【作者】 王敏；

【导师】 马永青；

【作者基本信息】 安徽大学 ， 材料科学与工程， 2019， 博士

【摘要】 四方对称的锐钛矿TiO₂具有稳定的物理化学性质,且具无毒性、价格低廉、储量丰富等优势,广泛应用于降解有机污染物、光催化水分解、电催化、太阳能电池等领域。锐钛矿TiO₂的导带主要是Ti的3d态构成,价带顶部主要来源于O的2p态,带隙宽度Eg约为3.2 eV。因此,激发电子-空穴对需要光子能量hv≥3.2 eV或波长λ≤387 nm,意味着TiO₂响应的波长范围在紫外光区域,少于太阳辐射总能量的5%,较低的太阳光利用率限制了TiO₂的实际应用。同时,TiO₂中光生电子-空穴对复合的时间间隔大约在10～100 ns,远快于它们被捕获和转移所需的时间（100 ns～1 ms）,导致大部分载流子在迁移到催化剂表面之前就复合而消失,致使催化降解性能降低。因此,如何抑制载流子复合成为亟待解决的问题。另外,纳米光催化剂颗粒小,而小粒子一般具有高比表面积,因此纳米光催化剂具有更多的催化活性位置,这对提高光催化效率有利。但小粒子在溶液中容易形成悬浊液,在水净化处理后难以沉降和分离,仍残留在水中,造成二次污染。催化剂的再回收利用问题也是具有挑战性的工作。为了解决上述问题,科研人员已经在TiO₂以及TiO₂基复合物的合成、改性以及应用等方面做了很多研究,得出了很多有意义的研究成果。但是对于TiO₂基复合物来说,其光催化反应是一个非常复杂的过程,影响光催化活性的参数很多,以往的研究报道也有很多相互矛盾、相互冲突的结论,仍存在很多亟待解决的难点问题,如在合成TiO₂基复合物时,如何实现两相间的紧密接触问题:其光催化反应机理问题等。在蒸汽热反应过程中醇类的醚化反应可以为Ti⁴⁺前驱体提供水解所需的水,具有水解速率可控的特点。本论文利用该特点做了以下的研究,主要研究内容如下:1.TiO₂/石墨烯复合物的蒸汽热合成与光催化性质研究。对于TiO₂/石墨烯复合物来说,实现两相之间的紧密接触仍然是一个巨大的挑战。在这项工作中,使用自主开发的蒸汽热法,以钛酸异丙酯为钛源,分别在异丙醇和去离子水蒸汽环境中合成了TiO₂/石墨烯复合物。对于异丙醇蒸汽环境中合成的样品,SEM/TEM结果显示TiO₂呈纳米片形貌,均匀且紧密地生长于石墨烯上:XRD和FT-IR结果表明TiO₂的晶格发生畸变,进一步证明了两相间的紧密接触。从价带XPS的结果中观察到晶格畸变导致TiO₂的价带态密度发生变化。UV-vis DRS、BET、PL的结果表明,石墨烯与TiO₂结合显著增强可见光吸收、增加比表面积、抑制光生载流子复合,进而提高光催化降解性能。ESR结果表明,·OH自由基对光催化降解的贡献更大。对于去离子水蒸汽环境中合成的样品,TiO₂呈粒子形貌,散乱分布于石墨烯上,形貌特征与以往文献报道的结果相似。没有观察到TiO₂的晶格畸变。石墨烯与TiO₂结合增强了可见光吸收,并促进了光生载流子分离,然而比表面积却减小,光催化性能没有得到提高。这些结果表明,两相间紧密接触是实现石墨烯与TiO₂协同增强效应的先决条件,而适当的合成方法至关重要。这种合成方法为进一步提高光催化性能铺平了道路,并提供了发现新性能的机会。2.TiO₂/石墨烯复合物蒸汽热合成与N掺杂研究。首先在异丙醇蒸汽环境中合成纳米片状TiO₂/石墨烯两相紧密接触的复合物,然后进行不同N/Ti摩尔比掺杂。掺杂N元素有以下几种存在状态:TiO₂晶格间隙中、替代TiO₂中的O元素、替代石墨烯中的C元素以及吸附于石墨烯和TiO₂粒子表面。N掺杂效应归纳如下:N掺杂使得TiO₂纳米片变成纳米粒子,伴随着复合物的比表面积减小;TiO₂晶粒大小随掺杂量增加先增加后减小;N掺杂导致Ti-O-Ti振动模发生改变;N掺杂对TiO₂中Ti和O元素的化学态产生影响,引入多种缺陷或杂质能级。这些因素的协同效应对光催化性能产生影响。仅当N/Ti摩尔比为2时,对亚甲基蓝的光催化降解性能有所增强,降解速率从7.7×10^-2 min^-1（未掺N样品）提高到9.6×10^-2 min^-1。分析ESR的结果得出,光生电子被缺陷和杂质俘获,光生空穴对光催化降解起主要作用。3.TiO₂/石墨烯复合物蒸汽热合成与S掺杂研究。首先在异丙醇的蒸汽环境中,利用多元醇的醚化反应提供Ti源水解所需的水,合成TiO₂/石墨烯复合物,实现TiO₂与石墨烯两相间紧密接触,且二氧化钛呈纳米厚度的片状形貌。然后,用硫脲作为S源,改变S/Ti摩尔比RS/Ti,利用蒸汽热法在水蒸汽环境中对TiO₂/石墨烯复合物进行不同程度S掺杂,制得系列S掺杂TiO₂/石墨烯复合物,并用XRD、TEM、FT-IR、XPS、UV-vis DRS、PL、BET、ESR等手段进行系统表征,研究了S掺杂对光催化降解性能的影响。观察到依赖S掺杂程度的形貌、化学态、晶体结构、电子结构以及光催化性能的变化。主要结论如下:（1）S掺杂使TiO₂纳米片变为纳米粒子。（2）掺杂的S元素以S^2-、S、S⁴⁺、S⁶⁺四种化学态存在,并对TiO₂晶格中Ti和O元素化学态产生影响。R_s/Ti=0.5时,仅有单质S存在,没有出现替位S元素。R_S/Ti≥1时,出现S^2-替代O^2-,S⁴⁺/S⁶⁺替代Ti⁴⁺。（3）S掺杂在TiO₂价带顶以上引入电子态。（4）TiO₂晶体结构产生畸变,R_S/Ti=3时晶格应变最大。（5）S掺杂使TiO₂对污染物亚甲基蓝（MB）的降解速率由7.7×10^-2min^-1增加到10.7×10^-2 min^-1,相应的样品具有最高数量的·OH和·O₂^-自由基,且·OH对降解起着更主要的作用。4.CoFe₂/CoFe₂O₄/TiO₂合成与光催化性质研究。这项工作的目的是研究磁芯材料与催化剂之间界面对光催化机理的影响。CoFe₂合金在所有二元合金中具有最高的磁化强度,有利于磁分离。首先,制备均匀分散的CoFe₂O₄纳米粒子,并利用金属有机盐热分解法包覆MgO。通过在H₂/N₂混合气体中还原,再在空气中氧化,完成从CoFe₂O₄/MgO到CoFe₂/MgO,再到CoFe₂/CoFe₂O₄/MgO的结构转变。其中MgO可以通过稀盐酸溶液酸洗。接下来,包覆TiO₂使CoFe₂O₄,CoFe₂/MgO,CoFe₂和CoFe₂/CoFe₂O₄磁性粒子的表面功能化,研究了它们的磁性、光催化性能、可分离循环性和温度效应。在没有模板和蚀刻剂的帮助下获得了具有摇铃结构的磁芯/TiO₂壳的纳米颗粒。这些光催化剂在亚甲基蓝（MB）降解中的光催化活性实验表明,不同催化剂表面负载TiO₂能力不同,光催化性能不同,具有纳米片状TiO₂和摇铃结构的磁芯/TiO₂壳的纳米颗粒表现出良好的光催化性能。CoFe₂/TiO₂样品具有93%的最佳降解效率,因为CoFe₂芯可以捕获TiO₂中的光生电子,抑制电子和空穴对的复合;同时具有最高的饱和磁化强度值42 emu/g,对光催化剂使用后的回收有利。除了光催化应用之外,这些强磁性颗粒易于被外部磁场驱动,在其他领域也可以发挥作用,如细胞分离和靶向给药等。更多还原

【Abstract】 The tetragonal symmetric anatase TiO₂ has been widely investigated in the fields of degradation of organic pollutants,photocatalytic water splitting,electrocatalysis and solar cell,because of its stable physicochemical properties,nontoxicity,low-cost,environment-friendly and abundant reserve.The conduction band of anatase TiO₂ is composed of the Ti 3d state and valence band is mainly contributed by the 0 2p states and the band gap energy（Eg）is about 3.2 eV.Therefore,the excitation of electron-hole pair requires photons energy hv≥3.2 eV or wavelength λ≤387 nm.It means that the photoresponse of TiO₂ ranges in the ultraviolet region whose energy is less than 5%of the total energy of solar spectrum.The low utilization of sunlight limits the practical application of TiO₂.Meanwhile the photo-generated electron-hole pairs will recombine and vanish away in 10～100 ns,much shorter than the time for electrons to be captured and transfer（100 ns～1 ms）,so the majority of carriers cannot migrate to the surface of the catalyst,where the photocatalytic reaction occurs,consequently reducing the photocatalytic reactivity.As a result,how to restrain the carrier recombination becomes the key issue.In addition,the TiO₂ photocatalyst usually has a small particle size;Small size is favorable to the photocatalytic reaction because of the high surface area and more reactive sites.However,small particles tend to form a suspension in the solution,which are difficult to recycle after water purification treatment,and remain in the water.This will cause the secondary pollution.The issue of catalyst recycling is also a challenging work.In order to solve these problems,researchers have made a lot of efforts in the synthesis,modification and application of TiO₂ and its composites,and obtained many meaningful research results.However,for TiO₂ composites,photocatalytic reaction is very complex,because there are many parameters that affect the photocatalytic activity.In previous reports,many conclusions are contradictory and conflicting.There are still many difficult problems to be solved,such as how to realize the intimate contact between two phases in the synthesis of TiO₂ composites,or revealing its photocatalytic reaction mechanism and so on.The etherification of polyol during vapor-thermal reaction provides water for the hydrolysis of T⁴⁺precursor,beneficial for controlling the hydrolysis rate.The research was carried out using this synthesis method,and the main research contents are as follows:1.Study on the vapor thermal synthesis and photocatalytic properties of TiO₂/graphene composites.For TiO₂/graphene,to achieve intimate contact between two phases remains a big challenge.In this work,using the self-developed vapor-thermal method,and titanium（Ⅳ）tetraisopropanolate as the titanium source,TiO₂/graphene composites were synthesized in the isopropyl alcohol and deionized water vapor.For the samples synthesized in isopropanol vapor,the SEM and TEM results show that TiO₂ exhibits the nano-sheet morphology,and TiO₂ nanosheets uniformly and intimately grow on graphene.The lattice distortion was observed in XRD and FTIR results,further confirming the intimate contact between two phases.The valence band density of states（DOS）of TiO₂ varies with increasing the mass of graphene due to the lattice distortion and intimate contact between two phases.The results of UV-vis DRS,BET and PL showed that the combination of graphene and TiO₂ significantly enhanced the visible light absorption,increased BET specific surface area,and inhibited the recombination of photo-generated carriers,thus improved photocatalytic degradation performance.The results of ESR indicated that ·OH radicals had a greater contribution to photocatalytic degradation.For the samples synthesized in the water vapor,TiO₂ exhibits the particle morphology,and TiO₂ particles randomly scattered on graphene,similar to the results reported in literatures.The lattice distortion of TiO₂ was not observed.Although the TiO₂/graphene enhanced the visible light absorption and promoted the separation of photogenic carriers,the specific surface area decreased and the photocatalytic performance was not improved.These results show that:the intimate contact between two phases is a prerequisite for achieving synergistic enhancement effects of graphene and TiO₂,which requires the improved synthesis method.This synthesis method further improves the photocatalytic performance and provides an opportunity to discover new properties.2.Study on vapor thermal synthesis and N doping of TiO₂/graphene composites.Firstly,intimatly contacted TiO₂/graphene composites were synthesized in the vapor environment of isopropanol,and then doped with N in different N/Ti molar ratios.The existence states of doped N elements were as follows:in the interstitial sites of TiO₂ lattice,substituting O element in TiO₂,substituting C element in graphene,and adsorbed at the surface of graphene and TiO₂ particles.The effects of N doping can be summarized as follows:N doping changed TiO₂ nano-sheet to nanoparticles,accompanying with the reduction of the specific surface area of composites;The paticle size of TiO₂ increased with the increase of N doping degree,then decreased;The vibration modes of Ti-O-Ti were changed by N doping;N doping affected the chemical states of Ti and O elements in TiO₂ and introduced a variety of defects or impurity levels.The synergistic effect of these factors affects the photocatalytic performance.Only when N/Ti=2,the photocatalytic degradation performance to methylene blue was enhanced,and the degradation rate was increased from 7.7×10^-2 min^-,（without N doping）to 9.6×10^-2 min^-1.The results of ESR analysis showed that photo-generated electrons were captured by defects and impurities,and photogenic holes played a major role in the photocatalytic degradation.3.Study on the vapor thermal synthesis and S doping of TiO₂/graphene composites.Firstly,TiO₂/graphene were synthesized in the vapor environment of isopropanol by using the polyol etherification reaction to provide water for the hydrolysis of Ti sources.The intimate contact between TiO₂ and graphene was achieved,and TiO₂ presented a nano-sheet morphology.Then,TiO₂/graphene composites were doped with S in the water vapor by changing the molar ratio of S/Ti with thiourea as the S source,and a series of S-doped TiO₂/graphene composites were obtained.The composites were systematically characterized by means of XRD,TEM,FT-IR,XPS,UV-vis DRS,PL,BET and ESR to study the influence of S doping on the photocatalytic degradation performance.The changes of morphology,chemical state,crystal structure,electronic structure and photocatalytic performance depending on S doping degree were observed.The main conclusions are as follows:（1）S-doping transformed TiO₂ nano-sheets into nano-particles;（2）there were four chemical states for the doped S elements,i.e.,S^2-,S,S⁴⁺ and S⁶⁺.Additionally,the S element also had an influence on the chemical states of Ti and O in the TiO₂ lattice.When R_S/Ti＝0.5,only elemental S was detected and there was no substitution of S for Ti and O in TiO₂.When R_S/Ti≥1,S₂^-replaced O^2-and S⁴⁺/S⁶⁺ replaced Ti⁴⁺;（3）S-doping introduced electronic states above the valence band maximum of TiO₂;（4）the crystal structure of TiO₂ was distorted,and the lattice strain was maximum when R_S/Ti=3;（5）S doping increased the degradation rate of TiO₂ to the methylene blue（MB）pollutant from 7.7×10^-2 min^-1 to 10.7×10^-2 min^-1.The corresponding S-doped samples had the highest amount of ·OH/·O₂^-free radicals,which plays an important role in the MB degradation.4.Study on the synthesis and photocatalytic properties of CoFe₂/CoFe₂O₄/TiO₂.The purpose of this work is to study the influence of the interface between the magnetic core material and the catalyst on the photocatalytic mechanism.The CoFe₂ alloy has the highest magnetization among all binary alloys,which is beneficial to magnetic separation.First,well-dispersed uniform CoFe₂O₄ nanoparticles were prepared and coated by MgO through thermal decomposition of a metal-organic salt in organic solvent.CoFe2/CoFe₂O₄/MgO architectures were synthesized following the route from CoFe₂O₄/MgO to CoFe₂/MgO by reducing in a H₂/N₂ mixture gas and subsequently to CoFe₂/CoFe₂O₄/MgO by oxidized in air.MgO can be dissolved by the HCl solution.Next,the surfaces of CoFe₂O₄,CoFe₂/MgO,CoFe₂ and CoFe₂/CoFe₂O₄ magnetic particles were functionalized by TiO₂,and their magnetism,photocatalytic performance,separability for recycling and temperature effects were investigated.The rattle-type particles with magnetic core and TiO₂ shell were obtained without the assistance of template and etchant.The photocatalytic activity of these photocatalysts in methylene blue（MB）degradation showed that the surface supporting capacity of TiO₂ of different catalysts was different,resulting in different photocatalytic performance.The nanosheet-shaped TiO₂ and rattle-type particles exhibited good photocatalytic performance.The CoFe₂/TiO₂ sample had the highest degradation efficiency of 93%because CoFe₂ can capture the photo-generated electrons in TiO₂,inhibiting the recombination of electron and hole;meanwhile,it has the highest magnetization value of 42 emu/g,beneficial for the recovery of catalyst after degradation.Besides the photocatalytic application,these strong magnetic particles,which can be guided easily by the external magnetic field,may be potential applications in other fields,such as the cell isolation and targeting drug.更多还原