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Adsorption properties of CH3COOH on (6,0), (7,0), and (8,0) zigzag, and (4,4), and (5,5) armchair single-walled carbon nanotubes: A density functional study

The behaviour of CH3COOH molecule adsorbed on the external surface of H-capped (6,0), (7,0), and (8,0) zigzag, and (4,4), and (5,5) armchair single-walled carbon nanotubes was studied by using density functional calculations. Geometry optimizations
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  ORIGINAL ARTICLE Adsorption properties of CH 3 COOH on (6,0),(7,0), and (8,0) zigzag, and (4,4), and (5,5) armchairsingle-walled carbon nanotubes: A densityfunctional study Maziar Noei  a, * , Ali-Akbar Salari  b , Mahsa Madani  b , Mina Paeinshahri  b ,Hossein Anaraki-Ardakani  a a Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran b Department of Chemistry, Shahr-E-Rey Branch, Islamic Azad University, Tehran, Iran Received 17 December 2012; accepted 19 November 2013Available online 28 November 2013 KEYWORDS Density functional theory(DFT);Acetic acid;Single-walled carbon nano-tubes (SWCNTs);Adsorption Abstract  The behaviour of CH 3 COOH molecule adsorbed on the external surface of H-capped(6,0), (7,0), and (8,0) zigzag, and (4,4), and (5,5) armchair single-walled carbon nanotubes was stud-ied by using density functional calculations. Geometry optimizations were carried out at theB3LYP/6-31G ** level of theory by using the Gaussian 03 suite of programs. We present the natureof the CH 3 COOH interaction in selected sites of the nanotubes. Adsorption energies correspondingto adsorption of the CH 3 COOH are calculated to be in the range 47–174 kcal mol  1 . The calculatedadsorption energies for CH 3 COOH in H-down orientation are higher than those in O-down and C-down orientation for all of the configurations. More efficient adsorption energies cannot beachieved by increasing the nanotube diameter. We also provide the effects of CH 3 COOH adsorp-tion on the electronic properties of the nanotubes. ª 2013 Production and hosting by Elsevier B.V. on behalf of King Saud University. Thisisanopenaccessarticle under the CC BY-NC-ND license ( 1. Introduction Since the discovery of carbon nanotubes (CNTs) Ijima (1991),carbon nanotubes (CNTs) have attracted great interest owingto their extraordinary structural, mechanical, chemical, physi-cal, and electronic properties (Ijima, 1991; Derycke et al.,2002; Liu et al., 1999). Numerous works have been performedtostudythepropertiesandapplicationsofthisfascinatingnovelmaterial (Zurek and Autschbach, 2004; Nojeh et al., 2003).Theyhaveawiderangeofapplicationinnanoelectronics,nano-scaling biotechnology and biosensors (Zhou et al., 2002; Zhenet al., 1999; Baughman et al., 1999; Gao et al., 2003). Becauseof their size, large surface area and hollow geometry, single-walled carbon nanotubes (SWCNTs) are being considered asprime materials for gas adsorption (Rawat et al., 2007;Zhao et al., 2002; Gordillo, 2007; Choi et al., 2004; Byl et al.,2003), biological, chemical, electromechanical sensors andnanoelectronic device (Yang et al., 2007; Gowtham et al., *Corresponding author. Tel.: +98 9125759236.E-mail address: (M. Noei).Peer review under responsibility of King Saud University. Production and hosting by Elsevier Arabian Journal of Chemistry (2017)  10 , S3001  –  S3006 King Saud University Arabian Journal of Chemistry  ª  2013 Production and hosting by Elsevier B.V. on behalf of King Saud University.This is an open access article under the CC BY-NC-ND license (  2007; Froudakis et al., 2003). For example, CNTs have beenexperientially investigated for the detection of gas molecules(Kongetal.,2000;Bekyarovaetal.,2004;Fengetal.,2005),or-ganic vapours (Li et al., 2003; Agnihotri et al., 2006), biomole-cules and different ions (Chen et al., 2001, 2004; Kam and Dai,2005).Thekeyelementthatmakesnanotubessopotentiallyuse-ful as electrochemical storages is their structure (Hirscher et al.,2001).Recentstudieshaveshownthatthephysicalpropertiesof single-wall carbon nanotubes (SWNTs) could be modified byadsorption of foreign atoms or molecules (Cheng et al., 2004;Peng et al., 2004; Cinke et al., 2003). For example, it has beenfound that the exposure to O 2 , NO 2  or NH 3  dramatically influ-encestheelectricalresistanceandthermoelectricpowerofsemi-conductingSWCNTs(Dagetal.,2003;Quangetal.,2006).Gasadsorption on carbon nanotubes and nanotube bundles is agreat issue for both essential research and applied applicationof nanotubes. The adsorptive characteristics of SWCNTs inthegasphaseenabledtheiruseasgassensorsofpollutantgases,storageoffuels(Liuetal.,1999),andremovalofhazardouspol-lutantsfrom gas streams (Long andYang, 2001a,b).The dopedor defective CNTs can give improved sensitivity when used indetecting molecules similar to CO, H 2 O, 1,2-dichlorobenzeneorgaseouscyanideandformaldehyde(PengandCho,2003;Fa-gan et al., 2004). The possibilities of using chemically dopedCNTs as highly sensitive gas sensors are also under intensiveinvestigation (Collins et al., 2000).Furthermore, there are evidences indicating the greatimportance of molecular gaseous orientation on the energyof adsorption (Paredes et al., 2003; Sorescu et al., 2001). Thiseffect is one of the major aspects of gas–nanotube interactionsthat has not been studied extensively. Sensitivity of CNTs tothe acetic acid (CH 3 COOH) has been indicated by quantummechanics calculations. The determination of the structure of adsorbed CH 3 COOH on CNT surfaces is important forunderstanding its bonding and reactivity in catalysis and othersurface phenomena. However, to our knowledge, no experi-ments and theoretical investigation have been reported onadsorption of CH 3 COOH on CNT surfaces. In this work, wetheoretically studied the adsorption of CH 3 COOH on variousadsorption sites of the three zigzag ( n ,0) ( n  = 4,6,8) and twoarmchair ( n , n ), ( n  = 4,5) carbon nanotubes. Also three orien-tations of CH 3 COOH molecules on the outside of the tubes,H-down, O-down, and C-down modes were considered. 2. Computational method In the present work, adsorption behaviours of the CH 3 COOHon SWCNTs were studied by using the representative modelsof (4,0), (6,0), and (8,0) zigzag, and (4,4), and (5,5) armchairsingle-walled CNTs in which the ends of nanotubes are satu-rated by hydrogen atoms. The hydrogenated (4,0), (6,0), and(8,0) zigzag, and (4,4), and (5,5) armchair single-walled CNTshave 48 (C 40 H 8 ), 72 (C 60 H 12 ), 80 (C 64 H 16 ), 72 (C 56 H 16 ), and 90(C 70 H 20 ) atoms, respectively. In the first step, the structureswere allowed to relax by all atomic geometrical optimizationat the DFT level of B3LYP exchange-functional and 6-31G ** basis set. The optimized structures have diameters   3.10,4.66, 6.22, 5.39, and 6.73 A ˚, respectively. The adsorption en-ergy of a CH 3 COOH on the CNT wall was calculated as fol-lows Eq. (1).E ad  ¼ð E CNT þ E CH 3 COOH Þ E ð CNT  CH 3 COOH Þ  ð 1 Þ where E CNT  is the energy of the optimized CNT structure, andE CH 3 COOH  is the energy of an optimized CH 3 COOH andE CNT  –  CH 3 COOH  was obtained from the scan of the potential en-ergy of the CNT–CH 3 COOH. All the calculations were carriedout by using the Gaussian 03 suite of programs (Frisch et al.,2003). 3. Results and discussion CH 3 COOH can approach the nanotube walls from outside(out), which is the most common case, and from inside (in).For the adsorption of the CH 3 COOH (H-down, O-down andC-down) on the CNTs, we considered two sites (i.e., C site di-rectly on top of carbon atoms, and B site on the middle of twonearest neighbour carbon atoms) as described in Fig. 1. Thenotation H-down, O-down and C-down denotes a CH 3 COOHperpendicular to the surface via H, O and C, respectively.We limited our analysis to the interaction of CH 3 COOHwith the nanotubes’ outer walls. Considering each site andconfiguration, we ended up with six different approaches of CH 3 COOH to the CNT walls. For each of these cases weinvestigated the CNT–CH 3 COOH potential energy surface(PES). The adsorption energies of the CH 3 COOH (H-down,O-down, and C-down) at the two sites on zigzag (4,0), (6,0),and (8,0) and armchair (4,4), and (5,5) single-walled CNTsare plotted in Fig. 2, and the adsorption energy with the equi-librium distance in each case is summarized in Table 1.In all pathways the potential is attractive, presenting a wellof maximum ca.   175 kcal mol  1 , which is characteristic of achemisisorption process. The calculations showed that the ob-tained adsorption energies depend on orientations and loca-tions of CH 3 COOH, and the interaction becomes rapidlyrepulsive as the molecule approaches the CNT wall. And thecalculated E ad  for CH 3 COOH in H-down is more than that Figure 1  Adsorption modes of a CH 3 COOH on CNTs: H-down (a), O-down (b), and C-down (c). S3002 M. Noei et al.  in O-down, and C-down. The most stable configuration of CH 3 COOH for H-down in the (4,0) CNT is the C site, theperpendicular approach of CH 3 COOH (H-down) to the (4,0)CNT wall on the upper carbon atom, and the current Figure 2  Adsorption energy curves of CH 3 COOH (H-down, O-down and N-down) adsorption at  C  , and  B  sites on zigzag (4,0), (6,0),and (8,0), and armchair (4,4), and (5,5) CNTs. Distances are in A ˚. Adsorption properties of CH 3 COOH on (6,0), (7,0), and (8,0) zigzag, and (4,4), and (5,5) armchair S3003  calculation shows that the adsorption energy for this site is  136.82 kcal mol  1 with equilibrium distance (rd) 1.0 A ˚. Themost stable configurations of CH 3 COOH for H-down in the(6,0), (8,0), (4,4), and (5,5) CNTs are the C, B, B, and C site,respectively. The current calculation showed that the adsorp-tion energies for these sites are   139.35,   174.31,   148.06,and   158.24 kcal mol  1 with equilibrium distance (rd) 1.0,1.0, 1.0 and 1.0 A ˚, respectively. We observed that when theCNT diameter increases, the E ad  of CH 3 COOH at eachparticular site of the interaction is different. For example,CH 3 COOH (H-down) binds on the B site of the (4,0) CNTwith   133.00 kcal mol  1 , whereas it binds on the B site of the (6,0), (8,0), (4,4), and (5,5) CNTs with   139.35,   126.15,  48.65, and   158.24 kcal mol  1 , respectively. An interestingconclusion that can be drawn from these pathways is thatonly the type of the tube (CNT) plays an important role indetermining the adsorption energy of the CH 3 COOH andnot the diameter of the tube as observed in previous cases.All the results are clearly demonstrated in Table 1. 3.1. Electronic properties Finally, we studied the influence of CH 3 COOH adsorptions onthe electronic properties of the CNTs. The calculated band gapenergies of the clean perfect (4,0), (6,0), (8,0), (4,4), and (5,5)single-walled CNTs are about 2.64, 1.13, 2.59, 2.72, and3.54 eV, respectively. The effects of the CH 3 COOH on adsorp-tion energies in the CNTs relate to their electronic structure.When the CH 3 COOH is adsorbed on the CNTs, the interac-tion between them being strong, the electronic properties of these tubes are changed obviously, and the band gap for theCH 3 COOH (8,0) zigzag CNT in the most stable configurationis calculated to be about 1.35 eV. Therefore, the adsorption of CH 3 COOH on the CNTs further decreases the band gap of thepristine CNTs, and increases their electrical conductance. 4. Conclusions We studied the adsorptions of CH 3 COOH on zigzag configu-rations of (4,0), (6,0), and (8,0) and armchair (4,4), and (5,5)SWCNTs by means of density functional theory (DFT) calcu-lations. On the basis of our calculations, it seems that pristineCNTs can be used as a CH 3 COOH storage medium as long asCH 3 COOH is adsorbed on the exterior walls of the CNTs be-cause of the high adsorption energy. We compared all theadsorption energy curves of CH 3 COOH interacting with allpossible sites of adsorption on nanotube walls in severalstructural configurations. For the CNTs the calculated E ad Table 1  Adsorption energy (kcal mol  1 ) and equilibrium distance (A ˚) of a CH 3 COOH molecule on zigzag (4,0), (6,0), and (8,0), andarmchair (4,4), and (5,5) CNTs. Model Mode Site C B (4,0) H-down Adsorption energy   136.82   133.00rd 1.0 1.0O-down Adsorption energy   78.06   111.57rd 1.5 1.5C-down Adsorption energy   67.77   109.35rd 2.0 2.0(6,0) H-down Adsorption energy   139.35   122.20rd 1.0 1.0O-down Adsorption energy   105.83   126.21rd 1.5 1.5C-down Adsorption energy   89.21   90.49rd 2.0 2.0(8,0) H-down Adsorption energy   126.15   174.31rd 1.0 1.0O-down Adsorption energy   100.83   108.59rd 1.5 1.5C-down Adsorption energy   89.48   83.83rd 2.0 2.0(4,4) H-down Adsorption energy   48.65   148.06rd 1.5 1.0O-down Adsorption energy   121.00   139.22rd 1.5 1.5C-down Adsorption energy   89.26   91.33rd 2.0 2.0(5,5) H-down Adsorption energy   158.24   47.38rd 1.0 1.5O-down Adsorption energy   157.00   147.24rd 1.5 1.5C-down Adsorption energy   80.17   95.60rd 2.0 2.0 S3004 M. Noei et al.  for CH 3 COOH in H-down is more than that in O-down and C-down. 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