Facile fabrication of nanoporous gold film electrodes

A novel one-step method has been developed for the fabrication of a three-dimensional (3D) nanoporous gold film (NPGF). The NPGF can be facilely made within 1 min from a pure gold substrate by applying a step potential just into the initial transition region of gold in an HCl medium. The pore formation and structural evolution have been revealed by scanning electron microscope, and the processes involve electrodissolution, disproportion, and deposition. The as-prepared 3D NPGF electrode has a large surface area and exhibits high catalytic activity in the electrooxidation of glucose. The NPGF electrode also shows excellent performance toward the electrooxidation of formic acid after being decorated with a tiny amount of Pt by electrodeposition.     

Top-down solid-phase fabrication of nanoporous cadmium oxide architectures

In this article, we have demonstrated one-step solid-phase transformation from high-quality cadmium carbonate microcrystals into highly nanoporous cadmium oxide. The high crystal quality of cadmium carbonate is critical for the successful fabrication of porous nanoarchitectures with predetermined morphology and well-controlled internal structure. This novel strategy has a good potential to prepare nanoporous materials at a large scale by using perfect monolithic carbonate crystals, and it is also useful to synthesize different nanoporous materials on metal-oxide-coated substrates. Meanwhile, this simple thermal transformation of cadmium carbonate into porous structures has further been extended to convert calcium carbonate into such porous structures.     

Nanostructured silicon surfaces via nanoporous alumina

Nanostructured silicon surfaces are generated using nanoporous alumina membranes as stamps to imprint PMMA films on silicon, followed by reactive ion etching (RIE).     

Infrared studies of silicon nitride

The FTS measurements of silicon nitride Si₃N₄ in the range 100–1400 cm⁻¹ have been made. There have been several varieties of IR spectra which can be ascribed to alpha silicon nitride. Tentative explanation of this effect suggesting a complex structure of alpha phase is proposed.     

Covalent biofunctionalization of silicon nitride surfaces

Covalently attached organic monolayers on etched silicon nitride (SixN4; x >/= 3) surfaces were prepared by reaction of SixN4-coated wafers with neat or solutions of 1-alkenes and 1-alkynes in refluxing mesitylene. The surface modification was monitored by measurement of the static water contact angle, XPS, IRRAS, AFM, and ToF-SIMS, and evidence for the formation of Si-C bonds is presented. The etching can be achieved by dilute HF solutions and yields both Si-H and N-H moieties. The resulting etched SixN4 surfaces are functionalized by terminal carboxylic acid groups in either of two ways: (a) via attachment of a 10-undecenoic acid 2,2,2-trifluoroethyl ester (trifluoro ethanol ester) and subsequent thermal acid hydrolysis; (b) through attachment of a photocleavable ester, and subsequent photochemical cleavage, as this would allow photopatterned functionalized SixN4. The carboxylic acids are successfully used for the attachment of oligopeptides (aspartame) and complete proteins using EDC/NHS chemistry. Finally, an amino-terminated organic monolayer can be formed by reaction of HF-treated SixN4 surfaces with a N-(omega-undecylenyl)phthalimide, which yields an amino-terminated surface upon deprotection with hydrazine.     

Oxidation kinetics of hot-pressed silicon nitride

A simplified model for evaluation of hot-pressed silicon nitride (HPSN) has been derived which accounts for the amount and type of grain boundary phase and for the composition at the Si₃N₄/oxide reaction interface. The model proved suitable for explaining the oxidation behaviour of a wide range of materials including MgO, CeO₂, (CeO₂ + SiO₂), (Y₂O₃ + SiO₂), (MgO + Y₂O₃) additives. Some deductions are also possible on how to improve the oxidation resistance of HPSN.     

Tailor-made functionalization of silicon nitride surfaces

This communication presents the first functionalization of a hydrogen-terminated silicon-rich silicon nitride (Si3Nx) surface with a well-defined, covalently attached organic monolayer. Properties of the resulting monolayers are monitored by measurement of the static water contact angle, X-ray photoelectron spectroscopy (XPS), and infrared reflection absorption spectroscopy (IRRAS). Further functionalization was performed by reaction of Si3Nx with a trifluoroethanol ester alkene (CH2=CH-(CH2)8CO2CH2CF3) followed by basic hydrolysis to afford the corresponding carboxylic acid-terminated monolayer with hydrophilic properties. These results show that Si3Nx can be functionalized with a tailor-made organic monolayer, has highly tunable wetting properties, and displays significant potential for further functionalization.     

Porogen approach for the fabrication of plasma-polymerized nanoporous polysiloxane films

Nanoporous polysiloxane films were fabricated by plasma polymerization of hexamethyldisiloxane mixed with cyclohexane under different conditions. The pores were generated through the elimination of carbonaceous aggregates (porogen) by annealing at 600 degrees C. Results of spectroscopic ellipsometry, Fourier transform infrared spectroscopy, and positron annihilation lifetime spectroscopy suggest that not only film porosity but also average pore size depends on the amount of the decomposable porogen. The pore size was controllable in a range between 0.6 and 1.0 nm in radius by proper selection of the substrate temperature and precursor composition.     

Electrocatalytic oxidation of glucose on nanoporous gold membranes

With characteristic of structural integrity and high surface area, nanoporous gold (NPG) prepared by dealloying method is proposed to be a highly sensitive catalyst for glucose electrooxidation. It can be found that a-NPG which obtained by electrochemical corrosion method has the highest sensitivity for glucose electrooxidation among the three studied samples. Under alkaline conditions, the catalytic current density of a-NPG is over 1.5 times and 17 times higher than that of f-NPG (prepared by free corrosion) and poly-Au electrode, respectively. Using a-NPG sample for glucose detection, the obtained minimum sensible concentration are 413 nM in alkaline media and 1 μM in neutral solutions. The a-NPG electrode also shows stable recovery and reproducibility characteristics. These results indicate that NPG may work as an efficient electrode material for electrochemical sensors and a promising catalyst for alkaline glucose fuel cells.     

Room-temperature ferromagnetism in silicon oxide/silicon nitride composite films

Room-temperature ferromagnetism has been observed in silicon oxide/silicon nitride composite films formed on Si substrates at different substrate temperatures, and the ferromagnetic properties of the samples have been found to depend on the silicon nitride content of the films. It is proposed that the ferromagnetism is related to the interface states between the silicon oxide particles and silicon nitride particles. The saturation magnetization (M_S) reached its maximum value in the film produced at a substrate temperature of 400 °C. A further study on the magnetic properties of the film has been carried out using first-principles calculations based on the density functional theory. The calculations suggest that the magnetic moments of the film originate from N 2p and Si 2p states in the vicinity of the hetoro-interface.     

Biological responses to silicon and nitrogen-rich PVD silicon nitride coatings

Most structural bioceramics are comprised of metallic oxides such as alumina and zirconia. They are generally considered to be completely bioinert, but a non-oxide ceramic, silicon nitride, achieves equivalent levels of mechanical reliability while being bioactive. Silicon nitride can not only stimulate cellular proliferation but it is also antipathogenic with demonstrated efficacy against Gram-positive and Gram-negative bacteria, fungi, and viruses. In this work, three physical vapor deposition coatings with different Si:N ratios (silicon-rich, stoichiometric, and nitrogen-rich) were deposited on mirror-polished silica glass substrates. The coatings were characterized by spectroscopic and microscopic techniques and tested in vitro against E. coli and KUSA-A1 mesenchymal cells. Results showed that nitrogen-enriched Si_xN_y has a strong antibacterial effect against E. coli and contributes to cellular proliferation while silicon-enriched Si_xN_y stimulates the production of bone tissue, with higher indexes for mineralization and quality. These results suggest that Si_xN_y's biological properties can be optimized for specific applications by carefully tuning its surface chemistry.     

Solid-state synthesis of luminescent silicon nitride nanocrystals

Silicon nitride nanocrystals (NCs) have been prepared via in situ nitridation of magnesium followed by a metathesis reaction with sol-gel derived silica particles. Highly luminescent, freestanding β-Si(3)N(4) NCs with complex surface chemistry dominated by Si-H and N-H moieties were isolated upon etching with hydrofluoric acid.     

Cell membranes suspended across nanoaperture arrays

We present a method for spreading large (>100 microm(2)) cell membrane fragments across nanoapertures in planar supports. Electron-beam and focused-ion-beam lithography were used to fabricate arrays of 50-600 nm diameter holes in free-standing silicon nitride (SiN) solid films 100-500 nm thick. By pressing adhering live cells onto the nanostructured SiN surface and then removing them, planar cell membrane sheets (CMSs) were transferred in a well-defined orientation onto the SiN support. We demonstrate the accessibility to both extracellular and intracellular surfaces of CMSs by targeting membrane constituents side-specifically with fluorescent markers. Our approach is of interest for studying ligand-receptor interactions using optical, electrical, and scanning probe techniques at the single-molecule level.     

Chemically modified opals as thin permselective nanoporous membranes

Thin-film opals comprising three layers of 440 nm diameter SiO2 spheres were assembled on Pt electrodes and modified with amino groups on the silica surface. Diffusion of anionic, cationic, and neutral redox species through the opals was studied by cyclic voltammetry. The chemically modified opal membranes demonstrate high molecular throughput and, at low pH, selectively block transport of a cationic redox species relative to that of anionic and neutral redox species. This permselective behavior is attributed to the electrostatic interactions that are enhanced by the tortuous pathway within the opal and by the high surface area of the chemically modified spheres.     

Ionic transport across tailored nanoporous anodic alumina membranes

Monodispersed silica particles with bimodal size distribution were successfully prepared through adding an ethanol (EtOH) solution containing tetraethylorthosilicate (TEOS) dropwise into an ammonia EtOH solution at a constant low rate. The effects of the reaction parameters such as ammonia/ethanol ratio, feeding rate of TEOS solution, reaction temperature, and time on the size and size distribution of the as-obtained particles were investigated. Based on these phenomena, a modified LaMer model of nucleation and growth mechanism was proposed to reasonably explain the formation of the as-obtained silica particles with bimodal size distribution. The as-prepared monodispersed silica particles with bimodal size distribution can be directly fabricated into binary colloidal crystals with small particles surrounding large particles by evaporation-induced cooperative self-assembly. This suggests that the method reported here provides a straightforward and effective route to the in situ fabrication of novel binary colloidal crystals and their replicated patterns in one reaction system.     

Kinetics and mechanism of oxidation of silicon nitride bonded silicon carbide ceramic

Thermogravimetry (TG) has been used to study the oxidation of a commercial silicon nitride bonded silicon carbide (SNBSC) ceramic. The oxidation was studied in air and carbon dioxide atmospheres between 800 and 1300°C. TG/mass spectrometry (MS) shows that the silicon nitride bonding phase oxidises first. The kinetics follow a multi-stage mechanism with diffusion control. Carbon dioxide was found to be a more powerful oxidant than air at temperatures above 1050°C.     

Thermodynamic analysis of the high-temperature stability of silicon nitride and silicon carbide

A thermodynamic analysis of the stability of Si₃N₄ and SiC is presented which can be employed to assess their suitability for use at high temperatures in various environments. Specifically, the decomposition and the volatilization of Si₃N₄ and SiC, and of SiO₂, which is the major constituent in the oxide scales formed on their surfaces in oxidizing atmospheres, are discussed in terms of ambient environment and temperature. The calculated values of the lowest oxygen partial pressure up to which a protective SiO₂ layer should be maintained on the surface of Si₃N₄ and SiC are also presented.