Effect of Pressure-Assisted Heat Treatment on Photoluminescence Emission of α-Bi2O3 Needles

Materials with high photoluminescence (PL) intensity can potentially be used in optical and electronic devices. Although the PL properties of bismuth(III) oxide with a monoclinic crystal structure (α-Bi2O3) have been explored in the past few years, methods of increasing PL emission intensity and information relating PL emission to structural defects are scarce. This research evaluated the effect of a pressure-assisted heat treatment (PAHT) on the PL properties of α-Bi2O3 with a needlelike morphology, which was synthesized via a microwaveassisted hydrothermal (MAH) method. PAHT caused an angular increase between the [BiO6]−[BiO6] clusters of α- Bi2O3, resulting in a significant increase in the PL emission intensity. The Raman and XPS spectra also showed that the α- Bi2O3 PL emissions in the low-energy region (below ∼2.1 eV) are attributed to oxygen vacancies that form defect donor states. The experimental results are in good agreement with first-principles total-energy calculations that were carried out within periodic density functional theory (DFT).This research was funded by The State of Sao Paulo Research Foundation (FAPESP), Research Award No. CEPID 2013/07296-2. Author S. Schmidt is grateful to the Coordination for the Improvement of Higher Education Personnel (Capes) for her doctoral scholarship. This work was also supported by Spanish MALTA-Consolider Ingenio 2010 Program (Project CSD2007-00045). The authors are also grateful to Prof. Juan Andrés for his collaboration in the preparation of this article

Toward an Understanding of the Growth of Ag Filaments on α-Ag2WO4 and Their Photoluminescent Properties: A Combined Experimental and Theoretical Study

A combined experimental and theoretical study was conducted on the structure and electronic properties of α-Ag2WO4 to clarify the nucleation and growth processes of Ag filaments on α-Ag2WO4 crystals induced by electron beam irradiation under electron microscopy. X-ray diffraction with Rietveld analysis, micro-Raman and Fourier-transform infrared spectroscopy were used to analyze the structural order/disorder of α-Ag2WO4 crystals. These complementary techniques indicated that the microwave-assisted hydrothermal method employed in the synthesis of α-Ag2WO4 crystals leads to the freezing of distorted [WO6] and [AgOy] (y = 2, 4, 6 and 7) clusters as the constituent polyhedra of α-Ag2WO4. On the basis of the theoretical and experimental results, we provide a complete assignment of the structure of α-Ag2WO4 and describe the relationship among the disorder, nucleation growth, rate of Ag formation, and photoluminescence behavior before and after the irradiation of the accelerated electron beam. Density functional theory (DFT) studies indicated significant changes in the order–disorder of the initial α-Ag2WO4electronic structure, with a decrease in the band gap value from 3.55 to 2.72 eV. The first stages of the electron irradiation on α-Ag2WO4 crystal were investigated by DFT calculations, and we have derived a mechanism to describe the formation and growth of Ag filaments during the electronic excitation of the [AgO2] cluster.This work is financially supported by the National Council for Scientific and Technological Development (CNPq), São Paulo Research Foundation (FAPESP), Prometeo/2009/053 (Generalitat Valenciana) and Ministerio de Economía y Competitividad (Spain), CTQ2012-36253-C03-02, and the Spanish–Brazilian program (PHB2009-0065-PC) for their financial support. TEM facilities were provided by LME-IQ-UNESP

The interplay between morphology and photocatalytic activity in ZnO and N-doped ZnO crystals

Photocatalytic materials can perform oxidative and reductive reactions over their surfaces when excited with light. Intrinsic characteristics of the material such as superficial area, morphological structure, and crystalline phase exposition play a fundamental role in the corresponding reaction paths. However, especially in doped semiconductors, as ZnO:N, less is known about how the synthesis parameters affect the morphologies and the photocatalytic activity simultaneously. To solve this issue, ZnO and ZnO:N samples were obtained using microwave-assisted hydrothermal and modified polymeric precursor methods of synthesis. Samples morphologies were characterized by TEM and FE-SEM. Crystallographic phases were observed by XRD and optical characteristics by DRS. XPS results confirmed the doping process. Degradation of Rhodamine-B and Cr(VI) reduction were employed as probe reactions to investigate their photocatalytic activity. Although the crystallographic structure of these powders maintains the ZnO hexagonal wurtzite structure, the optical properties and morphologies, and photocatalytic activities present different behaviors. Also, density functional theory calculations were employed to determine the specific features related to electronic structure, morphology, and photocatalytic activity. Different synthesis methods produce a singular behavior in the physicochemical properties of materials, and the doping effect produces various modifications in RhB degradation and Cr(VI) reduction for each synthesis method. Crystal face exposition and morphologies are related to the improvement in the photocatalytic activity of the materials.The authors acknowledge São Paulo Research Foundation (FAPESP), grants #2014/17343-0, #2015/04511-5, #2013/26671-9, #CEPID 2013/ 07296-2. National Council for Scientific and Technological Development – CNPQ, grant #444926/2014-3. J. A. acknowledges the financial support of the Spanish research funding projects: PrometeoII/2014/022 and ACOMP/2014/270 and ACOMP72015/1202 projects (Generalitat Valenciana), CTQ2015-652017-P and Salvador Madariaga program, PRX15/00261 (Ministerio de Economía y Competitividad) and Spanish-Brazilian Program (PHBP14-00020). The XPS facilities were provided by LNNano/CNPEM (Brazilian Nanotechnology Nation Laboratory, Proposal no. 20389)

Structural and electronic analysis of the atomic scale nucleation of Ag on α-Ag2WO4 induced by electron irradiation

In this work, we utilise a combination of theory, computation and experiments to understand the early events related to the nucleation of Ag filaments on α-Ag2WO4 crystals, which is driven by an accelerated electron beam from an electron microscope under high vacuum. The growth process and the chemical composition and elemental distribution in these filaments were analysed in depth at the nanoscale level using TEM, HAADF, EDS and XPS; the structural and electronic aspects were systematically studied in using first-principles electronic structure theory within QTAIM framework. The Ag nucleation and formation on α-Ag2WO4 is a result of the order/disorder effects generated in the crystal by the electron-beam irradiation. Both experimental and theoretical results show that this behavior is associated with structural and electronic changes of the [AgO2] and [AgO4] clusters and, to a minor extent, to the [WO6] cluster; these clusters collectively represent the constituent building blocks of α-Ag2WO4

Room-Temperature Triethylamine Sensor Based on Reduced Graphene Oxide/CeO2_2 Nanocomposites

Triethylamine (TEA) detection has been widely performed by using chemoresistive sensors. Nevertheless, chemoresistive sensors still exhibit limitations to be addressed in terms of power consumption and humidity interference, and developing efficient TEA sensors operated at low temperatures and humid conditions remains a challenge. Here, we present the effect of reduced graphene oxide (RGO) on the TEA-sensing performance of CeO$_2$ nanospheres at room temperature and a relative humidity (RH) range of 34–70%. We show that CeO$_2$ is a suitable sensing material for TEA detection at room temperature and humidity conditions; however, the modification with RGO greatly improves the TEA-sensing performance. The RGO/CeO$_2$ nanocomposite has higher sensitivity and selectivity to TEA than the bare CeO$_2$-based sensor, in addition to the low theoretical detection limit of 1 ppm at 70% RH. Moreover, we elucidate that humidity plays a positive role in the detection of TEA. Our findings elucidate that RGO positively affects the sensing performance of CeO$_2$ nanospheres, which can be attributed to the improvements in the baseline electrical resistance and enhancement of the active sites for TEA adsorption due to the RGO modification. This work provides a promising strategy for developing sensitive TEA sensors with practical applications

Efficient microwave-assisted hydrothermal synthesis of CuO sea urchin-like architectures via a mesoscale self-assembly

For the first time, we provide evidence for both heterogeneous nucleation at the mesoscopic scale and the progression of the non-classical crystallization process during the microwave synthesis of cupric oxide (CuO) with sea urchin-like morphology. This work sheds light on the general mechanism of the growth of CuO architectures.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Low-Temperature Carbon Dioxide Gas Sensor Based on Yolk−Shell Ceria Nanospheres

Monitoring carbon dioxide (CO$_2$) levels is extremely important in a wide range of applications. Although metal oxide-based chemoresistive sensors have emerged as a promising approach for CO$_2$ detection, the development of efficient CO$_2$ sensors at low temperature remains a challenge. Herein, we report a low-temperature hollow nanostructured CeO$_2$-based sensor for CO$_2$ detection. We monitor the changes in the electrical resistance after CO$_2$ pulses in a relative humidity of 70% and show the high performance of the sensor at 100 °C. The yolk–shell nanospheres have not only 2 times higher sensitivity but also significantly increased stability and reversibility, faster response times, and greater CO$_2$ adsorption capacity than commercial ceria nanoparticles. The improvements in the CO$_2$ sensing performance are attributed to hollow and porous structure of the yolk–shell nanoparticles, allowing for enhanced gas diffusion and high specific surface area. We present an easy strategy to enhance the electrical and sensing properties of metal oxides at a low operating temperature that is desirable for practical applications of CO$_2$ sensors

Reoxidation of graphene oxide: Impact on the structure, chemical composition, morphology and dye adsorption properties

The chemical composition, oxidation degree, and sheet size strongly affect the properties of graphene oxide (GO). Therefore, much effort has been directed to improve the synthesis of GO and control its structure. Herein, we report the reoxidation of GO using milder conditions of a modified Hummers’ method, including shorter reaction times and a reduced proportion of chemicals to obtain the reoxidized GO (Ox-GO). The reoxidation impact was evaluated by studying the materials’ adsorption performance towards methylene blue (MB) and rhodamine B (RB). Compared to GO, Ox-GO presents a similar C/O ratio, increased interlayer spacing, smaller sheets with holes, a higher exfoliation degree, and slight differences in each oxygenated functional group. Our measurements evidence that Ox-GO has an MB adsorption capacity more than 30% higher than GO under different conditions of dye concentration and pH. Moreover, Ox-GO also shows a notable improvement in the RB removal for a high dye concentration, where the removal capacity is almost 40% higher than that of GO. The enhancements in the dyes’ removal are attributed to the increased accessible surface area of Ox-GO, which provides more sites for dyes’ adsorption

Insight into Copper-Based Catalysts: Microwave-Assisted Morphosynthesis, In Situ Reduction Studies, and Dehydrogenation of Ethanol

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP