In this work, we have fabricated a graphene-wrapped MnO/carbon nanofibers (MnO/CNFs@G) membrane by a facile electrospinning technique followed by an ambient pressure chemical vapor deposition (APCVD) process. The resultant MnO/CNFs@G membrane with uniform MnO particle distribution in porous carbon nanofibers and with a graphene layer covering not only facilitates the transport of both electrolyte ions and electrons to the MnO surface, but also relieves the pulverization that originated from the large volume change of MnO during the charge/discharge cycles. Interestingly, the free-standing and binder-free MnO/CNFs@G membranes can deliver a high reversible capacity of 946.5 mA h g−1 when the current density is switched back to 0.1 A g−1 after 110 cycles. Even at a high rate (10 A g−1), the electrode can still keep 426.7 mA h g−1 after 5000 cycles with coulombic efficiency of above 99%. This is the best specific capacity and longest cycling life reported for the MnO composite film anodes. We believe that the approach based on CNFs and CVD graphene as a structural support for the transition metal oxide can be potentially extended to improve the electrochemical performance of other electrode materials in lithium ion batteries.

•Isotype g-C3N4 heterojunctions were synthesized using melamine as the single-source precursor.•Microwave-assisted molten-salt process was developed to form the heterojunctions.•The heterojunction consists of isotype triazine-/heptazine based g-C3N4 nanoplates.•The g-C3N4 heterojunctions have highly enhanced photocatalytic activity for HER.•The efficient separation of photogenerated e−–h+ pairs enhanced its photocatalytic performance.Rapid synthesis and construction of graphitic carbon nitride (g-C3N4) based heterojunctions, cost-effective metal-free photocatalysts for hydrogen evolution reaction (HER) under solar irradiation, is of highly practical significance. This work reports a one-pot microwave-assisted molten-salt (mw-ms) process to rapidly synthesize isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic HER performance using melamine as the single-source precursor. The typical sample (mw-ms-g-C3N4) was obtained by thermally polymerizing melamine molecules at 550 °C for 30 min in the media of eutectic KCl/LiCl salts under microwave irradiation in air. The analyses of phases, chemical compositions and microstructures indicate that the mw-ms-g-C3N4 sample consists of an isotype triazine-/heptazine based g-C3N4 heterojunction, taking on a plate-like morphology with a specific surface area (SBET) of 25.7 m2 g−1. Comparatively, the g-C3N4 sample synthesized via an electric-resistance molten-salt (er-ms) process at 550 °C for 240 min is composed of a triazine-based g-C3N4 phase with a SBET of 58.1 m2 g−1, whereas the samples obtained by electric-resistance heating (er, at 550 °C for 240 min) and microwave heating (mw, at 550 °C for 30 min) processes consist of a heptazine-based g-C3N4 phase. The mw-ms-g-C3N4 sample shows a photocatalytic HER rate of 1480 μmol g−1 h−1, which is 5 times that (300 μmol g−1 h−1) of the er-ms-g-C3N4 sample, 15 times that (95 μmol g−1 h−1) of the er-g-C3N4 sample and 23 times that (63 μmol g−1 h−1) of the mw-g-C3N4 sample, under the similar visible-light (λ ≥ 420 nm) irradiation. The typical apparent quantum yield of the mw-ms-g-C3N4 sample at 420 nm is up to 10.7%. The UV–vis DR spectra suggest that both the triazine-based g-C3N4 and heptazine-based g-C3N4 phases have a similar bandgap of ∼2.66 eV, whereas the Mott-Schottky analysis indicates that the triazine-based g-C3N4 phase has a more positive flat conductive potential (−0.90 V) than the triazine-based g-C3N4 phase (−1.22 V). Due to the suitable alignment of their energy bandgap structures, the isotype triazine-/heptazine based g-C3N4 hybrids in the mw-ms-g-C3N4 sample form a type II heterojunction of semiconductor/semiconductor, which provides a convenient carrier transfer path and leads to more efficient separation of photo-generated electron-hole pairs than the other samples. The synergistic effects of microwave heating and molten-salt liquid polycondensation provide a robust platform for rapid and large-scale construction of isotype g-C3N4/g-C3N4 heterojunctions as metal-free high-performance HER photocatalysts using a simple single-source precursor.We first develop a microwave-assisted molten-salt (KCl/LiCl) process to rapidly synthesize isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic activity in hydrogen evolution reaction using melamine as the single-source precursor.Download high-res image (310KB)Download full-size image

Morphology-controlled synthesis of copper vanadate nanocrystals is of great significance in electrochemical sensing applications. A facile hydrothermal process for synthesizing copper vanadate nanocrystals with various morphologies (e.g., nanoparticles, nanobelts and nanoflowers) was reported. Phase, morphology and electrochemical performance of the as-synthesized copper vanadate nanocrystals were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and cyclic-voltammogram (CV) techniques. The results revealed that the morphologies of the Cu3V2O7(OH)2·2H2O (CVOH) nanocrystals could be controlled by changing copper salts, surfactants and pH values. The CVOH samples showed enhanced electrochemical response to ascorbic acid. Comparatively, the CVOH nanobelts had the higher electrochemical sensing performance than those of CVOH nanoparticles and nanoflowers. The CVOH-nanobelts-modified GCEs had a linear relationship between the peak currents in their CVs and ascorbic acid concentration. The CVOH nanocrystals can be used as potential electrochemical active materials for the determination of ascorbic acid.

Hierarchical Fe2O3@WO3 nanocomposites with ultrahigh specific areas, consisting of Fe2O3 nanoparticles (NPs) and single-crystal WO3 nanoplates, were synthesized via a microwave-heating (MH) in situ growth process. WO3 nanoplates were derived by an intercalation and topochemical-conversion route, and the Fe2O3 NPs were in situ grown on the WO3 surfaces via a heterogamous nucleation. The water-bath-heating (WH) process was also developed to synthesize a Fe2O3@WO3 nanocomposite for comparison purposes. The techniques of X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the samples obtained. The results show that α-Fe2O3 NPs with a size range of 5–10 nm are uniformly, tightly anchored on the surfaces of WO3 nanoplates in the Fe2O3@WO3 samples obtained via the MH process, whereas the α-Fe2O3 NPs are not uniform in particle-sizes and spatial distribution in the Fe2O3@WO3 samples obtained via the WH process. The BET surface area of the 5wt%Fe2O3@WO3 sample derived by the MH process is as high as 1207 m2 g−1, 5.9 times higher than that (203 m2 g−1) of the corresponding WO3 nanoplates. The dramatic enhancement in the specific surface area of the Fe2O3@WO3 samples should be attributed to the hierarchical microstructure, which makes the internal surfaces or interfaces in aggregated polycrystals be fully outside surfaces via a house-of-cards configuration, where the single-layered and disconnected Fe2O3 NPs are tightly anchored on the surfaces of the WO3 nanoplates. The gas-sensing properties of the Fe2O3@WO3 sensors were investigated. The gas-sensors based on the Fe2O3@WO3 obtained via the MH process show a high response and selectivity to H2S at low operating temperatures. The 5%Fe2O3@WO3 sample shows the highest H2S-sensing response at 150 °C. Its response to 10 ppm H2S is as high as 192, 4 times higher than that of the WO3-nanoplate sensor. The improvement in the gas-sensing performance of the Fe2O3@WO3 nanocomposites can be attributed to the synergistic effect in compositions and the hierarchical microstructures with ultrahigh specific surface areas.

Hierarchical SnO2@rGO nanostructures with superhigh surface areas are synthesized via a simple redox reaction between Sn2+ ions and graphene oxide (GO) nanosheets under microwave irradiation. XRD, SEM, TEM, XPS, TG-DTA and N2 adsorption–desorption are used to characterize the compositions and microstructures of the SnO2@rGO samples obtained. The SnO2@rGO nanostructures are used as gas-sensing and electroactive materials to evaluate their property–microstructure relationship. The results show that SnO2 nanoparticles (NPs) with particle sizes of 3–5 nm are uniformly anchored on the surfaces of reduced graphene oxide (rGO) nanosheets through a heteronucleation and growth process. The as-obtained SnO2@rGO sample with a hierarchically sesame cake-like microstructure and a superhigh specific surface area of 2110.9 m2 g−1 consists of 92 mass% SnO2 NPs and ∼8 mass% rGO nanosheets. The sensitivity of the SnO2@rGO sensor upon exposure to 10 ppm H2S is up to 78 at the optimal operating temperature of 100 °C, and its response time is as short as 7 s. Compared with SnO2 nanocrystals (5–10 nm), the hierarchical SnO2@rGO nanostructures have enhanced gas-sensing behaviors (i.e., high sensitivity, rapid response and good selectivity). The SnO2@rGO nanostructures also show excellent electroactivity in detecting sunset yellow (SY) in 0.1 M phosphate buffer solution (pH = 2.0). The enhancement in gas-sensing and electroactive performance is mainly attributed to the unique hierarchical microstructure, high surface areas and the synergistic effect of SnO2 NPs and rGO nanosheets.

Hierarchical In2O3@WO3 nanocomposites, consisting of discrete In2O3 nanoparticles (NPs) on single-crystal WO3 nanoplates, were synthesized via a novel microwave-assisted growth of In2O3 NPs on the surfaces of WO3 nanoplates that were derived through an intercalation and topochemical-conversion route. The techniques of XRD, SEM, TEM and XPS were used to characterize the samples obtained. The gas-sensing properties of In2O3@WO3 nanocomposites, together with WO3 nanoplates and In2O3 nanoparticles, were comparatively investigated using inorganic gases and organic vapors as the target substances, with an emphasis on H2S-sensing performance under low concentrations (0.5–10 ppm) at 100–250 °C. The results show that the In2O3 NPs with a size range of 12–20 nm are uniformly anchored on the surfaces of the WO3 nanoplates. The amounts of the In2O3 NPs can be controlled by changing the In3+ concentrations in their growth precursors. The In2O3@WO3 (In/W = 0.8) sample has highest H2S-sensing performance operating at 150 °C; its response to 10 ppm H2S is as high as 143, 4 times higher than that of WO3 nanoplates and 13 times that of In2O3 nanocrystals. However, the responses of the In2O3@WO3 sensors are less than 13 upon exposure to 100 ppm of CO, SO2, H2, CH4 and organic vapors, operating at 100–150 °C. The improvement in response and selectivity of the In2O3@WO3 sensors upon exposure to H2S molecules can be attributed to the synergistic effect of In2O3 NPs and WO3 nanoplates, hierarchical microstructures and multifunctional interfaces.

•Au/SnO2@WO3 composites were formed by reducing HAuCl4 with SnCl2 on WO3 nanoplates.•Au/SnO2@plate-WO3 is high sensitive to H2S detection at low temperature of ∼50 °C.•Au/SnO2@plate-WO3 is highly selective to H2S detection in various gases or vapors.•Synergistic effect of Au/SnO2 and WO3 results in enhancement in H2S-sensing property.In order to improve the gas-sensing performance at low temperature, binary Au/SnO2 species were used to modify WO3 nanoplates, i.e., Au/SnO2@plate-WO3 composites, which were synthesized by in-situ reducing HAuCl4 with SnCl2 adsorbed on the surfaces of WO3 nanoplates derived via an intercalation and topochemical conversion route. XRD, XPS, SEM, TEM and UV–vis DR spectra were used to characterize the samples. The gas-sensing properties of the samples were evaluated using H2S as target gas. The Au/SnO2 nanoparticles with small sizes (several nanometers) are uniformly anchored on the surfaces of WO3 nanoplates. The response of the 0.5%Au/SnO2@plate-WO3 sensor to 10 ppm H2S is up to 220 at 50 °C, 28 times higher than that of the plate-WO3 sensor. The optimal operation temperature of the plate-WO3 and Au/SnO2@plate-WO3 sensor for H2S detection is about 150 °C. The responses of the Au/SnO2@plate-WO3 sensor to 100 ppm of CO, SO2, H2, CH4 and organic vapors are negligibly low (1.2–8.0) at low temperatures. The possible explanation for the high selectivity and response in H2S detection at low temperatures can be the synergistic effect of the binary Au/SnO2 nanoparticles and ultra-thin WO3 nanoplates in adsorption, reaction and diffusion of the gas molecules.

•Au@plate-WO3 nanocomposites were synthesized by a chemical process.•The Au@plate-WO3 sensors were highly selective to NO gases with low concentrations.•The Au@plate-WO3 sensors had the highest sensitivity operating at about 170 °C.•The optimum amount of Au nanoparticles was about 1 wt.%.•Au nanoparticles and the loose aggregates enhanced the NO-sensing performance.Au-modified WO3 nanoplates (Au@plate-WO3) were synthesized by chemically reducing HAuCl4 on the surfaces of two-dimensional WO3 nanoplates, which were derived from an intercalation–topochemical process. XRD, SEM, TEM, XPS and UV–vis DR spectra were used to characterize the WO3 nanoplates and Au@plate-WO3 nanocomposites. The gas-sensing properties of the WO3 nanoplates and Au@plate-WO3 nanocomposites were comparatively investigated using inorganic gases and organic vapors as the target gases, with an emphasis on exploring the response and selectivity of NO gases with low concentrations (0.5–10 ppm) at low operating temperature (130−250 °C). The results indicated that Au nanoparticles (Au NPs) enhance the low-temperature sensitivity and selectivity of the Au@plate-WO3 sensors for NO detection when compared with the performance of the WO3 sensors. The Au@plate-WO3 nanocomposite with 1 wt.% Au NPs has the best NO-sensing performance at the optimum operating temperature of ∼170 °C. In addition, the Au@plate-WO3 sensors show highly selective to NO gas among various inorganic gases (i.e., H2, SO2 and CO) and organic vapors (i.e., alcohol, acetone, methanal and benzene). The enhancement in sensitivity and selectivity for NO detection is probably due to the synergistic effect of Au NPs and the house-of-card structure of WO3 nanoplates.

For improving the low-temperature response and selectivity of WO3-based sensors, Ag nanoparticles (AgNPs) have been used to modify the WO3 nanoplates. Ag@plate-WO3 nanocomposites with various amounts of AgNPs were synthesized by growing AgNPs on WO3 nanoplates. XRD, SEM, TEM and XPS spectrum were used to characterize the Ag@plate-WO3 samples. The gas-sensing properties were evaluated at r.t. −250 °C using NO gases with various concentrations (0.5–50 ppm). AgNPs enhance the low-temperature response and selectivity of the Ag@plate-WO3 sensors for NO detection, and the amounts of AgNPs influence the NO-sensing performance of the Ag@plate-WO3 sensors. The sample with 0.5% AgNPs shows the best performance. Its optimum operating temperature is around 170 °C, but it shows response even at room temperature. The Ag@plate-WO3 sensors have a high selective response to NO gas, among various gases (i.e., H2 and CO) and organic vapors (i.e., alcohol, acetone, methanol and benzene). The morphologies of WO3 nanocrystals also influence the NO-sensing properties of the Ag@WO3 sensors, and the plate-like WO3 samples are better than the particle-like WO3 samples in improving NO-sensing performance. The NO-sensing enhancement should result from the synergistic effects of AgNPs and the loose house-of-card structure of plate-like WO3 aggregates.

The hierarchical photocatalysts of Ag/AgCl@plate–WO3 have been synthesized by anchoring Ag/AgCl nanocrystals on the surfaces of single-crystalline WO3 nanoplates that were obtained via an intercalation and topochemical approach. The heterogeneous precipitation process of the PVP–Ag+–WO3 suspensions with a Cl− solution added drop-wise was developed to synthesize AgCl@WO3 composites, which were then photoreduced to form Ag/AgCl@WO3 nanostructures in situ. WO3 nanocrystals with various shapes (i.e., nanoplates, nanorods, and nanoparticles) were used as the substrates to synthesize Ag/AgCl@WO3 photocatalysts, and the effects of the WO3 contents and photoreduction times on their visible-light-driven photocatalytic performance were investigated. The techniques of TEM, SEM, XPS, EDS, XRD, N2 adsorption–desorption and UV-vis DR spectra were used to characterize the compositions, phases and microstructures of the samples. The RhB aqueous solutions were used as the model system to estimate the photocatalytic performance of the as-obtained Ag/AgCl@WO3 nanostructures under visible light (λ ≥ 420 nm) and sunlight. The results indicated that the hierarchical Ag/AgCl@plate–WO3 photocatalyst has a higher photodegradation rate than Ag/AgCl, AgCl, AgCl@WO3 and TiO2 (P25). The contents and morphologies of the WO3 substrates in the Ag/AgCl@plate–WO3 photocatalysts have important effects on their photocatalytic performance. The related mechanisms for the enhancement in visible-light-driven photodegradation of RhB molecules were analyzed.

Molybdate-based inorganic–organic hybrid disks with a highly ordered layered structure were synthesized via an acid–base reaction of white molybdic acid (MoO3·H2O) with n-octylamine (C8H17NH2) in ethanol at room temperature. The thermal treatment of the as-obtained molybdate-based inorganic–organic hybrid disks at 550 °C in air led to formation of orthorhombic α-MoO3 nanoplates. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal analysis (TG–DTA), Fourier-transform infrared (FT–IR) spectra, Raman spectra, and a laser-diffraction grain-size analyzer were used to characterize the starting materials, the intermediate hybrid precursors and the final α-MoO3 nanoplates. The XRD, FT–IR and TG–DTA results suggested that the molybdate-based inorganic–organic hybrid compound, with a possible composition of (C8H17NH3)2MoO4, was of a highly ordered lamellar structure with an interlayer distance of 2.306(1) nm, and the n-alkyl chains in the interlayer places took a double-layer arrangement with a tilt angle of 51° against the inorganic MoO6 octahedra layers. The SEM images indicated that the molybdate-based inorganic–organic hybrids took on a well-dispersed disk-like morphology, which differed distinctly from the severely aggregated morphology of their starting MoO3·H2O powders. During the calcining process, the disk-like morphology of the hybrid compounds was well inherited into the orthorhombic α-MoO3 nanocrystals, showing a definite plate-like shape. The α-MoO3 nanoplates obtained were of a single-crystalline structure, with a side-length of 1–2 μm and a thickness of several nanometres, along a thickness direction of [010]. The above α-MoO3 nanoplates were of a loose aggregating texture and high dispersibility. The chemical sensors derived from the as-obtained α-MoO3 nanoplates showed an enhanced and selective gas-sensing performance towards ethanol vapors. The α-MoO3 nanoplate sensors reached a high sensitivity of 44–58 for an 800 ppm ethanol vapor operating at 260–400 °C, and their response times were less than 15 s.

A novel pseudo-morphotactic transformation route was developed to synthesize polycrystalline β-W2N nanoplates by thermally treating tungstate-based inorganic–organic hybrid nanobelts with a lamellar microstructure in an NH3 flow. The tungstate-based hybrid nanobelts were formed in a water-in-oil-microemulsion-like “commercial H2WO4 powders/n-octylamine/heptane” reaction system. The as-obtained hybrid nanobelts were thermally treated in an NH3 atmosphere at 650–800 °C for 2 h to form cubic β-W2N nanoplates. XRD, SEM, TEM, FT-IR and TG-DTA were used to characterize the precursors and their final products. The polycrystalline β-W2N nanoplates derived from hybrid nanobelts, with side lengths of several hundred nanometers, consist of small nanocrystals with an average grain size of 3.2 nm. The formation of β-W2N nanoplates involved two steps: decomposing tungstate-based hybrid nanobelts into WOy and W species and then nitridizing the active W-containing species to β-W2N nanocrystals in an NH3 flow. The platelike morphology of the β-W2N nanocrystals was inherited from the precursor of tungstate-based inorganic–organic hybrid nanobelts.A novel pseudo-morphotactic transformation route was developed to synthesize β-W2N nanoplates by thermally treating tungstate-based inorganic–organic hybrid nanobelts, the morphology of which was inherited to the β-W2N nanocrystals.Research highlights► We synthesize β-W2N nanoplates using inorganic–organic hybrid nanobelts as precursors. ► β-W2N nanoplates are formed at 650–800 °C for 2 h in an NH3 flow. ► β-W2N nanoplates consist of small nanocrystals. ► The plate-like morphology of β-W2N is inherited from its hybrid precursor. ► The pseudo-morphotactic transformation route is suitable for large-scale synthesis.

The aim of this paper was to provide a convincing experimental research to demonstrate a dissolution–reorganization mechanism for the formation of tungstate-based inorganic–organic hybrid nanobelts by comparatively investigating the reaction behaviors of H2WO4 and H2W2O7·xH2O with n-alkylamines (CmH2m+1NH2, m = 4–10). The formation of tungstate-based hybrid nanobelts derived from the reactions between n-alkylamines and H2WO4 with single-octahedral W–O layers was investigated with a detailed comparison with those between n-alkylamines and H2W2O7·xH2O with double-octahedral W–O layers. H2WO4 and H2W2O7·xH2O reacted with n-alkylamines, respectively, in reverse-microemulsion-like media. The obtained products were characterized by XRD, FT-IR, TG–DTA and SEM. The results indicated that the products derived from H2WO4 and those from H2W2O7·xH2O were similar in compositions, microstructures and morphologies. The structural analysis indicated the products were tungstate-based inorganic–organic hybrid one-dimensional belts with highly ordered lamellar structures by alternately stacking organic n-alkylammonium bilayers and inorganic single-octahedral W–O layers. The n-alkyl chains in the above hybrid nanobelts from H2WO4 and H2W2O7·xH2O took on a bilayer arrangement with tilt angles of 65° and 74°, respectively. The similarities in the microstructures of the products from H2W2O7·xH2O and H2WO4 demonstrated that the double-octahedral W–O layers of H2W2O7·xH2O were decomposed during the reactions. The changes of inorganic W–O layers and the morphologic changes of the tungstic-acid precursors before and after the reactions corroborated the dissolution–reorganization mechanism.

In this work, triclinic WO3 nanoplates and WO3 nanoparticles were comparatively investigated as sensing materials to detect acetone vapors. Single-crystalline WO3 nanoplates with large side-to-thickness ratios were synthesized via a topochemical conversion from tungstate-based inorganic–organic hybrid nanobelts, and the WO3 nanoparticles were obtained by calcining commercial H2WO4 powders at 550 °C. The acetone-sensing properties were evaluated by measuring the change in electrical resistance of the WO3 sensors before and after exposure to acetone vapors with various concentrations. The WO3 nanoplate sensors showed a high and stable sensitive response to acetone vapors with a concentration range of 2–1000 ppm, and the sensitivity was up to 42 for 1000 ppm of acetone vapor operating at 300 °C. The response and recovery times were as short as 3–10 s and 12–13 s, respectively, for the WO3 nanoplate sensors when operating at 300 °C. The acetone-sensing performance of the WO3 nanoplate sensors was more excellent than that of the WO3 nanoparticle sensors under a similar operating condition. The enhancement of the WO3 nanoplate sensors in the acetone-sensing property was attributed to the poriferous textures, single-crystalline microstructures and high surface areas of the aggregates consisting of WO3 nanoplates, which were more favorable in rapid and efficient diffusion of acetone vapors than the WO3 nanoparticles.

The paper described a novel approach toward WO3 nanocrystals by pyrolytically decomposing tungstate-based inorganic–organic hybrid nanobelts in air at 500–600 °C for 2 h. The above-mentioned hybrid nanobelts were derived via a reaction of layered H2W2O7·xH2O and n-octylamine in a reverse-micelle-like medium (H2W2O7·xH2O/n-octylamine/heptane). The as-obtained WO3 nanocrystals and their intermediate products were characterized by the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM & FE-SEM), thermoanalysis (TG–DSC), Fourier-transform infrared spectra (FT/IR), UV–vis absorption and X-ray photoelectron spectroscopy (XPS). The as-obtained WO3 nanocrystals had an apparent size of 20–50 nm, and took on a loose-aggregate-like morphology. The WO3 nanocrystals derived via the pyrolytic decomposition process were almost separated from each other and could be redispersed readily, while the WO3 nanocrystals obtained by the conventional acid-precipitation process tightly agglomerated into large particles with apparent sizes of several micrometers, without redispersibility even under an intense sonication treatment.

Molybdate-based inorganic–organic hybrid disks with a highly ordered layered structure were synthesized via an acid–base reaction of white molybdic acid (MoO3·H2O) with n-octylamine (C8H17NH2) in ethanol at room temperature. The thermal treatment of the as-obtained molybdate-based inorganic–organic hybrid disks at 550 °C in air led to formation of orthorhombic α-MoO3 nanoplates. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal analysis (TG–DTA), Fourier-transform infrared (FT–IR) spectra, Raman spectra, and a laser-diffraction grain-size analyzer were used to characterize the starting materials, the intermediate hybrid precursors and the final α-MoO3 nanoplates. The XRD, FT–IR and TG–DTA results suggested that the molybdate-based inorganic–organic hybrid compound, with a possible composition of (C8H17NH3)2MoO4, was of a highly ordered lamellar structure with an interlayer distance of 2.306(1) nm, and the n-alkyl chains in the interlayer places took a double-layer arrangement with a tilt angle of 51° against the inorganic MoO6 octahedra layers. The SEM images indicated that the molybdate-based inorganic–organic hybrids took on a well-dispersed disk-like morphology, which differed distinctly from the severely aggregated morphology of their starting MoO3·H2O powders. During the calcining process, the disk-like morphology of the hybrid compounds was well inherited into the orthorhombic α-MoO3 nanocrystals, showing a definite plate-like shape. The α-MoO3 nanoplates obtained were of a single-crystalline structure, with a side-length of 1–2 μm and a thickness of several nanometres, along a thickness direction of [010]. The above α-MoO3 nanoplates were of a loose aggregating texture and high dispersibility. The chemical sensors derived from the as-obtained α-MoO3 nanoplates showed an enhanced and selective gas-sensing performance towards ethanol vapors. The α-MoO3 nanoplate sensors reached a high sensitivity of 44–58 for an 800 ppm ethanol vapor operating at 260–400 °C, and their response times were less than 15 s.

Hierarchical In2O3@WO3 nanocomposites, consisting of discrete In2O3 nanoparticles (NPs) on single-crystal WO3 nanoplates, were synthesized via a novel microwave-assisted growth of In2O3 NPs on the surfaces of WO3 nanoplates that were derived through an intercalation and topochemical-conversion route. The techniques of XRD, SEM, TEM and XPS were used to characterize the samples obtained. The gas-sensing properties of In2O3@WO3 nanocomposites, together with WO3 nanoplates and In2O3 nanoparticles, were comparatively investigated using inorganic gases and organic vapors as the target substances, with an emphasis on H2S-sensing performance under low concentrations (0.5–10 ppm) at 100–250 °C. The results show that the In2O3 NPs with a size range of 12–20 nm are uniformly anchored on the surfaces of the WO3 nanoplates. The amounts of the In2O3 NPs can be controlled by changing the In3+ concentrations in their growth precursors. The In2O3@WO3 (In/W = 0.8) sample has highest H2S-sensing performance operating at 150 °C; its response to 10 ppm H2S is as high as 143, 4 times higher than that of WO3 nanoplates and 13 times that of In2O3 nanocrystals. However, the responses of the In2O3@WO3 sensors are less than 13 upon exposure to 100 ppm of CO, SO2, H2, CH4 and organic vapors, operating at 100–150 °C. The improvement in response and selectivity of the In2O3@WO3 sensors upon exposure to H2S molecules can be attributed to the synergistic effect of In2O3 NPs and WO3 nanoplates, hierarchical microstructures and multifunctional interfaces.