Effect of Oxygen Vacancies on the Reduction of Eu3+ in Mg3Ca3(PO4

21 Aug 2017 - Key Laborary of Special Function Materials and Structure Design, Ministry of Education, Department of Materials Science, School of Physi...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/IC

Effect of Oxygen Vacancies on the Reduction of Eu3+ in Mg3Ca3(PO4)4 in Air Atmosphere Hua Li and Yuhua Wang* Key Laborary of Special Function Materials and Structure Design, Ministry of Education, Department of Materials Science, School of Physical Science and Technology, National & local Joint Engineering Laboratory for Optical Conversion Materials and Technology, Lanzhou University, Tianshui South Road No. 222, Lanzhou, Gansu 730000, PR China S Supporting Information *

ABSTRACT: The paper reported the synthesis of mixed-valence Eu-doped Mg3Ca3(PO4)4 by solid-state reaction in the air atmosphere. The luminescence measurements indicated that the obtained phosphors exhibit a broad bluish-green fluorescence of Eu2+ and a sharp orange-red emission of Eu3+ excited by 395 nm. The abnormal reduction mechanism could be explained by a charge compensation model. A tunable luminescence was realized based on the reduction of Eu3+ to Eu2+ changed along with increasing the sintering time. The reduction of Eu3+ to Eu2+ in Mg3Ca3(PO4)4 host was effected by the oxygen vacancy defects when the samples sintered in different atmosphere and sintered for different lengths of time, and the oxygen vacancies which act as the electron traps were investigated with thermoluminescence. All of the results almost indicated the oxygen vacancy could weaken the reduction of Eu3+ to Eu2+. Our investigation of Mg3Ca3(PO4)4:Eu could provide a practical basis and theoretical basis to study the effect of oxygen vacancy on the emission of Eu ions.

activator in blue-emitting phosphors18−20 and long-persistence phosphors.21 It is necessary to reduce Eu3+ to Eu2+ in a host, because there is no natural material containing Eu2+. If the samples were prepared in reducing atmospheres, such as H2, H2/N2, or CO, the reduction could be realized in a matrix. However, in some special compounds, which has stiff threedimensional (3D) enclosed crystal structures, the reduction of Eu3+ to Eu2+ was observed in the air atmosphere. This special phenomenon was called “abnormal reduction” in many reports.22−31 Moreover, it is well-known that there are various types of lattice defects always present in materials, and there is still a lack of clarity about the defects (cation or anion vacancies), because it is difficult to identify and properly characterize them. Cation or anion vacancies can produce local potentials, which lead to the vacancies trapping the electrons or holes.32 The production of vacancies could be altered under different conditions in prepared materials. For example, in oxygenpoor atmosphere, the oxygen vacancies, which trap the electrons, are easily generated in oxide phosphor materials.33,34 As far as we know, very little has been done to study the effect of oxygen vacancy on the abnormal reduction in a phosphor. In order to learn more about the oxygen vacancy and the abnormal reduction, in this article, the reduction of Eu has

1. INTRODUCTION In recent years, studies on lanthanide ion (Ln3+)-doped materials have attracted increasing attention in light-emitting diodes (LEDs), field emission displays (FEDs), and plasma display panels (PDPs).1−4 The tunable emission color of phosphor for LED is a key point. In order to realize the tunable luminescence, many materials, which are based on energy transfer between the codoped multiactivators under UV excitation, have been successfully developed.5−10 As far as FED phosphors, most of the reported sulfide-based phosphors would contaminate the field emitter and hinder the electron emission, because the sulfide is easy to degrade and disperse into the vacuum, under electron beam bombardment.11−13 Compared with the sulfide phosphors, the oxide phosphors, which could escape these obstacles, have attracted a great deal of attention.14−17 Therefore, looking for materials with good stability and conductivity for FED phosphors has attracted much interest. In many phosphors, rare earth ions act as an effective luminescence centers because of their nonfull-4f configuration. Among the rare earth ions, Eu is interesting because of its adjustable valence and its emission is sensitive to the crystal field. The emission of Eu3+ ion is ascribed to the 4f−4f electronic transition, whereas the emission of Eu2+ presents an effective 5d−4f transition. Furthermore, Eu2+ has high absorption in the range of UV, tunable luminescence colors, and strong broad emission band, which makes Eu2+ used as © 2017 American Chemical Society

Received: May 25, 2017 Published: August 21, 2017 10396

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403

Article

Inorganic Chemistry

respectively. The 5% per mole doping is not reaching the quenching concentration.35,36,39 It is observed that the XRD diffraction peaks of all the samples are consistent with the Joint Committee on Powder Diffraction Standards card data (JCPDS card No. 73-1182) of Mg3Ca3(PO4)4, not merely the positions but also the relative intensities of the main diffraction peaks. From Figure 1(b), an obvious shift to the lower 2θ direction of the diffraction peak at 23.789 degree can be observed by changing the prepared conduction, due to the different content of Eu2+. Usually, the ionic radii of Ca2+ (1.06 Å) and Eu3+ (1.01 Å) are similar and both smaller than that of Eu2+ (1.20 Å). 3.1.2. Luminescent Properties of Sample MCPO-A. From Figure 2(a), the emissions in the range from 570 to 700 nm

been observed by our group in a kind of phosphate, Mg3Ca3(PO4)4:Eu prepared in air, which has not reported the abnormal reduction so far. The luminescence measurements indicate that both Eu 2+ and Eu 3+ ions exist simultaneously in the prepared Mg3Ca3(PO4)4:Eu phosphors, and their characteristic emissions can be observed clearly. The whole visible spectrum was covered by the superposition of the broad band 5d−4f fluorescence of Eu2+ and narrow 4f−4f emissions of Eu3+.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Powder Mg3Ca3(PO4)4:0.05Eu (MCPO:Eu) samples were prepared by high-temperature solid state reaction, and 0.05Eu is not the quenching concentration of the samples.35 The starting materials including (MgCO3)4·Mg(OH)2·5H2O [analytical reagent (AR)], CaCO3 (99%), (NH4)2HPO4 (AR), and Eu2O3 (99.99%) were ground in an agate mortar several times. Thoroughly mixed starting material was prefired at 700 °C for 2 h in an alumina crucible and slowly cooled to room temperature. Then, the powder was reground and sintered at 1000 °C for 4 h in the different atmosphere (air, N2, and 5H2/55N2) and cooled to room temperature. 2.2. Characterization. The X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (XRD, Bruker D2 PHASER) at a scanning rate of 2 min−1 and intervals of 0.02 in 2θ with Cu Kα radiation (λ = 1.5405 Å).The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were recorded using a fluorescence spectrophotometer (FLS-920T) equipped with a 450 W Xe light source and double excitation monochromators. The VUV luminescent spectra were measured at the VUV spectroscopy experimental station on beamline 3B1B, Beijing Synchrotron Radiation Facilities (BSRF). Low voltage CL spectra were obtained by using a modified Mp-Micro-S instrument. The morphologies of the asprepared samples were inspected by field emission scanning electron microscopy (FESEM, Hitachi, S-4800). The temperature-dependent luminescence measurements were carried out between 20 and 250 °C, in steps of about 25 °C and with a heating rate of 20 °C min−1, using the HORIBA JOBIN YVON Fluorlog-3 spectrofluorometer system. The thermoluminescence (TL) curves were measured by an FJ-427A1 m (Beijing Nuclear Instrument Factory) with a heating rate of 1 K s−1. Before the measurement, the samples were irradiated with ultraviolet light (254 nm) for 10 min. All the data were measured at room temperature except for the thermoluminescence curves.

Figure 2. (a) Emission spectrum of MCPO:0.05Eu sintered in air (MCPO-A); (b) in reduction atmosphere (MCPO-H).

belong to 4f−4f transitions of Eu3+. However, there are not only sharp emission peaks related with Eu3+, but also a broad band from 400 to 550 nm peaking at 443 nm in the emission spectrum excited by 395 nm. As is well-known, generally in a solid state compound, Eu2+ shows a broad band emission due to the 5d−4f transition, while Eu3+ gives a series of typical emission lines corresponding to 5D0 − 7FJ transitions. When excited by 395 nm, the host sample MCPO does not present any emission in the range from 400 to 750 nm. So the emission band peaking at 443 nm observed in Figure 2(a) is correlated with the doped Eu, which should be ascribed to the 5d−4f transition of Eu2+.37,38 For further confirmation of the presence of Eu2+ in MCPO:Eu prepared in the air (sample MCPO-A), we synthesized another sample MCPO:0.05Eu in a reducing atmosphere (sample MCPO-H), and measured its emission spectrum excited by 395 nm as shown in Figure 2(b). From Figure 2(b), one broad emission band of Eu2+ was situated at 443 nm. By comparison of the spectral characteristics in Figure 2(a) with (b), it is obvious that the shapes and positions of the emission bands are almost the same. Based on previous research work, in the host MCPO, the emission band of Eu2+ is from 400 to 550 nm.35,39 From the emission spectrum of MCPO-A, it can be observed that some quantity of Eu3+ can be reduced in MCPO-A prepared by solid-state reaction at high temperature in air atmosphere. From the emission spectrum of sample MCPO-H, the emission peaks of Eu3+ are distinct, indicating that the Eu3+ can not be completely reductive. The incomplete reduction might be caused by some defects that exist in the matrix lattice. The oxygen vacancies could be formed in the lattice matrix of MCPO, due to the poor oxygen

3. RESULTS AND DISCUSSION 3.1. Abnormal Reduction from Eu3+ to Eu2+ in MCPO. 3.1.1. XRD Patterns. Figure 1(a) shows the XRD profiles of the Mg3Ca3(PO4)4:0.05Eu prepared in air (MCPO-A sample) and a thermal reducing atmosphere 5H2/55N2 (MCPO-H),

Figure 1. (a) XRD profiles of the Mg3Ca3(PO4)4 (MCPO) and Mg3Ca3(PO4)4: 0.05Eu (MCPO-A, MCPO-H); (b) the movement of the diffraction peak at 23.789 degree. 10397

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403

Article

Inorganic Chemistry

Figure 3. Emission and excitation spectra of Eu3+ (a) and Eu2+ (b) in MCPO-A under UV and VUV sources, respectively.

atmosphere of the preparation conditions, which results in the incomplete reduction of Eu3+ even though it is in the reducing atmosphere. This will be explained in sections 3.1.3 and 3.1.4. The VUV−UV excited PL and PLE spectra of sample MCPO-A prepared in the air are shown in Figure 3. Whenever the sample was excited by VUV at 172 nm or UV at 395 nm, the emissions of both Eu2+ and Eu3+ could be observed, and the position of emission band (or lines) of Eu2+ (or Eu3+) excited by 172 nm is consistent with that excited by 395 nm. It is obvious that the 172 nm VUV light cannot efficiently excite the sample MCPO:Eu, which is in concert with the red excitation curve of Figure 3(a), in which the excitation intensity is very weak at ∼172 nm. In Figure 3(a) the emission lines of Eu3+ excited by 395 nm are composed of a group of typical 5D0 − 7FJ transitions and the main line is 5D0 − 7F2 transition at 612 nm. The 4f−5d transition emission of Eu2+ was located around 443 nm whenever the sample was excited at 395 nm (the green curve in Figure 3a) or 343 nm (the green curve in Figure 3b). As far as the VUV−UV PLE spectra of Eu3+ and Eu2+ are concerned, the excitation lines of Eu3+ monitored at 612 nm (the blue curve in Figure 3a) are composed of transitions from the ground 7F0 state to the excited 5H3 (326 nm), 5D4 (361 nm), 5L6 (395 nm), 5D3 (414 nm), and 5D2 (464 nm) levels, respectively.40 The predominant excitation line is 7F0 − 5L6 transition at 395 nm. In Figure 3(a), the broad band in the spectral region of 250−300 nm is attributed to a charge-transfer transition between oxygen ligand and Eu3+, and the intensity of the red curve is stronger than that of the blue one due to the higher energy of VUV than UV, which results in more electrons participating in the charge-transfer transition. Being monitored at 443 nm (the blue curve in Figure 3b), there is a broad band peaking at 343 nm in the range from 240 to 410 nm of the excitation spectrum, which results from the 4f−5d transition of Eu2+. From the results of the excitation and emission spectra, the Eu3+ could be partially reduced in sample MCPO-A prepared in the air at high temperature. Figure 4 shows the CL spectrum of sample MCPO-A. It is distinctly different with the PL spectrum. The emission of Eu3+ is much weaker than Eu2+ in the CL spectrum. The low-voltage electron-beam has high energy, and the electron-beam can affect the lattice matrix, and produce kinetic electrons and holes. According to the band theory, the electrons jump from the valence band to the conduction band under low-voltage electron-beam excitation, but the electrons are not stabilized. This will transition to the valence band in two forms. One of the forms is directly back to the valence band, and the other is

Figure 4. CL spectra of MCPO-A (accelerating voltage = 5 kV, filament current = 70 mA).

where the electron is captured by the defect and then released back to the valence band. In the lattice of MCPO, Eu3+ substitutes for Ca2+ generating a defect with a positive charge Eu·ca, and the defect level of Eu·ca is close to the bottom of the conduction band, so the defect can easily capture an electron when the electrons recede back to the valence band. The electron trapped by defect Eu·ca will release energy, which could excite the Eu over where the Eu3+ has been reducing to Eu2+ owing to capture of an electron. Therefore, the emission intensity of Eu2+ is much stronger than thatof Eu3+ in CL spectra. Compared with the emission spectrum excited by 172 nm (VUV) in Figure 3, the CL spectrum in Figure 4 has a similar spectral shape from 400 to 700 nm, but the emission intensity of CL spectra is much stronger due to the different mechanism, even if the VUV light (172 nm) also has a high energy. The similar spectral shapes that the emission band of Eu2+ produces are more obvious than the emission line of Eu3+, illustrating that the high energy can influence the emission of Eu3+ and Eu2+ through the way mentioned above. The intensities of the two spectra are very different. The CL spectrum has a higher intensity. The reasons for this could be that the energy of the electron-beam is higher than that of VUV light and the particle size of the electron is much bigger than a photon. So the lowvoltage electron-beam can give more electrons energy and motion, and thereby more electrons can take part in the jump 10398

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403

Article

Inorganic Chemistry

intensity of Eu3+ has a little increase although the emission intensity of Eu3+ decrease as the temperature increases from 20 to 225 °C. The thermoluminescence technique is very useful to reveal the nature of defects in insulators or semiconductors, which are produced by UV light.41,42 The aim of this section is to study the nature of the traps by performing TL measurements and to evidence the existence of oxygen vacancy in the sample MCPOA. The TL glow curve of the sample MCPO-A was shown in Figure 6, and the two peaks (Gaussian fit) in the TL grow

and energy transition, which leads to stronger intensity of the CL spectrum. 3.1.3. Thermal Luminescence of MCPO-A. The thermal quenching, as a crucial parameter for the application, is evaluated according to the dependence of the emission spectra of sample MCPO-A on the temperature from 20 to 250 °C, as shown in Figure 5(a). In the inset of Figure 5(a) are the

Figure 6. Thermoluminescence glow curves of the host MCPO and the sample MCPO-A.

curve, respectively, located at 323 K (T1) and 346 K (T2), correspond to the two types of traps (T1, T2) present in the sample. The black dashed curve (T′) in Figure 6 represents the TL growth curve of the air-sintered MCPO host. By comparing with curves T′ and T1, it could be observed that both samples have the same TL position (323 K), indicating that the same defect, respectively, exists in the MCPO-A and undoped sample MCPO lattice. The corresponding defect of the TL peak (T1) at 325 K could be ascribed to the oxygen vacancies. The results are similar to previous work which discussed the oxygen vacancy in Sr2SnO4:Sm3+.32 Trap T2 peaking at a higher temperature is considered to be ascribed to the defect Eu·ca. The result coincides with the thermal quenching result, that the oxygen vacancy can more easily release the electrons than the defect Eu·ca in the case of heating. 3.1.4. Discussion of the Abnormal Reduction and Oxygen Vacancy. The reduction of the trivalent rare earth ions by solid-state reaction at high temperature, as a general rule, needs a reducing atmosphere, such as NH3, activated carbon, CO, or H2/N2. The Eu2+ in a special host can be achieved in the air atmosphere, which is called abnormal reduction. This phenomenon has been studied and explained with the model of the charge compensation mechanism.31−33 There are the following observations: (1) the host compounds contain no oxidizing ions; (2) the divalent cations in the hosts were substituted by the trivalent dopant Eu3+; (3) the radius of substituted cation is similar to the divalent Eu2+; (4) the structure of the host compound must have the tetrahedral anion groups (SiO4, BO4, AlO4, SO4, or PO4), which is appropriate for the reduction. The compound MCPO has tetrahedral anion groups PO4 and a rigid crystal structure, so that the reduction of Eu3+ to Eu2+ occurs in the air in MCPO. As Eu3+ are substituted for Ca2+ and reduced to Eu2+, they are closely surrounded by PO4

Figure 5. (a) Temperature-dependent PL spectra of MCPO-A excited at 395 nm; the inset is the enlargements of (a) at around 443 and 612 nm (b). (c) Temperature dependence of the 443 and 612 nm peak intensities in MCPO-A obtained from the spectra in (a).

enlargements at around 443 and 612 nm, respectively. The data from Figure 5(b) are used to observe the change of emission intensity of Eu2+ and Eu3+, respectively. The PL intensity at 443 nm (Eu2+) and 613 nm (Eu3+) as two functions of temperature are plotted in Figure 5(c), respectively. When the temperature increases to 250 °C, the emission intensity is 0.57 of its initial intensity at room temperature 20 °C for Eu2+, and it is 0.5 for Eu3+. Compared with the reported phosphors, MCPO-A has relatively better thermostability, which is promising for applications. A peculiar phenomenon, as the temperature rises from 20 to 125 °C, the peak intensity at 443 nm increases as shown in Figure 5(c). It may result from the defects in the sample such as the oxygen vacancy and defect Eu·ca mentioned above. When the electrons escape from the potential well resulting from the oxygen vacancy, the defect Eu·ca with a positive effective charge can easily get the electron; then the Eu3+ becomes Eu2+ and emissions show the characteristic band of Eu2+. When temperature continues to rise, the emission intensity of Eu2+ decreases because the population of higher vibration levels, the density of phonons, and the probability of nonradiative transfer were increased due to increasing temperature. The defect Eu·ca also cannot hold the electron because of the high energy when the temperature increases from 225 to 250 °C, so the emission 10399

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403

Article

Inorganic Chemistry

excited by electron beam (processes 3, 4, and 5) and thermal (process 6). The reason the intensity of curve T1 is stronger than that of curve T′ in Figure 6 could also be explained by Figure 7. There is no defect Vca ″ in the undoped sample but V··O, and the V··O cannot trap the electrons from V″ca, so the number of electrons escaping from the V··O is less than the Eu-doped sample. Figure 7 just gives a general explanation, and there could be some other energy transfer process, such as the energy transfer from Eu2+ to Eu3+ or from the lattice to Eu3+, that result in the emission of Eu3+. 3.2. Effect of the Oxygen Vacancy on the Reduction of Eu3+ in MCPO. Oxide phosphor materials can easily produce oxygen vacancies by solid-state reaction in oxygenpoor atmosphere.36,37 To further study and prove the effect of oxygen vacancy on the reduction of Eu in MCPO host, the samples separately sintered in N2 and air atmosphere for different lengths of sintering time were prepared and discussed. 3.2.1. XRD Patterns. From Figure 8, it is observed that the relative intensities and positions of the main diffraction peaks for all the samples are consistent with Mg3Ca3(PO4)4 except for the sintered for 2 h sample in Figure 8(a). This indicates that the single phase cannot be obtained by sintering for 2 h. The other samples are all in the pure Mg3Ca3(PO4)4 phase. The movement of the diffraction peak at 23.789 degree was shown in (b) and (d), indicating the shift to lower 2θ direction due to similar reasons as in Figure 1(b). The different situations for the diffraction peak shift could also demonstrate the change of the ratio of Eu3+ to Eu2+. 3.2.2. Luminescent Properties of MCPO:Eu Separately Sintered in N2 and Air Atmosphere for Different Lengths of Time. Figure 9 shows the emission spectra excited at 395 nm of the MCPO:Eu samples sintered in the air atmosphere for different lengths of time. Each spectrum contains two parts, the broad emission of Eu2+ and the sharp peaks of Eu3+. All of the spectra cover the visible part of the whole spectrum. The inset of Figure 9(a) shows that the intensity ratio of the Eu2+ to Eu3+-related emission, designated as I(Eu2+)/I(Eu3+), changes with increasing sintering time despite that the doping amount of Eu remains constant. This indicates that the quantity of Eu2+ increases as the length of sintering time increased from 4 to 6 h, and then decrease from 6 to 20 h. The result is not consistent with the report of ref 32 which described the abnormal reduction of CaZr(PO4)2:Eu, and the reason could be the increase of oxygen vacancy content. In the equations mentioned previously, eq 3 is dominant when the length of sintering time increases from 4 to 6 h, but eq 5 is dominant when the length of sintering time continues to increase from 6 to 20 h, because the oxygen vacancy content increases with increasing sintering time. The CIE color coordinates are shown in Figure 9(b), which are calculated according to the data of the spectra in Figure 9(a). With increasing sintering time, the color coordinates range from (0.211, 0.132) to (0.422, 0.230), from bluish-violet moving toward the direction of the orange-red light region. This is ascribed to the decrease of Eu 2+ /Eu 3+ ratio corresponding with the inset of Figure 9(a). These results apparently indicate that the hue of the MCPO:Eu phosphors is tunable by controlling the length of sintering time. Figure 10 displays the PL spectra of the MCPO:0.05Eu samples sintered in the oxygen-deficient atmosphere (N2) for different lengths of time. Compared with the Figure 9(a), Figure 10 only has the profile which presents a few sharp peaks in the range from 570 to 700 nm, which is attributed to the 4f−

tetrahedra, which could effectively shield the attack of oxygen on Eu2+. Because the valence states of Eu3+ and Ca2+ are unequal, when trivalent Eu3+ are doped into MCPO, two Eu3+ would be needed to substitute for three Ca2+ (the total charge of two trivalent Eu3+ is equal to that of three Ca2+), as shown by eq 1. Hence, two positive defects of Eu·ca and one vacancy defect of Vca ″ with two negative charges would be created by each substitution of every two Eu3+ in MCPO.43 The two Eu·ca defects become acceptors of the electrons, while the vacancy Vca ″ would act as a donor of electrons. Consequently, the negative charges in the vacancy defects of Vca ″ would be transferred to the Eu3+ sites and reduce Eu3+ to Eu2+, due to thermal stimulation. The whole process can be expressed by the following eqs 1, 2, and 3:25 3Ca 2 + + 2Eu 3 + → 2Eu·Ca + V″Ca V″Ca →

× VCa

+ 2e

−

(1) (2)

× 2Eu·Ca + 2e− → 2EuCa

(3)

O2 − → V ··O

(4)

× V ··O + 2e− → VO

(5)

Moreover, by heat treatment under the poor oxygen atmosphere, the oxygen vacancy defects could be formed in some host (eq 4). Oxygen vacancy formation of the defect level is at the donor level, which is close to the bottom of the conduction band position, and the oxygen vacancy defect V··O is anion vacancy, whose effective charge is positive, so will attract electrons (eq 5). Based on the above analysis, the reduction from Eu3+ to Eu2+ is the defect Eu·ca obtaining one electron (eq 3). So the two processes (eq 3 and 5) are competitive. The oxygen vacancy can influence the reduction from Eu3+ to Eu2+. This phenomenon has been shown in Figure 2(b) in which the sample MCPO-H was sintered in an oxygen-poor atmosphere (H2/N2). The process of abnormal reduction (process 1), and the process in which the oxygen vacancy influences the reduction of the Eu3+ to Eu2+ in MCPO:Eu phosphor (process 2) during the preparation, were shown in Figure 7 (Section A). Process 2 weakens process 1 corresponding to eqs 5 and 3 when the number of defect V··O increases. In Figure 7, section B shows the effect of oxygen vacancy on the emission of Eu2+ in MCPO

Figure 7. Schematic diagram for the effect of the oxygen vacancy on the reduction from Eu3+ to Eu2+ (Process A) and on the emission of Eu2+ in materials. (CB − conduction band; VB − valence band; V··O − oxygen vacancy; V″Ca − Ca2+ vacancy; Eu·Ca− the defect Eu3+ substitute Ca2+; V×O − oxygen vacancy captured electrons). 10400

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403

Article

Inorganic Chemistry

Figure 8. XRD of samples MCPO:0.05Eu sintered in air (a) and N2 (c) atmosphere for different lengths of sintering time (2 h, 4 h, 6 h, 10 h, 20 h); movement of the diffraction peak at 23.789 degree (b,d).

4f transitions of Eu3+, except for the sample sintered for 4 h which has a weak emission of Eu2+. According to the research by Pei et al. the decrease of emission intensities of the Eu2+ with the change in preparation atmospheres from N2 to O2 was caused by the decrease of Eu2+ concentrations in the compounds, since the total amount of Eu is 1% and below the quenching concentrations of Eu2+ luminescence.27 However, it is different with those samples, because the oxygen vacancy defects can form easily in the oxygen-poor atmosphere for the host MCPO. The oxygen vacancy could weaken the reduction of Eu3+ to Eu2+ in MCPO:Eu samples, as we described earlier. According to some research results, it is known that the morphology of samples can influence luminescent properties, so that we described the morphology of the samples by FESEM. The FESEM pictures are shown in Figure S1 (Supporting Information). It is obvious that the particle size is not quite uniform and the particle shape is irregular, because the method of preparation is a solid-state reaction. By comparing the morphology of these samples, there is no great difference, which indicates that the change of emission of Eu in MCPO has nothing to do with the morphology. So, it is the oxygen vacancy which results in the change of the emission spectra as we discussed above.

4. CONCLUSION In summary, MCPO: 0.05Eu phosphors with the coexistence of Eu2+ and Eu3+ were obtained by solid-state reaction in the air atmosphere. The UV, VUV, and CL spectra show the samples’ optical property. The abnormal reduction of Eu3+ to Eu2+ is explained by a charge compensation model. The optical spectral results indicate that compared with the air-sintered sample, the emission intensity of Eu2+ in samples sintered in N2 atmosphere decreased significantly, which is no concern for the morphology. The Eu3+ cannot be reduced completely. This is manifested by relation of the reduction of Eu3+ to Eu2+ in the MCPO:Eu phosphor to the creation of the oxygen vacancies. By changing the length of sintering time, a tunable luminescence is identified on the basis of the variable ratio of Eu2+ to Eu3+-related emission under 395 nm UV light excitation. Therefore, the oxygen vacancy defects are confirmed to weaken the reduction of Eu3+ to Eu2+ in the MCPO lattice. The temperature-dependent PL spectra show the high thermal stability and good light color at high temperature. This study of MCPO:Eu could provide new ideas for design and fabrication of novel luminescent materials by changing the preparation conduction for LEDs, FEDs, and PDPs. 10401

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403

Inorganic Chemistry

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 931 8912772. fax: +86 931 8913554. ORCID

Yuhua Wang: 0000-0002-5982-8799 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work is supported by Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003), the National Natural Science Funds of China (Grant no. 51372105). At the same time, thanks for the support of Gansu Province Development and Reform Commission. Laboratory of Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

(1) Birkel, A.; Denault, K. A.; George, N. C.; Doll, C. E.; Hery, B.; et al. Rapid microwave preparation of highly efficient Ce3+-substituted garnet phosphors for solid state white lighting. Chem. Mater. 2012, 24, 1198−1204. (2) Liu, X. L.; Liu, Y. X.; Yan, D. T.; Zhu, H. C.; Liu, C. G.; et al. Single-phased white-emitting 12CaO·7Al2O3: Ce3+, Dy3+ phosphors with suitable electrical conductivity for field emission displays. J. Mater. Chem. 2012, 22, 16839−16843. (3) Wang, D. Y.; Chen, Y. C.; Huang, C. H.; et al. Photoluminescence investigations on a novel green-emitting phosphor Ba3Sc(BO3)3: Tb3+ using synchrotron vacuum ultraviolet radiation. J. Mater. Chem. 2012, 22, 9957−9962. (4) Abrams, B. L.; Holloway, P. H. Role of the surface in luminescent processes. Chem. Rev. 2004, 104, 5783−5801. (5) Yu, J.; Hao, Z.; Zhang, X.; Luo, Y.; Zhang, J. A new emission band of Eu2+ and its efficient energy transfer to Mn2+ in Sr2Mg3P4O15: Mn2+, Eu2+. Chem. Commun. 2011, 47, 12376−12378. (6) Chang, C. K.; Chen, T. M. Sr3B2O6: Ce3+, Eu2+: a potential single-phased white-emitting borate phosphor for ultraviolet lightemitting diodes. Appl. Phys. Lett. 2007, 91, 081902. (7) Guo, H.; Zhang, H.; Li, J.; Li, F. Blue-white-green tunable luminescence from Ba2Gd2Si4O13: Ce3+, Tb3+ phosphors excited by ultraviolet light. Opt. Express 2010, 18, 27257−27262. (8) Chung, H. Y.; Lu, C. H.; Hsu, C. H. Preparation and Photoluminescence Properties of Novel Color-Tunable MgY4Si3O13: Ce3+, Tb3+ Phosphors for Ultraviolet Light-Emitting Diodes. J. Am. Ceram. Soc. 2010, 93, 1838−1841. (9) Zhu, G.; Wang, Y. H.; Ci, Z. P.; Liu, B. T.; et al. Ca5La5(SiO4)3(PO4)3O2: Ce3+, Mn2+: a color-tunable phosphor with efficient energy transfer for white-light-emitting diodes. J. Electrochem. Soc. 2011, 158, J236−J242. (10) Geng, D. L.; Li, G. G.; Shang, M. M.; Yang, D. M.; et al. Color tuning via energy transfer in Sr 3 In(PO 4 ) 3 : Ce 3+ /Tb 3+ /Mn 2+ phosphors. J. Mater. Chem. 2012, 22, 14262−14271. (11) Tang, W. J. K.; Wang, K.; Bi, X. H.; Chen, D. H. Synthesis of CaTiO3: Pr, Al phosphors by a peroxide-based route and their photoluminescence properties. J. Mater. Sci. 2007, 42, 9915−9919. (12) Lee, S. K.; Chang, H. J.; Han, C. H.; Kim, H. J.; et al. Electronic Structures and Luminescence Properties of YNbO4 and YNbO4: Bi. J. Solid State Chem. 2001, 156, 267−273. (13) Park, J. H.; Park, B. W.; Park, K. W.; et al. Cathodoluminescent and thermal properties of carbon nanotube−ZnS: Cu, Al phosphor composites. Solid State Commun. 2008, 148, 573−576. (14) Ozawa, L. Cathodoluminescence and Photoluminescence: Theories and Practical Applications; CRC Press: Boca Raton, FL, 2007.

Figure 9. (a) PL spectrum (ex = 395 nm) of MCPO:0.05Eu samples sintered in air for different lengths of time (4 h, 6 h, 10 h, and 20 h); the inset is the time dependence of the ratio of peak intensity at 443 and 612 nm in samples obtained from the spectra in (a). (b) Standard CIE coordinate-color graph of samples in (a).

Figure 10. PL spectrum (ex = 395 nm) of MCPO:0.05Eu samples sintered in N2 atmosphere for different length of time (4 h, 6 h, 10 h, and 20 h).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01315. FESEM pictures of samples with different lengths of sintering time in the air and N2, respectively (PDF) 10402

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403

Article

Inorganic Chemistry

(37) Su, Q.; Liang, H. B.; Hu, T. D.; Tao, Y.; Liu, T. Preparation of divalent rare earth ions in air by aliovalent substitution and spectroscopic properties of Ln2+. J. Alloys Compd. 2002, 344, 132−136. (38) Nogami, M.; Yamazaki, T.; Abe, Y. Fluorescence properties of Eu3+ and Eu2+ in Al2O3−SiO2 glass. J. Lumin. 1998, 78, 63−68. (39) Zhang, X.; et al. The relationship between occupied sites and Mn2+ quantum efficiency in dual-emission Mg3Ca3(PO4)4:Eu2+, Mn2+ phosphors. Mater. Sci. Eng., B 2014, 187, 26−31. (40) Chang, N. C.; Gruber, J. B. Spectra and energy levels of Eu3+ in Y2O3. J. Chem. Phys. 1964, 41, 3227−3234. (41) Cooke, D.; Bennett, B.; et al. Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5: Ce3+. J. Lumin. 2004, 106 (2), 125−132. (42) Glodo, J.; Wojtowicz, A. Thermoluminescence and scintillation properties of LuAP and YAP. J. Alloys Compd. 2000, 300, 289−294. (43) Pei, Z.; Zeng, Q.; Su, Q. A study on the mechanism of the abnormal reduction of Eu3+→ Eu2+ in Sr2B5O9Cl prepared in air at high temperature. J. Solid State Chem. 1999, 145, 212−215.

(15) Li, G. G.; Xu, X. G.; Peng, C.; Shang, M. M.; et al. Yellowemitting NaCaPO4: Mn2+ phosphor for field emission displays. Opt. Express 2011, 19, 16423−16431. (16) Jeong, I. K.; Park, H. L.; Mho, S. L. Photoluminescence of ZnGa2O4 mixed with InGaZnO4. Solid State Commun. 1998, 108, 823−826. (17) Jiao, H.; Liao, F. H.; Tian, S. J.; Jing, X. P. Luminescent Properties of Eu3+ and Tb3+ Activated Zn3Ta2O8. J. Electrochem. Soc. 2003, 150, H220−H224. (18) Meijerink, A.; Nuyten, J.; Blasse, G. Luminescence and energy migration in (Sr, Eu)B4O7, a system with a 4f7-4f65d crossover in the excited state. J. Lumin. 1989, 44, 19−31. (19) Yamamoto, H.; Matsuzawa, T. Mechanism of long phosphorescence of SrAl2O4:Eu2+, Dy3+ and CaAl2O4:Eu2+, Nd3+. J. Lumin. 1997, 72, 287−289. (20) Kim, C.; Kwon, I.; et al. Phosphors for plasma display panels. J. Alloys Compd. 2000, 311, 33−39. (21) Wang, M.; Wang, D.; Lu, G. Research on fluorescence spectra and structure of single-phase 4SrO·7Al2O3:Eu2+ phosphor prepared by solid-state reaction method. Mater. Sci. Eng., B 1998, 57, 18−23. (22) Machida, K.; Adachi, G.; Shiokawa, J. Luminescence properties of Eu(II)-borates and Eu2+-activated Sr-borates. J. Lumin. 1979, 21, 101−110. (23) Zeng, Q. H.; Pei, Z. W.; Wang, S. B.; Su, Q. The reduction of Eu3+ in SrB6O10 prepared in air and the luminescence of SrB6O10:Eu. J. Alloys Compd. 1998, 275−277, 238−241. (24) Pei, Z. W.; Zeng, Q. H.; Su, Q. The application and a substitution defect model for Eu3+→ Eu2+ reduction in non-reducing atmospheres in borates containing BO4 anion groups. J. Phys. Chem. Solids 2000, 61, 9−12. (25) Su, Q.; Zeng, Q. H.; Pei, Z. H. Preparation of borates doped with divalent rare earth ions (RE2+) in air and spectroscopy of divalent rare earth ions (RE2+= Sm, Eu, Tm, Yb). Chin. J. Inorg. Chem. 2000, 16, 293−298. (26) Madhusoodanan, U.; Jose, M. T.; Lakshmanan, A. R. Development of BaSO4:Eu thermoluminescence phosphor. Radiat. Meas. 1999, 30, 65−72. (27) Peng, M. Y.; Pei, Z. W.; Hong, G. Y.; Su, Q. Study on the reduction of Eu3+→ Eu2+ in Sr4Al14O25:Eu prepared in air atmosphere. Chem. Phys. Lett. 2003, 371, 1−6. (28) Peng, M. Y.; Hong, G. Y. Reduction from Eu3+ to Eu2+ in BaAl2O4:Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl2O4:Eu. J. Lumin. 2007, 127, 735−741. (29) Liu, B. T.; Wang, Y. H.; Zhou, J.; Zhang, F.; Wang, Z. F. The reduction of Eu3+ to Eu2+ in BaMgAl10O17:Eu and the photoluminescence properties of BaMgAl10O17:Eu2+ phosphor. J. Appl. Phys. 2009, 106, 053102. (30) Peng, M. Y.; Pei, Z. W.; Hong, G. Y.; Su, Q. The reduction of Eu3+ to Eu2+ in BaMgSiO4:Eu prepared in air and the luminescence of BaMgSiO4; Eu2+ phosphor. J. Mater. Chem. 2003, 13, 1202−1205. (31) Gao, G. J.; Reibstein, S.; Peng, M. Y.; Wondraczek, L. Tunable dual-mode photoluminescence from nanocrystalline Eu-doped Li2ZnSiO4 glass ceramic phosphors. J. Mater. Chem. 2011, 21, 3156−3161. (32) Xu, X. H.; Wang, Y. H.; Gong, Y.; Zeng, W.; Li, Y. Q. Effect of oxygen vacancies on the red phosphorescence of Sr2SnO4: Sm3+ phosphor. Opt. Express 2010, 18, 16989−16994. (33) Dexter, D. L. A theory of sensitized luminescence in solids. J. Chem. Phys. 1953, 21, 836−850. (34) Trojan-Piegza, J.; Zych, E. Afterglow luminescence of Lu2O3:Eu ceramics synthesized at different atmospheres. J. Phys. Chem. C 2010, 114, 4215−4220. (35) Li, Y.; Deng, D.; Wang, Q.; Li, G.; et al. Luminescent properties of Mg3Ca3(PO4)4:Eu2+ blue-emitting phosphor for white light emitting diodes. J. Lumin. 2012, 132, 1179−1182. (36) Xia, Z. G.; Zhuang, J. Q.; Liao, L. B.; et al. Synthesis and luminescence properties of Ba2Gd(BO3)2Cl:Eu2+ phosphor. J. Electrochem. Soc. 2011, 158, J359−J362. 10403

DOI: 10.1021/acs.inorgchem.7b01315 Inorg. Chem. 2017, 56, 10396−10403