【作者】 徐剑峰；

【导师】 洪樟连；

【作者基本信息】 浙江大学 ， 材料学， 2016， 博士

【摘要】 进入新世纪以来,高速发展的科学技术推动着社会进步的同时,给人类赖以生存的地球环境带来了能源消耗以及环境污染两个大的挑战,对处于高速发展中的我国来说,环境污染问题更为严重,需要发展新的技术加以应对。自从Fujishima和Honda在1972第一次采用TiO2电极通过光电催化成功将水分解为氧气和氢气后,各国研究人员纷纷开始关注清洁、安全而又充足的太阳能。因此在今后的几十年中,基于半导体材料的光催化剂获得了能源与环境领域研究人员广泛的关注与重视,并开展了一系列系统而又重要的光催化研究。半导体光催化剂不仅提供了一种将太阳能有效转化为化学能的途径,而且基于光催化剂的超强降解能力,在光照下,能够通过其在液相中产生的强氧化性的羟基自由基降解大部分有毒有机污染物,一定程度上解决了环境污染的问题。在众多半导体光催化剂中,TiO2因其稳定的物理化学性质,无毒性,高效的紫外光活性以及低成本,成为了光催化领域中研究最多且最深入的材料之一,具有潜在的工业化价值。然而,由于固有带隙较宽,传统TiO2光催化剂对太阳光谱的催化响应范围仅仅限于紫外光波段,而紫外光的能量仅占光谱能量的5%,导致最终光能利用率十分低,结合较高的光生电子空穴复合率,极大限制了它的应用。为了扩宽TiO2纳米材料的光谱响应范围,同时抑制光生载流子的复合,最终达到提升其光催化活性的目的,以TiO2纳米材料为基的复合光催化剂成为了当前该领域重要的研究热点。本文首先对光催化原理及具有代表性TiO2光催化材料研究进行了综述,介绍了半导体光催化原理与过程要素、TiO2制备与测试分析方法,并对TiO2常用修饰改性方法进行了重点阐述。从拓宽光谱响应范围、有效抑制光生电子-空穴对复合以及解决催化剂固载化问题考虑,采用经溶剂热法、水热法等工艺制备得到的不同形貌锐钛矿结构TiO2作为基体,进行了表面复合修饰改性研究：主要利用原位光沉积法、浸溃法等湿化学方法进行两相复合结构制备,成功构筑制备了四种TiO2基的复合光催化剂。研究表明,复合结构有效促进了光生载流子的分离、提高了光能利用率,增强了对有机污染物甲基橙的降解效率,提高了光催化产氢速率。主要工作有以下三方面：一、构筑制备了TiO2@C/Au和TiO2@C/Ag两种复合光催化剂,拓宽了TiO2光谱响应范围、增加了光能利用率,探明了界面无定形碳层以及贵金属浓度对于光催化性能的影响。首先,采用溶剂热法,成功制备了包覆着具有不同表面结构的无定形碳层的锐钛矿相TiO2@C纳米颗粒,颗粒大小在15nm左右；随后,经过不同温度(70℃,180℃和360℃)热处理,成功获得了不同表面有序度和厚度的无定形碳层,测试分析证明热处理过程仅仅对表面碳层进行了调控,并没有对TiO2的粒径和结晶度产生显著影响。采用上述TiO2@C材料为原料,采用紫外光照生成的Ti3+离子原位还原HAuCl4和AgNO3,成功制备了贵金属含量不同的TiO2@C/Au和TiO2@C/Ag复合光催化剂。实验结果证明了有机钛源制备得到的TiO2含有对光生电子的分离和迁移产生重要作用的无定形碳层复合结构。在可见光降解甲基橙的实验中,TiO2@C/Au和TiO2@C/Ag样品均获得了可见光催化活性,并且发现随着贵金属的浓度增加,可见光催化活性出现了不同程度的下降。经讨论分析,主要原因可能是由于光生电子主要来自于贵金属纳米颗粒,而本实验中1.5n～2nm左右的纳米金和纳米银的表面等离子共振作用只有在较低浓度下才能发挥最佳作用。因此通过该方法,相较于其他已经报道的研究工作,我们可以采用较低浓度的贵金属源,获得更高的可见光催化活性。二、构筑制备了TiO2纳米线薄膜为基体复合纳米Au颗粒而成的TiO2@Au复合光催化剂,探明了纳米Au复合物浓度、纳米Au颗粒尺寸对光催化产氢效率的影响规律。采用碱热法在Ti上生成了锐钛矿ZiO2纳米线薄膜,然后利用湿化学浸渍法成功地在TiO2纳米线薄膜上负载了不同粒径和不同浓度的纳米Au颗粒。测试分析表明,该方法在成功提高TiO2@Au复合物的光催化产氢活性的情况下,能够有效解决催化剂固载问题。研究发现通过调控负载纳米Au颗粒的尺寸,可有效调控复合物的光催化活性,发现尺寸为10nm的Au颗粒具有最佳的光催化产氢促进作用,且此复合物具有很好的循环使用稳定性。研究发现,采用不同纳米Au复合物浓度制备的系列复合物样,呈现出催化产氢活性随着浓度增加而增强的特性规律。研究判定,纳米Au颗粒浓度在达到表面分散阈值之前,增加复合物中纳米Au颗粒含量可持续提升和促进光催化产氢活性。此外,通过在不同光源下的光催化产氢对比研究发现,在明确排除了可见光照射下的表面等离子共振作用效应起作用的情况下,加入可见光照射可一定程度地提升原有紫外光照下的产氢效率,表明传统观点认为主要是表面等离子体共振效应为主的纳米Au颗粒复合之后,对复合物光催化性能存在着更为复杂的作用机制。三、构筑制备了TiO2纳米管薄膜(TNF)为基体复合纳米碳颗粒的TiO2@Cdots复合薄膜光催化剂,显著提高了光催化活性,揭示了固载化的薄膜表面形貌与结构以及碳纳米点复合修饰改性在光催化分解水反应中的重要作用。首先,采用碱性水热法在10M的NaOH溶液中于130℃反应不同时间制备得到了具有不同多孔结构的TiO2纳米管薄膜,并对其光电性能进行了表征。水热反应15小时获得的TiO2薄膜(TNF-15)因其特有的表面多孔结构展现出了最佳的光电性能。以抗坏血酸为碳源利用溶剂热法制备得到了不同浓度的碳纳米点,采用TEM对其形貌、尺寸和分散性进行了表征；并通过紫外可见吸收谱和荧光光谱对碳纳米点的光学性能进行了测试,从而获得了一系列具有不同吸光性和荧光特性的碳纳米点。随后,采用简便的溶液浸渍法将碳纳米点负载到了具有最优表面结构的TNF-15薄膜上,构筑得到了一系列不同碳纳米点含量的固载化的TNF/Cdots复合光催化剂。最后,对制备得到的纯TiO2纳米管薄膜及其碳纳米点复合光催化剂进行了光催化产氢测试。研究结果表明,调节碳纳米点的含量可成功调节复合光催化剂的产氢效率,相对于纯相的TiO2纳米管薄膜催化剂,1000mg/L的碳纳米点复合后TNF/Cdots复合光催化剂的产氢效率提高了400%。通过制备优良的PEC电极,对比复合前后的光电流证实了碳纳米点的复合修饰使得光生电子-空穴对的复合几率大大降低,最终提高了光催化活性。更多还原

【Abstract】 Despite the high quality of life brought by the advanced science and technology since the 21st century, humans are still under undying warnings:the resources and self-healing ability of the earth is limited and increasing survival crisis is coming. Among these major issues, the energy and environmental crisis are the two of the most urgent problems faced by China and the developed countries.Since the first realization of water splitting using TiO2 as the electrode by Fujishima and Honda in 1972, this safe, clean and abundant solar energy has been concerned by researchers and thus the technology of photocatalysis have been sufficiently developed. Based on the photocatalysis process that photocatalyst can not only turn the solar energy into chemical energy,but also form hydroxide radical to realize the degradation of organic pollutants, scientists have proposed various semiconductor photocatalysts to solve the energy and environmental problems in some degree. Amog the semiconductor photocatalysts, TiO2 is one of the most studied materials virtually for its stable physical and chemical properties, nontoxicity, low price and high activity, which have the great potential to apply for the industry. However, because of its intrinsic wide bandgap, high recombination rate of excited electron-hole arid low charge transfer ability, the conventional TiO2 photocatalysts is limited to ultraviolet light. Thus the utilization of solar energy is quite low, resulting poor photocatalytic activity. In order to extend the utilization of solar energy and promote the charge transfer thus enhancing its photocatalytic activity, TiO2-based composited photocatalysts have become a hotspot of current research in this field.In this dissertation, the photocatalysis process and the representative photocatalyt (TiO2 materials) were briefly introduced firsly. And the mechanism of semiconductor photocatalysis and the preparation and analysis methods for TiO2 photocatalyst were also included. The existing modification technologies of TiO2 were summaried and the modification by noble metal and carbon materials were highlighted. In order to broaden the scope of the spectrum and inhibit photo-induced electron-hole pairs’ recombination of TiO2, the pristine TiO2 was modified by compositing noble metals and carbon materials. The pristine TiO2 matrixs were prepared through solvothermal and hydrothermal methods. The modification was realized by the in-situ deposition method and the dipping wet chemical method. Four kinds of TiO2-based photocatalyst were successfully designed and prepared in this way, obtaining significantly enhancement of the photocatalytic activity under UV or visible light. By designing of these modification structures, extended light response, promoted separation of photo-generated carriers and improved energy absorption efficiency were realized, all of which contributed to the highly enhanced photocatalytic activity. The main work is presented in the following three aspects:1, The prepared TiO2@C/Au and TiO2@ C/Ag composited photocatalysts exhibit extended light response and enhanced harvest of solar energy contributing to highly improved photocatalytic activity. The effect of interface amorphous carbon layer and the noble metal concentration on the photocatalytic activity was also studied. The pristine TiO2 samples with various surface structures of amorphous carbon layers were synthesized by solvethermal method assisted with heat treatment at different temperatures. The particle size and crystallization of the prepared anatase TiO2 at about 15nm wasn’t affected during the annealing process. Then the deposition of noble metal on the TiO2 particles was carried out by the Ti3+ on the TiO2 surface generated under UV light irradiation. The diameters of reduced Au and Ag nano particles arranged between 1.5 nm and 2 nm. By controlling the surface status of TiO2 through amorphous carbon layer adjusting, the concentration of Ti3+ was tunable which could result in tunable concentration of noble metal. This phenomenon verifys the fact that the amorphous carbon layer will great influence the separation and transport of photo induced electron and thus play an important role during the photocatalysis process.The degradation of methyl orange was carried out under visible light and both of TiO2@C/Au and TiO2@ C/Ag samples obtained efficient photocatalytic degradation. Besides, the photocatalytic activity was closely related to the concentrations of noble metal which could generate electron under visible light through surface plasmon resonance. And the final results depicted that the Au and Ag particles around 1.5-2nm would have efficient performance at quiet low concentration, correponding to TiO2-70-Au-10 and TiO2-70-Ag-7, respectively, In this way, we find an efficient method to extend the light response and promote the photocatalytic activity at low concentration with the interface engineering assistance.2, The prepared TNF/Au composites displayed enhanced photocatalytic water splitting performanc. The TiO2 nanowire film (TNF) was firstly fabricated by hydrothermal method and the Au nano particles was loaded on the surface of TNF by wet-impregnation method with tunable concentrations. The results shows that the surface plasmon resonance in this composites could not contribute to the realization of water splitting under visible light because of the high barrier potential at the interface inhibiting the transport of photo induced electron on the Au particles to conducting band of TiO2. The size effect of Au nano particle have a great influence on the photocatalytic water splitting performance for TNF/Au composites that Au particle of 10 nm exhibited the best performance at the same concentration with stable recycling ability compared to the activity of 20 nm and 30 nm.At the same time, the concentration effect of Au particle was also explored and the results revealed that the water splitting performance of the composites would be gradually improved with the concentraion before surface distribution valve value. The results of water splitting performance of the TNF/Au composites under different light sources displayed a new phenomenon that the photocatalytic activity in the ultraviolet light could be affected by irradiation of visible light without the assistance of surface plasmon resonance. While there is no hydrogen generation under white light LED, the performance under both of UV LED and white light LED is much better than pure UV LED revealing the complicated mechanism of photocatalysis for TNF/Au composites.3, The prepared TNF/Cdots with various concentrations of carbon dots by hydrothermal method conbined with wet-impregnation method obtained highly improved photocatalytic water splitting performance. The TiO2 nanotube films with various surface morphologies and structures were firstly fabricated by hydrothermal method at 130 ℃ for different reaction time. The TNF-15 samples displayed best performance for PEC analysis benefited from the extraordinary surface structure for light harvest and absorption. Secondly, the carbon dots solutions were prepared by solvethermal method at 160 ℃ using ascorbic acid and ethanol as carbon source and solvent, resepctively. These carbon dots of 5nm exhibited excellent distribution and optical properties.The pure TNF samples and TNF/Cdots composites were used to split water under 300 W xenon lamp. With the increasing content of carbon dots, the photocatlytic water splitting performance of the TNF/Cdots composites could be successfully adjusted. The improved PEC performance of TNF/Cdots composites furtherly testified the contribution of carbon dots to efficient separation and transport of photo induced electron and hole pairs. The TNF decorated by carbon dots of 1000 mg/1 obtained 400% enhancement of hydrogen generation compared to pure TNF samples.更多还原