Bouvy, Claire

Marine, Wladimir

Su, Bao-Lian

English

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.beZnO/mesoporous silica nanocomposites prepared by the reverse micelles and thecolloidal methods: photoluminescent properties and quantum size effectBouvy, Claire; Marine, Wladimir; Su, Bao-LianPublished in:Chemical Physics LettersPublication date:2007Document VersionPublisher's PDF, also known as Version of recordLink to publicationCitation for pulished version (HARVARD):Bouvy, C, Marine, W & Su, B-L 2007, 'ZnO/mesoporous silica nanocomposites prepared by the reverse micellesand the colloidal methods: photoluminescent properties and quantum size effect' Chemical Physics Letters, vol.438, pp. 67-71.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. 2019ARTICLE IN PRESSwww.elsevier.com/locate/cplettChemical Physics Letters xxx (2007) xxx–xxxZnO/mesoporous silica nanocomposites prepared by the reversemicelle and the colloidal methods: Photoluminescent propertiesand quantum size eﬀectClaire Bouvy a,1, Wladimir Marine b, Bao-Lian Su a,*a Laboratoire de Chimie des Mate´riaux Inorganiques (CMI), University of Namur (FUNDP), Rue de Bruxelles 61, B-5000 Namur, Belgiumb Centre de Recherche en Matie`re Condense´e et Nanosciences (CRMC-N), UPR 7251 CNRS, Faculte´ des Sciences de Luminy,Universite´ de la Me´diterrane´e, FranceReceived 26 January 2007; in ﬁnal form 14 February 2007AbstractQuantum size eﬀect was demonstrated in ZnO/mesoporous silica nanocomposites prepared by two diﬀerent routes: the reverse micellestrategy and the colloidal method owing to a ﬁne tuning of particles grown by the control of mesopore size. N2 adsorption–desorptionmeasurements, Atomic Absorption and Photoluminescent spectroscopy have evidenced the eﬃcient loading and growth of ZnO nano-particles in the channels of the mesoporous silica matrix. The present work illustrated that both methods are quite eﬃcient in the prep-aration of short wavelength (blue) laser nanocomposite. 2007 Published by Elsevier B.V.1. IntroductionOptoelectronic nanocomposites are a new class of mate-rials which exhibit very interesting and particular proper-ties and attract a growing attention due to their potentialapplications in information storage and optoelectronicdevices. This kind of nanocomposites are usually composedof two phases which are generally a semiconductor and aninorganic matrix [1]. The size of the semiconductor due tothe matrix eﬀect is in the nanometric size range resulting inquantum size eﬀect (QSE) which can lead to the enhance-ment of the band gap of the semiconductor [2]. Conse-quently, the emission wavelength of the semiconductor isblue-shifted, which is very relevant for the short wave-length design. However, the conception of optoelectronicnanocomposites still remains a great challenge since it isvery diﬃcult to obtain a particle size of semiconductor as0009-2614/$ - see front matter  2007 Published by Elsevier B.V.doi:10.1016/j.cplett.2007.02.061* 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 Scientiﬁque, 5 rued’Egmon, B-1000 Bruxelles, Belgium).Please cite this article in press as: C. Bouvy et al., Chem. Phys. Letdesired while preventing the aggregation of thenanoparticles.Very recently, we have demonstrated a very eﬃcientquantum size eﬀect and an important blue shift in the PLemission of ZnO/Faujasite X optoelectronic nanocompos-ites [3,4]. It was shown that the ZnO nanoparticles weregrown inside the 3D-channels of the Faujasite X withoutdestruction of the crystalline structure of the zeolite. How-ever, some modiﬁcations of the symmetry were observeddue to the eﬀective presence of the ZnO nanoparticles.Discovered in 1992 [5], silica mesoporous materials havesuccessfully been used in the conception of optoelectronicnanocomposites [6,7] and considered as very promisingmatrix because their channels, with diameter ranging from2 to 10 nm, can limit the size of the semiconductor nano-particles during their growth. In 2001, our laboratory hasreported the preparation of highly ordered silica mesopor-ous materials CMI-1 with hexagonal structure of the mes-ochannels [8] by employing a neutral surfactant whichallows a soft removal of the templating agent by simpleethanol extraction. This synthesis pathway is expected toprepare pure siliceous mesoporous materials with struc-t. (2007), doi:10.1016/j.cplett.2007.02.0612 C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxxARTICLE IN PRESStural and surface defects as less as possible. Preliminary PLstudies of CMI-1 mesoporous materials have clearly shownthe absence of the 1.9 eV PL band usually observed inamorphous SiO2 and associated with oxygen vacancies[9]. The absence of these defects is crucial for the prepara-tion of optoelectronic materials using silica as matrix sincethe optical transparency in visible light range is the primarycondition. This conﬁrmed our expectation that CMI-1mesoporous materials are ideal matrix for the preparationof optoelectronic nanocomposites.ZnO is of particular interest due to its large band gapvalue (3.37 eV) and its important exciton binding energy(60 meV) [10,11] which allows the excitonic emission tobe quite signiﬁcant even at room temperature (25 meV)[12]. Our recent studies have revealed for the ﬁrst time astrong two-photon random lasing eﬀect arising from thestimulated emission of ZnO nanoparticles conﬁned insidethe silica mesoporous CMI-1 matrix [13]. The mechanismof lasing implies the scattering of the stimulated emissionwithin the mesoporous matrix where active ZnO nanopar-ticles were grown [14].ZnS nanoparticles have also been incorporated intoCMI-1 matrix and the obtained nanocomposite has shownvery eﬃcient quantum size eﬀect resulting in an importantblue shift of the PL emission due to the nanometric size ofthe ZnS particles [15]. However, the distribution of ZnOand ZnS nanoparticle sizes is still relatively broad andthe improvement of the homodispersity could be achievedonly by the development of more eﬃcient methods to con-trol the nanoparticle growth during their incorporation.This Letter deals with two diﬀerent but original path-ways to load ZnO nanoparticles inside the channels of mes-oporous silica materials, the reverse micelle and thecolloidal method. The rapid growth of particles in size incolloidal solution is followed and the PL properties havebeen studied. The present letter reports our continuedeﬀort in the conception of optoelectronic nanocomposites.+abZn(ClO4)2 + NaOH Surfactant molecule, ZnO nanoparticles,Fig. 1. The preparation of nanocomposites by the reverPlease cite this article in press as: C. Bouvy et al., Chem. Phys. Let2. Experimental2.1. SynthesisThe reverse micelle method implies the preparation of aZnO reverse micelle solution by dissolving 33.3 g ofNaAOT (sodium bis(2-ethylhexyl)sulfosuccinate) in awater/iso-octane solution containing Zn(ClO4)2 andNaOH [16]. After homogenization, 0.5 g of the reversemicelle solution was slowly added in a micellar solution(Brij56 10% in water) at 70 C. To this mixture of two sur-factants, the silica source (tetramethoxysilane) was intro-duced dropwise. After the hydrothermal treatment, theethanol extraction and calcinations under N2 and then O2were carried out in order to remove the surfactantmolecules.For the colloidal method, a mesoporous silica materialwas ﬁrstly synthesized as described elsewhere [8]. A ZnOcolloidal suspension was prepared by mixing Zn(ClO4)2and NaOH aqueous solutions in ca. 90 ml of methanolunder vigorous stirring [17]. 0.5 g of the mesoporous silicawas impregnated with this colloidal solution during 30 sand was then immediately ﬁltered and washed several timeswith methanol.These two synthesis strategies are illustrated in Fig. 1.2.2. CharacterizationNitrogen adsorption–desorption isotherms wereobtained at 77 K over a wide relative pressure range from0.01 to 0.995 with a volumetric adsorption analyzer TRI-STAR 3000 manufactured by Micromeritics. The samplewas degassed further under vacuum for several hours at320 C prior to nitrogen adsorption measurements. Thepore diameter and the pore size distribution were deter-mined by the BJH (Barret, Joyner, Halenda) method [18].Small-angle X-ray diﬀraction patterns were recorded withTMOSMesochannel of silica matrix. se micelle method (a) and the colloidal method (b).t. (2007), doi:10.1016/j.cplett.2007.02.061C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxx 3ARTICLE IN PRESSa Panalytical X’Pert diﬀractometer equipped with a copperanode producing X-rays with a wavelength equal to1.54178 A˚. The morphology of the obtained mesoporousmaterials was studied using a Philips XL-20 scanning elec-tron microscope (SEM) with sample preparation by metal-lisation. Photoluminescent spectra were recorded at roomtemperature with an ArF laser (193 nm, 6.4 eV) as the exci-tation source and an iCCD camera as detector. The laserbeam was focused onto the sample with a spot size of1.36 · 102 cm2.3. Results and discussion3.1. Reverse micelle methodThe general characterization of the sample prepared bythe reverse micelle method is presented in Fig. 2. N2adsorption–desorption isotherm is type IV [19], character-istic of mesoporous materials (Fig. 2a), surface area is veryhigh (880 m2/g) and the pore size distribution is centered at2.8 nm (Fig. 2b). Small-angle XRD study revealed only onebroad diﬀraction peak within the 2h range 1.0–5.0, indi-cating that the mesopores are not periodically organized(Fig. 2c). Nevertheless, the observation of this diﬀractionpeak can be correlated with the regular repetition of thepore diameter with a very good homogeneity. The particle0.0 0.2 0.4 0.6 0.8 1.0100200300400500Volume adsorbed (cm³/g-STP)Relative pressure p0/p1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.50Intensity (arb. units)2 Theta (˚)c5.0aFig. 2. Textural (a) and (b), structural (c) and morphologic (d) propertiePlease cite this article in press as: C. Bouvy et al., Chem. Phys. Letmorphology was studied by scanning electron microscopy.The micrograph of Fig. 2d reveals that the sample preparedby the reverse micelle method exhibits undeﬁned morphol-ogies made of blocks and very small particles.PL studies point out a complex emission spectrum com-posed of one very broad band in the range of 2.5–5.5 eV(Fig. 3a). Our recent work about the PL properties of mes-oporous silica CMI-1 can help us to discrimiate the originof these emission bands [9]. Usually, with the 6.4 eV excita-tion, three contributions, at 3.0, 4.0 and 4.3 eV, can beobserved in the PL spectrum of a pure CMI-1 material,which are attributed to surface defects associated with sila-nol groups. The ﬁrst one, at ca. 3.0 eV, is ascribed to E 0centers (”Si) while the second, at ca. 4.0 eV, is believedto come from ”Si–O2 centers. The origin of the third PLband, at 4.3 eV, is not clear but is thought to come alsofrom silanol groups. After a Gaussian decompositionstrictly on the basis of the physical principle and accordingto the PL properties of ZnO and SiO2 species, four emis-sion bands at 3.0, 3.6, 4.0 and 4.3 eV can be distinguished.Three bands at 3.0, 4.0 and 4.3 eV can be immediatelyattributed to mesoporous silica matrix. The last PL bandis relatively wide and centered at 3.6 eV. This emissioncan obviously be assigned to ZnO nanoparticles growninside the channels of the CMI-1 matrix by the reversemicelle method (0.5 wt.% loading). This PL band is blue2 4 6 8 10 12 14 16 180.000.000.010.01dV/dD (cm³/g-Å)Pore diameter (nm)bd20s of ZnO/mesoporous silica prepared by the reverse micelle method.t. (2007), doi:10.1016/j.cplett.2007.02.0612.5 3.0 3.5 4.0 4.5 5.0 5.5PL intensity (arb. units) Photon energy (eV)abFig. 3. PL spectrum of ZnO/mesoporous silica nanocomposite preparedby the reverse micelle method (a) and the colloidal method (b) (excitationwavelength = 193 nm).1.5 2.0 2.5 3.0 3.5 4.00100200300400500600700800 1 min 10 min 15 min 25 min 75 min 90 min 24 hIntensity (arb. units)Photon Energy (eV)abcdefgabcdefgFig. 4. Evolution of the PL spectra of the ZnO colloid solution with time(excitation wavelength = 300 nm).4 C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxxARTICLE IN PRESSshifted in comparison to bulk ZnO (3.3 eV), indicating thatthe channels of the CMI-1 matrix have an eﬃcient limita-tion eﬀect on the size of ZnO nanoparticles during theirgrowth. This part of the work demonstrates clearly thatby the reverse micelle method, we can well control theZnO particle size and prepare short wavelength (blue) lasernanocomposites. Our further work of this part is toincrease the ZnO loading to enhance the intensity of theblue-shifted band emission.3.2. Colloidal methodPrior to the incorporation of ZnO nanoparticles insidethe CMI-1 matrix, we have prepared a ZnO colloidal solu-tion and performed ﬂuorescence measurements to followthe growth of the colloids (Fig. 4). The excitation wave-length was 300 nm. In the ﬁrst minutes, the emission ofthe ZnO solution is blue shifted in comparison to bulkZnO and this blue shift disappears quickly after 15 minmeasurements, indicating that the particles of ZnO arelarge enough to behave as bulk ZnO. This ﬁrst study onthe growth of ZnO in a colloidal solution demonstratedPlease cite this article in press as: C. Bouvy et al., Chem. Phys. Letthe instability of this colloidal solution and the spontane-ous aggregation of ZnO nanoparticles. It also suggests thatthe CMI-1 matrix must be impregnated inside the ZnO col-loidal solution as soon as the ZnO colloidal solution is pre-pared to ensure that the ZnO nanoparticles are smallenough to enter the mesochannels.On the basis of above work, the impregnation time ofCMI-1 silica matrix in the colloidal solution was ﬁxed at60 s.Fig. 5 depicts the textural, structural and morphologicproperties of the nanocomposite prepared by the colloidalmethod. The N2 adsorption–desoprtion curve is also a typeIV isotherm, characteristic of mesoporous materials(Fig. 5a) indicating that the mesoporosity is conserved dur-ing the incorporation of ZnO. The surface area and poresize of the sample are 780 m2/g and 2.7 nm, respectively(Fig. 5b). No diﬀraction peak can be observed in theSmall-angle XRD pattern within the 2h range of 1.0–5.0(Fig. 5c). As the pore size distribution is quite narrow,the absence of diﬀraction peak is assigned to the lack ofcontrast between the silica walls and the channels ﬁlledwith the ZnO nanoparticles (5 wt.% loading). The micro-graphs taken by scanning electron microscopy show spher-ical particles of 5 lm in diameter (Fig. 5d).The PL spectrum presented in Fig. 3b shows two broadPL bands and a narrower one in the range of 2.5–5.5 eV.After a Gaussian decomposition, four bands at 3.0, 3.2,3.8 and 4.3 eV can be observed. Two contributions of mes-oporous silica can be recognized at 3.0 and 4.3 eV. Usually,a contribution centered at 4.0 eV is also present but mayhave disappeared in our case due to the contact of theCMI-1 matrix with methanol during the incorporation pro-cess. On the other hand, the very narrow peak centered at3.2 eV is attributed to ‘big’ ZnO nanoparticles grown onthe external surface of the CMI-1 matrix. The last PLband, a broad band at 3.8 eV, is thought to come from‘small’ nanoparticles grown inside the channels of themesoporous silica materials and which exhibit a relativelyt. (2007), doi:10.1016/j.cplett.2007.02.0610.0 0.2 0.4 0.6 0.8 1.0100200300400Volume adsorbed (cm³/g-STP)Relative pressure p0/pa2 4 6 8 10 12 14 16 18 200.000.010.020.03dV/dD (cm³/g-Å)Pore diameter (nm)b1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Intensity (arb. units)2 Theta (°)c d5.0Fig. 5. Textural (a) and (b), structural (c) and morphologic (d) properties of ZnO/mesoporous silica prepared by the colloidal method.C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxx 5ARTICLE IN PRESSimportant blue shift due to the expected quantum sizeeﬀect. This part of the work suggests that the colloidalpreparation method can also give a better control of ZnOparticle size.4. ConclusionThe preparation of ZnO/mesoporous silica is achievedby the colloidal and the reverse micelle methods. Bothmethods lead to nanocomposites exhibiting strong quan-tum size eﬀect due to the very small particle size of ZnOincluded in the silica matrix. These methods can be eﬃcientin the preparation of short wavelength (blue) lasernanocomposites.AcknowledgmentsC. Bouvy thanks the FRIA (Fonds national de laRecherche Scientiﬁque) for doctoral fellowship and theﬁnancial support from the University of Namur (FUNDP)was gratefully acknowledged. This work was realized in aBelgian federal excellent research frame PAI-IUAP(INANOMAT, P6/17).References[1] A. Henglein, Chem. Rev. 89 (1989) 1861.Please cite this article in press as: C. Bouvy et al., Chem. Phys. Let[2] L.E. Brus, J. Chem. Phys. 80 (1984) 4403.[3] C. Bouvy, W. Marine, R. Sporken, B.L. Su, Chem. Phys. Lett. 428(2006) 312.[4] C. Bouvy, W. Marine, R. Sporken, B.L. Su, Colloid. Surf. A, in press,doi:10.1016/j.colsurfa.2006.12.043.[5] J.S. Beck et al., J. Am. Chem. Soc. 114 (1992) 10834.[6] W.H. Zhang, J.L. Shi, L.Z. Wang, D.S. Yan, Chem. Mater. 12 (2000)1408.[7] W.H. Zhang, J.L. Shi, H.R. Chen, Z.L. Hua, D.S. Yan, Chem.Mater. 13 (2001) 648.[8] J.L. Blin, A. Leonard, B.L. Su, Chem. Mater. 13 (2001) 3542.[9] C. Bouvy, W. Marine, R. Sporken, B.L. Su, Chem. Phys. Lett. 420(2006) 225.[10] C. Klingshirn, Phys. Stat. Sol. B 71 (1975) 547.[11] J.M. Hvam, Solid State Comm. 26 (1978) 987.[12] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T.Goto, Appl. Phys. Lett. 70 (1997) 2230.[13] A. Leonard, A. Vantomme, C. Bouvy, N. Moniotte, P. Mariaulle,B.L. Su, Nanopages 1 (2006) 1.[14] D. Wiersma, Nature 406 (2000) 132.[15] C. Bouvy, F. Piret, W. Marine, B.L. Su, Chem. Phys. Lett. 433 (2007)350.[16] J. Zhang, B. Han, Z. Hou, Z. Lin, J. He, T. Jiang, Langmuir 19 (2003)7616.[17] U. Koch, A. Fojtik, H. Weller, A. Henglein, Chem. Phys. Lett. 122(1985) 507.[18] E.P. Barret, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951)373.[19] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Am. Chem.Soc. 62 (1940) 1723.t. (2007), doi:10.1016/j.cplett.2007.02.061

ZnO/mesoporous silica nanocomposites prepared by the reverse micelles and the colloidal methods: photoluminescent properties and quantum size effect

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.beZnO/mesoporous silica nanocomposites prepared by the reverse micelles and thecolloidal methods: photoluminescent properties and quantum size effectBouvy, Claire; Marine, Wladimir; Su, Bao-LianPublished in:Chemical Physics LettersPublication date:2007Document VersionPublisher's PDF, also known as Version of recordLink to publicationCitation for pulished version (HARVARD):Bouvy, C, Marine, W & Su, B-L 2007, 'ZnO/mesoporous silica nanocomposites prepared by the reverse micellesand the colloidal methods: photoluminescent properties and quantum size effect', Chemical Physics Letters, vol.438, pp. 67-71.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. 2020ARTICLE IN PRESSwww.elsevier.com/locate/cplettChemical Physics Letters xxx (2007) xxx–xxxZnO/mesoporous silica nanocomposites prepared by the reversemicelle and the colloidal methods: Photoluminescent propertiesand quantum size effectClaire Bouvy a,1, Wladimir Marine b, Bao-Lian Su a,*a Laboratoire de Chimie des Mate´riaux Inorganiques (CMI), University of Namur (FUNDP), Rue de Bruxelles 61, B-5000 Namur, Belgiumb Centre de Recherche en Matie`re Condense´e et Nanosciences (CRMC-N), UPR 7251 CNRS, Faculte´ des Sciences de Luminy,Universite´ de la Me´diterrane´e, FranceReceived 26 January 2007; in final form 14 February 2007AbstractQuantum size effect was demonstrated in ZnO/mesoporous silica nanocomposites prepared by two different routes: the reverse micellestrategy and the colloidal method owing to a fine tuning of particles grown by the control of mesopore size. N2 adsorption–desorptionmeasurements, Atomic Absorption and Photoluminescent spectroscopy have evidenced the efficient loading and growth of ZnO nano-particles in the channels of the mesoporous silica matrix. The present work illustrated that both methods are quite efficient in the prep-aration of short wavelength (blue) laser nanocomposite. 2007 Published by Elsevier B.V.1. IntroductionOptoelectronic nanocomposites are a new class of mate-rials which exhibit very interesting and particular proper-ties and attract a growing attention due to their potentialapplications in information storage and optoelectronicdevices. This kind of nanocomposites are usually composedof two phases which are generally a semiconductor and aninorganic matrix [1]. The size of the semiconductor due tothe matrix effect is in the nanometric size range resulting inquantum size effect (QSE) which can lead to the enhance-ment of the band gap of the semiconductor [2]. Conse-quently, the emission wavelength of the semiconductor isblue-shifted, which is very relevant for the short wave-length design. However, the conception of optoelectronicnanocomposites still remains a great challenge since it isvery difficult to obtain a particle size of semiconductor as0009-2614/$ - see front matter  2007 Published by Elsevier B.V.doi:10.1016/j.cplett.2007.02.061* 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 Scientifique, 5 rued’Egmon, B-1000 Bruxelles, Belgium).Please cite this article in press as: C. Bouvy et al., Chem. Phys. Letdesired while preventing the aggregation of thenanoparticles.Very recently, we have demonstrated a very efficientquantum size effect and an important blue shift in the PLemission of ZnO/Faujasite X optoelectronic nanocompos-ites [3,4]. It was shown that the ZnO nanoparticles weregrown inside the 3D-channels of the Faujasite X withoutdestruction of the crystalline structure of the zeolite. How-ever, some modifications of the symmetry were observeddue to the effective presence of the ZnO nanoparticles.Discovered in 1992 [5], silica mesoporous materials havesuccessfully been used in the conception of optoelectronicnanocomposites [6,7] and considered as very promisingmatrix because their channels, with diameter ranging from2 to 10 nm, can limit the size of the semiconductor nano-particles during their growth. In 2001, our laboratory hasreported the preparation of highly ordered silica mesopor-ous materials CMI-1 with hexagonal structure of the mes-ochannels [8] by employing a neutral surfactant whichallows a soft removal of the templating agent by simpleethanol extraction. This synthesis pathway is expected toprepare pure siliceous mesoporous materials with struc-t. (2007), doi:10.1016/j.cplett.2007.02.0612 C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxxARTICLE IN PRESStural and surface defects as less as possible. Preliminary PLstudies of CMI-1 mesoporous materials have clearly shownthe absence of the 1.9 eV PL band usually observed inamorphous SiO2 and associated with oxygen vacancies[9]. The absence of these defects is crucial for the prepara-tion of optoelectronic materials using silica as matrix sincethe optical transparency in visible light range is the primarycondition. This confirmed our expectation that CMI-1mesoporous materials are ideal matrix for the preparationof optoelectronic nanocomposites.ZnO is of particular interest due to its large band gapvalue (3.37 eV) and its important exciton binding energy(60 meV) [10,11] which allows the excitonic emission tobe quite significant even at room temperature (25 meV)[12]. Our recent studies have revealed for the first time astrong two-photon random lasing effect arising from thestimulated emission of ZnO nanoparticles confined insidethe silica mesoporous CMI-1 matrix [13]. The mechanismof lasing implies the scattering of the stimulated emissionwithin the mesoporous matrix where active ZnO nanopar-ticles were grown [14].ZnS nanoparticles have also been incorporated intoCMI-1 matrix and the obtained nanocomposite has shownvery efficient quantum size effect resulting in an importantblue shift of the PL emission due to the nanometric size ofthe ZnS particles [15]. However, the distribution of ZnOand ZnS nanoparticle sizes is still relatively broad andthe improvement of the homodispersity could be achievedonly by the development of more efficient methods to con-trol the nanoparticle growth during their incorporation.This Letter deals with two different but original path-ways to load ZnO nanoparticles inside the channels of mes-oporous silica materials, the reverse micelle and thecolloidal method. The rapid growth of particles in size incolloidal solution is followed and the PL properties havebeen studied. The present letter reports our continuedeffort in the conception of optoelectronic nanocomposites.+abZn(ClO4)2 + NaOH Surfactant molecule, ZnO nanoparticles,Fig. 1. The preparation of nanocomposites by the reverPlease cite this article in press as: C. Bouvy et al., Chem. Phys. Let2. Experimental2.1. SynthesisThe reverse micelle method implies the preparation of aZnO reverse micelle solution by dissolving 33.3 g ofNaAOT (sodium bis(2-ethylhexyl)sulfosuccinate) in awater/iso-octane solution containing Zn(ClO4)2 andNaOH [16]. After homogenization, 0.5 g of the reversemicelle solution was slowly added in a micellar solution(Brij56 10% in water) at 70 C. To this mixture of two sur-factants, the silica source (tetramethoxysilane) was intro-duced dropwise. After the hydrothermal treatment, theethanol extraction and calcinations under N2 and then O2were carried out in order to remove the surfactantmolecules.For the colloidal method, a mesoporous silica materialwas firstly synthesized as described elsewhere [8]. A ZnOcolloidal suspension was prepared by mixing Zn(ClO4)2and NaOH aqueous solutions in ca. 90 ml of methanolunder vigorous stirring [17]. 0.5 g of the mesoporous silicawas impregnated with this colloidal solution during 30 sand was then immediately filtered and washed several timeswith methanol.These two synthesis strategies are illustrated in Fig. 1.2.2. CharacterizationNitrogen adsorption–desorption isotherms wereobtained at 77 K over a wide relative pressure range from0.01 to 0.995 with a volumetric adsorption analyzer TRI-STAR 3000 manufactured by Micromeritics. The samplewas degassed further under vacuum for several hours at320 C prior to nitrogen adsorption measurements. Thepore diameter and the pore size distribution were deter-mined by the BJH (Barret, Joyner, Halenda) method [18].Small-angle X-ray diffraction patterns were recorded withTMOSMesochannel of silica matrix. se micelle method (a) and the colloidal method (b).t. (2007), doi:10.1016/j.cplett.2007.02.061C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxx 3ARTICLE IN PRESSa Panalytical X’Pert diffractometer equipped with a copperanode producing X-rays with a wavelength equal to1.54178 A˚. The morphology of the obtained mesoporousmaterials was studied using a Philips XL-20 scanning elec-tron microscope (SEM) with sample preparation by metal-lisation. Photoluminescent spectra were recorded at roomtemperature with an ArF laser (193 nm, 6.4 eV) as the exci-tation source and an iCCD camera as detector. The laserbeam was focused onto the sample with a spot size of1.36 · 102 cm2.3. Results and discussion3.1. Reverse micelle methodThe general characterization of the sample prepared bythe reverse micelle method is presented in Fig. 2. N2adsorption–desorption isotherm is type IV [19], character-istic of mesoporous materials (Fig. 2a), surface area is veryhigh (880 m2/g) and the pore size distribution is centered at2.8 nm (Fig. 2b). Small-angle XRD study revealed only onebroad diffraction peak within the 2h range 1.0–5.0, indi-cating that the mesopores are not periodically organized(Fig. 2c). Nevertheless, the observation of this diffractionpeak can be correlated with the regular repetition of thepore diameter with a very good homogeneity. The particle0.0 0.2 0.4 0.6 0.8 1.0100200300400500Volume adsorbed (cm³/g-STP)Relative pressure p0/p1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.50Intensity (arb. units)2 Theta (˚)c5.0aFig. 2. Textural (a) and (b), structural (c) and morphologic (d) propertiePlease cite this article in press as: C. Bouvy et al., Chem. Phys. Letmorphology was studied by scanning electron microscopy.The micrograph of Fig. 2d reveals that the sample preparedby the reverse micelle method exhibits undefined morphol-ogies made of blocks and very small particles.PL studies point out a complex emission spectrum com-posed of one very broad band in the range of 2.5–5.5 eV(Fig. 3a). Our recent work about the PL properties of mes-oporous silica CMI-1 can help us to discrimiate the originof these emission bands [9]. Usually, with the 6.4 eV excita-tion, three contributions, at 3.0, 4.0 and 4.3 eV, can beobserved in the PL spectrum of a pure CMI-1 material,which are attributed to surface defects associated with sila-nol groups. The first one, at ca. 3.0 eV, is ascribed to E 0centers (”Si) while the second, at ca. 4.0 eV, is believedto come from ”Si–O2 centers. The origin of the third PLband, at 4.3 eV, is not clear but is thought to come alsofrom silanol groups. After a Gaussian decompositionstrictly on the basis of the physical principle and accordingto the PL properties of ZnO and SiO2 species, four emis-sion bands at 3.0, 3.6, 4.0 and 4.3 eV can be distinguished.Three bands at 3.0, 4.0 and 4.3 eV can be immediatelyattributed to mesoporous silica matrix. The last PL bandis relatively wide and centered at 3.6 eV. This emissioncan obviously be assigned to ZnO nanoparticles growninside the channels of the CMI-1 matrix by the reversemicelle method (0.5 wt.% loading). This PL band is blue2 4 6 8 10 12 14 16 180.000.000.010.01dV/dD (cm³/g-Å)Pore diameter (nm)bd20s of ZnO/mesoporous silica prepared by the reverse micelle method.t. (2007), doi:10.1016/j.cplett.2007.02.0612.5 3.0 3.5 4.0 4.5 5.0 5.5PL intensity (arb. units) Photon energy (eV)abFig. 3. PL spectrum of ZnO/mesoporous silica nanocomposite preparedby the reverse micelle method (a) and the colloidal method (b) (excitationwavelength = 193 nm).1.5 2.0 2.5 3.0 3.5 4.00100200300400500600700800 1 min 10 min 15 min 25 min 75 min 90 min 24 hIntensity (arb. units)Photon Energy (eV)abcdefgabcdefgFig. 4. Evolution of the PL spectra of the ZnO colloid solution with time(excitation wavelength = 300 nm).4 C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxxARTICLE IN PRESSshifted in comparison to bulk ZnO (3.3 eV), indicating thatthe channels of the CMI-1 matrix have an efficient limita-tion effect on the size of ZnO nanoparticles during theirgrowth. This part of the work demonstrates clearly thatby the reverse micelle method, we can well control theZnO particle size and prepare short wavelength (blue) lasernanocomposites. Our further work of this part is toincrease the ZnO loading to enhance the intensity of theblue-shifted band emission.3.2. Colloidal methodPrior to the incorporation of ZnO nanoparticles insidethe CMI-1 matrix, we have prepared a ZnO colloidal solu-tion and performed fluorescence measurements to followthe growth of the colloids (Fig. 4). The excitation wave-length was 300 nm. In the first minutes, the emission ofthe ZnO solution is blue shifted in comparison to bulkZnO and this blue shift disappears quickly after 15 minmeasurements, indicating that the particles of ZnO arelarge enough to behave as bulk ZnO. This first study onthe growth of ZnO in a colloidal solution demonstratedPlease cite this article in press as: C. Bouvy et al., Chem. Phys. Letthe instability of this colloidal solution and the spontane-ous aggregation of ZnO nanoparticles. It also suggests thatthe CMI-1 matrix must be impregnated inside the ZnO col-loidal solution as soon as the ZnO colloidal solution is pre-pared to ensure that the ZnO nanoparticles are smallenough to enter the mesochannels.On the basis of above work, the impregnation time ofCMI-1 silica matrix in the colloidal solution was fixed at60 s.Fig. 5 depicts the textural, structural and morphologicproperties of the nanocomposite prepared by the colloidalmethod. The N2 adsorption–desoprtion curve is also a typeIV isotherm, characteristic of mesoporous materials(Fig. 5a) indicating that the mesoporosity is conserved dur-ing the incorporation of ZnO. The surface area and poresize of the sample are 780 m2/g and 2.7 nm, respectively(Fig. 5b). No diffraction peak can be observed in theSmall-angle XRD pattern within the 2h range of 1.0–5.0(Fig. 5c). As the pore size distribution is quite narrow,the absence of diffraction peak is assigned to the lack ofcontrast between the silica walls and the channels filledwith the ZnO nanoparticles (5 wt.% loading). The micro-graphs taken by scanning electron microscopy show spher-ical particles of 5 lm in diameter (Fig. 5d).The PL spectrum presented in Fig. 3b shows two broadPL bands and a narrower one in the range of 2.5–5.5 eV.After a Gaussian decomposition, four bands at 3.0, 3.2,3.8 and 4.3 eV can be observed. Two contributions of mes-oporous silica can be recognized at 3.0 and 4.3 eV. Usually,a contribution centered at 4.0 eV is also present but mayhave disappeared in our case due to the contact of theCMI-1 matrix with methanol during the incorporation pro-cess. On the other hand, the very narrow peak centered at3.2 eV is attributed to ‘big’ ZnO nanoparticles grown onthe external surface of the CMI-1 matrix. The last PLband, a broad band at 3.8 eV, is thought to come from‘small’ nanoparticles grown inside the channels of themesoporous silica materials and which exhibit a relativelyt. (2007), doi:10.1016/j.cplett.2007.02.0610.0 0.2 0.4 0.6 0.8 1.0100200300400Volume adsorbed (cm³/g-STP)Relative pressure p0/pa2 4 6 8 10 12 14 16 18 200.000.010.020.03dV/dD (cm³/g-Å)Pore diameter (nm)b1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Intensity (arb. units)2 Theta (°)c d5.0Fig. 5. Textural (a) and (b), structural (c) and morphologic (d) properties of ZnO/mesoporous silica prepared by the colloidal method.C. Bouvy et al. / Chemical Physics Letters xxx (2007) xxx–xxx 5ARTICLE IN PRESSimportant blue shift due to the expected quantum sizeeffect. This part of the work suggests that the colloidalpreparation method can also give a better control of ZnOparticle size.4. ConclusionThe preparation of ZnO/mesoporous silica is achievedby the colloidal and the reverse micelle methods. Bothmethods lead to nanocomposites exhibiting strong quan-tum size effect due to the very small particle size of ZnOincluded in the silica matrix. These methods can be efficientin the preparation of short wavelength (blue) lasernanocomposites.AcknowledgmentsC. Bouvy thanks the FRIA (Fonds national de laRecherche Scientifique) for doctoral fellowship and thefinancial support from the University of Namur (FUNDP)was gratefully acknowledged. This work was realized in aBelgian federal excellent research frame PAI-IUAP(INANOMAT, P6/17).References[1] A. Henglein, Chem. 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ZnO/mesoporous silica nanocomposites prepared by the reverse micelles and the colloidal methods: photoluminescent properties and quantum size effect

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