We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO2 nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO2 (H:TiO2) nanowires were prepared by annealing the pristine TiO2 nanowires in hydrogen atmosphere at various temperatures in a range of 200–550 °C. In comparison to pristine TiO2 nanowires, H:TiO2 samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO2 samples have exceptionally low photocurrent saturation potentials of −0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO2 nanowire sample yields a photocurrent density of ∼1.97 mA/cm2 at −0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm2 from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ∼1...

Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting

Enriching the functionality of ferroelectric materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced information storage with diverse signal manipulation functions. We report experimental observations of robust intralayer ferroelectricity in two-dimensional (2D) van der Waals layered α-In2Se3 ultrathin flakes at room temperature. Distinct from other 2D and conventional ferroelectrics, In2Se3 exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical electric field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow bandgap (∼1.3 eV) of ferroelectric In2Se3, a prototypical nonvolatile memory device, which can be manipulated both by electric field and visible light illumination, is demonstrated for advancing data storage technologies.

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Intercorrelated In-Plane and Out-of-Plane Ferroelectricity in Ultrathin Two-Dimensional Layered Semiconductor In2Se3.

This work reports a facile method for preparing the new photoactive In2O3 films as well as their implementation in photoelectrochemical (PEC) application. We firstly investigated the relationship between oxygen vacancies (VO) and PEC performance and revealed a rule between them. We found that the optimized In2O3−n sample yielded a photocurrent density up to 3.83 mA/cm2 in 1 M Na2SO4 solution under the solar illumination. It also gave efficiency as high as 75% over 400 nm in the incident-photon-to-current-conversion efficiency (IPCE) spectrum, which is the best value for an In2O3 photoanode reported. Moreover, the PEC performance of these films is enhanced as the VO increased and then decreased with further increasing VO. This two-side effect means VO can favor the photoelectron activation, or act as recombination centers to prohibit the generation of photocurrent. Making highly photoactive In2O3 nanostructures in this work will open up new opportunities in various areas.

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Oxygen vacancies promoting photoelectrochemical performance of In(2)O(3) nanocubes.

In black and white: The hydrogenation of TiO(2) can extend its optical absorption into the visible and infrared region and change its color from white to black. Furthermore, the hydrogenated black TiO(2) exhibits excellent photocatalytic activity for the splitting of water to yield H(2).

A Highly Efficient Photocatalyst—Hydrogenated Black TiO2 for the Photocatalytic Splitting of Water

Thin film TiO2 photocatalyst deposited by reactive magnetron sputtering

Sub-band-gap photoresponse of Ti O 2 − x thin-film—electrolyte interface

Polycrystals of ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$ and ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ were prepared from high-purity elements at 1270 \ifmmode^\circ\else\textdegree\fi{}C. The optical-absorption spectra of these polycrystalline powders were measured in the wavelength region 300--3000 nm with use of an integrating sphere and photoacoustic spectrophotometer at 298 K. The optical energy gap is found to be 3.18 eV for the ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$ crystal and 2.60 eV for the ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ crystal. The optical-absorption peaks in the measurement of ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ crystal absorption spectrum at 3790, 6049, 13 725, 20 704, and 22 779 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are found to be the electron transition of the ${\mathrm{Co}}^{2+}$ ion with ${\mathit{T}}_{\mathit{d}}$ symmetry from the ground state $^{4}$${\mathit{A}}_{2}$${(}^{4}$F) to the excited states $^{4}$${\mathit{T}}_{2}$${(}^{4}$F), $^{4}$${\mathit{T}}_{1}$${(}^{4}$F), $^{4}$${\mathit{T}}_{1}$${(}^{4}$P), $^{2}$${\mathit{T}}_{2}$${(}^{2}$G), and $^{2}$E${(}^{2}$G). The crystal-field parameter and the Racah parameter, which were obtained from the optical-absorption peaks, are found to be Dq=379 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and B=560 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The $^{4}$${\mathit{T}}_{1}$${(}^{4}$P) state of the ${\mathrm{Co}}^{2+}$ ion splits into three state of ${\mathrm{\ensuremath{\Gamma}}}_{6}$, ${\mathrm{\ensuremath{\Gamma}}}_{8}$, and ${\mathrm{\ensuremath{\Gamma}}}_{8}$+${\mathrm{\ensuremath{\Gamma}}}_{7}$ by first-order spin-orbit coupling effects at 298 K. The value of the spin-orbit coupling parameter was \ensuremath{\lambda}=-436 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Optical absorption of ZnGa 2 S 4 and ZnGa 2 S 4 : Co 2 + crystals

Single crystals of ${\mathrm{ZnGa}}_{2}$${\mathrm{Se}}_{4}$:${\mathrm{Co}}^{2+}$ were grown by the chemical transport reaction method. The optical absorption of single crystals was investigated in the temperature range 20\char21{}300 K. The optical-absorption peaks due to the transition $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$${(}^{4}$P), $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$${(}^{4}$F), and $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{2}$${(}^{4}$F) of the ${\mathrm{Co}}^{2+}$ ion that has entered the substitutional tetrahedral sites appeared at 12 830, 6221, and 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at 289 K. The crystal-field parameter and the Racah parameter were given by Dq=400 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and B=469 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The transition $^{4}\mathrm{A}_{2}$${(}^{4}$F)${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$${(}^{4}$P) split into a triplet state by first-order spin-orbit\char21{}coupling effects in the temperature range 55\char21{}289 K, and the ${\ensuremath{\Gamma}}_{8}$+${\ensuremath{\Gamma}}_{7}$ level split into ${\ensuremath{\Gamma}}_{8}$ and ${\ensuremath{\Gamma}}_{7}$ levels by second-order spin-orbit\char21{}coupling effects of ${\mathrm{Co}}^{2+}$ ions in the low-temperature range below 55 K. The spin-orbit\char21{}coupling parameter was found to be \ensuremath{\lambda}=-363 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Wha-Tek Kim

Papers

Sub-band-gap photoresponse of Ti O 2 − x thin-film—electrolyte interface

Optical absorption of ZnGa 2 S 4 and ZnGa 2 S 4 : Co 2 + crystals

Optical absorption of ZnGa2Se

Optical properties of Ga2Se3:Co2+ single crystals

Optical properties of SbSi: Co and SbSeI: Co single crystals