Quantitative Luminescence Switching in CePO4: Tb by Redox Reactions

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Quantitative Luminescence Switching in CePO4:Tb by Redox Reactions Mami Kitsuda and Shinobu Fujihara* Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

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bS Supporting Information ABSTRACT: Luminescence-switching behaviors of CePO4: Tb3þ upon redox reactions were examined using KMnO4 and L(þ)-ascorbic acid as an oxidant and a reductant, respectively. Powdery nanorod-like CePO4:Tb3þ particles were synthesized by liquid-phase reactions of rare-earth ions and phosphate ions in aqueous solutions at low temperatures. According to X-ray diffractometry, X-ray photoelectron spectroscopy, and transmission electron microscopy, the oxidizing treatment of CePO4:Tb3þ was found to bring about partial oxidation of Ce3þ to Ce4þ and a small degree of structural damage in the particle surface, which could be recovered by the subsequent reducing treatment. Photoluminescence (PL) intensity of CePO4:Tb3þ due to 5D4 f 7FJ (J = 36) electronic transitions of Tb3þ was decreased with increasing KMnO4 concentration in an exponential manner, resulting from excitation cutoff by an intervalence Ce3þ f Ce4þ charge transfer and emission cutoff by a Ce3þ absorption red-shifted to the visible region. Reversely, PL intensity was increased also in the exponential manner by increasing the L(þ)-ascorbic acid concentration in reducing the previously oxidized CePO4:Tb3þ, thereby achieving the quantitative, repetitive luminescence switching. Our results demonstrate that the luminescence switching is regarded as a surface phenomenon without a whole change of structure and valence in the CePO4:Tb3þ material.

’ INTRODUCTION Inorganic luminescent materials, in which a small amount of rare-earth ions are incorporated as luminescent activators, have been widely used in lighting equipments and display devices.1 Control over color rendering properties and improvement of emission intensities have been major concerns for both academic and industrial researches in practical application of luminescent materials. Recently, tuning and switching of luminescent properties have attracted much attention with great expectations for a variety of fields of applications including chemical sensing and bioimaging. Some organic solid-state materials can actually show such interesting luminescent properties based on electrochemical reactions,2,3 oxidation of functional groups,4 changes of the molecular packing mode,5,6 or excited-state intramolecular proton-transfer processes.7 Our motivation lies in that luminescence switching should be realized in inorganic solid-state materials having excellent thermal, chemical, and mechanical stability for the use under more severe conditions. Furthermore, inorganic luminescent materials having additional new functions are expected to expand their application fields in the future.8 In our study, luminescence-switching materials are defined as materials of which luminescent properties can be finely or quantitatively controlled by physical or chemical interactions with external fields. CePO4:Tb3þ, one of the luminescent rareearth phosphates, seems to be a good candidate for achieving luminescence switching because it is known to exhibit bright green emissions and their quenching following a change in Ce3þ/Ce4þ redox states.9 In photoluminescence (PL) of r 2011 American Chemical Society

CePO4:Tb3þ, excitation occurs through optical absorption by Ce3þ having one 4f electron and subsequent energy transfer to Tb3þ. The excited Tb3þ ions emit green light when they return to the ground state. The green emissions are quenched when CePO4:Tb3þ is treated with oxidizing agents. The mechanism underlying this on/off switching of luminescence was thought to be the disturbed Ce3þ f Tb3þ energy transfer by the cerium ions oxidized to a tetravalent state, although a role of Ce4þ ions has not been understood well. Some researchers have proposed the use of this phenomenon in sensing applications with colloidal solutions of nanocrystalline CePO4:Tb3þ.1013 However, the relationship between the intensity of the Tb3þ emissions and the degree of Ce3þ/Ce4þ oxidation has not been clarified yet. Because the sensing application requires their highly quantitative relationship, it is of fundamental importance to develop not only on/off but also fine-tunable, reversible luminescence switching. In the present work, we attempted to clarify the mechanism of luminescence switching and subsequently establish a quantitative relationship between PL intensities and the degree of oxidation in CePO4:Tb3þ. We then examined the structure, the valence state, and the PL properties of CePO4:Tb3þ powders, which were treated with various concentrations of oxidizing KMnO4 solutions and reducing L(þ)-ascorbic acid solutions. It was found that only a few percent of Ce4þ worked well as luminescent Received: November 29, 2010 Revised: March 28, 2011 Published: April 11, 2011 8808

dx.doi.org/10.1021/jp201406y | J. Phys. Chem. C 2011, 115, 8808–8815

The Journal of Physical Chemistry C

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killers due to the occurrence of a Ce3þ f Ce4þ intervalence charge transfer that preceded the Ce3þ f Tb3þ energy transfer. Changes in the intensity of the Tb3þ green emissions could be fitted well by exponential functions as the KMnO4 or the L(þ)ascorbic acid concentration were varied stepwise at room temperature. Our results demonstrate that highly quantitative and fine-tunable luminescence switching is available in CePO4:Tb3þ, which is expected to be helpful for creating a novel type of solidstate photoelectrochemical sensors.14

’ EXPERIMENTAL SECTION Materials. Ce(NO3)3 3 6H2O, KMnO4, L(þ)-ascorbic acid, and hydrochloric acid (HCl, 35.037.0 vol %) were purchased from Wako Pure Chemical Industries, Co., Ltd., Japan. TbCl3 3 6H2O was purchased from Kanto Chemical Co., Inc., Japan. NaH2PO4 3 H2O was purchased from Aldrich, France. All chemicals were of analytical grade and used without further purification. Synthesis. Two kinds of synthesis methods were employed in the present work, aiming at obtaining samples having different morphologies. In a simple liquid-phase reaction method, 18 mmol of Ce(NO3)3 3 6H2O and 2.0 mmol of TbCl3 3 6H2O were dissolved in 500 mL of distilled water (Ce:Tb = 9:1 in mol). Separately, 40 mmol of NaH2PO4 3 H2O was dissolved in another 500 mL of distilled water. The amount of PO43 ions was set to be twice that of the rare-earth ions, taking account the following chemical equilibrium:

H2 PO4 T 2Hþ þ PO4 3

ð1Þ

White precipitates were formed just after mixing the solutions, which were aged under stirring for 3 h at room temperature. The precipitates were collected by centrifugation, washed with distilled water and ethanol, dried at room temperature, and finally ground into fine powders (designated “sample A”, hereafter). In an acid-assisted liquid-phase reaction method, 4.5 mmol of Ce(NO3)3 3 6H2O and 0.50 mmol of TbCl3 3 6H2O were dissolved in 500 mL of distilled water. 5.0 mL of HCl was then added to the resultant solution. Separately, 10 mmol of NaH2PO4 3 H2O was dissolved in another 500 mL of distilled water. The solutions were mixed (any precipitation could not be observed just after mixing), stirred for 30 min at room temperature, and aged at 60 °C for 20 h to obtain precipitates. The resultant white precipitates were collected by centrifugation, washed with distilled water and ethanol, dried at room temperature, and finally ground into fine powders (designated “sample B”, hereafter). Oxidation and Reduction. For redox reactions, a 0.05 M aqueous KMnO4 stock solution and a 0.05 M aqueous L(þ)ascorbic acid stock solution were prepared as an oxidant and a reductant, respectively. The solutions were used after dilution to various concentrations: 0.040.16 mM for the KMnO4 solution and 0.100.40 mM for the L(þ)-ascorbic acid solution. In the initial oxidizing treatment, each 0.30 g of the CePO4: Tb3þ powders prepared without (sample A) or with acid (sample B) was treated by 25 mL of the KMnO4 solution with varying concentrations between 0.04 and 0.16 mM for a partial oxidation. The solutions containing the CePO4:Tb3þ powders were stirred for 2 h at room temperature to promote the oxidation reaction. The oxidized powder samples were collected by centrifugation, washed with distilled water and ethanol, and dried at room temperature.

To investigate the reduction behavior, heavily oxidized powders were prepared beforehand by immersing 1.50 g of the CePO4:Tb3þ powder (sample A) in 125 mL of the 0.16 mM KMnO4 solution. In the reducing treatment of the resultant oxidized powder, 0.30 g of the powder was treated by 25 mL of the L(þ)-ascorbic acid solution with varying concentrations between 0.10 and 0.40 mM under stirring for 2 h at room temperature to promote the reduction reaction. The reduced powder samples were collected by centrifugation, washed with distilled water and ethanol, and dried at room temperature. Effects of the reaction time on the oxidation of CePO4:Tb3þ were also examined. 0.30 g of the CePO4:Tb3þ powder (sample A) was oxidized by 25 mL of weaker KMnO4 solutions with varying concentrations between 0.01 and 0.04 mM at room temperature for the partial oxidation. The reaction time under stirring was varied between 0.5 and 2 h. The oxidized samples were collected by centrifugation, washed with distilled water and ethanol, and dried at room temperature. Characterization. The phase identification of the powders was performed with an X-ray diffractometer using Cu KR radiation (Bruker AXS, type D8 Advance). The morphology and microstructure of the powders were observed with fieldemission scanning electron microscopy (FE-SEM; Hitachi, type S-4700) and field-emission transmission electron microscopy (FE-TEM; FEI, type TECNAI Spirits). BrunauerEmmett Teller (BET) specific surface areas were determined from nitrogen adsorption at 77 K with a micrometric analyzer (Shimadzu, type Tristar 3000). Valence and chemical states of cerium, oxygen, and terbium were analyzed by X-ray photoelectron spectroscopy (XPS; JEOL, type JPS-9000MX) using Mg KR radiation. PL spectra were measured at room temperature with a spectrofluorophotometer (JASCO, type FP-6500) using a xenon lamp (150 W) as a light source. A filter was used to remove a second-order peak of the excitation light in the PL measurement. Diffuse reflectance spectra were recorded with an ultraviolet (UV)-visible-near-infrared (NIR) spectrophotometer (JASCO, type V-670) using an integrating sphere unit (JASCO, type ISN-723).

’ RESULTS AND DISCUSSION Structural Characterization. The crystalline phases of the synthesized powders (samples A and B) were identified with the X-ray diffraction (XRD) analysis. Results are shown in Figure 1 as XRD patterns of the two samples. Diffraction peaks appearing in the pattern of both samples can be indexed with the hexagonal CePO4 phase (ICDD 34-1380). Minor peaks are also assigned to the rhabdophane phase (the hydrated CePO4 3 H2O; ICDD 35614). The crystallinity of sample B synthesized with acid is much higher than that of sample A without acid, judging from the peak width and the background noise. Figure 2 compares the morphology of the samples by FE-SEM and FE-TEM images. In the FE-SEM images with the low magnification, sample A exhibits morphology with spherical aggregated particles (Figure 2a), while sample B consists of well-defined acicular particles (Figure 2b). The FE-TEM image (Figure 2c) reveals that sample A is actually composed of nanorods as primary particles, which aggregate to the secondary spherical particles shown in the FE-SEM image. The acicular particles in sample B are observed to be approximately 2 μm in length and 10 nm in diameter (Figure 2d). These results indicate that the acidic condition in the synthesis makes the nucleation 8809

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Figure 3. Nitrogen adsorptiondesorption isotherms of samples A and B measured at 77 K.

Figure 1. XRD patterns of products obtained by the simple (sample A) and the acid-assisted (sample B) liquid-phase reaction method. A pattern of the hexagonal CePO4 is also shown as a reference.

Figure 4. XRD patterns of samples A (the as-prepared CePO4:Tb3þ powder), O (the oxidized CePO4:Tb3þ powder), and R (the oxidized and then reduced CePO4:Tb3þ powder).

Figure 2. FE-SEM images of (a) sample A and (b) sample B, and FETEM images of (c) sample A and (d) sample B.

rate lower, leading to the formation of the larger crystals. In the dissociation reaction expressed by eq 1, a higher concentration of protons in the solution displaces the equilibrium to the left side. The number of the PO43 ions to react with Ce3þ and Tb3þ is

then decreased under the acidic condition, resulting in a smaller degree of supersaturation of the solution. Because it took much longer time to obtain the precipitates (sample B), the supersaturation was supposed to be kept lower during the synthesis. Under the small degree of supersaturation, the crystal growth proceeds by heterogeneous nucleation on the existing crystals, which explains the well-developed acicular crystals in sample B. The nitrogen adsorptiondesorption isotherms of samples A and B are shown in Figure 3. Sample A shows a type IV isotherm with a type H2 hysteresis loop, indicative of the presence of mesopores without well-defined shapes. This porous structure comes from the aggregation of the small nanorods as shown in Figure 2c. Sample B also shows a type IV isotherm, although its hysteresis loop corresponds to a type H3, which is characteristic of mesoporous materials being comprised of agglomerates of plate-like particles with slit-shaped pores. The BET specific surface area of the samples was measured to be 127.9 and 34.96 m2 g1 for samples A and B, respectively. The surface area of sample A is much larger than that of sample B, resulting from the shape and size of the primary particles, as mentioned above. Effects of Redox Reactions on Structure and Valence States. In our experiments for investigating the redox-induced luminescence switching, it is of fundamental significance to clarify whether any structural change follows the redox reactions in the CePO4:Tb3þ crystal. We therefore examined the structure of heavily oxidized and reduced CePO4:Tb3þ powders. Samples were prepared as follows. An oxidized powder (designated “sample O”) was obtained by immersing 0.15 g of sample A in 25 mL of the high-concentration (0.05 M) KMnO4 solution for 1 h. A reduced powder (designated “sample R”) was obtained by immersing 0.15 g of sample O in 25 mL of the high-concentration 8810



Figure 5. (a) The Ce 3d and (b) the O 1s XPS spectra of samples A, O, and R. (c) The Tb 4d XPS spectra of the as-prepared TbPO4 powder (A) and the oxidized TbPO4 powder (B).

(0.05 M) ascorbic acid solution for 1 h. Figure 4 compares XRD patterns of samples A, O, and R. All of the patterns, including the peak position and the relative intensity, agree well with each other, thereby indicating that the crystalline CePO4:Tb3þ powders do not undergo any XRD-detectable structural change by the redox reactions. A possible change in the cerium valence state is another great concern in the redox reactions. The XPS analysis was then performed for samples A, O, and R, focusing on the core-level photoemission from cerium to examine its valence state. XPS spectra for the Ce 3d core region are shown in Figure 5a. Generally, the Ce 3d spectrum is composed of two multiplets corresponding to the spinorbit split 3d5/2 and 3d3/2 separated by approximately 18 eV.15 A criterion for judging the valence state of cerium is the fine structure of the 3d5/2 and 3d3/2 multiplets. Because the 4f state couples with the core hole in the final state of photoemission, a remarkable splitting in the 3d5/2 and 3d3/2 core photoemission spectrum is observed for rareearth systems.16 In mixed-valence cerium compounds, three different final states (4f0, 4f1, and 4f2) arise from the 4f-level relaxation around the core hole. The corresponding XPS peaks are denoted by v0 (Ce3þ, 4f2), v (Ce4þ, 4f2), v0 (Ce3þ, 4f1), v00 (Ce4þ, 4f1), and v000 (Ce4þ, 4f0) for 3d5/2 and u0 (Ce3þ, 4f2), u (Ce4þ, 4f2), u0 (Ce3þ, 4f1), u00 (Ce4þ, 4f1), and u000 (Ce4þ, 4f0) for 3d3/2.17,18 In sample A, there exist four peaks of v0, v0 , u0, and

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u0 in the spectrum, thereby indicative of the Ce3þ state in CePO4: Tb3þ. The broadness and position of the Ce3þ 3d peaks match well those reported in the literature for CePO4.15 In sample O, six minor peaks of v, v00 , v000 , u, u00 , and u000 can be distinguished as small shoulders in addition to the four main broad peaks (v0, v0 , u0, and u0 ). This result indicates that only a few parts of the Ce3þ ions are actually oxidized to Ce4þ even in the heavily oxidized sample O, and hence the full oxidation of CePO4:Tb3þ is not possible. As described below, however, the very small amount of Ce4þ in sample O can effectively quench the green emission from Tb3þ. In sample R, the Ce4þ-related shoulder peaks disappear, and the spectrum returns to its former state of sample A. Figure 5b shows XPS spectra for the O 1s region. All of the samples (A, O, and R) exhibit a single peak at 530.1530.3 eV, which is assigned to the lattice oxygen of CePO4. The O 1s core level peak for CePO4 is known to be shifted to the higher binding energy than that for CeO2 at 529 eV. This shift is due to the presence of CeOP bonds and the Ce3þ state.15 Additionally, a minor peak appears at 532.5 eV, which is attributable to the oxygen of the metalOH bonds at the particle surface. This peak has no relation to the redox reaction because it can be observed in all of the samples. For terbium, the most intense Tb 3d core region is located between 1230 and 1290 eV in binding energy and is difficult to be analyzed with the Mg KR line (1253.6 eV).19 We then attempted to examine the next most intense Tb 4d region (around 150 eV) for our samples. However, the small amount of the Tb ions in the CePO4:Tb3þ samples (10 at. % of Tb against Ce) did not allow us to distinguish the Tb 4d peaks in the XPS spectra. Because the purpose of the XPS analysis was to check the possible valence change in the redox reaction, TbPO4 powders were synthesized through the same procedure with sample A and used for analyzing the Tb 4d3/2 state. The oxidizing treatment was also done as for sample O. As shown in Figure 5c, the Tb 4d3/2 peak appearing at 149 eV is unchanged before and after the oxidizing treatment. Furthermore, there is no peak at 164 eV, which is a fingerprint for the presence of Tb4þ.19 These results indicate that the valence state of the terbium ions does not change by the oxidizing treatment. This can exclude the possibility of the Tb3þ oxidation as one of the reasons for the luminescence quenching. When a part of the Ce3þ ions are oxidized to the Ce4þ ions, charge compensation should be achieved possibly by incorporating negatively ionized oxygen species into the CePO4 lattice. Although it has been confirmed that the oxidation does not affect the crystal structure of CePO4:Tb3þ (see Figure 4), a microscopic change below the XRD detectable limit may damage the crystal lattice near the surface of the primary nanorod-like particles. The surface structure of the particles was then observed by FE-TEM for samples A, O, and R. As shown in Figure 6, clear lattice fringes are seen in samples A and R, while sample O exhibits an amorphous or poly microcrystalline state. Electron diffraction (ED) patterns support this result. Spot patterns are observed for samples A and R, indicative of the single-crystalline nature of the particles. In contrast, sample O shows diffraction rings coming from the polycrystalline nature. Thus, the oxidizing treatment induces the destruction of the particle surface, making it easier to incorporate the oxygen ions and change the Ce3þ/ Ce4þ valence states. What is more important is that the structural damage is recovered by the reducing treatment, which would lead to a reversible luminescence switching. Quantitative Luminescence Quenching by Oxidation. Results of the PL measurements for the CePO4:Tb3þ powders 8811



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Figure 7. PL excitation (λem. = 544 nm) and emission (λex. = 300 nm) spectra of sample A before and after the partially oxidizing treatment by the KMnO4 solution with the concentration between 0.04 and 0.16 mM.

Figure 8. Semilogarithmic plots of the integrated PL intensity, (a) Φi and (b) Φvis, against the KMnO4 concentration for the partially oxidized sample A.

Figure 6. FE-TEM images of samples (a) A, (b) O, and (c) R, and ED patterns of samples (d) A, (e) O, and (f) R.

are described here in detail. Figure 7 shows PL excitation and emission spectra of sample A before and after the partially oxidizing treatment by the KMnO4 solution with the concentration between 0.04 and 0.16 mM. Tb3þ has a 4f8 electronic configuration, and its ground state is expressed by 7FJ (J = 06). The lowest excitation level is the 5D4 state, and then green emissions follow 5D4 f 7FJ electronic transitions, as observed at 490 (7F6), 544 (7F5), 587 (7F4), and 621 nm (7F3) for sample A. The excitation occurs through optical absorption of Ce3þ with 2 F5/2 (4f1) f 2D (5d1) transition and subsequent energy transfer to Tb3þ. The excited 5d state is strongly split by the crystal field, resulting in the overlapping excitation bands.20 Fang et al.21

reported that the structure of the Ce3þ absorption was dependent on the size and shape of CePO4 materials. The absorption spectrum of CePO4 nanowires consisted of five peaks with maxima at 215, 240, 257, 273 (strongest), and 320 nm. On the contrary, the absorption spectrum of bulk CePO4 powders had four peaks with maxima appearing at 226, 253, 265, and 300 nm (strongest). The excitation spectrum of sample A in Figure 7 resembles that of the bulk samples reported by Fang et al. in that the peak of the longest wavelength (304 nm) shows the highest intensity. This may be due to the aggregation of nanorods in sample A, as shown in Figure 2a. After the partially oxidizing treatment, the PL intensity (both the excitation and the emission) decreases with increasing the KMnO4 concentration from 0.04 to 0.16 mM as seen from Figure 7 (see also Figure S1 with optical images). In addition, the excitation peak (the strongest one) is observed to be blue-shifted 8812



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Table 1. Estimated Amount of Ce4þ and the Integrated PL Intensity (Φi) for Sample A before and after the Partially Oxidizing Treatment by the KMnO4 Solution with the Concentration between 0.04 and 0.16 mM amount of Ce4þ (%)a KMnO4 concentration/mM

a

eq 4

eq 5

Φi (%) 100

(sample A)

0

0

0.04

0.439

0.263

45.2

0.08

0.878

0.527

23.5

0.12

1.32

0.790

11.1

0.16

1.76

1.05

4þ

The amount of Ce eq 4 or eq 5.

5.94

was calculated using the combination of eq 3 and

as the KMnO4 concentration increases. We evaluated the decrease in the PL intensity by integrating the 5D4 f 7F5 emission (Φi) between 530 and 570 nm or the whole 5D4 f 7 FJ (J = 36) emissions (Φvis) between 470 and 640 nm, and plotting them against the KMnO4 concentration for all of the samples. Figure 8 shows the relationship between Φi or Φvis and the KMnO4 concentration (denoted by c) as a semilogarithmic plot. The integrated intensity for sample A before the oxidizing treatment was normalized to 100%, and the Φi or the Φvis values were calculated as the percentage against it. It was then found that the relationship between Φ and c could be fitted well by an exponential function as follows: Φ ¼ A expð BcÞ

ð2Þ

where A and B were constants and determined to be (A, B) = (96.3, 17.7) and (96.6, 17.6) for Φi and Φvis, respectively. The similar A and B values for Φi and Φvis indicate that the intensity of all four emission peaks (5D4 f 7F3, 7F4, 7F5, and 7F6) is decreased in the same manner. Thus, the luminescence quenching of CePO4:Tb3þ can be quantitatively controlled. Our preliminary experiments revealed that the emission intensity from sample A (the exact composition of Ce0.9Tb0.1PO4) fell below the detectable level of the spectrofluorophotometer when 0.266 mmol of Ce3þ (corresponding to 0.295 mmol of Ce0.9Tb0.1PO4) was treated with 0.0025 mmol of KMnO4. Assuming the following oxidation of Ce3þ to Ce4þ and reduction of MnO4 to Mn2þ, at the most 4.70% of the Ce3þ ions in Ce0.9Tb0.1PO4 are oxidized to the Ce4þ ions. Ce3þ f Ce4þ þ e

ð3Þ

MnO4 þ 8Hþ þ 5e f Mn2þ þ 4H2 O

ð4Þ

Actually, however, the full reduction from Mn7þ to Mn2þ may not be completed in the nearly neutral KMnO4 aqueous solution, and the following reaction seems more reasonable:13 MnO4 þ 4H2 O þ 3e f Mn4þ þ 8OH

ð5Þ

In this case, only 2.82% of the Ce3þ oxidation comes to quench the emissions completely. Table 1 summarizes the Ce3þ f Ce4þ oxidation percentage together with the Φi values for sample A before and after the partially oxidizing treatment by the KMnO4 solution (0.040.16 mM). In any case, the amount of the Ce4þ ions is very small, which agrees with the XRD and XPS results as described above.

Figure 9. Diffuse reflectance spectra of sample A before and after the partially oxidizing treatment by the KMnO4 solution with the concentration between 0.04 and 0.16 mM.

Mechanism of Luminescence Quenching. The fact that the amount of Ce4þ is much smaller than expected in the oxidized CePO4:Tb3þ samples implies that the decreasing number of the Ce3þ ions should have much less influence on the hindered Ce3þ f Tb3þ energy transfer. Because we noticed that sample O was tinged with yellow and sample R returned to colorless (see Figure S2), we considered another plausible mechanism explaining the luminescence quenching based on the light absorption. Figure 9 compares diffuse reflectance spectra of sample A before and after the partially oxidizing treatment by the KMnO4 solution (0.040.16 mM). A strong absorption band observed in the UV region for sample A is ascribed to the optically allowed 2F5/2 (4f1) f 2D (5d1) transition of Ce3þ, and its peak position (309 nm) coincides with that of the excitation spectrum in Figure 7. The UV absorption band is broadened and enhanced with increasing the KMnO4 concentration from 0.04 to 0.16 mM, which is an opposite trend to what was observed with the excitation spectra. The Ce3þ absorption no longer leads to the excitation of Tb3þ in the oxidized CePO4:Tb3þ. This conflicting relationship is supposed to be caused by the occurrence of the intervalence charge transfer between Ce3þ and Ce4þ. Effects of Ce4þ on optical properties of lanthanide phosphates were intensively studied in LaPO4:Ce materials.2225 As Ce3þ was partially oxidized to Ce4þ in La0.95Ce0.05PO4, an intense absorption band appeared in the UVvis region at wavelengths below 450 nm. In other words, the cerium concentration as low as 5 at. % is enough to bring about the intense Ce3þ f Ce4þ charge transfer absorption. A similar situation is actually found in our CePO4:Tb3þ samples in which only a few percent of Ce3þ is oxidized to Ce4þ. Thus, the Ce3þ f Tb3þ energy transfer is effectively cut off by the Ce3þ f Ce4þ charge transfer, which is a more reasonable mechanism for the luminescence quenching. The yellow coloring of the oxidized samples is explained by the optical absorption band extended to 450 nm (violetblue in the visible region). In addition, the oxidized samples also show a slight decrease in the reflectance in the whole visible range. This kind of the absorption might be caused by the broadened 2F5/2 (4f1) f 2D (5d1) transition of Ce3þ in the environment with increased electron density.26 The force of attraction that the 4f electron experiences from the nucleus is screened by the inner electron core. The electron density donated by the oxygens, which are incorporated to compensate the higher positive charge of Ce4þ, serves to increase this screening further, allowing the 4f electron to escape more easily to the 5d level. This brings about the considerable red-shift in the optical absorption to the longer wavelength visible region.27 8813



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Figure 10. Semilogarithmic plots of the integrated PL intensity, Φvis, against the KMnO4 concentration for the partially oxidized samples A and B.

Taking a close look at the diffuse reflectance spectra, the maximum peak position of the UV absorption due to Ce3þ is blue-shifted from 309 (sample A) to 297 (0.04 mM) and 278 nm (0.080.16 mM). This trend was also observed in the excitation spectra in Figure 7. This blue shift is attributed to the distorted lattices in the oxidized samples with the incorporated oxygens, which may induce lower crystal field symmetry.21 The exponential manner of the luminescence quenching by the oxidation is ascribed to the excitation cutoff effect due to the Ce3þ f Ce4þ charge transfer in the UV region and, simultaneously, the emission cutoff effect due to the Ce3þ absorption in the visible region. Actually, both effects could be quantitatively evaluated by the absorptions, which increased in a nearly exponential manner, as deduced from the diffuse reflectance spectra in Figure 9. Further examination is ongoing to ensure this explanation by clarifying the relationship between the amount of the Ce4þ ions and the optical absorption behavior in CePO4: Tb3þ, which will be reported elsewhere. Effect of Morphology on Luminescence Quenching. The effect of the sample morphology on the luminescence quenching was examined next. As described above, sample A has the mesopores without well-defined shapes, whereas sample B has the slit-shaped pores. Figure 10 compares the semilogarithmic relationship between Φvis and the KMnO4 concentration for samples A and B. Sample B also shows the linear relationship with a larger negative gradient (B = 27.1 in eq 2). The difference in the gradient between samples A and B may arise from the shape of the pores, which would influence the diffusion of the charged MnO4 ions, rather than the specific surface area determined from the N2 adsorption. It is expected that sample B consisting of the larger acicular particles facilitates more the MnO4 diffusion and the subsequent oxidation at the particle surface, thereby quenching more the Tb3þ luminescence. On the other hand, the aggregated nanorods in sample A are less oxidized at the same MnO4 concentration. We have confirmed this hypothesis by synthesizing another sample having the smaller particle size by the acid-assisted procedure (see experimental details and results including Figures S3, S4, and S5 in the Supporting Information). When the morphology is almost the same, the sample having the larger specific surface area gives the larger gradient for the luminescence quenching in eq 2. In any case, the degree of the surface oxidation is considered to be a key factor for the luminescence quenching. Quantitative Luminescence Recovering by Reduction. The recovery of the quenched luminescence by the reducing treatment was examined for the heavily oxidized sample A

Figure 11. (a) PL excitation (λem = 544 nm) and emission (λex = 300 nm) spectra of the heavily oxidized sample A before and after the partially reducing treatment by the L(þ)-ascorbic acid solution with the concentration between 0.10 and 0.40 mM, and (b) a semilogarithmic plot of the integrated PL intensity, Φvis, against the L(þ)-ascorbic acid concentration.

powder. Figure 11a shows PL excitation and emission spectra of the heavily oxidized sample A before and after the partially reducing treatment by the L(þ)-ascorbic acid solution with the concentration between 0.10 and 0.40 mM. With increasing concentration, the PL intensity increases and finally reaches the same level as that of sample A before the oxidizing treatment. Thus, the luminescence switching of CePO4:Tb3þ can be reversibly controlled. A comparison of the PL excitation spectra in Figures 7 and 11 reveals that the structure of the excitation band is changed after the one cycle of the redox reaction. That is, the excitation peak located at the shorter wavelength (260 nm) is stronger than that at the longer wavelength (295 nm) in the reduced samples. This means that the aggregation of the nanorods is loosened and the spectral feature of the nanorods appears as reported by Fang et al.21 A semilogarithmic plot of Φvis against the L(þ)-ascorbic acid concentration is shown in Figure 11b. The integrated intensity for the sample after the fully reducing treatment was normalized to 100%. The plot also exhibits a well-fitted linear relationship (A = 4.52 and B = 7.78). Assuming the following oxidation of C6H8O6 to C6H6O6, the reduction of Ce4þ to Ce3þ can supposedly be completed at 0.40 mM of the L(þ)-ascorbic acid: C6 H8 O6 f C6 H6 O6 þ 2Hþ þ 2e

ð6Þ

Because the luminescence switching follows the exponential manner both in the oxidation and in the reduction of chemically stable CePO4:Tb3þ, it can be used as repeatable redox sensors under heavily oxidizing and reducing conditions. Time Response of Switching. Finally, time response of the luminescence switching was evaluated for sample A in the 8814



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’ AUTHOR INFORMATION Corresponding Author

*Phone: þ81 (0)45-566-1581. Fax: þ81 (0)45-566-1551. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the Izumi Science and Technology Foundation for financial support. Figure 12. Time dependence of the integrated PL intensity, Φvis, for the partially oxidized sample A with the weaker KMnO4 solutions (0.010.04 mM).

oxidizing treatment. The Φvis values for the Tb3þ emissions were plotted against time in the reaction with the KMnO4 solutions (0.010.04 mM). The weaker solutions were used to discriminate between the initial changes in the PL intensity. Results are shown in Figure 12. It is seen that the luminescence quenching is greatly promoted in the first 30 min for all of the concentrations. At the lower concentrations of 0.01 and 0.02 mM, the quenching seems not to be completed even after 2 h. On the contrary, the higher concentrations (0.03 and 0.04 mM) result in a faster completion of the quenching. These results also indicate that the oxidation reaction takes place predominantly at the particle surface. The luminescence switching is therefore regarded as a surface phenomenon and is not accompanied by the whole change of the structure and valence in the CePO4:Tb3þ material.

’ CONCLUSIONS The luminescence switching of the CePO4:Tb3þ material due to the redox reactions was investigated in terms of its quantitative evaluation. The samples were synthesized by the liquid-phase reactions and underwent the oxidation and the reduction by KMnO4 and L(þ)-ascorbic acid, respectively. The XRD, XPS, and FE-TEM observation revealed that the oxidation reaction of CePO4:Tb3þ was followed by the Ce3þ/Ce4þ valence change and the small degree of structural damage at the particle surface. The oxidizing and reducing treatment of CePO4:Tb3þ led to the exponential change of the photoluminescence intensity, which was explained by the excitation and the emission cutoff related to the Ce4þ amount in CePO4:Tb3þ. It was shown that the luminescence switching was quantitative, reversible, and repeatable, thereby indicative of its usefulness as the redox-sensing material. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1 for the appearance of the partially oxidized CePO4:Tb3þ powders under the UV light illumination and Figure S2 for the appearance of samples A, O, and R under the fluorescent lamp. Experimental procedures and results for the acicular CePO4:Tb3þ powder with a smaller particle size, including Figures S3 (nitrogen adsorptiondesorption isotherms), S4 (FE-SEM images), and S5 (semilogarithmic plots of Φvis against the KMnO4 concentration). This material is available free of charge via the Internet at http://pubs.acs.org.

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Quantitative Luminescence Switching in CePO4: Tb by Redox Reactions

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