Stabilization and Decomposition of Organic Matters by Nano-porous Metals

AbstractNanoporous (np-) metals affect chemical stability of various organic matters much more than their bulk counterparts. Self-assembly monolayers (SAMs) of 4-aminothiophenol are more stable on np-Au fabricated by dealloying Ag-Au alloy than on flat Au, which were elucidated by cyclic voltammetry (CV). The first-principles calculations indicate that atomic defect of Au surface, which is characteristic of ligament surface of np-Au, reduce the binding energy of thiol molecule.The stabilized SAMs on np-Au are also effective for enzyme immobilization. When laccase was immobilized on np-Au surface, its thermal stability was improved compared with nonfixed laccase. The higher stability of immobilized laccase is due to the synergistic effect of reduced conformational flexibility of the enzyme in nanopores and SAMs stability. CV with the working electrode of enzyme-immobilized np-Au revealed that the electron transfer between enzyme and electrode is successfully enhanced by SAMs. These aspects of enzyme-immobilized and SAMs-decorated np-Au can be applied to the electrodes of biofuel cell.Np-metals such as np-Au, np-Pd and np-Ni, on the other hand, remarkably decompose methyl orange (MO, a typical stable azo dye in the textile industry) in an aqueous solution, while MO is highly stable in aqueous solutions which include their bulk counterparts. Ultraviolet-visible light spectroscopy and high-performance liquid chromatography demonstrated that np-Au breaks the azo bond (–N=N–) in MO molecules. Defective surface of ligaments in np-metals plays an important role in this catalytic decomposition of MO

Fabrication of carbon nanotube/NiOx(OH)y nanocomposite by pulsed electrodeposition for supercapacitor applications

Nanocomposite films based on carbon nanotubes (CNTs)/nickel (Ni)-species were fabricated by electrophoretic deposition (EPD) of CNT, followed by pulsed electrodeposition of Ni and annealing in air. NiO/Ni(OH)2 particles with 7 ± 4 nm diameters were uniformly dispersed on the CNT networks. Cyclic voltammetry revealed that the CNT/NiOx(OH)y nanocomposite films had high specific capacitances of 1451, 1158, 998, 896 and 851 F g^{−1} per mass of active Ni species at 1, 10, 30, 50 and 100 mV s^{−1}, respectively, in the potential window of 0.4 V. An annealing time of 1 h (at 523 K) enhanced the pseudocapacity of deposited Ni species by the dehydration of Ni(OH)2 to form NiO. Longer annealing time reduced the capacitance because of sintering and subsequent decrease in surface area of Ni deposits. The capacitance of the fabricated CNT/NiOx(OH)y nanocomposite was maintained after 20, 000 cycles of potential sweeping

Oxygen reduction on bimodal nanoporous palladium-copper catalyst synthesized using sacrificial nanoporous copper

Nanoporous copper (NP-Cu), as a sacrificial support, was used for the synthesis of bimodal nanoporous palladium–copper (BNP-PdCu) for oxygen reduction reaction (ORR) electrodes in fuel cells. The catalytic performance of BNP-PdCu in ORR per electrochemical surface area was enhanced by the dissolution and removal of supporting NP-Cu, which indicates that the intrinsic catalytic properties of palladium are improved by the proposed synthesis strategy including galvanic replacement of copper with palladium, following copper dissolution. Cu remained on Pd surfaces even after dissolution of Cu. Additionally, significant local lattice contraction was observed at the ligament surface. First-principles calculations on the adsorbing oxygen species on Pd show that both lattice contraction and alloying with copper increase the binding energies of oxygen species to the Pd surface. The high ORR activity of the present BNP-PdCu is suggested to be mainly due to the Cu-ligand effect

Antibacterial activity of ultrathin platinum islands on flat gold against Escherichia coli

Nanoporous Au exhibits high antibacterial activity (AA) without releasing reactive oxygen species or metal ions, instead its AA depends on the work function (WF) because cell walls are affected by peculiar electronic states at the surface. Based on this mechanism, a flat surface without nanostructure should show high AA if the WF of the surface is suitably tuned. To verify this, ultrathin Pt islands with high WF was fabricated on flat Au by underpotential deposition (UPD) of copper and subsequent redox replacement with Pt, and the AA of the Pt/Au substrate on Escherichia coli was evaluated. The Pt/Au substrate showed higher AA than Pt and Au surfaces, and a positive relationship between AA and WF was demonstrated. In addition, first principles calculations were performed to investigate the mechanism for the high WF of the Pt/Au substrate. The findings suggest that the high WF of the Pt/Au substrate is at least partly due to charge transfer from Au to Pt

Electrochemical actuation of nanoporous Ni in NaOH solution

The fabrication of a bilayer-stacked Ni electrochemical actuator is reported. The stacked bilayer sheet composed of bulk and nanoporous Ni deformed when a potential of ± 1 V was applied in aqueous NaOH solution, which is the first report of nanoporous Ni exhibiting actuation characteristics. Actuation is considered to have a strong relationship with the electrical double layer, because cyclic voltammetric measurements suggested that the effect of oxygen adsorption on the surface was minor. The results suggest that nanoporous Ni can be used as an actuator, and it has potential in being applied for commercial use because of its low price and high availability

Inactivation of HeLa cells on nanoporous gold

Nanoporous metals strongly affect organic matter; however, there is a poor understanding of their effects on cells. The present work shows that HeLa cells on nanoporous gold (NPG) were less active than those on flat gold (FG) with no nanoporous structure. Initially, HeLa cells adhered to the NPG over a period of more than 10 h, then the adhered cells subsequently exhibited apoptosis that was not related to anoikis. ELISA analyses showed that the conformational change of fibronectin was more greatly induced by NPG than FG. First-principles calculations and molecular dynamics simulations were performed to investigate the conformational change in the RGD sequence and the integrin signaling. The simulations suggested that the extended form of integrin, with an open headpiece, was not generated owing to the conformational change of RGD, and the outside-in signals could not be intracellularly transmitted via the integrin binding to the fibronectin on NPG, resulting in cell death

Tensile Properties of Forged Mg-Al-Zn-Ca Alloy

Continuously-casted Mg-9Al-1Zn-1Ca (in mass%) alloy (Mg-Ca alloy) and Mg-9Al-1Zn alloys (Ca-free Mg alloy) were forged at 573 K and their mechanical properties were investigated by tension tests at ambient temperature and 573 K. The forged Mg-Ca alloy showed higher 0.2% proof stress than the forged Ca-free Mg alloy. The high strength for the Mg-Ca alloy was attributed not only to grain refinement by hot forging, but also to the strengthening mechanisms arising from the difference in thermal expansion and geometrical incompatibility between Mg matrix and second phase. The Ca addition decreased the elongation to failure; however, the decrease was reduced for the forged specimens, compared to the unforged specimen. This results from segmentation of the second phases by the hot forging. Also, the forged Mg-Ca alloy showed a large elongation of 284% at 573 K

Mechanical characterization of nanoporous Au modified with self-assembled monolayers

The surface of nanoporous Au was modified with self-assembled monolayers (SAMs) of 6-mercapto-1-hexanol and the hardness tests were performed on the SAM-modified and non-modified nanoporous Au to investigate the effects of SAM modification on the mechanical properties of nanoporous Au. In addition, the origin of the chemomechanical effects was investigated by first principles shear test simulations on an Au–S alloy. The SAM-modified nanoporous Au showed lower hardness than the non-modified nanoporous Au. The loading rate dependence tests showed that the activation volume was low for both, indicating that events of a short range play an important role in deformation of nanoporous Au, regardless of whether the nanoporous Au was modified with SAMs. It was suggested from the simulations that the lower hardness for the SAM-modified nanoporous Au is because movement of dislocation endpoints at the surface is facilitated by chemical effects of Au–S bonding

Effects of Homogenization Annealing on Dynamic Recrystallization

Compression tests were conducted at the temperature of 573 K with the true strain rates of 10 À3 -1 s À1 on as-cast and homogenized Mg6Al-2Ca-2RE (RE = rare earth) (in mass%) alloy specimens, and their dynamic recrystallization (DRX) behaviors were investigated. Strain hardening occurred after yielding, followed by strain softening. The flow stress of the as-cast specimen was higher than that of the homogenized specimen. The DRX grain size depended minimally on the Z-parameter in both of the as-cast and homogenized specimens. This is likely to be due to the particle-stimulated nucleation mechanism involving the second-phase particles. When the specimens were deformed to the true compressive strain of 1.6, non-recrystallized regions were not observed in the homogenized specimen; however, they were partially observed in the as-cast specimen. The grain size in the recrystallized region in the as-cast specimen was smaller than that in the homogenized specimen. Elemental analyses revealed Al segregation around the second-phase particles in the as-cast specimen. Therefore, it is suggested that DRX in the present Mg-Al-Ca-RE alloy is affected by not only the second-phase particles, but also the Al segregation

Fabrication, Microstructure, and Properties of Nanoporous Pd, Ni, and Their Alloys by Dealloying

Nanoporous metals can be fabricated by dealloying, which is one of the reactions that occur during the corrosion of alloys. Nanoporous gold has been widely investigated for several decades, and it has recently been found that other metals, such as platinum, palladium, nickel, and copper, can form nanoporous structures through the dealloying of binary alloys. This article mainly shows fabrication and properties of nanoporous palladium and nickel after introduction of nanoporous metals by referring to nanoporous gold as an example. It is necessary to select binary alloys with suitable elements, in which the dissolution of the less noble element and the aggregation of the nobler element at the solid/electrolyte interface are simultaneously allowed. Postprocessing by thermal or acid treatment alters the nanoporous structure. Various properties of nanoporous metals (including mechanical, catalytic, piezoelectric, hydrogenation, and magnetic ones) are different from those of bulk and nanocrystalline materials and nanoparticles because of their specific three-dimensional network structures consisting of nanosized pores and ligaments. Hydrogenation and magnetic properties are reviewed in terms of lattice strain at curved surfaces. These new metallic nanomaterials are now being investigated from the viewpoint of functional applications, and provide much room for study in various fields