1,720,965 research outputs found
Energy-efficient NO2 sensors based on two-dimensional layered 2H-WS2 nanosheets
No full textLayered transition metal dichalcogenides (TMDCs) are considered among the next-generation materials for gas sensing. Here, we report exfoliated 2 H-WS2 nanosheets for the fabrication of highly performing NO2 sensors. Thermal annealing at several temperatures was performed to investigate the oxidation of WS2. The long-term stability of 2 H-WS2 bulk was verified. Using droplet variation method, three batches of conductometric sensors from 2 H-WS2 dispersions were fabricated on electrical transducers, namely two layers (2 L), five layers (5 L) and ten layers (10 L) WS2 nanosheets. These sensors were tested towards low NO2 concentrations at different temperatures (Room Temperature (20 °C), 50 °C and 100 °C) and relative humidity (RH) levels (20%, 40%, 60%, 80% and 90% RH). 2 L-WS2 based sensor showed the highest response at room temperature (RT). Excellent repeatability (4 cycles) towards 1 ppm NO2 and long-term stability (more than two months) were achieved. Full selectivity towards NO2 (1 ppm) at RT was observed over NH3 (15 ppm), H2S (15 ppm), ethanol (30 ppm) and acetone (30 ppm). Our results confirm that low-power consumption devices with high sensitivity (even at high RH), long-term stability and excellent selectivity towards NO2 were fabricated using 2 H-WS2 nanosheets
Carbon nanotubes synthesis using siliceous breccia as a catalyst source
No full textIn this work, siliceous breccia, a natural rock powder composed essentially of SiO2 α-quartz, has been employed directly as a catalyst without any chemical treatment for the synthesis of carbon nanotubes (CNTs) via chemical vapor deposition (CVD). In addition to quartz, it contains dispersed micro-inclusions of iron oxide-hydroxides, goethite or hematite, which act as catalysts to dissociate the hydrocarbon precursors and form carbon nanostructures. The catalytic performance of this powder was evaluated for C2H4 decomposition at 750 °C, with and without H2 flux. Thermal oxidation stability and carbon yield were measured by means of thermogravimetric analysis. Structural and vibrational characterization of the resulting material was carried out by environmental scanning electron microscopy (ESEM) and micro Raman spectroscopy. ESEM images show that the H2 addition affects the CNT diameter. We observed that the addition of H2 to the C2H4 flux effectively removes amorphous carbon leading to taller growth of nanotubes. This concept provides a highly economical way for large-scale synthesis of CNTs
Room-Temperature NO2 Sensing of CVD-Modified WS2-WSe2 Heterojunctions
No full textTwo-dimensional (2D) semiconducting heterojunction chemical sensors are in high demand because of their enhanced response, stability, and selectivity. However, fine-tuning heterojunctions using vapor deposition growth still needs further research. Our present study focuses on the ambient pressure chemical vapor deposition (CVD) synthesis of hexagonal tungsten sulfide-tungsten selenide (WS2-WSe2) p-p heterojunctions (as a 2D-2D arrangement). We use the liquid-phase exfoliation method to disperse bulk WS2 and WSe2 and decorate large flakes of WS2 with smaller WSe2 nanosheets in CVD. Electron microscopy and related surface investigations reveal their homogeneity on drop-casting. Two drops from the exfoliated heterojunction dispersion were drop-cast on a transducer to study the NO2 response and related sensing properties. The sensor showed long-term stability (>2 months), even at high humidity levels (40-90%). The gas-sensing properties of this layered p-p heterojunction-based nanocomposite strongly suggest an affinity toward NO2 gas, leading to improved response, high stability, independent of humidity effects, and high selectivity
Natural Laterite as a Catalyst Source for the Growth of Carbon Nanotubes and Nanospheres
No full textCarbon nanotubes (CNTs), the closest structure to ideal one-dimensional (1D) conductors, have stimulated substantial interest in the last decades for many applications in the field of nanotechnology. Unfortunately, the high cost of efficient metal catalysts limits the large-scale exploitation of carbon nanomaterials' synthesis processed by chemical vapor deposition (CVD). However, minor or even trace amounts of metal or metal oxides in the ideal form to be used as catalysts can be easily found in almost all-natural materials. Herein, we report on the synthesis of carbon nanotubes and nanospheres obtained via CVD from a natural laterite, as a catalyst source. The synthesized nanostructures were carefully analyzed by X-ray diffraction (XRD), environmental scanning electron microscopy (ESEM), high-resolution transmission electron microscopy (HR-TEM), micro-Raman spectroscopy, and thermogravimetric analysis (TGA). In particular, we investigated and discussed the structural properties of the catalyst nanoparticles and of the produced carbon nanomaterials as well as the influence of temperature on the activity of the laterite based catalyst. At 700 °C, mainly CNTs grew, whereas at 800 °C carbon nanospheres start to form and they become clearly visible in the form of continuous networks of spheroidal structures in the samples grown at 900 °C. The obtained yields indicate that it could be possible to scale up the synthesis of CNTs to be used in technological applications, starting from natural mineral oxide sources.
Robust Room-Temperature NO2Sensors from Exfoliated 2D Few-Layered CVD-Grown Bulk Tungsten Di-selenide (2H-WSe2)
No full textWe report a facile and robust room-temperature NO2 sensor fabricated using bi- and multi-layered 2H variant of tungsten di-selenide (2H-WSe2) nanosheets, exhibiting high sensing characteristics. A simple liquid-assisted exfoliation of 2H-WSe2, prepared using ambient pressure chemical vapor deposition, allows smooth integration of these nanosheets on transducers. Three sensor batches are fabricated by modulating the total number of layers (L) obtained from the total number of droplets from a homogeneous 2H-WSe2 dispersion, such as ∼2L, ∼5-6L, and ∼13-17L, respectively. The gas-sensing attributes of 2H-WSe2 nanosheets are investigated thoroughly. Room temperature (RT) experiments show that these devices are specifically tailored for NO2 detection. 2L WSe2 nanosheets deliver the best rapid response compared to ∼5-6L or ∼13-17L. The response of 2L WSe2 at RT is 250, 328, and 361% to 2, 4, and 6 ppm NO2, respectively. The sensor showed nearly the same response toward low NO2 concentration even after 9 months of testing, confirming its remarkable long-term stability. A selectivity study, performed at three working temperatures (RT, 100, and 150 °C), shows high selectivity at 150 and 100 °C. Full selectivity toward NO2 at RT confirms that 2H-WSe2 nanosheet-based sensors are ideal candidates for NO2 gas detection
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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