Structural Investigations on the Hydrolysis and Condensation

J. Phys. Chem. B 2007, 111, 7519-7528

7519

Structural Investigations on the Hydrolysis and Condensation Behavior of Pure and Chemically Modified Alkoxides. 2. Germanium Alkoxides Venkata Krishnan,*,†,§ Silvia Gross,*,‡ Sonja Mu1 ller,† Lidia Armelao,‡ Eugenio Tondello,‡ and Helmut Bertagnolli† Institute of Physical Chemistry, UniVersity of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany, and Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle Ricerche, and Dipartimento di Scienze Chimiche, UniVersita` di PadoVa and Consorzio InteruniVersitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Via Marzolo 1, 35131 PadoVa, Italy ReceiVed: January 22, 2007; In Final Form: April 13, 2007

Structural investigations on the hydrolysis and condensation behavior of germanium alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that germanium alkoxides are monomeric in nature and undergo very fast hydrolysis and condensation reactions upon water addition. However, the chelation of germanium alkoxides by acetylacetone does not take place even 48 h after mixing, and any change in hydrolysis and condensation behavior is not observed after acetylacetone addition. When mixed with prehydrolyzed silicon alkoxide, the structures of germanium alkoxides are not modified. Both Si and Ge precursors are insensitive to the presence of each other in the reaction solution even after 48 h of aging. The addition of water to this mixture catalyzes the hydrolysis and condensation reactions very fast and leads to the formation of Ge-O-Ge (and consequently Si-O-Si) homocondensation products.

1. Introduction Germanium alkoxides are well-known and well-characterized compounds,1-8 and they have been extensively used as solgel precursors for GeO2 and GeO2-MOx mixed-oxide-based networks.9-14 In the preparation of complex materials it is important to investigate the early stages that govern the oxide network formation to yield a homogeneous and controlled mutual distribution of the constituents. To this aim, in the synthesis of multicomponent glasses by the sol-gel process, the experimental parameters that control the hydrolysis and condensation reactions of the molecular precursors have to be carefully studied and optimized. Different analytical techniques such as IR and proton and multinuclear NMR spectroscopy can be used to study the degree of association and physicochemical behavior of the alkoxides in solution.3,4,7,15,16 Among the various techniques, X-ray absorption fine structure (XAFS) spectroscopy, based on the absorption of X-rays by a particular atom, is a powerful technique for probing the environment of a specific atom regardless of the physical state of the sample.17 Extended X-ray absorption fine structure (EXAFS) spectroscopy provides information on the coordination number, the nature of scattering atoms surrounding the absorbing atom, the interatomic distance between the absorbing atom and the backscattering atoms, and the Debye-Waller factor, which account for the disorders due to static displacements and thermal vibrations.17 This technique enables a selective determination of the short range order, and * Authors to whom correspondence should be addressed. Fax: +39 049 827 5161 (S.G.); +1 215 573 2112 (V.K.). E-mail: [email protected] (V.K.); [email protected] (S.G.). † University of Stuttgart. § Current address: Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, PA 19104. ‡ Universita ` di Padova and Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM).

it is hence suitable to investigate the local atomic environment around a specific atom.18 Time-resolved XAFS measurements have also been assessed as a valuable tool to investigate the structural evolution of systems in solution, such as kinetics, complexation processes, or the reaction degree.19-21 By combining the local information provided by EXAFS with structural details on a longer range, a more precise description of the system evolution can be obtained. In this framework, Raman spectroscopy, which provides information on the structural modification of dynamic systems,22 was used to investigate the hydrolysis and polycondensation behavior of germanium alkoxide solutions. Moreover, the chemical modification of germanium alkoxides by the bidentate ligand acetylacetone was also investigated in the present work. In our earlier studies, we have analyzed the structure and the influence of chemical modifications on zirconium alkoxides.23-25 As mentioned in the first paper of this series,26 acetylacetone is a well-known stabilizing agent for different metal alkoxides, such as W(OEt)3,27 Zr(OR)4 (R ) OnPr, OiPr),28 Al(OiBu)3,29 and Ti(OR)4 (R ) OiPr, OnBu).30 It behaves as a chelating rather than as a bridging ligand, and it lowers the high reactivity of the metal alkoxides. While upon hydrolysis most of the alkoxy groups are quickly removed, the bidentate complexing ligands are hardly cleaved. Since the understanding of the hydrolysis and condensation behavior of pure and chemically modified alkoxides can remarkably contribute to improve the sol-gel synthetic routes, further efforts were performed in this framework. In one of our previous works, the hydrolysis and condensation behavior of single and mixed germanium alkoxides have been extensively studied by means of electrospray ionization (ESI) mass spectrometry.31 It has been pointed out that, under ESI conditions, the polycondensation reaction in germanium alkoxide solutions occurs through a cationic reaction, conversely to silicon and titanium alkoxides.9 In a further part

10.1021/jp0705424 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007

7520 J. Phys. Chem. B, Vol. 111, No. 26, 2007

Krishnan et al.

Figure 1. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for the germanium alkoxides measured at the Ge K-edge.

of this study, germanium alkoxide was mixed with prehydrolyzed silicon tetraethoxide (TEOS) to explore the possibility to prepare homogeneously mixed GeO2-SiO2 materials. Tetraethoxide was previously prehydrolyzed due to its slower hydrolysis and condensation kinetics with respect to germanium alkoxides. However, the formation of mixed Ge-O-Si species could not be demonstrated, even upon addition of a large excess (1:10) of tetraethoxysilane (TEOS). In the present work, structural investigations on the hydrolysis and condensation behavior of germanium alkoxides under solgel conditions are presented. Therefore, similar to the first paper of this series26 in this study time-resolved in situ EXAFS and Raman experiments were performed with the aim to investigate: (a) the structures of four different germanium alkoxides in solution; (b) the hydrolysis and condensation behavior of germanium alkoxide upon water addition; (c) the structure and hydrolytic behavior of germanium alkoxide in the presence of acetylacetone; (d) the structure of germanium alkoxide in the mixture with prehydrolyzed Si(OEt)4; (e) the effect of prehydrolyzed Si(OEt)4 on the hydrolysis and condensation behavior of the germanium alkoxide. To the best of our knowledge, this is the first comprehensive study on the hydrolysis and condensation behavior of germanium alkoxides as pure alkoxides and in the presence of a complexing ligand or other alkoxides by means of XAFS and Raman spectroscopic methods. 2. Experimental Details 2.1. Sample Preparation. The commercially available alkoxides Ge(OMe)4, Ge(OEt)4, Ge(OiPr)4, Ge(OnBu)4, and Si(OEt)4 purchased from ABCR GmbH, Karlsruhe, Germany, were

studied as solutions in anhydrous ethanol. The hydrolysis and condensation reactions were performed by adding to the alkoxide solutions a stoichiometric amount of acidified deionized water. Typical molar ratios in solutions were Ge(OR)x/EtOH/H2O/HCl ) 1:20:2:0.01. Acetylacetone (acac, 2,4-pentandione, Aldrich) was added to ethanolic solutions of the germanium alkoxide by using a 1:2 molar ratio of metal alkoxide/acac. The hydrolysis and condensation reactions were carried out by using the same molar ratios already used in the case of alkoxides that are not chemically modified. Si(OEt)4 was prehydrolyzed by reacting it with hydrochloric acid and water in a Si(OEt)4/EtOH/H2O/ HCl ) 1:20:1:0.01 molar ratio and by stirring this solution at room temperature for 12 h. A mixed solution of prehydrolyzed Si(OEt)4 and Ge(OR)4 (R ) OnBu, OEt) was prepared by mixing the prehydrolyzed TEOS and germanium alkoxide in ethanol using a Si/Ge molar ratio of 1:1. In all of the cases the solutions were stirred for about 5 min and then measured at different time intervals (vide infra). Due to the high moisture sensitivity of metal alkoxides, all procedures involved in preparation and structural characterization were performed under dry nitrogen atmosphere by using a Schlenk line. 2.2. Measurements and Analysis. The XAFS measurements of the samples were performed at the X-ray absorption spectroscopy (XAS) beamline of Angstroemquelle Karlsruhe (ANKA) at Forschungszentrum Karlsruhe (FZK), Karlsruhe, Germany. The measurements were performed at the Ge K-edge at 11 103 eV using a Si(111) double-crystal monochromator, and energy calibration was monitored using a Pt metal foil, having LIII-edge at 11 564 eV. All experiments were carried out in transmission mode with nitrogen-filled ion chambers at ambient conditions. The samples, prepared as ethanolic solutions, were filled into a transmission sample cell for liquids

Germanium Alkoxides

J. Phys. Chem. B, Vol. 111, No. 26, 2007 7521

TABLE 1: EXAFS-Obtained Structural Parameters for the Different Germanium Alkoxides sample

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

Ge(OMe)4

Ge-O Ge-C Ge-O Ge-C Ge-O Ge-C Ge-O Ge-C

5.9 ( 0.6 6.3 ( 0.9 5.7 ( 0.6 6.0 ( 0.9 5.7 ( 0.6 6.1 ( 0.9 5.8 ( 0.6 6.2 ( 0.9

1.74 ( 0.02 2.79 ( 0.03 1.75 ( 0.02 2.79 ( 0.03 1.75 ( 0.02 2.80 ( 0.03 1.75 ( 0.02 2.79 ( 0.03

0.055 ( 0.006 0.087 ( 0.013 0.055 ( 0.006 0.087 ( 0.013 0.055 ( 0.006 0.087 ( 0.013 0.055 ( 0.006 0.087 ( 0.013

2.388

2.98-15.04

21.42

1.486

2.98-15.03

16.71

1.386

2.97-15.03

24.46

0.968

2.99-15.03

16.16

Ge(OEt)4 Ge(OiPr)4 Ge(OnBu)4 a

Absorber-backscatterers. b Coordination number. c Interatomic distance. d Debye-Waller factor with its calculated deviation. e Fermi energy.

TABLE 2: EXAFS-Obtained Structural Parameters for a Ge(OnBu)4 and Water Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min

Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge

5.3 ( 0.5 4.9 ( 0.7 1.1 ( 0.2 5.3 ( 0.5 5.0 ( 0.7 1.1 ( 0.2 5.3 ( 0.5 5.4 ( 0.8 1.0 ( 0.2 5.2 ( 0.5 5.0 ( 0.8 1.1 ( 0.2 5.3 ( 0.5 4.9 ( 0.7 1.1 ( 0.2

1.74 ( 0.02 2.82 ( 0.03 3.16 ( 0.04 1.74 ( 0.02 2.82 ( 0.03 3.17 ( 0.04 1.74 ( 0.02 2.82 ( 0.03 3.16 ( 0.04 1.74 ( 0.02 2.82 ( 0.03 3.16 ( 0.04 1.74 ( 0.02 2.82 ( 0.03 3.16 ( 0.04

0.050 ( 0.005 0.097 ( 0.015 0.050 ( 0.010 0.050 ( 0.005 0.095 ( 0.014 0.050 ( 0.010 0.050 ( 0.005 0.102 ( 0.015 0.050 ( 0.010 0.050 ( 0.005 0.097 ( 0.015 0.050 ( 0.010 0.050 ( 0.005 0.095 ( 0.014 0.050 ( 0.010

1.673

2.95-15.04

20.36

1.479

2.94-15.03

19.84

1.836

2.95-15.04

20.97

1.541

2.94-15.03

19.31

1.558

2.94-15.03

20.14

20 min 40 min 60 min 80 min

a


Figure 2. Proposed molecular structure of Ge(OR)4.

specifically designed for XAFS measurements, and the concentrations of all of the samples were adjusted to yield an extinction of 1.5. The XAFS data analysis and Raman spectroscopic measurements were performed as described in the first paper of this series.26 For clarity reasons, the EXAFS spectra and the Fourier transformation plots are shifted along the ordinate axis in all cases. The exact zero position of the EXAFS spectra and that of the corresponding Fourier transformation can be obtained by observing the value on the ordinate axis at the intersection point. In the fitting procedure, the amplitude reduction factor (AFAC) was set to a value of 0.8, and the various parameters, i.e., coordination numbers, interatomic distances, Debye-Waller factor, and Fermi energy value, were determined by iterations for all of the cases. The term R-factor refers to the agreement of the fitted function to the experimental spectrum. 3. Results and Discussion 3.1. Structures of Different Germanium Alkoxides. The structural investigations were initially performed on four different germanium alkoxides, Ge(OMe)4, Ge(OEt)4, Ge(OiPr)4, and Ge(OnBu)4, to determine possible structural variations when going up on the homologous series. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for

Figure 3. Raman spectrum of Ge(OEt)4.

the four different germanium alkoxides, measured at the Ge K-edge, are shown in Figure 1, and the corresponding structural parameters are summarized in Table 1. The EXAFS spectra obtained for the different germanium alkoxides were identical. In all of the cases, two distinct shells could be fitted, one with about six oxygen backscatterers at 1.75 Å and the other with about six carbon backscatterers at 2.79 Å. These backscatterers correspond to the four terminal alkoxy groups and the two coordinated alcohol molecules. The results indicate that that the number of coordinated solvent molecules does not change when the R-group of the alkoxide changes. In addition, the alcoholysis reaction also does not reveal any remarkable structural differences. Furthermore, no nearest-


Krishnan et al.

Figure 4. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Ge(OnBu)4 and water mixture measured at different time intervals at the Ge K-edge.

neighbor germanium backscatterer could be demonstrated from the EXAFS measurements on the pure alkoxides, which indicates that all the four investigated germanium alkoxides are monomeric in nature irrespective of the chain length and branching of the alkoxy groups, which is in agreement with the data already reported in the literature.1 On the basis of the EXAFS investigations and considering the details on the nature and molecular complexity of germanium alkoxides reported in literature,1 the molecular structure proposed for Ge(OR)4 is depicted in Figure 2. As a representative example, the Raman spectrum of Ge(OEt)4 is shown in Figure 3. The peak at 620 cm-1 can be assigned to Ge-O vibrations, and the band around 875 cm-1 can be attributed to C-O vibrations.32 The peak at 1050 cm-1 is attributed to C-C vibrations, and the peaks between 2800 and 2950 cm-1 are ascribed to the C-H vibrations of the alkyl groups.32 3.2. Hydrolysis and Condensation Behavior of Germanium Alkoxide. The hydrolysis and condensation reactions were studied by adding to the ethanolic solution of germanium alkoxide a stoichiometric amount of acidified water. Soon after the addition of water to germanium alkoxide, the formation of a turbid solution could be observed, ascribable to the fast hydrolysis and condensation reactions. EXAFS measurements were performed on the solution immediately after mixing, and subsequent measurements were carried out at different time intervals. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for a Ge(OnBu)4 and water mixture measured at different time intervals are shown in Figure 4. For clarity reasons only representative spectra at three different time intervals are shown. The structural parameters are given in Table 2. The results show that Ge(OnBu)4 instantaneously undergoes hydrolysis and condensation reactions, as indicated by the

Figure 5. Raman spectra of Ge(OEt)4 and water measured at different time intervals along with pure Ge(OEt)4: (a) Ge(OEt)4, (b) Ge(OEt)4 + H2O (0 h), and (c) Ge(OEt)4 + H2O (24 h).

formation of a new germanium backscatterer at 3.16 Å, a distance typical of the Ge-O-Ge moiety. The spectra collected at different time intervals did not show any variation in the structural parameters in comparison with those obtained immediately after mixing. Raman spectroscopic investigations were performed to obtain deeper insights into the hydrolysis and condensation mechanisms. Studies were performed on germanium alkoxide and water at the same molar ratio (1:2) as used for the EXAFS investigations. Raman spectra of Ge(OEt)4 and water measured immediately and 24 h after mixing are shown in Figure 5 along with the spectrum of pure Ge(OEt)4. It is interesting to note

Germanium Alkoxides


Figure 6. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for Ge(OnBu)4-acac mixture measured at different time intervals, at the Ge K-edge.

TABLE 3: EXAFS-Obtained Structural Parameters for a Ge(OnBu)4-acac Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min

Ge-O Ge-C Ge-O Ge-C Ge-O Ge-C

5.9 ( 0.6 6.3 ( 1.0 5.6 ( 0.6 6.4 ( 1.0 5.6 ( 0.6 6.3 ( 1.0

1.74 ( 0.02 2.79 ( 0.03 1.75 ( 0.02 2.78 ( 0.03 1.75 ( 0.02 2.78 ( 0.03

0.050 ( 0.005 0.081 ( 0.012 0.063 ( 0.006 0.095 ( 0.014 0.059 ( 0.006 0.087 ( 0.013

0.694

2.98-15.03

25.54

3.848

2.97-15.01

32.28

2.824

2.96-15.00

24.78

210 min 825 min a


TABLE 4: EXAFS-Obtained Structural Parameters for a Ge(OnBu)4, acac, and Water Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min

Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge

5.1 ( 0.5 5.0 ( 0.8 1.0 ( 0.2 5.1 ( 0.5 5.1 ( 0.8 1.0 ( 0.2 5.1 ( 0.5 5.2 ( 0.8 1.0 ( 0.2 5.0 ( 0.5 5.0 ( 0.8 1.0 ( 0.2

1.75 ( 0.02 2.82 ( 0.03 3.16 ( 0.04 1.75 ( 0.02 2.82 ( 0.03 3.16 ( 0.04 1.75 ( 0.02 2.82 ( 0.03 3.16 ( 0.04 1.75 ( 0.02 2.82 ( 0.03 3.16 ( 0.04

0.050 ( 0.005 0.092 ( 0.014 0.055 ( 0.011 0.050 ( 0.005 0.097 ( 0.015 0.055 ( 0.011 0.050 ( 0.005 0.095 ( 0.015 0.055 ( 0.011 0.050 ( 0.005 0.095 ( 0.015 0.055 ( 0.011

0.706

2.98-15.03

19.92

1.200

3.00-15.03

20.78

0.919

2.95-15.03

20.55

0.939

2.99-15.03

20.99

35 min 70 min 170 min

a


that the intensity of the Ge-O peak at 620 cm-1 in pure Ge(OEt)4 decreases in the mixture with water, which could be attributed to the decrease in the concentration of the germanium species in the solution owing to condensation and subsequent precipitation. 3.3. Structure of Germanium Alkoxide in the Presence of Acetylacetone. To study the influence of chemical modifica-

tion on the reactivity of germanium alkoxide, acetylacetone was added to the germanium butoxide solution. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for a Ge(OnBu)4-acac mixture measured at different time intervals are shown in Figure 6, and the obtained structural parameters are summarized in Table 3.


Figure 7. Raman spectra of a Ge(OEt)4-acac mixture measured at different time intervals along with pure Ge(OEt)4: (a) Ge(OEt)4, (b) Ge(OEt)4 + acac (0 h), (c) Ge(OEt)4 + acac (24 h), and (d) Ge(OEt)4 + acac (48 h).

The EXAFS analysis yielded the same results for the measurements performed at different time intervals, and the obtained structural parameters resemble those of the pure Ge(OnBu)4. The coordination of the ligand with the metal could not be demonstrated even 825 min after mixing, indicating that Ge(OnBu)4 does not undergo chemical modification by acac. Raman spectroscopic measurements were performed on an alkoxide-acac mixture in the same molar ratio as used for EXAFS measurements, to further investigate the results observed from EXAFS analysis. The Raman spectra of a Ge(OEt)4-acac

Krishnan et al. mixture measured at different time intervals are shown along with the spectrum of Ge(OEt)4 in Figure 7. The spectra measured at different time intervals are characterized by the presence of the peak at 1605 cm-1 corresponding to the C-O stretching mode of the free enol form of acac.29 This peak should disappear upon the coordination of acac to the metal atom. However, the presence of this peak was pointed out in all of the spectra, thus indicating that no coordination took place. In agreement with the EXAFS investigations, no remarkable changes could be observed in the Raman spectra measured at different time intervals. Furthermore, the Raman studies indicate that the coordination of acac does not occur even 48 h after mixing. 3.4. Hydrolytic Behavior of Germanium Alkoxide in the Presence of Acetylacetone. Investigations on the hydrolysis and condensation reactions of germanium alkoxide in the presence of acetylacetone were performed to check whether the addition of the ligand induces any changes in the condensation behavior, even though the coordination has not occurred. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for a Ge(OnBu)4, acac, and water mixture measured at different time intervals are shown in Figure 8. For clarity reasons only three representative spectra measured at different time intervals are shown. The obtained structural parameters are summarized in Table 4. The EXAFS analysis shows that the hydrolysis and condensation reactions occur instantaneously upon the addition of water in the presence of acac as well, which could also be visually observed by the development of turbidity in the solution. The fast hydrolysis and condensation could be attributed to the noncoordination of the ligand to the germanium atom. The obtained structural parameters are in agreement with those

Figure 8. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Ge(OnBu)4, acac, and water mixture measured at different time intervals at the Ge K-edge.

Germanium Alkoxides

Figure 9. Raman spectra of a Ge(OEt)4-acac-water mixture measured at different time intervals along with a Ge(OEt)4-acac mixture: (a) Ge(OEt)4 + acac (0 h), (b) Ge(OEt)4 + acac + H2O (0 h), and (c) Ge(OEt)4 + acac + H2O (24 h).

measured in the analogous solution prepared without acac. Subsequent measurements performed at different time intervals also indicate no changes in the structural parameters in comparison with the measurements performed immediately after mixing. Raman spectroscopic measurements were performed on an alkoxide-acac mixture in the same molar ratio as used for EXAFS measurements, to further investigate the results observed from EXAFS analysis. The Raman spectra measured immediately and after subsequent time intervals following the addition of water to a Ge(OEt)4-acac mixture are shown along with the spectrum of a Ge(OEt)4-acac mixture in Figure 9.

J. Phys. Chem. B, Vol. 111, No. 26, 2007 7525 In the obtained spectra, the peak due to Ge-O vibrations at 620 cm-1 decreases in intensity owing to condensation and precipitation, similar to the previous investigations performed without acac. Furthermore, the peak due to the free enol form of acac at 1605 cm-1 could also be demonstrated in all of the measured spectra, which were measured at different time intervals. 3.5. Structure of Germanium Alkoxide Mixed with Prehydrolyzed Silicon Alkoxide. Structural investigations on the mixture of germanium alkoxide with prehydrolyzed Si(OEt)4 were carried out by means of EXAFS and Raman spectroscopy to determine whether the addition of a different alkoxide alters the structure of the metal alkoxide thereby building a mixed Ge-O-Si moiety. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for Ge(OnBu)4 mixed with prehydrolyzed Si(OEt)4 (measured at different time intervals) are shown in Figure 10, and the obtained structural parameters are summarized in Table 5. For clarity, only three representative spectra are shown. The EXAFS analysis indicates that the structural parameters for the mixture of alkoxides at different time intervals were similar to each other and in agreement with those of pure Ge(OnBu)4. The results indicate that the addition of prehydrolyzed Si(OEt)4 does not change the structure around the germanium atoms, and even after 1020 min after mixing the two alkoxides remain as separate entities in the solution. Furthermore, the formation of Ge-O-Si species could not be demonstrated from the EXAFS investigations. These EXAFS results were further confirmed by the Raman spectroscopic studies, which were performed on Ge(OEt)4 mixed with prehydrolyzed Si(OEt)4 in the same molar ratio. The Raman spectra recorded for Ge(OEt)4 mixed with prehydrolyzed Si-

Figure 10. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for Ge(OnBu)4 mixed with prehydrolyzed Si(OEt)4 measured at different time intervals at the Ge K-edge.


Krishnan et al.

TABLE 5: EXAFS-Obtained Structural Parameters for Ge(OnBu)4 Mixed with Prehydrolyzed Si(OEt)4 Measured at Different Time Intervals time 0 min 35 min 190 min 370 min 1020 min a

A-Bsa

Nb

rc (Å)

σd (Å)

Ge-O Ge-C Ge-O Ge-C Ge-O Ge-C Ge-O Ge-C Ge-O Ge-C

5.8 ( 0.6 6.2 ( 0.9 6.4 ( 0.6 5.6 ( 0.8 5.8 ( 0.6 6.8 ( 1.0 5.8 ( 0.6 6.7 ( 1.0 5.8 ( 0.6 6.6 ( 1.0

1.75 ( 0.02 2.80 ( 0.03 1.73 ( 0.02 2.76 ( 0.03 1.75 ( 0.02 2.81 ( 0.03 1.75 ( 0.02 2.81 ( 0.03 1.75 ( 0.02 2.80 ( 0.03

0.055 ( 0.006 0.089 ( 0.013 0.050 ( 0.005 0.084 ( 0.013 0.050 ( 0.005 0.097 ( 0.015 0.055 ( 0.006 0.097 ( 0.015 0.055 ( 0.006 0.097 ( 0.015

EFe (eV)

k-range (Å-1)

R-factor

0.259

2.96-15.02

15.94

2.662

2.99-15.00

38.92

0.001

2.99-15.02

17.79

-0.018

2.99-15.02

15.74

0.474

2.97-15.03

16.66


TABLE 6: EXAFS-Obtained Structural Parameters for a Ge(OnBu)4, Prehydrolyzed Si(OEt)4, and Water Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

0 min

Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge Ge-O Ge-C Ge-Ge

5.3 ( 0.5 5.9 ( 0.9 0.9 ( 0.2 5.3 ( 0.5 6.1 ( 0.9 0.8 ( 0.2 5.3 ( 0.5 6.3 ( 1.0 0.9 ( 0.2 5.4 ( 0.5 6.5 ( 1.0 0.9 ( 0.2 5.4 ( 0.5 6.8 ( 1.0 0.8 ( 0.2 5.4 ( 0.5 6.6 ( 1.0 0.8 ( 0.2 5.3 ( 0.5 6.6 ( 1.0 0.8 ( 0.2

1.74 ( 0.02 2.83 ( 0.03 3.17 ( 0.04 1.74 ( 0.02 2.84 ( 0.03 3.17 ( 0.04 1.74 ( 0.02 2.84 ( 0.03 3.17 ( 0.04 1.74 ( 0.02 2.84 ( 0.03 3.17 ( 0.04 1.74 ( 0.02 2.85 ( 0.03 3.17 ( 0.04 1.74 ( 0.02 2.84 ( 0.03 3.17 ( 0.04 1.74 ( 0.02 2.85 ( 0.03 3.17 ( 0.04

0.050 ( 0.005 0.102 ( 0.015 0.067 ( 0.013 0.050 ( 0.005 0.100 ( 0.015 0.067 ( 0.013 0.050 ( 0.005 0.102 ( 0.015 0.071 ( 0.014 0.050 ( 0.005 0.102 ( 0.015 0.074 ( 0.015 0.050 ( 0.005 0.102 ( 0.015 0.074 ( 0.015 0.050 ( 0.005 0.100 ( 0.015 0.071 ( 0.014 0.050 ( 0.005 0.102 ( 0.015 0.074 ( 0.015

40 min 80 min 120 min 300 min 420 min 640 min

a

EFe (eV)

k-range (Å-1)

R-factor

0.696

2.98-15.03

16.79

1.072

2.99-15.03

17.00

0.667

2.98-15.03

16.61

-0.384

2.97-15.02

16.85

-0.422

2.97-15.02

17.63

0.009

2.95-15.02

16.22

0.001

2.95-15.02

16.29


(OEt)4 at different time intervals are shown in Figure 11 along with pure Ge(OEt)4 and Si(OEt)4. In the Raman spectra of the mixture of alkoxides, the peak at 620 cm-1 characteristic of Ge-O vibrations in Ge(OEt)4 and the peak at 658 cm-1 characteristic of Si-O vibrations in Si(OEt)4 could be demonstrated.32 The spectra acquired at different time intervals did not show any remarkable changes in comparison with the spectrum obtained immediately after mixing. The Raman results indicate that even 48 h after mixing the alkoxides are both insensitive to the presence of each other in the ethanolic solution. 3.6. Hydrolytic Behavior of Germanium Alkoxide Mixed with Prehydrolyzed Silicon Alkoxide. Hydrolysis and condensation behavior of germanium alkoxide mixed with prehydrolyzed Si(OEt)4 was also investigated to study the influence of the addition of prehydrolyzed Si(OEt)4 on the formation of the condensation product, especially to investigate whether in the formed gel homo- or heterocondensation species are present. As mentioned in the Introduction, the formation of Ge-O-Si heterolinkages instead of Si-O-Si and Ge-O-Ge homobonds would lead, upon gelation, to more homogeneous and welldispersed materials. Si(OEt)4, which is remarkably less reactive than the germanium homologues, was prehydrolyzed to match their hydrolysis-condensation reactions. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for a

Figure 11. Raman spectra of Ge(OEt)4 mixed with prehydrolyzed Si(OEt)4 measured at different time intervals along with pure Ge(OEt)4 and Si(OEt)4: (a) Si(OEt)4, (b) Ge(OEt)4, (c) Si(OEt)4 + Ge(OEt)4 (0 h), (d) Si(OEt)4 + Ge(OEt)4 (24 h), and (e) Si(OEt)4 + Ge(OEt)4 (48 h).

Ge(OnBu)4, prehydrolyzed Si(OEt)4, and water mixture measured at different time intervals are shown in Figure 12, and

Germanium Alkoxides


Figure 12. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Ge(OnBu)4, prehydrolyzed Si(OEt)4, and water mixture measured at different time intervals at the Ge K-edge.

Figure 13. Raman spectra of a Ge(OEt)4, prehydrolyzed Si(OEt)4, and water mixture measured at different time intervals along with Ge(OEt)4 mixed with prehydrolyzed Si(OEt)4: (a) Si(OEt)4 + Ge(OEt)4 (0 h), (b) Si(OEt)4 + Ge(OEt)4 + H2O (0 h), and (c) Si(OEt)4 + Ge(OEt)4 + H2O (24 h).

the obtained structural parameters are presented in Table 6. For clarity reasons only three representative spectra are shown. The evaluation of the EXAFS spectra reveals that the hydrolysis and condensation reactions occur instantaneously upon the addition of water to the mixture of alkoxides, as confirmed by the immediate formation of a turbid solution. It is interesting to note that the obtained structural parameters are in agreement with those obtained when water was added to pure Ge(OnBu)4 in the same molar ratio. In particular, a higher backscatterer at a distance of 3.16 Å could be determined, which

could be unambiguously assigned to germanium, thus proving the formation of Ge-O-Ge species. Moreover, the subsequent measurements performed at different time intervals reveal no remarkable changes in the structural parameters in comparison with the measurements performed immediately after mixing. The EXAFS results indicate the formation of homocondensation species with Ge-O-Ge moieties, and no heterocondensated Ge-O-Si species were detected. This can be attributed to the differences in the reactivities of Ge(OnBu)4 and Si(OEt)4 with respect to their hydrolysis and condensation behavior. The EXAFS results were further confirmed by Raman spectroscopic investigations performed on Ge(OEt)4 under similar reaction conditions. The Raman spectra measured for a Ge(OEt)4, prehydrolyzed Si(OEt)4, and water mixture at different time intervals are shown in Figure 13 along with Ge(OEt)4 mixed with prehydrolyzed Si(OEt)4. In the Raman spectra of the mixture, the characteristic peaks belonging to Ge-O and Si-O vibrations at 620 and 658 cm-1, respectively, could be noticed. As observed in the earlier cases, the intensity of the peak at 620 cm-1 has decreased due to the decrease of the germanium species in the solution as a result of gelation and precipitation. Furthermore, vibrations of mixed Ge-O-Si at 680 cm-1 could not be detected.33 Both EXAFS and Raman results are in agreement with the ESI mass spectrometric investigations reported earlier,31 revealing only the formation of homocondensation products. 4. Conclusions EXAFS and Raman spectroscopic investigations on germanium alkoxides revealed that Ge(OMe)4, Ge(OEt)4, Ge(OiPr)4, and Ge(OnBu)4 are monomeric in nature and have similar structures, indicating that the alkoxy group does not influence the general structure of the germanium alkoxides. The results

7528 J. Phys. Chem. B, Vol. 111, No. 26, 2007 show that germanium alkoxides instantaneously undergo hydrolysis and condensation reactions upon water addition, leading to the formation of Ge-O-Ge polyoxocondensation oligomers. Moreover, the results indicate that germanium alkoxide is insensitive to the presence of acetylacetone as the chemical modification of the germanium species is not observed even 48 h after mixing. This behavior can be chiefly ascribed to the fact that Ge, belonging to the IV group of the main group elements, has a less marked tendency to expand its coordination number and to form stable coordination compounds as in the case of transition metals. In this regard, the formation of stable coordination compounds in the case of transition metals has been already pointed out in the first paper of this series,26 which is devoted to XAFS and Raman investigations on Hf and Ta alkoxides. In addition, the results also illustrate that hydrolysis and condensation reactions occur immediately upon the addition of water even in the presence of acetylacetone due to the noncoordination of the ligand with germanium alkoxide. Furthermore, the structure of germanium alkoxide is not altered in the mixture with prehydrolyzed Si(OEt)4, and even after 48 h both alkoxides remain as individual entities. Consequently, the hydrolysis and condensation of this mixture of alkoxides revealed solely the formation of homocondensation products, which can be attributed to the difference in the reactivity of germanium and silicon alkoxides. Acknowledgment. The Italian Rectors Conference (CRUI), Rome, Italy, and Deutscher Akademischer Austauschdienst (DAAD), Bonn, Germany, are gratefully acknowledged for funding the researchers’ mobility in the framework of a Vigoni Programme. S.G. gratefully thanks the DAAD also for financing the research period in Germany. L.A. is indebted to the research program FIRB RBNE033KMA, “Molecular Compounds and Hybrid Nanostructured Materials with Resonant and Nonresonant Optical Properties for Photonic Devices” for financial support. We thank ANKA at FZK, Karlsruhe, Germany, for the provision of synchrotron radiation for XAFS measurements. References and Notes (1) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. Metal Alkoxides; Academic Press: London, 1978. (2) Mehrotra, R. C. AdV. Inorg. Chem. Radiochem. 1983, 26, 269. (3) Mehrotra, R. C.; Batwara, J. M.; Kapoor, P. N. Coord. Chem. ReV. 1980, 31, 67.

Krishnan et al. (4) Aegerter, M. A. Sol-Gel: Science Technology, Proceedings of the Winter School on Glasses and Ceramics from Gels, Sa˜o Carlos, Brazil, Aug 14-19, 1989; World Scientific: Singapore, 1989. (5) Mehrotra, R. C. J. Sol-Gel Sci. Technol. 1998, 100, 1. (6) Mehrotra, R. C.; Singh, A.; Sogani, S. Chem. ReV. 1994, 94, 1643. (7) Gugliemi, M.; Carturan, G. J. Non-Cryst. Solids 1988, 100, 16. (8) Bradley, D. C.; Holloway, H. Can. J. Chem. 1962, 40, 62. (9) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990. (10) Armelao, L.; Fabrizio, M.; Gross, S.; Martucci, A.; Tondello, E. J. Mater. Chem. 2000, 5, 1147. (11) Sharma, S. K.; Matson, D. W.; Philpotts, J. A.; Rousch, T. L. J. Non-Cryst. Solids 1984, 68, 115. (12) Jang, J. H.; Koo, J.; Bae, B. S. J. Am. Ceram. Soc. 2000, 83, 1356. (13) Jang, J. H.; Koo, J.; Bae, B. S. J. Mater. Res. 2000, 15, 282. (14) Jang, J. H.; Koo, J.; Bae, B. S. J. Non-Cryst. Solids 1999, 259, 144. (15) Turova, N. Y.; Turevskaya, E. P.; Yanovskaya, M. I.; Yanovsky, A. I.; Kessler, V. G.; Tcheboukov, D. E. Polyhedron 1998, 17, 899. (16) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259. (17) Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. (18) X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES; Koningsberger, D. C., Prins, R., Eds.; Wiley: New York, 1988. (19) Kickelbick, G.; Feth, M. P.; Bertagnolli, H.; Puchberger, M.; Holzinger, D.; Gross, S. J. Chem. Soc., Dalton Trans. 2002, 3892. (20) Liu, C.; Frenkel, A. I.; Vairavamurthy, A.; Huang, P. M. Can. J. Soil Sci. 2001, 81, 337. (21) Tsodikov, M. V.; Kugel, V. Y.; Slivinskii, E. V.; Bondarenko, G. N.; Maksimov, Y. V.; Alvarez, M. A.; Hidalgo, M. C.; Navio, J. A. Appl. Catal., A 2000, 193, 237. (22) Long, D. A. Raman Spectroscopy; McGraw-Hill: New York, 1977. (23) Peter, D.; Ertel, T. S.; Bertagnolli, H. J. Sol-Gel Sci. Technol. 1994, 3, 91. (24) Peter, D.; Ertel, T. S.; Bertagnolli, H. J. Sol-Gel. Sci. Technol. 1995, 5, 5. (25) Bauer, M.; Gastl, C.; Koeppl, C.; Kickelbick, G.; Bertagnolli, H. Monatsh. Chem. 2006, 137, 567. (26) Krishnan, V.; Gross, S.; Mu¨ller, S.; Armelao, L.; Tondello, E.; Bertagnolli, H. J. Phys. Chem. B 2007, 111, 7501. (27) Unuma, H.; Tokoda, T.; Susuki, Y.; Furusaki, T.; Kodaira, K.; Hatsuchida, T. J. Mater. Sci. Lett. 1985, 5, 1248. (28) Schmidt, H.; Seiferling, B. Mater. Res. Soc. Symp. Proc. 1986, 73, 739. (29) Leaustic, A.; Babonneau, F.; Livage, J. Chem. Mater. 1989, 1, 248. (30) Deskibar, J. C. J. Mater. Sci. 1985, 20, 44. (31) Cristoni, S.; Armelao, L.; Gross, S.; Seraglia, R.; Tondello, E.; Traldi, P. Rapid Commun. Mass Spectrom. 2002, 16, 733. (32) Weidlein, J.; Mu¨ller, U.; Dehnicke, K. Schwingungsspektroskopie; Thieme: Stuttgart, Germany, 1988. (33) Duverger, C.; Ferrari, M.; Mazzoleni, C.; Montagna, M.; Pucker, G.; Turrell, S. J. Non-Cryst. Solids 1999, 245, 129.

Structural Investigations on the Hydrolysis and Condensation

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