Nanogravimetric Measurements Research Articles

(Invited) Ligand-Functionalised Conducting Polymer Films for Analysis of Metal Ions in Solution

Innumerable technological devices and applications utilise metals and metal ions that ultimately find their way into the environment. While many metals are required at trace level for good health, their presence at higher levels is generally detrimental to human health and to the environment at large. For example, higher levels of copper are associated with liver and kidney disease; of nickel with cancer, cardiovascular disease, neurological conditions and infant developmental disorders; and of cobalt with respiratory conditions. This motivates the need to detect and determine these species at trace level; a related goal (not addressed directly here) is their extraction for the purposes of remediation. Although numerous analytical techniques exist for the determination of metal ions, electrochemically-based devices have a number of advantages, including portability for remote application. This work explores underpinning fundamental electrochemistry and interfacial characterisation associated with the fabrication and performance of carboxylate ligand-functionalised conducting polymer films as metal ion sensing moieties immobilised on electrode surfaces.Typical chelating ligands for metal ions are relatively bulky compared to the electropolymerizable monomer units, for example based on pyrrole or thiophene, commonly used to generate electroactive polymer films. As a consequence, electropolymerization of ligand-functionalised monomers is not generally effective: steric hindrance simply impedes coupling of the monomer units. We therefore adopt the alternative approach of electropolymerizing monomers functionalised with relatively small labile groups, followed by hydrolysis of the labile groups and re-functionalisation with the desired ligand. Here we explore this strategy for pyrrole-, thiophene- and aniline-based monomers functionalised with 9-fluorenylmethoxycarbonyl (fmoc) or pentafluorophenyl groups via ester linkages. Following electrochemically controlled polymerization, the resulting polymer films were treated with base to remove the labile groups, then with Nα,Nα-bis-(carboxymethyl)-L-lysine to couple N-nitrilotriacetic acid functionalities via amide formation.The polymerization process and the subsequent electroactivity of each of the parent films were studied using voltammetric and nanogravimetric (QCM) techniques. The ultimate effectiveness of the surface synthetic sequence, effected by post-deposition functionalisation with the ligand species, was validated using FTIR spectroscopy. This was accomplished by monitoring the bands associated with the carbonyl functionality: this is initially part of an ester, then an acid and finally an amide, each of which has a characteristic frequency. In the case of initially pentafluorophenyl-functionalised monomers, the hydrolysis of this group is associated with the loss of the C-F functionality. Measurements as a function of time permit the dynamics of these processes to be followed. Additionally, nanogravimetric measurements allow the time course of as-deposited polymer hydrolysis and subsequent functionalisation to be followed and quantitatively compared with the reaction stoichiometry.The effectiveness of these ligand-functionalised films was explored in terms of their ability to take up nickel, cobalt and copper ions from aqueous solution. Coulometric and gravimetric (QCM) data at various points in the process and as a function of solution metal ion concentration permit the acquisition of isotherms for metal ion uptake. At high metal ion concentration, the ligand sites are saturated; this permits metal:ligand stoichiometry to be evaluated. The relative merits of Langmuir, Frumkin, Temkin and Freundlich isotherms are discussed and the viability of the films for metal ion sensing evaluated. Unsurprisingly, the Langmuir isotherm provides a relatively poor description of the variation of metal ion uptake with concentration; this is attributed to the rather simplistic assumptions of this model. Of the remaining models, the Frumkin isotherm provides the best description of metal ion uptake for all systems considered. The binding constants evaluated from the polymer isotherms are significantly smaller than for the same ligand in solution; this is presumably a consequence of steric effects in the surface-confined polymer environment. Of relevance to potential applications, the films can be regenerated, i.e. restored to metal-free state, by exposure to EDTA solution.

(Keynote) Nanoscale Structuring of Aniline-Based Electroactive Polymer Films By Co-Polymerisation and Particulate Inclusion

Electroactive polymer films derived from aniline and substituted derivatives have attracted huge interest, as a consequence of their electronic, optical, chemical and charge storage properties. Surprisingly, there has been limited translation of these characteristics into practical devices. Two factors that have impeded progress here are an inability to deliver and optimise multiple characteristics in a single material and the absence of control of internal film structure. The latter is particularly difficult to maintain for soft matter, whose molecular geometry and spatial disposition at the mesoscopic scale necessarily changes in response to solvent influx/efflux during redox switching. These solvation phenomena have been variously described in terms of activity (thermodynamic) effects, volume constraints (polymer mechanical properties) and mobility (kinetic) effects. Quite generally, the dominant factor depends on the timescale, a feature of the experiments reported here.In this presentation, we explore strategies aimed at understanding and controlling internal film structure and solvation for polymer films based on aniline-type monomers. From a chemical perspective, we explore the effects of three factors: the nature of the solvent / electrolyte, extension from homopolymers to co-polymers, and the inclusion of geometrically distinct particulate species to generate hybrid organic/inorganic materials.In the first instance, we compare film deposition and subsequent redox chemistry when exposed to a conventional protic solvent (water) or to the aprotic ionic liquid Ethaline (a mixture of choline chloride and ethylene glycol). As an intermediate case, we consider film deposition and redox chemistry in the protic ionic liquid Oxaline (a mixture of choline chloride and oxalic acid). Nanogravimetric (QCM) measurements during deposition from ionic liquid reveal that the polymer film has substantially greater solvent content than if deposited from aqueous medium. A recognised phenomenon for electroactive polymer films is an anomalous response for the first redox cycle; this is generally attributed to solvation effects. This phenomenon is manifested quite differently for polyaniline films in water and Ethaline. In water, film evolution occurs over a number of cycles, while in Ethaline (in which the film is more highly solvated) the transition is complete with a single cycle.In the second instance, we consider copolymerization of aniline with o-toluidine or with o-aminophenol. Counter ion coordination by the hydrogen bond donors results in substantive differences in the effectively transferred entity. In the cases of Ethaline and Oxaline, the counter ion is no longer a small, mobile chloride ion, but rather a large hydrogen bond donor coordinated species. In pursuit of possible environmental applications, we consider the extreme case of fluoride as a counter ion. Surprisingly, the relatively small changes in polymer composition effected by co-polymerisation with o-toluidine or o-aminophenol result in substantial changes in fluoride uptake.Finally, we explore the effect of inclusion of particulates, chosen so as to influence different aspects of film behaviour. Specifically, we chose multi-walled carbon nanotubes (MWCN), graphite flakes and molybdenum oxide as examples of 1D, 2D and 3D entities, respectively. Data will be presented to demonstrate their substantive effects on film structure and dynamics.The outcomes of these studies provide insights relevant to practical applications. A particular case is that of medium transfer experiments, in which a film is deposited from one medium (aqueous or ionic liquid), redox cycled in the other medium and then returned to the first medium. We discuss the responses in terms of a “nature vs nurture” competition, in which the “nature” of a film is dictated by the conditions (medium) of deposition, and “nurture” is represented by the conditions to which it is exposed. Nanogravimetric (EQCM) responses of films subjected to water / ionic liquid / water and ionic liquid / water / ionic liquid transfer experiments reveal that films do retain some memory of their deposition conditions, but that extended redox cycling results in slow evolution that reflects the immediate medium of exposure. We speculate that this is a consequence of slow polymer structural relaxation.

(Keynote) Nanoscale Structuring of Aniline-based Electroactive Polymer Films by Co-polymerisation and Particulate Inclusion

Electroactive polymer films derived from aniline and substituted derivatives have attracted huge interest, as a consequence of their electronic, optical, chemical and charge storage properties. Surprisingly, there has been limited translation of these characteristics into practical devices. Two factors that have impeded progress here are an inability to deliver and optimise multiple characteristics in a single material and the absence of control of internal film structure. The latter is particularly difficult to maintain for soft matter, whose molecular geometry and spatial disposition at the mesoscopic scale necessarily changes in response to solvent influx/efflux during redox switching. These solvation phenomena have been variously described in terms of activity (thermodynamic) effects, volume constraints (polymer mechanical properties) and mobility (kinetic) effects. Quite generally, the dominant factor depends on the timescale, a feature of the experiments reported here.In this presentation, we explore strategies aimed at understanding and controlling internal film structure and solvation for polymer films based on aniline-type monomers. From a chemical perspective, we explore the effects of three factors: the nature of the solvent / electrolyte, extension from homopolymers to co-polymers, and the inclusion of geometrically distinct particulate species to generate hybrid organic/inorganic materials.In the first instance, we compare film deposition and subsequent redox chemistry when exposed to a conventional protic solvent (water) or to the aprotic ionic liquid Ethaline (a mixture of choline chloride and ethylene glycol). As an intermediate case, we consider film deposition and redox chemistry in the protic ionic liquid Oxaline (a mixture of choline chloride and oxalic acid). Quantitative nanogravimetric (QCM) measurements during deposition from ionic liquid reveal that the polymer film has substantially greater solvent content than if deposited from aqueous medium. A recognised phenomenon for electroactive polymer films is an anomalous response for the first redox cycle; this is generally attributed to solvation effects. This phenomenon is manifested quite differently for polyaniline films in water and Ethaline. In water, film evolution occurs over a number of cycles, while in Ethaline (in which the film is more highly solvated) the transition is complete with a single cycle.In the second instance, we consider copolymerization of aniline with o-toluidine or with o-aminophenol. Counter ion coordination by the hydrogen bond donors results in substantive differences in the effectively transferred entity. In the cases of Ethaline and Oxaline, the counter ion is no longer a small, mobile chloride ion, but rather a large hydrogen bond donor-coordinated species. In pursuit of possible environmental applications, we consider the extreme case of fluoride as a counter ion. Surprisingly, the relatively small changes in polymer composition effected by co-polymerisation with o-toluidine or o-aminophenol result in substantial changes in fluoride uptake.Finally, we explore the effect of inclusion of particulates, chosen so as to influence different aspects of film behaviour. Specifically, we chose multi-walled carbon nanotubes (MWCNT), graphite flakes and molybdenum oxide particles as examples of 1D, 2D and 3D entities, respectively. Electrochemical and imaging data will be presented to demonstrate their substantive effects on film structure and dynamics.The outcomes of these studies provide insights relevant to practical applications. A particular case is that of medium transfer experiments, in which a film is deposited from one medium (aqueous or ionic liquid), redox cycled in the second and then returned to the first. We discuss the responses in terms of a “nature vs nurture” competition, in which the “nature” of a film is dictated by the conditions (medium) of deposition, and “nurture” is represented by the conditions to which it is exposed. Nanogravimetric (EQCM) responses of films subjected to water / ionic liquid / water and ionic liquid / water / ionic liquid transfer experiments reveal that films do retain some memory of their deposition conditions, but that extended redox cycling results in slow evolution that reflects the immediate medium of exposure. We discuss this in terms of slow polymer structural relaxation.

(Invited) Determination of Solvation Populations and Dynamics in Electroactive Polymer Films By Modelling Time-Resolved Neutron Reflectivity

Characterization of electroactive polymer films and their application in practical devices (electronic, chemical, sensing, energy storage, optical, actuator) tends to focus on the transfer of charged species (electrons and counter ions), since these are the entities most directly associated with the response to the electrochemical control function. However, transport rates of ions (“dopant”, in the context of conducting polymer systems) are dictated by solvent content of the film, which governs the local viscosity of the medium through which mobile species move. Further, the presence of solvent (and other small molecules) creates free volume that facilitates the dynamics of polymer motion, macroscopically seen as a plasticising effect on film viscoelasticity. In the light of the above, it is of value to determine the solvent content of electroactive polymer films; this is the focus of the present work. Experimentally, the challenge is that the solvent is (by choice) electrochemically “silent” and optically transparent, and its selective observation within a film exposed to a vast reservoir of bulk solution is not trivial. Nanogravimetric measurements made using the EQCM have provided innumerable insights into changes in film solvent population, e.g. in response to redox state changes, but the technique cannot provide absolute solvent population in the film. Spectroscopic techniques generally have difficulty in distinguishing solvent in a film from that in bulk solution, but reflectivity techniques do possess surface selectivity. Of these, ellipsometry has the advantage of rapid data acquisition, i.e. good time resolution, but modelling the data can present ambiguities. Here we use neutron reflectivity (NR) which, through the penetrating nature of neutrons, allows access to "buried" interfaces. Traditionally, the negative consequence of this weak interaction with matter is long data acquisition times; addressing this is the central feature of this presentation. Since neutrons interact with the nuclei in the system (cf. photons interacting with the electrons in ellipsometry), NR has the huge advantage of isotopic selectivity. This is commonly referred to as “contrast variation” and has the critical characteristic of allowing one to vary the visibility of a selected chemical component without (to a first approximation) altering the (electro)chemistry of the system. Most frequently, this is exploited by deuteration, since the neutron scattering lengths of H and D are very different. Correlation of the h- and d-system NR data sets removes ambiguities in modelling the data. We have previously used NR with solvent contrast variation to determine solvent volume fraction depth profiles, ΦS(z), in redox polymer films at equilibrium (fixed potential) and on long timescales during slow cycling. We now present data under dynamic conditions more relevant to fabrication or operation of a working device. This necessitates an order of magnitude improvement in time resolution. Instrumentally, the challenge can be met using more sophisticated detection systems, including event mode data capture, in which every incident/reflected neutron is detected individually. The dramatic increase in the size of data sets presents challenges in modelling the data, but the facility to (re-)select signal averaging time post experiment permits optimisation of competing aspirations of time resolution and signal-to-noise. This is key for single shot experiments, such as film deposition or evolution of film behaviour. Here we present neutron reflectivity data for aniline-, pyrrole- and thiophene-based polymer films. For these systems, we have the opportunity to exploit h- and d- variants of both solvent and monomer. Four significant insights are revealed. First, the data reveal that film deposition protocol (i.e. potentiostatic, potentiodynamic or galvanostatic control function) influences film solvation state and spatial distribution; while anecdotally suspected, this provides unambiguous quantitative evidence. Second, there are substantial spatial variations in film solvent content; this is a feature commonly not accommodated in models of modified electrodes. Third, for some systems, film solvent population and spatial disribution are "frozen" into the system during fabrication, so deposition protocol determines film characteristics in the longer term. Fourth, in selected cases the structure evolves with electrochemical manipulation, such that differently prepared films subsequently exposed to the same environment and history evolve to a common solvation profile. The implications of these phenomena for film design and handling are discussed for particular types of application.

Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt

The effect of elastic strain on catalytic activity of platinum (Pt) toward oxygen reduction reaction (ORR) is investigated through dealloyed Pt–Cu thin films; stress evolution in the dealloyed layer and the mass of the Cu removed are measured in real-time during electrochemical dealloying of (111)-textured thin-film PtCu (1:1, atomic ratio) electrodes. In situ stress measurements are made using the cantilever-deflection method, and nanogravimetric measurements are made using an electrochemical quartz crystal nanobalance. Upon dealloying via successive voltammetric sweeps between −0.05 and 1.15 V vs standard hydrogen electrode, compressive stress develops in the dealloyed Pt layer at the surface of thin-film PtCu electrodes. The dealloyed films also exhibit enhanced catalytic activity toward ORR compared with polycrystalline Pt. In situ nanogravimetric measurements reveal that the mass of dealloyed Cu is approximately 210 ± 46 ng/cm2, which corresponds to a dealloyed layer thickness of 1.2 ± 0.3 monolayers or 0.16 ± 0.04 nm. The average biaxial stress in the dealloyed layer is estimated to be 4.95 ± 1.3 GPa, which corresponds to an elastic strain of 1.47% ± 0.4%. In addition, density functional theory calculations have been carried out on biaxially strained Pt(111) surface to characterize the effect of strain on its ORR activity; the predicted shift in the limiting potentials due to elastic strain is found to be in good agreement with the experimental shift in the cyclic voltammograms for the dealloyed PtCu thin film electrodes.

(Invited) In Situ Stress and Nanogravimetric Measurements during Underpotential Deposition

The underpotential deposition (UPD) of metals onto foreign metal substrates involves monolayer or submonolayer formation at potentials that are positive of the reversible electrodeposition of the bulk metal. UPD is particularly important to the understanding of electrodeposition where processes in the UPD region can be expected to influence the growth and subsequent properties of bulk thin films. In electrocatalysis, sub-monolayers of metals such as Bi, Pb, and Tl on noble metal surfaces have shown enhanced catalytic activity for a variety of electroreduction processes. Modified catalytic activity has also been observed for Pd and Pt overlayers consisting of a few monolayers. For this reason UPD processes are being examined by a variety of in situ techniques, including wafer curvature, in order to better understand the relationship between the structure of the UPD adlayers and the electrocatalytic activity. Changes in surface stress can be measured by monitoring the curvature of a wafer or cantilever while in solution and under potential control. Surface stresses on the order of 0.01 N/m can be resolved, sufficient to study the adsorption of molecular monolayers onto the electrode surface. Equation (1) quantifies the change in surface stress associated with epitaxial overlayer growth1, ΔF = Δfo + Δfpzc + Δh + Σcohtf (1) where Δfo is the intrinsic surface stress difference between the overlayer and the substrate, Δfpzc is an electrocapillary term that reflects the change in capacitance and potential of zero charge (pzc) associated with changing the surface from the substrate metal to the overlayer metal, Δh is the interface stress, and ΔΣcohtf is the product of the coherency stress and the film thickness. For sub-monolayers, Eqn. (1) may also contain an additional term that captures the contribution of the compressive strain due to island size effects2. Although the misfit term in Eqn. (1) is often assumed to dictate the overall change in surface stress, this is not always the case. The upd of Al and Cu onto Au are clear examples where the stress change is compressive even though the lattice misfit is positive. In some cases, pseudomorphic film growth of several monolayers allows the coherency term to be distinguished from the interfacial terms, which are independent of film thickness. Measurements of this type have been made for the electrodeposition of Pd onto (111)-textured Au3, and have resulted in estimates of Δh for the Pd-Au interface to be about -1.2 N/m. This is similar in both sign and magnitude to values reported for Ag-Cu4. This talk will examine the surface stress changes associated with underpotential deposition, paying particular attention to the interfacial contributions, in addition to that associated with lattice misfit.

Redox-Driven Ion Exchange of Poly(3,4,-ethylenedioxythiophene) Films in Deep Eutectic Solvents

Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of a range of conducting polymers based on aromatic heterocyclic monomers, typified by thiophene, pyrrole and their derivatives. The substitution pattern in PEDOT has the dual advantages of shifting the potentials for polymerization and redox switching into a more convenient range (as compared to the parent thiophene) and of blocking attack at the 3- and 4- positions of the aromatic ring. Both of these attributes enhance its practical application for energy storage applications, such as batteries and supercapacitors.Quite generally, the rates of doping and undoping (charging and discharging, in the context of energy storage) of conducting polymers are limited not by the rate of electron transport along the polymer backbone, but by the transport rate(s) of the charge balancing ion(s), i.e. dopant(s). The dynamics of these processes have been widely studied during redox switching of PEDOT films immersed in diverse electrolyte media based on molecular solvent systems. Although the quantitative details vary with the electrolyte selected, the qualitative outcome is that anion transfer is commonly the dominant process accompanying PEDOT oxidation (p-doping), but that cation transfer (in the opposite direction) can be appreciable, and in any case solvent transfer is significant; in the context of nanogravimetric measurements (using the EQCM), the ion and neutral molecule transfers can be distinguished according to their correlation with transferred charge. Measurements of the absolute solvent population (cf. changes measured in an EQCM experiment) in many conducting polymer films reveal that this may represent ca. 50% by volume of the film: this facilitates both ion transfers and polymer spinal motions. While such studies have revealed many interesting phenomena, notably solvation-controlled viscoelastic phenomena, a number of limitations make conventional solvent media unsuitable for practical applications; these may include available potential window, safety, toxicity or flammability. This motivates research on the redox chemistry of electroactive polymers in novel media, such as room temperature ionic liquids. Amongst these, we have been exploring the properties of deep eutectic solvent (DES) media based on various formulations of quaternary ammonium salts (QAS), hydrogen bond donors (HBD) and metal salts, in which complexation chemistry amongst the components generates fully ionic media. From a practical perspective, this provides an essentially unlimited source of ions (dopant). From a fundamental perspective, the dramatic difference is that these fully ionic media contain no solvent to plasticise the polymer; the more subtle difference is that the question of permselectivity that prevails (or fails) at low (or high) electrolyte concentration in a conventional solvent-based medium now becomes redundant.Here we describe nanogravimetric acoustic wave (EQCM) studies of redox driven ion transfers for PEDOT films exposed to three DES formulations variously involving choline chloride (Ch+Cl-) as the QAS, ethylene glycol (EG) as the HBD and ZnCl2 as the metal salt. Viscosity effects associated with the DES are significant but, via acoustic admittance measurements, one can obtain film ion population change data. These are presented as a function of timescale (potential scan rate in a voltammetric experiment) for potentiostatically- and potentiodynamically-grown PEDOT films subsequently exposed to ZnCl2/EG, ZnCl2/acetamide and ZnCl2/Ethaline (where Ethaline is a 1:2 stoichiometric mixture of Ch+Cl- and EG). Significantly, for thicker films (consistent with practical applications), there is incomplete charge recovery at faster scan rates, with accompanying failure to restore the initial film ion composition. In multi-cycling experiments this leads to evolution of response; we discuss re-equilibration when the film is subsequently held in a resting state. Cycling over prolonged time intervals also results in evolution of film responses. The fundamental interpretation and practical consequences of these observations will be discussed.We thank EU FP7 for financial support (NMP3-SL-2008-226655)

Frontispiece

The frontispiece shows a schematic representation of a technique that simultaneously combines electrochemical impedance spectroscopy and nanogravimetric measurements by using an electrochemical quartz crystal nanobalance. This approach allows in situ characterization and better understanding of concurrent processes (adsorption, absorption, metal deposition, and catalytic reactions) at different electrode potentials or as a function of time. A full account can be found in the Minireview by A. Bandarenka and co-workers on page 348 ff.,.

Characterisation of Complex Electrode Processes using Simultaneous Impedance Spectroscopy and Electrochemical Nanogravimetric Measurements.

The methodology and illustrative examples of application are presented for a technique that simultaneously combines electrochemical impedance spectroscopy (EIS) and nanogravimetric measurements; the latter are implemented using a so-called electrochemical quartz crystal nanobalance (EQCN). The combination of EIS and EQCN provides a powerful method for the characterisation of many complex processes at electrochemical interfaces. This method gives in one relatively simple experiment more detailed information than is available from conventional electrochemical techniques. The combined measurements can be performed either as a function of time, at a constant electrode potential, or under potentiodynamic conditions, as a function of the electrode potential. Herein, we show how this can be applied to enable more accurate investigation of processes that occur at boundaries between electrodes and electrolytes. The application examples range from eletrocatalysis, in which evaluation of a catalyst is performed simultaneously with its formation, and the intercalation and electrodeposition of thin metal films to in situ characterisation of non-electroactive self-assembled monolayers during their formation.

Electrochemical nanogravimetric studies on the electropolymerization of indole and on polyindole

The electrochemical quartz crystal nanobalance has been used to study the formation and redox behavior of polyindole in acid media. It was found that the redox transformations of polyindole are accompanied with the sorption and desorption of counterions. A comparison is made with the electropolymerization of 4-aminoindole and with the behavior of poly(4-aminoindole), where completely different mass change curves have been detected. Based on the results of the nanogravimetric measurements a redox mechanism describing also the pH dependence is suggested.

Nanosphere Lithography as a Versatile Method to Generate Surface‐Imprinted Polymer Films for Selective Protein Recognition

AbstractA versatile approach based on nanosphere lithography is proposed to generate surface‐imprinted polymers for selective protein recognition. A layer of 750 nm diameter latex bead‐protein conjugate is deposited onto the surface of gold‐coated quartz crystals followed by the electrosynthesis of a poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) film with thicknesses on the order of the bead radius. The removal of the polymer bead‐protein conjugates, facilitated by using a cleavable protein‐nanosphere linkage is shown to result in 2D arrays of periodic complementary size cavities. Here it is demonstrated by nanogravimetric measurements that the imprinting proceeds further at molecular level and the protein (avidin) coating of the beads generates selective recognition sites for avidin on the surface of the PEDOT/PSS film. The binding capacity of such surface‐imprinted polymer films is ca. 6.5 times higher than that of films imprinted with unmodified beads. They also exhibit excellent selectivity against analogues of avidin, i.e., extravidin, streptavidin, and neutravidin, the latter being in fact undetectable. This methodology, if coupled with properly oriented conjugation of the macromolecular template to the nanoparticles, offers the possibility of site‐directed imprinting.

Electrochemical and nanogravimetric studies of palladium phthalocyanine microcrystals

The electrochemical quartz crystal nanobalance has been used to study the redox behavior of palladium phthalocyanine microcrystals attached to gold and platinum in aqueous solutions at different pH values. In order to investigate the substrate effect, paraffin impregnated graphite electrodes were also applied. It was found that the redox transformations of palladium phthalocyanine are accompanied with deprotonation-protonation as well as the sorption and desorption of counterions, respectively, the extent of which depends on the pH. Based on the results of the nanogravimetric measurements, a redox mechanism describing also the pH dependence is suggested. Simultaneously with the charge transfer processes, solid-solid phase transitions and water transport occur.

In Situ Studies of the Adsorption Kinetics of 4-Nitrobenzenediazonium Salt on Gold

Self-assembled organic layers are an important tool for modifying surfaces in a range of applications in materials science. Covalent modification of metal surfaces with aryldiazonium cations has attracted much attention primarily because this reaction offers a route for spontaneously grafting a variety of aromatic moieties from solution with high yield. We have investigated the kinetics of this process by performing real-time, in situ nanogravimetric measurements. The spontaneous grafting of 4-nitrobenzene diazonium salts onto gold electrodes was studied via quartz crystal microbalance (QCM) from aqueous solutions of the salt at varying concentrations. The concentration dependence of the grafting rate within the first 10 min is best modeled by assuming a reversible adsorption process with free energy comparable to that reported for arylthiols self-assembled on gold. Multilayer formation was observed after extended grafting times and was found to be favored by increasing bulk concentrations of the diazonium salt. Modified gold surfaces were characterized ex situ with cyclic voltammetry, infrared reflection absorbance spectroscopy, and X-ray photoemission spectroscopy. Based on the experimentally determined free energy of adsorption and on the observed grafting rates, we discuss a proposed mechanism for aryldiazonium chemisorption.

Underpotential Deposition of Tl on (111)-Textured Au: In Situ Stress and Nanogravimetric Measurements

The surface stress associated with the underpotential deposition (upd) of thallium (Tl) on (111)-textured Au is examined, using the wafer curvature method in acidic perchlorate supporting electrolyte. The process is also examined by independent nanogravimetric measurements using an electrochemical quartz crystal nanobalance (EQCN). We observe a sweep rate dependence for both the individual voltammetric waves and stress response, which we attribute to kinetically controlled surface alloying that occurs only at low coverage. Similar behavior has been reported for Pb upd on (111)-textured Au, but the kinetics for Tl are considerably slower and are very sensitive to the defect density of the Au(111) surface. At high coverage, a full Tl monolayer is formed; however, a dealloying step is required prior to completion of the monolayer. The stress hump that is coincident with the last voltammetric wave appears to be caused by the formation and removal of the surface alloy. This is confirmed by long-term potentiostatic pulsing experiments.

In Situ Stress and Nanogravimetric Measurements during Hydrogen Adsorption/Absorption on Pd Overlayers Deposited onto (111)-Textured Au

not Available.

Deposition of selenium thin layers on gold surfaces from sulphuric acid media: Studies using electrochemical quartz crystal microbalance, cyclic voltammetry and AFM

In this paper we report here new considerations about the relationship between the mass and charge variations ( m/ z relationship) in underpotential deposition (UPD), bulk deposition and also in the H 2Se formation reaction. Nanogravimetric experiments were able to show the adsorption of H 2SeO 3 on the AuO surface prior to the voltammetric sweep and that, after the AuO reduction, 0.40 monolayer of H 2SeO 3 remains adsorbed on the newly reduced Au surface, which was enough to gives rise to the UPD layer. The UPD results indicate that the maximum coverage with Se ads on polycrystalline gold surface corresponds to approximately 0.40 monolayer, in good agreement with charge density results. The cyclic voltammetry experiments demonstrated that the amount of bulk Se obtained during the potential scan to approximately 2 Se monolayers, which was further confirmed by electrochemical quartz crystal microbalance (EQCM) measurements that pointed out a mass variation corresponding of 3 monolayers of Se. In addition, the Se thin films were obtained by chronoamperometric experiments, where the Au electrode was polarized at +0.10 V during different times in 1.0 M H 2SO 4 + 1.0 mM SeO 2. The topologic aspects of the electrodeposits were observed in Atomic Force Microscope (AFM) measurements. Finally, in highly negative potential polarizations, the H 2Se formation was analyzed by voltammetric and nanogravimetric measurements. These finding brings a new light on the selenium electrodeposition and point up to a proposed electrochemical model for molecule controlled surface engineering.

In Situ Stress and Nanogravimetric Measurements During Hydrogen Adsorption/Absorption on Pd Overlayers Deposited onto (111)-Textured Au

The stress induced by electrochemical hydrogen adsorption and absorption in very thin palladium layers electrodeposited onto (111)-textured gold has been examined in 0.1 M H2SO4 by the cantilever curvature method and by comparing resonant frequency changes on AT- and BT-cut quartz crystals. A compressive surface stress change of about −0.25 N m−1 is measured in the hydrogen adsorption region. Compressive stress for hydrogen adsorption is not expected from charge distribution models for adsorbate-induced surface stress but is consistent with first-principles calculations in the literature as well as experimental data for H adsorbed on Pt(111). Cantilever measurements in the hydrogen absorption region are consistent with a maximum atomic H/Pd loading of 0.63 and give a compressive stress-thickness change of −0.45 N m−1 per monolayer of Pd, corresponding to a biaxial stress change of −2.0 GPa. A somewhat lower value of −0.80 GPa for a H/Pd loading of 0.68 was obtained from EQNB data using the double crystal ...

Large cation model of dissociative reduction of electrochromic WO3−x films

AbstractStudies of dissociative reduction processes of electrochromic WO3−x films were conducted to: (i) evaluate their utility for electroetching and (ii) determine their fundamental mechanistic features to reduce or eliminate their occurrence in normal optical switching and modulation operation of WO3−x films. We have found that while the small intercalating cations stabilize WO3−x structure, the large nonintercalating surfactant cations (Et4N+, CtMe3N+) contribute to the dissociative reduction. While these cations do not affect WO3−x structure of anodically protected films (E > 0.2 V), they cause surface lattice polarization on electron injection to the conduction band of WO3−x at lower electrode potentials, in the absence of intercalating cations. We have found that this process is limited to the surface and no structural damage occurs to the underlying film. The mechanistic aspects of the process have been discussed on the basis of experimental voltammetric and electrochemical quartz crystal nanogravimetric (EQCN) measurements and ab initio quantum mechanical calculations.

In Situ Stress and Nanogravimetric Measurements During Underpotential Deposition of Pd on (111)-Textured Au

In Situ Stress and Nanogravimetric Measurements During Underpotential Deposition of Pd on (111)-Textured Au

In Situ Stress and Nanogravimetric Measurements During Underpotential Deposition of Pb on (111)-Textured Au

The surface stress associated with the underpotential deposition (upd) of lead on (111)-textured Au is examined, using the wafer curvature method, in acidic perchlorate supporting electrolyte. The surface stress is correlated to 0, the fractional Pb coverage, by independent nanogravimetric measurements using an electrochemical quartz crystal nanobalance (EQNB). The gravimetric results are similar to the data found in the literature, showing an electrosorption valency of 2 and the formation of a hexagonal close-packed monolayer of Pb. The complete Pb monolayer causes an overall compressive surface stress change of about -1.2 N m -1 . The stress-coverage curve can be divided into two linear regions separated by a plateau. The region at low to intermediate coverage is caused by the formation of Au-Pb bonds which decrease the charge density and reduce the tensile surface stress inherent to the clean Au surface. In the second linear region at high coverage, the Pb adlayer compresses and behaves as a free-standing elastic film with a biaxial modulus close to that for Pb (111) in the bulk. The plateau that separates these two linear regions corresponds to the stress relaxation hump that appears in the stress-potential curve. This stress relaxation is attributed to island coalescence.