Aerogels containing strongly photoluminescing zinc oxide nanocrystals

Journal of Non-Crystalline Solids 238 (1998) 1±5

Aerogels containing strongly photoluminescing zinc oxide nanocrystals 1 C. Lorenz b

a,*

, A. Emmerling a, J. Fricke a, T. Schmidt b, M. Hilgendor€ b, uller b L. Spanhel b, G. M

a Physikalisches Institut der Universit at, Am Hubland, D-97074 W urzburg, Germany Lehrstuhl f ur Silicatchemie der Universit at, R ontgenring 10, D-97070 W urzburg, Germany

Received 7 September 1997

Abstract Sol±gel processing was used to prepare ZnO-doped silica-aerogels by hydrolysis and condensation of tetramethoxysilane (TMOS) in the presence of 6 nm sized nanocrystalline ZnO particles. Supercritical drying yielded strongly green±yellow ¯uorescing aerogels. Fluorescence excitation and emission spectra of the resulting ZnO particles in a silica matrix were preserved after the supercritical drying, in contrast to air sintering conditions under which the visible ZnO ¯uorescence is completely suppressed. Anomalous small angle X-ray scattering (ASAXS) was employed to evaluate the internal structure of these nanocrystalline precursors as well as the resulting nearly transparent aerogel samples. Ó 1998 Published by Elsevier Science B.V. All rights reserved.

1. Introduction Due to their special optical, electronic and photocatalytic characteristics small semiconductor clusters are of interest because of initial achievements in the small particle research on colloids [1] as well as by the theoretical analysis of nanocrystals [2]. Small semiconductor particles were investigated in solutions [1], zeolites and molecular sieves [3] as well as in conventional [4] and sol±gel glasses [5]. The aim of this research is synthesis of an aerogel in which the ZnO nanocrystals are eciently incorporated into a SiO2 matrix without * Corresponding author. Tel.: +49-931 8885161; fax: +49-931 8885181; e-mail: [email protected]. 1 This paper was presented at the 5th International Symposium on Aerogels, Montpellier, France, September 8±10, 1997.

destruction of their characteristic green±yellow photoluminescence.

2. Experimental 2.1. Sample preparation Aerogels were prepared by hydrolysis and condensation of methanol solutions of tetramethoxysilane (TMOS) and a highly concentrated 2±3 M ZnO colloid. The ZnO-sol with 6 nm sized crystallites was prepared as recently reported [6]. The alkoxysilane was used as received (ABCR, FLUKA, 97±99% purity). Gels were hydrolysed under neutral conditions; the amount of water necessary to hydrolyse all SiOCH3 groups was added. Sample designation and preparation conditions are denoted in Table 1.

0022-3093/98/$ ± see front matter Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 6 7 9 - 6

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C. Lorenz et al. / Journal of Non-Crystalline Solids 238 (1998) 1±5

Table 1 Sample designation and preparation conditions of the ZnO doped aerogels Sample

Si(OMe)4 g (mmol)

MeOH g (mmol)

Z9 A

5.07 (33.3) 3.30 (24.9) 2.53 (16.6) 1.27 (8.3) 0.52 (3.4) 0.52 (3.4) 0.52 (3.4) 0.52 (3.4)

2.07 (66.7) 3.46 (111.6) 4.95 (159.7) 6.29 (202.9) 0.21 (6.8) 0.47 (15.2) 1.01 (32.7) 2.57 (82.9)

Z9 B Z9 C Z9 D Z14 A Z14 B Z14 C Z14 D

The density of the alcogels of the samples Z9 was adjusted by the amount of methanol to obtain a target density of about 200 kg/m3 for samples indicated A, 150 kg/m3 for samples B, 100 kg/m3 for samples C and 50 kg/m3 for samples D. After gelation (gelation times for the Z9 samples were about half an hour, those for the Z14 samples about 6 h) samples were aged for 7 days at 30°C in closed glass vessels. Supercritical CO2 drying at 35°C was performed after solvent exchange with methanol at room temperature and ambient pressure for one week. Monolithic aerogels were obtained for samples Z9, while samples Z14 cracked. 2.2. Characterization Photoluminescence measurements were carried out at room temperature on a Perkin Elmer-LS 50 B spectrometer. Anomalous small angle X-ray scattering (ASAXS) is a powerful tool for investigating multicomponent systems [7]. Hereby, the strong energy dependence of the atomic form factor f …q; E† ˆ f 0 …E† ‡ if 00 …E† in the vicinity of an absorption edge is used to vary the scattering power of one component and thus the contrast with respect to the matrix. In the small angle scattering regime the q-dependence can be neglected. Due to resonances at an edge the real part f 0 is reduced;

2M ZnO-sol g (mmol) 1 (1.94) 1 (1.94) 1 (1.94) 1 (1.94) 5.43 (10) 5.43 (10) 5.43 (10) 5.43 (10)

wt ZnO (%)

H2 Obidist . g (mmol)

3 4 6 11 61 61 61 61

2.4 (133.3) 1.8 (100) 1.2 (66.7) 0.6 (33.3) 0.24 (13.3) 0.24 (13.3) 0.24 (13.3) 0.24 (13.3)

well above all absorption edges it is determined by the atomic number Z. The imaginary part f 00 is correlated to the absorption coecient. It drastically increases when passing the edge energy. Considering the SiO2 /ZnO system under investigation, the measured volume-speci®c scattering cross section I(q, E) can be written as I…q; E† ˆ fS2 iS …q† ‡ 2…ReffS fZ …E†g ‡ImffS fZ …E†g†iSZ …q† ‡ fZ2 …E†iZ …q† ‡ ffl2 ifl ; …1† where Re and Im denote the real and imaginary part, respectively, iS is the intensity of the silica matrix, iSZ the cross term resulting from mutual correlation between the zinc oxide cluster and the matrix; iZ is the scattering pro®le of the ZnO clusters and ifl an energy-dependent (f2fl ) ¯uorescence background. fS ˆ fSi + 2fO and fZ ˆ 34fZn + 10fO are the molecular form factors corresponding to the stoichiometric composition. As the dispersion of the atomic form factors can be calculated [8], it is thus possible to separate the single scattering contributions of Eq. (1) from measurements at at least four energies in the pre-edge region [9]. ASAXS measurements were performed at the synchrotron beamline JUSIFA at HASYLAB/ DESY, Hamburg at 9530, 9640, 9656 and 9658 eV near the Zn±K absorption edge of 9659 eV. En-

C. Lorenz et al. / Journal of Non-Crystalline Solids 238 (1998) 1±5

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0 ergies were chosen so that fZn decreases by two electrons when approaching the edge. After calibrating and correcting the raw data for parasitic e€ects the four scattering pro®les give a system of linear equations which can be solved.

3. Results Surface area, pore volume and pore diameter of ZnO doped SiO2 aerogels are summarized in Table 2. Low ZnO concentrations in the silica aerogels yielded the best monoliths whereas shrinkage and cracking was noted with increasing ZnO concentration. Bulk densities for the series Z9 are 50 kg mÿ3 higher than the target density. Nitrogen adsorption showed that ZnO doped aerogels have a BET surface area in the range from 85 to 480 m2 gÿ1 and average pore diameters between 10 and 18 nm. It is note worthy that the BET surfaces of the Z14 samples are 1/3 of those of Z9 samples prepared to the same target density. The nearly transparent aerogels show a strong green±yellow ¯uorescence. The corresponding photoluminescence excitation and emission spectra depicted in Fig. 1 are characteristic of all samples. Two excitonic transitions at around 350 and 380 nm and the corresponding emission band peaking at 550 nm are identi®able. Fig. 2(a) shows the total cross section together with the scattering contributions from the silica part and the ZnO particles derived from ASAXS for sample Z9 A and the scattering curve of the ZnO precursor colloid. The silica curve exhibits a pro®le similar to that of pure SiO2 aerogels of

Fig. 1. Corrected photoluminescence excitation and emission spectra of silica-ordered 6 nm sized ZnO nanocrystallites.

comparable density and pH with a fractal dimension of D ˆ 2.5. The ZnO gives a pattern which for q P 1 nmÿ1 is comparable to the one of the precursor sol. For q 6 1 nmÿ1 the slope is comparable to the one of the silica part. Sample Z14 A with a reduced fraction of SiO2 shows a di€erent behaviour (Fig. 2(b)): Here no fractal regime is visible. The ZnO crystallites yield a pattern similar to the precursor sol. 4. Discussion The photoluminescence data displayed in Fig. 1 coincide with the data collected on concentrated ZnO precursor sols which proves that the ZnO nanocrystallites have been incorporated into the

Table 2 Characteristic data of ZnO doped aerogels; surface areas are determined by BET nitrogen adsorption, mass loss by thermogravimetry, density at 30°C by determination of mass and volume for the monolithic samples Sample

BET surface area (m2 gÿ1 ) ‹25

Mass loss (%) ‹2

Density (kg mÿ3 ) ‹10

Pore volume (cm3 gÿ1 ) ‹0.08

Ave. pore diam. (nm) ‹1

Z9 A Z9 B Z9 C Z9 D Z14 A Z14 B Z14 C Z14 D

518 481 439 327 141 112 91 85

19 20 21 21 32 33 31 35

248 209 162 103 ) ) ) )

2.45 2.14 1.57 0.93 0.54 0.40 0.25 0.20

10 18 14 11 15 14 10 10

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C. Lorenz et al. / Journal of Non-Crystalline Solids 238 (1998) 1±5

Fig. 2. Total scattering intensity I of samples Z9 A (a) and Z14 (b) and the ASAXS derived cross sections iS and iZ of the silica and the zinc oxide fractions, respectively (from top to bottom). Additionally, the scattering pro®le of the ZnO precursor sol is shown for comparison (bottom curve).

silica matrix. Their structure, based on previous ¯uorescence excitation studies on ZnO [10] and on the more recent SAXS investigations on CdSe [11] colloids, can probably be explained in terms of mass fractal structures, possibly doubling their size during nucleation. At the low-q part of the scattering curve the individual ZnO entities are recognised to form nanocrystal-nanocrystal aggregates with a fractal dimension of 2 as a result of the high concentration in the sol. The high-q part of the scattering curve (1 < q < 10) indicates that the internal structure of the ZnO crystallites possibly is also mass fractal. However, the Koch-pyramidal structure as suggested for ``zinc blende'' metal sul®des and selenides in Ref. [11] does not necessarily apply to the hexagonal wurtzite ZnO system [10]. To clarify this point, more thorough theoretical structural calculations will be needed in the future to ®t the high q-part of the ZnO scattering curves. Nevertheless, from Fig. 2 we conclude that the structure of the ZnO nanocrystallites is in principle preserved when they are integrated in a silica gel network. Their aggregates, however, are dissolved

upon dilution in the aerogel starting solution. Hereafter the ZnO particles are homogeneously distributed, as they show the same long-range correlations as the constitutive silica network (Fig. 2(a)). By contrast, aerogels Z14 (Fig. 2(b)) with an extremely high content of ZnO show that even the nanocrystal-nanocrystal aggregates of the precursor colloid are conserved. These aggregates must then be ``glued'' together by silica particles in order to form a coherent network. Due to the low SiO2 content of this sample no particular silica network is to be expected. The silica curve resembles the one of a system of isolated particles or clusters. 5. Conclusion Preliminary results presented demonstrated that ZnO clusters can be successfully integrated into SiO2 aerogels. The primary structure of these clusters is comparable to that of the precursor solution as indicated by the anomalous scattering and photoluminescence results. Their aggregate state is preserved only for high concentrations. For

C. Lorenz et al. / Journal of Non-Crystalline Solids 238 (1998) 1±5

low concentrations, dissolution of the aggregates and a homogeneous incorporation of such clusters into the silica matrix was observed. References [1] A. Henglein, Top. Curr. Chem. 143 (1988) 115; Chem. Rev. 89 (1989) 1861 and references therein. [2] L.E. Brus, J. Phys. Chem. 90 (1986) 2555. [3] X. Liu, K. Iu, J.K. Thomas, Chem. Phys. Lett. 195 (1992) 163. [4] A.I. Ekimov, A.L. Efros, A.A. Onushchenko, Solid State Commun. 56 (1985) 921.

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[5] D.A. Hummel, I.L. Torriani, A.F. Craievich, N. de la Rosa Fox, A.Y. Ramos, O. Lyon, J. Sol±Gel Sci. Techn. 8 (1997) 285. [6] T. Schmidt, L. Spanhel, G. M uller, K. Kerkel, A. Forchel, Chem. Mater. 10 (1) (1998) 65. [7] H. Stuhrmann in: Topics in Current Chemistry, Springer, Heidelberg, 1988, Vol. 145, p. 151. [8] D.T. Cromer, D.A. Liebermann, Acta Cryst. A 37 (1981) 267. [9] H.-G. Haubold et al., Rev. Sci. Instrum. 60 (1989) 1943. [10] L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 28261. [11] V. Ptatschek, T. Schmidt, M. Lerch, G. M uller, L. Spanhel, A. Emmerling, J. Fricke, A.H. Foitzik, E. Langer, Ber. Bunsenges. Phys. Chem. 102 (1998) 85.