Vocationalising the school curriculum A case study of school management responses to the implementation of the national policy on education in Nigerian secondary schools

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Silica-Supported Cu2O Nanoparticles with Tunable Size for Sustainable Hydrogen Generation

Cu2O is a p-type semiconductor which attracts much attention for application in photovoltaics, photocatalysis and solar water splitting. However, Cu2O is not intrinsically stable under illumination in aqueous solutions, and the edge of the valence band is not positive enough to provide sufficient overpotential for water oxidation. The stability and band edge position of nanostructured materials depend on crystallite size. In this paper, we describe a new strategy to vary the size of Cu2O nanoparticles using mesoporous silica supports. First, CuO nanoparticles were obtained via impregnation-drying-heating. The size of the nanoparticles was tuned by varying either the concentration of Cu precursor or the pore diameter of the supporting silica. Subsequently, the CuO was converted to Cu2O without particle growth by gas-phase reduction with carbon monoxide. The visible light absorption of these nanoparticles depended on the copper oxide phase and crystallite size, leading to a direct band gap energy of 2.60 eV for 2 nm Cu2O nanoparticles compared to 1.94 eV for macrocrystalline Cu2O. Our results highlight a new synthesis strategy for the preparation of metal-oxide nanoparticles with controlled sizes of 2–15 nm that are not directly accessible by alternative synthesis techniques. The as-obtained 15 nm Cu2O nanoparticles were used for H2 evolution in a water-methanol mixture, the photocatalyst gave a H2 evolution rate of 11.5 × 10−3 μmol min−1 which corresponded to an internal quantum efficiency of 15.8% and an overall quantum efficiency of 3.5% for light between 310 and 710 nm. Finally, the nanoparticles were stable during three hours of light illumination

Silica-Supported Cu2O Nanoparticles with Tunable Size for Sustainable Hydrogen Generation

Cu2O is a p-type semiconductor which attracts much attention for application in photovoltaics, photocatalysis and solar water splitting. However, Cu2O is not intrinsically stable under illumination in aqueous solutions, and the edge of the valence band is not positive enough to provide sufficient overpotential for water oxidation. The stability and band edge position of nanostructured materials depend on crystallite size. In this paper, we describe a new strategy to vary the size of Cu2O nanoparticles using mesoporous silica supports. First, CuO nanoparticles were obtained via impregnation-drying-heating. The size of the nanoparticles was tuned by varying either the concentration of Cu precursor or the pore diameter of the supporting silica. Subsequently, the CuO was converted to Cu2O without particle growth by gas-phase reduction with carbon monoxide. The visible light absorption of these nanoparticles depended on the copper oxide phase and crystallite size, leading to a direct band gap energy of 2.60 eV for 2 nm Cu2O nanoparticles compared to 1.94 eV for macrocrystalline Cu2O. Our results highlight a new synthesis strategy for the preparation of metal-oxide nanoparticles with controlled sizes of 2–15 nm that are not directly accessible by alternative synthesis techniques. The as-obtained 15 nm Cu2O nanoparticles were used for H2 evolution in a water-methanol mixture, the photocatalyst gave a H2 evolution rate of 11.5 × 10−3 μmol min−1 which corresponded to an internal quantum efficiency of 15.8% and an overall quantum efficiency of 3.5% for light between 310 and 710 nm. Finally, the nanoparticles were stable during three hours of light illumination

A New Stability Criterion for the Hot Deformation Behavior of Materials:Application to the AZ31 Magnesium Alloy

In the present work, a new stability criterion is presented that uses the single sine hyperbolic equation of Garofalo for all values of the thermomechanical variables of the hot-working process. In this procedure, the efficiency and stability are calculated for each _e and T by means of this equation. This is carried out directly applying the Garofalo equation on Lyapunov conditions in the framework of the dynamic material model (DMM), which simplifies operations and minimizes errors. This procedure, therefore, is straightforward, starting with experimental data and reaching the new established Lyapunov stability criterion. It is an alternative to the stability conditions using Lyapunov criteria, as established by Malas and Gegel and Prasad, where the strain-rate-sensitivity exponent, m, was determined by fitting the curve strain rate, _e, vs stress, r, by means of a potential equation named power law, or by a polynomial of second or third degree, and calculating the slope of the logarithmic curve at each point using successive derivatives. In addition, a revision of various stability criteria and calculations of efficiency is conducted to delineate the framework of our new criterion. The developed method allows obtaining inequations that determine the more or less stable regions in the form of maps. These maps predict optimal temperatures and strain rates that are different from those given in the maps of Malas and Gegel and Prasad, although significant matches with various authors may also be observed. An analysis of maps for the magnesium alloy AZ31 based on various methods and models was performed to compare the predictions with the experimental results of other authors.This work is financially supported by CICYT, Spain, under Programs MAT2012-38962 and MAT2015-68919 (MINECO/FEDER).Peer Reviewe