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An unstable energy-unbalanced state such as superheating or supercooling is often unexpectedly observed because a factor of energy depends not only on the temperature but is a product of temperature (T) and entropy (S). Thus, at the same temperature, if the entropy is different, the total energy of the system can be different. In such cases, the temperature-change-rate cannot match the entropy-change-rate, which results in a hysteresis curve for the temperature/entropy relationship. Due to the difference between the temperature- and entropy-change-rates, properties of a material, such as the boiling and freezing points, can be extended from point to area. This study confirmed that depending on the heating rate, tin powders exhibit different melting points. Given the contemporary reinterpretation of many energy-non-equilibrium phenomena that have only been discussed on the basis of temperature, this study is expected to contribute to the actual expansion of scientific/engineering applications.
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Breakthrough process technologies have been introduced that can increase the chemical sensitivity of an interface at which reactions occur without significantly altering the physico-chemical properties of the material. Such an interfacial treatment method is based on amorphous-carbon as a base so that fluids can be deposited, and the desired thickness and quality of the deposition can be ensured irrespective of the interface state of the material. In addition, side effects such as diffusion and decreasing strength at the interface can be avoided. This is simpler than existing vacuum-based deposition technology and it has an unmatched industrial advantage in terms of economics, speed, accuracy, reliability, accessibility, and convenience. In particular, this amorphous-carbon interface treatment technology has been demonstrated to improve gas-sensing characteristics of NO2 at room temperature.
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Theoretical advances in science are inherently time-consuming to realise in engineering, since their practical application is hindered by the inability to follow the theoretical essence. Herein, we propose a new method to freely control the time, cost, and process variables in the fabrication of a hybrid featuring Au nanoparticles on a pre-formed SnO2 nanostructure. The above advantages, which were divided into six categories, are proven to be superior to those achieved elsewhere, and the obtained results are found to be applicable to the synthesis and functionalisation of other nanostructures. Furthermore, the reduction of the time-gap between science and engineering is expected to promote the practical applications of numerous scientific theories.
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A simple yet powerful flame chemical vapor deposition technique is proposed that allows free control of the surface morphology, microstructure, and composition of existing materials with regard to various functionalities within a short process time (in seconds) at room temperature and atmospheric pressure as per the requirement. Since the heat energy is directly transferred to the material surface, the redox periodically converges to the energy dynamic equilibrium depending on the energy injection time; therefore, bidirectional transition between the semiconductor/metal is optionally available. To demonstrate this, a variety of Sn-based particles were created on preformed SnO2 nanowires, and this has been interpreted as a new mechanism for the response and response times of gas-sensing, which are representative indicators of the most surface-sensitive applications and show one-to-one correspondence between theoretical and experimental results. The detailed technologies derived herein are clearly influential in both research and industry.
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The use of conventional doping methods requires consideration of not only the energy connection with the base material but also the limits of the type and doping range of the dopant. The scope of the physico-chemical change must be determined from the properties of the base material, and when this limit is exceeded, a large energy barrier must be formed between the base material and the dopant as in a heterojunction. Thus, starting from a different viewpoint, we introduce a so-called metallization of surface reduction method, which easily overcomes the disadvantages of existing methods while having the effect of doping the base material. Such new synthetic techniques enable sequential energy arrangements-gradients from the surface to the centre of the material-so that free energy transfer effects can be obtained as per the energies in the semiconducting band, eliminating the energy discontinuity of the heterojunction.
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A new gas sensor working in room temperature, which is compatible with silicon fabrication technology is presented. Porous silicon nanowires (NWs) were synthesized by metal-assisted chemical etching method and then TeO2 NWs branches were attached to their stem by thermal evaporation of Te powders in the presence of air. Afterwards TeO2 branched porous Si NWs were functionalized by Pt via sputtering followed by low temperature thermal annealing. Scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy collectively confirmed successful formation of TeO2 branched porous Si NWs functionalized by Pt nanoparticles. Their gas sensing properties in the presence of CO, C6H6 and C7H8 were tested at room temperature, for Si wafer, pristine porous Si NWs, pristine TeO2 branched porous Si NWs, and Pt functionalized TeO2 branched porous Si NWs sensors. Pt functionalized TeO2 branched porous Si NWs have higher responses to all tested gases than the other sensors. The origin of high response is discussed in detail. This new room temperature gas sensor can open a new aperture for development of gas sensors with minimum energy consumption which are compatible with silicon fabrication technology.
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We report the room-temperature sensing characteristics of Si nanowires (NWs) fabricated from p-Si wafers by a metal-assisted chemical etching method, which is a facile and low-cost method. X-ray diffraction was used to the the study crystallinity and phase formation of Si NWs, and product morphology was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After confirmation of Si NW formation via the SEM and TEM micrographs, sensing tests were carried out at room temperature, and it was found that the Si NW sensor prepared from Si wafers with a resistivity of 0.001-0.003 Ω.cm had the highest response to NO2 gas (Rg/Ra = 1.86 for 50 ppm NO2), with a fast response (15 s) and recovery (30 s) time. Furthermore, the sensor responses to SO2, toluene, benzene, H2, and ethanol were nearly negligible, demonstrating the excellent selectivity to NO2 gas. The gas-sensing mechanism is discussed in detail. The present sensor can operate at room temperature, and is compatible with the microelectronic fabrication process, demonstrating its promise for next-generation Si-based electronics fused with functional chemical sensors.
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We obtained extremely high and selective sensitivity to NO2 gas by fabricating graphene-SnO2 nanocomposites using a commercial microwave oven. Structural characterization revealed that the products corresponded to agglomerated structures of graphene and SnO2 particles, with small secondary SnOx (x ≤ 2) nanoparticles deposited on the surfaces. The overall oxygen atomic ratio was decreased with the appearance of an SnOx (x < 2) phase. By the microwave treatment of graphene-SnO2 nanocomposites, with the graphene promoting efficient transport of the microwave energy, evaporation and redeposition of SnOx nanoparticles were facilitated. The graphene-SnO2 nanocomposites exhibited a high sensor response of 24.7 for 1 ppm of NO2 gas, at an optimized temperature of 150 °C. The graphene-SnO2 nanocomposites were selectively sensitive to NO2 gas, in comparison with SO2, NH3, and ethanol gases. We suggest that the generation of SnOx nanoparticles and the SnOx phase in the matrix results in the formation of SnO2/SnO2 homojunctions, SnO2/SnOx (x < 2) heterojunctions, and SnO2/graphene heterojunctions, which are responsible for the excellent sensitivity of the graphene-SnO2 nanocomposites to NO2 gas. In addition, the generation of surface Sn interstitial defects is also partly responsible for the excellent NO2 sensing performance observed in this study.
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SiOx structures with different diameters of a few hundreds of nanometers and/or a few micrometers are prepared using applied thermal evaporation. Subsequently, Sn quantum dot-based SiOx architectures are synthesized via the continuous steps of the carbothermal reduction of SnO2, substitution of Sn(4+) for In(3+), thermal oxidation of Si, Sn sublimation, interfacial reaction, and diffusion reaction consistent with corresponding phase equilibriums. Several crystalline and spherical-shaped Sn quantum dots with diameters between 2 and 7 nm are observed in the amorphous SiOx structures. The morphological evolution, including hollow Sn (or SnOx) sphere and wire-like, worm-like, tube-like, and flower-like SiOx, occurs stepwise on the Si substrate upon increasing the given process energies. The optical characteristics based on confocal measurements reveal the as-synthesized SiOx structures, irrespective of whether crystallinity is formed, which all have visible-range emissions originating from the numerous different-sized and -shaped Sn quantum dots permeating into the SiOx matrix. In addition, photoluminescence emissions ranging between ultraviolet and red regions are in agreement with confocal measurements. The origins of the morphology- and luminescence-controlled amorphous SiOx with Sn quantum dots are also discussed.
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Balloon whisk-like and flower-like SiOx tubes with well-dispersed Sn and joining countless SiOx loops together induce intense luminescence characteristics in substrate materials. Our synthetic technique called "direct substrate growth" is based on pre-contamination of the surroundings without the intended catalyst and source powders. The kind of supporting material and pressure of the inlet gases determine a series of differently functionalized tube loops, i.e., the number, length, thickness, and cylindrical profile. SiOx tube loops commonly twist and split to best suppress the total energy. Photoluminescence and confocal laser measurements based on quantum confinement effect of the embedded Sn nanoparticles in the SiOx tube found substantially intense emissions throughout the visible range. These new concepts related to the synthetic approach, pre-pollution, transitional morphology, and permeable nanoparticles should facilitate progress in nanoscience with regard to tuning the dimensions of micro-/nanostructure preparations and the functionalization of customized applications.
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We coated zinc-oxide (ZnO)-branched tin oxide (SnO2) nanowires with a Pt shell layer via a sputtering method and subsequently annealed the composite to generate Pt nanoparticles. The spillover effect of Pt nanoparticles was expected to play a significant role in enhancing the response. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy revealed that the nanoparticles were comprised of a cubic Pt phase. A sensing test with NO2 gas revealed that the sensor response to NO2 gas was significantly increased, being related to the spillover effect of the Pt nanoparticles. As a result of the Pt-functionalization, the sensor response time and recovery time were decreased and increased, respectively. The high sensor response and fast response time make Pt-functionalized ZnO branched nanowires a promising candidate for gas sensors. The present work will be useful in exploring new areas of multiple-component nanosystems.
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In this paper, we detail improvements in the sensing properties of reduced graphene oxide (RGO), which were achieved through functionalization. The functionalization process utilizes graphene oxide suspensions, generating nanoparticles on the RGO surface mainly comprised of Ni(OH)2 phase. Raman spectra indicate that functionalization increases the degree of disorder in RGOs. NO2 gas sensing tests reveal an approximate increase of 154% in the sensor response of the RGOs after functionalization. Possible mechanisms for improving sensing responses via functionalization are discussed. The enhancement is due to the spillover effect, to the increase of the sensor surface by the catalytic particles, to the reduction of RGO conduction volume through the generation of depletion region, and to the resistance modulation of the heterojunctions.
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We have characterized and evaluated changes in graphene oxide (GO) induced by means of freeze-drying. In order to evaluate these changes, we investigated the effects of freeze-drying and chemical reduction processes on the structure, morphology, chemical composition, and Raman properties of GO and reduced GO. The freeze-dried GO had a pore structure, maintaining a pored morphology even after thermal annealing. The freeze-dried samples were composed of a single folded nanosheet or a few nanosheets stacked and folded. The oxygen-containing functional groups were removed not only during the freeze-drying but also during the reduction processes, with an accompanying decrease in the average size of the sp(2) carbon domain (i.e. an increase in the ID/IG value).
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We fabricated SnO2/cobalt (Co) core-shell nanowires by means of a two-step process, for their application as chemical sensors. For Co-functionalization, we synthesized SnO2-Co core-shell nanowires by the sputtering deposition of Co layers on the surface of networked SnO2 nanowires, subsequently transforming the continuous Co-shell layers into crystalline islands by thermal heating. While scanning electron microscopy (SEM) images of annealed core-shell nanowires exhibited a rough surface, transmission electron microscopy (TEM) images revealed that the roughness is related to the agglomeration of the sputtered Co layer. The X-ray diffraction (XRD) pattern and lattice-resolved TEM images coincidentally indicated that the agglomerated particles are comprised of a hexagonal Co phase. The NO2 sensing test revealed that the sensor response was enhanced by decoration with Co nanoparticles. In addition, both response and recovery times tended to decrease as a result of the Co-functionalization. This indicates that the Co-functionalized SnO2 nanowire sensors can be used to sense gases at very low concentrations. We discussed possible mechanisms for enhancing sensor properties by Co-functionalization. The NO2 gas sensing test demonstrated the ability of the Co-functionalization to provide higher sensitivity, shorter response time, and shorter recovery time than would bare SnO2 nanowires.
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We report the fabrication of thin (< 100 nm) hexagonal Zn nanowires in a conventional reactor, by heating a mixture of Zn and graphite powders. By material characterization, the products were identified as one-dimensional nanowires of serpent-like morphology with a hexagonal Zn phase. The main growth mechanism of the Zn nanowires was proposed to be a vapor-solid process, which was corroborated by the absence of any tip catalyst. Raman spectra of the Zn nanowires exhibited a prominent peak at around 570 cm(-1). X-ray photoelectron spectroscopy revealed that the surface of the Zn nanowires was clearly oxygen-deficient in comparison to that of ZnO nanowires. Photoluminescence analysis indicated that the Zn nanowires exhibited emission bands centered at 1.6, 2.0, 2.4, 3.0, and 3.3 eV, respectively.
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We fabricated SnO2/Pt core-shell nanowires by means of a two-step process, in which Pt layers were sputtered onto the surface of networked SnO2 nanowires. For Pt-functionalization, we have synthesized the SnO2-Pt core-shell nanowires by depositing Pt layers using a sputtering method on bare SnO2 nanowires, subsequently annealing and thus transforming the continuous Pt shell layers into Pt nanoparticles. The NO2 gas sensing test demonstrated the ability of the Pt functionalization to attain the higher sensitivity and faster response than bare SnO2 nanowires. The possible mechanisms for improvment of the sensing properties by Pt-functionalization were discussed.
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We report a significant enhancement in the gas sensing properties of In(2)O(3) nanowires by functionalizing their surfaces with Pt nanoparticles. For Pt-functionalization, In(2)O(3)-Pt core-shell nanowires are synthesized by the sputtering deposition of Pt layers on bare In(2)O(3) nanowires. Next, continuous Pt shell layers are transformed into Pt nanoparticles of cubic phase by heat treatment. In an O(2) gas sensing test, the Pt-functionalized In(2)O(3) nanowires reveal exceptionally higher sensitivity and faster response than bare In(2)O(3) nanowires.
