The Significant Role of Carboxylated Carbonaceous Fragments in the Electrochemistry of Carbon Nanotubes

Carbon nanotubes (CNTs) have been widely employed as electrode materials in diverse branches of electrochemistry, which are claimed to display dramatically improved electrochemical behaviour compared to the conventional carbon materials. But a series of recent publications have demonstrated that the electrocatalysis of CNTs might be due to the presence of some impurities, such as metallic catalysts, nanographitic particles and amorphous carbon. For this reason, CNTs are usually purified or treated with nitric acid or nitric and sulphuric acid prior to their versatile applications. However, the strong acidic and oxidative conditions are so aggressive that serious erosion of the tube structures has inevitably taken place, which creates defects on the sidewalls and gives rise to numerous molecular byproducts, commonly referred as carboxylated carbonaceous fragments (CCFs). The adsorption of CCFs on CNTs greatly alters the surface conditions of CNTs which may significantly impact on their electrochemical properties. To this end, we wish to disclose whether the electrocatalysis of the nitric acid purified CNTs is affected by the adsorption of the CCFs. Ascorbic acid (AA) and β‐nicotinamide adenine dinucleotide (NADH) as selected as the targeting benchmarks that are known to be insensitive to the presence of metallic impurities, which may guarantee the preclusion of the promoting contributions from the metallic catalysts resident in CNTs. We have demonstrated that the electrocatalytic activities of the CNTs are actually dominated by the adsorbed CCFs generated during the acidic pre‐treatment. After removal of the CCFs by base rinse, the electrocatalytic properties of CNTs are greatly deteriorated and degraded to the level similar to the conventional graphite powder. We believe this finding is particularly meaningful to uncover the mysterious electrocatalysis of CNTs.     

碳纳米管负载的铂催化剂对柠檬醛选择加氢及CO微分吸附热研究

碳纳米管(CNTs)是近年来发现的一种新型催化剂载体材料,将其作为α,β-不饱和醛的选择加氢的研究则报道较少．本文对柠檬醛[Citral,3,7-二甲基-2,6-辛二烯醛(3,7-Dimethyl-2,6-octadienal)]在Pt／CNTs和Pt／XC-72催化剂作用下的液相选择加氢进行了探索性研究．结果发现,碳纳米管(CNTs)负载的Pt催化剂具有生成不饱和醇的高选择性．     

Formation of fractal-like structures driven by carbon nanotubes-based collapsed hollow capsules

Carbon nanotubes (CNTs) based hollow capsules were obtained by degradation under acidic conditions of core-shell nanocomposites build up upon adsorption of multilayers of CNTs (shell) onto melamine-formaldehyde (MF) spheres (core). By evaporation of the dispersions obtained, polymeric fractal patterns from the degradation products of the MF core were formed onto silicon wafers. The proposed mechanism for the formation of these structures is based on the role of the capsules as arrangements of heterogeneities that facilitate the dewetting of the liquid polymeric films.     

Interaction of narrow carbon nanotubes with nitronium tetrafluoroborate salts

Density functional calculations have been performed to investigate the destruction of narrow carbon nanotubes (CNTs) under the attack of nitronium tetrafluoroborate salts. The dissociation of these salts in a solvent produces nitronium and tetrafluoroborate ions which coadsorb on the external surface of the tubes. It is shown that the ions bind strongly to both metallic and semiconducting narrow nanotubes, although stronger to the metallic ones. The nitronium cations bind to the CNTs through a charge transfer mechanism, whereas the tetrafluoroborate anions remain negatively charged upon adsorption on the nanotubes. The surface of the nanotubes gets substantially deformed around the adsorption site of the nitronium ion, but it is hardly changed around the adsorption site of the tetrafluoroborate ion. These results are the theoretical basis to explain the destruction of the narrow CNTs found in the experiments and also to unravel, in agreement with the experimental interpretation, the distinct role played by the nitronium and the tetrafluoroborate ions. The tetrafluoroborate ions contribute to separate the CNTs from the bundles into individual tubes, without affecting the tubes. The nitronium ions, in contrast, modify the electronic and geometrical structures of the narrow tubes leading eventually to their destruction. The implications for the selective removal of intermediate diameter metallic CNTs found in the experiments are also discussed. The adsorption of the neutral nitrogen dioxide molecule is also studied, and the results show that the weak interactions of this molecule with both metallic and semiconducting tubes cannot be used as a model for the strong attack of the nitronium ions to the narrow tubes. The sensor effect of the nanotubes toward adsorption of nitrogen dioxide is also discussed.     

Adsorption and desorption of atrazine on carbon nanotubes

The potential impact of carbon nanotubes (CNTs) on human health and the environment is receiving more and more attention. The high surface area of CNTs tends to adsorb a large variety of toxic chemicals, which may enhance the toxicity of CNTs and/or toxic chemicals. In this study, adsorption and desorption of atrazine on carbon nanotubes from aqueous solution were studied through batch reactors. The adsorption equilibrium isotherms were nonlinear and were fitted by Freundlich, Langmuir, and Polanyi-Manes models. It was found that the Polanyi-Manes model described the adsorption process better than other two isotherm models. Together with the "characteristic curve," the Polanyi adsorption potential theory is applicable to describe the adsorption process of atrazine on CNTs. Thermodynamic calculations indicated that the adsorption reaction of atrazine on CNTs is spontaneous and exothermic. The desorption data showed that no significant desorption hysteresis occurred. High adsorption capacity and adsorption reversibility of atrazine on CNTs suggest that CNTs have high health and environmental risks, whereas they have potential applications for atrazine removal from water.     

Selective adsorption and alignment behaviors of double- and multiwalled carbon nanotubes on bare Au and SiO2 surfaces

We present the study of selective adsorption and alignment behaviors of double- and multiwalled carbon nanotubes (dwCNTs and mwCNTs) on self-assembled monolayer (SAM) patterns, bare Au, and SiO2 surfaces. dwCNTs and mwCNTs exhibited stronger affinity to polar SAMs, bare Au, and SiO2 surfaces than to nonpolar SAM surfaces. Furthermore, we found the adsorption probability of smaller carbon nanotubes (CNTs) was higher than that of larger CNTs. As proof of concept, we successfully assembled and aligned dwCNTs and mwCNTs on Au and SiO2 substrates without relying on external forces and demonstrated wafer-scale fabrication of back-gate transistors based on dwCNTs with a high yield.     

Synthesis and modification of multi-walled carbon nano-tubes (MWCNTs) for water treatment applications

Synthesis of MWCNTs by chemical vapor deposition (CVD) of acetylene is investigated at different temperatures. Fe-Co/CaCO₃ catalyst/support prepared by wet impregnation method is used. CaCO₃ was found to be a good support as a high selective material for deposition of CNTs with high purity. The effect of temperature on catalyst/support phases and crystal size was identified by using XRD. The crystallite size was decreased with increase temperature. The effect of growing time and temperature on carbon yield was studied and the deposited MWCNTs increased with temperature. The structure and purity of synthesized CNTs at different temperatures was examined by TEM and the effect of temperature on the surface area of the synthesized MWCNTs was investigated, the surface area decreased as the temperature increased. The prepared CNTs were purified using chemical oxidation method and the effect of acid treatment on CNTs surface was examined by TEM and SEM. The function groups produced at CNTs surface were investigated by using FTIR spectroscopy also the effect of CNTs preparation temperature on FTIR spectra was studied. The functionalized CNTs were used for adsorption of some heavy metals and for removal of some organic dyes from water.     

碳纳米管对结晶紫的吸附研究

研究了碳纳米管对溶液中结晶紫的吸附性能,考察了溶液浓度、溶液p H、吸附时间和吸附温度等因素对吸附行为的影响,初步探讨了碳纳米管对结晶紫的吸附机理。结果表明,碳纳米管对结晶紫的吸附量随着溶液初始浓度的增大而升高;酸性条件有利于吸附的进行,最佳p H等于5;对结晶紫的吸附在3h达到平衡,吸附速率常数为779.76h-1;温度的变化对结晶紫的吸附量影响不大。通过Langmuir方程和Freundlich方程对吸附进行拟合,平衡吸附量Qe与平衡质量浓度Ce之间的关系符合Freundlich等温吸附方程所描述的规律,说明吸附过程以物理吸附为主。     

Removal of radiocobalt from aqueous solution by different sized carbon nanotubes

Batch adsorption technique was applied to study the adsorption of radiocobalt on multiwalled carbon nanotubes (CNTs) with deferent sizes. The aim of this work was to examine the effect of contact time, pH, solid content, foreign ions and CNT particle sizes on the removal of Co(II) ions from aqueous solutions by CNTs. The results indicated that the adsorption of Co(II) was strongly dependent on pH and the adsorption capacity was in inverse proportion to the particle sizes of CNTs. The adsorption of Co(II) was weakly affected by ionic strength and foreign ions. Ion exchange and surface complexation were the main adsorption mechanisms. The kinetics of Co(II) adsorption on CNTs was described well by pseudo-second-order model. The Langmuir and Freundlich models were applied to interpret the adsorption data. The results are important to understand the physicochemical behavior of Co(II) with CNTs, and for the application of CNTs in the preconcentration of radiocobalt from large volumes of aqueous solutions.     

Effect of pH and Temperature on Adsorption of Dimethyl Phthalate on Carbon Nanotubes in Aqueous Phase

Adsorptions of dimethyl phthalate (DMP) on carbon nanotubes (CNTs) in aqueous phase at various pH and temperatures were studied. The increase in pH results in the increase in adsorption coefficient. The adsorption is governed by the π-π electron interaction which is affected by the changes in pH of the medium. The outer diameter of the CNTs greatly influences the adsorption behavior of CNT for DMP. Under the same working temperature, the adsorption capacity of CNTs for DMP is inversely related to the average outer diameter of the CNT: single-walled SWCNT (1.4 nm)>multi-walled MWCNT10 (9.4 nm)>MWCNT30 (27.8 nm)>MWCNT40 (42.7 nm). The larger surface area of CNTs provides many active sites for adsorption of DMP molecules. The Freundlich model can describe well the adsorption isotherms of DMP on CNTs. The thermodynamic parameters of standard free energy, standard enthalpy (ΔH), and standard entropy changes are determined, showing that the adsorption of DMP on CNTs is an endothermic and spontaneous reaction. The ΔH value of 27.8 nm-sized MWCNT (22.69 kJ/mol) is higher than 1.4 nm-sized SWCNT (6.05 kJ/mol), inferring that the adsorption process becomes more endothermic with the increase in the outer diameter of CNTs.     

Surface characterization of adsorbents in ultrasound-assisted oxidative desulfurization process of fossil fuels

Surface properties of two different phases of alumina were studied through SEM images. Characterization of amorphous acidic alumina and crystalline boehmite by XRD explains the differences in adsorption capacities of each sample. Data from small angle neutron scattering (SANS) provide further results regarding the ordering in amorphous and crystalline samples of alumina. Quantitative measurements from SANS are used for pore size calculations. Higher disorder provides more topological traps, irregularities, and hidden grooves for higher adsorption capacity. An isotherm model was derived for adsorption of dibenzothiophene sulfone (DBTO) by amorphous acidic alumina to predict and calculate the adsorption of sulfur compounds. The Langmuir-Freundlich model covers a wide range of sulfur concentrations. Experiments prove that amorphous acidic alumina is the adsorbent of choice for selective adsorption in the ultrasound-assisted oxidative desulfurization (UAOD) process to produce ultra-low-sulfur fuel (ULSF).     

Highly selective adsorption of methanol in carbon nanotubes immersed in methanol-water solution

The systems of open-ended carbon nanotubes (CNTs) immersed in methanol-water solution are studied by molecular dynamics simulations. For the (6,6) CNT, nearly pure methanol is found to preferentially occupy interior space of the CNT. Even when the mass fraction (MF) of methanol in bulk solution is as low as 1%, the methanol MF within the CNT is still more than 90%. For CNTs with larger diameters, the methanol concentrations within CNTs are also much higher than those outside CNTs. The methanol selectivity decreases with increasing CNT diameter, but not monotonically. From microscopic structural analyses, we find that the primary reason for the high selectivity of methanol by CNTs lies on high preference of methanol in the first solvation shell near the inner wall of CNT, which stems from a synergy effect of the van der Waals interaction between CNT and the methyl groups of methanol, together with the hydrogen bonding interaction among the liquid molecules. This synergy effect may be of general significance and extended to other systems, such as ethanol aqueous solution and methanol/ethanol mixture. The selective adsorption of methanol over water in CNTs may find applications in separation of water and methanol, detection of methanol, and preservation of methanol purity in fuel cells.     

Donor–Acceptor Nanocarbon Ensembles to Boost Metal‐Free All‐pH Hydrogen Evolution Catalysis by Combined Surface and Dual Electronic Modulation

A combined surface and dual electronic modulation strategy is used to realize metal‐free all‐pH catalysis towards the hydrogen evolution reaction (HER) by coupling a N‐doped carbon framework (MHCF, electron acceptors) derived from MOFs with higher‐Fermi‐level pure carbon nanotubes (CNTs, electron donors), followed by surface modification with carboxyl‐group‐rich polymers. Although the three constituents are inactive, as‐assembled ternary membranes yield superior HER performance with low overpotentials and high durability (≤5 % activity loss over 100 h) at all pH values. The C adjacent to pyrrolic N in MHCF is the most active site and the induced directional interfacial electron transfer from CNTs to MHCF coupled with N‐driven intramolecular electron transfer in MHCF optimizes Gibbs free energy for hydrogen adsorption (ΔG_H*) near zero, while the polymer modulation enables local H⁺ enrichment in acidic media and enhanced water adsorption and activation in neutral and basic media.     

Adsorption of fluoride from water by amorphous alumina supported on carbon nanotubes

A new candidate for fluoride adsorption from water, amorphous Al₂O₃ supported on carbon nanotubes (Al₂O₃/CNTs), is reported in this Letter. The adsorption isotherms show that the best fluoride adsorption of Al₂O₃/CNTs occurs at a pH range of 5.0–9.0. The adsorption capacity for Al₂O₃/CNTs is about 13.5 times higher than that of AC-300 carbon, 4 times higher than that of γ-Al₂O₃ at equilibrium fluoride concentration of 12 mg/l. The broad range of the pH values and high adsorption capacity of Al₂O₃/CNTs make it very suitable for potential applications in fluoride removal from water.     

Investigations into the mechanism of adsorption of carbon nanotubes onto aminopropylsiloxane functionalized surfaces

The factors that control carbon nanotube (CNT) adsorption onto aminopropyl siloxane (APS)-derivatized surfaces were investigated using two distinct types of well-characterized films with significant differences in their detailed structures. Both types of APS films showed a marked increase in CNT adsorption relative to untreated SiO2 surfaces but differed in the amount of CNTs adsorbed. To gain insight into the factors governing adsorption, the surface coverage of the CNTs was monitored as a function of the pH during the deposition, the surfactant used to suspend the CNTs, and the type and amount of salt added to the deposition solution. The adsorption is shown to be governed by electrostatic and VDW forces. In the case of complimentarily charged surfaces, the adsorption is proposed to occur through an ion exchange mechanism.     

Influence of pH and surface oxygen-containing groups on multiwalled carbon nanotubes on the transformation and adsorption of 1-naphthol

The pH-dependent behavior, including the transformation of 1-naphthol by oxidative polymerization to form precipitates in solution and the adsorption of 1-naphthol onto carbon nanotubes (CNTs), was examined. Neglecting the precipitate loss of 1-naphthol and possibly of similar chemicals may result in the overestimation of their adsorption and inadequate interpretation of the underlying adsorption mechanisms. Surface oxygen-containing groups on CNTs and the dissociated species of these groups can interact with the dissociated and neutral species of 1-naphthol in a way similar to polymerization, thus promoting the adsorption of 1-naphthol onto CNTs. Adsorption onto CNTs may reduce the polymeric precipitates of 1-naphthol in solution by possibly decreasing aqueous 1-naphthol concentrations. These observations and the underlying mechanisms are important for predicting the fate and risks of naphthalene and carbaryl in the environment because 1-naphthol is a primary metabolite of naphthalene and carbaryl. In addition, it is possible to enhance the removal of 1-naphthol and similar chemicals by controlling the pH and designing specific surface functional groups for CNTs.     

Nitrogen adsorption characterization of aligned multiwalled carbon nanotubes and their acid modification

Aligned multiwalled carbon nanotube (CNT) arrays were synthesized by using an iron-based sol-gel catalyst and acetylene as the precursor. These CNTs show high purity, uniform diameters and pore-wall thickness. Low temperature nitrogen adsorption was employed to characterize the structural and surface properties of the as-synthesized sample and that modified with boiling concentrated nitric acid. The adsorption characteristics of the as-synthesized and modified CNTs were thoroughly investigated. High-resolution comparative alpha(s)-plot showed that the nitrogen adsorption on CNTs takes place via a multistage mechanism closely related to their structures. It was also found that the acid modification significantly increased the adsorption energy and enhanced the adsorption capacity under low pressures. High-resolution comparative method provided valuable insights about the surface and pore structures of CNTs.     

碳纳米管复合微球修饰L-色氨酸及其对LDL的吸附性能

本文以多孔碳纳米管/活性炭复合微球为载体, 以L-色氨酸为配基, 采用环氧氯丙烷偶联法, 制得修饰L-色氨酸的碳纳米管/活性炭复合微球(L-CNTs/AC)。采用扫描电镜、氮气吸附、傅立叶红外光谱、热分析、X射线光电子能谱等对复合微球进行表征;通过体外静态吸附法对其低密度脂蛋白(LDL)吸附能力进行初步研究。结果表明:环氧氯丙烷偶联法可接枝上L-色氨酸。复合微球中碳纳米管加入量越多, 对LDL的吸附能力越强;当碳纳米管加入量为45wt%时, 对LDL的吸附量达4.623 mg·g^-1, 是未添加碳纳米管的2.3倍多。这是因为碳纳米管不仅可促进复合微球中20~100 nm孔的形成, 而且还可促进复合微球配基修饰量的增多, 从而大大增强了复合微球对LDL的吸附能力。此复合微球可望开发成一种新型的血液灌流LDL吸附剂。     

Adsorption of direct dyes from aqueous solutions by carbon nanotubes: determination of equilibrium, kinetics and thermodynamics parameters

This study examined the feasibility of removing direct dyes C.I. Direct Yellow 86 (DY86) and C.I. Direct Red 224 (DR224) from aqueous solutions using carbon nanotubes (CNTs). The effects of dye concentration, CNT dosage, ionic strength and temperature on adsorption of direct dyes by CNTs were also evaluated. Pseudo second-order, intraparticle diffusion and Bangham models were adopted to evaluate experimental data and thereby elucidate the kinetic adsorption process. Additionally, this study used the Langmuir, Freundlich, Dubinin and Radushkevich (D-R) and Temkin isotherms to describe equilibrium adsorption. The adsorption percentage of direct dyes increased as CNTs dosage, NaCl addition and temperature increased. Conversely, the adsorption percentage of direct dyes decreased as dye concentration increased. The pseudo second-order model best represented adsorption kinetics. Based on the regressions of intraparticle diffusion and Bangham models, experimental data suggest that the adsorption of direct dyes onto CNTs involved intraparticle diffusion, but that was not the only rate-controlling step. The equilibrium adsorption of DR86 is best fitted in the Freundlich isotherm and that of DR224 was best fitted in the D-R isotherm. The capacity of CNTs to adsorb DY86 and DR224 was 56.2 and 61.3 mg/g, respectively. For DY86, enthalpy (DeltaH(0)) and entropy (DeltaS(0)) were 13.69 kJ/mol and 139.51 J/mol K, respectively, and those for DR224 were 24.29 kJ/mol and 172.06 J/mol K, respectively. The values of DeltaH(0), DeltaG(0) and E all indicate that the adsorption of direct dyes onto CNTs was a physisorption process.