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First investigations on the use of scanning tunnelling microscopy (STM) for the characterisation of porous membranes
Journal of Membrane Science, 67 (1992) 295-300 Elsevier Science Publishers B.V., Amsterdam
295
Short Communication
First investigations on the use of scanning tunnelling microscopy (STM) for the characterisation of porous membranes A. Chahboun*, R. Coratger”, F. Ajustro9, J. Beauvillain”, P. Aimarb and V. Sanchezb “CEMES/CNRS, 29 rue J. Marvig, B.P. 4347,31055 Toulouse Cedex (France) bURA CNRS de GEnie Chimique, L.G.C.E., 118, route de Narbonne, 31062 Toulouse Cedex (France) (Received September 30,199l; accepted in revised form November 14,199l)
Abstract A new microscopic technique, scanning tunnelling microscopy (STM ) , has been used to analyse the surface of microfiltration (MF) and ultrafiltration (UF) membranes. Some of the observations have been compared to those obtained with scanning electron microscopy. Although the agreement is good between the two methods for MF membranes, STM shows a better resolution, which is of major interest for the characterisation of UF membranes. Further, STM gives pictures of the surface in 3 dimensions. Keywords: porous membranes; scanning tunnelling microscopy; membrane structure
Introduction Ultrafiltration and microfiltration membranes are porous barriers used for the separation and concentration of proteins, colloids or particles from pharmaceutical, food or chemical industries. One of the problems process engineers have to face is to get relevant information for the choice of the membranes convenient for a given separation. Membrane manufacturers are also concerned with membrane characterisation for the development of better filters and for production control. A great deal of work has therefore been dedCorrespondence to: P. Aimar, URA CNRS de GEnie Chimique, L.G.C.E., 118, route de Narbonne, 31062, Toulouse Cedex, France.
0376-7388/92/$05.00
icated to the improvement of characterisation methods for artificial membranes. A large part of the efforts aims at determining the apparent size of the pores and the pore density. Three main categories of method may be mentioned. (a) Measurement of the retention of calibrated molecules used as tracers [ 1,2]. The information is directly available in terms of retention of solutes, but the translation into pore size and pore size distribution requires attention [ 31. (b) Measurements of the permeability of the filter for more or less complex systems (water, mercury or multi-phase fluids); these do not give direct information on molecule retentions. An apparent pore size, the significance of which can be different from the previous one, can be obtained. (c) Microscopic techniques which should al-
0 1992 Elsevier Science Publishers B.V. All rights reserved.
A. CHAHBOUN ET AL.
296
low a direct determination of the physical pore size, such as scanning electron microscopy (SEM) [ 41, or transmission electron microscopy TEM [ 51. However, these methods are not fully satisfactory because their resolution is poor (around 1 nm for SEM )i.e. in the range of the pore sizes of ultrafiltration membranes (1.5-15 nm) and of nanofiltration membranes ( < 3 nm). Another shortcoming, also mentioned by Cheryan and Merin [5] is the influence of the sample preparation on the results. A new technique, scanning tunnelling microscopy (STM) , has recently been developed by Binnig and co-workers [ 6,7]. On top of having a very high resolution, this technique has quickly proved to be inexpensive compared to other microscopic techniques and relatively simple to set-up. Another advantage are the relatively mild sample preparation conditions. One of its first achievements has been to give experimental evidence of the anomalous periodicity of silicium under ultra-vacuum conditions [ 81. More recently, the first applications of STM to biological materials appeared, with the observation of tobacco viruses [ 91. The scope of this note is to present the first observations of porous membranes using an STM built in our university [lo]. A series of Nuclepore membranes has been investigated by STM and SEM. These membranes are microfiltration membranes, but they present the advantage of offering a range of pore radii from micrometer-size down to nanometer-size. This provides the opportunity to calibrate the different methods available and to explore their limits. Results concerning a polysulfone membrane are also shown.
that is designed to ensure a good mechanical stability and a high reproducibility of the results. The tips are made from 0.1 mm tungsten wire thinned out in NaOH solution. The resolution qualified on a HOPG (highly ordered pyrolytic graphite) graphite surface, is 0.1 A horizontally and 0.01 A vertically. The scanning frequency lies between 0.1 and 1 Hz according to the type of surface (this gives an average of one minute per picture). The tunnelling voltage and current are 0.6 V and 0.2 nA respectively. The SEM apparatus is a commercial Cambridge S250 model. For both techniques, the membrane surface is covered with a carbon film (at least 2 nm thick), evaporated under 10m5 Torr. This thickness is estimated using a quartz microbalance. Results Figure 1 shows a SEM view of a 80 nm (pore size) Nuclepore membrane. This can be compared with Fig. 2 where a similar membrane (100 nm pore size) was observed by STM. Similar features are visible, although STM seems to give more accurate information on the local topography. The pore density estimated from the manual counting of several images either
Material and methods The apparatus is a lab-made, pocket size replica of the Binnig and Rohrer microscope. It is a triple axis displacement system, with an improved electromagnetic clamping system [ 111,
Fig. 1. Surface of a Nuclepore (0.08pm) membrane (SEM) .
USE OF SCANNING TUNNELLING MICROSCOPY FOR THE CHARACTERISATION OF POROUS MEMBRANES
Fig. 2. Surface of a Nuclepore (0.1 pm) membrane (STM).
Fig. 3. Topview and cross-section of a pore from a Nuclepore (0.08 pm) membrane by STM.
297
298
Fig. 4. Top view of a pore of a Nuclepore (0.015 pm) membrane by STM.
Fig. 5. Surface of a polysulfone membrane (MWCO: 100 kDa) after fouling by BSA by STM.
A. CHAHBOUN ET AL.
USE OF SCANNING TUNNELLING MICROSCOPY FOR THE CHARACTERISATION OF POROUS MEMBRANES
299
Fig. 6. Top view and cross-section of a pore of a polysulfone membrane (MWCO: 100 kDa) by STM.
from STM or from SEM gives an average of 6 x 1012 and 4.1 x 1Ol2 pores/m2 respectively, where the value published by Strathmann [ 121 is 6 x 1012.Although the two techniques give the same order of magnitude, the difference between the two figures has not yet been resolved. One of the possible explanations might be the damage caused by the beam voltage in SEM, as discussed in [ 131. Figure 3 shows a pore of a 80 nm membrane as observed by STM. The cross-section along the line l-2 measures 105 nm and the depth is 40 nm. The pore walls should be almost vertical, but the shape of the tip (an approximate pyramid) gives a drift in the signal and the an-
gle of the tip is likely to be the apparent angle of the pore wall observed here. The size of the pore entrance, as shown on this graph, is 7.5 divisions, i.e. around 78.8 nm, when the nominal size is 80. Figure 4 is a picture taken by STM of the surface of a 15 nm Nucleopore membrane. It is clear that the cross-section of the pore is not circular as it is for the 80 nm, but the average dimensions of the pore are consistent with the nominal characteristics. The same membrane observed by SEM would have given circular patterns, due to the diffusion of the electron beam by the pore. Whereas Nucleopore membranes are quite a
A. CHAHBOUN
good example to calibrate the method, their surface roughness and pore shape differ from what can be expected in polysulfone ultrafiltration membranes, for example. Figure 5 shows the surface of a polysulfone membrane (MWCO: 100 kDa) after ultrafiltration of BSA. Although smooth, the roughness is of significant importance compared to the average pore size. The detailed picture of a pore is given in Fig. 6. This pore is quite big for such a membrane (about 15 nm on cross-section). Apart from the tip-effect, the profile of the pore mouth is not as simple as before, and a pore equivalent radius is now difficult to define.
2
Conclusion
7
These first attempts to use scanning tunnelling microscopy provide new and consistent information on the surface of ultrafiltration membranes. STM could prove to be a very powerful tool for the investigation of e.g. the surface topography and the mechanisms of fouling since they give information even on parts of the membranes which are not permeable, and on the location of membrane modification. The major advantages of this technique over others are a fair resolution at nanometer level, and the ability to provide 3dimensional information on the surface topography of membranes. Further work is in progress in our laboratories to adapt the technique to the requirements of membrane surface characte- risation. References 1
S.I. Nakao and S. Kimura, Analysis of solute rejection in ultrafiltration, J. Chem. Eng. Jpn., 14 (1981) 32.
3
4
5
6
8
9
10
11
12
13
ET AL.
A.R. Cooper and D.S. van Derveer, Characterisation of ultrafiltration membranes by polymer transport measurements, Sep. Sci. Technol., 14 (1979) 551. P. Aimar, M. Meireles and V. Sanchez, A contribution to the translation of retention curves into pore size distribution for sieving membranes, J. Membrane Sci., 54 (1990) 321. K. Kamide and S.I. Manabe, Characterisation technique of straight through porous membranes, in: A.R. Cooper (Ed. ) , Ultrafiltration Membranes and Applications, Plenum Press, New York, NY, 1980, p. 173. M. Cheryan, and U. Merin, A study of the foulingphenomenon during ultrafiltration of cottage cheese whey, in: A.R. Cooper (Ed.), Ultrafiltration Membranes and Applications, Plenum Press, New York, NY, 1980, p. 619. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Surface studies by scanning tunnelling microscopy, Phys. Rev. Lett., 49 (1982) 57. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Tunnelling through a controllable vacuum gap, Appl. Phys. Lett., 40 (1982) 178. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, 7 X 7 reconstruction on Si (III) resolved in real space, Phys. Rev. Lett., 50 (1983) 120. A.M. Baro, R. Miranda, J. Alaman, N. Garcia, G. Binnig, H. Rohrer, Ch. Gerber and J.L. Carracosa, Determination of surface topography of biological specimen at high resolution by scanning tunnelling microscopy, Nature, 315 (1985) 253. R. Coratger, A. Claverie, F. Ajustron and J. Beauvillain, Scanning tunnelling microscopy of defects induced by carbon bombardment on graphite surfaces, Surf. Sci., 227 (1990) 7. R. Coratger, A. Claverie, F. Ajustron, J.C. Lacaze and C. Tremolliers, A stage for submicron displacements using electromagnetic coils and its application to scanning tunnelling microscopy, Rev. Sci. Instrum., 62 (1991) 830. H. Strathmann, Synthetic membranes and their preparation, in: M.C. Porter (Ed.), Handbook of Industrial Membrane Technology, Noyes Publications, Park Ridge, NJ, 1990, p. 68. A. Chahboun, R. Coratger, F. Ajustron, J. Beauvillain, P. Aimar and V. Sanchez, Comparative study of micro and ultrafiltration membranes using STM, AFM and SEM techniques, Ultramicroscopy (in press).
295
Short Communication
First investigations on the use of scanning tunnelling microscopy (STM) for the characterisation of porous membranes A. Chahboun*, R. Coratger”, F. Ajustro9, J. Beauvillain”, P. Aimarb and V. Sanchezb “CEMES/CNRS, 29 rue J. Marvig, B.P. 4347,31055 Toulouse Cedex (France) bURA CNRS de GEnie Chimique, L.G.C.E., 118, route de Narbonne, 31062 Toulouse Cedex (France) (Received September 30,199l; accepted in revised form November 14,199l)
Abstract A new microscopic technique, scanning tunnelling microscopy (STM ) , has been used to analyse the surface of microfiltration (MF) and ultrafiltration (UF) membranes. Some of the observations have been compared to those obtained with scanning electron microscopy. Although the agreement is good between the two methods for MF membranes, STM shows a better resolution, which is of major interest for the characterisation of UF membranes. Further, STM gives pictures of the surface in 3 dimensions. Keywords: porous membranes; scanning tunnelling microscopy; membrane structure
Introduction Ultrafiltration and microfiltration membranes are porous barriers used for the separation and concentration of proteins, colloids or particles from pharmaceutical, food or chemical industries. One of the problems process engineers have to face is to get relevant information for the choice of the membranes convenient for a given separation. Membrane manufacturers are also concerned with membrane characterisation for the development of better filters and for production control. A great deal of work has therefore been dedCorrespondence to: P. Aimar, URA CNRS de GEnie Chimique, L.G.C.E., 118, route de Narbonne, 31062, Toulouse Cedex, France.
0376-7388/92/$05.00
icated to the improvement of characterisation methods for artificial membranes. A large part of the efforts aims at determining the apparent size of the pores and the pore density. Three main categories of method may be mentioned. (a) Measurement of the retention of calibrated molecules used as tracers [ 1,2]. The information is directly available in terms of retention of solutes, but the translation into pore size and pore size distribution requires attention [ 31. (b) Measurements of the permeability of the filter for more or less complex systems (water, mercury or multi-phase fluids); these do not give direct information on molecule retentions. An apparent pore size, the significance of which can be different from the previous one, can be obtained. (c) Microscopic techniques which should al-
0 1992 Elsevier Science Publishers B.V. All rights reserved.
A. CHAHBOUN ET AL.
296
low a direct determination of the physical pore size, such as scanning electron microscopy (SEM) [ 41, or transmission electron microscopy TEM [ 51. However, these methods are not fully satisfactory because their resolution is poor (around 1 nm for SEM )i.e. in the range of the pore sizes of ultrafiltration membranes (1.5-15 nm) and of nanofiltration membranes ( < 3 nm). Another shortcoming, also mentioned by Cheryan and Merin [5] is the influence of the sample preparation on the results. A new technique, scanning tunnelling microscopy (STM) , has recently been developed by Binnig and co-workers [ 6,7]. On top of having a very high resolution, this technique has quickly proved to be inexpensive compared to other microscopic techniques and relatively simple to set-up. Another advantage are the relatively mild sample preparation conditions. One of its first achievements has been to give experimental evidence of the anomalous periodicity of silicium under ultra-vacuum conditions [ 81. More recently, the first applications of STM to biological materials appeared, with the observation of tobacco viruses [ 91. The scope of this note is to present the first observations of porous membranes using an STM built in our university [lo]. A series of Nuclepore membranes has been investigated by STM and SEM. These membranes are microfiltration membranes, but they present the advantage of offering a range of pore radii from micrometer-size down to nanometer-size. This provides the opportunity to calibrate the different methods available and to explore their limits. Results concerning a polysulfone membrane are also shown.
that is designed to ensure a good mechanical stability and a high reproducibility of the results. The tips are made from 0.1 mm tungsten wire thinned out in NaOH solution. The resolution qualified on a HOPG (highly ordered pyrolytic graphite) graphite surface, is 0.1 A horizontally and 0.01 A vertically. The scanning frequency lies between 0.1 and 1 Hz according to the type of surface (this gives an average of one minute per picture). The tunnelling voltage and current are 0.6 V and 0.2 nA respectively. The SEM apparatus is a commercial Cambridge S250 model. For both techniques, the membrane surface is covered with a carbon film (at least 2 nm thick), evaporated under 10m5 Torr. This thickness is estimated using a quartz microbalance. Results Figure 1 shows a SEM view of a 80 nm (pore size) Nuclepore membrane. This can be compared with Fig. 2 where a similar membrane (100 nm pore size) was observed by STM. Similar features are visible, although STM seems to give more accurate information on the local topography. The pore density estimated from the manual counting of several images either
Material and methods The apparatus is a lab-made, pocket size replica of the Binnig and Rohrer microscope. It is a triple axis displacement system, with an improved electromagnetic clamping system [ 111,
Fig. 1. Surface of a Nuclepore (0.08pm) membrane (SEM) .
USE OF SCANNING TUNNELLING MICROSCOPY FOR THE CHARACTERISATION OF POROUS MEMBRANES
Fig. 2. Surface of a Nuclepore (0.1 pm) membrane (STM).
Fig. 3. Topview and cross-section of a pore from a Nuclepore (0.08 pm) membrane by STM.
297
298
Fig. 4. Top view of a pore of a Nuclepore (0.015 pm) membrane by STM.
Fig. 5. Surface of a polysulfone membrane (MWCO: 100 kDa) after fouling by BSA by STM.
A. CHAHBOUN ET AL.
USE OF SCANNING TUNNELLING MICROSCOPY FOR THE CHARACTERISATION OF POROUS MEMBRANES
299
Fig. 6. Top view and cross-section of a pore of a polysulfone membrane (MWCO: 100 kDa) by STM.
from STM or from SEM gives an average of 6 x 1012 and 4.1 x 1Ol2 pores/m2 respectively, where the value published by Strathmann [ 121 is 6 x 1012.Although the two techniques give the same order of magnitude, the difference between the two figures has not yet been resolved. One of the possible explanations might be the damage caused by the beam voltage in SEM, as discussed in [ 131. Figure 3 shows a pore of a 80 nm membrane as observed by STM. The cross-section along the line l-2 measures 105 nm and the depth is 40 nm. The pore walls should be almost vertical, but the shape of the tip (an approximate pyramid) gives a drift in the signal and the an-
gle of the tip is likely to be the apparent angle of the pore wall observed here. The size of the pore entrance, as shown on this graph, is 7.5 divisions, i.e. around 78.8 nm, when the nominal size is 80. Figure 4 is a picture taken by STM of the surface of a 15 nm Nucleopore membrane. It is clear that the cross-section of the pore is not circular as it is for the 80 nm, but the average dimensions of the pore are consistent with the nominal characteristics. The same membrane observed by SEM would have given circular patterns, due to the diffusion of the electron beam by the pore. Whereas Nucleopore membranes are quite a
A. CHAHBOUN
good example to calibrate the method, their surface roughness and pore shape differ from what can be expected in polysulfone ultrafiltration membranes, for example. Figure 5 shows the surface of a polysulfone membrane (MWCO: 100 kDa) after ultrafiltration of BSA. Although smooth, the roughness is of significant importance compared to the average pore size. The detailed picture of a pore is given in Fig. 6. This pore is quite big for such a membrane (about 15 nm on cross-section). Apart from the tip-effect, the profile of the pore mouth is not as simple as before, and a pore equivalent radius is now difficult to define.
2
Conclusion
7
These first attempts to use scanning tunnelling microscopy provide new and consistent information on the surface of ultrafiltration membranes. STM could prove to be a very powerful tool for the investigation of e.g. the surface topography and the mechanisms of fouling since they give information even on parts of the membranes which are not permeable, and on the location of membrane modification. The major advantages of this technique over others are a fair resolution at nanometer level, and the ability to provide 3dimensional information on the surface topography of membranes. Further work is in progress in our laboratories to adapt the technique to the requirements of membrane surface characte- risation. References 1
S.I. Nakao and S. Kimura, Analysis of solute rejection in ultrafiltration, J. Chem. Eng. Jpn., 14 (1981) 32.
3
4
5
6
8
9
10
11
12
13
ET AL.
A.R. Cooper and D.S. van Derveer, Characterisation of ultrafiltration membranes by polymer transport measurements, Sep. Sci. Technol., 14 (1979) 551. P. Aimar, M. Meireles and V. Sanchez, A contribution to the translation of retention curves into pore size distribution for sieving membranes, J. Membrane Sci., 54 (1990) 321. K. Kamide and S.I. Manabe, Characterisation technique of straight through porous membranes, in: A.R. Cooper (Ed. ) , Ultrafiltration Membranes and Applications, Plenum Press, New York, NY, 1980, p. 173. M. Cheryan, and U. Merin, A study of the foulingphenomenon during ultrafiltration of cottage cheese whey, in: A.R. Cooper (Ed.), Ultrafiltration Membranes and Applications, Plenum Press, New York, NY, 1980, p. 619. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Surface studies by scanning tunnelling microscopy, Phys. Rev. Lett., 49 (1982) 57. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Tunnelling through a controllable vacuum gap, Appl. Phys. Lett., 40 (1982) 178. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, 7 X 7 reconstruction on Si (III) resolved in real space, Phys. Rev. Lett., 50 (1983) 120. A.M. Baro, R. Miranda, J. Alaman, N. Garcia, G. Binnig, H. Rohrer, Ch. Gerber and J.L. Carracosa, Determination of surface topography of biological specimen at high resolution by scanning tunnelling microscopy, Nature, 315 (1985) 253. R. Coratger, A. Claverie, F. Ajustron and J. Beauvillain, Scanning tunnelling microscopy of defects induced by carbon bombardment on graphite surfaces, Surf. Sci., 227 (1990) 7. R. Coratger, A. Claverie, F. Ajustron, J.C. Lacaze and C. Tremolliers, A stage for submicron displacements using electromagnetic coils and its application to scanning tunnelling microscopy, Rev. Sci. Instrum., 62 (1991) 830. H. Strathmann, Synthetic membranes and their preparation, in: M.C. Porter (Ed.), Handbook of Industrial Membrane Technology, Noyes Publications, Park Ridge, NJ, 1990, p. 68. A. Chahboun, R. Coratger, F. Ajustron, J. Beauvillain, P. Aimar and V. Sanchez, Comparative study of micro and ultrafiltration membranes using STM, AFM and SEM techniques, Ultramicroscopy (in press).