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attapulgite nanocomposite: Effect of upconversion and fluorine vacancy
Solar Energy 191 (2019) 251–262
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Photocatalytic nitrogen fixation over fluoride/attapulgite nanocomposite: Effect of upconversion and fluorine vacancy ⁎
Xiazhang Lia,b, , Chengli Hea, Shixiang Zuoa, Xiangyu Yana, Da Daia, Yuying Zhangb, Chao Yaoa, a b
T ⁎
Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, PR China Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Nitrogen fixation Upconversion Vacancy Attapulgite Z-scheme
Developing cost-effective photocatalyst which converts nitrogen to ammonia under mild conditions remains a great challenge from both academic and industrial perspective. In this work, we synthesize one dimensional attapulgite (ATP) mineral supported Pr3+:CeF3 nanocomposite by a microwave hydrothermal method. Results indicate that adequate doping of Pr3+ facilitates the conversion of visible light into ultraviolet light which expands the adsorption range of solar energy. The abundant fluorine vacancies which generate from the incorporation of partial metal ions of ATP into the crystal lattice of CeF3 act as active sites promoting the adsorption of N2 as well as weakening the N ≡ N triple bond in the photocatalysis reaction. The 40 wt% loading of Pr3+:CeF3 exhibits the highest photocatalytic nitrogen fixation performance attributed to the formation of Zscheme structure which not only improves the separation of photoexcited charge carriers, but also preserve the high redox potential to reduce nitrogen. This work provides a sustainable way for nitrogen fixation using solar energy based on natural mineral and beyond.
1. Introduction Although molecular dinitrogen occupies the major composition of the atmosphere on earth (~78 vol%), it is significantly difficult to be utilized mainly due to the fact that cleavage of the N ≡ N bond has a very large activation barrier (941 kJ/mol) (Du et al., 2015; Yang, J. et al., 2018). At present, the artificial nitrogen fixation method widely accomplished in the industry is the well-established Haber-Bosch process. However, the requirements are extremely high due to the harsh reaction conditions, not to mention the serious pollution (Qiu et al., 2018; Michalsky and Pfromm, 2011). Therefore, the development of more environment-friendly and lower energy consumption artificial nitrogen fixation process has important social significance. Alternatively, the photocatalytic nitrogen-fixing for ammonia production has attracted extensive attentions (Xu et al., 2018; Hirakawa et al., 2017; Xue et al., 2018; Shiraishi et al., 2018; Li et al., 2015). Currently, numerous photocatalysts have been designed for the conversion of N2 into NH3 under ambient conditions, such as Bi5O7I nanosheets (Bai et al., 2016), graphene oxide-supported transition metal (Yang, T. et al., 2018), ternary MoS2/C-ZnO composite (Xing et al., 2018), Au/TiO2-OV (Yang, J. et al., 2018), layered bismuth oxyhalides (Li et al., 2017), c-PAN on Bi2WO6 semiconductors (Zhang,
C. et al., 2018), etc., However, the light absorption of those efforts is still low. Fortunately, up-conversion with rare earth materials which convert low energy light to high energy light can provide another pathway to improve the utilization of solar energy. In particular as a typical functional rare earth fluoride with unique physical, chemical properties and up-conversion luminescence, CeF3 has been applied in various fields, such as photocatalytic reaction and optical components (Miao et al., 2015; Wan et al., 2016). However, limitation remains for the usage of a CeF3 as a photocatalyst due to its wide bandgap. Since CeF3 has low phonon energy and high chemical stability, making it as an ideal matrix for up-conversion material, numerous rare earth elements have been doped to improve the up-conversion luminescence properties of CeF3, such as Er3+ (Dantelle et al., 2005), Tb3+ (Chai et al., 2009), Pr3+ and Yb3+ (Runowski and Lis, 2016) etc. Among them Pr3+ has rich energy levels and close radius to Ce3+ (Balestrieri et al., 2014; Sudhakar et al., 2018), making it suitable for the generation of high-energy photons after doping with CeF3, which not only improve the up-conversion luminescence property but also increase the defects in CeF3 lattice as active sites (Chilkalwar and Rayalu, 2018; Coleman and Abdulmalik, 2008; Cao et al., 2019). Although lots of traditional carriers like TiO2 etc. demonstrate good photocatalytic performance after adding active components, their
⁎ Corresponding authors at: Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, PR China (X. Li). E-mail addresses: [email protected] (X. Li), [email protected] (C. Yao).
https://doi.org/10.1016/j.solener.2019.08.063 Received 23 May 2019; Received in revised form 21 August 2019; Accepted 25 August 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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for ultrasonic dissolution. The suspension was transferred to a microwave hydrothermal container and heated up to 160 °C for 70 min. The resulting samples were centrifuged and washed with deionized water and ethanol several times. Finally, a series of as-prepared Pr3+:CeF3/ ATP were obtained after drying at 80 °C.
relatively high prices lead to high costs for photocatalytic nitrogen fixation. As a natural clay mineral, attapulgite (ATP) is widely used in catalyst supports because of its cost-effectiveness, large specific surface area, superior adsorption properties and unique porous structure (Li, X. et al., 2018a; Li, X. et al., 2019). Intriguingly, the ATP also has semiconductor property due to the incorporation of iron oxide. Zhang (Zhang et al., 2016) reported that ATP modified by hydrochloric acid can be used for photocatalytic water splitting rendering its potential in solar energy area. Since the rapid recombination of photogenerated electron-hole pairs always occurs, constructing heterojunction between two different semiconductors is an effective way to separate photoexcited electrons and holes (Zhang, M. et al., 2018; Lou et al., 2018). The traditional heterojunctions including type I and type II subject to low redox potential when it comes to the migration of charge carriers. Alternatively, Z-type heterojunction not only effectively separate photogenerated electrons and holes, but also maintains high potential of band energy (Ma et al., 2018; Wen et al., 2018; You et al., 2018). In this work, a series of Pr3+:CeF3/ATP with various doping ratios and loading amounts are synthesized via microwave hydrothermal method. The utilization efficiency of solar energy can be well tackled by rare earth up-conversion luminescence, which convert visible light into high energy ultraviolet light. The Pr3+ doping improves the up-conversion luminescence performance and modulate the bandgap of CeF3. Furthermore, the fluorine vacancy for N2 activation and dissociation is investigated by electron spin resonance (ESR). The photocatalytic N2 fixation rate of Pr3+:CeF3/ATP reaches the similar order of magnitude as previous report (Chen et al., 2018). The formation of Z-scheme heterojunction and the photocatalytic N2 fixation mechanism is well studied.
2.3. Characterizations The phase of the samples was measured by a D/max 2500PC X-ray powder diffractometer (XRD), irradiated with a Cu-Kα target (λ = 1.5406 Å) at a scanning speed of 6° min−1 and a scan range of 5–80°. A Nicolet 460 FT-IR spectrometer was used to analyze the functional groups on samples surface and the scanning wavelength range was 500–4000 cm−1. The photo-response range of the samples was characterized via a Shimadzu UV-2500 UV–vis spectrophotometer. JEM-2100 transmission electron microscope (TEM) was used to observe the morphology of the samples. A LS45 fluorescence photometer was provided for photoluminescence (PL) test. X-ray photoelectron spectroscopy (XPS) was conducted on Quantum 2000 scanning ESCA microprobe instrument test. The electrochemical characterization (MottSchottky measurement), transient photocurrent responses and the electrochemical impedance spectra (EIS) were carried out with a PARSTAT 3000 using a conventional three-electrode and a 500 W Xe lamp. The measurement was performed using an aqueous 0.50 M Na2SO4 solution as electrolyte. Electron spin resonance (ESR) spectra were obtained on a Bruker A300 nano spectrometer at a low temperature (100 K). The morphology and the elements distribution of the samples were measured by ZEISS SUPRA-55 field emission scanning electron microscope (FE-SEM).
2. Experimental section 2.4. Photocatalytic nitrogen fixation reaction.
2.1. Materials and chemicals
A 300 W xenon lamp was used as simulated sunlight source using circulating water (flow rate 0.5 L/min) for cooling, respectively. 420 nm filter was used to remove UV light. The 0.789 g/L aqueous solution of ethanol was used as a hole trapping agent. 0.04 g catalyst was added to 100 mL aqueous ethanol solution with stirring. After dark adsorption, the solution was then placed under light source irradiation, and stirred with bubbling nitrogen (60 mL/min). 5 mL of the suspension was taken in a centrifuge tube every 1 h and centrifuged at 8000 r/min for 2 min, The NH4+ ion concentration in the product was analyzed by the Nessler’s reagent method at 420 nm combined with an UV-2600 spectrophotometer (Shimadzu).
Pr(NO3)3·6H2O, Ce(NO3)3·6H2O and NH4F were provided by Sinopharm Chemical Reagent Co., Ltd., China. Attapulgite was supplied by Jiangsu Nanda Zijin Technology Group Co., Ltd. (Changzhou, China). All reagents were analytical grade and used without further purification. 2.2. Preparation of Pr3+:CeF3/ATP The raw ATP was treated with formic acid first denoted as H-ATP. A certain proportion of Pr(NO3)3·6H2O, Ce(NO3)3·6H2O, NH4F (the molar ratio of Pr/Pr + Ce = x, x ranged from 0.1 mol% to 2.5 mol%, [Pr + Ce]:[F] = 1:3) and H-ATP (the mass ratio of [Pr3+:CeF3]:[HATP] was adjusted from 10 to 60 wt%) was added to deionized water
Fig. 1. XRD patterns of H-ATP, CeF3 and 0.1%, 0.5%, 1.0%, 1.5%, 2.5% Pr3+:CeF3/ATP (a), partial enlargement XRD patterns from 42.5° to 47.0° (b). 252
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raw ATP (JCPDS 82-1873). Compared with raw ATP, the intensity of HATP peaks was appreciably weakened. Moreover, the characteristic peaks at 20.8°, 21.5° correspond to the (1 2 1), (3 1 0) crystal planes of raw ATP disappear owing to the dissolution of octahedral structures (AleO, MgeO, and FeeO) in the ATP (Zhang et al., 2016). The diffraction peaks emerging at 24.4°, 25.0°, 27.8° are in consistence with the (0 0 2), (1 1 0) and (1 1 1) crystal planes of CeF3 (JCPDS 70-0002). In addition, no other peaks were observed in the XRD patterns, indicative of the well-crystallized CeF3 crystals. Fig. 1(b) showed the enlarged pattern of Fig. 1(a) from 42.5° to 47.0°. Compared with CeF3, the peaks of Pr3+:CeF3/ATP have an obvious shift towards higher angles, due to the fact that partial metal ions (Al3+, Fe3+) from H-ATP are doped into the CeF3 lattice. However, with the increasing of Pr3+ doping amount, the peaks in Fig. 1(b) have a slight shift towards higher angles, which is due to the fact that the Pr3+ (0.099 nm) radius is smaller than Ce3+ (0.101 nm). It is assumed that the diffusion of Al3+ and Fe3+ ions from H-ATP into CeF3 lattice plays a dominant role for the lattice distortion (Li, X. et al., 2018a; Bai et al., 2018). Fig. 2 showed the Fourier transform infrared spectroscopy (FT-IR) absorption spectra of the raw ATP, H-ATP and Pr3+:CeF3/ATP, respectively. It is clear that the FT-IR spectra are sensitive to small changes in the octahedral position occupied by ATP (Bai et al., 2018). In Fig. 2, the absorption band at 3663.7 cm−1 is caused by the stretch vibrations of structural eOH in high frequency region (3700–3200 cm−1) and the bands at 1209.4, 1656.9 cm−1 located within the middle frequency region (1700–1600 cm−1) which are attributed to the bend vibration of bound structural eOH in H-ATP (Zhang et al., 2016; Li, X. et al., 2018a). More absorption bands
Fig. 2. FT-IR spectra of the raw ATP, H-ATP and Pr3+:CeF3/ATP.
3. Results and discussion 3.1. Characterizations of Pr3+:CeF3/ATP The X-ray diffraction (XRD) patterns of the nanocomposite with different molar ratio of Pr3+ were displayed in Fig. 1. As is shown in Fig. 1 (a), the characteristic peaks at 8.5°, 19.9°, 28.0° and 35.8° correspond to the (1 1 0), (0 4 0), (4 0 0) and (1 6 1) crystal planes of the
Fig. 3. UV–Vis diffuse reflection spectra of raw ATP and H-ATP (a); Kubelka-Munk conversion of raw ATP and H-ATP (b); UV–Vis spectra of CeF3 and Pr3+:CeF3 (c), Kubelka-Munk conversion corresponding to CeF3 and Pr3+:CeF3 (d). 253
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Fig. 4. TEM images of H-ATP (a), 10 wt% Pr3+:CeF3/ATP (b), 20 wt% Pr3+:CeF3/ATP (c) 40 wt% Pr3+:CeF3/ATP (d) and 60 wt% Pr3+:CeF3/ATP (e) with corresponding SAED patterns inserted; HRTEM images of 40 wt% Pr3+:CeF3/ATP (f); EDS pattern of 40 wt% Pr3+:CeF3/ATP (g).
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Fig. 5. Up-conversion PL spectra of different doping amount of Pr3+:CeF3/ATP excited at 480 nm (a); PL spectra of different loading mount of Pr3+:CeF3/ATP excited at 320 nm (b).
Fig. 6. Mott-Schottky plots for 0.5 mol% Pr3+:CeF3 (a), H-ATP (b), and 40 wt% Pr3+:CeF3/ATP(c).
and H-ATP. Compared with the raw ATP, the H-ATP showed significant red shift and the band gap of H-ATP become narrow. The following equation is adopted to calculate the bandgap energy (Eg) of the samples (Acuña et al., 2017; Lebedev et al., 2019):
appeared in the ATP treated with formic acid compared with the raw ATP. The bands at 3544.8 cm−1 and 3400.1 cm−1 are attributed to the tensile vibration of the structure eOH. The absorption bands at 642.1 cm−1 and 985.1 cm−1 are mainly caused by the bending vibration of the Si (or Al)eO tetrahedron and the MeOH (M]Mg, Al, Fe) octahedron (Zhang et al., 2016). For the Pr3+:CeF3/ATP, a new absorption band appeared at 1384.60 cm−1, which may be ascribe to the bending vibration of the CeeOH tetrahedron, indicating that Pr3+:CeF3 is successfully loaded on H-ATP. Fig. 3 (a) showed the UV–vis diffuse reflection spectra of raw ATP
α hν = A(hν − Eg)2 where α is the absorption coefficient, h is Planck constant, ν is light frequency, A is a constant and Eg is band gap energy. The (Ahν)2 versus hν curves of raw ATP and H-ATP are shown in Fig. 3 (b). The Eg of raw ATP and H-ATP are determined to be approximately 3.81 eV and 255
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Fig. 7. The transient photocurrent responses, Light source: Xe lamp, 500 W (a) and the EIS of H-ATP, Pr3+:CeF3, Pr3+:CeF3/ATP with various loading amount (b).
Fig. 8. The VB-XPS of Pr3+:CeF3 (a); and H-ATP (b).
Information) showed the TEM images of 40 wt% Pr3+:CeF3/ATP and Fig. S1(b) was the dark field images corresponding to Fig. S1(a). The bright area corresponds to the Pr3+:CeF3 particles with excellent crystallinity. Fig. 4(g) showed an EDS pattern of 40 wt% Pr3+:CeF3/ATP, in which Mg, Al, Si, O, Fe elements were derived from ATP, and Pr, Ce, and F elements were derived from Pr3+:CeF3, wherein Cu, C were derived from the copper mesh and the carbon film. Fig. S2(a) (Supporting Information) showed an FT-SEM image of 40 wt% Pr3+:CeF3/ATP, where the Pr3+:CeF3 nanoparticles was uniformly dispersed on the rodshaped H-ATP. Elemental mapping of Pr, Ce, F, O, Si, Fe, Mg and Al elements in 40 wt% Pr3+:CeF3/ATP were displayed in Fig. S2 (Supporting Information), exhibiting the well-defined distribution of elements. In order to investigate the effects of different doping amounts of Pr3+ on up-conversion photoluminescence (PL), the up-conversion PL spectra of as-prepared Pr3+:CeF3/ATP samples with various doping ratios were measured under the excitation of 480 nm laser shown in Fig. 5(a). The emission intensity of the up-conversion PL spectra showed a strong peak centered at 278 nm, which increases with doping amounts from 0.1 mol% to 0.5 mol% of Pr3+, and decreases with increasing Pr3+ doping amounts from 0.5 mol% to 2.5 mol%. Therefore, the 0.5 mol% Pr3+:CeF3 demonstrates the best up-conversion capability which converts the visible light to UV light. As shown in Fig. S3 (Supporting Information), the Pr3+:CeF3/ATP samples with different doping ratios transfers the visible light of 443 nm, 466 nm, 593 nm to 260–286 nm UV light, which can be used by Pr3+:CeF3/ATP for photocatalytic nitrogen fixation. In general, the photoluminescence (PL) spectra reflects the recombination rate of photogenerated electron/hole pairs, and the high fluorescence intensity suggests a high degree of
3.59 eV by Kubelka-Munk conversion. Fig. 3 (c) showed the UV–vis plot of CeF3 and Pr3+:CeF3, respectively. The Pr3+:CeF3 was red-shifted compared to pure CeF3, indicating that a small amount of Pr3+ doped into the crystal lattice of CeF3 narrows the band gap energy. The peaks at 443 nm, 466 nm, 480 nm and 593 nm are attribute to the Pr3+ ions in CeF3 lattices which can absorb visible light. As shown in Fig. 3(d), the band gap energy of CeF3 is 3.29 eV, and the Eg of Pr3+:CeF3 is 3.18 eV. The transmission electron microscopy (TEM) and the field emission scanning electron microscope (FE-SEM) were provided to investigate the morphology and structure of the synthesized samples. Fig. 4(a) showed the TEM images of H-ATP and corresponding selected area electron diffraction (SAED) pattern (inset). For the as-prepared H-ATP, one-dimensional nanorod structures were clearly observed and the SAED pattern inserted indicates that the H-ATP are amorphous. The TEM images of Pr3+:CeF3/ATP with various loading amounts were displayed in Fig. 4(b)–(e).The Pr3+:CeF3 nanoparticles were uniformly loaded on the H-ATP with size ca.10 nm. The diffraction rings of in Fig. 4(b) and (c) correspond to the (1 1 1), (1 1 3) and (3 1 0) crystal planes of CeF3. With the increased loading of Pr3+:CeF3 particles, the intensity of diffraction rings of the corresponding SAED increased. The image inserted in Fig. 4(d) exhibited diffraction rings corresponding to (1 1 1), (1 1 3), (3 1 0) and (1 4 1) crystal planes. Comparatively, the SAED in Fig. 4(e) displayed one more diffraction ring corresponding to (1 5 1) crystal plane. The high-resolution TEM (HRTEM) images of 40 wt% Pr3+:CeF3/ATP shown in Fig. 4(f) exhibited a clear heterostructure between Pr3+:CeF3 and H-ATP. The nanoparticles were highly crystallized with the interplanar spacing of 0.22 nm and 0.36 nm corresponding to the (1 2 1) and (1 1 0) lattice planes of CeF3, in good agreement with XRD patterns in Fig. 1. Fig. S1(a) (Supporting 256
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Fig. 9. XPS spectra of H-ATP, Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP nanocomposite (a) survey of the samples, (b) Al 2p of H-ATP, (c) Fe 2p of H-ATP, (d) Ce 3d, (e) Pr 3d, (f) F 1 s, (g) Si 2p.
are equal to −0.21 eV, 0.02 eV and −0.09 eV (vs. NHE, PH = 7), respectively. The transient photocurrent responses of H-ATP, Pr3+:CeF3, 3+ Pr :CeF3/ATP with various loading amounts were performed to reveal the separation of photogenerated electrons and holes as shown in Fig. 7 (a). The photocurrent of the 40 wt% Pr3+:CeF3/ATP exhibited the highest intensity indicating more photogenerated carriers existed and superior separation performance of photogenerated electrons and holes. Fig. 7(b) demonstrated the electrochemical impedance spectra (EIS) Nyquist plots of H-ATP and Pr3+:CeF3/ATP with various loading amounts, investigating the migration and interfacial transfer/recombination rates of charge carriers (Pan et al., 2019). It is obvious that 40 wt% Pr3+:CeF3/ATP showed the smallest semicircle indicative of low charge transfer resistance and high conductivity, suggesting high efficient separation of photo-generated carriers and outstanding efficiency of charge immigration across the heterojunction interface, in good agreement with the results of transient photocurrent responses in Fig. 7(a). Fig. 8 showed the XPS valance band edge spectrum (VB-XPS) of Pr3+:CeF3, and H-ATP, which were obtained as 1.41 eV and 2.22 eV corresponding to the distance between valence band and Fermi level (Evf). In general, the flat band potential is equal to fermi level for n-type semiconductors (Li, R. et al., 2018). Therefore, the valance band (VB) potentials of Pr3+:CeF3 and H-ATP are 1.20 eV and 2.24 eV, respectively. The conduction band gap energy position (ECB) can be calculated by the following equation:
recombination (Zhang et al., 2018). The PL spectra of Pr3+:CeF3/ATP with different loadings excited at 320 nm laser were demonstrated in Fig. 5(b). The emission intensity increased when loading of Pr3+:CeF3 is lower than 40 wt%, indicating that the electrons and holes recombination rate of Pr3+:CeF3/ATP is accelerated (Li, X. et al., 2019). The 40 wt% Pr3+:CeF3/ATP exhibited the highest fluorescence emission intensity indicating the highest recombination rate of photogenerated electrons and holes. However, when the loading of Pr3+:CeF3 is larger than 40 wt%, the fluorescence emission intensity decreased, due to the fact that the recombination balance of electrons and holes are broken, leading to the decrease of recombination efficiency. The Mott-Schottky plots of 0.5 mol% Pr3+:CeF3, H-ATP and 40 wt% Pr3+:CeF3/ATP were shown in Fig. 6(a)–(c) respectively, which were obtained to investigate the band potential of as-prepared samples. The flat band potential (Efb) is calculated by using the Mott-Schottky equation as follows (Pan et al., 2019; Martínez-Vargas et al., 2019):
1 2NA RT ⎤ ⎡E − Efb − = 2 Nd Fεε0 ⎣ F ⎦ CSc where NA is the Avogadrós number (6.023 × 1023 mol−1), F is the Faraday constant (9.65 × 104 C mol−1), ε0 is the vacuum permittivity (8.8542 × 10−14F cm−1), ε is the dielectric constant of the semiconductor, R is the gas constant (8.314 J K−1 mol−1), T is the absolute temperature (298 K), and E (V) is the applied potential. It was observed in Fig. 6 that the slope in the Mott-Schottky pattern was positive, revealing that both of the 0.5 mol% Pr3+:CeF3 and H-ATP are n-type semiconductors (Martínez-Vargas et al., 2019). The flat band potentials of 0.5% Pr3+:CeF3, H-ATP and 40 wt% Pr3+:CeF3/ATP were calculated to be −0.41 eV, −0.18 eV and −0.29 eV (vs. Ag/AgCl, PH = 7). which
ECB = EVB − Eg According to the Eg of Pr3+:CeF3 and H-ATP obtained in Fig. 3, the 257
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Fig. 9. (continued)
characteristic peaks of both Al and Fe shifted toward to higher binding energy in Pr3+:CeF3/ATP composites compared to H-ATP, due to the fact that Al and Fe elements have greater electronegativity than Ce and the formation of Al-F, Fe-F bond partially replace Ce-F bond, suggestive of the formation of fluorine vacancies. Fig. 9(d) showed the XPS spectra of Ce 3d for the Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP. As for Pr3+:CeF3, the peaks of binding energy at 883.6, 886.5 and 902.4, 905.0 eV are attributed to Ce 3d5/2 and 3d3/2 of Ce3+, respectively (Xiang et al., 2016; Li, Xuejiao et al., 2019). The binding energies in the sample of 40 wt% Pr3+:CeF3/ATP showed a slightly shift to higher binding energy compared with Pr3+:CeF3, suggesting that some metal ions (like Al3+ and Fe3+) from H-ATP may be doped into the Pr3+:CeF3 lattice after Pr3+:CeF3 and H-ATP closely combined (Xiang et al., 2016). Fig. 9(e) showed the XPS spectrum of Pr 3d. The peaks at 934.8 and 943.8 eV are attributed to Pr 3d5/2 in Pr3+:CeF3 exhibiting the existence of Pr3+ in the composites (Gurgul et al., 2013). It was observed that the peaks in 40 wt% Pr3+:CeF3/ATP obviously shifted to higher binding energy compared to Pr3+:CeF3, suggesting the Pr3+:CeF3 and H-ATP are tightly conjunct by covalent bonds (Dalai et al., 2011). As displayed in Fig. 9(f), the same peak at 684.9 eV was found in both Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP which shifted towards the lower binding energy compared with standard peak of F 1s at 685.7 eV, indicating the possible effect of the formation Ce-F bond (Xiang et al., 2016). The peak of binding energy at 691.3 eV is mainly attributed to the existence of the bonding between Pr and F ion. Fig. 9(g) showed the Si 2p spectra of H-ATP and 40 wt% Pr3+:CeF3/ATP where binding energies were observed at 101.2 eV and 102.7 eV, respectively (Li, X. et al., 2018b; Chao et al., 2008). The difference of binding energy between H-ATP and 40 wt% Pr3+:CeF3/ATP reveals the
Fig. 10. ESR spectra measured at 100 K for CeF3 and 40 wt% Pr3+:CeF3/ATP.
ECB of Pr3+:CeF3 is calculated to be −1.98 eV and the ECB of H-ATP is calculated to be −1.35 eV. The X-ray photoelectron spectroscopy (XPS) was performed to investigate the valence states and the chemical compositions of the samples. Fig. 9(a) showed the survey scan of H-ATP, Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP. Fig. 9(b) and (c) showed the characteristic peaks in H-ATP and 40 wt% Pr3+:CeF3/ATP separately. Al and Fe elements were present in H-ATP and 40 wt% Pr3+:CeF3/ATP, which is consistent with the results of EDS pattern. It was observed that the 258
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Fig. 11. Photocatalytic nitrogen fixation activity of H-ATP, Pr3+:CeF3, and Pr3+:CeF3/ATP with various loading amounts (a) under visible light, λ greater than 420 nm, (b) under simulated sunlight, (c) under UV light, λ < 420 nm, (d) under visible light, with and without sacrificial agent over 40 wt% Pr3+:CeF3/ATP, (e) recycle performance over 40 wt% Pr3+:CeF3/ATP under visible light for five times. The error bars represent the standard deviations of three independent tests for each data point.
formation of FeOeSi bond in 40 wt% Pr3+:CeF3/ATP, attributed to the partial substitution of SieOeSi bond (Li, X. et al., 2018c). In order to investigate the defects in the as-prepared samples, low temperature electron spin resonance (ESR) test was performed. Fig. 10 showed the ESR spectra measured at 100 K for CeF3 and Pr3+:CeF3/ ATP. Generally, there is no ESR signal for pure CeF3, implying that there are no fluorine vacancies. However, a single Lorentzian line signal with a g value around 2 can be identified when the CeF3 possess fluorine vacancies (Vahakangas et al., 2015). It is obvious that a weak signal was observed in the CeF3, indicating that the as-prepared CeF3 has a little defect, and a strong ESR signal at g = 2.005 was observed in the sample of Pr3+:CeF3/ATP, which can be assigned to the abundant fluorine vacancies in Pr3+:CeF3/ATP. The fluorine vacancies may be due to the fact that partial metal Fe3+, Al3+ ions in H-ATP are doped
into the crystal lattice of CeF3, resulting in lattice defects, which is consistent with the analysis results of XPS in Fig. 9(b) and (c). 3.2. Photocatalytic nitrogen fixation performance The photocatalytic N2 fixation efficiencies under visible light were performed in Fig. 11(a). H-ATP, Pr3+:CeF3 and the Pr3+:CeF3/ATP nanocomposites with different mass loading were measured three times (n = 3) and the results were the average of three repeated tests (with error bars represented the standard deviations), which were displayed in Fig. 11. The separate H-ATP and Pr3+:CeF3 exhibited very low nitrogen fixation performance which were 8.89 ± 0.40 μmol/L and 46.38 ± 1.90 μmol/L (n = 3), respectively. After Pr3+:CeF3 combined with H-ATP, significant more N2 were fixed and converted into NH4+, 259
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which can be mainly ascribe to the enhanced light-harvesting capacity and the improved carrier separation efficiency (Xu et al., 2018). The photocatalytic N2 fixation efficiencies increased when the loading of Pr3+:CeF3 increased from 10 wt% to 40 wt%, while decreased from 40 wt% to 60 wt%. The 40 wt% Pr3+:CeF3/ATP nanocomposites was the most efficient in photocatalytic nitrogen fixation reaching as high as 629.16 ± 17.62 μmol/L (n = 3), within 4 h. Fig. 11(b) and (c) showed the photocatalytic nitrogen fixation efficiencies of Pr3+:CeF3/ATP under simulated sunlight and UV light (λ < 420 nm), respectively. The photocatalytic performance of Pr3+:CeF3/ATP was found following the order: UV light > simulated sunlight > visible light. As shown in Fig. 11(d), Pr3+:CeF3/ATP with ethanol as sacrificial agent demonstrated more ammonia production than that without sacrificial agent, which may be due to the fact that partial NH4+ ions are oxidized into NO3− ions by photogenerated holes. Fig. 11(e) showed the cycle experiment of Pr3+:CeF3/ATP, whose concentration of NH4+ can still reach 606.37 ± 16.67 μmol/L within 4 h at the fifth cycle suggestive of excellent reusability. The slight drop in NH4+ production might result from partial block of porous structure of ATP which affected the adsorption performance. Besides, Fig S4 showed the TEM image of 40 wt%
Fig. 12. Transient photocurrent response of 40 wt% Pr3+:CeF3/ATP under the Ar or N2 atmosphere, Light source: Xe lamp, 500 W.
Fig. 13. (a) Mechanism for the formation of Z-type heterojunction, (b) mechanism for up-conversion luminescence and photocatalytic nitrogen fixation. 260
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Pr3+:CeF3/ATP after using for five times, which was almost the same as that of the fresh sample. To further shed light on the effect of Pr3+:CeF3/ATP composites in photocatalytic nitrogen fixation, the transient photocurrent response experiments were carried out with Pr3+:CeF3/ATP when different gas (Ar, N2) flows were bubbled into the electrolyte solution. As displayed in Fig. 12, the photocurrents of Pr3+:CeF3/ATP under N2 atmosphere exhibited a significant decrease compared with the composites under Ar atmosphere, mainly due to the fact that the electrons are consumed by the N2 molecules and are prone to participate in the process of photocatalytic nitrogen fixation.
3.4. Conclusions In conclusion, Pr3+:CeF3/ATP nanocomposites were synthesized via a facile microwave hydrothermal method. The Pr3+:CeF3 nanoparticles having size ca.10 nm are uniformly loaded on the surface of one-dimensional nanorod ATP with strong interaction. The 0.5 mol% doping amount of Pr3+ demonstrates the strongest up-conversion property improving the utilization of solar light. Meanwhile, the metal ions (Al3+, Fe3+) species from modified ATP enter into the crystal lattice of CeF3 forming the fluorine vacancies, which serve as the active sites to weaken the N ≡ N triple bond for activation of N2. The Pr3+:CeF3/ATP nanocomposite with 40 wt% loading amount forms a well-defined Zscheme heterojunction which effectively separates photogenerated electrons and holes, and preserve more negative conduction band leading to the enhanced reducibility for photocatalytic N2 fixation.
3.3. Photocatalytic nitrogen fixation mechanism The Fig. 13(a) showed the formation of heterojunction between Pr3+:CeF3 and H-ATP before and after contact. Before contact, the EVB, Ef and ECB values of Pr3+:CeF3 are more negative than the values of HATP. After the two semiconductors are in contact, the electrons flow from the semiconductor with a high Fermi level (Ef) to the semiconductor with a low Fermi level. In fact, the Ef value of Pr3+:CeF3/ATP nanocomposites is −0.09 eV. Therefore, it can be inferred that the Ef value of H-ATP is upshifted from 0.02 eV to −0.09 eV and the Ef value of Pr3+:CeF3 is downshifted from −0.21 eV to −0.09 eV after contact between H-ATP and Pr3+:CeF3. A built-in electric field is formed on the interface between H-ATP and Pr3+:CeF3, and the energy band of H-ATP bend down, while the energy band of Pr3+:CeF3 bend upward. The photogenerated electrons on the H-ATP conduction band recombine with the holes in the valence band of Pr3+:CeF3 driven by the built-in electric field, leading to the Z-type heterojunction. It is reported that the standard redox potential for N2 to NH3 is −0.092 eV versus normal hydrogen electrode (NHE) (Devthade et al., 2018). Obviously, ECB of 0.5% Pr3+:CeF3 (−1.86 eV) is more negative than the N2/NH3 reduction potential. The existence of fluorine vacancies in Pr3+:CeF3/ATP act as active sites to weaken the N ≡ N triple bond and efficiently promote the participation of N2 in the reaction (Yang, J. et al., 2018). Fig. 13(b) showed the mechanism of up-conversion luminescence and photocatalytic nitrogen fixation. As illustrated in the energy level image of Pr3+ ion, the up-conversion capability of Pr3+:CeF3/ATP is attribute to the process of excited state absorption (ESA) and energy transfer UC (ETU). After absorbing the energy of two visible photons (ω1 and ω2), Pr3+ ions are transformed to excited intermediate state. Finally, the UC photons (ω) are released when excited electrons transfer back to the ground state via radiation relaxation. The 3P0, 3P1 and 1I6, etc., levels of Pr3+ ions can act as intermediate excited states to reach the process of ESA as the process of 3H4 → 3P0 → 4f5d (Gao et al., 2017). The rare earth element Pr3+ ions convert part of visible light into ultraviolet light with higher photon energy indirectly improving the utilization of sunlight. N2 is reduced to NH3 by the photogenerated electrons on the Pr3+:CeF3 conduction band, which finally dissolves in water as NH4+. The photogenerated holes in the valence band of H-ATP oxidize H2O to O2 and H+ ions and finally oxidize the sacrificial agent ethanol to oxidation products. The possible reactions occurring during the photocatalytic N2 fixation can be shown as following:
H − ATP + hv → e(−CB, H − ATP ) + h (+VB, H − ATP )
(1)
Pr 3 + : CeF3 + hv → e −
(2)
(CB,Pr3 + : CeF3)
+ h+
(VB,Pr3 + : CeF3)
N2 + e(−CB,Pr3 + : CeF ) + H+ → NH4+
(3)
H2 O + h(+VB, H − ATP ) → H+ + O2
(4)
ethanol + h(+VB, H − ATP ) → Oxidation products
(5)
3
Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgments This work was supported by the National Natural Science Foundation of China (51674043, 51702026), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), and Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX18_0951). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.08.063. References Acuña, K., Yáñez, J., Ranganathan, S., Ramírez, E., Pablo Cuevas, J., Mansilla, H.D., Santander, P., 2017. Photocatalytic degradation of roxarsone by using synthesized ZnO nanoplates. Sol. Energy 157, 335–341. Bai, Y., Yang, P., Wang, P., Fan, Z., Xie, H., Wong, P.K., Ye, L., 2018. Solid phase fabrication of Bismuth-rich Bi3O4ClxBr 1–x solid solution for enhanced photocatalytic NO removal under visible light. J. Taiwan Inst. Chem. Eng. 82, 273–280. Bai, Y., Ye, L., Chen, T., Wang, L., Shi, X., Zhang, X., Chen, D., 2016. Facet-dependent photocatalytic N2 fixation of bismuth-rich Bi5O7I nanosheets. ACS Appl. Mater. Interf. Balestrieri, M., Gallart, M., Ziegler, M., Bazylewski, P., Ferblantier, G., Schmerber, G., Chang, G.S., Gilliot, P., Muller, D., Slaoui, A., Colis, S., Dinia, A., 2014. Luminescent properties and energy transfer in Pr3+ doped and Pr3+-Yb3+ Co-doped ZnO thin films. J. Phys. Chem C 118 (25), 13775–13780. Cao, J., Nie, W., Huang, L., Ding, Y., Lv, K., Tang, H., 2019. Photocatalytic activation of sulfite by nitrogen vacancy modified graphitic carbon nitride for efficient degradation of carbamazepine. Appl. Catal. B 241, 18–27. Chai, R., Lian, H., Li, C., Cheng, Z., Hou, Z., Huang, S., Lin, J., 2009. In situ preparation and luminescent properties of CeF3 and CeF3:Tb3+ nanoparticles and transparent CeF3:Tb3+/PMMA nanocomposites in the visible spectral range. J. Phys. Chem. C 113 (19), 8070–8076. Chao, Z., Ji, C., Zhou, Y., Li, D., 2008. Ionic liquid-based “all-in-one” synthesis and photoluminescence properties of lanthanide fluorides. J. Phys. Chem. C 112 (27), 10083–10088. Chen, X., Li, N., Kong, Z., Ong, W.-J., Zhao, X., 2018. Photocatalytic fixation of nitrogen to ammonia: state-of-the-art advancements and future prospects. Mater. Horiz. 5 (1), 9–27. Chilkalwar, A.A., Rayalu, S.S., 2018. Synergistic plasmonic and upconversion effect of the (Yb, Er)NYF-TiO2/Au composite for photocatalytic hydrogen generation. J. Phys. Chem C 122 (46), 26307–26314. Coleman, P.G., Abdulmalik, D.A., 2008. Useful vacancies: positron beam interrogation of fluorine-vacancy complexes in semiconductor device structures. Appl. Surf. Sci. 255 (1), 71–74. Dalai, M.K., Kundu, R., Pal, P., Bhanja, M., Sekhar, B.R., Martin, C., 2011. XPS study of Pr1−xCaxMnO3 (x=0.2, 0.33, 0.4 and 0.84). J. Alloy. Compd. 509 (29), 7674–7676. Dantelle, G., Mortier, M., Vivien, D., Patriarche, G., 2005. Effect of CeF3 addition on the nucleation and upconversion luminescence in transparent oxyfluoride glass−ceramics. Chem. Mater. 17 (8), 2216–2222. Devthade, V., Gupta, A., Umare, S.S., 2018. Graphitic carbon nitride−γ-gallium oxide (GCN-γ-Ga2O3) nanohybrid photocatalyst for dinitrogen fixation and pollutant decomposition. ACS Appl. Nano Mater. 1 (10), 5581–5588. Du, Z., Denkenberger, D., Pearce, J.M., 2015. Solar photovoltaic powered on-site
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Photocatalytic nitrogen fixation over fluoride/attapulgite nanocomposite: Effect of upconversion and fluorine vacancy ⁎
Xiazhang Lia,b, , Chengli Hea, Shixiang Zuoa, Xiangyu Yana, Da Daia, Yuying Zhangb, Chao Yaoa, a b
T ⁎
Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, PR China Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalysis Nitrogen fixation Upconversion Vacancy Attapulgite Z-scheme
Developing cost-effective photocatalyst which converts nitrogen to ammonia under mild conditions remains a great challenge from both academic and industrial perspective. In this work, we synthesize one dimensional attapulgite (ATP) mineral supported Pr3+:CeF3 nanocomposite by a microwave hydrothermal method. Results indicate that adequate doping of Pr3+ facilitates the conversion of visible light into ultraviolet light which expands the adsorption range of solar energy. The abundant fluorine vacancies which generate from the incorporation of partial metal ions of ATP into the crystal lattice of CeF3 act as active sites promoting the adsorption of N2 as well as weakening the N ≡ N triple bond in the photocatalysis reaction. The 40 wt% loading of Pr3+:CeF3 exhibits the highest photocatalytic nitrogen fixation performance attributed to the formation of Zscheme structure which not only improves the separation of photoexcited charge carriers, but also preserve the high redox potential to reduce nitrogen. This work provides a sustainable way for nitrogen fixation using solar energy based on natural mineral and beyond.
1. Introduction Although molecular dinitrogen occupies the major composition of the atmosphere on earth (~78 vol%), it is significantly difficult to be utilized mainly due to the fact that cleavage of the N ≡ N bond has a very large activation barrier (941 kJ/mol) (Du et al., 2015; Yang, J. et al., 2018). At present, the artificial nitrogen fixation method widely accomplished in the industry is the well-established Haber-Bosch process. However, the requirements are extremely high due to the harsh reaction conditions, not to mention the serious pollution (Qiu et al., 2018; Michalsky and Pfromm, 2011). Therefore, the development of more environment-friendly and lower energy consumption artificial nitrogen fixation process has important social significance. Alternatively, the photocatalytic nitrogen-fixing for ammonia production has attracted extensive attentions (Xu et al., 2018; Hirakawa et al., 2017; Xue et al., 2018; Shiraishi et al., 2018; Li et al., 2015). Currently, numerous photocatalysts have been designed for the conversion of N2 into NH3 under ambient conditions, such as Bi5O7I nanosheets (Bai et al., 2016), graphene oxide-supported transition metal (Yang, T. et al., 2018), ternary MoS2/C-ZnO composite (Xing et al., 2018), Au/TiO2-OV (Yang, J. et al., 2018), layered bismuth oxyhalides (Li et al., 2017), c-PAN on Bi2WO6 semiconductors (Zhang,
C. et al., 2018), etc., However, the light absorption of those efforts is still low. Fortunately, up-conversion with rare earth materials which convert low energy light to high energy light can provide another pathway to improve the utilization of solar energy. In particular as a typical functional rare earth fluoride with unique physical, chemical properties and up-conversion luminescence, CeF3 has been applied in various fields, such as photocatalytic reaction and optical components (Miao et al., 2015; Wan et al., 2016). However, limitation remains for the usage of a CeF3 as a photocatalyst due to its wide bandgap. Since CeF3 has low phonon energy and high chemical stability, making it as an ideal matrix for up-conversion material, numerous rare earth elements have been doped to improve the up-conversion luminescence properties of CeF3, such as Er3+ (Dantelle et al., 2005), Tb3+ (Chai et al., 2009), Pr3+ and Yb3+ (Runowski and Lis, 2016) etc. Among them Pr3+ has rich energy levels and close radius to Ce3+ (Balestrieri et al., 2014; Sudhakar et al., 2018), making it suitable for the generation of high-energy photons after doping with CeF3, which not only improve the up-conversion luminescence property but also increase the defects in CeF3 lattice as active sites (Chilkalwar and Rayalu, 2018; Coleman and Abdulmalik, 2008; Cao et al., 2019). Although lots of traditional carriers like TiO2 etc. demonstrate good photocatalytic performance after adding active components, their
⁎ Corresponding authors at: Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, PR China (X. Li). E-mail addresses: [email protected] (X. Li), [email protected] (C. Yao).
https://doi.org/10.1016/j.solener.2019.08.063 Received 23 May 2019; Received in revised form 21 August 2019; Accepted 25 August 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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for ultrasonic dissolution. The suspension was transferred to a microwave hydrothermal container and heated up to 160 °C for 70 min. The resulting samples were centrifuged and washed with deionized water and ethanol several times. Finally, a series of as-prepared Pr3+:CeF3/ ATP were obtained after drying at 80 °C.
relatively high prices lead to high costs for photocatalytic nitrogen fixation. As a natural clay mineral, attapulgite (ATP) is widely used in catalyst supports because of its cost-effectiveness, large specific surface area, superior adsorption properties and unique porous structure (Li, X. et al., 2018a; Li, X. et al., 2019). Intriguingly, the ATP also has semiconductor property due to the incorporation of iron oxide. Zhang (Zhang et al., 2016) reported that ATP modified by hydrochloric acid can be used for photocatalytic water splitting rendering its potential in solar energy area. Since the rapid recombination of photogenerated electron-hole pairs always occurs, constructing heterojunction between two different semiconductors is an effective way to separate photoexcited electrons and holes (Zhang, M. et al., 2018; Lou et al., 2018). The traditional heterojunctions including type I and type II subject to low redox potential when it comes to the migration of charge carriers. Alternatively, Z-type heterojunction not only effectively separate photogenerated electrons and holes, but also maintains high potential of band energy (Ma et al., 2018; Wen et al., 2018; You et al., 2018). In this work, a series of Pr3+:CeF3/ATP with various doping ratios and loading amounts are synthesized via microwave hydrothermal method. The utilization efficiency of solar energy can be well tackled by rare earth up-conversion luminescence, which convert visible light into high energy ultraviolet light. The Pr3+ doping improves the up-conversion luminescence performance and modulate the bandgap of CeF3. Furthermore, the fluorine vacancy for N2 activation and dissociation is investigated by electron spin resonance (ESR). The photocatalytic N2 fixation rate of Pr3+:CeF3/ATP reaches the similar order of magnitude as previous report (Chen et al., 2018). The formation of Z-scheme heterojunction and the photocatalytic N2 fixation mechanism is well studied.
2.3. Characterizations The phase of the samples was measured by a D/max 2500PC X-ray powder diffractometer (XRD), irradiated with a Cu-Kα target (λ = 1.5406 Å) at a scanning speed of 6° min−1 and a scan range of 5–80°. A Nicolet 460 FT-IR spectrometer was used to analyze the functional groups on samples surface and the scanning wavelength range was 500–4000 cm−1. The photo-response range of the samples was characterized via a Shimadzu UV-2500 UV–vis spectrophotometer. JEM-2100 transmission electron microscope (TEM) was used to observe the morphology of the samples. A LS45 fluorescence photometer was provided for photoluminescence (PL) test. X-ray photoelectron spectroscopy (XPS) was conducted on Quantum 2000 scanning ESCA microprobe instrument test. The electrochemical characterization (MottSchottky measurement), transient photocurrent responses and the electrochemical impedance spectra (EIS) were carried out with a PARSTAT 3000 using a conventional three-electrode and a 500 W Xe lamp. The measurement was performed using an aqueous 0.50 M Na2SO4 solution as electrolyte. Electron spin resonance (ESR) spectra were obtained on a Bruker A300 nano spectrometer at a low temperature (100 K). The morphology and the elements distribution of the samples were measured by ZEISS SUPRA-55 field emission scanning electron microscope (FE-SEM).
2. Experimental section 2.4. Photocatalytic nitrogen fixation reaction.
2.1. Materials and chemicals
A 300 W xenon lamp was used as simulated sunlight source using circulating water (flow rate 0.5 L/min) for cooling, respectively. 420 nm filter was used to remove UV light. The 0.789 g/L aqueous solution of ethanol was used as a hole trapping agent. 0.04 g catalyst was added to 100 mL aqueous ethanol solution with stirring. After dark adsorption, the solution was then placed under light source irradiation, and stirred with bubbling nitrogen (60 mL/min). 5 mL of the suspension was taken in a centrifuge tube every 1 h and centrifuged at 8000 r/min for 2 min, The NH4+ ion concentration in the product was analyzed by the Nessler’s reagent method at 420 nm combined with an UV-2600 spectrophotometer (Shimadzu).
Pr(NO3)3·6H2O, Ce(NO3)3·6H2O and NH4F were provided by Sinopharm Chemical Reagent Co., Ltd., China. Attapulgite was supplied by Jiangsu Nanda Zijin Technology Group Co., Ltd. (Changzhou, China). All reagents were analytical grade and used without further purification. 2.2. Preparation of Pr3+:CeF3/ATP The raw ATP was treated with formic acid first denoted as H-ATP. A certain proportion of Pr(NO3)3·6H2O, Ce(NO3)3·6H2O, NH4F (the molar ratio of Pr/Pr + Ce = x, x ranged from 0.1 mol% to 2.5 mol%, [Pr + Ce]:[F] = 1:3) and H-ATP (the mass ratio of [Pr3+:CeF3]:[HATP] was adjusted from 10 to 60 wt%) was added to deionized water
Fig. 1. XRD patterns of H-ATP, CeF3 and 0.1%, 0.5%, 1.0%, 1.5%, 2.5% Pr3+:CeF3/ATP (a), partial enlargement XRD patterns from 42.5° to 47.0° (b). 252
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raw ATP (JCPDS 82-1873). Compared with raw ATP, the intensity of HATP peaks was appreciably weakened. Moreover, the characteristic peaks at 20.8°, 21.5° correspond to the (1 2 1), (3 1 0) crystal planes of raw ATP disappear owing to the dissolution of octahedral structures (AleO, MgeO, and FeeO) in the ATP (Zhang et al., 2016). The diffraction peaks emerging at 24.4°, 25.0°, 27.8° are in consistence with the (0 0 2), (1 1 0) and (1 1 1) crystal planes of CeF3 (JCPDS 70-0002). In addition, no other peaks were observed in the XRD patterns, indicative of the well-crystallized CeF3 crystals. Fig. 1(b) showed the enlarged pattern of Fig. 1(a) from 42.5° to 47.0°. Compared with CeF3, the peaks of Pr3+:CeF3/ATP have an obvious shift towards higher angles, due to the fact that partial metal ions (Al3+, Fe3+) from H-ATP are doped into the CeF3 lattice. However, with the increasing of Pr3+ doping amount, the peaks in Fig. 1(b) have a slight shift towards higher angles, which is due to the fact that the Pr3+ (0.099 nm) radius is smaller than Ce3+ (0.101 nm). It is assumed that the diffusion of Al3+ and Fe3+ ions from H-ATP into CeF3 lattice plays a dominant role for the lattice distortion (Li, X. et al., 2018a; Bai et al., 2018). Fig. 2 showed the Fourier transform infrared spectroscopy (FT-IR) absorption spectra of the raw ATP, H-ATP and Pr3+:CeF3/ATP, respectively. It is clear that the FT-IR spectra are sensitive to small changes in the octahedral position occupied by ATP (Bai et al., 2018). In Fig. 2, the absorption band at 3663.7 cm−1 is caused by the stretch vibrations of structural eOH in high frequency region (3700–3200 cm−1) and the bands at 1209.4, 1656.9 cm−1 located within the middle frequency region (1700–1600 cm−1) which are attributed to the bend vibration of bound structural eOH in H-ATP (Zhang et al., 2016; Li, X. et al., 2018a). More absorption bands
Fig. 2. FT-IR spectra of the raw ATP, H-ATP and Pr3+:CeF3/ATP.
3. Results and discussion 3.1. Characterizations of Pr3+:CeF3/ATP The X-ray diffraction (XRD) patterns of the nanocomposite with different molar ratio of Pr3+ were displayed in Fig. 1. As is shown in Fig. 1 (a), the characteristic peaks at 8.5°, 19.9°, 28.0° and 35.8° correspond to the (1 1 0), (0 4 0), (4 0 0) and (1 6 1) crystal planes of the
Fig. 3. UV–Vis diffuse reflection spectra of raw ATP and H-ATP (a); Kubelka-Munk conversion of raw ATP and H-ATP (b); UV–Vis spectra of CeF3 and Pr3+:CeF3 (c), Kubelka-Munk conversion corresponding to CeF3 and Pr3+:CeF3 (d). 253
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Fig. 4. TEM images of H-ATP (a), 10 wt% Pr3+:CeF3/ATP (b), 20 wt% Pr3+:CeF3/ATP (c) 40 wt% Pr3+:CeF3/ATP (d) and 60 wt% Pr3+:CeF3/ATP (e) with corresponding SAED patterns inserted; HRTEM images of 40 wt% Pr3+:CeF3/ATP (f); EDS pattern of 40 wt% Pr3+:CeF3/ATP (g).
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Fig. 5. Up-conversion PL spectra of different doping amount of Pr3+:CeF3/ATP excited at 480 nm (a); PL spectra of different loading mount of Pr3+:CeF3/ATP excited at 320 nm (b).
Fig. 6. Mott-Schottky plots for 0.5 mol% Pr3+:CeF3 (a), H-ATP (b), and 40 wt% Pr3+:CeF3/ATP(c).
and H-ATP. Compared with the raw ATP, the H-ATP showed significant red shift and the band gap of H-ATP become narrow. The following equation is adopted to calculate the bandgap energy (Eg) of the samples (Acuña et al., 2017; Lebedev et al., 2019):
appeared in the ATP treated with formic acid compared with the raw ATP. The bands at 3544.8 cm−1 and 3400.1 cm−1 are attributed to the tensile vibration of the structure eOH. The absorption bands at 642.1 cm−1 and 985.1 cm−1 are mainly caused by the bending vibration of the Si (or Al)eO tetrahedron and the MeOH (M]Mg, Al, Fe) octahedron (Zhang et al., 2016). For the Pr3+:CeF3/ATP, a new absorption band appeared at 1384.60 cm−1, which may be ascribe to the bending vibration of the CeeOH tetrahedron, indicating that Pr3+:CeF3 is successfully loaded on H-ATP. Fig. 3 (a) showed the UV–vis diffuse reflection spectra of raw ATP
α hν = A(hν − Eg)2 where α is the absorption coefficient, h is Planck constant, ν is light frequency, A is a constant and Eg is band gap energy. The (Ahν)2 versus hν curves of raw ATP and H-ATP are shown in Fig. 3 (b). The Eg of raw ATP and H-ATP are determined to be approximately 3.81 eV and 255
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Fig. 7. The transient photocurrent responses, Light source: Xe lamp, 500 W (a) and the EIS of H-ATP, Pr3+:CeF3, Pr3+:CeF3/ATP with various loading amount (b).
Fig. 8. The VB-XPS of Pr3+:CeF3 (a); and H-ATP (b).
Information) showed the TEM images of 40 wt% Pr3+:CeF3/ATP and Fig. S1(b) was the dark field images corresponding to Fig. S1(a). The bright area corresponds to the Pr3+:CeF3 particles with excellent crystallinity. Fig. 4(g) showed an EDS pattern of 40 wt% Pr3+:CeF3/ATP, in which Mg, Al, Si, O, Fe elements were derived from ATP, and Pr, Ce, and F elements were derived from Pr3+:CeF3, wherein Cu, C were derived from the copper mesh and the carbon film. Fig. S2(a) (Supporting Information) showed an FT-SEM image of 40 wt% Pr3+:CeF3/ATP, where the Pr3+:CeF3 nanoparticles was uniformly dispersed on the rodshaped H-ATP. Elemental mapping of Pr, Ce, F, O, Si, Fe, Mg and Al elements in 40 wt% Pr3+:CeF3/ATP were displayed in Fig. S2 (Supporting Information), exhibiting the well-defined distribution of elements. In order to investigate the effects of different doping amounts of Pr3+ on up-conversion photoluminescence (PL), the up-conversion PL spectra of as-prepared Pr3+:CeF3/ATP samples with various doping ratios were measured under the excitation of 480 nm laser shown in Fig. 5(a). The emission intensity of the up-conversion PL spectra showed a strong peak centered at 278 nm, which increases with doping amounts from 0.1 mol% to 0.5 mol% of Pr3+, and decreases with increasing Pr3+ doping amounts from 0.5 mol% to 2.5 mol%. Therefore, the 0.5 mol% Pr3+:CeF3 demonstrates the best up-conversion capability which converts the visible light to UV light. As shown in Fig. S3 (Supporting Information), the Pr3+:CeF3/ATP samples with different doping ratios transfers the visible light of 443 nm, 466 nm, 593 nm to 260–286 nm UV light, which can be used by Pr3+:CeF3/ATP for photocatalytic nitrogen fixation. In general, the photoluminescence (PL) spectra reflects the recombination rate of photogenerated electron/hole pairs, and the high fluorescence intensity suggests a high degree of
3.59 eV by Kubelka-Munk conversion. Fig. 3 (c) showed the UV–vis plot of CeF3 and Pr3+:CeF3, respectively. The Pr3+:CeF3 was red-shifted compared to pure CeF3, indicating that a small amount of Pr3+ doped into the crystal lattice of CeF3 narrows the band gap energy. The peaks at 443 nm, 466 nm, 480 nm and 593 nm are attribute to the Pr3+ ions in CeF3 lattices which can absorb visible light. As shown in Fig. 3(d), the band gap energy of CeF3 is 3.29 eV, and the Eg of Pr3+:CeF3 is 3.18 eV. The transmission electron microscopy (TEM) and the field emission scanning electron microscope (FE-SEM) were provided to investigate the morphology and structure of the synthesized samples. Fig. 4(a) showed the TEM images of H-ATP and corresponding selected area electron diffraction (SAED) pattern (inset). For the as-prepared H-ATP, one-dimensional nanorod structures were clearly observed and the SAED pattern inserted indicates that the H-ATP are amorphous. The TEM images of Pr3+:CeF3/ATP with various loading amounts were displayed in Fig. 4(b)–(e).The Pr3+:CeF3 nanoparticles were uniformly loaded on the H-ATP with size ca.10 nm. The diffraction rings of in Fig. 4(b) and (c) correspond to the (1 1 1), (1 1 3) and (3 1 0) crystal planes of CeF3. With the increased loading of Pr3+:CeF3 particles, the intensity of diffraction rings of the corresponding SAED increased. The image inserted in Fig. 4(d) exhibited diffraction rings corresponding to (1 1 1), (1 1 3), (3 1 0) and (1 4 1) crystal planes. Comparatively, the SAED in Fig. 4(e) displayed one more diffraction ring corresponding to (1 5 1) crystal plane. The high-resolution TEM (HRTEM) images of 40 wt% Pr3+:CeF3/ATP shown in Fig. 4(f) exhibited a clear heterostructure between Pr3+:CeF3 and H-ATP. The nanoparticles were highly crystallized with the interplanar spacing of 0.22 nm and 0.36 nm corresponding to the (1 2 1) and (1 1 0) lattice planes of CeF3, in good agreement with XRD patterns in Fig. 1. Fig. S1(a) (Supporting 256
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Fig. 9. XPS spectra of H-ATP, Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP nanocomposite (a) survey of the samples, (b) Al 2p of H-ATP, (c) Fe 2p of H-ATP, (d) Ce 3d, (e) Pr 3d, (f) F 1 s, (g) Si 2p.
are equal to −0.21 eV, 0.02 eV and −0.09 eV (vs. NHE, PH = 7), respectively. The transient photocurrent responses of H-ATP, Pr3+:CeF3, 3+ Pr :CeF3/ATP with various loading amounts were performed to reveal the separation of photogenerated electrons and holes as shown in Fig. 7 (a). The photocurrent of the 40 wt% Pr3+:CeF3/ATP exhibited the highest intensity indicating more photogenerated carriers existed and superior separation performance of photogenerated electrons and holes. Fig. 7(b) demonstrated the electrochemical impedance spectra (EIS) Nyquist plots of H-ATP and Pr3+:CeF3/ATP with various loading amounts, investigating the migration and interfacial transfer/recombination rates of charge carriers (Pan et al., 2019). It is obvious that 40 wt% Pr3+:CeF3/ATP showed the smallest semicircle indicative of low charge transfer resistance and high conductivity, suggesting high efficient separation of photo-generated carriers and outstanding efficiency of charge immigration across the heterojunction interface, in good agreement with the results of transient photocurrent responses in Fig. 7(a). Fig. 8 showed the XPS valance band edge spectrum (VB-XPS) of Pr3+:CeF3, and H-ATP, which were obtained as 1.41 eV and 2.22 eV corresponding to the distance between valence band and Fermi level (Evf). In general, the flat band potential is equal to fermi level for n-type semiconductors (Li, R. et al., 2018). Therefore, the valance band (VB) potentials of Pr3+:CeF3 and H-ATP are 1.20 eV and 2.24 eV, respectively. The conduction band gap energy position (ECB) can be calculated by the following equation:
recombination (Zhang et al., 2018). The PL spectra of Pr3+:CeF3/ATP with different loadings excited at 320 nm laser were demonstrated in Fig. 5(b). The emission intensity increased when loading of Pr3+:CeF3 is lower than 40 wt%, indicating that the electrons and holes recombination rate of Pr3+:CeF3/ATP is accelerated (Li, X. et al., 2019). The 40 wt% Pr3+:CeF3/ATP exhibited the highest fluorescence emission intensity indicating the highest recombination rate of photogenerated electrons and holes. However, when the loading of Pr3+:CeF3 is larger than 40 wt%, the fluorescence emission intensity decreased, due to the fact that the recombination balance of electrons and holes are broken, leading to the decrease of recombination efficiency. The Mott-Schottky plots of 0.5 mol% Pr3+:CeF3, H-ATP and 40 wt% Pr3+:CeF3/ATP were shown in Fig. 6(a)–(c) respectively, which were obtained to investigate the band potential of as-prepared samples. The flat band potential (Efb) is calculated by using the Mott-Schottky equation as follows (Pan et al., 2019; Martínez-Vargas et al., 2019):
1 2NA RT ⎤ ⎡E − Efb − = 2 Nd Fεε0 ⎣ F ⎦ CSc where NA is the Avogadrós number (6.023 × 1023 mol−1), F is the Faraday constant (9.65 × 104 C mol−1), ε0 is the vacuum permittivity (8.8542 × 10−14F cm−1), ε is the dielectric constant of the semiconductor, R is the gas constant (8.314 J K−1 mol−1), T is the absolute temperature (298 K), and E (V) is the applied potential. It was observed in Fig. 6 that the slope in the Mott-Schottky pattern was positive, revealing that both of the 0.5 mol% Pr3+:CeF3 and H-ATP are n-type semiconductors (Martínez-Vargas et al., 2019). The flat band potentials of 0.5% Pr3+:CeF3, H-ATP and 40 wt% Pr3+:CeF3/ATP were calculated to be −0.41 eV, −0.18 eV and −0.29 eV (vs. Ag/AgCl, PH = 7). which
ECB = EVB − Eg According to the Eg of Pr3+:CeF3 and H-ATP obtained in Fig. 3, the 257
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Fig. 9. (continued)
characteristic peaks of both Al and Fe shifted toward to higher binding energy in Pr3+:CeF3/ATP composites compared to H-ATP, due to the fact that Al and Fe elements have greater electronegativity than Ce and the formation of Al-F, Fe-F bond partially replace Ce-F bond, suggestive of the formation of fluorine vacancies. Fig. 9(d) showed the XPS spectra of Ce 3d for the Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP. As for Pr3+:CeF3, the peaks of binding energy at 883.6, 886.5 and 902.4, 905.0 eV are attributed to Ce 3d5/2 and 3d3/2 of Ce3+, respectively (Xiang et al., 2016; Li, Xuejiao et al., 2019). The binding energies in the sample of 40 wt% Pr3+:CeF3/ATP showed a slightly shift to higher binding energy compared with Pr3+:CeF3, suggesting that some metal ions (like Al3+ and Fe3+) from H-ATP may be doped into the Pr3+:CeF3 lattice after Pr3+:CeF3 and H-ATP closely combined (Xiang et al., 2016). Fig. 9(e) showed the XPS spectrum of Pr 3d. The peaks at 934.8 and 943.8 eV are attributed to Pr 3d5/2 in Pr3+:CeF3 exhibiting the existence of Pr3+ in the composites (Gurgul et al., 2013). It was observed that the peaks in 40 wt% Pr3+:CeF3/ATP obviously shifted to higher binding energy compared to Pr3+:CeF3, suggesting the Pr3+:CeF3 and H-ATP are tightly conjunct by covalent bonds (Dalai et al., 2011). As displayed in Fig. 9(f), the same peak at 684.9 eV was found in both Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP which shifted towards the lower binding energy compared with standard peak of F 1s at 685.7 eV, indicating the possible effect of the formation Ce-F bond (Xiang et al., 2016). The peak of binding energy at 691.3 eV is mainly attributed to the existence of the bonding between Pr and F ion. Fig. 9(g) showed the Si 2p spectra of H-ATP and 40 wt% Pr3+:CeF3/ATP where binding energies were observed at 101.2 eV and 102.7 eV, respectively (Li, X. et al., 2018b; Chao et al., 2008). The difference of binding energy between H-ATP and 40 wt% Pr3+:CeF3/ATP reveals the
Fig. 10. ESR spectra measured at 100 K for CeF3 and 40 wt% Pr3+:CeF3/ATP.
ECB of Pr3+:CeF3 is calculated to be −1.98 eV and the ECB of H-ATP is calculated to be −1.35 eV. The X-ray photoelectron spectroscopy (XPS) was performed to investigate the valence states and the chemical compositions of the samples. Fig. 9(a) showed the survey scan of H-ATP, Pr3+:CeF3 and 40 wt% Pr3+:CeF3/ATP. Fig. 9(b) and (c) showed the characteristic peaks in H-ATP and 40 wt% Pr3+:CeF3/ATP separately. Al and Fe elements were present in H-ATP and 40 wt% Pr3+:CeF3/ATP, which is consistent with the results of EDS pattern. It was observed that the 258
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Fig. 11. Photocatalytic nitrogen fixation activity of H-ATP, Pr3+:CeF3, and Pr3+:CeF3/ATP with various loading amounts (a) under visible light, λ greater than 420 nm, (b) under simulated sunlight, (c) under UV light, λ < 420 nm, (d) under visible light, with and without sacrificial agent over 40 wt% Pr3+:CeF3/ATP, (e) recycle performance over 40 wt% Pr3+:CeF3/ATP under visible light for five times. The error bars represent the standard deviations of three independent tests for each data point.
formation of FeOeSi bond in 40 wt% Pr3+:CeF3/ATP, attributed to the partial substitution of SieOeSi bond (Li, X. et al., 2018c). In order to investigate the defects in the as-prepared samples, low temperature electron spin resonance (ESR) test was performed. Fig. 10 showed the ESR spectra measured at 100 K for CeF3 and Pr3+:CeF3/ ATP. Generally, there is no ESR signal for pure CeF3, implying that there are no fluorine vacancies. However, a single Lorentzian line signal with a g value around 2 can be identified when the CeF3 possess fluorine vacancies (Vahakangas et al., 2015). It is obvious that a weak signal was observed in the CeF3, indicating that the as-prepared CeF3 has a little defect, and a strong ESR signal at g = 2.005 was observed in the sample of Pr3+:CeF3/ATP, which can be assigned to the abundant fluorine vacancies in Pr3+:CeF3/ATP. The fluorine vacancies may be due to the fact that partial metal Fe3+, Al3+ ions in H-ATP are doped
into the crystal lattice of CeF3, resulting in lattice defects, which is consistent with the analysis results of XPS in Fig. 9(b) and (c). 3.2. Photocatalytic nitrogen fixation performance The photocatalytic N2 fixation efficiencies under visible light were performed in Fig. 11(a). H-ATP, Pr3+:CeF3 and the Pr3+:CeF3/ATP nanocomposites with different mass loading were measured three times (n = 3) and the results were the average of three repeated tests (with error bars represented the standard deviations), which were displayed in Fig. 11. The separate H-ATP and Pr3+:CeF3 exhibited very low nitrogen fixation performance which were 8.89 ± 0.40 μmol/L and 46.38 ± 1.90 μmol/L (n = 3), respectively. After Pr3+:CeF3 combined with H-ATP, significant more N2 were fixed and converted into NH4+, 259
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which can be mainly ascribe to the enhanced light-harvesting capacity and the improved carrier separation efficiency (Xu et al., 2018). The photocatalytic N2 fixation efficiencies increased when the loading of Pr3+:CeF3 increased from 10 wt% to 40 wt%, while decreased from 40 wt% to 60 wt%. The 40 wt% Pr3+:CeF3/ATP nanocomposites was the most efficient in photocatalytic nitrogen fixation reaching as high as 629.16 ± 17.62 μmol/L (n = 3), within 4 h. Fig. 11(b) and (c) showed the photocatalytic nitrogen fixation efficiencies of Pr3+:CeF3/ATP under simulated sunlight and UV light (λ < 420 nm), respectively. The photocatalytic performance of Pr3+:CeF3/ATP was found following the order: UV light > simulated sunlight > visible light. As shown in Fig. 11(d), Pr3+:CeF3/ATP with ethanol as sacrificial agent demonstrated more ammonia production than that without sacrificial agent, which may be due to the fact that partial NH4+ ions are oxidized into NO3− ions by photogenerated holes. Fig. 11(e) showed the cycle experiment of Pr3+:CeF3/ATP, whose concentration of NH4+ can still reach 606.37 ± 16.67 μmol/L within 4 h at the fifth cycle suggestive of excellent reusability. The slight drop in NH4+ production might result from partial block of porous structure of ATP which affected the adsorption performance. Besides, Fig S4 showed the TEM image of 40 wt%
Fig. 12. Transient photocurrent response of 40 wt% Pr3+:CeF3/ATP under the Ar or N2 atmosphere, Light source: Xe lamp, 500 W.
Fig. 13. (a) Mechanism for the formation of Z-type heterojunction, (b) mechanism for up-conversion luminescence and photocatalytic nitrogen fixation. 260
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Pr3+:CeF3/ATP after using for five times, which was almost the same as that of the fresh sample. To further shed light on the effect of Pr3+:CeF3/ATP composites in photocatalytic nitrogen fixation, the transient photocurrent response experiments were carried out with Pr3+:CeF3/ATP when different gas (Ar, N2) flows were bubbled into the electrolyte solution. As displayed in Fig. 12, the photocurrents of Pr3+:CeF3/ATP under N2 atmosphere exhibited a significant decrease compared with the composites under Ar atmosphere, mainly due to the fact that the electrons are consumed by the N2 molecules and are prone to participate in the process of photocatalytic nitrogen fixation.
3.4. Conclusions In conclusion, Pr3+:CeF3/ATP nanocomposites were synthesized via a facile microwave hydrothermal method. The Pr3+:CeF3 nanoparticles having size ca.10 nm are uniformly loaded on the surface of one-dimensional nanorod ATP with strong interaction. The 0.5 mol% doping amount of Pr3+ demonstrates the strongest up-conversion property improving the utilization of solar light. Meanwhile, the metal ions (Al3+, Fe3+) species from modified ATP enter into the crystal lattice of CeF3 forming the fluorine vacancies, which serve as the active sites to weaken the N ≡ N triple bond for activation of N2. The Pr3+:CeF3/ATP nanocomposite with 40 wt% loading amount forms a well-defined Zscheme heterojunction which effectively separates photogenerated electrons and holes, and preserve more negative conduction band leading to the enhanced reducibility for photocatalytic N2 fixation.
3.3. Photocatalytic nitrogen fixation mechanism The Fig. 13(a) showed the formation of heterojunction between Pr3+:CeF3 and H-ATP before and after contact. Before contact, the EVB, Ef and ECB values of Pr3+:CeF3 are more negative than the values of HATP. After the two semiconductors are in contact, the electrons flow from the semiconductor with a high Fermi level (Ef) to the semiconductor with a low Fermi level. In fact, the Ef value of Pr3+:CeF3/ATP nanocomposites is −0.09 eV. Therefore, it can be inferred that the Ef value of H-ATP is upshifted from 0.02 eV to −0.09 eV and the Ef value of Pr3+:CeF3 is downshifted from −0.21 eV to −0.09 eV after contact between H-ATP and Pr3+:CeF3. A built-in electric field is formed on the interface between H-ATP and Pr3+:CeF3, and the energy band of H-ATP bend down, while the energy band of Pr3+:CeF3 bend upward. The photogenerated electrons on the H-ATP conduction band recombine with the holes in the valence band of Pr3+:CeF3 driven by the built-in electric field, leading to the Z-type heterojunction. It is reported that the standard redox potential for N2 to NH3 is −0.092 eV versus normal hydrogen electrode (NHE) (Devthade et al., 2018). Obviously, ECB of 0.5% Pr3+:CeF3 (−1.86 eV) is more negative than the N2/NH3 reduction potential. The existence of fluorine vacancies in Pr3+:CeF3/ATP act as active sites to weaken the N ≡ N triple bond and efficiently promote the participation of N2 in the reaction (Yang, J. et al., 2018). Fig. 13(b) showed the mechanism of up-conversion luminescence and photocatalytic nitrogen fixation. As illustrated in the energy level image of Pr3+ ion, the up-conversion capability of Pr3+:CeF3/ATP is attribute to the process of excited state absorption (ESA) and energy transfer UC (ETU). After absorbing the energy of two visible photons (ω1 and ω2), Pr3+ ions are transformed to excited intermediate state. Finally, the UC photons (ω) are released when excited electrons transfer back to the ground state via radiation relaxation. The 3P0, 3P1 and 1I6, etc., levels of Pr3+ ions can act as intermediate excited states to reach the process of ESA as the process of 3H4 → 3P0 → 4f5d (Gao et al., 2017). The rare earth element Pr3+ ions convert part of visible light into ultraviolet light with higher photon energy indirectly improving the utilization of sunlight. N2 is reduced to NH3 by the photogenerated electrons on the Pr3+:CeF3 conduction band, which finally dissolves in water as NH4+. The photogenerated holes in the valence band of H-ATP oxidize H2O to O2 and H+ ions and finally oxidize the sacrificial agent ethanol to oxidation products. The possible reactions occurring during the photocatalytic N2 fixation can be shown as following:
H − ATP + hv → e(−CB, H − ATP ) + h (+VB, H − ATP )
(1)
Pr 3 + : CeF3 + hv → e −
(2)
(CB,Pr3 + : CeF3)
+ h+
(VB,Pr3 + : CeF3)
N2 + e(−CB,Pr3 + : CeF ) + H+ → NH4+
(3)
H2 O + h(+VB, H − ATP ) → H+ + O2
(4)
ethanol + h(+VB, H − ATP ) → Oxidation products
(5)
3
Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgments This work was supported by the National Natural Science Foundation of China (51674043, 51702026), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), and Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX18_0951). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.08.063. References Acuña, K., Yáñez, J., Ranganathan, S., Ramírez, E., Pablo Cuevas, J., Mansilla, H.D., Santander, P., 2017. Photocatalytic degradation of roxarsone by using synthesized ZnO nanoplates. Sol. Energy 157, 335–341. Bai, Y., Yang, P., Wang, P., Fan, Z., Xie, H., Wong, P.K., Ye, L., 2018. Solid phase fabrication of Bismuth-rich Bi3O4ClxBr 1–x solid solution for enhanced photocatalytic NO removal under visible light. J. Taiwan Inst. Chem. Eng. 82, 273–280. Bai, Y., Ye, L., Chen, T., Wang, L., Shi, X., Zhang, X., Chen, D., 2016. Facet-dependent photocatalytic N2 fixation of bismuth-rich Bi5O7I nanosheets. ACS Appl. Mater. Interf. Balestrieri, M., Gallart, M., Ziegler, M., Bazylewski, P., Ferblantier, G., Schmerber, G., Chang, G.S., Gilliot, P., Muller, D., Slaoui, A., Colis, S., Dinia, A., 2014. Luminescent properties and energy transfer in Pr3+ doped and Pr3+-Yb3+ Co-doped ZnO thin films. J. Phys. Chem C 118 (25), 13775–13780. Cao, J., Nie, W., Huang, L., Ding, Y., Lv, K., Tang, H., 2019. Photocatalytic activation of sulfite by nitrogen vacancy modified graphitic carbon nitride for efficient degradation of carbamazepine. Appl. Catal. B 241, 18–27. Chai, R., Lian, H., Li, C., Cheng, Z., Hou, Z., Huang, S., Lin, J., 2009. In situ preparation and luminescent properties of CeF3 and CeF3:Tb3+ nanoparticles and transparent CeF3:Tb3+/PMMA nanocomposites in the visible spectral range. J. Phys. Chem. C 113 (19), 8070–8076. Chao, Z., Ji, C., Zhou, Y., Li, D., 2008. Ionic liquid-based “all-in-one” synthesis and photoluminescence properties of lanthanide fluorides. J. Phys. Chem. C 112 (27), 10083–10088. Chen, X., Li, N., Kong, Z., Ong, W.-J., Zhao, X., 2018. Photocatalytic fixation of nitrogen to ammonia: state-of-the-art advancements and future prospects. Mater. Horiz. 5 (1), 9–27. Chilkalwar, A.A., Rayalu, S.S., 2018. Synergistic plasmonic and upconversion effect of the (Yb, Er)NYF-TiO2/Au composite for photocatalytic hydrogen generation. J. Phys. Chem C 122 (46), 26307–26314. Coleman, P.G., Abdulmalik, D.A., 2008. Useful vacancies: positron beam interrogation of fluorine-vacancy complexes in semiconductor device structures. Appl. Surf. Sci. 255 (1), 71–74. Dalai, M.K., Kundu, R., Pal, P., Bhanja, M., Sekhar, B.R., Martin, C., 2011. XPS study of Pr1−xCaxMnO3 (x=0.2, 0.33, 0.4 and 0.84). J. Alloy. Compd. 509 (29), 7674–7676. Dantelle, G., Mortier, M., Vivien, D., Patriarche, G., 2005. Effect of CeF3 addition on the nucleation and upconversion luminescence in transparent oxyfluoride glass−ceramics. Chem. Mater. 17 (8), 2216–2222. Devthade, V., Gupta, A., Umare, S.S., 2018. Graphitic carbon nitride−γ-gallium oxide (GCN-γ-Ga2O3) nanohybrid photocatalyst for dinitrogen fixation and pollutant decomposition. ACS Appl. Nano Mater. 1 (10), 5581–5588. Du, Z., Denkenberger, D., Pearce, J.M., 2015. Solar photovoltaic powered on-site
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