The interaction of silanes with amorphous silicon oxide surfaces

ARTICLE IN PRESS

International Journal of Adhesion & Adhesives 26 (2006) 79–87 www.elsevier.com/locate/ijadhadh

The interaction of silanes with amorphous silicon oxide surfaces T. Choudhury, F.R. Jones University of Sheffield, Department of Engineering Materials, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK Available online 17 May 2005

Abstract g-Mercaptopropyltrimethoxy silane (SPS) has been adsorbed onto model surfaces from aqueous solution. Silica glass, acid-treated E-glass and E-glass have been employed to establish if the Si 2p core has electrons of significantly different binding energy in the siloxane structures associated with a hydrolysed trialkoxy monoalkyl silane and a silicate surface which enables differentiation to be achieved. It is shown that it is practical for silica surfaces including E-glass denuded of the network modifiers but not for E-glass surfaces. r 2005 Published by Elsevier Ltd. Keywords: A: Primers and coupling agents; B: Glass; B: Interfaces; C: X-ray photoelectron spectroscopy

1. Introduction Silane coupling agents are often used as adhesion promoters on silica-based minerals or glasses, such as E-glass. Most X-ray photoelectron spectroscopy (XPS) studies have employed silanes with appropriate labels such as the nitrogen in g-aminopropyltriethoxy silane (g-APS) [1–5] because of uncertainty in the assignment of a binding energy for the R–Si(OH)3 with respect to Si(OH)4 component. However, some authors [6,7] have assumed that differentiation can be readily achieved. The object of this paper is to establish the methodologies which allow the contributions of the hydrolysed monoalkyl substituted trialkoxy silane unit and the silica of the substrate to the XPS spectrum, to be identified. We have recently reported [3,4] the interaction of g-APS with an E-glass surface. Even with the resolution of a monochromatic source and the high performance of the ESCA 300 instrument, it has proved impossible to separate these contributions and interpretation had to Corresponding author. Tel.: +44 114 222 5477; fax: +44 114 222 5943. E-mail address: f.r.jones@sheffield.ac.uk (F.R. Jones).

0143-7496/$ - see front matter r 2005 Published by Elsevier Ltd. doi:10.1016/j.ijadhadh.2005.03.007

rely on a diagnostic element in the silane. For g-APS, the nitrogen can be readily employed to determine the extent of the interaction. Other authors [6–7] have used the difference between the O 1s and Si 2p binding energies to differentiate between the silicon from the silane adsorbate and that from the substrate. Model experiments have been undertaken in an attempt to separate the siloxane component in the Si 2p spectra, which arises from the silane deposit, from that of the glass substrate. g-Mercaptopropyltrimethoxy silane (SPS, A189) was chosen because the sulphur provides a ‘diagnostic’ element. Furthermore, in contrast to the gAPS, where extensive transfer between the substrate and deposit has been reported [3–5], the incorporation of aluminium ions from the glass into the coating has been shown to be absent [8]. High-purity silica glass was selected as the control substrate to maximise the probability of identifying the differing chemical environments. To provide a model glass substrate, E-glass was treated with nitric acid to produce a silica-rich surface. In this case Al, Ca, Na, K, Mg and Fe are extracted leaving a surface composed of mainly Si–O and Si–OH and can be considered to be representative of a hydrated silica, E-glass surface.

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

O1s

0

1200

(a)

C1s

10

1000 800 600 400 Binding Energy (eV)

Si2s Si2p1 O2s

20

200

20 15 10 5 0 (b)

1200

1000 800 600 400 Binding Energy (eV)

C1s S2s S2p Si2p O2s

2.2.3. Treated E-glass surfaces E-glass cullet (Owens-Corning Fibreglass, Wrexham) was melted and cast into rectangular blocks from which slides 10  10  2 mm3, were cut with a diamond impregnated wheel. These were polished down to a 1 mm thickness and to 1 mm surface finish, with diamond

30

O1s

2.2.2. Silanised silica glass surface Silica glass discs were immersed in a 3% solution (by weight) of A189 g-SPS in deionised water for 45 min, under agitation. Some samples, denoted ‘3%-coated’ were removed and dried in a vacuum oven at 50 1C for 12 h, for analysis. Warm-water-extracted samples were obtained by immersing the remaining samples in deionised water at 50 1C, for 12 h. One sample was removed and dried in a vacuum oven for 12 h prior to analysis. A hot-water-extracted sample was prepared by placing a warm-water-extracted sample in deionised water at 100 1C for 4 h and drying it in vacuum at 50 1C for 12 h. A ‘1.5%-coated’ sample was prepared using a 1.5% (by weight) of A189 SPS in a mixture of methylated spirit (20%) and deionised water (80%) analogously to the above.

3.1.1. Survey spectra Typical survey spectra for the uncoated silica glass and ‘3%-coated’ surfaces are shown in Figs. 1a and b. Deposition of the silane is confirmed by the introduction of the S 2s and S 2p peaks at 225 and 164 eV. The surface compositions in at.% for all of the surfaces have been calculated as described above and are shown in Table 1. A TOA of 451 was used for most of the analyses but the ‘3%-coated’ sample was also examined at a TOA of 101, because previous data suggested that a thinner

Oa Oa

2.2.1. Untreated silica glass surface Quartz silica glass discs were obtained from MultiLab Ltd. The discs were washed in isopropyl alcohol (IPA) in an ultrasonic bath prior to heat cleaning at 200 1C for 2 h. These were then stored in an airtight container.

3.1. Silanised silica glass

Oa Oa

2.2. Materials

3. Results

Ca

The analyses were carried out on a Scienta ESCA 300 spectrometer, using monochromatic Al Ka radiation at 14 keV and 200 mA (Daresbury Laboratory). A pass energy of 300 eV for the survey spectra and 150 eV for the high-resolution spectra scans of selected regions. The slit width of the electron analyser was set at 1.9 mm for the former and 0.5 mm for the latter. Take-off-angles (TOA) relative to sample surfaces of 451, 301 and 151 were employed. Charge compensation was accomplished with an electron flood gun operating at an electron energy of 4–6 eV. The Cls line at 285.0 eV was used to reference the spectra. The relative concentrations of the elements were calculated using appropriate atomic sensitivity factors specific to the Scienta ESCA 300. Curve fitting after background subtraction was conducted assuming a complex mixed Gaussian–Lorentzian peak shape. Asymmetry was implemented with a tail function dependent on photoelectron energy.

Ca

2.1. X-ray photoelectron spectroscopy (XPS)

paste. The samples were washed with deionised water and heated at 200 1C for 2 h. ‘Nitric acid-treated E-glass’ slides were prepared by immersing E-glass slides in nitric acid for 20 min, then washed twice in fresh nitric acid, washed in deionised water and heat cleaned at 200 1C for 2 h. ‘SPS-silanised nitric acid-treated’ E-glass slides were prepared by immersion in a 3% solution (by weight) of A189 in deionised water for 45 min under agitation, and dried in vacuum at 50 1C for 12 h. ‘SPS-treated’ E-glass surface was prepared analogously by immersion in a 3% A189 aqueous solution (i.e., without prior nitric acid treatment) for 1 h. The samples were dried in a vacuum at 50 1C for 12 h.

Counts × 103

2. Experimental

Counts × 103

80

200

Fig. 1. XPS survey spectra of (a) the uncoated silica glass surface and (b) the ‘3%-coated’ surface. ‘a’ refers to the Auger peaks for C and O.

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

81

Table 1 The surface composition (in at.%) of silane-treated silica glass determined from the survey spectra at a TOA of 451 and 101 (in parenthesis) Element

Si 2p

S 2p

C 1s

O 1s

Cu 2p

N 1s

Uncoated silica glass 1.5%-coated surface 3%-coated surface Warm-water (50 1C)-extracted 3%-coated surface Hot-water (100 1C)-extracted 3%-coated surface

26.2 16.2 22.6 (17.3) 18.8 19.1 (12.7)

— 5.2 2.5 (5.0) 2.0 2.3 (3.9)

16.1 46.4 26.2 (42.6) 35.2 31.9 (49.0)

57.7 31.9 48.4 (34.9) 42.4 44.0 (30.2)

— 0.3 0.3 (0.2) 0.6 0.8 (1.7)

— — — 1.0 1.9 (2.5)

The silane was g-mercaptopropyltrimethoxy silane (SPS).

Table 2 The surface composition (in at.%) of treated E-glass surfaces determined from the survey spectra at a TOA of 451 Sample

Si 2p (%)

S 2p (%)

C 1s (%)

O 1s (%)

Al 2p (%)

Ca 2p (%)

Untreated E-glass slide Nitric acid-treated E-glass SPS-silanised nitric acid-treated E-glass SPS-treated E-glass (without prior nitric acid treatment)

17.0 20.1 19.4 10.6

— — 3.4 8.8

19.4 29.5 32.0 53.5

53.6 50.4 45.2 27.1

6.8

3.3

— —

— —

coating may have been deposited. At a TOA of 451, the ‘1.5%-coated’ surface exhibited the maximum concentration of sulphur. Thus, a thicker coating was achieved using a 1.5%-solution of the SPS (A189) than from a 3%-solution. Since the ‘1.5%-coated’ surface was prepared with the A189 in a solution of alcohol and deionised water (20% MS and 80% water) whereas the ‘3%-coated’ surface was prepared from deionised water alone, the increased thickness can be attributed to differing degrees of hydrolysis and polymerisation. In previous experiments where g-APS was deposited from methanol, similar observations were made [9]. For both the ‘3%-coated’ and ‘1.5%-coated’ surfaces, the Si concentration is shown to be reduced on deposition of the silane because Si concentrations in the silane is lower than that in silica. Traces of copper and nitrogen were present on some of the surfaces. These were contaminants found to arise from a poor batch of deionised water, which was used to prepare the silane solutions. In the hot water-extracted surface, the Cu and N concentrations are seen to be higher at the take-off angle of 101, which corresponds to a shallower analysis depth confirming that the Cu and N signals are more likely to be surface contaminants rather than in the substrate. In subsequent experiments, these contaminants were absent as shown in Table 2. Access to the highresolution spectrometer was limited so this was checked on a different spectrometer. Extraction in water at elevated temperatures did not lead to a significant reduction in the S concentration. This shows that the deposit was strongly bound to the substrate and resistant to hydrolysis as a result of a high degree of cross linking.

3.1.2. The O 1s, C 1s, Si 2p and S 2p spectra Table 3 shows the binding energies for the elements O, C, Si and S with their possible chemical state identity. The chemical bonding of Si and S are of major interest in this study. These have been identified by comparison to reference assignments, as shown in Table 4. The O 1s XPS spectra, for the uncoated silica glass is shown in Fig. 2a. Only one component, at 532.9 eV, could be fitted to this peak which demonstrates that it can be attributed to the oxygen in the SiO2 chemical environment. However, since the FWHM is relatively broad (1.5 eV), other oxygen containing groups such as Si–OH, C–O, C–OH and adsorbed water, may contribute. For the A189 SPS coated surfaces, the major O 1s contribution still occurs at 532.9 eV but a minor (1% area) peak at 531.0 eV could be resolved, which could be attributed to CuO contamination, because of the presence of a small peak of Cu in the survey scans. Thus the O 1s peak shape shows very little change after silane treatment. These findings are summarised in Table 5. Fig. 2b shows the S 2p peak resulting from the ‘3%coated’ surface. No change in the peak shape was observed after extraction in water. The spectra exhibits a S 2p doublet at 163.7 and 164.9 eV which can be attributed to sulphur–hydrogen or sulphur–carbon bonds. This is concluded after comparison of the measured binding energies with the literature assignments, given in Table 4. Fig. 3a and b show the C 1s spectra together with their curve fits, for the uncoated and ‘3%-coated’ surfaces, respectively. The quantification of this curvefitting data is given in Table 6. The uncoated surface

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

82

Table 3 The experimental core line binding energies for SPS-coated silica glass surfaces Core line

BE (eV)

FWHM (eV)

Assignment

Comments

O 1s

531.0 532.9

1.5 1.5

Cu–O Mainly SiO2 and Si–OH, with (C–O, C–OH and H–OH, etc.)

Trace amounts. Not on uncoated surface Only on uncoated surface

C 1s

285.0 286.6 287.9 289.2

1.5 1.5 1.5 1.5

C–C, C–H C–O, C–OH O–C–O, O–CQO, Acid or ester bond

On all surfaces On all surfaces On uncoated but not coated surfaces On uncoated surface but not coated surfaces, reappears after aqueous extraction

Si 2p3/2 Si 2p1/2

103.5 104.2

SiO2 and Si–OH

On all surfaces

Si 2p3/2 Si 2p1/2

102.4 103.1

Si–C, Si–CH

Not on uncoated surfaces

S 2p3/2 S 2p1/2

163.7 164.9

S–C, S–H

Not on uncoated surfaces

Table 4 Si 2p and S 2p assignments Measured BE(eV)

Reference BE(eV)

102.4 102.4

102.4 101.8

Chemical environment

Control surface Polydimethylsiloxane [10] Silicone (Si 2p3/2) in: poly(dimethylsiloxane) [11]

CH3 ( Si

O

)n

CH3 163.7

163.5

( CH2

shows four distinct carbon environments, with the binding energies of 285.0, 286.6, 287.9 and 289.2 eV. The deposition of SPS from the 1.5% and 3% solutions, gave surfaces with only two C 1s environments at 285.0 and 286.6 eV. However, upon warm and hot water extraction, the component at 289.2 eV reappeared. This appears to suggest that the silane is partially removed by the extraction process exposing the carbonaceous contamination on the glass surface. The small reduction in the concentration of sulphur (Table 1) may therefore be significant. The Si 2p spectra for the uncoated silica glass and for ‘3%-coated’ surface, respectively, are shown in Fig. 4a and b. The curve fitting is extremely important because it shows that the silane component of the Si 2p spectra can be separated from that of the glass substrate. In the uncoated surface, the peaks at 103.5 and 104.2 eV represent the 2p doublets associated with SiO2 and Si–OH. However, the peak shape changed substantially after SPS treatment, with the introduction of a second set of peaks at 102.4 and 103.1 eV. These can be attributed to a siloxane bond (see Table 4). Table 7 gives

CH2

S )n

Sulphur (S 2p3/2) in: poly(ethylene sulphide)

the fraction of each component in the two Si states. These results show that the silane deposit (from a 3% solution) is not significantly removed on extraction for 12 h in warm water and a further 4 h in hot water. This is in agreement with the data presented in Table 1, obtained from the survey spectra, especially the concentration of the sulphur label. The reappearance of the component at 289.2 eV in the C 1s spectrum after extraction with water at elevated temperatures (Table 6) suggests that a small fraction of the silane deposit was removed. However, maintenance of Si spectrum is strongly indicative that this arises from a patchy carbon contamination which was introduced by the original polishing and/or washing procedures. This experiment does, however, indicate that it is possible to differentiate the silicon signals arising from the silica glass and that from the silane deposit. Recently, it has become recognised that an additive shift of 0.6 eV can be applied to silicones on introduction of an oxygen. Thus, SiO4 occurs at 103.6 eV with SiC4 at 101.2 eV. For the silane-coated surfaces, the component at 103.1 eV can be readily attributed to

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

83

25 532.9 eV 285.0 eV

3000 Counts

Counts × 103

20

15

2000

10

287.9 eV

5

1000 Binding Energy (eV)

(a)

0

(a)

286.6 eV

Binding Energy (eV)

5000

1800

285.0 eV

1600

2p3/2 (163.7 eV)

Counts

1400 2p1/2 (164.9 eV)

Counts

4000

1200

286.6 eV

2000

1000

1000

800

(b)

600

(b)

3000

Binding Energy (eV)

Fig. 3. The C 1s spectra curve fits for (a) the uncoated silica glass surface and (b) the ‘3%-coated’ SPS surface.

Binding Energy (eV)

Fig. 2. (a) The O 1s spectra from the uncoated silica surface and (b) the S 2p spectra from the ‘3%-coated’ surface with g-mercaptopropyltrimethoxy silane SPS.

Table 5 The O 1s curve-fitting data for SPS-coated silica glass surfaces Sample

532.9 eV (area fraction %)

531.0 eV (area fraction %)

Un-coated silica glass 1.5% coated 3% coated Warm-water (50 1C) extracted Hot-water (100 1C) extracted

100 99.0 98.8 97.4

— 1.0 1.2 2.6

96.7

3.3

SiOC3 which is the main environment in these hydrolysed trialkoxy silanes. The component at 102.4 eV (Fig. 4) can be attributed to both the Si 2p3/2 or SiO2C2 (which could be present as a contaminant) complicating the assignment, making differentiation difficult. 3.2. Silanisation of E-glass after acidic pre-conditioning 3.2.1. The nitric acid-treated E-glass surface Table 2 shows that nitric acid pre-treatment depletes the E-glass surface (to a depth greater than the analysis

depth of 5–10 nm) of components such as Al and Ca giving a surface which is dominated by Si and O 3.2.2. SPS-silanised E-glass surface The composition of the surface of the SPS-treated Eglass (without prior nitric acid treatment) are given in Table 2. The presence of the silane deposit is confirmed by the S 2p line. The Si 2p and O 1s concentrations decrease while the C 1s concentration increases. In contrast to g-APS [3–5], Al and Ca are absent from the spectrum inferring that either a relatively thick film has been deposited or that the interaction between the surface and SPS differs. In previous studies with g-APS, the Al was shown to migrate into the coating. 3.2.3. SPS-silanised nitric acid-pre-treated E-glass surface In Table 2, it can be seen that the concentration of sulphur on the surface of the nitric acid-pre-treated Eglass surface onto which SPS was deposited is much lower than the sample without the acid pre-treatment. This could be attributable to a slightly shorter deposition time of 45 min compared to 60 min. However, the sulphur concentration for the deposit on the silica glass surface also after 45 min was at a comparative level (albeit slightly lower) as shown in Table 1. This indicates that the thickness of the silane deposit on an E-glass surface is significantly larger than when deposited onto

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

84

Table 6 The C 1s curve-fitting data for the SPS-coated silica glasses Sample

285.0 eV fraction (area %)

286.6 eV fraction (area %)

287.9 eV fraction (area %)

289.2 eV fraction (area %)

Un-coated silica glass 1.5% coated 3% coated Warm-water (50 1C) extracted Hot-water (100 1C) extracted

56.5 72.1 72.9 82.1 75.0

36.0 27.9 27.1 14.2 21.5

3.8 — — — —

3.7

showing that the thickness of the deposit is such that the substrate does not contribute to the analysis.

8000 2p3/2, 103.5 eV

Counts

6000

4000

0 Binding Energy (eV) 5000

Counts

4000

2p3/2, 103.5 eV

3000 2p1/2, 104.5 eV

2000

2p1/2, 103.1 eV 2p1/2, 102.4 eV

1000 0 (b)

3.3. Binding energy assignments for the ‘model (nitric acid-pre-treated) E-glass surfaces’ before and after SPS treatment

2p1/2, 104.2 eV

2000

(a)

3.5 3.5

Binding Energy (eV)

Fig. 4. The Si 2p curve fits for (a) the doublets for the uncoated silica glass and (b) the ‘3%-coated’ surfaces and (b) shows the silicon states from the glass and the silane.

‘silica’-like surfaces, which demonstrates how the silane deposition is strongly surface dependent. Eldridge et al. [1] found that g-APS tended to be adsorbed as a monolayer, whereas Jones et al. [3,5] observed multilayer deposits on E-glass surfaces. With a thin silane deposit, the glass substrate can contribute to the analysis (in contrast to the SPSsilanised E-glass surface) which is confirmed by the small reduction (10% and 4%) in the O 1s and Si 2p concentrations (Table 1). For the E-glass surface (without nitric acid treatment) the concentrations of these elements are reduced 50% and 38%, respectively,

Table 8 reports the observed binding energies for the elements O, C, Si and S with their assignments, for the nitric acid-treated E-glass before and after SPS treatment. It can be seen that the silane contribution to the Si 2p binding energies can be resolved from that of the E-glass when the Al and Ca in E-glass have been extracted. The O 1s peak shows only the presence of one component at 532.9 eV; even after SPS treatment. Thus the peak at 532.9 eV can be assigned principally to Si–O and Si–OH bonds although C–O, C–OH may also contribute. In this case the Al–O and Ca–O contributions which complicate the assignment for an E-glass surface have been removed. 3.3.1. Quantification of the Si 2p spectra from the ‘model E-glass’ systems In Table 9, the results from the curve-fitted Si 2p spectra belonging to the ‘nitric acid-treated E-glass’ and the ‘SPS-silanised nitric acid-treated E-glass’ surfaces are compared. Approximately 29% of the Si contribution in the SPS-silanised nitric acid-treated surface originates from the siloxane deposit.

4. Discussion Table 10 lists the core binding energies obtained from the untreated E-glass and SPS-treated E-glass surface. Comparing the data from the model (nitric acid-pretreated) E-glass surfaces in Table 8 with those from true E-glass surface in Table 10, it can be seen that the only difference is in the binding energies for O 1s and the Si 2p spectra. The O 1s spectra from the untreated E-glass surface (Table 10) shows three peaks at 533.0, 532.0 and 531.0 eV, all with FWHM of 1.5 eV. The peak at

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

85

Table 7 The Si 2p curve-fitting data for the SPS-coated silica glass surface Sample

Silica glass contribution (SiO2 and Si–OH) (area %)

silane contribution (i.e. siloxane bond) (area %)

Un-coated silica glass 1.5% coated 3% coated Warm-water (50 1C) extracted Hot-water (100 1C) extracted

100 44.5 77.7 77.0 78.6

— 55.5 22.3 23.0 21.4

Table 8 The core line binding energies (relative to C 1s binding energy at 285.0 eV) for the model nitric acid-treated E-glass surface before and after SPS treatment Core line

BE (eV)

FWHM (eV)

Assignment

Comments

O 1s

532.9

1.5

SiO2 and Si–OH with a possible contribution from C–O, C–OH and H–OH

On both surfaces

C 1s

285.0 286.6 287.9

1.5 1.5 1.5

C–C, C–H C–O, C–OH O–C–O, O–CQO

289.4

1.5

Acid or ester bond

On both surfaces On both surfaces On nitric acid-treated E-glass surface On nitric acid-treated E-glass surface

Si 2p3/2 Si 2p1/2

103.5 104.2

SiO2 Si–OH

On both surfaces

Si 2p3/2

102.4

Si–C, Si–CH

Not on nitric acid-treated E-glass surface

Si 2p1/2

103.1

S 2p3/2

163.2

S–C, S–H

Not on nitric acid-treated E-glass surface

S 2p1/2

164.4

Table 9 The Si 2p curve-fitting data for the model silanised nitric acid treated E-glass surface Sample

E-glass contribution Si 2p peaks at 103.5 and 104.2 eV (%)

Silane contribution Si 2p peaks at 102.4 and 103.1 eV (%)

Nitric acid-treated E-glass SPS-silanised nitric acid-treated E-glass

100 70.8

— 29.2

533.0 eV has been attributed to mainly adsorbed water (H–OH), the peak at 532.0 eV has been attributed to mainly Si–O and Si–OH bonds and the peak at 531.0 eV has been attributed to Al–O and Ca–O bonds. But in Table 8 the O 1s spectrum, from the nitric acidtreated E-glass surface, only shows the presence of one peak at 532.9 eV also at a FWHM of 1.5 eV. The peak at 532.9 eV from the nitric acid-treated E-glass surface is expected to arise mainly from the Si–O and Si–OH. Since the surface does not contain any Al–O and Ca–O bonds, the O 1 s peak (Si–O and S–OH) cannot be

influenced by the presence of these inorganic hydroxides. Hence, the Si–O and Si–OH bonds appear at the slightly higher binding energy of 532.9 eV rather than at 532.0 eV for the untreated E-glass surface. The adsorbed water (H–OH) on the nitric acid-treated surface should not be influenced by the absence of Al–O and Ca–O type bonds. Thus, for the nitric acid-treated surface, the Si–O, Si–OH contribution cannot be separated from the H–OH, C–OH contributions and only one peak at 532.9 eV is observed. These assignments for the O 1s binding energy of the nitric acid-treated surface are

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

86

Table 10 The core line binding energies (relative to C 1s binding energy at 285.0 eV) for the E-glass surface before and after SPS treatment Core line

BE (eV)

O 1s

533 532 531

C 1s

FWHM (eV)

Assignment

Comments

1.5 1.5 1.5

H–OH (with C–OH, C–O) Si–O, Si–OH Al–O, Ca–O

On both surfaces

285 286.6 287.9 289.4

1.5 1.5 1.5 1.5

C–C, C–H C–O, C–OH O–C–O, CQO acid or ester

On both surfaces

Si 2p3/2 Si 2p1/2

102.4

103.1

Si–O and Si–OH, and Si–C, Si–CH

On both surfaces

S 2p3/2 S 2p1/2

163.7

164.9

S–C, S–H

On SPS-treated E-glass surface

Table 11 A comparison of the Si 2p binding energies for the E-glass systems Sample

Si 2p1/2 BE (eV)

FWHM (eV)

Si 2p3/2 BE (eV)

FWHM (eV)

Untreated E-glass SPS-silanised E-glass APS-silanised E-glass GPS-silanised E-glass

103.1 103.1 103.1 103.1

1.4 1.4 1.4 1.4

102.4 102.4 102.4 102.4

1.4 1.4 1.4 1.4

All binding energy and FWHM values are within an error of 70.2 eV.

experimentally confirmed by comparing them with those from the untreated silica glass surface given in Table 8, where only one O 1s peak at 532.9 eV is observed. In the Si 2p spectra, Table 8 shows that the contributions from the SPS deposit can be differentiated from the substrate silicon, in the nitric acid-treated Eglass surface. In contrast, Table 10 shows that the contributions in the Si 2p spectra from the SPS deposit, on the E-glass substrate (without acid treatment), cannot be separated from that of the substrate. In the acid-treated E-glass surface, the Si 2p3/2 line, from Si–O, Si–OH appears at the same binding energy of 103.5 eV found for the untreated silica glass surface (Table 2). When the nitric acid-treated E-glass surface is subsequently treated by SPS, the Si 2p3/2 line shows peak broadening resulting in the appearance of an extra peak at 102.4 eV. Since the binding energies are 1 eV apart, the contributions from the silane deposit can be fairly easily separated from that of the nitric acid-treated Eglass substrate. However, in the untreated E-glass surface (Table 10), the Si 2p3/2 appears at 102.4 eV, which is identical to the binding energy expected from the Si 2p3/2 line of the silane deposit. Thus on the E-glass surface, the Si contributions from the silane deposit cannot be separated from that of the substrate since both appear at the same binding energy and quantification is not possible.

Additional results for SPS, g-APS and GPS (glycidylpropyltrimethoxy silane) silanised untreated E-glass surfaces are compared in Table 11. For all three silanised surfaces, the Si 2p binding energies are identical to that of the untreated E-glass surface and cannot be differentiated.

5. Conclusions The Si contribution from the silane can be distinguished from the Si contribution from the substrate when deposited onto a silica glass surface. However, when E-glass is employed, the presence of Al–O and Ca–O complicates the spectrum and differentiation is no longer possible. This was confirmed by acid extraction of E-glass prior to silane deposition. The Si 2p3/2 and Si 2p1/2 peaks from the silica glass (Si–O, Si–OH bonds) appear at 103.5 and 104.2 eV, respectively. Thus, no separation between the Si–O and Si–OH bonds can be made. Treatment of the silica glass with g-mercaptopropyl trimethoxy silane, leads to a broadening of the Si 2p peak and curve fitting shows that the silane contribution (i.e., Si–C) appears at 102.4 eV for the 2p3/2 line and at 103.1 eV for the 2p1/2 line.

ARTICLE IN PRESS T. Choudhury, F.R. Jones / International Journal of Adhesion & Adhesives 26 (2006) 79–87

With an E-glass surface (i.e. without nitric acid treatment), the Si 2p3/2, and Si Sp1/2 peaks appear at 102.4 and 103.1 eV, respectively. Since these binding energies are identical to those expected from the silane, the contribution of either SPS, APS and g-glycidyl propyl trimethoxy, to the Si 2p spectra cannot be determined. The use of the Wagner approach [12] of D(O–Si), to separate Si–O, Si–OH bonds on the glass from the Si–C, Si–CH bonds from the deposit, must also be carefully interpreted.

Acknowledgements We thank EPSRC/DTI for the award of a Link Grant. The contributions of Federal Mogul companies, Owens-Corning Fibreglass Ltd., EPSRC for funding access to the RUSTI Facility at the Daresbury Laboratory, DERA (now Qinetiq) for discussions through the TTP programme is acknowledged. References [1] Eldridge BN, Buchwalter LP, Chess CA, Goldberg MJ, Goldblatt RD, Novak DP. A time-of-flight static secondary ion mass spectrometry and X-ray photoelectron spectroscopy study of 3-aminopropyltrihydroxy silane on water plasma treated chro-

[2]

[3] [4]

[5] [6]

[7] [8] [9]

[10] [11] [12]

87

mium and silicon surfaces’. In: Mittal KL, editor. Silanes and other coupling agents. Utrecht, The Netherlands: VSP; 1992. p. 305–21. Horner MR, Boerio FJ, Clearfield HM. An XPS investigation of the adsorption of amino silanes onto metal substrates. In: Mittal KL, editor. Silanes and other coupling agents. Utrecht, The Netherlands: VSP; 1992. p. 241–62. Wang D, Jones FR, Denison P. J Mater Sci 1993;28:2481–8. Choudhury T, Jones FR. The interaction of Resole and Novolak phenolic resins with g-aminopropyltriethoxy silane treated E-glass surface: A high resolution XPS and micromechanical study. In: Mittal KL, editor. Silane and other coupling agents, vol. 2. Utrecht, The Netherlands: VSP; 2000. p. 79–98. Wang D, Jones FR, Denison P. J Adhes Sci Tech 1992;6:79. Watson H, Mikkola PJ, Matisons JG, Rosenholm JB. Colloids and surfaces A: physicochemical and engineering aspects 2000;161:183–92. Watson H, Mikkola PJ, Rosenholm JB, Matisons JG. Colloid Polym Sci 2001;279:1020–7. Choudhury T, Jones FR. in preparation. Pawson D, Jones FR. The role of oligomeric silanes on interfacial shear strength of AR-glass-vinyl ester composites. In: Jones FR, editor. Proceedings of the international conference on interfacial phenomena in composites materials. London: Butterworths; 1989. p. 189–92. Briggs D. In: Briggs D, Seah MP, editors. Practical surface analysis. 2nd ed. Vol. 1. New York: Wiley; 1995. Beamson G, Briggs D. High resolution XPS of organic polymers, The Scienta ESCA 300 database. Wiley: New York; 1992. Wagner CD, Passaja DE, Hillery HF, Kinisky TG, Six HA, Jansen WT, Taylor JA. J Vac Sci Technol 1982;21:933.