SURFACE AND INTERFACE ANALYSIS, VOL. 15, 113-118 (1990)

Determination of Amino and Silanol Functional Groups on Glass via Chemical Derivatization and ESCA

Tuan A. Jlang* GTE Products Corporation, Towanda, PA 18848, USA

Ramaswaimy Gnanasekaran Department of Chemistry, Mansfield University, Mansfield, PA 16933, USA

The ESCA analysis of surface silanol (Si-OH) and amino (NH,) functional groups on glass is either not possible or very ambiguous Tbese functional groups can be identified positively by d n g chemical derivatization in conjunction with ESCA. In addition to specificity, the technique also offers a significant decrease in detection limit. Tbe NH, was derivaitized using penta-fluorobenzoldehyde (PFB) and the Si-OH was silylated with tridecaflfluor0-1,1,2,2- tetrahydrmrtyl-1-tricrichlor0- silane (TDFS). Quanti6cation of the NH, group can be accomplished with PFB deri- vatization. Only a relative comparison of the silanol group on glass can be carried out in TDFS silylation owing to the uncertainty in the completion of this reaction in the ESCA sampling depth.

Electron spectroscopy for chemical analysis (ESCA or XPS) has been used successfully to elucidate the struc- tural and chemical information of many different types of surfaces.'-4 Its ability to detect and quantify a number of functional groups is, however, limited owing to the finite instruiment resolution. Although deconvolu- tion or curve-fitting can be used to resolve the poorly separated components, results are generally ambiguous. The identification and quantification of functional groups can be enhanced significantly by chemical deri- vatization. In this method, the functional group of inter- est is specifically reacted with a reagent containing a 'tag' atom, which is not part of the surface studied. Quantitative analysis of the tag is then carried out using ESCA. In addition to a more positive identification of a specific functional group, derivatization also offers a sig- nificant decrease m the detection limit. The increased sensitivity is generally achieved through the enhanced stoichiometry of the reagent (the selected reagent gener- ally contains mart: than one atom of the tag element) and through the hiigh cross-section of the tag element.

Chemical derivatization has been used widely in facilitating the ESCA analysis of functional groups on polymeric and car bon-based materials. 5-9 Only limited work has, however, been performed on inorganic matrices." In this paper, we will discuss the ESCA analysis of silanol (Si-OH) and amino (NH,) functional groups on glass through derivatization. The quantifica- tion of surface silanol (Si-OH) and amino groups is critical in the synthesis of stationary phases in chroma- tography and in immobilized enzymes. Silica is used extensively as a supporting material for bonded station-

ary phases in chromatography. The presence of surface silanol groups makes the glass highly surface active, which can lead to poor peak shapes in chromatog- raphy." In other instances, a high surface silanol con- centration is desirable to bond silanes to silica through the silanol groups. In the field of immobilized enzymes, enzymes are immobilized on inorganic matrices con- taining amino groups through the glutaraldehyde coup- ling agent.'*

The amino group was derivatized using pentafluoro- benzaldehyde (PFB). Two reagents were used to deriva- tize the OH group: tridecafluoro-l,1,2,2- tetrahydrooctyl-1-trichlorosilane (TDFS) and tri- fluoroacetic anhydride (TFAA). Comparison of derivati- zation results between polymer and inorganic matrices will be made.

Reagents

The chemicals used were reagent grade. Pentafluoro- benzaldehyde (PFB), trifluoroacetic anhydride (TFAA) and other common chemicals were obtained from Aldrich Chemical Company.

Controlled pore glass (CPG) and aminopropyl- controlled pore glass (AMP-CPG) were supplied by Electro-Nucleonics Corporation.

Tridecafluoro- 1,1,2,2- tetrahydrooctyl- 1-1 -trichloro- silane (TDFS) and 3-aminopropyl trimethoxysilane (TMS) were obtained from Petrarch Systems.

The amino group was added to the controlled pore glass (CPG) through silylation. Controlled pore glass was boiled with concentrated HCl for 3 h. It was rinsed subsequently with deionized water and dried in an oven at 110°C for 2 h. One gram of the dried CPG was

Received 31 May 1989 Accepted 25 September 1989

0142-2421/90/02011~ ?iO5.00 0 1990 by John Wiley & Sons, Ltd

114 T. A. DANG AND R. GNANASEKARAN

mixed with 1.0 ml of a 10% (v/v) solution of 3- aminopropyl trimethoxysilane in 95% ethanol and left to stand for -5 min. The glass was then filtered, washed with 95% ethanol and placed in an oven at 110°C. The silylated glass samples were then extracted with ether for 12 h to remove any adsorbed silane.

Derivatization procedure

Derivatization of amino group. 1 g of aminopropyl- controlled pore glass (AMP-CPG) or silylated-CPG was refluxed with 2.5 ml of pentafluorobenzaldehyde (PFB) in 100 ml of pentane for 2 h. The glass was then rinsed with pentane followed by Soxhlet extraction with pentane for 12 h. Subsequent Soxhlet extraction with ether for another 12 h was also performed.

Derivatization of hydroxyl group. With tridecafluoro- 1,1,2,2-tetrahydrooctyl-l-trichlorosilane (TDFS), the silylation procedure described above is applied. With trifluoroacetic anhydride (TFAA), 2 g of the CPG were mixed with 50 ml of benzene, 7.5 ml of TFAA and 7.5 ml of pyridine. The mixture was stirred for 1.5 h. It was then filtered, washed with benzene and Soxhlet extracted with ether for 12 h.

ESCA analysis

ESCA studies were carried out on a Physical Elec- tronics Model 590 AM, ESCA-SAM. Aluminum Ko! (1486.6 eV) from the dual anode source was used for excitation. The spectrometer was operated at 15 kV and 300 W and the base pressure was below 5 x Torr. Binding energies were referenced to the C 1s line at 284.6 eV. Integrated peak areas were used for quantitat- ive analysis. The atomic sensitivities provided by Physi- cal Electronics were used for conversion to atomic percentage.

RESULTS AND DISCUSSION

Chemical derivatization of the amino group

The ESCA survey (general scan) spectrum of the com- mercial aminopropyl-controlled pore glass (AMP-CPG) is shown in Fig. l(a). Although Si and 0 are readily seen, the peak corresponding to N is hardly observed. Repeated signal averaging of the N 1s region at a higher resolution scan mode (multiplex) showed the pre- sence of an N species whose binding energy (399.2 eV) was in the binding energy range for an amine.13 Quanti- tative analysis carried out using the integrated peak areas of Si 2p, C Is, 0 1s and N 1s indicated that only 1.1 % of N was present in this sample. The N : Si atomic ratio was 0.04 (Table 1). The binding energies of Si 2p and C 1s were found to correspond to that of SiO, and hydrocarbon, respectively. The existence of other Si (silane) and C (C-N, C-Si) species in the AMP-CPG

6 AMP-CPG

s12s A

il, d u & d u u d

-1000 -900 -800 -700 -600 -500 -400 -300 -200 - 1 0 0 0 BINDING ENERGY, EV

7

AMP-CPG/ PFB

-1000 -900 -BOO -700 -600 -500 -400 -300 -200 -100 0 BINDING ENERGY, EV

Figure 1. ESCA survey spectra of: (a) commercial AMP-CPG; (b) commercial AMP-CPG reacted with PFB.

was not readily revealed from the Si 2p and C 1s ESCA spectra (Figs 2(a) and 3(a)). This is the result of a small shift in binding energy and the presence of only a trace level of amino groups in this commercial sample.

Derivatization of the amino group was carried out by forming a Schiff base with pentafluorobenzaldehyde (PFB)

I

I -0-Si-(CH,),-NH, + C,F,CHO --+

1 I

-O-Si-(CH,),-N=CH-C6F5 (1)

This reagent specifically reacts with amines (primary, secondary and hydrazines). Other nitrogen groups, such as amides, imines and nitrides, do not react with PFB. The derivatized aminopropyl glass was then Soxhlet extracted with pentane for 12 h to remove the unre- acted and adsorbed PFB. The ESCA survey spectrum of the derivatized AMP-CPG after pentane extraction is given in Fig. l(b). Peaks corresponding to F were identi- fied easily. Although derivatization did not cause any

DETERMINATION OF FUNCTIONAL GROUPS ON GLASS 115

Table 1. Summary of atomic ratio for various glass s,amples'

N:Si F : N

Commercial AMP-CPG 0.04 - Commercial AMP-CPG/PFB 0.038 8.0

Commercial ,AMP-CPG/PFB 0.04 5.0

CPG/PFB 0 0

(pentane extraction)

(pentane li ether extraction)

(pentane & ether extraction)

(pentane IP ether extraction) TMS-silylated CPG/PFB 0.24 4.4

a Ratios are reproducible to i5% (RSD).

significant variation in either the binding energy or peak shape of N :Is and Si 2p, the C 1s ESCA spectrum of the derivatized AMP-CPG was dramatically different from that of the original sample (Fig. 2(b)). It skews to the left, which suggests the existence of an additional peak (or peaks) whose binding energy is larger than that of hydrocarbon. Derivatization results in a large con- centration of C F group (287.5 eV), which readily accounts for the unsymmetrical C 1s shape observed. We can use curve-fitting to fit five different carbon species (hydrocarbon, C-Si, C-N, C-N and CF) into the C 1s ESCA spectrum of the reacted AMP-CPG; curve-fitting results can then be used to quantify the amino groups. It is, however, less ambiguous and much more quantitative by just determining the F level present.

Table 1 lists the atomic ratios N : Si and F : N for the derivatized and pentane-extracted AMP-CPG. As

( b )

( C )

TMS-CPG/ PFB

- 1 1 1 t

Binding Energy, eV 293 285 277

Figure 2. C 1s ESCA spectra of: (a) commercial AMP-CPG; (b) commercial AMP- CPG reacted with PFB; (c) TMS-silylated CPG reacted with PFB; (d) HCI-treated CPG silylated with TDFS.

I I I I 1

107 103 99

Binding Energy, eV

Figure 3. Si 2p ESCA spectra of: (a) commercial AMP-CPG or commercial AMP-CPG reacted with PFB; (b) TMS-silylated CPG reacted with PFB.

expected, derivatization did not change the N : Si atomic ratio. In the absence of surface adsorption, a complete derivatization would result in an F : N atomic ratio of five. The F : N atomic ratio of the derivatized AMP-CPG after pentane extraction was eight. The higher F : N ratio obtained in the pentane-extracted sample suggested that its surface contained some unre- acted and adsorbed PFB. To remove the adsorbed PFB, we further Soxhlet-extracted this sample with ether (a more polar solvent) for 12 h. The F : N atomic ratio of the derivatized AMP-CPG after pentane and ether extraction was now found to be five. To ascertain that the pentane/ether extraction has removed all adsorbed PFB from the glass surface, we have applied the exact derivatization and extraction procedures to the controlled pore glass. This sample does not contain any amino group. As shown in Fig. 4, no F was detected in the ESCA spectrum of the CPG that was reacted with PFB and extracted with pentane and ether. Thus, pentane/ether extractions have sufficiently removed all unreacted and adsorbed PFB from the glass surface.

It is noteworthy that an incomplete derivatization of the amino group in the AMP-CPG would result in an F : N atomic ratio of less than five. Since an F : N ratio of five was obtained, it suggests that in the ESCA sam- pling depth region the conditions applied have suc- cessfully derivatized all the amino groups on the glass.

Glass containing amino groups was also prepared in our laboratory through silylation. The controlled pore glass (CPG) was boiled in concentrated HCI for 3 h. This treatment would increase the OH concentration in the glass. The treated CPG was then silylated through the OH group with 3-aminopropyl trimethoxysilane (Reaction 2). The silylated-CPG was then reacted with PFB (Reaction 3).

116

J

TMS-CPG/ PFB - 0 is 6

FlS I F(KLL1 5 - -

\ n -

I _ !!! 4 - u - W - -

7

6

5

- \ W 4 - W

z - U

m 1 3

2

I

0

T. A. DANG AND R. GNANASEKARAN

NKVV) 01s

CPG/ PFB (PENTANE 8 ETHER EXT.)

1 S I L S

-1000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 BINDING ENERGY, EV

Figure 4. ESCA survey spectrum of CPG reacted with PFB (pentane and ether extraction).

-OH + (Me0)3-Si-(CH2)3-NH, + The ESCA survey spectrum of the derivatized silylated-CPG after pentane/ether extraction is shown in Fig. 5. Compared to the derivatized sample of com- mercial AMP-CPG, the N and F signals in the deriva- tized silylated-CPG were significantly more intense. The higher N intensity does indicate that we have suc- cessfully added more amino groups into the CPG. The derivatization of the amino groups has consequently resulted in a larger F level being detected. Another major difference between the two amino glass samples was illustrated in the Si 2p ESCA spectrum (Fig. 3). The

I I

-0-si--(CH2)3-NH2 (2)

I I

-O-Si-(CH2)3-NH2 + C6F,CH0 -+

I -0-si(CH2)3NPCHC6F5 I (3)

7 3

-1000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 BINDING ENERGY. EV

Figure 5. ESCA survey spectrum of TMS-silylated CPG reacted with PFB.

DETERMINATION OF FUNCTIONAL GROUPS ON GLASS 117

Si peak of the derivatized silylated-CPG can be resolved into two components, which correspond to ‘SiO,’ Si and ‘silane’ Si (OSi-C), respectively. ‘Silane’ Si was not apparent in the Si 2p of the commercial AMP-CPG owing to its small presence. The higher ‘silane’ Si content in the derivatized silylated-CPG is consistent with a larger amino concentration present in this com- pound. The higher NH, content also accounts for a large CF concentra,tion readily seen from the C 1s ESCA spectrum (Fig. 2(c)).

The amino level in the silylated-CPG was, indeed, found to improve by more than six-fold, as indicated in the N : Si atomic ratio (N : Si = 0.24). Only the SiO, Si was included in the calculation of this N : Si ratio. The F : N atomic ratio of the derivatized silylated glass is very close to five, which indicates that complete deriva- tization has been achieved in the ESCA sampling depth.

The NH, group on glass matrix can, thus, be identi- fied successfully and quantified through derivatization with PFB and subriequent analysis with ESCA. The method offers specificity and a twelve-fold increase in sensitivity. The sensitivity enhancement is achieved through the improvement in cross-section (cross-section of F is 2.3 times higher than that of N) and a five-fold enhancement in stoichiometry (there are five F for every NH, group present). Successful derivatization of the NH, group with PFB on polymer matrix has been carried out by Everhart and Reille~.~ Like glass matrix, the derivatization in the ESCA sampling depth was also shown to be completed in the polymer matrix. Soxhlet extraction with pentene is able to remove all unreacted and adsorbed PFB in the polymer m a t r i ~ . ~ It is, however, not sufficiant for the glass matrix; the high surface area of SiO, requires a subsequent ether extrac- tion.

Chemical derivatization of the silanol group

The OH group on glass cannot be identified directly by ESCA. Its presence does not result in any significant change in binding energy of either Si or 0. The OH group must react with a reagent containing a ‘tag’ atom, which is then measured by ESCA. We have used a number of reagents to derivatize the OH on glass. Tridecafluoro- 1,1,2,2- tetrahydrooctyl- 1-trichlorosilane (TDFS) reacts with the OH group on glass through sily- lation (Reaction 4). Although all three labile X groups of the silane molecule are available for silylation with the OH groups in glass, only one bond is generally formed from each silicon of the silane to the glass sub- strate. The other two either couple with other silane agents or remain unboundI4

-OH + C1,-Si-(CH,),-(CF,),-CF, +

I

I -O-Si-(CH2),-(CF,),-CF, (4)

The CPG was boiled in HCl before derivatization to increase the OH content. Figure qa ) shows a survey ESCA spectrum of the CPG silylated with TDFS. This sample was subjected to oven drying after derivatiza- tion. The presence 0 1 F in the silylated CPG is readily identified from the: survey spectrum. The high-

7

6

5

HCI TREATED CPG - ? ‘ Y

- 3 s 2 Y

2

I

0

TDFS

-1000 -900 -800 -700 -600 -500 -400 -300 -2W -100 0 BINDING ENERGY, EV

7

6

5

c w 4

W

2

4 3

- - Y

tn

2

I

0 -1000 -900 -800 -700 -600 -500 -400 -300 -200 - 1 0 0 0

BINDING ENERGY, EV

Figure 6. ESCA survey spectra of: (a) HCI-treated CPG silylated with TDFS; (b) heated CPG silylated with TDFS.

resolution C 1s scan of this sample (Fig. 2(d)) shows peaks corresponding to both CF, (290.8 eV) and CF3 (293.4 eV) groups. The ‘silane’ Si, which is also a com- ponent in the derivatized CPG, is, however, not readily shown in the Si 2p ESCA spectrum owing to its small presence. Since it is not possible to separate the sub- strate Si (SiO,) from the ‘silane’ Si, the concentration of F obtained from the F 1s peak was used to estimate the ‘silane’ Si content (Sisilanc = Si,,,,, - F/13). The concen- tration of the substrate Si can be determined as the dif- ference between total Si and the ‘silane’ Si estimated from the F concentration (Sisu, = Si,,,,, - F/13). The F : Si,,, atomic ratio for the oven-dried TDFS-reacted CPG was found to be 1.3. The oven-dried TDFS- silylated CPG was then subjected to Soxhlet extraction with ether. Ether extraction slightly reduced the F : Si,,, atomic ratio (1.0). The oven-drying step, thus, still leaves some unreacted and adsorbed TDFS on the glass surface. Extraction with ether is required for further removal of adsorbed silane.

A glass relatively free of OH was prepared by heating the CPG at 550°C for 1 h. Silylation with TDFS, fol- lowed by drying and ether extraction, was carried out for the heated glass. ESCA analysis of this sample did not show any F present (Fig. qb)). The absence of F clearly indicates that heating has successfully generated a surface that is relatively free of OH groups. Oven drying followed by ether extraction has sufficiently

118 T. A. DANG A N D R. GNANASEKARAN

removed all the unreacted and adsorbed TDFS from the glass surface.

The silanol groups on glass can, therefore, be identi- fied successfully via TDFS derivatization and sub- sequent ESCA analysis. In addition to a positive identification, the derivatization with TDFS also offers a significant enhancement in detection capability (13F for each OH group). Since ESCA quantification of the OH group without derivatization cannot be carried out, there is no means to check the completion of TDFS derivatization in the ESCA sampling region. This causes some uncertainty in the absolute quantification of the OH group. The relative comparison between samples can, however, still be carried out.

We also have reacted the silanol group in CPG with trifluoroacetic anhydride (TFAA). This derivatization has not been successful. Indeed, no F was detected in ESCA analysis of HC1-treated CPG that had been reacted with TFAA and Soxhlet extracted with ether.

Table 2 summarizes the derivatization results on glass and polymer matrices for both NH, and OH functional groups. On both matrices, the amino group can be iden- tified successfully and quantitated through PFB deriva- tization. Only a certain reagent can be used in analysis of the OH group on glass. On polymeric surfaces, quan- tification of the hydroxyl group (C-OH) can be obtained from reaction with TFAA.5-7 Barth et al. have reported the successful derivatization of the C-OH groups in carbon-overcoated disks using (tridecafluoro- 1,1,2,2- tetrahydroocty1)- 1-dimethyl- chloro~ilane.~ Their paper has, however, not shown the completion of silylation in the ESCA sampling depth, so absolute quantification of the C-OH is still uncertain. The TFAA reagent does not react with the Si-OH group on glass. Derivatization of the Si-OH group can be achieved only through TDFS silylation.

Table 2. Comparison of methods between matrices

Polymer Glass

PFB Identified = Yes Identified = Yes (NH,) Quantified =Yes Quantified = Yes

TFAA Identified = Yes Identified = No (OH) Quantified =Yes Quantified = No

Silane Identified =Yes Identified = Yes (OH) Quantified = - Quantified = Only relative

comparison

CONCLUSION

Chemical derivatization has significantly enhanced the ESCA analysis of amino and silano groups on glass. Derivatization of the NH, and Si-OH group can be obtained with PFB and TDFS, respectively. The tech- nique not only provides specificity but it also offers a considerable increase in sensitivity. Quantification of the NH, can be obtained from derivatization with PFB. Since the completion of the silylation reaction in the ESCA sampling depth has not been ascertained, only a relative comparison of the OH group can be carried out.

Acknowledgement

The authors would like to thank John L. Schoonover for helpful dis- cussions. This work was supported by GTE Products Corporation.

REFERENCES

1. T. L. Barr. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, ed. by D. Briggs and M. P. Seah, Chapt. 8, pp. 283-358. Wiley, Chichester (1983).

2. D. M. Hercules.Ana1. Chem. 58, 177a (1986). 3. T. A. Dang, L. Petrakis and D. M. Hercules, J. Phys. Chem. 88,

3209 (1 984). 4. D. Briggs, Practical Surface Analysis by Auger and X-ray

Photoelectron Spectroscopy, ed. by D. Briggs and M. P. Seah, Chapt. 9, pp. 35S396. Wiley, Chichester (1983).

5. D. S. Everhart and C. N. Reilley, Anal. Chem. 53,665 (1 981 ). 6. S. Zeggane and M. Delamar, Appl. Surf. Sci. 31,151 (1 988). 7. R. A. Dickie, J. S. Hamrnond, J. E. deVries and J. Holubka.

Anal. Chem. 54,2045 (1982). 8. T. P. Tougas and W. G. Collier, Anal. Chem. 59, 2269 (1 987).

9. G. Barth, R. D. Corrnia and L. A. Teasley, Solid State Techno/. January, 32,119 (1989).

10. Reflections, pp. 14-1 5. Surface Science Laboratories, Moun- tain View (1988).

11. H. H. Willard, L. L. Merrit. J. A. Dean and Settle, Instrumental Methods ofAnalysis, 6th Edn, p. 460. Wadsworth.

12. R. Gnanasekaran and H. A. Mottola, Anal. Chem. 57, 1005 (1985).

13. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg, Handbook of X-ray Photoelectron Spectro- scopy. Perkin Elmer, Eden Prairie (1979).

14. B. Arkles, Silicon Compounds Register and Review. p. 54. Petrarch Systems, Bristol (1 987).