Cheon Woo Moon

Wafer-scale transferable molybdenum disulfide thin-film catalysts for photoelectrochemical hydrogen production

We demonstrate that wafer-scale, transferable, and transparent thin-film catalysts based on MoS2, which consists of cheap and earth abundant elements, can provide a low onset potential of 1 mA cm−2 at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA cm−2 at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS2 thin films, which are grown via a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS2/p-Si heterojunction.

Organolead Halide Perovskites for Low Operating Voltage Multilevel Resistive Switching

Organolead halide perovskites are used for low-operating-voltage multilevel resistive switching. Ag/CH3 NH3 PbI3 /Pt cells exhibit electroforming-free resistive switching at an electric field of 3.25 × 10(3) V cm(-1) for four distinguishable ON-state resistance levels. The migration of iodine interstitials and vacancies with low activation energies is responsible for the low-electric-field resistive switching via filament formation and annihilation.

Superhydrophobic and antireflective nanograss-coated glass for high performance solar cells

We present a facile method for producing superhydrophobic nanograss-coated (SNGC) glass surfaces that possess both reduced reflectivity and self-cleaning properties at the air/glass interface. The refractive index of a CaF2 nanograss (NG) layer on a glass substrate, deposited by glancing angle vapor deposition, is 1.04 at 500 nm, which is the second-lowest value ever reported so far. The fluorinated NG layer gives rise to a high water contact angle (>150°) and very efficient cleaning out of dust with water drops. Using the dual functionalities of the SNGC glass, we demonstrate superhydrophobic and antireflective organic photovoltaic cells with excellent power conversion efficiency.

A wafer-scale antireflective protection layer of solution-processed TiO2 nanorods for high performance silicon-based water splitting photocathodes

Sustainable and efficient conversion of solar energy to transportable green energy and storable fuels, hydrogen, represents a solution to the energy crisis and reduces the consumption of fossil fuels, which are mainly responsible for the rise in global temperature. Solar water splitting using semiconductors, such as silicon, is promising to satisfy the global energy demand by producing hydrogen molecules. However, the solar to hydrogen conversion efficiency of a silicon photoelectrode is suppressed by overpotential, high reflectance and/or instability in liquid electrolytes. Herein, we report the synthesis of multifunctional solution-processed TiO2 nanorods on a 4-inch p-silicon wafer with controllable heights and diameters for highly efficient water splitting photocathodes. The solution-processed passivation layer of TiO2 nanorods reduces the overpotential of the silicon photocathode due to its catalytic properties. The TiO2 NRs also dramatically improves the light absorption of silicon due to the antireflective ability of the nanorods. The reflectance of silicon is decreased from 37.5% to 1.4% and enhances the saturated photocurrent density. The Pt-decorated (1–2.5 nm diameter) TiO2 nanorods/p-Si photocathodes show a short circuit current density of up to 40 mA cm−2, an open circuit voltage ∼440 mV and incident photon to current conversion efficiency of >90% using 0.5 M H2SO4 electrolyte with simulated 1 sun irradiation. The heterostructure photocathodes are stable for more than 52 h without noticeable degradation and an ideal regenerative cell efficiency of 2.5% is achieved.

Enhanced Endurance Organolead Halide Perovskite Resistive Switching Memories Operable under an Extremely Low Bending Radius

It was demonstrated that organolead halide perovskites (OHPs) show a resistive switching behavior with an ultralow electric field of a few kilovolts per centimeter. However, a slow switching time and relatively short endurance remain major obstacles for the realization of the next-generation memory. Here, we report a performance-enhanced OHP resistive switching device. To fabricate topologically and electronically improved OHP thin films, we added hydroiodic acid solution (for an additive) in the precursor solution of the OHP. With drastically improved morphology such as small grain size, low peak-to-valley depth, and precise thickness, the OHP thin films showed an excellent performance as insulating layers in Ag/CH3NH3PbI3/Pt cells, with an endurance of over 103 cycles, a high on/off ratio of 106, and an operation speed of 640 μs and without electroforming. We suggest plausible resistive switching and conduction mechanisms with current-voltage characteristics measured at various temperatures and with different top electrodes and device structures. Beyond the extended endurance, highly flexible resistive switching devices with a minimum bending radius of 5 mm create opportunities for use in flexible and wearable electronic devices.

Dominance of Plasmonic Resonant Energy Transfer over Direct Electron Transfer in Substantially Enhanced Water Oxidation Activity of BiVO4 by Shape-Controlled Au Nanoparticles

The performance of plasmonic Au nanostructure/metal oxide heterointerface shows great promise in enhancing photoactivity, due to its ability to confine light to the small volume inside the semiconductor and modify the interfacial electronic band structure. While the shape control of Au nanoparticles (NPs) is crucial for moderate bandgap semiconductors, because plasmonic resonance by interband excitations overlaps above the absorption edge of semiconductors, its critical role in water splitting is still not fully understood. Here, first, the plasmonic effects of shape-controlled Au NPs on bismuth vanadate (BiVO4 ) are studied, and a largely enhanced photoactivity of BiVO4 is reported by introducing the octahedral Au NPs. The octahedral Au NP/BiVO4 achieves 2.4 mA cm-2 at the 1.23 V versus reversible hydrogen electrode, which is the threefold enhancement compared to BiVO4 . It is the highest value among the previously reported plasmonic Au NPs/BiVO4 . Improved photoactivity is attributed to the localized surface plasmon resonance; direct electron transfer (DET), plasmonic resonant energy transfer (PRET). The PRET can be stressed over DET when considering the moderate bandgap semiconductor. Enhanced water oxidation induced by the shape-controlled Au NPs is applicable to moderate semiconductors, and shows a systematic study to explore new efficient plasmonic solar water splitting cells.

Toward High-Performance Hematite Nanotube Photoanodes: Charge-Transfer Engineering at Heterointerfaces

Vertically ordered hematite nanotubes are considered to be promising photoactive materials for high-performance water-splitting photoanodes. However, the synthesis of hematite nanotubes directly on conducting substrates such as fluorine-doped tin oxide (FTO)/glass is difficult to be achieved because of the poor adhesion between hematite nanotubes and FTO/glass. Here, we report the synthesis of hematite nanotubes directly on FTO/glass substrate and high-performance photoelectrochemical properties of the nanotubes with NiFe cocatalysts. The hematite nanotubes are synthesized by a simple electrochemical anodization method. The adhesion of the hematite nanotubes to the FTO/glass substrate is drastically improved by dipping them in nonpolar cyclohexane prior to postannealing. Bare hematite nanotubes show a photocurrent density of 1.3 mA/cm(2) at 1.23 V vs a reversible hydrogen electrode, while hematite nanotubes with electrodeposited NiFe cocatalysts exhibit 2.1 mA/cm(2) at 1.23 V which is the highest photocurrent density reported for hematite nanotubes-based photoanodes for solar water splitting. Our work provides an efficient platform to obtain high-performance water-splitting photoanodes utilizing earth-abundant hematite and noble-metal-free cocatalysts.

Comprehensive study on critical role of surface oxygen vacancies for 2DEG formation and annihilation in LaAlO3/SrTiO3 heterointerfaces

Here we report comprehensive study of 2DEG at a-LAO/STO interfaces in comparison with 2DEG at crystalline LaAlO3 (c-LAO)/STO interfaces. We observe that the oxygen deficient environment during the deposition of LAO overlayer is essentially required to create 2DEG at LAO/STO interface regardless of growth temperature from 25°C to 700°C, indicating that the oxygen-poor condition in the system is more important than the crystallinity of LAO layer. The critical thickness (2.6 nm) of 2DEG formation at a-LAO/STO heterostructure is thicker than (1.6 nm) that at c-LAO/STO. Upon ex-situ annealing at 300°C under 300 mTorr of oxygen pressure, 2DEG at a-LAO/STO interface is annihilated, while that in c-LAO/STO interface is still maintained. With combing these findings and scanning transmission electron microscope (STEM) analysis, we suggest that oxygen vacancies at the LAO surface is attributed to the origin of 2DEG formation at the LAO/STO and the crystallinity of the LAO overlayer plays a critical role in the annihilation of 2DEG at a-LAO/STO interface rather than in the formation of 2DEG. This work provides a framework to understand the importance of prohibiting the LAO surface from being oxidized for achieving thermally stable 2DEG at a-LAO/STO interface.

Magnetically retrievable nanocomposite adorned with Pd nanocatalysts: efficient reduction of nitroaromatics in aqueous media

Herein, we describe the fabrication of a magnetically retrievable nanocomposite adorned with highly active Pd nanoparticles (NPs) (MRN-Pd), which is useful for the efficient reduction of nitroaromatics in aqueous solution. The polymerization of pyrrole as the monomer in the presence of Pd salt and iron nanopowder generates Pd nanocatalysts and localizes the resultant Pd NPs discretely and uniformly on the polypyrrole framework comprising strongly magnetic MRN-Pd catalyst without the need for any reducing agent. The nitrogen-containing polymer enhances the interaction between the decorated Pd nanocatalysts and the polymer scaffold, endowing stability to the Pd NPs and maintaining their monodispersity. This prevents the possible aggregation of the MRN-Pd catalyst and promotes its reactivity for fast reduction processes. The unique features exhibited by the MRN-Pd catalyst result in excellent catalytic activity for the expeditious reduction of nitroaromatics under green reaction conditions at room temperature. Furthermore, the pronounced magnetic characteristics of the MRN-Pd catalyst allow its convenient separation and recycling from the reaction mixture. In addition, the MRN-Pd catalyst can be completely separated and recycled using a small magnet and reused for seven consecutive cycles of high-yield reduction of nitrobenzene (99–95%) in water, thus affording a highly retrievable and sustainable magnetic nanocomposite catalyst suitable for environmentally friendly processes. The MRN-Pd catalyst also presents high catalytic activity in other typical catalytic transformations requiring Pd nanocatalysts, such as the Suzuki and Heck cross-coupling reactions.

Substantially enhanced front illumination photocurrent in porous SnO2 nanorods/networked BiVO4 heterojunction photoanodes

BiVO4 is a promising photoanode for photoelectrochemical applications owing to its suitable band edge position for oxygen evolving reactions. High photocurrent under front illumination is very much essential to design tandem structures with a wireless configuration. However, the performance of BiVO4 under front illumination is limited due to poor charge transport properties. Here, we show that network-like BiVO4 coupled with porous SnO2 nanorods (NRs) is a promising model to enhance the front illumination performance. A very high photocurrent density of 5.6 mA cm−2 and 5.5 mA cm−2 has been obtained from the front and back illumination at 1.23 V vs. the reversible hydrogen electrode, respectively. We demonstrate that the appropriate nanostructuring of SnO2 NRs/BiVO4 is the underlying technology to tune the performance under directional illumination. The SnO2 NRs/BiVO4 exhibits a maximum incident photon to current efficiency of ∼80% under front and back illumination. A systematic study reveals that the optimized network like BiVO4 coated on porous SnO2 NRs synergistically boosts both the charge separation and transfer efficiencies of the photoanode resulting in a significantly high photocurrent for illumination on either side. This work provides a direction to achieve enhanced photocurrent during front and back side illumination in order to realize a wireless tandem configuration.