Room temperature sintering of polar ZnO nanosheets: II-mechanism.

In a previous work by the authors (A. Fernández-Pérez el al., Room temperature sintering of polar ZnO nanosheets: I-evidence, 2017, DOI: 10.1039/C7CP02306E), polar ZnO nanosheets were stored at room temperature under different atmospheres and the evolution of their textural and crystal properties during storage was followed. It was found that the specific surface area of the nanosheets drastically decreased during storage, with a loss of up to 75%. The ZnO crystals increased in size mainly through the partial merging of their polar surfaces at the expense of narrow mesoporosity, in a process triggered by the action of moisture, oxygen and, in their absence, by light. In the present work, a set of spectroscopic techniques (FTIR, Raman and XPS) has been used in an attempt to unravel the mechanism behind this spontaneous sintering process. The mechanism starts with the molecular adsorption of water, which takes place on Zn atoms close to oxygen vacancies on the (100) surface, where H2O dissociates to form two hydroxyl groups and to heal one oxygen vacancy. This process triggers the room temperature migration of Zn interstitials towards the outer surface of the polar region. What were previously interstitial Zn atoms now gradually occupy the mesopores, with interstitial oxygen being used to build up the O sublattice until total occupancy of the narrow mesoporosity is achieved.

Room temperature sintering of polar ZnO nanosheets: I-evidence.

Polar ZnO nanosheets of a high specific surface area (∼120 m2 g-1) were subjected to storage under different atmospheres at room temperature and analyzed for changes in their textural and crystal properties. During their storage under laboratory conditions (in closed transparent polypropylene vials kept under the light of the laboratory on worktop tables) the nanosheets lost up to 75% of their specific surface area in approximately two months, with most of the loss occurring during the first two weeks. The narrow mesoporosity (∼5 nm pore size) became filled with ZnO during the process. No loss or gain in weight was detected. The loss of specific surface area took place under all of the atmospheres assayed, in the following order: moist air (with or without light) > moist CO2-free atmosphere (with or without light and/or oxygen) > dry CO2-free oxygen-containing atmosphere (with or without light) > dry inert atmosphere (with light) > dry inert atmosphere (in the dark). During storage the ZnO crystals grew mainly by the partial merging of their polar surfaces in a process triggered by the action of moisture, oxygen and, in the absence of these two agents, light. The mechanism of this intriguing phenomenon will be analyzed in detail in the second part of this work.

Stainless steel wire mesh-supported ZnO for the catalytic photodegradation of methylene blue under ultraviolet irradiation.

The aim of this study was to assess the activity of catalysts formed by nanostructured zinc oxide supported on stainless steel wire mesh for the photocatalytic degradation of methylene blue under UV irradiation. Catalysts prepared by means of different low temperature synthesis methods, as described in a previous work (Vu et al., Mater. Res. Bull. 47 (2012) 1577-1586) were tested. A new activity parameter was introduced in order to compare the catalytic activity of the different catalysts. The best catalyst showed a catalytic activity higher than that of the reference material TiO(2) P25 (Degussa-Evonik). This high activity is attributed to a higher quantum yield derived from the small particle length of the ZnO deposited on the wire mesh. The photocatalytic degradation kinetics of methylene blue fitted a potential model with n orders ranging from 0.5 to 6.9. Reaction orders over 1 were attributed to catalyst deactivation during the reaction resulting from the photocorrosion of ZnO.

Fabrication of mesoporous SiO(2)-C-Fe(3)O(4)/gamma-Fe(2)O(3) and SiO(2)-C-Fe magnetic composites.

A synthetic method for the fabrication of silica-based mesoporous magnetic (Fe or iron oxide spinel) nanocomposites with enhanced adsorption and magnetic capabilities is presented. The successful in situ synthesis of magnetic nanoparticles is a consequence of the incorporation of a small amount of carbon into the pores of the silica, this step being essential for the generation of relatively large iron oxide magnetic nanocrystals ( approximately 10+/-3nm) and for the formation of iron nanoparticles. These composites combine good magnetic properties (superparamagnetic behaviour in the case of SiO(2)-C-Fe(3)O(4)/gamma-Fe(2)O(3) samples) with a large and accessible porosity made up of wide mesopores (>9nm). In the present work, we have demonstrated the usefulness of this kind of composite for the adsorption of a globular protein (hemoglobin). The results obtained show that a significant amount of hemoglobin can be immobilized within the pores of these materials (up to 180mgg(-1) for some of the samples). Moreover, we have proved that the composite loaded with hemoglobin can be easily manipulated by means of an external magnetic field.

Shape and size effects of ZnO nanocrystals on photocatalytic activity.

A wet-chemical method was employed to prepare zinc oxide nanocrystals having controlled morphology through thermal decomposition of a zinc precursor in self-assembled supramolecular structures in solvent under mild conditions. This solution method offers finer tailoring of the size and shape of the nanocrystals and is complementary to most reported physical methods. Understanding the morphological effects of pure or modified zinc oxide nanocrystals on photocatalytic activity is important in regard to enhanced solar energy capture and utilization but has been scarcely addressed in the past. The photocatalytic rate was found to have no dependence on ZnO particle size, but the shape factor seems to be of overriding importance. Hexagonal platelike nanocrystals were found to display at least 5 times higher activity than rod-shaped crystals, which clearly suggests that the polar (001) and (001) faces are more active surfaces than the nonpolar surfaces perpendicular to them.

Signatures of clustering in superparamagnetic colloidal nanocomposites of an inorganic and hybrid nature.

The individual and co-operative properties of inorganic and hybrid superparamagnetic colloidal nanocomposites that satisfy all the requirements of magnetic carriers in the biosciences and/or catalysis fields are been studied. Essential to the success of this study is the selection of suitable synthetic routes (aerosol and nanocasting) that allow the preparation of materials with different matrix characteristics (carbon, silica, and polymers with controlled porosity). These materials present magnetic properties that depend on the average particle size and the degree of polydispersity. Finally, the analysis of the co-operative behavior of samples allows for the detection of signatures of clustering, which are closely related to the textural characteristics of samples and the methodology used to produce the magnetic carriers.