Light-Induced Dynamic Restructuring of Cu Active Sites on TiO2 for Low-Temperature H2 Production from Methanol and Water.

The structure and configuration of reaction centers, which dominantly govern the catalytic behaviors, often undergo dynamic transformations under reaction conditions, yet little is known about how to exploit these features to favor the catalytic functions. Here, we demonstrate a facile light activation strategy over a TiO2-supported Cu catalyst to regulate the dynamic restructuring of Cu active sites during low-temperature methanol steam reforming. Under illumination, the thermally deactivated Cu/TiO2 undergoes structural restoration from inoperative Cu2O to the originally active metallic Cu caused by photoexcited charge carriers from TiO2, thereby leading to substantially enhanced activity and stability. Given the low-intensity solar irradiation, the optimized Cu/TiO2 displays a H2 production rate of 1724.1 µmol g-1 min-1, outperforming most of the conventional photocatalytic and thermocatalytic processes. Taking advantages of the strong light-matter-reactant interaction, we achieve in situ manipulation of the Cu active sites, suggesting the feasibility for real-time functionalization of catalysts.

Direct and Selective Photocatalytic Oxidation of CH4 to Oxygenates with O2 on Cocatalysts/ZnO at Room Temperature in Water.

Direct conversion of methane into methanol and other liquid oxygenates still confronts considerable challenges in activating the first C-H bond of methane and inhibiting overoxidation. Here, we report that ZnO loaded with appropriate cocatalysts (Pt, Pd, Au, or Ag) enables direct oxidation of methane to methanol and formaldehyde in water using only molecular oxygen as the oxidant under mild light irradiation at room temperature. Up to 250 micromoles of liquid oxygenates with ∼95% selectivity is achieved for 2 h over 10 mg of ZnO loaded with 0.1 wt % of Au. Experiments with isotopically labeled oxygen and water reveal that molecular O2, rather than water, is the source of oxygen for direct CH4 oxidation. We find that ZnO and cocatalyst could concertedly activate CH4 and O2 into methyl radical and mildly oxidative intermediate (hydroperoxyl radical) in water, which are two key precursor intermediates for generating oxygenated liquid products in direct CH4 oxidation. Our study underlines two equally significant aspects for realizing direct and selective photooxidation of CH4 to liquid oxygenates, i.e., efficient C-H bond activation of CH4 and controllable activation of O2.

Elemental Boron for Efficient Carbon Dioxide Reduction under Light Irradiation.

The photoreduction of CO2 is attractive for the production of renewable fuels and the mitigation of global warming. Herein, we report an efficient method for CO2 reduction over elemental boron catalysts in the presence of only water and light irradiation through a photothermocatalytic process. Owing to its high solar-light absorption and effective photothermal conversion, the illuminated boron catalyst experiences remarkable self-heating. This process favors CO2 activation and also induces localized boron hydrolysis to in situ produce H2 as an active proton source and electron donor for CO2 reduction as well as boron oxides as promoters of CO2 adsorption. These synergistic effects, in combination with the unique catalytic properties of boron, are proposed to account for the efficiency of the CO2 reduction. This study highlights the promise of photothermocatalytic strategies for CO2 conversion and also opens new avenues towards the development of related solar-energy utilization schemes.

Promoting Active Species Generation by Plasmon-Induced Hot-Electron Excitation for Efficient Electrocatalytic Oxygen Evolution.

Water splitting represents a promising technology for renewable energy conversion and storage, but it is greatly hindered by the kinetically sluggish oxygen evolution reaction (OER). Here, using Au-nanoparticle-decorated Ni(OH)2 nanosheets [Ni(OH)2-Au] as catalysts, we demonstrate that the photon-induced surface plasmon resonance (SPR) excitation on Au nanoparticles could significantly activate the OER catalysis, specifically achieving a more than 4-fold enhanced activity and meanwhile affording a markedly decreased overpotential of 270 mV at the current density of 10 mA cm(-2) and a small Tafel slope of 35 mV dec(-1) (no iR-correction), which is much better than those of the benchmark IrO2 and RuO2, as well as most Ni-based OER catalysts reported to date. The synergy of the enhanced generation of Ni(III/IV) active species and the improved charge transfer, both induced by hot-electron excitation on Au nanoparticles, is proposed to account for such a markedly increased activity. The SPR-enhanced OER catalysis could also be observed over cobalt oxide (CoO)-Au and iron oxy-hydroxide (FeOOH)-Au catalysts, suggesting the generality of this strategy. These findings highlight the possibility of activating OER catalysis by plasmonic excitation and could open new avenues toward the design of more-energy-efficient catalytic water oxidation systems with the assistance of light energy.

Nature-Inspired Environmental "Phosphorylation" Boosts Photocatalytic H2 Production over Carbon Nitride Nanosheets under Visible-Light Irradiation.

Inspired by the crucial roles of phosphates in natural photosynthesis, we explored an environmental "phosphorylation" strategy for boosting photocatalytic H2 production over g-C3N4 nanosheets under visible light. As expected, a substantial improvement was observed in the rate of H2 evolution to 947 µmol h(-1), and the apparent quantum yield was as high as 26.1% at 420 nm. The synergy of enhanced proton reduction and improved hole oxidation is proposed to account for the markedly increased activity. Our findings may provide a promising and facile approach to highly efficient photocatalysis for solar-energy conversion.

Porous-structured Cu2O/TiO2 nanojunction material toward efficient CO2 photoreduction.

Porous-structured Cu2O/TiO2 nanojunction material is successfully fabricated by a facile method via loading Cu2O nanoparticles on the network of a porous TiO2 substrate. The developed Cu2O/TiO2 nanojunction material has a size of several nanometers, in which the p-type Cu2O and n-type TiO2 nanoparticles are closely contacted with each other. The well designed nanojunction structure is beneficial for the charge separation in the photocatalytic reaction. Meanwhile, the porous structure of the Cu2O/TiO2 nanojunction can facilitate the CO2 adsorption and offer more reaction active sites. Most importantly, the gas-phase CO2 photoreduction tests reveal that our developed porous-structured Cu2O/TiO2 nanojunction material exhibits marked photocatalytic activity in the CH4 evolution, about 12, 9, and 7.5 times higher than the pure TiO2, Pt-TiO2, and commercial Degussa P25 TiO2 powders, respectively. The greatly enhanced activity can be attributed to the well designed nanojunction structure combined with the porous structure, which can simultaneously enhance the charge separation efficiency and facilitate the CO2 adsorption.

Photothermal conversion of CO2 into CH4 with H2 over Group VIII nanocatalysts: an alternative approach for solar fuel production.

The photothermal conversion of CO2 provides a straightforward and effective method for the highly efficient production of solar fuels with high solar-light utilization efficiency. This is due to several crucial features of the Group VIII nanocatalysts, including effective energy utilization over the whole range of the solar spectrum, excellent photothermal performance, and unique activation abilities. Photothermal CO2 reaction rates (mol h(-1) g(-1)) that are several orders of magnitude larger than those obtained with photocatalytic methods (µmol h(-1) g(-1)) were thus achieved. It is proposed that the overall water-based CO2 conversion process can be achieved by combining light-driven H2 production from water and photothermal CO2 conversion with H2. More generally, this work suggests that traditional catalysts that are characterized by intense photoabsorption will find new applications in photo-induced green-chemistry processes.

Photoelectrochemical properties of nanomultiple CaFe2O4/ZnFe2O4 pn junction photoelectrodes.

Nanomultiple CaFe2O4/ZnFe2O4pn junctions are prepared by a pulsed laser deposition method to explore their photoelectrochemical properties as the photoelectrodes. It is demonstrated that the multiple-pn-junction structure is favorable to enhancing the photocurrent density and the onset potential of the photoelectrode. Furthermore, the 20-junction photoelectrode-based PEC cell yields a high open circuit photovoltage of up to 0.97 V, which is much higher than that for a single pn junction photoelectrode PEC cell that yields an open circuit photovoltage of 0.13 V. A multiple-junction band structure model is assumed to describe the behavior of the CaFe2O4/ZnFe2O4 multiple-junction photoelectrodes. It is suggested that the open circuit photovoltage is dominated by the number of pn junctions in a multiple-junction photoelectrode and the carrier transfer inside the photoelectrode is improved by narrowing the single-layer thickness. These findings provide a new approach to designing the multiple-junction structure to improve the PEC properties of the photoelectrodes.

Photoassisted fabrication of zinc indium oxide/oxysulfide composite for enhanced photocatalytic H2 evolution under visible-light irradiation.

A photoassisted approach has been developed to synthesize a zinc indium oxide (Zn5In2O8)/oxysulfide composite through in situ sulfuration of vacancy-rich Zn5In2O8. It was found that vacancies have a considerable impact on the formation of the composite. The composite exhibited an increased photocatalytic H2 evolution activity under visible-light irradiation, which probably resulted from the enhanced ability to separate photoinduced electrons and holes. The H2 evolution rate over the composite was about 17 times higher when using vacancy-rich rather than conventional Zn5In2O8. This study provides a new method of improving the activity of photocatalysts.

An orthophosphate semiconductor with photooxidation properties under visible-light irradiation.

The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization. Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In(1-x)Ni(x)TaO(4) and (V-,Fe- or Mn-)TiO(2) (refs 7,8), non-metal-ion substitution as in TiO(2-x)N(x) and Sm(2)Ti(2)O(5)S(2) (refs 9,10) or solid-solution fabrication as in (Ga(1-x)Zn(x))(N(1-x)O(x)) and ZnS-CuInS(2)-AgInS(2) (refs 11,12). Here we report a new use of Ag(3)PO(4) semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.

Hydrogen production using zinc-doped carbon nitride catalyst irradiated with visible light.

Recently, graphitic carbon nitride (g-C3N4) has been investigated as a photocatalyst for water splitting and organic dye degradation. In this study, we have developed a simple soft-chemical method of doping Zn into g-C3N4 to prepare a metal-containing carbon nitride. The doping was confirmed by x-ray photoelectron spectroscopy, and diffusion reflectance spectra revealed a significant red shift in the absorption edge of Zn/g-C3N4. This hybrid material shows high photocatalytic activity and good stability for hydrogen evolution from an aqueous methanol solution under visible light irradiation (λ≥420 nm). The hydrogen evolution rate was more than 10 times higher for a 10%-Zn/g-C3N4 sample (59.5 µmol h-1) than for pure g-C3N4. The maximum quantum yield was 3.2% at 420 nm.

Nanoarchitectonics of a Au nanoprism array on WO3 film for synergistic optoelectronic response.

A layered photoelectrode consisting of a conductive indium tin oxide substrate, a WO3 nanocrystalline film and an array of Au nanoprisms was fabricated via a multistep process. Scanning electron microscopy and atomic force microscopy showed that the Au nanoprisms had a uniform size and shape and formed periodic hexagonal patterns on the WO3 film. The optical absorption of the photoelectrode combined the intrinsic absorption of WO3 and plasmonic absorption of Au. Using this photoelectrode, we investigated the effect of the Au nanoprism array on the optoelectronic conversion performance of the WO3 film. Photoelectrochemical measurement indicated that the array substantially enhanced the photocurrent in the WO3 film. Electrochemical impedance measurements revealed that the Schottky junctions formed between Au and WO3 can facilitate the separation of photogenerated carriers as well as the interfacial carrier transfer. In this study, we demonstrate that covering a semiconductor with plasmonic noble metal nanoparticles can improve its optoelectronic conversion efficiency.

Efficient photocatalytic decomposition of acetaldehyde over a solid-solution perovskite (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 under visible-light irradiation.

A green chemistry process for environmental purification is realized on a novel visible-light-active photocatalyst (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3. This mixed valent solid-solution perovskite with a modulated electronic structure possesses a strong oxidative potential for efficient photocatalytic decomposition of acetaldehyde into CO2 at ambient temperature.

Light-Enhanced Carbon Dioxide Activation and Conversion by Effective Plasmonic Coupling Effect of Pt and Au Nanoparticles.

Photocatalytic reduction of carbon dioxide (CO2) is attractive for the production of valuable fuels and mitigating the influence of greenhouse gas emission. However, the extreme inertness of CO2 and the sluggish kinetics of photoexcited charge carrier transfer process greatly limit the conversion efficiency of CO2 photoreduction. Herein, we report that the plasmonic coupling effect of Pt and Au nanoparticles (NPs) profoundly enhances the efficiency of CO2 reduction through dry reforming of methane reaction assisted by light illumination, reducing activation energies for CO2 reduction ∼30% below thermal activation energies and achieving a reaction rate 2.4 times higher than that of the thermocatalytic reaction. UV-visible (vis) absorption spectra and wavelength-dependent performances show that not only UV but also visible light play important roles in promoting CO2 reduction due to effective localized surface plasmon resonance (LSPR) coupling between Pt and Au NPs. Finite-difference time-domain simulations and in situ diffuse reflectance infrared Fourier transform spectroscopy further reveal that effective coupling LSPR effect generates strong local electric fields and excites high concentration of hot electrons to activate the reactants and intermediate species, reduce the activation energies, and increase the reaction rate. This work provides a new pathway toward the efficient plasmon-enhanced chemical reactions via reducing the activation energies by utilizing solar energy.

Superior Photocatalytic H2 Production with Cocatalytic Co/Ni Species Anchored on Sulfide Semiconductor.

Downsizing transition metal-based cocatalysts on semiconductors to promote photocatalytic efficiency is important for research and industrial applications. This study presents a novel and facile strategy for anchoring well-dispersed metal species on CdS surface through controlled decarboxylation of the ethylenediaminetetraacetate (EDTA) ligand in the metal-EDTA (M-EDTA) complex and CdS mixture precursor to function as a cocatalyst in the photocatalytic H2 evolution. Microstructure characterization and performance evaluation reveal that under visible light the resulting pentacoordinated Co(II) and hexacoordinated Ni(II) on CdS exhibits a high activity of 3.1 mmol h-1 (with turnover frequency (TOF) of 626 h-1 and apparent quantum efficiency (AQE) of 56.2% at 420 nm) and 4.3 mmol h-1 (with TOF of 864 h-1 and AQE of 67.5% at 420 nm), respectively, toward cocatalytic hydrogen evolution, and the cocatalytic activity of such a hexacoordinated Ni(II) even exceeds that of platinum. Further mechanistic study and theoretical modeling indicate that the fully utilized Co(II)/Ni(II) active sites, efficient charge transfer, and favorable kinetics guarantee the efficient activities. This work introduces a promising precursor, i.e., M-EDTA for planting well-dispersed transition metal species on the sulfide supports by a facile wet-chemistry approach, providing new opportunities for photocatalytic H2 production at the atomic/molecular scale.

Photophysical and photocatalytic properties of SrTiO3 doped with Cr cations on different sites.

Usually, SrTiO3 monodoped with Cr cations at the Ti4+ site hardly shows visible light photocatalytic activity. Revealing the origin of this issue is important for us to find an alternative approach to make SrTiO3 active under visible light irradiation. In this paper, two Cr-doped SrTiO3-(Sr0.95Cr0.05)TiO3 and Sr(Ti0.95Cr0.05)O3-were synthesized by a conventional solid-state reaction method, and their photophysical and photocatalytic properties were studied comparatively. It was found that both (Sr0.95Cr0.05)TiO3 and Sr(Ti0.95Cr0.05)O3 showed considerable absorption to visible light. However, their photocatalytic activities for H2 evolution from aqueous methanol solution under visible light irradiation were significantly different: the H2 evolution rate over (Sr0.95Cr0.05)TiO3 (approximately 21 micromol/h) was more than 100 times that over Sr(Ti0.95Cr0.05)O3 (approximately 0.2 micromol/h). X-ray photoelectron spectroscopy analysis results revealed that the Cr cations doped at the Sr2+ site were all trivalent state (Cr3+), while those doped at the Ti4+ site were mixed valent states (Cr3+ and Cr6+). The different photocatalytic activities of H2 evolution are supposed to closely relate to the different valent states of Cr doped at different sites (Sr2+ or Ti4+) in SrTiO3. Possible electronic structures of (Sr0.95Cr0.05)TiO3 and Sr(Ti0.95Cr0.05)O3 were proposed in relation to their photophysical and photocatalytic properties.

An Ag3PO4/nitridized Sr2Nb2O7 composite photocatalyst with adjustable band structures for efficient elimination of gaseous organic pollutants under visible light irradiation.

A new Ag3PO4/nitridized Sr2Nb2O7 (N: 0-6.18 wt%) heterojunction was designed to eliminate gaseous pollutants under visible light irradiation. The phase compositions, optical properties, and morphologies of the heterojunction photocatalysts were systematically investigated via powder X-ray diffraction, UV-visible absorption spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. Calculations of the electronic structure indicated that the top of the valance band of Sr2Nb2O7 could be raised by nitrogen doping. Therefore, the electronic structure of the Ag3PO4/nitridized Sr2Nb2O7 composite photocatalysts could be continually changed by controlling the amount of nitrogen in nitridized Sr2Nb2O7. Photocatalytic degradation of isopropyl alcohol (IPA) was carried out to test the photocatalytic activity of the heterojunction. The highest activity (CO2 evolution rate, 10.32 ppm h(-1)) was observed over the Ag3PO4/nitridized Sr2Nb2O7 heterojunction prepared by nitridation of Sr2Nb2O7 (SNO) at 1023 K. The CO2 evolution rate over the heterojunction was about 40 times higher than that over pure Ag3PO4 (CO2 evolution rate, 0.26 ppm h(-1)) under visible light irradiation. An investigation of the energy-band structure via valence band X-ray photoelectron spectroscopy indicated that the conduction band (CB) and valence band (VB) of Ag3PO4 are both more positive than those of nitridized Sr2Nb2O7, which facilitates the separation and transfer of photogenerated electrons and holes between the two photocatalysts. By continually adjusting the electronic structures, an optimal band gap for the nitridized Sr2Nb2O7 of 2.15 eV was obtained, and the potential of the valance band was +1.88 eV.

Photocatalytic CO2 conversion over alkali modified TiO2 without loading noble metal cocatalyst.

Surface modification of TiO2 with NaOH promoted the chemisorption, activation and photocatalytic CO2 reduction. An optimized loading amount of NaOH kept a good balance between CO2 chemisorption quantity and BET surface area of TiO2. This noble metal free method provides a simple pathway for effective multiple H(+)/e(-) CO2 photoreduction.

High-active anatase TiO2 nanosheets exposed with 95% {100} facets toward efficient H2 evolution and CO2 photoreduction.

We succeed in preparation of anatase TiO2 single crystals with marked photocatalytic activity via a facile and effective method. This TiO2 is composed of TiO2 ultrathin nanosheets (2 nm in thickness) with 95% of exposed {100} facet, which is considered to be the active facet for photocatalytic reaction. This percentage (95%) is the highest among previously reported {100} facet exposed anatase TiO2. More importantly, due to this high ratio, our developed TiO2 nanosheets showed marked photocatalytic activity, about 5 times higher activity in both H2 evolution and CO2 reduction than the reference sample, TiO2 cuboids with 53% of exposed {100} facet. For the TiO2 nanosheets, both the higher percentage of exposed {100} facets and larger surface area can offer more surface active sites in the photocatalytic reaction. On the other hand, the superior electronic band structure which results from the higher percentage of {100} facet is also beneficial for the higher activity. This study exemplifies that the facet engineering of semiconductors is one of the most effective strategies to achieve advanced properties over photofunctional materials for solar energy conversion.

An ion-exchange route for the synthesis of hierarchical In2S3/ZnIn2S4 bulk composite and its photocatalytic activity under visible-light irradiation.

In(2)S(3)/ZnIn(2)S(4) bulk composite was successfully synthesized through an ion-exchange route using NaInS(2) as a precursor. Compared with the constituent pure component (In(2)S(3) or ZnIn(2)S(4)), the photocatalytic H(2) evolution of the composite was greatly enhanced because of the efficient separation and migration of photoexcited carriers (electrons and holes) at the interface of the bulk composite.