Photochemical deposition of cobalt-based oxygen evolving catalyst on a semiconductor photoanode for solar oxygen production.

This study describes the photochemical deposition of Co-based oxygen evolution catalysts on a semiconductor photoanode for use in solar oxygen evolution. In the photodeposition process, electron-hole pairs are generated in a semiconductor upon illumination and the photogenerated holes are used to oxidize Co(2+) ions to Co(3+) ions, resulting in the precipitation of Co(3+)-based catalysts on the semiconductor surface. Both photodeposition of the catalyst and solar O(2) evolution are photo-oxidation reactions using the photogenerated holes. Therefore, photodeposition provides an efficient way to couple oxygen evolution catalysts with photoanodes by naturally placing catalysts at the locations where the holes are most readily available for solar O(2) evolution. In this study Co-based catalysts were photochemically deposited as 10-30 nm nanoparticles on the ZnO surface. The comparison of the photocurrent-voltage characteristics of the ZnO electrodes with and without the presence of the Co-based catalyst demonstrated that the catalyst generally enhanced the anodic photocurrent of the ZnO electrode with its effect more pronounced when the band bending is less significant. The presence of Co-based catalyst on the ZnO photoanode also shifted the onset potential of the photocurrent by 0.23 V to the negative direction, closer to the flat band potential. These results demonstrated that the cobalt-based catalyst can efficiently use the photogenerated holes in ZnO to enhance solar O(2) evolution. The photodeposition method described in this study can be used as a general route to deposit the Co-based catalysts on any semiconductor electrode with a valence band edge located at a more positive potential than the oxidation potential of Co(2+) ions.

Atomic plane-selective deposition of gold nanoparticles on metal oxide crystals exploiting preferential adsorption of additives.

A strategy of using preferential adsorption of additives for selectively depositing gold metal particles on specific crystallographic planes of Cu(2)O and ZnO crystals was demonstrated. Cu(2)O crystals and ZnO crystals were first electrochemically deposited on a conducting substrate, and gold particles were electrochemically or photochemically deposited on these crystals with the presence of appropriate additives. Preferential adsorption of sodium dodecyl sulfate on {111} planes of Cu(2)O crystals, which was previously used to form octahedral shapes of Cu(2)O crystals, effectively inhibited gold deposition on {111} planes of Cu(2)O, resulting in selective gold deposition on {100} planes. In a similar manner, the use of Cl(-) ions that adsorb selectively on polar {0001} planes of ZnO resulted in selective deposition of gold particles on the {01-10} planes of ZnO rods. The simple chemical method for selective metal deposition described in this study is based on utilizing preferential adsorption of additives as an in situ generated mask to protect certain atomic planes from metal deposition, which significantly simplifies the procedure to assemble precisely controlled composite structures.

Electrochemical synthesis of Zn-Al layered double hydroxide (LDH) films.

A new electrodeposition condition to produce Zn-Al LDH films was developed using nitrate solutions containing Zn (2+) and Al (3+) ions. Deposition was achieved by reducing nitrate ions to generate hydroxide ions on the working electrode. This elevates the local pH on the working electrode, resulting in precipitation of Zn-Al LDH films. The effect of deposition potential, pH of the plating solution, and the Zn (2+) to Al (3+) ratio in the plating solution on the purity and crystallinity of the LDH films deposited was systematically studied using X-ray diffraction and energy dispersive spectroscopy (EDS). The optimum deposition potential to deposit pure and well-ordered Zn-Al LDH films was E = -1.65V versus a Ag/AgCl in 4 M KCl reference electrode at room temperature using a solution containing 12.5 mM Zn(NO 3) 2.6H 2O and 7.5 mM Al(NO 3) 3.9H 2O with pH adjusted to 3.8. The resulting film contained 39 atomic %Al (3+) ions replacing Zn (2+) ions, leading to a composition of Zn 0.61Al 0.39(OH) 2(NO 3) 0.39. xH 2O. Increasing or decreasing the aluminum concentration in the plating solution resulted in the formation of aluminum- or zinc-containing impurities, respectively, instead of varying aluminum content incorporated into the LDH phase. Choosing an optimum deposition potential was important to obtain LDH as a pure phase in the film. When the potential more negative than the optimum potential is used, zinc metal or zinc hydroxide was deposited as a side product, whereas making the potential less negative than the optimum potential resulted in the formation of zinc oxide as the major phase. The pH condition of the plating solution was also critical, as increasing pH destabilizes the formation of the LDH phase while decreasing pH promoted deposition of other impurities.

Electrochemical synthesis of cobalt hydroxide films with tunable interlayer spacings.

An electrochemical synthetic condition is developed to produce cobalt hydroxide films with significantly increased basal spacings (d(001)>or= 25.0 A) by incorporating anionic surfactants (i.e. sodium dodecyl sulfate and 1-hexadecanesulfonate) into the interlayer regions via electrodeposition.

Elucidating the structure-dependent photocatalytic properties of Bi2WO6: a synthesis guided investigation.

Polycrystalline microspheres and single-crystalline microplates of Bi(2)WO(6) have been synthesized by ultrasonic spray pyrolysis. Herein, these materials are evaluated as photocatalysts for the visible light mediated degradation of rhodamine B, a model pollutant, and the results compared to those obtained with Bi(2)WO(6) prepared by traditional methods. The microplates, which displayed the best crystallinity and highest surface area, were anticipated to facilitate the greatest rate of dye photodegradation. However, the polycrystalline microspheres outperformed both the Bi(2)WO(6) microplates and traditional samples. To understand the origin of this result, the local and macroscale structures of the Bi(2)WO(6) samples were comprehensively characterized by spectroscopy techniques (diffuse reflectance, fluorescence, Raman, and X-ray photoelectron spectroscopy) as well as electron microscopy and diffraction. This analysis found that the enhanced performance of the Bi(2)WO(6) microspheres results from the expression of a hydrophilic surface, a low concentration of point defects, and a moderate surface area. This finding highlights the significant role synthesis plays in imparting structure and functionality to solid materials.

Anodic construction of lamellar structured ZnO films using basic media via interfacial surfactant templating.

Zinc oxide films with ordered lamellar structures were anodically deposited in basic media using an interfacial surfactant templating method. Sodium dodecyl sulfate (SDS) and lauric acid served as efficient structure-directing agents and incorporated a lamellar structure with d(001) = 3.5 and 2.8 nm, respectively, into ZnO films. When 0.03 M cetyltrimethylammonium bromide (CTAB) or tetramethylammonium bromide (TAB) was added as supporting electrolytes, the amount of SDS required to template the lamellar structures was decreased from 10 to 5 wt % and the basal spacing of the resulting lamellar structure was decreased to 3.1 nm. The effects created by the addition of CTAB and TAB are identical, indicating that the amphiphilic nature of CTAB does not play a major role in altering SDS assemblies. Investigation of the effect of various supporting cations and anions (e.g., NaCl, NaBr, NaI, Na2SO4) demonstrated that the effect seen with the addition CTAB and TAB is primarily due to the cationic groups reducing the repulsion of SDS head groups and enhancing interactions between anionic inorganic species (i.e., [Zn(OH)4]2-) and anionic SDS. Br- and I- ions also appear to have a slight effect on improving the ordering of interfacial SDS assemblies, while no apparent changes were observed when NaCl and Na2SO4 were added. These results indicate that it is not the increase in concentration of any salts but the specific type of cations and anions that can alter the interfacial SDS assemblies.

Electrochemical tailoring of lamellar-structured ZnO films by interfacial surfactant templating.

Zinc oxide films with ordered lamellar structures can be electrochemically produced by interfacial surfactant templating. This method utilizes amphiphile assemblies at the solid-liquid interface (i.e., the surface of a working electrode) as a template to electrodeposit inorganic nanostructures. To gain the ability to precisely tailor inorganic lamellar structures, the effect of various chemical and electrochemical parameters on the repeat distances, homogeneity, orientation, and quality of the interfacial amphiphilic bilayers were investigated. Surfactants with anionic headgroups (e.g., 1-hexadecanesulfonate sodium salt, dodecylbenzenesulfonate sodium salt, dioctyl sulfosuccinate sodium salt, mono-dodecyl phosphate, and sodium dodecyl sulfate) are critical because they incorporate Zn(2+) ions into their bilayer assemblies as counterions and guide the lamellar growth of ZnO films. Unlike surfactant structures in solution, the interfacial surfactant assemblies are insensitive to the surfactant concentration in solution. The use of organic cosolvents (e.g., ethylene glycol, dimethyl sulfoxide) can increase the homogeneity of bilayer assemblies when multiple repeat distances are possible in a pure aqueous medium. In addition, organic cosolvents can make the interfacial structure responsive to the change in bulk surfactant concentrations. The presence of quaternary alkylammonium salts (e.g., cetyltrimethylammonium bromide) as cationic cosurfactants improves the ordering of anionic bilayers significantly. Consequently, it also affects the orientation of lamellar structures relative to the substrate as well as the surface texture of the films. The quality of lamellar structures incorporated in ZnO films is also dependent on the deposition potentials that determine deposition rates. A higher degree of ordering is achieved when a slower deposition rate (I < 0.15 mA/cm(2)) is used. The results described here will provide a useful foundation to design and optimize synthetic conditions for the electrochemical construction of broader types of inorganic nanostructures.