Nasser Y. Mostafa

In vitro surface biocompatibility of high-content silicon-substituted calcium phosphate ceramics

AbstractThe present work investigates surface biocompatibility of silicon-substituted calcium phosphate ceramics. Different silicon-substituted calcium phosphate ceramic bodies were prepared from co-precipitated powders by sintering at 1300°C. The in vitro bioactivity of the ceramics was assessed in simulated body fluid (SBF) at 37°C for periods up to 4 weeks. The changes in the surface morphology and composition were determined by scanning electron microscopy (SEM) coupled with electron probe microanalysis and energy dispersive spectrometer (EDX). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to observe the change in ionic concentration of SBF after removal of the samples. The bioactivity of the ceramics increased with an increasing silicate ion substitution in a systematic way. The surface of ceramics with 2.23% silicon substitution was partially covered with apatite layer after one week, while ceramics with 8.1% silicon substitution were completely covered with apatite in the first week. The porous microstructure of high-concentration Si-substituted ceramics helps the dissolution of surface ions and the leaching process. This allows SBF to reach supersaturation in a short time and accelerate the deposition of apatite layer.

Aluminum Titania Nanoparticle Composites as Nonprecious Catalysts for Efficient Electrochemical Generation of H2

In this paper, we demonstrated, for the first time, aluminum titania nanoparticle (Al-TiO2 NP) composites with variable amounts of TiO2 NPs as nonprecious active catalysts for the electrochemical generation of H2. These materials were synthesized by mixing desired amounts of hydrogen titanate nanotubes (TNTs), fabricated here by a cost-effective approach at moderate hydrothermal conditions, with aluminum powder (purity 99.7%; size 35 μm). The mixture was compacted under an applied uniaxial stress of 300 MPa followed by sintering at 500 °C for 1 h. After sintering had been completed, all TNTs were found to convert to TiO2 NPs (average particle size 15 nm). Finally, Al-xTiO2 NP nanocomposites (x = 1, 3, 5, and 10) were obtained and characterized by scanning electron microscopy/energy-dispersive X-ray, X-ray diffraction, and X-ray photoelectron spectroscopy. The hydrogen evolution reaction (HER) activity of these materials was studied in 0.5 M H2SO4 at 298 K using polarization and impedance measurements. The nanocomposite of chemical composition Al-5% TiO2 NPs showed the best catalytic performance for the HER, with an onset potential (EHER), a Tafel slope (βc), and an exchange current density (j0) of -100 mV (RHE), 59.8 mV decade(-1), and 0.14 mA cm(-2), respectively. This HER activity is not far from that of the commercial platinum/carbon catalyst (EHER = 0.0 mV, βc = 31 mV dec(-1), and j0 = 0.78 mA cm(-2)). The best catalyst also exhibited good stability after 10000 repetitive cycles with negligible loss in current.

Characterization of low-purity clays for geopolymer binder formulation

The production of geopolymer binders from low-purity clays was investigated. Three low-purity clays were calcined at 750°C for 4 h. The calcined clays were chemically activated by the alkaline solutions of NaOH and Na2SiO3. The compressive strength was measured as a function of curing time at room temperature and 85°C. The results were compared with those of a pure kaolin sample. An amorphous aluminosilicate polymer was formed in all binders at both processing temperatures. The results show that, the mechanical properties depend on the type and amount of active aluminum silicates in the starting clay material, the impurities, and the processing temperature.

Room-Temperature Wet Chemical Synthesis of Au NPs/TiH2/Nanocarved Ti Self-Supported Electrocatalysts for Highly Efficient H2 Generation

Self-supported electrocatalysts are a new class of materials exhibiting high catalytic performance for various electrochemical processes and can be directly equipped in energy conversion devices. We present here, for the first time, sparse Au NPs self-supported on etched Ti (nanocarved Ti substrate self-supported with TiH2) as promising catalysts for the electrochemical generation of hydrogen (H2) in KOH solutions. Cleaned, as-polished Ti substrates were etched in highly concentrated sulfuric acid solutions without and with 0.1 M NH4F at room temperature for 15 min. These two etching processes yielded a thin layer of TiH2 (the corrosion product of the etching process) self-supported on nanocarved Ti substrates with different morphologies. While F--free etching process led to formation of parallel channels (average width: 200 nm), where each channel consists of an array of rounded cavities (average width: 150 nm), etching in the presence of F- yielded Ti surface carved with nanogrooves (average width: 100 nm) in parallel orientation. Au NPs were then grown in situ (self-supported) on such etched surfaces via immersion in a standard gold solution at room temperature without using stabilizers or reducing agents, producing Au NPs/TiH2/nanostructured Ti catalysts. These materials were characterized by scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), grazing incidence X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS). GIXRD confirmed the formation of Au2Ti phase, thus referring to strong chemical interaction between the supported Au NPs and the substrate surface (also evidenced from XPS) as well as a titanium hydride phase of chemical composition TiH2. Electrochemical measurements in 0.1 M KOH solution revealed outstanding hydrogen evolution reaction (HER) electrocatalytic activity for our synthesized catalysts, with Au NPs/TiH2/nanogrooved Ti catalyst being the best one among them. It exhibited fast kinetics for the HER with onset potentials as low as -22 mV vs. RHE, high exchange current density of 0.7 mA cm-2, and a Tafel slope of 113 mV dec-1. These HER electrochemical kinetic parameters are very close to those measured here for a commercial Pt/C catalyst (onset potential: -20 mV, Tafel slope: 110 mV dec-1, and exchange current density: 0.75 mA cm-2). The high catalytic activity of these materials was attributed to the catalytic impacts of both TiH2 phase and self-supported Au NPs (active sites for the catalytic reduction of water to H2), in addition to their nanostructured features which provide a large-surface area for the HER.