Electrochemical Quartz Crystal Microgravimetry Research Articles

Anion-Catalyzed Active Dissolution Model for the Electrochemical Adsorption of Bisulfate, Sulfate, and Oxygen on Gold in H2SO4 Solution

Several plausible mechanisms for the potential-dependent adsorption of bisulfate, sulfate, and oxygen on polycrystalline gold in H2SO4 (0.5 M) were evaluated, based on their fit to the experimental potentio dynamic polarization data using the anion-catalyzed active dissolution model (dc portion). The mechanism giving the best preliminary fit was further analyzed by modelling the electrochemical impedance spectroscopy data collected at various dc potentials in the electric double-layer and pre-oxide formation regions. Electrochemical quartz-crystal microgravimetry and optical ellipsometry provided additional evidence verifying the electrochemical adsorption of solution species on the gold surface. Surprisingly, dissolution was observed throughout the whole potential range under study, despite the electrode surfaces developing significant oxygen surface coverages. Key features of the experimental fractional surface coverages and current density were reproduced by the anion-catalyzed active dissolution model.

Mass and Charge Transfer in a Polymeric NiSalen Complex at Subzero Temperatures

Electrochemical energy storage systems have a wide range of commercial applications. They keep energy and power even at temperatures up to +60 °C. However, the capacity and power of such energy storage systems reduce sharply at negative temperatures due to the difficulty of counterion injection into the electrode material. The application of organic electrode materials based on salen-type polymers is a prospective approach to the development of materials for low-temperature energy sources. Poly[Ni(CH3Salen)]-based electrode materials synthesized from different electrolytes were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and quartz crystal microgravimetry at temperatures from -40 °C to 20 °C. By analyzing data obtained in various electrolyte solutions, it was shown that at subzero temperatures, the process of injection into the polymer film, together with slow diffusion within the film, predominantly limit the electrochemical performance of electrode materials based on poly[Ni(CH3Salen)]. It was shown that the deposition of the polymer from solutions with larger cations allow the enhancement of the charge transfer due to the formation of porous structures facilitating the counter-ion diffusion.

In-Situ EC-AFM Study of Electrochemical P-Doping of Polymeric Nickel(II) Complexes with Schiff base Ligands

Conductive electrochemically active metallopolymers are outstanding materials for energy storage and conversion, electrocatalysis, electroanalysis, and other applications. The hybrid inorganic–organic nature of these materials ensures their rich chemistry and offers wide opportunities for fine-tuning their functional properties. The electrochemical modulation of the nanomechanical properties of metallopolymers is rarely investigated, and the correlations between the structure, stiffness, and capacitive properties of these materials have not yet been reported. We use electrochemical atomic force microscopy (EC-AFM) to perform in-situ quantitative nanomechanical measurements of two Schiff base metallopolymers, poly[NiSalphen] and its derivative that contains two methoxy substituents in the bridging phenylene diimine unit poly[NiSalphen(CH3O)2], during their polarization in the electrolyte solution to the undoped and fully doped states. We also get insight into the electrochemical p-doping of these polymers using electrochemical quartz crystal microgravimetry (EQCM) coupled with cyclic voltammetry (CV). Combined findings for the structurally similar polymers with different interchain interactions led us to propose a correlation between Young’s modulus of the material, its maximum doping level, and ion and solvent fluxes in the polymer films upon electrochemical oxidation.

Anion-catalyzed active dissolution model for the electrochemical adsorption of bisulfate, sulfate, and oxygen on gold in H2SO4 solution

The mechanism of the electrochemical adsorption of bisulfate, sulfate, and oxygen on polycrystalline gold in H2SO4 (0.5 M) was modeled using a mechanistic analysis of potentiodynamic polarization and electrochemical impedance spectroscopy data acquired at multiple dc potentials in the electric double-layer and pre-oxide formation region. Electrochemical quartz-crystal microgravimetry and optical ellipsometry provided evidence for the potential-dependent adsorption of solution species on the gold surface. Also, despite the high surface coverages of oxygen on the electrode surfaces, dissolution was observed over the entire the potential region studied. The anion-catalyzed active dissolution model accounts well for the experimentally observed current density and fractional surface coverages.

Probing Adsorption of Uranium Species at Electrochemical Interfaces in Support of Environmental Radiochemistry

Special nuclear material, like enriched uranium and plutonium, can contaminate environmental soils and water because of weapons development and testing or improper waste storage. To remediate affected lands and design long-term waste storage solutions, it is crucial to understand uranium speciation in the environment which depends on its complexation, oxidation state, and associated adsorption mechanisms. These phenomena are complex and poorly understood because they depend on many variables including the environmental pH, water or soil composition, uranium concentration, and surface chemistry of the minerals or soils. In previous studies, the proposed mechanism for adsorption of the hexavalent uranyl species, UVIO2 2+, on metal-oxide surfaces follows these general steps: 1) adsorption of UVIO2 2+ on the mineral, followed by 2) reduction to UVIO2 2+ due to the coupled reaction of a redox active species within the mineral. To gain better mechanistic insight into coupled electron transfer and adsorption phenomena associated with uranyl species, we demonstrate here a novel technique to probe an electrochemical interface to detect adsorbed uranium species under reducing conditions. In this technique, we monitor the transient response of the electrode potential immediately after electrochemical reduction of UVIO2 2+ on a rotating disc electrode is halted. This potential relaxation rate, when analyzed, contains information on presence and coverage of surface adsorbed species. In combination with electrochemical quartz crystal microgravimetry (e-QCM), these experiments confirm the presence of an adsorbed layer of uranium ions during the reduction of UVIO2 2+. To simulate uranyl adsorption on minerals under environmental conditions, future work will aim to probe adsorption onto a metal-oxide film prepared on an e-QCM electrode under varying redox and pH conditions.

Adsorption-Mediated Electrochemical Atomic Layer Deposition of Gold

Electrochemical atomic layer deposition (e-ALD) is an effective method for achieving atomic monolayer-by-monolayer deposition of metals. In this presentation, we will discuss the mechanistic events underlying gold (Au) e-ALD. The e-ALD process involves lead underpotential deposition (PbUPD) followed by spontaneous redox replacement (SLRR) of PbUPD by Au. During Au e-ALD, a unique three-stage mass transient is observed. Using voltammetry, electrochemical quartz crystal microgravimetry (e-QCM), and chronoamperometry, we present an in-depth study of the characteristics of this mass transient during Au e-ALD. Adsorption of Au+3-ligand complex(es) (Au-L) on the Au surface is shown to play a key role in the Au-ALD process. The adsorbed Au-L species are reduced in stage I while a PbUPD adlayer is formed. The SLRR of PbUPD adlayer by nobler Au occurs in stage II, followed by the re-adsorption of the Au-L species in stage III. A quantitative analysis of the deposit mass under different conditions allows estimation of the average molecular weight of the adsorbed Au-L complexes. Ramifications of such adsorption-mediated deposition to thickness uniformity, conformality and film roughness will be discussed. Figure 1

Electrodeposition of Cu on CoTa Barrier in the Alkaline CuSO4-Ethylenediamine Solution

Direct Cu electroplating on the CoxTay layers in an alkaline CuSO4-Ethylenediamine (En) solution was carried out and the effects of plating time, plating potential and current density on the film properties were investigated. The electrodeposition mechanism of Cu in CuSO4-En solution at different pH values was systematically studied by cyclic voltammetry and electrochemical quartz crystal microgravimetry in combination with UV–visible spectrophotometry. The deposition mechanism of Cu in the electrolyte is closely related to pH value. At pH 5, the predominant reaction is the reduction of Cu2+ to metallic Cu; at pH 7 and pH 9, the predominant reactions are the reduction of CuEn2 2+ to metallic Cu. Results also show that increasing plating current density or potential can effectively increase Cu nucleation density on the CoxTay. A high nucleation density (about 2.2 × 1011 cm−2) of Cu on Co1Ta1 is obtained. The electroplated Cu film has a relatively strong (111) preferred orientation. These results suggest that the direct Cu electroplating on the Co1Ta1 alloy barrier with higher anti-corrosion than Co in the alkaline solution and high nucleation density provides a new approach for the future Cu interconnect.

(Invited) Photoelectrode Materials Design for Solar Fuels: Where Are We Now?

Solar water splitting offers an attractive solution for renewable energy storage. Ternary metal vanadates have drawn attention for use in photoelectrochemical (PEC) water oxidation due to their low band gap and photostability.1 For example, Cu2V2O7 with a ratio of CuO: V2O5 stoichiometry (2:1) is a visible light active n-type semiconductor with a low band gap (see below), and has seen use in PEC devices because of its high stability for PEC water oxidation.2 This paper describes the development of a hybrid electrochemical/thermal route for the preparation of metal vanadate thin films. Electrodeposition is a facile, low temperature method which can be deployed to prepare thin films of a target semiconductor. Also, the optoelectronic parameters (e.g., photocurrent, flat band potential) of the thin film semiconductors prepared by electrodeposition can be measured easily without any more processing steps. In addition, the thickness of a given thin film semiconductor, a crucial factor for PEC reactions, can be controlled easily in an electrodeposition process.A two-step electrodeposition strategy for ternary oxides3 was deployed in this study. Results will be presented for M-V-O ternary films (e.g., Cu2V2O7) on fluorine doped tin oxide (FTO) substrate. In the first step, a copper thin film was deposited cathodically from copper(II) nitrate in a non-aqueous medium to preclude copper oxide formation. In the second step, the copper thin film was stripped anodically in ammonium metavanadate (NH4VO3) aqueous solution to generate Cu+ and subsequent in situ precipitation with VO3 - species to yield a green CuVO3 film on the substrate. Electrochemical quartz crystal microgravimetry confirmed two- and one-electron stoichiometry for the first and second steps respectively.Finally, aspects related to film stability and charge transfer efficiency as they relates to solar water splitting, will be reviewed in the light of data from our group.

Β-Cu2V2O7 Thin Films By a Hybrid Electrochemical/Thermal Route: Preparation and Characterization

Solar water splitting offers an attractive solution for renewable energy storage. Ternary copper vanadates have drawn attention for use in photoelectrochemical (PEC) water oxidation due to their low band gap and photostability.1 In particular, Cu2V2O7 with a ratio of CuO: V2O5 stoichiometry (2:1) is a visible light active n-type semiconductor with a low band gap (see below), and has seen use in PEC devices because of its high stability for PEC water oxidation.2 This paper describes the development of a hybrid electrochemical/thermal route for the preparation of Cu2V2O7 thin films. Electrodeposition is a facile, low temperature method which can be deployed to prepare thin films of a target semiconductor. Also, the optoelectronic parameters (e.g., photocurrent, flat band potential) of the thin film semiconductors prepared by electrodeposition can be measured easily without any more processing steps. In addition, the thickness of a given thin film semiconductor, a crucial factor for PEC reactions, can be controlled easily in an electrodeposition process.A two-step electrodeposition strategy for ternary oxides3 was deployed in this study, to prepare Cu2V2O7 thin films on fluorine doped tin oxide (FTO) substrate. In the first step, a copper thin film was deposited cathodically from copper(II) nitrate in a non-aqueous medium to preclude copper oxide formation. In the second step, the copper thin film was stripped anodically in ammonium metavanadate (NH4VO3) aqueous solution to generate Cu+ and subsequent in situ precipitation with VO3 - species to yield a green CuVO3 film on the substrate. Electrochemical quartz crystal microgravimetry confirmed two- and one-electron stoichiometry for the first and second steps respectively.Compositional assays by X-ray energy-dispersive analyses (EDX), averaged from different spots of as-prepared sample, were consistent with a Cu/V ratio: 0.98 ± 0.02. X-ray diffraction pattern for the as-prepared film showed that it was amorphous. Then, the green amorphous CuVO3 thin film was oxidized to a dark reddish crystalline phase-pure β-Cu2V2O7 product by thermal anneal at 450 oC for 30 min in the presence of oxygen. The direct and indirect energy band gaps of β-Cu2V2O7 were measured by diffuse reflectance spectroscopy (DRS) and Tauc analyses yielded values of 2.26 ± 0.03 and 2.08 ± 0.02 eV respectively. The PEC activity of β-Cu2V2O7 was measured at different thicknesses to find the optimum photoelectrode film thickness for water oxidation; this was found to be ~250 nm. These new results will be discussed in the context of the prospects for this semiconductor candidate for PEC water oxidation.

An Insight into Ionic Conductivity of Polyaniline Thin Films.

The work addresses an issue of the conductivity phenomenon in conductive polymer thin films. Polyaniline was chosen as a broadly used and thoroughly investigated conductive polymer in order to test and show capabilities of the developed original approach based on impedance spectra analysis. A number of films of different thickness were deposited onto a Pt electrode surface and consequently investigated in aqueous solution containing perchloric acid as an electrolyte. The processes that occur in polyaniline film were studied by cyclic voltammetry, electrochemical quartz crystal microgravimetry (EQCM) and electrochemical impedance spectroscopy (EIS). The role of incorporated ions as charge carriers was investigated with respect to the control of the conductivity properties of the film. Along with detailed polyaniline behavior study, the work makes up a fundamental scientific impact on theoretical electrochemistry and electroanalytical techniques.

Electrosynthesis of MoTe2 Thin Films: A Combined Voltammetry-Electrochemical Quartz Crystal Microgravimetry Study of Mechanistic Aspects

Molybdenum telluride (MoTe2) belongs to the family of layered transition metal dichalcogenides (TMDs) with unique optical, optoelectronic, structural properties and potential applications in a wide array of technologies related to solar energy conversion, optoelectronics, lubrication, and hydrogen production. Here, electrodeposition is shown to be a facile method for the synthesis of MoTe2 in bulk (i.e., not exfoliated) form. The electrosynthesis of MoTe2 films and the underlying compound formation mechanism were investigated for the first time using linear sweep voltammetry (LSV) combined with electrochemical quartz crystal microgravimetry (EQCM). A Te-modified electrode in an electrolyte containing molybdenum precursor species, a MoOx-modified electrode in tellurium precursor-containing electrolyte and a variety of control experiments were employed to elucidate the electrodeposition mechanism of MoTe2 films. Electrogeneration of HTe− was the key step in MoTe2 film formation which occurred by the reaction of electrodeposited MoOx with HTe− generated by electroreduction of Te or HTeO2+. Thermodynamic aspects of this reaction are finally presented.

Electrodeposition of Silver Vanadate Films: A Tale of Two Polymorphs.

Two polymorphs of AgVO3 , namely the α- and β- forms, were prepared and their physical, structural, optical, electrochemical, and photoelectrochemical characteristics were compared using a battery of experimental and theoretical tools. A two-step method, previously developed in the our laboratory for the electrodeposition of inorganic semiconductor films, was applied to the electrosynthesis of silver vanadate (AgVO3 ) films on transparent, conducting oxide surfaces. In the first step, silver was cathodically deposited from a non-aqueous bath containing silver nitrate. In the second step, the silver film was anodically stripped in an aqueous medium containing ammonium metavanadate. The anodically generated silver ions at the interface underwent a precipitation reaction with the vanadate species to generate the desired product in situ. Each of these steps were mechanistically corroborated via the use of electrochemical quartz crystal microgravimetry, used in conjunction with voltammetry and coulometry. As-deposited films were crystalline and showed p-type semiconductor behavior. Theoretical insights are provided for the electronic origin of the α→β phase transformation in AgVO3 and the disparate optical band gaps of the two polymorphs. Finally, implications for the application of this material in solar cells are provided.

The Electrocatalytic Reduction of Aqueous Selenate As Mediated By Underpotential Deposition of Cu and Cd on Au Electrodes

In analogy with many other oxyanions, the electrochemical reduction of selenate, SeO4 2-(aq) in aqueous solutions on most electrode surfaces is typically characterized by slow kinetics. Typical reduction strategies employ excessive amounts of cationic reducing agents like Cu(I) and Fe(II) compounds [1, 2]. Recently, Cu UPD on Au(poly) was found not only to promote reduction of SeO4 2-(aq) in acidic electrolytes, but advantage was taken of the resulting Cu/Se stripping peaks to develop an exceedingly sensitive analytical method for SeO4 2-(aq) detection down to the nM range [3]. An example of some typical qSe vs. [Na2SeO4] calibration curves, where qSe represents the charge under the Se stripping peak, (see insert in Fig. 1), are shown in Fig. 1. The rates of SeO4 2-(aq) reduction as catalyzed by Cu UPD are very time dependent, indicative of a complex electrocatalytic mechanism. Insight into some aspects of this process has been gained from studies involving the low index faces of single crystal Au via hold/strip voltammetry coupled with Normal Incidence Differential Reflectance (NIDR) and in the case of Au(111), by Electrochemical Quartz Crystal Microgravimetry (EQCM) studies. It has been found that a minimum surface coverage of Cu UPD is required before any reduction of SeO4 2-(aq) occurs, and that reduction slows over time as the surface saturates with Se atoms. However, excessively high Cu UPD coverages were found to hinder SeO4 2-(aq) reduction on the Au(111) plane. Some simple kinetic models have been formulated to attempt to capture the observed behavior. Additionally, at fixed coverages of Cu UPD, rates of SeO4 2-(aq) reduction increase with decreasing applied potential. To this end, the possible electrocatalytic properties of other metals capable of undergoing UPD at potentials more negative than those of Cu were considered. The first candidate to be examined was Cd, a metal for which the work function in bulk form is much lower than that of Cu [4] and, thus, its UPD should occur at more negative potentials. In addition, Cd UPD on Au has shown electrocatalytic activity for the reduction of NO3 -(aq), yet another example of a difficult to reduce oxyanion, in aqueous electrolytes [5]. It was found that SeO4 2-(aq)reduction can indeed be catalyzed by Cd UPD on Au, as shown in Fig. 2. In contrast to Cu UPD catalyzed reduction, it was found that under certain conditions the reduction rates of SeO4 2-(aq) were time independent when mediated by Cd UPD, as shown in Fig 3.

Electroless Metal Deposition Using Multivalent Metal Ions As Reducing Agents

The possibility and conditions to use lower oxidation state metal ions as reducing agents for autocatalytic electroless metal deposition are discussed. The theoretical background of such kind reactions is presented in the view of thermodynamics. Kinetic data on electroless silver, copper, palladium and platinum deposition using Co(II)/Co(III) redox systems with different ligands are presented and discussed. Ti(III)/Ti(IV) redox systems with different ligands were shown also to be suitable reducing agents for reducing of Pt(IV), Pd(II), Ni(II) and Co(II) to metallic state and formation of continuous metal layer on the surface to be plated. The morphology of metal layers deposited using above-mentioned reducing agents was characterized by means of Field Emission Scanning Electron Microscopy (FESEM). Kinetics of metal deposition rate was investigated by means of the gravimetry or Electrochemical Quartz Crystal Microgravimetry (EQCM).

(Invited) Electrodeposition of Semiconductors: An Emerging and Hugely Popular Art

This perspective talk focuses on the history and status of electrodeposition for preparing thin films of inorganic semiconductors, metal/semiconductor junctions, or even nanocomposite materials containing different phases. The use of light brings an added dimension and versatility to the preparative strategy. Variants on the basic cathodic or anodic deposition protocols will be elaborated as are approaches that rely on the initial deposition and stripping of a precursor layer. Another topic is operando compositional analysis of the depositing layers using a combination of electrochemical techniques with other complementary probes such as electrochemical quartz crystal microgravimetry (EQCM) and AC impedance spectroscopy. Finally, state-of-the art and future challenges will be identified using examples drawn from the author’s and collaborators’ laboratories as well as other works.

Investigation of electroless deposition of cobalt films by EQCM in the presence of different amines

Electroless deposition (also called chemical deposition) is a commonly used chemical approach to prepare metallic layers on various substrates. Cobalt films were deposited on the copper electrode using dimethylamine borane (DMAB) as a reducing agent and different amines: diethylenetriamine (dien), triethylenetetraamine (trien), propylenediamine (prop), tetraethylenepentaamine (ttren), pentaethylenehexaamine (pnten). The deposition conditions of electroless deposition of cobalt films were investigated in more detail by means of electrochemical quartz crystal microgravimetry (EQCM). The effects of pH, temperature and concentration of the reactants on the deposition rate of cobalt films have been determined. It was found that the induction period of the deposition of cobalt films depends on solution composition, temperature, and the presence of different amines.

Electrosynthesis of Cobalt Telluride Thin Films by Voltammetry Combined with Electrochemical Quartz Crystal Microgravimetry

Cobalt telluride (CoTe) thin films were electrodeposited for the first time using alkaline solutions containing nitrilotriacetic acid (NTA), CoCl2, and K2TeO3. NTA was employed in order to shift the reduction potential of cobalt in the negative direction and to stabilize Co2+ ions in alkaline electrolytes via the formation of Co-NTA complexes. The electrodeposition mechanism was investigated by linear sweep voltammetry (LSV) combined with electrochemical quartz crystal microgravimetry (EQCM). The formation of CoTe is proposed to occur through the reaction of Co2+ with HTe−, which is generated by the reduction of TeO32− via a 6-electron reduction pathway. A concurrent pathway consists of the stepwise reduction of TeO32− to Te by a 4-electron process, followed by the 2-electron process reduction of Te to HTe−. The electrodeposited CoTe films were characterized by a variety of physical methods including scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, and X-ray photoelectron spectroscopy. An optical energy bandgap of ∼1.86 eV was obtained by diffuse reflectance spectroscopy.

Multivalent Metal Ions As Efficient Reducing Agents for Electroless Metal Deposition Processes

In this study the possibility and conditions to use lower oxidation state metal ions as reducing agents for autocatalytic electroless metal deposition are discussed. The theoretical background of such kind reactions is presented in the view of thermodynamics. Kinetic data on electroless silver, copper, palladium and platinum deposition using Co(II)/Co(III) redox systems with different ligands are presented and discussed. Ti(III)/Ti(IV) redox systems with different ligands were shown also to be suitable reducing agents for reducing of Pt(IV), Pd(II), Ni(II) and Co(II) to metallic state and formation of continuous metal layer on the surface to be plated. The morphology of metal layers deposited using above-mentioned reducing agents was characterized by means of Field Emission Scanning Electron Microscopy (FESEM). Kinetics of metal deposition rate was investigated by means of the gravimetry or Electrochemical Quartz Crystal Microgravimetry (EQCM).

Compositional Analysis of Electrodeposited Cobalt Selenide Thin Films Using Continuous Flow Electrochemical Quartz Crystal Microgravimetry

In this study, the composition of electrosynthesized cobalt selenide thin films was analyzed by electrochemical quartz crystal microgravimetry (EQCM) in a continuous flow mode. First, cobalt selenide films were electrodeposited on a Pt-EQCM electrode using a 0.1 M Na2SO4 electrolyte solution containing Co and Se precursor species at different potentials. A combination of sequential stripping and EQCM frequency monitoring protocol enabled mapping of the free Co, free Se, and CoSe content of the electrodeposited films. It was found that the composition of cobalt selenide film was very sensitive to the electrodeposition potential. The targeted CoSe content was maximum at −0.7 V and the free Co content was minimum at this potential. On the other hand, the free Se content decreased from ∼45% at −0.6 V to nearly 0% at −0.9 V. The film composition as established by the continuous flow analysis mode stood in stark contrast with the static (batch) mode where no free Co was observed in the films. Factors in these compositional differences as well as in the EQCM electrode geometry (face-down vs. face-up) are finally discussed.

Electrochemical processes in weakly acid solutions containing Cu(II)-glycine complexes

Rotating disc electrode (RDE) and linear potential sweep (LPS) voltammetry together with electrochemical quartz crystal microgravimetry (EQCM) were used for the investigation of cathodic processes in weakly acid solutions (pH 3–5) containing Cu(II)-glycine complexes and 0.3 M NaClO4 as a supporting electrolyte. The Cu2+ aqua-complex and monoligand complex CuL+ dominate in slightly acid Cu(II)-glycine solutions. Less than 1% of Cu(II) is converted to Cu(I) upon contact of solutions with a copper electrode. At pH > 4, the formation of surface layers containing Cu2O becomes possible.Due to the specific redistribution of species at the electrode surface, characteristic pre-waves appear on the RDE voltammograms, which are transformed into additional current peaks under LPS conditions. Using the model of mass transport of chemically interacting particles, changes in surface concentrations were estimated with further construction of linear Tafel plots normalized with respect to the surface concentration of the electrochemically active complex.The following values of kinetic parameters (the cathodic charge transfer coefficient, αc1, and the exchange current density, i01) were determined for the rate-determining step CuL+ + e → CuL: αc1 = 0.23 and i01 = 0.36 mA cm−2 (pH 3); αc1 = 0.44 and i01 = 0.14 mA cm−2 (pH 4). According to EQCM data, additional processes involving the formation of surface oxides arise at pH 5.