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超疏水是指水与相应表面的接触角大于150°,滚动角小于10°的现象[1]。自然界中的很多表面都是超疏水性的,其典型研究如荷叶表面的毛状体褶皱结构[2],实现飞檐走壁的壁虎脚部[3],呈现疏水‒亲水交替界面的沙漠甲虫背部[4],可以轻盈地在水面上行走的水黾脚部[5]等等。在仿生学研究的启发下[6],人们做了巨大的努力来理解潜在的机理并探索构建具有特定功能的人造超疏水材料的新方法。超疏水材料在自清洁、防腐蚀、防覆冰、油水分离和抗菌材料等领域具有巨大应用优势[7-9],常用于构建超疏水材料的低表面能材料主要有烷烃类化合物[10-12]、有机硅化合物[13-15]、含氟化合物[16-18]等,但这些材料普遍存在生产成本高、环境污染大、制备工艺复杂等问题[19],严重限制了超疏水涂层的工业生产应用。人们需要寻找一种广泛存在、性能优异、环境友好的低表面能材料来促进超疏水材料的发展应用,石墨烯就这样进入了人们的视野。
自2004年英国曼彻斯特大学的两位科学家安德烈·盖姆(Andre Geim)和康斯坦丁·诺沃消洛夫(Konstantin Novoselov)首次用机械剥离法制得石墨烯以来,石墨烯(G)以其特殊的物理化学性质,引起了全世界科学家的极大兴趣。石墨烯是一种由碳原子以sp2杂化轨道形成共价键连接而成的蜂窝状二维结构材料,是构成石墨材料的最基本单元[20]。石墨烯由于其高电导率、高导热系数、高比表面积、高透光率和优异的机械性能,广泛应用于航空航天、石油化工、海洋船舶等领域,同时由于其良好的疏水性,不仅更易合成超疏水性材料,还可提升复合材料的机械稳定性、电热学特性,延长使用寿命,保证使用效果,扩大应用范围。同时,由于石墨烯合成技术的快速发展(于基体外延催化生长、天然石墨的化学剥落和对石墨烯的功能化修饰),使得人们可以设计和制造各种具有不同功能的石墨烯基超疏水材料,在生产应用方面展现出巨大潜力。
鉴于此,本文从超疏水现象形成的原理入手,梳理总结了石墨烯基超疏水材料的制备方法,讨论了现实当中的优势应用,并对石墨烯基超疏水材料未来发展的重点研究方向进行了展望。
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润湿性是固体材料表面的重要属性之一,接触角是衡量润湿性主要参数,包括静态接触角(Static contact angle,SCA or CA)和滚动角(Sliding angle,SA)。理想固体表面的静态接触角
$ \theta $ 可用Young’s[21]方程来描述,如图1(a)。$$ {\cos}\theta =\left({\gamma }_{\rm{SV}}-{\gamma }_{\rm{SL}}\right)/{\gamma }_{\rm{LV}} $$ (1) 图 1 材料表面常见润湿性模型示意图。(a)Young’s模型;(b)Wenzel模型;(c)Cassie模型;(d)Wenzel‒Cassie共存模型
Figure 1. Schematic of common wettability models on material surfaces: (a) Young’s model; (b) Wenzel model; (c) Cassie model; (d) Wenzel-Cassie coexistence model
式中,
$ {\gamma }_{\rm{SV}} $ 、$ {\gamma }_{\rm{SL}} $ 和$ {\gamma }_{\rm{LV}} $ 分别表示固‒气、固‒液和液‒气三个界面的界面张力。此时,这3种张力相互作用处于平衡状态。根据Young’s方程的润湿性理论,人们发展总结出了Wenzel模型和Cassie模型两种理想模型来阐述超疏水现象产生的原因,并在实际问题中发现了两种模型共存的状况。1936年,Wenzel[22]研究表面粗糙度与疏水性的关系,提出了Wenzel模型。Wenzel模型指在大多数粗糙固体表面,我们都假设水滴始终填满粗糙表面上的缝隙,称为“非复合接触”。如图1(b)所示,水滴完全进入粗糙表面的缝隙孔洞中的状态称为Wenzel模式。Wenzel方程引入了粗糙度因子r,提出液滴在固体表面接触时的表观接触角方程:
$$ {\cos}{\theta }_{\rm{W}}=r\left({\gamma }_{\rm{SV}}-{\gamma }_{\rm{SV}}\right)/{\gamma }_{\rm{LV}}=r{\cos}\theta $$ (2) 式中,
$ {\theta }_{\rm{W}} $ 和$ \theta $ 分别为粗糙表面和光滑表面上的表观接触角,r为固体表面的粗糙度因子,反映固体表面的粗糙程度。由于r≥1,从式(2)可以看出,对于疏水表面(
$ \theta $ >90°),表面越粗糙,$ \theta $ 越大;而对于亲水表面($ \theta $ <90°),表面越粗糙,则$ \theta $ 越小。但是,当物体表面处于热力学不平衡状态时,Wenzel方程将不再适用,这是由于表面起伏不平,液体在表面展开时的振动能小于由于表面粗糙度不平而造成的势垒,液滴则处于某种亚稳定状态而不能达到Wenzel方程所需的平衡状态。1944年,Cassie等在研究表面特殊润湿性的基础上,改进Wenzel方程,提出Cassie模型。Cassie模型可以将粗糙不均匀表面设想为一个复合接触表面,假设固体表面是由两种物质A和B构成,两种不同成分表面以远小于液滴大小的尺寸面积分布在表面。如图1(c)所示,水滴和粗糙截留空气的固体表面接触时,一部分水滴与空气气垫相接触,另一部分与固体表面突起直接接触。Cassie方程如下:
$$ {\cos}{\theta }_{\rm{c}}={f}_{\rm{A}}{\cos}{\theta }_{\rm{A}}+{f}_{\rm{B}}{\cos}{\theta }_{\rm{B}} $$ (3) 式中,
$ {\theta }_{\rm{c}} $ 为材料表面的表观接触角,$ {f}_{\rm{A}} $ 、$ {f}_{\rm{B}} $ 分别表示成分A和B所占的单位表观面积分数($ {f}_{\rm{A}} $ +$ {f}_{\rm{B}} $ =1)。$ {\theta }_{\rm{A}} $ 、$ {\theta }_{\rm{B}} $ 分别为液体与固体表面和空气的本征接触角。由于水对空气的接触角$ {\theta }_{\rm{B}} $ =180°,因此上式可以变为:$$ {\cos}{\theta }_{\rm{c}}={f}_{\rm{A}}{\cos}{\theta }_{\rm{A}}-{f}_{\rm{B}} $$ (4) 在Cassie模型下,由于部分液体和空气气垫接触,因而抑制了水滴流动阻力,有利于水滴的滚动。自清洁性超疏水表面即是基于Cassie模型制备的。
通常,水滴置于一个粗糙的固体表面上时,既可能发生Wenzel模型接触,也可能发生Cassie模型接触,但由于2种模型均为理想状态下的模拟情况,而事实上水滴会自主以接触能量低的方式在表面达到稳定接触状态,形成Wenzel-Cassie共存模型,如图1(d)。并且,在实际问题中,固液相接触时有从Cassie向Wenzel模型的转化趋势,但由Wenzel向Cassie模型转变则比较困难[23]。
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目前,构筑石墨烯基超疏水表面的方法主要有2种,一是在疏水材料(石墨烯材料)表面上构建粗糙的微纳结构,二是在粗糙的微纳结构表面接枝低表面能物质(石墨烯)进行改性[24]。现有的石墨烯基超疏水表面制备技术主要有表面修饰法、沉积改性法、激光诱导法、涂覆法、层层自组装法[25-29]等。下面将对各种制备技术进行一一阐述。
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表面修饰法是以具有一定微纳结构或粗糙度的基材为基础,通过在基材表面接枝低表面能物质,以达到降低表面能的目的。表面修饰法是制造超疏水表面的常用方法,在基于石墨烯的超疏水表面制备方面已有较多的研究进展,其中以将石墨烯材料作为低表面能改性物质、以石墨烯材料作为基材和将石墨烯材料作为改性填料3个方向是目前研究的热点。
将石墨烯材料作为低表面能物质直接对已有粗糙表面改性,是最直接的石墨烯超疏水表面构筑方式。刘海东等[30]将改性氧化石墨烯接枝到聚氨酯泡沫上,利用改性氧化石墨烯中的伯氨基团与聚氨酯泡沫上的腈基进行原位接枝反应,制得石墨烯改性的聚氨酯泡沫超疏水泡沫。经测试,水接触角可达166.2°,且对聚氨酯泡沫的稳定性有较大提升。Mo等[31]在氧化石墨烯表面移植嫁接双氨基‒聚二甲基硅氧烷(NH2‒PDMS‒NH2)桥状大分子,形成弧形聚二甲基硅氧烷大分子桥接石墨烯(GO‒g-Arc PDMS),形成过程如图2(a)所示,其结构如图2(c)所示,制备出石墨烯基超疏水表面,其水接触角可达153.4°,滚动角为8.5°,展现出良好的超疏水特性。Liao等[32]将聚氨酯纤维表面接枝不同含量的改性3-巯基丙基三甲氧基硅烷(MPTMS)氧化石墨烯和乙烯基聚二甲基硅氧烷(V-PDMS),制备出超疏水mGO/PDMS杂化涂层。纤维表面呈现出微纳结构和分层粗糙度,如图3所示,随着氧化石墨烯比例的升高,静态水接触角先升高后降低,最高达157°,其呈现出优异的超疏水性能。将石墨烯材料作为基材进行改性以构筑超疏水表面,为石墨烯基超疏水材料的构建提供了新的方向。He等[33]将疏水性Fe3O4纳米颗粒锚固到石墨烯表面,开发了三维超疏水/亲油材料。该材料水接触角为164.1±1.3°,油接触角0°,当该材料服役于严苛的强腐蚀性/盐溶液、有机溶剂和高温/低温系统中,呈现出优异且稳定的超疏水性能。
此外,由于石墨烯特殊的片层状结构,将石墨烯作为填料制备成疏水涂层已成为目前研究的热点。Saharudin等[34]将石墨烯,氧化石墨烯和经甲氧基三甲基硅烷修饰的氧化石墨烯分别作为填料制备成超疏水涂层。当填料为石墨烯时,其呈现出优异的超疏水性能,其接触角为162°。通过上述研究表明,将石墨烯进行改性修饰处理后,均可有效提高石墨烯材料的疏水性能。
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沉积改性法指利用物理、化学或电化学方法在基体表面沉积一层具有微纳结构的表面材料,并采用多种方法尽可能降低所构筑表面的自由能,使之成为超疏水表面的方法。其中,电化学沉积和化学沉积以其沉积稳定性强、成本低和操作简单等特点而被广泛研究。
电化学沉积是通过阴极发生的还原反应,在样品表面沉积一层金属及其氧化物,以满足超疏水表面所需的粗糙度。以电化学沉积为基础,在电解液中加入石墨烯材料,可将石墨烯引入沉积层中形成复合涂层制备出石墨烯基超疏水表面。Tang等[35]通过电化学技术将石墨烯沉积到如图4(a)所示的不锈钢网表面,通过优化网径大小调控表面的润湿性,形成树枝状仿生学表面的石墨烯基超疏水材料,如图4(b)所示,其修饰后网面的水接触角大于150°。Bai与Zhang[36]基于松果形结构,采用镍预沉积和高电流相结合的电化学沉积方法,在不锈钢基底上制备出具有松果样微纳结构的新型氧化石墨烯(rGO/Ni)复合涂层,如图4(c)和(d)所示,该涂层具有良好的超疏水特性,其静态水接触角为162.7°±0.8°,滚动角为2.5°±1.0°。Ding等[37]通过电化学沉积技术在低碳钢表面制备了具有分层结构的自清洁超疏水镍‒石墨烯杂化膜,其表面静态水接触角高达160.4°±1.5°,滚动角低至4°±0.9°,表现出优异的超疏水性。Liang等[38]通过电镀和电泳沉积方法,在铁‒钨非晶态合金上制备了氧化石墨烯修饰的硅烷偶联剂Si-69(BTESPT)复合涂层,经X射线光电子能谱仪(XPS)和傅里叶红外吸收光谱仪(FTIR)表征表明BTESPT形成了明显的硅氧烷网络并与Fe-W衬底和GO板紧密连接,该表面具备较好的疏水性,水接触角达141.7°。Zhu等[39]采用一步式的高压电化学沉积技术在单晶硅晶片基板表面制备出三元石墨烯/非晶碳/镍碳基薄膜,该薄膜静态水接触角达158.98°,滚动角为2.75°,表现出优良的超疏水特性。Jena等[40]通过电化学沉积技术在碳钢表面制备出Ni还原氧化石墨烯肉豆蔻酸超疏水涂层。当电解液温度为45 ℃时,该涂层表面为松果状结构,平均接触角为174°±1.5°,最高可达179°,滚动角约为1°,呈现出优异的超疏水性,同时其与基体具有良好的结合强度。
图 4 (a)石墨烯沉积的不锈钢网面;(b)石墨烯修饰不锈钢网的扫描电镜图像[35];(c)松果状石墨烯复合涂层;(d)松果状石墨烯复合涂层放大图[36];(e)花瓣形态石墨烯[41]
Figure 4. (a) Graphene-deposited stainless steel mesh; (b) SEM of graphene-modified stainless steel mesh[35]; (c) pinecone-like graphene composite coating; (d) magnified pinecone-like graphene composite coating[36]; (e) petal morphology graphene[41]
化学沉积法是利用基底与溶液或气体进行的化学反应,从而在基底的表面形成所需的转化涂层或薄膜。Yoon等[41]采用化学气相沉积技术,以甲烷为碳源,在3D Cu结构表面生长出花瓣形态的石墨烯,如图4(e)所示,其接触角为154.2°,具有良好的超疏水性。Zheng等[42]通过化学气相沉积技术,以甲烷和氢气的混合气体为碳源,在铜箔表面生成石墨烯层,经过刻蚀处理,将石墨烯层与经仿生微结构表面处理的铝合金基体结合,制备出具有仿生图案的石墨烯基疏水表面,静态水接触角为130.8°±2°。Ong等[43]将化学气相沉积技术与电化学沉积技术相结合,制备出3D石墨烯‒碳纳米管(G‒CNT)杂化结构材料,碳纳米管的加入增加了石墨烯表面的粗糙度,提高了材料的疏水性能,水接触角为148°,此外其具有优异的亲油性,可吸收其自重51倍重量的汽油。
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激光诱导法是指以激光照射经预处理的基材表面,从而诱导表面发生物理化学变化而制备石墨烯基超疏水材料的方法。
通过激光照射特定材料表面,可以直接诱导合成石墨烯。Li等[44]以二氧化碳激光器对聚酰亚胺(PI)膜进行诱导照射制备出石墨烯涂层。研究表明,通过调控反应气氛,可控制石墨烯涂层的疏水性能。当反应气氛为Ar或H2时,所制备的石墨烯涂层表面接触角均大于150°,分别为152°和157°,表现出良好的超疏水性;而当气氛为O2或空气时,其表面均表现出超亲水性。Nasser等[45]采用同样的技术制备石墨烯涂层,通过调控激光能量密度来控制涂层表面的几何结构进而调控涂层表面的疏水性。当脉冲扫描速率低于DPI 200时,其表面形成团簇状的花瓣结构,此时涂层表面呈现出疏水性能,接触角为161.1°;随着脉冲扫描速率的升高,涂层表面粗糙度降低,当达到DPI 1000时,涂层表面接触角为0°,此时石墨烯表面呈现出超亲水性能。上述研究表面,通过调控反应氛围及激光能量的强度均可有效改善石墨烯表面的润湿性。
此外,激光扫描方式对石墨烯表面的疏水性能也具有一定的影响作用。Wu等[46]以芋头叶表面为模板,通过autoCAD建模控制激光的扫描路径,在适当的激光强度下,使用两步诱导法制备出石墨烯基超疏水涂层,其制备过程如图5(a)~(d)所示,其表面形成与芋叶结构相似的微观结构,如图5(e)~(f)所示,表面接触角达到151.5°。
图 5 简要流程图[46]。(a)激光诱导过程;(b)预碳化过程;(c)模型化诱导过程;(d)扫描激光束工作流程;(e)预碳化聚酰亚胺(PI)膜的光学图;(f)经模型碳化的光学图;(g)芋叶的扫描电镜图
Figure 5. Brief flow chart[46]: (a) laser induction process; (b) pre-carbonization process; (c) modeling induction process; (d) scanning laser beam workflow; (e) optical diagram of pre-carbonized PI film; (f) model carbonized optical image; (g) SEM image of taro leaf
激光不仅能诱导石墨烯合成,还可对石墨烯材料表面进行分子级别处理,调节石墨烯材料性质。Wang等[47]通过激光干涉调节GO薄膜的成分,去除亲水基团制备出石墨烯基超疏水表面,制备过程如图6(a)所示,其微观结构如图6(b)和(c)所示,其接触角高达156.7°,表现出优异的超疏水性能。
激光诱导法制备石墨烯超疏水涂层,也为仿生表面的制备提供了途径。Song等[48]通过激光刻蚀和转移技术在不锈钢表面制备了具有超疏水性和高附着力的玫瑰花瓣结构石墨烯薄膜,其接触角达154.3°,且在酸性和碱性条件下,涂层仍呈现出优异的疏水性能。弯艳玲等[49]通过激光烧蚀技术在铝合金表面制备石墨烯超疏水涂层,其接触角为154.4°,改善了铝合金表面的疏水性能,同时延缓了水滴的结冰时间。Jiang等[50]采用激光全息技术,利用干涉激光的诱导、烧蚀和还原作用,在玻璃基体上制备出有具有分层粗糙度的微米级光栅结构石墨烯纳米片层表面,该表面静态水接触角为155°。
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涂覆法是将含有疏水改性材料的涂料通过浸泡、喷涂、旋涂等方式直接叠加覆盖到基体表面,经过固化、干燥处理后,形成具有一定粗糙结构的涂层。涂覆法操作简单、成本低廉、制备周期短、易于扩展到任何基体表面,适合大规模商业制备,是未来的发展趋势。在实际应用中,涂覆方法又包括浸涂、喷涂、旋涂和滴涂。
浸涂是指将基体放置在含有疏水材料的溶液中浸泡完成涂覆过程,多用于软性、易渗透的织物纤维材料。Shateri-Khalilabad与Yazdanshenas[51]将织物浸入氧化石墨烯分散液中,通过对其进行还原改性处理,制备出石墨烯层超疏水表面织物,其水接触角达163°±3.4°,滚动角为7°具备良好的超疏水性。Zhang等[52]将聚氨酯海绵浸入石墨烯纳米片和纤维素纳米晶须的去离子水混合液中后,制得了静态水接触角超过150°的改性聚氨酯石墨烯海绵,具有稳定的超疏水性。Peng等[53]将三聚氰胺海绵浸入氧化石墨烯溶液中并进行干燥固化处理,而后浸入疏水性高岭土溶液中,制备出高岭土改性的氧化石墨烯聚氨酯海绵,涂层呈现出良好的超疏水特性,其接触角为156.5°。
喷涂是将含有疏水材料的涂料通过喷枪喷涂、等离子喷涂等方法完成涂覆过程,多用于硬质基体。Lü等[54]将碳纳米管和还原氧化石墨烯作为填料制成涂料,通过气枪喷涂形成多层结构涂层,该涂层接触角达161°±1°,滚动角为2°±1°,具有良好的超疏水性能,且经过5000次摩擦后其疏水性未发生改变,表现出稳定的超疏水性能。Uzoma等[55]通过两步喷涂系统制备出氟硅烷改性的石墨烯涂层,该涂层接触角大于152°,滚动角小于7°,呈现出良好的超疏水性。Zhang等[56]利用静电喷涂技术,将环氧树脂‒聚四氟乙烯石墨烯聚多巴胺‒二氧化硅‒全氟辛基三乙氧基硅烷等物质按比例混合,制备出改性石墨烯涂层,涂层微观形貌如图7所示,该涂层接触角为156.3°±1.5°,滚动角为3.5°±0.5°,石墨烯的改性改变了涂层的表面形貌,提高了涂层的超疏水性能。
旋涂是指以旋涂方式将含有疏水材料的溶液涂覆至基材表面形成疏水涂层,也多用于硬质基体。Wang等[57]将通过电化学法剥离的石墨烯配以聚二甲基硅氧烷和3-氨丙基三乙氧基硅烷制作成悬浊液,以旋涂方式涂覆在铝基材表面形成超疏水涂层。该涂层静态水接触角高达160°±2°,滚动角为9°,不仅具有较好的自洁性,且对水和沙的冲击具有较强抵抗力。Liu等[58],将石墨烯粉在乙醇中进行高功率超声处理获得悬浊液,使用旋涂法在铝合金上沉积了具有优异机械耐磨性和耐腐蚀性的超疏水石墨烯薄膜。其接触角为153.7°±2°。
滴涂是指将含有疏水材料的溶液滴在基材表面,使溶液主动渗透至基材中形成疏水涂层,在多孔结构中有所应用。陈宁宁等[59]在AZ91镁合金表面做微弧氧化处理后,少量多次滴涂石墨烯‒硬脂酸共混溶液形成复合膜层。经测试,其接触角高达162°,远高于单独由硬脂酸涂覆形成的涂层,且新涂层的耐蚀性得到进一步提升。
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层层自组装法是生产各种微米级、纳米级结构和超疏水涂层的首选方法之一,包括自组装和层层组装两个部分。
自组装是指利用原材料本身的特性主动发生的组装而形成涂层的方法,Wang等[60]利用对苯二胺和氧化石墨烯的自组装特性,在二氧化硅基底上制备了石墨烯超疏水涂层,如图8(a)所示,该研究表明,通过自组装技术,无需其他有机表面活性剂进一步修饰,可制备出具有超疏水性能的石墨烯涂层,涂层接触角达150.8°,滚动角5.2°。Zhang等[61]通过将含锆有机金属框架材料UIO-66-F4纳米粒子组装在rGO基质上,合成了类似于三明治状结构的UIO-66-F4@rGO杂化体,如图8(b),并以羟基‒氟代聚硅氧烷作为偶联剂,成功在多种材料表面上制备了石墨烯基超疏水涂层。其在海绵和滤纸上形成的超疏水涂层的静态水接触角分别达到169.3°±0.6°和155.3°±1.2°,不仅具有良好的超疏水性,且对腐蚀环境和物理损伤表现出良好的抗性。Sin等[62]先在硅基板上溅射400 nm的Cu,尔后通过电泳沉积将氟化石墨烯(FG)沉积到铜箔上,制备出石墨烯基超疏水涂层,如图8(c)。研究结果表明,在Cu和FG的界面处形成了C‒F‒Cu‒F‒C连接的复合材料,该材料显著增强了超疏水涂层的结合强度,为实现在多种形状基材上进行表面改性提供了解决方案。
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超疏水表面凭借其特殊的浸润性以及较大的液体接触角使得超疏水材料在生产及生活上具有十分广阔的应用前景,如自清洁、防覆冰、油水分离等,结合不同的材料和使用领域,超疏水表面表现出独特的性能。石墨烯超疏水表面作为超疏水表面的一类,相比传统由氟硅烷构造的超疏水表面,整体质量更轻,原料更加环保,价格更具潜力,且在以下几方面可有广泛应用。
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超疏水涂层表面不能被液滴浸润,而是呈球状或椭圆形滚走,液滴滚动的同时会带走附着在材料表面的污渍和灰尘等,从而实现材料表面的自清洁。
Zhu等[39]在单晶硅晶片基板表面制备出三元石墨烯/非晶碳/镍碳基薄膜,通过自清洁实验,其表面的黄色粉笔末可完全被水滴带走,展现出较为洁净的板面,如图9所示,相较于纯的类金刚石镀膜(DLC)和非晶碳/镍碳基薄膜呈现出优异的自清洁性能。目前,传统外墙涂料由于普遍存在防污能力差的缺点,较大限制了其应用的拓展,而制备出用于建筑外墙、高层建筑玻璃幕墙上的超疏水自清洁涂料,可有效地防止大气中污染颗粒堆积和侵蚀,提高表面的耐玷污性能。
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雨雪等自然天气不仅会给高压输电线路、变电站、基站、通信电缆等电力、通信设备带来覆冰现象,引起电力及信号的传输不稳定甚至是大面积倒塌损毁,也会在飞机表面积累污染物给飞行安全带来巨大隐患。传统的热力除冰、机械除冰、化学除冰方式存在能耗大、效率低、污染重等不利因素,而涂覆超疏水表面形成抗冰层能够有效地减少覆冰量及降低冰雪对基底的附着力,也可有效延缓液体的结冰时间,同时具有施工方便,大量节省人力物力资源等优势。Wang等[63]制备出具有焦耳热效应的聚碳酸酯石墨烯基超疏水材料,经过防覆冰测试,其可延迟335 s的冷冻时间,并使冰晶不易在表面积聚,其防覆冰性能是普通的6倍,同时通过釉冰消除试验,在75 s内可完全去除表面3 mm厚的釉冰,呈现出优异的防覆冰及快速除冰性能。此外,该涂层经过不同PH值的酸、碱、盐溶液中浸泡24 h后,呈现出优异的防腐和超疏水性能;分别经过12.5 kPa和2.5 kPa的法向压力移动20 cm循环磨耗样品40次和400次后,表现出优异的超疏水性能。Akhtar等[64],采用表面修饰法制备了氟化石墨烯防覆冰涂层,并在蓝宝石玻璃基体表面上测试了涂层的防覆冰性能,如图10所示,相比原基体和普通石墨烯表面,氟化石墨烯涂层大幅度延缓了基体的结冰时间,并且在−10~−5 ℃延缓能力尤为明显。
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超疏水表面用于油水分离在实验室中的研究已经相当成熟,各类具备超疏水表面的吸油材料都展现出优异的油水分离能力,且具有较好的循环利用性。但受制于原料成本高及工艺较为复杂等因素,需要研究人员将材料成本再降低,分离效率再提高,循环耐久性再提升,以不断满足日益发展的生产需要。Liao等[32]制备出油水分离的超疏水mGO/PDMS杂化涂层,在几秒内水面和底部的油已全部被顺利分离,如图11所示;涂层对石油醚、己烷、甲苯等多种油的分离效率均高于90%,对三氯甲烷的分离效率达到了99.8%,且在经历15个分离循环后仍能保持98.4%的高分离效率;此外,将其分别浸泡于水、己烷、甲苯、NaCl、酸和碱溶液中,浸泡于不同温度(30、60、90、120和150 ℃)6 h后,其接触角几乎无变化,仍保持在150°以上,呈现出优异的分离效率、可重复使用性、热稳定性和化学稳定性。Das等[65]通过表面改性技术合成了磁性活化二维超疏水氧化石墨烯(MASHGO)粉末,该粉末水接触角高达152°,具有良好的疏水亲油性,对各种天然以及合成油在水包油和油包水乳液呈现出良好的油水分离能力。如图11(c)~(e)所示,未添加MASHGO前图(c),油滴均匀分布在水中,添加MASHGO后(d)~(e),油滴被吸附并逐渐团聚,在以铷磁铁控制粉末移动时,油滴基本上全部从水中分离出。
图 11 石墨烯基超疏水聚氨酯材料油水分离测试图。(a)轻油;(b)重油[32];(c~e)分别为MASHGO添加前、中、后的油水分离实验图及局部扫描电镜图[65]
Figure 11. Graphene-based super-hydrophobic polyurethane material oil‒water separation test: (a) light oil; (b) heavy oil[32]; (c‒e) are the oil-water separation experiment and partial SEM before, during, and after MASHGO addition[65]
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金属防腐是科学和生活关注的重点方向,涂层防护是金属防腐中简便的方法。在金属材料上涂覆或构造超疏水表面,可以减少腐蚀性液体与金属表面的接触,隔绝水分和霉菌,起到很好的防护作用。在石墨烯超疏水表面的研究中也证实,添加石墨烯材料的超疏水涂层防腐蚀性能得到进一步提高。Sadeghian等[66]在铜表面制备出石墨烯基疏水杂化涂层,与裸铜腐蚀能力对比,其防腐蚀性能提高15倍,呈现出优异的防护性能。Asaldoust等[67]通过表面改性法在氧化石墨烯表面合成了磷酸锌化合物,制备成石墨烯基超疏水涂层。该涂层显著提高盐雾试验中低碳钢的耐蚀性能,如图12所示。
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超疏水涂层在抗菌方面也有很大的应用,主要应用在生物、医学、食品包装以及工业和船舶设备等领域,而且应用范围正在逐步扩大。Ouadil等[68]制备出石墨烯/银纳米颗粒(PET-G/Ag)改性疏水聚氨酯织物,研究结果表明,PET-G/Ag改性的疏水聚氨酯织物可明显改善原织物的疏水和抑菌性能,呈现出优异防护性能,其可为医疗、纺织和水处理等领域的抗菌材料发展提供技术支撑。Jiang等[69]用激光诱导法在玻璃基板上制备了石墨烯基超疏水涂层,该涂层在阳光照射时可在2 min内将表面温度加热至55 ℃,并在10 min内稳定在60 ℃,显示出高的热转化率和超疏水性能。通过超疏水和吸热升温的协同作用,可使阳光照射下细菌数量减少率超过99.99%,如图13,即使无阳光照射也仍能保持99.87%的细菌减少率。并且,该涂层可快速涂覆到各种基材上,在多种抗菌领域均有广泛应用前景。
图 13 大肠杆菌菌落图及其扫描电镜图[69]. 未经阳光照射的PDMS(a,d),玻璃(b,e)和石墨烯涂层(c,f);经10 min阳光照射的PDMS(g,j);玻璃(h,k);石墨烯涂层(i,l)
Figure 13. Colony of Escherichia coli and its SEM[69]: PDMS (a,d), glass (b,e), and graphene-coated glass (c,f) without sunlight glass; PDMS (g,j), glass (h,k), and graphene-coated glass (i,l) after 10 minutes of sunlight glass
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综上所述,本文基于超疏水现象,说明了石墨烯超疏水材料的发展状况,分析了超疏水理论模型,综述石墨烯基超疏水材料的制备技术的国内外研究现状,并对其应用现状进行了分析。总体来看,石墨烯超疏水材料在油水分离、自清洁、防覆冰、耐腐蚀、抗菌等方面表现出巨大的应用价值,尤其在新冠病毒全球流行的今年,石墨烯基超疏水涂层的协同抗菌能力若用于口罩制作,将对疫情防控产生较大积极影响[25]。目前,大规模生产和应用石墨烯超疏水涂层还存在一定困难,还有很多亟待克服的技术难题,主要体现在以下几个方面:
(1)在规模化生产方面,生产成本高,工艺繁琐复杂,是阻碍石墨烯超疏水涂层大规模应用的主要原因;
(2)在长效性服役方面,石墨烯超疏水材料表面的机械稳定性和化学稳定性仍待提高,涂层表面易因机械磨损而失效;
(3)在综合性防护方面,现代表面处理技术对涂层综合性能的要求逐步提升,而石墨烯超疏水表面在防护机理上的研究严重不足,阻碍其综合防护性能的进一步提升。
为进一步适应生产需要,研究人员可从以下几个方面着手进行探索。
(1)优化石墨烯基超疏水涂层的制备工艺及综合性能。石墨烯基超疏水涂层作为一种前景广阔的材料,对其应用阻碍最大的还是工艺和性能问题。工艺流程要进一步简化,生产成本要进一步降低,制备时间要进一步减少,机械强度要进一步增加,综合性能要进一步丰富,只有性能足够优异,才能推动生产力发展,促进产业升级,也才能有更多资源投入到未来探索中;
(2)深化石墨烯基超疏水表面的作用机理研究。现阶段的研究报道中,根据实验现象得出具体结论的实验性文献偏多,而对具体现象深入进行机理分析的探究性报道较少。缺少足够的理论支撑,会导致超疏水表面的研究无法摆脱盲目性的束缚,难以在更高维度上从设计角度出发制造所需材料;
(3)建立超疏水表面统一的测试及评价体系。超疏水表面除了以共性的水接触角、滚动角来评价外,在涂层机械强度,涂层与基体的结合强度等方面测试、评价的标准不一,这直接导致同一研究方向的不同文献缺乏可比性参数的现象发生。同时,评价体系的建立也有助于今后超疏水表面向商业化发展。
Research progress in the preparation and application of graphene-based superhydrophobic materials
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摘要: 超疏水表面是具有独特性能的一类表面,本身就具有广泛应用前景。石墨烯材料作为理化性质出众的一类材料,由于其高电导率、高导热系数、高比表面积、高透光率和有优异的机械性能,广泛应用于航空航天、石油化工、海洋船舶等领域。目前,基于石墨烯材料构建超疏水表面,是超疏水表面研究中一个较新的方向。本文对超疏水表面的原理进行了概述,重点总结归纳了石墨烯基超疏水材料制备技术的研究现状,包括表面修饰法、沉积改性法、激光诱导法、涂覆法、层层自组装法等,简要介绍了石墨烯超疏水材料在自清洁、油水分离、防覆冰、耐腐蚀、抗菌等领域的应用,并对石墨烯超疏水材料的下一步研究方向进行了展望。Abstract: Superhydrophobicity in the surface is a phenomenon in which the contact angle between the water and the corresponding surface is greater than 150° and the rolling angle is less than 10°. A superhydrophobic surface exhibits unique properties and has a wide range of application prospects in the field of self-cleaning, anti-corrosion, anti-icing, oil-water separation, and antibacterial agents. In addition to its unique self-cleaning properties, it can play a distinctive role in the fields of building maintenance, anti-biological corrosion in ship bodies, medical antibacterial agents, etc. At present, low-surface-energy materials commonly used to construct superhydrophobic materials mainly include alkane compounds, organosilicon compounds, and fluorine-containing compounds. However, these materials generally have problems of high production costs, large environmental pollution, and complex preparation processes, which severely restrict the industrial production and application of superhydrophobic coatings. Graphene is a two-dimensional honeycomb-structured material formed by the covalent bonding of carbon atoms through sp2 hybrid orbitals. It is the basic unit of graphite, and it is the thinnest two-dimensional material found so far. As a class of materials with outstanding physical and chemical properties, graphene materials have always received extensive attention because of its high electrical conductivity, high thermal conductivity, high specific surface area, high light transmittance, and excellent mechanical properties. Therefore, graphene has been considered a promising material in aerospace, petrochemical, marine ships, and other fields. The construction of superhydrophobic surfaces based on graphene is a relatively new direction in the research of superhydrophobic surfaces at present. Although graphene-based superhydrophobic materials have shown excellent performance in the laboratory, they have not been used on a large scale in industrial production. In this paper, the principles of superhydrophobic surfaces were summarized, focusing on the research status of graphene-based super-hydrophobic materials preparation technology, including surface modification, deposition modification, laser induction, dip-coating method, and layer-by-layer self-assembly. The applications of graphene-based super-hydrophobic materials in the fields of self-cleaning, oil-water separation, anti-icing, corrosion resistance, and anti- bacterial agents were also introduced. Finally, this paper presents the prospective future research directions of graphene-based super-hydrophobic materials.
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Key words:
- graphene /
- super-hydrophobic /
- surface /
- preparation /
- application
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图 4 (a)石墨烯沉积的不锈钢网面;(b)石墨烯修饰不锈钢网的扫描电镜图像[35];(c)松果状石墨烯复合涂层;(d)松果状石墨烯复合涂层放大图[36];(e)花瓣形态石墨烯[41]
Figure 4. (a) Graphene-deposited stainless steel mesh; (b) SEM of graphene-modified stainless steel mesh[35]; (c) pinecone-like graphene composite coating; (d) magnified pinecone-like graphene composite coating[36]; (e) petal morphology graphene[41]
图 5 简要流程图[46]。(a)激光诱导过程;(b)预碳化过程;(c)模型化诱导过程;(d)扫描激光束工作流程;(e)预碳化聚酰亚胺(PI)膜的光学图;(f)经模型碳化的光学图;(g)芋叶的扫描电镜图
Figure 5. Brief flow chart[46]: (a) laser induction process; (b) pre-carbonization process; (c) modeling induction process; (d) scanning laser beam workflow; (e) optical diagram of pre-carbonized PI film; (f) model carbonized optical image; (g) SEM image of taro leaf
图 11 石墨烯基超疏水聚氨酯材料油水分离测试图。(a)轻油;(b)重油[32];(c~e)分别为MASHGO添加前、中、后的油水分离实验图及局部扫描电镜图[65]
Figure 11. Graphene-based super-hydrophobic polyurethane material oil‒water separation test: (a) light oil; (b) heavy oil[32]; (c‒e) are the oil-water separation experiment and partial SEM before, during, and after MASHGO addition[65]
图 13 大肠杆菌菌落图及其扫描电镜图[69]. 未经阳光照射的PDMS(a,d),玻璃(b,e)和石墨烯涂层(c,f);经10 min阳光照射的PDMS(g,j);玻璃(h,k);石墨烯涂层(i,l)
Figure 13. Colony of Escherichia coli and its SEM[69]: PDMS (a,d), glass (b,e), and graphene-coated glass (c,f) without sunlight glass; PDMS (g,j), glass (h,k), and graphene-coated glass (i,l) after 10 minutes of sunlight glass
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