Lanthanide-Doped Layered Double Hydroxides Intercalated with

Publication Date (Web): September 10, 2009 ... E-mail: [email protected]. ... intercalated BPA guest molecules to Tb3+ centers in the host layers takes p...
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J. Phys. Chem. C 2009, 113, 17206–17214

Lanthanide-Doped Layered Double Hydroxides Intercalated with Sensitizing Anions: Efficient Energy Transfer between Host and Guest Layers Poernomo Gunawan and Rong Xu* School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore 637459 ReceiVed: June 23, 2009; ReVised Manuscript ReceiVed: July 25, 2009

Layered double hydroxides (LDHs) doped with Tb3+ ions in the brucite-like layers were prepared by a simple one-step coprecipitation method at ambient conditions. When 4-biphenylacetate (BPA) anions were intercalated in the interlayer space, a high concentration of Tb3+ up to around 19 wt % can be homogeneously incorporated in the octahedral lattice of LDHs. The luminescence study indicated that efficient energy transfer from the excited state of the intercalated BPA guest molecules to Tb3+ centers in the host layers takes place. Compared with LDHs without a photosensitizer, Tb3+-doped LDHs intercalated with BPA display much enhanced green luminescence from Tb3+ ions. Long lifetimes of around 1.3 ms and high quantum yields of 14-22% were obtained. In addition, the emission color of this type of hybrid materials can be tuned from blue to green by varying the amount of Tb3+ ions. As the compositions of metal cations and interlayer anions in the LDH structure can be easily varied, the inorganic-organic hybrid system reported here opens great opportunity for developing efficient and functional luminescent materials by simple wet chemical methods. Introduction Light-emitting materials based on lanthanide ions have been widely investigated in view of their potential applications in optoelectronic devices, optical communications, light conversion molecular devices, biological labeling, etc.1-3 Upon excitation, lanthanide ions emit sharp and intense luminescence based on their f-f electronic transitions in contrast to those from organic phosphors and quantum dots.4,5 Long luminescence lifetime in a realm of microsecond to millisecond can be frequently obtained with Tb3+ and Eu3+ containing materials, thus offering the possibility for advanced applications, e.g., time-resolved fluoroimmunoassays.6 However, the extinction coefficients of lanthanides are intrinsically low (less than 10 dm3/mol/cm) due to Laporte forbidden f-f transitions.7 As a result, direct excitation of the metal ion center is often cumbersome and inefficient. This situation can be circumvented by using a light conversion process involving energy transfer from excited states of suitable chromophores to lanthanides. The chromophore acts as a sensitizer (or antenna) and the relaxed excited state transfers its energy to the lanthanide ion, which is referred to as an acceptor.8 Energy transfer can only occur if the energy differences between the ground and excited states of the sensitizer and acceptor are nearly equal (resonance condition) and if a suitable interaction between the two exists.9 Considerable effort has been made in designing various kinds of sensitizing ligands, suchasmacrocyclicmolecules,10-13 polymers,14-16 anddendrimers,17,18 with their triplet state energies comparable to those of the lanthanide acceptor states for enhanced luminescence efficiency. On the other hand, the incorporation of lanthanide ions and lanthanide complexes in solid materials has been a subject of increasing attention for practical device applications. The frequently used inorganic matrices are lanthanide oxides (e.g.,Y2O319-23) and metal fluorides (e.g., NaYF424-26) which also act as sensitizers. Some common metal oxides, such as * To whom correspondence should be addressed. Phone: +65 67906713. Fax: +65 67947553. E-mail: [email protected].

SiO2,27,28 TiO2,28,29 and Fe2O3,30 have also been used to host a small percentage of the lanthanide ions in their crystal lattices. Inorganic materials possessing a lamellar structure with guest inclusion capability are of great scientific interest. The properties of such materials can be tuned flexibly by varying the host and/ or guest composition and the interactions between the two.3,31,32 To date, several types of layered compounds containing lanthanides have been reported. Gandara et al. prepared layered lanthanide hydroxides of pure cationic rare earth (Yb3+, Dy3+, Ho3+, Y3+) intercalated with polyaromatic disulfonates via a hydrothermal route.33 A series of layered hydroxynitrates and hydroxyhalides of various lanthanides were synthesized hydrothermally.34-37 The nitrate and halide anions in these compounds can be exchanged by organic carboxylate and sulfonate anions. Karmaoui et al. produced lanthanide oxide based lamellar hybrid materials intercalated with organic molecules (benzoate, biphenolate). The oxide layers were doped with optically active lanthanide ions, such as Tb3+, Eu3+, or Nd3+, to achieve green, red, or infrared luminescence, respectively.38,39 Although incorporation of Y3+ in layered double hydroxides (LDHs) was reported a long time ago,40 the use of LDHs as host materials for optically active lanthanides is still a relatively new topic. LDHs, as an important class of host-guest materials, have received great attention due to their versatile applications in catalysis, separation, electrochemistry, and the biomedical field.41-47 LDHs consist of positively charged brucite-like layers and interlayer anions and their compositions can be represented by the general formula [MII1-xMIIIx(OH)2]x+An-x/n · mH2O.48 One of the most striking features of LDHs lies in their compositional flexibility in accommodating numerous types of metal cations in the host layer and guest anions in the interlayer space. It has been reported that lanthanide complex anions of Eu3+, Gd3+, and Ce3+ can be intercalated in LDHs.49-52 The complex anions of Eu3+ in the interlayer space displayed similar luminescence features as those of the free complexes.49 A more recent study involved doping of Tb3+ cations in the brucite-like layer. The

10.1021/jp905884n CCC: $40.75  2009 American Chemical Society Published on Web 09/10/2009

Lanthanide-Doped Layered Double Hydroxides resulting Tb3+-LDHs emit green fluorescence in the absence of any organic sensitizer.53 In this work, Tb3+ was successfully doped in the octahedral lattice of the brucite-like layer by a simple coprecipitation method under ambient conditions. At the same time, a sensitizer anion, 4-biphenylacetate (BPA), was intercalated in the interlayer space. With such a molecular level assembly between the acceptor and sensitizer, efficient energy transfer occurred from the excited state of interlayer BPA anions to the Tb3+ ions in the host layer. The resulting materials exhibit much enhanced green luminescence compared to LDHs without a sensitizer. Furthermore, BPA displays blue emission, which allows the possibility of tuning the luminescence color by varying the compositions. This type of inorganic-organic hybrid system may offer interesting perspectives in developing efficient luminescent materials. Experimental Section Synthesis of Tb3+-Doped LDHs Intercalated with Sensitizer Anions. The synthesis of Tb3+-doped LDHs intercalated with sensitizer anions was carried out by a one-pot coprecipitation method. All chemicals used for the synthesis were of analytical grade and were used without further purification. Briefly, 20 mL of an aqueous solution containing magnesium nitrate hexahydrate (0.3 M, Mg(NO3)2 · 6H2O, 99%, BDH), aluminum nitrate nonahydrate (Al(NO3)3 · 9H2O, 99%, ACROS Organics), and terbium(III) chloride hexahydrate (TbCl3 · 6H2O, 99.9%, Sigma-Aldrich) was first prepared. The total concentration of aluminum and terbium cations was maintained at 0.15 M and the molar ratio of Tb3+:Al3+ was varied at 0, 0.05, 0.2, and 1.0. The mixed metal salt solution was added dropwise at room temperature to 80 mL of an aqueous solution containing 18 mmol of NaOH (>99%, pellet, Merck) and 9 mmol of 4-biphenylacetic acid, sodium salt (BPA, 98%, ACROS) under vigorous stirring. After complete addition, the mixture was aged for 48 h at 70 °C. Before precipitation, the alkaline solution was bubbled with nitrogen at 60 mL/min for 30 min to push away carbon dioxide from the reaction vessel. Purging was continued during the precipitation and aging period. At the end of aging, the precipitate was centrifuged and washed thoroughly with deionized water followed by drying at 70 °C overnight. The as-synthesized samples were denoted as NoTb-BPA (Tb3+: Al3+ ) 0), 0.05Tb-BPA (Tb3+:Al3+ ) 0.05), 0.2Tb-BPA (Tb3+: Al3+ ) 0.2), and 1.0Tb-BPA (Tb3+:Al3+ ) 1.0), according to the Tb3+:Al3+ molar ratio used in the precursor solutions. Tb3+doped LDHs intercalated with nitrate anions were prepared by using the same procedure except in the absence of BPA compound. The Tb3+:Al3+ molar ratio in the precursor solution was varied at 0.0, 0.05, and 0.2, and the resulting products were named NoTb-N, 0.05Tb-N, and 0.2Tb-N, respectively. Ion Exchange of Zeolite Na-Y with Tb(III) and Adsorption of BPA. The experimental procedure was similar to that reported by Wada et al.54 The ion exchange of Na+ with Tb3+ was carried out by stirring 200 mg of zeolite Na-Y (Zeolyst International, Netherland) in 25 mL of an aqueous solution of TbCl3 · 6H2O (0.115 M) at 80 °C for 16 h. The solid was collected by centrifugation and washed with deionized water followed by drying at 70 °C overnight. The Tb3+-exchanged zeolite sample (ZY-Tb) was then degassed under vacuum overnight to remove the entrapped water molecules. The adsorption of BPA was carried out by mixing the degassed powder with 0.3 g of 4-biphenylacetic acid, sodium salt (BPA) in 25 mL of ethanol. The suspension was stirred at room temperature for 20 h before centrifugation and drying. The obtained sample was named ZYTb-BPA.

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17207 Leaching Study of Tb3+ Ions from LDH Samples. During the leaching study, 25 mg of sample 0.2Tb-BPA was suspended in 25 mL of deionized water and subjected to ultrasonication for 2 h. The suspension was then incubated at room temperature with continuous shaking at 180 rpm for 5 d. The final suspension was then centrifuged at 12 000 rpm for 5 min. The supernatant and the solid particles were collected for determination of the concentration of Tb3+ ions in the solution and the percentage of Tb3+ ions leached out from the sample. The above procedure was also carried out for sample NoTb-BPA after being physically adsorbed with Tb3+ ions for comparison. The experimental details for the preparation of the physical mixture of sample NoTb-BPA and Tb3+ ions can be found in the Supporting Information. Characterization. The powder X-ray diffraction (XRD) patterns of as-prepared samples were recorded on a Bruker AXS D8 X-ray diffractometer with Cu KR (λ ) 1.5406 Å) radiation at 40 kV and 20 mA. High-resolution XRD was carried out with 2θ between 59° and 64° to determine the lattice parameter a based on the (110) diffraction peak at a scanning rate of 0.06 deg/min. Fourier transform infrared (FTIR) spectra were obtained on a Digilab FTS 3100 FTIR with a 4 cm-1 resolution and in the range of 400-4000 cm-1 with use of a standard KBr disk technique. Elemental analysis of metal compositions in LDH samples was performed by inductively coupled plasma (ICP) optical emission spectroscopy on a Perkin-Elmer ICP Optima 2000DV. The weight percentage of Tb3+ in zeolite samples was measured by using energy dispersive X-ray spectroscopy (EDS) supported by a scanning electron microscope (SEM, JEOL JSM 6700F). The weight percentages of carbon and nitrogen were measured in a Elementarvario CHNS elemental analyzer. Thermogravimetric analysis (TGA) was carried out by heating the dry powder samples at a rate of 10 deg/min with air flow at 200 mL/min over 25-800 °C in a TA Instrument SDT Q600. The weight percentage of BPA was measured with a UV-vis spectrometer (Shimadzu 2450) after a complete dissolution of the solid sample in a mixture of HCl aqueous solution (1.0 M) and ethanol of 1:1 volume ratio. The UV-visible diffuse reflectance spectra were obtained from the same UV-visible spectrometer. The elemental mapping of metals was conducted with a transmission electron microscope (TEM, JEOL 2100) fitted with an EDS detector. Photoluminescence Study. A Shimadzu RF-5301 PC spectrofluorophotometer with a 150 W xenon lamp was used to observe luminescence emission. For each sample, around 2 mg of the fine powder was dispersed in 10 mL of methanol. Ultrasonication was applied for about 1.5 h to make a homogeneous suspension. The excitation and emission slits used were 5 and 1.5 nm, respectively. The luminescence decay curves were collected with a Fluorolog Jobin-Yvon-SPEX equipped with a flash lamp source with both excitation and emission slits of 8 nm. Quantum yield of samples was determined by using eq I,7

QS ) QR

IS ODR nS2 IR ODS n2

(I)

R

where the subscripts S and R denote the sample under study and the reference, respectively. Q is the quantum yield, I is the integrated intensity, OD is the optical density at the excitation wavelength, and n is the refractive index. A solution of cresyl violet in methanol with a dilute concentration of 10-7 M was used as reference and it has a quantum yield of 0.54.55 The

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Figure 1. XRD patterns of Tb3+-doped LDH samples intercalated with (A) 4-biphenylacetate and (B) nitrate anions, the asterisk (*) indicates the hexagonal Tb(OH)3 (JCPDS no. 19-1325). (C and D) High-resolution XRD patterns around the (110) and (113) diffraction region for samples shown in panels A and B, respectively.

primary and secondary inner-filter effects on the fluorescence intensity were approximately corrected by eq II,7

Icorr ) Iobs10[OD(λex)+OD(λem)]/2

(II)

where OD(λex) and OD(λem) are the optical densities at the excitation and emission wavelength, respectively. Results and Discussion Physicochemical Properties of Tb3+-Doped LDHs. The XRD patterns of Tb3+-doped LDH samples are displayed in Figure 1. A single LDH crystalline phase was formed at all Tb3+ loadings when BPA was intercalated in the interlayer space (Figure 1A). Diffraction patterns of these samples can be indexed according to a 3R rhombohedral symmetry. Sample NoTb-BPA exhibits a sharp and well-resolved XRD pattern. The presence of Tb3+ ions during the coprecipitation process resulted in a less ordered stacking of the brucite-like layers and a lower crystallinity. When more Tb3+ ions were added, a reduction of the grain size in the c direction can be clearly observed as indicated by the broader basal peaks. The estimated number of brucite-like layers based on Scherrer formula using the (003) basal peak is 7.4, 3.9, 3.5, and 3.0 for samples NoTbBPA, 0.05Tb-BPA, 0.2Tb-BPA, and 1.0Tb-BPA, respectively (Table 1). Such crystallographic changes were probably induced by the incorporation of Tb3+ ions in the brucite-like layers. As the ionic radius of the Tb3+ ion (1.06 Å) is substantially larger than those of Mg2+ (0.86 Å) and Al3+ (0.67 Å) ions, the doping of the Tb3+ ion results in distorted crystal lattices and poor

TABLE 1: Lattice Parameters of As-Synthesized LDH Samples sample

a (Å)

size in the c-axisa (nm)

no. of layersb

NoTb-N 0.05Tb-N 0.2Tb-N NoTb-BPA 0.05Tb-BPA 0.2Tb-BPA 1.0Tb-BPA

3.035 3.039 3.042 3.032 3.035 3.038 3.070

17.5 6.7 6.6 15.3 8.4 8.0 7.9

20.2 7.6 7.7 7.4 3.9 3.5 3.0

a The mean crystalline size calculated from the full width at half-maximum (fwhm) of the (003) peak. b One layer consists of a brucite-like layer and an anion layer. It was calculated by dividing the size in the c-axis by d003.

crystallinity.56 In addition, a gradually increased basal spacing from 20.63 Å (NoTb-BPA) to 25.89 Å (1.0Tb-BPA) could be related to a smaller Coulombic interaction between the positively charged brucite-like layers and negatively charged interlayer anions due to a lower polarizing ability of Tb3+. A similar phenomenon was also observed by others when Ce3+ or Tb3+ cations were incorporated into the LDH lattices.53,57 Figure 1B displays the XRD patterns of Tb3+-doped LDHs intercalated with nitrate anions. A similar trend on the crystallographic properties can be observed when the Tb3+ concentration was increased. The high-resolution XRD patterns around the (110) diffraction region (Figure 1C,D) provide further evidence for the doping of Tb3+ ions in the brucite-like layers. First, the (110) peak intensity became weaker as more Tb3+ ions were added. Second, the position of the (110) peak shifted to lower 2θ values. Due to poor crystallinity, the (110) and (113) peaks were overlapped. The deconvolution of these two peaks was carried

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TABLE 2: Compositions and Chemical Formulas of As-Prepared LDH Samples sample NoTb-N 0.05Tb-N 0.2Tb-N NoTb-BPA 0.05Tb-BPA 0.2Tb-BPA 1.0Tb-BPA

Mg:(Al + Tb)a Tb:Ala % Tba % Cb % Nb % BPAc % H2Od (M) (M) (wt %) (wt %) (wt %) (wt %) (wt %) 2.01 2.04 2.05 1.97 2.33 2.17 2.14

0.06 0.21

3.33 9.25

0.06 0.27 1.18

2.02 7.89 19.54

0.43 0.88 1.06 37.84 35.89 37.04 28.18

3.67 2.94 2.68 0.10 0.10 0.13 0.47

46.40 44.53 48.23 34.54

10.22 12.05 12.79 13.26 6.85 9.73 7.77

chemical formula Mg0.67Al0.33(OH)2(NO3)0.23(CO3)0.05 · 0.48H2O Mg0.67Al0.31Tb0.018(OH)2(NO3)0.184(CO3)0.071 · 0.59H2O Mg0.69Al0.27Tb0.041(OH)2(NO3)0.18(CO3)0.066 · 0.68H2Oe Mg0.66Al0.34(OH)2(BPA)0.32(CO3)0.01 · 1.09H2O Mg0.70Al0.28Tb0.017(OH)2(BPA)0.28(NO3)0.009(CO3)0.007 · 0.49H2O Mg0.69Al0.25Tb0.07(OH)2(BPA)0.34 · 0.84H2O Mg0.68Al0.15Tb0.17(OH)2(BPA)0.24(NO3)0.05(CO3)0.013 · 0.64H2O

a The composition of metal ions obtained from ICP measurement. b The weight percentage of C and N obtained from CHN elemental analysis. c The weight percentage of BPA obtained from UV-vis analysis. d The weight loss percentage of dry sample obtained from TGA analysis from 25 to 200 °C. e On the basis of the elemental analysis results, this sample contains approximately 1.2 mol % of Tb(OH)3 impurity (i.e., Tb(OH)3:LDH ) 1.2%:1), which was taken into account during the determination of the LDH formula.

out by a computer program without setting any peak-width constraints.58 Figure S1 in the Supporting Information shows the deconvolution results of all the LDH samples. The lattice parameter a calculated based on the (110) peak position increases with the concentration of Tb3+ ions in the samples, as shown in Table 1. In particular, the increase in a becomes very obvious from 3.032 Å (without Tb3+) to 3.070 Å in sample 1.0Tb-BPA, which contained 17 mol % of Tb3+ ions (metal basis from elemental analysis results shown in Table 2). A straight line can be used to fit the parameter a with Tb% for BPA intercalated LDH samples by excluding the data from sample 0.2Tb-BPA (Figure S2A, Supporting Information). Similarly, a straight line was obtained by plotting a versus Ce% based on the data by Das et al. (Figure S2B, Supporting Information) except for one point.57 These results indicated that the relationship between the lattice parameter a and the metal composition in the brucite-like layer may not follow a simple linear curve. A more systematic change in the doping level would be required to elucidate the comprehensive correlations. On the basis of the above crystallographic results, it is suggested that Tb3+ ions were doped in the brucite-like layers of LDHs which were intercalated with BPA anions, rather than forming separate impurity phases, or being adsorbed on the surface of LDHs. However, the formation of an impurity phase of hexagonal Tb(OH)3 (JCPDS no. 19-1325, P63/m symmetry) was observed (marked with an asterisk) in sample 0.2Tb-N in the absence of BPA. Segregation of Tb(OH)3 was not found in the previous work by Musumeci et al. probably due to the hydrothermal condition applied during their synthesis.53 The FTIR results shown in Figure S3 in the Supporting Information are in agreement with the formation of the LDH compounds intercalated with either BPA or nitrate anions. The peaks at around 1560 and 1395 cm-1 are attributed to the antisymmetric and symmetric stretching vibration of the carboxylate group of BPA, respectively. The spectrum of sample 1.0Tb-BPA shows that besides BPA, nitrate anion is also present in the interlayer space as indicated by the absorption peak at 1384 cm-1. Further, the vibration of the M-O bond in this sample shows a red shift to 402 cm-1 compared to samples with a smaller amount of Tb3+ (448 cm-1), indicating a weaker M-O bond due to a lower polarizing ability of Tb3+ ions. The compositional analyses were carried out for all LDH samples and the results are consistent with the XRD and FTIR observations. The detailed results together with the estimated formulas were shown in Table 2. It is to be noted that the measured molar ratio of Tb3+:Al3+ in the solid samples is always larger than that in the precursor solutions, while the M2+:M3+ ratio is close to two in general. Both Fernandez et al. and Das et al. reported that the RE3+:Al3+ (RE ) Y and Ce) ratios in doped LDHs were smaller than those in the precursor

solutions.40,57 In the work by Musumeci et al., the Tb3+:Al3+ ratio was found similar to that in the precursor solution.53 The different results found in this work could be due to the varied synthesis conditions, in particular, a relatively long aging time of 48 h, which may facilitate the inclusion of the relatively large Tb3+ ions into the brucite-like layers. The elemental mappings of Mg, Al, and Tb in LDH samples intercalated with BPA anions were obtained by a TEM-EDS technique and are presented in Figure 2. The density of each element, which is indicated by the relative brightness and the intensity of the color, corresponds to its composition in the sample. Overall, these three elements exhibit a unanimous distribution of density throughout the particle under examination. It can be observed that upon varying the ratio of Al3+ to Tb3+, the relative densities of Al3+ and Tb3+ change accordingly. The results therefore suggest a successful incorporation and homogeneous doping of Tb3+ ions in the crystal lattice of LDHs. The immobilization of lanthanide ions, either through complexation with ligands or incorporation in a host material, is important because the ions are inherently toxic.59 Hence confinement of Tb3+ ions in the LDH lattice may improve their stability. On the basis of the leaching study, it was found that about 10.4% of the Tb3+ ion was released if Tb3+ was just physically adsorbed on sample NoTb-BPA. On the contrary, only 0.09% of Tb3+ leached out from sample 0.2Tb-BPA, confirming that Tb3+ ions in LDH samples are rather confined in the solid matrix with a good stability, instead of being adsorbed on the surface of the solids. A mortality study by Hirano et al. showed that the LD50 values for intravenously injected rare earth elements range from 10 to 100 mg/kg (body weight), depending on the type of rare earth elements and their compounds.60 The concentration of the leached Tb3+ ions from our sample 0.2Tb-BPA in the solution was found to be 0.07 mg/L after 5 d. With an average blood volume per body weight for a typical adult as 85 mL/kg,61 the estimated amount of leached Tb3+ ions is as low as around 6.0 × 10-3 mg/kg (body weight). Therefore, the incorporation of Tb3+ into LDHs certainly enhances its biocompatibility. Photoluminescence Properties of Tb3+-Doped LDHs. As one of the brightest line emitters of all the lanthanides, Tb3+ emits several spectral lines with the strongest emission from the intra-4f8 5D4 f 7F5 transition.14 The excitation spectra were monitored with this Tb3+ line (at 542 nm) and are shown in Figure 3. The spectra of samples 0.05Tb-BPA, 0.2Tb-BPA, and 1.0Tb-BPA are composed of bands in the UV region at around 233, 240, and 275 nm (Figure 3A). In contrast, sample 0.2Tb-N without BPA only exhibits a weak excitation band at around 233 nm (Figure 3B). As a control, samples NoTb-BPA and NoTb-N without Tb3+ ions obviously do not exhibit any appreciable excitation bands. Furthermore, it is important to

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Figure 2. TEM images and the corresponding EDS images of Mg, Al, and Tb elements in samples (A) 0.05Tb-BPA, (B) 0.2Tb-BPA, and (C) 1.0Tb-BPA.

point out that the intra-4f8 excitation lines at 380 (7F6 f 5D3) and 490 nm (7F6 f 5D4) are absent for all samples (not shown). Therefore, it clearly indicates that the luminescence was not due to direct excitation into Tb3+ intra-4f8 levels, but rather, through energy transfer. The BPA molecule contains a biphenyl group and the UV diffuse reflectance spectra (Figure S4, Supporting Information) show that BPA exhibits π-π* absorption with a maxima centered at around 270 nm coupled with a shoulder at around 230 nm. There exist only minor variations in the absorption wavelength and intensity when BPA is present in its sodium salt form, or in the interlayer space of LDH samples (NoTb-BPA and 0.2Tb-BPA). As a result, the observed excitation bands could be ascribed to the excited states of BPA anions, of which the absorbed energy is subsequently transferred to the Tb3+ centers.9 The bands at the shorter wavelengths (233 and 240 nm) could be partially contributed by the LDH lattice based on the appearance of a weak band at round 233 nm in the spectra of 0.2Tb-N. The excitation features of our samples thus demonstrate that the intercalated BPA anions play an important role on the luminescence properties. On the basis of the results from the excitation spectra, the emission spectra were recorded at both shorter and longer UV wavelengths of 233 (Figure 4A-F) and 275 nm (Figure 4G-L), respectively. All the spectra were normalized to their respective maximum intensities. The BPA anion in the interlayer space of the LDH sample (NoTb-BPA) is found to emit strong blue photoluminescence as shown by a broadband centered at around 420 nm (Figure 4A-G). After the Tb3+ ion was doped in the brucite-like lattice, the four characteristic sharp emission peaks

at 488, 542, 583, and 619 nm appear in the spectra of samples 0.05Tb-BPA, 0.2Tb-BPA, and 1.0Tb-BPA. These peaks are attributed to 4f8 orbital 5D4 f 7Fj (where j ) 6, 5, 4, 3) transitions of Tb3+ in the green region.14 It can be seen in panels B-D and H-J of Figure 4 that the green and blue emissions are superimposed. According to the Dieke diagram, the energy gap between the excited state 5D4 and the ground state 7F6 of Tb3+ is about 21 000 cm-1, which is equivalent to a photon energy at a wavelength of around 470 nm.62 Noting that the emission of the BPA anion has a slightly higher energy than this energy gap, it may yield good resonance that enables an efficient energy transfer. The high BPA:Tb3+ molar ratio of 16.5:1 in sample 0.05Tb-BPA explains the relatively strong blue emission (Figure 4B,H) due to excess BPA. The relative intensity of BPA emission decreases greatly in samples 0.2TbBPA and 1.0Tb-BPA in which the concentration of Tb3+ was increased, corresponding to a reduced molar ratio of BPA:Tb3+ to 4.8:1 and 1.4:1, respectively. It thus confirms that energy transfer from the excited states of BPA anions to Tb3+ centers in this hybrid system takes place, as shown in Scheme 1. The interlayer BPA anions located in a close proximity to the Tb3+ ions in the brucite-like layer act as an energy antenna for the green emission of Tb3+ ions. A similar energy transfer process was reported in lanthanide-based lamellar oxides containing interlayer aromatic molecules,38,39 as well as zeolite and mesoporous silica incorporated with lanthanides and organic photosensitizers.54,63 In contrast, sample 0.2Tb-N without a photosensitizer displays very weak luminescence from Tb3+ as shown in Figure 4E,K.

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Figure 4. Photoluminescence spectra of samples (A and G) NoTbBPA, (B and H) 0.05Tb-BPA, (C and I) 0.2Tb-BPA, (D and J) 1.0TbBPA, (E and K) 0.2Tb-N, and (F and L) ZY-Tb-BPA. The spectra in panels A-E, F, G-K, and L were recorded at λex of 233, 229, 275, and 260 nm, respectively. Figure 3. Excitation spectra (λem ) 542 nm) of Tb3+-doped LDH samples intercalated with (A) 4-biphenylacetate and (B) nitrate anions.

A blue emission band centered at around 410 nm is also present in these two spectra. It may be tentatively ascribed to the emission of the brucite-like lattice, as some evidence on the fluorescent property of CoAl- and ZnAl-LDHs in the absence of fluorescent substances was observed elsewhere.64,65 The origin of this emission could be due to the scattering of the incident beam by hydroxyl groups.66 The energy transfer between the brucite-like lattice and Tb3+ ions could take place to enable the green emission of Tb3+. It has to be noticed that the formation of Tb(OH)3 impurity phase in this sample not only reduces the concentration of Tb3+ ions in LDH lattice (as shown in Table 2), but also quenches Tb3+ luminescence.9 It has been proposed that the nanosized pore of zeolites offers an ideal environment for locating the lanthanide ions and photosensitizing molecules for controlled energy transfer.54 For comparison, the photoluminescent property of Tb3+-exchanged zeolite Y incorporated with BPA anions (ZY-Tb-BPA) was also investigated. The composition of this sample can be found in Table S1 in the Supporting Information. In particular, the molar BPA:Tb3+ ratio in this sample is quite low at around 0.16:1, because BPA anions can only be adsorbed on the surface of the zeolite framework. The emission spectra were obtained at 229 (Figure 4F) and 260 nm (Figure 4L) based on the maxima in the excitation spectrum of this sample (Figure S5, Supporting Information). It can be seen that the intensity ratio between the blue and green emissions in ZY-Tb-BPA is similar to that in sample 1.0Tb-BPA despite a much lower ratio of BPA:Tb3+ in the former. This therefore indicates that energy transfer is more efficient in LDH samples with regularly arranged layers of lanthanide ions and photosensitizer anions.

SCHEME 1: Sensitization of Tb3+ Centers in the Brucite-Like Layer of Layered Double Hydroxide by Energy Transfer from the Interlayer 4-Biphenylacetate Anions

It is noted that no substantially different emission features can be observed when the samples were excited at 233 and 275 nm. Nevertheless, a slightly higher signal-to-noise ratio can be seen in the spectra obtained at 275 nm. The photoluminescent colors displayed by the samples (Figures 5) are consistent with the emission spectra results. Overall, the excitation at the longer wavelength (275 nm) results in brighter fluorescence. Sample NoTb-BPA (Figure 5A,G) emits blue color, which corresponds to the emission of BPA molecules. Upon increasing the Tb3+ content, the color gradually turns to cyan (sample 0.05Tb-BPA) and green (samples 0.2Tb-BPA and 1.0Tb-BPA) due to an increased emission from Tb3+. Herewith, the emitted color can be tuned by varying the composition of Tb3+. For comparison, sample 0.2Tb-N shows only a faint green color at 275 nm (Figure 5K). The green luminescence from sample ZY-Tb-BPA is also quite weak.

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Gunawan and Xu TABLE 3: Lifetimes and Decay Parameters Obtained from the Decay Curves Obtained at Excitation Wavelength of 275 nm and Monitored with Emission Wavelength of 542 nm sample

Po

A1

τ1 (ms)

A2

τ2 (ms)

τav (ms)

0.05Tb-BPA 0.2Tb-BPA 1.0Tb-BPA ZY-Tb-BPA

0.009 0.002 0.002 0.016

0.833 0.925 0.964 0.892

1.263 1.293 1.337 1.109

0.163 0.083 0.042 0.110

0.326 0.309 0.345 0.439

1.22 1.27 1.33 1.08

because no emission arising from benzoate could be detected.38 The logarithmic decay graphs in our case were fitted by a double exponential equation with r2 > 0.9995.7

I(t) ) Po + A1 exp(-t/τ1) + A2 exp(-t/τ2)

(III)

where Po is the Poisson noise, τ1 and τ2 are the lifetimes for each component, and A1 and A2 are the intensity amplitudes at t ) 0. The average lifetime τav is given by

A1τ12 + A2τ22 τav ) A1τ1 + A2τ2

Figure 5. Photoluminescence pictures of samples (A and G) NoTbBPA, (B and H) 0.05Tb-BPA, (C and I) 0.2Tb-BPA, (D and J) 1.0TbBPA, (E and K) 0.2Tb-N, and (F and L) ZY-Tb-BPA. The pictures in panels A-E, F, G-K, and L were taken at λex of 233, 229, 275, and 260 nm, respectively.

Figure 6. Photoluminescence decay curves of Tb3+-doped LDH samples intercalated with 4-biphenylacetate and Tb3+-doped zeolite incorporated with the same anion.

The lifetimes of the 5D4 level of Tb3+ in LDH samples intercalated with BPA anions were monitored around the 5D4 f 7F5 transition (around 542 nm) at the excitation wavelength of 275 nm. The emission at this wavelength is superimposed by Tb3+ and BPA emissions, although the contribution from the latter is very minor especially for samples 0.2Tb-BPA and 1.0Tb-BPA. Consequently, it is expected to have curved multiexponential decay characteristics as shown in Figure 6. In a previous report, the decay curve of Tb3+-doped lanthanide oxides intercalated with benzoate is a single-exponential function

(IV)

In this case, the emission intensity is fractioned into two amplitudes with different magnitudes. They are represented by pre-exponential values A1 and A2, each of which corresponds to one of the decay times τ1 and τ2, respectively. The Poisson noise indicates a random fluctuation of the received emission signal and its magnitude becomes more significant with decreased signal.67 It can be generally observed from Table 3 that the larger fraction of the intensity has a long decay time ranging from 1.109 to 1.337 ms (τ1) with the corresponding pre-exponential value (A1) close to unity. This could be ascribed to the decay of Tb3+ emission as lanthanides are well-known for their inherent long lifetimes. In literature, a shorter decay time of 0.467 ms was reported for crystalline TbCl3 · 6H2O, in which the energy of the excited Tb3+ states can be lost via vibronic coupling to the -OH oscillators in coordinated water molecules.4 In our case, the hydroxyl group in the brucite-like layer seems to have a less quenching effect. The smaller amplitude corresponds to shorter decay times of 0.309 to 0.439 ms (τ2) and smaller A2 values close to zero. The assignment of this lifetime is not so straightforward since the organic phosphors usually exhibit short lifetimes of nanosecond order. The results obtained here may suggest back energy transfer from the excited Tb3+ states to BPA anions, giving rise to longer lifetimes for BPA molecules than usual. The back energy transfer phenomenon in the Tb3+ complex of salicylamide derivatives was previously reported.10 Nonetheless, the decay curves displayed by our samples could demonstrate complex phenomena occurring concurrently. The occurrence of energy transfer between BPA and Tb3+, as well as the quenching by the hydroxyl groups in the brucite-like layers and the solvent (methanol) may together affect the fluorescence decay profile.7 Furthermore, different crystallinity of LDH layers could influence the crystal field, which eventually affects the emission property and the lifetime.9 The photoluminescence quantum yield was determined at an excitation wavelength of 275 nm for LDH samples and 260 nm for ZY-Tb-BPA. The emission from both BPA and Tb3+ ions was taken into account during the calculations. The results obtained are in line with the proposed energy transfer mechanism between BPA and Tb3+ in the LDH samples. As expected, sample 0.2Tb-N exhibits a very low quantum yield of around

Lanthanide-Doped Layered Double Hydroxides 0.2% due to the lack of photosensitizer. The quantum yield reported by Musumeci et al. for a similar Tb3+-incorporated LDH sample was higher at 4.4%.53 This could be due to the presence of the Tb(OH)3 impurity phase and a lower crystallinity of our sample. In the presence of interlayer BPA sensitizer, the quantum yield of samples 0.05Tb-BPA, 0.2Tb-BPA, and 1.0TbBPA is significantly improved to 15.1%, 21.6%, and 14.0%, respectively. It is found that the optimum quantum yield was obtained at a moderate doping concentration of Tb3+ as in the case of sample 0.2Tb-BPA. With a higher Tb3+ concentration in sample 1.0Tb-BPA, the closer distance between Tb3+ cations could result in self-quenching and thus a lower quantum yield.9 Overall, the estimated quantum yields of our samples are comparable to those reported for other lanthanides doped in organic-inorganic hybrid materials.38,68 For comparison, the zeolite sample (ZY-Tb-BPA) has a quantum yield of 1.0%, which is similar to those obtained by Wada et al. for Tb3+doped zeolite with 4-acetylbiphenyl as the photosensitizer.54 The relatively low quantum yield compared to those of the LDH samples should be due to a lower concentration of BPA anions, as well as less efficient energy transfer in this sample. Finally, it is worth noting that among the lanthanides the Tb3+ ion exhibits a relatively large energy gap between its emissive and ground states. Although hydroxyl groups are recognized as the efficient quenchers to lanthanide luminescence, the extent of the quenching is inversely proportional to this energy gap.2 Quenching via nonradiative energy transfer to the crystal lattice becomes less efficient when a large quantum of energy must be dissipated.4 As a result, the hydroxyl groups of the LDH lattice do not cause a significant quenching effect to Tb3+ luminescence. Conclusions Incorporation of Tb3+ ions into the brucite-like layers of LDH was successfully achieved by using a simple coprecipitation technique in the presence of 4-biphenylacetate anions. Detailed characterizations confirmed a homogeneous distribution of Tb3+ ions in the octahedral lattice of LDH layers despite its relatively large ionic size. A high content of Tb3+ up to around 19 wt % was obtained without inducing phase segregation of Tb3+ ions. The luminescence study suggested that energy transfer from the excited state of the intercalated 4-biphenylacetate molecules to Tb3+ centers takes place, which greatly enhances the green luminescence from Tb3+ ions. Furthermore, as 4-biphenylacetate emits strong blue fluorescence, the emission color of the hybrid materials can be tuned from blue to green by varying the amount of Tb3+ ions. The obtained long lifetime and high quantum yield values infer that there is a good energy match between the donating level of BPA sensitizer and the accepting level of Tb3+ cations. This work indicates that lanthanide ions-doped LDHs intercalated with photosensitizer anions may be found as a new type of luminescent materials system with interesting properties. Furthermore, since the compositions of metal cations and interlayer anions in the LDH structure can be easily varied, the inorganic-organic hybrid system reported here opens great opportunity for developing efficient and functional luminescent materials by simple wet chemical methods. Acknowledgment. This work was supported by AcRF grants (RG30/07) from Ministry of Education, Singapore. Supporting Information Available: Experimental procedure for preparation of the physical mixture of sample NoTb-BPA and Tb3+ ions, composition of Tb3+-doped zeolite samples,

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