专论 正式出版 版本 5 Vol 9 (5) : 415-433 2018.
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光催化薄膜的亲水性及其应用
Hydrophilicity of Photocatalytic Films and its Applications
: 2018 - 05 - 17
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摘要&关键词
摘要:玻璃表面的雾化及建筑物表面的污染严重影响人们日常生活及工业。传统方法通常借助外力解决表面雾化及污染问题,如人工清洗,不仅维护成本高,而且施工难度大。光催化亲水薄膜作为一种具有特殊润湿性的表面,可高效持久地实现基底表面的防雾和自清洁功能。近年来,防雾和自清洁产品的市场需求不断扩大,因此光催化亲水薄膜的研究及系统性综述至关重要。本文详细阐述了光催化薄膜的亲水原理及亲水性评价方法,重点介绍了亲水性的改善方法,并对缺乏研究的亲水性与光催化分解有机污染物活性关系进行了简要概述,总结了国内外光催化薄膜亲水性的应用现状,分析了其存在问题及发展方向,旨在于为光催化薄膜亲水性的研究及其应用起到一定的指导作用。
关键词:Key words: Photocatalytic films; hydrophilicity; anti-fogging; self-cleaning;;
Abstract & Keywords
Abstract: Background, aim, and scope Nowadays, glass fogging and building exterior pollution have influenced all aspects of our social lives. For instance, fogging of rearview mirror would affect visibility and increase the occurrence of traffic accidents; the fouling of building walls and sculptures would affect appearance and even corrode the material. Traditionally the surface cleaning was performed manually, which is expensive and high risky, especially for skyscrapers cleaning. Recently, the photo-induced hydrophilic film, as one of the special wetting surface, has emerged as a self-cleaning surface that can be used for anti-fouling function with low maintenance cost and operation risk. The good water wettability can be obtained under solar light, which can effectively reduce the refraction and total reflection to achieve the anti-fogging effect. In addition, the dust and pollutants can be removed thoroughly by rainfall to prevent surface fouling. In this background, it is necessary to conduct a systematical review on photocatalytic hydrophilic film. Materials and methods In this review, we focused on the following aspects: (1) the hydrophilic theory of photocatalytic film; (2) the evaluation methods of hydrophilicity; (3) the relationship between photocatalytic activity and hydrophilicity; (4) the strategies of improving hydrophilicity; and (5) the application level of photocatalytic hydrophilic film. Results Firstly, taking TiO2 for example, four typical hydrophilic theories of photocatalytic film are summarized as following: (1) 2D/3D capillary effect: simulating spike-decorated tubes to porous materials with different cohesion and adhesive force of water inside and outside of thin tubes, the water molecules tend to infiltrate or rise in the thin tubes and crack of spikes, which macroscopically results in water wettability on the film surface; (2) photo-induced generation of surface vacancies: oxygen vacancies can be formed at the two coordinated oxygen bridging sites near the surface under UV irradiation, and the oxygen vacancies can increase the amount of hydroxyl radicals formed by the chemisorbed water molecules; (3) photo-induced reconstruction of Ti-OH bonds: the photo-generated holes (h+) of TiO2 can transport to the surface and be captured by lattice oxygen, which weaken the binding energy between Ti and lattice oxygen so that water molecules can rupture this bond resulting in the formation of new Ti-OH bonds; (4) photo-oxidation of adsorbed hydrocarbon: the adsorbed organic contaminants are decomposed by photocatalysis to obtain UV-induced hydrophilic surface that the water droplet slides to the clean surface and spreads completely. Secondly, the evaluation method for analyzing hydrophilic surface of photocatalytic film is described by measuring water contact angle, referring to ISO 15989. According to the international standard, the droplet of water (1~2 μL) is suspended at the end of the syringe needle followed by transferring to the specimen surface, and ten preferable points on the specimen are selected for contact angle measurements through contact angle meter to calculate the average as contact angle of the specimen. Thirdly, referring to several articles, the relationship between photocatalytic activity and hydrophilicity of photocatalytic film can promote each other. Furthermore, according to the influencing factors of hydrophilicity and four typical hydrophilic theories, strategies to improve hydrophilicity of photocatalytic film are summarised and generalized into regulating chemical composition and morphology structures. Finally, the practical applications of photocatalytic hydrophilic films are summarized. Many famous foreign companies (such as Pilkington, TOTO and PPG) have concentrated on investigating and manufacturing anti-fogging and self-cleaning products in the mid-1990s, while the relevant research started relatively late in China since 2000. So far, a few commercial products based on hydrophilic TiO2 films have gradually developed, including anti-fogging and self-cleaning glass. For example, the self-cleaning glass produced by Zhongke Nanotechnology Engineering Center Co.,Ltd., has been applied in the National Grand Theatre and car showrooms to realize self-cleaning effect. Disscusion For hydrophilic theories of photocatalytic film, 2D/3D capillary effect pays attention to surface morphology of films, while the other three theories lay emphasis on hydrophilic transformation under UV irradiation. ISO 15989 is the international standard to measure water-contact angle of films, and it is worth noting that the horizontal plane and measurement points should be chosen carefully to ensure data reliability. Photocatalytic activity and hydrophilicity of photocatalytic films can promote mutually. On one hand, the photocatalytic film can decompose organic contaminants and the water droplet slides to the clean surface to improve hydrophilicity. On the other hand, the improvement of hydrophilicity can increase hydroxyl groups on the surface of photocatalytic film to facilitate pollutant decomposition. The above mentioned strategies can improve hydrophilicity of photocatalytic films. For regulating chemical composition, the hydrophilicity can be effectively improved by widening light-response range and enhancing photocatalytic activities. For regulating morphology structures, the hydrophilicity can be promoted by constructing the porous or rough surface. Hence, the former strategy is suitable for practical applications by coating photocatalytic hydrophilic materials on glass and building surface, but the latter with rigorous process is mainly applied in the laboratory research. Combining photocatalysis with hydrophilic surface can facilitate practical applications, which is matured and industrialized in foreign countries. Although domestic research advances have been achieved in laboratory, it’s still insufficient in industrialization. Conclusions The photocatalytic hydrophilic films can realize anti-fogging and self-cleaning surfaces to overcome the disadvantages of traditional methods. Thus it shows promising application prospects in many fields. However, in terms of hydrophilic films in practical applications, some problems are still needed to be solved, including low utilization of sunlight, weak adhesion, short durability of photoinduced super-hydrophilicity, difficulties in obtaining super-hydrophilic surface at low temperature for industrialization. Recommendations and perspectives Therefore, it is needed to focus on solving the above problems, which will play an important role in accelerating the progress and leading the future development trend of photocatalytic hydrophilic films in anti-fogging and self-cleaning applications.
Keywords: Photocatalytic films; hydrophilicity; anti-fogging; self-cleaning
玻璃表面的雾化及建筑表面的污染,不仅影响人们的日常生活和建筑物美观及使用寿命,而且其清洁难度大、维护成本高,甚至会造成严重后果,例如,汽车挡风玻璃表面雾化会影响能见度,增加交通事故发生的几率;石材雕塑的表面污染不仅影响美观,甚至可能滋生细菌的生长,腐蚀破坏其结构等。因此,研究如何实现具有防雾和自清洁功能的表面至关重要。大量研究表明,亲水性光催化薄膜表现出良好防雾(Watanabe et al,1999;刘湘梅和贺军辉,2010;Du and He,2012)、自清洁(Linsebigler et al,1995;Premkumar,2004;Parkin and Palgrave,2005;Euvananont et al,2008;Fateh et al,2011)作用。Wang et al(1997,1998)报道了紫外光诱导下TiO2光催化薄膜的超亲水现象,即H2O与TiO2光催化薄膜表面接触,在受紫外光照射下接触角由几十度迅速变小,最后达到0±1°。停止光照后,将TiO2长时间置于黑暗中,接触角会逐步增大重新恢复至疏水状态,若再经紫外光照射又会转变为超亲水状态。这种亲水性光催化薄膜防雾的实现主要在于亲水性表面能有效减少雾滴对光线的折射和全反射,从而降低雾化度并增强材料透光率,达到防雾的效果。自清洁原理(Nishimoto and Bhushan,2013)主要体现在两方面:一方面是光催化作用,在光照下薄膜表面有机污染物可被降解,使水滴在洁净表面铺展达到亲水状态;另一方面是亲水性使水滴在薄膜表面易于铺展形成水膜,降低污染物的附着力,经风力和雨水冲刷去除污染物(图1)。因此,光催化薄膜表面的亲水性越强,则其防雾、自清洁效果越好。


图1   亲水表面的自清洁过程:从(a)到(b)(Nishimoto and Bhushan,2013)
Fig.1 Self-cleaning process on a superhydrophilic surface: from (a) to (b)
20世纪90年代中叶,很多发达国家的知名公司(如英国Pilkington公司、日本TOTO公司、美国PPG公司、德国GEA公司、VTA公司和UIC公司等)将光催化技术与亲水表面相结合,从事防雾和自清洁玻璃、陶瓷、涂料等的研发及生产,并逐步进入商业化水平。国内相关研究虽起步较晚,但也取得了显著进展,自2000年以来,多家企业和科研单位先后成功研制了自清洁建材玻璃,并陆续投入商品化生产,如中科纳米技术工程中心有限公司的常温固化纳米自清洁玻璃技术,结合玻璃深加工工艺完成大板面自清洁玻璃的制作,其在紫外光照射下表现出良好的亲水性,应用于国家大剧院和汽车展示厅玻璃等建设项目实现自清洁效果(封玉凤和王利新,2013)。近年来,随着防雾和自清洁产品等的市场需求不断增大,光催化亲水薄膜被广泛应用在日常生活、工农业、医药、建筑、军事等众多领域(表1),其亲水性优化及产业化的研究至关重要。因此,在掌握光催化薄膜的亲水性原理、亲水表面的测试方法及其影响因素的基础上,研究如何有效改善光催化薄膜的亲水性能,实现具有防雾、自清洁作用的功能表面意义重大。
表1   不同基底上的亲水薄膜性能及应用
性能
Properties
薄膜基底
Substrates
应用
Applications
参考文献
References
防雾
Antifogging
玻璃/塑料
Glass/plastic
车辆镜、医用镜、眼镜、农用棚膜
Vehicle mirrors, medical scopes, glasses, agricultural greenhouse films
Takagi et al,2001;Hashimoto et al,2005;Ohdaira et al,2007;周适明和奚邦为,2010;Lee et al,2010
自清洁
Self-cleaning
瓷砖/水泥/玻璃/塑料
Tile/cement/glass/plastic
建筑外墙、混凝土路面、汽车镜、窗户、户外帐篷
Building walls, concrete pavements, car mirrors, windows, outdoor tent
Cassar,2004;Bai,2005;Fujishima,2008;Chen and Poon,2009;Eiamchaia et al,2009;Liu and Jiang,2012;Nishimoto and Bhushan,2013
本文详细阐述了光催化薄膜亲水性的4种典型理论及其评价方法,重点介绍了光催化薄膜亲水性的改善方法,主要包括调控化学组成和调控形貌结构等,并对缺乏研究的亲水性与光催化去除有机污染物活性的关系进行了简要概述。此外,基于国内外光催化薄膜亲水性的研究及应用现状,本文分析了其存在问题及今后发展方向,旨在于为光催化薄膜亲水性的研究及其应用起到一定的指导作用。
1   光催化薄膜亲水性的原理及其评价方法
1.1   亲水性的典型理论
固体表面润湿行为是表面化学的一个重要特征,通常以接触角(CA)来表征液体对固体的润湿程度(Feng and Jiang,2006)。接触角是指在气、液、固三相交点处所作的气-液界面的切线与固-液交界线之间的夹角(θ),通常将固体与水接触角CA < 90°、CA < 5°的表面分别称为亲水、超亲水表面,而将固体与水接触角CA > 90°、CA > 150°的表面分别称为疏水、超疏水表面。
固体表面的润湿特性由表面化学组成及微观几何结构共同决定,接触角是材料表面润湿性的宏观表征。自然界中存在很多天然的亲水性表面,利用扫描电子显微镜(SEM)观测常见亲水植物的表面微观结构(图2):可见紫花琉璃草(图2a)与松萝凤梨(图2b)表面分布有大量的孔道结构,这些类似于海绵状的结构使其对水的吸收高达自身干重的20倍;天鹅绒竹竿叶片(图2c)表面分布着均匀一致的微米级乳突,而在紫叶芦莉草(图2d)的乳突上又分布着圆锥状突起,这些微通道结构能促使水在其表面快速铺展。这些独特的表面微结构赋予叶片优异的亲水性能(Koch and Barthlott,2009)。以为基础,研究者通过光刻蚀、热处理、溶剂刻蚀等方法改善材料表面的微结构以调控材料的亲水性能(图3)(Kunihiro et al,2000;Zhang et al,2005;Sun et al,2004;Yu et al,2002;Cebeci et al,2006;Du and He,2012)。表面化学组成是影响材料亲水性的另一重要因素,表面羟基的含量是主要影响因素(Yu et al,2001)。Prabhu et al(2017)利用溶剂热法合成还原石墨烯(rGO)与TiO2复合催化剂,并通过旋涂法合成复合薄膜,结果表明薄膜表面水接触角由复合前的34°降低为5°,亲水性能得到显著提升,主要由于rGO的添加增强了TiO2光致电子(e-)与空穴(h+)的分离,引起更多Ti4+被还原为Ti3+,产生大量氧空位,而氧空位可增强表面化学吸附水的含量,促进材料表面亲水性能。


图2   亲水植物表面的SEM图:(a)紫花琉璃草,(b)泥炭藓,(c)天鹅绒竹竿,(d)紫叶芦莉草(Koch and Barthlott,2009)
Fig.2 SEM micrographs of hydrophilic plant surfaces: (a) rhacocarpus purpurescens, (b) sphagnum squarrosum (c) calathea zebrine, (d) ruellia devosiana


图3   (A)-(D)分别为三角网状聚酰胺薄膜的FESEM图,聚N-异丙基丙烯酰胺(PNIPAAm)修饰后的硅片基底SEM图,多孔TiO2薄膜AFM图,聚丙烯氯化铵(PAH)/SiO2聚电解质多层薄膜的AFM图(上)及其接触角测试结果(下)(Zhang et al,2005;Sun et al,2004;Yu et al,2002;Cebeci et al,2006)
Fig.3 (A)-(D) Typical FESEM images of the triangular net-like structure of the polyamide film; SEM image of PNIPAAm films on the silicon substrates; AFM images of the surface of porous TiO2 thin film; AFM image of polyelectrolyte multilayer film prepared from (PAH 7.5/SiO2 8.0)12 with (PAH 4.0/SPS 4.0)4 adhesion layers (top) and their contact angle respectively (bottom)
近年来TiO2光催化薄膜由于其特殊的光诱导亲水现象及优异的光催化性能备受研究者的关注(图4)(Wang et al,1997,1998,1999;Watanabe et al,1999;Nakajima et al,2001;Sun et al,2001;Gao et al,2004)。Wang et al(1997)研究发现,TiO2多晶薄膜在紫外光照射前的水接触角为72±1°,而紫外光照射后水滴铺展其接触角降为0±1°。同时经紫外光照射的薄膜表面,甘油三醇或十六烷等油性液滴的接触角也明显减小。即紫外光照射后TiO2表面显现出优异的双亲特性,而在暗处放置一段时间后薄膜表面的接触角又逐渐恢复至光照前的状态。此外,其他半导体氧化物光催化材料如ZnO和SnO2等,当紫外光照射后水滴在其表面的接触角也会发生明显变化,且增加光强和延长光照时间都会显著加速材料表面亲水性的转变(Sun et al,2001;Miyauchi et al,2002)。基于大量光催化亲水薄膜的研究,关于其亲水性有以下四种典型的理论:(1)2D/3D毛细管效应;(2)光致表面空位的产生;(3)光致表面Ti-OH的重构;(4)光催化致表面有机物的分解。


图4   (A)-(E)分别为光照射前后,水滴在不同方法制备TiO2薄膜表面的铺展状态:(a)紫外光照射前的材料为疏水表面;(b)紫外光照射后的材料为亲水表面(Wang et al,1997;Sun et al,2001;Watanabe et al,1999;Wang et al,1999;Gao et al,2004)
Fig.4 (A)-(E) The spreading of water before and after irradiation on the TiO2 film prepared by different methods: (a) hydrophobic surface before UV irradiation; (b) highly hydrophilic surface after UV irradiation
1.1.1   2D/3D毛细管效应
当液体与含有细微缝隙的物体接触时,在浸润情况下液体沿缝隙渗入或上升的现象。在浸润情况下,缝隙越细,液体上升越高,即液体在细管状物体的内侧因为内聚力及附着力的差异,克服地心引力而向上升的现象称之为毛细管效应。对于表面较光滑对的材料,其接触角θ可通过Young’s方程式(1)表示,其中γSLγSγL 分别代表固-液、固-气、液-气界面间的表面张力。对于表面较粗糙的材料,Wenzel提出了接触角θ′用方程式(2)表示,其中γ为粗糙因子,由材料的实际面积与几何投影面积的比值决定。所以cosθ′cosθ之间的关系可用方程式(3)表示,由此可知表面粗糙度对材料的固有浸润性有一定影响,可进一步影响材料表面的亲水或疏水性能。
------------(1)
------------(2)
------------(3)
假设材料表面具有多维度的孔道结构,即在材料本体的表面会有较大的凸起,而在每个凸起表面又分布大量的小凸起结构,则可对粗糙材料表面进行近似模拟(图5)(Bico,2002)。对于表面能高的材料表面而言,其固有水接触角小于90°,假设材料表面不同维度的孔道尺寸小于其毛细高度,则基于毛细作用水滴可浸润材料的表面,同时大孔上的小孔可以进一步促进水的浸润及铺展。因此,基于材料表面多孔、微通道等结构的影响,其表观接触角远小于材料的固有接触角,使材料的表面亲水性得以改善,甚至具有超亲水性能。


图5   多维度孔道模拟多孔材料表面:材料表面有不同维度孔道,H2O浸润大孔及微孔道并促进水在表面铺展(Bico,2002)
Fig.5 Tube decorated with spikes, to simulate porous materials, then water molecules should invade both the tube itself and spikes to promote H2O wetting the film surface
1.1.2   光致表面空位的产生
Wang等人(1997,1998)提出TiO2光诱导亲水过程是由紫外光照射下形成了表面空位缺陷所致。TiO2表面为五配位Ti原子与二配位O原子结构,其反应活性高于本体内部的六配位Ti原子与三配位O原子的配位结构。在紫外光照射下,TiO2表面生成电子空穴对,这些活性物种与二配位处的桥位氧相互作用产生氧空位,同时将Ti4+转变为Ti3+。这些缺陷的存在有益于吸附空气中的水分子并产生-OH形成化学吸附层,进一步吸附空气中的水分子形成物理吸附层(图6)(Banerjee,2015;Highfield and Gratze,1988;Hugenschmidt et al,1994;Henderson,1996),在缺陷位置形成亲水微区,其它位置则为疏水微区。


图6   光诱导亲水过程:(a)缺陷位点的形成,(b)电子还原Ti(IV)-Ti(III)及空穴与桥位氧反应产生氧空位(Banerjee,2015;Highfield and Gratze,1988;Hugenschmidt et al,1994;Henderson,1996)
Fig.6 Photo-induced hydrophilic process: (a) formation processes of defective sites, (b) reduction Ti(IV) to Ti(III) by electrons and reaction of holes and bridge oxygen to create vacancies
随着TiO2表面电子空穴产生反应的进一步加强,在TiO2表面构成了均匀交替分布的纳米尺寸亲水微区和微疏水区。 Wang et al(1998)通过摩擦力显微镜明显观察到紫外光照射前后 TiO2表面明暗交替的区域,通过高倍率观察,亲水微区为30-80 nm的矩形结构,同时亲水微区平面较疏水微区平面高出0.4-6.0 nm,该尺寸单层化学吸附水表面或多层物理吸附水的高度基本一致(图7)。停止光照后,TiO2表面逐渐恢复到均匀的微观疏水结构。同时测试了新制备的TiO2薄膜及其在黑暗中储存7 d以及紫外光照射5 h后的傅立叶红外变换光谱图。结果表明,新制备的薄膜有化学吸附的羟基以及物理吸附羟基的振动峰,这是由于薄膜制备过程中高温煅烧过程造成了材料的表面缺陷所致,而缺陷有利于形成化学吸附水以及物理吸附水。但在黑暗中存储7 d后,这些特征峰又明显减弱,同时薄膜的接触角也明显增大,这归因于表面的羟基被空气中的氧所取代。光照5 h后,相应羟基的特征峰又增强,接触角降低,这是由于光照过程导致材料表面缺陷含量提高所致。原子力显微镜(AFM)结果显示亲水区沿着桥位氧分布(Wang et al,1998,1999)。对于金红石相的TiO2单晶,其含有较多氧桥键的(110)晶面比较于含量较少氧桥键的(001)面,有更加显著的光诱导亲水现象(图8)(Nakajima et al,2001;Wang et al,1999;Hugenschmidt et al,1994;Zhao et al,2013)。Nakajima et al(2000)也通过实验证实了多晶TiO2薄膜在紫外光照射下的亲水性及亲油性的双亲性转变过程,发现延长光照时间,薄膜表面的亲油性减弱,仅表现亲水性,这归因于TiO2亲水性转变的速率,(001)晶面完全转变为亲水性能所需要的时间为(110)晶面的三倍。多晶TiO2生长并无特定的生长方向,所以每个TiO2晶粒在光照下的亲水性转变速率不同,导致多晶薄膜亲水性的增强及亲油性的减弱。


图7   TiO2(110)表面的FFM图谱:(a)UV照射前,(b)UV照射后,(c)图b局部放大图,(d)图c局部放大图(Wang,1998)
Fig.7 FFM image of TiO2(110) surface (a) before UV illumination, (b) after UV illumination, (c) medium scale FFM image showing enlargement of framed area in (b), (d) further enlarged topographic image corresponding to framed area in (c)


图8   金红石单晶(110)和(001)晶面光照后,氧空位的形成及其对水分子的吸附(Nakajima,2001)
Fig.8 Schematic illustration: formation of oxygen defect and its adsorption of H2O on (110) and (001) of the single rutile phase
1.1.3   光致表面Ti-OH的重构
光照射条件下,材料表面Ti-OH的重构对TiO2薄膜亲水性能的转变有显著影响(图9),Sakai et al(2001)发现在紫外光照下TiO2产生空穴并迁移至表面被晶格氧所捕获削弱Ti-O键能,在断裂的Ti-O处,空气中水分子形成新的-OH,当停止光照时,表面的-OH开始产生H2O2或H2O和O2,表面重新恢复到疏水状态。同时,由于表面吸附的-OH为热力学不稳定状态,通过机械超声或摩擦等即可将亲水材料表面恢复至疏水状态(Sakai et al,1998,2001,2003;Hashimoto,2005;Kamei and Mitsuhashi,2000)。和频振动光谱(SFG)作为二阶非线性光谱,是表征表面及界面分子取向、结构及动力学信息的一种高效手段(Shen,1989,1998;Shultz et al,2002;Ma and Allen,2002;Kim and Cremer,2000),Wang et al(2003)通过SFG技术检测到光照过程中TiO2表面-OH含量的明显增多,进一步表明光诱导-OH的重构是改善材料表面亲水性的重要影响因素之一。


图9   UV光照下TiO2表面羟基含量的可逆变化示意图(Sakai et al,2001;Hashimoto,2005)
Fig.9 Schematic illustrations of reversible changes in the amount of hydroxyl groups on TiO2 film under UV light irradiation and in the dark
1.1.4   光催化表面有机物的分解
材料表面有机污染物的分解能有效改善其表面亲水状态(Watanabe et al,1999; Fujishima et al,2000;White et al,2003;Gao et al,2004;Nakamura et al,2003)。Wang et al (2003)通过表征TiO2表面光照前后的SFG振动光谱,结果表明光照前存在明显的CH-伸缩振动峰,光照之后该峰完全消失,说明光照过程引起了表面有机污染物的降解,从而材料的亲水性得以改善(图10)。Zubkov et al(2005)通过严格控制实验条件,以己烷为表面有机污染物,通过研究表面污染物的光催化降解与亲水性能改善之间的关系,TiO2表面的己烷降解是光诱导亲水性能改善的重要因素。随着光照时间的延长,表面污染物的覆盖率降低。当达到临界点时,表面有机物覆盖率为0,水滴随之迅速铺展(图11)。


图10   (a)TiO2薄膜在UV光照前后的SFG振动光谱图,(b)TiO2降解有机污染物示意图(Wang,2003)
Fig.10 (a) SFG vibrational spectra of TiO2 film before and after UV irradiation, (b) mechanism for degrading organic pollutants of TiO2 by UV irradiation


图11   亲水性转变过程示意图:TiO2(110)晶面的光催化氧化降解表面有机物(Zubkov,2005)
Fig.11 Schematic diagram of the sudden hydrophilic effect due to hydrocarbon photo-oxidation on TiO2 (110)
1.2   亲水性评价方法
测量水接触角法是评价薄膜亲水性认可度最高、应用最广的方法,通过测试薄膜表面水接触角大小判断亲/疏水性表面(Wang et al,2016)。如图12所示,将水滴滴在固体薄膜表面,固-液界面与液-气界面的夹角θ为水滴在材料表面的接触角,即水接触角。国际标准ISO 15989中对薄膜表面的水接触角测试进行了详细规定。依据ISO 15989的规定,薄膜表面水接触角的测试方法为:采用注射器将1~2 μL蒸馏水或超纯水注射并悬挂至针头,向上调节样品台,使水滴与待测样品表面接触,然后向下调节样品台,将水滴转移至待测样品表面(图13),再采用接触角仪测试水滴在样品表面的接触角,并在其表面选取10个点依次进行测试,计算10个点的平均值作为最终样品表面接触角取值。此外,国际标准ISO 27448-2009中对光催化材料的自清洁评价-水接触角测试方法也有详细的规定。


图12   基于液滴在固体表面对接触角的定义(Wang et al,2016)
Fig.12 Definition of contact angle based on a liquid drop on a solid surface


图13   ISO 15989规定材料表面水接触角的测试过程中液滴转移过程:(a)使液滴悬挂至注射器针口处;(b)液滴转移;(c)液滴转移至样品表面(1:医用注射器针头;2:水滴;3:样品表面)
Fig.13 Water droplet transfer technique in ISO 15989 of the water contact angle test on film surface: (a) droplet on syringe; (b) droplet transfer; (c) droplet on film surface (1: syringe needle; 2: water; 3: specimen surface)
2   亲水性与光催化分解有机物污染物活性的关系
光催化薄膜的亲水性与光催化分解有机物污染物活性之间呈现相辅相承的促进作用。当材料表面吸附有机污染物则表面亲水性下降,而光催化过程可分解有机污染物,水滴易于铺展至洁净表面,使材料表面恢复亲水状态;当材料亲水性较强,其表面易于吸附水分子并在光照作用下形成羟基自由基,有利于光催化过程的进行。Anheden et al(1996)对TiO2等半导体材料的光催化性能进行了深入的研究,其机理为材料受到光照射之后会产生e-与h+,光致e-与氧分子(O2)相互作用产生超氧自由基(·O2- );光致h+与水相互作用产生羟基自由基(·OH),最终这两种自由基与污染物相互作用将其进行分解。在该过程中有机污染物被分解,可改善基底材料表面的亲水性能,同时h+可与吸附水结合,增加材料表面-OH含量,进一步提升材料表面亲水性能(Ollis et al,1991;Schwarz et al,1997)。随后Guan(2005)合成了TiO2/SiO2复合薄膜对乙酸目标污染物进行降解,深入研究了材料表面光诱导亲水性与材料光催化活性的关系,结果表明随着材料亲水性的改善,材料对于乙酸的降解性能明显改善(图14),这归因于亲水性能的提高使得材料在光诱导过程中材料表面羟基含量明显提高,其最终改善材料的光催化分解有机污染物的性能。


图14   (a)TiO2/SiO2薄膜光照时间与乙酸浓度的关系,SiO2含量:a-0%,b-10%,c-20%,d-30%;(b)不同SiO2含量的TiO2/SiO2薄膜在光诱导光照的接触角变化(Guan,2005)
Fig.14 (a) Relationship between the content of acetic acid and irradiation time of TiO2 films containing SiO2: a-0%, b-10%, c-20%, d-30%; (b) Dependence of photo-induced change in the water contact angle of TiO2/SiO2 films
3   光催化薄膜的亲水性改善
光催化薄膜的亲水性越强,即水接触角越小,则薄膜表面的防雾、自清洁等性能越优异。而影响光催化薄膜亲水性的因素较多(周学东等,2005),如光照时间和强度、材料的不同晶面(Wang et al,1999)、膜厚度(Sirghi and Hatanaka,2003)、表面组成与结构、环境气氛、热处理条件等。依据薄膜亲水性典型理论的研究表明,通过调控表面化学组成(Zhang et al,2001;Sharma et al,2006;Meng and Sun,2009)、形貌结构(Yu et al,2002;Zhang et al,2006;Xi et al,2012;Wena et al,2013)或进行表面紫外光辐射(Watanabe et al,1999;Liu et al,2014;Rudakova et al,2015)等方法可有效改善光催化薄膜的亲水性能。
3.1   调控化学组成
3.1.1   掺杂
非金属掺杂主要包括N、C、S掺杂,以及S-N、C-N、N-F、C-N-F共掺杂。Irie et al(2003)报道了可见光诱导的N掺杂TiO2亲水性薄膜,讨论了亲水性与N掺杂量的关系。研究显示,在可见光强0.2 mW/cm2辐射下,TiO2-xNx薄膜的水接触角随着N掺杂量的增加而减小,薄膜的亲水性提高。而亲水性的改善主要归因于N掺杂替代O位,增强了薄膜对光子的吸收,提高可见光诱导亲水性能。Kontos et al(2011)基于改性溶胶-凝胶技术通过浸渍提拉法制备了N-F共掺杂TiO2薄膜,薄膜表面具有多孔粗糙结构,显现良好的可见光诱导亲水性。研究还发现,N-F共掺杂TiO2薄膜的亲水性转换和光催化活性有关。
金属掺杂主要包括过渡金属离子(Jiang and Gao,2002;Rudakova et al,2015)、镧系金属、贵金属掺杂。Mokhtarimehr et al(2013)通过溶胶-凝胶、浸渍提拉法制备了V掺杂TiO2-SiO2薄膜,具有可见光吸收性能。研究发现,V掺杂不仅能有效分离电子-空穴对,而且增加了表面羟基量,使得V掺杂TiO2-SiO2薄膜在可见光辐射下展现出超亲水性能。Zhang et al(2008)报道了Y2O3掺杂的TiO2薄膜,表面水接触角为8°,当日光灯照射60 min后,薄膜表现出超亲水性。由于Y2O3掺杂的TiO2薄膜中,Y3+离子取代Ti4+位置,在可见光照射下,薄膜表面形成大量氧空位,使得Y2O3掺杂的TiO2薄膜亲水性大幅度提高,在防雾和自清洁领域表现出良好的应用前景。Wang et al(2014)报道了旋涂镀膜法制备Cu掺杂的TiO2薄膜。研究发现,Cu掺杂TiO2薄膜的水接触角为5.1°,较TiO2薄膜的41.1°明显下降。掺杂后的薄膜亲水性提高,主要归因于TiO2晶格表面氧空位的产生及其表面吸附水分子产生的羟基。田守卫等(2007)通过溶胶-凝胶法和浸渍-提拉工艺在载玻片上负载了均匀、透明的WO3+掺杂纳米TiO2薄膜,探讨了WO3+掺杂量、热处理温度对薄膜可见光诱导亲水性的影响,考察了薄膜在停止光照后亲水性的变化。结果表明,当WO3+掺杂量为3 wt%,薄膜表面接触角为6°,与TiO2薄膜相比,WO3+掺杂纳米TiO2薄膜可见光诱导亲水性明显提高,停止光照后薄膜亲水性能维持更长时间。可见光诱导下亲水性的改善主要归因于WO3+掺杂能形成较多的电子-空穴对,空穴氧化薄膜表面的桥氧离子形成浓度较高的氧空位,易于吸附水分子并形成-OH,从而提高薄膜的亲水性能。
3.1.2   复合
光催化材料复合SiO2薄膜(Dohshi et al,2003;Novotna et al,2008;Horiuchi et al,2010;郭邵龙等,2016)因具有更持久的超亲水性和自清洁功能而备受关注。Machida et al(1999)通过喷涂TiO2和SiO2混合溶胶在瓷砖表面制备了SiO2-TiO2复合薄膜,并考察了SiO2添加量对复合薄膜光诱导亲水性的影响。结果表明,当SiO2添加量为10-30 mol%,复合薄膜具有一定的表面粗糙结构和良好的光催化活性,表现出光诱导超亲水性能。关凯书等(2003)采用溶胶-凝胶法在载玻片表面制备了均匀透明的TiO2-SiO2复合薄膜,考察了不同SiO2含量对复合薄膜的光催化活性和亲水性的影响。研究发现,TiO2薄膜水接触角为80°,而TiO2-SiO2复合薄膜的水接触角为40°,亲水性有明显的改善。其主要原因是SiO2与TiO2复合形成了Lewis酸,表面酸性的提高可增加表面羟基吸附量,有利于亲水性及光催化活性的提高。此外,表面酸性提高使薄膜表面对空气中H2O分子的吸附能力增强,在竞争吸附过程中,空气中污染物分子的吸附能力相对较弱,有利于保持表面的自清洁功能。Permpoon et al(2008)利用改性的TiO2晶体悬浮液与SiO2聚合溶液的混合物制备了TiO2/SiO2和SiO2/TiO2双层复合薄膜,其表面为多孔粗糙结构,无需光照可表现持久的超亲水性。研究还发现,由于SiO2表面对污染物的吸附能力较TiO2弱,薄膜表面的Si-OH较Ti-OH更加稳定,因此SiO2/TiO2复合薄膜较TiO2/SiO2能保持时间更久的超亲水性。
研究显示通过半导体复合也能有效改善薄膜亲水性。王青等(2009)采用混合溶胶法和旋涂技术制备了TiO2-ZnO复合薄膜,探讨了不同煅烧时间、煅烧温度、镀膜层数对其亲水性的影响及原因。研究结果表明,当煅烧温度为550℃,煅烧时间为2 h,薄膜层数为3层时,TiO2由锐钛矿相向金红石转变,薄膜的亲水性最佳。TiO2-ZnO复合薄膜晶粒粒径减小、表面粗糙度增加,其晶粒粒径约为25.2 nm,薄膜表面粗糙度约1.815 nm,水接触角为5°。由于复合形成了杂质能级,增强了光生载流子的分离,抑制了电子空穴对的复合有利于薄膜表面氧空穴和羟基的形成,从而提高了复合薄膜的亲水性。Eshaghi et al(2010)通过溶胶-凝胶法和浸渍提拉镀膜技术在玻璃基底上制备了TiO2-SiO2-In2O3复合薄膜,研究结果显示,TiO2-SiO2-In2O3复合薄膜的光诱导亲水性较纯TiO2薄膜显著提高,且亲水性在暗室存储能保持时间更久。由于复合金属氧化物表面易于形成酸位点,使其表面电荷利于吸附羟基基团,促进复合薄膜的亲水性提高。
此外,Anandan et al(2012)报道了TiO2/GO(石墨烯)复合薄膜,在日光灯照射下,GO可将TiO2导带上的电子转移走,从而提高了电子-空穴对分离效率,增强了TiO2/GO复合薄膜的光催化活性,使得其分解污染物能力增强,从而提高了亲水性能。研究发现,随着光照时间延长,异质结薄膜表面显现超亲水性,表现出良好的自清洁效果。Karimi et al(2014)在棉织物表面负载了TiO2/GO复合薄膜,在日光灯照射下,GO能转移TiO2导带上被激发的电子,有效阻止电子-空穴对的复合,提高了光催化分解有机污染物的能力,使得TiO2/GO复合薄膜具有超亲水性能,水接触角为4.2°,展现良好的防雾和自清洁效果。
3.1.3   构建异质结
Miyauchiet al(2000,2002)通过在TiO2薄膜上沉积WO3制备了TiO2/WO3异质结薄膜,由于WO3禁带宽度较窄,能够被可见光激发,从而使异质结薄膜在日光灯照射下具有较高的亲水性能。在可见光照射下,TiO2/WO3异质结薄膜易于形成表面氧空位,有利于吸附水分子而形成-OH,从而提高可见光诱导亲水性能。Patrocinio et al(2014)利用层层自组装(LBL)技术在氟掺杂SnO2透明导电玻璃(FTO)玻璃上构建了TiO2/WO3异质结薄膜,薄膜表面具有纳米多孔结构,紫外光辐照下表现出超亲水性,其水接触角为9°,在自然环境下置于暗处,表面的超亲水性能可持续96 h,具有良好的光催化活性和自清洁作用。Tian et al(2013)报道了Bi2MoO6/TiO2异质结双层薄膜,形成的多孔层状结构增大了薄膜表面的比表面积,异质结界面处的电子-空穴对能够得到有效分离。在可见光照射下,与TiO2相比,Bi2MoO6/TiO2异质结双层薄膜的光催化活性和自清洁性能显著提高。
3.2   调控形貌结构
依据2D/3D毛细管效应的亲水理论可知,表面形貌结构对光催化材料表面的亲水性改善至关重要,例如粗糙表面、多孔结构对形成超亲水表面具有积极作用(Tettey et al,2011)。研究表明,通过金属有机气相沉积(MOCVD)、添加模板剂、层层自组装(LBL)、热处理(Euvananont et al,2008)等方法可有效调控薄膜表面形貌,构造具有多孔结构的粗糙表面。
通过MOCVD技术调控表面形貌结构,改善光催化薄膜亲水性能。Lee et al(2000)在玻璃基底上沉积了TiO2薄膜,研究了亲水性与表面形貌的关系。结果表明,表面粗糙的TiO2薄膜水接触角为15°,暗室存储7 d后紫外光照射10分钟即可恢复亲水表面,而表面光滑的TiO2薄膜水接触角为70°,紫外光照2 h后无明显变化。其亲水性的提高主要归因于两个方面:一方面,在紫外光照射条件下,TiO2薄膜产生大量Ti3+,表面易于吸附-OH;另一方面,薄膜形成的粗糙表面能促进水滴的浸润和铺展。
通过添加模板剂调控表面形貌结构,改善光催化薄膜亲水性能。Huang et al(2011)采用溶胶-凝胶法制备了TiO2薄膜,通过在前驱液中加入适量稳定剂二乙醇胺和乙酰丙酮,然后高温煅烧处理,构建了多孔结构表面,表现出超亲水性能。林承朴等(2011)采用蒸发诱导自组装法制备了介孔TiO2薄膜,考察了模板剂F127用量对薄膜避光超亲水性的影响。结果表明,当F127用量最佳,经煅烧处理后,薄膜具有介孔结构,水接触角小于5°,且超亲水性在避光条件下可维持12 h。Wang et al(2011)采用溶胶凝-胶法制备了TiO2/SiO2复合薄膜,考察了不同聚乙二醇(PEG)添加量对薄膜表面粗糙度的影响。研究发现, PEG添加量为0.5 wt%,通过煅烧制备的复合薄膜粗糙度明显增加(图15a、15b),具有多孔结构,水接触角为3°。TiO2/SiO2表面呈现超亲水性,暗室存储60 d仍能保持超亲水表面,并具有防雾效果(图15c)。


图15   (a)未添加PEG的TiO2/SiO2复合薄膜SPM图;(b)添加0.5 wt% PEG的TiO2/SiO2复合薄膜SPM图; (c)负载添加0.5 wt% PEG的TiO2/SiO2复合薄膜的载玻片(上)和未负载薄膜的空白载玻片(下),分别在-15℃下冰箱存储3 h后暴露在潮湿室内的照片(室内环境:25 °C,湿度40%,空气)(Wang et al,2011)
Fig.15 (a) SPM images of TiO2/SiO2 bi-layer films; (b) SPM images of TiO2/SiO2 bi-layer films with 0.5 wt% PEG; (c) digital images of TiO2/SiO2 bi-layer films with 0.5 wt% PEG (upper) and of an uncoated glass slide (bottom) after cooling at about -15°C for 3 h in a refrigerator followed by exposure to humid laboratory (25 °C, RH: 40%, air)
通过LBL技术调控表面形貌结构,改善光催化薄膜亲水性能。Cebeci et al(2006)利用LBL技术在载玻片上构建了聚丙烯氯化铵(PAH)/SiO2多层薄膜,探讨了自组装条件对薄膜性能的影响。结果显示,薄膜表面呈3D纳米孔网状结构,水接触角小于5°,表现出超亲水性,具有防雾、抗反射作用。Wu et al(2007)利用LBL技术将TiO2和SiO2纳米颗粒分别组装到聚电解质层中,经煅烧形成低折射率SiO2堆积层和高折射率TiO2堆积层组成的复合薄膜,通过调控堆积层的层数和厚度改变其表面结构。由于纳米颗粒的堆积形成了三维粗糙结构,薄膜表面呈现超亲水状态,具有自清洁作用。Liu et al(2008)采用具有复杂结构的颗粒构建超亲水表面,通过将聚电解质和SiO2纳米颗粒选择性地组装到单分散聚苯乙烯(PS)微球上,制得有机/无机杂化颗粒。再通过LBL将这些颗粒与聚电解质组装在基底上,经煅烧得到由中空SiO2微球组成的多级多孔膜,具有超亲水性,表现出防雾效果。McDonald and Cui(2011)通过LBL和液相沉积技术在铜基底上修饰了TiO2薄膜。研究表明,铜基底表面水接触角为90°,而修饰TiO2薄膜后的水接触角为0°,主要归因于其比表面积的增大及粗糙的表面结构,因此薄膜表现出超亲水性能。
通过热处理调控表面形貌结构,改善光催化薄膜亲水性能。Yu et al(2002)采用凝胶-溶胶法制备了TiO2薄膜,探讨了煅烧温度对其亲水性能的影响。研究表明,薄膜表面为粗糙的介孔结构,其煅烧后的比表面积明显提高,表现出良好的光诱导亲水性。Zhang et al(2006)报道了TiO2-SiO2双层薄膜,通过以下步骤制备:(1)多层SiO2颗粒和聚二烯丙基二甲基氯化铵(PDDA)阳离子自组装沉积;(2)在PDDA/SiO2薄膜上,层层自组装沉积多层钛酸盐纳米片;(3)经500℃热处理去除聚合物制得TiO2薄膜层。薄膜由多孔SiO2底层和致密TiO2表层组成,呈现光诱导亲水性,具有良好的自清洁作用和抗反射功能。Park et al(2013)利用超声气相沉积技术制备了纳米TiO2薄膜,并在无紫外光照射条件下,研究了不同煅烧热处理温度与水接触角的关系(图16)。结果显示,随着煅烧温度的升高,表面粗糙度增强,水接触角不断减小,且500℃煅烧后,无需光照,薄膜表面就呈现超亲水性。


图16   无紫外光照射下,水接触角与煅烧处理温度的关系(Park et al,2013)
Fig.16 Relationship between water contact angle and annealing temperature under no exposure to UV light
3.3   其他
此外,通过光效应诱导、电极化及超声处理也能有效改善薄膜的亲水性。Sun et al(2001)利用喷雾热解法制备了ZnO和TiO2薄膜,考察了光照及热处理对其亲水性影响。结果显示,光照射前ZnO和TiO2表面水接触角分别为109°和54°,紫外光照射后两者水接触角均小于10°。Ma et al(2012)对制备的TiO2涂层进行了电极化处理提高其亲水性,电极化作用诱导涂层表面产生电场和电荷,形成Ti2+和Ti3+并产生氧空位,吸附水分子并形成大量-OH,使得涂层表面长期维持亲水性。Sakai et al(1998)在玻璃表面负载了TiO2薄膜,考察了薄膜在水中超声处理对表面亲水性的作用,在紫外光照射下,经水超声处理的薄膜表面的水接触角为0°。由于超声处理使表面吸附水形成了-OH,从而提高了薄膜的亲水性。
4   应用现状及发展趋势
光催化薄膜特殊的光诱导亲水性能可实现基底表面的防雾和自清洁,以太阳光、荧光灯的紫外光作激发源,光催化亲水薄膜被广泛应用于医用镜子、眼镜镜片、浴室镜子、挡风玻璃及侧/后视镜、家具表面、玻璃/陶瓷建筑材料、高速公路护墙等方面,具有防雾和自清洁等功能,市场巨大且前景广阔。
TiO2薄膜负载在玻璃表面具有良好的亲水性能、较高的折光率,使得玻璃透光度高、视觉清晰度好。因此,涂敷TiO2的玻璃产品具有很好的防雾效果,被广泛应用于各类镜子。Ohdaira et al(2007)比较了传统腹腔镜和有涂层TiO2腹腔镜的防雾性能,将两种不同的腹腔镜插入腹腔中,临床实验证明,涂有TiO2亲水薄膜的腹腔镜具有很好的防雾功能,无需从腹腔撤回腹腔镜即可成功完成手术。Funakoshi and Nonami(2007)将光催化TiO2薄膜负载在口腔镜上,具有超亲水性、高反射率、高透明度,已商业化生产。此外,大部分日本制造的车辆已经使用具有抗雾效果的侧视镜(图17)(Zhang et al,2012),TOTO公司还开发了表面负载亲水性TiO2薄膜的用户型侧光镜膜,产品已经投入市场。中国纳米技术工程中心有限公司采用常温固化自清洁玻璃技术,成功生产出具有自清洁性能的玻璃制品,应用于国家大剧院和汽车展厅等建筑项目,实现了建筑物外墙的自清洁(封玉凤和王利新,2013)。此外,中国三峡新材料公司、中国长春新世纪纳米技术研究所、西北永新化工股份有限公司以及上海常祥实业有限公司也成功研制出不同类型的光催化自清洁涂层,用于建筑物外墙以及太阳能光伏板表面实现自清洁。


图17   具有抗雾效果的侧视镜:传统侧视镜(左)及涂覆有TiO2的侧视镜(右)(Zhang et al,2012)
Fig.17 Anti-fogging effect of automobile side-view mirror: conventional mirror (left) and TiO2-coated mirror (right)
光催化材料与建筑材料结合使用最早起始于20世纪90年代。在日本,TiO2被广泛的 用于室外建筑材料,如瓷砖、玻璃、甚至帐篷等。基于TiO2的光催化降解污染物以及光诱导亲水的性能,这些产品在太阳光以及雨水的作用下可以长时间保持洁净状态。但是这并不意味着这些产品不会被污染,因为光催化过程以及亲水过程的改善与光照情况、雨水量以及污垢的积累速度等均有很大关系。但是这些光催化材料的使用可以延缓建筑物受到污染的速度,节省了清洗维护所需的时间以及费用。TOTO Inc.是日本主要生产自清洁瓷砖(Wang et al,1998)的公司,至2003已经有超过5000座建筑物使用了该公司的产品,其中最为著名的就是位于东京商业中心区的Maru(图18),利用屋顶水流以及自然光的作用,自清洁瓷砖具有明显的抗污自清洁效果。一般的外墙瓷砖需要5年清洗一次,而TOTO预测具有自清洁性能的瓷砖清洁周期超过20年,因此可以节约大量的维护费用。同时,光催化材料也被广泛的用于高分子聚合物薄膜表面。Taiyo Kangyo Co.是自清洁帐篷(Fujishima and Zhang,2006)在日本的主要生产厂家,帐篷的材料一般为具有柔性的聚合物膜制成,如氯乙烯(PVC),其清洗过程耗时又难于操作,在PVC表面涂覆TiO2很好的解决了这一问题,同时通过SiO2中间层的加入,避免了有机基地被TiO2光催化过程破坏。基于这一技术,该自清洁帐篷可以在户外持续使用4年(图19)。


图18   暴露一定时间后不同区域的外观图:(A)具有自清洁性能外墙;(B)普通外墙(Wang et al,1998)
Fig.18 Photograph showing alternating self-cleaning (A) and ordinary (B) exterior wall tiles that have been exposed for time


图19   左半边涂覆有TiO2的帐篷(Taiyo Kangyo Co.)在室外暴露2月后的外观图(Wang et al,2006)
Fig.19 Photograph of a tent (Taiyo Kangyo Co.) whose left half has been coated with a TiO2 self-cleaning layer after an outdoor exposure test for 2 months in the Museum of KAST
20世纪90年代中叶,日本对光催化技术与亲水表面相结合的技术研究已经较成熟,玻璃防雾和自清洁等建材进入商业化水平。随着我国经济的发展,环境问题日益凸显,光催化亲水膜具有强大的市场应用潜力及前景。而国内在这方面的技术水平仍较薄弱,主要停留在实验室研究阶段已获得的成果,未得到广泛的应用,产业化方面仍处于劣势。此外,对于光催化亲水薄膜的应用,仍存在太阳光利用率低、超亲水表面制备困难、光诱导亲水性耐久性差、低温制备及固化技术欠缺、产业化困难等问题,其研究将成为未来光催化亲水薄膜重点发展方向。
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稿件与作者信息
黄宇*
HUANG Yu*
huangyu@ieecas.cn
刘燕
LIU Yan
张静
ZHANG Jing
王震宇
WANG Zhen-yu
曹军骥
CAO Jun-ji
基金项目: 国家自然科学基金项目(41401567, 41573138)
Foundation Item: National Natural Science Foundation of China (41401567, 41573138)
出版历史
出版时间: 2018年5月17日 (版本5
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地球环境学报
Journal of Earth Environment