Three-dimensional gold nanowires with high specific surface area for simultaneous detection of heavy metal ions.

Traditional detection methods to detect heavy metal ions are time-consuming, complicated, and expensive. Here, we developed a simple electroless plating method to prepare three-dimensional gold nanowire (Au NW) films with high specific surface area. In an aqueous plating bath, tetrachloroauric acid, 4-dimethylaminopyridine and formaldehyde are used as precursor, ligand, and reducing agent, respectively. An electrochemical sensor based on a Au NWs/SPE could be applied for simultaneous detection of lead (Pb(II)), arsenic (As(III)), and mercury (Hg(II)) ions. The detection limits of Pb(II), As(III), and Hg(II) are 2.6, 1.5, and 4.2 µg L-1, all lower than the permissible limits of the WHO for drinking water (the permissible level of Pb(II) and As(III) is 10.0 µg L-1, and the permissible level of Hg(II) is 6.0 µg L-1), respectively. This work presents a simple and novel method to prepare gold nanowires for quick detection of trace heavy metal ions.

Observation of a novel double layer surface oxide phase on Ni3Al(111) at low temperature.

The Ni3Al(111) surface was characterized during oxidation within the temperature range of 690-800 K by in situ scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and auger electron spectroscopy (AES). Within this temperature range oxygen dosing always leads to the formation of a surface aluminum oxide layer while Ni atoms remain in their metallic state. The temperature however, affects the kinetics and the structure of the grown oxides. Above 790 K the known (√67 × âˆš67)R12.2° double layer oxide grows, which consists of two Al-O layers. Oxygen dosing at the lower temperature of 740 ± 10 K leads to a single layer oxide with only one Al-O plane. The lattice mismatch of the aligned oxygen and substrate lattices induce a (7 × 7) moiré pattern of this surface phase. Surprisingly, when lowering the sample temperature below 720 K during oxygen exposure, again a bilayer oxide grows on the Ni3Al(111) surface. The formation of this bilayer oxide starts with the growth of the single layer oxide that is subsequently covered by a second Al-O layer. At temperatures close to 720 K, the 2nd layer is ordered and a diffraction pattern is observed indicating a (4√3 × 4√3)R30° unit cell with regard to the oxygen lattice of the surface oxide. A structure model is presented that relates this so far unknown double layer oxide to the building principle of α-Al2O3. The respective growth kinetics and the availability of Al atoms dictate whether the single or the low temperature double layer oxide is formed. The related mass transport on the surface can be observed using in situ STM which allows the qualitative discussion of the growth kinetics. When lowering the oxide growth temperature below 700 K, the 2nd oxide layer is still formed ontop of the single layer oxide but in a disordered state so that the LEED pattern of the single layer oxide with a (7 × 7) moiré unit cell is again observed. This accounts for the confusing fact that the (7 × 7) moiré LEED pattern may indicate either the formation of a single or a low temperature double layer oxide.

Imaging the confined surface oxidation of Ni3Al(111) by in situ high temperature scanning tunneling microscopy.

The initial oxidation of Ni3Al(111) was imaged by in situ scanning tunneling microscopy (STM) at 700-750 K. At 740 K ± 10 K a moiré structure is formed as the major surface phase: high resolution STM data atomically resolve a top hexagonal lattice with a lattice constant of 2.93 ± 0.01 Å aligned or slightly rotated with respect to the substrate. Auger electron spectra acquired from the surface phase identify Al atoms in an oxidic environment together with Ni atoms unaffected by the oxidation of the Ni3Al(111) surface. A special mass balance analysis applied to STM images recorded during formation of the moiré structure allowed to extract the metal content of the surface phase. The moiré phase can be attributed to a single O/Al double layer of α-Al2O3 ontop of the Ni3Al(111) crystal. The surface double layer is laterally expanded by ∼7% with respect to α-Al2O3 and, relating to the next nearest neighbor distance of the substrate of 2.52 Å, it contains 0.73 ML oxygen and 0.49 ML aluminium atoms. The building principle of the surface phase is almost identical to the one of the reported Oi/Ali interface layer of the so called surface oxide, except for its rotational alignment with respect to the substrate as shown in a careful moiré analysis. It could be shown that this thinnest possible surface aluminum oxide layer is formed due to kinetic restrictions: the oxide grows within the first layer of the Ni3Al(111) surface ejecting 0.5 ML surface metal atoms, which are then converted into the surface oxide laterally separated at the ascending step edge of the same terrace. While the formation of the surface oxide is kinetically hindered most likely by the availability of Al adatoms, all rearrangement processes required for the surface oxide formation on each terrace are not rate limiting as identified by in situ STM. Instead, the local oxide growth rather follows the kinetics driven by the adsorption probability of the impinging oxygen molecules and provides the possibility to entirely cover whole Ni3Al(111) surface.

Controllable Synthesis of [11-2-2] Faceted InN Nanopyramids on ZnO for Photoelectrochemical Water Splitting.

Indium nitride (InN) is one of the promising narrow band gap semiconductors for utilizing solar energy in photoelectrochemical (PEC) water splitting. However, its widespread application is still hindered by the difficulties in growing high-quality InN samples. Here, high-quality InN nanopyramid arrays are synthesized via epitaxial growth on ZnO single-crystals. The as-prepared InN nanopyramids have well-defined exposed facets of [0001], [11-2-2], [1-212], and [-2112], which provide a possible routine for understanding water oxidation processes on the different facets of nanostructures in nanoscale. First-principles density functional calculations reveal that the nonpolar [11-2-2] face has the highest catalytic activity for water oxidation. PEC investigations demonstrate that the band positions of the InN nanopyramids are strongly altered by the ZnO substrate and a heterogeneous n-n junction is naturally formed at the InN/ZnO interface. The formation of the n-n junction and the built-in electric field is ascribed to the efficient separation of the photogenerated electron-hole pairs and the good PEC performance of the InN/ZnO. The InN/ZnO shows good photostability and the hydrogen evolution is about 0.56 µmol cm-2 h-1 , which is about 30 times higher than that of the ZnO substrate. This study demonstrates the potential application of the InN/ZnO photoanodes for PEC water splitting.

Self-Organized Growth of Two-Dimensional GaTe Nanosheet on ZnO Nanowires for Heterojunctional Water Splitting Applications.

Epitaxial two-dimensional GaTe nanosheets on ZnO nanowires were routinely prepared via a two-step chemical vapor deposition procedure. The epitaxial relationship and growth mechanism of the GaTe/ZnO core/shell structures were explored and attributed to a layer-overlayer model. The hybrid structures increased the surface area and the favorable p-n heterojunction enhanced the charge separation for photoelectrochemical performance in water splitting. The above synergistic effects boosted the photocurrent density from -0.3 mA cm-2 for the pristine ZnO nanowires to -2.5 mA cm-2 for the core/shell GaTe/ZnO nanowires at -0.39 V vs RHE under the visible light irradiation. This highlights the promise for utilization of GaTe nanosheet/ZnO nanowires as efficient photoelectrocatalyst for water splitting.

Electrochemical machining of gold microstructures in LiCl/dimethyl sulfoxide.

LiCl/dimethyl sulfoxide (DMSO) electrolytes were applied for the electrochemical micromachining of Au. Upon the application of short potential pulses in the nanosecond range to a small carbon-fiber electrode, three-dimensional microstructures with high aspect ratios were fabricated. We achieved machining resolutions down to about 100 nm. In order to find appropriate machining parameters, that is, tool and workpiece rest potentials, the electrochemical behavior of Au in LiCl/DMSO solutions with and without addition of water was studied by cyclic voltammetry. In waterless electrolyte Au dissolves predominantly as Au(I), whereas upon the addition of water the formation of Au(III) becomes increasingly important. Because of the low conductivity of LiCl/DMSO compared with aqueous electrolytes, high machining precision is obtained with moderately short pulses. Furthermore, the redeposition of dissolved Au can be effectively avoided, since Au dissolution in LiCl/DMSO is highly irreversible. Both observations render LiCl/DMSO an appropriate electrolyte for the routine electrochemical micromachining of Au.