Haoyu Wu

Two-Dimensional Mesoporous Carbon Nanosheets and Their Derived Graphene Nanosheets: Synthesis and Efficient Lithium Ion Storage

We report a new solution deposition method to synthesize an unprecedented type of two-dimensional ordered mesoporous carbon nanosheets via a controlled low-concentration monomicelle close-packing assembly approach. These obtained carbon nanosheets possess only one layer of ordered mesopores on the surface of a substrate, typically the inner walls of anodic aluminum oxide pore channels, and can be further converted into mesoporous graphene nanosheets by carbonization. The atomically flat graphene layers with mesopores provide high surface area for lithium ion adsorption and intercalation, while the ordered mesopores perpendicular to the graphene layer enable efficient ion transport as well as volume expansion flexibility, thus representing a unique orthogonal architecture for excellent lithium ion storage capacity and cycling performance. Lithium ion battery anodes made of the mesoporous graphene nanosheets have exhibited an excellent reversible capacity of 1040 mAh/g at 100 mA/g, and they can retain at 833 mAh/g even after numerous cycles at varied current densities. Even at a large current density of 5 A/g, the reversible capacity is retained around 255 mAh/g, larger than for most other porous carbon-based anodes previously reported, suggesting a remarkably promising candidate for energy storage.

Controlled Sn-doping in TiO2 nanowire photoanodes with enhanced photoelectrochemical conversion.

We demonstrate for the first time the controlled Sn-doping in TiO(2) nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO(2) and TiO(2), Sn dopants are incorporated into TiO(2) NWs by a one-pot hydrothermal synthesis with different ratios of SnCl(4) and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl(4) hydrolysis rate. The obtained Sn-doped TiO(2) (Sn/TiO(2)) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO(2) NWs is well controlled at a low level, that is, 1-2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO(2) NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm(2) at 0 V vs Ag/AgCl under 100 mW/cm(2) simulated sunlight illumination up to ~100% enhancement compared to our best pristine TiO(2) NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO(2) NWs in H(2) further improves their photoactivity with an optimized photoconversion efficiency of ~1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO(2) NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO(2) materials in a variety of solar energy driven applications.

Simultaneous etching and doping of TiO2 nanowire arrays for enhanced photoelectrochemical performance.

We developed a postgrowth doping method of TiO2 nanowire arrays by a simultaneous hydrothermal etching and doping in a weakly alkaline condition. The obtained tungsten-doped TiO2 core-shell nanowires have an amorphous shell with a rough surface, in which W species are incorporated into the amorphous TiO2 shell during this simultaneous etching/regrowth step for the optimization of photoelectrochemical performance. Photoanodes made of these W-doped TiO2 core-shell nanowires show a much enhanced photocurrent density of ~1.53 mA/cm(2) at 0.23 V vs Ag/AgCl (1.23 V vs reversible hydrogen electrode), almost 225% of that of the pristine TiO2 nanowire photoanodes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that the substantially improved performance of the dual W-doped and etched TiO2 nanowires is attributed to the enhancement of charge transfer and the increase of charge carrier density, resulting from the combination effect of etching and W-doping. This unconventional, simultaneous etching and doping of pregrown nanowires is facile and takes place under moderate conditions, and it may be extended for other dopants and host materials with increased photoelectrochemical performances.

Aligned NiO nanoflake arrays grown on copper as high capacity lithium-ion battery anodes

Transition metal oxides are promising candidates for lithium-ion battery electrodes, while their performances are generally limited by their poor electrical conductivity and cycling stability. In this paper, we report the growth of aligned, single-crystalline NiO nanoflake arrays directly on copper substrates by a modified hydrothermal synthesis and post-annealing. The close contact of NiO nanoflakes on a current collector (e.g. Cu) allows for efficient charge transport, and waives the need for adding ancillary conducting materials or binders. In addition, the mesopores inside the NiO nanoflakes and the spacing between the adjacent aligned nanoflakes provide efficient ion transport pathways as well as sufficient flexibility for electrode volume expansion. As proof-of-concept, anodes made of NiO nanoflakes directly grown on Cu showed a high capacity and excellent cycling stability. The capacity was retained at 720 mA h g−1 over 20 cycles at a current density of 100 mA g−1, almost equal to the theoretical value of NiO and much higher than the NiO products formed in the same growth solution. Even at a high discharge–charge rate of 1 A g−1 (1.5 C), the NiO nanoflakes grown on Cu were capable of retaining a capacity of 500 mA h g−1 over 40 cycles. Our report suggests that NiO nanoflakes may serve as a promising anode material for a high-power lithium-ion battery.

Natural graphite coated by Si nanoparticles as anode materials for lithium ion batteries

Nano-sized crystalline silicon particles, prepared by a laser-induced vapour deposition method, were coated onto the surface of particles of a modified natural graphite (SSG) by sonicated dispersion and a subsequent heat-treatment process. The microstructure of the Si-coated SSG was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the nanometer-scale Si particles were uniformly and completely coated on the surface of SSG particles, and both the Si and SSG particles existed in the crystalline state. The Si-coated SSG exhibits a much higher reversible capacity than pristine SSG, while keeping the good cycling performance of SSG material. The higher capacity can be ascribed to the alloying of Si with lithium. Because of the heat-treatment at 600 °C, used to achieve a good combination of Si with the SSG base, the cycling of the composites is very satisfactory. As a result, Si-coated SSG is a promising anode material for lithium ion batteries.

Multi-layered mesoporous TiO2 thin films with large pores and highly crystalline frameworks for efficient photoelectrochemical conversion

Mesoporous thin films with various compositions are unique architectures for photoelectrochemical (PEC) solar cells. In this paper, we report the synthesis of highly ordered, multi-layered, continuous mesoporous TiO2 thin films with uniform large pores, crystalline walls and tunable film thickness, via a ligand-assisted evaporation induced self assembly (EISA) method. A Ti(acetylacetone) precursor and a diblock copolymer PEO-b-PS are employed for the controlled assembly of the TiO2/template mesostructure, followed by a two-step pyrolysis that generates carbon residue as an intermediate protection layer to support the TiO2 framework and mesostructures during the crystallization. Other transition metal ion dopants (such as Cr, Ni and Co) can be facilely incorporated into the TiO2 frameworks by co-assembly of these metal acetylacetone precursors during the EISA process. The obtained TiO2 thin film possesses an ordered monoclinic mesostructure distorted from a (110)-oriented primitive cubic structure, uniform and tunable large pores of 10–30 nm, a large surface area of ∼100 m2 g−1 and a high crystallinity anatase wall. The film thickness can be well controlled from 150 nm to several microns to tune the absorption, with the capability of generating free-standing film morphologies. Furthermore, this designed architecture allows for effective post-deposition of other small-bandgap semiconductor nanomaterials inside the large, open and interconnecting mesopores, leading to significantly improved solar absorption and photoconversion. As a proof-of-concept, we demonstrate that the photoanodes made of 4.75 μm thick mesoporous TiO2 film with deposited cadmium sulfide quantum dots exhibit excellent performance in PEC water splitting, with an optimized photocurrent density of 6.03 mA cm−2 and a photoconversion efficiency of 3.9%. These multi-layered mesoporous TiO2-based thin films can serve as a unique architecture for PEC and other solar energy conversion and utilization.

Dislocation-driven CdS and CdSe nanowire growth.

We report the synthesis of CdS and CdSe nanowires (NWs) and nanoribbons (NRs) with gold catalysts by H(2)-assisted chemical vapor deposition. Nanopods and nanocones were obtained without catalysts at higher system pressure. Transmission electron microscopy (TEM) studies, including two-beam TEM and displaced-aperture dark-field TEM characterization, were used to investigate the NW growth mechanism. Dislocation contrast and twist contours have been routinely observed within the synthesized one-dimensional (1D) CdS and CdSe NWs, suggesting the operation of the dislocation-driven NW growth mechanism under our experimental conditions. The Burgers vectors of dislocations and the associated Eshelby twists were measured and quantified. We hypothesize that gold nanoparticles provide nucleation sites to initiate the growth of CdS/CdSe NWs and lead to the formation of dislocations that continue to drive and sustain 1D growth at a low supersaturation level. Our study suggests that the dislocation-driven mechanism may also contribute to the growth of other 1D nanomaterials that are commonly considered to grow via the vapor-liquid-solid mechanism.

WO3–reduced graphene oxide composites with enhanced charge transfer for photoelectrochemical conversion

Hybrid structures between semiconducting metal oxides and carbon with rational synthesis represent unique device building blocks to optimize the light absorption and charge transfer process for the photoelectrochemical conversion. Here we demonstrate the realization of a WO3-reduced graphene oxide (RGO) nanocomposite via hydrothermal growth of ultrathin WO3 nanoplates directly on fluorine-doped tin oxide (FTO) substrates, followed by in situ photo-reduction to deposit RGO layers on WO3 nanoplate surface. Photoanodes made of the WO3-RGO nanocomposites have achieved a photocurrent density of 2.0 mA cm(-2) at 1.23 V vs. reversible hydrogen electrode (RHE), which is among the highest reported values for photoanodes based on hydrothermally grown WO3. Electrochemical impedance spectroscopy reveals that the increase of photoactivity is attributed to the enhanced charge transfer by the incorporation of RGO, thus suggesting a general approach for designing other metal oxide-RGO hybrid architectures.

Synthesis of hierarchically nanoporous silica films for controlled drug loading and release

Films with well-controlled porous structures provide many exciting application opportunities in chemistry and biology. Here we report the synthesis of a highly uniform, hierarchically nanoporous silica film structure, and its application in drug loading and release for antibacterial surface coating. Templated by both sub-micron poly-styrene (PS) particles and a triblock copolymer (F127), this hierarchically nanoporous film has two distinct pore sizes of 200 nm and 7 nm. The 7-nm mesopores provide high surface area and thus high adsorption capacity for drug molecules, and the 200-nm macropores facilitate the adsorption rate of drug molecules, especially for molecules with comparable sizes to mesopores. Fluorescence measurement of rhodamine release demonstrates that this hierarchically porous film has a higher adsorption capacity, efficiency and much longer molecule releasing time window than both the inverse opal film and the mesoporous film. When loaded with Ampicillin, this hierarchically porous film shows over 8 times longer of inhibition of E. coli growth than both the inverse opal film and the mesoporous film. This simple and versatile process allows for fabrication of a variety of surface-coated, hierarchically nanoporous films with different chemical compositions and applications.

Unconventional 0-, 1-, and 2-dimensional single-crystalline copper sulfide nanostructures

We report the synthesis of several unconventional 0-, 1- and 2-dimensional copper sulfide nanostrucutures by the chemical vapor deposition method. The key factor for morphology and structure control of a variety of copper sulfide products is the tuning of deposition and growth temperature to fit for the surface energy barriers and promote different growth directions. At a high growth temperature (480 °C) that provides enough thermal energy, a 0-D octahedral copper sulfide single crystal structure was synthesized. At a slightly lower growth temperature (460 °C), a new 1-D copper sulfide nanorod structure with a nanocrystal head was discovered for the first time. At a much lower growth temperature (150 °C), 2-D copper sulfide nanoflakes with a single crystal hexagonal structure were obtained. These novel structural varieties of copper sulfide can lead to discovering more unconventional material structures and growth mechanisms of other transitional metal chalcogenides, and may allow for new copper sulfide based devices and applications.