Bouvy, Claire

Marine, Wladimir

Sporken, Robert

Su, Bao-Lian

English

Bouvy, C.

Marine, W.

Sporken, R.

Su, B.L.

HAL AMU

Photoluminescence properties and quantum size effect of ZnO nanoparticles confined inside a faujasite X zeolite matrix

Repository of the University of Namur

RESEARCH OUTPUTS / RÉSULTATS DE RECHERCHEAuthor(s) - Auteur(s) :Publication date - Date de publication :Permanent link - Permalien :Rights / License - Licence de droit d’auteur :Bibliothèque Universitaire Moretus PlantinInstitutional Repository - Research PortalDépôt Institutionnel - Portail de la Rechercheresearchportal.unamur.bePhotoluminescence properties and quantum size effect of ZnO nanoparticles confinedinside a faujasite X zeolite matrixBouvy, Claire; Marine, Wladimir; Sporken, Robert; Su, Bao-LianPublished in:Chemical Physics LettersPublication date:2006Document VersionPublisher's PDF, also known as Version of recordLink to publicationCitation for pulished version (HARVARD):Bouvy, C, Marine, W, Sporken, R & Su, B-L 2006, 'Photoluminescence properties and quantum size effect ofZnO nanoparticles confined inside a faujasite X zeolite matrix' Chemical Physics Letters, vol. 428, no. 4-6, pp.312-316.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 21. May. 2019www.elsevier.com/locate/cplettChemical Physics Letters 428 (2006) 312–316Photoluminescence properties and quantum size eﬀect ofZnO nanoparticles conﬁned inside a faujasite X zeolite matrixC. Bouvy a,1, W. Marine b, R. Sporken c, B.L. Su a,*a Laboratoire de Chimie des Mate´riaux Inorganiques (CMI), The University of Namur (FUNDP), Rue de Bruxelles 61, B-5000 Namur, Belgiumb CRMCN, UMR CNRS 6631, De´partement de Physique, Case 901, Faculte´ des Sciences de Luminy, Universite´ de la Me´diterrane´e,F-13288 Marseille, Cedex 9, Francec Laboratoire de Physique des Mate´riaux Electroniques (LPME), The University of Namur (FUNDP), Rue de Bruxelles 61, B-5000 Namur, BelgiumReceived 30 March 2006; in ﬁnal form 26 June 2006Available online 5 July 2006AbstractZnO nanoparticles were grown inside a faujasite matrix by wet chemistry method. The structural and textural results by N2 adsorp-tion, X-ray diﬀraction, 29Si and 27Al solid state NMR indicated successful incorporation of ZnO nanoparticles inside supercages of zeo-lite. After the incorporation, a slight structural symmetry change of the zeolite occurred. The photoluminescence of ZnO nanoparticles inzeolite matrix was studied by exciting the sample with an ArF laser (193 nm). A strong blue-shift of the emission band of ZnO, suggestinga strong quantum size eﬀect, is observed. It is also revealed an improved stability of excitons when ZnO nanoparticles are conﬁned withina zeolite matrix. 2006 Elsevier B.V. All rights reserved.1. IntroductionThe preparation of light-emitting nanostructures basedon semiconducting materials of nanometer scale hasrecently received considerable attention because of theunique optoelectronic properties appearing in this sizerange. In particular, Zinc Oxide, a wide band-gap II–VIsemiconductor (gap = 3.37 eV, 298 K), shows signiﬁcantquantum conﬁnement eﬀect (QSE) when its size reachesthe Bohr radius, ca. 1.8 nm.Moreover, ZnO is an interestingmaterial due to its many applications such as varistors [1,2],gas sensors [3,4], ceramics [5], electrical and optical devices[6,7] and exhibits photoluminescence gain and lasing eﬀect.However, the threshold for lasing was very high [8] and onlylow-dimensional nanostructures can facilitate lasing.Various methods have been employed to prepare ZnOnanoparticles with extremely small diameters including0009-2614/$ - see front matter  2006 Elsevier B.V. All rights reserved.doi:10.1016/j.cplett.2006.06.106* Corresponding author. Fax: +32 81 72 54 14.E-mail address: bao-lian.su@fundp.ac.be (B.L. Su).1 FRIA fellow (Fonds National de la Recherche Scienctiﬁque, rued’Egmont 5, B-1000 Bruxelles, Belgium).precipitation of colloids in solution [9], sol–gel methods[10], thermal decomposition methods [11], pulsed laserdeposition (PLD) [12] and metal-organic chemical vapourdeposition (MOCVD) [13]. However, in general it is verydiﬃcult to avoid the aggregation of ZnO nanoparticles dur-ing the preparation. This work aims to resolve this problemvia the growth of ZnO nanoparticles inside the channels ofa porous matrix. The faujasite X zeolite, with 3D channelsconnected by supercages of 1.3 nm in diameter, wasemployed for the conception of ZnO/faujasite nanocom-posites. The photoluminescence properties of the nano-composites have been evaluated and the optical characterof the nanocomposites has been correlated with the particlesize of ZnO.2. Experimental2.1. Nanocomposite preparationFaujasite X zeolite was synthesized by stirring a sodiumaluminate solution with a mixture of NaOH and KOH dis-solved in distilled water. After homogenisation, an aqueous0.0 0.2 0.4 0.6 0.8 1.0050100150200(b)(a)Adsorbed volume (cm³/g-STP)Relative pressure p/p0Fig. 1. Nitrogen adsorption–desorption isotherms of: (a) faujasiteX matrix (b) ZnO/FAU nanocomoposite.C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316 313sodium silicate solution was rapidly added to the mixtureunder strong agitation in a molar ratio SiO2/Al2O3 = 2.2.The resulting gel was aged at 70 C for 24 h before washingand drying. On the basis of the chemical analysis made byAtomic absorption spectrometry, the as-prepared zeolite,labelled here FAU, had a Si/Al ratio of 1 with a chemicalcomposition of Na62K34Si96Al96O192.The incorporation of ZnO was performed by impregna-tion of 0.5 g FAU into 20 ml of a 1 M Zn(NO3)2 aqueoussolution over 20 min. The powder was separated by centri-fugation, was dried and calcined under ﬂowing O2 for 6 hat 550 C. The obtained powder is labelled ZnO/FAU.The chemical analysis gives 36.2 wt% of ZnO impregnatedin faujasite zeolite, i.e. 0.362 g of ZnO for 1 g of zeolite.2.2. CharacterizationNitrogen adsorption–desorption isotherms were mea-sured at 77 K on a Micromeritics ASAP 2010 instrument.The samples were ﬁrst degassed under vacuum at 593 Kfor several hours. The Horvath–Kawazoe [14] methodwas used for pore size estimation. The surface areas weredetermined using the BET equation in the low pressureregion (0.05 6 p/p0 6 0.25). X-ray diﬀraction (XRD) pat-terns were recorded on a Philips PW1820 diﬀractometerusing Cu Ka radiation with wavelength of 1.54178 A˚. Solidstate magic angle spinning (MAS)-NMR spectra wereobtained with a Bruker MSL 500 spectrometer for 29Si(at 99.25 MHz) and a Bruker MSL 400 spectrometer for27Al (at 104.3 MHz). Photoluminescence spectra wererecorded at room temperature with an ArF laser(193 nm) as the excitation source and an iCCD camera asthe detector. The laser beam was focused onto the samplewith a spot size of 1.36 · 102 cm2. For the time-resolvedluminescence spectra, the excitation was made by theArF laser with a repetition rate of 5 Hz and pulse durationof 8 ns. The iCCD camera gate width was 5 ns and thedecay curve was measured from the maximum intensityof the ZnO PL band until its extinction at roomtemperature.3. Results and discussionNitrogen adsorption–desorption results (Table 1 andFig. 1) indicate that microporosity is well maintained afterincorporation of ZnO and the decrease in speciﬁc area con-ﬁrms the introduction of ZnO inside the channels. At thesame time, the mean pore diameter is not aﬀected by thepresence of ZnO nanoparticles inside the structure.Table 1Textural properties of the FAU X zeolite and the ZnO/FAU nanocompositeSpeciﬁc area (m2/g) Pore volume (cm3/g) Pore diamFAU 616 0.32 5.0ZnO/FAU 497 0.27 5.0Variation (%) 19 16As the decrease in microporous area is more signiﬁcantthan the total speciﬁc area (including the external surface)it is possible that the majority of the nanoparticles are con-ﬁned within the structure of the zeolite. On the other hand,both samples exhibit a hysteresis loop at relatively high rel-ative pressure due to capillary condensation eﬀects, gener-ally associated with the creation of mesopores [15,16]. Thepresence of the mesoporosity can lead to the formation ofZnO nanoparticles larger than the size of supercages(1.3 nm).Powder X-ray diﬀraction patterns of the parent zeoliteFAU and the ZnO/FAU nanocomposite are shown inFig. 2. For a better understanding, the XRD pattern ofcommercial ZnO is also shown. It can clearly be seen thatthe pattern of FAU and ZnO/FAU are somehow diﬀerentthus indicating some changes in the crystalline structure ofFAU. Change of the relative peaks intensity and small shiftof the diﬀraction angles to the higher values indicate thecompression of the FAU crystalline structure in the pres-ence of ZnO. It is observed that the ﬁrst diﬀraction line(111) at 6.15 (2h) is sharply increased in intensity whilesome other diﬀraction lines such as (220) at 10.11 (2h),(311) at 11.86 (2h) decrease signiﬁcantly their intensityafter incorporation of ZnO nanoparticles. This phenome-non is often observed in Na form faujasite zeolites afterion exchange with bivalent cations like Ca2+ since theintroduction of bivalent Ca2+ ions to replace monovalentNa+ ions (one Ca2+ for Na+ ions) will change the symme-try of crystalline structure from Fd3 (Na–FAU) to Fd3m(NaCa–FAU) [17]. This indicates that the formation ofeter (A˚) Microporous area (m2/g) Microporous volume (cm3/g)594 0.30451 0.2227 2410 20 30 40 50 60(c)(b)(a)Intensity (arb. units)2 theta (˚)Fig. 2. Powder XRD patterns of: (a) faujasite X zeolite matrix, (b) ZnO–FAU, and (c) commercial ZnO.314 C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316ZnO nanoparticles in faujasite zeolite will change the sym-metry of the system. It is also possible that ion exchangecan occur and some Zn2+ ions replace Na+ ions to playthe role of the compensating ions.However no ZnO peaks can be observed in the XRDpattern of ZnO/FAU thus meaning that no big particleswere formed both inside and outside the zeolite frameworkduring the growth process in conﬁrmation with the nitro-gen adsorption–desorption results. In order to gain a dee-per understanding of the structural changes observedwithin the faujasite crystallites after the incorporation ofZnO, 29Si and 27Al NMR were performed (Fig. 3 and Table2).For the 29Si NMR spectra the signal at 84 ppm wasassigned to Si atoms with 4 Al neighbours while for the27Al NMR spectra, the resonance centred around 60 ppmrepresents tetrahedrally coordinated Al. In both cases,except for the small shift of the signal position, an impor-tant broadening of the bands is observed. This broadeningcan be explained by the interaction created between theZnO nanoparticles and the FAU internal surface and alsoby the symmetrical changes observed in the XRD patterns.These observations led to a preliminary conclusion thatduring the growth of ZnO nanoparticles inside the fauja-site, textural properties were conserved and that somesymmetrical changes did occur but without the collapseof the zeolite framework. From this, it can be aﬃrmed thatthe growth of the nanoparticles has been well limited by thezeolite walls.PL results at room temperature are presented in Fig. 4.First, the emission spectrum of the faujasite zeolite matrix(Fig. 4a) under excitation at 193 nm (6.4 eV) is composedof one broad band around 4.18 eV and one smaller bandat 3.26 eV. The origin of this weak photoluminescence isstill unknown owing to an absence of information on theoptical properties of zeolites within the literature. Theemission spectrum of the nanocomposite is presented inFig. 4b. A very broad PL band can be seen between 2.75and 4.75 eV. The contribution to the band by the matrixhas either vanished or is hidden by the very intense photo-luminescence of ZnO/FAU. Two Gaussian functions areobtained by ﬁtting the emission spectrum. The results aregiven in Table 3.The obtained values of 3.62 and 3.98 eV, which aremuch higher than that of bulky ZnO (3.37 eV), can beattributed to ZnO nanoparticles with two diﬀerent sizes.Using the equation based on the eﬀective mass approxima-tion given by Brus in 1984 [18] a particle size of 1.7 m (forthe PL at 3.98 eV) and 2.5 nm (for 3.62 eV) has been deter-mined. It is believed that these values are an approximationand do not correspond exactly with the real particle size.This inaccuracy could be due to the use of values that weredetermined for bulk ZnO and not for nanosized ZnO, suchas the eﬀective masses and the optical dielectric constant.However, on the basis of the results obtained, it is sug-gested that some particles have grown inside the supercagesand give rise to the strongest quantum size eﬀect whilstother particles have continued to grow by the expansionof zeolite framework, inducing a change in structural sym-metry and thus yielding a smaller QSE. It is also possiblethat these larger particles could be formed in mesoporesas shown by N2 adsorption isotherms the presence of mes-opore type hysteresis loop (Fig. 1).Fig. 5 illustrates a time-resolved photoluminescencespectrum of the emission of ZnO nanoparticles. The inten-sity of the luminescence band decreases with time. Thisdecay can be ﬁtted to the exponential curve (y = A.exp(t/s)), with s = 8.93 ns. This value is rather high in compar--50 -60 -70 -80 -90 -100 -110 -12001x1072x1073x107Intensity (arb. units)Chemical shift (ppm)150 125 100 75 50 25 0 -25 -500.05.0x1051.0x1061.5x1062.0x1062.5x106Intensity (arb. units)Chemical shift (ppm)abFig. 3. (a) 29Si NMR spectra of FAU (solid line) and ZnO–FAU (dashedline) (b) 27Al NMR spectra of FAU (solid line) and ZnO–FAU (dashedline).Table 2Chemical shifts and widths of the NMR bands of FAU X zeolite andZnO–FAU nanocompositeFAU 29Si (ppm) 27Al (ppm)Chemical shift FWHM Chemical shift FWHM84 0.95 63 5.5ZnO–FAU 86 4.5 60 (+39) 7.52.5 3.0 3.5 4.0 4.5 5.0 5.510001500200025003000350040004500PL intensity (arb. units)Photon energy (eV)2.5 3.0 3.5 4.0 4.5 5.0 5.501x1042x1043x1044x1045x1046x104PL intensity (arb. units)Photon energy (eV)baFig. 4. PL spectra of the FAU X zeolite (a) and the ZnO–FAUnanocomposite (b) (Eexc = 6.4 eV).Table 3Results of the GAUSSIAN decomposition of the PL spectrum of the ZnO/FAU nanocompositeWavelength (eV) FWHM (eV)Peak 1 3.62 0.72Peak 2 3.98 0.51C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316 315ison to those reported in the literature [19–21], indicatingthe improved stability of excitons when the ZnO nanopar-ticles are conﬁned within a zeolite matrix.4. ConclusionsZeolite framework is eﬃcientmatrix to control the growthof ZnO nanoparticles. Nitrogen adsorption–desorptionresults reveal no important changes in the textural propertiesof the zeolite after the incorporation process. The presence ofZnO in zeolite changes the structural symmetry. PL resultsshow an important blue-shift of the ZnO emission band,which is attributed to the quantum size eﬀect due to the sizeof the ZnO nanoparticles formed inside the zeolite. Animproved stability of excitons is obtained by the conﬁnementof ZnO nanoparticles within a microporous zeolite matrix.Our next step is to tailor and to homogenize the particle sizeof ZnO in diﬀerent porousmatrix to design the light-emittingsystems and laser generators.AcknowledgementsC.Bouvy thanks theFRIA (FondsNational de laRecher-che Scientiﬁque) for a doctoral fellowship. This work wasrealized in the frame of Belgian federal government project0 5 10 15 20 25 30 35 4005001000150020002500300035004000ZnO PL band intensity (arb. units)Time (ns.)Fig. 5. Time-resolved photoluminescence decay of the ZnO PL band(points represent the experimental data and the line the exponential ﬁt).316 C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316PAI/IUAP 5/01. Financial supports from the University ofNamur (FUNDP) and from the European SOXESS net-work were gratefully acknowledged.References[1] D.R. Clarke, Journal of American Ceramic Society 82 (1999) 485.[2] N.T. Hung, N.D. Quang, S. Bernik, Journal of Material Research 16(2001) 2817.[3] Y. Shimizu, F.C. Lin, Y. Takao, M. Egashira, Journal of AmericanCeramic Society 81 (1998) 1633.[4] R. Paneva, D. Gotchev, Sensors and Actuator A: Physical 72 (1999)79.[5] L. Gao, Q. Li, W.L. Luan, Journal of American Ceramic Society 85(2002) 1016.[6] B.D. Yao, H.Z. Shi, H.J. Bi, L.D. Zhang, Journal of Physics:Condensed Matter 12 (2000) 6265.[7] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Applied PhysicsLetters 78 (2001) 407.[8] J.C. Johnson, H. Yan, R.D. Schaller, L.H. Haber, R.J. Saykally, P.D.Yang, Journal of Physical Chemistry B 105 (2001) 11387.[9] U. Koch, A. Fojtik, H. Weller, A. Henglein, Chemical Physics Letters122 (1985) 507.[10] C. Cannas, M. Casu, A. Lai, A. Musinu, G. Piccaluga, Journal ofMaterials Chemistry 9 (1999) 1765.[11] L. Jing, Z. Xu, J. Shang, X. Sun, W. Cai, H. Guo, Materials Scienceand Engineering A 332 (2002) 356.[12] I. Ozerov, D. Nelson, A.V. Bulgakov, W. Marine, M. Sentis, AppliedSurface Science 9789 (2003) 1.[13] B. Hahn, G. Heindel, E. Pschoor-Schoberer, W. Gebhardt, Semicon-ductor Science and Technology 13 (1998) 788.[14] G. Hortvath, K.J. Kawazoe, Journal of Chemical Engineering ofJapan 16 (1983) 470.[15] J. Rathousky, A. Zukal, N. Jaeger, G. Sschulz-Ekloﬀ, Journal ofChemical Society, Faraday Transactions 88 (1992) 489.[16] M. Wark, H. Kessler, G. Schulz-Ekloﬀ, Microporous Materials 8(1997) 241.[17] M.M.J. Treacy, J.B. Higgins, Collections of Simulated XRD PowderPatterns for Zeolites, Elsevier, Amsterdam, 2001.[18] L.E. Brus, Journal of Chemical Physics 80 (1984) 4403.[19] T. Hirai, Y. Harada, S. Hashimoto, K. Edamatsu, T. Itoh, Journal ofLuminescence 94–95 (2001) 261.[20] B. Guo, Z. Ye, K.S. Wong, Journal of Crystal Growth 253 (2003)252.[21] T. Hirai, Y. Harada, S. Hashimoto, T. Itoh, N. Ohno, Journal ofLuminescence 112 (2005) 196.

RESEARCH OUTPUTS / RÉSULTATS DE RECHERCHEAuthor(s) - Auteur(s) :Publication date - Date de publication :Permanent link - Permalien :Rights / License - Licence de droit d’auteur :Bibliothèque Universitaire Moretus PlantinInstitutional Repository - Research PortalDépôt Institutionnel - Portail de la Rechercheresearchportal.unamur.bePhotoluminescence properties and quantum size effect of ZnO nanoparticles confinedinside a faujasite X zeolite matrixBouvy, Claire; Marine, Wladimir; Sporken, Robert; Su, Bao-LianPublished in:Chemical Physics LettersPublication date:2006Document VersionEarly version, also known as pre-printLink to publicationCitation for pulished version (HARVARD):Bouvy, C, Marine, W, Sporken, R & Su, B-L 2006, 'Photoluminescence properties and quantum size effect ofZnO nanoparticles confined inside a faujasite X zeolite matrix', Chemical Physics Letters, vol. 428, no. 4-6, pp.312-316.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 23. Jun. 2020www.elsevier.com/locate/cplettChemical Physics Letters 428 (2006) 312–316Photoluminescence properties and quantum size effect ofZnO nanoparticles confined inside a faujasite X zeolite matrixC. Bouvy a,1, W. Marine b, R. Sporken c, B.L. Su a,*a Laboratoire de Chimie des Mate´riaux Inorganiques (CMI), The University of Namur (FUNDP), Rue de Bruxelles 61, B-5000 Namur, Belgiumb CRMCN, UMR CNRS 6631, De´partement de Physique, Case 901, Faculte´ des Sciences de Luminy, Universite´ de la Me´diterrane´e,F-13288 Marseille, Cedex 9, Francec Laboratoire de Physique des Mate´riaux Electroniques (LPME), The University of Namur (FUNDP), Rue de Bruxelles 61, B-5000 Namur, BelgiumReceived 30 March 2006; in final form 26 June 2006Available online 5 July 2006AbstractZnO nanoparticles were grown inside a faujasite matrix by wet chemistry method. The structural and textural results by N2 adsorp-tion, X-ray diffraction, 29Si and 27Al solid state NMR indicated successful incorporation of ZnO nanoparticles inside supercages of zeo-lite. After the incorporation, a slight structural symmetry change of the zeolite occurred. The photoluminescence of ZnO nanoparticles inzeolite matrix was studied by exciting the sample with an ArF laser (193 nm). A strong blue-shift of the emission band of ZnO, suggestinga strong quantum size effect, is observed. It is also revealed an improved stability of excitons when ZnO nanoparticles are confined withina zeolite matrix. 2006 Elsevier B.V. All rights reserved.1. IntroductionThe preparation of light-emitting nanostructures basedon semiconducting materials of nanometer scale hasrecently received considerable attention because of theunique optoelectronic properties appearing in this sizerange. In particular, Zinc Oxide, a wide band-gap II–VIsemiconductor (gap = 3.37 eV, 298 K), shows significantquantum confinement effect (QSE) when its size reachesthe Bohr radius, ca. 1.8 nm.Moreover, ZnO is an interestingmaterial due to its many applications such as varistors [1,2],gas sensors [3,4], ceramics [5], electrical and optical devices[6,7] and exhibits photoluminescence gain and lasing effect.However, the threshold for lasing was very high [8] and onlylow-dimensional nanostructures can facilitate lasing.Various methods have been employed to prepare ZnOnanoparticles with extremely small diameters including0009-2614/$ - see front matter  2006 Elsevier B.V. All rights reserved.doi:10.1016/j.cplett.2006.06.106* Corresponding author. Fax: +32 81 72 54 14.E-mail address: bao-lian.su@fundp.ac.be (B.L. Su).1 FRIA fellow (Fonds National de la Recherche Scienctifique, rued’Egmont 5, B-1000 Bruxelles, Belgium).precipitation of colloids in solution [9], sol–gel methods[10], thermal decomposition methods [11], pulsed laserdeposition (PLD) [12] and metal-organic chemical vapourdeposition (MOCVD) [13]. However, in general it is verydifficult to avoid the aggregation of ZnO nanoparticles dur-ing the preparation. This work aims to resolve this problemvia the growth of ZnO nanoparticles inside the channels ofa porous matrix. The faujasite X zeolite, with 3D channelsconnected by supercages of 1.3 nm in diameter, wasemployed for the conception of ZnO/faujasite nanocom-posites. The photoluminescence properties of the nano-composites have been evaluated and the optical characterof the nanocomposites has been correlated with the particlesize of ZnO.2. Experimental2.1. Nanocomposite preparationFaujasite X zeolite was synthesized by stirring a sodiumaluminate solution with a mixture of NaOH and KOH dis-solved in distilled water. After homogenisation, an aqueous0.0 0.2 0.4 0.6 0.8 1.0050100150200(b)(a)Adsorbed volume (cm³/g-STP)Relative pressure p/p0Fig. 1. Nitrogen adsorption–desorption isotherms of: (a) faujasiteX matrix (b) ZnO/FAU nanocomoposite.C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316 313sodium silicate solution was rapidly added to the mixtureunder strong agitation in a molar ratio SiO2/Al2O3 = 2.2.The resulting gel was aged at 70 C for 24 h before washingand drying. On the basis of the chemical analysis made byAtomic absorption spectrometry, the as-prepared zeolite,labelled here FAU, had a Si/Al ratio of 1 with a chemicalcomposition of Na62K34Si96Al96O192.The incorporation of ZnO was performed by impregna-tion of 0.5 g FAU into 20 ml of a 1 M Zn(NO3)2 aqueoussolution over 20 min. The powder was separated by centri-fugation, was dried and calcined under flowing O2 for 6 hat 550 C. The obtained powder is labelled ZnO/FAU.The chemical analysis gives 36.2 wt% of ZnO impregnatedin faujasite zeolite, i.e. 0.362 g of ZnO for 1 g of zeolite.2.2. CharacterizationNitrogen adsorption–desorption isotherms were mea-sured at 77 K on a Micromeritics ASAP 2010 instrument.The samples were first degassed under vacuum at 593 Kfor several hours. The Horvath–Kawazoe [14] methodwas used for pore size estimation. The surface areas weredetermined using the BET equation in the low pressureregion (0.05 6 p/p0 6 0.25). X-ray diffraction (XRD) pat-terns were recorded on a Philips PW1820 diffractometerusing Cu Ka radiation with wavelength of 1.54178 A˚. Solidstate magic angle spinning (MAS)-NMR spectra wereobtained with a Bruker MSL 500 spectrometer for 29Si(at 99.25 MHz) and a Bruker MSL 400 spectrometer for27Al (at 104.3 MHz). Photoluminescence spectra wererecorded at room temperature with an ArF laser(193 nm) as the excitation source and an iCCD camera asthe detector. The laser beam was focused onto the samplewith a spot size of 1.36 · 102 cm2. For the time-resolvedluminescence spectra, the excitation was made by theArF laser with a repetition rate of 5 Hz and pulse durationof 8 ns. The iCCD camera gate width was 5 ns and thedecay curve was measured from the maximum intensityof the ZnO PL band until its extinction at roomtemperature.3. Results and discussionNitrogen adsorption–desorption results (Table 1 andFig. 1) indicate that microporosity is well maintained afterincorporation of ZnO and the decrease in specific area con-firms the introduction of ZnO inside the channels. At thesame time, the mean pore diameter is not affected by thepresence of ZnO nanoparticles inside the structure.Table 1Textural properties of the FAU X zeolite and the ZnO/FAU nanocompositeSpecific area (m2/g) Pore volume (cm3/g) Pore diamFAU 616 0.32 5.0ZnO/FAU 497 0.27 5.0Variation (%) 19 16As the decrease in microporous area is more significantthan the total specific area (including the external surface)it is possible that the majority of the nanoparticles are con-fined within the structure of the zeolite. On the other hand,both samples exhibit a hysteresis loop at relatively high rel-ative pressure due to capillary condensation effects, gener-ally associated with the creation of mesopores [15,16]. Thepresence of the mesoporosity can lead to the formation ofZnO nanoparticles larger than the size of supercages(1.3 nm).Powder X-ray diffraction patterns of the parent zeoliteFAU and the ZnO/FAU nanocomposite are shown inFig. 2. For a better understanding, the XRD pattern ofcommercial ZnO is also shown. It can clearly be seen thatthe pattern of FAU and ZnO/FAU are somehow differentthus indicating some changes in the crystalline structure ofFAU. Change of the relative peaks intensity and small shiftof the diffraction angles to the higher values indicate thecompression of the FAU crystalline structure in the pres-ence of ZnO. It is observed that the first diffraction line(111) at 6.15 (2h) is sharply increased in intensity whilesome other diffraction lines such as (220) at 10.11 (2h),(311) at 11.86 (2h) decrease significantly their intensityafter incorporation of ZnO nanoparticles. This phenome-non is often observed in Na form faujasite zeolites afterion exchange with bivalent cations like Ca2+ since theintroduction of bivalent Ca2+ ions to replace monovalentNa+ ions (one Ca2+ for Na+ ions) will change the symme-try of crystalline structure from Fd3 (Na–FAU) to Fd3m(NaCa–FAU) [17]. This indicates that the formation ofeter (A˚) Microporous area (m2/g) Microporous volume (cm3/g)594 0.30451 0.2227 2410 20 30 40 50 60(c)(b)(a)Intensity (arb. units)2 theta (˚)Fig. 2. Powder XRD patterns of: (a) faujasite X zeolite matrix, (b) ZnO–FAU, and (c) commercial ZnO.314 C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316ZnO nanoparticles in faujasite zeolite will change the sym-metry of the system. It is also possible that ion exchangecan occur and some Zn2+ ions replace Na+ ions to playthe role of the compensating ions.However no ZnO peaks can be observed in the XRDpattern of ZnO/FAU thus meaning that no big particleswere formed both inside and outside the zeolite frameworkduring the growth process in confirmation with the nitro-gen adsorption–desorption results. In order to gain a dee-per understanding of the structural changes observedwithin the faujasite crystallites after the incorporation ofZnO, 29Si and 27Al NMR were performed (Fig. 3 and Table2).For the 29Si NMR spectra the signal at 84 ppm wasassigned to Si atoms with 4 Al neighbours while for the27Al NMR spectra, the resonance centred around 60 ppmrepresents tetrahedrally coordinated Al. In both cases,except for the small shift of the signal position, an impor-tant broadening of the bands is observed. This broadeningcan be explained by the interaction created between theZnO nanoparticles and the FAU internal surface and alsoby the symmetrical changes observed in the XRD patterns.These observations led to a preliminary conclusion thatduring the growth of ZnO nanoparticles inside the fauja-site, textural properties were conserved and that somesymmetrical changes did occur but without the collapseof the zeolite framework. From this, it can be affirmed thatthe growth of the nanoparticles has been well limited by thezeolite walls.PL results at room temperature are presented in Fig. 4.First, the emission spectrum of the faujasite zeolite matrix(Fig. 4a) under excitation at 193 nm (6.4 eV) is composedof one broad band around 4.18 eV and one smaller bandat 3.26 eV. The origin of this weak photoluminescence isstill unknown owing to an absence of information on theoptical properties of zeolites within the literature. Theemission spectrum of the nanocomposite is presented inFig. 4b. A very broad PL band can be seen between 2.75and 4.75 eV. The contribution to the band by the matrixhas either vanished or is hidden by the very intense photo-luminescence of ZnO/FAU. Two Gaussian functions areobtained by fitting the emission spectrum. The results aregiven in Table 3.The obtained values of 3.62 and 3.98 eV, which aremuch higher than that of bulky ZnO (3.37 eV), can beattributed to ZnO nanoparticles with two different sizes.Using the equation based on the effective mass approxima-tion given by Brus in 1984 [18] a particle size of 1.7 m (forthe PL at 3.98 eV) and 2.5 nm (for 3.62 eV) has been deter-mined. It is believed that these values are an approximationand do not correspond exactly with the real particle size.This inaccuracy could be due to the use of values that weredetermined for bulk ZnO and not for nanosized ZnO, suchas the effective masses and the optical dielectric constant.However, on the basis of the results obtained, it is sug-gested that some particles have grown inside the supercagesand give rise to the strongest quantum size effect whilstother particles have continued to grow by the expansionof zeolite framework, inducing a change in structural sym-metry and thus yielding a smaller QSE. It is also possiblethat these larger particles could be formed in mesoporesas shown by N2 adsorption isotherms the presence of mes-opore type hysteresis loop (Fig. 1).Fig. 5 illustrates a time-resolved photoluminescencespectrum of the emission of ZnO nanoparticles. The inten-sity of the luminescence band decreases with time. Thisdecay can be fitted to the exponential curve (y = A.exp(t/s)), with s = 8.93 ns. This value is rather high in compar--50 -60 -70 -80 -90 -100 -110 -12001x1072x1073x107Intensity (arb. units)Chemical shift (ppm)150 125 100 75 50 25 0 -25 -500.05.0x1051.0x1061.5x1062.0x1062.5x106Intensity (arb. units)Chemical shift (ppm)abFig. 3. (a) 29Si NMR spectra of FAU (solid line) and ZnO–FAU (dashedline) (b) 27Al NMR spectra of FAU (solid line) and ZnO–FAU (dashedline).Table 2Chemical shifts and widths of the NMR bands of FAU X zeolite andZnO–FAU nanocompositeFAU 29Si (ppm) 27Al (ppm)Chemical shift FWHM Chemical shift FWHM84 0.95 63 5.5ZnO–FAU 86 4.5 60 (+39) 7.52.5 3.0 3.5 4.0 4.5 5.0 5.510001500200025003000350040004500PL intensity (arb. units)Photon energy (eV)2.5 3.0 3.5 4.0 4.5 5.0 5.501x1042x1043x1044x1045x1046x104PL intensity (arb. units)Photon energy (eV)baFig. 4. PL spectra of the FAU X zeolite (a) and the ZnO–FAUnanocomposite (b) (Eexc = 6.4 eV).Table 3Results of the GAUSSIAN decomposition of the PL spectrum of the ZnO/FAU nanocompositeWavelength (eV) FWHM (eV)Peak 1 3.62 0.72Peak 2 3.98 0.51C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316 315ison to those reported in the literature [19–21], indicatingthe improved stability of excitons when the ZnO nanopar-ticles are confined within a zeolite matrix.4. ConclusionsZeolite framework is efficientmatrix to control the growthof ZnO nanoparticles. Nitrogen adsorption–desorptionresults reveal no important changes in the textural propertiesof the zeolite after the incorporation process. The presence ofZnO in zeolite changes the structural symmetry. PL resultsshow an important blue-shift of the ZnO emission band,which is attributed to the quantum size effect due to the sizeof the ZnO nanoparticles formed inside the zeolite. Animproved stability of excitons is obtained by the confinementof ZnO nanoparticles within a microporous zeolite matrix.Our next step is to tailor and to homogenize the particle sizeof ZnO in different porousmatrix to design the light-emittingsystems and laser generators.AcknowledgementsC.Bouvy thanks theFRIA (FondsNational de laRecher-che Scientifique) for a doctoral fellowship. This work wasrealized in the frame of Belgian federal government project0 5 10 15 20 25 30 35 4005001000150020002500300035004000ZnO PL band intensity (arb. units)Time (ns.)Fig. 5. Time-resolved photoluminescence decay of the ZnO PL band(points represent the experimental data and the line the exponential fit).316 C. Bouvy et al. / Chemical Physics Letters 428 (2006) 312–316PAI/IUAP 5/01. Financial supports from the University ofNamur (FUNDP) and from the European SOXESS net-work were gratefully acknowledged.References[1] D.R. Clarke, Journal of American Ceramic Society 82 (1999) 485.[2] N.T. Hung, N.D. 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Photoluminescence properties and quantum size effect of ZnO nanoparticles confined inside a faujasite X zeolite matrix

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