Tungsten oxide nanostructures peculiarity and photocatalytic activity for the efficient elimination of the organic pollutant.

The WO3 nanostructures were synthesized by a simple hydrothermal route in the presence of C14TAB and gemini-based twin-tail surfactant. The impact of using these special shape and size directing agents for the synthesis of nanostructures was observed in the form of different shapes and sizes. The WO3 web of chains type nanostructure was obtained using C14TAB in comparison to the cube-shaped nanoparticles through twin-tail surfactant. On contrary, the twin-tail surfactant provides sustainable and controlled growth of cube shape nanoparticles of size ~ 15 nm nearly half of the size ~ 35 nm obtained using conventional surfactant C14TAB, respectively. For the detailed structural features, the Williamson-Hall analysis method was implemented to find out the crystalline size and lattice strain of the prepared nanostructures. Owing to the strong quantum confinement effect, the WO3 cube-shaped nanoparticles with an optical band gap of 2.69 eV of the prepared nanoparticles showed excellent photocatalytic efficacy toward organic pollutant (fast green FCF) compared to the web of chain nanostructures with an optical band gap of 2.66 eV. The ability of the prepared systems to decompose the organic pollutant (fast green FCF) in water was tested under visible light irradiations. The percentage degradation was found to be 94% and 86% for WO3 cube-shaped nanoparticles and WO3 web of chains, respectively. The simplicity of the fabrication method and the high photocatalytic performance of the systems can be promising in environmental applications to treat water pollution.

Dimethylenebis-(tetra-decyldimethylammonium Bromide)-Driven Metal Nanoparticles: Hg2+ Sensing a Competency.

We report an excellent anisotropic Au nanoparticle-based colorimetric probe for the detection of Hg2+ ions with higher detection ability and selectivity. The manifestation of different morphologies of Au nanoparticles including round, triangular, rectangular, pentagonal, and hexagonal has been realized by the dimethylenebis-(tetra-decyldimethylammonium bromide) (14-2-14 Gemini surfactant) assisted one-step thermal reduction method where the average size of Au nanoparticles was 54.65 ± 44.3 nm. The growth and frequency of Au nanoparticles were enhanced as a function of Gemini surfactant's concentration. The detection limit as low as 1.8 nM was efficaciously achieved and was considerably lower than the required world standards defined the maximum allowable level of Hg2+ ions for health hazards. Notably, the Au nanoparticles showed visible detection for 100 µM Hg2+ ion by means of the change in the solution color from red to tarnish blue within 180 s followed by saturation in the absorption ratio (A LSPR/A TSPR). These results provide novel insight into the detection of the heavy metal ion using Gemini surfactant-assisted grown anisotropic metal nanoparticles. On the basis of obtained results, it is concluded that the size of metal nanoparticles is no longer critical for preparation of efficient selective chemoprobe; rather, growth of more number of edges provides a large number of sights for incoming moieties and plays an important role in improving the detection capability of the anisotropic metal nanoparticle irrespective of their large sizes. We believe that this work provides valuable insight into researchers working in the area of chemosensor applications.

Surface-area-controlled synthesis of porous TiO2 thin films for gas-sensing applications.

Surface-area-controlled porous TiO2 thin films were prepared via a simple sol-gel chemical route, and their gas-sensing properties were thoroughly investigated in the presence of typical oxidizing NO2 gas. The surface area of TiO2 thin films was controlled by developing porous TiO2 networked by means of controlling the TiO2-to-TTIP (titanium isopropoxide, C12H28O4Ti) molar ratio, where TiO2 nanoparticles of size ∼20 nm were used. The sensor's response was found to depend on the surface area of the TiO2 thin films. The porous TiO2 thin-film sensor with greater surface area was more sensitive than those of TiO2 thin films with lesser surface area. The improved sensing ability was ascribed to the porous network formed within the thin films by TiO2 sol. Our results show that surface area is a key parameter for obtaining superior gas-sensing performance; this provides important guidelines for preparing and using porous thin films for gas-sensing applications.

Realization of ppm-level CO detection with exceptionally high sensitivity using reduced graphene oxide-loaded SnO2 nanofibers with simultaneous Au functionalization.

We have realized the highly sensitive, selective ppm-level carbon monoxide (CO) detection based on graphene oxide (RGO) nanosheets-loaded SnO2 nanofibers with simultaneous Au functionalization. The interplay between RGO/Au and SnO2 in terms of transfer of charge carriers and modulation of potential barriers is responsible for the exceptionally high CO detectability.

Eicosyl ammoniums elicited thermal reduction alleyway towards gold nanoparticles and their chemo-sensor aptitude.

The construction of dimethylenebis(eicosyldimethylammonium bromide) surfactant-directed gold nanoparticles (NPs) has been accomplished via a one-pot thermal reduction of HAuCl4 with trisodium citrate. The effect of cationic twin-tail surfactants, dimethylenebis(hexadecyldimethylammonium bromide) (16-2-16), dimethylenebis(octadecyldimethylammonium bromide) (18-2-18) and dimethylenebis(eicosyldimethylammonium bromide) (20-2-20), and their concentrations on shape and size of Au nanoparticles was thoroughly investigated. The UV-Vis spectroscopy and transmission electron microscopy (TEM) results show that longer tail length surfactants act as shape-directing agents promoting diversified morphologies. The formation of multiple-shaped Au nanoparticles, such as round, hexagonal, pentagonal, triangular and rod-like, has been confirmed from microstructure analysis; among them, many triangular shapes enhanced at elevated levels of surfactant concentration. In addition, the triangular Au nanoparticles with truncated corners were changed to smooth corners as the hydrocarbon chain length increased from (18-2-18) to (20-2-20). The concentration and hydrocarbon tails of twin-tail surfactants strongly influence the size and structure of Au NPs. In addition, the Au NPs synthesized with the twin-tail surfactant (18-2-18) were found to be highly sensitive towards Hg(2+), which could be because of the preferential adsorption of Hg(0) on the lower energy facets of triangular-shaped Au NPs.

Grain-Size-Tuned Highly H2-Selective Chemiresistive Sensors Based on ZnO-SnO2 Composite Nanofibers.

We investigated the effect of grain size on the H2-sensing behavior of SnO2-ZnO composite nanofibers. The 0.9SnO2-0.1ZnO composite nanofibers were calcined at 700 °C for various times to control the size of nanograins. A bifunctional sensing mechanism, which is related not only to the SnO2-SnO2 nanograins, but also to the ZnO-SnO2 nanograins with surface metallization effect, is responsible for the grain-oriented H2-sensing properties and the selective improvement in sensing behavior to H2 gas compared to other gases. Smaller grains are much more favorable for superior H2 sensing in SnO2-ZnO composite nanofibers, which will be an important guideline for their use in H2 sensors. The one-dimensional nanofiber-based structures in the present study will be efficient in maximizing the sensing capabilities by providing a larger amount of junctions.

Low Temperature Sensing Properties of Pt Nanoparticle-Functionalized Networked ZnO Nanowires.

Networked ZnO nanowires were fabricated via a vapor-phase selective growth method. Pt nanoparticles were functionalized on the networked ZnO nanowires. In this study, for the functioanlization, γ-ray radiolysis was applied. By the method, Pt nanoparticles of - 10 nm in diameter were uniformly anchored on the surface of each ZnO nanowire. The sensing properties of the Pt-functionalized, networked ZnO nanowires were investigated in terms of NO2, CO and benzene at 100 degrees C. The sensing capability of the Pt-functionalized ZnO nanowires at that temperature supports their potential use in chemical gas sensors.

Chemiresistive sensing behavior of SnO2 (n)-Cu2O (p) core-shell nanowires.

We report the synthesis of SnO2-Cu2O n-p core-shell nanowires (C-S NWs) and their use as chemiresistive sensors for detecting trace amounts of gas. The n-p C-S NWs were synthesized by a two-step process, in which the core SnO2 nanowires were prepared by the vapor growth technique and subsequently the Cu2O shell layers were deposited by atomic layer deposition. A systematic investigation of the sensing capabilities of the n-p C-S NWs, particularly as a function of shell thickness, revealed the underlying sensing mechanism. The radial modulation of the hole-accumulation layer is intensified under shells thinner than the Debye length. On the other hand, the contribution of volume fraction to resistance modulation is weakened. By the combination of these two effects, an optimal sensing performance for reducing gases is obtained for a critical p-shell thickness. In contrast, the formation of p-shell layers deteriorates the NO2-sensing performance by blocking the expansion of the hole-accumulation layer due to the presence of p-n heterointerface.

Bifunctional Sensing Mechanism of SnO2-ZnO Composite Nanofibers for Drastically Enhancing the Sensing Behavior in H2 Gas.

SnO2-ZnO composite nanofibers fabricated using an electrospinning method exhibited exceptional hydrogen (H2) sensing behavior. The existence of tetragonal SnO2 and hexagonal ZnO nanograins was confirmed by an analysis of the crystalline phase of the composite nanofibers. A bifunctional sensing mechanism of the composite nanofibers was proposed in which the combined effects of SnO2-SnO2 homointerfaces and ZnO-SnO2 heterointerfaces contributed to an improvement in the H2 sensing characteristics. The sensing process with respect to SnO2-ZnO heterojunctions is associated not only with the high barrier at the junctions, but also the semiconductor-to-metallic transition on the surface of the ZnO nanograins upon the introduction of H2 gas.

O2 sensing dynamics of BiFeO3 nanofibers: effect of minor carrier compensation.

In this paper we investigate O(2) sensing dynamics in BiFeO(3) (BFO) nanofibers at various concentrations and temperatures, by using a combined experiment and computer simulation approach. Samples of pristine BFO, Ni-doped BFO, and Pb-doped BFO nanofibers were prepared. By incorporating Ni and Pb, additional acceptor states are introduced in BFO. Density functional theory calculations show that Ni prefers to substitute Fe site while Pb substitutes Bi site, resulting in a new deep donor originating from Ni interstitial defects, along with oxygen vacancies (V(o)). We find that both the sensing response and recovery time are shorter in samples made of pristine BFO nanofibers than in Ni- and Pb-doped nanofiber samples. We interpret the observed sensing dynamics through charge transport theory of the major (acceptors) and minor (donors) carriers, and found that the minor carrier compensation plays a significant role in determining the response and recovery time of the sensor device. This minor carrier compensation charge transport mechanism will provide new insights into more robust sensor development strategies, and into the research of ion-electron coupling in chemical dynamics of semiconductors.

Extraordinary improvement of gas-sensing performances in SnO2 nanofibers due to creation of local p-n heterojunctions by loading reduced graphene oxide nanosheets.

We propose a novel approach to improve the gas-sensing properties of n-type nanofibers (NFs) that involves creation of local p-n heterojunctions with p-type reduced graphene oxide (RGO) nanosheets (NSs). This work investigates the sensing behaviors of n-SnO2 NFs loaded with p-RGO NSs as a model system. n-SnO2 NFs demonstrated greatly improved gas-sensing performances when loaded with an optimized amount of p-RGO NSs. Loading an optimized amount of RGOs resulted in a 20-fold higher sensor response than that of pristine SnO2 NFs. The sensing mechanism of monolithic SnO2 NFs is based on the joint effects of modulation of the potential barrier at nanograin boundaries and radial modulation of the electron-depletion layer. In addition to the sensing mechanisms described above, enhanced sensing was obtained for p-RGO NS-loaded SnO2 NFs due to creation of local p-n heterojunctions, which not only provided a potential barrier, but also functioned as a local electron absorption reservoir. These mechanisms markedly increased the resistance of SnO2 NFs, and were the origin of intensified resistance modulation during interaction of analyte gases with preadsorbed oxygen species or with the surfaces and grain boundaries of NFs. The approach used in this work can be used to fabricate sensitive gas sensors based on n-type NFs.

Highly sensitive and selective H2 sensing by ZnO nanofibers and the underlying sensing mechanism.

We report, and propose a mechanism for, the exceptional hydrogen gas (H2) sensing ability of ZnO nanofibers. In comparison to SnO2 nanofibers, ZnO nanofibers show outstanding H2 gas response and unmistakable H2 selectivity. Different from the reducing gas effect observed in SnO2 nanofibers, a semiconductor-to-metal transition that occurs in the presence of H2 gas molecules is responsible for the exceptional response and selectivity of ZnO nanofibers to H2. Notably, the presence of nanograins within nanofibers further intensifies the resistance modulation observed due to this transition.

CuO/SnO2 Mixed Nanofibers for H2S Detection.

In this work we report the synthesis of copper oxide/tin oxide (CuO/SnO2) mixed nanofibers and their gas sensing properties in terms of H2S gas. The CuO/SnO2 mixed nanofibers were synthesized by electrospinning technique using two needles. Based on the thermogravimetric-differential thermal analysis, the calcination temperature was optimized at 700 degrees C to acquire both phases of CuO and SnO2. With this method, intermixed nanofibers of SnO2 and CuO were obtained. The sensing properties of the CuO/SnO2 mixed nanofibers to H2S are investigated as functions of operating temperature and gas concentration. The CuO/SnO2 mixed nanofibers were highly sensitive towards H2S with a response 522 for 10 ppm H2S and a response time 1 s at 300 degrees C. The semiconductor-metal transition of CuO due to H2S is likely to the reason of the high H2S response. The results evidently demonstrate that the CuO/SnO2 mixed nanofibers synthesized with double needles are a promising sensor material for detection of H2S.

Remarkable improvement of gas-sensing abilities in p-type oxide nanowires by local modification of the hole-accumulation layer.

This paper proposes a method for improving the reducing or oxidizing gas-sensing abilities of p-type oxide nanowires (NWs) by locally modifying the hole-accumulation channel through the attachment of p-type nanoparticles (NPs) with different upper valence band levels. In this study, the sensing behaviors of p-CuO NWs functionalized with either p-NiO or p-Co3O4 NPs were investigated as a model materials system. The attachment of p-NiO NPs greatly improved the reducing gas-sensing performance of p-CuO NWs. In contrast, the p-Co3O4 NPs improved the oxidizing gas-sensing properties of p-CuO NWs. These results are associated with the local suppression/expansion of the hole-accumulation channel of p-CuO NWs along the radial direction due to hole flow between the NWs and NPs. The approach proposed in this study provides a guideline for fabricating sensitive chemical sensors based on p-CuO NWs.

TiO2/ZnO inner/outer double-layer hollow fibers for improved detection of reducing gases.

TiO2/ZnO double-layer hollow fibers (DLHFs) are proposed as a superior sensor material in comparison to regular single-layer hollow fibers (HFs) for the detection of reducing gases. DLHFs were synthesized on sacrificial polymer fibers via atomic layer deposition of a first layer of TiO2 followed by a second layer of ZnO and by a final thermal treatment. The inner TiO2 receives electrons from the ZnO outer layer, which becomes more resistive due to the significant loss of electrons. This highly resistive ZnO layer partially regains its original resistivity when exposed to reducing gases such as CO, thus enabling more resistance variation in DLHFs. DLHFs are a novel material compared to HFs and can be successfully employed to fabricate chemical sensors for the accurate detection of reducing gases.

Effect of the wall thickness on the gas-sensing properties of ZnO hollow fibers.

ZnO hollow fibers (HFs) with a range of wall thicknesses were synthesized by electrospinning and atomic layer deposition. The effects of the wall thickness of the HFs on their sensing properties were examined using CO as a representative reducing gas. The thin-walled HFs showed improved sensor responses to CO compared to thick-walled HFs. Most importantly, despite the polycrystalline nature of HFs, their sensing abilities were determined mainly by the wall thickness, not by the size of the nanograins or crystalline quality. In particular, the resistance modulation was attributed mainly to radial suppression/broadening of the underlying conducting channel during adsorption/desorption of gas species on both the inner and outer surface.

Prominent reducing gas-sensing performances of n-SnO2 nanowires by local creation of p-n heterojunctions by functionalization with p-Cr2O3 nanoparticles.

A novel approach to improving the reducing gas-sensing properties of n-type nanowires (NWs), by locally creating p-n heterojunctions with p-type nanoparticles (NPs), is proposed. As a model system, this work investigates the sensing behaviors of n-SnO2 NWs functionalized with p-Cr2O3 NPs. Herein, n-SnO2 NWs demonstrate greatly improved reducing gas-sensing performance when functionalized with p-Cr2O3 NPs. Conversely, such functionalization deteriorates the oxidizing gas-sensing properties of n-SnO2 NWs. These phenomena are closely related to the local suppression of the conduction channel of n-type NWs, in the radial direction, beneath the p-n heterojunction by the flow of charge carriers. The approach used in this work can be used to fabricate sensitive reducing-gas sensors based on n-type NWs.

Dual functional sensing mechanism in SnO2-ZnO core-shell nanowires.

We report a dual functional sensing mechanism for ultrasensitive chemoresistive sensors based on SnO2-ZnO core-shell nanowires (C-S NWs) for detection of trace amounts of reducing gases. C-S NWs were synthesized by a two-step process, in which core SnO2 nanowires were first prepared by vapor-liquid-solid growth and ZnO shell layers were subsequently deposited by atomic layer deposition. The radial modulation of the electron-depleted shell layer was accomplished by controlling its thickness. The sensing capabilities of C-S NWs were investigated in terms of CO, which is a typical reducing gas. At an optimized shell thickness, C-S NWs showed the best CO sensing ability, which was quite superior to that of pure SnO2 nanowires without a shell. The dual functional sensing mechanism is proposed as the sensing mechanism in these nanowires and is based on the combination of the radial modulation effect of the electron-depleted shell and the electric field smearing effect.

Mechanism and prominent enhancement of sensing ability to reducing gases in p/n core-shell nanofiber.

We have devised a sensor system comprising p-CuO/n-ZnO core-shell nanofibers (CS nanofibers) for the detection of reducing gases with a very low concentration. The CS nanofibers were prepared by a two-step process as follows: (1) synthesis of core CuO nanofibers by electrospinning, and (2) subsequent deposition of uniform ZnO shell layers by atomic layer deposition. We have estimated the sensing capabilities of CS nanofibers with respect to CO gas, revealing that the thickness of the shell layer needs to be optimized to obtain the best sensing properties. It is found that the p-CuO/n-ZnO CS structures are suitable for detecting reducing gases at extremely low concentrations. The associated sensing mechanism is proposed on the basis of the radial modulation of an electron-depleted region in the shell layer.

Acceptor-compensated charge transport and surface chemical reactions in Au-implanted SnO2 nanowires.

A new deep acceptor state is identified by density functional theory calculations, and physically activated by an Au ion implantation technique to overcome the high energy barriers. And an acceptor-compensated charge transport mechanism that controls the chemical sensing performance of Au-implanted SnO2 nanowires is established. Subsequently, an equation of electrical resistance is set up as a function of the thermal vibrations, structural defects (Au implantation), surface chemistry (1 ppm NO2), and solute concentration. We show that the electrical resistivity is affected predominantly not by the thermal vibrations, structural defects, or solid solution, but the surface chemistry, which is the source of the improved chemical sensing. The response and recovery time of chemical sensing is respectively interpreted from the transport behaviors of major and minor semiconductor carriers. This acceptor-compensated charge transport mechanism provides novel insights not only for sensor development but also for research in charge and chemical dynamics of nano-semiconductors.