High-Throughput Single-Nanoparticle-Level Imaging of Electrochemical Ion Insertion Reactions

Nanoparticle electrodes are attractive for electrochemical energy storage applications because their nanoscale dimensions decrease ion transport distances and generally increase ion insertion/extraction efficiency. However, nanoparticles vary in size, shape, defect density, and surface composition, which warrants their investigation at the single-nanoparticle level. Here we demonstrate a nondestructive high-throughput electro-optical imaging approach to quantitatively measure electrochemical ion insertion reactions at the single-nanoparticle level. Electro-optical measurements relate the optical density change of a nanoparticle to redox changes of elements in the particle under working electrochemical conditions. We benchmarked this technique by studying Li-ion insertion in hexagonal tungsten oxide (h-WO₃) nanorods during chronoamperometry and cyclic voltammetry. Interestingly, the optically detected current response revealed underlying processes that are hidden in the conventional electrochemical current measurements. This imaging technique may be applied to 13 nm particles and a wide range of electrochemical systems such as electrochromic smart windows, batteries, solid oxide fuel cells, and sensors.

Small Nanosized Oxygen-Deficient Tungsten Oxide Particles: Mechanistic Investigation with Controlled Plasma Generation in Water for Their Preparation

Production of oxygen-deficient tungsten oxide nanoparticles with a diameter of around 10 nm have been successfully developed using a microwave-induced plasma in liquid technique. The prepared blue-green nanoparticles exhibit strong absorption in the visible region; thus, these could be efficient visible-light photocatalysts. The high-angle annular dark-field images revealed the dislocation of tungsten, which causes oxygen deficiencies.

Localized Charge Transfer in Two-Dimensional Molybdenum Trioxide

Molybdenum trioxide is an interesting inorganic system in which the empty 4d states have potential to hold extra electrons and therefore can change states from insulating opaque (MoO₃) to colored semimetallic (H_xMoO₃). Here, we characterize the local electrogeneration and charge transfer of the synthetic layered two-dimensional 2D MoO₃-II (a polymorph of MoO₃ and analogous to α-MoO₃) in response to two different redox couples, i.e., [Ru(NH₃)₆]³⁺ and [Fe(CN)₆]^3- by scanning electrochemical microscopy (SECM). We identify the reduction of [Ru(NH₃)₆]³⁺ to [Ru(NH₃)₆]²⁺ at the microelectrode that leads to the reduction of MoO₃-II to conducting blue-colored molybdenum bronze H_xMoO_3. It is recognized that the dominant conduction of the charges occurred preferentially at the edges active sites of the sheets, as edges of the sheets are found to be more conducting. This yields positive feedback current when approaching the microelectrode toward 2D MoO₃-II-coated electrode. In contrast, the [Fe(CN)₆]^4-, which is reduced from [Fe(CN)₆]^3-, is found unfavorable to reduce MoO₃-II due to its higher redox potential, thus showing a negative feedback current. The charge transfer on MoO₃-II is further studied as a function of applied potential. The results shed light on the charge transfer behavior on the surface of MoO₃-II coatings and opens the possibility of locally tuning of their oxidation states.

Ultra-large optical modulation of electrochromic porous WO3 film and the local monitoring of redox activity

Porous WO₃ films with ultra-high transmittance modulation were successfully fabricated on different substrates by a novel electrochemical deposited method.

Porous WO₃ films with ultra-high transmittance modulation were successfully fabricated on different substrates by a novel, facile and economical pulsed electrochemical deposited method with 1.1 s interval time between each pulse. The near ideal optical modulation (97.7% at 633 nm), fast switching speed (6 and 2.7 s), high coloration efficiency (118.3 cm² C^–1), and excellent cycling stability are achieved by the porous WO₃ on ITO-coated glass. The outstanding electrochromic performances of the porous WO₃ film were mainly attributed to the porous structure, which facilitates the charge-transfer, promotes the electrolyte infiltration and alleviates the expansion of the WO₃ during H⁺ insertion compared to that of the compact structure. In addition, the relationships between the structural and electrochemical activity of the electrochromic WO₃ films were further explored by the scanning electrochemical microscopy. These results testify that the porous structure can promote the infiltration of electrolyte and reduce the diffusion path, which consequently enhance the electrochemical activity.

Analytical expressions for quantitative scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM), is a recent analytical technique in electrochemistry, which was developed in the 1990s and uses microelectrodes to probe various surfaces. Even with the well-known disc microelectrodes, the system geometry is not as simple as in regular electrochemistry. As a consequence even the simplest experiments, the so-called positive and negative feedback approach curves, cannot be described with exact analytical expressions. This review gathers all the analytical expressions available in the SECM literature in steady-state feedback experiments. Some of them are claimed as general expressions, other are presented as approximate. Their validity is discussed in the light of the current understanding and computer facilities.

Distinction of the Two Phases of CuTCNQ by Scanning Electrochemical Microscopy

The two known phases of CuTCNQ and TCNQ (TCNQ = 7,7',8,8'-tetracyanoquinodimethane) have been probed by scanning electrochemical microscopy (SECM) in the feedback mode. The first use of this technique for distinguishing differences in the electronic properties of semiconductor phases exploits the large differences in conductivity that exist between CuTCNQ and the parent TCNQ material and also between the CuTCNQ phases I and II. However, the packing density of the individual CuTCNQ crystals in a film structure also is shown to influence the SECM feedback response. Finally, it is shown that films of pure phase II material or mixtures of the phases can be mapped using feedback mode SECM. The SECM method provides valuable insights for elucidating properties of semiconducting solids that are mounted on insulating substrates.

Nanostructuring and nanoanalysis by scanning electrochemical microscopy (SECM)

The generation and application of nanodes in SECM experiments are described. Nanodes are ultramicroelectrodes with an active disk diameter in the submicrometer range. We investigated the behaviour of these electrodes by testing their properties with SECM applications which were previously performed at the micrometer scale. The active diameter of the nanodes was determined using cyclic voltammetry and SECM. The nanoanalysis was conducted at two nano interdigitated arrays. The nanostructuring was demonstrated by galvanic and electroless silver deposition from solution and from the surface, respectively. Experiments with nanodes illustrate that they exhibit the same behaviour as ultramicroelectrodes, but are more sensitive to adsorption and dirt particles in the electrolyte solution.