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The glassy carbon electrode (GCE) embellished with porous Co3O4 as a non-enzymatic amperometric sensor is applied to the determination of glucose concentration. Spinel porous Co3O4 samples are synthesized by a facile coprecipitation method using sodium oxalate (SO) and ammonia water (AW) as different precipitants. The influence of the different precipitants on the morphology, structure, and catalytic performance is systematically studied. The electrochemical measurements reveal that the Co3O4 synthesized using sodium oxalate (SO) precipitant exhibits higher sensitivity of 1060.1 μA mM−1 cm−2 and better long-term stability with maintain in 90% after 30 days than the Co3O4 (37.25 μA mM−1 cm−2) synthesized using ammonia water (AW) precipitant. The limit of detection for Co3O4 (using SO as precipitant)/GCE is estimated to be 0.32 μM less than 0.51 μM of Co3O4 (using AW as precipitant)/GCE (S/N = 3). The reason may be that theCo3O4 (using SO as precipitant) has fluffy porous structure and higher specific surface area than the Co3O4 (using AW as precipitant), which can provide the large electroactive sites and improve the diffusion of electrolyte ions and reduces their diffusion resistance.

Spinel CuCo2O4 with 3D macro-/mesoporous structure has been successfully synthesized via template-free solution combustion method. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and N2 absorption/desorption have been done to obtain compositional and morphological information as well as BET surface area of the as-synthesized sample. Catalytic activities of the porous CuCo2O4 towards the thermal decomposition of ammonium perchlorate (AP) were investigated with differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques. The results show that the addition of spinel CuCo2O4 with 3D macro-/nanoporousstructure to AP remarkably decreases the high temperature decomposition (HTD) from 470.92 °C for pure AP to 308.74 °C for AP containing CuCo2O4 additive with a mass ratio of 2%. The as-synthesized 3D macro-/nanoporous spinel CuCo2O4 is the promising type of catalytic materials for AP-based composite solid rocket propellants. The simple synthesis method is conducive to large-scale industrial production for application in AP thermal decomposition.

Water-soluble rhodamine B assisted graphene quantum dots (RhB-GQDs) were synthesized by a facile one-pot hydrothermal method, using rhodamine B and ethylenediamine as nitrogen source. The as-synthesized RhB-GQDs have few layers configuration with an average size of 4.5 nm and good distribution. The fluorescence quenching approach was applied to the sensing of mercury ions in water. Rodamine B absorb on the surface of RhB-GQDs, and it can prevent RhB-GQDs from contacting with interfering cations. The presence of N atoms reduces the ability of interfering cations to form stable complex with carboxyl groups, and gives the RhB-GQDs good selectivity to mercury ions. In the case of low concentration, the fluorescence quenching efficiency of as-synthesized RhB-GQDs still has a good liner relationship with concentration. Due to the strong affinity of Hg2+ to amino and carboxyl on the surface of RhB-GQDs, the fluorescence of RhB-GQDs is quenched by nonradiative electron transfer between Hg2+ and the excited state of RhB-GQDs. Based on the fluorescence turn off process, the as-synthesized RhB-GQDs can be served as fluorescent platform for the sensitive and selective detection of Hg2+ with a detection limit of 0.16 nM.

Cobalt–nickel sulfide (NiCo2S4) was directly grown on nickel foam by a facile hydrothermal method. The effect of four different sulfur sources, the composite step and RGO density on the morphology and electrochemical performance of NiCo2S4 electrode for supercapacitor were investigated in detail. The results exhibited that under the same conditions, using the thioacetamide as the sulfur source, the electrochemical properties of the electrode material was the best, which combined with 0.1 g/L RGO in the second step of hydrothermal. It showed the highest capacity and longest cycling life, demonstrating a specific capacitance of 1733 F/g at a charge–discharge current density of 1 A/g and 87% capacitor retention rate after 5000 cycles.

Nickel cobalt oxide (NiCo2O4) was successfully grown on the nickel foam by a mild hydrothermal method combined with a simple annealing treatment, utilizing SDS, PVP, CTAB, and PVA as template agents. The effect of different template agents on the morphology and electrochemical performance of NiCo2O4 electrode for supercapacitor was further investigated in detail. The physicochemical properties of the NiCo2O4 materials were examined via the X-ray diffraction and the scanning electron microscopy. It could be seen from the XRD patterns that there were no other clear clutter peaks, which indicated that the purity of the material was relatively high. As shown by the SEM diagram, the prepared NiCo2O4 showed different morphology structures via the addition of the template agent, which exhibited high capacitance and cycling stability. Specially, the NiCo2O4 electrode with the usage of 0.75 g SDS, PVP, CTAB, PVA, and none template agents showed high capacitance of 1357, 1469, 1290.6, 1297.4, and 863.8 F g−1 at 1 A g−1. In addition, asymmetric supercapacitors (ASC) were assembled with the brilliant binary oxides as the positive electrode, activated carbon as negative electrode, and 6 mol/L KOH solution as electrolyte. The ASC device demonstrated a high energy density of 64.76 Wh kg−1 at a power density of 774.8 W kg−1. Remarkably, it still displayed desirable cycle retention of 84.39% over 5000 cycle numbers at a current density of 10 A g−1, and the excellent electrochemical performance suggested its potential application in electrode material for supercapacitor.

A sensitive nonenzymatic amperometric glucose sensor is described that relies on a glassy carbon electrode modified with a macro-/meso-porous NiCo2O4. NiCo2O4 with spinel structure has been prepared via a one-step solution combustion method. The material was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen absorption/desorption. An electrode was coated with the porous material and then displayed excellent electrocatalytic activity towards the direct oxidation of glucose in 0.15 M NaOH solution by cyclic voltammetry. Amperometric I-t curve demonstrated a sensitivity of 2100 μA·mM−1·cm−2 at an applied potential of 0.45 V (vs Hg/HgCl). The sensor has a linear response in the 0.001 to 1.0 mΜ glucose concentration range, a fast response time (3.9 s) and a low detection limit (0.38 μΜ).

Mesoporous ZnCo2O4 rods have been successfully prepared via oxalate co-precipitation method without any template. The nano-sized spinel crystallites connected together to form mesoporous structure by annealing homogeneous complex oxalates precursor at a low rate of heating. It is found that the low anneal rate plays an important role for the formation of mesoporous ZnCo2O4 rods. The effects of the heat temperature on the phase, morphology and catalytic properties of the products were studied. The XRD, SEM TEM, and N2 absorption/desorption have been done to obtain compositional and morphological information as well as BET surface area of the as-prepared sample. Catalytic activities of mesoporous ZnCo2O4 rods toward the thermal decomposition of ammonium perchlorate (AP) were investigated with differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques. The results show that the addition of ZnCo2O4 rods to AP dramatically reduces the decomposition temperature. The ZnCo2O4 rods annealed at 250 °C possesses much larger specific area and exhibits excellent catalytic activity (decrease the high decomposition temperature of AP by 162.2 °C). The obtained mesoporous ZnCo2O4 rods are promising as excellent catalyst for the thermal decomposition of AP.

In this work, an efficient and stable fluorescent probe for Al3+was established. The fluorescent probe based on the fluorescence 'turn-on' mode of zinc sulfide crystal composite zinc oxide quantum dots (ZnS/ZnO QDs). The ZnS/ZnO QDs were synthesized via two-step method using L-Cysteine (L-Cys) as a sulfur source and stabilizer. In the synthesis of ZnS/ZnO QDs, the fluorescence of zinc oxide quantum dots (ZnO QDs) decreased and its stability increased in aqueous solution after the addition of L-Cys. In addition, the as-synthesized ZnS/ZnO QDs shows fluorescent enhancement to Al3+. The ZnS/ZnO QDs based fluorescence 'turn-on' probe presented wide linear ranges (1 nM-8μM and 8-100μM). The availability of as-established sensing probe was also estimated by real water sample tests. Furthermore, the fluorescent enhancing mechanism was carried out by recording the fluorescent lifetime of samples, which might be related to the QDs dispersion and charge transfer weaken.

The mesoporous ZnCo2O4 nanosheets were synthesized via a facile hydrothermal method and subsequent thermal annealing treatment. The sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption isotherms (BET) to obtain information about its crystal structure, morphology, and BET surface area, respectively. The nanosheets present hexagonal shape with mesoporous structure in nature. The obtained mesoporous ZnCo2O4 nanosheets exhibit a high specific surface area of 191.64 m2/g. The electrochemical properties of the mesoporous ZnCo2O4 nanosheets were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) in 6 mol/L KOH aqueous electrolyte. The ZnCo2O4 nanosheets show a high specific capacity of 835.26 F/g at the current density of 1.0 A/g, as well as good cycle stability with specific capacitance remaining 73.28% of the initial value after 1000 continuous charge-discharge cycles under the current density of 8 A/g. These superior electrochemical features demonstrate that the mesoporous ZnCo2O4 nanosheet is a promising electroactive material for supercapacitor.

In this paper, a convenient solution combustion method to synthesis hierarchically porous Ag-loaded ZnO heterojunction nanocomposites with various Ag contents was reported. The analysis of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption were conducted to characterize the ingredients, microstructure and surface morphology. Ag and ZnO show separate phases due to the reduction of Ag + and the faster crystallization rate of ZnO. The as-synthesized powders were used as gas sensing materials to fabricate gas sensors. The gas-sensing performance was tested towards 100 ppm formaldehyde at an optimum operating temperature of 240 °C and Ag-loaded ZnO (with the different Ag atomic ratio) based gas sensors showed a superior gas-sensing performance comparing with pure ZnO. Especially 1 at% Ag-loaded ZnO has the highest gas response as 170.42. The possible gas-sensing mechanism was discussed in this paper. For example, the influence of surface area, the formation of heterostructure, Ag elements, adsorbed oxygen and so on.

SnO 2 -composite Pd nanoparticles (0, 2.5, 7.5, 10 mol% Pd loading) were synthesized via solvothermal method, followed by calcination. The structure, morphology, chemical state and specific surface area of the Pd-SnO 2 composite were analyzed with X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), N 2 physisorption, respectively. It is found that the composites consist of large amount of SnO 2 microspheres with average diameters up to hundreds of nanometers, and the microspheres are assembled by numerous nanoparticles with average sizes of about 8 nm. To demonstrate their potential application, gas sensors based on the as-synthesized Pd-SnO 2 composites were fabricated to test their sensing performances. The 10 mol% Pd-SnO 2 composite shows an excellent sensing response towards different concentrations of hydrogen at 200 °C. The highest sensing response is up to 315.34 for 3000 ppm hydrogen with a fast response-recovery time (4 s/10 s), which is over 8 times higher than that of pristine SnO 2 , and the lowest detection limit is down to 10 ppm. More significantly, it presents excellent selectivity and stability for hydrogen. The improved sensing response characteristics of the composite could be attributed to the chemical sensitization and electronic sensitization of Pd catalyst.

The natural coconut shell activated carbon (AC) was firstly treated with KOH in advance, and the Mn3O4/AC composites in various AC rations were prepared by the hydrothermal process and subsequent heat treatment. The resulting Mn3O4/AC composites were characterized by means of different techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and so on. Morphological studies reveal the hierarchical porous structure of Mn3O4/AC composites. Mn3O4/AC nanocomposite with the activated carbon quality percentage of 10% exhibits the highest specific capacitance of 491.2 F g−1 at the current density of 2 A g−1 in 6 M KOH aqueous electrolyte measured from the charge–discharge process. The value is nearly 6 times higher than that of the pure Mn3O4 nanoparticles as an electrode for supercapacitor. The good performance of the as-prepared Mn3O4/AC composites as electrode materials may be attributed to the synergistic effects of the activated carbon and the Mn3O4.

3D macro-/nanoporous spinel MCo2O4 (M = Zn, Ni, Cu and Fe) have been successfully fabricated by means of an untemplate and flash synthesis via self-sustained decomposition of metal-organic complexes The effects of the combustion agent to metal nitrate ratio on the combustion behavior, phase and morphology of the products were studied. Pure cobaltites with the hierarchically porous morphology were obtained directly through the combustion of the precursors under fuel-lean conditions in one-step without further heat treatment. The samples were characterized by X-ray diffraction, SEM/TEM imaging, mercury intrusion porosimetry, which indicate that the products possess hierarchical porous structure. A possible forming mechanism was given based on the series of experimental data.

In this paper, the NiCo2O4 nanoparticles were synthesized via a facile one-step solvothermal method without annealing treatment. XRD, TEM, XPS, and N2 adsorption/desorption were used to characterize the composition, morphology, and BET surface area of as-prepared sample, respectively. The analysis results indicated that the NiCo2O4 nanoparticles with spinel structure were successfully obtained. The NiCo2O4 sample was prepared into supercapacitors electrode materials. At room temperature, the electrode materials were investigated with cyclic voltammetry (CV) and galvanostatic charging-discharging in a 6 mol L−1 KOH aqueous electrolyte. Three-electrode systems, in which Hg/HgCl was employed as reference electrode, were used. The NiCo2O4 exhibited a specific capacitance of 717.9 F g−1 at the current density of 0.5 A g−1. Moreover, the specific capacitance remained 84% after 1000 continuous charge-discharge cycles under the current density of 10 A g−1.

Cerium oxide nanoparticles/multi-wall carbon nanotubes (MWCNTs) composites are synthesized by a facile hydrothermal method without any surfactant or template. The morphology and microstructure of samples are examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD), Raman spectrum and X-ray photoelectron spectroscopy (XPS). Electrochemical properties of the MWCNTs, the pure CeO2, and the CeO2/MWCNTs nanocomposites electrodes are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GDC) and electrochemical impedance spectroscopy (EIS) measurements. The CeO2/MWCNTs nanocomposite (at the mole ratio of 1:1) electrode exhibits much larger specific capacitance compared with both the MWCNTs electrode and the pure CeO2 electrode and significantly improves cycling stability compared to the pure CeO2 electrode. The CeO2/MWCNTs nanocomposite (at the mole ratio of 1:1) achieves a specific capacitance of 455.6 F g−1 at the current density of 1 A g−1. Therefore, the as prepared CeO2/MWCNTs nanocomposite is a promising electrode material for high-performance supercapacitors.

Monodisperse ZnFe2O4 nanospheres were succesuffully synthesized via a nonaqueous route in benzyl alcohol at low temperaure of 200 °C. It was found that the ZnFe2O4 is 7–16 nm in diameter with a large surface area of 87.40 m2/g. The gas sensor based on ZnFe2O4 nanospheres shows a high pontential for detecting low-ppm-level toluene, exhibiting a good linearity ranging from 1 to 100 ppm with a high response (100 ppm:9.98) and excellent selectivity.

Tin oxide microspheres were successfully obtained through a facile hydrothermal method without any polymer templates or surfactant. The as-synthesized SnO 2 microspheres are composed of large amount of small spheres with average diameters of about 250 nm, and every small sphere consists of numerous primary nanocrystallites with average sizes of about 8 nm. The resultant product was used as sensing material for gas sensor to detect the formaldehyde (HCHO) gas. The gas response, response and recovery time, selectivity and stability were carefully studied. It was found that the response value of the sensor to 100 ppm HCHO was 38.3 at the operating temperature of 200 °C. The gas sensor based on SnO 2 microspheres has excellent gas response, good response-recovery properties, linear dependence, repeatability and selectivity, making it to be a promising candidate for practical detectors for HCHO.

Nanoporous titanium dioxide was synthesized by a hydrothermal method without using of surfactant or template. The structure, morphology, surface chemical states and specific surface area were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption isotherms, respectively. The as-synthesized products are anatase-TiO2 with small grain size (about 12.27 nm) and high surface area (147.17 m2 g−1). The as-synthesized porous TiO2 powder was used to fabricate indirect-heating gas sensor whose gas-sensing characteristics toward acetone were investigated. At its optimal operation temperature, the sensor possesses a good sensitivity, selectivity, linear dependence, low detection limitation, and response/recovery, repeatability as well as long-term stability. Especially for the high sensitivity and fast response/recovery, its response reaches 25.97 for 500 ppm acetone, which is several times higher that of the reported TiO2-based sensors. The response and recovery times are only 13 and 8 s, respectively. Those values demonstrate the potential of using as-synthesized TiO2 for acetone gas detection, particularly in the dynamic monitoring. Apart from these, the mechanism related to the advanced properties was also investigated and presented.

As a new methanol sensing material, the composite of palladium-platinum-In 2 O 3 composited nanocrystalline SnO 2 was successfully obtained by a solid-phase reaction method. The phase composition and the unique structure of as-prepared SnO 2 -Pd-Pt-In 2 O 3 composite were comprehensively characterized using the techniques such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results of gas sensing properties indicate that the sensor based on the as-prepared SnO 2 -Pd-Pt-In 2 O 3 composite shows the ultra-high response to methanol at an optimal operating temperature of 160 °C when the mass ratio of SnO 2 , Pd, Pt and In 2 O 3 is 100:0.8:2:30. The gas response toward 100 ppm of methanol reaches as high as 320.73. The detection limit of the sensor is estimated to be 0.1 ppm. The sensor also exhibited good response-recovery properties, selectivity, remarkable repeatability and stability, suggesting its greatly promising application in gas sensing. This work could stimulate an effective means to fabricate methanol sensor with superior performance. At the same time, a possible sensing mechanism of the SnO 2 -Pd-Pt-In 2 O 3 composite is discussed.

ZnO hollow spheres with high crystallinity were prepared successfully via a simple template process using ZnCl 2 , carbamide and polystyrene spheres (PSS) as raw materials. The structural and morphological characterizations of the samples were carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), respectively. Indirect-heating sensor using ZnO hollow spheres as sensing materials was fabricated on an alumina tube with Au electrodes and Pt wires. The gas sensing properties of the as-synthesized ZnO hollow spheres for n -butanol were investigated. It is shown that the sensor exhibited good sensing performances, characterized by high response, very short response time, and stability to n -butanol gas at operating temperature of 385 °C. These results indicate that the ZnO hollow spheres are highly promising candidates for practical detectors for n -butanol.

Macro-/nanoporous Al-doping ZnO powders with enhanced gas sensing properties are prepared by a solution combustion method using Zn(CH 3 COO) 2 ·2H 2 O, Zn(NO 3 ) 2 ·6H 2 O, Al(NO 3 ) 3 ·9H 2 O, N 2 H 4 ·H 2 O and C 2 H 5 NO 2 as precursors. Characterization by TEM, SEM and BET techniques demonstrates that the Al-doping ZnO powders have a coral-like morphology with a hierarchically porous structure from macro pores, with the wall containing smaller mesopores. The gas-sensing characteristics of the ZnO samples with different Al-doping concentrations are investigated and the results indicate that the gas sensor prepared from 2.5 at% Al doping ZnO powders shows the highest gas sensitivity and selectivity towards n -butanol. A maximum gas response of 751.95 towards 100 ppm n -butanol is achieved at the operating temperature of 300 °C. The mechanisms of responses are also proposed considering the unique material structure.

A new high performance fluorescence assay for detection of Ag+ based on CdS quantum dots (QDs) using L-Aspartic acid (L-Asp) as a stabilizer was proposed in this work. The CdS quantum dots conjugation with L-Aspartic acid (L-Asp@CdS QDs) were successfully synthesized via a simple hydrothermal process. The QDs have a fluorescence emission band maximum at 595 nm with a quantum yield of 11%. The obtained CdS QDs exhibit a particle size of 1.63 &plusmn, 0.28 nm and look like quantum dot flowers. Basically, the fluorescence intensity of L-Asp@CdS QDs can be enhanced only upon addition of Ag+ and a redshift in the fluorescence spectrum was observed. Under optimum conditions, the fluorescence enhancement of L-Asp@CdS QDs appeared to exhibit a good linear relationship in between 100&ndash, 7000 nM (R2 = 0.9945) with the Ag+ concentration, with a detection limit of 39 nM. The results indicated that the L-Asp@CdS QDs were well used in detection for Ag+ as fluorescence probe in aqueous solution with high sensitivity and selectivity. Moreover, the sensing system has been applied in detection Ag+ in real water samples. The recovery test results were 98.6%~113%, and relative standard deviation (n = 5) is less than 3.6%, which was satisfactory.

A simple, rapid and high stability analytical method development for determination ascorbic acid (AA) is using the glassy carbon electrode (GCE) embellished with mesoporous CuCo2O4 rods as a nonenzymatic amperometric sensor. The mesoporous CuCo2O4 rods were prepared by hydrothermal method adopting NH4F as a surfactant to synthesize triumphantly are employed. The sample is pure spinel CuCo2O4 rods with mesoporous structure. It is excellent electrocatalytic performances for AA is measured by I-t curve. The sensitivity of the mesoporous CuCo2O4 rods sensor is up to 9.482 mA mM−1 cm−2 within linearity range of AA concentration (1–100 μM) and 2.474 mA mM−1 cm−2 within linearity range of AA concentration (100–1000 μM), respectively. The detection limit is down to 0.21 μM with correlation coefficient R2 = 0.99. The CuCo2O4/GCE AA sensor exhibits excellent electrochemical stability, repeatability and selectivity, and actual measurement for vitamin C tablets.

A series of NiO/RGO composites based on NiO nanoparticles anchored on layered RGO surfaces were proposed by the same hydrothermal method combined with different calcination temperatures (250, 300, 400 and 500 °C). The effects of calcination temperatures on the capacitive behaviors have been discussed by investigating the components, morphologies, surface conditions of the NiO/RGO composites. The specific capacitance values of NiO/RGO composites calcined at 250, 300, 400 and 500 °C are 950, 553, 375 and 205 F/g at the current density of 1 A/g and the corresponding capacitance retention are 91.3%, 83.9%, 71.9% and 67.3% after 1000 cycles at the current density of 10 A/g. The results suggest the calcination temperature plays an important role in the electrochemical performances of NiO/RGO composites and the electrochemical performances were deteriorated with the increasing calcination temperatures.

Mesoporous Co3O4 nanosheets were synthesized via a facile chemical coprecipitation method without using of surfactant or template. Mesoporous Co3O4 material was characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) to examine the morphology and microstructure to find out the cause for the highly sensitive sensing behavior. SEM and TEM analyses reveal that the Co3O4 nanosheets consisting of nanoparticles prepared here are porous in nature, with average pore sizes of approximately 50 nm or smaller. The sensor based on porous Co3O4 nanaosheets was used for detecting the volatile organic compounds (VOCs) including ethanol, methanol, acetone, isopropanol, formaldehyde and n-butanol. The results demonstrate that the sensor shows potential for detecting VOCs. The significant improvement of sensitivity is attributed to the porous structure, good contact and relatively small crystal size. The gas sensor shows high response values, fast response and recovery times towards VOCs. So, porous Co3O4 nanosheets are considered as promising sensor material for detecting VOCs.

Cd doped porous Co 3 O 4 nanosheets were successfully synthesized via a facile chemical coprecipitation method without using of surfactant or template. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), N 2 adsorption/desorption isotherms (BET) and X-ray photoelectron spectrum (XPS) for their composition, structure/morphology, and BET surface area, respectively. The nanosheets consisting of nanoparticles prepared here are porous in nature, with average pore sizes of approximately 50 nm or smaller. The electrochemical properties of the Cd doped porous Co 3 O 4 nanosheets were investigated by cyclic voltammetry (CV), galvanostatic charge–discharge, electrochemical impedance spectroscopy (EIS) in 6 mol/L KOH solution. The 5% Cd doped porous Co 3 O 4 nanosheets electrode shows the maximum specific capacitance as high as 737 F/g at the current density of 1 A/g, which is 69% higher than that of the undoped porous Co 3 O 4 nanosheets electrode. Furthermore, it has good specific capacitance retention of ca. 96% after 1000 continuous charge–discharge cycles. It shows a better cyclic stability than undoped porous Co 3 O 4 nanosheets electrode, indicating that the Cd doped porous Co 3 O 4 nanosheets is a promising electroactive material for supercapacitor.

In this work, Ag-functionalized In 2 O 3 /ZnO (IZO) nanocomposites with various contents were successfully fabricated by a nonaqueous route. The ZnO and In 2 O 3 showed separated phases and different sizes deriving from the faster growth of ZnO than In 2 O 3 using benzyl alcohol as the oxygen supplying agent. To demonstrate the usage of such Ag-functionalized IZO, the gas sensors have been fabricated and investigated for formaldehyde (HCHO) detection. The results reveal that the as-synthesized 3 wt% Ag-functionalized IZO samples exhibit high response of about 842.9 towards 2000 ppm HCHO at operating temperature of 300 °C. All sensors show rapid response and recovery. The highly sensing properties are attributed to the synergistic effects arising from the presence of these multiple functional materials, i.e. the special structure of IZO nanocomposites, the formation of the heterojunctions, the influence of Ag nanoparticles, and the mutual doping effect.

The aim of this paper is to develop easily manufactured and highly sensitive gas sensors for VOCs (volatile organic compounds) detection. Macro-/mesoporous 5 at% Al-doped ZnO (AZO) and Ag-AZO composite powders were prepared by a one-step solution combustion method and used to fabricate gas sensors. Both powders showed the same macro-/mesoporous morphology but have different grain sizes and separated silver phase due to the fast growth of ZnO and the reduction of Ag+. The capability of Ag-AZO was investigated for VOCs detection, including n-butanol, methanol, acetone, ethanol, isopropanol and formaldehyde. It is found that the added Ag greatly increases the gas response towards different VOCs. Particularly 2.5 at% Ag-AZO shows the most superior gas sensing performance to 100 ppm VOCs at an operating temperature of 240 °C, which is also the optimum operating temperature of other sensors. The results may be attributed to the synergistic effects of these doped and composite elements, the amount of adsorbed oxygen and the macro-/mesoporous morphology.

Here, pristine and WO3 decorated TiO2 nanoparticles were synthesized by a one-step hydrothermal method without the use of a surfactant or template and used to fabricate gas sensors. Various techniques were employed for the characterization of the structure and morphology of the as-prepared products. The gas-sensing characteristics of the fabricated sensors were investigated for various concentrations of xylene at different temperatures. At a low operation temperature of 160 °C, the sensors possess an excellent gas response, selectivity, linear dependence, low detection limitation, and repeatability as well as long-term stability. In particular, for the high gas response of the 10.0 mol% WO3 decorated TiO2 nanoparticles based sensor, its response reaches 92.53 for 10 ppm xylene, which is much higher than that of the pristine TiO2 based sensor. And the detection limit is 1 ppm. Those values demonstrate the potential of using WO3 decorated TiO2 nanoparticles for xylene gas detection, particularly with low concentration xylene. Apart from this, the mechanism related to the advanced properties was also investigated and presented.

Binder-free NiO@MnO 2 core-shell structure was deposited on Ni foam used as electrode of supercapacitor. The rod-like NiO core was prepared by thermal decomposition of NiC 2 O 4 synthesized by corrosion of Ni foam with oxalic acid under hydrothermal conditions. Sphere-like MnO 2 shell was deposited on the surface of NiO core through hydrothermal treatment with KMnO 4 solution. The structure, composition, morphology evolution and surface chemical state of the NiO@MnO 2 electrode were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The obtained NiO and NiO@MnO 2 electrodes were directly used as electrodes of supercapacitor, and their electrochemical performances were tested with cyclic voltammograms, galvanostatic charge/discharge and electrochemical impedance spectroscopy. The results show that the sphere-like MnO 2 shell leads to an impressive enhancement of the performance including the areal capacitance (AC) and cycling stability, particular in the AC. To be more specific, the NiO@MnO 2 exhibits a high AC of 3.584 F/cm 2 at a current density of 5 mA/cm 2 , which is much higher than that of NiO electrode whose AC is 1.483 F/cm 2 at the same current density, demonstrating the potential to use NiO@MnO 2 electrode for supercapacitor. At the same time, the related mechanisms for the superior performance were also discussed.

SnO2 microspheres in a composite with Pt nanoparticles (0, 0.5, 1.5, 2.5, 5.0 mol% Pt loading) were synthesized by a solvothermal method. The crystal structure, morphology, and specific surface area were thoroughly characterized. It is found that the Pt–SnO2 nanocomposites consist of a large amount of small spheres with average diameters up to hundreds of nanometers, and every small sphere is composed of numerous primary nanocrystallites with an average size of about 8 nm. Compared with the pristine SnO2, the presence of Pt nanoparticles has no influence on the growth behavior of the SnO2 microspheres. The gas sensors based SnO2 microspheres in a composite with Pt nanoparticles not only show a lower operating temperature and immensely enhanced responses, but also exhibit a faster response and recovery speeds and remarkable stability to methanol, especially the 5.0 mol% Pt–SnO2 nanocomposite. The gas sensor based on the 5.0 mol% Pt–SnO2 nanocomposite exhibits a response value of 190.88 to 100 ppm methanol at a low operating temperature of 80 °C, while the gas sensor based on pristine SnO2 only displayed a response value of 19.38 at an operating temperature of 200 °C. The reasonable explanation of the gas-sensing performance enhancement for the gas sensors based on Pt–SnO2 nanocomposites is attributed to the strong spillover effect of the Pt nanoparticles and the electronic interaction between Pt nanoparticles and SnO2 microspheres, both of which promoted the low temperature gas-sensing performance.

Here, CuO micro-sheets were successfully synthesized from Cu foil using the annealing procedure. Cupric oxalate (CuC2O4 xH2O) micro-sheets were firstly peeled off by immersing Cu foil in oxalic acid solution at room temperature, and then they were converted into CuO with preserved configuration after thermal treatment at 350 1C. Various techniques were employed for the characterization of the structure and morphology of as-prepared products. Results revealed that the samples were composed of a large amount of porous CuO micro-sheets, which were constructed by plenty of nano-sized primary particles. A gas sensor was fabricated using as-prepared CuO micro-sheets and was systematically investigated for its ability to detect n-butanol. Due to the porous structure of CuO micro-sheets, the sensor based on CuO micro-sheets manifests a remarkably improved sensing performance, including high response, good selectivity, excellent reproducibility and stability, and limit of detection as low as 10 ppm at 160 1C, suggesting its greatly promising applications in gas sensing.

Pd functionalized TiO2 nanoparticles were synthesized by a facile hydrothermal method. The structure, morphology, surface chemical states and surface area were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption isotherms, respectively. The as-synthesized pure and Pd functionalized TiO2 nanoparticles were used to fabricate indirect-heating gas sensor, and the gas-sensing characteristics towards butane were investigated. At the optimum temperature, the sensors possess good response, selectivity, response/recovery, repeatability as well as long-term stability. Especially for the high response, the response of 7.5 mol% Pd functionalized TiO2 nanoparticles based sensor reaches 33.93 towards 3000 ppm butane, which is about 9 times higher than that of pure TiO2 nanoparticles. The response and recovery time are 13 and 8 s, respectively. Those values demonstrate the potential of using as-synthesized Pd functionalized TiO2 nanoparticles as butane gas detection, particularly in the dynamic monitoring. Apart from these, a possible mechanism related to the enhanced sensing performance is also investigated.

Citric acid capped CdS quantum dots (CA-CdS QDs), a new assembled fluorescent probe for copper ions (Cu2+), was synthesized successfully by a simple hydrothermal method. In this work, the fluorescence sensor for the detection of heavy and transition metal (HTM) ions has been extensively studied in aqueous solution. The results of the present study indicate that the obtained CA-CdS QDs could detect Cu2+ with high sensitivity and selectivity. It found that the existence of Cu2+ has a significant fluorescence quenching with a large red shifted (from greenish-yellow to yellowish-orange), but not in the presence of 17 other HTM ions. As a result, Cu2S, the energy level below the CdS conduction band, could be formed at the surface of the CA-CdS QDs and leads to the quenching of fluorescence of CA-CdS QDs. Under optimal conditions, the copper ions detection range using the synthesized fluorescence sensor was 1.0 &times, 10‒8 M to 5.0 &times, 10‒5 M and the limit of detection (LOD) is 9.2 &times, 10‒9 M. Besides, the as-synthesized CA-CdS QDs sensor exhibited good selectivity toward Cu2+ relative to other common metal ions. Thus, the CA-CdS QDs has potential applications for detecting Cu2+ in real water samples.

An easy, inexpensive combustion route was designed to synthesize hierarchically porous WO3. The tungsten source was fresh peroxiotungstic acid by dissolving tungsten powder into hydrogen peroxide. To promote the combustion reaction, a combined fuel of both glycine and hydrazine hydrate was used. The microstructure was well-connected pores comprised of subunit nanoparticles. Upon exposing towards acetone gas, the porous WO3 based sensor exhibits high gas response, rapid response and recovery, and good selectivity in the range of 5–1000 ppm under working temperature of 300 °C. This excellent sensing performance was plausibly attributed to the porous morphology, which hence provides more active sites for the gas molecules' reaction.