Quartz Crystal Microbalance Technique Research Articles

The kinetics, thermodynamics and mineral crystallography of CaCO3 precipitation by dissolved organic matter and salinity

Calcium carbonate (CaCO3) precipitation is an important geochemical process. In the estuary zone and some arid shallow lakes, DOM (dissolved organic matter) and salinity are two frequent changing factors that may affect CaCO3 precipitation. The joint effect of DOM and salinity on CaCO3 precipitation kinetics and thermodynamics are still unclear. In this study, effects of DOM on CaCO3 precipitation process at 0.5‰ and 70‰ salinity were investigated by QCM (Quartz Crystal Microbalance) technique, real-time pH measurement and single-injection nanoliter ITC (isothermal titration calorimetry). The mineral crystallography was analyzed by SEM-EDS. Both DOM and salinity had inhibitory effect on CaCO3 precipitation. DOM had more pronounced inhibitory effect on CaCO3 precipitation at lower salinity. Regardless of DOM, 70‰ salinity inhibited CaCO3 precipitation >0.5‰ salinity. The CaCO3 precipitation kinetics followed the first-order kinetic model and the adhesion kinetics of the instantaneous nucleation and crystal growth stage could be well described by the exponential function. CaCO3 precipitation was an endothermic process and high salinity strongly hindered CaCO3 precipitation. The effect of DOM on the absorbed heat was significant at 0.5‰ salinity. At 70‰ salinity, regardless of the effect of DOM, CaCO3 precipitation rate was greatly slowed down because it needed much more heat. CaCO3 minerals were dominated by rhombohedral calcite while CaCO3 minerals were mainly shaped as spherical vaterite at 0.5‰ salinity and rhombohedral calcite at 70‰ salinity. The crystal phase changed during CaCO3 precipitation at 0.5‰ salinity. In conclusion, the presence of DOM had substantial impact on the micrograph of the CaCO3 minerals. The percentage of flawed crystals with rough surface increased significantly with increased DOM concentration.

Measuring of isothermal water vapor adsorption/desorption rate using QCM method and its mass transfer resistance of a layer coated with silica-gel micro particles in a moist air

A quartz crystal microbalance (QCM) technique was used to measure amounts of adsorbed and desorbed water vapor in a moist air to a thin layer of adsorbent consolidated with silica-gel micro particles on the quartz crystal coated with thin gold electrodes. First, measured relative isothermal equilibrium adsorption was confirmed to reproduce the catalogue data of the silica gel. Then, the adsorption and desorption rates to the layer were measured under a condition of a step change of the surrounding relative humidity. Overall mass transfer resistance which related to both the mass diffusion in the particle and the mass transfer in the boundary layer was revealed by the obtained time constants of these reactions. The resistance decreased with the increase of the velocity, and it became a negligible quantity in the boundary layer when the flow rate was above 0.1 m s−1. Finally, correlations were proposed to predict these resistances.

Membrane hydration number of Li+ ion in highly concentrated aqueous LiCl solutions

The hydration numbers of Li+ ions in various concentrated aqueous LiCl electrolytes have been determined using a novel physico-chemical environment of conducting polypyrrole polymer in order to investigate the size of the primary hydration shell of Li+ ions. Simultaneous cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) techniques were used. The primary (membrane) hydration numbers obtained are 2 M LiCl, 5.4–5.5; 2.5 M LiCl, 4.9–5.0; 3 M LiCl, 3.9–4.0; 4 M LiCl, 3.5–3.6; 5 M LiCl, 2.8–3.1; 6 M LiCl, 2.6–2.7; 7 M LiCl, 1.7–1.8. The values obtained are found to remain constant and independent of the concentrations of the electrolytes below ~ 2 M but to decrease considerably with increasing concentrations. The estimation of free bulk water implies that the structure of highly concentrated aqueous electrolytes is different than that in the standard concentrations and that electrolytes may contain two types of water molecules, namely tightly bound water molecules and loosely bound water molecules but hardly contain any free bulk water molecules.

Effect of the Micelle Opening in Self-assembled Amphiphilic Block Co-polymer Films on the Infiltration of Inorganic Precursors.

Infiltration of the polymer templates with inorganic precursors using the selective vapor-phase infiltration approach, or sequential infiltration synthesis (SIS), allows the design of materials with advanced properties. Swelling of the block co-polymer (BCP) templates enables the additional control of the structure, porosity, and thickness of the composite or inorganic materials. Here, we use the highly precise quartz crystal microbalance (QCM) technique to investigate quantitatively the effect of the micelle opening by swelling and inorganic precursor infiltrating on the evolution of porosity in amphiphilic BCPs. We show that swelling of the polystyrene- block-poly-4-vinyl pyridine (PS- b-P4VP) BCP in ethanol at 75 °C occurs rapidly and results in a stable polymer structure in 30 min. By using an alumina model system, we found that swelling enables access to all available polar domains of the PS- b-P4VP film leading to an increase in the SIS-infiltrated alumina mass as compared to the nonswelled BCP layer. Our results demonstrate that swelling of the 110 nm thick BCP template results in the formation of 192 nm thick alumina films with 2 times larger alumina mass and 4 times larger effective pore volume than in case of the nonswelled sample. In the case of the thicker polymer template, the difference due to swelling becomes even more substantial because the fraction of accessible polymer is increased much more than in thin films. Our findings provide important insights into the mechanism of the infiltration of the inorganic precursors into swelled and nonswelled, spin-coated BCP templates enabling the design of highly porousthick ceramic films by SIS.

Atomic layer deposition of cobalt(II) oxide thin films from Co(BTSA)2(THF) and H2O

In this work, we have studied the applicability of Co(BTSA)2(THF) [BTSA = bis(trimethylsilyl)amido] (THF = tetrahydrofuran) in atomic layer deposition (ALD) of cobalt oxide thin films. When adducted with THF, the resulting Co(BTSA)2(THF) showed good volatility and could be evaporated at 55 °C, which enabled film deposition in the temperature range of 75–250 °C. Water was used as the coreactant, which led to the formation of Co(II) oxide films. The saturative growth mode characteristic to ALD was confirmed with respect to both precursors at deposition temperatures of 100 and 200 °C. According to grazing incidence x-ray diffraction measurements, the films contain both cubic rock salt and hexagonal wurtzite phases of CoO. X-ray photoelectron spectroscopy measurements confirmed that the primary oxidation state of cobalt in the films is +2. The film composition was analyzed using time-of-flight elastic recoil detection analysis, which revealed the main impurities in the films to be H and Si. The Si impurities originate from the BTSA ligand and increased with increasing deposition temperature, which indicates that Co(BTSA)2(THF) is best suited for low-temperature deposition. To gain insight into the surface chemistry of the deposition process, an in situ reaction mechanism study was conducted using quadrupole mass spectroscopy and quartz crystal microbalance techniques. Based on the in situ experiments, it can be concluded that film growth occurs via a ligand exchange mechanism.

Haloalkanes and aromatic hydrocarbons sensing using Langmuir–Blodgett thin film of pillar[5]arene-biphenylcarboxylic acid

Here, a pillar[5]arene derivative including biphenylcarboxylic acid groups was designed for obtaining a macrocycle with an ideal cavity for volatile organic compounds. The pillar[5]arene -biphenylcarboxylic acid (P5-BPCA) based Langmuir-Blodgett (LB) thin films were produced onto 50 nm thick gold-coated glass and 3.5 MHz quartz crystal substrates to form a thin film chemical sensor element. Surface plasmon resonance (SPR) and Quartz crystal microbalance (QCM) techniques were employed to characterize all the P5-BPCA LB thin film layers. The mass of LB film layer loaded onto a quartz crystal and the resonance frequency shifts per layer were determined to be 711.71 ng per layer (2.68 ng mm−2) and 48.24 Hz per layer, respectively. The P5-BPCA LB thin film sensor element was exposed to various haloalkane and aromatic hydrocarbon vapors. The sensitivities of the P5-BPCA LB film sensor were determined between 1.776 and 3.976 Hz ppm−1. Sensitivity with detection limits were obtained between 0.754 and 1.689 ppm against organic vapors. The results showed that P5-BPCA LB thin film was highly selective with a large response to chloromethane vapor.

Tuning Nanoscale Friction by Applying Weak Magnetic Fields to Reorient Adsorbed Oxygen Molecules

Sliding friction levels of thin (1–2 monolayers) and thick (~10 monolayers) oxygen films adsorbed on nickel and gold at 47.5 K have been measured by means of a quartz crystal microbalance (QCM) technique. Friction levels for the thin (thick) films on nickel in the presence of a weak magnetic field were observed to be approximately 30% (50%) lower than those recorded in the absence of the external field. Friction levels for thin films on gold were meanwhile observed to be substantially increased in the presence of the field. Magnetically-induced structural reorientation (magnetostriction) and/or realignment of adlayer spins, which respectively reduce structural and magnetic interfacial corrugation and commensurability, appear likely mechanisms underlying the observed field-induced reductions in friction for the nickel samples. Eddy current formation in the gold substrates may account for the increased friction levels in this system. The work demonstrates the role of magnetic effects in model systems that are highly amenable to theoretical studies and modeling.

Mechanism and kinetics of uric acid adsorption on nanosized hydroxyapatite coating

Uric acid (UA) produced from purine metabolism is rather harmful to human health when its concentration is high. To better understand the application of hydroxyapatite (HAP) as an adsorbent for UA removal, quartz crystal microbalance (QCM) technique was employed to in situ investigate the adsorption behavior of UA on nanosized HAP coatings. This work was mainly focused on the mechanism and kinetics of UA adsorption. The obtained results showed that nanosized HAP coatings produced physical adsorption for UA, and the driving force of UA adsorption on HAP coatings was electrostatic interaction. The adsorption kinetic parameter estimated from the in situ frequency measurement was about 3.08 × 106 L/mol. The obtained information suggests that QCM measurement provides a useful method for monitoring the interaction between HAP and UA.

Dissipative Microgravimetry to Study the Binding Dynamics of the Phospholipid Binding Protein Annexin A2 to Solid-supported Lipid Bilayers Using a Quartz Resonator.

The dissipative quartz crystal microbalance technique is a simple and label-free approach to measure simultaneously the mass uptake and viscoelastic properties of the absorbed/immobilized mass on sensor surfaces, allowing the measurements of the interaction of proteins with solid-supported surfaces, such as lipid bilayers, in real-time and with a high sensitivity. Annexins are a highly conserved group of phospholipid-binding proteins that interact reversibly with the negatively charged headgroups via the coordination of calcium ions. Here, we describe a protocol that was employed to quantitatively analyze the binding of annexin A2 (AnxA2) to planar lipid bilayers prepared on the surface of a quartz sensor. This protocol is optimized to obtain robust and reproducible data and includes a detailed step-by-step description. The method can be applied to other membrane-binding proteins and bilayer compositions.

Dissipative Microgravimetry to Study the Binding Dynamics of the Phospholipid Binding Protein Annexin A2 to Solid-supported Lipid Bilayers Using a Quartz Resonator

The dissipative quartz crystal microbalance technique is a simple and label-free approach to measure simultaneously the mass uptake and viscoelastic properties of the absorbed/immobilized mass on sensor surfaces, allowing the measurements of the interaction of proteins with solid-supported surfaces, such as lipid bilayers, in real-time and with a high sensitivity. Annexins are a highly conserved group of phospholipid-binding proteins that interact reversibly with the negatively charged headgroups via the coordination of calcium ions. Here, we describe a protocol that was employed to quantitatively analyze the binding of annexin A2 (AnxA2) to planar lipid bilayers prepared on the surface of a quartz sensor. This protocol is optimized to obtain robust and reproducible data and includes a detailed step-by-step description. The method can be applied to other membrane-binding proteins and bilayer compositions.

Accessibility of the pores in highly porous alumina films synthesized via sequential infiltration synthesis

Inorganic nanoporous materials with highly accessible pores are of great interest for the design of efficient catalytic, purification and detection systems. Limited access to the pores is a common problem associated with traditional approaches for the synthesis of porous materials, affecting the functionality of the low-density structure. Recently, infiltration of a nanoporous polymer template with inorganic precursors followed by oxidative annealing was proposed as a new and efficient approach to creating porous inorganic structures with controlled thickness, composition and pore sizes. Here, we report an ultra-high accessibility of the pores in porous films prepared via polymer-swelling-assisted sequential infiltration synthesis (SIS). Using a quartz crystal microbalance technique, we show the increased solvent adsorbing capabilities of highly porous alumina films as a result of high interconnectivity of the pores in such structures. The directionality and highly interconnected nature of the pores are demonstrated in experiments with the partial blocking of pore access by the deposition of a single-layer graphene that is not transparent to solvent. 60% of the pores remain accessible when only 20% of the surface is exposed to solvent. Using humidity detection as an example, we also show that highly porous alumina produced by polymer-swelling-assisted SIS is a promising candidate for sensing applications.

A Tribological Study of γ-Fe2O3 Nanoparticles in Aqueous Suspension

In this work, the nano- and macroscale tribological properties of maghemite (γ-Fe2O3) nanoparticles in aqueous solution are compared and contrasted for alumina contacts as a function of nanoparticle concentration. A quartz crystal microbalance (QCM) technique was used for nanotribology measurements and a ball-on-disk method was used to measure macroscale friction coefficients. Statistical methods were employed to identify significant associations between the QCM and ball-on-disk measurements, employing selected candidate performance factors for each system. In particular, the macroscale response was parameterized by % reduction in the friction coefficient while candidate QCM “bulk” and “surface” performance factors were selected from functions of the frequency f and resistance Rm shifts upon addition of nanoparticles to the water surrounding the QCM. Incremental increases in concentration were performed and reductions in friction and drag forces were observed for concentrations up to 0.6 wt%, after which further reductions were not observed. The factor δRfilm/δffilm exhibited a linear correlation with the reduction in macroscale friction coefficient, defined as the ratio of the shift in resistance to the shift in frequency attributable to interfacial effects and changes in the glide plane location. Atomic force microscopy was also utilized to both qualitatively and quantitatively determine surface roughness before and after particle uptake, leading to the observations that particles are easily removed from the surface and do not significantly alter surface morphology.

Specific responses of surface plasmon resonance and quartz crystal microbalance techniques to the mass of adsorbates at solid-liquid interfaces

Surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) techniques are become increasingly popular in probing behaviors and properties of adsorbates at solid-liquid interfaces. Despite the numerous studies focusing on revealing various relations between their output signals and adsorbed mass, however, the mechanics of these specific relations remains elusive. In this paper, we showed that the universality of a proportional relation between SPR signal and surface mass stems from the catholicity of a linear relation between surface refractive index and surface concentration, and a small refractive index shift. The partial validity of a proportional relation between QCM signal and surface mass is due to that a linear relation between surface viscosity and surface concentration, and a small viscosity shift are not always true. The ratio of SPR and QCM signals to the surface mass is dominated by the refractive index increment and intrinsic viscosity, respectively. The weak sensitivity of refractive index increment to conformation, architecture, etc., makes the mass estimation from SPR technique very convenient. The strong sensitivity of intrinsic viscosity to these factors makes the mass evaluation from QCM technique very difficult, whereas endowing a powerful method in quantitatively probing the conformation of adsorbates at solid-liquid interfaces.

Hydrogen Sorption of Porous Tungsten Oxide Nanomaterial and Effect of a Pd Coating on its Storage Capacity Under Hydrogenation Cycles

The H2 uptake performance at room temperature of porous tungsten oxide nanomaterials with and without a catalytic Pd coating was studied by the quartz crystal microbalance technique. Tungsten oxide composed mainly of WO3 was synthesized by inert gas condensation method using He. The samples consisted of semi-amorphous nanomaterials of low crystallinity and high porosity, as revealed by SAED, TEM, XRD, Raman spectroscopy and N2 adsorption–desorption isotherms. The H2 uptake capacity of the porous oxide was studied under increasing H2 exposure pressures (1000–7000[Formula: see text]Pa), with and without Pd coating. After reaching a maximum value of 1.2 H2[Formula: see text]wt.%, at 1160[Formula: see text]Pa, the H2 uptake capacity of the Pd-coated oxide consistently decreased. Successive hydrogenation cycles were carried out on the Pd-coated oxide at 3300 Pa and 6000 Pa to evaluate the H2 uptake performance of the sample under this H2 loading and unloading process. It was found that the H2 uptake capacity decreased from around 1 to values below 0.53[Formula: see text]H2[Formula: see text]wt.%, which is the reference H2 storage capacity achieved by a 15[Formula: see text]nm-thick Pd film. We argued that water molecule formation in the Pd/oxide interface and sublayers negatively affects the H2 uptake capacity of the oxide under successive hydrogenation cycles at room temperature.

Nitrogen Codoped Unique Carbon with 0.4 nm Ultra-Micropores for Ultrahigh Areal Capacitance Supercapacitors.

A full understanding of ion transport in porous carbon electrodes is essential for achieving effective energy storage in their applications as electrochemical supercapacitors. It is generally accepted that pores in the size range below 0.5 nm are inaccessible to electrolyte ions and lower the capacitance of carbon materials. Here, nitrogen-doped carbon with ultra-micropores smaller than 0.4 nm with a narrow size distribution, which represents the first example of electrode materials made entirely from ultra-microporous carbon, is prepared. An in situ electrochemical quartz crystal microbalance technique to study the effects of the ultra-micropores on charge storage in supercapacitors is used. It is found that ultra-micropores smaller than 0.4 nm are accessible to small electrolyte ions, and the area capacitance of obtained sample reaches the ultrahigh value of 330 µF cm-2 , significantly higher than that of previously reported carbon-based materials. The findings provide a better understanding of the correlation between ultra-micropore structure and capacitance and open new avenues for design and development of carbon materials for the next generation of high energy density supercapacitors.

Spun films of perylene diimide derivative for the detection of organic vapors with host–guest principle

The optical characterization and chemical vapor sensing properties of 1,7-dibromo-N,N′-(bicyclohexyl)-3,4:9,10-perylene diimide thin film against to organic vapors were discussed in this study by using spin coating, UV–Vis spectroscopy, atomic force microscopy, surface plasmon resonance (SPR) and Quartz Crystal Microbalance (QCM) techniques. The perylene diimide thin films were fabricated with a refractive index values from 1.55 to 1.60 and thicknesses in the range between 15.80 and 26.32 nm using different spin speeds from 1000 to 5000 rpm. In this study, perylene diimide thin film sensor was exposed to dichloromethane, chloroform, carbon tetrachloride, tetrahydrofuran and ethyl acetate vapors by using both SPR and QCM techniques. Also, the swelling behaviors of the perylene diimide thin films prepared at different spin speeds were investigated with respect to dichloromethane vapor at the room temperature by using SPR data. Diffusion coefficients were found to be 11.34 × 10−17 (1000 rpm), 2.56 × 10−17 (3000 rpm) and 0.38 × 10−17 cm2 s−1 (5000 rpm) for dichloromethane vapor by using the Fick’s law of diffusion. It might be proposed that perylene diimide thin film optical chemical sensor element has a good sensitivity and selectivity for the dichloromethane vapor at room temperature.

Electrodeposition of Pt and Gd from the Same Ionic Liquid

Platinum rare earth alloys show several times higher electrocatalytic activity than pure platinum while still maintaining an excellent stability [1-3]. This has been demonstrated for polycrystalline bulk materials [1, 3], for nanoparticles prepared in a cluster source [2] and for thin sputtered films [4, 5]. Once exposed to aqueous solutions, dealloying leads to formation of a several layers thick Pt skin that is under compressive strain, causing the activity enhancement [1]. The controlled preparation of such alloys using scalable techniques is difficult due to the high reactivity of the rare earth elements [5]. Electrochemical deposition from ionic liquids should be a possibility [6]. The electrodeposition of a number of rare earth elements has been reported in the literature [7-10], even though not in all cases clear proof has been given that a pure metal has been obtained. Several studies showed that in TFSI based ionic liquids rare earth electrochemistry can be complicated, and that not always a metal can be easily achieved [11]. Our own experiments – usually applying the electrochemical quartz crystal microbalance technique - on the electrodeposition of yttrium from two ionic liquids did not lead to the desired metal deposition [6], while there was indication for deposition of lanthanum [6] and gadolinium from BMP TFSI using TFSI precursors. However, the deposition was very slow, and independent proof for actual metal deposition was not obtained. Motivated by a study reporting lanthanum electrodeposition from butylmethylimidazolium dicyanamide (BMIm DCA) at elevated temperatures [10], we decided to attempt the gadolinium deposition from this ionic liquid. While experiments at room temperature failed due to passivation of the counter electrode, the deposition worked well at 60°C using GdCl3 as precursor. Under these conditions a thick metal greyish layer was obtained that in contact with air rapidly reacted and became whitish. The electrodeposition of Pt has been accomplished in different ionic liquids in literature [12-14], and we also managed to deposit Pt from two ionic liquids using a –still water containing – H2PtCl6 precursor [6]. In this work we discuss the results obtained for Pt deposition using different water-free precursors in BMIm DCA, namely Pt(acac)2, PtCl2 and PtCl4, and in part using additives to control complex formation. From Pt(acac)2, electrodeposition of Pt was not achieved at all. Addition of Pt(acac)2 to a GdCl3 electrolyte did not result in alloy formation, but the electrodeposition of Gd was blocked as well. Using PtCl2 and PtCl4, indication for successful Pt deposition was obtained. The role of complex formation on Pt deposition and alloy deposition will be elaborated in this contribution. The project leading to this application has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 700127. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme and Hydrogen Europe and N.ERGHY.

Pre-Treatment Effects on Electrochemical Lithium Deposition/Dissolution Processesstudied By Electrochemical Quartz Crystal Microbalance

Introduction High capacity battery like lithium air battery (LAB) is demanded for electric car and clean energy battery. Lithium is known as an ideal anode material because of its high theoretical capacity (3860 mA h g-1) and the most negative reduction potential (-3.04 V vs. SHE). However, during charging process, lithium dendrite, which leads to short circuit and/or explosion, is produced. In order to avoid dendritic lithium deposition, the way using solid electrolyte interphase (SEI), which is formed by decomposition of electrolytes and/or solvents on the anode surface during charging/discharging processes, is expected. Relatively flat and flexible SEI with higher lithium ion conductivity and less electron conductivity leads to better battery performance [1]. Electrochemical quartz crystal microbalance (EQCM) technique provides us not only ng leveled mass change (Δm) at the electrode surface from frequency change but also electrode surface area change (ΔA) and/or density/viscosity change (Δρ/Δη) at the electrode/electrolyte solution interface from resonance resistance change (ΔR) [2-5]. In addition, we can get mass per electron (MPE) value from this technique and then, this technique is quite useful to study the electrochemical lithium deposition/dissolution processes. In this study, we focused on the "pre-SEI" formed by potential pre-cycling before lithium deposition on the Cu EQCM electrode. As compared with the case without pre-SEI, we found that much more ideal MPE values in the case with pre-SEI were obtained in the three electrolyte solutions [6]. Experimental Using three kinds of electrolyte solutions containing 1 M lithium salts, such as 1 M LiPF6 in tetraglyme (G4), 1 M LiTFSI in G4, and 1 M LiFSI in G4, we measured cyclic voltammograms (CVs) of Cu EQCM electrode and we got simultaneously Δm andΔR. In the case without pre-SEI, the potential was negatively scanned from open circuit potential (OCP) to -0.1 V (vs. Li/Li+), where lithium deposition is already started, and then cycled between -0.1 V and +0.35 V. In the case with pre-SEI, the potential was negatively scanned from OCP to 0 V, where lithium deposition is not started yet, cycled between 0 V and +0.35 V for two cycles, and then, cycled between -0.1 V and +0.35 V. All the electrochemical measurements were carried out in the glove box, in which argon gas was filled. The SEI components were evaluated by IR and XPS. Results and discussion When the electrode potential was negatively scanned from OCP, small cathodic peaks due to the electrochemical reductive decomposition of unavoidable water, unavoidable dissolved oxygen, and electrolytes were observed around +2.0 V, +1.5 V, and +0.5 V, respectively, in all the solutions, with increases of Δm and ΔR. During pre-SEI formation, Δm and ΔR also increased although no conspicuous current peaks were observed. From IR and XPS, components of the SEI prepared from the three electrolyte solutions were almost same as each other. On the basis of the MPE values during lithium deposition and dissolution, it is concluded that the formation of the pre-SEI layers, which have the appropriate thickness and lithium ion conductivity with a relatively flat surface, is effective in regulating the lithium deposition/dissolution processes.

(Invited) Ultrasound Application and Multi-Step Reactions in Electrodeposition of Refractory Metals

This contribution discusses the application of ultrasound in ionic liquids (ILs) and its effect on refractory metal deposition as well as the particular electrochemistry of NbCl5 in TFSI based ILs. A systematic study was carried out studying electrochemical reactions and electrolyte decomposition in the presence of ultrasound. Extended application of ultrasound induced partial IL decomposition and formation of solid precipitates. Electrochemical reactions in ILs were enhanced by ultrasound application. Especially the application of short ultrasound pulses at elevated temperatures increased electrochemical reaction rates. The reduction of NbCl5 from BMP and OMP TFSI ILs is characterized by five separate distinct reduction and corresponding oxidation steps. This system was studied in depth using the electrochemical quartz crystal microbalance technique, revealing strong changes in the electrolyte properties close to the interface.

(Invited) Ultrasound Application and Multi-Step Reactions in Electrodeposition of Refractory Metals

This contribution discusses the application of ultrasound in ionic liquids (ILs) and its effect on refractory metal deposition as well as the particular electrochemistry of NbCl5 in TFSI based ILs. While ILs have enabled the electrodeposition of materials not accessible from aqueous solutions [1, 2], the transfer to technological applications is still hampered by high costs, stringent requirements on humidity and oxygen levels, and aspects of integration into industrial scale processes. Further, there are metals that are difficult to deposit, particularly the refractory metals: The deposition of tantalum [3] and niobium [4, 5] has been accomplished, but not yet in the form of thick, crack-free layers with good adhesion and low impurity level. Titanium was only deposited in ultrathin layers [6, 7]. Research into fundamental properties of ILs and especially the interfacial properties has shed light on some causes for these difficulties, like the presence of strong cation-anion multilayers at the interface and partial reduction of the metal precursors, especially halides, resulting in subhalide precipitation [3, 8]. The application of ultrasound leads to strongly improved mass transport and thus can counteract the accumulation of halide ions and the depletion of metal precursor close to the interface. It also might disturb the interfacial IL layers by cavitation, facilitating thereby refractory metal deposition. In aqueous solutions, the application of ultrasound can significantly enhance the deposition rate and influence the morphology and grain size of deposited films [9]. ILs with their negligible vapor pressure represent a very different environment. Nevertheless, ultrasound has been applied in ILs [10-12], cavitation was reported [13, 14] and is also associated with locally extreme temperatures [13]. This induces the risk of electrolyte decomposition. Therefore a systematic study was carried out in order to study electrochemical reactions and electrolyte decomposition in the presence of ultrasound. Extended application of ultrasound induced partial IL decomposition and formation of solid precipitates. Changes in background current and conductivity were observed and the electrolytes characterized by different spectroscopic techniques. The enhancement of electrochemical reactions in ILs by ultrasound application was demonstrated for model reactions and by application to Ta deposition from BMP TFSI. Especially the application of short ultrasound pulses at elevated temperatures increased electrochemical reaction rates. The reduction of Ta and Nb precursors requires the transfer of five electrons for each ion. The exact speciation in the IL (complexation) and thus the counter ion of the metal precursor influence the redox potentials of these individual reduction steps. (Apparent) multi-electron transfer reactions may facilitate the deposition, sequential one or two-electron transfer steps increase the risk to form sparingly soluble intermediates and can lead to corrosion of as-deposited layers [15]. The reduction of NbCl5 from BMP and OMP TFSI ILs [15] is characterized by five separate distinct reduction and corresponding oxidation steps. This system was studied in depth using the electrochemical quartz crystal microbalance technique, among others, and revealed strong changes in the electrolyte properties close to the interface.