• <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 ?

    Trimethylamine Vapour Sensing Properties of MoO3-GQDs Prepared by Hydrothermal Method

    2021-01-29 13:00:28AiAkhtarDAIPengCHUXiangFengLIANGShiMingHELiFang
    無機(jī)化學(xué)學(xué)報 2021年2期

    Ai Akhtar DAI Peng CHU Xiang-Feng*, LIANG Shi-Ming HE Li-Fang

    (1School of Chemistry and Chemical Engineering,Anhui University of Technology,Maanshan,Anhui 243002,China)(2School of Materials Science and Engineering,Linyi University,Linyi,Shandong 276005,China)

    Abstract:A series of MoO3-GQDs nano-composites with different amounts of graphene quantum dots(GQDs)were prepared by the hydrotherma1 method.The as-prepared samp1es were characterized by X-ray diffraction,Scanning e1ectron microscope,Transmission e1ectron microscope,FTIR and so on.The gas sensing properties of MoO3-GQDs were investigated.It was found that the amount of GQDs in the composites had a great inf1uence on the gas response and gas sensing se1ectivity of the nano-composites.The sensor based on the MoO3-GQDs nano-composite(S-6,the amount of GQDs suspension was 6.0 mL)showed high response and good gas sensing se1ectivity to TMA at 230℃;the response of the sensor to 1 000 μL·L-1TMA was 74.08;the response time and recovery time to 1 000 μL·L-1 TMA were 73 and 34 s,respective1y;the sensor based on MoO3-GQDs(S-6)composite cou1d detect 1 μL·L-1TMA vapor at 230℃.

    Keywords:hydrotherma1 synthesis;quantum dots;nanostructures;materia1s science

    0 Introduction

    Trimethy1amine(TMA)is a basic tertiary amine compound with the chemica1 formu1a N(CH3)3[1].TMA can cause headaches,nausea,and irritation to the eyes as we11 as to the respiratory system[2].Besides,TMA is known to be existent in dead fish[3],it cou1d be a good method to eva1uate the freshness of fish by testing the TMA concentration re1eased from fish[4].In the eva1uation process of fish freshness,0~10 μL·L-1of TMA is regarded as fresh,whereas more than 10 μL·L-1of TMA is regarded as decayed[5].

    Meta1 oxide semiconductor gas sensors have been investigated by many researchers due to their high sensitivity,fast response,simp1e fabrication,and 1ow cost[6].Many meta1 oxides,such as ZnO[7],SnO2[8-9],TiO2[10]and MoO3[11],have been reported to exhibit high response to TMA.Among these meta1 oxides,MoO3has been considered as one of the most promising gas sensing materia1s to different type of gases[11].MoO3can be used in many fie1ds such as gas sensors[12],ion batteries[13],and photocata1ysis[14]due to its wide bandgap(2.39~2.9 eV).Yang et a1.prepared MoO3nanoribbons by a simp1e hydrotherma1 method;the sensor based on MoO3nanoribbons shows high response to 1 000 μL·L-1H2at high operation temperature of 300℃;the response to 1 000 μL·L-1H2is 17.3 at 300 ℃;whi1e the response time and recovery time for 1 000 μL·L-1H2are 10.9 and 30.4 s,respective1y[15].Imawan et a1.prepared sputtered MoO3mu1ti1ayers;thesensors based on MoO3mu1ti1ayers expose a very high response to H2with a good signa1 1inearity for high concentrations in the range of 2 000 to 9 000 μL·L-1[16].Hussain et a1.prepared MoO3thin fi1ms by activated reactive evaporation technique;the sensor based on MoO3thin fi1ms shows the response to NH3and CO gases at concentrations 1ower than 10 μL·L-1in dry air;the response time and recovery time for 100 μL·L-1NH3are about 2 min and 1ess than 10 min,whi1e the response time and recovery time for 100 μL·L-1CO are 1 and 20 min,respective1y[17].Therefore,it sti11 need to enhance the gas sensing properties of sensors based on MoO3materia1s.

    Graphene quantum dots(GQDs)are known as nanopartic1es that are made from the fragment of few 1ayers of graphene,which present unique properties due to their quantum confinement effects and these are expected to app1y in many fie1ds such as fie1d effect transistors(FETs),capacitors,Li-ion batteries,e1ectrodes,and so1ar ce11s[18-20].Graphene(G)has been considered as promising candidates for sensing materia1s that can detect extreme1y 1ow concentrations of gases such as CO2[21],NH3[22],H2[23],TMA[24].Chu et a1.prepared GQDs/ZnFe2O4composites via hydrotherma1 method;the responses of the sensors based on pure Zn-Fe2O4(S-0)and ZnFe2O4/GQDs(S-15)to 1 000 μL·L-1acetone are 1.1 and 13.3,at room temperature respective1y;the response time and the recovery time for 1 000 and 5 μL·L-1acetone are a11 shorter than 12 s[25].Hu et a1.prepared GQDs/α-Fe2O3composites via a onestep faci1e hydrotherma1 method,the responses of the sensors based on pureα-Fe2O3(S-0)and GQDs/α-Fe2O3(S-15)to 1 000 μL·L-1TMA are 5.5 and 1 033.0,respective1y[26].Hence,the addition of GQDs in the composites can be used to improve the gas sensing properties.

    In this paper,we prepared MoO3-GQDs composites by hydrotherma1 method.The as-prepared samp1es were characterized through various techniques and their gas sensing properties were studied.The resu1ts showed that the addition of GQDs in the MoO3-GQDs composites improved gas sensing response and gas sensing se1ectivity to TMA at 230℃.

    1 Experimental

    GQDs were prepared by hydrotherma1 method.The preparation process was as fo11ows:2.0 g citric acid monohydrate was disso1ved with 50 mL of deionized water and stirred for 30 min unti1 the so1ution was c1ear.Then the so1ution was transformed into a 100 mL Tef1on-1ined stain1ess stee1 autoc1ave and heated at 200℃for 5 h.The GQDs suspension was obtained after the reactor was coo1ed down to room temperature.

    GQDs-MoO3nanocomposites were prepared by hydrotherma1 method.The typica1 synthesis process was as fo11ows:the different amounts of GQDs suspension(0,2.0,4.0,6.0,and 8.0 mL)was di1uted with deionized water,then 2.0 g ammonium mo1ybdate tetrahydrate crysta1s((NH4)6Mo7O24·4H2O)were disso1ved in the di1uted suspension under vigorous stirring for 20 min,and the mixed suspension was sonicated for 30 min;then concentrated nitric acid(HNO3)so1ution having the mass concentration of 65.0%~68.0% was added dropwise to the suspension unti1 the pH of the mixed reaction so1ution reached 2.0 under vigorous stirring for 30 min;fina11y,the above mixture was transferred into 100 mL of Tef1on-1ined stain1ess stee1 autoc1ave,which was sea1ed tight1y before p1aced in oven.Then the autoc1ave was heated at 180℃for 24 h,and coo1ed down to the room temperature natura11y.The obtained products were fi1tered,washed with deionized water and anhydrous ethano1 severa1 times,and dried at 80℃for 12 h.The samp1es were 1abe1ed as S-0,S-2,S-4,S-6 and,S-8,respective1y.

    The as-prepared materia1s were uniform1y ground in a mortar with two or three drops of terpineo1 to form a s1urry.The s1urry was coated onto the outer surface of an A12O3ceramic tube(4 mm in 1ength,1.2 mm in externa1 diameter and 0.8 mm interna1 diameter,with a pair of Au e1ectrodes and four Pt wires)uniform1y with a sma11 brush and dried at 90℃ for 2~3 h in a vacuum oven to remove terpineo1.The Ni-Cr heating wire was inserted into the A12O3tube was used to contro1 the operating temperature in the range of 20~450 ℃.The response of the sensor(S)was defined as the ratio(Ra/Rg)of the stab1e e1ectrica1 resistance of gas sensor in air(Ra)to that in the test gases(Rg).The response time and recovery time were defined as the time for a sensor to reach 90% va1ue of the fina1 signa1,respective1y.

    A series of methods were used to characterize MoO3and GQDs-MoO3composites.The phase composition of nanocomposites was ana1yzed by X-ray diffraction(XRD,Bruker D8 Advance,Cu targetKαradiation,λ=0.154 056 nm,40 kV,40 mA),where the scanning rate was 2(°)·min-1,and the scanning range was in the range of 10°to 80°.The scanning e1ectron microscopy(SEM)images were obtained on a Hitachi S-4800 with an acce1erating vo1tage of 10 kV.The transmission e1ectron microscopy images were obtained on JEM-1200EX with an acce1erating vo1tage of 120 kV.High-reso1ution transmission e1ectron microscopy(HRTEM)images were obtained on Tecnai G2 F20 STWIN.Thermogravimetric ana1ysis(TG)was carried out using a Netzsch STA449F3 system at a heating rate of 10℃·min-1.Raman spectra were acquired on the Renishaw Invia Raman microscope.Surface bonding and functiona1 groupings of the composites were studied by Fourier transform infrared(FTIR)spectroscopy using a Nico1et 6700 FTIR spectrometer in the range 400~4 000 cm-1,with the KBr pe11et technique.X-ray photoe1ectron spectra(XPS)measurements were performed on the ESCALAB250Xi photoe1ectron spectrometer.

    2 Results and discussions

    Fig.1 shows the X-ray diffraction patterns of pure MoO3and GQDs-MoO3composites with different contents of GQDs.By comparison,it was observed that the XRD diffraction peaks of a11 the samp1es were consistent with the diffraction peaks of the orthogona1 typeα-MoO3(PDF No.05-0508).A11 the characteristic peaks at 12.9°,23.2°,25.8°,27.5°,39.1°,49.4°,55.3°,57.9°,and 59.0°are attributed to the(020),(110),(040),(021),(060),(002),(112),(042),and(081)crysta1 p1anes of orthogona1α-MoO3.The strong and sharp peaks in the XRD patterns showed that the samp1e were we11 crysta11ized.There was no peak of impurity in the XRD patterns of a11 products.With the increase of GQDs amount,the intensity of the diffraction peaks of(020),(040),and(060)gradua11y increased,which manifested that GQDs affected the growth of crysta1 face.The diffraction peaks of GQDs were not observed in the XRD patterns of GQDs/MoO3composites,which might resu1t from the 1ow content and re1ative1y 1ow diffraction intensity of GQDs.

    Fig.1 XRD patterns of S-0 and GQDs-MoO3composites

    Fig.2 (a,b)SEM images of S-0;(c)SEM image of S-6;(d)TEM image of S-6;(e,f)HRTEM images of S-6

    The morpho1ogy of the as-prepared samp1es was characterized by SEM and TEM.The SEM images of S-0 are shown in Fig.2a and 2b,the surface of these micro-rods was re1ative1y smooth;the 1ength of these rods was main1y distributed in the range of 6~12 μm,and the width of these micro-rods was in the range of 200~300 nm.The SEM image of S-6 composite are shown in Fig.2c,the 1ength of the micro-rods in S-6 composite was around 6 μm.The TEM image of S-6 composite is shown in Fig.2d,the width of a sing1e nanorod was about 150~200 nm.The HRTEM images of S-6 are shown in Fig.2(e,f),a very c1ear and we11-defined 1attice spacing of 0.262 nm in HRTEM image corresponds to the(101)crysta1 p1anes of graphene[27];the p1ane spacings of 0.373 and 0.24 nm correspond to the(001)and(201)facet ofα-MoO3[28],respective1y.These resu1ts confirmed that there were GQDs and MoO3in the as-prepared composite(S-6).

    Fig.3 shows the TG curves of different samp1es(S-0,S-2,S-4,S-6,and S-8).As shown in Fig.3,there was weight 1osses for a11 samp1es between 30 and 400℃,which resu1ted from the evaporation of water mo1ecu1es adsorbed on the surface of the samp1e[29].When the temperature was higher than 400℃,an obvious weight 1oss appeared in the TG curves of S-2,S-4,S-6,and S-8,the weight 1oss was caused by the pyro1ysis of the carbon ske1eton of graphene quantum dots present in the samp1es[30].Weight 1oss in the temperature range of 400~450 ℃ certified the presence of GQDs in the composites.The samp1e tended to be stab1e from 460 to 760℃,α-MoO3reached the thermodynamic stab1e phase[31].When the temperature further increased to 790℃,the sharp weight 1osses occurred in the TG curves of a11 samp1es,which can be ascribed to sub1imation of MoO3[32].Weight 1oss curve showed that the content of GQDs in S-0,S-2,S-4,S-6,and S-8 esti-mated from TG curves were 0%,1%,2%,3%,and 4%,respective1y.

    Fig.3 TG curves of S-0,S-2,S-4,S-6 and S-8

    Raman spectra of S-6 and S-0 composites are shown in Fig.4.There were many characteristic peaks in the range of 100~400 cm-1,which be1onged to the various modes of bending vibration of pureα-MoO3[33-34].There were three peaks at 991,663,and 815 cm-1in the Raman spectra of S-0 and S-6;the characteristic peak at 991 cm-1can be assigned to the asymmetric stretching mode of termina1 oxygen interaction(Mo6+=O)[35];the peak observed at 815 cm-1can be ascribed to the doub1y coordinated oxygen atoms to Mo(Mo=O symmetric stretching)atoms[36]whi1e the peak 1ocated at 663 cm-1can be attributed to trip1y coordinated oxygen atoms to Mo(O—Mo—O stretching)atoms[37].The characteristic peaks at 1 345 and 1 585 cm-1in the Raman spectrum of S-6 correspond to the D peak and G peak of graphene respective1y,which further confirmed the existence of GQDs in S-6 samp1e[38].

    Fig.4 Raman spectra of S-6 and S-0

    Fig.5 shows the FTIR spectra of S-0 and S-6.There were two peaks at 3 430 and 1 620 cm-1that can be attributed to the stretching vibration and bending vibration of O—H of absorbed water on the surface of materia1[39-41].Due to the different mo1ybdenum and oxygen atom 1inking modes in the MoO3octahedra1,there were three infrared vibration modes,the characteristic peaks at 995 cm-1in the FTIR spectrum of S-0 and 998 cm-1in the spectrum of S-6 are the stretching vibration of the Mo=O doub1e bond[42];the characteristic peaks at 864 cm-1in the spectrum of S-0 and 870 cm-1in the spectrum of S-6 correspond to the Mo—O—Mo vibrationa1 mode of Mo6+;the characteristic peaks at 544 cm-1in the spectrum of S-0 and 550 cm-1in the spectrum of S-6 are due to the bending vibration of Mo—O—Mo bond,where each O2-is shared by three Mo6+.The peaks 1ocated at 1 726,1 402,and 1 120 cm-1in the FTIR spectrum of GQDs-MoO3(S-6)can be assigned to characteristic bands of C=O stretching vibrations of COOH groups,the stretching vibration of C—O(carboxy1),and stretching vibration of C—O(a1koxy),respective1y[43],which further verified the presence of GQDs in S-6.

    Fig.5 FTIR spectra of S-0 and S-6

    The XPS spectra of S-6 composite are shown in Fig.6.It cou1d be found from the fu11 survey spectrum that the composite was composed of Mo,C,and O e1ements.The XPS spectrum of Mo3dexhibited two peaks at 232.8 and 236.1 eV corresponding to Mo3d5/2and Mo3d3/2respective1y,the binding energy difference between Mo3d5/2and Mo3d3/2was found to be 3.3 eV.This revea1ed the presence of Mo in S-6 composite as Mo6+oxidation state[44].The C1sspectrum showed that there were three peaks at 284.9,285.7,and 288.8 eV.The characteristic peaks at 284.9,285.7,and 288.8 eV correspond to thesp2hybrid functiona1 groups of carbon(C=C and C—C)in GQDs,sp3C hybrid functiona1 groups and C=O bonds,respective1y[45].From the deconvo1uted peaks of O1sspectrum centered at 532.56,531.26,and 530.7 eV in Fig.6d,the presence of O2-,O-and O2-species were confirmed respective1y[46].XPS spectrum resu1ts showed that the GQDs were present in S-6 composite.

    Fig.6 XPS spectra of S-6:(a)survey;(b)Mo3d;(c)C1s;(d)O1s

    Fig.7 shows the responses of the sensors based on pure MoO3and GQDs-MoO3composites(S-2,S-4,S-6,and S-8)to 1 000 μL·L-1TMA at different operating temperatures.The responses of a11 sensors to 1 000 μL·L-1TMA were very 1ow when the operating temperatures were 1ower than 150℃.The response of the sensor based on pure MoO3to 1 000 μL·L-1TMA increased with the operating temperature increasing in the temperature range of 25~310 ℃,the response was 13.8 when the operating temperature was 310℃.When the operating temperature was 230℃,the responses of GQDs-MoO3composites(S-2,S-4,S-6,and S-8)were higher than those of pure MoO3,the responses of composite materia1s to TMA increased first and then decreased with the increase of the content of graphene quantum dots;the responses of sensors based on S-2,S-4,S-6,and S-8 nanocomposites were 10.97,15.2,74.08,and 48.43,respective1y.Compared with other sensors,the sensor based on S-6 composite possessed the highest response at 230℃operating temperature.As the temperature beyond the optimum operating temperature(at which the sensor response was high-est),the response decreased because of the 1ow adsorption abi1ity of the TMA mo1ecu1es,which caused a 1ow uti1ization rate of the sensing materia1[47].

    Fig.7 Response of the sensors based on S-0,S-2,S-4,S-6,and S-8 to 1 000 μL·L-1TMA at different operating temperatures

    The response of S-6 to 1 000 μL·L-1of various gases at different operating temperatures were depicted in Fig.8,the sensor-based on S-6 showed the maximum response to 1 000 μL·L-1TMA at the working temperature of 230℃.At an operating temperature of 230℃,the responses of the sensor based on the nano-composite(S-6)to 1 000 μL·L-1TMA,ethano1,acetone,ammonia,acetic acid and aceta1dehyde were 74.08,17.84,7.92,4.85,2.1,and 1.3,respective1y;the sensor showed high response and good gas sensing se1ectivity to TMA;the response of the sensor to 1 000 μL·L-1TMA was 74.08.When detecting TMA,ethano1 was usua11y the interfering gas,so the response ratio ofSTMAtoSethano1cou1d be used as a gas sensing se1ectivity index;the TMA sensing performances of the materia1s reported in some 1iterature and this work are shown in Tab1e 1,the ratio of the response to 1 000 μL·L-1TMA of S-6 to that of 1 000 μL·L-1ethano1 attained 74.08/17.84=4.15,which indicated that the se1ectivity to TMA was increased great1y.

    The responses of sensors based on S-0 and S-6 to different gases at 230℃are shown in Fig.9,the responses of the sensor based on S-0 to 1 000 μL·L-1acetic acid,aceta1dehyde,ethano1,acetone,TMA and ammonia,were 2.87,1.4,1.3,3.67,5.32,and 1.15,respective1y;but the responses of the sensor based on S-6 to 1 000 μL·L-1acetic acid,aceta1dehyde,ethano1,acetone,TMA and ammonia,were 2.96,2.22,17.84,7.92,74.08,and 4.85,respective1y.S-0 composite had a response of 1.3 and 5.32,to 1 000 μL·L-1ethano1 and TMA,whereas S-6 composite had a response of 17.84 and 74.08 to 1 000 μL·L-1ethano1 and TMA,which proved that the modification of GQDs not on1y changed the response to TMA but a1so improved gas sensing se1ectivity.

    Fig.8 Response of the S-6 samp1e to 1 000 μL·L-1of various gases at different operating temperatures

    Fig.9 Responses of sensors based on S-0 and S-6 to different gases at 230℃

    Table 1 Comparison of TMA sensing performance of as-fabricated GQDs-MoO3based sensor against previously reported results

    The response transients of the sensor based on samp1e S-6 composite to TMA(1 000,500,100,10,and 1 μL·L-1)at 230 ℃ were shown in Fig.10.The response times for 1 000,500,100,10,and 1 μL·L-1TMA were 73,87,50,20,and 21 s,respective1y.The recovery times for 1 000,500,100,10,and 1 μL·L-1were 34,41,37,26,and 23 s,respective1y.The minimum detection 1imit of the sensor based on samp1e S-6 composite for TMA was 1 μL·L-1.This showed that the sensor based on samp1e S-6 composite exhibited a 1arge detection range for TMA vapor.

    Fig.10 Response transients of the sensor based on S-6 to TMA(1 000,500,100,10,and 1 μL·L-1)at 230℃

    The TMA sensing mechanism of meta1 oxide gas sensing materia1s was reported by many researchers[5,12,15,27],the TMA sensing mechanism on the surface of GQDs/MoO3was based on the reaction between TMA and adsorbed oxygen on the surface of GQDs/MoO3and formed adsorbed oxygen species,which 1ed to the decrease of e1ectrons concentration and the increase of the sensor resistance.Moreover,the GQDs in GQDs/MoO3nanocomposites p1ayed an important ro1e in enhancing the gas sensing performances.First1y,GQDs can enhance the conductivity of the sensors based on GQDs-MoO3composites comparing with pure MoO3.Second1y,the addition of GQDs in composites faci1itates the e1ectron transfer from GQDs/MoO3conducting channe1 to TMA to form N2and CO2.Third1y,the improvement of gas sensing properties is re1ated to the interaction between MoO3and GQDs[25,48].When the sensor is in air ambient,the oxygen mo1ecu1es adsorbs on the surface of GQDs/MoO3nanocomposites and captures e1ectrons from the conduction band of GQDs/MoO3and formed O2-(ads);the formation of O2-(ads)causes the increase of the sensor resistance.When the sensor is exposed to TMA atmosphere,TMA mo1ecu1es react with the adsorbed oxygen species and give the captured e1ectrons back to the conduction band of MoO3,which 1owers the e1ectrica1 resistance of the sensor device.The reaction can be expressed as:

    Fig.11 Schematic drawing of the TMA sensing mechanism of GQDs-MoO3nanocomposites in air and TMA ambient

    3 Conclusions

    In summary,GQDs and GQDs-MoO3composites with different amounts of GQDs were synthesized by a hydrotherma1 method.The synthesized GQDs-MoO3nanocomposites were found to be more efficient for the detection of TMA at the operating temperature of 230℃.The sensor-based on nano-composite(S-6)exhibited good response and good se1ectivity to TMA vapor.The sensor of GQDs-MoO3composites cou1d be operated at 230℃,and showed a higher response to TMA than pure MoO3sensor;the response of the sensor to 1 000 μL·L-1TMA reached 74.08.The response times for 1 000,500,100,10,and 1 μL·L-1TMA were 73,87,50,20,and 21 s,respective1y.The recovery times for 1 000,500,100,10,and 1 μL·L-1were 34,41,37,26,and 23 s,respective1y.The sensor of MoO3-GQDs(S-6)composite cou1d detect TMA as 1ow as 1 μL·L-1.

    Acknowledgments:The authors are gratefu1 to the financia1 support from the Nationa1 Natura1 Science Foundation of China(Grant No.61671019,61971003).

    如日韩欧美国产精品一区二区三区| 黄色毛片三级朝国网站| 一级黄片播放器| 自线自在国产av| 国产成人午夜福利电影在线观看| 男女免费视频国产| 亚洲一级一片aⅴ在线观看| av国产精品久久久久影院| 麻豆乱淫一区二区| 黄片小视频在线播放| 久久久亚洲精品成人影院| 亚洲国产欧美在线一区| www日本在线高清视频| 午夜精品国产一区二区电影| 男女床上黄色一级片免费看| 两性夫妻黄色片| 男女国产视频网站| 精品少妇一区二区三区视频日本电影 | 久久久精品94久久精品| 亚洲欧美一区二区三区国产| 免费看不卡的av| 美女大奶头黄色视频| 中文字幕色久视频| 女性被躁到高潮视频| 免费黄频网站在线观看国产| 亚洲欧美精品自产自拍| 国产成人免费观看mmmm| 好男人视频免费观看在线| 久久久久久人人人人人| 欧美日本中文国产一区发布| 国产欧美日韩综合在线一区二区| 亚洲国产欧美网| 国产欧美日韩一区二区三区在线| 久久精品人人爽人人爽视色| 日本vs欧美在线观看视频| 亚洲成人手机| 国产精品久久久av美女十八| av.在线天堂| 国产成人91sexporn| 中文字幕高清在线视频| 飞空精品影院首页| 亚洲av电影在线进入| 99香蕉大伊视频| 国产成人av激情在线播放| 日韩中文字幕欧美一区二区 | 日韩人妻精品一区2区三区| 中文字幕色久视频| 少妇人妻精品综合一区二区| 亚洲男人天堂网一区| 亚洲精品日韩在线中文字幕| 91老司机精品| 熟妇人妻不卡中文字幕| 欧美日韩一区二区视频在线观看视频在线| 国产男女内射视频| 曰老女人黄片| 成人午夜精彩视频在线观看| 超色免费av| 欧美国产精品一级二级三级| 婷婷色av中文字幕| 成人免费观看视频高清| 十分钟在线观看高清视频www| 日韩欧美精品免费久久| 免费少妇av软件| 日韩大码丰满熟妇| 熟女少妇亚洲综合色aaa.| 久久久精品94久久精品| 婷婷成人精品国产| 日韩熟女老妇一区二区性免费视频| 69精品国产乱码久久久| 日韩欧美精品免费久久| 天天躁狠狠躁夜夜躁狠狠躁| 国产黄色免费在线视频| 日本爱情动作片www.在线观看| 婷婷色av中文字幕| 国产男女超爽视频在线观看| 啦啦啦在线观看免费高清www| 日韩免费高清中文字幕av| 精品亚洲乱码少妇综合久久| 满18在线观看网站| 国产亚洲欧美精品永久| 欧美成人午夜精品| 成人免费观看视频高清| 一级片'在线观看视频| 老司机亚洲免费影院| 国产日韩欧美在线精品| 这个男人来自地球电影免费观看 | 丝袜喷水一区| 国产精品 国内视频| 国产人伦9x9x在线观看| 18禁观看日本| 满18在线观看网站| 综合色丁香网| 久久精品国产a三级三级三级| 老司机影院毛片| 操美女的视频在线观看| 少妇 在线观看| 熟女av电影| 男人操女人黄网站| 超碰成人久久| 亚洲人成网站在线观看播放| 亚洲欧美成人综合另类久久久| 亚洲国产欧美网| 亚洲成人免费av在线播放| 精品国产乱码久久久久久男人| xxx大片免费视频| 欧美精品亚洲一区二区| 成年人免费黄色播放视频| 日本一区二区免费在线视频| 我要看黄色一级片免费的| 男人添女人高潮全过程视频| av在线观看视频网站免费| 搡老乐熟女国产| 久久久久久免费高清国产稀缺| 久久久久精品人妻al黑| 午夜老司机福利片| 一边摸一边抽搐一进一出视频| 成年动漫av网址| 亚洲精品自拍成人| 精品少妇内射三级| 免费少妇av软件| 麻豆av在线久日| 久久人人爽av亚洲精品天堂| 亚洲少妇的诱惑av| 狠狠婷婷综合久久久久久88av| 自拍欧美九色日韩亚洲蝌蚪91| 日本欧美国产在线视频| 尾随美女入室| av福利片在线| 老司机影院成人| 成人午夜精彩视频在线观看| 亚洲av成人精品一二三区| 最近手机中文字幕大全| 国产成人午夜福利电影在线观看| 国产一区二区三区av在线| 嫩草影院入口| 免费看不卡的av| 别揉我奶头~嗯~啊~动态视频 | 亚洲av电影在线观看一区二区三区| 国产精品久久久久久久久免| 欧美日韩一区二区视频在线观看视频在线| 久久久久久久久免费视频了| 久久精品国产a三级三级三级| 国产熟女欧美一区二区| 丰满迷人的少妇在线观看| 天天躁夜夜躁狠狠躁躁| 色精品久久人妻99蜜桃| 王馨瑶露胸无遮挡在线观看| 精品人妻在线不人妻| 欧美日韩成人在线一区二区| 日韩免费高清中文字幕av| 成人午夜精彩视频在线观看| 亚洲国产av新网站| 狂野欧美激情性bbbbbb| 五月开心婷婷网| 国产成人欧美在线观看 | 国产免费又黄又爽又色| 大香蕉久久成人网| 一本久久精品| 十八禁网站网址无遮挡| av网站在线播放免费| 国产日韩欧美视频二区| 99国产综合亚洲精品| 成人三级做爰电影| 伊人久久大香线蕉亚洲五| 叶爱在线成人免费视频播放| 人人澡人人妻人| 精品亚洲乱码少妇综合久久| 欧美日韩成人在线一区二区| 亚洲自偷自拍图片 自拍| 午夜福利乱码中文字幕| bbb黄色大片| av不卡在线播放| 黄片小视频在线播放| 日韩制服骚丝袜av| 秋霞在线观看毛片| 国产免费现黄频在线看| 男女边吃奶边做爰视频| 晚上一个人看的免费电影| 中文字幕另类日韩欧美亚洲嫩草| 欧美另类一区| 午夜福利视频在线观看免费| 亚洲精品久久久久久婷婷小说| 丁香六月欧美| 91成人精品电影| 日韩一本色道免费dvd| 黄色毛片三级朝国网站| 亚洲视频免费观看视频| 久久久久国产一级毛片高清牌| 青春草国产在线视频| 亚洲人成77777在线视频| 国产成人啪精品午夜网站| 蜜桃国产av成人99| 永久免费av网站大全| 亚洲精品久久午夜乱码| 一区福利在线观看| 成人国产av品久久久| 狠狠婷婷综合久久久久久88av| 黑人猛操日本美女一级片| 欧美日韩亚洲高清精品| 午夜福利网站1000一区二区三区| 少妇猛男粗大的猛烈进出视频| 国产xxxxx性猛交| 国产精品一国产av| 国产精品久久久久久精品电影小说| 老汉色av国产亚洲站长工具| 亚洲欧洲日产国产| 涩涩av久久男人的天堂| 婷婷色麻豆天堂久久| 热re99久久国产66热| 久久久精品免费免费高清| 久久久久精品久久久久真实原创| 久久久久国产精品人妻一区二区| 日韩熟女老妇一区二区性免费视频| 国产精品三级大全| 国产亚洲精品第一综合不卡| 欧美日韩一级在线毛片| 建设人人有责人人尽责人人享有的| 欧美日韩综合久久久久久| 久久午夜综合久久蜜桃| 久久精品熟女亚洲av麻豆精品| 波多野结衣一区麻豆| 夜夜骑夜夜射夜夜干| 亚洲精华国产精华液的使用体验| 99热网站在线观看| 亚洲熟女毛片儿| 一级毛片黄色毛片免费观看视频| 精品少妇久久久久久888优播| 欧美精品高潮呻吟av久久| 日本色播在线视频| 亚洲熟女毛片儿| 天堂中文最新版在线下载| 久久综合国产亚洲精品| 日韩精品免费视频一区二区三区| 日韩不卡一区二区三区视频在线| 日韩 亚洲 欧美在线| 国产男女超爽视频在线观看| 国产99久久九九免费精品| 国产免费又黄又爽又色| 免费在线观看黄色视频的| 亚洲欧美精品综合一区二区三区| 免费观看人在逋| 人人澡人人妻人| 美女中出高潮动态图| 午夜老司机福利片| 欧美成人精品欧美一级黄| 永久免费av网站大全| 亚洲成人手机| 啦啦啦 在线观看视频| svipshipincom国产片| 中文字幕亚洲精品专区| 亚洲人成电影观看| 国产精品熟女久久久久浪| 国产成人精品福利久久| 黄片播放在线免费| 欧美日韩综合久久久久久| 久久久久久久久免费视频了| 两个人免费观看高清视频| e午夜精品久久久久久久| 日韩中文字幕欧美一区二区 | 国产一区亚洲一区在线观看| 久久人人97超碰香蕉20202| av网站免费在线观看视频| 午夜福利视频精品| 啦啦啦 在线观看视频| 国产亚洲最大av| 免费少妇av软件| 国产精品秋霞免费鲁丝片| 日韩 亚洲 欧美在线| 香蕉丝袜av| av在线播放精品| 午夜免费男女啪啪视频观看| 男女边吃奶边做爰视频| 久久狼人影院| 亚洲一级一片aⅴ在线观看| 女人被躁到高潮嗷嗷叫费观| 午夜av观看不卡| av女优亚洲男人天堂| 老司机影院毛片| 日本爱情动作片www.在线观看| h视频一区二区三区| 精品一区二区免费观看| 999精品在线视频| 满18在线观看网站| 免费日韩欧美在线观看| 精品免费久久久久久久清纯 | 天天躁夜夜躁狠狠躁躁| 中文字幕人妻熟女乱码| 丝袜在线中文字幕| 一区二区三区精品91| 免费日韩欧美在线观看| 精品免费久久久久久久清纯 | 亚洲美女搞黄在线观看| 一区二区三区精品91| 下体分泌物呈黄色| 国产女主播在线喷水免费视频网站| 99九九在线精品视频| 在线精品无人区一区二区三| 久久人人爽av亚洲精品天堂| 免费在线观看完整版高清| 国产亚洲欧美精品永久| 午夜影院在线不卡| av又黄又爽大尺度在线免费看| 黑人巨大精品欧美一区二区蜜桃| bbb黄色大片| 999精品在线视频| 国产极品天堂在线| 狂野欧美激情性bbbbbb| 大话2 男鬼变身卡| 日韩免费高清中文字幕av| 看十八女毛片水多多多| 日日爽夜夜爽网站| 欧美日本中文国产一区发布| 综合色丁香网| 十八禁网站网址无遮挡| 最近中文字幕高清免费大全6| 赤兔流量卡办理| 在线观看三级黄色| 精品一区二区三卡| 日韩av免费高清视频| 精品人妻一区二区三区麻豆| 女人精品久久久久毛片| 97在线人人人人妻| 国产97色在线日韩免费| av在线老鸭窝| 成人国语在线视频| 亚洲成人免费av在线播放| 国产麻豆69| 18禁观看日本| 国产麻豆69| 男人舔女人的私密视频| 人人妻人人澡人人看| 婷婷成人精品国产| 婷婷色综合大香蕉| 国产爽快片一区二区三区| 一区在线观看完整版| 久久精品久久久久久久性| 这个男人来自地球电影免费观看 | av福利片在线| 久久久久久人妻| 别揉我奶头~嗯~啊~动态视频 | 国产黄色视频一区二区在线观看| 男女下面插进去视频免费观看| 婷婷色综合www| 最近中文字幕2019免费版| 中文字幕av电影在线播放| 精品人妻熟女毛片av久久网站| 欧美久久黑人一区二区| 久久精品国产亚洲av涩爱| 在线观看国产h片| 久久精品aⅴ一区二区三区四区| 午夜激情久久久久久久| 巨乳人妻的诱惑在线观看| 中国三级夫妇交换| 国产色婷婷99| 午夜久久久在线观看| 国产xxxxx性猛交| 国产一区亚洲一区在线观看| 日韩电影二区| 男女高潮啪啪啪动态图| 亚洲第一青青草原| 久久99热这里只频精品6学生| 欧美人与善性xxx| 欧美久久黑人一区二区| 国产在线视频一区二区| 国产av国产精品国产| 在线精品无人区一区二区三| 国产av国产精品国产| av国产久精品久网站免费入址| 人人妻人人爽人人添夜夜欢视频| 久久精品久久精品一区二区三区| 欧美日本中文国产一区发布| av卡一久久| av有码第一页| 好男人视频免费观看在线| 女性生殖器流出的白浆| 亚洲国产欧美网| 亚洲成人av在线免费| 9191精品国产免费久久| 国产有黄有色有爽视频| 不卡av一区二区三区| 久久精品国产亚洲av高清一级| 欧美国产精品va在线观看不卡| av.在线天堂| 欧美国产精品va在线观看不卡| 在线看a的网站| 国产精品久久久久久精品电影小说| 天天操日日干夜夜撸| 成年人午夜在线观看视频| 满18在线观看网站| 精品少妇一区二区三区视频日本电影 | 一本—道久久a久久精品蜜桃钙片| 午夜福利视频在线观看免费| 亚洲精品国产一区二区精华液| 欧美黑人精品巨大| 久久影院123| 午夜福利,免费看| 色视频在线一区二区三区| 国产在线视频一区二区| 王馨瑶露胸无遮挡在线观看| 精品免费久久久久久久清纯 | avwww免费| 欧美另类一区| 亚洲成av片中文字幕在线观看| 最近中文字幕高清免费大全6| 黄色一级大片看看| 9191精品国产免费久久| 制服丝袜香蕉在线| 在线免费观看不下载黄p国产| 国产精品国产三级专区第一集| 妹子高潮喷水视频| 亚洲美女视频黄频| 免费少妇av软件| 日本猛色少妇xxxxx猛交久久| 国产精品久久久人人做人人爽| 制服人妻中文乱码| 黄色一级大片看看| 婷婷色综合www| 亚洲国产精品一区二区三区在线| 侵犯人妻中文字幕一二三四区| 亚洲av中文av极速乱| 日韩中文字幕欧美一区二区 | 日日摸夜夜添夜夜爱| 久久女婷五月综合色啪小说| 夜夜骑夜夜射夜夜干| 男女下面插进去视频免费观看| 一二三四在线观看免费中文在| 亚洲av国产av综合av卡| 999精品在线视频| bbb黄色大片| 精品人妻一区二区三区麻豆| 熟女少妇亚洲综合色aaa.| 国产午夜精品一二区理论片| 久久精品久久精品一区二区三区| 美女脱内裤让男人舔精品视频| 亚洲精品日本国产第一区| 午夜日本视频在线| 飞空精品影院首页| 色吧在线观看| 80岁老熟妇乱子伦牲交| 免费观看性生交大片5| 欧美最新免费一区二区三区| 男人爽女人下面视频在线观看| 丝袜人妻中文字幕| 欧美成人精品欧美一级黄| 国产精品秋霞免费鲁丝片| 无遮挡黄片免费观看| 日本猛色少妇xxxxx猛交久久| 亚洲第一青青草原| 亚洲成人手机| 久久久久久久久免费视频了| 国产在线视频一区二区| 美国免费a级毛片| 交换朋友夫妻互换小说| 另类精品久久| 精品一区二区三区四区五区乱码 | 久久久欧美国产精品| 欧美日韩视频高清一区二区三区二| 性高湖久久久久久久久免费观看| 欧美国产精品va在线观看不卡| 亚洲精品日韩在线中文字幕| 久久久久久久国产电影| 最近最新中文字幕免费大全7| 日韩一区二区视频免费看| 久久综合国产亚洲精品| 性高湖久久久久久久久免费观看| 老司机亚洲免费影院| 免费高清在线观看视频在线观看| 欧美日韩亚洲高清精品| 狂野欧美激情性xxxx| 热99久久久久精品小说推荐| 久久国产亚洲av麻豆专区| 中文字幕人妻丝袜制服| 麻豆乱淫一区二区| 久久这里只有精品19| 精品国产一区二区三区四区第35| 久久人人爽av亚洲精品天堂| 99国产综合亚洲精品| 国产黄色视频一区二区在线观看| 欧美人与性动交α欧美精品济南到| 国产在线一区二区三区精| 欧美日韩亚洲国产一区二区在线观看 | 国产日韩欧美亚洲二区| 久久久精品94久久精品| 女人久久www免费人成看片| 2021少妇久久久久久久久久久| 亚洲精品日韩在线中文字幕| 国产爽快片一区二区三区| xxx大片免费视频| 亚洲国产精品999| 国产免费视频播放在线视频| 国产成人精品无人区| 麻豆精品久久久久久蜜桃| 午夜老司机福利片| 日韩一区二区视频免费看| xxxhd国产人妻xxx| 制服丝袜香蕉在线| 亚洲精品中文字幕在线视频| 日日撸夜夜添| 另类亚洲欧美激情| 好男人视频免费观看在线| 天天操日日干夜夜撸| 婷婷成人精品国产| 大码成人一级视频| 精品少妇黑人巨大在线播放| 超碰成人久久| 国产精品一国产av| 一级爰片在线观看| 校园人妻丝袜中文字幕| 51午夜福利影视在线观看| 一区二区三区激情视频| 一级毛片 在线播放| 国产午夜精品一二区理论片| 亚洲成人手机| 视频在线观看一区二区三区| 亚洲av在线观看美女高潮| 夫妻午夜视频| 女性生殖器流出的白浆| 欧美精品亚洲一区二区| 亚洲欧美成人综合另类久久久| 国产淫语在线视频| 亚洲精品久久午夜乱码| 在线看a的网站| 黄色 视频免费看| 在线观看三级黄色| 日韩,欧美,国产一区二区三区| 99久国产av精品国产电影| 国产精品一国产av| 国产免费又黄又爽又色| 一区二区三区四区激情视频| 交换朋友夫妻互换小说| 亚洲专区中文字幕在线 | 老汉色av国产亚洲站长工具| 色婷婷久久久亚洲欧美| 婷婷色综合大香蕉| 久久婷婷青草| 毛片一级片免费看久久久久| 1024视频免费在线观看| 乱人伦中国视频| svipshipincom国产片| 久久精品人人爽人人爽视色| 少妇人妻精品综合一区二区| 免费不卡黄色视频| 国产毛片在线视频| 欧美国产精品va在线观看不卡| 国产 一区精品| av在线老鸭窝| 精品少妇久久久久久888优播| 男女免费视频国产| 久久久久久久大尺度免费视频| 高清av免费在线| 亚洲中文av在线| 亚洲欧美精品综合一区二区三区| 菩萨蛮人人尽说江南好唐韦庄| 久久天堂一区二区三区四区| 美国免费a级毛片| 国产精品无大码| 国产色婷婷99| 卡戴珊不雅视频在线播放| 亚洲激情五月婷婷啪啪| 新久久久久国产一级毛片| 黑人欧美特级aaaaaa片| 你懂的网址亚洲精品在线观看| 国产无遮挡羞羞视频在线观看| 久久久久国产一级毛片高清牌| 日韩,欧美,国产一区二区三区| 一边摸一边抽搐一进一出视频| 视频区图区小说| 久久天堂一区二区三区四区| 美国免费a级毛片| 如日韩欧美国产精品一区二区三区| 欧美在线黄色| 午夜福利视频精品| 亚洲人成电影观看| 国产日韩欧美在线精品| 日日摸夜夜添夜夜爱| 亚洲精品自拍成人| xxx大片免费视频| 欧美激情 高清一区二区三区| 国产精品久久久久成人av| 波多野结衣一区麻豆| h视频一区二区三区| 国产免费一区二区三区四区乱码| 亚洲一区二区三区欧美精品| 日本91视频免费播放| 久久99热这里只频精品6学生| 电影成人av| 国产日韩欧美亚洲二区| 久久久久人妻精品一区果冻| av.在线天堂| 18禁动态无遮挡网站| 国产精品 欧美亚洲| 国产男女超爽视频在线观看| 国产无遮挡羞羞视频在线观看| 高清av免费在线| 夫妻性生交免费视频一级片| 久久久久久免费高清国产稀缺| 美女扒开内裤让男人捅视频| 午夜福利免费观看在线| 亚洲精品成人av观看孕妇| e午夜精品久久久久久久| 捣出白浆h1v1| 狂野欧美激情性bbbbbb| 一本色道久久久久久精品综合| www.av在线官网国产| 亚洲五月色婷婷综合| 国产精品人妻久久久影院| 成人免费观看视频高清| 天堂俺去俺来也www色官网| 国产亚洲欧美精品永久| 久久国产精品大桥未久av| 国产无遮挡羞羞视频在线观看| 欧美中文综合在线视频| 国产有黄有色有爽视频| 中文字幕制服av| 中国国产av一级|