The impact of surface coverage on the kinetics of electron transfer through redox monolayers on a silicon electrode surface

Electrochimica Acta 186 (2015) 216–222

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

The impact of surface coverage on the kinetics of electron transfer through redox monolayers on a silicon electrode surface Simone Ciampia,1,* , Moinul H. Choudhurya , Shahrul Ainliah Binti Alang Ahmada , Nadim Darwisha,2 , Anton Le Brunb , J.Justin Goodinga,* a b

School of Chemistry and the Australian Centre for NanoMedicine, University of New South Wales, Sydney 2052, Australia Bragg Institute, Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC NSW 2232, Australia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 August 2015 Received in revised form 29 September 2015 Accepted 22 October 2015 Available online 28 October 2015

The impact of the coverage of ferrocene moieties, attached to a silicon electrode modified via hydrosilylation of a dialkyne, on the kinetics of electron transfer between the redox species and the electrode is explored. The coverage of ferrocene is controlled by varying the coupling time between azidomethylferrocene and the distal alkyne of the monolayer via the copper assisted azide-alkyne cycloaddition reaction. All other variables in the surface preparation are maintained identical. What is observed is that the higher the surface coverage of the ferrocene moieties the faster the apparent rates of electron transfer. This surface coverage-dependent kinetic effect is attributed to electrons hopping between ferrocene moieties across the redox film toward hotspots for the electron transfer event. The origin of these hotspots is tentatively suggested to result from minor amounts of oxide on the underlying silicon surface that reduce the barrier for the electron transfer. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Hydrosilylation Si(100) monolayers electrode kinetics

1. Introduction Electrode surfaces that are modified with an organic monolayer [1–3] have attracted an enormous amount of academic interest as the platform for molecular scale devices and are behind the development of fields as diverse as biosensing [4,5], molecular electronic circuitry [6,7] and energy conversion [8]. In particular, quite exquisite control over the lateral and vertical spacing of surface-bound redox species relative to the electrode surface has helped in understanding the subtleties of electrode kinetics [9,10]. However, most of the work to date has been performed on gold electrodes with alkanethiol chemistry [11]. An expansion of these surface chemistry systems towards silicon electrodes would have tremendous technological implications [12,13]. This is because (i) the electronics properties of the substrate are easily tuned in predictable fashion [14] and (ii) technological implementations of these surface systems are compatible with the well-established

* Corresponding author. E-mail addresses: [email protected] (S. Ciampi), [email protected] (J.J. Gooding). 1 Present Address: ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Wollongong 2500, Australia 2 Present Address: Departament de Química Física, Universitat de Barcelona, Diagonal 645, Barcelona 08028, Spain http://dx.doi.org/10.1016/j.electacta.2015.10.125 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

semiconductor industry [15]. However, as for any non-oxide semiconductor [16], anodic decomposition poses serious practical limitations. Until recently the most notable limitation of Si-based electrodes was the lack of a stable surface chemistry that is compatible with electrodes operating in aqueous media [17]. A major breakthrough came with the discovery by Chidsey and co-workers that very stable monolayers could be formed on hydrogen terminated silicon surfaces via the hydrosilylation (i.e., the insertion of hydrogen and silicon) of organic molecules possessing terminal alkenes or alkynes giving an Si

C-bonded organic monolayer [18–20]. This surface chemistry has allowed redox monolayers to be formed on silicon electrodes surfaces although in most cases significant silicon oxide, which has a deleterious influence on the surface electrochemistry, was still observed [21,22]. More recently, close to ideal surface electrochemistry for ferrocene-terminated monolayers on Si(111) [23] and Si(100) [24,25] have been reported. In a study by us, there was no observable oxide formed, as determined using X-ray photoelectron spectroscopy (XPS) on the Si(100) surface even after 200 redox cycles at potentials that would oxidise bare silicon in aqueous solution [24]. The ability to perform electrochemistry at silicon electrodes without the formation of oxide removes a key barrier for the implementation of silicon-based devices in sensing, electrocatalysis and molecular circuitry. However, thus far studies into electron transfer at redox monolayers on silicon electrode are very

S. Ciampi et al. / Electrochimica Acta 186 (2015) 216–222

much in their infancy [13] with no discussion of the impact of the surface density of redox species. The seminal 1990 work by Chidsey on redox-active self-assembled monolayers on gold electrodes carried a message of warning relevant to this latter point [26]. Chidsey provided experimental evidence of electron hopping between adjacent ferrocene units; in the word of the author “evidence for a defect-mediated pathway for electron transfer at high coverage”. In line with a hopping mechanism between adjacent ferrocene units, Chidsey did observe an increase in the apparent rate constant with increasing coverages of ferrocene. In contrast, at least three recent reports present data for surfacebound systems that exhibits slower electron transfer kinetics with an increase in surface coverage of the redox species at monolayer modified electrodes. For “metallic” electrodes an explanation invoking disorder was put forward by Hamers and co-workers; i.e. at sufficiently low coverages conformational disorder in the redox film leads to fast tunneling through space becoming the dominant charge transport mechanism [27]. This coverage/kinetic feature is also in line with a report of porphyrin-containing films anchored to gold electrodes [28]. For Si–O-bound layers directly grafted on Si (100) electrodes a similar trend in a decreasing rate constant for electron transfer with increasing surface coverage of the redox species was attributed by Bocian and co-workers to drops in counter-ion mobility at high coverages [29]. It is important to note that, in general, variations in the amount of monolayer coverage bring about both kinetic effects as well as thermodynamic ones. For instance, in Bocian and co-workers report [29], as the coverage increased, the apparent redox potential of the Fc/Fc+ couple shifted anodically. This is a general phenomenon, where space considerations allow the formation of mixed-valence complexes [30], hence shifts in potential or peak broadening in voltammetric curves, are observed. The purpose of the work presented in this paper is to explore the impact of the coverage of ferrocene-terminated monolayers on silicon electrodes on the kinetics of electron transfer. We begin to address what happens to the kinetic/coverage interplay when precautions are taken to (i) limit through-space tunneling, as at metallic electrodes and (ii) when we select a redox species that displays minimal thermodynamic effects with changes in coverage. We explore this via chemical derivatization of oxide free Si (100) electrode surfaces using the hydrosilylation of 1,8-

217

nonadiyne, 1, which we have shown previously forms well defined monolayers with a distal alkyne that is amenable to further modification with azide species according to the copper assisted azide-alkyne cycloaddition reaction (CuAAC [31], see Scheme 1) [32,33]. The azido derivative used here is azidomethylferrocene, 2 and the redox coverage is controlled by adjusting the time of the CuAAC reaction. 2. Experimental 2.1. Materials 1,8-Nonadiyne (1, Alfa Aesar, 97%) was redistilled from sodium borohydride (Sigma-Aldrich, 99+%) under reduced pressure (80  C, 10–12 Torr) and stored under a high purity argon atmosphere (H2O < 10 ppb, O2 < 5 ppb) prior to use. Milli-QTM water (>18 MV cm) was used to prepare solutions and for chemical reactions. Dichloromethane, and ethanol for surface cleaning procedures were redistilled prior to use. Azidomethylferrocene 2 was prepared according to literature procedures [33]. Milli-QTM water (>18 MV cm) was used to prepare solutions for chemical reactions, electrochemical measurements and surface cleaning procedures. Highly-doped (p++, HD) prime grade double-side polished silicon wafers, 100-oriented (<100>  0.9 ), p-type (boron-doped), 500  25 mm thick, 0.007–0.009 V cm resistivity, were obtained from Virginia Semiconductors, Inc. (VA, USA). 2.2. Surface modification 2.2.1. Acetylene-functionalized Silicon(100) Surface (S-1) Assembly of the acetylenylated Si(100) surface by covalent attachment of the diyne 1 followed a previously reported procedure [24,32]. In brief, silicon wafers were cut into pieces (approximately 10  20 mm), cleaned for 30–40 min in hot Piranha solution (100  C, 1 vol 30% by mass aqueous hydrogen peroxide/ 3 vol sulfuric acid), before being transferred first to an aqueous fluoride solution (2.5% hydrofluoric acid, 1.5 min). Subsequently, the samples were transferred, taking extra care to exclude air completely from the reaction vessel (a custom-made Schlenk flask), to a degassed (through a minimum of 4 freeze–pump–thaw cycles) sample of diyne 1. The samples were kept under a stream of

Scheme 1. Thermal hydrosilylation of 1,8-nonadiyne 1 at a Si(100)-H electrode (S-1). Attachment of azidomethylferrocene 2 via CuAAC “click” reactions to yield redox-active Si(100) electrodes (S-2).

218

S. Ciampi et al. / Electrochimica Acta 186 (2015) 216–222

argon (H2O < 10 ppb, O2 < 5 ppb) while the reaction vessel was immersed in an oil bath set to 165  C for 3 h. The flask was then opened to the atmosphere, and the functionalized surface samples (S-1) were rinsed several times with dichloromethane and rested for a 12-h period in a sealed vial at +4  C under dichloromethane, before being either analyzed or further reacted with the redox molecule 2. 2.2.2. CuAAC attachment of azidomethylferrocene onto the acetylenyl surface (S-2) To a reaction vial containing the alkyne-functionalized silicon surface were added (i) azidomethylferocene (2, 10 mM, 2propanol/water, 2:1), (ii) copper(II) sulfate pentahydrate (0.8 mol % relative to the azide) and (iii) sodium ascorbate (80 mol% relative to the azide). Reactions were carried out in air, at room temperature, under ambient light, and stopped by removal of the samples from the reaction vessel after a reaction time that varied from ca. 30 s to ca. 45 min. The prepared surface-bound [1,2,3]-triazoles (S-2) were rinsed consecutively with copious amounts of water and ethanol, and then rested at room temperature for a 1-min period in a 0.5 M hydrochloric acid solution. Samples were then rinsed with copious amounts water and ethanol before being analyzed. 2.3. Surface analysis 2.3.1. XPS measurements X-ray photoelectron spectra were obtained on an ESCALAB 220iXL spectrometer fitted with a monochromatic Al Ka source (1486.6 eV), a hemispherical analyzer and a 6 multichannel detector. Spectra of Si 2p (93–109 eV), C 1s (279–295 eV), N 1s (392–408 eV) and Fe 2p (700–735 eV) were recorded in normal emission (u = 90 ) with the analyzing chamber operating below 10
9 mbar. The atomic compositions were corrected for atomic sensitivities and measured from high-resolution scans. Atomic sensitivity factors are often instrument-sensitive and so the atomic sensitivities obtained from the Avance software interfaced with the spectrometer were 0.817 for Si 2p, 1.000 for C 1s, 1.800 for N 1 s, 16.420 for Fe 2p. The resolution of the spectrometer is ca. 0.6 eV as measured from the Ag 3d5/2 signal (full width at half maximum, fwhm) with 20 eV pass energy. High-resolution scans were run with 0.1 eV step size, dwell time of 100 ms and the analyzer pass energy set to 20 eV. After background subtraction using the Shirley routine, spectra were fitted with Voigt functions (a convolution of Lorentzian and Gaussian profiles) as described previously [24,32,33]. All energies are binding energies expressed in eV, obtained by applying to all samples a rigid shift to bring the binding energy of the C 1 s peak to a value of 285.0 eV. The spectrometer silica detection limit can be approximated to ca. 0.06 monolayers equivalents [34]. 2.3.2. X-ray reflectometry X-ray reflectivity (XRR) profiles of the self-assembled surfaces were measured under ambient conditions on a Panalytical Ltd X'Pert Pro Reflectometer using Cu Ka X-ray radiation (l = 1.54056 Å). The X-ray beam was focused using a Göbel mirror and collimated with 0.2 mm pre-sample slit and a post-sample parallel plate collimator. Reflectivity data were collected over the angular range 0.05  u  5.00 , with a step size of 0.010 and counting times of 10 s per step. Prior to measurements, samples were stored under argon and exposed to air for approximately 10 min in order to be aligned on the reflectometer. From the experimental data, structural parameters of the self-assembled structures were refined using the MOTOFIT analysis software with reflectivity data presented as a function of the momentum transfer vector normal to the surface Q = 4p(sinu)/l [35]. The Levenberg-

Marquardt method was used to minimize x2 values in the fitting routines. Single-layer or two-layer models were used to fit the observed data. XRR data for S-1 samples (Fig. S1, Supplementary data) are in agreement with previous reports [32,36,37]. 2.3.3. Electrochemical characterization All electrochemical experiments were performed in a PTFE three-electrode cell with the modified silicon surface as the working electrode, a platinum mesh (ca. 1200 mm2) as the counter electrode, and Ag|AgCl in 3 M NaCl as the reference electrode. The counter electrode used in SECM experiments was a platinum wire. All potentials are reported versus the reference electrode. A rectilinear cross-section Viton1 gasket defined the geometric area of the working electrode to 24.6 mm2. The bulk of the back-side of the silicon sample was exposed with emery paper and rubbed with gallium indium eutectic. A planar copper electrode pressed to the sample backside served as an ohmic contact. The cell was enclosed in a grounded Faraday cage during all measurements. Cyclic voltammetry (CV) and square-wave (SW, square-wave amplitude was 25 mV) voltammetry measurements were performed using a BAS 100B electrochemical analyzer (Bioanalytical Systems, Inc., W. Lafayette, IN). Electrochemical impedance measurements (EIS) were performed with a Solartron 1255B (Farnborough, UK) frequency response analyzer interfaced to a Solartron 1287 potentiostat/galvanostat module. Impedance data were collected at 60 frequencies in the frequency range of 0.01 Hz to 0.1 MHz. An ac potential amplitude of 15 mV root mean square was added to the dc potential (Edc) of the working electrode. CV and EIS measurements for ferrocenyl films (S-2) were performed in aqueous perchlorate-based electrolytes (1.0 M HClO4). Both the in-phase (Z0 ) and out-of-phase impedance (Z00 ) were extracted at the same time from the data and analyzed with the ZView 3.1 and ZPlot softwares (Scribner Associates, Inc.). The formalism developed by Laviron was used to obtain kinetic information from EIS [38,39], namely the apparent electron-transfer rate constant, ket, for the electron-transfer process between tethered redox groups and the conducting substrate. Measurements of redox kinetics by the analysis of the quasi-reversible maximum in square-wave voltammetry followed the method developed by Lovri c [40]. All electrochemical experiments were performed at room temperature (23  2  C) in air. 3. Results The experimental variation in electron transfer kinetics (ket) as a function of ferrocene coverage is shown in Fig. 1a. The important features to note from Fig. 1a are as the coverage of ferrocene units increases the value of ket shows an average increase but also a greater scatter in the data. The coverage of ferrocene derivatives on the surface was determined for over 50 independently prepared and analyzed S-2 samples by integration of the current-voltage trace in the cyclic voltammograms (CVs), while electrochemical impedance spectroscopy (EIS) data were used to determine the kinetic parameters for the electron transfer process. The equivalent circuits used to fit the EIS data is shown in Fig. S2 (Supplementary data). The near ideal surface CVs are shown in Fig. 2a and c. The coverage of the ferrocenyl derivative determined from CV ranges over two orders of magnitude from 2. 10
12 mol cm
2, as the low end, up to the value of 2.8  10
10 mol cm
2 for a highly-packed layer of redox ferrocenyl derivatives (Fig. 1a). Due to both the high levels of dopant used in this study (p++,ca. 1019 cm
3) and the sufficiently anodic redox potential of ferrocene, space-charge effects are unlikely to be involved. This is because in the case of a p-type semiconductor (here p++), accumulation is achieved by making the potential of the electrode positive with respect to flatband potential (EFB). Using as reference the data of Föll and co-

S. Ciampi et al. / Electrochimica Acta 186 (2015) 216–222

Fig. 1. Electrochemical characterization of S-2 samples on p++ Si(100) electrodes. Semilogarithmic plots of (a) ket versus surface coverage (G ), and (b) evolution of the cyclic voltammetry-determined full width at half maximum (Efwhm) versus G (scan rate = 100 mV s
1).

219

workers [41], and by taking into account the density of acceptors calculated from the nominal resistivity data of the silicon wafers used (ca. 1.1 1019 cm
3 for p++), the value of EFB can be estimated [42] to be around
30 mV vs. Ag|AgCl|3 M NaCl. Representative EIS data used to estimate kinetics parameters are presented as Bode plots in Fig. 2b and d (data for two surfaces of similar surface coverage but quite different kinetics), which allows for a very rapid visualization of the kinetic differences. At an AC frequency that approaches the time constant of the redox reaction, the slope of Z vs f plots goes to zero and the value of phase angle reach a minimum. For a more quantitative treatment of the kinetic properties of S-2 samples we employed a well-established modeldependent formalism that describes the relationship between the EIS circuit elements (Cads, adsorption pseudo-capacitance; Rct, charge-transfer resistance, circuit A, Fig. S2, Supplementary data) and the kinetic parameters of strongly adsorbed electro-active species [38,39,43,44]. It is noteworthy that Cads, as well as Cdl (double-layer capacitance), showed frequency-dependent behavior and were therefore treated as constant-phase elements (CPEs). For a CPE the impedance is equal to 1/(jv)’Q, in which v is the angular frequency, Q is a parameter with the units of V
1cm
2 s, and ’ is an exponential term with values between 0 and 1. CPE defines inhomogeneity in the electrochemical system, such as kinetic dispersion [45]. In our redox monolayer systems the CPEs behave very similar to a capacitor, as the power-law modifiers have values 0.95  w  0.99, where ’ equal to unity indicates an ideal capacitor. By approximating the value of Cads by the corresponding Q value, the apparent electron-transfer rate constant, ket, can be simply expressed as ket = (2RctCads)
1. Most importantly, the value of ket increases over one order of magnitude from 18 s
1 (at 2.8  10
12 mol cm
2) to as high as 198 s
1 (at 2.1 10
10 mol cm
2) as the surface coverage of ferrocene species increases. Crucial to the claims of this paper is to rule out that the increase in ket with coverage presented in Fig. 1a is not a mere artefact of the

Fig. 2. Electrochemical characterization of S-2 samples prepared via CuAAC reactions of azidomethylferrocene 2 with S-1 electrodes. Shown are representative cyclic voltammograms ((a) and (c), n = 100 mV s
1) and EIS Bode plots ((b) and (d)) for samples that differ in ferrocene coverage by less than 5% (data presented here refers to the points highlighted as solid symbols in Fig. 1a). EIS data were obtained are at an applied potential Edc = E1/2. Frequency range was from 100 kHz to 100 mHz. EIS data were interpreted by curve fitting the data to the equivalent circuit of Fig. S2 (Supplementary data). All symbols are experimental data, and solid lines are best fits to the data (x2 < 0.0005). The refined ket values were 159 s
1 (Rs = 9.5 V, Cdl = 1.6 mF (w = 0.94), Cads = 38.5 mF (w = 0.98), Rct = 81.6 V, G = 2.28  10
12 mol cm
2) and 96 s
1 (Rs = 9.8 V, Cdl = 1.4 mF (w = 0.95), Cads = 36.9 mF (w = 0.98), Rct = 141.9 V, G = 2.37  10
12 mol cm
2).

220

S. Ciampi et al. / Electrochimica Acta 186 (2015) 216–222

EIS analysis. For instance, variations in the RC circuit could arise from a combination of kinetic and coverage effects. Toward this point we have validated the EIS kinetic data with information that can be extracted from the analysis of square-wave voltammetry [40] (SWV, Fig. 3 and S3-S4, Supplementary data). The kinetics parameters extracted from SWV are known to be independent on the amount of adsorbed molecules [40] and are found to be within a 10% difference from those obtained from the modelling of EIS data. The quality and packing of the monolayers were determined using XPS, EIS and XRR. XPS data indicate no oxide formation (Fig. S5, Supplementary data). XPS spectroscopic shifts for the Si 2p region introduced by Si(1)
Si(4) oxides are well-documented [46–48]. The silicon oxide content (102–104 eV) was below the spectrometer detection limit, data supporting the S-2 monolayer high quality and its ability to prevent appreciable oxidation of the Si substrate. EIS data gives complementary insights on the monolayer quality (Fig. S6, Supplementary data). At cathodic applied dc potentials (Edc = 0, * in Fig. 3), the phase angle of the Bode plots adopts a sigmoid shape that starts near 0 at high frequency and shifts close to 90 at low frequencies (<100 Hz). The lower-frequency points of an ideal RC series would have to be close to 90 , this is because the response is dominated by the doublelayer capacitance charging. The EIS data suggest a blocking or insulating S-2 monolayer that can be modelled using circuit B in Fig. S2 (Supplementary data), a circuit that describes a polarizing electrode [44] with terms Rs and Cdl in series, where Rs is the solution resistance and Cdl represents the double-layer capacitance. In other words, such a phase angle indicates minimal ion penetration through the monolayer [49]. Circuit B does not model a charge-transfer reaction. Curve fitting results in an interfacial capacity, Cdl, of 4.2  0.1 mF cm
2, a value in good agreement with those obtained by Fawcett and Ulman for alkanethiol SAMs on Au (111) in similar electrolytes at the potential of zero charge [50]. Because the applied potential is ca. 380 mV more cathodic than the

formal potential, it is not surprising that tethered ferrocenyl units are not undergoing a charge-transfer reaction because the monolayer is in its fully reduced form. Essentially, at potentials sufficiently more cathodic of E1/2, Rct increases to infinity (>50 MV), and minimal ion penetration results in only Rs and Cdl terms in the equivalent circuit. Regardless of the ferrocene coverage value, as extracted from CV data the phase angle of the Bode plots shifts near 90 at frequencies of <1 Hz (Fig. 2b, 2d and Fig. S6d, Supplementary data). It is likely that both the convenient positioning of the apparent formal potential (E1/2, 375  7 mV) in a region where ionic permeability is limited [49], together with the high monolayer quality, are contributing to the low-leakage character of the monolayer. In agreement with previous reports [24,25], the specular XRR profiles from S-2 samples were simulated using a one-layer model (Fig. S7, Supplementary data) which indicated a very dense organic layer (SLD 13.5(1)  10
6 Å
2) [51]. This is consistent with the base layer obtained from the grafting of diyne 1 retaining its order after the CuAAC step. It is important to stress that the experimental Efwhm (129  7 mV) of the CV waves [52], even at relatively high sweep rates (100 mV s
1) is independent of the coverage value (Fig. 1b) [53]. Interestingly, given the majority of fundamental electrochemical studies reporting on surface redox systems prepared by single-step immobilization procedures, and with the co-adsorption of “diluent” molecules being often required to achieve nearly ideal voltammetry [26], the close-to-ideal redox behavior of S-2 samples was largely unexpected. On the basis of the conclusions of previous reports [54], we propose that the presence of a triazole link in the structure of construct S-2 does impart a sufficiently high polarity to molecules in the film that aids in preventing the formation of aggregates. This aspect is a key in ensuring that the change in redox coverage does not bring about appreciable thermodynamic effects.

Fig. 3. Representative plots of SW voltammograms for a S-2 sample (G = 2.28  10
10 mol cm
2) and analysis of the ‘quasi-reversible maxima’ [40]. Dependence of the squarewave (SW) current maxima on frequency ((a)-(c)) and plot of the current maxima-to-SW frequency ratio with respect to the SW frequency (d). The frequency was changed between 10 and 1000 Hz. The SW amplitude was set to 25 mV. The SWV-determined ket value is ca. 125 s
1, which is in good agreement with the EIS-derived value (114 s
1, Fig. S4, Supplementary data).

S. Ciampi et al. / Electrochimica Acta 186 (2015) 216–222

4. Discussion Two questions that arise from the electrochemical data presented above; (i) why do the experimental electron transfer kinetics increase with ferrocene coverage and (ii) why is there much greater dispersion in the experimental value of ket at high ferrocene coverage than at low coverage? Turning first to the question of why ket increases with coverage; it is likely that this relates to lateral hopping of electrons between adjacent redox sites across the organic layer. Certainly such a phenomenon has been suggested by Chidsey (vide supra) [26], described in mathematically by Blauch and Saveant in terms of percolation theory [55], and such a theory on electron hopping has been later invoked by Riveros et al. to explain, at a qualitative level, kinetic anomalies for a monolayer systems closely related to ours [56]. Thus, it can be hypothesized that the extent of surface hopping to fast sites, hotspots hereafter, could be linked to the observed variation in kinetics with changes in ferrocene coverages (Fig. 1a). Hortholary and Coudret have also reported an experiment that is particularly relevant to our discussion. They formed a monolayer system containing two redox species; a ferrocene- and a ruthenium-derivative along with an inert diluent molecule. The ruthenium species has a 20 fold faster ket than the ferrocene and the relative surface density of the two redox species can be varied systematically. As the ratio of ferrocene-to-ruthenium increases, there was a significant increase in ket for the ferrocene but little change in ket for the ruthenium complex. If we regard the ruthenium species as hotspot to electron transfer, then it becomes apparent how electron-hopping in conjunction with the presence of putative hotspots may account for the ket vs. coverage trend we have measured for S-2 samples (Fig. 1a). However, the question for our study is now what these hotspots are likely to be. It is possible, as we have in fact previously suggested via molecular junction experiments, that miniscule amounts of silicon oxide can counterintuitively decrease the Schottky barrier in their vicinity [57]. We have previously shown how this barrier-lowering effect can still be present for organic monolayers that are formed on silicon surfaces even when no oxide is measurable by laboratory XPS instruments (e.g. Fig. S5, Supplementary data). An increase of ket as a result of minor oxidative defects of the substrate is consistent with the greater variability in ket measured for the higher surface coverage data (Fig. 1a). This is because the way the high coverage surfaces are prepared is by using longer coupling times, i.e. longer exposure to oxidative preparative conditions (cuprous species in water-based systems). Thus if defects do occur, then faster ket values are expected. However, the nature of these hotspots, their number and the electron transfer kinetics at different hotspots are ill-defined at this stage and hence, modelling the behaviour using for instance published models [58] does not seem prudent until there is some control over these hostpots. 5. Conclusions Herein we have explored the electron transfer kinetics from a covalently attached ferrocenyl derivative, through an organic monolayer Si-C bound to a Si(100) electrodes as a function of the coverage of the redox species. The experimental value of ket is shown to increase markedly with surface coverage of the redox species which we tentatively explain as a result of surface hopping of electrons to electron transfer hotspots. We propose these hotspots are small oxidative defects in the monolayer where a minor amount of silicon oxide forms which reduces the barrier to electron transfer between the organic monolayer and the underlying silicon surface. If the hypothesis of miniscule amounts of silicon oxide forming electron transfer hotspots is correct what are the implications? We feel that such minor amounts of silicon

221

oxide does not have a deleterious effect on performance of charge storage or electrocatalytic devices, as we have shown that there is no apparent change in oxide levels over prolonged voltammetry performed in aqueous solution [24,25,59]. However, in studies where well defined electron transfer kinetics is important, even better layers are required. These will be the subject of further work by us. Acknowledgements This research was supported by the Australian Research Council's Discovery Projects Funding Scheme (DP1094564 and DP150103065). We would also like to thank Professor Christian Amatore for useful discussions on this data. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http:// Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.10.125. References [1] M.S. Wrighton, Surface functionalization of electrodes with molecular reagents, Science 231 (1986) 32. [2] J.J. Gooding, F. Mearns, W. Yang, J. Liu, SAMs into the 21st century, Electroanalysis 15 (2003) 81. [3] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology, Chem. Rev. 105 (2005) 1103. [4] Y. Xiao, F. Patolsky, E. Katz, J.F. Hainfeld, I. Willner, Plugging into Enzymes: Nanowiring of Redox Enzymes by a Gold Nanoparticle, Science 299 (2003) 1877. [5] E.L.S. Wong, J.J. Gooding, Charge Transfer through DNA: A Selective Electrochemical DNA Biosensor, Anal. Chem. 78 (2006) 2138. [6] G. Ashkenasy, D. Cahen, R. Cohen, A. Shanzer, A. Vilan, Molecular Engineering of Semiconductor Surfaces and Devices, Acc. Chem. Res. 35 (2002) 121. [7] N. Darwish, A.C. Aragonès, T. Darwish, S. Ciampi, I. Díez-Pérez, MultiResponsive Photo- and Chemo-Electrical Single-Molecule Switches, Nano Lett. 14 (2014) 7064. [8] R. Har-Lavan, I. Ron, F. Thieblemont, D. Cahen, Toward metal-organic insulatorsemiconductor solar cells, based on molecular monolayer self-assembly on nSi, Appl. Phys. Lett. 94 (2009) 043308. [9] D.M. Adams, L. Brus, C.E.D. Chidsey, S. Creager, C. Creutz, C.R. Kagan, P.V. Kamat, M. Lieberman, S. Lindsay, R.A. Marcus, R.M. Metzger, M.E. Michel-Beyerle, J.R. Miller, M.D. Newton, D.R. Rolison, O. Sankey, K.S. Schanze, J. Yardley, X. Zhu, Charge Transfer on the Nanoscale: Current Status, J. Phys. Chem. B 107 (2003) 6668. [10] N. Darwish, M.N. Paddon-Row, J.J. Gooding, Surface-Bound Norbornylogous Bridges as Molecular Rulers for Investigating Interfacial Electrochemistry and as Single Molecule Switches, Acc. Chem. Res. 47 (2014) 385. [11] J.J. Gooding, S. Ciampi, The Molecular Level Modification of Surfaces: From Self-Assembled Monolayers to Complex Molecular Assemblies, Chem. Soc. Rev. 40 (2011) 2704. [12] M.C. Hersam, N.P. Guisinger, J.W. Lyding, Silicon-based molecular nanotechnology, Nanotechnology 11 (2000) 70. [13] B. Fabre, Ferrocene-Terminated Monolayers Covalently Bound to HydrogenTerminated Silicon Surfaces. Toward the Development of Charge Storage and Communication Devices, Acc. Chem. Res. 43 (2010) 1509. [14] M.U. Herrera, T. Ichii, K. Murase, H. Sugimura, Use of Diode Analogy in Explaining the Voltammetric Characteristics of Immobilized Ferrocenyl Moieties on a Silicon Surface, ChemElectroChem 2 (2015) 68. [15] P. Ball, Material witness: Silicon still supreme, Nat. Mater. 4 (2005) 119. [16] A.J. Bard, M.S. Wrighton, Thermodynamic Potential for the Anodic Dissolution of n-Type Semiconductors: A Crucial Factor Controlling Durability and Efficiency in Photoelectrochemical Cells and an Important Criterion in the Selection of New Electrode/Electrolyte Systems, J. Electrochem. Soc. 124 (1977) 1706. [17] Developing stable redox molecular devices on Si(100) has long been a key step toward the development of hybrid semiconductor/organic devices. The stabilization to surface corrosion has proven challenging and it would normally require inert atmospheres and organic solvents-based electrolyte solutions. [18] M.R. Linford, C.E.D. Chidsey, Alkyl monolayers covalently bonded to silicon surfaces, J. Am. Chem. Soc. 115 (1993) 12631.

222

S. Ciampi et al. / Electrochimica Acta 186 (2015) 216–222

[19] M.R. Linford, P. Fenter, P.M. Eisenberger, C.E.D. Chidsey, Alkyl Monolayers on Silicon Prepared from 1-Alkenes and Hydrogen-Terminated Silicon, J. Am. Chem. Soc. 117 (1995) 3145. [20] S. Ciampi, J.B. Harper, J.J. Gooding, Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si-C bonds: surface preparation, passivation and functionalization, Chem. Soc. Rev. 39 (2010) 2158. [21] K. Huang, F. Duclairoir, T. Pro, J. Buckley, G. Marchand, E. Martinez, J.-C. Marchon, B. De Salvo, G. Delapierre, F. Vinet, Ferrocene and Porphyrin Monolayers on Si(100) Surfaces: Preparation and Effect of Linker Length on Electron Transfer, ChemPhysChem 10 (2009) 963. [22] E.J. Faber, W. Sparreboom, W. Groeneveld, L.C.P.M. de Smet, J. Bomer, W. Olthuis, H. Zuilhof, E.J.R. Sudhölter, P. Bergveld, A. van den Berg, pH Sensitivity of Si-C Linked Organic Monolayers on Crystalline Silicon Surfaces, ChemPhysChem 8 (2007) 101. [23] B. Fabre, F. Hauquier, Single-Component and Mixed Ferrocene-Terminated Alkyl Monolayers Covalently Bound to Si(111) Surfaces, J. Phys. Chem. B 110 (2006) 6848. [24] S. Ciampi, P.K. Eggers, G. Le Saux, M. James, J.B. Harper, J.J. Gooding, Silicon (100) Electrodes Resistant to Oxidation in Aqueous Solutions: An Unexpected Benefit of Surface Acetylene Moieties, Langmuir 25 (2009) 2530. [25] S. Ciampi, M. James, P. Michaels, J.J. Gooding, Tandem Click Reactions at Acetylene-Terminated Si(100) Monolayers, Langmuir 27 (2011) 6940. [26] C.E.D. Chidsey, C.R. Bertozzi, T.M. Putvinski, A.M. Mujsce, Coadsorption of ferrocene-terminated and unsubstituted alkanethiols on gold: electroactive self-assembled monolayers, J. Am. Chem. Soc. 112 (1990) 4301. [27] R.E. Ruther, Q. Cui, R.J. Hamers, Conformational Disorder Enhances Electron Transfer Through Alkyl Monolayers: Ferrocene on Conductive Diamond, J. Am Chem. Soc. 135 (2013) 5751. [28] K.M. Roth, D.T. Gryko, C. Clausen, J. Li, J.S. Lindsey, W.G. Kuhr, D.F. Bocian, Comparison of Electron-Transfer and Charge-Retention Characteristics of Porphyrin-Containing Self-Assembled Monolayers Designed for Molecular Information Storage, J. Phys. Chem. B 106 (2002) 8639. [29] K.M. Roth, A.A. Yasseri, Z. Liu, R.B. Dabke, V. Malinovskii, K.-H. Schweikart, L. Yu, H. Tiznado, F. Zaera, J.S. Lindsey, W.G. Kuhr, D.F. Bocian, Measurements of Electron-Transfer Rates of Charge-Storage Molecular Monolayers on Si(100). Toward Hybrid Molecular/Semiconductor Information Storage Devices, J. Am. Chem. Soc 125 (2003) 505. [30] W.L. Davis, R.F. Shago, E.H.G. Langner, J.C. Swarts, Synthesis and electrochemical properties of a series of ferrocene-containing alcohols, Polyhedron 24 (2005) 1611. [31] M. Meldal, C.W. Tornøe, Cu-Catalyzed Azide-Alkyne Cycloaddition, Chem. Rev. 108 (2008) 2952. [32] S. Ciampi, T. Böcking, K.A. Kilian, M. James, J.B. Harper, J.J. Gooding, Functionalization of Acetylene-Terminated Monolayers on Si(100) Surfaces: A Click Chemistry Approach, Langmuir 23 (2007) 9320. [33] S. Ciampi, G. Le Saux, J.B. Harper, J.J. Gooding, Optimization of Click Chemistry of Ferrocene Derivatives on Acetylene-Functionalized Silicon(100), Surfaces Electroanalysis 20 (2008) 1513. [34] L.J. Webb, N.S. Lewis, Comparison of the Electrical Properties and Chemical Stability of Crystalline Silicon(111) Surfaces Alkylated Using Grignard Reagents or Olefins with Lewis Acid Catalysts, J. Phys. Chem. B 107 (2003) 5404. [35] A. Nelson, Co-refinement of multiple-contrast neutron/X-ray reflectivity data using MOTOFIT, J. Appl. Crystallogr. 39 (2006) 273. [36] H. Liu, F. Duclairoir, B. Fleury, L. Dubois, Y. Chenavier, J.-C. Marchon, Porphyrin anchoring on Si(100) using a beta-pyrrolic position, Dalton Trans. (2009) 3793. [37] M. James, T.A. Darwish, S. Ciampi, S.O. Sylvester, Z. Zhang, A. Ng, J.J. Gooding, T. L. Hanley, Nanoscale condensation of water on self-assembled monolayers, Soft Matter 7 (2011) 5309. [38] E. Laviron, A. c. polarography and Faradaic impedance of strongly adsorbed electroactive species. Part I. Theoretical and experimental study of a quasi-

[39]

[40]

[41]

[42]

[43] [44]

[45]

[46] [47]

[48] [49]

[50]

[51] [52]

[53] [54]

[55] [56]

[57]

[58]

[59]

reversible reaction in the case of a Langmuir isotherm, J. Electroanal. Chem. 97 (1979) 135. E. Laviron, The a. c. polarography and faradaic impedance of strongly adsorbed electroactive species. Part III. Theoretical complex plane analysis for a surface redox reaction, J. Electroanal. Chem. 105 (1979) 35. Š. Komorsky-Lovri c, M. Lovri c, Measurements of redox kinetics of adsorbed azobenzene by a quasireversible maximum in square-wave voltammetry, Electrochim. Acta 40 (1995) 1781. S. Ottow, G.S. Popkirov, H. Föll, Determination of flat-band potentials of silicon electrodes in HF by means of ac resistance measurements, J. Electroanal. Chem. 455 (1998) 29. T.L.S.L. Wijesinghe, S.Q. Li, D.J. Blackwood, Influence of Doping Density on the Current-Voltage Characteristics of p-Type Silicon in Dilute Hydrofluoric Acid, J. Phys. Chem. C 112 (2007) 303. S.E. Creager, T.T. Wooster, A New Way of Using ac Voltammetry To Study Redox Kinetics in Electroactive Monolayers, Anal. Chem. 70 (1998) 4257. A.D. Abhayawardhana, T.C. Sutherland, Heterogeneous Proton-Coupled Electron Transfer of an Aminoanthraquinone Self-Assembled Monolayer, J. Phys. Chem. C 113 (2009) 4915. D.A. Brevnov, H.O. Finklea, H. Van Ryswyk, Ac voltammetry studies of electron transfer kinetics for a redox couple attached via short alkanethiols to a gold electrode, J. Electroanal. Chem. 500 (2001) 100. F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff, G. Hollinger, Microscopic structure of the SiO2/Si interface, Phys. Rev. B 38 (1988) 6084. G.F. Cerofolini, C. Galati, S. Reina, L. Renna, Grafting of 1-alkynes to hydrogenterminated (100) silicon surfaces, Appl. Phys. A: Mater. Sci. Process. 80 (2005) 161. S. Ciampi, B. Guan, N. Darwish, P.J. Reece, J.J. Gooding, Redox-Active Monolayers in Mesoporous Silicon, J. Phys. Chem. C 116 (2012) 16080. N. Darwish, P.K. Eggers, S. Ciampi, Y. Zhang, Y. Tong, S. Ye, M.N. Paddon-Row, J.J. Gooding, Reversible potential-induced structural changes of alkanethiol monolayers on gold surfaces, Electrochem. Commun. 13 (2011) 387. R.P. Janek, W.R. Fawcett, A. Ulman, Impedance Spectroscopy of Self-assembled monolayers on Au(111): evidences for complex double-layer structure in aqueous NaClO4 at the potential of zero charge, J. Phys. Chem. B 101 (1997) 8550. SLD for X-rays is obtained by multiplying the electron density (e
/Å3) of the material by the factor 2.82  10–5 Å. An entropically-calculated dispersion value of 90.6mV is expected for an array of identical, independent and isolated ferrocene molecules, and therefore the voltammetric data in this report supports the formation of a well-behaved redox film as the product of the multi-step procedure depicted in Scheme 1. Similar considerations hold for the dependency of E1/2 over the coverage. N.K. Devaraj, R.A. Decreau, W. Ebina, J.P. Collman, C.E.D. Chidsey, Rate of Interfacial Electron Transfer through the 1,2,3-Triazole Linkage, J. Phys. Chem. B 110 (2006) 15955. D.N. Blauch, J.M. Saveant, Dynamics of electron hopping in assemblies of redox centers. Percolation and diffusion, J. Am Chem. Soc. 114 (1992) 3323. G. Riveros, S. Meneses, G.C. Escobar, B. Chornik, Electron transfer rates of alkylferrocene molecules forming incomplete monolayer on silicon electrodes, J. Chil. Chem. Soc. 55 (2010) 61. O. Seitz, T. Bocking, A. Salomon, J.J. Gooding, D. Cahen, Importance of monolayer quality for interpreting current transport through organic molecules: Alkyls on oxide-free Si, Langmuir 22 (2006) 6915. D. Mandler, P.R. Unwin, Measurement of lateral charge propagation in polyaniline layers with the scanning electrochemical microscope, J. Phys. Chem. B 107 (2003) 407. S. Ciampi, M. James, M.H. Choudhury, N.A. Darwish, J.J. Gooding, The detailed characterization of electrochemically switchable molecular assemblies on silicon electrodes, Phys. Chem. Chem. Phys. 15 (2013) 9879.