Achieving highly-efficient H2S gas sensor by flower-like SnO2-SnO/porous GaN heterojunction

Zeng Liu(刘增) Ling Du(都灵) Shao-Hui Zhang(张少辉) Ang Bian(边昂)Jun-Peng Fang(方君鹏) Chen-Yang Xing(邢晨阳) Shan Li(李山)Jin-Cheng Tang(汤谨诚) Yu-Feng Guo(郭宇锋) and Wei-Hua Tang(唐为华)

1College of Integrated Circuit Science and Engineering,Nanjing University of Posts and Telecommunications,Nanjing 210023,China

2National and Local Joint Engineering Laboratory for RF Integration and Micro-Packing Technologies,Nanjing University of Posts and Telecommunications,Nanjing 210023,China

3School of Electronic and Information Engineering,Jinling Institute of Technology,Nanjing 211169,China

4Institute of Microscale Optoelectronics,Shenzhen University,Shenzhen 518060,China

5Key Laboratory of Luminescence and Optical Information of Ministry of Education,Institute of Optoelectronic Technology,Beijing Jiaotong University,Beijing 100044,China

6School of Integrated Circuits,Tsinghua University,Beijing 100084,China

7Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education,College of Physics and Optoelectronic Engineering,Shenzhen University,Shenzhen 518060,China

8State Key Laboratory of Information Photonics and Optical Communications,School of Science,Beijing University of Posts and Telecommunications,Beijing 100876,China

Keywords: gas sensor,SnO2-SnO,porous GaN,heterojunction

1.Introduction

H2S usually refers to extremely harmful manure and sewer gas in human daily life.[1]At low concentrations close to 5 ppm,it smells like rotten eggs.At a high concentration of more than 50 ppm,it will irritate the eyes and the respiratory system.The sense of odors disappears as the olfactory nerves have been paralyzed when the gas concentration reaches above 100 ppm.Therefore,efficient gas sensors with a fast response with regard to the high-sensitivity detection of H2S in the surrounding environment are urgently required.

Nowadays,gas sensors play a vital role in most domains of intelligent sensing technology.Among all kinds of gas sensors,semiconductor metal oxides(MOs)[2-6]sensors(such as SnO2,ZnO,and WO3)have always been the spotlight due to their low cost, out-bound sensitivity, low power consumption and easiness of fabrication.By and large,the MOs sensors are divided into n-type MOs[7,8]and p-type MOs.[9,10]Generally,n-type MOs are more advantageous for use as gas sensors than p-type MOs such as higher carrier mobilities.As a typical ntype semiconductor,the application of SnO2(Eg=3.6 eV)gas sensor has been investigated for a long time.Despite the advantages of SnO2over other materials,such as higher sensitivity and faster response time,practical applications of SnO2are still subject to the limitations of temperature and water vapor poisoning,[11,12]even poor selectivity.[13]These constraints have severely affected the further promotion of SnO2-based gas sensors.Thereafter,it is found that SnO2/p-type metal oxides hybrid sensing materials can effectively overcome some shortcomings.[14-17]

Traditional MOs sensors are composed of the sensitive layer depositing on substrate Al2O3or Si,[18,19]and a miniature heater is usually installed on the back of substrate.Compared with Al2O3just only as support substrate, Si has been confirmed to be responsive to several gases.[20-22]With changes in the working environment of gas sensors (including high pressure, strong corrosion, etc.) and the future development trend of multi-sensor integration,[23-25]GaN shows higher stability and superiority.GaN,known as a wide direct bandgap (3.39 eV) material, has a superior thermal conductivity, a large saturation electron drift velocity and excellent thermostability apart from good acid and alkali resistance.Accordingly, GaN based MO gas sensors has been noticed.Especially, since 2016, Pan’s team[26-28]proposed a variety of GaN nanostructures with different shapes, sizes, and depths through electrochemistry, photoelectrochemistry, direct photochemistry,and metal-assisted photochemistry etching methods,which had been successfully applied to various biochemical sensors, constructed surface-enhanced raman scattering(SERS) substrates, and photoluminescence, and porous GaN(PGaN) was especially noticeable among them.On the one hand, the PGaN is not only more stable than porous Si, but also has response to certain gases.[29,30]On the other hand,the porous structure itself is more propitious for the gas flow and offers larger specific surface(beneficial to the sensitive material attached).[31]In previous work,[32]we reported a heterojunction of ZnO/PGaN and directly applied to a diode-type humidity sensor,except for the high response value,the response and release time within a wide range of relative humidity(RH)levels have also been enhanced.

In order to develop practically useful H2S sensors,herein,SnO2-SnO/PGaN heterojunction sensor was fabricated by a facile spraying method, and involving hydrothermal preparation of flower-like SnO2-SnO and electrochemical wet etching of GaN throughout the assembly process.The prepared SnO2-SnO composites with PGaN substrate could effectively enhance the gas response to the H2S and reduce the working temperature.A highly plausible charge transport mechanism based on physical and chemical adsorbed H2S layers was raised to elucidate the tested gas sensing response at lowering temperature.

2.Experimental details

2.1.Chemicals

Anhydrous ethanol (C2H5OH), acetone (C3H6O), sulfuric acid (H2SO4, 98%), urea (H2NCONH2) and SnCl4·2H2O were supplied by Sinopharm Chemical Reagents Co., Ltd,GaN pitaxial wafers were purchased from Suzhou Nanowin Science and Technology Co., Ltd, and ionic liquid (ILs)[EMIM][OTF]was purchased from Shanghai Chengjie Chemical Co.,Ltd.

2.2.Materials preparation

2.2.1.PGaN preparation

Single-crystal n-type GaN (0001) films were grown on sapphires(0001)by hydride vapor phase epitaxy(HVPE).The size of GaN slice is 1.0 cm×0.3 cm and the Si-doped GaN layer is 5 µm thick.The PGaN substrate was prepared by a photo-assisted electrochemical etching (Fig.1(a)).Before etching, the GaN sample was sequentially ultrasonic cleaned by acetone,absolute ethanol and deionized(DI)water,then the surface was dried with high-purity N2,also performed UV/O3vacuum plasma treatment to enhance the hydrophilicity and improve the binding capacity between the etching solution and GaN through surface modification,as shown in Fig.1(b).The silver lead and GaN were connected by a conductive silver paste to form an ohmic contact.Meanwhile, the GaN sample and Pt plate were put into[EMIM][OTF]as the anode and cathode in the illumination with a 300 W xenon lamp.The etching voltage was 5 V and the etching time was 20 min.After etching, PGaN was immersed in 1 M oxalic acid solution for 3 h to clean the surface and then rinsed with deionized water.

Fig.1.Schematic diagram of (a) the photo-assisted electrochemical etching GaN and (b) the GaN epitaxial wafer was pre-treated using UV/O3 vacuum plasma treatment.

2.2.2.SnO2-SnO composites preparation

We synthesized the SnO2-SnO complexes by one-step hydrothermal method.In typical synthetics, 0.108 g of urea and 0.132 g of SnCl4·2H2O were dissolved into 30 mL of DI water under modest stirring for around 30 min until a homogeneous emulsion formed.Then, the mixed solution was transferred to 50 mL hydrothermal reactors in which the hydrothermal reaction was performed at 180°C for 18 h.Next, the SnO2-SnO composites could be formed by washing with DI water and anhydrous ethanol by centrifuging at 2500 rpm,and finally dried at 70°C for 2 days in air.

2.3.Fabrication of the heterojunction

The etched PGaN was fixed on a heating platform at a controlled temperature of 90°C, and the SnO2-SnO/ethanol dispersion was sprayed onto the PGaN surface by using the spraying gun to maintain a relatively constant moving speed and distance.Obviously, it was essential to maintain the balance between the spraying gun pressure/the moving speed and the distance.Then the Ag electrode was placed on the surface of the PGaN as the bottom electrode, and the Ag electrode at the other end was placed on the SnO2-SnO layer as the top electrode to construct the FSS/PGaN heterojunction.As a contrast,the SnO2-SnO layer was sprayed on Al2O3in the same manner.

2.4.Gas sensing measurements

The as-obtained gas sensor was placed in an inert, airtight telflons chamber in which H2S,NO2and H2were carried by zero grade dry air.The specific testing equipment and the whole testing process could be learned about from the previous work.[33]

3.Results and discussion

3.1.Preparation and characterization of the PGaN substrate and the SnO2-SnO sensing materials

In this study, the PGaN substrate was prepared by a photo-assisted electrochemical etching.In the photo-assisted electrochemical etching process,[28]two-thirds of the GaN epitaxial wafer were immersed in the[EMIM][OTF]under the influence of light and electric energy.The electrons were excited from the valence band to the conduction band, thereby forming holes in the valence band.[34]As the reaction proceeded, the holes were internally transferred to the interface between GaN and the ILs,and interacted with the active anion at the interface.The triple bond between the Ga atom and the N atom was broken,and Ga formed a compound with the anion in the form of Ga3+.N atoms were combined with each other to generate N2.The reactions from the GaN and the ILs could be described as follows:

The SEM images of PGaN were shown in Fig.2(a), and the PGaN possessed an orderly and uniform pores distribution with average size about 50 nm.The inset in Fig.2(a)showed the etching depth was about 830 nm,and the calculated average etching rate is 41.5 nm/min.The FSS gas sensing materials were prepared via one-step hydrothermal method.During the hydrothermal reaction process, H2NCONH2played a critical role during the emergence of the SnO2-SnO complexes, which hydrolyzed to form NH4+and CO2-3 when the temperature was higher than 90°C.With further increase in temperature, H2NCONH2produced CO2-3 and OH, respectively.Due to the limited amount of oxygen in the solution and the air of teflon-lined vessel, only a bit amount portion of SnO could be turned into SnO2to bring into being SnO2-SnO composites.The main reactions from the Sn2+and Sn4+could be described in supporting information (Fig.S1).Figures 2(b) and 2(c) showed SEM images of the SnO2-SnO hybrids at different scales.It could be seen that the sample presented an uneven spheroidal structure, consisting of nanosheets and nanoparticles.The sample displayed a 3D flower-like structure between 100-200 nm, assembled from nanosheets about 8-10 nm in thickness.The high-resolution TEM(HRTEM)showed the detailed microstructural and crystallographic properties of the as-prepared SnO2-SnO composites.Figures 2(d) and 2(e) exhibited the layered structure of the nanosheets sample.The HRTEM image in Fig.2(f)revealed the well-defined lattice fringes with the interplanar spacings of 0.27 nm, 0.24 nm and 0.33 nm, corresponding to the (110) and (002) lattice planes of SnO and (110) lattice plane of SnO2,respectively.

Fig.2.(a)SEM images of the PGaN.Inset: cross-sectional SEM of etched PGaN.(b),(c)SEM images of the SnO2-SnO composites at different scales.Inset: partially enlarged flower-like SnO2-SnO.(d), (e) TEM images of the SnO2-SnO composites at different scales.(f)HRTEM images of the SnO2-SnO composites.

Fig.3.(a)XRD patterns of the SnO2-SnO composites.(b)Full XPS spectrum of the SnO2-SnO composites.(c)EDS elemental analysis for SnO2-SnO/GaN heterojunction.

The components and crystal structures of the as-prepared sample were characterized by x-ray diffraction analysis techniques (XRD) and scanning electron microscopy (including of EDS and XPS).Figure 3(a) exhibited the XRD pattern of the heterojunction-structured SnO2-SnO composites,showing the peaks were consistent with of that of standard cassiterite(SnO2) and romarchite (SnO).In the XRD pattern of SnO2-SnO,the intensity and full width at half maxima(FWHM)of the diffraction peaks from the SnO2were much weaker and wider than those from the SnO component,indicating that the SnO2nanocrystals were much smaller than the SnO nanoparticles.

XPS measurements were performed to determine the chemical states of the SnO2-SnO complexes.Figure 3(b)showed the XPS in the range of 0-1000 eV binding energy.Visibly,there were no other impurities peaks observed except for Sn,O and C.The specific spectrum of Sn and O could be exhibited in Fig.S1.The XPS results were consistent with the XRD measured pattern results.Above XPS and XRD characterization both indicated that the SnO2-SnO complexes has been successfully prepared through the facile one-step hydrothermal reaction process.Figure 3(c) displayed that the EDS elemental analysis confirmed the existence of Sn,O,Ga and N elements from the FSS/PGaN heterojunction.In total,XRD and XPS measurements were consistent with these electron microscopy results,indicating that the p-n heterostructure of SnO2-SnO was formed by large SnO layered crystals surrounded by small SnO2nanoparticles.

For further confirming the structure of SnO2-SnO/PGaN,nitrogen adsorption and desorption measurements and BJH pore size distribution analysis were performed.As shown in Fig.S2, the specific surface area and pore size construction of SnO2-SnO and SnO2-SnO/PGaN were comparably evaluated.The specific surface areas of SnO2-SnO and SnO2-SnO/PGaN were 158.31 m2·g-1and 218.29 m2·g-1, respectively.This result displayed that the composite had a larger surface area and stronger gas adsorption than PGaN.While SnO2-SnO/PGaN displayed a type IV isotherm with a H3 hysteresis loop in Fig.S2(a), the SnO2-SnO/PGaN composite also exhibited dual desorption characteristics of SnO2-SnO and PGaN,which indicated that the composite possessed pore structures of SnO2-SnO and PGaN.The SnO2-SnO/PGaN composite possessed the hierarchical pores consisting of micropores and mesoporous from the PGaN and tightly packing of SnO2-SnO nano-flowers (Fig.S2(b)).In addition, the SnO2-SnO/PGaN composite also contained some additional mesopores and macropores that came from the interspaces between SnO2-SnO and PGaN.The above pores of SnO2-SnO/PGaN acted as direct channels of diffusion for target gas molecules and promoted rapid adsorption/desorption of gas molecules at the interface.All results demonstrated that the SnO2-SnO grew on the PGaN surface and formed the composite, which was an ideal composite for enhancing gas performances.

3.2.H2S sensing performance test of the SnO2-SnO/PGaN

The working temperature is one of the significant sensing properties of sensitive materials.Figure 4(a)demonstrated the response curves of the SnO2-SnO/PGaN and SnO2-SnO/Al2O3at 50 ppm,100 ppm and 200 ppm H2S as a function of working temperature.As could be seen, the response value(sensitivityS)was closely related to the working temperature.The sensitivity first reached a peak and then decreased as the temperature increased further.Also,the sensitivity also increased as the concentration of H2S increases,which showed sets of sensitivity of 6.3,12.2 and 23.5 at 50 ppm,100 ppm and 200 ppm,respectively.

The sensitivity in this work is defined as

whereRaandRgare the resistances in air and H2S gas,respectively.

Apparently,it was closely related to the presence of PGaN substrates.Specifically,heterojunction generated between the SnO2-SnO and PGaN,the high electron mobility of the porous GaN itself both accelerated the electron transfer rate, which were the main drivers for lowering the sensor operating temperature.The dynamic sensing response and release properties of the SnO2-SnO/PGaN for H2S detection were investigated.Figures 4(b) and 4(c) displayed the sensing responses of the SnO2-SnO/PGaN as a function of the concentration of H2S(from 10 ppm to 50 ppm and from 80 ppm to 200 ppm)at 150°C, indicating response effect with steadily increasing sensitivity.Meanwhile,the linear relationship of sensitivity with H2S concentration (from 10 ppm to 200 ppm) was also confirmed in Fig.4(d).In addition, the resistance of the sensor was fully restored when it was exposed to fresh air throughout the test.The response and release times are 47 s and 37 s, respectively (Fig.4(e)).The sensing selectivity of the SnO2-SnO/PGaN for H2S was also studied.As shown in Fig.4(f)with the SnO2-SnO/PGaN sensor exposed to 50 ppm H2S, NO2, H2and C2H5OH at 150°C, the response(S)to H2S was distinctly far greater than those to NO2,H2and C2H5OH gases,demonstrating the clearly distinguishable selectivity of the dual heterojunction(SnO2-SnO p-n and SnO2-SnO/PGaN heterojunction) nanostructure for H2S detection.

Fig.4.(a) Sensing response curves of FSS/PGaN and pure SnO2-SnO to 50 ppm H2S as a function of the working temperature at different H2S concentrations.Sensing response curves of SnO2-SnO/PGaN exposure to H2S with different concentrations varying from(b)10 ppm to 50 ppm and(c)80 ppm to 200 ppm.(d)Variation curves of response(S)with different gas concentrations.(e)One response/release cyclic curve of SnO2-SnO/PGaN at 50 ppm H2S.(f)Gas selectivity comparative curves of FSS/PGaN to 50 ppm H2S,H2,NO2 and C2H5OH at 150 °C.

Fig.5.(a)Histogram of the response value varied with H2S concentration under different RH.(b)Response curves of sensors based on PGaN,FSS or FSS/PGaN at 50 ppm H2S.

Relative humidity(RH)has a great influence on different types of gas sensors, and for this regard, we have compared the gas response performance under different humidity conditions, as shown in the Fig.5(a).The results showed that the sensitivity of the sensor exhibited a decreasing trend with increasing RH, and the decreasing trend became more conspicuous with increasing H2S concentration.Specifically,the sensitivities were 23.5,22.0,20.1 and 18.2 when the RH were 0%, 33%, 57% and 76% at 200 ppm, respectively.The effect of RH on sensitivity was mainly due to hopping and Grotthus transport mechanisms.[35]In addition,Fig.5(b)presented the response curves at 50 ppm H2S for the PGaN, FSS and FSS/PGaN-based sensors,where the pure PGaN-based sensor gave extremely weak response value for H2S, much smaller than the FSS and FSS/PGaN-based sensors.However,the gas sensing materials of FSS and FSS/PGaN-based sensors were both flower-like SnO2-SnO,their morphology and microstructure were similar,and the sensitivity and the response speed of FSS/PGaN were yet higher than FSS-based sensor.Therefore,the heterojunction effect between FSS and PGaN was highlighted, and the signal amplification function of the heterojunction made the property of FSS/PGaN-based sensor exceed a single FSS or PGaN-based devices.

The repeatability is an important factor for practical application of gas sensor.Simultaneously, five reversible and similar cyclic peaks of the FSS/PGaN-based gas sensor toward 50 ppm H2S was exhibited at 150°C in Fig.6(a), indicating superb response/release property.Long-term stability is another significant performance parameter of gas sensor in real application.Figure 6(b) illustrated a negligible fluctuation ratio～2.5%relative to initial response value after more than 6 months continuous monitoring, demonstrating that asprepared sensor was of prominent long-term stability and relatively reliable application prospects.

Fig.6.(a)Response(S)-time curve of 5 response/release cyclic peaks.(b)Histogram of gas response values(S)recorded for half a year.

By considering the rare reports of pure SnO or SnO2-SnO based H2S gas sensor(mostly reported SnO2-SnO based NO2or H2sensor), Table 1 demonstrated the comparison of the operating temperature and response/release time of the sensor in this work with SnO2and other MO based H2S sensors reported.Thus,the construction of FSS/PGaN based H2S sensor was a major breakthrough in this domain.The FSS/PGaN based H2S sensor showed improved overall performance involving all parameters as opposed to other H2S sensors.The response time was 65 s when the gas concentration changed from 0 ppm to 50 ppm,which was closely related to the time for H2S gas to penetrate into the PGaN layer.The as-obtained sensor displayed potential for rapid gas sensing over a wide range of H2S concentration variations.It is worth mentioning that the release time was 27 s when H2S concentration was converted from 50 ppm to 0 ppm.The release time of H2S gas sensor was faster than the response time, which was mostly dominated by the desorption time of H2S gas from the FSS layer.Moreover, the release time of FSS/PGaN based H2S gas sensor was still faster than that of some SnO2and other MO based H2S gas sensors reported in the prior literature of others.Given that the FSS/PGaN based H2S sensor had only two components,including of high sensitivity and the special PGaN substrate.In concrete terms, for one side, high sensitivity could be mainly ascribed to the enhanced dispersity of FSS.For other side,the characteristic of high electron mobility of PGaN could accelerate the electron transfer rate to make SnO2-SnO quickly react with H2S.Therefore,the FSS/PGaN heterojunction presented potential for fast H2S sensing.

Table 1.Main performance parameters comparison of H2S gas sensor.

3.3.Gas sensing mechanism of the SnO2-SnO/PGaN sensor with heterojunctions

In most gas sensing response systems,gas sensing mechanisms are generally analyzed in terms of adsorption/desorption model (including oxygen sorption models, chemical adsorption/desorption and physical adsorption/desorption),bulk resistance control mechanism and gas diffusion control mechanism.[42]In our experimental system, we focused on PGaN intrinsic property (gas diffusion control mechanism)and the adsorption-desorption model(mainly oxygen adsorption model and chemisorption-desorption dominated) of the p-n heterostructure formed by the MO.

From the above studies,the SnO2-SnO/PGaN sensor exhibited improved sensing properties compared with the SnO2-SnO/Al2O3sensor.For gas diffusion control mechanism,this mechanism was more focused on the gas diffusion process,and the most important factor affecting this process was the morphology of the material.Gas diffusion in the whole process was a dynamic transition between the surface chemical reaction (SCR) and the rate-determining step (RDS), and the condition depended on transition temperature stage,pore canal model[43]and hollow sphere model[44-46]gave a good account of the relationship between these two responses and their operating intervals.The gas diffusion control mechanism was limited by the quantitative analysis of multiple factors and needed to be optimized, but it did not affect the index used by the model to evaluate the effect of the MO-based gas sensor morphology on performance.

Fig.7.(a) Schematic diagram of the heterostructure of the n-SnO2 nanoparticles on the surface of the p-SnO nanosheets with a p-n junction.(b)H2S sensing mechanism of SnO2-SnO/PGaN heterojunction.

The sensing mechanism of the SnO2-SnO/PGaN was meant to be heterojunction nanostructure for the adsorptiondesorption model.The n-type SnO2and p-type SnO developed a heterojunction structure at the interface first,as illustrated in Fig.7(a).[47,48]SnO2and SnO have forbidden band widths of 3.6 eV and 2.9 eV, respectively.The creation of a depletion area is caused by the p-n junction.[1]The schematic illustration of the SnO2-SnO/PGaN sensor was shown in Fig.7(b).SnO2-SnO/PGaN gas sensitivity was recognized to be altered by the interaction between gas species and adsorbed oxygen.Upon energization, the electrons were excited from SnO2-SnO composites and formed electron-hole pairs.Excited electrons could contribute to turning adsorbed oxygen to O2-and O.The thickness of the electron depletion layer increased with large amounts of adsorbed oxygen surged, and thus affected the operating temperature.Because of chemisorption, H2S molecules adsorbed on the SnO2-SnO during H2S gas exposure,activating the H2S reaction and leading in a reduction in resistance,

The excited electrons were transferred from the conduction band of SnO2-SnO to PGaN.The PGaN as an electrons acceptor suppressed charge recombination and resulted in more charge carriers to form reactive species.The sprayed SnO2-SnO composites and the PGaN substrate formed together the continuous conductive pathways between the Ag electrodes.Meanwhile, the excellent transfer capability of PGaN contributed to the heightened electron transfer.The porous structure of GaN was more conducive to gas diffusion, compared with the Al2O3substrate, which contributed plentiful adsorption of H2S molecules.Therefore, the SnO2-SnO/PGaN sensor exhibited better performance than the SnO2-SnO/Al2O3sensor.

The above is mainly the analysis of the sensor chemisorption-desorption model, while in most situations,physical adsorption-desorption has little effect on gas sensing performance and rarely discussed further, which only interferes with the directional movement of the surface carrier,leading to a slight increase in resistance.

4.Conclusion

In summary,an FSS/PGaN heterojunction was fabricated and the properties of the sensor based on the heterojunction have been investigated in-depth for the detection of H2S.At the concentration of 50 ppm H2S,the sensor exhibited excellent sensitivity,a fast response time of 65 s and a release time of 27 s at 150°C.Compared with SnO2-SnO/Al2O3sensor,SnO2-SnO/PGaN sensor showed the optimum temperature as low as 150°C.Meanwhile, SnO2-SnO/PGaN-based gas sensor showed enhanced H2S sensing performance that could be attributed to the structure of PGaN itself and the heterojunction between SnO2-SnO and PGaN.The study presented that,in addition to constructing sensitive p-n junction semiconductor materials,optimizing the substrate of sensitive materials is also a considerable manner to construct gas sensors with excellent sensing performance.

Acknowledgements

Project supported by the Natural Science Research Start-up Foundation of Recruiting Talents of Nanjing University of Posts and Telecommunications (Grant Nos.XK1060921115 and XK1060921002), Young Scientists Fund of the National Natural Science Foundation of China (Grant No.62204125), the National Key R&D Program of China (Grant No.2022YFB3605404), and the Natural Science Foundation of Guangdong Province,China(Grant No.2019A1515010790).