• <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 ?

    Porous reduced graphene oxide for ultrasensitive detection of nitrogen dioxide

    2023-03-14 06:52:46ZengyongChuMinXiaoQichaoDongGuochenLiTianjiaoHuYeZhangZhenhuaJiang
    Chinese Chemical Letters 2023年1期

    Zengyong Chu,Min Xiao,Qichao Dong,Guochen Li,Tianjiao Hu,Ye Zhang,Zhenhua Jiang

    College of Liberal Arts and Science,National University of Defense Technology,Changsha 410073,China

    Keywords:Gas sensors Nitrogen dioxide Graphene oxide Photo-fenton reaction Porous reduced graphene oxide

    ABSTRACT The defect engineering in graphene plays a significant role for the application of gas sensors.In this work,we proposed an efficient method to prepare ultrasensitive gas sensors based on the porous reduced graphene oxide (PRGO).Photo-Fenton etching was carried out on GO nanosheets in a controlled manner to enrich their vacancy defects.The resulting porous graphene oxide (PGO) was then drop-coated on interdigital electrodes and hydrothermal reduced at 180 °C.Controllable reduction was achieved by varying the water amount.The gas sensor based on PRGO-5 min-6 h exhibited superior sensing and selective performance toward nitrogen dioxide (NO2),with an exceptional high sensitivity up to 12 ppm-1.The theoretical limit of detection is down to 0.66 ppb.The excellent performance could be mainly attributed to the typical vacancy defects of PRGO.Some residue carboxylic groups on the edges could also facilitate the adsorption of polar molecules.The process has a great potential for scalable fabrication of high-performance NO2 gas sensors.

    Detection of volatile toxic gas is extremely important for air pollution monitoring and personal safety protection.Nitrogen dioxide (NO2) can be produced by many pathways,such as car exhaust,burning of fossil fuels,and emissions from industrial complexes[1,2].As a volatile gas,NO2can undergo photochemical reactions with other pollutants or water,generating ozone or acid rain [3].In addition,NO2with the concentration higher than 1 ppm is dangerous to the human’s respiration system [4].Furthermore,according to the U.S.Environmental Protection Agency (EPA),the annual threshold limit of NO2is only 53 ppb [5].Thus,high performance sensors with high sensitivity and ultralow limit of detection (LOD)are extremely necessary to detect trace amounts of NO2.

    In the past decades,various materials including semiconducting metal oxides [6],conducting polymers [7] and carbon nanomaterials [2] have been developed to promote the performance of chemiresistor-type gas sensors.Especially,graphene and its derivatives had attracted the most attention due to large specific surface area [8],versatile surface chemistry [9].In addition,graphene derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) could be synthesized on a large scale,which bring the gas sensors to a bright future.

    Generally,the charge transfer between adsorbed molecules and graphene governs the sensing mechanism.However,pristine 2D graphene [10] and rGO [11] displays poor gas sensitivity and limited reversibility.Many strategies have been put forward to improve their gas sensing performance.On the one hand,microstructure regulation such as chemical modification [12],porous defects[13],and heteroatom-doping [14] can enhance their sensing performance due to the increased effective adsorption sites.For example,sulfonated rGO and ethylenediamine-modified rGO exhibited 16.4 and 4.3 times enhanced responses respectively [12].In addition,there have been many theoretical calculations based on first principles to study the effects of defects and functional groups on graphene gas sensing [2,15-17].It was found that compared with intrinsic graphene,the defective and the carboxyl-functionalized graphene had higher binding energy and larger charge-transfer with NO2molecules.The porous reduced graphene oxide (PRGO)prepared by solution etching showed a sensitivity of 2.44 ppm-1in NO2detection [13].However,the sensors could seldom detect NO2molecules at concentrations lower than 20 ppb and their sensitivities are rarely above 10 ppm-1.Furthermore,the reagents used for reduction such as hydroquinone [18],NaHSO3[19],EDA[12],H3BO3[14] and hydrazine [13] inevitably caused environmental pollution or serious safety concerns.Reduction using Vitamin C (VC) is environmentally friendly,but the sensitivity of the VCreduced hydrogel was only 0.03 ppm-1[20].

    Fig.1.Schematic illustration of the fabrication process.

    In this work,we report a simple method for the efficient introduction of vacancy defects and the controllable reduction of functional groups.We firstly prepared porous graphene oxide (PGO)viathe photo-Fenton reaction,which is widely used for treating organic waste waters [21,22].The process was much faster than the solution etching using hydrogen peroxide at 100 °C [13].Large vacancy defects could be efficiently introduced by the photo-Fenton etching.The obtained PGO dispersion was then drop-coated onto the interdigital electrode (IE) and hydrothermal reduced at 180 °C[23].Some carboxylic groups could be retained after the hydrothermal reduction.The combination of vacancy defects and carboxylic groups boosts the gas sensing performance to an exceptional sensitivity and selectivity toward NO2.

    Fig.1 illustrates the fabrication process of PRGO.Aqueous GO dispersion was treated under UV light in the presence of Fenton reagents (Fe2+/H2O2) [21].For a typical photo-Fenton etching process,GO aqueous dispersion,H2O2solution,FeSO4solution and deionized water were added dropwise into a quartz tube.The mixed solution was irradiated under a UV lamp (365 nm,200 W)at room temperature.The product was dialyzed in the ultra-pure water to remove impurities.PGO dispersion was obtained and labeled as PGO-t1,wheret1is the reaction time.PGO-t1solution was transferred onto an IE surface with a microsyringe,dried and suspended in a 50 mL stainless steel autoclave containing a small amount of deionized water at the bottom.The material was hydrothermal reduced at 180°C.Reduction to PRGO was carried out in an on-chip manner.The final product was labeled as PRGO-t1-t2,wheret2is the reduction time.All the gas sensing tests were performed on a commercial testing platform.The detailed experimental procedures can be found in Supporting information.

    The response of the gas sensor was derived from the relative conductance change (ΔG/G0).

    whereG0is the original conductivity in N2,andGis the changing conductivity during exposure to NO2.

    Compared with the solution etching method using hydrogen peroxide at 100 °C [13],the etchingviaphoto-Fenton reaction is much faster.As shown in Fig.2a,the aqueous GO solution is becoming more and more transparent within 20 min.The dispersion became fully transparent after UV irradiation for 60 min,as the GO nanosheets could be completely degraded to CO2[22].The process of the photo-Fenton etching can be monitored by AFM.Figs.2b-f show the AFM images of the PGO-t1.After irradiation for 5 min,several small holes could be observed.As the etching time increases,more and more holes appear on the surface of the nanosheets,and the size of the holes is also becoming larger.Being etched for 25 min,the nanosheets could transform to graphene quantum dots (GQDs) (Fig.S1 in Supporting information).This is consistent with the results reported by Zhouet al.[21].

    Fig.2.Morphologies of PGO-t1.(a) Optical image of GO dispersions with different photo-Fenton reacting times.AFM images of (b) GO (PGO-0 min),(c) PGO-5 min,(d) PGO-10 min,(e) PGO-15 min and (f) PGO-20 min.SEM images of (g) GO and (h)PGO-5 min.(i) Pore size distribution of PGO-5 min.

    The degradation mechanism of GO to GQDsviathe photo-Fenton reaction has ever been released by Baiet al.[24].The abundant oxygen-containing groups make GO well dispersed in water.Under the photo-assisted catalysis of Fe3+/Fe2+,H2O2can be decomposed into hydroxyl radicals (·OH),which has a high redox potential (E0(HO·/H2O)=2.73 V) [22].Hydroxyl radicals will further oxidize GOviaboth the conversion of oxygen moieties to higher oxidation states and the electrophilic addition to unsaturated bonds.So the etching is going on along the oxygencontaining groups and makes the GO nanosheets holey and/or tiny.

    Figs.2g and h present the SEM images of GO and PGO-5 min and Fig.2i is the pore size distribution of PGO-5 min.Discrete circular pores could be obviously observed from the SEM image,which has a distribution range from 1 nm to 18 nm.The average pore size is about 8.9 nm.

    As shown in Fig.3,chemical analysis of PGO-t1was performed using XPS,FTIR and Raman.Fig.3a are the XPS full-scan profiles of GO and PGO-t1,in which only C and O peaks are observed.Fe signals are absent,indicating a fully removal during the dialysis process.Figs.3b-f show the C 1s spectra of GO and PGO-t1.There are four peaks observed for GO located at 284.8,286.4,287.2 and 288.5 eV,corresponding to C=C/C–C in aromatic rings,C–O(epoxy and alkoxy),C=O (carbonyl) and COOH (carboxyl) groups respectively [21].As the etching goes on,the intensity of the peaks corresponding to alkoxy,epoxy,and the carbonyl groups rapidly decreased,but the intensity of the peak corresponding to carboxyl groups remained relatively stable.It indicates that the etching is violent and could convert the alkoxy,epoxy,and the carbonyl groups into carboxyl groups,and further oxidize the carboxyl groups to CO2[24].The amount of carboxyl groups could remain stable at a proper stage.

    Fig.3.Chemical analysis of PGO-t1.(a) XPS full profiles.High-resolution C 1s spectra of (b) GO,(c) PGO-5 min,(d) PGO-10 min,(e) PGO-15 min and (f) PGO-20 min.(g)Raman spectra.(h) FT-IR spectra.

    Fig.3g shows the FT-IR spectra of GO and PGO-t1.Compared with the peak corresponding to C–O stretching (1058 cm-1),the peaks related to carboxyl groups including C=O stretching (1726 cm-1) and C–OH stretching (1228 cm-1) gradually increase with the progress of photo-Fenton etching,which also indicates the conversion of alkoxy groups and epoxy groups to carboxyl groups.Furthermore,Fig.3h shows the Raman spectra of GO and PGO-t1.The G peak (~1580 cm-1) is related to the E2gvibrational mode and the D peak (~1350 cm-1) is activated by the presence of defects.The values ofID/IGof PGO-t1were calculated in the range of 1.02 to 1.05,which is relatively stable and hard to get useful information [25].

    Hydrothermal could convert GO to RGO.Fig.4 shows the chemical analysis of PRGO-t1-t2using Raman and XPS.Fig.4a is a field emission SEM image of a typical sensor assembled from PRGO-5 min-6 h supported on an IE.The 2D layer of PRGO-5 min-6 h bridges the strip of the IE.Fig.4b shows the Raman spectra of RGO-1 h and PRGO-t1–1 h.The values ofID/IGwere calculated to be in the range of 1.03 to 1.14,slightly higher than the values of PGOt1illustrated in Fig.3h,indicating a slight recovery of sp2clusters upon 1 h reduction [12].

    Fig.4c shows the conductance responses of RGO-1 h and PRGOt1–1 h toward 5 ppm NO2.NO2gas sensing of graphene and its derivatives is mainly due to the change of carrier density caused by the adsorption of gas molecules [10,11].The residual carboxyl group in PRGO generates holes in the conduction band.NO2molecules (p-type dopants) will increase the hole concentration and significantly improve the conductance value [26].In Fig.4c,the response value of PRGO-5 min-1 h is the highest,up to 318%,34 times higher than that of RGO-1 h.As the etching time increases,the response increased sharply from the bottom value of RGO-1 h to the peak value of PRGO-5 min-1 h,then varied slowly.

    The above trend can be explained by the density change of vacancy defects.The increase in vacancy defect density and the carboxyl groups on the edge can enhance the adsorption of NO2molecules,which has been proved by theoretical simulation calculations [2,15-17].The amount of the sensing layer strongly affects the performance of the PRGO-based gas sensor.The thinner of the PRGO layers,the better is the sensing performance (Fig.S2 in Supporting information),which is consistent with the simulation results [27].In thicker samples,holes are effectively confined near the surface.In addition,the response of PRGO is also related to the reduction degree.Figs.4d-i are the XPS full scan profile and C 1s spectra of PRGO-5 min-t2.The alkoxy and epoxy groups were rapidly reduced during the reduction process.However,the strength of the peak corresponding to carboxyl groups did not change so significantly,indicating a slower reduction than those of the alkoxy and epoxy groups [28].In addition,during the process of hydrothermal reduction (Figs.S3a-d in Supporting information),the degree of reduction increases with the increased amount of deionized water [23].The higher of the reduction,the higher is the sensing performance (Fig.S4 in Supporting information).

    Fig.5 illustrates the typical gas sensing performance of PRGO.Fig.5a shows the response of the gas sensor based on PRGO-5 mint2toward 5 ppm NO2.As the reduction time increases,the response first increased and then decreased,reaching a peak at the reduction time of 6 h.This result is related to the combined effect of the amount of residue carboxyl groups and electrical conductivity.Figs.4d-i have shown that some weaker alkoxy and epoxy groups could be reduced to increase overall conductance; while more stable carboxyl groups were reduced at a slower speed.They ultimately lead to a peak response at the reduction time of 6 h.Namely,the highest response,up to 1170%,was obtained in the sample of PRGO-5 min-6 h.

    Fig.4.Chemical analysis of PRGO-t1-t2.(a) SEM image of IE-supporting PRGO-5 min-6 h.(b) Raman spectra of RGO-1 h and PRGO-t1–1 h.(c) Comparison of the responses of RGO-1 h and PRGO-t1–1 h toward 5 ppm NO2.(d) XPS full profiles of PRGO-5 min-t2.High-resolution C 1s spectra of (e) PRGO-5 min-1 h,(f) PRGO-5 min-2 h,(g) PRGO-5 min-4 h,(h) PRGO-5 min-6 h and (i) PRGO-5 min-8 h.

    Fig.5.Gas sensing performance of PRGO-t1-t2.(a) Responses of PRGO-5 min-t2 versus reduction time.(b) Responses of PRGO-5 min-6 h at different temperatures.(c) I-V curves of PGO-5 min and PRGO-5 min-6 h in pure N2.(d) Cyclic responses of PRGO-5 min-6 h upon exposure to NO2.(e) Responses of PRGO-5 min-6 h versus NO2 concentrations.(f) Exponential fitting of the responses of PRGO-5 min-6 h toward 2 ppm NO2 (τ =283 s).(g) Analyses of recovery time of PRGO-5 min-6 h (t80=1060 s); (h) Response of PRGO-5 min-6 h toward 5 ppb NO2.(i) Responses of PRGO-5 min-6 h toward other gasses.All the feeding time of the target gas was 300 s.

    Table 1 Performance comparison of the NO2 sensors based on different types of graphene materials.a

    The response of PRGO is also related to the testing temperature.Theoretically,an appropriate increase in temperature can accelerate the electron transfer process from PRGO to NO2.For example,when the temperature increases from 25 °C to 100 °C,the resistance of the gas sensor based on PRGO-5 min-6 h decreased by 5.5 times (Fig.S5 in Supporting information).Fig.5b shows the effect of the testing temperature on the response of PRGO-5 min-6 h toward 5 ppm NO2.With the increase of temperature,the response increases firstly and then decreases.Especially,the response increases from 1170% to 4300% as the testing temperature increased from 50°C to 100°C,an enhancement of about 2.7 times.However,when the temperature is higher than 100°C,the desorption of NO2gas molecules dominates,and the response decreases [20].Fig.5b indicates a good repeatability of the gas sensor.The reduction degree will definitely affect the stability of the gas sensor at an elevated temperatures,namely,PRGO-5 min-6 h will be much more stable than PRGO-5 min-1 h.When tested at the temperature of 100 °C,the conductance of PRGO-5min-1 h increases slowly (Fig.S6 in Supporting information).It is due to that its limited reduction was further progressed.The conductance of PRGO-5 min-6 h is much higher than that of its un-reduced state,PGO-5 min (Fig.5c).

    Fig.5d shows the response of PRGO-5 min-6 h toward NO2gas with a concentration ranging from 100 ppb to 5 ppm.The testing temperature is 100°C.The response of the sensor toward 100 ppb and 5 ppm NO2is 126% and 4300% respectively.As shown in Fig.5e,the sensor exhibits good linearity in the concentration range of 5 ppb–2 ppm.The slope of the linear fitting curve is defined as the sensitivity of the gas sensor.Table 1 lists the comparison of different types of graphene materials in recent years [12,14,20,29-35].We found that they could seldom detect NO2at the concentrations lower than 20 ppb and their sensitivity are rarely above 10 ppm-1.Fig.5e shows that our gas sensor has an exceptionally high sensitivity,climbing to 12 ppm-1(a response of 2400%to 2 ppm NO2at 100°C),which is,to our knowledge,the highest value of the RGO-based NO2gas sensors.Moreover,when tested toward NO2with randomly varying concentrations,the sensor still exhibits excellent performance (Fig.S7 in Supporting information).In terms of LOD,the signal value needs to be at least three times the noise,so as to distinguish the signal clearly.According to this rule,the theoretical LOD of PRGO-5 min-6 h was calculated using a sensitivity of 12 ppm-1and the root-mean-square (RMS) noise(LOD=3RMS/sensitivity).The theoretical LOD of PRGO-5 min-6 h is as low as 0.66 ppb (Table 1,Fig.S8 and Table S1 in Supporting information),which is lower than most of the current graphenebased NO2sensors.

    whereτandG∞are the time constant and steady-state conductivity value respectively.

    The fitting results are shown in Fig.5f,which indicates that the time constant of the (PRGO-5 min-6 h)-based sensor isτ=283 s(toward 2 ppm NO2).The recovery time is another index to evaluate the recovery speed of the sensor.Here we uset80to define the recovery time,which is the time required for the signal change approaching to 80% of the total response during the recovery process.As shown in Fig.5g,thet80recovery time is 1060 s.We also found thatτandt80were dependent on the concentration of NO2,for example,when sensing toward a higher concentration of 5 ppm NO2,τandt80were calculated as 189 and 1275 s (Figs.S9b and d in Supporting information),respectively,indicating a faster response speed and a slower recovery speed.It can be explained by the kinetics of the adsorption and desorption process.Higher concentrations can accelerate the adsorption speed,but it takes longer time to be desorbed.As a comparison,a sensor based on RGO-6 h was also tested toward 5 ppm NO2,whoseτandt80are 236 and 1828 s (Figs.S9a and c in Supporting information) respectively.Both are longer than PRGO-5 min-6 h.It indicates that both the response and recovery process are accelerated after the photo-Fenton etching and water-vapor reduction.

    Compared with PRGO-5 min-6 h,hydrothermal reduced at 180°C for 6 h,whose recovery time is 1275 s (Fig.S9d),the recovery times of PRGO-5 min-12 h (hydrothermal reduced at 180°C for 12 h) and PRGO-5 min-6 h/200 °C (hydrothermal reduced at 200°C for 6 h) are 288 s and 196 s (Figs.S9e and f in Supporting information),respectively.The recovery speed is increased by 3.42 and 5.5 times,respectively.That is to say,extending the reduction time or increasing the reduction temperature can effectively improve the recovery speed due to the further reduction of the carboxyl groups[23].However,the peak response of the sensors based on PRGO-5min-12 h and PRGO-5 min-6 h/200 °C drop to 1900% and 740%,respectively.So the existence of carboxyl groups also plays a great role in increasing the sensitivity of the sensors.

    Since the theoretical LOD of PRGO-5 min-6 h is as low as 0.66 ppb,we put the sensor to an atmosphere with the experimentallyultralow concentration of 5 ppb NO2.As shown in Fig.5h,the sensor exhibits a clearly distinguishable signal corresponding to a value change of 6.8%,which experimentally proves the excellent detecting ability of the gas sensor to trace amount of NO2.

    The selectivity of a gas sensor is its relatively much higher sensitivity to one target gas than to the other gasses.As shown in Fig.5i,the selectivity of the gas sensor based on PRGO-5 min-6 h was comparatively studied toward 5 ppm NO2(4300%),5 ppm NO(2500%),5 ppm NH3(80%),5 ppm HCHO (6.5%),5 ppm H2S (<1%),100 ppm C2H2(-6.52%),and 100 ppm CO (6.8%).At ppm or higher levels,the sensor is dominantly sensitive to NO2than to the other common gasses like NH3,HCHO,C2H2,CO.It is nearly inert to H2S.The relatively high sensitivity toward NO is because NO is very similar to NO2from the aspects of the polar structures and the molecular properties.

    The response to 100 ppm C2H2is negative in the opposite direction,while the response to 5 ppm NH3is positive in the same direction with NO2.This is somewhat different to the reported results of other graphene- or RGO-based gas sensors [29].We tested the response of RGO which was water-vapor reduced from GO in the same way as PRGO-5 min-6 h,as well as the response of a graphene synthesized by mechanical exfoliation.The former is also positive but the latter is negative (Fig.S10 in Supporting information).This means that the poorly reduced state of GO (i.e.,the defects of graphene) make the response behavior reverse.Theoretical calculations have shown that,compared with NH3,the adsorption energy between NO2molecular and graphene vacancy is much stronger [15,36,37].So,the vacancy defects of PRGO,as well the residue functional groups like carboxyl,selectively enhance their interaction with NO2molecules,through enhanced induced forces or hydrogen bonds (Fig.S11 in Supporting information).

    In a summary,we have fabricated a high-performance NO2gas sensor based on defective PRGO through a fast and controllable way.The photo-Fenton reaction was selected for fast etching vacancy defects and the hydrothermal reduction was used to tune the functional groups.PRGO-5min-6 h exhibits superior sensing performance toward NO2,with an exceptional sensitivity up to 12 ppm-1.The theoretical LOD is as low as 0.66 ppb and the measured sensitivity is 6.8% toward 5 ppb NO2.The good sensing performance is mainly due to the vacancy defects and carboxyl groups.The sensor also exhibits excellent properties such as good selectivity,good linearity,and wide linear range.This work not only provides a new method to fabricate high-performance gas sensors,but also presents a new view to optimize the gas sensing properties of RGO with chemical modification and defects regulation.

    Declaration of competing interest

    The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

    Acknowledgment

    The research was financially supported by National Natural Science Foundation of China (No.52073302).

    Supplementary materials

    Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2022.02.003.

    特级一级黄色大片| 国产乱人伦免费视频| 中文字幕人妻熟人妻熟丝袜美| 欧美日韩亚洲国产一区二区在线观看| 干丝袜人妻中文字幕| 午夜激情欧美在线| 在线看三级毛片| 国产成人aa在线观看| 日韩亚洲欧美综合| 成人av在线播放网站| 国产日本99.免费观看| 欧美丝袜亚洲另类 | 成人毛片a级毛片在线播放| 国产精品一区二区三区四区免费观看 | 99国产精品一区二区蜜桃av| 日本撒尿小便嘘嘘汇集6| 亚洲在线观看片| 一个人看的www免费观看视频| 无遮挡黄片免费观看| 九九热线精品视视频播放| 亚洲四区av| 亚洲第一电影网av| 欧美日韩综合久久久久久 | 狂野欧美白嫩少妇大欣赏| 黄色丝袜av网址大全| 国产毛片a区久久久久| 色在线成人网| 少妇裸体淫交视频免费看高清| 一进一出抽搐动态| 在线国产一区二区在线| 亚洲 国产 在线| 国产欧美日韩精品亚洲av| www.www免费av| 亚洲久久久久久中文字幕| 搞女人的毛片| 亚洲成人中文字幕在线播放| 观看美女的网站| 在线看三级毛片| 国产精品人妻久久久久久| 一进一出抽搐gif免费好疼| 我的女老师完整版在线观看| 日韩欧美精品免费久久| 精品久久久噜噜| 日本黄色视频三级网站网址| 久久久成人免费电影| 岛国在线免费视频观看| 亚洲图色成人| 欧美三级亚洲精品| 国产精品三级大全| 简卡轻食公司| 18+在线观看网站| ponron亚洲| АⅤ资源中文在线天堂| 麻豆国产av国片精品| 亚洲人成网站高清观看| 国产高潮美女av| 亚洲精品色激情综合| a在线观看视频网站| 欧美国产日韩亚洲一区| 精品福利观看| 高清日韩中文字幕在线| 午夜免费激情av| 国产久久久一区二区三区| 国内精品一区二区在线观看| 3wmmmm亚洲av在线观看| 日本 欧美在线| 蜜桃久久精品国产亚洲av| 欧美精品啪啪一区二区三区| 18禁黄网站禁片免费观看直播| 嫩草影院新地址| 干丝袜人妻中文字幕| 免费av观看视频| 波多野结衣高清无吗| 久久久久久国产a免费观看| 亚洲国产色片| 日日夜夜操网爽| 国产爱豆传媒在线观看| 99九九线精品视频在线观看视频| 国产亚洲欧美98| 亚洲无线观看免费| 少妇丰满av| 亚洲国产精品成人综合色| 极品教师在线视频| 婷婷精品国产亚洲av| 国产伦精品一区二区三区视频9| 久久久久久久精品吃奶| 色av中文字幕| 熟女电影av网| 久久久久久久午夜电影| 免费看av在线观看网站| 亚洲经典国产精华液单| 免费av毛片视频| 日韩欧美精品免费久久| 日日摸夜夜添夜夜添小说| 日韩欧美 国产精品| АⅤ资源中文在线天堂| 国产69精品久久久久777片| 国产伦在线观看视频一区| 久久人人精品亚洲av| 中文字幕熟女人妻在线| 国产精品伦人一区二区| 国产精品98久久久久久宅男小说| 亚洲一级一片aⅴ在线观看| 一本一本综合久久| 国内精品久久久久精免费| 悠悠久久av| a级毛片免费高清观看在线播放| 热99re8久久精品国产| 中文字幕高清在线视频| 一级a爱片免费观看的视频| 欧美zozozo另类| 欧美另类亚洲清纯唯美| 欧美三级亚洲精品| 色综合站精品国产| 中出人妻视频一区二区| 亚洲av二区三区四区| 一进一出好大好爽视频| 国产精华一区二区三区| 看黄色毛片网站| 亚洲av中文av极速乱 | 麻豆成人av在线观看| 久久久成人免费电影| 午夜福利视频1000在线观看| 国产激情偷乱视频一区二区| 午夜免费成人在线视频| 亚洲五月天丁香| 国产大屁股一区二区在线视频| 在线天堂最新版资源| 天堂动漫精品| 中文字幕精品亚洲无线码一区| 亚洲成av人片在线播放无| 国产精品无大码| 一区二区三区四区激情视频 | 精品免费久久久久久久清纯| 日韩高清综合在线| 一级av片app| 欧美国产日韩亚洲一区| 欧美黑人巨大hd| 麻豆成人午夜福利视频| 99久国产av精品| 国产高清三级在线| 黄色视频,在线免费观看| 桃红色精品国产亚洲av| 很黄的视频免费| 欧美国产日韩亚洲一区| 看黄色毛片网站| 国产精品亚洲一级av第二区| 国产三级中文精品| 免费不卡的大黄色大毛片视频在线观看 | 亚洲无线在线观看| 超碰av人人做人人爽久久| www.www免费av| 一进一出抽搐动态| 国产大屁股一区二区在线视频| 熟妇人妻久久中文字幕3abv| 99久久久亚洲精品蜜臀av| 成人三级黄色视频| 女生性感内裤真人,穿戴方法视频| 日本 欧美在线| 欧美色欧美亚洲另类二区| 国内精品久久久久精免费| 免费无遮挡裸体视频| 国产一区二区三区av在线 | 人妻制服诱惑在线中文字幕| 男女啪啪激烈高潮av片| 精品久久久久久,| 黄色一级大片看看| 国产高潮美女av| 丰满人妻一区二区三区视频av| 嫩草影院入口| 亚洲美女搞黄在线观看 | 99热这里只有精品一区| 亚洲国产日韩欧美精品在线观看| 亚洲中文字幕日韩| www日本黄色视频网| 国产欧美日韩精品亚洲av| 一个人观看的视频www高清免费观看| 午夜激情欧美在线| 成人美女网站在线观看视频| 看十八女毛片水多多多| 久久人人精品亚洲av| 国产一区二区在线av高清观看| 一个人观看的视频www高清免费观看| 又爽又黄无遮挡网站| 小蜜桃在线观看免费完整版高清| 悠悠久久av| 久久久久久久久久黄片| 欧美xxxx黑人xx丫x性爽| 午夜福利成人在线免费观看| 国产精品乱码一区二三区的特点| ponron亚洲| 五月伊人婷婷丁香| 国产主播在线观看一区二区| 欧美日韩中文字幕国产精品一区二区三区| 国模一区二区三区四区视频| 少妇人妻精品综合一区二区 | 久久人人爽人人爽人人片va| 国产成人aa在线观看| 成人高潮视频无遮挡免费网站| 日韩欧美三级三区| 麻豆精品久久久久久蜜桃| 国语自产精品视频在线第100页| 精品午夜福利在线看| 97超级碰碰碰精品色视频在线观看| 白带黄色成豆腐渣| 听说在线观看完整版免费高清| 一卡2卡三卡四卡精品乱码亚洲| 日本黄大片高清| 免费大片18禁| 99热精品在线国产| 他把我摸到了高潮在线观看| xxxwww97欧美| 女同久久另类99精品国产91| 极品教师在线免费播放| 大又大粗又爽又黄少妇毛片口| 久久人人精品亚洲av| 高清毛片免费观看视频网站| 国产伦在线观看视频一区| 国产91精品成人一区二区三区| 午夜激情福利司机影院| 久久精品91蜜桃| 国内毛片毛片毛片毛片毛片| 老师上课跳d突然被开到最大视频| 大型黄色视频在线免费观看| 亚洲精华国产精华精| 国产在线男女| 亚洲av美国av| 亚洲男人的天堂狠狠| 日本免费一区二区三区高清不卡| 夜夜爽天天搞| 亚洲无线观看免费| 国产精品国产三级国产av玫瑰| 国产亚洲欧美98| 一区二区三区激情视频| 少妇猛男粗大的猛烈进出视频 | 亚洲第一电影网av| 一级a爱片免费观看的视频| 在现免费观看毛片| 老熟妇乱子伦视频在线观看| 久久精品综合一区二区三区| 老师上课跳d突然被开到最大视频| 免费看a级黄色片| 国模一区二区三区四区视频| 中国美白少妇内射xxxbb| 我的女老师完整版在线观看| 极品教师在线视频| 亚洲真实伦在线观看| 中国美女看黄片| 中国美白少妇内射xxxbb| 精品乱码久久久久久99久播| 蜜桃亚洲精品一区二区三区| 成人av一区二区三区在线看| 免费搜索国产男女视频| 成人永久免费在线观看视频| 国产男人的电影天堂91| 级片在线观看| 久久精品国产自在天天线| 一进一出抽搐动态| 亚洲成人免费电影在线观看| 一区二区三区四区激情视频 | 中文在线观看免费www的网站| 欧美又色又爽又黄视频| 精品欧美国产一区二区三| 欧美激情久久久久久爽电影| 嫩草影院精品99| 乱码一卡2卡4卡精品| 深爱激情五月婷婷| 真人做人爱边吃奶动态| 在线免费观看的www视频| 男人狂女人下面高潮的视频| 22中文网久久字幕| 1024手机看黄色片| 亚洲国产高清在线一区二区三| 午夜亚洲福利在线播放| 欧美日本视频| 国产一区二区激情短视频| 亚洲第一电影网av| avwww免费| aaaaa片日本免费| 在线天堂最新版资源| 国产高清有码在线观看视频| 18+在线观看网站| 欧美黑人欧美精品刺激| 如何舔出高潮| 精品人妻视频免费看| 毛片女人毛片| 国产高清不卡午夜福利| 久久亚洲真实| 国产精品一区二区三区四区免费观看 | 女生性感内裤真人,穿戴方法视频| 日本三级黄在线观看| 国产三级在线视频| aaaaa片日本免费| av在线蜜桃| 毛片一级片免费看久久久久 | 国产男人的电影天堂91| 观看免费一级毛片| 99热网站在线观看| 欧洲精品卡2卡3卡4卡5卡区| 国产 一区精品| 亚洲人与动物交配视频| 麻豆久久精品国产亚洲av| 亚洲aⅴ乱码一区二区在线播放| 国产在线男女| 亚洲国产欧美人成| 精品国内亚洲2022精品成人| 美女黄网站色视频| 亚洲天堂国产精品一区在线| 男女做爰动态图高潮gif福利片| 97超级碰碰碰精品色视频在线观看| 我的老师免费观看完整版| 一个人观看的视频www高清免费观看| 亚洲无线观看免费| 听说在线观看完整版免费高清| 免费在线观看日本一区| 深夜精品福利| 国产aⅴ精品一区二区三区波| 91av网一区二区| 午夜激情欧美在线| 欧美+亚洲+日韩+国产| 久久久色成人| 国内精品美女久久久久久| 一个人观看的视频www高清免费观看| 国产成人一区二区在线| 偷拍熟女少妇极品色| 成人一区二区视频在线观看| 国产精品人妻久久久影院| 午夜福利成人在线免费观看| 亚洲成人中文字幕在线播放| 国产精品伦人一区二区| 亚洲精品日韩av片在线观看| 性欧美人与动物交配| 欧美+亚洲+日韩+国产| 精品一区二区三区视频在线| 国内精品美女久久久久久| a在线观看视频网站| 免费无遮挡裸体视频| 精品免费久久久久久久清纯| 欧美绝顶高潮抽搐喷水| 日本爱情动作片www.在线观看 | av天堂在线播放| 乱系列少妇在线播放| 欧美三级亚洲精品| 在现免费观看毛片| 我的老师免费观看完整版| 男插女下体视频免费在线播放| 亚洲av五月六月丁香网| 亚洲国产精品成人综合色| 日本熟妇午夜| 欧美成人性av电影在线观看| 成人高潮视频无遮挡免费网站| 一级毛片久久久久久久久女| 欧美人与善性xxx| 极品教师在线免费播放| 欧美+日韩+精品| 亚洲最大成人av| 成人一区二区视频在线观看| 长腿黑丝高跟| 欧美潮喷喷水| 国产一区二区在线av高清观看| 午夜精品一区二区三区免费看| 精品乱码久久久久久99久播| 99热这里只有是精品50| 国产欧美日韩精品亚洲av| 99热这里只有是精品50| 天堂√8在线中文| 99热这里只有是精品50| АⅤ资源中文在线天堂| 日韩一区二区视频免费看| 国产av不卡久久| 中文字幕久久专区| 亚洲熟妇熟女久久| 国产欧美日韩精品一区二区| 国产成人福利小说| 日韩欧美在线乱码| 俄罗斯特黄特色一大片| 搞女人的毛片| 日韩av在线大香蕉| 麻豆成人av在线观看| 午夜福利在线观看吧| 亚洲经典国产精华液单| 日韩高清综合在线| 国产一级毛片七仙女欲春2| 高清毛片免费观看视频网站| 亚洲中文字幕一区二区三区有码在线看| 桃红色精品国产亚洲av| 国产精品一区二区性色av| 真人一进一出gif抽搐免费| 亚洲第一区二区三区不卡| 免费观看的影片在线观看| 亚洲电影在线观看av| 桃红色精品国产亚洲av| 色哟哟哟哟哟哟| 国产欧美日韩一区二区精品| 午夜福利高清视频| 少妇人妻精品综合一区二区 | 色综合婷婷激情| 亚洲精品国产成人久久av| 日韩在线高清观看一区二区三区 | 成人美女网站在线观看视频| 有码 亚洲区| 日日摸夜夜添夜夜添小说| 国产一区二区激情短视频| 真人做人爱边吃奶动态| 18禁在线播放成人免费| 日韩国内少妇激情av| 天美传媒精品一区二区| 国产乱人伦免费视频| 中国美女看黄片| 久久香蕉精品热| 毛片女人毛片| 日韩一本色道免费dvd| 欧美xxxx性猛交bbbb| 一区二区三区高清视频在线| 18禁黄网站禁片午夜丰满| 亚洲精品亚洲一区二区| 久久久国产成人精品二区| 成人三级黄色视频| 亚洲图色成人| 亚洲一级一片aⅴ在线观看| 亚洲在线自拍视频| 男女边吃奶边做爰视频| 亚洲性久久影院| 国产高清有码在线观看视频| 一级黄色大片毛片| 亚洲欧美日韩高清专用| 嫩草影视91久久| 美女高潮的动态| 精品久久久久久久久亚洲 | 国产亚洲91精品色在线| 免费一级毛片在线播放高清视频| 狂野欧美激情性xxxx在线观看| 91狼人影院| av视频在线观看入口| 亚洲精品一卡2卡三卡4卡5卡| 午夜福利在线观看吧| 欧美三级亚洲精品| 国产成人av教育| 亚洲精品在线观看二区| 成熟少妇高潮喷水视频| 免费电影在线观看免费观看| 免费观看精品视频网站| 88av欧美| 又爽又黄a免费视频| av国产免费在线观看| 日韩 亚洲 欧美在线| 在线天堂最新版资源| 狠狠狠狠99中文字幕| 日韩欧美在线乱码| 国产国拍精品亚洲av在线观看| 国产精品久久电影中文字幕| 亚洲在线自拍视频| 亚洲精品日韩av片在线观看| 很黄的视频免费| 两个人的视频大全免费| 少妇猛男粗大的猛烈进出视频 | 国产精品av视频在线免费观看| 91午夜精品亚洲一区二区三区 | 亚洲图色成人| 2021天堂中文幕一二区在线观| 给我免费播放毛片高清在线观看| 又黄又爽又免费观看的视频| 超碰av人人做人人爽久久| 国产在线男女| 国产精品人妻久久久影院| 久久久久精品国产欧美久久久| 久久精品久久久久久噜噜老黄 | 亚洲国产精品久久男人天堂| av中文乱码字幕在线| 人妻少妇偷人精品九色| 国产视频一区二区在线看| 嫩草影院新地址| 波多野结衣巨乳人妻| 亚洲一级一片aⅴ在线观看| 老司机福利观看| 男女啪啪激烈高潮av片| 毛片女人毛片| 日韩,欧美,国产一区二区三区 | 嫩草影院入口| 亚洲最大成人中文| 乱人视频在线观看| 少妇的逼水好多| 长腿黑丝高跟| 国产成人a区在线观看| 日韩欧美一区二区三区在线观看| 91麻豆av在线| 亚洲乱码一区二区免费版| 成人性生交大片免费视频hd| 国产一级毛片七仙女欲春2| 久久久久九九精品影院| 黄色日韩在线| 女人十人毛片免费观看3o分钟| 国产精品亚洲美女久久久| 国内毛片毛片毛片毛片毛片| 欧美高清成人免费视频www| 色哟哟·www| 乱码一卡2卡4卡精品| 国产 一区精品| 三级毛片av免费| 亚洲人成网站在线播放欧美日韩| 日日干狠狠操夜夜爽| 韩国av一区二区三区四区| 国国产精品蜜臀av免费| 久久久精品大字幕| 丰满人妻一区二区三区视频av| 欧美高清性xxxxhd video| eeuss影院久久| 久久精品国产99精品国产亚洲性色| 欧美日本亚洲视频在线播放| xxxwww97欧美| 亚洲精品一区av在线观看| 国产麻豆成人av免费视频| 三级国产精品欧美在线观看| 一本一本综合久久| 国产精品一区二区性色av| 亚洲欧美清纯卡通| 日韩中文字幕欧美一区二区| 午夜免费激情av| 婷婷亚洲欧美| 国产一区二区激情短视频| 成人亚洲精品av一区二区| 久久久久久九九精品二区国产| 亚洲四区av| 我的老师免费观看完整版| 国产淫片久久久久久久久| 成人av一区二区三区在线看| 一进一出抽搐动态| 午夜福利在线在线| 性欧美人与动物交配| 美女大奶头视频| 女的被弄到高潮叫床怎么办 | xxxwww97欧美| 91av网一区二区| 亚洲欧美精品综合久久99| 精品久久久久久久久亚洲 | 3wmmmm亚洲av在线观看| 成熟少妇高潮喷水视频| 婷婷六月久久综合丁香| 国产大屁股一区二区在线视频| 99热只有精品国产| 人妻丰满熟妇av一区二区三区| 国产男人的电影天堂91| 99久久精品一区二区三区| 91久久精品国产一区二区三区| 国产亚洲精品综合一区在线观看| 国产av不卡久久| 久久久久精品国产欧美久久久| 国产精品福利在线免费观看| 一边摸一边抽搐一进一小说| 午夜免费男女啪啪视频观看 | 国产免费一级a男人的天堂| 搡老妇女老女人老熟妇| 亚洲在线观看片| 日本 av在线| 麻豆精品久久久久久蜜桃| 人妻夜夜爽99麻豆av| 日本爱情动作片www.在线观看 | 午夜福利欧美成人| 国产v大片淫在线免费观看| 3wmmmm亚洲av在线观看| 亚洲av电影不卡..在线观看| 精品国产三级普通话版| 夜夜夜夜夜久久久久| 国产在线男女| 婷婷精品国产亚洲av| 国模一区二区三区四区视频| 12—13女人毛片做爰片一| aaaaa片日本免费| 91av网一区二区| 亚洲不卡免费看| 88av欧美| 久久久久久久久中文| 免费观看人在逋| 国产精品一区二区三区四区免费观看 | 黄色日韩在线| 国产真实乱freesex| 2021天堂中文幕一二区在线观| 亚洲av电影不卡..在线观看| 国产免费av片在线观看野外av| 国产aⅴ精品一区二区三区波| 十八禁网站免费在线| 国产精品野战在线观看| 丰满乱子伦码专区| 搞女人的毛片| 嫩草影院入口| 亚洲一区二区三区色噜噜| 99热6这里只有精品| 美女免费视频网站| 黄色丝袜av网址大全| 免费不卡的大黄色大毛片视频在线观看 | a级毛片a级免费在线| av视频在线观看入口| 99视频精品全部免费 在线| 最近在线观看免费完整版| 波多野结衣高清无吗| 性欧美人与动物交配| 婷婷六月久久综合丁香| 精品久久久久久久久久免费视频| 亚洲av日韩精品久久久久久密| 久久久久免费精品人妻一区二区| 亚洲熟妇熟女久久| 国产激情偷乱视频一区二区| 美女xxoo啪啪120秒动态图| 婷婷精品国产亚洲av| 免费观看的影片在线观看| 日韩精品青青久久久久久| 91精品国产九色| 在线免费观看的www视频| 中文字幕av在线有码专区| 一个人免费在线观看电影| 精品不卡国产一区二区三区| 午夜精品在线福利| 亚洲真实伦在线观看| 精品99又大又爽又粗少妇毛片 | 精品久久久久久久久久久久久|