Highly Enhanced and Switchable Photoluminescence Properties in

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Highly Enhanced and Switchable Photoluminescence Properties in Pillared Layered Hydroxides Stabilizing Ce3+ Jinghua Wu, Jianbo Liang, Renzhi Ma, and Takayoshi Sasaki J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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The Journal of Physical Chemistry

Highly

Enhanced

Photoluminescence

and Properties

Switchable in

Pillared

Layered Hydroxides Stabilizing Ce3+

Jinghua Wu, Jianbo Liang, Renzhi Ma, Takayoshi Sasaki* International Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan * Fax: (+81) 29-854-9061; E-mail: [email protected]

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Abstract: We have developed pillared layered rare earth hydroxides showing a reversible photoluminescence switching via reducing/oxidizing processes. An air-stable Ce3+-based host, Ce2(OH)4SO4·2H2O, was successfully synthesized via a homogeneous alkalization protocol to precipitate Ce3+ ions from a solution of the relevant salt. Structural analysis revealed that the compound consists of cationic layers of {[Ce(OH)2(H2O)]+}∞, linked by sulfate bidentate ligands to construct a layered framework architecture. Tb3+ ion was incorporated into this host lattice to form a solid solution across the full compositional range. At an optimized doping of ~30%, the characteristic green emission was enhanced by ~20 times, being promoted by the efficient energy transfer from Ce3+ to Tb3+. The emission could be drastically diminished upon the action of the KMnO4 oxidizing reagent, which induced the transformation of Ce3+ to Ce4+. Characterizations by X-ray diffraction and X-ray photoelectron spectroscopy showed that the oxidation of Ce3+ occurs without degradation of the crystalline framework. The emission could be recovered to its original intensity by a reducing treatment with ascorbic acid. This photoluminescence switching behavior was eye-detectable and exhibited high reversibility. Keywords: layered rare earth hydroxides, pillared structure, phosphors based on Ce3+, photoluminescence switching.


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Introduction Rare earth (RE) elements are widely used in various optical and optoelectronic applications such as lighting, displays, and bio-labeling because of their versatile and tunable photoluminescence properties.1-3 Upon excitation, RE metal ions emit a sharp and intense luminescence derived from the f-f or f-d electronic transitions that exhibit well-defined color purity in contrast to those of organic phosphors and quantum dots.4-6 Therefore, for decades, intensive studies motivated by both fundamental scientific importance and practical applications have been conducted to develop new fluorescent inorganic materials to accommodate RE ions. Ce3+ is unique among the RE ions due to its parity-allowed electric-dipole 4f-5d transitions.7 In many materials, energy transfer from an excited element to emissive element has been designed to attain enhanced photoluminescence. Ce3+ is expected to be useful as such a sensitizer and is actually applied in traditional green emission materials such as LaCeTb-PO4.8 However, stable accommodation of Ce3+ in an inorganic layered host lattice remains challenging because of its tendency to undergo facile oxidation into Ce4+. Recently, a new class of layered rare earth hydroxides (LREHs) constructed from cationic hydroxide layers and anionic guest species has been developed.9-16 The combination of the unique chemistry based on the lanthanide series and the reactivity derived from the layered structure has given rise to a range of valuable applications of these hydroxides as new anion-exchangers, photoluminescent materials, catalysts, and biomedical agents.17-22 The general chemical formula of LREH compounds can be expressed as RE2(OH)6-m(An-)m/n·xH2O (1 ≤ m ≤ 2, An- represents the n-valence anion), in which the ideal phases with m = 1 and m = 2 represent two distinctive types of host layer topologies. The m = 1 compounds are typical layered hosts, accommodating exchangeable monovalent anions such as Cl- and NO3- in the interlayer galleries. By contrast, the m = 2 phases are characterized by their rigid pillared or more open framework structures. In these structures, SO42- ions or organic moieties with sulfate groups typically act as bidentate ligands to connect the adjacent hydroxide layers. Considerable efforts have been devoted to vary the guest and/or host composition and/or the guest-host interactions to tune or enhance performance and specific applications of these newly discovered layered host compounds. Nevertheless, in all LREHs reported so 3 ACS Paragon Plus Environment


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far, the RE elements are still limited from Pr to Lu. It is highly desirable to extend LREHs to other structures and RE elements to create novel properties. In particular, the incorporation of Ce3+ into the LREH structure to widen their range of optical applications is of great interest. However, in our previous study, we found that the Ce3+ ion was difficult to stabilize in the m = 1 phase because it tends to be oxidized into Ce4+ in air. Nevertheless, we successfully synthesized Ce3+-based framework compounds of the m = 2 type as a major product using n-propyldisulfate as a ligand, even though the sample still contained a minor amount of CeO2. Such an impurity by-product resulted in a degraded luminescence property.13 Recently, Xiang et al. reported the doping of Ce3+ into layered yttrium hydroxide (LYH:Ce) nanoplates through a polyethylenimine (PEI)-assisted hydrothermal route. 23 However, to the best of our knowledge, a stable single phase Ce3+-based LREH has not been reported. Controlling the optical properties in response to external stimuli such as temperature, pressure, electromagnetic field, or environmental chemical species is an important issue for technology development. Such so-called smart optical materials have received enormous attention as candidate materials for advanced optoelectronic devices and sensors. Photochromic glasses, which have been integrated in smart windows as a key component allowing automatic tuning of indoor light conditions, are a well-known example of such smart materials.24,25 Fundamental studies have been motivated by a long-standing interest in the exploration of novel smart optical functionalities. The luminescence switching behaviors, in which the luminescence signal of a phosphor can be reversibly switched on and off upon redox stimuli, have been of particular interest.26, 27 Such smart behaviors are mostly observed in organic molecules with a redox site, for which the light emission can be controlled by the state of the redox active site via its communication with the emission center. However, organic materials generally suffer from low thermal stability and often lose their sensitivity in harsh surroundings, hindering their applications in specific conditions.28 In this regard, inorganic phosphors will be advantageous for designing highly stable luminescence switches. In particular, once Ce3+ is incorporated into the LREH structure, we may expect the switching behavior that is controllable by the Ce3+/Ce4+ redox process. 4 ACS Paragon Plus Environment

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In this study, we found that the m = 2 type pillared structure can stabilize Ce3+ to produce the Ce2(OH)4SO4·nH2O compound. Structural analysis revealed that Ce ions are accommodated in a truly trivalent state, which may be ascribed to the involvement of SO42- in the strong coordination of Ce3+. Furthermore, a solid-solution series of the Tb/Ce system, (TbxCe1-x)2(OH)4SO4·nH2O, was successfully synthesized across the entire compositional range. Enhanced green emission from Tb3+ was observed via the Ce3+ excitation,

indicating

efficient

energy

transfer.

The

eye-detectable

luminescence-switching behavior was demonstrated by reversibly controlling the oxidation state of the Ce ion through chemical redox processes.

Experimental Sections Chemicals. Ce(NO3)3·6H2O (purity: ~99.99%) and Tb(NO3)3·5H2O (purity: ~99.99%) were

purchased

from

Aldrich.

Na2SO4,

KMnO4,

ascorbic

acid,

and

hexamethylenetetramine (HMT) with purity >99.5% were obtained from Wako Pure Chemical Co. All the reagents were used as received without further purification. Milli-Q filtered water with resistivity > 18 MΩ·cm was used throughout. Synthesis. A Ce2(OH)4SO4·2H2O sample was synthesized by applying the homogeneous alkalization process. In a typical synthesis, Ce(NO3)3·6H2O (5 mmol), Na2SO4 (2.5 mmol) and HMT (3 mmol) were dissolved in 1000 mL of water. The solution was magnetically stirred and heated to the boiling temperature under N2 flow, yielding a white precipitate. The mixture was further aged at the refluxing condition for 6 h and then recovered by filtration. The sample was washed with copious amounts of water and ethanol, conditioned at a controlled humidity (~70%) to a constant weight, and stored in a sealed bottle for further characterization and application. To obtain a Ce-Tb co-doped sample, a mixed solution of Ce(NO3)3·6H2O and Tb(NO3)3·5H2O in a target molar ratio (total amount: 5 mmol) was used, keeping the other synthesis parameters fixed. The synthesis and recovery were done in the same way as the procedures described above. Photoluminescence Switching Behaviors. The photoluminescence switching behavior of the Ce-Tb co-doped sample upon action with oxidizing and reducing reagents was examined by monitoring the emission spectra. In the oxidation process, 1 g of the powder sample was dispersed in 100 mL of KMnO4 solution (1 mM). The mixture was stirred for 5 ACS Paragon Plus Environment


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2 h at room temperature, recovered by centrifugation, washed and dried following the procedure described above in the synthesis section. In the reduction process, the sample was dispersed in a solution of ascorbic acid (25 mM, 100 mL) for 2 h and was then recovered. This process was repeated in alternating turns by characterizing the sample. Characterization. Powder X-ray diffraction (XRD) data were collected by a Rigaku Rint-2200 diffractometer with monochromatic Cu Kα radiation (λ = 0.15405 nm). The sample morphology was examined using a JSM-6010LA scanning electron microscope (SEM). Fourier transform infrared (FT-IR) spectra were recorded using the KBr pellet method

on

a

Varian

7000e

FT-IR

spectrophotometer

equipped

with

a

liquid-nitrogen-cooled MCT detector. Excitation and emission photoluminescence spectra were obtained at room temperature using a Hitachi F-4500 fluorescence spectrometer. X-ray photoelectron spectroscopy (XPS) data were recorded using the theta probe angle-resolved X-ray photoelectron spectrometer (ARXPS) system (Thermo Electron).

Results and Discussion Refluxing a solution containing Ce(NO3)3, Na2SO4, and HMT at N2 atmosphere yielded a white precipitate. The product was air-stable, and retained the white color after the filtration and drying procedures at ambient conditions. SEM images (Figure S1a) show that the recovered powder consists of platelet-like hexagonal-shaped microcrystals with a lateral size of up to several micrometers. The powder XRD data (Figure S1b) confirmed that the sample was phase pure. Sharp and intense basal diffraction series corresponding to a spacing of 0.84 nm were observed, a typical feature of a lamellar structure. All diffraction peaks can be incorporated into a monoclinic system with lattice parameters of a = 0.638(1) nm, b = 0.3890(8) nm, c = 1.680(2) nm, and β = 90.28(1)˚. Note that the unit cell dimensions are very close to those for other rare earth hydroxide sulfates,12,29 including Tb2(OH)4SO4·2H2O, indicating their isostructural relationship. The indices show the systematic absence of k + l = 2n+1, indicating an A-centered unit cell. The crystal structure was refined to a satisfactory result as illustrated in Figure 1 and Table S1. It was confirmed that this compound is isomorphous with the series of rare earth hydroxide sulfates. In the unit cell, the Ce atoms occupy the 4i sites surrounded by 6 ACS Paragon Plus Environment

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nine oxygen atoms: six from the hydroxyl groups, two from the water molecules, and one from the sulfate group. The Ce-O coordination environment can be described as a distorted trigonal prism, in which the oxygen atom of the sulfate group acts as the capping point. These Ce-centered coordination polyhedra are joined with six surrounding neighbors by sharing the µ3-OH groups and µ2-H2O molecules and extend along the ab plane into the corrugated two-dimensional {[Ce(OH)2(H2O)]+}∞. The sulfate groups connect the {[Ce(OH)2(H2O)]+}∞ layers to form a rigid 3D pillared structure. The sulfate groups are present in two orientations, leaving penetrated channels in the crystal structure if the arrangement is ordered.

Figure 1. (a) Rietveld plot for Ce2(OH)4SO4·2H2O. Red, green, and blue lines in (a) represent experimental, calculated, and difference profiles, respectively. (b) Bonding scheme of Ce(OH)6(H2O)2 polyhedron and SO4 tetrahedron. (c) Polyhedral representation of layer architecture. Based on crystallographic data, the Ce atom bond valence sum was calculated as 3.13, comparable to that in Ce2(SO4)3·4H2O (3.07) and Ce(OH)3 (3.03). This result provides evidence that Ce atoms are accommodated in the host layer as trivalent cations. It is well known that the 4f1 Ce3+ tends to transform into the 4f0 Ce4+, losing one electron. In other 7 ACS Paragon Plus Environment


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words, many phases based on Ce3+ are not stable at ambient conditions. A Ce(OH)3 solid is highly active to oxygen in the presence of water and spontaneously transforms into CeO2 if left in air. For example, in our previous efforts to obtain different series of Ce3+-containing LREHs,

Ce2(OH)5NO3·2H2O

and

Ce2(OH)5Cl·2H2O,

by

the

homogeneous alkalization method, the as-prepared samples instantaneously turned yellow when exposed to air (a clear evidence of oxidization) and transformed into CeO2 during filtration. In contrast to the previous results, we found that the Ce2(OH)4SO4·2H2O solid was stable even when the synthesis and filtration were conducted in air. The structural stability and the resistance to the oxidation are most likely due to the rigid pillared structure that is different from the structures of the m = 1 phases of RE2(OH)5NO3·nH2O or RE2(OH)5Cl·nH2O. In those compounds, the RE ions have two coordination environments. One RE ion is surrounded by seven hydroxyls and one water molecule, forming a {RE(OH)7(H2O)} dodecahedron. The other RE ion is bonded to eight hydroxyls and one water molecule, forming a {RE(OH)8(H2O)} monocapped square antiprism with one capping position occupied by a water molecule. The REO8 and REO9 polyhedra are joined in alternating rows, forming two-dimensional corrugated layers. Anions are accommodated in 2D galleries of the rare earth hydroxide layers through electrostatic interactions and can be exchanged with various other anionic species. By contrast, all the RE ions occupy the {RE(OH)8(H2O)} monocapped square antiprism site in the RE2(OH)4SO4·2H2O system, suggesting that they are more tightly bound in the host lattice through the coordination of SO42-. The Ce-O (SO42-) interatomic distance is 0.254 nm, comparable to those for Ce-O(OH) (0.239-0.251 nm) and Ce-O(H2O) (0.261-0.262 nm). Figure 2 shows the obtained FT-IR spectrum that confirms the rigid coordination of SO42- to Ce3+. It is known that four fundamental modes (ν1, ν2, ν3, and ν4) of a free SO42- ion with the Td point group are Raman active, whereas only the ν3 and ν4 modes are IR active.30 The samples in this study exhibited the ν1 and ν2 modes, strongly suggesting the lowering of the symmetry to C2ν. Notably, the ν3 and ν4 modes are split into three separate peaks, which is a signature of the SO42- ions chelating as trans-bidentate ligands.31 Such a distortion is indicative of the strong involvement of the SO42- ions in the coordination to Ce3+. The robust and dense coordination environment realized with the SO42- group is believed to be crucial for the 8 ACS Paragon Plus Environment

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stabilization of Ce3+. Actually, it is well known that the SO42- group acts as a robust ligand in a number of framework structures.32-34 The sharp peaks at 3597 and 3472 cm-1 are attributed to the stretching modes of -OH groups without hydrogen bonding.35 The broad band at approximately 3200 cm-1 is assigned to the -OH groups associated with hydrogen bonding as well as the water molecules in the gallery. The peak at approximately 1677 cm-1 is due to the bending mode of the water molecules. The bands at approximately 772 and 539 cm-1 can be assigned to the stretching vibration of the M-O due to the presence of -OH and the coordinated water.36-38

Figure 2. FT-IR spectrum of Ce2(OH)4SO4·2H2O. Based on the isostructural aspect of Ce- and Tb- phases, solid-solution compounds incorporated binary RE elements, that is, the Ce-Tb co-doped series were synthesized by using conditions similar to those used to obtain Ce2(OH)4SO4·2H2O. Figure 3 depicts the XRD patterns and SEM images of the selected samples with Tb3+ compositions of 0, 0.3, 0.7 and 1.0. All diffraction peaks can be indexed based on the monoclinic system. No impurity peaks were detected, suggesting that Ce3+ has been successfully co-precipitated with Tb3+ into a pure solid-solution phase of the layered hydroxide. The successful synthesis is likely due to the slow and gradual precipitation promoted by HMT hydrolysis as well as the stabilization by the coordination with the sulfate group. The in-plane 100 peak was progressively shifted to the high angle side with increasing Tb content (Figure S2); this can be ascribed to the smaller ionic radius for Tb3+ (92.3 pm) than for Ce3+ (103.4 pm). Figures 3b and 3c show the refined lattice parameters a and b of the host 9 ACS Paragon Plus Environment


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layer as a function of Tb3+ content. The linear trends that follow Vegard’s law provide clear evidence that Ce3+ and Tb3+ are successfully incorporated into the host lattice to form homogeneous solid solution series across the entire compositional range. For a typical sample synthesized at a Ce molar fraction of 0.7, Tb and Ce contents were determined by chemical analysis as 19.4 wt% and 39.5 wt%, respectively. The Ce3+/Tb3+ molar ratio of 2.31 is close to that for the starting solution (2.33), indicating that these ions were quantitatively precipitated. SEM images (Figure 3d-g, Figure S3) show that the samples had a morphology of elongated microplatelets, apparently different from the wide platelets for the Ce pure phase and the slim tablets for the Tb pure phase. The microplatelets tend to become thinner with increasing Tb3+ content. In addition, as can be seen in FT-IR spectra of (TbxCe1-x)2(OH)4SO4·nH2O solid-solution (Figure S4), some characteristic bands such as the -O-H stretching mode and the triply split ν3 mode of the SO42- groups shifted to higher wavenumber progressively with increasing Tb content, suggesting stronger interactions between the hydroxyl and sulfate groups with the Tb3+ due to its smaller ionic radius of Tb3+.39

Figure 3. XRD patterns (a) and typical SEM images (d-g) of the as-prepared (TbxCe1-x)2(OH)4SO4·nH2O samples (x = 0, 0.30, 0.7, 1.0), enlarged view of the pattern in the 2θ range of 20-50° is shown for the sample of x = 0.7. (b, c) In-plane lattice parameters, a and b, as a function of the Tb content.

Photoluminescence Properties


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The obtained pillared framework compounds exhibited photoluminescence properties at room temperature (Figure 4). The Ce2(OH)4SO4·2H2O sample showed a broad emission band with a maximum at 360 nm (Figure S5). This ultraviolet emission band is similar to that in CePO4, Ce doped CaSO4 and several Ce3+ based complexes and is attributed to the characteristic 4f-5d transition of Ce3+ ions.40 The excitation spectrum was dominated by a broad band with a maximum at approximately 265 nm that can be assigned to the lowest Ce3+ 4f-5d transition.41 These photoluminescence features also support the incorporation of stable Ce3+ ions in the pillared structure. For the pure Tb sample, Tb2(OH)4SO4·2H2O, emission peaks in the 400-700 nm range were identified as the 5D4-7FJ (J = 3-6) transitions of the Tb3+ ion: 5D4-7F6 at 484 nm, 5D4-7F5 at 545 nm, 5

D4-7F4 at 589 nm, and 5D4-7F3 at 626 nm.42 The 5D4-7F5 emission peak was the strongest

in the measured range, indicating that the material exhibits the typical Tb green light emission. In the excitation spectra, two strong bands superimposed on a series of straight lines of Tb3+ were observed at approximately 220 and 245 nm. These two bands are assigned to the low-spin (LS, spin-allowed) and high-spin (HS, spin-forbidden) inter-configurational f-d transitions in Tb3+, consistent with the previously reported spectral features.43

Figure 4. Room-temperature emission spectra (a) and excitation spectra (b) of TbxCe1-x(OH)2SO4·nH2O solid solutions excited at 265 nm and monitored at 545 nm. The photoluminescence features of the Tb–Ce solid solution series were examined in a representative example of (Tb0.30Ce0.70)2(OH)4SO4·2H2O. Upon excitation at 265 nm, the 11 ACS Paragon Plus Environment


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compound showed sharp 5D4-7FJ emission lines for Tb3+ ion. Compared with the Tb2(OH)4SO4·2H2O sample, the emission was enhanced approximately by a factor of 20 compared with the intensity of the 5D4-7F5 line. This enhancement may be accounted for by the energy transfer from Ce3+ to Tb3+. Similar enhancement was reported for Tb doped CePO4 phosphor.8 Ultraviolet Ce3+ emission was observed in addition to the Tb3+ emission lines, indicating the incomplete energy transfer from Ce3+ to Tb3+. The excitation spectrum monitored by the Tb3+ 5D4-7F5 line showed a broad band from 200 to 300 nm, markedly different from the pure Tb sample with its dominant low-spin and high-spin inter-configurational f-d transitions. This broad band indicates that the excitation was governed by the Ce3+ ions, providing additional evidence that Tb3+ emission is promoted by energy transfer from Ce3+ to Tb3+. Furthermore, the intensity of the excitation spectrum varied with the Tb3+ ion content and reached an optimum value for x = 0.3. The strong intensity suggests that the energy transfer process from Ce3+ to Tb3+ is important in the solid solution.

Figure 5. Emission intensity at 360 nm and 545 nm of TbxCe1-x(OH)2SO4·nH2O solid solutions as a function of Tb3+ content when excited at 265 nm. (Inset) Schematic illustration of the distribution of the rare earth ions in the host layer when the Ce3+: Tb3+ ratio is 2:1 (a) and 1:2 (b).

The emission intensity of the Tb-Ce solid solution series was studied as a function of Tb content (Figure 5). For all the examined samples, no apparent difference was observed for the line position and relative intensity of the Tb3+ emission peaks. The Ce3+ and Tb3+ emissions were evaluated using the peak intensities at 360 nm and 545 nm (5D4-7F5 line), 12 ACS Paragon Plus Environment

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respectively. As can be seen from Figure 5, the Ce3+emission intensity decreased sharply with increasing Tb3+ fraction; this is ascribed to the efficient energy transfer from Ce3+ to Tb3+. The intensity decreased continuously and was almost extinguished when the Tb3+ fraction reached 60%, indicating the nearly complete energy transfer from Ce3+ to Tb3+ at this stoichiometry. Unlike the phenomena in the ordinary 3D crystals, the maximum Tb3+ emission yield of approximately 30% is much higher than the usually observed value of less than 10%. The maximum emission at x = 0.3 and the complete extinction at x = 0.6 may be understood based on the 2D hydroxide host layers of the quasi-hexagonally arranged RE ions. At the critical stoichiometry of x = 1/3 (Ce:Tb = 2:1), Ce3+ ions occupy the vertex of a hexagon in the host layer, and all Tb3+ ions can have Ce3+ ions at the nearest neighbor sites, realizing a favorable environment for efficient energy transfer. By contrast, at x = 2/3 (Ce:Tb = 1:2), Ce3+ ions do not have neighboring Ce3+ and are fully surrounded by Tb3+ ions. This situation may be responsible for the complete extinction of Ce3+ emission. In addition, the concentration quenching effect is not dominant in the present solid-solution series up to the Tb content of 60%, which may be associated with the unique two-dimensional structure. The energy transfer from Ce3+ to Tb3+ occurs predominantly between the nearest neighbor ions. It is reasonable to expect that these ions are locally arranged in an ordered fashion.[44] Photoluminescence Switching Behaviors Interestingly, the luminescence properties of the Ce-based materials could be switched by using the Ce3+/Ce4+ redox couple. The (Tb0.3Ce0.7)2(OH)4SO4·2H2O sample was used as a typical sample for the examination of this behavior because of its intense Tb emission. Typical oxidizing and reducing regents such as KMnO4 and ascorbic acid were chosen to control the luminescence properties. The white powder rapidly turned to pale yellow when brought into contact with the KMnO4 solution. Extending the reaction period or increasing the concentration of KMnO4 resulted in a brown powder, and the color of the powder could be returned to white by treatment with an ascorbic acid solution. Emission spectra were recorded after each treatment (see Figure 6a). It is seen that the Tb emission, typically the 5D4-7F5 line at 545 nm, was drastically lowered to only 1/8 of the original value upon the oxidation with KMnO4 and could be recovered to nearly initial intensity upon the reduction with ascorbic acid. This process could be 13 ACS Paragon Plus Environment


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recognized by the naked eye under UV irradiation in a dark room, as shown in Figure 6b. The sample displayed visible green emission when illuminated by an ultraviolet lamp, which is denoted as the luminescence “on” state. After treatment with the KMnO4 solution, the emission was relatively weak and only showed a darkened appearance under UV irradiation. This state is denoted as the luminescence “off” state. To elucidate this process, a schematic illustration is presented in Figure S6. By repeating the reaction alternatively with the oxidizing and reducing agents, the sample could be switched between the “on” or “off” state, yielding a behavior similar to that of the molecular redox switches. To demonstrate the reproducibility of the “on/off” processes, three cycles of oxidation and reduction were repeated. As shown in Figure S7, this switching behavior is reversible with only a slight decay. This result may offer a promising possibility of using this inorganic luminescent material in redox-responsive optical devices.

Figure 6. (a) Tb emission spectra of the original sample (left), oxidized sample (middle), and reduced sample (right); (b) Appearance of the original sample (left), oxidized sample (middle), and reduced sample (right) under the visible light (up) and the UV light (down).

The photoluminescence switching behavior involves the variation of oxidation state of Ce ions. XPS analysis was performed for the original, oxidized and reduced samples to 14 ACS Paragon Plus Environment

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elucidate a valence state change induced by the redox reactions (Figure S8). Generally, the Ce 3d spectrum is composed of two multiplets (u and v) corresponding to the spin orbit split 3d5/2 and 3d3/2 core holes.45,46 The as-synthesized sample showed corresponding peaks resolved from the pair of spin-orbit doublets, identified as arising from the Ce3+ state. The higher binding energy peaks, u’ and v’, are located at 904.5 eV and 885.7 eV and can be assigned to the Ce 3d94f1O2p6 final state, whereas the lower binding energy peaks, u0 and v0, located at 900.4 eV and 882.0 eV, can be assigned to the Ce 3d94f2O2p5 final state.47 A small peak was also detected at 917.0 eV, which is attributed to Ce4+, suggesting unavoidable oxidation at the surface of the sample even before the oxidation treatment.48 After the treatment in KMnO4 solution, the peak at 917.0 eV became stronger and the peaks at 880 – 890 and 900 – 910 eV broadened. Deconvolution of the peak profiles suggests that three new components u’’, v’’ and v’’’, located at 908.0, 898.2 and 890.0 eV, respectively, emerged in addition to the main original peaks due to the Ce3+ state.49,50 These new peaks may be due to the Ce4+ 3d final states. These results indicate that some fraction of the Ce3+ ions were actually oxidized to Ce4+ after contact with the KMnO4 solution. Upon reduction, the peak intensity at 917 eV clearly decreased, and the spectral profile returned to the original state of the as-synthesized sample, confirming the reversible nature of the luminescence switching redox process for the (TbxCe1-x)2(OH)4SO4·2H2O solid solutions. The XPS spectra for the O 1s region are shown in Figure S8b. In the original samples, peaks at 533.5, 532.0, and 531.0 eV may be assigned to H2O, sulfate species and the hydroxyl groups, respectively.51 Additionally, the small peak at 530.0 eV may be attributed to the lattice oxygen and should be related to the trace Ce4+. In the oxidized sample, this component obviously increased. This change may be due to the transformation from Ce-OH to lattice oxygen (O2-) associated with the oxidation of the Ce3+. XRD data revealed that the pillared structure remained virtually unchanged upon cycling (Figure S9). All characteristics of the patterns, including the peak position and the relative intensity, are in good agreement with each other, indicating that the redox processes led to negligible or substantially topotactic structural changes. The rigid structure of the Ce-Tb framework compounds is responsible for this feature and demonstrates their promising potential as efficient “on/off” luminescence sensors. 15 ACS Paragon Plus Environment


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Conclusions RE

hydroxide

compounds

with

a

rigid

pillared

structure

composed

of

{[RE(OH)2(H2O)]+}∞ layers and interconnecting SO42- ions have been proven useful as a unique host for accommodating luminescent RE centers. In particular, Ce3+ ions can be stabilized in the host layer due to the robust coordination from the sulfate ions. Each sulfate ion in the gallery is covalently bonded with two lanthanide ions from the neighboring two layers, leading to an outstanding structure stability in contrast to other LREHs with exchangeable anions. Based on this host lattice, Tb3+ ions were successfully incorporated to produce a homogeneous solid solution. At the optimized doping, the characteristic Tb green emissions were enhanced by ~20 times due to the promotion of the efficient energy transfer from Ce3+ to Tb3+. Luminescence-switching behaviors upon redox reactions were also examined using KMnO4 and ascorbic acid as oxidizing and reducing agents, respectively.

Acknowledgment. This work was supported by the World Premier International Center Initiative (WPI Initiative) on Materials Nanoarchitectonics, MEXT, Japan.

Supporting Information. . Refined structure parameters, complete XRD pattern and SEM image of the Ce-Tb hydroxide solid solutions, XPS spectra of the original, oxidized and reduced samples, XRD patterns of the samples after repeating the oxidation/reduction cycles three times, schematic illustration of the photoluminescence switching behavior.

Corresponding Author E-mail: *[email protected]

Notes The authors declare no competing financial interest.


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F.-Y.;

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S.-M.

Cerium

Dioxide/polyaniline


Core–Shell


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Highly Enhanced and Switchable Photoluminescence Properties in

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