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Novel Tm3+ and Yb3+ co-doped bismuth tungstate up-conversion photocatalyst with greatly improved photocatalytic properties
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111864
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Novel Tm3+ and Yb3+ co-doped bismuth tungstate up-conversion photocatalyst with greatly improved photocatalytic properties ⁎
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Zhe Shena, Hong Lia, Hongshun Haoa,b, , Zhengguang Chena, Hongman Houb, , Gongliang Zhangb, Jingran Bib, Shuang Yana, Guishan Liua, Wenyuan Gaoa a b
Department of Inorganic Nonmetallic Materials Engineering, Dalian Polytechnic University, Dalian 116034, China Liaoning Key Lab for Aquatic Processing Quality and Safety, Dalian Polytechnic University, Dalian 116034, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bi2WO6 Photocatalysis Up-conversion Rare earth Optical properties
Tm3+ and Yb3+ co-doped Bi2WO6 up-conversion photocatalysts (Tm3+/Yb3+:Bi2WO6) were synthesized via coprecipitation route at different calcination temperatures. Doping Tm3+ and Yb3+ into Bi2WO6 lattice and substituting for the fractional Bi3+ were successfully confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. The morphology and optical property of the obtained samples were characterized by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS) and up-conversion luminescence spectra (UC-PL). The photocatalytic results revealed that the Tm3+/Yb3+:Bi2WO6 exhibited a much superior photocatalytic activity than the pure Bi2WO6 and commercial Degussa P25 for the degradation of rhodamine B (RhB), and the degradation rate reached 97.6% in 25 min. The effects of Tm3+ and Yb3+ on the photocatalytic activity of Bi2WO6 were studied in detail. The improved photocatalytic activity of Bi2WO6 could be attributed to the efficient energy transfer between Tm3+/Yb3+ and Bi2WO6 via infrared to visible up-conversion and the reduced recombination rate of photogenerated electron-hole pairs through co-doping of Tm3+/Yb3+ on Bi2WO6 nanopaticles.
1. Introduction Environmental problems caused by the excessive use of organic dye pollutants have seriously threatened human existence and sustainable development of society [1]. Photocatalysis is a green and energy saving technology for the elimination of the organic pollutants in the environment and it has received worldwide attention [2–4]. Currently, various semiconductor photocatalysts such as TiO2, ZnO, BiVO4 and Bi2MO6 have been developed [5–7]. Because of its low cost, nontoxicity, high chemical stability and strong oxidizing abilities, TiO2 has been the most widely studied and applied in many fields, especially for the degradation of organic pollutants in wastewater [8]. However, due to its large band gap energy of 3.2 eV, TiO2 can only respond in the ultraviolet light that occupies only about 4% energy of the total solar spectrum, but visible light accounts for 43% of sunlight spectrum. In order to efficiently use solar energy, many efforts have been focused on the extension of the response of photocatalysts from ultraviolet light into visible light region. Doping metallic (Pt, Au and Ag) and nonmetallic (S, N and B) elements into conventional semiconductor photocatalysts, such as In2O3 [9], NaTaO3 [10], and Bi4Ti3O12 [11],
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decreases the band gap energy and mostly takes advantage of the available energy in the sunlight [12–15]. Unfortunately, the efficient utilization of sunlight energy is not capable of solving the severe energy and environmental crises. Therefore, it is reasonable to fabricate new materials with high-efficiency energy transformation that can transform near infrared (NIR) light to the visible and ultraviolet light. This desirable process is known as up-conversion (UC) luminescence [16,17]. Since the initial report of up-conversion process by Auzel et al in 1960, up-conversion luminescence has attracted considerable attention and become focus of the research. Rare earth ions doped UC nanoparticles can effectively convert low energy radiation (NIR light) into higher energy radiation (visible light) by means of multiple absorption or energy transfer process. Up-conversion materials have demonstrated wide application in solid-state lasers, biomedical diagnostics, optical temperature sensors, solar cells and other fields. Recently, related studies had revealed that up-conversion process could be employed for the effective photocatalysts. Incorporating up-conversion agent into the traditional photocatalysts for the improvement of photocatalytic efficiency has been reported in many literatures [18–21]. Bi2WO6, as a typical layered Aurivillius oxide consisted of
Corresponding authors at: Liaoning Key Lab for Aquatic Processing Quality and Safety, Dalian Polytechnic University, Dalian 116034, China E-mail addresses: [email protected] (H. Hao), [email protected] (H. Hou).
https://doi.org/10.1016/j.jphotochem.2019.111864 Received 9 January 2019; Received in revised form 25 April 2019; Accepted 15 May 2019 Available online 16 May 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111864
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perovskite-like slab of WO6, owing to its excellent nonlinear dielectric, ion-conductive, luminescent and catalytic properties, has aroused great interest [22]. Various research has shown that Bi2WO6 could exhibit strong photocatalytic efficiency for water splitting and the degradation of organic compounds such as acetaldehyde, acetic acid, 4-chlorophenol and rhodamine B (RhB). However, pure Bi2WO6 possesses the narrow photo-response range and high combination rate of photogenerated electron-hole pairs in the photocatalytic reaction. Fortunately, up-conversion refers to absorption of multiple low energy photons into the intermediate level to form high energy photons, which efficiently increased the number of photons for a desired energy transfer. In the photocatalysis process, the higher energy photons can be absorbed to generate more electrons that could degrade the organic pollutants. For example, Er3+ and Yb3+ co-doped TiO2-xFx, as a light scattering layer, has been applied into dye sensitized solar cell (DSSC) [23], to harvest higher light so as to achieve the satisfied DSSC efficiency. Visible light responsive photocatalysts including Er3+/Yb3+ codoped Bi2MoO6 [24], Ho3+/Yb3+ and Er3+/Yb3+co-doped Bi2WO6 [25] have been reported in recent years. However, Tm3+/Yb3+: Bi2WO6 up-conversion photocatalyst has not been studied yet. Herein, in this work, we report the synthesis of visible light responsive Tm3+/Yb3+:Bi2WO6 up-conversion photocatalyst by co-precipitation method for the first time. The samples were synthesized with different calcination temperatures and Tm3+ and Yb3+ contents. Moreover, we obtained the excellent photocatalyst with optimal calcination temperature and Tm3+ and Yb3+ contents. By harvesting NIR and low energy photons, Tm3+ and Yb3+ emiting UV and visible light that matches well with the band gap of Bi2WO6. The effects of Tm3+ and Yb3+ on the photocatalytic properties were studied in detail. Under visible light irradiation, Tm3+/Yb3+:Bi2WO6 up-conversion photocatalyst exhibited higher photocatalytic efficiency of degrading RhB than undoped Bi2WO6.
with a low heating rate (5 °C min−1) until the weight loss of the material was negligible. Pure Bi2WO6 was prepared by the same process without the addition of rare earth ions.
2. Experimental
The photocatalytic activities of the obtained samples were evaluated by the degradation of RhB. Using a 500 W Xe lamp as simulated sunlight source. In addition, a 500 W Xe lamp was used as the near-infrared light source where the λ < 800 nm light were filtered out during nearinfrared light photocatalysis. Before irradiation, 70 mg of photocatalyst was added into RhB aqueous solution (50 mL,10−5 mol·L-1) under magnetically stirring for 30 min in the darkness to achieve adsorptiondesorption equilibrium between the photocatalyst and RhB solution. In the irradiation process, 4 mL of suspension was taken out at given time intervals and centrifuged to remove the remaining photocatalyst particles. The concentration of RhB was calculated by analyzing the measured absorbance at 554 nm with a UV–vis spectrometer (UV-1800, Shimadzu). Besides, in order to investigate the stability of the photocatalyst, the recycling test was also conducted. After the above photocatalytic measurements, the remnant RhB solution was centrifuged. The obtained powders were dried at 80 °C for 10 h and repeated the photocatalytic experiments for four times.
2.3. Characterization methods The phase and crystallinity of the prepared samples were characterized by X-ray diffraction (XRD-7000, SHIMADZU) with Cu Kα radiation source (λ = 0.1546 nm) at a scanning rate of 5° /min in the range of 20-70°. The morphologies and sizes of the samples were observed with a field-emission scanning electron microscopy (FESEM, JSM-7800 F) and transmission electron microscopy (TEM, JEOLJEM2100). The composition and chemical states of the samples were identified by X-ray photoelectron spectroscopy (XPS, Thermo scientific K-Alpha). All the XPS data were calibrated using C1 s peak at 284.6 eV. The ultraviolet visible diffuse reflectance spectra were measured by an UV–vis spectrophotometer (UV–vis, Lambda35). The up-conversion luminescence spectra in the range of 500–700 nm were obtained by a fluorescence spectrometer (F-7000, Hitachi, Japan) equipped with 980 nm laser diode excitation source. In order to compare the intensity of up-conversion luminescence, the different samples were measured under the following identical conditions: the powders were filled into a sample tank with a diameter of 10 mm and a thickness of 2 mm and fixed on the sample table. The laser emission point was focused onto the sample with a 7 cm focal length lens and the focused laser beam had a spot diameter of 375 mm at 1/e2 of the maximum intensity. EX Slit and EM Slit were 5.0 nm and 10 nm, respectively, and scan speed was 40 nm/min. The incident angle of laser beam was maintained at 90° and the excitation power was 250 mW. All of the measurements were performed at room temperature. 2.4. Photocatalytic measurements
2.1. Materials and reagents All the chemicals used were analytical pure reagents and without further purification. Bi(NO3)3·5H2O (99.99%), Na2WO4·2H2O (99.99%), Tm2O3 (99.9%), Yb2O3(99.9%), nitric acid, sodium hydroxide, ethanol absolute and rhodamine B (RhB) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China). Aqueous sodium hydroxide solution was used to adjust the pH value of the solution. Nitrate solutions of Tm3+ and Yb3+ were prepared by dissolving the corresponding metal oxide in double dilute nitric acid at 90 °C. Degussa P25 (˜80％ anatase and ˜20％ rutile) was provided by Degussa (China) Co., Ltd. Ethanol absolute and deionized water were used during washing process. 2.2. Synthesis of photocatalyst
3. Results and discussion
The samples were synthesized by co-precipitation method. In this typical procedure, Bi(NO3)3·5H2O (1.21 g, 2.5 mmol) and Na2WO4·2H2O (0.41 g, 1.25 mmol) were dissolved in nitric acid solution (50 mL, 1.0 mol·L−1) and deionized water (50 mL), respectively. Rare earth (Tm3+ and Yb3+) nitrate solutions were added to Bi (NO3)3·5H2O solution. Then, Na2WO4·2H2O solution was dropwise added to the above solution under vigorous stirring. The pH value of the obtained suspension was adjusted to 5.5 by using concentrated NaOH solution (2 mol·L−1). The mixture was stirred and kept in a water bath at 80 °C for 2 h in order to obtain a slow evaporation of water until the formation of a white solid. The precursor was cooled to room temperature naturally and individually centrifuged and washed with ethanol absolute and deionized water three times, respectively, dried at 80 °C for 12 h. The as-synthesized samples were obtained by calcination of the powder in air by thermal treatments at 400, 500, 600 and 700 °C
3.1. Characterization on Tm3+/Yb3+:Bi2WO6 photocatalyst Bi2WO6, as one of the most important members in the layered Aurivillius oxide family, belongs to perovskite layers crystal structure consisting of arranging alternately in the form of bismuth oxide (Bi2O2)2+ layers and WO6 octahedron layers. This layered structure is beneficial to increase the separation efficiency of photogenerated electrons (e−) and holes (h+). As presented in Fig. 1, the Tm3+ and Yb3+ ions are expected to partly substitute the Bi3+ ions in (Bi2O2)2+ layers when introduced into the crystal lattice of Bi2WO6. It indicates that the unit cell is composed of (TmxYbyBi2-x-yO2)2+ groups and WO6 groups. Moreover, it remains the well-layered structure with single phase of orthorhombic after co-doping. 2
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111864
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Fig. 1. The crystal structure of Bi2WO6.
comparison and analysis of the XRD patterns of pure ad doped Bi2WO6 samples shows the shifts of diffraction peak position to higher 2θ angles. It indicates that the variation in the unit cell volume is due to the substitution of Bi3+ by Tm3+ and Yb3+. The lattice substitution of Bi3+ in the (Bi2O2)2+ by Tm3+ and Yb3+ rely on the matching of oxidation. Little decrease in cell volume can be explained by the substitution of smaller ionic radii of Tm3+/Yb3+ (88 pm/86.8 pm) for the larger Bi3+ (117 pm) ions. This result revealed that the Tm3+ and Yb3+ ions have been incorporated into the crystal lattice of Bi2WO6 and substituted for the fractional Bi3+ ions. In addition, the effect of doping content on the crystalline phase was also described by the changes of XRD patterns. A huge increase in the intensity ratio of (131)/(200) can be found in these samples with Tm3+ doping content of 1.0 mol% and Yb3+ of 20 mol%. It illustrates the formation of better crystalline. However, the ratio value decreases gradually with the increasing doping contents. The change of the intensity ratio suggests that the variation is preferred orientation growth with the incorporation of dopant ions. The general morphologies and particle sizes of the pure Bi2WO6 and 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 powders are observed by FESEM. Fig. 5(a,b) presented images of the different magnification of the pure Bi2WO6. It reveals that there are some aggregated particles blocks, which is caused by large surface energy and many dangling bonds. Most of the particles appear uniformly spherical and average diameter of about 80–120 nm. Fig. 5(c,d) shows images of the different magnification of Tm3+/Yb3+:Bi2WO6. It can be seen that the presence of Tm3+ and Yb3+ ions has influenced certainly the morphology of the samples. By contrast, size of the particles is in the range of about 8060 nm. This change can be attributed to the introduction of Tm3+ and Yb3+ ions which easily occupies vacancies of WO6 octahedron structure, further limits the growth of grain and ultimately decreases the degree of crystallinity. It may help improve the photocatalytic activity. In order to comprehend the relations between doping rare earth ions and metal oxides, the composition and the chemical states of 1.0 mol %Tm3+/20 mol%Yb3+:Bi2WO6 powder are confirmed by the X-ray photoelectron spectroscopy (XPS). The elements of Bi, W, O, Tm, Yb and C can be found in the survey spectra in Fig. 6(a). Fig. 6(b–d) displays the high-resolution XPS spectra of the three basic elements (Bi, W and O). The binding energy of Bi 4f7/2 is 159.2 eV, which is 0.4 eV
Fig. 2. XRD patterns of the as-prepared samples at different calcination temperatures for 4 h.
The 1.0 mol% Tm3+/20 mol% Yb3+:Bi2WO6 samples prepared by co-precipitation crystallization method at different calcination temperatures (400–700 °C) were characterized by X-ray diffraction (XRD), and the typical diffraction patterns are shown in Fig. 2. It can be clearly seen from the XRD patterns that the high degree of crystallinity can be obtained at different calcination temperatures and no peaks about thulium and ytterbium or other impurities can be observed. This indicates that Tm3+ and Yb3+ co-doping in the co-precipitation route does not change the crystalline phase of Bi2WO6 owing to the low contents of Tm3+ and Yb3+. The diffraction peaks of all as-prepared samples can be well assigned to the pure russellite phase with a = 5.4568 Å, b = 16.4355 Å, and c = 5.43823 Å, which are in accordance with that of JCPDS card No. 39-0256. When the calcination temperature is 600 °C, the higher diffraction peaks obviously illustrate that the crystalline phase would be better. The calcination temperature of subsequent experimental sample will be set at 600 °C. Figs. 3(a,b) and 4(a,b) show the XRD patterns of the pure and Tm3+/Yb3+:Bi2WO6 with different doping content. A detailed 3
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111864
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Fig. 3. (a, b). XRD patterns and enlarged patterns for selected 2θ ranges of the pure and Tm3+/Yb3+:Bi2WO6 samples with 20 mol% Yb3+ and different Tm3+ doping content.
photogenerated electron-hole pairs. As a result, the visible light photocatalytic activities were improved. The band gap energy (Eg) of the prepared samples can be calculated by using the equation αhν=A(hνEg)n/2 where α, ν, Eg, h and A represent the absorption coefficient, light frequency, band gap energy, Planck constant and proportionality constant, respectively. The value of n is determined by the types of the transition in a semiconductor, which is n = 1 for direct transition and n = 4 for an indirect transition. The band gap energies of the pure Bi2WO6 and 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 samples are estimated to be 2.65 eV and 2.53 eV. It indicates a shift of the absorption edge to longer wavelength as well as the increase of the light absorption in the visible light region by Tm3+ and Yb3+ co-doping. Compared with pure Bi2WO6, two new absorption bands at 685 nm and 796 nm, respectively, are observed in Tm3+/Yb3+:Bi2WO6 samples. It can be attributed to the 4f electronic transitions from the 3H6 ground state to 3 F2,3 and 3H4 states of Tm3+, respectively. It is well known that the separation of photogenerated electron-hole pairs on the surface of a crystal is the driving force for the degradation of dye [26]. The effect of Tm3+/Yb3+ ions on the recombination of the photogenerated electron-hole pairs of Bi2WO6 is studied with photoluminescene (PL) spectra. Fig. 8 shows the PL spectra of pure Bi2WO6 and 1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6 within the wavelength range of 410–600 nm excited at 330 nm. Compared with pure Bi2WO6, the PL spectra intensity of Tm3+/Yb3+:Bi2WO6 exhibits greatly decrease. It indicates that the combination rate of photogenerated electron-hole pairs is suppressed effectively due to the Tm3+/Yb3+ codoped on Bi2WO6.
higher than that of pure Bi2WO6 (158.9 eV). It can be caused by the increase of electron density around the Bi3+ ions when doped with Tm3+ and Yb3+ ions. The binding energies of 35.6 eV and 37.7 eV for W 4f7/2 and W 4f5/2, respectively, can be assigned to a W6+ oxidation state. Obvious O 1s peak located at around 529.6 eV corresponds to the lattice oxygen. As shown in Fig. 6(e, f), the value of Tm 4d is 175.6 eV, but the reported binding energy is 182.18 eV in thulium oxide. The shift of binding energy of Tm 4d to the low energy position is due to the decrease of electron density around Tm3+ ion. Similarly, Yb3+ peak appeared at 185.4 eV. These results confirm that Ho3+ and Yb3+ ions are successfully doped into the Bi2WO6 lattice. Moreover, the molar ratio of Yb/Tm in Tm3+/Yb3+:Bi2WO6 is calculated to be about 12.56 according to the XPS analysis, close to the theoretical value of 13.33. The greatly improved photocatalytic activity of Tm3+/ 3+ Yb :Bi2WO6 may be due to the change of band gap energy and the transfer of energy. It is generally known that the separation of photogenerated electron-hole pairs and then the transfer of electrons and holes to the surface of the photocatalyst contribute to the photocatalytic reaction. The efficient separation of photogenerated electron-hole pairs depends on the absorbed light by the photocatalyst. The absorbed light by the pure Bi2WO6 and Tm3+/Yb3+:Bi2WO6 were measured using UV–vis diffuse reflection spectrum in the wavelength range of 300–800 nm. Fig. 7 displays the UV–vis absorption spectra of the pure Bi2WO6 and x (x = 0.2, 0.5, 1.0, 1.5, 2.0) mol%Tm3+/20 mol% Yb3+:Bi2WO6 samples. All of the samples exhibit strong absorption in visible light region where may generate photoexcited electrons for improved photocatalytic degradation of RhB. The valence band from the electronic structure of Bi2WO6 is formed by the hybrid orbital of O 2p and Bi 6 s and the conduction band of W 5d and very little Bi 6 s. It is conducive to the absorption of visible light and the separation of
Fig. 4. (a, b). XRD patterns and enlarged patterns for selected 2θ ranges of the pure and Tm3+/Yb3+:Bi2WO6 samples with1.0 mol% Tm3+ and different Yb3+ doping content. 4
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111864
Z. Shen, et al.
Fig. 5. Typical FESEM images of the different magnification of the pure Bi2WO6 (a,b) and 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 (c,d) powders.
wavelength and can successively transfer energy to Tm3+. However, Tm3+ itself can not absorb 980 nm photons and can not undergo upconversion process. Therefore, the co-doping effect of Tm3+ and Yb3+ can effectively harvest energy around 980 nm by Bi2WO6. Considering the more efficient energy transfer from Yb3+ to Tm3+, the content of Yb3+ should be higher than that of Tm3+. We can see from Fig. 9(a,b) that with the increase of Tm3+ mole fraction from 0.2% to 1.0%, all of the up-conversion emission intensities gradually increase, but all of the up-conversion emission intensities gradually decrease when the mole fraction of Tm3+ is up to 2.0%. As the concentration of Tm3+ increase gradually, the number of excited Tm3+ increase correspondingly and the luminescence intensity of nanopowders also gradually strengthen.
3.2. Up-conversion photoluminescence and the energy transfer mechanism Up-conversion luminescence spectra of the x%Tm3+/20% Yb :Bi2WO6 (x = 0.2, 0.5, 1.0, 1.5, 2.0 mol) under 980 nm continuous laser excitation and the variable relationship of all of the emissions intensity with the mole fraction of Tm3+ are shown in Fig. 9. The emission peaks at 477 nm (blue emission), 545 nm (green emission), 648 nm (red emission), and 701 nm (red emission) can be attributed to 1 G4→3H6, 1D2→3H5 and 1G4→3F4, 3F3→3H6 transitions of Tm3+, respectively [27–29]. The blue and the green up-conversion emissions are much higher than the red emissions. Since Yb3+ has a very large absorption cross section area at 980 nm, it is very efficiently excited at this 3+
Fig. 6. XPS spectrum of Tm3+/Yb3+:Bi2WO6, survey (a), Bi 4f (b), W 4f (c), O 1s (d), Tm 4d (e) and Yb 4d (f). 5
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111864
Z. Shen, et al.
Fig. 7. UV–vis diffuse reflectance spectra of BiWO6 with the various mole percentages of Tm3+ (a); Inset shows the corresponding band gap energies (b).
increases from 5% to 20%, the up-conversion luminescence of nanopowders on 477, 545, 648 and 701 nm gradually increases with the increase of Yb3+ concentration. When the Yb3+ mole fraction increases to 60%, its luminescence intensity reduces greatly relative to the highest value. Before reaching the optimal doping content of Yb3+, as the increase of Yb3+ concentration, theabsorbed photons increase on 980 nm and then Tm3+get more energy which result in the strength of luminescence intensity. With the concentration of Yb3+ continues to increase, the photons energy absorbed by Yb3+ is transferred to surface defects and organic vibration groups via bridging effect between Yb3+Yb3+ and surface defects, resonance energy transfer and then the energy is lost by non-radiative process, so the up-conversion luminescence intensity decrease, which is one of the main mechanisms of the Yb3+ concentration quenching. As a result, the overall luminescence intensity decreases. Up-conversion mechanism in Tm3+/Yb3+ co-doped system has been widely discussed. The up-conversion luminescence strongly depends on energy transfer between up-conversion agent (Tm3+/Yb3+) and the host materials (Bi2WO6). Intense blue up-conversion emission centers at 477 nm, which is assigned to 1G4→3H6 energy level transition. Green luminescence emission around 545 nm originates from 1 D2→3H5 radiative transition. Weak red up-conversion emissions center at 648 nm and 701 nm corresponding to 1G4→3F4 and 3F3→3H6 energy level transitions, respectively. For a better understanding of the up-conversion process, the pump power dependence of the up-conversion emission is measured. For small up-conversion rates, the luminescence intensity, IUC, follows the relation IUC∝Pinn, where Pin is the incident pump power and n is the number of photons required to populate the emitting state. The plot of IUC versus Pin in double logarithmic scale yields a straight line with a slope n. As shown in Fig. 11 the value of n is close to 2 which indicates that the up-conversion is governed by the two photonic processes.
Fig. 8. Photoluminescene spectra of pure Bi2WO6 and 1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6.
When the mole fraction of Tm3+ up to 1.0%, the up-conversion luminescence intensity reaches maximum value, and then gradually weaken. It can be ascribed to the decrease of distance between adjacent ions and the enhancement of the interaction between ions due to the increase of Tm3+ concentration, generating the concentration quenching and cross-relaxation effects and eventually causing the decrease of luminescence intensity [30,31]. Based on the above research, up-conversion luminescence spectra of the 1.0 mol%Tm3+/y%Yb3+:Bi2WO6 (y = 5, 10, 20, 40, 60 mol) under 980 nm continuous laser excitation and the variable relationship of all of the emission intensities with the mole fraction of Yb3+ are displayed in Fig. 10. As seen from Fig. 10(a), all of the spectra have the same position as that of Fig. 9(a). Similarly, when the mole fraction of Yb3+
Fig. 9. Up-conversion luminescence spectra of x%Tm3+/20 mol%Yb3+:Bi2WO6 with different Tm3+ mole fraction, λex = 980 nm (a); The inset shows the variation of the blue, green and red up-conversion luminescence with Tm3+ mole fraction (b). 6
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111864
Z. Shen, et al.
Fig. 10. Up-conversion luminescence spectra of 1.0 mol%Tm3+/y%Yb3+:Bi2WO6 with different Yb3+ mole fraction, λex = 980 nm (a); The inset shows the variation of the blue, green and red up-conversion luminescence with Yb3+ mole fraction (b).
are prevalently used as UC mechanisms in RE doped UC materials. Yb3+ ions could be excited from the ground state 5F7/2 to 5F5/2 level by the pump light. Subsequently, the three successive energy transfer (ET) processes from Yb3+ to Tm3+ populate the 3H5, 3F2, and 1G4 energy levels of the Tm3+ ions, but the 1D2 level of the Tm3+ ions cannot be populated directly via the fourth energy transfers from Yb3+ ions because of the large energy mismatch (about 3500 cm−1) between the 1G4 and the 1D2 levels. And then the efficient cross relaxation process 3F2 (or 3F3) + 3H4 / 3H6 + 1D2 between two Tm3+ ions may result in the population of the 1D2 level. Simultaneously, some of excited Tm3+ ions fall to the lower energy level, which lead to the radiative transitions of 1 D2 / 3H6 (364 nm), 1D2 / 3F4 (454 nm), 1G4 / 3H6 (478 nm), 1D2 / 3H5 (542 nm), 1G4 / 3F4 (652 nm), 3F3 / 3H6 (698 nm), respectively. Due to the narrow energy gap of Bi2WO6, most of the fluorescence emission energy can be further utilized by the photocatalyst to form photoelectrons (e−) and holes (h+), which are generally considered to play a vital role in the photocatalytic process. In addition, the effect on fluorescence intensity by concentration quenching should be noticed. The tendency of the blue emission intensity sharp fall in high Tm3+/ Yb3+ concentration cases can be obviously observed in the spectra of up-conversion luminescence. In order to further explore the effect of upconversion luminescence on the photocatalytic degradation process, the photocatalytic experiment under near-infrared light irradiation was showed in Fig. 12(d). The RhB degradation degree for 1.0 mol%Tm3+/ 20 mol%Yb3+:Bi2WO6 under 120 min with near-infrared light irradiation is 21.56%. In contrast, the RhB degradation is almost not been examined for Bi2WO6 with near-infrared irradiation. It could be determined that upconversion luminescence has a certain promoting effect on photocatalysis. Sequentially, the decreased emission energy result in a weakening of the subsequent photocatalytic process. The photocatalytic degradation process by all samples followed the firstorder rate law, -ln (Ct/C0) as a function of reaction time t is exhibited in Fig. 12(e–g), and the rate constant of 1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6 is estimated to be 0.11589 min−1 under simulated sunlight irradiation and 0.00163 min-1 under near-infrared irradiation respectively. In particular, the photocatalytic ability for different dye concentrations has been also given in Fig. 12(h). 1.0 mol%Tm3+/ 20 mol%Yb3+:Bi2WO6 exhibits the prominent photocatalytic capacity with dye concentrations of 2 × 10-5 mol/L, but decreasing gradually with the increment of the dye concentrations. Meanwhile, in order to study the further application, the cycle degradation results are revealed in Fig. 12 (i). There is a slightly deactivation even though the catalyst has been recycled fourth times, which demonstrated the stability of the photocatalysts.
Fig. 11. Dependence of the integrated up-conversion intensities (IUC) on the pump power (Pin). The straight lines are the fittings obtained by linear regression to the experimental data for each wavelength.
3.3. Photocatalytic test The photocatalytic activity of the as-prepared samples was investigated by the degradation of a model dye RhB solution under the visible light irradiation. Fig. 12(a) shows the UV–vis absorption spectra of RhB by 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 after different photocatalytic durations. The RhB absorption peak maximized at 554 nm decreases rapidly with the increase of irradiation time and it completely disappears after about 20 min. Further irradiation causes no absorption peak, which indicates the complete degradation of RhB. A series of color changes of the 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 sample are shown in the inset of Fig. 12(a). It is also consistent with the corresponding changes of the absorption measurements. To differentiate the photocatalytic degradation of Bi2WO6 doped with different Tm3+ and Yb3+ contents, these results are revealed in Fig. 12(b,c). From the above results, the excellent photocatalytic activity is observed when the concentration of Tm3+ and Yb3+ are 1.0 mol% and 20 mol%, respectively. The photocatalytic activities of the Tm3+/Yb3+:Bi2WO6 photocatalysts increase gradually with the Tm3+ and Yb3+ concentrations until the Tm3+ and Yb3+ concentrations are 1.0 mol% and 20 mol% respectively. However, when the Tm3+ and Yb3+ concentrations are more than 1.0 mol% and 20 mol%, the photocatalytic activity decreases sharply, which is consistent with the trend of fluorescence intensity in the inset of Figs. 9 and 10. This can be explained by the following reasons: this involves the mainly possible synergic reinforcement between up-conversion luminescence and photocatalytic processes in the Tm3+/Yb3+ doped materials as shown in the inserted schematic diagram of Fig. 13. Excited state absorption (ESA) and energy transfer (ET)
4. Conclusion In summary, Tm3+/ Yb3+ co-doped Bi2WO6 has been successfully prepared by a facile co-precipitation route and a sinter process. The 7
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Fig. 12. (a) The UV–vis spectral change of RhB over 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 sample with irradiation time; (b,c) The photocatalytic degradation of RhB with different Tm3+and Yb3+ concentration under simulated sunlight irradiation; (d) The photocatalytic degradation of RhB with1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6 sample under near-infrared irradiation; (e–g) The kinetic curves of RhB degradation with different Tm3+and Yb3+ concentration; (h) The photocatalytic degradation for different RhB concentration;(i) Cycling experiment of the photocatalytic degradation of RhB with 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6.
effect on fluorescence intensity by doping concentration has been investigated in detail, which laid a foundation for subsequent upconversion photocatalysis studies, and the result showed that the excellent
photocatalytic activity was consistent with the tendency of the blue emission intensity. The optimal doping concentrations for Tm3+ and Yb3+ which achieve maximum luminescence intensities has a
Fig. 13. The photocatalytic mechanism for the Tm3+/Yb3+-Bi2WO6 upconversion photocatalyst. 8
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significant impact on photocatalytic ability. This is not only reflected in the near-infrared photocatalytic performance, but also in the narrower band gap after doping rare earth ions and the lower hole-electron recombination rate. This work will provide some references for the further study of rare earth doping.
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Acknowledgements This work was supported by National Key R&D Program of China (No. 2017YFC1600403), Natural Science Foundation of Liaoning (No.20170540075) and China Scholarship Council (No. 201708210045). References [1] Huanli Wang, Lisha Zhang, Zhigang Chen, Junqing Hu, Shijie Li, Zhaohui Wang, Jianshe Liu, Xinchen Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234–5244. [2] Chuncheng Chen, Wanhong Ma, Jincai Zhao, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chemical Society Reviews. 39 (2010) 4206–4219. [3] Hao Li, Jie Li, Zhihui Ai, Falong Jia, Lizhi Zhang, OxygenVacancy-mediated photocatalysis of BiOCl: reactivity,Selectivity and perspective, Angew. Chemie Int. Ed. 57 (2018) 122–138. [4] V. Sara Thoi, Nikolay Kornienko, Charles G. Margarit, Peidong Yang, Christopher J. Chang, Visible-light photoredox catalysis: selective reduction of carbon dioxide to carbon monoxide by a nickel N-heterocyclic carbene−isoquinoline complex, Am. Chem. Soc. 135 (2013) 14413–14424. [5] Yangen Zhou, Yongfan Zhang, Mousheng Lin, Jinlin Long, Zizhong Zhang, Huaxiang Lin, Jeffrey C.-S. Wu, Xuxu Wang, Monolayered Bi2WO6 nanosheets mimicking heterojunction interface with open surfaces for photocatalysis, Nat. Commun. 6 (2015) 8340–8348. [6] Guangcheng Xi, Jinhua Ye, Synthesis of bismuth vanadate nanoplates with exposed {001} facets and enhanced visible-light photocatalytic properties, Chem. Commun. 46 (2010) 1893–1895. [7] Rajesh Adhikari, Gobinda Gyawali, Sung Hun Cho, R. Narro-García, Tohru Sekino, Soo Wohn Lee, Er3+/Yb3+co-doped bismuth molybdate nanosheets upconversion photocatalyst with enhanced photocatalytic activity, J. Solid State Chem. 209 (2014) 74–81. [8] Miguel Pelaez, Nicholas T. Nolan, Suresh C. Pillai, Michael K. Seery, Polycarpos Falaras, Athanassios G. Kontos, Patrick S.M. Dunlop, Jeremy W.J. Hamilton, J.Anthony Byrne, Kevin O’Shea, Mohammad H. Entezari, Dionysios D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B 125 (2012) 331–349. [9] Meiling Shao, Haitao Chen, Ming Shen, Wuxia Chen, Synthesis and photocatalytic properties of In2O3 micro/nanostructures with different morphologies, Colloids Surf. A Physicochem. Eng. Asp. 529 (2017) 503–507. [10] Yu-Liang Liu, Chuan-Lu Yang, Mei-Shan Wang, Xiao-Guang Ma, You-Gen Yi, Tedoped perovskite NaTaO3 as a promising photocatalytic material for hydrogen production from water splitting driven by visible light, Mater. Res. Bull. 107 (2018) 125–131. [11] Yongbao Liu, Gangqiang Zhu, Jianzhi Gao, Mirabbos Hojamberdiev, Runliang Zhu, Xiumei Wei, Quanmin Guo, Peng Liu, Enhanced photocatalytic activity of Bi4Ti3O12 nanosheets by Fe3+-doping and the addition of Au nanoparticles: photodegradation of Phenol and bisphenol A, Appl. Catal. B 200 (2017) 72–82. [12] Jin Wang, De Nyago Tafen, James P. Lewis, Zhanglian Hong, Ayyakkannu Manivannan, Mingjia Zhi, Ming Li, Wu Nianqiang, Origin of photocatalytic activity of nitrogen-doped TiO2 nanobelts, J. Am. Chem. Soc. 131 (2009) 12290–12297.
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Novel Tm3+ and Yb3+ co-doped bismuth tungstate up-conversion photocatalyst with greatly improved photocatalytic properties ⁎
T
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Zhe Shena, Hong Lia, Hongshun Haoa,b, , Zhengguang Chena, Hongman Houb, , Gongliang Zhangb, Jingran Bib, Shuang Yana, Guishan Liua, Wenyuan Gaoa a b
Department of Inorganic Nonmetallic Materials Engineering, Dalian Polytechnic University, Dalian 116034, China Liaoning Key Lab for Aquatic Processing Quality and Safety, Dalian Polytechnic University, Dalian 116034, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bi2WO6 Photocatalysis Up-conversion Rare earth Optical properties
Tm3+ and Yb3+ co-doped Bi2WO6 up-conversion photocatalysts (Tm3+/Yb3+:Bi2WO6) were synthesized via coprecipitation route at different calcination temperatures. Doping Tm3+ and Yb3+ into Bi2WO6 lattice and substituting for the fractional Bi3+ were successfully confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. The morphology and optical property of the obtained samples were characterized by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS) and up-conversion luminescence spectra (UC-PL). The photocatalytic results revealed that the Tm3+/Yb3+:Bi2WO6 exhibited a much superior photocatalytic activity than the pure Bi2WO6 and commercial Degussa P25 for the degradation of rhodamine B (RhB), and the degradation rate reached 97.6% in 25 min. The effects of Tm3+ and Yb3+ on the photocatalytic activity of Bi2WO6 were studied in detail. The improved photocatalytic activity of Bi2WO6 could be attributed to the efficient energy transfer between Tm3+/Yb3+ and Bi2WO6 via infrared to visible up-conversion and the reduced recombination rate of photogenerated electron-hole pairs through co-doping of Tm3+/Yb3+ on Bi2WO6 nanopaticles.
1. Introduction Environmental problems caused by the excessive use of organic dye pollutants have seriously threatened human existence and sustainable development of society [1]. Photocatalysis is a green and energy saving technology for the elimination of the organic pollutants in the environment and it has received worldwide attention [2–4]. Currently, various semiconductor photocatalysts such as TiO2, ZnO, BiVO4 and Bi2MO6 have been developed [5–7]. Because of its low cost, nontoxicity, high chemical stability and strong oxidizing abilities, TiO2 has been the most widely studied and applied in many fields, especially for the degradation of organic pollutants in wastewater [8]. However, due to its large band gap energy of 3.2 eV, TiO2 can only respond in the ultraviolet light that occupies only about 4% energy of the total solar spectrum, but visible light accounts for 43% of sunlight spectrum. In order to efficiently use solar energy, many efforts have been focused on the extension of the response of photocatalysts from ultraviolet light into visible light region. Doping metallic (Pt, Au and Ag) and nonmetallic (S, N and B) elements into conventional semiconductor photocatalysts, such as In2O3 [9], NaTaO3 [10], and Bi4Ti3O12 [11],
⁎
decreases the band gap energy and mostly takes advantage of the available energy in the sunlight [12–15]. Unfortunately, the efficient utilization of sunlight energy is not capable of solving the severe energy and environmental crises. Therefore, it is reasonable to fabricate new materials with high-efficiency energy transformation that can transform near infrared (NIR) light to the visible and ultraviolet light. This desirable process is known as up-conversion (UC) luminescence [16,17]. Since the initial report of up-conversion process by Auzel et al in 1960, up-conversion luminescence has attracted considerable attention and become focus of the research. Rare earth ions doped UC nanoparticles can effectively convert low energy radiation (NIR light) into higher energy radiation (visible light) by means of multiple absorption or energy transfer process. Up-conversion materials have demonstrated wide application in solid-state lasers, biomedical diagnostics, optical temperature sensors, solar cells and other fields. Recently, related studies had revealed that up-conversion process could be employed for the effective photocatalysts. Incorporating up-conversion agent into the traditional photocatalysts for the improvement of photocatalytic efficiency has been reported in many literatures [18–21]. Bi2WO6, as a typical layered Aurivillius oxide consisted of
Corresponding authors at: Liaoning Key Lab for Aquatic Processing Quality and Safety, Dalian Polytechnic University, Dalian 116034, China E-mail addresses: [email protected] (H. Hao), [email protected] (H. Hou).
https://doi.org/10.1016/j.jphotochem.2019.111864 Received 9 January 2019; Received in revised form 25 April 2019; Accepted 15 May 2019 Available online 16 May 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
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perovskite-like slab of WO6, owing to its excellent nonlinear dielectric, ion-conductive, luminescent and catalytic properties, has aroused great interest [22]. Various research has shown that Bi2WO6 could exhibit strong photocatalytic efficiency for water splitting and the degradation of organic compounds such as acetaldehyde, acetic acid, 4-chlorophenol and rhodamine B (RhB). However, pure Bi2WO6 possesses the narrow photo-response range and high combination rate of photogenerated electron-hole pairs in the photocatalytic reaction. Fortunately, up-conversion refers to absorption of multiple low energy photons into the intermediate level to form high energy photons, which efficiently increased the number of photons for a desired energy transfer. In the photocatalysis process, the higher energy photons can be absorbed to generate more electrons that could degrade the organic pollutants. For example, Er3+ and Yb3+ co-doped TiO2-xFx, as a light scattering layer, has been applied into dye sensitized solar cell (DSSC) [23], to harvest higher light so as to achieve the satisfied DSSC efficiency. Visible light responsive photocatalysts including Er3+/Yb3+ codoped Bi2MoO6 [24], Ho3+/Yb3+ and Er3+/Yb3+co-doped Bi2WO6 [25] have been reported in recent years. However, Tm3+/Yb3+: Bi2WO6 up-conversion photocatalyst has not been studied yet. Herein, in this work, we report the synthesis of visible light responsive Tm3+/Yb3+:Bi2WO6 up-conversion photocatalyst by co-precipitation method for the first time. The samples were synthesized with different calcination temperatures and Tm3+ and Yb3+ contents. Moreover, we obtained the excellent photocatalyst with optimal calcination temperature and Tm3+ and Yb3+ contents. By harvesting NIR and low energy photons, Tm3+ and Yb3+ emiting UV and visible light that matches well with the band gap of Bi2WO6. The effects of Tm3+ and Yb3+ on the photocatalytic properties were studied in detail. Under visible light irradiation, Tm3+/Yb3+:Bi2WO6 up-conversion photocatalyst exhibited higher photocatalytic efficiency of degrading RhB than undoped Bi2WO6.
with a low heating rate (5 °C min−1) until the weight loss of the material was negligible. Pure Bi2WO6 was prepared by the same process without the addition of rare earth ions.
2. Experimental
The photocatalytic activities of the obtained samples were evaluated by the degradation of RhB. Using a 500 W Xe lamp as simulated sunlight source. In addition, a 500 W Xe lamp was used as the near-infrared light source where the λ < 800 nm light were filtered out during nearinfrared light photocatalysis. Before irradiation, 70 mg of photocatalyst was added into RhB aqueous solution (50 mL,10−5 mol·L-1) under magnetically stirring for 30 min in the darkness to achieve adsorptiondesorption equilibrium between the photocatalyst and RhB solution. In the irradiation process, 4 mL of suspension was taken out at given time intervals and centrifuged to remove the remaining photocatalyst particles. The concentration of RhB was calculated by analyzing the measured absorbance at 554 nm with a UV–vis spectrometer (UV-1800, Shimadzu). Besides, in order to investigate the stability of the photocatalyst, the recycling test was also conducted. After the above photocatalytic measurements, the remnant RhB solution was centrifuged. The obtained powders were dried at 80 °C for 10 h and repeated the photocatalytic experiments for four times.
2.3. Characterization methods The phase and crystallinity of the prepared samples were characterized by X-ray diffraction (XRD-7000, SHIMADZU) with Cu Kα radiation source (λ = 0.1546 nm) at a scanning rate of 5° /min in the range of 20-70°. The morphologies and sizes of the samples were observed with a field-emission scanning electron microscopy (FESEM, JSM-7800 F) and transmission electron microscopy (TEM, JEOLJEM2100). The composition and chemical states of the samples were identified by X-ray photoelectron spectroscopy (XPS, Thermo scientific K-Alpha). All the XPS data were calibrated using C1 s peak at 284.6 eV. The ultraviolet visible diffuse reflectance spectra were measured by an UV–vis spectrophotometer (UV–vis, Lambda35). The up-conversion luminescence spectra in the range of 500–700 nm were obtained by a fluorescence spectrometer (F-7000, Hitachi, Japan) equipped with 980 nm laser diode excitation source. In order to compare the intensity of up-conversion luminescence, the different samples were measured under the following identical conditions: the powders were filled into a sample tank with a diameter of 10 mm and a thickness of 2 mm and fixed on the sample table. The laser emission point was focused onto the sample with a 7 cm focal length lens and the focused laser beam had a spot diameter of 375 mm at 1/e2 of the maximum intensity. EX Slit and EM Slit were 5.0 nm and 10 nm, respectively, and scan speed was 40 nm/min. The incident angle of laser beam was maintained at 90° and the excitation power was 250 mW. All of the measurements were performed at room temperature. 2.4. Photocatalytic measurements
2.1. Materials and reagents All the chemicals used were analytical pure reagents and without further purification. Bi(NO3)3·5H2O (99.99%), Na2WO4·2H2O (99.99%), Tm2O3 (99.9%), Yb2O3(99.9%), nitric acid, sodium hydroxide, ethanol absolute and rhodamine B (RhB) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China). Aqueous sodium hydroxide solution was used to adjust the pH value of the solution. Nitrate solutions of Tm3+ and Yb3+ were prepared by dissolving the corresponding metal oxide in double dilute nitric acid at 90 °C. Degussa P25 (˜80％ anatase and ˜20％ rutile) was provided by Degussa (China) Co., Ltd. Ethanol absolute and deionized water were used during washing process. 2.2. Synthesis of photocatalyst
3. Results and discussion
The samples were synthesized by co-precipitation method. In this typical procedure, Bi(NO3)3·5H2O (1.21 g, 2.5 mmol) and Na2WO4·2H2O (0.41 g, 1.25 mmol) were dissolved in nitric acid solution (50 mL, 1.0 mol·L−1) and deionized water (50 mL), respectively. Rare earth (Tm3+ and Yb3+) nitrate solutions were added to Bi (NO3)3·5H2O solution. Then, Na2WO4·2H2O solution was dropwise added to the above solution under vigorous stirring. The pH value of the obtained suspension was adjusted to 5.5 by using concentrated NaOH solution (2 mol·L−1). The mixture was stirred and kept in a water bath at 80 °C for 2 h in order to obtain a slow evaporation of water until the formation of a white solid. The precursor was cooled to room temperature naturally and individually centrifuged and washed with ethanol absolute and deionized water three times, respectively, dried at 80 °C for 12 h. The as-synthesized samples were obtained by calcination of the powder in air by thermal treatments at 400, 500, 600 and 700 °C
3.1. Characterization on Tm3+/Yb3+:Bi2WO6 photocatalyst Bi2WO6, as one of the most important members in the layered Aurivillius oxide family, belongs to perovskite layers crystal structure consisting of arranging alternately in the form of bismuth oxide (Bi2O2)2+ layers and WO6 octahedron layers. This layered structure is beneficial to increase the separation efficiency of photogenerated electrons (e−) and holes (h+). As presented in Fig. 1, the Tm3+ and Yb3+ ions are expected to partly substitute the Bi3+ ions in (Bi2O2)2+ layers when introduced into the crystal lattice of Bi2WO6. It indicates that the unit cell is composed of (TmxYbyBi2-x-yO2)2+ groups and WO6 groups. Moreover, it remains the well-layered structure with single phase of orthorhombic after co-doping. 2
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Fig. 1. The crystal structure of Bi2WO6.
comparison and analysis of the XRD patterns of pure ad doped Bi2WO6 samples shows the shifts of diffraction peak position to higher 2θ angles. It indicates that the variation in the unit cell volume is due to the substitution of Bi3+ by Tm3+ and Yb3+. The lattice substitution of Bi3+ in the (Bi2O2)2+ by Tm3+ and Yb3+ rely on the matching of oxidation. Little decrease in cell volume can be explained by the substitution of smaller ionic radii of Tm3+/Yb3+ (88 pm/86.8 pm) for the larger Bi3+ (117 pm) ions. This result revealed that the Tm3+ and Yb3+ ions have been incorporated into the crystal lattice of Bi2WO6 and substituted for the fractional Bi3+ ions. In addition, the effect of doping content on the crystalline phase was also described by the changes of XRD patterns. A huge increase in the intensity ratio of (131)/(200) can be found in these samples with Tm3+ doping content of 1.0 mol% and Yb3+ of 20 mol%. It illustrates the formation of better crystalline. However, the ratio value decreases gradually with the increasing doping contents. The change of the intensity ratio suggests that the variation is preferred orientation growth with the incorporation of dopant ions. The general morphologies and particle sizes of the pure Bi2WO6 and 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 powders are observed by FESEM. Fig. 5(a,b) presented images of the different magnification of the pure Bi2WO6. It reveals that there are some aggregated particles blocks, which is caused by large surface energy and many dangling bonds. Most of the particles appear uniformly spherical and average diameter of about 80–120 nm. Fig. 5(c,d) shows images of the different magnification of Tm3+/Yb3+:Bi2WO6. It can be seen that the presence of Tm3+ and Yb3+ ions has influenced certainly the morphology of the samples. By contrast, size of the particles is in the range of about 8060 nm. This change can be attributed to the introduction of Tm3+ and Yb3+ ions which easily occupies vacancies of WO6 octahedron structure, further limits the growth of grain and ultimately decreases the degree of crystallinity. It may help improve the photocatalytic activity. In order to comprehend the relations between doping rare earth ions and metal oxides, the composition and the chemical states of 1.0 mol %Tm3+/20 mol%Yb3+:Bi2WO6 powder are confirmed by the X-ray photoelectron spectroscopy (XPS). The elements of Bi, W, O, Tm, Yb and C can be found in the survey spectra in Fig. 6(a). Fig. 6(b–d) displays the high-resolution XPS spectra of the three basic elements (Bi, W and O). The binding energy of Bi 4f7/2 is 159.2 eV, which is 0.4 eV
Fig. 2. XRD patterns of the as-prepared samples at different calcination temperatures for 4 h.
The 1.0 mol% Tm3+/20 mol% Yb3+:Bi2WO6 samples prepared by co-precipitation crystallization method at different calcination temperatures (400–700 °C) were characterized by X-ray diffraction (XRD), and the typical diffraction patterns are shown in Fig. 2. It can be clearly seen from the XRD patterns that the high degree of crystallinity can be obtained at different calcination temperatures and no peaks about thulium and ytterbium or other impurities can be observed. This indicates that Tm3+ and Yb3+ co-doping in the co-precipitation route does not change the crystalline phase of Bi2WO6 owing to the low contents of Tm3+ and Yb3+. The diffraction peaks of all as-prepared samples can be well assigned to the pure russellite phase with a = 5.4568 Å, b = 16.4355 Å, and c = 5.43823 Å, which are in accordance with that of JCPDS card No. 39-0256. When the calcination temperature is 600 °C, the higher diffraction peaks obviously illustrate that the crystalline phase would be better. The calcination temperature of subsequent experimental sample will be set at 600 °C. Figs. 3(a,b) and 4(a,b) show the XRD patterns of the pure and Tm3+/Yb3+:Bi2WO6 with different doping content. A detailed 3
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Fig. 3. (a, b). XRD patterns and enlarged patterns for selected 2θ ranges of the pure and Tm3+/Yb3+:Bi2WO6 samples with 20 mol% Yb3+ and different Tm3+ doping content.
photogenerated electron-hole pairs. As a result, the visible light photocatalytic activities were improved. The band gap energy (Eg) of the prepared samples can be calculated by using the equation αhν=A(hνEg)n/2 where α, ν, Eg, h and A represent the absorption coefficient, light frequency, band gap energy, Planck constant and proportionality constant, respectively. The value of n is determined by the types of the transition in a semiconductor, which is n = 1 for direct transition and n = 4 for an indirect transition. The band gap energies of the pure Bi2WO6 and 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 samples are estimated to be 2.65 eV and 2.53 eV. It indicates a shift of the absorption edge to longer wavelength as well as the increase of the light absorption in the visible light region by Tm3+ and Yb3+ co-doping. Compared with pure Bi2WO6, two new absorption bands at 685 nm and 796 nm, respectively, are observed in Tm3+/Yb3+:Bi2WO6 samples. It can be attributed to the 4f electronic transitions from the 3H6 ground state to 3 F2,3 and 3H4 states of Tm3+, respectively. It is well known that the separation of photogenerated electron-hole pairs on the surface of a crystal is the driving force for the degradation of dye [26]. The effect of Tm3+/Yb3+ ions on the recombination of the photogenerated electron-hole pairs of Bi2WO6 is studied with photoluminescene (PL) spectra. Fig. 8 shows the PL spectra of pure Bi2WO6 and 1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6 within the wavelength range of 410–600 nm excited at 330 nm. Compared with pure Bi2WO6, the PL spectra intensity of Tm3+/Yb3+:Bi2WO6 exhibits greatly decrease. It indicates that the combination rate of photogenerated electron-hole pairs is suppressed effectively due to the Tm3+/Yb3+ codoped on Bi2WO6.
higher than that of pure Bi2WO6 (158.9 eV). It can be caused by the increase of electron density around the Bi3+ ions when doped with Tm3+ and Yb3+ ions. The binding energies of 35.6 eV and 37.7 eV for W 4f7/2 and W 4f5/2, respectively, can be assigned to a W6+ oxidation state. Obvious O 1s peak located at around 529.6 eV corresponds to the lattice oxygen. As shown in Fig. 6(e, f), the value of Tm 4d is 175.6 eV, but the reported binding energy is 182.18 eV in thulium oxide. The shift of binding energy of Tm 4d to the low energy position is due to the decrease of electron density around Tm3+ ion. Similarly, Yb3+ peak appeared at 185.4 eV. These results confirm that Ho3+ and Yb3+ ions are successfully doped into the Bi2WO6 lattice. Moreover, the molar ratio of Yb/Tm in Tm3+/Yb3+:Bi2WO6 is calculated to be about 12.56 according to the XPS analysis, close to the theoretical value of 13.33. The greatly improved photocatalytic activity of Tm3+/ 3+ Yb :Bi2WO6 may be due to the change of band gap energy and the transfer of energy. It is generally known that the separation of photogenerated electron-hole pairs and then the transfer of electrons and holes to the surface of the photocatalyst contribute to the photocatalytic reaction. The efficient separation of photogenerated electron-hole pairs depends on the absorbed light by the photocatalyst. The absorbed light by the pure Bi2WO6 and Tm3+/Yb3+:Bi2WO6 were measured using UV–vis diffuse reflection spectrum in the wavelength range of 300–800 nm. Fig. 7 displays the UV–vis absorption spectra of the pure Bi2WO6 and x (x = 0.2, 0.5, 1.0, 1.5, 2.0) mol%Tm3+/20 mol% Yb3+:Bi2WO6 samples. All of the samples exhibit strong absorption in visible light region where may generate photoexcited electrons for improved photocatalytic degradation of RhB. The valence band from the electronic structure of Bi2WO6 is formed by the hybrid orbital of O 2p and Bi 6 s and the conduction band of W 5d and very little Bi 6 s. It is conducive to the absorption of visible light and the separation of
Fig. 4. (a, b). XRD patterns and enlarged patterns for selected 2θ ranges of the pure and Tm3+/Yb3+:Bi2WO6 samples with1.0 mol% Tm3+ and different Yb3+ doping content. 4
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Fig. 5. Typical FESEM images of the different magnification of the pure Bi2WO6 (a,b) and 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 (c,d) powders.
wavelength and can successively transfer energy to Tm3+. However, Tm3+ itself can not absorb 980 nm photons and can not undergo upconversion process. Therefore, the co-doping effect of Tm3+ and Yb3+ can effectively harvest energy around 980 nm by Bi2WO6. Considering the more efficient energy transfer from Yb3+ to Tm3+, the content of Yb3+ should be higher than that of Tm3+. We can see from Fig. 9(a,b) that with the increase of Tm3+ mole fraction from 0.2% to 1.0%, all of the up-conversion emission intensities gradually increase, but all of the up-conversion emission intensities gradually decrease when the mole fraction of Tm3+ is up to 2.0%. As the concentration of Tm3+ increase gradually, the number of excited Tm3+ increase correspondingly and the luminescence intensity of nanopowders also gradually strengthen.
3.2. Up-conversion photoluminescence and the energy transfer mechanism Up-conversion luminescence spectra of the x%Tm3+/20% Yb :Bi2WO6 (x = 0.2, 0.5, 1.0, 1.5, 2.0 mol) under 980 nm continuous laser excitation and the variable relationship of all of the emissions intensity with the mole fraction of Tm3+ are shown in Fig. 9. The emission peaks at 477 nm (blue emission), 545 nm (green emission), 648 nm (red emission), and 701 nm (red emission) can be attributed to 1 G4→3H6, 1D2→3H5 and 1G4→3F4, 3F3→3H6 transitions of Tm3+, respectively [27–29]. The blue and the green up-conversion emissions are much higher than the red emissions. Since Yb3+ has a very large absorption cross section area at 980 nm, it is very efficiently excited at this 3+
Fig. 6. XPS spectrum of Tm3+/Yb3+:Bi2WO6, survey (a), Bi 4f (b), W 4f (c), O 1s (d), Tm 4d (e) and Yb 4d (f). 5
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Fig. 7. UV–vis diffuse reflectance spectra of BiWO6 with the various mole percentages of Tm3+ (a); Inset shows the corresponding band gap energies (b).
increases from 5% to 20%, the up-conversion luminescence of nanopowders on 477, 545, 648 and 701 nm gradually increases with the increase of Yb3+ concentration. When the Yb3+ mole fraction increases to 60%, its luminescence intensity reduces greatly relative to the highest value. Before reaching the optimal doping content of Yb3+, as the increase of Yb3+ concentration, theabsorbed photons increase on 980 nm and then Tm3+get more energy which result in the strength of luminescence intensity. With the concentration of Yb3+ continues to increase, the photons energy absorbed by Yb3+ is transferred to surface defects and organic vibration groups via bridging effect between Yb3+Yb3+ and surface defects, resonance energy transfer and then the energy is lost by non-radiative process, so the up-conversion luminescence intensity decrease, which is one of the main mechanisms of the Yb3+ concentration quenching. As a result, the overall luminescence intensity decreases. Up-conversion mechanism in Tm3+/Yb3+ co-doped system has been widely discussed. The up-conversion luminescence strongly depends on energy transfer between up-conversion agent (Tm3+/Yb3+) and the host materials (Bi2WO6). Intense blue up-conversion emission centers at 477 nm, which is assigned to 1G4→3H6 energy level transition. Green luminescence emission around 545 nm originates from 1 D2→3H5 radiative transition. Weak red up-conversion emissions center at 648 nm and 701 nm corresponding to 1G4→3F4 and 3F3→3H6 energy level transitions, respectively. For a better understanding of the up-conversion process, the pump power dependence of the up-conversion emission is measured. For small up-conversion rates, the luminescence intensity, IUC, follows the relation IUC∝Pinn, where Pin is the incident pump power and n is the number of photons required to populate the emitting state. The plot of IUC versus Pin in double logarithmic scale yields a straight line with a slope n. As shown in Fig. 11 the value of n is close to 2 which indicates that the up-conversion is governed by the two photonic processes.
Fig. 8. Photoluminescene spectra of pure Bi2WO6 and 1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6.
When the mole fraction of Tm3+ up to 1.0%, the up-conversion luminescence intensity reaches maximum value, and then gradually weaken. It can be ascribed to the decrease of distance between adjacent ions and the enhancement of the interaction between ions due to the increase of Tm3+ concentration, generating the concentration quenching and cross-relaxation effects and eventually causing the decrease of luminescence intensity [30,31]. Based on the above research, up-conversion luminescence spectra of the 1.0 mol%Tm3+/y%Yb3+:Bi2WO6 (y = 5, 10, 20, 40, 60 mol) under 980 nm continuous laser excitation and the variable relationship of all of the emission intensities with the mole fraction of Yb3+ are displayed in Fig. 10. As seen from Fig. 10(a), all of the spectra have the same position as that of Fig. 9(a). Similarly, when the mole fraction of Yb3+
Fig. 9. Up-conversion luminescence spectra of x%Tm3+/20 mol%Yb3+:Bi2WO6 with different Tm3+ mole fraction, λex = 980 nm (a); The inset shows the variation of the blue, green and red up-conversion luminescence with Tm3+ mole fraction (b). 6
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Fig. 10. Up-conversion luminescence spectra of 1.0 mol%Tm3+/y%Yb3+:Bi2WO6 with different Yb3+ mole fraction, λex = 980 nm (a); The inset shows the variation of the blue, green and red up-conversion luminescence with Yb3+ mole fraction (b).
are prevalently used as UC mechanisms in RE doped UC materials. Yb3+ ions could be excited from the ground state 5F7/2 to 5F5/2 level by the pump light. Subsequently, the three successive energy transfer (ET) processes from Yb3+ to Tm3+ populate the 3H5, 3F2, and 1G4 energy levels of the Tm3+ ions, but the 1D2 level of the Tm3+ ions cannot be populated directly via the fourth energy transfers from Yb3+ ions because of the large energy mismatch (about 3500 cm−1) between the 1G4 and the 1D2 levels. And then the efficient cross relaxation process 3F2 (or 3F3) + 3H4 / 3H6 + 1D2 between two Tm3+ ions may result in the population of the 1D2 level. Simultaneously, some of excited Tm3+ ions fall to the lower energy level, which lead to the radiative transitions of 1 D2 / 3H6 (364 nm), 1D2 / 3F4 (454 nm), 1G4 / 3H6 (478 nm), 1D2 / 3H5 (542 nm), 1G4 / 3F4 (652 nm), 3F3 / 3H6 (698 nm), respectively. Due to the narrow energy gap of Bi2WO6, most of the fluorescence emission energy can be further utilized by the photocatalyst to form photoelectrons (e−) and holes (h+), which are generally considered to play a vital role in the photocatalytic process. In addition, the effect on fluorescence intensity by concentration quenching should be noticed. The tendency of the blue emission intensity sharp fall in high Tm3+/ Yb3+ concentration cases can be obviously observed in the spectra of up-conversion luminescence. In order to further explore the effect of upconversion luminescence on the photocatalytic degradation process, the photocatalytic experiment under near-infrared light irradiation was showed in Fig. 12(d). The RhB degradation degree for 1.0 mol%Tm3+/ 20 mol%Yb3+:Bi2WO6 under 120 min with near-infrared light irradiation is 21.56%. In contrast, the RhB degradation is almost not been examined for Bi2WO6 with near-infrared irradiation. It could be determined that upconversion luminescence has a certain promoting effect on photocatalysis. Sequentially, the decreased emission energy result in a weakening of the subsequent photocatalytic process. The photocatalytic degradation process by all samples followed the firstorder rate law, -ln (Ct/C0) as a function of reaction time t is exhibited in Fig. 12(e–g), and the rate constant of 1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6 is estimated to be 0.11589 min−1 under simulated sunlight irradiation and 0.00163 min-1 under near-infrared irradiation respectively. In particular, the photocatalytic ability for different dye concentrations has been also given in Fig. 12(h). 1.0 mol%Tm3+/ 20 mol%Yb3+:Bi2WO6 exhibits the prominent photocatalytic capacity with dye concentrations of 2 × 10-5 mol/L, but decreasing gradually with the increment of the dye concentrations. Meanwhile, in order to study the further application, the cycle degradation results are revealed in Fig. 12 (i). There is a slightly deactivation even though the catalyst has been recycled fourth times, which demonstrated the stability of the photocatalysts.
Fig. 11. Dependence of the integrated up-conversion intensities (IUC) on the pump power (Pin). The straight lines are the fittings obtained by linear regression to the experimental data for each wavelength.
3.3. Photocatalytic test The photocatalytic activity of the as-prepared samples was investigated by the degradation of a model dye RhB solution under the visible light irradiation. Fig. 12(a) shows the UV–vis absorption spectra of RhB by 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 after different photocatalytic durations. The RhB absorption peak maximized at 554 nm decreases rapidly with the increase of irradiation time and it completely disappears after about 20 min. Further irradiation causes no absorption peak, which indicates the complete degradation of RhB. A series of color changes of the 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 sample are shown in the inset of Fig. 12(a). It is also consistent with the corresponding changes of the absorption measurements. To differentiate the photocatalytic degradation of Bi2WO6 doped with different Tm3+ and Yb3+ contents, these results are revealed in Fig. 12(b,c). From the above results, the excellent photocatalytic activity is observed when the concentration of Tm3+ and Yb3+ are 1.0 mol% and 20 mol%, respectively. The photocatalytic activities of the Tm3+/Yb3+:Bi2WO6 photocatalysts increase gradually with the Tm3+ and Yb3+ concentrations until the Tm3+ and Yb3+ concentrations are 1.0 mol% and 20 mol% respectively. However, when the Tm3+ and Yb3+ concentrations are more than 1.0 mol% and 20 mol%, the photocatalytic activity decreases sharply, which is consistent with the trend of fluorescence intensity in the inset of Figs. 9 and 10. This can be explained by the following reasons: this involves the mainly possible synergic reinforcement between up-conversion luminescence and photocatalytic processes in the Tm3+/Yb3+ doped materials as shown in the inserted schematic diagram of Fig. 13. Excited state absorption (ESA) and energy transfer (ET)
4. Conclusion In summary, Tm3+/ Yb3+ co-doped Bi2WO6 has been successfully prepared by a facile co-precipitation route and a sinter process. The 7
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Fig. 12. (a) The UV–vis spectral change of RhB over 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6 sample with irradiation time; (b,c) The photocatalytic degradation of RhB with different Tm3+and Yb3+ concentration under simulated sunlight irradiation; (d) The photocatalytic degradation of RhB with1.0 mol%Tm3+/20 mol% Yb3+:Bi2WO6 sample under near-infrared irradiation; (e–g) The kinetic curves of RhB degradation with different Tm3+and Yb3+ concentration; (h) The photocatalytic degradation for different RhB concentration;(i) Cycling experiment of the photocatalytic degradation of RhB with 1.0 mol%Tm3+/20 mol%Yb3+:Bi2WO6.
effect on fluorescence intensity by doping concentration has been investigated in detail, which laid a foundation for subsequent upconversion photocatalysis studies, and the result showed that the excellent
photocatalytic activity was consistent with the tendency of the blue emission intensity. The optimal doping concentrations for Tm3+ and Yb3+ which achieve maximum luminescence intensities has a
Fig. 13. The photocatalytic mechanism for the Tm3+/Yb3+-Bi2WO6 upconversion photocatalyst. 8
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significant impact on photocatalytic ability. This is not only reflected in the near-infrared photocatalytic performance, but also in the narrower band gap after doping rare earth ions and the lower hole-electron recombination rate. This work will provide some references for the further study of rare earth doping.
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