Pu-Wei Wu

Ni Inverse Opals for Water Electrolysis in an Alkaline Electrolyte

A vertical electrophoretic deposition technique was employed to prepare polystyrene (PS) colloidal crystals with negligible crystallographic defects. The colloidal crystals were plated with Ni, followed by selective removal of PS microspheres to fabricate inverse opals. With adjustments in relevant processing parameters, we were able to obtain inverse opals in multiple layers with excellent surface uniformities. The inverse opals were used as the electrode material for water electrolysis. Results from current-potential polarizations in 1 M KOH aqueous solution indicated that the catalytic abilities for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) increased with the layers of inverse opals. Estimation of the effective current densities determined that the entire area of the inverse opals was available for electrochemical reactions. In galvanostatic measurements for both OER and HER, the inverse opals demonstrated stable voltage profiles without notable degradations for 120 h. Compared with a planar Ni plate, the inverse opals exhibited a significantly reduced overpotential for the water electrolysis.

Facile Electrochemical Fabrication of Large-Area ZnO Inverse Opals with Reduced Defects

We demonstrate a facile fabrication scheme to construct large-area ZnO inverse opals on an indium-tin-oxide substrate with significantly reduced defects and improved stoichiometry. The fabrication steps involve the preparation of colloidal template via vertical electrophoresis of polystyrene (PS) microspheres in 720 nm radius, followed by galvanostatic electrodeposition of ZnO in the interstitial voids among the PS microspheres. Subsequently, the sample undergoes a heat-treatment in air at 500 C for different time to remove the colloidal template, leaving an integral ZnO skeleton with thickness adjusted by the electrodeposition time. Since the colloidal template is deliberately designed with desirable thickness and considerable uniformity, the electroplating of ZnO can be achieved in a reduced ethanol/water ratio that allows faster growth rate and improved ZnO structure. Scanning electron microscope images demonstrate negligible microstructural defects for the ZnO inverse opals. X-ray diffraction reveals a wurtzite hexagonal lattice with (002) preferred orientation. In addition, both X-ray photoelectron spectroscopy and photoluminescence spectra suggest improved crystallinity and stoichiometry with increasing heat-treatment time. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3552604] All rights reserved.

Silver-Carbon Nanocapsule Electrocatalyst for Oxygen Reduction Reaction

The types of carbonaceous material and electrocatalyst are the critical components in fabrication of gas diffusion electrodes for fuel cells and metal-air batteries. In this study, carbon nanocapsule (15-30 nm) and Ag-carbon nanocapsule powders were synthesized and characterized for their electrochemical performances as oxygen reduction electrocatalyst in alkaline electrolyte using commercial noncatalyzed gas diffusion electrode as substrate. The i-V polarization response of the carbon nanocapsule demonstrated enhanced electrocatalytic capability over those of XC-72R and vapor growth carbon fiber, delivering 0.80 V at 200 mA/cm 2 . In addition, Ag-carbon nanocapsule powders exhibited a value of 0.99 V at 200 mA/cm 2 , surpassing commercially available Mn-catalyzed, and MnCo-catalyzed gas diffusion electrodes. Galvanostatic discharge of these Ag-carbon nanocapsule powders from 10 to 200 mA/cm 2 confirmed their stability and sustainability. This report identified a class of carbon material that not only exhibits electrocatalytic capability itself but also provides opportunity as substrate for known electrocatalysts.

Effect of Crystallinity on the Optical Reflectance of Cylindrical Colloidal Crystals

We fabricate cylindrical colloidal crystals (CCCs) via electrophoretic depositions of polystyrene microspheres on a carbon fiber in a concentric arrangement. Images from optical and scanning electron microscope indicate that the CCCs of 460 nm microspheres reveal a semicrystalline structure in close-packed lattice, while the CCCs of 660 exhibit an amorphous arrangement. In optical reflectance spectra, both CCCs demonstrate a dispersive photonic bandgap as compared to that from planar counterparts. The curved contour for the CCCs renders incident light interacting with reflecting planes at slightly different angles, leading to a notable reduction in the reflectance intensity and minor relocation of photonic bandgap contingent on their crystallinity.

Rapid Galvanostatic Determination on Levelers for Superfilling in Cu Electroplating

4-Amino-2,1,3-benzothiadiazole and 6-aminobenzo-thiazole were studied as potential levelers for Cu plating in submicrometer trenches. We conducted galvanostatic measurements on rotating disk electrodes at various concentrations and recorded their respective voltages at 200 and 430 rpm. A profile of voltage difference between them at various concentrations revealed a volcano curve in which superfilling was expected to occur for 4-amino-2,1,3-benzothiadiazole but not for 6-aminobenzo-thiazole. Direct scanning electron microscopy observations on the plated Cu confirmed predictions from the volcano curves. A diffusion- adsorption mechanism was proposed for these two compounds to explain their leveling abilities on Cu superfilling.

Controlling the Spontaneous Precipitation of Silver Nanoparticles in Sol-Gel Materials ⁄

The mechanisms responsible for spontaneous silver precipitation in silver-doped sol-gel materials are identified. The chemistry of the solvent phase is found to be the critical factor in controlling this phenomenon. The addition of HCl as catalyst leads to the formation of AgCl and subsequent formation of silver upon light exposure. Another factor leading to silver precipitation is the reducing capability of methanol radicals. Silver precipitation is inhibited by simply washing out the pore solvents by a solvent exchange method.

Electrowetting of Superhydrophobic ZnO Inverse Opals

ZnO inverse opals are fabricated by electrophoresis of polystyrene (PS) microspheres (720 nm in diameter) on a ITO glass to form a close-packed colloidal crystal, followed by potentiostatic deposition of ZnO in the interstitial voids among the PS microspheres and chemical removal of the PS colloidal template. By adjusting the electrodeposition time, we obtain semi-layered and multilayered ZnO inverse structures with significantly reduced defects and considerable surface uniformity. The semi-layered ZnO inverse opals display a bowl-like morphology with individual cavities isolated from each other. In contrast, the multi-layered ZnO inverse opals exhibit a three-dimensional skeleton with hexagonally-arranged cavities interconnected to each other. After surface coating of perfluorodecyltriethoxysilane, both samples reveal a superhydrophobic nature with contact angle larger than 150 . In electrowetting measurements, the contact angles are decreasing with increasing applied voltages. The droplet on the semi-layered ZnO inverse opals demonstrates a notable transition from the Cassie-Baxter state to the Wenzel one. However, the droplet on the multi-layered ZnO inverse opals indicates three distinct regimes; Cassie-Baxter state, mixed Cassie-Baxter/Wenzel state, and Wenzel state. Repelling pressure of the entrapped air in the cavities is estimated to explain the observed contact angle variation upon the applied voltage for both samples. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3594723] All rights reserved.

Fabrication of Large-Area Colloidal Crystals by Electrophoretic Deposition in Vertical Arrangement

Fabrication of large-area colloidal crystals (400 μm 2 ) with superb uniformity in multiple layers (∼ 128 layers) from SiO 2 microspheres (∼400nm) was achieved by electrophoresis in vertical arrangement. With a large difference in electrode areas and selection of ethanol as the solvent, we investigated self-assembly with SiO 2 suspensions at various pH and electric fields of 20-100 V cm -1 . Colloid formations with negligible vacancies and small-angle grain boundaries were obtained at a suspension of pH ≈ 10 and electric field of 20 V cm -1 . Our fabrication scheme enables construction of colloidal crystals with significantly reduced structural disorders as compared to conventional routes.

Rapid Fabrication of Cylindrical Colloidal Crystals and Their Inverse Opals

We employed an electrophoretic deposition technique to prepare cylindrical colloidal crystals (CCCs) from polystyrene (PS) microspheres of 460 and 660 nm diameters using a carbon fiber (CF) with 7 μm diameter as the substrate. Measurements on the CCC diameter demonstrated growth rates that slowed down as time progressed. Scanning electron microscope images confirmed that the CCCs of 460 nm microspheres formed a face-centered cubic close-packed lattice, whereas an amorphous structure appeared for the CCCs of 660 nm. Subsequently, both CCCs underwent a potentiostatic electroplating to deposit Ni into the interstitial voids among the PS microspheres. After chemical removal of the PS microspheres, we fabricated cylindrical inverse opals (CIOs) in various diameters. As expected, better crystallinity was found on the CIOs of 460 nm microspheres as opposed to the 660 nm ones. The electrical resistivity for both CIOs exhibited a substantial reduction over that of the CF. Our CCCs and CIOs revealed considerable structure stability with excellent surface uniformity. The fabrication scheme enables rapid preparations of CCCs and CIOs in desirable lengths and diameters.

Pulse Electrodepositions of PtRu on Large-Area Carbon Nanotubes for Enhancement of Methanol Electro-Oxidation

We employed an electroless deposition technique to prepare Ni seeds uniformly on a carbon cloth, followed by carbon nanotube CNT formation at an elevated temperature. Subsequently, a pulse electrodeposition was used to impregnate PtRu nanoparticles on the CNT structure. Similar procedures were performed on carbon supports including Vulcan XC72R, BP2000, and carbon nanocapsules CNCs for comparison purposes. Diffraction patterns from an X-ray confirmed a PtRu alloyed phase. An analysis from inductively coupled plasma-mass spectrometry indicated that the PtRu composition was relatively unchanged. Images from scanning and transmission electron microscopes revealed dense CNTs throughout the carbon cloth with nanoparticulate PtRu evenly impregnated. Considerable PtRu aggregations were found on the CNCs and BP2000. We observed a significantly improved coulomb efficiency and an electrochemically active surface area for the CNT-grown carbon cloth. In both apparent current density and mass activity for methanol electro-oxidation, the CNT-grown carbon cloth revealed the largest values. We attribute the performance enhancement to the large-area CNT structure that allowed facile access of electrolytes.