Hydrothermal synthesis of one-dimensional α-MoO3 nanomaterials and its unique sensing mechanism for ethanol

In gas sensor applications, the availability of highly sensitive and rapid response/recovery detector for ethanol gas is sparse. One-dimensional orthogonal crystalline molybdenum trioxide nanomaterials were synthesized by an economical and environmentally friendly hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy spectroscopy (EDS) were used to investigate the structure and morphology of the nanometer materials. The relevant characterization shows that nanobelts are highly crystalline layered structures with a width of about 200 nm and a length of a few micrometers. The synthesized ethanol gas sensors based on α-MoO₃ semiconductor material show the highest response at 350 °C. Gas sensitivity tests indicated that α-MoO₃ nanobelts respond well to 50 ～ 600 ppm ethanol at optimal operating temperatures. The selectivity test among various reducing gases shows that the sensor responds better to ethanol compared to other gases such as xylene, NO₂, CO, and H₂ gases. This excellent sensing performance is attributed to the unique sensing mechanism formed in the layered MoO₃ nanobelts through the catalytic reaction between ethanol and MoO₃ lattice oxygen and adsorbed oxygen. The sensing mechanism of the co-catalytic effect of lattice oxygen and adsorbed oxygen on ethanol is also discussed in depth.     

A highly sensitive nonenzymatic glucose sensor based on nickel oxide–carbon nanotube hybrid nanobelts

In this study, we demonstrated a highly sensitive electrochemical sensor for the determination of glucose in alkaline aqueous solution by using nickel oxide single-walled carbon nanotube hybrid nanobelts (NiO–SWCNTs) modified glassy carbon electrode (GCE). The hybrid nanobelts were prepared by the deposition of SWCNTs onto the Ni(SO₄)_0.3(OH)_1.4 nanobelt surface, followed by heat treatment at different temperatures ranging from 400 °C to 600 °C. The NiO–SWCNTs hybrid nanobelts modified electrode prepared at 500 °C displays enhanced electrocatalytic activity towards glucose oxidation, revealing a synergistic effect between the NiO and the deposited SWCNTs. The as-fabricated nonenzymatic glucose sensor exhibits excellent glucose sensitivity (2,980 μA cm^?2 mM^?1), lower detection limit (0.056 μM, signal/noise [S/N] ratio?=?3), and wider linear range (0.5–1,300 μM). Moreover, the sensor has been successfully used for the assay of glucose in serum samples with good recovery, ranging from 96.4 % to 102.4 %.     

Ammonia gas sensor based on a spinel semiconductor,Co<Subscript>0.8</Subscript>Ni<Subscript>0.2</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanomaterial

Thick film of nanocrystalline Co_0.8Ni_0.2Fe₂O₄ was obtained by sol–gel citrate method for gas sensing application. The synthesized powder was characterized by X-ray diffraction (XRD) and transmission electron microscopy. The XRD pattern shows spinel type structure of Co_0.8Ni_0.2Fe₂O₄. XRD of Co_0.8Ni_0.2Fe₂O₄ revels formation of solid solution with average grain size of about 30 nm. From gas sensing properties it observed that nickel doping improves the sensor response and selectivity towards ammonia gas and very low response to LPG, CO, and H₂S at 280 °C. Furthermore, incorporation of Pd improves the sensor response and stability of ammonia gas and reduced the operating temperature upto 210 °C. The sensor is a promising candidate for practical detector of ammonia.     

Deposition of In2O3 nanofibers on polyimide substrates to construct high-performance and flexible trimethylamine sensor

Flexible trimethylamine sensor has been realized based on In₂O₃ nanofibers via electrospinning and a deposition technique. The web-like In₂O₃ nanofibers with high length-to-diameter ratios are benefit for gas adsorption and desorption. High trimethylamine sensing properties are observed. The sensors can detect trimethylamine gas down to 1 ppm at 80 °C with the response up to 3.8. Additionally, rapid response (6 s) and recovery (10 s) behavior can also be obtained. Good reliability and flexibility are observed in 100 bending/extending cycles. Our results open a new route to construct flexible gas sensors in practice.     

Optimization and modeling of Zn2SnO4 sensitivity as gas sensor for detection benzene in the air by using the response surface methodology

In this paper, the performance of the benzene gas detection sensor in the air is optimized by an experimental design method. So in this work, Nanostructured thin films of ZnO and Zn₂SnO₄ were prepared in wurtzite form via a facile atmospheric pressure chemical vapor deposition (CVD) method, using metallic zinc and tin precursors. Characterization of the gas sensor was performed by using Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and surface area analysis (using BET method). The results show that Zn₂SnO₄ nanowire network exhibited good sensitivity at 299 °C temperature to low concentrations (100 ppb) of Benzene which can be potentially used as a resistive gas sensor. Ultimately modeling and optimization of Zn₂SnO₄ sensor performance to detect benzene by surface response method in design expert11 software has been done. Also, the effect of each parameter on the sensitivity of the sensor was analyzed by analysis of variance (ANOVA). Moreover, the performance efficiency of the Zn₂SnO₄ sensor is estimated with the reliable correlations obtained in the modeling. The two parameters selected to optimize the performance of the gas sensor include the operating temperature of the sensor and the concentration of the sensor. Comparison of the modeling results and the predicted values for the sensor sensitivity to benzene shows 97.60% excellent agreement.     

Flexible all-biomass gas sensor based on doped carbon quantum dots/nonwoven cotton with discriminative function

Carbon quantum dots (CQDs) co-doped with N, P and S derived from expired milk was prepared by a simple hydrothermal method. By dipping pure cotton face towel (PCFT) into CQDs ink, a flexible all-biomass CQDs/PCFT sensor was prepared for the first time. Due to the heteroatom doping, extremely small particle size of CQDs and excellent permeability of CQDs/PCFT film, the flexible CQDs/PCFT sensor showed the high sensitivity and bending stability. In the range of 0–60° bending states, the responses of CQDs/PCFT sensor to four target analytes changed by less 5.0%. After 3000 bending of 60°, the maximum change of the response to the target analytes was only 6.4%. Interestingly, due to the abundant functional groups and defects of CQDs, the flexible CQDs/PCFT sensor displayed sensing curves of different shapes for different target analytes. In this way, by establishing a database of sensing curves of target analytes, multiple analytes can be detected discriminatively by relying only on single sensor with the help of image recognition. This work provided a reference for the development of cotton fiber based all biomass flexible gas sensor.

Graphical abstract

     

Nanocrystalline spinel zinc-substituted cobalt ferrite thick film an efficient ethanol sensor

This study considered Zn-substituted cobalt ferrite (Zn_xCo_1-xFe₂O₄ (x = 0.0–1.0) (ZCF)) thick films structural, morphological, and electrical properties; and gas sensing performance. The ZCF thick film sensor was screen printed on a glass substrate and tested for different analyte gases, including H₂, H₂S, CO₂, Cl₂, NH₃, LPG, and C₂H₅OH. We used X-ray photoelectron spectrometry to investigate composition, chemical state, iron/cobalt or zinc ratio, and cation distribution within Zn-substituted cobalt spinel ferrite tetrahedral and octahedral sites without impurities. FESEM and HR-TEM confirmed grain dimensions between 0.13 and 0.23 μm and porous, nearly spherical to flake-like morphology for the ZCF samples. Sample DC resistivity reduced with increasing temperature, confirming semiconductor nature. Thick film ZCF composition achieved highest the gas response and selectivity to 100 ppm ethanol at room temperature (30 °C). Overall results confirmed that flake-like ZCF sensors could be effective ethanol gas sensors.     

Novel water based cerium acetate precursor solution for the deposition of epitaxial cerium oxide films as HTSC buffers

A novel water based precursor solution using ethylenediaminetetraacetic acid (H₄EDTA) as a complexant and acetic acid and ethylenediamine (EDA) as additional components to obtain CeO₂ buffer layers on Ni (5%W) tapes is described in detail. The influence of complexation behavior in the formation of transparent and homogenous sols and gels by the combination of cerium acetate, acetic acid and H₄EDTA has been studied. The optimal growth conditions for cerium oxide were Ar-5% H₂ gas processing atmosphere, solution concentration levels of 0.2–0.4 M, a dwell time of 60 min at 900 °C and 5–30 min at 1,050 °C. X-ray diffraction, SEM, spectroscopic ellipsometry and pole figures were used to characterize the CeO₂ films. Highly textured CeO₂ layers were obtained.     

Sensing mechanism of Pd-doped SnO2 sensor for LPG detection

In the present work, studies have been made to analyze the sensitivity, response, recovery time and sensing mechanism of Pd-doped thick film SnO₂ sensor for detection of LPG. To achieve this, thick film Pd-doped (0.25 and 1% by weight in available Indium doped SnO₂ thick film paste supplied by ESL, USA) along with an undoped (Indium doped) SnO₂ sensors were fabricated on a 1″ × 1″ alumina substrate. It consists of a gas sensitive layer (doped SnO₂), a pair of electrodes underneath the gas sensing layer serving as a contact pad for sensor. Also, a heater element on the backside of the substrate was printed for generating appropriate operating temperature at the substrate necessary for acquiring gas sensing properties. The sensor doped with 1% palladium showed the maximum sensitivity of 72% at 350 °C for 0.5% concentration of LPG. Possible detailed sensing mechanism of Pd-doped SnO₂ sensor for LPG detection has been proposed.     

Influence of the thermal treatment in the crystallization of NiWO4 and ZnWO4

NiWO₄ and ZnWO₄ were synthesized by the polymeric precursor method at low temperatures with zinc or nickel carbonate as secondary phase. The materials were characterized by thermal analysis (TG/DTA), infrared spectroscopy, UV–Vis spectroscopy and X-ray diffraction. NiWO₄ was crystalline after calcination at 350 °C/12 h while ZnWO₄ only crystallized after calcination at 400 °C for 2 h. Thermal decomposition of the powder precursor of NiWO₄ heat treated for 12 h had one exothermic transition, while the precursor heat treated for 24 h had one more step between 600 and 800 °C with a small mass gain. Powder precursor of ZnWO₄ presented three exothermic transitions, with peak temperatures and mass losses higher than NiWO₄ has indicating that nickel made carbon elimination easier.     

Synthesis of Bi/Sr doped zinc sulphide by spray pyrolysis technique: Effects of doping tempertures on solar cell application

With the aim of determining the best synthesizing substrate temperature that will improve the optical properties of Bi/Sr doped ZnS thin film, spray coated Bi/Sr doped ZnS thin films were deposited at a varying glass substrate temperature of 200 °C–350 °C using an interval of 50 °C. A constant volume of 40 ml of precursor solution was created by adding 10 ml of each of the following solutions: 0.045 M solution of zinc acetate dihydrate C4H6O4Zn.2H2, 0.1 M solution of thioacetamide CH3CSNH2, 0.02 M solution of bismuth nitrate Bi(NO3)3.5H2O, and 0.07 M solution of Strontium hydroxide Sr(OH).₂. UV–Visible Spectrophotometry, scanning electron microscope (SEM), EDX, X-ray diffraction (XRD), photoluminescence, and Fourier transform infrared (FTIR) were all used to investigate the samples. 53.84 and 193.26% increment in carrier concentration and mobility, a 36.36% and 17.77% reduction in resistivity, and a band gap were obtained at a doping temperature of 300 °C. An open-circuit voltage (Voc) of 0.30 V and a power conversion efficiency of 0.58% were achieved. It was established that a doping temperature of 300 °C on Bi/Sr doped ZnS thin films can be used to lower the band gap of ZnS for solar cell applications.     

High ethanol sensitivity of Palladium/TiO2 nanobelt surface heterostructures dominated by enlarged surface area and nano-Schottky junctions

TiO₂ nanobelts were prepared by the hydrothermal growth method. The surface of the nanobelts was coarsened by selective acid corrosion and functionalized with Pd catalyst particles. Three nanobelt samples (TiO₂ nanobelts, surface-coarsened TiO₂ nanobelts and Pd nanoparticle/TiO₂ nanobelt surface heterostructures) were configured as gas sensors and their sensing ability was measured. Both the surface-coarsened nanobelts and the Pd nanoparticle-decorated TiO₂ nanobelts exhibited dramatically improved sensitivity to ethanol vapor. Pd nanoparticle-decorated TiO₂ nanobelts with surface heterostructures exhibited the best sensitivity, selectivity, working temperature, response/recovery time, and reproducibility. The excellent ethanol sensing performance is attributed to the large surface area and enhancement by Schottky barrier-type junctions between the Pd nanoparticles and TiO₂ nanobelts.     

Lattice engineering route for self-assembled nanohybrids of 2D layered double hydroxide with 0D isopolyoxovanadate: chemiresistive SO2 sensor

Tuning the interior chemical composition of layered double hydroxides (LDHs) via lattice engineering route is a unique approach to enable multifunctional applications of LDHs. In this regard, the exfoliated 2D LDH nanosheets coupled with various guest species lead to the lattice-engineered LDH-based multifunctional self-assembly with precisely tuned chemical composition. This article reports the synthesis and characterization of mesoporous zinc–chromium-LDH (ZC-LDH) hybridized with isopolyoxovanadate nanohybrids (ZCiV) via lattice-engineered self-assembly between delaminated ZC-LDH nanosheets and isopolyoxovanadate (iPOV) anions. Electrostatic self-assembly between 2D ZC-LDH monolayers and 0D iPOV significantly altered structural, morphological, and surface properties of ZC-LDH. The structural and morphological study demonstrated the formation of mesoporous interconnected sheet-like architectures composed of restacked ZCiV nanosheets with expanded surface area and interlayer spacing. In addition, the ZCiV nanohybrid resistive elements were used as a room-temperature gas sensor. The selectivity of ZCiV nanohybrid was tested for various oxidizing (SO₂, Cl₂, and NO₂) gases and reducing (LPG, CO, H₂, H₂S, and NH₃) gases. The optimized ZCiV nanohybrid demonstrated highly selective SO₂ detection with the maximum SO₂ response (72%), the fast response time (20 s), low detection limit (0.1 ppm), and long-term stability at room temperature (27 ± 2 °C). Of prime importance, ZCiV nanohybrids exhibited moderately affected SO₂ sensing responses with high relative humidity conditions (80%–95%). The outstanding SO₂ sensing performance of ZCiV is attributed to the active surface gas adsorptive sites via plenty of mesopores induced by a unique lattice-engineered interconnected sheet-like microstructure and expanded interlayer spacing.     

Fabrication of a non-enzymatic glucose sensor field-effect transistor based on vertically-oriented ZnO nanorods modified with Fe2O3

Herein, we report a non-enzymatic glucose sensor field-effect transistor (FET) based on vertically-oriented zinc oxide nanorods modified with iron oxide (Fe₂O₃-ZNRs). Compared with ZnO-based non-enzymatic glucose sensors, which show poor sensing performances, modification of ZnO with Fe₂O₃ dramatically enhances the sensing behavior of the fabricated non-enzymatic FET glucose sensor due to the excellent electrocatalytic nature of Fe₂O₃. The fabricated non-enzymatic FET sensor showed excellent catalytic activity for glucose detection under optimized conditions with a linear range up to 18 mM, detection limits down to ~ 12 μM, excellent selectivity, good reproducibility and long-term stability. Moreover, the fabricated FET sensor detected glucose in freshly drawn mouse whole blood and serum samples. The developed FET sensor has practical applications in real samples and the solution-based synthesis process is cost effective.     

Synthesis and formation mechanism of TiO2/Al2O3 nanobelts by electrospinning

Poly(vinyl pyrrolidone) (PVP)/[Ti(SO₄)₂ + Al(NO₃)₃] composite nanobelts were prepared via electrospinning technology, and TiO₂/Al₂O₃ nanobelts were fabricated by calcination of the prepared composite nanobelts. The samples were characterized by thermogravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). XRD results show that the composite nanobelts were amorphous in structure, and pure phase TiO₂/Al₂O₃ nanobelts were obtained by calcination of the relevant composite nanobelts at 950°C for 8 h. SEM analysis indicates that the surface of as-prepared composite nanobelts was smooth, the widths of the composite fibers were in narrow range, and the mean width was ca. 8.9 ± 2.1 μm, thickness was about 255 nm, and there is no cross-linking among nanobelts. The width of TiO₂/Al₂O₃ nanobelts was ca. 1.3 ± 0.1 μm and the thickness was about 105 nm. TG-DTA analysis reveals that the N,N-dimethylformamide (DMF), organic compounds and inorganic salts in the composite nanobelts were decomposed and volatilized totally, and the weight of the sample kept constant when sintering temperature was above 900°C, and the total weight loss percentage was 81%. FTIR analysis manifests that crystalline TiO₂/Al₂O₃ nanobelts were formed at 950°C. The possible formation mechanism of the TiO₂/Al₂O₃ nanobelts was preliminarily discussed.     

Synthesis of Cu2Mo6S8 powders and thin films from intermediate oxides prepared by polymeric precursor method

Powders and thin films of the copper molybdenum sulfide Cu₂Mo₆S₈ were synthesized from intermediate oxides prepared by polymeric precursor method based on Pechini process. In the case of the thin films, deposition was performed onto R-plane sapphire single crystal by spin coating. The influence of temperature and duration of the 3 step heat treatment cycle (calcination, sulfurization and reduction) were investigated to optimize the synthesis conditions. The first step of calcination under air atmosphere performed for 3 h at 450 °C and 400 °C is suitable to obtain the intermediate oxides powders and thin films, respectively. The sulfurization treatment at 600 °C for 2 h under H₂S/H₂ gas flow followed by reduction at 650 °C for 4 h under H₂ gas flow allowed to obtain Cu₂Mo₆S₈ in powder or thin film form. In the last case, a multilayer process led to dense and homogeneous films. Moreover, the insertion and superconducting behaviour of the final powders allowed to validate the Cu₂Mo₆S₈ synthesis by this moderate temperature process.     

Nano silver coated patterned silica thin film by sol–gel based soft lithography technique

We report the fabrication of nano silver coated patterned silica thin film by sol–gel based soft lithography technique. Initially, silica gel film on soda lime silica glass was prepared by dipping technique from a silica sol of moderate silica concentration. A PolydimethylSiloxane elastomeric stamp containing the negative replica of the patterns of commercially available compact disc was used for embossing the film and the embossed film was cured up to 450 °C in pure oxygen atmosphere for oxide film. Finally, a precursor solution of AgNO₃ in water containing polyvinyl alcohol as an organic binder was made and used for coating on the patterned silica film by dipping technique and cured the sample up to 450 °C in reducing gas atmosphere to obtain nano silver layer. The formation of only cubic silver (~4.0 nm) and both cubic silver (~5.2 nm) and silver oxide (~3.6 nm) crystallites at 350 and 450 °C film curing temperatures respectively were confirmed by XRD measurements. The % of nano silver metal and silver oxide were 75.4 and 24.6 respectively. The nano-structured surface feature was visualized by FESEM whereas AFM revealed the high fidelity grating structure of the films. Presence of both spherical and rectangular structure (aspect ratio, 2.37) of nano silver/silver oxide was confirmed by TEM. The films were also characterized by UV–Vis spectral study. The patterned film may find application in chemical sensor devices.     

Thermal,XRD, and magnetization studies on ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> and NiAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinels,synthesized by citrate precursor method and annealed at 450 and 650 °C

Two aluminate spinel materials (ZnAl₂O₄ and NiAl₂O₄) were synthesized by the citrate precursor method. The citrate precursors consisting of coprecipitated citrates of Zn²⁺ or Ni²⁺ and aluminum were first subjected to thermal analysis (TG-DSC) for determining the optimum temperature for annealing. Two step decomposition was observed incorporating dehydration and formation of the aluminate. The second step gives an endo peak (−2937 J/g) at 356 °C in the DSC curve of the coprecipitated nickel(II) citrate–aluminum citrate gel in O₂ atmosphere. Kinetic/mechanistic analysis of the TG data has also been carried out and values of E _a, ΔS ^#, ΔG ^#, and A were approximated. On the basis of the findings, 450 °C has been chosen for annealing of the gels. Annealing has also been done at 650 °C for 1 h in muffle furnace in an attempt to obtain nanometric particles of aluminates (MAl₂O₄) {M = Ni, Zn} and to find out their magnetic properties which could render them useful for chemical sensing applications, etc. The TG-DSC curves of various powders which were obtained on annealing at the two temperatures did exhibit thermal instability when carried out in N₂ atmosphere. NiAl₂O₄ and ZnAl₂O₄ spinels (particle size 17 and 34 nm, respectively) are obtained in pure crystalline phase at 650 °C. ZnAl₂O₄ prepared this way shows coercivity values of 470 and 58.37 G and NiAl₂O₄, 107 and 23.24 G when annealed at 450 and 650 °C, respectively. ZnAl₂O₄ prepared by a polymer precursor method and annealed at 1000 °C, has earlier been reported to have coercivity value of 469 G. Thus, the citrate precursor method is good for the synthesis of ZnAl₂O₄, producing single phase nanocrystalline powder of high quality and crystallinity. The value of magnetization was found to be small in the present case for the NiAl₂O₄ spinel obtained at 450 °C.     

ITO films on glass substrate by sol–gel technique: synthesis,characterization and optical properties

The present paper extensively demonstrates synthesis, characterization and optical properties of semiconductor indium tin oxide (ITO) thin films on glass substrate using sol–gel technique for gas sensor applications. Turbidity, pH values, wettability and rheological properties of the prepared solutions were measured to determine solution characteristics by turbidimeter, pH meter, contact angle goniometer and rheometer machines prior to coating process. Thermal, structural, microstructural, mechanical and optical properties of the coatings were characterized by differential thermal analysis–thermogravimetry (DTA/TG), fourier transform infrarared, X-ray diffraction (XRD), scanning electron microscopy, scratch tester, refractometer and spectrophotometer. Four different solutions were prepared by changing solvent concentration. Turbidity, pH, contact angle and viscosity values of the solutions were convenient for coating process. Glass substrates were coated using the solutions of InCl₃, SnCl₂, methanol and glacial acetic acid. The obtained gel films were dried at 300 °C for 10 min and subsequently heat-treated at 500 °C for 10 min in air. The oxide thin films were annealed at 600 °C for 60 min in air. DTA/TG results revealed that endothermic and exothermic reactions are observed at temperature between 70 and 560 °C due to solvent removal, combustion of carbon based materials and oxidation of Sn and In. The spectrum of ITO precursor film annealed at 500–600 °C shows an absence of absorption bands corresponding to organics and hydroxyls. In₂Sn₂O_7−x phase was dominantly found as well as SnO₂ with low intensity from XRD patterns. It was found that surface morphologies of the film change from coating island with homogeneous structures to regular surface and thinner film structures with increasing solvent concentration. The films prepared from the solutions with 8 mL methanol have better adhesion strength to the glass substrate among other coatings. Refractive index, thickness and band gap of ITO thin films were determined to be 1.3171, 0.625 μm and 3.67, respectively.     

Sensing characteristics of aged zirconia-based hydrogen sensor utilizing Zn–Ta-based oxide sensing-electrode

We report here the enhanced sensing characteristics to H₂ for a potentiometric sensor using an yttria-stabilized zirconia (YSZ) solid electrolyte and a ZnO(+ 84 wt.% Ta₂O₅) sensing electrode (SE) after aging at 500 °C. The emf response toward 400 ppm H₂ was found to gradually increase up to − 800 mV after 40 days operation (aging) and was stabilized at this value until the 90th day. The aged and stabilized sensor exhibited highly sensitive response to H₂, with minor responses toward other examined gases such as NOx and HCs. The 90% response time toward 100 ppm H₂ was approximately 70 s. The H₂ sensitivity of the stabilized sensor was hardly affected by changes in water vapor as well as O₂ concentration, with repeatable and reproducible responses to H₂.