Available online at www.sciencedirect.com
Materials Letters 62 (2008) 1649 – 1651 www.elsevier.com/locate/matlet
Optical properties of zinc oxide nanocrystals embedded in mesoporous silica T.S. Vaishnavi a , Prathap Haridoss a,⁎, C. Vijayan b a
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India b Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India Received 6 June 2007; accepted 21 September 2007 Available online 29 September 2007
Abstract This paper reports on the synthesis and characterization of a nanocomposite incorporating zinc oxide (ZnO) in mesoporous silica (MPS), a host medium for stabilizing the nanoparticles. The composite is prepared using a chemical route involving in-situ synthesis of ZnO in pores of host material in an acidic medium. The samples are characterized by X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS), high resolution Transmission Electron Microscopy (TEM), UV–Vis absorption and Photoluminescent (PL) spectroscopy. The UV–Vis spectra and the PL spectra of the composite show blue shift in the position of the absorption band, indicating quantum confinement effect. On excitation at 320 nm, the samples exhibit a relatively narrow UV fluorescence emission band around 350 nm and an exciton recombination luminescence peak around 405 nm. The generally observed defectrelated green emission is not observed, possibly being too low in intensity when compared to UV/blue emission. © 2007 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Nanomaterials; ZnO; Mesoporous silica; Optical properties
1. Introduction The discovery of mesoporous materials has led to a new era of inclusion chemistry in the field of nanoscience and technology. Among porous materials, mesoporous silica since its first report [1] continues to attract recent attention as an efficient medium for embedding nanoparticles [2–6]. Silica mesoporous material of type MCM-41, a member of M41S family exhibits a hexagonal arrangement of uniform mesopores and the dimensions of the pores can be engineered in the range from 2 to 30 nm [7] by the use of different template molecules. Semiconductor nanoparticles such as PbS, CdS and ZnO incorporated in the pores of the mesoporous solids [2–4] have received considerable attention in the past decade in view of their applications in optoelectronic devices. In general, photoluminescent property is found to be enhanced by incorporating the nanoparticles in the mesoporous host matrix [5,6]. Zinc oxide is a multifunctional semiconductor material that has applications in ultraviolet/blue emission devices [8], laser
⁎ Corresponding author. Tel.: +91 44 2257 4771; fax: +91 44 2257 0545. E-mail address:
[email protected] (P. Haridoss). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.051
diodes [9], solar cells [10] and in acoustic devices [11]. It's multifunctionality is due to features such as a wide band gap (3.37 eV), a large exciton binding energy (∼60 meV) at room temperature, piezoelectric property, and its surface properties [12]. ZnO is a prominent excitonic material, suited for light emission applications since its bound electron hole pairs have a high radiative recombination probability [13]. Many strategies have been employed to incorporate ZnO in MPS host material including electrochemical deposition [14], static intercalation technique [6], and using monolithic MPS host by sol–gel method [4]. The use of hydrophobic zinc complex as the precursor [6] has lead to a very homogeneous distribution of ZnO in MPS. While there are several reports on ZnO loaded in MPS the UV emission in PL spectra were not significantly blue shifted relative to that of bulk ZnO. These reports also indicate defect-related emissions in the green region of the spectrum. In the present work, growth, characterization and study of optical properties of ZnO embedded in MPS have been carried out. ZnO is grown in-situ while the porous structure of the MPS is formed in an acidic medium. The techniques of optical absorption and PL spectroscopy are used to determine the band gap and to analyze the emission properties. The UV emission is found to be significantly blue shifted when compared to similar spectra
1650
T.S. Vaishnavi et al. / Materials Letters 62 (2008) 1649–1651
Fig. 1. XRD patterns of ZnO in mesoporous silica. The two patterns are from samples prepared using two different reactant concentrations.
reported in literature while the defect-related emissions in the green region of the spectrum are not detected. 2. Experimental details
Fig. 3. PL spectra of MS-ZnO 1 and MS samples when excited with 320 nm radiation.
comparison another sample is prepared in the same condition but without the addition of the zinc precursor, which results in mesoporous silica without any nanoparticle incorporation, and is given the sample code MS.
2.1. Preparation of ZnO/MPS composite 2.2. Characterization of composite samples The preparation of MPS is essentially the same as reported in the Ref. [15], using cetyl trimethyl ammonium bromide (CTAB) as the structure directing agent, in an acidic medium. In this method a solution is prepared using 8.35 g of tetraethoxy silane (TEOS) in 16 g of ethanol and 4 g of 0.1 M HCl. The solution is stirred thoroughly, and in this solution one other solution is introduced which contains 3 g of CTAB and 0.75 g of zinc acetate in 16 g of ethanol. The mixture is rigorously stirred for about 30 min. and then dried in an oven at 80 °C for about 10 h. The dried powder sample is then heat treated at 550 °C for about 12 h, in air, to get rid of the surfactant CTAB and for the zinc precursor to get oxidized to form ZnO. This sample is indicated as MS-ZnO 1. For
Fig. 2. UV–Vis absorption spectrum of MS-ZnO 1 and MS samples.
XRD studies of the samples are carried out on a Bruker axs D8Discover diffractometer using Cu-Kα radiation. Compositional analysis is obtained using FEI Model quanta 200 SEM-EDS. Optical absorption is taken using a CARY 5E UV–Vis spectrophotometer. PL was carried out using SPECTRAQ Fluorolog spectrophotometer. HRTEM micrographs were recorded using a JEOL 3010, operating at 200 keV.
Fig. 4. HRTEM image showing the morphology of the ZnO in mesoporous silica. The inset shows lattice fringes.
T.S. Vaishnavi et al. / Materials Letters 62 (2008) 1649–1651
3. Results and discussion Fig. 1 shows the XRD pattern of the MS-ZnO 1 nanocomposite sample. As the concentration of ZnO was low in the composite, prominent peaks were not detected using the XRD technique. To confirm ZnO nanocrystallite formation, another sample (MS-ZnO 2) was prepared under the same condition with slight higher concentration of zinc acetate (i.e. 1 g, instead of 0.75 g, of zinc acetate was used). The XRD pattern of the sample with higher concentration, also shown in Fig. 1, unambiguously shows the presence of hexagonal wurtzite crystals. To further confirm the existence of ZnO in the mesoporous silica, elemental composition analysis was performed for MS-ZnO 1 using SEM-EDS. It is found that the atomic ratio of Si/O/Zn is about 31:68:1, thus indicates the existence of zinc though present in small quantity. EDS data together with the XRD result confirms the ZnO presence. To study the influence of the particle size of ZnO (by incorporation in the host matrix) on its electronic band gap, an UV–Vis absorption spectrum is recorded and is shown in the Fig. 2. The MS-ZnO 1 sample shows an absorption band around 350 nm which is shifted towards the lower wavelength region when compared to the bulk bandgap (375 nm) of ZnO. Such a shift is most likely due to quantum confinement effect. For comparison, the absorption spectrum of MS sample is also shown in the same figure. A sharp peak around 220 nm is observed in both samples and this can be attributed to the mesoporous silica absorption in the lower wavelength region. Room temperature photoluminescence measurement was carried out with an excitation wavelength of 320 nm for both MS and MS-ZnO 1 samples, and the spectrum is shown in Fig. 3. From the graph, it is clear that MS does not contribute to the luminescence and the observed intensity of the MS-ZnO 1 sample can be attributed solely due to the ZnO nanoclusters present in the system. In general, the nature of the PL spectrum is known to depend on the method of synthesis employed [5,6,14]. Usually ZnO shows two bands in the PL emission spectrum, one in the UV region and the other in the green region of the electromagnetic spectrum. The UV emission is attributed to the near band gap luminescence of the ZnO nanoparticles while the green emission originates due to the surface states present due to oxygen vacancies in the nanoparticles. In the PL spectrum observed in the present work, shown in Fig. 3, there are two peaks, at 355 nm and at 401 nm for the ZnO incorporated sample. The peak at 355 nm is due to band gap luminescence, which has been shifted when compared to the bulk ZnO band gap, attributed to the quantum confinement effect of ZnO nanoparticles in MPS. The peak observed at 401 nm could be due to the interface traps in the depletion region arising as a result of the existence of ZnO–SiO2 interface in the material as reported [16]. The oxygen related defect peak in the green region around 525 nm is quenched in the spectrum.
1651
HRTEM micrographs obtained from MS-ZnO 1 are shown in the Fig. 4. This figure shows the morphology of the MS-ZnO 1 at lower magnification while lattice fringes observed in the same sample are shown as an inset in the same figure. The presence of lattice fringes indicates that the MPS has retained its crystallinity even after the incorporation of the ZnO nanoparticles.
4. Conclusions Quantum confinement effects are observed under experimentally accessible conditions in the case of ZnO nanoparticles embedded in MPS host matrix, prepared by a surfactant-assisted chemical process. The sample is found to be a very good PL emitter at 355 nm and free of oxygen defect-related emission. HRTEM micrographs show an ordered structure for sample, indicating that the presence of ZnO nanoparticles in the host structure of MPS has not affected the crystalline nature of mesoporous silica. Mesoporous silica provides an excellent and convenient matrix for stabilizing embedded ZnO nanoparticles. References [1] M. Ogawa, J. Am. Chem. Soc. 116 (1994) 7941. [2] D. Buso, P. Falcaro, S. Costacurta, M. Guglielmi, A. Martucci, Chem. Mater. 17 (2005) 4965. [3] M. Wark, H. Wellmann, J. Rathousky, Thin Solid Films 458 (2004) 20. [4] B. Yao, H. Shi, H. Bi, L. Zhang, J. Phys., Condens. Matter 12 (2000) 6265. [5] C. Bouvy, W. Marine, Bao-Lian Su, Chem. Phys. Lett. 438 (2007) 67. [6] L.I. Burova, D.I. Petukhov, A.A. Eliseev, A.V. Lukashin, Yu.D. Tretyak, ov, Superlattices Microstruct. 39 (2006) 257. [7] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, et al., J. Am. Chem. Soc. 114 (1992) 10834. [8] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, et al., Solid State Commun. 103 (1997) 459. [9] H. Cao, J.Y. Xu, E.W. Seelig, R.P.H. Chang, Appl. Phys. Lett. 76 (2000) 2997. [10] O. Kluth, B. Rech, H. Wagner, 17th European Photovoltaic Solar Energy Conference, 2001. [11] Wen-Ching Shih, Mu-Shiang Wu, J. Cryst. Growth 137 (1994) 319. [12] Wolfgang H. Hirschwald, Acc. Chem. Res. 18 (1985) 228. [13] U. Koch, A. Fojtik, H. Weller, A. Henglein, Chem. Phys. Lett. 122 (1985) 507. [14] F. Gao, S.P. Naik, Y. Sasaki, T. Okubo, Thin Solid Films 495 (2006) 68. [15] D. Zhao, P. Yang, Nick Melosh, J. Feng, B.F. Chmelka, G.D. Stucky, Adv. Mater. 10 (1998) 1380. [16] H.G. Chen, J.L. Shi, H.R. Chen, J.N. Yan, Y.S. Li, Z.L. Hua, et al., Opt. Mater. 25 (2004) 79.
