This article describes the development of a kind of full carbon-based humidity sensor fabricated on the paper substrate by handwriting. The electrodes were written by commercial pencils, and the sensitive layer was drawn with an oxidized multiwalled carbon nanotubes (o-MWCNTs) ink marker. The resultant devices exhibit good reproducibility and stability during the dynamic measurement. The response of the optimized paper-based sensor exhibits about five times higher than sensors fabricated on the ceramic substrate, which is owing to the hydrophilic property of the paper substrate. The structure of the sensitive layer formed by dispersing sensitive materials in the porous surface of paper substrates alleviates the inner stress in the process of bending. The response of printing paper-based sensors only shows the 6.7% decay even under an extremely high bending degree.Keywords: drawing pencil electrodes; drawing sensitive layer; flexible sensor; humidity sensing; sensor on paper;

•Novel organic-inorganic hybrid materials SBA-15-PSSx have been prepared via a free radical polymerization.•The dispersibility and stability of hydrophilic groups in humidity sensing materials could be guaranteed by chemical modification method.•The obtained sensor shows particularly rapid response.•The impedance analysis under different frequencies is used to support equivalent circle modes of the sensor at different RH.Mesoporous silicas loading hydrophilic materials composites were widely used in humidity sensors during the last decade. However, the physical mixing process used for loading the hydrophilic materials in the channel of mesoporous silicas is hard to guarantee their uniform dispersion, and there is no strong force to bind the host and guest materials together. To solve above problems, novel vinyl functionalized mesoporous silicas (SBA-15-vinyl) were chemically modified with hydrophilic sodium p-styrenesulfonate (SSS) with free radical polymerization. The chemical and porous structures of the obtained hybrid materials (SBA-15-PSSx) were characterized by fourier transform infrared spectroscopy (FT-IR), elemental analyses (EA), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and N2 adsorption-desorption. The humidity sensing properties of the sensors based on SBA-15-vinyl and SBA-15-PSS indicate the introduction of PSS could effectively enhance the sensing properties of SBA-15-vinyl. The impedance of the optimized sensor changed by more than three orders of magnitude over the relative humidity (RH) range of 11%–95%, with rapid response to RH change.

•Novel organic-inorganic hybrid materials SBA-15-propyl-S-DMCx have been prepared via a thiol-ene click reaction.•The dispersibility and stability of hydrophilic groups in hybrid materials could be ensured by chemical modification.•The conduction mechanism of the optimal sensor is analyzed in detail.Mesoporous silica based composites are widely used for humidity sensors. But a physical mixing is needed to load humidity sensitive materials in the mesoporous silica materials, so the uniform dispersion of humidity active materials could not be guaranteed. A chemically modification method has been used to ensure the uniform dispersion and stable structure of the humidity active units in this work. Novel organic-inorganic hybrid materials have been prepared via modifying the sulfhydryl functionalized mesoporous silica SBA-15 (SBA-15-propyl-SH) with hydrophilic unit methacrylatoethyl trimethyl ammonium chloride (DMC) via a thiol-ene click reaction. Mesoporous silica skeletons could ensure the stability at high humidities and hydrophilic groups modified on the skeletons behave as humidity sensitive parts. The content of DMC on the silica could be tuned by the feed ratios. The obtained hybrid materials keep the mesoporous characteristics of silica, which is beneficial for the transport of water molecules in adsorption and desorption processes. The humidity sensing properties of the sensors based on SBA-15-propyl-SH and SBA-15-propyl-S-DMCx were investigated in details. The impedance modulus of the optimized sensor changes by more than three orders of magnitude over the relative humidity (RH) range of 11% to 95%, with prompt response to RH change (11 s and 60 s for adsorption and desorption processes, respectively).A novel mesoporous silica based organic-inorganic hybrid material was synthesized via a thiol-ene click chemistry, which exhibited great humidity sensing properties and long-term stability.Download high-res image (110KB)Download full-size image

•A novel organic-inorganic composites LiCl/PETMP-DVB with cross-linked structure were prepared.•A way of in situ loading LiCl by click chemistry can ensure the uniform distribution of LiCl.•The sensor with 8 wt% LiCl/PETMP-DVB shows a little hysteresis and fast response to humidity change.•The equivalent circuits under optimal working frequency were detailed by combining with the phase angle.A physical mixing is usually needed in order to load lithium chloride (LiCl) in a matrix. A method of in situ loading LiCl in the cross-linked polymer framework by click reaction is demonstrated in this work. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and divinylbenzene (DVB) were taken as the monomers to construct stable cross-linked polymer matrix, while different contents of LiCl were introduced in the polymerization process for preparing humidity sensitive composites. The cross-linked composites with uniform distribution of LiCl were obtained. Humidity sensors based on the composites were fabricated and their sensing properties were systematically investigated. The optimum mass ratio of LiCl was determined to be 8 wt%, the corresponding sensor owns a variation of impedance modulus beyond two orders of magnitudes from 11% to 95% relative humidity (RH), a maximum humidity hysteresis of ∼1.5% RH, and a short response and recovery time of 3.5 and 63 s, respectively. The humidity sensing mechanism of the obtained sensor was explored by the complex impedance spectroscopy and the equivalent circuit.Download high-res image (124KB)Download full-size image

A catalyst-free Friedel–Crafts alkylation reaction has been developed to synthesize hierarchically porous polymeric microspheres (HPPMs) with phloroglucin and dimethoxymethane. HPPMs with uniform size were obtained and the size can be tuned by the concentration of raw materials. The chemical structure and hierarchical porous characteristic of HPPMs were characterized in detail. HPPMs were then loaded with humidity sensitive material LiCl to construct composites for humidity sensor. The optimum sensor based on 3 wt % LiCl-loaded HPPMs shows high sensitivity at the relative humidity (RH) atmosphere of 11–95%, small hysteresis, enhanced durability and rapid response. The sensitive mechanism was discussed through the investigation of complex impedance plots.Keywords: cross-linked; Friedel−Crafts alkylation; hierarchically porous polymeric microspheres; humidity sensor; LiCl loaded

A nanoporous organic polymer based on triphenylamine was synthesized, and the structure and morphology of the resultant polytriphenylamine (PTPA) were described by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption/desorption analysis. PTPA was acted as the host to load the guest Fe(NO3)3 for preparing humidity sensitive composites. Compared with pure PTPA sensor, Fe(NO3)3/PTPA sensors showed improved humidity sensitive properties, especially the 20 wt% Fe(NO3)3/PTPA sensor. The impedance of the 20 wt% Fe(NO3)3/PTPA sensor changed four orders of magnitude over the whole humidity range, with a good linearity, litter hysteresis, rapid response and good long-time stability. The complex impedance plots and direct current (DC) reverse polarity method were used to research the mechanism of the optimized sensor.

A crosslinked polyelectrolyte was synthesized by reaction of lithium benzene-1,3,5-tris(olate) and terephthalic aldehyde using a one-step hydrothermal process. The structure and morphology of the resultant polymer poly(lithium benzene-1,3,5-tris(olate)) (PLBTO) were investigated by Fourier transform infrared spectrum (FTIR), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A humidity sensor based on PLBTO was fabricated and humidity sensitive properties were explored. The impedance of PLBTO sensor changed four orders of magnitude with a good linearity over a relative humidity (RH) range from 33% to 95%. The conductive mechanism of the sensor was discussed based on complex impedance plots. The results indicate the quantity of adsorption water molecules would determine the category of conductive particles, and cause the impedance change under different RH. Moreover, the PLBTO sensor showed little hysteresis, rapid response/recovery and good long-term stability, which have benefited from the crosslinked structure of PLBTO.

A microporous organic polymer based on 1,3,5-trihydroxybenzene was synthesized, and the structure and properties of the resultant polymeric organic framework (POF) were determined by a series of characterizations. The POF acted as a host to load different amounts of LiCl for the preparation of humidity sensitive materials. Compared with the pure POF sensor, the LiCl/POF sensors showed improved humidity sensitive properties, especially the 4 wt% LiCl/POF sensor. The impedance of the optimum sensor changed by three orders of magnitude, with good linearity over the whole humidity range. Moreover, the optimum sensor showed little hysteresis, good long-term stability and a rapid response time. In order to explore the conductive mechanism, the complex impedance plots of the optimum sensor were discussed.

Au-loaded ZnO hollow nanospheres have been successfully synthesized by using carbon nanospheres as sacrificial templates. This simple strategy could be expected to be extended for the fabrication of similar metal–oxide loaded hollow nanospheres using different precursors. The structural and morphological characteristics of the resultant product were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The hollow nanospheres are porous, with the diameters ranging from 220 to 280 nm. To demonstrate the usage of such Au-loaded ZnO nanomaterial, a chemical gas sensor has been fabricated and investigated for NH3 detection. The Au-loaded ZnO sensor exhibits excellent sensing performances compared with hollow ZnO and compact ZnO sensors. The dynamic transients of the Au-loaded ZnO sensors demonstrated both their fast response (0.8–1.5 s) and recovery (3–4 s) towards NH3 gases. The combination of ZnO hollow structure and catalytic activity of Au loaded gives a very attractive sensing behavior for applications as real-time monitoring gas sensors with fast responding and recovering speed.

A new type of rattle-type structure with a porous shell was prepared via a simple template strategy in the case of SnO2, which showed high response and good selectivity to ethanol.

A new type of ring-like architecture with a lamellar structure was prepared via a simple hydrothermal strategy in the case of PdO–NiO, which showed a low operating temperature, a high response and rapid response/recovery to CO gas.

A novel sensing material of NiO-doped SnO2 polyhedra was prepared and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX) spectroscopy and transmission electron microscopy (TEM). Gas sensing properties of the sensor fabricated from the as-prepared NiO-doped SnO2 were systematically investigated and compared with those of pure SnO2. The NiO-doped SnO2 sensor exhibits a response of 6.7 to 30 ppm ethanol at 280 °C with the response and recovery times shorter than 0.6 and 10 s, respectively, which are much better than pure SnO2. The enhanced sensing performances to ethanol are mainly attributed to the formation of p–n heterojunctions between p-type NiO and n-type SnO2 and the increased alkalinity of SnO2 by NiO doping.

Ba0.8Sr0.2TiO3/Poly (vinylpyrrolidone) (BST/PVP) composite fibers were successfully synthesized via electrospinning. The ceramic nanofibers were obtained after calcining the composite at 800 °C for 2 h. The morphology and structure of the BST fibers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results reveal that the as-synthesized BST nanofibers show a diameter of 50–150 nm with the length over 0.1 mm, and a well-defined perovskite crystal structure. The electrical properties of the as-synthesized BST nanofibers were investigated through an impedance-type humidity sensor. The nanofibers exhibited excellent humidity sensing properties at room temperature. The possible sensing mechanism was proposed.Research Highlights► We synthesized the BST ceramic nanofibers via electrospinning and calcination method. ► We fabricated a humidity sensor based on as-synthesized BST nanofibers. ► The sensor exhibited excellent humidity sensing properties at room temperature.

Hierarchical hollow Au-loaded NiO hybrid microspheres have been synthesized with good uniformity by a surfactant-free hydrothermal route and subsequent heat treatment. This method requires high concentrations of a nickel precursor (0.2 M) and introduction of a trace amount of Au nanoparticles into the reaction system. The hierarchical hollow Au-loaded NiO hybrid microsphere comprises several nanorods and nanosheets. To demonstrate the usage of such hierarchical hybrid nanomaterials, a gas sensor has been fabricated from the as-synthesized Au-loaded NiO microspheres and investigated for acetone detection. The Au-loaded NiO sensor exhibits significantly improved sensing performances in terms of high sensitivity, low detection limit, better selectivity, rapid response, and good reproducibility in comparison with pure NiO. The effects of Au loading on the acetone-sensing properties of hierarchical NiO microspheres have been investigated.

Hollow NiO–SnO2 nanospheres have been fabricated via a simple one-pot template-free method. The synthesis is based on solvothermal treatment of stannate and nickel nitrate as the precursor in a mixed solvent of heptane–ethanol. The hollow nanospheres show diameters of about 200–300 nm with the wall thickness of about 50 nm, and the shell consists of numerous small nanoparticles. The gas sensor based on hollow NiO–SnO2 nanospheres exhibits high response and quick response–recovery to NH3, which are much better compared with sensors based on solid NiO–SnO2 nanospheres. The enhanced sensor performances are attributed to the larger surface area and fast gas diffusion.

A new type of spherically multilayered core–shell structure was prepared via a simple hard template strategy in the case of ZnO. The structure and morphology characteristics of the resultant product were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The ZnO microspheres with hollow interior and porous shells are multilayered structures with diameters ranging from 0.4 to 3.5 μm. Further investigation of the formation mechanism reveals that the preheating program is vital to the formation of the multishelled structures. To demonstrate the usage of such a multilayered nanomaterial, a chemical gas sensor has been fabricated and investigated for toluene detection. The sensor exhibits excellent sensing performances in terms of high response, low detection limit, rapid response-recovery, and superior selectivity.

The α-Fe2O3 hierarchical nanostructures have been successfully synthesized via a simple solvothermal method. The as-prepared samples are loose and porous with flowerlike structure, and the subunits are irregularly shaped nanosheets. The morphology of the α-Fe2O3 structures was observed to be tunable as a function of reaction time. To demonstrate the potential applications, we have fabricated a gas sensor from the as-synthesized hierarchical α-Fe2O3 and investigated it for ethanol detection. Results show that the hierarchical α-Fe2O3 sensor exhibits significantly improved sensor performances in comparison with the compact α-Fe2O3 structures. The enhancement of sensing properties is attributed to the unique porous and well-aligned nanostructure.Keywords: gas sensor; hierarchical structures; rapid response; α-Fe2O3;

Au-loaded ZnO hollow nanospheres have been successfully synthesized by using carbon nanospheres as sacrificial templates. This simple strategy could be expected to be extended for the fabrication of similar metal–oxide loaded hollow nanospheres using different precursors. The structural and morphological characteristics of the resultant product were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The hollow nanospheres are porous, with the diameters ranging from 220 to 280 nm. To demonstrate the usage of such Au-loaded ZnO nanomaterial, a chemical gas sensor has been fabricated and investigated for NH3 detection. The Au-loaded ZnO sensor exhibits excellent sensing performances compared with hollow ZnO and compact ZnO sensors. The dynamic transients of the Au-loaded ZnO sensors demonstrated both their fast response (0.8–1.5 s) and recovery (3–4 s) towards NH3 gases. The combination of ZnO hollow structure and catalytic activity of Au loaded gives a very attractive sensing behavior for applications as real-time monitoring gas sensors with fast responding and recovering speed.

A new type of spherically multilayered core–shell structure was prepared via a simple hard template strategy in the case of ZnO. The structure and morphology characteristics of the resultant product were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The ZnO microspheres with hollow interior and porous shells are multilayered structures with diameters ranging from 0.4 to 3.5 μm. Further investigation of the formation mechanism reveals that the preheating program is vital to the formation of the multishelled structures. To demonstrate the usage of such a multilayered nanomaterial, a chemical gas sensor has been fabricated and investigated for toluene detection. The sensor exhibits excellent sensing performances in terms of high response, low detection limit, rapid response-recovery, and superior selectivity.

A new type of ring-like architecture with a lamellar structure was prepared via a simple hydrothermal strategy in the case of PdO–NiO, which showed a low operating temperature, a high response and rapid response/recovery to CO gas.

A new type of rattle-type structure with a porous shell was prepared via a simple template strategy in the case of SnO2, which showed high response and good selectivity to ethanol.