Plasmon‐Assisted Water Splitting Using Two Sides of the Same SrTiO3 Single‐Crystal Substrate: Conversion of Visible Light to Chemical Energy

A plasmon‐induced water splitting system that operates under irradiation by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO₃) single‐crystal substrate. The water splitting system contains two solution chambers to separate hydrogen (H₂) and oxygen (O₂). To promote water splitting, a chemical bias was applied by regulating the pH values of the chambers. The quantity of H₂ evolved from the surface of platinum, which was used as a reduction co‐catalyst, was twice the quantity of O₂ evolved from an Au‐nanostructured surface. Thus, the stoichiometric evolution of H₂ and O₂ was clearly demonstrated. The hydrogen‐evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon‐induced charge separation at the Au/SrTiO₃ interface promotes water oxidation and the subsequent reduction of a proton on the backside of the SrTiO₃ substrate. The chemical bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.     

Novel Au/La‐SrTiO3 microspheres: Superimposed Effect of Gold Nanoparticles and Lanthanum Doping in Photocatalysis

Novel multielement Au/La‐SrTiO₃ microspheres were synthesized by a solvothermal method using monodisperse gold and La‐SrTiO₃ nanocrystals as building blocks. The porous Au/La‐SrTiO₃ microspheres had a large surface area of 94.6 m² g^?1. The stable confined Au nanoparticles demonstrated strong surface plasmon resonance effect, leading to enhanced absorption in a broad UV/Vis/NIR range. Doping of rare‐earth metal La also broadened the absorption band to the visible region. Both the conduction and valence bands of Au/La‐SrTiO₃ microspheres thus show favorable potential for proton reduction under visible light. The superimposed effect of Au nanoparticles and La doping in Au/La‐SrTiO₃ microspheres led to high photocurrent density in photoelectrochemical water splitting and good photocatalytic activity in photodegradation of rhodamine B. The photocatalytic activities are in the order of the following: Au/La‐SrTiO₃ microspheres>Au/SrTiO₃ microspheres>La‐SrTiO₃ microspheres>SrTiO₃ microspheres.     

Selective Dinitrogen Conversion to Ammonia Using Water and Visible Light through Plasmon‐induced Charge Separation

The generation of ammonia from atmospheric nitrogen and water using sunlight is a preferable approach to obtaining ammonia as an energy carrier and potentially represents a new paradigm for achieving a low‐carbon and sustainable‐energy society. Herein, we report the selective conversion of dinitrogen into ammonia through plasmon‐induced charge separation by using a strontium titanate (SrTiO₃) photoelectrode loaded with gold nanoparticles (Au‐NPs) and a zirconium/zirconium oxide (Zr/ZrO_x) thin film. We observed the simultaneous stoichiometric production of ammonia and oxygen from nitrogen and water under visible‐light irradiation.     

Hot‐Electron‐Transfer Enhancement for the Efficient Energy Conversion of Visible Light

Great strides have been made in enhancing solar energy conversion by utilizing plasmonic nanostructures in semiconductors. However, current generation with plasmonic nanostructures is still somewhat inefficient owing to the ultrafast decay of plasmon‐induced hot electrons. It is now shown that the ultrafast decay of hot electrons across Au nanoparticles can be significantly reduced by strong coupling with CdS quantum dots and by a Schottky junction with perovskite SrTiO₃ nanoparticles. The designed plasmonic nanostructure with three distinct components enables a hot‐electron‐assisted energy cascade for electron transfer, CdS→Au→SrTiO₃, as demonstrated by steady‐state and time‐resolved photoluminescence spectroscopy. Consequently, hot‐electron transfer enabled the efficient production of H₂ from water as well as significant electron harvesting under irradiation with visible light of various wavelengths. These findings provide a new approach for overcoming the low efficiency that is typically associated with plasmonic nanostructures.     

Protoelectrochemical properties of the single-crystal SrTiO3 doped in the surface region

The photoelectrochemical properties of the single crystal SrTiO₃, doped in the surface region are studied. It is found that the doped SrTiO₃ with Cr, Co, Pt and Rh give a relatively large photoresponse to visible light. The dark anodic currents which will be due to the resonance tunnelling or hopping mechanism are observed at the doped electrodes with the above metal cations. Therefore, it is concluded that the visible light response is mainly attributable to the formed impurity levels and/or structure defetcs by the doping metal cations near the conduction band of SrTiO₃. The above doped electrodes also bring the large cathodic photocurrent or the dark cathodic current due to the O₂ reduction, except for the Co doped electrode. This will show that the impurity levels act as the active site of O₂ reduction.     

Constructing Ordered Three‐Dimensional TiO2 Channels for Enhanced Visible‐Light Photocatalytic Performance in CO2 Conversion Induced by Au Nanoparticles

As a typical photocatalyst for CO₂ reduction, practical applications of TiO₂ still suffer from low photocatalytic efficiency and limited visible‐light absorption. Herein, a novel Au‐nanoparticle (NP)‐decorated ordered mesoporous TiO₂ (OMT) composite (OMT‐Au) was successfully fabricated, in which Au NPs were uniformly dispersed on the OMT. Due to the surface plasmon resonance (SPR) effect derived from the excited Au NPs, the TiO₂ shows high photocatalytic performance for CO₂ reduction under visible light. The ordered mesoporous TiO₂ exhibits superior material and structure, with a high surface area that offers more catalytically active sites. More importantly, the three‐dimensional transport channels ensure the smooth flow of gas molecules, highly efficient CO₂ adsorption, and the fast and steady transmission of hot electrons excited from the Au NPs, which lead to a further improvement in the photocatalytic performance. These results highlight the possibility of improving the photocatalysis for CO₂ reduction under visible light by constructing OMT‐based Au‐SPR‐induced photocatalysts.     

Visible‐Light‐Induced Electron Transport from Small to Large Nanoparticles in Bimodal Gold Nanoparticle‐Loaded Titanium(IV) Oxide

A key to realizing the sustainable society is to develop highly active photocatalysts for selective organic synthesis effectively using sunlight as the energy source. Recently, metal‐oxide‐supported gold nanoparticles (NPs) have emerged as a new type of visible‐light photocatalysts driven by the excitation of localized surface plasmon resonance of Au NPs. Here we show that visible‐light irradiation (λ>430 nm) of TiO₂‐supported Au NPs with a bimodal size distribution (BM‐Au/TiO₂) gives rise to the long‐range (>40 nm) electron transport from about 14 small (ca. 2 nm) Au NPs to one large (ca. 9 nm) Au NP through the conduction band of TiO₂. As a result of the enhancement of charge separation, BM‐Au/TiO₂ exhibits a high level of visible‐light activity for the one‐step synthesis of azobenzenes from nitrobenzenes at 25 °C with a yield greater than 95 % and a selectivity greater than 99 %, whereas unimodal Au/TiO₂ (UM‐Au/TiO₂) is photocatalytically inactive.     

Photocatalytic Activity of Au/TiO2 Photocatalysts for H2 Evolution: Role of the Au Nanoparticles as a Function of the Irradiation Wavelength

An investigation of hydrogen production with a series of Au/TiO₂ photocatalysts reveals that the Au nanoparticles play different roles depending on the wavelength of the light irradiation. Under visible‐light irradiation, the photoactivity is primarily controlled by the intensity of the Au surface plasmon band, whereas under UV irradiation the Au nanoparticles act as co‐catalysts with TiO₂.     

Control of Surface Plasmon Resonance of Au/SnO2 by Modification with Ag and Cu for Photoinduced Reactions under Visible‐Light Irradiation over a Wide Range

Gold particles supported on tin(IV) oxide (0.2 wt % Au/SnO₂) were modified with copper and silver by the multistep photodeposition method. Absorption around λ=550 nm, attributed to surface plasmon resonance (SPR) of Au, gradually shifted to longer wavelengths on modification with Cu and finally reached λ=620 nm at 0.8 wt % Cu. On the other hand, the absorption shifted to shorter wavelength with increasing amount of Ag and reached λ=450 nm at 0.8 wt % Ag. These Cu‐ and Ag‐modified 0.2 wt % Au/SnO₂ materials (Cu‐Au/SnO₂ and Ag‐Au/SnO₂) and 1.0 wt % Au/SnO₂ were used for mineralization of formic acid to carbon dioxide in aqueous suspension under irradiation with visible light from a xenon lamp and three kinds of light‐emitting diodes with different wavelengths. The reaction rates for the mineralization of formic acid over these materials depend on the wavelength of light. Apparent quantum efficiencies of Cu‐Au/SnO₂, Au/SnO₂, and Ag‐Au/SnO₂ reached 5.5 % at 625 nm, 5.8 % at 525 nm, and 5.1 % at 450 nm, respectively. These photocatalysts can also be used for selective oxidation of alcohols to corresponding carbonyl compounds in aqueous solution under visible‐light irradiation. Broad responses to visible light in formic acid mineralization and selective alcohol oxidation were achieved when the three materials were used simultaneously.     

Synthesis and Study of Plasmon‐Induced Carrier Behavior at Ag/TiO2 Nanowires

Nanocomposites of Ag/TiO₂ nanowires with enhanced photoelectrochemical performance have been prepared by a facile solvothermal synthesis of TiO₂ nanowires and subsequent photoreduction of Ag⁺ ions to Ag nanoparticles (AgNPs) on the TiO₂ nanowires. The as‐prepared nanocomposites exhibited significantly improved cathodic photocurrent responses under visible‐light illumination, which is attributed to the local electric field enhancement of plasmon resonance effect near the TiO₂ surface rather than by the direct transfer of charge between the two materials. The visible‐light‐driven photocatalytic performance of these nanocomposites in the degradation of methylene blue dye was also studied, and the observed improvement in photocatalytic activity is associated with the extended light absorption range and efficient charge separation due to surface plasmon resonance effect of AgNPs.     

TiO2 Nanotube Arrays Modified with Cr‐Doped SrTiO3 Nanocubes for Highly Efficient Hydrogen Evolution under Visible Light

In recent decades, solar‐driven hydrogen production over semiconductors has attracted tremendous interest owing to the global energy and environmental crisis. Among various semiconductor materials, TiO₂ exhibits outstanding photocatalytic properties and has been extensively applied in diverse photocatalytic and photoelectric systems. However, two major drawbacks limit practical applications, namely, high charge‐recombination rate and poor visible‐light utilization. In this work, heterostructured TiO₂ nanotube arrays grafted with Cr‐doped SrTiO₃ nanocubes were fabricated by simply controlling the kinetics of hydrothermal reactions. It was found that coupling TiO₂ nanotube arrays with regular SrTiO₃ nanocubes can significantly improve the charge separation. Meanwhile, doping Cr cations into SrTiO₃ nanocubes proved to be an effective and feasible approach to enhance remarkably the visible‐light response, which was also confirmed by theoretical calculations. As a result, the rate of photoelectrochemical hydrogen evolution of these novel heteronanostructures is an order of magnitude larger than those of TiO₂ nanotube arrays and other previously reported SrTiO₃/TiO₂ nanocomposites under visible‐light irradiation. Furthermore, the as‐prepared Cr‐doped SrTiO₃/TiO₂ heterostructures exhibit excellent durability and stability, which are favorable for practical hydrogen production and photoelectric nanodevices.     

Three‐Dimensional Ordered Assembly of Thin‐Shell Au/TiO2 Hollow Nanospheres for Enhanced Visible‐Light‐Driven Photocatalysis

An Au/TiO₂ nanostructure was constructed to obtain a highly efficient visible‐light‐driven photocatalyst. The design was based on a three‐dimensional ordered assembly of thin‐shell Au/TiO₂ hollow nanospheres (Au/TiO₂‐3 DHNSs). The designed photocatalysts exhibit not only a very high surface area but also photonic behavior and multiple light scattering, which significantly enhances visible‐light absorption. Thus Au/TiO₂‐3 DHNSs exhibit a visible‐light‐driven photocatalytic activity that is several times higher than conventional Au/TiO₂ nanopowders.     

Integration of Plasmonic Effects and Schottky Junctions into Metal–Organic Framework Composites: Steering Charge Flow for Enhanced Visible‐Light Photocatalysis

A wide range of light absorption and rapid electron–hole separation are desired for efficient photocatalysis. Herein, on the basis of a semiconductor‐like metal–organic framework (MOF), a Pt@MOF/Au catalyst with two types of metal–MOF interfaces integrates the surface plasmon resonance excitation of Au nanorods with a Pt‐MOF Schottky junction, which not only extends the light absorption of the MOF from the UV to the visible region but also greatly accelerates charge transfer. The spatial separation of Pt and Au particles by the MOF further steers the formation of charge flow and expedites the charge migration. As a result, the Pt@MOF/Au presents an exceptionally high photocatalytic H₂ production rate by water splitting under visible light irradiation, far superior to Pt/MOF/Au, MOF/Au and other counterparts with similar Pt or Au contents, highlighting the important role of each component and the Pt location in the catalyst.     

Preparation and photocatalytic activity of visible light-active sulfur and nitrogen co-doped SrTiO3

Nitrogen and sulfur co-doped SrTiO₃ was prepared by high energy grinding of the mixture of SrTiO₃ and thiourea. A new band gap in visible light region (522 nm) corresponding to 2.37 eV could be formed by the co-doping. The photocatalytic activity for nitrogen monoxide oxidation of SrTiO₃ in visible light region especially in the long wavelength range (λ > 510 nm) could be improved greatly. Under the irradiation of light with wavelength larger than 510 nm, the photocatalytic activity of nitrogen and sulfur co-doped SrTiO₃ was 10.9 times greater than that of pure SrTiO₃. The high visible light photocatalytic activity of this substance may be due to the formation of a new band gap that enables to absorb visible light effectively.     

Au nanorods decorated TiO2 nanobelts with enhanced full solar spectrum photocatalytic antibacterial activity and the sterilization file cabinet application

TiO₂ photocatalysts have been widely studied and applied for removing bacteria, but its antibacterial efficiency is limited to the ultraviolet (UV) range of the solar spectrum. In this work, we use the gold (Au) nanorods to enhance the visible and near-infrared (NIR) light absorption of TiO₂ NBs, a typical UV light photocatalyst, thus the enhancement of its full solar spectrum (UV, visible and NIR) photocatalytic antibacterial properties is achieved. Preliminary surface plasmon resonance (SPR) enhancement photocatalytic antibacterial mechanism is suggested. On one hand, transverse and longitudinal SPR of Au NRs is beneficial for visible and NIR light utilization. On the other hand, Au NRs combined with TiO₂ NBs to form the heterostructure, which can improve the photogenerated carrier separation and direct electron transfer increases the hot electron concentration while Au NRs as the electron channel can well restrain charge recombination, finally produces the high yield of radical oxygen species and exhibits a superior antibacterial efficiency. Furthermore, we design a sterilization file cabinet with Au NR/TiO₂ NB heterostructures as the photocatalytic coating plates. Our study reveals that Au NR/TiO₂ NB heterostructure is a potential candidate for sterilization of bacteria and archives protection.     

Mesoporous Au/TiO2 Nanocomposite Microspheres for Visible‐Light Photocatalysis

Mesoporous Au/TiO₂ nanocomposite microspheres have been synthesized by using a microemulsion‐based bottom‐up self‐assembly (EBS) process starting from monodisperse gold and titania nanocrystals as building blocks. The microspheres had large surface areas (above 270 m² g^?1) and open mesopores (about 5 nm), which led to the adsorption‐driven concentration of organic molecules in the vicinity of the microspheres. Au nanoparticles, which were stably confined within the microspheres, enhanced the absorption over the broad UV/Vis/NIR spectroscopic range, owing to their strong surface plasmon resonance (SPR); as a result, the Au nanoparticles promoted the visible‐light photo‐induced degradation of organic compounds.     

Noble Metals Can Have Different Effects on Photocatalysis Over Metal–Organic Frameworks (MOFs): A Case Study on M/NH2‐MIL‐125(Ti) (M=Pt and Au)

M‐doped NH₂‐MIL‐125(Ti) (M=Pt and Au) were prepared by using the wetness impregnation method followed by a treatment with H₂ flow. The resultant samples were characterized by powder X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), X‐ray absorption fine structure (XAFS) analyses, N₂‐sorption BET surface area, and UV/Vis diffuse reflectance spectroscopy (DRS). The photocatalytic reaction carried out in saturated CO₂ with triethanolamine (TEOA) as sacrificial agent under visible‐light irradiations showed that the noble metal‐doping on NH₂‐MIL‐125(Ti) promoted the photocatalytic hydrogen evolution. Unlike that over pure NH₂‐MIL‐125(Ti), in which only formate was produced, both hydrogen and formate were formed over Pt‐ and Au‐loaded NH₂‐MIL‐125(Ti). However, Pt and Au have different effects on the photocatalytic performance for formate production. Compared with pure NH₂‐MIL‐125(Ti), Pt/NH₂‐MIL‐125(Ti) showed an enhanced activity for photocatalytic formate formation, whereas Au has a negative effect on this reaction. To elucidate the origin of the different photocatalytic performance, electron spin resonance (ESR) analyses and density functional theory (DFT) calculations were carried out over M/NH₂‐MIL‐125(Ti).The photocatalytic mechanisms over M/NH₂‐MIL‐125(Ti) (M=Pt and Au) were proposed. For the first time, the hydrogen spillover from the noble metal Pt to the framework of NH₂‐MIL‐125(Ti) and its promoting effect on the photocatalytic CO₂ reduction is revealed. The elucidation of the mechanism on the photocatalysis over M/NH₂‐MIL‐125(Ti) can provide some guidance in the development of new photocatalysts based on MOF materials. This study also demonstrates the potential of using noble metal‐doped MOFs in photocatalytic reactions involving hydrogen as a reactant, like hydrogenation reactions.     

Photosensing Device Composed of Bilayer Membranes of Polymer-Pendant Ru(bpy)3 2+ and Prussian Blue

A photosensing device was fabricated based on a photochemical electron transfer reaction. A graphite electrode was coated with bilayer membranes of polymer-pendant Ru(bpy)₃ ²⁺ and Prussian Blue (PB) to give a bilayer-coated device. It was irradiated with visible light in an aqueous electrolyte and the induced photocurrent was measured as a function of the applied potential. The excitation of PB was responsible for the anodic photocurrent, while the excitation of the Ru complex mainly induced the cathodic photocurrent by an electron-transfer mechanism.     

Electrochemical approach to evaluate the mechanism of photocatalytic water splitting on oxide photocatalysts

Photoelectrochemical measurements of TiO₂, NaTaO₃, and Cr or Sb doped TiO₂ and SrTiO₃ photocatalysts were carried out in H₂ and O₂ saturated electrolytes in order to evaluate the reverse reactions during water photolysis. The poor activity of TiO₂ as a result of reverse photoreactions of O₂ reduction and H₂ oxidation was revealed with the respective high cathodic and anodic photocurrents. The rise in the photocurrents at NaTaO₃ after La doping was in harmony with the doping-induced increase in the photocatalytic activity. NiO loading suppresses the O₂ photoreverse reactions, which declines photocatalytic activity, and/or promotes the photo-oxidation of water, because the O₂ photo-reduction current was scarcely observed near the flatband potential. Photocurrents of O₂ reduction and H₂ oxidation were observed under visible light for the Cr and Sb doped SrTiO₃ and TiO₂, respectively. These phenomena are in harmony with the previous reports on the photocatalysts examined with sacrificial reagents.