Nageh K. Allam

Photoelectrochemical and water photoelectrolysis properties of ordered TiO2 nanotubes fabricated by Ti anodization in fluoride-free HCl electrolytes

Described is the synthesis of TiO2 nanotube array films by anodization of Ti foil in HCl electrolytes containing different H2O2 concentrations. Highly ordered nanotube arrays up to 860 nm in length, 15 nm inner pore diameter, and 10 nm wall thickness were obtained for one hour anodizations using a 0.5 M HCl aqueous electrolyte containing 0.1–0.5 M H2O2 concentrations for anodization potentials between 10–23 V. The use of ethylene glycol as the electrolyte medium significantly alters the anodization kinetics and resulting film morphologies; nanotube bundles several microns in length achieved for anodization potentials between 8 V and 18 V in only a few minutes. The nanotube arrays obtained from the ethylene glycol electrolytes show relatively higher photocurrents, ≈0.8 mA cm−2 under AM 1.5. Under 100 mW cm−2 AM 1.5 illumination a 500 °C annealed 1 cm2 nanotube array sample, obtained by anodization of a Ti foil sample in ethylene glycol + 0.5 M HCl + 0.4 M H2O2 electrolyte, demonstrates a hydrogen evolution rate of approximately 391 μL h−1 by water photoelectrolysis, time-power normalized evolution rate of 3.9 mL W−1 h−1, with water splitting confirmed by the 2 : 1 ratio of evolved hydrogen to oxygen.

TiO2 Nanotube/CdS Hybrid Electrodes: Extraordinary Enhancement in the Inactivation of Escherichia coli

Titanium dioxide nanotubes offer distinct advantages over films of the same material in the production of hydroxyl radicals and subsequent inactivation of Escherichia coli in wastewater. However, their visible light absorption capabilities are limited. Semiconducting nanocrystals of cadmium sulfide have been used to increase the sensitivity of TiO(2) nanotubes to visible light. A small applied potential, using CdS-coated TiO(2) nanotube arrays, allowed for total inactivation of E. coli in hitherto record short time.

Room Temperature One-Step Polyol Synthesis of Anatase TiO2 Nanotube Arrays: Photoelectrochemical Properties

We describe an optimized anodization process to fabricate anatase TiO2 nanotube arrays at room temperature using a polyol electrolyte. The critical roles of water and anodization voltage are investigated, and mechanistic actions are considered. The as-fabricated electrodes show low efficiency when used as photoanodes to photoelectrochemically split water; however, their efficiency is remarkably enhanced upon their annealing at temperatures as low as 300 degrees C. The efficiency of the annealed samples was found to increase with the anatase content in the as-anodized electrodes, suggesting that these pre-existing crystallites can act as seeding layers that enhance the nucleation and growth of further anatase crystallites.

Enhanced Photoassisted Water Electrolysis Using Vertically Oriented Anodically Fabricated Ti−Nb−Zr−O Mixed Oxide Nanotube Arrays

Self-ordered, highly oriented arrays of titanium-niobium-zirconium mixed oxide nanotube films were fabricated by the anodization of Ti(35)Nb(5)Zr alloy in aqueous and formamide electrolytes containing NH(4)F at room temperature. The nanostructure topology was found to depend on the nature of the electrolyte and the applied voltage. Our results demonstrate the possibility to grow mixed oxide nanotube array films possessing several-micrometer-thick layers by a simple and straightforward electrochemical route. The fabricated Ti-Nb-Zr-O nanotubes showed a ∼17.5% increase in the photoelectrochemical water oxidation efficiency as compared to that measured for pure TiO(2) nanotubes under UV illumination (100 mW/cm(2), 320-400 nm, 1 M KOH). This enhancement could be related to a combination of the effect of the thin wall of the fabricated Ti-Nb-Zr-O nanotubes (10 ± 2 nm) and the formation of Zr oxide and Nb oxide layers on the nanotube surface, which seems to slow down the electron-hole recombination in a way similar to that reported for Grätzel solar cells.

Vertically oriented Ti-Pd mixed oxynitride nanotube arrays for enhanced photoelectrochemical water splitting.

In recent years, considerable efforts have been made to design and discover photoactive nanostructured materials that can be used as anodes in water photoelectrolysis cells. Herein, we report on the growth of a novel photoanode material composed of self-ordered, vertically oriented nanotube arrays of titanium-palladium mixed oxynitride films via anodization of Ti-Pd alloy in an electrolyte solution of formamide containing NH(4)F at room temperature, followed by annealing in an ammonia atmosphere. The nanostructure topology was found to depend on both the anodization time and the applied voltage. Our results demonstrate the ability to grow mixed oxynitride nanotube array films that are several micrometers thick. The Ti-Pd oxynitride nanotube array films were utilized in solar-spectrum water photoelectrolysis, demonstrating a photocurrent density of 1.9 mA/cm(2) and a ∼5-fold increase in the photoconversion efficiency under AM 1.5 illumination (100 mW/cm(2), 1.0 M KOH) compared to pure TiO(2) nanotubes fabricated and tested under the same conditions. The obtained efficiency is among the highest reported values for a TiO(2) nanotube-based photoelectrochemical cell. This enhancement in the photoconversion efficiency is related to the synergistic effects of Pd alloying, nitrogen doping, and the unique structural properties of the fabricated nanotube arrays.

Bacteriorhodopsin/TiO2 nanotube arrays hybrid system for enhanced photoelectrochemical water splitting

In recent years, considerable efforts have been made to improve the performance of photoactive nanostructured materials for water splitting applications. Herein, we report on the assembly and use of a bacteriorhodopsin (bR)/TiO2 nanotube array hybrid electrode system. Photoanode materials composed of ∼ 7μm long self-ordered and vertically oriented nanotube array of titanium dioxide films were fabricated via the anodization of Ti foil in formamide electrolytes containing NH4F at room temperature followed by sensitization of the electrodes with bR. The stability of bR on the TiO2 surface was found to depend on the pretreatment process of the TiO2 films. Our results demonstrate the opportunity to fabricate fairly stable bR/TiO2 hybrid electrodes that can be used as photoanodes for photoelectrochemical water splitting. Under AM 1.5 illumination (100 mW/cm2), the hybrid electrodes achieved a photocurrent density of 0.65 mA/cm2 which is a ∼ 50% increase over that measured for pure TiO2 nanotubes (0.43 mA/cm2) fabricated and tested under the same conditions. In the presence of a redox electrolyte, the photocurrent increased to 0.87 mA/cm2. To the best of our knowledge, this is the first report on the use of bR/TiO2 hybrid electrodes in photoelectrochemical water oxidation cells. We believe the proton pumping property of bR can be used in a variety of applications, especially those related to third generation photovoltaic cells.

Interface Architecture Determined Electrocatalytic Activity of Pt on Vertically Oriented TiO2 Nanotubes

The surface atomic structure and chemical state of Pt is consequential in a variety of surface-intensive devices. Herein we present the direct interrelationship between the growth scheme of Pt films, the resulting atomic and electronic structure of Pt species, and the consequent activity for methanol electro-oxidation in Pt/TiO(2) nanotube hybrid electrodes. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed to relate the observed electrocatalytic activity to the oxidation state and the atomic structure of the deposited Pt species. The atomic structure as well as the oxidation state of the deposited Pt was found to depend on the pretreatment of the TiO(2) nanotube surfaces with electrodeposited Cu. Pt growth through Cu replacement increases Pt dispersion, and a separation of surface Pt atoms beyond a threshold distance from the TiO(2) substrate renders them metallic, rather than cationic. The increased dispersion and the metallic character of Pt results in strongly enhanced electrocatalytic activity toward methanol oxidation. This study points to a general phenomenon whereby the growth scheme and the substrate-to-surface-Pt distance dictates the chemical state of the surface Pt atoms, and thereby, the performance of Pt-based surface-intensive devices.

Nanostructured tantala as a template for enhanced osseointegration

The goal of current dental and orthopedic biomaterials research is to design implants that induce controlled and guided tissue growth, and rapid healing. In addition to acceleration of normal wound healing phenomena, these implants should result in the formation of a characteristic interfacial layer with adequate biomechanical properties. To achieve these goals, however, a better understanding of events at the bone-material interface is needed, as well as the development of new materials and approaches that promote osseointegration. Here we present novel nanostructured nanoarrays from tantala that can promote cell adhesion and differentiation. Our results suggest that tantala nanotube arrays enhance osteoblast cell adhesion, proliferation and differentiation. The routes of fabrication of tantala nanotube arrays are flexible and cost-effective, enabling realization of desired platform topologies on existing non-planar orthopedic implants.

Morphological and structural characterization of single-crystal ZnO nanorod arrays on flexible and non-flexible substrates

Summary We report a facile synthesis of zinc oxide (ZnO) nanorod arrays using an optimized, chemical bath deposition method on glass, PET and Si substrates. The morphological and structural properties of the ZnO nanorod arrays were investigated using various techniques such as field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) measurements, which revealed the formation of dense ZnO nanorods with a single crystal, hexagonal wurtzite structure. The aspect ratio of the single-crystal ZnO nanorods and the growth rate along the (002) direction was found to be sensitive to the substrate type. The lattice constants and the crystallite size of the fabricated ZnO nanorods were calculated based on the XRD data. The obtained results revealed that the increase in the crystallite size is strongly associated with the growth conditions with a minor dependence on the type of substrate. The Raman spectroscopy measurements confirmed the existence of a compressive stress in the fabricated ZnO nanorods. The obtained results illustrated that the growth of high quality, single-crystal ZnO nanorods can be realized by adjusting the synthesis conditions.

Novel MWCNTs/graphene oxide/pyrogallol composite with enhanced sensitivity for biosensing applications

A novel and highly sensitive biosensor employing graphene oxide nano-sheets (GO), multiwalled carbon nanotubes (MWCNTs), and pyrogallol (PG) was fabricated and utilized for the sensitive determination of omeprazole (OME). The morphological and structural features of the composite material were characterized using different techniques. The modified electrode showed a remarkable improvement in the anodic oxidation activity of OME due to the enhancement in the current response compared to the bare carbon paste electrode (CPE). Sensor composition and measurement conditions were optimized using an experimental design. Differential pulse voltammetry (DPVs) exhibited two expanded linear dynamic ranges of 2.0×10-10-6.0×10-6M and 6.0×10-6-1.0×10-4M for OME at pH 7 with a detection limit of 1.02×10-11M. The practical analytical utilities of the modified electrode were demonstrated by the accurate determination of OME in pharmaceutical formulation and human serum samples with mean recoveries of 100.97% and 98.58%, respectively. The results clearly revealed that the proposed sensor is applicable to clinical analysis, quality control and routine determination of drugs in pharmaceutical formulations.