【作者】 葛性波；

【导师】 丁轶；

【作者基本信息】 山东大学 ， 物理化学， 2011， 博士

【摘要】 近年来,纳米多孔金属因为其独特的结构、性能成为了纳米材料家族中的重要一员,并在电子、光学、催化工业中得到了广泛的应用。进一步优化、发展这类材料的制备方法,研究它们的结构、性能,拓展它们的应用对纳米材料科学及工业应用都具有重要意义。由于工业界对新型能源材料,如高效率的燃料电池催化剂的迫切需求,本论文致力于制备超低金属载量的纳米多孔Au、Pt/Au、Pd/Au薄膜,并研究它们的结构与电催化性能,探索它们在电化学传感及燃料电池等领域的应用前景。主要结果如下：1.以Au/Ag合金薄膜为前躯体,通过在浓硝酸中腐蚀溶解掉Ag,成功制备了纳米多孔Au (NP-Au)薄膜。制备的NP-Au薄膜的厚度约为100 nm,其金属载量约为0.1mg/cm2。以NP-Au为基底,制备了纳米多孔的Pt/Au (NP-Pt/Au)薄膜,我们采用化学镀和欠电位沉积加置换的方法分别在NP-Au表面均匀地修饰上Pt原子层,通过控制反应条件,其厚度可以从次原子层到数个原子层内得到精确调控。成分分析表明该薄膜的Pt载量从1到25μg/cm2间得到精确调控。运用扫描电子显微镜(SEM)和高分辨透射电子显微镜(HR-TEM)研究了NP-Au和NP-Pt/Au薄膜的结构,发现它们具有三维双连续的海绵状结构,使大量活性原子暴露在表面。研究了不同化学环境下Pt在NP-Au表面的生长过程,HR-TEM、电化学的结果表明Pt在NP-Au表面以岛状的方式生长。利用循环伏安法(CV)研究了这些纳米多孔金属薄膜的基本电化学行为,并探索了NP-Au对NO2-氧化的电催化性能,发现其对该反应具有高灵敏的响应。计时电流的研究表明NO2-在NP-Au电极上从1μM到1mM都具有良好的线性关系,表明NP-Au薄膜可能在电化学传感方面得到应用。2.系统研究了NP-Pt/Au薄膜对甲醇、甲酸的电氧化,氧还原等重要电化学反应的催化性能,并以该薄膜为催化剂进行了质子交换膜燃料电池测试。以NP-Pt/Au薄膜为阳极催化剂,利用CV等方法评估了不同Pt载量的NP-Pt/Au薄膜的电催化活性,发现高Pt载量的NP-Pt/Au薄膜的催化特性与商业Pt/C相似,但具有更高的本征活性,低Pt载量的NP-Pt/Au薄膜则表现出比Pt/C更高的质量比催化活性。利用SEM、HR-TEM、X射线光电子能谱(XPS)、电化学CO扫除等表面科学手段研究了该薄膜的表面结构、性能,分析了其高催化活性的起因。将该薄膜作为阳极催化剂,进行了H2/空气,直接甲醇、甲酸燃料电池测试,探索了其理想的工作条件参数,探讨了其工业应用的可能。3.探索了NP-Pt/Au薄膜的结构稳定性,尤其是其在低温加热时的结构、性能的演化。将具有不同Pt载量的NP-Pt/Au薄膜在100到400℃之间加热不同时间,并利用SEM、HR-TEM、XPS和电化学技术研究了其结构的演化及表面原子的化学状态,探讨了温度等因素对其结构的影响。结果表明NP-Pt/Au薄膜在300℃仍能保持其原有的纳米多孔结构,但其表面的Pt纳米粒子与Au衬底之间发生了相互扩散并形成了表面合金薄层,使得其表面原子的排列和化学状态发生了明显的改变。讨论了结构演化对其催化性能的影响,电化学测试显示发生表面Pt原子重排的NP-Pt/Au薄膜对甲酸电氧化具有更高的催化活性。4.制备了纳米多孔Pd/Au (NP-Pd/Au)薄膜。通过发展、优化NP-Pt/Au的制备工艺,成功在将Pd沉积到NP-Au表面。XPS结果显示其表面Pd为金属态,HR-TEM研究表明Pd原子层在NP-Au表面为层状模式的外延生长,从而形成了连续核壳结构的NP-Pd/Au薄膜。探索了该薄膜的基本电化学性质及在酸性介质中对甲酸的电催化性能,CV测试结果表明NP-Pd/Au薄膜的电催化活性比商业Pd/C催化剂的电催化活性提高了两倍多。并以该薄膜为阳极催化剂进行了直接甲酸燃料电池测试。更多还原

【Abstract】 Nanoporous metal is one of the most important nanomaterials and widely used in electronics, optic, catalysis, due to its unique structure and property. Exploiting new method to fabricate it, further studying its structure, and exploring its applications are of special significance for nanomaterial science and modern industry. On the other hand, the novel material for energy conversion, such as effective fuel cell catalysts, recently is highly desirable for industry. Here we focus on fabricating nanoporous Au, Pt/Au, Pd/Au membranes with ultra-low metal loading. We also investigated their structure and electrocatalytic activity and explored their applications for the electrochemical detection and fuel cells. The results are as follows:1. Nanoporous Au (NP-Au) membrane was made by dealloying Au/Ag alloy in concentrated HNO3. Upon silver dissolution, gold atoms left behind will self-organize into an interconnected network of pores and ligaments. The as-prepared (NP-Au) membrane is about 100 nm in thickness, with a metal loading about 0.1 mg/cm2. On the basis of the NP-Au, we prepared nanoporous Pt/Au (NP-Pt/Au) membrane by plating an atomically thin layer of Pt over NP-Au substrate. Two routes were developed to fabricate NP-Pt/Au membrane, i.e. interface electroless plating technique and electrochemical under potential deposition combining redox replacement method. By controlling the reaction process, the deposited Pt overlayers can be tuned from sub-monolayer to several monolayers. Composition analysis suggests the Pt loadings in NP-Pt/Au membrane are in the range of 1 to 25μg/cm2. The structure of NP-Au and NP-Pt/Au membranes were studied via scanning electron microscope (SEM) and high-resolution transmission electron microscope (HR-TEM), which exhibits a three-dimensional continuous porous structure, exposing a large number of active Pt atoms on the surface. The HR-TEM and electrochemical study suggest that Pt overlayers epitaxially grow on NP-Au surfaces, adopting an islanding growth mode. Cyclic voltammetry (CV) was also performed to study their electrochemical behaviors in acidic medium. As an electrode material, the electrocatalytic activity of NP-Au towards NO2- oxidation was evaluated. NP-Au exhibits sensitive responses to this reaction. Amperometric study showed a linear relationship for NO2- determination in a concentration range from 1 uM to 1 mM. These results suggest that NP-Au has potential applications in electrochemical sensor.2. We systematically studied the electrocatalytic activity of NP-Pt/Au membrane towards a series of important fuel cell reactions, including methanol, formic acid oxidation and oxygen reduction. CV was earried out to evaluate the activity of a series of NP-Pt/Au membranes with the various Pt loadings. While the heavily plated samples (high Pt loadings) were found to display an similar electrocatalysis behavior and better activity with that of commercial Pt/C electrocatalyst, the slightly plated samples (low Pt loadings) display an enhanced mass-normalized activity towards these reactions. To reveal the origin of the observed activities, SEM, HR-TEM, X-ray photoelectron spectroscopy (XPS), and electrochemical CO stripping were combined to characterize the surface structure and property of NP-Pt/Au membrane. Using the NP-Pt/Au membrane as an anodic catalyst, we also performed the H2/air, direct methanol, formic acid fuel cells test and optimized the work parameter.3. We studied the stability and structure evolution of NP-Pt/Au membrane during thermal annealing at relatively low temperatures. A series of NP-Pt/Au samples with various Pt loadings were annealed in an electronic oven under various temperatures ranging from 100 to 400℃. The annealing time, according to the experimental requirement, varies from 2 to 48 h. SEM, HR-TEM, XPS, and electrochemical techniques were combined to characterize the surface structures and chemical state of the annealed NP-Pt/Au membranes. The results suggest that the NP-Pt/Au membranes preserve initial nanoporous structure at the temperature as high as 300℃. But the surface Pt nanoislands smoothed out and alloyed with the Au substrate to form a thin alloy layer coating on NP-Au, resulting in obvious change of surface atom arrangement. The effect from this change on the electrocatalytic property was evaluated. The rearrangement Pt atoms were found to show an enhanced activity for formic acid oxidation.4. By developing the fabrication method, we succeed depositing Pd on NP-Au surface to form nanoporous Pd/Au (NP-Pd/Au) membrane. XPS study suggests that the surface Pd atoms are metallic and HR-TEM observation demonstrates the Pd layer epitaxially deposits on NP-Au surfaces with a layer growth mode. The electrochemical behavior and the electrocatalytic property of the as prepared NP-Pd/Au membrane were characterized, which exhibits a more than two times activity towards formic acid oxidation in acidic medium than that of commercial Pd/C catalyst. Further test was performed on direct formic acid fuel cell using NP-Pd/Au membrane as anodic catalyst.更多还原