• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Rational construction and triethylamine sensing performance of foam shaped α-MoO3@SnS2 nanosheets

    2022-03-14 09:30:56XinhuiDongQingHnYruKngHiongLiXinyuHungZhengtoFngHuiminYunAhmeElzthryZongtoChiGungleiWuWnfengXie
    Chinese Chemical Letters 2022年1期

    Xinhui Dong,Qing Hn,Yru Kng,Hiong Li,Xinyu Hung,Zhengto Fng,Huimin Yun,Ahme A.Elzthry,Zongto Chi,?,Gunglei Wu,?,Wnfeng Xie,??

    aSchool of Material Science &Engineering,Institute of Materials for Energy and Environment,State Key Laboratory of Bio-Fibers and Eco-Textiles,Qingdao University,Qingdao 266071,China

    bSchool of Electronics and Information,Qingdao University,Qingdao 266071,China

    cCollege of Physics and Electronic Engineering,Qilu Normal University,Ji’nan 250200,China

    dMaterials Science and Technology Program,College of Arts and Sciences,Qatar University,PO Box 2713,Doha,Qatar

    ABSTRACT Owing to their high surface area,stable structure and easy fabrication,composite nanomaterials with encapsulation structures have attracted considerable research interest as sensing materials to detect volatile organic compounds.Herein,a hydrothermal route is designed to prepare foam shaped α-MoO3@SnS2 nanosheets that exhibit excellent sensing performance for triethylamine(TEA).The developed sensor,based on α-MoO3@SnS2 nanosheets,displays a high response of 114.9 for 100 ppm TEA at a low working temperature of 175°C with sensitivity higher than many other reported sensors.In addition,the device shows a wide concentration detection range(from 500 ppb to 500 ppm),good stability after exposure to air for 80 days,and excellent selectivity.The superior sensing characteristics of the developed sensor are attributed to the high crystallinity of α-MoO3/SnS2,excessive and accessible active sites provided by the good permeability of porous SnS2 shells,and the excellent conductivity of the encapsulation heterojunction structure.Thus,the foam shaped α-MoO3@SnS2 nanosheets presented herein have promising practical applications in TEA gas sensing devices.

    Keywords:MoO3 SnS2 Encapsulation heterojunction Triethylamine Sensing performance

    In addition to the maturity and commercial promotion of 5 G technology,the internet of things(IoT),artificial intelligence and wearable electronics are bound to cause significant development in the industry-university research cooperation.Among them,smart sensors are widely used in IoT,wearable devices and artificial intelligence fields owing to their excellent performance in the realtime acquisition,feedback and analysis of large amounts of data[1,2].Thus,sensor technology has broad market demand and application prospects.In particular,research on gas sensors has attracted research interest because of the gradual increase in human environmental awareness and the demand for a better environment[3].Gas sensors can directly detect dangerous situations caused by toxic and harmful components in the air[4].Therefore,the design and fabrication of gas sensors with good sensitivity,fast response,a low detection limit,excellent selectivity and effective cost are also highly desirable[5].

    Among the several types of available gas sensors,such as oxide semiconductor[6],catalytic combustion[7],thermal conductivity cell[8],electrochemical[9]and solid electrolyte[10],metal oxide semiconductor(MOS)gas sensors have been extensively studied owing to their controllability,high sensitivity and good stability[11,12].According to literature,MOS-based sensors play an important role in monitoring toxic and harmful gases,such as triethylamine(TEA)[13],toluene[14],acetone[15,16],CO[17,18],H2[19],H2S[20],NH3[21],NOx[22],ethanol[23–26]and formaldehyde[27].Of the various available metal oxide semiconductors,such as MoO3[28–30],ZnO[31],SnO2[32],WO3[33],Fe2O3[34],Co3O4[35,36]and In2O3[37–39],MoO3has unique advantages as a traditional sensing material in gas monitoring owing to its special electrical characteristics,excellent high stability,high reactivity and surface effect[40].For example,chemical sensing performance can be significantly enhanced from 7 to 33viathe introduction of 2DMoO3nanosheets compared with sensors using bulk MoO3[41].In 2019,Zhuet al.fabricated hollow MoO3microcages that exhibited larger ethanol gas response than that of solid polyhedrons[42].In their study,porous ultrathinα-MoO3nanosheets with oxygen vacancies were obtainedviathe solvothermal approach and their sensor demonstrated the fastest response to trimethylamine(TMA)at 133°C(the response of the sensor was 198 ppm to 50 ppm TMA)[43].Very recently,α-MoO3/BiVO4composites with a heterojunction were synthesizedviathe hydrothermal method and the results showed that the response of theα-MoO3/BiVO4composite to 20 ppm TEA was 1.86 times and 15 times higher than those ofα-MoO3and BiVO4,respectively,at 125°C[44].

    TEA,a volatile organic compound(VOC),is extensively used in catalysts,preservatives,curing agents,synthetic dyes and industrial raw materials[45,46].However,the toxic,volatile,flammable and explosive nature of TEA is disadvantageous in its practical applications[47].Once an industrial leakage occurs,it is extremely easy to cause serious injuries or destruction to the public environment and human body[48].Therefore,developing a TEA gas sensor with fast response,wide detection limitation,good selectivity,low working temperature and long-term stability is important and urgent for industrial and agricultural production activities.Thus far,TEA sensors based on MOSs,such as ZnO[49],SnO2[50],Fe2O3[51,52],V2O5[53],In2O3[54]and ZnCo2O4[55],have been widely investigated.Although it has been found that molybdenum oxide(MoO3)also has TEA sensing properties,the potential benefits of MoO3to TEA gas warrant further exploration[56].SnS2is a multifunctional narrow bandgap(2.2 eV)n-type semiconductor that has been widely used in various fields,such as photoelectric,photoelectrochemical and lithium-ion batteries(LIBs)[57–59].Very recently,SnS2nanomaterials have attracted substantial research attention because they are good candidates for the synthesis of gas sensing nanocomposites with other MOSs[60].For example,Guet al.reported that a SnO2/SnS2heterojunction based chemiresistive gas sensor exhibited excellent sensitivity and selectivity to different concentrations of NO2,from 1 ppm to 8 ppm,at 80°C[61].Yanet al.reported that SnS2/rGO nanohybrids show ultrasensitive room temperature ppb-level NO2gas sensing performance[62],and Yanet al.demonstrated that Schottky-contacted n-type SnS2gas sensors reveal excellent device sensitivities,as high as 13,000%for 9 ppm and 97% for 1 ppb NO2[63].To the best of our knowledge,the development of a novel TEA sensing MoO3@SnS2material with excellent sensing performance at low operating temperature is still a major challenge,despite the considerable progress in composite of SnS2and other metal oxides.

    Herein,MoO3@SnS2composites with encapsulation structures were obtainedviaa two-step hydrothermal method wherein the thickness of the SnS2shells was manipulated by controlling the second hydrothermal reaction time.Accordingly,the sensing performances of different MoO3@SnS2composites were carefully studied.Furthermore,gas sensing measurements revealed that MoO3@SnS2composites with encapsulation structures display excellent gas sensing performance as compared to individual MoO3and SnS2.

    Theα-MoO3nanosheets were successfully synthesized by the following processes,which have reported in detail in our previous works[64].First,1 g(NH4)2MoO4,300 mg NH4F,100 mg NaOH,and 3 g C6H12O6were dissolved in 50 mL distilled water,followed by vigorous stirring for 30 min.Subsequently,the obtained solution was transferred into a 60 mL Teflon-lined autoclave and heated at 120°C for 12 h.Next,the obtained suspension was washed several times with absolute ethanol and deionized water,to remove redundant ions,and centrifuged at 6900 rpm for 30 min.Then,the product was calcined at 400°C for 2 h to obtain the final gray precursor.Then we synthesized foam shaped MoO3@SnS2nanosheets through a second hydrothermal reaction.In this process,360 mg of the obtained gray precursor,175.3 mg SnCl4·5H2O,6 mL CH3COOH and excess CH3CSNH2were first dissolved into 40 mL ethanol,followed by vigorous stirring for 30 min.The resulting solution was then transferred into a 60 mL Teflon-lined autoclave and heated at 160 °C for 2,6 and 10 h.Then,the obtained suspension was washed several times with absolute ethanol and deionized water,respectively.After drying at 60°C overnight in a vacuum chamber,the MoO3@SnS2composites were obtained.For convenience,we named the three composites MS2,MS6 and MS10 according to their second step hydrothermal reaction times of 2,6 and 10 h,respectively.

    Fig.1.(a)Schematic illustration of the synthesis processes for MoO3@SnS2 nanocomposites and an as-fabricated gas sensor.(b–d)Field-emission scanning electron microscopy(FE-SEM)images of α-MoO3 nanosheets and(e–g)FE-SEM images of the MS6 composite at increasing magnification.

    The as-prepared sensing material was mixed with ethanol and continuously grounded to form a slurry.Then,the slurry was pasted onto a ceramic tube with a brush to form a thin and uniform sensing material coating.Four Pt wires and a pair of Au electrodes were pre-installed on the ceramic tube to facilitate the collection of electrical signals.A Ni–Cr coil pierced through the ceramic tube was used as a heater.Next,the Pt wires and the Ni–Cr heater were soldered on the pedestal of the gas sensor.The response(Rs)of the sensor was calculated byRs=Ra/Rg,whereRaandRgare the resistances in fresh air and target gas environment,respectively.The response and recovery times were defined as the time taken by the sensor to reach 90% of the total resistance variation[65].

    Fig.1a provides a schematic of the overall process of material synthesis and device preparation.Herein,we first prepared theα-MoO3precursorviathe hydrothermal method and then annealed at 400°C.Thereafter,dark MoO3@SnS2composites with encapsulation structures were obtained by the second hydrothermal reaction route.Finally,pureα-MoO3and MoO3@SnS2sensors were fabricated using brush-coating technology.FE-SEM was used to examine the microstructure and morphology of as-prepared samples.As Figs.1b and c show,several MoO3nanosheets were successfully synthesized with irregular sheet-like profiles.To scrutinize the morphology of the MoO3nanosheets,high-magnification FE-SEM was employed.The average thickness of anα-MoO3nanosheet was~200 nm and its surface are rather smooth(Fig.1d).Interestingly,MS6 exhibits markedly different morphology,though the overall profile indicates nanosheet structure(Figs.1e and f).Additionally,the thickness of the nanosheets increases from 200 nm to 400 nm,which is attributed to the encapsulation of SnS2to the positive and negative facets ofα-MoO3nanosheets.In addition,the thickness of SnS2layer was determined by the second hydrothermal reaction time.According to a rough estimation,the average thickness of a SnS2single face is about 100 nm.Tremendous changes also occurred in the morphology of the MoO3@SnS2composite,from its original smooth surface to a foam-shape(Fig.1g),which considerably increases the specific surface ratio.This can increase gas absorption capacity and create more active centers,which are beneficial to sensing performance.

    Fig.2.(a)XRD pattern of pure α-MoO3 precursor and MS6 nanocomposites.XPS spectra of MS6 indicated(b)Mo 3d,(c)O 1s,(d)Sn 3d and(e)S 2p.(f)TEM and(g)HR-TEM images of MS6 composite.

    XRD patterns of MoO3precursor and MS6 composite were measured because of the distinguished difference sensing performance(Fig.2a).In this work,MoO3is the reference material,MS6 exhibits the best sensing performance among MoO3,MS2,MS6 and MS10.For pure MoO3nanosheets,diffraction peaks were located at 2θ=12.8°,23.3°,27.3°,33.7° and 38.9° that can be ascribed to the(200),(101),(210),(111)and(600)planes.These results agree well with standard diffraction patterns of orthorhombicα-MoO3(JCPDS No.89-7112).Furthermore,peaks corresponding toα-MoO3and SnS2were detected in the XRD patterns of the MS6 composite as theα-MoO3nanosheets are coated by ultrathin SnS2sheets.Five small peaks are observed at 2θ=15.0°,29.2°,36.4°,48.0°and 50.0°,corresponding to the(002),(101),(103),(105)and(110)planes,respectively,in the hexagonal phase of SnS2(JCPDS card no.89-2357).No diffraction peaks from impurities were detected.Furthermore,it was found that the intensity of the diffraction peaks ofα-MoO3is stronger than those of SnS2due to the higher crystallinity ofα-MoO3.These results confirm that MoO3@SnS2nanocomposites with high crystallinity were successfully synthesized.

    High-resolution XPS measurements were performed to further analyze the chemical components and valence states of the MoO3@SnS2composites.The peaks of Mo 3d,O 1s,Sn 3d,and S 2p can be clearly identified in the XPS spectrum of MoO3@SnS2(Figs.2b–e).This indicates that the final product only contains Mo,O,Sn and S.In Fig.2b,peaks at 235.8 and 232.7 eV belong to the doublet Mo 3d3/2and Mo 3d5/2,respectively,which is attributed to the Mo6+ofα-MoO3phase[66].Fig.2c displays the O 1s XPS spectrum.Two peaks at 530.6 and 531.5 eV indicate two independent types of O species in MoO3@SnS2.According to literature,the peak at 530.6 eV could be ascribed to surface lattice oxygen in MoO3@SnS2nanocomposites and the peak at 531.5 eV could be surface absorbed oxygen species,such as O?,O2?and O2?,that are in oxygen deficient regions within the matrix of MoO3@SnS2.As shown in Fig.2d,there are two sharp peaks in the XPS spectrum of Sn 3d,at 495.69 and 487.27 eV,which are the peaks of Sn 3d3/2and Sn 3d5/2,respectively[67].In Fig.2e,the S 2p spectra was assigned to the binding energy of S 2p1/2(163.65 eV)and S 2p3/2(162.25 eV),which contributed to the S2?in the SnS2.

    Figs.2f and g display TEM and HR-TEM images of the MS6 nanocomposite.It can be clearly seen that the SnS2phase is connected to the surface of theα-MoO3phase.The 0.329 nm fringe spacing corresponds to the(111)plane of SnS2and the 0.382 nm fringe spacing fits well with the(110)plane ofα-MoO3.We further verified the constituent elements and corresponding ratio of MoO3@SnS2nanocompositesviaenergy dispersive X-ray(EDS)analysis(Fig.S1 in Supporting information).The EDS results reveal that the MS6 composites are composed of Mo,O,S and Sn,and the weight ratio meets the chemical formula of MoO3@SnS2.On the other hand,the elements are uniformly distributed on the surface of MoO3@SnS2,which was identified by EDS mapping in Fig.S1.This means thatα-MoO3nanosheets have been successfully coated with the SnS2ultrathin sheets.

    The schematic internal circuit of the homemade sensor was displayed(Fig.S2 in Supporting information).In order to investigate the thermal stability of MoO3@SnS2composites,the TG curve of MS6 was measured,as shown in Fig.3a.Clearly,the weight of the sample begins to decrease after 250°C,which can be attributed to the loss of absorbed water and the decomposition of residual reagents,such as NH4F.Thereafter,a significant decrease can be observed in weight between 350°C and 430°C,which should be related to the oxidation behavior of SnS2to SnO2in the MoO3@SnS2composite.In the next stage(430–740°C),no obvious changes are observed in the TG curve,indicating formation of thermally stable composite.In contrast,there is another sudden weight loss after 740°C,which is ascribed to the melt and sublimation behavior of MoO3.Thus,it is demonstrated that the MS6 composite can effectively work below 350°C.

    Fig.3.(a)TG curve of the MS6 composite and(b)sensing performance of pure α-MoO3 precursor,MS2,MS6 and MS10 samples for 100 ppm TEA gas at different operating temperatures.(c)Dynamic response curves of MoO3,MS2,MS6 and MS10 sensors for TEA,from 0.5 ppm to 500 ppm,at 175°C.(d)response versus gas concentrations for MoO3,MS2,MS6 and MS10 sensors.(e)Repeatability test(five periods)of MoO3,MS2,MS6 and MS10 sensors for 100 ppm of TEA at 175°C.(f)Fitting curve between concentration and response used to obtain LoD.

    The operating temperature is an important factor for a gas sensor.Fig.3b shows the relationship between the sensitivity and the temperature of the device.The sensing properties of pureα-MoO3precursor and MoO3@SnS2composites were tested for 100 ppm TEA gas over a wide range of temperatures,from room temperature to 275°C.All samples exhibit an inverted V-type curve with the increase of temperature;however,the MS6 composite displays the highest sensor response of 114.9 at 175°C.On the one hand,the inverted V-type curve(i.e.,the response of the sensor to TEA increases first and then decreases with the increase of temperature)can be attributed to the sensor material being insufficiently active at low temperature and the TEA molecules not having suffi-cient energy to overcome the activation energy barrier and surface adsorption of the oxygen reaction.With the increasing temperature,the material activity is enhanced,the TEA gains energy,and the sensing performance is improved.In contrast,as the temperature continues to increase,the sensing material has difficulty absorbing the test gas,which results in desorption phenomenon on the surface of the material and degradation of sensing performance at high temperature.Therefore,175°C is defined as the optimum working temperature for the MoO3@SnS2sensor.

    Fig.4.(a,b)Response and recovery characteristics of the pure α-MoO3,MS2,MS6 and MS10 sensors for 100 ppm TEA at 175°C.(c)Bar chart of the long-term stability of the MS6 sensor and(d)selectivity of the MoO3 and MS6 sensors to different gases at 175°C.The formation mechanism of the electron depletion layer at the n-n heterojunction energy band structure(e)before contact of n-type MoO3 and n-type SnS2 and(f)after contact of n-type MoO3 and n-type SnS2.(g)Schematics of the gas sensing reaction mechanism of MoO3@SnS2 nanocomposite.

    Fig.3c depicts the dynamic response curves of the pureα-MoO3,MS2,MS6 and MS10 sensors at 175°C for different concentrations of TEA vapor,ranging from 500 ppb to 500 ppm.The results reveal that the response values of all MoO3@SnS2composite sensors climb significantly with increasing TEA concentration while the pureα-MoO3sensor shows sluggish rising performance.Among the samples,the MS6 composite sensor shows the highest response(Fig.3d).The sensing response of the MS6 composite is as high as 234.7 at 500 ppm TEA,which is 16.88 times that of the pureα-MoO3sensor.Even when the concentration of TEA decreases to 0.5 ppm,the response of the MS6 sensor reaches 1.38.This indicates that MS6 has good sensitivity and response to TEA at a low temperature of 175°C.For comparison,the specific response values ofα-MoO3and MS6 sensors to different concentrations are listed in Table S1(Supporting information).From the Table S1,it can be concluded that MS6 sensors have a much higher response to TEA than pureα-MoO3sensors;therefore,MS6 sensors have great application prospects.Additionally,the sensing performance comparison between our sensor and recent literature results are summarized in Table S2(Supporting information).It is worth noting that the MS6 sensor exhibits the highest response in comparison with those reported.In Fig.3e,the repeatability of pureα-MoO3,MS2,MS6 and MS10 composites is evaluated.These results were collected when the pureα-MoO3,MS2,MS6,and MS10 sensors were exposed to fresh air and TEA target gas(100 ppm)at the optimal working temperature of 175°C.The results show that bothα-MoO3and MoO3@SnS2composites have excellent response and recovery stability after five cycles.The limit of detection(LoD)of TEA gas is studied by linear extrapolation of the response sensitivity as a function of TEA concentration LoD(Fig.3f).The calculating formula of the LoD is:LoD=3 ×(standard deviation/slope of responseversusconcentration plot).An ultra-low TEA detection concentration of 177.06 ppb was predicted for MS6.

    Fig.4a shows the dynamic response curves of all sensors for 100 ppm TEA at 175°C.Compared to theα-MoO3sensor,the response time(τres)and recovery time(τrec)of MS2,MS6 and MS10 sensors are shortened(see the bar chart in Fig.4b).Overall,MoO3@SnS2composite sensors exhibit fast recovery performance;that is,MS2 is 23 s,MS6 is 21 s,and MS10 is 32 s.Additionally,the MS6 sensor exhibits the shortestτresof 51 s and theτresof the pureα-MoO3sensor is 57 s,which is slower than that of MS6.This is because the heterostructure interface formed between MoO3and SnS2can activate conducting electrons and accelerate electron transfer behavior.Additionally,an optimal Mo/Sn weight ratio is another beneficial reason for shorter response time.

    From the perspective of industrial applications,a good sensor should have long-term stability.Thus,the stability of MS6 to 100 ppm TEA was evaluated at 175°C over 80 days,as shown in Fig.4c.It is evident that the maximum deviation of the response for the MS6 gas sensor to TEA is less than 10%,which exhibits good stability after 80 days.Selectivity another important function of a gas sensor.In order to study the selectivity of pureα-MoO3and MS6 composite sensors,the responses toward 100 ppm benzene,acetone,acetic acid,methanol,ethanol,ammonia and TEA gases at 175°C were investigated,as shown in Fig.4d.Obviously,both pureα-MoO3and MS6 are sensitive to TEA compared to other gases,which is very attractive for the detection of trace amounts of TEA.Meanwhile,the measured response of the MS6 sensor to TEA is remarkably larger than that of pureα-MoO3,demonstrating the gas sensing performance ofα-MoO3has been effectively enhanced by loading of SnS2ultrathin nanosheets.

    MoO3and SnS2are n-type semiconductors with bandgaps of about 3.3 eV and 2.2 eV(Fig.4e),respectively,that have been intensively investigated as gas sensors[68].As shown in Fig.4f,when MoO3and SnS2contact each other,the intrinsically excited electrons(e?)flow from MoO3to SnS2due to the higher Fermi level(Ef)of MoO3.As the number of e?in the MoO3conduction band increases,the system reaches an equilibrium Fermi level(Ef).Thus,energy-band bending and an additional electron depletion layer(EDL)at the interface between MoO3and SnS2are formed.When the MS6 sensor is exposed to fresh air,the change in resistance of the sensor is replaced by the absorption and desorption process of oxygen molecules(O2)on the surface of the sensing material;the absorbed O2is ionized by capturing conducting electrons from MoO3@SnS2heterojunctions[69].Then,reactive oxygen ions(O2?,O2?or O?)are produced(Fig.4g).In this process,O2acts as the electron acceptor,leading to the creation of an EDL and the increase of sensor resistance.Once exposed to the TEA gas atmosphere at a suitable temperature,the TEA gas molecules will react with the reactive oxygen ions on the surface,then,the released electrons will go back into the conduction band(Eg).Consequently,the EDL becomes narrower,and the sensor resistance decreases.The reactive processes can be expressed by the following formulas:

    In summary,herein,MoO3@SnS2composites were successfully prepared by a facile hydrothermal method.The sensing performance,based onα-MoO3nanosheets and MoO3@SnS2sensors,was carefully investigated with respect to TEA gas.The resultant MS6 composite exhibits superior sensing performance compared to pureα-MoO3nanosheets.The sensor based on MS6 nanosheets displays a high response of 114.9 for 100 ppm TEA at a working temperature of 175°C;the sensitivity is much higher than those reported for other sensors.In addition,the MS6 device shows a wide concentration detection range,from 500 ppb to 500 ppm,very good stability after 80 days exposed in air,and excellent selectivity.The extraordinary performance is ascribed to a synergistic coupling effect between high crystallineα-MoO3/SnS2heterojunctions,encapsulation design,and accessible large pores on the surface of SnS2.This study demonstrates a new avenue to effectively construct gas sensing materials with encapsulation nanostructuresviaa metal oxide and sulfide.

    Declaration of competing interest

    The authors report no declarations of interest.

    Acknowledgments

    This work was financially supported by the National Natural Science Foundation of China(No.51227804).This work was also funded by the Postdoctoral Scientific Research Foundation of Qingdao,National College Students Innovation and Entrepreneurship Training Program of China(No.G201911065028),College Students Innovation and Entrepreneurship Training Program of Qingdao University(Nos.X201911065058,X202011065056).Natural Science Foundation of Shandong Province(No.ZR2019YQ24),Taishan Scholars and Young Experts Program of Shandong Province(No.tsqn202103057),the Qingchuang Talents Induction Program of Shandong Higher Education Institution(Research and Innovation Team of Structural-Functional Polymer Composites).The authors would like to thank Kehui Han from Shiyanjia Lab(www.shiyanjia.com)for the SEM and XRD analysis.

    Supplementary materials

    Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2021.06.022.

    亚洲美女视频黄频| 色播亚洲综合网| 搡女人真爽免费视频火全软件| 久久国产乱子免费精品| 男女啪啪激烈高潮av片| 日本黄大片高清| 国产精品一区二区在线观看99 | 淫秽高清视频在线观看| 五月玫瑰六月丁香| www.色视频.com| av国产免费在线观看| 人人妻人人澡欧美一区二区| 国产一区二区在线观看日韩| 国产精品女同一区二区软件| 色视频www国产| 日本一本二区三区精品| 国产成人精品一,二区| 久久久久国产网址| 亚洲在久久综合| 午夜福利成人在线免费观看| 久久精品国产亚洲av天美| 狂野欧美激情性xxxx在线观看| 成人毛片a级毛片在线播放| 一个人看的www免费观看视频| 亚洲最大成人中文| 精华霜和精华液先用哪个| 国产一区二区在线av高清观看| 春色校园在线视频观看| 国产久久久一区二区三区| 男女那种视频在线观看| www.av在线官网国产| 中文欧美无线码| a级毛片免费高清观看在线播放| 国产亚洲精品久久久com| 亚洲美女搞黄在线观看| 夫妻性生交免费视频一级片| 菩萨蛮人人尽说江南好唐韦庄 | 国产精品久久久久久久久免| 日本熟妇午夜| 免费播放大片免费观看视频在线观看 | 国产精品久久久久久精品电影| 一区二区三区四区激情视频| 成年av动漫网址| 久久鲁丝午夜福利片| 村上凉子中文字幕在线| 午夜亚洲福利在线播放| 亚洲国产成人一精品久久久| 白带黄色成豆腐渣| 九九久久精品国产亚洲av麻豆| 亚洲aⅴ乱码一区二区在线播放| 极品教师在线视频| 天天一区二区日本电影三级| 久久亚洲精品不卡| 男人的好看免费观看在线视频| 成年av动漫网址| 国产精品国产高清国产av| 91av网一区二区| 综合色丁香网| 亚洲av电影不卡..在线观看| 精品不卡国产一区二区三区| 亚洲av男天堂| 一本一本综合久久| 国产日韩欧美在线精品| 国产高潮美女av| 亚洲av福利一区| 禁无遮挡网站| 午夜视频国产福利| 国产午夜精品久久久久久一区二区三区| a级毛片免费高清观看在线播放| 亚洲欧美一区二区三区国产| 91精品一卡2卡3卡4卡| 中文字幕av成人在线电影| 麻豆国产97在线/欧美| 久久久国产成人免费| 精品国内亚洲2022精品成人| 久久欧美精品欧美久久欧美| 天美传媒精品一区二区| 欧美高清性xxxxhd video| 伦理电影大哥的女人| 婷婷色综合大香蕉| 欧美成人一区二区免费高清观看| 国产成人精品一,二区| 身体一侧抽搐| 日韩强制内射视频| 91精品国产九色| 国产免费男女视频| www.色视频.com| 国语对白做爰xxxⅹ性视频网站| 国产在线一区二区三区精 | 深爱激情五月婷婷| 午夜精品国产一区二区电影 | 午夜福利成人在线免费观看| 看十八女毛片水多多多| 在线免费观看的www视频| 一本久久精品| 国产激情偷乱视频一区二区| 日韩欧美精品v在线| 岛国在线免费视频观看| 中文在线观看免费www的网站| 欧美日韩一区二区视频在线观看视频在线 | 丝袜喷水一区| 日韩av不卡免费在线播放| 欧美一区二区国产精品久久精品| 超碰97精品在线观看| 伊人久久精品亚洲午夜| 日韩大片免费观看网站 | 欧美激情久久久久久爽电影| 日本午夜av视频| 日本黄大片高清| 韩国高清视频一区二区三区| 国产精品国产三级国产专区5o | 亚洲经典国产精华液单| 在线a可以看的网站| 青春草视频在线免费观看| 国产在视频线在精品| 联通29元200g的流量卡| 人人妻人人看人人澡| 国产黄片美女视频| 我的老师免费观看完整版| 免费看a级黄色片| 亚洲欧洲国产日韩| 99久久中文字幕三级久久日本| 久久久久久大精品| 亚洲国产精品成人综合色| 99久久精品国产国产毛片| 国产精品国产三级国产专区5o | 国产亚洲5aaaaa淫片| 精品人妻偷拍中文字幕| 成人国产麻豆网| 免费观看人在逋| 99热这里只有精品一区| 麻豆久久精品国产亚洲av| 久久久久久国产a免费观看| 男女视频在线观看网站免费| 亚洲精品一区蜜桃| 国产爱豆传媒在线观看| 亚洲欧美日韩卡通动漫| 日韩视频在线欧美| av线在线观看网站| 国产免费福利视频在线观看| 日韩成人av中文字幕在线观看| 中文字幕亚洲精品专区| 1000部很黄的大片| 综合色av麻豆| 亚洲美女搞黄在线观看| 91久久精品电影网| 精品久久久久久久久av| 久久精品国产99精品国产亚洲性色| 大香蕉久久网| 国产av不卡久久| 最近手机中文字幕大全| 亚洲图色成人| 在线观看av片永久免费下载| 天美传媒精品一区二区| 尤物成人国产欧美一区二区三区| 日韩人妻高清精品专区| 99国产精品一区二区蜜桃av| 国产大屁股一区二区在线视频| 两个人的视频大全免费| 校园人妻丝袜中文字幕| 国产免费又黄又爽又色| 日本与韩国留学比较| 欧美成人a在线观看| 国产在视频线在精品| 身体一侧抽搐| 中国国产av一级| 欧美xxxx性猛交bbbb| 亚洲不卡免费看| 狠狠狠狠99中文字幕| 国产黄色视频一区二区在线观看 | 国产毛片a区久久久久| 国产一级毛片七仙女欲春2| 久久国产乱子免费精品| 亚洲av电影在线观看一区二区三区 | 午夜福利在线观看免费完整高清在| 99热网站在线观看| 性色avwww在线观看| av线在线观看网站| 国产免费又黄又爽又色| 大香蕉久久网| 黄片wwwwww| 观看美女的网站| 三级毛片av免费| 日韩一区二区视频免费看| 精品国产一区二区三区久久久樱花 | 色视频www国产| 亚洲人成网站高清观看| 国产精品一区二区在线观看99 | 高清视频免费观看一区二区 | 欧美日韩一区二区视频在线观看视频在线 | 99久国产av精品国产电影| 免费一级毛片在线播放高清视频| 高清午夜精品一区二区三区| www日本黄色视频网| 最近中文字幕高清免费大全6| 中文字幕制服av| 日韩国内少妇激情av| 高清午夜精品一区二区三区| 亚洲成人精品中文字幕电影| 床上黄色一级片| 国产精品久久视频播放| 男人狂女人下面高潮的视频| 日日摸夜夜添夜夜添av毛片| 在线a可以看的网站| 成年版毛片免费区| 国产精品国产三级专区第一集| 欧美精品国产亚洲| 久久6这里有精品| 特大巨黑吊av在线直播| 特级一级黄色大片| 精品不卡国产一区二区三区| 神马国产精品三级电影在线观看| 中国国产av一级| 日韩 亚洲 欧美在线| 特大巨黑吊av在线直播| 91精品一卡2卡3卡4卡| 国产乱人视频| 亚洲国产精品专区欧美| 精品免费久久久久久久清纯| 99久久人妻综合| 在线天堂最新版资源| 国产真实伦视频高清在线观看| 最近中文字幕高清免费大全6| 一卡2卡三卡四卡精品乱码亚洲| av线在线观看网站| 亚洲国产最新在线播放| 亚洲欧美清纯卡通| 久久久久性生活片| 一个人看视频在线观看www免费| 村上凉子中文字幕在线| 亚洲怡红院男人天堂| 午夜免费激情av| 三级毛片av免费| 国产免费又黄又爽又色| 男人舔奶头视频| 亚洲自偷自拍三级| 99热6这里只有精品| 内射极品少妇av片p| 亚洲欧美日韩无卡精品| 成人漫画全彩无遮挡| a级毛色黄片| 久久人妻av系列| 国产精品一区二区性色av| 激情 狠狠 欧美| 亚洲精品456在线播放app| 久久精品国产亚洲av涩爱| 亚洲精品,欧美精品| 变态另类丝袜制服| 男插女下体视频免费在线播放| 丝袜喷水一区| 国产一级毛片七仙女欲春2| 亚洲国产欧洲综合997久久,| 亚洲精品国产av成人精品| 久久久精品大字幕| 国产精品一区二区性色av| 1024手机看黄色片| 三级国产精品片| 老师上课跳d突然被开到最大视频| 国产精品国产高清国产av| 91av网一区二区| av国产久精品久网站免费入址| 韩国av在线不卡| 一二三四中文在线观看免费高清| 国产乱人偷精品视频| 白带黄色成豆腐渣| 国内揄拍国产精品人妻在线| 综合色丁香网| 亚洲美女搞黄在线观看| 成人高潮视频无遮挡免费网站| 久久久久精品久久久久真实原创| 深爱激情五月婷婷| 一级av片app| 内射极品少妇av片p| 亚洲成人精品中文字幕电影| 国产一区二区在线av高清观看| 99热全是精品| 久久精品国产亚洲av涩爱| 精品一区二区免费观看| 国语对白做爰xxxⅹ性视频网站| 中文字幕制服av| 欧美人与善性xxx| 爱豆传媒免费全集在线观看| 2021少妇久久久久久久久久久| 亚洲av电影不卡..在线观看| 免费av观看视频| 纵有疾风起免费观看全集完整版 | 不卡视频在线观看欧美| 哪个播放器可以免费观看大片| 卡戴珊不雅视频在线播放| 蜜桃久久精品国产亚洲av| 免费看光身美女| 久久婷婷人人爽人人干人人爱| 国产黄色视频一区二区在线观看 | 我要看日韩黄色一级片| 国产真实乱freesex| 小蜜桃在线观看免费完整版高清| 久久亚洲精品不卡| 亚洲va在线va天堂va国产| 国产一区有黄有色的免费视频 | 国产伦精品一区二区三区四那| eeuss影院久久| 青青草视频在线视频观看| 女的被弄到高潮叫床怎么办| 国产成人freesex在线| 日韩一本色道免费dvd| 国国产精品蜜臀av免费| 少妇人妻精品综合一区二区| 级片在线观看| 久久久久久久久久成人| 久久久久精品久久久久真实原创| 亚洲精品国产av成人精品| 观看免费一级毛片| 波多野结衣高清无吗| 18禁在线播放成人免费| 亚洲一区高清亚洲精品| 网址你懂的国产日韩在线| 久久精品夜夜夜夜夜久久蜜豆| 嫩草影院新地址| 国产单亲对白刺激| 两性午夜刺激爽爽歪歪视频在线观看| 哪个播放器可以免费观看大片| 嫩草影院入口| 小蜜桃在线观看免费完整版高清| 国产三级在线视频| 日日摸夜夜添夜夜爱| 免费黄色在线免费观看| 亚洲av二区三区四区| 亚洲av免费在线观看| 日本一二三区视频观看| 中文字幕久久专区| 国产精品福利在线免费观看| 免费大片18禁| 国产91av在线免费观看| 在线天堂最新版资源| av在线天堂中文字幕| 国产av一区在线观看免费| 日韩av在线免费看完整版不卡| 少妇猛男粗大的猛烈进出视频 | 欧美bdsm另类| 在现免费观看毛片| 一个人看视频在线观看www免费| 精品久久久久久电影网 | 男女啪啪激烈高潮av片| 国产免费视频播放在线视频 | 亚洲人成网站在线观看播放| 91aial.com中文字幕在线观看| 日本爱情动作片www.在线观看| 欧美高清性xxxxhd video| 日本免费在线观看一区| 综合色丁香网| 老师上课跳d突然被开到最大视频| 婷婷色麻豆天堂久久 | 国产午夜精品一二区理论片| 精品人妻视频免费看| 日日摸夜夜添夜夜爱| 亚洲天堂国产精品一区在线| 我的老师免费观看完整版| 乱人视频在线观看| 国产乱人偷精品视频| 国产精品一区二区性色av| 91久久精品国产一区二区三区| 久久久国产成人精品二区| .国产精品久久| 长腿黑丝高跟| 成人鲁丝片一二三区免费| 99久久九九国产精品国产免费| 五月玫瑰六月丁香| 99热这里只有是精品在线观看| 欧美激情久久久久久爽电影| 亚洲综合色惰| 欧美色视频一区免费| 国产成人freesex在线| 亚州av有码| 成年av动漫网址| 国产精品1区2区在线观看.| 国产成人精品婷婷| 晚上一个人看的免费电影| 精品熟女少妇av免费看| 国产黄色小视频在线观看| 又黄又爽又刺激的免费视频.| 国产精品国产三级专区第一集| 亚洲国产最新在线播放| 91精品一卡2卡3卡4卡| 在线免费观看的www视频| 国产精品一二三区在线看| 亚洲精品国产av成人精品| 亚洲国产日韩欧美精品在线观看| 久久99热这里只有精品18| 久久人人爽人人片av| 高清午夜精品一区二区三区| 69人妻影院| 99久久精品一区二区三区| 久久久久久久久大av| 男人狂女人下面高潮的视频| 天天躁夜夜躁狠狠久久av| 午夜老司机福利剧场| 成人午夜精彩视频在线观看| 两个人的视频大全免费| 免费观看a级毛片全部| 日日撸夜夜添| 高清毛片免费看| 日日啪夜夜撸| 中文乱码字字幕精品一区二区三区 | 狂野欧美白嫩少妇大欣赏| 欧美区成人在线视频| 亚洲欧美日韩卡通动漫| 尾随美女入室| av又黄又爽大尺度在线免费看 | 亚洲精品乱码久久久久久按摩| 午夜精品在线福利| 国产av码专区亚洲av| 丰满少妇做爰视频| 欧美人与善性xxx| 噜噜噜噜噜久久久久久91| 免费观看精品视频网站| 高清日韩中文字幕在线| 内地一区二区视频在线| 久久99精品国语久久久| 亚洲欧美中文字幕日韩二区| 永久网站在线| 亚洲va在线va天堂va国产| 99久久九九国产精品国产免费| 欧美一区二区亚洲| 人人妻人人看人人澡| 亚洲av成人av| 久久精品国产鲁丝片午夜精品| av在线观看视频网站免费| av女优亚洲男人天堂| 22中文网久久字幕| 麻豆av噜噜一区二区三区| 哪个播放器可以免费观看大片| 蜜桃久久精品国产亚洲av| 亚洲av.av天堂| 啦啦啦观看免费观看视频高清| 一级毛片电影观看 | 亚洲国产欧洲综合997久久,| 久久久欧美国产精品| 天美传媒精品一区二区| 一级黄色大片毛片| 精品一区二区三区视频在线| 国产淫片久久久久久久久| 女人十人毛片免费观看3o分钟| 秋霞伦理黄片| 免费不卡的大黄色大毛片视频在线观看 | 18禁在线播放成人免费| 国产成人午夜福利电影在线观看| 日韩精品有码人妻一区| 久热久热在线精品观看| 国产精品福利在线免费观看| 青春草国产在线视频| 亚洲精品久久久久久婷婷小说 | 老女人水多毛片| 国产三级在线视频| 国产精品一区www在线观看| 最近中文字幕2019免费版| 丝袜美腿在线中文| 久久精品国产亚洲av涩爱| 嫩草影院精品99| 日韩视频在线欧美| 日日撸夜夜添| 中文字幕免费在线视频6| 黄片无遮挡物在线观看| 在线免费观看不下载黄p国产| 国产免费福利视频在线观看| 插阴视频在线观看视频| 亚洲经典国产精华液单| 日本免费a在线| 欧美高清成人免费视频www| 精品久久久久久久久av| 国产色婷婷99| 免费电影在线观看免费观看| 亚洲国产精品久久男人天堂| 久久精品久久久久久噜噜老黄 | 自拍偷自拍亚洲精品老妇| 国产人妻一区二区三区在| 1000部很黄的大片| 久久精品国产99精品国产亚洲性色| 成人国产麻豆网| 久久久精品大字幕| 舔av片在线| 看十八女毛片水多多多| 麻豆成人午夜福利视频| av播播在线观看一区| 99热这里只有精品一区| 又粗又硬又长又爽又黄的视频| 日日摸夜夜添夜夜添av毛片| 91精品伊人久久大香线蕉| 天堂中文最新版在线下载 | 激情 狠狠 欧美| 精品酒店卫生间| 男插女下体视频免费在线播放| 又粗又爽又猛毛片免费看| 一级av片app| 国产精品国产三级国产av玫瑰| 久久久久久久久中文| 91久久精品国产一区二区成人| 夜夜看夜夜爽夜夜摸| 狂野欧美白嫩少妇大欣赏| 男人舔女人下体高潮全视频| 午夜精品一区二区三区免费看| 久久久精品欧美日韩精品| 国产黄片美女视频| 久久久久网色| 国产一区二区三区av在线| 中文在线观看免费www的网站| 一级毛片电影观看 | 深爱激情五月婷婷| 在线a可以看的网站| 欧美xxxx性猛交bbbb| 精华霜和精华液先用哪个| 国产精品麻豆人妻色哟哟久久 | 婷婷色av中文字幕| 一本—道久久a久久精品蜜桃钙片 精品乱码久久久久久99久播 | 大香蕉97超碰在线| 日产精品乱码卡一卡2卡三| 色尼玛亚洲综合影院| 变态另类丝袜制服| 国产毛片a区久久久久| 国产老妇伦熟女老妇高清| 亚洲成人精品中文字幕电影| 日本一本二区三区精品| 综合色丁香网| 亚洲中文字幕一区二区三区有码在线看| 免费在线观看成人毛片| 亚洲国产精品专区欧美| 18+在线观看网站| 午夜福利在线观看吧| 日韩av在线免费看完整版不卡| 欧美极品一区二区三区四区| 高清av免费在线| 九草在线视频观看| 午夜爱爱视频在线播放| 亚洲欧洲国产日韩| 小蜜桃在线观看免费完整版高清| 好男人视频免费观看在线| 日韩精品青青久久久久久| 国产久久久一区二区三区| a级毛片免费高清观看在线播放| 色综合色国产| 国产精品一区www在线观看| www日本黄色视频网| 中文字幕久久专区| 女人十人毛片免费观看3o分钟| 纵有疾风起免费观看全集完整版 | 欧美一级a爱片免费观看看| 免费看美女性在线毛片视频| 亚洲第一区二区三区不卡| 欧美成人a在线观看| 国产乱人视频| 精品99又大又爽又粗少妇毛片| 亚州av有码| 精品少妇黑人巨大在线播放 | 国产三级中文精品| 午夜精品国产一区二区电影 | 日韩人妻高清精品专区| 欧美高清性xxxxhd video| 一级毛片电影观看 | 乱人视频在线观看| 在线天堂最新版资源| 麻豆精品久久久久久蜜桃| 丰满人妻一区二区三区视频av| 亚洲,欧美,日韩| 一本一本综合久久| 26uuu在线亚洲综合色| 亚洲av中文字字幕乱码综合| 校园人妻丝袜中文字幕| 三级男女做爰猛烈吃奶摸视频| 日本欧美国产在线视频| 看非洲黑人一级黄片| 欧美潮喷喷水| 色播亚洲综合网| 神马国产精品三级电影在线观看| 高清在线视频一区二区三区 | 午夜视频国产福利| 97热精品久久久久久| 村上凉子中文字幕在线| 久久久久久久亚洲中文字幕| 久久6这里有精品| 亚洲精品乱码久久久久久按摩| 日韩欧美国产在线观看| 国产黄色小视频在线观看| 国产精品综合久久久久久久免费| 亚洲精品一区蜜桃| 舔av片在线| 亚洲中文字幕一区二区三区有码在线看| 超碰97精品在线观看| 精品久久久久久久久av| 精品不卡国产一区二区三区| 男人狂女人下面高潮的视频| 国产成人福利小说| 亚洲经典国产精华液单| 麻豆av噜噜一区二区三区| 又粗又硬又长又爽又黄的视频| 国语自产精品视频在线第100页| 麻豆精品久久久久久蜜桃| 国产男人的电影天堂91| h日本视频在线播放| 免费在线观看成人毛片| 亚洲国产欧美在线一区| 国产精品一区二区三区四区免费观看| 最近2019中文字幕mv第一页| 色综合亚洲欧美另类图片| 内射极品少妇av片p| 最近2019中文字幕mv第一页| av免费在线看不卡| 精品少妇黑人巨大在线播放 | 久久精品夜夜夜夜夜久久蜜豆| 欧美日本视频| 亚洲精品国产av成人精品| eeuss影院久久| 国产精品不卡视频一区二区| 国产亚洲av片在线观看秒播厂 | 色吧在线观看| 精品少妇黑人巨大在线播放 | 亚洲av成人精品一二三区|