Efficient plasmonic water splitting by heteroepitaxial junction-induced faceting of gold nanoparticles on an anatase titanium(IV) oxide nanoplate array electrode.

Plasmonic photocatalysts represented by gold nanoparticle (NP)-loaded titanium(IV) oxide (Au/TiO2) can be promising solar-to-fuel converters by virtue of their response to visible-to-near infrared light. Hitherto, Au/rutile (R)-TiO2 has been recognized as exhibiting photocatalytic activity higher than that of Au/anatase (A)-TiO2. Herein, we demonstrate that the high potential of A-TiO2 as the Au NP support can be brought out through atomic level interface control. Faceting of Au NPs is induced by a heteroepitaxial junction on an A-TiO2(001) nanoplate array (Au/A-TiO2 NPLA). Photoexcitation towards the Au/A-TiO2 NPLA electrode generates current for the water oxidation reaction at λ < 900 nm with a maximum efficiency of 0.39% at λ = 600 nm, which is much larger than the values reported so far for the usual electrodes. The striking activity of the Au/A-TiO2 NPLA electrode was rationalized using a potential-dependent Fowler model. This study presented a novel approach for developing solar-driven electrodes for green and sustainable fuel production.

Near Infrared Light-to-Heat Conversion for Liquid-Phase Oxidation Reactions by Antimony-Doped Tin Oxide Nanocrystals.

Effective utilization of the sunlight for chemical reactions is pivotal for dealing with the growing energy and environmental issues. So far, much effort has been focused on the development of semiconductor photocatalysts responsive to UV and visible light. However, the near infrared and infrared (NIR-IR) light occupying ∼50 % of the solar energy has usually been wasted because of the low photon energy insufficient for the band gap excitation. Antimony doping into SnO2 (ATO) induces strong absorption due to the conduction band electrons in the NIR region. The absorbed light energy is eventually converted to heat via the interaction between hot electrons and phonons. This Concept highlights the photothermal effect of ATO nanocrystals (NCs) on liquid-phase oxidation reactions through the NIR light-to-heat conversion. Under NIR illumination even at an intensity of ∼0.5 sun, the reaction field temperature on the catalyst surface is raised 20-30 K above the bulk solution temperature, while the latter is maintained near the ambient temperature. In some reactions, this photothermal local heating engenders the enhancement of not only the catalytic activity and selectivity but also the regeneration of catalytically active sites. Further, the photocatalytic activity of semiconductors can be promoted. Finally, the conclusions and possible subjects in the future are summarized.

Crystallographic interface control of the plasmonic photocatalyst consisting of gold nanoparticles and titanium(iv) oxide.

A big question in the field of plasmonic photocatalysis is why a typical photocatalyst consisting of gold nanoparticles and rutile titanium(iv) oxide (Au/R-TiO2) usually exhibits activity much higher than that of Au/anatase TiO2 (Au/A-TiO2) under visible-light irradiation. Shedding light on the origin should present important guidelines for the material design of plasmonic photocatalysts. Au nanoparticles (NPs) were loaded on ordinary irregular-shaped TiO2 particles by the conventional deposition precipitation method. Transmission electron microscopy analyses for the Au/TiO2 particles ascertain that faceting of Au NPs is induced on R-TiO2 by using a domain-matching epitaxial junction with the orientation of (111)Au//(110)R-TiO2 , whereas non-faceted hemispherical Au NPs are exclusively formed on A-TiO2. The faceting probability of Au NPs (P f) on R-TiO2 increases with decreasing Au particle size (d Au) to reach 14% at d Au = 3.6 nm. A clear positive correlation between the photocatalytic activity and P f in several test reactions indicates that the heteroepitaxial junction-induced faceting of Au NPs is the principal factor for governing the plasmonic photocatalytic activity of Au/TiO2. In light of this finding, R-TiO2 nanorods with a high percentage (95%) of {110} facets were hydrothermally synthesized and used for the support of Au NPs. Consequently, the P f value increases to as much as 94% to enhance the photocatalytic activity with respect to that of Au/R-TiO2 with P f = 14% by factors of 2.2-4.4 depending on the type of reaction.

Photothermal Oxidation of Cinnamyl Alcohol with Hydrogen Peroxide Catalyzed by Gold Nanoparticle/Antimony-Doped Tin Oxide Nanocrystals.

Gold nanoparticles with different mean sizes were formed on antimony-doped tin oxide nanocrystals by the temperature-varied deposition-precipitation method (Au/ATO NCs). Au/ATO NCs possess strong absorption in the near-infrared region due to Drude excitation in addition to the localized surface plasmon resonance (LSPR) of AuNPs around 530 nm. Au/ATO NCs show thermally activated catalytic activity for the oxidation of cinnamyl alcohol to cinnamaldehyde by hydrogen peroxide. The catalytic activity increases with a decrease in the mean Au particle size (dAu ) at 5.3 nm≤dAu ≤8.2 nm. Light irradiation (λex >660 nm, ∼0.5 sun) of Au/ATO NCs increases the rate of reaction by more than twice with ∼95 % selectivity. Kinetic analyses indicated that the striking enhancement of the reaction stems from the rise in the temperature near the catalyst surface of ∼30 K due to the photothermal effect of the ATO NCs.

Noble Metal-Free Inorganic Photocatalyst Consisting of Antimony-Doped Tin Oxide Nanorod and Titanium oxide for Two-Electron Oxygen Reduction Reaction.

This study reports a noble metal-free robust inorganic photocatalyst for H2 O2 synthesis via two-electron oxygen reduction reaction (ORR). Antimony-doped tin oxide nanorods were heteroepitaxially grown from rutile TiO2 seed crystals with an orientation of (001)ATO //(001)TiO2 (ATO-NR//TiO2 ,//denotes heteroepitaxial junction) by a hydrothermal method. UV-light irradiation of ATO-NR//TiO2 particles stably and continuously produces H2 O2 from aerated aqueous solution of ethanol. Electrochemical measurements using rotating electrodes show that Sb-doping into SnO2 greatly enhances the electrocatalytic activity for two-electron ORR. The striking photocatalytic activity of ATO-NR//TiO2 stems from the effective charge separation, electrocatalytic activity for two-electron ORR, low catalytic activity for H2 O2 decomposition, and extraordinary robustness.

Highly Active and Renewable Catalytic Electrodes for Two-Electron Oxygen Reduction Reaction.

This study has shown that antimony-doped tin oxide (ATO) works as a robust "renewable catalyst" for the electrochemical synthesis of hydrogen peroxide (H2O2) from water and oxygen. Antimony doping into SnO2 gives rise to remarkable electrocatalytic activity for two-electron oxygen reduction reaction (2e--ORR) by water with a volcano-type relation between the activity and doping levels (xSb). Density functional theory simulations highlight the importance of an isolated Sb atom of ATO inducing the high activity and selectivity for 2e--ORR due to the effects of O2 adsorption enhancement, decrease in the activation energy, and lowering the adsorptivity of H2O2. Electrolysis by a normal three-electrode cell using ATO (xSb = 10.2 mol %) at -0.22 V (vs reversible hydrogen electrode) stably and continuously produces H2O2 with a turnover frequency of 6.6 s-1. This remarkable activity can be maintained even after removing the surface layer of ATO by argon-ion sputtering.

Bottom-up formation of gold truncated pyramids smaller than 10 nm on SrTiO3 nanocubes: an application for plasmonic water oxidation.

An atomically commensurate interface gives rise to Au truncated pyramids < 10 nm on single-crystalline SrTiO3 nanocubes (NCs) in a simple deposition-precipitation process without a surface modifier, and the resulting hybrid nanocrystals exhibit a high level of photocatalytic activity for a plasmonic oxygen evolution reaction at light wavelengths (λex) ≤ 1200 nm.

Ammonium ion-promoted electrochemical production of synthetic gas from water and carbon dioxide on a fluorine-doped tin oxide electrode.

In situ generated Sn nanoparticles on fluorine-doped tin oxide act as an electrocatalyst for the CO2 reduction reaction to efficiently and stably produce synthetic gas from water and carbon dioxide with the reaction rate drastically enhanced by the addition of ammonium ions.

Au-Ag alloy nanoparticle-incorporated AgBr plasmonic photocatalyst.

A solid-phase photochemical method produces Au-Ag alloy nanoparticles (NPs) with a sharp size distribution and varying composition in AgBr crystals (Au-Ag@AgBr). These features render Au-Ag@AgBr promising as a material for the plasmonic photocatalyst further to provide a possibility of elucidating the action mechanism due to the optical tunability. This study shows that the visible-light activity of Au-Ag@AgBr for degradation of model water pollutant is very sensitive to the alloy composition with a maximum at the mole percent of Au to all Ag in AgBr (y) = 0.012 mol%. Clear positive correlation is observed between the photocatalytic activity and the quality factor defined as the ratio of the peak energy to the full width at half maximum of the localized surface plasmon resonance band. This finding indicates that Au-Ag@AgBr works as a local electromagnetic field enhancement-type plasmonic photocatalyst in which the Au-Ag NPs mainly promotes the charge separation. This conclusion was further supported by the kinetic analysis of the light intensity-dependence of external quantum yield.

Highly Efficient and Selective Oxidation of Ethanol to Acetaldehyde by a Hybrid Photocatalyst Consisting of SnO2 Nanorod and Rutile TiO2 with Heteroepitaxial Junction.

Single-crystal SnO2 nanorods were grown on rutile TiO2 with a heteroepitaxial relation of SnO2 {001}/TiO2 {001} (SnO2 -NR#TiO2 ) by a hydrothermal reaction. Resulting compressive lattice strain in the SnO2 -NR near the interface induces a continuous increase in the a-axis length extending over 60 nm to relax towards the [001] direction from the root to the tip. UV-light irradiation of the robust SnO2 -NR#TiO2 stably progresses the selective oxidation of ethanol to acetaldehyde with an external quantum yield of 25.6 % at excitation wavelength (λex )=365 nm under ambient temperature and pressure. Spectroscopic analyses and density functional theory simulation results suggested that the extremely high photocatalytic activity stems from the smooth interfacial electron transfer from TiO2 to SnO2 -NR through the high-quality junction and subsequent efficient charge separation due to the lattice strain-induced unidirectional potential gradient of the conduction band minimum in the SnO2 -NR.

Red-Light-Driven Water Splitting by Au(Core)-CdS(Shell) Half-Cut Nanoegg with Heteroepitaxial Junction.

A key material for artificial photosynthesis including water splitting is heteronanostructured (HNS) photocatalysts. The photocatalytic activity depends on the geometry and dimension, and the quality of junctions between the components. Here we present a half-cut Au(core)-CdS(shell) (HC-Au@CdS) nanoegg as a new HNS plasmonic photocatalyst for water splitting. UV-light irradiation of Au nanoparticle (NP)-loaded ZnO (Au/ZnO) at 50 °C induces the selective deposition of hexagonal CdS on the Au surface of Au/ZnO with an epitaxial (EPI) relation of CdS{0001}/Au{111}. The subsequent selective dissolution of the ZnO support at room temperature yields HC-Au@CdS with the Au NP size and EPI junction (#) retained. Red-light irradiation (λex = 640 nm) of HC-Au@#CdS gives rise to continuous stoichiometric water splitting with an unprecedentedly high external quantum yield of 0.24%.

Rapid Removal and Mineralization of Bisphenol A by Heterosupramolecular Plasmonic Photocatalyst Consisting of Gold Nanoparticle-Loaded Titanium(IV) Oxide and Surfactant Admicelle.

The establishment of technology for rapid and complete removal and mineralization of harmful phenolic compounds from water is of great importance for environmental conservation. Visible-light irradiation (λ > 430 nm, light intensity integrated from 420 to 485 nm = 6.0 mW cm-2) of Au nanoparticle (NP)-loaded TiO2 (Au/TiO2) in dilute aqueous solutions of bisphenol A (BPA) and p-cresol (PC) causes degradation of the phenols. The addition of trimethylstearylammonium chloride (C18TAC) enhances the adsorption of BPA on Au/TiO2 to greatly increase the rate of reaction. Consequently, 10 µM phenols are completely removed from the solutions within 2.5 h irradiation, and prolonging irradiation time to 24 h quantitatively oxidizes BPA to CO2. Dynamic light scattering ζ-potential measurements indicate that a C18TAC bilayer or admicelle is formed on the Au/TiO2 particle surface at C18TAC concentration >50 µM. The action spectrum for reaction shows that this reaction is driven by the Au NP localized surface plasmon resonance excitation-induced interfacial electron transfer from Au to TiO2. We propose a possible reaction scheme on the basis of the experimental results including intermediate analysis.

Photocatalytic Synthesis of CdS(core)-CdSe(shell) Quantum Dots with a Heteroepitaxial Junction on TiO2 : Photoelectrochemical Hydrogen Generation from Water.

A major challenge in chemistry for the synthesis of hetero-nanostructures is to build up atomically commensurate interfaces for smooth interfacial charge transfer. Photodeposition of CdSe on a CdS-preloaded mesoporous TiO2 nanocrystalline film yields CdS(core)-CdSe(shell) quantum dots (CdS@CdSe/mp-TiO2 ) with a heteroepitaxial nanojunction at 298 K. Two-electrode quantum-dot-sensitized photoelectrochemical (QD-SPEC) cells with the structure photoanode |0.25 M Na2 S, 0.35 M Na2 SO3 (solvent=water)| cathode were fabricated. The CdS@CdSe QD-SPEC cell affords a solar-to-current efficiency (STCE) of 0.03 % without external bias under illumination of simulated sunlight (λ>430 nm, AM 1.5, one sun). By applying 0.1 V between the electrodes, the STCE increases up to 0.048 %, far surpassing the CdS/mp-TiO2 and CdSe/mp-TiO2 photoanode cells. The CdS-CdSe interfacial analysis by high-resolution transmission electron microscopy and the band energy analysis taking the size quantization and the electrolyte effect indicate that the excellent performance of the CdS@CdSe/mp-TiO2 photoanode originates from the effective charge separation due to the cascade-like band edge alignment and the heteroepitaxial junction between CdS and CdSe QDs. In addition, high surface coverage of TiO2 with QDs can contribute to the reduction in the loss of the electron transport from TiO2 to the electron collecting electrode.

Gold(Core)-Lead(Shell) Nanoparticle-Loaded Titanium(IV) Oxide Prepared by Underpotential Photodeposition: Plasmonic Water Oxidation.

Underpotential photodeposition of Pb yields an ultrathin shell layer on the Au(111) surface of Au nanoparticle(NP)-loaded TiO2 (Au/TiO2 ) with heteroepitaxial nanojunctions. The localized surface plasmon resonance of Au/TiO2 undergoes no damping with the Pb-shell formation, and the Pb shell offers resistance to aerobic oxidation. Mesoporous films comprising the Au(core)-Pb(shell) NP-loaded TiO2 and unmodified Au/TiO2 were formed on fluorine-doped tin oxide (FTO) electrode. Using them as the photoanode, photoelectrochemical cells were fabricated, and the photocurrent was measured under illumination of simulated sunlight. The photocurrent for water splitting is dramatically enhanced by the Pb-shell formation. The photoelectrochemical measurements of the hot-electron lifetime and density functional theory calculations for model clusters indicate that the Pb-shell effect originates from the charge separation enhancement.

Electron Filtering by an Intervening ZnS Thin Film in the Gold Nanoparticle-Loaded CdS Plasmonic Photocatalyst.

In the gold nanoparticle (Au NP)-loaded CdS film on fluorine-doped tin oxide electrode (Au/CdS/FTO), the localized plasmonic resonance excitation-induced electron injection from Au NP to CdS has been proven by photoelectrochemical measurements. Formation of ZnS thin films between the Au NP and CdS film leads to a drastic increase of the photocurrent under visible-light irradiation (λ > 610 nm) in a 0.1 M NaClO4 aqueous electrolyte solution due to the electron filtering effect. The photocurrent strongly depends on the thickness of the ZnS film, and the maximum value is obtained at a thickness as thin as 2.1 nm. Furthermore, the ZnS overlayer significantly stabilizes the photocurrent of the CdS/FTO electrode in a polysulfide/sulfide electrolyte solution even under the excitation of CdS (λ > 430 nm). This work presents important information about the design for new plasmonic photocatalysts consisting of plasmonic metal NPs and chalcogenide semiconductors with high conduction band edge.

Reaction Mechanism of the Multiple-Electron Oxygen Reduction Reaction on the Surfaces of Gold and Platinum Nanoparticles Loaded on Titanium(IV) Oxide.

Au and Pt nanoparticles with varying mean particle size and comparable loading amounts were loaded on the surface of TiO2 (Au/TiO2 and Pt/TiO2, respectively). The photocatalytic activities of Au/TiO2 and Pt/TiO2 for the oxygen reduction reaction (ORR) in an aerated aqueous solution containing 4% ethanol were compared under ultraviolet-light irradiation at 298 K. The initial rate of H2O2 generation (or H2O2 formation rate) in the Au/TiO2 system is much greater than that in the Pt/TiO2 system regardless of the metal particle size. To clarify the origin for the striking difference in the activity, the photocatalytic ORR on the model slabs (M28/(TiO2)32 and M50/(TiO2)96, M = Au and Pt) was simulated by density functional theory (DFT) calculations taking the solvation effect into consideration. The DFT calculations clearly show that regardless of the cluster size, H2O2 formation more easily occurs structurally and energetically for the Au/TiO2 system, whereas H2O is generated with the O-O bond cleavage in the Pt/TiO2 system.

Gold-Nanoparticle-Loaded Carbonate-Modified Titanium(IV) Oxide Surface: Visible-Light-Driven Formation of Hydrogen Peroxide from Oxygen.

Gold nanoparticle-loaded rutile TiO2 with a bimodal size distribution around 10.6 nm and 2.3 nm (BM-Au/TiO2 ) was prepared by the deposition precipitation and chemical reduction (DP-CR) technique. Visible-light irradiation (λ>430 nm) of the BM-Au/TiO2 plasmonic photocatalyst yields 35 µm H2 O2 in aerated pure water at irradiation time (tp )=1 h, and the H2 O2 concentration increases to 640±60 µm by the addition of 4 % HCOOH as a sacrificing electron donor. Further, a carbonate-modified surface BM-Au/TiO2 (BM-Au/TiO2 -CO3 (2-) ) generates a millimolar level of H2 O2 at tp =1 h with a quantum efficiency (Φ) of 5.4 % at λ=530 nm under the same conditions. The recycle experiments confirmed the stable performance of BM-Au/TiO2 .

Size-Dependence of the Activity of Gold Nanoparticle-Loaded Titanium(IV) Oxide Plasmonic Photocatalyst for Water Oxidation.

Mesoporous TiO2 nanocrystalline film was formed on fluorine-doped tin oxide electrode (TiO2 /FTO) and gold nanoparticles (NPs) of different sizes were loaded onto the surface with the loading amount kept constant (Au/TiO2 /FTO). Visible-light irradiation (λ>430 nm) of the Au/TiO2 /FTO photoanode in a photoelectrochemical cell with the structure of photoanode|0.1 m NaClO4 aqueous solution|Ag/AgCl (reference electrode)|glassy carbon (cathode) leads to the oxidation of water to oxygen (O2 ). We show that the visible-light activity of the Au/TiO2 /FTO anode increases with a decrease in Au particle size (d) at 2.9≤d≤11.9 nm due to the enhancement of the charge separation and increasing photoelectrocatalytic activity.

Room-temperature selective oxidation of 2-naphthol to BINOL using a Au/SrTiO3-H2O2 catalytic system.

Gold nanoparticle-loaded SrTiO3 (Au/SrTiO3) effectively catalyzes the selective C-C bond formation between 2-naphthol molecules to yield BINOL in the presence of H2O2, and the catalytic activity increases with decreasing Au particle size (d) in the range 2.6 ≤ d ≤ 8.9 nm.

Mechanism of the Multiple-Electron Oxygen Reduction Reaction in the Presence of the Binuclear Cu(acac)2 Complex.

The potential of the electron for the serial oxygen reduction reaction is calculated by DFT in an aqueous solution in the presence and absence of Cu(acac)2 complex. The study provides interesting information about the rational design of complex-semiconductor hybrid photocatalysts and cathodes for polymer electrolyte membrane fuel cells.