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Rapid annealing: A novel processing technique for Cr:ZnAl2O4 nanoparticles
Journal of Alloys and Compounds 728 (2017) 484e489
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Rapid annealing: A novel processing technique for Cr:ZnAl2O4 nanoparticles Samvit G. Menon a, K.S. Choudhari a, S.A. Shivashankar b, C. Santhosh a, Suresh D. Kulkarni a, * a b
Department of Atomic and Molecular Physics, Manipal University, Manipal, Karnataka, 576104, India Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, Karnataka, 560012, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2017 Received in revised form 1 September 2017 Accepted 2 September 2017 Available online 5 September 2017
Nanocrystalline Cr:ZnAl2O4 was synthesized for the first time by rapid thermal annealing the corresponding hydroxide precipitate in just 10 min. The XRD patterns showed that synthesis between 500 C and 700 C had smaller crystallites (~2.4 e 4 nm), while at 800 C and 900 C, had ~10 nm crystallites. HRTEM images of the 900 C sample showed the well-developed lattice fringes confirming the high crystallinity. The particles were polyhedral having an average size of ~16 nm. The diffuse reflectance spectra showed characteristic 4A2g / 4T1g and 4A2g / 4T2g transitions, while the photoluminescence emission spectrum comprised of the zero phonon line (R-line) along with other multi-phonon side bands that are characteristic of the spin e parity forbidden transitions of the Cr3þ ion. The emission spectra and photoluminescence lifetimes are comparable with reports on sensing and imaging applications. Our work demonstrates the suitability of this technique to quickly synthesise phosphors for a variety of applications. © 2017 Elsevier B.V. All rights reserved.
Keywords: A. Ceramics A. Nanostructured materials B. Nanofabrication C. Optical properties D. Luminescence
1. Introduction In view of their superior thermal and chemical stability, enhanced optical and electrical properties, mechanical strength, etc., nanocrystalline metal oxides have been widely explored in catalysis, displays, sensing, lasing, etc. [1e6]. Nanocrystalline ZnAl2O4, with a wide bandgap of 3.8 eV is known for its range of potential applications in photocatalysis, as phosphor hosts, for fabricating UV-optoelectronic devices, optomechanical stress sensors, and electroluminescent displays [7,8]. Several solid state and wet chemical recipes exist for the synthesis of the crystalline ZnAl2O4 spinel [9e11]. However, obtaining high-quality and phasepure spinels requires that most synthesis methods be followed by a high-temperature annealing process where temperatures can go up to 1200 C. Annealing, at relatively slow heating rates, is a widely employed post-processing technique for glass and ceramics, including spinels [11e15]. Annealing is usually achieved through heating elements made of silicon carbide, molydisilicide, or chromium-iron-
* Corresponding author. E-mail address: [email protected] (S.D. Kulkarni). http://dx.doi.org/10.1016/j.jallcom.2017.09.026 0925-8388/© 2017 Elsevier B.V. All rights reserved.
aluminium alloys, which bring about structural changes in the material through convective heating. It is already well established that annealing influences the material properties for the application of interest, be it as a catalyst, phosphor, or a magnetic device. Rapid annealing (RA) can instantly raise the temperatures to several hundred degrees within minutes; a similar process, rapid thermal annealing (RTA) with heating rates ranging from 5 C/sec to 25 C/sec [16e19], is used extensively in the processing of integrated circuits [16,19,20]. RTA has been used for the fabrication of silicide layers [21], dopant activation of thin films, and to “heal” damage due to ion implantation [20]. RTA is particularly useful in thin film fabrication because it minimises interaction between the film and substrate. While RTA is an industrially well-established thin film processing technique, here we report on employing rapid annealing (RA), with the ramp up rates of ~150e200 C/min, in the synthesis of ceramic powders. Even though the ramp up rates are lower than in RTA, the use of RA in the synthesis of ceramic powders such as Cr3þ doped ZnAl2O4 (Cr:ZAO), has not been explored previously. In fact, there are only a few reports on the RA of metal oxide powders [22]. Since RA can significantly reduce the synthesis/processing time of oxides, it may potentially be of technological importance. In view of the lack of literature on the use of RA for the synthesis of spinel/
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ZnAl2O4 nanoparticles, it is important to investigate the effects of RA on the structural and optical properties of such materials. Here, we explore the use of RA for the synthesis of Cr:ZAO, starting from the co-precipitate of the respective hydroxides. Cr:ZAO is a wellknown red/NIR emitting phosphor [13] and we have investigated its emission properties in detail for different RA parameters. We show that 10 min of RA is sufficient for the synthesis of Cr:ZAO nanoparticles, making RA potentially useful for the rapid synthesis of nanoparticles of other metal oxides. 2. Experimental 2.1. Synthesis of Cr:ZAO The aim was to prepare ZnAl1.95Cr0.05O4 nanoparticles starting from zinc acetate dihydrate (Zn(CH3COO)2.2H2O) (Merck), aluminium nitrate nonahydrate (Al(NO3)3.9H2O) (Merck), and chromium chloride hexahydrate (CrCl3.6H2O) (Merck). Aqueous solutions of the corresponding salts taken in 1:1.95:0.05 mol ratio (Zn: Al: Cr) were mixed and precipitated using a 2 M solution of NaOH at a pH of 9.5, under constant stirring for 30 min. The precipitate was vacuum-filtered, washed and dried in an oven at 60 C for 2 h, before subjecting it to RA at temperatures between 500 and 900 C for 10 min, employing a ramp rate of ~200 C/min. The corresponding heating profile (time vs. temperature) of the furnace for different temperature set-points are shown ESI, Fig. S1. The details of RA system are reported elsewhere [22]. The precipitate was evenly spread over a quartz boat that ensured uniform heating. The samples synthesized at temperatures from 500 Ce900 C were labelled A e E, respectively. 2.2. Characterisation X-Ray diffraction (XRD) patterns of the powder samples were collected using a Rigaku Ultima-IV diffractometer (Cu-Ka radiation).
485
Fourier transform infrared (FT-IR) spectra were recorded using Jasco FT/IR 6200 spectrometer. Bandgaps were deduced from diffuse reflectance spectra of the samples obtained from Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer with a 150 mm integrating sphere attachment. Diffuse reflectance measurements were made between 800 and 200 nm using BaSO4 as the reflectance standard. Crystallinity and morphology of the powders were examined by high-resolution transmission electron microscopy (HR-TEM, JEOL 3010). Photoluminescence spectra were obtained at room temperature for the various samples (Jasco FP8300 fluorescence spectrometer equipped with a 450 W Xenon lamp and a high-speed chopper). The luminescence decay curves were processed by single-exponential fit to derive the lifetime in each sample. 3. Results and discussion 3.1. X-ray diffraction The effect of RA on the formation of Cr:ZAO nanoparticles was investigated at different temperatures between 500 and 900 C for 10 min, reached at a ramp rate of ~200 C/min. The crystallinity and phase purity of the samples were determined by X-ray diffraction. The XRD patterns of the samples (Fig. 1a) show peaks corresponding only to the spinel phase. The broad peaks observed for sample A are due to the very small size of the crystallites (2 nm). As expected, crystallinity improves as the synthesis temperature is raised, causing the narrowing of the peaks. The change in crystallite size, bandgap, and lattice parameters are listed in Table 1. The lattice parameter approaches the reported values as the temperature is raised (Table 1) [23,24]. The increase in the average crystallite size with temperature follows an exponential trend (Fig. 1b), suggesting that the significant crystal growth occurs between 700 C and 900 C. It is worth noting that mere 10 min of RA led to well crystallized Cr:ZAO nanoparticles starting from
Fig. 1. (a) XRD patterns of Cr-doped ZnAl2O4 rapid-annealed at different temperatures and (b) plot showing the variation in crystallite size with temperature.
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Table 1 Changes in material properties with different synthesis temperatures. Sample Synthesis Temperature ( C)
Bandgap Crystallite (eV) Size (nm)
Lattice Lifetime Parameter (Å) (ms)
A B C D E
3.03 3.03 3.05 3.05 3.06
8.1555 8.1565 8.1442 8.1320 8.1053
500 600 700 800 900
2.4 3.41 4 6.1 10.5
12.81 12.13 12.61 23.19 25.43
hydroxides. This is remarkable considering that Cr:ZAO is a ceramic material and the other reported methods use much longer synthesis and/or processing times [25]. Synthesis of ZnAl2O4 nanoparticles by co-precipitation is known to produce ZnO as an impurity phase, due to the amphoteric nature of ZnO [23]. Further, Al3þ salts are known to form amphoteric Al(OH)3. Al3þ and Cr3þ salts are known to precipitate together25 at a pH of 9, whereas Zn2þ precipitates above a pH of 8.5 [23]. Simultaneous precipitation of all component ions is a prerequisite for coprecipitation synthesis and hence a pH of 9.5 was selected. The XRD patterns revealed no additional impurity peaks, attesting the formation of only the spinel phase. 3.2. Transmission electron microscopy (TEM) High-resolution TEM images were used to assess the fine-scale morphology of sample E and to obtain the particle size distribution (Fig. 2). Fig. 2a and b shows HRTEM images displaying the lattice fringes for the (511), (220) and (400) planes, indicating that each crystallite is a single crystal. The selective area electron diffraction (SAED) pattern shows slightly diffuse rings that are characteristic of the crystallites that are quite small even after annealing at 900 C for 10 min (Fig. 2c). However, the crystallites appear polyhedral with characteristics of the cubic symmetry of the spinel structure. The particle size histogram (Fig. 2d) is seen to follow a Gaussian profile with an average size of 16 nm.
signatures of Al3þ in tetrahedral coordination evidencing the presence of partial inversion which is expected in nanometric spinel crystals [22,34]. 3.4. Diffuse reflectance spectroscopy The diffuse reflectance spectra of the co-precipitate before RA and the samples A, C, and E (Fig. 4a) show a broad band at 370 nm and a shoulder at 540 nm, which are the characteristic 4A2g(F) / 4 T1g(F) and 4A2g(F) / 4T2g(F) absorption transitions of the Crþ3 ions, due to the spin-allowed d-d transitions in the host lattice [13]. The optical (direct) bandgap of samples A and E determined24, 35 from the Tauc plot of the Kubelka-Munk function are shown in Fig. 4b. It is clear that the bandgap does not change significantly with temperature; it is 3.03 eV and 3.06 eV for samples A and E respectively. These values are in agreement with previous reports [13]. 3.5. Photoluminescence spectroscopy The room temperature excitation spectra (lem ¼ 690 nm) of the samples comprise two broad peaks, at 420 nm (n1 e 23810 cm1) and 540 nm (n2 e 18519 cm1), with a weak shoulder at 405 nm, the intensity of which increases with the synthesis temperature (Fig. 5). The ground state, 4F, and the excited states, 4P and 2G, of the d3 Cr3þ ion, split into several doublet and quartet states, such as 4A2g, 4T2g, 4 T1g, 2Eg, 2T1g, etc., in an octahedral field [35]. The ground state, t23 , of Cr3þ complex in an octahedral field gives rise to 4A2, 2E, 2T1, and 2 T2 states, and the lowest energy state among these, the 4A2 state, forms the ground state of the ion [36]. The first excited state, t22 e1 state gives rise to the 4T2 and 4T1 states, which result in the n1 and n2 spin-allowed transitions, 4A2g / 4T1g and 4A2g / 4T2g [37e39]. The weak shoulder on the 420 nm band is due to trigonal distortions that further split the excited states of the host ZnAl2O4 lattice [40,41]. The additional splitting of triply degenerate quartet terms (in the cubic approximation) result in asymmetric excitation bands or sometimes even double peaks [42]. The 4T1 state splits
3.3. Fourier transform infrared spectroscopy For ZnAl2O4, low-frequency bands observed in the range of 1000e400 cm1 can be attributed to the vibration modes of AleO bonds [26]. For Al3þ in an octahedral symmetry (AlO6), AleO stretching and bending modes are observed in the regions between 500 and 700 cm1 and 330-450 cm1, respectively [27,28]. However, in the case of crystallographic inversion in the spinel lattice, Al3þ becomes tetrahedrally co-ordinated (AlO4), and corresponding AleO stretching modes appear between 700 and 850 cm1 whereas the bending modes would be between 250 and 320 cm1 [27,28]. The FT-IR spectra (Fig. 3a) of the as-prepared precipitate and the samples AeD showed broad and unresolved bands between 400 and 1000 cm1. For samples C and D, the bands just begin to resolve themselves, whereas well-resolved bands are obtained for sample E. The broad band for sample A, between 400 and 1000 cm1 can be attributed to the AleOH vibrations, indicating the possible presence of Al(OH)3 even after annealing at 500 C [29]. For sample E, the bands at 672 and 565 cm1 are due to the symmetric stretching, while the weak band at 503 cm1 is due to the asymmetric stretching mode of octahedral Al3þ. The bands at 449 and 492 cm1 (Fig. 3b) can be attributed to the stretching modes of the ZneO [30,31]. The low-frequency bands obtained between 400 and 430 cm1 for sample E have been attributed to the vibrations of tetrahedral Al3þ which are blue-shifted because of their small crystallite size [27,32,33]. The weak bands at 711 and 726 cm1 in sample E are
Fig. 2. (a) TEM and (b) HR-TEM images of RA910 sample and its (c) SAED pattern along with (d) particle size distribution histogram.
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Fig. 3. FT-IR spectrum of (a) Cr:ZAO nanoparticles synthesized by RA and (b) FT-IR of sample E (Full range).
into the 4T1(t22 e1 ) and 4T1(t21 e2 ) states resulting in 4A2 / 4T1(t22 e1 ) and 4A2 / 4T1(t21 e2 ) transitions, of which the number of latter transitions are much lower, as signified by the weak shoulder at about 400 nm. However, the effects of trigonal distortion are not significant vis-a-vis reported work [13]. For samples A and B, the 4A2g / 4T2g transition band is observed at 549 nm and 550 nm, respectively, which is shifted by about Dl ¼ 10 nm relative to sample E, which has a lmax ¼ 540 nm. This shift could be a result of the changes in the lattice constant of the two samples. Essentially, the Zn2þ - O2 - Al3þ - O2 separations control the peak positions of the 4A2g / 4T2g transitions. The lattice constants of samples A and B are larger than the others, suggesting that the higher ionic separation results in the observed spectral shift towards higher wavelengths [43,44]. The emission bands (Fig. 6) observed correspond to the general perturbed or unperturbed 2E / 4A2 Cr3þ transitions. These perturbations are caused by crystal defects and can manifest in the form of separate bands [45]. The emission spectra comprise a zero phonon line (ZPL), or the R e line at about 688 nm for all samples, and multi-phonon side bands on either side of the ZPL for samples D and E [13,46,47]. This ZPL is the resultant of unresolved R1 and R2 bands, formed due to the splitting of the 2E state which, in turn, is a consequence of spin-orbit coupling and trigonal distortions. For a ZnAl2O4 host, these two bands are separated by about 7e10 cm1 [46] and have only been observed [48,49] at low temperatures.
Fig. 5. Excitation spectra of samples A e E at an emission wavelength of 688 nm, and (inset) Temperature vs intensity plot of this excitation spectrum.
The band at 699 nm for samples D and E is known as the N2-line and is ascribed to the short-range perturbations of the octahedral sites surrounding Cr3þ. This is a structure-dependent line and has energy lower than the R1-R2 doublet [42]. The intensity of the N2 line is lower than that of the R-line for these samples, probably
Fig. 4. (a) The diffuse reflectance spectra of the co-precipitate sample along with samples A, C, and E, and (b) the Tauc plots of sample A and sample E.
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Fig. 6. Emission spectrum of samples A e E with the temperature vs intensity plot of the R-line in the inset.
because of fewer anti-site defects at higher annealing temperatures. However, at lower annealing temperatures (samples A - C), these N lines are diffuse, unresolved, and are much lower in intensity, perhaps because of fewer Cr3þ ions being at the octahedral site of the ZnAl2O4 lattice. The intensity of the R-line increases with temperature (Fig. 6 inset), as in our previous results [13]. Spectroscopic parameters and properties of the Cr3þ ion are a function of its local environment. d3 systems have fivefold degeneracy and split into t2g and e2g energy levels in a crystal field of a specified strength. The strength of the crystal field splitting parameter, 10.15Dq, is given by n1. From the excitation spectra of the samples, the corresponding Tanabe-Sugano diagrams can be plotted, which helps in determining the different spectroscopic parameters. The excitation spectrum also helps in calculating the electronic repulsion parameter, the Racah parameter (B), from the equation [50e52]:
15ðn 8Þ n2 10n
Dq ¼ B
(1)
Fig. 7. PL decay curves of samples A e E measured using a 550 nm wavelength. Table 2 Lifetime comparisons of Cr: ZAO with different synthesis techniques. Synthesis Method
Measurement Temperature
Lifetime (ms)
References
Electro-spinning Co-precipitation Flux growth Sol-gel Co-Precipitation and Rapid Annealed
Room temperature Room temperature 77 K 10 K Room temperature
23.47 31.19 29.0 32.7 25.43
[45] [13] [49] [55] Current study
688 nm emission line for samples D and E (Fig. 7). The excitation wavelengths employed were 550 nm for samples A e C, and 540 nm for samples D and E. The lifetimes of samples A e C did not change significantly and ranged between 12.1 and 12.8 ms (Table 1). However, the lifetimes for samples D and E is significantly longer: 23.19 ms for sample D and 25.43 ms for sample E, showing that the synthesis temperature influences emission lifetimes directly. These values, even though slightly lower, are quite comparable with the lifetimes measured in samples synthesized by other methods (Table 2) [13], affirming the novelty and effectiveness of the RA technique.
where
g¼
E A2g / T1g E A2g / T2g Dq
4. Conclusions
(2)
These equations can be solved to give the Racah parameter:
B¼
2n21 þ n22 3n1 n2 15n2 27n1
(3)
The free ion electron repulsion parameter (Bfree) for the Cr3þ ion is 918 cm1 [43]. In the present study, the Racah parameter, B, is found to be 490 cm1, which is much smaller than Bfree. This discrepancy may be attributed to the nephelauxetic effect, wherein metal electrons are delocalized over molecular orbitals comprised of both ligands and metal ions. It is speculated that this effect is stronger because of the rapid annealing process. The Dq/B value at the region where the 2E and 4T2 curves intersect on the TanabeSugano diagrams gives insights into field strengths with values greater than 2.3, implying high crystal field strengths. In this work, the Dq/B value is found to be 3.78, confirming that that Cr3þ ions are in strong crystal fields and that the photoluminescence emissions are associated with their corresponding spin-parityforbidden transitions [53,54]. The emission lifetimes were measured by recording the decay profiles of the 690 nm emission line for samples A e C and of the
High-quality nanocrystalline Cr:ZnAl2O4 can be prepared within 10 min by rapid annealing of the coprecipitate. Crystallites were very small at low temperatures, with significant crystal growth taking place beyond 800 C, as confirmed by XRD. HRTEM of the sample synthesized at 900 C shows that the nanocrystals are mostly polyhedral, with an average size of 16 nm. Room temperature photoluminescence excitation and emission spectra of the samples are comparable to those of samples obtained by other methods, with the zero-phonon line and the defect-induced N2 line appearing at 688 nm and 699 nm, respectively. The measured high Dq/B value of 3.78 suggests that the dopant Cr3þ ions are in a strong crystal field, causing emissions related to spin-parity-forbidden transitions. The lifetime for the 900 C sample was measured as 25.43 ms, long enough for bioimaging, lasing, and sensing applications. The rapid annealing technique, with modest ramp rates, is found to be effective in yielding powder samples in minutes, with properties comparable to those prepared using methods that are much longer. Acknowledgments We gratefully acknowledge the financial support from SERB, DST
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Govt. of India, Manipal University, DST-FIST Program and I-CUP, the Indian Cluster for Ultrafast Photonics for the following projects: SB/ S2/CMP-017/2014; SB/FT/CS-123/2013; SR/FST/PSI-174/2012; and Prn.SA/Photonics-UFL/2008. Samvit G. Menon thanks Manipal University for providing the MU-Ph.D. scholarship. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2017.09.026. References [1] H. Grabowska, M. Zawadzki, L. Syper, Gas phase alkylation of 2hydroxypyridine with methanol over hydrothermally synthesised zinc aluminate, Appl. Catal. A Gen. 314 (2006) 226e232. [2] Y. Nakayama, P.J. Pauzauskie, A. Radenovic, R.M. Onorato, R.J. Saykally, J. Liphardt, P. Yang, Tunable nanowire nonlinear optical probe, Nature 447 (2007) 1098e1101. [3] Z. Nie, A. Petukhova, E. Kumacheva, Properties and emerging applications of self-assembled structures made from inorganic nanoparticles, Nat. Nanotechnol. 5 (2010) 15e25. [4] C. Yan, A. Dadvand, F. Rosei, D.F. Perepichka, Near-IR photoresponse in new up-converting CdSe/NaYF4:Yb, Er nanoheterostructures, J. Am. Chem. Soc. 132 (2010) 8868e8869. [5] M. Zawadzki, Synthesis of nanosized and microporous zinc aluminate spinel by microwave assisted hydrothermal method (microwaveehydrothermal synthesis of ZnAl2O4), Solid State Sci. 8 (2006) 14e18. [6] F. Zhang, Y. Wan, T. Yu, F. Zhang, Y. Shi, S. Xie, Y. Li, L. Xu, B. Tu, D. Zhao, Uniform nanostructured arrays of sodium rare-earth fluorides for highly efficient multicolor upconversion luminescence, Angew. Chem. Int. Ed. 46 (2007) 7976e7979. [7] H. Matsui, C.N. Xu, H. Tateyama, Stress-stimulated luminescence from ZnAl2O4: Mn, Appl. Phys. Lett. 78 (2001) 1068. [8] G. Mueller, Electroluminescence II (Semiconductors and Semimetals), vol. 65, Academic Press, New York, 2000. [9] Z. Bi, R. Zhang, X. Wang, S. Gu, B. Shen, Y. Shi, Z. Liu, Y. Zheng, Synthesis of zinc aluminate spinel film through the solid-phase reaction between zinc oxide film and a-alumina substrate, J. Am. Ceram. Soc. 86 (2003) 2059e2062. [10] Z.Z. Chen, E.W. Shi, Y.Q. Zheng, B. Xiao, J.Y. Zhuang, Hydrothermal synthesis of nanosized CoAl2O4 on ZnAl2O4 seed crystallites, J. Am. Ceram. Soc. 86 (2003) 1058e1060. [11] X. Duan, D. Yuan, X. Wang, H. Xu, Synthesis and characterization of nanocrystalline zinc aluminum spinel by a new sol-gel method, J. Sol-Gel Sci. Technol. 35 (2005) 221e224. re, B. Viana, Order and dis[12] N. Basavaraju, K.R. Priolkar, D. Gourier, A. Bessie order around Cr3þ in chromium doped persistent luminescent AB2O4 spinels, Phys. Chem. Chem. Phys. 17 (2015) 10993e10999. [13] S.G. Menon, D.N. Hebbar, S.D. Kulkarni, K. Choudhari, C. Santhosh, Facile synthesis and luminescence studies of nanocrystalline red emitting Cr: ZnAl2O4 phosphor, Mater. Res. Bull. 86 (2017) 63e71. [14] S.G. Menon, S.D. Kulkarni, K. Choudhari, Diffusion-controlled growth of CuAl2O4 nanoparticles: effect of sintering and photodegradation of methyl orange, J. Exp. Nanosci. 11 (2016) 1227e1241. [15] S.G. Menon, K.S. Choudhari, S.A. Shivashankar, S. Chidangil, S.D. Kulkarni, Microwave solution route to ceramic ZnAl2O4 nanoparticles in 10 minutes: inversion and photophysical changes with thermal history, N. J. Chem. 41 (2017) 5420e5428. [16] J.K. Kim, H.J. Cheong, Y. Kim, J.Y. Yi, H.J. Bark, S. Bang, J. Cho, Rapid-thermalannealing effect on lateral charge loss in metaleoxideesemiconductor capacitors with Ge nanocrystals, Appl. Phys. Lett. 82 (2003) 2527e2529. [17] K.-K. Kim, H.-S. Kim, D.-K. Hwang, J.-H. Lim, S.-J. Park, Realization of p-type ZnO thin films via phosphorus doping and thermal activation of the dopant, Appl. Phys. Lett. 83 (2003) 63e65. [18] K.K. Kim, S. Niki, J.Y. Oh, J.O. Song, T.Y. Seong, S.J. Park, S. Fujita, S.W. Kim, High Electron Concentration and Mobility in Al-doped n-ZnO Epilayer Achieved via Dopant Activation Using Rapid-thermal Annealing, 2005. [19] B.H. Lee, L. Kang, R. Nieh, W.J. Qi, J.C. Lee, Thermal stability and electrical characteristics of ultrathin hafnium oxide gate dielectric reoxidized with rapid thermal annealing, Appl. Phys. Lett. 76 (2000) 1926e1928. n, C. Balocchi, X. Errazu, R. Avila, G. Piderit, Rapid thermal annealing [20] N. Beltra of zirconia films deposited by spray pyrolysis, J. Electron. Mater. 27 (1998) L9eL11. [21] R. Thakur, K. Schuegraf, P. Fazan, H. Rhodes, RTP: manufacturing perspective, Solid State Technol. 39 (1996) 99e104. [22] R. Sai, S.D. Kulkarni, S.S. Bhat, N.G. Sundaram, N. Bhat, S. Shivashankar, Controlled inversion and surface disorder in zinc ferrite nanocrystallites and their effects on magnetic properties, RSC Adv. 5 (2015) 10267e10274. [23] L. Cornu, M. Gaudon, V. Jubera, ZnAl2O4 as a potential sensor: variation of luminescence with thermal history, J. Mater. Chem. C 1 (2013) 5419e5428.
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Express 1 (2014) 045029. , M. Malinowski, W. Stre,k, Site selection spectroscopy of Cr3þ in [40] P. Deren MgAl2O4 green spinel, J. Lumin. 68 (1996) 91e103. [41] R. Macfarlane, Analysis of the spectrum of d3 ions in trigonal crystal fields, J. Chem. Phys. 39 (1963) 3118e3126. [42] W. Mikenda, A. Preisinger, N-lines in the luminescence spectra of Cr3þ-doped spinels (I) identification of N-lines, J. Lumin. 26 (1981) 53e66. [43] J.E. Huheey, E.A. Keiter, R.L. Keiter, O.K. Medhi, Inorganic Chemistry: principles of Structure and Reactivity, Pearson Education India, 2006. [44] P.T. Lanh, N.N. Long, T.T. Loan, Some physical properties of ZnAl2O4:Cr3þ (Co2þ) powders prepared by hydrothermal method, in: Journal of Physics: Conference Series, IOP Publishing, 2009, p. 012053. [45] G. Dong, X. Xiao, M. Peng, Z. Ma, S. Ye, D. Chen, H. Qin, G. Deng, Q. Liang, J. Qiu, Synthesis and optical properties of chromium-doped spinel hollow nanofibers by single-nozzle electrospinning, RSC Adv. 2 (2012) 2773e2782. re, B. Viana, The [46] N. Basavaraju, K.R. Priolkar, D. Gourier, S.K. Sharma, A. Bessie importance of inversion disorder in the visible light induced persistent 3þ luminescence in Cr doped AB2O4 (A¼ Zn or Mg and B¼ Ga or Al), Phys. Chem. Chem. Phys. 17 (2015) 1790e1799. [47] W. Mikenda, N-lines in the luminescence spectra of Cr3þ-doped spinels (III) partial spectra, J. Lumin. 26 (1981) 85e98. re, S. Jacquart, K. Priolkar, A. Lecointre, B. Viana, D. Gourier, ZnGa2O4: [48] A. Bessie Cr3þ: a new red long-lasting phosphor with high brightness, Optics Express 19 (2011) 10131e10137. [49] D. Wood, G. Imbusch, R. Macfarlane, P. Kisliuk, D. Larkin, Optical spectrum of Cr3þ ions in spinels, J. Chem. Phys. 48 (1968) 5255e5263. [50] M. Casalboni, V. Ciafardone, G. Giuli, B. Izzi, E. Paris, P. Prosposito, An optical study of silicate glass containing and ions, J. Phys. Condens. Matter. 8 (1996) 9059. [51] J.R. Perumareddi, Electronic spectra of quadrate chromium (III) complexes, Coord. Chem. Rev. 4 (1969) 73e105. [52] F. Rasheed, K. O'Donnell, B. Henderson, D. Hollis, Disorder and the optical spectroscopy of Cr3þ-doped glasses. II. Glasses with high and low ligand fields, J. Phys. Condens. Matter. 3 (1991) 3825. [53] G. Blasse, W. van der Weg, An investigation of Cr3þ luminescence in borate glasses, J. Phys. C Solid State Phys. 18 (1985) 3379. ska, L. Arizmendi, Nephelauxetic effect in [54] A. Suchocki, S. Biernacki, A. Kamin luminescence of Cr3þ-doped lithium niobate and garnets, J. Lumin. 102 (2003) 571e574. [55] H. Luc, T. Nguyen, V. Nguyen, A. Suchocki, A. Kaminska, V. Le, V. Nguyen, T. Luong, Luminescence spectra of ZnAl2O4:Cr3þ spinel nanopowders, Acta Phys. Pol. A 104 (2003) 581e587.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Rapid annealing: A novel processing technique for Cr:ZnAl2O4 nanoparticles Samvit G. Menon a, K.S. Choudhari a, S.A. Shivashankar b, C. Santhosh a, Suresh D. Kulkarni a, * a b
Department of Atomic and Molecular Physics, Manipal University, Manipal, Karnataka, 576104, India Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, Karnataka, 560012, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2017 Received in revised form 1 September 2017 Accepted 2 September 2017 Available online 5 September 2017
Nanocrystalline Cr:ZnAl2O4 was synthesized for the first time by rapid thermal annealing the corresponding hydroxide precipitate in just 10 min. The XRD patterns showed that synthesis between 500 C and 700 C had smaller crystallites (~2.4 e 4 nm), while at 800 C and 900 C, had ~10 nm crystallites. HRTEM images of the 900 C sample showed the well-developed lattice fringes confirming the high crystallinity. The particles were polyhedral having an average size of ~16 nm. The diffuse reflectance spectra showed characteristic 4A2g / 4T1g and 4A2g / 4T2g transitions, while the photoluminescence emission spectrum comprised of the zero phonon line (R-line) along with other multi-phonon side bands that are characteristic of the spin e parity forbidden transitions of the Cr3þ ion. The emission spectra and photoluminescence lifetimes are comparable with reports on sensing and imaging applications. Our work demonstrates the suitability of this technique to quickly synthesise phosphors for a variety of applications. © 2017 Elsevier B.V. All rights reserved.
Keywords: A. Ceramics A. Nanostructured materials B. Nanofabrication C. Optical properties D. Luminescence
1. Introduction In view of their superior thermal and chemical stability, enhanced optical and electrical properties, mechanical strength, etc., nanocrystalline metal oxides have been widely explored in catalysis, displays, sensing, lasing, etc. [1e6]. Nanocrystalline ZnAl2O4, with a wide bandgap of 3.8 eV is known for its range of potential applications in photocatalysis, as phosphor hosts, for fabricating UV-optoelectronic devices, optomechanical stress sensors, and electroluminescent displays [7,8]. Several solid state and wet chemical recipes exist for the synthesis of the crystalline ZnAl2O4 spinel [9e11]. However, obtaining high-quality and phasepure spinels requires that most synthesis methods be followed by a high-temperature annealing process where temperatures can go up to 1200 C. Annealing, at relatively slow heating rates, is a widely employed post-processing technique for glass and ceramics, including spinels [11e15]. Annealing is usually achieved through heating elements made of silicon carbide, molydisilicide, or chromium-iron-
* Corresponding author. E-mail address: [email protected] (S.D. Kulkarni). http://dx.doi.org/10.1016/j.jallcom.2017.09.026 0925-8388/© 2017 Elsevier B.V. All rights reserved.
aluminium alloys, which bring about structural changes in the material through convective heating. It is already well established that annealing influences the material properties for the application of interest, be it as a catalyst, phosphor, or a magnetic device. Rapid annealing (RA) can instantly raise the temperatures to several hundred degrees within minutes; a similar process, rapid thermal annealing (RTA) with heating rates ranging from 5 C/sec to 25 C/sec [16e19], is used extensively in the processing of integrated circuits [16,19,20]. RTA has been used for the fabrication of silicide layers [21], dopant activation of thin films, and to “heal” damage due to ion implantation [20]. RTA is particularly useful in thin film fabrication because it minimises interaction between the film and substrate. While RTA is an industrially well-established thin film processing technique, here we report on employing rapid annealing (RA), with the ramp up rates of ~150e200 C/min, in the synthesis of ceramic powders. Even though the ramp up rates are lower than in RTA, the use of RA in the synthesis of ceramic powders such as Cr3þ doped ZnAl2O4 (Cr:ZAO), has not been explored previously. In fact, there are only a few reports on the RA of metal oxide powders [22]. Since RA can significantly reduce the synthesis/processing time of oxides, it may potentially be of technological importance. In view of the lack of literature on the use of RA for the synthesis of spinel/
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ZnAl2O4 nanoparticles, it is important to investigate the effects of RA on the structural and optical properties of such materials. Here, we explore the use of RA for the synthesis of Cr:ZAO, starting from the co-precipitate of the respective hydroxides. Cr:ZAO is a wellknown red/NIR emitting phosphor [13] and we have investigated its emission properties in detail for different RA parameters. We show that 10 min of RA is sufficient for the synthesis of Cr:ZAO nanoparticles, making RA potentially useful for the rapid synthesis of nanoparticles of other metal oxides. 2. Experimental 2.1. Synthesis of Cr:ZAO The aim was to prepare ZnAl1.95Cr0.05O4 nanoparticles starting from zinc acetate dihydrate (Zn(CH3COO)2.2H2O) (Merck), aluminium nitrate nonahydrate (Al(NO3)3.9H2O) (Merck), and chromium chloride hexahydrate (CrCl3.6H2O) (Merck). Aqueous solutions of the corresponding salts taken in 1:1.95:0.05 mol ratio (Zn: Al: Cr) were mixed and precipitated using a 2 M solution of NaOH at a pH of 9.5, under constant stirring for 30 min. The precipitate was vacuum-filtered, washed and dried in an oven at 60 C for 2 h, before subjecting it to RA at temperatures between 500 and 900 C for 10 min, employing a ramp rate of ~200 C/min. The corresponding heating profile (time vs. temperature) of the furnace for different temperature set-points are shown ESI, Fig. S1. The details of RA system are reported elsewhere [22]. The precipitate was evenly spread over a quartz boat that ensured uniform heating. The samples synthesized at temperatures from 500 Ce900 C were labelled A e E, respectively. 2.2. Characterisation X-Ray diffraction (XRD) patterns of the powder samples were collected using a Rigaku Ultima-IV diffractometer (Cu-Ka radiation).
485
Fourier transform infrared (FT-IR) spectra were recorded using Jasco FT/IR 6200 spectrometer. Bandgaps were deduced from diffuse reflectance spectra of the samples obtained from Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer with a 150 mm integrating sphere attachment. Diffuse reflectance measurements were made between 800 and 200 nm using BaSO4 as the reflectance standard. Crystallinity and morphology of the powders were examined by high-resolution transmission electron microscopy (HR-TEM, JEOL 3010). Photoluminescence spectra were obtained at room temperature for the various samples (Jasco FP8300 fluorescence spectrometer equipped with a 450 W Xenon lamp and a high-speed chopper). The luminescence decay curves were processed by single-exponential fit to derive the lifetime in each sample. 3. Results and discussion 3.1. X-ray diffraction The effect of RA on the formation of Cr:ZAO nanoparticles was investigated at different temperatures between 500 and 900 C for 10 min, reached at a ramp rate of ~200 C/min. The crystallinity and phase purity of the samples were determined by X-ray diffraction. The XRD patterns of the samples (Fig. 1a) show peaks corresponding only to the spinel phase. The broad peaks observed for sample A are due to the very small size of the crystallites (2 nm). As expected, crystallinity improves as the synthesis temperature is raised, causing the narrowing of the peaks. The change in crystallite size, bandgap, and lattice parameters are listed in Table 1. The lattice parameter approaches the reported values as the temperature is raised (Table 1) [23,24]. The increase in the average crystallite size with temperature follows an exponential trend (Fig. 1b), suggesting that the significant crystal growth occurs between 700 C and 900 C. It is worth noting that mere 10 min of RA led to well crystallized Cr:ZAO nanoparticles starting from
Fig. 1. (a) XRD patterns of Cr-doped ZnAl2O4 rapid-annealed at different temperatures and (b) plot showing the variation in crystallite size with temperature.
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Table 1 Changes in material properties with different synthesis temperatures. Sample Synthesis Temperature ( C)
Bandgap Crystallite (eV) Size (nm)
Lattice Lifetime Parameter (Å) (ms)
A B C D E
3.03 3.03 3.05 3.05 3.06
8.1555 8.1565 8.1442 8.1320 8.1053
500 600 700 800 900
2.4 3.41 4 6.1 10.5
12.81 12.13 12.61 23.19 25.43
hydroxides. This is remarkable considering that Cr:ZAO is a ceramic material and the other reported methods use much longer synthesis and/or processing times [25]. Synthesis of ZnAl2O4 nanoparticles by co-precipitation is known to produce ZnO as an impurity phase, due to the amphoteric nature of ZnO [23]. Further, Al3þ salts are known to form amphoteric Al(OH)3. Al3þ and Cr3þ salts are known to precipitate together25 at a pH of 9, whereas Zn2þ precipitates above a pH of 8.5 [23]. Simultaneous precipitation of all component ions is a prerequisite for coprecipitation synthesis and hence a pH of 9.5 was selected. The XRD patterns revealed no additional impurity peaks, attesting the formation of only the spinel phase. 3.2. Transmission electron microscopy (TEM) High-resolution TEM images were used to assess the fine-scale morphology of sample E and to obtain the particle size distribution (Fig. 2). Fig. 2a and b shows HRTEM images displaying the lattice fringes for the (511), (220) and (400) planes, indicating that each crystallite is a single crystal. The selective area electron diffraction (SAED) pattern shows slightly diffuse rings that are characteristic of the crystallites that are quite small even after annealing at 900 C for 10 min (Fig. 2c). However, the crystallites appear polyhedral with characteristics of the cubic symmetry of the spinel structure. The particle size histogram (Fig. 2d) is seen to follow a Gaussian profile with an average size of 16 nm.
signatures of Al3þ in tetrahedral coordination evidencing the presence of partial inversion which is expected in nanometric spinel crystals [22,34]. 3.4. Diffuse reflectance spectroscopy The diffuse reflectance spectra of the co-precipitate before RA and the samples A, C, and E (Fig. 4a) show a broad band at 370 nm and a shoulder at 540 nm, which are the characteristic 4A2g(F) / 4 T1g(F) and 4A2g(F) / 4T2g(F) absorption transitions of the Crþ3 ions, due to the spin-allowed d-d transitions in the host lattice [13]. The optical (direct) bandgap of samples A and E determined24, 35 from the Tauc plot of the Kubelka-Munk function are shown in Fig. 4b. It is clear that the bandgap does not change significantly with temperature; it is 3.03 eV and 3.06 eV for samples A and E respectively. These values are in agreement with previous reports [13]. 3.5. Photoluminescence spectroscopy The room temperature excitation spectra (lem ¼ 690 nm) of the samples comprise two broad peaks, at 420 nm (n1 e 23810 cm1) and 540 nm (n2 e 18519 cm1), with a weak shoulder at 405 nm, the intensity of which increases with the synthesis temperature (Fig. 5). The ground state, 4F, and the excited states, 4P and 2G, of the d3 Cr3þ ion, split into several doublet and quartet states, such as 4A2g, 4T2g, 4 T1g, 2Eg, 2T1g, etc., in an octahedral field [35]. The ground state, t23 , of Cr3þ complex in an octahedral field gives rise to 4A2, 2E, 2T1, and 2 T2 states, and the lowest energy state among these, the 4A2 state, forms the ground state of the ion [36]. The first excited state, t22 e1 state gives rise to the 4T2 and 4T1 states, which result in the n1 and n2 spin-allowed transitions, 4A2g / 4T1g and 4A2g / 4T2g [37e39]. The weak shoulder on the 420 nm band is due to trigonal distortions that further split the excited states of the host ZnAl2O4 lattice [40,41]. The additional splitting of triply degenerate quartet terms (in the cubic approximation) result in asymmetric excitation bands or sometimes even double peaks [42]. The 4T1 state splits
3.3. Fourier transform infrared spectroscopy For ZnAl2O4, low-frequency bands observed in the range of 1000e400 cm1 can be attributed to the vibration modes of AleO bonds [26]. For Al3þ in an octahedral symmetry (AlO6), AleO stretching and bending modes are observed in the regions between 500 and 700 cm1 and 330-450 cm1, respectively [27,28]. However, in the case of crystallographic inversion in the spinel lattice, Al3þ becomes tetrahedrally co-ordinated (AlO4), and corresponding AleO stretching modes appear between 700 and 850 cm1 whereas the bending modes would be between 250 and 320 cm1 [27,28]. The FT-IR spectra (Fig. 3a) of the as-prepared precipitate and the samples AeD showed broad and unresolved bands between 400 and 1000 cm1. For samples C and D, the bands just begin to resolve themselves, whereas well-resolved bands are obtained for sample E. The broad band for sample A, between 400 and 1000 cm1 can be attributed to the AleOH vibrations, indicating the possible presence of Al(OH)3 even after annealing at 500 C [29]. For sample E, the bands at 672 and 565 cm1 are due to the symmetric stretching, while the weak band at 503 cm1 is due to the asymmetric stretching mode of octahedral Al3þ. The bands at 449 and 492 cm1 (Fig. 3b) can be attributed to the stretching modes of the ZneO [30,31]. The low-frequency bands obtained between 400 and 430 cm1 for sample E have been attributed to the vibrations of tetrahedral Al3þ which are blue-shifted because of their small crystallite size [27,32,33]. The weak bands at 711 and 726 cm1 in sample E are
Fig. 2. (a) TEM and (b) HR-TEM images of RA910 sample and its (c) SAED pattern along with (d) particle size distribution histogram.
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487
Fig. 3. FT-IR spectrum of (a) Cr:ZAO nanoparticles synthesized by RA and (b) FT-IR of sample E (Full range).
into the 4T1(t22 e1 ) and 4T1(t21 e2 ) states resulting in 4A2 / 4T1(t22 e1 ) and 4A2 / 4T1(t21 e2 ) transitions, of which the number of latter transitions are much lower, as signified by the weak shoulder at about 400 nm. However, the effects of trigonal distortion are not significant vis-a-vis reported work [13]. For samples A and B, the 4A2g / 4T2g transition band is observed at 549 nm and 550 nm, respectively, which is shifted by about Dl ¼ 10 nm relative to sample E, which has a lmax ¼ 540 nm. This shift could be a result of the changes in the lattice constant of the two samples. Essentially, the Zn2þ - O2 - Al3þ - O2 separations control the peak positions of the 4A2g / 4T2g transitions. The lattice constants of samples A and B are larger than the others, suggesting that the higher ionic separation results in the observed spectral shift towards higher wavelengths [43,44]. The emission bands (Fig. 6) observed correspond to the general perturbed or unperturbed 2E / 4A2 Cr3þ transitions. These perturbations are caused by crystal defects and can manifest in the form of separate bands [45]. The emission spectra comprise a zero phonon line (ZPL), or the R e line at about 688 nm for all samples, and multi-phonon side bands on either side of the ZPL for samples D and E [13,46,47]. This ZPL is the resultant of unresolved R1 and R2 bands, formed due to the splitting of the 2E state which, in turn, is a consequence of spin-orbit coupling and trigonal distortions. For a ZnAl2O4 host, these two bands are separated by about 7e10 cm1 [46] and have only been observed [48,49] at low temperatures.
Fig. 5. Excitation spectra of samples A e E at an emission wavelength of 688 nm, and (inset) Temperature vs intensity plot of this excitation spectrum.
The band at 699 nm for samples D and E is known as the N2-line and is ascribed to the short-range perturbations of the octahedral sites surrounding Cr3þ. This is a structure-dependent line and has energy lower than the R1-R2 doublet [42]. The intensity of the N2 line is lower than that of the R-line for these samples, probably
Fig. 4. (a) The diffuse reflectance spectra of the co-precipitate sample along with samples A, C, and E, and (b) the Tauc plots of sample A and sample E.
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Fig. 6. Emission spectrum of samples A e E with the temperature vs intensity plot of the R-line in the inset.
because of fewer anti-site defects at higher annealing temperatures. However, at lower annealing temperatures (samples A - C), these N lines are diffuse, unresolved, and are much lower in intensity, perhaps because of fewer Cr3þ ions being at the octahedral site of the ZnAl2O4 lattice. The intensity of the R-line increases with temperature (Fig. 6 inset), as in our previous results [13]. Spectroscopic parameters and properties of the Cr3þ ion are a function of its local environment. d3 systems have fivefold degeneracy and split into t2g and e2g energy levels in a crystal field of a specified strength. The strength of the crystal field splitting parameter, 10.15Dq, is given by n1. From the excitation spectra of the samples, the corresponding Tanabe-Sugano diagrams can be plotted, which helps in determining the different spectroscopic parameters. The excitation spectrum also helps in calculating the electronic repulsion parameter, the Racah parameter (B), from the equation [50e52]:
15ðn 8Þ n2 10n
Dq ¼ B
(1)
Fig. 7. PL decay curves of samples A e E measured using a 550 nm wavelength. Table 2 Lifetime comparisons of Cr: ZAO with different synthesis techniques. Synthesis Method
Measurement Temperature
Lifetime (ms)
References
Electro-spinning Co-precipitation Flux growth Sol-gel Co-Precipitation and Rapid Annealed
Room temperature Room temperature 77 K 10 K Room temperature
23.47 31.19 29.0 32.7 25.43
[45] [13] [49] [55] Current study
688 nm emission line for samples D and E (Fig. 7). The excitation wavelengths employed were 550 nm for samples A e C, and 540 nm for samples D and E. The lifetimes of samples A e C did not change significantly and ranged between 12.1 and 12.8 ms (Table 1). However, the lifetimes for samples D and E is significantly longer: 23.19 ms for sample D and 25.43 ms for sample E, showing that the synthesis temperature influences emission lifetimes directly. These values, even though slightly lower, are quite comparable with the lifetimes measured in samples synthesized by other methods (Table 2) [13], affirming the novelty and effectiveness of the RA technique.
where
g¼
E A2g / T1g E A2g / T2g Dq
4. Conclusions
(2)
These equations can be solved to give the Racah parameter:
B¼
2n21 þ n22 3n1 n2 15n2 27n1
(3)
The free ion electron repulsion parameter (Bfree) for the Cr3þ ion is 918 cm1 [43]. In the present study, the Racah parameter, B, is found to be 490 cm1, which is much smaller than Bfree. This discrepancy may be attributed to the nephelauxetic effect, wherein metal electrons are delocalized over molecular orbitals comprised of both ligands and metal ions. It is speculated that this effect is stronger because of the rapid annealing process. The Dq/B value at the region where the 2E and 4T2 curves intersect on the TanabeSugano diagrams gives insights into field strengths with values greater than 2.3, implying high crystal field strengths. In this work, the Dq/B value is found to be 3.78, confirming that that Cr3þ ions are in strong crystal fields and that the photoluminescence emissions are associated with their corresponding spin-parityforbidden transitions [53,54]. The emission lifetimes were measured by recording the decay profiles of the 690 nm emission line for samples A e C and of the
High-quality nanocrystalline Cr:ZnAl2O4 can be prepared within 10 min by rapid annealing of the coprecipitate. Crystallites were very small at low temperatures, with significant crystal growth taking place beyond 800 C, as confirmed by XRD. HRTEM of the sample synthesized at 900 C shows that the nanocrystals are mostly polyhedral, with an average size of 16 nm. Room temperature photoluminescence excitation and emission spectra of the samples are comparable to those of samples obtained by other methods, with the zero-phonon line and the defect-induced N2 line appearing at 688 nm and 699 nm, respectively. The measured high Dq/B value of 3.78 suggests that the dopant Cr3þ ions are in a strong crystal field, causing emissions related to spin-parity-forbidden transitions. The lifetime for the 900 C sample was measured as 25.43 ms, long enough for bioimaging, lasing, and sensing applications. The rapid annealing technique, with modest ramp rates, is found to be effective in yielding powder samples in minutes, with properties comparable to those prepared using methods that are much longer. Acknowledgments We gratefully acknowledge the financial support from SERB, DST
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