Ballistic Phonon Transport in Holey Silicon

When the size of semiconductors is smaller than the phonon mean free path, phonons can carry heat with no internal scattering. Ballistic phonon transport has received attention for both theoretical and practical aspects because Fourier’s law of heat conduction breaks down and the heat dissipation in nanoscale transistors becomes unpredictable in the ballistic regime. While recent experiments demonstrate room-temperature evidence of ballistic phonon transport in various nanomaterials, the thermal conductivity data for silicon in the length scale of 10–100 nm is still not available due to experimental challenges. Here we show ballistic phonon transport prevails in the cross-plane direction of holey silicon from 35 to 200 nm. The thermal conductivity scales linearly with the length (thickness) even though the lateral dimension (neck) is as narrow as 20 nm. We assess the impact of long-wavelength phonons and predict a transition from ballistic to diffusive regime using scaling models. Our results support strong persistence of long-wavelength phonons in nanostructures and are useful for controlling phonon transport for thermoelectrics and potential phononic applications

Surfactant-Free, Large-Scale, Solution–Liquid–Solid Growth of Gallium Phosphide Nanowires and Their Use for Visible-Light-Driven Hydrogen Production from Water Reduction

Colloidal GaP nanowires (NWs) were synthesized on a large scale by a surfactant-free, self-seeded solution–liquid–solid (SLS) method using triethylgallium and tris(trimethylsilyl)phosphine as precursors and a noncoordinating squalane solvent. Ga nanoscale droplets were generated in situ by thermal decomposition of the Ga precursor and subsequently promoted the NW growth. The GaP NWs were not intentionally doped and showed a positive open-circuit photovoltage based on photoelectrochemical measurements. Purified GaP NWs were used for visible-light-driven water splitting. Upon photodeposition of Pt nanoparticles on the wire surfaces, significantly enhanced hydrogen production was observed. The results indicate that colloidal surfactant-free GaP NWs combined with potent surface electrocatalysts could serve as promising photocathodes for artificial photosynthesis

Atomic Structure of Ultrathin Gold Nanowires

Understanding of the atomic structure and stability of nanowires (NWs) is critical for their applications in nanotechnology, especially when the diameter of NWs reduces to ultrathin scale (1–2 nm). Here, using aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM), we report a detailed atomic structure study of the ultrathin Au NWs, which are synthesized using a silane-mediated approach. The NWs contain large amounts of generalized stacking fault defects. These defects evolve upon sustained electron exposure, and simultaneously the NWs undergo necking and breaking. Quantitative strain analysis reveals the key role of strain in the breakdown process. Besides, ligand-like morphology is observed at the surface of the NWs, indicating the possibility of using AC-HRTEM for surface ligand imaging. Moreover, the coalescence dynamic of ultrathin Au NWs is demonstrated by in situ observations. This work provides a comprehensive understanding of the structure of ultrathin metal NWs at atomic-scale and could have important implications for their applications

Widely Tunable Distributed Bragg Reflectors Integrated into Nanowire Waveguides

Periodic structures with dimensions on the order of the wavelength of light can tailor and improve the performance of optical components, and they can enable the creation of devices with new functionalities. For example, distributed Bragg reflectors (DBRs), which are created by periodic modulations in a structure’s dielectric medium, are essential in dielectric mirrors, vertical cavity surface emitting lasers, fiber Bragg gratings, and single-frequency laser diodes. This work introduces nanoscale DBRs integrated directly into gallium nitride (GaN) nanowire waveguides. Photonic band gaps that are tunable across the visible spectrum are demonstrated by precisely controlling the grating’s parameters. Numerical simulations indicate that in-wire DBRs have significantly larger reflection coefficients in comparison with the nanowire’s end facet. By comparing the measured spectra with the simulated spectra, the index of refraction of the GaN nanowire waveguides was extracted to facilitate the design of photonic coupling structures that are sensitive to phase-matching conditions. This work indicates the potential to design nanowire-based devices with improved performance for optical resonators and optical routing

Cysteine–Cystine Photoregeneration for Oxygenic Photosynthesis of Acetic Acid from CO₂ by a Tandem Inorganic–Biological Hybrid System

Tandem “Z-scheme” approaches to solar-to-chemical production afford the ability to independently develop and optimize reductive photocatalysts for CO₂ reduction to multicarbon compounds and oxidative photocatalysts for O₂ evolution. To connect the two redox processes, molecular redox shuttles, reminiscent of biological electron transfer, offer an additional level of facile chemical tunability that eliminates the need for solid-state semiconductor junction engineering. In this work, we report a tandem inorganic–biological hybrid system capable of oxygenic photosynthesis of acetic acid from CO₂. The photoreductive catalyst consists of the bacterium <i>Moorella thermoacetica</i> self-photosensitized with CdS nanoparticles at the expense of the thiol amino acid cysteine (Cys) oxidation to the disulfide form cystine (CySS). To regenerate the CySS/Cys redox shuttle, the photooxidative catalyst, TiO₂ loaded with cocatalyst Mn­(II) phthalocyanine (MnPc), couples water oxidation to CySS reduction. The combined system <i>M. thermoacetica</i>–CdS + TiO₂–MnPc demonstrates a potential biomimetic approach to complete oxygenic solar-to-chemical production

Room-Temperature Dynamics of Vanishing Copper Nanoparticles Supported on Silica

In heterogeneous catalysis, a nanoparticle (NP) system has immediate chemical surroundings with which its interaction needs to be considered, as nanoparticles are typically loaded onto certain supports. Beyond what is known about these interactions, dynamic atomic interactions between the nanoparticle and support could result from the increased energetics at the nanoscale. Here, we show that the dynamic response of atoms in copper nanoparticles to the underlying silica support at room temperature and ambient atmosphere results in the complete disappearance of supported nanoparticles over the course of only a few weeks. A quantitative study of copper nanoparticles at various size regimes (6–17 nm) revealed the significance of size-dependent nanoparticle energetics to the interaction with the support. Extended X-ray absorption fine structure is used to show that copper atoms could readily diffuse into the support to be locally surrounded by oxygen and silicon with structurally disordered outer coordination shells. Increased energetic states at the nanoscale and the energetically favorable configuration of individual copper atoms within silica, identified through EXAFS, are suggested as the cause of nanoparticle disappearance. This unexpected observation opens up new questions as to how nanoparticles interact with surrounding environments that could fundamentally change our conventional view of supported nanoparticle systems

Synthesis of Carbohydrates from Methanol Using Electrochemical Partial Oxidation over Palladium with the Integrated Formose Reaction

Electrochemically derived multicarbon products are a golden target for valorization of captured carbon dioxide due to the potential of turning a waste product into useful commodity chemicals with renewable energy sources. As a tantalizing approach toward their synthesis, the formose reaction utilizes catalytic condensation of formaldehyde to generate carbohydrates. While a sustainable approach to artificial carbohydrate production through electrochemical generation of formaldehyde is desirable, to date, it has not been fully realized. Here, we study the electrocatalytic conversion of methanol to formaldehyde on palladium with faradaic efficiency of over 90% at 0.9 V vs Ag/AgCl and with the partial current density of nearly 3 mA cm–2 at 1.6 V vs Ag/AgCl. We observe the concurrent generation of palladium oxides as a consequence of the high overpotentials employed, which may partially explain the higher selectivity toward the partial oxidation product. Moreover, we demonstrate that formaldehyde produced electrochemically from methanol is feasible for formose reactions without the need for further purification, achieving 21–28% carbon conversion to carbohydrates. This process, therefore, represents a potential avenue for the electrochemical generation of formaldehyde and its utilization in generating multicarbon products inaccessible by other electrocatalytic means

Salt-Induced Self-Assembly of Bacteria on Nanowire Arrays

Studying bacteria–nanostructure interactions is crucial to gaining controllable interfacing of biotic and abiotic components in advanced biotechnologies. For bioelectrochemical systems, tunable cell–electrode architectures offer a path toward improving performance and discovering emergent properties. As such, <i>Sporomusa ovata</i> cells cultured on vertical silicon nanowire arrays formed filamentous cells and aligned parallel to the nanowires when grown in increasing ionic concentrations. Here, we propose a model describing the kinetic and the thermodynamic driving forces of bacteria–nanowire interactions

Plasmon-Enhanced Photocatalytic Activity of Iron Oxide on Gold Nanopillars

Photocatalytic water splitting represents a promising way to produce renewable hydrogen fuel from solar energy. Ultrathin semiconductor electrodes for water splitting are of particular interest because the optical absorption occurs in the region where photogenerated charge carriers can effectively contribute to the chemical reactions on the surface. It is therefore important to manipulate and concentrate the incident light so that more photons can be absorbed within the thin film. Here we show an enhanced photocurrent in a thin-film iron oxide photoanode coated on arrays of Au nanopillars. The enhancement can be attributed primarily to the increased optical absorption originating from both surface plasmon resonances and photonic-mode light trapping in the nanostructured topography. The resonances can be tuned to a desirable wavelength by varying the thickness of the iron oxide layer. A net enhancement as high as 50% was observed over the solar spectrum

Photoelectrochemical Properties of TiO₂ Nanowire Arrays: A Study of the Dependence on Length and Atomic Layer Deposition Coating

We report that the length and surface properties of TiO₂ nanowires can have a dramatic effect on their photoelectrochemical properties. To study the length dependence, rutile TiO₂ nanowires (0.28–1.8 μm) were grown on FTO substrates with different reaction times (50–180 min) using a hydrothermal method. Nanowires show an increase in photocurrent with length, and a maximum photocurrent of 0.73 mA/cm² was measured (1.5 V <i>vs</i> RHE) for 1.8 μm long nanowires under AM 1.5G simulated sunlight illumination. While the incident photon to current conversion efficiency (IPCE) increases linearly with photon absorptance (1–10^{–α×length}) with near band gap illumination (λ = 410 nm), it decreases severely at shorter wavelengths of light for longer nanowires due to poor electron mobility. Atomic layer deposition (ALD) was used to deposit an epitaxial rutile TiO₂ shell on nanowire electrodes which enhanced the photocatalytic activity by 1.5 times (1.5 V <i>vs</i> RHE) with 1.8 μm long nanowires, reaching a current density of 1.1 mA/cm² (61% of the maximum photocurrent for rutile TiO₂). Additionally, by fixing the epitaxial rutile shell thickness and studying photoelectrochemical (PEC) properties of different nanowire lengths (0.28–1.8 μm), we found that the enhancement of current increases with length. These results demonstrate that ALD coating improves the charge collection efficiency from TiO₂ nanowires due to the passivation of surface states and an increase in surface area. Therefore, we propose that epitaxial coating on materials is a viable approach to improving their energy conversion efficiency