【作者】 王文广；

【导师】 余家国；

【作者基本信息】 武汉理工大学 ， 材料物理与化学， 2012， 博士

【摘要】 人类进入21世纪后,随着环境污染的日益严重,环境污染的控制与治理已经成为人类社会面临的亟待解决的重大问题。在众多治理环境污染的材料中,以二氧化钛为代表的氧化物半导体光催化材料以其独特的性能成为一种理想的环境污染清洁材料。但是,以二氧化钛半导体为基础的光催化技术还存在着量子效率低,太阳能利用率低等关键科学技术难题,极大地制约着它在工业上的广泛应用。针对上述难题,为加速光催化技术的实用化进程,提高二氧化钛的光催化活性和新型光催化材料的稳定性,本论文通过对二氧化钛进行复合改性以降低光生电子-空穴对的复合率,并考察了一种新型光催化材料的光腐蚀机理,取得了以下成果：第一,通过对C2H4(NH2)2, NaOH和Ni (NO3)2的混合溶液进行水热处理合成出由纳米片组装而成的分等级结构的花状β-Ni(OH)2。所制备的β-Ni(OH)2粉末在400℃煅烧5小时后则转变成NiO,而其形貌没有发生大的改变。此外,通过将β-Ni(OH)2粉末分散在Ti(OC4H9)4和C2H5OH的混合溶液中并加热至100度使乙醇蒸发,最后于400度煅烧5小时,得到表面均匀沉积了Ti02纳米颗粒的NiO超结构。所制备的NiO/TiO2p-n结超结构在降解对氯苯酚的过程中表现出比相同条件下制备的TiO2粉末和NiO粉术更高的光催化活性。其光催化活性的显著提高可归因为诸多因素,包括分等级多孔结构的形成,TiO2粒子在NiO超结构表面的均匀分布以及p-n结的形成。当p型NiO和n型TiO2复合时,它们之间会形成许多p-n结(内建电场)。在紫外光照射下,二氧化钛受激发产生电子-空穴对。在内建电场的作用下,二氧化钛价带上的光生空穴会加速往p型NiO上迁移,而电子向n型TiO2上迁移,使得光生电子-空穴对得到了有效的分离。此外,与传统的粉末光催化剂相比,NiO/TiO2复合结构在光催化反应结束后更容易通过离心或沉降法从悬浮液中分离出来并重复利用。经过多次降解对氯苯酚的循环实验后,NiO/TiO2复合体系的光催化活性没有明显的降低,表明NiO/TiO2样品具有很好的稳定性且不容易发生光腐蚀。第二,石墨烯是一种单层石墨,它拥有独特的二维结构,高导电率,优异的电子迁移率和非常大的比表面积,成本低廉,可以被大批量生产。因此,它被认为是一种优异的催化剂载体。最近,基于石墨烯的半导体光催化材料由于具有增强的光催化活性而引起了广泛的关注。我们以钛酸四丁酯为钛源,通过简单的一步水热法制备出低石墨烯负载(0-0.2wt.%)的分等级大孔/介孔TiO2-石墨烯复合材料。所制备的复合样品在光催化降解空气中的丙酮时表现出增强的光催化活性。石墨烯的含量对光催化活性有显著的影响,我们确定了石墨烯的最佳负载量。当石墨烯达到最佳负载量(0.05wt.%)时,所制备的复合材料具有最高的光催化活性,其速率常数分别是纯二氧化钛和商业P25(Degussa公司生产)的1.7倍和1.6倍。YiO2-石墨烯复合材料光催化活性的增强是因为石墨烯是优异的电子受体和传输体,它减少了电荷载流子的复合从而提高了光催化活性。瞬时光电流响应测试进一步证实了光生电子从Ti02到石墨烯的转移过程。第三,以甲醇为牺牲剂,通过在反应体系中直接添加Ni(NO3)2作为助催化剂,研究了商业P25二氧化钛粉末在氙灯光照下的光催化产氢活性。研究了Ni(NO3)2的浓度对二氧化钛在甲醇水溶液中光催化产氢活性的影响。研究结果表明,在添加Ni(NO3)2助催化剂后,Ti02的光催化产氢活性得到了明显的提高。本实验中,Ni(NO3)2(?)勺最佳浓度为0.32mo1.%,此时的光催化产氢速率为2547μmolh-1g-1,相应的量子效率为8.1%。二氧化钛光催化产氢活性的显著提高是因为二氧化钛表面沉积着被还原的Ni0。光催化活性提高的机理是：由于Ni2+/Ni的电极电势的位置(Ni2++2e-=Ni,E°=-0.23V)要比锐钛矿Ti02的导带位置(-0.26V)稍微低一些,同时又要比H+/H2的电极电势的位置(2H++2e-=H2,矿=-0.00v)高一些。这就有利于二氧化钛导带的电子转移到Ni2+上,并将部分Ni2+还原成Ni0,而Ni0可以促进电荷的分离并作为光催化分解水产氢的助催化剂,从而提高了二氧化钛的光催化产氢活性。第四,以硝酸银和Na2HPO4为前驱体,通过简单的沉淀法合成出Ag3P04球形颗粒。新制备的样品在可见光下降解罗丹明B时表现出高的光催化活性。其光催化活性随着循环次数的增加先升高而后降低。根据X射线衍射、X射线光电子能谱、紫外-可见吸收光谱、扫描电子显微镜以及透射电子显微镜等一系列表征结果,我们第一次提出了一种Ag3PO4光催化性能的增强与失活的机理。经过四次循环的Ag3PO4球形颗粒具有最高的光催化活性,这是由于沉积在Ag3PO4表面的Ag纳米颗粒扮演着电子捕获中心的角色,它可以阻止光生电子和空穴的复合。进一步增加循环次数,Ag3PO4的光催化活性下降,这是由Ag3PO4表面的Ag薄层的屏蔽效应导致的。本实验研究结果对Ag3PO4球形颗粒的光稳定性提供了新的解释,并有可能适用于阐述其它光催化活性复合材料的失活机理。更多还原

【Abstract】 In the21st century, environmental pollution has become increasingly serious. Pollution control and management has been the major issue and must be resolved. Among various pollution control technologies, titanium dioxide, as a representative oxide semiconductor photocatalytic material, is an ideal environmental pollution cleaning product due to its unique properties. However, there still exists some key scientific and technological problems with titanium dioxide semiconductor based photocatalytic technology such as low quantum efficiency and can only absorb a small fraction of solar energy, which greatly restricts its widely practical applications in industry. In view of the above problems, in order to promote the practical application of photocatalytic technology, improve the photocatalytic activity of titanium dioxide and stability of novel photocatalyst, we successfully prepared a serious of titanium dioxide-based composite photocatalysts to reduce the recombination rate of photogenerated electron-hole pairs. Meanwhile, the photocorrosion mechanism of a novel photocatalyst was also studied. The main points could be summarized as follows:Firstly, hierarchical flowerlike (3-Ni(OH)2superstructures composed of intermeshed nanoflakes are synthesized by hydrothermal treatment with a mixed solution of C2H4(NH2)2, NaOH and Ni (NO3)2. The as-prepared β-Ni(OH)2superstructures could be easily changed into NiO superstructures without great morphology change by calcination at400℃for5h. Furthermore, the TiO2nanoparticles can be homogeneously deposited on the surface of NiO superstructures by dispersing (3-Ni(OH)2powders in Ti(OC4H9)4-C2H5OH mixed solution and then vaporizing to remove the ethanol at100℃, and finally calcination at400℃for5h. The prepared NiO/TiO2p-n junction superstructures show much higher photocatalytic activity for photocatalytic degradation of p-chlorophenol aqueous solution than conventional TiO2powders and NiO superstructures prepared under the same experimental conditions. An obvious enhancement in the photocatalytic activity can be related to several factors, including formation of hierarchical porous structures, dispersion of TiO2particles on the surface of NiO superstructures, and production of a p-n junction. When p-type NiO and n-type TiO2integrate, a number of p-n junctions will be formed. Under UV light illumination, photons strike TiO2and create free electron-hole pairs in the electric field, the photo-generated holes at the valence band of TiO2will be accelerated towards the p-type NiO and the electrons towards the n-type with the effect of the inner electric field. Thus, the photogenerated electron-hole pairs will be easily separated. Further results show that NiO/TiO2composite superstructures can be more readily separated from the slurry system by filtration or sedimentation after photocatalytic reaction and re-used, compared with conventional powder photocatalysts. After many recycling experiments for the photodegradation of p-chlorophenol, the NiO/TiO2composite sample does not exhibit any great activity loss, confirming that NiO/TiO2sample is stable and not photocorroded.Secondly, graphene, a single layer of graphite, possesses a unique two-dimensional structure, high conductivity, superior electron mobility and extremely high specific surface area, and can be obtained on a large scale at low cost. Thus, it has been regarded as an excellent catalyst support. Recently, graphene-based semiconductor photocatalysts have attracted more attention due to their enhanced photocatalytic activity. In this work, hierarchical macro/mesoporpous TiO2-graphene composites with low loadings (0-0.2wt.%) of graphene were first produced by a simple one-step hydrothermal method using tetrabutyl titanate as the titanium precursor. The prepared composite samples presented enhanced photocatalytic activity in photodegradation of acetone in air. Graphene content exhibited an obvious influence on photocatalytic activity and the optimal graphene addition content was determined. At the optimal graphene concentration (0.05wt.%), the prepared composites showed the highest photocatalytic activity, exceeding that of pure TiO2and Degussa P-25by a factor of1.7and1.6, respectively. The enhanced photocatalytic activity is due to graphene as an excellent electron acceptor and transporter, thus reducing the recombination of charge carriers and enhancing the photocatalytic activity. The transient photocurrent response experiment further confirmed the transfer of photogenerated electrons from TiO2to graphene.Thirdly, photocatalytic H2-production activity on Degussa P25TiO2powder (P25) was evaluated by directly using Ni(NO3)2as a cocatalyst and methanol as a scavenger under Xenon light irradiation. The effect of Ni(NO3)2concentration on the photocatalytic hydrogen production rates of the TiO2in methanol aqueous solution was investigated. The results showed that the photocatalytic H2-production activity of TiO2was significantly enhanced by adding Ni(NO3)2aqueous solution. The optimal Ni(NO3)2concentration was found to be0.32mol.%, giving H2-production rate of2547μmolh-1g-1with quantum efficiency (QE) of8.1%. This high photocatalytic H2-production activity is due to the reduced Ni0on the surface of TiO2. The enhanced mechanism is because the potential of Ni2+/Ni (Ni2++2e-=Ni,E0=-0.23V) is slightly lower than conduction band (CB)(-0.26V) of anatase TiO2, meanwhile higher than the reduction potential of H+/H2(2H++2e-=H2, E0=-0.00V), which favors the electrons transfer from CB of TiO2to Ni2+and the reduction of partial Ni2+to Ni0. The function of Ni0is to help the charge separation and to act as co-catalyst for water reduction, thus enhancing the photocatalytic H2-production activity.Forthly, Ag3PO4spherical particles were synthesized by a facile precipitation method using silver nitrate and Na2HPO4as precursors. The as-prepared samples showed enhanced photocatalytic activity toward rhodamine-B (RhB) degradation under visible-light. The photocatalytic activity first increased and then decreased with increasing recycle times. Based on systematic characterization results obtained using X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, scanning electron microscopy and transmission electron microscopy, a possible mechanism responsible for the enhancement and deactivation of Ag3PO4photocatalytic performance was for the first time proposed. Ag3PO4spherical particles recycled for four times showed the highest photocatalytic activity in that Ag nanoparticles deposited on Ag3PO4acted as the electron trapping centers to prevent photogenerated electron-hole pairs from recombination. Further increasing recycle times decreased photocatalytic activity owing to the shielding effect by Ag layers on the surface of Ag3PO4. Results from current study shed new light on the photo stability of Ag3PO4spherical particles and are potentially applicable to other photocatalytically active composites.更多还原