title: Scanning electrochemical microscopy. 59. Effect of defects and structure on electron transfer through self-assembled monolayers.
abstract: Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.
authors.affiliation: Center for Electrochemistry, Chemistry and Biochemistry Department, The University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: Abolfazl
authors.initials: A
authors.lastname: Kiani
authors.forename: Mario A
authors.initials: MA
authors.lastname: Alpuche-Aviles
authors.forename: Paul K
authors.initials: PK
authors.lastname: Eggers
authors.forename: Michael
authors.initials: M
authors.lastname: Jones
authors.forename: J Justin
authors.initials: JJ
authors.lastname: Gooding
authors.forename: Michael N
authors.initials: MN
authors.lastname: Paddon-Row
authors.forename: Allen J
authors.initials: AJ
authors.lastname: Bard
mesh_terms: D000473:Alkanes / Q000737:chemistry; D004563:Electrochemistry; D004583:Electrons; D005296:Ferrous Compounds / Q000737:chemistry; D006046:Gold / Q000737:chemistry; D007700:Kinetics; D008567:Membranes, Artificial; D008628:Mercury / Q000737:chemistry; D000075163:Metallocenes; D008855:Microscopy, Electron, Scanning / Q000379:methods; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013499:Surface Properties
chemical_list: D000473:Alkanes; D005296:Ferrous Compounds; D008567:Membranes, Artificial; D000075163:Metallocenes; D013438:Sulfhydryl Compounds; D006046:Gold; D008628:Mercury; C004998:ferrocene