[show abstract][hide abstract] ABSTRACT: Herein, we describe the growth of Si nanowires (NWs) in the vapor phase of an organic solvent medium on various substrates (Si, glass, and stainless steel) upon which an indium layer was evaporated. Variation of the reaction time allowed NW length and density to be controlled. The NWs grew via a predominantly root-seeded mechanism with discrete In catalyst seeds formed from the evaporated layer. The NWs and substrates were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The suitability of the indium seeded wires as anode components in Li batteries was probed using cyclic voltammetric (CV) measurements. The route represents a versatile, glassware-based method for the formation of Si NWs directly on a variety of substrates. KEYWORDS: silicon nanowires, indium, Li ion anode material ■ INTRODUCTION Si nanowires (NWs) have attracted considerable research interest because of their suitability for a wide range of emerging applications from transistor gate channels in the semiconductor industry to high density light trapping antennae in photo-voltaics. 1−3 More recently, Si NWs have been developed as a viable candidate material for Li ion battery anodes due their ability to withstand the volume expansion associated with Li cycling (not possible in bulk), allowing the high specific capacity (4200 mAhr g −1) to be exploited. 4 Practical integration of Si NWs in energy conversion and storage applications requires their formation in high density and at low cost if grid competitive devices are to be realized. A wide range of metal catalyzed protocols have been developed in conventional chemical vapor deposition (CVD) processes, which facilitate Si NW growth by vapor−liquid− solid (VLS) or vapor−solid−solid (VSS) mechanisms. 5−7 These reports can largely be divided into three general categories based on the state of the NW catalyst. Type A catalytic materials possess a single eutectic point with high Si solubility (typically >10%) and include Ag, 8 Al, 9 and the archetypal NW catalyst, Au. 10,11 Type B catalysts (e.g., In, Ga) 12−15 also show one dominant eutectic point; however, the composition is typically <1% Si whereas type C catalysts are silicide forming metals with complex phase diagrams that allow either VLS or VSS growth depending on the reaction temperature. 16 Indium in particular is an attractive type B catalyst for Si NW growth as its melting point, of just 156.6 °C, facilitates low reaction temperatures, while the incorporation of In atoms into the NW lattice can also be used to impart p-type doping. 17−19 The incorporation of dopants also increase the electrical conductivity of the NWs, which can improve their suitability for Li ion cells due to enhanced charge/discharge rates versus their undoped analogues. 20 While vacuum based CVD processes allow precise tuning of dimensions 21 and composition 22 and doping of the nanostruc-tures, 23 more recently developed organic solvent based syntheses have gained attention for their potential to produce high density NWs at low cost. 24−26 The challenge is that the boiling point of conventional solvents provides for a very narrow temperature window (typically <400 °C), limiting this technique mainly to compound semiconductors that nucleate at lower temperatures. 27 This obstacle can be overcome by operating within pressurized supercritical fluid systems, 25,28,29 or alternatively, by selecting a suitable metal catalyst and conducting the NW growth within a high boiling point organic solvent (HBS). 30,31 While Ge NWs can be routinely nucleated in HBS systems, 30,32 Si NW growth is more difficult and the only success to date involved the use of a highly reactive
Chemistry of Materials 05/2012; · 8.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: Vertical nanorod assembly over three dimensions is shown to result in the formation of Moiré interference patterns arising from rotational offsets between respective monolayer sheets. Six distinct patterns are observed in HRTEM and angular dark-field STEM (DF-STEM) images, allowing the exact angle of rotation to be determined from their respective size and repeat order. At large rotation angles approaching 30°, the aperiodicity in the structure of the nanorod supercrystals becomes apparent, resulting in 12-fold ordering characteristics of a quasicrystal. The rotational offsets are further elucidated from Fourier transform and small angle electron diffraction, allowing interpretation of several multilayers when combined with DF-STEM and SEM. Pattern formation owing to angular rotation is differentiated from those occurring from a lateral shift, providing an important insight into the complex multilayered structures in assembled rods that may have an impact on their collective electronic or photonic properties. We also show how random tetrapods when present at low concentrations in colloidal nanorod solutions act as termination points for 2D sheet crystallization, impacting the size and shape of the resultant assemblies. The occurrence of Moiré patterns in rod assemblies demonstrates the extraordinary order achievable in their assembly and offers a nondestructive technique to precisely map the placement of each nanorod in this important nanoarchitecture.
[show abstract][hide abstract] ABSTRACT: A highly efficient and reproducible approach for effective Pt nanoparticles dispersion and excellent decoration (inside/outside) of functionalised carbon nanofibers (f-CNF) is presented. The surface morphological, compositional and structural characterisations of the synthesised Pt(19.2)/f-CNF(80.8) material were examined using transmission electron microscopy (TEM/STEM/DF-STEM), energy-dispersive X-ray spectrometry (EDS), thermogravimetric analysis (TGA/DTG), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was employed in order to confirm the typical electrochemical response for Pt. The aim of the work was to improve the utility of both the supporting matrix (via the use of both inner/outer surfaces of nanofibers) and precious Pt, together with the sensitive glucose determination. TEM data indicated successful nanoparticle decoration with average Pt particle size 2.4 nm. The studies demonstrated that utilisation of the inner surface of the nanofibers, together with the modified outer surface characteristics using chemical treatment, enables excellent decoration, effective dispersion and efficient impregnation of Pt nanoparticles on carbon nanofibers. Pt(19.2)/f-CNF(80.8) exhibited excellent amperometric response (sensitivity = 22.7 μAmM(-1)cm(-2) and LoD = 0.42 μM) towards direct glucose sensing, over the range 0-10 mM glucose, in neutral conditions (pH 7.4). The improved carbon surface area for nanoparticle decoration, inner surface structure and morphology of nanofibers together with the presence of functional groups provided strong interactions and stability. These features together with the effective nanoparticle dispersion and decoration resulted in excellent catalytic response. The decorated nanoscaled material (Pt(19.2)/f-CNF(80.8)) is capable of large scale production, providing sensing capability in neutral conditions, while eliminating the temperature sensitivity, pH and lifetime issues associated with glucose enzymatic sensors and holds great promise in the quantification of glucose in real clinical samples.
The Analyst 03/2012; 137(7):1639-48. · 4.23 Impact Factor
[show abstract][hide abstract] ABSTRACT: A conducting polymer was used for the immobilization of various transition metal ion-substituted Dawson-type polyoxometalates (POMs) onto glassy carbon electrodes. Voltammetric responses of films of different thicknesses were stable within the pH domain 2-7 and reveal redox processes associated with the conducting polymer, the entrapped POMs and incorporated metal ions. The resulting POM doped polypyrrole films were found to be extremely stable towards redox switching between the various redox states associated with the incorporated POM. An amperometric sensor for hydrogen peroxide detection based upon the POM doped polymer films was investigated. The detection limits were 0.3 and 0.6 μM, for the Cu(2+)- and Fe(3+)-substituted POM-doped polypyrrole films respectively, with a linear region from 0.1 up to 2 mM H(2)O(2). Surface characterization of the polymer films was carried out using atomic force microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy.
The Analyst 12/2011; 137(3):624-30. · 4.23 Impact Factor
[show abstract][hide abstract] ABSTRACT: Germanium nanowires (Ge NWs) have found significant emerging applications spanning the solar, semiconductor, and storage industries. 1À5 The combination of high mobility and a large Bohr excitonic radius makes Ge NWs the semiconductor material of choice either for next generation, on-chip gate architec-tures or as pÀn junction absorber arrays in photovoltaics. 1,2 Ge NWs have shown promise in Li battery anodes as a result of their ability to withstand volume expansion on lithium insertion coupled with high theoretical capacities (1600 mA h/g) and increased room temperature diffusivity (compared to Si). 3 The conventional seeding protocol where a metal nanoparticle forms a eutectic melt with the semiconductor has several advantages in forming NWs, particularly for discrete applications, as precise control over diameter and, in some cases, length is possible. 4À6 The metal seed acts as a sink for the growth species either as a liquid 7 (vapor liquid solid (VLS)) or as a solid 8À10 (vapor solid solid (VSS)) with work showing that the preferred orientations and defects in the metal seed can be transferred to the semiconductor NW. 11 Metal seeded growth has shifted toward solid catalysts consisting of Cu, 12 Ni, 13,14 and Fe 15 in an effort to limit the dif-fusion of metal atoms into the NWs associated with the use of Au, which can severely impact the electrical properties of the NWs. 16,17 The emergence of storage and photovoltaic applica-tions places new demands on synthetic protocols for Ge NWs, with production directly from the current collector, in high yield and low cost, desirable. In directly seeded growth, high density is difficult to achieve, as temperature driven agglomeration can limit the number of catalytic sites available while also leading to a diameter spread in the NWs formed. 18À20 The emergence of self-catalytic 15,21À25 growth systems for Ge NW formation has allowed high yield growth, often from low cost precursors. 26 Using the correct conditions, spontaneous NW formation with defect free morphologies and tight diameter distributions have been achieved. 27 Self-catalytic approaches (without the direct incorporation of discrete nanoparticle seeds) have also been successfully extended to the formation of Ge NWs directly on Fe, 15 Ta, and W substrates. 25 The most interesting candidate for NW growth is copper, because of its use as a current collector in lithium ion batteries. While VSS seeding from copper nanocrystals is known, direct growth from bulk copper would be very attractive, as it potentially offers a route toward binder free cells at low cost, with sufficient gravimetric density for commer-cial viability. 28À30 Here, we present the highly dense growth of Ge NWs in a self-catalyzed process through the thermal decomposition of diphe-nylgermane (DPG) on copper foil in the vapor phase of a high boiling point solvent. The NWs are grown without the incor-poration of discrete metal nanoparticles. Rather, we show that the in situ formation of Cu 3 Ge acts as a catalyst for the formation of extremely dense NW mats with a very low diameter variation. This is the first direct observation of metal germanide tips in a self-induced, VSS process from bulk metals, offering important
Chemistry of Materials 10/2011; · 8.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: Pt based mono/bi/tri-metallic nanocomposites on different carbon based supports (activated carbon (AC), carbon nanotubes (CNTs) and carbon nanofibers (CNFs)) were synthesised and Pt surface enrichment achieved. The overall theoretical metallic content (Pt + Au + Sn) was 20% (w/w) in all mono/bi/tri-metallic nanocomposites and was found to be uniformly distributed in the supporting matrix (80%). The surface morphology and composition of the synthesised materials was characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), while cyclic voltammetry was employed in order to confirm their typical metallic electrochemical characteristics. Electrochemical measurements indicated that Pt(2)Au(1)Sn(1) trimetallic catalysts demonstrated a significantly higher electrochemically active surface area relative to activated carbon supported PtAu based bimetallic counterparts. The results show that the CNT based trimetallic catalyst (Pt(2)Au(1)Sn(1)/CNT) showed greatest electroactive surface area (49.3 m(2)/g) and current density for methanol oxidation in acidic (490 mA mg(-1) Pt) as well as basic (1700 mA mg(-1) Pt) conditions. Results demonstrated that in comparison to Au/C and Sn/C (no/negligible response), the presence of a small amount of Pt in the Au and Sn based nanocomposites, significantly modified the catalytic properties. The activated carbon supported bimetallic (Pt(1)Au(3)/C) catalyst showed reasonably good response (260 mA mg(-1) Pt) among all bimetallic nanomaterials examined. The current response achieved for Pt(2)Au(1)Sn(1)/CNT was 1.9 times (in acidic media) and 2.1 times (in basic media) that for synthesised Pt/C in terms of per mg Pt activity. Overall the methanol oxidation studies demonstrated that the presence of Au and Sn in Pt based catalysts strongly indicated their capacity to reduce the precious Pt content required for this application, demonstrating the role of Au in improving current/potential response and signifying the importance of supporting matrices.
[show abstract][hide abstract] ABSTRACT: Novel Co-Ni based catalysts on activated carbon support were prepared using NaBH4 as a reducing agent in aqueous conditions and examined with respect to direct amperometric uric acid detection. The surface morphology and composition of the synthesised materials were examined using transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was employed in order to confirm the typical metallic electrochemical response of the Co, Ni and Co-Ni based materials. The combination of metal hydroxides/oxides and nanoparticles in the carbon supported Co-Ni based materials were found to play a key role in uric acid determination. Upon surface confinement of the Co1Ni1/C material, uric acid sensitivity 248.2 µA mM−1 cm−2 and limit of detection 0.08 µM at Eapp=+0.4 V vs. Ag/AgCl was found by hydrodynamic amperometry over the range 0–250 µM (r2=0.9992). The sensor provided linear and reproducible behaviour over a wide range (25–575 µM) of uric acid. The composite materials showed excellent selectivity with respect to commonly found interferences with no response even at 10-fold concentration of urea, glucose and oxalate, and minimal influence of ascorbic acid (2 fold concentration). Overall, these materials are excellent candidates for direct uric acid detection in a stable, sensitive and very specific fashion over relevant physiological ranges, eliminating the pH, temperature sensitivity and lifetime issues associated with enzyme based systems. The materials are very promising for a range of applications including wound care/management and as non-enzymatic disposable uric acid test strips.
[show abstract][hide abstract] ABSTRACT: Nanocomposites of ethylene glycol protected platinum nanoparticles were prepared in the presence of activated carbon (AC), multi-walled carbon nanotubes (MWNTs) and carbon nanofibres (CNFs) at 20% (w/w) Pt loading and their potential in non-enzymatic glucose sensing evaluated. Physical and electrochemical characterization of these hybrid materials was enabled using transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and cyclic voltammetry. The average platinum nanoparticle diameters, as determined from TEM and XRD measurements, were 2 ± 1 to 3 ± 1 nm. The electrochemically active surface area of the platinum nanoparticles were found to be 91, 78 and 128 m2 g−1 for Pt-C, Pt-MWCNT and Pt-CNF respectively, as determined by the hydrogen adsorption/desorption phenomenon, using cyclic voltammetry in H2SO4. The nanomaterials were applied to the direct non-enzymatic quantization of glucose over its physiological range in the absence of the enzyme glucose oxidase. Hydrodynamic amperometric at Eapp = 0.55 V vs. Ag/AgCl in phosphate buffer (pH 7.4) was employed and the materials responded linearly to glucose (at pH 7.4, 298 K) over the range 2–20 mM (R2 = 0.99) with sensitivity 1.07, 1.10 and 0.52 μA mM−1 cm−2 for Pt-C, Pt-MWCNT and Pt-CNF respectively.
[show abstract][hide abstract] ABSTRACT: Silicon nanocrystals were synthesized at high temperatures and high pressures by the thermolysis of diphenylsilane using a combination of supercritical carbon dioxide and phosphonic acid surfactants. Size and shape evolution from pseudo-spherical silicon nanocrystals to well-faceted tetrahedral-shaped silicon crystals with edge lengths in the range of 30-400 nm were observed with sequentially decreasing surfactant chain lengths. The silicon nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering spectroscopy.