 K show that CO₂ adsorbs preferentially on oxygen vacancy (V_O) sites. Since the reaction of CO₂ with oxygen adatoms (surface hydroxyl groups) may lead to the formation of carbonate (bicarbonate), O₂ (H₂O) was predosed to form oxygen adatom (hydroxyl) covered TiO₂ surfaces. On the oxygen precovered surfaces, CO₂ molecules were found to preferentially bind on the Ti sites next to oxygen adatoms (O_a’s) and form CO₂/O_a complexes, while on hydroxylated surfaces no interactions were observed between CO₂ and hydroxyl groups. CO₂ binding to O_a’s is weak as revealed by the dissociation of the CO₂/O_a complexes at 50 K where CO₂ diffuses away along the Ti row. The weak binding indicates that CO₂ is bound to O_a only via dispersion forces. Temperature dependent studies (100 - 150 K) show that the CO₂ binding energy next to O_a’s is smaller by ~20 mV than that on V_O’s. At 50 K, however, the adsorption of CO₂ on V_O is partially hindered by the higher adsorption barrier. CO₂ molecules diffusing between two CO₂/O_a complexes are found to move fast compared to the STM sampling rate and are imaged as a time average of all CO₂ binding configurations on Ti sites. DFT studies reveal the rotation-tumbling mechanism for CO₂ diffusion with a very low diffusion barrier (~ 50 meV) in agreement with the experiment.

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X.L. is grateful for the support of the Linus Pauling Distinguished Postdoctoral Fellowship Program at PNNL. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PN NL). PNNL is a multiprogram national laboratory operated for DOE by Battelle.

We have investigated initial C-H bond selectivity in the activation of propane and n-butane on PdO(101) both experimentally and computationally. Temperature-programmed experiments using different propane isotopologues reveal a strong preference toward primary C-H bond cleavage of propane on PdO(101); about 90% of the propane molecules which react do so by primary C-H bond activation. Direct measurements of the initial dissociation probability of various n-butane isotopologues also demonstrate a high selectivity for primary C-H bond activation of n-butane on PdO(101) at low coverages. Unlike propane, however, TPRS experiments show that the preference for primary C-H bond cleavage of n-butane diminishes with increasing molecular coverage. Calculations using dispersion-corrected DFT reproduce the selectivity toward primary C-H bond cleavage of propane and n-butane on PdO(101), and predict that alkane C-H bond scission occurs heterolytically on the oxide surface. The calculations suggest that greater substituent polarization in the 1-alkyl transition structures is responsible for the lower energy barriers for primary vs. secondary C-H bond activation of alkanes on PdO(101).

Since the recent discovery of production of electronic flows in Schottky diodes with nanosized “top” metal layer due to ballystic metal-to-semiconductor transport of hot electrons formed by the surface exoergic chemical reaction, e.g. [1], this effect attracts attention of scientists owing to its fundamental and practical potential. Here we invastigate a new system of that kind, namely Pd-(n)GaP planar Schottky diod (15 nm Pd-layer) placed in the atmosphere of atomic hydrogen. We found the steady-state current flow through the system under consideration in perpendicular direction to the metal surface on which the hydrogen atoms stationary recombine into molecules.

We elaborated a new approach to detect production of the inequilibrium charge carriers via nonadiabatic channel by observing the current-voltage characteristic of the Schottky diode in the presence and absence of the atomic flux incident on the structure. The nonequilibrium nature of the additional carriers is confirmed by kinetics measurements: the current drops to its initial value in the absense of atoms practically momentarily once the atoms are “switched off” and jumps immediately to its excited value when atoms are “switched on” (at the given temperature of the structure and the fixed forward voltage bias on the structures). We were able to draw some quantitative information about the processes of generation of nonequilibrium electron-hole pairs in the reaction of recombination of hydrogen atoms on Pd-surface and their transport in the metal film. The short circuit current is expressed in terms of yield of the chemoexcited carriers and probability of their survival while traveling through the Pd-film.

For a 1V forward bias the current drastically grows from 3 nA to 950 uA; thus the bias allows gaining chemicurrent value as large as five orders of magnitude. This result can be of significant importance for the practical applications of the nonequilibrium chemiconductance and chemicurrents in Schottky nanostructures including sensing and chemical-to-electricity energy conversion.

[1] B. Georgen, H.Nienhaus, W.H. Weinberg, E. McFarland. Science, 294, 2521 (2003)