Influence of relative humidity and ozone on atmospheric silver corrosion

Corrosion Science 77 (2013) 69–76

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Influence of relative humidity and ozone on atmospheric silver corrosion R. Wiesinger a,⇑, I. Martina a, Ch. Kleber a,b, M. Schreiner a,c a

Institute of Science and Technology in Art, Academy of Fine Arts, 1010 Vienna, Austria Centre for Electrochemical Surface Technology, 2700 Wr. Neustadt, Austria c Institute for Chemical Technologies and Analytics, Vienna University of Technology, 1060 Vienna, Austria b

a r t i c l e

i n f o

Article history: Received 6 March 2013 Accepted 19 July 2013 Available online 29 July 2013 Keywords: A. Silver C. Atmospheric corrosion C. Oxidation B. Raman spectroscopy B. XRD B. AFM

a b s t r a c t The interaction of highly pure polycrystalline silver samples with ozone (500 ppb) was investigated under certain relative humidity (RH) content (0%, 50% and 90%) in synthetic air. All experiments were performed at room temperature (22 °C) and atmospheric pressure. Highly surface sensitive methods were used to investigate chemical, morphological and structural changes and composition of the corrosion products formed at different RH and reaction times. Silver is oxidized by ozone forming Ag2O/ AgO(AgIAgIIIO2) surface species. The oxide formation and corrosion rate is dependent on the RH content in the atmosphere, showing that silver is most susceptible to ozone oxidation at 50% RH. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Metals such as gold, silver, copper or iron and their alloys have played a key role in the course of civilization. The golden, silver, bronze and iron age of human history signalled a major intellectual leap for each civilization that achieved it. Silver and its alloys have been valued as a precious metal, used for many objects of our cultural heritage such as statues, ornaments, jewellery, high-value utensils, currency coins and silverware because of its working properties and pleasant appearance. The luxuriousness, lustre and intrinsic value of silver have made it a material of admiration and desire for centuries. Today, silver metal is also widely used in electrical contacts and conductors, mirrors, optical devices and as a substrate for catalysis. Exactly those features which make silver so attractive for many applications are also its weaknesses. Unfortunately, the chemical stability of silver and its alloys is not as high as of gold, and exposed to different atmospheres the features of silver can be affected or even destroyed by the interaction of their surfaces with the ambient atmosphere. Conventional atmospheric parameters that affect silver comprise weathering factors (temperature, moisture, radiation, wind velocity, etc.), air pollutants and aerosols [1–3]. Nowadays increasing concentration of anthropogenic caused corrosive gases such as H2S, SO2, CO2 and O3 present in urban environments is challenging corrosion scientists as well as ⇑ Corresponding author. Address: Academy of Fine Arts, Institute of Science and Technology in Art, Schillerplatz 3, 1010 Vienna, Austria. Tel.: +43 1 588168616; fax: +43 1 588168699. E-mail address: [email protected] (R. Wiesinger). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.07.028

scientists and conservators dealing with art objects in museums, private collections or archaeological findings [4–12]. Therefore, it is of fundamental interest to understand the chemistry occurring on the surfaces in order to be able to control material degradation in the near future. This implies to study the occurring reactions in situ and in a time-resolved way in order to develop methods and strategies to reduce or even stop and prevent those atmospheric attacks [13–20]. Atmospheric corrosion of silver comprises complex reactions due to different physical and chemical surface reactions. As many macroscopic long-term investigations of the atmospheric corrosion of silver have been devoted in the past, the knowledge of the exact chemical reactions occurring on the surface during the early stages of corrosion is still rudimentary. Besides identifying the exact reaction mechanisms on the surface the study of synergistic effects in order to decrease or even stop corrosion on silver surfaces is of great importance. Many studies have focused on tarnishing of silver due to sulphur containing pollutants [7,8,12,21,22], and it could be shown that the presence of a strong oxidizer such as ozone increases the corrosion rate of silver if H2S or SO2 are present in the ambient atmosphere, but the specific mechanism for the interaction of the oxidizer has not been identified yet [5,10,20]. The chemistry of the ozone–silver interaction has been already considered in previous studies, especially the thermodynamic aspects of the O2/Ag and O3/Ag systems. At standard conditions (ambient pressure and temperature), silver is not oxidized by O2, as molecular or atomic oxygen is just chemisorbed on the surface [23–26]. In literature, [23,27–29] three different chemisorbed

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atomic oxygen species are described, namely Oa (weakly chemisorbed surface atomic oxygen), Ob (atomic oxygen dissolved in subsurface) and Oc (strongly chemisorbed surface atomic oxygen). If silver is exposed to high ozone concentrations (>ppm) the surface is immediately oxidized to form Ag2O. Furthermore, it is stated that prolonged exposure to ozone oxidizes Ag2O to AgO (AgIAgIIIO2) [23,24,30–38]. Other groups [5,24,37] investigated the influence of ozone and UV irradiation on bare silver and Ag powder by colorimetric reduction, SEM, XPS and Raman spectroscopy showing the formation of Ag2O and AgO (AgIAgIIIO2) surface species. Furthermore, studies [5,10] showed that the combined exposure of ozone and UV light causes rapid corrosion due to the photo-dissociation of ozone into atomic oxygen which reacts promptly with Ag to Ag2O. In the presence of UV radiation the corrosion rate seems to be proportional to the ozone concentration. Most interestingly the reaction of UV and ozone on silver seems to be hardly affected by the relative humidity. This behaviour is contrary to other degradation processes in atmospheric corrosion where the increase of relative humidity also causes an increased corrosion product formation. As the effects of ozone in combination with RH on silver are still inchoate a detailed study using highly sensitive analytical methods was performed to investigate the early stages of atmospheric corrosion behaviour of highly pure polycrystalline silver exposed to controlled atmospheres containing different amounts of RH (0%, 50% and 90%) and 500 ppb ozone. The experiments were performed for 24 h at room temperature (RT 22 °C) and atmospheric pressure. In order to obtain information concerning the chemical reactions and the surfaces species formed Raman spectroscopy, X-ray diffraction (XRD) and Scanning Kelvin Probe (SKP) were applied. For topographic information of the corrosion products Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) analysis was performed. A Quartz Crystal Microbalance (QCM) could be used for in situ gravimetric information of changing chemistry on the sample surface and bulk composition.

2. Experimental 2.1. Silver samples For the experiments silver square foils (25  25 mm2), 2.0 mm thickness, highly pure (99.9985%, Alfa AesarÒ) were used. The silver foils were freshly abraded using SiC-paper of various grits up to 4000 mesh, then polished with diamond paste (1 lm, StruersÒ), and finally washed with ethanol (p.a.) in an ultrasonic bath to remove any remnant of the abrading and polishing procedure.

2.2. Weathering experiments The weathering system consists on one hand of an air generating unit [20] and the weathering cell. The air generating unit provides pure synthetic air which can be batched with any relative humidity content and corrosive gas. The ozone was generated by an Ozonisator (Topchem GmbH, Germany, Model Airmaster OMX 500) and the ozone concentration in the gas stream was monitored by an ozone sensor (Aeroqual Limited, New Zealand, Model AQL S200). The weathering cell consists of polymethylmethacrylate (PMMA-PLEXIGLASÒ) with gas in- and outlets; the outer dimension are 5  5  3.3 cm3 with an inner volume of 40 cm3 and a dwell time of the gas mixture in the cell of 3.2 s. For the weathering experiments a concentration of 500 ppb O3 at 3 different RH contents (0%, 50% and 90%) was chosen. The samples were weathered for 24 h.

2.3. Micro Raman spectroscopy The instrumental system used is an integrated confocal microRaman system (Horiba Jobin Yvon, Germany, LabRAM ARAMIS) equipped with three internal lasers as excitation wavelength sources: Nd:YAG (532 nm), HeNe (632.8 nm) and Diode (785 nm). Measurements were performed using the Nd:YAG (532 nm) laser with a real output power of 1.48 mW; the Olympus BX41 confocal microscope is equipped with Leitz objectives of 10, 50 LWD (Long Working Distance), 50, and 100. The samples were observed with the 50 LWD objectives which give laser spots with a diameter of 0.87 lm, respectively. Acquisition and basic treatment of spectra were performed with LabSpec software (Horiba Jobin Yvon) and OriginPro 7.5: no baseline was subtracted, nor any smoothing operation was done in the recorded spectra. 2.4. X-ray diffraction To identify any crystalline corrosion products on the silver surface XRD (Philips X’Pert) was available. The samples were measured with a diffractometer (STOE GmbH, Germany) in grazing incidence X-ray diffraction (GIXD) geometry. The diffractometer is equipped with a Cu Ka X-ray tube and a Ge monochromator. The incidence angle for each measurement was 2° and the measurement time 2000 s per sample. The maximal penetration of the Cu X-rays in the pure silver is around 10 lm. Since the incidence angle was rather small, it can be assumed that the penetration depth was significantly smaller than 10 lm. Qualitative XRD analysis was done using the WinXPOW software and JCPDS (Joint Committee on Powder Diffraction Standard Data). 2.5. Scanning Kelvin probe The contact potential difference (CPD) of the silver samples was measured with a Scanning Kelvin Probe (SKP) system by KP Technology Ltd. (Caithness, UK). For the experiments a stainless steel tip (reference electrode) was used scanning a projected area uniformly. The tip moved 0.400 lm per step. The measurements of all the samples were performed applying the same gradient and the sample tip space within 1 lm. The scan area was 10 point 10 point, equivalent to 1600 lm2, where one point to point distance was 160 lm (100 steps). 2.6. Atomic force microscopy The topography studies of the weathered silver samples could be performed with an AFM (Atos, Germany, Topometrix ExplorerTM). Silicon cantilevers with integrated pyramidal silicon nitride tips (spring constant 0.05 N/m, resonance frequencies 15–30 kHz) were applied. The software to control the microscope was ThermoMicroscopes SPML 5.01. For data evaluation WSxM [39] (Nanotec Electronica S.L., Spain) was used. The software allows the generation of two- and three-dimensional images, estimation of particle sizes, and the possibility of height and distance measurements as well as the calculation of the rms (root mean square)roughness (Rq) for comparing the different treated surfaces of silver before and after weathering. It is calculated by Eq. (1) the following equation:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 Pn i¼1 ðZ i  Z Þ Rq ¼ n

ð1Þ

where Zi is the height value of each single data point in the image, Z the mean value of all height values in the image and n the number of data points within the image.

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2.7. Scanning electron microscopy The corrosion product morphology was investigated using a Philips XL30 environmental scanning microscope/field emission gun (ESEM/FEG). For the collection of micrographs the electron gun was operated at 10 kV accelerating voltage. 2.8. Quartz crystal microbalance To obtain gravimetric information of the occurring surface reactions in situ a Quartz Crystal Microbalance (Maxtek Inc., USA, QCM) could be applied. The deployed AT-cut quartz crystals (diameter 2.54 cm; thickness 333 lm) have a resonance frequency of 5 MHz, which change in proportion to mass changes<0.4 ng cm2 [40]. 3. Results and discussion 3.1. Chemical Identification by Raman spectroscopy, SKP and XRD Freshly polished polycrystalline silver samples were exposed to synthetic air and 500 ppb ozone with different RH content (0%, 50% and 90%) for 24 h at ambient pressure and RT. Raman analysis was performed after 3, 6 and 24 h for the differently weathered samples. Fig. 1 presents the Raman spectra obtained which contain bands at 95, 230–248, 350–370, 430, 490, 622, 810, 950, 1043 and 1140 cm1. Table 1 summarizes the appearing Raman bands which can be attributed to different chemisorbed molecular and atomic oxygen as well as silver oxide species. This assignment was made with reference to vibrational data reported for oxygen species on silver. The Ag reference which corresponds to a freshly polished and untreated silver surface shows two bands at 95 and 230– 248 cm1 which can be attributed to the Ag lattice and T0 modes of Ag+ cations [41–43] e.g. to chemisorbed molecular oxygen species. Even if the reference sample was measured immediately after the polishing and cleaning procedure, the measurements took place at an ambient atmosphere explaining a certain amount of molecular oxygen on the reference surface. This is also in accordance with literature [18,23] where it is stated that silver surfaces, when exposed to ambient conditions, form a layer consisting of chemisorbed oxygen. No further bands related to other species such as carbonate or other impurities are visible in the reference spectra. The sample exposed to synthetic air, 500 ppb ozone and 0% RH shows one additional band (compared to the Ag reference) after already 3 h of weathering at 1043 cm1. After 6 h of weathering additional bands at 622, 810, and 950 cm1 appear. The band at 1043 cm1 is either assigned to Ag2O/AgO species or to superoxide Ag[OAO] present on the surface as Ag+O 2 [13]. In our case it is suspected that this band could also be a combination band (430 + 622 = 1052 cm1) which is assigned to chemisorbed atomic oxygen. The band at 622 cm1 is related to subsurface atomic oxygen (Ob) but also to another dioxygen species, nominally O2 or 2 Ag[OAO]2, adsorbed in the subsurface sites and constituted by already formed features (Ag2O or AgO) [13,44,45]. Literature suggests that the bands at 950 cm1 may be related to molecular chemisorbed oxygen species [23], described as surface superoxo 1 species (O is typical for strongly chemi2 ). The band at 810 cm sorbed surface atomic oxygen and is a m(AgAO) stretching vibration [23,25,27–29]. As polycrystalline silver surfaces contain a significant amount of defects which are known to stabilize chemisorption [18,46] and also facilitate diffusion of atomic oxygen into the subsurface and bulk [18,23] we conclude that the band at

Fig. 1. Raman spectroscopy of polycrystalline silver samples exposed to synthetic air, 500 ppb ozone and different amounts of RH (0%, 50% and 90%) for 24 h. Intermediate measurements were performed after 3 and 6 h of weathering.

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Table 1 Assignment and chemical species of Raman shift for various silver oxide and chemisorbed oxygen species [13,23–25,29,32,36–38,41–43,48,59]. Raman shift (cm1)

Assignment

Species

Description

95 230–248

Ag lattice T‘ modes Ag+ cation m(AgAO2)

Ag Ag+ O 2

350–370

d(AgAO)

Oc

430

m(AgAO)

Oc

m(AgAO)

AgO (AgIAgIIIO2) Oa

Ag lattice Ag+ cation Molecular oxygen species chemisorbed Atomic oxygen strongly chemisorbed Atomic oxygen strongly chemisorbed Mode for Ag+ cations in AgO structure Atomic oxygen surface chemisorbed Stretching mode of bulk Ag2O Atomic oxygen dissolved in subsurface layer Dioxygen species, adsorbed in the subsurface sites Atomic oxygen strongly chemisorbed Molecular oxygen species chemisorbed Dioxygen species

490

Ag2O

m(AgAO)

Ob

m(OAO)

O2 2

810

m(AgAO)

Oc

950

m(OAO)

O 2

1043

m(AgAO) + m(OAO)

2 O 2 /O2 Ag2O

622

AgO 1140

m(AgAO2) + m(OAO)

O 2

Stretching mode of bulk Ag2O Mode for Ag+ cations in AgO structure Molecular oxygen species

622 cm1 is caused by oxygen atoms between the first layers of the Ag surface, as oxygen atoms in deeper domains will not be detected by Raman technique. After 24 h of exposure to synthetic air and ozone in dry conditions three more bands at 1140, 430–490, and 350–370 cm1 appear, which are also related to strongly chemisorbed surface molecular and atomic oxygen, but also to Ag2O/AgO species. Additionally, a shoulder at 930 cm1 is visible which has already been observed in previous studies and related to atomic oxygen involved in Ag@O [45]. According to literature silver when exposed to ozone (5 mol.%) [24] shows Raman bands at 490 cm1 assigned to the (AgAO) stretching mode of Ag2O and AgO which was only visible after 24 h of weathering in our experiments. In a previous study [43] highly pure Ag2O powder was analysed with Raman spectroscopy showing Raman shifts at 230–250, 342, 430, 487, 565, 933–950, 1072 and 1100 cm1. There the conclusion was drawn that it is rather difficult to differentiate between chemisorbed atomic oxygen and Ag2O species with Raman spectroscopy in this case. Due to our Raman measurements it can be concluded that a polycrystalline silver surface exposed to dry synthetic air and ozone under ambient pressure and RT mainly chemisorbs molecular and atomic oxygen during the first few hours of exposure, caused by dissociative adsorption of ozone. These atomic oxygen species form surface oxides consisting of ionic or nucleophilic oxygen and Agd+ species [47]. At the same time, ozone oxidizes silver directly to form Ag2O surface species. Comparing those results with the investigations where additionally RH was present, the following observations were made: the sample exposed to synthetic air, 500 ppb ozone and 90% RH shows small bands at 230–248, 622, 930–950, 1043 cm1 after 3 h of weathering which are related to mainly chemisorbed molecular oxygen species but also atomic subsurface oxygen and Ag2O/ AgO species. After 6 h of weathering the band at 810 cm1 appears, which is related to strongly chemisorbed atomic oxygen. After 24 h

bands in the region of 350–490 cm1 related to Oc, Ag2O are visible. These results clearly demonstrate that the RH seems to favour the formation of strongly chemisorbed surface oxygen Oc (810 cm1) which is in accordance with other studies [28,29]. Furthermore, the bands at 950 and 1043 cm1 show higher intensities at 90% compared to 0% RH suggesting an increased formation of superoxo but also Ag2O/AgO species. The sample weathered in 50% RH and ozone shows high intensities after already 3 h of weathering of the bands at 810 and 1043 cm1 (strongly chemisorbed atomic oxygen and Ag2O species) but also the bands at 622 and 950 cm1 (subsurface atomic oxygen and molecular oxygen chemisorbed) are quite well developed. After 24 h of weathering the same Raman bands are visible similar to the sample in 90% RH. The high intensities of the band at 1043 cm1, especially after 24 h of weathering may be due to certain surface enhancement effects caused by restructuring of surface due to the strong interaction of oxygen species causing a roughness favourable for SERS effects [27,42,48] and not necessarily by a higher amount of silver oxide species on the surface. Interesting is the relation of the bands at 810 and 950 cm1 (Oc and O 2 ) of the samples weathered in 50% and 90% RH. In the 50% RH spectra the band at 810 cm1 has similar intensities in the experiments carried out for 3, 6 and 24 h, while the band at 950 cm1 increases with continuous weathering. After 24 h both bands have nearly the same intensities. The sample exposed to 90% RH and ozone shows very low intensities for both bands after 3 and 6 h of weathering, leading to a small band at 810 cm1 and a high band at 950 cm1 after 24 h of exposure. This suggests that an Ag surface when exposed to ozone and 50% RH favours the formation of strongly chemisorbed atomic oxygen species, while the molecular chemisorbed oxygen is formed over time. At 90% RH obviously the formation of Oc and O 2 is a lot slower compared to 50% RH favouring the formation of molecular chemisorbed oxygen species after 24 h of weathering. SKP (Scanning Kelvin Probe) was used to calculate the contact potential difference (CPD) and work function (WF) of the differently weathered samples. The WF is a very sensitive indicator of different surface states, meaning bound and adsorbed species which induce substantial variations of energy required to remove an electron from the Fermi level [44,49–52]. Fig. 2 shows the SKP results scanned over an area of 1600  1600 lm2 of the untreated Ag reference and the samples weathered in synthetic air, 500 ppb ozone and at different RH for 24 h. It has to be mentioned that the primary output of the SKP measurements is the CPD, not the work function. Table 2 presents the mean CPD for 10  10 measurements per sample. Calculating the WF is quite delicate as it requires precise

Fig. 2. CPD over an area of 1600  1600 lm2 of the untreated Ag reference and the samples weathered in synthetic air, 500 ppb ozone and different RH for 24 h.

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knowledge of the work function of the reference electrode under measurement conditions. For our experiments a gold reference was used, as it is less prone to surface reactions, but still even smallest amounts of physisorption of surface adsorbates can produce a reduction of the gold work function. The values for absolute work function seen in Table 2 were calculated by assuming a gold WF of 5.1 eV (as it is common in literature). These values have to be handled with care and are considered to be merely indicative. The value of the CPD of the freshly polished Ag reference was found to be 127 mV, corresponding to a WF value of 4.98 eV. According to literature [51,52] the WF of a polycrystalline silver surface measured by CPD and photoelectric measurements ranges between 4.3 and 4.8 eV. The presence of ozone and RH on polycrystalline Ag rises the CPD e.g. also the WF. The highest WF increase of 0.53 eV was obtained for the sample exposed to 50% RH, followed by the sample weathered in 90% RH (DWF = 0.43 eV). The sample exposed in dry atmosphere shows a WF increase of 0.32 eV. According to literature Ag2O and AgO surface species have WF values in the range of 5.1–5.8 eV [51], which also corresponds to the obtained results in our study. On the other hand, also adsorbed and chemisorbed oxygen will lead to small increases of WF. Due to the area scan it can also be concluded that the surface chemical species are quite uniformly distributed. According to these results the sample exposed to an ozone atmosphere in 50% RH forms the highest amount of chemisorbed molecular and atomic oxygen as well as Ag2O/AgO species which is in accordance with the Raman spectroscopy results. This already highlights that silver is obviously most susceptible to ozone attack in atmospheres containing 50% RH, followed by 90% and 0% RH. XRD (X-ray diffraction) was used to examine the crystalline corrosion products formed as a function of freshly polished polycrystalline Ag sample exposure to 500 ppb ozone and different amounts of RH (0%, 50% and 90%) for 24 h. A comparison of the XRD patterns presented in Fig. 3 with the JCPDS (Joint Committee on Powder Diffraction Standard Data) and the literature [30,32– 34,36,37,53,54] permitted the identification of silver surface species formed. The polycrystalline Ag reference sample (untreated) depicts diffraction peaks at 2b angles of 38.2° Ag(1 1 1), 44.3° Ag(2 0 0), 64.2° Ag(2 2 0), 77.5° Ag(3 1 1), and 82.1° Ag(2 2 2). The sample exposed to ozone and 0% RH shows besides the diffraction patterns of polycrystalline cubic Ag (Reference) two additional weak peaks at 27.2° and 32.8° for cubic Ag2O. These two peaks can also be observed on the sample exposed to 90% RH and ozone. The sample exposed to 50% RH and ozone shows 5 additional peaks typical for Ag2O and AgO surface species. XRD analysis determined that exposing silver foils to 500 ppb ozone and 0% or 90% RH for 24 h leads to the formation of crystalline Ag2O surface species. In 50% RH and ozone atmosphere besides Ag2O also AgO surface species could be detected. Also concerning these results the sample exposed to 50% RH and ozone is oxidized most. 3.2. Topographical and morphological characterisation by SEM and AFM The changes in morphology of polycrystalline silver samples exposed to synthetic air, 500 ppb ozone and different amounts of RH

Table 2 Measured mean CPD and calculated WF for the Ag samples exposed to ozone and RH for 24 h at ambient pressure and RT. 500 ppb O3 (24 h)

Mean CPD (mV)

WF (eV)

Ag reference 0% RH 50% RH 90% RH

127 ± 5 452 ± 14 661 ± 9 561 ± 5

4.98 5.30 5.51 5.41

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Fig. 3. XRD patterns of the polycrystalline Ag reference and the Ag samples exposed to 500 ppb ozone and 0%, 50% and 90% RH after 24 h of weathering.

(0%, 50% and 90%) for 24 h were investigated by SEM shown in Fig. 4. The Ag reference micrographs show surface damage due to the sample preparation procedure. The silver surface is characterized by uniform small scratches and residues due to the polishing with diamond paste. Exposure to ozone had a strong effect on both, the visual appearance as well as the surface morphology of the silver surface. The sample exposed to ozone at 0% RH shows a uniform distribution of roundly shaped surface species of diameters ca. 30–90 nm after 24 h of weathering. It has to be noted that the scratches due to the polishing are still visible and that the corrosion products are in fact not covering the whole surface. This is not the case for the samples exposed to ozone under 50% and 90% RH, where the corrosion film completely covers the silver surfaces. The sample weathered under 90% RH also shows uniformly distributed round and agglomerated amorphous appearing features. Comparing the morphology of 0–90% RH sample the topography looks quite similar, only that the 90% RH silver sample is completely covered by the corrosion products, suggesting a thicker corrosion layer compared to the 0% RH sample but a similar reaction mechanism. In contrast to these observations, the sample exposed to 50% RH shows a different surface mainly covered by agglomerated amorphous appearing features with a few round features embedded or on top of it. These round features have diameters of approximately 30–180 nm. Comparing the micrographs of all the silver surfaces weathered the sample exposed to ozone and 50% RH shows the most corroded surface, which is in coherence with the already obtained results by Raman, SKP and XRD. AFM (Atomic Force Microscopy) measurements were carried out to directly assess the influence of ozone and RH on the film deposition, surface morphology and roughness. Fig. 5 presents the AFM images scanned over a 5  5 lm2 (2D) and 1  1 lm2 (3D) area. As already observed with SEM the untreated polycrystalline silver sample (Ag reference) shows scratches of different depths due to the polishing procedure. Fig. 5 makes clear that the amount of RH in the ozone containing atmospheres highly influences the morphology of the silver oxide surface films. Table 3 summarizes the roughness (rms) and the mean height of the samples at different RH contents.

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Fig. 4. Scanning electron micrographs of the freshly polished Ag sample (Ag reference), and the samples exposed to 500 ppb ozone for 24 h with 0%, 50% and 90% RH. All images were collected at an acceleration voltage of 10 kV, at magnifications of 10000 (upper row) and 40000 (lower row).

Fig. 5. AFM images scanned over a 5x5 lm2 (2D) and 1x1 lm2 (3D) area. Ag samples were exposed to synthetic air, 500 ppb ozone and 0, 50 and 90% RH for 24 h.

The 3D images already show that the sample exposed to 50% RH has the biggest agglomerated surface features followed by 90% and 0% RH, this is confirmed by the highest obtained roughness and mean height. Concerning roughness and mean height the 0% RH sample has the second highest values. Even if the surface of the 0% RH sample has quite small uniform features, the silver surface still shows the scratches from polishing and is not yet completely covered by the corrosion film, which is also visible in the SEM images. This fact yields higher rms and mean height values compared to the 90% RH sample, where the surface features are already quite big, but cover uniformly the scratched surface. Table 3 Calculated roughness and mean height for the Ag samples after exposure for 24 h to synthetic air, 500 ppb ozone and different amounts of RH. Ag synth. air, 500 ppb O3, 24 h

Roughness (rms)

Mean height (nm)

Reference 0% RH 50% RH 90% RH

9.874 18.557 32.825 15.110

49.58 63.24 120.82 49.21

3.3. Gravimetric investigation by QCM Fig. 6 presents the in situ QCM profiles obtained from silver samples exposed to synthetic air with 0%, 50% and 90% RH and 500 ppb ozone for 24 h. Table 4 summarizes the obtained mass increase of the 3 different samples after 3, 6 and 24 h. Furthermore, the water layer thickness on the sample surface was calculated by turning off the RH for 1 min during the weathering. The water layer thickness was calculated to be 0.381 nm for the sample in 50% RH and 4.42 nm for the sample exposed to 90% RH. Analysing the curve slope of the sample exposed to ozone in 0% RH a constant nearly linear mass increase can be observed. Supposing a linear inclination for this curve a slope of 0.0002 was calculated. The sample exposed to ozone in 90% RH containing atmosphere shows a fast mass increase in the first 70 min of weathering and then proceeds also in a linear matter with a slope of 0.0029. This linear mass gain could not be observed for the sample exposed to 50% RH. For the first 600 min of weathering the mass gain can also be described to be linear with a slope of 0.0032 which is quite similar to the slope of the sample in 90%

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Fig. 6. In situ mass gain obtained by QCM measurements of silver samples exposed to synthetic air, 500 ppb ozone and 0%, 50% or 90% RH for 24 h at ambient pressure and RT.

Table 4 Mass gain of the Ag samples after 3, 6 and 24 h. Ag

Dm (lg/cm2)

Dm (lg/cm2)

Dm (lg/cm2)

500 ppb O3 0% RH 50% RH 90% RH

3h 0.227 0.411 1.717

6h 0.285 0.597 2.252

24 h 0.485 8.086 5.384

RH (k = 0.0029). After 600 min of weathering the mass gain progresses in a logarithmic manner and proceeds again nearly linear until the end of the experiment. For this sharp mass gain after 600 min a slope of 0.0095 was calculated. This results in a 3 faster corrosion rate for the sample exposed to ozone in 50% RH compared to the sample in 90% RH. After 24 h of weathering the highest mass increase is obtained by the sample exposed to ozone and 50% RH, followed by 90% and 0% RH (Table 4). These in situ results prove the results obtained with the other applied methods, that the sample in 50% RH atmosphere forms the thickest corrosion layer. These in situ QCM results show that the silver oxidation process seems to be similar for the samples exposed to ozone in 0% or 90% RH, while in 50% RH the surface degradation is different over time. It can be concluded that during the first few hours of weathering (0–10 h) the chemisorption and oxidation rate is higher with increasing RH content of the surrounding atmosphere. After around 10 h of weathering the chemical degradation process of the sample exposed to ozone and 50% RH changes obviously due to a faster oxidation process.

3.4. Summary of the surface reaction mechanisms Summarizing all the results the following surface reaction mechanisms can be suggested. If a silver surface is exposed to ozone in a dry atmosphere the surface forms chemisorbed species as well as silver oxides. The formed silver oxides and chemisorbed species occupy the silver surface leading to a very slow proceeding corrosion product formation. These formed surface species seems to act as a passivation layer, preventing a further fast oxidation due to ozone. This mechanism is different for Ag samples exposed to ozone in humidified atmosphere. When exposed to ambient air, ozone and RH silver samples are immediately covered by a thin film of oxide or hydroxide species. Furthermore, water will be physisorbed on this hydroxylated surface forming an adlayer, whereby the number of such adlayers is proportional to the humidity content of the

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ambient atmosphere. Additionally, corrosive gases, such as O3, H2S, SO2, etc. [2,3,55] may form solvated species or react with water or other species present in the water layer, and contribute to the degradation in a variety of chemical reactions. For a bulk water reaction the ratio of the area to volume is of the order of 100 (m1) compared to 109 (m1) for a water adlayer system. On the basis of these facts for a certain concentration of species in a water solution, the relative influence of surface species on chemical reactions will be much greater in a water adlayer system compared to an ordinary bulk water system [56,57]. Concerning our experiments the presence of humidity obviously favours the formation of chemisorbed and silver oxide surface species. On the other hand the experiments showed that Ag exposed to ozone and 50% RH is a lot more susceptible to oxidation compared to atmospheres with 90% RH. This concludes that the different amounts of RH and in fact number of water monolayers highly affect the corrosion mechanism. According to literature [58] 3 or more water monolayers on a surface approach bulk water properties, which is the case for RH contents of 65%, depending on the substrate. This could have been the case in our experiments where in 90% RH the surface water layers might show bulk water properties, slowing down the diffusion of ozone to the metal surface on one hand, and intermediate reactions of ozone in the water film on the other hand. In 50% RH atmosphere, which is equivalent to 2 water monolayers it is more an adlayer system. Another point to consider is that an already formed thin film of a corrosion product or a hydroxylated oxide can be considered as extended structures bearing surface functional groups. Those groups possess a high diversity of interactions for species in a water layer [56]. It was proven [57] that metal oxides are covered with surface hydroxyl groups with an average surface density of the order of 5  1014 hydroxyls/cm2 of the oxide. Furthermore, the hydroxyl groups can release and take up hydrogen ions or be replaced by other ligands to form surface complexes. This could be an explanation for the sudden mass increase obtained by QCM of the sample in ozone atmosphere at 50% RH after 10 h of weathering. At this point of time obviously a certain amount of oxides was already formed, reacting with the RH to hydroxylated oxides which promotes further formation of silver oxide species.

4. Conclusions The interaction of highly pure polycrystalline silver samples with ozone was investigated under different RH contents in synthetic air at ambient pressure and RT to elucidate the mechanisms behind the chemistry of both oxygen and ozone-exposed silver metal and silver oxide surface species. (i) Silver reacts with ozone in dry atmosphere either by direct oxidation to form Ag2O or by dissociative adsorption of O3, producing strongly chemisorbed surface and subsurface atomic oxygen but also chemisorbed molecular oxygen species. (ii) The experiments showed that silver surfaces exposed to synthetic air, 500 ppb ozone and different amounts of RH mainly chemisorb molecular and atomic oxygen species during the first hours of weathering with a subsequent oxidation and formation of Ag2O/AgO surface species. (iii) The presents of RH in the ozone atmosphere enhances the chemisorption and oxidation process of silver. (iv) Highly interesting is the fact that silver is most susceptible to ozone oxidation at RH contents of 50%, followed by 90% and 0%, which is contrary to other corrosive gases in contact with silver, where an increase of RH is directly related to a higher corrosion rate.

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This observation makes clear that silver corrosion mechanisms involving O3 cannot be compared to other corrosive gases, where an increasing RH content is directly related to an increase of the corrosion rate. References

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