北京大学 电分析 课件 扫描探针显微镜及其应用 elect8a.ppt

College of Chemistry and Molecular Engineering Peking University Email: Phone: 62759394,Scanning Probing Microscopy (SPM),Yuanhua Shao,扫描探针显微镜及其应用,Outline: 1. Introduction 2. Instrumentation 3. Theory 4. Applications,REFERENCES: 1. R.M.Wightman and D.O.Wipf, in Electroanalytical Chemistry, Vol:15, (A.J.Bard, Ed.), Marcel Dekker, New York, 1988, p.267 2. A.J.Bard, F.Fan, J.Kwak and O.Lev, Anal.Chem., 1989, 61, 132 1989, 61, 1221. 3. M.V.Mirkin, Anal.Chem., 1996, 68, 177A 4. M.V.Mirkin, Mikrochim.Acta., 1999, 130, 127 5. A.J.Bard, F.Fan and M.V.Mirkin, in Electroanalytical Chemistry, Vol:18 (A.J.Bard, Ed.), Marcel dekker, New York, 1993, p.243 6. 扫描力显微术，白春礼，田芳和罗克，科学出版社，2000,STM 2. AFM 3. SECM,Figure. (a) Representation of tunneling between tip and sample atoms. Shaded portions denote electron distributions. (b) Tip attached to three Piezo elements used to position the tip and scan it across a surface,STM,itun = (constant) V exp (-2x) = V/Rtun,Where V is the tip-substrate bias, x is the distance between tip and surface, 1 -1, And Rtun is the effective resistance of the tunneling gap, typically 109 to 1011 ohms.,Figure. Cell for electrochemical STM. Upper: schematic diagram. Lower: Nanoscope III cell, top view.,Figure. STM images of HOPG. Gray-scale images at low resolution (left) and higher resolution topographic plot (right),Figure. (a) STM images of (A-D) mixed adsorbed layers Of PP and FePP on HOPG. Taken with a wax-coated Pt Tip in 0.05 M NaBO Solution containing FePP and PP in the ratio of (A)0:1 (B)1:4 (C) 4:1 and (D) 1:0. The HOPG substrate potential Was 0.41 V vs. SCE; the tip/ Substrate bias was 0.1 V; And the tunneling current was 30 pA. (b)(E-H) The Corresponding cyclic Voltammograms for A-D, Respectively, at a sweep rate Of 0.2 V/s.,AFM,Figure. Electrochemical cell for AFM for Nanoscope III.,Scanning Electrochemical Microscopy(SECM),Scanning electrochemical microscopy(SECM) was introduced in 1989 by Bard and co- workers at the University of Texas at Austin who published a series of papers describing the instrumentation theory, principles and applications of the technique. SECM is a novel in-situ electrochemical technique with its spatial resolution between optical microscopy and STM. Since its introduction, it has provided considerable insights into, and an understanding of, localized surface reactivity at a variety of solid/liquid interfaces ranging from biomaterials, polymers and minerals to electrode surfaces. Ion and electron transfer processes occurring at the liquid/liquid interface have also been studied.,2. Instrumentation A. Basic Apparatus The basic SECM apparatus consists of four parts: tip position controller; electrochemical cell( including tip, substrate, counter and reference electrodes); bipotentiostat and data acquisition system B. Microprobes The information obtainable from SECM measurements depends mostly on the type and size of the used microprobe. (1) Amperometric tips: solid UMEs, micro- to nano-meter disk UMEs (2) Potentiometric tips: Ion-selective microelectrodes (3) Dual-functional tips: Antimony (Sb), dual-channel tips (4) Tips based on Charge-transfer across liquid/liquid interface: electron transfer, ion transfer processes(Micropipettes),iT, = 4nFDCa,3. Theory A. Modes of Operation The SECM can be used in a variety of ways, e.g., as an electrochemical tool to study heterogeneous and homogeneous reactions, as an imaging device (microscope), and for microfabrication. These applications make use of different modes of the SECM operation. (1) Amperometric feedback mode: (2)Generation/Collection mode and potentiometric measurements: (3) Penetration Mode: (4) Ion transfer feedback mode:,B. Kinetic Measurements and Imaging Surface Reactivity There are two important advantages of SECM for heterogeneous kinetic measurements: (1)high mass-transfer rate allowing one to study fast reactions under steady-state conditions; (2)availability of powerful means for probing the mechanism and physical localization of interfacial reaction(High Spatial resolution).,The mass transfer rate in SECM is a function of the tip-substrate distance: d a, m D/a d a, m D/d D = 1x10 -5 cm 2/s , d=0.1m, m =1cm/s 10cm/s High Spatial Resolution: about 30 50 nm limit for conducting substrates and 2nm for insulating substrates,3. Theory of the SECM The considerable complexity of SECM theory is due to the combination of a cylindrical diffusion to the micro-tip electrode with a thin-layer diffusion space. The general solution of the diffusion problem for an uncomplicated quasi-reversible non-steady-state process in SECM was obtained as a system of two-dimensional integral equations. Two limiting cases, a diffusion-controlled process and one with totally irreversible kinetics, were treated numerically. These results and the theory for complex processes. Including homogeneous chemical stages and adsorption-desorption kinetics, have been reviewed.,The theory for steady-state (time-independent) processes is simpler. The knowledge of the shape of the iT - L ( the “approach“ )curve for a diffusion-controlled process is critical for both imaging and quantitative kinetic measurements because it allows one to establish the distance scale. The dimensionless current-distance curves were obtained numerically for both insulating and conductive substrates and several values of RG ( RG = rg/a, where rg is the radius of the glass insulator plus the radius of the electrode a), assuming a tip held at the potential where the reaction is diffusion controlled, equal coefficients and an infinitely large substrate. For RG = 10, an analytical expression for a conductive substrate can be fit to the numerical results to yield the equation: IT(L) = iT/IT, =0.78377/L + 0.3315exp(-1.0672/L) + 0.68 (1),Where L = d/a is the normalized distance between the conductive substrate and the tip of radius a; this fits the iT -d curve over a L interval from 0.05 to 20 to within 0.7%. For an insulating substrate, similar equation(2) is slightly less accurate (to within 1.2%); however, the longer expression (equation 3) is accurate to within 0.5% over the same L interval IT(L) = 1/0.292 + 1.5151/L + 0.6553exp(-2.4053/L) (2) IT(L) = 1/0.15 + 1.5385/L + 0.58exp(-1.14/L) + 0.0908exp(L-6.3)/(1.017L) (3),4. Applications A. Study of heterogeneous reaction kinetics B. Images C. Micro-fabrications,Tip Steady-state voltammograms for the oxidation of 5.8mM FC in 0.52M TBABF4 in MeCN at a 1.08m radius Pt tip. Solid lines calculated with k0= 3.70.6cm/s and =0.37 0.02.,Scanning electron micrograph of GaAs (Cr doped)etched in three places for 5,6, and 20 min in a 0.02MHBr and 0.1MHCl solution with a 25m Pt microelectrode,Image of mica treated with DNA(2.96kbp) specimens on mica taken in humid air,Single Molecule detection,Idealized schematic illustration of the tip geometry and the tip-substrate configuration used.,Schematic representation of the principles of SECM with micro-ITIES,SECM image obtained using a micro-ITIES probe (5-m tip) . Substrate was silicon with parallel platinum bands.,J.Electroanal.Chem., 1997, 439, p137-143,Positive feedback,Negative Feedback,Approach a big L/L interface,Approach a solid substrate,RG = Rg/a,Bard et al. have developed the theory for RG 10 cases. However, for micropipettes as the tips for SECM, the RGs are usually 2.,Using software package PDEase (SPDE, Inc.), we have solved the theoretical i - d curves for RG 2.,J.Phys.Chem B., 1998, 102, p9915-9921,RG = Rg/a =b/a,L = d/a,Aqueous Phase,Organic Phase,Current-distance curves obtained with different concentrations of TEACl in the aqueous phase. (1)0, (2)0.4, and (3)10 mM.,TEA+ (w) = TEA+(o),RG =1.1,Constant height mode gray scale images of pores in polycarbonate membrane,TCNQ + Fe(CN)64- = TCNQ.- + Fe(CN)63-,Fe(CN)63- (w) + e = Fe(CN)64-(w),E1o =0.41V vs SHE,TCNQ (o) + e = TCNQ.-(o),E2o =0.22V vs SHE,ow = owo +(RT/nF)lnai(o)/ai(w),ow E2o - E1o (-190mV),owTBA+o = -225mV,Reverse Electron Transfer Reactions,Approach curve for the system: H2O, TPAsCl, 1mM Fe(CN)63- | DCE, 1mM TPAsTPB, 10mM TCNQ; showing the electron transfer driven by the phase transfer catalyst TPAs+ .From top to bottom, the experent curve are shown for Kp=10,5,2,1 . A:TPAs system. B: TBA system.,Dependence of the effective ET rate constant on concentration of TCNQ in DCE. The k value were used to fitthe approach curev with a=10m and . From top to bottom the Kp value was shown for 10,5,2. A:TPAs syetem. B:TBA system.,Dependence of the effective ET rate constant on potential drop across the ITIES. A:TPAs. B:TBA.,SECM Studies of ET Reactions across Ice/Liquid Interface,Figure Experimental approach curves of the system (line) and the theoretical curves (dot) at -30C. The ice phase contained 5mM K4Fe(CN)6, 0.1M KCl, and 0.01M TBACl. The DCE phase contained 1mM Fc. From top to bottom, the concentrations of TBATPB in DCE phase were 1, 5, 10, 20, 40, 100 and 200mM, respectively. The rates of electron transfer k/(cm.s-1.M-1) were 10.1, 3.62, 2.45, 1.83, 1.01, 0.65, 0.47, All of the approach rates were 1m/s.,Combination of SECM and polarized liquid/liquid interfaces,Fc (o) + Fe(CN)63- (w) = Fc+ (o) + Fe(CN)64-,Figure Experimental approach curves (dot) fitted with theoretical values (line). The DCE phase contained 0.01M TBATPBCl and 0.2mM ferrocene. The aqueous phase contained 0.1M LiCl + xM K3Fe(CN)6 + yM K4Fe(CN)6. (D)x = y = 0.2mM. From top to bottom, the external-potential were -0.10V, -0.05V, 0V, 0.05V, 0.10V, 0.20V and 0.30V. All of the approach rates were 1m/s.,Investigation of Modified Electrodes,Surface pKa,Figure: Schematic diagram of the application of SECM to probe facilitated ion transfer at an externally polarized Liquid/Liquid interface,Probing facilitated ion transfer at an externally polarized L/L interface,Study of simple ion, facilitated ion and electr