1.Introduction

In 2004,Geim and Novoselov used the mechanical stripping method from graphene exfoliated into only one carbon atom layer two-dimensional graphene structure[1].The twodimensional graphene prepared by SiC reduction method,expansion method,and chemical vapor deposition(CVD)is more stable[2],which has excellent electrical properties,thermal properties[3,4].The two-dimensional graphene has been applied in optical devices,chemical sensors,new energy battery et al.[5-7].However,the two-dimensional graphene contains only one kind of element carbon.Later,scientists discovered three kinds of two-dimensional structure materials[8],such as six party boron nitride,molybdenum disulfide,and silicon carbide,which expanded the two-dimensional material members.In 2011,Gogotsi and Barsoum discovered a new type of two-dimensional material,including transition metals,carbides,or nitrides.It is found that the corresponding twod imensional layered compound Ti3C2can be prepared by HF etching of Ti3AlC2[9].They also used the same method to etch the MAX phase materials,and successfully prepared twodimensional transition metal carbide nanosheets[10,11].In order to emphasize the role of selective etching of 3A element etching agent HF on MAX phase materials,a new material similar to the two-dimensional structure of graphene is named MXene material.The surface of MXene contains a large number of active groups such as hydroxyl group,oxygen ion and fluorine ion[12-14].The two-dimensional MXene materials have good stability[15-17],electronic properties[18],magnetic properties[19]and mechanical properties[20].In addition,MXene has a wide range of applications,such as energy storage,catalysis,adsorption,hydrogen storage,sensors,and new polymer reinforced composites.At present,the research work about MXene is mainly focused on lithium ion batteries[21,22],supercapacitors,and fuel cells[23-25].In addition,some special interests have been investigated on the catalytic performances of MXene materials,such as highperformance oxygen evolution,efficient electrocatalyst for hydrogen evolution,single-atom catalyst for CO oxidation,etc.[26-30].Moreover,in recent years our groups have prepared some novel MXene/Ag composites,MXene-based nano flower-shaped TiO2/C composites,and MXene/magnetic iron oxide nanocomposites,demonstrating unexpected electrocatalytic activity,extraordinary long cycle lifetime lithium storage,and catalytic activity for dehydrogenation of sodium alanates,respectively[31-34].However,it is still challenging to design and obtain MXene-based composites with facile and effective process in self-assembled technique.

特征点识别出后需计算特征值，接着对这些特征值与袖带充气式电子血压计测量的SBP和DBP分别进行相关性分析，并选择相关性比较大的特征值作为回归变量。再接着将这些回归变量分别与SBP和DBP进行线性回归分析，建立血压与这些特征值之间的回归方程。

On the other hand,the layer-by-layer(LbL)self-assembly technique is regarded as a simple and effective surface modification technology.The general driving interactions of the layer assembly process are hydrogen bond,electrostatic interaction,coordination bond,π-π stacking,hydrophobic interaction and molecular recognition[35-44].The LbL assembly technique has many advantages,such as simple process and mild preparation condition.It can be expected that a combination of MXene-based composites involved in surface modification and LbL self-assembled method should be especially advantageous owing to controllable shell layer,moderate nanostructures,and enhanced chemical properties.

思考：学生完成此问题时，要熟练掌握动点问题，相似三角形问题，直角三角形存在性等问题。如果教师在出这个题目之前有分解的小练，复杂问题定会迎刃而解。

使用改进的液压缸进行了5次实验，得到滤波后的压力载荷波形如图 8所示。载荷曲线上不再有波动信号，且曲线的变化趋势与半正弦波形状基本吻合。这说明采用“滤波板+滤波孔”的滤波方式可取得较好的滤波效果。5次实验得到的脉冲载荷峰值及脉冲载荷脉宽数据，如表1所列。

In the work presented here, we reported the synthesis andcharacterization of functionalized core-shell MXene compositesmade by LbL assembled strategy via amine-containingpolyethyleneimine (PEI) and carboxyl-containing poly acrylicacid (PAA). We investigated the characteristic core-shellstructures by using various morphological and spectral characterizationmethods. The targeted MXene-COOH@(PEI/PAA)n@AuNPs nanocomposites show excellent catalyticproperties for catalytic reduction of nitro compounds, such as2-NA and 4-NP [45]. In addition, the prepared MXene-COOH@(PEI/PAA)n@AuNPs nanocomposites also exhibitoutstanding stability and repeatability. This present work suggeststhat MXene-based composites by LbL method can bepromising catalysts candidates, demonstrating great potentialsin the fields of composites catalyst materials and wastewatertreatment.

2.Experimental

2.1.Materials

Polyethylene polyimide(PEI,50%hydrolyzed,average M.W.4300-6500),poly(acrylic acid)(PAA,M.W.～40,000),and chloroacetic acid were purchased from Aladdin Shanghai Industrial Corporation and Alfa Aesar Chemicals.Ti3C2(MXene)was synthesized by immersing of Ti3AlC2powders in 50%concentrated HF solution for 40 h at room temperature.2-Nitrophenol(abbreviated as 2-NA),4-nitrophenol(abbreviated as 4-NP)were obtained from Aladdin Shanghai Industrial Corporation.Deionized water was prepared and used through the experiments. Chloroauric acid tetrahydrate(HAuCl4⋅4H2O)and sodium borohydride(NaBH4)were obtained from Alfa Aesar Chemicals.Ethanol(C2H5OH)was provided by Sinopharm Chemical Reagent Co,Ltd(Beijing,China).

2.2.Preparation of composite MXene-COOH@(PEI/PAA)n@AuNPs

In addition,in order to investigate the organized nanostructures in the obtained composite materials,XRD curves of the synthesized MXene composite materials were measured and shown in Fig.3.The peaks with 2θ values of 8.90°,18.24°,and 27.65° can be corresponding to the characteristic(002),(006),and(008)planes[30-32].After modification with carboxyl groups,the X-ray pattern of MXene-COOH displays the obvious shiftofstrong peak to 7.02°corresponding to the(002)diffraction peak with an increased layer distance of 1.26 nm as well as another changed peak to 28.80°corresponding to the(008)re flection peak.At the same time,the(006)reflection peak disappeared.This suggested some chemical reaction occurred in MXene sheets with the modification of carboxyl groups.Next,after LbL-assembled process,there appears a wide convex protrusion in the range of 12°-30° for the obtained MXene-COOH@(PEI/PAA)2and MXene-COOH@(PEI/PAA)10 composites.And the(002)re flection peak shifted to new position of 6.53°with an increased layer distance of 1.35 nm.The obvious change can be assigned to the assembled PEI/PAA multilayer shell structures due to the formation of strong intermolecular hydrogen bonds and electronic forces in the presence of free amino and carboxyl groups in polymeric molecules.The above obtained results clearly indicated that the hierarchical core-shell self-assembled composites have been successfully prepared.Next,the characteristic(111),(200),(220)crystal peaks of gold nanoparticles are also found after modification of gold nanoparticles,indicating the successful synthesis of gold nanoparticle-containing core-shell nanocomposites materials.At the same time,the disappearance of(002)re flection peak originated from MXene sheet in MXene-COOH@(PEI/PAA)2@AuNPs and MXene-COOH@(PEI/PAA)10@AuNPs composites could be speculated as the oxidation of MXene sheet in the modification treatment of gold nanoparticles.

2.3.Catalytic performance

In conclusion,new hierarchical core-shell structured MXene-COOH@(PEI/PAA)n@AuNPs nanocomposites were prepared by layer-by-layer assembly and modification of gold nanoparticles.The obtained MXene-based nanocomposite catalysts show good catalytic effect on nitro compounds,such as 2-NA and 4-NP.In addition,after 8 successive catalytic cycles,the prepared catalysts demonstrated good catalytic efficiency,well stability and repeatability.The present research work provides a new exploration for the preparation of MXenebased nanocomposite materials for catalytic applications.

观察两组病人治疗5 d后会阴部IAD治疗效果及IAD愈合时间。治疗效果判断标准：治愈为皮肤完全恢复正常；好转为皮肤潮红明显改善，无出血、水疱、渗液，糜烂基本愈合；无效为临床症状未改善甚至有加重[7]。

2.4.Characterization

As shown in Fig.4,the TG curves of MXene and composite materials were investigated.From the figure we can clearly f i nd that the heat loss of pristine MXene is least at 650°C with the quality conservation value of 94%.After modification of carboxyl groups,the quality conservation decreased slightly with valueof92.5%.In addition,afterlayer-by-layer assembly,the obtained core-shell nanocomposites MXene-COOH@(PEI/PAA)2and MXene-COOH@(PEI/PAA)10 demonstrated obvious mass decrement.It can be concluded that the heat loss of MXene-COOH@(PEI/PAA)10is greatest without gold modification,indicating that the thermal stability of MXene-COOH@(PEI/PAA)10is the worst.However,after gold nanoparticles modification,the quality conservation of formed MX ene-COOH@(PEI/PAA)10@AuNPscomposite showed the value of 58.8%,demonstrating the improved thermal stability.

3.Results and discussion

3.1.Structural characterization of composites

Next,the obtained core-shell MXene-based composites were measured to investigate the interfacial composition using XPS technique.The survey data of XPS spectra from the prepared composites in Fig.5 demonstrate the characteristic elemental peaks,suchasC1s,O1s,Ti2p,N1s,andAu4f.After chemical modification of carboxyl groups and layer-by-layer assembly of nanocom posites,the N 1s and Au 4f elemental peaks appears in XPS data,indicating the successful modi fication of gold nanoparticles in the shell layers via LbL selfassembly process.Next,the deconvolution data of C 1s,O 1s,Ti 2p,N 1s and Au 4f peaks for the MXene-COOH@(PEI/PAA)10@AuNPs composite material were analyzed and demonstrated,asshowninFig.6.Forthe peakde convolution of MXene C 1s core levels in Fig.6a,the peak centered at 280.7 and 283.2 eV could be assigned to the C-Ti the C-C bonds[13,47].In addition,for C 1s deconvolution of MXene-COOH@(PEI/PAA)10@AuNP scomposite materialin Fig.6b,newdeconvoluted peaksat positions of284.2,285.0and 287.5 eV were assigned to C-N,C-O and O=C-O oxygencontaining bonds,respectively[48,49].Moreover,the O(1s)peak in MXene(Fig.6c)could be deconvoluted into two main Gaussian component peaks after subtraction treatment of Shirley background.The concentrated peaks of 529.3 and 530.8 eV can be assigned to the Ti-O-Ti(bond of TiO2oxides)and the C-Ti-OH bonds[13].For O 1s of the prepared composite in Fig.6d,new peak deconvolution at 532.5 eV can be assigned to oxygen of C=O in composites.This means that the surface of core-shell composite was still functional and porous,which show an advantage for nextcatalytic application.In addition,as shown in Fig.6e,the obvious peak at～458.8 eV from original MXene samples is assigned to the Ti(IV)2p3/2belonging to TiO2oxides[13],while the lower binding energy peaks belong to the MXene.The broad peak ranging from 454 to 457 eV suggests the possible presence of low valence Ti species,i.e.Ti(II)(～454.7 eV)and Ti(III)(～455.6 eV)[50].While selfassembly process onto MXene-based core-shell composite can lead to the initial conversion from Ti(III)to the termination of Ti(IV)species,as shown in Fig.6f.Furthermore,the N 1s peak in Fig.6g indicated the appearance of amine group at about 399.2 eV and N+segments at about 401.0 eV,demonstrating the self-assembly of PEI molecules in composite via both covalent bonding and weak interaction force to PAA molecules and MXene layer sheets.The core-shell composite shows Au 4f peaks at 82.3 eV and 86.1 eV in Fig.6h,further con firming the presence of Au nanoparticles almost in Au(0)(metallic)state.

Fig.2 shows the transmission electron microscopy(TEM)images of the obtained MXene-based nanocomposites with different structures.As shownin Fig.2a-b,the diameter size of composite with 2-bilayer shell is less than that with 10-bilayer shell.On the surface of the composite,there appeared someflocculent substance that forms a shell wrapped outside.Fig.2c-d show the TEM images of MXene-COOH@(PEI/PAA)2@AuNPs and MXene-COOH@(PEI/PAA)10@AuNPs composites,respectively.It can beclearly observed that the gold nanoparticles loaded on surface of the 2-bilayer nanocomposite are much less than that of the 10-bilayer nanocomposite.It suggested that the obtained nanocomposite with 10-bilayer shell seemed more suitable to next catalytic experiments than that with 2-bilayer shell due to larger accommodation of gold nanoparticles and expected better thermal stability.

50 mg samples are dissolved in 50 mL deionized water and the magnetic agitation for 5 min,which makes it a good suspension.Then,in the case of magnetic agitation,the solution is dripped with 10 mL HAuCl4⋅4H2O solution,followed by the addition of 10 mL fresh NaBH4solution in the mixture to make it fully responsive for 20 min.After 20 min of reaction,the MXene-COOH@(PEI/PAA)n@AuNPs composite suspension was washed 7 times with deionized water.Finally,the MXene-COOH@(PEI/PAA)n@AuNPsnanocomposites were obtained by lyophilizer at-50°C temperature for 2-3 days.

All the present synthesized materials were obtained by lyophilizer at-50°C temperature via a lyophilizer(FD-1C-50,Beijing Boyikang Experimental Instrument Co.,Ltd.,China)to completely remove water over 2-3 days.The morphologies were characterized by a transmission electron microscopy (TEM,HT7700,High-Technologies Corp.,Ibaraki,Japan).Thermogravimetry(TG)characterizations were carried out using a NETZSCH STA 409 PC Luxx simultaneous thermal analyzer (Netzsch Instruments Manufacturing Co,Ltd,Seligenstadt,Germany)in an argon gasatmosphere.X-ray diffraction (XRD)analysiswas performed on an X-ray diffractometer equipped with a Cu Kα X-ray radiation source and a Bragg diffraction setup(SMART LAB,Rigaku,Akishima,Japan).X-ray photoelectron spectroscopy(XPS)was measured on an ESCALAB 250Xi XPS(Thermo Fisher Scientific,San Jose,CA,USA)using 200 Wmonochro mated Al Kα radiation.FT-IR spectra were obtained by Fourier infrared spectroscopy(Thermo Nicolet Corporation,Madison,WI,USA)via the KBr tablet method.

Fig.1.Schematic illustration of the fabrication and catalysis of MXene-COOH@(PEI/PAA)n@AuNPs composites.

Fig.2.(a,b)TEM images of the prepared MXene-COOH@(PEI/PAA)2,and MXene-COOH@(PEI/PAA)10samples;(c,d)TEM images of the prepared MXene-COOH@(PEI/PAA)2@AuNPs,and MXene-COOH@(PEI/PAA)10@AuNPs samples.

Fig.3.XRD curves of as-prepared materials:MXene,MXene-COOH,MXene-COOH@(PEI/PAA)2, MXene-COOH@(PEI/PAA)10, MXene-COOH@(PEI/PAA)2@AuNPs, and MXene-COOH@(PEI/PAA)10@AuNPs,respectively.

Fig.4.TG curves of MXene,MXene-COOH,MXene-COOH@(PEI/PAA)2,MXene-COOH@(PEI/PAA)10,MXene-COOH@(PEI/PAA)2@AuNPs,and MXene-COOH@(PEI/PAA)10@AuNPs samples,respectively.

From the flow chart in Fig. 1, it clearly indicates that theexperimental process is divided into two parts. The firstpart is mainly the preparation of MXene-COOH@(PEI/PAA)n @AuNPs composite materials. The first part includes thepreparation of composite materials after layer-by-layer assembly.Another part is catalysis of nitro compounds of the obtainedMXene-COOH@(PEI/PAA)n@AuNPs composites, such as 2-NAand 4-NP. Herein, we selected two composite materials withdifferent shell layers for gold nanoparticle modifications toform MXene-COOH@(PEI/PAA)2@AuNPs and MXene-COOH@(PEI/PAA)10@AuNPs composites, which wereused to catalyze nitro compounds.

Fig.5.XPS pro files of the MXene,MXene-COOH,MXene-COOH@(PEI/PAA)10,and MXene-COOH@(PEI/PAA)10@AuNPs samples,respectively.

3.2.Catalytic capacity of composites

The catalytic activity of novel metal nanoparticle is usually evaluated by catalytic reduction of nitro compounds[45,46].Fresh NaBH4(20 mL,0.01 M)solution was added to 2-NA(2 mL,5 mM)or 4-NP(2 mL,5 mM)at room temperature,and then MXene-COOH@(PEI/PAA)n@AuNPs composites(300 μL,1 mg/ml)suspension was added to the mixed solution to catalyze the above reduction reaction.In a certain period of time,the absorbance of the supernatant was measured by UV-visible spectrum.After adding fresh sodium borohydride solution,2-NA or 4-NP had obvious characteristic absorption peaks at 415 and 402 nm,respectively.In the present research system,MXene-COOH@(PEI/PAA)2@AuNPs and MXene-COOH@(PEI/PAA)10@AuNPs composites were investigated for catalytic reduction of 2-NA or 4-NP.

The UV visible spectra of 4-NP and 2-NA catalyzed by the MXene-COOH@(PEI/PAA)2@AuNPs nanocomposites are shown in Fig.7.As shown in Fig.7a,the fully catalytic reduction of 2-NA required 60 min.In the 2-NA catalytic reduction experiment,the linear relationship between ln(Ct/C0)and time(t)is a pseudo- first-order reaction(Fig.7c).After 8 cycles of catalysis,the catalytic efficiency of the same sample can still reach 70%,as shown in Fig.7e.In the absence of a catalyst,the 2-NA is mixed with a fresh sodium borohydride solution and the mixture is yellow in color.However,after adding the catalyst materials,the color of the mixed solution gradually changed from yellow to colorless,as shown in Fig.8b,indicating that the reaction of 2-NA in solution could be completely catalyzed.

从第一模态的时间序列(图2b)看出,感热通量有明显的年际和年代际变化,其中1983年、1998年和2012年有明显的转折,第一模态时间序列与气候态时间序列变化较为相似,其相关系数为0.4(通过99%置信度的显著性检验),因此第一模态的时间序列可以很好地代表高原感热通量随时间变化。由于感热通量具有明显的年际和年代际变化,为此对第一模态的时间序列进行了小波变换分析。从分析结果(图2c)看,高原感热通量具有4～5 a的主周期和8 a的副周期。

Fig.6.XPS pro files of MXene and MXene-COOH@(PEI/PAA)10@AuNPs nanocomposites:a and b,C1s;c and d,O1s;e and f,Ti2p;g,N1s;h,Au4f.

Fig.7.(a and b)Catalytic reduction of 2-NA or 4-NP with MXene-COOH@(PEI/PAA)2@AuNPs composite;(c and d)The relationship between ln(Ct/C0)and the reaction time(t)of 2-NA or 4-NP catalyzed by the MXene-COOH@(PEI/PAA)2@AuNPs nanocomposites catalyst;(e and f)The recyclable catalysis capacities of MXene-COOH@(PEI/PAA)2@AuNPs composite for the reduction reaction of 2-NA or 4-NP by NaBH4.

Moreover,we also studied the catalytic capacities of MXene-COOH@(PEI/PAA)10@AuNPs composites for 2-NA or 4-NP with a mixture of fresh NaBH4solution,as shown in Fig.9.As shown in Fig.9a,the composite fully catalyzes the mixture of 2-NAwith fresh NaBH4in 36 min.The ln(Ct/C0)and time(t)of the composite are linearly related,as shown in Fig.9c.The MXene-COOH@(PEI/PAA)10@AuNPs composites can completely catalyze the mixture of 4-NP with fresh NaBH4in 30min(Fig.9b).Inaddition,ln(Ct/C0)andtime(t)conformtothe pseudo- first-order reaction,as shown in Fig.9d.By catalytic cyclicrea ctionof2-NAor4-NP with fresh NaBH4solution,itcan be concluded that after 8 catalytic cycles,the catalytic efficiency stillc anstill beabove90%,indicating that the com positehas good recycling and suita blesta bility,as shownin Fig.9e-f.Inaddition,the MXene-COOH@(PEI/PAA)10@AuNPs composites catalyzed 4-NP and fresh NaBH4mixed solution,with similar color change from dark yellow to colorless,as shown in Fig.8a.After consecutive 8 cycles,the composite MXene-COOH@(PEI/PAA)10@AuNPs showed better stability and catalytic capacities than MXene-COOH@(PEI/PAA)2@AuNPs with less LbL assembled shell structures,demonstrating the obvious signi ficance of LbL self-assembly for the improvement of porous structures and anchored sites on surface in composite materials[51-54].It should be noted that easy aggregations between AuNPs prevent widespread applications.In recent years,various structures and composites with AuNPs have been designed and investigated.In our previous reported systems,the AuNPsmodified electrospun membrane nanocomposites[55]and diamond-based core-shell nanostructures[56]demonstrated eco-friendly prepared process and superior catalytic properties, as well as magnetically recyclable capacities. In present research work, the AuNPs-incorporated MXene-based core-shell nanocomposites show sufficient anchored sites and large spacial capacity for accommodation of well-dispersed AuNPs, which helps to avoid agglomeration and improve catalytic performances.

Fig.8.(a)4-NP changes in color before and after catalytic reactions;(b)2-NA changes in color before and after catalytic reactions.

Fig.9.(a and b)Catalytic reduction of 2-NA or 4-NP with MXene-COOH@(PEI/PAA)10@AuNPs composite;(c and d)The relationship between ln(Ct/C0)and the reaction time(t)of 2-NA or 4-NP catalyzed by the MXene-COOH@(PEI/PAA)10@AuNPs nanocomposites catalyst;(e and f)The recyclable catalysis capacities of MXene-COOH@(PEI/PAA)10@AuNPs composite for the reduction reaction of 2-NA or 4-NP by NaBH4.

4.Conclusions

The catalytic properties of the MXene-COOH@(PEI/PAA)n@AuNPs composites were evaluated mainly by reductivenitro compounds[46].The prepared MXene-COOH@(PEI/PAA)n@AuNPs composites suspension 10 mL was added to 2-NA or 4-NP with fresh NaBH4mixture.In a certain period of time,the absorbance of supernatant was measured by UV-Vis spectroscopy.In addition,we havestudied the cyclic stability of composite catalyzed nitro compounds.

In addition,under the same catalytic conditions,the MXene-COOH@(PEI/PAA)2@AuNPs composites suspension catalyzes the reduction of the mixture of 4-NP and fresh NaBH4.The mixture was completely catalytically reduced requires 57 min,far short of need in the catalytic 2-NA time,as shown in Fig.7b.The linear relationship between ln(Ct/C0)and time(t)is shown in Fig.7d.The catalytic reduction of 4-NP is also a pseudo- first-order reaction.As shown in Fig.7f,after a continuous catalytic reduction of 8 cycles,the catalytic efficiency was 74%.Therefore,it seemed that the catalytic effect for 4-NP was better than that of 2-NA.

Conflict of interest

There is no con flict of interest.

起终点站的配线设置主要有站前折返、站后折返和灯泡线折返3种形式。岛式车站一般采用站前折返形式，侧式车站一般采用站后折返形式，而灯泡线折返形式一般在较大广场或终点道路宽度条件受限的条件下采用。从减少有轨电车与地面道路交通的干扰考虑，有轨电车的起终点的位置更应结合城市建筑(如交通枢纽、大型商圈和人流密集区等)的布局统筹规划，车站配线亦应结合站位综合等因素进行合理选择。

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China(Nos.21473153 and 51771162),Support Program for the Top Young Talents of Hebei Province,China Postdoctoral Science Foundation(No.2015M580214),the Scientific and Technological Research and Development Program of Qinhuangdao City(No.201701B004),and Undergraduate Training Programs for Innovation and Entrepreneurship of Yanshan University(No.CXXL2017227).

此外，在加速布局海外业务的过程中，探索适应国际化发展战略的海外税务管理体系迫在眉睫且意义深远。我们应对挑战、化解难题的思路是合理优化公司控股架构、管控税务风险、降低海外所得税成本。但是，通盘管控就需要全局判断和全球视野。也是这个时候，我切身感受到“厚积薄发”四个字的魅力。此前在德勤为制造业、金融业、能源业等不同领域企业提供审计和税务咨询服务的专业经验、职业敏感度以及全球性的广阔视野等积累集中释放，于无形中对我当时全情投入的海外税务管理形成了强有力的支持。

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