【作者】 刘洋；

【导师】 全燮；

【作者基本信息】 大连理工大学 ， 环境工程， 2014， 博士

【摘要】 光催化产氢技术是一种利用光激发半导体产生的光生电荷分解水产生氢气的技术。这一技术有望转化太阳能为氢能来替代日渐枯竭且污染环境的化石能源。该技术的核心是高效光催化剂的开发。但是,传统产氢光催化剂的光生载流子容易复合且其表面反应速度慢,导致光能转化效率低。因此,开发载流子分离效率高且表面反应速度快的新型高效光催化剂是该领域的研究重点。近几年出现的二维纳米材料MoS2和石墨烯具有原子层厚度且载流子迁移速率高的特点,有利于光生电荷的分离与迁移。另外,二维纳米材料大的比表面积和边缘活性点位对表面反应有利,是解决传统产氢光催化剂面临问题的首选材料。本论文基于上述分析设计并制备了三类基于二维纳米MoS2或(和)石墨烯的产氢光催化材料,通过瞬态吸收谱、表面光电压谱、电子自旋共振波谱以及与典型产氢光催化剂对比等手段和方式考察了其电荷分离效率和表面反应速率,并测试了其产氢性能,主要研究成果如下：利用溶剂热、水热和光还原等方法制备了CdS/Gr、Au/Gr/TiO2和Au/GO三种基于石墨烯的光催化剂。透射电镜和X射线衍射表征结果表明,石墨烯不但能抑制CdS的团聚,而且有利于提高其结晶程度。瞬态吸收谱和表面光电压谱测试结果显示,CdS/Gr的光生电子寿命为87.86μs,是CdS(57.86μs)的1.5倍,表明石墨烯能够提高光生载流子分离效率。当石墨烯含量为8wt%时CdS/Gr的光催化产氢速率最大,为44μmol-h"1,是CdS的2.9倍。CdS/Gr在重复实验中能够保持良好的稳定性。添加Pt助催化剂后,CdS/Gr/Pt催化剂的产氢速率相对于典型的CdS/Pt提高了近100%,表明石墨烯是贵金属Pt的潜在替代物；利用电子自旋共振波谱对不同波长范围入射光照射下Au/Gr/TiO2的光生电子迁移途径进行的分析结果表明,Au/Gr/TiO2的光生电子存在由Au到Ti02再到石墨烯的矢量传递。表面光电压谱测试结果表明,与Au/TiO2相比,上述光生电子的矢量传递使得Au/Gr/TiO2中的光生电子和空穴可以被有效的分离。光催化产氢实验表明,在Ti02和TiO2/Gr无法产氢的纯水中,Au/Gr/TiO2的产氢速率为44μmol·h-1;而在甲醇中Au/Gr/TiO2的产氢速率为712μmol·h-1,是Au/TiO2的2倍；将石墨烯经过表面修饰得到的石墨烯氧化物直接用于光催化产氢的实验结果表明,石墨烯氧化物具有光催化产氢的能力,其产氢速率为0.04μmol·h-1,而Au/GO的光催化产氢速率约为3μmol·h-1。这主要是由于纳米Au的等离子共振效应可以提高可见光吸收效率。采用水热法在温和条件下合成了基于MoS2的产氢光催化剂MoS2/CdS。研究发现,合成温度、时间和MoS2含量分别为200℃、24h和6.9wt%时,MoS2/CdS的产氢速率最高,达到192μmol·h-1,是CdS的17倍。此时,MoS2/CdS的产氢速率高于CdS/Pt,显示MoS2具备提高表面产氢反应速率的能力。此外,瞬态吸收谱结果显示MoS2/CdS的光生电子寿命为167.03μs,约为CdS的3倍,说明MoS2/CdS可以提高光生载流子分离效率。在以Na2S和Na2SO3作为牺牲剂的重复实验中,MoS2/CdS显示出良好的稳定性。将牺牲剂替换为甲酸,MoS2/CdS仍能高效地产氢。该方法在MoS2/CdS的制备过程中不仅可以避免其他合成方法中的高温煅烧步骤(>400℃),还可以避免使用有毒的硫化物前驱体。利用水热法制备了基于石墨烯和MoS2的产氢光催化剂MoS2/Gr/CdS。研究结果表明,MoS2/Gr中MoS2和Gr的质量分别为95wt%和5wt%时,MoS2/Gr对CdS产氢速率的促进作用最大,此时MoS2/Gr/CdS的最高产氢速率为390.7μmo1·h-1。瞬态吸收谱表征结果显示,此时MoS2/Gr/CdS的光生电子寿命为183.12μs。MoS2/Gr/CdS的光生电子寿命和产氢速率明显高于MoS2/CdS和CdS/Gr,说明MoS2/Gr可以同时在光生电子和空穴分离效率和表面产氢反应速率两方面促进CdS的光催化产氢性能。此外,MoS2/Gr/CdS在连续产氢测试中显示出良好的稳定性。综上所述,将二维纳米材料石墨烯和MoS2用于光催化产氢,可以显著提高光催化剂的光生载流子分离效率和表面产氢反应速率,为提高产氢反应的光能量利用效率提供了新途径,有助于推动光催化产氢技术的发展。更多还原

【Abstract】 Photocatalytic hydrogen production technology is the use of the photogenerated charges of semiconductor to split water into hydrogen. This technology is expected to convert solar energy to hydrogen energy to replace the daily exhausting and environmental unfriendly fossil energy. The kernel of this technology is development and application of highly efficient photocatalyst. However, traditional photocatalysts suffer from photogenerated charge recombination and low surface reactive rates, which makes traditional photocatalysts far from the application requirements. Therefore, development of highly efficient photocatalysts with high photogenerated carrier seperation efficiency and surface reaction rate is a focus of research in the field of photocatalysis. Recently, two-dimensional nano MoS2and graphene is emerging as the prior choice to sovle the problems facing traditional photocatalyst becouse their single atomic layer structure and high carrier mobility rate could facilitate the photogenerated carrier seperation and transfer as well as their huge surface and active sites on the edges could speed the surface reaction rate. Based on the above analysis, three kind of photocatalyst based on graphene and (or) MoS2were synthesised. The photogenerated carrier seperation efficiency, surface reaction rate and hydrogen production were studied by the methods of transient absorption spectroscopy (TA), surface potovoltage spectroscopy and electron paramagnetic resonance spectrum as well as the comparison analysis of the as-prepared photocatalyst and typical photocatalyst. In this dissertation, several works have been done as follows:Three gaphene based photocatalyst CdS/Gr, Au/Gr/TiO2and Au/GO have been synthesized by solvent-thermal, hydrothermal and photoreduction methods. The results of TEM and XRD revealed that graphene inhibited the aggragation and promoted the crystallization of CdS. The results of transient absorption spectroscopy and surface potovoltage spectroscopy (SPV) showed that, the photogenerated carrier lifetime of CdS/Gr is87.86μs, which is1.5times as long as that of CdS (57.86μs), indicating the enhancement of photogenerated charge seperation. When the weight ratio of graphene was8wt%, the highest rate44μmol·h-1was obtained which was2.9times as high as that of CdS. Moreover, CdS/Gr showed good stability in the continuous experiment. After deposition of Pt on this photocatalyst, the hydrogen production rate of CdS/Gr/Pt is100%higher than that of CdS/Pt, indicating that graphene is a potential substitute of Pt; Electron paramagnetic resonance spectrum was employed to study the photogenerated electron transfer under incident light irradiation with different wavelenghs. The results showed that there was a vector transfer from Au to TiO2and then to graphene. The results of SPV revealed that, compared with that of Au/TiO2, the photogenerated electrons of Au/Gr/TiO2could be separated more effectively. The results of hydrogen production experiments showed that, the hydrogen production rate of Au/Gr/TiO2and was44mol·h-1in water where no hydrogen was detected over TiO2and TiO2/Gr; In methanol, the hydrogen production rate was712mol·h-1,2times as high as that of Au/TiO2; Graphene oxide (GO) was directly used as a photocatalyst in photocatalytic hydrogen production. The results showed that GO could produce hydrogen from pure water and its photocatalytic hydrogen production rate was0.04μmol·h-1. Au nanoparticles with surface plasmon resonance effect could absorb visible light, and thus improve the photocatalytic hydrogen production rate of GO, and this rate was about3umol·h-1. Moreover, Au/GO showed good stability in the continuous experiment.MOS2based photocatalyst MoS2/CdS has been fabricated through a green hydrothermal method. The research results showed that the highest hydrogen production rate was192μmol·h-1which was17times as high as that of CdS when the synthetic temperature and time as well as the ratio of MoS2were200℃,24h and6.9wt%. The results of transient absorption spectroscopy showed that the photogenerated electron lifetime of asprepared MoS2/CdS was167.03μs, about3times as long as that of CdS, indicating the enhanced photogenerated charge seperation. The hydrogen production rate of MoS2/CdS was higher than that of Pt/CdS, which showed the advantage of MoS2with supporting active sites for hydrogen production and presented the potential of MoS2as a cheap cocatalyst. The MoS2/CdS photocatalyst remained good stability in Na2S and Na2SO3solution in the continuous experiments. Moreover, this photocatalyst showed high activity for hydrogen production in the solution of formic acid, one of the main sources of acid rain, indicating it can realize the pollutant degradation and hydrogen production simutanously. In addition, this preparation method avioded high temperature calcination step (>400℃) and toxic sulfur source.Graphene and MoS2based photocatalyst MoS2/Gr/CdS has been synthesized for H2production. The results showed that MoS2/Gr with95wt%MoS2and5wt%graphene could maximize the photocatalytic hydrogen production rate of CdS. The photocatalytic hydrogen production rate and photogenerated carrier lifetime of MoS2/Gr/CdS were390.7μmol·h-1and183.12μs. Compared with MoS2/CdS and CdS/Gr, the higher photocatalytic hydrogen production rate and longer photogenerated carrier lifetime of MoS2/Gr/CdS indicated that both photogenerated carrier separation and surface reaction rate of CdS could be improved by MoS2/Gr. Moreover, MoS2/Gr/CdS showed good stability in the continuous experiment.In summary, two-dimensional MoS2and graphene could significantly improve the photogenerated charge separation efficiency and surface reactive rates of photocatalyst. The application of them in photocatalytic hydrogen production paves the way to enhance the ultilization efficency of solar light and promotes the development of photocatalytic hydrogen production technology.更多还原