Photocatalytic and Photoluminescence Properties of CoreâShell SiO2

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Photocatalytic and photoluminescence properties of coreshell SiO2@TiO2:Eu3+,Sm3+ and its etching products Meiqi Chang, Yanhua Song, Jie Chen, Lei Cui, Zhan Shi, Ye Sheng, and Haifeng Zou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02285 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Sustainable Chemistry & Engineering

Photocatalytic

and

photoluminescence

properties

of

core-shell SiO2@TiO2:Eu3+,Sm3+ and its etching products Meiqi Changa, Yanhua Songa, Jie Chena, Lei Cuia, Zhan Shib, Ye Shenga, Haifeng Zou*a a

College of Chemistry, Jilin University, No.2699, Qianjin Street, Changchun 130012, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, No.2699, Qianjin Street, Changchun 130012, PR China E-mail address:[email protected]. b

Abstract A novel multifunctional core-shell, yolk-shell SiO2@TiO2: Eu3+/Sm3+ and hollow TiO2: Eu3+/Sm3+ structures were successfully synthesized. The relationship between morphology/ions doping and multifunctional properties including photoluminescence and photocatalysis has been discussed in detail for the first time by systematic characterization techniques, including SEM, TEM, XRD, PL, UV-vis, FT-IR, BET and XPS. Upon ultraviolet (UV) excitation, the products show the characteristic red and orange-red emission lines of Eu3+ and Sm3+, respectively. In addition, Judd-Ofelt intensity parameters (Ω2, Ω4) were used to investigate the symmetry and coordination state of Eu3+ ions in the products with different morphologies. The relative luminescence intensities were in the order: yolk-shell > core-shell > hollow structure. This phenomenon can be ascribed to the unique yolk-shell configuration, which possesses appropriate interior cavity used for multiple reflections and scattering, allowing more efficient utilization of the light. Moreover, compared with yolk-shell and hollow structure, the core-shell spheres

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exhibit excellent photoactivity for the degradation of MO, because the existence of Ti-O-Si bonds increases the surface acidity of the sample, thereby activating the oxidation reaction. In addition, the electron activation in the TiO2 matrix can be facilitated through intermediate oxygen atoms. And products doped with different RE3+ ions exhibited distinguishable photocatalytic performance. The surfactant-free method provides a promising route towards the development of multifunctional materials for many applications in photocatalysis and photoluminescence. KEYWORDS: Surfactant-free solvothermal method, Rare earth doping, Multifunctional materials, Etching process, Ti-O-Si bond, Multiple reflections Introduction As a promising material for photoluminescence, catalysts, solar cells, anatase TiO2 has been intensively studied because of its excellent electrical,

optical

and

photochemical

properties.1-12

As

such,

photo-degradation and luminescence properties of pure TiO2 matrix have been investigated for a long time. However, single-component matrix still has some problems: nanoparticles agglomerated easily; the capping ligands which used for maintaining colloidal stability would occupy the active sites of TiO2, which limited its practical application.13 Therefore, the improvement of the performance of titanium dioxide is still a serious challenge. ACS Paragon Plus Environment

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At present, morphology control and ions/compounds doping are considered as two of important methods among various strategies. Compared to pure TiO2, SiO2@TiO2 hybrid materials have recently attracted attention as catalysts through the generation of new catalytically active sites for a variety of reactions. And as one kind of hard template, SiO2 can be etched with NaOH or NH4OH which is helpful for formation of materials with intricate morphologies. In addition, the high surface energy (001) of TiO2 leads to easy aggregation which hinders their photocatalytic activity, SiO2 spheres as a supporting materials could solve this problem. As you know, structure–function relationship is essential to understand the improved performance deeply. It is worth mentioning that core-shell and yolk-shell structures are suitable for scale-dependent applications because of their improved physical and chemical properties.13-19 Yolk–shell structure is a derivative of the core-shell structure, which consists of three parts: core, cavity, and shell structure (core@void@shell configuration). The utilization of light is effectively improved because of multiple light scattering and reflection effects within the interior cavity, which is beneficial for catalytic activity and luminescence efficiency.20-22 Recently, rare earth ions doped nanophosphors have been widely used in flat displays, optical devices and bio-imaging probes.23-31 Meanwhile, as you know, RE3+ doping as an efficient method is beneficial to the



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improvement of photocatalytic performance, the introduction of RE3+ could suppress the recombination of photo-generated electrons and holes (PGEH), and generate the O2-• radical ions when the trapped electrons transferred to the adsorbed O2 molecules. Moreover, incorporation of RE3+ could increase the concentration of dye which attached to the surface of the catalyst.32-36 Accordingly, the Eu3+/Sm3+ doped SiO2@TiO2 core-shell and yolk-shell hybrid spheres have been prepared through deposition of TiO2 on the surface of SiO2 cores through solvothermal method without assistance of surfactant, followed by selective etching process. Through changing the etching time, the space of interior cavity can be adjusted, and hollow structure were prepared as a comparison. The relationship between morphology/ions doping and multifunctional properties including photoluminescence and photocatalysis has been discussed in detail for the first time. The luminescence properties have been discussed by PL spectra and decay kinetics, yolk-shell SiO2@TiO2:Eu3+/Sm3+ hybrid spheres within the appropriate interior cavity exhibited excellent performance, which can be attributed to efficient use of light, the light can be used more efficiently due to multiple reflections and scattering effects. The photocatalytic activities of products were in the order: core-shell > hollow > yolk-shell structure. The Ti-O-Si bonds and light absorption effect are responsible for this phenomenon.


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Experimental Materials Tetraethyl orthosilicate (TEOS) was from Internet Aladdin Reagent Database Inc. Tetrabutyl titanate (TBOT), absolute ethanol, isopropyl alcohol (IPA), ammonium hydroxide (NH4OH, 28%), and Sm2O3, Eu2O3 (99.99%) were purchased from Beijing Chemical Co. All chemicals were analytical-grade and used directly without further purification. The Eu2O3 (Sm2O3) powders were dissolved in dilute HNO3 solution under heating until the solution becomes transparent and the pH is equal to 2-3, thereby obtaining the Eu(NO3)3 (Sm(NO3)3) aqueous solution. Deionized water was used for all treatment processes.

Preparation The fabrication processes of SiO2@TiO2:Eu3+/Sm3+ hybrid spheres and hollow TiO2:Eu3+/Sm3+ structures mainly include two steps, which were showed in Scheme 1. Synthesis of the SiO2 spheres Silica spheres were prepared using the modified Stöber method. In a typical procedure, 0.35 mL of NH4OH and 2.5 mL H2O were added into 20 mL IPA under stirring, then 5 mL of TEOS were added dropwise to the above solution and stirred for an additional 10 h over which a



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colloidal suspension formed. The white products were centrifuged, washed with deionized water and absolute ethanol several times, and the finally SiO2 powders were obtained by drying at 60 °C for 12 h.

Synthesis of the SiO2@TiO2:Eu3+ (Sm3+) hybrid materials and hollow TiO2:Eu3+ (Sm3+) materials 0.25 g as-prepared SiO2 powders were ultrasonically dispersed in a mixed solution containing ethanol (37.5 mL) and deionized water (0.75 mL) for 20 min. Then 1.25 mL TBOT and 0.2 mL (0.5mol/L) Eu(NO3)3 (Sm(NO3)3) (molar ratio of Eu3+(Sm3+)/Ti4+ = 2.7%, and the actual doping amount detected by EDX spectra are shown in Figure S1(S2) and Table S1(S2)) were added to the above solution and stirred for an additional 20 min. Then the mixture was transferred into a Teflon stainless steel autoclave, aged at 140 °C for 3 h and cooled to room temperature. The washing and drying conditions are the same as above mentioned, the products were designated as non-STE (non-STS). In addition, non-STE (non-STS) annealed at 700 °C in air for 3 h were designated as STE (STS). Yolk-shell SiO2@TiO2:Eu3+ (Sm3+) and hollow TiO2:Eu3+ (Sm3+) structures were prepared through etching process. The STE (STS) hybrid materials were etched with NH4OH solution (1 M) in a water bath at 60 °C for different time and the washing and drying conditions are the same


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as above mentioned. SiO2@TiO2:Eu3+ and SiO2@TiO2:Sm3+ etched with 2 h, 4 h, 6 h, 14 h were designated as STE-2, STE-4, STE-6, TE, STS-2, STS-4, STS-6, TS, respectively. Non-doped products (SiO2@TiO2) were designated as ST-2, ST-4, ST-6, TE. In addition, non-STE annealed at 800 °C, 900 °C and 1000 °C in air for 3 h were designated as STE-800, STE-900 and STE-1000, respectively.

Synthesis of the TiO2:Eu3+ materials In order to investigate the effect of Ti-O-Si bond on the phase transformation of TiO2, pure TiO2:Eu3+ materials have been prepared as follows: 1.25 mL TBOT was added dropwise in a mixture of 0.75 mL deionized water and 37.5 mL ethanol. Then 0.2 mL (0.5mol/L) Eu(NO3)3 were added to the mixture and stirred for 20 min. The final solution was diverted to an autoclave and maintained at 140 °C for 3 h. the washing and drying conditions are the same as above mentioned, the as-prepared samples calcined at 700, 800, 900 and 1000 °C were designated as pTE-700, pTE-800, pTE-900, pTE-1000. Characterization The

scanning

electron

microscope

(Hitachi

S-4800)

with

an

energy-dispersive X-ray spectrometer (EDX) and transmission electron microscopy (FEI Tecnai G2S-Twin) with a field emission gun operating at 200 kV were used to investigate the morphology, size and element



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distribution of the products. Image visualization software (Image J) was adopted to measure the diameters of samples. The crystalline structure and phase of materials were recorded by XRD (Rigaku D/max-B II) using Cu-Kα radiation (λ = 0.15405 nm), scanning range (2θ): 10°-70°. The vibration in functional groups of core-shell, yolk-shell and hollow structures were investigated through Fourier transform-infrared radiation (FT-IR) spectrometer (SHIMADZU, 1.50SU1, Japan). The mixture of the dried KBr powder and samples (weight ratio = 100:1) was ground and then pressed into a pellet through hydraulic press. The spectral resolution was 4 cm-1. The UV–vis absorbance spectra was recorded with a UV–Vis-NIR spectrophotometer (SHIMADZU, UV-3600) equipped with an integrating sphere using BaSO4 as a reflectance standard to obtain spectra over the 300–600 nm. XPS spectra were obtained through a VG ESCALAB 250 spectrometer using Mg Ka (1253.6 eV) as excitation source. The binding energies of all samples were corrected to C 1s peak at 284.6 eV, and the relevant experimental parameters are listed below: take-off angle = 90°, spot size = 500 µm. For the survey spectra, the pass energy = 100 eV, the energy step size = 1.00 eV. For High-resolution spectra, the pass energy = 30 eV, the energy step size = 0.05 eV. XPS Peak 4.1 software using a 70:30 Gaussian: Lorentzian peak shape and a Shirley background function was used to perform the spectral deconvolutions. Before the fitting process, the difference between the


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standard value of C (284.6 eV) and the actual binding energy provided by the data should be calculated. Then other elements need to add this difference to complete the charge calibration process. Fluorescence spectrophotometer (Jobin Yvon FluoroMax-4, 150 W xenon lamp) was used to record the luminescence spectra. The surface photocurrent (SPC) measurements were performed on the system through a monochromatic light source consists of 500 W xenon lamp (CHFXQ500 W, Global xenon lamp power) and double-prism monochromatic (Hilger and Watts, D300), a low-frequency light chopper-based (SR540, 23 Hz) lock-in amplifier (SR830-DSP), sample cell, comb-like electrode and computer.

Photocatalytic activity Photocatalytic decomposition of methyl orange (MO) under white light irradiation (a Xenon lamp (CHFXQ 500 W, Global Xenon Lamp Power)) was carried out to investigate the activities of core-shell, yolk-shell and hollow structures. Before the irradiation, 20 mg of sample was ultrasonically dissolved in a quartz beaker containing 20 mL of the MO solution (20 mg/L) for 5 min, and the absorption–desorption equilibrium between MO and the surface of samples was established through magnetically stirring for 1 h in the dark. 1 mL of solution were extracted every 5 minutes and centrifuged. The degradation rates for each time period can be obtained by comparing the absorption band maximum



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values (464 nm) of the UV-Vis spectra. In addition, P25 was also examined using the same parameters as a comparison. Results and discussion Morphology and structure properties As shown in Figure 1a, the as-synthesized sample consists of smooth and monodispersed spheres with an average diameter of about 310 nm, XRD pattern of product showed that only a broad peak at about 22–23° corresponding to amorphous SiO2 emerged, which was presented in Figure 1b. SEM and TEM images of SiO2@TiO2:Eu3+ core-shell structures were shown in Figure 2(a), it is noted that the diameter of spheres increases to 450~550 nm after coating with TiO2, and titanium dioxide is present in the form of nanoparticles. The as-prepared samples exhibit yolk-shell structure after etching for 2 h, the interior cavity size is 45 nm. (Figure 2(b)). As the extension of etching time, the diameters of SiO2 spheres became smaller, the interior cavity sizes became larger and larger, which are about 60nm and 80nm for STE-4 and STE-6, respectively, which were shown in Figure 2(c) and (d). However, the etching process would more or less destroy the structural integrity of the material, some TiO2 nanoparticles are present independently. When the etching time was fixed at 14 h, hollow structure can be obtained (Figure 2(e)). So it can be concluded that the space of interior cavity is adjustable through changing ACS Paragon Plus Environment

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the etching time. The crystallinity and crystalline phases are two important factors for improving the photocatalysis and photoluminescence properties. The increase of separation and migration rate of PGEH depends on the improvement of crystallinity. Moreover, the existence of highly crystalline anatase phase is helpful for improving the luminescence intensities. The photocatalytic and photoluminescence efficiencies of anatase are higher than those of the other two minerals: rutile and brookite. Therefore, it is vital to confirm the crystal phase and crystalline degree of samples. In our research, the diffraction peaks of hybrid materials closely relate to the crystalline phase of the TiO2 because of amorphous state of silica.37 Figure 3(a-b) displays the XRD patterns of STE, STE-2, STE-4, STE-6, TE and STS, STS-2, STS-4, STS-6, TS. The peaks at 2θ = 25.5°, 38.0°, 48.1°, 54.1°, 62.9° and 69.1° were assigned to the (101), (112), (200), (105), (204) and (220) planes of the anatase, respectively. The diffraction peaks of all samples could be assigned to anatase TiO2 (JCPDS 73-1764) possessing a tetragonal structure (space group: I41/amd (141), a = 3.776 Å, b =3.776 Å, c = 9.486 Å), and no impurity phases appeared, which indicated that the effect of the introduction of Eu3+/Sm3+ on the crystal structure is negligible due to the low doping concentration. The sharp diffraction peaks mean the excellent crystallinity of TiO2 shell. The particle sizes of STE series were showed



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in Table S3. XRD measurements of SiO2@TiO2:x%Eu3+ (0, 1, 2.7, 5, 7, 9) core-shell materials were performed to investigate the effect of doping concentration on crystallite size, which were shown in Figure S3. The corresponding parameters are shown in Table S4. It can be noticed that the doped products possess smaller crystallite sizes compared with that of pure non-doped samples, moreover, the sizes decreased with an increase in the Eu3+ ions concentration. This reduction can be ascribed to separation of Eu3+ ions at the grain boundary, which avoids the agglomeration of the grains, thereby inhibiting the growth of the grains.38

UV-vis spectra and FT-IR spectra analysis The optical properties of core-shell, yolk-shell and hollow structures were distinguished by UV-vis diffuse absorbance spectra, which were shown in Figure 4(a). As displays, all samples had no absorption in visible light region but exhibited a broadband absorption below 390 nm, because of the relatively high band gap energy of anatase and the excitation of electrons from the valence band to the conduction band, respectively. The morphological changes would affect light absorption effect of samples, hollow structures exhibited the highest UV light absorption among the five samples. In addition, the band gap energy (Eg) of STE, STE-2, STE-4, STE-6 and TE are 3.26 eV, 3.25 eV, 3.27 eV, 3.24 eV, 3.29 eV,


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respectively (Figure S4). There is almost no considerable change in Eg values, which is within the experimental error. S. Ullah et al. have reported that the Eg value can be altered through changing the crystal size.39 Therefore, the crystal sizes of STE, STE-2, STE-4, STE-6 and TE were calculated from the Scherrer equation to study the size effect, which were showed in Table S4. The etching process would reduce the particle size, but the degree is insignificant, and the value difference < 1.32 nm. Therefore, the invariance of band gap values can be attributed to small differences in particle size. As shown in Figure 4(b), FT-IR results of the core-shell, yolk-shell and hollow structures were compared. Peaks at 1118 cm−1 and 800 cm−1 were observed in STE, STE-2, STE-4 and STE-6 hybrid materials, which are assigned to the asymmetric (vasSi-O-Si) and symmetric Si-O-Si (ṽsSi-O-Si) stretching modes, respectively.40 And the main peaks at 481 cm−1 correspond to Ti–O–Ti bridging stretching modes emerged in all samples, confirming the formation of TiO2.41 Most importantly, a stretching vibration at a wave number of 943 cm-1 was detected, which is not only attributed to the dangling Si-O (ṽSi-Od) stretching modes but also assigned to Ti-O-Si bond.42-43 It is important to note that the bond intensities become weaker with the extension of etching time, therefore, it can be speculated that the etching process disrupted the Ti-O-Si bond. However, for TE, only Ti-O-Ti bond appeared, which indicated that the



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complete etching of SiO2 and the formation of hollow TiO2:Eu3+.

BET analysis Nitrogen adsorption–desorption isotherms were performed to compare the porous properties of the products with different morphologies, as shown in Figure 5(a-e). All products exhibited Type IV isotherms containing a hysteresis loop, which belong to the mesoporous material. The pore size distributions are similar, showing a majority of mesoporous in the ~12.4 nm, which is due to the formation of gaps between TiO2 particles (insets in Figure 5). And the specific surface areas (SBET) of STE, STE-2, STE-4, STE-6 and TE are 41.11, 45.89, 48.30, 48.84 and 67.07 m2/g, respectively.

Luminescence properties The emission spectra of STE, STE-2, STE-4, STE-6 and TE under the excitation at 393 nm are shown in Figure 6(a), all spectra exhibit the characteristic emission peaks of Eu3+ ions, which can be attributed to the intra-4f transition (D0-7FJ (J = 0, 1, 2, 3, 4)). The major emissions are 591 nm and 612 nm corresponding to the magnetic dipole transition (5D0–7F1, MD) and electric dipole transition (5D0–7F2, ED). The ratio of integrated emission intensity (5D0–7F2/5D0–7F1) called the asymmetric ratio (R), which can provide the relevant information about crystal field


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environments.44 When the Eu3+ ions locate at low symmetries without inversion centre, 5D0–7F2 transition is dominant. However,

5

D0–7F1

transition is the dominant transition when Eu3+ ions occupy an inversion symmetry site. It can be observed from the emission spectra that the intensity of MD transition is weaker than that of ED transition, indicating the presence of Eu3+ ions with low symmetry site. It is clearly shown that the variation of etching time did not change the peak position and shape, but the R values decreased with the extension of etching time, which proved that the morphology change would affect the symmetry, and most of Eu3+ ions entered into the crystal lattice sites having centre with inversion symmetry in the TE. In order to further investigate the site symmetry of Eu3+ ions, Judd-Ofelt parameters were calculated in the Table 1. The magnetic dipole transition rate (A0-1) is independent of host matrix and the value is assumed approximately 50 s−1.45 The radiative transition probabilities A0–2,4 can be calculated through the following formula (formula (1)): A0 J =

64π 4υ J3 2 n (n 2 + 2)2 e 3h( 2 J + 1) 9

∑ Ωλ λ =2,4

5

D0 U ( λ ) 7 F0

2

(1)

where υJ is the wavenumber of 5D0–7FJ transition, which is the reciprocal of the wavelength, e is the elementary charge, which equals to 1.6 × 10-19, h is the Planck's constant (6.63 × 10-34), and n is the refractive index of 5

the sample.

D 0 U（λ） 7 FJ

2

are the square reduced matrix elements, which

are constants and the values for J = 2 and 4 are 0.00324, 0.00229, ACS Paragon Plus Environment


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respectively. Therefore, A0-2, A0-4, Ω2, and Ω4 can be obtained through the equation (2) : 3

A0J e 2 υJ (n 2 + 2)2 = Ωλ 5D 0 U (λ ) 7FJ A01 S md υ13 9n 2

2

=

∫ I J dυ ∫ I dυ

(2)

1

Herein, the A0J/A01 can be obtained through calculating the ratio of spectra integral intensity, the Ω2 values reveal the local symmetry environment of Eu3+ ions and the covalency between Eu3+ and O2-. The decrease of value of Ω2 indicated that the symmetry of Eu3+ ions has increased and the decrease in covalency of the Eu-O bonds. It can be seen that the Ω2 values decrease with the extension of etching time, indicating the decrease of symmetry of Eu3+ ions. With the etching time increased, most of the Eu3+ ions entered into the crystal lattice sites. Similar results are reflected by the asymmetry ratio. While the parameter Ω4 is the embodiment of ligand electron density, and the electron density would decreases with Ω4 value increases. The luminescence decay curves of the STE, STE-2, STE-4, STE-6 and TE are shown in Figure 6(b). All curves conform to double-exponential fit: I(t) = I0 + A1exp(-t/t1) + A2exp(-t/t2)

(3)

where I(t) and I0 are the luminescence intensities, A1 and A2 are the fitting parameters, and t1 and t2 are the rapid and slow decay times, respectively. Confirming the multiplicity of the location of Eu3+ ions. Furthermore, the


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average decay lifetimes t are calculated through the following formula (formula (4)): t = (A1t12 + A2t22)/(A1t1 + A2t2)

(4)

Thus the average lifetimes were determined to be 0.44, 0.47, 0.48, 0.51, 0.42 ms for the STE, STE-2, STE-4, STE-6 and TE, respectively. The variation tendency of lifetime is consistent with that of the luminescence intensity. As you know, luminescence intensity is proportional to radiative transition probability, and the lifetime is inversely proportional to total radiative transition probability.46 Therefore, core-shell structures possess the highest non-radiative transition probability. Figure 7(a) showed the excitation and emission spectra of STS series. The excitation spectra (the inset) monitored at 610 nm consist of a broad band absorption with a maximum at 357 nm which is ascribed to the anatase host absorption and several weak lines in the range of 400–475 nm which are attributed to the f–f transition of the Sm3+ ions, the difference in intensity illustrated that the host sensitized emission is the main energy transfer mode. All emission spectra are composed of several lines centered at 583, 610, 661 and 725 nm which can be ascribed to the 4

G5/2-6HJ (5/2, 7/2, 9/2, 11/2) transitions, respectively. Among these, the

strongest emission peak centered at 610 nm is the strong red-emitting transition of a partly magnetic dipole and partly electric dipole nature, 4

G5/2-6H5/2 is MD natured and 4G5/2-6H9/2 is purely ED natured. The Sm3+



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ions with high symmetry existed in all samples because of the relative stronger intensity of magnetic dipole transition. The luminescence decay curves of the STS, STS-2, STS-4, STS-6 and TS are shown in Figure 7(b). All of them also can be well fitted using a double-exponential function, and the average lifetimes are 0.23, 0.63, 0.65, 0.67 and 0.13 ms, respectively. The lifetime values and luminescence intensity were in the order: TS < STS< STS-2 < STS-4 < STS-6, which is consistent with the luminescence intensity trend of Eu3+ ions. Normally, the introduction of a large number of surface defects occurs in the products with a large specific surface area, which would cause fluorescence quenching. Although TE hollow structures possess excellent light absorption capacity, the highest specific surface area lead to a decrease in fluorescence intensity. It is well known that the existence of Ti-O-Si bond would repair unsaturated bonds which come from surface of TiO2, thereby reduce selection rules of radiative transitions, which is helpful for improving luminescence. However, the core-shell structure possessing the largest amount of Ti-O-Si bonds and relatively low specific surface area exhibited weaker luminescence intensity compared with yolk-shell structure, which can be attributed to the morphological

differences.

Yolk-shell

nanostructures

within

the

appropriate interior cavity allow multiple reflections and scattering of light, allowing more efficient utilization of the light, thereby improving


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luminescence intensity. In addition, Figure 8 shows the emission spectra of non-doped SiO2@TiO2 and TiO2, under excitation at 325 nm, all products exhibited a broad band emission with a maximum intensity at around 440 nm, It is worth mentioning that the sharp peak at 450 nm arise from laser. As is known, for non-doped TiO2 host matrix, the generation of luminescence is due to the combination of PGEH. Its intensity is related to the recombination rate which can be used to reveal the differences in the photocatalytic efficiencies. The corresponding luminescence intensity trend of non-doped samples is consistent with that of the STE series, which indicated that hollow structures possess a much lower recombination rate of PGEH.

Photocatalytic activity In order to investigate the effects of morphology and different RE3+ doping on the photocatalytic activity, methyl orange dye molecules were used to evaluate the photocatalytic performance under white light illumination. The photocatalytic degradation of MO (degradation efficiency = 38.5%) with Degussa P25 was conducted for comparison. Changes in the concentration of MO of STE, STE-2, STE-4, STE-6 and TE can be obtained through monitoring the UV–vis spectra, which were shown in Figure S5(a-e). The absorption intensity of MO decreased



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gradually with the increase of irradiation time for all samples, the degradation efficiencies are 99.5%, 54%, 93.5%, 30%, 98.3% after 30 min irradiation, respectively (Figure 9(a)). In addition, a kinetic research was used, all products were fitted through Langmuir–Hinshelwood first-order kinetics, ln(C0/C) = kt, where k is apparent reaction rate constant, which were determined as 0.159, 0.025, 0.088, 0.009, 0.131, 0.017 min-1 for STE, STE-2, STE-4, STE-6, TE and P25, respectively ( Figure 9(b)). It is obvious that STE materials with the largest rate constant have the highest catalytic activity, and the photocatalytic activities were in the order: STE > TE > STE-4 > STE-2 > P25 >STE-6. Figure S6(a) shows the UV-vis spectra for the catalytic degradation by STS, the strong adsorption peak gradually decreased and almost completely disappeared after irradiation for 30 min which possessed the highest photodegradation efficiency compared with the yolk-shell and hollow structure. As shown in Figure S6(b-e), the degradation efficiencies are 75%, 93%, 61% and 94%, respectively. Compared with the P25, all samples exhibit better photocatalytic performance (Figure 10(a)). And the k values were in the order: STS > TS > STS-4 > STS-2 > STS-6 > P25. (Figure 10(b)) As you know, the enhancement of catalytic activity is the result of a combination of many factors, such as high crystallinity, excellent surface properties, high specific surface area and high utilization efficiency of


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light. Therein, the improvement of specific surface area is an important process because of the formation of more reaction sites, which has been discussed deeply in many literatures.12, 47-48 However, in this paper, the SBET values of core-shell and yolk-shell structures are similar. Importantly, the core-shell structure spheres with the lowest specific surface areas possess the best photocatalytic activities compared with samples with hollow and yolk-shell structure. Such unusual phenomenon revealed that the increase of the BET value is hardly responsible for the improvement in photocatalytic activity of core-shell structures. And the crystallinity properties are very nearly the same (Figure 3). So in this case, such dramatic change can’t simply be attributed to the difference in specific surface area. Based these experiment results, the relationship between morphology and photocatalytic performance has been discussed as follows: Core-shell structure with relatively strong Ti–O–Si bonds exhibited the highest photocatalytic activity. The bonds generate additional surface acidity which is conducive to the oxidation of MO, in addition, intermediate oxygen atoms between Ti and Si increased the probability of electron activation and in turn improved the photocatalytic activity of core-shell structure.49-50 It is worth mentioning that the hollow structure also exhibited excellent activity, which can be ascribed to strong light-harvesting capacity and large specific surface area. The former contributes to the increase in the number of PGEH, and the latter



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conducive to the increase of the reduction site on the TiO2 surface. In addition, multiple light reflection and scattering effects emerged in the yolk–shell structures, the light source would be more effectively utilized. It is noted that the photocatalytic efficiency is not proportional to inner-sphere diameter, yolk-shell spheres etched for 4 h with an appropriate inner-sphere diameter show the highest activity compared with the samples etched for 2 h and 6 h, when the etching time increased, the hybrid spheres possess a lower activity than STE-4, possibly due to the smaller inner cores that reduce multiple reflection within their interior cavity. Therefore, the appropriate space of interior cavity is essential. The similar phenomenon has been proposed by Lu et al.51 As you know, there are three crucial steps for the photocatalytic process: the generation of PGEH; the separation and migration of PGEH; the occurrence of surface reduction reactions.37 Herein, the photocurrent intensity is the embodiment of electron-hole migration ability. Therefore, the surface photocurrent measurements were carried out to investigate the dynamic properties of the PGEH,52 and the corresponding spectra were displayed in Figure 11. The response intensity of STE is the strongest, which indicated that the migration ability of PGEH is superior to that of other product.53 The change trend of intensity is consistent with that of degradation efficiency, which provided strong evidence for explaining the difference in photocatalytic performance.


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The Ti–O–Si bonds could be further confirmed through comparing the crystallization behaviour of pure TiO2:Eu3+ and SiO2@TiO2:Eu3+ core-shell structure, as shown in Figure 12. Although the calcination temperature was raised to 1000℃, the rutile phase does not appear in the composite (Figure 12(b)), however, rutile, anatase and Eu2Ti2O7 phase can be detected in the XRD pattern of TiO2:Eu3+, as shown in Figure 12(a). This phenomenon can be attributed to the inhibition of grain growth by the Ti-O-Si bonds, and the anatase phase is stabilized. In addition, some of the tetrahedral Si sites were replaced by Ti atoms during the coating process, the interaction between tetrahedral Ti atoms and octahedral Ti species suppresses the formation of rutile phase. Also, the presence of SiO2 can inhibit the nucleation of TiO2 by locking the Ti-O bond.54 It should be noted that the formation of Eu2Ti2O7 occured at 900℃ for the TiO2:Eu3+ samples, however, emerged at 1000℃ for SiO2@TiO2:Eu3+ core-shell samples. The high calcination temperature facilitated the movement of the Eu3+ ions toward the interface, resulting in a stable Eu2Ti2O7 structure. The existence of the Eu2Ti2O7 promotes the formation of rutile phase because the mass gathered in the nucleation area, which is beneficial for phase transformation.55 In addition, TEM mapping measurements of STE-700 and STE-900 were performed to investigate the effect of calcination temperature on Eu distribution which were shown in Figure S8, which exhibited uniform distributions of O, Ti



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and Si components throughout the SiO2@TiO2:Eu3+ composites, and core-shell structure can be further confirmed. Most importantly, regardless of whether the calcination temperature is 700 or 900℃, all images displayed a uniform distribution of Eu element, therefore, it can be concluded that the change in calcination temperature does not destroy the homogeneity of the Eu3+ ion distribution. In order to quantify the Ti-O-Si bond and study its effect on the catalytic performance, XPS analysis were performed. The Ti 2p spectra of pTE-700 and STE series were shown in Figure 13a. The spectrum of pTE-700 consists of two major peaks at 464.2 eV and 458.6 eV, corresponding to the binding energy of Ti 2p1/2 and Ti 2p3/2, respectively. The two symmetric peaks with a separation of 5.6 eV proving the presence of the tetravalent Ti. However, for SiO2@TiO2:Eu3+ composite systems, the peak position shifts towards higher binding energy compared with that of TiO2:Eu3+ because of the presence of Ti-O-Si bond, which would reduce the electron density around Ti atom.56 Similar results can be observed in the STS series, as shown in Figure S9(a). The binding energies of

Ti 2p3/2 for STE, STS-2, STS-4, STS-6 and TS are 458.7, 458.8, 459.0, 458.9, 458.9 eV, respectively. It is worth noted that the FWHM of pTE-700, STE, STE-2, STE-4 and STE-6 are 1.09, 1.19, 1.94, 1.85 and 1.60 eV, respectively. The Ti 2p lines of STE series are broader compared with that of pTE-700, which also confirmed the existence of


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Ti-O-Si bond.39 The difference of binding energy between the O 1s (Ti-O-Ti) and Ti 2p3/2 levels was showed in Table S5 to investigate the effect of changing morphology on binding energy. Although the morphological changes caused red shift of Ti 2p3/2 and O 1s, the difference of the value is significantly lower.

The peak at 102.1 eV is associated with the binding energy of Si 2p (Figure 13(b)), corresponding to the Si4+ oxidation state, which is less than that for pure SiO2 (103.4 eV).57 This phenomenon can be attributed to the presence of TiO2 shell. The O 1s spectra are shown in Figure 13(c). All spectra can be de-convoluted into three peaks. For STE, the major peak at 530.2 eV corresponds to Ti-O-Ti bond, and peaks at 531.9 eV and 532.6 eV are Ti–O–Si and Si–O–Si bonds, respectively. However, the peak of Ti-O-Ti bond shifts to 530.6 eV for STE-2, STE-4 and STE-6, the slight increase of binding energy indicated that the etching process affected the Ti-O-Ti bond. For STS series, the similar shift of the Ti-O-Ti bond binding energy for yolk-shell samples with respect to core-shell samples has appeared, as shown in Figure S9. According to Rasalingam et al.,58 the percentage surface contents of Ti–O–Ti, Ti–O–Si and Si–O–Si of STE series were calculated through quantifying Gaussian/Lorentzian shape

GL

(70:30)

curves

with

equivalent

eV

at

the

full-width-half-maximum (FWHM). And the results were indicated in Table 2. And the corresponding information about STS series were showed in Table S6. Apparent surface coverage (ASC) was used to



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compensate for the differences in BET values, and ASC values of Ti–O–Si are calculated. Core-shell structures with the best photocatalytic activity possess the highest ASC value. The amount of Ti-O-Si bonds decreased with the increase of etching time, indicating that the etching process has a damaging effect on the Ti-O-Si bonds, which is consistent with the results of FT-IR. In addition, the O 1s spectrum of TiO2:Eu3+ as a reference is shown in Figure 13(d), the peak at 529.8 eV is ascribed to Ti-O-Ti bond, and the weak peak at 531.8 eV can be assigned to physically adsorbed oxygen or OH groups on TiO2 surface.59-60 The Ti–O–Si bonds generate surface acidity site, which serves to activate the oxidation of MO always exist in the binary composite oxide. So, what type of site does the core-shell structures belong to? Tanabe et al. have proposed the relationship between the electronic valence model and the acidity sites.61 They assumed that when the cation of the doped oxide enters the lattice of the main oxide, the original coordination number remains unchanged, therefore, the oxygen atoms have a new coordination number for the dopant cation, resulting in a charge imbalance, which can be calculated through the number of charge of each individual bond to the dopant cation multiplied by the number of bonds. As is well known, SiO2 is tetrahedrally coordinated and titania is octahedrally coordinated. When Ti atom enters SiO2 lattice, the coordination number remained six for Ti, the four positive charges of Ti divided by six bonds, the two electrons of


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oxygen divided by two bonds, therefore, the charge difference for all the bonds is 6 × (4/6 − 2/2)= −2. Herein, Brönsted sites formed. If Si atom enters TiO2 lattice, the four positive charges of Si divided by four bonds, and the two electrons of oxygen divided by three bonds, the charge imbalance is 4/3, which would promote the generation of Lewis sites. The doping configurations and the corresponding acidity sites are shown in Figure 14(a-b). For the SiO2@TiO2 core-shell structure, TiO2 nanoparticles are coated on SiO2 spheres, Ti as a dopant cation may enter the SiO2 matrix, in this case, Brönsted sites are presumed to exist in this composite system. Therefore, the Ti-O-Si bonds are present in two forms: SiO2 matrix due to Ti doping and the interface between SiO2 and TiO2 layers. In addition, rare earth ions (Pr3+, Y3+, Eu3+ and Er3+) doped samples applied for photocatalytic have been discussed in previous report,34 which can inhibit the recombination of PGHE. Therefore, Eu3+ and Sm3+ doping have been discussed in the paper. Photocatalytic performance of non-doped SiO2@TiO2 and TiO2 was conducted as a reference (Figure S7). As shown in Figure 15(a), the degradation efficiencies are 89%, 42%, 78%, 28%, 80% after 30 min irradiation for ST, ST-2, ST-4, ST-6 and T, respectively. The decomposition rate constants are determined as 0.070, 0.017, 0.050, 0.010, and 0.052 min-1, respectively (Figure 15(b)) .



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Obviously, the introduction of rare earth ions would improve the photocatalytic performance. Eu3+/Sm3+ ions interact with the surface of catalysts forming electrons trap states, and the oxidant superoxide radical ion (O2−•) generated since electrons are captured by oxygen molecules, which were shown in following equations ((5) and (6)): Sm3+/Eu3+ + e- → Sm2+/Eu2+

(5)

Sm2+/Eu2+ + O2 → Sm3+/Eu3+ + O2−•

(6)

The Sm3+/Eu3+ ions near TiO2 surface could trap photo-generated electrons, and the Sm3+/Eu3+ was reduced to Sm2+/Eu2+. Then, the Sm2+/Eu2+ could be oxidized back to Sm3+/Eu3+ by the adsorbed oxygen and generate O2−•. Therefore, it can be concluded that the Sm3+/Sm2+ (Eu3+/Eu2+) pair has a dual effect: separation of electrons and holes and generation of O2−•. It’s worth noting that samples doped with different rare earth ions will cause different catalytic performance. As you see, the performances of STE, STE-4 and TE are better than that of STS, STS-4 and TS, respectively. However, Eu3+ doped yolk-shell spheres etching for 2 h and 6 h have lower activities compared with STS-2 and STS-6, respectively. The different performance may be attributed to different electron configuration of Eu3+ and Sm3+, host sensitization process between matrix and Sm3+ ions and morphology. Normally,

separation

of

PGEH

is

benefit


for

improving

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photocatalytic activities, however, the radiative recombination probability of PGEH decreased, the luminescence intensity decreased. In our research, luminescence and photocatalytic properties are affected by many common factors, including specific surface area, crystallinity, Ti-O-Si bond and light utilization, as shown in Scheme 2. However, the contribution degree of factors is variant for different properties. For photocatalytic properties, the Ti-O-Si bonds and light utilization are considered to be two important influencing factors. For luminescence properties, the light utilization is major factor. In this work, the core-shell and hollow structure is favorable for catalysis, while the yolk-shell structure is more suitable for luminescence. Conclusion The SiO2@TiO2:Eu3+/Sm3+ hybrid multifunctional materials and its etching products have been successfully synthesized. All samples possess anatase phase which is beneficial for enhancing catalysis activities and luminescence efficiency. And the multifunctional properties including photoluminescence and photocatalysis of products with different structures have been discussed in detail for the first time. The yolk-shell SiO2@TiO2:Eu3+/Sm3+

hybrid

spheres

exhibited

the

strongest

luminescence intensities because of multiple reflections and scattering effect. Considering the formation of Ti-O-Si bonds and light absorption capacity, the relative photocatalytic activities were in the order:



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core-shell > hollow > yolk-shell structure. In addition, the positive effect of RE3+ ions on the photocatalytic activity of the products is demonstrated in depth. It is believed that the as-prepared samples with different structures, as multifunctional materials, can be widely applied in the fields of photocatalysis and photoluminescence.

Supporting information EDX spectra, XRD patterns, emission spectra, UV-vis absorption spectra, TEM, XPS spectra

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21171066 and 21671078), Jilin Province Science

and

Technology

Development

Plan

Item

(Grant

No.

20170312002ZX), the Opening Research Fund of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2016-06), and the Special Project of Provincial School Construction Plan of Jilin Province (SXGJSF2017-3).

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Scheme 1. Preparation of SiO2@TiO2:Eu3+ (Sm3+) hybrid spheres and its etching products.

Figure 1. (a) SEM image and (b) XRD pattern of SiO2 spheres, the inset is corresponding diameter distribution.


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Figure 2. SEM images of (a) STE, (b) STE-2, (c) STE-4, (d) STE-6 and (e) TE. Upper inset: high-magnification TEM image.



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Figure 3. XRD patterns of (a) STE, STE-2, STE-4, STE-6, TE and (b) STS, STS-2, STS-4, STS-6, TS, respectively. Along with the standard data for the anatase TiO2 (JCPDS 73-1764) as references.

Figure 4. (a) UV-vis absorbance spectra and (b) FI-IR spectra of STE, STE-2, STE-4, STE-6 and TE.


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Figure 5. Nitrogen adsorption–desorption isotherms of (a) STE, (b) STE-2, (c) STE-4 and (d) STE-6 and (e) TE. The insets are corresponding pore size distribution.

Figure 6. Emission spectra (a) and decay curves (b) of STE, STE-2, STE-4, STE-6, TE, respectively. The inset is corresponding asymmetric ratio versus different etching time. Table 1 Spectral parameters of STE, STE-2, STE-4, STE-6 and TE. Sample

A0-1(s-1)

A0-2(s-1)

A0-4(s-1)

Ω2 (10-20 cm2)

Ω4 (10-20 cm2)

R

STE STE-2 STE-4 STE-6

50 50 50 50

221.0 202.0 195.5 193.0

32.2 35.0 37.2 34.0

4.83 4.62 4.47 4.42

1.59 1.72 1.83 1.67

4.42 4.04 3.91 3.86



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TE

50

150.5

44.9

3.44

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2.21

3.01

Figure 7. Emission spectra (a) and decay curves (b) of STS, STS-2, STS-4, STS-6, TE, respectively.The inset is corresponding excitation spectra.

Figure 8. Emission spectra of STS, STS-2, STS-4, STS-6, TE respectively.


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Figure 9. (a) Photodegradation of MO by STE, STE-2, STE-4, STE-6 and TE (b) kinetic linear simulation curves of MO photodegradation with different photocatalysts under white light irradiation.

Figure 10. (a) Photodegradation of MO by STS, STS-2, STS-4, STS-6 and TS (b) kinetic linear simulation curves of MO photodegradation with different photocatalysts under white light irradiation.



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Figure 11. The SPC spectra of STE, STE-2, STE-4, STE-6 and TE.

Figure 12. XRD patterns of (a) pTE-700, pTE-800, pTE-900, pSTE-1000 and (b) STE-700, STE-800, STE-900, STE-1000, respectively. Along with the standard data for the anatase TiO2 (JCPDS 73-1764), rutile TiO2 (JCPDS 87-0710) and Eu2Ti2O7 (JCPDS 87-1852) as references.


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Figure 13. XPS spectra of (a) Ti 2p (b) Si 2p, (c) O 1s for STE, STE-2, STE-4, STE-6 and TE (d) O 1s for pTE-700.

Table 2. O 1s binding energies (B.E.), FWHM of core electrons and apparent surface coverage (ASC) of Ti–O–Si species for STE, STE-2, STE-4, STE-6.

Sample

O 1s B.E.(eV)

% Surface content

ASC

Ti-O-Ti Ti-O-Si Si-O-Si

(m2/g)

Ti-O-Ti

Ti-O-Si

Si-O-Si

STE

530.2

531.9

532.6

31.0

26.9

42.1

11.1

STE-2

530.6

532.1

532.9

35.9

21.8

42.3

10.0

STE-4

530.6

532.1

532.6

34.3

19.2

46.5

9.4

STE-6

530.6

532.1

532.6

30.0

17.7

52.3

8.5



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Figure 14. Schematic model of Ti-O-Si bonds and corresponding acidity sites. (a) Si in TiO2, (b) Ti in SiO2.

Figure 15. (a) Photodegradation of MO by ST, ST-2, ST-4, ST-6 and T (b) kinetic linear simulation curves of MO photodegradation with different photocatalysts under white light irradiation.


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Scheme 2. Influence factors of luminescence and photocatalytic properties.



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For Table of Contents Use Only

SiO2@TiO2:RE3+ multifunctional materials with different morphologies were prepared through surfactant-free method, the luminescence and photocatalytic properties have been discussed.


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Photocatalytic and Photoluminescence Properties of CoreâShell SiO2

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