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Applications of Polymer, Composite, and Coating Materials

Novel Construction of Morphology-tunable C-N/SnO2/ZnO/ Au Microspheres with Ultra-sensitivity and High-selectivity for Triethylamine under Various Temperature Detections Hui Li, Shushu Chu, Qian Ma, Yuan Fang, Junpeng Wang, Quande Che, Gang Wang, and Ping Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22357 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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ACS Applied Materials & Interfaces

Novel Construction of Morphology-tunable C-N/SnO2/ZnO/Au Microspheres with Ultra-sensitivity and High-selectivity for Triethylamine under Various Temperature Detections Hui Li, Shushu Chu, Qian Ma*, Yuan Fang, Junpeng Wang, Quande Che, Gang Wang, Ping Yang* School of Material Science and Engineering, University of Jinan, 250022, Jinan, P. R. China *To whom correspondence should be addressed. KEYWORDS: C-N/SnO2/ZnO/Au composites, microspheres, electrospinning, gas sensor, triethylamine ABSTRACT: Morphology-tunable C-N/SnO2-based hierarchical microspheres with good gas sensitivity for triethylamine (TEA) have been fabricated via a facile electrospinning and subsequent calcination process. The reaction temperature and modifying calcining technology played the dominant role for the morphological evolution from precursor fibers to microspherical shapes and the formation of C-N decorated SnO2 phase composition. C-N/SnO2/ZnO composites with tunable crystallinity, microstructure, and gas sensing performance were strictly dependent on the adding amount of Zn element. Fascinatingly, the constructed CN/SnO2/ZnO/Au composites can not only precisely regulate the crystal size, dispersion status, loading position and content of Au nanoparticles, but also display excellent gas sensing properties with ultra-sensitivity and high-selectivity under various temperature detections. The response of C-N/SnO2/ZnO/Au composites can reach up to approximately 1970 calculated to be 121.6 and 23.6 times for 50 ppm TEA molecules at the optimal conditions compared with C-N/SnO2 and C-N/SnO2/ZnO microspheres, respectively, actually representing the highest response value at high temperature reported to date. The superior long-aging stability of sensing behaviors and phase structures can be also observed after one month. More importantly, novel C-N/SnO2/ZnO/Au sensors were employed for availably detecting low concentration volatiles released from the storage procedure of fishes at 80oC, indicating the practical application in chemical detectors and biosensors at low temperature. The novel gas sensing mechanisms derived primarily from the combination of phase compositions, morphologies, and unique surface/interface transfer processes of CN/SnO2/ZnO/Au composites are presented and investigated in details, which will contribute to the design and development of other semiconductor-based composite sensors.

INTRODUCTION Triethylamine (TEA) is widely used as preservatives, catalysts, synthetic dyes, and energy fuels, besides acting as an important raw material for industrial organic synthesis. TEA can be given off from microbial degradation of many marine organisms whose concentration is markedly dependent on freshness, and its volatile gases will bring serious environmental pollution and a great damage on respiratory systems, causing pulmonary edema and even death.1-3 Because of its potential hazards, Occupational Safety and Health Administration (OSHA) stipulate that the threshold limit of TEA concentration is 10 ppm in air.4-6 Thus it is urgent to develop novel TEA gas sensors with low workingtemperature, high sensitivity, and low limit of detection for accomplishing practical application in industrial processes, fish processing industries, and complex other surrounding conditions. Currently, oxide semiconductor sensors with superior gas sensing performances referring to high sensitivity and selectivity have been considered as the promising materials for the practical application of TEA detecting. Notwithstanding certain contributions have been made by SnO2-based semiconductor materials for gas sensors due to their low-cost, easy fabrication, and good chemical and thermal stability, the normally displayed high working temperature and low gas

response significantly limits further development in the practical application.7-9 Adjusting the morphological characteristics and incorporating various additive materials are considered as valid approach to further enhance the sensing performances. It has been paid more and more attentions to the incorporation of C or N components into SnO2-based semiconductors with different micro/nanostructures in recent years. For examples, Hu et al.10 presented that 2D C3N4-tin oxide gas sensors fabricated by one-step synthesis exhibited 22 times enhancement of sensing sensitivity compared with pure SnO2 sensors. R. Leghrib et al.11 revealed that NCNTs/SnO2 sensors had much higher gas sensing response for NO2 detection at room temperature than the blank SnO2 and N-substituted CNT sensors. Nevertheless, there are rare researches referring to both carbon and nitrogen components distributed evenly within the matrix material for improving the gas sensing behaviors. Actually, the introducing C and N into suitable microstructures of oxide compounds could be beneficial for significantly modifying the surface status, the electron transport modulation process, the magnitude of adsorption sites, and the distribution of defect structures.12,13 The synthesis of C-N/SnO2 microspheres with tunable sizes holds great practical and theoretical significance because of the great potential for use in many crucial areas, such as gas sensing, catalysis, water treatment, and energy storage. However, it is well-known that no investigation has been

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reported concerning to C-N/SnO2 microspheres prepared by facile electrospinning method, even though the formation process of beads sometimes can be observed due to the avoidable fiber imperfection. In addition, the introduction of ZnO and Au into SnO2-based materials can be considered to play an important role on the enhanced gas sensing properties by adjusting the micro-morphology, crystallinity, crystal size, composition, crystal growth habits, and interfacial mass transfer.14-16 In the present work, for the ultra-sensitive and high-selective gas sensing to TEA, morphology-tunable and uniform C-N/SnO2/ZnO/Au microspheres have been originally designed and fabricated by a facile procedure combined of electrospinning, calcination process, and in situ reducing process, contributing to the suitable and effective actual detection of volatiles during storage of fishes.

EXPERIMENTAL SECTION Materials Zinc nitrate hexahydrate (Zn(NO3)2.6H2O), chloroauric acid (HAuCl4.4H2O), L-Lysine (C6H14N2O2) and Sodium borohydride (NaBH4) were taken from Sinopharm Chemical Reagent Company. Polyvinylpyrrolidone (MW 1,300,000) was obtained from Aladdin Industrial Corporation (China). Tin(IV) chloride (SnCl4.5H2O) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Ethanol (anhydrous, AR) and dimethylformamide (DMF, anhydrous, 99.8 %) were purchased from Tianjin Fuyu Fine chemical Co., Ltd. Materials were used as received without any further purification. Preparation of C-N/SnO2 and C-N/SnO2/ZnO microspheres: In a typical procedure, different amounts of SnCl4.5H2O and Zn(NO3)2.6H2O were dissolved in the mixed solvent containing 1 mL of ethanol and 4 mL of DMF by vigorously stirring for 30 min. Then, 0.7 g of PVP was added to the above solution stirring for 4 h. The obtained solution was then transformed into a plastic 5 mL syringe with a spinneret of 0.8 mm. The distance between needle tip and collector was 18 cm and a voltage of 18 kV was applied during electrospinning. The obtained precursor fibers were placed into a 25 mL crucible until taking up 80 % of the volume. Subsequently, the crucible was sealed and heated to different annealing temperatures for 2 h at a heating rate of 5 oC/min, and then cooled to room temperature. As the comparative materials, pure SnO2 porous fibers were synthesized by replacing the precursor fibers into a 25 mL crucible without sealing and proceeding the same heat treatment process. C-N/SnO2 microspheres were named as Sample 1, Sample 2, Sample 3, and Sample 4, according to different calcination temperatures. C-N/SnO2/ZnO microspheres were named as Sample 5, Sample 6, Sample 7, and Sample 8, based on different Zn components. The detailed preparation conditions were listed in Table S1. Preparation of C-N/SnO2/ZnO/Au microspheres: C-N/SnO2/ZnO/Au microspheres were prepared by a simple in situ reducing process. First, 50 mg C-N/SnO2/ZnO, 1 mL of 0.01 M L-lysine and different amounts of 0.05 M chloroauric acid (HAuCl4.4H2O) were dispersed into 25 mL deionized water by vigorously stirring for 15 min. Then 5 mL of 0.01 M NaBH4 solution was

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slowly added in the above solution, stirring for 30 min. The obtained samples were washed several times with deionized water and ethanol, and dried at 65 oC for 6 h. In this process, L-lysine is an adhesive and NaBH4 is a reducing agent. CN/SnO2/ZnO/Au microspheres were named as Sample 9, Sample 10, and Sample 11, due to different Au-loaded amounts. The detailed preparation conditions were listed in Table S2. Materials preparation, characterization, and sensing measurement Electrostatic spinning machine (FM-1206, Beijing Future Material Sci-tech Co., Ltd) was applied to prepare different samples. A field-emission scanning electron microscope (QUANTA 250 FEG, FEI, USA) was used to investigate the morphology and Energy Dispersive Spectrometer (EDS) analysis of samples. The samples were observed by transmission electron microscopy (TEM/HRTEM, Tecnai F20, FEI). X-ray diffraction (XRD, D8-ADVANCE of Bruker Corporation) was used to examine crystal structures and phase composition of samples. Raman spectra, UV-Vis diffuse reflectance spectra (DRS), and XPS spectra were operated by UV-Vis spectrometer (Hitachi U-4100), high resolution Raman spectrometer (LabRAM HR Evolution, HORIBA JOBIN YVON SAS), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250), respectively. The specific surface area and pore size distribution of samples were observed by a multifunction adsorption instrument (MFA-140, Builder Company, Beijing). Gas sensitivity test was measured on the equipment of CGS-4TPs (Beijing Elite Tech Co., Ltd).

RESULTS AND DISCUSSION

Figure 1. SEM images of C-N/SnO2 samples annealed at different temperatures: (a) 400oC, (b) 500oC, (c) 600oC, and (d) 700oC. Figure 1 presents SEM images of C-N/SnO2 samples annealed at different temperatures. The dominant morphology of Sample 1 is clearly crimped fiber, without the formation of typical microsphere structures. However, there are small blocks appearing in several locations of fibrous adhesion or fiber intersection, providing the basis for forming process of microspheres. As seen in Figure 1 (b), as increasing the annealing temperature to 500oC, the microstructure tends to take on the dispersion of plentiful microsphere shapes, with the presence of slight crimped fibers. Figure 1(c) indicates that Sample 3 has the characteristics of uniform microsphere


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structures, mainly composed of tunable crystalline nanosheets with the size distribution of 4.5-6.0 μm. It is obvious that the microsphere structure can be destroyed and changed to granular aggregate as the annealing temperature arising to 700°C because of the overheating phenomenon in Figure 1 (d). Thus, the morphological evolution from precursor fibers to microspheres can be primarily dependent on the annealing temperature based on the modified precursor system and calcination behavior. For C-N/SnO2 microsphere, precursor fibers are placed into a 25 mL crucible until taking up 80% of the volume. Subsequently, the crucible is sealed and heated. The anoxic atmosphere makes the organic matter of the precursor fiber carbonize rather than immediately decompose. The precursor fibers crossly curled each other with many nodes formed, as shown in Figure 1 (a) in the manuscript. These nodes continue to grow and break the fibers, and eventually the original fibers transforming to microsphere structure. As seen in Figure 1 (a) and (b) in the manuscript, it is difficult to observe particles in one-dimensional fibers. Therefore, the presence of C-N components can effectively prevent growing and aggregation of SnO2 naonoparticles, and induce the self-assembly and growth process of microspheres during the high-temperature treatment. As shown in Figure S1, all diffraction peaks of tetragonal rutile SnO2 in XRD patterns are gradually strengthened with the increase of annealing temperature, indicating that SnO2 phase composition with tunable crystallinity can be successfully obtained. With annealing temperature reaching up to 700oC, the peaks of SnO impurity phase can be also observed (marked as red dots). Gas sensing performance tests of different C-N/SnO2 microspheres are shown in Figure S2. Compared with other samples, Sample 3 exhibits good gas selectivity for TEA with the highest response of 16.2 at the optimal operating temperature of 360oC, along with the response/recovery times of 30/28 s, respectively.

Figure 2. (a)-(e) Low magnification SEM images of Sample 3, Sample 5, Sample 6, Sample 7, and Sample 8; (f)-(j) High magnification SEM images of Sample 3, Sample 5, Sample 6, Sample 7, and Sample 8; (k) the scheme for the formation mechanism of microspheres. Considering the similar crystallization and matching degree of some special crystal faces of SnO2 and ZnO, Zn component is introduced as regulating agent to adjust the phase composition and morphology, for further improving the physicochemical properties of SnO2-based gas sensors. Figure 2 (a) depicts the low-magnification SEM images of C-N/SnO2

sample obtained at 600oC, from which a number of microspheres with good dispersity and uniform size with average diameter of 5.5 μm can be clearly observed. It is also verified that the microsphere are composed of many nanosheets as shown from the high magnification SEM image in Figure 2 (f). After introducing a small amount of Zn component into C-N/SnO2, the as-synthesized Zn-doped CN/SnO2 samples are still maintained the microsphere morphologies as displayed in Figure 2 (b), (c), (g), and (h). The particle surface become smoother with a reduction in size as increasing the adding content of Zn source, along with the structural unit of nanosheet transforming to thicker and smaller nanorods. When the Sn:Zn molar ratio is 93:7 (Figure 2 (d) and (i) ), a mass of nanorods can be organized to form hollow microstructures, manifesting that large doping amount of Zn component change the migration process of original ions. Upon further increasing the doping concentration of Zn component, nanorods tend to insignificantly form small blocks during the annealing process as shown in Figure 2 (e) and (j). In general, the low concentration of Zn component is not enough to destroy the crystalline nature of the matrix material so that the microsphere morphology of Zn-doped C-N/SnO2 composite is maintained and optimized. If the amount of Zn component exceeds a certain limit, the original structures will be changed from microspheres to coarse blocks. Figure 2 (k) briefly describes the attractive formation process of Zn-doped C-N/SnO2 microspheres under the proper conditions. For CN/SnO2/ZnO microspheres, the addition of the Zn component can optimize C-N/SnO2 microspheres, and the processes such as forming nodes and breaking fibers are similar with CN/SnO2 composites. However, due to the ion migration and ionic interaction of various components and the formation of multi-phase composition, the addition of Zn element can obviously affect the evolution of structural units of microspheres. It is also demonstrated that the excess Zn components can result in the destruction of microsphere structure. The morphological evolution induced by Zn component can contribute to the modulation of electron transport and surface absorption of composites. As shown in Figure S3 (a)-(c), pure SnO2 fibers without C and N components have been fabricated in opening air condition for comparing with C-N/SnO2. It is clear from Figure S3 (a) and (b) that pure SnO2 fibers have characteristic morphology of porous and uniform nanofibers, of which the phase structure belongs to the tetragonal rutile SnO2 (JCPDS 41-1445). Raman patterns reveal no information traces of D and G band in Figure S3 (c), also representing the formation of pure SnO2 sample. Compared with Sample 3, pure SnO2 nanofibers display lower response of 8.3, and longer response/recovery times of 42/33 s, respectively, which indicates the existence of C and N decorated SnO2 particles has significant effect on the enhanced gas sensing properties (Figure S3 (d)). Figure S4 (a) shows that the radius of the semicircle of C-N/SnO2 is much smaller than SnO2 in impedance plots, indicating the improvement of charge transport nature of C-N/SnO2 products. It is clear that the introducing of C and N makes charge transfer resistance of the materials become smaller. Resistance curve of SnO2 and CN/SnO2 (Sample 3) to 50 ppm TEA gas in and off at 360oC are shown in Figure S4 (b). The higher Ra value and lower Rg value of C-N/SnO2 can be observed. The introducing of C and N can efficiently conduct electrons and trap or return more



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electrons, signally leading to the relative lager Ra or rapid reduction of Rg values for the enhanced gas sensing performance. In order to demonstrate that the doping of Zn component into C-N/SnO2 microspheres is an effective way to enhance the gas sensing performances of SnO2-based sensors, the corresponding gas sensing performances of Sample 5-8 are investigated in Figure S5. The result reveals that Zn-doped CN/SnO2 microspheres possess superior gas sensing properties than C-N/SnO2 sensors. For example, the response value of Sample 6 is 83.5 as well as the response/recovery times are 21/16 s at 360oC for 50 ppm TEA, respectively (Figure S6). Typically, the enhanced gas sensing performance of Sample 6 can be ascribed to the combination of phase composition, unique morphology, surface state, and interface effect.

Figure 3. (a) Energy dispersive X-ray spectroscopic (EDS) images of Sample 10 and the inset of (a) corresponding to SEM image of Sample 10; (b)-(g) EDS mapping images of Sample 10; (h) and (i-j) TEM and HRTEM images of Sample 10. To realize effective detection of TEA gases, Au loaded CN/SnO2/ZnO microspheres are deemed as an effective approach. EDS spectrum in Figure 3 (a) reveals that CN/SnO2/ZnO/Au microspheres consist of Sn, Zn, O, Au, C, and N elements. EDS mapping images corresponding to the inset of single C-N/SnO2/ZnO/Au microsphere in Figure 3 (a) further demonstrate that Sn, Zn, O, Au, C, and N elements can evenly disperse over the entire particle area (Figure 3 (b)-(g)). TEM and HRTEM are employed to further investigate the structural features of C-N/SnO2/ZnO/Au microspheres. As shown in Figure 3 (h), Au nanoparticles referring to the average size of 8-15 nm are uniformly deposited on the surface of C-N/SnO2/ZnO composites. The HRTEM images (Figure 3 (i-j) ) reveal that three lattice planes with the interplanar distances of 3.34, 2.47, and 2.36 Ao correspond well to (110) planes of SnO2, (101) planes of the ZnO, and (111) planes of Au, respectively, proving that C-N/SnO2/ZnO microspheres can act as good substrate for decorating Au nanoparticles.

Figure 4. (a) XRD patterns of Sample 3, Sample 6, and Sample 10; (b) Raman patterns of Sample 3, Sample 6, and Sample 10; (c) UV-vis diffuse absorption spectra of Sample 3, Sample 6, and Sample 10; (d) N2 adsorption-desorption isotherm curves of Sample 3 and Sample 6. XRD patterns of Sample 3, Sample 6, and Sample 10 are shown in Figure 4 (a). Two obvious phase compositions of ZnO (JCPDS 89-1397) and tetragonal rutile SnO2 (JCPDS 411445) can be observed, without other impurity diffraction peaks. There are no diffraction peaks of Au phase in XRD patterns mainly due to the small loading amount of Au component. In Figure 4 (b), typical vibrational peaks of SnO2 can be observed without ZnO vibrational peaks resulted from weak vibrational peaks of ZnO or less ZnO components. Two typical carbon peaks at 1350 and 1600 cm-1 indicates that the resulting samples contain C component, and another two peaks at 1460 and 1526 cm-1 are assigned to N-C vibrational peaks, demonstrating that C and N components are successfully introduced.17 Figure 4 (c) shows that Sample 10 has an intense absorption in 500-600 nm ascribed to the surface plasmon absorption band of Au, confirming the successful fabrication of C-N/SnO2/ZnO microspheres decorated with tunable Au nanoparticles.18 N2 adsorptiondesorption isotherm curves and pore size distribution curves are described in Figure 4 (d). The specific surface values of Sample 3, Sample 6, and Sample 10 are 14.6, 36.5, and 40.7 m2/g, and the pore diameters are 2.8, 2.2, and 2.0 nm, respectively, suggesting the optimal absorption ability of Sample 10 for gas sensing application.


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Figure 5. Survey (a), Sn 3d (b), Zn 2p (c), O 1s (d) and Au 4f (e) XPS spectra of Sample 10. To analyze the chemical composition and states, XPS spectra of Sample 10 is studied in Figure 5. The representative peaks of Sn, Zn, O, C, and N can be clearly observed in survey spectrum in Figure 5 (a). Sn 3d and Zn 2p XPS spectrum displays the valence of Sn and Zn are +4 and +2, respectively. Moreover, the energy gap of 8.3 and 22.9 eV between two predominant peaks are exactly closed to the values reported in the standard spectrum of Sn 3d and Zn 2p, respectively (Figure 5 (b, c)).19,20 O 1s spectrum (Figure 5 (d)) consists of three peaks centered at 530.2, 531.7, and 533.0 eV, representing lattice oxygen species (OL), the oxygen vacancy species (OV), and the chemisorbed and dissociated oxygen species (OC), respectively. OC can be originated from chemisorbing on the surface of synthesized materials and facilitate the reaction with test gas molecules, conducing to the enhancement of gas sensing behavior.21 In Figure 5 (e), Au 4f spectrum has two separate peaks located at 82.8 and 86.6 eV corresponding to Au 4f7/2 and Au 4f5/2, respectively. In comparison with bulk Au, a visible shift between two peaks is primarily attributed to the strong electronic interaction between Au nanoparticles and C-N/SnO2/ZnO microspheres.22 Figure S7 (a) reveals four peaks of C 1s spetrum at 283.6, 284.3, 284.8, and 288.1 eV, which can be attributed to the C=C, C-C, C-N, and C=O bonds, respectively. The presence of C-N clearly indicates the existence of C and N ingredients. The weak N signals are also detected in the survey spectrum, mainly derived from PVP raw material.23 As seen in Figure S7 (b), N 1s spectrum displays the pyridinic N, pyrrolic N, and graphitic N located at 398.4, 399.5, and 400.5 eV, respectively.24 The results of XPS measurement of Sample 10 are in good accordance with EDS and EDS mapping results in Figure 3, confirming the presence of Sn, Zn, O, C, N, and Au for composite.

Figure 6. Gas sensing performance of different CN/SnO2/ZnO/Au samples: (a) The selectivity comparison for 50 ppm different gases at 280oC. (b) Response curves at different temperature for 50 ppm TEA. (c) Response curves for different concentrations TEA at 280oC. (d) Response and recovery time curves for 50 ppm TEA at 280oC. A series of sensors based on C-N/SnO2/ZnO/Au microspheres have been prepared by choosing Sample 6 as effective substrate for loading Au nanoparticles in our case. As shown in Figure 6 (a) and (b), the gas selectivity is investigated in terms of reducing gases including ethanol, methanol, acetone, TEA, benzene, toluene, and ammonia. The highest selective gas can be focused on TEA at the optimal temperature of 280oC. In addition, the responses of different composites signally increase with the temperature increasing in the range of 80-280oC. For example, the response values of Sample 9, Sample 10, and Sample 11 can reach up to 720, 1970, and 1260 for 50 ppm TEA at 280oC, respectively. Whereas the responses of these sensors severely decrease as the operating temperature is higher than 280oC. High response and excellent selectivity of sensors for TEA mainly attribute to attractive force between N atom in TEA and Sn4+ (or Zn2+) on surfaces of materials and active C-N bond in TEA molecule.25,26 Figure 6 (c) reveals that the responses of all samples increase with the concentration of TEA increasing from 10 to 100 ppm. It is found that Sample 10 always exhibits higher response values than other samples under the same measurement concentration, which indicates the appropriate Au loading on the microsphere can easily lead to a significant increase in the gas sensitivity and selectivity of materials. Figure 6 (d) shows response/recovery times of Sample 9, Sample 10, and Sample 11 are 30/7, 18/6, and 47/7 s, respectively, demonstrating the important reference values for the practical application prospect.



Figure 7. Response curves for different concentrations TEA at 80°C. Until now, there have been rare investigations on the exceptionally high sensitivity to TEA at low temperature for SnO2-based composites. It is attractive that the designed CN/SnO2/ZnO/Au composite sensors have a valid response of approximate 55 for 50 ppm TEA at 80oC, as the striking breakthrough with respect to response value of TEA detection at low temperature below 100oC. Figure S8 (a) and (b) reveal that Sample 10 exhibits the superior selective and the good stability after 6 cycles of gas to TEA for 50 ppm TEA at 80oC. Figure 7 presents that response of materials increase with the increasing concentrations upon exposure to 0.5-100 ppm of TEA, and Sample 10 displays much higher response than other samples at every concentration. In addition, Figure S9 indicates that the response/recovery times of Sample 10 are 95/20 s for 50 ppm TEA at 80oC. The inset of Figure 7 gives a result of the responses of Sample 9-11 under low TEA concentration of 0.5-5 ppm. It is clear that the response of Sample 10 can be up to 3.4 for 0.5 ppm TEA, suggesting the promise of sensors employed for detecting low concentration of the target gas in practice. Figure S10 (a) and (b) reveal the resistance curve and response curve of Sample 10 to 50 ppm TEA after 6 cycles of gas in and off at 280oC, indicating good concentration stability. No obvious fluctuations of gas sensing responses of Sample 10 in the large range of relative humidity (RH %) from 10 to 90 % and no significant degradation of responses in the testing period of 30 days can be observed at 80 and 280oC conditions in Figure S10 (c) and (d), reflecting the excellent humidity applicability and long term stability of C-N/SnO2/ZnO/Au sensors under series of extreme environmental applications. Meanwhile, SEM image of the inset of Figure S10 (d) shows no morphological difference between the as-prepared C-N/SnO2/ZnO/Au microspheres and the ones after 30 days testing, accounting for the good physicochemical stability of the sensors.

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Figure 8. The response (a), and sensing transients’ resistance (b) of Sample 10 to 5 mL volatiles from 200 g Carassius auratus after storage for different times (0.25, 1, 3, 5, and 7 day) at the working temperature of 80oC; (c) The linear relationship of log (S-1)-log (c) plot to 0.5-5 ppm TEA based on Sample 10 sensors at 80oC. C-N/SnO2/ZnO/Au microspheres based on Sample 10 are employed to detect targeted odors from a Carassius auratus (about 200 g) at different storage times which is stored in a closed chamber at room temperature.27,28 The response, and sensing transients’ resistance of Sample 10 to 5 mL volatiles from 200 g Carassius auratus after storage for different times (0.25, 1, 3, 5, and 7 day) are shown in Figure 8 (a) and (b). An accelerating decrease of the resistance and the opposite continuous increase of the response can be regularly monitored with the storage time prolonging to 7 day, corresponding to the deterioration process of the dead fish. The response of the gas obtained after storing period of 0.25, 1, 3, 5, and 7 day are approximate 2.0, 4.3, 6.4, 8.9, and 11.0, respectively, as shown in Figure 8 (a). Figure 8 (b) reveals sensing transients’ resistance of Sample 10 to 5 mL volatiles derived from 200 g Carassius auratus after storage for different times (0.25, 1, 3, 5 and 7 day), corresponding to the resistance in Figure S11 and response in Figure 8 (a). Significantly, C-N/SnO2/ZnO/Au microspheres actually display fast transient response characteristics and fine reversibility to the volatiles of fish. Based on the response curves for different concentrations in Figure 7, Figure 8 (c) describes the linear relationship of log (S-1)-log (c) plot to 0.5-5 ppm TEA at 80oC, where c represents TEA concentration, and S represents the response for corresponding TEA concentration, indicating that Sample 10 sensors exhibits excellent linear relationship with the TEA concentration. The linear equation is described as y = 0.68x + 0.59. The responses of detected fish at different times are substituted into the above equation, and then the amount of released TEA gas can be evaluated to be 0.13, 0.8, 1.6, 2.8, and 4.0 ppm at 0.25, 1, 3, 5, and 7 day, respectively. This result indicates that CN/SnO2/ZnO/Au composite sensors will provide a particular promising approach for the instantaneous detection of fish freshness. In general, gas sensing mechanism of SnO2-based materials for TEA can be explained by the theory of electron depletion layer.29-31 When the sensor is exposed to air, oxygen molecules are adsorbed to the surface of sensor, and then trap electrons to


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form Oδ- (O2−, O−, and O2−), thereby forming a thicker electron depletion layer on the surface of materials which results in increased resistance of materials. The detail process is shown as following: O2(ads) (1)

+

e−

→

Oδ-(ads)

Figure 9. The schemes of the gas sensing mechanism of CN/SnO2/ZnO/Au microspheres: (a) electron depletion layer of sensors in O2 and TEA; (b) Resistance changes based on electrons leaving materials or returning to materials; (c) electron transfer paths of materials; (d) Electronic transfer in details. When the sensor is exposed to the reducing TEA gas, the interaction of TEA molecules with Oδ- releases electrons back to materials, leading to the thickness of electron depletion layer decrease, and hence the resistance of materials decrease (Figure 9 (a) and (b)). The description is shown as following: TEA + Oδ- → CO2 + H2O + NO2 + e−

(2)

C-N/SnO2/ZnO/Au microspheres with superior gas sensing performance may be primarily attributed to the combination of phase compositions, morphologies, and unique surface/interface transfer processes. The construction of CN/SnO2/ZnO/Au compositions has significant influence on the morphological evolution, adsorption capacity, and electron transfer process of composites. The uniform dispersity and high stability of the typical microsphere structure with large specific surface area can provide a number of reactive sites on the whole surface for absorbing gas molecules.32 The introduction of C and N components into composite could not only offer high electrical conductivity for accelerating electron transfer process, but also change the electron transfer paths and electronic interaction to promote the preferable interconnection between gases and sensors, which contributes to improve response and response/recovery times and increase the response value of sensors (Figure 9 (c)). When the sensor based on C-N/SnO2/ZnO/Au microspheres is exposed to air, oxygen molecules are adsorbed to the surface of sensor. Since C-N components are easy to conduct electrons, excited electrons inside materials can rapidly transfer the surface and effectively participate in the surface reaction so that more electrons are trapped, resulting in higher Ra. Because more electrons participate in the surface reaction, more oxygen species are formed, so that more TEA molecules are oxidized and more electrons are released. The rapid flow of released electrons through the C and N components causes the resistance of the materials to significantly drop, thereby obtaining lower Rg value and higher response value. Furthermore, it is confirmed that C-N/SnO2/ZnO composites could be the promising and efficient sensor substrates for

enhancing the gas sensing properties by taking advantage of the sensitization effect of Au nanoparticles. Due to their different work functions (Ws(ZnO) = 5.2 eV, Ws(SnO2) = 4.55 eV), electron affinities (χ(ZnO) = 4.5 eV, χ(SnO2) = 4.32 eV), and band gaps (Eg (ZnO) = 3.37 eV, Eg (SnO2) = 3.6 eV), the electrons will transfer from SnO2 to ZnO until their Fermi levels equalize.33 This process creates an electron depletion layer on the surface of SnO2 and further bends the energy band, then leads to a higher resistance state and hence the formation of heterojunctions can improve gas responses. Sensitization of Au nanoparticles, such as chemical sensitization and electron sensitization greatly promotes the gas sensitivity of C-N/SnO2/ZnO/Au microspheres. Compared with C-N/SnO2/ZnO microspheres, C-N/SnO2/ZnO/Au composites can catalyze and accelerate the dissociation of oxygen molecules on the surface due to the chemical sensitization effects of Au, thereby generating excess oxygen species and greatly improving the reaction of targeted gases and oxygen species for promoting the gas sensitivity. In addition, since the Fermi level of Au is relatively lower, electrons on the conduction bands of SnO2 and ZnO are easily transferred to Au. As C-N/SnO2/ZnO/Au microspheres emerge in contact with oxygen in air, it will create a thicker electron depletion layer because of electron sensitization of Au, resulting in higher resistance. Since Equation (S = Ra/Rg) is used to calculate the gas sensitivity, the Ra value is higher, the response is higher.16 Furthermore, Au nanoparticles deposited uniformly on the surface of composite materials can be connected to C-N components which can change the electronic transmission paths and accelerate electron transport behaviors, leading to a substantial enhanced gas sensing performance (Figure 9 (c) and (d)). Meanwhile, C-N/ZnO samples can be prepared by using a similar method to C-N/SnO2 (Sample 3), and C-N/ZnO and Sample 3 are subjected to be decorated with Au nanoparticles through the similar method to that of Sample 10. As shown in Figure S12 (a), the morphology of C-N/ZnO samples has dramatically been changed with many inhomogenous large and small pieces stack as bulk structures. The corresponding EDS mapping images in Figure S12 (b-f) reveal that CN/ZnO/Au samples consist of Zn, O, Au, C, and N elements with uniform distribution in the entire area. XRD and Raman patterns in Figure S12 (g) and (h) further indicate the successful formation of C-N/ZnO/Au composites. Figure S13 shows gas-sensing responses of C-N/SnO2/Au and CN/ZnO/Au for 50 ppm TEA at 280°C. The response of CN/SnO2/Au (345) is 2.8 times higher than C-N/ZnO/Au (120) for 50 ppm TEA at 280°C, illustrating that the construction of C-N/SnO2 microspheres is beneficial for the enhanced sensing properties. However, the response of Sample 10 (1970) is much higher than C-N/SnO2/Au and C-N/ZnO/Au, further confirming the importance of the accurate construction of phase compositions on the intrinsic improvement of gassensing performances of materials. Compared to other typical sensors used for detecting TEA (Table S3), the promising sensors based on C-N/SnO2/ZnO/Au composites can exhibit the highest response value, relatively faster response/recovery time, and relatively low working temperature.

CONCLUSIONS In summary, we successfully synthesized C-N/SnO2 and CN/SnO2/ZnO microspheres with well-dispersed



microstructures and uniform sizes via a single-spinneret electrospinning and subsequent calcination process. The phase structure and morphological evolution can be mainly adjusted by changing the introducing of Zn component and heat treatment behavior. C-N/SnO2/ZnO composites could be the promising and efficient sensor substrates for loading Au nanoparticles. In view of gas sensing performance, the responses of C-N/SnO2, C-N/SnO2/ZnO, and CN/SnO2/ZnO/Au microspheres are 16.2, 83.5, and 1970, with the response/recovery times of 30/28, 21/16, and 18/6 s for 50 ppm TEA at the operating conditions, respectively. It is intriguing that the response of C-N/SnO2/ZnO/Au microspheres with the optimal gas sensing performance can reach up to 55 for 50 ppm TEA at 80oC. The superior sensing performances are mainly attributed to the combination of phase compositions, morphologies, and unique surface/interface transfer processes of C-N/SnO2/ZnO/Au composites, effectively leading to more surface active sites and special electronic effect. Therefore, these sensors based on C-N/SnO2/ZnO/Au microspheres with superior response can be employed as the promising composite substitutes for monitoring and detecting odors from rotten fishes in practices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. XRD patterns of different C-N/SnO2 samples; Sensing performance of different C-N/SnO2 samples; Characterization and gas-sensing performance of SnO2 without C and N; Impedance plots of SnO2 and C-N/SnO2 and resistance curve of SnO2 and CN/SnO2 (Sample 3) to 50 ppm TEA gas in and off at 360oC; Sensing performance of different C-N/SnO2/ZnO samples; The response and recovery time curves for 50 ppm TEA at 360oC of Sample 6; C 1s (a) and N 1s (b) XPS spectra of Sample 10; Gas sensing performance of different C-N/SnO2/ZnO/Au samples at 80oC; The response and recovery time curves for 50 ppm TEA at 80oC of Sample 10; The gas-sensing performances of Sample 10; The resistance of Sample 10 to 5 mL volatiles from 200 g Carassius auratus after storage for different times (0.25, 1, 3, 5, and 7 day) at the working temperature of 80oC; SEM image, the corresponding to EDS mapping images, XRD pattern and Raman pattern of C-N/ZnO/Au; Gas-sensing responses of C-N/SnO2/Au and C-N/ZnO/Au for 50 ppm TEA at 280oC; The detailed preparation conditions of C-N/SnO2 and C-N/SnO2/ZnO; The detailed preparation conditions of C-N/SnO2/ZnO/Au; Comparison of some recently developed highly sensitive and selective sensors for TEA (PDF).

AUTHOR INFORMATION Corresponding Author

E-mail: [email protected] [email protected] Fax: +86-531-87974453, Tel: +86-531-89736225 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the projects from the National Natural Science Foundation of China (51402123), and the

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project from Shenzhen Gangchuang Building Material Co.,Ltd., and the National Training Program of Innovation and Entrepreneurship for Undergraduates (201610427017).

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