Justin Z. Wu

An Advanced Ni–Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation

Highly active, durable, and cost-effective electrocatalysts for water oxidation to evolve oxygen gas hold a key to a range of renewable energy solutions, including water-splitting and rechargeable metal-air batteries. Here, we report the synthesis of ultrathin nickel-iron layered double hydroxide (NiFe-LDH) nanoplates on mildly oxidized multiwalled carbon nanotubes (CNTs). Incorporation of Fe into the nickel hydroxide induced the formation of NiFe-LDH. The crystalline NiFe-LDH phase in nanoplate form is found to be highly active for oxygen evolution reaction in alkaline solutions. For NiFe-LDH grown on a network of CNTs, the resulting NiFe-LDH/CNT complex exhibits higher electrocatalytic activity and stability for oxygen evolution than commercial precious metal Ir catalysts.

High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation

Stabilizing Silicon Solar-driven water splitting has potential as an energy storage mechanism to supplement the direct conversion of sunlight to electricity. A submersed integrated device has been proposed both to absorb the light and to catalyze the reaction, but stability has been a problem. Kenney et al. (p. 836; see the Perspective by Turner) found that a nickel coating, thin enough to let light through, could protect a silicon absorber in the alkaline environment of a lithium/potassium borate electrolyte. The nickel also functioned as the oxidation catalyst, and the lithium ions helped to establish a protective film structure in situ. A nickel coating renders silicon stable in alkaline lithium borate electrolyte for solar water-splitting applications. [Also see Perspective by Turner] Silicon’s sensitivity to corrosion has hindered its use in photoanode applications. We found that deposition of a ~2-nanometer nickel film on n-type silicon (n-Si) with its native oxide affords a high-performance metal-insulator-semiconductor photoanode for photoelectrochemical (PEC) water oxidation in both aqueous potassium hydroxide (KOH, pH = 14) and aqueous borate buffer (pH = 9.5) solutions. The Ni film acted as a surface protection layer against corrosion and as a nonprecious metal electrocatalyst for oxygen evolution. In 1 M aqueous KOH, the Ni/n-Si photoanodes exhibited high PEC activity with a low onset potential (~1.07 volts versus reversible hydrogen electrode), high photocurrent density, and durability. The electrode showed no sign of decay after ~80 hours of continuous PEC water oxidation in a mixed lithium borate–potassium borate electrolyte. The high photovoltage was attributed to a high built-in potential in a metal-insulator-semiconductor–like device with an ultrathin, incomplete screening Ni/NiOx layer from the electrolyte.

Ultrafast fluorescence imaging in vivo with conjugated polymer fluorophores in the second near-infrared window

In vivo fluorescence imaging in the second near-infrared window (1.0-1.7 μm) can afford deep tissue penetration and high spatial resolution, owing to the reduced scattering of long-wavelength photons. Here we synthesize a series of low-bandgap donor/acceptor copolymers with tunable emission wavelengths of 1,050-1,350 nm in this window. Non-covalent functionalization with phospholipid-polyethylene glycol results in water-soluble and biocompatible polymeric nanoparticles, allowing for live cell molecular imaging at >1,000 nm with polymer fluorophores for the first time. Importantly, the high quantum yield of the polymer allows for in vivo, deep-tissue and ultrafast imaging of mouse arterial blood flow with an unprecedented frame rate of >25 frames per second. The high time-resolution results in spatially and time resolved imaging of the blood flow pattern in cardiogram waveform over a single cardiac cycle (~200 ms) of a mouse, which has not been observed with fluorescence imaging in this window before.

Controlled Chlorine Plasma Reaction for Noninvasive Graphene Doping

We investigated the chlorine plasma reaction with graphene and graphene nanoribbons and compared it with the hydrogen and fluorine plasma reactions. Unlike the rapid destruction of graphene by hydrogen and fluorine plasmas, much slower reaction kinetics between the chlorine plasma and graphene were observed, allowing for controlled chlorination. Electrical measurements on graphene sheets, graphene nanoribbons, and large graphene films grown by chemical vapor deposition showed p-type doping accompanied by a conductance increase, suggesting nondestructive doping via chlorination. Ab initio simulations were performed to rationalize the differences in fluorine, hydrogen, and chlorine functionalization of graphene.

Thermally Limited Current Carrying Ability of Graphene Nanoribbons

We investigate high-field transport in graphene nanoribbons (GNRs) on SiO(2), up to breakdown. The maximum current density is limited by self-heating, but can reach >3 mA/μm for GNRs ~15 nm wide. Comparison with larger, micron-sized graphene devices reveals that narrow GNRs benefit from 3D heat spreading into the SiO(2), which enables their higher current density. GNRs also benefit from lateral heat flow to the contacts in short devices (<~0.3 μm), which allows extraction of a median GNR thermal conductivity (TC), ~80 W m(-1)K(-1) at 20 °C across our samples, dominated by phonons. The TC of GNRs is an order of magnitude lower than that of micron-sized graphene on SiO(2), suggesting strong roles of edge and defect scattering, and the importance of thermal dissipation in small GNR devices.

Chirality enriched (12,1) and (11,3) single-walled carbon nanotubes for biological imaging.

The intrinsic band gap photoluminescence of semiconducting single-walled carbon nanotubes (SWNTs) makes them promising biological imaging probes in the second near-infrared (NIR-II, 1.0-1.4 μm) window. Thus far, SWNTs used for biological applications have been a complex mixture of metallic and semiconducting species with random chiralities, preventing simultaneous resonant excitation of all semiconducting nanotubes and emission at a single well-defined wavelength. Here, we developed a simple gel filtration method to enrich semiconducting (12,1) and (11,3) SWNTs with identical resonance absorption at ~808 nm and emission near ~1200 nm. The chirality sorted SWNTs showed ~5-fold higher photoluminescence intensity under resonant excitation of 808 nm than unsorted SWNTs on a per-mass basis. Real-time in vivo video imaging of whole mouse body and tumor vessels was achieved using a ~6-fold lower injected dose of (12,1) and (11,3) SWNTs (~3 μg per mouse or ~0.16 mg/kg of body weight vs 1.0 mg/kg for unsorted SWNTs) than a previous heterogeneous mixture, demonstrating the first resonantly excited and chirality separated SWNTs for biological imaging.

WS2 nanoflakes from nanotubes for electrocatalysis

AbstractNext-generation catalysts for water splitting are crucial towards a renewable hydrogen economy. MoS2 and WS2 represent earth-abundant, noble metal cathode alternatives with high catalytic activity at edge sites. One challenge in their development is to nanostructure these materials in order to achieve increased performance through the creation of additional edge sites. In this work, we demonstrate a simple route to form nanostructured-WS2 using sonochemical exfoliation to break interlayer and intralayer bonds in WS2 nanotubes. The resulting few-layer nanoflakes are ∼100 nm wide with a high density of edge sites. WS2 nanoflakes are utilized as cathodes for the hydrogen evolution reaction (HER) and exhibit superior performance to WS2 nanotubes and bulk particles, with a lower onset potential, shallower Tafel slope and increased current density. Future work may employ ultra-small nanoflakes, dopant atoms, or graphene hybrids to further improve electrocatalytic activity.

Fluorescence Imaging In Vivo at Wavelengths beyond 1500 nm

Compared to imaging in the visible and near-infrared regions below 900 nm, imaging in the second near-infrared window (NIR-II, 1000-1700 nm) is a promising method for deep-tissue high-resolution optical imaging in vivo mainly owing to the reduced scattering of photons traversing through biological tissues. Herein, semiconducting single-walled carbon nanotubes with large diameters were used for in vivo fluorescence imaging in the long-wavelength NIR region (1500-1700 nm, NIR-IIb). With this imaging agent, 3-4 μm wide capillary blood vessels at a depth of about 3 mm could be resolved. Meanwhile, the blood-flow speeds in multiple individual vessels could be mapped simultaneously. Furthermore, NIR-IIb tumor imaging of a live mouse was explored. NIR-IIb imaging can be generalized to a wide range of fluorophores emitting at up to 1700 nm for high-performance in vivo optical imaging.

Three-dimensional imaging of single nanotube molecule endocytosis on plasmonic substrates

Investigating the cellular internalization pathways of single molecules or single nano objects is important to understanding cell-matter interactions, and to applications in drug delivery and discovery. Imaging and tracking the motion of single molecules on cell plasma membranes require high spatial resolution in three dimensions. Fluorescence imaging along the axial dimension with nanometre resolution has been highly challenging, but critical to revealing displacements in transmembrane events. Here, utilizing a plasmonic ruler based on the sensitive distance dependence of near-infrared fluorescence enhancement of carbon nanotubes on a gold plasmonic substrate, we probe ~10 nm scale transmembrane displacements through changes in nanotube fluorescence intensity, enabling observations of single nanotube endocytosis in three dimensions. Cellular uptake and transmembrane displacements show clear dependences to temperature and clathrin assembly on cell membrane, suggesting that the cellular entry mechanism for a nanotube molecule is via clathrin-dependent endocytosis through the formation of clathrin-coated pits on the cell membrane.

Short channel field-effect transistors from highly enriched semiconducting carbon nanotubes

AbstractSemiconducting single-walled carbon nanotubes (s-SWNTs) with a purity of ∼98% have been obtained by gel filtration of arc-discharge grown SWNTs with diameters in the range 1.2–1.6 nm. Multi-laser Raman spectroscopy confirmed the presence of less than 2% of metallic SWNTs (m-SWNTs) in the s-SWNT enriched sample. Measurement of ∼50 individual tubes in Pd-contacted devices with channel length 200 nm showed on/off ratios of >104, conductances of 1.38–5.8 μS, and mobilities in the range 40–150 cm2·V/s. Short channel multi-tube devices with ∼100 tubes showed lower on/off ratios due to residual m-SWNTs, although the on-current was greatly increased relative to the devices made from individual tubes.