Abstract. Introduction

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1 [7]- Cosofret, V.V. nd Buck, R.P., Anlyst, 109 (198) 1321 [8]- Peng T., Yng Z. nd Lu R., Yoxue-Xuebo, 25 (1990) 277; Anl. Abs. 503G00092 [9]-Wng J., BonkdrM. nd Pck M.M., Anl. Chim. Act, 192 (1987) 215 [10]- Bell F. nd Anderson J.L., Anlyst, 110 (1985) 193 [11]- Grci J.M., Jimenez, A.I., Jimenez, F. nd Aris, J.J., Anl. Letters, 25 (1992) 1511 [12]- Anlyticl Methods Committee, Anlyst, 112 (1987) 199 SELF-ASSEMBLED MONOLAYERS OF FERROCENYLTfflOLS ON GOLD. AN ELECTROCHEMICAL AND OPTICAL CHARACTERISATION L. M. Abrntes 1 *, G. Jin 2, M. Klji 3 nd A. S. Vin 1 ' 3 'icate Deprtmento de Químic e Bioquímic d Fculdde de Ciêncis d Universidde de Lisbo, Cmpo Grnde 1700 Lisbo, Portugl 2 Lbortory of Lser Reserch, Institute of Mechnics, Chinese Acdemy of Sciences, Beijing , P. R. Chin 3 Deprtment of Chemistry, University of Wles, Bngor, Gwynedd LL57 2UW.UK Received, June 15, 1999 Accepted, September 17, 1999 Abstrct Short chin ferrocenylkylthiols, (C5H 5 )Fe(C5H)CO(CH2) n SH (n= 3, 5 nd 7), were self-ssembled on gold (111). The influence of the chin length nd dsorption time on stbility, surfce coverge nd order of the monolyers were ssessed electrochemiclly nd by the use of opticl ellipsometry. The redox behviour nd structure of the SAMs were strongly dependent on the chin length. Long dsorption times were required to obtin ordered monolyers with high surfce coverge. Ellipsometry indicted tht the thicknesses of the lyers were dependent on the surfce concentrtion of the ferrocene derivtives. The thickness of the monolyer with the longer lkyl chin ws estimted to be 2.9 nm with complex refrctive index h= Í. Imging Ellipsometry llowed the visulistion of the lterl thickness distribution of monolyer deposited on gold. Keywords: Self-Assembled Monolyers, Au, Ferrocenylkylthiols, Voltmmetry, Ellipsometry. Introduction Self-Assembled Monolyers (SAMs) re prepred by the spontneous dsorption of orgnic molecules onto substrte from homogeneous solution. Verstility, ese of preprtion, stbility nd orgnistion contributed to the gret populrity of self-ssembly in the lst two decdes [1]. Alknethiols used in the formtion of SAMs generlly contin three importnt prts [2]: (i) the surfce ctive hed group, which binds strongly to the substrte, (ii) the lkyl chin, responsible for the stbility nd order of the lyer due to vn der Wls chin interction, nd (iii) functionlity. The ltter plys prmount role in controlling the redox behviour of the monolyer nd cn be tilor-mde to crete the desired interfcil microenvironment. Self-ssembly of thiol derivtives on gold is believed to occur in two stges [1-3]. Physisorption of the thiol, which fter short time on the surfce, forms strong covlent bond with the substrte vi the formtion of thiolte, Portugáli? Electrochimic Act, 17(1999)

2 ccompnied by the loss of proton. This is followed by slow chnges in the monolyer properties, suggesting n pproch to more highly ordered stte. Since their discovery, SAMs exhibit interesting properties pointing to gret promise in severl res, such s chemicl sensors, electronic devices, electroctlysis, nd lubriction. These modified surfces re lso idel models for the study of interfcil rectivity nd fundmentl chrge trnsfer phenomen [2-], In this work, short chin ferrocenyllkylthiols with crbonyl functionlity, (C 5 H5)Fe(C5H)CO(CH 2 )nsh (n = 3, 5 nd 7), were ssembled on Au (111) surfces. Effect of the dsorption time nd lkyl chin length, on the structure nd order of the monolyers, were studied by cyclic voltmmetry nd ellipsometry. There re few ellipsometric studies on ferrocene derivtive monolyers [5, 6], nd the mjority of such studies were performed on long chin compounds (n > 10). In this pper we show tht it is possible to deduce lyer thicknesses for short chin monolyers (n = 5 nd 7). The dt indicte strong dependence on the order nd pcking of the SAMs. Experimentl Chemicls. The ferrocene derivtives (Figure 1) were synthesised using previously described method [7]. Perchloric cid (Aristr, BDH) nd bsolute ethnol were used s received. Ultr pure wter ws obtined from n Elgster UHQ U purifiction system (nominl resistivity 18MQ cm t 25 C). o FcCO(CH 2) 3SH (FcC) FcCO(CH 2) 5SH (FcC6) FcCO(CH 2) 7SH (FcC8) heted in the cold prt of bunsen flme nd quenched in ultr pure wter. This tretment produces flt gold surfce with strong Au (111) chrcteristics [8] Monolyer Preprtion. After pproprite surfce tretment, electrodes were rinsed with ethnol nd immersed in ethnolic solutions contining the compounds of interest t concentrtion of 1-2 mm. The immersion time vried between 5 minutes nd three dys. After self-ssembly, the electrodes were rinsed prior to use with copious mounts of fresh bsolute ethnol nd ultr pure wter. Electrochemistry. Cyclic voltmmetry ws performed using DT 2101 Hi-Tek potentiostt, PP RI Hi-Tek Wveform Genertor, nd Yokohm 3023 X-Y recorder. The experiments were crried out in one comprtment Teflon cell, with n Au wire spirl counter electrode nd sturted clomel reference electrode (SCE). The Au slide ws clmped ginst n o-ring, which defined the geometric re of the working electrode (0.6 cm 2 ). The electrolyte solution (HC M) ws degssed for 15 minutes before the electrochemicl mesurements were initited. Glsswre nd electrochemicl cell were bthed in freshly prepred 1:1 mixture of concentrted sulphuric cid nd nitric cid (Fisher Chemicls), thoroughly rinsed with ultr pure wter nd trnsferred to stem bth for 20 min. Ellipsometry. Ex-situ ellipsometric results were obtined using rotting nlyser type SENTECH Ellipsometer (SENTECH Instruments GmbH, Berlin, Germny) fitted with He-Ne lser (632.8 nm). The mesurements were crried out t n ngle of incidence of 70. The gold electrodes were dried with N 2 nd mesured before nd fter immersion in the coting solution. Imging ellipsometry [9] ws performed in conventionl null ellipsometer where n expnded light bem is used insted of the conventionl nrrow bem. The source ws Xenon lmp with nm interference filter nd collimting system to ensure uniform intensity distribution over the bem re. The photomultiplier or photodiode detector ws replced by CCD video cmer (752 x 582 pixels) nd suitble optics llowing more thn x 10 5 surfce res on 15 x 25 mm 2 smple surfce to be mesured t the sme time. The video signl is digitised, cptured nd stored in greyscle formt. Results nd Discussion Fig 1. Structures of the ferrocenyllkylthiols used s electrode modifiers. Fc corresponds to ferrocene, (C5H 5 )Fe(C 5 H). Au Substrtes. The gold slides used were mde by the evportion of 200 nm Au on glss slides (Tempx AF 5) which hve been coted with Cr lyer (2- nm). Prior to use, the gold slides were Electrochemicl Studies. As expected, the redox behviour exhibited by the monolyers ws wholly dominted by the ferrocene/ferricinium couple. Effect of chin length nd deposition time on the stbility, pcking nd order of the monolyers were nlysed by evluting the following prmeters: 1) surfce coverge, r Fc, obtined by the integrtion of the re under the redox peks of ferrocene; 2) hlf

3 wve potentils (Em); 3) full width t hlf mximum (AEFWHM) nd ) redox pek seprtion ((AEp). vlues for the three compounds re very smll, they seem to indicte tht short chin monolyers re more esily oxidised thn the correspondent longer ones. On the other hnd, the voltmmetric pek seprtion decreses significntly with the number of CH2 groups. The vlue for compound FcC8 ws very close to the idel 0 mv, for dsorbed species showing reversible electron trnsfer process. The smll AE P vlues obtined for monolyers of FcC6 nd FcC8 indicte tht the ferrocene is surfce-confined nd tht the rte of electron trnsfer is reltively fst. Tble 1. Effect of chin length on the electrochemistry of the investigted SAMs. Compound r Fc (molcm 2 ) E(mV) AEFWHM (mv) AEp(mV) E/mV(vsSCE) E/mV (vs SCE) E/mV (SCE) Figure 2. Representtive cyclic voltmmogrms of SAMs prepred from 1-2 rm solutions of compounds FcC (), FcC6 (b) nd FcC8 (c), fter hours immersion. The voltmmogrms were recorded t 100 mv/s. Figure 2 shows the typicl cyclic voltmmogrms obtined for SAMs of compounds FcC, FcC6 nd FcC8, prepred under the sme experimentl conditions. Only slight decrese (5-10%) of the surfce coverge, with no chnges in the shpe or position of the voltmmogrms, ws detected fter repetitive potentil cycles. This behviour illustrtes the inherent stbility of these monolyers. As is chrcteristic for n dsorbed redox centre, the nodic pek current for the three monolyers studied ws found to increse linerly with the sweep rte [10]. However, there ws significnt difference between the shpe of the voltmmetric peks for monolyer of compound FcC when compred to compounds FcC6 nd FcC8. The observed redox behviour is intimtely relted to the structure nd orgnistion of the monolyer. The electrochemicl dt obtined from the voltmmetric curves re summrised in Tble 1. Results confirmed tht longer lkyl chins dsorbed preferentilly nd in more ordered structure thn the equivlent shorter chin compounds [1], The mount of molecules dsorbed on the surfce for the sme deposition time nd solution concentrtion increses with the chin length. The coverge of the monolyer formed by compound FcC8 is.7xl0' 10 mol cm" 2, if clculted using roughness fctor of 1.2 determined by the iodine chemisorption method [11]. This vlue is very close to the mximum theoreticl vlue of.5xl0" 10 mol cm" 2 estimted by Chidsey et l. [5] for ferrocene derivtive monolyer, bsed on close pcked lyer of 6.6 A dimeter spheres. The hlf wve potentil vlues for the ferrocene redox process of compounds FcC, FcC6 nd FcC8 were positively shifted from the vlues observed for ferrocene monolyers without the crbonyl group (c. 300 mv vs. SCE) [1]. This is expected due to the presence of the electron withdrwing crbonyl group in the compounds used here. Although the differences between experimentl E1/2 FcC 8.6x10"" 565 FcC6 3.2xl0' FcC8 5.7xl0" Brod nd ill-defined oxidtion pek. The vlue of full width t hlf mximum of the oxidtion pek provides qulittive mesure of the reltive interction between redox centres ttched to the monolyer. For n idel cse, where the interctions between the electroctive dsorbtes re miniml, vlue of 90.3/n mv t C cn be expected [10]. The redox pek for oxidtion of the ferrocene groups belonging to the monolyer of compound FcC8 ws shrper thn the peks for the other monolyers nd the vlue observed for AEFWHM ws very close to the idel vlue. This reinforces the presence of very ordered nd pcked lyer of reltively short lkyl chin, with miniml lterl interctions between the ferrocene moieties. On the other hnd, the shpe of the voltmmogrm of FcC cn be explined in terms of different dsorption sites [1,10]. It hs been suggested tht the reltively hydrophobic ferrocenes occupy sites both inside nd on the surfce of the SAM, conformtion tht would cuse multiple forml potentils to exist in the ensemble of redox centres. Electrochemicl prmeters obtined for different dsorption times of compound FcC6 re summrised in tble 2. The results clerly show tht t millimolr concentrtion, longer deposition time leds to higher surfce coverge. After three dys the mount of compound deposited on the gold ws identicl to the observed fter hours. Both the hlf wve potentil nd pek seprtions do not chnge significntly with the deposition time. This fct seems to indicte tht the mechnism of electron trnsfer is somewht independent of the mount of compound t the electrode surfce for given chin length. The vlues of AEFWHM were lrger thn the theoreticl vlue, which re possibly due to the existence of ttched ferrocene til molecules with different microenvironments within the monolyer. However, longer deposition times nd higher surfce coverge produced...» 80

4 shrper oxidtion peks. This behviour indictes the presence of more orgnised lyer with incresing dsorption time. Tble 2. Electrochemicl dt obtined for monolyer of FcC6 t different times of dsorption. Adsorption time r Fc (molcm 2 ) E m (mv) ' AEFWHM (mv) AEp (mv) 5 min 1.x10 ' min 2.3x lh 2.3xl0" h 2.5x10' h 3.2x10"' Brod nd ill-defined oxidtion pek Ellipsometric Studies Ex situ ellipsometry ws pplied s convenient nd powerful men of evluting the verge monolyer thickness nd for following the dsorption process. The thickness of the monolyers ws estimted from the chnge in the ellipsometric prmeters between the bre gold nd the monolyer covered surfces. Before depositing the lyer the substrtes were mesured nd their complex refrctive indices («= n - i k, where n is the refrctive index nd k the extinction coefficient) clculted from the opticl prmeters *F nd A, using Fresnel's nd Drude's [12] equtions. The vlues of n, k nd thickness of the monolyers deposited on gold could be then estimted from the vlues of 5^ nd 5A, using computer progrm bsed on three-phse model. Since the determintion of three unknowns from two mesurble prmeters is mthemticlly impossible, one of the unknowns hd to be resonbly introduced to deduce the other two. Since the lyers were not trnsprent (k smll but different from zero), the clcultions were not strightforwrd, nd the refrctive index nd lyer thickness were deduced in two steps. First ssuming n s 1.5, which is the vlue estimted for the index of refrction of lknethiols [1,5], k nd the thickness were clculted. The other step, consisted of predicting the thicknesses for compounds, lso bsed on previous studies performed for lkylthiols (20 A for CH 3 (CH 2 )i 0 SH; 1.07 A per CH 2 nd 7 A for ferrocene group [6]), compring them with the previous vlues, nd introducing series of thicknesses close to the verge to evlute n nd k vritions. Ellipsometric dt obtined for the monolyers of ferrocenyllkylthiols re shown in tble 3, nd re finl result of the two steps mentioned bove. The predicted thicknesses for the monolyers formed by compounds FcC, FcC6 nd FcC8 re 2.1, 2.3 nd 2.5 nm respectively. After hours immersion, the electrochemicl dt indictes tht the monolyers re pcked nd ordered. The experimentl thickness vlues obtined for monolyers of FcC6 nd FcC8, were slightly lrger thn the predicted ones, but still in good greement with them. However, immersion for hours resulted in thinner films, which is indictive of the fct tht the dsorption process occurs in two stges. The first step consists of rpid formtion of the monolyer followed by slow pproch to more orgnised stge [1], where the molecules become more perpendiculr to the gold surfce nd the ferrocene centres mintin well-defined distnce from the surfce. Our electrochemicl results hve shown tht further thiol dsorption occurs during the orgnistion stge. Resonble vlues for complex refrctive index nd thickness of FcC monolyer, could not be obtined fter hours immersion. Furthermore, the verge thickness deduced for this monolyer fter hours immersion, ws higher thn the vlues found for the longer chin compounds. This fct is most probbly relted with the low coverge nd loosely pcked structure of the monolyer s observed in the voltmmetric studies. One possible fctor contributing to these observtions is the thiol chemisorption s thiolte, which my produce chnges in electron density t the gold surfce, shifting its opticl constnts reltive to the bre unmodified substrte [1,6,13]. Such effect could be coverge dependent, which cn explin its lrger influence in the ellipsometric prmeters on SAMs formed from FcC. Tble 3. Ellipsometric dt obtined for the three SAMs under study fter nd hours of immersion in 1-2 mm solutions of FcC, FcC6 nd FcC8., Adsorption Time, ~,,,. Compound 7. n k Thickness (nm) (hours) FcC FcC6 FcC8 No resonble vlues were obtined Imging ellipsometry ws performed on monolyer formed by FcC6 (figure 3). This recent technique provides the visulistion of the thickness distribution of lyer deposited on substrte. The distribution is mesured s difference in the reflected light intensity between the bre substrte nd the lyer, with the thickness being proportionl to the squre root of the intensity. Imging ellipsometry does not give bsolute vlues of thickness, so n independent mesure of the opticl prmeters in the conventionl ellipsometer ws performed. The complex refrctive index nd the lyer thickness obtined for the monolyer visulised in figure 3 were respectively Í nd 2.6 nm

5 Figure 3. Imge (15x20 mm 2 ) of thickness distribution of monolyer of FcC6 () on Au (111) (b) obtined with imging ellipsometry. 10. R. C. Sbpthy, S. Bhttchryy, M. C. Levy, W. E. Clelnd Jr., C. H. Hussey, Lngmuir, 1 (1998) J. F. Rodriguez, T. Mebrhtu, M. P. Sorig, J. Electronl. Chem., 233 (1987) R. Greef New techniques for the study of electrodes nd their rections, Ed by R. G. Compton, vol. 29 of Comprehensive Chemicl Kinetics, Elsevier, Amsterdm, M. D. Porter, T. B. Bright, D. L. Allr, nd C. E. D. Chydsey, J. Am. Chem. Soc, 109 (1987) Conclusions We hve demonstrted tht the redox behviour nd structure of short chin monolyers is strongly dependent on the chin length, even when they differ of two methylene groups. Longer lkyl chins produce more oriented nd pcked monolyers. At millimolr solution concentrtion long dsorption times (up to hours) were required to obtin more orgnised SAMs with higher surfce coverge. This result, together with ellipsometric thicknesses obtined t different deposition times, confirm tht dsorption of thiol molecules on gold occurs in two steps. We re currently investigting using in situ ER spectroscopy nd in situ ellipsometry, possible structurl chnges occurring within the monolyers of short chin ferrocenyllkylthiols. Received, June 15, 1999 Accepted, August 12, 1999 References 1 H. O. Finkle, in Electronlyticl chemistry: A series of Advnces, vol. 19, ed. by A. J. Brd nd I. Rubinstein, Mrcell Dekker Inc., New York (1996). 2 S. Arnold, Z. Q. Feng, T. Kkiuchi, W. Knoll, K. Niki, J. Electronl. Chem., 38 (1997) G. Che, Z. Li, H. Zhng, C, R Cbrer, J. Electronl. Chem., 53 (1998) T. Kondo, M. Tkechi, Y. Sto, K. Uoski, J. Electronl. Chem., 381 (1995) C. E. D. Chidsey, C. R. Bertozzi, T. M. Putvinski, nd A. M. Mujsce, J. Am. Chem. Soc, 112 (1990) T. Uhtsuk, Y. Sto, nd K. Uoski, Lngmuir, 10 (199) A. Jones, PhD Thesis, University of Wles, Bngor (1999) 8. W. Hiss, D. Lckey, J. K. Sss, K. H. Besocke, J. Chem. Phys., 95 (1991) G. Jin, P. Tengvll, I. Lundstrom, H. Arwin, Anlyticl Biochemistry, 232 (1995)

UNIVERSITY OF MALTA DEPARTMENT OF CHEMISTRY. CH237 - Chemical Thermodynamics and Kinetics. Tutorial Sheet VIII

UNIVERSITY OF MALTA DEPARTMENT OF CHEMISTRY CH237 - Chemicl Thermodynmics nd Kinetics Tutoril Sheet VIII 1 () (i) The rte of the rection A + 2B 3C + D ws reported s 1.0 mol L -1 s -1. Stte the rtes of