- Email: [email protected]
Electron microscopic investigation of mesoporous SBA-2
. . . . . .
,,,
lJt.t
~..,,ul l.u,v~,
t..,,~,l~,,lll,,lu
Ulltl
k.,,Otl, l~,l ,V/~l~
1"1" 1
A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
379
Electron microscopic investigation o f mesoporous SBA-2 Wuzong Zhou*, Alfonso E. Garcia-Bennett, Hazel M.A. Hunter and Paul A. Wright School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK.
Microstructure of mesoporous SBA-2 has been investigated by using transmission electron microscopy and scanning electron microscopy. The material consists of two phases based on hexagonal close-packed and cubic close-packed supercages. These two phases coexist in domains and synthesis of monophasic specimen has not so far been achieved. Three morphologies, i.e. solid spheres, hollow spheres and flat plates, have been recorded and their formation mechanisms are discussed.
1. INTRODUCTION Mesoporous silica SBA-2 was first reported in 1995 [1]. The material was believed to consist of discrete supercages in a hexagonal close-packed (hcp) arrangement and the space group was determined to be P63/mmc. In 1998, based on transmission electron microscopic (TEM) studies, we proposed [2] that two types of mesopores, a group of straight pores along the [100] direction and another group of zigzag pores parallel to the [001 ] zone axis, connect the supercages in the hcp structure (Fig. l a). We also revealed a new phase designated STAC-1, which has a structure with cubic close-packed (ccp) supercages. The latter phase is also connected by two-dimensional mesopores (Fig. 1b). Since then, very few reports dealing with the structures of these materials have been released, and, up to date, the above two models provide the best approaches to the real structures of the hcp and ccp phases. However, uncertainty exists regarding the pore connectivity. Once well ordered materials are prepared, better structural models will be developed from electron microscopy using the so-called direct determination method [3, 4]. Nevertheless, the co-existence of the hexagonal and cubic forms is not in doubt. One of the difficulties in the structural studies of these materials is that it is hard to obtain large domains of the monophasic hcp or ccp phase. The TEM images we used in our previous report [2] show large enough monophasic domains of these two phases for image simulations in order to determine the mesopore systems. However, there are usually some stacking faults showing a mixture of the ABCABC and ABAB ordering along the c axis of the hexagonal unit cell (Fig. 2). In fact, the real structure contains much smaller domains and their orientations can be random. The present work is therefore focused on these domain structures and on the microstructure-related morphologies of the particles.
380
Fig. 1 Schematic drawing of the channel structures of (a) the hcp phase and (b) the ccp phase in mesoporous specimen SBA-2. For comparison with the hcp phase, a hexagonal unit cell is also chosen for the ccp phase.
2. EXPERIMENTAL
The synthetic method is the same as that reported previously [2]. Gemini quaternary ammonium surfactant was used as template. The ratio of surfactant, TMAOH (tetramethylammonium hydroxide), TEOS (tetraethyl orthosilicate) and water was 0.05 : 0.5 : 1 : 1.50. The reaction pH was adjusted to 11 with 1M HCI. Aider 2 h stirring at room temperature, the specimen was recovered by filtration, washed with distilled water, and dried in air at room temperature. The powder sample was calcined at 500 ~ to Fig. 2 TEM image of a large domain of the ccp phase. Stacking faults are indicated by remove surfactant molecules. Initial characterization of the specimens white arrows. was by X-ray powder diffraction (XRD) method using a Philips PW 1830 diffractometer equipped with a secondary monochromator. A 20 range from 1.5 to 8 ~ was normally scanned over 2 h. TEM images were obtained on a Jeol JEM-~)0 CX and a Jeol JEM-2010 electron microscopes, both operating at 200 kV. Specimen was prepared by spreading the powder on a holey carbon f i l l supported on a Cu grid, followed by transferring it into the chamber of the microscope. Structural images were recorded at magnifications from 24,000X to 80,000X. Scanning electron microscopic (SEM) images were recorded on a Jeol JSM-5600 scanning electron microscope operating at various accelerating voltages from 1 kV to 30 kV. The powder sample was deposited on a double-sided carbon adhesive disc sitting on a specimen stub. The specimen was then directly transferred into the SEM chamber without any coating treatments. An accelerating voltage with minimum beam charge was then chosen.
3. RESULTS AND DISCUSSION
XRD profiles of the samples agreed with the previous results for SBA-2 [1 ] and may be indexed onto a hexagonal unit cell with a = 4.90 and c = 8.04 nm. However, some variation in
381 the peak intensities indicated possible existence of the ccp component [5]. Three principal morphologies were found in the specimen after calcination. One is large hollow sphere with about 50 to 150 ~tm in diameter and the thickness of shells is about 1 to 2 ~tm directly measured from the SEM images of some holes on the hollow balls (Fig. 3a, b). The second morphology is small solid sphere with the diameter in a range of 2 to 3 lxm. Some individual solid spheres can be seen on the surface of the hollow ball. The third morphology is sheet-like plate as shown in Fig. 3c. The particle becomes transparent under the electron beam, indicating that it is very thin along the incident beam direction. It was also noticed that these flat plates usually have sharp edges. Some small spherical particles, 2 to 3 ~tm in diameter, can also be seen on the surface of the plate (Fig. 3c). TEM images of most small spheres show a multi-domain structure (Fig. 4). Some domain boundaries are highlighted in Fig. 4b. It can be seen that these domains intergrow together with random orientations. The projections of domains 1, 2 and 6 can be considered to be [100] of the hcp phase. However, their c axes rotate around the a axis as shown in Fig. 4b. Domain 5 shows mainly the ccp phase with a few stacking faults, the image contrast pattern is similar to that shown in Fig. 2. Domain 4 is also a ccp phase viewed down the [110] direction of the cubic unit cell. This domain structure is beneficial to the formation of the spherical morphology.
Fig. 3 (a) and (b) SEM images of the synthesized SBA-2 specimen, showing two principal morphologies, hollow ball and solid sphere. The diameter of the hollow ball shown is about 150 ~tm and that of the small spheres is about 1 to 2 ~tm. The cross section of the shell of the hollow ball is marked by two white arrows in (b). (c) TEM image of a fiat plate obtained at a low magnification with some solid spheres on the surface.
382
Fig. 4 (a) A TEM image of part of a small spherical particle. (b) A copy of (a) with the domain boundaries marked by white lines.
Fig. 5 Enlarged TEM image of the domain 2 in Fig. 4b. The sequence of layer-packing is indicated.
Fig. 6 TEM image of a solid sphere showing a single ccp phase when viewed down the [110] direction of the cubic unit cell.
383 A close examination of individual domains in Fig. 4 reveals that stacking faults are very common inside the domains. For example, the area 2 in Fig. 4b looks like a monophasic domain with the projection along the [100] zone axis of the hcp phase. However, an examination of sequence of the layer-arrangement along the c axis enables us to find many stacking faults so that it becomes a mixed phase of the hcp phase and ccp phase. Consequently, identification of this domain to either the hcp phase or the ccp phase is not justifiable (Fig. 5). This structural feature is similar to the intergrowth of zeolites FAU/EMT [6,7]. The hcp/ccp irregular intergrowth happens often because the lattice energies of these two phases are very close. Refinement of the synthetic conditions in order to produce either pure hcp or pure ccp phase is difficult, but not impossible. In the same specimen presented above, we occasionally observed indeed some particles that seem to be monophasic. For example, Fig. 6 is a TEM image from a small solid sphere. The structure has been identified as the ccp phase and the view direction is along the [ 110] zone axis. No domain structure can be seen in this particle. Direct TEM examination on the hollow balls is difficult due to their large size and the spherical shape. To can'y out TEM structural studies, the hollow balls were selected under optical microscope. The specimen was then ground for a few minutes and most hollow spheres were crushed into fragments. TEM images of these fragments show again a multidomain property and a uniform thickness (Fig. 7). The size of domains in the particle shown in Fig. 7a is about 5 nm or more and they do not have regular shapes. The domains in Fig. 7b, on the other hand, show a regular but distorted hexagonal shape with domain size of about 2 nm in diameter. These domains are close-packed on the shell plan, forming a larger hexagonal pattern. A possible formation mechanism is that in the ab plans of the hcp phase exist some clusters of ccp phase as shown in the inset of Fig. 7b. These clusters are partially ordered in the ab plans to form hexagonal pattern. The thin particle shown in Fig. 3c is observed from a specimen before grinding. Its more regular shape and lower thickness distinguish itself from the fragments of the hollow balls. In fact, the flat plate shown in Fig. 3c was most likely to be originally a part of larger sheet. Some TEM images showed indeed much larger plates with several cracks. TEM images at a high magnification show that the sheet-like particle seems to be monophasic, although some local defects are still visible (Fig. 8). Selected area electron diffraction (SAED) pattern from an area of a few micrometer in diameter (see the inset of Fig. 8) confirms its monophasic property and shows a hexagonal pattern. Therefore the incident beam was perpendicular to either the (001) plane of the hcp phase or the { 111 } planes of the ccp phase. According to the models proposed in our first paper about SBA-2 [2], the ideal mesopore networks in the hcp and ccp phases are both 2-dimensional instead of 3-dimensional (Fig. 1). In the case of latter, the 2-dimensional network contain mesopore-connected supercages is the (111) plane of the cubic unit cell, and there are no other mesopores acting as bridges between them. Consequently, the interaction of the micellar network in between these (111) planes must be much weaker in comparison with the intraplane interaction. It is therefore not surprising to see that the flat plates are perpendicular to the [111] zone axis of the cubic unit cell.
384
Fig. 7 TEM images of some fragments from hollow spherical particles. A multi-domain structure can be easily observed. Examples of typical domains in (a) and (b) are highlighted. The inset of (b) shows schematic drawing of a ccp cluster in the hcp network.
385
Fig. 8. TEM image at high magnification obtained from a sheet-like particle as seen in Fig. 3c. The inset is the corresponding SAED pattern.
4. CONCLUSION According to the SEM and TEM observations, synthesized SBA-2 specimens have three morphologies. Most solid small spheres consist of irregular domains with random orientations. This morphology must relate to a spherical micelle packing arrangement. Although each domain shows structural homogeneity, it lacks long-range ordering and otten contains irregular intergrowth of the hcp and ccp components with size in a nanometer scale. Large domains and even single domain spheres were occasionally observed, implying that the formation of monophasic spheres is possible. We believe that hollow spherical balls of silicate form from assembly of micellar and silicate condensation on surface of some bubbles. These particles therefore can move to the liquid surface during the reaction process and were indeed observed by optical microscopy. During calcination, hollow balls undergo considerate damage, which resulted in the formation of irregular openings that enabled us to measure the thickness of the shells (Fig. 3b). TEM images revealed that the shells of the hollow particles have also a domain structure (Fig. 7). In addition to the stacking faults along the c axis of the hexagonal unit cell as shown in Fig. 5, some very small 3-dimensional domains were also observed (see Fig. 7b). The flat plates, which probably formed in the liquid/air interface, are monophasic and the orientation is well selective to be normal to the [ 111 ] axis of the cubic unit cell. If our model
386 for the STAC-1 [2] is correct, all the mesopores in the flat plates would be parallel to the planes and there would be no pores across the plates. Further studies are being carried out in these laboratories. During the preparation of this report, we have performed part of systematic investigations of the synthetic conditions for SBA-2 and found that we were already approaching the goal of producing single-phase materials. Introducing bubbles into the reaction system, we obtained much larger yield of hollow balls of silicate.
REFERENCES
1. Q. Hue, R. Leon, P. M. Petroff and G. D. Stucky, Science, 268, 1324 (1995). 2. W. Zhou, H. M. A. Hunter, P. A. Wright, Q. F. Ge and J. M. Thomas, J. Phys. Chem., 102, 6933 (1998). 3. O. Terasaki, personal communication, (2001). 4. Y. Sakamoto, M. Kaneda, O. Terasaki, D. Y. Zhao, J. M. Kim, G. Stucky, H. J. Shin and R. Ryoo, Nature, 408, 449 (2000). 5. H.M.A. Hunter, A. E. Garcia-Bennett, I. D. Shannon, W. Zhou and P. A. Wright, J. Mater. Chem., in press (2001) 6. J. M. Thomas and G. R. Millward, J. CherrL Soc. Chem. Commun., 1380 (1982). 7. J.M. Thomas, O. Terasaki, P. L. Gai, W. Zhou and J. Gonzalez-Calbet, Accounts Chem. Res., 34, 583 (2001).
,,,
lJt.t
~..,,ul l.u,v~,
t..,,~,l~,,lll,,lu
Ulltl
k.,,Otl, l~,l ,V/~l~
1"1" 1
A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
379
Electron microscopic investigation o f mesoporous SBA-2 Wuzong Zhou*, Alfonso E. Garcia-Bennett, Hazel M.A. Hunter and Paul A. Wright School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK.
Microstructure of mesoporous SBA-2 has been investigated by using transmission electron microscopy and scanning electron microscopy. The material consists of two phases based on hexagonal close-packed and cubic close-packed supercages. These two phases coexist in domains and synthesis of monophasic specimen has not so far been achieved. Three morphologies, i.e. solid spheres, hollow spheres and flat plates, have been recorded and their formation mechanisms are discussed.
1. INTRODUCTION Mesoporous silica SBA-2 was first reported in 1995 [1]. The material was believed to consist of discrete supercages in a hexagonal close-packed (hcp) arrangement and the space group was determined to be P63/mmc. In 1998, based on transmission electron microscopic (TEM) studies, we proposed [2] that two types of mesopores, a group of straight pores along the [100] direction and another group of zigzag pores parallel to the [001 ] zone axis, connect the supercages in the hcp structure (Fig. l a). We also revealed a new phase designated STAC-1, which has a structure with cubic close-packed (ccp) supercages. The latter phase is also connected by two-dimensional mesopores (Fig. 1b). Since then, very few reports dealing with the structures of these materials have been released, and, up to date, the above two models provide the best approaches to the real structures of the hcp and ccp phases. However, uncertainty exists regarding the pore connectivity. Once well ordered materials are prepared, better structural models will be developed from electron microscopy using the so-called direct determination method [3, 4]. Nevertheless, the co-existence of the hexagonal and cubic forms is not in doubt. One of the difficulties in the structural studies of these materials is that it is hard to obtain large domains of the monophasic hcp or ccp phase. The TEM images we used in our previous report [2] show large enough monophasic domains of these two phases for image simulations in order to determine the mesopore systems. However, there are usually some stacking faults showing a mixture of the ABCABC and ABAB ordering along the c axis of the hexagonal unit cell (Fig. 2). In fact, the real structure contains much smaller domains and their orientations can be random. The present work is therefore focused on these domain structures and on the microstructure-related morphologies of the particles.
380
Fig. 1 Schematic drawing of the channel structures of (a) the hcp phase and (b) the ccp phase in mesoporous specimen SBA-2. For comparison with the hcp phase, a hexagonal unit cell is also chosen for the ccp phase.
2. EXPERIMENTAL
The synthetic method is the same as that reported previously [2]. Gemini quaternary ammonium surfactant was used as template. The ratio of surfactant, TMAOH (tetramethylammonium hydroxide), TEOS (tetraethyl orthosilicate) and water was 0.05 : 0.5 : 1 : 1.50. The reaction pH was adjusted to 11 with 1M HCI. Aider 2 h stirring at room temperature, the specimen was recovered by filtration, washed with distilled water, and dried in air at room temperature. The powder sample was calcined at 500 ~ to Fig. 2 TEM image of a large domain of the ccp phase. Stacking faults are indicated by remove surfactant molecules. Initial characterization of the specimens white arrows. was by X-ray powder diffraction (XRD) method using a Philips PW 1830 diffractometer equipped with a secondary monochromator. A 20 range from 1.5 to 8 ~ was normally scanned over 2 h. TEM images were obtained on a Jeol JEM-~)0 CX and a Jeol JEM-2010 electron microscopes, both operating at 200 kV. Specimen was prepared by spreading the powder on a holey carbon f i l l supported on a Cu grid, followed by transferring it into the chamber of the microscope. Structural images were recorded at magnifications from 24,000X to 80,000X. Scanning electron microscopic (SEM) images were recorded on a Jeol JSM-5600 scanning electron microscope operating at various accelerating voltages from 1 kV to 30 kV. The powder sample was deposited on a double-sided carbon adhesive disc sitting on a specimen stub. The specimen was then directly transferred into the SEM chamber without any coating treatments. An accelerating voltage with minimum beam charge was then chosen.
3. RESULTS AND DISCUSSION
XRD profiles of the samples agreed with the previous results for SBA-2 [1 ] and may be indexed onto a hexagonal unit cell with a = 4.90 and c = 8.04 nm. However, some variation in
381 the peak intensities indicated possible existence of the ccp component [5]. Three principal morphologies were found in the specimen after calcination. One is large hollow sphere with about 50 to 150 ~tm in diameter and the thickness of shells is about 1 to 2 ~tm directly measured from the SEM images of some holes on the hollow balls (Fig. 3a, b). The second morphology is small solid sphere with the diameter in a range of 2 to 3 lxm. Some individual solid spheres can be seen on the surface of the hollow ball. The third morphology is sheet-like plate as shown in Fig. 3c. The particle becomes transparent under the electron beam, indicating that it is very thin along the incident beam direction. It was also noticed that these flat plates usually have sharp edges. Some small spherical particles, 2 to 3 ~tm in diameter, can also be seen on the surface of the plate (Fig. 3c). TEM images of most small spheres show a multi-domain structure (Fig. 4). Some domain boundaries are highlighted in Fig. 4b. It can be seen that these domains intergrow together with random orientations. The projections of domains 1, 2 and 6 can be considered to be [100] of the hcp phase. However, their c axes rotate around the a axis as shown in Fig. 4b. Domain 5 shows mainly the ccp phase with a few stacking faults, the image contrast pattern is similar to that shown in Fig. 2. Domain 4 is also a ccp phase viewed down the [110] direction of the cubic unit cell. This domain structure is beneficial to the formation of the spherical morphology.
Fig. 3 (a) and (b) SEM images of the synthesized SBA-2 specimen, showing two principal morphologies, hollow ball and solid sphere. The diameter of the hollow ball shown is about 150 ~tm and that of the small spheres is about 1 to 2 ~tm. The cross section of the shell of the hollow ball is marked by two white arrows in (b). (c) TEM image of a fiat plate obtained at a low magnification with some solid spheres on the surface.
382
Fig. 4 (a) A TEM image of part of a small spherical particle. (b) A copy of (a) with the domain boundaries marked by white lines.
Fig. 5 Enlarged TEM image of the domain 2 in Fig. 4b. The sequence of layer-packing is indicated.
Fig. 6 TEM image of a solid sphere showing a single ccp phase when viewed down the [110] direction of the cubic unit cell.
383 A close examination of individual domains in Fig. 4 reveals that stacking faults are very common inside the domains. For example, the area 2 in Fig. 4b looks like a monophasic domain with the projection along the [100] zone axis of the hcp phase. However, an examination of sequence of the layer-arrangement along the c axis enables us to find many stacking faults so that it becomes a mixed phase of the hcp phase and ccp phase. Consequently, identification of this domain to either the hcp phase or the ccp phase is not justifiable (Fig. 5). This structural feature is similar to the intergrowth of zeolites FAU/EMT [6,7]. The hcp/ccp irregular intergrowth happens often because the lattice energies of these two phases are very close. Refinement of the synthetic conditions in order to produce either pure hcp or pure ccp phase is difficult, but not impossible. In the same specimen presented above, we occasionally observed indeed some particles that seem to be monophasic. For example, Fig. 6 is a TEM image from a small solid sphere. The structure has been identified as the ccp phase and the view direction is along the [ 110] zone axis. No domain structure can be seen in this particle. Direct TEM examination on the hollow balls is difficult due to their large size and the spherical shape. To can'y out TEM structural studies, the hollow balls were selected under optical microscope. The specimen was then ground for a few minutes and most hollow spheres were crushed into fragments. TEM images of these fragments show again a multidomain property and a uniform thickness (Fig. 7). The size of domains in the particle shown in Fig. 7a is about 5 nm or more and they do not have regular shapes. The domains in Fig. 7b, on the other hand, show a regular but distorted hexagonal shape with domain size of about 2 nm in diameter. These domains are close-packed on the shell plan, forming a larger hexagonal pattern. A possible formation mechanism is that in the ab plans of the hcp phase exist some clusters of ccp phase as shown in the inset of Fig. 7b. These clusters are partially ordered in the ab plans to form hexagonal pattern. The thin particle shown in Fig. 3c is observed from a specimen before grinding. Its more regular shape and lower thickness distinguish itself from the fragments of the hollow balls. In fact, the flat plate shown in Fig. 3c was most likely to be originally a part of larger sheet. Some TEM images showed indeed much larger plates with several cracks. TEM images at a high magnification show that the sheet-like particle seems to be monophasic, although some local defects are still visible (Fig. 8). Selected area electron diffraction (SAED) pattern from an area of a few micrometer in diameter (see the inset of Fig. 8) confirms its monophasic property and shows a hexagonal pattern. Therefore the incident beam was perpendicular to either the (001) plane of the hcp phase or the { 111 } planes of the ccp phase. According to the models proposed in our first paper about SBA-2 [2], the ideal mesopore networks in the hcp and ccp phases are both 2-dimensional instead of 3-dimensional (Fig. 1). In the case of latter, the 2-dimensional network contain mesopore-connected supercages is the (111) plane of the cubic unit cell, and there are no other mesopores acting as bridges between them. Consequently, the interaction of the micellar network in between these (111) planes must be much weaker in comparison with the intraplane interaction. It is therefore not surprising to see that the flat plates are perpendicular to the [111] zone axis of the cubic unit cell.
384
Fig. 7 TEM images of some fragments from hollow spherical particles. A multi-domain structure can be easily observed. Examples of typical domains in (a) and (b) are highlighted. The inset of (b) shows schematic drawing of a ccp cluster in the hcp network.
385
Fig. 8. TEM image at high magnification obtained from a sheet-like particle as seen in Fig. 3c. The inset is the corresponding SAED pattern.
4. CONCLUSION According to the SEM and TEM observations, synthesized SBA-2 specimens have three morphologies. Most solid small spheres consist of irregular domains with random orientations. This morphology must relate to a spherical micelle packing arrangement. Although each domain shows structural homogeneity, it lacks long-range ordering and otten contains irregular intergrowth of the hcp and ccp components with size in a nanometer scale. Large domains and even single domain spheres were occasionally observed, implying that the formation of monophasic spheres is possible. We believe that hollow spherical balls of silicate form from assembly of micellar and silicate condensation on surface of some bubbles. These particles therefore can move to the liquid surface during the reaction process and were indeed observed by optical microscopy. During calcination, hollow balls undergo considerate damage, which resulted in the formation of irregular openings that enabled us to measure the thickness of the shells (Fig. 3b). TEM images revealed that the shells of the hollow particles have also a domain structure (Fig. 7). In addition to the stacking faults along the c axis of the hexagonal unit cell as shown in Fig. 5, some very small 3-dimensional domains were also observed (see Fig. 7b). The flat plates, which probably formed in the liquid/air interface, are monophasic and the orientation is well selective to be normal to the [ 111 ] axis of the cubic unit cell. If our model
386 for the STAC-1 [2] is correct, all the mesopores in the flat plates would be parallel to the planes and there would be no pores across the plates. Further studies are being carried out in these laboratories. During the preparation of this report, we have performed part of systematic investigations of the synthetic conditions for SBA-2 and found that we were already approaching the goal of producing single-phase materials. Introducing bubbles into the reaction system, we obtained much larger yield of hollow balls of silicate.
REFERENCES
1. Q. Hue, R. Leon, P. M. Petroff and G. D. Stucky, Science, 268, 1324 (1995). 2. W. Zhou, H. M. A. Hunter, P. A. Wright, Q. F. Ge and J. M. Thomas, J. Phys. Chem., 102, 6933 (1998). 3. O. Terasaki, personal communication, (2001). 4. Y. Sakamoto, M. Kaneda, O. Terasaki, D. Y. Zhao, J. M. Kim, G. Stucky, H. J. Shin and R. Ryoo, Nature, 408, 449 (2000). 5. H.M.A. Hunter, A. E. Garcia-Bennett, I. D. Shannon, W. Zhou and P. A. Wright, J. Mater. Chem., in press (2001) 6. J. M. Thomas and G. R. Millward, J. CherrL Soc. Chem. Commun., 1380 (1982). 7. J.M. Thomas, O. Terasaki, P. L. Gai, W. Zhou and J. Gonzalez-Calbet, Accounts Chem. Res., 34, 583 (2001).