Enhanced Luminescence of BPO4 by Mixing with SiO2 and Al2O3

Jan 16, 2008 - mechanisms of PSG-derived BPO4-xSiO2 and BPO4-xAl2O3 samples were ... The PSG process, which uses common metal salts (nitrates,...
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J. Phys. Chem. C 2008, 112, 2183-2192

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Enhanced Luminescence of BPO4 by Mixing with SiO2 and Al2O3 Cuimiao Zhang, Cuikun Lin, Chunxia Li, Zewei Quan, Xiaoming Liu, and Jun Lin* State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed: October 16, 2007; In Final Form: NoVember 8, 2007

In this paper, BPO4-xSiO2 (x: SiO2/BPO4 molar ratio, 0-70%) and BPO4-xAl2O3 (x: Al2O3/BPO4 molar ratio, 0-20%) powder samples were prepared by the Pechini-type sol-gel (PSG) process using glycerol and poly(ethylene glycol) as additives. 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), diffuse reflection spectra, photoluminescence (PL) excitation and emission spectra, kinetic decay, and X-ray photoelectron spectra (XPS), respectively. It was found that the Pechini-type sol-gel-derived BPO4-xSiO2 annealed at 1000 °C and BPO4-xAl2O3 annealed at 960 °C exhibited bright bluish-white emissions centered at 428 and 413 nm, respectively. The luminescence decay curve analysis indicates that each sample has two kinds of lifetimes (more than 0.4 ms and less than 10 ns) and two types of kinetic decay behaviors, which can be fitted into a double-exponential function and a single-exponential function, respectively. This can be attributed to two kinds of emission centers in samples. The luminescent mechanisms of PSG-derived BPO4-xSiO2 and BPO4-xAl2O3 samples were 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 agents 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 alkoxidebased sol-gel process. For instance, most of the metal alkoxides * Author to whom any correspondence should be addressed. E-mail: [email protected].

suffer from high cost, unavailability, toxicity, and a fast hydrolysis rate. (Thus, it is difficult to control the homogeneity of different components during experimental processes.)10 Furthermore, the materials prepared by this process can be made into powders and 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, coreshell structured phosphors, and pigments.11 Alternatively, defects can influence the properties of materials greatly, such as dielectric, luminescent, and photochemical properties.12 The polycrystalline powder of BPO4 is well known as a catalyst for a range of organic chemical reactions.13-16 Recently, we found that PSG process-derived BPO4 exhibits 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 BPO4based materials to enhance the luminescent intensity and investigate the luminescence mechanisms. In this paper, two kinds of stable, efficient, and environmentally friendly luminescent materials based on SiO2 and Al2O3-mixed BPO4 (without rare earth or transition metal ions as activators) were prepared by the PSG process. These two kinds of materials have higher luminescence intensities and are more environmentally friendly than Ba2+-doped BPO4. The luminescent materials based on BPO4-xSiO2 and BPO4-xAl2O3 (x ) SiO2/BPO4 or Al2O3/BPO4 molar ratio) show an intense bluish-white emission (350-600 nm, λmax ) 428 nm for the former and 350-575 nm, λmax ) 413 nm for the latter) under a wide range of UV light excitation (200-400 nm). Possible mechanisms have been proposed to explain the observed luminescent phenomena.

10.1021/jp710046x CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008

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2. Experimental Section All of the chemicals were of analytical grade reagents purchased from Beijing Chemical Corporation and used without further purification. Preparation of BPO4-xSiO2. BPO4-xSiO2 (x ) 0-70%) 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 (v/v ) 1:4) 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 tetraethoxysilane (TEOS) were dropped into the solution and subsequently mixed with a certain amount of glycerol. Transparent sol was obtained after mixing the above two solutions and 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 1000 °C for 3 h to produce the final samples. Preparation of BPO4-xAl2O3. BPO4-xAl2O3 (x ) 0-20%) powders were also prepared by the PSG process.9-11 Stoichiometric weights of (NH4)2HPO4 and Al(NO3)3‚9H2O were dissolved in water under vigorous stirring. Then H3BO3 (50 mol % excess to compensate the evaporation of B at high temperature), citric acid as the chelating agent, and a certain amount of polyethylene glycol (PEG, 20 000) as the cross-linking agent were added to the solution and then mixed with a certain amount of glycerol reagent. The molar ratio of citric acid to (NH4)2HPO4 was 2:1. The following process was the same as that of BPO4-xSiO2 except that the annealing temperature for BPO4xAl2O3 was 960 °C. Characterization. The X-ray diffraction (XRD) of the powder samples was examined on a Rigaku-Dmax 2500 diffractometer using Cu Ka radiation (λ ) 0.15405 nm). Fourier transform infrared (FT-IR) spectra were measured with a PerkinElmer 580B infrared spectrophotometer with the KBr pellet technique. The morphology and composition of the samples were inspected using a field emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energydispersive X-ray spectrum (EDX, JEOL JXA-840). The X-ray photoelectron spectra (XPS) were taken on a VG ESCALAB MK II electron spectrometer using Mg KR (1253.6 eV) as the exciting source. Diffuse reflectance spectra were carried out on a Hitachi F-4100 spectrophotometer from 200 to 800 nm. 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) using 310 nm lasers (pulse width ) 4 ns) as the excitation source (Continuum Sunlite OPO). The crystal parameters and bond lengths of BPO4, BPO4-xSiO2, and BPO4xAl2O3 were calculated using CASTEP code.17 All measurements were performed at room temperature. 3. Results and Discussion XRD. Figure 1 shows the XRD patterns for the synthesized BPO4-xSiO2: (a) x ) 0, (b) x ) 30, (c) x ) 50, (d) x ) 60% 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 other phase can be detected, indicating that the SiO4 groups have been dissolved successfully in the BPO4 host. However, with the

Figure 1. XRD patterns for PSG-derived BPO4 (a), BPO4-30% SiO2 (b), BPO4-50% SiO2 (c), BPO4-60% SiO2 (d), and the standard data of BPO4 (JCPDS 34-0132) as a reference. All of the samples were obtained after annealing at 1000 °C for 3 h.

Figure 2. XRD patterns for PSG-derived BPO4 (a), BPO4-1% Al2O3 (b), BPO4-5% Al2O3 (c), BPO4-10% Al2O3 (d), BPO4-15% Al2O3 (e), and JCPDS 34-0132 for BPO4 as a reference. All of the samples were obtained after annealing at 960 °C for 3 h.

Figure 3. FT-IR spectra for PSG-derived BPO4 (a), BPO4-50% SiO2 (b), and BPO4-10% Al2O3 (c).

increase of mixing concentration (x) of SiO2 in BPO4, the XRD peak intensity becomes weaker. This is related to the crystal structure of BPO4. Pure BPO4 crystallizes in a tetragonal lattice, with space group I4 (No. 82).18 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 11a).19 A decrease in XRD peak intensity with increasing concentration of SiO2 indicates that SiO4 groups may replace the BO4 and/or PO4 in lattice sites.20 The XRD patterns of BPO4-xAl2O3 samples prepared by the PEG process are shown in Figure 2, (a) BPO4, (b) BPO4-1% Al2O3, (c) BPO4-5% Al2O3, (d) BPO4-10% Al2O3, and (e) BPO4-

Enhanced Luminescence of BPO4

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Figure 4. FESEM micrograph (a), EDX (b) for PSG-derived BPO4-50% SiO2, and FESEM micrograph (c), EDX (d) for PSG-derived BPO4-10% Al2O3.

15% Al2O3, respectively. As seen clearly in Figure 2, the diffraction peaks of the PSG-derived BPO4 and BPO4-xAl2O3 samples are consistent with those of the standard data of BPO4 (JCPDS No. 34-0132) and can be indexed accordingly when the content of Al2O3 is very low. However, with the increase of the concentration (x) of Al2O3 in BPO4, the XRD patterns show a few other weak peaks between 20 and 24° and the peak intensity increases with the growing x value. It can be concluded that AlO4 or AlO5 have been dissolved successfully in the BPO4 host when the mixing concentration was low. Nevertheless, a little AlPO4 may be formed with the increase of Al2O3 concentration. Tables 1 and 2 show the lattice parameters, bond lengths (BO, P-O), and the distance B-P of BPO4-xSiO2 (x ) 0, 30, 50, 60%; 1000 °C) and BPO4-xAl2O3 (x ) 0, 5, 10, 15%; 960 °C), respectively, which were calculated on the basis of XRD data. Basically, from Tables 1 and 2, it can be seen that the lattice parameters and bond lengths first increase with increasing SiO2 or Al2O3 concentration (x), reaching a maximum at x ) 50% (SiO2) or x ) 10% (Al2O3), then decrease with further increasing SiO2 or Al2O3 concentration. From the fact that ionic radii of Si4+ (r ) 42 pm) and Al3+ (r ) 50 pm) are bigger than those of B3+ (r ) 20 pm) and P5+ (r ) 34 pm), it is deduced that part of SiO4 or AlO4 will replace the BO4 and/or PO4 in the host lattice. The rest of the SiO2 or Al2O3 may occupy the interstitial sites of the BPO4 lattice. This is the reason that introduction of SiO2 or Al2O3 induces the slight change of the lattice parameters and bond lengths for BPO4. FT-IR. Figure 3 shows the FT-IR spectra of the BPO4 (a), BPO4-50% SiO2 (b), and BPO4-10% Al2O3 (c) samples. In Figure 3a, 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.19 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.19 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,21 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 temperatures (900-1000 °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, the characteristic vibrations of Si-O bonds are observed at 470, 800, and 1100 cm-1, respectively.19 In Figure 3b, the characteristic vibration of Si-O bonds at 468 cm-1 can be observed. It indicates that SiO2 has been introduced in the BPO4 lattice. But we cannot distinguish other bond vibrations (800 and 1100 cm-1) of Si-O. This is because the band in the range of 7001000 cm-1 (centered at 933 cm-1, including 800 cm-1) is assigned to the asymmetric stretching vibration of the BO4 tetrahedron and the band in the range of 1045-1150 cm-1 (including 1100 cm-1) is attributed to the asymmetric stretching vibrations of PO4 groups. Comparing with Figure 3a and b, there is a weak peak at 731 cm-1 in Figure 3c, which is assigned to the absorption of AlO4. This suggests that a small amount of PO4 and BO4 groups in BPO4 have been replaced by the AlO4 groups.21 FESEM, EDX, and XPS. To confirm the component and the morphology, we subjected BPO4-xSiO2 and BPO4-xAl2O3 samples to FESEM, EDX, and XPS analysis. Figure 4 shows

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Figure 7. (a) Excitation and emission spectra for BPO4 and BPO450% SiO2 annealed at 1000 °C; (b) excitation and emission spectra for BPO4 and BPO4-10% Al2O3 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.

Figure 5. Survey XPS spectra for PSG-derived BPO4 (a), BPO4-50% SiO2 (b), and BPO4-10% Al2O3 (c) samples.

Figure 6. Diffuse reflection spectra of PSG-derived BPO4 (a), BPO450% SiO2 (b), and BPO4-10% Al2O3 (c).

Figure 8. CIE chromaticity diagram showing the emission color for BPO4-50% SiO2 (point a) and BPO4-10% Al2O3 (point b).

the FESEM images and the energy-dispersive X-ray spectra (EDX) of the BPO4-50% SiO2 (a and b) and BPO4-10% Al2O3

(c and d) samples, respectively. From Figure 4a and c, we can see that the BPO4-50% SiO2 and BPO4-10% Al2O3 samples are

Enhanced Luminescence of BPO4

Figure 9. (a) Emission intensity of PSG-derived BPO4-xSiO2 as a function of SiO2 concentration (x, molar ratio); (b) the emission intensity of PSG-derived BPO4-xAl2O3 as function of Al2O3 concentration (x, molar ratio).

composed of aggregated particles ranging from 500 to 1000 nm in size. EDX examination confirms the presence of P, O, B, C, and Si (Al) from the samples (Si from the silicon substrate in Figure 4d). In Figure 4b, the Si signals observed in the EDX spectrum may be contributed from the overlapped effects of the silicon substrate and Si dopants. The Si signals can also be observed in the FT-IR (Figure 3b) and XPS spectra (Figure 5b) of BPO4-xSiO2, which further confirm the existence of Si in sample. The detected carbon impurities (C, also observed in the IR and XPS spectra) are from the sample prepared by the PSG process in which some organic solvents and additives (glycerol and poly(ethylene glycol)) were employed. Figure 5 shows the survey XPS spectra of the PEG-derived BPO4 (a), BPO4-50%SiO2 (b), and BPO4-10%Al2O3 (c) samples in the bingding energy range of 0-990 eV. In Figure 5a, it can be seen that the PEG-derived BPO4 contains B, P, and O elements and no other elements are detected besides carbon. Moreover, the carbon element is also detected in Figure 5b and c. This demonstrates that the carbon impurities also exist on the surface of PEG-derived BPO4, BPO4-50%SiO2, and BPO410%Al2O3 samples. In addition, Figure 5b shows that the Si element exists in BPO4-50%SiO2. The inset in Figure 5 shows the narrow scan spectrum of Al2p at 74.4 eV, which indicates the presence of Al element in the BPO4-10%Al2O3 sample. Photoluminescence Properties. Under UV-light irradiation, PEG-derived BPO4 shows a very weak blue luminescence; however, BPO4-50% SiO2 and BPO4-10% Al2O3 (as representative of BPO4-xSiO2 and BPO4-xAl2O3) exhibit bright bluish-

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Figure 10. (a) Emission spectra for BPO4-50% SiO2: 900 °C (black line), 960 °C (green line), 1000 °C (red line), and 1100 °C (blue line); (b) emission spectra for BPO4-10% Al2O3: 800 °C (black line), 900 °C (green line), 960 °C (blue line), and 1000 °C (red line).

white luminescence. Furthermore, from Figure S1, we can see that the PL emission intensity of BPO4-50% SiO2 (red line) and BPO4-10% Al2O3 (black line) are much stronger than that of BPO4/6 mol % Ba2+ (blue line).11b Diffuse reflection spectra (Figure 6) show that pure BPO4 (black line) has a weak absorption band from 200 to 350 nm with two maxima at 231 and 281 nm, BPO4-50% SiO2 (red line) has a strong absorption from 200 to 600 nm with two maxima at 314 and 362 nm, and BPO4-10% Al2O3 (blue line) also has a strong absorption from 200 to 600 nm with two maxima at 332 and 376 nm. Figure 7a shows the excitation and emission spectra of the PSG-derived BPO4 (blue line), BPO4-50% SiO2 (red line), and the solid-state (SS) obtained BPO4 (black line), respectively. The SS-derived BPO4 do not show any luminescence (Figure 7a, black line). The slight emission is from the xenon lamp background in measurement process. For the PSG-derived BPO4, the emission spectrum consists of a weak broad band ranging from 380 to 500 nm with a maximum at 413 nm, and the corresponding excitation spectrum, which agrees with the diffuse reflection spectrum (Figure 6a), includes a broad band from 200 to 330 nm with a maximum at 307 nm. However, the emission spectrum of the PSG-derived BPO4-50% SiO2 sample (Figure 7a, red line) shows a very strong emission band ranging from 350 to 600 nm (centered at 428 nm), whose integral intensity is 10 times as large as that of pure BPO4 (without SiO2). Basically in agreement with the diffuse reflection spectrum (Figure 6b), the excitation spectrum of BPO4-50% SiO2 (Figure 7a, red line) contains a very strong and broad band ranging from 200 to 400 nm with a maximum at 313 nm. The excitation for BPO4-50% SiO2 is greatly broadened and intensified in comparison with that of pure BPO4. The BPO4-50% SiO2

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Figure 11. Structure models of BPO4: ideal (a), interstitial carbon (c), oxygen vacancy (e), and the calculated band structures according to the models of ideal BPO4 (b), interstitial carbon-defective BPO4 (d), and oxygen vacancy defective BPO4 (f).

TABLE 1: Cell Constants and Bonding Lengths of PSG-Derived BPO4-xSiO2 (x ) 0, 30, 50, 60%, 1000 °C) Calculated Using CASTEP Code

TABLE 2: Cell Constants and Bonding Lengths of PSG-Derived BPO4-10% Al2O3 (x ) 0, 5, 10, 15%, 960 °C) Calculated Using CASTEP Code

x (%)

0

30

50

60

x (%)

0

5

10

15

a (Å) b (Å) B-O (Å) P-O (Å) B-P (Å)

4.3317 6.6396 1.437 1.544 2.730

4.3368 6.6375 1.441 1.547 2.734

4.3453 6.6383 1.447 1.553 2.740

4.3149 6.6368 1.425 1.530 2.716

a (Å) b (Å) B-O (Å) P-O (Å) B-P (Å)

4.3391 6.6473 1.442 1.554 2.735

4.3418 6.6571 1.444 1.561 2.738

4.3516 6.6615 1.451 1.571 2.745

4.3344 6.6432 1.439 1.548 2.732

sample can be excited easily by the light from 200 to 400 nm, leading to the strong bluish-white emission. The chromaticity coordinates (CIE) of BPO4-50% SiO2 are x ) 0.1595 and y ) 0.1385 (point a in Figure 8). BPO4-10% Al2O3 also exhibits a bright bluish-white luminescence, but the color is blue-shifted, which is proven by the emission spectrum and the chromaticity coordinates (CIE) (point b in Figure 8). The CIE of BPO4-10% Al2O3 are x ) 0.1564 and y ) 0.1133. Figure 7b shows the excitation and emission spectra of PSG-derived BPO4 (blue line), BPO4-10% Al2O3 (red

line), and SS-derived BPO4 (black line), respectively. For BPO410% Al2O3, the emission spectrum consists of a very strong broad band ranging from 350 to 600 nm with a maximum at 413 nm (Figure 7b, red line), and the excitation spectrum (Figure 7b, red line), which basically agrees with the diffuse reflection spectrum (Figure 6c, blue line), includes a broad band ranging from 200 to 400 nm with two maxima at 308 and 372 nm. It is very different from the spectrum of pure BPO4. To understand and optimize the luminescent properties of BPO4-xSiO2 and BPO4-xAl2O3 luminescent materials, we

Enhanced Luminescence of BPO4 performed experiments with different mixing concentrations of SiO2 and Al2O3 and annealing temperatures. It is found that the shape and PL intensity for the emission spectra vary to some extent with the change of the SiO2 or Al2O3 concentration and annealing temperature, as shown in Figures 9 and 10. This indicates that the environments of the emission centers change slightly with these factors. In Figure 9a, the PL intensity first increases with increasing SiO2 concentration (x), reaching a maximum at x ) 50%, and then decreases quickly with further increasing SiO2 concentration. This phenomenon also exists in BPO4-xAl2O3 (Figure 9b). Thus, the optimum mixing concentration of SiO2 is determined to be 50% while the optimum mixing concentration of Al2O3 is 10% for obtaining the strongest PL emission intensity. The excitation and emission spectra of BPO4xSiO2 and BPO4-xAl2O3 have much overlap, 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. Thus, it is understandable that there is an optimum concentration of SiO2 or Al2O3, which brings about the emission centers, for obtaining the strongest PL emission intensity. In addition, Figure 10 shows the influence of annealing temperature on the PL emission intensity of BPO4-xSiO2 and BPO4-xAl2O3 samples. Apparently, in Figure 10, the optimum annealing temperature for BPO4-xSiO2 is 1000 °C, while the optimum annealing temperature for BPO4-xAl2O3 sample is 960 °C. The luminescence decay curves for BPO4, BPO4-50% SiO2, and BPO4-10% Al2O3 are shown in Figure S2. It indicates that each kind of sample (BPO4, BPO4-50% SiO2, and BPO4-10% Al2O3) has two kinds of lifetimes (a long lifetime and a short lifetime) and two types of decay curves. The two types of decay curves were obtained by independent measurements. The luminescence decay curves of Figure S2 (a, c, and e) can be fitted into 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), BPO4-50% SiO2 (detected at 428 nm), and BPO4-10% Al2O3 (detected at 413 nm) are determined to be 0.414, 0.482, and 0.514 ms, respectively, according to the equation τ ) (A1τ12 + A2τ22)/(A1τ1 + A2τ2).22 However, the luminescence decay curves of Figure S2 (b, d, and f) cannot be fitted into a double-exponential function but into a singleexponential function as I(t) ) A exp(-t/τ) (where τ is the 1/e lifetime), from which the short lifetimes for BPO4, BPO4-50% SiO2, and BPO4-10% Al2O3 are determined to be 4.1, 5.1, and 5.2 ns, respectively. In view of the luminescence decay curves, it can be concluded that the two types of kinetic decay behaviors and two kinds of lifetimes may be attributed to two absolutely different luminescence centers (mechanisms) in the samples. Possible Luminescence Mechanisms. Because Si4+ and Al3+ are not able to show luminescence themselves, the observed luminescence from BPO4, BPO4-xSiO2, and BPO4-xAl2O3 samples should be related to defect and/or impurities in the systems as reported previously in other analogues.4-8,23,24 The greatly different luminescence lifetimes and kinetic decay behaviors in the BPO4, BPO4-50% SiO2, and BPO4-10% Al2O3 samples indicate that the emission centers in these samples are absolutely different. There are at least two emission centers in these samples. Here, it is assumed that the carbon-related impurities play a key role in the luminescence of BPO4, BPO4-xSiO2, and BPO4xAl2O3, and we have the following evidence to support the above luminescence mechanisms. First, the profile of the

J. Phys. Chem. C, Vol. 112, No. 6, 2008 2189 emission spectra (emission range: 380-500 nm, λmax ) 413 nm for BPO4 (PSG); 350-600 nm, λmax ) 428 nm for BPO450% SiO2; 350-550 nm, λmax ) 413 nm for BPO4-10% Al2O3, respectively) and the PL short lifetimes (4.1 ns for BPO4, 5.1 ns for BPO4-50% SiO2, and 5.2 ns for BPO4-10% Al2O3) 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 < 10 ns) based on TEOS and TMOS incorporating a variety of carboxylic acids, in which the luminescence is attributed to the carbon impurities built into the -Si-O-network. 4b Second, the carbon impurities have been detected by FT-IR spectra (Figure 3), EDX spectra for BPO4-50% SiO2 and BPO4-10% Al2O3 (Figure 4b and 4d), and an X-ray photoelectron spectrum (XPS) for BPO4 (Figure 5). It has been studied that carbon impurities in TiO2 result in modest variations of the band gap and induce several localized occupied states in the gap that will modify the absorption and emission properties.4a,6, 24 The same situation might also occur in our samples. Third, our group has reported the luminescence properties of Ba2+-doped BPO4 materials.11b The lifetimes of BPO4 and BPO4/Ba2+ are determined to be 6.4 and 10.8 ns, respectively. The possible luminescence mechanism of BPO4 was attributed to carbon impurities. The luminescence properties of BPO4/Ba2+ samples were induced by carbon impurities and peroxyl radicals. The peroxyl radicals could be confirmed by the EPR signals. No obvious EPR signal was observed for the undoped BPO4, whereas BPO4/Ba2+ exhibited a strong and sharp EPR band at g ) 2.001, which was caused by peroxyl radicals.11b Compared with our present work, the short lifetimes of BPO4, BPO4-50% SiO2, and BPO4-10% Al2O3 are very similar to the undoped BPO4 because no obvious EPR signal is observed and the short lifetimes are also less than 10 ns. So we can assume that the short lifetime luminescence is induced by carbon impurities. Finally, to further check the effects of carbon impurities, we prepared the BPO4 sample through the solidstate reaction (SS) process (mixing H3BO3 and (NH4)2HPO4 powders and sintering them at high temperature (1000 or 960 °C), without using any organic solvents and additives; thus, no carbon impurities were involved). These SS-derived BPO4 samples did not show any luminescence under the same excitation conditions (the excitation and emission spectra cannot be detected as shown in Figure 7, black line). From the above results, we can conclude that the short lifetime luminescence properties are caused mainly by the carbon impurities in the host lattice. In the preparation of BPO4, BPO4-xSiO2, and BPO4xAl2O3 via the PSG process, citric acid (CA) was used to coordinate B and P, forming -O-B-COO-(COH)-COOHtype and -O-P-COO-(COH)-COOH-type complexes. The -COOH groups in the complexes further reacted with the glycerol and PEG (cross-linking agents that contain a large amount of -CH2 and -OH groups), which were employed as additives. In the subsequent annealing process, most of the organic species decomposed into H2O and CO2 and escaped from the system, but minor amounts of them may decompose to create carbon impurity defects for BPO4, BPO4-xSiO2, and BPO4-xAl2O3. Because the ionic radius of C4+ is very small (16 pm), the carbon impurities probably occupy the interstitial sites of the lattice. The interstitial carbon impurities are assumed to be the luminescent species in the host lattice.4-8,24 These organic species are attached to the -O-B-O-P-O- networks, such as -O-B-O-C-O-P-, -O-B-O-C-R, -O-P-O-C-R, and so forth. The bond energies of B-O, P-O, C-O, and C-C are ∼806, ∼597, ∼357.7, and ∼345.6 kJ/mol, respectively. Note that the bond energies of B-O and

2190 J. Phys. Chem. C, Vol. 112, No. 6, 2008 P-O are larger than those of the C-O and C-C bonds and the bond energies lead to a significant dependence of the order of bond cleavage on the chemical and thermal environment and remove a portion of the network oxygen along with carbon. For example

-O-B-O-C-R + -O-P-O-C-R f BPO4-x + R‚ x/2O (1) -O-B-O-CV O-P-O- f -O-B-O-C‚ + ‚O-P-Oor -O-B-OV C-O-P-O- f -O-B-O‚ + ‚C-O-P-O- (2) where x is the net oxygen vacancy created by the annealing process, and the vertical arrows show the possible bond cleavage site. The above bond cleavages occur at a temperature where healing of the fragmented bonds and normalization of the coordination state are thermodynamically prohibited. Charge unbalances created at these sites must be rectified by the localization of electrons and electron holes. The residual fragmented bonds are apparently the precursors for various centers. These defect centers induce an electron being localized in a 2p orbital of the single bonded carbon and the single bonded oxygen. This would give rise to photoluminescence through a strong electron-photon coupling.4a,6,24-26 In addition, the greatly different luminescence properties from those induced by carbon impurities (including lifetimes and kinetic decay behaviors) are observed. These luminescence lifetimes are much longer, and the luminescence decay curves can be fitted to a double-exponential function as I(t) ) A1 exp(-t/τ1) + A2 exp(-t/τ2). It is well known that the annealing process can cause recovery of the structure defects and eliminate the nonradiative combination centers efficiently.27 In general, defects such as oxygen vacancies are known to be the most common defects and usually act as radiative centers in luminescence processes. Therefore, the long lifetime luminescence properties can be ascribed to other kinds of defects different from the carbon impurities: the oxygen defects in the BPO4, BPO4-50% SiO2, and BPO4-10% Al2O3 lattices (like B-VOP, VO is an oxygen vacancy). This can be confirmed by the longer lifetimes (0.414, 0.482, 0.514 ms), a clear sign of the recombination nature of the luminescence.28 Among the oxygen vacancies, V‚O is assumed to be the recombination center for the luminescence emission,29 which has an effectively monovalent positive charge with respect to the regular O2- site. Actually, after such a recombination the effectively neutral V ×O center will be formed, whose energy is very close to the conduction band edge because of the correlation energy of the two electrons.30 Therefore, photons with an energy of 3.0 eV (413 nm) will not be emitted through a transition of an electron from the conduction band to a V‚O level because such a transition occurs effectively between the conduction band edge and the V ×O level. However, a mechanism in which the recombination of a conduction band electron with a V‚‚O center (an oxygen vacancy containing no electrons, having an effective divalent positive charge with respect to the normal O2- site) can yield such a blue emission of the photoexcited BPO4 particles can be proposed. Each recombination luminescence includes the following processes: ionization, migration, recombination, and emission. The excitation of the BPO4 sample starts with the creation of an electron-hole pair using photons or electrons with energy exceeding the BPO4 band gap. The electron is promoted from the valence band to the conduction band, leaving

Zhang et al. a hole in the valence band. The active hole formed can be trapped at the V‚O center directly to form the V‚‚O center or at the surface of the particle. Then the surface-trapped hole may transfer back into the particle to recombine with an electron in a deep trap (V‚O) to form the V‚‚O center. Thereafter, recombination of a V‚‚O center with a conduction band electron gives rise to the blue emission. This can be consistent with the longer lifetime (0.414, 0.482, 0.514 ms). Schematically, as representative of PSG-derived BPO4, it can be described as follows:

BPO4 + hV (4.04 eV) f BPO4(e′CB + h‚VB)

(3)

V‚O + h‚VB f V‚‚O

(4)

BPO4(e′CB + h‚VB) f BPO4(e′CB + h‚surface)

(5)

V‚O + h‚surface f V‚‚O

(6)

V‚‚O + e′CB f V‚O + hVo (3.0 eV)

(7)

After photoexcitation, all of the above processes can take place in the BPO4 particles. Process 7 is the radiative process, which gives the blue emission in BPO4. Furthermore, after the addition of SiO2 or Al2O3 to the host lattice of BPO4, the PL emission intensity is enhanced dramatically. The mechanism of this phenomenon is not clear until now. However, it is speculated that the introduction of SiO2 or Al2O3 in BPO4 will disturb the frame structure of BO4 and PO4 tetrahedra (the slight change of the lattice constants and bonding lengths: Tables 1 and 2) to some extent, which can also be evidenced by the XRD (Figures 1 and 2) and the slight change of the constants and bond lengths (Tables 1 and 2). Thus, the oxygen vacancies may be formed more easily than pure BPO4 in the process of calcination. As the luminescent centers of the bluish-white emission in BPO4-xSiO2 or BPO4-xAl2O3, the oxygen defects will increase with increasing of the content of SiO2 or Al2O3, and the emission intensity can be enhanced accordingly. However, at higher concentrations of SiO2 (>50%) or Al2O3 (>10%) the energy transfer among the emission centers will result in quenching of the luminescence (Figure 9). Finally, the band structures of defective BPO4 were calculated using CASTEP code based on the density-function theory (DFT), and the exchange and correlation have been treated by the generalized gradient approximation (GGA) within the scheme due to Perdew-Burke-Ernzerhof (PBE).31 This method has been applied to many materials, such as SrAl2O4:Eu2+ nanocrystals, defective ZnO, aluminum hydroxide, and so forth.32 In this calculation, the structure models of ideal and defective BPO4, that is, interstitial carbon (Ci) and oxygen vacancy (VO), are established, as shown in Figure 11a, c, and e. The calculated band gap for ideal and defective BPO4 are given in Figure 11b, d (Ci), and f (VO). Supporting Information Figures S3-5 show the calculated partial PDOS plots of ideal BPO4, Ci-BPO4, and VO-BPO4, respectively. The calculated band gap for ideal BPO4 is larger than that of defective CiBPO4 and VO-BPO4. Both the interstitial carbon (Ci) and the oxygen vacancy (VO) can cause great change in the band structure (size of band gap, site of conduction band and valence band, and band structure). In an ideal BPO4 lattice, only B-O and P-O bonds (all B and P sites being four-coordinate and all O sites being two-coordinate) are present with a calculated band gap around 8.57 eV (theoretical value is around 8.9 eV) and will not show any luminescence.16 The presence of structural defects will introduce electronic states into the band gap,

Enhanced Luminescence of BPO4

J. Phys. Chem. C, Vol. 112, No. 6, 2008 2191 new efficient and environmentally friendly bluish-white luminescent materials. Acknowledgment. This project is financially supported by the foundation of “Bairen Jihua” of Chinese Academy of Sciences, the National Natural Science Foundation of China (50572103,20431030),andtheMOSTofChina(No.2003CB314707, 2007CB935502).

Figure 12. Energy band diagram showing the possible defects and emission processes in PSG-derived BPO4, BPO4-xSiO2, and BPO4xAl2O3.

resulting in luminescence of this material. Determined by the calculated partial PDOS plots shown in Supporting Information Figure S4, the energy levels located above and close to the Fermi energy level in Figure 11d are ascribed to the energy level of the Ci center. It is to be noted that the P element also contributes to these energy levels. In Figure 11f, the energy levels located below and close to the Fermi energy level (determined by the calculated partial PDOS plots; Supporting Information Figure S5) are attributed to the energy levels of the VO center. From Figure S5, we can note that B and P elements also contribute to the energy levels by the Fermi energy level. Figure 11d and f shows that the sizes of the calculated band gap for Ci and VO are very similar. This indicates that the emission wavelength arising from the VO center is also similar to that arising from the Ci center, which is in agreement with the proposed luminescent mechanisms of visible emission for BPO4, BPO4xSiO2, and BPO4-xAl2O3 particles. In summary, the whole emission process in our PSG-derived BPO4, BPO4-xSiO2, and BPO4-xAl2O3 is shown schematically in Figure 12. In addition, it should be noted that the calculated band gaps may be underestimated using this first-principle energy band calculation, probably because of many factors such as the action of the crystal field, the structure model, the defect density, the limits of method itself, and so forth.33 These factors are expected to give some contributions to the change of the calculated energy level (the calculated band gap is around 8.57 eV in Figure 11b, which is less than the theoretic value of the ideal BPO4 around 8.9 eV). Here they were not considered in view of the difficulty of doing so correctly. Nevertheless, the results of calculation can well explain the luminescence phenomenon and the proposed mechanisms. 4. Conclusions BPO4, BPO4-xSiO2 (x ) 0-70%), and BPO4-xAl2O3 (x ) 0-20%) powder samples have been prepared successfully by the Pechini-type sol-gel process using glycerol and poly(ethylene glycol) as additives followed by high-temperature annealing. The pure BPO4 shows a weak blue emission (413 nm); however, BPO4-xSiO2 and BPO4-xAl2O3 samples exhibit bright bluish-white emission (centered at 428 and 413 nm, respectively). The blue or bluish-white emission may be ascribed to the carbon impurities and the oxygen vacancies in the host lattice, respectively. Mixing SiO2 or Al2O3 with BPO4 results in the increase of the oxygen-related defects (most probably the oxygen vacancies) that enhance the emission intensity greatly and make the band of the emission spectrum broad. The optimum mixing concentrations of SiO2 and Al2O3 are 50% and 10%, respectively. These phosphors can potentially be used as

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