Yu-Kuei Hsu

Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures

Nature routinely produces nanostructured surfaces with useful properties, such as the self-cleaning lotus leaf, the colour of the butterfly wing, the photoreceptor in brittlestar and the anti-reflection observed in the moth eye. Scientists and engineers have been able to mimic some of these natural structures in the laboratory and in real-world applications. Here, we report a simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region. Reflection is suppressed for a wide range of angles of incidence and for both s- and p-polarized light. The antireflection properties of the silicon result from changes in the refractive index caused by variations in the height of the silicon nanotips, and can be simulated with models that have been used to explain the low reflection from moth eyes. The improved anti-reflection properties of the surfaces could have applications in renewable energy and electro-optical devices for the military.

High-cell-voltage supercapacitor of carbon nanotube/carbon cloth operating in neutral aqueous solution

Dense and entangled carbon nanotubes (CNTs) directly grown on the carbon cloth (CC) have been employed as hierarchical supercapacitor electrodes in neutral aqueous electrolyte of Na2SO4. XPS and Raman results show the CNTs with large amounts of oxygen functionalities and high degree of disorder, respectively, after treatment of electrochemical oxidation. The voltammograms and galvanostatic charge/discharge are performed to probe the capacitive characteristics of CNTs/CC electrode. Moreover, the symmetric supercapacitor of CNTs/CC exhibits an excellent cyclability over 10 000 cycles and high energy density of 27.8 Wh kg−1, with a wide cell potential of 2 V and a large specific capacitance of 210 F g−1.

Novel Iron Oxyhydroxide Lepidocrocite Nanosheet as Ultrahigh Power Density Anode Material for Asymmetric Supercapacitors

A simple one-step electroplating route is proposed for the synthesis of novel iron oxyhydroxide lepidocrocite (γ-FeOOH) nanosheet anodes with distinct layered channels, and the microstructural influence on the pseudocapacitive performance of the obtained γ-FeOOH nanosheets is investigated via in situ X-ray absorption spectroscopy (XAS) and electrochemical measurement. The in situ XAS results regarding charge storage mechanisms of electrodeposited γ-FeOOH nanosheets show that a Li(+) can reversibly insert/desert into/from the 2D channels between the [FeO6 ] octahedral subunits depending on the applied potential. This process charge compensates the Fe(2+) /Fe(3+) redox transition upon charging-discharging and thus contributes to an ideal pseudocapacitive behavior of the γ-FeOOH electrode. Electrochemical results indicate that the γ-FeOOH nanosheet shows the outstanding pseudocapacitive performance, which achieves the extraordinary power density of 9000 W kg(-1) with good rate performance. Most importantly, the asymmetric supercapacitors with excellent electrochemical performance are further realized by using 2D MnO2 and γ-FeOOH nanosheets as cathode and anode materials, respectively. The obtained device can be cycled reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, further delivering a maximum power density of 16 000 W kg(-1) at an energy density of 37.4 Wh kg(-1).

Novel ZnO/Fe2O3 Core–Shell Nanowires for Photoelectrochemical Water Splitting

A facile and simple fabrication of Fe2O3 as a shell layer on the surface of ZnO nanowires (NW) as a core-shell nanoelectrode is applied for the photoelectrochemical (PEC) splitting of water. An ZnO NW array of core diameter ∼80 nm was grown on a fluorine-doped tin-oxide (FTO) substrate with a hydrothermal method; subsequent deposition and annealing achieved a shell structure of the Fe2O3 layer of thickness a few nm. Fe2O3 in the α phase and ZnO in the wurtzite phase were identified as the structures of the shell and core, respectively, through analysis with X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The ZnO/Fe2O3 core-shell NW showed an excellent PEC response to the oxidation of water, and also benefited from a negative shift of onset potential because of an n/n heterojunction structure. A detailed energy diagram of the ZnO/Fe2O3 core-shell NW was investigated with a Mott-Schottky analysis. This novel core-shell nanostructure can hence not only exhibit a great potential for the solar generation of hydrogen, but also offer a blueprint for the future design of photocatalysts.

Plasmonic Ag@Ag3(PO4)1−x nanoparticle photosensitized ZnO nanorod-array photoanodes for water oxidation

We report the new design of a high-activity model for photocatalytic nanosystem comprising an Ag core covered with an approximately 2 nm thick nanoshell of Ag3(PO4)1–x (Ag@Ag3(PO4)1–x) on the ZnO NRs that are visible-light-sensitive photofunctional electrodes with strong photooxidative capabilities to evolve O2 from water. The maximum photoconversion efficiency that could be successfully achieved was 2%, with a significant photocurrent of 3.1 mA cm−2. Furthermore, in addition to achieving a maximum IPCE value of 90%, it should be noted that the IPCE of Ag@Ag3(PO4)1−x photosensitized ZnO photoanodes at the monochromatic wavelength of 400 nm is up to 60%. Our photoelectrochemical performances are comparable to those of many oxide-based photoanodes in recent reports. The improvement in photoactivity of PEC water-splitting may be attributed to the enhanced near-field amplitudes resulting from localized surface plasmon resonance (LSPR) of Ag–core and absorption edge of the Ag3(PO4)1−x nanoshell, which increase the rate of formation of electron–hole pairs at the nearby surface of Ag3(PO4)1−x nanoshell and ZnO nanorod, thus enlarging the amount of photogenerated charge contributing to photocatalysis. The capability of developing highly photoactive Ag@Ag3(PO4)1−x-photosensitized ZnO photoanodes opens up new opportunities in various photocatalytic areas, particularly solar-hydrogen fields.

Nanostructured Zinc Oxide Nanorods with Copper Nanoparticles as a Microreformation Catalyst

The use of hydrogen for energy generation has attracted significant attention in recent years as a clean, sustainable, and transportable alternative fuel, and this interest has consequently sparked a rapid global development of hydrogen fuel cells for electric power generation. Catalytic reformation of hydrocarbons, with careful attention to avoid storage and safety issues, 3] is currently the predominant process for hydrogen generation. One of the leading and most promising techniques for hydrogen generation is catalytic reformation of methanol. 5] Cu/ZnO-based catalysts are, therefore, of great importance for industrial scale catalytic production of reformate hydrogen. Owing to their wide commercial relevance, Cu/ZnO-based catalysts, prepared by several preparation routes, are being extensively investigated, and substantial improvements in their efficiency of catalytic activity brought about by addition of suitable promoter/ support, combination with effective component, and implementation of new preparation techniques, have been reported. 7–12] Unfortunately, use of Cu/ZnO-based heterostructures as reforming catalysts is still lacking to date. This inspired us to design a core–shell nanostructured catalyst consisting of a ZnO nanorod (NR) core and an outer shell of copper nanoparticles (NPs), that is, NR@NPs, for achieving high efficiency of catalytic conversion. The idea of using microreformers is also highly attractive for several applications, such as on-board hydrogen sources for small vehicles and portable fuel cells. 14] However, two key issues have hindered the realization of microreformers for catalysis, namely, poor adhesion between the catalyst layer and the microchannels and poor utilization of catalyst layer deposited in the form of thick film. Notwithstanding, several approaches investigated to overcome these issues, catalyst immobilization, and its efficient utilization inside the microchannel remains a challenge. 17] Most of these approaches involve a two-step process, wherein active catalysts are prepared in the first step, followed by its immobilization on the surface of the microchannels in the second step. Herein, we report a simple and reliable method for integrating in-situ synthesis of catalyst and its immobilization for microreformer applications. The ZnO NR arrays were first grown on a microchannel reactor using a simple template-free aqueous approach. A simple mixture of copper salts, aqueous media, and ZnO NR arrays at low temperature subsequently resulted in spontaneous formation of cable-like nanostructures. As the ZnO NR@Cu NP nanocomposites were synthesized in-situ directly on the microreactor, the arrayed ZnO@Cu nanocomposites were strongly anchored onto the microchannel. The strong mechanical anchorage of nanostructured catalysts on the surface of microchannel was shown by the observation that no material loss occurred after sonication in the water for several hours. The interaction between Cu NPs and ZnO NRs was studied by several analytical techniques, including electron microscopy (EM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and temperature-programmed reduction (TPR). The structure of the microreformer design based on the ZnO NR@Cu NP nanocomposite is illustrated in the Supporting Information (Scheme S1), along with photographs comparing the microchannels before and after the deposition of the ZnO NR@Cu NP nanocomposite (Supporting Information, Figure S1). One of the most significant advantages of the core–shell nanocomposites, which are clearly distinct from traditional catalysts, is the large surface area they offer for effective surface contact between the reactants and catalysts. Figure 1a shows a cross-sectional SEM image of vertically aligned ZnO NRs grown on the inner surface of the microchannel. The size of the NRs range from 35 to 50 nm in diameter and around 5 mm in height, as determined directly from the SEM micrograph. The TEM image of a typical NR is shown in Figure 1b, indicating an uneven surface with stacking faults (marked with arrows), which is shown in greater detail in Figure 1c. Figure 1d shows a typical TEM image of ZnO@Cu hybrid nanocomposites at a copper decoration concentration of 2 mm. Close attachment of Cu NPs on the ZnO NR cores can be clearly observed. More detailed TEM images with EDX elemental mapping of copper and zinc are shown in Figure 2, which further confirms the attachment of Cu NPs on the ZnO NR cores. Furthermore, a high-resolution TEM image (Figure 1e) yielded the spacing of the {111} lattice [*] Dr. L.-C. Chen, Dr. K.-H. Chen Center for Condensed Matter Sciences, National Taiwan University Taipei 106 (Taiwan) E-mail: chenlc@ntu.edu.tw

Birnessite-type manganese oxides nanosheets with hole acceptor assisted photoelectrochemical activity in response to visible light

A cost-effective and simple electroplating technique has been developed to prepare layered manganese oxide (MnO2) arrays as a promising material for solar hydrogen production and waste-water cleaning through photoelectrochemical (PEC) process. The microstructure of these MnO2 nanosheets can be referenced to Birnessite-type, as characterized by Raman spectra and transmission electron microscopy. The bandgap energy of the as-grown nanosheets determined from UV-vis spectroscopy is about 2.1 eV. Mott–Schottky plots show the flat band potential of the MnO2 nanosheets to be −0.01 V and a donor concentration of 4.68 × 1020 cm−3. Remarkable photocurrent in response to visible light is observed in the presence of hole acceptors, such as sodium formate and methanol, which efficiently suppress the recombination loss of electron–hole pairs from localized d–d transitions within manganese ion. Meanwhile, the transient photocurrent–time responses and the effect of different hole acceptors on PEC activity are studied with an increase of respective hole acceptor concentration, and the results reveal the critical role in the process of absorption and decomposition of the hole acceptor. Significantly, the MnO2 nanosheets exhibit an incident photon-to-electron conversion efficiency of 7% in response to the monochromatic wavelength of 400 nm, which is comparable to that from hematite (α-Fe2O3). These results demonstrate the nanoporous MnO2 nanosheets have great potential in solar hydrogen applications and organic pollutant cleaning.

Hierarchical Cu2O photocathodes with nano/microspheres for solar hydrogen generation

A hierarchical p-type Cu2O film with nano/microspheres on copper foil is successfully synthesized via a facile and cost-effective liquid reduction route through transformation of a lotus-like CuO/Cu(OH)2 nanosheet/nanowire structure. Various sizes of sphere-like Cu2O are transformed from CuO nanosheets and Cu(OH)2 nanowires by chemically reducing the oxides from Cu2+ to Cu1+ in a solution of ascorbic acid at a low temperature of 60 °C. Mott–Schottky analysis shows the flat band potential of the sphere-like Cu2O film to be −0.11 V and an acceptor concentration of 2.7 × 1020 cm−3. A direct band gap of 2.02 eV in Cu2O film is determined by incident photon-to-electron conversion efficiency measurements. Significantly, this hierarchical Cu2O photocathode exhibits remarkable photoelectrochemical activity in visible light. These results demonstrate that the Cu2O film with nano/microspheres has great potential in solar hydrogen applications.

Microwave-activated CuO nanotip/ZnO nanorod nanoarchitectures for efficient hydrogen production

Microwave treatment of CuO nanotip/ZnO nanorod catalyst precursors has been demonstrated to remarkably enhance their activity in methanol reforming reactions. Studies using XRD, Raman, XPS, XAS, and HRTEM analyses conclude that comparative to conventional heating, microwave treatment significantly enhances the activity and stability of the catalysts, which might be attributed to defect and microstrain formation and strong metal support interaction at the Cu/ZnO interface.

Generation properties of coherent infrared radiation in the optical absorption region of GaSe crystal

We report a study of the effect of optical absorption on generation of coherent infrared radiation from mid-IR to THz region from GaSe crystal. The infrared-active modes of epsilon-GaSe crystal at 236 cm(-1) and 214 cm(-1) were found to be responsible for the observed optical dispersion and infrared absorption edge. Based upon phase matching characteristics of GaSe for difference-frequency generation (DFG), new Sellmeier equations of GaSe were proposed. The output THz power variation with wavelength can be properly explained with a decrease of parametric gain and the spectral profile of absorption coefficient of GaSe. The adverse effect of infrared absorption on (DFG) process can partially be compensated by doping GaSe crystal with erbium ions.