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Apr 28, 2015 - −0.7 eV as calculated with the PBE functional) to CSA copper sites on ... Its binding energies on CSA sites range from −1.7 to −2...
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DFT Study of Adsorption of Benzotriazole on CuO Surfaces Anton Kokalj, and Sebastijan Peljhan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01677 • Publication Date (Web): 28 Apr 2015 Downloaded from http://pubs.acs.org on May 6, 2015

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DFT Study of Adsorption of Benzotriazole on Cu2O Surfaces Anton Kokalj∗ and Sebastijan Peljhan† Department of Physical and Organic Chemistry Joˇzef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia April 26, 2015 Abstract The adsorption of benzotriazole—an outstanding corrosion inhibitor—was characterized on Cu2 O surfaces using density-functional-theory calculations. Cu2 O surfaces contain two distinct Cu sites, coordinatively-saturated (CSA) and coordinatively-unsaturated (CUS). The following surfaces were considered: (i) Cu2 O(111), (ii) Cu2 O(111)-w/o-CuCUS which lacks the CUS copper sites, and (iii) Cu2 O(110):CuO which is a CuO terminated Cu2 O(110). Neutral benzotriazole (BTAH) binds weakly, by about −0.4 and −0.7 eV as calculated with PBE functional, to CSA copper sites on Cu2 O(111)-w/o-CuCUS and Cu2 O(110):CuO, respectively. These binding energies are comparable to those obtained on metallic Cu(111) and Cu(110). In contrast, BTAH binds rather strongly to CUS copper sites by about −1.5 eV, which is even stronger as the bonding to very low coordinated surface defects on metallic Cu surfaces. Indeed, this bonding is so strong that it overcompensates a thermodynamic deficiency of stoichiometric Cu2 O(111). Hence, the BTAH covered Cu2 O(111) is thermodynamically more stable than the BTAH covered Cu2 O(111)-w/o-CuCUS . Similarly as reported previously for metallic Cu surfaces, also on Cu2 O surfaces a deprotonated benzotriazole bonds much stronger than the neutral BTAH. Its binding energies on CSA sites range from −1.7 to −2.0 eV (depending on details), while on CUS sites they are about −2.8 eV (as measured with respect to BTA⊙ radical in the gas phase). Despite this relatively strong bonding, dissociation of BTAH (N–H bond cleavage) is slightly endothermic on Cu2 O surfaces. The impact of van der Waals dispersion interactions on adsorption bonding was also addressed and their main effect is to strengthen the molecule–surface bonding on average by about 0.4 eV, but otherwise the relative stability trends among various adsorption forms largely remain the same. The observation that benzotriazole binds much stronger to coordinatively unsaturated copper sites on oxidized surfaces and to under-coordinated defects on metallic Cu surfaces (compared to regular sites) may indicate its ability to passivate reactive surface sites.

We have recently characterized in detail the adsorption of benzotriazole on oxide-free surfaces of copper by means of density-functional-theory (DFT) calculations. 5–9 Oxidefree surfaces of copper are more relevant at acidic pH, but in other conditions the copper surfaces are oxidized 10 (e.g., the oxidation of Cu to Cu2 O can be described by the following potential–pH dependence, 11 UCu2 O/Cu = 0.461 − 0.0591·pH, in V vs SHE (standard hydrogen electrode)). It has been often proposed that Cu2 O has an important role, acting as a substrate for efficient adsorption of benzotriazole on copper surfaces. The Cu/Cu2 O/Cu(I)BTA multilayer structure is the result of the adsorption of benzotriazole on Cu immersed in aqueous solution of NaCl, and the Cu(I)BTA complex was claimed to be polymeric, 2,4,12,13 although recently a dimer Cu(BTA)2 structure was proposed. 14–16

∗ Corresponding

23;

Fax:

Author: Anton Kokalj, Tel: +386-1-477-35+386-1-251-93-85, E-mail: [email protected], URL:

http://www-k3.ijs.si/kokalj/ † Current address: BIA Separations, Mirce 21, SI-5270 Ajdovˇ sˇ cina

1

Introduction Though copper is a widely used noble metal, its surface is prone to corrosion in various water solutions and to oxidation in air. Benzotriazole (BTAH) has been used as a corrosion inhibitor for copper and its alloys for a long time. 1–3 There is a number of reports—see, e.g., a recent review of Finˇsgar and Miloˇsev 4 and references therein— concerning the nature of the BTAH inhibiting behavior. Nevertheless, the structure of the adsorbed BTAH molecules still comes up for discussion and it is not completely resolved.

In a recent study of our group, 17 the influence of the benzotriazole on the thickness of the Cu2 O oxide layer formed on Cu immersed in 3 % NaCl solution was estimated by 1

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obtained (experimental value is 112 GPa). 32 2.2

most stable surface at given conditions (T, p) is the one with the lowest γsurf (T, p). It has been shown that to a first approximation total energies can be used to represent Gibbs free energies of solids in eq (1). 20,36 Hence γsurf (T, p) can be approximated by:

Surface structures and free energies

Cu2 O surfaces, considered in this work, are shown in Figure 1. The Cu and O sites of Cu2 O(111) are labeled as CuCSA , CuCUS , Oup , and Odn . The first two labels were already defined above, whereas the Oup and Odn stand for the O ions located above and below the plane of surface Cu ions (hence the superscripts “up” and “dn”). For the Cu2 O(111)-w/o-CuCUS , the O ions below the CuCUS vacancies are labeled as Osub (“sub” stands for subsurface). The surface CuO layer of Cu2 O(110):CuO consists of Cusurf and Osurf sites (“surf” stands for surface; for non-relaxed ideal bulk-cut structure both sites are located at the same height on the surface plane). The Cusurf sites are coordinatively saturated. Although non-stoichiometric Cu2 O surfaces may display sizable magnetic moment (e.g., about 1 µB per CuCUS vacancy), 35 several test calculations revealed that the effect of magnetism on the total energy is marginal (about 30 meV per CuCUS vacancy); furthermore in adsorption calculations even this small effect largely cancels out, hence all the presented results refer to spin unpolarized slab calculations. Surface free energies were calculated with symmetric (1 × 1)–slabs that have both surfaces equivalent. The inplane lattice spacing was fixed to the calculated equilibrium bulk lattice parameter, while other degrees of freedom were relaxed. The BZ integrations were performed using 4 × 4 × 1 and 6 × 4 × 1 uniformly shifted k-meshes for (1×1)–Cu2 O(111) and (1×1)–Cu2 O(110) type structures, respectively. The Cu2 O(111) type surfaces were calculated with symmetric slabs of different thicknesses, ranging from 7 to 13 O–Cu–O trilayers (the structure of O–Cu–O trilayer is evident from the left inset of Figure 1), whereas Cu2 O(110):CuO was described with slabs ranging from 6 to 12 CuO–Cu bilayers (the structure of CuO–Cu bilayer can be seen on the right inset of Figure 1) plus an extra CuO layer on the bottom side of the slab (hence the name Cu2 O(110):CuO). Stabilities of different Cu2 O surfaces were evaluated using ab initio thermodynamic approach, 36 by considering the surface in ambient of oxygen atmosphere, which acts as an oxygen reservoir. Because surfaces are modeled with symmetric slabs having two equivalent surfaces, the surface free energy at given (T, p) can be written as:

γsurf (T, p) ≈ where

NCu ) and 2 ≈ 2µCu (T, p) + µO (T, p).

stoich ∆NO = (NO − bulk ECu 2O

(2b) (2c)

bulk The Eslab and ECu are total energies of the slab and of 2O stoich the formula unit of Cu2 O bulk, respectively. The ∆NO is the number of excess O atoms, i.e., this number accounts for the non-stoichiometry of the slab: it is positive if the ratio Cu:O < 2, negative if Cu:O > 2, and zero for stoichiometric slabs. For a given surface, the surface free energy was estimated by fitting the equation: stoich Eslab (NL ) = 2Aγsurf + NL EL + ∆NO µO

(3)

for several large enough values of NL (as specified above), where NL is the number of either O–Cu–O trilayers for (111) slabs or CuO–Cu bilayers for (110) slabs, and EL is the total energy of either trilayer or bilayer. When plotting the γsurf (T, p) as a function of oxygen chemical potential (µO ), a viable range of µO should be considered. The oxygen poor limit (Olean ) can be defined as a point where the bulk Cu2 O decomposes into bulk Cu and O2 gas, whereas at oxygen rich limit (Orich ) oxygen gas condenses on the surface. The Olean and Orich limits are hence chosen according to eqs (4a) and (4b), respectively. 20,36 bulk bulk bulk µmin and µmax O = ECu2 O − ECu Cu = ECu , 1 1 bulk 1 µmax = EO2 and µmin Cu = [ECu2 O − EO2 ], O 2 2 2

(4a) (4b)

bulk where ECu and EO2 are total energies of Cu atom in the Cu-bulk and isolated O2 molecule, respectively. Half the total energy of O2 molecule is chosen as the zero reference for µO (p, T ), i.e., ∆µO = µO − 12 EO2 , hence at Olean limit the ∆µO = −1.27 eV and at Orich limit the ∆µO = 0 eV.

2.3 1 γsurf (T, p) = [Gslab (T, p, NCu , NO ) 2A − NCu µCu (T, p) − NO µO (T, p)],

1 NCu bulk stoich [Eslab − ECu2 O − ∆NO µO (T, p)], 2A 2 (2a)

Adsorption calculations

Molecules were adsorbed on the top side of the slab and the thickness of the vacuum region—the distance between the top of the ad-molecule and the adjacent slab—was set to about 20 ˚ A. Dipole correction of Bengtsson 37 was applied to cancel an artificial electric field that develops along the direction normal to the slab due to periodic boundary conditions imposed on the electrostatic potential.

(1)

where A is the area spanned by a supercell, Gslab is the Gibbs free energy of the slab, µCu and µO are the chemical potentials of copper and oxygen, while NCu and NO are the number of Cu and O atoms. Thermodynamically the 3

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TABLE 3: Adsorption energies (Eads and εtot ads ) for high-coverage structures of BTAH(ads) ; the values corrected with lateral dispersion correction are stated in parentheses. The reported lateral molecule–molecule dispersion interactions (Edisp ) are lateral calculated with the original PBE-D functional (the PBE-D’ calculated Edisp values are about 30% smaller in magnitude). lateral lateral The reported dispersion corrected Eads values are calculated as Eads (PBE-D′ ) − Edisp (PBE-D′ ) + Edisp (PBE-D); εtot ads values are corrected analogously. lateral coverage adsorption mode shown in Eads εtot Edisp ads (10−3 ˚ A−2 ) Figure (eV/molecule) (meV/˚ A2 ) (eV/molecule) Cu2 O(111) 30.5 N2@CUS +H1· · · Oup 6a −1.68a (−2.11a ) −51.2a (−64.4a ) (−0.12) Cu2 O(111)-w/o-CuCUS 30.5 N2@CUS +H1· · · Osub 6b −0.33 (−0.94) −10.1 (−28.7) (−0.14) Cu2 O(110):CuO 24.9 N2+H1· · · O 6c −0.63 (−1.01) −15.7 (−25.1) (−0.08) a With respect to high symmetry Cu O(111) substrate of Figure 1a. In contrast, the CUS binding energies in Tables 1 and 2 2 are calculated with respect to low symmetry substrate with relaxed CuCUS site.

surface

that the much stronger bonding of BTAH to CuCUS compared to CuCSA overcompensates the thermodynamic deficiency of the bare stoichiometric Cu2 O(111). There is an alternative, though related, way of estimating whether a stronger molecular bonding to an unstable surface site is sufficient to compensate its instability, i.e., by taking into account the energy needed for its formation. For example, the formation energy of the CuCUS site can be calculated as: CUS Eform = ECuCUS @slab − Eslab − µCu ,

would consider molecular adsorption free energy as a function of molecular chemical potential, µmol . Correspondingly, the one dimensional phase diagram of Figure 6d would be replaced by a two dimensional phase diagram, i.e., γsurf = f (µO , µmol ). Such treatment is however beyond the scope of the current paper. 3.4

on

high-

(12) The discussion in the preceding Section 3.3 was based on PBE data. The effect of dispersion correction on the molecule–surface bonding for standalone molecules at low coverage (cf. Section 3.2) is to enhance the bonding by about 0.3 to 0.4 eV. However, for high-coverage phases, such as those considered in Figure 6, the dispersion forces do not affect only the molecule–surface bonding, but also lateral molecule–molecule interactions due to proximity of neighboring molecules. Lateral molecule–molecule disperlateral sion interactions (Edisp ) were estimated by performing a set of single-point calculations as follows:

where ECuCUS @slab and Eslab are total energies of slab with and without the CuCUS atom, respectively, whereas µCu is the chemical potential of Cu atom. Currently we consider a substrate in ambient of oxygen atmosphere and the µCu in range between that of Cu and Cu2 O bulk as given by eqs (4a) and (4b); note that chemical potentials of oxygen min and copper are related, hence the µmax Cu and µCu correspond to oxygen-lean and oxygen-rich limits, respectively. min By plugging the µmax Cu and µCu into eq (12) one obtains the range of formation energies between these two limits. For a single CuCUS per (2 × 2)–Cu2 O(111)-w/o-CuCUS supercell, the CuCUS formation energy is in range between 0.42 and 1.06 eV for oxygen-lean and -reach limits, respectively. If the CuCUS site is to be stabilized by the stronger molecular bonding, then the molecule must bind to it by the corresponding amount stronger than to CuCSA site. form form < Eb,CSA , where Eb,CUS is the This implies that Eb,CUS CUS binding energy corrected by the Cu formation energy: form CUS Eb,CUS = Eb + Eform .

Effect of dispersion interactions coverage BTAH(ads) phases

PBE+D lateral PBE Edisp = Elateral − Elateral ,

(14)

and Elateral

(13)

# " n X 1 Ei,BTAH , EnBTAH − = n i=1

(15)

where PBE+D stands for dispersion corrected PBE functional (either original PBE-D or reparametrized PBE-D’). The EnBTAH is the total energy of the layer of BTAH molecules with structure kept the same as in the adsorption system and n is the number of molecules per supercell. The Ei,BTAH is the total energy of isolated molecule having the same structure as the ith molecule in the layer. The reason that the lateral molecule–molecule dispersion interactions are calculated by eq (14) and not simply by PBE+D Elateral of eq (15) is due to a large permanent molecular PBE+D dipole moment of benzotriazole, because the Elateral contains also the resulting lateral dipole–dipole contributions. PBE These are canceled out by the Elateral term in eq (14).

According to Figure 6d, the BTAH adsorbed to CuCUS site bonds sufficiently strong to stabilize it. The corresponding form Eb,CUS is in range between −1.09 and −0.45 eV, which is indeed more exothermic than the Eb,CSA of −0.39 eV over the whole range. In contrast, for BTA(ads) the ⊙form ⊙ Eb,CUS ∈ [−2.38, −1.74] eV and Eb,CSA = −1.97 eV. ⊙form ⊙ Hence the Eb,CUS is more exothermic than Eb,CSA over the 56% span of the range. The BTA(ads) thus stabilizes the CuCUS site only under more oxygen-lean conditions. It should be mentioned that the current treatment of the stability issue is approximate. A more rigorous treatment 10

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The lateral molecule–molecule dispersion interactions, estimated by eq (14) using the original PBE-D functional (see note 43), are −0.12, −0.14, and −0.08 eV/molecule for the high-coverage structures of Figure 6a,b,c, respectively. Although these values are smaller in magnitude than molecule–surface dispersion interactions, they nevertheless contribute about 20 to 30 % to the overall dispersion enhancement of the adsorption energy. Lateral molecule–molecule dispersion interactions were also found important for high-coverage BTAH phases on metallic Cu surfaces. 44 Table 3 compares the PBE and dispersion corrected high-coverage adsorption energies. Dispersion correction enhances the adsorption energy magnitudes by sizable 0.43, 0.61, and 0.38 eV/molecule for structures of Figure 6a,b,c, respectively. The main effect of dispersion interactions on the phase-diagram of Figure 6d is that they downshift the “BTAH@substrate” lines, but the phase diagram remains dominated by the Only at BTAH@Cu2 O(111) structure of Figure 6a. oxygen-rich limit the BTAH@Cu2 O(111)-w/o-CuCUS and BTAH@Cu2 O(110):CuO structures become competitive. The corresponding dispersion corrected phase-diagram is shown in Figure S1 in the Supporting Information. 4

dispersion interactions, their main effect is to stabilize adsorption structures on average by about 0.4 eV/molecule, but otherwise the relative stability trends among various adsorption forms largely remain the same. A comparison between the current study and previous DFT studies of benzotriazole on metallic Cu surfaces reveals that the bonding of benzotriazole to Cu2 O surface is not too different from that on metallic Cu surfaces, though some differences are significant. While neutral BTAH binds considerably stronger to CuCUS sites of Cu2 O(111) than to metallic Cu surfaces, deprotonated BTA binds significantly weaker to coordinatively saturated Cu sites on Cu2 O surfaces than to metallic Cu surfaces. Only the bonding of BTA to coordinatively unsaturated CuCUS sites is comparable in strength to that on Cu(111), but it is still weaker than the bonding to either more open Cu(hkl) surfaces or under-coordinated surface defects on metallic Cu surfaces. While the behavior of benzotriazole on surfaces of copper has been extensively considered by experimental techniques (e.g., see Review 4), the issue has been considerably less investigated by means of explicit DFT modeling, which can provide a complementary picture. In the context of corrosion inhibition the bonding to both oxide-free and oxidized surfaces is important, yet the bonding of benzotriazole has been considered by DFT mainly on metallic Cu surfaces. The current study may be thus seen as the first step toward the completion of a comprehensive DFT picture of benzotriazole–copper bonding; this picture is incomplete unless also the bonding to oxidized surfaces is scrutinized in sufficient detail. In this respect, the bonding of inhibitor molecules to very thin oxide layers supported on metals is also relevant and should be considered, first because this may correspond to realistic situation and second because it is known that the reactivity of such thin oxide films can be very different from the reactivity of bulk oxide surfaces. 45 Although the inhibitor–surface bonding is not synonymous with corrosion inhibition, it may nevertheless provide some useful information, in particular, because the molecule must adsorb in order to become inhibitor. 46

Conclusions Benzotriazole is an outstanding corrosion inhibitor for copper and its bonding to Cu2 O surfaces—used as a model of oxidized copper—has been characterized by means of DFT calculations. Namely, copper surfaces are usually oxidized unless the pH is sufficiently low. Under such non-acidic pH conditions a significant fraction of benzotriazole exists in deprotonated form, hence the bonding of both, neutral and deprotonated forms, has been currently addressed (for obvious modeling reasons the calculations were performed at solid/vacuum interface). Benzotriazole bonds via triazole nitrogen atoms to Cu sites on surfaces of Cu2 O. We showed that deprotonated (neutral) benzotriazole forms at least (at most) two N–Cu bonds. In addition, neutral BTAH also forms a relatively strong hydrogen bond with the surface O ion, i.e., the N1–H1· · · O bonding. Deprotonated benzotriazole binds considerably stronger to Cu2 O surfaces than the neutral BTAH. The PBE binding energies of the former are in range between −1.7 to −2.0 eV on coordinatively saturated Cu sites and about −2.8 eV on unsaturated CuCUS sites (as measured with respect to isolated BTA⊙ radical), whereas neutral BTAH binds by −0.4, −0.7, and −1.5 eV on coordinatively saturated CuCSA of Cu2 O(111)-w/o-CuCUS , Cusurf of Cu2 O(110):CuO, and coordinatively unsaturated CuCUS sites, respectively. The bonding of benzotriazole to the latter site is so much stronger than to saturated sites that it overcompensates a thermodynamic deficiency of stoichiometric Cu2 O(111), making it more stable than the Cu2 O(111)-w/o-CuCUS . As for the role of van der Waals

Supporting Information Figure S1 and Table S1. Figure S1 shows the stabilityphase diagram corrected with dispersion interactions, whereas Table S1 tabulates the PBE and PBE-D’ calculated adsorption data. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments This work has been supported by the Slovenian Research Agency (Grants No. J1-2240 and P2-0148). The authors thank Dunja Peca for useful discussions and careful reading of the manuscript. 11

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[1] Cotton, J. B.; Scholes, I. R. Benzotriazole and Related Compounds as Corrosion Inhibitors for Copper, Br. Corros. J. 1967, 2, 1–5. [2] Poling, G. W. Reflection Infra-Red Studies of Films Formed by Benzotriazole on Cu, Corros. Sci. 1970, 10, 359–37. [3] Antonijevi´c, M. M.; Petrovi´c, M. B. Copper Corrosion Inhibitors. A Review, Int. J. Electrochem. Sci. 2008, 3, 1–28. [4] Finˇsgar, M.; Miloˇsev, I. Inhibition of Copper Corrosion by 1,2,3-Benzotriazole: A Review, Corros. Sci. 2010, 52, 2737–2749. [5] Kokalj, A.; Peljhan, S. Density Functional Theory Study of ATA, BTAH, and BTAOH as Copper Corrosion Inhibitors: Adsorption onto Cu(111) from Gas Phase, Langmuir 2010, 26, 14582–14593. [6] Kokalj, A.; Peljhan, S.; Finˇsgar, M.; Miloˇsev, I. What Determines the Inhibition Effectiveness of ATA, BTAH, and BTAOH Corrosion Inhibitors on Copper?, J. Am. Chem. Soc. 2010, 132, 16657–16668. [7] Peljhan, S.; Kokalj, A. DFT Study of Gas-Phase Adsorption of Benzotriazole on Cu(111), Cu(100), Cu(110), and Low Coordinated Defects Thereon, Phys. Chem. Chem. Phys. 2011, 13, 20408–20417. [8] Peljhan, S.; Koller, J.; Kokalj, A. The Effect of Surface Geometry of Copper on Adsorption of Benzotriazole and Cl. Part I, J. Phys. Chem. C 2014, 118, 933–943. [9] Kokalj, A.; Peljhan, S.; Koller, J. The Effect of Surface Geometry of Copper on Dehydrogenation of Benzotriazole. Part II, J. Phys. Chem. C 2014, 118, 944–954. [10] Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; NACE, Cebelcor: Houston, Texas, 2nd ed.; 1974. [11] Tromans, D.; Sun, R. Anodic Polarization Behavior of Copper in Aqueous Chloride/Benzotriazole Solutions, J. Electrochem. Soc. 1991, 138, 3235–3244. [12] Cotton, J. B. Control of Surface Reactions on Copper by Means of Organic Reagents. In Proceedings of the 2nd International Congress on Metallic Corrosion; N.A.C.E.: New York, USA, 1963. [13] Morito, N.; Su¨etaka, W. Infrared and Ultraviolet-Visible Reflection Spectra of the Surface Films on Copper Treated with Benzotriazole, J. Jpn. Inst. Metals 1971, 35, 1165– 1170. [14] Grillo, F.; Tee, D. W.; Francis, S. M.; Fruchtl, H.; Richardson, N. V. Initial Stages of Benzotriazole Adsorption on the Cu(111) Surface, Nanoscale 2013, 5, 5269– 5273. [15] Grillo, F.; Tee, D. W.; Francis, S. M.; Fr¨ uchtl, H. A.; Richardson, N. V. Passivation of Copper: Benzotriazole Films on Cu(111), J. Phys. Chem. C 2014, 118, 8667– 8675. [16] Chen, X.; H¨ akkinen, H. Divide and Protect: Passivating Cu(111) by Cu-(benzotriazole)2 , J. Phys. Chem. C 2012, 116, 22346–22349. [17] Finˇsgar, M.; Peljhan, S.; Kokalj, A.; Kovaˇc, J.; Miloˇsev, I. Determination of the Cu2 O Thickness on

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