Observation of the surface stress induced in microcantilevers by electrochemical redox processes

Revealing nanomechanical deformation at the interface and degradation in all-thin-film inorganic electrochromic devices

Recently, progress in electrochromic (EC) devices has been made in optimizing electrode and device configurations and performance. However, the ion insertion/de-insertion induced charge transfer (CT) nanomechanical effect has remained unexplored, i.e., repetitive electrode size changes at the nanoscale and stress/strain generated during electrochemical cycling, which is the focus of this work due to its intimate correlation with the elastic and plastic deformation at the interface. Considering the intervalence electrons, excellent electrochemical kinetics, and dramatic color changes, tungsten oxide (WO₃) and nickel oxide (NiO) films are configured as the EC cathode and anode materials, respectively, within a full device. Upon extended cycles (>10 000), the void generation and delamination that occurred at the interface account for performance decay. Encouraged by the findings, nanoindentation mechanical tests and electrical kelvin probe force microscopy were employed to investigate the CT induced effects at the interface. There is a dramatic increase of up to 45% in the elastic Young's modulus in colored/charged WO₃ at ∼40 mC cm^-2. The correlation between CT and synergistic mechanical effect is interpreted by the Lippman equation. Interestingly, despite the charged state (colored; lithiated) with a relatively flat morphology bringing an ∼3.4 times higher electrostatic surface potential, the electrical work function unexpectedly decreases, arising from the dominant effect of the dipole layer potential over the chemical potential. The interatomic cohesive energy and equilibrium distance increase bury the seeds for mechanical deformation in the long run. This work provides fundamental insights into electro-chemo mechanics and interdisciplinary concerted interfacial effects at the nano/atomic level. The dependence of surface potential, stress, work function, and cohesive energy on electrochemical kinetics has been interpreted.

Anomalous interfacial stress generation during sodium intercalation/extraction in MoS2 thin-film anodes

Although the generation of mechanical stress in the anode material is suggested as a possible reason for electrode degradation and fading of storage capacity in batteries, only limited knowledge of the electrode stress and its evolution is available at present. Here, we show real-time monitoring of the interfacial stress of a few-layer MoS₂ system under the sodiation/desodiation process using microcantilever electrodes. During the first sodiation with a voltage plateau of 1.0 to 0.85 V, the MoS₂ exhibits a compressive stress (2.1 Nm^-1), which is substantially smaller than that measured (9.8 Nm^-1) during subsequent plateaus at 0.85 to 0.4 V due to the differential volume expansion of the MoS₂ film. The conversion reaction to Mo below 0.1 V generates an anomalous compressive stress of 43 Nm^-1 with detrimental effects. These results also suggest the existence of a separate discharge stage between 0.6 and 0.1 V, where the generated stress is only approximately one-third of that observed below 0.1 V. This approach can be adapted to help resolve the localized stress in a wide range of electrode materials, to gain additional insights into mechanical effects of charge storage, and for long-lifetime battery design.

Scanning electrochemical microscopy monitoring in microcantilever platforms

The deflection of cantilever systems may be performed by an indirect electrochemical method that consists of measuring the local cantilever activity and deflection in a feedback generation-collection configuration of the SECM. This is illustrated during the electrochemically assisted adsorption of Br onto a gold-coated cantilever, either in its pristine state or previously coated with a thin organic barrier. It is further extended to the adsorption of an antibody in a heterogeneous immunoassay at an allergen-coated microcantilever platform. In both reactions, the cantilever deflection is qualitatively detected from the SECM tip current measurement and a quantitative estimate is obtained through modeling. This electroanalytical strategy provides an alternative approach to standard optical detection. It can overcome some limitations of the optical method by allowing electrochemical characterization of nonconductive cantilevers and appropriate use for closed systems.

Redox actuation of a microcantilever driven by a self-assembled ferrocenylundecanethiolate monolayer: an investigation of the origin of the micromechanical motion and surface stress

The electrochemically induced motion of free-standing microcantilevers is attracting interest as micro/nanoactuators and robotic devices. The development and implementation of these cantilever-based actuating technologies requires a molecular-level understanding of the origin of the surface stress that causes the cantilever to bend. Here, we report a detailed study of the electroactuation dynamics of gold-coated microcantilevers modified with a model, redox-active ferrocenylundecanethiolate self-assembled monolayer (FcC(11)SAu SAM). The microcantilever transducer enabled the observation of the redox transformation of the surface-confined ferrocene. Oxidation of the FcC(11)SAu SAM in perchlorate electrolyte generated a compressive surface stress change of -0.20 +/- 0.04 N m(-1), and cantilever deflections ranging from approximately 0.8 microm to approximately 60 nm for spring constants between approximately 0.01 and approximately 0.8 N m(-1). A comparison of the charge-normalized surface stress of the FcC(11)SAu cantilever with values published for the electrochemical oxidation of polyaniline- and polypyrrole-coated cantilevers reveals a striking 10- to 100-fold greater stress for the monomolecular FcC(11)SAu system compared to the conducting polymer multilayers used for electroactuation. The larger stress change observed for the FcC(11)SAu microcantilever is attributable to steric constraints in the close-packed FcC(11)SAu SAM and an efficient coupling between the chemisorbed FcC(11)S- monolayer and the Au-coated microcantilever transducer (vs physisorbed conducting polymers). The microcantilever deflection vs quantity of electrogenerated ferrocenium obtained in cyclic voltammetry and potential step/hold experiments, as well as the surface stress changes obtained for mixed FcC(11)S-/C(11)SAu SAMs containing different populations of clustered vs isolated ferrocenes, have permitted us to establish the molecular basis of stress generation. Our results strongly suggest that the redox-induced deflection of a FcC(11)SAu microcantilever is caused by a monolayer volume expansion resulting from collective reorientational motions induced by the complexation of perchlorate ions to the surface-immobilized ferroceniums. The cantilever responds to the lateral pressure exerted by an ensemble of reorienting ferrocenium-bearing alkylthiolates upon each other rather than individual anion pairing events. This finding has general implications for using SAM-modified microcantilevers as (bio)sensors because it indicates that the cantilever responds to collective in-plane molecular interactions rather than reporting individual (bio)chemical events.

Microcantilever-Based Sensors: Effect of Morphology, Adhesion, and Cleanliness of the Sensing Surface on Surface Stress

The surface stress response of micromechanical cantilever-based sensors was studied as a function of the morphology, adhesion, and cleanliness of the gold sensing surface. Two model systems were investigated: the adsorption of alkanethiol self-assembled monolayers at the gas-solid interface and the potential-controlled adsorption of anions at the liquid-solid interface. The potential-induced surface stress, on a smooth and continuous polycrystalline Au(111)-textured microcantilever in 0.1 M HClO4, is in excellent agreement with macroscopic Au(111) single-crystal electrode results. It is shown that ambient contaminants on the sensing surface dramatically alter the surface stress-potential response. This observation can be misinterpreted as evidence that for polycrystalline Au(111) microcantilever electrodes, surface stress is dominated by surface energy change. Results for anions adsorption on gold are in contrast to the gas-phase model system. We demonstrate that the average grain size of the gold sensing surface strongly influences the magnitude of the surface stress change induced by the adsorption of octanethiol. A 25-fold amplification of the change in surface stress is observed on increasing the average gold grain size of the sensing surface from 90 to 500 nm.

The Origin of Surface Stress Induced by Adsorption of Iodine on Gold

The cantilever technique for the measurement of film stress on both macroscopic and microscopic cantilevers is validated, then applied to the experimental determination of film stress induced by the adsorption of a monolayer of iodine onto a gold substrate. A model is proposed that relates the change in the interatom potential upon chemisorption of iodine onto gold to the measured film stress. Excellent agreement is found with the experimentally determined value. This result gives insight into the origins of film stress that is observed in all thin film and coating applications.

Comments on “Observation of the surface stress induced in microcantilevers by electrochemical redox processes” by F. Tian, J.H. Pei, D.L. Hedden, G.M. Brown, T. Thundat

Critical remarks have been formulated in connection with the surface stress measurements reported by Tian et al. It has been shown that due to the shortcomings of the experimental procedure some conclusions of the authors are hardly acceptable.