Hyun Joon Kang

Phosphate doping into monoclinic BiVO4 for enhanced photoelectrochemical water oxidation activity.

Phos-phorus is a typical dopant for silicon or germanium to make itan n-type semiconductor. However, it has been rarely used asdopant for semiconductor photocatalysts. This is rathersurprising because other non-metallic elements, such as N,C, and S, have been widely used as anionic dopants forphotocatalysts to reduce their band-gap energies.

Photocatalytic and Photoelectrochemical Water Oxidation over Metal‐Doped Monoclinic BiVO4 Photoanodes

The visible-light-induced water oxidation ability of metal-ion-doped BiVO(4) was investigated and of 12 metal ion dopants tested, only W and Mo dramatically enhanced the water photo-oxidation activity of bare BiVO(4); Mo had the highest improvement by a factor of about six. Thus, BiVO(4) and W- or Mo-doped (2 atom %) BiVO(4) photoanodes about 1 μm thick were fabricated onto transparent conducting substrate by a metal-organic decomposition/spin-coating method. Under simulated one sun (air mass 1.5G, 100 mW cm(-2)) and at 1.23 V versus a reversible hydrogen electrode, the highest photocurrent density (J(PH)) of about 2.38 mA cm(-2) was achieved for Mo doping followed by W doping (J(PH) ≈ 1.98 mA cm(-2)), whereas undoped BiVO(4) gave a J(PH) value of about 0.42 mA cm(-2). The photoelectrochemical water oxidation activity of W- and Mo-doped BiVO(4) photoanodes corresponded to the incident photon to current conversion efficiency of about 35 and 40 % respectively. Electrochemical impedance spectroscopy and Mott-Schottky analysis indicated a positive flat band shift of about 30 mV, a carrier concentration 1.6-2 times higher, and a charge-transfer resistance reduced by 3-4-fold for W- or Mo-doped BiVO(4) relative to undoped BiVO(4). Electronic structure calculations revealed that both W and Mo were shallow donors and Mo doping generated superior conductivity to W doping. The photo-oxidation activity of water on BiVO(4) photoanodes (undoped<W doped<Mo doped) was in accordance with the results from electrochemical impedance spectroscopy, Mott-Schottky analysis, and theoretical electronic structural calculations. Thus, Mo or W doping enhanced the photocatalytic and photoelectrochemical water oxidation activity of monoclinic BiVO(4) by drastically reducing its charge-transfer resistance and thereby minimizing photoexcited electron-hole pair recombination.

Improved photoelectrochemical activity of CaFe2O4/BiVO4 heterojunction photoanode by reduced surface recombination in solar water oxidation.

A bismuth vanadate photoanode was first fabricated by the metal-organic decomposition method and particles of calcium ferrite were electrophoretically deposited to construct a heterojunction photoanode. The characteristics of the photoanodes were investigated in photoelectrochemical water oxidation under simulated 1 sun (100 mW cm(-2)) irradiation. Relative to the pristine BiVO4 anode, the formation of the heterojunction structure of CaFe2O4/BiVO4 increased the photocurrent density by about 60%. The effect of heterojunction formation on the transfer of charge carriers was investigated using hydrogen peroxide as a hole scavenger. It was demonstrated that the heterojunction formation reduced the charge recombination on the electrode surface with little effect on bulk recombination. The modification with an oxygen evolving catalyst, cobalt phosphate (Co-Pi), was also beneficial for improving the efficiency of CaFe2O4/BiVO4 heterojunction photoanode mainly by increasing the stability.

A versatile photoanode-driven photoelectrochemical system for conversion of CO2 to fuels with high faradaic efficiencies at low bias potentials

A photoanode-driven photoelectrochemical system consisting of a WO3 photoanode under bias potential and Cu or Sn/SnOx as the cathode for the reduction of CO2 has been studied under visible light irradiation. The bias potentials typically required for the onset of oxygen evolution current at the photoanode were sufficient for the efficient reduction of CO2 at the metallic/composite counter electrodes. Using Cu as a cathode electrocatalyst, faradaic efficiencies of 67% for CH4 and 71.6% for all carbon-containing products were achieved. With Sn/SnOx, a combined faradaic efficiency (CO + HCOOH) of 44.3% was obtained at +0.8 V. The 2-electrode potential between the counter electrode and working electrode for the WO3 driven system was less than the lowest bias potential reported so far for conventional photocathode-driven systems. The results demonstrate for the first time that the intrinsically more stable photoanode-driven systems could accomplish the reduction of CO2 with higher efficiencies relative to the conventional photocathode-driven systems.

Solution-based fabrication of ZnO/ZnSe heterostructure nanowire arrays for solar energy conversion

We report a method for synthesizing ZnO/ZnSe heterostructure nanowire arrays for use in photoelectrochemical (PEC) water splitting. The surfaces of ZnO nanowires immobilized on a conducting glass substrate were modified to form ZnO/ZnSe heterostructure nanowire arrays through a reaction with an aqueous sodium selenite and hydrazine solution. ZnO/ZnSe heterostructure nanowires with different morphologies were synthesized by varying solution concentrations and reaction times. The ZnO nanowire/ZnSe nanoparticle heterostructures (ZS1) were synthesized by a dissolution–recrystallization mechanism. At longer reaction times and higher solution concentrations, the nanostructure arrays transformed into ZnO nanowire/ZnSe nanosphere heterostructure arrays (ZS2) via Ostwald ripening. ZnO/ZnSe heterostructure arrays (ZS1 and ZS2) yielded higher photocurrents than the pristine ZnO nanowire arrays in a PEC water splitting test under AM 1.5G simulated solar light. The ZnO/ZnSe heterostructure array photoanodes exhibited absorption in the visible spectrum (<550 nm in wavelength) with a high incident-photon-to-current-conversion efficiency (IPCE) of up to 47% (ZS1) or 57% (ZS2) at 0.0 V vs. Ag/AgCl. The photoanode yielded a relatively high photocurrent density of 1.67 mA cm−2 (ZS1) or 2.35 mA cm−2 (ZS2) at 0.3 V compared to the ZnO nanowire arrays (0.125 mA cm−2). Structural differences between ZS1 and ZS2 yielded different PEC performances. A comparison to ZS2 revealed that ZS1 exhibited a higher photocurrent density under a low applied potential (from −0.78 V to −0.07 V) and a lower photocurrent density under a high applied potential (above −0.07 V).

Phase transition-induced band edge engineering of BiVO4 to split pure water under visible light

Significance Hydrogen has been recognized as one of the most promising energy carriers for the future, because it can generate enormous energy by clean combustion chemistry without any greenhouse gas emissions. Water splitting under visible light irradiation is an ideal route to cost-effective, large-scale, and sustainable hydrogen production, but it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band gap energy, appropriate band positions, and photochemical stability. To create this rare photocatalyst, we engineered the band edges of BiVO4 by simultaneously substituting In3+ for Bi3+ and Mo6+ for V5+ in the host lattice of monoclinic BiVO4, which induced partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4. Through phase transition-induced band edge engineering by dual doping with In and Mo, a new greenish BiVO4 (Bi1-XInXV1-XMoXO4) is developed that has a larger band gap energy than the usual yellow scheelite monoclinic BiVO4 as well as a higher (more negative) conduction band than H+/H2 potential [0 VRHE (reversible hydrogen electrode) at pH 7]. Hence, it can extract H2 from pure water by visible light-driven overall water splitting without using any sacrificial reagents. The density functional theory calculation indicates that In3+/Mo6+ dual doping triggers partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4, which sequentially leads to unit cell volume growth, compressive lattice strain increase, conduction band edge uplift, and band gap widening.

Isomorphic Ti substitution into SBA-15 without Ti loss and with lower TiO2 segregation.

A convenient method has been discovered to incorporate Ti atoms isomorphically into a SBA-15 lattice without Ti loss. By hydrolysis of a Ti precursor near neutral pH instead of conventional acidic conditions, Ti loss was almost eliminated and its segregation to form TiO2 particles was suppressed while the mesoporous structure remained intact.

NiFeO x co-catalyzed BiVO 4 photoanode for improved photoelectrochemical water splitting

【PEC (photoelectrochemical) water splitting for

Carbonate-coordinated cobalt co-catalyzed BiVO4/WO3 composite photoanode tailored for CO2 reduction to fuels

O_2/H_2

Palladium oxide as a novel oxygen evolution catalyst on BiVO4 photoanode for photoelectrochemical water splitting

production is one of the promising but difficult way to utilize solar energy. Among photocatalytic materials for PEC water oxidation,