High thermal stability and blue-violet emitting phosphor CaYAlO4:Ti4+ with enhanced emission by Ca2+ vacancies

Journal Pre-proof 4+ High thermal stability and blue-violet emitting phosphor CaYAlO4:Ti with enhanced 2+ emission by Ca vacancies Mao Xia, Yongli Zhang, Minghui Li, Dingqiang Li, Yuan Zhong, Simin Gu, Nan Zhou, Zhi Zhou PII:

S1002-0721(19)30118-8

DOI:

https://doi.org/10.1016/j.jre.2019.04.015

Reference:

JRE 581

To appear in:

Journal of Rare Earths

Received Date: 10 February 2019 Revised Date:

27 April 2019

Accepted Date: 28 April 2019

Please cite this article as: Xia M, Zhang Y, Li M, Li D, Zhong Y, Gu S, Zhou N, Zhou Z, High thermal 4+ 2+ stability and blue-violet emitting phosphor CaYAlO4:Ti with enhanced emission by Ca vacancies, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.04.015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

High

thermal

stability

and

blue-violet

emitting

phosphor

CaYAlO4:Ti4+ with enhanced emission by Ca2+ vacancies Mao Xiaa, b, c, **, Yongli Zhanga, b, **, Minghui Lia, Dingqiang Lia, Yuan Zhonga, b, Simin Gua, b, Nan Zhoua, b, *, Zhi Zhoua, b, * a

College of Science, Hunan Agricultural University, Changsha, 410128, P. R. China

b

Hunan Optical Agriculture Engineering Technology Research Center, 410128, P. R. China

c

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, P. R. China

** Mao Xia and Yongli Zhang contributed equally to this work. 1

1

Corresponding authors: Prof. Dr. Nan Zhou E-mail: [email protected] Prof. Dr. Zhi Zhou E-mail: [email protected] 2 Foundation item National Natural Science Foundation of China (No. 21706060, 51703061); Natural Sciences Foundation of Hunan Province (No. 2017JJ3103); Youth Project of Hunan Education Department (No. 17B118); Hunan Provincial Engineering Technology Research Center for Optical Agriculture (No. 2018TP2003).

Abstract: Blue-violet light could not only enhance the total content of biomass and glucoside but also enrich the taste of the fruit. Thus, it is meaningful to study the blue-violet luminescent materials for plant cultivation. In this study, titanium (
) -activated CaYAlO4 (CYAO) phosphors are synthesized by conventional high-temperature solid-state reaction. X-ray powder diffraction is employed to analyze the crystal-structure of CYAO. It is found that the doped Ti4+ ions did not change obviously the crystal structure of phosphors. Upon 246 nm excitation, CaYAl1 4+ phosphors exhibit broad blue-violet emission band peaking at 395 nm, －xO4:xTi which can be attributed to the charge transfer of Ti4+－O2-. Moreover, this phosphor exhibits strong thermal stability. The luminescence emission intensity at 150℃ maintained about 91 mol% of its initial value at room temperature. Additionally, the electron transition process and concentration quenching mechanism of CaYAl1 － 4+ is discussed in detail. The excellent luminescent properties indicating that xO4:xTi CaYAl1 － xO4:xTi4+ phosphor may have a promising application in indoor plant cultivation. Keywords: blue-violet-emitting; Ca2+ vacancies; phosphors; plant growth lighting; Ti4+-activated

1. Introduction: In recent years, with the increasingly serious environmental problems, indoor plant cultivation has attracted widely attention due to its significant advantages, such as pesticide-free, controllable growth environment and high yield [1-4]. As we all know, plants need an appropriate conditions for growth including light, heat, water, gas, fertilizer, etc,. Suitable environmental conditions are beneficial to plant growth. Light is a key parameter plant growth, since the photosynthesis, growth, flowering and fruiting of plant are greatly affected by it. As for the artificial light sources using in indoor plant growth, phosphor-converted light emitting-diodes (pc-LEDs) have become one of the major artificial sources for plant growth, owing to of its more superior merits than fluorescent lamps and incandescent lamps including long lifetime, energy economization, high luminous efficiency and eco-friendly. Therefore, as an essential part of pc-LED, phosphors determine the spectra, luminescence intensity, correlated color temperature and lifetime of luminescent device directly. The mainly absorption bands of plant pigments are far-red (700－760 nm), red (640－700 nm) and blue-violet (350－500 nm) regions [5-6]. The effect of far-red, red and blue-violet

on plant growth has been widely reported, far-red and red light can promote the flowering and fruiting of plants [7]. Meanwhile, the blue-violet light can facilitate plant growth and increase the total content of lariciresinol diglucoside, dehydroabiol glucoside and biomass, contributes to improve the taste of the fruit flavor [8-9]. Thus, it is meaningful to study the effect of blue-violet light on plant growth. So far, some blue-violet emission phosphors have been studied, such as MYSi4N7:Eu2+, (Sr, Zn)3(PO4)2:Tl3+ and SrZnO2:Pb2+ [10-12]. However, there are still some problems existing in the investigation of blue-violet emission phosphors. Firstly, the price of relevant raw materials is expensive and the conditions of synthesis are harsh, so, it is difficult to be on the market. Secondly, the commonly toxic metal activator (Pb, Tl) ions are harmful to the environment and human body, and have been banned. Accordingly, it is necessary to develop a high-performance, eco-friendly blue-violet phosphor for plant growth lighting. The CaYAlO4 (CYAO) has a tetragonal K2NiF4 structure and belongs to Abma space group, which is composed of interleaved layers with a perovskite and rock-salt structure [13]. The unit cell of CYAO was composed by [Ca/YO9] and [AlO6] polyhedrons which connected each other to form the framework [14]. Over the last few decades, Tb3+, Ho3+, Eu3+, Yb3+ and Mn4+ single-doped tetragonal K2NiF4-like CYAO phosphor have been reported and which can be widely used in field emission display, laser and up-conversion fields [13-17]. Thus, the CYAO substrate may have a promising application in LED plant growth lighting, due to their superior advantages such as high color purity, chemical stability and low environmental toxicity. In recent years, transition metal (TM) ions such as Mn4+, Fe3+, Cr3+, Ti4+ have attracted wide interest as an activator because of its outstanding spectral property, abundant reserves and inexpensive [18-22]. Manganese-activated phosphors show a deep-red emission arising from the 2Eg→4A2g spin-forbidden transition. Typically, they exhibit good luminescence stability and non-photon reabsorption phenomenon, which are crucial for practical usage [18-19]. In addition, chromium-activated phosphors have caused extensive attention for its outstanding luminescence properties in different matrix (MgAl2O4, Y3Al5O12 and ZnGa2O4) [20-22]. Therefore, as a member of TM ions, the titanium ions attract our attention as activator in many materials. The emission peak position of Ti4+-doped phosphors rest with the crystal field environment. And its blue-violet emission and the band gaps can be adjusted in the host. Moreover, the titanium doped phosphors could be efficiently excited by ultraviolet (UV) light which means that they can avoid the occurrence of re-absorption effect. Nevertheless, the Ti4+-activated phosphors were rarely reported and the luminescence performance cannot be guaranteed due to its naked 3d orbit [23]. Ti4+ is considered to be able to achieve blue-violet luminescence but many properties have not been studied at present. For instance, the blue-violet luminescent

materials can be obtained by Ti4+ doping CAZO (Ca3Al4ZnO10 and Ca14Al10Zn6O35) [24-25]. The strong blue-violet emission phosphors have great prospects for plant applications, owing to its excellent optical properties and low environmental toxicity. But poor chemical and thermal stability limited its applications. Recently, it was demonstrated that the spectral adjustment in some substrates caused by the formation of the oxygen or cation vacancies defect. For instance, Wu et al. reported a giant enhancement of luminescence intensity that was realized by a cluster model involving the cation and oxygen vacancies, which is a new strategy for exploring novel influence factor of the emission characteristics [26-27]. Moreover, Liu et al. reported that charge compensation models improve successfully the emission intensity of Eu3+ doped CaMoO4 [28]. Thus, the luminescence properties of CYAO: Ti4+ phosphors may be improved by building cation vacancies to compensate for charges. In this work, for acquiring a promising bright blue-violet emission phosphor in the application of plant growth fluorescent lighting, Ti4+-doped CYAO phosphors are obtained by using a simple solid-state reaction process. The luminescence properties and crystal structure of CYAO: Ti4+ phosphors are investigated by constructing cation vacancies to compensate for charges. The FT-IR spectra, UV-vis absorption properties and lifetimes are studied in detail. 2. Experimental sections 2.1. Synthesis of samples The CaYAlO4:xTi4+ (abbreviated as: CaYAl1－xO4:xTi4+; x = 0.01, 0.02, 0.03, 0.05, 0.07, 0.09, molar ratio) phosphors were successfully prepared by conventional high-temperature solid-state reaction (HT-SSR) method in air. A mixture of solid analytical reagents, namely, CaCO3, Al2O3 (0.999), Y2O3 (0.999) and TiO2 were used as starting materials. Subsequently, the above raw materials were weighted according to the chemical composition ratio of 2: (1-2x):1:2x with the adding of 2 wt% boric acid, mixed with absolute ethanol in an agate mortar. After grinding process, these samples were obtained at 1300 ℃ for 4 hours (speed rate is 5 °C /min) in a sealed box-type furnace. Next, these samples were cooled to room temperature naturally, further fine reground to obtain sub-micron particles samples for soon afterwards characterizations. A series of Ca2+ vacancies phosphors of CayYAl0.97O4:0.03Ti4+ (y = 1.0000, 0.9925, 0.9850, 0.9775, 0.9700 and 0.9625) were prepared by the similarly method. 2.2. Measurements The phase purity and crystal structure compositions of as-synthesized CYAO: Ti phosphors were analyzed using an X-Ray Diffraction (XRD) (D/Max-2200/pc, 4+

Rigaku) with Cu-Kα radiation (λ = 0.15406 nm), setting the 2θ from 10° to 80° at a speed of 5°·min-1, and operating at 40 mA and 40 kV. The micro-morphologies of the phosphors were acquired by using a Scanning Electron Microscope (SEM) (Nova NanoSEM230, FEI). The photoluminescence excitation (PLE) and emission spectra (PL), quipped with a Temperature controlled reactor (TAP-02, Orient KOJI) at excitation light source of a 150 W pulse Xe lamp. The decay curves were measured via FLS920 fluorescence spectrometer (Edinburgh, UK) with a build-in millisecond flashgun. UV-vis absorption spectra and Fourier Transform Infrared Spectrometer (FT-IR) of the fine ground samples were measured by Hitachi U-3310 spectrophotometer and ALPHA infrared spectrometry, respectively. Unless specifically requested, all the characterizations were recorded on room temperature. 3. Results and discussion 3.1. Structure characterization The XRD patterns for the CYAO:xTi4+ (x=0.01, 0.02, 0.03, 0.05, 0.07 and 0.09,) phosphors and the standard card of CYAO (JCPDS card No. 50－0426) are shown in Fig. 1. Compare with the standard card, the as-prepared phosphors are almost crystallized in a purity phase, but the XRD patterns show an unidentified phase with the increase of Ti4+ concentration. Moreover, no significant changes are observed in the diffraction peak of phosphors, indicated that the incorporated Ti4+ ion have no obvious effect on the crystal structure. In addition, the distinct tetragonal crystal structure system is investigated according to the previous report [29-30]. The crystal structure has a highly condensed skeleton which is composed by several Ca/YO9 polyhedron and AlO6 octahedron. It is reported that the effective ion radius of six-coordinated Ti4+ ions (CN=6, r=0.0605 nm) are closer Al3+ ions (CN=6, r=0.0535 nm) than Ca2+ ions (CN=6, r=0.1000 nm) and Y3+ ions (CN=6, r=0.0900 nm), Based on the consideration of ionic radii matching, the dopant Ti4+ ions should prefer to substitute the lattice sites of Al3+ in CYAO [31]. As shown in Fig. 1(b), the diffraction peaks change distinctly toward low-angle with the increase of the doped Ti4+ concentration, demonstrating that the occurrence of lattice expansion after doping Ti4+ ions. As mentioned above, the Al3+ site are more easily substituted by Ti4+ ions, but the problem of charge imbalance is also generated at the same time. To solve the question, the Ca2+ vacancy is engineered. (Insert Fig. 1.) Furthermore, SEM images of the CYA0.97O:0.03Ti4+ samples had been measured and shown in Fig. 1(c, d). The irregular and smooth surface morphology of as-prepared samples particles implied that their anomalous systemic arteries at around 9 µm. Meanwhile, to better understand the structural features, the crystal structure of

CYAO:Ti4+ and the schematic representation of Ti4+ replacing Al3+ in host cells are separately exhibited in Fig. 1 (e-g). From this figure, the host space lattice has two types of coordination cation environments. One is AlO6 octahedral composed of Al and oxygen atoms, the other is the Ca and Y atoms in a ratio of 1:1 to occupy randomly the 4e site in the center of (Ca/Y)O9 polyhedron (Fig. 1 (f, g)). Oxygen atoms have two sites denoted as O1 and O2, and distributed on the 4c and the 4e site, respectively. Hence, the typical crystal structure of CYAO can be expressed as a highly compressed cytoskeleton [13]. Moreover, it is found that the highly compressed Ca2+ sites as compared with the local environment of Al3+ sites in the cytoskeleton of CYAO, in which the (Ca/Y)O9 polyhedrons are caged by AlO6 octahedrons and form a compactly rigid structure, indicates Ti4+ ions prefer external Al-ions [13, 32]. (Insert Fig. 2.) The FT-IR spectra of the as-prepared CayYAl0.97O4:0.03Ti4+ phosphors are shown in Fig. 2. The peak position of CYA0.97O:0.03Ti4+ phosphors with Ca2+ vacancies had a slight offset by comparison to the non-vacancies CYAO structure. A broad band located at 1638 cm-1 can be ascribed to the stretching vibrations of O-H bond from H2O molecule. The peak at 1399 cm−1 is allocated to the Al-O stretching vibrations, while the peaks at 1087 cm−1 and 988 cm−1 are attributed to the bending vibration of Al-O bands at the AlO6 group [33-34]. The metal–oxygen stretching vibrations are related to the inorganic network. The peaks at 866 cm-1 can be attributed to Ca-O bond vibrations. The absorption bands around 530 and 620 cm−1 are characteristic of the AlO6 group which built up the CYAO crystalline [35-37]. The functional groups or chemical bonds did not significantly change after introducing Ca2+ vacancies. 3.2. Luminescence properties of CYAO: Ti4+ phosphor (Insert Fig. 3.) The photo-luminescence excitation (PLE) and photo-luminescence (PL) spectra for CYAO:Ti4+ with various Ti4+ concentrations are shown in Fig. 3(a). The emission spectra of the CYAO:Ti4+ showed a broad blue-violet band ranging from 320 to 460nm, which is attributed to the charge transfer transition of Ti4+－O2-. The emission region of this sample matched well with the absorption spectra of plant pigments, indicating this material has promising application in plant growth lighting. The emission peaks are discovered at 395 nm, which can be attributed to the recombination transition between trapped electrons and the Ti4+－O2- charge transfer transition when excited at 246 nm. In addition, the emission spectra could be decomposed into two individual peaks by the means of Gaussian fitting. The

maximum peaks are discovered at 392 nm and 435 nm, which should can be ascribed to Ti－O charge transfer and the recombination of trapped electrons and holes, respectively [7, 38]. Moreover, when monitored at 395 nm, the as-prepared phosphors showed a narrow excitation band in deep UV region from 220 nm to 280 nm and the excitation peak located at 246 nm. The excitation spectra are separated to be two bands peaking at 240 nm and 250 nm by Gaussian fitting [8]. The emission intensity of CYA1－xO:xTi4+ phosphors increased at first and then, decreased rapidly with the increase of Ti4+ concentration which is mainly caused by concentration quenching effect (see Fig. 3(b, c)). It is obviously observed that (Fig. 3(b)) the emission spectra have the highest emission intensity when 3 mol% Ti4+ is incorporated. Moreover, the emission wavelength had a slight red shift (about 2 nm) as shown in Fig. 3(b), which can be explained by following equation [39]: ଵ

௥ర

‫ܦ‬௤ = ܼ݁ ଶ ோఱ ଺

(1)

where Dq, Z, e, r and R stand for the crystal field strength, energy level separation, anion charge, electron charge, size of a center ion and the bond length between a center ion and ligands, respectively. The effective ionic radii for six-coordinated Al3+ (r = 0.054 nm, CN = 6) is smaller than that of Ti4+ (r = 0.0605 nm, CN =6), thus, when the Al3+ sites are substituted by Ti4+ ions, the bond distance (RTi－O) between Ti4+ and O2− increases. In this situation, the value of Dq would increase, resulting in the reduced energy gap between the lowest excited state level and the ground state. As a result, the red shift is observed in the emission spectra of this phosphor. (Insert Fig. 4.) In addition, the slight valence mismatch between dopant aluminum (III) and acceptor titanium (VI) could lead to the decrease of luminescence of CYAO: Ti4+ phosphor. A widely-reported approach to weaken this tendency is introducing the oxygen or cation vacancies into the crystal structure of phosphors [26-28]. Therefore, in this study, Ca2+ vacancy is regarded as research subject, and the impact of it on the luminescent properties of CYAO:Ti4+ is discussed. The emission spectra of CayYAl0.97O4:0.03Ti4+ phosphors are shown in Fig. 4. The emission intensity improved firstly and the optimal Ca2+ concentration is confirmed to be 0.97, after that it decreased rapidly (see Fig. 4 insert). Therefore, the luminescence intensity of CYAO: Ti4+ can be enhanced via Ca2+ vacancy through charge compensation. (Insert Fig. 5.) The UV-vis absorption spectra of CYAO, CYA0.97O:0.03Ti4+ and C0.985YA0.97O:0.03Ti4+ samples are shown in the Fig. 5(a). As for CYAO host material, the strong absorption could be detected at from 200 nm to 270 nm (label as: band A). With the substitution of Ti4+ ions, an additional absorption band in the region

of 270－420 nm (label as: band B) is detected which should be assigned to the self-absorption of TiO6 octahedron. Both band A and B are consistent with the excitation spectrum, which owing to the host absorption and charge transfer spin-allowed transitions of Ti4+ － O2-, respectively. In addition, the absorption intensity of bands B are enhanced due to Ca2+ vacancies, which is probably related to the charge compensation. Moreover, the energy gap of these phosphors is calculated by following equation [40-44]. (ߙℎߥ)ଶ = ‫(ܣ‬ℎߥ − ‫ܧ‬௚ )

(2)

where α, h, ν and A stand for the absorption coefficient, the Planck constant, the frequency and constant, respectively. Eg is the band gap energy. As shown in Fig. 5(b), the value of band gap for the three samples are estimated to be 4.48 eV, 4.40 eV and 4.18 eV, respectively. Obviously, the energy gap shrunk from 4.48 eV to 4.18 eV with the increase of Ti4+ content in CYAO host. This change of energy gap could be explained by the improved electronegativity of this phosphor. (Insert Fig. 6.) To further understand of the influence of Ca vacancies on the photoluminescence properties, the decay curves are determined under the excitation of 246 nm and monitoring of 395 nm (Fig. 6). In addition, the lifetimes of these phosphors could be well fitted by mono-exponential decay model, which can be expressed as follow: [42-43] ି௧

‫ܫ‬௧ = ‫ܫ‬଴ × exp ቀ த ቁ

(3)

where It and I0 represents the background intensity at time t and 0, τ is the luminescence lifetimes. As shown in the Fig. 6, the luminescence lifetime of CYA0.97O:0.03Ti4+ and C0.985YA0.97O:0.03Ti4+ samples are finally calculated to be 3.742 and 4.454 ms, respectively. As expected, the C0.985YA0.97O:0.03Ti4+ sample has a higher luminescence lifetime and luminescence intensity. (Insert Fig. 7.) In addition, thermal stability is another key parameter for the application of phosphors applied in pc-LED. The Fig. 7 shows the temperature-dependent emission spectra of CYA1－xO:xTi4+ phosphor. The temperatures range is set from 25 ℃ to 275 ℃ and is excited by 246 nm. The emission intensity is remained stable at first and followed by the decrease with the increase of temperature, which can be attributed to thermal quenching effect and defect energy level [44]. The emission intensity of CYAO:Ti4+ maintained 91.23 mol% of the initial value at 150℃ (Fig. 7). Furthermore, the activation energy (Ea) can be calculated on the basis of following Arrhenius formula [45-46]:

ூ೅ ூబ

= [1 + ‫ ݌ݔ݁ܦ‬ቀ

ିாೌ ௄்

ቁ]ିଵ

(4)

where IT and I0 is the initial emission intensity at a given measuring temperature and room temperature (25
), respectively. D is a dependence constant depending on phosphors, K is Boltzmann constant (K = 8.617×10-5 eV/K). The slope of the fitted line is determined. Thus, the value of Ea for this phosphor is calculated to be 0.991 eV. (Insert Fig. 8.) In order to illustrate the electron transitions and energy level of CYAO:Ti4+ phosphor, a simple model illustrating is given and shown in Fig. 8. Under UV light excitation, a great deal of electrons is excited from ground state to the conduction band and excited state. Then some excited electrons could be captured through the defect energy and followed by relaxing to the Ti4+ －O2- charge transfer state. Meanwhile, other excited electrons returned to the charge transfer band via the non-radiation relaxation. Then returned to the ground state level later and emitted blue-violet light as observed by us. 4. Conclusion In summary, a series of blue-violet-emitting phosphors CaYAl1－xO4:xTi4+ are prepared by typical high-temperature solid-state reaction method. The optimal Ti4+ ion doping concentration of as-synthesized phosphor is determined to 0.03 mol, and the corresponding concentration quenching mechanism is confirmed to be the d－d interaction. Additionally, this phosphor possesses a good thermal stability, which could maintain 91 mol% of its initial emission in room temperature. The emission intensity of samples can be improved by introducing Ca2+ vacancies due to the charge compensation mechanism. Finally, the emission region of this phosphor matched well with the absorption of plant pigments, implying that CYA1－xO: xTi4+ phosphor have a potential application in indoor plant cultivation. Acknowledgements The authors would like to thank the funding support from Hunan Provincial Engineering Technology Research Center for Optical Agriculture (Grant No. 2018TP2003) and the outstanding Youth Project of Hunan Education Department (Grant No. 17B118) for their financial support. References 1. Olle M, Viršilė A. The effects of light-emitting diode lighting on greenhouse plant growth and quality. Agr Food Sci. 2013;22(2):223.

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TOC:

The figure illustrates the electron transitions and energy level of CYAO: Ti4+ phosphor. Under UV light excitation, a great deal of electrons is excited from ground state to the conduction band and excited state. Then some excited electrons could be captured through the defect energy and followed by relaxing to the Ti4+ -O2- charge transfer state. Meanwhile, other excited electrons returned to the charge transfer band via the non-radiation relaxation. Then returned to the ground state level later and emitted blue-violet light as observed by us.

Research Highlights 1. A blue-violet emitting CaYAl1-xO4: xTi4+ phosphor was synthesized. 2. The luminescence intensity could be improved by creating the Ca2+ vacancy. 3. The phosphor has a high thermal stability at 150

maintains 91%.

4. The luminescence enhancement mechanism was investigated in detail.