FeCo/MgO催化生长体相单壁碳纳米管的直径调控

Modulating the Diameter of Bulk Single-Walled Carbon Nanotubes Grown by FeCo/MgO Catalyst

单壁碳纳米管的直径可控生长是碳纳米管生长与应用领域的重要问题。直径在0.9–1.2 nm范围内的碳纳米管非常适合应用于近红外荧光生物成像领域和量子器件单光子光源之中。本文使用FeCo/MgO催化剂生长出了直径在这一范围内的体相单壁碳纳米管,并研究了催化剂制备和CVD生长条件对碳纳米管直径的影响。催化剂前驱体的制备是获得小尺寸催化剂颗粒的关键步骤。在浸渍过程中,使用难水解的金属硫酸盐作为前驱体、降低浸渍pH以及加入络合剂分子都会抑制溶液干燥过程中金属盐的水解,从而控制催化剂的尺寸,使其适合于生长出直径可控的单壁碳纳米管。在CVD生长过程中,使用乙醇作为碳源、使用较低的碳氢比例也有利于小直径碳纳米管的生长。

The diameter-controlled growth of single-walled carbon nanotubes(SWNTs)is one of the key issues of SWNT synthesis and application.To guarantee that SWNTs grow with desired diameters,it is necessary to control catalyst size and modulate growth conditions.SWNTs with diameters of 0.9–1.2 nm are highly desirable for near-infrared fluorescence bioimaging and serving as effective single-photon sources for the development of quantum devices.Herein,we used an FeCo/MgO catalyst to grow bulk SWNTs with diameters in the range and studied the influence of catalysts and chemical vapor deposition(CVD)growth conditions on the diameter of SWNTs.The preparation of catalyst precursors is a key step in obtaining catalyst nanoparticles of small size.In the impregnation process,we used three different types of metal salts,namely,sulfates,acetates,and nitrates,to prepare the catalysts.The metal sulfates,which exhibit the weakest hydrolysis ability,were found to grow SWNTs with the smallest diameters.Lowering the immersion pH,which suppresses the hydrolysis of metal ions,was also favorable for growing smaller SWNTs.Moreover,the addition of complexing agent molecules such as ethylenediaminetetraacetic acid during the impregnation process,which inhibits the hydrolysis of metal ions as well,further confined the diameter distribution of the resultant SWNTs.During the solution drying process,metal salts hydrolyze into metal hydroxides and oxides.Under mild hydrolysis conditions,the produced hydroxide and oxide particles are smaller and more likely to be uniformly distributed on the surface of the supports.Therefore,it is more favorable to produce catalysts with controlled sizes under mild hydrolysis conditions,which are preferred for diameter control of the resultant SWNTs.In the CVD growth process,we used either ethanol or methane as the carbon source and found that,under our experimental conditions,the SWNTs grown from ethanol had smaller diameters than those from methane.The hydrogen content in the CVD process also affects diameter distribution of SWNTs.As the carbon-to-hydrogen ratio decreased,SWNTs with larger diameters disappeared,and the number of SWNTs with smaller diameters increased.During the CVD process,the carbon-to-hydrogen ratio determines the carbon feeding rate to the catalysts.At a low carbon feeding rate,catalysts of large sizes are underfed and unable to grow SWNTs,whereas smaller catalysts are in a favorable condition for growth.Therefore,the average diameter of the SWNTs decreased as the carbon-to-hydrogen ratio decreased.