Coryl Jing Jun Lee

Effect of oxygen evolution catalysts on hematite nanorods for solar water oxidation

Photochemical deposition of Co and Ni based oxygen evolution catalysts on hematite nanorods cathodically shifted the onset potential of photocurrent near to the flat band potential of hematite. A 9.5 fold enhancement in the photocurrent density at 0.86 V vs. RHE compared to the parent hematite photoanode was observed with the Ni-Bi/Fe(2)O(3) photoanode.

A highly active hydrogen evolution electrocatalyst based on a cobalt–nickel sulfide composite electrode

A novel Co9S8–NixSy/Ni foam composite material was synthesized through the thermal decomposition of a cobalt–thiourea molecular precursor onto a 3D metallic support. The obtained electrode exhibited good activity toward the hydrogen evolution reaction in an alkaline medium, requiring a small overpotential of 163 mV at a current density of 10 mA cm−2, which is one of the lowest ever reported among transition metal sulfide materials.

The effect of crystallinity on photocatalytic performance of Co3O4 water-splitting cocatalysts

Cocatalysts, when loaded onto a water splitting photocatalyst, accelerate the gas evolution reaction and improve the efficiency of the photocatalyst. In this paper, we report that the efficiency of the photocatalyst is enhanced using an amorphous cobalt oxide cocatalyst. The WO3 film, when loaded with amorphous or nanocrystalline Co3O4, shows an improvement of up to 40% in photocurrent generation and 34% in hydrogen gas evolution. The effect of cocatalyst crystallinity on performance was systematically studied, and we found that the photocurrent deteriorates with the conversion of the cocatalyst to a highly crystalline phase at an annealing temperature of 500 °C. The mechanism of this effect was studied in detail using electrochemical impedance spectroscopy, and the enhancement effect produced by the amorphous cocatalyst is attributed to the large density of unsaturated catalytically active sites in the amorphous material.

Amorphous ruthenium nanoparticles for enhanced electrochemical water splitting

This paper demonstrates an optimized fabrication of amorphous Ru nanoparticles through annealing at various temperatures ranging from 150 to 700 °C, which are used as water oxidation catalyst for effective electrochemical water splitting under a low overpotential of less than 300 mV. The amorphous Ru nanoparticles with short-range ordered structure exhibit an optimal and stable electrocatalytic activity after annealing at 250 °C. Interestingly, a small quantity of such Ru nanoparticles in a thin film on fluorine-doped tin oxide glass is also effectively driven by a conventional crystalline silicon solar cell that has excellent capability for harvesting visible light. Remarkably, it achieves an overall solar-to-hydrogen efficiency of 11.3% in acidic electrolyte.

Effect of La-Doping on optical bandgap and photoelectrochemical performance of hematite nanostructures

La-doped hematite nanotubes are fabricated by electrospinning of a sol–gel solution consisting of La(III) acetylacetonate hydrate/polyvinylpyrrolidone(PVP)/ferric acetylacetonate, and subsequent sintering at 500 °C for 5 h in air. Further grinding of these nanotubes affords La-doped hematite nanoparticles. FESEM EDX indicates that the La content is 3.66 mol% in La-doped hematite. HRTEM and XRD reveal that La3+ cations are doped into the hematite crystal lattice. UV-Vis diffuse reflectance shows increased light absorption for La-doped hematite, with the bandgap reduced from 2.58 eV to 2.46 eV. EIS and four-probe characterization demonstrate that La-doping reduces charge transfer resistance and increases the electrical conductivity, thus leading to improved charge transportation. Photoelectrochemical (PEC) water splitting studies show that under 100 mW cm−2 simulated solar irradiation, La-doped hematite nanoparticles demonstrate a net photocurrent density up to 0.112 and 0.270 mA cm−2 at 1.23 and 1.60 V vs. RHE, which are 187% and 63% higher than pristine hematite nanoparticles, respectively. The effect of La-doping on improving electrical conductivity, light absorption, and PEC performance is mainly attributed to the intensification of crystal orientation along the (110) plane and the lattice expansion caused by the La3+ cations, which have much larger radii and are more electron-rich than Fe3+.

Giant Blistering of Nanometer-Thick Al2O3/ZnO Films Grown by Atomic Layer Deposition: Mechanism and Potential Applications

Giant circular blisters of up to 300 μm diameter and 10 μm deflection have been produced on nanometer-thick Al2O3-on-ZnO stacks grown by atomic layer deposition at 150 °C followed by annealing at elevated temperatures. Their shape changes upon varied ambient pressures provide evidence that their formation is related to an anneal-induced outgassing combined with their impermeability. The former mainly occurs in the bottom ZnO layer that recrystallizes and releases residual hydroxide ions at elevated temperatures while the latter is dominantly contributed by the pinhole-free Al2O3 layer on top. Vibrations at a resonant frequency of ∼740 kHz are mechanically actuated and optically probed from an individual blister. By modulating the thickness and stacking sequence of Al2O3 and ZnO, we further demonstrate a localized circular film swelling upon electron-beam irradiation and its recovery after reducing the irradiation flux. The elastic blistering and the recoverable swelling of the nanometer-thick films represent a miniaturized event-driven mechanical system for potential functioning applications.

New Insights into Planar Defects in Layered α-MoO3 Crystals

The observation of regular ( h0 l) planar defects in α-MoO3 crystals can be traced back to over 60 years ago. Two mechanisms have been proposed to interpret the formation of the planar defects. One is related to the diffusion of oxygen vacancies because of thermal-driven release of oxygen atoms in vacuum and the consequent crystallographic shear of α-MoO3. The other is associated with redox reactions of moisture and/or hydrocarbons that give rise to H xMoO3 precipitates. Here, we report that regularly spaced (302) planar defects can be introduced into α-MoO3 belt crystals by heating in liquid sulfur at 300 °C. These defects are undetectable by both atomic force microscopy and scanning electron microscopy at the crystal surface. Raman scattering enhancement and weakening have been observed for different phonon modes of α-MoO3 at the (302) planar defects as probed from the (010) surface. Their comparisons with the Raman scattering enhancements at the edges and the argon-plasma-induced Raman spectral evolutions of the as-grown α-MoO3 belt crystals provide new insights into the planar defects with regard to their formation and characteristics.