Download pdf - TEM/AFM Investigation of Size and Surface Properties of Nanocrystalline Ceria

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Copyright copy 2005 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 5 1101ndash1107 2005

TEMAFM Investigation of Size andSurface Properties of Nanocrystalline Ceria

S Gupta1 P Brouwer1 S Bandyopadhyay1lowast S Patil2 R Briggs3 J Jain4 and S Seal21School of Materials Science and Engineering The University of New South Wales Sydney 2052 Australia

2Department of Advanced Materials Processing and Analysis Centre University of Central Florida FL 32816 USA3Rochester Institute of Technology USA

4University of Notre Dame USA

A series of ceria nanoparticles were synthesized by using a microemulsion method The effect ofrelative concentration of surfactantwater on the size and the surface roughness of ceria nanopar-ticles was examined using transmission electron microscopy (TEM) and atomic force microscopy(AFM) respectively The investigation confirmed a relationship between the size and the roughnessproperties of the nanoceria as a function of the water to surfactant ratio With increasing dilutionof the surfactant the size distribution became narrow such that average particle size decreasedlinearly as the ratio increased without affecting lower size threshold of particles (sim10 nm) The sur-face roughness on the other hand was found to increase with increasing water to surfactant ratioimplying diluted surfactant would provide rougher surface of ceria nanoparticles The informationcan be used to tailor the adhesion properties of nanoceria by optimizing the size distribution as wellas surface roughness as a function of water to surfactant ratio

Keywords Ceria Nanoparticles AFM TEM Microemulsion SolndashGel Surfactant Roughness

1 INTRODUCTION

Due to increasing economic and environmental concernsthere is increasing demand to prepare high performancematerials such as Thermal Barrier Coatings (TBC) in orderto enhance the efficiency of many industrial processesPopular TBC materials such as yttria-stabilized zirconia(YSZ) are approaching to their performance limits leadingto development of alternative materials1 Ceramics oxidessuch as perovskite heavy atom doped zirconia and ceriabased materials are some of the promising materials beingdeveloped which are believed to display higher perfor-mance when compared to commonly used thermal barriercoating materials2ndash4 Due to high versatility such as higherphase stability thermal properties and lower oxygen dif-fusivity ceria is one of the most promising high perfor-mance materials5ndash6 Ceria containing materials find widerange of applications including in metallurgy industry toimprove oxidative resistance of steel6 catalysis functional

lowastAuthor to whom correspondence should be addressed

ceramics and as solid electrolyte materials for fuel cells7

Most of the properties of the nanosized materials are oftenfound to be different from those of micron-sized particlessuch as higher surface area and to depend on size shapeand composition Therefore there is increasing emphasisin developing nano-ceria as potential catalytic materials

Recently solndashgel or microemulsion technology hasbecome a popular technique to prepare nano-phase mate-rials including ceria7ndash12 In solndashgel processing a polarsolvent is added to a mixture of non-polar solution and sur-factant the surfactant molecules get coordinated withthe help of water molecules to form spherical globules(micelles) resulting in synthesis of nanoparticles Subse-quent densification stages often produce quite uniformmicrostructure high packing density with limited flawsThe process conditions such as the nature of the surfac-tant the relative concentration of reactants and the cal-cination temperature can influence the size of the ceriananoparticles8 For example different surfactant types andtheir relative concentration and calcination temperaturewere shown to influence the particle size distribution8

J Nanosci Nanotech 2005 Vol 5 No 7 1533-4880200551101007$1700+25 doi101166jnn2005151 1101

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TEMAFM Investigation of Size and Surface Properties of Nanocrystalline Ceria Gupta et al

Several applications of cerium oxide can be furtherimproved by enhancing their adhesion properties to thesubstrate which is strongly influenced by the size and thesurface roughness of their nanophase particles Thereforeit is imperative to design nanoparticles of controlled prop-erties such as the size and the surface properties8

In the present study a range of nano-phase ceria par-ticles were synthesized using microemulsion method byvarying water to surfactant ratios as detailed elsewhere6

The main aim of this particular study was to investigate theeffect of surfactant concentration on the size distributionand the roughness properties of nano-ceria particles syn-thesized via solndashgel route Transmission electron micro-scope (TEM) was used to measure the particle size andtheir distribution while Atomic force microscope (AFM)was used to characterize the surface characteristics of ceriaparticles

2 EXPERIMENTAL SECTION

Synthesis of Nano-Ceria Microemulsion method wasused to prepare cerium oxide nanoparticles (5 mM con-centration as characterized by ICP measurement at Uni-versity of Notredame USA) by using water toluene andsurfactant AOT (sodium bis(2-ethylhexyl) sulfosuccinate)at Department of Advanced Materials Processing andAnalysis Centre University of Central Florida USA6 Dif-ferent sols were synthesized by varying the surfactant con-centration used in the process while all the other chemicalconditions were kept same Molar water to AOT ratiosused in the preparation of samples R1 R2 and R3 were41 82 and 164 respectively Subsequently the sols wereseparated and calcined at 600 C to evaporate toluene andburn off the surfactant to provide nanoparticles of ceriumoxide in powder formICP Analysis Sample (1 ml) was transferred to a pre-

cleaned screw-capped Teflon beaker and dried on hot plateset at 90 C To dried mass 2 ml of concentrated nitricacid was added and the beaker was left overnight on hotplate with lid on Next day the sample was dried and re-dissolved in 10 ml of 2 nitric acid The analysis forcerium was carried out using the Element 2 (FinneganMAT Bremen Germany) sector field high resolutioninductively coupled plasma mass spectrometer (ICP-MS)Despite high resolution capability of the instrument a lowresolution mode was used for optimal sensitivity Sulfurand sodium were analyzed using Optima 3300 XL (Perkin-Elmer Norwalk CT) inductively coupled plasma atomicemission spectrometer (ICP-AES) A three minute washusing 5 nitric acid was employed between samples inboth cases Samples were introduced using a peristalticpump in conjunction with an autosampler Single elementstandard solutions purchased from Inorganic Ventures(Lakewood NJ) were utilized to prepare calibration andinternal standard solutions Analysis was performed using

external calibration procedure Internal standards wereincluded for matrix and instrumental drift corrections13ndash14

Procedural blanks were analyzed to check for any contri-bution from the reagentsTEM Measurements The transmission electron mic-

roscope (TEM) is commonly used for characterizationof nanoscale properties of advanced materials17810

A Philips CM200 (TEM) equipped with EDAX detectorand SIS CCD camera at the University of New SouthWales Australia was used to analyse the particle sizeA small amount of cerium oxide powder was mixed with10 ml of ethanol followed by stirring in Sanophon ultra-sonic stirrer for about three minutes Ceria suspensiondroplets were dripped onto the surface of 3 mm carboncoated copper grid by using a pipette followed by dryingin air for five minutes to evaporate ethanol The carbonfilm of copper grid did not interfere in the analysis as itwas effectively transparent to electrons due to very thinsize For each sample at least seven images were acquiredat more than two scales of magnifications Each image isprocessed using TEM computer software in order to obtainparticle size information

Due to agglomeration of ceria particles in the TEMimage it was difficult to distinguish the boundary of eachceria particle For instance in many images dark and greyspots were often observed which could be related to over-lapping of particles in horizontal plane as well as stackingin vertical direction as image brightness depends on theextent of electron transmission which in turn is influencedby the particle thickness or density In each image parti-cles were selected on the basis of their visible distinctionof the surface boundaries as well as representing aver-age smallest and largest particle Average particle size wasobtained by measuring maximum width in two mutuallyperpendicular directions of each particle For each sam-ple all the data set of size measurements was combinedto correlate the number particles occurring in a given sizerange on the basis of logarithmic correlation between theparticle size and the number of particles The correlationwas not extrapolated outside the minimum and maximummeasured size range of the sample The size distributionestimated by this approach appeared to be consistent withqualitative assessment based on visual observation of largenumber of particles in the TEM imagesAFM Analysis Atomic Force Microscopy (AFM) is

one of the most powerful tools to study the surface charac-teristics of nanophase materials15ndash21 The nanophase prop-erties of materials can be analyzed by using atomic forcemicroscope in several operating modes such as non-contact(NC-AFM) tapping mode (TM-AFM) etc In past indi-rect techniques such as X-ray photoelectron spectroscopy(XPS) have also been used to analyze surface propertiesof nanomaterials at the atomic level18 However the AFMhas not been used frequently for direct observations of theatomic-scale surface structures of cerium oxide particu-larly for discrete particles17ndash18 Non-contact mode of AFM

1102 J Nanosci Nanotech 5 1101ndash1107 2005

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Gupta et al TEMAFM Investigation of Size and Surface Properties of Nanocrystalline Ceria

Glass Slide

Mica

CeriumOxideParticles

Ethanol

Fig 1 Schematic of the sample assembly used for AFM analysis ofceria particles

has been used for bulk samples rather than surface proper-ties of segregated nanoparticles17ndash18 Recently we studiedthe effect of single rubber components and their blendson surface morphology at atomic levels using AFM in atapping mode21 Due to our previous experience and wellknown advantages tapping mode atomic force microscopy(TM-AFM) was used to study the surface properties ofindividual ceria particles Due to high sensitivity of theAFM standardization of experimental parameters is criti-cal to make a reliable interpretation of the measurementsTherefore the specimen preparation was one of the chal-lenging tasks in itself For sample preparation a thin micasheet which was stuck on to a glass slide with an adhesiveas illustrated in Figure 1 Top layer of mica was peeledoff to expose a fresh smooth layer of mica A dropletof ethanol-ceria suspension was dripped onto the freshlypeeled mica surface using a pipette The ceria droplets onmica surface were dried in air for about five minutes toevaporate ethanol in order to obtain an array of ceria par-ticles for AFM analysis

The Atomic Force Microscopic measurements were con-ducted using tapping mode in air at ambient temperature(25 C) using the AFM (Digital Dimension 3000) facility atthe University of New South Wales Australia Topographic(height) and phase images were recorded simultaneouslyScanning was carried out using etched silicon tips probe(TESP) each with a nominal tip radius of curvature of 5to 10 nm and spring constant in the range of 20ndash100 NmThe cantilever was oscillated at its resonance frequencywhich ranged between 200 and 400 kz The set point ratioof the cantilever was between 08 and 09 for all the scansThe cantilever was uncoated single beam and 125 micronlong with 17 sidewall angle 25 front and 10 back angleAll the images contained 256 data points More than fiveimages of each sample were acquired and were analyzedusing Nanoscope software

All background in AFM images excluding ceria par-ticles was removed by using the Nanoscope programA minimum filter was slowly increased until individualparticles could be clearly distinguished Nanoscope pro-gram allowed to measure average roughness value of aselected region within a particle during section analysisA large box was avoided as it could include lateral sideof a particle resulting in unexpectedly large variance andmisleading information Higher roughness value indicatesgreater roughness of a particle or selected region insidethe particle from section analysis Surface irregularities

were measured by drawing a cross-sectional line betweentwo points in vertical or horizontal directions Roughnessbetween the two points was measured by carefully select-ing the points in the selected image segment using rough-ness analysis function by calculating average roughness(Ra) indicative of difference between the highest and low-est points on the surface relative to the mean plane

3 RESULTS AND DISCUSSIONS

(a) Effect of Surfactant Concentration on Size Distri-bution TEM Study Figure 2 shows the images of ceriaparticles of three set of ceria particles illustrated at twomagnifications Residual ethanol was seen in images asshown by arrow close to large lighter circle in Figure 2awhich can be clearly distinguished from the adjacent ceriaparticles Some times uneven surfaces appeared as ridgemarks in the images as illustrated in dotted square inFigure 2e During the image processing the software canbe used to avoid such unwanted effects in the image suchas sample stage and carbon grid In addition overlappingand clustering of particles also occurred which were alsoavoided during image analysis In general similarity ofcolor intensity of the ceria particles imply that most of theparticles might have similar phase and density (Fig 2)

a

c

e

b

d

f

100 nm

100 nm

100 nm 50 nm

50 nm

50 nm

Fig 2 TEM images of ceria nanoparticles of samples R1 (a b) R2(b c) and R3 (d e) Samples R1 R2 and R3 were prepared by varyingmolar ratios of H2OAOT in a range of 41 82 and 164 respectively

J Nanosci Nanotech 5 1101ndash1107 2005 1103

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Darker spots in the center of each image show several agg-lomerates ranging from 50 nm to several hundred nano-meters Smaller particles were frequently seen around theedges of the agglomerates such that their boundaries couldbe clearly distinguished At present ethanol appears to besatisfactory dispersing medium for the examination of dis-crete phase of nano-ceria particles in the TEM Howeveragglomeration tendency of nanoparticles can be furtherreduced if a better alternative dispersing medium is found

The spherical morphology for the particles is expecteddue to formation of the nanoparticles in the sphericalmicelles of the microemulsion Nanometer-sized spheri-cal micelles in the microemulsion behave as nanoreactorsfor nanoparticles synthesis Due to the spherical shape ofthe micelles the nanoparticles achieve spherical morphol-ogy which is maintained during heating of the sol thoughthe particle size growth takes place Figures 2a 2c and2e compares the size of ceria particles in samples R1R2 and R3 respectively at the same magnification whileFigures 2b 2d and 2f compares the particle sizes of thesame samples at higher magnification On the basis ofvisual examination of TEM images a clear reduction inthe size distribution of ceria particles can be seen as thewater to surfactant ratio is increased from sample R1 tosample R3 Figure 3 compares the size distribution of ceriaparticles based on the TEM images

Figure 3 shows that in sample R3 with least surfac-tant concentration ([H2O][AOT] = 164) the particles varyfrom 10 to 40 nm in size and provides the most narrowsize distribution Sample R3 most of the ceria particleswere less than 40 nm in size Particle size of sample R1with most surfactant concentration ([H2O][AOT] = 41)varies from 10 to 75 nm providing highest range of par-ticle size distribution The size range of ceria particlesin sample R2 synthesized using with intermediate sur-factant concentration was found to be smaller than thatof sample R3 and greater than that associated with sam-ple R3 This observation was also found to be consistentwith visual observations of a large number of particlesin the TEM images Despite limited statistics the TEMresults clearly demonstrate the differences in the particlesize distribution as a consequence of surfactant dilution

100

75

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0Par

ticle

Fre

quen

cy (

)

lt15 20ndash30 30ndash40 40ndash50 50ndash60 60ndash70 70ndash80 80ndash90 90ndash100

Size range (nm)

R3 R2 R1

Fig 3 Comparison of percentage of ceria particles in various size ranges of samples R1 R2 and R3 which were prepared by using different waterto surfactant ratios of 41 82 and 164 respectively

by water during synthesis process Figure 3 further showsthat while increasing water to surfactant ratio the propor-tion of smaller particles is increased however the ratio didnot influence the lower threshold limit of their size Thisimplies that with increasing water dilution there is greatertendency to form monosize ceria particles

The same information has been plotted in Figure 4to establish a quantitative relationship between surfactantconcentration and the average particle size Figure 4 showsthat the average size of ceria particles decreases linearlyas the water to surfactant (H2OAOT) ratio is increasedIt may be noted that this linear correlation is applicablefor the limited range of surfactant concentration studiedand might not necessarily extrapolated linearly to otherrange of surfactant concentration The standard deviationof particle size in each sample is indicated as Y error barsin Figure 4 Even though same deviation is indicated inthe graph the standard deviation of size of particles withlarger surfactant concentration was greater which couldbe attributed to measurement difficulties associated withfrequent agglomeration of particles when compared to thedeviation associated with samples with lower concentra-tion The broken horizontal line indicates the lower limitsof the nanoparticles which is the same for all three sam-ples and is of the order of 10 nm

Water to surfactant ratio (R = [H2O][AOT]) plays animportant role in particle size of the nanoparticles formedusing microemulsion method Increasing R value increasesthe particle size22 Although the particle size is controlledduring chemical synthesis the particle size grows fur-ther after heating the sol at higher temperatures to burnoff the surfactant and obtain the powder The nanoparti-cles are surrounded by the surfactant molecules impartingsteric hindrance to agglomeration which restricts the par-ticle size growth during heating of the sol keeping thesize in nanometer range Therefore higher surfactant con-centration is expected to restrict the particle size growthmore Particles obtained using microemulsion processhave uniform shape and narrow size distribution23 butthe particles will grow differently during heating of thesol due to difference in steric hindrance by the surfac-tant molecules proportional to its concentration in the sol


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100

60

80

40

20

00 50 100 150 200

Lower size threshold

y = ndash026x + 6141

R2 = 098

Molar H2OAOT ratio

Ave

rage

par

ticle

siz

e (n

m)

Fig 4 Variation of average particle size of ceria with R (H2OAOT)Lower size threshold range of particles is shown as broken horizontalline

The low amount of surfactant might not be sufficient tomaintain the narrow particle size distribution obtained inmicroemulsion method Thus the difference in particlesize distribution for different R values can be attributedto the difference in steric hindrance to agglomeration pro-vided by the surfactant molecules during the heating ofthe sol to obtain powder The current trend of reductionon particle size range with decreased surfactant concen-tration is opposite to our previous experience with syn-thesis of nano-zirconia by solndashgel process in which theparticle size was found to decrease with increasing surfac-tant concentration of a hydroxypropyl cellulose (HPC)24

The exact mechanisms of the surfactant concentration onparticle size of nano-ceria is not clear at this stage how-ever a similar trend of formation of finer nanoparticleswith diluted surfactant has been reported in past25 Previ-ous studies have also highlighted the strong influence ofthe nature and amount of surfactants used in the solndashgelprocess such that different surfactant could provide oppos-ing impact on the size distribution for the synthesis of thesame nanoparticles7

(b) Effect of Surfactant Concentration on SurfaceRoughness AFM Study Due to distinct surface morphol-ogy of mica ethanol and ceria particles in the AFMimages ceria particles could be easily distinguished fromthe stage surface and mica background Two dimensionaltopographic image of mica phase indicated a smooth sur-face with out any variation in the AFM height and phaseimages The AFM image of residual ethanol on the drymica surface was also seen implying that dried ethanolmight be providing sufficient adhesion to hold the ceriaparticles intact during application of AFM tapping forceOften a series of streaks or peaks are seen when the AFMtip is moving through dried surface with residual ethanolphase These streaks and elongated images could occur

a

b

0 403 nm 403 nm0Data type2 range

Data type2 range

Height3000 nm

Phase4500 de

100

200

300

400nm

Fig 5 (a) TM-AFM height and phase images of ceria particles of sam-ple R2 and (b) 3D surface plot of the same particles

due to the movement of the particles improper scanningparameters very small scan size debris dragged alongwith the tip or the broken tip Sometimes multiple imageswere seen which could be related to insufficient dryingor lose particles and were not selected for further surfaceanalysis Figure 5a shows typical TM-AFM height andphase images illustrated through ceria particles of sampleR2 Figure 5b shows typical three dimensional view ofceria nanoparticles Figure 6 provides representative plotsof section analysis of particles in three samples to mon-itor height variation with size In each analysis three setof cursors were selected to measure three set distancesbetween two positions in the selected region

Figure 7 provides the correlation between average rough-ness and the surfactant concentration used during synthesisof these particles Figure 7 shows that average roughnessof ceria particles increases with increasing dilution ratio(H2OAOT) in an exponential manner There are twomechanisms proposed for the growth of oxide particleswhen formed from chemical reactions for example Nano-particles grow by aggregation of reacting species on thesurface of the nuclei26 The reacting species diffuse tothe surface of the nuclei where they attach themselves tothe drowning particle surface by completing the chemicalreaction According to the second hypothesis the chemicalreactions initially generate small nuclei which aggregatequickly to form a metastable colloidal sol27 Due to Brow-nian motion these metastable particles grow further by


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a

b

c

0

0

0

100 200 300 400

100

100

200

200

300 400

ndash10

0+

100

0nm

ndash10

0+

100

0

nmndash2

50

+25

00nm

nm

nm

nm

Fig 6 (a) Illustration of measurement of cross-sectional profile of ceriaparticles of sample R1 Y-axis represents the variation of vertical dimen-sion in the selected regions of particles (b) and (c) indicate similar illus-tration for samples R2 and R3 respectively

coagulation with smaller sized particles It is believed thatthe particles grown by these two mechanisms display dif-ferent surface morphologies such that the first mechanismgiving smooth while second mechanism giving roughsurface morphologies28

The difference in the surface roughness of the particlescan also be influenced by the difference in the particlesize growth mechanism According to current understand-ing particle size growth mechanism is expected to changefrom the first mechanism to the second one at higher sur-factant concentration In the experiments carried out in thisstudy the R value is varied by changing the amount of sur-factant (AOT) in the microemulsion system while keeping

10

08

06

04

02

000 50 100 150 200

Molar H2OAOT ratio

Ave

rage

rou

ghne

ss fa

ctor

y = 020Ln(x) ndash 048

R2 = 095

Fig 7 Variation of average roughness factor of ceria nanoparticles withR (H2OAOT)

all the other conditions same With increasing R valuethe micelle size increases which will decrease the proba-bility of aggregation of the nuclei during particle growthgiving the smoother surface Although above mechanismsdo not provide a satisfactory explanation of the roughnesstrend indicated by ceria nanoparticles used in this studyThe study does conclude that the surface roughness ofthe particles can be varied by varying the R value in themicroemulsion system

In previous section it was seen that as a consequence ofincreasing dilution sample R3 contained greater propor-tion of finer particles The AFM results indicated that theaverage roughness of nanoceria particles in sample R3 wasthe highest Therefore combining both TEM and AFMobservations the results imply that roughness of finer ceriaparticles synthesized via solndashgel route could be highercompared to larger particles Further studies are expectedto confirm the correlation between particle size and rough-ness by selecting mono size set of nanoparticles or byanalyzing a large number of particles particularly to clar-ify the mechanisms of evolution of surface roughnesswith decreasing size of nanoceria particles synthesized viasolndashgel route However the results are sufficiently clearenough to demonstrate that with increasing water to sur-factant ratio of the microemulsion it could be possible todecrease the size range of ceria nanoparticles as well asimprove their surface roughness and both could improvethe adhesion behavior and hence their potential for furtherapplication

4 CONCLUSIONS

A series of nanoceria particles were synthesized via mic-roemulsion route by varying surfactant concentration Thesize distribution and surface properties of discrete ceriaparticles were characterized by TEM and AFM This studyconfirmed the relationship between the size and the rough-ness properties of the ceria particles as a function of sur-factant dilution by water Following conclusions weremade

1 The study demonstrates that atomic level surface pro-perties of discrete ceria nanoparticles can be char-acterized by using tapping mode Atomic ForceMicroscopy

2 The TEM study showed that during synthesis of nano-ceria using microemulsion technique increasing dilu-tion of surfactant resulted in a narrow range of sizedistribution such that average particle size decreasedlinearly increasing water to surfactant ratio Underthe present processing conditions the surfactant dilu-tion did not have any significant effect on lower sizethreshold of particles formed

3 The AFM study showed that average roughness ofceria particles increased with increasing water to sur-factant ratio The study implies that by controlling


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particle size of synthesized ceria the atomic levelsurface roughness can also be improved Thus thesurfactant dilution of microemulsion during synthesisof nanoceria would promote the formation of finerand rougher particles The information can be used totailor the adhesion behavior of nano-phase by opti-mizing the effect of surfactant dilution on size distri-bution as well as surface roughness

Acknowledgments Authors would like to acknowl-edge the financial support provided by NSF REU (EEC0136710) and NSF US-Australia (EEC 0139614)

References and Notes

1 S Shukla S Seal R Vij S Bandyopadhyay and Z Rahman NanoLett 2 989 (2002)

2 M B Beardsley in International Thermal Spray Conference andExposition (ITSC) Orlando USA (2003)

3 D Clarke in International Thermal Spray Conference andExposition (ITSC) Orlando USA (2003)

4 J Ilavasky in International Thermal Spray Conference and Exposi-tion (ITSC) Orlando USA (2003)

5 D Stover in International Thermal Spray Conference and Exposi-tion (ITSC) Orlando USA (2003)

6 S Patil S C Kuiry S Seal and R Vanfleet J NanoparticleResearch 4 433 (2002)

7 Y Li J Ding J Chen X Xu B Wei J Liang and D Wu MaterRes Bull 37 317 (2002)

8 Y He B Yang and G Cheng Mater Lett 57 1880 (2003)9 S Shukla S Seal R Vij and S Bandyopadhyay Rev Adv Mater

Sci 4 1 (2003)10 F Zhang S Chan J E Spanier E Apak Q Jin R D Robinson

and I P Herman Appl Phys Lett 80 127 (2002)11 S Qiu J Dong and G Chen Powder Technology 113 9 (2000)

12 E J Kim and S H Hahn Mater Sci Eng A 303 24 (2001)13 M A Schneegurt J C Jain J A Menicuccu Jr S Brown D F

Garafalo M Quallick C R Neal and C F Kulpa Jr Environ SciTechnol 35 3786 (2001)

14 R A K Srivastava and J C Jain J Neurol Sci 196 45 (2002)15 R S Rajeev S K De A K Bhowmick G J P Kao and

S Bandyopadhyay J Mater Sci 36 2621 (2001)16 R S Rajeev S K De A K Bhowmick B Gong

S Bandyopadhyay J Adhesion Science and Technology 16 1957(2002)

17 K Fukui Y Namai and Y Iwasawa Appl Surf Sci 188 252(2002)

18 Y Namai K Fukui and Y Iwasawa Catalysis Today 85 79 (2003)19 S Anandhan P P De S K De S Bandyopadhyay and A K

Bhowmick J Mater Sci 38 2793 (2003)20 R S Rajeev A K Bhowmick S K De and S Bandyopadhyay

J Appl Polym Sci 89 1211 (2003)21 A Ghosh R S Rajeev A K Bhattacharya A K Bhowmick S K

De B Wolpensinger and S Bandyopadhyay Rubber Chemistry andTechnology Rubber Division ACS USA 76 220 (2003)

22 Takeshi Kawai Yuki Usui Kijiro and Kon-No Colloids and Sur-faces A Physicochemical and Engineering Aspects 149 39 (1999)

23 Toshiyuki Masui Kazuyasu Fujiwara Yumin Peng Takao SakataKen-ichi Machida Hirataro Mori and Gin-ya Adachi J Alloys andCompounds 269 116 (1998)

24 S Shukla S Seal and R Vanfleet J SolndashGel Sci Technol 27 119(2003)

25 S K Malhotra P Singh and A Thirunavukkarasu in Proceed-ings of International Conference on Nanomaterials SynthesisCharacterization and Application Tata McGraw-Hill PublicationNew Delhi (2004) p 94

26 J K Bailey and M L Mecartney Materials Research Society Sym-posium Proceedings (1990) Vol 180 p 153

27 J Livage M Henry J P Jolivet and C Sanchez MRS Bulletin 1518 (1990)

28 V Nagpal R Davis and J Riffle in Proceedings of the ACS Divi-sion of Polymeric Materials Science and Engineering ACS Bookand Journal Division Washington DC (1992) Vol 67 p 235

Received 28 January 2005 Accepted 2 February 2005


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Several applications of cerium oxide can be furtherimproved by enhancing their adhesion properties to thesubstrate which is strongly influenced by the size and thesurface roughness of their nanophase particles Thereforeit is imperative to design nanoparticles of controlled prop-erties such as the size and the surface properties8

In the present study a range of nano-phase ceria par-ticles were synthesized using microemulsion method byvarying water to surfactant ratios as detailed elsewhere6

The main aim of this particular study was to investigate theeffect of surfactant concentration on the size distributionand the roughness properties of nano-ceria particles syn-thesized via solndashgel route Transmission electron micro-scope (TEM) was used to measure the particle size andtheir distribution while Atomic force microscope (AFM)was used to characterize the surface characteristics of ceriaparticles

2 EXPERIMENTAL SECTION

Synthesis of Nano-Ceria Microemulsion method wasused to prepare cerium oxide nanoparticles (5 mM con-centration as characterized by ICP measurement at Uni-versity of Notredame USA) by using water toluene andsurfactant AOT (sodium bis(2-ethylhexyl) sulfosuccinate)at Department of Advanced Materials Processing andAnalysis Centre University of Central Florida USA6 Dif-ferent sols were synthesized by varying the surfactant con-centration used in the process while all the other chemicalconditions were kept same Molar water to AOT ratiosused in the preparation of samples R1 R2 and R3 were41 82 and 164 respectively Subsequently the sols wereseparated and calcined at 600 C to evaporate toluene andburn off the surfactant to provide nanoparticles of ceriumoxide in powder formICP Analysis Sample (1 ml) was transferred to a pre-

cleaned screw-capped Teflon beaker and dried on hot plateset at 90 C To dried mass 2 ml of concentrated nitricacid was added and the beaker was left overnight on hotplate with lid on Next day the sample was dried and re-dissolved in 10 ml of 2 nitric acid The analysis forcerium was carried out using the Element 2 (FinneganMAT Bremen Germany) sector field high resolutioninductively coupled plasma mass spectrometer (ICP-MS)Despite high resolution capability of the instrument a lowresolution mode was used for optimal sensitivity Sulfurand sodium were analyzed using Optima 3300 XL (Perkin-Elmer Norwalk CT) inductively coupled plasma atomicemission spectrometer (ICP-AES) A three minute washusing 5 nitric acid was employed between samples inboth cases Samples were introduced using a peristalticpump in conjunction with an autosampler Single elementstandard solutions purchased from Inorganic Ventures(Lakewood NJ) were utilized to prepare calibration andinternal standard solutions Analysis was performed using

external calibration procedure Internal standards wereincluded for matrix and instrumental drift corrections13ndash14

Procedural blanks were analyzed to check for any contri-bution from the reagentsTEM Measurements The transmission electron mic-

roscope (TEM) is commonly used for characterizationof nanoscale properties of advanced materials17810

A Philips CM200 (TEM) equipped with EDAX detectorand SIS CCD camera at the University of New SouthWales Australia was used to analyse the particle sizeA small amount of cerium oxide powder was mixed with10 ml of ethanol followed by stirring in Sanophon ultra-sonic stirrer for about three minutes Ceria suspensiondroplets were dripped onto the surface of 3 mm carboncoated copper grid by using a pipette followed by dryingin air for five minutes to evaporate ethanol The carbonfilm of copper grid did not interfere in the analysis as itwas effectively transparent to electrons due to very thinsize For each sample at least seven images were acquiredat more than two scales of magnifications Each image isprocessed using TEM computer software in order to obtainparticle size information

Due to agglomeration of ceria particles in the TEMimage it was difficult to distinguish the boundary of eachceria particle For instance in many images dark and greyspots were often observed which could be related to over-lapping of particles in horizontal plane as well as stackingin vertical direction as image brightness depends on theextent of electron transmission which in turn is influencedby the particle thickness or density In each image parti-cles were selected on the basis of their visible distinctionof the surface boundaries as well as representing aver-age smallest and largest particle Average particle size wasobtained by measuring maximum width in two mutuallyperpendicular directions of each particle For each sam-ple all the data set of size measurements was combinedto correlate the number particles occurring in a given sizerange on the basis of logarithmic correlation between theparticle size and the number of particles The correlationwas not extrapolated outside the minimum and maximummeasured size range of the sample The size distributionestimated by this approach appeared to be consistent withqualitative assessment based on visual observation of largenumber of particles in the TEM imagesAFM Analysis Atomic Force Microscopy (AFM) is

one of the most powerful tools to study the surface charac-teristics of nanophase materials15ndash21 The nanophase prop-erties of materials can be analyzed by using atomic forcemicroscope in several operating modes such as non-contact(NC-AFM) tapping mode (TM-AFM) etc In past indi-rect techniques such as X-ray photoelectron spectroscopy(XPS) have also been used to analyze surface propertiesof nanomaterials at the atomic level18 However the AFMhas not been used frequently for direct observations of theatomic-scale surface structures of cerium oxide particu-larly for discrete particles17ndash18 Non-contact mode of AFM


RE

SE

AR

CH

AR

TIC

LE


Glass Slide

Mica


Ethanol








a

c

e

b

d

f

100 nm

100 nm

100 nm 50 nm

50 nm

50 nm



RE

SE

AR

CH

AR

TIC

LE





100

75

50

25

0Par

ticle

Fre

quen

cy (

)


Size range (nm)

R3 R2 R1






RE

SE

AR

CH

AR

TIC

LE


100

60

80

40

20

00 50 100 150 200


y = ndash026x + 6141

R2 = 098

Molar H2OAOT ratio

Ave

rage

par

ticle

siz

e (n

m)





a

b


Data type2 range

Height3000 nm

Phase4500 de

100

200

300

400nm





RE

SE

AR

CH

AR

TIC

LE


a

b

c

0

0

0

100 200 300 400

100

100

200

200

300 400

ndash10

0+

100

0nm

ndash10

0+

100

0

nmndash2

50

+25

00nm

nm

nm

nm




10

08

06

04

02

000 50 100 150 200

Molar H2OAOT ratio

Ave

rage

rou

ghne

ss fa

ctor


R2 = 095




4 CONCLUSIONS






RE

SE

AR

CH

AR

TIC

LE


































RE

SE

AR

CH

AR

TIC

LE


Glass Slide

Mica


Ethanol








a

c

e

b

d

f

100 nm

100 nm

100 nm 50 nm

50 nm

50 nm



RE

SE

AR

CH

AR

TIC

LE





100

75

50

25

0Par

ticle

Fre

quen

cy (

)


Size range (nm)

R3 R2 R1






RE

SE

AR

CH

AR

TIC

LE


100

60

80

40

20

00 50 100 150 200


y = ndash026x + 6141

R2 = 098

Molar H2OAOT ratio

Ave

rage

par

ticle

siz

e (n

m)





a

b


Data type2 range

Height3000 nm

Phase4500 de

100

200

300

400nm





RE

SE

AR

CH

AR

TIC

LE


a

b

c

0

0

0

100 200 300 400

100

100

200

200

300 400

ndash10

0+

100

0nm

ndash10

0+

100

0

nmndash2

50

+25

00nm

nm

nm

nm




10

08

06

04

02

000 50 100 150 200

Molar H2OAOT ratio

Ave

rage

rou

ghne

ss fa

ctor


R2 = 095




4 CONCLUSIONS






RE

SE

AR

CH

AR

TIC

LE


































RE

SE

AR

CH

AR

TIC

LE





100

75

50

25

0Par

ticle

Fre

quen

cy (

)


Size range (nm)

R3 R2 R1






RE

SE

AR

CH

AR

TIC

LE


100

60

80

40

20

00 50 100 150 200


y = ndash026x + 6141

R2 = 098

Molar H2OAOT ratio

Ave

rage

par

ticle

siz

e (n

m)





a

b


Data type2 range

Height3000 nm

Phase4500 de

100

200

300

400nm





RE

SE

AR

CH

AR

TIC

LE


a

b

c

0

0

0

100 200 300 400

100

100

200

200

300 400

ndash10

0+

100

0nm

ndash10

0+

100

0

nmndash2

50

+25

00nm

nm

nm

nm




10

08

06

04

02

000 50 100 150 200

Molar H2OAOT ratio

Ave

rage

rou

ghne

ss fa

ctor


R2 = 095




4 CONCLUSIONS






RE

SE

AR

CH

AR

TIC

LE


































RE

SE

AR

CH

AR

TIC

LE


100

60

80

40

20

00 50 100 150 200


y = ndash026x + 6141

R2 = 098

Molar H2OAOT ratio

Ave

rage

par

ticle

siz

e (n

m)





a

b


Data type2 range

Height3000 nm

Phase4500 de

100

200

300

400nm





RE

SE

AR

CH

AR

TIC

LE


a

b

c

0

0

0

100 200 300 400

100

100

200

200

300 400

ndash10

0+

100

0nm

ndash10

0+

100

0

nmndash2

50

+25

00nm

nm

nm

nm




10

08

06

04

02

000 50 100 150 200

Molar H2OAOT ratio

Ave

rage

rou

ghne

ss fa

ctor


R2 = 095




4 CONCLUSIONS






RE

SE

AR

CH

AR

TIC

LE


































RE

SE

AR

CH

AR

TIC

LE


a

b

c

0

0

0

100 200 300 400

100

100

200

200

300 400

ndash10

0+

100

0nm

ndash10

0+

100

0

nmndash2

50

+25

00nm

nm

nm

nm




10

08

06

04

02

000 50 100 150 200

Molar H2OAOT ratio

Ave

rage

rou

ghne

ss fa

ctor


R2 = 095




4 CONCLUSIONS






RE

SE

AR

CH

AR

TIC

LE


































RE

SE

AR

CH

AR

TIC

LE
































