Tunable photoluminescence of europium-doped layered double hydroxides intercalated by coumarin-3-carboxylate

Abstract

We report a highly tunable photoluminescence of rare-earth (europium, Eu³⁺) doped layered double hydroxides (LDHs) intercalated by an organic photofunctional anion, courmarin-3-carboxylate (C3C). Novel Eu³⁺-doped LDHs intercalated by C3C (ZnAlEu–LDH–C3C) with high purity and crystallinity have been successfully synthesized via an interlayer ion exchange process using Eu³⁺ doped LDH (ZnAlEu–LDH–NO₃) as a precursor. A bilayer arrangement with nearly perpendicular orientation was deduced for the C3C anions in the LDH interlayer space. Compared with the precursor, ZnAlEu–LDH–C3C shows a strong UV absorbance and an interesting photoluminescence function, possibly due to an interfacial energy transfer process between the interlayer C3C anions and the Eu³⁺ within the LDH lattices. The photoluminescent spectra denote a lower symmetry of the coordinating environment around Eu³⁺ within ZnAlEu–LDH–C3C. More importantly, the photoluminescence of ZnAlEu–LDH–C3C could be highly tuned by simply adjusting the structural constituents (including the content of Eu³⁺ or C3C) or the excited wavelength. These findings open new avenues to prepare tunable photoluminescent materials and further have potential applications for optical devices.

---

1 Introduction

In recent years, photoluminescent materials have attracted significant academic and industrial interest due to their wide applications in various fields such as optoelectronic devices, optical communications, light conversion molecular devices and biological labeling, etc.^1–5 From the perspective of practical applications, those materials having tunable photoluminescent properties such as emission colour and intensity have drawn special attentions.^6–9 Besides the conventional inorganic rare-earth elements and organic luminescent molecules, layered inorganic materials have attracted much of current interests. One of the most successful examples is perhaps layered double hydroxides (LDHs).

LDHs, also known as anionic clays or hydrotalcite-like compounds, consist of positively charged brucite-like nanolayers and charge-balanced interlayer anions, which have become a hot topic in the recent decade.^10–15 Their compositions are commonly expressed by the general formula [M_1−x²⁺M_x³⁺ (OH)₂]^x+A_x/z^z−·mH₂O, in which M²⁺ and M³⁺ are divalent (Mg²⁺, Zn²⁺, Co²⁺, Fe²⁺, Ni²⁺, etc.) and trivalent (Al³⁺, Cr³⁺, Fe³⁺, etc.) metal cations occupying in the octahedral positions within the hydroxide layers, respectively; A^z− can be almost any organic or inorganic anions balancing the positive charges of the hydroxide layers; x is generally believed to be between 0.20 and 0.33.¹⁶ Due to their high versatility in structural constituents and unique anion exchange ability in the interlayer space, LDHs have been widely applied in a variety of fields such as catalysts,^17,18 pharmaceuticals,¹⁹ adsorbents^20,21 and nano-fillers for polymer nanocomposites.^22–25

In order to achieve LDHs with photoluminescent function, rare-earth doping,^26–29 intercalation of rare-earth complexes³⁰ or organic photofunctional anions^31,32 are the three main ways reported so far. Rare-earth doped LDHs have been found as new photoluminescent materials.^26–29 Due to the presence of the structural rare-earth metal ions such as europium (Eu³⁺) in the lattice of LDH host layers, typical ⁵D₀ → ⁷F_J (J = 0−4) transitions of Eu³⁺ were clearly observed in the photoluminescence spectra.^26–29 The second method is to intercalate the organic complexes of rare-earth with ethylenediaminetetraacetate (EDTA) or nitrilotriacetate (NTA) via ion-exchange reaction in the interlayer.³⁰ The photoluminescence properties of the LDH hybrids containing Eu(III) complexes were found to be significantly affected by the charge intensity of the interlayer anions.³⁰ Thirdly, the photoluminescence function of LDHs can be realized by guest intercalation of organic photofunctional anions such as dyes and sensitizers into the LDH host interlayer.^31,32 The incorporation of the guest photofunctional anions into the LDH host matrices offers a stable and robust environment which enhances the mechanical, thermal and photo-stabilities of the organic species. And the homogeneous distribution of the organic photofunctional anions in the solid matrices at molecular level will suppress aggregation, and therefore mitigate the fluorescence quenching.^31,32

Theoretically, combination of LDH host doping of rare-earth ions within lattices and guest intercalation of organic photoluminescent anions in the interlayer should provide more fascinating prospects. Unfortunately, in sharp contrast to the studies using the above three methods, such publications are very scarce. Coumarin is an important type of laser dyes, which is extensively applied in photoluminescence. Nevertheless, the studies of coumarin-intercalated LDHs are very limited. Indeed, in a literature survey, the report of photoluminescence of Eu³⁺-doped LDHs intercalated by coumarin has not been found so far.

Herein, we report novel Eu³⁺-doped ZnAl–LDHs intercalated by courmarin-3-carboxylate (C3C) anions (ZnAlEu–LDH–C3C). First, the samples of ZnAlEu–LDH–C3C were synthesized by an interlayer ion exchange procedure using Eu³⁺ doped LDH as precursor. Then, their structure and morphology were characterized by X-ray diffraction (XRD), Fourier-transfer infrared (FTIR) spectroscopy, element analysis and scanning electron microscopy (SEM). Finally, the photoluminescent properties, especially the tunable photoluminescence properties, and the corresponding mechanism were studied.

2 Experimental

2.1 Materials

Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, NaNO₃, Eu₂O₃, nitric acid, ammonia water and hexamethylenetetramine (HMT) were purchased from Sinopharm Chemical Reagent Co., Ltd. Coumarin-3-carboxylic acid was bought from Sigma-Aldrich Co. LLC. All of the reagents were of analytical reagent (A.R.) pure grade, and used without further purification. Distilled water was used in all experimental procedures.

2.2 Synthesis of ZnAlEu–LDH precursors

The precursors of Eu³⁺ doped ZnAl–LDH with nitrate anions as counterions in the interlayer space (ZnAlEu–LDH–NO₃) were synthesized using an HMT hydrolysis method. In brief, 2.636 g Eu₂O₃ powder was dissolved in excess concentrated nitric acid (0.10 mol L⁻¹). After partly removing the nitric acid by distillation, distilled water was added to formed a 50 mL solution of Eu(NO₃)₃ with concentration of 0.30 mol L⁻¹. Subsequently, Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, NaNO₃ and HMT were dissolved in distilled water with the above Eu(NO₃)₃ solution to yield a 40 mL aqueous solution. The total metal concentration was 0.15 mol L⁻¹, and Zn : (Al + Eu) molar ratios were set to 2 : 1 with various ratios of Eu/Al. And the concentration of NaNO₃ and HMT solutions were 0.50 and 0.075 mol L⁻¹, respectively. Then, the mixed solution was transferred into a stainless autoclave with inner Teflon vessel, and hydrothermally treated at 110 °C for 24 h. After being cooled at ambient temperature, the solid was filtered and washed with distilled water in N₂ atmosphere for at least three times, and finally dried in vacuum oven at 50 °C for 24 h. The obtained ZnAlEu–LDH–NO₃ samples are labelled as LN0, LN1, LN2 and LN3 with an increase of Eu³⁺ content, where LN0 represents a ZnAl–LDH–NO₃ without Eu³⁺ doping.

2.3 Preparation of ZnAlEu–LDH–C3C

The hybrids of ZnAlEu–LDH–C3C were prepared by interlayer anion exchange process. By adjusting the pH value to ∼7.0, 2.138 g of coumarin-3-carboxylic acid was dissolved in dilute ammonia water under ultra-sonication. The concentration of the obtained C3C solution (100 mL) was 0.01 mol L⁻¹. Then, about 0.050 g of ZnAlEu–LDH–NO₃ precursor was added in 10 mL C3C solution. Subsequently, the mixture was stirred at room temperature for 3 days. The obtained solid was filtered, washed with distilled water and anhydrous ethanol for several times, and finally dried in vacuum oven at 50 °C for 24 h. According to the abbreviations of the above precursors, the corresponding ZnAlEu–LDH–C3C samples were signed as LC0, LC1, LC2 and LC3, respectively.

2.4 Characterizations

Powder XRD measurements were conducted on a Rigaku D/max 2400 diffractometer with Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 4° min⁻¹. The FTIR spectra were collected on a Perkin-Elmer System 2000 FTIR spectrophotometer after 64 scans within the range of 4000−400 cm⁻¹. Content analysis of metals was performed by inductively coupled plasma (ICP) atomic emission spectroscopy on a SPECTRO ARCOS EOP Axial View Inductively Coupled Plasma Spectrometer (SPECTRO Analytical Instruments GmbH). CHN elemental analysis was carried out using a Flash EA1112 elemental analysis instrument. SEM images were taken on a HITACHI S-4800 scanning electron microscope with an acceleration voltage of 15 kV. Solid ultraviolet-visible (UV-vis) diffuse reflectance spectra were carried out using a U-3900 spectrophotometer (HITACHI). The photoluminescence excitation and emission spectra were recorded using a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as the excitation source. Confocal laser scanning microscopic (CLSM) images were obtained on an OLYMPUS FV1000-IX81 microscope using excitation wavelength of 320–380 nm (Hg lamp).

3 Results and discussion

3.1 Structural and morphological characterizations

The precursors of ZnAlEu–LDH–NO₃ were synthesized by an HMT hydrolysis method. Their crystalline structure was characterized by XRD technique. As shown in Fig. 1a, a series of intense and sharp diffraction peaks, characteristic of ordered two-dimensional (2D) alignment ordered structure, appear. The corresponding structural parameters including the characteristic reflection positions, the corresponding distances and the refined lattice parameters are illustrated in Table 1. Note that all of the peaks can be indexed according to a 3R rhombohedral symmetry.^33,35 In addition, there are no noticeable Bragg reflections of any impurity phases such as Eu(OH)₃, whose characteristic peaks were marked with star symbols. These results suggest the success of Eu³⁺ ions doping within the LDH nanosheets rather than forming separate impurity phases or being adsorbed on the LDH surfaces. Moreover, due to the slightly larger ionic radius of the Eu³⁺ ion (95 pm) compared to the Zn²⁺ (74 pm) and Al³⁺ (67 pm) ions,³⁴ the d-spacing values of the basal reflections essentially increase with the increasing of the Eu³⁺ content. Accordingly, the cell parameter c has an obvious increase via doping with Eu³⁺ (Table 1). These XRD results demonstrate the success of the insertion of Eu³⁺ ions into the LDH lattices and the preparation of the ZnAlEu–LDH–NO₃.³⁴

Fig. 1 XRD patterns of (a) ZnAlEu–LDH–NO₃ and (b) ZnAlEu–LDH–C3C. The number of 0–3 represents the increase of Eu³⁺ content, where 0 stands for without Eu³⁺.

Table 1 Lattice parameters of ZnAlEu–LDH–NO₃ and ZnAlEu–LDH–C3C samples

Sample^a	d003 (nm)	d110 (nm)	a (nm)	c (nm)
a LNn and LCn (n = 0–3) denote ZnAlEu–LDH–NO3 and ZnAlEu–LDH–C3C with an increased Eu^3+ content.
LN3	0.897	0.154	0.307	2.691
LN2	0.897	0.154	0.307	2.691
LN1	0.885	0.153	0.307	2.654
LN0	0.881	0.153	0.306	2.643
LC3	1.768	0.153	0.306	5.304
LC2	1.760	0.153	0.306	5.281
LC1	1.746	0.153	0.306	5.239
LC0	1.746	0.153	0.306	5.239

---

Fig. 2 and S1a (ESI†) show the FTIR spectra of the precursors of ZnAlEu–LDH–NO₃. A strong absorption band at 1384 cm⁻¹ (characteristic of ν₃ stretching vibration mode of nitrate anions)³⁶ can be clearly observed, demonstrating the presence of nitrate in the LDH interlayer. And the band at ∼826 cm⁻¹ is attributed to the stretching mode that confirms the presence of NO₃⁻ in the LDH interlayer with D3h symmetry.³⁷ In addition, the broad absorption band ranged from 3750 to 2700 cm⁻¹ is ascribed to the stretching modes of the structural hydroxyl groups for the LDH platelets and the interlayer water molecules.³⁸ And the weak band at 1620 cm⁻¹ results from the O–H bending vibration of the hydroxyl groups.³⁹ Additionally, the band at approximately 426 cm⁻¹ arises from O–M–O vibrations in the brucite-like layers, while the band at 610 cm⁻¹ is attributed to the lattice vibration modes corresponding to the translation vibrations of M–OH.^37,40

Fig. 2 FTIR spectra of ZnAlEu–LDH–NO₃ (LN3), ZnAlEu–LDH–C3C (LC3) and C3C.

After the interlayer ion exchange of NO₃⁻ by C3C anions, the positions of the basal reflections for all samples shift to low 2θ values (Fig. 1b). Thus, the basal spacings (d₀₀₃) for ZnAlEu–LDH–C3C (LC) are much larger than those for the corresponding ZnAlEu–LDH–NO₃ (LN), as summarized in Table 1. The dramatically increase of the interlayer distance and cell parameter c demonstrates the success of the nitrate replacement by the organic anions (C3C), which has larger ion radius than the nitrate anions. Furthermore, the intense and sharp (00l) diffraction peaks reveal that there is no obvious decrease of the crystallinity. More importantly, please note that there are reflections of neither impurity phases formed after ion exchange nor the residues of the original ZnAlEu–LDH–NO₃. Table 2 displays the chemical formulae of ZnAlEu–LDH–C3C samples calculated based on the measured data of ICP and CHN elemental analyses. The absence of nitrogen element in the ZnAlEu–LDH–C3C samples (LC1, LC2 and LC3) provides further proof that the interlayer nitrate anions had been completely replaced. Therefore, we conclude that the interlayer NO₃⁻ had been fully exchanged by the C3C anions, and ZnAlEu–LDH–C3C were successfully synthesized.

Table 2 Chemical formulae of the ZnAlEu–LDH–C3C samples

Sample	Chemical formula
LC3	Zn1.648Al0.925Eu0.075(OH)5.463(C10H5O4)0.833·2.241H2O
LC2	Zn1.751Al0.998Eu0.002(OH)5.711(C10H5O4)0.791·1.778H2O
LC1	Zn1.842Al0.999Eu0.001(OH)5.857(C10H5O4)0.826·2.158H2O
LC0	Zn1.861Al1.000(OH)5.779(C10H5O4)0.943·2.498H2O

---

Besides the proofs of XRD and chemical formulae, the success of C3C intercalation is also confirmed by FTIR spectra, as shown in Fig. 2 and S1b (ESI†). For the ZnAlEu–LDH–C3C samples, the bands characteristic of C3C can be clearly observed, and obvious band shifts appear due to the strong interfacial interaction between C3C anions and LDH nanosheets. For example, the band at 1703 cm⁻¹ is the vibration of the carboxylic groups for ZnAlEu–LDH–C3C,⁴¹ shifting from 1745 cm⁻¹ for C3C. The band shift is as high as 42 cm⁻¹. As for the stretching vibrations of carbonylic groups, obvious band-shift can be seen as well, i.e. from 1684 (for C3C)^41,42 to 1607 cm⁻¹ (for ZnAlEu–LDH–C3C). The C–C stretching modes at 1610 and 1568 cm⁻¹ for C3C move to 1568 and 1384 cm⁻¹, respectively. Additionally, the bands characteristic of phenyl group vibrations for C3C, between 833 and 646 cm⁻¹,⁴¹ shift to 814, 762 and 619 cm⁻¹ for ZnAlEu–LDH–C3C. These significant band-shifts for pure C3C to low wavenumbers for ZnAlEu–LDH–C3C may be ascribed to the presence of strong interfacial interactions including hydrogen bonding between carboxylate of C3C anion and hydroxyl groups on LDH sheets, electrostatic interactions between the guest anions and the positive layers, or even π–π interactions among the intercalated C3C anions.⁴²

On the basis of the above results of XRD, FTIR and chemical formulae, a schematic model showing the alignment of the C3C anions in the interlayer of ZnAlEu–LDH–C3C was proposed in Scheme 1. If we take LC3 as an example, after deducting the distance of a single LDH platelet (0.48 nm)⁴³ from its basal spacing (1.768 nm), the height occupied by the C3C anions could be obtained (1.288 nm). The longitude length of a C3C anion is ∼0.65 nm, calculated by the software of ChemBio 3D Ultra 12.0 via MM2 energy minimization. Assuming that the two oxygen atoms of carboxylate are combined directly with the metal hydroxide nanosheets, it can be deduced that the C3C anions pack as a bilayer arrangement with nearly perpendicular orientation in the interlayer, as depicted in Scheme 1. As for the other samples of LC0−2, their basal spacings are only a little smaller than that of LC3. Thus, for all of the ZnAlEu–LDH–C3C samples (LC0−3), the C3C anions adopt a bilayer arrangement with nearly perpendicular orientation in the interlayer space.

Scheme 1 Schematic structural model showing the alignment of C3C anions in the interlayer of ZnAlEu–LDH–C3C (LC3).

Additionally, the morphologies of both ZnAlEu–LDH–NO₃ (LN0−3) and ZnAlEu–LDH–C3C (LC0−3) measured by SEM are displayed in Fig. 3. Clearly, sheet-like aggregates with a diameter of about 300–600 nm are prevalent. No distinct differences can be recognized for the morphology between ZnAlEu–LDH–NO₃ and ZnAlEu–LDH–C3C.

Fig. 3 SEM images of ZnAlEu–LDH–C3C ((a) LC3, (b) LC2, (c) LC1 and (d) LC0) and ZnAlEu–LDH–NO₃ ((e) LN3, (f) LN2, (g) LN1 and (h) LN0).

3.2 Photoluminescence properties

Fig. 4 shows the room temperature photoluminescence excitation and emission spectra of the LN3 and LC3 samples. For ZnAlEu–LDH–NO₃ sample (LN3), the excitation spectrum was collected by monitoring the ⁵D₀ → ⁷F₂ emission lines (613 nm), and the emission spectrum was measured by the intra-⁴f₆ (396 nm) direct excitation. In Fig. 4a, a series of sharp peaks resulting from f–f transitions within the intra-⁴f₆ electronic configuration of Eu³⁺ ions can be clearly seen. In the emission spectra (Fig. 4b), the emission peaks at 577, 589, 613, 652 and 698 nm can be assigned to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3 and 4) transition of Eu³⁺, respectively.^41,42 Furthermore, the red luminescence of ZnAlEu–LDH–NO₃ was confirmed by the red emission CLSM image photographed under UV irradiation (Fig. 6d). Indeed, the intensities and splitting of the ⁵D₀ → ⁷F_J transition emission peaks, which strongly depend on the local symmetry of the crystal field, are usually employed to probe the environment of the Eu³⁺ ions.^30,44 More importantly, the intensity ratio of the electric-dipole transition (⁵D₀ → ⁷F₂) to the magnetic-dipole transition (⁵D₀ → ⁷F₁) reveals valuable information on the local environment of the anions coordinating with the Eu³⁺ ions.^40,45 As shown in Fig. 4b, for the ZnAlEu–LDH–NO₃, the emission intensities of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transition are essentially equal, suggesting a high symmetry of the coordinating environment around Eu³⁺. Besides, the obvious split peaks for ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ bands further demonstrates the high symmetry of the crystal field around the Eu³⁺ ions.^30,44

Fig. 4 Photoluminescence (a) excitation and (b) emission spectra of ZnAlEu–LDH–NO₃ (LN3) and ZnAlEu–LDH–C3C (LC3).

As for the ZnAlEu–LDH–C3C sample (LC3), in the photoluminescence excitation spectra (Fig. 4a), all of the characteristic excited peaks for the Eu³⁺ ion f–f transition disappear. Instead, a strong broad band ranged from 320 to 400 nm occurs, which can be ascribed to the transition from the ground state (S₀) to the first excited state (S₁) (π, π*) of the interlayer C3C anions. This confirms that the excited energy absorbing centre transfers from Eu³⁺ ions within the LDH nanosheets to the interlayer C3C anions.³⁴ Furthermore, this deduction is strongly supported by the UV-vis diffuse reflectance spectra (Fig. 5 and S2 in ESI†). In the UV-vis spectrum for ZnAlEu–LDH–NO₃, there are only two weak absorption bands at 300 and 392 nm, resulting from the LDH crystal lattices and the Eu³⁺ ions, respectively. In sharp contrast, the ZnAlEu–LDH–C3C shows a very strong and broad band over almost the whole UV region, essentially similar to the C3C itself. In addition, in the photoluminescence emission spectra (Fig. 4b), besides the characteristic emission peaks of Eu³⁺ (⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁), ZnAlEu–LDH–C3C also displays a new broad band at 470 nm, which may be contributed by the emission of the C3C anions.

Fig. 5 UV-vis spectra of ZnAlEu–LDH–NO₃ (LN3), ZnAlEu–LDH–C3C (LC3) and C3C.

Based on the above results, an interfacial energy transfer process (Scheme 2) was put forward to discuss the possible photoluminescent mechanism, which played an important role in photoluminescent phenomena.^33,34 In brief, the energy of light is mainly absorbed by the interlayer C3C anion first. Then, it is partially transferred to the Eu³⁺ ions within the LDH lattices. As a result, a typical red luminescence is emitted. Meanwhile, the residual energy excites the C3C anions, resulting in a blue luminescence. The hydrogen bonding between the carboxylate in C3C anions and the hydroxyl groups of the LDH nanosheets may act as an energy transfer bridge. Similar energy transfer mechanism has been reported for lanthanide-based lamellar oxides containing aromatic molecules in the interlayer.^46,47

Scheme 2 Interfacial energy transfer process from the interlayer the C3C anions to the Eu³⁺ ions within the LDH lattices.

Moreover, the intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transition peak for ZnAlEu–LDH–C3C is distinctly different from that for ZnAlEu–LDH–NO₃. Its hypersensitive ⁵D₀ → ⁷F₂ transition peak at 613 nm (LC3) appears to be dominant, suggesting that there is no inversion centre in the ZnAlEu–LDH–C3C. The unsplit peak of ⁵D₀ → ⁷F₂ and the emission peak of ⁵D₀ → ⁷F₁ further prove that the symmetry of the crystal field around the Eu³⁺ ions becomes lower after the LDH intercalation of C3C anions.

3.3 Tunable photoluminescence

Compared with common inorganic materials, LDHs have obvious advantages of a variety of tunable structural parameters, such as metal cations within layer lattices and exchangeable anions in the galleries. Thus, it is reasonable to achieve highly tunable photoluminescence by simply adjusting the LDH structure and constituents. Fig. 6 clearly shows that the photoluminescent colour could be effectively tuned by simply alteration of the LDH constituents. In sharp contrast to the weak glaucous luminescence (as shown in Fig. 6a and e) of the pristine un-doped ZnAl–LDH–NO₃ (LN0),⁴⁸ the incorporation of Eu³⁺ ions (LN3) endows it a bright red luminescence (as shown in Fig. 7a and d), which is mainly contributed by the characteristic emission peaks of Eu³⁺ ions (⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁). On the other hand, the intercalation of C3C anions in the LDH interlayer results in an intensely palatinate blue luminescence (as shown in Fig. 6a and c). The intercalation is also an excellent way to tune the photoluminescence properties of C3C, which only has a weak violet luminescence (as shown in Fig. 7a and f). The reason may be attributed to the orientation of C3C anions in the galleries. As a result, the ZnAlEu–LDH–C3C displays a brilliant steel pink luminescence (as shown in Fig. 7a and b), which is mixed of red and palatinate blue. Indeed, all of ZnAlEu–LDH–C3C show similar shape in photoluminescence emission spectra, and the intensity varies with the Eu³⁺ content (Fig. S3 in ESI†). Therefore, the luminescence colour of LDHs can be effectively tuned by simply changing the structural constituents of both Eu³⁺ with the LDH lattices and the C3C anions in the interlayer.

Fig. 6 (a) Photoluminescence emission spectra and (b–f) CLSM images of (b) LC3, (c) LC0, (d) LN3, (e) LN0 and (f) C3C.

Fig. 7 The effect of wavelength on photoluminescence spectra of ZnAlEu–LDH–C3C (LC3). (a) Photoluminescence excitation spectra at different emission wavelength and their intensity ratio (inner). (b) Photoluminescence emission spectra excited at varied wavelength.

On the other hand, Fig. 7 presents the effects of excited wavelength on the photoluminescence excitation and emission spectra of the ZnAlEu–LDH–C3C. In Fig. 7a, the photoluminescence excitation spectra were recorded by monitoring the Eu^{3+ 5}D₀ → ⁷F₂ emission lines (613 nm) and the C3C characteristic emission (470 nm), respectively. From the changes of the intensity ratio of Eu³⁺ to C3C emission with wavelength (Fig. 7a inset), their contributions to the macrocosmic photoluminescence can be clearly seen. Distinctly, the wavelength of 370 nm could excite the highest intensity of Eu³⁺ emission, while the emission of C3C is evidently dominant at 400 nm excited wavelength. Furthermore, a variety of photoluminescence emission spectra exited at different wavelength are shown in Fig. 7b, confirming the success of tunable photoluminescence for ZnAlEu–LDH–C3C by simply changing the excited wavelength.

4 Conclusions

ZnAlEu–LDH–C3C with Eu³⁺ doped within LDH lattices and organic photo-functional anions of C3C intercalated in the interlayer space have been successfully prepared via an interlayer ion exchange process using the precursor of ZnAlEu–LDH–NO₃. Their structural characterizations of XRD, FTIR spectra and chemical formulae demonstrate the high purity and high crystallinity for both ZnAlEu–LDH–NO₃ and ZnAlEu–LDH–C3C. A bilayer arrangement with nearly perpendicular orientation was proposed for the alignment of C3C anions in the LDH interlayer space. SEM images reveal that no obvious damage or breakage of the ZnAlEu–LDH–NO₃ sheets occurred during the ion exchange procedure. Compared with ZnAlEu–LDH–NO₃, obvious un-split peaks and high intensity ratio of emission peaks at 613 to 589 nm denote a lower symmetry of the coordinating environment around Eu³⁺ in the ZnAlEu–LDH–C3C. In contrast with ZnAlEu–LDH–NO₃, ZnAlEu–LDH–C3C shows a strong UV absorbance and an interesting photoluminescence function, possibly due to an interfacial energy transfer process between the interlayer C3C anions and the Eu³⁺ within the LDH lattices. Furthermore, the photoluminescence of ZnAlEu–LDH–C3C was found to be highly tunable by simply adjusting the structural constituents (i.e. the contents of Eu³⁺ and C3C) or the excited wavelength. And the photoluminescence can be directly and clearly seen by naked eye in the CLSM images. These findings may open new avenues to prepare novel tunable photoluminescent materials and have potential applications for optical devices.

Acknowledgements

The authors thank National Natural Science Foundation of China (no. 51073162). G. Chen acknowledges Youth Innovation Promotion Association, CAS.

Notes and references

1. Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2011, 112, 1126.
2. W. S. Ojo, S. Xu, F. Delpech, C. Nayral and B. Chaudret, Angew. Chem., Int. Ed., 2012, 51, 738.
3. D. Wang, H. Gao, E. Roze, K. Qu, W. Liu, Y. Shao, S. Xin and Y. Wang, J. Mater. Chem. C, 2013, 1, 5772.
4. N. C. Tansil, L. D. Koh and M. Han, Adv. Mater., 2012, 24, 1388.
5. K. Sato, S. Yokosuka, Y. Takigami, K. Hirakuri, K. Fujioka, Y. Manome, H. Sukegawa, H. Iwai and N. Fukata, J. Am. Chem. Soc., 2011, 133, 18626.
6. M. J. Teng, X. R. Jia, S. Yang, X. F. Chen and Y. Wei, Adv. Mater., 2012, 24, 1255.
7. C.-C. Huang, H.-Y. Liao, Y.-C. Shiang, Z.-H. Lin, Z. Yang and H.-T. Chang, J. Mater. Chem., 2009, 19, 755.
8. B. R. C. Maki Kinami and C. Weder, Chem. Mater., 2006, 18, 946.
9. S. J. Toal, K. A. Jones, D. Magde and W. C. Trogler, J. Am. Chem. Soc., 2005, 127, 11661.
10. Q. Wang and D. O'Hare, Chem. Rev., 2012, 112, 4124.
11. T. Hibino and W. Jones, J. Mater. Chem., 2001, 11, 1321.
12. Z. Hu and G. Chen, RSC Adv., 2013, 3, 12021.
13. S. P. Lonkar, A. Leuteritz and G. Heinrich, RSC Adv., 2013, 3, 1495.
14. Z. Zhang, G. Chen and K. Xu, Appl. Clay Sci., 2013, 72, 206.
15. K. Xu, G. Chen and J. Shen, Chin. Chem. Lett., 2012, 23, 805.
16. F. Cavani, F. Trifirò and A. Vaccari, Catal. Today, 1991, 11, 173.
17. C. Hu, X. Zhang, L. Xu, B. Mu, W. Zu and E. Wang, Appl. Clay Sci., 1998, 13, 495.
18. F. Malherbe, C. Forano, B. Sharma, M. P. Atkins and J. P. Besse, Appl. Clay Sci., 1998, 13, 381.
19. V. Bugatti, G. Gorrasi, F. Montanari, M. Nocchetti, L. Tammaro and V. Vittoria, Appl. Clay Sci., 2011, 52, 34.
20. X. Lv, Z. Chen, Y. Wang, F. Huang and Z. Lin, ACS Appl. Mater. Interfaces, 2013, 5, 11271.
21. A. De Martino, M. Arienzo, M. Iorio, F. Vinale, M. Lorito, P. D. Prenzler, D. Ryan and H. K. Obied, Appl. Clay Sci., 2011, 53, 737.
22. T. Cao, K. Xu, G. Chen and C.-Y. Guo, RSC Adv., 2013, 3, 6282.
23. P. Fu, K. Xu, H. Song, G. Chen, J. Yang and Y. Niu, J. Mater. Chem., 2010, 20, 3869.
24. J. Liu, G. Chen and J. Yang, Polymer, 2008, 49, 3923.
25. K. Xu, G. Chen and J. Shen, Appl. Clay Sci., 2013, 75–76, 114.
26. T. Posati, F. Costantino, L. Latterini, M. Nocchetti, M. Paolantoni and L. Tarpani, Inorg. Chem., 2012, 51, 13229.
27. M. Dominguez, M. E. Perez-Bernal, R. J. Ruano-Casero, C. Barriga, V. Rives, R. A. S. Ferreira, L. D. Carlos and J. Rocha, Chem. Mater., 2011, 23, 1993.
28. H. Chen and W.-G. Zhang, J. Am. Ceram. Soc., 2010, 93, 2305.
29. Y. Zhao, J.-G. Li, F. Fang, N. Chu, H. Ma and X. Yang, Dalton Trans., 2012, 41, 12175.
30. C. Li, L. Wang, D. G. Evans and X. Duan, Ind. Eng. Chem. Res., 2009, 48, 2162.
31. R. Liang, R. Tian, W. Shi, Z. Liu, D. Yan, M. Wei, D. G. Evans and X. Duan, Chem. Commun., 2013, 49, 969.
32. S. Cho, S. Jung, S. Jeong, J. Bang, J. Park, Y. Park and S. Kim, Langmuir, 2013, 29, 441.
33. P. Gunawan and R. Xu, J. Phys. Chem. C, 2009, 113, 17206.
34. X. R. Gao, M. Hu, L. X. Lei, D. O'Hare, C. Markland, Y. Sun and S. Faulkner, Chem. Commun., 2011, 47, 2104.
35. M. Khaldi, A. DeRoy, M. Chaouch and J. P. Besse, J. Solid State Chem., 1997, 130, 66.
36. F. Yang, B. Y. Xie, J. Z. Sun, J. K. Jin and M. Wang, Mater. Lett., 2008, 62, 1302.
37. A. A. A. Ahmed, Z. A. Talib and M. Z. bin Hussein, Appl. Clay Sci., 2012, 56, 68.
38. T. Wen, X. Wu, X. Tan, X. Wang and A. Xu, ACS Appl. Mater. Interfaces, 2013, 5, 3304.
39. L. Vieille, I. Rousselot, F. Leroux, J. P. Besse and C. Taviot-Gueho, Chem. Mater., 2003, 15, 4361.
40. Y. Feng, D. Li, Y. Wang, D. G. Evans and X. Duan, Polym. Degrad. Stab., 2006, 91, 789.
41. I. Georgieva, I. Kostova, N. Trendafilova, V. K. Rastogi and W. Kiefer, J. Mol. Struct., 2010, 979, 115.
42. L. Li, L. Zhang, Z. Wen and D. Chen, Chin. J. Chem., 2010, 28, 171.
43. T. Itoh, N. Ohta, T. Shichi, T. Yui and K. Takagi, Langmuir, 2003, 19, 9120.
44. Y. Chen, F. Li, S. Zhou, J. Wei, Y. Dai and Y. Chen, J. Solid State Chem., 2010, 183, 2222.
45. L. Hu, R. Ma, T. C. Ozawa and T. Sasaki, Chem.–Asian J., 2010, 5, 248.
46. M. Karmaoui, R. A. S. Ferrerira, A. T. Mane, L. D. Carlos and N. Pinna, Chem. Mater., 2006, 18, 4493.
47. M. Karmaoui, L. Mafra, R. A. S. Ferrerira, J. Rocha, L. D. Carlos and N. Pinna, J. Phys. Chem. C, 2007, 111, 2539.
48. Z. Zhang, G. Chen and K. Xu, Ind. Eng. Chem. Res., 2013, 52, 11045.

---

Footnote

† Electronic supplementary information (ESI) available: FTIR and UV-vis spectra of all of the ZnAlEu–LDH–NO₃ and ZnAlEu–LDH–C3C samples, PL spectra of all ZnAlEu–LDH–C3C samples. See DOI: 10.1039/c3ra46930a

---