Wen-eh Yu

Transparent electrodes of ordered opened-end TiO2-nanotube arrays for highly efficient dye-sensitized solar cells

We introduce a version of a dye-sensitized solar cell (DSSC), in which the transparent electrode comprises an opened-end TiO2-nanotube (TiNT) film oriented perpendicular to the fluorine-doped tin oxide (FTO) conductive glass. This electrode is fabricated using a facile process involving flaking the anodic TiNT film off the Ti-metal substrate to open its closed bottom and strongly adhering it onto FTO glass by a nanocrystalline TiO2 (NP-TiO2) thin underlayer. Apart from the enhanced light-harvesting and electron-collecting efficiencies, the use of opened-end TiNT film allows the redox electrolyte (iodide, I− and triiodide, I3−) easy access to the NP-TiO2 underlayer. As compared to the closed-end TiNT-based DSSC, the opened-end TiNT-based device exhibited an increase in one-sun efficiency from 5.3% to 9.1%, corresponding to 70% enhancement.

Selective hydrogen production from formic acid decomposition on Pd-Au bimetallic surfaces.

Pd-Au catalysts have shown exceptional performance for selective hydrogen production via HCOOH decomposition, a promising alternative to solve issues associated with hydrogen storage and distribution. In this study, we utilized temperature-programmed desorption (TPD) and reactive molecular beam scattering (RMBS) in an attempt to unravel the factors governing the catalytic properties of Pd-Au bimetallic surfaces for HCOOH decomposition. Our results show that Pd atoms at the Pd-Au surface are responsible for activating HCOOH molecules; however, the selectivity of the reaction is dictated by the identity of the surface metal atoms adjacent to the Pd atoms. Pd atoms that reside at Pd-Au interface sites tend to favor dehydrogenation of HCOOH, whereas Pd atoms in Pd(111)-like sites, which lack neighboring Au atoms, favor dehydration of HCOOH. These observations suggest that the reactivity and selectivity of HCOOH decomposition on Pd-Au catalysts can be tailored by controlling the arrangement of surface Pd and Au atoms. The findings in this study may prove informative for rational design of Pd-Au catalysts for associated reactions including selective HCOOH decomposition for hydrogen production and electro-oxidation of HCOOH in the direct formic acid fuel cell.

Fabrication of open-ended high aspect-ratio anodic TiO2 nanotube films for photocatalytic and photoelectrocatalytic applications.

A facile process is introduced to flake high aspect-ratio anodic TiO(2) nanotube (TiNT) arrays off Ti substrates and then chemically remove the bottom caps to obtain open-ended TiNT films that exhibit high activity to photocatalytic degradation of methylene blue and efficient hydrogen production from photoelectrocatalytic water splitting.

Rough conical-shaped TiO2-nanotube arrays for flexible backilluminated dye-sensitized solar cells

Conical-shaped anodic TiO2-nanotube (TiNT) arrays with length of a few tens of micrometer and rough tube walls were fabricated for use in flexible backilluminated dye-sensitized solar cells (DSSCs) which exhibited conversion efficiency of 4.3% under AM1.5 back side illumination. As compared to TiO2 nanoparticles/Ti-based DSSC, TiNT/Ti-based DSSC showed enhanced light-harvesting efficiency, rapid electron-transport rate, prolonged electron lifetime, and reduced dark current, leading to an increase in efficiency by ∼30%.

Effect of anodic TiO2 powder as additive on electron transport properties in nanocrystalline TiO2 dye-sensitized solar cells

Highly crystalline anodic TiO2 (AO-TiO2) powder was obtained by grinding of annealed TiO2 nanotube arrays grown using anodization of Ti foil. The influence of AO-TiO2 as additive in nanocrystalline TiO2 film on electron transport properties affecting the performance of these dye-sensitized solar cell was investigated using electrochemical impedance spectroscopy and the open-circuit voltage decay technique under AM 1.5 illumination. As a result of the enhanced charge-collection and light harvesting efficiencies, an increase of above 20% in photocurrent density was observed after the addition of AO-TiO2.

Structure Revealing H/D Exchange with Co-Adsorbed Hydrogen and Water on Gold.

A fundamental understanding of the interactions between coadsorbed water and hydrogen on metallic surfaces is critical to many chemical processes including catalysis and electrochemistry. Here, we report on the strong and intricate interactions between coadsorbed H/D and water on the close-packed (111) surface of gold. Deuterium isotopic labeling shows H/D exchange in H-D2O and D-H2O systems, indicating water dissociation and suggesting a nonrandom scrambling process by revealing the origin of hydrogen evolution (from surface H atoms or from water molecules) during annealing. In this reaction, the protonation of the H-bonding ice network (i.e., the formation of (H2O)nH(+)) is energetically favorable and is responsible for water dissociation. Density functional theory (DFT) modeling suggests that the thermodynamics and structure of the protonated clusters are predominant factors for yielding the traceable H2 desorption features from the surface interaction with H atoms, providing insights into reaction mechanisms.

Glycan arrays on aluminum-coated glass slides.

We have developed a novel method of immobilizing glycans onto aluminum-coated glass (ACG) slides for potential use in disease diagnosis and drug discovery. The quality of these sugar chips can be assessed by mass spectrometry and fluorescence measurements with high sensitivity. The unique properties of ACG slides include: 1) the metal oxide layer on the surface can be activated for grafting organic compounds such as modified oligosaccharides; 2) the surface remains electrically conductive, and the grafted oligosaccharides can be simultaneously characterized by mass spectrometry and carbohydrate-binding assay; and 3) the slides are more sensitive than transparent glass slides in binding analysis. To demonstrate this, we synthesized a model compound of mannose with a built-in photocleavable linker bound to the ACG slide surface. The molecular weight of the grafted mannose was identified by mass spectrometry, and the slide was subjected to biotinylated ConA binding followed by Cy3-tagged streptavidin detection. This method was further extended to the preparation of glycan arrays containing lactose and the cancer antigen Globo H.

Layer-controlled two-dimensional perovskites: synthesis and optoelectronics

Solution-processed hybrid organic–inorganic metal halide perovskites are emerging as one of the most promising candidates for low-cost photovoltaics and optoelectronics. Moreover, two-dimensional (2D) forms of these materials induce a dielectric quantum confinement effect that drastically increases the exciton binding energy. Previous studies on two-dimensional (2D) hybrid perovskites have been focused on the thinnest counterparts, namely, the zero-layer and monolayer species, in which the photoluminescence quantum yield is typically low (<10%), thereby limiting their applications in optoelectronics. Recent advances in colloidal synthesis have suggested that precise control over the layer numbers can be realized on a large scale, offering another degree of freedom in tailoring the optoelectronic properties. Herein, we reviewed the photophysical properties, synthetic routes, and potential technology opportunities of layer-controlled 2D hybrid perovskites.

Highly selective, facile NO2 reduction to NO at cryogenic temperatures on hydrogen precovered gold.

We have discovered that NO(2) is reduced to NO at 77 K by hydrogen precovered gold in vacuum. Here, we investigate the partial reduction of NO(2) to NO on an atomic-hydrogen populated model gold catalyst for a more fundamental understanding of the surface chemistry of hydrogenation. Gold-based catalysts have been found to be active for many hydrogenation reactions, but few related fundamental studies have been conducted. Our experimental results reveal a high catalytic activity for gold: indeed, NO(2) is reduced to NO with 100% conversion and 100% selectivity at temperatures lower than 120 K. Density functional theory calculations and reflection-absorption infrared spectroscopy measurements indicate that HNO(2) and N(2)O(3) are intermediates which are highly dependent on surface hydrogen concentrations; subsequent hydrogenation of HNO(2) and dissociation of N(2)O(3) upon annealing induces the production of NO and H(2)O.

Preparation of Au/TiO2 for catalytic preferential oxidation of CO under a hydrogen rich atmosphere at around room temperature

Au/TiO2 prepared in a pH adjusted gold solution by a deposition-precipitation method can possess high activity and stability for selective CO oxidation in a H2 rich stream at ambient temperature.