Journal of the Ceramic Society of Japan 128 [11] 875-882 2020 -Japan DOI http://doi.org/10.2109/jcersj2.20124 JCS FULL PAPER Hydrothermal synthesis of luminescent niobate thin rods Masanori HIRANO1,³, Tokifumi IWATA1, Katsuyuki KOMAKI1, Hiroyuki. IWATA2 and Kouki TANAKA1 1 Department of Applied Chemistry, Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota, Aichi 470–0392, Japan 2 Department of Electrical and Electronics Engineering, Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota, Aichi 470–0392, Japan 3+ 3+ 3+ Luminescent columbite-type CaNb2O6 crystals undoped and doped with Eu (CaNb2O6:Eu ) and Tb 3+ (CaNb2O6:Tb ) have been synthesized at 240 °C using hydrothermal method from precursor solutions of inorganic salts. Microscopic examinations reveal that the morphology of the columbite crystals is like thin rods or needles (width: 0.1­0.3 ¯m, length 5­10 ¯m). The red, green, and blue light-emitting CaNb2O6-based materials have been prepared. Under the UV light excitation, the as-prepared CaNb2O6 presents a broad and 7¹ blue photoluminescence (PL) peaked at 452 nm, originated from the niobate octahedral group [NbO6] . The 3+ columbite-type CaNb2O6 with 12 mol % Eu synthesized hydrothermally at 240 °C shows the strongest PL: 5 7 5 7 3+ weak orange and strong red light, attributed to the D0¼ F1 and D0¼ F2 transitions of Eu , respectively. The 5 7 5 7 3+ intensities of blue­green and green PL bands originated from the D4¼ F6 and D4¼ F5 transitions of Tb , 3+ 3+ respectively for the as-prepared CaNb2O6:Tb reach the maximum at 10 mol % Tb , and their PL intensities have been improved by heating at 1000 °C in air. ©2020 The Ceramic Society of Japan. All rights reserved. 3+ 3+ Key-words : CaNb2O6, Columbite, Eu ,Tb , Photoluminescence, Hydrothermal synthesis [Received May 22, 2020; Accepted July 30, 2020] rials as nanocrystals from aqueous precursor solutions at 1. Introduction relatively low temperatures.21)­24) As for the direct synthe- Calcium niobate (CaNb2O6) is a best-known member of sis of CaNb2O6 using wet chemical synthesis method at orthorhombic columbite group, MNb2O6 in which M is low temperatures, there have been a very few studies. In calcium, magnesium, or transition metal elements. The the case of biphasic liquid method, heating at 700 °C in air materials based on the columbite group have attracted was necessary for the crystallization of CaNb2O6 from considerable attention because of their high potentials and amorphous niobate gels.25) The hydrothermal synthesis interesting characteristics such as microwave dielectric reaction can replace the calcination or heat treatment for properties,1)­4) photocatalytic activities5),6) including pho- crystallization. There are a few reports on the formation of 7) 8)­12) 5) tocatalytic water splitting, and phosphors. As a crystalline CaNb2O6 via solvothermal and hydrothermal 6),26) coherent light source the niobate is useful in applications routes. The as-prepared samples with CaNb2O6 com- to holography and laser host material,13),14) on the other position that were formed by the solvothermal method hand. were amorphous, and subsequent heating at 600 °C in air In order to improve the performance and properties of was necessary for the crystallization of columbite phase.5) inorganic materials, the investigation on their synthesis In that hydrothermal method,6) two times of long period of techniques is an efficient way.15),16) Therefore, various hydrothermal treatments more than 1 day and for 2 days investigations on the preparation of CaNb2O6 using solid- were also necessary to synthesize crystalline CaNb2O6,on state reaction,4),7),12) flux growth,17) laser-heated pedestal the other hand. Those studies were all done for the syn- 10) 18) 19) growth, glycothermal, sol­gel, and coprecipitation thesis of pure CaNb2O6, moreover aiming at photocatalyst. method20) have been carried out. In recent years, from a Few studies have ever been conducted on the direct viewpoint of green processing, great attention has also synthesis of luminescent CaNb2O6 columbite phosphors been devoted to a direct synthesis of fine inorganic mate- with high crystallinity doped with activators under hydro- rials at low temperatures. Hydrothermal synthesis tech- thermal conditions. nique is well known to be able to prepare inorganic mate- Here, we report on the hydrothermal synthesis of red, green, and blue light-emitting CaNb2O6-based thin rods. ³ Corresponding author: M. Hirano; E-mail: hirano@aitech. Luminescent columbite-type CaNb2O6 fine crystals un- + + ac.jp doped and doped with Eu3 and Tb3 have been synthe- ‡ Preface for this article: DOI http://doi.org/10.2109/jcersj2. sized under mild hydrothermal conditions from precursor 128.P11-1 solutions of inorganic salts, i.e., Ca(NO3)2, NbCl5, EuCl3, ©2020 The Ceramic Society of Japan 875 This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by-nd/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. JCS-Japan Hirano et al.: Hydrothermal synthesis of luminescent niobate thin rods and TbCl3. The effects of hydrothermal treatment con- and excitation spectra of samples were done using a ditions and the concentrations of Eu3+ and Tb3+ on the spectrofluorometer (F-2700, Hitachi High-Tech, Japan) crystallization, structure, and PL properties of CaNb2O6 with Xe lamp. Powder samples were excited with 260 or with columbite phase have been investigated. 393 nm radiation from a 150 W xenon lamp. The emission wavelength was scanned from 300 to 800 nm at a scanning 2. Experimental rate of 60 nm/min. For the measurements of excitation Two CaNb2O6-based metal complex oxides whose cation spectra, the excitation wavelength was scanned from 220 ratios were Ca/Eu/Nb = 1.00 ¹ x/x/2.00 and Ca/Tb/ to 400 nm at the same scanning rate, and emissions were Nb = 1.00 ¹ x/x/2.00 were formed under hydrothermal monitored at 450 nm (for pure CaNb2O6), 610 nm (for 3+ 3+ conditions. Reagent-grade Ca(NO3)2·4H2O, EuCl3·6H2O, CaNb2O6:Eu ), and 546 nm (for CaNb2O6:Tb ). TbCl3·6H2O, and NbCl5 were used as starting materials. An aqueous solution dissolving the starting materials 3. Results and discussion selected from Ca(NO3)2, EuCl3, and TbCl3 and an ethanol 3.1 Crystallization of columbite-type CaNb2O6 3© 3© solution dissolving NbCl5 were mixed in a Teflon con- thin rods doped with Eu and Tb tainer while stirring, thus a precursor solution was prepared. The hydrothermal treatment of precursor solutions add- The pH of the solution was controlled by the addition of ed with different amounts of aqueous ammonia was con- aqueous ammonia to have acidic (pH 2.7) to weakly basic ducted at 240 °C for 5 h to investigate the effect of the pH (pH 8.7) conditions. This precursor solution with total condition of the precursor solutions on the preparation of 3 cation concentration of 0.10­0.30 mol/dm (: Ca/Nb = CaNb2O6. Under the acidic hydrothermal condition, the 1/2) in the Teflon container was then placed in a stainless- formed solid precipitates were almost amorphous-like. 3+ steel vessel. In the preparation of samples doped with Eu Almost a single phase of orthorhombic CaNb2O6 columbite 3+ or Tb , the site of Ca in the chemical formula, CaNb2O6 with relatively high crystallinity was detected in the sample was partially substituted with Eu or Tb (; substitution formed under weakly basic conditions at pH 8.7, on the atomic ratio is referred to as x). The vessel was tightly other hand. Thus, the hydrothermal treatment was carried sealed and it was heated at 180­240 °C for 5­24 h while out under weakly basic conditions at various temperatures rotating at 1.5 rpm. After the hydrothermal treatment, pre- for 5 h. The XRD patterns of precipitates formed hydro- cipitates formed in the Teflon container were washed with thermally at various treatment temperatures are shown in distilled water until the pH value of the rinsed water Fig. 1. The precipitates formed hydrothermally at 180 and became 7.0, separated from the solution by centrifugation, 200 °C were almost amorphous. At 220 °C, the hydro- and dried in an oven at 60 °C. The as-prepared powder was thermal crystallization of CaNb2O6 phase from amor- heated in an alumina crucible at heating rate 200 °C/h, held phous precipitate was observed. To promote the growth of at 1000­1300 °C for 1 h in air, and then cooled to room columbite-type CaNb2O6 hydrothermal treatment at 240 °C temperature in a furnace. The powder sample after heating for 5 h under weakly basic condition is suggested to be in air was pulverized in an alumina mortar. necessary. It is known that CaNb2O6 has a columbite struc- The observation of morphology and structure of the sam- ples was carried out using transmission electron micros- copy (TEM; model JEM-2100Plus, JEOL, Tokyo, Japan) and selected area electron diffraction (SAED). The powder X-ray diffraction (XRD) measurements for sample powders were done using Cu K¡ radiation (XRD; model RINT- 2000, Rigaku, Tokyo, Japan). The measurement of the crystallite size of orthorhombic columbite phase was con- ducted from the line broadening of 131 diffraction peak, according to the Scherrer equation, DXRD = K­/¢ cos ª, where ª is the Bragg angle of diffraction lines; K is a shape factor (K = 0.9 in this work); ­ is the wavelength of incident X-rays, and ¢ is the corrected half-width given by 2 2 2 ¢ = ¢m ¹ ¢s , where ¢m is the measured half-width and ¢s is the half-width of a standard sample. The molar ratio (mole percent) of Eu/Ca and Tb/Ca in the samples was determined via an inductively coupled plasma emission spectrometer (ICP; model SPS-3100, SII NanoTechnology Inc., Japan) The ultraviolet­visible (UV­Vis) absorption spectra of the prepared powders were recorded at room temperature in air by means of UV­Vis spectrophotometer with an Fig.