The reaction of copper metal with benzotriazole

Applications of Surface Science 3 (1979) 229 235 © North-Holland Publishing Company

THE REACTION OF COPPER METAL WITH BENZOTRIAZOLE A.R. SIEDLE ~, R.A. VELAPOLDI and N. ERICKSON Institute for Materials Research, National Bureau of Standards, Washington, D.c. 20234, USA Received 30 October 1978 Revised manuscript received 21 December 1978

Intense, visible luminescence, attributed to charge transfer excited states, was observed in Cu(l) complexes of benzotrjazole but not in the Cu(lI) analogues. Microspectrofluorimetry was used to character cuprous benzotriazole, a surface phase formed from bulk copper and benzotriazole in dichloromethane.

1. Introduction Corrosion inhibitors are widely used to modify the structural and mechanical properties of bulk metals and their surfaces. Knowledge of chemical reactions between the inhibitor and the metal surface are critical to understanding the mechanism(s) of corrosion inhibition. Benzotriazole, 1, reacts with metallic copper to form

a surface film which can act as a barrier to cathodic reactions [1,2,3]. This paper reports the use of a new technique, m.icrospectrofluorirnetry, for the study of surface reactions and its use to investigate the reaction of copper with 1. Previously, we [4,5] reported the isolation of pure Cu(l) and Cu(II) derivatives of benzotriazole and the observation that the Cu(l) compounds, but not the Cu(II) analogues, ex.hibited an intense luminescence in the visible region. The absorption process was characterized as a metal -+ ligand d —~ ~ charge transfer process. The excited state lifetimes of (benzotriazole)CuC1, 2, and cuprous benzotriazole, 3, at 300 K are 3.6 and 0.7 ps respectively. Although spin labels may be inappropriate for heavy atom charge transfer emitters, such assignment suffices for light elements Present address: Central Research Laboratories, 3M Company, St. Paul, Minnesota 55101 ,USA.

A.R. Siedle ct aL

230

/ Reaction ofcopper metal nil/I

henzotria:ole

such as copper and we refer to the emission process as spin forbidden charge transfer phosphorescence. Since visible emission is not observed for Cu(I I) derivatives of 1, the process is clearly oxidation state specifIc ~. The 3d [7] electron configuration of Cu(l1) has low lying energy levels with z~Ecorresponding to transitions in the infrared. This provides a radiationless pathway to drain energy from the high energy charge transfer excited state. Thus, although emission could occur in the infrared region, it is unlikely since this level is close to the ground state and the probability of emission is decreased substantially [7] compared to the radiative emission from the charge transfer excited state in Cu(I) coniplexes. Microspectrofluorimetry, which combines a conventional spectrofluorimeter with microscope optics, is well suited to the study of sniall, luminescent areas. In general, only milligrani quantities of solids are reciuired to obtain an emission Spectrum. However, when the quantum yield for luminescence is lugh. as it is in the case 2. corof copper(l) benzotriazole compounds, spectra from areas as small at 20 pM responding to nano- or picogram quantities, have been obtained. In addition, since the sample is held on a mechanical stage, specific areas may be examined reproducibly to within a few micrometers. Relative to oilier surface sensitive techniques. microspectrofluorimetry is nondestructive providing that photodecoinposition of the sample does not occur, does not require that the sample he analyzed under vacuum and offers, in appropriate cases, low detection limits and good spatial resolution.

2. Experimental ~ 2. 1.

Instrumentation

The microspectrofluorimeter used in this work has been partially described [71. It consists of a Leitz “Ortholux One” microscope equipped with a vertical illuminator [81 that allows sample excitation by incident irradiation, fig. I . The exciting radiation was produced by a 150W xenon source. The radiation was filtered through a “red suppression” BG 38 filter that gives broad band transmittance front 320 to 670 nm. In this work, a Ploem illuminator barrier filter-dicliroic mirror combination designated I -~ 1 was generally used. Tius combination provided for sample excitation with a broad band of radiation from 320 to 380 nm (~0.0l transmittance wavelengths) with maximum transmittance of ~360 nm (transmittance ~0.6). To Similar effects are obtained in copper complexes of other heterocyclic amines, e.g. pyrazoic, imidazole, and tetrazole, ref. 151. In order to adequately describe materials and experimental procedures, it was occasionally necessary to identify commercial products by manufacturer’s name or label. In no instances does such identification imply endorsement by the National Bureau of Standards, nor does it imply that the particular product or equipment is necessarily the best available for that purpose.

AR. Siedle eta!. /Reaction of copper metal with henzotriazole

PM

231

X-Y REC X-IN

b~~Tt~T~

MONO[

X-Y FO —MA

—

-IN

R AMP V TO F PS

s-MT

~‘

MCA LAS

,,~,MO

CL

TT

r’~

[DM-FC

XES

R PMT

1. Block diagram of microspectrofluorimeter: XES — xenon source; LAS — laser source; RPMT — reference photomultiplier tube; SAM — sample; MA — measuring aperture; DM—FC — dichroic mirror—barrier filter combination; MO — monocular; MT — measuring telescope. X—Y FO — XY focusing optics; MONO — monochromator; PMT — photomultiplier; RAMP — ratio amplifier; X—Y REC — X—Y recorder; V to F — voltage to frequency converter; PS — pulse shaper; MCA — multichannelanalyzer;TT — teletype;CL — crystal clock. Fig.

assure that the emission spectrum did not depend on the excitation wavelength, barrier filter-dichroic mirror combinations designated 2—2 and 3—3 were occasionally used to give broad band excitation at 350—450 nm and 375—500 nm, respectively. The emission spectra obtained using the 1—2, 2—2, and 3—3 optical combination were equivalent excluding barrier filter perturbations. The radiation emitted by the sample passed through a photometer head (Leitz MPV), X—Y focusing optics, a 0.1 M grating monochromator (8 nm bandpass/l mm slit, Schoeffel Instrument Co., Westwood, N.J.), and was then measured by a photomultiplier tube (EMI 9659 QB extended red, S20 response). The signal was amplified (current to voltage amplification) and fed directly to an X—Y recorder or a voltage to frequency converter, The pulses from the V—F converter were shaped and gated into a multichannel analyzer operated in the multiscaling mode. This instrument produced emission spectra that were uncorrected for instrument and sample parameters. X-ray photoelectron spectra were obtained on AEI or Hewlett-Packard instruments, Binding energies are referenced to the carbon is level at 285.0 eV.

232

A.R. Siedle eta!.

/ Reaction ofcopper metal wit/i henzotriazole

Commercial oxygen-free high conductivity copper was cut into 1 X0.5 inch strips which were abraded to provide a clean surface. These strips were placed into capped vials containing 10% (w/v) solutions of benzotriazole in spectrograde solvents which had been previously purged with nitrogen. After immersion for 24 hours, the samnpies were removed, rinsed with pure solvent and brie/it’ air dried (cuprous benzotriazole is somewhat air sensitive and undergoes surface oxidation) and examined with the microspectrofluorirneter. Copper powder was treated with hydrogen at 400°Cand stored in an argon tilled drybox.

3. Results and discussion The surface of clean elemental copper underwent a reaction at room temperature with benzotriazole in dichlorotnethane or acetonitrile solutions. Abraded strips of the metal became coated with a dull, yellow-green fIlms that exhibited a bright red-orange luminescence when excited by ultraviolet radiation. The films obtained using ethanol or ethyl acetate as solvents exhibited a similar luminescence but it was less intense and uniform. Portions of the copper metal which were not abraded did not react to give a luminescent species. When viewed with the microspectrofluoritneter, small, luminescent crater-like areas were observed which are attributed to pinhole defects in the oxide—-hydroxide-—carbonate layer with which the metal is coated. The observation that a lutninescent film is formed constitutes prima fade evidence for a Cu(l)-benzotriazole species. Additional and supportive evidence was obtained from mnicrospectrofluorimetry. Fig. 2, curve A displays the uncorrected emis-

I

400

450

I

500

I

I

I

550

600

650

WAVELENGTH, nm lig. 2. Uncorrected emission spectra of A, benzotriazole-copper surface phase; B, pure cuprous benzotriazole.

Al?. Siedlc et aL

/ Reaction of copper metal nit/i benzotriarolc

233

sion spectrum of the film obtained using benzotriazole in dichloroniethane and curve B the spectrum of cuprous benzotriazole. The two curves appear to be congruent and have identical maxima to within the experiniental error of ±2nni. We therefore conclude that the reaction of copper surfaces with benzotriazole produces films of cuprous benzotriazole. T’lie absence ofa reaction with organic solvents, particularly dichhoromethiane, is noteworthy. Since chlorinated hydrocarbons are known to corrode copper, presumably with formation of CuC1 [9], it would not have been surprising to find the CuCI complex of 1 on the surface. TIme eniission spectrum of (C 6H5N3)CuCI, 2, however, is blue shifted by 51 nm relative to cuprous benzotriazole and no additional entission at shorter wavelengths, due to 2, in curve A is apparert. Control experiments disclosed that 1 and 2 did not react in dichiloromethiane and thus, henzotriazohe is not a strong enough base to deprotonate 2. It seems unlikely that 2 is an intermediate in the formation of cuprous benzotriazohe althiought we recognize that its reactivity as a surface species could conceivably be greater than that of the bulk material. X-ray photoelectron spectral data, table 1, are fully consistent with the conclusions drawn froni the microspectrofluorimetric data. The Cu core levels in copper(II) hen zotriazole are about 2.5 eV below those in the Cu(h) analogue, a value much larger titan the estimated experimental error of ±0.2eV. Thus, distinction between the two formal valence states of copper in this system appears to be straightforward. The Cu binding energies of the surface films formed from a dicliloromethane solution of 1 and copper are clearly indicative of Cu(l) and no shake up satellites, characteristic of divalent copper [101 were observed. The ESCA spectrum reveals that the surface does not contain chlorine, which is also consistent with the microspectrofhuorirnetric data. The reaction of metallic copper with benzotriazole in the absence of solvent was also studied. When hydrogen-treated copper powder was heated in vacuo with molten 1, the melt took on the yellow color of cuprous benzotriazole. The uncorrected emission spectruni of the solidified reaction mixture exhibited maxima at 460 and 590 nm which are attributed to 1 and cuprous benzotriazole respectively. The small

Table I X-ray photoelectron spectral data a) Level

Surface film

C6H4N31-I CuC1

C6H4N3Cu

(C6H4N3)2Cu

Cu 2P372 Cu 3P112 Cu 3P312 NIS

647.5

647.5

646.7

649.7

207.0 209.8 114.9

207.5 210.0 115.3

206.8 209.5 115.8

209.2 211.8 115.2

—

—86.8

—

—

—84.6

—

—

Cl 2P112 Cl 2P312 a) Binding energies

—

with carbon is level at 285.0 eV.

234

AR. Siedle eta!. / Reaction of copper metal wit/i henzotria:ole

shift of the longer wavelength peak relative to that of 1 is attributed to a matrix effect Of the currently available surface-sensitive analytical techniques, X-ray photoelectron spectroscopy (ESCA) provides the niost detailed molecular characterization. Chemical shifts depend largely on interactions between valence level electrons and core electrons where binding energies are measured in the ESCA experiment. Whilst it is often relatively easy to characterize the oxidation state of an element by measuring its core level binding energies, it is less easy to deduce, from the same data, finer details of chemical interest such as coordination environnient. For example, the data in table 1, copper in both cuprous benzotriazole and the henzotriazole cuprous chloride have similar binding energies and ESCA cannot distinguish what must sorely be chemically nonequivalent nitrogens in these compounds. Spectrofluorimetry deals with transitions among valence level electrons and is inherently suited to provide information about bonding in which these electrons participate. In this regard, the level of chemical detail achieved by microspectrofluorimetry is probably comparable to that obtained by inelastic tunneling spectroscopy, but it has the added advantage of the ability to scan small areas of a sample. The surface reactions of henzotriazole with copper substrate has been studied by several groups. Poling [11] used infrared spectroscopy to demonstrate the probable formation of cuprous benzotriazole on copper mirrors. More recently, surfacesensitive techniques have been employed. Roberts [12], using ESCA. found evidence for tIme formation of Cu(l)-benzotriazole surface species when cuprous oxide was treated with water solutions of 1 . Later, Chiadwick and 1-lashiemi [131 found that sonic cuprous benzotriazole surface species was formed on both cuprous oxide and metallic copper. Our ESCA and microspectrofluorimetric data indicate that copper undergoes a reaction with molten benzotriazole or its solutions in organic solvents to produce films of cuprous benzotriazole. We find no evidence for participation of solvent in this reaction. Cuprous benzotriazohe is easily oxidized and turns green in air. Air oxidation may lead to a highly cross-linked, amorphous film of a polymeric cupric benzotriazole, possible structures of which have been discussed by Roberts [121, which may protect the underlying copper from further reaction. The unknown surface species formed on cuprous oxide is probably not cuprous benzotriazole. When pure, single crystal of Cu 20 were exposed to I in dichloromethane or to its sodium salt in water, the crystals remained clear, red, amid transparent; no luminescent surface phase was detected by microspectrofluorimetry. The precise role of surface oxides, as distinct from bulk, crystalline Cu2 0, has yet to be elucidated. It is quite possible that nonstoichionietric or surface oxides may exhibit a reactivity different from the crystalline phase and/or that an oxide layer may lead, through chemisorption, to a hmigh local concentration of benzotriazohe, thus accelerating the reaction with adjacent or underlying metal.

AR. Siedle eta!. /Reaction of copper metal wit/i benzotriazo!e

235

Acknowledgment We are grateful to Prof. iN. Demas for providing the photochemical data in advance of publication.

References lilt. Dugdale and J.B. Cotton, Corros. Sci. 3 (1963) 69. [21 J.B. Cotton and I.R. Scholes, Brit. Corros. J. 2 (1967) 1. [3] SM. Mayana and THY. Setty, Corros. Sci. 95 (1975) 625. [4] AR. Siedle, R. Velapoldi and N. Erickson, Inorg. Nuci. Chem. Lett. 15 (1978) 33. [51 J.N. Demas, S. Buell, R. Velapoldi and A.R. Siedle, manuscript in preparation. 161 A. Heller, J. Mol. Spectry. 28 (1968) 208. 171 R.A. Velapoldi, J.C. Travis, W.A. Cassat and W.T. Yap, J. Microsc. 103 (1975) 293. [81 J.S. Ploem, Z. Wiss. Mikrosk. 68(1967)129.

191 J. Kruger, private communication. [101 D.C. Frost, A. Ishtani and C.A. McDowell, Mol. Phys. 24(1972)861. [111 B.W. Poling, Corros. Sci. 10(1970) 359. [121 R.F. Roberts, J. Electron Spectry. 4 (1974) 273. [131 D. Chadwick and T. 1-lashemi, J. Electron Spectry. 10(1977)79.