Min Hyung Lee

Optically- and thermally-responsive programmable materials based on carbon nanotube-hydrogel polymer composites

A simple approach is described to fabricate reversible, thermally- and optically responsive actuators utilizing composites of poly(N-isopropylacrylamide) (pNIPAM) loaded with single-walled carbon nanotubes. With nanotube loading at concentrations of 0.75 mg/mL, we demonstrate up to 5 times enhancement to the thermal response time of the nanotube-pNIPAM hydrogel actuators caused by the enhanced mass transport of water molecules. Additionally, we demonstrate the ability to obtain ultrafast near-infrared optical response in nanotube-pNIPAM hydrogels under laser excitation enabled by the strong absorption properties of nanotubes. The work opens the framework to design complex and programmable self-folding materials, such as cubes and flowers, with advanced built-in features, including tunable response time as determined by the nanotube loading.

p-Type InP Nanopillar Photocathodes for Efficient Solar-Driven Hydrogen Production†

Water splitting by using sunlight for the production of hydrogen yields a storable product, which can be used as a fuel. There is considerable research into H2 generation, namely the reduction of protons to H2 in aqueous solution using semiconductor photocathodes. To maximize the photoelectrochemical (PEC) performance, the selection of the active materials and device configurations should be carefully considered. First, the short-circuit current density (Jsc) should be maximized by choosing materials with high optical absorption coefficients and low carrier recombination rates, both in the bulk and at the surface. The reflectance should be minimized by using surface nanotexturing to further improve light absorption. The onset potential (Eos) of the PEC device versus the reversible H /H2 redox potential should be maximized. Finally, the surface energy needs to be controlled to minimize the accumulation of gas bubbles on the surface of the photoelectrode. Light absorbers with band gaps in the range of 1.1–1.7 eV provide both a good match to the terrestrial solar spectrum and a significant fraction of the 1.23 eV free energy required to split water. Overpotentials associated with the electron transfer to (solvated) protons in aqueous solution should be minimized by improving carrier transport from semiconductor to electrolyte by decorating the semiconductor with cocatalysts, tuning band edges, and decreasing contact resistance. p-Type Si has been extensively investigated as a photocathode for photochemical hydrogen production. Planar Si has relatively low short-circuit current densities under AM1.5 G illumination, approximately 10 mAcm 2 (reference [9]), compared to what can be achieved in a pn junction solar cell (> 35 mAcm ). Nanostructuring and incorporation of cocatalysts have been used to raise the short-circuit current density to over 30 mAcm . A recent study using np Si radial junction microwires reported an Eos value of 0.54 V and an Jsc value of 15 mA, leading to an overall efficiency near 6%. The onset potential observed to date for p-Si photocathodes is less than half of the value required for overall water splitting (1.23 V). This low onset potential limits the performance of tandem or “Z-scheme” approaches, which would function without external bias, as it limits the potential overlap required for spontaneous water splitting. An ideal photocathode for use in a solar-driven hydrogen production system without bias should have both a high current density and a favorable open-circuit potential versus the reversible H/H2 redox couple. Herein, we employ nanotextured p-InP photocathodes in conjunction with a TiO2 passivation layer and a Ru cocatalyst to increase both Jsc and Eos values under H2 evolution conditions. InP has a number of attractive attributes as a photocathode: 1) Its band gap of 1.3 eV is well-matched to the solar spectrum; InP-based solar cells have achieved AM1.5 G efficiencies of up to 22%. 2) The conduction band edge of InP is slightly above the water reduction potential, thus electron transfer is favorable in this system. 3) The surface-recombination velocity of untreated InP is low (ca. 10 cms 1 for n-type and 10 cms 1 for p-type), which is particularly important for nonplanar devices with high surface areas, such as those explored in this study. For these reasons, InP has been studied previously as a photocathode for both water splitting and CO2 reduction. [18–20] Specifically, Heller and Vadimsky reported attractive PEC performances with current densities up to 28 mAcm 2 and conversion efficiencies of approximately 12% in InP photocathodes. Motivated by these results, we use InP as a model material system to elucidate the role of surface nanotexturing on the PEC device performance. We find that nanotextured InP photocathodes exhibit drastically enhanced performances compared to our planar cells that were processed using identical conditions. We examine the various effects of nanotexturing [*] M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, J. W. Ager, Prof. A. Javey Material Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) E-mail: jwager@lbl.gov ajavey@berkeley.edu M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, Prof. A. Javey Electrical Engineering and Computer Sciences University of California, Berkeley, CA 94720 (USA) M. H. Lee, T. S. Matthews, J. W. Ager, Prof. A. Javey Joint Center for Artificial Photosynthesis Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)

Hemispherically Aggregated BaTiO3 Nanoparticle Composite Thin Film for High-Performance Flexible Piezoelectric Nanogenerator

We report high-performance flexible nanogenerators (NGs) based on a composite thin film, composed of hemispherically aggregated BaTiO3 nanoparticles (NPs) and poly(vinylidene fluoride-co-hexafluoropropene) P(VDF-HFP). The hemispherical BTO-P(VDF-HFP) clusters were realized by a solvent evaporation method, which greatly enhanced piezoelectric power generation. The flexible NGs exhibit high electrical output up to ∼75 V and ∼15 μA at the applied force normal to the surface, indicating the important role of hemispherical BTO clusters. Besides, the durability and reproducibility of the NGs were tested by cyclic measurement under bending stage, generating the output of ∼5 V and ∼750 nA. The approach we introduce here is simple, cost-effective, and well-suited for large-scale high-performance flexible NG fabrication.

Triboelectric Charging Sequence Induced by Surface Functionalization as a Method To Fabricate High Performance Triboelectric Generators

Two different materials, apart from each other in a triboelectric series, are required to fabricate high performance triboelectric generators (TEGs). Thus, it often limits the choices of materials and causes related processing issues for TEGs. To address this issue, we report a simple surface functionalization method that can effectively change the triboelectric charging sequence of the materials, broadening material choices and enhancing the performance of TEGs. Specifically, we functionalized the surfaces of the polyethylene terephthalate (PET) films either with poly-l-lysine solution or trichloro(1H,1H,2H,2H-perfluorooctyl) silane (FOTS). Consequently, the PET surfaces were modified to have different triboelectric polarities in a triboelectric series. The TEGs, fabricated using this approach, demonstrated the maximum Vopen-circuit (Voc) of ∼330 V and Jshort-circuit (Jsc) of ∼270 mA/m(2), respectively, at an applied force of 0.5 MPa. Furthermore, the functionalized surfaces of TEGs demonstrated superior stability during cyclic measurement over 7200 cycles, maintaining the performance even after a month. The approach introduced here is a simple, effective, and cost-competitive way to fabricate TEGs, which can also be easily adopted for various surface patterns and device structures.

Lithium-Doped Zinc Oxide Nanowires–Polymer Composite for High Performance Flexible Piezoelectric Nanogenerator

We present a method to develop high performance flexible piezoelectric nanogenerators (NGs) by employing Li-doped ZnO nanowires (NWs). We synthesized Li-doped ZnO NWs and adopted them to replace intrinsic ZnO NWs with a relatively low piezoelectric coefficient. When we exploited the ferroelectric phase transition induced in Li-doped ZnO NWs, the performance of the NGs was significantly improved and the NG fabrication process was greatly simplified. In addition, our approach can be easily expanded for large-scale NG fabrication. Consequently, the NGs fabricated by our simple method exhibit the excelling output voltage and current, which are stable and reproducible during periodic bending/releasing measurement over extended cycles. In addition, output voltage and current up to ∼ 180 V and ∼ 50 μA, respectively, were obtained in the large-scale NG. The approach introduced here extends the performance limits of ZnO-based NGs and their potentials in practical applications.

Roll-to-roll anodization and etching of aluminum foils for high-throughput surface nanotexturing.

A high-throughput process for nanotexturing of hard and soft surfaces based on the roll-to-roll anodization and etching of low-cost aluminum foils is presented. The process enables the precise control of surface topography, feature size, and shape over large areas thereby presenting a highly versatile platform for fabricating substrates with user-defined, functional performance. Specifically, the optical and surface wetting properties of the foil substrates were systematically characterized and tuned through the modulation of the surface texture. In addition, textured aluminum foils with pore and bowl surface features were used as zeptoliter reaction vessels for the well-controlled synthesis of inorganic, organic, and plasmonic nanomaterials, demonstrating yet another powerful potential use of the presented approach.

Piezoelectric performance enhancement of ZnO flexible nanogenerator by a CuO–ZnO p–n junction formation

The piezoelectric potential screening by large excess electrons in nominally undoped ZnO has limited the energy conversion efficiency of the ZnO nanogenerators (NGs). In this study, we report a simple and effective approach to enhance the piezoelectric output performance of the ZnO NGs by forming a CuO–ZnO heterostructure. By depositing a ZnO thin film on the pre-deposited CuO thin film, which forms a p–n junction, excess electrons in ZnO can be effectively reduced. Thus, the piezoelectric potential generated in ZnO by an applied force can be less affected. Using this approach, we obtained an output voltage up to ∼7.5 V and a maximum current of 4.5 μA cm−2 measured under the forward connection, which is a 7-fold higher output voltage and an approximately one order of magnitude higher current density by comparison to the ZnO NGs without a CuO layer. Our results clearly demonstrate the effectiveness of a CuO–ZnO heterostructure for realizing high performance flexible energy harvesting devices.

Morphology‐Directed Selective Production of Ethylene or Ethane from CO2 on a Cu Mesopore Electrode

The electrocatalytic conversion of CO2 to value-added hydrocarbons is receiving significant attention as a promising way to close the broken carbon-cycle. While most metal catalysts produce C1 species, such as carbon monoxide and formate, the production of various hydrocarbons and alcohols comprising more than two carbons has been achieved using copper (Cu)-based catalysts only. Methods for producing specific C2 reduction outcomes with high selectivity, however, are not available thus far. Herein, the morphological effect of a Cu mesopore electrode on the selective production of C2 products, ethylene or ethane, is presented. Cu mesopore electrodes with precisely controlled pore widths and depths were prepared by using a thermal deposition process on anodized aluminum oxide. With this simple synthesis method, we demonstrated that C2 chemical selectivity can be tuned by systematically altering the morphology. Supported by computational simulations, we proved that nanomorphology can change the local pH and, additionally, retention time of key intermediates by confining the chemicals inside the pores.

Control of density and LSPR of Au nanoparticles on graphene

In this study, we introduce the tunable density and localized surface plasmon resonance (LSPR) of plasmonic gold (Au) nanoparticles which were formed on monolayer graphene at room temperature, based on the difference of the reduction potential between graphene and the Au(3+) precursor. The size of the Au nanoparticles was ~40 nm, which is very desirable to provide an optical enhancement effect by LSPR in the full visible range. It is demonstrated that the density of the Au nanoparticles was modulated by the surface energy of the graphene on the substrate as well as the concentration of the Au(3+) precursor. Furthermore, the cycle number of the reduction process strongly affected the distribution of the nanoparticle size and their optical properties. The LSPR of the plasmonic Au nanoparticles was red-shifted from 560 to 620 nm and its full width at half maximum broadened as the Au(3+) precursor concentration was increased and the cyclic reduction process progressed. Based on the optical enhancement of the plasmonic Au nanoparticles and the extraordinary physical characteristics of graphene, the Au/graphene assembly may offer a promising optoelectronic platform for next-generation flexible optical electronics or biosensors.

Formation of Triboelectric Series via Atomic-Level Surface Functionalization for Triboelectric Energy Harvesting

Triboelectric charging involves frictional contact of two different materials, and their contact electrification usually relies on polarity difference in the triboelectric series. This limits the choices of materials for triboelectric contact pairs, hindering research and development of energy harvest devices utilizing triboelectric effect. A progressive approach to resolve this issue involves modification of chemical structures of materials for effectively engineering their triboelectric properties. Here, we describe a facile method to change triboelectric property of a polymeric surface via atomic-level chemical functionalizations using a series of halogens and amines, which allows a wide spectrum of triboelectric series over single material. Using this method, tunable triboelectric output power density is demonstrated in triboelectric generators. Furthermore, molecular-scale calculation using density functional theory unveils that electrons transferred through electrification are occupying the PET group rather than the surface functional group. The work introduced here would open the ability to tune triboelectric property of materials by chemical modification of surface and facilitate the development of energy harvesting devices and sensors exploiting triboelectric effect.