Local Heating Induced Single-Crystalline Phase Control in Electrochemical Synthesis of Nanomaterials.

Development of synthetic strategies selectively yielding single crystals is desired owing to the facet-dependent chemical reactivities. Recent advances in electrochemical materials synthesis yielded nanomaterials that are surfactant-free, however, typically in polycrystalline forms. In this work, an electrochemical synthetic strategy selectively yielding single-crystalline nanoparticles by implementation of surface-selective heating of the working electrode is developed. Single crystals of copper, silver, gold, and platinum are afforded, and the crystallinity verified by electron diffraction and chemical reactivity studies. Notably, Cu (100) surface prepared by electrochemical synthesis yielded high single product selectivity when applied to electrochemical CO2 reduction catalysis.

Driving Triplet State Population in Benzothioxanthene Imide Dyes: Let's twist!

Controlling the formation of photoexcited triplet states is critical for many (photo)chemical and physical applications. Here, we demonstrate that a permanent out-of-plane distortion of the benzothioxanthene imide (BTI) dye promotes intersystem crossing by increasing spin-orbit coupling. This manipulation was achieved through a subtle chemical modification, specifically the bay-area methylation. Consequently, this simple yet efficient approach expands the catalog of known molecular engineering strategies for synthesizing heavy atom-free, dual redox-active, yet still emissive and synthetically accessible photosensitizers.

Membraneless Ionic Liquid Droplet Nanoprobe for Oxygen Sensing and Gas Phase Scanning Electrochemical Microscopy.

A novel membraneless oxygen sensing nanoprobe was developed based on a hanging drop ionic liquid electrochemical cell. An ultrasmall (<500 nm) working electrode and small volume electrochemical cell allowed for an impressively low detection limit of ∼13 ppm and a response time less than 100 ms, which is unusually fast for an electrochemical gas sensor. The oxygen sensor was stable for hours of operation and, owing to the membraneless design, was easily regenerable when fouled. The pulled capillary form factor of the nanoprobe was found compatible with scanning probe techniques, the demonstration of which was made by application as a tip electrode in gas phase scanning electrochemical microscopy (SECM). In the SECM experiments, the oxygen nanoprobe exhibited micrometer scale spatial resolution with ease. This unique probe design developed here may potentially be engineered into versatile sensors for various volatile molecules other than oxygen, such as those pertinent to hazard analysis and biomedical diagnosis.

Stoichiometry-Controlled Synthesis of Nanoparticulate Mixed-Metal Oxyhydroxide Oxygen Evolving Catalysts by Electrochemistry in Aqueous Nanodroplets.

Mixed-metal oxyhydroxides-especially those of Ni and Fe-are one of the most active classes of materials known for catalyzing the oxygen evolution reaction (OER). Here, nanoparticulate mixed metal oxyhydroxides (of Ni, Fe, and Co) were prepared on an electrode surface by electrochemical reaction of a precursor solution encapsulated in aqueous nanodroplets (AnDs), with each of the droplets containing 10 s of attoliters of fluid. Electrode reactions and synthesis can be monitored in situ by electrochemistry as single AnD stochastically lands and interacts with the working electrode. Resultant metal oxyhydroxide nanoparticles can be size and composition controlled precisely by modulating the precursor solution stored in the AnD. Nanoparticulate metal oxyhydroxides were implemented as catalysts for the OER and exhibited superior catalysis compared to their thin-film counterparts, demonstrating a hundred-thousand-fold enhancement in atom efficiency at comparable turnover rates.

Direct Observation of C2O4•- and CO2•- by Oxidation of Oxalate within Nanogap of Scanning Electrochemical Microscope.

Oxalate oxidation in the presence of different oxidized luminophores leads to the emission of light and has been studied extensively in electrogenerated chemiluminescence (ECL). The proposed mechanism involves the initial formation of the oxalate radical anion, C2O4•-. The ensuing decomposition of C2O4•- produces a very strong reductant, CO2•-, which reacts with the oxidized luminophores to generate excited states that emit light. Although the mechanism has been proposed for decades, the experimental demonstration is still lacking, because of the complexity of the system and the short lifetimes of both radical anions. To address these issues, we studied oxalate oxidation at platinum ultramicroelectrodes (UMEs) in anhydrous N, N-dimethylformamide (DMF) solution by nanoscale scanning electrochemical microscopy (SECM) with the tip generation/substrate collection (TG/SC) mode. A Pt nanoelectrode was utilized as the SECM generator for oxalate oxidation, while another Pt UME served as the SECM collector and was used to capture the generated intermediates. We studied the influence of the gap distance, d, on the substrate current ( is). The results indicate that, when 73 nm < d < 500 nm, the species captured by the substrate were primarily CO2•-, while C2O4•- was the predominant intermediate measured when d was below 73 nm. A half-life of 1.3 µs for C2O4•- was obtained, which indicates a stepwise mechanism for oxalate oxidation. The relevance of these observations to the use of oxalate as the coreactant in ECL systems is also discussed.

Activation of the Basal Plane in Two Dimensional Transition Metal Chalcogenide Nanostructures.

Achieving a molecular level understanding of chemical reactions on the surface of solid-state nanomaterials is important, but challenging. For example, the fully saturated basal plane is believed to be practically inert and its surface chemistry has been poorly explored, while two-dimensional (2D) layered transition-metal chalcogenides (TMCs) display unique reactivities due to their unusual anisotropic nature, where the edges consisting of unsaturated metals and chalcogens are sites for key chemical reactions. Herein, we report the use of Lewis acids/bases to elucidate the chemical reactivity of the basal plane in 2D layered TMCs. Electrophilic addition by Lewis acids (i.e., AlCl3) selectively onto sulfides in the basal plane followed by transmetalation and subsequent etching affords nanopores where such chemical activations are initiated and propagated from the localized positions of the basal plane. This new method of surface modification is generally applicable not only to various chemical compositions of TMCs, but also in crystal geometries such as 1T and 2H. Nanoporous NbS2 obtained by this method was found to have an enhanced electrochemical energy storage capacity, offering this chemical strategy to obtain functional 2D layered nanostructures.

Surface Interrogation Scanning Electrochemical Microscopy for a Photoelectrochemical Reaction: Water Oxidation on a Hematite Surface.

To understand the pathway of a photoelectrochemical (PEC) reaction, quantitative knowledge of reaction intermediates is important. We describe here surface interrogation scanning electrochemical microscopy for this purpose (PEC SI-SECM), where a light pulse to a photoactive semiconductor film at a given potential generates intermediates that are then analyzed by a tip generated titrant at known times after the light pulse. The improvements were demonstrated for photoelectrochemical water oxidation (oxygen evolution) reaction on a hematite surface. The density of photoactive sites, proposed to be Fe4+ species, on a hematite surface was successfully quantified, and the photoelectrochemical water oxidation reaction dynamics were elucidated by time-dependent redox titration experiments. The new configuration of PEC SI-SECM should find expanded usage to understand and investigate more complicated PEC reactions with other materials.

A Study of the Mechanism of the Hydrogen Evolution Reaction on Nickel by Surface Interrogation Scanning Electrochemical Microscopy.

The hydrogen evolution reaction (HER) on Ni in alkaline media was investigated by scanning electrochemical microscopy under two operating modes. First, the substrate generation/tip collection mode was employed to extract the "true" cathodic current associated with the HER from the total current in the polarization curve. Compared to metallic Ni, the electrocatalytic activity of the HER is improved in the presence of the low-valence-state oxide of Ni. This result is in agreement with a previous claim that the dissociative adsorption of water can be enhanced at the Ni/Ni oxide interface. Second, the surface-interrogation scanning electrochemical microscopy (SI-SECM) mode was used to directly measure the coverage of the adsorbed hydrogen on Ni at given potentials. Simulation indicates that the hydrogen coverage follows a Frumkin isotherm with respect to the applied potential. On the basis of the combined analysis of the Tafel slope and surface hydrogen coverage, the rate-determining step is suggested to be the adsorption of hydrogen (Volmer step) in the investigated potential window.

Assessment of the Stability and Operability of Cobalt Phosphide Electrocatalyst for Hydrogen Evolution.

Transition metal phosphides have been investigated heavily as hydrogen evolution reaction (HER) catalysts. One of the most active transition metal phosphides, CoP, has been tested for its stability and operability under mild conditions that it may be exposed to in its applications (photoelectrochemistry and artificial photosynthesis). Surface-interrogation scanning electrochemical microscopy (SI-SECM) revealed that CoP HER catalyst is vulnerable to oxidation (by oxygen and chemical oxidants). The degradation mechanism was shown to be surface oxidation by dioxygen, followed by acid etching of the oxidized layer. The compositional integrity (unity ratio of cobalt and phosphorus) was maintained throughout the film decomposition progress.

Surface Interrogation Scanning Electrochemical Microscopy of Ni(1-x)Fe(x)OOH (0 < x < 0.27) Oxygen Evolving Catalyst: Kinetics of the "fast" Iron Sites.

Nickel-iron mixed metal oxyhydroxides have attracted significant attention as an oxygen evolution reaction (OER) catalyst for solar fuel renewable energy applications. Here, we performed surface-selective and time-dependent redox titrations to directly measure the surface OER kinetics of Ni(IV) and Fe(IV) in NiOOH, FeOOH, and Ni(1-x)Fe(x)OOH (0 < x < 0.27) electrodes. Most importantly, two types of surface sites exhibiting "fast" and "slow" kinetics were found, where the fraction of "fast" sites in Ni(1-x)Fe(x)OOH matched the iron atom content in the film. This finding provides experimental support to the theory-proposed model of active sites in Ni(1-x)Fe(x)OOH. The OER rate constant of the "fast" site was 1.70 s(-1) per atom.

Surface interrogation of CoP(i) water oxidation catalyst by scanning electrochemical microscopy.

Despite exhaustive spectroscopic investigations on the CoPi oxygen-evolving catalyst over the past several years, little is known about the surface cobalt sites and intermediates in direct contact with water that are responsible for the actual catalysis. Many studies thus far have been limited to ex situ characterizations or bulk film measurements, often in the absence of solvent. Here we describe an investigation of the CoPi catalyst by surface interrogation scanning electrochemical microscopy (SI-SECM). This method should allow us to selectively study surface atoms separately from the bulk in a solvent-filled environment. By SI-SECM, independent titrations of surface Co(III) and Co(IV) were performed, yielding a direct measurement of the surface active-site density of a CoPi electrode (11 Co/nm(2)). The pseudo-first-order reaction rate constants of Co(III) and Co(IV) with water were determined to be 0.19 and >2 s(-1), respectively, through time-dependent titration measurements.

A Liquid Junction Photoelectrochemical Solar Cell Based on p-Type MeNH3PbI3 Perovskite with 1.05 V Open-Circuit Photovoltage.

A liquid junction photoelectrochemical (PEC) solar cell based on p-type methylammonium lead iodide (p-MeNH3PbI3) perovskite with a large open-circuit voltage is developed. MeNH3PbI3 perovskite is readily soluble or decomposed in many common solvents. However, the solvent dichloromethane (CH2Cl2) can be employed to form stable liquid junctions. These were characterized with photoelectrochemical cells with several redox couples, including I3(-)/I(-), Fc/Fc(+), DMFc/DMFc(+), and BQ/BQ(•-) (where Fc is ferrocene, DMFc is decamethylferrocene, BQ is benzoquinone) in CH2Cl2. The solution-processed MeNH3PbI3 shows cathodic photocurrents and hence p-type behavior. The difference between the photocurrent onset potential and the standard potential for BQ/BQ(•-) is 1.25 V, which is especially large for a semiconductor with a band gap of 1.55 eV. A PEC photovoltaic cell, with a configuration of p-MeNH3PbI3/CH2Cl2, BQ (2 mM), BQ(•-) (2 mM)/carbon, shows an open-circuit photovoltage of 1.05 V and a short-circuit current density of 7.8 mA/cm(2) under 100 mW/cm(2) irradiation. The overall optical-to-electrical energy conversion efficiency is 6.1%. The PEC solar cell shows good stability for 5 h under irradiation.

Switching Transient Generation in Surface Interrogation Scanning Electrochemical Microscopy and Time-of-Flight Techniques.

In surface interrogation scanning electrochemical microscopy (SI-SECM), fine and accurate control of the delay time between substrate generation and tip interrogation (tdelay) is crucial because tdelay defines the decay time of the reactive intermediate. In previous applications of the SI-SECM, the resolution in the control of tdelay has been limited to several hundreds of milliseconds due to the slow switching of the bipotentiostat. In this work, we have improved the time resolution of tdelay control up to ca. 1 µs, enhancing the SI-SECM to be competitive in the time domain with the decay of many reactive intermediates. The rapid switching SI-SECM has been implemented in a substrate generation-tip collection time-of-flight (SG-TC TOF) experiment of a solution redox mediator, and the results obtained from the experiment exhibited good agreement with that obtained from digital simulation. The reaction rate constant of surface Co(IV) on oxygen-evolving catalyst film, which was inaccessible thus far due to the lack of tdelay control, has been measured by the rapid switching SI-SECM.

Observation of nanometer-sized electro-active defects in insulating layers by fluorescence microscopy and electrochemistry.

We report a method to study electro-active defects in passivated electrodes. This method couples fluorescence microscopy and electrochemistry to localize and size electro-active defects. The method was validated by comparison with a scanning probe technique, scanning electrochemical microscopy. We used our method for studying electro-active defects in thin TiO2 layers electrodeposited on 25 µm diameter Pt ultramicroelectrodes (UMEs). The permeability of the TiO2 layer was estimated by measuring the oxidation of ferrocenemethanol at the UME. Blocking of current ranging from 91.4 to 99.8% was achieved. Electro-active defects with an average radius ranging between 9 and 90 nm were observed in these TiO2 blocking layers. The distribution of electro-active defects over the TiO2 layer is highly inhomogeneous and the number of electro-active defect increases for lower degree of current blocking. The interest of the proposed technique is the possibility to quickly (less than 15 min) image samples as large as several hundreds of µm(2) while being able to detect electro-active defects of only a few tens of nm in radius.

Single-Nanoparticle Collision Events: Tunneling Electron Transfer on a Titanium Dioxide Passivated n-Silicon Electrode.

Single-nanoparticle collisions were observed on an n-type silicon electrode (600 µm diameter) passivated by a thin layer of amorphous TiO2, where the current steps occurred by tunneling electron transfer. The observed collision frequency was in reasonable agreement with that predicted from theory. The isolated electrode, after a collision experiment, with a Pt/TiO2/n-Si architecture was shown to retain the photoelectrochemical properties of n-Si without photocorrosion or current decay. The Pt/TiO2/n-Si electrode produced 19 mA cm(-2) of photocurrent density under 100 mW cm(-2) irradiation from a xenon lamp during oxygen evolution without current fading for over 12 h.

Enhanced photoelectrochemical water oxidation on bismuth vanadate by electrodeposition of amorphous titanium dioxide.

n-BiVO4 is a promising semiconductor material for photoelectrochemical water oxidation. Although most thin-film syntheses yield discontinuous BiVO4 layers, back reduction of photo-oxidized products on the conductive substrate has never been considered as a possible energy loss mechanism in the material. We report that a 15 s electrodeposition of amorphous TiO2 (a-TiO2) on W:BiVO4/F:SnO2 blocks this undesired back reduction and dramatically improves the photoelectrochemical performance of the electrode. Water oxidation photocurrent increases by up to 5.5 times, and its onset potential shifts negatively by ∼500 mV. In addition to blocking solution-mediated recombination at the substrate, the a-TiO2 film-which is found to lack any photocatalytic activity in itself-is hypothesized to react with surface defects and deactivate them toward surface recombination. The proposed treatment is simple and effective, and it may easily be extended to a wide variety of thin-film photoelectrodes.

Electrochemical Synthesis of Hollow Nanoparticles via Anodic Transformation of Metastable Core-Shell Precursors.

Recent advances in electrosynthesis of nanomaterials expanded structural and compositional variations accessible by the electrochemical method; however, reliably synthesizable morphological variety fall shy of that available by conventional solvothermal synthesis. In this communication, electrochemical preparation of surfactant-free hollow nanoparticles is demonstrated. By anodic conversion of core-shell precursors with metastable cores, hollowed nickel nanoparticles with uniform dimensions were synthesized and characterized. Implementation of TEM grids as the working electrodes, identical location tracking of the morphological evolution of single particles to anodic stimulus has been demonstrated. The synthesized nanoparticles were employed as catalysts for the alkaline hydrogen evolution reaction and exhibited catalytic rates that compare favorably to the Pt/C benchmark. This marks the first pure electrochemical synthesis of hollow nanoparticles and shall contribute to the structural diversification of electrosynthesized nanomaterials.

Electrofluorochromic Hydrogels by Oligothiophene-Based Color-Tunable Fluorescent Dye Doping.

Soft electronic materials hold great promise for advancing flexible functional devices. Among the various soft materials available, hydrogels are particularly attractive for soft electronic device development due to their inherent properties, including transparency, shape adaptability through swelling/deswelling, and self-healing capabilities. Transparent hydrogels contribute to the creation of advanced smart devices such as sensors, smart windows, and anticounterfeiting technologies. Poly(vinyl alcohol) hydrogels are used as a platform for creating electrofluorochromic (EFC) devices in combination with oligothiophene-conjugated benzothiazole derivatives (OCBs) as fluorescent emitters. OCBs demonstrated excited-state intramolecular proton transfer (ESIPT) behavior with a large Stokes shift and emission changes responsive to solvent polarity and pH stimuli. Even in the solid state, OCBs exhibited strong fluorescence emission across a wide range of colors from blue to red, making them exceptionally well-suited for EFC device development. Their quantum yields in the powder state were obtained between 2.3% and 19.9%. Through the incorporation of OCBs into a PVA hydrogel (OCB@PVA), we achieved the successful fabrication of flexible EFC devices, including electronic paper and smart panels. When electric potentials (-2.4 and +2.4 V) were applied in OCB@PVA, fluorescence color changes were observed by an electrochemically induced pH change owing to electrohydrolysis of water. These devices demonstrated the potential of OCB@PVA hydrogels in the realm of flexible electronics. They could be used to create innovative and versatile devices with stimuli-responsive fluorescence properties.

Real-Time Monitoring and 3D Mapping of Trace Vapors of Explosive Nitroaromatic Compounds at Room Temperature by Gas-Phase Scanning Electrochemical Microscopy.

Development of sensing technologies for trace vapors of nitroaromatic compounds (NACs) is highly desired due to the toxic and explosive nature of the target molecules. Here, a NAC sensor based on a membraneless ionic liquid electrochemical cell was developed and applied for room-temperature trace vapor detection. Submicrometer working electrode dimensions yielded maximized portability and cost efficiency and extremely short time scales for molecular identification. The nanoprobe exhibited detection limitscomparable to those of state-of-the-art NAC sensors. The most noteworthy feature was the fast response to trace vapors, allowing for real-time detection of NACs without sample pretreatment. The pulled capillary form factor of the developed sensor enabled its application as tip electrodes in gas-phase scanning electrochemical microscopy (SECM). With the degree of freedom in three dimensions, mapping of the differential vapor pressure of NACs was possible, leading to potential application of the probe in sniffing out the source of explosive gas dissemination.

Prolonged hydrogen production by engineered green algae photovoltaic power stations.

Interest in securing energy production channels from renewable sources is higher than ever due to the daily observation of the impacts of climate change. A key renewable energy harvesting strategy achieving carbon neutral cycles is artificial photosynthesis. Solar-to-fuel routes thus far relied on elaborately crafted semiconductors, undermining the cost-efficiency of the system. Furthermore, fuels produced required separation prior to utilization. As an artificial photosynthesis design, here we demonstrate the conversion of swimming green algae into photovoltaic power stations. The engineered algae exhibit bioelectrogenesis, en route to energy storage in hydrogen. Notably, fuel formation requires no additives or external bias other than CO2 and sunlight. The cellular power stations autoregulate the oxygen level during artificial photosynthesis, granting immediate utility of the photosynthetic hydrogen without separation. The fuel production scales linearly with the reactor volume, which is a necessary trait for contributing to the large-scale renewable energy portfolio.