UV-activated gas sensing properties of ZnS nanorods functionalized with Pd

Pd-functionalized ZnS nanorods were prepared for use as gas sensors. Scanning electron microscopy revealed the diameters and lengths of the nanorods ranging from 30 to 80 nm and from 2 to 5 μm, respectively. The diameter of Pd nanoparticles ranged from 2 to 5 nm. Transmission electron microscopy revealed that ZnS nanorods and Pd nanoparticles were monocrystalline and amorphous, respectively. The responses of multiple networked ZnS nanorods sensors to 1–5 ppm NO₂ were increased substantially by a combination of Pd functionalization and UV irradiation. Pristine ZnS nanorod sensors at room temperature in the dark showed a response (∼100%) almost independent of NO₂ concentration in a NO₂ concentration range of 1–5 ppm. Pristine ZnS nanorod sensors showed enhanced responses of 214–603% to 1–5 ppm NO₂ at room temperature under UV illumination. Pd-functionalized ZnS nanorods sensors showed further enhanced responses of 355–1511% to 1–5 ppm NO₂ at room temperature under UV illumination. The NO₂ gas sensing mechanism of the Pd-functionalized ZnS nanorods sensors under UV illumination is discussed in depth.     

Facile synthesis of SnO2–ZnO core–shell nanowires for enhanced ethanol-sensing performance

The design of core–shell heteronanostructures is powerful tool to control both the gas selectivity and the sensitivity due to their hybrid properties. In this work, the SnO₂–ZnO core–shell nanowires (NWs) were fabricated via two-step process comprising the thermal evaporation of the single crystalline SnO₂ NWs core and the spray-coating of the grainy polycrystalline ZnO shell for enhanced ethanol sensing performance. The as-obtained products were investigated by X-ray diffraction, scanning electron microscopy, and photoluminescence. The ethanol gas-sensing properties of pristine SnO₂ and ZnO–SnO₂ core–shell NW sensors were studied and compared. The gas response to 500 ppm ethanol of the core–shell NW sensor increased to 33.84, which was 12.5-fold higher than that of the pristine SnO₂ NW sensor. The selectivity of the core–shell NW sensor also improved. The response to 100 ppm ethanol was about 14.1, whereas the response to 100 ppm liquefied petroleum gas, NH₃, H₂, and CO was smaller, and ranged from 2.5 to 5.3. This indicates that the core–shell heterostructures have great potential for use as gas sensing materials.     

Highly sensitive ethanol sensors based on nanocrystalline SnO2 thin films

This paper reports on a simple and inexpensive ultrasonic spray pyrolysis method to synthesize agglomerate-free nanosized SnO₂ particles with a size smaller than 10 nm. Scanning electron microscopy, transmission electron microscopy and high resolution X-ray diffraction studies were used to characterize the morphology, crystallinity, and structure of the SnO₂ particles. Under the optimized experimental conditions, the prepared SnO₂ sensor shows the high response (S = 491) towards 100 ppm ethanol gas at 300 °C, linearity in the range of 100–500 ppm, quick response time (2 s), recovery time (60 s) and selectivity against other gases. The response of the sensor was monitored in a 250–450 °C temperature range. The seven fold enhancement in gas response and selective detection of C₂H₅OH in the presence of other gases such as CH₃OH and CH₃CHOHCH₃ are the significant points in this investigation. These results demonstrate that pure nanocrystalline SnO₂ thin film can be used as the sensing material for fabricating high performance ethanol sensors.     

Influence of CuO catalyst in the nanoscale range on SnO2 surface for H2S gas sensing applications

The dispersal of CuO catalyst on the surface of the semiconducting SnO₂ film is found to be of vital importance for improving the sensitivity and the response speed of a SnO₂ gas sensor for H₂S gas detection. Ultra-thin CuO islands (8 nm thin and 0.6 mm diameter) prepared by evaporating Cu through a mesh and subsequent oxidation yield a fast response speed and recovery. Ultimately nanoparticles of Cu (average size = 15 nm) prepared by a chemical technique using a reverse micelle method involving the reduction of Cu(NO₃)₂ by NaBH₄ exhibited significant improvement in the gas sensing characteristics of SnO₂ films. A fast response speed of ∼14 s and a recovery time of ∼60 s for trace level ∼20 ppm H₂S gas detection have been recorded. The sensor operating temperature (130° C) is low and the sensitivity (S = 2.06 × 10³) is high. It is found that the spreading over of CuO catalyst in the nanoscale range on the surface of SnO₂ allows effective removal of excess adsorbed oxygen from the uncovered SnO₂ surface due to spill over of hydrogen dissociated from the H₂S-CuO interaction.     

Functionalized Pd/ZnO Nanowires for Nanosensors

A method for surface doping and functionalization of ZnO nanowires (NWs) with Pd (Pd/ZnO) in a one‐step process is presented. The main advantage of this method is to combine the simultaneous growth, surface doping, and functionalization of NWs by using electrochemical deposition (ECD) at relatively low temperatures (90 °C). Our approach essentially reduces the number of technological steps of nanomaterial synthesis and final nanodevices fabrication with enhanced performances. A series of nanosensor devices is fabricated based on single Pd/ZnO NWs with a radius of about 80 nm using a FIB/SEM system. The influence of Pd nominal composition in Pd/ZnO NW on the H₂ sensing response is studied in detail and a corresponding mechanism is proposed. The results demonstrate an ultra‐high response and selectivity of the synthesized nanosensors to hydrogen gas at room temperature. The optimal concentration of PdCl₂ in the electrolyte to achieve extremely sensitive nanodevices with a gas response (S_H2) ≈ 1.3 × 10⁴ (at 100 ppm H₂ concentration) and relatively high rapidity is 0.75 µM. Theoretical calculations on Pd/ZnO bulk and functionalized surface further validated the experimental hypothesis. Our results demonstrate the importance of noble metal presence on the surface due to doping and functionalization of nanostructures in the fabrication of highly‐sensitive and selective gas nanosensors operating at room temperature with reduced power consumption.     

Catalyst free rutile phase TiO2 nanorods as efficient hydrogen sensor with enhanced sensitivity and selectivity

Using a low-cost hydrothermal method, we demonstrated the fabrication of phase pure rutile phase high-density vertically aligned TiO₂ nanorods-based catalyst-free hydrogen (H₂) gas sensor. The synthesized TiO₂ nanorods on FTO are decorated with the aluminum interdigitated electrode pattern for electrical measurements. TiO₂ nanorods-based hydrogen sensor showed the optimum response of ∼53.18% at 150 ppm H₂ concentration relative to air at 100 °C. The measured response and recovery time of TiO₂ nanorods are 85 and 620 s, respectively. The TiO₂ nanorods-based H₂ gas sensor showed a relatively better response, good reproducibility, and stability at moderate temperatures, i.e., 50 and 100 °C. The electrochemical impedance measurements showed a small variation in the surface characteristics of TiO₂ nanorods before and after exposing H₂ gas. The carrier lifetime at 50 °C and 100 °C at 150 ppm are 5 μs and 3 μs, respectively. Interestingly, H₂ selectivity is also observed against H₂S, CO, and NH₃ gases, suggesting that high-density vertically aligned TiO₂ nanorods can be a good candidate for efficient hydrogen sensing at relatively low temperatures.     

Enhanced ethanol sensing properties of TiO2/ZnO core–shell nanorod sensors

TiO₂-core/ZnO-shell nanorods were synthesized using a two-step process: the synthesis of TiO₂ nanorods using a hydrothermal method followed by atomic layer deposition of ZnO. The mean diameter and length of the nanorods were ～300 nm and ～2.3 μm, respectively. The cores and shells of the nanorods were monoclinic-structured single-crystal TiO₂ and wurtzite-structured single-crystal ZnO, respectively. The multiple networked TiO₂-core/ZnO-shell nanorod sensors showed responses of 132–1054 % at ethanol (C₂H₅OH) concentrations ranging from 5 to 25 ppm at 150 ^°C. These responses were 1–5 times higher than those of the pristine TiO₂ nanorod sensors at the same C₂H₅OH concentration range. The substantial improvement in the response of the pristine TiO₂ nanorods to C₂H₅OH gas by their encapsulation with ZnO may be attributed to the enhanced absorption and dehydrogenation of ethanol. In addition, the enhanced sensor response of the core–shell nanorods can be attributed partly to changes in resistance due to both the surface depletion layer of each core–shell nanorod and the potential barriers built in the junctions caused by a combination of homointerfaces and heterointerfaces.     

Detection of hydrogen with SnO2-coated ZnO nanorods

SnO₂-coated ZnO nanorods on c-plane sapphire substrates were synthesized by pulsed laser deposition. The thickness of the polycrystalline SnO₂ was ∼10 nm, as determined by high-resolution transmission electron microscopy, while the diameter of the ZnO nanorods was ∼30 nm. The sensitivity of the SnO₂/ZnO structures to hydrogen was tested by depositing Ti/Au Ohmic contacts on a random array of the nanorods and measuring the current at fixed voltage. There was no response to 500 ppm H₂ in N₂ at room temperature, but we obtained a sensitivity of ∼70% at 400 °C. The SnO₂/ZnO structures exhibit drift in their recovery characteristics and for sequential detection of hydrogen, as generally reported for SnO₂ thin film sensors.     

Influence of SnO2 coating and thermal annealing on the structure and luminescence properties of CuO nanorods

CuO-core/ SnO₂-shell one-dimensional nanostructures have been fabricated by thermal oxidation of a copper foil and then atomic layer deposition of SnO₂. The structure and optical properties of the nanostructures have been investigated by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, photoluminescence (PL) spectroscopy, and energy-dispersive X-ray analysis techniques. The nanostructures are found to have the form of nanorods, with the diameter of the CuO cores being in the range from a few tens to a few hundreds of nanometers, the thickness of the SnO₂ shells being ～15 nm, and with a length of a few tens of micrometers. The CuO cores and the SnO₂ shells of the as-synthesized nanorods have crystalline monoclinic CuO and amorphous SnO₂ structures, respectively, but the SnO₂ shells are found to crystallize to tetragonal SnO₂ on thermal annealing. The PL emission intensity of the CuO nanorods has been slightly increased by SnO₂ coating. The PL emission of the SnO₂-coated CuO nanorods is somewhat increased and the emission peak position is red-shifted from 550 to 580 nm by annealing in a reducing atmosphere. On the other hand, the PL emission is significantly increased and the emission peak position is shifted from 550 nm further to around 595 nm by annealing in an oxidative atmosphere. In addition, the origins of the PL enhancements in the nanorods by coating and annealing are discussed.     

One-pot synthesis of crystalline SnO2 nanoparticles and their low-temperature ethanol sensing characteristics

Crystalline SnO2 nanoparticles(NPs) with a diameter less than 6 nm are synthesized using potassium stannate trihydrate as the precursor in a basic system.The synthesized NPs can detect ethanol at a ppm level even at 100℃.Furthermore,the NPs have good selectivity to ethanol.The excellent ethanol sensing performances are attributed to the small size effect according to the space-charge model.     

Preparation of PdAu/C-Sb2O5·SnO2 electrocatalysts by borohydride reduction process for direct formic acid fuel cell

Pd/C-Sb₂O₅·SnO₂ and PdAu/C-Sb₂O₅·SnO₂ electrocatalysts with different PdAu atomic ratio (90:10, 70:30, and 50:50) were prepared by borohydride reduction method, and characterized by X-ray diffraction, transmission electron microscopy, cyclic voltammetry, chronoamperommetry, and performance test on direct formic acid fuel cell at 100 °C. X-ray diffraction showed for Pd/C-Sb₂O₅·SnO₂ the presence of Pd face-centered cubic (fcc) system, while for PdAu/C-Sb₂O₅·SnO₂ it showed the presence of Pd fcc phase, PdAu fcc alloys and a segregated phases fcc Pd-rich and Au-rich phases. TEM micrographs and histograms for all electrocatalysts showed that the nanoparticles where not well dispersed on the support and some agglomerates were present. The electrochemical studies showed that PdAu/C-Sb₂O₅·SnO₂ (70:30) had superior performance for formic acid electro-oxidation at 25 °C compared to others electrocatalysts prepared while PdAu/C-Sb₂O₅·SnO₂ (90:10) showed superior performance in direct formic acidic fuel cell at 100 °C. These results indicated that the addition of 10–30 % Au to Pd favor the electro-oxidation of formic acid. This effect could be attributed to the synergy between the constituents of the electrocatalyst (metallic Pd and Au, SnO₂, and Sb₂O₅·SnO₂).     

Recovery properties of hydrogen gas sensor with Pd/titanate and Pt/titanate nanotubes photo-catalyst by UV radiation from catalytic poisoning of H2S

Recovery properties after H₂S catalytic poisoning of catalytic-type gas sensor with photo-catalysts and UV radiation have been examined. Each sensing material of the sensor consists of Pd, Pt supported on γ-Al₂O₃ and Pd/titanate, Pt/titanate nanotubes or TiO₂ particles. Pd/titanate and Pt/titanate nanotubes photo-catalyst were synthesized by hydrothermal synthesis method. All the sensors were deactivated after 500 ppm H₂S exposure for 20 h. The sensors with Pd/titanate or Pt/titanate nanotubes showed regenerated voltage response under UV radiation. However the sensor with TiO₂ particles showed negligible regenerated voltage response. Regenerated voltage response with Pd/titanate or Pt/titanate nanotubes may stem from location of Pd or Pt catalyst on the titanate nanotube photo-catalyst.     

Enhanced ethanol sensing properties of TeO2/In2O3 core–shell nanorod sensors

TeO₂/In₂O₃ core–shell nanorods were fabricated using thermal evaporation and sputtering methods. The multiple networked TeO₂/In₂O₃ core–shell nanorod sensor showed responses of 227–632%, response times of 50–160 s, and recovery times of 190–220 s at ethanol (C₂H₅OH) concentrations of 50–250 ppm at 300 °C. The response values are 1.6–2.9 times higher and the response and recovery times are also considerably shorter than those of the pristine TeO₂ nanorod sensor over the same C₂H₅OH concentration range. The origin of the enhanced ethanol sensing properties of the core–shell nanorod sensor is discussed.     

Detection of hydrogen at room temperature with catalyst-coated multiple ZnO nanorods

A variety of different metal catalyst coatings (Pt, Pd, Au, Ag, Ti and Ni) deposited on multiple ZnO nanorods have been compared for their effectiveness in enhancing sensitivity for detecting hydrogen at room temperature. Pt-coated nanorods show a relative response of up to 8% in room-temperature resistance upon exposure to a hydrogen concentration in N₂ of 500 ppm. This is a factor of two larger than that obtained with Pd and more than an order of magnitude larger than that achieved with the remaining metals. The power levels for these sensors were low, ∼0.4 mW for the responses noted above. Pt-coated ZnO nanorods easily detected hydrogen down to 100 ppm, with a relative response of 4% at this concentration after 10-min exposure. The nanorods show a return to their initial conductance upon switching back to a pure-air ambient with time constants of the order of a few minutes at room temperature. This slow response at room temperature is a drawback in some applications, but the sensors do offer low-power operation and ppm detection sensitivity. PACS 78.66.Hf; 73.61.Ga; 73.40.Qv     

Enhancement of ethanol sensing of TeO2 nanorods by Ag functionalization

Gas sensors based on Ag–TeO₂ composite nanorods were fabricated using thermal evaporation and sputtering techniques. The morphology, structure and phase composition of the as-prepared nanofibers were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD), respectively. TEM and XRD showed that the nanorods and nanoparticles on them were tetragonal-structured single crystal TeO₂ and a mainly amorphous phase, respectively. The multiple-networked bare TeO₂ nanorod sensors exhibited a response of ～219% at 25 ppm C₂H₅OH at 300 °C, whereas the Ag-functionalized TeO₂ nanorod sensors showed a response of ～808% under the same conditions. The mechanism by which the sensing properties of the TeO₂ nanorods were enhanced by functionalization with Ag is also discussed.     

Semiconducting SnO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> (S-T) composites for detection of SO<Subscript>2</Subscript> gas

SnO₂-TiO₂ (S-T) composites with different molar ratios were prepared by mechanical mixing followed by sintering at 700 °C for 4 h in air. The structural and microstructural properties of the composites were investigated using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). S-T composites were investigated by introducing SO₂ to test their chemical stability using PXRD and SEM coupled with energy dispersive X-ray (EDX) analysis. The sensing performance was measured at different temperatures using various SO₂ concentrations (10–100 ppm). A composite comprising 25 mol% of SnO₂ and 75 mol% TiO₂ (S25-T75) exhibited the highest sensitivity comparing to other S-T composites studied under the presently investigated conditions. t ₉₀ (90 % of response time) was found to be ～5 min for thick pellet (~2 mm in thickness). SO₂ sensing mechanism has been explained through the band structure model.     

Synthesis and enhanced acetone gas-sensing performance of ZnSnO<Subscript>3</Subscript>/SnO<Subscript>2</Subscript> hollow urchin nanostructures

A kind of novel ZnSnO₃/SnO₂ hollow urchin nanostructure was synthesized by a facile, eco-friendly two-step liquid-phase process. The structure, morphology, and composition of samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption–desorption techniques. The results revealed that many tiny needle-like SnO₂ nanowires with the average diameter of 5 nm uniformly grew on the surface of the ZnSnO₃ hollow microspheres and the ZnSnO₃/SnO₂ hollow urchin nanostructures with different SnO₂ content also were successfully prepared. In order to comprehend the evolution process of the ZnSnO₃/SnO₂ hollow urchin nanostructures, the possible growth mechanism of samples was illustrated via several experiments in different reaction conditions. Moreover, the gas-sensing performance of as-prepared samples was investigated. The results showed that ZnSnO₃/SnO₂ hollow urchin nanostructures with high response to various concentration levels of acetone enhanced selectivity, satisfying repeatability, and good long-term stability for acetone detection. Specially, the 10 wt% ZnSnO₃/SnO₂ hollow urchin nanostructure exhibited the best gas sensitivity (17.03 for 50 ppm acetone) may be a reliable biomarker for the diabetes patients, which could be ascribed to its large specific surface area, complete pore permeability, and increase of chemisorbed oxygen due to the doping of SnO₂.     

Pd nanoparticle-mediated acetone sensing performance improvement of SnO2 substrate: A combined DFT and experimental study

The article presents a combined theoretical and experimental study attempting to show how Pd nanoparticles (NPs) loading onto SnO₂ substrate improves the acetone gas sensing performance. Pristine nanostructured SnO₂ and Pd nanoparticles (Pd NPs) loaded SnO₂ substrates have been prepared, characterized, and their acetone sensing performances have been measured. Experimental measurements have shown that Pd NP loading onto SnO₂ suppresses the interfering effects of ethanol, water vapors, etc., and enhances the acetone sensor response, reversibility, response/recovery speeds, and signal-to-noise ratio. Various parameters like the adsorption energy, HOMO–LUMO energy gap, charge distribution, polarizability change, electrophilicity index, global hardness, etc., of several model systems, have been computed by using DFT. The computed parameters have been correlated with the conductivity, local reactivity, sensor response and selectivity, response/recovery times, etc., of the systems to understand the molecular-level effects of the Pd NP loading onto the SnO₂ on the gas sensing process.     

The improvement of gas-sensing properties of SnO<Subscript>2</Subscript>/zeolite-assembled composite

SnO₂-impregnated zeolite composites were used as gas-sensing materials to improve the sensitivity and selectivity of the metal oxide-based resistive-type gas sensors. Nanocrystalline MFI type zeolite (ZSM-5) was prepared by hydrothermal synthesis. Highly dispersive SnO₂ nanoparticles were then successfully assembled on the surface of the ZSM-5 nanoparticles by using the impregnation methods. The SnO₂ nanoparticles are nearly spherical with the particle size of ~?10 nm. An enhanced formaldehyde sensing of as-synthesized SnO₂-ZSM-5-based sensor was observed whereas a suppression on the sensor response to other volatile organic vapors (VOCs) such as acetone, ethanol, and methanol was noticed. The possible reasons for this contrary observation were proposed to be related to the amount of the produced water vapor during the sensing reactions assisted by the ZSM-5 nanoparticles. This provides a possible new strategy to improve the selectivity of the gas sensors. The effect of the humidity on the sensor response to formaldehyde was investigated and it was found the higher humidity would decrease the sensor response. A coating layer of the ZSM-5 nanoparticles on top of the SnO₂-ZSM-5-sensing film was thus applied to further improve the sensitivity and selectivity of the sensor through the strong adsorption ability to polar gases and the “filtering effect” by the pores of ZSM-5.     

Polypyrrole–NiO hybrid nanocomposite films: highly selective,sensitive, and reproducible NO2 sensors

Polypyrrole–nickel oxide (PPy–NiO) hybrid nanocomposite thin-film sensor was prepared by spin-coating method on glass substrate. The PPy–NiO hybrid nanocomposite thin film sensors were used to study room temperature gas-sensing properties for oxidizing (NO₂, Cl₂) as well as reducing (NO₂, H₂S, C₂H₅OH, NH₃, and Cl₂) gases. It was revealed that PPy–NiO (50 %) hybrid nanocomposite thin-film sensor could detect NO₂ at low concentration (100 ppm) with very high selectivity (47 % compared with Cl₂) and high sensitivity (47 %), with better stability (90 %) and reproducibility. The response and recovery times were changed significantly with NO₂ concentration.