- Email: [email protected]
Y2O3:Eu nanocable arrays
Journal of Physics and Chemistry of Solids 137 (2020) 109215
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Competition of emission and lattice defects participating in energy transfer for ZnO:Tb/Y2O3:Eu nanocable arrays Lei Yang *, Qianwen Liu, Hongying Zheng, Siyuan Zhou, Wei Zhang College of Materials Science and Engineering, Hunan University, Changsha, 410082, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanostructures Sol–gel growth Electron microscopy Optical properties
Lattice defects participating in energy-transfer behaviour and competition of emissions for rare-earth-doped nanocable arrays are investigated in this article. ZnO:Tb3þ/Y2O3:Eu3þ nanocable arrays embedded in anod ized aluminium are synthesized via two-step template assembly approach. The outer diameter of the nanocable is about 61–71 nm. The core diameter is about 21–41 nm. The competition of emissions and lattice defects involving energy-transfer behaviour are found. With adjustment of the molar ratio of doping ion to host lattice cation to 0.01–0.05, the luminescence intensity increases first and then decreases because of concentration quenching. Although the chromaticity coordinates of nanocable arrays varies as doping concentration changes, they stay in the yellow region. In this process, the oxygen vacancy, interstitial zinc and interstitial oxygen serve as media in the energy-transfer process. Simultaneously, energy transfers from oxygen vacancy to Eu3þ and Tb3þ are also found, but energy transfer between Eu3þ and Tb3þ is not found. These nanocable arrays will have wide application in photoelectronic devices and luminescence materials.
1. Introduction Nanostructure is a type of significant structure which has aroused much attention because of a large number of applications in photo electronic devices and luminescence materials [1–12]. Compared with ordinary micrometer and nanomaterials, nanocable arrays can provide better display resolution whilst not reducing luminous intensity. The main emission band of Tb3þ and Eu3þ belongs to primary colours [2–15], from which a large number of colours can be tailored. Although mixing fluorescent powders can also tailor the luminescence colours, it is difficult to obtain stable and uniform emissions. Nanocables have huge advantages which avoid these defects. As competition of emissions is unavoidable for rare earth (RE)-doped nanocables, slight concentration fluctuations of doping ion may have a great effect on fluorescence properties, which may exert a great effect on the luminescence colour of nanocables. Although mixing light has been proposed previously [2,3], the competition of emissions is not adequately studied. Only several data cannot adequately reflect the ef fect of RE concentration fluctuation on luminescence properties. The competition of emissions severely influences the stability of the emission colour. Because of the concentration quenching effect, the luminescence intensity does not simply increase with the increase in doping
concentration, resulting in the variation of the luminescence colour of nanocables in the RGB colour space. However, it is found that the luminescence colour of all of our samples stay in the yellow region with the change in the doping concentration. Because of the wide band gap of pure Y2O3 and ZnO, ultraviolet light is necessary to excite these oxides. After analysis of many luminescence curves, we find the energy transfer from interstitial oxygen and interstitial zinc to RE ions, besides oxygen vacancy, which suggests that visible emissions of two kinds of RE ions can be obtained by visible-light stimulation. In this process, energy transfer from the oxygen vacancy to luminescence centres of Tb3þ and Eu3þ simultaneously promotes visible-light excitation. In this study, ZnO:Tb3þ/Y2O3:Eu3þ nanocable arrays are synthesized by growing nanowires in prepared nanotubes embedded in anodized aluminium (AAO) template. Competition of emissions and lattice defect involving energy transfer are found. The assembly method has great significance in the control of the outer diameter and composition of nanocables. These nanocable arrays will have wide application pros pects in luminescence and nanodisplay devices.
* Corresponding author. E-mail address: [email protected] (L. Yang). https://doi.org/10.1016/j.jpcs.2019.109215 Received 3 June 2019; Received in revised form 25 September 2019; Accepted 25 September 2019 Available online 26 September 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
L. Yang et al.
Journal of Physics and Chemistry of Solids 137 (2020) 109215
Fig. 1. Schematic of the two-step assembly process (a). A typical profile SEM image (b) and TEM image (c) of the nanocable arrays. A typical TEM image of a single RE-doped nanocable (d) with HAADF image and corresponding 2D elemental mapping. TEM-EDS profiles (e) and HRTEM (f) and corresponding RFFT patterns (g) of nanocables of Y2O3:Eu0.03/ZnO:Tb0.03.
2. Experimental
In order to control the constituents of the nanocable, the outer shell and the inner core of the nanocables were assembled in two steps. In the first step, an electrolytic bath made up of polymethyl methacrylate was used. The middle hole on the bath wall was secluded by a blank AAO. After the water leakage was checked, a 0.1 M solution of Eu(NO3)3 and Y(NO3)3 was added to the left cell. Subsequently, a 0.1 M solution of Na2C2O4 and H2C2O4 was added to the right cell. The molar ratio of Eu3þ to Y3þ was changed in the range of 0.01–0.05. A 5 V direct current voltage was applied to the two cells. The graphite immersed in the mixed solution of lanthanide nitrate was connected to the positive pole. After the circuit was switched on, RE3þ and C2O24 were driven by an electric field. They were allowed to meet in the pore canals of the AAO, and the mixture of xEu2(C2O4)3⋅yY2(C2O4)3 was precipitated. Other ions, such as Naþ and NO3 , passed through the AAO canals continuously. These ions could keep the canals unimpeded for a long time. Since the drift velocity of ions in the canal centre was much higher than that near the canal brink, the newly generated oxalates were prone to be pushed to the canal brink and then finally adsorbed to the wall. It contributed to the fabrication of tube structure first. After electro-deposition for 5 days, the template was taken out, washed and dried. In the second assembly step, Tb(NO3)3 was prepared by dissolving 0.0183 g of Tb2O3 in nitric acid. After the pH was adjusted, Tb(NO3)3 was poured into a 100 ml volumetric flask and diluted with the solution to volume, and then mixed uniformly to obtain
2.1. Preparation of nanocable arrays Using two-step template assembly method, we synthesized ZnO: Tb3þ/Y2O3:Eu3þ nanocable arrays by growing nanowires in prepared nanotubes embedded in AAO template. The AAO template was obtained by anodic oxidation of high-purity (99.999%) aluminium in aqueous solution of oxalic acid [2,3]. RE nitrate was prepared by dissolving Eu2O3 (99.99%) or Tb2O3 (99.99%) in concentrated nitric acid. In a typical preparation, 5.65 g of Y2O3 and 0.088 g of Eu2O3 (Eu/Y molar ratio of 0.01) was dissolved in concentrated nitric acid. The Eu/Y molar ratio could be adjusted from 0.01 to 0.05. A small amount of H2O2 so lution was added to help RE oxide dissolve. After evaporating excess HNO3, the pH of the lanthanide nitrate solution was adjusted to neutral by using ammonia and dilute nitric acid. The mixture was poured into a 500 ml volumetric flask and diluted with the solution to volume, and then mixed uniformly to obtain a 0.1 M mixture of Y(NO3)3 and Eu (NO3)3. Subsequently, 2.25 g of H2C2O4 and 3.35 g of Na2C2O4 were dissolved in deionized water and mixed uniformly. The mixture was poured into a 500 ml volumetric flask and diluted with the solution to volume, and then mixed uniformly to obtain a 0.1 M mixture of C2O4 and H2C2O4. Fig. 1(a) shows the assembly process of nanocable arrays. 2
L. Yang et al.
Journal of Physics and Chemistry of Solids 137 (2020) 109215
Fig. 2. Xe-lamp-excited fluorescence spectra (a) and 488 nm laser induced fluorescence spectra of the as-prepared nanocable samples at different doping levels. (b) Simultaneous increase in the doped-ion concentration, (c) fixed Eu concentration and (d) fixed Tb concentration. (e) Chromaticity coordinates are calculated from Table 1.
0.01 M Tb(NO3)3. Zn(NO3)2∙6H2O (0.594 g) was dissolved in deionized water. The solution was diluted to 100 ml to form 0.02 M Zn(NO3)2. Tb (NO3)3 (2 ml, 0.01 M, Tb3þ/Zn2þ molar ratio of 0.01), 0.3 g of urea, and 5 ml of dispersion agent (ethylene glycol) were dissolved. The Tb/Zn molar ratio could be adjusted from 0.01 to 0.05. After the solution was mixed well, AAO with embedded xEu2(C2O4)3⋅yY2(C2O4)3 nanotubes was placed flat at the bottom of the beaker. The beaker mouth was sealed by adhesive tape and then was kept at 80 � C for 72 h. Along with the hydroxide generated in the hydrolysis process, the solution became cloudy gradually. The hydroxide sols diffused into the pores of the AAO, and inner cores of nanocables formed. Finally, the AAO was taken out, washed, dried and then annealed in air at 700 � C for about 4 h.
3. Results and discussion 3.1. Characterization of morphological features, size, chemical composition and crystal structure Fig. 1(b) shows a typical profile SEM image of nanocable arrays embedded in the AAO template. Large-scale and ordered onedimensional (1D) threadiness nanostructure is observed. However, the inner core and outer shell of the nanostructure is not distinguished. Fig. 1(c) shows a typical profile TEM image of nanocable arrays after the as-prepared AAO sample was treated with 5% of NaOH solution for 8 h 1D nanostructures with different grey levels are observed. These indi cate that the core and shell of nanocable are composed of different chemical starting materials. The outer diameter of nanocable is about 61–1 nm, and the inner core diameter is about 21–41 nm. A single nanocable TEM image in Fig. 1(d) reveals the apparent cable structure. It is made up of a dark inner core surrounded by a grey outer shell. The outer diameter of the nanoscale cable is about 66 nm, and the diameter of the core area is only about 36 nm. HAADF-STEM and two-dimensional (2D) element mapping in the inset of Fig. 1(d) exhibit the different element compositions of the inner core and bigger shell. The chemical elements of the smaller core are mainly made up of Zn, Tb and O, and the elements of greater threadiness are composed of Y, Eu and O. The EDS spectra shown in Fig. 1(e) further confirm the existence of Eu, Tb, Zn, Y and O. HRTEM in Fig. 1(f) gives detailed insight into the ZnO:Tb3þ/Y2O3:Eu3þ nanocable structure. The lattice fringes of the core area and outer shell area fit well the corre sponding crystal lattices of ZnO and Y2O3, respectively. The RFFT of HRTEM shown in Fig. 1(g) further explains the existence of ZnO and Y2O3. EDS, HRTEM and RFFT illustrate that the ZnO:Tb3þ/Y2O3:Eu3þ nanocable structure has been synthesized.
2.2. Characterization techniques The morphology features, size and shape of the as-prepared nano cables were characterized by scanning electron microscopy (SEM) using a JSM-6700F. The size, morphological features, chemical composition and lattice structure of the nanocables were studied by transmission electron microscopy (TEM) using a Tecnai G2 F20 S-TWIN (200 kV operating voltage), energy-dispersive spectrometry (EDS), high-angle annular dark-field scanning TEM (HAADF-STEM) and high-resolution TEM (HRTEM). Reduced fast Fourier transform (RFFT) of nanocables was obtained by using Gatan Microscopy Suite (GMS) software. GMS software was also used to measure the lattice spacing of nanocables in the HRTEM images and RFFT patterns. Laser-stimulated luminescence at room temperature was measured on a near-field optical system. Xelamp-excited emission and excitation spectra of nanocables were recorded by a Hitachi luminescence spectrometer (F 4600).
3.2. Emission spectra Fig. 3
2(a) shows
the
room-temperature
Xe lamp
excitated
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Journal of Physics and Chemistry of Solids 137 (2020) 109215
0.04, respectively. Fig. 2(c) shows the room-temperature photoluminescence spectra obtained at 488 nm by laser induction with increasing doping molar ratio of Tb3þ. At fixed doping molar ratio of Eu3þ and increasing doping molar ratio of Tb3þ, green emissions of Tb3þ increase first and then decrease. The strongest green emission appears at a Tb/Zn doping mole ratio of 0.03. Fig. 2(d) shows the room-temperature laser-induced photoluminescence spectra with increasing doping concentration of Eu3þ at a Tb3þ doping molar ratio of 0.04. With increasing doping concentration of Eu3þ, red emissions of Eu3þ increase first and then decrease. The turning point of the Eu/Y mole ratio is at 0.04. The CIE (Commission Internationale de L’Eclairage) system characterizes col ours by two colour coordinates, x and y, on a chromaticity diagram [2, 3]. The calculated chromaticity coordinates for the nanocables with the change in the molar ratio of Tb3þ/Eu3þ are shown in Table 1. These indicate that with the increase in doping level of Eu, x presents an in crease. Similarly, with increase in doping level of Tb, the calculated y value shows an increase. The calculated chromaticity coordinates for all measured curves are shown in Fig. 2(e). The colour of all of the nano cables stays in the yellow region.
Table 1 Chromaticity coordinates at different doping levels. Tb3þ/ Eu3þmolar ratio
Colour coordinates x
y
1:1 1:2 1:3 1:4 1:5 2:1 2:2 2:3 2:4 2:5 3:1 3:2 3:3
0.49278 0.46865 0.39292 0.40207 0.52223 0.49376 0.51629 0.46607 0.51510 0.44099 0.39148 0.38966 0.54411
0.37771 0.46087 0.49566 0.51537 0.37698 0.42940 0.40718 0.41488 0.37978 0.43225 0.5004 0.47081 0.38252
Tb3þ/ Eu3þmolar ratio
Colour coordinates x
y
3:4 3:5 4:1 4:2 4:3 4:4 4:5 5:1 5:2 5:3 5:4 5:5
0.48465 0.51015 0.39508 0.41626 0.48193 0.48882 0.43703 0.37234 0.40541 0.41122 0.43974 0.44544
0.40462 0.41363 0.53375 0.45734 0.45163 0.42422 0.41714 0.51662 0.52133 0.47864 0.4404 0.43342
photoluminescence spectra at 250 nm, with simultaneous increase in the doping molar ratio. Except for several multiple frequency peaks, two obvious luminescence bands appear at 380–480 nm and 608–622 nm. The 380–480 nm luminescence band breaks up into two sub-bands. The 380 nm sub band originates from the recombination of near-band gap exciton, whilst we can interpret the 480 nm sub-band as lattice-defectrelated emission [13]. The red 608–622 nm luminescence peak is from the 5D0→7F2 transition of Eu3þ [13–16]. As the main luminescence band of Tb3þ overlaps with multiple frequency peaks, we do not observe the peak at about 540 nm. Fig. 2(b) shows spectra obtained with 488 nm laser photo luminescence at room temperature with simultaneous increase in the doped ion. Both emissions of red Eu3þ and green Tb3þ are observed, whilst no crystal-defect-correlated emission is observed. With the simultaneous increase in doping molar ratio, both green emissions of Tb3þ and red emission of Eu3þ increase first and then decrease. The turning point appears at Tb/Zn and Eu/Y mole ratios of 0.02–0.03 and
3.3. Excitation spectra The excitation spectra of the nanocables under different monitoring wavelengths are shown in Fig. 3(a)–(d). Under a monitoring wavelength of 610 nm, a strong excitation band appears at 240–255 nm except at strong multiple frequencies (Fig. 3(a)). It can be explained as a chargetransfer band from the 2p electron orbital of O2 to the 4f electron orbital of Eu3þ [15]. In order to present local weak luminescence peaks, amplified excitation spectra in the wavelength region of 350–600 nm are shown in Fig. 3(b). Except for the nanocable sample with 5:5 Tb/Eu mole ratio, the excitation peak of other cable samples are relatively weak. For the nanocable with 5:5 Tb/Eu mole ratio, except for several f–f transitions of Eu3þ at about 361, 381, 393, 465 and 533 nm, a broad
Fig. 3. Wide-scope (a), local (b), and 610 nm monitored excitation spectra and wide-scope (c), local (d), and 540 nm monitored excitation spectra of the as-prepared nanocable samples at different doping levels. 4
L. Yang et al.
Journal of Physics and Chemistry of Solids 137 (2020) 109215
Fig. 4. The energy diagram of ZnO:Tb/Y2O3:Eu nanocable.
weak peak is observed at about 494–530 nm, which can be explained by the excitation of oxygen vacancy (VO) [17]. For the sample with 3:3 Tb/Eu mole ratio, besides the weak peak tips of Eu3þ f–f transitions and VO excitation, a slightly strong peak appears at 554 nm, which can be explained by excitation of interstitial oxygen of Oi [13]. These two weak-defect relative excitation peaks indicate the energy and electron transfer from crystal defects to Eu3þ. When moni tored at 540 nm, except at strong multiple frequencies, two discrimi nable strong excitation bands appear at 200–210 and 210–230 nm (Fig. 3(c)). The amplified excitation spectra monitored at 540 nm in Fig. 3(d) show the curves in the wavelength region of 350–500 nm. There are three obvious weak peaks appearing at about 373, 449 and 494 nm, which can be explained by the f–f transition of Tb3þ, interstitial zinc of Zni and Vo [13,17]. The two defect-relative peaks indicate the energy transfer from crystal lattice defects to Tb3þ. Interestingly, energy can be transferred from the VO level to Eu3þ and Tb3þ simultaneously.
3.4. Energy-transfer process The energy diagram of the nanocable from the emission and exci tation spectra is shown in Fig. 4. Two sets of energy systems are found to be composed of ZnO:Tb3þ and Y2O3:Eu3þ. Exciting these two ions can obtain the emission of these two RE ions. Interestingly, the nanocable system irradiated at 488–500 nm is promoted to the energy level of VO. In the inner-core area, energy transfer of VO→Tb3þ occurs. In the outershell area, energy transfer from VO to Eu3þ takes place. Since VO-related green emission cannot be observed, the energy transfer of VO→RE should be very efficient. In this process, electron transfer from VO to two kinds of RE ions and then electron transitions of both Tb 5D4→7FJ and Eu 5 D0→7FJ take place, producing green and red emissions. 4. Conclusions ZnO:Tb3þ/Y2O3:Eu3þ nanocable arrays embedded in AAO are pre pared via two-step template-assembly approach. The outer diameter of nanocable is about 61–71 nm, and the inner core diameter is about 5
L. Yang et al.
Journal of Physics and Chemistry of Solids 137 (2020) 109215
21–41 nm. Under excitation of a Xe lamp at 250 nm, two obvious luminescence bands appear at 380–480 nm and 608–622 nm, which originated from the recombination of near-band-gap exciton of host ZnO, defect-correlated emissions in the ZnO lattice, and 5D0→7F2 tran sition of Eu3þ occurred. Under excitation at 488 nm by a laser, both emissions of red Eu3þ and green Tb3þ are observed. With the change in the doping molar ratio of RE to host cation from 0.01 to 0.05, the colour coordinates of nanocable arrays stably stay in the yellow region. During monitoring at 610 nm, except for several f–f transition peaks of Eu3þ at about 361, 381, 393, 465 and 533 nm, a broad peak and a slightly strong peak appear at 500–530 nm and 554 nm, which can be explained by the excitation of VO and Oi, respectively. Monitoring at 540 nm indicated three obvious weak peaks at about 373, 449 and 494 nm, which can be described by f–f transition of Tb3þ, excitation of Zni and Vo. The lattice defects of VO, Oi and Zni indicate the energy transfer from crystal defects to RE ions. Energy transferred simultaneously from VO to Eu3þ and Tb3þ is also found.
[2] L. Yang, Y.J. She, S.H. Zhao, S.H. Yue, Q. Wang, A.P. Hu, W. Zhang, Synthesis and optical properties modulation of ZnO/Eu2O3 nanocable arrays, J. Appl. Phys. 108 (2010) 104301. [3] L. Yang, J.Z. Dong, Z.C. Jiang, A.L. Pan, X.J. Zhuang, Visible light stimulating dualwavelength emission and O vacancy involved energy transfer behavior in luminescence for coaxial nanocable arrays, J. Appl. Phys. 115 (2014) 224308. [4] Z.M. Zhang, Yi Ning, X.S. Fang, From nanofibers to ordered ZnO/NiO heterojunction arrays for self-powered and transparent UV photodetectors, J. Mater. Chem. C 7 (2019) 223–229. [5] S. Choudhary, Structural, optical, dielectric and electrical properties of (PEO–PVP)–ZnO nanocomposites, J. Phys. Chem. Solids 121 (2018) 196–209. [6] F. Teng, K. Hu, W. Ouyang, X.S. Fang, Photoelectric detectors based on inorganic ptype semiconductor materials, Adv. Mater. 30 (2018) 1706262. [7] B. Zhao, F. Wang, H.Y. Chen, L.X. Zheng, L.X. Su, D.X. Zhao, X.S. Fang, An ultrahigh responsivity (9.7 mA W 1) self-powered solar-blind photodetector based on individual ZnO–Ga2O3 heterostructures, Adv. Funct. Mater. 27 (2017) 1700264. [8] R. Raji, K.G. Gopchandran, Fast photocatalytic degradation of sulforhodamine B using ZnO:Cu nanorods, J. Phys. Chem. Solids 113 (2018) 39–49. [9] K. Hu, F. Teng, L.X. Zheng, P.P. Yu, Z.M. Zhang, H.Y. Chen, X.S. Fang, Binary response Se/ZnO p-n heterojunction UV photodetector with high on/off ratio and fast speed, Laser Photonics Rev. 11 (2017) 1600257. [10] W.X. Ouyang, F. Teng, M.M. Jiang, X.S. Fang, ZnO Film UV photodetector with enhanced performance: heterojunction with CdMoO4 microplates and the hot electron injection effect of Au nanoparticles, Small 13 (2017) 1702177. [11] Y. Ning, Z.M. Zhang, F. Teng, X.S. Fang, Novel transparent and self-powered UV photodetector based on crossed ZnO nanofiber array homojunction, Small 14 (2018) 1703754. [12] M. Shkira, S. Khan, A.M. El-Toni, A. Aldalbahi, I.S. Yahia, S. AlFaify, Structural, morphological, opto-nonlinear-limiting studies on Dy:PbI2/FTO thin films derived facilely by spin coating technique for optoelectronic technology, J. Phys. Chem. Solids 130 (2019) 189–196. [13] L. Yang, J.Z. Dong, Y.J. She, Z.C. Jiang, L.D. Zhang, H.B. Yu, Self-purification construction of interstitial O in the neighbor of Eu3þ ions to act as energy transfer bridge, Appl. Phys. Lett. 104 (2014), 033109. [14] L. Yang, Y.H. Tang, A.P. Hu, X.H. Chen, K. Liang, L.D. Zhang, Raman scattering and luminescence study on arrays of ZnO doped with Tb3þ, Physical B 403 (2008) 2230–2234. [15] H.W. Song, B.J. Chen, H.S. Peng, J.S. Zhang, Light-induced change of charge transfer band in nanocrystalline Y2O3:Eu3þ, Appl. Phys. Lett. 81 (2002) 1776. [16] L. Yang, H.Y. Zheng, Q.W. Liu, S.Y. Zhou, W. Zhang, The doping site analysis and control of Eu3þ in ZnO:Eu crystal lattice, J. Lumin. 204 (2018) 189–194. [17] L. Yang, Z.C. Jiang, J.Z. Dong, A.L. Pan, X.J. Zhuang, The study on crystal defectsinvolved energy transfer process of Eu3þ doped ZnO lattice, Mater. Lett. 129 (2014) 65–67.
Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements This work was supported by the Hunan Provincial Natural Science Foundation of China (grant no. 2018JJ2030) and the Industrial Science and Technology Commissioner of Changsha (China, CSKJ2017-46). References [1] X.D. Wang, Z.D. Li, J. Shi, Y.H. Yu, One-dimensional titanium dioxide nanomaterials: nanowires, nanorods, and nanobelts, Chem. Rev. 114 (2014) 9346–9384.
6
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Competition of emission and lattice defects participating in energy transfer for ZnO:Tb/Y2O3:Eu nanocable arrays Lei Yang *, Qianwen Liu, Hongying Zheng, Siyuan Zhou, Wei Zhang College of Materials Science and Engineering, Hunan University, Changsha, 410082, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanostructures Sol–gel growth Electron microscopy Optical properties
Lattice defects participating in energy-transfer behaviour and competition of emissions for rare-earth-doped nanocable arrays are investigated in this article. ZnO:Tb3þ/Y2O3:Eu3þ nanocable arrays embedded in anod ized aluminium are synthesized via two-step template assembly approach. The outer diameter of the nanocable is about 61–71 nm. The core diameter is about 21–41 nm. The competition of emissions and lattice defects involving energy-transfer behaviour are found. With adjustment of the molar ratio of doping ion to host lattice cation to 0.01–0.05, the luminescence intensity increases first and then decreases because of concentration quenching. Although the chromaticity coordinates of nanocable arrays varies as doping concentration changes, they stay in the yellow region. In this process, the oxygen vacancy, interstitial zinc and interstitial oxygen serve as media in the energy-transfer process. Simultaneously, energy transfers from oxygen vacancy to Eu3þ and Tb3þ are also found, but energy transfer between Eu3þ and Tb3þ is not found. These nanocable arrays will have wide application in photoelectronic devices and luminescence materials.
1. Introduction Nanostructure is a type of significant structure which has aroused much attention because of a large number of applications in photo electronic devices and luminescence materials [1–12]. Compared with ordinary micrometer and nanomaterials, nanocable arrays can provide better display resolution whilst not reducing luminous intensity. The main emission band of Tb3þ and Eu3þ belongs to primary colours [2–15], from which a large number of colours can be tailored. Although mixing fluorescent powders can also tailor the luminescence colours, it is difficult to obtain stable and uniform emissions. Nanocables have huge advantages which avoid these defects. As competition of emissions is unavoidable for rare earth (RE)-doped nanocables, slight concentration fluctuations of doping ion may have a great effect on fluorescence properties, which may exert a great effect on the luminescence colour of nanocables. Although mixing light has been proposed previously [2,3], the competition of emissions is not adequately studied. Only several data cannot adequately reflect the ef fect of RE concentration fluctuation on luminescence properties. The competition of emissions severely influences the stability of the emission colour. Because of the concentration quenching effect, the luminescence intensity does not simply increase with the increase in doping
concentration, resulting in the variation of the luminescence colour of nanocables in the RGB colour space. However, it is found that the luminescence colour of all of our samples stay in the yellow region with the change in the doping concentration. Because of the wide band gap of pure Y2O3 and ZnO, ultraviolet light is necessary to excite these oxides. After analysis of many luminescence curves, we find the energy transfer from interstitial oxygen and interstitial zinc to RE ions, besides oxygen vacancy, which suggests that visible emissions of two kinds of RE ions can be obtained by visible-light stimulation. In this process, energy transfer from the oxygen vacancy to luminescence centres of Tb3þ and Eu3þ simultaneously promotes visible-light excitation. In this study, ZnO:Tb3þ/Y2O3:Eu3þ nanocable arrays are synthesized by growing nanowires in prepared nanotubes embedded in anodized aluminium (AAO) template. Competition of emissions and lattice defect involving energy transfer are found. The assembly method has great significance in the control of the outer diameter and composition of nanocables. These nanocable arrays will have wide application pros pects in luminescence and nanodisplay devices.
* Corresponding author. E-mail address: [email protected] (L. Yang). https://doi.org/10.1016/j.jpcs.2019.109215 Received 3 June 2019; Received in revised form 25 September 2019; Accepted 25 September 2019 Available online 26 September 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
L. Yang et al.
Journal of Physics and Chemistry of Solids 137 (2020) 109215
Fig. 1. Schematic of the two-step assembly process (a). A typical profile SEM image (b) and TEM image (c) of the nanocable arrays. A typical TEM image of a single RE-doped nanocable (d) with HAADF image and corresponding 2D elemental mapping. TEM-EDS profiles (e) and HRTEM (f) and corresponding RFFT patterns (g) of nanocables of Y2O3:Eu0.03/ZnO:Tb0.03.
2. Experimental
In order to control the constituents of the nanocable, the outer shell and the inner core of the nanocables were assembled in two steps. In the first step, an electrolytic bath made up of polymethyl methacrylate was used. The middle hole on the bath wall was secluded by a blank AAO. After the water leakage was checked, a 0.1 M solution of Eu(NO3)3 and Y(NO3)3 was added to the left cell. Subsequently, a 0.1 M solution of Na2C2O4 and H2C2O4 was added to the right cell. The molar ratio of Eu3þ to Y3þ was changed in the range of 0.01–0.05. A 5 V direct current voltage was applied to the two cells. The graphite immersed in the mixed solution of lanthanide nitrate was connected to the positive pole. After the circuit was switched on, RE3þ and C2O24 were driven by an electric field. They were allowed to meet in the pore canals of the AAO, and the mixture of xEu2(C2O4)3⋅yY2(C2O4)3 was precipitated. Other ions, such as Naþ and NO3 , passed through the AAO canals continuously. These ions could keep the canals unimpeded for a long time. Since the drift velocity of ions in the canal centre was much higher than that near the canal brink, the newly generated oxalates were prone to be pushed to the canal brink and then finally adsorbed to the wall. It contributed to the fabrication of tube structure first. After electro-deposition for 5 days, the template was taken out, washed and dried. In the second assembly step, Tb(NO3)3 was prepared by dissolving 0.0183 g of Tb2O3 in nitric acid. After the pH was adjusted, Tb(NO3)3 was poured into a 100 ml volumetric flask and diluted with the solution to volume, and then mixed uniformly to obtain
2.1. Preparation of nanocable arrays Using two-step template assembly method, we synthesized ZnO: Tb3þ/Y2O3:Eu3þ nanocable arrays by growing nanowires in prepared nanotubes embedded in AAO template. The AAO template was obtained by anodic oxidation of high-purity (99.999%) aluminium in aqueous solution of oxalic acid [2,3]. RE nitrate was prepared by dissolving Eu2O3 (99.99%) or Tb2O3 (99.99%) in concentrated nitric acid. In a typical preparation, 5.65 g of Y2O3 and 0.088 g of Eu2O3 (Eu/Y molar ratio of 0.01) was dissolved in concentrated nitric acid. The Eu/Y molar ratio could be adjusted from 0.01 to 0.05. A small amount of H2O2 so lution was added to help RE oxide dissolve. After evaporating excess HNO3, the pH of the lanthanide nitrate solution was adjusted to neutral by using ammonia and dilute nitric acid. The mixture was poured into a 500 ml volumetric flask and diluted with the solution to volume, and then mixed uniformly to obtain a 0.1 M mixture of Y(NO3)3 and Eu (NO3)3. Subsequently, 2.25 g of H2C2O4 and 3.35 g of Na2C2O4 were dissolved in deionized water and mixed uniformly. The mixture was poured into a 500 ml volumetric flask and diluted with the solution to volume, and then mixed uniformly to obtain a 0.1 M mixture of C2O4 and H2C2O4. Fig. 1(a) shows the assembly process of nanocable arrays. 2
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Fig. 2. Xe-lamp-excited fluorescence spectra (a) and 488 nm laser induced fluorescence spectra of the as-prepared nanocable samples at different doping levels. (b) Simultaneous increase in the doped-ion concentration, (c) fixed Eu concentration and (d) fixed Tb concentration. (e) Chromaticity coordinates are calculated from Table 1.
0.01 M Tb(NO3)3. Zn(NO3)2∙6H2O (0.594 g) was dissolved in deionized water. The solution was diluted to 100 ml to form 0.02 M Zn(NO3)2. Tb (NO3)3 (2 ml, 0.01 M, Tb3þ/Zn2þ molar ratio of 0.01), 0.3 g of urea, and 5 ml of dispersion agent (ethylene glycol) were dissolved. The Tb/Zn molar ratio could be adjusted from 0.01 to 0.05. After the solution was mixed well, AAO with embedded xEu2(C2O4)3⋅yY2(C2O4)3 nanotubes was placed flat at the bottom of the beaker. The beaker mouth was sealed by adhesive tape and then was kept at 80 � C for 72 h. Along with the hydroxide generated in the hydrolysis process, the solution became cloudy gradually. The hydroxide sols diffused into the pores of the AAO, and inner cores of nanocables formed. Finally, the AAO was taken out, washed, dried and then annealed in air at 700 � C for about 4 h.
3. Results and discussion 3.1. Characterization of morphological features, size, chemical composition and crystal structure Fig. 1(b) shows a typical profile SEM image of nanocable arrays embedded in the AAO template. Large-scale and ordered onedimensional (1D) threadiness nanostructure is observed. However, the inner core and outer shell of the nanostructure is not distinguished. Fig. 1(c) shows a typical profile TEM image of nanocable arrays after the as-prepared AAO sample was treated with 5% of NaOH solution for 8 h 1D nanostructures with different grey levels are observed. These indi cate that the core and shell of nanocable are composed of different chemical starting materials. The outer diameter of nanocable is about 61–1 nm, and the inner core diameter is about 21–41 nm. A single nanocable TEM image in Fig. 1(d) reveals the apparent cable structure. It is made up of a dark inner core surrounded by a grey outer shell. The outer diameter of the nanoscale cable is about 66 nm, and the diameter of the core area is only about 36 nm. HAADF-STEM and two-dimensional (2D) element mapping in the inset of Fig. 1(d) exhibit the different element compositions of the inner core and bigger shell. The chemical elements of the smaller core are mainly made up of Zn, Tb and O, and the elements of greater threadiness are composed of Y, Eu and O. The EDS spectra shown in Fig. 1(e) further confirm the existence of Eu, Tb, Zn, Y and O. HRTEM in Fig. 1(f) gives detailed insight into the ZnO:Tb3þ/Y2O3:Eu3þ nanocable structure. The lattice fringes of the core area and outer shell area fit well the corre sponding crystal lattices of ZnO and Y2O3, respectively. The RFFT of HRTEM shown in Fig. 1(g) further explains the existence of ZnO and Y2O3. EDS, HRTEM and RFFT illustrate that the ZnO:Tb3þ/Y2O3:Eu3þ nanocable structure has been synthesized.
2.2. Characterization techniques The morphology features, size and shape of the as-prepared nano cables were characterized by scanning electron microscopy (SEM) using a JSM-6700F. The size, morphological features, chemical composition and lattice structure of the nanocables were studied by transmission electron microscopy (TEM) using a Tecnai G2 F20 S-TWIN (200 kV operating voltage), energy-dispersive spectrometry (EDS), high-angle annular dark-field scanning TEM (HAADF-STEM) and high-resolution TEM (HRTEM). Reduced fast Fourier transform (RFFT) of nanocables was obtained by using Gatan Microscopy Suite (GMS) software. GMS software was also used to measure the lattice spacing of nanocables in the HRTEM images and RFFT patterns. Laser-stimulated luminescence at room temperature was measured on a near-field optical system. Xelamp-excited emission and excitation spectra of nanocables were recorded by a Hitachi luminescence spectrometer (F 4600).
3.2. Emission spectra Fig. 3
2(a) shows
the
room-temperature
Xe lamp
excitated
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Journal of Physics and Chemistry of Solids 137 (2020) 109215
0.04, respectively. Fig. 2(c) shows the room-temperature photoluminescence spectra obtained at 488 nm by laser induction with increasing doping molar ratio of Tb3þ. At fixed doping molar ratio of Eu3þ and increasing doping molar ratio of Tb3þ, green emissions of Tb3þ increase first and then decrease. The strongest green emission appears at a Tb/Zn doping mole ratio of 0.03. Fig. 2(d) shows the room-temperature laser-induced photoluminescence spectra with increasing doping concentration of Eu3þ at a Tb3þ doping molar ratio of 0.04. With increasing doping concentration of Eu3þ, red emissions of Eu3þ increase first and then decrease. The turning point of the Eu/Y mole ratio is at 0.04. The CIE (Commission Internationale de L’Eclairage) system characterizes col ours by two colour coordinates, x and y, on a chromaticity diagram [2, 3]. The calculated chromaticity coordinates for the nanocables with the change in the molar ratio of Tb3þ/Eu3þ are shown in Table 1. These indicate that with the increase in doping level of Eu, x presents an in crease. Similarly, with increase in doping level of Tb, the calculated y value shows an increase. The calculated chromaticity coordinates for all measured curves are shown in Fig. 2(e). The colour of all of the nano cables stays in the yellow region.
Table 1 Chromaticity coordinates at different doping levels. Tb3þ/ Eu3þmolar ratio
Colour coordinates x
y
1:1 1:2 1:3 1:4 1:5 2:1 2:2 2:3 2:4 2:5 3:1 3:2 3:3
0.49278 0.46865 0.39292 0.40207 0.52223 0.49376 0.51629 0.46607 0.51510 0.44099 0.39148 0.38966 0.54411
0.37771 0.46087 0.49566 0.51537 0.37698 0.42940 0.40718 0.41488 0.37978 0.43225 0.5004 0.47081 0.38252
Tb3þ/ Eu3þmolar ratio
Colour coordinates x
y
3:4 3:5 4:1 4:2 4:3 4:4 4:5 5:1 5:2 5:3 5:4 5:5
0.48465 0.51015 0.39508 0.41626 0.48193 0.48882 0.43703 0.37234 0.40541 0.41122 0.43974 0.44544
0.40462 0.41363 0.53375 0.45734 0.45163 0.42422 0.41714 0.51662 0.52133 0.47864 0.4404 0.43342
photoluminescence spectra at 250 nm, with simultaneous increase in the doping molar ratio. Except for several multiple frequency peaks, two obvious luminescence bands appear at 380–480 nm and 608–622 nm. The 380–480 nm luminescence band breaks up into two sub-bands. The 380 nm sub band originates from the recombination of near-band gap exciton, whilst we can interpret the 480 nm sub-band as lattice-defectrelated emission [13]. The red 608–622 nm luminescence peak is from the 5D0→7F2 transition of Eu3þ [13–16]. As the main luminescence band of Tb3þ overlaps with multiple frequency peaks, we do not observe the peak at about 540 nm. Fig. 2(b) shows spectra obtained with 488 nm laser photo luminescence at room temperature with simultaneous increase in the doped ion. Both emissions of red Eu3þ and green Tb3þ are observed, whilst no crystal-defect-correlated emission is observed. With the simultaneous increase in doping molar ratio, both green emissions of Tb3þ and red emission of Eu3þ increase first and then decrease. The turning point appears at Tb/Zn and Eu/Y mole ratios of 0.02–0.03 and
3.3. Excitation spectra The excitation spectra of the nanocables under different monitoring wavelengths are shown in Fig. 3(a)–(d). Under a monitoring wavelength of 610 nm, a strong excitation band appears at 240–255 nm except at strong multiple frequencies (Fig. 3(a)). It can be explained as a chargetransfer band from the 2p electron orbital of O2 to the 4f electron orbital of Eu3þ [15]. In order to present local weak luminescence peaks, amplified excitation spectra in the wavelength region of 350–600 nm are shown in Fig. 3(b). Except for the nanocable sample with 5:5 Tb/Eu mole ratio, the excitation peak of other cable samples are relatively weak. For the nanocable with 5:5 Tb/Eu mole ratio, except for several f–f transitions of Eu3þ at about 361, 381, 393, 465 and 533 nm, a broad
Fig. 3. Wide-scope (a), local (b), and 610 nm monitored excitation spectra and wide-scope (c), local (d), and 540 nm monitored excitation spectra of the as-prepared nanocable samples at different doping levels. 4
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Journal of Physics and Chemistry of Solids 137 (2020) 109215
Fig. 4. The energy diagram of ZnO:Tb/Y2O3:Eu nanocable.
weak peak is observed at about 494–530 nm, which can be explained by the excitation of oxygen vacancy (VO) [17]. For the sample with 3:3 Tb/Eu mole ratio, besides the weak peak tips of Eu3þ f–f transitions and VO excitation, a slightly strong peak appears at 554 nm, which can be explained by excitation of interstitial oxygen of Oi [13]. These two weak-defect relative excitation peaks indicate the energy and electron transfer from crystal defects to Eu3þ. When moni tored at 540 nm, except at strong multiple frequencies, two discrimi nable strong excitation bands appear at 200–210 and 210–230 nm (Fig. 3(c)). The amplified excitation spectra monitored at 540 nm in Fig. 3(d) show the curves in the wavelength region of 350–500 nm. There are three obvious weak peaks appearing at about 373, 449 and 494 nm, which can be explained by the f–f transition of Tb3þ, interstitial zinc of Zni and Vo [13,17]. The two defect-relative peaks indicate the energy transfer from crystal lattice defects to Tb3þ. Interestingly, energy can be transferred from the VO level to Eu3þ and Tb3þ simultaneously.
3.4. Energy-transfer process The energy diagram of the nanocable from the emission and exci tation spectra is shown in Fig. 4. Two sets of energy systems are found to be composed of ZnO:Tb3þ and Y2O3:Eu3þ. Exciting these two ions can obtain the emission of these two RE ions. Interestingly, the nanocable system irradiated at 488–500 nm is promoted to the energy level of VO. In the inner-core area, energy transfer of VO→Tb3þ occurs. In the outershell area, energy transfer from VO to Eu3þ takes place. Since VO-related green emission cannot be observed, the energy transfer of VO→RE should be very efficient. In this process, electron transfer from VO to two kinds of RE ions and then electron transitions of both Tb 5D4→7FJ and Eu 5 D0→7FJ take place, producing green and red emissions. 4. Conclusions ZnO:Tb3þ/Y2O3:Eu3þ nanocable arrays embedded in AAO are pre pared via two-step template-assembly approach. The outer diameter of nanocable is about 61–71 nm, and the inner core diameter is about 5
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Journal of Physics and Chemistry of Solids 137 (2020) 109215
21–41 nm. Under excitation of a Xe lamp at 250 nm, two obvious luminescence bands appear at 380–480 nm and 608–622 nm, which originated from the recombination of near-band-gap exciton of host ZnO, defect-correlated emissions in the ZnO lattice, and 5D0→7F2 tran sition of Eu3þ occurred. Under excitation at 488 nm by a laser, both emissions of red Eu3þ and green Tb3þ are observed. With the change in the doping molar ratio of RE to host cation from 0.01 to 0.05, the colour coordinates of nanocable arrays stably stay in the yellow region. During monitoring at 610 nm, except for several f–f transition peaks of Eu3þ at about 361, 381, 393, 465 and 533 nm, a broad peak and a slightly strong peak appear at 500–530 nm and 554 nm, which can be explained by the excitation of VO and Oi, respectively. Monitoring at 540 nm indicated three obvious weak peaks at about 373, 449 and 494 nm, which can be described by f–f transition of Tb3þ, excitation of Zni and Vo. The lattice defects of VO, Oi and Zni indicate the energy transfer from crystal defects to RE ions. Energy transferred simultaneously from VO to Eu3þ and Tb3þ is also found.
[2] L. Yang, Y.J. She, S.H. Zhao, S.H. Yue, Q. Wang, A.P. Hu, W. Zhang, Synthesis and optical properties modulation of ZnO/Eu2O3 nanocable arrays, J. Appl. Phys. 108 (2010) 104301. [3] L. Yang, J.Z. Dong, Z.C. Jiang, A.L. Pan, X.J. Zhuang, Visible light stimulating dualwavelength emission and O vacancy involved energy transfer behavior in luminescence for coaxial nanocable arrays, J. Appl. Phys. 115 (2014) 224308. [4] Z.M. Zhang, Yi Ning, X.S. Fang, From nanofibers to ordered ZnO/NiO heterojunction arrays for self-powered and transparent UV photodetectors, J. Mater. Chem. C 7 (2019) 223–229. [5] S. Choudhary, Structural, optical, dielectric and electrical properties of (PEO–PVP)–ZnO nanocomposites, J. Phys. Chem. Solids 121 (2018) 196–209. [6] F. Teng, K. Hu, W. Ouyang, X.S. Fang, Photoelectric detectors based on inorganic ptype semiconductor materials, Adv. Mater. 30 (2018) 1706262. [7] B. Zhao, F. Wang, H.Y. Chen, L.X. Zheng, L.X. Su, D.X. Zhao, X.S. Fang, An ultrahigh responsivity (9.7 mA W 1) self-powered solar-blind photodetector based on individual ZnO–Ga2O3 heterostructures, Adv. Funct. Mater. 27 (2017) 1700264. [8] R. Raji, K.G. Gopchandran, Fast photocatalytic degradation of sulforhodamine B using ZnO:Cu nanorods, J. Phys. Chem. Solids 113 (2018) 39–49. [9] K. Hu, F. Teng, L.X. Zheng, P.P. Yu, Z.M. Zhang, H.Y. Chen, X.S. Fang, Binary response Se/ZnO p-n heterojunction UV photodetector with high on/off ratio and fast speed, Laser Photonics Rev. 11 (2017) 1600257. [10] W.X. Ouyang, F. Teng, M.M. Jiang, X.S. Fang, ZnO Film UV photodetector with enhanced performance: heterojunction with CdMoO4 microplates and the hot electron injection effect of Au nanoparticles, Small 13 (2017) 1702177. [11] Y. Ning, Z.M. Zhang, F. Teng, X.S. Fang, Novel transparent and self-powered UV photodetector based on crossed ZnO nanofiber array homojunction, Small 14 (2018) 1703754. [12] M. Shkira, S. Khan, A.M. El-Toni, A. Aldalbahi, I.S. Yahia, S. AlFaify, Structural, morphological, opto-nonlinear-limiting studies on Dy:PbI2/FTO thin films derived facilely by spin coating technique for optoelectronic technology, J. Phys. Chem. Solids 130 (2019) 189–196. [13] L. Yang, J.Z. Dong, Y.J. She, Z.C. Jiang, L.D. Zhang, H.B. Yu, Self-purification construction of interstitial O in the neighbor of Eu3þ ions to act as energy transfer bridge, Appl. Phys. Lett. 104 (2014), 033109. [14] L. Yang, Y.H. Tang, A.P. Hu, X.H. Chen, K. Liang, L.D. Zhang, Raman scattering and luminescence study on arrays of ZnO doped with Tb3þ, Physical B 403 (2008) 2230–2234. [15] H.W. Song, B.J. Chen, H.S. Peng, J.S. Zhang, Light-induced change of charge transfer band in nanocrystalline Y2O3:Eu3þ, Appl. Phys. Lett. 81 (2002) 1776. [16] L. Yang, H.Y. Zheng, Q.W. Liu, S.Y. Zhou, W. Zhang, The doping site analysis and control of Eu3þ in ZnO:Eu crystal lattice, J. Lumin. 204 (2018) 189–194. [17] L. Yang, Z.C. Jiang, J.Z. Dong, A.L. Pan, X.J. Zhuang, The study on crystal defectsinvolved energy transfer process of Eu3þ doped ZnO lattice, Mater. Lett. 129 (2014) 65–67.
Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements This work was supported by the Hunan Provincial Natural Science Foundation of China (grant no. 2018JJ2030) and the Industrial Science and Technology Commissioner of Changsha (China, CSKJ2017-46). References [1] X.D. Wang, Z.D. Li, J. Shi, Y.H. Yu, One-dimensional titanium dioxide nanomaterials: nanowires, nanorods, and nanobelts, Chem. Rev. 114 (2014) 9346–9384.
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