Effects of Sintering Temperature on the Morphology and Photoluminescence of Eu3+ Doped Zinc Molybdenum Oxide Hydrate
Synthesis of shape controlled and rare-earth doped ZnMoO4 nanostructures on a large scale with low costs is a present challenge in nanotechnology. The precursor of Eu3+ doped zinc molybdenum oxide hydrate (Zn5Mo2O11·5H2O) was synthesized at room temperature via the coprecipitation method. The influences of the sintering temperature on the microstructures and photoluminescence (PL) of the precursor were investigated by means of X-ray diffraction, scanning electron microscopy, thermal gravimetry, differential scanning calorimetry, energy dispersive X-ray spectroscopy, diffuse reflectance spectroscopy, and PL spectrophotometry. It is found that Eu3+ doped ZnMoO4 nanostructures can be derived by sintering the precursor at a relatively low temperature of about 400°C. Our results have demonstrated that Eu3+ doped ZnMoO4 nanostructures can be cost-effectively derived by sintering the precursor at a relatively low temperature.
Zinc molybdate tetraoxide (ZnMoO4), which can be utilized as cryogenic scintillating bolometers for neutrinoless double beta decay detection [1–3], has attracted interest in the last years as the host material for rare-earth ions [4–8]. As is well known, triclinic ZnMoO4 is composed of a series of distorted Mo-O tetrahedrons, Zn-O pentahedrons, and Zn-O octahedrons. In a distorted Mo-O tetrahedron, the Mo atom is surrounded by four oxygen atoms. Similarly, the Zn atom in a Zn-O pentahedron is surrounded by five oxygen atoms, and the Zn atom in a Zn-O octahedron is surrounded by six oxygen atoms . After substituting the Zn2+ sites in ZnMoO4, the rare-earth ions in the ZnMoO4 crystal lattice can give off the characteristic emissions of the rare-earth dopants upon ultraviolet excitation. Therefore, triclinic ZnMoO4 is considered as an excellent host material for rare-earth dopants. For example, Zhou et al. prepared red phosphor Eu3+ doped ZnMoO4 for light-emitting diode application by solid-state reaction method at 900°C ; Chengaiah et al. synthesized white phosphor Dy3+ doped ZnMoO4 using the solid-state reaction method at 900°C ; Ju et al. prepared green phosphor Tb3+ doped ZnMoO4 via sintering precipitation at 800°C . Although characteristic emissions of the rare-earth dopants can be clearly identified in the rare-earth doped ZnMoO4, the high temperature treatment makes the reported approaches suffer from the drawback of being highly energy-consuming [4, 7, 8, 10]. Thus synthesis of rare-earth doped ZnMoO4 nanostructures on a large scale with low costs is a present challenge in nanotechnology, and it is necessary to develop an energy-saving and cost-effective route to prepare rare-earth doped ZnMoO4.
The precipitation synthesis is an energy-saving and cost-effective method for mass-producing nanomaterials . Eu3+ is a typical rare-earth dopant; the most prominent feature of Eu3+ is its red emission at 616 and 628 nm due to its 5D0 → 7F2. So we selected Eu3+ as a representative of rare-earth dopants to study the photoluminescence (PL) properties of rare-earth doped ZnMoO4. Normally the dopant concentration of rare-earth element in ZnMoO4 varies in the range of 0.1–30 mol% [4–8]. It is known that the intensity of the characteristic emissions of Eu3+ in host materials depends on the Eu3+ concentration, but no much new insight can be provided by varying the concentration of Eu3+ dopant except PL quenching at high doping concentration [4, 8]. This is the reason why we simply selected 1 mol% of Eu3+ as dopant in the host materials. In this paper, we reported that Eu3+ doped ZnMoO4 nanostructures could be derived by sintering the precursor of Eu3+ doped zinc molybdenum oxide hydrate (Zn5Mo2O11·5H2O, ZMO) at a relatively low temperature of about 400°C. The precursor Eu3+ doped ZMO was synthesized via the precipitation method at room temperature. The effects of the sintering temperature on the morphology and PL of the precursor were investigated.
2. Materials and Characterizations
The precursor of Eu3+ doped ZMO was synthesized at room temperature via the precipitation method. Hexaammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24·4H2O], zinc nitrate hexahydrate [Zn(NO3)2·6H2O], europium nitrate hexahydrate [Eu(NO3)3·6H2O], and sodium hydroxide (NaOH) were used as the starting materials. In a cleaned beaker, aqueous solution was prepared by dissolving (NH4)6Mo7O24·4H2O (0.01 mol) and Eu(NO3)3·6H2O (0.0007 mol) into 100 mL deionized water. In a similar way, aqueous solution was prepared by dissolving Zn(NO3)2·6H2O (0.175 mol) into 100 mL deionized water. Under vigorous stirring with a magnetic bar, aqueous solution of NaOH (0.08 mol) was dropped into solution in order to hydrolyze (NH4)6Mo7O24. White precipitate was formed in the beaker when solution was slowly added to the hydrolyzed solution. The pH value of the solution in the beaker was adjusted to 7 by adding extra aqueous solution of NaOH. The precipitate was filtered, washed with water, and dried in an oven at 60°C for overnight. The dried precipitate, which was identified as Eu3+ doped ZMO, was divided into four shares for subsequent sintering in an air-filled furnace at 200, 400, 600, and 800°C for 2 h, respectively. The dopant concentration of Eu3+ in each sample was 1 mol%.
X-ray diffraction (XRD) curves were recorded on an X-ray diffractometer (D/max 2500 PC, Rigaku Corporation, Japan) using Cu Kα radiation (λ = 0.15405 nm). The voltage applied to the Cu target in the XRD machine was 40 kV. The scanning electron microscope (SEM) (S-4800, Hitachi, Japan) was employed to analyze the morphology and elemental composition of the resultant compounds. The accelerating voltage applied to the electron gun in the SEM was 15 kV. The SEM was coupled with a silicon drifted detector as the X-ray analyzer for the energy dispersive X-ray spectroscopic (EDX) analysis. The weight change and heat flow of the dried precipitate were measured with a simultaneous thermal analyzer (Pyris Series STA 6000, Perkin Elmer). Al2O3 crucibles were used as the sample containers in the thermogravimetry (TG) analysis and the differential scanning calorimetry (DSC) analysis. The N2 purge gas flow rate was approximately 19.8 mL/min. The samples were run using a scanning rate of 10°C/min in the TG and DSC characterizations.
The diffuse reflectance spectrum of Eu3+ doped ZnMoO4 was measured with a UV-vis spectrometer (UV3600, Shimazu). The PL spectra of the resultant compounds were recorded with a spectrophotometer (Tianjin Gangdong Ltd., China). The 325 nm laser line from a helium-cadmium laser was utilized as the excitation source for the PL measurement. The PL lifetime measurement of Eu3+ doped ZnMoO4 was performed at room temperature on a picosecond fluorescence lifetime spectrometer (LifeSpec II, Edinburgh Instruments). The 375 nm excitation source was supplied with a picosecond pulsed diode laser, which was operating at a repetition rare of 10 MHz and having a pulse width of 55 ps. The photoexcitation spectrum of Eu3+ doped ZnMoO4 was measured with the fluorescence spectrometer LS45 (Perkin Elmer) by fixing the monitoring wavelength at 615 nm.
3. Results and Discussions
3.1. XRD Analysis
Figure 1 depicts the XRD curves of the as-prepared precursor before (a) and after thermal annealing at 200°C (b). The duration of thermal annealing is 2 h, and the materials in (a) and (b) are Eu3+ doped ZMO. The XRD curve of the as-prepared precursor is presented in Figure 1(a), where one can observe well-defined peaks corresponding to the single phase of ZMO. The diffraction peaks at 12.266, 17.170, 23.466, 26.578, 29.062, 31.680, 33.562, 33.955, 34.714, and 38.489° can be assigned to the reflections from the 003, 101, 104, 015, 110, 113, 107, 021, 202, and 116 crystallographic planes of ZMO (JCPDS No. 30-1486), respectively. The standard XRD data of hexagonal ZMO is shown at the bottom of Figure 1 for comparison (JCPDS No. 30-1486). According to the JCPDS No. 30-1486, the lattice parameters of standard hexagonal ZMO are known as nm and = 2.16 nm. The lattice parameters of the Eu3+ doped ZMO can be determined by whole pattern fitting and Rietveld refinement of the XRD curves in Figure 1. As listed by the second row in Table 1, the lattice parameters of the Eu3+ doped ZMO without annealing are found to be nm, nm, α = β = 90°, and γ = 120°. It can be seen clearly that the lattice parameters of Eu3+ doped ZMO agree well with those of standard ZMO. Figure 1(b) shows that the Eu3+ doped ZMO preserves its crystal structure upon heating at 200°C for 2 h. The lattice parameters of the annealed precursor are listed as the third row in Table 1. Using Scherrer equation, we calculated the mean grain sizes of the crystalline domains as a function of the annealing temperature. The shape factor in the Scherrer equation took the typical value of 0.9. On the basis of the (015) peak broadening in Figure 1, the mean sizes were derived to be around 74.2 and 54.4 nm for the as-prepared precursor before and after thermal annealing at 200°C, respectively.
Figure 2 represents the XRD spectra of Eu3+ doped ZnMoO4 compounds obtained by thermally annealing the precursor at 400°C (a), 600°C (b), and 800°C (c) for 2 h. As marked by the vertically aligned dash lines, the 12 diffraction peaks are located at 2θ = 13.219, 15.994, 19.235, 22.877, 24.179, 26.129, 26.633, 27.597, 30.054, 32.253, 33.388, and 33.876°. These 12 peaks in Figure 2 can be assigned to the reflections from the , 110, 011, 11, 120, 02, 20, 21, 1, 012, 300, and 21 planes of triclinic ZnMoO4, respectively [4, 5, 7, 8, 10, 11]. The standard XRD data of triclinic ZnMoO4 are presented at the bottom of Figure 2 for comparison. The lattice parameters of standard triclinic ZnMoO4 are = 0.9625 nm, = 0.6965 nm, = 0.8373 nm, α = 103.28°, β = 96.30°, and γ = 106.72° (JCPDS No. 35-0765). In our case, the lattice parameters of Eu3+ doped ZnMoO4 can be determined by whole pattern fitting and Rietveld refinement, as listed as the fourth, fifth, and sixth rows in Table 1. It is clear that the lattice parameters of Eu3+ doped ZnMoO4 vary a little bit away from the standard parameters when the annealing temperature increases from 400 to 800°C. A careful check on the XRD curves (a)–(c) reveals that the resultant ZnMoO4 is in its pure triclinic phase when the sintering temperatures are 400 and 600°C. But the case is changed when the sintering temperature is elevated to 800°C. A secondary phase appears, as marked with the solid blue dots upon the XRD curve (c). The diffraction peaks of the secondary phase are located at 11.837, 11.800, 21.187, 23.836, 24.850, 28.126, and 34.756°, which are consistent with those of Zn3Mo2O9 (JCPDS No. 30-1484). These results demonstrate that the secondary phase Zn3Mo2O9 coexists with the primary phase ZnMoO4 when the sintering temperature is elevated to 800°C. On the basis of the (120) peak broadening in Figure 2, the mean sizes of Eu3+ doped ZnMoO4 are calculated to be 42.7, 270.7, and 812.1 nm for the precursors annealed at 400, 600, and 800°C, respectively. It is important to realize that the Scherrer formula provides a lower bound on the particle size. The Scherrer equation is limited to nanoscale particles only; it is not accurately applicable to grains larger than about 200 nm. The reason for this is that a variety of factors can contribute to the width of a diffraction peak besides instrumental effects and crystallite size; the most important of these are usually inhomogeneous strain and crystal lattice imperfections.
The most prominent result in Figures 1 and 2 is that Eu3+ doped ZMO can be converted into Eu3+ doped ZnMoO4 at a specific temperature between 200 and 400°C. The ZnMoO4 belongs to the triclinic crystal system with space group P. One unit cell of triclinic ZnMoO4 is composed of six molecular weights of ZnMoO4 . In the ZnMoO4 structure, the Mo6+ ions occupy three nonequivalent positions being surrounded by four oxygen ions with approximately tetrahedral coordination; meanwhile Zn2+ ions occupy sites with 5- and 6-fold coordination. As we know, the ionic radius of Eu3+ ( = 94.7 pm when coordination number (CN) = 6) is close to that of Zn2+ ( = 90 pm when CN = 6) . Therefore, the four coordinated Mo6+ ( = 41 pm when CN = 4) sites are too small for Eu3+ ( = 94.7 pm) to occupy. Thus we believe that Eu3+ ions prefer to occupy the Zn2+ site. That is the reason why the diffraction patterns of the ZnMoO4 are not affected too much by Eu3+ doping at the concentration of 1 mol%.
3.2. SEM Characterization
Figure 3 shows the SEM micrographs of the as-prepared precursors after thermal annealing at 200°C (a), 400°C (b), 600°C (c), and 800°C (d), respectively. The luminescent materials in (a) are Eu3+ doped ZMO whilst those in (b–d) are Eu3+ doped ZnMoO4 compounds. As shown in Figure 3(a), the size of Eu3+ doped ZMO nanoparticles varies from 20 to 200 nm. Figure 3(b) shows that the Eu3+ doped ZnMoO4 nanostructures are a mixture of nanoparticles and nanoplates when the sintering temperature is 400°C. The typical side length of the nanoplates is around 300 nm whilst the thickness of the nanoplates varies from 20 to 50 nm. If the sintering temperature is increased further to 600°C, Eu3+ doped ZnMoO4 nanoplates are formed. As illustrated in Figure 3(c), all nanoparticles are converted into nanoplates whose well-defined crystal facets can be clearly seen. The typical thickness of Eu3+ doped ZnMoO4 nanoplates is around 20–90 nm, and their side length ranges from 300 nm to 1 μm. When the sintering temperature is elevated to 800°C, the Eu3+ doped ZnMoO4 phosphors are developed into micrometer-sized crystals. The largest size of the Eu3+ doped ZnMoO4 microcrystal is around 10 μm. It is worth noting that the melting temperature of ZnMoO4 is 1003 ± 5°C and the crystallization temperature of its melt is 975 ± 5°C. So bulk crystal is formed when the sintering temperature is higher than 1000°C, which is not shown here. The data in Figure 3 demonstrate that the morphology of Eu3+ doped ZnMoO4 is highly dependent on the sintering temperature. Particularly, it is found that ZnMoO4 nanoplates can result when the sintering temperature is in the range of 400 and 600°C, suggesting that there is a preferential direction of crystal growth. Peng et al. proposed a schematic diagram to illustrate the formation of ZnMoO4 nanoplates . Moreover, as has been observed in Figure 3, the size of Eu3+ doped ZnMoO4 gets larger when the sintering temperature is higher; meanwhile the surface area of Eu3+ doped ZnMoO4 phosphor is significantly reduced. During the sintering process, atomic diffusion drives powder surface elimination. The driving force for the densification is the change in free energy from the decrease in surface area and lowering of the surface free energy. As a result of the sintering, new but lower-energy solid-solid interfaces are formed with a total decrease in free energy. This is one of the main reasons why sintering at a higher temperature enlarges the size of Eu3+ doped ZnMoO4 with the result of significantly reduced surface area.
Through the SEM pictures in Figure 3, we calculated the statistics of the particle sizes as a function of temperature. The particle size of ZMO in Figure 3(a) spans from 20 to 200 nm with its most probable size at around 85 nm. After having excluded the large-sized nanoplates from particle size statistics, the particle size of Eu3+ doped ZnMoO4 in Figure 3(b) spans from 20 to 50 nm with its most probable size at around 40 nm. In a similar way, the length of Eu3+ doped ZnMoO4 plates in Figure 3(c) varies from 300 nm to 1 μm with its most probable size at about 420 nm. Finally, the particle sizes of Eu3+ doped ZnMoO4 microcrystals in Figure 3(d) are distributed in the range of 1–10 μm with their most probable size being about 3 μm.
3.3. Thermogravimetric Analysis
TG analysis measures the change in weight of a sample as it is heated. The phase transition from Eu3+ doped ZMO to Eu3+ doped ZnMoO4 can be verified by thermogravimetric analysis. Figure 4(a) shows the TG curve of the precursor Eu3+ doped ZMO. The samples were heated up to 600°C using a scanning rate of 10°C/min in the TG and DSC characterizations. From Figure 4(a), one can find that precursor has one distinct weight loss event that occurs at around 250°C during heating from 50 to 600°C. The total weight loss is determined to be 10.45% for Eu3+ doped ZMO. The weight loss of the precursor suggests that something happens to the Eu3+ doped ZMO. DSC measures the heat difference with the change of temperature, so a direct evidence for the phase transition can be given by the DSC analysis. Figure 4(b) is the DSC curve of the Eu3+ doped ZMO. As shown by Figure 4(b), there is a sharp peak at about 267°C when Eu3+ doped ZMO is heated from 50 to 600°C, indicating that the phase transition takes place at around 267°C. Based on the TG and DSC results along with the XRD data in Figures 1 and 2, the formation processes of Eu3+ doped ZMO could be summarized by the following reaction scheme:Equation (1) describes the hydrolysis of the molybdenum salt in the basic solution whilst (2) illustrates the formation of ZMO. Upon heating, ZMO undergoes a reaction where hydrate is released from the material when sintering at a temperature higher than 267°C. Our recorded weight loss (10.45%) is extremely comparable with the theoretical value of water in ZMO (11.5%). Therefore, the weight loss of the precipitate in Figure 4 can be attributed to the decomposition of Eu3+ doped ZMO at 267°C. Besides the characterizations in nitrogen, we performed the TG and DSC characterizations for the sample under air purge gas at the heating rate of 10°C/min. No obvious differences were recorded in the TG/DSC curves when measured in air and in nitrogen. The reason for the no differences rests on the fact that our sample is inert to both oxygen and nitrogen.
3.4. PL Spectral Analysis
Figure 5 displays the PL spectra of Eu3+ doped ZMO after thermal annealing at 200, 400, 600, and 800°C for 2 h. The luminescent materials in (a) are the Eu3+ doped ZMO whilst those in (b–d) are Eu3+ doped ZnMoO4 compounds. Each PL spectrum consists of broadband emissions centered at around 550 nm and a series of sharp emissions peaking at 592, 612, and 650 nm. After the Eu3+ doped ZMO has been annealed at 200°C for 2 h, the sharp emissions in Figure 5(a) peak at 592, 612, and 650 nm, corresponding to the electronic transitions 5D0 → 7F1, 5D0 → 7F2, and 5D0 → 7F3 of Eu3+ ions in the host ZMO [4–6]. Figure 5(b) shows that quite similar PL spectrum has been recorded for Eu3+ doped ZnMoO4. The most important feature in Figures 5(a) and 5(b) is that the transition 5D0 → 7F2 (at 612 nm) is strong whilst the other two transitions 5D0 → 7F1 (at 592 nm) and 5D0 → 7F3 (at 650 nm) are relatively weak. As is well known, the 5D0 → 7F1 line originates from magnetic dipole transition whilst the 5D0 → 7F2 line results from the electric dipole transition. In terms of the Judd–Ofelt theory, the magnetic dipole transition is permitted, but the electric dipole transition is allowed only on condition that the Eu ion occupies a site without an inversion center. Subsequently, the 5D0 → 7F1 transitions should be relatively strong if Eu3+ ions occupy inversion center sites. Otherwise, the 5D0 → 7F2 transitions should be strong when Eu3+ occupies a center of asymmetry in the host lattice. The results in Figures 5(a) and 5(b) indicate that most of Eu3+ ions do not occupy inversion center sites either in ZMO or in ZnMoO4. The lack of inversion symmetry and the break of parity selection rules in ZMO and ZnMoO4 make the 5D0 → 7F2 electric dipole transition strongest among all these transitions.
The second most important feature shared by Figures 5(a) and 5(b) is that the transitions 5D0 → 7F1 and 5D0 → 7F2 of Eu3+ in Figures 5(a) and 5(b) are structureless. After substituting the Zn2+ sites in ZnMoO4, Eu3+ ions in the ZnMoO4 crystal lattice experience different crystal field strength due to the different chemical environments around the substitution sites. There are, in fact, multiple emission lines that can be expected in each PL spectrum due to the crystal field splitting of the ground state of the Eu3+ ions. A literature survey reveals that the splitting of the characteristic emissions of Eu3+ ions is not well considered for rare-earth doped ZnMoO4 [4–8], but line splitting has been reported in Eu3+ doped tellurite glasses , double borate Ca3Gd2(BO3)4 , NaYP2O7 , and Y2Sn2O7 . Although the splitting of the characteristic emissions of Eu3+ can be expected, the split emissions of Eu3+ are not observed in Figure 4(b) for Eu3+ ZnMoO4, suggesting that the local crystal fields around the Eu3+ sites are not distinctly differentiated. Fortunately, the splitting of the characteristic emission of Eu3+ can be observed when the sintering temperature is 600°C or higher. As depicted in Figures 5(c) and 5(d), the characteristic emission of Eu3+ at 612 nm is split into four fine lines peaking at 606, 611, 615, and 620 nm; meanwhile the emission at 592 nm is split into two fine lines peaking at 590 and 595 nm. Zooms in the region 585–630 nm are shown as insets in Figures 5(c) and 5(d) to show the emission splitting of the transitions 5D0 → 7F1 and 5D0 → 7F2. The energy difference between the two split lines of the transitions 5D0 → 7F1 is 142 cm−1 whereas those of the split lines of the transitions 5D0 → 7F2 are 135, 106, and 131 cm−1, respectively. The splitting of the Eu3+ emissions can be thought of as a consequence of the change in the local environment; thus the splitting of Eu3+ emissions in ZnMoO4 suggests a sufficient variation in the local crystal field around Eu3+ ions in ZnMoO4. We can see that sintering is effective in changing the local crystal field around the rare-earth ions .
The third most important feature in Figure 5 rests on the fact that a broadband emission appears in each PL spectrum with its center at around 550 nm. We can attribute it to the intrinsic defects in Eu3+ doped hosts. Generally speaking, photoexcitation creates electrons in the conduction band and holes in the valence band of Eu3+ doped ZMO and Eu3+ doped ZnMoO4; then the photoexcited carriers are relaxed through the band-edge free excitonic recombination or through the intrinsic defects in the hosts. In the case of ZnMoO4, the band-edge excitonic recombination can be excluded since the bandgap of ZnMoO4 is about 4 eV [9, 10]. Consequently, the defects in ZnMoO4 should be the candidate of the broadband PL centered at about 550 nm. As we know, Zn, O, and Mo vacancies are the most common intrinsic defects in ZnMoO4. At the current stage, we may tentatively assign the broadband PL centered at about 550 nm to these intrinsic vacancies in ZnMoO4.
Color coordinates are important parameters to quantitatively describe the emission color for luminescent materials, and the CIE chromaticity coordinates can be calculated from the PL spectral data of luminescent materials [17–20]. Figure 6 represents the CIE chromaticity diagram of the PL color of thermally annealed ZMO at 200°C (a), 400°C (b), 600°C (c), and 800°C (d). The luminescent materials in (a) are Eu3+ doped ZMO whilst those in (b–d) are Eu3+ doped ZnMoO4 compounds. As shown in Figure 6, the color coordinates are (0.495, 0.335), (0.437, 0.407), (0.482, 0.451), and (0.444, 0.396) when the Eu3+ doped ZMO is thermally annealed at 200, 400, 600, and 800°C, respectively. Accordingly, the luminescent color is pink for the Eu3+ doped ZMO after annealing at 200°C for 2 h; meanwhile the luminescent color is yellowish orange when the Eu3+ doped ZMO is annealed at 400, 600, and 800°C for 2 h. Although the luminescent color of Eu3+ doped ZMO is different from that of Eu3+ doped ZnMoO4, it is clear that the PL color of Eu3+ doped ZnMoO4 varies not too much with the annealing temperature.
3.5. EDX and PL Excitation Spectra of ZnMoO4:Eu3+
It is important to demonstrate the presence of Eu3+ ions in Eu3+ doped ZnMoO4. EDX is an analytical technique used for the elemental analysis of a specimen. Figure 7(a) depicts the EDX spectrum of Eu3+ doped ZnMoO4 nanoplates. The Eu3+ doped ZnMoO4 nanoplates are derived by sintering the Eu3+ doped ZMO at 600°C for 2 h. As can be seen in Figure 7, the X-ray emission peaks are located at 0.53, 1.02, 2.30, 8.61, and 9.57 keV, corresponding to the characteristic X-ray emissions of O(Kα1,2), Zn(Lα1,2), Mo(Lα1), Zn(Kα1), and Zn(Kβ1,3), respectively. These data indicate the presence of Zn, O, and Mo in the phosphor, which is consistent with the XRD analysis. In particular, we also recorded the X-ray emission peaks of element Eu in the sample. As marked by the vertical arrows in Figure 7(a), the X-ray emission peaks at 5.85 and 6.46 keV can be assigned to Eu(Lα1) and Eu(Lβ1), respectively.
Due to its superior signal-to-noise ratio, PL excitation spectroscopy is one useful method to investigate the electronic levels of materials with low absorption. Figure 7(b) depicts the PL excitation spectrum of Eu3+ doped ZnMoO4 nanoplates that are derived by sintering the precursor at 600°C for 2 h. The monitoring wavelength was fixed at 615 nm. It can be seen in Figure 7(b) that the PL excitation spectrum consists a broadband centered at about 300 nm and some sharp absorption peaks. The broad excitation band from 220 to 350 nm is ascribed to the O-Mo charge transfer transition whereas the sharp lines in the 360–500 nm range are intraconfigurational 4f-4f transitions of Eu3+ ions in the host lattice. Additionally, the broad excitation band from 220 to 350 nm could partly be attributed to the O-Eu charge transfer transition. On one hand, the strong excitation peaks at 393 and 464 nm can be attributed to the 7F0 → 5L6 and 7F0 → 5D2 transitions of Eu3+, respectively. On the other hand, the three weak peaks at 361, 381, and 413 nm can be attributed to the transitions from 7F0 to 5D4, 5L7, and 5D3, respectively [4–6]. These characteristic transitions indicate that the Eu3+ ions are incorporated into ZnMoO4.
3.6. Absorption Spectra of ZnMoO4:Eu3+
Doping ZnMoO4 with Eu3+ ions introduces extra defects in the host material. Figure 8 shows the absorption spectra of the undoped ZnMoO4 and Eu3+ doped ZnMoO4 nanoplates. The undoped ZnMoO4 was utilized as a control sample. Both the undoped ZnMoO4 and Eu3+ doped ZnMoO4 nanoplates were derived by sintering their precursors at 600°C for 2 h. It can be seen in Figure 8 that two absorption bands are present in each curve, among which one starts its absorption in the range 310–390 nm whilst the other starts its absorption in the range 250–310 nm. When compared to the transmission spectrum of ZnMoO4 single crystals , we can assign the strong absorption band in the range of 310–390 nm to defects in ZnMoO4; meanwhile we can assign the intense absorption in the range of 250–310 nm to the electron transition from the valence band to the conduction of the ZnMoO4. Thus, Figure 8 shows clearly that doping ZnMoO4 with Eu3+ ions can significantly increase the population density of the defects in the host. Assuming that the cation Eu has +3 charge, the Kröger-Vink notation for the defect reactions of Eu3+ in ZnMoO4 can be described by the following equation:In (3), the masses, sites, and charges are balanced for both intrinsic and extrinsic defects. Equation (3) indicates that an Eu3+ ion substitutes one Zn site in ZnMoO4, yielding one positively charged defect and another positively charged interstitial defect . Thus doping ZnMoO4 with Eu3+ ions can significantly increase the population density of the defects in the host. On the other hand, triclinic ZnMoO4 is an indirect semiconductor . In a parabolic band structure, the optical bandgap and absorption coefficient of an indirect semiconductor can be calculated bywhere α is the linear absorption coefficient of the material, is the photon energy, is a proportionality constant, and is the optical bandgap. The inset of Figure 8 shows the plot of against for the undoped ZnMoO4. The optical bandgap value of the undoped ZnMoO4 nanoplates can be calculated to be 3.70 eV. This optically derived bandgap value agrees roughly with the theoretically calculated bandgap value of 3.79 eV . A quite similar bandgap value (3.72 eV) is derived for Eu3+ doped ZnMoO4 nanoplates. Moreover, we note that the absorption in the inset starts at about 3.1 eV. As documented in the literature, the bandgap value of ZnMoO4 was estimated in the range of 4.17–5.35 eV on the basis of the luminescence excitation and reflectivity spectra . In our case, the intensity of the absorption band in the range of 310–390 nm can be effectively modified by Eu3+ doping, as shown in Figure 8. Consequently, we tentatively assign this absorption band to defects in ZnMoO4.
3.7. Time-Resolved PL of ZnMoO4:Eu3+ Nanocrystals
As discussed above, the broad PL band centered at about 550 nm belongs to the intrinsic defect emission whereas the characteristic emission of Eu3+ at 615 nm can be classified as the extrinsic defect emission. Quantitatively speaking, the two kinds of emissions should have different decay behaviors. Figure 9 depicts the semilogarithmic plots of the room temperature PL decay curve of Eu3+ doped ZnMoO4 nanoplates under the excitation of 375 nm. The Eu3+ doped ZnMoO4 nanoplates were derived by sintering the precipitate at 600°C for 2 h. The emission wavelength for the PL decay in Figure 9(a) was monitored at 550 nm, while the emission wavelength for the PL decay in Figure 9(b) was fixed at 615 nm. It can be seen in Figure 9(a) that the PL decay profile exhibits double exponential nature. The best fit to the experimental data in Figure 9(a) can be obtained using two exponentials with the decay time constants = 0.96 ± 0.11 ns and = 10.23 ± 1.09 ns. It is noted that τ1 is at the limit of the measurement capabilities of the instrument and therefore it merely represents the order of the short decay time constant. When compared to the long PL lifetime (3.9 μs) of the ZnMoO4 single crystals grown by the Czochralski method , the observation of the quite short PL decay time for our Eu3+ doped ZnMoO4 nanoplates gives further evidence on the defect emission at 550 nm for the Eu3+ doped ZnMoO4 nanoplates. As shown in Figure 9(b), the PL decay profile of Eu3+ doped ZnMoO4 nanoplates exhibits single exponential nature. The best fit to the experimental data in Figure 9(b) can be obtained using one exponential with the decay time constants τ1 = 218.29 ± 17.57 ns for the PL emission at 615 nm. A comparison of the two decay curves in Figure 9 demonstrates that the PL decay behavior of the broad PL band centered at about 550 nm is dramatically different from that of the characteristic emission of Eu3+ at 615 nm.
The effects of sintering temperature on the crystal structure, morphology, and PL properties of the precursor are investigated; both the morphology and the PL spectrum of Eu3+ doped ZnMoO4 nanocrystals are found to be highly dependent on the sintering temperature of the precursor. XRD, SEM, and thermogravimetric analyses have shown that Eu3+ doped ZnMoO4 nanostructures can be derived by controlling the annealing temperature in the range of 267–800°C. As the sintering temperature increases from 267 to 800°C, the morphology of Eu3+ doped ZnMoO4 evolves from nanoparticles at 400°C to nanoplates at 600°C and eventually to microcrystals at 800°C. Moreover, a broad PL band with its center at around 560 nm is recorded for Eu3+ doped ZnMoO4; the characteristic emissions of Eu3+ at 592 and 612 nm are split into several fine lines when the sintering temperature is beyond 600°C. Our results have demonstrated that Eu3+ doped ZnMoO4 can be derived cost-effectively by sintering the precursor of Eu3+ doped ZMO at a relatively low temperature of about 400°C.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors would like to acknowledge the financial support from National Natural Science Foundation of China (nos. 11574036, 11304025, and 11604028).
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