Evidence of nanopores in sol gel based TiO 2 and TiN ultrafiltration membranes

Transcription

1 Materials Chemistry and Physics 63 (2000) Evidence of nanopores in sol gel based TiO 2 and TiN ultrafiltration membranes G. Tomandl a, M. Mangler a, E. Pippel b, J. Woltersdorf b, a University of Mining and Technology, Institute of Ceramic Materials, D Freiberg, Gustav-Zeuner-Str. 3, Germany b Max Planck Institute of Microstructure Physics, D Halle, Weinberg 2, Germany Received 26 August 1999; accepted 30 August 1999 Abstract An essential criterion for applying TiO 2 and TiN ultrafiltration (UF) membranes in the cross-flow filtration is the pore size distribution in the active filter layer. This layer is very thin ( m) compared to the entire cross-section of the membrane. Therefore the pores inside the layer cannot be determined by means of the classical method of N 2 -adsorption, but only by high resolution techniques which are demanding (as the pore sizes amount to only a few nanometers), and require intricate preparation methods. However, adsorption measurements are easy to perform in so-called unsupported layers which are manufactured by the crystallization of the gel in a glass dish. Using high resolution electron microscopy and image analysis methods we have directly evidenced (i) the pore and particle sizes and (ii) the very good agreement of the results of the adsorption measurements in unsupported layers with the real pore size distributions in the UF layer of the multilayer membranes. Converting the volume distribution of the spatial pore diameters from the N 2 -adsorption measurements into a number distribution of the chord lengths allows the comparison of both methods. In addition, scanning electron microscopy combined with energy dispersive X-ray microanalysis revealed the thicknesses of the UF layers of the multilayer membranes to be 130 nm for TiO 2, and 550 nm for TiN. No sol particles had penetrated into the multilayer supports Published by Elsevier Science S.A. All rights reserved. Keywords: Ultrafiltration membranes; TEM; HVEM; HREM; SEM 1. Introduction A sol gel method of producing ceramic TiO 2 and TiN ultrafiltration (UF) membranes was developed yielding oxidic or nitridic layers as a function of the sintering atmospheres. The resulting microstructure and the pore size distribution of the layers, which are particularly important for potential applications to the cross-flow UF, are adjusted via a wide range of parameters as, e.g., the particle size of the sol, the kind and quantity of the binder, the drying and sintering regime. The deliberate optimization of the process parameters during the membrane preparation requires a subtle characterization of the resulting porosity. The active filter layer in the multilayer membranes is very thin ( m) compared to the entire cross-section of the membrane (about 1.5 mm). Thus, the proportion of the nanoporous layer amounts only to 10 4 of the whole membrane and the pores inside the layer cannot be determined by means of the classical method of the N 2 -adsorption, but only by high resolution techniques and intricate specimen preparation methods. Nevertheless, adsorption measurements are easy to perform in the powder material of so-called unsupported layers of arbitrary thick- Corresponding author. ness which are manufactured by the crystallization of the gel in a glass dish. Therefore, the paper has principally two aims: (i) to evidence the pore and particle sizes directly using high resolution electron microscopy and image analysis methods, and (ii) to compare the results of the adsorption measurements in unsupported layers with the real pore size distributions in the UF layer of the multilayer membranes. In addition, the degree of the TiO 2 TiN transformation during the ammonolysis was to be examined by X-ray investigations of the lattice parameters of pure unsupported membranes, and energy dispersive X-ray microanalysis was applied for the determination of the titanium distribution in the fine-grained alumina supports. 2. Processing route for UF membranes The membrane production comprises the following three steps [1,2]: 1. Sol preparation: A titanium alkoxide is hydrolyzed in a mixture of water and an acid or organic base before the precipitate is peptized at room temperature. The particle size of the sol depends on the molar ratio of water to metal alkoxide to acid/base, whereas the time of pep /00/$ see front matter 2000 Published by Elsevier Science S.A. All rights reserved. PII: S (99)

2 140 G. Tomandl et al. / Materials Chemistry and Physics 63 (2000) tization is influenced solely by the ratio of acid or base to metal alkoxide. A large amount of acid or base accelerates the reaction. The elasticity of the sol is enhanced by adding an organic binding substance, which also prevents the infiltration of the support with solid particles during the subsequent coating process. 2. Film preparation: The UF films are produced on tubular multilayer ceramic supports by depositing the mixture of sol and binder on these substrates. A part of the solvent leaves the mixture due to capillary forces. Thus the solid particles are deposited on the surface of the support. Then the surplus mixture is rejected. After careful and slow drying a gel film is forming on the support. 3. Sintering: The films were heat-treated in different atmospheres: TiO 2 (anatase) membranes were sintered in air at 550 C. The anatase layers are transformed into TiN by further sintering in ammonia (ammonolysis) between 900 and 1100 C via the following reaction kinetics: NH N H 2 (1) 2TiO 2 + H 2 Ti 2 O 3 + H 2 O (2) Ti 2 O 3 + 2NH 3 2TiN + 3H 2 O (3) 3. Specimen preparation and electron microscope equipment The TEM samples were prepared by cutting thin slices, polishing, dimple-grinding them to about 10 m, and finally, by Ar-ion milling, thus providing specimens for high resolution imaging, with only a few nanometers in thickness and a tolerable surface roughness [3]. Some of these steps were hard tricky owing to some characteristic features of these samples: the curvature of the tubular multilayer ceramic supports, the varying porosity at the different microstructural levels, the weak bonding between the UF membranes and the supporting alumina ceramics, the high brittleness of the multilayer system and thermomechanical stresses occurring during the processing route and by the preparation technique. The samples have been investigated down to the atomic scale using high voltage electron microscopy (HVEM) and high resolution electron microscopy (HREM). In addition, cross-sections were studied by analyzing fracture surfaces using scanning electron microscopy (SEM) combined with energy dispersive X-ray microanalysis (surface mode and line scan) to determine exactly the thickness of the deposited UF films, the interfacial morphology between the fine-grained alumina support and the films, and the penetration depth of the sol into the supporting ceramics. For the investigations we used the high voltage electron microscope Jeol-JEM run at 1000 kv, the high resolution Philips CM 20 FEG field emission electron microscope run at 200 kv, and the scanning electron microscope Jeol JSM 6300F (with field emission gun). The latter was equipped with a Voyager II EDXS detector system (Tracor). Fig. 1. SEM image of the system TiO 2 membrane/alumina support (cross-section; left: TiO 2, right: fine-grained Al 2 O 3 ). 4. SEM results Figs. 1 and 2 show SEM images of cross-sections of TiO 2 and TiN UF membranes, respectively, on fine-grained alumina supports. The thickness of the membrane layers (left) is nearly 130 nm for TiO 2, and 550 nm for TiN. The grain size of the neighbored alumina material is m. SEM could not reveal single pores neither did it provide information about the pore structure of the UF layers. EDXS line scans with the Al K and the Ti K signals along the dotted lines (as indicated in the figures) yield the intensity curves inserted in Figs. 1 and 2. (The Ti counts are enlarged by a factor of 20 so that the small intensity peaks represent the noise level.) In case of TiO 2 the intensity curves make evident that a maximum of the Ti intensity corresponding to the thin TiO 2 membrane occurs due in front of the Al onset. Fig. 2. SEM image of the system TiN membrane/alumina support (cross-section; left: TiN, right: fine-grained Al 2 O 3 ).

3 In addition, EDXS spectra over an area of 1 m below the TiO 2 measured with 100 s registration time do not at all indicate any titanium. Therefore, it can be concluded that the binder used prevented the penetration of the sol into the supporting ceramic. Analogous statements can be given for the TiN films, which are four times thicker. G. Tomandl et al. / Materials Chemistry and Physics 63 (2000) HREM analyses Fig. 3 demonstrates in a plane view the relatively homogeneous structure of the TiO 2 UF films. The TiO 2 particles with diameters of nm are quite densely packed, enclosing oblong or polygonal pores of a few to ten nanometers, with the larger ones especially in the triple points between the particles. The evaluation of the size distribution of the pores (as shown in the HREM image of Fig. 3) using image analysis methods [4] results in a bimodal distribution of the chord lengths of the pores (cf. Fig. 4). Maximum values are found at 4 and 9 nm. The second maximum may be partially due to crystallites, positioned to the electron beam in such a way that they do not show any orientation contrast. Therefore, these crystallites possibly are often considered as pores. The topography and the relative orientation of the individual TiO 2 crystallites can be seen in the atomic plane resolved Fig. 5. The distinct particles exhibit different types of atomic planes due to the difference in orientation. The systematic imaging in such resolution should detect even smallest pores and could enable the statistical evaluation of the pore size distribution. Fig. 4. Chord length distribution of the pores in a TiO 2 -UF membrane s 0 (logl) = frequency distribution, S 0(L) = cumulative distribution of chord length L. In some cases, the atomic plane resolved imaging reveals single crystals with crystallographically defined contours, just in the very contrast position, as a few examples in Fig. 5 show. Similar results are provided by the HREM analysis of the TiN UF films as demonstrated in Fig. 6. However, the TiN particles exhibit more distinctly the tendency to form a closed continuous layer, resulting in a pronounced polygonal habitus of the crystals (with again the size of some ten nanometers), and, consequently, in more narrow pores with a width of often only a few nanometers and propagating along the faceted crystal planes. Fig. 3. TEM image of the grains and pores in a TiO 2 UF film.

4 142 G. Tomandl et al. / Materials Chemistry and Physics 63 (2000) Fig. 5. Atomic plane resolved image of the arrangement of TiO 2 crystallites in an UF membrane. 6. X-ray diffraction measurements To examine the degree of the TiO 2 TiN transformation during the ammonolysis, the lattice parameters of pure unsupported membranes were determined by X-ray investigations. In order to prepare those membranes the sol was deposited on glass. After the sol gel transition, the gel layer was removed from the glass and sintered. For the unsupported TiN layers, a mixed phase of TiO 1 x N x was revealed (x can be varied experimentally in a wide range). Checking the lattice parameters of TiO and TiN enabled the degree of the conversion of TiO 2 into TiN to be assessed. In the membranes, the amount of TiN depends on the temperature of the ammonolysis: with higher temperatures the amount of TiN is increasing as demonstrated in Fig. 7. But an increase in temperature also leads to growing pores and a lower specific surface. The compromise was best at 1050 C. 7. N 2 -adsorption measurements In addition, the pore sizes of unsupported membranes have been examined via N 2 adsorption measurements. This method enables the estimation of the pore size distribution within the UF layer without an extensive sample preparation. Considering adsorption and condensation the volume frequency distribution of the pore sizes can be determined by using the Barrett Joyner Halenda (BJH) method [5]. Fig. 8 shows the respective results for TiN and TiO 2 UF membranes. For TiO 2 layers, this method yielded an average pore size of 10 nm, and for TiN membranes it was 15 nm. However, the interpretation requires an identical microstructure of both the multilayer membranes and the unsupported layers as no direct measurement of the supported membranes is possible: The pore volume fraction of the UF layer compared to the support and the coarse-grained layers is too low.

5 G. Tomandl et al. / Materials Chemistry and Physics 63 (2000) Fig. 6. TEM image of the grains and pores in a TiN UF film. Fig. 7. Lattice parameters of the mixed phase TiO 1 x N x as a function of the temperature of the ammonolysis. Furthermore, it can be assumed that the relatively short sintering time at low temperatures does not cause any shrinkage, not even in the unsupported membranes. 8. Comparison of the HREM analyses with N 2 -adsorption measurements The results of both the image analyses of a TiO 2 membrane (cf. Fig. 4) and the N 2 -adsorption measurements (cf. Fig. 8) are not directly comparable. While the adsorption measurements reveals a volume distribution of pore diameters, the image analysis results in a number distribution of chord lengths of the pores. The cumulative chord length distribu- Fig. 8. Pore size distribution in TiN and TiO 2 UF membranes (BJH-desorption branch), resulting from the N 2 adsorption measurements. tion S 0 (L), (L denotes chord length) can be calculated from the volume frequency distribution p3 (logd) (D denotes pore diameter) by the numerical evaluation of the equation S 0 (L) = 1 c logl [ p 3 (logd) ( D 2 L 2) ] D 3 d logd (c denotes normalization factor) which is applicable solely to spherical pores [6]. As shown in Fig. 9, the results of such an estimation are in good agreement with the adsorption measurements.

6 144 G. Tomandl et al. / Materials Chemistry and Physics 63 (2000) films; the pores between these particles could be clearly detected. The crystallite sizes were found to be nm for TiO 2 and nm for TiN-layers. A comparison between the pore size distributions for TiO 2 -membranes obtained by image analyses and by N 2 -adsorption measurements showed a good agreement after conversion into comparable graphs. Acknowledgements Fig. 9. Comparison of the HREM analyses with N 2 -adsorption measurements by calculating the chord length distribution of the pores from their volume diameter distribution, obtained with N 2 -adsorption. 9. Conclusions Via SEM investigations and EDXS measurements the thickness of the UF layers of the membranes was determined to be 130 nm for TiO 2 and 550 nm for TiN, respectively. No penetration of the sol particles into the multilayer supports could be detected. Using HREM analyses it was possible to image single crystallites in the UF The authors would like to thank Dr. W. Erfurth for his assistance in the SEM studies and to Dr. R. Kohl and Mrs. C. Siewert for preparing the specimens and performing several measurements. References [1] R. Kohl, G. Tomandl, A. Larbot, L. Cot, Proceedings Annual meeting of the German Ceramics Society, Bayreuth, 1992, pp [2] R. Kohl, G. Tomandl, A. Larbot, L. Cot, Proceedings Annual meeting of the German Ceramics Society, Weimar, 1993, pp [3] J. Woltersdorf, E. Pippel, Pract. Metallogr. 29 (1992) [4] J. Serra, Image Analysis and Mathematical Morphologie, Academic press, London, [5] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids, Academic Press, New York, [6] G. Tomandl, in press.