Brigitte Sieber

Synthesis and photocatalytic activity of iodine-doped ZnO nanoflowers

The paper reports on the preparation and photocatalytic activity of thin films of iodine-doped ZnO nanoflowers deposited on glass substrate using a simple growth process based on hydrothermal synthesis. Addition of iodic acid (5–20 vol%) in the reaction mixture allows the introduction of iodine ions in the form of I− or IO3− in the ZnO lattice, as suggested by X-ray photoelectron spectroscopy. Doping ZnO nanostructured films with iodine did not impact their morphology, while it has a significant influence on their optical properties. Indeed, the nanostructured ZnO films, prepared in the presence of iodic acid, display a large increase of the visible luminescence, which reaches a maximum at a concentration of 10 vol%. Finally, the photocatalytic activity of the ZnO nanostructured films for the photodegradation of a model pollutant, rhodamine B, was evaluated under UV and visible light irradiation. While under UV light irradiation, both undoped and iodine-doped ZnO films show a similar behavior, the photocatalytic performance of iodine-doped under visible light irradiation is significantly enhanced in comparison to that of undoped ZnO.

Hydrothermal synthesis, phase structure, optical and photocatalytic properties of Zn2SnO4 nanoparticles.

Zinc stannate (Zn2SnO4 or ZTO) nanoparticles were synthesized via hydrothermal method using NaOH as a mineralizer. X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) of the synthesized ZTO nanoparticles revealed the formation of highly pure ZTO phase with the spinel-like structure. The nanoparticles have spherical shape with an average size of about 25 nm. The Raman spectrum of the sample was dominated by the A(1g) vibration mode of pure ZTO phase. From UV-Vis measurement, a band gap E(g) of 3.465 eV was determined. The photocatalytic activity of the ZTO nanoparticles was evaluated for the photodegradation of rhodamine B (RhB) under visible light irradiation. The influence of catalyst concentration and irradiation time on the photocatalytic process was investigated. The ZTO catalyst showed the best photocatalytic performance at a concentration of 0.2 g/L, and the photodecomposition of RhB followed first-order kinetics with a rate constant k=0.0249 min(-1). The ZTO-assisted photocatalytic degradation of RhB occurred via two competitive processes: a photocatalytic process and a photosensitized process. The detection of hydroxyl radicals by fluorescence measurements suggests that these species play an important role in the photocatalytic process.

Magnetic Co3O4/reduced graphene oxide nanocomposite as a superior heterogeneous catalyst for one-pot oxidative esterification of aldehydes to methyl esters

A magnetically separable hybrid material consisting of Co3O4 nanoparticles supported on reduced graphene oxide (Co3O4/rGO) was synthesized through a simple co-reduction process of graphene oxide (GO) and cobalt chloride (CoCl2) using sodium borohydride (NaBH4). The Co3O4/rGO heterogeneous catalyst exhibited a high-performance for the oxidative esterification of aldehydes to the corresponding methyl esters using tert-butyl hydroperoxide (TBHP) as an oxidant. Owing to the synergistic effect of rGO support, the hybrid catalyst exhibited superior catalytic activity to the corresponding cobalt oxide catalyst. Importantly, the synthesized hybrid possesses good magnetic properties, which enable facile recovery of the catalyst by using an external magnet.

Hydrothermal synthesis of ZTO/graphene nanocomposite with excellent photocatalytic activity under visible light irradiation.

A facile and efficient one-step hydrothermal approach for the synthesis of Zn2SnO4 nanoparticles/reduced graphene oxide (ZTO/rGO) nanocomposites using zinc acetate, tin chloride and graphene oxide (GO) as precursors, and sodium hydroxide as reducing agent has been developed. This approach allows simultaneous reduction of GO and growth of spinel ZTO nanoparticles (NPs) on the rGO sheets. The morphology and microstructure characterizations of ZTO/rGO nanocomposites revealed that this method leads to close interfacial contact of ZTO NPs and rGO and efficient dispersion of ZTO NPs on the surface of rGO sheets. The photocatalytic activity of the ZTO/rGO nanocomposite was investigated for the reduction of rhodamine B under visible light irradiation. Compared to pure ZTO NPs, ZTO/rGO nanocomposite exhibited superior photocatalytic activity with a full degradation of rhodamine B within 15min. The enhanced photocatalytic performance of ZTO/rGO was mainly attributed to excellent electron trapping and effective adsorption properties of rGO.

Structural and Luminescence Properties of Highly Crystalline ZnO Nanoparticles Prepared by Sol--Gel Method

ZnO nanoparticles were synthesized using sol–gel method. The structural and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), Raman spectroscopy, and photoluminescence (PL). XRD analysis demonstrates that the nanoparticles have the hexagonal wurtzite structure and the particle size is increased with annealing temperature. The average size of the nanoparticles was determined by SEM as well as XRD data and found to be ~50 nm after annealing at 800 °C. A sharp, strong and dominant UV emission with a suppressed green emission has been observed at 300 and 10 K, indicating the good optical properties of ZnO nanoparticles. The 10 K UV band is dominated by a neutral-donor bound exciton, and the surface-related SX emission at 3.31 eV is evidenced.

High photocatalytic activity of plasmonic Ag@AgCl/Zn2SnO4 nanocomposites synthesized using hydrothermal method

Ag@AgCl/Zn2SnO4 (ZTO) nanocomposites were successfully prepared by a hydrothermal method. The morphology, structure, composition, and optical properties of the developed composites were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, UV-visible (UV-vis) spectrophotometry, X-ray photoelectron spectroscopy (XPS), and photoluminescence techniques. All analysis confirmed the anchoring of Ag@AgCl on ZTO. The photocatalytic activity of the Ag@AgCl/ZTO nanocomposites was evaluated for the photodegradation of rhodamine B (RhB) dye, phenol and bisphenol A (BPA) in aqueous solution, under visible light irradiation. An important improvement of the catalytic activity was observed using the nanocomposites as compared to ZTO solely. The photocatalytic enhancement can be attributed to a plasmonic effect at the interface between Ag@AgCl and ZTO. Thus, the good catalytic performance of the nanocomposites combined with their simple synthesis could provide a facile way to achieve highly efficient photocatalysts.

Enhancement of the intensity ratio of ultraviolet to visible luminescence with increased excitation in ZnO nanoparticles deposited on porous anodic alumina

Porous anodic alumina (PAA), with large pore diameters (up to 100?nm), was used as a substrate for the growth of ZnO nanostructures. Almost unstrained ZnO nanoparticles of less than 8?nm in size were prepared by the spin-on method and annealed at 500??C.Excitonic recombination is found to dominate the ultraviolet band. The ZnO nanoparticles have a high deep level emission at low excitation which becomes negligible at high excitation, as deduced from photoluminescence measurements. This large increase of the ultraviolet to visible luminescence ratio with excitation is ascribed to the variation of the ultraviolet and of the visible luminescence with excitation intensity. Non-radiative Auger recombination becomes dominant at high excitation. It is also shown that the PAA substrate improves the internal quantum efficiency of the ZnO nanoparticles achieved by the sol?gel method. At low excitation, the luminescence intensity drops by a factor of 3 between 13 and 300?K, leading to an estimate of the internal quantum efficiency as high as 30%.

Resistance of group III nitrides irradiated with a 10 keV electron beam; comparison of the cathodoluminescence emission of GaN quantum dots, quantum wells and (Al)GaN epitaxial layers

We present a comparative study of the in situ modifications induced by a 10?keV electron beam in gallium nitride (GaN) quantum wells and quantum dots as well as in ELOG (epitaxial laterally overgrown GaN) and AlGaN epilayers. Cathodoluminescence (CL) experiments were performed to investigate the room temperature evolution of the optical properties as a function of the beam current density. Higher dot resistance is much more apparent when the beam current density is larger than about 6?A?cm?2. Recombination enhanced diffusion of defects is involved in the degradation of the CL signal. The two serial mechanisms inferred to be present in the degradation process are described.

Ferromagnetism induced in ZnO nanorods by morphology changes under a nitrogen–carbon atmosphere

We use thermal carbonization with acetylene and nitrogen to treat hydrothermally grown ZnO nanorods on silicon substrates. The method is found to be strongly temperature dependent. Treatment temperatures below 800 °C do not induce any morphological changes of the rods, while temperatures above 800 °C cause significant erosion of the rods leading to hollow- and tubular-like structures. The temperature dependent weight decrease confirms the observation of erosion and X-ray photoelectron spectroscopy (XPS) measurements indicate significant decrease in Zn content. Raman spectra evidence the presence of a diamond-like carbon layer around the rods. The coupling of resonant and non-resonant Raman results with photoluminescence measurements allow us to suggest that both nitrogen and carbon are present within the ZnO lattice. The samples treated above 800 °C are also found to be ferromagnetic and the magnetization increases as the treatment temperature increases (up to 1.45 emu cm−3 at 5 K). Thermal annealing in nitrogen atmosphere does not cause either morphological changes or ferromagnetism, suggesting that the erosion results from the additional carbon source (acetylene) of the treatment. Complexes formed by carbon and nitrogen atoms at the surface of the treated and eroded samples are believed to be at the origin of the ferromagnetism.

Structural and luminescence properties of highly crystalline ZnO nanoparticles prepared by sol-gel method

ZnO nanoparticles were synthesized using sol–gel method. The structural and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), Raman spectroscopy, and photoluminescence (PL). XRD analysis demonstrates that the nanoparticles have the hexagonal wurtzite structure and the particle size is increased with annealing temperature. The average size of the nanoparticles was determined by SEM as well as XRD data and found to be 50nm after annealing at 800 C. A sharp, strong and dominant UV emission with a suppressed green emission has been observed at 300 and 10K, indicating the good optical properties of ZnO nanoparticles. The 10K UV band is dominated by a neutral-donor bound exciton, and the surface-related SX emission at 3.31eV is evidenced. # 2012 The Japan Society of Applied Physics