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Enhanced visible-light-driven photocatalytic performance of AgNbO3 cubes with a high-energy (001) facet
Journal of Physics and Chemistry of Solids 135 (2019) 109083
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Enhanced visible-light-driven photocatalytic performance of AgNbO3 cubes with a high-energy (001) facet
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Bifen Gao∗, Desheng Hu, Caixia Xiao, Dongxu Li, Bizhou Lin, Yilin Chen, Yun Zheng Department of Applied Chemistry, College of Materials Science & Engineering, Huaqiao University, Xiamen, 361021, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Semiconductors Surface properties Crystal structure Microstructure
AgNbO3 cubes with an exposed high-energy (001) facet were prepared by hydrothermal treatment of Ag2O–Nb2O5–NH4HF2 aqueous suspension. The crystal phase, surface microstructure, and optical properties of the samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and UV–vis diffuse reflectance spectroscopy. The energy band structure and charge carrier separation efficiency were investigated by photoelectrochemical measurements. In comparison with AgNbO3 powder, whose surface was dominated by a low-energy (114) facet, the AgNbO3 cubes had superior visible-lightdriven photocatalytic activity for degradation of tetracycline because of the greater redox ability and more efficient separation of photogenerated electrons and holes. The surface F− ions had no influence on the photocatalytic reactions. Radical trapping experiments demonstrated that the superoxide radical played a decisive role in the photodegradation of tetracycline.
1. Introduction Semiconductor photocatalysts have attracted worldwide attention for their promising application in the fields of environmental remediation and energy generation. The catalytic activity of a semiconductor-based photocatalyst can be significantly influenced by its surface structure that is directly exposed to the reaction media. Recently, engineering the surface structure of a photocatalyst to a desirable crystal plane to optimize the photocatalytic reactivity has been actively pursued [1–8]. Yang et al. [1] first reported the synthesis of micron-sized anatase TiO2 crystals with 47% exposed (001) facets and demonstrated that the (001) facets were much more reactive than the thermodynamically stable (101) facets for the production of H2 from water [1]. Han et al. [2] observed that an anatase TiO2 sheet showed much higher activity for the degradation of methyl orange than commercial P25 because of the exposure of a high percentage of the (001) facets. BiVO4 sheets with a preferred (010) surface orientation exhibited superior photocatalytic activity for water oxidation and degradation of rhodamine B [3,4]. Unique ZnO nanocrystals with a high (0001) facet population showed better photocatalytic efficiency for decomposition of methylene blue and rhodamine B [5,6]. The photocatalytic activity of the (001) facets of BiOBr nanosheets for the degradation of salicylic acid and rhodamine B was found to be higher than that of the (010) facets [7]. All of these results demonstrate that the
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photocatalytic performance is highly correlated with the crystal facet of the photocatalyst. Acquisition of a high percentage of reactive facets by crystal facet engineering is highly desirable to increase the photocatalytic reactivity. AgNbO3, which has a band gap of about 2.8 eV [9], is an attractive candidate material for visible-light-driven photocatalysts for degradation of organic pollutants and hydrogen generation. Li et al. [9–11] reported that AgNbO3 can decompose 2-propanol and other organic compounds. Photocatalytic O2 evolution from an aqueous silver nitrate solution and H2 formation from a mixture of water and methanol vapor over AgNbO3 under visible-light irradiation were also observed [12]. However, the photocatalytic performance of AgNbO3 still needs to be improved. Furthermore, in reported studies, AgNbO3 was generally synthesized by solid-state reaction [9], the sol-gel method [13], and salt flux [14]. The as-prepared AgNbO3 samples possessed irregular and polyhedral structures, so that the shape and facet effects on their photocatalytic properties remained unclear. In 2012, Chang et al. [15] reported the hydrothermal synthesis and phase transition of AgNbO3 cubes; however, the surface microstructure and photocatalytic performance of the AgNbO3 cubes was not explored. The impact of surface microstructure on photocatalytic activity was not clarified. In this work, cubic AgNbO3 crystals with an exposed (001) facet were synthesized by facile hydrothermal reactions. The photocatalytic activity of the as-obtained AgNbO3 sample for degradation of the
Corresponding author. E-mail address: [email protected] (B. Gao).
https://doi.org/10.1016/j.jpcs.2019.109083 Received 15 January 2019; Received in revised form 28 June 2019; Accepted 29 June 2019 Available online 29 June 2019 0022-3697/ © 2019 Published by Elsevier Ltd.
Journal of Physics and Chemistry of Solids 135 (2019) 109083
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antibiotic tetracycline was investigated. The effect of an exposed highenergy (001) facet on the photocatalytic reaction was explored and the photocatalytic mechanism is proposed. 2. Experimental 2.1. Synthesis of AgNbO3 AgNbO3 cubes were synthesized by a facile hydrothermal process. In a typical synthesis, 0.9268 g Ag2O, 1.0632 g Nb2O5, and 0.6845 g NH4HF2 were dispersed in 70 mL H2O under vigorous stirring. The mixture was then transferred to a 100 mL Teflon-lined stainless steel autoclave and kept at 200 °C for 60 h. After reaction, the precipitates were collected, washed with distilled water, and dried at 60 °C overnight. The as-obtained AgNbO3 sample was denoted as AgNbO3 (HT). For comparison, AgNbO3 powders were prepared by the conventional solid-state reaction. For this, 1 mmol Nb2O5 and 1 mmol Ag2O were thoroughly mixed and ground with the addition of methanol. After being dried under infrared light, the mixture was calcined sequentially at 850 °C for 5 h and 1100 °C for 10 h. Finally, the powder was allowed to cool naturally to room temperature. The sample obtained by the solid-state reaction was denoted as AgNbO3 (SSR).
Fig. 1. X-ray diffraction patterns of AgNbO3 (HT) and AgNbO3 (SSR).
monitoring the absorbance at 358 nm with a UV–vis spectrophotometer (UNIC UV-2600).
2.2. Characterization
3. Results and discussion
The crystal phase of the catalyst was characterized by powder X-ray diffraction (XRD) with a Rigaku D/max-2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å). The morphology and surface microstructure of the samples were observed by field-emission scanning electron microscopy (SEM; Hitachi S-4800) and transmission electron microscopy (TEM; JEOL JEM-2010). Diffuse reflectance spectra were obtained with a Shimadzu 2550 spectrometer with BaSO4 as the reflectance standard. X-ray photoelectron spectroscopy measurements were performed with a Thermo Fisher Scientific ESCALAB25 spectrometer with an nonmonochromatic Al Kα (1486.6 eV) X-ray source. All spectra were calibrated to the binding energy of the adventitious C 1s peak at 284.6 eV. Nitrogen adsorption-desorption isotherms were acquired with a Quantochrome autosorb iQ analyzer. The samples were outgassed for 2 h at 200 °C before measurement.
3.1. Crystal phase Fig. 1 shows XRD patterns of the AgNbO3 samples obtained by the hydrothermal and solid-state reactions. For both samples, all the diffraction peaks can be indexed to the orthorhombic perovskite AgNbO3 phase (JCPDS card no. 52-0405). No peak of other impurities was detected. The diffraction peaks of AgNbO3 (SSR) are stronger and sharper than those of AgNbO3 (HT), implying higher crystallinity of the former. 3.2. Morphology characterization SEM and TEM images of AgNbO3 (HT) are shown in Fig. 2a and c, respectively. It can be clearly seen that the AgNbO3 (HT) sample is composed of cubes with a side length of about 600 nm. The high-resolution TEM (HRTEM) image in the inset in Fig. 2c shows an interplanar lattice spacing of 0.280 nm, corresponding to the (020) plane of the orthorhombic perovskite AgNbO3 phase. The selected-area electron diffraction pattern indicates the single-crystal nature of the AgNbO3 (HT) cubes. As shown in Fig. 2d, the diffraction spots of the (020) and (110) facets and the 45.2° angle between the two facets can be well ascribed to perovskite-structured AgNbO3. The TEM analyses imply that the zone axis perpendicular to the top surface of the cube is [001]. The preceding structural information suggests that the exposed upper and bottom surfaces of the cube are the (001) plane. Fig. 2b presents an SEM image of AgNbO3 (SSR). Obviously, this sample consists of particles with irregular shape and a wide size distribution. Furthermore, the particles aggregate and form large clumps. An HRTEM image of AgNbO3 (SSR) is shown in the inset in Fig. 2b. The lattice spacing of 0.277 nm is attributed to the (114) plane of the orthorhombic AgNbO3 phase. Generally, the thermodynamically stable plane would grow slowly during crystal growth and result in high exposure and high density in the final crystal. In the HRTEM characterization of AgNbO3 (SSR), the lattice spacing of the (114) plane can be well observed in most particles examined, indicating the high exposure ratio of the (114) facet. On the other hand, the (114) peak has the strongest intensity among the diffraction peaks, suggesting high density of the (114) plane in the synthetic crystal. Taking the high exposure of the (114) facet in the HRTEM characterization of AgNbO3 (SSR) and the strongest intensity of the (114) peak into account, we speculate that (114) might be the most thermodynamically stable crystal plane. To minimize the total
2.3. Photoelectrochemical measurements Mott-Schottky plots, photocurrent data, and electrochemical impedance spectra were collected from a conventional three-electrode cell with a CHI660E electrochemical workstation. The measurements were performed in 0.2 M Na2SO4 electrolyte under visible light irradiation (λ ≥ 400 nm). AgNbO3, Pt plate, and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. The working electrode was prepared as follows: 10 mg AgNbO3 was dispersed in 2 mL distilled water under sonication for 15 min to obtain a suspension. Subsequently, the suspension was distributed on the conductive surface of fluorine-doped tin oxide glass to form a photocatalyst film with an area of 1 cm2. The film was dried under infrared light before use. 2.4. Photocatalytic performance The photocatalytic activity was evaluated by the degradation of tetracycline with an initial concentration of 10 ppm. The suspension of photocatalyst (100 mg) in tetracycline aqueous solution (100 mL) was stirred in the dark for 40 min to achieve adsorption-desorption equilibrium. Subsequently, the photocatalytic degradation was performed by continuous stirring of the mixture under visible light irradiation (λ ≥ 400 nm). The light source was a xenon lamp (300 W) equipped with a filter to remove UV light. The suspension was withdrawn at a given time interval. After centrifugation to remove the catalyst particles, the concentration of the residual tetracycline was determined by 2
Journal of Physics and Chemistry of Solids 135 (2019) 109083
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Fig. 2. Field-emission scanning electron microscopy image of (a) AgNbO3 (HT) and (b) AgNbO3 (SSR). (c) Transmission electron microscopy image and (d) selected-area electron diffraction pattern of AgNbO3 (HT). The insets in (b) and (c) show high-resolution transmission electron microscopy images of AgNbO3 (SSR) and AgNbO3 (HT), respectively. The inset in (d) shows the angle between the (020) and (110) facets in the AgNbO3 (HT) cube.
blue-shifted to 425 nm. An absorption tail until 600 nm is also observed for AgNbO3 (HT). The optical band gap of the samples was deduced from the following equation: (Ahν)2 = hν – Eg, where h, ν, Eg, and A are Planck's constant, the incident photon frequency, the band gap, and the absorption coefficient, respectively [18]. As shown in the inset in Fig. 3, the band gaps of AgNbO3 (SSR) and AgNbO3 (HT) are 2.82 and 2.92 eV, respectively. The different optical properties of the two samples may be due to their different surface structures.
surface energy during synthesis, AgNbO3 (SSR) might adopt a (114) facet as the basal surface. However, it is also well known that the crystal growth habit can be controlled by the specific adsorption of ions to a particular crystal plane, thereby inhibiting the growth of these facets by lowering their surface energy [16,17]. In the case of AgNbO3 (HT), the F− ions from the raw material NH4HF2 might adsorb on the (001) facet and lower its surface energy, resulting in higher exposure of the (001) facet [2]. The exposure ratio of the (001) facet is approximately 33% in the AgNbO3 (HT) cubes.
3.4. Photoelectrochemical measurements
3.3. Optical properties
The electronic band structure of the AgNbO3 samples was determined by the flat-band potential and the optical band gap. The flatband potential of a semiconductor can be obtained from the intercept of the Mott-Schottky plot. As shown in Fig. 4, the positive slope of the Mott-Schottky plot indicates that AgNbO3 is an n-type semiconductor. The flat-band potentials of AgNbO3 (SSR) and AgNbO3 (HT) are −0.29 and −0.35 V (versus the reversible hydrogen electrode at pH 7), respectively. In general, the energy difference between the conduction band edge potential and the flat-band potential is assumed to be 0.1–0.3 V [19]. In this work, this energy difference is assumed to be 0.1 V to calculate the conduction band edge potential of AgNbO3, which is −0.39 V for AgNbO3 (SSR) and −0.45 V for AgNbO3 (HT). From these findings together with the optical absorption analysis, we can deduce that the valence band edge potentials of AgNbO3 (SSR) and AgNbO3 (HT) are 2.43 and 2.47 V, respectively. Apparently, the conduction band edge potential of AgNbO3 (HT) is more negative than that of AgNbO3 (SSR), while the valence band edge potential of AgNbO3 (HT) is more positive than that of AgNbO3 (SSR). It can be expected that the redox abilities of charge carriers in AgNbO3 (HT) are greater than those in AgNbO3 (SSR). The photocurrent was measured to evaluate the efficiency of separation of photogenerated charge carriers. Both AgNbO3 (HT) and AgNbO3 (SSR) show a transient photocurrent on visible light
UV–vis diffuse reflectance spectra of AgNbO3 (HT) and AgNbO3 (SSR) are presented in Fig. 3. Both samples exhibit strong absorption in the UV region. The absorption-edge onset of AgNbO3 (SSR) is at 440 nm, whereas the absorption-edge onset of AgNbO3 (HT) is slightly
Fig. 3. UV–vis diffuse reflectance spectra of AgNbO3 (HT) and AgNbO3 (SSR). The inset shows the band gap energy of the samples. 3
Journal of Physics and Chemistry of Solids 135 (2019) 109083
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Fig. 5. (a) Photocatalytic degradation of tetracycline by AgNbO3 (HT) and AgNbO3 (SSR). (b) Specific photocatalytic activity of the catalysts. The inset shows repeated photocatalytic degradation of tetracycline in the presence of AgNbO3 (HT). N–TiO2, nitrogen-doped TiO2.
irradiation. However, the gaps between the photocurrent and the dark current (ΔI) of the two samples are quite different. The average ΔI value of AgNbO3 (HT) is approximately 4.6 times higher than that of AgNbO3 (SSR). The higher photocurrent indicates more efficient separation of photogenerated electrons and holes in AgNbO3 (HT), which is beneficial for photocatalytic reactions. Electrochemical impedance spectra of AgNbO3 (SSR) and AgNbO3 (HT) film electrodes are shown in Fig. 4c. It is apparent that the AgNbO3 (HT) electrode has a lower impedance than the AgNbO3 (SSR) electrode, indicating more effective separation of photogenerated electron-hole pairs and rapid interfacial charge transfer. The electrochemical impedance spectroscopy results are consistent with the photocurrent analyses. 3.5. Photocatalytic performance
Fig. 4. (a) Mott-Schottky plots of the AgNbO3 film electrodes in 0.2 M Na2SO4 aqueous solution. (b) Transient photocurrent of AgNbO3 (HT) and AgNbO3 (SSR) irradiated with visible light (λ ≥ 400 nm). (c) Electrochemical impedance spectra of AgNbO3 (SSR) and AgNbO3 (HT) film electrodes. RHE, reversible hydrogen electrode.
The photocatalytic performance of AgNbO3 was evaluated by the degradation of tetracycline, which forms a colorless solution, under visible light irradiation (λ ≥ 400 nm). As can be seen from Fig. 5a, no photolysis of tetracycline was observed under the experimental conditions. AgNbO3 (SSR) exhibits some photocatalytic activity because of its 4
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visible light response. The photodegradation efficiency of AgNbO3 (SSR) is approximately 21% after irradiation for 2.5 h AgNbO3 (HT) shows better performance than AgNbO3 (SSR). In the same time of 2.5 h, AgNbO3 (HT) achieves a photocatalytic efficiency of 66%, which is three times that of AgNbO3 (SSR). The photocatalytic performance of AgNbO3 was also compared with that of nitrogen-doped TiO2 (N–TiO2), a well-known visible-light-driven photocatalyst. N–TiO2 was synthesized according to Ref. [20]. Since N–TiO2 has a much larger surface area than AgNbO3 (SSR) and AgNbO3 (HT), Fig. 5b presents the specific photocatalytic efficiency per unit surface area of the catalysts. It can be seen that the photocatalytic activities of AgNbO3 (SSR) and N–TiO2 are comparable, whereas AgNbO3 (HT) shows much higher activity than N–TiO2. To investigate the stability of AgNbO3 (HT), which is important for practical application, the photocatalytic reaction was performed five times under the same conditions; the corresponding results are shown in the inset in Fig. 5b. The photocatalytic efficiency shows a slight decrease after two cycles, while the photocatalytic efficiency in the fifth cycle is 87% of that in the first cycle. In addition, AgNbO3 (HT) exhibits a similar crystal phase and similar morphology before and after the photocatalytic reaction (as shown in Fig. S4). All the results imply high stability of the AgNbO3 (HT) catalyst. 3.6. Photocatalytic mechanism As shown above, AgNbO3 (HT) exhibits significantly higher efficiency for degradation of tetracycline than AgNbO3 (SSR). It has been reported that the surface area, adsorbed surface species, and surface microstructure of the catalyst play important roles in photocatalytic reactions. However, the impact of the surface area can be excluded in our system, since AgNbO3 (HT) (2.28 m2/g) and AgNbO3 (SSR) (2.10 m2/g) have comparable specific surface areas. If the surface species are taken into consideration, the surface F− ions may have an impact on photocatalytic reactions. Liu et al. [21] reported that the degradation of methylene blue and phenol over BiPO4 was greatly promoted by F doping. F-doped TiO2 also showed higher activity for degradation of acetone than did commercial P25 [22]. Since NH4HF2 was used as a raw material in the synthesis of AgNbO3 (HT), it is inevitable that F− ions adsorbed on the surface of AgNbO3 (HT). Therefore, the impact of F− ions on the degradation of tetracycline was examined by comparing the photocatalytic performances of the AgNbO3 (HT) cubes with and without F− ions. To remove the surface F− ions, the AgNbO3 (HT) cubes were washed with basic solution and acid solution in sequence as described in Ref. [23]. The washed sample is denoted as AgNbO3 (W). X-ray photoelectron spectroscopy measurements show that the surface F− ions of AgNbO3 (HT) were almost removed by the washing process. As presented in Fig. 6a, the F 1s region of AgNbO3 (HT) is composed of two contributions. The main peak, at 685.2 eV, is attributed to the F atom physically adsorbed on the surface of AgNbO3, while the signal at 690.1 eV may originate from a F atom in a Nb–F bond formed during the hydrothermal reaction [22,24,25]. After washing with basic solution and acid solution, the F 1s signal in both states disappeared, confirming the depletion of F− ions in AgNbO3 (W). At the same time, the crystal phase and the well-defined surface structure of the AgNbO3 cube were not destroyed by washing according to the SEM image and XRD patterns (Fig. 6b and c). The photocatalytic performance of AgNbO3 (HT) and AgNbO3 (W) for degradation of tetracycline is presented in Fig. 6d. Obviously, the two samples have comparable photocatalytic efficiency, which implies that the presence of F− ions does not have an impact on the photocatalytic reaction. Our result disagrees with other reported work on F-doped TiO2, which may be due to the different location of F− ions in the catalyst [22–25]. In Fdoped TiO2, the F− ion dopants in the TiO2 lattice will trap photogenerated electrons because of their strong electronegativity, reducing the recombination of charge carriers and promoting the enhancement of photocatalytic efficiency. However, in our work, most of the F− ions
Fig. 6. (a) F 1s spectra of AgNbO3 (HT) and AgNbO3 (W). (b) Scanning electron microscopy image of AgNbO3 (W). (c) X-ray diffraction patterns of AgNbO3 (W) and AgNbO3 (HT). (d) Photocatalytic degradation of tetracycline by AgNbO3 (W) and AgNbO3 (HT) under visible light irradiation. 5
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Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgements This work was supported by the Natural Science Foundation of Fujian Province of China (no. 2017J01014), the Scientific Research Funds of Huaqiao University (no. 600005-Z17Y0060), and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment of Fuzhou University (no. SKLPEE-201803). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2019.109083.
Fig. 7. Effects of different scavengers on the photocatalytic degradation of tetracycline by AgNbO3 (HT). AO, ammonium oxalate; BQ, benzoquinone; TBA, tert-butyl alcohol.
References
are at the surface of AgNbO3 instead of being in the lattice, so the F− ions could not capture electrons efficiently and had no impact on the photocatalytic activity. According to the above analyses, it is deduced that the surface microstructure might play a decisive role in the photocatalytic degradation of tetracycline by AgNbO3 (HT). As shown by photoelectrochemical measurements, the AgNbO3 (HT) cubes with an exposed (001) facet exhibit higher charge carrier separation efficiency and greater redox ability of photogenerated electrons and holes, leading to superior photocatalytic activity. To further explore the contributions of the main oxidation species in the photocatalytic degradation of tetracycline, active species trapping experiments were performed. Three scavengers (benzoquinone for O2•radicals, tert-butyl alcohol for •OH radicals, and ammonium oxalate for holes) were used in the photocatalytic process [26,27]. The results are presented in Fig. 7. It can be clearly seen that the photodegradation ratio of tetracycline changes only slightly on addition of tert-butyl alcohol or ammonium oxalate. However, the photocatalytic efficiency greatly decreases in the presence of benzoquinone, which suggests that O2•- radicals, formed by adsorbed O2 trapping the photoinduced electrons, play a crucial role in photocatalytic degradation of tetracycline. According to the above results, it can be well understood why AgNbO3 (HT) has higher photocatalytic activity than AgNbO3 (SSR). The surface of AgNbO3 (SSR) is dominated by low-energy (114) facets, whereas the surface of AgNbO3 (HT) is partially covered by high-energy (001) facets. To decrease the surface energy, AgNbO3 (HT) might tend to adsorb more oxygen molecules and therefore be beneficial for the production of O2•- radicals. In addition, the conduction band edge potential of AgNbO3 (HT) is more negative than that of AgNbO3 (SSR), which also promotes the capture of photogenerated electrons by oxygen molecules.
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4. Conclusion AgNbO3 cubes with a high-energy (001) facet were synthesized by a facile hydrothermal method. In comparison with AgNbO3 powder synthesized by solid-state reaction, AgNbO3 cubes had a larger band gap, more positive valence band edge potential, and more negative conduction band edge potential. The exposed (001) facet was beneficial for the transfer and separation of charge carriers. The synergetic effects of greater redox ability and greater separation efficiency of charge carriers result in the superior photocatalytic performance of AgNbO3 cubes for the degradation of tetracycline. The surface F− ions had no influence on the photodegradation of tetracycline. The radical capture experiments showed that O2•- radicals were the dominant oxidation species in the photocatalytic reactions.
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Enhanced visible-light-driven photocatalytic performance of AgNbO3 cubes with a high-energy (001) facet
T
Bifen Gao∗, Desheng Hu, Caixia Xiao, Dongxu Li, Bizhou Lin, Yilin Chen, Yun Zheng Department of Applied Chemistry, College of Materials Science & Engineering, Huaqiao University, Xiamen, 361021, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Semiconductors Surface properties Crystal structure Microstructure
AgNbO3 cubes with an exposed high-energy (001) facet were prepared by hydrothermal treatment of Ag2O–Nb2O5–NH4HF2 aqueous suspension. The crystal phase, surface microstructure, and optical properties of the samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and UV–vis diffuse reflectance spectroscopy. The energy band structure and charge carrier separation efficiency were investigated by photoelectrochemical measurements. In comparison with AgNbO3 powder, whose surface was dominated by a low-energy (114) facet, the AgNbO3 cubes had superior visible-lightdriven photocatalytic activity for degradation of tetracycline because of the greater redox ability and more efficient separation of photogenerated electrons and holes. The surface F− ions had no influence on the photocatalytic reactions. Radical trapping experiments demonstrated that the superoxide radical played a decisive role in the photodegradation of tetracycline.
1. Introduction Semiconductor photocatalysts have attracted worldwide attention for their promising application in the fields of environmental remediation and energy generation. The catalytic activity of a semiconductor-based photocatalyst can be significantly influenced by its surface structure that is directly exposed to the reaction media. Recently, engineering the surface structure of a photocatalyst to a desirable crystal plane to optimize the photocatalytic reactivity has been actively pursued [1–8]. Yang et al. [1] first reported the synthesis of micron-sized anatase TiO2 crystals with 47% exposed (001) facets and demonstrated that the (001) facets were much more reactive than the thermodynamically stable (101) facets for the production of H2 from water [1]. Han et al. [2] observed that an anatase TiO2 sheet showed much higher activity for the degradation of methyl orange than commercial P25 because of the exposure of a high percentage of the (001) facets. BiVO4 sheets with a preferred (010) surface orientation exhibited superior photocatalytic activity for water oxidation and degradation of rhodamine B [3,4]. Unique ZnO nanocrystals with a high (0001) facet population showed better photocatalytic efficiency for decomposition of methylene blue and rhodamine B [5,6]. The photocatalytic activity of the (001) facets of BiOBr nanosheets for the degradation of salicylic acid and rhodamine B was found to be higher than that of the (010) facets [7]. All of these results demonstrate that the
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photocatalytic performance is highly correlated with the crystal facet of the photocatalyst. Acquisition of a high percentage of reactive facets by crystal facet engineering is highly desirable to increase the photocatalytic reactivity. AgNbO3, which has a band gap of about 2.8 eV [9], is an attractive candidate material for visible-light-driven photocatalysts for degradation of organic pollutants and hydrogen generation. Li et al. [9–11] reported that AgNbO3 can decompose 2-propanol and other organic compounds. Photocatalytic O2 evolution from an aqueous silver nitrate solution and H2 formation from a mixture of water and methanol vapor over AgNbO3 under visible-light irradiation were also observed [12]. However, the photocatalytic performance of AgNbO3 still needs to be improved. Furthermore, in reported studies, AgNbO3 was generally synthesized by solid-state reaction [9], the sol-gel method [13], and salt flux [14]. The as-prepared AgNbO3 samples possessed irregular and polyhedral structures, so that the shape and facet effects on their photocatalytic properties remained unclear. In 2012, Chang et al. [15] reported the hydrothermal synthesis and phase transition of AgNbO3 cubes; however, the surface microstructure and photocatalytic performance of the AgNbO3 cubes was not explored. The impact of surface microstructure on photocatalytic activity was not clarified. In this work, cubic AgNbO3 crystals with an exposed (001) facet were synthesized by facile hydrothermal reactions. The photocatalytic activity of the as-obtained AgNbO3 sample for degradation of the
Corresponding author. E-mail address: [email protected] (B. Gao).
https://doi.org/10.1016/j.jpcs.2019.109083 Received 15 January 2019; Received in revised form 28 June 2019; Accepted 29 June 2019 Available online 29 June 2019 0022-3697/ © 2019 Published by Elsevier Ltd.
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antibiotic tetracycline was investigated. The effect of an exposed highenergy (001) facet on the photocatalytic reaction was explored and the photocatalytic mechanism is proposed. 2. Experimental 2.1. Synthesis of AgNbO3 AgNbO3 cubes were synthesized by a facile hydrothermal process. In a typical synthesis, 0.9268 g Ag2O, 1.0632 g Nb2O5, and 0.6845 g NH4HF2 were dispersed in 70 mL H2O under vigorous stirring. The mixture was then transferred to a 100 mL Teflon-lined stainless steel autoclave and kept at 200 °C for 60 h. After reaction, the precipitates were collected, washed with distilled water, and dried at 60 °C overnight. The as-obtained AgNbO3 sample was denoted as AgNbO3 (HT). For comparison, AgNbO3 powders were prepared by the conventional solid-state reaction. For this, 1 mmol Nb2O5 and 1 mmol Ag2O were thoroughly mixed and ground with the addition of methanol. After being dried under infrared light, the mixture was calcined sequentially at 850 °C for 5 h and 1100 °C for 10 h. Finally, the powder was allowed to cool naturally to room temperature. The sample obtained by the solid-state reaction was denoted as AgNbO3 (SSR).
Fig. 1. X-ray diffraction patterns of AgNbO3 (HT) and AgNbO3 (SSR).
monitoring the absorbance at 358 nm with a UV–vis spectrophotometer (UNIC UV-2600).
2.2. Characterization
3. Results and discussion
The crystal phase of the catalyst was characterized by powder X-ray diffraction (XRD) with a Rigaku D/max-2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å). The morphology and surface microstructure of the samples were observed by field-emission scanning electron microscopy (SEM; Hitachi S-4800) and transmission electron microscopy (TEM; JEOL JEM-2010). Diffuse reflectance spectra were obtained with a Shimadzu 2550 spectrometer with BaSO4 as the reflectance standard. X-ray photoelectron spectroscopy measurements were performed with a Thermo Fisher Scientific ESCALAB25 spectrometer with an nonmonochromatic Al Kα (1486.6 eV) X-ray source. All spectra were calibrated to the binding energy of the adventitious C 1s peak at 284.6 eV. Nitrogen adsorption-desorption isotherms were acquired with a Quantochrome autosorb iQ analyzer. The samples were outgassed for 2 h at 200 °C before measurement.
3.1. Crystal phase Fig. 1 shows XRD patterns of the AgNbO3 samples obtained by the hydrothermal and solid-state reactions. For both samples, all the diffraction peaks can be indexed to the orthorhombic perovskite AgNbO3 phase (JCPDS card no. 52-0405). No peak of other impurities was detected. The diffraction peaks of AgNbO3 (SSR) are stronger and sharper than those of AgNbO3 (HT), implying higher crystallinity of the former. 3.2. Morphology characterization SEM and TEM images of AgNbO3 (HT) are shown in Fig. 2a and c, respectively. It can be clearly seen that the AgNbO3 (HT) sample is composed of cubes with a side length of about 600 nm. The high-resolution TEM (HRTEM) image in the inset in Fig. 2c shows an interplanar lattice spacing of 0.280 nm, corresponding to the (020) plane of the orthorhombic perovskite AgNbO3 phase. The selected-area electron diffraction pattern indicates the single-crystal nature of the AgNbO3 (HT) cubes. As shown in Fig. 2d, the diffraction spots of the (020) and (110) facets and the 45.2° angle between the two facets can be well ascribed to perovskite-structured AgNbO3. The TEM analyses imply that the zone axis perpendicular to the top surface of the cube is [001]. The preceding structural information suggests that the exposed upper and bottom surfaces of the cube are the (001) plane. Fig. 2b presents an SEM image of AgNbO3 (SSR). Obviously, this sample consists of particles with irregular shape and a wide size distribution. Furthermore, the particles aggregate and form large clumps. An HRTEM image of AgNbO3 (SSR) is shown in the inset in Fig. 2b. The lattice spacing of 0.277 nm is attributed to the (114) plane of the orthorhombic AgNbO3 phase. Generally, the thermodynamically stable plane would grow slowly during crystal growth and result in high exposure and high density in the final crystal. In the HRTEM characterization of AgNbO3 (SSR), the lattice spacing of the (114) plane can be well observed in most particles examined, indicating the high exposure ratio of the (114) facet. On the other hand, the (114) peak has the strongest intensity among the diffraction peaks, suggesting high density of the (114) plane in the synthetic crystal. Taking the high exposure of the (114) facet in the HRTEM characterization of AgNbO3 (SSR) and the strongest intensity of the (114) peak into account, we speculate that (114) might be the most thermodynamically stable crystal plane. To minimize the total
2.3. Photoelectrochemical measurements Mott-Schottky plots, photocurrent data, and electrochemical impedance spectra were collected from a conventional three-electrode cell with a CHI660E electrochemical workstation. The measurements were performed in 0.2 M Na2SO4 electrolyte under visible light irradiation (λ ≥ 400 nm). AgNbO3, Pt plate, and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. The working electrode was prepared as follows: 10 mg AgNbO3 was dispersed in 2 mL distilled water under sonication for 15 min to obtain a suspension. Subsequently, the suspension was distributed on the conductive surface of fluorine-doped tin oxide glass to form a photocatalyst film with an area of 1 cm2. The film was dried under infrared light before use. 2.4. Photocatalytic performance The photocatalytic activity was evaluated by the degradation of tetracycline with an initial concentration of 10 ppm. The suspension of photocatalyst (100 mg) in tetracycline aqueous solution (100 mL) was stirred in the dark for 40 min to achieve adsorption-desorption equilibrium. Subsequently, the photocatalytic degradation was performed by continuous stirring of the mixture under visible light irradiation (λ ≥ 400 nm). The light source was a xenon lamp (300 W) equipped with a filter to remove UV light. The suspension was withdrawn at a given time interval. After centrifugation to remove the catalyst particles, the concentration of the residual tetracycline was determined by 2
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Fig. 2. Field-emission scanning electron microscopy image of (a) AgNbO3 (HT) and (b) AgNbO3 (SSR). (c) Transmission electron microscopy image and (d) selected-area electron diffraction pattern of AgNbO3 (HT). The insets in (b) and (c) show high-resolution transmission electron microscopy images of AgNbO3 (SSR) and AgNbO3 (HT), respectively. The inset in (d) shows the angle between the (020) and (110) facets in the AgNbO3 (HT) cube.
blue-shifted to 425 nm. An absorption tail until 600 nm is also observed for AgNbO3 (HT). The optical band gap of the samples was deduced from the following equation: (Ahν)2 = hν – Eg, where h, ν, Eg, and A are Planck's constant, the incident photon frequency, the band gap, and the absorption coefficient, respectively [18]. As shown in the inset in Fig. 3, the band gaps of AgNbO3 (SSR) and AgNbO3 (HT) are 2.82 and 2.92 eV, respectively. The different optical properties of the two samples may be due to their different surface structures.
surface energy during synthesis, AgNbO3 (SSR) might adopt a (114) facet as the basal surface. However, it is also well known that the crystal growth habit can be controlled by the specific adsorption of ions to a particular crystal plane, thereby inhibiting the growth of these facets by lowering their surface energy [16,17]. In the case of AgNbO3 (HT), the F− ions from the raw material NH4HF2 might adsorb on the (001) facet and lower its surface energy, resulting in higher exposure of the (001) facet [2]. The exposure ratio of the (001) facet is approximately 33% in the AgNbO3 (HT) cubes.
3.4. Photoelectrochemical measurements
3.3. Optical properties
The electronic band structure of the AgNbO3 samples was determined by the flat-band potential and the optical band gap. The flatband potential of a semiconductor can be obtained from the intercept of the Mott-Schottky plot. As shown in Fig. 4, the positive slope of the Mott-Schottky plot indicates that AgNbO3 is an n-type semiconductor. The flat-band potentials of AgNbO3 (SSR) and AgNbO3 (HT) are −0.29 and −0.35 V (versus the reversible hydrogen electrode at pH 7), respectively. In general, the energy difference between the conduction band edge potential and the flat-band potential is assumed to be 0.1–0.3 V [19]. In this work, this energy difference is assumed to be 0.1 V to calculate the conduction band edge potential of AgNbO3, which is −0.39 V for AgNbO3 (SSR) and −0.45 V for AgNbO3 (HT). From these findings together with the optical absorption analysis, we can deduce that the valence band edge potentials of AgNbO3 (SSR) and AgNbO3 (HT) are 2.43 and 2.47 V, respectively. Apparently, the conduction band edge potential of AgNbO3 (HT) is more negative than that of AgNbO3 (SSR), while the valence band edge potential of AgNbO3 (HT) is more positive than that of AgNbO3 (SSR). It can be expected that the redox abilities of charge carriers in AgNbO3 (HT) are greater than those in AgNbO3 (SSR). The photocurrent was measured to evaluate the efficiency of separation of photogenerated charge carriers. Both AgNbO3 (HT) and AgNbO3 (SSR) show a transient photocurrent on visible light
UV–vis diffuse reflectance spectra of AgNbO3 (HT) and AgNbO3 (SSR) are presented in Fig. 3. Both samples exhibit strong absorption in the UV region. The absorption-edge onset of AgNbO3 (SSR) is at 440 nm, whereas the absorption-edge onset of AgNbO3 (HT) is slightly
Fig. 3. UV–vis diffuse reflectance spectra of AgNbO3 (HT) and AgNbO3 (SSR). The inset shows the band gap energy of the samples. 3
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Fig. 5. (a) Photocatalytic degradation of tetracycline by AgNbO3 (HT) and AgNbO3 (SSR). (b) Specific photocatalytic activity of the catalysts. The inset shows repeated photocatalytic degradation of tetracycline in the presence of AgNbO3 (HT). N–TiO2, nitrogen-doped TiO2.
irradiation. However, the gaps between the photocurrent and the dark current (ΔI) of the two samples are quite different. The average ΔI value of AgNbO3 (HT) is approximately 4.6 times higher than that of AgNbO3 (SSR). The higher photocurrent indicates more efficient separation of photogenerated electrons and holes in AgNbO3 (HT), which is beneficial for photocatalytic reactions. Electrochemical impedance spectra of AgNbO3 (SSR) and AgNbO3 (HT) film electrodes are shown in Fig. 4c. It is apparent that the AgNbO3 (HT) electrode has a lower impedance than the AgNbO3 (SSR) electrode, indicating more effective separation of photogenerated electron-hole pairs and rapid interfacial charge transfer. The electrochemical impedance spectroscopy results are consistent with the photocurrent analyses. 3.5. Photocatalytic performance
Fig. 4. (a) Mott-Schottky plots of the AgNbO3 film electrodes in 0.2 M Na2SO4 aqueous solution. (b) Transient photocurrent of AgNbO3 (HT) and AgNbO3 (SSR) irradiated with visible light (λ ≥ 400 nm). (c) Electrochemical impedance spectra of AgNbO3 (SSR) and AgNbO3 (HT) film electrodes. RHE, reversible hydrogen electrode.
The photocatalytic performance of AgNbO3 was evaluated by the degradation of tetracycline, which forms a colorless solution, under visible light irradiation (λ ≥ 400 nm). As can be seen from Fig. 5a, no photolysis of tetracycline was observed under the experimental conditions. AgNbO3 (SSR) exhibits some photocatalytic activity because of its 4
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visible light response. The photodegradation efficiency of AgNbO3 (SSR) is approximately 21% after irradiation for 2.5 h AgNbO3 (HT) shows better performance than AgNbO3 (SSR). In the same time of 2.5 h, AgNbO3 (HT) achieves a photocatalytic efficiency of 66%, which is three times that of AgNbO3 (SSR). The photocatalytic performance of AgNbO3 was also compared with that of nitrogen-doped TiO2 (N–TiO2), a well-known visible-light-driven photocatalyst. N–TiO2 was synthesized according to Ref. [20]. Since N–TiO2 has a much larger surface area than AgNbO3 (SSR) and AgNbO3 (HT), Fig. 5b presents the specific photocatalytic efficiency per unit surface area of the catalysts. It can be seen that the photocatalytic activities of AgNbO3 (SSR) and N–TiO2 are comparable, whereas AgNbO3 (HT) shows much higher activity than N–TiO2. To investigate the stability of AgNbO3 (HT), which is important for practical application, the photocatalytic reaction was performed five times under the same conditions; the corresponding results are shown in the inset in Fig. 5b. The photocatalytic efficiency shows a slight decrease after two cycles, while the photocatalytic efficiency in the fifth cycle is 87% of that in the first cycle. In addition, AgNbO3 (HT) exhibits a similar crystal phase and similar morphology before and after the photocatalytic reaction (as shown in Fig. S4). All the results imply high stability of the AgNbO3 (HT) catalyst. 3.6. Photocatalytic mechanism As shown above, AgNbO3 (HT) exhibits significantly higher efficiency for degradation of tetracycline than AgNbO3 (SSR). It has been reported that the surface area, adsorbed surface species, and surface microstructure of the catalyst play important roles in photocatalytic reactions. However, the impact of the surface area can be excluded in our system, since AgNbO3 (HT) (2.28 m2/g) and AgNbO3 (SSR) (2.10 m2/g) have comparable specific surface areas. If the surface species are taken into consideration, the surface F− ions may have an impact on photocatalytic reactions. Liu et al. [21] reported that the degradation of methylene blue and phenol over BiPO4 was greatly promoted by F doping. F-doped TiO2 also showed higher activity for degradation of acetone than did commercial P25 [22]. Since NH4HF2 was used as a raw material in the synthesis of AgNbO3 (HT), it is inevitable that F− ions adsorbed on the surface of AgNbO3 (HT). Therefore, the impact of F− ions on the degradation of tetracycline was examined by comparing the photocatalytic performances of the AgNbO3 (HT) cubes with and without F− ions. To remove the surface F− ions, the AgNbO3 (HT) cubes were washed with basic solution and acid solution in sequence as described in Ref. [23]. The washed sample is denoted as AgNbO3 (W). X-ray photoelectron spectroscopy measurements show that the surface F− ions of AgNbO3 (HT) were almost removed by the washing process. As presented in Fig. 6a, the F 1s region of AgNbO3 (HT) is composed of two contributions. The main peak, at 685.2 eV, is attributed to the F atom physically adsorbed on the surface of AgNbO3, while the signal at 690.1 eV may originate from a F atom in a Nb–F bond formed during the hydrothermal reaction [22,24,25]. After washing with basic solution and acid solution, the F 1s signal in both states disappeared, confirming the depletion of F− ions in AgNbO3 (W). At the same time, the crystal phase and the well-defined surface structure of the AgNbO3 cube were not destroyed by washing according to the SEM image and XRD patterns (Fig. 6b and c). The photocatalytic performance of AgNbO3 (HT) and AgNbO3 (W) for degradation of tetracycline is presented in Fig. 6d. Obviously, the two samples have comparable photocatalytic efficiency, which implies that the presence of F− ions does not have an impact on the photocatalytic reaction. Our result disagrees with other reported work on F-doped TiO2, which may be due to the different location of F− ions in the catalyst [22–25]. In Fdoped TiO2, the F− ion dopants in the TiO2 lattice will trap photogenerated electrons because of their strong electronegativity, reducing the recombination of charge carriers and promoting the enhancement of photocatalytic efficiency. However, in our work, most of the F− ions
Fig. 6. (a) F 1s spectra of AgNbO3 (HT) and AgNbO3 (W). (b) Scanning electron microscopy image of AgNbO3 (W). (c) X-ray diffraction patterns of AgNbO3 (W) and AgNbO3 (HT). (d) Photocatalytic degradation of tetracycline by AgNbO3 (W) and AgNbO3 (HT) under visible light irradiation. 5
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Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgements This work was supported by the Natural Science Foundation of Fujian Province of China (no. 2017J01014), the Scientific Research Funds of Huaqiao University (no. 600005-Z17Y0060), and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment of Fuzhou University (no. SKLPEE-201803). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2019.109083.
Fig. 7. Effects of different scavengers on the photocatalytic degradation of tetracycline by AgNbO3 (HT). AO, ammonium oxalate; BQ, benzoquinone; TBA, tert-butyl alcohol.
References
are at the surface of AgNbO3 instead of being in the lattice, so the F− ions could not capture electrons efficiently and had no impact on the photocatalytic activity. According to the above analyses, it is deduced that the surface microstructure might play a decisive role in the photocatalytic degradation of tetracycline by AgNbO3 (HT). As shown by photoelectrochemical measurements, the AgNbO3 (HT) cubes with an exposed (001) facet exhibit higher charge carrier separation efficiency and greater redox ability of photogenerated electrons and holes, leading to superior photocatalytic activity. To further explore the contributions of the main oxidation species in the photocatalytic degradation of tetracycline, active species trapping experiments were performed. Three scavengers (benzoquinone for O2•radicals, tert-butyl alcohol for •OH radicals, and ammonium oxalate for holes) were used in the photocatalytic process [26,27]. The results are presented in Fig. 7. It can be clearly seen that the photodegradation ratio of tetracycline changes only slightly on addition of tert-butyl alcohol or ammonium oxalate. However, the photocatalytic efficiency greatly decreases in the presence of benzoquinone, which suggests that O2•- radicals, formed by adsorbed O2 trapping the photoinduced electrons, play a crucial role in photocatalytic degradation of tetracycline. According to the above results, it can be well understood why AgNbO3 (HT) has higher photocatalytic activity than AgNbO3 (SSR). The surface of AgNbO3 (SSR) is dominated by low-energy (114) facets, whereas the surface of AgNbO3 (HT) is partially covered by high-energy (001) facets. To decrease the surface energy, AgNbO3 (HT) might tend to adsorb more oxygen molecules and therefore be beneficial for the production of O2•- radicals. In addition, the conduction band edge potential of AgNbO3 (HT) is more negative than that of AgNbO3 (SSR), which also promotes the capture of photogenerated electrons by oxygen molecules.
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4. Conclusion AgNbO3 cubes with a high-energy (001) facet were synthesized by a facile hydrothermal method. In comparison with AgNbO3 powder synthesized by solid-state reaction, AgNbO3 cubes had a larger band gap, more positive valence band edge potential, and more negative conduction band edge potential. The exposed (001) facet was beneficial for the transfer and separation of charge carriers. The synergetic effects of greater redox ability and greater separation efficiency of charge carriers result in the superior photocatalytic performance of AgNbO3 cubes for the degradation of tetracycline. The surface F− ions had no influence on the photodegradation of tetracycline. The radical capture experiments showed that O2•- radicals were the dominant oxidation species in the photocatalytic reactions.
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