Vertical graphene nanosheets coated with gold nanoparticle arrays: Effect of inter-particle spacing on optical response

Vertical graphene nanosheets have advantages over their horizontal counterparts, primarily due to the larger surface area available in the vertical systems. Vertical sheets can accommodate more functional particles, and due to the conduction and optical properties of thin graphene, these structures can find niche applications in the development of sensing and energy storage devices. This work is a combined experimental and theoretical study that reports on the synthesis and optical responses of vertical sheets decorated with gold nanoparticles. The findings help in interpreting optical responses of these hybrid graphene structures and are relevant to the development of future sensing platforms.

Probing the effect of charge transfer enhancement in off resonance mode SERS via conjugation of the probe dye between silver nanoparticles and metal substrates

The charge transfer-mediated surface enhanced Raman scattering (SERS) of crystal violet (CV) molecules that were chemically conjugated between partially polarized silver nanoparticles and optically smooth gold and silver substrates has been studied under off-resonant conditions. Tyrosine molecules were used as a reducing agent to convert silver ions into silver nanoparticles where oxidised tyrosine caps the silver nanoparticle surface with its semiquinone group. This binding through the quinone group facilitates charge transfer and results in partially oxidised silver. This establishes a chemical link between the silver nanoparticles and the CV molecules, where the positively charged central carbon of CV molecules can bind to the terminal carboxylate anion of the oxidised tyrosine molecules. After drop casting Ag nanoparticles bound with CV molecules it was found that the free terminal amine groups tend to bind with the underlying substrates. Significantly, only those CV molecules that were chemically conjugated between the partially polarised silver nanoparticles and the underlying gold or silver substrates were found to show SERS under off-resonant conditions. The importance of partial charge transfer at the nanoparticle/capping agent interface and the resultant conjugation of CV molecules to off resonant SERS effects was confirmed by using gold nanoparticles prepared in a similar manner. In this case the capping agent binds to the nanoparticle through the amine group which does not facilitate charge transfer from the gold nanoparticle and under these conditions SERS enhancement in the sandwich configuration was not observed.

Nanohybrids of silver particles on clay platelets delaminate Pseudomonas biofilms

Aim To evaluate the effectiveness of novel nanohybrids, composed of silver nanoparticles and nanoscale silicate platelets, to clear Pseudomonas aeruginosa biofilms. Materials & methods The nanohybrids were manufactured from an in situ reduction of silver salts in the silicate platelet dispersion, and then applied to biofilms in vitro and in vivo. Results In reference to the biocidal effects of gentamycin, the nanohybrids mitigated the spreading of the biofilms, and initiated robust cell death and exfoliation from the superficial layers of the biofilms in vitro. In vivo, the nanohybrids exhibited significant therapeutic effects by eliminating established biofilms from infected corneas and promoting the recovery of corneal integrity. Conclusion All of the evaluations indicate the high potency of the newly developed silver nanoparticle/nanoscale silicate platelet nanohybrids for eliminating biofilms.

Integrating efficient filtration and visible-light photocatalysis by loading Ag-doped zeolite Y particles on aluminia nanofiber membrane

Filtration membrane technology has already been employed to remove various organic effluents produced from the textile, paper, plastic, leather, food and mineral processing industries. To improve membrane efficiency and alleviate membrane fouling, an integrated approach is adopted that combines membrane filtration and photocatalysis technology. In this study, alumina nanofiber (AF) membranes with pore size of about 10 nm (determined by the liquid-liquid displacement method) have been synthesized through an in situ hydrothermal reaction, which permitted a large flux and achieved high selectivity. Silver nanoparticles (Ag NPs) are subsequently doped on the nanofibers of the membranes. Silver nanoparticles can strongly absorb visible light due to the surface plasmon resonance (SPR) effect, and thus induce photocatalytic degradation of organic dyes, including anionic, cationic and neutral dyes, under visible light irradiation. In this integrated system, the dyes are retained on the membrane surface, their concentration in the vicinity of the Ag NPs are high and thus can be efficiently decomposed. Meanwhile, the usual flux deterioration caused by the accumulation of the filtered dyes in the passage pores can be avoided. For example, when an aqueous solution containing methylene blue is processed using an integrated membrane, a large flux of 200 L m-2 h-1 and a stable permeating selectivity of 85% were achieved. The combined photocatalysis and filtration function leads to superior performance of the integrated membranes, which have a potential to be used for the removal of organic pollutants in drinking water.

Self-organized Au nanoarrays on vertical graphenes : an advanced three-dimensional sensing platform

A three-dimensional surface enhanced Raman scattering (SERS)/plasmonic sensing platform based on plasma-enabled, catalyst-free, few-layer vertical graphenes decorated with self-organized Au nanoparticle arrays is demonstrated. This platform is viable for multiple species detection and overcomes several limitations of two-dimensional sensors.

Rapid, simultaneous activation of thin nanowire growth in low-temperature, low-pressure chemically active plasmas

Multiscale numerical modeling of the species balance and transport in the ionized gas phase and on the nanostructured solid surface complemented by the heat exchange model is used to demonstrate the possibility of minimizing the Gibbs-Thompson effect in low-temperature, low-pressure chemically active plasma-assisted growth of uniform arrays of very thin Si nanowires, impossible otherwise. It is shown that plasma-specific effects drastically shorten and decrease the dispersion of the incubation times for the nucleation of nanowires on non-uniform Au catalyst nanoparticle arrays. The fast nucleation makes it possible to avoid a common problem of small catalyst nanoparticle burying by amorphous silicon. These results explain a multitude of experimental observations on chemically active plasma-assisted Si nanowire growth and can be used for the synthesis of a range of inorganic nanowires for environmental, biomedical, energy conversion, and optoelectronic applications.

Simulation of ion flux distribution in conductive and nonconductive nanotip patterns

The distribution of flux of carbon-bearing cations over nanopatterned surfaces with conductive nanotips and nonconductive nanoislands is simulated using the Monte-Carlo technique. It is shown that the ion current is focused to nanotip surfaces when the negative substrate bias is low and only slightly perturbed at higher substrate biases. In the low-bias case, the mean horizontal ion displacement caused by the nanotip electric field exceeds 10 nm. However, at higher substrate biases, this value reduces down to 2 nm. In the nonconductive nanopattern case, the ion current distribution is highly nonuniform, with distinctive zones of depleted current density around the nanoislands. The simulation results suggest the efficient means to control ion fluxes in plasma-aided nanofabrication of ordered nanopatterns, such as nanotip microemitter structures and quantum dot or nanoparticle arrays. © World Scientific Publishing Company.

Fabrication of metal-nanoparticle-modified semiconducting copper--and silver--TCNQ materials as substrates for the reduction of chromium (VI) using thiosulfate ions at ambient temperature

The formation of readily recoverable and reusable organic semiconducting Cu- and AgTCNQ (TCNQ=7,7,8,8-tetracyanoquinodimethane) microstructures decorated with Pt and Pd metallic nanoparticles is described for the effective reduction of CrVI ions in aqueous solution at room temperature using both formic acid and an environmentally friendly thiosulfate reductant. The M-TCNQ (M=metal) materials were formed by electrocrystallisation onto a glassy carbon surface followed by galvanic replacement in the presence of H2PtCl6 or PdCl2 to form the composite material. It was found that loading of the surface with nanoparticles could easily be controlled by changing the metal salt concentration. Significantly, the M-TCNQ substrates facilitated the formation of well-isolated metal nanoparticles on their surfaces under appropriate galvanic replacement conditions. The semiconductor–metal nanoparticle combination was also found to be critical to the catalyst performance, wherein the best-performing material was CuTCNQ modified by well-isolated Pt nanoparticles with both formic acid and thiosulfate ions as the reductant.

Bacterial kinetics-controlled shape-directed biosynthesis of silver nanoplates using Morganella psychrotolerans

We show for the first time that by controlling the growth kinetics of Morganella psychrotolerans, a silver-resistant psychrophilic bacterium, the shape anisotropy of silver nanoparticles can be achieved. This is particularly important considering that there has been no report that demonstrates a control over shape of Ag nanoparticles by controlling the growth kinetics of bacteria during biological synthesis. Additionally, we have for the first time performed electrochemistry experiments on bacterial cells after exposing them to Ag(+) ions, which provide significant new insights about mechanistic aspects of Ag reduction by bacteria. The possibility to achieve nanoparticle shape control by using a "green" biosynthesis approach is expected to open up new exciting avenues for eco-friendly, large-scale, and economically viable shape-controlled synthesis of nanomaterials.

Aqueous phase synthesis of copper nanoparticles : a link between heavy metal resistance and nanoparticle synthesis ability in bacterial systems

We demonstrate aqueous phase biosynthesis of phase-pure metallic copper nanoparticles (CuNPs) using a silver resistant bacterium Morganella morganii. This is particularly important considering that there has been no report that demonstrates biosynthesis and stabilization of pure copper nanoparticles in the aqueous phase. Electrochemical analysis of bacterial cells exposed to Cu2+ ions provides new insights into the mechanistic aspect of Cu2+ ion reduction within the bacterial cell and indicates a strong link between the silver and copper resistance machinery of bacteria in the context of metal ion reduction. The outcomes of this study take us a step closer towards designing rational strategies for biosynthesis of different metal nanoparticles using microorganisms.

Photocatalysis on supported gold and silver nanoparticles under ultraviolet and visible light irradiation

Studies of the optical properties and catalytic capabilities of noble metal nanoparticles (NPs), such as gold (Au) and silver (Ag), have formed the basis for the very recent fast expansion of the field of green photocatalysis: photocatalysis utilizing visible and ultraviolet light, a major part of the solar spectrum. The reason for this growth is the recognition that the localised surface plasmon resonance (LSPR) effect of Au NPs and Ag NPs can couple the light flux to the conduction electrons of metal NPs, and the excited electrons and enhanced electric fields in close proximity to the NPs can contribute to converting the solar energy to chemical energy by photon-driven photocatalytic reactions. Previously the LSPR effect of noble metal NPs was utilized almost exclusively to improve the performance of semiconductor photocatalysts (for example, TiO2 and Ag halides), but recently, a conceptual breakthrough was made: studies on light driven reactions catalysed by NPs of Au or Ag on photocatalytically inactive supports (insulating solids with a very wide band gap) have demonstrated that these materials are a class of efficient photocatalysts working by mechanisms distinct from those of semiconducting photocatalysts. There are several reasons for the significant photocatalytic activity of Au and Ag NPs. (1) The conduction electrons of the particles gain the irradiation energy, resulting in high energy electrons at the NP surface which is desirable for activating molecules on the particles for chemical reactions. (2) In such a photocatalysis system, both light harvesting and the catalysing reaction take place on the nanoparticle, and so charge transfer between the NPs and support is not a prerequisite. (3) The density of the conduction electrons at the NP surface is much higher than that at the surface of any semiconductor, and these electrons can drive the reactions on the catalysts. (4) The metal NPs have much better affinity than semiconductors to many reactants, especially organic molecules. Recent progress in photocatalysis using Au and Ag NPs on insulator supports is reviewed. We focus on the mechanism differences between insulator and semiconductor-supported Au and Ag NPs when applied in photocatalytic processes, and the influence of important factors, light intensity and wavelength, in particular estimations of light irradiation contribution, by calculating the apparent activation energies of photo reactions and thermal reactions.

Physisorption-induced electron scattering on the surface of carbon-metal core-shell nanowire arrays for hydrogen sensing

Palladium is sputtered on multi-walled carbon nanotube forests to form carbon-metal core-shell nanowire arrays. These hybrid nanostructures exhibited resistive responses when exposed to hydrogen with an excellent baseline recovery at room temperature. The magnitude of the response is shown to be tuneable by an applied voltage. Unlike the charge-transfer mechanism commonly attributed to Pd nanoparticle-decorated carbon nanotubes, this demonstrates that the hydrogen response mechanism of the multi-walled carbon nanotube-Pd core-shell nanostructure is due to the increase in electron scattering induced by physisorption of hydrogen. These hybrid core-shell nanostructures are promising for gas detection in hydrogen storage applications.

Enhancement of optical absorption in thin-film solar cells through the excitation of higher-order nanoparticle plasmon modes

Recent research in the rapidly emerging field of plasmonics has shown the potential to significantly enhance light trapping inside thin-film solar cells by using metallic nanoparticles. In this article it is demonstrated the plasmon enhancement of optical absorption in amorphous silicon solar cells by using silver nanoparticles. Based on the analysis of the higher-order surface plasmon modes, it is shown how spectral positions of the surface plasmons affect the plasmonic enhancement of thin-film solar cells. By using the predictive 3D modeling, we investigate the effect of the higher-order modes on that enhancement. Finally, we suggest how to maximize the light trapping and optical absorption in the thin-film cell by optimizing the nanoparticle array parameters, which in turn can be used to fine tune the corresponding surface plasmon modes.

Carbon saturation of arrays of Ni catalyst nanoparticles of different size and pattern uniformity on a silicon substrate

The kinetics of saturation of Ni catalyst nanoparticle patterns of the three different degrees of order, used as a model for the growth of carbon nanotips on Si, is investigated numerically using a complex model that involves surface diffusion and ion motion equations. It is revealed that Ni catalyst patterns of different degrees of order, with Ni nanoparticle sizes up to 12.5 nm, exhibit different kinetics of saturation with carbon on the Si surface. It is shown that in the cases examined (surface coverage in the range of 1-50%, highly disordered Ni patterns) the relative pattern saturation factor calculated as the ratio of average incubation times for the processes conducted in the neutral and ionized gas environments reaches 14 and 3.4 for Ni nanoparticles of 2.5 and 12.5 nm, respectively. In the highly ordered Ni patterns, the relative pattern saturation factor reaches 3 for nanoparticles of 2.5 nm and 2.1 for nanoparticles of 12.5 nm. Thus, more simultaneous saturation of Ni catalyst nanoparticles of sizes in the range up to 12.5 nm, deposited on the Si substrate, can be achieved in the low-temperature plasma environment than with the neutral gas-based process.

Size-selected Ni catalyst islands for single-walled carbon nanotube arrays

Many properties of single-walled carbon nanotube (SWCNT) arrays are determined by the size and surface coverage of the metal catalyst islands from which they are nucleated. Methods using thermal fragmentation of continuous metal films frequently fail to produce size-uniform islands. Hybrid numerical simulations are used to propose a new approach to controlled self-assembly of Ni islands of the required size and surface coverage using tailored gas-phase generated nanocluster fluxes and adjusted surface temperatures. It is shown that a maximum surface coverage of 0.359 by 0.96-1.02 nm Ni catalyst islands can be achieved at a low surface temperature of 500 K. Optimized growth of Ni catalyst islands can lead to fabrication of size-uniform SWCNT arrays, suitable for numerous nanoelectronic applications. This approach is deterministic and is applicable to a range of nanoassemblies where high surface coverage and island size uniformity are required.