Huisu Jeong

Tailoring n‑ZnO/p-Si Branched Nanowire Heterostructures for Selective Photoelectrochemical Water Oxidation or Reduction

We report the fabrication of three-dimensional (3D) branched nanowire (NW) heterostructures, consisting of periodically ordered vertical Si NW trunks and ZnO NW branches, and their application for solar water splitting. The branched NW photoelectrodes show orders of magnitudes higher photocurrent compared to the bare Si NW electrodes. More interestingly, selective photoelectrochemical cathodic or anodic behavior resulting in either solar water oxidation or reduction was achieved by tuning the doping concentration of the p-type Si NW core. Specifically, n-ZnO/p-Si branched NW array electrodes with lightly doped core show broadband absorption from UV to near IR region and photocathodic water reduction, while n-ZnO/p(+)-Si branched NW arrays show photoanodic water oxidation with photoresponse only to UV light. The photoelectrochemical stability for over 24 h under constant light illumination and fixed biasing potential was achieved by coating the branched NW array with thin layers of TiO2 and Pt. These studies not only reveal the promise of 3D branched NW photoelectrodes for high efficiency solar energy harvesting and conversion to clean chemical fuels, but also developing understanding enabling rational design of high efficiency robust photocathodes and photoanodes from low-cost and earth-abundant materials allowing practical applications in clean renewable energy.

3D Branched Nanowire Photoelectrochemical Electrodes for Efficient Solar Water Splitting

We report the systematic study of 3D ZnO/Si branched nanowire (b-NW) photoelectrodes and their application in solar water splitting. We focus our study on the correlation between the electrode design and structures (including Si NW doping, dimension of the trunk Si and branch ZnO NWs, and b-NW pitch size) and their photoelectrochemical (PEC) performances (efficiency and stability) under neutral conditions. Specifically, we show that for b-NW electrodes with lightly doped p-Si NW core, larger ZnO NW branches and longer Si NW cores give a higher photocathodic current, while for b-NWs with heavily doped p-Si NW trunks smaller ZnO NWs and shorter Si NWs provide a higher photoanodic current. Interestingly, the photocurrent turn-on potential decreases with longer p-Si NW trunks and larger ZnO NW branches resulting in a significant photocathodic turn-on potential shift of ~600 mV for the optimized ZnO/p-Si b-NWs compared to that of the bare p-Si NWs. A photocathode energy conversion efficiency of greater than 2% at -1 V versus Pt counter electrode and in neutral solution is achieved for the optimized ZnO/p-Si b-NW electrodes. The PEC performances or incident photon-to-current efficiency are further improved using Si NW cores with smaller pitch size. The photoelectrode stability is dramatically improved by coating a thin TiO2 protection layer using atomic-layer deposition method. These results provide very useful guidelines in designing photoelectrodes for selective solar water oxidation/reduction and overall spontaneous solar fuel generation using low cost earth-abundant materials for practical clean solar fuel production.

Fabrication of an Efficient Light‐Scattering Functionalized Photoanode Using Periodically Aligned ZnO Hemisphere Crystals for Dye‐Sensitized Solar Cells

Since the pioneering work of O’Regan and Grätzel in 1991[1] numerous studies have investigated dye-sensitized solar cells (DSSCs) as an alternative next generation solar cell. This evolution has continued to progress, and solar light-to-electricity conversion efficiencies (power conversion efficiency, PCE) have now exceeded 11%, which was attained with a 12 μm thick TiO2 nanoparticulate photoanode.[2] DSSCs have recently attracted increasing attention as an ideal photovoltaic concept; the advantages of DSSCs are their low-cost, transparency, color rendition, eco-friendly process, biocompatibility and simplicity.[3,4] Generally, improvements in overall PCE have focused on increasing the photovoltage through the modification of the oxide layer, improving the photocurrent with new dye molecules, and increasing the stability by controlling the cell configurations.[4,5] A transparent mesoporous TiO2 nanoparticulate layer is a well-known photoanode material used in conventional DSSCs. However, the small size of TiO2 nanoparticles (diameter ∼ 20 nm) makes this layer transparent to visible light, and thus weakly light scattering due to the small particle size. As a result, a substantial amount of the incident light passes through the TiO2 nanoparticulate layer without being captured and utilized to produce photocurrent. Many studies have focused on capturing more light from the photoanode layer by using sub-micron poly-dispersed oxide particle aggregates, which act as effective scattering centers,[6,7] and/or by using gradient scattering layers consisting of TiO2 nano-particles with different radii along the light path.[8] Although the utilization of the large size aggregates within the photoanode film with a thickness of ∼9 μm and a cell area of

Palladium-Decorated Hydrogen-Gas Sensors Using Periodically Aligned Graphene Nanoribbons

Polymer residue-free graphene nanoribbons (GNRs) of 200 nm width at 1 μm pitch were periodically generated in an area of 1 cm(2) via laser interference lithography using a chromium interlayer prior to photoresist coating. High-quality GNRs were evidenced by atomic force microscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy measurements. Palladium nanoparticles were then deposited on the GNRs as catalysts for sensing hydrogen gases, and the GNR array was utilized as an electrically conductive path with less electrical noise. The palladium-decorated GNR array exhibited a rectangular sensing curve with unprecedented rapid response and recovery properties: 90% response within 60 s at 1000 ppm and 80% recovery within 90 s in nitrogen ambient. In addition, reliable and repeatable sensing behaviors were revealed when the array was exposed to various gas concentrations even at 30 ppm.

Enhanced Light Absorption of Silicon Nanotube Arrays for Organic/Inorganic Hybrid Solar Cells

By combining nanoimprint lithography technique and a two-step lift-off process, a Si nanotube array is fabricated and applied as a light absorber for n-Si/PEDOT:PSS hybrid solar cells. The light is effectively trapped within the nanotubes and the device reveals a Jsc of 29.9 mA · cm(-2) and a power conversion efficiency of 10.03%, which is an enhancement of 13.4% compared to the cell having the best-known Si architecture of nanocones as a light absorber to date.

All‐Solution‐Processed Transparent Thin Film Transistor and Its Application to Liquid Crystals Driving

All-solution-processed transparent thin film transistors (TTFTs) are demonstrated with silver grid source/drain electrodes, which are fabricated by printing and subsequent silver nanoparticles solution coating, which allows continuous processing without using high vacuum systems. The silver grid electrode shows a reasonable transmittance in visible range, moderate electrical conductance and mechanical strength. The TTFTs are employed to drive liquid crystal cells and demonstrate a successful switching operation.

Functional analysis of a hybrid endoglucanase of bacterial origin having a cellulose binding domain from a fungal exoglucanase

A cellulose binding domain (CBD) of an endo-β-l,4-glucanase (Ben) from the bacteriumBacillus subtilis BSE616 was replaced with the CBD of exoglucanase I (TexI) from the fungusTrichoderma viride HK-75. The resultant hybrid enzyme Ben’-CBDTexI, comprising the catalytic domain (Ben’) of Ben and the CBD (CBDTexI) of TexI, was highly expressed at 20% of the total protein inEscherichia coli. The molecular mass of the hybrid enzyme was estimated to be ca. 38 kDa by SDS-PAGE, which was in good agreement with that calculated from 305 amino acids of Ben and 42 amino acids of CBDTexI. The hybrid enzyme exhibited almost the same activity as that of the original Ben toward soluble substrates, such as cellooligosaccharides. The hybrid enzyme showed higher binding ability and hydrolysis activity toward microcrystalline cellulose (Avicel), even though the length of the CBD of TexI was four times smaller than that of Ben. The hybrid enzyme was more resistant to tryptic digestion than the original Ben. The efficient binding ability of the hybrid enzyme to Avicel permitted purification of the enzyme using an Avicel-affinity column to the extent of ca. 90% purity.

Printing of sub-100-nm metal nanodot arrays by carbon nanopost stamps.

This work reports an efficient method to fabricate hexagonally patterned metal nanodot arrays at the sub-100-nm scale, which is based on contact printing via novel nanometer-scaled stamps. Vertically aligned carbon nanoposts, supported by hexagonally ordered nanochannels of anodic aluminum oxide templates, are employed as the stamping platform to directly transfer controlled metal nanodot arrays. Using the fabrication platform, a number of patterned metal nanodot arrays made of Au, Cu, Ni, Ag, Pt, Al, and Ti can be contact-printed over large substrate areas in ambient conditions. The size, density, and interdistance of the printed nanodots are controllable with a tight correspondence to the mother stamp geometries, which can be precisely tuned by modifying the pore dimensions of the alumina matrixes. An advanced example of contact printing of metal nanoparticles is successfully demonstrated by the controlled formation of nanodot arrays in a specific area.

Large‐Area Fabrication of Periodic Sub‐15 nm‐Width Single‐Layer Graphene Nanorings

A periodically aligned array of graphene nanorings (GRNRs) with a sub-15 nm linewidth at a pitch of 450 nm is fabricated with a large area, 9 cm(2) , through conventional nanoimprint lithography coupled with sophisticated metal deposition and plasma-etching processes. The existence of the single-layer GRNRs is verified by various techniques.

Spontaneous nanoscale polymer solution patterning using solvent evaporation driven double-dewetting edge lithography

We develop an innovative solution processable edge lithography, which we call double-dewetting edge lithography (DDEL). The polymer solution spontaneously dewets the hydrophobic regions and covers only hydrophilic regions on a surface energy-engineered substrate, which is achieved by a combination of conventional photolithography and a subsequent hydrophobic treatment of the exposed areas. Then, the secondary dewetting occurs through a coffee stain effect during the solvent evaporation, leaving polymer edge patterns behind. The whole double-dewetting phenomenon is complete within 1 s. This technique is a fast, cost-effective and easy direct solution patterning method, which enables nanoscale polymer edge patterns to be produced from various micron-scale platforms including lines, angular and irregular shapes.