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An electrochemical quartz crystal microbalance study of platinum phthalocyanine thin films
Journal of Electroanalytical Chemistry 633 (2009) 339–346
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An electrochemical quartz crystal microbalance study of platinum phthalocyanine thin films Richard J.C. Brown a,*, Dan J.L. Brett b, Anthony R.J. Kucernak c a
Analytical Science Team, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK Department of Chemical Engineering, University College London, London WC1E 7JE, UK c Department of Chemistry, Imperial College, London SW7 2AZ, UK b
a r t i c l e
i n f o
Article history: Received 18 May 2009 Received in revised form 29 June 2009 Accepted 2 July 2009 Available online 7 July 2009 Keywords: Platinum phthalocyanine Electrochemical quartz crystal microbalance Thin films Anion doping
a b s t r a c t An electrochemical quartz crystal microbalance study of platinum phthalocyanine thin films in aqueous media is presented. It has been shown that these films exhibit significant mass changes upon electrochemical cycling. These mass changes have been shown to be controlled by the ingress and egress of solvated anions during oxidation and reduction of the film, respectively. The mass changes within the film are shown to tend to steady state after a prolonged period of cycling. Chrono-amperometric studies have additionally shown that the movement of cations also has a role in the electrochemistry and mass transport within these films, but generally these events occur on a shorter timescale than those involving the solvated anions, and are hidden during most potential cycling studies. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Phthalocyanines (Pcs) represent a very important class of organic macrocycle [1]. Metallophthalocyanines (MPcs) have found particular uses as electrocatalysts for a variety of reactions. The most common electrocatalytic application for these MPcs has been in oxygen reduction, with the aim being their use in fuel cells or batteries [2,3]. As a function of the central transition metal in the phthalocyanine complex (MPc), van Veen found that the order of activity for oxygen reduction in both acid and base is: Fe > Co Ru > Mn > Pd Pt > Zn [4]. This result has lead to the almost exclusive study of the FePc and CoPc complexes in acid and alkaline aqueous solutions [5,6]. However, the stability of these firstrow complexes is not high under operating conditions, especially in acidic electrolytes, and this has frustrated attempts to use their excellent electrocatalytic properties to sustain competitive current densities. MPcs have also received some attention as targets for photodynamic therapy in cancer treatment [7], and in display devices because of their electrochromic properties [8]. The incorporation of different metals into the core of the phthalocyanine ring can affect the chemical and physical properties of the compound. In contrast to FePc and CoPc precious metal phthalocyanines, such as PtPc, PdPc and IrPc are known to exhibit remarkable thermal * Corresponding author. Tel.: +44 20 8943 6409; fax: +44 20 8614 0423. E-mail address: [email protected] (R.J.C. Brown). 0022-0728/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2009.07.002
stability and to resist oxidation. They have been shown to offer good performance as oxygen reduction electrocatalysts [9–12]. Under certain treatment regimes ‘activated’ precious metal phthalocyanine complexes are produced that are more active than FePc or CoPc [13]. However, platinum phthalocyanine (PtPc) is somewhat difficult to prepare in useable quantities. Recently, though, we have presented studies of the electrochemistry [14], spectroscopy [15], photo-electrochemistry [16], and optical second harmonic generation [17] of PtPc in aqueous media – information on electrochemical studies in non-aqueous media is also available [18]. We have previously suggested [14] that the PtPc thin films investigated exhibited unusual quasi-reversible electrochemistry, and cyclic voltammograms that develop with repeated scanning, with a large first scan discrepancy. This is similar to redox and conducting polymer films, whose growth and electrochemistry is mediated by the ingress and egress of anions and solvent from the films during cyclic voltammetry, where some literature is available [19]. We now present an electrochemical quartz crystal microbalance (EQCM) study to measure minute mass changes within the PtPc thin film during electrochemistry, and show that the electrochemistry of the PtPc is mediated by changes in the mass of the film, assumed to be the ingress and egress of solvated anions. This novel work further elucidates the physical and chemical characteristics of PtPc. (The closest comparable work has looked at EQCM studies of conducting and redox polymers [20].)
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2. Experimental All solutions used were prepared gravimetrically from ultrapure chemical reagents (Fisher) and deionised water (18.2 MX cm, Millipore, MilliQ). A standard three-compartment, three-electrode configuration was used with some alterations as described below. A platinum electrode flag was used as a counter electrode, and a saturated calomel electrode acted as the reference electrode, against which all voltages are henceforth quoted. Sulfuric acid solutions have been used for the experiments described (unless otherwise stated) as these are known to provide the best intercalation conditions for these PtPc films [14]. All solutions were de-oxygenated prior to use with argon (99.999+%, BOC) and an argon atmosphere was maintained above the solution during analysis. The working electrode comprised one side of a quartz crystal microbalance (QCM) device and consisted of a 100 nm gold layer vacuum deposited on both sides of AT-cut quartz crystal resonator (International Crystal Manufacturing Co. Inc., Oklahoma City); the deposited metal electrode formed a circle with a diameter of approximately 5 mm. The resonant frequency of these crystals was nominally 10 MHz. The connection was made with a coiled steel wire that was in turn soldered to an additional wire to which the QCM terminals could then be easily attached. The crystals were bonded to the open end of a small sample tube with silicone sealant in such a way that the steel was protected from solution contact and the second gold side of the crystal was exposed only to the atmosphere trapped within the sample tube. The experimental set-up used has been previously described [21]. The working electrode output from an EcoChemie Autolab PGSTAT 10 potentiostat was connected to the working electrode input on an in-house built oscillator circuit. This circuit controlled the potential of the working electrode side of the gold covered quartz crystal and additionally measures the resonant frequency of oscillation of the crystal. The frequency measured by the oscillator circuit was the difference between the resonant frequencies of the crystal in question and a standard reference crystal housed within the oscillator circuit. The oscillator circuit then output this signal to a frequency-to-voltage converter module which in turn delivered voltage to an analogue-to-digital input on the potentiostat, so that the current and mass responses may be measured simultaneously against voltage. At all times an oscilloscope was used to monitor the output of the oscillator circuit and determine the integrity of the quartz crystal resonator. In this way it was possible to determine if any given resonator failed. The PtPc was synthesised as previously described [15]. Mechanical abrasion was used to apply the PtPc layers onto the QCM surface. This was undertaken by rubbing the QCM electrode gently in a figure-of-eight motion onto excess PtPc powder placed onto a clean glass surface. At the end of the procedure, excess PtPc was gently brushed off the surface of the QCM, leaving a dark purple tinge to the previously gold surface. This procedure is known to deposit layers with an approximate thickness of 200 nm [17,22]. For the quartz crystals and the equipment used in this study the theoretical sensitivities were 16 Hz/mV and 4.4 ng/(cm2 Hz). The system was calibrated shortly before the experiments were undertaken using electro-deposition of copper from solution and found to yield an overall sensitivity of 18 ng/mV, which is henceforth used in all voltage to mass change conversions. The oscillating frequency change associated with processes occurring at the surface of the EQCM can be converted to a mass change using the well known Sauerbrey equation [23]. Whether it is reasonable to discuss the system in terms of absolute mass changes is debatable because of the non-ideal behaviour of QCMPtPc films, especially when solvated. This notwithstanding, the
most important observations from this study are qualitative, and their impact is not reduced by them not being rigorously quantitative. 3. Results and discussion 3.1. Bare gold It is of important to characterise the EQCM response of the bare gold electrode, since this will be superimposed on all subsequent investigations involving phthalocyanine films. For this purpose an exemplar response from a bare gold electrode is shown in Fig. 1. As the potential is scanned positively from 0 V the mass is seen to increase slowly as (bi)sulfate ions are adsorbed to the gold surface. This increase slows as gold oxide formation occurs at about 1.1 V. On the negative sweep the gold oxide layer is removed by 0.75 V with a concomitant mass decrease and an associated small hysteresis. The mass then continues to decrease with little hysteresis as (bi)sulfate de-associates from the surface. However, in the example shown in Fig. 1, the mass change due to oxide formation and removal is not particularly clear. The history of the electrode may affect the shape of the response seen and the magnitude of the contributions made from oxide formation/removal and (bi)sulfate association/de-association but the recorded response compares favourably with literature examples [24,25]. It should be noted that although the EQCM system has the mass resolution stated above, the resolution of the Autolab analogue-todigital converter is such that the mass signal shown in Fig. 1 shows quantisation steps of approximately 5 ng. However, the mass changes associated with PtPc electrochemistry are 50- to 100-fold larger than for bare gold, therefore background Au electrochemistry and signal processing limitations have little effect on the results presented in the study of the PtPc films. 3.2. PtPc cyclic voltammetry The electrochemistry of PtPc has been previously described [14]. In summary, PtPc films applied to gold and glassy carbon surfaces were shown to exhibit relatively complex electrochemistry in aqueous media. Upon potentiodynamic cycling three anodic peaks (first: 0.81 V, second: 1.10 V and third: 1.27 V) and two cathodic peaks (the pair of the second anodic peak: 1.12 V and the pair of the third anodic peak: 0.59 V) were seen to develop over time. These processes were ascribed to phthalocyanine ring oxidation and reduction processes. It was proposed that this phthalocyanine ring electrochemistry was controlled by anion doping and intercalation within the film, and that the large separations between cor-
Fig. 1. Current-potential (grey) and mass-potential (black) curves for a new, bare gold QCM electrode in 0.2 M sulfuric acid. Scan rate 50 mV/s.
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responding anodic and cathodic processes were manifestations of barrier potential to anion movement within the film (often referred to as unusual quasi-reversibility [26]). A large reduction peak at 0.59 V is attributed to the mass expulsion of anions from the film. Moreover, a distinct first scan discrepancy in the electrochemical response was observed, proposed to be owing to the initial intercalation of anions within the virgin film. (The mechanically abraded PtPc films have high density as confirmed by SEM examination [22], however, upon the initial oxidation of the film, with the associated intercalation of solvated anions, the porosity of the film will increase substantially as channels are opened up between PtPc microparticles.) The electrochemistry of the film was then seen to evolve over subsequent scans asymptotically approaching an equilibrium state. Whilst the evidence from the electrochemical study to support this thesis was compelling, it was not definitive. Thus additional studies to examine the mass changes within the PtPc films during the electrochemical processes were required to provide corroborating evidence. The EQCM and electrochemical response for the first scan of a PtPc layer in 0.2 M sulfuric acid is shown in Fig. 2. As consistent with the electrochemical observations [14] the mass change shows a very large increase on initial intercalation of the film after about 1.0 V. The mass increase then slows slightly after the first PtPc oxidation peak at 1.1 V but continuous anion ingress is seen during the second oxidation peak (at 1.3 V) up until 1.4 V. On the reverse scan a small mass decrease is observed as the first PtPc reduction peak is reached at 1.1 V and then a slow mass increase is seen before another drop coinciding with the second PtPc reduction peak at 0.55 V. Correlating the EQCM frequency response to a mass change, the scan finishes with a mass excess of 1500 ng compared to the initial (pre-intercalation) weight. In the interpretation of this first scan, caution must be exercised since it is expected that during the first scan the layer will under-go the majority of its major and irreversible changes in structure and visco-elastic properties. It is known that changes in the rigidity of a surface film will cause non-ideality in the EQCM response and resonant frequency changes may not be easily ascribable purely to mass changes within the film. Nevertheless the EQCM response shows three interesting general points. Firstly, increases in mass occur with the oxidation of the film suggesting net solution and anion ingress – similarly to the predictions made during electrochemical studies [14]. Secondly, and conversely, mass decreases occur with the reduction of the film, suggesting a net egress of solution and anions during the reductive sweep. Finally, there is an irreversible mass increase associated with the first scan due to the irreversible doping of anion and solvent into the film.
Subsequent scans on the same system begin to adopt to the same typical shape, although there is still variation with scan number in terms of the mass change caused by oxidation and reduction and the overall mass properties of the film at the beginning of each scan. Scans from a typical example are shown in Fig. 3. It is interesting to note that the mass of the film after the second scan is significantly less than after the first scan. Furthermore the reduced weight of the film after the second scan until the end of the tenth scan decreases slightly with each scan. This is not inconsistent with small losses of material from the film with scanning. The actual loss of mass would seem to rule out isolation of parts of the film losing electric connection and becoming electro-inactive as an explanation. Perhaps the more significant aspect of the study is the much more rapid decrease in the mass of the fully oxidised film with repeated scanning. One explanation of this would be that the film is approaching a reversible state with scanning. The use of dilute acid is likely to cause large quantities of solvent (in this case water) to be involved in the doping and undoping process. It is possible that excess water is incorporated into the film during the first scan of the virgin film and that subsequent scans see a net loss of mass because water is expelled as the film relaxes after initial swelling. That is to say that during scans 2–10 there is still a slow egress of excess solvent, which entered the film during the first scan, and anions from the film until an equilibrium is reached where the ratios of the cathodic to anodic charge passed approach unity with scanning, at which stage there will be a reversible change in mass upon cycling – a situation approximately represented by scan 10 of the example in Fig. 3, shown in Fig. 4. Indeed little change is observed to this profile in subsequent scans. Another interesting feature of the development of the film with scanning is the appearance of the first PtPc oxidation peak at 0.7 V. This peak has been previously observed and is known to grow with repetitive scanning and is associated with the oxidation of the PtPc film at, or near to, the electrode/PtPc interface [14]. By scan 10 (shown in Fig. 4) on the gold EQCM electrode, the peak is very apparent and now contributes significantly to the mass increase during the scan. The response at scan 10 is now one of continuous intercalation with three distinct regions of mass change (with different gradients) on the positive scan corresponding to the three oxidation regimes of the PtPc film. That the overall mass increases with oxidation and decrease with reduction indicates a net movement of sulfate and bisulfate ions into and out of the film, probably accompanied by solvent, during these processes. Increasing the sulfuric acid concentration to 3.0 M induces some subtle changes in the EQCM response. As is already known, the concentration of the electrolyte affects the peak potentials for
Fig. 2. Current-potential (grey) and mass-potential (black) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The first scan is displayed, at a scan rate of 50 mV/s.
Fig. 3. Mass-potential curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The second (top) to tenth (bottom) successive scans are shown at a scan rate of 50 mV/s.
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Fig. 4. Current-potential (grey) and mass-potential (black) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The tenth successive scan is shown, at a scan rate of 50 mV/s.
Fig. 6. Mass-potential curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 3.0 M sulfuric acid. Successive scans for 1 (bottom) to 10 (top) are shown, at a scan rate of 50 mV/s.
the PtPc oxidations and reductions as a result of the change in the molar free energy for the anion insertion process (which has a dependence on anion concentration) [14] and hence the mass changes occur at slightly different potentials – a developed profile is shown in Fig. 5. The most significant feature of the studies using 3.0 M sulfuric acid is the development of the mass response with scanning which shows an important difference from the response seen with 0.2 M sulfuric acid. The first 10 scans of the mass response in 3.0 M sulfuric acid are shown in Fig. 6. Similarly to the development seen using 0.2 M sulfuric acid a large mass increase is seen upon initial ‘break-in’ of the film during the first scan. However, apart from the larger mass change observed, the first scan shows the same characteristics as seen with the 0.2 M sulfuric acid response. The other interesting aspect of these results is the increase in mass of both the reduced and the oxidised film with repetitive scanning, apparently converging on a maximum value. Clearly the mass shows an increase upon oxidation and a decrease upon reduction, proving again that the redox processes are mediated mainly by the net movement of sulfate and bisulfate ions. However the similarity of the first scan to subsequent scans and the continuing increase in film mass with scanning would suggest that less solvent is dragged into the film initially with the sulfate ions as one might expect in a more concentrated background electrolyte. This effect is not inconsistent with a slower, more controlled film swelling and structural development with the 3.0 M acid leading to a film with a maximum of irreversibly intercalated anions. This occurs without the need to expel solvent after the first scan, as is seen in the 0.2 M acid. The total intercalation mass
changes between experiments can only be compared tentatively since the mass of (active) PtPc on the electrode is likely to vary slightly between experiments. However, to obtain some qualitative comparison between systems it is instructive to look at the ranges of mass change observed with sequential scanning for the two acid concentrations. This is shown in Fig. 7. Several features are of interest: firstly, the mass gains in the 3.0 M sulfuric acid are much greater – even for mass values that are not fully quantitative this is significant. Secondly, the range of mass change within one scan is greater for the 3.0 M sulfuric acid (about double that of the 0.2 M acid). Thirdly, as previously discussed, the films exposed to the 3.0 M sulfuric acid continue to increase in mass with sequential scanning, asymptotically tending to an equilibrium state. The films exposed to the 0.2 M sulfuric acid also tend asymptotically to an equilibrium state; however, now this is on a baseline of decreasing mass, following the maximum reached after the first scan. The response of these films in other common electrolytes has been tested and was found to be broadly similar [22].
Fig. 5. Current-potential (grey) and mass-potential (black) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 3.0 M sulfuric acid. The tenth scan is shown, at a scan rate of 50 mV/s.
3.3. Mass–charge relationships In view of the proposal that the changes in film mass being due to the ingress and egress of anions and solvent, it is instructive to also examine the mass–charge relationships during scanning. This is done for the first scans in 0.2 M and 3.0 M sulfuric acid in Fig. 8 and for the tenth scans in 0.2 M and 3.0 M sulfuric acid in Fig. 9.
Fig. 7. The ranges of mass change observed with sequential scanning between 0 V and 1.4 V, at 50 mV/s, for PtPc layers deposited on gold electrodes by mechanical abrasion in 0.2 M sulfuric acid (grey bars) and 3.0 M sulfuric acid (black bars). The mass change prior to scanning, the bottom end of the mass ranges at scan one, is zero in both cases (not shown on the plot).
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Fig. 8. Mass–charge curves for the first scan of a PtPc layer deposited on a gold electrode by mechanical abrasion from 0 V to 1.4 V and back to 0 V, at a scan rate of 50 mV/s in 0.2 M sulfuric acid (grey squares) and 3.0 M sulfuric acid (black squares). Selected potentials during the scan are identified on the plot. Each square represents a different potential during the scan.
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with evidence of the involvement of significant solvent transfer during the redox chemistry of similar polymer thin films [27], because the anion migration and solvent diffusion into and out of the film are likely to occur on different timescales. On the tenth scan this hysteresis is evident until after the second oxidation peak at about 1.1 V, indicating that the third oxidation peak is not so intimately connected with solvent transport. Both curves in Fig. 9 also exhibit a small positive offset in the charge domain, this is much smaller than on the first scan, but still noticeable. Whilst this irreversible oxidation decreases further with further scanning the cathodic charge passed is always slightly less than the anodic charge, even though the mass changes show conflicting behaviour (small mass decreases and small mass increases for the 0.2 M and 3.0 M acids, respectively). This observation has previously been attributed to small quantities of material within the PtPc film becoming electrically disconnected upon prolonged scanning [14]. The mass fluxes of anions and solvent into the film during scanning may be estimated from Figs. 2 to 5. If we assume that the vast majority of mass increase occurs between 0.8 V and 1.4 V then on the first scan fluxes of approximately 160 ng/s and 300 ng/s are observed for PtPc films in the 0.2 M and 3.0 M sulfuric acid, respectively. By the tenth scan these fluxes have decreased to approximately 60 ng/s and 90 ng/s for the 0.2 M and 3.0 M sulfuric acid, respectively. If we propose that the portion of the curves in Fig. 9 showing hysteresis are dominated by solvent transfer, whilst the portion of the curves where no hysteresis is observed is dominated by anion movement, we can estimate the relative contribution of the anion and solvent flux by the relative changes in film mass in these portions of the curves. This suggests that solvent transfer contributes approximately 75% of the mass flux for electrochemically developed PtPc films in both 3.0 M and 0.2 M sulfuric acid. 3.4. PtPc chrono-amperometry
Fig. 9. Mass–charge curves for the tenth scan of a PtPc layer deposited on a gold electrode by mechanical abrasion from 0 V to 1.4 V and back to 0 V, at a scan rate of 50 mV/s in 0.2 M sulfuric acid (grey squares) and 3.0 M sulfuric acid (black squares). Selected potentials during the scan are identified on the plot. Each square represents a different potential during the scan.
These plots present a demonstrative link between the mass variation observed and charge variation of the PtPc thin film. For the first scans shown in Fig. 8 there is little change in mass or charge prior to 1.0 V as no PtPc oxidation is occurring. As charge flows during oxidation of the PtPc films between 1.0 V and 1.4 V mass also increases in a relatively monotonic fashion. The profile of both curves of the negative potential scan is broadly similar with the majority of change in mass and charge occurring during the reduction PtPc electrochemistry between 1.0 V and 0 V. Both plots finish the first scan with positive offsets in both the mass and charge domains. This is proposed as supporting evidence of a significant irreversibility in the oxidation of the PtPc film during the first scan with concomitant increase in mass owing to irreversibly incorporated solvated anions, more highly solvated in the case of the 3.0 M acid. Fig. 9 shows the analogous situation, but for the tenth scan. As the first oxidation peak of the PtPc at 0.7 V develops so there is a larger change in the mass increase and charge passed prior to 1.0 V. This notwithstanding, the majority of change in the mass– charge profile occurs between 1.0 V and 1.4 V on the positive scan and between 1.0 V and 0 V on the negative scan. There is a clear hysteresis in these curves (which shows a scan rate dependence – data not shown). This has been has been previously associated
The results presented thus far are instructive and provide an insight into the operation of the phthalocyanine films during oxidation and reduction mechanisms. However, cyclic voltammetric analysis of such a system makes even qualitative deconvolution of a response into the different intercalation regimes, i.e. solvent, anion and cation ingress or egress, difficult. A convenient method of analysing the processes occurring within the film is the use of potential step chrono-amperometry and measurement of the corresponding mass changes. In this way, the rate of change of potential is eliminated as an obfuscating element in interpretation. The two different characteristics of the second and third PtPc redox pairs are shown clearly in the 0.2 M sulfuric acid electrolyte by stepping from the reduced film (at 0 V) to the partially oxidised (1.18 V) (Fig. 10) and the fully oxidised film (1.4 V) (Fig. 11). The removal of the scan rate dependency of the mass-potential response reveals some interesting and unexpected features. Partial deconvolution of the processes occurring within the film and to the film itself may be qualitatively elucidated when the additional time dependency of the potential sweep in cyclic voltammetry is removed. The first general features to note are the large current and mass changes induced by stepping past the second PtPc redox process pair of peaks at approximately 1.1 V. Steps involving the third PtPc redox process peak pair at approximately 1.3 V involve only minor changes – it is known [14] that the third pair of PtPc redox peaks account for only about one-fifth of the charge transfer involved compared with the second pair of redox peaks. The mass responses seen in the cyclic voltammetric studies for the third redox pair are very slight, and may be contributed to by long time-constant processes associated with the second redox pair, especially at a scan rate of 50 mV/s. In fact, the proposed notion
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Fig. 10. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The first step at 1 s is from 0 V to 1.18 V, the second step at 11 s is from 1.18 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sulfuric acid.
Fig. 11. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The first step at 1 s is from 0 V to 1.4 V, the second step at 11 s is from 1.4 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sulfuric acid.
of continuous intercalation may well only be a manifestation of the timescale ambiguity of ion/solvent movement induced by voltage scanning. It has been proposed in conducting polymer films [28] that given the time resolved deconvolution offered by a potential step, the order of the relative contribution to mass change is ion transfer > solvent transfer > film reconfiguration. This seems to be a reasonable assumption to extend to phthalocyanine films. Additionally, with the electrolytes under consideration it is with good basis that one assumes the cations (Na+ and H+) move faster within the film than the sulfate counter ion owing to their smaller sizes and solvation radii. The chrono-amperometry responses for both electrolytes exhibits movements in both directions in the mass response, albeit on different timescales, which unequivocally indicates the presence of two (or more) mobile species during the redox processes. These data are particularly instructive for analysing a system where the electrochemical processes are not well defined and characterised (such as the PtPc film with sodium sulfate electrolyte). As observed from the cyclic voltammetric studies, oxidation of the film produces an overall increase in mass and reduction an overall decrease. These mass changes are therefore necessarily dominated by anion and concomitant solvent movement. As seen in Fig. 10, partial oxidation of the film results in an overshoot in the mass and then a small mass decrease owing most probably to solvent
expulsion or film rearrangement or both. On reduction of the film, a sharp mass increase is observed before the expected anion expulsion. We suggest that this peak is due to the fast influx of solvated protons into the film upon reduction. On full oxidation of the film in Fig. 11 the initial mass increase is followed by a slower mass increase. On this long timescale and after all faradaic current has decayed, this mass change is probably an osmotic phenomenon and/ or film reconstruction. Salt draining, involving ion pairs, is also likely to be involved in causing mass changes within the film [28]. On re-reduction only a very small mass increase is observed. This is again most likely to be proton ingress on short timescales. When one examines the mass responses of the peaks individually, one unexpectedly observes a slight overall decrease in mass for the third redox pair, as shown in Fig. 12. This indicates that this peak is associated with the egress of solvated protons, rather than additional anion ingress. It would seem that the second pair of redox peaks is mediated predominantly by solvated anion movement, whereas the third pair of redox peaks involve significant solvated cation movement. This is an effect that is apparently masked under cyclic voltammetric conditions. To investigate the cation effect further, sodium sulfate was used as the electrolyte instead of sulfuric acid in an attempt to slow down the movement of cations and increase the mass per unit charge observed in order see their effect more clearly. The electrochemistry of the PtPc films on the gold electrodes in 0.2 M sodium sulfate is shown in Fig. 13. The potential step of the PtPc film in sodium sulfate from the fully reduced form (0 V) to the fully oxidised form (1.3 V) and back produces a total mass increase and decrease, respectively – this is shown in Fig. 14. Both processes exhibit aspects relating to two or more mobile species because of movements in both directions of the mass response, on different timescales. Upon oxidation the mass increases dramatically, as sulfate is incorporated within the film, but exhibits an overshoot. The mass then decreases slightly. Once again, this is consistent with solvent exclusion from the film and/or film reconstruction. On reduction the mass decreases rapidly as anions are expelled, but then continues to decrease slowly in the absence of any faradaic current indicating a slow diffusional removal of solvent from the film. The absence of an initial small positive mass peak on the reductive step proves that this feature in Figs. 10 and 11 was most likely due to solvated proton movement. However, when one examines the effects of each peak individually the involvement of the Na+ cations becomes apparent. This is displayed in Fig. 15. Upon stepping from 0 V to 0.71 V, past a peak similar in character to the first redox pair of PtPc films in sulfuric acid, a small slow
Fig. 12. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. First step at 1 s, 0 V to 1.18 V; second step at 11 s, 1.18 V to 1.4 V; third step at 21 s, 1.4 V to 1.02 V; forth step at 31 s, 1.02 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sulfuric acid.
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Fig. 13. Current-potential curve for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sodium sulfate. The 5th scan is shown, at a scan rate of 50 mV/s.
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contributor. The subsequent decrease in mass is most likely caused by solvent expulsion as the film relaxes. Reduction of the film by potential step to 0.93 V gives a sharp decrease in mass owing to sulfate expulsion. Stepping past the final two reduction peaks one again obtains significant mass decrease but with an increase occurring early on in the mass-time transient. This is thought to be due to the additional ingress of sodium cations into the film – occurring over a longer timescale to the movement of the solvated protons. Similarly to the explanations proffered above, this response is thought to be a result of solvated cation ingress and anion egress. The use of sodium as a cation has lengthened the timescale of solvated cation movement and made the motion of the cation much more apparent within the film and potential stepping studies – any involvement was hidden in Fig. 14. Whether this is an effect of the increased solvated size and weight of the cation is unclear.
4. Conclusion
Fig. 14. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sodium sulfate. First step at 1 s, 0 V to 1.3 V; second step at 21 s, 1.3 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sodium sulfate.
Fig. 15. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sodium sulfate. First step at 1 s, 0 V to 0.71 V; second step at 11.5 s, 0.71 V to 1.3 V; third step at 21.5 s, 1.3 V to 0.93 V; fourth step at 32 s, 0.93 V to 0.43 V; fifth step at 42 s, 0.43–0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sodium sulfate.
mass decrease is observed. We suggest this is predominantly due to solvated cation expulsion from the film. Stepping past the main oxidation peak to 1.3 V one observes a mass increase with a very significant overshoot. The overall mass increase shows that solvated sulfate ingress into the film is once again the largest
The EQCM studies have revealed much about the behaviour of the PtPc films. It has been shown that anions and cations both have important roles to play in charge balance during redox processes. It seems that anion and concomitant solvent shell movement is the controlling factor in mass changes during redox processes. Hence one sees a net mass increase upon oxidation and a decrease upon reduction. However, cations and solvent are also involved in mass variations. This was shown clearly by EQCM potential step measurements. The non-monotonic mass changes observed during potential step EQCM experiments have shown that two or more mobile species take part in charge balancing within the film during redox processes. The potential step regime produced a time resolved mass response where the kinetics of the individual ionic species were more clearly resolved than in cyclic voltammetry. The EQCM, however, is relatively insensitive to the movement of protons and hence larger sodium cations were used in the electrolyte to observe the role of the cation in charge balancing during a potential step. Additional crystal immitance measurements [22], not displayed here, have shown that the phthalocyanine films can be considered ‘rigid’ before electrochemical modulation but show changes in visco-elasticity upon electrochemical intercalation. This notwithstanding, it is thought that the frequency changes observed during redox processes can still be interpreted qualitatively as an indicator of film mass change and swelling. The concentration and the nature of the electrolyte used has a large part to play in determining the extent of film swelling and mass change. This is due to the differing mass and swelling contributions from water with respect to the electrolyte concentration and the variation in the sizes of the anions (and their solvent shell) intercalating the film for different electrolytes. A proposed mechanism for the behaviour in 0.2 M sulfuric acid electrolyte is that there is an initial over-swelling of the film upon intercalation and the mass of the reduced film actually decreases with repetitive scanning. However, for a PtPc film in 3.0 M sulfuric acid there is no over-swelling during the first scan and the mass of the reduced film continues to increase with repetitive scanning as there is a net increase of electrolyte incorporated within the PtPc film. Examination of mass–charge profiles for the electrochemistry of these PtPc films has provided some additional evidence for irreversible oxidation of the film during the first scan, and the involvement of both anions and solvent in the intercalation process during subsequent scanning. The observation of a hysteresis in a limited portion of the mass–charge curves have allowed an estimation of the relative contribution of anions and solvent to the mass flux during redox cycling to be proposed.
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The results presented here serve to corroborate some of the conclusions drawn from previous electrochemical and photo-electrochemical studies on these films [14,15]. Acknowledgement The UK Department for Innovation, University and Skills’ funding of the National Measurement System Chemical & Biological Metrology Programme is gratefully acknowledged. References [1] N.B. McKeown, Phthalocyanine Materials: Synthesis, Structure and Function, Cambridge University Press, Cambridge, 1998. [2] A. Hamnett, Philos. Trans. R. Soc. London Ser. A 354 (1996) 1653. [3] R. Janinski, J. Electrochem. Soc. 112 (1965) 526. [4] J.A.R. van Veen, C. Visser, Electrochim. Acta 24 (1979) 921. [5] J.A.R. van Veen, J.F. van Baar, C.J. Kroese, J.G.F. Coolegem, N. de Wit, H.A. Colijn, Ber. Bunsenges. Phys. Chem. 85 (1981) 693. [6] E. Yeager, D.A. Scherson, C.A. Fierro, ACS Symp. Ser. 288 (1985) 535. [7] T. Nyokong, J. Porphyrins Phthalocyanines 12 (2008) 1005. [8] M. Kimura, K. Yasuta, N. Adachi, Y. Tatewaki, T. Fukawa, H. Shirai, Chem. Lett. 38 (2009) 82.
[9] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Electroanal. Chem. 160 (1984) 359. [10] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Electroanal. Chem. 239 (1988) 273. [11] A.L. Bouwkamp-Wijnoltz, W. Visscher, J.A.R. van Veen, Electrochim. Acta 39 (1994) 1641. [12] A.L. Bouwkamp-Wijnoltz, B.J. Palys, W. Visscher, J.A.R. van Veen, J. Electroanal. Chem. 406 (1996) 195. [13] S.J. Cooper, G.A. Hards, D. Thompsett, Johnson Atthey, Catalyst Material, United States Patent 5,316,990, May 31, 1994. [14] R.J.C. Brown, A.R. Kucernak, Electrochim. Acta 46 (2001) 2573. [15] R.J.C. Brown, A.R. Kucernak, N.J. Long, C. Mongay-Batalla, New J. Chem. 28 (2004) 676. [16] R.J.C. Brown, A.R. Kucernak, J. Solid State Electrochem. 9 (2005) 459. [17] R.J.C. Brown, A.R. Kucernak, A.G. Taylor, Thin Solid Films 476 (2005) 373. [18] J.H. Jiang, A. Kucernak, Electrochim. Acta 45 (2000) 2227. [19] S.A. Campbell, Y.L.S. Breakspear, F.C. Walsh, J.R. Smith, Trans. Inst. Met. Finish. 85 (2007) 237. [20] G.A. Snook, G.Z. Chen, J. Electroanal. Chem. 612 (2008) 140. [21] C.P. Wilde, S.V. De Cliff, K.C. Hui, D.J.L. Brett, Electrochim. Acta 45 (2000) 3649. [22] R.J.C. Brown, Ph.D. Thesis, Department of Chemistry, Imperial College, London, 2000. [23] G. Sauerbrey, Z. Phys. 155 (1959) 222. [24] S. Chen, B. Wu, C. Cha, J. Electroanal. Chem. 431 (1997) 243. [25] S. Bruckenstein, M. Shay, J. Electroanal. Chem. 188 (1995) 131. [26] S.W. Feldberg, I. Rubenstein, J. Electroanal. Chem. 240 (1988) 1. [27] S. Ramirez, A.R. Hillman, J. Electrochem. Soc. 145 (1998) 2640. [28] A.R. Hillman, N.A. Hughes, S. Bruckenstein, Analyst 119 (1994) 167.
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
An electrochemical quartz crystal microbalance study of platinum phthalocyanine thin films Richard J.C. Brown a,*, Dan J.L. Brett b, Anthony R.J. Kucernak c a
Analytical Science Team, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK Department of Chemical Engineering, University College London, London WC1E 7JE, UK c Department of Chemistry, Imperial College, London SW7 2AZ, UK b
a r t i c l e
i n f o
Article history: Received 18 May 2009 Received in revised form 29 June 2009 Accepted 2 July 2009 Available online 7 July 2009 Keywords: Platinum phthalocyanine Electrochemical quartz crystal microbalance Thin films Anion doping
a b s t r a c t An electrochemical quartz crystal microbalance study of platinum phthalocyanine thin films in aqueous media is presented. It has been shown that these films exhibit significant mass changes upon electrochemical cycling. These mass changes have been shown to be controlled by the ingress and egress of solvated anions during oxidation and reduction of the film, respectively. The mass changes within the film are shown to tend to steady state after a prolonged period of cycling. Chrono-amperometric studies have additionally shown that the movement of cations also has a role in the electrochemistry and mass transport within these films, but generally these events occur on a shorter timescale than those involving the solvated anions, and are hidden during most potential cycling studies. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Phthalocyanines (Pcs) represent a very important class of organic macrocycle [1]. Metallophthalocyanines (MPcs) have found particular uses as electrocatalysts for a variety of reactions. The most common electrocatalytic application for these MPcs has been in oxygen reduction, with the aim being their use in fuel cells or batteries [2,3]. As a function of the central transition metal in the phthalocyanine complex (MPc), van Veen found that the order of activity for oxygen reduction in both acid and base is: Fe > Co Ru > Mn > Pd Pt > Zn [4]. This result has lead to the almost exclusive study of the FePc and CoPc complexes in acid and alkaline aqueous solutions [5,6]. However, the stability of these firstrow complexes is not high under operating conditions, especially in acidic electrolytes, and this has frustrated attempts to use their excellent electrocatalytic properties to sustain competitive current densities. MPcs have also received some attention as targets for photodynamic therapy in cancer treatment [7], and in display devices because of their electrochromic properties [8]. The incorporation of different metals into the core of the phthalocyanine ring can affect the chemical and physical properties of the compound. In contrast to FePc and CoPc precious metal phthalocyanines, such as PtPc, PdPc and IrPc are known to exhibit remarkable thermal * Corresponding author. Tel.: +44 20 8943 6409; fax: +44 20 8614 0423. E-mail address: [email protected] (R.J.C. Brown). 0022-0728/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2009.07.002
stability and to resist oxidation. They have been shown to offer good performance as oxygen reduction electrocatalysts [9–12]. Under certain treatment regimes ‘activated’ precious metal phthalocyanine complexes are produced that are more active than FePc or CoPc [13]. However, platinum phthalocyanine (PtPc) is somewhat difficult to prepare in useable quantities. Recently, though, we have presented studies of the electrochemistry [14], spectroscopy [15], photo-electrochemistry [16], and optical second harmonic generation [17] of PtPc in aqueous media – information on electrochemical studies in non-aqueous media is also available [18]. We have previously suggested [14] that the PtPc thin films investigated exhibited unusual quasi-reversible electrochemistry, and cyclic voltammograms that develop with repeated scanning, with a large first scan discrepancy. This is similar to redox and conducting polymer films, whose growth and electrochemistry is mediated by the ingress and egress of anions and solvent from the films during cyclic voltammetry, where some literature is available [19]. We now present an electrochemical quartz crystal microbalance (EQCM) study to measure minute mass changes within the PtPc thin film during electrochemistry, and show that the electrochemistry of the PtPc is mediated by changes in the mass of the film, assumed to be the ingress and egress of solvated anions. This novel work further elucidates the physical and chemical characteristics of PtPc. (The closest comparable work has looked at EQCM studies of conducting and redox polymers [20].)
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2. Experimental All solutions used were prepared gravimetrically from ultrapure chemical reagents (Fisher) and deionised water (18.2 MX cm, Millipore, MilliQ). A standard three-compartment, three-electrode configuration was used with some alterations as described below. A platinum electrode flag was used as a counter electrode, and a saturated calomel electrode acted as the reference electrode, against which all voltages are henceforth quoted. Sulfuric acid solutions have been used for the experiments described (unless otherwise stated) as these are known to provide the best intercalation conditions for these PtPc films [14]. All solutions were de-oxygenated prior to use with argon (99.999+%, BOC) and an argon atmosphere was maintained above the solution during analysis. The working electrode comprised one side of a quartz crystal microbalance (QCM) device and consisted of a 100 nm gold layer vacuum deposited on both sides of AT-cut quartz crystal resonator (International Crystal Manufacturing Co. Inc., Oklahoma City); the deposited metal electrode formed a circle with a diameter of approximately 5 mm. The resonant frequency of these crystals was nominally 10 MHz. The connection was made with a coiled steel wire that was in turn soldered to an additional wire to which the QCM terminals could then be easily attached. The crystals were bonded to the open end of a small sample tube with silicone sealant in such a way that the steel was protected from solution contact and the second gold side of the crystal was exposed only to the atmosphere trapped within the sample tube. The experimental set-up used has been previously described [21]. The working electrode output from an EcoChemie Autolab PGSTAT 10 potentiostat was connected to the working electrode input on an in-house built oscillator circuit. This circuit controlled the potential of the working electrode side of the gold covered quartz crystal and additionally measures the resonant frequency of oscillation of the crystal. The frequency measured by the oscillator circuit was the difference between the resonant frequencies of the crystal in question and a standard reference crystal housed within the oscillator circuit. The oscillator circuit then output this signal to a frequency-to-voltage converter module which in turn delivered voltage to an analogue-to-digital input on the potentiostat, so that the current and mass responses may be measured simultaneously against voltage. At all times an oscilloscope was used to monitor the output of the oscillator circuit and determine the integrity of the quartz crystal resonator. In this way it was possible to determine if any given resonator failed. The PtPc was synthesised as previously described [15]. Mechanical abrasion was used to apply the PtPc layers onto the QCM surface. This was undertaken by rubbing the QCM electrode gently in a figure-of-eight motion onto excess PtPc powder placed onto a clean glass surface. At the end of the procedure, excess PtPc was gently brushed off the surface of the QCM, leaving a dark purple tinge to the previously gold surface. This procedure is known to deposit layers with an approximate thickness of 200 nm [17,22]. For the quartz crystals and the equipment used in this study the theoretical sensitivities were 16 Hz/mV and 4.4 ng/(cm2 Hz). The system was calibrated shortly before the experiments were undertaken using electro-deposition of copper from solution and found to yield an overall sensitivity of 18 ng/mV, which is henceforth used in all voltage to mass change conversions. The oscillating frequency change associated with processes occurring at the surface of the EQCM can be converted to a mass change using the well known Sauerbrey equation [23]. Whether it is reasonable to discuss the system in terms of absolute mass changes is debatable because of the non-ideal behaviour of QCMPtPc films, especially when solvated. This notwithstanding, the
most important observations from this study are qualitative, and their impact is not reduced by them not being rigorously quantitative. 3. Results and discussion 3.1. Bare gold It is of important to characterise the EQCM response of the bare gold electrode, since this will be superimposed on all subsequent investigations involving phthalocyanine films. For this purpose an exemplar response from a bare gold electrode is shown in Fig. 1. As the potential is scanned positively from 0 V the mass is seen to increase slowly as (bi)sulfate ions are adsorbed to the gold surface. This increase slows as gold oxide formation occurs at about 1.1 V. On the negative sweep the gold oxide layer is removed by 0.75 V with a concomitant mass decrease and an associated small hysteresis. The mass then continues to decrease with little hysteresis as (bi)sulfate de-associates from the surface. However, in the example shown in Fig. 1, the mass change due to oxide formation and removal is not particularly clear. The history of the electrode may affect the shape of the response seen and the magnitude of the contributions made from oxide formation/removal and (bi)sulfate association/de-association but the recorded response compares favourably with literature examples [24,25]. It should be noted that although the EQCM system has the mass resolution stated above, the resolution of the Autolab analogue-todigital converter is such that the mass signal shown in Fig. 1 shows quantisation steps of approximately 5 ng. However, the mass changes associated with PtPc electrochemistry are 50- to 100-fold larger than for bare gold, therefore background Au electrochemistry and signal processing limitations have little effect on the results presented in the study of the PtPc films. 3.2. PtPc cyclic voltammetry The electrochemistry of PtPc has been previously described [14]. In summary, PtPc films applied to gold and glassy carbon surfaces were shown to exhibit relatively complex electrochemistry in aqueous media. Upon potentiodynamic cycling three anodic peaks (first: 0.81 V, second: 1.10 V and third: 1.27 V) and two cathodic peaks (the pair of the second anodic peak: 1.12 V and the pair of the third anodic peak: 0.59 V) were seen to develop over time. These processes were ascribed to phthalocyanine ring oxidation and reduction processes. It was proposed that this phthalocyanine ring electrochemistry was controlled by anion doping and intercalation within the film, and that the large separations between cor-
Fig. 1. Current-potential (grey) and mass-potential (black) curves for a new, bare gold QCM electrode in 0.2 M sulfuric acid. Scan rate 50 mV/s.
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responding anodic and cathodic processes were manifestations of barrier potential to anion movement within the film (often referred to as unusual quasi-reversibility [26]). A large reduction peak at 0.59 V is attributed to the mass expulsion of anions from the film. Moreover, a distinct first scan discrepancy in the electrochemical response was observed, proposed to be owing to the initial intercalation of anions within the virgin film. (The mechanically abraded PtPc films have high density as confirmed by SEM examination [22], however, upon the initial oxidation of the film, with the associated intercalation of solvated anions, the porosity of the film will increase substantially as channels are opened up between PtPc microparticles.) The electrochemistry of the film was then seen to evolve over subsequent scans asymptotically approaching an equilibrium state. Whilst the evidence from the electrochemical study to support this thesis was compelling, it was not definitive. Thus additional studies to examine the mass changes within the PtPc films during the electrochemical processes were required to provide corroborating evidence. The EQCM and electrochemical response for the first scan of a PtPc layer in 0.2 M sulfuric acid is shown in Fig. 2. As consistent with the electrochemical observations [14] the mass change shows a very large increase on initial intercalation of the film after about 1.0 V. The mass increase then slows slightly after the first PtPc oxidation peak at 1.1 V but continuous anion ingress is seen during the second oxidation peak (at 1.3 V) up until 1.4 V. On the reverse scan a small mass decrease is observed as the first PtPc reduction peak is reached at 1.1 V and then a slow mass increase is seen before another drop coinciding with the second PtPc reduction peak at 0.55 V. Correlating the EQCM frequency response to a mass change, the scan finishes with a mass excess of 1500 ng compared to the initial (pre-intercalation) weight. In the interpretation of this first scan, caution must be exercised since it is expected that during the first scan the layer will under-go the majority of its major and irreversible changes in structure and visco-elastic properties. It is known that changes in the rigidity of a surface film will cause non-ideality in the EQCM response and resonant frequency changes may not be easily ascribable purely to mass changes within the film. Nevertheless the EQCM response shows three interesting general points. Firstly, increases in mass occur with the oxidation of the film suggesting net solution and anion ingress – similarly to the predictions made during electrochemical studies [14]. Secondly, and conversely, mass decreases occur with the reduction of the film, suggesting a net egress of solution and anions during the reductive sweep. Finally, there is an irreversible mass increase associated with the first scan due to the irreversible doping of anion and solvent into the film.
Subsequent scans on the same system begin to adopt to the same typical shape, although there is still variation with scan number in terms of the mass change caused by oxidation and reduction and the overall mass properties of the film at the beginning of each scan. Scans from a typical example are shown in Fig. 3. It is interesting to note that the mass of the film after the second scan is significantly less than after the first scan. Furthermore the reduced weight of the film after the second scan until the end of the tenth scan decreases slightly with each scan. This is not inconsistent with small losses of material from the film with scanning. The actual loss of mass would seem to rule out isolation of parts of the film losing electric connection and becoming electro-inactive as an explanation. Perhaps the more significant aspect of the study is the much more rapid decrease in the mass of the fully oxidised film with repeated scanning. One explanation of this would be that the film is approaching a reversible state with scanning. The use of dilute acid is likely to cause large quantities of solvent (in this case water) to be involved in the doping and undoping process. It is possible that excess water is incorporated into the film during the first scan of the virgin film and that subsequent scans see a net loss of mass because water is expelled as the film relaxes after initial swelling. That is to say that during scans 2–10 there is still a slow egress of excess solvent, which entered the film during the first scan, and anions from the film until an equilibrium is reached where the ratios of the cathodic to anodic charge passed approach unity with scanning, at which stage there will be a reversible change in mass upon cycling – a situation approximately represented by scan 10 of the example in Fig. 3, shown in Fig. 4. Indeed little change is observed to this profile in subsequent scans. Another interesting feature of the development of the film with scanning is the appearance of the first PtPc oxidation peak at 0.7 V. This peak has been previously observed and is known to grow with repetitive scanning and is associated with the oxidation of the PtPc film at, or near to, the electrode/PtPc interface [14]. By scan 10 (shown in Fig. 4) on the gold EQCM electrode, the peak is very apparent and now contributes significantly to the mass increase during the scan. The response at scan 10 is now one of continuous intercalation with three distinct regions of mass change (with different gradients) on the positive scan corresponding to the three oxidation regimes of the PtPc film. That the overall mass increases with oxidation and decrease with reduction indicates a net movement of sulfate and bisulfate ions into and out of the film, probably accompanied by solvent, during these processes. Increasing the sulfuric acid concentration to 3.0 M induces some subtle changes in the EQCM response. As is already known, the concentration of the electrolyte affects the peak potentials for
Fig. 2. Current-potential (grey) and mass-potential (black) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The first scan is displayed, at a scan rate of 50 mV/s.
Fig. 3. Mass-potential curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The second (top) to tenth (bottom) successive scans are shown at a scan rate of 50 mV/s.
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Fig. 4. Current-potential (grey) and mass-potential (black) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The tenth successive scan is shown, at a scan rate of 50 mV/s.
Fig. 6. Mass-potential curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 3.0 M sulfuric acid. Successive scans for 1 (bottom) to 10 (top) are shown, at a scan rate of 50 mV/s.
the PtPc oxidations and reductions as a result of the change in the molar free energy for the anion insertion process (which has a dependence on anion concentration) [14] and hence the mass changes occur at slightly different potentials – a developed profile is shown in Fig. 5. The most significant feature of the studies using 3.0 M sulfuric acid is the development of the mass response with scanning which shows an important difference from the response seen with 0.2 M sulfuric acid. The first 10 scans of the mass response in 3.0 M sulfuric acid are shown in Fig. 6. Similarly to the development seen using 0.2 M sulfuric acid a large mass increase is seen upon initial ‘break-in’ of the film during the first scan. However, apart from the larger mass change observed, the first scan shows the same characteristics as seen with the 0.2 M sulfuric acid response. The other interesting aspect of these results is the increase in mass of both the reduced and the oxidised film with repetitive scanning, apparently converging on a maximum value. Clearly the mass shows an increase upon oxidation and a decrease upon reduction, proving again that the redox processes are mediated mainly by the net movement of sulfate and bisulfate ions. However the similarity of the first scan to subsequent scans and the continuing increase in film mass with scanning would suggest that less solvent is dragged into the film initially with the sulfate ions as one might expect in a more concentrated background electrolyte. This effect is not inconsistent with a slower, more controlled film swelling and structural development with the 3.0 M acid leading to a film with a maximum of irreversibly intercalated anions. This occurs without the need to expel solvent after the first scan, as is seen in the 0.2 M acid. The total intercalation mass
changes between experiments can only be compared tentatively since the mass of (active) PtPc on the electrode is likely to vary slightly between experiments. However, to obtain some qualitative comparison between systems it is instructive to look at the ranges of mass change observed with sequential scanning for the two acid concentrations. This is shown in Fig. 7. Several features are of interest: firstly, the mass gains in the 3.0 M sulfuric acid are much greater – even for mass values that are not fully quantitative this is significant. Secondly, the range of mass change within one scan is greater for the 3.0 M sulfuric acid (about double that of the 0.2 M acid). Thirdly, as previously discussed, the films exposed to the 3.0 M sulfuric acid continue to increase in mass with sequential scanning, asymptotically tending to an equilibrium state. The films exposed to the 0.2 M sulfuric acid also tend asymptotically to an equilibrium state; however, now this is on a baseline of decreasing mass, following the maximum reached after the first scan. The response of these films in other common electrolytes has been tested and was found to be broadly similar [22].
Fig. 5. Current-potential (grey) and mass-potential (black) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 3.0 M sulfuric acid. The tenth scan is shown, at a scan rate of 50 mV/s.
3.3. Mass–charge relationships In view of the proposal that the changes in film mass being due to the ingress and egress of anions and solvent, it is instructive to also examine the mass–charge relationships during scanning. This is done for the first scans in 0.2 M and 3.0 M sulfuric acid in Fig. 8 and for the tenth scans in 0.2 M and 3.0 M sulfuric acid in Fig. 9.
Fig. 7. The ranges of mass change observed with sequential scanning between 0 V and 1.4 V, at 50 mV/s, for PtPc layers deposited on gold electrodes by mechanical abrasion in 0.2 M sulfuric acid (grey bars) and 3.0 M sulfuric acid (black bars). The mass change prior to scanning, the bottom end of the mass ranges at scan one, is zero in both cases (not shown on the plot).
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Fig. 8. Mass–charge curves for the first scan of a PtPc layer deposited on a gold electrode by mechanical abrasion from 0 V to 1.4 V and back to 0 V, at a scan rate of 50 mV/s in 0.2 M sulfuric acid (grey squares) and 3.0 M sulfuric acid (black squares). Selected potentials during the scan are identified on the plot. Each square represents a different potential during the scan.
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with evidence of the involvement of significant solvent transfer during the redox chemistry of similar polymer thin films [27], because the anion migration and solvent diffusion into and out of the film are likely to occur on different timescales. On the tenth scan this hysteresis is evident until after the second oxidation peak at about 1.1 V, indicating that the third oxidation peak is not so intimately connected with solvent transport. Both curves in Fig. 9 also exhibit a small positive offset in the charge domain, this is much smaller than on the first scan, but still noticeable. Whilst this irreversible oxidation decreases further with further scanning the cathodic charge passed is always slightly less than the anodic charge, even though the mass changes show conflicting behaviour (small mass decreases and small mass increases for the 0.2 M and 3.0 M acids, respectively). This observation has previously been attributed to small quantities of material within the PtPc film becoming electrically disconnected upon prolonged scanning [14]. The mass fluxes of anions and solvent into the film during scanning may be estimated from Figs. 2 to 5. If we assume that the vast majority of mass increase occurs between 0.8 V and 1.4 V then on the first scan fluxes of approximately 160 ng/s and 300 ng/s are observed for PtPc films in the 0.2 M and 3.0 M sulfuric acid, respectively. By the tenth scan these fluxes have decreased to approximately 60 ng/s and 90 ng/s for the 0.2 M and 3.0 M sulfuric acid, respectively. If we propose that the portion of the curves in Fig. 9 showing hysteresis are dominated by solvent transfer, whilst the portion of the curves where no hysteresis is observed is dominated by anion movement, we can estimate the relative contribution of the anion and solvent flux by the relative changes in film mass in these portions of the curves. This suggests that solvent transfer contributes approximately 75% of the mass flux for electrochemically developed PtPc films in both 3.0 M and 0.2 M sulfuric acid. 3.4. PtPc chrono-amperometry
Fig. 9. Mass–charge curves for the tenth scan of a PtPc layer deposited on a gold electrode by mechanical abrasion from 0 V to 1.4 V and back to 0 V, at a scan rate of 50 mV/s in 0.2 M sulfuric acid (grey squares) and 3.0 M sulfuric acid (black squares). Selected potentials during the scan are identified on the plot. Each square represents a different potential during the scan.
These plots present a demonstrative link between the mass variation observed and charge variation of the PtPc thin film. For the first scans shown in Fig. 8 there is little change in mass or charge prior to 1.0 V as no PtPc oxidation is occurring. As charge flows during oxidation of the PtPc films between 1.0 V and 1.4 V mass also increases in a relatively monotonic fashion. The profile of both curves of the negative potential scan is broadly similar with the majority of change in mass and charge occurring during the reduction PtPc electrochemistry between 1.0 V and 0 V. Both plots finish the first scan with positive offsets in both the mass and charge domains. This is proposed as supporting evidence of a significant irreversibility in the oxidation of the PtPc film during the first scan with concomitant increase in mass owing to irreversibly incorporated solvated anions, more highly solvated in the case of the 3.0 M acid. Fig. 9 shows the analogous situation, but for the tenth scan. As the first oxidation peak of the PtPc at 0.7 V develops so there is a larger change in the mass increase and charge passed prior to 1.0 V. This notwithstanding, the majority of change in the mass– charge profile occurs between 1.0 V and 1.4 V on the positive scan and between 1.0 V and 0 V on the negative scan. There is a clear hysteresis in these curves (which shows a scan rate dependence – data not shown). This has been has been previously associated
The results presented thus far are instructive and provide an insight into the operation of the phthalocyanine films during oxidation and reduction mechanisms. However, cyclic voltammetric analysis of such a system makes even qualitative deconvolution of a response into the different intercalation regimes, i.e. solvent, anion and cation ingress or egress, difficult. A convenient method of analysing the processes occurring within the film is the use of potential step chrono-amperometry and measurement of the corresponding mass changes. In this way, the rate of change of potential is eliminated as an obfuscating element in interpretation. The two different characteristics of the second and third PtPc redox pairs are shown clearly in the 0.2 M sulfuric acid electrolyte by stepping from the reduced film (at 0 V) to the partially oxidised (1.18 V) (Fig. 10) and the fully oxidised film (1.4 V) (Fig. 11). The removal of the scan rate dependency of the mass-potential response reveals some interesting and unexpected features. Partial deconvolution of the processes occurring within the film and to the film itself may be qualitatively elucidated when the additional time dependency of the potential sweep in cyclic voltammetry is removed. The first general features to note are the large current and mass changes induced by stepping past the second PtPc redox process pair of peaks at approximately 1.1 V. Steps involving the third PtPc redox process peak pair at approximately 1.3 V involve only minor changes – it is known [14] that the third pair of PtPc redox peaks account for only about one-fifth of the charge transfer involved compared with the second pair of redox peaks. The mass responses seen in the cyclic voltammetric studies for the third redox pair are very slight, and may be contributed to by long time-constant processes associated with the second redox pair, especially at a scan rate of 50 mV/s. In fact, the proposed notion
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Fig. 10. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The first step at 1 s is from 0 V to 1.18 V, the second step at 11 s is from 1.18 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sulfuric acid.
Fig. 11. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. The first step at 1 s is from 0 V to 1.4 V, the second step at 11 s is from 1.4 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sulfuric acid.
of continuous intercalation may well only be a manifestation of the timescale ambiguity of ion/solvent movement induced by voltage scanning. It has been proposed in conducting polymer films [28] that given the time resolved deconvolution offered by a potential step, the order of the relative contribution to mass change is ion transfer > solvent transfer > film reconfiguration. This seems to be a reasonable assumption to extend to phthalocyanine films. Additionally, with the electrolytes under consideration it is with good basis that one assumes the cations (Na+ and H+) move faster within the film than the sulfate counter ion owing to their smaller sizes and solvation radii. The chrono-amperometry responses for both electrolytes exhibits movements in both directions in the mass response, albeit on different timescales, which unequivocally indicates the presence of two (or more) mobile species during the redox processes. These data are particularly instructive for analysing a system where the electrochemical processes are not well defined and characterised (such as the PtPc film with sodium sulfate electrolyte). As observed from the cyclic voltammetric studies, oxidation of the film produces an overall increase in mass and reduction an overall decrease. These mass changes are therefore necessarily dominated by anion and concomitant solvent movement. As seen in Fig. 10, partial oxidation of the film results in an overshoot in the mass and then a small mass decrease owing most probably to solvent
expulsion or film rearrangement or both. On reduction of the film, a sharp mass increase is observed before the expected anion expulsion. We suggest that this peak is due to the fast influx of solvated protons into the film upon reduction. On full oxidation of the film in Fig. 11 the initial mass increase is followed by a slower mass increase. On this long timescale and after all faradaic current has decayed, this mass change is probably an osmotic phenomenon and/ or film reconstruction. Salt draining, involving ion pairs, is also likely to be involved in causing mass changes within the film [28]. On re-reduction only a very small mass increase is observed. This is again most likely to be proton ingress on short timescales. When one examines the mass responses of the peaks individually, one unexpectedly observes a slight overall decrease in mass for the third redox pair, as shown in Fig. 12. This indicates that this peak is associated with the egress of solvated protons, rather than additional anion ingress. It would seem that the second pair of redox peaks is mediated predominantly by solvated anion movement, whereas the third pair of redox peaks involve significant solvated cation movement. This is an effect that is apparently masked under cyclic voltammetric conditions. To investigate the cation effect further, sodium sulfate was used as the electrolyte instead of sulfuric acid in an attempt to slow down the movement of cations and increase the mass per unit charge observed in order see their effect more clearly. The electrochemistry of the PtPc films on the gold electrodes in 0.2 M sodium sulfate is shown in Fig. 13. The potential step of the PtPc film in sodium sulfate from the fully reduced form (0 V) to the fully oxidised form (1.3 V) and back produces a total mass increase and decrease, respectively – this is shown in Fig. 14. Both processes exhibit aspects relating to two or more mobile species because of movements in both directions of the mass response, on different timescales. Upon oxidation the mass increases dramatically, as sulfate is incorporated within the film, but exhibits an overshoot. The mass then decreases slightly. Once again, this is consistent with solvent exclusion from the film and/or film reconstruction. On reduction the mass decreases rapidly as anions are expelled, but then continues to decrease slowly in the absence of any faradaic current indicating a slow diffusional removal of solvent from the film. The absence of an initial small positive mass peak on the reductive step proves that this feature in Figs. 10 and 11 was most likely due to solvated proton movement. However, when one examines the effects of each peak individually the involvement of the Na+ cations becomes apparent. This is displayed in Fig. 15. Upon stepping from 0 V to 0.71 V, past a peak similar in character to the first redox pair of PtPc films in sulfuric acid, a small slow
Fig. 12. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sulfuric acid. First step at 1 s, 0 V to 1.18 V; second step at 11 s, 1.18 V to 1.4 V; third step at 21 s, 1.4 V to 1.02 V; forth step at 31 s, 1.02 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sulfuric acid.
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Fig. 13. Current-potential curve for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sodium sulfate. The 5th scan is shown, at a scan rate of 50 mV/s.
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contributor. The subsequent decrease in mass is most likely caused by solvent expulsion as the film relaxes. Reduction of the film by potential step to 0.93 V gives a sharp decrease in mass owing to sulfate expulsion. Stepping past the final two reduction peaks one again obtains significant mass decrease but with an increase occurring early on in the mass-time transient. This is thought to be due to the additional ingress of sodium cations into the film – occurring over a longer timescale to the movement of the solvated protons. Similarly to the explanations proffered above, this response is thought to be a result of solvated cation ingress and anion egress. The use of sodium as a cation has lengthened the timescale of solvated cation movement and made the motion of the cation much more apparent within the film and potential stepping studies – any involvement was hidden in Fig. 14. Whether this is an effect of the increased solvated size and weight of the cation is unclear.
4. Conclusion
Fig. 14. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sodium sulfate. First step at 1 s, 0 V to 1.3 V; second step at 21 s, 1.3 V to 0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sodium sulfate.
Fig. 15. Current-time (grey line) and mass-time (black line) curves for a PtPc layer deposited on a gold electrode by mechanical abrasion in 0.2 M sodium sulfate. First step at 1 s, 0 V to 0.71 V; second step at 11.5 s, 0.71 V to 1.3 V; third step at 21.5 s, 1.3 V to 0.93 V; fourth step at 32 s, 0.93 V to 0.43 V; fifth step at 42 s, 0.43–0 V. The inset shows the positions of the potential steps with respect to the PtPc cyclic voltammogram in 0.2 M sodium sulfate.
mass decrease is observed. We suggest this is predominantly due to solvated cation expulsion from the film. Stepping past the main oxidation peak to 1.3 V one observes a mass increase with a very significant overshoot. The overall mass increase shows that solvated sulfate ingress into the film is once again the largest
The EQCM studies have revealed much about the behaviour of the PtPc films. It has been shown that anions and cations both have important roles to play in charge balance during redox processes. It seems that anion and concomitant solvent shell movement is the controlling factor in mass changes during redox processes. Hence one sees a net mass increase upon oxidation and a decrease upon reduction. However, cations and solvent are also involved in mass variations. This was shown clearly by EQCM potential step measurements. The non-monotonic mass changes observed during potential step EQCM experiments have shown that two or more mobile species take part in charge balancing within the film during redox processes. The potential step regime produced a time resolved mass response where the kinetics of the individual ionic species were more clearly resolved than in cyclic voltammetry. The EQCM, however, is relatively insensitive to the movement of protons and hence larger sodium cations were used in the electrolyte to observe the role of the cation in charge balancing during a potential step. Additional crystal immitance measurements [22], not displayed here, have shown that the phthalocyanine films can be considered ‘rigid’ before electrochemical modulation but show changes in visco-elasticity upon electrochemical intercalation. This notwithstanding, it is thought that the frequency changes observed during redox processes can still be interpreted qualitatively as an indicator of film mass change and swelling. The concentration and the nature of the electrolyte used has a large part to play in determining the extent of film swelling and mass change. This is due to the differing mass and swelling contributions from water with respect to the electrolyte concentration and the variation in the sizes of the anions (and their solvent shell) intercalating the film for different electrolytes. A proposed mechanism for the behaviour in 0.2 M sulfuric acid electrolyte is that there is an initial over-swelling of the film upon intercalation and the mass of the reduced film actually decreases with repetitive scanning. However, for a PtPc film in 3.0 M sulfuric acid there is no over-swelling during the first scan and the mass of the reduced film continues to increase with repetitive scanning as there is a net increase of electrolyte incorporated within the PtPc film. Examination of mass–charge profiles for the electrochemistry of these PtPc films has provided some additional evidence for irreversible oxidation of the film during the first scan, and the involvement of both anions and solvent in the intercalation process during subsequent scanning. The observation of a hysteresis in a limited portion of the mass–charge curves have allowed an estimation of the relative contribution of anions and solvent to the mass flux during redox cycling to be proposed.
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The results presented here serve to corroborate some of the conclusions drawn from previous electrochemical and photo-electrochemical studies on these films [14,15]. Acknowledgement The UK Department for Innovation, University and Skills’ funding of the National Measurement System Chemical & Biological Metrology Programme is gratefully acknowledged. References [1] N.B. McKeown, Phthalocyanine Materials: Synthesis, Structure and Function, Cambridge University Press, Cambridge, 1998. [2] A. Hamnett, Philos. Trans. R. Soc. London Ser. A 354 (1996) 1653. [3] R. Janinski, J. Electrochem. Soc. 112 (1965) 526. [4] J.A.R. van Veen, C. Visser, Electrochim. Acta 24 (1979) 921. [5] J.A.R. van Veen, J.F. van Baar, C.J. Kroese, J.G.F. Coolegem, N. de Wit, H.A. Colijn, Ber. Bunsenges. Phys. Chem. 85 (1981) 693. [6] E. Yeager, D.A. Scherson, C.A. Fierro, ACS Symp. Ser. 288 (1985) 535. [7] T. Nyokong, J. Porphyrins Phthalocyanines 12 (2008) 1005. [8] M. Kimura, K. Yasuta, N. Adachi, Y. Tatewaki, T. Fukawa, H. Shirai, Chem. Lett. 38 (2009) 82.
[9] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Electroanal. Chem. 160 (1984) 359. [10] C. Paliteiro, A. Hamnett, J.B. Goodenough, J. Electroanal. Chem. 239 (1988) 273. [11] A.L. Bouwkamp-Wijnoltz, W. Visscher, J.A.R. van Veen, Electrochim. Acta 39 (1994) 1641. [12] A.L. Bouwkamp-Wijnoltz, B.J. Palys, W. Visscher, J.A.R. van Veen, J. Electroanal. Chem. 406 (1996) 195. [13] S.J. Cooper, G.A. Hards, D. Thompsett, Johnson Atthey, Catalyst Material, United States Patent 5,316,990, May 31, 1994. [14] R.J.C. Brown, A.R. Kucernak, Electrochim. Acta 46 (2001) 2573. [15] R.J.C. Brown, A.R. Kucernak, N.J. Long, C. Mongay-Batalla, New J. Chem. 28 (2004) 676. [16] R.J.C. Brown, A.R. Kucernak, J. Solid State Electrochem. 9 (2005) 459. [17] R.J.C. Brown, A.R. Kucernak, A.G. Taylor, Thin Solid Films 476 (2005) 373. [18] J.H. Jiang, A. Kucernak, Electrochim. Acta 45 (2000) 2227. [19] S.A. Campbell, Y.L.S. Breakspear, F.C. Walsh, J.R. Smith, Trans. Inst. Met. Finish. 85 (2007) 237. [20] G.A. Snook, G.Z. Chen, J. Electroanal. Chem. 612 (2008) 140. [21] C.P. Wilde, S.V. De Cliff, K.C. Hui, D.J.L. Brett, Electrochim. Acta 45 (2000) 3649. [22] R.J.C. Brown, Ph.D. Thesis, Department of Chemistry, Imperial College, London, 2000. [23] G. Sauerbrey, Z. Phys. 155 (1959) 222. [24] S. Chen, B. Wu, C. Cha, J. Electroanal. Chem. 431 (1997) 243. [25] S. Bruckenstein, M. Shay, J. Electroanal. Chem. 188 (1995) 131. [26] S.W. Feldberg, I. Rubenstein, J. Electroanal. Chem. 240 (1988) 1. [27] S. Ramirez, A.R. Hillman, J. Electrochem. Soc. 145 (1998) 2640. [28] A.R. Hillman, N.A. Hughes, S. Bruckenstein, Analyst 119 (1994) 167.