Directly electrospinning submillimeter continuous fibers on tubes to fabricate H2S detectors with fast and high response

Xuto Ning,Dou Tng,Ming Zhng

a Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials,School of Materials and Environmental Engineering,Hunan University of Humanities,Science and Technology,Loudi,China

b College of Resources & Environment,Hunan Agricultural University,Changsha,410128,China

c School of Physics and Electronics,Hunan University,Changsha,410082,China

Keywords:Submillimeter continuous fibers H2S detectors In-situ fabrication

ABSTRACT The fast and high response detection of neurotoxic H2S is of great importance for the environment.In this paper,directly electrospinning technology on the ceramic tube is developed to improve the response of H2S detector based on superlong SnO2 fibers.The submillimeter continuous fibers are deposited directly on ceramic tubes by in-situ electrospinning method and can keep morphology of fibers during calcination.By employing this technology,CuO-doped SnO2 fiber H2S detectors are fabricated,and 10%atom CuO-doped SnO2 H2S detector shows the highest response of 40 toward 1 ppm H2S at 150 °C while the response is only 3.6 for the H2S detector prepared in traditional route.In addition,the in-situ electrospinning H2S detectors show faster response and recovery compared to the H2S detectors fabricated by the conventional way.The high and fast response of H2S detectors based on in-situ electrospinning can be ascribed to the continuous fiber structure and CuO modification.The present in-situ electrospinning technology may provide a new strategy for the development of other gasdetectors and bio-detectors with fast and high response.

1.Introduction

Hydrogen sulfide(H2S)is not only a common raw material and byproduct in industry,but it is also an useful gas for laboratories[1,2].Although H2S is important in chemical industry and experimental study,its toxicity should not be ignored.It is virulent and irritant to human respiratory tract,eyes and central nervous system even at low concentration.According to the U.S.Occupational Safety and Health Administration,the permissible exposure concentration of H2S for human is about 10 ppm[3].In addition,the lung will suffer from serious injury if it was exposed to 190 ppm H2S for about 6 min.Therefore,the H2S detection has drawn a lot of attention and the rapid,reliable,low detection limited H2S gas detector is in urgent demand.

Metal oxide semiconductors(SnO2[4,5],TiO2[6],ZnO[7],NiO[8],CuO[9],WO3[10]et al.)and their composites(CuO–SnO2[11],SnO2–Co3O4[12],In2O3–SnO2[13],Co3O4–TiO2[14],Cr2O3–ZnO[15],CuO–ZnO[16],In2O3–ZnO[17]et al.)which appear low-cost,high sensitive and reliable superiority,have been widely used in H2S detection.Among those studies,CuO doped SnO2has been found to be a typical material for H2S sensing since 1991[18].Many other researchers also have fabricated CuO modified H2S detectors based on traditional electrospinning-coated method[19–21].During the traditional procedure to fabricate H2S detectors,grinding to obtain the paste is necessary process,and the microstructure of most of sensitive materials would be broken,resulting in the decrease of nano-effect.Unlike the coating technology,in-situ fabrication technology with various advantages has drawn much attention in last few years.For example,Yudong Zhu et al.fabricated H2S detectors by in-situ growth ZnO nanosheets on the surface of the ceramic tube[22].Chemical bath deposition also was employed to prepare sensitive materials and fabricate detectors[23].An electrostatic spray deposition method was developed to assemble H2S detector,which has a high response at low temperature[24].Recently,our group fabricated a rapid H2S detector by an in-situ electrospinning method[25].According to the work mechanism of metal oxide semiconductor detectors,the sensitive layers with high surface area and large porosity is conducive to the high and fast response of gas detectors[26].Therefore,it is of great significant to develop new strategy to in-situ synthesize sensitive layers for high performance detectors.

Electrospinning is a simple method to fabricate one-dimensional materials,which has been widely used due to their unique aspect ratioand surface structure[27–29].For example,Li and co-authors synthesized SnO2–CuO nanofibers using electrospinning and fabricated the sensors for CO,which showed high sensitivity and selectivity at a relative low temperature[20].Kim et al.reported the H2S sensors based on electrospun nanofibers of CuO–SnO2on Si wafers[21].However,the fast and high response of H2S detectors should be improved further,especially developing the new strategy to fabricate gas detector combing in-situ technology and electrospinning because they can avoid the pulverization of nanofiber sensitive materials and make sure the controllable temperature.Hence,to promote the development of H2S detector,we develop an in-situ electrospinning strategy to prepare the detector.Comparing with traditional electrospinning-coating way,the device made in this way had shown a superior performance in low concentrations at a low temperature.

Fig.1.The fabricate procedures of detectors via in-situ electrospinning method.

2.Experimental section

All the chemicals used in this study were supplied by Sinopharm Chemical Reagent Co.Ltd.The deionized water with a resistivity larger than 18 MΩ/cm was prepared by the Milli-Q Water System.

2.1.Electrospinning

The precursor solutions for electrospinning were got as follows[30].Firstly,2 ml N,N-dimethylformamide(DMF)solutions of 1 mmol of metal salt and 4 ml DMF solutions of 0.43 g polyacrylonitrile(PAN)were prepared with mechanical stirring and ultrasound to obtain homogeneous and transparent dispersion,respectively.Then metal salt solution was added dropwise into PAN solution,followed by vigorously stirring for at 2 h.Series of precursor for electrospinning were obtained by such steps with solute what were comprised of SnCl2?2H2O and Cu(CH3COO)2in the mole ratio of 9.5:0.5,9:1,8.5:1.5,respectively.The precursor solution was transferred into a 5 mL plastic syringe with a stainless-steel needle.The flow rate was fixed at approximate 0.4 mL/h.A ceramic tube was employed as the collector,which was placed about 12 cm between beneath the needle.The needle was connected to a direct current(DC)power with high-voltage of approximate 8 kV.The matched group materials were obtained by direct electrospun on the surface of aluminum foil followed by annealing at 500°C for 0.5 h.

2.2.Characterization of sample

The surface morphologies were observed by a scanning electron microscopy(SEM,Hitachi S-4800).A powder diffractometer(XRD,Rigaku D/MAX-2500)with Cu Kαradiation was used to analyse the crystalline phases of samples.The microstructures of the samples were also characterized using transmission electron microscopy(TEM,JEOL JEM-2100F microscope).

Fig.2.(a–h)SEM images of 10% CuO–SnO2.(a),(b),(e)and(g)show the morphology of fibers on detectors fabricated via in-situ electrospinning route.(c),(d),(f)and(h)display the morphology of fibers on detectors fabricated by traditional coated.

2.3.Fabrication and measurements of gas detector

The fabrication and testing principle of the gas detectors in traditional way was similar to our previous reports with following procedures[24].Firstly,the materials were mixed with terpineol to form the pastes.Secondly,alloy coil tubes of Ni–Cr were fixed in the center of tubes and served as heaters to control the operating temperature.Thirdly,the devices were age for at least for two days before testing.Nevertheless,the new method can be such simple for manufacture that can be made into devices directly.Tied up Pt wire on both ends of the ceramic tube obtained by in-situ electrospinning way and equip it with alloy coil tubes of Ni–Cr as heaters,as illustrated in Fig.1.All devices had age for at least for two days before testing.For convenience,the detectors were marked as Ex and Cx in which x was the mole ratio of CuO,and Detector-Ex and Detector-Cx represented the detectors fabricated by in-situ electrospinning and the conventional process,respectively.For example,Detector-E10 represent the detector prepared via an in-situ electrospinning of precursor solution contained 10%CuO and 90%SnO2.

The sensing performances were evaluated by a gas sensing measurement system(NS-4003,made by Zhong-Ke Micro-nano Co Ltd).The gas response behavior of detector was investigated under laboratory conditions(50 RH%,25°C).The detector response was defined as S––Ra/Rg,where Raand Rgwere the resistance in air and detected gas,severally.The response and recovery times(Tresand Trec)of a gas detector aredefined as the time it takes for the resistance to reach 90%of its steadystate value after introduction or removal of the target gas,respectively.

Fig.3.XRD patterns of 10% CuO–SnO2.

3.Results and discussions

3.1.Sample characterization

CuO-doped SnO2detectors were fabricated using an in-situ electrospinning strategy(Fig.1).The metal salts and PAN were dissolved in DMF to form the solution what were comprised of SnCl2?and Cu(CH3-COO)2(CuAc2)in the mole ratio of 9.5:0.5,9:1,and 8.5:1.5.The ceramic(alumina)tube linked with a motor took the place of the aluminum foil used as collectors.Then CuAc2-SnCl2-PAN fibers were deposited on ceramic tubes and oxidized in the air at 500°C with a heating rate of 2°C/min in a muffle furnace.Finally,the detector was obtained by tying platinum wires and adding heating wires.For convenience,the detectors with Cu mole rate of 5%,10%,15% were marked as Detector-E5,Detector-E10,Detector-E15,respectively.For comparison,the matched group obtained from the same condition expect Al foil as collector were marked as Detector-C5,Detector-C10,Detector-C15,respectively.The similar procedures can be found in previous reports[31,32].

The morphologies and microstructures of sample have been investigated based on scanning electron microscopy(SEM)and transmission electron microscope(TEM).Fig.2b and d shows the typical appearance of detectors prepared by in-situ electrospinning and traditional coating at very low magnification.No significant difference is found between the two detectors.When observing at a larger resolution,the distinguishing feature of detector fabricated by in-situ electrospinning is successive and long fibers(more than 100 μm,Fig.2a)while the fibers are fractured on the detectors prepared using traditional coating way(Fig.2c).The comparison between Fig.2e and f further confirms that the in-situ electrospinning can ensure the continuity of fibers even at a high resolution,and the fibers on traditional detectors are broken into fibers shorter than 2 μm.Fig.2g and h displays the enlarged SEM images.

of fibers on detectors fabricated by two methods.Both of them show the fibers with similar diameters.The difference among them is that the fibers structures of detectors prepared by in-situ electrospinning are remained while the traditional-coating wreck the architecture.Although the reduction of the size to the dimension comparable to the thickness of charge depletion layer can lead to a dramatic improvement in sensitivity and speed of response[33],the detectors based on fibers with similar diameters were employed to investigate the effect of the fabrication method on their performance.

Fig.4.TEM images of 10%CuO–SnO2.(a)and(c)show the low-magnification images;(b)and(d)display the high-magnification images of a particular area.

XRD was implemented to verify the crystal structure of sample prepared via electrospinning named 10%CuO–SnO2.As shown in Fig.3,the obvious peaks at 26.6,33.9,37.9,and 51.8°can be indexed to SnO2(JCPDS 41–1445)with cassiterite phase.In addition,the relative strong of XRD patterns also match well with the standard peaks of cassiterite SnO2,confirming that the main phase of 10%CuO–SnO2is SnO2.Some slightly peaks at 35.5 and 38.7°are observed.Although those peaks are not obvious because of the small account of Cu,they can be attributed to the(002)and(111)planes of CuO(JCPDS 45–937,tenorite).In addition,none of the characteristic diffraction peaks of SnO,CuSnO3and CuO can be observed from the XRD patterns,indicating the exist form of cupper and tin is only CuO and SnO2,respectively.Therefore,SnO2fibers modified by CuO are synthesized via an electrospinning route and subsequent annealing treatment.

TEM and high-resolution TEM(HRTEM)provide further insight into the microstructure and morphology of sample.The porous structure can be easily seen from the low magnification TEM image in Fig.4a and c.Fig.4a confirms that the diameter of 10% CuO–SnO2fibers is about 320 nm,and there are some tiny particles(e.g.white frame in Fig.4a and c)on the fiber with vertical stripe.It can be observed from HRTEM that the nanofiber is comprised of subunits of particles and the size of CuO particle is about 10 nm.The high-resolution image in Fig.4b shows clear lattice fringes of nanoparticle outside the fiber,which can be indexed to the(110)plane of SnO2.And the lattice fringes in the fibers arise from the(002)plane of CuO,suggesting the presence of both SnO2and CuO in fibers.Fig.4c and d displays another nanoparticle on the porous fiber.Two types of lattice fringes are observed,which can be ascribed to the(002)plane of CuO and(110)plane of SnO2.Different from those in Fig.4b,both SnO2and CuO in Fig.4d are outside the fiber.Those phenomena indicate that the distribution both SnO2and CuO are homogenous.In addition,the lattice resolved HRTEM depicts the interplanar distance of 0.25 nm for(002)plane of CuO and 0.33 nm for(110)plane of SnO2which matched the XRD result.Furthermore,the coexistence of SnO2and CuO that possibly facilitating the p-n junction formation was also identified by the high magnification TEM photograph Fig.4b.

3.2.Gas sensing properties

Interest rose in our mind that which gas detector based on CuO–SnO2performs better,the in-situ electrospinning detectors(Detector-Ex)or theconventional electrospinning-coating group(Detector-Cx)?H2S was chosen as target gas because CuO–SnO2not only is a typical material for H2S detecting,but also attribute to its importance in our life while flammable and poisonous in its nature.Fig.5a shows the sensitivities of the two groups at different temperatures to 1 ppm H2S.Obviously,the optimum operation temperature is 150°C for all detectors,and 10%CuO–SnO2show high response than CuO–SnO2with Cu content of 5%and 15%for both Detector-Ex and Detector-Cx.This is meaning that 10%CuO–SnO2have suitable grain size for gas sensing,which is similar with published article[34].As the increasement of doping content of CuO,carrier concentration increase and grain size enlarge(Fig.S1).Both of about two converse effects would result in the decrease of the detectors’response.Thus 10% CuO doping is the most suitable choice for H2S detecting.The response of Detector-E10 to 1 ppm H2S at room temperature also were collected,as shown in Fig.S2.The response was low and slow,showing the inferior activity to H2S at room temperature.

Fig.5.Gas sensing performance of CuO–SnO2 detectors via in-situ electrospinning and conventional method:(a)Response of Detector-C5,Detector-C10,Detector-C15,Detector-E5,Detector-E10 and Detector-E15 to 1 ppm H2S with different temperature;(b)Response of Detector-C10 and Detector-E10 to various concentrations of H2S at 150 °C;(c)Typical instant performance of Detector-E10 and Detector-C10 to 1 ppm H2S;(d)Response and(e)recovery time of Detector-E10 and Detector-C10 to 10 ppm H2S at different temperature;(f)Selectivity test of Detector-E10 and Detector-C10 at 150 °C towards 10 ppm H2S and other gases.

As shown in Fig.5c,Detector-E10 represents the highest response(as high as 30),while the Detector-C10 has a response about 4.The similar phenomena are also found in the other comparison groups.As temperature rises,the sensitivities descend and the gaps between the groups decrease.The detection limit of detectors were evaluated and results is shown in Fig.S3.The response of Detector-E10 and Detector-C10 are 4.4 and 2.0 for 0.5 ppm H2S.The response of Detector-E10 decrease to 1.5 for in 0.2 ppm H2S while there are few responses for Detector-C10,suggesting a detection limit of 0.2 ppm.Those phenomena suggest that the in-situ electrospinning method does have great effect on the increase of response at lower temperature which could be attributed to the continuity of nanofibers for about 1 mm.

It’s easy to understand that the undestroyed structure of Detector-Ex is convenient for gas diffusion,which signified that more gas could react with CuO doped SnO2nanofibers.Furthermore,it could be confirmed indirectly that the result form current-voltage test(I–V test)as shown in Fig.6a and b that Detector-Ex always show the higher resistance than Detector-Cx.I–V test was done under the air condition where O2absorbed in the surface of P-type nanofiber and combined free electrons to form O2-Or O-which resulting the increase of resistance[35].Therefore,the Detector-Ex with complete and structure shows higher resistance than Detector-Cx because there more O2absorbed in the nanofiber of Detector-Ex.After long-time test,the Detector-E10 remain the well structure of long-nanofiber network(Fig.S4b)and Detector-C10 remain still keep the morphologies of wrecked fiber(Fig.S4a),revealing the great stability of CuO–SnO2detectors.

The gas detector performance to various H2S concentrations at 150°C is as also researched.As shown in Fig.5b,the response of the detectors improved with increasing concentration from 500 ppb to 10 ppm.The responses are approximate 4.5,30,76,185,323 to H2S at concentrations of 0.5,1,2,5 and 10 ppm,respectively.According to the previous report[36],the relationship between the H2S concentration(C)and the response(S)can be described as follow:

Fig.6.(a)I–V test and(b)Resistance variation with temperature of Detector-C10 and Detector-E10;(c)Mechanism model of a single nanofiber and energy band diagram.

where a is a coefficient.The corresponding picture is shown in Fig.S5.The special mathematical relation of between the response and the concentration is of benefit for the practical application.

Fig.5d and e reveal the response time and recovery time of Detector-C10 and Detector-E10 toward 10 ppm H2S at different temperature respectively.It is obviously that the Detector-E10 has a shorter response and recovery time than Detector-C10 at 150°C,which is benefit from the undestroyed architectural structure for gas diffusion.In warmer atmosphere,the gas molecules are more energetic,the structure effect decrease and temperature plays a more important role which result the gap between Detector-E10 and Detector-C10 is decreases.As the description Fig.2e and f depicts,the devices fabricated by in-situ electrospinning method have more electron transport paths and the paths are much longer.For the detectors with fractured fibers,there are countless joints existing and poor contact may appear.Some fibers cannot connect with the others to form an efficacious route for electron transformation,causing that the active sites on those fibers can be invalid.Therefore,the whole efficiency of teleportation is inferior.While for Detector-Ex,the nanofibers are long enough to contact with the two electrodes and connected with each other well.In this way,all fibers can be accounted to the electrical transmission route.Therefore,when exposing to equal gas molecules in a certain extent,the response and recovery of the Detector-Ex is much faster than the Detector-Cx.In addition,the recovery time between Detector-E10 and Detector-C10 don’t reduce as remarkable as the response time at 150°C.This should account of the sluggish process that CuS converts back to CuO at low temperature.

The Fig.5f shows the response of Detector-E10 to NO,CO,CH4,C2H5OH,SO2,and H2S at the same concentration(10 ppm).The response to H2S is 323,which is much higher than others,implying the high selectivity to H2S.The response of both Detector-E10 and Detector-C10to 1 ppm H2S at different time were evaluated,as shown in Fig.S6.Both of them show the decrease in terms of the response to 0.5 ppm H2S.The response of Detector-E10 after 90 days can retain 91%.

To further evaluate the performance of CuO doped SnO2gas detector via in-situ electrospining,we compare it with existing reference about H2S detector based on Cu/SnO2and listed on Table 1.In briefly,this work shows a balanced performance to H2S(quite high response,quick response and fairly fast recovery)which provide another well option for mass production.Certainly,the selectivity and the speed of response of detectors can be improved further.Modifying the sensitive materials with the catalyst for H2S may be a possible strategy to achieve those aim.

Table 1Sensing parameter comparison between H2S detectors based on Cu/SnO2.

3.3.Gas sensing mechanism

The sensing mechanism of CuO–SnO2materials has been researched for many years[43,46,47].Seen from Fig.5c,the devices performance in line with earlier studies shows an n-type performance.The decrease of resistance when exposed to H2S can be explained as follows.SnO2is a typical n-type material and O2gas adsorbed on its surface combined with free electrons converted into O2-,O-,O2-which forming a depletion layer that play crucial role in gas sensing[48],while CuO is a typical P-type material which majority carrier is holes[49].As the schematic shown in Fig.6c,CuO is randomly dispersed in the nanofibers composed of a mixture of SnO2and CuO.A thick depletion layer as a p-n structure is formed between CuO and SnO2,and an energy band bending is formed,which lead to the increased initial resistance.When the CuO-doped SnO2fibers contact with H2S gas,the CuO is converted into CuS as shown in equation(1).In addition,CuS is reported as a zero-band gap material,which means the PN junction between CuO and SnO2convert into Schottky contact between CuS and SnO2as shown in Fig.6c.

Therefore,the energy band barrier transforms from PN junction barrier into Schottky barrier shown as resistance decrease sharply in macro scale.The resistance increases back to the original value once this H2S atmosphere is removed.This is because CuS is oxidized and converted back to CuO in the presence of oxygen by the following reaction(2)

In addition,I–V test was chosen to verify our suggestion about the different performance between detectors fabricated via different methods.I–V test was done in air where O2was absorbed and combined with free electrons in the existing form of O2-,O-or O2-at the surface of nanofibers.The larger resistance of Detector-E10 could be attributed to the larger and deeper depletion layer formed by more absorbed O2,because the undestroyed structure in Detector-E10 is more beneficial for gas diffusion.

4.Conclusions

In summary,CuO doped SnO2nanofibers with continuous structure can be fabricated via an in-situ electrospinning method,which is realized by adding rotating ceramic tube on the collection board.The implement of such a method can get rid of the manual grind and coat procedure.Among the three comparing groups distinguished by mole ratio of CuO and SnO2,the detector based on the fibers with mole ratio of 10% CuO shows the best performance.And compared to detectors prepared in tradition,the devices produced in new means show preferable sensing performance in low H2S concentration at 150°C.For 1 ppm H2S detecting,the response is elevated to about octuplet,the response time is shortened about 70%and the recovery time is lessened to be half.These excellent results may attribute to the complete architectural structure for facilitating gas diffusion and absorbing.Therefore,the in-situ process used in this paper may be available for other analogous materials obtained by electrospinning.

Notes

The authors declare no competing financial interest.

Acknowledgements

The work was supported by National Natural Science Foundation of China(51772082,51804106,51572078,51772086 and 51872087).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.nanoms.2021.07.005.