Growth kinetics of vanadium pentoxide nanostructures under hydrothermal conditions

The work reported here involved a study of the growth kinetics of V2O5nH2O nanostructures under hydrothermal conditions. The coarsening process of V2O5nH2O nanoribbons was followed by subjecting the as-prepared suspensions to hydrothermal treatments at 80 °C for periods ranging from 0 to 7200 min. X-ray diffraction (XRD) confirms that the hydrothermal treatments at 80 °C caused no significant modification of the long-range order structure of samples subjected to different periods of hydrothermal treatment. Field emission scanning transmission electron microscope (FE-STEM) was used to analyze the morphology and width distribution of the nanostructures. The results indicated that the crystal growth mechanism in the [1 0 0] direction of vanadium pentoxide 1D nanostructure under hydrothermal conditions is well described by the oriented attachment (OA) mechanism. This evidence was supported by HRTEM images showing the existence of defects at the interface between nanostructures, which is characteristic of the oriented attachment (OA) mechanism.FAPESPCNPqLNL

An efficient synthesis route of 'Na IND.2''V IND.6''O IND.16.n''H IND.2'O nanowires in hydrothermal conditions

Na2V6O16·nH2O nanowires were synthesized using an environmentally friendly and one-step low-temperature hydrothermal route. The synthesis involved the hydrothermal reaction between V2O5, H2O2 and NaOH, without the addition of any organic surfactants or inorganic ions. X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and thermal gravimetric analysis (TGA) techniques were used to characterize the structure, morphology, chemical composition and thermal stability of the nanostructured samples. High purity hydrated Na2V6O16·nH2O nanowires of diameters around 20–30 nm can be more efficiently obtained when subject to hydrothermal treatment at 140 °C for 24 h. The variation in the electronic and local atomic structure was analyzed using X-ray absorption spectroscopy (XAS). Based on the results obtained from the present study, a mechanism for the formation of Na2V6O16·nH2O nanowires in hydrothermal conditions was proposed.FAPESPCNP

CeO2 nanoparticles synthesized by a microwave-assisted hydrothermal method: evolution from nanospheres to nanorods

Ceria (CeO2) plays a vital role in emerging technologies for environmental and energy-related applications. The catalytic efficiency of ceria nanoparticles depends on its morphology. In this study, CeO2 nanoparticles were synthesized by a microwave-assisted hydrothermal method under different synthesis temperatures. The samples were characterized by X-ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, electron paramagnetic resonance spectroscopy and by the Brunauer-Emmett-Teller method. The X-ray diffraction and Raman scattering results indicated that all the synthesized samples had a pure cubic CeO2 structure. Rietveld analysis and Raman scattering also revealed the presence of structural defects due to an associated reduction in the valence of the Ce4+ ions to Ce3+ ions caused by an increasing molar fraction of oxygen vacancies. The morphology of the samples was controlled by varying the synthesis temperature. The TEM images show that samples synthesized at 80 degrees C consisted of spherical particles of about 5 nm, while those synthesized at 120 degrees C presented a mix of spherical and rod-like nanoparticles and the sample synthesized at 160 degrees C consisted of nanorods with 10 nm average diameter and 70 nm length. The microwave-assisted method proved to be highly efficient for the synthesis of CeO2 nanoparticles with different morphologies.FAPESPCNP

A novel ozone gas sensor based on one-dimensional (1D) α-Ag2WO4 nanostructures

This paper reports on a new ozone gas sensor based on α-Ag2WO4 nanorod-like structures. Electrical resistance measurements proved the efficiency of α-Ag2WO4 nanorods,which rendered good sensitivity even for a low ozone concentration (80 ppb), a fast response and a short recovery time at 300 C, demonstrating great potential for a variety of applications.CNPqFAPESP (13/07296-2)FAPESP (13/09573-3)Generalitat Valenciana (PROMETEO/2009/053)Ministerio de Ciencia e Innovación(CTQ2009-14541-C02)Programa de Cooperación Científica con IberoamericaMinisterio de Educación (PHB2009-0065-PC

Modification and crystallization of BaOB2O3RO2(R=Ti, Si) glassy system using a CO2 laser radiation source

Este trabalho apresenta um estudo sobre a modificação e cristalização superficial dos sistemas vítreos 40BaO-45B2O3-15TiO2 (BBT) e 42BaO-42B2O3-16SiO2 (BBS) utilizando a radiação proveniente de um laser modulado de CO2. Três diferentes parâmetros experimentais foram avaliados: a potência do laser, o tempo de exposição à radiação e a presença de micropartículas da fase ?-BaB2O4 (?-BBO) na superfície da amostra. As modificações causadas pela exposição das amostras à radiação laser foram caracterizadas através das técnicas de perfilometria, microscopia óptica, espectroscopia micro-Raman e difração de raios X. De uma maneira geral, os resultados mostram que a escolha adequada da potência do laser e do tempo de exposição à radiação leva a uma modificação superficial ou a uma modificação seguida da cristalização superficial da região exposta a radiação. A presença de micropartículas da fase ?-BBO na superfície da amostras influi de maneira significativa no processo de modificação e cristalização superficial. A presença dessas micropartículas induziu a cristalização da fase ?-BBO com um relativo alto grau de orientação preferencial na direção (006), permitindo a observação visual do efeito de geração de segundo harmônico em algumas amostras. Estudos de micro-fotoluminescência mostraram que o íon Sm3+ quando adicionado a matriz vítrea BBT apresenta um alto grau de desordem mesmo em amostras cristalizadas. No entanto, para a matriz BBS, este estudo mostra uma possível incorporação do íon terra-rara na região cristalizada.This work describes a study regarding the permanent modification and/or crystallization of 40BaO-45B2O3-15TiO2 (BBT) and 40BaO-45B2O3-16SiO2 (BBS) glass compositions when exposed to a continuous CO2 laser beam radiation. The effect of three experimental parameters was evaluated: the laser power, the radiation time exposure and the presence of micro-particles of the ?-BaB2O4 crystalline phase on the glass sample surface prior to radiation. The geometry of permanent modification and the crystallization process were characterized by using the perfilometry, optical microscopy, Micro-Raman spectroscopy and X-ray diffraction (XRD) techniques. The results show that the surface modification and crystallization can be controlled by a suitable choice of the laser power and radiation time exposure. In all samples, it was observed that the presence of ?-BaB2O4 micro-particles on the sample surface affects significantly equally the permanent modification and the crystallization process. The preferential orientation of the ?-BaB2O4 on the (006) plane direction allowed the observation of a relative high intense second harmonic generation effect. The micro-photoluminescence study shows that the Sm3+ ion in BBT glass sample present a high degree of disorder, even in the crystallized samples. On the other hand, when added to the BBS glass sample, it seems that the Sm3+ ions are incorporated onto the crystallized region

Synthesis of Vanadium Oxide Nanoparticles obtained by the decomposition of peroxide

A utilização de novas rotas de síntese de materiais nanoestruturados tem levado à obtenção de materiais apresentando novas propriedades e aplicações. O presente trabalho teve como principal objetivo realizar a síntese e a caracterização de nanopartículas de óxido de vanádio obtidas pelo método da decomposição de peróxido utilizando o tratamento hidrotermal. Por meio do controle do tempo e da temperatura de síntese, foi possível obter nanoestruturas de pentóxido de vanádio com diferentes fases cristalinas e morfologias. Os resultados de espectroscopia Raman e difração de raios X (DRX) mostraram que o tratamento hidrotermal da solução em temperaturas de até 180oC, durante 2 horas, leva à formação da fase V2O5nH2O monoclínica, apresentando uma orientação preferencial na direção 00l. A partir desta temperatura, o material obtido passou a ter a fase ortorrômbica perdendo sua orientação preferencial. Através das medidas de termogravimetria (TG) foi observado que, dependendo da condição de síntese, as amostras apresentam diferentes quantidades de moléculas de água (H2O) intercaladas em sua estrutura. Imagens de microscopia eletrônica mostraram que as nanoestruturas obtidas podem possuir a forma de fitas, fios ou bastões. Utilizando a técnica de espectroscopia de absorção de raios X (XAS), foi possível verificar a presença de íons de V4+ nas amostras de fase monoclínica, o que não foi observado naquelas que possuem estrutura ortorrômbica. Pela técnica de XAS também foi possível constatar que as amostras de estrutura ortorrômbica possuem menor grau de desordem ao redor do átomo de vanádio. O estudo do mecanismo de crescimento da nanoestrutura de V2O5nH2O mostrou que ocorre um crescimento lateral, o qual pode ser descrito pelo mecanismo denominado Coalescência Orientada ou Oriented attachment (OA). Finalmente, através da decomposição do peróxido de vanádio, foi possível obter compostos vanadatos contendo cátions de Na+ em sua estrutura.The use of new routes for nanostructured materials synthesis has lead to materials with new properties and applications. This PhD project aimed to realize the synthesis and the characterization of vanadium pentoxide nanoparticles obtained by peroxide decomposition using the hydrothermal treatment. By controlling synthesis time and temperature it was possible to achieve vanadium pentoxide nanoparticles with different crystalline phases and orphologies. The results of Raman spectroscopy and X-ray diffraction (XRD) showed that the hydrothermal treatment of the solution at temperatures up to 180oC for two hours leads to formation of the V2O5nH2O monoclinic phase with a preferential orientation on 00l direction. From this temperature on the material obtained began to present the orthorhombic phase misleading its preferential orientation. Through thermogravimetry (TG) measurements it was seen that depending on the synthesis condition the samples contains different amounts of water molecules (H2O) interleaved in their structure. Images of transmission electron microscopy (TEM) showed that the V2O5nH2O compound could have different morphologies such as ribbons, wires and rods. From X-Ray Absorption spectroscopy (XAS) measurements, it was observed the presence of V4+ ions in the monoclinic phase samples, which was not observed on orthorhombic structure samples. Through the XAS technique was also observed that the orthorhombic structure samples have a lower degree of disorder around the vanadium atom. Regarding the growth mechanism of these nanostructures, it was possible to verify that has been a wide growth which can be explained by the mechanism denominated Oriented Attachment (OA). Finally, through vanadium peroxide decomposition vanadates compounds containing Na+ cation in its structure were carried out

Indirect doping of microstructures fabricated by two-photon polymerization with gold nanoparticles

Nanoplasmonics and metamaterials sciences are rapidly growing due to their contributions to photonic devices fabrication with applications ranging from biomedicine to photovoltaic cells. Noble metal nanoparticles incorporated into polymer matrix have great potential for such applications due to their distinctive optical properties. However, methods to indirectly incorporate metal nanoparticles into polymeric microstructures are still on demand. Here we report on the fabrication of two-photon polymerized microstructures doped with gold nanoparticles through an indirect doping process, so they do not interfere in the two-photon polymerization (2PP) process. Such microstructures present a strong emission, arising from gold nanoparticles fluorescence. The microstructures produced are potential candidates for nanoplasmonics and metamaterials devices applications and the nanoparticles production method can be applied in many samples, heated simultaneously, opening the possibility for large scale processes.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Long-range and short-range structures of cube-like shape SrTiO3 powders: microwave-assisted hydrothermal synthesis and photocatalytic activity

We report herein a detailed study on the influence of microwave-assisted hydrothermal (MAH) treatment time on both long and short range structures around Ti atoms of SrTiO3 powders. Few studies have been carried out on short-order structural properties as well as the relationship between the local order and the SrTiO3 photocatalytic properties. We use X-ray diffraction to determine the long-range structure, while the local environment around the Ti atom is probed with X-ray absorption spectroscopy and the vibration frequencies are investigated by Raman spectroscopy. The faster crystallization of SrTiO3 powders provided by the MAH system resulted in large distortions of Ti-O bond lengths which remain unchanged even for a longer MAH treatment time. Despite the long-range structure being associated with ideal TiO6 clusters, X-ray absorption spectroscopy measurements identified the presence of undercoordinated TiO5 clusters. Compared with the reference bulk SrTiO3, the hierarchical SrTiO3 cube-like shape showed enhanced photocatalytic activity, which was associated with the presence of these TiO5 clusters. Field emission scanning electron microscopy (FE-SEM) revealed that the superstructures based on a cube-like shape are formed by an assembly process, becoming well defined as a function of MAH treatment time.FAPESPMCTI/CNPq (70/2008)Generalitat Valenciana (Prometeo/2009/053)MICINN (CTO2009-14541-C02)Programa Iberoamericano de Ciencia y Tecnología para el Desarrollo (CYTED)MECD (PHB2009-0065-PC

Optical and Luminescent Properties of CdSe/ZnS and TiO2 Semiconductor Quantum Dots Embedded into PMMA Layers

International audienceOptical quality PMMA thin film layers including different concentrations of 3nm CdSe/ZnS and 5 nm TiO 2 semiconductor quantum dots have been made by spin coating method. Their optical properties show evident consequences of the confinement effects with wavelength bandgap shifting for both types of QDs. The confinement effects are more pronounced for CdSe/ZnS QDs than for TiO 2 QDs. Layers including CdSe/ZnS QDs exhibit a strong luminescent behavior centred on 560nm with a stock-shift in front of the first exciton absorption peak