Color-tunable smectite : Eu(L)₂ (L = Phen, Bpy) hybrid materials from red to blue: internal bond rotation and reduction by hydrazine

Abstract

Eu(Phen)₂³⁺ complexes are successfully intercalated into the interlayer space of smectite minerals with three different layer charges, whereas the Bpy molecules are not intercalated even at the higher solution temperature of 200 °C, probably due to the internal bond rotation of the aryl–aryl single bond, resulting in the variable van der Waals thickness of the guest molecule (Bpy). In the red-emitting Smectite : Eu(Phen)₂³⁺ hybrid materials, the orientation model of the Eu(Phen)₂³⁺ complex in which the plane of the aromatic ring of the Phen molecule is parallel to the silicate layer (possibly slightly tilted) is proposed based on the variation of the d₍₀₀₁₎ basal spacings before and after Eu(Phen)₂³⁺ intercalation. Furthermore, the novel blue emission in smectite : Eu(Phen)₂ hybrid materials is obtained using hydrazine solution, for the first time. In combination with the PL and XPS results of smectite : Eu(Phen)₂ hybrid materials, it is evident that the Eu³⁺ of the Eu(Phen)₂³⁺ complex stabilized into the interlayer space of the smectite mineral is reduced to Eu²⁺ by hydrazine. The solution chemistry using hydrazine to gain the lower valence state of metal ions allows us to explore new inorganic–organic hybrid materials that are particularly susceptible to high temperature.

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1 Introduction

Natural layered aluminosilicates (LAS) have been widely studied because of their rich inclusion chemistry due to the large surface area and ion exchange properties. Among LAS, the smectite group is classified into the several members, such as montmorillonite, beidellite, nontronite, hectorite, and saponite depending on the types of the structural isomorphous substitution in which some Si⁴⁺ in the tetrahedron sites or Al³⁺ in the octahedron sites are replaced by other cations with lower valency.¹ The prominent characteristic of smectite group minerals is that the cations introduced into interlayer space are exchangeable with other ions, in particular, the surface active agents (organic cations) such as alkylammonium ions, which induces the surface of clay minerals to be organophilic. Miller et al., studied that Na-saturated Wyoming-montmorillonite sample was treated with Eu³⁺, Ho³⁺, and Yb³⁺ ions and observed that the three lanthanide ions diffused into the hexagonal rings of surface oxygens regardless of the radius of the cation.² It is well-known that the various activators in luminescent materials, such as Nd³⁺, Pr³⁺, Sm³⁺, Eu³⁺, Ce³⁺, Tb³⁺, Yb³⁺ etc., have been widely used for flat panel displays and optoelectronic devices.^3–6 Among them, the trivalent europium ions (Eu³⁺) have been considered as a very important and useful activator which exhibits red-emission with a line spectrum around 610 nm.^7–9 The hydrated Eu³⁺ ions introduced into the inter-laminar space of smectite minerals, are stabilized by electrostatic force between the inorganic layers, which results in the weak luminescence either in solution or in a solid state. This problem can be solved by complexation using certain organic which enhances the luminescence intensity of the tri-positive lanthanide, Ln(III) ions. The enhanced luminescence has been explained by a ligand to metal energy transfer mechanism.^10,11 Among the organic ligands used in complexation between Eu³⁺ and organic molecules, 2,2′-bipyridine (Bpy) and 1,10-phenanthroline (Phen) have been widely used because of its simple molecular structure and strong fluorescent emission.^12–15 The lanthanide complexes, RE(L)₂³⁺ (RE = Eu³⁺ and Tb³⁺; L = Bpy and Phen) were intercalated into Na⁺-smectite minerals and its luminescent properties were analyzed by several groups.^16–20 However, all the samples (smectites : RE(L)₂³⁺) showed a weak emission with the lack of the excitation spectra compared with those of the pristine lanthanide-complexes. This problem may be ascribed to the inadequate experiments and can be overcome by the following conditions: (i) the enough reaction time and temperature are needed for the bulky RE(L)₂³⁺ complexes (∼10 Å) to be introduced into the interlayer space of smectite minerals, (ii) two step process is necessary, in first the intercalation of organic ligands (Ls) into Na⁺-smectites, in second the ion-exchange reaction between RE³⁺ ions and Na(L)₂⁺-smectites, (iii) the layer charge density of smectite minerals should be estimated in order to relate with the luminescent properties of RE(L)₂³⁺ complexes. The intercalation strategy related to the inadequate experiments (i.e., (ii)) was reported by Li et al., and they proposed a simple way of preparing luminescent host–guest materials by electrostatic adsorption of ionic lanthanide complexes on the negative charge-bearing surfaces of zeolite L crystals.²¹ It is well-known that hydrazine as a reductant has been used to control the valence state of metal ions in a solution as previously reported, such as “Ag⁺ ion reduction with hydrazine”,²² “Ni²⁺ ion reduction with hydrazine”,²³ “Eu²⁺ formation in EuAlO₃ nano-phosphor using hydrazine”,²⁴ “reduction of tris(Phen)Fe³⁺ with hydrazine”,²⁵ and “Eu²⁺ formation in apatites and borosilicates using hydrazine”.²⁶ Despite the extensive studies on the Eu(L)_n³⁺-intercalated LAS hybrid materials, the luminescent behavior of Eu²⁺ stabilized into the interlayer space of LAS minerals has not been reported, to the best of our knowledge. It should be pointed out that it is very difficult to obtain fully reduced Eu²⁺ in metal oxides based on the standard reduction potential (Eu³⁺/Eu²⁺ = −0.36 V vs. SHE), implying that the reduction of Eu³⁺ to Eu²⁺ needs an annealing process at high temperature (≥1000 °C) under a reducing atmosphere, such as H₂ or H₂–Ar mixture gas. In the inorganic–organic hybrid materials, however, the solution chemistry using hydrazine is indispensable to obtain the lower valence state of metal ions because of the decomposition of organic species at high temperature. Herein, therefore, we present the luminescent properties of Eu(L)₂³⁺ (L = Bpy and Phen) complexes stabilized into the interlayer space of smectite minerals with three different layer charge densities: montmorillonite from Wyoming in U.S.A. (MW), montmorillonite from Gampo in Korea (MG), beidellite from Unterrupsroth in Germany (BU). Furthermore, the internal bond rotation effect (using Bpy and Phen ligand) and reduction effect (using hydrazine solution) are discussed.

2 Experimental

2.1 Purification of smectite raw-minerals

Three smectite raw-minerals were obtained, montmorillonite from Wyoming area in U.S.A. (MW), montmorillonite from Gampo area in Korea (MG), and beidellite from Unterrupsroth area in Germany (BU). For the removal of carbonates, the minerals were treated with buffer solution of sodium acetate–acetic acid at pH 5 in a boiling water bath, then the organic matters were removed using 6% H₂O₂ solution in a boiling water bath. The minerals were converted to the sodium form by treatment with 1 M NaCl solution, and then the excess salts were removed by dialysis. Finally, the sodium saturated minerals were collected as the size fraction of smaller than 2 μm by centrifuge and freeze-dried.

2.2 Layer charge density of smectite minerals

For the determination of the layer charge densities of smectite minerals using n-alkylammonium method,^27,28 n-alkylammonium chloride solutions were prepared by potentiometric titration of n-alkylamine with diluted hydrochloric acid at 65 °C and the pH of the solution was adjusted to about 6. The concentrations of the solutions were as follows: about 2 N for N_c = 4 and 6 (N_c is the number of carbon in n-alkylamine), about 0.5 N for N_c = 8 and 10, about 0.1 N for N_c = 12 and 14, and about 0.05 N for N_c = 16 and 18. For cation exchange, the minerals were immersed into the n-alkylammonium solution at 70 °C for 72 h. After decantation of the supernatant liquid and washing the minerals, each sample was reacted twice again for the same period, then washed to remove excess alkylamine and alkylammonium salts with ethanol–water (1 : 1) until no-AgCl precipitate is formed in supernatant liquid. After drying samples in an oven at 80 °C, the samples were carefully ground, and dried under high-vacuum (≤10⁻² Torr) at 70 °C for 24 h.

2.3 Syntheses of smectite : Eu(L)₂³⁺ (L = Bpy and Phen) hybrids

Na⁺-smectites (1.5 g) were treated with Bpy (or Phen, 1 g) in absolute ethanol (100 mL) at 70 °C for 7 days. In the reaction, the solutions were intermittently sonicated. After decantation of the supernatant liquid and washing the minerals, the samples were reacted twice again for the same period, then washed to remove excess Bpy and Phen until no absorption bands due to the free Bpy and Phen molecules was found in the supernatant liquid using UV/visible spectrophotometer. After drying samples in an oven at 80 °C, the samples were carefully ground, and mixed with Eu(NO₃)₃·6H₂O (1 g) in absolute ethanol solution at 70 °C for 7 days with intermittent sonication. After decantation of the supernatant liquid and washing the minerals, the samples were reacted twice again for the same period, then washed to remove excess Eu³⁺ and NO₃⁻ until no red-emission due to the Eu(L)₂³⁺ complexes was found in the supernatant liquid using UV-lamp. After drying samples in an oven at 80 °C, the samples were carefully ground, and dried under high-vacuum (≤10⁻² Torr) at 70 °C for 24 h.

2.4 Materials characterization

The powder X-ray diffraction patterns of smectite minerals were characterized by X-ray diffractometer (XRD 6000 model, Shimadzu) using Cu K_α radiation at 30 kV and 30 mA. The morphologies of smectite minerals were observed by Field Emission Scanning Electron Microscopy (FE-SEM) using Hitachi S-4200 under V_acc. = 15 kV. Specimens for electron microscope were coated with Pt–Rh for 180 s under vacuum. The photoluminescence excitation and emission spectra were collected on a Fluorometer (FS-2 model, Scinco) with a 150 W xenon lamp under an operating voltage of 350 V. The reflectance spectra were recorded using UV-visible spectrophotometer (UV-2600, Shimadzu) with BaSO₄ as a reference. Fourier-transform infrared spectroscopy (FT-IR) was performed using a FT-IR spectrophotometer (IRTracer-100, Shimadzu) with a KBr medium (KBr 200 mg + sample 1 mg). The valence states of the Eu ions were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a monochromatic Al K_α X-ray source (hν = 1486.6 eV) at Busan Center of Korea Basic Science Institute (KBSI). The obtained binding energies (BEs) were calibrated with that of the adventitious carbon (C 1s) core level peak at 284.6 eV as a reference.

3 Results and discussion

3.1 Layer charge density of smectite minerals

The structural formulas of three smectite minerals are, as follows:

M_0.70⁺[(Al_2.99Mg_0.52Fe_0.42³⁺Fe_0.01²⁺)(Si_7.97Al_0.03)O₂₀(OH)₄] for montmorillonite from Wyoming (MW),²⁹ M_0.80⁺[(Al,Mg)₄(Si₈)O₂₀(OH)₄] for montmorillonite from Gampo (MG),³⁰ M_1.22⁺[(Al_3.74Mg_0.24Fe_0.04³⁺)(Si_7.00Al_1.00)O₂₀(OH)₄] for beidellite from Unterrupsroth (BU).³¹ The layer charges (eq./(Si,Al)₄O₁₀) of three smectite minerals are determined by n-alkylammonium method^27,28 and presented in Fig. 1. n-Alkylammonium ions introduced into the interlayer space of smectite mineral are intercalated in the flat monolayers with a spacing of 13.5 Å. With increasing N_c of alkylammonium, the bilayers are formed with a basal spacing of 17.5 Å due to the more area demand between the area required for alkylammonium ions (A_n) and the equivalent area per unit charge (A_e). Thus, the layer charge can be estimated as a mean value between the last N_c of the monolayer and the first N_c of the bilayer. The layer charge for MW is determined to be 0.27 eq./(Si,Al)₄O₁₀, 0.29 eq./(Si,Al)₄O₁₀ for MG, and 0.36 eq./(Si,Al)₄O₁₀ for BU. It is evident that BU has the highest layer charge and the most homogeneous charge distribution among three minerals. It should be noted that the structural charge of BU mineral arises mainly from the tetrahedral layers near by the interlayer space, on the other hand, that of the MW and MG mineral arises mainly from the octahedral layer. In the characterization of smectite group minerals, the layer charge of smectite minerals is very important because the amount of RE(L)₂³⁺ complexes intercalated into the interlayer space of smectite minerals is directly proportional to the luminescent properties of smectite : RE(L)₂³⁺ hybrid materials, to the best of our knowledge.

Fig. 1 Variation of d₍₀₀₁₎ basal spacing determined from XRD patterns of RNH₃⁺–smectite complexes (R = C_nH_2n+1) as a function of the carbon number in n-alkylammonium.

3.2 Host–guest reaction between smectite minerals and organic ligands (Bpy and Phen)

In order to verify the host–guest reaction between smectite minerals and organic ligands (Bpy and Phen) as a function of reaction temperature, three smectite minerals were treated with Bpy and Phen in a sealed quartz tube for 7 days. After washing and drying the samples, the d₍₀₀₁₎ basal spacings of the samples were analyzed using XRD as presented in Fig. 2. Three samples showed the same tendency. Compared with d₍₀₀₁₎ basal spacing of BU raw (∼12 Å), Bpy molecules cannot be intercalated into the interlayer space of BU because the basal spacing of d₍₀₀₁₎ is only ∼13 Å even at high temperature at 200 °C. On the other hand, Phen molecules easily enter the interlayer space with the increased d₍₀₀₁₎ basal spacing of ∼18 Å even at low temperature of 70 °C. It is presumed that the reactivity difference between Bpy and Phen molecule may be ascribed to the distinct molecular structure of two ligands. It is well-known that there is free rotation about the carbon–carbon single bond in organic molecules such as ethane, propane, butane etc. For example, in the ethane molecule, there are two conformations, “the eclipsed and staggered conformation” converted by rotation about C–C single bond with a potential energy barrier of about 12.5 kJ mol⁻¹.³² It is generally accepted that there are the cis and trans conformers of 2,2′-bipyridine (Bpy), in which the stable both conformers exist in the neutral and its protonated cation. The potential energy barriers for cis/trans interconversion of Bpy molecule were estimated by several groups.^33–36 Howard reported that the barrier for cis/trans interconversion in Bpy was determined to be 5.6 kJ mol⁻¹ using ab initio self-consistent field (SCF) level calculation.³⁴ Thus, it is clearly explained that the Bpy molecule cannot be intercalated into the interlayer space of smectite minerals due to the internal bond rotation of the aryl–aryl single bond, in other words, because of the variable van der Waals thickness of the guest molecule. On the contrary, the Phen molecules are easily intercalated with the increased basal spacing of d₍₀₀₁₎ because the Phen molecule have a fixed dimension. The proposed host–guest reaction between Bpy (or Phen) and smectite group mineral is schemed in Fig. 3. Fig. 4 shows the morphology of smectite minerals before and after the intercalation of Eu(Phen)₂³⁺ complex. It is evident that the raw-minerals (for BU, MG, and MW) show the severely coherent sheet-flakes, whereas after the intercalation of Eu(Phen)₂³⁺ complex, the smaller and swelled sheet-fragments are observed, implying that Eu(Phen₂)³⁺ complexes are successfully introduced into the inter-layer space of smectite minerals. The variation of d₍₀₀₁₎ basal spacings of the samples corroborates the discussion on the SEM morphology of smectite minerals before and after the intercalation of Eu(Phen)₂³⁺ complex. Fig. 5 shows XRD patterns of smectite : Eu(Phen)₂ hybrid minerals. For the raw-minerals, the d₍₀₀₁₎ basal spacing values are approximately 12.5 Å in three samples with the hydrated Na⁺ ions, and subsequently by introduction of Phen molecules, the d₍₀₀₁₎ basal spacings are increased to be about 5 Å relative to that of the raw-minerals with the hydrated Na⁺ ions. Remarkably, after the intercalation of Eu(Phen)₂ complex in the inter-layer space of smectite minerals, the d₍₀₀₁₎ basal spacings are increased to be about 4 Å relative to that of the raw-minerals with the hydrated Na⁺ ions. Assuming that Phen has a planar structure, the molecular dimensions calculated from the bond length and bond angle are, as follows: 9.1 Å (long axis) × 4.6 Å (short axis) and 3.0 Å (thickness).³⁷ Evidently, an orientation in which the plane of the aromatic ring of Phen molecule is parallel to the silicate layer (possibly slightly tilted) is substantiated when we compare the d₍₀₀₁₎ basal spacing of smectite : Eu(Phen)₂³⁺ with that of smectite raw-mineral as depicted in Fig. 6. In order to verify the well-intercalated Eu(Phen)₂³⁺ complexes into the inter-layer space of smectite minerals, FT-IR measurements were performed and presented in Fig. 7. For the Phen molecule, the IR modes between 1600 and 1500 cm⁻¹ can be assigned to the aromatic skeleton vibrations of C=C bonds and one around 1420 cm⁻¹ assigned to the stretching vibrations of C=N bonds. Additionally, two modes around 847 cm⁻¹ and 735 cm⁻¹ are associated with out of plane of the C–H bonds of the aromatic cycle.^20,38 For the BU-raw mineral, the IR mode at 1640 cm⁻¹ is assigned to H₂O bending and those at 1026 cm⁻¹ and 470 cm⁻¹ are assigned to the stretching vibration of Si–O bonds and Si–O–Si bending vibrations, respectively. The modes at 915 cm⁻¹ and 527 cm⁻¹ are assigned to the Al–Al–OH bending vibrations in the octahedral layer and Al–O–Si bending vibrations, respectively.³⁹ From the careful examination of IR modes between Phen and BU-raw mineral, it is evident that Eu(Phen)₂³⁺ complexes are stabilized into the inter-layer space of the BU mineral because the IR modes of Phen molecule are prominent in BU : Eu(Phen)₂³⁺ hybrid material (at 1589, 1512, 1420, 844, and 734 cm⁻¹). The elemental analysis results (C, H, N, and Eu) are summarized in Table 1. For 1,10-phenanthroline (Phen), the calculated C/N and C/H ratios are 5.14 and 17.87, respectively. Comparing with the experimental values, it is presumed that the increased H percentages are attributed to the adsorbed water and ethanol molecules, and the discrepancy of C/N ratios between calculated and experimental value originates from the Phen molecules adsorbed on the surface of smectite minerals. Nevertheless, the weight percentages of C and N are in good agreement with the magnitude of layer charge in smectite minerals, i.e., the higher the layer charge is, the larger the amount of C and N (wt%). Additionally, the Eu contents of the three hybrid materials determined by ICP, are gradually increased along with the series of smectite minerals (MW → MG → BU) depending on the layer charge, which is well matched with the elemental analysis results of CHN.

Fig. 2 Variation of d₍₀₀₁₎ basal spacing as a function of solution temperature in BU-Bpy and BU-Phen reaction.

Fig. 3 Proposed intercalation scheme for Bpy and Phen molecule into smectite mineral.

Fig. 4 SEM image of smectite minerals before and after the intercalation of Eu(Phen)₂ complex.

Fig. 5 XRD patterns of smectite : Eu(Phen)₂ hybrid materials.

Fig. 6 Proposed orientation of Eu(Phen)₂ complex intercalated into the interlayer space of smectite mineral. Gray sphere: C atom, red sphere: N atom, cyan sphere: Eu³⁺ ion.

Fig. 7 IR spectra of Phen, BU : Eu(Phen)₂, and BU-raw.

Table 1 Elemental analysis^a results for smectite : Eu(Phen)₂ hybrid materials

Compound	C (wt%)	N (wt%)	H (wt%)	C/N ratio (calcd)	Eu content, wt% (calcd)
a The elemental analysis (CHN) result for sulfanilic acid as a standard was obtained, as follows: C-41.6%, H-4.1%, N-8.1%, S-18.5%, O-26.2%.
BU : Eu(Phen)2	9.20	1.80	1.93	5.11 (5.14)	4.1 (4.8)
MG : Eu(Phen)2	8.61	1.66	1.63	5.18 (5.14)	3.5 (3.9)
MW : Eu(Phen)2	7.78	1.60	1.50	4.86 (5.14)	3.3 (3.6)

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3.3 Photoluminescent properties of smectite : Eu(Phen)₂³⁺ hybrid materials

Eu(L)₂³⁺ (L = Bpy and Phen) complexes were collected as power form, and its PL spectra were obtained as presented in Fig. 8. The broad bands in the range of 200–400 nm are associated with the Eu³⁺–O²⁻ charge transfer band (CTB). The small peaks from 400 to 550 nm correspond to the intra-4f transitions of Eu³⁺ in two complexes. The emission spectra monitored under 303 nm excitation reveal that two complexes emit a red-light as the line spectra at approximately 610 nm are observed. For the Eu(Phen)₂³⁺-intercalated smectites, the excitation and emission spectra are well coincident with that of Eu(Phen)₂³⁺ complex as shown in Fig. 9, indicating that Eu(Phen)₂³⁺ complex is successfully intercalated into the inter-layer space of smectite minerals. Interestingly, the maximum emission intensity at approximately 610 nm is proportional to the layer charge density of smectite minerals (the layer charge density for BU is 0.36 eq./(Si,Al)₄O₁₀; for MG, 0.29; for MW, 0.26) because the higher the layer charge is, the more the amount of intercalated Eu(Phen)₂³⁺ complex. The reflectance spectra of smectite : Eu(Phen)₂ hybrid materials are shown in Fig. 10. For the Eu(L)₂³⁺ (L = Phen, Bpy) complexes, the broad absorption bands between 200 nm and 400 nm can be assigned to the Eu³⁺–O²⁻ CTB, and the sharp lines between 380 and 600 nm correspond to the intra-4f transitions of Eu³⁺: ⁷F₀ → ⁵L₆ (395 nm), ⁷F₀ → ⁵D₃ (416 nm), ⁷F₀ → ⁵D₂ (464 nm), ⁷F₀ → ⁵D₁ (536 nm), which coincides well with the excitation spectra (Fig. 8).⁴⁰ For the smectite : Eu(Phen)₂ hybrid materials, the broad absorption bands are also observed between 200 and 400 nm, associated with the Eu³⁺–O²⁻ CTB in which the absorption bands are more intense than that of BU-raw. The new broad bands between 400 and 600 nm may be ascribed to the non-radiative absorption because it does not reflect the emission behavior of smectite : Eu(Phen)₂ hybrid materials as shown in Fig. 9.

Fig. 8 Excitation and emission spectra of Eu(Phen)₂ and Eu(Bpy)₂ complex.

Fig. 9 Excitation and emission spectra of smectite : Eu(Phen)₂ hybrid materials.

Fig. 10 Diffuse reflectance spectra of smectite : Eu(Phen)₂ hybrid materials.

3.4 Blue emission of BU : Eu(Phen)₂ by hydrazine solution

To verify the formation of Eu²⁺, the optimal amount of Eu(Phen)₂³⁺ complex was dissolved in absolute-ethanol, and its solution was monitored under 365 nm UV-light (at the left in Fig. 11). It is evident that the red-emission is ascribed to the Eu³⁺ in the complex, as desired. Surprisingly, the color of solution is dramatically changed to deep-blue after adding hydrazine (at the right in Fig. 11), most likely due to the formation of Eu²⁺ induced by hydrazine. To obtain PL spectra as a function of the hydrazine concentration, the red-emitting Eu(Phen)₂³⁺ complex (10 mmol) was dissolved into an absolute ethanol solution (40 mL), and subsequently hydrazine was added into the solution. The variation of the emission spectrum of Eu(Phen)₂³⁺ solution before and after hydrazine treatment corroborates that hydrazine induces Eu³⁺ to reduce to Eu²⁺ as shown in Fig. 12. The emission spectrum of Eu(Phen)₂³⁺ solution shows the maximum emission intensity at 615 nm, associated with the red-emission (at the left in Fig. 12). After adding hydrazine to the solution, the line spectrum centered at 615 nm is disappeared and the broad band with the maximum intensity at 387 nm is observed, corresponding to the blue-emission (at the right in Fig. 12). Additionally, the blue emission intensity is maximized at 5% hydrazine concentration as indicated in Fig. 12. This preliminary experiment warrants further examination of the red-emitting smectite : Eu(Phen)₂³⁺ hybrid materials because there has been not reported on the Eu²⁺ species stabilized in the inter-layer space of clay minerals, to the best of our knowledge. BU : Eu(Phen)₂³⁺ hybrid materials were treated with hydrazine in absolute ethanol solution at room temperature for 2 days with intermittent sonication. After centrifuge and vacuum-drying (≤10⁻³ Torr) for the reactant, the PL spectra of the powder samples were obtained as presented in Fig. 13. The emission spectra show the broad bands with a maximum intensity around 403 nm, corresponding to the blue-emission associated with the spin allowed 4f⁶5d → 4f⁷ transition of Eu²⁺ activator ion.^41–44 Furthermore, the fact that the emission intensities are changed depending on the concentration of hydrazine, reveals that hydrazine plays an important role in the blue-emission of BU : Eu(Phen)₂ hybrid material. As presented in Fig. 13, the small red-emission peaks around 615 nm are still remained below hydrazine 5% concentration (in 2% and 5%), whereas the perfect blue-emission bands are observed at the hydrazine concentration of more than 10%. From the careful examination of the PL spectra, it is presumed that the hydrazine 10% solution is an optimal one in this experimental condition, indicating that BU-raw mineral could be deteriorated in the higher concentration than hydrazine 10%-solution. This experimental result reveals that the hydrazine molecules diffuse into the inter-layer space of smectite mineral and reduce Eu³⁺ to Eu²⁺ in smectite : Eu(Phen)₂³⁺ hybrid material. To examine the valence state of the Eu ions before and after hydrazine treatment in the BU : Eu(Phen)₂ compound, XPS measurements were performed. All the XPS spectra were fitted after a Shirley background correction, and their binding energies were determined. It is well-known that the Eu²⁺ species are thermodynamically unstable based on the standard reduction potential (Eu³⁺/Eu²⁺ = −0.36 V vs. standard hydrogen electrode (SHE)).⁴⁵ Thus, it should be noted that the Eu²⁺ species in the interlayer space of smectite mineral are destined to be easily oxidized by oxidizing agents, such as H₂O and O₂, particularly under X-ray irradiation in the XPS measurements. The XPS 3d binding energies of the reference compounds could be precisely assigned as shown in Fig. 14: Eu³⁺ 3d_5/2 (1133.7 eV) and Eu³⁺ 3d_3/2 (1163.5 eV) for Eu₂O₃; Eu³⁺ 3d_5/2 (1134.1 eV) and Eu³⁺ 3d_3/2 (1163.8 eV) for Eu(Phen)₂³⁺. The small difference in the Eu³⁺ 3d binding energies between Eu₂O₃ and Eu(Phen)₂³⁺ is most likely due to the different ligand atoms (O and N). Interestingly, in the Eu(Phen)₂³⁺ complex (see Fig. 14b), the weak peaks at approximately 1127.1 and 1153.7 eV are observed corresponding to the Eu²⁺ 3d_2/5 and 3d_3/2, respectively.^46–48 Furthermore, the electrostatically bound Eu(Phen)₂³⁺ complex into the interlayer space of beidellite (for BU : Eu(Phen)₂ compound in Fig. 15a) has an effect on the valence state of Eu species by means of a ligand-to-metal charge-transfer (LMCT), i.e., the charge-transfer from the beidellite mineral with the negative surface charge to the Eu³⁺ ions of Eu(Phen)₂³⁺ complex. Thus, a LMCT results in the partial reduction of the Eu³⁺ ions in the complexes. As observed in Fig. 15a, it is evident that the Eu 3d binding energies at 1125.4 (Eu²⁺ 3d_5/2) and 1154.9 eV are assigned as Eu²⁺ 3d_5/2 and Eu²⁺ 3d_3/2, respectively. After hydrazine-treatment for the BU : Eu(Phen)₂ compound (see Fig. 15b), the fact that the intensities of Eu²⁺ 3d_5/2 and Eu²⁺ 3d_3/2 peak are more intensified relative to those of Eu³⁺ 3d peaks, warrants the evidence of reduction from Eu³⁺ to Eu²⁺ using hydrazine solution, which is well matched with the blue-emission as observed in Fig. 13. The Commission International de I'Eclairage (CIE) coordinates for smectite : Eu(Phen)₂ hybrid materials, are measured under UV irradiation at 300 nm and presented in Fig. 16. On the basis of the CIE coordinates, it is evident that smectite : Eu(Phen)₂ hybrid materials emit red-light and its colors are slightly shifted to the white-zone compared with that of Eu(L)₂³⁺ (L = Phen, Bpy) complexes. Surprisingly, the CIE coordinate of BU : Eu(Phen)₂ hybrid material after hydrazine treatment is moved to the blue region, directly indicating that the Eu³⁺ ions are reduced to Eu²⁺ by hydrazine. The solution chemistry using hydrazine to gain the lower valence state of metal ions allows us to explore new inorganic–organic hybrid materials that are particularly susceptible to high temperature.

Fig. 11 Color tunability of Eu(Phen)₂ complex solution before and after hydrazine treatment.

Fig. 12 Emission spectra of Eu(Phen)₂ complex solution before and after hydrazine treatment.

Fig. 13 Excitation and emission spectra of BU : Eu(Phen)₂ hybrid material after hydrazine treatment.

Fig. 14 Eu 3d XPS spectra of reference compounds, Eu₂O₃ and Eu(Phen)₂.

Fig. 15 Eu 3d XPS spectra of BU : Eu(Phen)₂ before and after hydrazine (10%) treatment.

Fig. 16 CIE chromaticity for smectite : Eu(Phen)₂ hybrid materials.

4 Conclusions

The layer charges of three smectite minerals are determined by n-alkylammonium method: 0.27 eq./(Si,Al)₄O₁₀ for MW, 0.29 eq./(Si,Al)₄O₁₀ for MG, 0.36 eq./(Si,Al)₄O₁₀ for BU. In the host–guest reaction between smectite minerals and organic ligands (Bpy and Phen), the Bpy molecule cannot be intercalated into the interlayer space of smectite minerals due to the internal bond rotation of the aryl–aryl single bond, resulted in the variable van der Waals thickness of the guest molecule. On the contrary, the Phen molecules are easily intercalated with the increased basal spacing of d₍₀₀₁₎ because the Phen molecule have a fixed dimension. Based on the variation of the d₍₀₀₁₎ basal spacings before and after Eu(Phen)₂³⁺ intercalation, the orientation model of Eu(Phen)₂³⁺ complex in which the plane of the aromatic ring of Phen molecule is parallel to the silicate layer (possibly slightly tilted) is proposed. The excitation and emission spectra of smectite : Eu(Phen)₂ hybrid materials reveal that the red-emitting behavior is ascribed to the characteristic of Eu³⁺ activator ion. Furthermore, in the combination with the results from the layer charge and PL measurement, it is evident that the maximum emission intensity around 610 nm is proportional to the layer charge density of smectite minerals because the higher the layer charge is, the more the amount of intercalated Eu(Phen)₂³⁺ complex. In search for color-tunable smectite : Eu(Phen)₂ hybrid materials, the novel blue emission is obtained using hydrazine solution, for the first time, implying that the valence state of Eu³⁺ in Eu(Phen)₂³⁺ complex stabilized into the interlayer space of smectite mineral is reduced to Eu²⁺ by hydrazine. The solution chemistry using hydrazine to gain the lower valence state of metal ions allows us to explore new inorganic–organic hybrid materials that are particularly susceptible to high temperature.

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