Tiverios Vaimakis

Preparation of Fluorine-Doped TiO2 Photocatalysts with Controlled Crystalline Structure

Nanocrystalline F-doped TiO2 powders were prepared by sol-gel route. The thermal behavior of the powders was recorded by DTA/TG technique. The crystalline phase of the fluorinated TiO2 powders was determined by X-ray diffraction technique. It was demonstrated that F-doping using CF3COOH favors the formation of rutile along with anatase phase even at low temperature. Moreover, the rutiles phase content increases with the increase of the quantity of the fluorine precursor in the starting solution. The surface area of the powders and the pore size distribution were studied by N2 adsorption-desorption using BET and BJH methods. X-ray photoelectron spectroscopy (XPS) revealed that the fluorine is presented in the TiO2 powders mainly as metal fluoride in quantities ∼16 at %. The F-doped TiO2 showed a red-shift absorption in UV-vis region which was attributed to the increased content of rutile phase in the powders. The powders exhibited enhanced photocatalytic activity in decomposition of acetone.

Study of TiO2 anatase nano and microstructures with dominant {001} facets for NO oxidation

IntroductionTiO2 anatase nanoplates and hollow microspheres were fabricated by a solvothermal–hydrothermal method using titanium isopropoxide as a titanium precursor and hydrofluoric acid as a capping agent in order to enhance the formation of the {001} crystal facets of the anatase nanocrystals.MethodsThese different morphological structures of TiO2 anatase can be achieved by only changing the solvent, keeping the amount of the precursor and of the capping agent identical during the solvothermal–hydrothermal process.Results and discussionAfter calcination of the samples, the adsorbed fluoride atoms on the {001} crystal facets of the TiO2 anatase nanocrystals were completely removed from their surface according to XPS analysis. The calcined TiO2 anatase structures were higher crystallized and the specific surface area of the catalysts increased, enhancing their photocatalytic activity in comparison to the non-calcined TiO2 anatase structures. All TiO2 anatase samples with adsorbed as well as non-adsorbed fluoride atoms on their {001} crystal facets, exhibited a higher photonic efficiency than Degussa P25, which was used as a reference.ConclusionThe fluoride free TiO2 anatase nanoplates exhibited the best photocatalytic activity in oxidizing the NO gas to NO2 and NO3−.

Adsorption of direct yellow 27 from water by poorly crystalline hydroxyapatite prepared via precipitation method

Abstract In the present study, hydroxyapatite powders were prepared by modified precipitation method and characterized by XRD, FT-IR and N2 adsorption–desorption techniques. The prepared nonporous particles were organized to agglomerate with mesoporous structure and consisted of low crystallinity Ca-deficient hydroxyapatite and in amorphous phase. The commercial direct yellow 27 was selected as a model dye in order to examine the adsorption capacity of hydroxyapatite at room temperature. The adsorption isotherms are transformed from L-type to S-type curve, in Giles classification, by increasing the pH values. Equilibrium data fitted very well to the Langmuir model, signifying the energetic homogeneity of hydroxyapatite surface adsorption sites. The dye sorption kinetics was fairly described by the pseudo-first-order kinetic model.

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H2bihyat. Water-Assisted Activation of the MoVI═O Bond and Reversible Dimerization of cis-[MoVIO2(bihyat)] to [MoVI2O4(bihyat)2(H2O)2]

Reaction of the N,N-disubstituted bis(hydroxylamino) ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H(2)bihyat) with cis-[Mo(VI)O(2)(acac)(2)] in tetrahydrofuran resulted in isolation of the mononuclear compound cis-[Mo(VI)O(2)(bihyat)] (1). The treatment of Na(2)Mo(VI)O(4)·2H(2)O with the ligand H(2)bihyat in aqueous solution gave the dinuclear compounds cis-[Mo(VI)(2)O(4)(bihyat)(2)(H(2)O)(2)] (2) and trans-[Mo(VI)(2)O(4)(bihyat)(2)(H(2)O)(2)] (3) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum(VI) compounds were determined by X-ray crystallography. Compound 1 has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat(2-) ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo(2)(VI)O(2)(μ(2)-O(2))}(4+) moiety. The potentiometry revealed that the Mo(VI)bihyat(2-) species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3-5. A subtle change in the coordination environment of the five-coordinate compound 1 with ligation of a weakly bound water molecule trans to the oxido ligand (1w) renders the equatorial oxido group in 1w more nucleophilic than that in 1, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds 2 and 3 are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (1 → 2/3). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum(VI) species, vibrational spectra, and energetics of the metal-ligand interaction for the three molybdenum(VI) compounds 1-3 have been studied by means of density functional theory calculations.

2-Dimensional Clay/Reduced Graphene Oxide Ordered Heterostructures Dispersible in Water via a One-Step Hydrothermal Route

A facile and green method for trapping the hydrophobic reduced graphene oxide between the hydrophilic Kunipia F layers in order to attain stable aqueous dispersions of reduced graphene oxide is described. Initially stable aqueous dispersions of hydrophilic clay intercalated with hydrophilic graphene oxide sheets were formed providing well-organized heterostructures, as it was revealed by scanning electron microscopy images. These structures were preserved in the product obtained after hydrothermal treatment where the hydrophilic graphene oxide was converted to hydrophobic reduced graphene oxide. Ultraviolet measurements revealed the aforementioned conversion which was accompanied by a characteristic change in color from yellow-brown to black in the corresponding aqueous dispersions of these hybrids before and after hydrothermal treatment. The stability of these homogeneous dispersions was confirmed by Zeta Potential measurements implying interactions both in cases of graphene oxide and reduced graphene oxide with clay sheets that made feasible the effective interstratification of graphene-clay layered materials. In these stable dispersions chemistry in aqueous environment could be fully utilized making possible their incorporation e.g., as fillers to hydrophilic polymeric matrices extending thus the limits of application.

Preparation of Hydroxyapatite Nanoparticles Using a Modified Precipitation Method

Hydroxyapatite (HAp) is the primary constituent of the human hard tissues. The preparation of fine and well sinterable HAp powder is the most important step in fabrication of bioceramics. HAp nanoparticles were prepared using rapid increase of the pH value, by adding an amount of concentrated NH4OH solution, of a well-mixed solution of Ca(H2PO4)2·H2O and CaCl2 with stoichiometric atomic ratio of Ca/P. This modified precipitation method is called “pH shock wave method”. The XRD and FTIR analyses indicated that the precipitated powder was crystallized HAp. The HAp was stable during calcination at 550°C, while a significant amount of β-Ca3(PO4)2 appear at 800°C. The TEM micrograph showed arbitrary morphology of nanoparticles, with average grain size of ∼80–200 nm in length, and of ∼10–30 nm in width and thickness. The adsorption isotherm of HAp was typical for nonporous particles. The specific surface area of powder was 74 m2·g-1. The chemical analysis indicates the formation of nonstoichiometric hydrated HAp with empirical formula Ca8.86(HPO4)1.28(PO4)4.72(OH)·7.50H2O and atomic ratio of Ca/P equal to 1.477. The lattice parameters were different from standard ones, due to the presence of HPO4 2- groups in the powder. The sizes of the crystallites responsible for the Bragg reflection of (002) and (300) planes were 31.1 and 14.9 nm, respectively, indicating anisotropic growth with maximum along the direction. The HAp particles form aggregates, which average diameter is 1–5 μm. in the interspaces between the aggregated particles, the water remained entrapped after drying. The thermogravimetric analysis confirmed the above mentioned conclusions. The formation of β-Ca3(PO4)2 took place at about 780°C. Sintering of a unixially compressed cylindrical specimen, took place mainly in the temperature range from 800 to 1150 °C.

Physical Properties of Photo-Aged Graphene/Polypropylene Nanocomposites

In the present research, graphene nanoplatelets/polypropylene (GNP/PP) nanocomposites were prepared by melt mixing and were subjected to accelerated ageing. The effect of graphene on the morphology and physical properties of aged GNP/PP nanocomposites was investigated. The incorporation of graphene to non-aged PP matrix led to changes in its crystal conformation, decreased the ultraviolet-visible (UV-Vis) transmittance and tensile strain and increased the elastic modulus. The ageing of non-reinforced PP increased the β-phase of PP and caused the formation of cracks on its surface, while voids were observed in its cross-section. The aged PP was also characterized by significantly lower UV-Vis transmittance, thermal stability and tensile strain, but increased elastic modulus compared to non-aged PP. Graphene retarded the ageing of PP matrix, according to Fourier Transform Infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) results. In the aged GNP/PP nanocomposite, the morphology did not present any changes and the examined properties were maintained to similar values with that of non-aged GNP/PP nanocomposite.