SiO2@TiO2:Eu3+ and Its Derivatives: Design, Synthesis, and

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SiO2@TiO2:Eu3+ and its derivatives: design, synthesis and properties Meiqi Chang, Yanhua Song, Jie Chen, Lei Cui, Ye Sheng, Zhan Shi, and Haifeng Zou Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01149 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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Crystal Growth & Design

SiO2@TiO2:Eu3+ and its derivatives: design, synthesis and properties Meiqi Changa, Yanhua Songa, Jie Chena, Lei Cuia, Ye Shenga, Zhan Shib, Haifeng Zoua,* a

College of Chemistry, Jilin University, Changchun 130012, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China E-mail address:[email protected]. b

ABSTRACT SiO2@TiO2:Eu3+ core-shell structure was synthesized by a surfactant-free solvothermal method, and its luminescence intensities increased through coating

SiO2

layer.

Furthermore,

SiO2@TiO2:Eu3+@SiO2

core

double-shell structures with different coating thickness have been prepared to discuss the relationship between luminescence intensity and thickness. And the luminescence enhancement mechanisms including surface repair effect and excitation/emitted light absorption effect were proposed. Moreover, the luminescence properties and photocatalytic behaviours of SiO2@TiO2:Eu3+ core-shell structure and its derivatives: SiO2@TiO2:Eu3+ yolk-shell structure and TiO2:Eu3+ hollow structure have been investigated in detail. It is worth mentioning that yolk-shell and hollow structures could be obtained through changing the order of calcination and etching process of SiO2@TiO2:Eu3+ core-shell structure. Herein, hollow structure showed the highest luminescence intensity and core-shell structure exhibited excellent photocatalytic efficiency for the degradation of methyl orange.

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INTRODUCTION Titanium dioxide is a versatile material due to its photoreactivity, high chemical stability, unique optical, electrical and photochemical properties, and relatively low-cost, which can be utilized in many applications such as photoluminescence, photocatalysis and dye-sensitized solar cells.1-10 Among the three crystal phases of titanium dioxide, the photocatalytic and photoluminescence efficiency of anatase is well known to be better than those of rutile and brookite, because of its higher reduction potential, lower recombination rate of electron-hole pairs and higher electron mobility.2 However, bulk TiO2 nanoparticles usually have a spontaneous aggregation and exhibit low utilisation of light which would limit its application. At present, many applications depend not only on the properties of TiO2 itself but also on the modifications of the TiO2 material. Among them, core-shell materials have been proved to improve the catalytic and luminescence performance of TiO2 host material because of its unique configuration and properties. Among various core materials, SiO2 has received much attention because of its thermal stability and good absorption, and can be etched with alkaline solution which is conducive to the formation of special morphologies. In addition, as a shell material, SiO2 can repair the surface defects of host material, thereby improving its performance. Recently,

rare-earth

activated

oxide

compounds


have

attracted

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considerable attention due to their potential applications in optoelectronic devices, catalysts, and bio-imaging probes.11-20 However, there is a challenge to improve the luminescent intensity and decay time efficiency effectively because of surface defect effect, which would cause fluorescence quenching. Many efforts have been devoted to solve this problem. For example, Wang et al reported that red upconversion luminescence is selectively enhanced by about 7 times via Fe3+ co-doping into NaYF4:Yb,Er nanocrystalline lattice.21 Lin et al have reported that YOF:Eu3+ nano/micro-crystals with different morphologies possess different luminescence intensity.22 Bednarkiewicz et al have reported that 40-fold up-conversion intensity enhancement may be obtained for the α-NaYF4:Yb3+,Tb3+@NaYF4 compared with the α-NaYF4:Yb3+,Tb3+ nanoparticles.23 Among them, heterogeneous structure coating methods are attracting extensive attentions because of its different chemical properties and dual functional properties. SiO2 layer possesses wide transparency range, and silicate species would penetrate into surface pores and condense there during the addition of the outer protective silica coating.24 These features make silica become a suitable surface modified material which would reduce the effect of surface defect on the fluorescence intensity.25 Some literatures about phosphors with enhanced luminescence coated with SiO2 layer have been reported.26-28 And TiO2:Eu3+@SiO2 luminescence enhancement materials have been



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reported in our previous work.29 However, the detailed mechanism of luminescence enhancement and the dependence of the luminescence intensity on the coating thickness have not been involved previously. Moreover, TiO2 hollow structure have received much attention because of their low effective density, efficient light utilization and high specific surface area. However, the difference in preparation methods may bring about disparate properties. Yin et al have proposed “silica-protected calcination” process,24 which could maintain the hollow sphere morphology, restrict the growth of TiO2 and prevent interparticle coalescence. As you know, the morphology has a great influence on luminescence and photocatalysis properties.11,24,30,31 Herein, SiO2@TiO2:Eu3+ core-shell structure and its derivatives: SiO2@TiO2:Eu3+@SiO2 core double-shell structure, SiO2@TiO2:Eu3+ yolk-shell structure and TiO2:Eu3+ hollow structure were synthesized. It is important that silica coating has a dual effect: repair of surface defects and maintenance of morphological integrity. The luminescence properties of SiO2@TiO2:Eu3+@SiO2 with different coating thickness have been discussed and luminescence enhancement mechanism was proposed. Moreover, products with different structures can be obtained by merely changing the order of calcination and etching, And luminescence and catalytic properties of core-shell, yolk-shell and hollow structures were discussed in detail.


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EXPERIMENTAL SECTION Reagents All chemicals were analytical grade and were used as receive. Tetraethyl orthosilicate (TEOS) was bought from Internet Aladdin Reagent Database Inc. Tetrabutyl titanate (TBOT), absolute ethanol, isopropyl alcohol, ammonium hydroxide, cetyltrimethylammonium bromide (CTAB) and Eu2O3 (99.99%) were purchased from Beijing Chemical Co. The Eu(NO3)3 aqueous solution was obtained by dissolving Eu2O3 in dilute HNO3 solution under heating with vigorously agitation. Deionized water was used in all experiments.

Synthesis of the SiO2 spheres Silica spheres are synthesized by modified Stöber method. First, 0.35 mL of NH4OH solution (28%) and 2.5 mL of deionized H2O were added into 20 mL isopropyl alcohol under stirring, Then 5 mL of TEOS was added dropwise into the above solution at room temperature under magnetic stirring. The mixture was stirred for an additional 10 h over the transparent solution turned opaque. Finally, the products were centrifugally separated from the suspension and washed with water and ethanol for several times, and then dried in air at 60 °C for 12 h.



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Synthesis of the SiO2@TiO2:Eu3+ core-shell spheres SiO2@TiO2:Eu3+ core-shell spheres were fabricated using facile solvothermal method. In a typical procedure, 0.25 g as-prepared SiO2 powders and 0.75 mL deionized water were dispersed ultrasonically in 37.5 mL ethanol for 20 min. After that, 1.25 mL of TBOT and 0.148 mL of Eu(NO3)3 (0.5mol/L) (the molar ratio of Eu3+ /Ti4+ was 2%) were added dropwise to the mixture under vigorous stirring, the mixture was stirred vigorously for 20 min and then transferred into a Teflon stainless steel autoclave with a capacity of 50 mL and maintained at 140 °C for 3 h, and then cooled to room temperature naturally. Finally, the white product washed with deionized water and absolute ethanol four times, which was designated as ST. And the dried sample calcined at 700 °C for 3 h was designated as STc. In order to discuss concentration quenching effect, varying concentrations of Eu(NO3)3 solutions (the molar ratio of Eu3+ /Ti4+ = 1, 5, 7 and 9%) were added, which were designated as STc-1%, STc-5%, STc-7% and STc-9%, respectively.

Synthesis of the SiO2@TiO2:Eu3+@SiO2 core-double-shell spheres SiO2 layer was coated on the surface of ST/STc by the sol-gel method. Typically, the as-prepared ST/STc templates (0.12 g) and CTAB (0.33 g) were dispersed in a mixture of deionized water (24 mL) and ethanol (36 mL) under vigorous stirring, then 0.38 mL of aqueous ammonia was


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added into the above solution, after stirring for 10 min, different amount of TEOS has been added, the above solution was stirred for 4 hours and isolated by centrifugation, washed four times with deionized water and ethanol, dried at 60 °C for 12 h, and finally calcined at 700 °C for 3 h. By controlling the amount of TEOS at 0.165, 0.33 and 0.66 mL, the as-prepared products were denoted as STSc-1, STSc-2, STSc-3/STcSc-1, STcSc-2, STcSc-3, respectively.

Synthesis of the yolk-shell SiO2@TiO2:Eu3+ and hollow TiO2:Eu3+ spheres Products obtained through different methods have been shown in Scheme 1. SiO2@TiO2:Eu3+ spheres were calcined at 700 °C for 3 h and then etched with NH4OH solution (1 M) in a water bath at 90 °C for 4 h, which were designated as T-c-e. SiO2@TiO2:Eu3+ spheres were etched with NH4OH solution (1 M) in a water bath at 90 °C for 4 h firstly, and then calcined at 700 °C for 3 h, which were designated as T-e-c. In addition, in order to study the effect of SiO2 layer on the morphology and particle size of the samples, STSc-2 core-double-shell spheres were etched with NH4OH solution (1 M) in a water bath at 90 °C for 4 h, which were designated as TS-c-e. Characterization X-ray diffraction (XRD) patterns data were recorded on a Rigaku



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D/max-B II X-ray diffractometer with Cu-Kα radiation (λ = 0.15405 nm), scans were made from 10° to 70° (2θ). The morphology and size of the as-obtained samples were characterized on a Hitachi S4800 scanning electron microscope 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. Fourier transform-infrared radiation (FT-IR) spectrometer (SHIMADZU, 1.50SU1, Japan) was used to identify the vibration in functional groups presented in the samples. The Brunauer–Emmett–Teller (BET) specific surface area of the sample was determined through nitrogen adsorption (Micromeritics, ASAP 2010). The UV–vis diffuse absorbance spectra were acquired with a UV-3600 spectrophotometer (SHIMADZU). X-ray photoelectron spectra (XPS) were taken using a VG ESCALAB 250 electron energy spectrometer with Mg Ka (1253.6 eV) as the X-ray excitation source. Photoluminescence (PL) excitation and emission spectra were recorded with a Jobin Yvon FluoroMax-4 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source.

Photocatalytic Tests Photocatalytic decomposition of methyl orange (MO) was carried out in a quartz beaker containing a suspension of 20 mg of sample in 20 mL of MO solution (10 mg/L) under white light irradiation (a Xenon lamp


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(CHFXQ 500 W, Global Xenon Lamp Power)). Before the irradiation, the solution was mixed thoroughly in an ultrasonic water bath for 5 min and then magnetically stirred for 1 h in the dark to ensure the absorption–desorption equilibrium between the organic molecules and the catalyst surfaces. 1 milliliter of solution was drawn from the mixture at intervals of 5 min and centrifuged. The solutions were analyzed by UV-Vis spectra.

RESULTS AND DISCUSSION SiO2@TiO2:Eu3+ core-shell spheres SiO2 spheres with an average size of 310 nm were used as core templates, the SEM micrographs for the as-formed SiO2@TiO2:Eu3+ with different amount of TBOT are shown in Figure 1(A-F), Obviously, just a few TiO2 nanoparticles have adhered to the surface of SiO2 spheres when the amount of TBOT equals to 0.125 mL. By increasing the amount of TBOT, the samples consist of well separated spherical particles with a narrow size distribution and more and more TiO2 nanoparticles have coated on the SiO2 spheres. The results indicated that it is possible to tune the coating thickness by merely varying the amount of TBOT. And X-ray diffraction studies were carried out to investigate the crystallization process. The change of diffraction peaks relates to the phase transition of TiO2 due to the amorphous state of SiO2.3 As shown in Figure 2(a),



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ST-0.125 and ST-2.5 both displayed a broad peak between 15° and 35°, which corresponds to the amorphous phase of SiO2. The diffraction peak intensity of titanium dioxide is much lower than that of amorphous phase of silica because of small amount of TiO2. When the amount increased to 0.5 mL, the ST-0.5 showed peaks at 2θ = 25.7, 38.4 and 48.3, which are attributed to the (101), (004) and (200) plane diffraction of the anatase, respectively. And peaks intensities increase as the coating amount increases. In addition, in order to study the relationship between the luminescence intensity and coating amount, the emission spectra of ST with different amount of TiO2 have been measured as displayed in Figure 2(b). Upon excitation at 393 nm, all samples exhibit red emission peaks at 578, 591, 612, 650, and 699 nm, corresponding to the 5D0–7FJ (J = 0, 1, 2, 3, 4) transitions of the Eu3+ ions, respectively. It should be mentioned that the emission peaks positions of all samples are consistent, however, the emission intensities are different. ST-1.25 exhibit the strongest emission intensity, which indicated that adequate TiO2 host matrix is the prerequisite for improving the luminescence intensity. It is well known that the crystal field environments of Eu3+ ions can be understand through comparison of the intensity of the magnetic dipole transition with that of the electric dipole transition. Herein, the electric dipole transition is dominant, which suggests that Eu3+ ions occupy low-symmetry sites without an inversion center in the TiO2 host matrix.


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SiO2@TiO2:Eu3+@SiO2 core-double-shell spheres The schematic 1 illustrates the major steps employed in the present work. More specifically, the synthesis consists of the following steps: (1) preparation of SiO2 spheres; (2) coating SiO2 with TiO2:Eu3+ via solvothermal method to form SiO2@TiO2:Eu3+core-shell structure; (3) coating the calcined /non-calcined core-shell structure with different thickness of mesoporous SiO2 layers accompanied by subsequent calcination process, and photoluminescence intensities can be improved inordinately. Figure 3 shows the SEM images of the composite materials: (a) ST, (b) STSc-1, (c) STSc-2, (d) STSc-3. TiO2 is deposited on the surface of the SiO2 spheres in the form of nanoparticles, as shown in Figure 3(a). The affinity could be attributed to the chelating effect of the deprotonated hydroxyl groups on SiO2 surface to unprotected Ti4+ sites.32 And the surface becomes smooth when silica is coated on ST (Figure 3(b-d)), moreover, the diameter of SiO2@TiO2:Eu3+@SiO2 increases with the increase of coating thickness of SiO2 layer. However, unsupported SiO2 spheres emerge and increase as the coating amount increases, this phenomenon can be attributed to the addition of excess silica which would nucleated independently. XRD patterns of ST, STSc-1, STSc-2 and STSc-3 are shown in



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Figure 4(A). All samples exhibited anatase phase (JCPDS no. 89-4921). Moreover, the intensities of peaks decrease with an increase of coating thickness of SiO2 layer, and the diffraction peaks of STSc series are broad compared to those of ST, which indicated that the SiO2 protecting layer would inhibit the growth of anatase grains. Simultaneously, the calcined SiO2@TiO2:Eu3+

core-shell

structure

and

the

corresponding

SiO2@TiO2:Eu3+@SiO2 composites displayed the similar crystal structure, as shown in Figure 4(B). In addition, the Scherrer equation is used to calculate the crystallite sizes: Dhkl = Kλ/βcosθ. Dhkl is the size along the (hkl) direction, K = 0.941, λ = 0.15405 nm, θ and β are the diffraction angle and full width at half-maximum. The corresponding parameters and sizes are listed in Table 1. It can be seen that crystallite sizes of both ST and STc decrease when SiO2 deposited on their surface, which further proved the grain inhibition effect of SiO2 layer. In order to investigate the dependence of the luminescence intensity on the SiO2 layer and coating thickness, the emission spectra of ST, STSc-1, STSc-2 and STSc-3 have been measured, as shown in Figure 5(a). It is obvious that all samples share an identical spectral pattern with distinct differences in the intensity. The emission peaks at 577, 591(596), 612, 651 and 702 nm are ascribed to the 5D0–7FJ (J= 0, 1, 2, 3, 4) transitions of the Eu3+ ions, respectively. It is clear that the ST core-shell structures show the weakest emission intensity, however, the intensity of


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Eu3+ increases 1.5, 6.6 and 2.4 fold in contrast to the corresponding SiO2@TiO2:Eu3+ structure after coating the SiO2 layer with different thickness, which demonstrates that appropriate coating thickness is essential for the improvement of luminescence intensity. Simultaneous, the luminescence intensities of the calcined SiO2@TiO2 core-shell composite (STc) and the corresponding coating products (STcSc-1, STcSc-2, STcSc-3) exhibit similar variation trend, as shown in Figure 5(b). Compared with STc, red down-conversion luminescence has been enhanced by 1.3, 2.2 and 1.8 times when the amounts of TEOS are 0.165 mL, 0.33 mL and 0.66 mL, respectively. Although the crystallinity of STcSc-2 is much better than that of STSc-2 which can be seen through increased intensity and sharpness of the XRD peaks, the luminescence intensity of STSc-2 is stronger, which can be attributed to the surface defect effect. As you know, SiO2 coating could prevent the destruction of the morphology and the formation of surface defect during calcination, it is important to note that there has intimate relationship between emission intensity and surface defect. The first calcination process for STcSc-2 promote the process of particle crystallization, thereby reducing surface defects, so the role of SiO2 layer on STSc-2 is more obvious. It is well known that the surface defect would lead to non-radiative recombination and luminescence quenching. Thus the number of electron-hole pairs via non-radiative recombination in STSc-2 is less than that in STcSc-2. As a



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result, the photoluminescence intensity of STSc-2 is higher than that of STcSc-2 (Figure 5(c)). The decay curves of the Eu3+ ions for the STSc-2 and STcSc-2 are performed at room temperature which have been shown in Figure 5(d). Both the curves can be fitted to a double exponential function as I(t) = I0 + A1exp(−t/τ1) + A2exp(−t/τ2), where τi (i = 1, 2) is the decay lifetime. The average lifetimes are determined to be 0.72 ms and 0.46 ms for the STSc-2 and STcSc-2, respectively. The longer lifetime of the former can be mainly attributed to the decrease of surface defect and the non-radiative transition rate due to the SiO2 coating, which is in accordance with the result of PL intensity. In order to have a better view of the products properties, FT-IR spectra have been measured to investigate the surface chemistry of core-shell structures (ST, STc) and core-double-shell structures (STSc-1, STSc-2, STSc-3, STcSc-1, STcSc-2, STcSc-3). As shown in Figure 6, the absorption bands at 1093, 800 and 463 cm-1 are observed in all spectra which can be ascribed to the anti-symmetric stretching vibration and symmetrical stretching vibration of Si−O–Si and stretching vibration of Ti−O–Ti, respectively.33,34 In addition, the peak at 962 cm−1 corresponds to stretching vibration of Ti–O–Si bond was observed,35 the bond intensities become stronger when coating the silica layer on core-shell structure, which implies that silica is strongly attached to the surface of the TiO2 shell by covalent bonds.


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It can be seen that the luminescence intensities of SiO2-coated doped samples increase to different extent compared with those of non-coated products. According to the experimental results above and relevant theoretical analysis, we have proposed the possible luminescence enhancement mechanism: (1) As you know, the electronic transitions between these 4fN levels will cause luminescence. However, due to incomplete shielding effect of 5s, 5p orbitals, the optical properties of Eu3+ are very sensitive to their local coordination environment occupied. The phosphor surfaces are chemically unstable and exist many unsaturated bonds, moreover, structure disorder and surface defects are inevitable when doping Eu3+ ions into the lattices of TiO2 NPs, which will decrease the luminescence efficiency of the phosphors. Silica coating has been practiced to alleviate these problems through stabilize the surface of SiO2@TiO2:Eu3+, repair the unsaturated bonds and eliminate the defect and accordingly improve the luminescence intensity and quantum efficiency. In brief, the SiO2 shell will protect the internal structure from the interference effect arise from outer environment, and weaken the surface effects of nanoparticles. (2) The Eu3+ ions in matrix have multiple sites, including interior and surface layer, as shown in Figure 7. The luminescence of Eu3+ ions originates from intra-4fn transitions influenced by crystalline fields. Red-emitting can be observed from Eu3+ ions within the interior structure



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because of the perturbation by the crystalline fields of the matrix. However, low luminescence efficiency are obtained from isolated Eu3+ ions due to the forbidden character of the 4fn transitions, meanwhile, some Eu3+ ions which near the surfaces also keep the “dormant” state due to less perturbation effect by the degenerated crystalline fields of the interfaces. Thus, before coating with SiO2 layer, because of the weak perturbation effect of the crystal field in the interface system, some Eu3+ ions near or on the surface layer don’t produce radiative transitions. However, the interface between TiO2 and SiO2 layer will generate ligand field after coating, in essential, the ligand field equivalent to the Ti-O-Si bond which formed in the interface. The evidences for the existence of ligand field come from FT-IR spectra and Ω2 values, respectively. The peak at 962 cm-1 confirmed the presence of Ti-O-Si bond, which can be observed in Figure 6, the bond can repair unsaturated bonds which reduce selection rules of radiative transitions, thereby the state of Eu3+ ions convert from “dormant” to “activated”,27 moreover, the spectral parameter Ω2 reflects the asymmetry of the local environment at the Eu3+ ions sites, which have been listed in Figure 5. All Ω2 values of coated hybrid materials are smaller than that of non-coated materials, which means that the symmetric nature of Eu3+ get better and the perturbation effect of the crystal field in the interface becomes larger, which enhance luminescence intensity effectively.


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Nevertheless, we have found an interesting phenomenon that the luminescence intensities increase when the amount of TEOS changes from 0.165 to 0.33 mL, but then decrease when the coating thickness increases to 0.66 mL, which means that negative factor must be considered. Before the excitation light arrived at rare earth ions, SiO2 layer would absorb part of light, meanwhile, emitted light absorption effect existed in the SiO2 layer which also reduce the utilization of light. The excitation and emitted light absorption effect become stronger when the thickness of SiO2 layer increases. Therefore, the change tendency of luminescence intensities can be attributed to the interaction of two opposing mechanisms. One mechanism is advantageous, namely, the SiO2 layer repair the unsaturated bonds and eliminate the defect and the state of Eu3+ ions convert from “dormant” to “activated” because of the existence of ligand field. And the effect of ligand fields would reach a maximum when the coating thickness increases to a certain extent, however, the further growth of thickness has no more effects on the luminescence intensities enhancement when the layer has provided appropriate ligand field. The other is the excitation and emitted light absorption effect which is unfavorable for luminescence. In conclusion, the positive factors are in a dominant position when the coating thickness is relatively small, which result in the enhancement of luminescence intensities. As the coating layer reaches a certain thickness, enough ligand



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field has been provided, at this time, the optimum luminescence intensity has been obtained. However, when the coating thickness further increases, the ligand field has no more effects and the excitation and emitted light absorption effect get stronger, which weaken the luminescence intensity.

Hollow

TiO2:Eu3+,

yolk-shell

SiO2@TiO2:Eu3+

and

core-shell

SiO2@TiO2:Eu3+ spheres In the preparation process of hollow TiO2:Eu3+ and yolk-shell SiO2@TiO2:Eu3+ spheres, the order of calcination and etching, as well as the presence or absence of SiO2 coating layer have different effects on the morphology and crystallinity of the samples. As shown in Figure 8, hollow structure can be observed when the core-shell structure was firstly etched before calcination process (T-e-c). (Figure 8(b)) However, when the SiO2@TiO2:Eu3+ spheres were firstly calcined before etching, yolk-shell SiO2@TiO2:Eu3+ spheres were obtained, as shown in Figure 8(a). Why different structures can be obtained through adjusting the order of etching and calcination? This phenomenon can be attributed to the effect of the calcination process on the SiO2 structure. The pre-calcination process improves the degree of condensation of Si-O-Si bonds, therefore, a longer etch time is needed to remove SiO2 completely compared with the sample was firstly etched before calcination process. However, TS-c-e spheres possess hollow structure (Figure 8(c)), it can be speculated that


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the coated SiO2 layer can reduce the condensation degree of Si-O-Si of SiO2 spheres in the calcination process. It is worth noting that the morphology of T-c-e has been destroyed, which is due to the structural rearrangement of TiO2:Eu3+ nanoparticles accompanied with the excessive crystallization and grain growth during calcination process.24 In addition, it is found that the SiO2 coating layer would protect the integrity of morphology effectively. In addition, the compositions of core-shell, yolk-shell and hollow structures were analyzed by EDX, as shown in Figure S1, which confirmed the presence of Ti, Si, O, and Eu elements in all products. The result indicated that a small amount of SiO2 still existed in the hollow structure because of the re-deposition of the silica during the etching process.36 which may act as bind material to maintain the integrity of the hollow structure. FT-IR spectra of T-c-e, T-e-c, TS-c-e and STc have been shown in Figure 9. The absorption bands observed around 471 cm-1 in all spectra are attributed to stretching mode Ti–O bond, confirming the formation of TiO2. Moreover, the peak at 1108 cm−1 corresponds to anti-symmetric stretching vibration of Si–O–Si bond still exists in the T-e-c and TS-c-e hollow spheres, which further confirmed the re-deposition of the silica. The results are consistent with that of EDX. In addition, the weak shoulder peak at 957 cm−1 can be observed in the spectra of T-c-e and STc, which is ascribed to the vibration of Si–O–Ti.



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The chemical compositions of the products were further analyzed by XPS, as shown in Figure 10. There are two symmetric peaks in the Ti 2p region, which are attributed to the Ti 2p3/2 and Ti 2p1/2. The binding energies of Ti 2p3/2, Ti 2p1/2 and O 1s are listed in Table 2. The splitting data are 5.6 eV, indicating a oxidation state of Ti4+ in the anatase TiO2. The O 1s spectra are shown in Figure 10(b), All spectra can be de-convoluted into three peaks, demonstrating that there are three kinds of O chemical states. Take T-c-e as an example, the major peak at 530.1 eV corresponds to Ti-O-Ti bond, peaks at 531.9 eV and 532.9 eV are attributed to Ti-O-Si and Si-O-Si bond, respectively. Similarly, Si-O-Si bond also existed in the hollow structure (T-e-c, TS-c-e). Which are consistent with the results of EDX and FT-IR. XRD are employed to investigated the crystallization behavior of hollow TiO2:Eu3+, yolk-shell and core-shell SiO2@TiO2:Eu3+ spheres, as shown in Figure 11. All products exhibit anatase phase. The average crystal sizes were 15.53, 14.15, 14.87 and 15.73 nm for T-c-e, T-e-c, TS-c-e and STc, respectively (Table 3). When coating the SiO2 layer on the SiO2@TiO2:Eu3+ spheres, the silica would penetrate into the surface pores and condense, which inhibited the growth of TiO2 grains during calcination process. Normal, the first etching process will dissolve SiO2, leaving the internal space to promote the growth of TiO2 grains during calcination process. However, interestingly, the crystal sizes of T-e-c are


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relative smaller compared with that of T-c-e. It is speculated that alkaline conditions have a certain destructive effect on grain size of TiO2. In addition, the calcination process increases condensation degree of Ti-O-Ti bonds, which means that TiO2 calcined structure is more stable compared with non-calcined samples, therefore, the destruction effect of the calcined samples by alkali is relatively small. The UV–Vis absorbance spectra are shown in Figure 12(a). All products exhibited optical absorption in the UV region, which can be ascribed to the band-band transition of TiO2 and relatively high band gap energy. The indirect optical band gap could be estimated through extrapolating the linear portion of the (F(R)hv)1/2 versus photon energy hv curve to zero, where F(R) is the Kubelka-Munk function: F(R) = (1-R)2/2R, and R is reflectance. The band gap energy were estimated to be 3.26, 3.28, 3.25 and 3.26 eV for T-c-e, T-e-c, TS-c-e and STc, respectively. (Figure 12(b)) It is worth noting that there is almost no considerable change in optical band gap, which is within the experimental error. Considering the relationship between band gap energy and crystal size,37 the invariance of band gap energy can be attributed to small differences in crystal size. Figure 13(a) shows the excitation (inset) and emission spectra of the T-c-e, T-e-c, TS-c-e and STc, respectively. The excitation spectrum monitored at the 5D0-7F2 transition consists of Eu3+ ions characteristic



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excitation peaks (382, 393, 414, 464, and 525 (534) nm), which are due to the intra-4f6 transitions from the 7F0 to 5G2, 5L6, 5D3, 5D2 and 5D1 levels. And several sharp peaks in the emission spectra are assigned to the 5

D0–7FJ (J = 0, 1, 2, 3, 4) transitions of the Eu3+ ions, respectively.

However, it should be noted that the products possess different luminescence intensities. Obviously, the T-e-c samples exhibit the highest emission intensity, the TS-c-e takes the second place and the STc possesses the lowest emission intensity. As you know, the particle size of the sample has a crucial role on luminescence intensity. The smaller the particle size, the more grain boundary barrier, which inhibits separation of photogenerated electron and hole and increases the radiative recombination probability of electron−hole pairs, luminescence intensity can be improved. As a result, the T-e-c spheres with the smallest crystal sizes present the strongest emission intensity. In conclusion, the emission intensity could be arranged as a function of morphologies: hollow structure > yolk-shell structure > core-shell structure. In addition, the luminescence properties of SiO2@TiO2:Eu3+ core-shell structure with different doping concentration have been discussed. It is clearly seen that the emission intensities increase with the increase in concentration of Eu3+, reaching a maximum at 7% and then decrease due to concentration quenching phenomena (Figure S2). Figure 13(b) displays the decay curves of the Eu3+ ions in the T-c-e,


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T-e-c, TS-c-e and STc, respectively. All of the curves can be well fitted into a double exponential function, implying that the lattice coordination environments of Eu3+ ions are different. The short lifetime and long lifetime correspond to the distorted lattice sites near the surface and the ordered lattice environment of Eu3+. And the average lifetimes for T-c-e, T-e-c, TS-c-e and STc are 0.46, 0.76, 0.48 and 0.44 ms, respectively. The lifetime is the inverse of the sum of the radiative transition probability and the non-radiative transition probability, and luminescence intensity is proportional to radiative transition probability.38 It is clear that the variation tendency of lifetimes is consistent with that of the luminescence intensities. Therefore, the non-radiative transition probability of Eu3+ in STc is the largest and lead to the shortest decay time. In order to investigate the photocatalytic properties of hollow TiO2:Eu3+, yolk-shell and core-shell SiO2@TiO2:Eu3+ spheres, the photocatalytic behaviors of STc, T-e-c, T-c-e and TS-c-e are studied for degradation of MO dye solutions under white light irradiation. Considering the pyrolysis and photolysis of MO, the blank test without catalyst has been performed (Figure S3). The degradation efficiency is 4.4%, which indicated that MO molecule is relative stable under white light irradiation. The maximal absorption peaks located at 464 nm in UV-vis spectra are used to evaluate their photodegradation at given time interval. As shown in Figure S4, the characteristic absorption intensity of



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MO decreases gradually with the irradiation time increases for all samples, and the degradation rates are 98%, 59%, 70%, 33% after 30 min irradiation, respectively (Figure 14(a)). The apparent first-order kinetic equation ln(C0/C) =kt is used to fit experimental data. And the decomposition rate constants (k) are displayed in Figure 14(b). The decomposition rate constant of STc is 10 times greater than that of TS-c-e. And the photodegradation rates of these products obey the following order: STc > T-e-c > T-c-e > TS-c-e. As you know, increased number density of redox reaction sites can be obtained through increasing the specific surface area of catalysts, which is helpful for improving the photocatalysis activities.39 Figure 15 presents the nitrogen adsorption–desorption isotherms of samples, which shows that the BET values of STc, T-c-e, T-e-c and TS-c-e are 41.11, 66.28, 144.89 and 51.77 m2/g, respectively. For hollow and yolk-shell structure, the photocatalytic behaviors obey the following order: T-e-c > T-c-e > TS-c-e. Herein, BET specific surface areas are considered as the key factors affecting the photocatalytic activities of samples. However, STc with the smallest BET surface area exhibited the best photocatalytic activities. The improved catalytic activities should be attributed to the existence of Ti–O–Si bonds in core-shell structure. The Ti–O–Si bonds generate additional surface acidity which serves to activate the oxidation of MO, in addition, intermediate oxygen atoms in the Ti–O–Si bonds can


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further facilitate the electron activation in the TiO2.40,41Although the Ti–O–Si bonds also exist in the T-c-e, the amount is reduced due to the etching process. In addition, the effect of doping concentration on catalytic performance is investigated. As shown in Figure 16(a), the degradation efficiencies are 80%, 93%, 98%, 80%, 67% and 57% after 30 min irradiation for STc-0%, STc-1%, STc-2%, STc-5%, STc-7% and STc-9%, respectively. The decomposition rate constants are determined as 0.052, 0.086, 0.130, 0.052, 0.037 and 0.027 min-1, respectively (Figure 16(b)). The improved photocatalytic activity can be ascribed to the introduction of Eu3+ ions, which would inhibit the recombination of photogenerated electrons and holes, and the trapped electrons can be transferred to the adsorbed O2 molecules to generate O2−•, as shown in following steps: Eu3+ + e- → Eu2+ Eu2+ + O2 → Eu3+ + O2−• The Eu3+/Eu2+ pair has a dual effect: separation of electrons and holes and generation of O2−•. However, the catalytic efficiency is not proportional to the dopant concentration. 2% is the optimum concentration, the efficiency reduced when the concentration further increased, because excess Eu3+ ions might cover the active sites or act as a recombination center, resulting in a reduction in separation efficiency of electrons and holes.42



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CONCLUSIONS In summary, SiO2@TiO2:Eu3+ core-shell structure was synthesized by a surfactant-free solvothermal coating method, moreover, the luminescence intensity of SiO2@TiO2:Eu3+@SiO2 core double-shell structure increased compared with that of SiO2@TiO2:Eu3+. And the relationship between luminescence intensity and coating thickness of SiO2 has been investigated. The luminescence enhancement mechanism including two opposing mechanisms was proposed and discussed in detail. Moreover, yolk-shell SiO2@TiO2:Eu3+ and hollow TiO2:Eu3+ can be obtained through changing the order of calcination and etching. The luminescence and photocatalytic behaviours of hollow structure, yolk-shell structure and core-shell structure have been discussed. It is observed that hollow TiO2:Eu3+ structure is more suitable for luminescence and core-shell SiO2@TiO2:Eu3+ structure exhibits the best photocatalytic activities, which suggests that functional-improved materials can be obtained through adjusting the morphology of samples.

ASSOCIATED CONTENT Supporting Information EDX spectra, emission spectra, UV-vis absorption spectra ACKNOWLEDGMENTS This work was financially supported by the National Natural Science ACS Paragon Plus Environment

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Foundation of China (Grant No. 21171066 and 21671078), Jilin Province Science

and

Technology

Development

Plan

Item

(Grant

No.

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

REFERENCES (1) Li, S. X.; Chen, J.; Zheng, F. Y.; Li, Y. C.; Huang, F. Y. Nanoscale 2013, 5, 12150. (2) Joo, J. B.; Zhang, Q.; Dahl, M.; Lee, I.; Goebl, J.; Zaera, F.; Yin, Y. D. Energy Environ. Sci. 2012, 5, 6321. (3) Joo, J. B.; Lee, I.; Dahl, M.; Moon, G. D.; Zaera, F.; Yin, Y. D. Adv. Funct. Mater. 2013, 23, 4246. (4) Chang, M. Q.; Song, Y. H.; Sheng, Y.; Chen, J.; Guan, H. X.; Shi, Z.; Zhou, X. Q.; Zheng, K. Y.; Zou, H. F. J. Phys. Chem. C 2017, 121, 2369. (5) Chen, H. Y.; Zhang, T. L.; Fan, J.; Kuang, D. B.; Su, C. Y. Acs Appl. Mater. Inter. 2013, 5, 9205. (6) Zheng, X. L.; Kuang, Q.; Yan, K. Y.; Qiu, Y. C.; Qiu, J. H.; Yang, S. H. Acs Appl. Mater. Inter. 2013, 5, 11249. (7) Wu, W. Q.; Xu, Y. F.; Rao, H. S.; Su, C. Y.; Kuang, D. B. J. Am. Chem. Soc. 2014, 136, 6437. (8) Yan, M.; Zou, H. F.; Zhao, H.; Song, Y. H.; Zheng, K. Y.; Sheng, Y.; Wang, G. J.; Huo, Q. S. Crystengcomm 2014, 16, 9216. (9) Dou, J.; Li, Y. F.; Xie, F. Y.; Ding, X. K.; Wei, M. D. Cryst. Growth Des. 2016, 16, 121. (10) Chen, C. D.; Xu, L. F.; Sewvandi, G. A.; Kusunose, T.; Tanaka, Y.; Nakanishi, S.; Feng, Q. Cryst. Growth Des. 2014, 14, 5801. (11) Zhao, Q.; Zheng, Y. H.; Guo, N.; Jia, Y. C.; Qiao, H.; Lv, W. Z.; You, H. P. Crystengcomm 2012, 14, 6659. (12) Reszczynska, J.; Grzyb, T.; Sobczak, J. W.; Lisowski, W.; Gazda, M.; Ohtani, B.; Zaleska, A. Appl. Catal. B-Environ. 2015, 163, 40. (13) Reszczynska, J.; Grzyb, T.; Wei, Z. S.; Klein, M.; Kowalska, E.; Ohtani, B.; Zaleska-Medynska, A. Appl. Catal. B-Environ. 2016, 181, 825. (14) Ma, Q. L.; Wang, J. X.; Dong, X. T.; Yu, W. S.; Liu, G. X. Adv. Funct. Mater. 2015, 25, 2436. (15) Liu, Y. W.; Ma, Q. L.; Yang, M.; Dong, X. T.; Yang, Y.; Wang, J. X.; Yu, W. S.; Liu, G. X. Chem. Eng. J. 2016, 284, 831. (16) Li, D.; Ma, Q. L.; Xi, X.; Dong, X. T.; Yu, W. S.; Wang, J. X.; Liu, G. X. Chem. Eng. J. 2017,



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309, 230. (17) Zheng, C. X.; Li, D.; Ma, Q. L.; Song, Y.; Dong, X. T.; Wang, X. L.; Yu, W. S.; Wang, J. X.; Liu, G. X. Chem. Eng. J. 2017, 310, 91. (18) Hou, Z. Y.; Deng, K. R.; Li, C. X.; Deng, X. R.; Lian, H. Z.; Cheng, Z. Y.; Jin, D. Y.; Lin, J. Biomaterials 2016, 101, 32. (19) Gai, S. L.; Li, C. X.; Yang, P. P.; Lin, J. Chem. Rev. 2014, 114, 2343. (20) Yang, D. M.; Ma, P. A.; Hou, Z. Y.; Cheng, Z. Y.; Li, C. X.; Lin, J. Chem. Soc. Rev. 2015, 44, 1416. (21) Tang, J.; Chen, L.; Li, J.; Wang, Z.; Zhang, J.; Zhang, L.; Luo, Y.; Wang, X. Nanoscale 2015, 7, 14752. (22) Zhang, Y.; Li, X. J.; Geng, D. L.; Shang, M. M.; Lian, H. Z.; Cheng, Z. Y.; Lin, J. Crystengcomm 2014, 16, 2196. (23) Prorok, K.; Bednarkiewicz, A.; Cichy, B.; Gnach, A.; Misiak, M.; Sobczyk, M.; Strek, W. Nanoscale 2014, 6, 1855. (24) Joo, J. B.; Zhang, Q.; Lee, I.; Dahl, M.; Zaera, F.; Yin, Y. D. Adv. Funct. Mater. 2012, 22, 166. (25) Chang, M.; Sheng, Y.; Song, Y.; Chen, J. Phys. Chem. Chem. Phys. 2017, 19, 17063. (26) Zhang, J.; An, L.; Wang, S. J. Alloy Compd. 2009, 471, 201. (27) Lü, Q.; Li, A.; Guo, F.; Sun, L.; Zhao, L. Nanotechnology 2008, 19, 205704. (28) Lü, Q.; Guo, F.; Sun, L.; Li, A.; Zhao, L. J. Appl. Phys. 2008, 103, 123533. (29) Chang, M. Q.; Song, Y. H.; Sheng, Y.; Chen, J.; Zou, H. F. Phys. Chem. Chem. Phys. 2017, 19, 17063. (30) Dong, P. Y.; Wang, Y. H.; Li, H. H.; Li, H.; Ma, X. L.; Han, L. L. J. Mater. Chem. A 2013, 1, 4651. (31) Cho, J. S.; Yang, K. M.; Kang, Y. C. Crystengcomm 2014, 16, 6170. (32) Zhao, W.; Feng, L. L.; Yang, R.; Zheng, J.; Li, X. G. Appl. Catal. B-Environ. 2011, 103, 181. (33) Ye, M. M.; Zhang, Q.; Hu, Y. X.; Ge, J. P.; Lu, Z. D.; He, L.; Chen, Z. L.; Yin, Y. D. Chem-Eur. J. 2010, 16, 6243. (34) Dong, W. J.; Zhu, Y. J.; Huang, H. D.; Jiang, L. S.; Zhu, H. J.; Li, C. R.; Chen, B. Y.; Shi, Z.; Wang, G. J. Mater. Chem. A 2013, 1, 10030. (35) Hamad, H.; Abd El-Latif, M.; Kashyout, A. E. H.; Sadik, W.; Feteha, M. New J. Chem. 2015, 39, 3116. (36) Leshuk, T.; Linley, S.; Baxter, G.; Gu, F. Acs Appl. Mater. Inter. 2012, 4, 6062. (37) Ullah, S.; Ferreira-Neto, E. P.; Pasa, A. A.; Alcantara, C. C. J.; Acuna, J. J. S.; Bilmes, S. A.; Ricci, M. L. M.; Landers, R.; Fermino, T. Z.; Rodrigues, U. P. Appl. Catal. B-Environ. 2015, 179, 333. (38) Wang, L. L.; Wang, Q. L.; Xu, X. Y.; Li, J. Z.; Gao, L. B.; Kang, W. K.; Shi, J. S.; Wang, J. J. Mater. Chem. C 2013, 1, 8033. (39) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851. (40) Qi, K. H.; Chen, X. Q.; Liu, Y. Y.; Xin, J. H.; Mak, C. L.; Daoud, W. A. J. Mater. Chem. 2007, 17, 3504. (41) Kibombo, H. S.; Peng, R.; Rasalingam, S.; Koodali, R. T. Catal. Sci. Technol. 2012, 2, 1737. (42) Huang, H. W.; Liu, K.; Chen, K.; Zhang, Y. L.; Zhang, Y. H.; Wang, S. C. J. Phys. Chem. C. 2014, 118, 14379.


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Figure 1. SEM images of SiO2@TiO2:Eu3+ core–shell nanostructure with different amount of TBOT (a) 0.125 mL, (b) 0.25 mL, (c) 0.5 mL, (d) 0.75 mL, (e) 1 mL, (f) 1.25 mL.



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Figure 2. XRD patterns and emission spectra of SiO2@TiO2:Eu3+ core–shell nanostructure with different amount of TBOT.

Scheme 1. Schematic illustration of the procedure for the synthesis of SiO2@TiO2:Eu3+@SiO2 core–shell nanostructure and yolk-shell SiO2@TiO2:Eu3+ and hollow TiO2:Eu3+ spheres.


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Figure 3. SEM images of (a) ST, (b) STSc-1, (c) STSc-2, (d) STSc-3.

Figure 4. XRD patterns of (A) ST, STSc-1, STSc-2, STSc-3 and (B) STc, STcSc-1, STcSc-2, STcSc-3. Table 1. Crystallite sizes and the corresponding parameters. Sample

2θ θ

β

Crystallite size

Sample

2θ θ

β

Crystallite size

ST STSc-1 STSc-2 STSc-3

25.321 25.373 25.527 25.424

0.570 0.582 0.645 0.659

14.85 14.57 13.16 13.05

STc STcSc-1 STcSc-2 STcSc-3

25.476 25.476 25.373 25.424

0.545 0.581 0.567 0.559

15.73 14.72 15.00 15.18



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Figure 5. (a) Emission spectra of ST, STSc-1, STSc-2 and STSc-3. (b) Emission spectra of STc, STcSc-1, STcSc-2 and STcSc-3. (c) Emission spectra of STSc-2 and STcSc-2. (d) Decay curves of the 5D0-7F2 emission of Eu3+ in the STSc-2 and STcSc-2 samples.

Figure 6. FT-IR spectra of (a) ST, STSc-1, STSc-2, STSc-3, and (b) STc, STcSc-1, STcSc-2, STcSc-3.


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Figure 7. The schematic diagram of luminescent enhancement mechanism.

Figure 8. TEM images of (a) T-c-e, (b) T-e-c, (c) TS-c-e.



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Figure 9. FT-IR patterns of T-c-e, T-e-c, TS-c-e and STc.

Figure 10. XPS spectra of (a) Ti 2p and (b) O 1s for T-c-e, T-e-c, TS-c-e and STc. Table 2. Ti 2p and O 1s binding energies (B.E.) T-c-e, T-e-c, TS-c-e and STc. Sample T-c-e T-e-c TS-c-e STc

Ti 2p B.E.(eV) Ti 2p3/2 458.8 459.0 459.0 458.9

Ti 2p1/2 464.4 464.6 464.6 464.5

O 1s B.E.(eV) Ti-O-Ti 530.1 530.4 530.5 530.2


Ti-O-Si 531.9 531.9 532.1 531.9

Si-O-Si 532.9 533.0 533.1 532.7

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Figure 11. XRD patterns of T-c-e, T-e-c, TS-c-e and STc. Table 3. Crystallite sizes and the corresponding parameters of T-c-e, T-e-c, TS-c-e and STc. Sample

2θ θ

β

Crystallite size

T-c-e T-e-c TS-c-e STc

25.476 25.321 25.476 25.476

0.549 0.604 0.573 0.545

15.53 14.14 14.87 15.73

Figure 12. (a) UV−vis diffuse absorbance spectra of T-c-e, T-e-c, TS-c-e and STc, (b) Kubelka−Munk plots and band gap energy estimation of samples for indirect transition.



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Figure 13. (a) Emission spectra of T-c-e, T-e-c, TS-c-e and STc, and the typical excitation spectrum (inset). (b) Decay curves of the 5D0-7F2 emission of Eu3+ in the T-c-e, T-e-c, TS-c-e and STc.

Figure 14. (a) Photodegradation of MO by different photocatalysts, (b) kinetic linear simulation curves of MO photodegradation with different photocatalysts under white light irradiation.


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Figure 15. Nitrogen adsorption–desorption isotherms of T-c-e, T-e-c, TS-c-e and STc.

Figure 16. (a) Photodegradation of MO by different photocatalysts, (b) kinetic linear simulation curves of MO photodegradation with different photocatalysts under white light irradiation.



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For Table of Contents Use Only SiO2@TiO2:Eu3+ and its derivatives: design, synthesis and properties Meiqi Changa, Yanhua Songa, Jie Chena, Lei Cuia, Ye Shenga, Zhan Shib, Haifeng Zoua,* a

College of Chemistry, Jilin University, Changchun 130012, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China b

E-mail address:[email protected].

The luminescence and photocatalytic properties of SiO2@TiO2:Eu3+ and its derivatives have been discussed.


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SiO2@TiO2:Eu3+ and Its Derivatives: Design, Synthesis, and

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