TL

Na

b

a

ARRAA

KZLIPP

1

oTpfmip[m

trsa2ic

RS

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 80– 89

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jou rna l h om epa ge: www.elsev ier .com/ locate /co lsur fa

wo-dimensional arrangement of monodisperse ZnO particles withangmuir–Blodgett technique

. Ábraháma, D. Seboka, Sz. Pappb, L. Korösib, I. Dékánya,b,∗

University of Szeged, Department of Physical Chemistry and Materials Science, H-6720, Szeged, Aradi vt. 1, HungarySupramolecular and Nanostructured Materials Research Group of The Hungarian Academy of Sciences, H-6720, Szeged, Aradi vt. 1., Hungary

r t i c l e i n f o

rticle history:eceived 31 January 2011eceived in revised form 6 March 2011ccepted 8 March 2011vailable online 16 March 2011

eywords:

a b s t r a c t

Monodisperse, spherical ZnO particles with different sizes were synthesized in diethylene glycol. Particleswere characterized by means of TEM and SEM images, XRD, nitrogen adsorption–desorption isothermsand small angle X-ray scattering methods. The ZnO nanoparticles were used to prepare Langmuir-films(at the air/water interface) and Langmuir–Blodgett films (on solid supports). Langmuir-films were char-acterized by surface pressure vs. surface area isotherms. Single and multilayer Langmuir–Blodgett filmswere prepared for optical interference, photonic band gap and photoluminescence investigation. We

nOangmuir–Blodgettnterferencehotonic band gaphotoluminescence

improved our previous optical interference model, special features of ordered films of uniform spheri-cal particles were integrated into the model to calculate effective refractive index and thickness of thefilms. Photonic band gap of single layer was determined from transmittance spectra and film parameterswere calculated such as effective refractive index of the film and volume fraction of partilces in the film.Photoluminescence measurements showed correspondence between primary crystallite size of particlesand the wavelength of photoluminescence emission maxima.

. Introduction

Monodisperse particles have great importance in fabricationf highly ordered nanostructured materials and photonic crystals.he well-ordered structure provides good control over the desiredroperties of the material. Several synthesis methods are availableor the preparation of ZnO nanomaterials [1–3], but only a few

ethods result in monodisperse, spherical colloid particles, whichs essential for several applications. Preparation of uniform ZnOarticles in aqueous media was used by Chittofrati and Matijevic4], Zhong and Matijevic [5], Jiang et al. [6] and a microemulsion

ethod was used by Masashi et al. [7].Polyol-mediated preparation of metallic and metal-oxide par-

icles in the micrometer and submicrometer size range waseported to result in uniform, spherical particles with narrowize-distribution [8–10]. Jezequel et al. [11] reported first timebout preparation of monodisperse spheres of ZnO particles of

00–400 nm in diameter in diethylene glycol (DEG) media. They

nvestigated the effect of several parameters, such as water con-entration, nature of the solvent, salt concentration, maximum

∗ Corresponding author at: Supramolecular and Nanostructured Materialsesearch Group of The Hungarian Academy of Sciences, University of Szeged, H-6720zeged, Aradi vt. 1, Hungary. Tel.: +36 62 544210; fax: +36 62 544042.

E-mail address: i.dekany@chem.u-szeged.hu (I. Dékány).

927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2011.03.025

© 2011 Elsevier B.V. All rights reserved.

reaction temperature and heating rate, as well. They found that thepresence of water is essential in the formation of ZnO, the optimalresult was obtained with the molar ratio H2O/Zn = 2 (accordinglyZn(CH3COO)2 × 2H2O is used for the synthesis). DEG was proved tobe the best solvent for the formation of spherical particles, whilemaximum salt concentration of 0.18 mol/L was observed for parti-cles in the submicrometer size range. Heating rate was reported tobe the key parameter for tuning the size of the particles. Seelig et al.[12] described a two-step reaction process for ZnO formation indiethylene glycol. In the first step a polydisperse sol was prepared,and after centrifugation some aliquot of the supernatant was addedto the reaction mixture in the second step. They were able to controlthe particle diameter in the range of 100–600 nm with the amountof primary supernatant added. Tay et al. [13] investigated theformation mechanism of ZnO particles during the DEG-mediatedsynthesis. Their researches showed that particle growth occurs bythe aggregation of nano-entities, which are individually unstable.Spherical configuration of the aggregated sub-units is believed tobe the minimum of the surface free energy, therefore the greateststability.

Uniform colloidal particles are usually prepared for optical,especially for photonic crystal applications. Numerous tehniques

have been developed to fabricate different periodic, orderedstructures including lithographic techniques [14], optical inter-ference methods [15], and colloidal self-assembly: sedimentation[12], spin-coating [16], dip-coating [17], convective self-assembly

: Phy

[tna(ptiLt[lp

cLgtBLoss

2

2

w(4ploprcr

rww

2

(MbC(cwdc

2

dmctsm

N. Ábrahám et al. / Colloids and Surfaces A

18] and Langmuir–Blodgett technique [19]. Self-assembly basedechniques reqire high quality building blocks, namely theanoparticles must be monodisperse and spherical in shape tochieve really a good quality photonic material. Langmuir–BlodgettLB) technique is a relatively simple and cheap method for the per-aration of ordered structures. The colloidal crystal is obtained byhe consecutive deposition of single layers of the uniform build-ng blocks. High quality photonic crystals can be fabricated by theB technique from different sized silica particles [19,20]. Prepara-ion of LB- films of ZnO particles was reported by Naszalyi et al.21,22], they fabricated complex LB-films with introducing silicaayers between ZnO layers and invesitgated the optical properties,hotocatalytical activity and mechanical stability of the films.

The aim of our work was to synthesize monodisperse, spheri-al ZnO colloidal particles with various sizes in autoclave, prepareB films and characterize the optical interference, photonic bandap and photoluminescence properties of the films. We presenthe investigation of ZnO particles with TEM and SEM images, XRD,ET and SAXS techniques. The preparation and characterization ofangmuir-films at the air/water interface are detailed. Investigationf Langmuir–Blodgett films on solid supports with SEM, UV–vispectroscopic and photoluminescence methods are also demon-trated.

. Experimental

.1. Preparation of ZnO particles

Particle synthesis was carried out by the method of Jezequel [11]ith some modifications. Particles were prepared in an autoclave

Parr, 5500) at atmospheric conditions. 1.1 g (in case of 234 nm and57 nm ZnO particles) or 2.2 g (in case of 301 nm and 349 nm ZnOarticles) zinc acetate dihydrate (Fluka, a.r.) and 100 ml of diethy-

ene glycol (Molar Chemicals, purum) were put into the autoclave,ne valve was opened for air, and settings were as follows: tem-erature: 160 ◦C; heating rate: ∼3.5 ◦C/min; stirrer: 400 rpm. Theeaction mixture was heated to 160 ◦C, maintained 1 h and thenooled down in half an hour. The resultant white sol was stored atoom temperature.

Powder samples were obtained by repeated centrifugation andedispersion in ethanol (Molar Chemicals, a.r.). After the thirdashing cycle the supernatant was discarded and the particlesere dried in air at 105 ◦C for 12 h.

.2. Preparation of Langmuir and Langmuir–Blodgett films

Langmuir films at the air/water interface and LB films on solidglass, quartz, Si wafer) substrates were prepared in a Kibron

icroTroughS Langmuir-trough. Spreading sols were obtainedy mixing the original sol with chloroform (Sigma–Aldrich,hromasolv®, ≥99.9%) in the volume ratio 1:1. Surface pressure�) vs. surface area (A) compression isotherms were recorded at aompression speed of 815 mm2/min. Solid substrates for LB filmsere cleaned in fresh piranha solution (at least 1 h) and rinsed witheionized water before film preparation. Films were withdrawn atonstant pressure (� = 10 mN/m), only in the upstroke direction.

.3. Characterization of ZnO particles

Size and size-distribution function of ZnO particles wereetermined from TEM pictures (Hitachi S-4700 field electronicroscope, transmission mode). Samples were prepared on

arbon-coated copper grids from the compressed Langmuir film athe air/water interface. Size distribution was calculated by mea-uring the diameter of at least 150 particles. Scanning electronicroscopic pictures (Hitachi S-4700 field electron microscope)

sicochem. Eng. Aspects 384 (2011) 80– 89 81

were taken to observe the suface morphology of the particles.XRD measurement of powder samples were carried out to charac-terize the crystal structure of the particles and to determine theprimary crystallite size. XRD measurements were performed onPhilips PW 1830 diffractometer using Cu K� radiation at a volt-age of 40 kV and a current of 35 mA. Scherrer equation was usedto determine average crystallite diameter from half width of thediffraction peaks: d = (k�)/( cos�), where d is the mean crystallitesize of the powder, � is the wavelength of Cu K�, is the full width athalf-maximum, � is the Bragg diffraction angle and k is a constant.N2 adsorption–desorption isotherms (Micrometrics, Gemini 2375)were measured to calculte BET surface area and porosity of pow-der samples. Small angle X-ray scattering (SAXS) measurementsof powder samples were taken on a Philips PW 1820 diffractome-ter, CuK( radiation (( = 0.154 nm) being used at 40 kV and 30 mA.The primary beam was directed through a Ni-filter into a compactKratky camera, type KCEC/3. The beam width and its thickness were15 mm and 40 �m, respectively. The measurements were done invacuum atmosphere. The powdered sample was placed in a sam-ple holder with thickness of 0.5 mm. The intensity of the scatteredradiation was measured by a position sensitive detector (PDS 50 M)controlled by ASA software, in an angle range of 2� = 0.05–8◦.Absorption intensities (As, Ab) were determined by the so-calledmoving slit method. The I (h) scattering function measured in theKratky camera has to be normalized and then background scat-tering has to be taken into consideration, in order to be able todetermine structural parameters.

2.4. Characterization of Langmuir and Langmuir–Blodgett films

Surface pressure vs. surface area isoterms were evaluated bydetermining the collapse pressure (�c), collapse area (Ac) and thecontact cross-sectional area (Ak) of the particles (see Fig. 4a.). Onecan calculate the area occupied by one single particle (A1) in theLangmuir-film using the following equation [22]:

A1 = Ak

N= Akd3��p

6m(1)

where N is the number of particles in the Langmuir-film, m is themass of the particles in the film, d is the mean particle diameterand �p is the density of particles. As a comparison it is possibleto calculate the theoretically expected area for one particle withthe area of the hexagon including the particle (AH), which can beexpressed as follows [22]:

AH =√

32

d2 (2)

Structure of the multilayered Langmuir–Blodgett films and par-ticle ordering was studied by a field emission scanning electronmicroscope (Hitachi S-4700). For the investigation of the opti-cal and photonic properties transmittance spectra (Ocean OpticsUSB4000) were measured during the preparation of multilayeredfilms on glass slides after each deposited layer. Reflectance spec-tra were recorded with an Ocean Optics (Ocean Optics USB4000)spectrophotometer equipped with a goniometer with fiber opticattachment. Reflection was investigated with polychromatic, non-polarized light at incident angle of 45◦. Photoluminescence (PL)measurements of quartz supported films were performed with aHoriba Jobin Yvon FluoroMax-4 fluorescence spectrometer using

350 nm excitation wavelength. The measurement layout was setto an incident angle of � = 30◦ for the excitation source, the direc-tion of the detection of the emitted ligth was � = 60◦ (� is the anglebetween the ligth and the surface normal).

82 N. Ábrahám et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 80– 89

F he pics

3

3

etostph

Fd

ig. 1. (a)–(d) TEM images of ZnO particles, mean particle diameter is marked on turface structure of particles.

. Results and discussion

.1. Nanoparticle synthesis

Preparation of ZnO colloidal particles was described by Jezequelt al. [11] in a one-step reaction. Seelig et al. [12] tried to reproduceheir method, but they could not make monodisperse particles inne step, so they developed their two-step method. We tried to

ynthesize the ZnO particles by adapting the method of Seelig, buthe result was a bidisperse sol. We decided to achieve monodis-erse particles in an autoclave, where the temperature and theeating rate can be controlled accurately. The synthesis was suc-

ig. 2. Size-distribution functions of ZnO colloidal spheres calculated from particleiameter values resulting from TEM.

tures, (e) SEM image and f. TEM image of 234 nm ZnO particles demonstrating the

cesfull in one step after finding the right parameters. The key is thatthe autoclave must be opened to air, otherwise the particles growup to a few �m in diameter. Ashtaputre et al. [23] carried out thepreparation of ZnO particles in autoclave, they used high pressure,added some water to the reaction mixture and obtained 1–5 �mdoughnut-shaped particles.

3.2. Particle characterization

TEM pictures (Fig. 1a–d.) were taken to determine the size andsize-distribution (Fig. 2) of particles synthesized in the autoclave.The images show that particles are spherical in shape and all thesols contain particles with narrow size-distribution (standard devi-ation is less than 5%). SEM picture of 234 nm ZnO particles (Fig. 1e)was taken to demonstrate the particle morphology and synthe-sis mechanism: the particles were formed by the aggregation ofcolloidal sub-units, which results in a rough surface and remark-

able particle porosity. The results of the obtained data from TEM,XRD, and BET measurements for the different sized ZnO spheres arecollected in Table 1 XRD diffraction pattern (Fig. 3a) exhibits thecharacteristic peaks of hexagonal wurtzit crystal structure (JCPDS

Table 1Characterization of ZnO particles with TEM, XRD and N2 adsorption–desorptionmethods.

d (nm) 234 ± 11 301 ± 12 349 ± 14 457 ± 16

dprimary (nm) 9.7 14.5 12.3 9.6aS

BET (m2/g) 57 66 79 71�pT (g/cm) 3.83 3.54 3.41 3.63p (%) 32 37 39 35N 10,389 6620 16,904 77,238

N. Ábrahám et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 80– 89 83

Fp

3rwmmisTtnpoh

(Sp) determined by SAXS is much higher than the results obtained

ig. 3. (a) XRD diffractograms, (b) N2 adsorption and desorption isotherms and (c)oresize distribution function for the different sized ZnO powder samples.

6-1451, marked in Fig. 3a). Crystallite size determined from Scher-er equation based on line broadening reveals that the particlesere formed by the aggregation of well crystallized 9–14 nm pri-ary particles. N2 adsorption–desorption isotherms (Fig. 3b.) wereeasured to evaluate BET surface area, particle density and poros-

ty of particles (calculated values are collected in Table 1.). The poreize distribution (Fig. 3c.) shows that particles have mesopores.he high value of particle porosity originates from the aggregationype growth mechanism. It is possible to calculate the approximateumber of primary crystallites for one particle if we suppose that

rimary cristallites are spherical in shape and the volume fractionf primary cristallites in one particle is 0.74 (this value is used forexagonally close packed systems). The number of primary crys-

Fig. 4. (a) Surface pressure (�) vs. surface area (A) isotherms of different sized ZnOparticles, (b) series of isotherms of 301 nm ZnO particles with different spread mass.

tallites (N) can be expressed as follows:

N = V · 0.74Vprimary

= r3 · 0.74rprimary

3(3)

where V is the volume of one particle, Vprimary is the volume of oneprimary crystallite, r is the radius of particles and rprimary is theradius of primary crystallites (obtained from XRD). The results arecollected in Table 1. The number of primary crystallites in case ofdifferent particle sizes are quite diverse, which can be due to thedifferent tendencies of primary crystallite size and particle size.

The parameters determined from SAXS measurements were col-lected in Table 2. Mass fractal dimension (Dm, 1 ≤ Dm ≤ 3) describesthe porosity of the sample: if Dm = 1, the sample has very highporosity, it consists of almost only pores, if Dm = 3 the sample iscompact, it has no pores. The Dm value of our samples is around2.5–2.7, in case of 457 nm ZnO particles it is 1.85. The higher poros-ity result does not agree with the nitrogen adsorption–desorptionresults (see as

BET values in Table 1). The reason for this can be thepresence of closed pores of the particles which are not accessible forN2 molecules. Surface fractal dimension (Ds, 2 ≤ Ds ≤ 3) describesthe structure of the surface of the sample: if Ds = 2 the surface issmooth, if Ds = 3 the surface is very rough and fragmented. Our sam-ples have Ds value between 2.3 and 2.5. The specific surface area

with BET method, which can be due to closed particle porosity.The tendency of Sp values for different sized ZnO particles agreeswell with Dm values, 457 nm ZnO particles have higher porosity

84 N. Ábrahám et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 80– 89

Fig. 5. Langmuir–Blodgett films of 234 nm, 301 nm, 349 nm and 457 nm

Table 2SAXS parameters determined for the different sized ZnO samples.

d (nm) 234 nm 301 nm 349 nm 457 nm

Dma 2.5 2.7 2.7 1.85

Dsb 2.3 2.5 2.4 2.4

Kp (Cps/nm3)c 43.2 42.1 38.7 41.58M1 (Cps/nm2)d 177 1145 191 642Kp/M1 0.244 0.193 0.202 141S/V (nm2/nm3)e 0.244 0.192 0.201 0.29SP (m2/g)f 115 92 105 142lc (nm)g 11.3 10.5 10.8 9.05L1 (nm)h 11.9 14.7 10.4 8.96L2 (nm)i 23.0 29.0 27.6 19.12

a Mass fractal dimension.b Surface fractal dimension.c Porod constant.d First moment of the scatterig function.e Relative inner surface.f Specific surface area.g Correlation length (inhomogenity length for the entire powder sample).

ta(sm

series of �–A isotherms were measured in case of 301 nm ZnO

TP

h Inhomogenity length for phase 1.i Inhomogenity length for phase 2.

han others which involves the highest mass fractal dimensionnd specific surface area. The inhomogenity length for ZnO phase

L1) in the samples is in the order of 10 nm. These values corre-pond well with the primary crystallite size resulted from XRDeasurements (dprimary in Table 1). Inhomogenity length for air

able 3arameters determined from Langmuir balance experiments.

d (nm) �c (mN/m) Ac (mm2/mg)

234 nm 30.3 1646

301 nm 26.5 1493

349 nm 29.1 1238

457 nm 30.5 910

ZnO particles: top view of single layers (scale bar equals to 5 �m).

phase (L2) is higher than the pore sizes determined from nitrogenadsorption–desorption measurements. These two values shouldnot be equal, because the inhonogenity length includes the airbetween the particles beside the pores.

3.3. Langmuir-film preparation, characterization

Formation of 2D arrays of ZnO particles at the air/water interfacewas characterized through Langmuir-balance experiments. Surfacepressure (�) vs. surface area (A) isotherms were recorded to inves-tigate the behaviour of the particles at the interface. Fig. 4a shows�–A isotherms of different sized ZnO particles and the determi-nation of the collapse pressure (�c), collapse area (Ac) and thecontact cross-sectional area (Ak). Table 3 contains the results forthe different samples. Isotherms of different sized ZnO particlefilms have almost the same shape: after a well-defined, steeplyrising part comes collapse and the surface pressure is furtherincreasing but with a smaller gradient. Theoretical (AH) and exper-imental (A1) calculations (Eqs. (1) and (2)) of the area for oneparticle in the Langmuir-film were compared for different sizedZnO particles (Table 3). The good agreement of the results meanwell ordered, close packed particles at the air/water interface. A

particles with different spread amount (Fig. 4b). Contact cross-sectional area (Ak) was determined from the isotherm, Fig. 4b insetshows the Ak values as a function of the spread mass of ZnO. The

Ak (mm2/mg) A1 (nm2/db) AH (nm2/db)

1964 5.05 × 104 4.74 × 104

1664 8.41 × 104 7.85 × 104

1377 1.06 × 105 1.05 × 15

1047 1.9 × 105 1.81 × 105

N. Ábrahám et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 80– 89 85

lc

3

Ssofbles

3L

iwmm1to

rctoot(tiitopdfi

IR∼a r + 2rr (1 − r ) cos�

+ r (1 − r ) (9)

Fig. 6. UV–vis transmittance spectra of multilayered films of 234 nm ZnO.

inear dependence reveals good control over the measurement pro-ess.

.4. Structure of Langmuir–Blodgett films (SEM)

Single layered Langmuir–Blodgett films were prepared oni wafer substrate to investigate particle ordering in the solidupported films. Fig. 5 contains pictures of single layer filmsf different sized ZnO particles. It can be seen that particlesorm a single layer, no aggregated or collapsed domains cane found. Particle ordering is very good but not perfect, the

ayers are consisting of hcp ordered domains. We can furtherstablish that larger particles are easier to assemble in orderedtructure.

.5. Optical and photoluminescence characterization ofangmuir–Blodgett films

Langmuir–Blodgett films of our ZnO particles show differentnteresting features. Multilayered films of 234 nm ZnO particles

ere characterized by transmittance and reflectance measure-ents. Film preparation was followed by transmittance measure-ent after each deposited layer. The spectra recorded in case of

–5 layers is collected in Fig. 6. The decrease in transmittance andhe appearance of reflection with increasing layer thickness can bebserved.

Optical interference properties of our films were studied byeflectance measurements. We have established a simple opti-al model based on the interference on plane parallel plates forhe theoretical calculation of reflection intensity. The essentialsf the method is finding the optimal values of the parametersf our model in order to achieve the best fit for the experimen-ally recorded spectra. Our model considers particle characteristicsaverage diameter, porosity, refractive index) and film proper-ies (particle–particle distance, film thickness, effective refractivendex), as well. Details of calculation regarding on the reflectancentensity is published in our previous work [24]. The model ofhat work was extended with the special calculation possibilities ofrdered films of monodispersed colloid spheres, which covers the

recise determination of film parameters, such as particle–particleistance, film thickness and volume fraction of the particles in thelm.

Fig. 7. Schematic representation of the parameters used in our optical model.

The initial step of our method is to calculate the refractive indexof particles (np) with Lorentz–Lorenz equation [25]:

n2p − 1

n2p + 2

=∑

i

fin2

i− 1

n2i

+ 2= fZnO

n2ZnO − 1

n2ZnO + 2

(4)

where nZnO is the bulk refractive index of ZnO, fZnO is the volumefraction of ZnO in one single particle. The wavelength dependentbulk refractive index of ZnO was calculated with an analytical for-mula [24]:

nZnO = a + b

�2

where a and b are constants (a = 1.84, b = 26,667), � is the wave-length of the light. The volume fraction of ZnO in one single particle(fZnO) can be expressed as a function of the porosity:

fZnO = (100 − p)100

(5)

where p is particle porosity in %. Thickness of the film (H) can becalculated as follows [25]:

H = d + (k − 1)

√d2 − D2

3(6)

where d is the average diameter of the particles, k is the number oflayers and D is the distance between the centre of two neighbouringparticle in the film (Fig. 7.)

If we assume hexagonally close packed order in the film, thevolume fraction of the particles in the film (fp) can be expressedwith the following equation:

fp = kd3�

3√

3D2H(7)

The effective refractive index of the film (neff) can be determinedwith Lorentz–Lorenz equation:

n2eff − 1

n2eff + 2

= fpn2

p − 1

n2p + 2

(8)

The reflection intensity (IR) is proportional to the followingexpression (the details of the deduction were published previously[24]):

2

[2 ′ 2 4�neffH cos ˇ ′2 2 2

]

where a is the amplitude of the incident ray, r and r′ are ampli-tudes of reflections on the air/film and film/substrate interfaces, ˇ

86 N. Ábrahám et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 80– 89

FLfi

ia

r

r

otFlslavtfi

Table 4Film thickness (H) and particle volume fraction (fp) values resulted from the opticalcalculations at different layer numbers (k).

k 1 2 3 4 5

H (nm) 234 409 586 764 940fP 0.4475 0.5202 0.5562 0.5735 0.5822

TRi

ig. 8. (a) Measured and fitted reflectance spectra of multilayeredangmuir–Blodgett films of 234 nm ZnO particles, (b) effective refractive indices oflms resulted from optical fitting.

s the angle of refraction and � is the wavelength of the light. Themplitudes of reflections can be calculated [24]:

=(

neff − nair

neff + nair

)2(10)

′ =(

ns − neff

ns + neff

)2(11)

Results of optical interference calculation of multilayered filmsf 234 nm ZnO particles are demonstrated in Fig. 8. Fig. 8a showshe measured and the corresponding fitted reflectance spectra andig. 8b includes effective refractive indices as a function of wave-ength at different layer numbers. The calculated spectra for theingle layer has good fit for the experimental curve in a wide wave-ength region (∼400–850 nm). The lower limit of wavelength is

round 400 nm, because ZnO has increasing absorbance below thisalue. For the double layer it is also a wide wavelength region wherehe fitting is appropriate (∼400–800 nm). Calculations in case oflms consisting of three or more layers have accepable results in

able 5esults of photonic band gap calculation: first order photonic band gap (�max,1), second ord

ndex of ZnO (nZnO, bulk), porosity of particles (p), volume fraction of particles in the film (

d (nm) �max,l

(nm)�max,2

(nm)neff nZnO, bulk

(at �max

301 mn 719 408 1.355 2.000

349 mn 820 460 1.318 1.966

457 mn – 598 1.309 1.915

Fig. 9. Transmittance spectra of single layer Langmuir–Blodgett films of 301 nm,349 nm and 457 nm ZnO particles.

a much more narrow interval (∼430–570 nm). Accordingly, onlythe results regarding these wavelength intervals can be accepted.The effective refractive indices show an increasing tendency withincreasing layer numbers. The explanation of this is the increas-ing volume fraction of ZnO particles in the film: particles sit in thegaps of the former layer. Film thickness (H) and volume fraction (fp)values are collected in Table 4. Our model gives excellent resultsfor single and double layers, but less convenient for higher layernumbers. More reasons can be responsible for it: the first and mostserious factor is that the absorbance of the film is enhanced withincreasing layer numbers due to the increasing amount of ZnO. Onthe other hand the small defects in the film structure accumulateat increased layer numbers and results in less ordered structureswhich cannot be described perfectly with our model. At this pointwe should mention that the disadvantage of the incomplete orderof the particles can be probably overcome with other techniques.Self-assemby of the particles with other methods (for example theconvective self-assembly [18]) could result in almost perfect crys-tals if hitting on the optimal parameters of the film formation. Itwould be interesting to test our model for such a well-orderedcrystal. Considering all the observations we can conclude that theapplied model describes the samples well, the calculations particu-larly for lower layer numbers engage properly for the experimentalresults.

Single layer Langmuir–Blodgett films of 301 nm, 349 nm and452 nm ZnO particles were characterized by UV–vis transmittancemeasurement (Fig. 9). First and second order photonic band gap(PBG) positions of the different sized ZnO samples are marked in

er photonic band gap (�max,2), effective refractive index of film (neff), bulk refractivefp) and interparticle distance in the monolayer (D).

,2)p (%) np fp D (nm)

36.72 1.633 0.563 31239.18 1.688 0.541 36935.13 1.594 0.522 492

: Physicochem. Eng. Aspects 384 (2011) 80– 89 87

tlib[

�

�

wstle(liwT

otbfwt

n

wtmc

n

spetpi

pptcaUspsavisptatfrstnic

N. Ábrahám et al. / Colloids and Surfaces A

he figure and listed in Table 5. The first order PBG is at longer wave-engths than 900 nm in case of 457 nm particles, so it cannot be seenn the spectra. Theoretically expected wavelengths of the potonicand gaps of the films can be calculated by two simple equations26]:

max,1 = 2 Lneff (12)

max,2 = Lneff (13)

here �max,1 and �max,2 are the first and second order photonictop band positions, L is the lattice parameter and neff is the effec-ive refractive index of the film. The lattice parameter for singleayers is the thickness of the film. In our case the film thickness isqual to the particle diameter. It can be seen from Eqs. (12) and13) that the wavelength of the first order PBG is twice the wave-ength of the second order PBG. Transmittance spectra of our filmsndicate different relationship, the first order PBG appears at lower

avelengths than expected from the value of the second order PBG.he reason for this is not yet clear.

Effective refractive indices of films were calculated from secondrder photonic band gap positions (these neff values correspond tohe wavelength of the second order maxima). Several methods cane found in the literature for the calculation of refractive indices,or photonic crystal materials it is often used to be calculated as theeighted average of the compounds of the film, and the weight is

he volume fraction [26]:

eff = fpnp + fairnair (14)

here fp and np are the volume fraction and refractive index ofhe particles, fair and nair are the same parameters for the air. This

ethod can be extended to calculate the refractive index of parti-les.

p = fZnOnZnO + fairnair (15)

Table 5 contains the bulk refractive indices of ZnO at the corre-ponding wavelengths (�max,2), porosity of particles (p), calculatedarticle refractive indices (with Eqs. (5) and (15)). At this pointffective refractive indices resulted from the measured transmit-ance spectra can be used to determine the volume fraction of thearticles (Eq. (14)) and the interparticle distance (Eq. (7), consider-

ng that H = d) in the film. Table 5 contains the results.Photoluminescence (PL) properties of the different sized ZnO

articles were studied by measuring the emision of the quartz sup-orted single layer Langmuir–Blodgett films. Fig. 10a demonstrateshe recorded emission intensities as a function of the wavelength inase of 350 nm excitation source. A strong and wide visible emissionppears around 570 nm, and a weak, sharp UV emission at 390 nm.sualy nanostructured ZnO materials exhibit UV and visible emis-

ion. The UV emission is due to the radiative recombination ofhotogenerated excitons, the origin of the visible part of the emis-ion is not completely understood yet, it is related to surface statesnd defects in the crystal structure (mostly connected with oxigenacancies) [27,28]. The weak, sharp UV emission appears at 390 nmn case of all samples. The position of the visible emission peak ishifted for different samples. Since our particles are composed ofrimary crystallites, it is evident to look for a relationship betweenhe primary crystallite size and the PL emision features (wavelengthnd intensity). Figs. 10b and c show the wavelength and intensity ofhe visible emission as a function of pimary cristallite size (obtainedrom XRD). We obtained that increasing primary crystallite sizeesults in red shift in the emission maxima and decrease in emis-ion intensity, which agrees well with earlier results [29]. We have

o consider the effect of the photonic structure on the photolumi-escence emission. It is known that photonic stop band lowers the

ntensity of the emission if the wavelengths are overlapping. In ourase all the emissions are positioned around 570 nm. This emission

Fig. 10. (a) PL spectra of quartz supported single layer Langmuir–Blodgett films ofZnO particles with various sizes, dependence of (b) PL emission wavelength and (c)intensity on primary crystallite size (the solid lines are guide to the eyes).

cannot be compared with he PBG wavelengths determined fromFig. 9, because these values are obtained at � = 0◦ incident angle (�is the angle to the surface normal). Since photoluminescence emis-sion has angular dependence in case of photonic crystals [30] wehave to calculate the photonic band gap corresponding to � = 60◦

with the following equations [30]:√

�max,2 = L n2

eff − sin2 � (16)

�max,1 = 2L

√n2

eff − sin2 � (17)

8 : Phys

wfiagsfitiaab

4

iocdasNfiwprmormo

A

HPtbF

A

s

eoa

I

wbi

�

wisf

�

[

[

[

[

[

8 N. Ábrahám et al. / Colloids and Surfaces A

here � is the angle to the surface normal. As a result for � = 60◦ therst order band gaps for 301 nm, 349 nm and 457 nm ZnO particlesre respectively 594 nm, 651 nm and 842 nm, the second order bandaps are 297 nm, 326 nm and 421 nm. It is clear that none of theecond order band gaps overlap with the PL emission. Only therst order band gap of the film of 301 nm ZnO particles is closeo the emission maxima and this sample has the lowest emissionntensity. We suppose that this is mainly due to particle size effectnd not the effect of the PBG, because the first order stop bandsre much weaker and wider dispersed than the second order stopands.

. Conclusions

We have synthesized monodispersed, spherical ZnO particlesn autoclave in diethylene glycol media adapting the methodf Jezequel et al. [11]. The size and size-distribution of parti-les was determined from TEM images, surface structure wasemonstrated with SEM and TEM pictures, particle features suchs primary cristallite size, specific surface area, porosity, den-ity and several SAXS parameters were determined by XRD,2 adsorption–desorption and SAXS measurements. Langmuir-lms at the air water interface were prepared and characterizedith surface pressure and surface area isotherms. Solid sup-orted Langmuir–Blodgett films were prepared and characterizedegarding optical interference, photonic band gap and photolu-inescence. We have improved our former optical model (based

n interference on plane parallel plates) for the calculation ofeflectance intensity as a function of wavelength. Fitting for theeasured spectra provided effective refractive index and thickness

f films.

cknowledgement

The authors are very thankfull for the finantial support of theungarian Scientific Fund (OTKA) Nr. K 73307 and NK 73672. Theroject named ÁMOP-4.2.1/B-09/1/KONV-2010-0005 – Creatinghe Centre of Excellence at the University of Szeged” is supportedy the European Union and co-financed by the European Regionalund.

ppendix A.

Calculation methods for small angle X-ray scattering by disperseystems [31–34]:

When X-rays are scattered by colloidal particles due to differ-nces in electron density caused by inhomogeneities, the intensityf scattered radiation is a function of the angle of scattering (�)nd the scattering vector (h):

(h) = �2(0)V

∫ ∞

0

4�r2�0(r)sin hr

hrdr (18)

here V is the volume of the system in which X-rays are scatteredy electrons. In the above equation the value of �2(0) can be defined

n the following way [31–33]:

2(0) = 1V

∫ ∞

0

(�e(r) − �e)2d3r (19)

here �e(r) is the local electron density at a given point (r) and �e

s the average electron density. When the concept of electron den-ity fluctuation is introduced: �(r) = �e(r) − �e, then the correlation

unction in Eq. (18) can be given:

0(r) = �2(r)�2(0)

(20)[

icochem. Eng. Aspects 384 (2011) 80– 89

The correlation function contains significant information on thegeometry and structural arrangement of the scattering particles.

The following relationship holds for the tailing region of the scat-tering function, where hR > 1 (the so-called Porod-range) [34–39]:

I(h) = �2(0) 2 �S

h4(21)

where S is the surface area of the particles. The relative specificsurface area of the particles (relative to unit volume V) is [33–35]:

S

V= �

lim I(h) h4

h→∞Q

= �Kp

Q(22)

where Kp is the tail end constant.The specific surface area is:

Sp = S/V × 103

d(23)

where d (g cm−3) is the density of the disperse system.Correlation length can also be calculated directly from the scat-

tering function, in case if the following integrations are known[36–39]:

lc = �

∫ ∞0

I(h)hdh

Q(24)

From the middle section of the scattering curve the fractaldimension of the particles can be determined [36] in the case ifthe scattering curve is linear in the relatively wide range of h: log I(h) = p log h. From the value of the tangent p, the surface fractal Ds

is calculated by the relationship Ds = p + 5, the value of which has tofall within the range of 2 ≤ Ds ≤ 3.

References

[1] J. Nemeth, G. Rodriguez-Gattorno, D. Diaz, A.R. Vázquez-Olmos, I. Dékány, Syn-thesis of ZnO Nanoparticles on a clay mineral surface in dimethyl sulfoxidemedium, Langmuir 20 (2004) 2855–2860.

[2] E. Pál, V. Hornok, A. Oszkó, I. Dékány, Hydrothermal synthesis of prism-likeand flower-like ZnO and indium-doped ZnO structures, Colloid Surf A: Physic-ochem. Eng. Aspects 340 (2009) 1–9.

[3] I. Dékány, L. Túri, Á. Szucs, Z. Király, Preparation of semiconductor and transitionmetal nanoparticles on colloidal solid supports, Colloid Surf A: Physicochem.Eng. Aspects 141 (1998) 405–417.

[4] A. Chittofrati, E. Matijevic, Uniform particles of zinc-oxide of different mor-phologies, Colloid Surf. 48 (1990) 65–78.

[5] Q.P. Zhong, E. Matijevic, Preparation of uniform zinc oxide colloids by controlleddouble-jet precipitation, J. Mater. Chem. 6 (1996) 443–447.

[6] H. Jiang, J.Q. Hu, F. Gu, C.Z. Li, Large-scaled, uniform, monodispersed ZnO col-loidal microspheres, J. Phys. Chem. C 112 (2008) 12138–12141.

[7] I. Masashi, S. Keigo, K. Keisuke, T. Hiroshi, Y. Sakabe, Monodispersed and well-crystallized zinc oxide nanoparticles fabricated by microemulsion method, J.Am. Chem. Soc. 91 (2008) 3850–3855.

[8] C. Feldmann, Polyol-mediated synthesis of nanoscale functional materials,Solid State Sci. 7 (2005) 868–873.

[9] C. Feldmann, H.-O. Jungk, Polyol-mediated preparation of nanoscale oxide par-ticles, Angew. Chem. Int. Ed. 40 (2001) 359–362.

10] Fine particles, synthesis, characterization and mechanisms of growth, in: T.Sugimoto (Ed.), Surfactant Science Series, 92, Marcel Dekker Inc, New York,2000, pp. 359–362.

11] D. Jezequel, J. Guenot, N. Jouini, F. Fivet, Preparation and morphological char-acterizaton of fine, spherical, monodisperse particles of ZnO, Mater. Sci. Forum152–153 (1994) 339–342.

12] E.W. Seelig, B. Tang, A. Yamilov, H. Cao, R.P.H. Chang, Self-assembled 3D pho-tonic crystals from ZnO colloidal spheres, Materials Cemistry and Physics 80(2003) 257–263.

13] Y.Y. Tay, S. Li, F. Boey, Y.H. Cheng, M.H. Liang, Growth mechanism of sphericalZnO nanostructures synthesized via colloid chemistry, Physica B 394 (2007)372–376.

14] (a) S. Noda, Two- and three-dimensional photonic crystals in III–V semicon-ductors, MRS Bull. 26 (2001) 618–621;

(b) S.Y. Lin, J.G. Fleming, E. Chow, Two- and three-dimensional photonic crystalsbuilt with VLSI tools, MRS Bull. 26 (2001) 627–631.

15] T. Kondo, S. Matsuo, S. Juodkazis, H. Misawa, Femtosecond laser interferencetechnique with diffractive beam splitter for fabrication of three-dimensionalphotonic crystals, Appl. Phys. Lett. 79 (2001) 725–727.

: Phy

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

N. Ábrahám et al. / Colloids and Surfaces A

16] H. Wang, K.P. Yan, J. Xie, M. Duan, Fabrication of ZnO colloidal photonic crystalby spin-coating method, Mater. Sci. Semicond. Process. 11 (2008) 44–47.

17] E. Pál, A. Oszkó, P. Mela, M. Möller, I. Dékány, Preparation of hexagonally alignedinorganic nanoparticles from diblock copolymer micellar systems, Colloid Surf.A 331 (2008) 213–219.

18] P. Jiang, J.F. Bertone, K.S. Hwang, V.L. Colvin, Single-crystal colloidal multilayersof controlled thickness, Chem. Mater. 11 (1999) 2132–2140.

19] M. Szekeres, O. Kamalin, P.G. Grobet, R.A. Schoonheydt, K. Wostyn, K. Clays, A.Persoons, I. Dékány, Two-dimensional ordering of Stöber silica particles at theair/water interface, Colloid Surf. A 227 (2003) 77–83.

20] S. Reculusa, S. Ravaine, Colloidal photonic crystals obtained by theLangmuir–Blodgett technique, Appl. Surf. Sci. 246 (2005) 409–414.

21] L. Naszályi Nagy, N. Ábrahám, E. Hild, D. Cot, A. Ayral, Z. Hórvölgyi, ComplexLangmuir–Blodgett films of SiO2 and ZnO nanoparticles with advantageousoptical and photocatalytical properties, Langmuir 24 (2008) 12575–12580.

22] L. Naszályi Nagy, N. Ábrahám, E. Hild, A.L. Kovács, A. van der Lee, V. Rouessac,D. Cot, A. Ayral, Z. Hórvölgyi, Zinc oxide LB films with improved antireflective,photocatalytic and mechanical properties, Progr. Coll. Polym. Sci. 135 (2008)107–118.

23] S Ashtaputre, S. Marathe, S. Kulkarni, Doughnut-shaped zinc oxide particles, J.Mater. Sci. 42 (2007) 9990–9994.

24] D. Sebok, T. Szabó, I. Dékány, Optical properties of zinc peroxide and zinc oxidemultilayer nanohybrid films, Appl. Surf. Sci. 255 (2009) 6953–6962.

25] E. Hild, A. Deák, L. Naszályi, Ö. Sepsi, N. Ábrahám, Z. Hórvölgyi, Use of the opticaladmittance function to stimulate and evaluate transmittance spectra of graded-index colloidal films, J. Opt. A: Pure Appl. Opt. 9 (2007) 920–930.

26] M. Szekeres, O. Kamalin, R.A. Schoonheydt, K. Wostyn, K. Clays, A. Persoons,I. Dékány, Ordering and optical properties of monolayers and multilayers of

silica spheres deposited by the Langmuir–Blodgett method, J. Mater. Chem. 12(2002) 3268–3274.

27] L. Zhang, L. Yin, C. Wang, N. lun, Y. Qi, D. Xiang, Origin of visible photolumi-nescence of ZnO quantum dots: defect-dependent and size-dependent, J. Phys.Chem. C 114 (2010) 9651–9658.

[

sicochem. Eng. Aspects 384 (2011) 80– 89 89

28] N. Ábrahám, I. Dékány, Size-dependent photoluminescence properties ofbare ZnO and polyethylene imine stabilized ZnO nanoparticles and theirLangmuir–Blodgett films, Colloids Surf. A: Physicochem. Eng. Aspects 364(2010) 26–33.

29] L. Irimpan, V.P.N. Nampoori, P. Radhakrishnan, Size dependent fluorescencespectroscopy of nanocolloids of ZnO, J. Appl. Phys. 102 (2007) 063524.

30] A.N. Gruzintsev, G.A. Emelchenko, V.M. Masalov, Photoluminescence of ZnOinfiltrated into a three-dimensional photonic crystal, Semiconductors 43(2009) 1017–1022.

31] O. Glatter, O. Kratky, Small-Angle X-ray Scattering, Academic Press, New York,1982.

32] A. Guinier, G. Fournet, Small-Angle Scattering of X-rays, J Wiley, New York,1955.

33] G. Porod, Die Röntgenkleinwinkelstreuung von dichtgepackten kolloiden Sys-temen I. Teil (X-ray low angle scattering of dense colloid systems. Part I),Kolloid-Zeitschrift 124 (1951) 83–114.

34] O. Kratky, Diffuse Röntgenkleinwinkelstreuung Bestimmung von Größe undGestalt von Kolloidteilchen und Makromolekülen, Angew Chem. 72 (1960)467–482.

35] O. Kratky, P. Laggner, X-Ray Small-Angle Scattering, Encyclopedia Phy. Sci.Technol. 14 (1987) 693.

36] P.W. Schmidt, D. Avnir, D. Levy, A. Hohr, Small-angle x-ray scatter-ing from the surfaces of reversed-phase silicas: power-law scatteringexponents of magnitudes greater than four, J. Chem. Phys. 94 (1991)1474–1479.

37] J.A. Hurd, D.W. Schaefer, J.E. Martin, Surface and mass fractals in vapor-phaseaggregates, Phys. Rev. A 35 (1987) 2361–2364.

38] P. Zipper, A. Janosi, E. Wrentschur, P.A. Abuja, Small-angle and wide-angle X-ray

scattering investigations of injection-molded polypropylene, J. Appl. Crystallo-grap. 24 (1991) 702–708.

39] M. Kriechbaum, G. Degovics, J. Tritthardt, Fractal structure of Portland cementpaste during age hardening analyzed by small-angle X-ray scattering, Prog.Colloid Polim. Sci. 79 (1989) 101–105.