Theoretical Studies Of The Functionalized One-dimensional Nanotubes

The carbon nanotube (CNT), which was discovered in 1991, possesses the quasi-one-dimensional structure. The unique properties of carbon nanotube render it have wide potential applications in energy storage, sensor, and so on. Recently, many other nanotubes, such as boron nitride nanotubes (BNNT) and silicon carbide nanotubes (SiCNT), have been synthesized. The investigation on their structures, properties and the reactions might play an important role on the exploration of their potential applications.With the progress in density functional theory (DFT) and its numerical methods, DFT based first-principles calculation has become a routine method for condensed matter theory, quantum chemistry and material science. In the present paper, using DFT calculations, we examine the possibility of the potential applications of several functionalized nanotubes, including CNT, BNNT, and SiCNT, in the detection of gas molecule, the design of field emitter. Our result might provide the guidance for the related studies in experiment. The obtained results are summarized as follows:(1) As is known, CNT has high-ratio surface, thus it can be used to detect the presence of some molecular gases, including O2 and NO2. However, some toxic gases (such as CO) can't be sensed by CNT because of the weak adsorption on CNT. In this paper, we investigate the interaction between Rh-decorated (8,0) SWCNT CO gases by using density functional theory (DFT) methods. The results indicate the adsorption of Rh has activated its nearest carbon atoms. Both the Rh-site and the"activated"C-sites are considered as reactivity sites for the adsorption of CO gases. We found that the adsorption energy of CO gas on Rh site is larger than that of the"activated carbon atoms". The adsorption of CO and Rh and the"activated carbon atom"could lead to certain charge transfer and the changes of electronic properties of Rh-CNT. Thus, in view of the charge transfer, the Rh-CNT is a good candidate for the detection of CO. Yet, the Rh-decorated SWCNT is not reusable for CO gases detecting due to the large binding energy. These calculation results are useful not only to explain the sensing mechanisms but also to evaluate the potential for SWCNTs-based sensing materials at room temperature.(2) Since the atoms of the edges of open nanotubes have high reactivity, the chemical functionalization of the open nanotube may modify their electronic and magnetic properties. In this paper, we have systematically studied the effects of several gaseous adsorbates (H2, N2, O2, and H2O) on the electronic properties of open edges of boron nitride nanotubes (BNNTs) by using density functional theory calculations. The results indicate that all of the molecules, except N2, dissociate and chemisorb on open BNNT edges with large adsorption energies. Additionally, the electronic and magnetic properties of the BNNTs with open edges could be modified to various degrees due to the adsorption, which are dependant on the kinds of the adsorbates. Finally, we note the magnetic properties of the adsorbed N atoms of N-rich-ended BNNT are significantly decreased because the dangling bond is saturated.(3) The CNT-based field emission display is the most potential in application. BNNT possesses good stability and exhibits significant resistance to oxidation at high temperature. Interestingly, the BNNT with open edges has unique electronic and magnetic properties. In the present work, through density functional theory (DFT) calculations, we present the first attempt on the effects of O2 and H2O adsorption on the field-emission properties of an open-ended (8, 0) BNNT. The two adsorbates can adsorb at the tips of the open-ended BNNT with large adsorption energies and significant charge transfer. An applied electric field of 1 eV/? at the tube tip (a) significantly increases the adsorption energy to stabilize the adsorbates, and (b) alters the emission properties such as ionization potential (IP) or band-gap. The IP of the open N-rich-ended BNNT is lowered, thereby making it easier to lose electrons. However, there is a slight increase of the IP for the open B-rich-end BNNT. Our results would be useful not only to better understand the property of open-ended BNNTs, but also to design more efficient field emitters of molecular electronic devices in experiments.(4) Single-walled carbon nanotubes functionalized by the transition metal have attracted considerable interest due to their potential applications in hydrogen storage, gas sensor and nanodevice design. Silicon carbide nanotbues (SiCNTs), which have been synthized in 2001, possess unique properties and wide applications. Aiming to deeply understand the properties of SiCNT and further explore its potential, we first study by using DFT methods the adsorption of various transition metals on the outer surface of SiCNT. The results suggest that these metal atoms can be chemisorbed on the SiCNT and the binding energy ranges from 1.17 eV (Cu-adsorption) to 3.18 eV (Ti-adsorption). Moreover, the band structures and the density of states near the Fermi leves are significantly modified due to the adsorption of transitiom metals. Interestingly, these changes are only depended on the kinds of transition metals, while not their adsorption-sites. An exception is the Ti-adsorption: for the adsorption of Ti on H-site, SiCNT is converted from semiconductor to metal-materials. However, when Ti is adsorbed on C-site of SiCNT, it still keep semiconducting conductor. Finally, an amount of charge transfer is found and the SiCNT might be magenic materials because of the adsorption of transition metal.(5) Theoretical studies indicate that the SiCNT has higher reactivity than those of CNT and BNNT. Thus, some gases might be adsorbed on the surface of SiCNT, while not on CNT or BNNT. To explore the possibility of the SiCNTs as potential gas sensors for CO2-detection, we study the CO2 adsorption on various zigzag (n,0) (n = 6, 8, 10, 12, and 18) SiCNTs by density functional theory (DFT) calculations. It is found that tube diameter and CO2 coverage play important roles in the tube-CO2 interaction. Additionally, the average adsorption energy of CO2 on the SiCNTs is less than 1 eV and the charge transfer is about 0.20 e. Because of the sufficient charge transfer and high concentration of CO2, SiCNT could be a perfect material for efficiently detecting the CO2 molecule.(6) Effective cleavage of the N-H and O-H bonds at the"metal-free"centers has attracted considerable attention due to the fundamental and industrial importance. In this paper, we show by density functional theory calculations that the pure silicon carbide nanotube (SiCNT) can effectively cleave the N-H bond of ammonia and O-H bond of H-OX (X = H, CH3 and C2H5). It is shown that both the N-H and O-H bond cleavage undergoes two evolution steps: (i) molecular chemisorption of NH3 or H-OX followed by (ii) activated N-H bond of NH3 or O-H cleavage of H-OX. For the N-H bond cleavage of ammonia, the adsorption energy (-1.199 eV) and the subsequent H-transfer barrier (0.842 eV) from Si-atom to the neighboring C-atom indicate a zero total N-H splitting barrier by the Si+-C? center of SiCNT. Similarly, the SiCNT can also barrierlessly split the O-H bonds of H-OX with a considerably larger exothermicity (~-1.800 eV) than that of the N-H bond cleavage (-1.370 eV). Subsequently, the resultant NH2- or OX-groups and H-atom can be converted to useful organic compounds with the presence of CO, which leads to the recovery of pure SiCNT. Our work for the first time demonstrate that the pure SiCNT is very promising in metal-free cleavage and conversion of the N-H or O-H bonds, while would greatly put forward the potential application of SiCNTs.