功能化石墨烯纳米复合材料的制备及其光催化产氢性能研究

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英文题名：Preparation of Functionalized Graphene Nanocomposite Materials and Their Photocatalytic Hydrogen Generation Properties 作者：牟志刚 论文级别：博士 学科专业名称：物理化学 中文关键词：石墨烯 ; 功能化 ; 纳米复合物 ; 光催化 ; 氢气 英文关键词：Graphene ; Functionalization ; Nanocomposite ; Photocatalysis ; Hydrogen 学位年度：2013 导师：杨平 学科代码：070304 学位授予单位：苏州大学 论文提交日期：2013-09-01

摘要

现今，能源短缺和环境污染的日益严重，人类寻找新的可持续的清洁能源来取代常规能源显得尤为迫切。通过太阳能转化为化学能是解决全球能源问题的重要途径。利用光催化剂太阳能分解水制氢可以实现太阳能的转化。制得的氢气具有清洁、经济、环境友好等优点。对于一个高效的光催化剂，为了提高光催化反应效率，必须要有长寿命的电荷载流子，较少的电荷陷阱中心，合适的能带结构和光稳定性。在过去几十年内，人们通过各种方法来提高光催化活性。包括：催化剂结构设计，表面沉积助催化剂，掺杂、染料敏化、催化剂复合等。石墨烯材料由于其特有的二维碳结构为人们提高光催化效果提供了一个新途径。本论文制备了不同类型的功能化石墨烯纳米复合材料，研究了它们的光催化分解水产氢性能。在这些光催化体系中，石墨烯不仅可以充当电子的接受体和传递体，而且氧化石墨烯（GO）或者还原的氧化石墨烯（RGO）本身可以作为一个产氢光催化剂。本论文主要内容概括如下：
     (1) GO或者RGO是具有一定带宽的半导体，带宽大小随着石墨烯氧化程度的不同而变化。部分氧化的石墨烯材料光照条件下直接可用来光催化分解水产氢。然而GO和RGO不能充分的利用可见光，在可见光下光催化产氢活性很低。基于染料敏化半导体的思想，考虑将曙红（EY）通过非共价方式功能化石墨烯（EY-RGO）。研究发现EY与RGO表面可以通过氢键和-堆积作用结合。染料非共价功能化石墨烯具有不破坏石墨烯本身的共轭结构的优点。紫外可见吸收光谱，拉曼光谱，荧光光谱以及光电化学实验表明在RGO表面的EY分子可以作为“捕光天线”，吸收光并且将激发电子有效地传递到石墨烯。EY-RGO可以用作分解水产氢光催化剂。在30h紫外-可见光和10h可见光照射下，其在10vol%三乙醇胺水溶液中平均产氢速率分别为3.35mmol·g-1·h-1和0.40mmol·g-1·h-1，EY-RGO的光催化活性要高于RGO，GO和EY-GO。修饰了Pt纳米粒子形成的EY-Pt/RGO可进一步提高光催化活性。
     (2)通过一步沉积法合成了RuO2/TiSi2/RGO复合物，石墨烯由于超大的表面积和优良的导电性能，可以作为半导体的载体和电子接受体，而RuO2作为助催化剂。该复合物通过扫描电镜，X-射线粉末衍射，红外光谱，X射线光电子能谱，紫外-可见漫反射，光电流响应与电化学阻抗进行了表征。结果表明：石墨烯与RuO2和TiSi2颗粒有很好的相互作用，三者之间直接的物理连接有利于光生电荷多途径的传输和分离。该复合物增强了在可见区的吸收能力。当RuO2和RGO负载量都为1wt%时，RuO2/TiSi2/RGO在纯水和可见光(λ420nm)照射下的光催化产氢速率为97.5mol·h-1·g-1，高于RuO2/TiSi2（71.9mol·h-1·g-1）和纯TiSi2（56.3mol·h-1·g-1）。在半导体TiSi2中引入石墨烯与RuO2时，实现了电子多途径的传递，降低了电子空穴复合几率，增加了氢气生成的活性位点，因此提高了光催化性能。当反应体系加热到较高温度时可以释放出O2，实现全面的分解水。
     (3)以GO和尿素为原料通过固态热反应合成了N掺杂石墨烯。N掺杂量可以简单通过改变氧化石墨和尿素的比例来调节。在相对较低的比例（如wGO/wurea=0.3），N的掺杂量相对较高（~10at.%），而在相对较高的比例时（如wGO/wurea=1或者0.5），N的掺杂量相对较低（wGO/wurea=1时，N的掺杂量大约为3.2at.%；wGO/wurea=0.5时，N的掺杂量大约为6.5at.%），而且在N掺杂的同时GO发生了明显的还原效应。含氧量在相同反应温度下要低于文献报道的在H2气氛或者Ar气氛中的含氧量，表明在尿素作用下GO更容易被还原。氧化石墨中的含氧官能团和尿素中的氨基基团对形成N掺杂石墨烯晶格中的C-N键起着重要的作用。X射线光电子能谱表明了GO与尿素固态热反应制备N掺杂石墨烯的形成机理：随着热反应温度逐渐升高，N的化学结构有一个逐渐的热转变过程。即从氨基N转变到吡咯N，再到吡啶N，最终到石墨N。N掺杂石墨烯的电导率可以达到40S·cm-1，该数值高于氧化石墨烯5个数量级。在较高温度下导电率增加很快，其可能原因为：一是温度越高，石墨烯的共轭程度越高，二是通过N的掺杂减小了氧化石墨烯中存在的结构缺陷。该研究为后续N掺杂石墨烯在光催化体系应用研究提供了基础。
     (4)以N掺杂的石墨烯（NGR）为TiO2纳米粒子的载体制备了NGR/TiO2复合物。TEM图谱显示大约为8nm的TiO2纳米粒子紧密附着在NGR片层表面，而RGO/TiO2复合物中TiO2纳米粒子直径大约为20nm，表明TiO2和NGR之间形成了强烈的结合效应。NGR中的含氮基团可以成为TiO2纳米粒子的成核和连接位点。XPS图谱和拉曼图谱表明TiO2和NGR之间存在强烈的电子相互作用。光电流响应和电化学阻抗和循环伏安曲线结果表明NGR有较高的电导性，相比与RGO更有利于光生电子的传递。该复合物相比于纯TiO2，RGO/TiO2复合物显示了较高的光催化产氢活性。同时，实验结果表明Pt纳米粒子修饰的NGR/TiO2复合物（Pt/NGR/TiO2）光催化产氢的活性和稳定性也明显高于Pt/TiO2和Pt/RGO/TiO2.
Nowadays, energy shortage and environmental contamination are serious issues andthus seeking for renewable and clean energy is an urgent task. The production of chemicalfuels by solar energy conversion has been considered as one of the major strategies forsolving the global energy problem. Photocatalytic water splitting for hydrogen productionhas been considered as an ultimate solution for clean, economical, and environmentallyfriendly production of hydrogen by using solar energy. For an efficient photocatalyst,longlived charge carriers, fewer charge trapping centers, proper energy level offsets, andstability against light are highly desirable for improving the photocatalytic reactivity.During the past decade, a variety of strategies, including textural design, doping, noblemetal loading, and surface sensitization of semiconductor photocatalysts, have beenemployed to improve the photocatalytic performance of photocatalysts. The discovery ofgraphene has opened up a new way to improve photocatalytic performance owing to aunique sp2hybrid carbon network. In this dissertation, we prepared various functionalizedgraphene-based nanomaterials for photocatalytic H2generation from water. In thisphotocatalytic systems, graphene can act as an electron acceptor. In addition, grapheneoxide (GO) or reduced graphene oxide (RGO) can also act as a photocatalyst for hydrogenproduction. The main points could be summarized as follows:
     (1) RGO/GO is a semiconductor with finite bandgap which can be tuned based on thedifferent oxidation level. RGO/GO itself has ability of extraction of hydrogen from waterin the presence of light. However, RGO/GO is not effective for absorbing visible light andhas a low photocatalytic activity for H2generation under visible light irradiation. Based ona similar consideration for dye sensitization of semiconductor, RGO is functionalized by eosin Y (EY) in aqueous media to form the stable aqueous EY functionalized graphene(EY-RGO) suspension through noncovalent modification. EY molecules are built on thesurface of the RGO via hydrogen bonding and-stacking interactions without causingdamage to the electronic properties of the RGO. UV-vis, Raman, fluorescence spectra, andphotoelectrochemical measurements reveal that EY molecules attached on the surface ofthe RGO play a role of an antenna: harvesting irradiation light to give more efficientphotoinduced electron transfers from EY to RGO. The EY-RGO is photocatalytic activefor water reduction to produce hydrogen. The average production rate of H2for thephotocatalyst (wEY/wRGO=1) in a10vol%triethanolamine aqueous solution can reach3.35mmol·g-1·h-1and0.40mmol·g-1·h-1under30h UV-vis and10h visible light irradiation,respectively. The photocatalytic activity of EY-RGO is superior to that of RGO, GO, andEY-GO. Modification EY-RGO with Pt nanoparticles can further improve photocatalyticactivity.
     (2) A novel composite composed of TiSi2, graphene and RuO2nanoparticles was fabricatedby one-pot deposition method. Graphene can act as an excellent supporting matrix forsemiconductors and as the electron acceptor due to its high specific surface area andsuperior electron mobility. RuO2serves as a cocatalyst in the system. The resultingRuO2/TiSi2/RGO composite was characterized by scanning electron microscopy, X-raydiffraction, Fourier transform infrared spectra, X-ray photoelectron spectroscopy, UV-visdiffuse reflectance spectra, photoelectrical response and electrochemical impedance spectra.The results indicated three components in the composite were effectively contacted, thusfacilitating the photogenerated charges transfer and separation through multiple routes. Thecomposite enhanced the absorption ability in the visible range. The composite shows ahigher rate of H2evolution (97.5mol·h-1·g-1) than that of the composite RuO2/TiSi2(71.9mol·h-1·g-1) and pure TiSi2(56.3mol·h-1·g-1) under visible-light irradiation (420nm)when the contents of graphene and RuO2were both1wt%. Incorporation of graphene andRuO2into TiSi2diversifies the electron transfer process, increases H2evolution active siteand reduces the probability of electron–hole recombination. O2can be evolved in high temperatures for the oxidation of water by holes in the valence band of TiSi2，leading tooverall water splitting.
     (3) Nitrogen doped graphene was synthesized from graphite oxide and urea by thermalsolid-state reaction. The nitrogen content in the graphene lattice may be tuned by simplychanging the ratio of reagents GO and urea. The sample prepared at a low ratio of GO andurea (wGO/wurea=0.3) has a relative high nitrogen content (~10at.%); while the sampleprepared at a high ratio of GO and urea (wGO/wurea=1or0.5) usually has a low nitrogencontent (~3.2at.%for wGO/wurea=1;~6.5at.%for wGO/wurea=0.5). Moreover, GOannealed with urea shows an evident reduction effect. The oxygen content in our samplesis lower than those of GO annealed in H2or in Ar at the same temperature, indicating GOcan be reduced more easily in the presence of urea. Oxygen-containing functional groupsin GO and amino-groups of urea are suggested to be essential for forming C-N bonds in thegraphene lattice. XPS investigations demonstrate a forming mechanism of N-dopedgraphene prepared by thermal solid-state reaction of GO and urea: a gradual thermaltransformation of nitrogen bonding configurations from amide form nitrogen to pyrrolic,then to pyridinic, and finally to “graphitic” nitrogen in graphene sheets with increasingannealing temperature. The electrical conductivity of the sample can reach ca.40S·cm-1,which is5orders of magnitude higher than that of graphene oxide. The increase inconductivity of the sample annealed at higher temperature may be attributed to bettergraphitization of C=C π-conjugation of the graphene basal plane and the decreased defectsformed within the plane associated with incorporation of nitrogen. This research providedthe foundation for the photocatalytic application of nitrogen doped graphene.
     (4) N-doped graphene (NGR)/TiO2nanocomposites was prepared using nitrogen dopedgraphene as a supporting matrix for TiO2nanoparticles. TEM image shows TiO2nanoparticles with an average diameter of ca.8nm were fairly well attached to the NGRsheets, while the average diameter of TiO2nanoparticles is ca.20nm for RGO/TiO2composite, indicating stronger coupling between TiO2and N-doped sites on the NGR thanRGO. Nitrogen-containing groups in graphene may serve as favourable nucleation and anchor sites for TiO2nanocrystals. The XPS spectra and Raman spectra of NGR/TiO2nanocomposites indicated the strong electronic interaction exist between TiO2and NGR.Photocurrent responses, electrochemical impedance spectra and cyclic voltammogramsresults show NGR has higher conductivity and higher carrier transfer rate than RGO. TheNGR/TiO2composite shows higher photocatalytic activity for hydrogen generationcompared with pure TiO2and RGO/TiO2. Furthermore, platinum nanoparticles modifiedNGR/TiO2composites (Pt/NGR/TiO2) enhanced the photocatalytic activity and stabilitycompared with Pt/TiO2and Pt/RGO/TiO2.

引文

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