Photoluminescence and Photocatalysis Properties of Dual-Functional

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Photoluminescence and Photocatalysis Properties of Dual-Functional Eu Doped Anatase Nanocrystals 3+

Meiqi Chang, Yanhua Song, Ye Sheng, Jie Chen, Hongxia Guan, Zhan Shi, Xiuqing Zhou, Keyan Zheng, and Haifeng Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11013 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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The Journal of Physical Chemistry

Photoluminescence and Photocatalysis Properties 3+ Dual-Functional Eu Doped Anatase Nanocrystals

of

Meiqi Changa, Yanhua Songa, Ye Shenga, Jie Chena, Hongxia Guana, Zhan Shib, Xiuqing Zhoua, Keyan Zheng a, 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 Red color-emitting TiO2:Eu3+ nanophosphors have been synthesized through hydrothermal process. Samples with different morphologies have been obtained though changing pH values of solution. The structural and luminescence properties of samples were investigated through scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–vis spectra and photoluminescence (PL) spectra, respectively. Most importantly, surface photovoltage spectra were used to discuss the charge transfer process and further explain the luminescence phenomenon of different morphologies for the first time. The critical transfer distance for the Eu3+ ions in TiO2 matrix was calculated and we could conclude that the concentration quenching process was attributed to multipole–multipole interaction. The relative intensity ratios of 5D0-7F2 transition to 5D0-7F1 transition of TiO2:x%Eu3+ (x = 2, 4, 6, 8, 10, 12) have been calculated to understand the lattice symmetry and coordination environment of Eu3+ ions. The spectral characteristics and site symmetry have been discussed through the Judd–Ofelt parameters. Moreover, the investigation of photocatalytic ability of TiO2:x%Eu3+ showed that the photodegradation efficiencies decrease with an increase in the amount of Eu3+ for the degradation of methyl orange under white light irradiation, and the photodegradation efficiency of TiO2:2%Eu3+ was 99% after 25 min, which is much more higher than that for TiO2:12%Eu3+ (25%).

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1. Introduction Recently, RE3+ doped nanophosphors are attracting considerable attentions because of their potential applications in optoelectronic devices, flat plane displays, bioimaging and luminescence probes.1-11 Compared to dyes and quantum dots, RE3+ doped nanophosphors have showed unique luminescent properties, such as long decay time, narrow emission lines, large stokes shift and so on.12, 13 Meanwhile, with the development of nanotechnology, dimensionality, size and band gap engineering of the materials have been regarded as important factors which can modulate the optical properties of RE3+ doped nanophosphors. For example, Yang et al have reported Eu-doped ZnO with different morphologies, and the influence of morphology, diameter, and uniformity on luminescence intensity were discussed.14 Lin et al have synthesized YOF micro-rods and nanospheres through changing the pH values of the original solution, moreover, the morphology-dependent luminescence properties have been investigated in detail.15 Due to unique optical, electrical, and photochemical properties of TiO2 because of its unique band gap characteristics, TiO2 is suggested to be a promising matrix for the luminescence of RE3+ ions. Among the various lanthanide doped materials, the trivalent europium ions are attractive because of the high colour purity and luminescence efficiency.13 And the luminescence properties of Eu3+ ions are sensitive to their coordination environment and the crystal structure. Therefore, Eu3+ ions are well-established as a spectroscopic probe to survey the structure and nature of the chemical bonds in luminescent materials. Moreover, we can get some information about the polarization behavior and coordination environment of the Eu3+ ions through Judd–Ofelt parameters.16-18 Moreover, TiO2 possesses excellent oxidation capacity, chemical stability and long-term


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photostability, these features make it become an ideal candidate material for photocatalysis.19, 20 Recently, many researchers have paid a great deal of attention to synthesize semiconductor-based heterosturctures which are helpful for enhanced photocatalytic activity. For example, Ag/TiO2 heterostructures have been prepared by electrospinning and solvothermal method,21 and ZnO/NiO hollow nanofibers are successfully fabricated by electrospinning process, which possesses improved photocatalytic activity compared with pure TiO2.22 However, according to our best knowledge, photocatalytic properties of TiO2:Eu3+ have been rarely reported. As you know, TiO2 doped with RE3+ has proved to be an efficient process to improve the photocatalytic properties of TiO2 matrix because the f-orbitals of the RE3+ can form complexes with organic contaminants,23, 24 moreover, matrix modified with RE3+ ions inhibits electron–hole recombination.25 Herein, we have prepared the dual functionality of TiO2:Eu3+ nanocrystals, possessing both photoluminescence and photocatalytic properties. Specifically, a facile hydrothermal process were adopted to prepare TiO2:Eu3+ nanocrystals with different morphologies by changing the pH values of the original solution and the relationship between morphology and luminescence properties has been discussed in detail. It is worth mentioning that surface photovoltage spectra were used to explain the luminescence phenomenon of different morphologies for the first time. The growth mechanism has been proposed. The intensity parameters (Ω2, Ω4) and various other radiative parameters have been calculated through Judd–Ofelt theory, which is helpful for understanding the coordination environment of Eu3+ in TiO2 matrix. Moreover, the photocatalytic performances of TiO2:x%Eu3+ prepared at pH values of 9.2 were evaluated by using MO as a representative dye pollutant under white light irradiation, and the photodegradation efficiencies decrease with an increase in the amount of Eu3+.



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2. Experimental 2.1 Materials Tetrabutyl titanate (TBOT), triethanolamine (TEOA), sodium dodecyl benzene sulfonate (SDBS), absolute ethanol, HCl, NH3⋅H2O, Eu2O3 (99.99%) were purchased from Beijing Chemical Co. All chemicals were analytical-grade reagents and used directly without further purification. The Eu(NO3)3 aqueous solution was obtained by dissolving stoichiometric amounts of Eu2O3 in dilute HNO3 solution under heating with magnetic stirring. Deionized water was used for all treatment processes. 2.2 Synthesis of the TiO2:Eu3+ nanophosphors Firstly, 1.76 mL TBOT was dissolved in 1.33 mL TEOA to form a stable compound of Ti4+, after vigorous stirring for 10 min, 15mL deionized water was introduced to form a white precipitate, then 0.6 g of SDBS and varying concentrations of Eu(NO3)3 (2, 4, 6, 8, 10 and 12 mol% compared to the concentration of titania) was mixed with the above solution. The pH values of the solution were subsequently adjusted by adding HCl or NH3⋅H2O. (PH=9.2 obtained through adding HCl, PH=9.7 obtained without adding HCl or NH3⋅H2O, PH= 10.3, 11.0, 11.5, 12.0 obtained through adding NH3⋅H2O). After additional agitation for 30 min, the final solution was put into a Teflon stainless steel autoclave kept at 180 °C for 12 h. The autoclave was then cooled to room temperature naturally. The white products were centrifuged with deionized water and absolute ethanol several times, and then collected and dried in air at 60 °C for 12 h. The finally products were then annealed at 600 °C in air for 3 h. 2.3 Characterization The samples morphologies were characterized through scanning electron microscope (SEM, S-4800, Hitachi). The crystalline structure of the products was identified with a Rigaku D/max-B


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II X-ray diffractometer with Cu-Kα radiation (λ = 0.15405 nm) in a wide range of angle 2θ (10° -70 ° ). The UV–vis diffuse reflectance spectra were recorded using Shimazu UV-3600 spectrophotometer in the wavelength range 250-800 nm. Photoluminescence (PL) excitation and emission spectra were measured using Jobin Yvon FluoroMax-4 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The lock-in amplifier-based SPV measurement was carried out on home-made systems. Typically, the SPV measurement system includes a 500 W xenon lamp (LSH-X500, Zolix), a lock-in amplifier (SR830-DSP, Stanford) with a light chopper (SR540, Stanford), a grating monochromator (Omni-5007, Zolix), a photovoltaic cell, and a computer. A low chopping frequency of 23 Hz was used in the conventional testing.

2.4 Photocatalytic activity

Photocatalytic decomposition of methyl orange (MO) was performed in a quartz beaker containing a suspension of 20 mg of sample in 20 mL of MO solution (20 mg/L) under white light irradiation (a Xenon lamp (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 keep the absorption–desorption equilibrium between the MO molecules and TiO2:x%Eu3+ (x = 2, 4, 6, 8, 10 and 12) surfaces. One milliliter of solution were extracted and centrifuged to separate the catalysts every five minutes. The centrifuged solutions were analyzed by recording the variations of absorbance of MO at 464 nm in the UV–vis spectra. As a comparison, the photocatalytic activity of P25 was also measured using the same parameters.

3. Results and discussion



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3.1 Phase Identification The crystallinity, composition and phase purity of the TiO2:2%Eu3+ were first examined by XRD. Figure 1 shows the XRD patterns of TiO2:2%Eu3+ obtained at different pH values. (9.2, 9.7, 10.3, 11.0, 11.5, 12.0). It can be seen that the diffraction peaks of all the samples match closely the anatase phase of TiO2 according to the JCPDS card No.89-4921 and no additional peaks for others phases can be identified, indicating the high purity and crystallinity of the products. The XRD datas were indexed on a tetragonal structure (space group: I41/amd (No. 141)) having cell parameters a = 3.777 Å, b =3.777 Å, c = 9.501 Å.

3.2 Morphology analysis

Generally, the successful synthesis of nanomaterials depends on the structure of the raw materials and the control of external conditions, such as surfactants, temperature, pH values and so on. Figure 2 shows the SEM images of TiO2:2%Eu3+ prepared at different pH values. We can notice that the pH values of original solution have important effects on the morphology and particle size of the product. As shown in Figure 2a, the obtained product prepared at pH = 9.2 was composed of uniform nanorods of about 45 nm in length and 30 nm in width. As the pH value was increased to 9.7 and 10.3 (Figure 2b, 2c), the sizes have increased to 100∗30, 150∗30, respectively. It can be seen that the mean aspect ratios of products increase with the pH values increase. As you know, A high pH value means a high concentration of hydroxide ions, which will lead to rapid formation of nuclei, the products with high aspect ratio can be obtained. When the pH value is increase to 11.0 and 11.5 (Figure 2d, 2e), the nanorods aggregate gradually and straw-sheaf structure have formed. When pH = 12.0 (Figure 2f), flower-like microstructures composed of a conjunction of nanorods can be obtained through aggregate and self-assembled process.


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3.3 Formation mechanism Based on the pH-change experiments, a possible formation mechanism of TiO2:2%Eu3+ nanoparticles, nanorods and micro-flowers is illustrated in Figure 3. In general, the crystal growth can be classified into two important steps: nucleation and growth. Firstly, nucleation process is the premise of crystal growth stage. Secondly, crystal growth is governed by kinetically and thermodynamically controlled process. In this article, the stable complex precursor has formed in the initial stage, TEOA as a stabilizer of Ti4+ which prevent its rapid hydrolysis. Because the hydrothermal process provides a high temperature and high pressure environment which weaken the chelating ability of complex, TEOA molecules were released gradually. Meanwhile, the Ti(OH)4 gel formed. And Eu3+ ions were introduced into Ti(OH)4 gel matrix through forming the Ti-O-Eu bond.26 The growth of TiO2 happened after the decomposition of the Ti(OH)4 gel.27 The surfactants cause a slower growth of the absorbed plane and a faster growth of the unabsorbed plane through preferential absorbed on some crystal facets. SDBS can selectively adsorb on the planes parallel to the c-axis and change the surface free energy, so the TiO2 nuclei aggregated to grow along this direction through assembly and finally nanorods have formed. However, when we change the pH values, morphology become different. Under low pH values, the chelating capabilities of SDBS are effectively inhibited, the nanorods with low aspect ratio can be obtained. And the aspect ratios of nanorods increase with pH values increase. When the hydroxide ions are sufficient, the nanorods aggregated into straw-sheaf structure and flower-like microstructures along their cross-sectional diameter direction through self-assembly process.

3.4 UV–vis spectra

In order to investigate the optical absorption properties of Eu doped titania samples, the diffuse



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reflectance spectra were investigated in the range of 250–800 nm. Figure 4a shows the UV–vis DRS spectra of TiO2:2%Eu3+ prepared at different pH values. All samples show strong DRS signals in the range of 250–400 nm, which belong to the band-band transition of TiO2 nanocrystals. We observed that the all TiO2:2%Eu3+ samples had no absorption in visible light region because of their relatively high bandgap energy. The optical absorption band gap is the lowest energy of the transition from the valence band (ground state) to the conduction band (exciting state), which can be obtained through the following equation:

where α represent the absorption coefficient, hv is the photo energy, B is a constant relative to the material, Eg is the band gap, and the exponent n of 2 and 1/2 is related to the direct and indirect transition of the electron, respectively.28 Anatase is an indirect-allowed transition semiconductor, the value of n is 1/2, therefore, the band gap of TiO2:2%Eu3+ with different pH values can be obtained through extrapolating the linear portion of the (αhv)1/2 versus hv curve to zero. The Eg of TiO2:2%Eu3+ are calculated to be about 3.17 eV, 3.20 eV, 3.22 eV, 3.23 eV, 3.25 eV, 3.26 eV for samples obtained at different pH values, respectively, which can be seen in Figure 4b-4g. The band gap shifts to high energy as the pH values increases. Therefore, we can conclude that morphology, doping effect and defect chemistry will have an unavoidable effect on the band gap values. 3.5 Luminescence Properties In order to study the relationship between the morphologies and luminescence intensity, the excitation and emission spectra of the TiO2:2%Eu3+ nanocrystals obtained at different pH values have been performed which can be seen in Figure 5. As for pH = 9.2, monitored with 611 nm


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emission of Eu3+ (5D0-7F2), the excitation spectrum (Figure 5a) shows characteristic excitation peaks of Eu3+ ions (384, 394, 413, 463, and 525 (531) nm), which are ascribed to the intra-4f6 transitions from the 7F0 to 5G2, 5L6, 5D3, 5D2, and 5D1. Upon excitation at 394 nm, a group of lines peaking at 578, 591, 611, 651 and 691 (702) nm can be observed at the emission spectrum, which correspond to the 5D0-7FJ (J = 0, 1, 2, 3, 4) transitions, respectively.27 Because of small energy gaps between the 5L6, 5D3, 5D2, 5D1 and 5D0 levels, luminescence results from radiation transition could be observed due to 5D0→7FJ (J = 0–4) transitions when the excitation energy was transferred to the 5D0 level non-radiatively. In general, the magnetic dipole transition (5D0-7F1) is insensitive to the local symmetry in the crystal field, which is dominant if Eu3+ ions occupy an inversion symmetry site, while the electric dipole transition (5D0-7F2) is hypersensitive to the surrounding coordination environment of the Eu3+ ions, it will become the dominant transition if Eu3+ ions hold the site without inversion symmetry. Moreover, we can observe that the 5D0-7F2 transitions is the strongest emission, which indicates that Eu3+ ions prefer to occupy a relative low symmetry site29. It is well known that TiO2 possesses a tetragonal structure (space group: I41/amd), which offers the Ti4+ ions a crystal site with a D2d point symmetry. However, the introduction of larger Eu3+ ions (0.098 nm) replaced small Ti4+ ions (0.061 nm), which would create oxygen vacancies and cause the lattice distortions in the TiO2 matrix, which makes the symmetry of Eu3+ ions change from D2d to lower symmetry site. The emission spectra of the TiO2:2%Eu3+ obtained at different pH values can be seen in Figure 5b, all of which have similar peak position, however, the fluorescence intensities were significantly different. Largest surface area can be obtained when lots of nanorods self-assembled into micro-flowers structure. And straw-sheaf split is smaller than that of micro-flowers, it possesses a relative lower surface area. The nanorods own lowest surface



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area. Generally speaking, the large surface area means the existence of a larger number of defects, which may result in non-radiative recombination of electrons and holes and a decrease in luminescence intensity. Therefore, the luminescence intensity were in the order: nanorods> straw-sheaf structure> micro-flower structure. In addition, the unique flower-like hierarchical architectures and straw-sheaf structure cause reflection loss of luminescence and reduce the luminescence efficiency, which can further explain the phenomenon. As we know, the smaller the particle size, the more grain boundary barrier, which has affected the transport and separation of photogenerated electron and hole seriously, and increases the radiative recombination probability of electron-hole pairs, so the luminescence intensity can be improved effectively. The SPV spectroscopy provides information of the carrier separation and transfer behavior of TiO2 semiconductors with the aid of incident light.30-32 In order to obtain the photovoltage signal, the following steps are needed: The absorption of incident light will cause the formation and separation of photogenerated charge carriers, which produces an induced photovoltage. The surface potential barrier of the sample before and after the light irradiation will affect the signal of the surface photovoltage. Based on the principle of SPV, the SPV response intensity is related to the degree of charge separation. In other words, the SPV response intensity implies the separation capability of photogenerated electron-hole pairs. The stronger the surface photovoltage signal, the faster the photogenerated carrier separation. The SPV responses of the samples with different morphology are shown in Figure 6. The signal of all samples appeared below 400 nm attributed to the electronic transition from the O 2p orbital to the Ti 3d orbital. We can notice that the responses increase in some degree with the pH values increase, which means that separation capability of


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photogenerated electron-hole pairs is weakest when the pH values is lowest. The result obtained from surface photovoltage spectra are consistent with luminescence properties about samples with different pH values. 3.6 Concentration quenching of Eu3+ in TiO2:Eu3+ In order to discuss concentration quenching effect, decay behaviors and spectral properties, we adopted a series of TiO2:x%Eu3+ samples which prepared at pH = 9.2. The effect of the doping concentration on PL emission intensities has been investigated, as shown in Figure 7a. It was clearly seen that the emission intensities increase with the increase in concentration of Eu3+, reaching a maximum at 10 mol% and then decrease due to concentration quenching phenomena relying on the non-radiative energy transfer between the adjacent activator ions (Figure.7b). The effective energy transfer distance between Eu3+ ions called a critical transfer distance (Rc) which given by Blasse33: 1

 3V  3  R c = 2  4πNX c  Where V is the volume of the unit cell, N is the number of cations in the unit cell and Xc is the critical concentration. In the case of the TiO2:10%Eu3+, values of V, Xc and N are 135.54 Å3, 0.1 and 4, respectively. Therefore, the Rc value of sample is calculated to be 8.7 Å. For the non-radiative energy transfer, multipole–multipole interaction mechanism and exchange interaction mechanism are involved.34 Because Eu3+-Eu3+ distance is greater than 4 Å, thus the latter is invalid. Therefore, energy transfer among Eu3+ ions in the TiO2:10%Eu3+ will be caused by electric multipolar interaction only. The relative intensity ratio of electric dipole transition to magnetic dipole transition can be used as probe to understand the symmetry and coordination environment of Eu3+ ions in the TiO2 host lattice.35, 36 This parameter called the asymmetric ratio



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(R).

∫ R= ∫

637

602 602

581

I 0-2 dλ I 0-1 dλ

where I0-1 and I0-2 are the respective integrated intensities of electric dipole transition and magnetic dipole transition of Eu3+, respectively. The consequence of that is explained as follows: the high R values indicate that the Eu3+ ions are located in a distorted lattice environment and the products possess good colour purity.37 The variation of R values with Eu3+ ion concentration (Eu3+ = 2, 4, 6, 8, 10 and 12 mol%) is shown in Figure 7c. It is clear that the asymmetric ratios for samples decrease with increase in Eu3+ concentration. A decrease in the ratios is attributed to the increase in the symmetry around the Eu3+ ions. Figure 7d shows the XRD patterns of the TiO2:x%Eu3+ (x = 2-12). The crystalline phases of all samples can be indexed to the anatase phase of TiO2, however, the intensities of the XRD diffraction peaks decrease with the increase of Eu3+ doping concentration. And no traces of impurities are detected when samples with higher Eu3+ amount, which indicate that the host matrix have larger capacity of Eu3+ ions.38 3.7. Decay time The decay behaviors for 5D0-7F2 (612 nm) of TiO2:x%Eu3+ with x = 2-12 were investigated at room temperature excited by 393 nm, which can be seen in Figure 8a, the curves of all samples can be fitted well by second-order exponential function: I(t) = I0 + A1exp(-t/t1) + A2exp(-t/t2) Where I(t) and I0 are the luminescence intensities at times t and 0, A1 and A2 are constants, t is the time, and t1 and t2 are two components of the decay time. The short lifetime indicates that the Eu3+ ions exist at the distorted lattice sites near the surface, however, the Eu3+ ions with long lifetime


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may locate at the ordered lattice environment.39 Furthermore, the average decay lifetimes (t) can be calculated as t=(A1t12 + A2t22)/(A1t1 + A2t2) Therefore, the average lifetimes of Eu3+ ions in TiO2:x%Eu3+ (x = 2, 4, 6, 8, 10, 12) can be determined to be 0.669, 0.890, 1.023, 1.173, 1.109 and 1.023 ms, respectively (Figure 8c). And as shown in Figure 8b, the average lifetimes increase with the increase of the Eu3+ concentration initially, reaching the maximum at the Eu3+ concentration of 8%, and then dropped gradually. In contrast, the PL intensity of the 5D0-7F2 transition reaching the maximum at the concentration of 10%. This phemomenon is not consistent with previous reports in which the phosphors with the strongest luminescence intensity possess the longest lifetime.40-42 And Dong et al has reported similar extraordinary phenomenon.43, 44 The amount of luminescent centers, surface defects, and morphology have important effects on the decay behavior of the phosphors.45, 46 Therefore, we can conclude that the extraordinary result should be the end product of all these factors. 3.8. Judd–Ofelt intensity parameters

The Judd–Ofelt parameters which obtained through emission spectra are useful tools to understand the symmetry and coordination environment of Eu3+ ions in the TiO2 matrix.47-49 In this part, J–O parameters (A01, A02, A04, Ω2 and Ω4) were calculated and according to this parameters, other radiative properties have been predicted.

The magnetic dipole transition rate (A01) of 5D0-7F1 can be written as：

A01 =

64π 4υ13n 3Smd 3h( 2 J + 1)

Where A01 is the Einstein’s coefficient of spontaneous emission between the 5D0 and 7F1 energy



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level, which has a value of 50 s-1 and as a reference to calculate A0J (J = 2, 4).50 Smd is the strength of the 5D0-7F1 transition, which does not vary with the coordination environment and being equal to 9.6*10-42 units,51 n refers to the refractive index of the product and h represent the Planck's constant. The radiative emission rates (A0J) can be written as:

A0 J =

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

∑ Ωλ λ

5

D0 U ( λ ) 7 FJ

2

=2 , 4

Where e is the charge, νJ is the transition wavenumber,

5

D0 U（λ） 7 FJ

2

(J = 2, 4) represent the

square reduced matrix elements which independent of the surrounding environment of the Eu3+ ions,52 and the values are 0.00324 and 0.00229. A02, A04, Ω2 and Ω4 can be obtained through the following equation:

A0 J e 2 υ J ( n 2 + 2) 2 = Ωλ A01 Smd υ13 9n 2 3

5

D0 U ( λ ) 7 FJ

2

=

∫ I dυ ∫ I dυ J

1

The Judd-Ofelt parameters can be used to calculate some important radiative properties such as radiative transition probability (ARAD), total transition probability (AT), radiative lifetime (τrad) and branching ratio (β0J). The ARAD is defined as the sum of the radiative emission rates A0J:

ARAD = ∑ A0 J J

The AT could be estimated by using following equation:

AT =

1

τ obs

= ARAD + ANRAD

Where τobs is experimental luminescence lifetime of 5D0 and ANRAD refers to non-radiative transition rate. The τrad is related to the ARAD and can be expressed as:


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τ rad =

1 ARAD

The branching ratio (β0J) for the transitions starting from the 5D0 level to its lower levels can be determined by：

β0 J =

A0 J ∑ A0 J

The WNR is low non-radiative relaxation rate and can be determined by:

WNR =

1

τ exp

−

1

τ rad

Due to non-radiative transition of 5D0 level, the value of τrad are higher than that of experimental lifetime (τexp (τobs)). Quantum efficiency (η) is another important characteristic parameter, which is the ratio of the experimental lifetime (τobs) to the calculated radiative lifetime (τrad) and can be expressed as:

η=

τ obs ARAD = τ rad ARAD + ANRAD

The Judd–Ofelt parameters and relative radiative properties of TiO2: Eu3+ with different doping concentration are displayed in Figure 9. The change of Ω2 values reveals the information about the coordination environment of Eu3+ and covalency between Eu3+ ions and O2- ions. The increase of value of Ω2 parameter indicated that the symmetry of Eu3+ ions has decreased and the Eu-O bonds with high covalence have formed. The 5D0-7F0 transition appeared when the Eu3+ ions occupy the Cs, C1, C2, C3, C4, C6, C2V, C3V, C4V, and C6V symmetries according to the ED selection rule.53 Therefore we inferred that the Eu3+ ions would existed at C2V, C2, Cs and C1 symmetry sites in anatase TiO2 through combining the branching rules of the 32 point groups and Ω2 values. The



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parameter Ω4 is related to the ligands electron density and not necessarily related to the symmetry of rare earth ions. And the ligand electron density would decreases with Ω4 value increases. 3.9 Photocatalytic activities The photocatalytic performances of TiO2:x%Eu3+ (x = 2, 4, 6, 8, 10, 12) were performed through degrading methyl orange (MO) under white light irradiation. Temporal changes in the concentration of MO, as monitored the absorbance of MO at 464 nm in the UV–vis spectra over the TiO2:x%Eu3+, are shown in Figure 10(a-f). For TiO2:2%Eu3+, we can observe a large decrease in the absorbance with prolonged irradiation time and the absorption band completely vanished after white light illumination for 25 min. It can be seen that Figure 10(g) shows the photocatalytic performances of the TiO2:x%Eu3+ under white light irradiation. In addition, Degussa P25 TiO2 was chosen as the reference photocatalysts for comparison. We can notice that the photodegradation efficiency follows the order: TiO2:2%Eu3+ > TiO2:4%Eu3+ > TiO2:6%Eu3+ > TiO2:8%Eu3+ > TiO2:10%Eu3+ > P25> TiO2:12%Eu3+. Most samples exhibited better activity than P25 except for TiO2:12%Eu3+. In particular, the photodegradation efficiencies decrease with an increase in the amount of Eu3+. Generally, the photocatalytic activity of sample is dependent on multiple factors, such as crystallinity, surface area, particle size, the amount of Eu3+ and so on. As you know, lanthanide ions doping would inhibit the recombination of photogenerated electrons and holes, meanwhile, concentrate the organic pollutant onto the TiO2 surface, which is an efficient method for improving the photocatalytic activity,54 however, appropriate doping amount is necessary. In our work, excessive Eu3+ ions would reduce the catalytic efficiency, which is in agreement with the results reported by Reszczynska.55 In addition, crystallinity is inversely proportional to the number of defects, the defects is detrimental to the improvement of


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photocatalytic performance of samples because of increased recombination probability of electrons and holes. As indicated in Figure 7d, the crystallinity of samples decreases with the amount of Eu3+ increases, thus the TiO2:2%Eu3+ exhibited the highest photocatalytic activities. And the catalytic mechanism of sample is as follows: The electrons which migrate on the surface of TiO2 react with oxygen molecule, which yield high active species such as hydroxyl radical and superoxide radical anion. Meanwhile, photogenerated holes are scavenged by water molecules and hydroxyl ions which absorbed into TiO2 surface, which can mineralize MO molecules deeply.

Conclusions In summary, TiO2:Eu3+ red phosphors with different morphologies were synthesized by the facile hydrothermal method through changing the pH values of original solution. The evolutionary mechanisms of the samples have been analyzed in detail. Most importantly, surface photovoltage spectra were used to explain the luminescence phenomenon of different morphologies for the first time. And the fluorescence and decay properties of phosphors with different europium ion concentrations have been discussed systematically. The luminescence intensities increase with Eu3+ up to 10 mol% and decrease gradually because of the concentration quenching effect. The critical transfer distance (Rc) of Eu3+ ions was calculated and the multipole interaction is dominant of quenching mechanism. The decay curves of TiO2:x%Eu3+ (x = 2-12) exhibit double-exponential feature. The J–O parameters and other radiative properties have been calculated to analyse spectral characteristics. Moreover, the photocatalytic performances of TiO2:x%Eu3+ nanorods indicate that the photodegradation efficiencies decrease with an increase in the amount of Eu3+ for the degradation of MO under white light irradiation, we can conclude that



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the good crystallinity and appropriate doping amount of Eu3+ are benefit for enhancing photocatalytic activity.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21171066 and 51272085). Project 2016154 Supported by Graduate Innovation Fund of Jilin University. References 1.

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Figure 1. XRD patterns of TiO2:2%Eu3+ obtained at different pH values. The standard data for TiO2 (JCPDS 89-4921) are also presented in the figure for comparison.

Figure 2. SEM patterns of TiO2:2%Eu3+ prepared at pH values of 9.2 (a), 9.7 (b), 10.3 (c), 11.0 (d), 11.5 (e), and 12.0 (f).


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Figure 3. Formation mechanism of the TiO2:2%Eu3+.



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Figure 4. (a) UV–vis diffuse reflectance spectra of TiO2:2%Eu3+ obtained at different pH values, (b-g) Kubelka-Munk plots and bandgap energy estimation of samples for indirect transition,(f) pH value vs. energy gap.


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Figure 5. (a) Excitation and (b) emission spectra of the TiO2:2%Eu3+ obtained at different pH values.



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Figure 6. SPV spectra of the TiO2:2%Eu3+ obtained at different pH values.

Figure 7. (a) PL emission spectra (b) intensity of 5D0-7F2 transition vs. concentration (c) asymmetric ratio vs. concentration graph and (d) XRD patterns of TiO2:x%Eu3+ (x = 2-12) nanophosphors prepared at pH = 9.2.


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Figure 8. (a) Lifetime decay curve, (b) tav vs. concentration and (c) fitting parameters of the decay time of TiO2:x%Eu3+ (x = 2-12) nanophosphors prepared at pH = 9.2.



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Figure 9. J-O and spectral parameters of TiO2:x%Eu3+ (x = 2-12) nanophosphors prepared at pH = 9.2.

Figure 10. (a-f) The absorption spectra of the MO aqueous solution between 400 to 600 nm with different irradiation time in the presence of the TiO2:x%Eu3+ (x = 2, 4, 6, 8, 10, 12) prepared at pH = 9.2. (g) Photodegradation of MO by different photocatalysts under white light irradiation.


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Photoluminescence and Photocatalysis Properties of Dual-Functional

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