Hany A. El-Sayed

Controlled Interconversion of Nanoarray of Ta Dimples and High Aspect Ratio Ta Oxide Nanotubes

We report the controlled formation of either high-aspect-ratio Ta(2)O(5) nanotubes or an organized nanoarray of Ta dimples by Ta anodization in a single H(2)SO(4) + HF solution. Dimpled Ta is the stable surface morphology in the first few seconds, followed by the growth of dense and fully vertically aligned Ta(2)O(5) nanotubes (up to 2.5 microm long). After 2 min, the dimpled surface morphology reappears, related to the build-up of a resistive Ta fluoride surface layer.

Formation of Dimpled Tantalum Surfaces from Electropolishing

Electropolishing of tantalum in concentrated acids has recently been reported to result in highly ordered arrays of nanoscale dimples. In this paper we study in more detail the electrochemical conditions affecting the formation of regularly dimpled surfaces on tantalum. Solution aging and anodization potential were found to play important roles. The chronoamperometric curves at different anodization potentials give some insight into the changing balance of processes leading to dimpling during electropolishing. We have found that dimpled tantalum can be reproducibly prepared within the anodization potential range of 10-20 V. Higher voltages lead to anisotropic etching of the Ta grains and the resulting surface nanostructure is no longer ordered. Overall, the dimple-formation process can be said to be robust and tunable, with great promise for nanotemplating applications.

Versatile fabrication of self-assembled metallic nanoparticle arrays

One of the challenging aspects of nanotechnology is the development of an effective and potentially universal method to place nanoparticles (NPs) into spatially well-defined, ordered, defect-free arrays. This can be achieved using “top-down” approaches, such as optical, electron beam, focused ion-beam and scanning probe lithography, or “bottom-up” approaches based on self-assembly. Here, we report the simple and rapid electrochemical generation of periodic surface defects, used to fabricate metallic NP arrays having good feature size and spacing control over a large area, without involving costly and time-consuming nanolithographic methods. Our high-throughput nanofabrication approach combines electrochemical anodization to quickly and reproducibly form a highly ordered Ta-based nanotemplate, in the form of inverted hemispherical caps (dimples), with the simplicity of thin metallic film dewetting techniques, forming a self-assembled metallic (individual metals or alloy) NP array. These can be used as nanoelectrode arrays that may have useful applications in analytical chemistry, biosensing, and electrocatalysis.

The impact of fabrication conditions on the quality of Au nanoparticle arrays on dimpled Ta templates

Highly ordered dimpled Ta (DT) nanotemplates, prepared by electrochemical anodization of Ta, were recently reported to be ideally suited for the fabrication of a Au nanoparticle (NP) array using a Au thin film dewetting method. Here, we provide guidance and understanding of the effect of the DT fabrication and Au film deposition steps on the characteristics of the resulting NP array. Specifically, the optimum anodization time, voltage and solution composition are established, and the thickness of the sputter-deposited metal film is shown to be a very important parameter in achieving the desired single Au NP per dimple. The resulting high quality Au NP arrays are demonstrated to be electrochemically addressable, with the total Au surface area, measured electrochemically for large-scale samples, agreeing with the calculated area, based on scanning electron microscope determination of average particle shape and distribution. As the NP formation process proceeds via confined thin film dewetting, the protocol developed here should be applicable to the formation of NP arrays of a range of other metals and alloys.

A method for the formation of Pt metal nanoparticle arrays using nanosecond pulsed laser dewetting

Nanosecond pulsed laser dewetting of Pt thin films, deposited on a dimpled Ta (DT) surface, has been studied here in order to form ordered Pt nanoparticle (NP) arrays. The DT substrate was fabricated via a simple electrochemical anodization process in a highly concentrated H2SO4 and HF solution. Pt thin films (3–5 nm) were sputter coated on DT and then dewetted under vacuum to generate NPs using a 355 nm laser radiation (6–9 ns, 10 Hz). The threshold laser fluence to fully dewet a 3.5 nm thick Pt film was determined to be 300 mJ/cm2. Our experiments have shown that shorter irradiation times (≤60 s) produce smaller nanoparticles with more uniform sizes, while longer times (>60 s) give large nanoparticles with wider size distributions. The optimum laser irradiation time of 1 s (10 pulses) has led to the formation of highly ordered Pt nanoparticle arrays with an average nanoparticle size of 26 ± 3 nm with no substrate deformation. At the optimum condition of 1 s and 500 mJ/cm2, as many as 85% of the dewetted...

Silver Nanoparticles-Decorated Titanium Oxynitride Nanotube Arrays for Enhanced Solar Fuel Generation

We demonstrate, for the first time, the synthesis of highly ordered titanium oxynitride nanotube arrays sensitized with Ag nanoparticles (Ag/TiON) as an attractive class of materials for visible-light-driven water splitting. The nanostructure topology of TiO2, TiON and Ag/TiON was investigated using FESEM and TEM. The X-ray photoelectron spectroscopy (XPS) and the energy dispersive X-ray spectroscopy (EDS) analyses confirm the formation of the oxynitride structure. Upon their use to split water photoelectrochemically under AM 1.5 G illumination (100 mW/cm2, 0.1 M KOH), the titanium oxynitride nanotube array films showed significant increase in the photocurrent (6 mA/cm2) compared to the TiO2 nanotubes counterpart (0.15 mA/cm2). Moreover, decorating the TiON nanotubes with Ag nanoparticles (13 ± 2 nm in size) resulted in exceptionally high photocurrent reaching 14 mA/cm2 at 1.0 VSCE. This enhancement in the photocurrent is related to the synergistic effects of Ag decoration, nitrogen doping, and the unique structural properties of the fabricated nanotube arrays.

Ionic Conductivity Measurements—A Powerful Tool for Monitoring Polyol Reduction Reactions

The reduction of metal precursors during the polyol synthesis of metal nanoparticles was monitored by ex situ ionic conductivity measurements. Using commonly used platinum precursors (K2PtCl6, H2PtCl6, and K2PtCl4) as well as iridium and ruthenium precursors (IrCl3 and RuCl3), we demonstrate that their reduction in ethylene glycol at elevated temperatures is accompanied by a predictable change in ionic conductivity, enabling a precise quantification of the onset temperature for their reduction. This method also allows detecting the onset temperature for the further reaction of ethylene glycol with HCl produced by the reduction of chloride-containing metal precursors (at ≈120 °C). On the basis of these findings, we show that the conversion of the metal precursor to reduced metal atoms/clusters can be precisely quantified, if the reaction occurs below 120 °C, which also enables a distinction between the stages of metal particle nucleation and growth. The latter is demonstrated by the reduction of H2PtCl6 in ethylene glycol, comparing ionic conductivity measurements with transmission electron microscopy analysis. In summary, ionic conductivity measurements are a simple and straightforward tool to quantify the reduction kinetics of commonly used metal precursors in the polyol synthesis.