A density functional study on the acidity properties of pristineand modified SiC nano-sheets

Ali Ahmadi Peyghan a, Saeed Amir Aslanzadeh b, Maziar Noei c,n

a Young Researchers and Elite Club, Central Tehran Branch, Islamic Azad University, Tehran, Iranb Technical Vocational University, Yaftabad, Tehran, Iranc Collage of Chemical Engineering, Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran

a r t i c l e i n f o

Article history:Received 5 August 2013Received in revised form11 December 2013Accepted 3 March 2014Available online 12 March 2014

Keywords:AmmoniaSensorAciditySiC nanosheet

a b s t r a c t

Surface acidity is an important property that is frequently used to characterize the reactivity of surfaces.NH3 is well known as a probe molecule in determining the acidity of surfaces experimentally. Here, theadsorption of an NH3 molecule at different sites of a SiC nanosheet was investigated based on densityfunctional theory framework. It was found that NH3 can be weakly adsorbed on the surface of thepristine with the adsorption energy (Ead) of about 3.5 kcal/mol with no sensible effect on the electronicproperties of the sheet. Replacing a Si or C atom with both Al and B atoms increases the Ead, indicatingthat the acidity of SiC sheet can be controlled by doping of B and Al atoms. Relative magnetic order of theacidity for different surfaces was found to be: AlC, a C atomwas replaced with a Al, (Ead¼86.4 kcal/mol)⪢AlSi (Ead¼39.2 kcal/mol)4BC (Ead¼30.9 kcal/mol)4BSi (Ead¼12.3 kcal/mol)⪢pristine h-SiC (Ead¼3.5kcal/mol). Moreover, it was found that when a C atom of the sheet was doped by an Al atom, HOMO–LUMO gap of the sheet becomes highly sensitive to NH3 adsorption so that it is decreased by about 31.3%after the adsorption process. It shows that the AlC-doped sheet may be used in the detection of NH3 gasmolecules.

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1. Introduction

Single-walled carbon nanotube (CNT) with its outstandingmechanical, chemical, electrical and one-dimensional structuralproperties has attracted considerable interest since its discoveryby Iijima in 1991 [1–3]. In addition to CNT, there are also someother nanotubes that are found experimentally, such as siliconcarbide (SiC) [4–6]. The outstanding physical, chemical, andthermal properties of SiC enable it to be employed in hightemperature, high frequency, and harsh environments [7]. Nano-structures of SiC, including nanotubes, nanowires and nanorodshave created enormous interest, and the properties of thesenanostructures are different from the bulk SiC. There are somedifferences between silicon and carbon, for example, Si has lowerelectronegativity, weaker bonds (except when bonded to veryelectronegative atoms), it is kinetically more reactive, and it haslarger atomic radius, etc. These differences lead to differences inthe properties of silicon compared with carbon.

The higher reactivity of SiC nanotubes (SiCNTs) on the exteriorsurface compared with CNTs results in facilitating the side wall

decoration and more stabilization at high temperatures [8]. It hasalso anticipated by theoretical methods that, for hydrogen storage,SiCNTs are more effective than CNTs [9]. Because of these advan-tages of SiCNTs, several methods have been employed to synthe-size them [10,11]. Sun et al. [11] were the first ones whosynthesized the silicon carbide nanotubes (SiCNTs) via the reactionof SiO with the multi-walled CNTs. After the reaction, half of thecarbon atoms of nanotubes were replaced by Si atoms. Theinterlayer distances of the resulting multi-walled SiCNTs weresignificantly larger than those of CNTs.

Similar to CNTs, SiCNTs can be considered to be formed byrolling the SiC sheet. Graphene-like SiC sheet (h-SiC) with sp2

bond may be implied in electronic devices in the future.In contrast to the half-metal behavior of graphene, h-SiC possessesa polar Si–C bond [12] which promises the potential application ofh-SiC in this silicon age of semiconductors. The stability character-istics of the h-SiC under uniaxial compression have been studied[13]. Two types of sheet can be studied. The first one includesalternating C and Si atoms and the other one C–C and Si–Si pairs.By theoretical methods, it has been represented that the first typeis energetically more favorable [14–18]. Using density functionaltheory (DFT) calculations with local density approximation (LDA),the SiC graphitic monolayer and SiCNTs were studied by Hudaet al. [19]. They showed that graphene-like SiC layers can exist.

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Physica B

http://dx.doi.org/10.1016/j.physb.2014.03.0060921-4526 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ98 9125759236.E-mail address: noeimaziar@gmail.com (M. Noei).

Physica B 443 (2014) 54–59

Wong et al. [20] used atomic force microscopy in conjunction withthe lithography technique to estimate the strength of SiC nanorods(NRs) and multiwalled CNTs (MWCNTs). They showed thatMWNTs were about two times as stiff as SiC NRs. However, theultimate strengths of the MWNTs were less than those of SiCNRs.Wang have found that the SiC sheet would be a promisingcandidate to detect the HCOH gas using DFT study [21]. It is foundthat the C atom of the SiC sheet is the active adsorption site andthe HCOH molecule prefers the C atom to the O and H atoms closeto the SiC sheet. However, we could not find any study whichinvestigates the acidity behavior of h-SiC.

Surface acidity is an important property that is frequently usedto characterize the reactivity of surfaces. Ammonia (NH3) is widelyused as a probe molecule in determining the Lewis and Brønstedacidity of surfaces experimentally. The binding energy to a particularsite on the surface serves as a measure of its acid strength [22]. Mostsurfaces have several types of reactive sites, and with the addition ofa dopant, the number of such sites can further increase [23].Theoretical methods are well suited to numerically probe thestrength of the binding of NH3 against these numerous reactive sites,and they provide rich information about the reactivity of the surface[24,25]. In this study, the interaction of NH3 with h-SiC will betheoretically investigated based on the analysis of structures, ener-gies, electronic properties, etc. We want to gain fundamental insightsinto the effect of boron and aluminum doping on the adsorptionproperties of the h-SiC toward NH3, and how these effects can changethe acidity of h-SiC.

2. Computational methods

A SiC sheet that consisted of 39 silicon and 39 carbon atomswas selected, in which the end atoms were saturated withhydrogen atoms to reduce the boundary effects (Fig. 1). The fullgeometry optimizations and property calculations on the pristine,B-doped and Al-doped sheets in the presence and absence ofa NH3 molecule were performed using three parameter hybridgeneralized gradient approximation with the dispersion correctedB3LYP (B3LYP-D) functional [26] and the 6-31G basis set includingthe d-polarization function (denoted as 6-31G (d)) as implemen-ted in the GAMESS suite of program [27]. GaussSum program has

been used to obtain the DOS results [28]. The B3LYP densityfunctional has been previously shown to reproduce experimentalproprieties and it has been commonly used for nanostructures[29–32]. We have defined the adsorption energy (Ead) as follows:

Ead¼E(h-SiC)þE(NH3)�E(NH3/h-SiC)þE(BSSE) (1)

where E(NH3/h-SiC) is the total energy of the adsorbed NH3

molecule on the h-SiC surface, and E(h-SiC) and E(h-SiC) are thetotal energies of the pristine h-SiC, and NH3 molecule, respectively.E(BSSE) is the basis set superposition error (BSSE) corrected for allinteraction energies. Counterpoise correction of Boys and Bernardiwas used to correct BSSE effects [33]. The canonical assumption forFermi level (EF) is that in a molecule (at T¼0 K) it lies approxi-mately in the middle of the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO)energy gap (Eg). It is noteworthy to mention that, in fact, what liesin the middle of the Eg is the chemical potential, and since thechemical potential of a free gas of electrons is equal to its Fermilevel as traditionally defined, herein, the Fermi level of theconsidered systems is at the center of the Eg.

3. Results and discussion

3.1. Pure h-SiC

The optimized structure and geometry parameters of thepristine h-SiC are shown in Fig. 1a, in which Si–C bond length is1.78 Å, which is in good accordance with previous results [12]. Thecharge analysis using the NBO method indicates that about 1.010 echarges are transferred from the Si atom to its adjacent carbonatom within the sheet, indicating partially the ionic character ofthe Si–C bonds in the sheet. In order to obtain the most stableconfiguration of single NH3 adsorbed on the h-SiC, various possibleinitial adsorption geometries including single (hydrogen or nitrogen)and double (N–H) bonded atoms to Si and C atoms on differentadsorption sites are considered. However, only one structure wasobtained after the relaxation process (Fig. 1b). More detailed infor-mation from the simulation of the different NH3/h-SiC system,including values of Ead, Eg and the charge transfer (QT) for thisconfiguration is listed in Table 1. This configuration stands for the

Fig. 1. Structure of optimized (a) h-SiC, (b) NH3/h-SiC and their density of state (DOS) plot. Bonds are in Å.

A. Ahmadi Peyghan et al. / Physica B 443 (2014) 54–59 55

covalent bonding between the nitrogen atom of NH3 molecule andthe silicon atom of the sheet with a distance of 2.01 Å. The adsorptionof the NH3 on the Si site in h-SiC is best rationalized by the fact thatin h-SiC, the HOMO is localized on the C sites, and the LUMO arelocated on the Si sites. As a result, ammonia adsorption (Lewis Basewhich acts as a probe molecule in experimentally determining theacidity of surfaces) is adsorbed strongly on Lewis acid sites (Si site). Inthis configuration, a net charge of about 0.143 electrons transfersfrom the molecule to the sheet and its corresponding calculated Eadvalue (Table 1) is about 1.5 kcal/mol, which suggests the low aciditybehavior of pristine h-SiC.

In the following, we have studied the influence of the NH3

adsorption on the electronic properties of the sheet. For the bareh-SiC (Fig. 1a), it can be concluded that it is a semiconductingmaterial with an Eg of 0.20 eV. By referring to Fig. 1b, it is revealedthat although both conduction and valence levels (also Fermilevel) slightly move to higher energy, the Eg of the sheet remainsconstant after the NH3 adsorption. The calculated DOS shows thatthe NH3 adsorption on h-SiC can be generally classified as a certaintype of “electronically harmless modification”.

3.2. B-doped h-SiC

The effects of replacing either C or Si atom of the sheet witha B atom on the geometrical structure and electronic properties ofthe h-SiC, and also on the acidity of surface were investigated. Asshown in Fig. 2, the geometric structures of the B-doped h-SiC areslightly distorted, but the planarity of the sheet is not affected byboron doping. The average bond lengths of B atom center withneighboring Si and C atoms (in BC or BSi) are about 1.86 and 1.61 Å,respectively. Here, we denote B impurities as BC or BSi, implyingthat B atom is replaced with a C atom or an Si atom in the pristineh-SiC. The doping energy (Edop) is also calculated as follows:

Edop¼E(h-SiC)þE(C or Si)�E(doped-h-SiC)�E(doping atom) (2)

where E (doped-h-SiC) is the total energy of the boron- oraluminum-doped h-SiC, and E (doping atom) and E (C or Si) arereferred to as the energy of a single doping atom (B or Al) and C (orSi) atom, respectively. When a B atom is doped in the sheet, theEdop is calculated to be �106.6 kcal/mol for BC, but that is found tobe �11.9 kcal/mol for BSi. These negative values indicate that bothof the doping processes are endothermic and also suggest that theBSi-doping may be a more energetically-favorable process thanBC-doping one.

Subsequently, we have explored NH3 adsorption on theB-doped sheet by locating the molecule above the B atom withdifferent initial orientations including N, or H atom of themolecule which is close to B. After careful relax optimization ofinitial structures similar to pristine h-SiC, only one final structurewas obtained for each of the BC and BSi doped sheets which areshown in Fig. 2. For the BC doped sheet, conduction level shifts tohigher energies after boron doping, but conduction level remainsconstant, so the Eg value of the system is increased to 0.54 eV. TheEg value for BSi doped sheet significantly increased from 0.20 to0.76 eV. However, unlike BC (in which Fermi level of the sheetincreased from �3.58 to �3.35 eV), its Fermi level is decreased to�3.95 eV.

Finally, we have explored the acidity behavior of h-SiC via NH3

adsorption on the doped sheet by locating the molecule above the

Table 1Calculated adsorption energy of NH3 (Ead in kcal/mol), HOMO energies (EHOMO),LUMO energies (ELUMO), HOMO–LUMO energy gap (Eg), Fermi level (EF) of systems(Figs. 1–5) in eV.

System EadbQT (e) EHOMO EF ELUMO Eg

aΔEg (%)

h-SiC – – �3.68 �3.58 �3.48 0.20 –

NH3/h-SiC 3.5 0.143 �3.62 �3.52 �3.42 0.20 0.0BC doped h-SiC – – �3.62 �3.35 �3.08 0.54 –

NH3/BC doped h-SiC 30.9 0.221 �3.51 �3.25 �3.00 0.51 5.5BSi doped h-SiC – – �4.33 �3.95 �3.57 0.76 –

NH3/BSi doped h-SiC 12.3 0.178 �4.39 �4.00 �3.61 0.78 2.6AlC doped h-SiC – – �4.35 �4.09 �3.84 0.51 –

NH3/AlC doped h-SiC 86.4 0.334 �4.16 �3.98 �3.81 0.35 31.3AlSi doped h-SiC – – �4.40 �4.10 �3.81 0.59 –

NH3/AlSi doped h-SiC 39.2 0.239 �4.33 �4.04 �3.76 0.57 3.4

a QT is defined as the total NBO charge on the NH3.b Change of Eg of the sheet after NH3 adsorption.

Fig. 2. Structure of optimized (a) BC doped h-SiC, (b) BSi doped h-SiC and their density of state (DOS) plot. Bonds are in Å.

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B atom with different initial orientations. We identified onedistinct adsorptive configuration of NH3 for each B-doped sheet,as shown in Fig. 3. Configuration P (NH3/BC-h-SiC complex) givesrise to an Ead of 30.9 kcal/mol, which is more exothermic than theEad value of configuration Q (NH3/BSi-h-SiC complex, Ead¼12.3 k-cal/mol). On comparison, NH3 adsorption on B-doped sheet isenergetically more favorable than that on the pristine h-SiC,indicating that the boron doping may improve the Lewis-acidityof the sheet. In configuration P, nitrogen atom of the molecule waslocated on the top of neighboring Si atom with the corresponding

bond length of 2.02 Å, while NH3 was adsorbed on the top of BSiimpurity through its hydrogen atoms in configuration Q (Fig. 3). Tofurther understand the electronic properties, the DOS plots for dopedsheets were compared before and after the adsorption of NH3.It should be noted that herein, Eg stands for SOMO (singly occupiedmolecular orbital)/LUMO energy gaps for the open shell systems.Table 1 and Fig. 3 show that the conduction, valence and Fermi levels(EF) of B-doped h-SiC changed after the adsorption of NH3, but the Egdoes not significantly. In other words, the electronic properties of theBC and BSi doped h-SiCs are negligibly changed by the NH3 adsorption.

Fig. 3. Structure of optimized (P) NH3/BC doped h-SiC, (Q) NH3/BSi doped h-SiC and their density of state (DOS) plot. Bonds are in Å.

Fig. 4. Structure of optimized (a) AlC doped h-SiC, (b) AlSi doped h-SiC and their density of state (DOS) plot. Bonds are in Å.

A. Ahmadi Peyghan et al. / Physica B 443 (2014) 54–59 57

3.3. Al-doped h-SiC

We first study the doping of a single Al atom to h-SiC to formAl-doped sheet. Again, we denote Al impurities as AlC or AlSi,implying that Al atom is replaced with a C atom or Si atom in thepristine h-SiC (Fig. 4). With the optimized structures, the Edop valuesof the AlC and AlSi doped-sheets are about �253.1 and �70.0 kcal/mol, respectively. Next, NH3 molecule has been considered to beadsorbed on the surface of the Al-doped h-SiC. We probeda number of different adsorption sites on the sheet to find thelowest-energy configuration for the adsorbate/adsorbent system.Adsorption configuration of NH3 on Al-doped sheet completely iscompletely different from B-doped one. During the optimization,the NH3 reoriented in such a way that its N atom moved closer tothe Al site in both of R (AlC doped h-SiC) and S (AlSi doped h-SiC)configurations with Ead of 86.4 and 39.2 kcal/mol, and the corre-sponding interaction distance between the Al atom of doped-sheetand the NH3 is 2.02 and 2.04 Å, respectively (Fig. 5).

Larger Ead between the Al-doped h-SiC and the moleculeindicates that the doping of Al can significantly improve thereactivity of the nanosheet toward the NH3. The NH3 binds to theexposed Al atom which is electron-deficient and can receiveelectrons from the lone pair orbitals of nitrogen. In other words,unlike boron atoms, the hybridization of the Al atom is close to sp3

and it can have a coordination number of 4. This site that can acceptmore electrons is referred to as “Lewis acid sites” (and conversely,N atom of the molecule is termed as “Lewis base”). Why is thereaction of NH3 with AlC-h-SiC more favorable than that of the AlSi-h-SiC? To answer this question, we investigated frontier molecularorbital (FMO) analysis of the Al doped sheets (Fig. 6). The chargetransfer between the NH3 and the sheet, which is estimated byusing the NBO analysis, indicated that the unfilled SOMO orbitals ofAlC and AlSi doped sheet accept 0.334 and 0.239 e from NH3

molecule. On the other hand, SOMO profile of AlC-h-SiC locatedmainly on the doped area (unlike AlSi-h-SiC in which SOMO profilewas located in the edge of the sheet). As a result, the HOMO of NH3

which is located on N atom donates electrons preferentially to theSOMO centered on the AlC site (Fig. 6). In short, for the pristine anddoped sheet systems, the obtained acidity diminishes in the seriesAlC⪢AlSi4BC4BSi⪢pristine h-SiC.

The nature of the sheet's DOS near the Fermi level is critical tothe understanding of electrical transport through these materials.Therefore, we have drawn DOS plots for the Al-doped sheet withand without NH3. Similar to boron doping, calculated DOS of AlC

Fig. 5. Structure of optimized (R) NH3/AlC doped h-SiC, (S) NH3/AlSi doped h-SiC and their density of state (DOS) plot. Bonds are in Å.

Fig. 6. SOMO profiles of (a) AlC doped h-SiC, (b) AlSi doped h-SiC.

A. Ahmadi Peyghan et al. / Physica B 443 (2014) 54–5958

and AlSi doped sheet is shown in Fig. 4, indicating that valence andconduction levels changed significantly and their Eg value changedto 0.51 and 0.59 eV, respectively, which resulted in reducedconductivity. By referring to Fig. 5 R (as the most stable config-uration), HOMO drastically move to lower energies so that Eg ofthe AlC-h-SiC decreased from 0.51 to 0.35 eV (by about 31.3%change). However, in configuration S both conduction and valencelevels of AlSi-h-SiC remain almost constant and Eg was slightlydecreased from 0.59 to 0.57 eV after NH3 adsorption. This changein electronic properties is negligible, indicating that the electronicproperties of AlSi-h-SiC are insensitive to the NH3 molecule.

4. Conclusion

Surface acidity is an important property that is frequently usedto characterize the reactivity of surfaces. The adsorption ofammonia on the SiC nanosheet was investigated using densityfunctional theory calculations. Interest in ammonia adsorption isaroused because of its use as a probe molecule in determining theacidity of surfaces experimentally. The objective of this study wasto understand the modification of surface acidity via the boron andaluminium dopings at the fundamental level. Relative magneticorder of the acidity for different surfaces was found to be: AlC(Ead¼86.4 kcal/mol)⪢AlSi (Ead¼39.2 kcal/mol)4BC (Ead¼30.9kcal/mol)4BSi (Ead¼12.3 kcal/mol)⪢pristine h-SiC (Ead¼3.5 kcal/mol), indicating that the acidity of h-SiC can be controlled bydoping of B and Al atoms.

References

[1] S. Iijima, Nature 354 (1991) 56.[2] L. Pan, W. Chen, Q. Sun, X. Hu, F. Wang, Y. Jia, Physica B 406 (2011) 2772.

[3] G. Yu, G. Tang, Y. Jia, L. Wu, Physica B 407 (2012) 813.[4] A. Loiseau, F. Willaime, N. Demoncy, N. Schramcheko, G. Hug, C. Colliex,

H. Pascard, Carbon 36 (1998) 743.[5] T. Taguchi, N. Igawa, H Yamamoto, S. Jitsukawa, J. Am. Ceram. Soc. 88 (2005) 459.[6] V. Tondare, C. Balasubramanian, S. Shende, D. Joag, V. Godbole, S. Bhoraskar,

M. Bhadhade, Appl. Phys. Lett. 80 (2002) 4813.[7] Y.T. Yang, R.X. Ding, J.X. Song, Physica B 406 (2011) 216.[8] V.E. Chelmokov, A.L. Syrkin, Mater. Sci. Eng., B 46 (1997) 248.[9] G. Mpourmpakis, G.E. Froudakis, G.P. Lithoxoos, J. Samios, Nano Lett. 6 (2006) 1581.[10] C. Pham-Huu, N. Keller, G. Ehret, M.J. Ledoux, J. Catal. 200 (2001) 400.[11] X.H. Sun, C.P. Li, W.K. Wong, N.B. Wong, J. Am. Chem. Soc. 124 (2002) 14464.[12] L. Wang, Appl. Surf. Sci. 258 (2012) 6688.[13] A. Ansari, S. Rouhi, M. Mirnezhad, M. Aryayi, Physica E 53 (2013) 22.[14] M. Menon, E. Richter, A. Mavrandonakis, G.E. Froudakis, A.N. Andriotis, Phys.

Rev. B: Condens. Matter 69 (2004) 115322.[15] B. Baumeier, P. Krüger, J. Pollmann, Phys. Rev. B: Condens. Matter 76 (2007)

085407.[16] K.M. Alam, A.K. Ray, Nanotechnology 18 (2007) 495706.[17] K.M. Alam, A.K. Ray, Phys. Rev. B: Condens. Matter 77 (2008) 035436.[18] I.J. Wu, G.Y. Guo, Phys. Rev. B: Condens. Matter 76 (2007) 035343.[19] M.N. Huda, Y. Yan, M.M. Al-Jassim, Chem. Phys. Lett. 479 (2009) 255.[20] E.W. Wong, P.E. Sheenhan, C.M. Lieber, Science 277 (1997) 1971.[21] L. Wang, Appl. Surf. Sci. 258 (2012) 6688.[22] H. Yao, Y. Chen, Y. Wei, Z. Zhao, Z. Liu, C. Xu, Surf. Sci. 606 (2012) 1739.[23] A. Joshi, A. Rammohan, Y. Jiang, S. Ogunwumi, J. Mol. Struct. THEOCHEM 329

(2009) 36.[24] I. Onal, S. Soyer, S. Senkan, Surf. Sci. 600 (2006) 2457.[25] M. Elanany, M. Koyama, M. Kubo, E. Broclawik, A. Miyamoto, Appl. Surf. Sci.

246 (2005) 96.[26] S. Grimme, J. Comput. Chem. 25 (2004) 1463.[27] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen,

S. Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis,J.A. Montgomery, J. Comput. Chem. 14 (1993) 1347.

[28] N. O’Boyle, A. Tenderholt, K. Langner, J. Comput. Chem. 29 (2008) 839.[29] A.A. Peyghan, M. Noei, S. Yourdkhani, Superlattices Microstruct. 59 (2013) 115.[30] R. Wanbayor, V. Ruangpornvisuti, Appl. Surf. Sci. 258 (2012) 3298.[31] F.J. Owens, Mater. Lett. 61 (2007) 1997.[32] V.M. de Menezes, S.B. Fagan, I. Zanella, R. Mota, Microelectron. J. 40 (2009) 877.[33] S.F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553.

A. Ahmadi Peyghan et al. / Physica B 443 (2014) 54–59 59