Solid-State Redox Solutions: Microfabrication and Electrochemistry

Scanning Electrochemical Probe Lithography for Ultra-Precision Machining of Micro-Optical Elements with Freeform Curved Surface

Two challenges should be overcome for the ultra-precision machining of micro-optical element with freeform curved surface: one is the intricate geometry, the other is the hard-to-machining optical materials due to their hardness, brittleness or flexibility. Here scanning electrochemical probe lithography (SECPL) is developed, not only to meet the machining need of intricate geometry by 3D direct writing, but also to overcome the above mentioned mechanical properties by an electrochemical material removal mode. Through the electrochemical probe a localized anodic voltage is applied to drive the localized corrosion of GaAs. The material removal rate is obtained as a function of applied voltage, motion rate, scan segment, etc. Based on the material removal function, an arbitrary geometry can be converted to a spatially distributed voltage. Thus, a series of micro-optical element are fabricated with a machining accuracy in the scale of 100 s of nanometers. Notably, the spiral phase plate shows an excellent performance to transfer parallel light to vortex beam. SECPL demonstrates its excellent controllability and accuracy for the ultra-precision machining of micro-optical devices with freeform curved surface, providing an alternative chemical approach besides the physical and mechanical techniques.

Polyvinylpyrrolidone-Bridged Prussian Blue/rGO Composite as a High-Performance Cathode for K-Ion Batteries

Prussian blue (PB) is a very promising cathode for K-ion batteries but its low electronic conductivity and deficiencies in the framework aggravate electrochemical performances. Compositing with conductive reduced graphene oxide (rGO) is an effective solution to address this problem. Nevertheless, little attention was paid to the loss of oxygen-containing functional groups on the rGO substrate during the compositing process, which weakens the interaction between PB and rGO and leads to poor electrochemical performance of PB/rGO. Herein, this interaction effect associated with surface functional groups is first openly debated. Two commonly used carbon substrates, graphene oxide (GO) and rGO, are investigated. A more stable interaction between PB and GO contributes to a higher capacity retention (91.8%) than that of PB/rGO (69.7%) after 300 cycles at a current density of 5 C. Meanwhile, polyvinylpyrrolidone (PVP) is employed to repair the weak interaction between PB and rGO substrates. PB is anchored to the rGO surface through the stable covalent linking of amide groups in PVP. A superior rate capability of 72 mA h g^-1 at 10 C and an improved capacity retention of 96.5% over 800 cycles at 5 C are obtained by as-prepared PB/PVP-rGO. This study provides a deeper understanding of fabricating PB/carbon composites with a robust connection.

Vertical Diffusion of Ions within Single Particles during Electrochemical Charging

Determining the trajectory of ionic transport and diffusion within single electroactive nanomaterials is critical for understanding the charging kinetics and capacity fading associated with ion batteries, with implications for rational design of excellent-performance electrode materials. While the horizontal pathway of mass transport has been feasibly investigated by optical superlocalization methods and electron microscopes, determination on the vertical trajectory has proven a more challenging task. Herein, we developed dual-angle total internal reflection microscopy by simultaneously introducing different angle-dependent illumination depths to trace the optical centroid shifts of nano-objects in the vertical dimension. We first demonstrated the proof of concept by resolving the vertical moving trails of a nanosphere doing Brownian motion and subsequently explored the picture of mass transport in the interior of single Prussian blue (PB) particles during electrochemical cycling. The results indicated that the vertical centroids of single PB particles remained unchanged when ions were inserted or extracted, suggesting an outside-in ionic transport pathway instead of bottom-up trajectory that one would intuitively expect.

Electrochemical micro/nano-machining: principles and practices

Micro/nano-machining (MNM) is becoming the cutting-edge of high-tech manufacturing because of the ever increasing industrial demands for super smooth surfaces and functional three-dimensional micro/nano-structures in miniaturized and integrate devices, and electrochemistry plays an irreplaceable role in MNM.

Positionable vertical microfluidic cell based on electromigration in a theta pipet

A microscale vertical fluidic cell system has been implemented, based on a simple theta pipet pulled to a sharp point (ca. 10-20 μm diameter for the studies herein) and positioned with a high degree of control on a surface. The dual channel arrangement allows an electric field to be generated between an electrode in each compartment of the pipet that can be used to control the electromigration of charged species between the two compartments, across a thin liquid meniscus in contact with the substrate of interest. By visualizing the interfacial region using laser scanning confocal microscopy, the adsorption of fluorescently-labeled materials on surfaces is monitored quantitatively in real time, exemplified through studies of the adsorption of anionic microparticles (1.1 μm diameter) on positively and negatively charged substrate surfaces of poly-L-lysine (PLL) and poly-L-glutamic acid (PGA), respectively, on glass. These studies highlight significant electrostatic effects on adsorption rates and also that the adsorption of these particles is dominated by the three phase meniscus/solid/air boundary. The technique is easily modified to the case of a submerged substrate, resulting in a much larger deposition area. Finite element method modeling is used to calculate local electric field strengths that are used to understand surface deposition patterns. To demonstrate the applicability of the technique to live biological substrates, the delivery of fluorescent particles directly to the surface of a single root hair cell of Zea mays is demonstrated. The mobile pipet allows deposition to be directed to specific regions of the cell, allowing discrete sites to be labeled with particles. Finally, the technique is used to study the uptake of fluorescent polymer molecules to single root hair cells, with quantitative analysis of the adsorption rates of vinyl-sulfonic acid copolymers, with varying rhodamine B content.

Scanning electrochemical cell microscopy: a versatile technique for nanoscale electrochemistry and functional imaging

Scanning electrochemical cell microscopy (SECCM) is a new pipette-based imaging technique purposely designed to allow simultaneous electrochemical, conductance, and topographical visualization of surfaces and interfaces. SECCM uses a tiny meniscus or droplet, at the end of a double-barreled (theta) pipette, for high-resolution functional imaging and nanoscale electrochemical measurements. Here we introduce this technique and provide an overview of its principles, instrumentation, and theory. We discuss the power of SECCM in resolving complex structure-activity problems and provide considerable new information on electrode processes by referring to key example systems, including graphene, graphite, carbon nanotubes, nanoparticles, and conducting diamond. The many longstanding questions that SECCM has been able to answer during its short existence demonstrate its potential to become a major technique in electrochemistry and interfacial science.