Photocatalytic activity enhancement of two-step and one-pot synthesis of Pd/ZnO nanocomposites: an experimental and DFT study.

Pd/ZnO nanocomposites were successfully synthesized by means of one and two pot synthesis and applied in the photodegradation of Rh6G. The nanocomposites were characterized by XRD, SEM, TEM, FTIR and micro-Raman spectroscopies. It was found the presence of PdZn2, PdO and agglomerated particles in the support surface for the Palladium-based nanocomposites fabricated by one-pot route; the two-step method allowed the formation of spherical Pd nanoparticles, with homogeneous distribution in the nanocomposite matrix, with an average size of 2.16 nm. The results show higher photocatalytic efficiency for the samples fabricated under the two-step approach compared to the one-pot synthesis. Based on experimental results, density functional theory (DFT) calculations were carried out to understand the enhancement photocatalytic of Pd/ZnO nanocomposites. To achieve it, the ZnO (001) and (101) surfaces were built and decorated by different Pd coverages. The theoretical results indicated two different photocatalytic mechanisms. In ZnO (001) case, the electrons flowed from surface to Pd, generating the superoxide radical anion (⋅O2-). Furthermore, the density of states of the ZnO (001) surface was modified by impurity Pd-d states at proximity to the conduction states, which may work as electron acceptors states. On the other hand, we found that the electrons flow from Pd to ZnO (101) surface, inducing the formation of ⋅OH and ⋅O2- for the degradation of Rh6G. The density of states of the ZnO (101) revealed a reduction in its bandgap, due to Pd-d states localized above valence states. Hence, our theoretical results suggest that the Pd-d states may facilitate the mobility of electrons and holes in (001) and (101) surfaces, respectively, reducing the rate of charge recombination.

Automated method for the determination of the band gap energy of pure and mixed powder samples using diffuse reflectance spectroscopy.

An automated method to determine the band gap energy (E g ) of pure and mixed powder compounds using diffuse reflectance spectroscopy is presented. This method is based on a five-step algorithm that mimics the judgment made by an expert analyst in identifying the linear segments in Tauc plots and subsequent estimation of the E g value. It is demonstrated that the method to estimate E g by intersecting the straight-line fit of the Tauc segment with the photon energy axis is not appropriate for those samples containing more than one optical absorbing phase because systematic underestimation of the E g value results. The automated method accounts for such cases by introducing a base line function. The robustness of the implemented algorithm was tested using three model systems, ZnO-Al2O3, ZnO-CoO and ZnO-CdO. The estimated E g 's using the automated method differ in less than 1% than those obtained by its manual counterpart.

First principles calculations of the electronic and chemical properties of graphene, graphane, and graphene oxide.

The electrical and chemical properties of graphene (C(24)H(12)), graphane (C(24)H(24)) and graphene oxide (C(54)H(17)+O+(OH)(3)+COOH) were studied through the density functional theory (DFT) at level of Local Density Approximation (LDA) using a model C(n)H(m) like. The optimized geometry, energy gap and chemical reactivity for the proposed carbon 2D models are reported. It was found that while the graphene and graphane structures have semiconductor behavior, the graphene oxide behaves as semi-metal. However, a transition from semi-mental to semiconductor is predicted if the carboxyl group (COOH) is removed from such structure. The chemically active sites are analyzed on the basis of the electrophilic Fukui functions for each structure.

Low temperature photoluminescence characteristics of chemically synthesized indium doped zinc oxide nanostructures.

Photoluminescence (PL) emission and excitation (EPL) spectra of un-doped and indium (1%) doped 1D zinc oxide nanostructures are studied at different temperatures. The nanostructures reveal a blue emission band attributed to localized donor states. Indium doping enhances the blue emission. While at low temperatures (< 50 K) PL spectra are dominated by the emission attributed to the recombination of excitons bound to neutral donors (D(0),X), at higher temperatures (>100 K), defect related emissions in the visible range dominate over the excitonic emission. Temperature dependence measurements on the doped sample reveal that (D(0),X) emission energies obey the Varshni's formula with fitting constants alpha = 8.4 +/- 0.3 x 10(-4) eV/K and beta = 650 +/- 40 K. The (D(0),X) emission intensity decays exponentially with temperature.

Incorporation of Sb in ZnO nanostructures through hydrothermal process.

Incorporation of dopants in optoelectronic semiconductor nanostructures has been a matter of great interest in recent times. While such doping has been performed almost routinely using physical methods, use of low-cost chemical techniques for that purpose is still rare. We incorporated antimony in zinc oxide (ZnO) nanostructures through a low temperature hydrothermal method. In as-grown nanostructures, antimony remains partially in Sb2O3 phase. On thermal annealing at 500 degrees C, it dissociates and antimony incorporates into ZnO mainly by substituting zinc from the crystal lattice. Incorporation of Sb drastically modifies the morphology of the ZnO nanostructures. While incorporation of Sb in low concentration promotes the formation of uniform prismatic ZnO nanorods probably due to catalytic effect, high concentration of Sb causes the formation of rounded shaped nanoparticles due to high interfacial compressive stress. Incorporated Sb in the ZnO nanostructures remains inhomogeneously distributed. The optical band gap of the ZnO nanostructures increases a bit for lightly doped samples but it decreases for heavy doping.