Research status and analysis of cement and geopolymer hydrophobic composites
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摘要: 混凝土的长期耐久性问题是其面临的主要问题之一,造成耐久性破坏的主要原因是水在混凝土的多孔结构中的迁移,使有害离子更容易进入基材内部。采用超疏水材料对水泥及地聚物进行改性复合处理,赋予其超疏水特性,避免水分在其孔隙中的传输,从而防止有害离子的迁移及侵蚀,增强混凝土的耐久性。本文总结了目前研究当中对于水泥及地聚物胶凝材料超疏水改性方法,包括整体改性和表面改性两种;归纳了整体改性方式中超疏水改性剂加入到水泥及地聚物混凝土中的改性机制,以及与无机物基体中的连接键合方式;概括了目前研究当中表面改性常用的改性方法,包括喷涂法、浸渍法、模板法等,并分析了表面改性机制。与表面改性所得到的超疏水复合涂层相比,整体改性的水泥及地聚物基复合材料在实际应用场景当中具有更大的优势。此外,分析了疏水改性后对复合材料润湿性、防水性、抗压性能以及防腐性能的影响规律,发现其抗压强度降低了约20%~60%。最后,阐述了水泥及地聚物复合材料的疏水改性研究中存在的一些问题并对未来的研究方向进行了展望,建议从体积型超疏水、提高抗压强度、成本控制及疏水外加剂在材料内部实现均匀化分散等方面进行研究。Abstract: The long-term durability of concrete is one of the main problems it faces, and the main cause of durability damage is the migration of water in the porous structure of concrete, which makes it easier for harmful ions to enter the interior of the substrate. Modifying the cement and geopolymer with superhydrophobic materials is a valid method to avoid the transmission of water, thereby preventing the migration of harmful ions and increased its durability of concrete. This review summarized the superhydrophobic modification methods of cement and geopolymer cementitious materials and categorized into superhydrophobic surface and bulk modification; the modification mechanism of superhydrophobic modifier added to cement and geopolymer and their bonding mode with inorganic matrix in the internal modification method. Besides, the modification methods commonly used for surface modification in the present research are summarized, divided into external coating, maceration, template method, etc., and the surface modification mechanism is analyzed. Compared with the superhydrophobic composite coatings, the monolithically modified cement and geopolymer matrix composites have greater advantages in practical application scenarios. In addition, the effects of hydrophobic modification on the wettability, waterproofing, compressive properties and anti-corrosion properties of composites are concluded, and their compressive strength was reduced by about 20%~60%. Finally, some problems in the research of hydrophobic modification of cement and geopolymer composites are described and the future research direction is prospected, and it is suggested to carry out research on volumetric super-hydrophobicity, improvement of compressive strength, cost control, and homogeneous dispersion of hydrophobic admixture inside the material.
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近年来,随着工业化进程的迅猛发展,自然水体中抗生素、重金属等污染情况日益突出,抗生素-重金属复合污染物的综合毒性和迁移性对环境和人类存在巨大威胁[1]。诺氟沙星(NF)作为喹诺酮类抗生素被广泛应用于医疗及畜牧业等领域。然而,NF的过量使用导致大量残留物渗透到自然水体中,对水生态系统和人类健康造成严重危害[2]。此外,水体中重金属Cr(VI)污染问题也日益严重,因其难以降解并随食物链富集在人类体内,极大地危害人类的生命健康[3]。因此,抗生素和重金属复合污染体系的降解研究对水环境治理和生态修复有着重大的意义。但是,传统的生物降解法、吸附法和芬顿氧化法存在高耗能、低效率、多副产物及后期运行维护困难等诸多问题,而近年出现的光催化降解提供了一种经济、高效、环境友好的污染物处理方法[4]。在光催化过程中,具有适合能级的光催化剂吸收光子后,在价带(VB)和导带(CB)中产生空穴电子对(hVB+-eCB−),空穴可以与吸附的水分子反应产生•OH或直接氧化吸附的有害抗生素,将其矿化成生物毒性更小甚至无毒的物质;还原性电子将高毒性Cr(VI)还原为Cr(Ⅲ)[5-6],可将废水中的重金属转化为有用的资源。
CdS量子点(CdS QDs,<10 nm)由于量子束缚效应产生独特的电学、光学和催化特性,同时合适的能带结构和较短的电荷传输路径,显著降低了电子和空穴的复合效率,因而被视为最具有应用前景的可见光驱动光催化材料之一[7]。然而,CdS QDs存在表面吸附能力弱、光催化过程中易团聚、重复利用率低等缺点,影响其光催化性能。目前,通过将CdS QDs与其他半导体材料(TiO2[8]、Bi2MoO6[9]、Bi2WO6[10]、ZnIn2S4[11]、C3N4[12])复合或将其分散于载体上(MXene[13]、金属有机框架材料(MOFs)[14]、共价有机框架材料(COFs)[15]、碳材料(Carbon)[16]),能有效克服以上问题,从而提高材料的光催化性能。但是,繁琐的合成过程和载体材料不理想的锚定能力仍然会导致CdS QDs的团聚。寻找一种简便的合成工艺和高效的CdS QDs复合光催化材料仍具有吸引力和挑战性。
金属有机凝胶(MOGs)是一类具有刺激响应性、高比表面积、快速传质及良好表面亲和性的多孔材料,在荧光传感[17-18]、手性识别[19]、催化[20-21]、质子传导[22]及吸附[23]等领域具有广阔的应用前景。Liu等[20]通过溶剂热反应合成了两种含有三嗪分子的MOGs (JLUE-MOG-1/2)光催化剂,利用Fenton氧化和光催化的协同作用,实现对四氯化碳的高效降解;Zhou等[24]将磷钼酸(PMA)引入MOG-Cr中制备PMA@MOG-Cr光催化剂,通过各组分的协同效应,大大改善对有机染料的吸附及光催化能力。然而,到目前为止,关于MOGs作为光催化剂在可见光下降解复合污染物的报道还非常少。
与金属有机框架(MOFs)材料不同,MOGs兼具MOFs的微孔结构和颗粒堆叠形成的介孔结构,多级孔道和高比表面积使其能吸附金属离子并将其均匀分散地限制在孔道内。因此,我们设计以金属有机凝胶(MOG-Al)为基体,通过浸渍-硫化方法得到CdS QDs@MOX(Al)异质结光催化剂。MOX(Al)的基质作用与CdS QDs的敏化作用使异质结催化剂具有较大的比表面积和可见光响应性能,为催化反应提供更多的界面活性点位和传输路径。此外,表征并研究了所制备材料的相纯度、晶体结构和光学性能;探讨了其在可见光下协同降解NF和还原Cr(VI)的催化活性和稳定性;结合材料的电化学性能和活性物种捕捉实验,阐明光催化剂内部的载流子迁移路径及NF和Cr(VI)的可能降解机制。
1. 实验材料及方法
1.1 原材料
九水合硝酸铝(Al(NO3)3·9H2O)、无水乙醇、均苯三甲酸(H3BTC)、四水合硝酸镉(Cd(NO3)2·4H2O)、硫代乙酰胺(TAA)、重铬酸钾和诺氟沙星(NF),所用试剂均为分析纯,购于麦克林生化科技有限公司(中国);实验用水均为去离子水。
1.2 催化剂的制备
1.2.1 MOX(Al)的制备
将Al(NO3)3·9H2O(1.5 mmol)与H3BTC(1 mmol)溶于15 mL乙醇,混合液转移至50 mL水热釜中,在120℃烘箱中保持2 h,冷却至室温,得到淡黄色湿凝胶,记为MOG(Al);为方便表征,将湿凝胶在60℃真空干燥过夜后研磨得到干凝胶颗粒,记为MOX(Al)。
1.2.2 CdS QDs@MOX(Al)的制备
将MOG(Al)湿凝胶浸入含有一定量Cd(NO3)2·4H2O(0.5 mmol、0.75 mmol、1.0 mmol、1.25 mmol)的乙醇溶液中,在避光环境中密闭浸渍48 h;用乙醇清洗湿凝胶表面后真空干燥,得到白色干凝胶颗粒。将干凝胶颗粒加入到硫代乙酰胺(TAA,1.5 mmol)乙醇溶液中,超声分散30 min后,将混合溶液转移至水热釜中,在180℃烘箱中保持3 h;冷却至室温后离心分离,用乙醇多次洗涤收集到的固体,干燥后研磨得到一系列不同CdS QDs含量的MOX(Al)颗粒,分别记为0.5-CdS QDs@MOX(Al)、0.75-CdS QDs@MOX(Al)、1.0-CdS QDs@MOX(Al)、1.25-CdS QDs@MOX(Al),如表1所示;为了进行对照实验,按照之前的报道制备了纯CdS颗粒[25]。
Sample Mole ratio of Cd2+ : MOX(Al) 0.5-CdS QDs@MOX(Al) 0.50 0.75-CdS QDs@MOX(Al) 0.75 1.0-CdS QDs@MOX(Al) 1.00 1.25-CdS QDs@MOX(Al) 1.25 1.3 催化剂表征
X射线衍射光谱(XRD)使用布鲁克D8粉末X射线衍射仪,在5°~60°的2θ范围内使用Cu-Kα靶(λ=0.15405 nm)测定;X射线光电子能谱(XPS)采用赛默飞公司250 Xi能谱仪对样品的表面组成和元素化合态进行测定,所有的结合能都采用284.6 eV处不定碳的C1s峰校准;在p/p0=0.0~1.0的范围内,采用贝士德PS2000 H表面积孔隙度分析仪测定氮气吸附-脱附等温曲线,所有样品在423 K真空下脱气12 h;日本电子JEM2100 F透射电子显微镜(TEM)对催化剂形貌进行表征;紫外-可见漫反射光谱(UV-Vis)利用安捷伦Cary5000型光谱仪测定样品在200~800 nm范围内的吸收光谱,以BaSO4作为基准物;采用上海元析UV-9000 S型紫外可见分光光度计测定污染物的吸光度。
1.4 电化学测试
通过瞬态光电流(TPC)响应和电化学阻抗谱(EIS)测量所制备的纯MOX(Al)、CdS和CdS QDs@MOX(Al)颗粒的表面电荷分离和转移效率。实验在CHI760 E电化学工作站上进行,石英池中采用传统的三电极配置:制备样品FTO薄膜作为工作电极,Ag/AgCl电极作为参比电极,Pt作为对电极。采用Na2SO4(0.5 mol/L)水溶液、含有5 mmol/L Fe(CN)63−/4−的KCl(0.1 mol/L)混合水溶液分别作为TPC和EIS的电解质。在没有光照的情况下,Na2SO4(0.5 mol/L)水溶液作为电解质,在1000 Hz固定频率和50 mV振幅下测量Mott-Schottky曲线,以估算样品的平带位置(EFB)。
1.5 光降解实验
采用配有420 nm滤光片的300 W氙灯作为光源,在室温下进行可见光降解实验。在氙灯光源下放置带有循环冷却水的夹套石英皿作为光反应器。将0.04 g催化剂加入到60 mL Cr(VI)与NF的混合水溶液中(pH=5.78),在避光条件下磁力搅拌30 min达到吸附-脱附平衡后,进行光照反应90 min,每隔10 min取0.5 mL样品经0.45 µm的水系滤膜注入石英比色皿中,通过紫外-可见分光光度计测定其吸光度;反应后的催化剂经乙醇洗涤3次干燥后进行再生,用其进行重复性实验。
根据朗伯-比尔定律,降解率w可按下式计算:
w=(C0−Ct)/C0×100% (1) 其中:C0 为污染物的初始浓度(mg/kg);Ct 为t时刻污染物的反应浓度(mg/kg);Cr(VI)、NF溶液的初始浓度分别为40 mg/L、100 mg/L。
通过伪一级动力学模型对各体系中污染物的降解速率常数进行拟合,其表达式为
ln(C0/Ct)=kt (2) 其中:k为表观速率常数(min−1);t为反应时间(min)。
2. 结果与讨论
2.1 催化剂组成与结构分析
采用XRD对材料的晶体结构进行表征,如图1所示。MOX(Al)的XRD特征衍射峰属于典型的馒头峰,结晶度较低,衍射峰宽且弱,晶体结构介于无定形络合物和高度有序的MOFs之间。MOX(Al)材料由MIL-100(Al)纳米颗粒堆叠组成[26]。CdS QDs@MOX(Al)复合材料的XRD图谱可以清楚地识别CdS的存在。在26.45°、43.87°和51.95°处的特征衍射峰分别归属于(111)、(220)和(311)晶面,所有的峰都与立方体晶相CdS的标准光谱(JCPDS 89-0440)一致,且无其他明显杂峰。同时,随着材料中CdS含量的增加,CdS的特征峰强度逐渐增强,MOX(Al)的特征峰强度逐渐降低。但其衍射峰位置几乎没有变化,说明MOX(Al)和CdS QDs的相互作用没有改变光催化剂本身的晶体相结构。
为进一步确定CdS QDs@MOX(Al)复合材料中CdS的颗粒尺寸及分散性,对样品进行TEM分析,如图2所示。从TEM图像(图2(a)、图2(b))中可以观察到球形CdS QDs均匀地分散在不定形凝胶颗粒中,通过Digital Micrograph软件计算CdS颗粒的平均尺寸为7.23 nm(3~12 nm),这证实了CdS QDs的形成[27]。HRTEM图像(图2(d))显示CdS QDs@MOX(Al)上有清晰的晶格条纹,层间距d为0.32 nm,与CdS(111)晶面的平面间距(JCPDS 89-0440)一致。MOX(Al)紧密包裹着CdS QDs,有利于形成异质界面来增强电子的转移和界面电荷的分离,可以有效防止CdS QDs的光腐蚀,提高光催化剂的活性和稳定性。
利用X射线光电子能谱(XPS)分析CdS QDs与MOX(Al)的相互作用及元素的化学价态,如图3所示。从图3(a)可知,相比于纯MOX(Al),复合材料1.0-CdS QDs@MOX(Al)的全谱图中能明显观察到归属于C1s、Al2p、O1s、Cd3d、S2s、S2p的特征峰,说明复合材料中CdS的存在。进一步分析Cd、S、Al元素的特征峰结合能,如图3(b)~3(d)所示。图3(b)中结合能在411.6 eV和404.8 eV的两个特征峰对应于Cd2+的Cd3d3/2和Cd3d5/2[27];S2p的高分辨谱图如图3(c)所示,在162.5 eV和161.2 eV处有两个强峰,归属于S2p1/2和S2p3/2,表明S元素在复合材料中主要以S2−的形式存在,以上进一步证实凝胶中CdS QDs的形成。Al2p光谱中对应于Al与有机配体中O原子结合(Al—O)的峰值发生约0.2 eV的正移(图3(d)),同时复合材料1.0-CdS QDs@MOX(Al)中Cd3d3/2和Cd3d5/2相比于文献[15,28]中纯CdS的特征峰(411.9 eV和405.2 eV)出现0.3 eV的偏移,表明CdS和MOX(Al)之间的异质结作用,部分电子通过MOX(Al)中有机配体的氧从Cd2+转移到Al3+[29],这有可能使CdS QDs更加分散和稳定。
为了获得样品的孔隙结构和比表面积,使用比表面分析仪进行测试,图4为样品的N2吸附-脱附等温线。纯MOX(Al)和CdS QDs@MOX(Al)复合材料都表现出典型的IV型曲线。在低压区(0~0.2)的吸附量急剧增加,表明材料中存在着微孔;而在相对压力p/p0为0.6~0.9区间内呈现出H3型滞后环,证明了介孔结构的存在。图4插图的孔径分布显示出样品分别在1 nm和6 nm左右有较大的孔体积,同样说明了复合材料具有多级孔道结构。表2列出了所制备样品的比表面积(BET)、孔体积(BJH)和平均孔径(BJH)。其中MOX(Al)和1.0-CdS QDs@MOX(Al)的比表面积、孔体积和孔径分别为1302.72 m2/g和1052.8 m2/g、1.4345 cm3/g和1.2485 cm3/g、4.79 nm和4.23 nm,由于CdS QDs在凝胶中高度分散性,使复合材料仍然保持多孔结构,并拥有较大的比表面积和孔体积,为污染物的吸附及降解提供更多的活性位点和传输路径。
Sample Surface aeraa/(m2·g−1) Pore volumeb/(cm3·g−1) Pore diameterc/nm MOX(Al) 1302.72 1.4345 4.79 0.5-CdS QDs@MOX(Al) 1288.56 1.3614 4.74 0.75-CdS QDs@MOX(Al) 1198.52 1.2617 4.35 1.0-CdS QDs@MOX(Al) 1052.80 1.2485 4.23 1.25-CdS QDs@MOX(Al) 1021.23 1.2111 3.97 Notes: a—BET multi-point method specific surface; b—BJH method desorption (Cylindrical pore model, 2.0-49.6 nm) pore volume; c—BJH method desorption (Cylindrical hole model) average hole diameter. 图5是样品的紫外-可见漫反射光谱(UV-Vis DRS)。可以看出,纯MOX(Al)和纯CdS的吸收边带分别在340 nm和530 nm左右;而复合材料的吸收边缘均发生红移,扩展到580 nm处。说明CdS与MOX(Al)间相互作用形成异质结构,拓宽了复合材料的可见光响应范围,并增强了光吸收能力。根据Kubelka-Munk公式[30]计算可知,MOX(Al)和CdS的带隙值(禁带宽度)分别为3.56 eV和2.25 eV(图5插图)。
αhv=A(hv−Eg)n/2 (3) 其中:α为吸收系数;h为普朗克常数;v为入射光频率;A为常数;Eg为带隙宽度值;n值为1。
2.2 光催化活性和稳定性分析
在模拟可见光下协同降解NF/Cr(VI)复合污染物,评估CdS QDs@MOX(Al)系列光催化剂的光催化活性,如图6所示。空白实验表明,在无光催化剂的情况下,NF/Cr(VI)复合污染物没有明显降解,体系较稳定。暗吸附过程中,各样品对NF和Cr(VI)都有一定的吸附效果并达到吸附平衡。其中纯MOX(Al)表现出最好的吸附效果,得益于其高比表面积和多级孔道结构。在可见光光照后,纯MOX(Al)由于较弱的光吸收能力造成其光催化活性较低;而纯CdS在复合体系中表现弱吸附、强光催化能力,但空穴-电子对难以有效分离,使其光催化活性显著下降。与之相比,CdS QDs@MOX(Al)复合材料呈现出较好的光催化能力,随着CdS QDs含量的增加,光催化活性先增加后降低,其中1.0-CdS QDs@MOX(Al)的光催化活性最高。值得注意的是,1.25-CdS QDs@MOX(Al)呈现出较弱的吸附和催化能力,应该是过量CdS QDs降低了复合材料的比表面积和孔体积,以致活性位点减少[31];同时CdS QDs团聚生长形成纳米颗粒,限制了光生载流子的有效转移。
通过伪一级动力学模型来描述NF和Cr(VI)降解过程的降解速率,如图7所示。MOX(Al)、CdS、0.5-CdS QDs@MOX(Al)、0.75-CdS QDs@MOX(Al)、1.0-CdS QDs@MOX(Al)和1.25-CdS QDs@MOX(Al)对NF(Cr(VI))的降解表观速率常数k分别为3.25(1.74)×10−3、3.74(4.24)×10−3、4.06(5.52)×10−3、7.24(5.71)×10−3、19.8(14.93)×10−3和2.73 (4.54)×10−3 min−1。其中,1.0-CdS QDs@MOX(Al)表现出最优的光催化降解效率,分别是纯MOX(Al)和CdS的6.1(8.5)倍和5.3(3.5)倍。复合材料光催化活性的显著增强,主要归因于MOX(Al)和CdS QDs之间的协同作用。MOX(Al)作为基质为复合材料提供高比表面积和多级孔道来吸附更多污染物;CdS QDs的敏化作用提高复合材料的光吸收能力,在界面处产生更多的空穴-电子对;二者匹配的能级结构形成Type-II型异质结,加速界面处电荷转移并降低光生载流子复合率[32],更多的活性基团用于降解污染物。
图8对比了NF和Cr(VI)在单独及复合体系中的降解率。单一污染物体系中,CdS QDs@MOX(Al)对NF和Cr(VI)的降解率分别为62.5%和54.25%;而在NF/Cr(VI)复合体系中,NF和Cr(VI)的降解率分别提高到80.1%和79.51%。表3中比较了已报道的不同催化剂催化性能。结果表明,复合体系中NF的氧化和Cr(VI)的还原具有协同效应。Cr(VI)的还原可以消耗激发电子,而NF的氧化可以消耗空穴,这两个过程协同作用有效地阻止了光生电子和空穴的复合,从而提高了复合体系的降解效率[33]。
Photocatalyst/Amount(mg) Pollutants/V(mL)/C0(mg·L−1) Light source Time/h Efficiency/% Ref. GTSA/25 Cr(VI)/35/50 UV mercury light 3.0 79 [6] 3%CdS QDs/BiOI/Bi2MoO6/20 NF/20/20 Xe lamp 1.0 93 [9] 15%Co9S8/g-C3N4/20 Cr(VI)/50/10 500 W Xe lamp 3.0 87 [34] Bi2S3/Bi2WO6/20 Cr(VI)/20/10 500 W Xe lamp 1.0 88 [35] ZnO/Cu2O NF/20/10 Light intensity 50 mW/cm2 4.0 86 [36] 1.0-CdS QDs@MOX(Al)/40 Cr(VI)/60/40 300 W Xe lamp 1.5 79.5 This work 1.0-CdS QDs@MOX(Al)/40 NF/60/100 300 W Xe lamp 1.5 80.1 This work Notes: GTSA—Thiourea/sodium alginate; V—Volume. 为了研究光催化剂的稳定性,进行重复性实验,结果如图9所示。在循环使用4次后,1.0-CdS QDs@MOX(Al)对NF和Cr(VI)的降解率略有降低。光催化效率的降低可能是由于回收过程中光催化剂的损失。为了进一步研究光催化剂的稳定性,对重复使用的催化剂进行了XRD分析,如图10所示。光催化剂的晶体结构没有明显变化,仍存在较强的CdS衍生峰。表明催化过程中还原Cr(VI)和氧化NF的协同作用迅速消耗光生电子和空穴,有效抑制了CdS的光腐蚀,展现出复合材料较好的稳定性[34]。
2.3 光催化降解机制分析
2.3.1 主要活性物种分析
为了评估各活性物种在CdS QDs@MOX(Al)-Cr(VI)-NF体系中降解污染物的贡献,在待降解污染物溶液中分别加入2 mL的异丙醇(IPA)、三乙醇胺(TEOA)、氮氧自由基哌啶醇(TEMPO)作为羟基自由基(•OH)、空穴(h+)、超氧自由基(•O2−)的捕获剂,其他操作与光降解实验过程一致。实验结果如图11所示,当在体系中分别加入TEMPO、TEOA、IPA时,NF和Cr(VI)的降解率分别为59.78%、26.85%、79.06%和55.22%、71.6%、78.1%。在不加任何捕获剂时,1.0-CdS QDs@MOX(Al)复合材料对NF和Cr(VI)的降解率分别为80.1%和79.5%。添加IPA对降解过程并没有明显的抑制作用,表明•OH在光催化过程中贡献不大;添加TEMPO后•O2−被捕获,使NF和Cr(VI)的降解率都有所下降;而在添加TEOA后,光生h+被TEOA消耗,NF的降解率显著下降,同时由于部分NF被吸附到催化剂孔道中不能被矿化降解,占据活性位点,使Cr(VI)的降解率略有下降。由此可知,h+和•O2−作为主要活性物种存在于光催化体系中。
2.3.2 光催化活性增强的机制
为了研究光催化剂的界面电荷传输效率,进行了电化学阻抗(EIS)和瞬态光电流(TPC) 响应测试,如图12(a)、图12(b)所示。EIS Nyquist图上的圆弧半径反映了在电极表面发生的电荷转移速率,电弧半径的减少表明界面电荷转移和光生电子-空穴对的有效分离[37]。与MOX(Al)和CdS相比,1.0-CdS QDs@MOX(Al)复合材料的圆弧半径显著减小,表明CdS QDs@MOX(Al)的界面电荷转移电阻更小。同时,从光电流-时间曲线可以发现,在连续的开/关光循环中,1.0-CdS QDs@MOX(Al)的光电流密度明显高于MOX(Al)和CdS,这意味着光生电荷在CdS QDs@MOX(Al)复合材料中得到了有效分离。因此,1.0-CdS QDs@MOX(Al)在表面具有比其他样品更好的界面电荷转移能力,这与它在污染物降解过程中具有较高的光催化活性是一致的。
为了了解光催化反应过程中光诱导电子的传输方向,采用莫特-肖特基(M-S)曲线估算了MOX(Al)和CdS的平带电位(EFB),如图12(c)所示。M-S曲线的正斜率表明MOX(Al)和CdS均为n型半导体[38]。参比于Ag/AgCl电极,MOX(Al)和CdS的EFB值分别为−0.79 eV和−0.88 eV。对于n型半导体,根据公式(4),计算出MOX(Al)和CdS的ECB值分别为−0.60 eV和−0.69 eV(vs NHE)。此外,结合MOX(Al)和CdS的带隙宽度值,根据式(5)计算得到样品的价带位置,MOX(Al)和CdS的EVB值分别为2.96 eV和1.56 eV。为更直观的观察MOX(Al)和CdS的能带位置,判断电荷的迁移路径,在图12(d)中绘制了MOX(Al)和CdS的能带结构示意图。
ENHE=EAg/AgCl+0.19eV (4) EVB=ECB+Eg (5) 其中:ENHE为氢电极电势;EAg/AgCl为Ag/AgCl电极电势;EVB为价带电势;ECB为导带电势;Eg为半导体禁带宽度(eV)。
基于上述讨论,提出了可见光(λ>420 nm)下CdS QDs@MOX(Al)复合材料对复合污染物光降解活性增强的合理机制,如图13所示。大量的CdS QDs均匀地分散在界面接触紧密的MOX(Al)中,在可见光照射下,CdS受光照激发产生光诱导电子-空穴对。由于 CdS的ECB (−0.69 eV)负于MOX(Al)的ECB (−0.60 eV),CdS导带中的光生电子转移至MOX(Al)的导带中,并且其CB位置比O2/•O2−的标准氧化还原电位(−0.046 eV vs NHE)更负,使部分光生电子将吸附的O2还原为•O2−[39]。MOX(Al)导带上富集的光生电子将Cr(VI)还原成Cr3+,•O2−将NF氧化成降解产物。同时,MOX(Al)价带上的空穴被传输到CdS表面,价带上的聚集空穴直接将NF氧化成降解产物。因此,CdS QDs@MOX(Al)光催化活性的提高可归因于Type-II型异质结的形成,并通过0D/3D纳米复合结构产生更多的界面内建电场,从而实现光生电子-空穴对分离效率的提高及光生载流子的有效迁移。
3. 结 论
(1) 通过凝胶限域法成功制备出CdS量子点@金属有机凝胶(CdS QDs@MOX(Al))异质结光催化剂,表征证明光催化剂具有较高的比表面积和可见光吸收能力,为污染物的吸附及降解提供更多的活性位点。
(2) 1.0-CdS QDs@MOX(Al)对诺氟沙星(NF)/Cr(VI)复合污染物体系表现出优异的光催化活性,降解过程符合伪一级动力学模型,表观速率常数k分别是纯MOX(Al)和CdS的6.1(8.5)倍和5.3(3.5)倍。相比于单一污染物体系,复合体系中氧化还原过程的协同效应,有效提高了复合材料的催化效率和抗光腐蚀性。
(3) 活性物种捕获实验表明h+和•O2−是CdS QDs@MOX(Al)-Cr(VI)-NF体系中的主要活性物种。结合材料的电化学性能分析表明, CdS QDs与MOX(Al)构筑的0D/3D纳米复合结构,产生更多的界面内建电场,界面间Type-II型异质结构,加速了光生电子-空穴对的分离效率及光生载流子的有效迁移。
致谢:本实验的分析表征工作由吉林化工学院分析测试中心协助完成,在此表示感谢。
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表 1 不同改性方法对水泥及地聚物材料表面改性的优劣对比
Table 1 Comparison of advantages and disadvantages of different modification methods for surface modification of cement and geopolymer materials
Cementitious types Coating preparation Method Test method WCA/(°) Surface coating of cementitious materials DC-30 (contains mainly octane-silane and siloxane)[27] External coating Abrasion of 200 grit sandpaper for 20 m under a load of 2.5 kpa 158 Aqueous solution of sodium laurate[33] Maceration — 150 Sandpaper and polydimethylsiloxane [35] Template — 142 Triethoxyoctysilane and diatomaceous earth low
surface materials [36]External coating Sandpaper for 18.00 m under 24.50 kpa load 158 Organosilicon functionalized Al2O3 + solid resins materials [37] External coating 400 cm abrasion of sandpaper under 200 g load 165 Surface coating of geopolymer materials Polymethylhydrosiloxane [40] External coating — 161 Polydimethylsiloxane solution containing
polytetrafuoroethylene /
stearic acid and fly ash [39]Maceration Polydimethylsiloxane -only coatings have higher adhesion than polydimethylsiloxane coatings containing fly ash 159 Notes: WCA—water contact angle 表 2 整体改性对超疏水水泥及地聚物材料的水化/聚合作用的影响
Table 2 Effect of Integral Modification on the Hydration/Polymerization of Superhydrophobic Cementitious and Geopolymer Materials
Cementitious types Coating preparation Test method Effects on hydration/polymerization WCA/(°) ordinary silicate cement[62] non-toxic lauric acid and covering metal mesh XRD Integral superhydrophobic concrete has fewer hydration products than ordinary silicate concrete. 153 magnesium oxychloride based cement [48] hydroxyl-terminated polydimethylsil-oxane XRD、SEM Nanoscale needle-like phases are covered by hydrophobic silicone rubber. >150 ordinary silicate cement[50] functionalization of SiO2 with fluorine-free silanes XRD、FTIR Silanes in superhydrophobic powders react with cement hydration products to slow down the cement hydration rate. 153.8 ordinary silicate cement[61] Stearic acid modified fly ash TGA Fly ash can increase the water-cement ratio and facilitate cement hydration, providing more nucleation sites for cement hydration. 93.2 ordinary silicate cement[64] nano-silica and isobutyl-triethoxysilane isothermal calorimeter Silane can mitigate the loss of flowability caused by nano-silica to some extent, while nano-silica can completely compensate for the delay of silane in the early hydration process. 153.5 fly ash based polymer materials [53] polymethylhydrosiloxane Hot Plate Method Thermal conductivity decreases with increasing amount of polymethylhydrosiloxane, and the higher the porosity, the lower the bulk density. 161 fly ash based polymer materials [58] polymethylhydrosiloxane TEM Grafting of poly(methylhydrosiloxane) is not exactly proportional to the amount of geopolymer produced. 152 fly ash-slag base polymer materials [56] isooctyltriethoxysilane BSE、EDX Silanes slow down the hydration kinetics, while the increase in the modulus of the alkali activator inhibits the formation of hydration products. 118.1 slag mortar [57] polydimethylsiloxane MIP、SEM、EDS Polydimethylsiloxane increases internal defects in the body. 128 表 3 不同改性剂的超疏水水泥基材料的接触角和滑动角
Table 3 WCA and SA of superhydrophobic cementitious materials with different modifiers
Cementitious types Hydrophobic Modifiers Modification type WCA/(°) SA/(°) ordinary silicate cement [62] 0.8 wt% non-toxic lauric acid Integral 153 10 magnesium oxychloride based cement [48] 6 wt% hydroxyl-terminated polydimethylsiloxane Integral >150 <10 high belite sulphoaluminate cement [65] lauric acid Integral 153.2 — ordinary silicate cement [66] 1H, 1H, 1H, 2H-perfluorodecyl-triethoxysilane Surface 163.3 — ordinary silicate cement [67] contains mainly octane-silane and siloxane Surface 160±1 6.5±0.5 ordinary silicate cement[68] hydrophobic silica nanoparticles Surface 160 1.7 Notes: SA—sliding angle 表 4 不同改性剂的超疏水地聚物材料的接触角和滑动角
Table 4 WCA and SA of superhydrophobic geopolymer materials with different modifiers
Geopolymer types Hydrophobic Modifiers Modification type WCA/(°) SA/(°) calcined clay and slag [69] 5 wt% polydimethylsiloxane Integral 120 — fly ash [70] 5 wt% stearic acid Integral 96.67 — metakaolin[71] 5 wt% polydimethylsiloxane Integral 127.5 — metakaolin [40] polymethylhydrosiloxane Surface 161 2 dust/silicate cement [72] polydimethylsiloxane Surface 154.1 6.1 表 5 超疏水水泥及地聚物复合材料的吸水率降低程度和原因
Table 5 Extent and causes of water absorption reduction in superhydrophobic cement and geopolymer composites
categories Materials and Dosages Modification type water absorption/% reason ordinary silicate cement 1 wt% stearic acid[74] Integral −86% Surface water is rejected by the surface. 1.4 wt%
cetyltrimethoxysilane[76]Integral −86% The internal inorganic mineralized layer further prevents water intrusion through a relatively dense micro/nano-scale two-layer structure. 5 wt% SiO2 silica solution[30] Surface −90% Hydration products and some unhydrated nanoparticles can clog capillaries, thus blocking water transfer paths. geopolymer materials 60 wt% iron ore tailings +1.5 wt% stearic acid[79] Integral −43% The particle size of superhydrophobic iron ore tailing is much smaller than that of sand, and the fine superhydrophobic iron ore tailing can easily fill up the pores of the mortar, making the mortar more dense and leading to a decrease in water absorption. 10 wt% hydrophobic metakaolin + polydimethylsiloxane [55] Integral −26%~−27.6% Weakening of the capillary's ability to absorb and hold water. polydimethylsiloxane + polypropylene fiber [81] Integral −70% Polypropylene fiber easily adsorbs polydimethylsiloxane but does not easily trap water vapor, blocking the water vapor diffusion channel. 表 6 超疏水水泥及地聚物材料的腐蚀电位和腐蚀电流密度
Table 6 Corrosion potential and corrosion current density of superhydrophobic cement and geopolymer materials
Concrete material name Hydrophobic Modifiers Ecror /V Icorr/(A·cm−2) hardened cement mortar [47] polydimethylsiloxane − 0.07204 4.10×10−8 superhydrophobic surface for concrete [29] stearic acid − 0.28225 — superhydrophobic concrete [82] containing silane and siloxane — 7.702×10−6 superhydrophobic concrete [52] lauric acid −0.477 1.26×10−5 superhydrophobic concrete [96] stearic acid −0.173 5.921×10−7 superhydrophobic iron ore tailings [79] stearic acid −0.321 1.451×10−6 Notes:Ecror is a mixed electrode potential determined by the cathode and anode reactions on the corroded surface; Icorr is the amount of electricity per unit area per unit time of cathodic protection on a metal electrode. -
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目的
混凝土的长期耐久性问题是其面临的主要问题之一,造成耐久性破坏的主要原因是水在混凝土的多孔结构中的迁移,使有害离子更容易进入基材内部。为了提高混凝土的应用耐久性,受“荷叶效应”的启发,通过在水泥及地聚物复合材料的内外表面构建微纳结构或覆盖低表面自由能物质的方式对复合材料进行疏水改性,疏水添加剂可以在材料表面形成疏水膜,从而阻止水的渗透。
方法本文根据目前研究当中的两种主要超疏水改性方式,表面改性和整体改性两个方面入手,介绍了以硅烷改性剂为主的水泥及地聚物复合材料的疏水改性机制。水泥及地聚物材料常用的表面改性方法包括喷涂法、浸渍法、模板法;而整体改性是引入疏水性填料或是加入有机改性剂,同时还可以通过调整混凝土的配合比如水灰比、砂石粒径分布等,改变混凝土的孔隙结构,以提高疏水性能。并总结分析了疏水改性后对复合材料润湿性、防水性、抗压性能以及防腐性能的影响规律。
结果水泥或地聚物复合材料的超疏水表面改性的基本原理是提高材料表面粗糙度及降低表面自由能。提高表面粗糙度可以通过覆盖铜网、砂纸或是加入纳米颗粒来获得,本文所阐述的降低材料表面的自由能主要是通过加入有机硅化合物来实现的。其硅烷改性的基本原理为硅烷可以与大气中的水分或混凝土表面孔隙中的水发生反应,通过三个烷氧基的水解形成硅烷中的含硅烷醇的基团,然后缩聚成低聚物,通过硅烷和混凝土之间的羟基键合,与材料表面形成共价连接。而水泥材料的整体疏水改性主要是水泥与水接触后发生水化反应,超疏水有机改性剂当中的硅烷有机物可以与水泥中的硅酸盐矿物发生反应通过Si—O—Si键合形成连续的有机膜,将水化产物结合成一个整体网络;地聚物复合材料是Si—O和Al—O在碱性环境中下断裂后再重组缩聚,在碱性激发剂的作用下,原料中的硅酸根和铝酸根发生解聚释放出硅酸根离子和铝酸根离子,随着反应的进行,水化硅酸根和铝酸根离子开始相互聚合,形成N—A—S—H凝胶,有机硅化合物与N—A—S—H通过Si—O—Si或Si—O—Al键合,N—A—S—H被疏水基团包围,使复合材料具有整体疏水性。通过对水泥基材料和地聚物材料进行表面改性或整体改性后其水接触角均表现出不同程度的增加;其孔隙率在对复合材料进行疏水改性后也出现了不同程度的增加,但也出现了孔隙率降低的情况;值得注意的是改性后材料的吸水率出现了大幅度降低的情况,尤其是水泥基材料的吸水率最高降低了约90%,阻隔了水分子进入到基体中,提高了材料的耐久性;并且通过对整体改性后的水泥或地聚物复合材料在3.5 wt%NaCl电解液中进行腐蚀试验,发现改性后的复合材料的防腐性能也有所增加;但对两种复合材料进行整体疏水改性后,材料的抗压强度表现出降低的趋势,其抗压强度降低了约20%~60%,这也是目前整体疏水改性所面临的问题之一。
结论通过对水泥和地聚物复合材料的疏水改性的研究现状进行总结,归纳了目前超疏水改性水泥及地聚物胶凝材料的改性方法及机制,并对改性后的复合材料的性能包括润湿性、防水性、抗压性能和防腐性能进行了分析评价。经对比发现,不同方法制备的超疏水复合材料可以使防水性得到较大提升,但也会导致其他性能的下降,例如抗压强度等。可以从体积型超疏水、提高抗压强度、成本控制及疏水外加剂在材料内部实现均匀化分散等方面进行研究。