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Electrodeposition of high aspect ratio LaPO4 and LaPO4:Ln3+ (Ln3 + = Ce3+, Tb3+) nanostructures
Electrochemistry Communications 17 (2012) 79–81
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrodeposition of high aspect ratio LaPO4 and LaPO4:Ln 3+ (Ln 3 + = Ce 3+, Tb 3+) nanostructures Hui Wang, Run Liu ⁎, Liying Liu, Xiaofang Shi, Zhude Xu Department of Chemistry, Zhejiang University, Hangzhou, 310027, China
a r t i c l e
i n f o
Article history: Received 7 December 2011 Received in revised form 15 January 2012 Accepted 31 January 2012 Available online 8 February 2012 Keywords: Electrodeposition Lanthanide orthophosphate Morphology Photoluminescence
a b s t r a c t LaPO4 and LaPO4:Ln3+(Ln 3+ = Ce 3+, Tb3+) nanostructures composed of interlaced high-aspect-ratio nanorods have been electrodeposited on indium tin oxide (ITO) substrate in aqueous solutions near room temperature without any surfactant, catalyst, or template. The deposits obtained at 1.6 V vs SCE and 80 °C are composed of interlaced nanorods with the length up to 300–500 nm and the diameter from 10 nm to 20 nm. The HRTEM image and selected area electron diffraction (SAED) of LaPO4 nanorods indicate that the LaPO4 nanorods are single crystal hexagonal structure. The energy dispersive spectroscopy mapping indicates that the doped Tb 3+ ions are distributed homogeneously in the nanorods. The LaPO4:Ce3+:Tb3 + (Ce3+, 10 mol% and Tb 3+, 20 mol%) nanostructures exhibit strong green emission at room temperature. The method is a novel and facile route for the fabrication of nanophosphor structures or films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Lanthanide (Ln) compounds have been extensively applied in luminescence and display, such as lighting, field emission display, cathode ray tubes, and plasma display panels [1–3]. Among various hosts for lanthanide dopants, LaPO4 is an appropriate one with high chemical stability and high light yields of the doped materials. LaPO4:Ce 3+:Tb 3+ phosphor is particularly interesting because it has been commercially used as an excellent green phosphor for fluorescent lamps and plasma display panels [4]. It is also expected that nanosize lanthanide compounds can increase the luminescent quantum efficiency and display resolution as a result of both their marked shape-specific and quantum confinement effects [5,6]. Up to the present, various methods have been employed to synthesized LaPO4 and Ln-doped LaPO4 nanocrystals, such as reverse micelle [7], direct precipitation [8], hydrothermal method [9], highboiling-solvent technique [10], microwave-assisted synthesis with some ionic liquids as the reaction media [11] and so on [12]. However, there are very few works to directly form Ln-doped LaPO4 nanostructures or films on substrate using solution methods [13,14]. Electrodeposition is a well known solution method that can be applied in the growth of semiconductor films or nanostructures from aqueous or nonaqueous solutions [15–18]. The unique features of electrodeposition include simplicity, low process temperature, low cost, and capability of controlling grain size, morphology and composition of the deposited films.
⁎ Corresponding author. Tel.: + 86 571 87953390. E-mail address: [email protected] (R. Liu). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.01.033
Here, we develop a facile electrodeposition method to grow LaPO4 and LaPO4:Ln 3+(Ln 3+ = Ce 3+, Tb 3+) nanostructures on indium tin oxide (ITO) substrate by electrochemical generate acid in aqueous solutions near room temperature without any surfactant, catalyst, or template. The ITO electrode is used as anode electrode. The local pH at ITO electrode is decreased by electrochemically oxidizing H2O, and then the La-EDTA complexes which are close to the ITO electrode are decomposed to release free La 3+ to react with PO43 − to form LaPO4 deposits onto ITO electrode. The doped LaPO4:Tb and LaPO4: Ce 3+:Tb 3+ deposits have been obtained using the same way. 2. Experimental section 2.1. Electrochemical deposition All chemical reagents are of analytic purities and were purchased from Sinopharm Chemical Reagent (China) without further purification. The electrochemical deposition of LaPO4 deposits was conducted in a solution consisting of 0.01 M La(NO3)3, 0.01 M EDTA, 0.02 M Na3PO4 and 0.1 M KCl. The solution of Na3PO4 was added into beaker after the forming of stable La-EDTA complex and the final pH was adjusted to 6.00 using 0.1 M HNO3. The solution for the electrodepositon of LaPO4:Ln 3+ (Ln 3+ = Ce 3+, Tb 3+) deposits were prepared using the solution above by adding certain proportion of rare earth ions. The electrodeposition procedure was carried out on a CHI 601C potentiostat connected with a classical three-electrode electrochemical cell. The working electrode was a glass coated with indium tin oxide (ITO) (≈20 Ω cm), the counter electrode was a platinum sheet, and the reference electrode was a saturated calomel electrode (SCE). Before electrodeposition, the ITO substrate was cleaned with
80
H. Wang et al. / Electrochemistry Communications 17 (2012) 79–81
acetone in an ultrasonic bath for 15 min and then rinsed with distilled water. The applied potential was 1.4 V to 1.8 V vs SCE and the solution temperature ranged from 30 to 80 °C for 2 h. 2.2. Characterization X-ray diffraction measurements were done to investigate the purity of the samples and the crystallite sizes using a high resolution Philips X'Pert MRD diffractometer with Cu Kα radiation (λ = 0.15406 nm) in the scan range of 10–60 . The morphology of the deposits was studied by a field-emission scanning electron microscopy (FE-SEM, Sirion-100) with incident electron energy of 25 kV. In order to increase the conductivity of LaPO4, platinum powders were sputtered on the surface of the deposits before. The Tb 3+-doped LaPO4 deposits were analyzed by energy dispersive spectroscopy (EDS) to determine the composition and element distribution. TEM images were collected on a Tecnai G2 F30 transmission electron microscopy operating at an accelerating voltage of 300 kV. The emission spectra of the LaPO4:Ln 3+(Ln 3+ = Ce 3+, Tb 3+) deposits were recorded on a Hitachi F-2500 luminescence spectrophotometer equipped with 150 W xenon lamp as the excitation source. 3. Results and discussion 3.1. Structure and morphologies of the deposits The composition and phase purity of the deposits were examined by X-ray diffraction. XRD patterns of LaPO4 , LaPO4:Tb 3+(Tb 3+, 20 mol%) and LaPO4:Ce 3+:Tb 3+(Ce 3+, 10 mol% and Tb 3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C are shown in Fig. 1. The XRD patterns are found to match well with the hexagonal phase of LaPO4 (JCPDS no. 75-1881). No other phase can be found in the patterns except ITO substrate. The high and shape peaks indicate that the deposits are well crystallized. Owing to the small sizes of LaPO4, the diffraction peaks are slightly broadened. The grain size were estimated from the values of FWHM (full width half maximum) of the above diffraction peaks using the Scherrer formula and the average crystalline size are around 15 nm, 13 nm and 16 nm for LaPO4, LaPO4:Tb 3+ and LaPO4:Ce 3+:Tb 3+ respectively. The scanning electron microscopy (SEM) images of LaPO4, LaPO4: Tb 3+(Tb 3+, 20 mol%) and LaPO4:Ce 3+:Tb 3+ (Ce 3+, 10 mol% and Tb 3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C are shown in Fig. 2. One can see that deposits are typically composed of interlaced nanorods with the length up to 300–400 nm and the diameter from 10 nm to 20 nm. In order to learn the distribution of elements in the doped LaPO4 deposits, LaPO4 deposits doped Tb 3+ were characterized by EDS. The EDS mapping result is shown in Fig. 2d, which demonstrates that terbium ions have been uniformly distributed in the deposits.
Fig. 1. XRD patterns of LaPO4, LaPO4:Tb3+(Tb3+, 20 mol%) and LaPO4:Ce3+:Tb3+ (Ce3+, 10 mol% and Tb3+, 20 mol%) deposits electrodeposited at 1.6 V vs SCE and 80 °C.
Fig. 2. SEM images of LaPO4(a), LaPO4:Tb3+(Tb3+, 20 mol%)(b) and LaPO4:Ce3+: Tb3+(Ce3+, 10 mol% and Tb3+, 20 mol%) (c) deposits electrodeposited at 1.6 V vs SCE and 80 °C. (d) EDS mapping of the LaPO4:Tb3+ (Tb3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C.
The LaPO4 nanorods obtained at 1.6 V vs SCE and 80 °C were further characterized by TEM, HRTEM and a fast Fourier transform pattern (FFT). Fig. 3a display the TEM images of LaPO4, which are typical nanorods, confirming the above SEM observation. The highresolution TEM images (Fig. 3b and c) taken from a single LaPO4 nanorod show the clearly resolved planes of (001) and (100). The (001) planes are oriented parallel to the nanorod growth axis, indicating that the direction of nanorod growth is along the c axis. The SAED pattern (Fig. 3d) taken from a single nanorod can be indexed as a hexagonal LaPO4 single crystal recorded from the [010] zone axis. 3.2. Possible mechanism of electrodeposited LaPO4 nanostructures It is known that EDTA will form stable coordination compound with metal ions in neutral aqueous solution. However, when solution pH lowers than 2.0, EDTA, existing as H6Y 2 +, H5Y + or H4Y (Y = EDTA), cannot form stable complex with metal ions. In the system of Ln 3+-EDTA (Ln = La, Ce, Tb), the dissolving Ln 3+ ions
Fig. 3. (a) Low-magnification TEM images of LaPO4 nanorods electrodeposited at 1.6 V vs SCE and 80 °C (b) TEM image of a single LaPO4 nanorod, (c) HRTEM image of a single LaPO4 nanorod (d) SAED pattern recorded from the [010] zone axis.
H. Wang et al. / Electrochemistry Communications 17 (2012) 79–81
81
transitions of Tb3+ ions. In this nanophosphor Ce 3+ ions behave as sensitizer and Tb3+ ions act as luminescent center. Compared with LaPO4: Tb (Tb3+, 20 mol%), the intensity of 5D4 → 7F5 significantly increases because of the energy transfer between Ce 3+ and Tb3+. 4. Conclusions
Fig. 4. PL emission spectra of LaPO4:Tb3+ (Tb3+, 20 mol%) and LaPO4:Ce3+,Tb3+ (Ce3+, 10 mol% and Tb3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C.
coordinate with EDTA to form stable Ln 3+-EDTA coordination compound in neutral solution. As a result, Ln 3+ could co-exist with PO43 − in the solution without forming precipitation initially. As long as applied anodic potentials are larger than 1.4 V vs SCE (in this work), the H2O will be oxidized (E o = 1.0 V vs SCE) and released H + and O2. When the released hydrogen ions lower pH at the surface of the ITO working electrode, Ln 3+ ions are set free from the Ln 3+EDTA coordination compound and then react with PO43 − ions to form LnPO4 deposits. Overall, the possible reaction mechanism can be summarized as following expressions: þ
H2 O→1=2O2 þ 2H þ 2e
ð1Þ
In summary, hexagonal phase LaPO4 and LaPO4:Ln 3+ (Ln 3+ = Ce 3+, Tb 3+) nanostructures typically composed of interlaced nanorods with the length up to 300–500 nm and the diameter from 10 nm to 20 nm have been obtained by a facile electrochemical deposition method. The selected area electron diffraction (SAED) of LaPO4 nanorods indicates that the LaPO4 nanorods are single crystal structures. The energy dispersive spectroscopy mapping indicates that the doped Tb 3+ ions are distributed homogeneously in the LaPO4 nanorods. The LaPO4:Ce 3+, Tb 3+(Ce 3+, 10 mol% and Tb 3+, 20 mol%) nanostructures exhibit strong green emission at room temperature. The electrodeposition method could be extended to grow other nanophosphor structures or coatings. Acknowledgements This work was financially supported by the National Basic Research Program of China (Grant No. 2011CB936003), the National Natural Science Foundation of China (20973153) and Zhejiang Provincial Natural Science Foundation of China (Y4090012). References
3þ
þ
3þ
Ln −ðEDTAÞ þ xH →Ln
3þ
Ln
3−
þ PO4
¼ LnPO4 ↓
4−x
þ Hx EDTA
ð2Þ
ð3Þ
3.3. Luminescent properties The PL emission spectra of LaPO4:Tb 3+ (Tb 3+, 20 mol %) and LaPO4:Ce 3+:Tb 3+ (Ce 3+, 10 mol% and Tb 3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C are shown in Fig. 4. The emission spectrum of LaPO4:Tb3+ (Tb3+, 20 mol%, indexed as dash line) under the 250 nm excitation consists of three peaks at about 542 nm, 583 nm, 623 nm. The peaks are related to 5D4 → 7F5, 5D4 → 7F4 and 5D4 → 7F3 transitions of Tb3+ ions, respectively [9]. The peak at 542 nm is the strongest, which emits intense green light. The emission spectrum of the Ce3+/Tb3+coactivated LaPO4 film is also displayed in Fig. 4(indexed as solid line). The excitation wavelength is 250 nm. The peaks ranged from 450 nm to 650 nm are correspond to the 5D4 → 7FJ (J = 5, 4, 3)
[1] J. Dhanaraj, R. Jagannathan, T.N. Kutty, C. Lu, J. Phys. Chem. B 105 (2001) 11098. [2] B. Dubertret, P. Skouride, D.J. Norris, V. Noireaus, A.H. Brivanlou, A. Libchaber, Science 298 (2002) 1759. [3] R.S. Meltzer, S. Feofilov, B.M. Tissue, Phys. Rev. B 102 (1999) R14012. [4] G. Blasse, B.C. Grabmaier, Luminescence Materials, Springer-Verlag, Berlin, 1994 Chapters 4 and 5. [5] K. Kompe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Moller, M. Haase, Angew. Chem. Int. Ed. 42 (2003) 5513. [6] Y.P. Fang, A.W. Xu, W.F. Dong, Small 1 (2005) 967. [7] P.C. Filho, O.A. Serra, J. Phys. Chem. C 115 (2011) 636. [8] M. Yang, H.P. You, K. Liu, Y.H. Zheng, N. Guo, H.J. Zhang, Inorg. Chem. 49 (2010) 4996. [9] Y.P. Fang, A.W. Xu, R.Q. Song, H.X. Zhang, L.P. You, J.C. Yu, H.Q. Liu, J. Am. Chem. Soc. 125 (2003) 16025. [10] W. Feng, L.D. Sun, Y.W. Zhang, C.H. Yan, Coord. Chem. Rev. 254 (2010) 1038. [11] R.X. Yan, X.M. Sun, X. Wang, Q. Peng, Y.D. Li, Chem. Eur. J. 11 (2005) 2187. [12] G. Buhler, C. Feldmann, Angew. Chem. Int. Ed. 45 (2006) 4864. [13] K. Rajesh, P. Shajesh, O. Seidel, P. Mukundan, K.G.K. Warrier, Adv. Funct. Mater. 17 (2007) 1682. [14] P. Schuetz, F. Caruso, Chem. Mater. 14 (2002) 4509. [15] G. Hodes, Electrochemistry of Nanomaterials, Wiley, New York, 2001. [16] J.A. Switzer, H.M. Kothari, P. Poizot, S. Nakanishi, E.W. Bohannan, Nature 425 (2003) 490. [17] D. Lincot, Thin Solid Films 487 (2005) 40. [18] M.J. Siegfried, K.-S. Choi, Angew. Chem. Int. Ed. 44 (2005) 3218.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrodeposition of high aspect ratio LaPO4 and LaPO4:Ln 3+ (Ln 3 + = Ce 3+, Tb 3+) nanostructures Hui Wang, Run Liu ⁎, Liying Liu, Xiaofang Shi, Zhude Xu Department of Chemistry, Zhejiang University, Hangzhou, 310027, China
a r t i c l e
i n f o
Article history: Received 7 December 2011 Received in revised form 15 January 2012 Accepted 31 January 2012 Available online 8 February 2012 Keywords: Electrodeposition Lanthanide orthophosphate Morphology Photoluminescence
a b s t r a c t LaPO4 and LaPO4:Ln3+(Ln 3+ = Ce 3+, Tb3+) nanostructures composed of interlaced high-aspect-ratio nanorods have been electrodeposited on indium tin oxide (ITO) substrate in aqueous solutions near room temperature without any surfactant, catalyst, or template. The deposits obtained at 1.6 V vs SCE and 80 °C are composed of interlaced nanorods with the length up to 300–500 nm and the diameter from 10 nm to 20 nm. The HRTEM image and selected area electron diffraction (SAED) of LaPO4 nanorods indicate that the LaPO4 nanorods are single crystal hexagonal structure. The energy dispersive spectroscopy mapping indicates that the doped Tb 3+ ions are distributed homogeneously in the nanorods. The LaPO4:Ce3+:Tb3 + (Ce3+, 10 mol% and Tb 3+, 20 mol%) nanostructures exhibit strong green emission at room temperature. The method is a novel and facile route for the fabrication of nanophosphor structures or films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Lanthanide (Ln) compounds have been extensively applied in luminescence and display, such as lighting, field emission display, cathode ray tubes, and plasma display panels [1–3]. Among various hosts for lanthanide dopants, LaPO4 is an appropriate one with high chemical stability and high light yields of the doped materials. LaPO4:Ce 3+:Tb 3+ phosphor is particularly interesting because it has been commercially used as an excellent green phosphor for fluorescent lamps and plasma display panels [4]. It is also expected that nanosize lanthanide compounds can increase the luminescent quantum efficiency and display resolution as a result of both their marked shape-specific and quantum confinement effects [5,6]. Up to the present, various methods have been employed to synthesized LaPO4 and Ln-doped LaPO4 nanocrystals, such as reverse micelle [7], direct precipitation [8], hydrothermal method [9], highboiling-solvent technique [10], microwave-assisted synthesis with some ionic liquids as the reaction media [11] and so on [12]. However, there are very few works to directly form Ln-doped LaPO4 nanostructures or films on substrate using solution methods [13,14]. Electrodeposition is a well known solution method that can be applied in the growth of semiconductor films or nanostructures from aqueous or nonaqueous solutions [15–18]. The unique features of electrodeposition include simplicity, low process temperature, low cost, and capability of controlling grain size, morphology and composition of the deposited films.
⁎ Corresponding author. Tel.: + 86 571 87953390. E-mail address: [email protected] (R. Liu). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.01.033
Here, we develop a facile electrodeposition method to grow LaPO4 and LaPO4:Ln 3+(Ln 3+ = Ce 3+, Tb 3+) nanostructures on indium tin oxide (ITO) substrate by electrochemical generate acid in aqueous solutions near room temperature without any surfactant, catalyst, or template. The ITO electrode is used as anode electrode. The local pH at ITO electrode is decreased by electrochemically oxidizing H2O, and then the La-EDTA complexes which are close to the ITO electrode are decomposed to release free La 3+ to react with PO43 − to form LaPO4 deposits onto ITO electrode. The doped LaPO4:Tb and LaPO4: Ce 3+:Tb 3+ deposits have been obtained using the same way. 2. Experimental section 2.1. Electrochemical deposition All chemical reagents are of analytic purities and were purchased from Sinopharm Chemical Reagent (China) without further purification. The electrochemical deposition of LaPO4 deposits was conducted in a solution consisting of 0.01 M La(NO3)3, 0.01 M EDTA, 0.02 M Na3PO4 and 0.1 M KCl. The solution of Na3PO4 was added into beaker after the forming of stable La-EDTA complex and the final pH was adjusted to 6.00 using 0.1 M HNO3. The solution for the electrodepositon of LaPO4:Ln 3+ (Ln 3+ = Ce 3+, Tb 3+) deposits were prepared using the solution above by adding certain proportion of rare earth ions. The electrodeposition procedure was carried out on a CHI 601C potentiostat connected with a classical three-electrode electrochemical cell. The working electrode was a glass coated with indium tin oxide (ITO) (≈20 Ω cm), the counter electrode was a platinum sheet, and the reference electrode was a saturated calomel electrode (SCE). Before electrodeposition, the ITO substrate was cleaned with
80
H. Wang et al. / Electrochemistry Communications 17 (2012) 79–81
acetone in an ultrasonic bath for 15 min and then rinsed with distilled water. The applied potential was 1.4 V to 1.8 V vs SCE and the solution temperature ranged from 30 to 80 °C for 2 h. 2.2. Characterization X-ray diffraction measurements were done to investigate the purity of the samples and the crystallite sizes using a high resolution Philips X'Pert MRD diffractometer with Cu Kα radiation (λ = 0.15406 nm) in the scan range of 10–60 . The morphology of the deposits was studied by a field-emission scanning electron microscopy (FE-SEM, Sirion-100) with incident electron energy of 25 kV. In order to increase the conductivity of LaPO4, platinum powders were sputtered on the surface of the deposits before. The Tb 3+-doped LaPO4 deposits were analyzed by energy dispersive spectroscopy (EDS) to determine the composition and element distribution. TEM images were collected on a Tecnai G2 F30 transmission electron microscopy operating at an accelerating voltage of 300 kV. The emission spectra of the LaPO4:Ln 3+(Ln 3+ = Ce 3+, Tb 3+) deposits were recorded on a Hitachi F-2500 luminescence spectrophotometer equipped with 150 W xenon lamp as the excitation source. 3. Results and discussion 3.1. Structure and morphologies of the deposits The composition and phase purity of the deposits were examined by X-ray diffraction. XRD patterns of LaPO4 , LaPO4:Tb 3+(Tb 3+, 20 mol%) and LaPO4:Ce 3+:Tb 3+(Ce 3+, 10 mol% and Tb 3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C are shown in Fig. 1. The XRD patterns are found to match well with the hexagonal phase of LaPO4 (JCPDS no. 75-1881). No other phase can be found in the patterns except ITO substrate. The high and shape peaks indicate that the deposits are well crystallized. Owing to the small sizes of LaPO4, the diffraction peaks are slightly broadened. The grain size were estimated from the values of FWHM (full width half maximum) of the above diffraction peaks using the Scherrer formula and the average crystalline size are around 15 nm, 13 nm and 16 nm for LaPO4, LaPO4:Tb 3+ and LaPO4:Ce 3+:Tb 3+ respectively. The scanning electron microscopy (SEM) images of LaPO4, LaPO4: Tb 3+(Tb 3+, 20 mol%) and LaPO4:Ce 3+:Tb 3+ (Ce 3+, 10 mol% and Tb 3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C are shown in Fig. 2. One can see that deposits are typically composed of interlaced nanorods with the length up to 300–400 nm and the diameter from 10 nm to 20 nm. In order to learn the distribution of elements in the doped LaPO4 deposits, LaPO4 deposits doped Tb 3+ were characterized by EDS. The EDS mapping result is shown in Fig. 2d, which demonstrates that terbium ions have been uniformly distributed in the deposits.
Fig. 1. XRD patterns of LaPO4, LaPO4:Tb3+(Tb3+, 20 mol%) and LaPO4:Ce3+:Tb3+ (Ce3+, 10 mol% and Tb3+, 20 mol%) deposits electrodeposited at 1.6 V vs SCE and 80 °C.
Fig. 2. SEM images of LaPO4(a), LaPO4:Tb3+(Tb3+, 20 mol%)(b) and LaPO4:Ce3+: Tb3+(Ce3+, 10 mol% and Tb3+, 20 mol%) (c) deposits electrodeposited at 1.6 V vs SCE and 80 °C. (d) EDS mapping of the LaPO4:Tb3+ (Tb3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C.
The LaPO4 nanorods obtained at 1.6 V vs SCE and 80 °C were further characterized by TEM, HRTEM and a fast Fourier transform pattern (FFT). Fig. 3a display the TEM images of LaPO4, which are typical nanorods, confirming the above SEM observation. The highresolution TEM images (Fig. 3b and c) taken from a single LaPO4 nanorod show the clearly resolved planes of (001) and (100). The (001) planes are oriented parallel to the nanorod growth axis, indicating that the direction of nanorod growth is along the c axis. The SAED pattern (Fig. 3d) taken from a single nanorod can be indexed as a hexagonal LaPO4 single crystal recorded from the [010] zone axis. 3.2. Possible mechanism of electrodeposited LaPO4 nanostructures It is known that EDTA will form stable coordination compound with metal ions in neutral aqueous solution. However, when solution pH lowers than 2.0, EDTA, existing as H6Y 2 +, H5Y + or H4Y (Y = EDTA), cannot form stable complex with metal ions. In the system of Ln 3+-EDTA (Ln = La, Ce, Tb), the dissolving Ln 3+ ions
Fig. 3. (a) Low-magnification TEM images of LaPO4 nanorods electrodeposited at 1.6 V vs SCE and 80 °C (b) TEM image of a single LaPO4 nanorod, (c) HRTEM image of a single LaPO4 nanorod (d) SAED pattern recorded from the [010] zone axis.
H. Wang et al. / Electrochemistry Communications 17 (2012) 79–81
81
transitions of Tb3+ ions. In this nanophosphor Ce 3+ ions behave as sensitizer and Tb3+ ions act as luminescent center. Compared with LaPO4: Tb (Tb3+, 20 mol%), the intensity of 5D4 → 7F5 significantly increases because of the energy transfer between Ce 3+ and Tb3+. 4. Conclusions
Fig. 4. PL emission spectra of LaPO4:Tb3+ (Tb3+, 20 mol%) and LaPO4:Ce3+,Tb3+ (Ce3+, 10 mol% and Tb3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C.
coordinate with EDTA to form stable Ln 3+-EDTA coordination compound in neutral solution. As a result, Ln 3+ could co-exist with PO43 − in the solution without forming precipitation initially. As long as applied anodic potentials are larger than 1.4 V vs SCE (in this work), the H2O will be oxidized (E o = 1.0 V vs SCE) and released H + and O2. When the released hydrogen ions lower pH at the surface of the ITO working electrode, Ln 3+ ions are set free from the Ln 3+EDTA coordination compound and then react with PO43 − ions to form LnPO4 deposits. Overall, the possible reaction mechanism can be summarized as following expressions: þ
H2 O→1=2O2 þ 2H þ 2e
ð1Þ
In summary, hexagonal phase LaPO4 and LaPO4:Ln 3+ (Ln 3+ = Ce 3+, Tb 3+) nanostructures typically composed of interlaced nanorods with the length up to 300–500 nm and the diameter from 10 nm to 20 nm have been obtained by a facile electrochemical deposition method. The selected area electron diffraction (SAED) of LaPO4 nanorods indicates that the LaPO4 nanorods are single crystal structures. The energy dispersive spectroscopy mapping indicates that the doped Tb 3+ ions are distributed homogeneously in the LaPO4 nanorods. The LaPO4:Ce 3+, Tb 3+(Ce 3+, 10 mol% and Tb 3+, 20 mol%) nanostructures exhibit strong green emission at room temperature. The electrodeposition method could be extended to grow other nanophosphor structures or coatings. Acknowledgements This work was financially supported by the National Basic Research Program of China (Grant No. 2011CB936003), the National Natural Science Foundation of China (20973153) and Zhejiang Provincial Natural Science Foundation of China (Y4090012). References
3þ
þ
3þ
Ln −ðEDTAÞ þ xH →Ln
3þ
Ln
3−
þ PO4
¼ LnPO4 ↓
4−x
þ Hx EDTA
ð2Þ
ð3Þ
3.3. Luminescent properties The PL emission spectra of LaPO4:Tb 3+ (Tb 3+, 20 mol %) and LaPO4:Ce 3+:Tb 3+ (Ce 3+, 10 mol% and Tb 3+, 20 mol%) electrodeposited at 1.6 V vs SCE and 80 °C are shown in Fig. 4. The emission spectrum of LaPO4:Tb3+ (Tb3+, 20 mol%, indexed as dash line) under the 250 nm excitation consists of three peaks at about 542 nm, 583 nm, 623 nm. The peaks are related to 5D4 → 7F5, 5D4 → 7F4 and 5D4 → 7F3 transitions of Tb3+ ions, respectively [9]. The peak at 542 nm is the strongest, which emits intense green light. The emission spectrum of the Ce3+/Tb3+coactivated LaPO4 film is also displayed in Fig. 4(indexed as solid line). The excitation wavelength is 250 nm. The peaks ranged from 450 nm to 650 nm are correspond to the 5D4 → 7FJ (J = 5, 4, 3)
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