Effects of Electrochemical Tailoring of Monolayers on a Catalytic

Nov 30, 2015 - Effects of Electrochemical Tailoring of Monolayers on a Catalytic Redox Entity: An ON–OFF Phenomenon Regulated by the Surrounding ...
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Effects of Electrochemical Tailoring of Monolayers on a Catalytic Redox Entity: An ON−OFF Phenomenon Regulated by the Surrounding Medium Alagar Raja Kottaichamy,† Harish Makri Nimbegondi Kotresh,‡ Mruthyunjayachari Chattanahalli Devendrachari,† Ravikumar Thimmappa,† Bhuneshwar Paswan,† Omshanker Tiwari,† Vimanshu Chanda,† Pramod Gaikwad,† and Musthafa Ottakam Thotiyl*,† †

Indian Institute of Science Education and Research (IISER) Pune, Dr. Homi Bhabha Road, Pashan, Pune 411008, India Department of Chemistry, Acharya Institute of Technology, Soldevanahalli, Bangalore-560107, India

‡

S Supporting Information *

ABSTRACT: Here we report an ON−OFF effect on electrocatalytic amplification when the molecules are connected to each other on an electrode surface. When the molecular connection is achieved, the redox potential of the catalytic redox entity is significantly upshifted with concurrent experience of a more electron-withdrawing atmosphere, and the electron transfer to and from the catalytic center is accelerated, rendering the molecules with strong electroreducing character; however, the overall outcome is dictated by the surface charge. Outer sphere redox probes and X-ray photoelectron spectroscopy evidently revealed that a positive surface charge is preserved in acidic and neutral media after molecular connection which is in parallel with the corresponding electrocatalytic amplifications toward oxygen reduction reaction (ORR), suggesting the electrodics after molecular connection is extremely pH dependent. Surface charge present after molecular connection is significantly neutralized by the abundant hydroxyl groups in alkaline media, making the central metal ion’s atmosphere less electron deficient and downshifting its redox potential (compared to the case in acidic and neutral media), thereby reducing the disparity in electroactivity before and after molecular connection. This study underlines that after molecular connection surface charge is the key as it can turn on and turn off the redox entity responsible for electrochemical amplifications. The solvent dependence and redox potential upshift outlined here for monolayer electrodes are nullified during bulk polymerization, indicating the circumstances leading to molecular connection are different from bulk polymerization. Since the surface charge can be modulated by connecting the molecules and tuning the surrounding media, the proposed strategy brings forward a way to tune the interfacial activity and is expected to have implications in electrocatalysis, selective sensing, ion screening, and so on. amplification. Hereafter we use the term “molecular connection” to denote the attempt to connect the molecules. To demonstrate this effect we have chosen widely studied molecular electrocatalysts cobalt tetraaminophthalocyanine (1,8,15,22- tetraaminophthalocyanatocobalt(II) (α TACoPc)) and (2,9,16,23-tetraaminophthalocyanatocobalt(II) (β TACoPc)) as the modifying molecules where amino groups are in the alpha (α TACoPc) and beta (β TACoPc) position of the phenyl ring (Scheme S1, Supporting Information), respectively, and then studied how the interfacial activity of modified electrodes toward fundamentally and technologically important oxygen reduction reaction (ORR)11 is tuned after molecular connection. We found that central metal ion redox potential after molecular connection is strongly influenced by the ORR

1. INTRODUCTION Surface modification of electrode surfaces to tune the electrochemical/chemical activity has been widely realized in electrocatalysis, chemical and biosensors, corrosion protection, etc.1−6 Many versatile strategies have been employed for electrode modifications like self-assembly, covalent modification, simple adsorption, the Langmuir−Blodget technique, layer-by-layer assembly, and so on.7−10 Regardless of the methodology employed for surface modification, often the modified electrodes have the modifying molecule electronically connected at their point of anchoring; however, at their point of termination they are in the open state (molecules are free at their termination point, Scheme 1). Here we show that irrespective of the methodology expended for surface modification connecting the molecules at their point of termination in an attempt to provide contact between the molecules has an ON−OFF effect on the interfacial activity © 2015 American Chemical Society

Received: August 17, 2015 Revised: November 16, 2015 Published: November 30, 2015 28276

DOI: 10.1021/acs.jpcc.5b07997 J. Phys. Chem. C 2015, 119, 28276−28284

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The Journal of Physical Chemistry C Scheme 1. Schematics of the Molecular Connection Process at the Point of Termination of Moleculesa

a

At the end of the connection process molecules are connected at their termination point. The structure of α and β TACoPc is shown.

media with acidic and neutral media turning on the catalytic site responsible for electrocatalytic enhancement, while the alkaline medium is turning it off. It should be noted that the attempt to connect the molecules in a monolayer is indeed a monolayer polymerization reaction, yet we use the term molecular connection to distinguish it from a usual electropolymerization reaction from the bulk where the number of molecules on the electrode surface increases with respect to the charge passed as opposed to the former where the number of molecules on the electrode surface is the same irrespective of the molecular connection process. Further, it is not our intention to design a strategy for monolayer polymerization but to study its ON− OFF effect on electrocatalytic amplification and the associated responsible factors. Figure 1. Cyclic voltammograms for β TACoPc-modified Au (a) and GC (b) electrodes in N2-saturated 0.5 M H2SO4 at a scan rate of 20 mV/s. Analogous voltammograms for α TACoPc-modified Au (c) and GC electrodes (d).

2. RESULTS AND DISCUSSION 2.1. Monolayer Molecular Connection by Electrochemical Cycling. α TACoPc and β TACoPc were synthesized and characterized as per the literature.12,13 We chose Au and Glassy Carbon (GC) electrodes as the substrates because of the difference in the molecular modifying mechanism, in the former it being self-assembly due to Au− N covalent bond formation compared to simple adsorption (π−π interaction) in the latter,14−16 in order to establish that molecular connection influences the electrocatalytic activity irrespective of the nature of the modifying process and is an effect solely of connecting the molecules at their point of termination. Monolayer molecular connection is carried out by cycling the potential in 0.5 M H2SO4 medium between 0 and 1.0 V vs RHE (see experimental details for more details). When the modified electrodes were subjected to potential cycling in 0.5 M H2SO4 (Figure 1), the wave observed at ∼0.9 V in the first cycle disappeared in the subsequent cycle with a concomitant appearance of a down-shifted redox peak corresponding to the phthalocyanine ring at ∼0.71 V.17 When we attempted to wire the molecules in neutral and alkaline media (results not shown), the illustrated behavior in Figure 1 was absent, suggesting that an acidic medium (pH < 4) is inevitable for molecular connection for the present set of molecules. Nevertheless, the behavior illustrated in Figure 1 is

indicative of increased delocalization of electrons over the ring which is possible only if the molecules are connected to each other during potential cycling like in a typical electropolymerization reaction.18−20 However, unlike in an electropolymerization reaction, in the present case, since the molecules are a part of a monolayer, they are largely immobile and hence can be connected to each other only two dimensionally at their limited points of available contacts, and their numbers before and after molecular contacts remain the same. Figure S1 (Supporting Information) manifested the redox pair due to the central metal ion17,21,22 (in the range 300−700 mV) with only marginal peak separations (39 mV for β TACoPc vs 23 mV for α TACoPc) whose peak currents linearly increased with the potential scan rate (Figure S2 (Supporting Information)), indicating a surface-confined redox species which is further clear from the close to unity slope of log (current) vs log (scan rate) plot. The molecular coverage for β TACoPc was calculated based on the charge passed for the central metal ion redox transformation (before molecular connection), and it turned out to be 1.36 ± 0.013 × 10−10 mol/ 28277

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The Journal of Physical Chemistry C cm2 and 1.11 ± 0.02 × 10−10 mol/cm2 for Au and GC electrodes, respectively. It is reported that if the coverage values are greater than 1.1 × 10−10 mol/cm2 a perpendicular orientation is preferred over a flat orientation.23−27 On the basis of this it is reasonable to argue that on Au the molecules prefer a perpendicular orientation over a flat orientation on GC. The ratio of coverage between Au and GC is found to be ∼1.25, suggesting that for a perpendicular orientation the monolayer can be very much disordered. The coverage observed on α TACoPc was found to be lower compared to β TACoPc, and this was attributed earlier by some researchers for the difference in their orientation especially on Au electrodes17,27 (α TACoPc surface coverage was 8.66 ± 0.02 × 10−11 and 6.79 ± 0.018 × 10−11 mol/cm2 on Au and GC electrodes, respectively). The lower coverage observed on the GC electrode suggests that the modifying molecules in the case of simple adsorption are lying flat on the electrode surface due to π−π interaction between the carbon skeleton and the modifying molecule.14−16 To prove the monolayer nature of the adsorbed molecules we carried out analysis using Amatore’s model (see experimental details). From the Nyquist plot of a bare Au electrode and modified Au electrode (Figure S3) and the Z′ (real part) vs 1/(ω)1/2 plots (Figure S4), we estimated the coverage values close to 0.96 (Table S1). According to Amatore’s model, θ (coverage) value >0.9 reflects a monolayer situation,28,29 and therefore the obtained value of 0.96 indicates a monolayer of modified molecules on Au and GC electrodes. A comparison of central metal ion redox potential (by the term redox potential we mean the formal potential of Co(II)/ Co(III) redox couple) before and after the molecular connection process is presented in Figure 2, clearly

region, suggesting increased delocalization of electrons and charges (see the discussion of Figures 4 and 5). Coverage values turned out to be ∼1.7 times higher after molecular connection (for example, after molecular connection of β TACoPc on Au, the coverage estimated was 2.54 ± 0.016 × 10−10 against 1.36 ± 0.013 × 10−10 mol/cm2 before molecular connection), and since the coverage should be the same irrespective of molecular connection, the amplification in the redox signals of the central metal ion suggests that the charge transport rate to and from the central metal ion is significantly enhanced, caused probably by the increased delocalization of charges and the closeness of metal ion centers after molecular connection. This is further probed by electrochemical impedance spectroscopy, and the complex plane impedance plot (Figure 3) demonstrates a decrease in the semicircle diameter after molecular connection. It is well-known that the diameter of a semicircle in the Nyquist plot is directly proportional to charge transfer resistance (RCT).30,31 RCT values are extracted by fitting the Nyquist plot by a R(C(RW)) electronic circuit model (Table S2) using Zsimpwin software. RCT values decrease noticeably after molecular connection, indicating a facile movement of charges after molecular connection compared to the case before connection. Since RCT is inversely related to exchange current density which talks about the actual rate of electron transfer, the data in Figure 3 unambiguously establish that after molecular connection the rate of electron transfer is accelerated back and forth in the central metal ion, in line with the voltammetric results presented in Figure 2. The result presented demonstrates unambiguously that it is solely an effect of connecting the molecules since the number of molecules on the surface should be essentially the same before and after molecular connection. These point to the fact that after molecular connection the catalytic redox entities are perhaps brought closer together making the monolayer rather compact, and the electron transfer back and forth in the catalytic redox entity is accelerated with a concurrent experience of a more electron-withdrawing atmosphere, which in turn upshifts its redox potential. This is an important observation because the central metal ion is the electrocatalysis governing entity as far as these molecules are concerned. The chemical changes after molecular connection on Au and GC electrodes were probed by XPS. The N (1s) XPS spectra of electrodes modified with α TACoPc and β TACoPc unambiguously show the presence of a protonated −NH− signal at ∼401.5 eV after molecular connection which was otherwise absent on electrodes before molecular connection (Figure 4).32,33 After molecular connection, the Co(III) peak intensity at 781.6 eV in the Co (2p3/2) XPS spectrum was significantly enhanced compared to the one before connection on either electrode (Figure 5),34,35 suggesting the proximity of the central metal ion in TACoPc is now more electron withdrawing than before connection in line with the electrochemistry data presented in Figure 2 corroborating further the presence of protonated −NH− after molecular connection. The voltammetric, impedance, and XPS results suggest that −NH2 groups in TACoPc were protonated and oxidized during oxidative potential scan in acidic medium, and it provides a conducive atmosphere for the molecules to connect each other at their point of termination as shown in Scheme 2. Since other possibilities of molecular connection are equally possible, at this stage the exact location of molecular connection could not be identified, and therefore we can only confirm the molecular connection. However, when

Figure 2. Cyclic voltammograms of the Co(II)/Co(III) redox pair for β TACoPc-modified Au (a) and GC (b) electrodes in N2-saturated 0.5 M H2SO4 at a scan rate of 20 mV/s before and after molecular connection. Analogous voltammograms for α TACoPc-modified Au (c) and GC electrodes (d).

demonstrating that the redox peaks are well-defined and amplified and the redox potential is upshifted significantly (60− 100 mV for β TACoPc vs 20−35 mV for α TACoPc) after molecular connection. The upshifting of central metal ion redox potential proposes that the circumstances surrounding it is more electron withdrawing than before connection. Analogous results to Figure 2 were obtained in a neutral medium (phosphate buffer, pH = 7.2) before and after molecular connection in 0.5 M H2SO4 (Figure S5). Further, the capacitive contribution is significantly enhanced throughout the potential 28278

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Figure 3. Complex plane impedance plot at a bias voltage of 450 mV vs RHE for β TACoPc-modified Au (a) and GC (b) electrodes before and after molecular connection. Electrolyte is O2-saturated 0.1 M H2SO4. Analogous plots for α TACoPc-modified Au (c) and GC electrodes (d).

The surface charge due to oxidative anchoring of the −NH2 group of one molecule on the neighboring molecule is further probed by highly reversible redox species such as ferricyanide and hexaamine ruthenium. After molecular connection, the peak currents for negatively charged ferricyanide redox species were considerably amplified in an acidic medium (Figure 6). Given the fact that ferricyanide is an outer sphere and highly reversible redox species, the behavior exemplified here is indicative of an electrostatic pull by a positively charged electrode surface. If this is true, then a reversible positively charged outer sphere redox couple should be repelled away from the electrode surface, and the peak current should decrease which indeed turned out to be true when the electrochemical cell was substituted with hexaammine ruthenium (Figure S6, Supporting Information). This surface charge may be one of the responsible factors for the increased nonfaradaic contribution in Figure 2 after molecular connection. Taking together the XPS data and the electrochemical signatures, it can be concluded that the central metal ion’s redox potential is positively shifted; the surface charge is significantly positive; and the catalytic metal ion centers are probably closer together after the molecular connection process, all of which is expected to render the catalytic center with strong electroreducing character.36,37 However, the surface charge plays a crucial role in dictating the overall effect despite the molecular connection as explained below 2.2. Oxygen Reduction Reaction. The modified electrodes before and after molecular connection are then attempted for ORR in neutral, acidic, and basic media. Linear sweep voltammograms of β TACoPc-modified Au and GC electrodes in phosphate buffer (pH = 7.2) before subjecting it to electrochemical molecular connection (Figure 7a,b, red trace) demonstrate a clear catalytic wave in the presence of oxygen. The unmodified Au and GC electrodes under identical conditions required a larger overpotential for ORR (Figure S7 (Supporting Information)), indicating that β TACoPc has a catalytic effect toward ORR as widely reported by others.17,27 It seems the ORR proceeds through a two-step pathway: first its conversion to peroxide and then into water because of the presence of two well-separated waves. This indeed turned out to be true when we supplemented H2O2 to the ORR reaction vessel, and the second peak was amplified without significantly affecting the first (Figure S8, Supporting Information). Yet there is a huge difference between oxygen and peroxide

Figure 4. N 1s XPS spectrum for α TACoPc (a, b) modified Au and β TACoPc (c, d) modified Au electrodes before (a and c) and after molecular connection (b and d).

Figure 5. Co (2p) XPS spectra for α TACoPc (a, b) modified Au and β TACoPc (c, d) modified Au electrodes before (a and c) and after molecular connection (b and d).

compared to a drop-casted electrode this molecular connection should be happening less randomly in a monolayer-modified electrode as explained below. 28279

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Scheme 2. Schematics for the Position-Dependent Electrochemical Monolayer Molecular Connection of α TACoPc- and β TACoPc-Modified Electrodes at Their Point of Terminations

Figure 6. Cyclic voltammograms for 2 mM ferricyanide on β TACoPcand α TACoPc-modified Au and GC electrodes at a scan rate of 20 mV/s before and after molecular connection. Electrolyte is N2saturated 0.1 M H2SO4.

Figure 7. Linear sweep voltammograms of β TACoPc and α TACoPc before (red trace) and after (black trace) molecular connection at a scan rate of 20 mV/s. Electrolyte is oxygen-saturated 0.1 M phosphate buffer (pH = 7.2). (a, c) On Au and (b, d) on GC electrodes.

reduction processes, and often they displayed unequal peak heights. To understand whether the thermal decomposition of peroxide is responsible for this behavior, we have carried out scan rate dependent studies. We estimated the ratio of the charge under the overall ORR LSV and the charge under the first oxygen reduction peak as a function of scan rate (Figure S9 Supporting Information). If thermal decomposition prevails, a lower ratio should be favored at lower scan rates as opposed to higher ratio at the higher scan rates. Contrary to this expectation, the ratio was found to be higher for the lower scan rates compared to the higher scan rates (Figure S10). Therefore, we presume that it is due to the electron transfer difficulty by the present set of molecular electrocatalysts for the transformation of peroxide to water rather than the thermal decomposition in the time limit of the experiments. Nevertheless, for the modified electrodes after molecular connection when attempted for ORR in phosphate buffer (pH = 7.2) (Figure 7 (black traces)), a marked catalytic shift toward lower

overpotentials was observed with enhanced peak currents, demonstrating electrocatalytic tuning of the interface. The comparative electrochemical parameters in phosphate buffer (pH = 7.2) before and after molecular connection are presented in Table S3 (Supporting Information), affirming this argument. Analogous results were obtained with α TACoPcmodified Au and GC electrodes (Figure 7c,d, black traces) suggesting that irrespective of the nature of the molecule the effect of connecting molecules at their termination point improves the electrocatalytic performance dramatically. Surface charge turned out to be positive in phosphate buffer (Figure S11, Supporting Information) as in an acidic medium (Figure 6). When the ORR medium was changed to H2SO4, parallel results to Figure 7 were obtained (Figure 8) demonstrating that the exemplified electrocatalytic tuning after connection of molecules toward ORR is not an isolated event in phosphate buffer, pointing to interfacial tuning of the molecular architecture at the electrode surface. 28280

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TACoPc was observed (Figure S13). In such electrodes the monomers are randomly oriented, nullifying the effect of any preferential orientation, yet the marked positive shift in ORR onset on the α TACoPc drop-casted electrode forces us to believe that orientation alone cannot contribute to the difference between α and β TACoPc monolayer-modified electrodes toward ORR as proposed by Abraham John,17 and the relative contribution of changes in electronic density around the central metal ion and ring due to substitution at different positions should be probed in a separate detailed investigation to further shed light on these. Nonetheless, the clear catalytic shift toward lower overpotential during the ORR in acidic and neutral media illustrated with the electrodes after molecular connection hint that the catalytic center responsible for this amplification is in the ON state. When the ORR medium was changed to alkaline, the illustrated electrocatalytic tuning observed in acidic and neutral pH was only marginal (Figure S14 (Supporting Information)), suggesting that the catalytic center creditworthy for the same does not maintain the same activity as in acidic and neutral media. It was a surprising observation because the ORR is reported to be very efficient in alkaline medium.39,40 It should be noted that electrocatalysis was observed in alkaline medium; however, interfacial tuning of electrocatalytic activity as demonstrated for acidic and neutral media after molecular connection was not noticed. When we probed the surface charge of the electrode with an outer sphere ferricyanide redox probe in alkaline medium, an almost an unchanged electrochemical signal was observed even after the molecular connection process (Figure S15 (Supporting Information)), contrary to the results in acidic and neutral media, demonstrating the surface charge is significantly neutralized under alkaline conditions; however, molecular connection is preserved as observed from the increased nonfaradaic contribution (Figure S16 (Supporting Information)). Central metal ion redox potential in alkaline medium demonstrated only a marginal positive shift after molecular connection (∼5−8 mV) (Figure S16 (Supporting Information)) which is further supported by the Co (2p) and N (1s) XPS spectra (Figure 10)

Figure 8. Linear sweep voltammograms of β TACoPc and α TACoPc before and after molecular connection at a scan rate of 20 mV/s. Electrolyte is oxygen-saturated 0.1 M H2SO4. (a) On Au and (b) on GC electrodes.

Exchange current density and Tafel slope values reflecting the intrinsic rate of the ORR are extracted from their respective Tafel slope measurements before and after molecular connection in H2SO4 medium (Figure 9 and Table S4,

Figure 9. Tafel slope measurements of α TACoPc and β TACoPc before (black trace) and after (red trace) molecular connection. Electrolyte is O2-saturated 0.1 M H2SO4. (a, c) on α TACoPc- and β TACoPc-modified Au and (b, d) on α TACoPc- and β TACoPcmodified GC electrodes.

Supporting Information). Tafel slope values comparable to the ideal 120 mV/dec are observed on monolayer-modified electrodes before molecular connection, suggesting the ratelimiting step involves the transfer of one electron. A closer look suggests that the Tafel slopes decrease and exchange current densities, reflecting that the true rate of the electrochemical reaction noticeably increases after molecular connection. The significant improvement of parameters reflecting the intrinsic activity of the electrochemical reaction proves beyond doubt that molecular connection modifies the catalytic centers, thereby improving the rate of the reaction significantly toward ORR. Nevertheless, a comparison of α TACoPc- and β TACoPcmodified Au and GC electrodes in phosphate buffer (pH = 7.2) (Figure S12 (Supporting Information)) demonstrated a larger anodic shift (∼190 mV) on α TACoP-modified electrodes and is well pronounced even before molecular connection. This is quite a dramatic effect given the lower surface coverage of α TACoPc compared to β TACoPc on either electrode. The difference in activity between α TACoPc and β TACoPc was argued to be due to their difference in orientation by Abraham John and other researchers.17,27,38 When drop-casted α and β TACoPc-modified GC electrodes were used for ORR an analogous positive shift in ORR (∼116 mV) in favor of α

Figure 10. Co (2p) XPS spectrum for α TACoPc-modified Au electrode after molecular connection and then treating with alkali (a). Corresponding N (1s) XPS spectrum (b).

of the modified electrodes (α TACoPc) after molecular connection and then treating with alkali. As shown in Figure 10, the relative intensity of the Co3+ signal has descended (compared to Figure 5b), and the protonated N (1s) signal has disappeared (compared to Figure 4b) when the wired electrodes are treated with alkali, supporting the neutralization of surface charges in alkaline medium. Abundant hydroxyl groups present in alkaline medium may neutralize the positive charges on the N atom by eliminating the charge balancing counterion. This may make the central metal ion in a less 28281

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O2 binding affinity and electron transfer rate after molecular connection; however, the surface charge plays a critical role in dictating the overall outcome and therefore the surrounding ORR media. In the XPS spectra, free −NH2 signals were richly present even after molecular connection (Figure 4); therefore, it can be presumed that only a few −NH2 groups on the molecules participate in the connection process (Scheme 2) presumably due to the immobility of the assembled molecules on the electrode surface. From the rich presence of free-NH2 groups even after the connection process, we envisage that the molecular connection proposed here is somewhat different from a polymerization reaction as the former is occurring only in two dimensions involving very few −NH2 groups as opposed to a polymerization reaction which is more likely a 3D reaction involving the majority of −NH2 groups on the molecules. The process of molecular connection at the monolayer level is expected to bring the catalytic redox entities on the electrode surface closer together and make the monolayer more compact as opposed to their open state, and more likely it may facilitate the interaction of one O2 molecule with two metal centers, thereby enhancing the electron transfer rate and the catalytic activity, provided the surface charge is preserved. We are in the process of executing in situ Raman and FTIR spectroelectrochemistry to shed more light on this.

electron-deficient atmosphere and may bring down its redox potential thereby decreasing the disparity on electrocatalysis before and after molecular connection. This evidence that the surface charge plays a crucial role in electrocatalytic tuning of the interface and molecular connection alone does not activate it. Taken together it is reasonable to conclude that the catalytic redox entity responsible for tuning the interfacial activity is turned off in alkaline medium. ORR in different media unambiguously reveals that there is an ON−OFF effect on electrocatalytic tuning of the interface after molecular connection, with acidic and neutral media favoring the ON state against the OFF state in alkaline conditions. Therefore, surface charge is the key as it can turn on and turn off the catalytic site responsible for electrochemical amplifications. In a different direction this brings the possibilities of ion screening and selective sensing using the same sets of molecules, as the surface charge of the modified electrodes can be modulated by connecting the molecules and tuning the surrounding media. It will be beneficial at this stage to compare the performance of drop-casted α and β TACoPc GC electrodes before and after electropolymerization to distinguish the effect of molecular connections from a bulk electropolymerization reaction. Dropcasted TACoPc electrodes, after electropolymerization by subjecting them into potentiodynamic cycling as in Figure 1, demonstrated an amplification in their ORR current signals; however, a positive shift in ORR onset potentials as observed for the monolayer-modified electrodes was not observed, and the ORR after polymerization (compared to the case before polymerization) was largely independent of the surrounding media (Figure S17, Supporting Information). A comparison of drop-casted electrode Co(II)/Co(III) redox potential (Figure S18 (Supporting Information)) did not reveal a noticeable upshift (5−15 mV); however, Faradaic and non-Faradaic contributions demonstrated a marked gain. This upshift is of abysmal magnitude when compared to a monolayer-modified electrode (60−100 mV, Figure 2). These reflect that the case of drop-casted electrodes after electropolymerization is different from the monolayer-modified electrodes. A drop-casted electrode can have random orientation of monomers, and therefore the polymerization should occur more irregularly. In the case of monolayer-modified electrodes as the modifying molecules are more organized, the molecular connection during the polymerization can be less random compared to a dropcasted electrode. We do not deny that molecular connection is indeed a monolayer polymerization; however, we believe it is occurring less randomly as compared to a drop-casted electrode. This may help in the better delocalization of electrons over the monolayer monomers compared to dropcasted electrodes, explaining the circumstances leading to molecular connection are different from bulk polymerization. The aftermath is the monolayer-modified electrodes after molecular connection is strongly influenced by the surrounding media which is not the case for drop-casted electrodes (Figure S17 (Supporting Information)). It is known that electron-withdrawing substituents at the periphery of phthalocyanines improve the electrocatalytic activity toward ORR as the electron-deficient metal ion center can strongly bind to O2 molecules, thereby aiding its bond scission at a lower overpotential.41,42 As shown in Figures 2−5 after molecular connection, the proximity of the central metal ion becomes electron deficient, upshifting its redox potential. Therefore, it can be concluded that the electronic density around the central metal ion is decreased, thereby enhancing its

3. CONCLUSIONS We have successfully demonstrated an ON−OFF effect toward electrocatalytic amplification after electrochemical monolayer molecular tailoring. The results indicate that after molecular connection the catalytic redox entity’s redox potential is upshifted; electron transfer rate back and forth in the catalytic center is accelerated; and individual metal ion centers are perhaps brought closer together, rendering the molecules with strong electroreducing character; however, the overall effect is dictated by the surface charge. Outer sphere redox probes, XPS and ORR, demonstrated that the surrounding media has an ON−OFF effect on the catalytic redox entity responsible for electrocatalytic tuning after molecular connection, with acidic and neutral media activating the ON state against the OFF state preferred by an alkaline medium. The nullification of electrocatalytic tuning in alkaline medium is attributed to neutralization of surface charges which in turn brings down the central metal ion redox potential (compared to acidic and neutral conditions). Therefore, surface charge is the key as it can turn on and turn off the catalytic site responsible for electrochemical amplifications. The upshift of redox potential after bulk polymerization is abysmal in the case of drop-casted electrodes compared to a monolayer-modified electrode after molecular connection with a consequent disparity in their solvent dependence; therefore, the circumstances leading to monolayer molecular connection are different from bulk polymerization. Since the surface charge can be modulated by connecting the molecules in the monolayer and tuning the surrounding media, the proposed strategy brings forward a way to tune the interfacial activity and is expected to have implications in electrocatalysis, selective sensing, ion screening, and so on.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07997. 28282

DOI: 10.1021/acs.jpcc.5b07997 J. Phys. Chem. C 2015, 119, 28276−28284

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Experimental details and supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91(020)25908261. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS MOT is indebted to MHRD India and DST India for financial support. REFERENCES

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