Junie Jhon M. Vequizo

Solar-driven Z-scheme water splitting using tantalum/nitrogen co-doped rutile titania nanorod as an oxygen evolution photocatalyst

A visible-light-driven water-splitting system that involves two-step photoexcitation (Z-scheme) was constructed using rutile TiO2 nanorod doped with Ta and N (TiO2:Ta/N) as an O2 evolution photocatalyst. The Ta-doped TiO2 nanorods, prepared by a solvothermal synthesis, underwent nitridation to possess visible-light absorption under mild conditions, even at 623 K under an ammonia flow. The TiO2:Ta/N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation for producing O2 in the presence of reversible electron acceptors (IO3− or Fe3+), while TiO2:N exhibited negligible activity. The results of time-resolved infrared absorption spectroscopy indicated that co-doping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Simultaneous H2 and O2 evolution via water splitting was achieved using a combination of RuO2-modified TiO2:Ta/N, Ru-loaded SrTiO3:Rh and an Fe3+/Fe2+ redox couple under visible-light irradiation (λ > 420 nm) and under AM 1.5G simulated sunlight.

Electrodeposition of SnO2 Thin Films from Aqueous Tin Sulfate Solutions

Tin oxide (SnO2) thin films were successfully electrodeposited on indium tin oxide (ITO) coated glass substrate from an acidic aqueous solution containing SnSO4 at room temperature. Oxygen bubbling was employed so that dissolved oxygen serves as oxygen precursor. With O2 bubbling and short deposition time, transparent films were obtained. The composition ratios of the films were measured by Auger electron spectroscopy. The n-type conductivity and the photosensitivity of the films were confirmed from photoelectrochemical measurement.

Enhancement of photoelectrochemical activity of SnS thin-film photoelectrodes using TiO2, Nb2O5, and Ta2O5 metal oxide layers

Tin sulfide (SnS) fine photoelectrodes fabricated by three-step pulsed electrodeposition were active for H2 evolution. The incident-photon-conversion-efficiency increases from 900 nm and offers a good fit with the absorption spectrum. The activity was enhanced by 3.4, 3.0, and 1.8 times compared to bare SnS by loading Nb2O5, TiO2, and Ta2O5, respectively. Nb2O5 was most efficient because its conduction band is low enough to facilitate effective electron transfer from SnS; it also has sufficiently high potential for H2 evolution. The overall activity is determined by the competitive interfacial electron transfer between SnS/metal-oxide and metal-oxide/water. Therefore, constructing appropriate heterojunctions is necessary for further improving photoelectrochemical systems.

Electrodeposition of Ga--O Thin Films from Aqueous Gallium Sulfate Solutions

Ga–O based thin films were electrodeposited on fluorine-doped tin oxide (FTO)-coated glass substrate at room temperature from aqueous gallium sulfate solution with hydrogen peroxide (H2O2). Effects of different deposition parameters such as deposition voltage, amount of H2O2 and deposition time were investigated and presented. Nearly smooth and crack-free morphologies were attained at -1.0 V vs SCE deposition potential. As-deposited films showed O to Ga ratio of 2.0, which signified GaOOH formation. Thermal annealing of the as-deposited films in ambient air at 500–600 °C reduced the O/Ga ratio closer to stoichiometric gallium oxide (Ga2O3) and retained the morphology of Ga–O thin films. As-prepared films with ~0.2 µm thickness had 80% transparency in the visible wavelength range.

Undoped Layered Perovskite Oxynitride Li2LaTa2O6N for Photocatalytic CO2 Reduction with Visible Light

Abstract Oxynitrides are promising visible‐light‐responsive photocatalysts, but their structures are almost confined with three‐dimensional (3D) structures such as perovskites. A phase‐pure Li2LaTa2O6N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium‐rich oxide precursor. Li2LaTa2O6N exhibited high crystallinity and visible‐light absorption up to 500 nm. As opposed to well‐known 3D oxynitride perovskites, Li2LaTa2O6N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2LaTa2O6N possesses a lower density of mid‐gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

Nitrogen/fluorine-codoped rutile titania as a stable oxygen-evolution photocatalyst for solar-driven Z-scheme water splitting

Nitrogen/fluorine-codoped rutile TiO2 (R-TiO2:N,F) was newly synthesized, and its photocatalytic activity for water oxidation was evaluated. R-TiO2:N,F could be prepared by nitridation of the rutile TiO2 (R-TiO2) and (NH4)2TiF6 mixture at 773 K. The prepared samples produced O2 from aqueous AgNO3 solution under visible light irradiation, while R-TiO2 nitrided at the same temperature without any fluorine source showed negligible activity. The highest activity was obtained with the sample prepared at the (NH4)2TiF6/R-TiO2 ratio of 15/85, exhibiting water oxidation activity even in the presence of a reversible electron acceptor such as IO3− or Fe3+ with the aid of a RuO2 cocatalyst. Stoichiometric water splitting into H2 and O2 was achieved using a mixture of Ru/SrTiO3:Rh and RuO2/TiO2:N,F in the presence of [Co(bpy)3]3+/2+ (bpy = 2,2′-bipyridine) as a shuttle redox mediator without noticeable degradation of activity under visible light and even under AM1.5G simulated sunlight. Transient absorption spectroscopy revealed that appropriate nitrogen/fluorine codoping reduces the density of mid-gap states working as deep traps of photogenerated electrons, and increases the number of free electrons compared to only nitrogen-doped R-TiO2. Experimental results highlighted that the photocatalytic activity of R-TiO2:N,F could be enhanced by improving visible-light absorption capability through N/F codoping while suppressing the density of deep trap sites.

Near infrared light induced plasmonic hot hole transfer at a nano-heterointerface

Localized surface plasmon resonance (LSPR)-induced hot-carrier transfer is a key mechanism for achieving artificial photosynthesis using the whole solar spectrum, even including the infrared (IR) region. In contrast to the explosive development of photocatalysts based on the plasmon-induced hot electron transfer, the hole transfer system is still quite immature regardless of its importance, because the mechanism of plasmon-induced hole transfer has remained unclear. Herein, we elucidate LSPR-induced hot hole transfer in CdS/CuS heterostructured nanocrystals (HNCs) using time-resolved IR (TR-IR) spectroscopy. TR-IR spectroscopy enables the direct observation of carrier in a LSPR-excited CdS/CuS HNC. The spectroscopic results provide insight into the novel hole transfer mechanism, named plasmon-induced transit carrier transfer (PITCT), with high quantum yields (19%) and long-lived charge separations (9.2 μs). As an ultrafast charge recombination is a major drawback of all plasmonic energy conversion systems, we anticipate that PITCT will break the limit of conventional plasmon-induced energy conversion.Hot hole transfer has applications in plasmonics, photocatalysis, and light harvesting, but is often limited by low quantum yields and short-lived charge separation times. Here, Lian et al. overcome these limitations in heterostructured nanocrystals and proposed a new hot hole transfer mechanism.

Binary flux-promoted formation of trigonal ZnIn2S4 layered crystals using ZnS-containing industrial waste and their photocatalytic performance for H2 production

The accumulation of solid waste due to rapid industrialization has a negative impact on the environment. In this study, ZnS-containing waste from the mining-metallurgy industry was utilized for the synthesis of trigonal ZnIn2S4 layered crystals by a flux method using various binary fluxes: CaCl2 : InCl3, SrCl2 : InCl3, BaCl2 : InCl3, NaCl : InCl3, KCl : InCl3, and CsCl : InCl3. Among the binary fluxes used, KCl : InCl3 was found to be the most favorable for the synthesis of phase-pure trigonal ZnIn2S4 layered crystals. The XRD and SEM results revealed that the flux-grown ZnIn2S4 crystals have a trigonal structure and a morphology composed of large stacked layers. Unexpectedly, the UV-vis diffuse reflectance spectrum exhibited the onset of the absorption edge at approximately 700 nm for trigonal ZnIn2S4 crystals. The photocatalytic activities for H2 production of the Pt-photodeposited ZnIn2S4 samples grown using CaCl2 : InCl3, NaCl : InCl3, and KCl : InCl3 fluxes were evaluated. Trigonal ZnIn2S4 crystals grown using the KCl : InCl3 flux in this study exhibited higher photocatalytic activity for H2 evolution (132 μmol h−1) than previously reported hexagonal ZnIn2S4 synthesized by a hydrothermal method due to the decreased defect density and higher crystallinity achieved by the binary flux method. The presence of secondary crystalline phases (ZnS and In2S3) in ZnIn2S4 crystals grown using NaCl : InCl3 and CaCl2 : InCl3 fluxes positively impacted the photocatalytic activity and exhibited photocatalytic H2 evolution rates of 188 and 232 μmol h−1, respectively, because of the efficient separation and transfer of photogenerated charge carriers. The decay of the transient absorption of electrons in three samples at 2000 cm−1 was monitored by transient absorption spectroscopy, confirming that the lifetime of free electrons becomes longer depending on the binary flux used: KCl : InCl3 < NaCl : InCl3 < CaCl2 : InCl3. The binary flux method applied in this study demonstrates that accumulated solid industrial wastes can be turned into beneficial photocatalytic materials.