Electrochemical quartz crystal microbalance study of formation and redox transformations of poly(diphenylamine)

The electropolymerization of diphenylamine (DPA) dissolved in 2 mol dm⁻³ H₂SO₄ has been studied on a gold electrode by cyclic electrochemical quartz crystal microbalance measurements. Similar experiments have been accomplished with DPA microcrystals attached to a gold electrode, in the presence of aqueous acidic media. In both cases the redox transformations of the poly(diphenylamine) films formed on Au have been investigated in solutions of different pHs. The cyclic voltammograms and the mass changes refer to two closely spaced redox reactions resulting in the formation of cation radicals and dications, respectively. The irregular, broad shape of the voltammetric waves may be explained by presence of other electrochemically active sites beside the linear diphenylbenzidine structure. The results indicate a pH and potential-dependent relative contribution of anions and hydrogen ions to the overall ion exchange process. The expulsion of protons is the predominant ion transport process at the beginning of the oxidation and the incorporation of anions occurs at a later stage of oxidation. The sorption/desorption of solvent molecules also contributes to the mass change observed during the redox transformations of the polymer.     

Electrochemical quartz crystal microbalance study of redox transformations of TCNQ microcrystals in concentrated LiCl solutions

Microcrystals of 7,7,8,8-tetracyanoquinodimethane (TCNQ) immobilized on gold surface have been investigated by combined cyclic voltammetry/chronoamperometry and piezoelectric nanogravimetry at an electrochemical quartz crystal microbalance (EQCM) in dilute and concentrated LiCl solutions. It is proved that TCNQ microcrystals even when TCNQ is reduced to its highly soluble Li⁺TCNQ⁻ salt can be studied in aqueous solutions when the water activity is decreased to a low level. At high electrolyte concentrations the voltammetric and chronoamperometric responses obtained for TCNQ microcrystals show the theoretically predicted pattern characteristic to the solid-solid phase transformation under the control of the nucleation and growth kinetics. The utilization of the idea presented in this work, i.e., using electrolyte of high concentrations (or decreasing the water activity by adding an indifferent component to the liquid phase in high concentration) can open up new vistas in the study of redox transformations of immobilized microcrystals which are soluble in water.     

The role of ion and solvent transport during the redox process of conducting polymers

The understanding of the redox behavior of conducting polymers is essential for a successful application of these so-called synthetic metals as functional coatings. The redox process involves the exchange of ions and solvent molecules. This so called doping/dedoping process involves changes of the mechanical and the electronic structure of the polymer. This paper discusses investigations at poly(3,4-ethylenedioxythiophene (PEDOT) and poly(pyrrole) (Ppy) by the electrochemical quartz crystal microbalance (EQCM) technique and electrochemical impedance spectroscopy (EIS). In the case of PEDOT a determination of the anion and the solvent fluxes was possible, and it was found that most anions replace solvent molecules upon their incorporation. The doping/dedoping mechanism of Ppy is more complicated. Here, the first redox cycles are characterized by a complex interplay of cation, anion and solvent fluxes with irreversible changes of the polymer structure. However, in combination with EIS new insights of the ion and solvent exchange and its influence on the electronic properties can be achieved.     

A voltammetric and nanogravimetric study of Te underpotential deposition on Pt in perchloric acid medium

This work describes the study of Te underpotential deposition on Pt in acid media using cyclic voltammetry, rotating ring-disc electrode and electrochemical quartz crystal microbalance techniques. The voltammetric results indicate the presence of two dissolution peaks in the positive scan with a total charge density of 420 μC cm⁻². These phenomena are attributed to the deposition of one Te monolayer with the occupancy of two active Pt sites by each ad-atom. This is confirmed by rotating ring-disc electrode results. The electrochemical quartz crystal microbalance (EQCM) experiments yielded the small mass variation of −32 ng cm⁻² (while the theoretical one is −140.4 ng cm⁻² for a complete Te monolayer). This low value can be attributed to the simultaneous adsorption of water, perchlorate anions and the formation of platinum oxide.     

Correlation between conductance and mass changes during p-doping of 2-methoxynaphthalene films

The redox response of 2-methoxynaphthalene films electrosynthesized in two different organic solvents, acetonitrile (ACN) and nitrobenzene (NB) has been studied by different in situ electrochemical techniques: the in situ conductance technique, electrochemical quartz crystal microbalance (EQCM) and electrochemical voltage spectroscopy (EVS). In situ measurements of conductance as a function of the potential during p-doping of 2-methoxynaphthalene films, electrosynthesized in TBAPF₆-ACN and TBAPF₆-NB, show that the conductance properties are strongly dependent on the solvent used during electrosynthesis, resulting in higher conductance values for films electrosynthesized in NB solutions. The EQCM technique has been used to correlate the frequency changes (mass changes) at the electrode surface with conductance changes during p-doping of the different films. The molar mass of the species involved in the charging-discharging reactions has been estimated from the EQCM results. For the determination of the electrochemical bandgap of the two different films, cyclic voltammetry (CV) and EVS were applied. Films electrosynthesized in NB solutions have a lower value of the bandgap (1.34 eV) than films electrosynthesized in ACN solutions (2.00 eV).     

Monitoring of formation and redox transformations of poly(Methylene blue) films using an electrochemical quartz crystal microbalance

An electrochemical quartz crystal microbalance and cyclic voltammetry have been used to monitor the growth of poly(methylene blue) films on gold during potential cycling between −0.6V and 1.0V vs. sce in sodium phosphate buffer (pH 8.2) and Na₂SO₄ solutions. Both the adsorption/desorption of monomer methylene blue and the formation of poly(methylene blue) film can be followed by microgravimetry. The response of the polymer films is significantly separated from that of the monomer species in solution. It has been proved that anion sorption/desorption occur during the redox transformation of the polymer, while cations play no role. Preliminary results on the complex protonation/deprotonation equilibria are also discussed.     

Effects of Electrolyte Concentration on Surfactant Adsorption to a QCM Immersed in Surfactant + Electrolyte Solutions

Abstract

Surfactant adsorption from aqueous electrolyte solutions onto metal surfaces was characterized through the use of a Quartz Crystal Microbalance (QCM). The need for a better understanding of the surfactant adsorption process became apparent in previous studies by Morton et al., directed toward the development of a thermodynamically-based model of oil removal from metal surfaces. These modeling efforts utilized existing data on surfactant adsorption data, yet required the estimation of surfactant adsorption phenomena, such as the transition between monolayer adsorption and multi-layer adsorption and the location of the critical micelle concentration (CMC). Experimental techniques utilizing the quartz crystal microbalance (QCM) have been shown to be highly reliable for measuring slight changes, on the order of a nanogram, in the adsorption behavior of surfactants from aqueous solutions. The current study demonstrates that the addition of low concentrations (< 100 mM) of sodium chloride to aqueous solutions of ionic surfactant can have a significant effect on the adsorption of the surfactant to a gold surface. An analysis of the QCM measurements as well as a discussion of the effects of salt addition on various solution properties will be presented. Impacts of the current results to previously published work as well as potential applications will be discussed.     

EQCM study of the electrodeposition of manganese in the presence of ammonium thiocyanate in chloride-based acidic solutions

The influence of ammonium thiocyanate (NH₄SCN) on the mechanism of manganese electrodeposition from a chloride-based acidic solution was investigated by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). The EQCM data were represented as plots dΔm dt⁻¹ versus E, known as massograms. Because massograms are not affected by interference from the hydrogen evolution reaction, they clearly show the manganese reduction and oxidation processes. By comparing the voltammograms with their corresponding massograms, it was possible to differentiate mass changes due to faradaic processes from those due to non-faradaic processes. Morphology, chemical composition and structure of the manganese deposits formed in different potential ranges were analyzed by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and X-ray diffraction (XRD). The results showed that in the absence of NH₄SCN, Mn(OH)_2(s) is formed in the potential range −1.1 to −0.9 V due to the hydrogen evolution reaction in this region. At more cathodic potentials, the deposition of β-manganese and the inclusion of Mn(OH)_2(s) into the deposit occur; both of these species underwent dissolution by non-faradaic processes during the anodic scan. In the presence of NH₄SCN, the formation of α- and γ-manganese was observed. When the potential was ≤−1.8 V and [NH₄SCN] exceeded 0.3 M, the α-manganese phase was favored.     

Electrodeposition and dissolution of yttrium-hexacyanoferrate layers

The electrodeposition and dissolution of yttrium-hexacyanoferrate [YHCNFe(II)] were investigated by electrochemical quartz crystal microbalance technique (EQCM). The electrodeposition was carried out by potential cycling or stepping from solutions of Y(NO₃)₃ and K₃[Fe^III(CN)₆] of different concentrations. The ratio of the reactants was also varied. No deposition was found in dilute solutions (c < 10⁻³ mol dm⁻³). The increase of concentrations led to an intense deposition of YHCNFe(II) in the course of reduction of [Fe^III(CN)₆]³⁻. At high concentrations of the reactants a coagulation deposition of YHCNFe(III) at open-circuit has also been detected. During the reduction the first phase is the nucleation which requires saturation or oversaturation in respect to the reacting species near the gold surface. The growth phase is much faster than the formation of nuclei, and its rate depends on the concentration and the concentration ratio of the species. The composition of the deposits has been determined by total reflection X-ray fluorescence (TXRF) spectrometry. From the molar ratio of atomic constituents (K, Y and Fe) of the slightly soluble deposit (solubility: 5 × 10⁻⁵ mol dm⁻³) formed after reduction of Fe(III) a formula K_0.46Y_1.18[Fe^II(CN)₆] can be derived. This value is in good accordance with the molar mass calculated from the results of EQCM experiments which also revealed that the deposit contains ca. 2 mol H₂O/mol YHCNFe(II). The solubility of YHCNFe(III) is substantially higher (s = 2 × 10⁻³ mol dm⁻³), and according to the results of TXRF measurements, its composition is Y[Fe^III(CN)₆]. The reoxidation of YHCNFe(II) takes place in two steps. The first one is a partial oxidation which is accompanied by the desorption of K⁺ ions from the layer. During further oxidation a fast dissolution occurs due to the high solubility of YHCNFe(III).     

Electrochemical copolymerisation of luminol with aniline: A new route for the preparation of self-doped polyanilines

Electrochemical copolymerisation of luminol and aniline from acidic aqueous medium onto gold electrodes has been investigated. Cyclic voltammetry in combination with electrochemical quartz crystal microbalance (EQCM) have been used to study both the in situ growth and redox switching process. In monomer free solution, the deposited polymers are stable and electrochemically active but distinct behaviour is shown by poly(luminol-aniline) films obtained from solutions with different monomers concentration ratio. In acidic medium, the current-voltage profiles range from a polyluminol (one pair of redox couple) to a polyaniline like redox conversion (three redox couples) as the aniline concentration increases. Unlike polyaniline, all prepared copolymers display well expressed electroactivity in sodium carbonate medium (pH 8), which also extends with the aniline content. The self-doping role assured by luminol moiety in the copolymer is also retrieved from the simultaneously recorded EQCM data.     

Interaction of BSA protein with copper evaluated by electrochemical impedance spectroscopy and quartz crystal microbalance

The interaction of bovine serum albumin (BSA) protein with copper in phosphate buffer solution has been studied by a combination of electrochemical impedance spectroscopy (EIS) close to the open circuit potential, with simultaneous monitoring by the electrochemical quartz crystal microbalance (EQCM), in order to throw light on BSA adsorption. Copper films were electroplated onto gold quartz crystals and mounted in the EQCM. Experiments were conducted in the presence and absence of dissolved oxygen and of BSA and the results show the influence of O₂ on the protein/metal interaction and also show specific interactions between BSA and copper. The good reproducibility obtained in these experiments suggests future application to other systems and which should lead to a better understanding of the use of such types of protein as corrosion inhibitors.     

An electrochemical quartz crystal microbalance study into the deposition of manganese dioxide

An electrochemical quartz crystal microbalance (EQCM) has been used to study the effects of electrolyte composition (MnSO₄ and H₂SO₄ concentrations) and anodic current density on the electrodeposition of manganese dioxide. The EQCM, in tandem with the electrode voltage during deposition, has been used to characterize features of the deposition mechanism such as the relative lifetime of the Mn(III) intermediate, the rate of soluble Mn(III) hydrolysis to form MnOOH, and the porosity of the resultant manganese dioxide deposit as a function of crystal nucleation. The connection between the results obtained here and commercial electrodeposition of manganese dioxide were also discussed.     

Novel preparation and electrochemical study of the dithiocarbamate polymer/thiuram disulfide redox system

Four dithiocarbamic polymers have been prepared by reacting a linear poly(ethylenimine) polymer (MW 50 000) with CS₂. Depending on the reaction conditions the resulting polymeric products contained, up to two sulfur atoms per one nitrogen atom (40% w/w). These polymers are soluble in dimethylsulfoxide and insoluble in acetonitrile solutions. In both solvents they are electrochemically active, in the latter case as cast film on glassy carbon or platinum electrodes. Specifically, upon oxidation the polymers turn into highly crosslinked (S–S bonds) form. The crosslinked polymer preserves partial electroactivity, although lower than expected due to its rigid structure but, more importantly, it is permeable to solvents and can act as an active matrix for electroactive species from the bulk solution. The switch between anionic and non-ionic (crosslinked) forms of the dithiocarbamic polymer results in transport of the solvent and lithium counterions between the film and the bulk solution.     

Investigations of ultrathin polypyrrole films: Formation and effects of doping/dedoping processes on its optical properties by electrochemical surface plasmon resonance (ESPR)

In the present work an investigation of the effects of the electropolymerization mode on the optical properties associated to the doping/dedoping processes of nanometric films of polypyrrole (PPy) is reported, monitoring in situ and in real time using simultaneously surface plasmon resonance and electrochemical techniques (ESPR). The electropolymerization of pyrrole was performed by potentiostatic, potentiodynamic and galvanostatic methods and the use of the ESPR technique showed that the electropolymerization mode is essential to the stability of polymer and the reversibility of its optical properties during the doping and dedoping processes. Thus, the optical properties of oxidized and reduced film were obtained by nonlinear least square fitting using Fresnel equations for a four-layer system. Then, the values of the real and imaginary parts of the complex dielectric constant for PPy fims were correlated with the polymer doping level. Finally, quartz crystal microbalance measurements were also applied to obtain correlation between doping/dedoping processes and the changes in the real and imaginary parts of the dielectric constant of the polypyrrole film, showing that the doping and dedoping processes in the polypyrrole film can act directly on its optical properties while the ESPR technique can give the same information indirectly.     

Quantitative characterization of protective films grown on copper in the presence of different triazole derivative inhibitors

The protective films developed on copper by anodic polarization in a borate-buffered solution containing benzotriazole (BTAH), 1-hydroxybenzotriazole (BTAOH) or 3-amino 1, 2, 4-triazole (ATA) have been characterized using coulometric experiments and an electrochemical quartz crystal microbalance (EQCM). The combination of these two techniques has allowed the CuO and Cu₂O layers and the cuprous-organic layer to be analyzed quantitatively. In the presence of BTAOH, the oxide layers were very similar to those formed in inhibitor-free solution and BTAOH appeared to be adsorbed on the oxide film. In the presence of BTAH, a thick Cu₂O film was covered by a Cu-BTA film containing 8% Cu⁺ ions. Cupric oxide appeared on Cu₂O areas uncovered by Cu-BTA. In the presence of ATA, the Cu₂O layer was very thin and the greater part of Cu⁺ ions (75%) was involved in a thick Cu-ATA film.     

Electrosorption of hydrogen into palladium-rhodium alloys

Hydrogen electrosorption into Pd-Rh alloys has been studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. Pd-Rh electrodes were prepared as thin alloy electrodeposits on Au wires. The influence of electrode potential on the amount of electrosorbed hydrogen was investigated. One can distinguish potential regions of α- and β-phase existence as well as the region of α-β phase transition. For alloys containing less than 20 at.% Rh in the bulk the hydrogen-to-metal ratio, H/(Pd + Rh), is greater than for pure Pd. The maximum value of the H/(Pd + Pt) ratio is 0.80 for an alloy containing in the bulk 94 at.% Pd and 6 at.% Rh. Due to a smaller lattice parameter of the Pd-Rh alloy with respect to pure Pd the α-β phase transition occurs at lower potentials than in Pd. The hysteresis is observed in chronoamperometric absorption and desorption experiments; the effect of hysteresis decreases with the increase in Rh content. Additionally, preliminary results are presented concerning the electrochemical quartz crystal microbalance measurements of hydrogen absorption/desorption into/from Pd-Rh alloys in comparison with Pd.     

鸟嘌呤和腺嘌呤吸附和电氧化行为的电化学石英晶体微天平研究

基于压电电化学石英晶体微天平(EQCM)技术,研究了腺嘌呤(A)和鸟嘌呤(G)在裸金电极和多壁碳纳米管(MWCNTs)修饰金电极上的吸附和电氧化行为。结果表明,在金电极上,A的吸附量和氧化电流均比G大,而在MWCNTs/Au电极上,A和G可类似地吸附,但其氧化峰电位负移且氧化峰电流增大,表明MWCNTs对A和G的氧化具有催化作用。实验发现,A和G在裸金电极上氧化的电子转移数分别为5.4(RSD=±2.3%)和1.9(RSD=±1.3%),而在MWCNTs/Au电极上的氧化电子转移数分别为5.1(RSD=±1.8%)和1.5(RSD=±1.6%)。     

Influence of organic additives on the initial stages of copper electrodeposition on polycrystalline platinum

Effects of two additives, thiourea and saccharin, on copper electrodeposition from acid sulphate solutions were investigated by different electrochemical methods (cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance) as well as by different observation techniques (scanning electron microscopy and atomic force microscopy). The morphology of copper coating obtained with thiourea leads to a smooth and bright deposit whereas it is only slightly modified by saccharin. The electrochemical reactions of copper electrodeposition were modified by the formation of complexes between thiourea and copper. However, in presence of saccharin, the kinetics and morphology of copper coating remain unchanged. From X-ray photoelectron spectroscopy analysis, thiourea was found to react with copper or copper ions by strong bond formation between the sulphur atoms of thiourea molecule and copper. This is the evidence of the adsorption of thiourea on the coating. Moreover, the thiourea action starts in the initial stages, allowing a homogeneous nuclei size and a large nuclei density. Finally, the nucleation mechanism of electrodeposition appears to be modified according to the additives used.     

Optical, electrochemical and electrogravimetric behavior of poly(1-methoxy-4-(2-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) films

Poly(1-methoxy-4-(2-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) films spin-cast from chloroform solution were characterized by cyclic voltammetry, in situ spectroelectrochemistry, and electrochemical quartz crystal microbalance (EQCM) measurements. The MEH-PPV films exhibited two well-defined redox waves during p- and n-electrochemical doping, and a mass gain at the end of each cycle that is characteristic of the movement of ions and solvent into and out the film. The electronic energy gap of MEH-PPV was estimated as 2.35 eV from the anodic and cathodic onset potentials obtained by cyclic voltammetry, in good agreement with the value of optical energy gap determined from the absorption edge, 2.21 eV. The films also showed a reversible and stable electrochromic transition at a positive range of applied potentials from reddish-orange (neutral form) to brownish-green (oxidized form).     

Characterization of Adsorption Behavior of Sucrose Monolaurate on Gold Substrate Using the Quartz Crystal Microbalance (QCM)

One class of biodegradable surfactants, sucrose fatty acid esters (SEs), has attracted wide interest in the food and cosmetic industries due to their excellent biocompatibility, biodegradability and lower pH- and temperature-sensitivity. The objective of this study was to evaluate the adsorption behavior of pure sucrose monolaurate (SML) at the solid–liquid interface as a function of concentration, 0.02–0.5 % w/w, and medium pH, as these can be the pre-determinants of their performance. The quartz crystal microbalance (QCM), an effective and versatile surface adsorption tool with customizable surfaces, was used for the first time in real-time characterization of the adsorption behavior. Adsorption was fast, between 1 and 3 min, with a typical one-step monolayer adsorption mechanism occurring at most concentrations and medium pH used, thus justifying the application of the Sauerbrey equation to determine the deposited mass. The adsorption isotherm was likely Langmuirian and the SML deposition on gold was significantly higher, at and beyond 0.1 % w/w, in de-ionised water (pH 6.9) than in aqueous buffer (physiological pH 5.5) due to the interaction of water molecules with the adsorbed layer. The results from this work highlight the importance of media for studying the adsorption behaviors of SEs and serve as a reference for future investigations on different functionalized surfaces.