【作者】 周新文；

【导师】 孙世刚；

【作者基本信息】 厦门大学 ， 物理化学， 2008， 博士

【摘要】 纳米材料具有不同于常规材料的特殊性质,在科学技术的发展中具有重要地位。纳米材料的合成,尤其是形状和结构控制合成、纳米材料的表征及其特殊性能的研究是纳米材料科学的前沿课题。CoPt和NiPt纳米材料由于其独特的磁学性能、高效的催化性能和特殊的光学性能而备受关注。本论文运用化学还原和电位置换法制备了不同结构的CoPt和NiPt纳米粒子,系统研究了它们的磁学、电催化和特殊红外光学性能。主要研究内容和结果如下:(1)一维链状CoPt纳米材料的合成及其性能研究。运用化学还原和电位置换法制备了两种不同结构的一维链状CoPt纳米材料,其一为实心结构(CoPt-a),另一种为空心结构(CoPt-b)。磁性研究指出,CoPt-a和CoPt-b纳米材料均表现出超顺磁性,对应的阻塞温度T_B分别为10.0 K和9.0 K。5 K时测得矫顽力分别为520 Oe和740 Oe;电化学循环伏安结果给出,CoPt-a/GC和CoPt-b/GC纳米粒子电极对CO的氧化都具有较好的电催化活性。与本体Pt电极相比,CO氧化峰电位分别提前了140 mV和160 mV。还测得CoPt-b纳米粒子对甲醇氧化具有很高的电催化活性,其氧化电流密度是商业Pt/C催化剂的1.9倍,稳定性与商业Pt/C催化剂相当。根据实验结果,CoPt-b纳米粒子对甲醇氧化有较好的催化活性可归因于Co元素的协同作用和其特殊的空心结构。原位红外光谱结果表明,当CoPt-a和CoPt-b纳米粒子负载在中等反射率的GC基底或高反射率的Au基底上时,无论是在固/液界面还是固/气界面,吸附态CO均给出异常红外效应(AIREs)的光谱特征。不同反射率的基底会影响增强红外吸收强度、半峰宽、Stark系数等参数,但不会改变AIREs光谱特征。进一步证明AIREs是一维CoPt纳米材料本身的特性,与基底无关。首次观察到一维纳米材料同样具有异常红外效应。研究结果有助于深入认识低维纳米材料异常红外性能的本质。(2)单分散NiPt纳米粒子合成及其性能研究。首次将电位置换法成功地应用到较难合成的NiPt体系,制备了单分散的NiPt纳米粒子。在实验的基础上,提出了合成机理。结果指出,实验体系的温度、反应物的滴加顺序和反应物比例是决定产物组成和结构的关键因素。NiPt纳米粒子同样表现出超顺磁性行为,测得阻塞温度T_B为7.8 K,5 K时矫顽力为517 Oe;原位红外光谱研究结果给出,在固/液界面和固/气界面、在中等反射率的GC基底和高反射率的Au基底上获得的结果光谱和单光束光谱中,吸附在NiPt纳米粒子上的CO均给出类Fano红外效应特征。同时表现出很强的红外增强吸收,测得在GC基底和Au基底上的红外增强因子分别为73和112。本研究首次在化学法合成的单分散NiPt纳米粒子上观察到类Fano红外效应,进一步验证了类Fano红外效应同样是低维纳米材料特殊的红外光学性能,对深入认识低维纳米材料特殊红外性能本质亦具有重要价值。(3)空心结构NiPt和CoPt纳米粒子对甲醇电催化氧化性能研究。首次运用电位置换法和共还原法得到空心结构(NiPt-a)和实心结构(NiPt-b)纳米粒子。电化学循环伏安研究结果指出,NiPt-a纳米粒子对甲醇氧化具有很高的电催化活性,其氧化电流密度是商业化Pt/C催化剂的1.9倍,同时具有较好的稳定性。NiPt纳米粒子对甲醇氧化有较好催化活性主要归因于双功能机理,也与纳米粒子的化学效应、电子效应及其特殊的空心结构相关联。运用原位红外光谱在分子水平研究了空心结构CoPt和NiPt纳米粒子上甲醇的电化学氧化过程。在空心结构CoPt纳米粒子上,检测到毒性中间体CO及其给出的异常红外效应光谱特征;而在空心结构NiPt纳米粒子上观察到类Fano红外效应,与直接用CO做为分子探针得到的结果一致。空心结构纳米粒子具有较高的催化活性,同时能提高金属催化剂利用率。因此,研究结果对发展燃料电池新型Pt基纳米催化剂具有指导意义。(4)核壳结构CoPt Pt纳米粒子的合成、自组装及其红外光学性能研究。运用改进的化学还原和电位置换法得到了CoPt纳米粒子,以HRTEM、XPS、XRD等方法表征确认制备的纳米粒子为核壳结构。通过液/液界面自组装得到CoPt Pt纳米粒子单层薄膜。透射红外光谱结果指出,吸附态CO在单分散无序堆积的CoPt_d/Si上给出增强红外吸收,而在纳米粒子薄膜CoPt_a/Si上给出类Fano红外效应。在CoPt_a/Si上吸附态CO的谱峰强度比在CoPt_d/Si上增强了约5倍。研究结果为合成核壳结构纳米粒子提供了一种新方法,进一步发展了液/液界面自组装方法。本论文研究结果对深入认识CoPt和NiPt纳米粒子的特殊性能,探索低维纳米材料特殊红外性能的本质具有重要意义,同时对研制直接甲醇燃料电池新型电催化剂具有重要应用价值。更多还原

【Abstract】 Synthesis,especially the controlled synthesis,characterization and properties of nanomaterials are the most important tasks in nanoscience because they have unusual properties that could not possessed by corresponding bulk materials.CoPt and NiPt nanomaterials have attracted multidisciplinary attentions because of their special magnetic, catalytic and optical properties.In this paper,CoPt and NiPt nanomaterials with different structure were prepared by chemical reduction and galvanic displacement reaction,and their magnetic,electrocatalytic and anomalous IR properties were studied.The main experiments and results are given as follow:（1） Synthesis and properties of one-dimension（1D） chain-like CoPt nanomaterials.Two kinds of 1D CoPt nanomaterials were prepared by galvanic displacement reaction.One is a solid structure（CoPt-a）,the other is a hollow structure（CoPt-b）.The results of magnetic measures showed that both CoPt-a and CoPt-b are superparamagnetic.The blocking temperatures of CoPt-a and CoPt-b are 10.0 K and 9.0 K.The coercivities are 520 Oe and 740 Oe obtained at 5 K,respectively.The results of CV demonstrated that the 1D chain-like CoPt-a and CoPt-b nanomaterials exhibit better electrocatalytic properties for CO oxidation than that of bulk Pt does in 0.1 M H₂SO₄.The current peak potentials of CO oxidation are shifted negatively by 140 mV and 160 mV comparison with bulk Pt electrode,respectively.It is clearly that the catalytic activity of the hollow CoPt-b nanoparticles is much higher than that of commercial Pt/C catalyst for electrooxidation of methanol.The oxidation density on CoPt-b nanoparticles is nearly 1.9 times than that on Pt/C catalyst.The remarkably high activity of CoPt-b nanomaterials may come from the effect of the cooperating function of Co element and the special hollow structure.The fact that both CoPt-a and CoPt-b nanomaterials loaded on GC or Au substrates produce abnormal infrared effects（AIREs） has confirmed that the anomalous IR features were generated mainly by the 1D chain-like CoPt nanomaterials, and that the influence of the substrate materials on the IR spectral features may be neglected in the present study.The current study demonstrated that 1D nanomaterials also show AIREs for the first time,and thrown a new insight into understanding the origin of anomalous IR properties observed on low-dimensional nanomaterials.（2） Synthesis and properties of monodispersed NiPt nanomaterials.Galvanic displacement reaction was introduced to synthesize monodispersed NiPt nanoparticles successfully for the first time.The size,composition and morphology can be well controlled by varying reaction temperature and ratio of the reactants.A probable mechanism for the formation of the NiPt nanoparticles was proposed based on a series of assistant experiments. The obtained NiPt nanopaeticles are superparamagnetic too.The blocking temperature is 7.8 K with a coercivity of 517 Oe at 5 K.In situ electrochemical FTIRS employing CO adsorption as probe reaction demonstrated that NiPt/GC or NiPt/Au electrodes exhibit characteristics of Fano-like infrared effects either in result spectra or single spectra.The substrate materials do not affect significantly the anomalous IR features,as illustrated by the similar anomalous IR features observed for CO adsorbed on both NiPt/GC and NiPt/Au electrodes.The results demonstrated that dispersed NiPt nanomaterials obtained by chemical method display characteristics of Fano-like infrared effects for the first time,and are of significant academic importance in understanding the origin of anomalous IR properties observed on low-dimensional nanomaterials.（3） Properties of methanol electrooxidation of CoPt and NiPt nanomaterials with a hollow structure.Two kinds of NiPt nanomaterials were prepared by galvanic displacement reaction.One is a hollow structure（NiPt-a）,the other is a solid structure（NiPt-b）.The results of CV demonstrated that the electrocatalytic activity of NiPt-a nanoparticles is much higher than that of commercial Pt/C catalyst for the electrooxidation of methanol.The oxidation density on NiPt-a nanoparticles is nearly 1.9 times than that on Pt/C catalyst.The remarkably high activity of the NiPt nanomaterials for oxidation of methanol may come from the bifunctional mechanism,electron effect,chemical effect and the special hollow structure of the nanomaterials.The electrooxidation of methanol on CoPt-b and NiPt-a nanoparticles was studied on the molecular level by in situ FTIR.The poisonous intermediates CO observed on CoPt-b/GC electrode displays AIREs while shows Fano-like infrared effects on NiPt-a/GC electrode.The results are of significant academic importance in developing catalysts of fuel cell. （4） Synthesis,self-assembly and IR optical properties of core-shell CoPt nanomaterials. We have synthesized spherical CoPt nanoparticles by modified galvanic displacement reaction,successfully.The results of HRTEM,XPS,XRD indicated that product has a core-shell structure,and can be signed as CoPt@Pt.An ordered monolayer film of CoPt nanoparticles was obtained on the Si wafer by a L/L interface technique.In situ FTIR transmission spectrum study indicated that the ordered self-assembled monolayer film of CoPt nanoparticles（CoPt_a） shown Fano-like infrared effects while the multilayer disordered CoPt nanoparticles（CoPt_d） give an enhanced IR adsorption.The results afford a new method to synthesize nanomaterials with a core-shell structure,and develop the Liquid/Liquid self-assembly technology.The results in the current paper have contributed to understand deeply the special properties of CoPt,NiPt nanomaterials,and are of significant academic importance in understanding the origin of the anomalous IR properties of low-dimensional nanomaterials,as well as developing of new type DMFC electrocatalysts.更多还原