Energy transfer from TiO2 nanocrystals to Tb3+ ions incorporated into silica

Available online at www.sciencedirect.com

Optical Materials 30 (2008) 725–729 www.elsevier.com/locate/optmat

Energy transfer from TiO2 nanocrystals to Tb3+ ions incorporated into silica Magdalena Zalewska, Andrzej M. Kłonkowski

*

Faculty of Chemistry, University of Gdan´sk, Sobieskiego 18, 80-952 Gdan´sk, Poland Available online 30 March 2007

Abstract Titania nanocrystals about 100 nm in size have exhibited a phenomenon related to the quantum size effect. The presence of nanocrystals in a three-component material (viz. Tb3+ ions and TiO2 nanoparticles entrapped in a silica matrix) suggests energy transfer from excited TiO2 nanocrystals to emitting Tb3+ ions takes place and consequently improve luminescence emission intensity. The effect is possible when semiconductor nanoparticles and Tb3+ ions are immobilized in a porous matrix by the impregnation method.  2007 Elsevier B.V. All rights reserved. PACS: 34.30+h; 78.55.m; 78.60; 78.66.J Keywords: Energy transfer; TiO2 nanocrystals; Tb(III); Photoluminescence; Silica xerogel

1. Introduction Much research of the last three decades has been focused on luminescent materials containing trivalent lanthanide ions, Ln(III) [1–3], with major applications in emissive displays, fluorescent lamps and some X-ray detector systems are based on luminescent materials as well. A number of luminescent materials have found their way into practical applications and in many cases lanthanide phosphors noticeably improved their performance [4]. The photoluminescent properties of Eu(III) and Tb(III) ions make them other potential candidates for use in luminescent materials [5–9]. At the same time, semiconductor nanocrystals have been widely studied for their fundamental properties [10–12] and applications, mostly as tunable emitters for light emitting diodes (LEDs) [13], lasers [14] and sensors [15]. Semiconductor nanoparticles have unique size-dependent optical properties and are of great interest for applications in optoelectronics, photovoltaics and biological sensing [11,16– 19]. *

Corresponding author. Tel.: +48 58 345 04 00; fax: +48 58 345 04 72. E-mail address: [email protected] (A.M. Kłonkowski).

0925-3467/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.02.021

Various chemical synthetic methods have been developed to prepare such nanoparticles. Wet chemical synthesis can be realized with the so-called ‘stabilizers’, which cap the surface of nanoparticles during their growth, or by confinement in nanoreactors [20]. Among various cage-shaped functional materials used in nanoreactors, such as diblock copolymers [21] and vesicles [22], we have applied reverse micelle [23] to control the growth of nanoparticles. If a semiconductor crystal becomes small enough to approach the material’s exciton Bohr radius, the electron energy levels can no longer be treated as quasi-continuous; they must be treated as discrete. This situation of discrete energy levels is referred to as quantum confinement and a quantum dot is present under these conditions, which has serious repercussions for the absorptive and emissive behavior of semiconductor materials. The band gap of a quantum dot is always energetically wider and thus the electron recombination produces radiation with quanta of energy greater than the bulk material. In quantum dots, the band gap is controlled simply by changing the dot’s size. As the emission frequency of a dot is dependent on the band gap, it is possible to control the energy output of a dot with extreme precision. This allows us to transfer excitation energy from excitons in

726

M. Zalewska, A.M. Kłonkowski / Optical Materials 30 (2008) 725–729

semiconductor quantum dots to Ln(III) ions in luminescent materials. Particular attention has been paid in recent years to the photoluminescent properties of doped semiconductor nanocrystals, due to their interesting optical, magnetic and photo-physical properties [24–26]. Wide band gap semiconductor nanocrystals doped with lanthanide(III) ions may maintain the advantages of nanocrystal emitters. There is a number of reports of semiconductors doped with lanthanide Ln(III) ions such as Tb(III), Sm(III) or Eu(III) [27–31]. However, it has been a challenge to dope all nanocrystals simultaneously [32]. To meet this challenge, we have used the reverse micelle method, which has allowed us to obtain semiconductor nanocrystals that can be doped on their surface by impregnation with Tb(III) ions. Thus, in our case the term doping should be considered as describing other locations of emissive ions as well, most likely bound to the surface with capping ligands or absorbed on the nanocrystal surface [33]. Recently, we have successfully prepared materials consisting of Tb(III) ions and ZnO [35] and ZnS:Mn2+ [36] nanoparticles exhibiting very strong luminescence properties of Tb(III) ions using the same sol–gel processing as Nogami et al. [34]. The sol–gel method is appropriate for synthesizing porous xerogel matrices which can be impregnated with nanosized semiconductors and Ln(III) ions. Interesting photo-physical, photochemical and surficial properties of TiO2 nanoparticles have been investigated and reported in earlier papers (cf. [37–40]). This study is focused on the photoluminescent properties of materials consisting of TiO2 nanocrystals and terbium(III) ions immobilized by impregnation in a porous silica xerogel matrix. 2. Experimental 2.1. Preparation 2.1.1. Silica xerogel A silica sol was prepared according to the standard sol– gel procedure [41] from an initial mixture of tetramethosilane (TMOS, Aldrich Co.), distilled water ([TMOS]: [H2O] = 1:4), methanol as a diluent, and NH3(aq) from POCh, Poland, as a catalyst. The mixture was stirred vigorously at room temperature. The sol was allowed to gel for 3 days and subsequently dried. The obtained xerogel was heated for 3 h at 120 C to remove ammonia and methanol, as well as some of the water from the pores. The dry xerogel was then ground in a mortar and passed through standard sieves. Xerogel particles 0.25–0.50 mm in size were used in the next preparation procedure. 2.1.2. Nanosized titanium dioxide Nanoparticles of TiO2 were prepared in reverse micelles by hydrolysis and condensation of titanium(IV) isopropoxide [42]. First, a mixture consisting of 0.02 M HClO4 added to 0.2 M dioctyl sulfosuccinate sodium (AOT, Aldrich Co.)

as a surfactant in n-heptan (from POCh, Poland) was stirred. At this stage, a given W = [water]/[surfactant] value was achieved and 0.1 cm3 of Ti(OC3H7)4 in 2-propanol solution (0.068 M) was added in drops during mild stirring to 20 cm3 of the reverse micelle solution, at room temperature. To complete the growth of nanoparticles, the final mixture was aged before use for 3–4 h. TiO2 nanoparticles of various diameters were synthesized when the W parameter was equal to 1, 5 and 10. 2.1.3. Luminescent materials In general, materials were synthesized by the impregnation method. In each case, a silica xerogel matrix was immersed in a methanol sol of TiO2 for one day. The TiO2 sol was prepared as follows: 0.04 g TiO2 nanoparticles were stirred in 5 cm3 of dried 5 · 104 M methanol. Then, the material with immobilized TiO2 was immersed in 5 cm3 of a 5 · 105 M methanol solution of TbCl3 for the same period of time. After each immersion step the material was decanted, rinsed with methanol and dried for a day at room temperature. Finally, a three-component material consisting of TiO2 nanoparticles and Tb3+ ions entrapped in silica xerogel was obtained. 2.2. The apparatus X-ray diffraction (XRD) analysis of powdered TiO2 samples was performed using a Siemens D5000 diffractometer with Cu Ka1 radiation. Crystallite sizes were estimated with the Scherrer formula. Photoluminescence and photoluminescence excitation spectra were measured using a Perkin–Elmer LS 50B spectrofluorometer with a reflection spectra attachment. The spectra were corrected with respect to the apparatus’ response. The photoluminescence experiments were performed at room temperature. 3. Results 3.1. TiO2 nanoparticles The XRD patterns shown in Fig. 1 exhibit narrow diffraction peaks of nanosized TiO2 prepared by the reverse micelle method. The diffraction peak positions are attributable to the structure of rutile. Photoluminescence spectra were recorded at room temperature for the sample with the smallest TiO2 nanoparticles (90 nm in diameter). Its excitation spectrum (kem = 400 nm) consists of three bands. The TiO2 nanopowder irradiated with light of the wavelength related to the exciton position (kexc = 238 nm) exhibits the most intensive emission (cf. the spectra in Fig. 2). For the samples with various nanoparticle diameters, photoluminescence emission spectra excited at 238 nm exhibit a red shift of the central band with increasing diameter. Simultaneously, the band intensity decreases (Fig. 3).

M. Zalewska, A.M. Kłonkowski / Optical Materials 30 (2008) 725–729

727

120

8

100

6

4 (a)

3 2

(b)

1 (c) 0 30

40

50

60

70

80

Emission intensity I / arb. unit

(a)

5

3

Intensity I / 10 arb.unit

7

2θ / degree

80 (b) (c) 60

40

20

Fig. 1. XRD patterns of TiO2 nanoparticles of diameters estimated as (a) 90, (b) 110 and (c) 120 nm. 0 350

400

450

500

Wavelength λ / nm 120 excitation

Fig. 3. Photoluminescence emission spectra of TiO2 nanocrystals (a) 90, (b) 110 and (c) 120 nm in diameter. Excitation wavelength of 238 nm.

emission (a)

100

80

100

60

40 250 20

(b)

274 (c)

0 200

300

400 500 600 Wavelength λ / nm

emission

excitation

80 Intensity I / arb. unit

Intensity I / arb. unit

238 nm

(b)

60

230 nm 40

(a)

700

Fig. 2. Photoluminescence of 90 nm TiO2 nanoparticles. Excitation spectrum monitored at 400 nm. Emission spectra obtained under (a) 238, (b) 250 and (c) 274 nm excitation.

The emission quenching is distinct for both larger TiO2 nanoparticles, in comparison with the smallest ones.

20

0 200

251

300

400

500

600

700

Wavelength λ / nm

3.2. Tb(III) ions and nanosized TiO2 in SiO2 matrix

Fig. 4. Photoluminescence of a material consisting of TiO2 nanoparticles (diameter 90 nm) and Tb(III) ions entrapped in silica xerogel. Excitation spectrum monitored at 543 nm. Emission spectra obtained under (a) 355 and (b) 238 nm excitations.

The material consisting of the smallest TiO2 nanoparticles, 90 nm in diameter, and Tb(III) ions immobilized in silica xerogel has an excitation spectrum for kem = 543 nm (related to the 5D4 ! 7F5 transition, see Fig. 4). A band attributed to the 7F6 ! 5L9 transition in Tb(III) ions is situated at 355 nm [43]. There are also two sharp exciton

bands at 230 and 251 nm. Two emission spectra excited with kexc = 230 and 355 nm are shown on the right side of Fig. 4. Excitation by radiation of the lowest wavelength (related to the exciton band position) causes the highest emission intensity.

728

M. Zalewska, A.M. Kłonkowski / Optical Materials 30 (2008) 725–729

100

(a)

Emission intensity I / arb. unit

(b) 80

(c)

60

(a) (b)

40 (c)

20

0 400

500

600

700

Wavelength λ / nm

Fig. 5. Photoluminescence emission spectra of a material consisting of TiO2 nanocrystals and Tb(III) ions immobilized in silica xerogel. Nanocrystals’ diameters: (a) 90, (b) 110 and (c) 120 nm. Excitation wavelength of 238 nm.

In the experiment with luminescent materials based on Tb(III) ions, the diameter of the excited TiO2 nanoparticles increases, which causes a shift of the Tb(III) emission bands towards greater wavelengths and simultaneous, gradual emission quenching (Fig. 5). 4. Discussion The size of titania nanoparticles was estimated by parameters of the XRD patterns (Fig. 1) using the Scherrer equation [44]. The nanoparticles were (a) 90, (b) 110 and (c) 120 nm in diameter. In the case of the smallest nanoparticles, the excitation spectrum of luminescence monitored at 400 nm exhibited a dominating sharp band at 238 nm (Fig. 2). Among the band positions applied to luminescence excitation of the semiconductor particles, kexc = 238 nm distinctly enhanced the intensity of the 400 nm emission band. The position of the former band suggests that the semiconductor nanoparticles exhibit higher values of the energy gap, Eg, than bulk TiO2 (Eg = 3.15 eV) [45]. This band shifts towards greater wavelengths for the larger nanosized samples with their increasing diameter (Fig. 3), a phenomenon based on the quantum size effect corresponding to decrease of semiconductor energy gap with the nanoparticle size [46]. In the experiment with the three-component material (TiO2 nanocrystals, Tb(III) ions and a silica xerogel support) luminescence emission spectra were recorded using excitation radiation, when kexc corresponded to kmax of both exciton bands or the Tb(III) absorption band

(Fig. 4). The greatest enhancement of Tb(III) emission was observed when the spectrum was obtained under the 230 nm excitation, corresponding the lowest exciton position. This effect is evidence of energy transfer from excited semiconductor quantum dots to the emitting Tb(III) ions. Thus, the energy of the band gap in TiO2 is in resonance with the 7F6 ! 5L7 transition energy in Tb3+ ions [47]. In the improved spectrum, there are particularly pronounced increases in the intensity of bands attributed to 5 D3 ! 7F5 at 415 nm, as well as the 5D4 ! 7F6 and 7F5 transitions at 488 and 543 nm, respectively. Additionally, a large and sharp band of TiO2 emission is peaked at 400 nm, due to transition from one of the trap levels in the energy gap to the valence band [48]. A comparison of the exciton and emission band positions for ZnO nanoparticles suggests that the energy due to electron-hole recombination in semiconductor nanoparticles is partly non-radiatively lost and then used primarily for excitation to the 5T3 and 5T4 emitting states in Tb(III) ions. The observed luminescence emission spectrum with characteristic sharp bands is obviously attributable to transitions from these excited levels to the ground one in Tb(III). The remaining portion of energy is emitted from ZnO nanoparticles. Parallel to the red shift of the TiO2 emission band, in Fig. 5 is seen shifting related to Tb(III) emission bands. This effect is due to a reduction in the crystal field strength around the Tb(III) ions placed on the surface of TiO2 nanocrystals with increasing diameter. Our experiments have shown that effective emitting materials can be prepared when components active in luminescence, such as semiconductor quantum dots and Ln(III) ions, are incorporated into supports by impregnation. At the same time, when optically active components are added to the reaction mixture with alkoxides in the sol–gel procedure, only minor enhancement of emission intensity has been observed. Since the enhancement of materials emission intensity is based on the energy transfer from excited semiconductor nanoparticles to Ln(III) ions, it requires a close contact between these two components. Such condition of the support surface is ensured by the used impregnation method, as adding components during gelation in the sol–gel method immobilizes the species isolated from each other at a distance. 5. Conclusions TiO2 nanoparticles cannot be treated as quantum dots because their diameter exceeds the conventional size range (2–10 nm). In spite of this, phenomena based on the quantum size effect are observed, which means that the samples exhibit a higher value of the optical energy gap, Eg, than bulk TiO2 (Eg = 3.15 eV) and, consequently, the emission band of the TiO2 samples is shifted towards greater wavelengths as the size of their nanoparticles increases. Photoluminescence and photoluminescence excitation spectra have been measured for SiO2 doped with Tb3+ ions

M. Zalewska, A.M. Kłonkowski / Optical Materials 30 (2008) 725–729

and TiO2 nanocrystals. The spectra clearly demonstrate that the increase in emission intensity of Tb3+ ions is due to an effective excitation energy transfer from free electron-hole pairs in nanosized TiO2 particles to Tb3+. Our experiments have shown that the effective energy transfer from TiO2 nanocrystals to Tb(III) ions is possible if both optically active components are entrapped in a silica xerogel matrix by impregnation. In this case, there is close contact between the excited nanoparticles and the emitting lanthanide(III) ions. Acknowledgement Financial support from the Ministry of Education and Science (Grant No. 1308/T09/2005/29) is gratefully acknowledged. References [1] G. Blasse, B.C. Grabmaier, Luminescence Materials, Springer-Verlag, Heidelberg, 1994. [2] T. Ju¨stel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084. [3] A.O. Yoshima, in: Hiap L. Ong (Ed.), Electroluminescent Display, vol. 1, World Scientific, Singapore, 1995. [4] C.A. Kodaira, H.F. Brito, O.L. Malta, O.A. Serra, J. Lumin. 101 (2003) 11. [5] W.T. Carnal, G.L. Goodman, K. Rajank, R.S. Rana, J. Chem. Phys. 90 (1989) 3443. [6] J. Ho¨lsa¨, P. Porcher, J. Chem. Phys. 75 (1981) 2108. [7] M.A. Bizeto, V.R.L. Constantino, H.F. Brito, J. Alloys Compd. 311 (2000) 159. [8] O.A. Serra, E.J. Nassar, C.A. Kodaira, C.R. Neri, P.S. Calefi, I.L.V. Rosa, Spectrochim. Acta A 54 (1998) 2077. [9] J.-C.G. Bu¨nzli, G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earth Science: Theory and Practice, Elsevier, Amsterdam, 1989, Chapter 7. [10] L.E. Brus, J. Chem. Phys. 79 (1983) 5566. [11] A.P. Alivisatos, Science 271 (1996) 933. [12] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [13] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [14] V.I. Klimov, A.A. Mikhailovsky, S. Xu, A. Malko, J.A. Holligsworth, C.A. Leatherdale, H.J. Eisler, M.G. Bawendi, Science 290 (2000) 314. [15] Y.A. Nazzal, L. Qu, X. Peng, M. Xiao, NanoLetters 3 (2003) 819. [16] A. Henglein, Top. Curr. Chem. 143 (1988) 113. [17] M.L. Steigerwald, L.E. Brus, Acc. Chem. Res. 23 (1990) 183. [18] N. Chestnoy, T.D. Harris, L.E. Brus, J. Phys. Chem. 90 (1986) 3393. [19] M.G. Bawendi, L.M. Steigerwald, L.E. Brus, Ann. Rev. Phys. Chem. 41 (1990) 477.

729

[20] X.C. Wu, A.M. Bittner, K. Kern, J. Phys. Chem. B 109 (2005) 230. [21] H. Zhao, E.P. Douglas, B.S. Harrison, K.S. Schanze, Langmuir 17 (2001) 8428. [22] H.S. Zhou, I. Honma, H. Komiyama, J.W. Haus, J. Phys. Chem. 97 (1993) 895. [23] M. Pileni, Catal. Today 58 (2000) 151. [24] A. Wolf, D.D. Awschalow, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [25] R.N. Bhargava, J. Lumin. 70 (1996) 85. [26] F.V. Mikulec, M. Kuno, M. Bennati, D. Hall, R. Griffin, M.G. Bawendi, J. Am. Chem. Soc. 122 (2000) 2532. [27] R.S. Kane, R.E. Cohen, R. Silbey, Chem. Mater. 90 (1999) 11. [28] M. Ihara, T. Igarashi, T. Kusunoki, K. Ohno, J. Electrochem. Soc. 147 (2000) 2355. [29] T. Kushida, A. Kurita, M. Watanabe, Y. Kanematsu, J. Lumin. 87– 89 (2000) 466. [30] W. Chen, W.J.O. Malm, V. Zwiller, Y. Huang, S. Liu, R. Wallenberg, J.O. Bovin, L. Samuelson, Phys. Rev. B 61 (2000) 11021. [31] M. Morita, D. Rau, H. Fujii, K. Minami, S. Murakami, M. Baba, M. Yoshita, H. Akiyama, J. Lumin. 87–89 (2000) 478. [32] N. Pradhan, D. Goorskey, J. Thessing, X. Peng, J. Am. Chem Soc. 127 (2005) 17586. [33] C. Tiseanu, R.K. Mehra, R. Kho, M. Kumke, Chem. Phys. Lett. 377 (2003) 131. [34] M. Nogami, T. Enomoto, T. Hayakawa, J. Lumin. 97 (2002) 147. [35] A.M. Kłonkowski, M. Zalewska, B. Kos´cielska, J. Non-Cryst. Solids 352 (2006) 4183. [36] M. Zalewska, B. Kuklin´ski, J. Mahlik, E. Grzanka, B. Pałosz, J. Jezierska, M. Grynberg, A.M. Kłonkowski, J. Lumin., submitted to publication. [37] Y. Sorek, R. Reisfeld, A.M. Weiss, Chem. Phys. Lett. 244 (1995) 371. [38] P.M. Kumar, S. Badrinaryanan, M. Sastry, Thin Solid Films 358 (2000) 122. [39] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 (1988) 5196. [40] Y. Wang, H. Cheng, L. Zhang, Y. Hao, J. Ma, B. Xu, W. Li, J. Mole. Catal. A: Chem. 151 (2000) 205. [41] C.J. Brinker, G.W. Scherer, Sol–Gel Science, The Physics and Chemistry of Sol Gel Processing, Academic Press, London, 1990. [42] P.A. Sant, P.V. Kamat, Phys. Chem. Chem. Phys. 4 (2002) 198. [43] C. Tiseanu, L. Frunza, M. Kumke, Physica B 352 (2004) 358. [44] A.S. Edelstein, R.C. Cammarata (Eds.), Nanomaterials. Synthesis, Properties and Applications, Inst. Physics Publ., Bristol, 1998, p. 74, and 234. [45] M. Ga¨rtner, V. Dremov, P. Mu¨ller, H. Kisch, Chem. Phys. Chem. 6 (2005) 714. [46] K.J. Klabunde (Ed.), Nanoscale Materials in Chemistry, WileyInterscience, New York, 2001, p. 64. [47] Y. Masumoto, in: S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC, Boca Raton, FL, 1999, p. 78. [48] K.K. Nanda, S.N. Sahu, Solid State Commun. 111 (1999) 6.