Enhanced photocatalytic activities of Nd-doped TiO2 under visible light using a facile sol-gel method

Journal of Rare Earths xxx (xxxx) xxx

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Enhanced photocatalytic activities of Nd-doped TiO2 under visible light using a facile sol-gel method* Jicai Liang a, b, Jingya Wang a, Kexian Song b, Xiaofeng Wang c, Kaifeng Yu a, *, Ce Liang a, * a Key Laboratory of Automobile Materials, Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China b Roll Forging Research Institute, Jilin University, Changchun 130025, China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130025, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 December 2018 Received in revised form 11 March 2019 Accepted 31 July 2019 Available online xxx

Titanium dioxide nanoparticles modified with neodymium in the range of 1 mol% to 5 mol% were prepared with template-free sol-gel method. The structures of obtained samples were characterized by X-ray powder diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and diffuse reflectance spectroscopy. The photocatalytic activity of the obtained samples was evaluated by photodegradation of methyl orange in aqueous solution under ultravioletevisible (l > 350 nm) and visible (l > 420 nm) irradiation. The experimental results show that the 1 mol% Nd-doped TiO2 exhibits the highest photocatalytic activity, of which the degradation can reach to 96.5% under visible irradiation. According to the XRD results, the pristine samples are combined with anatase TiO2 and rutile TiO2, while the Nd-doped TiO2 samples are anatase TiO2 only. This transformation has made an obvious promotion of photocatalyst activity after modification. © 2019 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

Keywords: Nd-doped titanium dioxide Solegel method Visible light photocatalyst Mechanism of up-conversion Rare earths

1. Introduction With the increasing consciousness of environmental protecting in the world today, the pollutant degradation has received appreciable interest in science research. The great advantages of using semiconductors as photocatalyst have attracted people's attention in the latest two decades. Among these semiconductors, titanium dioxide (TiO2) has been wildly used in the photocatalysis field owing to its prominent superiorities, such as non-toxic, economy and easy preparing and so on.1e3 Meanwhile, it possesses incomparable performance over other traditional water treatment process due to the superior effect on the degradation of refractory organic matter, thus can deal with various organic and inorganic pollutants, exhibiting a broad application prospect among various kinds of semiconductor.4e7 Despite these intense advances,

* Foundation item: Project supported by the National Natural Science Foundation of China (51275203), Key Scientific and Technological Project of Jilin Province (20140204052GX, 20180201074GX), China Postdoctoral Science Foundation (2017M611321); Project of Education Department of Jilin Province (JJKH20180130KJ) and Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University (2019-8). * Corresponding authors. E-mail addresses: [email protected] (K. Yu), [email protected] (C. Liang).

photocatalysis using TiO2 still faces many practical challenges considering its prominent defect that the wild bandgap (3.2 eV for anatase TiO2) makes it only sensitive under ultraviolet light (UVlight).8e11 And the photogenerated electronehole pair formed under irradiation will recombine in seconds, which also becomes an obstacle in the photocatalytic progress. Therefore, how to raise the catalytic sensitivity under visible light and prolong the existence time is the major problem to increase the photocatalytic ability. Among different researches for enhancing the degradation ability of organic pollutant under visible light, modification is one of the most effective methods. In particular, doping with rare earth elements (RE), mainly lanthanide series, has become a new hot research spot in recent years.12e15 Xia et al. doped La3þ ions into titanium nanotubes with a two electrode system to enhance the photocatalytic activities under full-wave absorption to degrade the methyl blue.16 Stojadinovi
c et al. used a plasma electrolytic oxidation process method to coat terbium element into titanium surface as a photocatalyst. But the degradation of the coatings under simulate light only reach around 80% after doping.17 Lin et al. used a facial sol-gel method to enhance the photocatalytic activities of titanium by co-modified Eu and Au element.18 Sun et al. doped Nd into TiO2 with the sol-gel method and then prepared solid superacid photocatalysts of sulfated Nd-doped TiO2 by an incipient

https://doi.org/10.1016/j.jre.2019.07.008 1002-0721/© 2019 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

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wetness impregnation technique. The photocatalytic activities were evaluated by using methylene blue aqueous solution as a model contaminant under visible light irradiation.19 In this paper, Nd-doped TiO2 (abbreviated as Nd-TiO2 in the following) is obtained in a sol-gel method, and used as photocatalyst to degrade methyl orange. The photocatalyst properties were detected by the decomposition of methyl orange, and the results indicated that NdTiO2 shows an outstanding photocatalytic property. The degradation could reach to 96.5% under simulation sunlight light in 120 min, and the organic dye has been degraded completely in an hour under the UV-vis light irradiation. 2. Experimental 2.1. Materials Tetrabutyl titanate (TBOT),Nd(NO3)3$H2O(99.99%), nitric acid (65%), ethanol, PEG-4000, glacial acetic acid, are used as received without further purification. Deionised water (0.05 mS) is used for all reactions and treatment processes. Experimental instruments include beakers, titration funnels, electronic scales, stainless steel autoclaves, centrifuge, 500 W UV light lamp, spherical xenon lamp (AHD350W), UV-visible spectrophotometer (UVe6100PC) and so on. All the chemicals were used as received without further purification. The Nd doped TiO2 will be called Nd-TiO2 in the paper. 2.2. Preparation of Nd-TiO2 The sol-gel method is used to prepare Nd-TiO2 nanoparticles. First 25 mL glacial acetic acid is dissolved into 50 mL absolute ethanol, then 10 mL TBOT is dissolved into the mixture solution drop by drop slowly under magnetic stirring for 30 min, the mixture is named as sol-A. Then a certain amount of Nd(NO3)3$H2O is dissolved into 30 mL deionized water, and the solution is called solution-B. The solution is stirring under magnetic stirring for another 30 min, and then the solution-B is added into sol-A drop by drop slowly, after that the mixture is stirring for 30 min. Then 0.07 g PEG-4000 is added into the mixture stirs for another 30 min and 1.2 mL nitric acid is added into the above mixture. Then the sol is put under room temperature for 8 h till it transformed into wet gelatin. And then the wet gel is dried under 60  C for 24 h in an oven to obtain the dried gel. Finally the sample is calcined for 2 h in the muffle oven under the temperature of 500  C to obtained the Nddoped TiO2. And the pristine TiO2 is prepared by the same method without the addition of Nd(NO3)3$H2O.

2.4. Photocatalytic experiments The final products (50 mg) were dissolved in 50 mL (20 mg/L) methyl orange (MO) solution. The mixtures were vigorous stirred for half an hour in a dark environment. Then 6 mL mixture was weighed, and the photoreaction was carried out under UV light for 30 min, during which the mixtures were measured every 5 min. After 3 min centrifuging at a rotate speed of 12000 r/min, the supernate was placed in a quartz colorimetric utensil. Finally, the absorbance values were measured by an ultraviolet spectrophotometer. In this experiment, the photoreaction was conducted and lasted for 120 min in the irradiation of simulated sunlight which was supplied by a spherical xenon lamp. During this period, 4 mL samples were measured every 20 min. Other experimental steps are the same as the photocatalytic performance test under UV light.

3. Results and discussion 3.1. X-ray powder diffraction analysis X-ray powder diffraction (XRD) was used to detect the phase composition of the Nd-doped TiO2, the XRD patterns are shown in Fig. 1. All the samples were calcined at 500  C for 2 h in the muffle oven. It can be observed that the pristine TiO2 consisted of a mixture of anatase and rutile phase with a mass ratio of 8:2. And the mass percentage of anatase phase and rutile phase was calculated by the RIR method as the equation given below.20

WA ¼

IA IR þ

(1)

IR K RA

WR ¼ 1  WA

(2)

In this equation, IA and IR indicate the highest intensities of anatase and rutile phase, and K indicates the adiabatic method coefficient. WA and WR indicate the contents of the phases in the sample. Which IA is 94.3%, WA is 80% and thus WR is 20%. The presented peaks can correspond to the structure of anatase and rutile phase. The absence of rutile phase represents that heat treatment at 500  C could transform anatase phase into rutile phase. Fig. 1 presents a group of lines at 2q values of 25.37, 37.88 ,

2.3. Characterization X-ray diffraction (XRD) patterns are collected on a Bruker D8 Advance X-ray diffraction meter. Surface areas are detected by the BET measurements. Scanning electron microscopy (SEM) images are taken on a Tecnai G2 S-Twin F20 microscope. AXIS-Ultra X-ray photoelectron spectroscopy (XPS, Kratos, England) analysis is performed to determinate the elemental composition and oxidation sate of materials using Al Ka X-ray at 15 kV and 15 mA. The standard C 1s peak (Binding energy, Eb ¼ 284.80 eV) is used to eliminate the static charge effects. Diffuse reflectance UV-vis absorption spectra (UV-DRS) of the materials were obtained using a UV-2400 spectrophotometer (Shimadzu, Japan). The photocatalytic activity of the sample was measured by monitoring the degradation of methylene blue at ambient temperature using a 500 W UV light lamp/350 W xenon lamp (AHD350W), and the optical properties of the films were characterized by using a UV- visible spectrophotometer (UVe6100PC).

Fig. 1. XRD patterns of the obtained samples.

Please cite this article as: Liang J et al., Enhanced photocatalytic activities of Nd-doped TiO2 under visible light using a facile sol-gel method, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.07.008

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48.12 and 68.79 , which are attributed to the anatase phase, responding to the lattice plane (101), (004), (105), (116). And the group of lines at 2q values of 27.48 , 36.13 , 41.30 , 54.37 and 56.69 are attributed to the phase of rutile, responding to the lattice plane (110), (101), (111), (211) and (220).21 The modified TiO2 has displayed anatase phase only, and thus shown higher photocatalytic reactive owing to the formation of the bond between rare element and TiO2. Meanwhile, the existence of the bond between rare element and TiO2 could restrain the generation of rutile phase.22 No characteristic peaks of neodymium oxide are found in the XRD patterns due to the very small amount of neodymium oxide which highly dispersed in the TiO2, when simply Nd3þ ions incorporated in the crystalline of TiO2. The absence peaks of neodymium oxide could be ascribed to the tiny content and wide dispersity of Nd3þ ions, which has made it difficult to be detected. Nd3þ is present in the form of an amorphous phase, or the Nd3þ ions are adsorbed on the titania surface or placed inside the titania lattice (titania doping).23,24 The particle size was calculated by the Debye-Scherrer equation. The primary size of the samples are listed in Table 1. The size of the undoped titanium is 51.98 nm while the size decreased to 15.55 nm after 1 mol% neodymium modification and the sizes of other doped samples were at the range of 11e13 nm. It is indicated that the modification of neodymium could inhibit the growth of the crystal size. Furthermore, compared with the pristine titanium the surface area increased to 122.05 m2/g when the doping content turned to 5 mol%, which indicated that the surface area increased with the decrease of the crystal size, which has also been demonstrated by some literature.25e27 3.2. X-ray photoelectron spectroscopy analysis To detect the elemental contents and chemical character of elements in the surface layer of pristine TiO2 and Nd modification TiO2, the X-ray photoelectron spectroscopy (XPS) was used to examine the samples. The detected oxygen, titanium, carbon and neodymium elements are corresponding to the XPS spectra of O 1s, Ti 2p, C 1s and Nd 3d, respectively. The XPS data are summarized in Fig. 2, where deconvoluted spectra are presented as well. The content of the elements is summarized in Table 2. The real content of the modification was detected by the content of neodymium in the samples and the units of the content were atomic mass (at%). The C 1s, Ti 2p and O 1s core level spectra have revealed the chemical character of carbon, titanium and oxygen species respectively. Three chemical states are separated in the C 1s spectrum, of which the peaks are 284.6, 286 and 288.7 eV, corresponding to CeC, CeO and C¼O bonds. And two chemical states of titanium separated in the Ti 2p spectrum at the BE of Ti 2p3/2 peak close to 458.4 and 456.8 eV can be identified as Ti4þ and Ti3þ, respectively. The reduction of small amount of Ti3þ could be observed in all titanium samples, which is attributed to the oxygen defects in the titania lattice. Three chemical states could be separated in the O 1s spectrum. The most intensive peak located at 530.1 eV indicated the presence of oxygen in the titania lattice (TiOlattice), while the peak of 530.6 eV respected the bond of titanium

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and oxygen in the surface of the samples (Ti-Osurf) and the peak of 532.5 eV indicated the presence of the hydroxyl groups (eOH).28,29 Pristine TiO2 and 1 mol% to 3 mol% Nd-doped TiO2 samples have shown that the major peak of Nd element appears at BE of 974.1 eV, and 4 mol% and 5 mol% Nd-doped TiO2 samples have shown that the doped Nd element existed as Nd3þ and two major peaks centred at 971.4 and 984 eV are corresponding to Nd 3d5/2 and Nd 3d3/2 orbitals, respectively. The existence Nd atoms have abstracted electrons from neighbouring Ti4þ cations strongly, which has made the migration of electrons from Ti ions and resulted in the decrease of the density of the outer electron cloud of Ti ions.30,31 This kind of migration is corresponding to the increased generation of Ti3þ cations, and the existence of cations has contributed a lot to the photocatalytic activity by retarding the recombination of hþ and e. It could be concluded that the modification of Nd has made a positive effect on the photocatalytic progress by creating a Schottky barrier at the metal-semiconductor junction.32 3.3. Scanning electron microscope analysis The scanning electron microscopy (SEM) was used to characterize the morphology of the obtained photocatalysts, in order to understand the modification effect of the RE elements intuitively. Fig. 3 shows the SEM images of pristine TiO2 and Nd-TiO2 with different modified contents. Through these SEM patterns we could observe that aggregation happened in all photocatalysts. Compared with the pristine TiO2 (Fig. 3(a)), which possesses a group of bulks with sharp but irregular shape, some nanoparticles appear on the surface of samples after modification. The size of the obtained photocatalysts is in the range of 3e5 mm. The elements in 1mol% Nd-doped TiO2 sample were analyzed by SEM-EDS method, and the result is shown in Fig. 4, in which the presence of carbon, titanium, oxygen and neodymium could be observed. The further information of the surface chemical properties is provided by the following TEM and XPS analysis. 3.4. Transmission electron microscope analysis The transmission electron microscopy (TEM) patterns of 1 mol% Nd-doped TiO2 sample are shown in Fig. 5. It can be observed that the titanium nanoparticles with the size of about 10e20 nm are grew around the neodymium particle. The larger radius of neodymium particle makes it attach on the surface of titanium nanoparticles rather than enter into titanium lattice to replace the Ti4þ cations, which is related to the larger surface area of the doped samples.25,26 3.5. BET surface area analysis Brunauer-Emmett-Teller (BET) method was used to detect the surface area and crystallite size of the samples, and the curves of the N2 sorption-desorption isotherms and pore size distribution are shown in Fig. 6 below. The N2 sorption-desorption isotherm curves of the modified samples belong to the IUPAC classification

Table 1 Chemical composition, textural and optical properties of investigated photocatalysts. No.

Catalyst

XPS The real content of Nd

Physisorption SBET(m2/g)

Pore size(nm)

DRS UVeVis Indirect band gap (eV)

Particle size (nm)

1 2 3 4 5 6

Pristine TiO2 1 mol% Nd/Ti 2 mol% Nd/Ti 3 mol% Nd/Ti 4 mol% Nd/Ti 5 mol% Nd/Ti

e 3.56 3.19 2.36 2.56 4.71

86.1 114.3 106.2 116.9 120.3 122.05

2.82 2.93 3.42 3.95 4.02 4.46

2.95 2.60 2.53 2.51 2.58 2.47

51.980 15.554 12.485 11.630 13.195 11.253

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Fig. 2. X-ray photoelectron spectroscopy of the obtained samples.

Table 2 The content of the element of samples according to the XPS data (at%). Catalyst

C

Ti

O

Pristine TiO2 1 mol% Nd-TiO2 2 mol% Nd-TiO2 3 mol% Nd-TiO2 4 mol% Nd-TiO2 5 mol% Nd-TiO2

45.31 27.86 35.91 44.52 53.37 27.25

13.8 19.34 14.67 11.9 8.59 18.64

40.89 49.23 46.23 41.22 35.54 49.41

of type IV, the H4 hysteresis loop has indicated the existence of the mesoporous of the Nd-doped samples, while the sorptiondesorption isotherm of the pristine TiO2 does not have a hysteresis loop. The nitrogen adsorption-desorption isothermal curves show well-defined adsorption steps. At low relative pressure (p/

p0 < 0.4) the isotherms exhibit a high adsorption, which indicates the powders are mesoporous. At a middle relative pressure (p/ p0 ¼ 0.4e0.8), the curves exhibit a H4 hysteresis loop, which can further confirm the powders are mesoporous photocatalysts. The specific surface areas (SBET), average pore diameters (DP) of all samples are summarized in Table 1. With the increase of the surface area, the size of the crystallite of the samples has decreased. According to the literature reports, the radius of RE3þ ions is larger than titanium and thus made ions unable enter into the interior of TiO2 to replace Ti4þ but formed the bond with titanium on the surface of TiO2.25,26 The REeOeTi bond on the surface has impeded the size growth of TiO2, which is related to the decrease of crystallite size.27 This phenomenon is consistent with TEM analysis results that TiO2 nanoparticles are grown around the neodymium particles chunk.

Fig. 3. Scanning electron microscope patterns of pristine TiO2 (a), 1 mol% Nd-doped TiO2 (b) and 5 mol% Nd-doped TiO2 (c).

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Fig. 4. SEM-EDX patterns of 1 mol% Nd-doped TiO2 sample. (a) C Ka1_2; (b) O Ka1; (c) Ti Ka1; (d) Nd Ma.

Fig. 5. TEM patterns of 1 mol% Nd-doped TiO2 sample.

Fig. 6. (a) N2 adsorption-desorption isotherms curves; (b) Pore size distribution curves of samples.

3.6. Diffuse reflectance spectroscopy Diffuse reflectance spectra (DRS) were used to understand the optical absorption properties of the photocatalytic processes under light irradiation in the range of 200e900 nm, and the result is shown in Fig. 3(a). All the samples possess a broad intense

absorption below 400 nm, which could be attributed to the electron excitation from the valence band to the conduction band in TiO2. The indirect band gap energies of the samples were calculated by the equation: Eg ¼ 1239.8/l, where Eg is the band gap (eV) and l (nm) is the wavelength of the absorption edges in the spectrum. The calculated band gap energy of the pristine TiO2 is 2.95 eV,

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which is similar to the theoretical band gap value of rutile phase titanium (3.0 eV), and the band gap of the other samples is listed in Table 1 as well, the reduce of the band gap could be interpreted into the existence of anatase phase of the doped samples. Meanwhile, the photocatalysis properties of TiO2 are influenced by the number of photo-generated holes generated under irradiation that the charge could transfer from the valance band to the conduction band (O 2p/Ti 3d). And the process is more sensitive under ultraviolet light region (UV-light) below 400 nm, which limited the application of photocatalysis under visible light.33 The DRS results have indicated that modification of Nd element does influence the optical absorption properties, the redshift phenomenon has been observed in the 1 mol%, 2 mol% to 5 mol% Nddoped TiO2 samples, which indicated that the Nd-doped samples have showed a higher absorption intensity in the same wavelength compared with the pristine TiO2. This phenomenon could be explained by the charge transition. The 4f states seemed to take part in the process of the chemical bonding with the oxygen 2p states which pushed the valence bond into a lower energy lever and finally led the decrease of the band gap. Nd3þ cations have eight kinds of atomic states as shown in Fig. 7(b), corresponding to 4I9/2, 4 I11/2, 4I13/2, 4I15/2, 4F3/2, 4F5/2, 4G5/2, 4G7/2 and 4G9/2. The major absorption peak in the visible region located at 586 nm, corresponding to transitions of 4G7/2 to 2G7/2.34 The charge transfer occurred between the valence band of TiO2 and Nd3þ ion levels after the modification, and thus the new energy level makes the redshift phenomenon.35 There are four absorption bands in the visible region typical for neodymium located at 520, 585, 745 and 805 nm. They correspond to the transitions from the 4I9/2 ground state to the excited states of 4G7/2 and 2K13/2, 4G5/2 and 2G7/2, 4S3/2 and 4F7/2, 4F5/2 and 2H9/2.36,37 In all of the studied Nd-modified samples, the intensity of these absorption bands is similar. 3.7. Photocatalytic activity The photocatalytic activity of the obtained samples was investigated by the degradation of the methyl orange (MO) in an aqueous solution under UVeVis light (l > 350 nm) and visible light (l > 420 nm) conditions. The pristine TiO2 sample prepared by the sol-gel method in this paper is used as the reference sample. According to the degradation curves of samples under UV light (l > 350 nm) as shown in Fig. 8(a), after 60 min UV-light irradiation,

the samples modified with 1 mol% Nd displays the best degradation of 98% which is much higher than the self-degradation (degrade process of methyl orange without catalyst under irradiation). The degradations of all the samples are similar at the range of 98%, the promotion of the modified samples could be interpreted as the decrease of the band gap energies listed in Table 1, and the band gap of the 1mol% Nd-doped titanium is 2.60 eV, which is less than the theoretical band gap of anatase (3.2eV). With a narrow band gap, TiO2 could respond to visible light and increase its light absorption, which may be beneficial for improving the photo absorption and photocatalytic activity of TiO2.24 Sun et al. prepared Nd-doped titanium with the sol-gel method and modified it by the sulfate, the degradation of methyl blue could only reach maximum 70% after 600  C calcining temperature, with the neodymium doping content of 0.25 at%.19 Parnicka et al.38 degraded the phenol with the Nddoped titanium with the hydrothermal method, and phenol degradation could only reach 28.03% after 60 min irradiation under visible light at the range of 0.1% Nd-TiO2. Shao et al.39 prepared neodymium doped titanium, the best activity of which is achieved when the ratio of Nd to Ti is 3 wt%e4 wt%, in which case MB is completely degraded under UV irradiation for 160 min, for the sample of 6% modification the real content of the neodymium is only 0.81% from the EDS data. Fig. 8(b) shows the photocatalytic degradation performance of Nd-doped TiO2 under visible light irradiation. In comparison to the self-degradation of methyl orange, the prepared TiO2 sample has possessed good photocatalytic performance. Nevertheless, the degradation of the pristine TiO2 under UV irradiation could be ignored considering that the absorbance basically did not change after irradiation. According to the XRD results, the phase of the pristine TiO2 is a mixture of anatase and rutile phase, thus the poor photocatalytic activities properties of rutile phase correspond to the low degradation of pure TiO2.40 The modified TiO2 can almost completely degrade methylene orange in 120 min under the visible light irradiation, in which 1 mol% Nddoped TiO2 is the optimal doping concentration due to the highest degradation rate of about 96.5%. The degradation can be calculated by the formula below 41:

Degradation:

C0  C  100% C

(3)

The degradation rates of samples from pristine TiO2 to 5 mol% Nd-doped TiO2 were calculated by the formula and the results are

Fig. 7. (a) (1) Diffuse reflect spectra patterns of the obtained samples at the range of a:pristine TiO2; (2) 1 mol% Nd-doped TiO2; (3) 2 mol% Nd-doped TiO2; (4) 3 mol% Nd-doped TiO2; (5) 4 mol% Nd-doped TiO2; (6) 5 mol% Nd-doped TiO2; (b) Simplified energyelevel diagrams and excitation path for the upconversion of emission at ultraviolet light Nd3þ under visible light excitation.

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Fig. 8. (a) Degradation of the methyl orange under UV-light irradiation; (b) Degradation of the methyl orange under visible light irradiation.

Table 3 Reaction rate constant and degradation efficiency of samples. No.

Catalyst

Reaction rate constant k (min1)

Degradation efficiency (visible light) (%)

1 2 3 4 5 6

Pristine TiO2 1 mol% Nd-doped 2 mol% Nd-doped 3 mol% Nd-doped 4 mol% Nd-doped 5 mol% Nd-doped

e 0.0115 0.0144 0.0135 0.0137 0.0124

0 96.5 94.4 94.9 94.79 95.4

shown in Table 3. The reaction rate constant k of all samples are calculated by the formula below and also listed in Table 3,42:

In

C0 ¼ kt C

(4)

According to the results, the sample modified with 1 mol% Nd3þ has shown the lowest reaction rate constant of 0.0115 min1, and the constant of pristine TiO2 could not be calculated with the zero percent degradation efficiency.

used 1 mol% Nd-TiO2 as the specimen while keeping the same reaction conditions (50 mg catalyst þ 50 mL methylene orange (20 mg/L), 50 min of solar irradiation). Catalyst was recovered using centrifuge after each experiment. Prior to next experiment, catalyst was dried in an air oven and powdered well using mortar and passel. They retained their high catalytic activity after being recycled three times. These results indicated that Nd-TiO2 has high chemical stability and is recoverable and reusable.

3.9. Discussion of the mechanism 3.8. Recycling and stability of the samples The chemical stability of the samples is an important property. In this experiment the aqueous stability was estimated by the MCC1 leaching method, the 1 mol% Nd-doped titanium was chosen as the specimen which used the Teflon PFA as the containers with a 60 mL capacity performed at 90  C after being polished, washed and dried. The concentrations of Nd were obtained by inductively coupled plasma-mass spectrometry (ICP-MS) analysis using an Agilent 7700  spectrometer. The normalized leaching rate (LRi), which is substantially based on the geometrical surface area, directly reveals the chemistry stability of relevant radioactive nuclides stabilization.43 The normalized elemental leaching rates (LRi) were calculated as the following formula: LRi ¼ ci·Vi/fi·As·Dt. Where fi is the mass fraction of element i in the sample, V the volume of the leachates, As the geometric surface area of the sample and Dt the duration of the experimental days. The geometric surface area of the specimen is about 400 mm2, the value of As/Vi is 2.70 me1 in this experiment, which satisfied the requirement of standard MCC-1 static leaching test. The leaching rate of the Nd is listed in Fig. 9, the LRNd is about 2.7  105 g/(m2∙d) at the first 7 days which increases to 4.5  105 g/(m2∙d) after 28 d. According to the requirement of HLW management it should be safe over geological timescales (typically hundreds of thousands of years) and the chemical stability of simulated waste forms is affected by both the temperature and pH,44 thus the stability of the Nd element should be further studied. To assess the recycling and stability, a degradation experiment of 3 consecutive processes was designed which

The photocatalysis mechanism of Nd-doped TiO2 under the simulate sunlight irradiation is shown in Fig. 10. The reaction of electrons in the conduction band of TiO2 is probably responsible for the formation of O2. Electrons from the conduction band of TiO2, generated by the excitation of the Nd3þ, migrate to the surface of

Fig. 9. Normalized elemental leaching rates of Nd in the 1 mol% Nd-TiO2 sample for 1e28 d. As/Vi ¼ 2.70 me1.

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Fig. 10. The photocatalysis mechanism of Nd-doped TiO2 under simulate sunlight irradiation.

the semiconductor where they are involved in the formation of O2, as presented in the following reactions45: Nd-doped TiO2 /eeþhþ

(5)

Nd3þþee/Nd2þ

(6)

Nd3þþhþ/Nd4þ

(7)

eeþO2/·O2

(8)

·O2þH2O/HO·2þOHe

(9)

HO·2þH2O/H2O2þ$OHe

(10)

H2O2/2OH·

(11)

OH·þMO/degraded products

(12)

From these reactions, we could explain the modification effect of Nd3þ, among the transition metals, Nd3þ can inhibit electron/hole recombination best by trapping both photo-generated electrons and holes and thus creating Nd2þ and Nd4þ, respectively, to enhance the photocatalytic efficiency.

4. Conclusions In this paper, Nd-doped TiO2 photocatalysts were prepared by sol-gel method, of which the photocatalytic activities were detected by the degradation of methyl orange in an aqueous solution under the irradiation of the UV-Vis light and visible light. The size of obtained samples is about 3 mm calculated by XRD results. The pristine TiO2 is a mixture of anatase and rutile phase, and the TiO2 transfers into anatase phase only after the modification of neodymium. And this transformation improves the photocatalytic activities greatly according to the fact that the modification effect of neodymium could not only decrease the band gap of the pristine TiO2 but also take part in the formation of the bond between Nd element and TiO2. The optimum Nd-doped amount of modification is 1 mol%, the degradation of which is the highest both under UV light and visible light irradiation. The redshift phenomenon among the modified samples according to the DRS results indicates that the doped neodymium has improved the absorption intensity at the wavelength of 200e300 nm. The surface chemical properties of the obtained samples were detected by X-ray diffuse spectra, which indicates that the interaction between the Nd atoms and Ti4þ is related to the generation of Ti3þ ions, and it impedes the recombination of photonic electron hole pairs and leads to the improved photocatalytic activities.

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Please cite this article as: Liang J et al., Enhanced photocatalytic activities of Nd-doped TiO2 under visible light using a facile sol-gel method, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.07.008