Benzotriazole Films on Cu(111) - American Chemical Society

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Passivation of Copper: Benzotriazole Films on Cu(111) Federico Grillo,* Daniel W. Tee, Stephen M. Francis, Herbert A. Früchtl, and Neville V. Richardson EaStCHEM and School of Chemistry, University of St. Andrews, St. Andrews, KY16 9ST, U.K. S Supporting Information *

ABSTRACT: Benzotriazole (BTAH) has been used as a copper corrosion inhibitor since the 1950s. However, the molecular level detail of how adsorption and surface passivation occur remains a matter of debate. BTAH adsorption on a Cu(111) single crystal has been investigated from medium coverage to multilayer using scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), high resolution electron energy loss (HREEL) spectroscopy and supporting density functional theory (DFT) calculations. Both physisorbed and chemisorbed phases are observed. One extended and highly ordered self-assembled metal−organic phase is seen at saturation coverage and above. A metastable phase is also observed. Complete desorption occurs at ca. 600 K. Those structures are critically discussed in the light of some of the various adsorption models reported in the literature and an alternative adsorption model is proposed. These results allow a further understanding of the interaction between benzotriazole and copper and, in turn, may help understanding the mechanism for protection of copper and copper alloys from corrosion, substantially contributing to a long-standing debate.

1. INTRODUCTION The search for efficient and economical methods to protect metal and alloys from corrosion is a key factor in industrial manufacturing. Benzotriazole (BTAH, Scheme 1) has been well-known as a corrosion inhibitor for copper since the 1950s.1 However, the mechanisms of adsorption and passivation are still a matter of debate and, to date, there is no universal agreement on the details of BTAH adsorption, orientation, and assembly on copper surfaces.2−5 It has been reported that BTAH molecules adsorb upright via the nitrogen atoms in acidic solution, forming ordered molecular rows on Cu(111) surfaces, and its molecular orientation is found to influence electrochemical reactions, hence its corrosion inhibiting properties.6 BTAH’s characteristic ability to act as an etching and corrosion inhibitor is thought related to the formation of a stable Cu(I)−BTA layer that is thought to behave as a water repellent physical barrier, impeding ionic movement across the surface, thus quenching the adsorption of water and other polar organic molecules, conferring some degree of surface oxidation passivation.7 DFT calculations show that BTAH can physisorb or weakly chemisorb on Cu(111), through nitrogen sp2 lone pairs. Chemisorption can be stabilized by forming fairly strong intermolecular N−H---N hydrogen bonds and two N−Cu chemisorption bonds to the surface; however C−H---N hydrogen bonds are not predicted to form.8 It was also proposed that the actual film is probably a hybrid, hydrogen bonded network embedded with some sections of a [Cu(I)− BTA]n polymeric layer with a 1:1 Cu-BTA stoichiometry and a bidentate structure.8 Rather than a [BTA−Cu]n polymer,4 [BTA−Cu−BTA] discrete units9 have been suggested to play a fundamental role in passivating the Cu(111) surface, because of © 2014 American Chemical Society

Scheme 1. Benzotriazole (BTAH) Chemical Structure with Approximate Molecular Dimensionsa

a

Nitrogen atoms are labelled for ease of reference in the discussion.

their ability to strongly and densely pack, therefore limiting the number of surface sites available for coordination with further species. Nevertheless Peljhan et al. claimed that polymeric and dimeric structures are almost degenerate, the polymeric one being slightly more stable.10 Recently we reported on the onset of BTAH adsorption on a pristine Cu(111) surface, highlighting that benzotriazole adsorbs as an anionic species (BTA−), coordinating with copper adatoms to form CuBTA, as a minority of the adsorbed species. The majority of the adsorbates is represented by the dimeric species, Cu(BTA)2, which adsorbs with the dimer’s plane at a small tilt to the surface normal. Dimers laterally interact to form chains, whose Received: November 22, 2013 Revised: March 31, 2014 Published: March 31, 2014 8667

dx.doi.org/10.1021/jp411482e | J. Phys. Chem. C 2014, 118, 8667−8675

The Journal of Physical Chemistry C

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Figure 1 shows enhanced high resolution electron energy loss spectra collected following exposure of a clean Cu(111) surface

sections appear to diffuse in a concerted manner, up to a coverage of ca. 0.5 ML (monolayer). It is worth noting that the copper surface formed a (2 × 1) reconstruction,11 indicating that there is a fairly strong interaction between the triazolate group and the copper, both in the form of surface adatoms as well as atoms in the surface plane. In this study, we report on the adsorption of BTAH on a pristine Cu(111) single crystal surface to saturation coverage and beyond, in an ultrahigh vacuum environment, investigated using complementary surface sensitive techniques, in order to gain a better understanding of the BTAH adsorption properties. In turn, this allows us to derive information regarding the effectiveness of benzotriazole in protecting the copper surface; in fact, a saturated layer has been reported to be the most favorable situation in which BTAH can act as a passivating physical barrier.7 The results obtained via STM, showing extended, self-assembled, and densely packed layers, vibrational spectroscopy (HREELS), and temperature-programmed desorption (TPD) measurements and supported by DFT calculations, allowed us to derive adsorption geometries, which are discussed in light of some of the models previously reported in the literature, and an adsorption mechanism is proposed.

Figure 1. Enhanced HREEL spectra of BTAH/Cu(111) at increasing coverage: (a) ca. 0.5 ML, (b) ca. 1 ML, (c) ca. 2.5 ML, and (d) ca. 2.5 ML annealed to 420 K.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS Experiments were performed in ultrahigh vacuum chambers with a base pressure better than 5 × 10−10 mbar and equipped with low energy electron diffraction (LEED) and scanning tunneling microscopy (VT-STM, Omicron) or high resolution electron energy loss spectroscopy (HREELS, VSW HIB 1000 double pass spectrometer). The copper single crystal was cleaned by Ar+ ion sputtering and annealing cycles until a clean surface, characterized by a sharp (1 × 1) LEED pattern and large, flat terraces in STM, was observed. STM images were acquired in constant current mode, at room temperature, using electrochemically etched W tips and were processed using the WSxM software package.12 BTAH dosing was carried out by electrically heating a quartz crucible containing the compound to ca. 298 K.13,14 These conditions yielded a deposition rate of ca. 0.5 ML min−1, which was reduced to ca. 0.1 ML min−1 by the introduction of a skimmer between the crucible and the sample, as determined by TPD measurements. The Cu(111) crystal was kept at room temperature during exposure to BTAH. HREELS measurements were carried out in the specular direction (θi = θf = 45°) with a primary beam energy of 8 eV and a typical elastic peak resolution of ca. 50 cm−1 (6.2 meV fwhm). A maximum likelihood based resolution enhancement method15,16 was used to recover the spectra from the instrumental broadening, leading to an improved resolution of ca. 40 cm−1 fwhm. Spectra were normalized to the intensity of the elastic peak. TPD experiments were performed on a separate UHV chamber with a base pressure better than 1 × 10−10 mbar allowing the crystal to be heated while in direct line of sight of a quadrupole mass spectrometer. Vibrational spectra for gas phase species were calculated using the Gaussian 03 software package,17 with the B3LYP hybrid exchange correlation functional and the 6-311G basis set and no symmetry constraints.

to BTAH. HREELS experimentally observed modes, previously obtained RAIRS results,11 comparison with calculated modes and their assignments are summarized in Table S1, Supporting Information. With reference to the ca. 0.5 ML spectrum, Figure 1a, the most intense peaks are attributed to the C−H stretching mode at ca. 3100 cm−1, whereas vibrational modes of the aromatic ring are seen at ca. 1460 cm−1. The energy loss peak at ca. 1155 and 1295 cm−1 are assigned to two N−N stretching modes of the triazole ring in a Cu(I)---N coordination environment.18 The peak at ca. 970 cm−1 is attributed to the C−H wagging mode. The peak at ca. 730 cm−1 is assigned to a N1−N2 stretching mode; the shoulder at ca. 765 cm−1 to a CH out-ofplane bend, while the low intensity shoulder at ca. 840 cm−1 is assigned to a combination of an aromatic ring breathing mode and an N2−Cu stretching vibration. The energy loss peaks at ca. 555 cm−1 and ca. 420 cm−1 are due to out-of-plane deformations of the aromatic ring, while less intense peaks at ca. 350 and ca. 300 cm−1 correspond to modes with a contribution from for Ni---Cu stretches. The observed modes give a clear indication that BTAH molecules adsorb essentially upright or with a small tilt angle with respect to the surface normal. Moreover, the absence of an N−H stretch (free NH) at ca. 3500 cm−1, or an N−H stretch at ca. 2800 cm−1 (H-bonded NH), or the N−H wagging modes in the 680−630 cm−1 range, confirm that the adsorbed molecules are in the anionic state (BTA−) thereby preventing any intermolecular hydrogen bonds involving the triazole ring. It is worth noting also the absence of a CN1−H in plane bending mode which, for upright adsorption, would be expected to be seen at ca. 1402 cm−1; however, this mode strongly mixes with various CC stretching and C−H bending modes, therefore might be difficult to identify. Nevertheless, HREELS data provide information on the final adsorbed state only, therefore it cannot be inferred when deprotonation occurs, i.e., whether the molecule is deprotonated during adsorption or whether deprotonation takes place as a second step after adsorption.

3. RESULTS AND DISCUSSION 3.1. HREELS. Infrared spectroscopy of the initial stages of BTAH adsorption on Cu(111) is reported elsewhere;11 here, 8668



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is first seen at ca. 350 K, which is attributed to desorption of physisorbed species. The maximum desorption temperature for these physisorbed species is ca. 400 K, corresponding to a desorption energy of ca. 125 kJ mol−1 (ca. 1.29 eV), calculated using the Redhead equation, using a pre-exponential factor of 10−16 s−1,19 and assuming first order desorption kinetics. Literature adsorption energy values for various configurations are reported in Table 1 for comparison. The best agreement with the tabulated values is with the physisorbed H-bonded BTAH in the neutral form, for which a calculated adsorption energy of 1.23 eV per molecule is reported.4 It is worth noting that such a species would not be easily identifiable in the HREEL spectra of the as prepared surfaces because of the dipole selection rules as explained above, nor would it be in the spectra recorded after annealing to ca. 420 K, since these species would have already desorbed. Another explanation for this low temperature desorption peak may be desorption from a second layer of a Cum(BTA)n complex, as clarified later. The onset of the main desorption peak is seen at ca. 550 K and shifts to higher temperatures by only a few degrees with increasing exposure. The maximum desorption temperature for this chemisorbed state is ca. 595 K, corresponding to a desorption energy of ca. 188 kJ mol−1 (ca. 1.94 eV). Interestingly, this value is similar to the desorption temperature of BTAH adsorbed on Ni(111).25 As shown by the data in Table 1, a better agreement is seen with the chemisorbed structures of monomer, dimer and necklace type polymer, involving adatoms, even though the calculated energies are 0.5−1 eV higher than those experimentally derived. The N−Cu interaction is reported to be quite strong,26,27 and metal− organic frameworks based upon it seem inherently stable, independent of the substrate upon which they are prepared.27 Since all expected desorption fragments follow the same trend, those are likely generated from the same chemical species cracking within the mass spectrometer rather than as a consequence of surface mediated decomposition. However, whether or not desorption occurs as copper benzotriazolate, in the form of a monomer, CuBTA, the dimer, Cu(BTA)2, or any different stoichiometric ratio, Cum(BTA)n, could not be determined because masses 182 amu (monomer) and 299 amu (dimer) fall outside the quadrupole detection limit. From SIMS experiments conducted on copper foils treated with glycine, hydrogen peroxide, and BTAH, Deshpande et al.21 identified only mass 299 amu and not 182 amu. Levin et al.28 identified signals from the intact benzotriazole and several heavier masses ascribed to ions of the CuxCNy type. Complexes of the Cun(BTA)n±1 type were also identified.18,28 Oertel et al. made analogous observations for desorption and fragmentation of a similar molecule, tolyltriazole, which was reported to desorb from Cu(111) at ca. 580 K.29 In the light of those results and our former report, where the majority of the adsorbed species was identified as the dimer,11 we are inclined to consider that the major higher temperature desorbing species is the dimer. It is worth noting that the submonolayer spectrum, red trace in Figure 2, shows already an increase in baseline and the high temperature peak corresponding to chemisorption, but not a clear physisorption peak, whereas both physisorbed and chemisorbed peaks are visible in the 0.5 ML spectrum. This implies that physisorption and chemisorption are concomitant events. Moreover, for coverages ranging from ca. 1 to ca. 3 ML, the amount of physisorbed species increases with increasing

Kokalj et al.19 have reported that dehydrogenation of weakly chemisorbed BTAH neutral molecules is unfavorable on an ideal Cu(111) surface because the process requires a high activation energy (1.14 eV, which cannot be overcome at room temperature) and because desorption would preferentially occur, with a calculated activation energy ≤0.6 eV. However, it was also reported that on more open surfaces and undercoordinated defects, which is a more realistic description, the activation energy barrier for dehydrogenation becomes smaller and desorption energies larger, so that dehydrogenation can occur. These HREELS results are in agreement with our earlier RAIRS results11 and also with some of the models proposed by other authors2−9 where a slightly tilted adsorption configuration of BTAH molecules was derived. The comparison between experimental and calculated spectra shows clear similarities between the observed modes and those calculated for Cu(BTA)2, less so for CuBTA, as shown in Figure S1. However, a parallel adsorption geometry for neutral BTAH cannot be ruled out completely, since, according to the surface selection rule, an N1−H stretch parallel to the surface would be largely dipole inactive; moreover, the other vibrations expected for such an adsorption configuration, buckling modes of the 6and 5- membered rings, are all weak and likely to be masked by those generated by upright species. In fact, the energy loss spectrum recorded after dosing BTAH on Au(111), where flatlying adsorption is expected, shows essentially those skeletal modes at ca. 415 cm−1, out-of-plane deformation of the triazo ring, 505 cm−1, out-of-plane deformation of the aromatic ring, and 745 cm−1, CH out-of-plane mode.20 The possible presence of BTAH species is further discussed in the TPD section. At increased coverage, Figure 1b,c (ca.1−2.5 ML), in general the spectra increase in intensity in a proportional way. A very low intensity energy loss peak appears at ca. 240 cm−1, which is attributed to a strong N2---Cu(111) surface vibrational interaction replacing the other two vibrations attributed to Ni−Cu type stretching modes at 300−350 cm−1 which become progressively less intense as the coverage increases. Annealing the adlayer to ca. 420 K, Figure 1d, has the effect of desorbing loosely bound species, as will be clarified in section 3.2, and induces ordering within the adsorbed layer, as confirmed by STM. These conclusions follow from the further general increase in intensity of the full spectrum and, more importantly, by the changes in relative intensity of the bands at 1155 cm−1, 1295 cm−1, which have been calculated as characteristic of N1-Cu coordination for both the N1 monomer and the dimer, and 1460 cm−1. 3.2. TPD. TPD data were collected following the adsorption of BTAH onto Cu(111) at room temperature as a function of increasing exposure. The cracking pattern of the gas-phase BTAH species observed during exposure of benzotriazole to the copper (111) surface, comprises ion fragments from the BTAH molecule identified as CN (26 amu), C3N− (50 amu), C6H4N− (90 amu), and C6H4N3 (118 amu), similar to a previous report.21 The cracking pattern reported in the NIST database also show mass 64 amu.22 Other masses observed were H2 (2 amu), N2 (14 and 28 amu), H2O (18 amu), and CO2 (44 amu). Since all the ion masses generated by molecular fragmentation follow the same pattern (see Figure S2), only the most intense fragments, masses 50, 64, and 90 amu were monitored during desorption for which a heating rate ca. 5 K s−1 was used. As shown in Figure 2, an increase in the baseline 8669



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Figure 2. Mass 90 amu TPD traces for BTAH/Cu(111) at increased coverage.

Table 1. Calculated Adsorption Energies (eV)a system BTAH on Cu(111) 4 × 4 × 4 BTAH/Cu(111) 1/16 ML BTAc/Cu(111) 1/16 ML BTAH/Cu(111) 1/16 ML BTAH/Cu(111) [BTA−Cu]n/Cu(111) BTAH/Cu(111)

BTAH chem. on Cu(111) 6 × 6 × 4 BTAH phys. on Cu(111) 6 × 6 × 4 BTA−Cu monomer BTA−Cu−BTA dimer [BTA−Cu]n BTAH/Cu(111) BTAc/Cu(111) BTAH2c/Cu(111) BTAH/Cu(111)

adsorption mode

Eads/eV

bridge N2−N3 top N2 bridge N2−N3 top N2 bridge N2−N3 stand-alone (flat) H-polymer (flat) “necklace” polymer top N2 bridge N2−N3 (SB) top N3 physisorbed upright on 2 Cu atoms flat N1 to adatom, N2 to surface N1 to adatom, N2 and N3 to surface N2 to surface, N1 and N3 to adatom bridge N2−N3 bridge N2−N3 top N2 physisorbed

−0.37 −0.40 −0.37 −0.40 −2.78 −0.72 −1.23/molecule −2.98/molecule −0.43/−0.53 Eads∞ −0.44/−0.60 Eads∞ −0.34/−0.49 Eads∞ −0.10/−0.7b −0.43 −0.051/−0.102 −2.43/BTAc −2.86/BTAc −3.00/BTAc −0.5 −2.8 −0.8 −0.7b

ref 3 4

5, 23

8 13

24

Eads∞ indicates zero coverage adsorption energy to consider the effects of dipole−dipole interactions. bAccounts for van der Waals’ interaction. Indicates radical species.

a c

3.3. STM. As we previously reported,11 at low coverage BTAH adsorbs readily on clean Cu(111) forming Cu(BTA)2 and CuBTA, which tend to condense into chains, whose sections diffuse in a concerted manner, while the copper surface reconstructs in a (2 × 1) superstructure. A different kind of nondiffusing molecular chains was previously reported by Vogt et al. for BTAH adsorption on Cu(110) in H2SO4.32 The intermediate coverage regime (ca. 0.5 ML), is characterized by a higher density of molecular chain features similar to those observed in the low coverage regime. Figure 3 shows a typical STM image where some molecular chains (a) and a pseudohexagonal structure (b) can be identified. The areas between adsorbates show the three rotational domains of the (2 × 1) surface reconstruction as revealed by

exposure; however the desorption peak related to the chemisorbed species has approximately the same integral regardless of the exposure, within the 1−3 ML range. Therefore, we are inclined to attribute this second feature to a chemisorbed complete single layer. Multilayers may occur for the as prepared layers. However, as soon as a heating commences, regardless of the initial coverage, physisorbed species desorb leaving only a single layer coverage, which is related to the higher desorption feature. Formation of a multilayered structure has been postulated by some authors. Poling30 and later Ling et al.31 proposed a multilayer film formation for BTAH in acidic solution, adding that continuous growth is controlled by the ability of Cu(I) atoms to diffuse from the surface through the BTAH layer. 8670



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Figure 3. As prepared molecular features observed at medium coverage BTAH on Cu(111), 30 × 30 nm2, −0.5 V, 0.55 nA: (a) molecular chain; (b) pseudohexagonal packing; (c) six-membered starlike feature; (d) junction. Black arrows indicate the three rotational domains of the (2×1)-Cu(111) surface reconstruction.

Figure 4. STM image showing a magnification of the pseudohexagonal phase (b) in Figure 3; 10 × 7.8 nm2; −0.7 V; 0.24 nA; the hexagonal unit cell is outlined in blue. One orientation of the basic building block is highlighted in the oval a) and in the inset with superposed molecular models where adsorption sites are indicative only. The central dimer, the right (left) outermost dimers, and the right (left) brighter monomer features form a structure reminiscent of those previously observed.11 It is not clear whether the upright species are Cu(N1)− BTA or Cu(N2)−BTA, nor their rotational angle. The model is made assuming Cu(N2)−BTA because considered to have “higher” physical contrast, with the caveat that geometrical arguments are always questionable in STM. Blurred dimers formally belong to adjacent, rotated basic building blocks. Atoms in inset: Cu lattice, white circles; Cu incorporated atom, orange; N, blue; C, dark gray; H, light gray.

the striped features parallel to ⟨110⟩ directions, with a separation of ca. 0.44 nm, corresponding to the distance of the next nearest neighboring row in the ⟨112⟩ directions,11 indicated by double-headed arrows. BTAH readily coordinates initially with freely diffusing copper atoms. A large number of diffusing species such as free copper atoms and copper complexes, which are weak enough to be mobile under the STM tip,11 is the cause for the low image quality typically seen when scanning as prepared surfaces at midcoverage regimes. When the supply of free atoms is exhausted, further BTAH molecules interact by removing surface atoms, thereby stabilizing a (2 × 1) reconstructed surface. The removal of surface atoms allows coordination to proceed further and those copper atoms incorporate into the organic matrix to form the more complex structures described below. Because of surface mediated electronic effects33,34 and lateral intermolecular interactions induced by the increased density of adsorbates,5 chains do not diffuse as readily as in the low coverage regime. Metastable structures, such as four-dimer clusters or five- and six-membered star-like features, one example of which is shown in Figure 3c, can be seen occasionally. Nevertheless, the adsorbed layer seems very stable overall. Figure 3d indicates a structure reminiscent of the junctions between dimeric chains described elsewhere.11 The dominant molecular arrangement is the pseudohexagonal packing, shown in more detail in Figure 4, which has a large unit cell, ca. 20.4 nm2, covering ca. 360 surface atoms, and an average vector length of ca. 4.2 nm. The basic building block, highlighted in the oval in Figure 4a, comprises two different chemical species which are identified as three Cu(BTA)2 dimers, features of low contrast aligned along ⟨110⟩ directions and two upright CuBTA species, brighter features, as shown in the inset in Figure 4. The dimers are oriented along close packed directions, with the two outermost ones parallel to each other and the central one rotated by 60°. Even though the dimers’ orientation can be assigned confidently enough using a step edge as a reference, the molecular/dimer adsorption sites in the model shown in the inset are indicative only, because of flexibility regarding the exact position of the molecular species which the system seems

to offer.11 For the upright species no preferential orientation can be inferred from the contrast in the STM image, though their appearance suggests that they may be free to rotate/ vibrate more than the dimers, giving them a less well-defined appearance. These basic building blocks are thought to share structural similarities with the strongly bound species, the socalled junctions, seen at low coverage.11 This is highlighted in the inset in Figure 4, where the central dimer and the two dimers on the right (left) form a triangular structure enclosing an upright feature. Three of these building blocks, rotated by 60° relative to each other, interlock to form the more complex unit cell structure highlighted with blue lines in Figure 4, which encompasses distorted triangular and hexagonal pores. Benzotriazole is able to coordinate with copper in a multitude of different structures. An hexagonal layered phase, formed by nearly planar layers of BTA− anions linked to copper via each of their nitrogen atoms and bridged via additional BTA− molecules on polycrystalline copper foils, was reported by Carron et al.35 on the basis of SER measurements. Recently Salorinne et al.18 reported on the role of BTA−Cu(I)−BTA units in protecting Cum(BTA)n clusters. Annealing to ca. 373 K promotes desorption or anchoring of any remaining loosely bound species and a decrease in the number of chains, favoring condensation of dimers and monomers into the pseudohexagonal phase. This phase persists up to desorption at ca. 600 K; however, heating is not necessary, since ordering may occur spontaneously in a matter of hours at room temperature. Chen and Häkkinen9 computed several adsorption geometries based on the Cu(BTA)2 complex and the [CuBTA]n polymer concluding that the overalyers 8671



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Figure 5. Nominal 1 ML coverage of BTAH/Cu(111): (a) as prepared, 40 × 40 nm2, −0.98 V, 0.37 nA; (b) after annealing to 373 K, 70 × 70 nm2, −0.94 V, 0.56 nA; (c) after annealing to 423 K, 100 × 100 nm2, −1.51 V, 0.21 nA.

Figure 6. Nominal 2.5 ML coverage of BTAH/Cu(111): (a) as prepared, 40 × 40 nm2, −0.98 V, 0.29 nA; (b) after annealing to 423 K, 70 × 70 nm2, −1.28 V, 0.37 nA; (c) after annealing to 498 K, 80 × 80 nm2, −0.95 V, 0.55 nA.

dimers, or entire basic units, are present and act as a source of lattice dislocations causing the adsorbed layer to modify its orientation. In fact, upper and lower terraces level structures are often not in registry with each other. The structures formerly attributed to etched pits are no longer seen. Step edges appear irregular and decorated by dimers which have yet to rearrange into the hexagonal lattice. After the Cu(111) surface has been exposed to BTAH long enough to produce a layer corresponding to ca. 2.5 ML (STM topography in Figure 6a and HREELS in Figure 1c), the as prepared surface exhibits features which appear quite different from those observed for the monolayer coverage (Figure 5a). In fact a layered structure seems to have formed, whose uppermost level is characterized by some local ordering with dimers arranged in a chain-like structures reminiscent of the preparations at submonolayer coverage. Also in this case no orientational preference with regard to the underlying lattice can be established clearly. This is mainly because the short sections of chains, which appear to have some degree of ordering, seem to constitute a second layer adsorbed on the first layer, which appears nearly featureless. The hexagonal phase however occurs spontaneously in a matter of a few hours without annealing or as a result of a short annealing to ca. 373 K. Along with the hexagonal structure already described, a further packing configuration appears after heating the preparation to 423 K (Figure 6b), with the two structures merging into each other. This second structure is produced only when annealing oversaturated preparations, whereas preparations close to the monolayer regime evolve solely into the hexagonal phase (Figure 5c). Further annealing of the higher coverage surface to ca. 500 K promotes evolution to the hexagonal phase only (Figure 6c).

formed by the dimers, which can densely pack, can interact with the surface more strongly than those based upon the polymer. This agrees with the present experimental findings, which show that the basic adsorbed species are dimers or monomers. However, there is a disagreement with the conclusions drawn by Peljhan et al. who calculated the polymeric form to be slightly more stable,10 since in the study described in this paper molecular features are well separated, no evidence was found for an upright polymeric structure. At one monolayer coverage (STM topography in Figure 5a and HREELS in Figure 1b) local ordering of elongated features parallel to each other with a separation of ca. 1.1 nm is observed, consistent with the observation made for the submonolayer coverage. Features are offset with respect to each other; moreover there is no clear relationship with the main surface crystallographic directions. Annealing the surface to 373 K has the effect of increasing the ordering of the adlayer. On upper level terraces, molecular chains begin to reorganize into the hexagonal phase, whereas on lower level terraces this phase seems to have fully formed (Figure 5b). Wider scan images suggest that the lower level terraces shown in Figure 5b are likely to be etch pits. Diffusion of Cu(I) species has been postulated by some authors30,31 in order to explain 3D growth of BTAH adlayers in acidic solution. A mobile Cu species coordinating to a strong ligand such BTAH forming monomers and dimers, which have some mobility at low coverage, may explain the formation of etched pits. However, the corrosion inhibition activity shown by BTAH is ascribed to a strong interaction of the adsorbed organic or organometallic layer with the surface.7 The hexagonal phase entirely covers the surface upon annealing to 423 K (Figure 5c), although some defects, such as missing 8672



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flat, or perhaps at a small angle to the surface. Previously, the presence of both flat-lying and upright species on Cu(110) was derived by Park et al.,13,14 on the basis of coverage dependent UPS measurements, whereby the first molecular layer had adsorbed flat-lying, whereas the second was considered upright and polymerized. Even though a flat lying adsorption configuration would be difficult to distinguish in the vibrational spectra, because laying almost parallel to the surface,20 the occurrence of both upright and flat lying structures may account for the changes in relative intensity of the energy loss peaks in the HREEL spectrum following annealing of high coverage surfaces shown in Figure 1 c,d. Within the darker contrast stripe, copper atoms may be represented by the small brighter intensity maxima and BTA− by the darker areas. Incorporation of metal atoms into organic layers has been reported to have different contrasts when under STM investigation: in some cases metal atoms can show as apparent depressions,38,39 whereas in others they exhibit brighter contrast.40,41 Domains of alternating dark and bright striped features were previously observed upon adsorption of BTAH on Cu(111) in HClO4,6 with a spacing of ca. 1.2 nm and variation of contrast within the stripes consistent with the adsorption of π−stacked aromatic molecules. A darker contrast for flat-lying adsorbed molecules, accompanied by higher contrast for adatoms, was also reported by Dougherty et al.42 as a result of scanning under high tunneling currents or invoking a chemical reaction occurring on the tip. The second phase appears to be less thermodynamically favorable and can be observed only in conjunction with the hexagonal one; hence the hexagonal phase is the most thermodynamically stable one. In agreement with the TPD measurements, desorption of the ad-layer occurs above 600 K (±20 K), when an almost clean and unreconstructed Cu(111) surface reappears.

Therefore different initial coverages, exhibiting different initial features, result in the same phase, going through different steps, as can be seen by comparing Figure 5 and Figure 6. More examples are reported in Figures S4 and S5. With reference to the structure highlighted in Figure 7, the unit cell of this structure measures ca. 2.56 × 1.8 nm2 and comprises of two upright Cu(BTA)2 dimers and some extra features.

4. CONCLUSIONS An ultrahigh vacuum investigation has been carried out to study the adsorption of BTAH to a Cu(111) single crystal from submonolayer to saturation and above using complementary surface sensitive techniques. Several different and complex structures are formed. Not surprisingly therefore many different adsorption models, most of which advocate polymeric species, have been proposed in the past years. From the data obtained a mechanism of adsorption can be derived, whereby BTAH may physisorb flat-lying as a neutral molecule on the Cu(111) surface and form mobile species. Deprotonation and coordination with free copper atoms at step edges and surface defects readily occurs, allowing chemisorbed upright species to form, as revealed by vibrational spectroscopy. Analysis of STM images reveals that these species are likely to be dimers, Cu(BTA)2, at step edges and CuBTA monomers, surrounded by dimers, at surface defects. However, deprotonation is an activated process on flat terraces. Upon defective terraces some Cu(BTA)2 upright species can arrange in a parallel fashion and are chemisorbed weakly enough to diffuse under the influence of the STM tip. Starting from medium coverage, STM images show that a regular yet incommensurate pseudohexagonal pattern begins to form. This is also the structure observed at the one monolayer regime, to which the surface passivation character is associated. As prepared multilayers appear to have differing and coverage dependent morphologies; the layers beyond the first one, which is chemisorbed, appear to be only physisorbed. A metastable

Figure 7. The second phase observed: 10 × 10 nm2; −0.7 V; 0.25 nA. Molecular models are superposed for clarity. Atoms in figure: Cu incorporated atom, orange; N, blue; C, dark gray; H, light gray.

The dimer’s longer axis is oriented along, or perhaps a few degrees off, one of the ⟨110⟩ crystallographic directions, with a peak-to-peak separation of ca. 1.8 nm, corresponding to one of the unit cell dimensions. A line profile taken over the dimers along this direction, Figure 7(I), shows three intensity maxima for each dimer; the internal one may be attributed to the incorporated copper, which is difficult to discriminate in the topographic image because of the dynamic range limits. The peak-to-peak shorter separation between parallel lines of dimers is ca. 0.96 nm, Figure 7(II), a little shorter than the distance between dimers in the low coverage molecular chains, 1.1−1.2 nm,11 but the pairing of the dimers here may be more compact because of the increased coverage. Nevertheless, this separation is much larger than ca. 0.33 nm,36,37 typical separation reported for π-stacked aromatic molecules adsorbing with the aromatic rings normal to the surface, implying that the one observed results from a balance of surface mediated, intermolecular interactions. The longer distance to the next repeating feature is ca. 1.85 nm. Within this spacing some extra features can be seen. Those are tentatively attributed to molecular species lying 8673



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structure, which is believed to comprise dimers and a flat lying polymeric species, coexisting with the pseudohexagonal one, may be generated upon annealing multilayer preparations. TPD measurements show that physisorption and chemisorpiotn are concomitant from the lowest coverage. Weakly adsorbed species desorb at ca. 350−375 K, whereas the chemisorbed layer is stable up to ca. 550 K, when desorption begins to occur. HREELS and STM show increased ordering upon annealing to ca. 420 K, when the physisorbed layer is completely desorbed. Full desorption of the molecular adlayer is seen at annealing above ca. 600 K. These results contribute to understanding further the interaction between benzotriazole and copper and, in turn, the mechanism at the basis of the protection of copper and copper alloys surfaces from corrosion.

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(8) Jiang, Y.; Adams, J. B. First Principle Calculations of Benzotriazole Adsorption onto Clean Cu(111). Surf. Sci. 2003, 529, 428−442. (9) Chen, X.; Häkkinen, H. Divide and Protect: Passivating Cu(111) by Cu-(benzotriazole)2. J. Phys. Chem. C 2012, 116, 22346−22349. (10) Peljham, 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. (11) Grillo, F.; Tee, D. W.; Francis, S. M.; Früchtl, H.; Richardson, N. V. Initial Stages of Benzotriazole Adsorption on the Cu(111) Surface. Nanoscale 2013, 5, 5269−5273. (12) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (13) Park, Y.; Choi, S. H.; Noh, H.; Kuk, Y. Nano-Interconnection for Microelectronics and Polymers with Benzotriazole. Proc. SPIE 2006, 6172, 617214−1. (14) Park, Y.; Noh, H.; Kuk, Y.; Cho, K.; Sakurai, T. Thickness Dependent Ultraviolet Photoemission Spectroscopy and Scanning Tunneling Microscopy Study of BTA Molecular Layers on Cu(110). J. Korean Phys. Soc. 1996, 29, 745−749. (15) Frederick, B. G.; Nyberg, G. L.; Richardson, N. V. Spectral Restoration in HREELS. J. Electron Spectrosc. Relat. Phenom. 1993, 64/ 65, 825−834. (16) Frederick, B. G.; Richardson, N. V. Comment on “Ultrahigh resolution electron energy loss spectroscopy”. Phys. Rev. Lett. 1994, 73, 772. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, Revision E.01 Gaussian, Inc.: Wallingford CT, 2004. (18) Salorinne, K.; Chen, X.; Troff, R. W.; Nissinen, M.; Häkkinen, H. One-pot Synthesis and Characterization of Subnanometre-Size Benzotriazolate Protected Copper Clusters. Nanoscale 2012, 4, 4095− 4098. (19) Kokalj, A.; Peljham, S.; Koller, J. The Effect of Surface Geometry of Copper on Dehydrogenation of Benzotriazole. Part II. J. Phys. Chem. C 2014, 118, 944−954. (20) Grillo, F.; Richardson, N. V. Manuscript in preparation. (21) Deshpande, S.; Kuiry, S. C.; Klimov, M.; Seala, S. Elucidating Cu-Glycine and BTA Complexations in Cu-CMP Using SIMS and XPS. Electrochem. Solid-State Lett. 2005, 8, G98−G101. (22) NIST Mass Spec Data Center; Stein, S. E. (director) Mass Spectra. In NIST Chemistry WebBook; Linstrom, P. J. and Mallard, W. G., Eds.; NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD, http:// webbook.nist.gov (retrieved April 6, 2013). (23) Kokalj, A.; Kovačević, N.; Peljhan, S.; Finšgar, M.; Lesar, A.; Milošev, I. Triazole, Benzotriazole, and Naphthotriazole as Copper Corrosion Inhibitors: I. Molecular Electronic and Adsorption Properties. ChemPhysChem 2011, 12, 3547−3555. (24) Kovačević, N.; Kokalj, A. Chemistry of the Interaction between Azole Type Corrosion Inhibitor Molecules and Metal Surfaces. Mater. Chem. Phys. 2012, 137, 331−339. (25) Chusuei, C. C.; de la Peña, J. V.; Schreifels, J. A. Contour Temperature Programmed Desorption for Monitoring Multiple Chemical Reaction Products. Rev. Sci. Instrum. 1999, 70, 3719−3722. (26) Tait, S. L.; Langner, A.; Lin, N.; Chandrasekar, R.; Fuhr, O.; Ruben, M.; Kern, K. Assembling Isostructural Metal-Organic Coordination Architectures on Cu(100), Ag(100) and Ag(111) Substrates. ChemPhysChem. 2008, 9, 2495−2499. (27) Li, Y.; Shubina, T. E.; Chen, M.; Shi, Z.; Schmid, M.; Steinrück, H.-P.; Gottfried, J. M.; Lin, N. Coordination and Metalation Bifunctionality of Cu with 5,10,15,20-Tetra(4-pyridyl)porphyrin: Toward a Mixed-Valence Two-Dimensional Coordination Network. J. Am. Chem. Soc. 2012, 134, 6401−6408.

ASSOCIATED CONTENT

S Supporting Information *

Comparison between experimental HREEL and calculated gas phase vibrational spectra, additional TPD traces, and additional STM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(F.G.) E-mail: [email protected]. Telephone: +44 (0) 1334 46 2273. Fax: +44 (0) 1334 46 3808. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ECK Technology Limited is acknowledged for financial support. Dr. C. J. Baddeley is thanked for the use of the TPD equipment and fruitful discussions. EaStCHEM is thanked for computational support via the EaStCHEM Research Computing Facility.

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REFERENCES

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Benzotriazole Films on Cu(111) - American Chemical Society

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