- Email: [email protected]
γ radiolyzed amorphous silica: A study with 29Si CP-MAS NMR spectroscopy
ARTICLE IN PRESS
Radiation Physics and Chemistry 77 (2008) 267–272 www.elsevier.com/locate/radphyschem
g radiolyzed amorphous silica: A study with 29 Si CP-MAS NMR spectroscopy Franco Cataldoa,, Donatella Capitanib, Noemi Proiettib, Pietro Ragnib a
Lupi Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy CNR, Institute of Chemical Metodologies, Area della Ricerca di Roma 1, 00016 Monterotondo Scalo, Rome, Italy
b
Received 29 March 2007; accepted 11 May 2007
Abstract Precipitated amorphous silica has been g radiolyzed at 333, 666 and 1000 kGy in air. The structural changes undergone during irradiation have been studied by 29Si CP-MAS NMR spectroscopy. The spectra suggest that silica looses the hydrogen atoms attached to the silanol groups and the resulting silanol radicals undergo a crosslinking reaction by forming peroxysiloxane groups or by reacting with E0 centres yielding siloxane groups. r 2007 Elsevier Ltd. All rights reserved. Keywords: Amorphous precipitated silica; Irradiation; Gamma radiolysis; Peroxysilanols; Crosslinking
1. Introduction Amorphous silica can be divided into two main classes: precipitated amorphous silica and fumed silica (Franta, 1989). Precipitated amorphous silica is mainly used as reinforcing filler for plastics and elastomers while fumed silica finds a much broader range of applications in numerous different fields (Bergna and Roberts, 2006; Franta, 1989). Precipitated silica is produced by a wet process where sodium silicate is treated with sulphuric acid (Iler, 1979; Franta, 1989). In contrast fumed silica is produced for instance in the thermal hydrolysis of SiCl4 in a hydrogen flame (Iler, 1979; Franta, 1989). Considerable attention has been dedicated to the action of high-energy radiation on silica glasses (Griscom, 2006) but incomparably less and relatively old works can be found in literature concerning the action of radiation on amorphous silica (Barter and Wagner, 1964; Boehm, 1966; Devine, 1990; LaVerne and Tonnies, 2003). Corresponding author.
E-mail address: cdcata@flashnet.it (F. Cataldo). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.05.010
29
Si CP-MAS NMR spectroscopy; Chemical structure changes; Silanols;
Recently, a stimulating work on the action of g radiation on fumed silica and on fumed silica filled elastomers (Patel et al., 2006) has shown that amorphous fumed silica irradiated at 50 kGy in air develops a strong electron spin resonance (ESR) spectrum due to the formation of trapped paramagnetic species. These species affect also the interaction between the silica surface and the polymer matrix which hosts silica as filler and can contribute to the reinforcing effect of the radiation-treated silica rubberbased composite. The previous works on the radiation treatment of carbon black Cataldo (2001a, b) have shown that similar effects occur in the carbon black interaction with elastomer matrix. The formation of paramagnetic centres in radiation-treated silica and the enhancement of its absorption and chemisorption power toward other molecules and macromolecules is known for a long time (Barou and Wolkenstein, 1982). The work of Patel et al. (2006) has also shown that the changes induced in fumed silica by g radiation can be followed by 29Si CP-MAS NMR spectroscopy. This prompted us to study with the same technique the effects of gamma radiation on amorphous precipitated silica, the most common silica employed as reinforcing filler in elastomers.
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
268
2. Experimental
HO
OH Si
2.1. Materials Amorphous precipitated silica (in powder form) Ultrasil 7000 from Degussa has been used in the present study. The silica was characterized by a nitrogen surface area of 175 m2/g, a water content of 6.5% as measured by the weight loss at 115 1C, a pH in water of 6.8 and by a tap density of 320 g/L. The SiO2 content was 497% and the main impurities are Na (about 1% expressed as Na2O), Al (0.2% expressed as Al2O3), Fe (300 ppm expressed as Fe2O3) and 0.06% of Cl and 0.9% of S (expressed as SO3).
O
OH O O
Si
O
Si
O
Si
O
Si
Si
2.3.
Si CP-MAS NMR
Samples were powdered and packed into 4 mm zirconia rotors and sealed with Kel-F caps. Solid state 29Si CPMAS NMR spectra were performed at 39.76 MHz on a Bruker AMX-200 spectrometer. The spin rate was 5 kHz, the p/2 pulse width was 4 ms and the recycling time was 4 s. Contact times were 5 and 10 ms. Spectra were obtained by using 1024 data points in the time domain, zero filled and Fourier transformed to a size of 1024 data points. The chemical shift was externally referred to tetramethylsilane. A full simulation of the 29Si CP-MAS NMR were carried out with the SHAPE2000 software package developed by Prof. Michele Vacatello, Naples University, Italy. 3. Results and discussion 3.1. 29Si CP-MAS NMR: a tool for determination of silica structure and reaction The most common silica chemical sites in the surface or in bulk are reported in Scheme 1. In silica each silicon atom is almost always connected to four oxygen atoms with a tetrahedral geometry. In amorphous silica the tetrahedrons are connected chaotically in a three-dimensional net. The oxygen atoms in their turns can be connected to other silicon atoms or, if they are on the surface of the silica particle, are connected with hydrogen atoms. As shown in Scheme 1, on the silica surface the OH groups attached to a silicon atom can occur in a geminal configuration or can be isolated. In the first case the site is called Q2 and in the latter case Q3. A variant of
O
H
Si O
O
O
O
Si
Si
Si
Si O O Si
O
Si
O
Si
O
O Si
Si O
O
O
O
O
Si Q4 - Bulk Si site
29
Si
H
Q3*- Vicinal & Bridged Si - OH
Q3 - Isolated Si - OH
2.2. Irradiation Three samples were irradiated in glass ampoules in presence of air using a 60Co g cell, with a dose rate 6 kGy/h. The absorbed dose administered to the three samples was 333, 666 kGy and 1 MGy. The samples remained white in colour even after 1 MGy absorbed dose. No discolouration was observed.
O
Q2 - Geminal Si - OH
Q4 - Siloxane Scheme 1.
isolated OH groups is when they occur vicinal and bridged (see Scheme 1); in this case we can talk about the Q3* sites. Finally, other two sites can be expected: a completely oxygen bridged silicon atom connected to other silicon atoms in the bulk of the solid and a siloxane bridge when the site occurs on the surface; both are called Q4 sites. 29 Si CP-MAS NMR spectroscopy is a powerful tool for the investigation of the surface structure and reactions of silica gels (Pesek and Leigh, 1994; Patel et al., 2006). In fact, well resolved 29Si CP-MAS NMR can be obtained showing the distribution of silicon sites in different environments. The Q2 silicon sites with geminal OH groups resonate at about 91 ppm. Q3 silicon sites bearing isolated OH group resonate at about 101 ppm; Q4 silicon sites bound to four OSi moieties and bearing no OH groups resonate at about 109 ppm. Due to the relaxation properties, using a proper contact time t ¼ 10 ms, the intensity of the signals due to both Q2 and Q3 is suitable for a quantitative treatment. This is not true for Q4 silicon atoms, since Q4 units close to the surface Si–OH groups mostly contribute to the signal intensity. In the present work we have applied 29Si CP-MAS NMR technique for studying the modifications occurring in precipitated amorphous silica during gamma radiation treatment by comparing the resulting spectra with that recorded on the same precipitated amorphous silica sample before the radiation processing.
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
a
269
b
-60
-80
-100
-120
-140
-160
-60
-180
c
-80
-100
-120
-140
-160
-180
d
-60
-80
-100
-120
-140
-160
-180
-60
-80
-100 -120 -140 -160 -180
Fig. 1. 29Si CP-MAS NMR spectra (recorded at contact time t ¼ 10 ms). The spectrum (a) was recorded on precipitated amorphous silica before its irradiation. The spectra (b)–(d) were recorded at 333, 666 and 1000 kGy, respectively. Gradual net growth of the signal at 110 ppm due to the Q4 silicon sites can be observed. The deconvoluted spectra (solid lines) have been overlapped to the experimental spectra (dotted lines).
In Fig. 1(a) the 29Si CP-MAS NMR spectrum of a reference, pristine silica, is reported. There are silicon atoms in three different environments: 110 ppm due to Q4 units 101 ppm due to Q3 units 90 ppm, very weak, due to Q2 units. After silica irradiation with a dose of 333 kGy a net increase of the intensity of the signal due to Q4 units in comparison to Q3 units is observed as shown in Fig. 1(b). At higher doses of 666 and 1000 kGy a further increase of the signal due to Q4 is observed as shown in Figs. 1(c) and (d). These spectral data were acquired with a contact time of 10 ms. To confirm these trends, 29Si CP-MAS NMR spectra were also recorded with contact time of 5 ms. These spectra are shown in Fig. 2. Again, an increase of the Q4 signal in comparison to the Q3 was observed. From the simulation of the experimental spectra, the area of resonance due to the Q2, Q3 and Q4 sites has been obtained. The deconvoluted spectra are superimposed to the experimental as shown in Figs. 1 and 2. The values of the areas are reported in Tables 1 and 2. It must be pointed out that, because crosspolarization is not a directly
quantitative technique, the obtained ratios R ¼ A(Q4)/ A(Q3) must not be thought as absolute values but are only indicative of a trend. Two types of silanol groups are present in silica. The isolated silanols are much more abundant than the geminal silanols (see Scheme 1). Tables 1 and 2 show that the NMR signal associated to geminal silanols (Q2) disappear under radiation processing while the NMR signal associated to isolated silanols (Q3) is reduced in its intensity. The changes of the silanol groups are accompanied by an increase of the signal due to the Q4 sites. In Fig. 3 the ratio R at two different NMR contact times has been reported against the adsorbed dose. In both cases the ratio R increases by increasing the absorbed dose, suggesting that silica radiolysis causes the transformation of Q3 sites into the Q4 sites. 3.2. Chemical changes occurring in precipitated amorphous silica during radiolysis The irradiation of silica involves the formation of a number of defects. These defective structures are not fully understood and sometimes, as in the case of the E0 centres, are interpreted in different ways by different authors (Agnello et al., 2002). E0 centres are often interpreted as an
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
270
b
a
-60
-80
-100
-120
-140
-160
-180
-60
-80
-100
-120 ppm
-140
-160
-180
d
c
-60
-80
-100
-120 ppm
-140
-160
-180
-60
-80
-100
-120 ppm
-140
-160
-180
Fig. 2. 29Si CP-MAS NMR spectra (recorded at contact time t ¼ 5 ms). The spectrum (a) was recorded on precipitated amorphous silica before its irradiation. The spectra (b)–(d) were recorded at 333, 666 and 1000 kGy, respectively. Gradual net growth of the signal at 110 ppm due to the Q4 silicon sites can be observed. The deconvoluted spectra (solid lines) have been overlapped to the experimental spectra (dotted lines). Table 1 Area of the resonances due to Q4, Q3, Q2 and the ratio R ¼ AðQ4 Þ/AðQ3 Þ obtained by applying a deconvolution procedure to 29Si CP-MAS NMR (t ¼ 10 ms) D (kGy)
A(Q4)
A(Q3)
A(Q2)
R ¼ A(Q4)/A(Q3)
0 333 666 1000
0.46 0.52 0.59 0.70
0.51 0.45 0.40 0.30
0.03 0.03 0.01 0.00
0.90 1.15 1.48 2.33
Table 2 Area of the resonances due to Q4, Q3, Q2 and the ratio R ¼ AðQ4 Þ=AðQ3 Þ obtained by applying a deconvolution procedure to 29Si CP-MAS NMR (t ¼ 5 ms) D (kGy)
A(Q4)
A(Q3)
A(Q2)
R ¼ A(Q4)/A(Q3)
0 333 666 1000
0.25 0.41 0.45 0.53
0.72 0.58 0.54 0.47
0.03 0.01 0.01 0.00
0.35 0.71 0.83 1.13
unpaired electron localized on a silicon atom bonded to three oxygen atoms (Imai and Hirashima, 1994; Griscom, 2006): O O O
Si
Another common defect is the non-bridging oxygen hole center (NBOHC), a hole trapped on a non-bridging oxygen (Imai and Hirashima, 1994; Griscom, 2006). The paramagnetic defects formed by the action of radiation in the bulk of silica at low dose may remain trapped but at higher dose tend to recombine or diffuse to the surface, making the surface to react with the paramagnetic centres formed directly on the surface (Patel et al., 2006). The sites where electrons accumulate are chemically active and may interact with surface water or silanol species liberating hydrogen. Hydrogen evolution is dependent on silica surface area. Larger yields are achieved with high surface areas (LaVerne and Tonnies, 2003). Many authors have reported that silica radiolysis involves always the production of hydrogen (Barter and Wagner, 1964; Shelby, 1994; Chemerisov et al., 2001; LaVerne and Tonnies, 2003; Van Ginhoven et al., 2006). Hydrogen derives both from water and from silanol radiolysis: Si
OH
Si
O
+
H
Hydrogen is released in atomic form and can remain trapped in the silica network or can migrate and combine with other hydrogen atoms to form molecular hydrogen or can react with radiation-induced defects formed in silica (Chemerisov et al., 2001; Van Ginhoven et al., 2006).
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
271
2.5 2.25 2 y = 0.0014 x + 0.7721 R2 = 0.915
1.75
Q4/Q3
1.5 1.25 1 y = 0.0008 x + 0.3741 R2 = 0.9732
0.75 0.5
at 10 ms at 5 ms
0.25 0 0
100
200
300
400 500 600 700 RADIATION DOSE (kGy)
800
900
1000
1100
Fig. 3. Area ratio R ¼ Q4/Q3 taken from Tables 1 and 2. It can be observed that the ratio R grows both at NMR contact time t ¼ 5 and 10 ms.
Silanol radicals are active species which may favour the chemisorption of molecular species on silica surface (Ermatov, 1959; Barou and Wolkenstein, 1982; Novak et al., 2003). Alternatively, silanol radicals may produce peroxy siloxanes (Van Ginhoven et al., 2006; Griscom, 2006) and their 29Si NMR signals should resonate in the same frequency range of the resonance due to Q4 units, causing its broadening and enhancement. O O
2
Si
O O
O
O
O Si
O
O
Si
O
O O
Alternatively, a silanol radical can react with an E0 centre: O O O
O Si
O
+
.Si
O O
O O O
O Si
O
Si
O
NMR causes its narrowing (Patel et al., 2006). This is in contrast with the results of the present work on precipitated amorphous silica. However, there are substantial differences in the morphology of fumed silica surface and amorphous silica surface. In fact, the former has a surface silanol density of 2–3 silanol groups/nm2 while amorphous precipitated silica usually has 6 silanols/nm2 (Iler, 1979; Bergna and Roberts, 2006). Therefore a more effective crosslinking can be expected from precipitated amorphous silica. Another interesting aspect which differentiates fumed silica from amorphous precipitated silica is their trend to crystallize at 1000 1C. Fumed silica does not crystallize even after 7 days at 1000 1C while precipitated amorphous silica crystallizes in 20 min at the mentioned temperature. This radical difference in the thermal and crystallization behaviour may be ascribed not only to the differences in the surface group density but also to the bulk chemical structure of the two silicas.
O
4. Conclusions Such kind of crosslinks between the surfaces not only explain the enhancement of the Q4 NMR signal and the decrease of the Q3 or Q2 signals but also can explain the reason why the density of silica glasses increases with the radiation dose (Ruller and Friebele, 1991; Piao et al., 2000; An et al., 2006). Curiously, the irradiation of fumed silica instead of leading to an increase of the Q4 signal at the 29Si CP-MAS
Irradiation of precipitated amorphous silica with g rays causes an increase of the amount of the Q4 silicon sites and a decrease of the Q3 and Q2. The sites are illustrated in Scheme 1. These changes have been detected by 29Si CP-MAS NMR and have been interpreted in terms of radiation-induced crosslinking of the silanols groups present on the silica surface with liberation of hydrogen.
ARTICLE IN PRESS 272
F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
References Agnello, S., Gelardi, F.M., Boscaino, R., Cannas, M., Boizot, B., Petite, G., 2002. Intrinsic defects induced by beta irradiation in silica. Nucl. Instrum. Methods Phys. Res. B 191, 387–391. An, Q., Zheng, L., Luo, S.N., 2006. Vacancy-induced densification of silica glass. J. Noncryst. Solids 352, 3320–3325. Barou, V., Wolkenstein, T., 1982. Effet de l’Irradiation sur les Proprie´te´s des Semi-Conducteurs. Mir, Moscow, Russia. Barter, C., Wagner, C.D., 1964. Generation of acidity in silica gel by ionizing radiation. J. Phys. Chem. 68, 2381–2383. Bergna, H.E., Roberts, W.O., 2006. Colloidal Silica. Fundamentals and Applications. CRC Press, Taylor & Francis, Boca Raton, FL, USA. Boehm, H.P., 1966. Chemical identification of surface groups. In: Eley, D.D., Herman, P., Weisz, P.B. (Eds.), Advanced Catalysis, vol. 16, pp. 225–247. Cataldo, F., 2001a. Effects of g radiation treatment on the reinforcing properties of carbon black. Int. J. Polym. Mater. 50, 29–46. Cataldo, F., 2001b. Effects of fillers radiation pre-treatments on their rubber absorption power and on the reinforcing properties of rubber compounds. Polym. Int. 50, 828–834. Chemerisov, S.C., Werst, D.W., Trifunac, A.D., 2001. Formation, trapping and kinetics of H atoms in wet zeolites and mesoporous silica. Radiat. Phys. Chem. 60, 405–410. Devine, R.A.B., 1990. Radiation damage and the role of structure in amorphous silica. Nucl. Instr. Methods Phys. Res. B 46, 244–251. Ermatov, S.E., 1959. Variation in the absorption properties of silica gel under the action of gamma irradiation. Sov. Phys. 129, 1259–1260. Franta, I. (Ed.), 1989. Elastomers and Rubber Compounding Materials. Elsevier, Amsterdam, Holland.
Griscom, D.L., 2006. Self-trapped holes in pure-silica glass: a history of their discovery and characterization and an example of their critical significance to industry. J. Noncryst. Solids 352, 2601–2617. Iler, R.K., 1979. The Chemistry of Silica. Solubility, Polymerization, Colloid and Surface Properties and Biochemistry. Wiley, New York, USA. Imai, H., Hirashima, H., 1994. Intrinsic- and extrinsic-defect formation in silica glasses by radiation. J. Noncryst. Solids 179, 202–213. LaVerne, J.A., Tonnies, S.E., 2003. H2 production in the radiolysis of aqueous SiO2 suspensions and slurries. J. Phys. Chem. B 107, 7277–7280. Novak, E., Hancz, A., Erdohelyi, A., 2003. Methanol adsorption on g-irradiated SiO2 surface. Radiat. Phys. Chem. 66, 27–33. Patel, M., Morrell, P.R., Murphy, J.J., Skinner, A., Maxwell, R.S., 2006. Gamma radiation induced effects on silica and on silica–polymer interfacial interactions in filled polysiloxane rubber. Polym. Degrad. Stabil. 91, 406–413. Pesek, J.J., Leigh, I.E., 1994. Chemically Modified Surfaces. Royal Society of Chemistry, Cambridge, UK. Piao, F., Oldham, W.G., Haller, E.E., 2000. The mechanism of radiationinduced compaction in vitreous silica. J. Noncryst. Solids 276, 61–71. Ruller, J.A., Friebele, E.J., 1991. The effect of gamma-irradiation on the density of various types of silica. J. Noncryst. Solids 136, 163–172. Shelby, J.E., 1994. Hydrogen, hydroxyl, hydride and water in amorphous SiO2. Protonic species in vitreous silica. J. Noncryst. Solids 179, 138–147. Van Ginhoven, R.M., Hjalmarson, H.P., Edwards, A.H., Tuttle, B.R., 2006. Hydrogen release in SiO2: sources sites and release mechanism. Nucl. Instrum. Methods Phys. Res. B 250, 274–278.
Radiation Physics and Chemistry 77 (2008) 267–272 www.elsevier.com/locate/radphyschem
g radiolyzed amorphous silica: A study with 29 Si CP-MAS NMR spectroscopy Franco Cataldoa,, Donatella Capitanib, Noemi Proiettib, Pietro Ragnib a
Lupi Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy CNR, Institute of Chemical Metodologies, Area della Ricerca di Roma 1, 00016 Monterotondo Scalo, Rome, Italy
b
Received 29 March 2007; accepted 11 May 2007
Abstract Precipitated amorphous silica has been g radiolyzed at 333, 666 and 1000 kGy in air. The structural changes undergone during irradiation have been studied by 29Si CP-MAS NMR spectroscopy. The spectra suggest that silica looses the hydrogen atoms attached to the silanol groups and the resulting silanol radicals undergo a crosslinking reaction by forming peroxysiloxane groups or by reacting with E0 centres yielding siloxane groups. r 2007 Elsevier Ltd. All rights reserved. Keywords: Amorphous precipitated silica; Irradiation; Gamma radiolysis; Peroxysilanols; Crosslinking
1. Introduction Amorphous silica can be divided into two main classes: precipitated amorphous silica and fumed silica (Franta, 1989). Precipitated amorphous silica is mainly used as reinforcing filler for plastics and elastomers while fumed silica finds a much broader range of applications in numerous different fields (Bergna and Roberts, 2006; Franta, 1989). Precipitated silica is produced by a wet process where sodium silicate is treated with sulphuric acid (Iler, 1979; Franta, 1989). In contrast fumed silica is produced for instance in the thermal hydrolysis of SiCl4 in a hydrogen flame (Iler, 1979; Franta, 1989). Considerable attention has been dedicated to the action of high-energy radiation on silica glasses (Griscom, 2006) but incomparably less and relatively old works can be found in literature concerning the action of radiation on amorphous silica (Barter and Wagner, 1964; Boehm, 1966; Devine, 1990; LaVerne and Tonnies, 2003). Corresponding author.
E-mail address: cdcata@flashnet.it (F. Cataldo). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.05.010
29
Si CP-MAS NMR spectroscopy; Chemical structure changes; Silanols;
Recently, a stimulating work on the action of g radiation on fumed silica and on fumed silica filled elastomers (Patel et al., 2006) has shown that amorphous fumed silica irradiated at 50 kGy in air develops a strong electron spin resonance (ESR) spectrum due to the formation of trapped paramagnetic species. These species affect also the interaction between the silica surface and the polymer matrix which hosts silica as filler and can contribute to the reinforcing effect of the radiation-treated silica rubberbased composite. The previous works on the radiation treatment of carbon black Cataldo (2001a, b) have shown that similar effects occur in the carbon black interaction with elastomer matrix. The formation of paramagnetic centres in radiation-treated silica and the enhancement of its absorption and chemisorption power toward other molecules and macromolecules is known for a long time (Barou and Wolkenstein, 1982). The work of Patel et al. (2006) has also shown that the changes induced in fumed silica by g radiation can be followed by 29Si CP-MAS NMR spectroscopy. This prompted us to study with the same technique the effects of gamma radiation on amorphous precipitated silica, the most common silica employed as reinforcing filler in elastomers.
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
268
2. Experimental
HO
OH Si
2.1. Materials Amorphous precipitated silica (in powder form) Ultrasil 7000 from Degussa has been used in the present study. The silica was characterized by a nitrogen surface area of 175 m2/g, a water content of 6.5% as measured by the weight loss at 115 1C, a pH in water of 6.8 and by a tap density of 320 g/L. The SiO2 content was 497% and the main impurities are Na (about 1% expressed as Na2O), Al (0.2% expressed as Al2O3), Fe (300 ppm expressed as Fe2O3) and 0.06% of Cl and 0.9% of S (expressed as SO3).
O
OH O O
Si
O
Si
O
Si
O
Si
Si
2.3.
Si CP-MAS NMR
Samples were powdered and packed into 4 mm zirconia rotors and sealed with Kel-F caps. Solid state 29Si CPMAS NMR spectra were performed at 39.76 MHz on a Bruker AMX-200 spectrometer. The spin rate was 5 kHz, the p/2 pulse width was 4 ms and the recycling time was 4 s. Contact times were 5 and 10 ms. Spectra were obtained by using 1024 data points in the time domain, zero filled and Fourier transformed to a size of 1024 data points. The chemical shift was externally referred to tetramethylsilane. A full simulation of the 29Si CP-MAS NMR were carried out with the SHAPE2000 software package developed by Prof. Michele Vacatello, Naples University, Italy. 3. Results and discussion 3.1. 29Si CP-MAS NMR: a tool for determination of silica structure and reaction The most common silica chemical sites in the surface or in bulk are reported in Scheme 1. In silica each silicon atom is almost always connected to four oxygen atoms with a tetrahedral geometry. In amorphous silica the tetrahedrons are connected chaotically in a three-dimensional net. The oxygen atoms in their turns can be connected to other silicon atoms or, if they are on the surface of the silica particle, are connected with hydrogen atoms. As shown in Scheme 1, on the silica surface the OH groups attached to a silicon atom can occur in a geminal configuration or can be isolated. In the first case the site is called Q2 and in the latter case Q3. A variant of
O
H
Si O
O
O
O
Si
Si
Si
Si O O Si
O
Si
O
Si
O
O Si
Si O
O
O
O
O
Si Q4 - Bulk Si site
29
Si
H
Q3*- Vicinal & Bridged Si - OH
Q3 - Isolated Si - OH
2.2. Irradiation Three samples were irradiated in glass ampoules in presence of air using a 60Co g cell, with a dose rate 6 kGy/h. The absorbed dose administered to the three samples was 333, 666 kGy and 1 MGy. The samples remained white in colour even after 1 MGy absorbed dose. No discolouration was observed.
O
Q2 - Geminal Si - OH
Q4 - Siloxane Scheme 1.
isolated OH groups is when they occur vicinal and bridged (see Scheme 1); in this case we can talk about the Q3* sites. Finally, other two sites can be expected: a completely oxygen bridged silicon atom connected to other silicon atoms in the bulk of the solid and a siloxane bridge when the site occurs on the surface; both are called Q4 sites. 29 Si CP-MAS NMR spectroscopy is a powerful tool for the investigation of the surface structure and reactions of silica gels (Pesek and Leigh, 1994; Patel et al., 2006). In fact, well resolved 29Si CP-MAS NMR can be obtained showing the distribution of silicon sites in different environments. The Q2 silicon sites with geminal OH groups resonate at about 91 ppm. Q3 silicon sites bearing isolated OH group resonate at about 101 ppm; Q4 silicon sites bound to four OSi moieties and bearing no OH groups resonate at about 109 ppm. Due to the relaxation properties, using a proper contact time t ¼ 10 ms, the intensity of the signals due to both Q2 and Q3 is suitable for a quantitative treatment. This is not true for Q4 silicon atoms, since Q4 units close to the surface Si–OH groups mostly contribute to the signal intensity. In the present work we have applied 29Si CP-MAS NMR technique for studying the modifications occurring in precipitated amorphous silica during gamma radiation treatment by comparing the resulting spectra with that recorded on the same precipitated amorphous silica sample before the radiation processing.
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
a
269
b
-60
-80
-100
-120
-140
-160
-60
-180
c
-80
-100
-120
-140
-160
-180
d
-60
-80
-100
-120
-140
-160
-180
-60
-80
-100 -120 -140 -160 -180
Fig. 1. 29Si CP-MAS NMR spectra (recorded at contact time t ¼ 10 ms). The spectrum (a) was recorded on precipitated amorphous silica before its irradiation. The spectra (b)–(d) were recorded at 333, 666 and 1000 kGy, respectively. Gradual net growth of the signal at 110 ppm due to the Q4 silicon sites can be observed. The deconvoluted spectra (solid lines) have been overlapped to the experimental spectra (dotted lines).
In Fig. 1(a) the 29Si CP-MAS NMR spectrum of a reference, pristine silica, is reported. There are silicon atoms in three different environments: 110 ppm due to Q4 units 101 ppm due to Q3 units 90 ppm, very weak, due to Q2 units. After silica irradiation with a dose of 333 kGy a net increase of the intensity of the signal due to Q4 units in comparison to Q3 units is observed as shown in Fig. 1(b). At higher doses of 666 and 1000 kGy a further increase of the signal due to Q4 is observed as shown in Figs. 1(c) and (d). These spectral data were acquired with a contact time of 10 ms. To confirm these trends, 29Si CP-MAS NMR spectra were also recorded with contact time of 5 ms. These spectra are shown in Fig. 2. Again, an increase of the Q4 signal in comparison to the Q3 was observed. From the simulation of the experimental spectra, the area of resonance due to the Q2, Q3 and Q4 sites has been obtained. The deconvoluted spectra are superimposed to the experimental as shown in Figs. 1 and 2. The values of the areas are reported in Tables 1 and 2. It must be pointed out that, because crosspolarization is not a directly
quantitative technique, the obtained ratios R ¼ A(Q4)/ A(Q3) must not be thought as absolute values but are only indicative of a trend. Two types of silanol groups are present in silica. The isolated silanols are much more abundant than the geminal silanols (see Scheme 1). Tables 1 and 2 show that the NMR signal associated to geminal silanols (Q2) disappear under radiation processing while the NMR signal associated to isolated silanols (Q3) is reduced in its intensity. The changes of the silanol groups are accompanied by an increase of the signal due to the Q4 sites. In Fig. 3 the ratio R at two different NMR contact times has been reported against the adsorbed dose. In both cases the ratio R increases by increasing the absorbed dose, suggesting that silica radiolysis causes the transformation of Q3 sites into the Q4 sites. 3.2. Chemical changes occurring in precipitated amorphous silica during radiolysis The irradiation of silica involves the formation of a number of defects. These defective structures are not fully understood and sometimes, as in the case of the E0 centres, are interpreted in different ways by different authors (Agnello et al., 2002). E0 centres are often interpreted as an
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
270
b
a
-60
-80
-100
-120
-140
-160
-180
-60
-80
-100
-120 ppm
-140
-160
-180
d
c
-60
-80
-100
-120 ppm
-140
-160
-180
-60
-80
-100
-120 ppm
-140
-160
-180
Fig. 2. 29Si CP-MAS NMR spectra (recorded at contact time t ¼ 5 ms). The spectrum (a) was recorded on precipitated amorphous silica before its irradiation. The spectra (b)–(d) were recorded at 333, 666 and 1000 kGy, respectively. Gradual net growth of the signal at 110 ppm due to the Q4 silicon sites can be observed. The deconvoluted spectra (solid lines) have been overlapped to the experimental spectra (dotted lines). Table 1 Area of the resonances due to Q4, Q3, Q2 and the ratio R ¼ AðQ4 Þ/AðQ3 Þ obtained by applying a deconvolution procedure to 29Si CP-MAS NMR (t ¼ 10 ms) D (kGy)
A(Q4)
A(Q3)
A(Q2)
R ¼ A(Q4)/A(Q3)
0 333 666 1000
0.46 0.52 0.59 0.70
0.51 0.45 0.40 0.30
0.03 0.03 0.01 0.00
0.90 1.15 1.48 2.33
Table 2 Area of the resonances due to Q4, Q3, Q2 and the ratio R ¼ AðQ4 Þ=AðQ3 Þ obtained by applying a deconvolution procedure to 29Si CP-MAS NMR (t ¼ 5 ms) D (kGy)
A(Q4)
A(Q3)
A(Q2)
R ¼ A(Q4)/A(Q3)
0 333 666 1000
0.25 0.41 0.45 0.53
0.72 0.58 0.54 0.47
0.03 0.01 0.01 0.00
0.35 0.71 0.83 1.13
unpaired electron localized on a silicon atom bonded to three oxygen atoms (Imai and Hirashima, 1994; Griscom, 2006): O O O
Si
Another common defect is the non-bridging oxygen hole center (NBOHC), a hole trapped on a non-bridging oxygen (Imai and Hirashima, 1994; Griscom, 2006). The paramagnetic defects formed by the action of radiation in the bulk of silica at low dose may remain trapped but at higher dose tend to recombine or diffuse to the surface, making the surface to react with the paramagnetic centres formed directly on the surface (Patel et al., 2006). The sites where electrons accumulate are chemically active and may interact with surface water or silanol species liberating hydrogen. Hydrogen evolution is dependent on silica surface area. Larger yields are achieved with high surface areas (LaVerne and Tonnies, 2003). Many authors have reported that silica radiolysis involves always the production of hydrogen (Barter and Wagner, 1964; Shelby, 1994; Chemerisov et al., 2001; LaVerne and Tonnies, 2003; Van Ginhoven et al., 2006). Hydrogen derives both from water and from silanol radiolysis: Si
OH
Si
O
+
H
Hydrogen is released in atomic form and can remain trapped in the silica network or can migrate and combine with other hydrogen atoms to form molecular hydrogen or can react with radiation-induced defects formed in silica (Chemerisov et al., 2001; Van Ginhoven et al., 2006).
ARTICLE IN PRESS F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
271
2.5 2.25 2 y = 0.0014 x + 0.7721 R2 = 0.915
1.75
Q4/Q3
1.5 1.25 1 y = 0.0008 x + 0.3741 R2 = 0.9732
0.75 0.5
at 10 ms at 5 ms
0.25 0 0
100
200
300
400 500 600 700 RADIATION DOSE (kGy)
800
900
1000
1100
Fig. 3. Area ratio R ¼ Q4/Q3 taken from Tables 1 and 2. It can be observed that the ratio R grows both at NMR contact time t ¼ 5 and 10 ms.
Silanol radicals are active species which may favour the chemisorption of molecular species on silica surface (Ermatov, 1959; Barou and Wolkenstein, 1982; Novak et al., 2003). Alternatively, silanol radicals may produce peroxy siloxanes (Van Ginhoven et al., 2006; Griscom, 2006) and their 29Si NMR signals should resonate in the same frequency range of the resonance due to Q4 units, causing its broadening and enhancement. O O
2
Si
O O
O
O
O Si
O
O
Si
O
O O
Alternatively, a silanol radical can react with an E0 centre: O O O
O Si
O
+
.Si
O O
O O O
O Si
O
Si
O
NMR causes its narrowing (Patel et al., 2006). This is in contrast with the results of the present work on precipitated amorphous silica. However, there are substantial differences in the morphology of fumed silica surface and amorphous silica surface. In fact, the former has a surface silanol density of 2–3 silanol groups/nm2 while amorphous precipitated silica usually has 6 silanols/nm2 (Iler, 1979; Bergna and Roberts, 2006). Therefore a more effective crosslinking can be expected from precipitated amorphous silica. Another interesting aspect which differentiates fumed silica from amorphous precipitated silica is their trend to crystallize at 1000 1C. Fumed silica does not crystallize even after 7 days at 1000 1C while precipitated amorphous silica crystallizes in 20 min at the mentioned temperature. This radical difference in the thermal and crystallization behaviour may be ascribed not only to the differences in the surface group density but also to the bulk chemical structure of the two silicas.
O
4. Conclusions Such kind of crosslinks between the surfaces not only explain the enhancement of the Q4 NMR signal and the decrease of the Q3 or Q2 signals but also can explain the reason why the density of silica glasses increases with the radiation dose (Ruller and Friebele, 1991; Piao et al., 2000; An et al., 2006). Curiously, the irradiation of fumed silica instead of leading to an increase of the Q4 signal at the 29Si CP-MAS
Irradiation of precipitated amorphous silica with g rays causes an increase of the amount of the Q4 silicon sites and a decrease of the Q3 and Q2. The sites are illustrated in Scheme 1. These changes have been detected by 29Si CP-MAS NMR and have been interpreted in terms of radiation-induced crosslinking of the silanols groups present on the silica surface with liberation of hydrogen.
ARTICLE IN PRESS 272
F. Cataldo et al. / Radiation Physics and Chemistry 77 (2008) 267–272
References Agnello, S., Gelardi, F.M., Boscaino, R., Cannas, M., Boizot, B., Petite, G., 2002. Intrinsic defects induced by beta irradiation in silica. Nucl. Instrum. Methods Phys. Res. B 191, 387–391. An, Q., Zheng, L., Luo, S.N., 2006. Vacancy-induced densification of silica glass. J. Noncryst. Solids 352, 3320–3325. Barou, V., Wolkenstein, T., 1982. Effet de l’Irradiation sur les Proprie´te´s des Semi-Conducteurs. Mir, Moscow, Russia. Barter, C., Wagner, C.D., 1964. Generation of acidity in silica gel by ionizing radiation. J. Phys. Chem. 68, 2381–2383. Bergna, H.E., Roberts, W.O., 2006. Colloidal Silica. Fundamentals and Applications. CRC Press, Taylor & Francis, Boca Raton, FL, USA. Boehm, H.P., 1966. Chemical identification of surface groups. In: Eley, D.D., Herman, P., Weisz, P.B. (Eds.), Advanced Catalysis, vol. 16, pp. 225–247. Cataldo, F., 2001a. Effects of g radiation treatment on the reinforcing properties of carbon black. Int. J. Polym. Mater. 50, 29–46. Cataldo, F., 2001b. Effects of fillers radiation pre-treatments on their rubber absorption power and on the reinforcing properties of rubber compounds. Polym. Int. 50, 828–834. Chemerisov, S.C., Werst, D.W., Trifunac, A.D., 2001. Formation, trapping and kinetics of H atoms in wet zeolites and mesoporous silica. Radiat. Phys. Chem. 60, 405–410. Devine, R.A.B., 1990. Radiation damage and the role of structure in amorphous silica. Nucl. Instr. Methods Phys. Res. B 46, 244–251. Ermatov, S.E., 1959. Variation in the absorption properties of silica gel under the action of gamma irradiation. Sov. Phys. 129, 1259–1260. Franta, I. (Ed.), 1989. Elastomers and Rubber Compounding Materials. Elsevier, Amsterdam, Holland.
Griscom, D.L., 2006. Self-trapped holes in pure-silica glass: a history of their discovery and characterization and an example of their critical significance to industry. J. Noncryst. Solids 352, 2601–2617. Iler, R.K., 1979. The Chemistry of Silica. Solubility, Polymerization, Colloid and Surface Properties and Biochemistry. Wiley, New York, USA. Imai, H., Hirashima, H., 1994. Intrinsic- and extrinsic-defect formation in silica glasses by radiation. J. Noncryst. Solids 179, 202–213. LaVerne, J.A., Tonnies, S.E., 2003. H2 production in the radiolysis of aqueous SiO2 suspensions and slurries. J. Phys. Chem. B 107, 7277–7280. Novak, E., Hancz, A., Erdohelyi, A., 2003. Methanol adsorption on g-irradiated SiO2 surface. Radiat. Phys. Chem. 66, 27–33. Patel, M., Morrell, P.R., Murphy, J.J., Skinner, A., Maxwell, R.S., 2006. Gamma radiation induced effects on silica and on silica–polymer interfacial interactions in filled polysiloxane rubber. Polym. Degrad. Stabil. 91, 406–413. Pesek, J.J., Leigh, I.E., 1994. Chemically Modified Surfaces. Royal Society of Chemistry, Cambridge, UK. Piao, F., Oldham, W.G., Haller, E.E., 2000. The mechanism of radiationinduced compaction in vitreous silica. J. Noncryst. Solids 276, 61–71. Ruller, J.A., Friebele, E.J., 1991. The effect of gamma-irradiation on the density of various types of silica. J. Noncryst. Solids 136, 163–172. Shelby, J.E., 1994. Hydrogen, hydroxyl, hydride and water in amorphous SiO2. Protonic species in vitreous silica. J. Noncryst. Solids 179, 138–147. Van Ginhoven, R.M., Hjalmarson, H.P., Edwards, A.H., Tuttle, B.R., 2006. Hydrogen release in SiO2: sources sites and release mechanism. Nucl. Instrum. Methods Phys. Res. B 250, 274–278.