新型掺氮金刚石薄膜结构及CO_(2)电化学还原性能研究

Study on the structure and CO_(2) electrochemical reduction properties of novel nitrogen-doped diamond films

掺氮金刚石薄膜不仅同已被广泛研究的掺硼金刚石(BDD)薄膜一样拥有优异的电化学性能,而且具有更加丰富的的结构和物相组成,是一种十分有潜力的CO_(2)电化学还原电极材料。本文采用微波等离子体化学沉积技术通过改变沉积温度(750和850℃)制备获得两种形态结构及组成特点显著不同的新型掺氮金刚石薄膜,SEM、XRD分析表明750℃下生长的薄膜仅含金刚石相且晶粒尺寸在30 nm以下,晶界占比极大,结合Raman光谱图中宽化的D峰和G峰,表明所得膜材为典型的掺氮超纳米金刚石膜(UNCD);而850℃制备的薄膜由金刚石和石墨相组成,极薄的石墨片垂直交叉排列呈“蜂窝”状,尺寸极小金刚石晶粒分布于蜂孔内,为典型的掺氮超纳米金刚石/多层石墨烯复合薄膜(UNCD/MLG)。电化学测试结果显示,两种新型掺氮金刚石薄膜均具有宽的电势窗(~3 V),可以有效地抑制CO_(2)还原的析氢竞争反应;同时两种电极都可以有效电催化还原CO_(2),生成甲酸和CO,甲酸和CO最大法拉第效率分别为38.37%和15.32%。其中850℃生长的UNCD/MLG薄膜具有更好的电化学活性,更高的CO法拉第效率并可产生多电子转移产物甲烷。这得益于其本身“蜂巢”状结构不仅使表面积极大地增加,同时适中的sp^(3)-C/sp^(2)-C比例可以有效结合CO_(2)还原的中间体,增加了产物的多样性。对于掺氮金刚石薄膜材料而言,这种由掺氮超纳米金刚石和多层石墨烯组成的具有丰富孔洞结构的复合膜是一种更有潜力的电催化CO_(2)还原电极材料。

Nitrogen doped diamond film is a very potential electrode material for CO_(2)electrochemical reduction.It not only has excellent electrochemical performances like boron doped diamond film which has been widely studied,but also has more abundant structure and phase composition.In this paper,two novel nitrogen-doped diamond films are prepared by microwave plasma chemical deposition technology through changing deposition temperatures(750℃and 850℃).SEM and XRD analysis show that the average grain size of the film grown at 750℃is less than 30 nm which results in the proportion of grain boundary being very large,and as-film only contains diamond phase.Matched with the widen G peak and D peak in the Raman spectroscopy,it could be proved that this film is a typical N-doped ultra-nano diamond film(UNCD).For the film prepared at 850℃,the composed phases include diamond and graphite along with the extremely thin graphite sheets vertically cross arranging to forming a honeycomb shape structure,where distributed many extremely small diamond grains.All these composition and structure characteristics indicates this is a typical nitrogen doped ultra-nano diamond/multilayer graphene composite film(UNCD/MLG).Electrochemical test results predict that the two novel nitrogen-doped diamond films both have wide potential window(about 3 V),which can effectively inhibit the competitive reaction of hydrogen evolution during CO_(2).At the same time,both electrodes could effectively electrocatalytically reduce CO_(2) to generate formic acid and CO,which maximum Faraday efficiencies could achieve 38.37%and 15.32%,respectively.But comparatively,the UNCD/MLG film grown at 850°C has better electrochemical activity,higher CO Faraday efficiency and can produce multi-electron transfer product methane,which could attribute to its own“honeycomb-like”structure that increases the surface area,and at the same time,the suitable sp^(3)-C/sp^(2)-C ratio can effectively combine with the intermediate of CO_(2) reduction,and further increase the diversity of products.So for nitrogen-doped diamond film materials,this composite film with abundant pore structure is a more potential electrode material using in CO_(2) electrochemical reduction.