Physica E 54 (2013) 115–117

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Physica E

1386-94http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/physe

A DFT study of H2 adsorption on functionalized carbon nanotubes

Hamed Soleymanabadi a, Jamal Kakemamb,n

a Central Tehran Branch, Islamic Azad University, Tehran, Iranb Mahabad Branch, Islamic Azad University, Mahabad, Iran

H I G H L I G H T S

G R A P H I C A L A

� H2 adsorption on different functio-nalized carbon nanotubes wasstudied.

� In contrary to −BH2 group, the −NH2

and −OH groups increase the bind-ing energy of H2.

� The chemical functionalization leadsto a dramatic HOMO–LUMO gapopening.

77/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.physe.2013.06.015

esponding author. Tel.: +989 14 9481540.ail address: kakemam.jamal@gmail.com (J. Ka

B S T R A C T

The molecular hydrogen adsorption on carbon nanotubes functionalized with −NH2, −OH, −BH2 groupswas investigated by means of density functional calculations.

a r t i c l e i n f o

Article history:Received 25 April 2013Received in revised form5 June 2013Accepted 10 June 2013Available online 25 June 2013

Keywords:DFTHydrogen storageBandgapAdsorption

a b s t r a c t

Hydrogen storage on carbon nanotubes (CNTs) is a key research issue, attracting a lot of interest aroundthe world. Unfortunately, pristine CNTs present very low hydrogen adsorption capacities. Here, themolecular hydrogen adsorption on CNTs functionalized with −NH2, −OH, −BH2 groups was investigatedby means of density functional calculations. In contrary to −BH2 group, the −NH2 and −OH functionalgroups induce an electric dipole moment on the H2 molecule, resulting in more efficient binding of H2 tothe functionalized CNTs. The H2 binding affinity is improved from about 0.72 kJ/mol (on the pristine (8, 0)CNT surface) to about 6.47–7.17 kJ/mol on the −NH2 and −OH functionalized CNTs. Additionally, thechemical functionalization leads to a dramatic HOMO–LUMO energy gap opening and also significantincrease of the work function of the CNT. This will raise the potential barrier of the electron emissionfrom the tube surface, making the field emission difficult.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Hydrogen has been recognized as an ideal energy sourcebecause its usage makes neither air pollution nor greenhouse-gas emissions. Extensive theoretical and experimental studieshave been recently reported to obtain highly efficient hydrogenstorage materials, including various nanotubes, metals or alloys,metal organic frameworks and other nanostructure materials[1–4]. Carbon nanotubes (CNTs) present a unique tubular structurewith high specific surface, thermal and chemical stability [5].

ll rights reserved.

kemam).

Hydrogen storage on CNTs has been a key research issue, attract-ing a lot of interest around the world [6].

The binding affinity of H2 to the CNTs plays a crucial role indetermining their adsorption capacity under ambient conditions.Unfortunately, recent researches have indicated that pristine CNTspresent very low hydrogen adsorption capacities because of theirweak van der Waales interaction with molecular H2 [7]. So far,several strategies have been introduced to overcome this problemsuch as creating defects or doping impurity atoms into the tubewall, chemical functionalization and metal decorating [8–12]. Tsaiet al. [11] have applied acidic etching and Pt particle decoration tomodify the hydrogen adsorption behavior CNTs. Their experimen-tal results showed that acidic etching could increase the surfacedefect density and lead to open-up of the caps of CNTs, resulting in

H. Soleymanabadi, J. Kakemam / Physica E 54 (2013) 115–117116

an increase in the active adsorption site for physical sorption of H2.Theoretical studies of Sankaran et al. [12] have shown that theeffective hydrogenation of CNTs is possible with activation centresand the boron doped CNTs are able to activate the hydrogen in afacile manner compared to pure CNTs. For effective hydrogenstorage the boron atoms should be incorporated geometricallyand chemically into the carbon network.

It is reasonable to expect that the chemical functionalization oftube surface may provide active sites for H2 adsorption, enhancingthe hydrogen storage capacity by increasing the binding energiesand number of adsorbing H2 molecules. It has been shown thatdepending on the particular process, CNTs can be functionalizedwith a large variety of chemical groups including amines, hydro-xyl, and carboxylic acids, etc. [13–16]. The aim of this work is tostudy the effect of −NH2, −OH and −BH2 functional groups on theH2 adsorption on a single walled CNT.

2. Computational methods

Geometry optimizations, natural bond orbitals (NBO) anddensity of states (DOS) analyses were performed on a (8, 0) zigzagpristine and different functionalized CNTs with and without a H2

molecule, using B3LYP level of theory with 6–31 G (d) basis set asimplemented in GAMESS suite of program [17]. The B3LYP hasbeen widely used and proved to be accurate enough for extensivesystems including nanostructured materials [18–22]. Reducing theboundary effects, atoms at the open ends of the tube weresaturated with hydrogen atoms. The adsorption energy (Ead) ofH2 molecule is defined as follows:

Ead ¼ EðH2=adsorbentÞ−EðadsorbentÞ−EðH2Þ þ EBSSE

where E(H2/adsorbent) is the total energy of the adsorbed H2

molecule on definite adsorbent including pristine or −NH2, −OH,and −BH2 functionalized CNTs. EBSSE (Table 1) is the basis setsuperposition error (BSSE) corrected for the all interaction ener-gies. The energy gap in energy levels (Eg) of a system was definedas Eg¼ELUMO–EHOMO (or SOMO); ELUMO is the energy of the lowest

Table 1Energy of adsorption of H2 on the pristine and functionalized CNTs (Ead, kJ/mol),energy of basis set superposition error (EBSSE, kJ/mol), and Mulliken point charge ofthe nearest H atom of H2 (QH1) to the tube surface and that of the other one (QH2) ine (Fig. 2). HOMO (SOMO)/LUMO gap (Eg), energies of HOMO, LUMO and Fermi level(EF) in eV for the studied systems (Fig. 1) before H2 adsorption.

System Ead EBSSE QH1 QH2 EHOMO EF ELUMO Eg

Pristine −0.72 0.06 – – −3.65 −2.93 −3.36 0.29

NH2 −6.47 0.13 0.030 −0.052 −4.30 −3.53 −2.82 1.48OH −7.17 0.15 0.024 −0.047 −4.32 −3.41 −2.95 1.37BH2 −0.62 0.05 0.008 −0.009 −4.37 −3.73 −2.67 1.70

Fig. 1. Optimized structures of pristine and different sidewa

unoccupied molecular orbital and EHOMO (or SOMO) is the energy ofthe highest or the singly occupied molecular orbital (for open shellsystems).

3. Results and discussion

Partial views of the optimized structures of the pristine andfunctionalized CNTs were shown in Fig. 1. First, let us consider theinfluence of the functional groups on the electronic properties ofthe pristine CNT. As shown in Table 1, LUMO and HOMO aresignificantly shifted up and down, respectively, in the all functio-nalized CNTs. The Eg is increased from 0.29 eV in pristine CNT toabout 1.37–1.70 eV in the functionalized forms. The NBO analysisshows that the gap opening in chemically functionalized tubes isassociated with the sp2 to sp3 rehybridization of the adsorbing Catom which splits aromatic π bonding among carbon atoms.However, gap opening in the functionalized single walled CNTswould serve as a scalable alternative to separate them fortransistor applications [23].

Meanwhile, the sidewall functionalization has a dramatic effecton the Fermi level energy of the CNT in such a way it is reducedfrom −2.93 eV to about −3.73 to −3.41 eV. The canonical assump-tion for Fermi level is that in a molecule (at T¼0 K) it liesapproximately in the middle of the Eg. It is noteworthy to mentionthat, in fact, what lies in the middle of the Eg is the chemicalpotential, and since the chemical potential of a free gas ofelectrons is equal to its Fermi level as traditionally defined, herein,the Fermi level of the considered systems is at the center of the Eg.However, reduction in Fermi level energy leads to an increase inwork function that is important in field emission applications. Thework function can be found using the standard procedure bycalculating the potential energy difference between the vacuumand the Fermi levels, which is the minimum energy required for anelectron to be removed from the Fermi level to the vacuum. Theincrease in work function indicates that the field emission proper-ties of the CNT are impeded upon the sidewall functionalization.Furthermore, this will raise the potential barrier of the electronemission for the tube, and make the field emission difficult.

Fig. 2 shows the optimized structures of the H2 adsorbed on thepristine and functionalized CNTs. Henwood et al. [7] have shownthat the most stable H2/CNT complex is that in which the H2 islocated on the center of a hexagonal ring of the tube surface, beingparallel with the tube axis. As shown in Fig. 2, after relaxoptimization, the distances between H atoms of H2 and two Catoms of the CNT surface are about 3.38 and 3.42 Å with adsorp-tion energy of −0.70 kJ/mol. Comparing all of the H2 physisorp-tions on the nanotube's wall in several structural configurations(Fig. 2, Table 1), one can undoubtedly conclude that more efficientbinding of hydrogen can be attained with −NH2 and −OH functio-nalized CNTs. The adsorption energies are about −6.47 and−7.17 kcal/mol in the −NH2 and −OH functionalized CNTs, beingmuch more than that of the pristine CNT.

ll functionalized carbon nanotubes. Distances are in Å.

Fig. 2. Optimized structures of H2 adsorbed on the pristine and OH-, BH2- and NH2-functionalized carbon nanotubes. Distances are in Å.

Fig. 3. Molecular electrostatic potential surface shows the perturbation of chargeson the adsorbed H2 molecule on the NH2-functionalized CNT. The surface is definedby the 0.0004 electrons/b3 contour of the electronic density. Color ranges, in a.u.:red, more positive than 0.010; yellow, between 0.010 and 0; green, between 0 and−0.015; blue, more negative than −0.015. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

H. Soleymanabadi, J. Kakemam / Physica E 54 (2013) 115–117 117

The electronegativity and point charges of the atoms of func-tional groups are the key, as they increase the binding energy ofhydrogen. The point charges on the functional groups induce anelectric dipole moment on the H2 molecule, resulting in moreefficient binding. As shown by calculated molecular electrostaticpotential surface (Fig. 3) for adsorbed H2 molecule on the NH2-CNT, the distribution of change has been perturbed by functionalgroup in such a way the nearest H atom to the tube surfacebecomes more positive than the other.

We have also calculated Mulliken point charge on the two Hatoms of the adsorbed H2 molecule in the three functionalizedcases. As shown in Table 1, the Ead and the point charges arerelated. The −NH2 and −OH groups caused larger point charges onthe H atoms in comparison with −BH2 group, thereby inducinglarger dipole moment on the H2 molecule which leads to astronger interaction. However, −BH2 group cannot significantlyinduce a dipole moment on the H2, therefore, the Ead in this case iseven somewhat less negative than pristine CNT. We believe thatsidewall functionalization of CNTs with proper groups may pro-vide a pathway to find new materials possessing higher hydrogenstorage capacities.

4. Conclusion

Using density functional theory calculations, we showed thatthe functional groups of −NH2 and −OH can considerably increasethe binding affinity of H2 molecules to CNTs due to inducing alarge electric dipole moment on the molecular H2. The H2 bindingaffinity is improved from about 0.72 kJ/mol (on the pristine (8, 0)CNT surface) to about 6.47–7.17 kJ/mol. Thus, we think that thesegroups can improve hydrogen storage capacity of the CNTs.

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