uv-shielding ceramic nanoparticles synthesised by...

Journal of the Australian Ceramic Society, 38 [1] (2002), 15-19.

UV-SHIELDING CERAMIC NANOPARTICLES SYNTHESISED BYMECHANOCHEMCIAL PROCESSING

T. TSUZUKI, J. S. ROBINSON and P.G. MCCORMICKAdvanced Nano Technologies Pty Ltd, 112 Radium street, Welshpool,6106, WA, Australia

email: [email protected]

SUMMARY:ZnO, TiO2 and CeO2 are known as UV-shielding ceramic materials that have advantages over organic UV absorbersfor their photo-stability and non-hazardous nature to human bodies. However, they normally cause lowtransparency in the visible-light range due to light scattering by large particles, which is undesirable for manytransparent UV-blocking applications in cosmetic and plastic industries. Light-scattering efficiency of particles canbe drastically reduced by decreasing the particle sizes down below 100 nm. This paper reviews recent investigationon the synthesis of ZnO and CeO2 nanoparticles by mechanochemical processing. The resulting particles had asignificantly low degree of agglomeration, having mean particles sizes of ~ 25 nm and ~ 10 nm, respectively. Theaqueous suspensions of the nanoparticles showed strong absorption in UV-light range and high transmittance in thevisible-light range. Mechanochemical processing offers possibility of industrial-scale production of transparent UV-shielding ceramic particles for many applications.

KEYWORDS: Nanoparticles, mechanochemical processing, UV-shield, ultrafine powder, ZnO, CeO2

INTRODUCTIONIn recent years, the demand for effective UV-shielding materials has significantly increased incosmetics, plastics and paints industries. Theceramic ultraviolet-shielding agents such as ZnO,TiO2 and CeO2 have advantages over organicagents for their photo-stability and non-hazardousnature to human bodies. However, since theceramic agents are normally in particulate forms, itis difficult to obtain high UV-shielding propertieswhile maintaining high transparency in the visiblelight range. This problem can be overcome byreducing the particle size down below 100 nm,since the light scattering efficiency decreasesproportional to the 6th powder of particle diameter[1,2].Several methods such as gas-phase processing,aerosol techniques and wet chemical processinghave been used for commercial production ofceramic nanoparticles. However, these methodslack a solid barrier between particles duringsynthesis, leading to the formation of largeaggregates that significantly reduce transparency inthe visible light region.Mechanochemical processing is a novel methodthat enables the production of agglomeration-freenanoparticles. The process involves the mechanicalactivation of solid-state displacement reactions atlow temperatures in a ball mill [3]. Milling ofprecursor powders leads to the formation of ananoscale composite structure of the startingmaterials which react during milling or subsequentheat treatment to form separated nanocrystals of thedesired phase within a solid matrix. Suchmechanochemically formed nanocompositeparticles can be further processed into dispersednano powders, simply by selective removal of the

matrix phase [4]. Of significance is the fact thatthis technique allows the formation of separatednanoparticles embedded in a solid matrix, leadingto agglomeration-free ultrafine particles.ZnO and CeO2 have bandgap energiescorresponding to ~365 and ~380 nm, and thusabsorb UV light having wavelength shorter than thebandgap energies. As such, they are known asexcellent UV-blocking agents. In this paper, thesynthesis of agglomeration-free ZnO and CeO2

nanoparticles by mechanochemical processing isreported.

METHODS AND PROCEDURESThe mixtures of starting materials ZnCl2 (Cerac,99.5+%, -8 mesh), CeCl3 (Cerac, 99.9%, -20 mesh),Na2CO3 (Aldrich, 99%, -20 mesh), NaOH (Aldrich,99.99% in pellet forms) and NaCl (Sigma, 99.8%,beads) were sealed in a hardened steel vial (AISI440C stainless steel) with hardened steel balls of6.4 mm in diameter, under a high purity argon-gasatmosphere. Milling was performed with a Spex8000 mixer/mill using a ball-to-powder mass ratioof 10 : 1. Heat treatment of the as-milled powderwas carried out in air in a porcelain crucible for 1hour. Removal of the salt phase was carried out bywashing the powder with de-ionised water, using anultrasonic bath and a centrifuge.The crystallite size and crystal structure of thepowder were examined via X-ray diffractometry(XRD) using a Siemens D5000 X-ray diffractionion spectrophotometer with Cu-Ka radiation. Themean crystallite size was estimated from the widthof diffraction peaks using the Scherrer equation [5].The microstructure of the powder was examined viatransmission electron microscopy (TEM) using aJEOL 2000FXII (a beam energy of 80 keV). For


TEM studies, the washed powder was dispersed inmethanol using an ultrasonic bath and a drop of thesuspension was placed on a copper grid coated withholey carbon film. The specific surface area wasmeasured via Brunauer-Emmett-Teller nitrogen-gasabsorption method (BET) at 77 K, using aMicromeritics Gemini 2360 Surface Area Analyser.Simultaneous differential thermal analysis (DTA)and thermogravimetric analysis (TGA) were carriedout using a Rigaku Thermoflex thermal analysissystem under a constant air flow of 2 cc/min with aheating rate of 20°C/min. Inductively CoupledPlasma Atomic Emission Spectroscopy (ICP-AES)was carried out for the trace analysis of Na, Cl andFe which are possible contaminant elements due tothe use of NaCl diluent and steel balls/vial. UV-Vis specular transmittance was measured using aVarian Cary 300 Bio UV-Vis spectrophotometeralong with a quartz cuvette having an optical pathlength of 10 mm.

RESULTS AND DISCUSSIONSynthesis of ZnO nanoparticlesA stoichiometric mixture of the starting powders,corresponding to the reaction equation of ZnCl2 +Na2CO3 + 8.6NaCl Æ ZnCO3 + 10.6NaCl wasmilled [6]. NaCl was added to the reactants so thatthe volume ratio of the ZnCO3:NaCl in the productphase was 1:10. The reaction during milling wasstudied by XRD. As the milling time increased to 4hours, the diffraction peaks associated with ZnCl2

and Na2CO3 decreased. With longer milling times,only peaks associated with NaCl were present,indicating that amorphization of the other phasesoccurred during milling (Fig. 1(a)) [7,8].

1 0 2 0 3 0 4 0 5 0 6 0 7 0

Inte

nsity

(arb

. uni

ts)

2q (degrees)

(a) as-milled

(b) annealed

(c) washed

x : ZnO

x x

x

xx x x

xx

o : NaCl

o

x

o

o

oo o

o

oo

oo o

x x

Fig. 1: XRD spectra of the ZnCl2 + Na2CO3 +8.6NaCl powder mixture.

TG/DTA analysis of the as-milled powder showedthat weight loss of 6% occurred in the temperaturerange of 170 – 380oC, corresponding to the reactionZnCO3 Æ ZnO + CO2(gas). The as-milled powderwas heat-treated at 400oC, and subsequently

washed. XRD study revealed the formation of ZnOin NaCl (Fig. 1(b)) during heat treatment. Asshown in Fig. 1(c), XRD pattern of the washedsample only consisted of peaks corresponding toZnO.TEM study showed that the ZnO powder consistedof well-separated 10 – 40 nm size particles havingequiaxed morphology (Fig. 2). Dark field imagingshowed that each particle was a single crystal.Particle size distribution obtained from TEM studyhad the mean particle size of 26.2 nm and standarddeviation of 8.6 nm. This narrow size distributionis a typical feature of mechanochemicallysynthesised nanoparticles [9].

Fig. 2: TEM image of ZnO nanoparticles

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800

BET

size

(nm

)

Temperature (oC)Fig. 3: BET mean particle size of ZnO as a function

of heat treatment temperature.

BET surface area of the ZnO powder was 47.3m2/g, which corresponds to a spherical particle sizeof 27 nm. The mean crystallite size estimated fromthe XRD peak width at 2q = 36o was 28.7 nm. Theparticle sizes estimated by XRD, BET and TEMstudies were in good agreement with each other,indicative of insignificantly agglomeration in thepowder [10].Milling without NaCl diluent phase for 4 hours andsubsequent heat treatment at 400°C resulted in ZnOparticles of 100- 1000 nm in size having a highdegree of agglomeration. Therefore, in order toform separate nanoparticles, it is necessary for thevolume fraction of the nanoparticles phase to be


sufficiently low to prevent the grain growth duringmilling and heat treatment.ICP-AES measurements detected the presence ofiron (0.041 wt%), sodium (0.06 wt%), and chlorine(0.076 wt%) in the ZnO nanopowder.Fig. 3 shows the BET size as a function of heattreatment temperature. As shown in the figure, Itwas possible to control the mean particle size in therange of 25 – 90 nm by changing the heat treatmenttemperature. Heat treatment above the meltingpoint of NaCl (801oC) caused extensiveagglomeration and growth of primary particles.For UVVis measurements, the resulting particleswere placed in deionised water, and was dispersedusing an ultrasonic probe for 15 min. Dispex-A40of 10 wt% relative to ZnO was added as adispersant prior to the sonication. Fig. 4 shows theUVVis specular transmittance of 0.01 wt% aqueoussuspensions. The particles having BET mean sizesof 26, 50 and 90 nm were used for themeasurements. For comparison, commercialpigment-grade ZnO (BET size 250 nm) was alsomeasured.

0

2 0

4 0

6 0

8 0

100

200 300 400 500 600 700 800Wavelength (nm)

Tran

smitt

ance

(%)

90 nm

250 nm

26 nm50 nm

Fig. 4: UV-Vis spectra of ZnO nanoparticleaqueous suspensions (0.01 wt%). BET mean

particle sizes are indicated in the figure.

The “whitening” effect of the particulatesuspension stems from the light scattering byparticles. Mie theory predicts that light scatteringefficiency decreases proportional to the 6th powderof particle diameter for the particles smaller than1/10 of light wavelength [1,2]. Due to this reason,transparency of the suspension in the visible-lightrange (wavelength 400 – 750 nm) drasticallyincreased as particle size decreased from 250 nm to26 nm, as shown in Fig. 4. On the other hand, UV-blocking characteristic showed only slightdependence on particle size, mainly becausebandgap absorption of UV-light (wavelength < 400nm) is independent of particle size.Since the transmittance in the visible-light range issensitive to particle size, it is also sensitive toagglomeration states [11]. Therefore, the hightransparency in the visible light range in Fig. 8 isanother indication of agglomeration-free feature ofthe nanoparticles.

Synthesis of CeO2 nanoparticlesFor the synthesis of CeO2, a stoichiometric mixtureof the starting powders, corresponding to thereaction equation of CeCl3 + 3NaOH + 12NaCl ÆCe(OH)3 + 15NaCl was milled for 4 hours [10].XRD study revealed that the as-milled powderconsisted of Ce(OH)3 and NaCl (Fig. 5(a)).TG/DTA analysis of the as-milled powder showeda gradual weight loss of 6% in the temperaturerange of 25 – 400oC, corresponding to the thermaldecomposition of reaction Ce(OH)3 into CeO2. Theas-milled powder was calcined at 500oC andsubsequently washed to remove NaCl. XRD studyrevealed occurrence of the formation of CeO2 inNaCl during calcinations (Fig. 1(b)), and that thewashed sample consisted of only the CeO2 phase.The mean crystallite size estimated from diffractionpeak widths was 10.2 nm.

1 0 2 0 3 0 4 0 5 0 6 0 7 0

Inte

nsity

(arb

. uni

ts)

2q (degrees)

xx

xx

x

x

x

x

xx

x

: NaCl

O O

O : CeO2

+ + ++ ++ +

+

O

(a) as-milled

(b) heat -treated

: Ce(OH)3

+

O

(c) washed OO O

OO

O

Fig. 5: XRD spectra of the CeCl3 + 3NaOH +12NaCl l powder mixture.

TEM analysis showed that the CeO2 particles havesizes of 5 - 40 nm (Fig. 6). The BET surface areaof the powder was 83 m2/g, which corresponds to aspherical particle size of 10 nm.

Fig. 6: TEM image of CeO2 nanoparticles


ICP-AES measurements detected the presence ofiron (0.023 wt%), sodium (0.244 wt%), andchlorine (0.045 wt%) in the CeO2 nanopowder.Fig. 7 shows BET particle size and XRD crystallitesize as a function of heat treatment temperature.Below the melting point of NaCl, BET particlesizes were nearly the same as XRD crystallite sizes,indicative of agglomeration-free nanoparticles.When the powder was heat treated above 801oC,BET particle size became larger than XRDcrystallite size. This result is due to the fact that theNaCl matrix phase became liquid above 801oC andthus nanoparticles were no longer separated fromeach other, causing agglomeration.

1 0

100

1000

400 500 600 700 800 900 1000 1100

BET

XRD

Size

(nm

)

Temperature (oC )Fig.7: BET particle size and XRD crystallite size ofCeO2 as a function of heat treatment temperature.

Fig. 8 shows the UV-Vis specular transmittancecurve of ~ 10 nm CeO2 aqueous suspension(0.005wt%) containing Dispex-A40 of 10wt%relative to CeO2. The spectrum of 30 nm ZnOaqueous suspension (0.01wt%) was also shown inFig. 8 for comparison. It is evident that both ZnOand CeO2 have an excellent UV light absorbingproperties, and that ZnO has broader UV-bandabsorption characteristics than CeO2.

0

20

40

60

80

100

200 300 400 500 600 700Wavelength (nm)

Spec

ular

tan

smitt

ance

(%

)

ZnO 30 nm0.01wt%

C e O2 10 nm0.005wt%

UV Visible

Fig. 8. UV-Vis spectra of ZnO and CeO2

nanoparticle aqueous suspension

Since the refractive index of CeO2 is higher (2.3)than that of ZnO (2.0), the light-scatteringefficiency of CeO2 is higher than that of ZnO, whenthe particle sizes are the same [12]. Therefore,0.005wt% CeO2 aqueous suspension showed

similar transmittance% as 0.01wt% ZnO aqueoussuspension even though the concentrations andmean particle sizes were different.

CONCLUSIONSThe synthesis of ZnO and CeO2 nanoparticles bymechanochemical processing was demonstrated.The resulting nanoparticles had mean particle sizesof less than 30 nm, and narrow particle sizedistributions. BET particle size, TEM particle sizeand XRD crystallite size were in good agreementwith each other, indicative of a substantially lowdegree of particle agglomeration.Mechanochemical processing enables building upnanoparticles through a solid-state chemicalreaction in a nanoscopically homogeneousenvironment, leading to the formation ofstructurally and morphologically uniformnanoparticles having a narrow size distribution[13]. On the other hand, a simple grinding processis a “top down” approach for reducing particle size,well known to yield a wide particle-sizedistribution. Wet chemical precipitation and gas-condensation techniques tend to form largeagglomeration due to the lack of solid matricesbetween particles, resulting in a bimodal particle-size distribution. Therefore, mechanochemicalprocessing has a significant advantage over thoseconventional technologies for the production ofnanopowders having a narrow particle-sizedistribution.The aqueous suspensions of the nanoparticlesshowed specular transmittance of less than 10% inthe UV-light range and higher than 80% in thevisible-light range, which is ideal for manytransparent UV-shielding applications. The hightransparency was only achieved due to the smallparticle sizes less than 30 nm.Mechanochemical processing offers possibility ofindustrial-scale production of transparent UV-shielding ceramic particles for many applications.

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uv-shielding ceramic nanoparticles synthesised by...

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