Structural and Bluish-White Luminescent Properties of Li+-Doped

Dec 31, 2008 - were performed at room temperature. 3. Results and Discussion ..... Li+ under the excitation of a 365 nm UV lamp in a dark room. Struct...
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J. Phys. Chem. C 2009, 113, 1580–1588

Structural and Bluish-White Luminescent Properties of Li+-Doped BPO4 as a Potential Environmentally Friendly Phosphor Material Cuimiao Zhang, Hongzhou Lian, Deyan Kong, Shanshan Huang, and Jun Lin* State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: October 7, 2008; ReVised Manuscript ReceiVed: NoVember 8, 2008

Many efforts have been devoted to exploring novel luminescent materials that not contain expensive or toxic elements, or do not need a mercury vapor plasma source. In this paper, BPO4 and Li+-doped BPO4 powder samples were prepared by the Pechini-type sol-gel (PSG) process. The structure and optical properties of the resulting samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), photoluminescence (PL) excitation and emission spectra, kinetic decay, and X-ray photoelectron spectra (XPS), respectively. It was found that PSG -derived Li+-doped BPO4 annealed at 960 °C exhibited bright bluish-white emission centered at 416 nm. The luminescence decay curves analysis indicates that each sample has two kinds of lifetimes (5.9 ns and 0.529 ms) and two types of kinetic decay behaviors which can be fitted into a single-exponential function and a double-exponential function, respectively. This can be attributed to two kinds of emission centers in samples. The luminescent mechanisms of the samples are discussed in detail. 1. Introduction Luminescent materials have found a wide variety of applications, including information displays, lighting, X-ray-intensifying and scintillation (e.g., for medical X-ray detection monitors), and so on.1,2 Most of the commercially available lamp phosphors require excitation by short-wavelength ultraviolet (UV) light for operation, a requirement resulting in the extensive use of a mercury vapor plasma in fluorescent lighting products.3 Nevertheless, the usage of mercury vapor will give rise to environmental contamination in disposing of junk.4a In addition, the activators used in fluorescent lamps or cathodoluminescent display phosphors are often expensive or nonenvironmentally friendly elements, such as Ag (in ZnS:Ag+) and lanthanides.1-3 For these reasons, a class of stable, efficient, and less-toxic photoluminescent materials is strongly desired.4b So now, much effort has been devoted to exploring novel luminescent materials that do not contain expensive or toxic elements or do not need mercury vapor plasma as the excitation source.4-9 The Pechini-type sol-gel process (PSG, also known as the polymerizable-complex technique) is well-known and used extensively for the design and synthesis of advanced functional and engineering materials, including powders, films, fibers, and monoliths of almost any shape, size, and chemical composition. The PSG process, which uses common metal salts (nitrates, acetates, etc.) as precursors and citric acid (CA) as the chelating ligand of metal ions and polyhydroxy alcohols such as ethylene glycol (EG) or poly(ethylene glycol) as cross-linking agent to form a polymeric resin on a molecular level, reduces the segregation of particular metal ions and ensures compositional homogeneity.9 This method can overcome most of the difficulties and disadvantages that occur frequently in the alkoxides based sol-gel process. For instance, most of the metal alkoxides suffer from high cost, unavailability, toxicity, and a fast * Author to whom any correspondence should be addressed at the Changchun Institute of Applied Chemistry. E-mail: [email protected].

hydrolysis rate (thus, it is difficult in control the homogeneity of different components during experimental processes).10 Furthermore, the materials prepared by this process can be made into powders, thin films, and others.9 In the past 5 years, we have extended the application of the Pechini-type sol-gel process to the systematic synthesis of various kinds of optical materials, including luminescent powders and thin films, core-shell structured phosphors, and pigments.11 The polycrystalline powder of BPO4 is well-known as a catalyst for a range of organic chemical reactions,12a the single crystal of BPO4 has been investigated as a potential nonlinear optical material,12b and Li-doped BPO4 is an important material as a ceramic electrolyte for Li-ion rechargeable batteries.13 However, to our knowledge, no attention has been paid to the luminescence properties of Li+-doped BPO4 as a kind of luminescent material. On the other hand, defects can influence the properties of materials greatly, such as dielectric properties, photochemical properties, and luminescent properties.14 Recently, we found that the PSG process-derived BPO4 exhibited a weak purple-colored emission caused by the carbon impurities under UV irradiation, and doping it with 6 mol % Ba2+ produces a material that emits an efficient bluish-white light, which is induced by carbon impurities and peroxyl radical defects, respectively.11b Thereafter, we pay more attention to the BPO4-based materials to enhance the luminescent intensity and investigate the luminescence mechanisms. In this paper, one of the stable, efficient, and environmentally friendly luminescent materials based on Li+-doped BPO4 (without rare earth or transition metal ions as activators) was prepared by the PSG process. This kind of material has higher luminescence intensities and is more environmentally friendly than Ba2+-doped BPO4. The luminescent materials based on BPO4:x Li+ show an intense bluishwhite emission (360-580 nm, λmax ) 416 nm) under a wide range of UV light excitation (200-400 nm). Furthermore,

10.1021/jp808868r CCC: $40.75  2009 American Chemical Society Published on Web 12/31/2008

Structural Properties of Li+-Doped BPO4

Figure 1. The XRD patterns for PSG-derived BPO4 (a), BPO4:2% Li+ (b), BPO4:6% Li+ (c), and BPO4:10% Li+ (d), as well as the standard data of BPO4 (JCPDS 34-0132) as a reference. All of the samples were obtained after annealing at 960 °C for 3 h.

possible mechanisms have been proposed to explain the observed luminescent phenomena. 2. Experimental Section All of the chemicals were of analytical grade reagents purchased from Beijing Chemical Corporation and used without further purification. Pure BPO4 and B1-xPO4:x Li+ (x: molar ratio, 0 < x e 0.1) powders were prepared by the Pechini-type sol-gel (PSG) process.9-11 Stoichiometric weights of (NH4)2HPO4 and superfluous H3BO3 (50 mol % excess to compensate the evaporation of B at high temperature) were dissolved in a water/ethanol solution under vigorous stirring. Then citric acid as the chelating agent and a certain amount of polyethylene glycol (PEG, 20000) as the cross-linking agent were added to the solution. The molar ratio of (NH4)2HPO4 to citric acid was 1:2. Then, stoichiometric weights of LiNO3 were added into the solution, and subsequently mixed with a certain amount of glycerol. Transparent sol was obtained after stirring for 10 h. Then the samples were dried at 100 °C to form gel immediately. The obtained gel was preheated at 500 °C for 3 h, fully ground, and annealed at 960 °C for 3 h to produce the final samples. In addition, to investigate the luminescent mechanisms, other alkali metal ions (Na+ and K+) were also doped into BPO4 host in the same way as that for Li+. Characterization. The X-ray diffraction (XRD) of the powder samples was examined on a Rigaku-Dmax 2500 diffractometer with use of Cu Ka radiation (λ ) 0.15405 nm). Fourier transform infrared (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. The morphology and composition of the samples were inspected by using a field emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energy-dispersive X-ray spectrum (EDX, JEOL JXA-840). The X-ray photoelectron spectra (XPS) were taken on a VG ESCALAB MK II electron spectrometer with Mg KR (1253.6 eV) as the exciting source. The photoluminescence (PL) excitation and emission spectra were taken on an F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Luminescence lifetimes were measured with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) with use of 310 nm lasers (pulse width ) 4 ns) as the excitation source (Continuum Sunlite OPO). The crystal parameters and bond lengths of BPO4:x Li+, BPO4:6% Na+, and BPO4:6% K+ were calculated by using the CASTEP code. All measurements were performed at room temperature. 3. Results and Discussion XRD. Figure 1 shows the XRD patterns for the synthesized pure BPO4 (a), BPO4:2% Li+ (b), BPO4:6% Li+ (c), and

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Figure 2. The FT-IR spectra for PSG-derived BPO4 (a) and BPO4: 6% Li+(b).

TABLE 1: Cell Constants and Bonding Lengths of PSG-Derived B1-xPO4:x Li+ (x ) 0%, 4%, 6%, 8%, 10%, 960 °C) and the Standard Data of BPO4 Calculated with the CASTEP Code PSG- BPO4:2% BPO4:6% BPO4:10% standard data BPO4 Li+ Li+ Li+ of BPO4 a (Å) c (Å) B-O (Å) P-O (Å) B-P (Å)

4.3391 6.6473 1.442 1.554 2.735

4.3355 6.6416 1.437 1.545 2.731

4.3405 6.6402 1.438 1.546 2.732

4.3285 6.6319 1.435 1.542 2.726

4.3425 636415 1.439 1.546 2.733

BPO4:10% Li+ (d), as well as the standard data for BPO4 (JCPDS No. 34-0132), respectively. The diffraction peaks of the four samples can be indexed as a pure tetragonal phase, which coincide well with the standard data of BPO4 (JCPDS No. 34-0132). No peak shift and other phase can be detected, indicating that the Li+ ions have been dissolved successfully in the BPO4 host. Pure BPO4 crystallizes in a tetragonal phase, with space group I4 (No. 82).15 The crystal structure of BPO4 is built by sharing the corner of the BO4 and PO4 tetrahedral, leading to a continuous, three-dimensional framework (Figure 10a).16 Li+-doped BPO4 has two possible defect models, which show the existence of a boron vacancy balanced by interstitial Li+ ions,13 shown here using the Kro¨ger-Vink notation:

LiB′′ + 2Lii

(a)

VB′′′ + 3Lii

(b)

Furthermore, Figure S1 in the Supporting Information shows the two possible structure models of Li+-doped BPO4 [LiB′′ + 2Lii (a), VB′′′ + 3Lii (c)] and the calculated band structures according to the models of Li+-doped BPO4 [LiB′′ + 2Lii (b), VB′′′ + 3Lii (d)]. The introduction of Li+ ions can decrease the band gap and the level of conduction band compared with the ideal BPO4 (see Figure 10b). Determined calculated partial PDOS plots of Li are shown in Figure S2 in the Supporting Information. From Figure S2 (Supporting Information), we note that the Li element also contributes to the Fermi energy level and conduction band. Table 1 shows the lattice parameters, bond lengths (B-O, P-O), and the distance B-P of BPO4:x Li+ (x ) 0%, 2%, 6%, 10%) which are calculated on the basis of XRD data. Basically, from Table 1, it can be seen that the lattice parameters and bond lengths first increase with increasing Li+ concentration (x), reaching a maximum at x ) 6%, then decrease with further increasing Li+ concentration. Furthermore, Table 2 shows the lattices parameters, bond lengths (B-O, P-O), and the distance B-P of BPO4:6% Li+, BPO4:6% Na+, and BPO4:6% K+ according to the XRD data (the XRD patterns of BPO4:6% Na+

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Zhang et al.

TABLE 2: Cell Constants and Bonding Lengths of Prepared BPO4:6% Li+, BPO4:6% Na+, BPO4:6% K+, and the Standard Data of BPO4 Calculated with the CASTEP Code x (%)

BPO4:6% Li+

BPO4:6% Na+

BPO4:6% K+

a (Å) c (Å) B-O (Å) P-O (Å) B-P (Å)

4.3405 6.6402 1.438 1.546 2.732

4.3385 6.6363 1.437 1.545 2.731

4.3277 6.6287 1.434 1.542 2.726

standard data of BPO4 4.3425 636415 1.439 1.546 2.733

and BPO4:6% K+ are shown in Figure S3 in the Supporting Information). As seen clearly in Table 2, the lattice parameters and bond lengths decrease in the order Li+, Na+, and K+. Because the effective ionic radii of the Li+ (r ) 0.59 Å, Coordination Number: CN ) 4) ion is much smaller than that of the Na+ (r ) 0.99 Å, CN ) 4) or K+ (r ) 1.37 Å, CN ) 4) ion, it is expected that structural disturbance of BPO4 by Li+ is lighter than that by other alkali metal ions, which is the reason that the lattice parameters of Na+/K+-doped BPO4 departure greater from standard data for BPO4 than for Li+-doped BPO4 samples. From the fact that the effective ionic radii of Li+ (r ) 0.59 Å, CN ) 4) are bigger than those of B3+ (r ) 0.11 Å, CN ) 4) and P5+ (r ) 0.17 Å, CN ) 4), it is deduced that some Li+ will replace the B3+ and/or P5+ atoms in the BPO4 host lattice; however, the rest of the Li+ ions may occupy the interstitial sites of the BPO4 lattice. The deduction is consistent with two possible defect models (VB′′′ + 3Lii and LiB′′ + 2Lii).13 This is also the reason that introduction of Li+ induces the slight change of lattice parameters and bond length for BPO4. FT-IR. Figure 2 shows the FT-IR spectra of the pure BPO4 (a) and BPO4:6% Li+ (b) samples. In Figure 2a, there are two different chemical bonds present in the structure of BPO4: stronger P-O and some weaker B-O. Two groups of bands in the range of 564-633 cm-1 (centered at 568 and 633 cm-1) are due to the bending vibrations of PO4 groups.16 The band at 1100 cm-1 is ascribed to the asymmetric stretching vibrations of PO4 groups, and the band with a maximum at 933 cm-1 corresponds to the asymmetric stretching vibration of the BO4 tetrahedron.16 The band at 3216 cm-1 is ascribed to the O-H vibrations of H2O absorbed in the samples. The weak band with a maximum at 1467 cm-1 is due to the C-O vibration,17 indicating that the carbon impurities have not been removed completely in the PSG-derived samples despite the fact that the samples have been annealed at high temperature (960 °C). The carbon impurities can also be detected by the energy-dispersive X-ray spectrum (EDX) and X-ray photoelectron spectra (XPS, see next section). Furthermore, in Figure 2b, each band that also exists in Figure 2a can be observed aside from a little change for the center position. This indicates that Li+ ions have been introduced in the BPO4 lattice and the carbon impurities also exist in the Li+-doped BPO4. FESEM, EDS, and XPS. To confirm the component and the morphology, we subjected the BPO4:x Li+ sample to FESEM, EDS, and XPS analysis. Panels a and b of Figure 3 show the FESEM images and the energy-dispersive X-ray spectrum (EDX) of the BPO4:6% Li+ (Figure 3b) sample, respectively. From Figure 3 we can see that the BPO4:6% Li+ samples are composed of aggregated particles ranging from 500 to 1000 nm in size. EDX examination confirms the presence of P, O, B, C, and Si from the samples (Si from the silicon substrate in Figure 3b). The detected carbon impurities (C, also observed in the IR and XPS spectrum) are from the sample prepared by the PSG process in which some organic solvents and additives (glycerol and poly(ethylene glycol)) were employed.

Figure 3. FESEM micrograph (a) and EDS (b) for PSG-derived BPO4:6%Li+.

Figure 4a shows the survey XPS spectrum of the PSG-derived BPO4 (black line) and BPO4:6% Li+ (red line) samples in the binding energy range of 0 to 900 eV. It can be seen that the PSG-derived BPO4 and BPO4:6% Li+ samples contain B, P, and O elements and no other elements are detected except for carbon. The carbon impurities may result from the PSG process in which some organic solvents and additives [glycerol and poly(ethylene glycol)] were employed. Figure 4b shows the XPS spectra of O1s core level peaks of BPO4 (black line) and BPO4:6% Li+ (red line), respectively. The O1s peak of BPO4 is located at 531.6 eV; however, the O1s peak of BPO4:6% Li+ decreases and shifts to 532 eV, as shown in Figure 4c. The decrease and shift of the peak (O1s) may be correlated with the oxygen vacancies or oxygen, which is very common in the samples with high surface energy.18a Photoluminescence Properties. Under UV-light irradiation, the PSG-derived BPO4 shows a very weak blue luminescence; however, BPO4:6% Li+ (as representative of BPO4:x Li+) exhibits bright blue luminescence. Furthermore, from Figure S4 in the Supporting Information, we can see that the PL emission intensity of BPO4:6% Li+ (green line) is much stronger than that of BPO4:6% Ba2+ (blue line),11b BPO4-50% SiO2 (red line), and BPO4-10% Al2O3 (black line).19 Figure 5 shows the excitation (a) and the emission (b) spectra of the PSG-derived BPO4 (blue line), BPO4:6% Li+ (red line), and the solid-state (SS) obtained BPO4 (black line), respectively. The SS-derived BPO4 do not show any luminescence (Figure 5b, black line). The slight emission is from the xenon lamp background in the measurement process. For the PSG-derived BPO4, the emission spectrum (Figure 5b, blue line) consists of a weak broadband ranging from 380 to 500 nm with a maximum at 413 nm. However, the emission spectrum of the PSG-derived

Structural Properties of Li+-Doped BPO4

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Figure 6. CIE chromaticity diagram showing the emission color for BPO4:6% Li+. The inset is the luminescent photograph of BPO4:6% Li+ under the excitation of a 365 nm UV lamp in a dark room. Figure 4. (a) The survey XPS spectra for PSG-derived BPO4 (black line) and BPO4:6% Li+ (red line) samples; (b) XPS narrow scan spectra of O1s core level peaks of PSG-derived BPO4 (black line) and BPO4:6% Li+ (red line) samples, respectively.

Figure 5. Excitation spectra (a) and emission spectra (b) for BPO4 and BPO4:6% Li+ annealed at 960 °C (PSG ) Pechini-type sol-gel process; SS ) solid-state reaction). All spectra were measured under the same instrumental conditions, so they are comparable.

BPO4:6% Li+ sample (Figure 5b, red line) shows a very strong emission band ranging from 360 to 570 nm (centered at 416 nm), whose PL intensity is much higher than that of pure BPO4. The excitation for BPO4:6% Li+ (Figure 5a, red line) is greatly broadened and intensified in comparison with that of pure BPO4.

The BPO4:6% Li+ sample can be excited easily by light from 200 to 400 nm, leading to the strong bluish-white emission. The chromaticity coordinates (CIE) of BPO4:6% Li+ are x ) 0.1583 and y ) 0.1030 (Figure 6). The inset of Figure 6 is the luminescent photograph of BPO4:6% Li+ under the excitation of a 365 nm UV lamp in a dark room. To understand and optimize the luminescent properties of B1-xPO4:x Li+ luminescent materials, we performed experiments with different doping concentration of Li+, the amount of glycerol additive, and the other alkali metal ions (Na+ and K+). It is found that the shape and PL intensity for the emission spectra clearly vary with the change of Li+ concentration, the amount of glycerol additive, and the doping ions of the alkali metals, as shown in Figures 7 and 8, respectively. Figure 7 shows the integrated PL intensity changes with altering Li+doping concentration and volume of glycerol additive. The PL intensity first increases with increasing Li+ concentration (x) and volume of glycerol additive, reaching a maxima at x ) 0.06 and 5 mL of glycerol, respectively, and then decreases quickly with further increasing of Li+ concentration and glycerol volume. Thus, the optimum doping concentration of Li+ is 6% and the glycerol volume is 5 mL for obtaining the strongest PL emission intensity. The excitation and emission spectra of BPO4:6% Li+ have an overlap (350-400 nm), so there should be a great probability of energy transfer among the emission centers. This energy transfer will result in luminescence quenching when the concentration of the emission center is high enough.1 Thus, it is understandable that there is optimum concentration of Li+ and optimum volume of glycerol additive (which bring about the emission centers) for obtaining the strongest PL emission intensity. Additionally, to research the luminescent mechanism, other alkali metals ions (Na+ and K+) were doped into BPO4 host in the same way as that for Li+. Figure 8 shows the emission spectra of BPO4:6% Li+ (red line), BPO4:6% Na+ (blue line), and BPO4:6% K+ (black line). Under the UV (311 nm) excitation, all the samples display similar luminescence properties, but BPO4:6% Li+ have the highest PL intensity. Because the Li+ ion is smaller than the other alkali

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Figure 7. (a) The emission intensity of PSG-derived BPO4:x Li+ as a function of Li+ concentration (x, molar ratio); (b) the emission intensity of PSG-derived BPO4:6% Li+ as function of volume (glycerol additive, mL).

Figure 8. Comparison for the emission spectra of sol-gel derived BPO4:6% Li+ (red line), BPO4:6% Na+ (blue line), and BPO4:6% K+ (black line) under UV 311 nm excitation.

metal ions (Na+ and K+), it is assumed that Li+ ions are easily built into the interstitial sites of the BPO4 lattice. The luminescence decay curves for BPO4 and BPO4:6% Li+ are shown in Figure 9. This indicates that each kind of sample (BPO4 and BPO4:6% Li+) has two kinds of lifetimes (a long lifetime and a short lifetime) and two kinds of decay curves. The two kinds of decay curves were obtained by respective measurement. The luminescence decay curves of Figure 9a,c can be fitted into a single-exponential function as I(t) ) A exp(-t/τ) (where τ is the 1/e lifetime), from which the short lifetimes for BPO4 (detected at 413 nm) and BPO4:6% Li+ (detected at 416 nm) are determined to be 4.1 and 5.9 ns, respectively. However, the luminescence decay curves of Figure 9b,d cannot be fitted into a single-exponential function, but into

Zhang et al. a double-exponential function as I(t) ) A1 exp(-t/τ1) + A2 exp(-t/τ2), where τ1 and τ2 are the fast and slow components of the luminescent lifetimes, and A1 and A2 are the fitting parameters, respectively. The long lifetimes for BPO4 (detected at 413 nm) and BPO4:6% Li+ (detected at 416 nm) are determined to be 0.414 and 0.529 ms, respectively, according to the equation τ ) (A1τ12 + A2τ22)/(A1τ1 + A2τ2).18b,19 In addition, the decay curves and lifetime features for Na+- and K+-doped BPO4 samples can be assigned similarly as for those of BPO4:6% Li+. In view of the luminescence decay curves, it can be concluded that two types of kinetic decay behaviors and two kinds of lifetimes are attributed to two absolutely different luminescent centers in the samples.19 Possible Luminescence Mechanisms. Since B3+, P5+, and + Li themselves are nonluminous, the observed luminescence from BPO4 and BPO4:x Li+ samples should be related to defect and/or impurities in the systems as reported previously in other analogues.4-8,11b,19 The two kinds of luminescence lifetimes and two types of kinetic decay behaviors in these samples indicate that there are at least two absolute different emission centers existing in each sample. Here, we assumed that the carbon-related impurities play a key role in the luminescence of BPO4 and BPO4:6% Li+, and have the following evidence to support the above luminescence mechanisms. First, the profile of the emission spectra (emission range: 380-500 nm, λmax ) 413 nm for BPO4 (PSG); emission range: 360-570 nm, λmax ) 416 nm for BPO4:6% Li+, respectively) and the PL short lifetimes (4.1 ns for BPO4, 5.9 ns for BPO4:6% Li+) are similar to those of sol-gel-derived SiO2 gels (emission range: 350-700 nm, λmax varies between 405 and 550 nm depending on the preparation conditions; PL lifetimes