Showing papers by "Peidong Yang published in 2013"

A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting

Abstract: Artificial photosynthesis, the biomimetic approach to converting sunlight’s energy directly into chemical fuels, aims to imitate nature by using an integrated system of nanostructures, each of which plays a specific role in the sunlight-to-fuel conversion process. Here we describe a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual b...

Electrodeposited cobalt-sulfide catalyst for electrochemical and photoelectrochemical hydrogen generation from water.

Abstract: A cobalt-sulfide (Co-S) film prepared via electrochemical deposition on conductive substrates is shown to behave as an efficient and robust catalyst for electrochemical and photoelectrochemical hydrogen generation from neutral pH water. Electrochemical experiments demonstrate that the film exhibits a low catalytic onset overpotential (η) of 43 mV, a Tafel slope of 93 mV/dec, and near 100% Faradaic efficiency in pH 7 phosphate buffer. Catalytic current densities can approach 50 mA/cm(2) and activity is maintained for at least 40 h. The catalyst can also be electrochemically coated on silicon, rendering a water-compatible photoelectrochemical system for hydrogen production under simulated 1 sun illumination. The facile preparation of this Co-S film, along with its low overpotential, high activity, and long-term aqueous stability, offer promising features for potential use in solar energy applications.

Large-scale synthesis of transition-metal-doped TiO2 nanowires with controllable overpotential.

Abstract: Practical implementation of one-dimensional semiconductors into devices capable of exploiting their novel properties is often hindered by low product yields, poor material quality, high production cost, or overall lack of synthetic control. Here, we show that a molten-salt flux scheme can be used to synthesize large quantities of high-quality, single-crystalline TiO2 nanowires with controllable dimensions. Furthermore, in situ dopant incorporation of various transition metals allows for the tuning of optical, electrical, and catalytic properties. With this combination of control, robustness, and scalability, the molten-salt flux scheme can provide high-quality TiO2 nanowires to satisfy a broad range of application needs from photovoltaics to photocatalysis.

Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel N-Heterocyclic Carbene–Isoquinoline Complex

Abstract: The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene-amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni((Pr)bimiq1)](2+) (1c, where (Pr)bimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (E(cat) = -1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s(-1), respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion.

Mesoporous Co3O4 as an electrocatalyst for water oxidation

Abstract: Mesoporous Co3O4 has been prepared using porous silica as a hard template via a nanocasting route and its electrocatalytic properties were investigated as an oxygen evolution catalyst for the electrolysis of water. The ordered mesostructured Co3O4 shows dramatically increased catalytic activity compared to that of bulk Co3O4. Enhanced catalytic activity was achieved with high porosity and surface area, and the water oxidation overpotential (η) of the ordered mesoporous Co3O4 decreases significantly as the surface area increases. The mesoporous Co3O4 also shows excellent structural stability in alkaline media. After 100 min under 0.8 V (versus Ag/AgCl) applied bias, the sample maintains the ordered mesoporous structure with little deactivation of the catalytic properties.

Atomic Layer Deposition of Platinum Catalysts on Nanowire Surfaces for Photoelectrochemical Water Reduction

Abstract: The photocathodic hydrogen evolution reaction (HER) from p-type Si nanowire (NW) arrays was evaluated using platinum deposited by atomic layer deposition (ALD) as a HER cocatalyst. ALD of Pt on the NW surface led to a highly conformal coating of nanoparticles with sizes ranging from 0.5 to 3 nm, allowing for precise control of the Pt loading in deep submonolayer quantities. The catalytic performance was measured using as little as 1 cycle of Pt ALD, which corresponded to a surface mass loading of ∼10 ng/cm2. The quantitative exploration of the lower limits of Pt cocatalyst loading reported here, and its application to high-surface-area NW photoelectrodes, establish a general approach for minimizing the cost of precious-metal cocatalysts for efficient and affordable solar-to-fuel applications.

Energy and environment policy case for a global project on artificial photosynthesis

Abstract: A policy case is made for a global project on artificial photosynthesis including its scientific justification, potential governance structure and funding mechanisms.

Photocatalytic generation of hydrogen from water using a cobalt pentapyridine complex in combination with molecular and semiconductor nanowire photosensitizers

Abstract: Recently, a family of cobalt pentapyridine complexes of the type [(R-PY5Me2)Co(H2O)])(CF3SO3)2, (R = CF3, H, or NMe2; PY5Me2 = 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine) were shown to catalyze the electrochemical generation of hydrogen from neutral aqueous solutions using a mercury electrode. We now report that the CF3 derivative of this series, [(CF3PY5Me2)Co(H2O)](CF3SO3)2 (1), can also operate in neutral water as an electrocatalyst for hydrogen generation under soluble, diffusion-limited conditions on a glassy carbon electrode, as well as a photocatalyst for hydrogen production using either molecular or semiconductor nanowire photosensitizers. Owing to its relatively low overpotential compared to other members of the PY5 family, complex 1 exhibits multiple redox features on glassy carbon, including a one-proton, one-electron coupled oxidative wave. Further, rotating disk electrode voltammetry measurements reveal the efficacy of 1 as a competent hydrogen evolution catalyst under soluble, diffusion-limited conditions. In addition, we establish that 1 can also generate hydrogen from neutral water under photocatalytic conditions with visible light irradiation (λirr ≥ 455 nm), using [Ru(bpy)3]2+ as a molecular inorganic chromophore and ascorbic acid as a sacrificial donor. Dynamic light scattering measurements show no evidence for nanoparticle formation for the duration of the photolytic hydrogen evolution experiments. Finally, we demonstrate that 1 is also able to enhance the hydrogen photolysis yield of GaP nanowires in water, showing that this catalyst is compatible with solid-state photosensitizers. Taken together, these data establish that the well-defined cobalt pentapyridine complex [(CF3PY5Me2)Co(H2O)]2+ is a versatile catalyst for hydrogen production from pure aqueous solutions using either solar or electrical input, providing a starting point for integrating molecular systems into sustainable energy generation devices.

Oriented assembly of polyhedral plasmonic nanoparticle clusters

Abstract: Shaped colloids can be used as nanoscale building blocks for the construction of composite, functional materials that are completely assembled from the bottom up. Assemblies of noble metal nanostructures have unique optical properties that depend on key structural features requiring precise control of both position and connectivity spanning nanometer to micrometer length scales. Identifying and optimizing structures that strongly couple to light is important for understanding the behavior of surface plasmons in small nanoparticle clusters, and can result in highly sensitive chemical and biochemical sensors using surface-enhanced Raman spectroscopy (SERS). We use experiment and simulation to examine the local surface plasmon resonances of different arrangements of Ag polyhedral clusters. High-resolution transmission electron microscopy shows that monodisperse, atomically smooth Ag polyhedra can self-assemble into uniform interparticle gaps that result in reproducible SERS enhancement factors from assembly to assembly. We introduce a large-scale, gravity-driven assembly method that can generate arbitrary nanoparticle clusters based on the size and shape of a patterned template. These templates enable the systematic examination of different cluster arrangements and provide a means of constructing scalable and reliable SERS sensors.

Cleaved-coupled nanowire lasers

Abstract: The miniaturization of optoelectronic devices is essential for the continued success of photonic technologies. Nanowires have been identified as potential building blocks that mimic conventional photonic components such as interconnects, waveguides, and optical cavities at the nanoscale. Semiconductor nanowires with high optical gain offer promising solutions for lasers with small footprints and low power consumption. Although much effort has been directed toward controlling their size, shape, and composition, most nanowire lasers currently suffer from emitting at multiple frequencies simultaneously, arising from the longitudinal modes native to simple Fabry–Perot cavities. Cleaved-coupled cavities, two Fabry–Perot cavities that are axially coupled through an air gap, are a promising architecture to produce single-frequency emission. The miniaturization of this concept, however, imposes a restriction on the dimensions of the intercavity gaps because severe optical losses are incurred when the cross-sectional dimensions of cavities become comparable to the lasing wavelength. Here we theoretically investigate and experimentally demonstrate spectral manipulation of lasing modes by creating cleaved-coupled cavities in gallium nitride (GaN) nanowires. Lasing operation at a single UV wavelength at room temperature was achieved using nanoscale gaps to create the smallest cleaved-coupled cavities to date. Besides the reduced number of lasing modes, the cleaved-coupled nanowires also operate with a lower threshold gain than that of the individual component nanowires. Good agreement was found between the measured lasing spectra and the predicted spectral modes obtained by simulating optical coupling properties. This agreement between theory and experiment presents design principles to rationally control the lasing modes in cleaved-coupled nanowire lasers.

Femtosecond M2,3-Edge Spectroscopy of Transition-Metal Oxides: Photoinduced Oxidation State Change in α-Fe2O3

Abstract: Oxidation-state-specific dynamics at the Fe M2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe2O3 thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe3+ and photoinduced Fe2+ state confirm this assignment and exclude the alternative d–d excitation. The Fe2+ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.

Bacterial recognition of silicon nanowire arrays.

Abstract: Understanding how living cells interact with nanostructures is integral to a better understanding of the fundamental principles of biology and the development of next-generation biomedical/bioenergy devices. Recent studies have demonstrated that mammalian cells can recognize nanoscale topographies and respond to these structures. From this perspective, there is a growing recognition that nanostructures, along with their specific physicochemical properties, can also be used to regulate the responses and motions of bacterial cells. Here, by utilizing a well-defined silicon nanowire array platform and single-cell imaging, we present direct evidence that Shewanella oneidensis MR-1 can recognize nanoscale structures and that their swimming patterns and initial attachment locations are strongly influenced by the presence of nanowires on a surface. Analyses of bacterial trajectories revealed that MR-1 cells exhibited a confined diffusion mode in the presence of nanowires and showed preferential attachment to the nanowires, whereas a superdiffusion mode was observed in the absence of nanowires. These results demonstrate that nanoscale topography can affect bacterial movement and attachment and play an important role during the early stages of biofilm formation.

Zigzag Inversion Domain Boundaries in Indium Zinc Oxide-Based Nanowires: Structure and Formation

Abstract: Existing models for the crystal structure of indium zinc oxide (IZO) and indium iron zinc oxide (IFZO) conflict with electron microscopy data. We propose a model based on imaging and spectroscopy o...

Energy and Environment Policy Case for a Global Project on Artificial Photosynthesis

Abstract: A policy case is made for a global project on artificial photosynthesis including its scientific justification, potential governance structure and funding mechanisms.

Ta3N5 nanowire bundles as visible-light-responsive photoanodes.

Abstract: Solar energy is one of the most promising renewable energy sources to replace fossil fuels. Using sunlight to split water enables the storage of solar energy in the chemical bonds of hydrogen. Since Fujishima and Honda first reported water splitting using a TiO2 electrode, [3] metal oxides have been extensively studied as photoanodes for water oxidation. However, valence bands of oxides have strong oxygen 2p character. As a result, the valence band maximum (VBM) is usually substantially lower than the water oxidation potential, which leads to a significant loss in the efficiency of the oxygen evolution reaction. To reduce this energy loss, researchers have proposed a strategy of partially or completely replacing oxygen with other anions, such as nitrogen, to raise the VBM. Using this strategy, several oxynitride/nitride semiconductors, such as InxGa(1 x)N, [5,6] TaON, Ta3N5, [10, 11] CaTaO2N, and SrNbO2N, [12, 13] have recently been identified as promising photoanode materials. Among these semiconductors, Ta3N5 is attractive because of its band gap of 2.1 eV, which is similar to Fe2O3 (2.2 eV). This band gap can achieve a maximum solar-to-hydrogen (STH) efficiency of about 15 %. Also, the VBM of Ta3N5 is about 0.8 eV higher than the VBM of Fe2O3, [1,15] which could reduce efficiency losses at the photoanode. Although Ta3N5 has an advantageous band structure, it suffers from its chemical and functional instability in aqueous solution. One possible reason for this instability is the self-oxidation of N3 species from the accumulation of photo-generated holes. One-dimensional nanostructures such as nanotubes and nanowires have shown great promise for photoelectrochemical and photovoltaic applications. Such structures allow minority carriers to easily migrate to the surface along the radial direction, while maintaining efficient charge collection along the longitudinal direction. There have been few reports on the successful synthesis of single crystalline one-dimensional Ta3N5 nanostructures. Recently, Li et al. and Zhen et al. have demonstrated the fabrication of vertically aligned Ta3N5 nanorod arrays with promising photoelectrochemical performance and improved stability. Here, we report a facile high-temperature bottom-up synthetic method for producing Ta3N5 nanowire bundles (NWBs), which exhibit great promise as a visible-light-responsive photoanode. The synthesis of the Ta3N5 NWBs was performed in two steps (see the Supporting Information for details). The first step is a molten salt synthesis of K6Ta10.8O30 micro/nanowires, which was modified from a previous report. After cleaning the products with hot deionized water and hydrochloric acid, an insoluble white powder was obtained. Scanning electron microscopy (SEM) imaging revealed that the K6Ta10.8O30 powder is comprised of one-dimensional structures (Figure 1 a) with diameters ranging from several hundred nanometers to micrometers. The second step is the conversion from K6Ta10.8O30 to Ta3N5 using a nitridation procedure. After annealing the K6Ta10.8O30 micro/nanowires in ammonia at 900 8C for 6 h in a tube furnace, the white powder was converted into a red powder. SEM imaging of this red powder shows that the one-dimensional morphology was maintained after nitridation (Figure 1 b). A closer inspection revealed a morphological change on a smaller length scale: each of the thick micro/nanowires was composed of bundles of thinner nanowires with diameters around 20 nm (Figure 1 c). The absorption spectrum has an edge around 620 nm (Figure 1 d), which matches the band gap of previously reported absorption measurements of Ta3N5. [20, 21] Powder X-ray diffraction performed on the sample before and after nitridation (Figure 1 e) confirmed the conversion from K6Ta10.8O30 to Ta3N5. The structure of these nanowires was further investigated by transmission electron microscopy (TEM). As the Ta3N5 NWBs are too thick for direct TEM imaging, we sonicated the bundles in ethanol for 10 min before drop-casting them onto a TEM grid. After sonication, while most of the bundles remained intact, several individual nanowires were found. TEM imaging (Figure 2 a) confirms the observation from SEM that the thin Ta3N5 nanowires have diameters around 20 nm. High-resolution TEM (Figure 2 b) shows the single-crystalline nature of the thin Ta3N5 nanowires. Based [a] C. H. Wu, Dr. C. Hahn, Prof. Dr. P. Yang Department of Chemistry University of California Berkeley, CA 94720 (USA) [b] S. B. Khan, A. M. Asiri, S. M. Bawaked, Prof. Dr. P. Yang Center of Excellence for Advanced Materials Research (CEAMR) King Abdulaziz University Jeddah 21589, P.O. Box 80203 (Saudi Arabia) Fax: (+1) 510-642-7301 E-mail : p_yang@berkeley.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201300717.

Multinozzle Emitter Arrays for Ultrahigh-Throughput Nanoelectrospray Mass Spectrometry

Abstract: The present invention provides for a structure comprising a plurality of emitters, wherein a first nozzle of a first emitter and a second nozzle of a second emitter emit in two directions that are not or essentially not in the same direction; wherein the walls of the nozzles and the emitters form a monolithic whole. The present invention also provides for a structure comprising an emitter with a sharpened end from which the emitter emits; wherein the emitters forms a monolithic whole. The present invention also provides for a fully integrated separation of proteins and small molecules on a silicon chip before the electrospray mass spectrometry analysis.

Effect of Thermal Annealing in Ammonia on the Properties of InGaN Nanowires with Different Indium Concentrations

Abstract: The utility of an annealing procedure in ammonia ambient is investigated for improving the optical characteristics of InxGa1–xN nanowires (0.07 ≤ x ≤ 0.42) grown on c-Al2O3 using a halide chemical vapor deposition method. Morphological studies using scanning electron microscopy confirm that the nanowire morphology is retained after annealing in ammonia at temperatures up to 800 °C. However, significant indium etching and composition inhomogeneities are observed for higher indium composition nanowires (x = 0.28, 0.42), as measured by energy-dispersive X-ray spectroscopy and Z-contrast scanning transmission electron microscopy. Structural analyses, using X-ray diffraction and high-resolution transmission electron microscopy, indicate that this is a result of the greater thermal instability of higher indium composition nanowires. The effect of these structural changes on the optical quality of InGaN nanowires is examined using steady-state and time-resolved photoluminescence measurements. Annealing in ammoni...

New Designs For Spectral Control In Nanowire Lasers

Abstract: The miniaturization of optoelectronic devices is essential for the continued success of photonic technologies. Nanowires have been identified as potential building blocks that mimic conventional photonic components such as interconnects, waveguides, and optical cavities at the nanoscale. Semiconductor nanowires with high optical gain offer promising solutions for lasers with small footprints and low power consumption. Although much effort has been directed toward controlling their size, shape, and composition, most nanowire lasers currently suffer from being low-quality cavities, a problem that is universal in nanocavities because miniature features induce significant optical loss. Because of this difficulty, it remains an outstanding challenge to rationally control the spectra of room-temperature nanolasers. This talk will focus on the development of experimental and theoretical strategies to manipulate the lasing modes of a semiconductor nanowire while also improving the performance of the laser. Lasing operation at a controlled single UV wavelength at room temperature was achieved using a simple coupling scheme. The measured lasing spectra and laser threshold are in good agreement with the calculated spectral modes and threshold gain. This agreement between theory and experiment provides design principles for next-generation nanowire lasers.