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Surface studies of hydroxylated multi-wall carbon nanotubes

机译：羟基化多壁碳纳米管的表面研究

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CVD grown MWCNTs, of typical diameter 5 to 50 nm and with approximately 15-20 concentric graphene layers in the multi-walls, have been surface functionalised using the Fenton hydroxylation reaction. HRTEM reveals little physical difference between the treated and untreated materials; images from both exhibit similar multi-wall structure and contain evidence for some low-level disruption of the very outermost layers. Raman spectra from the two types of nanotubes are almost identical displaying the disorder (D) peaks at approximately 1350 cm~(-1) and graphite (G) peaks at approximately 1580 cm~(-1), characteristic of graphene-based carbon materials, in approximately equal intensity ratios. Equilibrium adsorption data for nitrogen at 77 K leads to BET surface areas of 60.4 m~2 g~(-1) for the untreated and 71.8 m~2g~(-1) for the hydroxylated samples; the increase in area being due to separation of the tube-bundles during function-alization. This is accompanied by a decrease in measured porosity, mostly at high relative pressures of nitrogen, i.e. where larger (meso 2-5 nm and macro >5 nm) pores are being filled, which is consistent with an attendant loss of inter-tube capillarity. X-ray photoelectron spectroscopy (XPS) shows that hydroxylation increases the nanotube surface oxygen level from 4.3 at.% to 22.3 at.%; chemical shift data indicate that approximately 75% of that oxygen is present as hydroxyl (-OH) groups. Water vapour adsorption by the hydroxylated surfaces leads to Type II isotherms which are characteristic of relatively high numbers of hydrogen bonding interactions compared to the untreated materials which exhibit Type III curves. This difference in polar surface energy is confirmed by calorimetric enthalpies of immersion in water which are -54 mJ m~(-2) for the untreated and -192 mJm~(-1) for the hydroxylated materials. The treated materials therefore have significantly increased water wettability/dispersivity and a greater potential for cross-linking with matrix compounds. The mechanism by which hydroxylation occurs i.e. free radical (OH~*) attack and subsequent electrophilic addition at C=C bonds in the graphene basal planes, is discussed.
机译：CVD生长的MWCNT，通常直径为5至50 nm，并且在多壁中具有约15-20个同心石墨烯层，已使用Fenton羟基化反应进行了表面功能化。 HRTEM揭示了已处理和未处理材料之间的物理差异很小。两者的图像都显示出相似的多壁结构，并且包含最外层的一些低水平破坏的证据。两种纳米管的拉曼光谱几乎相同，显示出约1350 cm〜（-1）的无序（D）峰和约1580 cm〜（-1）的石墨（G）峰，这是石墨烯基碳材料的特征，强度比大致相等。氮在77 K下的平衡吸附数据导致未处理的BET表面积为60.4 m〜2 g〜（-1），而羟基化样品的BET表面积为71.8 m〜2g〜（-1）。面积的增加归因于功能化过程中管束的分离。这伴随着测量孔隙率的降低，主要是在较高的氮气相对压力下，即在较大的（中等2-5 nm且宏观> 5 nm的）孔隙被填充的情况下，这与随之而来的管间毛细血管损失相一致。 。 X射线光电子能谱（XPS）表明，羟基化将纳米管表面的氧含量从4.3at。％增加到22.3at。％；化学位移数据表明，大约75％的氧以羟基（-OH）的形式存在。羟基化表面对水蒸气的吸附导致II型等温线，与未经处理的具有III型曲线的材料相比，II型等温线具有较高数量的氢键相互作用。极性表面能的这种差异通过浸没在水中的量热焓得到确认，对于未处理的量，该焓为-54 mJ m〜（-2），对于羟基化物质为-192 mJm〜（-1）。因此，经处理的材料具有显着提高的水润湿性/分散性以及与基质化合物交联的更大潜力。讨论了发生羟基化的机制，即自由基（OH〜*）攻击以及随后在石墨烯基础平面中C = C键处的亲电加成。

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