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Synthesis and characterization of Ce-doped HfO2 nanoparticles in molten chlorides
Journal of Alloys and Compounds 692 (2017) 448e453
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Synthesis and characterization of Ce-doped HfO2 nanoparticles in molten chlorides E. Mendoza-Mendoza*, J.S. Quintero-García, B.A. Puente-Urbina, ndez, L.A. García-Cerda** O.S. Rodríguez-Ferna n en Química Aplicada, Departamento de Materiales Avanzados, Blvd. Enrique Reyna Hermosillo 140, 25294 Saltillo, Coahuila, Mexico Centro de Investigacio
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2016 Received in revised form 25 August 2016 Accepted 11 September 2016 Available online 12 September 2016
Ce-doped HfO2 nanoparticles were successfully prepared in molten chlorides. Nanoparticles were obtained at 380 C during 1 h in a LiCl/KCl molten flux. The Ce-doped HfO2 nanoparticles showed sizes below 7 nm and spherical shapes. Complete stabilization of cubic phase was achieved by adding 10 mol % of Ce. XRD, Raman and HRTEM techniques confirmed that the Ce-doped HfO2 nanoparticles have a cubic structure. Furthermore, this simple, efficient and high yield (>95%) method represents an alternative route for the synthesis of fluorite-type cubic phases and should be useful for doping or co-doping HfO2 with other cations. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hafnia Doping Nanoparticles Green synthesis Molten chlorides
1. Introduction There is an increasing scientific and technological interest worldwide for producing hafnia (HfO2) nanoparticles and films, due to its interesting physicochemical properties such as a relatively high dielectric constant (22e25) with respect to silica [1], large bandgap (5.3e5.9 eV) [2], transparency in the visible range [3], high thermal and chemical stability [4], large heat of formation (271 kcal/mol) [5], high neutron absorption coefficient [6], etc. These properties are suitable for using in many fields like new generation high-k gate dielectrics in microelectronic devises [7e9], scintillators [10], X-ray phosphors [11], luminescent activators [12], high temperature refractory materials and thermal barrier coatings [13,14], oxygen sensors [15] and electrolytes for solid oxide fuel cells (SOFC) [16]. Pristine HfO2 has tree crystalline phases at ambient pressure. The monoclinic phase with a space group P21/c is the stable
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (E. Mendoza-Mendoza), luis. [email protected] (L.A. García-Cerda). http://dx.doi.org/10.1016/j.jallcom.2016.09.122 0925-8388/© 2016 Elsevier B.V. All rights reserved.
crystalline structure at room temperature, which transforms to tetragonal structure with a space group P42/nmc at ~1720 C and finally becomes cubic structure with space group Fm3m at ~2600 C [17]. The volume expansion caused by the tetragonal to monoclinic transformation induces largest stresses, which normally cause cracking of pure HfO2 upon cooling from high temperature [18]. Several technological applications depend on the stabilization of the high-temperature phases of HfO2 powders or films at room temperature. For example, HfO2 is one of the most promising candidates for high k materials applications, showing a higher permittivity in the cubic (k ¼ 29) or in the tetragonal (k ¼ 70) structures than in the monoclinic phase [19]. HfO2 is an interesting material for using as luminescent host lattice doped with rare earths (RE) ions. RE ions are used as optical activators since they show narrow bands of emission and absorption, due to intraconfigurational transitions 4f / 4f, with emissions peaked at well-defined wavelengths. Some RE ions as Tb3þ, Eu3þ, Ce3þ, Er3þ, etc., have been used as dopant in HfO2 for obtaining luminescent materials [11,20,21]. E. Zych and colleagues have been deeply investigated the optical properties of diverse host lattice doped with RE ions, including HfO2, for using as phosphor and scintillator materials [22,23]. In addition, cubic phase of hafnia has a very low thermal conductivity which has led to its use as a thermal barrier
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coating in jet and diesel engines [14]. The stabilization of cubic HfO2 has been previously reported by several groups via element doping with aliovalent cations, such as Y, Mn, Al, Eu, Tb, Ce, etc. [14,20,24e27]. The nature of HfeO bond (predominantly covalent) favors the sevenfold coordination of Hf4þ cation in monoclinic phase. When Ce3þ is introduced as dopant, the stabilization of cubic phase is achieved due to the changes in the coordination number from seven to eightfold and is accompanied by the formation of vacancies in the oxygen sublattice for satisfying electroneutrality requirements [28]. Pure and doped HfO2 has been prepared by several routes in last years, mainly solid and liquid state reaction systems, such as the conventional solid state reaction [24], sol-gel [26,29], pechini [28], metathesis [30], solvothermal [31], sonochemical [32], etc. These methods have been successful in stabilizing and reducing the synthesis temperature and particle sizes of cubic HfO2. The solid route requires high temperature and long reaction time for increasing the diffusion of species and the obtained particles presents a broad size distribution and in some cases the product is not homogenous. The solution routes are relatively large scale and high yield in terms of the quantity of desired products generated without substantial amounts of harmful by-products, they can frequently use water as the solvent medium and run at reasonably low temperatures [33]. However, in some of these methods, the reactions take place in closed isolated system conditions, require sophisticated equipment, long reaction times and high consumption of electric power. Nowadays, great emphasis is placed in the implementation of simple, cost-effective, scale-up efficient and cleaner methodologies; particularly those which reduce or even avoid the generation of hazardous substances and minimizing waste production and energy consumption, called “green chemistry” [33,34]. Herein it is reported the synthesis of Ce-doped HfO2 nanoparticles with fluorite-type cubic structure via a relatively environmental-friendly methodology, using molten chlorides as inorganic media. The method combines a mechanically assisted metathesis reaction and molten salts. The purpose of the first is twofold; on the one hand to generate in-situ alkaline metal chlorides, and on the other hand, to obtain a Ce and Hf-containing precursor suitable for obtaining of Ce-doped HfO2 pure phases. In previous work, we reported the successful preparation of cubic HfO2 without using a stabilizer cation at 380 C and 3 h of reaction time [35]. The present contribution shows the complete stabilization of cubic HfO2 at 380 C using low concentration of Ce (10% mol), short reaction time (1 h) and a high yield (>95%). Furthermore this method represents an alternative for the synthesis of powders with a fluorite-type cubic structure and should be useful for doping or co-doping HfO2 with other cations.
2. Experimental procedure The starting materials used were: HfCl4 (Aldrich, 98%), CeCl3$7H20 (Aldrich, 99.9%), LiOH$H2O (Aldrich, 98%) and KOH (Aldrich, pellets, 99.99%). A typical experiment was conducted according to the following procedure shown schematically in Fig. 1: appropriate amounts of HfCl4 and CeCl3 was mixed with a mixture of LiOH/KOH (0.592:0.408 M ratio), then the reaction mixture was transferred to a ZrO2 jar together with 20 mm diameter ZrO2 balls as grinding media and milled in air for 1 h in a planetary ball mill (Pulverisette-6) using a rotating disk speed of 350 rpm and a ballsto-powder mass ratio of 10:1. The amount of reactants was saltbalanced such that there were no hydroxides or chlorides excess (total metal chlorides/lithium-potassium hydroxide molar ratio z 1/4) according to Equation (1):
449
Fig. 1. Flowchart of the experimental procedure followed in this work.
yCeCl3 $7H2 O þ ð1 yÞHfCl4 þ f½3y þ ½ð1 yÞ4 gAOH/Cey Hfð1yÞ O2d þ f½3y þ ½ð1 yÞ4gACl þ xH2 O (1) where A ¼ Li and K and y ¼ 0.086 and 0.10 mol of CeCl3. The resulting activated precursor material was dried 1 h at 100 C to reduce moisture and to minimize violent gas evolution on melting and finally loaded into alumina crucibles, fired in air at 380 C during different times (1, 3, 6 and 12 h) using a furnace (heating rate 5 C/min) and cooled thereafter to room temperature. As-obtained solidified melts were washed in deionized water to separate the alkali metal chlorides from the water insoluble fraction, which was then collected by centrifugation, dried 1 h at 100 C and characterized by several techniques. Phase identification was carried out by X-ray powder diffraction in a Siemens D5000 diffractometer using CuKa radiation (l ¼ 1.5418 Å) operated at 35 kV and 25 mA at a scan rate of 0.02 (2q/s). Thermal studies of precursor materials were realized by DSC analysis in a Setaram Labsys Evo 1600 thermoanalyser; approximately 10 mg of the samples were heated from room temperature to 600 C at a rate of 5 C/min. Raman spectra were measured with Xplora One Spectrometer, using a 785 nm line of the Arþ laser as the excitation source. The size and morphology of the synthesized particles were observed directly using a high-resolution transmission electron microscope (HRTEM TITAN 80-300). The crystallite size was estimated by using the Scherer's; D ¼ Kl/bcosq, where D is the mean crystallite size along the (h k l) direction, l is the X-ray wavelength (lcu ¼ 1.544056 Å), b is the peak full width at half minimum intensity (FWHM), q is the angle of diffraction, and the Scherer's constant K is conventionally set to 1. 3. Results and discussion Fig. 2(a) shows the X-ray diffraction pattern obtained from mechanical milling the starting stoichiometric mixture of HfCl4, CeCl3 and LiOH/KOH. After milling, the crystalline phases identified by XRD were tetragonal lithium and cubic potassium chlorides. The main reflection of the first phase has been labeled with the solid sphere located around 33 (2q) for (202) Miller index (PDF 731273), whereas the second phase was correlated with XRD pattern
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Fig. 2. X-ray diffraction pattern (a) and DSC/TGA curve (b) obtained after milling the reaction mixture of HfCl4, CeCl3 and LiOH/KOH.
found in the JCP-ICDD for KCl (PDF-75-0296) and included at the bottom for easily comparison. A metathesis or double displacement reaction took place on milling as was proved by the detection of crystalline chlorides. This chemical reaction involves the exchange of atomic/ionic species between reactants; in this case, Hf and Ce cations have been displaced by Li and K ions in order to obtain crystalline LiCl and KCl. These crystalline obtained products have high-lattice energy and similar atomic bonds. As is shown in the figure it was difficult to detect the resulting Ce and Hf-containing species by XRD because of lacking of long-range atomic ordering, and thus being amorphous as denoted by a broad background in the region of 25e38 (2q). Since both HfCl4 and CeCl3 show their most intense reflections far from the maximum of the amorphous area, ∽31 (2q), [i.e. 14.81 (PDF 85-0171) and 13.73 (PDF 77-0154) respectively], it follows from the XRD pattern that neither nonreacted crystalline hafnium nor cerium chlorides are presented. Reported XRD pattern of HfCl4 is also shown at the bottom in the figure as reference. Interestingly, the center of this maximum lies very close to the position of the main reflection peak of HfO2 [30.36 (2q) for (111) Miller index (PDF 53-0560)]. Therefore, it might be assumed that the metathesis chemical reaction was complete, and also, is possible the existence of an amorphous phase of HfO2 in the milled precursor. Further evidences of the existence of a metathesis reaction and the subsequent LiCl and KCl formation are inferred from the thermal analysis curve of milled precursor, Fig. 2(b). In DSC curve, the presence of the intense endothermic event around 354 C unrelated to weight loss is associated with the melting point of LiCl/KCl eutectic mixture [36], supporting the idea that milling promotes metathesis reaction. An additional endothermic event taking place around 85 C is due to a dehydration process in the sample. Notice there is no evidence of residuary Li or K hydroxides on the thermal curve; they must be represented by an endothermic pick around ~406 and 462 C for KOH and LiOH respectively; confirming the only existence of the eutectic mixture in molten media. LiCl/KCl inorganic solvent is chemically inert and does not suffer from drastic evaporation loss up to 1000 C. This thermal stability was confirmed by analyzing the TGA behavior of the precursor. Herein, the total mass loss in this milled precursor from room temperature to 600 C was around 11%. Some issues can
be observed on this curve: the increasing of mass at the beginning of thermal program as a result of the hygroscopic behavior of chlorides ̴(2%), the accentuated loss weight related to dehydration process, which is also related to the first endothermic event around 85 C, and the inexistence of loss mass at the melting point temperature. A metathesis or displacement reaction has become in last years an alternative route for the rapid synthesis of important solid-state compounds. Matovi c et al. [30], reported the preparation of pristine HfO2 by metathesis from the reaction between HfCl4 with NaOH. In this study, they detected the presence of two broad peaks in the Xray diffraction pattern of the sample, revealing that the as-prepared precursor was amorphous, which indicated that the proposed metathesis reaction does not lead to the formation of NaNO3 and NaCl crystalline phases at room temperature. In order to intensify crystallization on the samples, the powders were annealed from 600 to 1500 C, showing almost complete crystallization of monoclinic HfO2 at 1500 C. Similar results were reported during the preparation of Y-doped HfO2 (5, 10 y 15 mol % of Y). These results are opposite to this study. Herein, the LiCl and KCl crystalline species were generated at room temperature via mechanically assisted metathesis reaction and then detected by XRD. Fig. 3 shows the X-ray diffraction patterns of doped HfO2 nanoparticles, with 8.6 (a) and 10 mol % (b) of Ce. The samples were obtained at 380 C and fired during several times. For the sample with lowest Ce content, firing 12 h, a pair of crystalline phases was detected: cubic and monoclinic HfO2. The first one can be associated with the XRD characteristic pattern included at the bottom (cHfO2, PDF 53-0560), whereas the low-intensity set of reflections for second were labeled by empty squares around 24.23, 28.42, 31.65, 40.85 and 55.84 (2q), also related to (011), (111), (111), (211) and (130) crystalline planes (m-HfO2, PDF 74-1560). Reducing reaction time leads to detection of very weak-intensity reflections for monoclinic phase, even complicating its identification; mainly in 3 h and where (001) and (111) planes were the only observed. On the contrary, cubic phase showed four diffractions picks which were directly correlated to (111), (002), (022) and (113) Miller index, corresponding to the characteristic reflections of a fluoritetype cubic structure. Even if the cubic phase showed the highest
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451
Fig. 3. X-ray diffraction patterns of Ce-doped HfO2 nanoparticles with 8.6 (a) and 10 mol % of Ce (b), firing at 380 C for different times.
intensity ratio comparing to monoclinic phase, the powder was constituted by a mixture of phases. Increasing the content of Ce to 10 mol % leads to the complete stabilization of the cubic HfO2 as is shown in (b). The set of reflections detected in the XRD pattern and labeled with Miller Index on brackets correspond very well to a fluorite-type cubic structure, even further reducing reaction time to only 1 h. LiCl was detected in 3 h sample due to an inappropriate dissolution of reaction media during the washing operation. The main reflection of this undesirable phase has been labeled with a solid circle in the figure; LiCl was not identified in 1 h sample, which suggests obtaining pure cubic phase. The crystallite size was estimated from Scherrer's formula. Using this equation, the mean crystallite size of the obtained Cedoped HfO2 at 1 h, calculated from broadening of the (111), was 4.90 nm. It is well known that the stabilization of cubic hafnia is achieved so often at high-temperatures by element doping. The ideal ionic size ratio required for eight-coordination oxides (8CN) as in fluorite-type structures is 0.732. Evaluating the ionic radii for cation and anion in the appropriated coordination number for cubic 2 HfO2 [IR4þ Hf (VIII) ¼ 0.83 Å and IRO (IV) ¼ 1.38 Å], according to Shannon Database Ionic Radii [37], it results in a too lower ratio, 2 IR4þ Hf (VIII)/IRO (IV) ¼ 0.60, and thus the obtaining of monoclinic instead of cubic phase is totally expected. If the ionic ratio is increased by adding larger ionic size cations than Hf4þ, Ce 3þ [IRCe (VIII) ¼ 1.14 Å] for example, the stabilization of cubic phase is possible. This explains why slightly increasing Ce contents lead to the full stabilization of the cubic phase in this study. To this point, it has shown that the followed two-step methodology easily allows obtaining Ce-doped HfO2 with a fluorite-type structure. Nanoparticles were obtained at 380 C during 1 h in LiCl/ KCl. Furthermore, this method represents an alternative route for the synthesis of cubic HfO2 powders and might be useful for doping or co-doping with other elements. Prior to preparing Ce-doped samples this route was also used for synthesizing pure HfO2, which is considering the target material in this study with the purpose of easily compare the differences in the structural characteristics of cubic and monoclinic phases obtained by XRD and Raman analysis. Fig. 4(a) shows the diffraction pattern obtained for HfO2 powder fired at 450 C for 12 h. The crystalline
phase detected was the room-temperature stable monoclinic polymorph, which was in agreement with XRD pattern found in JCP-ICDD (PDF-75-0296) for the structure P21/c. The crystallite size calculated using Scherer's formula from the broadening of the (111) line was about 14.65 nm. Raman technique is sensitive to the polarization of the oxygen ions in the crystalline structure, and therefore, it was used in this study to corroborate the symmetry of HfO2 particles. Fig. 4(b) shows Raman spectra of pure and Ce-doped HfO2. The measurements were realized for the samples previously represented in Figs. 4(a) and 3(b). The bands in pure hafnia, labeled as m-HfO2 in the figure, were recorded around 109, 131, 145, 160, 239, 351, 324, 332, 379, 395, 495, 516, 547, 574, 635 and 667 cm1. These 16 bands, in the region between 100 and 700 cm1, were correlated with the characteristic vibrational modes of monoclinic HfO2 [38]. The case of Ce-doped HfO2 sample, labeled as c-HfO2, an exclusive vibrational mode was observed about 454 cm1, which is the F2g Raman active mode representative of the fluorite-type structure [39]. Although there are several publications of the Raman spectra for monoclinic HfO2, these reports are not in good agreement with the bands locations. The observed shifts in these reports are related so often to polarization phenomena in single HfO2 crystals [40,41]. Nevertheless, all of them are in agreement with the fact that 18, 6 and 1 are the Raman vibrational modes of the monoclinic, tetragonal and cubic structures correspondingly [42]. These results confirmed the existence of pure monoclinic and cubic HfO2 phases in samples as was previously determined by X-ray diffraction technique. HRTEM micrographs of 10% mol Ce sample prepared at 380 C for 3 h show that the powders are agglomerates composed of spherical particles, Fig. 5(a). The distribution of particle sizes varies in a narrow range from 4.8 to 6.2 nm as can be observed in some measured particles, (b). These measuring values were in agreement with 4.9 nm average crystallite size obtained from Scherrer's formula. Electron diffraction pattern recorded on the sample was appropriated indexed to Fm3m structure, (c). The obtained highresolution images showed clear lattice fringes, (d). This observation indicates the highly crystalline nature of the synthesized Cedoped HfO2 nanoparticles. The lattice fringes spacing in this figure are about 2.9 and 2.5 Å. These values correspond to the
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Fig. 4. X-ray diffraction pattern of pure HfO2 particles prepared at 450 C for 12 h and after removing LiCl/KCl (a). Raman spectra of Ce-doped and pure HfO2 powders (b).
Fig. 5. HRTEM (a, b and c) and SAED (b) images of Ce-doped HfO2 nanoparticles obtained at 380 C during 3 h.
separation between the (111) and (200) planes of the cubic structure of HfO2. EDS microanalysis confirmed the presence of Ce in samples, and also verified the inexistence of Zr contamination during milling (see Supplementary information). One of the main concerns when using mechanical milling as one of the synthesis steps for obtaining materials is the probable contamination of the samples because of wearing and debris of the milling tools; however the experimental settings used in this work
(e.g., short milling time, ZrO2 containers and balls, moderated rotating disk speed and hydrated metal chlorides and hydroxides as starting chemicals) were selected to minimize this problem. The followed methodology for preparing Ce-doped HfO2 nanoparticles has some outstanding features. First, it is simple, rapid, includes few reagents and does not involve the use of expensive chemicals (e.g., alkoxides) and thus, the entire process is costeffective. Secondly, the synthesis was carried out in a short
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reaction time (1 h), thus a saving energy is achieved, and does not need any sophisticated or costly equipment nor special handlings precautions because is realized under normal atmosphere; although there is still some room for improvement, the yield is very high (>95%) and the process can be easily adapted for technological applications. Third, the process is two step and not toxic, because not involve the use of organic solvents and neither generates volatile or toxic by-products. 4. Conclusions Ce-doped HfO2 nanoparticles with cubic structure were successfully synthesized at low-temperature and short reaction time (380 C/1 h), using a eutectic mixture of LiCl/KCl as molten flux. The complete stabilization of cubic phase was achieved by adding 10 mol % of Ce. XRD, Raman and HRTEM techniques showed that crystallites in powder sample have Fm3m structure. Ce-doped hafnia powders are forming agglomerates of spherical particles of about 5 nm size. The proposed metathesis reaction fulfilled two goals efficiently; obtaining an appropriated Ce and Hf-containing precursor and generating in-situ alkaline chlorides fluxes. Furthermore, the followed methodology represents an alternative to the solid and solution routes used for synthesizing cubic HfO2 nanoparticles and should be useful for doping or co-doping other fluorite-type structures. Acknowledgments Financial support for the work was provided by CONACYT through the project number 133991. The authors thank Laboratorio nicos, CONACYT grant 232753. E. Nacional de Materiales Grafe Mendoza-Mendoza thanks to CONACYT for the scholarship number 203766 for the postdoctoral position. Also thanks to R. Rangel for Raman measurements and E. Díaz for TEM images. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.09.122. References [1] R.K. Nahar, V. Singh, A. Sharma, Study of electrical and microstructure properties of high dielectric hafnium oxide thin film for MOS devices, J. Mater. Sci. Electron. 18 (2007) 615e619. [2] M.C. Cheynet, S. Pokrant, F.D. Tichelaar, J.L. Rouviere, Crystal structure and band gap determination of HfO2 thin films, J. Appl. Phys. 101 (2007) 054101. [3] J. Khoshman, A. Khan, M. Kordesch, Amorphous hafnium oxide thin films for antireflection optical coatings, Surf. Coat. Technol. 202 (2008) 2500e2502. [4] K.J. Hubbard, D.G. Schlom, Thermodynamic stability of binary oxides in contact with silicon, J. Mater. Res. 11 (1996) 2757e2761. [5] H. Ikeda, S. Goto, K. Honda, M. Sakashit, A. Sakai, S. Zaima, Y. Yasuda, Structural and electrical characteristics of HfO2 films fabricated by pulsed laser deposition, Jpn. J. Appl. Phys. 41 (2002) 2476. [6] J.A. Valdez, I.O. Usov, J. Won, M. Tang, R.M. Dickerson, G.D. Jarvinen, K.E. Sickafus, 10 MeV Au ion irradiation effects in an MgOeHfO2 ceramiceceramic (CERCER) composite, J. Nucl. Mater. 393 (2009) 126e133. [7] M. Filipescu, N. Scarisoreanu, V. Craciun, B. Mitu, A. Purice, A. Moldovan, V. Ion, O. Toma, M. Dinescu, High-k dielectric oxides obtained by PLD as solution for gates dielectric in MOS devices, Appl. Surf. Sci. 253 (2007) 8184e8191. [8] J. Robertson, High dielectric constant gate oxides for metal oxide Si transistors, Rep. Prog. Phys. 69 (2006) 327e396. [9] G.D. Wilk, R.M. Wallace, J.M. Anthony, High-kappa gate dielectrics: current status and materials properties considerations, J. Appl. Phys. 89 (2001) 5243e5275. [10] M. Kirm, J. Aarik, M. Jurgens, I. Sildos, Thin films of HfO2 and ZrO2 as potential scintillators, Nucl. Instrum. Meth. Phys. Res. Sect. A 537 (2005) 251e255. [11] C. LeLuyer, M. Villanueva-Ibanez, A. Pillonnet, C. Dujardin, HfO2:X (X ¼ Eu, Ce, Y) sol gel powders for ultradense scintillating materials, J. Phys. Chem. A 122
453
(2008) 10152e10155. [12] S. Lange, V. Kiisk, V. Reedo, M. Kirm, J. Aarik, I. Sildos, Luminescence of RE-ions in HfO2 thin films and some possible applications, Opt. Mater. 28 (2006) 1238e1242. [13] V.B. Glushkova, M.V. Kravchinskaya, HfO2-based refractory compounds and solid solutions, Ceram. Int. 11 (1985) 56e65. [14] M.R. Winter, D.R. Clarke, Thermal conductivity of yttria-stabilized zirconiaehafnia solid solutions, Acta Mater. 54 (2006) 5051e5059. [15] N. Izu, T. Itoh, W. Shin, I. Matsubara, N. Murayama, The effect of hafnia doping on the resistance of ceria for use in resistive oxygen sensors, Sens. Actuators B Chem. 123 (2007) 407e412. [16] M.F. Trubelja, V.S. Stubican, Ionic conductivity in the hafniaeR2O3 systems, Solid State Ion. 49 (1991) 89e97. [17] J. Wang, R. Stevens, Hafnia and hafnia-toughened ceramics, J. Mater. Sci. 27 (1992) 5397e5430. [18] X. Luo, W. Zhou, S.V. Ushakov, A. Navrotsky, A.A. Demkov, Monoclinic to tetragonal transformations in hafnia and zirconia: a combined calorimetric and density functional study, Phys. Rev. B 80 (2009) 134119. [19] X. Zhao, D. Vanderbilt, First-principles study of structural, vibrational, and lattice dielectric properties of hafnium oxide, Phys. Rev. B 65 (2002) 233106. n, T. Rivera-Montalvo, J. Guzm lito, [20] E. Montes, P. Cero an, M. García-Hipo n, R. García-Salcedo, C. Falcony, Thermoluminescent characA.B. Soto-Guzma 3þ terization of HfO2:Tb synthesized by hydrothermal route, Appl. Radiat. Isot. 83 (2014) 196e199. [21] J.Z. Wang, Z.Q. Shi, Y. Shi, L. Pu, L.J. Pan, R. Zhang, Y.D. Zheng, Z.S. Tao, F. Lu, Broad excitation of Er luminescence in Er-doped HfO2 films, Appl. Phys. A 94 (2009) 399e403. [22] E. Zych, Luminescence and scintillation of inorganic phosphor materials, in: H.S. Nalwa, L.S. Rohwer (Eds.), Handbook of Luminescence, Display Materials and Devices, American Scientific Publishers, CA, USA, 2003. ski, Monoclinic HfO2:Eu X-ray phosphor, [23] A. Wiatrowska, E. Zych, L. Kҿpin Radiat. Meas. 45 (2010) 493e496. [24] L. Gao, L. Zhou, J. Feng, L. Bai, C. Li, Z. Liu, J.L. Soubeyroux, Y. Lu, Manganese doped hafnia, Ceram. Int. 38 (2012) 2305e2311. [25] R. O'Connor, T. Kauerauf, H. Arimura, L.A. Ragnarsson, Stress induced defect generation implications of doping HfO2 with Al, Microelectron. Eng. 109 (2013) 54e56. [26] A. Lauria, I. Villa, M. Fasoli, M. Niederberger, A. Vedda, Multifunctional role of rare earth doping in optical materials: nonaqueous sol-gel synthesis of stabilized cubic HfO2 luminescent nanoparticles, ACS Nano 7 (2013) 7041e7052. [27] S. Chen, Z. Liu, L. Feng, X. Che, Influence of cerium-doping on the structural and electrical properties of hafnium oxide gate dielectric, J. Mater. Sci. Mater. Electron. 25 (2014) 749e753. lvez-Barboza, L.A. Gonz [28] S. Ga ales, B.A. Puente-Urbina, E.M. Saucedo-Salazar, L.A. García-Cerda, Preparation and characterization of Ce-doped HfO2 nanoparticles, J. Alloys Compd. 643 (2015) S62eS66. [29] P. Rauwel, E. Rauwel, C. Persson, M.F. Sunding, A. Galeckas, One step synthesis of pure and monoclinic HfO2 nanoparticles: correlating the structure to the electronic properties of the two polymorphs, J. Appl. Phys. 112 (2012) 104107. [30] B. Matovi c, D. Bǔcevac, M. Prekajski, V. Maksimovi c, D. Gautam, K. Yoshida, T.B. Yano, Synthesis and characterization of nanometric yttrium-doped hafnia solid solutions, J. Eur. Ceram. Soc. 32 (2012) 1971e1976. [31] Roo, K. Keukeleere, J. Feys, P. Lommens, Z. Hens, E. Driessche, Fast, microwave-assisted synthesis of monodisperse HfO2 nanoparticles, J. Nanopart. Res. 15 (2013) 1778. [32] A. Ramadoss, S.J. Kim, Synthesis and characterization of HfO2 nanoparticles by sonochemical approach, J. Alloys Compd. 544 (2012) 115e119. [33] Y. Mao, T.J. Park, F. Zhang, H. Zhou, S.S. Wong, Environmentally friendly methodologies of nanostructure synthesis, Small 3 (2007) 1122e1139. [34] X. Liu, N. Fechler, M. Antonietti, Salt melt synthesis of ceramics, semiconductors and carbon nanostructures, Chem. Soc. Rev. 42 (2013) 8237. [35] J.S. Quintero-García, B.A. Puente-Urbina, L.A. García-Cerda, O.S. Rodríguezndez, E. Mendoza-Mendoza, A “green methodology” approach to the Ferna synthesis of HfO2 nanoparticles, Mater. Lett. 159 (2015) 520e524. [36] A.S. Basin, A.B. Kaplun, A.B. Meshalkin, N.F. Uvarov, The LiCl-KCl binary system, Russ. J. Inorg. Chem. 53 (2008) 1509e1511. [37] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A 32 (1976) 751e767. [38] M. Modreanu, J. Sancho-Parramon, O. Durand, B. Servet, M. Stchakovsky, C. Eypert, C. Naudin, A. Knowles, F. Bridou, M.F. Ravet, Investigation of thermal annealing effects on microstructural and optical properties of HfO2 thin films, Appl. Surf. Sci. 253 (2006) 328e334. [39] V.G. Keramidast, W.B. White, Raman spectra of oxides with the fluorite structure, J. Chem. Phys. 59 (1973) 1561e1562. [40] B.K. Kim, H.O. Hamaguchi, Raman spectrum of 18o-labelled hafnia, Mater. Res. Bull. 32 (1997) 1367e1370. ris, A.P. Mirgorodsky, T. Merle-Me jean, Comparative [41] P.E. Quintard, P. Barbe lattice-dynamical study of the raman spectra of monoclinic and tetragonal phases of zirconia and hafnia, J. Am. Ceram. Soc. 85 (2002) 1745e1749. [42] X. Zhao, D. Vaderbilt, Phonon and lattice dielectric properties of zirconia, Phys. Rev. B 65 (2002) 233106.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis and characterization of Ce-doped HfO2 nanoparticles in molten chlorides E. Mendoza-Mendoza*, J.S. Quintero-García, B.A. Puente-Urbina, ndez, L.A. García-Cerda** O.S. Rodríguez-Ferna n en Química Aplicada, Departamento de Materiales Avanzados, Blvd. Enrique Reyna Hermosillo 140, 25294 Saltillo, Coahuila, Mexico Centro de Investigacio
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2016 Received in revised form 25 August 2016 Accepted 11 September 2016 Available online 12 September 2016
Ce-doped HfO2 nanoparticles were successfully prepared in molten chlorides. Nanoparticles were obtained at 380 C during 1 h in a LiCl/KCl molten flux. The Ce-doped HfO2 nanoparticles showed sizes below 7 nm and spherical shapes. Complete stabilization of cubic phase was achieved by adding 10 mol % of Ce. XRD, Raman and HRTEM techniques confirmed that the Ce-doped HfO2 nanoparticles have a cubic structure. Furthermore, this simple, efficient and high yield (>95%) method represents an alternative route for the synthesis of fluorite-type cubic phases and should be useful for doping or co-doping HfO2 with other cations. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hafnia Doping Nanoparticles Green synthesis Molten chlorides
1. Introduction There is an increasing scientific and technological interest worldwide for producing hafnia (HfO2) nanoparticles and films, due to its interesting physicochemical properties such as a relatively high dielectric constant (22e25) with respect to silica [1], large bandgap (5.3e5.9 eV) [2], transparency in the visible range [3], high thermal and chemical stability [4], large heat of formation (271 kcal/mol) [5], high neutron absorption coefficient [6], etc. These properties are suitable for using in many fields like new generation high-k gate dielectrics in microelectronic devises [7e9], scintillators [10], X-ray phosphors [11], luminescent activators [12], high temperature refractory materials and thermal barrier coatings [13,14], oxygen sensors [15] and electrolytes for solid oxide fuel cells (SOFC) [16]. Pristine HfO2 has tree crystalline phases at ambient pressure. The monoclinic phase with a space group P21/c is the stable
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (E. Mendoza-Mendoza), luis. [email protected] (L.A. García-Cerda). http://dx.doi.org/10.1016/j.jallcom.2016.09.122 0925-8388/© 2016 Elsevier B.V. All rights reserved.
crystalline structure at room temperature, which transforms to tetragonal structure with a space group P42/nmc at ~1720 C and finally becomes cubic structure with space group Fm3m at ~2600 C [17]. The volume expansion caused by the tetragonal to monoclinic transformation induces largest stresses, which normally cause cracking of pure HfO2 upon cooling from high temperature [18]. Several technological applications depend on the stabilization of the high-temperature phases of HfO2 powders or films at room temperature. For example, HfO2 is one of the most promising candidates for high k materials applications, showing a higher permittivity in the cubic (k ¼ 29) or in the tetragonal (k ¼ 70) structures than in the monoclinic phase [19]. HfO2 is an interesting material for using as luminescent host lattice doped with rare earths (RE) ions. RE ions are used as optical activators since they show narrow bands of emission and absorption, due to intraconfigurational transitions 4f / 4f, with emissions peaked at well-defined wavelengths. Some RE ions as Tb3þ, Eu3þ, Ce3þ, Er3þ, etc., have been used as dopant in HfO2 for obtaining luminescent materials [11,20,21]. E. Zych and colleagues have been deeply investigated the optical properties of diverse host lattice doped with RE ions, including HfO2, for using as phosphor and scintillator materials [22,23]. In addition, cubic phase of hafnia has a very low thermal conductivity which has led to its use as a thermal barrier
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coating in jet and diesel engines [14]. The stabilization of cubic HfO2 has been previously reported by several groups via element doping with aliovalent cations, such as Y, Mn, Al, Eu, Tb, Ce, etc. [14,20,24e27]. The nature of HfeO bond (predominantly covalent) favors the sevenfold coordination of Hf4þ cation in monoclinic phase. When Ce3þ is introduced as dopant, the stabilization of cubic phase is achieved due to the changes in the coordination number from seven to eightfold and is accompanied by the formation of vacancies in the oxygen sublattice for satisfying electroneutrality requirements [28]. Pure and doped HfO2 has been prepared by several routes in last years, mainly solid and liquid state reaction systems, such as the conventional solid state reaction [24], sol-gel [26,29], pechini [28], metathesis [30], solvothermal [31], sonochemical [32], etc. These methods have been successful in stabilizing and reducing the synthesis temperature and particle sizes of cubic HfO2. The solid route requires high temperature and long reaction time for increasing the diffusion of species and the obtained particles presents a broad size distribution and in some cases the product is not homogenous. The solution routes are relatively large scale and high yield in terms of the quantity of desired products generated without substantial amounts of harmful by-products, they can frequently use water as the solvent medium and run at reasonably low temperatures [33]. However, in some of these methods, the reactions take place in closed isolated system conditions, require sophisticated equipment, long reaction times and high consumption of electric power. Nowadays, great emphasis is placed in the implementation of simple, cost-effective, scale-up efficient and cleaner methodologies; particularly those which reduce or even avoid the generation of hazardous substances and minimizing waste production and energy consumption, called “green chemistry” [33,34]. Herein it is reported the synthesis of Ce-doped HfO2 nanoparticles with fluorite-type cubic structure via a relatively environmental-friendly methodology, using molten chlorides as inorganic media. The method combines a mechanically assisted metathesis reaction and molten salts. The purpose of the first is twofold; on the one hand to generate in-situ alkaline metal chlorides, and on the other hand, to obtain a Ce and Hf-containing precursor suitable for obtaining of Ce-doped HfO2 pure phases. In previous work, we reported the successful preparation of cubic HfO2 without using a stabilizer cation at 380 C and 3 h of reaction time [35]. The present contribution shows the complete stabilization of cubic HfO2 at 380 C using low concentration of Ce (10% mol), short reaction time (1 h) and a high yield (>95%). Furthermore this method represents an alternative for the synthesis of powders with a fluorite-type cubic structure and should be useful for doping or co-doping HfO2 with other cations.
2. Experimental procedure The starting materials used were: HfCl4 (Aldrich, 98%), CeCl3$7H20 (Aldrich, 99.9%), LiOH$H2O (Aldrich, 98%) and KOH (Aldrich, pellets, 99.99%). A typical experiment was conducted according to the following procedure shown schematically in Fig. 1: appropriate amounts of HfCl4 and CeCl3 was mixed with a mixture of LiOH/KOH (0.592:0.408 M ratio), then the reaction mixture was transferred to a ZrO2 jar together with 20 mm diameter ZrO2 balls as grinding media and milled in air for 1 h in a planetary ball mill (Pulverisette-6) using a rotating disk speed of 350 rpm and a ballsto-powder mass ratio of 10:1. The amount of reactants was saltbalanced such that there were no hydroxides or chlorides excess (total metal chlorides/lithium-potassium hydroxide molar ratio z 1/4) according to Equation (1):
449
Fig. 1. Flowchart of the experimental procedure followed in this work.
yCeCl3 $7H2 O þ ð1 yÞHfCl4 þ f½3y þ ½ð1 yÞ4 gAOH/Cey Hfð1yÞ O2d þ f½3y þ ½ð1 yÞ4gACl þ xH2 O (1) where A ¼ Li and K and y ¼ 0.086 and 0.10 mol of CeCl3. The resulting activated precursor material was dried 1 h at 100 C to reduce moisture and to minimize violent gas evolution on melting and finally loaded into alumina crucibles, fired in air at 380 C during different times (1, 3, 6 and 12 h) using a furnace (heating rate 5 C/min) and cooled thereafter to room temperature. As-obtained solidified melts were washed in deionized water to separate the alkali metal chlorides from the water insoluble fraction, which was then collected by centrifugation, dried 1 h at 100 C and characterized by several techniques. Phase identification was carried out by X-ray powder diffraction in a Siemens D5000 diffractometer using CuKa radiation (l ¼ 1.5418 Å) operated at 35 kV and 25 mA at a scan rate of 0.02 (2q/s). Thermal studies of precursor materials were realized by DSC analysis in a Setaram Labsys Evo 1600 thermoanalyser; approximately 10 mg of the samples were heated from room temperature to 600 C at a rate of 5 C/min. Raman spectra were measured with Xplora One Spectrometer, using a 785 nm line of the Arþ laser as the excitation source. The size and morphology of the synthesized particles were observed directly using a high-resolution transmission electron microscope (HRTEM TITAN 80-300). The crystallite size was estimated by using the Scherer's; D ¼ Kl/bcosq, where D is the mean crystallite size along the (h k l) direction, l is the X-ray wavelength (lcu ¼ 1.544056 Å), b is the peak full width at half minimum intensity (FWHM), q is the angle of diffraction, and the Scherer's constant K is conventionally set to 1. 3. Results and discussion Fig. 2(a) shows the X-ray diffraction pattern obtained from mechanical milling the starting stoichiometric mixture of HfCl4, CeCl3 and LiOH/KOH. After milling, the crystalline phases identified by XRD were tetragonal lithium and cubic potassium chlorides. The main reflection of the first phase has been labeled with the solid sphere located around 33 (2q) for (202) Miller index (PDF 731273), whereas the second phase was correlated with XRD pattern
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Fig. 2. X-ray diffraction pattern (a) and DSC/TGA curve (b) obtained after milling the reaction mixture of HfCl4, CeCl3 and LiOH/KOH.
found in the JCP-ICDD for KCl (PDF-75-0296) and included at the bottom for easily comparison. A metathesis or double displacement reaction took place on milling as was proved by the detection of crystalline chlorides. This chemical reaction involves the exchange of atomic/ionic species between reactants; in this case, Hf and Ce cations have been displaced by Li and K ions in order to obtain crystalline LiCl and KCl. These crystalline obtained products have high-lattice energy and similar atomic bonds. As is shown in the figure it was difficult to detect the resulting Ce and Hf-containing species by XRD because of lacking of long-range atomic ordering, and thus being amorphous as denoted by a broad background in the region of 25e38 (2q). Since both HfCl4 and CeCl3 show their most intense reflections far from the maximum of the amorphous area, ∽31 (2q), [i.e. 14.81 (PDF 85-0171) and 13.73 (PDF 77-0154) respectively], it follows from the XRD pattern that neither nonreacted crystalline hafnium nor cerium chlorides are presented. Reported XRD pattern of HfCl4 is also shown at the bottom in the figure as reference. Interestingly, the center of this maximum lies very close to the position of the main reflection peak of HfO2 [30.36 (2q) for (111) Miller index (PDF 53-0560)]. Therefore, it might be assumed that the metathesis chemical reaction was complete, and also, is possible the existence of an amorphous phase of HfO2 in the milled precursor. Further evidences of the existence of a metathesis reaction and the subsequent LiCl and KCl formation are inferred from the thermal analysis curve of milled precursor, Fig. 2(b). In DSC curve, the presence of the intense endothermic event around 354 C unrelated to weight loss is associated with the melting point of LiCl/KCl eutectic mixture [36], supporting the idea that milling promotes metathesis reaction. An additional endothermic event taking place around 85 C is due to a dehydration process in the sample. Notice there is no evidence of residuary Li or K hydroxides on the thermal curve; they must be represented by an endothermic pick around ~406 and 462 C for KOH and LiOH respectively; confirming the only existence of the eutectic mixture in molten media. LiCl/KCl inorganic solvent is chemically inert and does not suffer from drastic evaporation loss up to 1000 C. This thermal stability was confirmed by analyzing the TGA behavior of the precursor. Herein, the total mass loss in this milled precursor from room temperature to 600 C was around 11%. Some issues can
be observed on this curve: the increasing of mass at the beginning of thermal program as a result of the hygroscopic behavior of chlorides ̴(2%), the accentuated loss weight related to dehydration process, which is also related to the first endothermic event around 85 C, and the inexistence of loss mass at the melting point temperature. A metathesis or displacement reaction has become in last years an alternative route for the rapid synthesis of important solid-state compounds. Matovi c et al. [30], reported the preparation of pristine HfO2 by metathesis from the reaction between HfCl4 with NaOH. In this study, they detected the presence of two broad peaks in the Xray diffraction pattern of the sample, revealing that the as-prepared precursor was amorphous, which indicated that the proposed metathesis reaction does not lead to the formation of NaNO3 and NaCl crystalline phases at room temperature. In order to intensify crystallization on the samples, the powders were annealed from 600 to 1500 C, showing almost complete crystallization of monoclinic HfO2 at 1500 C. Similar results were reported during the preparation of Y-doped HfO2 (5, 10 y 15 mol % of Y). These results are opposite to this study. Herein, the LiCl and KCl crystalline species were generated at room temperature via mechanically assisted metathesis reaction and then detected by XRD. Fig. 3 shows the X-ray diffraction patterns of doped HfO2 nanoparticles, with 8.6 (a) and 10 mol % (b) of Ce. The samples were obtained at 380 C and fired during several times. For the sample with lowest Ce content, firing 12 h, a pair of crystalline phases was detected: cubic and monoclinic HfO2. The first one can be associated with the XRD characteristic pattern included at the bottom (cHfO2, PDF 53-0560), whereas the low-intensity set of reflections for second were labeled by empty squares around 24.23, 28.42, 31.65, 40.85 and 55.84 (2q), also related to (011), (111), (111), (211) and (130) crystalline planes (m-HfO2, PDF 74-1560). Reducing reaction time leads to detection of very weak-intensity reflections for monoclinic phase, even complicating its identification; mainly in 3 h and where (001) and (111) planes were the only observed. On the contrary, cubic phase showed four diffractions picks which were directly correlated to (111), (002), (022) and (113) Miller index, corresponding to the characteristic reflections of a fluoritetype cubic structure. Even if the cubic phase showed the highest
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451
Fig. 3. X-ray diffraction patterns of Ce-doped HfO2 nanoparticles with 8.6 (a) and 10 mol % of Ce (b), firing at 380 C for different times.
intensity ratio comparing to monoclinic phase, the powder was constituted by a mixture of phases. Increasing the content of Ce to 10 mol % leads to the complete stabilization of the cubic HfO2 as is shown in (b). The set of reflections detected in the XRD pattern and labeled with Miller Index on brackets correspond very well to a fluorite-type cubic structure, even further reducing reaction time to only 1 h. LiCl was detected in 3 h sample due to an inappropriate dissolution of reaction media during the washing operation. The main reflection of this undesirable phase has been labeled with a solid circle in the figure; LiCl was not identified in 1 h sample, which suggests obtaining pure cubic phase. The crystallite size was estimated from Scherrer's formula. Using this equation, the mean crystallite size of the obtained Cedoped HfO2 at 1 h, calculated from broadening of the (111), was 4.90 nm. It is well known that the stabilization of cubic hafnia is achieved so often at high-temperatures by element doping. The ideal ionic size ratio required for eight-coordination oxides (8CN) as in fluorite-type structures is 0.732. Evaluating the ionic radii for cation and anion in the appropriated coordination number for cubic 2 HfO2 [IR4þ Hf (VIII) ¼ 0.83 Å and IRO (IV) ¼ 1.38 Å], according to Shannon Database Ionic Radii [37], it results in a too lower ratio, 2 IR4þ Hf (VIII)/IRO (IV) ¼ 0.60, and thus the obtaining of monoclinic instead of cubic phase is totally expected. If the ionic ratio is increased by adding larger ionic size cations than Hf4þ, Ce 3þ [IRCe (VIII) ¼ 1.14 Å] for example, the stabilization of cubic phase is possible. This explains why slightly increasing Ce contents lead to the full stabilization of the cubic phase in this study. To this point, it has shown that the followed two-step methodology easily allows obtaining Ce-doped HfO2 with a fluorite-type structure. Nanoparticles were obtained at 380 C during 1 h in LiCl/ KCl. Furthermore, this method represents an alternative route for the synthesis of cubic HfO2 powders and might be useful for doping or co-doping with other elements. Prior to preparing Ce-doped samples this route was also used for synthesizing pure HfO2, which is considering the target material in this study with the purpose of easily compare the differences in the structural characteristics of cubic and monoclinic phases obtained by XRD and Raman analysis. Fig. 4(a) shows the diffraction pattern obtained for HfO2 powder fired at 450 C for 12 h. The crystalline
phase detected was the room-temperature stable monoclinic polymorph, which was in agreement with XRD pattern found in JCP-ICDD (PDF-75-0296) for the structure P21/c. The crystallite size calculated using Scherer's formula from the broadening of the (111) line was about 14.65 nm. Raman technique is sensitive to the polarization of the oxygen ions in the crystalline structure, and therefore, it was used in this study to corroborate the symmetry of HfO2 particles. Fig. 4(b) shows Raman spectra of pure and Ce-doped HfO2. The measurements were realized for the samples previously represented in Figs. 4(a) and 3(b). The bands in pure hafnia, labeled as m-HfO2 in the figure, were recorded around 109, 131, 145, 160, 239, 351, 324, 332, 379, 395, 495, 516, 547, 574, 635 and 667 cm1. These 16 bands, in the region between 100 and 700 cm1, were correlated with the characteristic vibrational modes of monoclinic HfO2 [38]. The case of Ce-doped HfO2 sample, labeled as c-HfO2, an exclusive vibrational mode was observed about 454 cm1, which is the F2g Raman active mode representative of the fluorite-type structure [39]. Although there are several publications of the Raman spectra for monoclinic HfO2, these reports are not in good agreement with the bands locations. The observed shifts in these reports are related so often to polarization phenomena in single HfO2 crystals [40,41]. Nevertheless, all of them are in agreement with the fact that 18, 6 and 1 are the Raman vibrational modes of the monoclinic, tetragonal and cubic structures correspondingly [42]. These results confirmed the existence of pure monoclinic and cubic HfO2 phases in samples as was previously determined by X-ray diffraction technique. HRTEM micrographs of 10% mol Ce sample prepared at 380 C for 3 h show that the powders are agglomerates composed of spherical particles, Fig. 5(a). The distribution of particle sizes varies in a narrow range from 4.8 to 6.2 nm as can be observed in some measured particles, (b). These measuring values were in agreement with 4.9 nm average crystallite size obtained from Scherrer's formula. Electron diffraction pattern recorded on the sample was appropriated indexed to Fm3m structure, (c). The obtained highresolution images showed clear lattice fringes, (d). This observation indicates the highly crystalline nature of the synthesized Cedoped HfO2 nanoparticles. The lattice fringes spacing in this figure are about 2.9 and 2.5 Å. These values correspond to the
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Fig. 4. X-ray diffraction pattern of pure HfO2 particles prepared at 450 C for 12 h and after removing LiCl/KCl (a). Raman spectra of Ce-doped and pure HfO2 powders (b).
Fig. 5. HRTEM (a, b and c) and SAED (b) images of Ce-doped HfO2 nanoparticles obtained at 380 C during 3 h.
separation between the (111) and (200) planes of the cubic structure of HfO2. EDS microanalysis confirmed the presence of Ce in samples, and also verified the inexistence of Zr contamination during milling (see Supplementary information). One of the main concerns when using mechanical milling as one of the synthesis steps for obtaining materials is the probable contamination of the samples because of wearing and debris of the milling tools; however the experimental settings used in this work
(e.g., short milling time, ZrO2 containers and balls, moderated rotating disk speed and hydrated metal chlorides and hydroxides as starting chemicals) were selected to minimize this problem. The followed methodology for preparing Ce-doped HfO2 nanoparticles has some outstanding features. First, it is simple, rapid, includes few reagents and does not involve the use of expensive chemicals (e.g., alkoxides) and thus, the entire process is costeffective. Secondly, the synthesis was carried out in a short
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reaction time (1 h), thus a saving energy is achieved, and does not need any sophisticated or costly equipment nor special handlings precautions because is realized under normal atmosphere; although there is still some room for improvement, the yield is very high (>95%) and the process can be easily adapted for technological applications. Third, the process is two step and not toxic, because not involve the use of organic solvents and neither generates volatile or toxic by-products. 4. Conclusions Ce-doped HfO2 nanoparticles with cubic structure were successfully synthesized at low-temperature and short reaction time (380 C/1 h), using a eutectic mixture of LiCl/KCl as molten flux. The complete stabilization of cubic phase was achieved by adding 10 mol % of Ce. XRD, Raman and HRTEM techniques showed that crystallites in powder sample have Fm3m structure. Ce-doped hafnia powders are forming agglomerates of spherical particles of about 5 nm size. The proposed metathesis reaction fulfilled two goals efficiently; obtaining an appropriated Ce and Hf-containing precursor and generating in-situ alkaline chlorides fluxes. Furthermore, the followed methodology represents an alternative to the solid and solution routes used for synthesizing cubic HfO2 nanoparticles and should be useful for doping or co-doping other fluorite-type structures. Acknowledgments Financial support for the work was provided by CONACYT through the project number 133991. The authors thank Laboratorio nicos, CONACYT grant 232753. E. Nacional de Materiales Grafe Mendoza-Mendoza thanks to CONACYT for the scholarship number 203766 for the postdoctoral position. Also thanks to R. Rangel for Raman measurements and E. Díaz for TEM images. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.09.122. References [1] R.K. Nahar, V. Singh, A. Sharma, Study of electrical and microstructure properties of high dielectric hafnium oxide thin film for MOS devices, J. Mater. Sci. Electron. 18 (2007) 615e619. [2] M.C. Cheynet, S. Pokrant, F.D. Tichelaar, J.L. Rouviere, Crystal structure and band gap determination of HfO2 thin films, J. Appl. Phys. 101 (2007) 054101. [3] J. Khoshman, A. Khan, M. Kordesch, Amorphous hafnium oxide thin films for antireflection optical coatings, Surf. Coat. Technol. 202 (2008) 2500e2502. [4] K.J. Hubbard, D.G. Schlom, Thermodynamic stability of binary oxides in contact with silicon, J. Mater. Res. 11 (1996) 2757e2761. [5] H. Ikeda, S. Goto, K. Honda, M. Sakashit, A. Sakai, S. Zaima, Y. Yasuda, Structural and electrical characteristics of HfO2 films fabricated by pulsed laser deposition, Jpn. J. Appl. Phys. 41 (2002) 2476. [6] J.A. Valdez, I.O. Usov, J. Won, M. Tang, R.M. Dickerson, G.D. Jarvinen, K.E. Sickafus, 10 MeV Au ion irradiation effects in an MgOeHfO2 ceramiceceramic (CERCER) composite, J. Nucl. Mater. 393 (2009) 126e133. [7] M. Filipescu, N. Scarisoreanu, V. Craciun, B. Mitu, A. Purice, A. Moldovan, V. Ion, O. Toma, M. Dinescu, High-k dielectric oxides obtained by PLD as solution for gates dielectric in MOS devices, Appl. Surf. Sci. 253 (2007) 8184e8191. [8] J. Robertson, High dielectric constant gate oxides for metal oxide Si transistors, Rep. Prog. Phys. 69 (2006) 327e396. [9] G.D. Wilk, R.M. Wallace, J.M. Anthony, High-kappa gate dielectrics: current status and materials properties considerations, J. Appl. Phys. 89 (2001) 5243e5275. [10] M. Kirm, J. Aarik, M. Jurgens, I. Sildos, Thin films of HfO2 and ZrO2 as potential scintillators, Nucl. Instrum. Meth. Phys. Res. Sect. A 537 (2005) 251e255. [11] C. LeLuyer, M. Villanueva-Ibanez, A. Pillonnet, C. Dujardin, HfO2:X (X ¼ Eu, Ce, Y) sol gel powders for ultradense scintillating materials, J. Phys. Chem. A 122
453
(2008) 10152e10155. [12] S. Lange, V. Kiisk, V. Reedo, M. Kirm, J. Aarik, I. Sildos, Luminescence of RE-ions in HfO2 thin films and some possible applications, Opt. Mater. 28 (2006) 1238e1242. [13] V.B. Glushkova, M.V. Kravchinskaya, HfO2-based refractory compounds and solid solutions, Ceram. Int. 11 (1985) 56e65. [14] M.R. Winter, D.R. Clarke, Thermal conductivity of yttria-stabilized zirconiaehafnia solid solutions, Acta Mater. 54 (2006) 5051e5059. [15] N. Izu, T. Itoh, W. Shin, I. Matsubara, N. Murayama, The effect of hafnia doping on the resistance of ceria for use in resistive oxygen sensors, Sens. Actuators B Chem. 123 (2007) 407e412. [16] M.F. Trubelja, V.S. Stubican, Ionic conductivity in the hafniaeR2O3 systems, Solid State Ion. 49 (1991) 89e97. [17] J. Wang, R. Stevens, Hafnia and hafnia-toughened ceramics, J. Mater. Sci. 27 (1992) 5397e5430. [18] X. Luo, W. Zhou, S.V. Ushakov, A. Navrotsky, A.A. Demkov, Monoclinic to tetragonal transformations in hafnia and zirconia: a combined calorimetric and density functional study, Phys. Rev. B 80 (2009) 134119. [19] X. Zhao, D. Vanderbilt, First-principles study of structural, vibrational, and lattice dielectric properties of hafnium oxide, Phys. Rev. B 65 (2002) 233106. n, T. Rivera-Montalvo, J. Guzm lito, [20] E. Montes, P. Cero an, M. García-Hipo n, R. García-Salcedo, C. Falcony, Thermoluminescent characA.B. Soto-Guzma 3þ terization of HfO2:Tb synthesized by hydrothermal route, Appl. Radiat. Isot. 83 (2014) 196e199. [21] J.Z. Wang, Z.Q. Shi, Y. Shi, L. Pu, L.J. Pan, R. Zhang, Y.D. Zheng, Z.S. Tao, F. Lu, Broad excitation of Er luminescence in Er-doped HfO2 films, Appl. Phys. A 94 (2009) 399e403. [22] E. Zych, Luminescence and scintillation of inorganic phosphor materials, in: H.S. Nalwa, L.S. Rohwer (Eds.), Handbook of Luminescence, Display Materials and Devices, American Scientific Publishers, CA, USA, 2003. ski, Monoclinic HfO2:Eu X-ray phosphor, [23] A. Wiatrowska, E. Zych, L. Kҿpin Radiat. Meas. 45 (2010) 493e496. [24] L. Gao, L. Zhou, J. Feng, L. Bai, C. Li, Z. Liu, J.L. Soubeyroux, Y. Lu, Manganese doped hafnia, Ceram. Int. 38 (2012) 2305e2311. [25] R. O'Connor, T. Kauerauf, H. Arimura, L.A. Ragnarsson, Stress induced defect generation implications of doping HfO2 with Al, Microelectron. Eng. 109 (2013) 54e56. [26] A. Lauria, I. Villa, M. Fasoli, M. Niederberger, A. Vedda, Multifunctional role of rare earth doping in optical materials: nonaqueous sol-gel synthesis of stabilized cubic HfO2 luminescent nanoparticles, ACS Nano 7 (2013) 7041e7052. [27] S. Chen, Z. Liu, L. Feng, X. Che, Influence of cerium-doping on the structural and electrical properties of hafnium oxide gate dielectric, J. Mater. Sci. Mater. Electron. 25 (2014) 749e753. lvez-Barboza, L.A. Gonz [28] S. Ga ales, B.A. Puente-Urbina, E.M. Saucedo-Salazar, L.A. García-Cerda, Preparation and characterization of Ce-doped HfO2 nanoparticles, J. Alloys Compd. 643 (2015) S62eS66. [29] P. Rauwel, E. Rauwel, C. Persson, M.F. Sunding, A. Galeckas, One step synthesis of pure and monoclinic HfO2 nanoparticles: correlating the structure to the electronic properties of the two polymorphs, J. Appl. Phys. 112 (2012) 104107. [30] B. Matovi c, D. Bǔcevac, M. Prekajski, V. Maksimovi c, D. Gautam, K. Yoshida, T.B. Yano, Synthesis and characterization of nanometric yttrium-doped hafnia solid solutions, J. Eur. Ceram. Soc. 32 (2012) 1971e1976. [31] Roo, K. Keukeleere, J. Feys, P. Lommens, Z. Hens, E. Driessche, Fast, microwave-assisted synthesis of monodisperse HfO2 nanoparticles, J. Nanopart. Res. 15 (2013) 1778. [32] A. Ramadoss, S.J. Kim, Synthesis and characterization of HfO2 nanoparticles by sonochemical approach, J. Alloys Compd. 544 (2012) 115e119. [33] Y. Mao, T.J. Park, F. Zhang, H. Zhou, S.S. Wong, Environmentally friendly methodologies of nanostructure synthesis, Small 3 (2007) 1122e1139. [34] X. Liu, N. Fechler, M. Antonietti, Salt melt synthesis of ceramics, semiconductors and carbon nanostructures, Chem. Soc. Rev. 42 (2013) 8237. [35] J.S. Quintero-García, B.A. Puente-Urbina, L.A. García-Cerda, O.S. Rodríguezndez, E. Mendoza-Mendoza, A “green methodology” approach to the Ferna synthesis of HfO2 nanoparticles, Mater. Lett. 159 (2015) 520e524. [36] A.S. Basin, A.B. Kaplun, A.B. Meshalkin, N.F. Uvarov, The LiCl-KCl binary system, Russ. J. Inorg. Chem. 53 (2008) 1509e1511. [37] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A 32 (1976) 751e767. [38] M. Modreanu, J. Sancho-Parramon, O. Durand, B. Servet, M. Stchakovsky, C. Eypert, C. Naudin, A. Knowles, F. Bridou, M.F. Ravet, Investigation of thermal annealing effects on microstructural and optical properties of HfO2 thin films, Appl. Surf. Sci. 253 (2006) 328e334. [39] V.G. Keramidast, W.B. White, Raman spectra of oxides with the fluorite structure, J. Chem. Phys. 59 (1973) 1561e1562. [40] B.K. Kim, H.O. Hamaguchi, Raman spectrum of 18o-labelled hafnia, Mater. Res. Bull. 32 (1997) 1367e1370. ris, A.P. Mirgorodsky, T. Merle-Me jean, Comparative [41] P.E. Quintard, P. Barbe lattice-dynamical study of the raman spectra of monoclinic and tetragonal phases of zirconia and hafnia, J. Am. Ceram. Soc. 85 (2002) 1745e1749. [42] X. Zhao, D. Vaderbilt, Phonon and lattice dielectric properties of zirconia, Phys. Rev. B 65 (2002) 233106.