描述
开 本: 16开包 装: 精装是否套装: 否国际标准书号ISBN: 9787030346582丛书名: 纳米科学与技术大全；3
此书共有5卷，由国际专家组写作而成，内容涉及材料科学、物理学、生命科学、化学等领域；每篇文章的写作都兼具学术性、批判性与可读性，内容深入浅出，前后呼应，是一本跨学科领域研究者们不可或缺的有价值的参考资料。
纳米科学与技术大全3：纳米结构表面（导读版）适合化学、物理学、材料科学、生物学、工程学等领域的研究生及科研人员参考，对于纳米研究实验室、学术机构，涉及纳米和生物材料、材料科学等方面的专业组织、公司、企业等也是不可多得的参考资料。
3.01.1 Preparation of Gold Catalysts
3.01.2 Catalytic CO Oxidation
3.01.2.1 Effect of Preparation Method
3.01.2.2 Effect of Particle Size and Support
3.01.2.3 Reaction Mechanism
3.01.3 Catalytic Oxidation of Organic Compounds
3.01.4 Catalytic Reduction of Organic Compounds
3.01.5 Gold/Semiconductor Photocatalysts
3.01.6 Gold Photocatalysts
3.01.7 Conclusion
References
3.02 纳米结构的受控组装
3.02.1 Introduction
3.02.2 Fundamentals of Directing Nanoscale Assembly at Surfaces
3.02.2.1 Noncovalent Interactions between Molecules
3.02.2.1.1 Hydrogen bonding
3.02.2.1.2 Metal-organic coordination
3.02.2.2 Molecule-Surface Interactions
3.02.2.2.1 Common surfaces for studies of molecular assembly
3.02.3 Patterned Bonding between Molecules and Surfaces
3.02.3.1 Chemical Chain Reactions
3.02.3.2 Selectively Patterned Surfaces
3.02.4 Guiding Supramolecular Assembly
3.02.4.1 Hydrogen-Bonded Architectures
3.02.4.1.1 Overview
3.02.4.1.2 Basic geometries in hydrogen-bonded structures
3.02.4.2 Metal-Organic Coordination
3.02.5 Templated Physisorption: Molecular Organization via Self-Assembled Inclusion Networks
3.02.5.1 Designing Host?Guest Networks at Surfaces
3.02.5.2 Patterning Arrays of Fullerenes
3.02.5.3 Patterning Other Molecules
3.02.6 Covalently Bonded Structures:Surface-Confined Polymerization
3.02.6.1 Polymer Lines
3.02.6.1.1 UV light and STM tip-induced polymerization of diacetylenes
3.02.6.1.2 Electrochemical formation of polythiophenes
3.02.6.1.3 Addition polymerization of carbenes
3.02.6.1.4 Polyphenylene lines via Ulmann dehalogenation
3.02.6.2 Two-Dimensional Polymers
3.02.6.2.1 Porphyrin networks
3.02.6.2.2 Condensation polymerization via dehydration
3.02.7 Conclusions and Outlook
References
3.03 有序纳米颗粒超结构的生物介导组装
3.03.1 Introduction
3.03.2 Synthesis and Biofunctionalization of Nanoparticles
3.03.2.1 Wet Chemistry Synthesis of Nanoparticles
3.03.2.2 Functionalization of Nanoparticles with Biomolecules
3.03.2.2.1 N:1 functionalized nanoparticles
3.03.2.2.2 1:1 functionalized nanoparticles
3.03.2.2.3 Anisotropically functionalized nanoparticles
3.03.3 Interactions between Biofunctionalized Nanoparticles
3.03.3.1 Specific Chemical Bonding Interactions
3.03.3.2 Nonspecific Physical Bonding Interactions
3.03.4 Assembly of Ordered Nanoparticle Superstructures
3.03.4.1 Nanoparticle Molecules
3.03.4.2 Nanoparticle Superlattices
3.03.4.2.1 Programmable DNA-based assembly
3.03.4.2.2 Nonspecific DNA-based assembly
3.03.4.3 Assembly of Nanoparticles by Other Biomolecules
3.03.5 Characterization
3.03.5.1 Microscopy
3.03.5.2 Gel Electrophoresis
3.03.5.3 Optical Spectroscopy
3.03.5.4 Small-Angle X-ray Scattering
3.03.5.5 Atomic Force Microscopy
3.03.6 Summary and Outlook
References
3.04 表面上的手性分子
3.04.1 Introduction
3.04.1.1 Definition of Chirality
3.04.1.2 Nomenclature of Chirality-the R,S Convention
3.04.2 Surface Chirality following Molecular Adsorption
3.04.2.1 Achiral Molecules on Achiral Surfaces
3.04.2.2 Chiral Molecules on Achiral Surfaces
3.04.2.2.1 Adsorption without substrate modification
3.04.2.2.2 Chiral substrate modification
3.04.2.3 Chiral Amplification and Recognition
3.04.2.3.1 Chiral amplification in two dimensions
3.04.2.3.2 Chiral recognition
3.04.2.4 Chiral Molecules on Chiral Surfaces
3.04.2.4.1 Chiral substrate geometries
3.04.2.4.2 Adsorption of chiral molecules on chiral surfaces
3.04.3 Kinetics of Desorption Processes
3.04.3.1 Achiral Surfaces
3.04.3.2 Chiral Surfaces
3.04.3.3 Effect of Chiral Templating/Modification on Achiral Surfaces
3.04.4 Chiral Heterogeneous Catalysis
3.04.5 Conclusions
References
3.05 金属纳米结构光学
3.05.1 Introduction
3.05.1.1 Scope of the Chapter
3.05.2 Surface Plasmon Polaritonic Crystals
3.05.2.1 Optical Properties
3.05.2.1.1 Smooth film surface plasmon polaritons
3.05.2.1.2 Surface plasmon polaritonic crystals
3.05.2.1.3 Plasmonic cavities as an SPP crystal basis
3.05.2.1.4 Mechanisms of enhanced optical transmission(EOT)through SPCs
3.05.2.2 Sample Fabrication and Experimental Configuration
3.05.2.3 Polarization Properties of SPCs
3.05.2.4 Dynamic Control of the Optical Properties of Plasmonic Crystals
3.05.2.4.1 Electronically controlled surface plasmon dispersion and optical transmission through metallic hole arrays using liquid crystal
3.05.2.4.2 Magneto-optical control of surface plasmon polariton Bloch modes
3.05.2.4.3 Light-controlled optical transmission through nonlinear surface plasmonic crystals
3.05.3 Metallic Nanorod Arrays
3.05.3.1 Plasmonic Nanorod Assembly
3.05.3.2 Optical Properties
3.05.3.2.1 Eigenmodes of nanorod arrays:spectral properties
3.05.3.2.2 Eigenmodes of nanorod arrays:spatial field distribution
3.05.3.3 Assembly of Core-Shell Nanorods
3.05.3.4 Applications
3.05.3.4.1 Tunable exciton-plasmon coupling strength in J-aggregate hybridized Au nanorod assemblies
3.05.3.4.2 Refractive index sensing
3.05.3.4.3 Electrically switchable nonreciprocal transmission
3.05.3.4.4 Light-controlled transmission through nanorod arrays embedded in a nonlinear dielectric
3.05.4 Conclusion
References
3.06 表面纳米光子学理论
3.06.1 Introduction
3.06.2 Background
3.06.2.1 Basic Electromagnetic Theory
3.06.2.2 Metal Films,Apertures,and Periodic Structures
3.06.3 Theoretical and Computational Methods
3.06.3.1 The Finite-Difference Time-Domain(FDTD)Method
3.06.3.2 The Rigorous Coupled-Wave Analysis(RCWA)
3.06.3.3 The Modal Expansion Method
3.06.4 Isolated Apertures in Metal Films
3.06.4.1 Isolated Slits
3.06.4.2 Isolated Holes
3.06.5 Periodic Nanostructured Metal Films
3.06.5.1 Hole Arrays
3.06.5.2 Pillar Arrays
3.06.5.3 Other Periodic Systems
3.06.6 Summary and Outlook
References
3.07 高性能LED的构建及优化:光子晶体及光子带隙结构的光提取及发射控制
3.07.1 Introduction
3.07.2 Basic Background
3.07.3 Large Band-Gap Nitride LEDs for the Green-Blue to Ultraviolet Spectral Range
3.07.4 Contact Formation
3.07.5 PhC Structures:The Royal Road to Enhanced Light Extraction from LEDs
3.07.6 The Grating Output Coupler:A Simplified Approach to PhC Light Extraction
3.07.6.1 Analysis of the Waveguide Grating Coupler
3.07.7 White LEDs
3.07.8 Conclusions
References
3.08 液晶纳米结构光学超材料
3.08.1 Introduction
3.08.1.1 Electromagnetic Fundamentals of Metamaterials
3.08.1.2 Complex Refractive Index of Lossy Metamaterials
3.08.1.3 Refractive Index of a General Medium
3.08.1.4 Phase-Space **** Description of the Complex Refractive Index
3.08.1.5 Reflection and Transmission Properties at the Air-Metamaterial Interface
3.08.2 Review of Liquid-Crystal Optical Physics
3.08.2.1 General Overview
3.08.2.2 An Example of Direct Current+Optical Field Tuning of LC Index
3.08.3 LC Nanodispersed Metamaterials
3.08.3.1 Dispersion of Nanoparticles in Aligned NLCs
3.08.3.1.1 Solid silver nanospheres
3.08.3.1.2 Silver-coated silica nanoshells
3.08.3.1.3 Polaritonic-silver core-shell spheres
3.08.3.2 Optical Properties of Metamaterials:Au versus Ag
3.08.3.3 Enhancement and Control of Dielectric Anisotropy and Absorption
3.08.3.4 Shifting the Frequency Response
3.08.3.5 Reduction of Losses with a Gain Medium
3.08.3.6 Some Experimental Results
3.08.4 Periodic Nanostructures Containing LCs
3.08.4.1 LC-Based Tunable FSSs
3.08.4.2 LC-Infiltrated PCs
3.08.5 Conclusion
References
3.09 纳米结构与表面增强拉曼光谱
3.09.1 Introduction
3.09.2 Localized Surface Plasmon Resonance Spectroscopy
3.09.2.1 Theory
3.09.2.1.1 Solution to Maxwell?s equations:Mie theory
3.09.2.1.2 Relationship between the dielectric function and nanoparticle extinction
3.09.2.1.3 Electric-field decay
3.09.2.1.4 Discrete dipole approximation for nonspherical particles
3.09.2.1.5 Experimental methods
3.09.2.2 Nanofabrication
3.09.2.2.1 Chemical syntheses
3.09.2.2.2 Laser ablation
3.09.2.2.3 Nanostructured films
3.09.2.2.4 Lithographic techniques
3.09.2.3 Characteristics of the LSPR
3.09.2.3.1 Dependence on nanoparticle size and shape
3.09.2.3.2 Sensitivity to external environment
3.09.2.3.3 Distance dependence
3.09.2.3.4 Coupling among nanoparticles
3.09.2.4 Functionalization and Stabilization of Nanoparticles
3.09.2.4.1 Thermal stability
3.09.2.4.2 Stability to laser excitation
3.09.2.4.3 Solvent stability
3.09.3 Surface-Enhanced Raman Spectroscopy
3.09.3.1 Background
3.09.3.1.1 Chemical enhancement mechanism of SERS
3.09.3.1.2 Electromagnetic enhancement mechanism of SERS
3.09.3.1.3 Calculating SERS enhancement factors
3.09.3.2 Experimental Consequences of the EM Mechansim
3.09.3.2.1 Distance dependence of SERS
3.09.3.2.2 Excitation-wavelength dependence of SERS
3.09.3.2.3 Excitation-wavelength dependence of SERRS
3.09.3.3 Single-Molecule SERS
3.09.3.3.1 A frequency domain existence proof of SMSERS
3.09.3.3.2 Surface dynamics in SMSERS
3.09.3.3.3 Structure and enhancement factors of SMSERS hot spots
3.09.3.3.4 Excitation-wavelength dependence of SMSERS
3.09.3.4 SERS Sensing Applications
3.09.3.4.1 In vivo glucose sensing by SERS
3.09.3.4.2 Application of SERS to art conservation
3.09.3.4.3 SERS for chemical and biological warfare agent detection
3.09.4 Future Directions
3.09.4.1 New Plasmonic Materials for SERS
3.09.4.2 Novel Nanostructures
3.09.4.3 Tip-Enhanced Raman Spectroscopy
3.09.5 Conclusion
References
3.10 纳米结构超导体的有效磁通钉扎
3.10.1 Introduction
3.10.2 Vortex Physics
3.10.3 Artificial Vortex Pinning:Defect Classification
3.10.4 APCs in YBCO Thin Films and CCs
3.10.4.1 Vacuum Deposition Methodologies
3.10.4.1.1 Thin-film growth by PLD:Microstructure and pinning
3.10.4.1.2 APC in YBCO films: Surface decoration by nanodots
3.10.4.1.3 APC in YBCO films and conductors:Self-organized nanocomposites
3.10.5 Chemical Deposition Methodologies
3.10.5.1 Thin-Film Growth by Chemical Routes:Microstructure and Pinning
3.10.5.2 APCs in YBCO Films: Nanodots Decoration
3.10.5.3 APCs in YBCO Films and Conductors:Nanocomposites
3.10.6 Conclusions and Future Perspectives
References
3.11 纳米结构的倍频效应
3.11.1 Introduction
3.11.2 Fundamentals of Second Harmonic Generation
3.11.3 Particles from Noncentrosymmetrical Material
3.11.3.1 General Theory
3.11.3.2 Volume Contribution
3.11.3.3 Surface Contribution
3.11.3.4 Magnetic Particles
3.11.4 Particles from a Centrosymmetrical Material
3.11.4.1 Particles with Noncentrosymmetrical Shape
3.11.4.2 Particles with Centosymmetrical Shape
3.11.5 Metallic Particles
3.11.5.1 Theoretical Approach
3.11.5.2 Origin of the SH Response
3.11.5.3 Resonance Enhancement
3.11.5.4 Aggregation
3.11.5.5 Other Metallic Nanostructures
3.11.6 Arrays of Metallic Particles
3.11.6.1 Regular Arrays of Metallic Nanoparticles
3.11.6.2 Random Metallic Structures
3.11.6.3 Conclusions
References
3.12 纳米结构表面的摩擦学
3.12.1 Introduction
3.12.2 Thin Lubricating Films with Ordered Molecular Structures
3.12.2.1 Challenges to the Lubrication in Microscopic Systems
3.12.2.2. Preparation of LB Films and SAMs
3.12.2.3 Tribological Properties of Ordered Molecular Films
3.12.2.3.1 Dependence on load and velocity
3.12.2.3.2 Effects of chain length and functional group
3.12.2.3.3 Improvements of wear resistance
3.12.2.3.4 Applications and patterned molecule films
3.12.2.4 Thin-Film Lubrication and Confined Liquids
3.12.2.4.1 Rheological properties of confined liquids
3.12.2.4.2 Thin-film lubrication
3.12.3 Tribology of Biological Systems
3.12.4 Tribology of Patterned or Textured Surfaces
3.12.4.1 Surface Texturing and Tribological Applications
3.12.4.1.1 Techniques of LST
3.12.4.1.2 Study on tribological properties of textured surfaces
3.12.4.1.3 Key applications
3.12.4.2 Stiction and Adhesion on Textured Surfaces
3.12.4.2.1 Stiction and adhesion at head/disk interfaces
3.12.4.2.2 Surface texture in MEMS applications
3.12.4.2.3 Understanding adhesion on structured surfaces
3.12.4.3 Wettability on Nanostructured Surfaces
3.12.4.3.1 Wettability and effects of surface roughness
3.12.4.3.2 Creating superhydrophobicity through surface structure
3.12.4.3.3 Applications to fluid drag reduction
3.12.5 Tribology of Nanocomposites
3.12.5.1 Tribology of Polymer-Based Nanocomposites
3.12.5.2 Tribology of Superhard Nanocomposite Coatings
3.12.5.3 Self-Lubricating Nanocomposite Coatings
References
3.13 纳米摩擦学及润滑材料的纳米级涂层
3.13.1 Introduction
3.13.2 Carbon-Based Nanolubricants
3.13.3 Lubricant Additives in Boundary Lubrication
3.13.4 Carbonaceous Films
3.13.4.1 C60 Film
3.13.4.2 CNT Film
3.13.4.3 Tribology of OLC
3.13.4.3.1 Isolated OLC
3.13.4.3.2 OLC film
3.13.5 Summary and Conclusions
References
3.14 碳纳米材料及无机纳米材料的功能化和溶液化
3.14.1 Introduction
3.14.2 Functionalization and Solubilization of Carbon Nanostructures
3.14.2.1 Fullerene(C60)
3.14.2.2 Nanodiamond
3.14.2.2.1 Covalent functionalization
3.14.2.2.2 Noncovalent functionalization
3.14.2.3 Carbon Nano-Onions
3.14.2.4 Carbon Nanotubes
3.14.2.4.1 Covalent functionalization
3.14.2.4.2 Defect functionalization
3.14.2.4.3 Noncovalent functionalization
3.14.2.4.4 Endrohedral filling
3.14.2.5 Graphene
3.14.2.5.1 Covalent functionalization
3.14.2.5.2 Noncovalent functionalization
3.14.2.6 Functionalization of Various Carbon Nanostructures:A Comparison
3.14.2.7 Comparison of Functionalization Methods
3.14.3 Functionalization and Solubilization of Inorganic Nanostructures
3.14.3.1 Metal Nanostructures
3.14.3.1.1 In situ functionalization
3.14.3.1.2 Postsynthesis functionalization
3.14.3.1.3 Functionalization through biphasic synthesis
3.14.3.1.4 Functionalization using fluorous chemistry
3.14.3.1.5 Functionalization using click chemistry
3.14.3.1.6 Other functionalization methods
3.14.3.2 Metal-Oxide Nanostructures
3.14.3.2.1 In situ functionalization
3.14.3.2.2 Postsynthesis functionalization
3.14.3.2.3 Silane-induced functionalization
3.14.3.2.4 Functionalization using polymers
3.14.3.3 Metal-Chalcogenide Nanostructures
3.14.3.3.1 In situ functionalization
3.14.3.3.2 Postsynthesis functionalization
3.14.3.3.3 Functionalization using fluorous chemistry
3.14.3.3.4 Silane-induced functionalization
3.14.3.3.5 Functionalization using polymers
3.14.3.4 Metal Nitride Nanostructures
3.14.4 Conclusions
References
X Chen and H Zhu, Queensland University of Technology, Brisbane, QLD, Australia a 2011 Elsevier B.V. All rights reserved.
3.01.1 Preparation of Gold Catalysts 1
3.01.2 Catalytic CO Oxidation 3
3.01.2.1 Effect of Preparation Method 3
3.01.2.2 Effect of Particle Size and Support 3
3.01.2.3 Reaction Mechanism 4
3.01.3 Catalytic Oxidation of Organic Compounds 5
3.01.4 Catalytic Reduction of Organic Compounds 7
3.01.5 Gold/Semiconductor Photocatalysts 7
3.01.6 Gold Photocatalysts 8
3.01.7 Conclusion 8
References 8
Inthepast,goldwasregardedascatalyticallyinac-tive.ThischangedafterthediscoverythatgoldisanexcellentcatalystforCOoxidationatlowtempera-ture[1].Nowtheresearchonthecatalyticactivityofgoldhasbecomeahottopic.Thischapterfocusesonrecentprogressinthefield,especiallyintheprepara-tionofhighlydispersedgoldcatalysts,thereactionmechanismforCOoxidation,andapplicationsincatalysisandevenphotocatalysis.
3.01.1 Preparation of Gold Catalysts
Duetothefactthatthemeltingpointofgoldismuchlower(1336K)thanotherelementsinthePtgroup(Pd:1823K;Pt:2042K),goldnanoparticleswerenotashighlydispersedinthesupportasothersupportednoblemetals.Theconventionalimpregnationmethod(IMP)isnotsuitabletosynthesizehighlydispersedgoldcatalysts.IntheIMPmethod,ametal-oxidesupportisimmersedinanaqueoussolu-tionofHAuCl4andthenthesolutionisheatedtodispersegoldcrystallitesoverthesupportsurfaces.ThedriedprecursoriscalcinedinairorreducedinanH2stream.However,duringtheheatingprocesstheinteractionbetweenHAuCl4crystallitesandthemetaloxidesupportisweak,andthesizeofAuparticlesobtainedbyIMPmethodislargerthan30nmasshowninFigure1[2].Inordertoproducegoldwithdiametersbelow10nmonavarietyofmetaloxidesupports,severalnewtechniquesmentionedbelowhavebeendeveloped.
1.
Coprecipitation[3]:AnaqueoussolutionofHAuC14andametalnitrateisaddedintoanaqueoussolutionofNa2CO3.Thenhydroxideorcarbonateprecipitateisobtained.Theprecipitationiswashed,dried,andfinallycalcinedinairtoobtainthecatalystsample.
2.
Co-sputtering[4]:Goldnanoparticlesaresimultaneouslysputter-depositedonasubstratetoproduceathinfilminanoxygenatmosphere,andthenthefilmisannealedinair.
3.
Chemicalvapordeposition(CVD)[5]:Thevaporofanorganicgoldcompoundisintroducedontoanevacuatedmetaloxidesupport.Theadsorbedgoldcompoundispyrolyzedinairtoobtainsmallgoldparticles.Thismethodcanbeappliedtoawidevarietyofmetaloxides,whiletheliquid-phasemethodsareusuallynotvalidtoacidicmetaloxideslikeSiO2.
4.
Deposition-precipitation(DP)[6]:Figure2showsthedetailedprocedureproposedbyHaruta[7].ThepHofanaqueoussolutionofHAuC14isadjustedtoafixedvalueintherangeof6?10withdiluteNaOHsolutionduetotheamphotericpropertiesofAu(OH)3.Duringtheprocesstheconcentration(around103M)andtemperatures(323?363K)ofthesolutionneedtobecontrolledcarefully.Thenthepartiallyhydrolyzedspeciescanreactonasupportsurface,resultinginthedepositionofAu(OH)3onthesurfacesofsupportedmetaloxides.
TheremarkableinfluenceofthepHonthepar-ticlesizeofAupreparedbyDPisshowninFigure3whenTiO2isthesupport[8].AbovepH6mainspeciesofAuinsolutionistransformedfrom
Figure2Flowchartoftheprocedureinthedeposition-precipitation(DP)method.FromHarutaM(2002)CatalysisofgoldNanoparticlesdepositedonmetaloxides.Cattech6:102.
AuC14 ? toAu(OH)nCl4n(n.13)andthemeandiameterofgoldparticlesinthecalcinedcatalystsbecomessmallerthan4nm[9].Oneadvantageofthemethodisthatthemetaloxidesupportimmersedinthesolutioncanbeinanyform,suchasapowder,bead,honeycomb,orthinfilm.However,oneoftheconstraintsofDPisthatitsapplicationcanbeonlytometaloxideswhoseisoelectricpoints(IEPs)areabove5.GoldhydroxidecannotbedepositedonoxideswithlowIEP,suchasSiO2,andWO3.
Diameter of Au particles (nm)
20
15
10
5
0
2 4 6 81012 pH of Au solution
Figure3ThemeandiameterofAuparticlesasafunctionofthepHofHAuCl4solutionfortheAu/TiO2catalystspreparedbythedeposition-precipitation(DP)method.AucontentintheHAuCl4solutioncorrespondedto13wt%withrespecttoTiO2.Calcinationwasconductedinairat400C. From Haruta M, Ueda A, Tsubota S, and Sanchez RM (1996) Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts. Catalysis Today 29: 443.
5. NaBH4reduction[10]:Zhongetal.[10]reportedanefficientmethodforthepreparationofhighlydis-persedsupportedAucatalysts.First,thesupportisplacedin10mldeionizedwater,andthenHAuCl4andlysineareadded.ThepHofthesuspensionwasadjustedto5?6withNaOH.ThesuspensionwassubjectedtosonicationinordertofacilitatedispersionanddepositionoftheAucolloidsontothecatalystsupport,andduringthesonicationfreshlypreparedNaBH4wasinjectedinstantly.Thesuspensionimme-diatelyturneddarkincolorandwaswashedwithdeionizedwaterbyusingacentrifuge.MostoftheobtainedAuparticlesarebelow5nmandwerehighlydispersed.
Thefivemethodsabovecanproducehemisphericalgoldparticleswhicharestronglyattachedtometaloxidesupportsattheirflatplanesandmorestablethansphericalparticlesatrelativelyhightemperature.Thiscanbeascribedtostrongeraffi-nitywithmetaloxidesupportsofthehydroxidic,oxidic,ororganicprecursors[7].Moreover,thesemethodscanalsobeclassifiedintotwocategories.Thefirstisbasedonthepreparationofwell-mixedprecursors?hydroxide,oxide,andmetalmixturesofAuandthemetalcomponentofthesupport?byDP,coprecipitation,co-sputtering,andNaBH4reduction.Theseprecursormixturescanbetrans-formedintometallicAuparticlesattachedtothecrystallinemetaloxides(suchas-Fe2O3,Co3O4,andZrO2)duringthecalcinationprocess.ThesecondtechniqueistoutilizethedepositionoradsorptionofAucompounds,suchasorganogoldcomplexesbyCVD.
3.01.2 Catalytic CO Oxidation
TheoxidationreactionofCOtoCO2bygoldcatalystreceivedagreatdealofinitialpublicity.Inthe1980sHarutaetal.reportedthatsupportedgoldcatalystscanexhibiteffectiveactivitytooxidizeCOatverylowtemperatures(showninFigure4)significantlybelow273K[2,11,12].Thispropertyhasnotbeenobservedbyothermetals.Theyfoundthat
-Fe2O3wasanexcellentsupportandsuggestedthatpreparationmethodwascrucialtohighcatalyticactivity[11].Byelectronmicroscopystudiestheyfoundthattheactivesiteinthecatalystsaresmallgoldnanoparticleswithdiametersofabout2?4nm[2].Thisdiscoveryoftheremarkablecatalyticactivityforlow-temperatureCOoxidationstartedtherapidgrowthinstudiesrelatedtoheterogeneouscatalysiswithgoldnanoparticles.Today,manyresearchgroupscontinuetopreparegoldcatalystsinvariouswaysandstudytheircatalyticactivityunderdifferentconditions.
3.01.2.1 Effect of Preparation Method
Thepreparationmethodplaysacrucialrolefordeterminingcatalyticactivity.TheIMPmethodisconsideredasadisadvantage,bywhichpoorCOoxidationcatalystsareobtained[12],unlesspretreat-mentofgoldsampleoccurs.Sequentialreduction?oxidation?reductiontreatmentscanconsiderablyenhancetheCOoxidationcatalyticactivity,clearlyillustratingthesignificanceofthepretreatmentpro-cedure[13].Bamwendaetal.[14]observedgreateractivityforgoldcatalystssynthesizedbytheDPmethodthanthoseforthecatalystssynthesizedbyCVDandIMPmethods,becausetheDPmethodyieldshemisphericalgoldparticleswithstronginteractionwiththesupportwhileCVDandIMPmethodsobtainsphericalparticlessimplyloadedonthesupport.Alsothecoprecipitation,DP,andCVDmethodsproducehighlydispersedgoldparticleswithameansizebelow5nmasactivecatalystsforCOoxidation.Thestrongcontactandhighdisper-sionofAuparticlestothesupportisindispensableforthecatalysis.AccordingtoYuanandcoworkers[15],highlydispersedactivegoldcatalystscanalsobesynthesizedbyreactingAu?phosphinecomplexwithas-precipitatedwetmetalhydroxidesupports.However,conventionalmetaloxideandhydroxidesupportswerefoundtobeunsuitable[16].Bymodifyingthesynthesisprocedureusingadifferentcomplexandsolvent,ortreatingwithatemperature-programmedreduction?oxidationprocedure,itispossibletoobtainhighlydispersedactivegoldcata-lystsevenonaconventionalsupportsuchasTiO2(P-25;Degussa)[17].
Furthermore,ithasbeenshownthattheactivityofgoldcatalystswithareductiontreatmentat773Kwasfoundtoberelativelygreat[18].ReductiontreatmentathightemperaturecanleadtosomecrystaltransformationofTiO2fromtheanataseformtotherutileform[18].Thehigh-temperaturereductionisalsoknowntoinducealowCOoxida-tionactivityinTiO2[13].However,itcanbespeculatedthatthedecompositionduringtheAuprecursorreduction?calcinationtreatmentcanincreasetheinteractionbetweenAuparticlesandtheTiO2support[17].Dominguezsuggestedthattheinfluenceofpretreatmenttemperatureisthegenerationofstructuralvacanciesbytheeliminationofcarbonates,whichcanmodifythegoldoxidationstate[19].
3.01.2.2 EffectofParticleSizeandSupport
Asmentionedabove,significantcatalyticactivitywasobservedforsmallAuparticles[13,20,21],indicatingthatgoldparticlesizeiscrucialtodeterminethecatalyticactivity.Lopezetal.[20]reportedthattheCOoxidationratefor2?4nmparticlesismorethantwoordersofmagnitudelargerthanfor20?30nmparticles.Theyproposedthatthemaineffectofdecreasingthegoldparticlesizeistoincreasetheconcentrationoflow-coordinatedAuatoms.FurtherevidencefortheparticlesizeeffecthasbeenobtainedviaCOoxidationstudiesondifferentcatalysts[21,22].InthesestudiesAuclus-terssupportedonTiO2thinfilmswerepreparedunderultrahighvacuum(UHV)conditionswithaveragemetalclustersizesthatvariedfrom2.5to
6.0 nm,whilethecatalyticactivitymeasurementswereperformedinareactorcontiguoustothesur-faceanalysischamber.ThespecificratesofreactionweredependentontheAuclustersizewithamax-imumoccurringat3.2nm[22].ScanningtunnelingmicroscopyandspectroscopystudiesrevealedametaltononmetaltransitionastheAuclustersizeapproachedat4nm,indicatingthattheelectronicstructure of the Au clusters was crucial to the CO oxidation activity.
ThecontributionofthesupportisalsoknowntobeimportantindeterminingtheCOoxidationactiv-ity[13,14,23].Ribeiro[24]comparedcatalyticactivityofgoldparticlessupportedonAl2O3,ZrO2,and10%ZrO2/Al2O3inCOoxidation.Au/ZrO2samplesexhibitedthebestperformance.However,Grunwaldtetal.[25]reportedthattheAu/TiO2catalystshowedsignificantlyhigheractivitythantheAu/ZrO2catalyst,althoughtheparticlesizeonbothsupportswascomparable.TheuncalcinedAu/TiO2alsoexhibitedhighactivity,whereastheuncalcinedAu/ZrO2wasinactiveunderthesameconditions.Theysuggestedthedifferentnumberofthelow-coordinatedgoldsites,anddifferentinteractionsbetweengoldandoxidesupportscanleadtodifferentactivityonthetwosupports[23].GoldclustersonCeO2werefoundtobecatalyticallyactiveat353KforCOoxidation[26].Au/Nano-Mn2O3materialwasobservedtoexhibithighactivitiesforlow-temperatureCOoxidation[27],andthereactionratesforCOoxidationwerecomparablewiththehighlyactiveAucatalystssupportedonotheroxides.Schubertetal.[28]madeathoroughcomparisonofgoldcatalystsondifferentsupportmaterials.Itissuggestedthatmetaloxide-supportedAucatalystscanbegroupedintotwocategorieswithrespecttoCOoxidation,whichdependonthesupportmaterialanddifferalsointhereactionmechanism:goldcatalystswithinertsupportmaterials,suchasSiO2,Al2O3,orMgO,areintrinsicallylessactive.Catalystswitharelativelyhighactivitycanbepreparedonthesesupportsaswell,butonlyifgoldexistsinahighlydispersedstate.Thesecatalystsshowastrongdependenceonthemetalparticlesizeandlosetheiractivityrapidlywithincreasingsizeofthegoldparticles.Forthesesystemsoxygenadsorptionoccursdirectlyonthegoldparticles,eitherondefectsites(steps,edges,andkinks)orfacilitatedbyvariationsintheelectronicstructureofsmallmetalparticles.Ontheotherhand,AucatalystssupportedonreducibletransitionmetaloxidessuchasFe2O3exhibitasignificantlyenhancedactivityforCOoxidation,whichisattributedtotheirabilitytoprovidereactiveoxy-gen.Theexistenceofanoxygenreservoironthesupportreducesthedependenceoftheturnoverfrequencyonthegoldparticlediameter,sinceoxy-gendissociationisnolongerratelimitingand,asaconsequence,theturnoverfrequency(TOF)isnotgovernedbyparticlesizeeffectsassuggestedforinertsupportmaterials.This,however,makestheperformanceprobablysensitivetowardthemicrocrys-tallinestructureofthemetal?supportinterfacesothattheactivityofsuchsystemsoftendependscruciallyuponthepretreatmentmethod.ItispostulatedthattheindependenceoftheTOFfromtheAuparticlesizeappliesonlytolowmetalloadings,wherethemetalparticlesaresufficientlydistantfromeachotherandtheoxygensupplyisnotratelimiting.
Goldcatalystssupportedonmetalhydroxidesalsoshowedhighcatalyticactivitiesinlow-temperatureCOoxidation.AmongtheobtainedAucatalysts,thoseonMn(OH)2andCo(OH)2weremosthighlyactiveevenat203K[13].ThoseonFe(OH)3andTi(OH)4alsocatalyzedCOoxidationatlowtem-peratures(203?273K).ThecatalystsonmetalhydroxidesexhibitedmuchgreaterCOoxidationactivitythanthoseonthecorrespondingmetaloxides[13,29].
3.01.2.3 Reaction Mechanism
Therehasbeenmuchdiscussionabouttheactivesiteandreactionmechanismofgoldcatalysts.In2000BondandThompson[30]suggestedthatAu0,Auxt,andthemetaloxidesupportallcon-tributetoactivity.TheyproposedamodelwhereAuatomsattheinterfacebetweenthegoldandthesupportaretheactiveoxidationcenters.Theseperipheralgoldatomsareresponsiblefortheoxy-genactivationinCOoxidation.HaoandGates
[31] haveshownthattheactivatedAu/MgOcat-alystcontainedgoldclustersandthesewereprincipallyAu0.ThroughIRcharacterizationofthecatalystunderCOoxidationconditions,itcanbesuggestedthatCO2oxidizedAu0 insup-portedclusterstoformAuxt sites.TheAu/MgOsamplecontainedmixturesofAu0 and Auxt spe-ciesundercatalyticreactionconditions.Theyinferredthatoneorbothofthesespeciesmightbeinvolvedinthecatalyticreaction,andtheactivegoldspeciesmightcontainboth,perhapswiththechargedspeciesbeingatthemetaloxidesupportinterface.Asimilarconclusionwasmadethroughotherstudies[23,26,32],whichsuggestedthatgoldcanbeoxidizedtoAut inthepresenceofCOandAuxt/Au0 redoxcouplesareactiveinlow-temperatureCOoxidation.
Catalyticactivityofwell-orderedmonolayersandbilayersofgoldatomsthatcompletelycovertheTiO2supporthasbeenstudiedbyChenandGoodman[33].Experimentalresultsfoundthatbilayersofgoldweremoreactivethanamonolayer,indicatingthatthecombinationofthefirst-andsecond-layerAusitesisnecessarytopromotereactionbetweenCOandO2.Theinteractionofthefirst-layerAuwithTi3t of the support, yielding Aux,likelyiscrucialforoxygenactivation.However,COhasbeenshowntoadsorbstronglyontheAubilayerstructure.ChoudharyandGoodman[34]haveshowntheevidencetosupportthemodelsystemsproposedinChen’sstudybystudyingAu/TiO2catalystactivity.Ontheotherhand,goldclustersexhibitmoreactivityforCOoxi-dationthanthemononucleargoldspecies[35].Basedontheanalysisofscanningtransmissionelectronmicroscopy,Herzing[36]proposedthatactivesitesongoldnanoparticleswithhighcatalyticactivityforcarbonmonoxideoxidationarethebilayerclustersthatareabout0.5nmindiameterandcontainonly10goldatoms.
Electronicstructuresofgoldnanoparticlessup-portedonTiO2havebeeninvestigatedbyOkazaki
[37] utilizingelectronholographymethods,scanningtunnelingmicroscopy,andfirst-principlescalcula-tions.ThedependenceofthemeaninnerpotentialofthegoldnanoparticlesonTiO2onthesizeofgoldparticleshasbeenobserved.Meaninnerpotentialisgivenasthezero-orderFouriercoefficientofthe
(a)
Au(001)
crystalpotential.Innerpotentialsofgoldnanoparti-cleswithsizebecome5nmlargerthanthebulkvalue(showninFigure4).Theseauthorssuggestedthatthistendencyforthepotentialofthegoldnanocrys-talstoincreasetheirsizecorrelateswellwiththecatalyticbehavioroftheseparticlesforCOoxidation.
3.01.3 Catalytic Oxidation of Organic Compounds
SupportedAucatalystscanalsocatalyzemanyreac-tionsinvolvingvariousorganiccompounds.Turner
[38] showedthatsmall55-atomgoldclusters(1.4nm)supportedoninertmaterials(BN,SiO2,andC)wereefficientcatalystsfortheselectiveoxidationofstyrenetobenzaldehyde.TheyfoundasignificantsizethresholdincatalyticactivitybecausecatalyticactivitydecreasedoncetheAuparticlewasabove2nmindiameter.TheAu4f7/2bindingenergywas
1.1 eVhigherthanthevaluecharacteristicofbulkAu,indicatingthatcatalyticactivityofgoldnanoclustersarisesfromthealteredelectronicstructure.Theoxi-dationresultsappearedtocoincidewiththereportbyHughesforAu/Ccatalyst[39].HarutahasreportedthatgoldsupportedonTiO2canexhibitexcellent
(b)
TiO2 1nm 1nm
(c) [001] (d) Phase image Simulation HREM image V0:50V
[111] [111]
(111)
(100) (010) Top half of the truncated octahedron 1nm
Figure4(a)High-resolutiontransmissionelectronmicroscopy(HRTEM)imageand(b)phaseimagereconstructedfromthehologramofanAuparticleonaTiO2support.Thediameterofthegoldparticleisunder2nm.Thephaseimageisamplified30-foldandsuperimposedontheHRTEMimage.(c)Schematic3Dmodel,whoseshapeisthetophalfofthetruncatedoctahedron.
(d) Simulatedphaseimageofthemodelwithasuperimposedreconstructedphaseimage.OkazakiK,IchikawaS,MaedaY,HarutaM,andKohyamaM(2005)ElectronicstructuresofAusupportedonTiO2.AppliedCatalysisA291:45.
selectivityinthepartialoxidationofpropylene,propane,andiso-butanetopropyleneoxide,acetone,andtert-butanol,respectively[40].Tanaka[2]foundthatgoldiseffectiveforthepartialoxidationofpropylenetoproducepropyleneoxideentirely,whilePdandPtareselectivetoobtainpropaneinthepartialoxidation.Theselectiveproductsofpro-pyleneoxidationchangefrompropyleneoxidetopropanewhentheAuloadingincreasesfrom0.05to
0.1 wt%.TheresultsindicatedthatonAuparticleshydrogenwasactivatedwithsurfaceadsorbedoxy-gentoformactivehydroperoxospecies,while,onPdandPtnanoparticles,hydrogendissociatedandledtoahydrogenationreaction.Theoxidationreactionoccursatgoldparticleswithdiameteraround
2.0 nm,whichwasattributedtodissociatedhydrogenmoleculesintoactivespeciesongoldnanoparticles.TheyalsofoundthatO2additioncanenhancecata-lyticactivity.
Supportedgoldnanoparticlesaremoreactiveandselectivefortheoxidationofalcohols[41?56].HighpHofthereactionsystemplaysacrucialroleinenhancingthecatalyticactivity.TheeffectofbasicityistoprovideanOH. anion and form a Au?OH. site,whichisessentialforhydrogenabstractionfromalco-hol[56].Withgoldalonethereactionprocessisdifficult,thoughBiellaandRossireportedthatgoldcatalystsarealsoactiveforoxidationofgas-phasecompoundswithoutbaseaddition[57].Goldoncar-bonwasobservedtobeactiveforoxidationofvariousalcohols,suchaspropenol,butanol,andethyleneglycol[58].Suetal.[59]demonstratedthatthecom-binationofgoldnanoparticlesongalliasupportwaseffectiveforthesolvent-freeoxidationofbenzylalcoholwithmolecularoxygen.Benzaldehydewasobtainedwithhighselectivityunder403K.TheAu/Ga2O3catalystwasstableandthealcoholcon-versionstillremainedbetterthan98%afterrecyclingfourtimes.Withbasicaddition,glycerolisoxidizedtoglycericacidentirelyusing1%Au/charcoaland1%Au/graphitecatalystundermildreactioncondi-tions(60C,waterassolvent)[60?62].UnderthesameconditionsglycericacidwasobtainedwithsupportedPdandPtcatalysts,butthemainproductswerenondesiredby-products,suchasCO2,HCHO,andHCOOH.Moreover,variousgoldcatalysts(sup-portedonTiO2,MgO,Al2O3,andFe2O3)exhibitactivityfortheselectiveoxidationofaminoalcoholstoaminoacids[63].OnlyafewpercentofN-oxideby-productcanbedetected.
Alloyingprocesscanalsoenhancethecatalyticactivity.Enacheetal.[64]showedthatalloyingPdwiththeAuinsupportedAu/TiO2catalysts(Au-richcorewithPd-richshell)obtainedveryhighturnoverfrequenciesfortheoxidationofalcoholsandimprovedtheselectivity.Au?VandAu?NbcatalystswerealsofoundtobeveryactiveintheoxidationofmethanoltoCO2[65].
Supportisacrucialfactortocontrolgoldcatalystactivity.Onthestudyofbenzylalcoholoxidation,thesignificantlyenhancedactivityachievedovertheAu/Ga2O3catalysthasbeenattributedtoastronginteractionbetweengoldnanoparticlesandGa2O3supportaswellastheenhanceddehydrogenationcapabilities[59].CormaandcoworkersshowedthatAu/CeO2catalystisactiveforthesolvent-freeselec-tiveoxidationofalcoholstoaldehydesandketonesandaldehydestoacids[66,67].ThecatalystTOFwas12500h1 fortheconversionof1-phenylethanolintoacetophenoneat160C,whichwashigherthanthecatalyticactivitieswithsupportedPdcatalystsundersamereactionconditions[68].Theinteractionbetweengoldandoxidesupportleadstoproducepositivelychargedgoldthroughchargetransferfromgoldtosupport.ThealcoholthenreactswiththeLewisacidsitesofAu/CeO2togiveametalalkoxide,andsubsequentlyundergoesarapidhydridetransferfromC?Htoproduceketone.Tsunoyamaetal.[69]reportedthatgoldnanoclusterssupportedonpolymers(poly(N-vinyl-2-pyrrolidone),PVP)cancatalyzebenzylalcoholoxidationinwateratambienttemperature.Thereactantscanaccessthegoldparticlesurface,duetotheinteractionthroughmultiplecoordinationofPVP.
Thesestudieshavebeenextendedtothesugaroxidation.Highcatalyticactivityabouttheoxidationofglucosehasbeenobserved[70,71],andgoldparticlesizeseemstobethemajorfactorinfluencingthecata-lyticreactionactivity.TheadditionofPdorPttotheAu/Ccatalystscanenhancereactionratefortheselec-tiveoxidationofd-sorbitoltogluconicandgulonicacids[72].Basheeretal.[73]haveshownthatasimplecapillaryreactorcanbeusedfortheselectiveoxidationofglucose,thusindicatingthatoxidationsinaflowreactorarefeasibleusingasupportedgoldcatalyst.
Ishidafound[74]thatthegoldcatalyticmechanismforglucoseoxidationseemstobedifferentbetweengas-phaseandliquid-phasereactions.Thesupporteffectisthemostimportantforthegas-phasereac-tions,whileAuparticlesizeiscriticalintheliquidphase.Therate-determiningstepintheliquid-phasereactionwassuggestedtobetheoxidationofglucose,becauseglucosecanbeadsorbedongoldsurfaceeasily[73].Thereactionisfirstorderwithrespecttooxygen.
Goldcatalystscanalsocatalyzealdehydeoxida-tionreactions.Jiaetal.reportedthatcatalyticactivitiesofgoldcatalystsbasedondifferentsupportsobtainedbyDPmethodcanbeinvestigatedintheoxidationofHCHO[75].AllthesecatalystsshowedgoodactivityforthesetworeactionsandtheAu/CeO2catalystexhibitedthehighestactivityforthetworeactionswiththe100%-conversiontempera-tureatabout258KforCOoxidationand353KforHCHOoxidation,respectively.Furthermore,cata-lystsderivedfromtheas-precipitatehydroxidesexhibitedhigheractivitythanthatderivedfromcor-respondingoxidesupports.Itisdemonstratedthatcatalystsfromas-precipitatehydroxidecouldmakethegolddispersehomogenousonthecatalystsurfaceandthusincreasethenumberofactivesites.
3.01.4 Catalytic Reduction of Organic Compounds
Supportedgoldcatalystscanshowhighactivityandselectivityforthereductionoforganiccompounds.Guzmanetal.[76]studiedtheethenehydrogenationwithmononucleargoldsupportedonMgOpowderatatmosphericpressureand353K.EXAFSandXANESdataprovidedevidenceforAu3t asthepredominantsurfacegoldspeciesduringcatalysis,andIRspectroscopyidentifiedethylgoldspeciesasthereactiveintermediate.Otheralkenes,alkynesand,-UnsaturatedAldehydesalsohavebeeninvestigatedinheterogeneousgold-catalysedreduction[77?80].
Functionalizedanilinesareindustriallyimportantintermediates[81].Cormaetal.[82,83]obtainedthemthroughselectivereductionofnitrogroupscatalyzedbygoldnanoparticlessupportedonTiO2orFe2O3,evenotherreduciblefunctionswerepresent.Thesetwogoldcatalystsexhibitedabove98%conversionsof3-nitrostyreneoxidationwith96%selectivityto3-vinylaniline.NeitherPtnorPdcatalystswereselec-tivefortheoxidation.Theyproposedthatadirectandfaststepfromnitrobenzenetophenylhydroxylaminecontributestothereductionreaction;thenphenylhy-droxylamineisreducedtoanilineslowly[84].
3.01.5 Gold/Semiconductor Photocatalysts
Inthewholeenergyoftheincomingsolarspectrum,ultravioletradiationaccountsforlessthan4%,whilethevisiblelight(wavelength>400nm)constitutesaround43%ofsolarenergy[85].Hence,oneofthegreatchallengesforcatalysisstudyistodevisenewcatalyststhatpossesshighactivitywhenilluminatedbyvisiblelight.Itwillallowustousesunlight,theabundantandcleanenergysourcewithlowcost,todrivechemicalreactions.
Asiswellknown,semiconductorscangenerateelectron?holepairswhenlightirradiationenergyisenoughtoovercomethebandgap.Thenthedegra-dationoftheorganiccompoundsproceeds[86].However,semiconductorsaseffectivephotocatalystshaveabigdrawbackinthattheyfailtoutilizevisiblelight,duetothebandgap.AsthebandgapofTiO2semiconductorisabout3.2eV,electron?holepairsanddegradationoforganiccompoundscanonlyoccurintheUV-illuminatedprocess,inwhichthewavelengthisshorterthanthatofvisiblelight.
Oneofthemethodstoextendthevisible-lightactivityofsemiconductorphotocatalystsisthesur-facemodificationwithgold[87?90].Oncegoldparticlescontactwiththesemiconductorsurface,thegoldFermilevelshiftsclosetotheFermilevelofthesemiconductor.Thentheelectronsgeneratedfromthesemiconductorunderlightirradiationaretransferredtogoldnanoparticlesresultingineffec-tivechargeseparation.Moreover,oxygencantraptheelectronsfromgoldnanoparticlesreadilyandenhancethephotocatalyticactivity.
ThemechanismissupportedbySonawane’sreport[87].BystudyingthinfilmsofAu/TiO2preparedbyasimplesol?geldipcoatingmethod,Sonawaneshowedthatthephotocatalyticactivityofphenoldecomposi-tionbyAu/TiO2photocatalystwasimprovedby2?2.3timesthatofundopedTiO2.SimilarexperimentalresultsonthephotocatalyticactivityofAu/TiO2thinfilmswerereportedforthereactionofmethylenebluedegradation[88].TheeffectofthedopedgoldtoincreaseinphotoactivitymaybeattributedtotheimprovementofthechargeseparationprocessthroughelectronmigrationfromTiO2conductionbandtogoldsurface.Thetransferofphotoelectronsresultsinthedecreaseofrecombinationofelectronsandholes;consequently,theAu/TiO2samplespreparedbywashingtreatmentshowedhigherphotocatalyticactivityformethylorangephotodegradationthanthatpreparedbyrotaryevaporation[89].Suchamechan-ismisalsosupportedbythestudyofWuetal.onthemechanismofmethanolreformingonAu/TiO2photocatalyst[90].Fourbasicstepsareinvolvedinthereformingreaction:(1)photogenerationofexcitedelectronsinthesemiconductorconductionband;(2)theelectronstransfertogoldparticlesandreducetheprotonstoproducehydrogen;(3)theholesoxidizeH2OandCH3OH,anditsreactionintermediatepro-ductsadsorbedonTiO2;and(4)thefinalintermediateHCOOHisoxidizedtoCO2.
3.01.6 Gold Photocatalysts
Thegoldnanoparticleshaveintensiveabsorptionofvisiblelightbecauseofthesurface-plasmonreso-nance(SPR)effect[91,92].Surfaceplasmonsaresurfaceelectromagneticwavesthatpropagateinadirectionparalleltothemetal/dielectric(ormetal/vacuum)interface.Theelectromagneticfieldofinci-dentlightcoupleswiththeoscillationsoftheconductionelectronsinthegoldparticles,resultinginstrongfieldenhancementofthelocalelectromag-neticfieldsneartheroughsurfaceofgoldnanoparticles[93].Theenhancedlocalfieldstrengthcanbeover500timeslargerthantheappliedfieldforthestructureswithsharpapices,edges,concavecur-vature(e.g.,nanowires,cubes,triangularplates,andnanoparticlejunctions)[94].TheSPRabsorptionmaycauserapidheatingofthenanoparticles[95,96].
Recently,wereportedanewfinding[97]:whenilluminatedwithvisiblelight,goldnanoparticlesonoxidesupportsexhibitedsignificantactivityforoxida-tionofformaldehydeandmethanolinairatroomtemperature(25C).Theturnoverfrequency,beingabout1.2103moleculesofHCHO(Auatom)1s 1,wascomparabletothefrequenciesfortheCOoxida-tion,onthegoldcatalysts,byheatingthereactionsystemto80Corabove.Thecatalyticactivityisfoundtobedependentontheintensityoflightirradia-tion,whichindicatesundoubtedlythatthereactionisdrivenbyvisiblelight.Theauthorstentativelyproposeareactionmechanismforthelight-drivencatalyticoxidation.TheirradiationofincidentlightwithwavelengthintherangeoftheSPRbandmayresultintwoconsequences:First,thelightabsorptionbythegoldnanoparticles[91,92],whichcouldheatthesenanopar-ticlesupquickly[85,86].Second,theinteractionbetweentheoscillatinglocalelectromagneticfieldsandpolarmoleculesalsoassistsactivatingthemole-cules.Theactivatedpolarorganicmoleculesreactwithoxygenincloseproximity.Astheoxygenadsorbedongoldnanoparticlesortheactivesupportsistheoxida-tionagent,theadsorptionofoxygenontheoxidesupportincreasestheconcentrationofoxygenaroundtheorganicmoleculesonthegoldnanoparticlesandthustheopportunitytoreactwiththemolecules,accel-eratingtheoxidation.Theproposedreactionmechanismisdistinctlydifferentfromthatoccuringinthereactioncatalyzedbysemiconductorphotocata-lysts.Moreover,thefindingthatsupportedgoldnanoparticlescanabsorbvisiblelightandexhibitsig-nificantactivityrevealsthattheroleofthegoldnanoparticlesinthephotocatalystsofTiO2dopedwithgoldhasnotbeenexplainedcorrectly,sincethephotocatalyticactivityofthegoldnanoparticlesthem-selvesundervisiblelighthasnotbeenrecognized.Thefindinghighlightsanewdirectionofcatalysisandher-aldssignificantchangesintheeconomicsandenvironmentalimpactofthechemicalproduction.
3.01.7 Conclusion
Thecatalyticandphotocatalyticnatureofgoldnano-particleshasbeenfoundtobeimportantinoxidationandselectiveoxidationreactions,withpropercontroloftheparticlesizeandthesuitableselectionofoxidesupportmaterials.Thesefeaturescanprovideimpor-tantopportunitiesfortheapplicationofgoldincatalysis.Fundamentalresearchaimingatinterpret-ingthecatalyticactivityandselectivityofgoldmaterialsatanatomicscaleisofgreatinterest.Thegoldcatalystsandphotocatalystshavemoderateredoxability,andthecatalyticprocesseswithgoldcatalystsgenerallyworkundermoderateconditions.Thesepropertiesmakethecatalystsandphotocata-lystsattractiveforapplicationsoverawiderangefromfinalchemicalsynthesisandenvironmentalremediation.Therefore,thepropertiesofgoldnano-particlespresentfutureopportunitiesforindustrialandenvironmentalapplications,especiallyfortheprocessesusingsunlight,themostabundantenergyintheworldtodrivereactionsanddegradepollutants.
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