Jina Choi

Photocatalytic production of hydrogen on Ni/NiO/KNbO3/CdS nanocomposites using visible light

The photocatalytic production of H2 from water splitting was demonstrated on Ni/NiO/KNbO3/CdS nanocomposites using visible light irradiation at wavelengths >400 nm in the presence of isopropanol. The inherent photocatalytic activity of bulk-phase CdS was enhanced by combining Q-sized CdS with KNbO3 and Ni deposited on KNbO3. Enhanced activity is most likely due to effective charge separation of photogenerated electrons and holes in CdS that is achieved by electron injection into the conduction band of KNbO3 and the reduced states of niobium (e.g., Nb(IV) and Nb(III)) are shown to contribute to enhanced reactivity in the KNbO3 composites by mediating effective electron transfer to bound protons. We also observed that the efficient attachment of Q-size CdS and the deposition of nickel on the KNbO3 surface increases H2 production rates. Other factors that influence the rate of H2 production including the nature of the electron donors and the solution pH were also determined. The Ni/NiO/KNbO3/CdS nanocomposite system appears to be a promising candidate for possible practical applications including the production of H2 under visible light.

Combinatorial doping of TiO 2 with platinum (Pt), chromium (Cr), vanadium (V), and nickel (Ni) to achieve enhanced photocatalytic activity with visible light irradiation

Titanium dioxide (TiO_2) was doped with the combination of several metal ions including platinum (Pt), chromium (Cr), vanadium (V), and nickel (Ni). The doped TiO_2 materials were synthesized by standard sol-gel methods with doping levels of 0.1 to 0.5 at.%. The resulting materials were characterized by x-ray diffraction (XRD), BET surface-area measurement, scanning electron microscopy (SEM), and UV-vis diffuse reflectance spectroscopy (DRS). The visible light photocatalytic activity of the codoped samples was quantified by measuring the rate of the oxidation of iodide, the rate of degradation of methylene blue (MB), and the rate of oxidation of phenol in aqueous solutions at λ > 400 nm. 0.3 at.% Pt-Cr-TiO_2 and 0.3 at.% Cr-V-TiO_2 showed the highest visible light photocatalytic activity with respect to MB degradation and iodide oxidation, respectively. However, none of the codoped TiO_2 samples were found to have enhanced photocatalytic activity for phenol degradation when compared to their single-doped TiO_2 counterparts.

Photoelectrochemical performance of multi-layered BiOx–TiO2/Ti electrodes for degradation of phenol and production of molecular hydrogen in water

Multi-layered BiO(x)-TiO(2) electrodes were used for the oxidation of chemical contaminants coupled with the production of H(2) characterized by a synergistic enhancement. The BiO(x)-TiO(2) electrodes were composed of a mixed-metal oxide array involving an under layer of TaO(x)-IrO(x), a middle layer of BiO(x)-SnO(2), and a top layer of BiO(x)-TiO(2) deposited in a series on both sides of Ti foil. Cyclic voltammograms showed that the BiO(x)-TiO(2) electrodes had an electrocatalytic activity for oxidation of phenol that was enhanced by 70% under illumination with AM 1.5 light. When the BiO(x)-TiO(2) anode was coupled with a stainless steel cathode in a Na(2)SO(4) electrolyte with phenol and irradiated with UV light at an applied DC voltage, the anodic phenol oxidation rate and the cathodic H(2) production rates were enhanced by factors of four and three, respectively, as compared to the sum of each light irradiation and direct DC electrolysis. These synergistic effects depend on the specific electrode composition and decrease on TaO(x)-IrO(x) and BiO(x)-SnO(2) anodes in the absence of a top layer of BiO(x)-TiO(2). These results indicate that the BiO(x)-TiO(2) layer functions as the key photo-electrocatalyst. The heavy doping level of Bi (25 mol%) in TiO(2) increases the electric conductivity of the parent TiO(2).

Photoelectrochemical hydrogen production on silicon microwire arrays overlaid with ultrathin titanium nitride

p-Si wire arrays overlaid with an ultrathin titanium nitride (TiN) film are developed and demonstrated to be an efficient and robust photocathode for hydrogen production. Arrays of vertically aligned 20 μm long p-Si microwires of varying diameters (1.6–14.6 μm) are fabricated via a photolithographic technique, and then the wires are coated with a TiN nanolayer 2–20 nm thick by low-temperature plasma-enhanced atomic layer deposition. The optimized heterojunction consisting of 1.6 μm-thick wires covered by 10 nm thick TiN exhibits significantly improved performance for hydrogen evolution reaction under simulated sunlight (AM 1.5G, 100 mW cm−2). It displays a photocurrent onset potential of ∼+0.4 V vs. reversible hydrogen electrode (RHE), and a faradaic efficiency of nearly 100% at 0 V vs. RHE over 20 h of reaction. Time-resolved photoluminescence decay reveals that the lifetime (τ) of the photogenerated charge carriers in the optimized wire/TiN heterojunction is ∼60% shorter than those using thicker wires, suggesting significantly faster charge transfer. Such remarkable performance is attributed to enhanced transfer of the minority carriers in the radial direction of the wires. TiN performs the triple roles of antireflection, protection of the Si surface, and electrocatalysis of hydrogen production. Finite-difference time-domain simulation reveals a significant increase in the absorptance of wire arrays with TiN film, and that long wavelength photons are more effectively absorbed by the wire/TiN arrays.