SiO2纳米管的合成及表面功能修饰研究

在氨水的负催化作用下,用酒石酸氢铵作模板剂,原硅酸四乙酯(TEOS)经水解、缩聚,生成SiO2纳米管.利用硅烷偶联剂KH-570对SiO2纳米管进行表面修饰,将乙烯基(—CH=CH2)引入SiO2纳米管表面,改善其表面功能性.通过扫描电镜(SEM)、透射电镜(TEM)、X射线衍射(XRD)和傅立叶红外(FT-IR)等分析测试手段研究了SiO2纳米管的形貌结构及其表面修饰性能.结果表明:溶液的pH值是影响SiO2纳米管形貌的最主要因素,最合适的酸碱度是4.5≤pH≤5.0;纳米管平均外径300 nm,内径100 nm,长度10μm以上,管径均匀,管壁表面光滑平坦,没有出现裂纹等缺陷,为无定形结构.KH-570功能修饰前后的纳米管形貌没有发生任何变化.     

水热法合成MOS2/CNT同轴纳米管

采用水热法合成了MoS2/CNT同轴纳米管．对合成的MoS2/CNT同轴纳米管用高分辨透射电镜(HRTEM)、X射线衍射(XRD)和电子能谱(EDS)方法对其进行了鉴定和表征，发现所制备的MoS2/CNT同轴纳米管结构完好，在CNT的外表面包覆了3～4层MoS2管．初步分析了MoS2/CNT的形成机理．     

受限于单壁碳纳米管中水分子结构、能量以及振动频率的密度泛函研究

利用密度泛函理论中的杂化泛函M06-2X研究了受限于不同管径单壁碳纳米管(SWCNT)内水分子团簇(H2O)n=3~6的结构、能量以及振动频率.结果显示由于SWCNT的限域效应,水分子团簇的几何构型与在真空相比发生了巨大变化,如受限(H2O)6能形成单链锯齿型水分子构型.随着管直径的增大,纳米管与水之间的相互作用逐渐减弱,但水分子之间的氢键相互作用能变化不大.对比受限和真空下水分子O—H振动频率发现,绝大部分O—H的振动频率由于碳纳米管与水的相互作用而发生了红移.AIM理论分析显示O—H振动频率的红移应归因于其电子密度的减小.这也表明碳纳米管绝非简单的几何限制效应,而是与水分子之间存在弱电子相互作用,主要包括H…π氢键作用和O…π轨道作用,从而导致水分子的小部分电荷转移到了SWCNT上.     

BN纳米管内含C纳米管——结构与电学性质

运用密度泛函理论的PW91/DNP方法对C(6,0)@BN(n,0)体系的结构与稳定性进行了研究, 发现最适合与C(6,0)纳米管形成的嵌套体系的锯齿型BN纳米管是BN(15,0)和BN(16,0), 在形成的C(6,0)@BN(15,0) 和 C(6,0)@BN(16,0)中, 碳壁与氮化硼壁之间的距离分别为0.36和0.40 nm. 在最稳定的C(6,0)@BN(16,0)体系中, 发现内层碳纳米管的电子结构并未受到外层氮化硼纳米管的影响, 然而氮化硼纳米管的能隙缩小了0.5 eV. 对C(6,0)@BN(16,0)的轨道分析表明, 碳纳米管与氮化硼纳米管之间的作用力为范德华力.     

Li和Al掺杂的MgO(001)表面负载W_3O_9团簇的构型与电子结构

采用基于第一性原理的分子动力学和量子力学相结合的方法,对W3O9团簇在经Li和Al原子掺杂的MgO(001)表面的负载构型、稳定性以及体系的电子结构进行了系统研究.结果表明,当掺杂发生在表层时,杂质原子的类型对W3O9团簇的负载构型有显著影响.对于缺电子的Li掺杂,负载后W3O9团簇环状构型并不稳定,转化为链状结构;而Al原子的掺杂则使得MgO(001)表面电子富余,此时W3O9团簇存在平躺和垂直两种吸附方式,二者能量稳定性相近,其中前者存在同时与三个W原子成键的帽氧结构.当掺杂发生在次表层时,两种掺杂体系W3O9的负载构型相似,团簇仍保持环状结构并倾向于采用垂直方式沉积在表面上.与Li掺杂体系相比,富电子的Al掺杂可显著增强W3O9与MgO(001)表面之间的结合能力,负载后有较多电子从表面转移到团簇中特定的W原子上,这将对W3O9团簇的催化性能产生显著影响.     

有限长锯齿型碳纳米管的电子结构与管径效应

在B3LYP／6—31G／／PM3水平上对一系列有限长锯齿型碳纳米管(n，0)(n=6-11)的构型和电子结构进行了研究．结果表明，最高占据轨道(HOMO)与最低空轨道(LUMO)间的能隙(Eg)随着管径的增大出现随奇偶n值的振荡变化，随着管长的增加呈单调减小的趋势；前线轨道波函数的成键结构随管径的变化而变化，但并不随管长的变化而改变，HOMO的成键性质与几何结构之间存在对应关系．     

单壁碳纳米管上毒性苯气体净化的分子模拟(英文)

采用巨正则系综蒙特卡罗(GCMC)方法研究了空气中微量苯组分在单臂碳纳米管(SWNTs)上的吸附净化.模拟表明,具有较大孔径的(20,20)纳米管比较适合吸附纯苯蒸汽,而对于移除空气中的毒性苯物质,苯的吸附选择性分别在(12,12)纳米管及4.0 MPa时和(18,18)纳米管及0.1 MPa时出现最小值和最大值.为了解释这一异常行为,我们进一步分析了N2-O2-C6H6混合物的局部密度分布、吸附分子构型和概率密度分布,发现(18,18)纳米管内外完全被苯分子占据,而对于(12,12)纳米管,由于存在更强的吸附质-吸附剂相互作用,空气分子更倾向于吸附在管与管之间的间隙.此外,吸附分子的空间有序参数表明大多数苯分子采取"平躺"在纳米管表面的定位,而线性的N2和O2分子则多数平行于孔轴方向.最后研究了温度和苯分子主体相浓度对分离效果的影响.我们发现较大孔中的选择性随着温度的增加比小孔下降更加明显.与此对比,主体相苯浓度对小孔中的选择性起到更加重要的作用.     

Rh(111)表面上CO和O共吸附的密度泛函理论计算研究

采用密度泛函理论对Rh(111)表面上CO和O的吸附和共吸附进行了系统的研究,计算了三类不同的共吸附结构.从吸附能和化学位移的角度,通过与已有实验结果对比,推断出可能存在的吸附构型.CO和O之间存在较大的排斥作用,在表面上竞争吸附.电子结构分析发现,这种排斥作用来源于CO和O之间与Rh的d轨道成键的竞争.用密度泛函理论计算的化学位移与实验测量结果一致,说明化学位移的理论计算能辅助对表面结构的预测.     

饱和、不饱和(N-杂环化)低聚硅烷的结构和光谱性质

用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)对线型(饱和N-杂环化)和(苯并N-杂环化)低聚硅烷的电子结构和吸收光谱性质以及溶剂效应进行了比较研究.对各体系的基态电子结构在B3LYP/6-31G(d,p)水平上进行了全优化,讨论了电荷分布和前线分子轨道性质.在获得基态稳定构型的基础上,用B3LYP/6-311+G(d)方法计算了电子吸收光谱的性质,探讨了主链的线型增长和溶剂对电子吸收光谱的影响.结果表明,随着主链的增长,低聚硅烷的电子结构发生明显扭曲,在(苯并N-杂环化)聚硅烷中形成了邻近苯并N-杂环之间π-π堆积作用,有利于结构的稳定.两类低聚硅烷的吸收光谱都随着主链的增长而发生明显的红移,(苯并N-杂环化)聚硅烷最大吸收光谱红移幅度要比(饱和N-杂环化)聚硅烷大得多.溶剂效应使得光谱略向短波长移动,溶剂的极性改变对吸收波长的影响不明显.     

MgO缺陷和不规则表面吸附Cl2的电子结构研究

采用从头算程序对MgO表面 3种不同配位位置吸附Cl2 的构型进行优化 ,并用扩展休克尔紧束缚 (EHT)晶体轨道方法对MgO的缺陷和不规则表面吸附Cl2的可能构型进行能带计算 ,讨论了吸附前后能带组成和成键性质的变化。研究表明 :MgO表面吸附Cl2 将更趋向于吸附在O原子上而非Mg原子上 ,而且在 3种配位中MgO表面三配位氧最有利于吸附Cl2 ;吸附时 ,电子从O原子转移到Cl2 分子的反键轨道 ,但是各种吸附构型的MgO表面对Cl2 的吸附作用均比较微弱 ,是典型的物理吸附。     

Electronic properties of capped, finite-length armchair carbon nanotubes in an electric field

This study investigates the electronic properties of finite-length armchair carbon nanotubes in an electric field (E) using a single-pi-band tight-binding model. Three different tip configurations are considered, namely, open ends with hydrogen terminations (H-terminations), one end capped with half of C60 fullerene and the other end open with H-terminations, and both ends capped with half of C60 fullerene. In general, the electronic states and energy gaps of low-energy electronic structures are highly sensitive to changes in the direction and magnitude of the applied electric field and to the tip configuration. The present results show that the electric field induces a strong modulation of the state energies and energy gaps of the current nanotubes, changes their energy spacings, and prompts the occurrence of semiconductor-metal transitions (SMTs). It is found that the SMTs occur more frequently as the direction of the electric field approaches the symmetry axis or when its magnitude becomes sufficiently large. The present results also indicate that the Fermi levels and energy gaps of the three nanotubes considered in this study are strongly influenced by the cap configuration. Finally, the convergent decay behavior of the energy gap which is observed as the length of the nanotube is increased is also strongly dependent on the tip configuration.     

Kinetics of water filling the hydrophobic channels of narrow carbon nanotubes studied by molecular dynamics simulations

The kinetics of water filling narrow single-walled carbon nanotubes was studied using molecular dynamics simulations. The time required to fully fill a nanotube was linear with respect to the tube length. We observed that water molecules could enter into nanotubes of different lengths, either from one end or from both ends. The probability of having a nanotube filled completely from both ends increased exponentially with the tube length. For short tubes, filling usually proceeded from only one end. For long tubes, filling generally proceeded from both tube ends over three stages, i.e., filling from one end, filling from both ends, and filling from both ends with the dipole reorientation of water molecules to give a concerted ordering within the fully filled tube. The water molecules in the partially filled nanotube were hydrogen bonded similarly to those in the fully filled nanotube. Simulations for the reference Lennard-Jones fluid without hydrogen bonds were also performed and showed that the filling behavior of water molecules can be attributed to strong intermolecular hydrogen bonding.     

Co-adsorption of CO molecules at the open ends of MgO nanotubes

Equilibrium geometries, stabilities, and electronic properties of the adsorption of a CO molecule along with two or three CO co-adsorptions at the open ends of MgO nanotubes have been investigated through density functional calculations. It was found that the interaction of CO molecule with ends of the tube is much stronger than that of with its exterior surface. It was also found that adsorption of the second CO molecule at the end of the tube is independent of the first adsorbed CO molecule, while the prior adsorption process impacts the third adsorption. This phenomenon was described based on the frontier molecular orbital analysis, showing that the third CO molecule has to interact with high energetic unoccupied orbitals instead of the LUMO. Furthermore, it was revealed that similar to CO adsorption on the exterior surface of the tube, the adsorption at its open ends is electronically harmless.     

Oxygen gas-induced lip-lip interactions on a double-walled carbon nanotube edge

We have investigated adsorption of an O(2) molecule on a double-walled carbon nanotube (DWCNT) edge using density functional theory calculations. An O(2) molecule adsorbs exothermally without an adsorption barrier at open nanotube edges that are energetically favorable with a large adsorption energy of about -9 eV in most cases. Dissociative adsorption of an O(2) molecule induces various spontaneous lip-lip interactions via the bridged carbon atoms, generating the closed tube ends. This explains why the DWCNTs are chemically more stable than the single-walled nanotubes during observed field emission experiments. The field emission takes place via the localized states of the bridged carbon atoms, not via those of the adsorbed oxygen atoms particularly in the armchair nanotubes. We also find that some O(2) precursor states exist as a bridge between tube edges.     

Amphoteric doping of carbon nanotubes by encapsulation of organic molecules: electronic properties and quantum conductance

In order to investigate and optimize the electronic transport processes in carbon nanotubes doped with organic molecules, we have performed large-scale quantum electronic structure calculations coupled with a Green's function formulation for determining the quantum conductance. Our approach is based on an original scheme where quantum chemistry calculations on finite systems are recast to infinite, non-periodic (i.e., open) systems, therefore mimicking actual working devices. Results from these calculations clearly suggest that the electronic structure of a carbon nanotube can be easily manipulated by encapsulating appropriate organic molecules. Charge transfer processes induced by encapsulated organic molecules lead to efficient n- and p-type doping of the carbon nanotube. Even though a molecule can induce p and n doping, it is shown to have a minor effect on the transport properties of the nanotube as compared to a pristine tube. This type of doping therefore preserves the intrinsic properties of the pristine tube as a ballistic conductor. In addition, the efficient process of charge transfer between the organic molecules and the nanotube is shown to substantially reduce the susceptibility of the pi electrons of the nanotube to modification by oxygen while maintaining stable doping (i.e., no dedoping) at room temperature.     

Chemical and biochemical sensing with modified single walled carbon nanotubes

The nano dimensions, graphitic surface chemistry and electronic properties of single walled carbon nanotubes make such a material an ideal candidate for chemical or biochemical sensing. Carbon nanotubes can be nondestructively oxidized along their sidewalls or ends and subsequently covalently functionalized with colloidal particles or polyamine dendrimers via carboxylate chemistry. Proteins adsorb individually, strongly and noncovalently along nanotube lengths. These nanotube-protein conjugates are readily characterized at the molecular level by atomic force microscopy. Several metalloproteins and enzymes have been bound on both the sidewalls and termini of single walled carbon nanotubes. Though coupling can be controlled, to a degree, through variation of tube oxidative pre-activation chemistry, careful control experiments and observations made by atomic force microscopy suggest that immobilization is strong, physical and does not require covalent bonding. Importantly, in terms of possible device applications, protein attachment appears to occur with retention of native biological structure. Nanotube electrodes exhibit useful voltammetric properties with direct electrical communication possible between a redox-active biomolecule and the delocalized pi system of its carbon nanotube support.     

Field emission properties of N-doped capped single-walled carbon nanotubes: a first-principles density-functional study

The geometrical structures and field emission properties of pristine and N-doped capped (5,5) single-walled carbon nanotubes have been investigated using first-principles density-functional theory. The structures of N-doped carbon nanotubes are stable under field emission conditions. The calculated work function of N-doped carbon nanotube decreases drastically when compared with pristine carbon nanotube, which means the enhancement of field emission properties. The ionization potentials of N-doped carbon nanotubes are also reduced significantly. The authors analyze the field emission mechanism in terms of energy gap between the lowest unoccupied molecular orbital and the highest occupied molecular orbital, Mulliken charge population, and local density of states. Due to the doping of nitrogen atom, the local density of states at the Fermi level increases dramatically and donor states can be observed above the Fermi level. The authors' results suggest that the field emission properties of carbon nanotubes can be enhanced by the doping of nitrogen atom, which are consistent with the experimental results.     

Effect of impurity on electronic properties of carbon nanotubes

We have studied the effect of impurity on electronic properties of single-walled carbon nanotubes using Density Functional Theory. Electronic band structures and density of states of (4, 4) and (7, 0) carbon nanotubes in the presence of different amount of B and N impurities were calculated. It was found that these impurities have significant effect on the conductivity of carbon nanotubes. The metallic (4, 4) nanotube remains to be metallic after doping with B and N. The electronic properties of small gap semiconducting (7, 0) tube can extensively change in the presence of impurity. Our results indicate that B-doped and N-doped (7, 0) carbon nanotubes can be p-type and n-type semiconductors, respectively.     

Statistical properties of carbon nanostructures

We look at modeling carbon nanostructures from a theoretical graph network view, where a graph has atoms at a vertex and links represent bonds. In this way, we can calculate standard statistical mechanics functions (entropy, enthalpy, and free energy) and matrix indices (Wiener index) of finite structures, such as fullerenes and carbon nanotubes. The Euclidean Wiener index (topographical index) is compared with its topological (standard) counterpart. For many of these parameters, the data have power law behavior, especially when plotted versus the number of bonds or the number of atoms. The number of bonds in a carbon nanotube is linear with the length of the nanotube, thus enabling us to calculate the heat of formation of capped (5,5) and (10,10) nanotubes. These properties are determined from atomic coordinates using MATLAB routines.     

椅型碳纳米管吸附氧原子的密度泛函理论研究

利用密度泛函B3LYP对有限长扶手椅形单壁碳纳米管(3,3),(4,4)和(5,5)吸附O原子的几何结构、电子属性、反应能和红外光谱进行了系统地理论研究,获得了一些有意义的结果,主要包括如下4个方面:(1)2个O原子吸附在管外壁垂直于管轴的C—C键形成开环的轮烯结构,吸附在管内壁形成环氧结构;(2)O原子吸附在管外壁要比吸附在管内壁具有较大的能隙和吸附反应能;(3)与单壁碳纳米管管外壁吸附1个O原子相比,2个O原子吸附在管外壁具有较大的吸附反应能;(4)B3LYP得到的C—O伸缩振动频率与实验一致.