ORIGINAL PAPER

A periodic density functional theory study of tetrazole adsorptionon anatase surfaces: potential application of tetrazole ringsin dye-sensitized solar cells

Alireza Najafi Chermahini & Behzad Hosseinzadeh &

Alireza Salimi Beni & Abbas Teimouri &Mahmood Moradi

Received: 3 October 2013 /Accepted: 13 November 2013 /Published online: 13 February 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract A density functional theory (DFT) method (period-ic DMol3) with full geometry optimization was used to studythe adsorption of tautomeric forms of tetrazole on anataseTiO2 (101), (100), and (001) surfaces. It was found that theadsorption of tetrazole on the TiO2 surfaces does not proceedvia a dissociative process, and negative shifts in the Fermilevel of TiO2 were noted upon N-containing heterocycle ad-sorption. The configuration of the tetrazole during adsorptionand the corresponding adsorption energies on different sur-faces and sites were estimated. In addition, it was found thattetrazole may be adsorbed on TiO2 surfaces through an inter-action between a cation on the surface and a lone pair on theN1 or N2 atom of the tetrazole molecule. The results indicatethat the adsorption of tetrazole through the N2 position (lead-ing to the 1H tautomer) on an anatase TiO2 surface is favoredover adsorption through the N1 position. In addition, it wasobserved that the photocatalytic activity of tetrazole-dopedTiO2 is higher than that of a pure anatase TiO2 surface.

Keywords Anatase titanium dioxide . Tetrazole . Densityfunctional theory . Adsorption . Dye-sensitized solar cells

Introduction

Titanium dioxide (TiO2) has been extensively studied formany years as a model metal oxide with a wide range ofapplications in catalysis, photochemistry, and electrochemis-try [1]. TiO2 has excellent properties, such as chemical inert-ness, regeneration ability, thermal stability, and low cost,which have made it a key candidate for use in environmentalremediation.

The most stable polymorph of TiO2 is rutile [2], but anataseand brookite are also common, especially in nanoscale naturaland synthetic samples. Anatase is particularly attractive be-cause of its catalytic and photocatalytic activities [1, 3, 4].However, it is difficult to obtain good-quality anatase samplesbecause the surface properties and reactivity of this materialare not well understand. Typically, the anatase phase containsequal amounts of its two main faces, (101) and (100)/(010),although (001) is also present [5, 6]. Parkinson and co-workers have reported that the sensitization efficiency of the(101) face is approximately an order of magnitude higher thanthat of the (001) face [7]. On the other hand, it has beensuggested that the minority (001) surface is more reactive thanthe other two surfaces; for this reason, the minority surfaceplays a key role in the reactivity of anatase nanoparticles [6, 8,9]. There are many well-known industrial applications ofanatase TiO2 nanocrystals, for instance in dye-sensitized solarcells [10], as photocatalysts [11], and as the support forheterogeneous catalysts [12]. The drawback of using TiO2 asa photocatalyst is its poor utilization of the energy fromsunlight to drive the photocatalytic reaction. Several attemptsto enhance the photocatalytic activity of TiO2 in visible light

A. N. Chermahini (*)Department of Chemistry, Isfahan University of Technology,Isfahan 84156-83111, Irane-mail: anajafi@cc.iut.ac.ir

B. Hosseinzadeh :A. S. Beni (*)Department of Chemistry, Faculty of Science, Yasouj University,Yasouj 75918-74831, Irane-mail: salimibeni@yu.ac.ir

A. TeimouriChemistry Department, Payame Noor University,19395-4697 Tehran, Iran

M. MoradiDepartment of Physics, Faculty of Science, Yasouj University,Yasouj 75918-74831, Iran

J Mol Model (2014) 20:2086DOI 10.1007/s00894-014-2086-y

have been reported, for example by modifying the TiO2 bandgap through the induction of oxygen vacancies [12], by re-placing oxygen sites with nitrogen ones [13], by substitutingTi sites with Cr, Fe, or Yb sites [14, 15], and by adsorbingorganic dye molecules that act as sensitizers on the surface ofthe TiO2 [16, 17]. The former approach is mainly applied inphotocatalysts, and the latter in dye-sensitized solar cells[18–24].

There have been many experimental and theoretical stud-ies of the adsorption of small molecules on various surfaceplanes of different TiO2 phases. In recent years, many theo-retical researchers have investigated related aspects of thisprocess, such as the adsorption of H2O and NH3 on anataseTiO2 (101) and (001) surfaces using cluster models anddensity functional theory (DFT) [25], the oxidation of COon a stoichiometric anatase TiO2 (001) surface using ab initiocalculations [26], the adsorption of H2O and O2 on the ana-tase TiO2 (100) surface [27], and the adsorption of H2O on theanatase TiO2 (100), (010), and (001) surfaces using a semi-empirical MINDO (semiempirical molecular orbital) meth-od [28]. In a more recent study, Wanbayor investigated theadsorption of CO, H2, N2O, NH3, and CH4 gases on anataseTiO2 (001) and (101) surfaces by performing two-dimensional periodic slab model DFT calculations [29]. In

addition, the adsorption of caffeic acid [30], thiophen [31],phosphonic acid [32], and formaldehyde [33] on TiO2 sur-faces has been investigated.

Tetrazole (CN4H2) and its derivatives are exceedingly im-portant compounds due to their roles in various industrial andmedical applications [34]. In medicine, the tetrazole ring ispresent in a wide range of drugs, as it acts as a powerfulsubstituent that is isosteric with the carboxylic group, –CO2H–, and has the advantage of being more metabolicallystable than the latter [35–38]. Tetrazoles are also applied asligands in coordination chemistry [39–41], and as explosives

Fig. 1 Geometry-optimized structures of anatase TiO2: a (101) surface, b (100) surface, and c (001) surface

Table 1 Geometric and energetic parameters of the bare surfaces (hkl)before (Eunrl) and after (Erel) relaxation

(hkl)

(001) (100) (101)

K-points sampled 3× 3×1 3×2×1 2×3×1

Surface area (Å2) 57.03 71.64 77.11

Eunrl (Ha m−2) 24006.3076 24006.08 24006.13

Erel (Ha m−2) 24006.3085 24006.39 24006.43

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and rocket propellants [42–44]. Another important applicationof tetrazoles is in the preparation of imidoylazides as a sourceof nitrenes, which are used as reactive intermediates in organicsynthesis [45, 46]. In line with our ongoing program to studyvarious aspects of tetrazole compounds [46–50], in the presentpaper we present the results of calculations we performed togain a deeper understanding of the adsorption of tetrazoletautomers on anatase TiO2 (101), (100), and (001) surfaces,and the effect of this adsorption on the structure and electronicproperties of TiO2. These calculations utilized a periodic slabmodel and a density functional theory (DFT) approach. Sincea tetrazole ring is similar to a carboxylic acid, the ultimate goalof the present work was to assess the potential applicability oftetrazole when incorporated into the structures of dyes used indye-sensitized solar cells.

Computational details

All DFT calculations were performed with the DMol3 pro-gram package in Materials Studio (Accelrys, Inc., San Diego,CA, USA), which was run on personal computers. In theDMol3 method [51–53], physical wavefunctions are expandedin terms of accurate numerical basis sets. A double-numericalquality basis set with polarization functions (DNP) was used.It should be noted that the DNP basis set included a double-zeta quality basis set that added a p-type and d-type polariza-tion function to hydrogen and heavier atoms, respectively.This is comparable to the 6-31G** Gaussian basis set [54],but DNP is more accurate than a Gaussian basis set of thesame size [55, 56]. The Perdew–Wang 91 (PW91) form wasused for the generalized gradient approximation (GGA) forthe exchange and correlation potential [57]. To improve com-putational performance, a Fermi smearing of 0.005 hartrees (1hartree = 27.2114 eV) and a global orbital cutoff of 4.5 Åwereemployed. The tolerances of the energy, gradient, and dis-placement convergences were 2×10−5 hartrees, 4×10−3

hartrees Å−1, and 5×10−3 Å, respectively. The electronicself-consistent field was converged to 1×10−5 eV. As illus-trated in Fig. 1, periodic surface slabs with a thickness of sixstoichiometric layers and 20-Å vacuum regions between theslabs were used for the anatase TiO2 (101), (100), and (001)surfaces. The molecules were adsorbed on just one side of theslab, while each slab contained 24 Ti atoms and 48 O atoms.The k-points sampled were 2×3×1, 3×2×1, and 3×3×1 forthe (101), (100), and (001) surfaces, respectively. The outer-most atoms on the anatase surfaces were twofold (O2c) andthreefold (O3c) coordinated oxygens and fivefold (Ti5c) coor-dinated titanium. In addition, the (101) surface had outermostsixfold (Ti6c) coordinated titanium atoms (Fig. 1a).

All of the atoms in the molecule and the surface wereallowed to relax during the optimization process. The opti-mized lattice parameters for the anatase TiO2 crystal were a=

Table 2 Adsorption energies, Ti–N bond distances, Fermi energies, andFermi energy shifts (Δνf) upon the adsorption of tetrazole at the anataseTiO2 surfaces

Adsorbate Ead (kcal mol−1) Ti–N distance(Å)

Fermienergy(eV)

Δνf

(101) surface

Clean surface 5.4211

1H-tetrazole −27.39 2.34 5.3666 −0.05452H-tetrazole −26.73 2.32 5.5285 0.1074

(100) surface

Clean surface 5.4783

1H-tetrazole −25.74 2.35 5.4440 −0.03432H-tetrazole −23.07 2.34 5. 5511 0.0728

(001) surface

Clean surface 5.4001

1H-tetrazole −51.83 2.29 5.2839 −0.11622H-tetrazole −37.98 2.28 5.5272 0.1271

Table 3 Bond lengths (in Å) intetrazole and for different adsorp-tion modes of tetrazole at variousanatase surfaces

Structure N1–H1 N1–N2 N2–N3 N3–N4 N4–C5 N1–C5 C5–H2

Surface (101)

1H-tetrazole 1.049 1.347 1.306 1.346 1.330 1.339 1.082

2H-tetrazole 1.045 1.335 1.318 1.324 1.347 1.337 1.082

Surface (100)

1H-tetrazole 1.032 1.343 1.303 1.351 1.330 1.340 1.082

2H-tetrazole 1.030 1.330 1.319 1.322 1.350 1.334 1.082

Surface (001)

1H-tetrazole 1.099 1.341 1.314 1.339 1.338 1.334 1.083

2H-tetrazole 1.065 1.333 1.311 1.331 1.345 1.338 1.081

Free molecule

1H-tetrazole 1.014 1.356 1.297 1.367 1.318 1.348 1.082

2H-tetrazole 1.015 1.329 1.334 1.315 1.357 1.333 1.081

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3.776, b=3.776, c=9.486, which are in close agreement withthe corresponding experimental values: a=b =3.785 Å, andc=9.514 Å [58].

The adsorption energy (ΔEads) for the adsorption of themolecule on the clean surface was calculated as follows:

ΔEads ¼ Eadsorbate=surface� Eadsorbate þ Esurfaceð Þ; ð1Þ

where Eadsorbate/surface is the energy of the adsorbate on theTiO2 surface and Eadsorbate and Esurface are the energies of anisolated molecule of adsorbate and the clean TiO2 surface,respectively.

Results and discussion

N-unsubstituted tetrazoles can exist in two tautomeric forms,the 1H and 2H isomers, which are in equilibrium in the gasphase and solution. It has been reported that the 2H tautomeris the predominant form in the gas phase [47]. The geometriesand energetic parameters of the bare (001), (100), and (100)surfaces of TiO2 before and after relaxation are presented inTable 1.

The calculated relaxed surface energies were 24006.43,24006.39, and 24006.31 hartrees m−2 for the (101), (100),and (001) surfaces, respectively (see Table 1). Thus, thestability increases in the following order: (001) < (100) <(101), which confirms the results reported in other theoreticalstudies [20, 29]. In addition, the adsorption energies, Ti–Ndistances (in Å), Fermi energies, and the Fermi energy shiftsupon the adsorption of tetrazole at the anatase TiO2 surfacesconsidered in this work are compiled in Table 2.

Moreover, the geometric parameters of the free tetrazoletautomers and those adsorbed on the different surfaces aretabulated in Table 3.

Adsorption of tetrazole tautomers at the (101) surface

We first investigated the adsorption of tetrazole tautomers onthe anatase (101) surface. A closer look at Fig. 2 indicates thatmolecular adsorption occurs via an N atomwith a lone pair—either N2 or N1—on the unsaturated fivefold-coordinatedtitanium (Ti5c) atom (Fig. 2a and b) for the 1H and 2Htautomers, respectively.

In addition, it was observed that the adsorption process isnot a dissociative process. Moreover, the Ti–N bond length ofthe adsorbed 2H-tetrazole listed in Table 2 is slightly shorterthan that of the 1H-tetrazole. The adsorption energy values forthe 1H and 2H isomers were found to be 27.39 and26.73 kcal mol−1, respectively. It is clear that there is littledifference in adsorption energy between the adsorption of 1H-tetrazole and that of 2H-tetrazole on the (101) surface, al-though the values indicate that the bond between 1H-tetrazoleand the TiO2 (101) surface is slightly stronger than that be-tween 2H-tetrazole and the TiO2 (101) surface. From Table 3,it is evident that tetrazole adsorption induces a substantialchange in the geometry of the tetrazole ring. For example, forthe 1H and 2H-tetrazole tautomers after adsorption, the N–Hbond lengths decreased from 1.049 and 1.045 to 1.014 and1.015 Å, respectively. In addition, the N1–N2 bond lengthsincreased from 1.347 and 1.335 to 1.356 and 1.329 Å for the1H and 2H isomers, respectively.

Figure 3 shows the density of states of pure anatase TiO2

(101) and those of each adsorbate–surface structure. It is clearthat the valence band of pure TiO2 consists mainly of the 2p

Fig. 2 Geometry-optimizedstructures of a 1H-tetrazoleadsorbed on an anatase TiO2

(101) surface, and b 2H-tetrazoleadsorbed on an anatase TiO2(101)surface

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and 2s states of O and the 3d states of Ti. In the uppermostvalence band, the O2p states are predominantly found be-tween −5 and 0 eV, while the O2s states are found in the rangefrom −18.10 to −15.52 eV. The Ti3d states gave rise to somebands in the energy range from −5.10 to −3.14 eV.

The lowest conduction band is dominated by Ti3d states.Meanwhile, hybridization between the Ti3d and O2p levels inthe valence band can be observed, which agrees well withpreviously calculated results [32]. The total densities of states(TDOSs) of TiO2 doped with different amounts of tetrazoleare shown in Fig. 3. In that figure, the vertical dotted line at0 eV is the Fermi level and the other dotted line is set to thelocation of the conduction band for comparison. It can be seenthat the valence band does not undergo any obvious shift uponadsorption. However, the conduction band shifts to a lowerenergy level, which causes the band gap to narrow. In

addition, the valence and conduction bands broaden whentetrazole is incorporated into the TiO2 surface, which increasesthe mobility of the photogenerated hole–electron pairs. Thehigher the mobility of these photogenerated carriers (includingholes and electrons), the better the performance of thephotocatalyst. As a result, the photocatalytic activity oftetrazole-doped TiO2 is higher than that of pure anataseTiO2. As can be seen from Fig. 4, the peak at −17.0 eV inthe PDOS derives from the 2s orbitals of O along with a smallcontribution from the 3d orbitals of Ti. It is also important tonote that the top of the valence band is dominated by the O2porbitals.

On the other hand, the conduction band contains a signif-icant contribution from the Ti3d orbitals along with a smallcontribution fromO2p states [25, 26]. The N2s and N2p stateshybridize with O2p states in the energy regions from −18.23

Fig. 3 Calculated DOSs of anatase TiO2 (101) surfaces: a bare, b with 1H-tetrazole adsorbed, c with 2H-tetrazole adsorbed

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to −14.96 eVand from −4.89 to −0.54 eV in the valence band.The valence bands are broadened through the hybridization ofO2p and N2s and N2p states. This hybridization also inducessubstantial dispersion in the valence band, which can

influence the mobility of photogenerated holes. Therefore,photogenerated holes should be more mobile in N-dopedTiO2 than in pure TiO2, which enhances photocatalytic activ-ity. As can be seen from Fig. 5a and b, the Ti3d states shifts to

Fig. 4 Plot of partial density of states for undoped TiO2

Fig. 5 Geometry-optimizedstructures of a 1H-tetrazoleadsorbed on an anatase (100)surface and b 2H-tetrazoleadsorbed on an anatase (100)surface

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lower energy after the tetrazole ring is adsorbed onto the Ti inthe TiO2 surface, causing the band gap to narrow.

According to Table 2, the adsorption of a tetrazole ring viathe 1H isomer shifts the Fermi energy of TiO2 to a more

negative value, but this shift is to a more positive value for2H-tetrazole. The Fermi energy shifts are −0.0545 eV and0.1074 eV for the 1H-tetrazole and 2H-tetrazole rings,respectively.

Fig. 6 Calculated DOSs of anatase (100) surfaces: a bare, b with 1H-tetrazole adsorbed, c with 2H-tetrazole adsorbed

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The calculated densities of states (DOSs) for the baresurface and for adsorbate–surface structures are depicted inFig. 3. The DOS of the bare anatase (101) surface is consistentwith previous works that focused on the same surface [24] aswell as bulk anatase [25, 26]. The conduction bands are splitinto two components, t2g (<2.5 eV of bare TiO2) and eg(>2.5 eV of bare TiO2) orbitals, due to crystal field effects[25, 26].

Adsorption at the (100) surface

Figure 5 illustrates the optimized geometries of tetrazoleadsorbed onto an anatase (100) surface. As can be seen fromTable 2, the (100) surface exhibits the same trends in therelative Ead and in Ti–N bond distances in N-containingheterocycles [34]. Figure 6 shows the DOSs for a bare (100)surface of anatase and adsorbate–surface structures. This fig-ure shows that an absence of tetrazole does not affect themagnitude of the band gap, but it does induce a new state inthe valance band. For this reason, the optical and electronicproperties of the bare surface and the doped surface are thesame. The conduction of 2H-tetrazole (Fig. 6c) was found tobe greater than that of 1H-tetrazole (Fig. 6b) because of thehybridization of various orbitals in its valance band.

Adsorption at the (001) surface

Figure 7 shows the optimized geometries of N-containingheterocycles adsorbed at an anatase (001) surface. Unlikeadsorption at the (101) and (100) surfaces, in this case all ofthe adsorbates are rotated azimuthally upon geometryoptimization.

Moreover, the top TiO2 layers were drastically displaced.This seems to be because the (001) surface is less stable thanthe (101) and (100) surfaces. The trends in the relative Ead andTi–N bond distances for tetrazole rings adsorbed at the (001)surface (Table 2) differed from the corresponding results forthe (101) and (100) surfaces. Figure 8 shows the calculatedDOSs for a bare (001) surface and adsorbate–surface struc-tures. The shape of the DOS for the (001) surface also differedfrom those for the (101) (Fig. 3) and (100) (Fig. 5) surfaces forboth bare and heterocycle-adsorbed structures. In Fig. 8b, theabsence of 1H-tetrazole causes the band gap to increase byabout 0.74 eV, which decreases the conductivity.

Conclusions

We calculated the DOSs of TiO2 surfaces in the absence andpresence of tetrazole in its 1H and 2H tautomeric forms.When

Fig. 7 Geometry-optimizedstructures of a 1H-tetrazoleadsorbed at an anatase (001)surface and b 2H-tetrazoleadsorbed at an anatase (001)surface

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1H-tetrazole is adsorbed at a (101) or (001) surface, the bandgap increases, but when it is adsorbed at a (100) surface thereis no change in the band gap energy as compared to the

bare surface. The band gap energies for 2H-tetrazole adsorbedat the (100) and (001) surfaces are similar to that for thebare TiO2.

Fig. 8 Calculated DOSs of anatase (001) surfaces: a bare, b with 1H-tetrazole adsorbed, c with 2H-tetrazole adsorbed

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Acknowledgments This work was carried out with financial supportfrom Yasouj University and Isfahan University of Technology.

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