DSpace at VNU: Synthesis and spectroscopic properties of ZnAl 2O 4:Co 2+ via a hydrothermal process

e-Journal of Surface Science and Nanotechnology e-J Surf Sci Nanotech Vol (2011) 526-530 27 December 2011 Conference - IWAMN2009 - Synthesis and Spectroscopic Properties of ZnAl2 O4 :Co2+ via a Hydrothermal Process∗ Trinh Thi Loan,† Nguyen Ngoc Long, and Le Hong Ha Faculty of Physics, Hanoi university of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnum (Received 11 December 2009; Accepted 18 March 2011; Published 27 December 2011) The ZnAl2 O4 :Co2+ powders with the low Co2+ dopant concentrations (0.5 at.%) have been synthesized by hydrothermal method using the following precursors: zinc chloride (ZnCl2 ), aluminium chloride (AlCl3 ·6H2 O), cobalt nitrate (Co(NO3 )2 ), urea (CO(NH2 )2 ), ethanol (C2 H5 OH) and sodium hydroxide (NaOH) The effect of the amounts of NaOH, CO(NH2 )2 and thermal treatment conditions on formatting the ZnAl2 O4 structure and their optical properties were investigated The results showed that the structure and the size of ZnAl2 O4 crystallites strongly depended on the amount of NaOH, CO(NH2 )2 and thermal treatment conditions The photoluminescence and photoluminescence excitation spectra of the samples strongly depended on the thermal treatment conditions For the samples which did not undergo thermal treatment, their optical properties almost did not depend on the amounts of CO(NH2 )2 For the samples undergone thermal treatment at 800 ◦ C and 1200 ◦ C, the photoluminescence and photoluminescence excitation spectra were totally changed They strongly depended on the excitation wavelength and amount of urea in the initial sample synthesis process [DOI: 10.1380/ejssnt.2011.526] Keywords: ZnAl2 O4 :Co2+ spinel; Hydrothermal method; Structural properties; Photoluminescent properties I INTRODUCTION Zinc aluminate ZnAl2 O4 spinel is well-known direct wide-bandgap semiconductor, whose optical bandgap has been to be about 3.8 eV [1, 2] ZnAl2 O4 has been widely used for transparent conductors, dielectrics Besides these, ZnAl2 O4 is also widely used in many catalytic reactions, such as cracking, hydrogenation, dehydrogenation and dehydration, in various chemical and petrochemical industries [3] ZnAl2 O4 spinel has a unique combination of attractive properties such as high thermal stability, better diffusion and ductility; hence it is used as high temperature material and optical coating Recently, ZnAl2 O4 has also attracted interest as an important phosphor host material for applications in thin film electroluminescent displays, mechano-optical stress sensors and stress imaging devices [4] There are many methods to prepare ZnAl2 O4 powders, such as the solid-state reaction method, sol-gel, chemical co-precipitation and hydrothermal synthesis In this work, Co2+ -doped ZnAl2 O4 powders (0.5 at.%) were synthesized by hydrothermal method In order to obtain a ZnAl2 O4 single phase, the effect of the amounts of NaOH, CO(NH2 )2 and thermal treatment conditions on formatting the ZnAl2 O4 structure and their optical properties were investigated II EXPERIMENTAL Co2+ -doped ZnAl2 O4 powders were prepared via a hydrothermal process The starting materials used were ZnCl2 , AlCl3 ·6H2 O, Co(NO3 )2 , CO(NH2 )2 , C2 H5 OH, and NaOH Firstly, 7.4229 g of AlCl3 ·6H2 O, 2.0340 g of ZnCl2 and 0.909 g of CO(NH2 )2 were dissolved in 25 ml of distilled water Then, 0.75 ml of 2M Co(NO3 )2 was added to the above solution, followed by steady stirring ∗ This paper was presented at the International Workshop on Advanced Materials and Nanotechnology 2009 (IWAMN2009), Hanoi University of Science, VNU, Hanoi, Vietnam, 24-25 November, 2009 † Corresponding author: loan.trinhthi@gmail.com FIG 1: XDR patterns for the samples synthesized at T = 200 ◦ C for 96 h with 0.9009 g of CO(NH2 )2 , and different volumes of 10 M solution of NaOH (⋆) AlO(OH), (×) ZnAl-LDH, (◦) ZnO, and (+) ZnAl2 O4 for 30 An appropriate quantity of 10 M solution of NaOH and 10 ml of C2 H5 OH were added in to the last solution, followed by continuous steady stirring The solution was then transferred into Teflon-lined steel autoclaves and kept at reaction temperature 200 ◦ C for 96 h Thereafter, solid material was filtered off, washed with distilled water and ethanol, and then dried in air at 100◦ C for 24 h The resulting powder was annealed at 800◦ C and 900◦ C in air for h The crystal structure and morphology of the synthetic samples were characterized by a Siemens D5005 X-ray diffraction (XRD) diffractometer and a JEOL JEM 1010 transmission electron microscope (TEM), respectively In order to determine the elemental composition of the produced samples, the energy dispersive spectra (EDS) were recorded by means of an equipment EDS ISIS 300 OXFORD attached to the JEOLJSM 5410 LV scanning electron microscope Photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra were measured at room temperature using a Fluorolog FL3-22 spectrofluorometer with a Xenon lamp of 450 W being used as an excitation source c 2011 The Surface Science Society of Japan (http://www.sssj.org/ejssnt) ISSN 1348-0391 ⃝ 526 e-Journal of Surface Science and Nanotechnology Volume (2011) TABLE I: The dependence of the lattice constants and the average crystalline sizes on the amount of CO(NH2 )2 mCO(NH2 )2 (g) 0.4505 0.9009 1.8018 2.7027 3.6036 4.5045 a (˚ A) 8.11 ± 0.01 8.11 ± 0.02 8.12 ± 0.01 8.11 ± 0.01 8.11 ± 0.01 8.10 ± 0.01 8.10 ± 0.01 D (nm) 06.0 10.0 10.2 13.8 19.7 53.0 57.6 FIG 2: XDR patterns for the samples synthesized at T = 200 ◦ C for 96 h with 10 ml of 10 M solution of NaOH, and different amount of CO(NH2 )2 FIG 4: XDR patterns for the samples with different amount of CO(NH2 )2 heat-treated at 1200◦ C for h FIG 3: XDR patterns of the samples with different amounts of CO(NH2 )2 heat-treated at 800 ◦ C for h III RESULTS AND DISCUSSION The XRD patterns of samples prepared from 0.9009 g of CO(NH2 )2 , 10 ml of C2 H5 OH and different volumes of 10 M solution of NaOH, undergone hydrothermal process at temperature T = 200◦ C for 96 h are shown in Fig As can be seen from the XRD patterns, the formation of ZnAl2 O4 phase is very sensitive to the solution NaOH For the samples prepared with volume of 10 M NaOH solution (VNaOH (10M) ) equal to ml, the observed XRD pattern can be indexed to the AlO(OH) phase No diffraction peaks from other impurities were detected For the samples with VNaOH (10M) = ml, the XRD pattern is composed of diffraction peaks corresponding to the ZnAl2 O4 phase and some weak peaks of the AlO(OH) phase and ZnAl layered double hydroxide (ZnAl-LDH) phase Pure ZnAl2 O4 spinel phase can be obtained only with the VNaOH (10M) = 10 ml The XRD pattern of the samples with VNaOH (10M) = 12 ml exhibits several distinct peaks corresponding to the ZnAl2 O4 , ZnAl-LDHs and ZnO phases For the samples with VNaOH (10M) = 15 ml, all the diffraction peaks can be well indexed to a hexagonal wurtzite structure of ZnO The representative XDR patterns for the samples synthesized at T = 200 ◦ C for 96 h with 10 ml of 10 M NaOH solution, 10 ml of C2 H5 OH and different amounts of CO(NH2 )2 are presented in Fig The results showed that all the samples were the ZnAl2 O4 spinel monophase and the half-width of the XDR lines strongly depended on the amount of CO(NH2 )2 The lattice constants and the average crystalline sizes of all samples calculated from the XRD patterns are shown in Table I With increasing the amount of CO(NH2 )2 , the lattice parameters almost keep constant, while the size nanocrystals increases FIG 5: TEM micrograph of the sample synthesized at T = 200◦ C for 96 h with g CO(NH2 )2 , and 10 ml of 10 M NaOH solution http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) 527 Loan, et al Volume (2011) FIG 6: EDS spectrum of the sample synthesized at T = 200◦ C for 96 h with 3.6036 g of CO(NH2 )2 , and 10 ml of 10 M NaOH solution Figures and show the XDR patterns of the samples with different amounts of CO(NH2 )2 heat-treated at 800◦ C and 1200◦ C, respectively As seen from the XDR patterns, with increasing heat-treatment temperature, the intensity of the diffraction peaks increases, which indicates an increase of the crystalline size It can be noticed that all the samples with different amounts of CO(NH2 )2 heat-treated at 800◦ C are the ZnAl2 O4 spinel monophase But when heat-treatment carried out at 1200◦ C, for the samples with g and 4.5045 g of CO(NH2 )2 , in addition to the diffraction peaks of the ZnAl2 O4 spinel phase, the Al2 O3 phase peaks are also observed The lattice constants and the average crystalline sizes of all the samples calculated from the XRD patterns are shown in Table II The results showed that lattice constants of the ZnAl2 O4 crystallites very weakly depended on the amount of CO(NH2 )2 and thermal treatment conditions, while the average crystalline sizes strongly depended on the amount of CO(NH2 )2 and thermal treatment conditions Figure shows the typical morphology of ZnAl2 O4 samples synthesized at T = 200◦ C for 96 h with g CO(NH2 )2 , 10 ml of C2 H5 OH and 10 ml of 10 M NaOH solution The average grain sizes are about nm, which is consistent with the result of the XRD analysis The EDS spectrum of sample synthesized at T = 200◦ C for 96 h with 3.6036 g of CO(NH2 )2 , 10 ml of C2 H5 OH and 10 ml of 10 M NaOH solution is presented in Fig The result showed that this sample is only composed of Zn, Al and O elements Co element is not detected due to a relatively low doping concentration The PL and PLE spectra for the samples synthesized at T = 200 ◦ C for 96 h with 10 ml of 10 M NaOH solution, 10 ml of C2 H5 OH, and different amounts of CO(NH2 )2 are depicted in Figs and 8, respectively The results show that the PL and PLE spectra of the samples did not depend on the amount of CO(NH2 )2 The PL and PLE spectra point out that the Co2+ ions locate in tetrahedral sites in the ZnAl2 O4 host crystals When the wide-bandgap oxide materials are doped with cobalt, they exhibit a luminescence in the visible region 528 FIG 7: PL spectra for the samples synthesized at T = 200◦ C for 96 h with 10 ml of 10 M solution of NaOH, and different amounts of CO(NH2 )2 , excited by the wavelength of 535 nm (a) g, (b) 0.4505 g, (c) 1.8018 g, (d) 2.7027 g, and (e) 3.6036 g FIG 8: PLE spectra for the samples synthesized at T = 200◦ C for 96 h with 10 ml of 10 M solution of NaOH, and different amounts of CO(NH2 )2 , recorded at 689 nm (a) g, (b) 0.4505 g, (c) 1.8018 g, (d) 2.7027 g, and (e) 3.6036 g due to the emission transitions within Co2+ ions It is known that divalent cobalt ions occupy tetrahedrally coordinated sites in spinel crystals and demonstrate a bright red luminescence [5] The crystal-field splitting of energy levels of Co2+ (3d7 ) in a tetrahedral field is similar to that of Cr3+ (3d3 ) in an octahedral field, but the value of crystal field parameter is significantly smaller in the octahedral case [6] Therefore, according to the TanabeSugano diagram, the luminescence sharp lines at 663, 675, 689, 698, and 708 nm are assigned to the E(2 G) → A2 (4 F) zero-phonon transition and its vibronic transitions of tetrahedral Co2+ ions in the ZnAl2 O4 host crystals The broad band centered at 535 nm in the PLE spectra assigned to the A2 (4 F) → T2 (4 F) transition of tetrahedral Co2+ ions in the ZnAl2 O4 host crystals Figures and 10 show the PL spectra of the samples with different amounts of CO(NH2 )2 heat-treated at 800◦ C, excited by the wavelength of 406 and 546 nm, respectively Compared with the samples which did not undergo thermal treatment, the PL spectra of the sam- http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume (2011) TABLE II: The dependence of the lattice constants and the average crystalline sizes on the amount of CO(NH2 )2 with different heat-treatment temperature T = 800◦ C mCO(NH2 )2 (g) 1.8018 2.7027 4.5045 a (˚ A) 8.08 ± 0.01 8.09 ± 0.01 8.09 ± 0.01 8.09 ± 0.01 T = 1200◦ C D (nm) 20.8 32.6 41.1 58.2 a (˚ A) 8.09 ± 0.01 8.09 ± 0.01 8.09 ± 0.01 8.09 ± 0.01 D (nm) 84.8 77.0 73.2 88.0 FIG 9: PL spectra for the samples heat-treated at 800◦ C for h with different amounts of CO(NH2 )2 , excited by the wavelength of 406 nm (a) g, (b) 0.4505 g, (c) 1.8018 g, (d) 2.7027 g, and (e) 3.6036 g FIG 11: PL spectra for the samples heat-treated at 1200◦ C for h with different amounts of CO(NH2 )2 , excited by the wavelength of 406 nm (a) 0.0450, 1.8018, and 2.7027 g, (b) 0, 3.6036, and 4.5045 g FIG 10: PL spectra for the samples heat-treated at 800◦ C for h with different amounts of CO(NH2 )2 , excited by the wavelength of 546 nm (a) g, (b) 0.4505 g, (c) 1.8018 g, (d) 2.7027 g, and (e) 3.6036 g ples undergone thermal treatment are totally different It is seen from Figs and 10 that the PL spectra of the samples strongly depended on the excitation wavelength The PL spectra of samples under excitation wavelength at 406 nm consist of seven sharp lines at 665, 675, 686, 698, 708, 717, and 723 nm and a broad emission band with the maximum located near 657 nm wavelength Whereas under excitation wavelength at 546 nm, the PL spectra of samples only consist of a broad emission band with two peaks about at 646 and 656 nm The sharp line at 686 nm is associated to the E(2 G) → A2 (F) zero-phonon transition within tetrahedral Co2+ ions in the ZnAl2 O4 host crystals The sharp lines at 665, 675 nm are associated to the E(2 G) → A2 (F) transition with absorption of phonons, the sharp lines at 698, 708, 717, and 723 nm are associated to the E(2 G) → A2 (F) transition with emission of phonons The emission band with two peaks at 646 nm and 656 nm (Fig 10) and the broad emission band centered at 657 nm (Fig 9) are attributed to the T1 (4 P)→ A2 (4 F) transition in a tetrahedral field of Co2+ ions The PL spectra excited by 546 nm for the samples heattreated at 1200◦ C and 800◦ C for h, basically exhibited the same fine structure The PL spectra of samples also consist of a broad emission band with two peaks about at 646 and 656 nm However, it is seen from Fig 11, under http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) 529 Loan, et al Volume (2011) at 399 and 414 nm for the excitation wavelength of 693 nm (line (b) in Fig 12) detected in the PLE spectra are assigned to the A2 (4 F) → T1 (4 F) transitions The bands at 406, 476, 546, 572 nm are assigned to A2 (4 F) → T2 (2 H), A2 (4 F) → T1 (2 P), A2 (4 F) → A1 (2 G) and A2 (4 F) → T1 (4 P) transitions, respectively The bands at 593 and 620 nm are assigned to vibronic transitions of band at 572 nm [7] IV FIG 12: PLE spectra for the samples with 3.6036 g of CO(NH2 )2 , heat-treated at 1200◦ C for h recorded at different wavelengths (a) 686 nm, (b) 693 nm, and (c) 660 nm excitation wavelength of 406 nm, in the PL spectra for the samples heat-treated at 1200 ◦ C is not observed the broad emission band with the maximum at 665 nm like as in the samples heat-treated at 800 ◦ C Beside, there are significant differences between the intensity ratio of the 686 nm line to the 693 nm line for the sample group with 0.0450 g, 1.8018 g, 2.7027 g of CO(NH2 )2 and the sample group with g, 3.6036 g, 4.5045 g of CO(NH2 )2 This difference may be due to the existence of the Al2 O3 phase in the samples with g, 3.6036 g and 4.5045 g of CO(NH2 )2 Figure 12 shows the PLE spectra for the samples heattreated at 1200 ◦ C for h recorded at different wavelength The bands at 392 and 414 nm for the excitation wavelength of 686 nm (line (a) in Fig 12) or the bands [1] S K Sampath and J F Codaro, J Am Ceram Soc 81, 649 (1998) [2] S K Sampath, D G Kanhere, and R Pandey, J Phys Condens Matter 11, 3635 (1999) [3] G Ritter, H Noller, and J A Lercher, J Chem Soc., Faraday Trans 78, 2239 (1982) [4] V Singh, R P S Chakradhar, J L Rao, and D K Kim, J Lumin 128, 394 (2008) [5] Y V Volk, A M Malyarevich, K V Yumashev, I P Alek- 530 CONCLUSION The effect of the amounts of NaOH, CO(NH2 )2 and thermal treatment conditions on formatting the ZnAl2 O4 structure and their optical properties have been investigated The results showed that the structure and the size of ZnAl2 O4 crystallites strongly depended on the amount of NaOH, CO(NH2 )2 and thermal treatment conditions The PL and PLE spectra of the samples strongly depended on the thermal treatment conditions For the samples which did not undergo thermal treatment, their optical properties almost did not depend on the amounts of CO(NH2 )2 For the samples undergone thermal treatment at 800◦ C and 1200◦ C, the PL and PLE spectra were totally changed They strongly depend on the excitation wavelength and amount of CO(NH2 )2 in the initial sample synthesis process Acknowledgments This work is financially supported by Ministry of Science and Technology of Vietnam (Project No 103.02.51.09 from NAFOSTED) seeva, O S Dymshits, A V Shashkin, and A A Zhilin, J Non-Cryst Solids 353, 2408 (2007) [6] X Duan, D Yuan, X Cheng, H Sun, Z Sun, X Wang, Z Wang, D Xu, and M Lv, J Am Ceram Soc 88, 399 (2005) [7] J Ferguson, D L Wood, and L G Van Unitert, J Chem Phys 51, 2904 (1969) http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) ... bands at 392 and 414 nm for the excitation wavelength of 686 nm (line (a) in Fig 12) or the bands [1] S K Sampath and J F Codaro, J Am Ceram Soc 81, 649 (1998) [2] S K Sampath, D G Kanhere, and. .. increase of the crystalline size It can be noticed that all the samples with different amounts of CO(NH2 )2 heat-treated at 800◦ C are the ZnAl2 O4 spinel monophase But when heat-treatment carried... constants and the average crystalline sizes of all the samples calculated from the XRD patterns are shown in Table II The results showed that lattice constants of the ZnAl2 O4 crystallites very weakly