Ultralow-Overpotential Acidic Oxygen Evolution Reaction Over Bismuth Telluride-Carbon Nanotube Heterostructure with Organic Framework.

The state-of-the-art iridium and ruthenium oxides-based materials are best known for high efficiency and stability in acidic oxygen evolution reaction (OER). However, the development of economically feasible catalysts for water-splitting technologies is challenging by the requirements of low overpotential, high stability, and resistance of catalysts to dissolution during the acidic oxygen evolution reaction . Herein, an organometallic core-shell heterostructure composed of a carbon nanotube core (CNT) and bismuth telluride (Bi2Te3) shell (denoted as nC-Bi2Te3) is designed and use it as a catalyst for the acidic OER. The proposed catalyst achieves an ultralow overpotential of 160 mV at 10 mA cm-2 (geometrical), thereby outperforming most of the state-of-the-art precious-metal-based catalysts. The low Tafel slope of 30 mV dec-1 and charge transfer resistance (RCT) of 1.5 Ω demonstrate its excellent electrocatalytic activity. The morphological and chemical compositions of nC-Bi2Te3 enable the generation of ─OH functional group in the Bi─Te sections formed via a ligand support, which enhances the absorption capacity of H+ ions and increases the intrinsic catalytic activity. The presented insights regarding the material composition-structure relationship can help expand the application scope of high-performance catalysts.

Condensing water vapor to droplets generates hydrogen peroxide.

It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A, 116, 19294-19298 (2019)] that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2 production in condensate microdroplets showed that H2O2 was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H2O2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2 generation. We also found that the H2O2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.

Tunable Dielectric and Thermal Properties of Oxide Dielectrics via Substrate Biasing in Plasma-Enhanced Atomic Layer Deposition.

The ability to control the properties of dielectric thin films on demand is of fundamental interest in nanoscale devices. Here, we modulate plasma characteristics at the surface of a substrate to tune both dielectric constant and thermal conductivity of amorphous thin films grown using plasma-enhanced atomic layer deposition. Specifically, we apply a substrate bias ranging from 0 to ∼117 V and demonstrate the systematic tunability of various material parameters of Al2O3. As a function of the substrate bias, we find a nonmonotonical evolution of intrinsic properties, including density, dielectric constant, and thermal conductivity. A key observation is that the maximum values in dielectric constant and effective thermal conductivity emerge at different substrate biases. The impact of density on both thermal conductivity and dielectric constant is further examined using a differential effective medium theory and the Clausius-Mossotti model, respectively. We find that the peak value in the dielectric constant deviates from the Clausius-Mossotti model, indicating the change of oxygen fraction in our thin films as a function of substrate bias. This finding suggests that the increased local strength of plasma sheath not only enhances material density but also controls the dynamics of microstructural defect formation beyond what is possible with conventional approaches. Based on our experimental observations and modeling, we further build a phenomenological relation between dielectric constant and thermal conductivity. Our results pave invaluable avenues for optimizing dielectric thin films at the atomic scale for a wide range of applications in nanoelectronics and energy devices.

Photo-annealed amorphous titanium oxide for perovskite solar cells.

Electron selective layers are important to the efficiency, stability and hysteresis of perovskite solar cells. Photo-annealing is a low-cost, roll-to-roll-compatible process that can be applied to the post-treatment fabrication of sol-gel based metal oxide layers. Here, we fabricate an amorphous titanium oxide electron selective layer at a low temperature in a dry atmosphere using a UV light annealing system and compare it with a thermal annealing process. Active oxygen species are created by using UV light to promote hydrolysis and condense the TiO2 precursor, which removes organic ligands effectively. The photo-annealed TiO2-based perovskite solar cell has a power conversion efficiency of 19.37% without hysteresis.

Spontaneous generation of hydrogen peroxide from aqueous microdroplets.

We show H2O2 is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 µm to 20 µm in diameter). Production of H2O2, as assayed by H2O2-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of H2O2 The generated H2O2 concentration was ∼30 µM (∼1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O2 or bubbling O2 decreased the yield of H2O2 in microdroplets, indicating that pure water microdroplets directly generate H2O2 without help from O2 either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for H2O2 formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH- at or near the surface of the water microdroplet. This catalyst-free and voltage-free H2O2 production method provides innovative opportunities for green production of hydrogen peroxide.

Point defect-reduced colloidal SnO2 electron transport layers for stable and almost hysteresis-free perovskite solar cells.

The commercialization of perovskite solar cells has been investigated, but the instability of their light-absorbing layers remains a problem. We demonstrate that the use of colloidal SnO2 nanoparticles prevents perovskite light absorber decomposition, reduces the hysteresis index to 0.1%, and increases the power conversion efficiency to 19.12%.

Electrochemical generation of sulfur vacancies in the basal plane of MoS2 for hydrogen evolution.

Recently, sulfur (S)-vacancies created on the basal plane of 2H-molybdenum disulfide (MoS2) using argon plasma exposure exhibited higher intrinsic activity for the electrochemical hydrogen evolution reaction than the edge sites and metallic 1T-phase of MoS2 catalysts. However, a more industrially viable alternative to the argon plasma desulfurization process is needed. In this work, we introduce a scalable route towards generating S-vacancies on the MoS2 basal plane using electrochemical desulfurization. Even though sulfur atoms on the basal plane are known to be stable and inert, we find that they can be electrochemically reduced under accessible applied potentials. This can be done on various 2H-MoS2 nanostructures. By changing the applied desulfurization potential, the extent of desulfurization and the resulting activity can be varied. The resulting active sites are stable under extended desulfurization durations and show consistent HER activity.

1-D structured flexible supercapacitor electrodes with prominent electronic/ionic transport capabilities.

A highly efficient 1-D flexible supercapacitor with a stainless steel mesh (SSM) substrate is demonstrated. Indium tin oxide (ITO) nanowires are prepared on the surface of the stainless steel fiber (SSF), and MnO2 shell layers are coated onto the ITO/SSM electrode by means of electrodeposition. The ITO NWs, which grow radially on the SSF, are single-crystalline and conductive enough for use as a current collector for MnO2-based supercapacitors. A flake-shaped, nanoporous, and uniform MnO2 shell layer with a thickness of ~130 nm and an average crystallite size of ~2 nm is obtained by electrodeposition at a constant voltage. The effect of the electrode geometry on the supercapacitor properties was investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and a galvanostatic charge/discharge study. The electrodes with ITO NWs exhibit higher specific capacitance levels and good rate capability owing to the superior electronic/ionic transport capabilities resulting from the open pore structure. Moreover, the use of a porous mesh substrate (SSM) increases the specific capacitance to 667 F g(-1) at 5 mV s(-1). In addition, the electrode with ITO NWs and the SSM shows very stable cycle performance (no decrease in the specific capacitance after 5000 cycles).

Direct printing synthesis of self-organized copper oxide hollow spheres on a substrate using copper(II) complex ink: gas sensing and photoelectrochemical properties.

The direct printing synthesis of metal oxide hollow spheres in the form of film on a substrate is reported for the first time. This method offers facile, scalable, high-throughput production and device fabrication processes. The printing was carried out via a doctor-blade method using Cu(II) complex ink with controllable high viscosity based on formate-amine coupling. Following only thermal heating in air, well-defined polycrystalline copper oxide hollow spheres with a submicrometer diameter (≤1 µm) were formed spontaneously while being assembled in the form of a film with good adhesion on the substrate. This spontaneous hollowing mechanism was found to result from the Kirkendall effect during oxidation at elevated temperature. The CuO films with hollow spheres, prepared via direct printing synthesis at 500 °C, led to the creation of a superior p-type gas sensor and photocathode for photoelectrochemical water splitting with completely hollow cores, a rough/porous shell structure, a single phase, high crystallinity, and no organic/polymer residue. As a result, the CuO hollow-sphere films showed high gas responses and permissible response speeds to reducing gases and high photocurrent density compared to conventional CuO powder films and the values previously reported. These results exemplify the successful realization of a high-throughput printing fabrication method for the creation of superior nanostructured devices.

TiO2 nanocrystals shell layer on highly conducting indium tin oxide nanowire for photovoltaic devices.

We demonstrated a highly efficient conducting indium tin oxide (ITO) core-TiO2 nanocrystals shell nanowire array for a photoelectrode in dye-sensitized solar cells with regard to light harvest and charge collection. The TiO2 shell layer, consisting of anatase nanocrystals of ~2 nm, were successfully formed on a single crystalline ITO nanowire prepared via a vapor transport method using repetitive TiCl4 aqueous solution treatments at 50 °C. We found that the nanocrystal size and number of Cl(-) ions remaining on the formed shell layer critically influence the dye loading properties. Moreover, these factors can be controlled by means of a post-annealing process. We also found that the dye loading and the back electron transport from the conductive ITO nanowire to the electrolyte mainly determine the final cell performance. The proposed double-shell layer structure consisting of dense and porous layers showed significantly improved cell performance.