Recoverable and Unrecoverable Bi3+-Related Photoemissions

Oct 5, 2016 - The single particle diagnosis further reveals significant intensity reduction and peak shift for nearly half of the ScVO4:Bi3+ particles...
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Recoverable and Unrecoverable Bi3+-Related Photoemissions Induced by Thermal Expansion and Contraction in LuVO4:Bi3+ and ScVO4:Bi3+ Compounds Fengwen Kang,†,‡ Mingying Peng,*,† Dang Yuan Lei,*,‡ and Qinyuan Zhang† †

The China-Germany Research Center for Photonic Materials and Device, The State Key Laboratory of Luminescent Materials and Devices, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China ‡ Department of Applied Physics, The Hong Kong Polytechnic University Hong Kong, China ABSTRACT: Most substances as thermodynamic law explicitly states will expand or contract upon heating or cooling, respectively, which sometimes may lead to changes in their crystallographic microstructures and therefore unexpected physicochemical and optoelectronic properties. Here, we report an efficient yellow photoemission from a compound of LuVO4:Bi3+, whose peak intensity and position after 11 rounds of yoyo experiments of heating and cooling can recover to their initial states. In sharp contrast, ScVO4:Bi3+, though crystallographically isomorphous to LuVO4, exhibits a completely different scenario, and it, submitted to the same thermal treatment, shows unrecoverable changes in both peak position and intensity of the red emission. In order to unravel why bismuth responds so differently upon the same thermal stimuli in the two isomorphous compounds, in situ high-temperature X-ray diffraction (HT-XRD), Rietveld refinement, static and dynamic high resolution photoluminescence, scanning electron microscopy, and single particle diagnosis techniques, as well as density functional theory (DFT) calculations have been employed to illustrate the microstructural changes along with environmental temperature. In situ HT-XRD measurements and consequent Rietveld refining analysis clearly illustrates that thermal expansion and contraction can induce permanent crystallographic microstructure changes, e.g., unrecoverable expansion of lattice cell in ScVO4:Bi3+ rather than LuVO4:Bi3+. Such expansion can be considered as an evidence for the removal of oxygen vacancy, which can be promoted by the accelerated oxygen diffusion rate as temperature increases. This, as DFT computation implies, can slightly increase the band gap of ScVO4:Bi3+, and it eventually leads to the unrecoverable blueshift and intensity loss of the red emission peak. The single particle diagnosis further reveals significant intensity reduction and peak shift for nearly half of the ScVO4:Bi3+ particles but not for all randomly selected LuVO4:Bi3+ particles. The diagnosis approach therefore provides a new strategy to distinguish and select the particles with desirable luminous intensity and color purity from a mass of powder mixture and in the meantime potentially gives new insights into unusual luminescence properties in phosphors.

1. INTRODUCTION For crystal phosphors used in white LEDs, particularly in high power LEDs working at temperatures higher than 200 °C, it has been commonly accepted as an essential criterion whether they can maintain desirable photoemission stability at high temperature. All existing phosphors, however, possess a temperature threshold for their structural and luminescent stability. Once the external temperature exceeds the threshold, the photoemission thermal quenching will become unavoidable in most cases. In general, the thermal quenching temperature (T50%), at which the photoemission intensity of a phosphor reduces to half of its intensity at room temperature, serves as a technological standard to evaluate the photoemission stability of the phosphor. For instance, the single-phased white light emitting X2-type Y2SiO5:Eu3+,Bi3+ phosphor has a thermal quenching temperature T50% = 293 °C, and it demonstrates good resistance to external thermal impact.1 So far, hundreds and thousands of phosphors have been discovered, and some of them are available on the market. Nevertheless, their thermal quenching temperatures are © 2016 American Chemical Society

dramatically different. Here are some examples on the emission color, work temperature, and emission intensity as compared to their room-temperature intensity for typical highly efficient nitrides phosphors: AlN:Eu 2+ (blue, 240 °C, ∼50%), 2 Sr2Si5N8:Eu2+ (red, 150 °C, ∼86%),3 Ca-α-sialon (yellow, 230 °C, ∼60%),4 and β-sialon (green, 150 °C, ∼90%).5 At higher temperature, some peculiar photoemission phenomena which always do not appear at room or lower temperature may show up along luminescence quenching processes, for example, (i) monotonic blue shift of photoemission in α-sialon:Yb2+,6 BaAl2Si2O8:Eu2+,7 CaWO4:Bi,8 and CaMoO4:Bi;9 (ii) coexistence of blue-shift and red-shift of Eu2+ emission in Sr2Si5N8:Eu2+;10 (iii) shift of multiple emissions toward different directions in ZnO;11 (iv) anomalous initial photoemission enhancement followed by subsequent quenching in colloidal Received: July 26, 2016 Revised: October 2, 2016 Published: October 5, 2016 7807

DOI: 10.1021/acs.chemmater.6b03062 Chem. Mater. 2016, 28, 7807−7815

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Chemistry of Materials quantum dots,12 LuVO4:Bi13 and KZnF3:Mn;15 (v) reversible or irreversible luminescence degradation in X2-Y2SiO5:Eu,Bi 1 or ZnS/CdS:Mn/ZnS,14 respectively; (vi) broadening line width of photoemission in YAG:Ce16 and Sr8(Si4O12)Cl8:Eu2+;17 and (vii) temperature-dependent energy transfer from one dopant to another in Na2SrMg(PO4)2:Eu2+,Mn2+18 or from host to dopant in ScVO4:Bi,19 and so forth. Moreover, tunable emission colors can be achieved easily by controls over photoemission shift or relative intensity ratio of multiple emission bands. For this, energy transfer processes have to be managed precisely among host, activators, and sensitizers in crystals,19−23 for instance, BaAl2Si2O8:Eu2+,7 LuVO4:Bi3+,13 and ScVO4:Bi3+.19,20 Because of the intricate nature of thermal quenching processes, different models have been proposed on the basis of experimental and theoretical results to qualitatively explain the underlying mechanisms, such as multiple phonon relaxation, thermally activated crossover, or trapping etc.12 However, among them, the configurational coordinate model is the most popular, and it shows that photoemission quenching becomes predominant only as thermal energy is comparable to or higher than activation energy Ea. Here, Ea can be experimentally derived from an Arrhenius equation,20,24 I(T) = I0/[1 + A exp(−Ea/κT)], where I0 stands for the initial photoemission intensity, T is the temperature in Kelvin, I(T) is the integrated intensity at temperature T, A is a constant parameter, and κ is the Boltzmann constant (8.617 × 10−5 eV·K−1). For example, the model can well explain the abnormal thermal quenching, energy transfer from host to Bi3+ as well as population redistribution between the excited states 3P0 and 3P1 of Bi3+ in ScVO4 and LuVO4 as temperature increases.13,19,20 The Arrhenius equation indicates that the activation energy is critical to evaluate the thermal behavior of a phosphor and that it physically depends on changes of crystallographic microstructures induced by thermal expansion and contraction as environmental temperature changes. So far, to our knowledge, few reports have focused on the correlation between the temperature-dependent photoemission behaviors of doped crystalline compounds and their thermally induced structural variations. In this work, we report that after 11 rounds of thermal expansion and contraction, the efficient yellow emission from a Bi3+ doped compound LuVO4:Bi3+ can restore to its initial state in emission wavelength and intensity, while the strong red orange emission from ScVO4:Bi3+ cannot, though these zircon-type compounds are isomorphous to each other. ScVO4:Bi3+ after the heat treatments exhibits the permanent blueshift and intensity loss of the emission. To understand why bismuth behaves so differently in them upon the same thermal stimuli, multiple characterizations have been made such as in situ HT-XRD, Rietveld refinement, static and dynamic high resolution photoemission, scanning electron microscopy, single particle diagnosis technique, etc. The results show that LuVO4:Bi3+ keeps intact after cycle experiments of heating and cooling and that ScVO4:Bi3+ shows unrecoverable structure changes, i.e., lattice cell expansion. Density functional theory (DFT) calculations unravel that the expansion is due to the annihilation of oxygen vacancy. This broadens the bandgap of ScVO4:Bi3+ and consequently leads to the blue shift and intensity loss of the red emission. The single particle diagnosis has been used to monitor the change of each particle in morphology and luminescence, and it reveals significant intensity reduction and peak shift for nearly half of ScVO4:Bi3+ particles but not for all randomly selected LuVO4:Bi3+ particles. In the end, schemes are

proposed for the recoverable and unrecoverable luminescence processes of Bi3+ in the vanadate compounds.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Bismuth doped vanadate compounds were prepared by standard solid state reaction at high temperature. The nominal concentration of Bi3+ dopant ions was kept as 1.00% since it is not higher than the critical concentration.13,19,20 The compound of ScVO4:Bi3+ was synthesized by mixing as-purchased raw chemicals of Sc2O3 (99.99%), NH4VO3 (99.95%), and Bi2O3 (99.999%) at 1100 °C for 4 h in air. LuVO4:Bi3+ was synthesized in a procedure similar to that of ScVO4:Bi3+. The starting material changed from Sc2O3 to Lu2O3 (99.99%). Prior to final sintering at 1100 °C, all mixed powders were preheated at 900 °C for 1 h, which was followed by an intermediate grinding for 15 min to improve sample homogeneity. Final products were ground once again for consequent measurements. To obtain microparticles, ∼0.02 g of compounds of ScVO4:Bi3+ and LuVO4:Bi3+ were dispersed in 10 mL of distilled water, followed by ultrasonic treatment for 3 min. Afterward, one drop of liquid supernatant (∼50 μL) of the solution was dripped onto a Pyrex glass substrate. Because of the surface tension of the drop, the dissolved particles were spread as a monolayer on the upper surface of the substrate, and they were left to naturally dry in air overnight for the single particle diagnosis. 2.2. Characterization Details. In situ high temperature X-ray diffraction (HT-XRD) patterns for all the samples were measured with a high resolution X’Pert Pro diffractometer equipped with a HTK-16 heating platform, a Cu Kα radiation source (λ = 1.5418 Å) at 40 kV and 30 mA. The temperature changed from 25 to 600 °C, with a heating rate of 10 °C·min−1. The samples were held for 30 min at each temperature point for data acquisition. Sample microstructure and morphology were recorded with a Hitachi S-3700N scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer. High resolution static and dynamic photoemission spectra over a temperature range of 25−300 °C were recorded by using an Edinburgh Instruments FLS 920 equipped with a red sensitive photomultiplier Hamamatsu R928 P and a technique of single photon counting in a Peltier cooled house. A 450 W xenon lamp free of ozone was used for the steady state measurements. A pulsed xenon lamp μF900 with an average power of 60 W was used for the dynamic state measurements. For the high temperature luminescence measurements, samples were placed into a circular copper substrate connected to a temperature control setup (Tian Jin OrientKOJI) and heated up at a rate of 4 °C·min−1 and kept at each temperature for 1 h. All the photoemission spectra were calibrated over the lamp intensity and the detector spectral response.

3. RESULTS AND DISCUSSION 3.1. Recoverable and Unrecoverable Bi3+ Photoemissions in Vanadate Compounds. Figure 1a, i and ii illustrates the Bi3+ photoemission spectra of LuVO4:Bi3+ upon excitation at 254 nm at different temperatures. The sample exhibits brilliant yellow lights peaking at 576 nm at room temperature. As temperature gradually rises up from 25 to 300 °C, the photoemission peak blue shifts from 576 to 552 nm with a decrease of intensity by ∼60%, and meanwhile, the band has been broadened significantly. The full width at half-maximum (fwhm) changes from 160 to 178 nm, and it is mainly due to the thermal Doppler effect. As the LuVO4:Bi3+ sample is cooled back to 25 °C, the photoemission intensity and position can recover to their initial values (see Figure 1a, i and ii), and more remarkably, this reversible photoemission response is persistent even after 11 rounds of heating and cooling yoyo experiments (see Figure 1a, iii). This indicates the excellent resistance of LuVO4:Bi3+ to thermal degradation. No permanent irreversible degradation appears in the compound. Consistent with previous reports,19,20 an efficient red emission can be found from ScVO4:Bi3+. Similar to LuVO4:Bi3+, the red 7808

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cooling, and after that, they reach a constant state (see Figure 1b, iii). As compared to the initial state at 25 °C, the loss of photoemission intensity is ∼21% in ScVO4:Bi3+ after the first eight cycles of heating and cooling. On basis of Figure 1, the thermal quenching temperatures T50% and the activation energy Ea values can be evaluated on the basis of the Arrhenius equation, i.e., ∼263 °C and 0.297 eV for LuVO4:Bi3+, and ∼250 °C and 0.286 eV for ScVO4:Bi3+. These values are consistent with previous studies.13,20 The quenching temperatures are comparable to nitride phosphors mentioned in the Introduction, and they are even superior to commercial yellow YAG:Ce16 or (Sr,Ba)2SiO4:Eu2+ phosphors.23 3.2. Crystallographic Microstructural Changes upon Heating and Cooling. In this section, we will study how microstructures of these compounds change upon heating and cooling, and this can help understand why thermally induced blue shift and intensity loss can occur in ScVO4:Bi3+ other than LuVO4:Bi3+. Figure 2a shows the lattice profile of LnVO4 (Ln = Lu or Sc) viewed along axis c. Both compounds comprise one type of Ln and one type of V, which are coordinated by eight and four oxygen atoms, respectively. The polyhedra of [LnO8] and

Figure 1. Photoemission spectra of (a) LuVO4:Bi3+ and (b) ScVO4:Bi3+ at different temperatures during yoyo experiments of (i) heating up from 25 to 300 °C and (ii) cooling back from 300 to 25 °C. Panels iii in a and b show the relative integrated emission intensities for the two samples at 25 and 300 °C after different cycles of heating and cooling. The excitation wavelength is 265 nm.

emission peak of ScVO4:Bi3+ blue shifts from 635 to 621 nm as the temperature increases from 25 to 300 °C, and the fwhm band is broadened from ∼128 nm to ∼145 nm (see Figure 1b, i). However, as compared to LuVO4:Bi3+, the wavelength shift and the width increase become smaller in ScVO4:Bi3+. The integrated emission intensity decreases by ∼85%. Obviously, the loss is higher than that of LuVO4:Bi3+. Surprisingly, the ScVO4:Bi3+ sample fails to recover the photoemission intensity and position after cooling back to 25 °C. The peak slightly blue shifts to 632 nm from 635 nm, and the intensity can recover only to 82.5% of its initial intensity (see Figure 1b, i and ii). The intensities at 25 and 300 °C keep decreasing until the eighth cycle of heating and

Figure 2. (a) Crystallographic structure of LnVO4 (LnLu, Sc) viewed along axis c. (b) Distances among Lu, V, and O atoms; (a) was drawn on the basis of ICSD#78083 and (b) on the basis of ICSD#78073 for LuVO4 and ScVO4, respectively. Coordination environments of Ln and V atoms are illustrated at the bottom of a. 7809

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respectively. It is obvious that gradually heating up LuVO4:Bi3+ from 25 to 600 °C induces a shift of the diffraction peak from 33.61° to 33.43° and simultaneously broadens the peak from fwhm of 0.21° to 0.26°. Similar results appear in ScVO4:Bi3+ where the (020) peak shifts from 26.11° to 25.79° and fwhm increases from 0.08° to 0.17°. However, cooling these samples from 600 °C back to 25 °C can restore the (112) peak of LuVO4:Bi3+ to its original angle but not the (020) peak of ScVO4:Bi3+ (see Figure 3 and Figure 4a). To quantify crystallographic microstructural changes, Rietveld refinement has been utilized to analyze HT-XRD data at different temperatures. This was performed with a FullProf Suite Program (version July-2011). The refinement started with the crystallographic data of LuVO4 from Inorganic Crystal Structure Database (ICSD) card No.78083 and ScVO4 from ICSD No.78073, and it converges well for each refining to the XRD pattern at each temperature. Goodness of fit (GOF) is always not higher than 2.30. Table 1 exemplarily lists the refinement parameters for the samples heated at 200 or 500 °C, respectively. For LuVO4:Bi3+ at 500 °C, the refinement produced the profile factor Rp = 5.32%, weighted profile factor Rwp = 7.03%, expected weighted profile factor Rexp = 4.13%, Bragg factor RBragg = 3.17%, crystallographic factor RF = 1.73% and GOF = 2.18; while for ScVO4:Bi3+ at 200 °C, Rp = 6.48%, Rwp = 9.79%, Rexp = 5.46%, RBragg = 5.23%, RF = 2.01%, and GOF = 2.22 (see Table 1). The calculated results have been compiled in Figure 4 and Figure 5, which include the simulated XRD patterns, lattice parameters, cell volume, and bond lengths at different temperatures. The comparison between experimental and theoretical XRD patterns confirms once again the samples are single-phased compounds, which belong to the tetrahedral crystal system with a space group I41/amd (Figure 4b, i and c, i). Generally, as Figure 3 shows, diffraction peaks shift toward lower or higher angle as temperature increases or decreases, respectively. As Bragg’s law says, the shift means that the interplanar distances become larger or shorter, and the cell expands or contracts accordingly. Figure 4 and Figure 5 vividly illustrate such changes. As the temperature increases up to 600 °C, the lattice parameters a, b, c, cell volume (V), and bond lengths increase linearly for LuVO4:Bi3+ and ScVO4:Bi3+, and they decrease also linearly but in different ways for the two compounds as cooled back to 25 °C (see Figure 4 and Figure 5). For LuVO4:Bi3+, these values can be recoverable to the initial state before the cycle experiment within experimental errors, while for ScVO4:Bi3+, none of the a, b, c, and V can (see Figure 4). After such heating and cooling, all of them are larger than before at the same temperature. The change of c is more significant than a or b. As a consequence, V slightly expands. 3.3. Lattice Cell Expansion of ScVO4:Bi3+ after Heating and Cooling Cycles. The slight expansion of the lattice cell is induced permanently after heating and cooling cycles, and we think this structural degradation should be responsible for the unrecoverable Bi3+ emission in ScVO4:Bi3+. How does the structural degradation happen in the compound? DFT calculations have been made with the Vienna ab initio simulation package (VASP), and the results illustrate that as one bismuth atom substitutes for one Sc atom, the band gap can be reduced to 2.68 eV from 2.70 eV.19,20,29 Meanwhile, the formation energy of an oxygen vacancy can be lowered by 0.8 eV. This means oxygen can be more easily lost in the doped compound as compared to the sample without Bi doping.30,31 Our calculation shows a new gap state lying 1.33 eV below the conduction band bottom if one oxygen is lost in the vicinity of bismuth. This can explain why red

[VO4] are connected via common oxygen atoms and are featured with different distances between the central cations (e.g., Ln or V) and O ligands. For instance, there are two different distances between neighboring Sc and V atoms, 3.1174 and 3.3840 Å, and also two different Sc−O bond lengths, which are 2.4100 and 2.2516 Å, respectively (see Figure 2b). Since all of the atoms are linked with each other directly or indirectly such as Ln···O···V, V···O···V, and Ln···O···Lu etc., slight change in one linkage when responding to an external stimulus can affect the other bonds accordingly. To in situ monitor the structural change of ScVO4:Bi3+ and LuVO4:Bi3+ along heating and cooling processes, their HT-XRD patterns were collected and depicted as Figure 3,

Figure 3. In situ HT-XRD patterns of (a) LuVO4:Bi and (b) ScVO4:Bi at different temperatures during cycle experiments of heating and cooling between 25 and 600 °C. Standard ICSD data are included at the bottom of each figure for reference. The narrower figure on the right is the magnified part of the figure on the left.

where standard ICSD data for these compounds are included at the bottom of each panel for reference. The coincidence of the measured diffraction peaks with standard ICSD patterns indicates that all samples are single phase, crystallizing in tetrahedral space group I41/amd. Applied temperature within the range of 25−600 °C does not give rise to the secondary phase, and it also cannot lead to structural decomposition even at 600 °C. This indicates that both compounds are very stable and that they have good resistance to thermal impact. To illustrate the structural changes with temperature more clearly, we list the diffraction peaks which correspond to (112) of LuVO4:Bi3+ and (020) of ScVO4:Bi3+ in the right panels of Figure 3a and b, 7810

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for vacancy sites can at the same time lead to the expansion of the lattice cell as can be seen from Figures 2 to 5. The volume expansion can elongate the distance between bismuth and surrounding atoms and weaken their interaction and improve the degeneracy of excited levels of Bi3+. This can also contribute to the emission blue shift. The removal process of oxygen vacancy happens gradually and completes after nine rounds of heating and cooling. This is why the photoemission stops blue shifting afterward (see Figure 1b). 3.4. Schemes for Different Luminescence Processes of Bi3+ in the Vanadates. Figure 6 illustrates the schemes for recoverable Bi3+ photoemissions from LuVO4:Bi3+ (a) and unrecoverable Bi3+ photoemissions from ScVO4:Bi3+ (b). When the environmental temperature exceeds 25 °C, the emission from VO43− groups cannot be observed in the two compounds even when excited at their intrinsic absorptions, such as λex = 254 or 265 nm. What we observed is always the efficient emissions from Bi3+. This means the efficient energy transfers (ET) from the VO43− groups to Bi3+ (see Figure 6). Once the VO43− groups are excited into their charge transfer state as Figure 6 shows, they will donate their energy via “ET” processes to the Bi3+ ions. Consequently, electrons residing on the ground states 1S0 of Bi3+ ions can be lifted preferably to the excited state 3P1. They relax subsequently to the bottom of 3P1 or 3P0 levels, and some of them afterward release energy in form of yellow or red emissions peaking at 576 or 635 nm, respectively. As the temperature rises, the coupling interaction between Bi3+ and host could be strengthened by the enhanced vibration of the Bi−O bond and it also could be weakened by the elongation of Bi−O bond length due to the expansion of the lattice cell. Whether the overall interaction increases or not depends on the trade-off between the Bi−O bond vibration and elongation (see Figures 4 and 5). In vanadates, we suspect the bond elongation should dominate over the vibration. Thus, the interaction will be weakened as the temperature increases to 300 °C, and it will lift the energy level of the excited states as Figure 6 shows, and the emissions of Bi3+ blue shift to 552 and 621 nm for the compounds LuVO4:Bi3+ and ScVO4:Bi3+, respectively. Once the temperature decreases back to 25 °C, the crystal structure of LuVO4:Bi3+ can restore to its initial state and along with its photoemission. Because of oxygen vacancy removal, the lattice cell of ScVO4:Bi3+ expands after the cycles of heating and cooling, and it leads to the unrecoverable blue shift of the emission to 632 nm as compared to its original state (see Figures 1 and 6). 3.5. Microscopic Imaging and Spectroscopy of Bi3+ Doped Vanadates by a Single Particle Diagnosis Approach. Xie et al. and his co-workers in 2014 first reported a single particle diagnosis approach, and now it has developed as an efficient approach to discover novel luminescent materials from samples where multiple crystal phases coexist.25−28 With this approach, Xie et al. discovered a series of new blue-emitting phosphors for solid state lighting such as Sr3Si8‑xAlxO7+xN8‑x:Eu2+26 and Sr2.95Eu0.05Si5.5Al2.5O9.5N5.5,26 and they recently separated a new blue phosphor BaSi4Al3N9:Eu2+ and a yellow phosphor BaSi4Al3N9:Eu2+ from a ternary system Ba3N2−Si3N4−AlN where many crystalline phases can survive.25 Clearly, this technique is powerful to distinguish particles from particles at the microscopic scale and reveal what we cannot detect easily in bulk materials. Inspired by these findings, we built a data acquisition system on the basis of a single particle diagnosis technique to collect Bi3+ photoemissions from each particle of LuVO4:Bi3+ or ScVO4:Bi3+ at room temperature, as shown in Figure 7. This could be helpful

Figure 4. Typical refined XRD results of LuVO4:Bi (a) and LuVO4:Bi (b) as well as 2θ dependence of the diffraction peak of (i) (112) of LuVO4:Bi (c) and (ii) (020) of ScVO4:Bi (c) on temperature. For LuVO4:Bi, (a) (i) XRD pattern (x) of LuVO4:Bi at 500 °C, Rietveld refining results (−), Bragg reflections (|), and the profile difference between observed and calculated values (−). (ii−iv) Dependence of lattice parameters (a, b, and c) and cell volume on temperature during heating and cooling processes. For ScVO4:Bi, (b) (i) XRD pattern (x) of ScVO4:Bi at 200 °C, Rietveld refining results (−), Bragg reflections (|), and the profile difference between observed and calculated values (−). (ii−iv) Dependence of lattice parameters (a, b, and c) and cell volume on temperature during heating and cooling processes. The slope for each linear fitting has been inserted into each panel.

emission of Bi can be observed in ScVO4:Bi3+. The oxygen loss will lead to the slight collapse around bismuth and therefore the overall shrinkage of the cell. As ScVO4:Bi3+ is heated up in air, the oxygen diffusion rate can be accelerated along with temperature increase, and it, therefore, can fill the oxygen vacancy sites gradually. This leads to the decrease of oxygen vacancy content, which, in turn, increases the bandgap of the sample and eventually the blueshift of the photoemission of Bi3+ (see Figure 1). The substitution of oxygen 7811

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Chemistry of Materials Table 1. Exemplary Refinement Results for LuVO4:Bi3+ and ScVO4:Bi3+ at 500 and 200 °C, Respectivelya LuVO4:Bi3+

LuVO4 temperature ICSD no. crystal system space group cell parameters (Å) cell volume (Å3) Z profile factor Rp weighted profile factor Rwp expected weighted profile factor Rexp Bragg factor RBragg crystallographic factor RF GOF a

ScVO4

500 °C no. 78083

ScVO4:Bi3+ 200 °C

no. 78073 tetragonal I41/amd (141) a = b = 7.0981 a = b = 6.0780 c = 6.2996 c = 6.1345 317.39 282.03(1) 4 5.32% 7.03% 4.13% 3.17% 1.73% 2.18

a = b = 7.0254 c = 6.2347 307.72

a = b = 6.7891 c = 6.1351 282.78 6.48% 9.79% 5.46% 5.23% 2.01% 2.22

Data from ICSD standard cards are included for reference.

Figure 5. Bond length dependence of V−O, Ln-O1, and Ln-O2 on temperature in LnVO4:Bi3+: (a) LnLu and (b) LnSc. The bond lengths are calculated on the basis of the refined results of Figure 4.

still be detected from them (see Figure 8), comparable to bulk samples.13,19,29 This indicates that these samples have good tolerance to moisture. After being dispersed onto the glass substrate, particles with different sizes can be observed under bright field (see panel ii in Figure 8a and c). We randomly selected the LuVO4:Bi3+particles with different morphologies (numbered them as particles 1 to 6) to investigate whether the morphologies can influence the photoemission of Bi3+. They exhibit the same bright yellow color and the same emission peak as those exposed to an illumination of 254 nm, but the photoemission intensity depends highly on the size of these particles. Larger particles exhibit stronger emissions. So we see the emission intensity is gradually enhanced in the order of particles 3 → 2→5 → 4→1 → 6 where the particle

to see the difference in photoluminescence from each individual particle before and after cycles of heat treatments, and therefore better understand why bismuth behaviors exhibit such different responses in the two samples. In this system, when incandescent or UV lights are incident onto samples on glass substrate, reflected or emitted lights will be collected by an objective lens. A beam splitter will split the collected lights into two halves. One half will reach a digital camera and sample images can thus be recorded. Another half will go to a CCD spectrometer, and photoemission spectra can be collected from it (see Figure 7). The images and spectra in Figure 8 were measured in such a way for particles of LuVO4:Bi3+ and ScVO4:Bi3+. Though the phosphor particles have been kept inside water for 30 days before deposition on glass, bright yellow and red emissions can 7812

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Figure 7. Home-built data acquisition system on the basis of single particle diagnosis technique to collect Bi3+ photoemissions from each particle of LuVO4:Bi3+ or ScVO4:Bi3+ at room temperature.

images show, the particle size lies between 2 and 10 μm, and the particle surface keeps smooth all the time (see Figure 9). This implies such heat treatment does not lead to permanent changes in LuVO4:Bi3+ either in photoemission or morphology. Different from LuVO4:Bi3+, two kinds of ScVO4:Bi3+ particles can be distinguished upon exposure to 254 nm illumination. We randomly selected these kinds of particles and coded the red ones as R1 to R5 and the yellow ones as Y1 to Y5 (see Figure 8c). The emission spectra were collected for these particles and listed as Figure 8d. The red particles exhibit an emission peaked at 640 nm, while the yellow particles exhibit the emission at 630 nm. Both emission intensities of the red and yellow particles depend strongly on particle size. Similar to LuVO4:Bi3+, the larger particle always shows stronger emission intensities in ScVO4:Bi3+. For instance, the emission of the larger particle Y4 is about 9 times stronger than the smaller Y2 (see Figure 8c and d). This comparison points out a possibility for achieving efficient phosphors by controlling particle sizes. When ScVO4:Bi3+ particles were heated at 600 °C for 3 h, R1 to R5 and Y1 to Y5 show different responses to the thermal stimuli. The emissions from the red particles all exhibit a blue shift, and the peak shifts from 640 to 630 nm (see Figure 8d). In contrast, the emissions from the yellow particles show no peak shift. This means that heat treatment can convert all of the red particles into the yellow particles. We believe that the red particles should accommodate much higher content of oxygen vacancy than the yellow particles. The annealing can promote the removal of these defects by accelerating the diffusion rate of oxygen at 600 °C and eventually improve the crystal local ordering, which is reflected by the narrowed photoemission line width. For the red particles, oxygen vacancy defects are randomly distributed throughout the samples, and they can lower the local ordering to some degree. As a consequence, their fwhm is ∼171 nm, which is much broader than ∼128 nm for the yellow particles (see Figure 8d). The broadening in red particles should be due to the disorderly distribution of oxygen vacancy defects in the lattice. For all the particles, despite the shift of emission peak, the intensities are all weakened after the heat treatment at 600 °C (see Figure 8d). We also examined the sample morphologies with SEM images, which confirm that the size of the particles does not change significantly and that it varies from 1 to 7 μm. However, small particles show up on the surface of the particles after the high-temperature treatment. As we compare Figure 1b and Figure 8d more carefully, we can easily find that the peaks of bulk samples are slightly different from those of the single particles before or after the heat treatment. This is because Figure 1b shows the emission spectra averaged over different particles, while Figure 8d shows the emission spectra from individual particles. After the first round of heating up to 300 °C and cooling down to 25 °C, ScVO4:Bi3+ shows the emission at 632 nm (see Figure 1b) rather than 630 nm (see Figure 8d). This is due to the incomplete removal of oxygen vacancy defects at such lower temperatures.

size increases in sequence. The intensity of particle 6 is more than 3 times stronger than that of particle 3 (see Figure 8b). As these particles were heated at 600 °C for 3 h and cooled down to 25 °C, they showed similar emission spectra to those before the heat treatment (see Figure 8b). We calculated the ratio of photoemission intensity for each particle before and after the heat treatment, that is Ibef/Iaft, which remains unchanged as Figure 8b depicts. We also did not observe the morphology change for samples before and after the heat treatment. As SEM

4. CONCLUSIONS AND OUTLOOK In summary, we found that after 11 rounds of thermal expansion and contraction, the efficient yellow emission from a Bi3+ doped compound of LuVO4:Bi3+ can be recoverable to its initial state in emission wavelength and intensity, while the red orange emission from ScVO4:Bi3+, crystallographically isomorphous to LuVO4:Bi3+, cannot. Comprehensive characterizations such as in situ HT-XRD, Rietveld refinement, static and dynamic high resolution photoemission, scanning electron microscopy, single

Figure 6. Schemes for recoverable Bi3+ photoemissions from LuVO4:Bi3+ (a) and unrecoverable Bi3+ photoemissions from ScVO4:Bi3+ (b); ET denotes energy transfer processes.

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Figure 8. (a) Photographs of LuVO4:Bi3+ particles recorded (i) upon excitation at 254 or 365 nm and (ii) under bright field; microscopic fluorescence images of LuVO4:Bi3+ particles (iii) before and (iv) after heat treatment at 600 °C upon excitation at 254 nm; particles are coded as 1 to 6. (b) (i) Photoemission spectra of all randomly selected particles 1−6 of LuVO4:Bi3+ before (left) and after (right) heat treatment at 600 °C; (ii) the intensity ratio of emission from particles 1−6 of LuVO4:Bi3+ before and after heat treatment at 600 °C. (c) Photographs of ScVO4:Bi3+ particles recorded (i) upon excitation at 254 or 365 nm and (ii) under bright field; microscopic fluorescence images of ScVO4:Bi3+ particles (iii) before and (iv) after heat treatment at 600 °C upon excitation at 254 nm; particles are coded as Y1 to Y5, and R1 to R5, which give out emissions at 630 and 640 nm, respectively. (d) (i) Photoemission spectra of all randomly selected particles R1 to R5 of ScVO4:Bi3+ before (left) and (right) after heat treatment at 600 °C; (ii) photoemission spectra of all randomly selected particles Y1 to Y5 of ScVO4:Bi3+ before heat treatment at 600 °C; (iii) photoemission intensity of particles R1 to R5, and Y1 to Y5 before and after heat treatment at 600 °C.

shift and intensity loss of the red emission. Configuration coordination schemes are proposed for the recoverable and unrecoverable luminescence process of Bi3+ in these compounds. The single particle diagnosis also reveals significant intensity reduction and peak shift for ScVO4:Bi3+ particles but not for LuVO4:Bi3+ particles. Prospectively, the approach by single particle diagnosis allows us to distinguish and select crystalline particles with outstanding luminous properties such as higher efficiency, better color purity, etc. from a bundle of particles different in crystal phases, morphologies, or sizes. Meanwhile, it can serve as a new technique to bring us new insights into some peculiar luminescence behaviors we cannot observe in ensemble measurements and therefore help establish a direct relationship between luminescence properties, crystalline phase, size, and morphology. Consequently, this is expected to guide us in finding more efficient phosphors in a more effective way.

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Figure 9. SEM images of LuVO4:Bi3+ and ScVO4:Bi3+ before and after heat treatment at 600 °C. Scale bars have been denoted in the figure.

AUTHOR INFORMATION

Corresponding Authors

particle diagnosis technique, etc. unravel that LuVO4:Bi3+ keeps intact after cycle experiments of heating and cooling, yet ScVO4:Bi3+ shows unrecoverable structure changes, i.e., lattice cell expansion. This, as DFT calculations show, is due to the annihilation of oxygen vacancy. It therefore increases the bandgap of ScVO4:Bi3+ and consequently leads to the blue

*(M.P.) Tel: +86 20 22236910. E-mail: pengmingying@scut. edu.cn. *(D.Y.L.) Tel: +852 27665691. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7814

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51322208, 51672085, and 51472088), Guangdong Natural Science Foundation for Distinguished Young Scholars (Grant No. S20120011380), the Department of Education of Guangdong Province (Grant No. 2013gjhz0001), Fundamental Research Funds for the Central Universities, Key Program of Guangzhou Scientific Research Special Project (Grant No. 201607020009), Hundred, Thousand and Ten Thousand Leading Talent Project in Guangdong Program for Special Support of Eminent Professionals, and the Hong Kong Research Grants Council (GRF Grant No. 15301414).

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DOI: 10.1021/acs.chemmater.6b03062 Chem. Mater. 2016, 28, 7807−7815