Zhao Wang(王昭), Shu-Xing Fan(范樹興), and Wei Tang(唐偉)

School of Physics and Electronics,Shandong Normal University,Jinan 250358,China

SnO2/Co3O4 nanofibers (NFs) are synthesized by using a homopolar electrospinning system with double jets of positive polarity electric fields. The morphology and structure of SnO2/Co3O4 hetero-nanofibers are characterized by using field emission scanning electron microscope(FE-SEM),transmission electron microscope(TEM),x-ray diffraction(XRD),and x-ray photoelectron spectrometer(XPS).The analyses of SnO2/Co3O4 NFs by EDS and HRTEM show that the cobalt and tin exist on one nanofiber,which is related to the homopolar electrospinning and the crystallization during sintering.As a typical n-type semiconductor,SnO2 has the disadvantages of high optimal operating temperature and poor reproducibility.Comparing with SnO2,the optimal operating temperature of SnO2/Co3O4 NFs is reduced from 350 °C to 250 °C,which may be related to the catalysis of Co3O4. The response of SnO2/Co3O4 to 100-ppm ethanol at 250 °C is 50.9, 9 times higher than that of pure SnO2,which may be attributed to the p—n heterojunction between the n-type SnO2 crystalline grain and the p-type Co3O4 crystalline grain. The nanoscale p—n heterojunction promotes the electron migration and forms an interface barrier. The synergy effects between SnO2 and Co3O4,the crystalline grain p—n heterojunction,the existence of nanofibers and the large specific surface area all jointly contribute to the improved gas sensing performance.

Keywords: SnO2/Co3O4 nanofibers(NFs),homopolar double jets electrospinning,gas sensors,nanoscale p—n heterojunction

1. Introduction

Volatile organic compounds (VOCs) are important precursors for the formation of secondary pollutants such as fine particulate matter 2.5(PM2.5)and ozone(O3),which lead to atmospheric environmental problems such as haze and photochemical smog. It does great harm to the natural environment and human health. Therefore,it is necessary to develop a generalized gas sensor for VOCs, in order to carry out comprehensive prevention and control of VOCs pollution.

Chemical resistance gas sensor based on metal oxide semiconductors (MOSs)[1,2]has been widely studied in the detection of toxic and explosive gases because of its simple operation, low cost and high sensitivity. So far, a large number of MOSs have been studied,such as SnO2,[3—6]ZnO,[7—10]In2O3,[11]and CuO.[12]Tin oxide(SnO2)based on the change of resistance to detect VOCs has attracted special attention due to its high sensitivity and good stability.[13,14]However, the pure SnO2gas sensor has some disadvantages, such as poor selectivity, high operating temperature, long recovery time,etc.[15,16]Therefore, the doping and compounding of SnO2gas sensor,such as noble metal elements or other metal oxide semiconductors, organic compounds,etc., make SnO2-based gas sensor show better gas sensing characteristics than pure SnO2gas sensor.[17—21]

After mixing the n-type SnO2with other metal—oxide semiconductors, the formation of heterojunction can adjust the barrier height, expand the electron depletion layer, and produce a synergistic effect on the interface, to improve the gas sensing characteristics of SnO2-based gas sensors.[22—26]By compounding with different metal—oxide semiconductors,gas sensing materials with different gas sensing characteristics may be obtained. The Co3O4is a p-type semiconductor. Owing to the large energy gap between Co3O4band gap(1.63 eV)[27]and SnO2band gap(3.8 eV),[28,29]the p—n heterojunction is formed after SnO2and Co3O4have been combined, which leads the carriers to migrate between SnO2and Co3O4, thus forming the depletion layer and the high barrier layer.[30,31]This makes the target gas produce more hot spots when it contacts the gas sensing material. The p—n heterojunction is formed by the combination of MOSs with different conductive types. The p—n heterojunction can regulate the band gap of semiconductor,thereby forming the impurity energy levels, which can change the electrical properties of the material surface.

The formation of nanoscale p—n heterojunction promotes the additional oxygen to adsorb on the surface,and Co3O4also has a catalytic activity, which may improve the gas sensing characteristics at low temperatures. Therefore, Co3O4/SnO2composite can be used as a good gas sensing nanocomposite. It is reported that the SnO2/Co3O4nanoscale heterojunction has been formed, and the detection of acetone has been greatly improved.[32,33]Co-doped SnO2nanofibers showed the highest response toward 100-ppm ethanol.[1]The0.5SnO2/0.5Co3O4sensor has a good response to C6H6at 350°C.[30]

The one-dimensional nanofiber material prepared by the electrospinning technology can significantly improve the response of the VOCs gas sensor due to its large specific surface area and high porosity. The homopolar doublejet electrospinning technology can mix the precursor solutions of two different metal oxide semiconductors,which is different from the traditional solution mixed electrospinning[34—36]and double jets of positive and negative polarity electric fields.[37]

The homopolar doublejet electrospinning technology can effectively ensure that the nanoscale heterojunction forms between the two kinds of precursor solutions,and prevent the different precursor solutions from clustering. The SnO2is compounded with Co3O4in this work, and its gas sensing function is systematically investigated. Themorphologies, chemical compositions and element chemical states of the samples are characterized by XRD, SEM, TEM, EDS, and XPS. The SnO2/Co3O4nanofibers (NFs) have better gas sensing characteristics than the SnO2NFs, such as lower detection limit,lower operating temperature and better selectivity. The gas sensing properties and mechanism of SnO2/Co3O4NFs are also discussed.

2. Experiment

2.1. Fabrication of SnO2/Co3O4 NFs-based gas sensors

Analytical grade chemicals without further purification were used in the experiment. The SnO2/Co3O4NFs were prepared by two stages including electrospinning and annealing. The electrospinning precursors of SnO2and Co3O4were incorporated into two identical 10-ml syringes, respectively. The syringe contains 6 ml of ethanol, 7 ml of 2Ndimethylformamide(DMF),900 mg of polyvinyl pyrrolidone(PVP,Mw=1.3×106),900 mg of Co(NO3)2·6H2O or 700 mg of SnCl2·2H2O after having been stirred magnetically for 10 h.The molar ratio of Co(NO3)2·6H2O to SnCl2·2H2O was 1:1.The PVP/SnCl2·2H2O and PVP/Co(NO3)2·6H2O composite precursors were prepared by homopolar double jets electrospinning. The ratio of Sn to Co could be controlled by setting different propulsion speeds. The schematic diagram of the homopolar doublejet electrospinning device is shown in Fig. 1. In the electrospinning process, the distance between the syringe tip and the aluminum plate collector was fixed at 25 cm, and the potential between the needle tip and the aluminum plate was maintained at 20 kV. The platform moved back and forth with a speed of 20 cm/min. The calcining precursor was collected on the aluminum plate collector and calcined in the crucible. In the calcination process in the muffle furnace, it was annealed at 300°C for 2 h with a heating rate of 2°C/min to ensure the volatilization of PVP, and at 500°C for 2 h to be crystallized,and SnO2/Co3O4NFs were collected after natural cooling. Four kinds of SnO2/Co3O4NFs,i.e.,Sn:Co=1:6,Sn:Co=1:3,Sn:Co=1:1,and Sn:Co=3:1 were prepared at different propulsion speeds of syringe. The obtained samples were named SnO2/Co3O4-1,SnO2/Co3O4-2, SnO2/Co3O4-3, and SnO2/Co3O4-4, respectively. The pure SnO2and Co3O4NFs were prepared by the same process with only one syringe.

Fig.1. Schematic illustration of homopolar doublejet electrospinning setup.

2.2. Characterization

The crystalline structures of samples were recorded by Bruker D8 Advance x-ray powder diffraction(XRD),diffractometer with CuKαradiation and a wavelength of 1.54176over the 2θrange from 10°—80°in steps of 0.03°. The sample morphology was studied using a Sigma500 field emission scanning microscopy(FE-SEM)and Fei Tecnai G2 F20 field emission transmission electron microscope(FE-TEM).The elemental distribution of the sample was characterized by x-ray energy dispersive spectrometer (EDS) equipped with SEM.The x-ray photoelectron spectrometer (XPS) data were obtained by using Thermpfisher EscaLab 250Xi.

2.3. Measurement of gas sensing properties

The calcined sample was mixed with deionized water in an agate mortar and grinded it into a paste in a certain direction. The paste was evenly coated on the ceramic tube and dried overnight at room temperature to obtain gas sensors.The gas sensitivity was tested by an intelligent gas sensing analysis system(CGS-8).Owing to the existence of nanoscale p—n heterojunction between SnO2/Co3O4NFs, the resistance of the composite increased and exceeded the upper limit of CGS-8 resistance measurement (500 MΩ). Therefore, the paste was coated on the planar platinum electrode and the gas sensitivity was tested by using the comprehensive gas sensing test platform CGS-MT.The VOCs(the inference gases for gas selectivity testing include methanol, formaldehyde, acetone, ammonia, and ethanol) selectivity, the optimum operating temperature and the response(1 ppm—100 ppm concentration)ofthe prepared gas sensor were studied. All sensors were aged at 300°C for 10 h/time for 3 times prior to measurement.

3. Results and discussion

3.1. Microstructures

Figures 2(a)—2(c) show the morphology of SnO2NFs,Co3O4NFs, and SnO2/Co3O4NFs, respectively. Figure 2 shows that all NFs are relatively uniform with diameters in a range of 100 nm—150 nm. In Fig. 2(c), it is impossible to judge whether the nanofibers are SnO2/Co3O4NFs or single NFs, so the SnO2/Co3O4NFs are characterized by EDS, and the results are shown in Fig.3. Tin,cobalt and oxygen appear on one nanofiber. It can be found from Figs.3(c)—3(e)that the elements of Sn, Co and O are uniformly distributed, but the content of Co is clearly less than that of Sn.

The TEM images of SnO2/Co3O4NFs in Figs. 4(a) and 4(b) show that the sizes and morphologies are similar to the results of SEM. In order to further confirm the composites of SnO2and Co3O4on the same nanofiber, the samples are characterized by HRTEM as shown in Fig. 4(c). The lattice fringes can be observed and the lattice spacings are 0.287 nm and 0.24 nm,corresponding to the(220)plane and(311)plane of Co3O4, respectively. It can be seen from Fig. 4(c) that the lattice fringes with 0.325 nm and 0.328 nm correspond to the crystal plane (110) of SnO2. The lattice fingerprint of SnO2/Co3O4NFs is consistent with the XRD pattern(Fig.6).

Fig.2. SEM images of(a)SnO2,(b)Co3O4,(c)SnO2/Co3O4 NFs.

Fig.3. EDS of(a)SnO2/Co3O4 and(b)its mapping,EDS of(c)O element,(d)Sn element,and(e)Co element.

Fig.4. (a)and(b)TEM images of SnO2/Co3O4 NFs and(c)HRTEM of lattice fringes of SnO2/Co3O4 NFs.

The SnO2/Co3O4NFs have been compounded in the process of double jets electrospinning,which can be proved by the fact that the initially spun nanofibers collected by aluminum plate are blue. Homopolar doublejet electrospinning makes the two materials contact alternately during collection. Under the influence of positive high voltage electrode, the reaction of PVP/SnCl2·2H2O and PVP/Co(NO3)2·6H2O in doublejet electrospinning can occur as follows:

where[Co(6H2O)]2+is an octahedral complex,and(CoCl4)2?is a tetrahedral complex. The spectral absorption intensity of the tetrahedral complex is higher than that of the octahedron,so the composite fiber shows light blue.

In the calcination process, the electrospinning precursors form a hollow nanofiber structure due to the Kirkendall effect.[38—40]As shown in Fig. 5, tin and cobalt ions on the surface of the nanofibers are oxidized,while the internal ones are not oxidized due to hypoxia.The PVP on the surface of the nanofibers is calcined and volatilized,and the PVP inside is in a molten state. The PVP concentration difference between the inside and outside of the nanofiber is formed,while the diffusion rate of the molten PVP is faster, carrying tin and cobalt ions to the surface of the nanofibers to be calcined,forming a hollow nanofiber structure and further compounding.

Fig.5. Schematic diagram of calcination process model(a)prior to calcination,(b)calcining,(c)as-calcined.

As shown in Fig. 6, the crystal structures of SnO2NFs, Co3O4NFs and SnO2/Co3O4NFs are characterized by XRD. The diffraction peaks of pure SnO2are observed to be at 2θof 26.579, 33.864, 37.941, 38.965, 42.625, 51.756,54.740,57.804,61.861,64.714, and 65.943,which belong to(110), (101), (200), (111), (210), (211), (220), (002), (310),(112),and(301)crystal planes of tetragonal rutile SnO2(PDF No. 99-0024), respectively. The results show that the (111),(220), (311), (222), (400), (511), and (440) crystal faces of Co3O4correspond to the 2θvalues of 18.999,31.270,36.845,38.547, 44.808, 59.354, and 65.233, respectively, and the absorption peaks match well with those in the standard figure of Co3O4(PDF No. 74-2120). There are characteristic diffraction peaks of Co3O4in the curves of SnO2/Co3O4-1,SnO2/Co3O4-2, SnO2/Co3O4-3, and SnO2/Co3O4-4, which indicates that Co3O4has been successfully loaded on SnO2.No diffraction peaks of other phases are detected, implying high purity of the sample.

Fig. 6. XRD patterns of SnO2NFs, SnO2/Co3O4-4, SnO2/Co3O4-3,SnO2/Co3O4-2,SnO2/Co3O4-1,and Co3O4 NFs.

The chemical compositions of the SnO2NFs,Co3O4NFs,SnO2/Co3O4-4 are characterized by XPS as shown in Fig. 7.All the spectra are corrected by the standard C 1s binding energy at 284.8 eV. The survey XPS spectra in Fig. 7(a) indicate the existence of Sn, Co, O, and C in SnO2/Co3O4-4. It is found that the peak position of the binding energy for each of Sn and Co is shifted,which may be due to the formation of nanoscale p—n heterojunction, which produces new hot spots and makes more oxygen anion adsorb on the surface, resulting in the change of binding energy. The Sn 3d spectrum is shown in Fig.7(b),and the SnO2/Co3O4NFs signal located at 495.0 eV and 486.5 eV are assigned to Sn 3d3/2and Sn 3d5/2,respectively. The Co 2p and O 1s are separated into peaks as shown in Figs.7(c)and 7(d). The Co 2p spectrum in Fig.7(c)has two main peaks(796.9 eV and 780.8 eV)and two satellite peaks (802.3 eV and 786.4 eV). The two main peaks are assigned to Co 2p1/2and Co 2p3/2,respectively. Both Co 2p1/2and Co 2p3/2can be divided into two peaks corresponding to Co2+and Co3+.[41,42]The crystal phase of Co3O4is further confirmed. The O 1s spectrum in Fig. 7(d) can be divided into three peaks corresponding to three components: lattice oxygen(Olatt530.22 eV),adsorbed oxygen(Oads530.52 eV),and surface oxygen(Osurf531.72 eV).[42,43]The surface oxygen species of SnO2and SnO2/Co3O4-4 are shown in Table 1.The amount of Oadsis very important for the sensing performance of sensor material.The Oadsof SnO2/Co3O4-4(37.4%)is much higher than that of pure SnO2(21.3%),which plays a positive role in improving the gas sensing performance. This may be due to the catalytic performance of Co3O4promoting the oxygen adsorption of SnO2.In Table 1,the atomic percentages of SnO2and SnO2/Co3O4-4 are analyzed by XPS,which shows that the ratio of Sn to Co is very close to the theoretical ratio.

Fig.7.XPS about(a)survey spectrums of SnO2/Co3O4-4,SnO2,and Co3O4;(b)Sn 3d of SnO2/Co3O4-4 and SnO2;(c)Co 2p of SnO2/Co3O4-4,(d)O 1s of SnO2 and Co3O4-4.

Table 1. Relative percentages of three oxygen components,atomic percentages of SnO2 and SnO2/Co3O4-4.

3.2. Sensing properties

The response (R) of the SnO2and SnO2/Co3O4-2,SnO2/Co3O4-3,SnO2/Co3O4-4 are defined asR=Ra/Rg.The response (R) of Co3O4and SnO2/Co3O4-1 are defined asR=Rg/Ra, whereRais the stable resistance in air, andRgis the stable resistance in probe gas.

Fig.8. Curves of response of SnO2, Co3O4, and SnO2/Co3O4 NFs sensors to 100-ppm ethanol versus operating temperature.

Fig.9. Transient ethanolsensing properties of gas sensors based on Co3O4,SnO2/Co3O4-1, SnO2/Co3O4-2, SnO2/Co3O4-3, SnO2/Co3O4-4, and SnO2 at operating temperature 250 °C.

The operating temperature strongly influences the response property of a semiconductor gas sensor.Figure 8 shows the gas sensing response of SnO2, Co3O4, and SnO2/Co3O4NFs to 100-ppm ethanol versus operating temperature. Optimum operating temperature of SnO2sensor to 100-ppm ethanol is 350°C, which is a typical disadvantage of SnO2sensor. Compared with the optimum operating temperature of the SnO2sensor, Co3O4sensor’s is lower, only 175°C. The SnO2/Co3O4NFs shows the maximum response at 250°C,and the SnO2/Co3O4-4 has the highest response at 250°C.The SnO2/Co3O4-4 reduces the optimum operating temperature based on SnO2gas sensor.

Figure 9 shows the dynamic curves of response and recovery transient properties of Co3O4, SnO2/Co3O4-1,SnO2/Co3O4-2, SnO2/Co3O4-3, SnO2/Co3O4-4, and SnO2NFs gas sensors, respectively. The gas sensing properties are tested with ethanol at 250°C. In Fig. 9 from the left to the right are the responses at 1 ppm, 2 ppm, 5 ppm, 10 ppm,20 ppm, 30 ppm, 40 ppm, 50 ppm, 100 ppm, respectively.The conduction types of Co3O4and SnO2/Co3O4-1 are p-type,and the conduction types of SnO2/Co3O4-2, SnO2/Co3O4-3,SnO2/Co3O4-4, and SnO2are n-type. The type of conduction depends on the content of n-SnO2and p-Co3O4in the sample. When the ratio of SnO2to Co3O4is 1:1 (the ratio of Sn to Co is 1:3 for SnO2/Co3O4-2), the sensing behavior of the composite is of n-type, it is due to the contribution of electrons being greater than that of holes.[32]The SnO2/Co3O4NFs sensor can return to the initial resistance when exposed to air after different concentrations of ethanol have been injected,which indicates that the Co3O4/SnO2NFs sensor has good reproducibility. The resistance of SnO2NFs changes differently after the response has been recovered. Serving as a kind of gas sensor, the SnO2NFs have the disadvantage of poor reproducibility. When the ethanol concentration is 100 ppm,the response of SnO2/Co3O4-4, SnO2, and Co3O4are of 50.60,5.79, and 1.36, respectively. The synergistic effect of SnO2and Co3O4makes SnO2/Co3O4-4 have higher response and better reproducibility. It can be seen from the Fig. 9 that the resistance of composite material is much greater than that of single material,which may be due to the narrowing of electron transmission channel caused by the formation of nanoscale p—n heterojunction.

Figure 10 shows the selectivity of Co3O4, SnO2/Co3O4-1, SnO2/Co3O4-2, SnO2/Co3O4-3, SnO2/Co3O4-4, and SnO2to 40-ppm inference gases at 250°C; the inference gases for gas selectivity testing include ethanol, methanol, formaldehyde, acetone, and ammonia. The sensor of SnO2/Co3O4-4 shows the highest response to ethanol. The ethanol response of SnO2/Co3O4-4 is 10.80,which is higher than that of SnO2(3.23),and Co3O4(1.14),respectively. The sensing characteristics are summarized in Table 2.

Fig. 10. Response of gas sensors based on the Co3O4, SnO2/Co3O4-1, SnO2/Co3O4-2, SnO2/Co3O4-3, SnO2/Co3O4-4, and SnO2 to 40-ppm formaldehyde, methanol, ethanol, ammonia, and acetone at operating temperature 250 °C.

Table 2. Summary of sensing characteristics.

3.3. Gas sensing mechanism

The gas sensing characteristics of the sensor are related to the initial resistance of semiconductor metal oxides.The influence of temperature and nanoscale heterojunction on oxygen adsorption will change the initial resistance of semiconductor metal oxides. The possible sensing mechanism model for SnO2/Co3O4-based sensors is shown in Fig.11.

When the p-Co3O4is exposed to air, oxygen molecules adsorb on the surface of the Co3O4and capture electrons from the electron hole pair of Co3O4,forming negative oxygen ion(,O?,O2?);the overall reaction can be represented as

where ads. is the abbreviation for adsorbed. The hole concentration increases,and the hole accumulation layer(Fig.11(a))is formed on the surface of Co3O4.After contacting the reducing gas, the electron released from the desorption of negative oxygen ion forms a holeelectron pair. The electron released can be explained as[44]

The hole concentration on the surface of the semiconductor decreases. That is why the resistance of Co3O4increases when Co3O4contacts the reducing gas. When n-SnO2is exposed to air,oxygen molecules adsorb on the surface of the SnO2,oxygen molecules capture electrons from the surface of SnO2to form negative oxygen ion (O2?, O?, O?2), so electron depletion layer(Fig.11(a))is formed on the surface of SnO2. After contacting the reducing gas,the negative oxygen ion desorbs,and the electron concentration on the surface of the semiconductor increases.That is why the resistance of SnO2decreases when SnO2contacting the reducing gas.

Fig.11. (a)Sensing mechanism of SnO2 and Co3O4 in air and in VOCs,(b)schematic diagram of nanoscale p-Co3O4/n-SnO2 heterojunction,n-SnO2/n-SnO2 homojunction and p-Co3O4/p-Co3O4 homojunction in air and in VOCs,and(c)energy band diagram of SnO2-Co3O4 system.

When the temperature is raised, the concentration of semiconductor carriers will increase, and more negative oxygen ions will be adsorbed on the sensor surface, so the response will be enhanced after having contacted the reducing gas. However, when the temperature is too high, the affinity energy of gas molecules is less than the work function of the metal—oxide—semiconductor surface. Then the electrons will transfer from the adsorbed negative ions to the semiconductor surface,and the adsorbed negative ions will reduce the hot spots on the semiconductor surface due to the loss of electrons,and thus the response is low after having contacted the reducing gas. Therefore,different semiconductor—metal—oxide sensors have different optimal operating temperatures.[45—47]

According to the HRTEM, nanoscale p-Co3O4/n-SnO2heterojunctions, n-SnO2/n-SnO2homojunctions, and p-Co3O4/p-Co3O4homojunctions exist in SnO2/Co3O4NFs.The schematic diagram is shown in Fig.11(b). The nanoscale p—n heterojunction plays an important role in improving the gas sensing properties of composite materials.[26,48,49]The Fermi energy levels of n-SnO2and p-Co3O4are different.[30,32,33,50]When the two kinds of NFs are combined,the electrons move from n-SnO2(work function 4.9 eV)to p-Co3O4(work function 6.1 eV) due to the Fermi energy level equilibrium effect.[51]The electron depletion layer is formed on the SnO2side, and the hole is transferred from p-Co3O4to n-SnO2. The hole depletion layer is formed on the Co3O4side. The space charge layer of n-SnO2/p-Co3O4is formed,and the energy bands on both sides are bent(Fig.11(c)),resulting in the potential barrier,which greatly narrows the electron transport channel and reduces the conductivity,[52]as shown in Fig.11(c). The existence of space charge layer can enhance the adsorption capacity of oxygen and make nanoscale p—n heterojunction become a hot spot after having contacted the reducing probe gas. When the SnO2/Co3O4sensors contact the reducing gas, the electrons released from the reactions of reducing gas and the desorption of negative oxygen ion will result in great resistance change,thereby reducing the potential barrier height.

4. Conclusions

The SnO2/Co3O4NFs with the hollow structure are prepared by homopolar doublejet electrospinning. The SnO2/Co3O4NFs gas sensor has good ethanolsensing properties at 250°C. The formation mechanism of SnO2/Co3O4NFs’ hollow structure is analyzed. The results show that the SnO2/Co3O4NFs have better reproducibility than the SnO2.The optimum operating temperature of SnO2/Co3O4NFs is 250°C, which is 100°C lowers than that of SnO2NFs. The mechanism of gas sensing is analyzed. The SnO2/Co3O4NFs form nanoscale p—n heterojunction, which produces new hot spots to adsorb oxygen and improves its initial resistance. In addition, the method of homopolar doublejet electrospinning can be used for preparing other composite materials by changing the content of doping in the future work,which will be conducive to the quantitative research of the enhanced gas sensing performance of electrochemical gas sensors.