An Interfacial Europium Complex on SiO2 Nanoparticles: Reduction-Induced Blue Emission System

In this study, Eu-coated SiO2 nanoparticles have been prepared, consisting of an interfacial complex of Eu and 1,10-phenanthroline (phen) at the solid surfaces of the SiO2/Eu nanostructures. The as-prepared SiO2/Eu/phen nanoparticles exhibits sharp red emission via energy transfer from the phen to the EuIII. After sintering at 200 °C in air, the emission is tuned from red to blue. The blue emission is originated from EuII. This reduction-induced emissive phenomenon resulted from the electron-donating environment created by the surrounding phen and SiO2, which is the first reported fabrication of a stable EuII-based emissive material using mild conditions (reaction in air and at low temperature) and an organic-inorganic hybrid nanostructure. The existence of two different stable oxidation states with characteristic emissions, blue emissive EuII and red emissive EuIII, suggests significant potential applications as novel luminescent materials with inorganic-organic hybrid structures.

Interfacial nanostructures formed by organic and inorganic materials have great potential to exhibit novel properties not displayed by the original components. The suitable design of such interfaces is the key to fabricating functional organic-inorganic hybrid materials for applications in photonics [1][2][3][4] , electronics 5-7 , magnetic devices 8,9 and catalyses 10,11 . Our current research is focused on constructing functional interfaces through the complexation of organic and inorganic materials, with the aim of stabilizing and tuning the photo-physical properties associated with luminescent materials, such as lanthanide compounds.
Here we demonstrate the development of a novel reduction-induced emission system by forming interfacial europium (Eu) complexes on SiO 2 nanoparticles. The variations in the luminescence properties of Eu ions with changes in their valence state have been widely investigated in many host materials, and it is known that divalent and trivalent Eu ions luminesce in the blue and red spectral regions, respectively. The divalent Eu ion (Eu II ) shows a broad emission band assigned to the allowed 4f 6 5d → 4f 7 electric dipole transition, a phenomenon that has been reported in inorganic host materials such as sulfates, phosphates, borates, silicates and aluminates 12 . The trivalent Eu ion (Eu III ) shows some narrow emission bands assigned to the electric dipole forbidden (Laporte forbidden) transition of the inner-shell 4f orbitals. The existence of two different oxidation states with characteristic emissions as well as the high emission efficiency of Eu II and the high colour purity of Eu III is predicted to allow the fabrication of novel luminescent materials for a wide range of applications, in the event that a process is found that allows the desired species to be readily selected.
There are no natural sources containing Eu II . The emission of Eu II is of significantly higher intensity than that of Eu III in any inorganic host material. Thus, in order to prepare luminescent materials containing Eu II , it is necessary to reduce Eu III to Eu II in an appropriate matrix, using a reducing atmosphere at temperatures above 1000 °C [13][14][15][16][17][18] . Although some reports show that this reduction occurs even in air, high temperatures are still necessary, along with a rigid inorganic crystal structure as a host [19][20][21] . In contrast, Eu III emission can be easily and efficiently obtained by complexation with organic compounds. In such cases, an organic compound having a high absorption coefficient transfers its photo-excitation energy to the Eu ions. The organic compound in the complex not only plays an important role as an energy donor for the Eu ions, but is also able to control the structures and arrangements of the emissive substances at a molecular level through coordination bonds. It is thus expected that a combination of an inorganic matrix with organic compounds will produce novel luminescent materials in which the performance of the Eu ion is enhanced.
In the present study, we have prepared SiO 2 nanoparticles coated with Eu ions and discovered that an interfacial complex of Eu and 1,10-phenanthroline (phen) forms at the solid surfaces of the SiO 2 /Eu nanostructures, as illustrated in Fig. 1. SiO 2 nanoparticles were chosen as host materials because of their high availability and thermal stability. This hybrid nanostructure at the interface between inorganic and organic compounds induced significant visible light emission from two different stable oxidation states of the Eu ions.

Results And Discussion
To coat the SiO 2 surfaces with Eu ions, SiO 2 nanoparticles (20 ~ 50 nm) were immersed in a 50 mM ethanol solution of EuCl 3 at 70 °C for 30 min, resulting in coating of Eu ions on the SiO 2 surfaces (this material hereafter referred to as SiO 2 /Eu). This colloidal suspension containing the SiO 2 and Eu ion was subsequently dropped onto a quartz substrate that was then dried at 110 °C for 15 min. In previous reports, interfacial complexation with anthraquinone or cyclopentadiene has been demonstrated at the solid surface of TiO 2 , and has been shown to function as an excellent visible light absorber for photoelectric conversion 22,23 . Accordingly, the SiO 2 /Eu nanoparticles were immersed in an ethanol solution of phen at 75 °C for 60 min to form a new material (SiO 2 /Eu/phen). The phen molecule is known to coordinate to Eu ions in a bidentate fashion through bonding of two nitrogen atoms 24,25 . In the present work, phen was also assumed to form an interfacial complex with Eu ions at the solid surfaces of the SiO 2 /Eu nanostructures. X-ray photoelectron spectroscopy (XPS) measurements demonstrated the formation of coordination bonds between phen molecules and Eu ions on the SiO 2 /Eu nanoparticles. In the resulting data, the N1s XPS band of phen at 396.7 eV was shifted to 399.7 eV on the higher energy side, indicating that the phen coordinated to the Eu ions on the SiO 2 nanoparticles (Fig. S1) 26 . The phen molecule never coordinates the surface of SiO 2 nanoparticles without Eu ions. It suggests that Eu ions exist on the surface of SiO 2 nanoparticle and form the complexes at the interface between SiO 2 nanoparticles and phen ligands. Fig. 2 shows scanning electron microscope (SEM) images of SiO 2 nanoparticles coated with Eu ions. Compared with SiO 2 nanoparticles (Fig. S2), the SiO 2 /Eu particles exhibit close packing between themselves. The average SiO 2 particle size of 20 nm was almost unchanged by the addition of the Eu ions to the surface, indicating that the ions formed a nano-ordered thin film layer. Energy dispersive X-ray spectroscopy (EDS) was used to determine the elemental compositions, as shown in Fig. 2c. The EDS spectrum confirmed the presence of Si, O, and Eu. Since no Cl peaks were evident around 2.6 keV, we may conclude that Eu oxides or hydroxides were formed on the SiO 2 nanoparticles through the colloidal suspension process.

Figure 1. Schematic illustration of SiO 2 nanoparticles coated with Eu ions followed by complexation with phen at the interface.
Based on the synchrotron X-ray powder diffraction (XRPD) patterns in Fig. 3a, the SiO 2 nanoparticles had an amorphous structure with a broad peak at approximately 15° 27 , with an additional diffraction peak at 7.1° observed in the SiO 2 /Eu pattern (Fig. 3b). The additional peak indicates the formation of Eu  crystal shells around the SiO 2 nanoparticles. The crystal size, D, of the Eu shells was estimated using the Scherrer equation, D = 0.9λ / βcosθ 28 , where λ is the X-ray wavelength and β is the full width in radians at half-maximum (FWHM) of the diffraction peak at the Bragg angle of θ. The Eu nanocrystal shell on the SiO 2 nanoparticles was determined to be 1.3 nm in size, which is consistent with the SEM image. The nanostructure of the SiO 2 /Eu particles was evidently not affected by the coordination with phen, as determined from XRPD analysis and SEM images (Figs. S3 and S4). Fig. 4 presents the luminescence spectra of SiO 2 /Eu/phen nanoparticles. Under UV light, the nanoparticles generated a bright red emission originating from the ff transitions of Eu III . Since the SiO 2 / Eu without phen cannot emit in any wavelength regions, this red emission may occur through energy transfer from the phen to the Eu III at the interface of the SiO 2 /Eu nanoparticles. Interestingly, the emission colour of the SiO 2 /Eu/phen nanoparticles could be tuned from red to blue by sintering at 200 °C; after sintering for 60 min, the emission colour was completely changed to blue. The as-prepared SiO 2 /Eu/ phen exhibited sharp emission bands at 578.2, 589.5, 611.1, 651.1 and 700.5 nm, assigned to the 5 D 0 → 7 F 0 , 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , 5 D 0 → 7 F 3 and 5 D 0 → 7 F 4 transitions of Eu III , respectively. Excitation spectra monitored at the ff emission band position correspond to the π π * transition of phen (Fig. S5), confirming that energy transfer from the phen to the Eu III was occurring within the nanoparticles. The red emission band was decreased following sintering at 200 °C and replaced by a broad blue emission band at approximately 434 nm (Fig. S6). This broad emission was the result of the allowed electric dipole 4f 6 5d → 4f 7 transition of Eu II , via an allowed transition of Eu II rather than energy transfer from the phen. The Eu3d XPS bands in Fig. 5 provided evidence for the reduction of Eu III to Eu II at the interface between the SiO 2 and the phen. Eu3d XPS bands corresponding to Eu III were observed at 1135.4 and 1165.2 eV in the case of the as-prepared SiO 2 /Eu/phen nanoparticles, while the sintered nanoparticles generated corresponding bands at lower energies (1126.2 and 1155.8 eV), assigned to Eu II 29,30 .
It is noteworthy that the interfacial SiO 2 /Eu/phen structure allowed the blue emissive Eu II to be prepared in air and at the relatively low temperature of 200 °C. The morphology and thermal stability of the nanoparticles did not change before and after the sintering process, as shown in TGA and SEM images (Figs. S7 and S8). This transition to Eu II was also stable; the blue emission properties of the material were maintained for more than three months. This reduction phenomenon of Eu was not observed in SiO 2 /Eu nanoparticles without phen and a pure Eu complex with phen. The reduction of Eu III to Eu II in a specially prepared matrix with a rigid inorganic crystal structure following high temperature treatment in air has been reported, and has been explained by a charge compensation model [31][32][33] . In the present reduction system, Eu ions were present at the interfaces between SiO 2 nanoparticles and phen ligands, and the SiO 2 surface acted as a rigid host while the phen functioned as an electron donor through coordination bonds. Thus, the reduced Eu II state can be formed and protected from reaction with oxygen by the surrounding SiO 2 and phen. To the best of our knowledge, this is the first reported fabrication of a stable Eu II -based emissive material using mild conditions (reaction in air and at low temperature) and an organic-inorganic hybrid nanostructure.
To quantitatively assess the effect of phen on the reduction of Eu ions at the interface, the absolute luminescence quantum yields, φ ff , and lifetimes, τ ff , of the SiO 2 /Eu/phen nanoparticles were estimated. The as-prepared SiO 2 /Eu/phen generated a Eu III ff emission with φ ff = 5.3% and τ ff = 486.3 μ s (Fig. S9). The total emission quantum yield of Eu III sensitized by the ligand phen (φ ff ) was determined by the triplet yield of the ligand (φ ISC ), the efficiency of energy transfer (η EnT ) and the efficiency of the metal centred luminescence (η Ln ), as follows 34 .
Because of the nπ * character of the ligand and the high spin-orbit coupling constants of the lanthanide ion, it can be assumed that φ ISC was approximately 1 35,36 . The value of η Ln can be calculated from the observed emission lifetime (τ ff ) and the radiative rate constant (k R ) of the lanthanide ion, as shown below.
The k R value of the emissive excited state, 5 D 0 , is the sum of the spontaneous emission probabilities, A(0, J), to the lower 7 F J levels in Eu III , and can in turn be calculated from the following equation.
Here, I Total /I(0, 1) is the ratio of the total integrated intensity of the corrected Eu III emission spectrum to the intensity of the 5 D 0 → 7 F 1 band. In this case, we obtained a value of 6.21 for I Total /I(0, 1). The spontaneous emission probability of the magnetic dipole 5 D 0 → 7 F 1 transition, A(0, 1), is virtually independent of the ligand field or the environment of the ions, and can be determined directly from the theoretically calculated dipole strength as follows.
Here, σ is the energy gap between the excited ( 5 D 0 ) and the final ( 7 F 1 ) states (σ = 16963 cm −1 ), n is the refractive index (1.5 for the solid state metal-organic complex) 37 and S MDT (J, J') is the magnetic dipole strength 38 . The latter parameter has been calculated theoretically for the 5 D 0 → 7 F 1 transition of Eu III and found to have a value of 884 × 10 −8 Debye 2 , 39 leading to 45.7 s −1 for A(0, 1).
The calculated values for k R and η Ln (from Equations 2 and 3) obtained using the experimentally determined values of τ ff and I Total /I(0, 1) were 283.2 s −1 and 0.384, respectively. The k R value was less than that of a pure complex with phen (ex. 569.6 s −1 in [Eu(phen) 2 (NO 3 ) 3 ], Fig. S10 and Table S1), indicating that a more highly symmetrical environment was present in the vicinity of the Eu III ions in the SiO 2 /Eu nanoparticles. These highly symmetrical conditions allow the Eu ion to function as a stable inorganic emissive compound. Thus, this organic-inorganic hybrid material simultaneously exhibits the photochemical and structural properties of both organic and inorganic materials, which may be responsible for the unusual reduction of Eu III to Eu II in air at a low temperature. From Equation 1, the value of the energy transfer efficiency, η EnT , from phen to Eu III on the SiO 2 /Eu nanoparticles is estimated to be 0.385. In contrast, the value of η EnT is almost 1 in the solid state molecular structure [Eu(phen) 2 (NO 3 ) 3 ]. The considerably lower η EnT of the SiO 2 /Eu/phen nanostructure suggests the existence of an alternative energy migration pathway, such as a ligand (phen) to metal (Eu III ) charge transfer (LMCT) [40][41][42][43] . The presence of an LMCT pathway indicates the ability of the phen to donate electrons to Eu ions, which may result in the unusual reduction of Eu III to Eu II through a thermally activated process. In this reduction-induced emission system, SiO 2 nanoparticles as host materials is supposed to keep the valence of Eu II at the interface.
After sintering at 200 °C, a blue emission band due to the Eu II appeared around 435 nm with τ = 11.59 μ s (Fig. S9). The φ of the Eu II in the SiO 2 /Eu/phen eventually reached a value of 7.2%, higher than that of the Eu III state because of the allowed electric dipole 4f 6 5d → 4f 7 transition. Following sintering at 200 °C, Eu III emission was no longer observed. The surfaces of the SiO 2 /Eu particles were less likely to be attacked by oxygen because of the coordination with phen, and therefore the Eu II state in the SiO 2 /Eu/phen was very stable and the ions were prevented from being re-oxidized, even in air. The reduction-induced emission phenomena are summarized by the energy diagram in Fig. 6.

Conclusions
In conclusion, we discovered a novel emission phenomenon associated with the reduction process in an interfacial complex formed on inorganic nanoparticles. In this study, a Eu-coated SiO 2 nanostructure was developed, consisting of an interfacial complex of Eu and phen at the solid surfaces. The as-prepared SiO 2 /Eu/phen nanoparticles exhibited sharp red emission via energy transfer from the phen to the Eu III . After sintering at 200 °C in air, the emission was tuned from red to blue. The blue emission resulted from Eu II , indicating that the unusual reduction of Eu III to Eu II under mild conditions was successfully accomplished for the first time. The existence of two different stable oxidation states with characteristic emissions, blue emissive Eu II and red emissive Eu III , suggests significant potential applications as novel luminescent materials with inorganic-organic hybrid structures. For instance, our colour-tunable SiO 2 nanoparticles with Eu ions, as having less toxicity, will be greatly beneficial for biological and biomedical applications. Additionally, this redox-active interface between inorganic and organic materials may provide a new photon energy conversion system such as artificial photosyntheses and solar cells. Studies are now underway to fabricate a photoelectron conversion system based on the Eu interfacial complex by way of using a metal oxide with an appropriate redox potential, such as mesoscopic TiO 2 .

Methods
Sample preparation. SiO 2 /Eu nanoparticles were prepared by the sol-gel method. SiO 2 nanoparticles (QS-20, Tokuyama Co.) were suspended in ethanol (10 wt%) followed by the addition of a 50 mM ethanol solution of EuCl 3 (Kanto Chemicals Co., Inc.) at 70 °C for 30 min. The resulting colloidal suspension containing SiO 2 and Eu ions was dropped onto a quartz substrate that had been sequentially cleaned ultrasonically in acetone, isopropanol and ultra-pure water (10 min in each solvent). Following treatment at 110 °C for 15 min, SiO 2 nanoparticles coated with Eu III oxides or hydroxides were obtained. To generate complexation at the particle surfaces, the glass substrate holding the nanoparticles was immersed in a 1 mM ethanol solution of phen (Kanto Chemicals Co., Inc.) at 75 °C for 60 min. After drying, the nanoparticles were further sintered at 200 °C in air.
Apparatus. SEM images were obtained on a ZEISS ULTRA 55 microscope equipped with a secondary in-lens electron detector, together with a Bruker-QUANTAX detector for EDS studies. X-ray photoelectron spectroscopy (XPS) was performed using a KRATOS AXIS ULTRA DLD equipped with a monochromatic Al-Kα X-ray source (1253.6 eV); the binding energies were calibrated at the Au 4f level (84.0 eV). Synchrotron X-ray powder diffraction (XRPD) patterns were obtained with a large Debye-Scherrer camera installed at the SPring-8 BL02B2 beamline, using an imaging plate as the detector 44 and an incident X-ray wavelength of 0.99933 Å. Luminescence spectra were recorded on a Horiba Jobin-Ybon Fluorolog 3-22 with a UV cut filter. The emission decay curves were acquired using a Quantaurus-Τ au C11367-12 (Hamamatsu Photonics K. K.) with excitation via a xenon flash lamp with a band-path filter (λ ex = 280 nm). Fluorescence quantum yields were measured by using a C9920-02 Absolute PL Quantum Yield Measurement System (Hamamatsu Photonics K. K.) [45][46][47] .