SURFACE AND INTERFACE ANALYSIS, VOL. 25, 593È595 (1997)

AFM and STM Characterization ofMembranesTiO

2—Ultrafiltration

Dirk Beyer,* Hannes Richter and Gerhard TomandlInstitute for Ceramic Materials, TU Mining Academy Freiberg/Saxony, G.-Zeuner-Str.3, D-09596, Freiberg, Germany

Atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) were used to investigate the surfaceof ultraÐltration membranes with pore sizes of ¿10 nm. It is widely accepted that conventional AFM withTiO

2or Si tips is not able to resolve pores in supported ceramic membranes with a diameter of Æ40 nm. In thisSi3N

4study it should be tested whether it is possible to lower this border for further characterization of nanoÐltrationmembranes. Here we present di†erent ways of imaging and discuss alternative sample preparation techniques.

1997 by John Wiley & Sons, Ltd.(

Surf. Interface Anal. 25, 593È595 (1997)No. of Figures : 6 No. of Tables : 0 No. of Refs : 10

KEYWORDS: atomic force microscope, lateral force microscope, scanning tunnelling microscope, ultraÐltration membranesurface, replica techniques

INTRODUCTION

Scanning probe microscopy (SPM) techniques such asscanning tunnelling microscopy (STM) and atomic forcemicroscopy (AFM) are powerful tools for the exami-nation of ceramic membrane surfaces. In particular,AFM has become one of the most important methodsin the study of ceramic surfaces, which are usually elec-trically non-conducting. In contrast, STM is applicableonly with electrical conducting membrane surfaces suchas TiN, otherwise it requires metallic coatings or con-ducting liquid Ðlms. Therefore, it is challenging to useAFM for the detection of the pores in ceramic ultraÐl-tration membranes that have a typical pore diameter of10È100 nm. Such membranes exhibit an asymmetricstructure consisting of a macroporous support coatedwith intermediate layers and the Ðnal ultraÐltrationlayer that determines the Ðltration properties of themembrane. As an example, Fig. 1 shows a typical scan-ning electron microscopy (SEM) image of the cross-section of a membrane investigated in this study.TiO2The membranes prepared in our institute have aTiO2pore size of 8È10 nm and represent already the changeto nanoÐltration.

Recently much progress has been done in the charac-terization of ultraÐltration membranes with AFM,1h6including new instrumental methods like pneumaticscanning force microscopy.7 It is widely accepted thatcommon AFM techniques are not able to resolve poresin supported ceramic membranes with a diameter of\40 nm because the normally used and Si tipsSi3N4have a typical tip diameter of that size.

The aim of our investigation is to test whether it ispossible to lower this border for further character-

* Correspondence to : D. Beyer, Institute for Ceramic Materials,TU Mining Academy Freiberg/Saxony, G.-Zeuner-Str.3, D-09596,Freiberg, Germany.

ization of nanoÐltration membranes that have a poresize of \1 nm. In this paper we report the results ofboth AFM and STM studies on ultraÐltrationTiO2membranes and discuss alternative methods such aslateral force microscopy (LFM) and replica techniques.

MATERIALS AND METHODS

The ultraÐltration membranes were prepared atTiO2our institute by common solÈgel techniques.8 A com-mercial scanning probe microscope (TMX 2000, Dis-coverer, TopoMetrix) was used, equipped with a 75 lmand a 8 lm scanner for AFM and a 8 lm scanner forSTM. Either the height signal (topography) or the inter-nal sensor signal (cantilever deÑection) was acquired.Oxide-sharpened or conventional Si tips pur-Si3N4chased from L.O.T. were used. Typically, the scan speedwas set to 2È4 Hz for AFM and 1 Hz for STM. The

Figure 1. Scanning electron microscopy image of the cross-section of a ultrafiltration membrane with support and inter-TiO

2mediate layers.

CCC 0142È2421/97/070593È03 $17.50 Received 16 September 1996( 1997 by John Wiley & Sons, Ltd. Accepted 21 February 1997

594 D. BEYER, H. RICHTER AND G. TOMANDL

STM imaging was performed on Pt-coated samples (5nm) and LFM was performed using standard Si tips.

RESULTS AND DISCUSSION

A three-dimensional AFM image of the investigatedultraÐltration membrane is shown in Fig. 2. ThisTiO2Ðgure shows an area of 2000 ] 2000 nm and gives a

representative impression of the surface structure of themembrane. The particles are mostly spherical andTiO2have a mean diameter nm. From geo-dmean\ 60 ^ 8metrical considerations one can now easily derive atheoretical pore diameter in the range 8È15 nm, whichcorresponds surprisingly well with integral methods (N2adsorption/desorption). However, the pores themselvescannot be resolved and the measured particle diameterscertainly contain some broadening due to the tipÈsample interaction. In order to acquire more safety wehave performed an STM investigation of the samemembrane surface. The membranes possess lowTiO2conductivity and cannot be imaged directly by STM. Inaddition, STM shows the same broadening e†ectdescribed for AFM but, due to a di†erent tip shape,with a di†erent systematic error. Figure 3 shows a top-view image of the membrane coated with a thin plati-num layer of 5 nm. In spite of the relatively low qualityof the image, the observed average particle diameter isin good agreement with AFM investigations. On theother hand, coating with heavy metals such as gold orplatinum may obscure details of the pore structure andlimit the e†ectiveness of the STM examination. Mea-surements under conducting liquid Ðlms that couldavoid these problems are currently in progress.

Additional information about the surface morphol-ogy of the ultraÐltration membranes can be obtainedwith replicas of the surface. Pores in the membranes are

now visible as peaks in the replica and should be easilydetectable. A replica of the membrane surface was pre-pared as shown in Fig. 4. An easily soluble bu†er layer(Cu) and the platinum replica are deposited onto thesurface of the membrane. The replica is releasedTiO2by dissolving the bu†er layer in HCl (15%). As can beseen from Fig. 5, the platinum replica shows an inverseimage of the membrane surface but gives no clear hintsfor pores in the membrane. The bu†er layer and thereplica layer itself should penetrate deep enough intothe surface of the membrane, which is obviously not thecase. Other groups have reported grain sizes of 5È10 nmin Au/Pd replicas,9 which limit the resolution of replicatechniques with heavy metals. Further developmentsusing polymers, which have been successfully appliedfor the Ðlling up of nanoporous glasses,10 are inprogress.

A di†erent, very e†ective mode when imaging mem-brane surfaces is lateral force microscopy. Lateral forcemicroscopy is a modiÐcation of standard topographicimaging in which the sideways forces on the ample areimaged. It shows changes in material as well asenhanced contrast on sharp edges and steps. Figure 6represent a three-dimensional LFM plot of the mem-brane (reverse scan direction) that emphasizes the grainboundaries and the pores (here, visible as peaks) of themembrane surface. The e†ectiveness in detecting poreswith LFM depends strongly on the surface roughness.A curved top layer of the membrane hides pores on the

Figure 4. Schematic diagram of the preparation of the membranereplicas using heavy metals.

Figure 5. Three-dimensional AFM image of a platinum membrane replica.

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 593È595 (1997)

AFM AND STM OF TiO2 595

Figure 6. Three-dimensional LFM plot of the membrane, which emphasizes grain boundaries and pores.

sloping side of the area under investigation. Those porescan be imaged by performing both forward and back-ward scanning. Up to now, LFM in combination withcommon AFM contact techniques represents the mostadvanced imaging mode for the investigation of ultraÐl-tration membrane surfaces.

CONCLUSIONS

Standard SPM techniques such as AFM and STM canbe successfully applied to surface studies of ceramic

membranes as long as the pore size of the membranedoes not reach the region of nanoÐltration, whichremains a serious obstacle. Replica techniques withpolymers and LFM seem to be promising methods toshow pores of a few nanometres. The performance ofSTM could be improved by imaging under conductingliquid Ðlms, which provides the most reliable measure-ment of the particle size of the membranes. Further-more, AFM techniques have future applications ofimaging ceramic membranes in operation under liquidsin order to study the liquid-membrane surface inter-actions and their inÑuence on the Ðltration process.

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( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 593È595 (1997)