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Page 1: Preparation and size determination of monodisperse silica microspheres for particle size certified reference materials

Powder Technology 207 (2011) 232–237

Contents lists available at ScienceDirect

Powder Technology

j ourna l homepage: www.e lsev ie r.com/ locate /powtec

Preparation and size determination of monodisperse silica microspheres for particlesize certified reference materials

Sheng-Li Chen ⁎, Guimei Yuan, Chun-Tian HuState Key Laboratory of Heavy Oil Processing and Department of Chemical Engineering, China University of Petroleun-Beijing, Beijing 102249, China

⁎ Corresponding author. Tel.: +86 10 89733396.E-mail address: [email protected] (S.-L. Chen).

0032-5910/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.powtec.2010.11.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 August 2010Received in revised form 17 October 2010Accepted 3 November 2010Available online 9 November 2010

Keywords:Reference materialsParticles size measurementMonodisperse SiO2 microspheres

Five samples of monodisperse SiO2 microspheres used for certified reference materials for particle size weresynthesized by the seed-growth method, that is, through hydrolysis and condensation of tetroethylortho-silicate in a mixture of ethanol and water, using amoniua as catalyst. Their nominal diameters were 0.15 μm,0.3 μm, 0.5 μm, 1 μm, and 2 μm. The average diameters of these microspheres were accurately determinatedby SEM array-sizing method, and the microspheres with 1 μm and 2 μm nominal diameters were determinedby both SEM array-sizing method and optical array-sizing method, while the particle size distribution of thesespheres was determined by SEM. The measurement errors for each sample were quantitatively assessed. TheSEMmagnification was calibrated with a SEM calibration standard traceable to CNIM (China National Instituteof metrology), while the optical microscopy magnification was calibrated with a NIST stage micrometer (SRM2800). The average diameters determined by SEM and optical microscopy were identical to each other. Forcomparison, the average diameter of thesemicrospheres was alsomeasured by other independent techniques-light scattering and photo-correlation methods, and the obtained data agreed well with that obtained by thearray sizing method.

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The particle size certified referencematerials (CRM) arewidely usedfor the calibration of particle-size measuring instruments, includingelectron microscopes (SEM and TEM), optical microscopes, light-scattering instruments, photo-correlation instruments and so on. Theparticle size CRM are usually made of monodisperse polymer spheres(usually polystyrene spheres) because of their easy preparation. Forexample, the National Institute of Standard and Technology (NIST) ofthe US and the Community Bureau of Reference (BCR) of the EuropeUnion use the polymer spheres as the materials of the particle sizestandard reference materials (SRM) and the particle size CRM [1–3].However, because the polymer spheres swellwhen dispersed in organicsolvent, melt or decompose when heated to high temperature, and areprone to be eaten bybacteria, particle size CRMmade of polymer cannotbe used in organic solvents or at high temperature, and anti-bacterialagents have to be added into theparticle size SRM.Another shortcomingof the SRM made of polymer is that they cannot be used to calibrateparticle-size measuring instruments based on centrifuge force, becauseof the very small density difference between polymer and water. Theabove problems will disappear if silica spheres instead of polymerspheres are used as the particle size SRM. Silica spheres have highermechanical and chemical stability and higher tolerance to organic

solvents than polymer spheres. And the silica spheres have higherdensity than polymer spheres, making them suitable for the calibrationof centrifuge-particle sizing equipments and providing more contrastthan polymer spheres in optical and electron beans. Furthermore, silicacan stand the attack of bacteria and has excellent biocompatibility thanpolymer, making the SiO2 spheres be widely used in life sciencesresearch. Therefore, it is of great significance to develop particle sizestandard materials made of silica microsphere.

Although some scientific companies have particle size standardmaterials of silica microsphere available for sale, e.g. the ThermoFisher Scientific Inc (former Duke company)[4] and the PolyscienceCo.[5], these silica standard materials are not national referencematerials, and the uncertainty of the diameter of their silica micro-spheres is relatively large. Reported in this paper is the synthesis andsize determination of the silica microspheres used as China nationalparticle size certified reference materials. The nominal diameters ofthese reference materials are 2 μm, 1 μm, 0.5 μm, 0.3 μm and 0.15 μm,and their diameter was accurately determined to be 2061±18 nm,943±9 nm, 459±5 nm, 281±4 nm and 164±3 nm respectively.The China certified reference materials catalog numbers of thesefive types of SiO2 microspheres are GBW12012a, GBW12013a,GBW12014a, GBW12015a and GBW12016a respectively.

2. Synthesis of monodisperse silica spheres

The monodisperse silica spheres used as the particle size referencematerials were prepared by seeded growth method. The silica seeds

Page 2: Preparation and size determination of monodisperse silica microspheres for particle size certified reference materials

233S.-L. Chen et al. / Powder Technology 207 (2011) 232–237

were Ludox AS-40 silica sol or other small size monodisperse silicaspheres. The Ludox AS-40 was purchased from Du Pont, with anaverage diameter of 33.2 nm, and a relative standard deviation indiameter of 15%. The synthesis of seeds other than Ludox AS-40was performec by the stöber method [6]. The procedure is brieflydescribed as follows: ethanol solution containing 3–10 M H2O and0.5–2 M NH3 was measured and put into a reactor, which was kept at25 °C and agitated by a mechanical stirrer. Another ethanol solutioncontaining TEOS (tetraethylorthosilicate) was quickly added into thereactor. The TEOS concentration in the synthesis mixture was 0.22 M.After the TEOS was consumed in around 2–3 hours, the reactionsuspension was centrifuged and washed with DI water for severaltimes to remove NH3, ethanol and impurities. Thus the monodispersesilica seeds were obtained.

In the seeded growth experiment, a specific amount of silica seedswas put into the NH3–H2O–Ethanol solution, then the ethanol con-taining TEOS was drop-wisely added into the reactor, and the TEOSfeeding rate was less than the hydrolysis and condensation rates ofthe TEOS in the reactor [7], so that the TEOS concentration in thereactor was always less than 0.22 M. In order to make sure no for-mation of new particles during the silica-seeds growth, the totalsurface area of seeds in the reactor was less than the critical surfacearea of nucleation [8]. The other procedures of the seeded growthexperiment were similar to that of the seed synthesis. The mono-disperse silica spheres with the nominal diameter of 0.15 μm and0.3 μmwere prepared by using the Ludox AS-40 silica sol as the seeds.The silica spheres with the nominal diameter of 0.5 μm and 1 μmwereprepared by using the 0.3 μm and 0.6 μm silica spheres as seedsrespectively, which were prepared by the stöber method. The silicaspheres with the nominal diameter of 2 μm were prepared through atwo-step seeded growth process. The first step is to prepare 1.4 μmsilica spheres using 0.7 μm SiO2 spheres as the seeds, which wassynthesized by the stöber method. The second step is to prepare the2 μm SiO2 spheres using the 1.4 μm SiO2 spheres as the seeds. Thereaction conditions were the same for all the seeded growth ex-periments, i.e. 25 °C, 2 M NH3 and 6 M H2O.

3. Size determination of monodisperse silica spheres

So far, five techniques have been used for measuring the averageparticle size, and they are: light scattering method, TEM method,optical row-length measurements of spheres arranged in twodimensional arrays, electronical row-lengthmeasurements of spheresarranged in two dimensional arrays by Metrology Electron Microsco-py (MEM) and differential mobility analysis method. The lightscattering method was once used to measure the average particlesize of NIST SRM 1690 with nominal diameter of 1 μm [1]. The TEMmethod was once used to measure the particle size of NIST SRM1691with nominal diameter of 0.3 μm [9]. The optical row-length mea-surement of spheres arranged in two dimensional arrays was onceused to measure the average particle size of NIST SRM with nominaldiameter larger than 1 μm (including SRM 1690, SRM 1962, SRM1960, SRM 1965 and SRM 1961) [10–12] and the average particle sizeof all the certified reference materials of the Europe Union[3]. Theelectronical row-length measurement by MEM was used to measurethe average particle size of SRM 1961 and SRM 1962[10,12]. Thedifferential mobility analysis method was used to measurement theaverage size of SRM 1963 and SRM 1964[13–15].

The row-length measurements of spheres arranged in two di-mensional arrays (array sizing) was selected as the average sizedetermination method in our work, and this method was extendedfrommeasuringmicrometer sized spheres tomeasuring submicrometersized spheres. The row-length measurements were performed with anoptical microscope (for the certified reference materials with nominaldiameter size equal or larger than 1 μm) and/or a scanning electronmicroscope (SEM) (for all the standard particles). The magnification of

the optical microscope was calibrated with a NIST stage micrometer(SRM 2800), the extended uncertainty of which is 20 nm in the lengthrange of 0–100 μm. The length scale of the NISTmicrometer is traceableto He-Ne laserwavelength. Themagnification of the SEMwas calibratedwith a SEM calibration specimen Model 702CE, made by AdvancedSurface Microscopy Inc. The nominal pitch period of the Model 702CEspecimen is 700 nm andwas accurately measured to be 700.8±0.5 nmby CNIM (China National Institute of Metrology) with He–Ne laserdiffraction technique. Because the optical microscope and SEMcalibration specimen are highly accurate and monodisperse silicaspheres can form close-packed, two-dimensional hexagonal arrays,the array sizing is able to produce accurate values for the averagediameter. The standard deviation of the size distributionwas calculatedthrough measuring SEM images of separate particles.

For preparation of one layer arrays of close-packed spheres, a slideand/or a polished silicon chip was first carefully cleaned with aconcentrated K2CrO4–H2SO4 solution and rinsedwith DI water. Then adrop of the particle suspension was put onto the microscope slideand/or the silicon chip and the particle suspension drop was smearedout by vibration. After that, the slide and/or silicon chip was placedinto a container, the humidity of whichwas controlled to be ~90%. Theevaporating water swept the particles together and the surfacetension caused small scale motion, leading to one layer ordered array,with rows 10–60 spheres long.

The arrays were measured by a scaled microscope and/or a scaledSEM. The microphotography was performed on a PIP9.0 particle-sizeinstrument quipped with OLYMPUS CX21 optical microscope (OmecInstruments, Zhuhai, China). The magnification calibration and theimage distortion were measured by using NIST stage micrometer SRM2800, and it was found that the scale difference in different directions(image distortion) was less than 0.1%, and the scale changes fromrepeated focusing were also less than 0.1%. In compared with othermeasurement errors, the error caused by the image distortion isnegligible. More than 30 arrays were measured, and for each array, 5–10 row lengths were measured. The row length varied from 14 to 30spheres. The average diameters of silica spheres with nominaldiameters of 1 μm and 2 μm were measured by the optical method.

The scaled SEM was used to measure the arrays assembled onsilicon chips. The SEM measurement was performed on FEI Quanta200 F. The magnification calibrations in differtnt direactions wereperformed by using a two-dimensional calibration specimen Model702CE, and the pitch period of which was accurately measured to be700.8±0.5 nm by CNIM. The magnifications in two directionsperpendicular to each other were calculated. The acceleration voltageof the SEM was 10 kV. The SEM images of all the samples and thecalibration specimen were taken at the same SEM operation condi-tions. The scale changes from repeated focusing were less than 0.1%.

The average diameters of the prepared SiO2 spheres were alsomeasured by independent techniques — static light scattering anddynamic light scattering (photon-correlation) methods. The staticlight scattering measurement and dynamic light scattering measure-ment were performanced on HORIBA LA-920 and Malven ZetasizerNano-ZS respectively.

4. Results

The SEM images of the prepared silica microspheres with nominaldiameter of 0.15 μm, 0.3 μm, 0.5 μm, 1 μm and 2 μm were shown inFigs. 1–5 respectively. The prepared silica microspheres used for theparticle size CRM were highly uniform. The SEM image andmicrophotography of the arrays of SiO2 spheres with nominaldiameter of 2 μm are shown in Figs. 6 and 7, respectively. For com-parison, the images of the SEM calibration specimen and the stagemicrometer were also inserted in Figs. 6 and 7 respectively. The SiO2

spheres were highly ordered on both slides and silicon chips. Theuncertainty of the array sizing method involves the uncertainty of the

Page 3: Preparation and size determination of monodisperse silica microspheres for particle size certified reference materials

Fig. 1. SEM image of the SiO2 microspheres certified reference materials with nominaldiameter of 0.15 μm.

Fig. 2. SEM image of the SiO2 microspheres certified reference materials with nominaldiameter of 0.3 μm.

Fig. 3. SEM image of the SiO2 microspheres certified reference materials with nominaldiameter of 0.5 μm.

Fig. 4. SEM image of the SiO2 microspheres certified reference materials with nominaldiameter of 1 μm.

Fig. 5. SEM image of the SiO2 microspheres certified reference materials with nominaldiameter of 2 μm.

Fig. 6. SEM image of the ordered-packed SiO2 certified reference materials withnominal diameter of 2 μm on silicon chips.

234 S.-L. Chen et al. / Powder Technology 207 (2011) 232–237

Page 4: Preparation and size determination of monodisperse silica microspheres for particle size certified reference materials

Fig. 7. The microphotography of the ordered-packed SiO2 standard reference materialswith nominal diameter of 2 μm on slides.

235S.-L. Chen et al. / Powder Technology 207 (2011) 232–237

stage micrometer (for optical measurement) or the SEM calibrationspecimen (for SEM measurement), the uncertainties of the imagemeasurements and the random sampling uncertainty. The uncertain-ties of the imagemeasurements include themeasurement uncertaintyof calibration specimen and the measurement uncertainty of SiO2

spheres row length. The source of uncertainties for the average sizedetermination of the certified referece spheres in the array sizing

11.. The uncertainty of the calibration

The unc

calibr

The unc

spher

2. The uncertainties resulted from the image measurement

2u

3. Random measure uncertainty, 3u

UUnncceerr--

ttaaiinnttyy

Fig. 8. Source of uncertainties for the average size determination of

Table 1The determination results of SiO2 microspheres with nominal diameter of 0.15 μm.

Techniquea Equipment Mean diameter(μm)

Size distributC.V.(%)

SEM Quanta 200F 0.164 3.97DLS Malven. Nanosizer 0.169 –

a SEM—SEM array sizing. DLS—dynamic light scattering method.

Table 2The determination results of SiO2 microspheres with nominal diameter of 0.3 μm.

Techniquea Equipment Mean diameter(μm)

Size distributC.V.(%)

SEM Quanta 200F 0.281 2.96DLS Malven. Nanosizer 0.287 –

a SEM—SEM array sizing. DLS—dynamic light scattering method.

method is shown in Fig. 8. The u1 u21 u22 and u3 were calculatedaccording to Eqs. (1)–(4).

u1 =Ln

n ⋅ L0mΔL0 ð1Þ

u21 =Ln

n ⋅ L0m1fΔL0m ð2Þ

u22 =1

n ⋅ fΔLnm ð3Þ

u3 =σffiffiffiffiffim

p ð4Þ

ΔL0 is the uncertainty of the calibration specimen (SRM 2800 foroptical measurement and Model 702CE for SEM measurement). ΔL0mandΔLnm are the uncertainty of imagemeasurement of the calibrationspecimen and the SiO2 sphere's row length respectively. L0m and Ln arethe image's lengths of the calibration specimen and the SiO2 sphere'srow respectively. The n is the number of the SiO2 spheres in one row.The m is the total number of the measured SiO2 spheres. f ismagnification of the SEM or optical images. The σ is the standarddeviation of the SiO2 particle size distribution, which is obtained byEq. (5) based on individual sphere diameter in the SEM image and theaverage diameter determined by the array sizing.

σ = D

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi∑D2

iSEM− ð∑DiSEMÞ2n

n−1

s=DSEM ð5Þ

specimen, 1u

ertainty from image measurement of the

ation specimen, 21u

ertainty from image measurement of the SiO2

es row length, 22u

the SiO2 standard reference spheres in the array sizing method.

ion Uncertainty (μm)

u1 u2 u3 U

u21 u22

0.0 0.0003 0.0011 0.0001 0.003– – – – –

ion Uncertainty (μm)

u1 u2 u3 U

u21 u22

0.0 0.0005 0.0015 0.0003 0.004– – – – –

Page 5: Preparation and size determination of monodisperse silica microspheres for particle size certified reference materials

Table 3The determination results of SiO2 microspheres with nominal diameter of 0.5 μm.

Techniquea Equipment Mean diameter(μm)

Size distributionC.V.(%)

Uncertainty (μm)

u1 u2 u3 U

u21 u22

SEM Quanta 200F 0.459 3.28 0.0 0.0005 0.0021 0.0003 0.005SLS LA-920 0.457 – – – – – –

a SEM—SEM array sizing. SLS—static light scattering method.

Table 4The determination results of SiO2 microspheres with nominal diameter of 1 μm.

Techniquea Equipment Mean diameter(μm)

Size distributionC.V.(%)

Uncertainty (μm)

u1 u2 u3 U

u21 u22

OPT PIP9.0 0.947 0.0001 0.0008 0.006 0.0004 0.012SEM Quanta 200F 0.945 3.23 0.0 0.001 0.0042 0.0008 0.009SLS LA-920 0.943 – – – – –

a OPT—Optical array sizing. SEM—SEM array sizing. SLS—static light scattering.

Table 5The determination results of SiO2 microspheres with nominal diameter of 2 μm.

Techniquea Equipment Mean diameter(μm)

Size distributionC.V.(%)

Uncertainty (μm)

u1 u2 u3 U

u21 u22

OPT PIP9.0 2.068 0.0002 0.0018 0.0052 0.0006 0.012SEM Quanta 200 2.060 2.26 0.0 0.002 0.0084 0.001 0.018SLS LA-920 2.087 – – – – –

a OPT—Optical array sizing. SEM—SEM array sizing. SLS—static light scattering.

236 S.-L. Chen et al. / Powder Technology 207 (2011) 232–237

where,D is the average diameter determined by the array sizing;DiSEM

is the diameter of particle i in the SEM images; DSEM is the averagediameter of all the particles in the SEM images.

The total uncertainty in the average size determination is given byEq. (6), with the extended coefficient being 2.

U = 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiu21 + u2

21 + u222 + u2

3

qð6Þ

The final determination results of SiO2 microspheres with differentnominal diameters are shown in Tables 1–5. Static light scattering anddynamic light scattering (photon-correlation) methods were alsoused to measure the average diameters of the SiO2 spheres and thedata are also shown in Tables 1–5. Considering the uncertainty ofthese methods, the average diameters measured with NIST traceableoptical microscope and CNIM traceable SEM are identical with each

Table 6The determination results of the SiO2 microspheres.

The CRMcatalognumber

Nominaldiameter(μm)

Mean diameter anduncertaintya

(μm)

Size distribution

Std. Dev. (μm) C.V. (%)

GBW12012a 2 2.061±0.018 0.047 2.26GBW12013a 1 0.943±0.009 0.0305 3.23GBW12014a 0.5 0.459±0.005 0.0151 3.28GBW12015a 0.3 0.281±0.004 0.0083 2.96GBW12016a 0.15 0.164±0.003 0.0066 4.00

a Confidence level: 95 or extended coefficient of the uncertainty is 2.

other. The data obtained by light scattering and photon-correlationare also in good agreement with that by array sizing.

For the SiO2 spheres with nominal diameters of 1 μm and 2 μm, theaverage values of the optical array sizing diameter and the SEM arraysizing diameter are used as the certified diameters.

The determination results of all the samples are listed in Table 6.The China certified reference materials catalog numbers of these fivetypes of SiO2 microspheres are GBW12012a, GBW12013a, GBW12014a,GBW12015a and GBW12016a respectively.

5. Conclusion

SiO2 certified reference materials for particle size with nominaldiameters of 0.15 μm, 0.3 μm, 0.5 μm, 1 μm, and 2 μm were synthe-sized through seeded experiments and their average diameters wereaccurately determined by SEM-array sizing and/or optical-arraysizing. The measurement error for each sample was quantitativelyassessed. The average diametersmeasuredwith NIST traceable opticalmicroscope and CNIM traceable SEM are identical with each other,and the static light-scattering diameter and the dynamic light-scattering diameter are also in good agreement with that by arraysizing.

Acknowledgements

This research work was supported by National Basic ResearchProgram of China (Grant No: 2006CB932601) and China NationalNature Science Foundation (Grant No: 20976192).

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