Mesoporous silica photoluminescence properties in samples with different pore size

Mesoporous silica photoluminescence properties in samples with different pore size

Materials Science and Engineering C 23 (2003) 1073 – 1076 www.elsevier.com/locate/msec Mesoporous silica photoluminescence properties in samples with...

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Materials Science and Engineering C 23 (2003) 1073 – 1076 www.elsevier.com/locate/msec

Mesoporous silica photoluminescence properties in samples with different pore size A. Anedda, C.M. Carbonaro, F. Clemente *, R. Corpino, P.C. Ricci Dipartimento di Fisica Universita` di Cagliari and INFM UdR Cagliari, SP no 8, Km 0,700, I-09042 Monserrato (CA), Italy

Abstract Ultraviolet excited photoluminescence of porous silica under vacuum conditions is investigated in samples with different pore diameters. By exciting in the 180 – 300 nm range, a manifold luminescence activity in the 300 – 600 nm is found. The reconstructed three-dimensional photoluminescence excitation pattern shows how the distribution of the emissions changes by varying the pore size of the samples. Two different centers are singled out and their relative concentration is suggested to be a function of the pore size of the samples. D 2003 Elsevier B.V. All rights reserved. Keywords: Porous silica; Hydroxyl; Silanol; Optical properties; Sol – gel; Surface reactivity

1. Introduction Due to its peculiar properties, sol – gel synthesized porous silica (PS) is turning into a key material for both technological and theoretical applications [1 –7]. Besides being an ideal host for the study of chemical and physical behavior of confined molecules [3,4], PS is an appealing material for the development of new luminescent devices and optical sensors [1,2,5 – 7]. The sol –gel process allows tailoring the porosity of the samples in terms of pore size, interconnection and distribution, leading to the synthesis of an extremely resourceful material [5,6,8 – 10]. Owing to the high specific surface and the open and highly interconnected porous structure, PS properties are dominated by surface activity and reactivity influencing its performances [8 – 12]. H-related species in the form of SiOH and SiH structures terminate the silica walls making the material extremely reactive for polar molecules and hydrophilic [3,8 –10]. The structural properties of the silica backbone are almost independent on the porosity [8,9]. Aiming to engineer porous SiO2 based luminescent devices and sensors, the knowledge of the optical activity of the material itself is a mandatory task to achieve. In addition, the similarities between porous silicon green/blue emissions and ultraviolet (UV) excited photoluminescence (PL) of porous silica have given a further spur to the study of PS luminescence. * Corresponding author. Tel.: +39-070-6754755; fax: +39-070-510171. E-mail address: [email protected] (F. Clemente). 0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2003.09.077

In this work we present a comparative study of the UV PL of porous silica samples with different porosity in a wide range of excitation wavelengths. The photoluminescence emission is found to change with the pore diameter. Through the analysis of the reconstructed photoluminescence excitation patterns (PLE), two different emitting centers have been singled out. A surface location of the luminescent centers and their correlation with surface hydroxyls is proposed.

2. Experimental We performed room temperature photoluminescence (PL) measurements in a vacuum ultraviolet (VUV) facility with a vacuum < 10 5 Torr. The light source was a MgF2sealed Deuterium lamp (Hamamatsu mod. L1835) with a continuous emission in the 150 –300 nm range. The light was dispersed by a 0.3-m scanning monochromator (McPherson mod. 218) with a spectral bandwidth 5.0 nm. The photoluminescence signal was dispersed by a spectrograph (ARC-SpectraPro 275) and detected by an intensified array (EG and G 1420). The spectral bandpass was 2.4 nm. The reported spectra were recorded applying a short wavelength cut-off filter (WG295) and corrected for the optical transfer function of the system. Photoluminescence excitation spectra obtained by collecting the PL emission at different excitation energies are corrected for the spectral variations of the excitation light intensity and for the sample absorption [13,14].

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Porous silica samples produced by Geltech (US) via sol – gel route are provided in disks (diameter 6 mm, thickness 2 mm). The first set of samples (hereafter A samples) has a pore diameter distribution sharply peaked at around 3.2 nm (5% of standard deviation), a pore volume of 0.488 cm3g 1 with a specific surface area of 594 m2g 1 and a density of 1.2 gcm 3. The second set of samples (hereafter B samples) has a pore diameter distribution sharply peaked at around 18.2 nm (5% of standard deviation), a pore volume of 1.208 cm3g 1 with a specific surface area of 264 m2g 1 and a density of 0.6 gcm 3 [15].

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Fig. 2. 3D PLE pattern with projected contour plot of B samples. Spectra have been arbitrarily normalized to the maximum of the most intense emission. Lines on the contour plot are guides for the eyes.

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The 3D-PLE spectrum of A samples with the projected contour plot is reported in Fig. 1. By spanning the excitation in the 180 –300 nm range, a composite photoluminescence emission is detected in the 300– 600 nm range. PL emission is sharply peaked around 350 nm with an excitation maximum at about 230 nm. Fig. 2 shows the 3D PLE spectrum of B samples with the contour plot projection. The PLE is broadly distributed in the 300– 600 nm range and its maximum is located at about 430 nm when exciting at 250 nm. For a better comparison the PLE have been arbitrarily normalized to the most intense PL maximum in each set of samples. The PL emissions of the two sets of samples excited at 250 nm are shown in Fig. 3. Photoluminescence spectra are composite and their shape and width suggest a manifold PL activity. The experimental data have been reproduced by a standard best-fit procedure [16]; the number of

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Wavelength (nm) Fig. 3. PL spectra excited at 250 nm with standard best-fit deconvolution for samples A(a) and B(b). Markers are the experimental data, dashed lines are the gaussian components and the solid line is the resulting fit.

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gaussian functions used in the fitting procedure was increased until no further significant improvement of the v2 occurred, i.e. the best fit with the smallest number of parameters. Concerning A samples, the PL excited at 250 nm is resolved with two gaussian bands peaked at 355 and 426 nm with full width at half maximum (FWHM) of 65 and 135 nm, respectively (Fig. 3a). The gaussian component peaked at 355 nm is more intense than the 426 nm one. When excited at 250 nm, the PL emission of B samples has two components at 357 (FWHM = 60 nm) and 430 nm (FWHM = 135 nm), the latter being the most intense component (Fig. 3b). Fig. 4 displays the excitation spectrum of the emissions at 355 and 430 nm in the two sets of samples. The emission at 355 nm has an excitation channel peaked at 235 nm both in A and B samples (Fig. 4a). The 430 nm emission has a composite excitation spectrum whose main peak is located at about 245 nm in A samples and at 255 nm in B samples

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Wavelength (nm) Fig. 4. Excitation spectra of selected emissions: 355 (a) and 430 nm (b) for A and B samples.

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(Fig. 4b). A slight dependence of the 430 nm excitation spectrum on the porosity is found.

4. Discussion It is presently accepted that the PL emissions in high surface silicas (e.g., porous monoliths, nanoparticles, nanopowders) originates from surface centers; however a precise attribution of the emissions to specific defects has not yet been established [11 – 14,17 – 23]. Different emissions ranges under different excitations have been investigated and several hypotheses on the nature of the luminescent defects have been advanced: hydrogen-related defects in the form of SiUH and SiUOH have been related to the emission in the green region [17 – 19] while different kinds of adsorbed water in silica nanoparticles [20] and carbonylrelated centers have been associated to mesoporous silica blue luminescence [23]. In particular, a correlation of surface hydroxyl species with the emission at 335 nm has been recently proposed [12,21]. Moreover the dependence of the emissions on the surface equilibrium condition was proven [13]. Starting on the assumption of a surface location of the emitting centers, we investigated the photoluminescence emission in samples with different pore size. Mesoporous silica monoliths have an absorption band in the 180 –300 nm range [11,13]. By spanning the excitation within this range, a composite PL emission is found in the 300 – 600 nm range; comparing the reconstructed 3D-PLE of samples with different pore diameters we find that the spectral distribution of the photoluminescence signal is affected by the pore diameter of the samples. As shown in Fig. 1, the 3D-PLE of A samples is dominated by an emission peaked around 350 nm with a preferential excitation peaked at about 230 nm. A similar band has been observed in oxidized porous silicon, Si nanostructures and silica nano-powders; various assignments have been proposed, all related to surface centers [17 –23]. Conversely, B samples have a PL signal broadly distributed in the 300 – 600 nm range. A manifold PL activity with the maximum at about 430 nm is detected: its excitation peak is located at about 250 nm. The broad distribution of the PL emission suggests a manifold activity as evidenced by the reconstruction of the photoluminescence spectra with gaussian components. The standard best-fit deconvolution of the emissions excited at 250 nm for A and B samples, (Fig. 3) indicates that two gaussian components with similar spectral properties (peak position and FWHM) are found in both the sets of samples but with reversed intensity ratios (Fig. 3a and b). From the analysis of the excitation spectra of the gaussian components, we deduce the presence of at least two different emitting centers (Fig. 4a and b). The first one emits at about 355 nm with an excitation peak at 230 nm, the second one at 430 nm best excited at 250 nm.

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The relative concentration of the emitting species changes with the mean pore diameter of the samples: the first one, emitting at higher energy is dominant in the photoluminescence activity of A samples (Figs. 1 and 3a). The second one is the leading specie in B samples (Figs. 2 and 3b). A tentative interpretation of the data here presented can be given in terms of surface curvature of the pores: as proven by Raman spectroscopy analysis, the interaction, distribution and relative concentration of surface hydroxyls is influenced by the pore diameter of the samples [8,9]. Changing PS porosity changes the surface of the material but does not affect the structural properties of the silica skeleton of the matrix. Smaller pores promote the hydrogenbonding interaction between surface silanols: a correlation between emission around 350 nm with interacting SiOH species, further confirmed by literature [12], can explain the . data here presented. Moreover, surface >Si –OH have been associated to an absorption at about 220 nm in the same samples [14]. Samples with larger pores, namely B samples, have a reduced interaction between H-related surface groups leading to a diminished PL activity in the higher energy range. The attribution of the 430 nm band to a specific center could be given in terms of SiH surface defects. Silyl . radicals in the form of >Si – H (‘‘>’’ represents the bonds to silica backbone and ‘‘.’’ indicates an unpaired electron) have been related to an absorption band peaked at about 250 nm in high surface silica [14].

5. Conclusions By exciting in the 180 – 300 nm range, sol – gel synthesized porous silica emits in the 300– 600 nm with a pore size dependent spectral distribution. PLE spectra of samples with pores of 3.2 nm are dominated by an emission peaked at about 350 nm with a sharply distributed excitation channel centered at 230 nm. Samples with pores of 18.2 nm have a broadly distributed photoluminescence with the maximum at 430 nm and best excited at 250 nm. Two different centers are distinguished and their assignment to hydrogen related surface groups is proposed.

Acknowledgements This study has been supported by National Research Project of MIUR (Ministero dell’Istruzione, dell’Universita` e della Ricerca) and by INFM (Istituto Nazionale per la Fisica della Materia) of Italy. References [1] V. Popov, Y.O. Roizin, E. Rysiakiewick-Pasek, K. Marczuk, Opt. Mater. 2 (1993) 249. [2] M.H. Huang, A. Choudrey, P. Yang, Chem. Commun. (2000) 1063. [3] V. Crupi, G. Malsano, D. Majolino, P. Migliardo, V. Venuti, J. Chem. Phys. 109 (1998) 7394. [4] M.W. Shafer, D.D. Awschalom, J. Warnock, G. Ruben, J. Appl. Phys. 61 (1987) 4339. [5] R. Reisfeld, Opt. Mater. 16 (2001) 1. [6] G. Schulz-Ekloff, D. Wo¨hrle, B. van Duffel, R.A. Schoonheydt, Microporous Mesoporous Mater. 51 (2002) 91. [7] C. McDonagh, P. Bowe, K. Mongey, B.D. MacCraith, J. Non-Cryst. Solids 306 (2002) 138. [8] J. Brinker, G.W. Sherer, Sol – Gel Science—The Physics and the Chemistry of Sol – gel Processing, Academic Press, San Diego, 1990. [9] L. Klein, Sol – gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes, Noyes Publications. [10] L.L. Hench, J.K. West, Chem. Rev. 90 (1990) 33. [11] N. Chiodini, F. Meinardi, F. Morazzoni, A. Paleari, R. Scotti, D. Di Martino, Appl. Phys. Lett. 76 (2000) 3209. [12] A. Anedda, C.M. Carbonaro, F. Clemente, R. Corpino, S. Grandi, P. Mustarelli, A. Magistris, J. Non-Cryst. Solids 322 (1 – 3) (2003) 68. [13] A. Anedda, C.M. Carbonaro, F. Clemente, R. Corpino, F. Raga, A. Serpi, J. Non-Cryst. Solids 322 (1 – 3) (2003) 95. [14] G. Pacchioni, L. Skuja, D.L. Griscom (Eds.), Defects in SiO2 and Related Dielectrics: Science and Technology, Kluwer Academic Publishers, Dordrecht, 2000, pp. 339 – 370. [15] Geltech (US), Technical Report. [16] Numerical recipes in C, W.H. Press et al., Cambridge University, Cambridge. [17] Y.D. Glinka, S.-H. Lin, Y.-T. Chen, Appl. Phys. Lett. 75 (1999) 778. [18] Y.D. Glinka, S.-H. Lin, Y.-T. Chen, Phys. Rev., B 62 (2000) 4733. [19] Y.D. Glinka, S.-H. Lin, L.P. Hwang, Y.-T. Chen, Appl. Phys. Lett. 77 (2000) 3968. [20] Y.D. Glinka, S.N. Naumenko, V.M. Ogenko, A.A. Chuiko, Opt. Spectrosc. (USSR) 71 (1992) 250. [21] B. Yao, H. Shi, X. Zhang, L. Zhang, Appl. Phys. Lett. 78 (2001) 174. [22] G.G. Qin, J. Lin, J.Q. Duan, G.Q. Yao, Appl. Phys. Lett. 69 (1996) 1689. [23] M.R. Ayers, A.J. Hunt, J. Non-Cryst. Solids 217 (1997) 229.