Volume 33 Issue 4
Sep.  2020
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Li-rong Wang, Ting-ting Hou, Yue Xin, Wen-kun Zhu, Shu-yi Yu, Zi-cheng Xie, Shu-quan Liang, Liang-bing Wang. Large-Scale Synthesis of Porous Bi2O3 with Oxygen Vacancies for Efficient Photodegradation of Methylene Blue[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 500-506. doi: 10.1063/1674-0068/cjcp2001009
Citation: Li-rong Wang, Ting-ting Hou, Yue Xin, Wen-kun Zhu, Shu-yi Yu, Zi-cheng Xie, Shu-quan Liang, Liang-bing Wang. Large-Scale Synthesis of Porous Bi2O3 with Oxygen Vacancies for Efficient Photodegradation of Methylene Blue[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 500-506. doi: 10.1063/1674-0068/cjcp2001009

Large-Scale Synthesis of Porous Bi2O3 with Oxygen Vacancies for Efficient Photodegradation of Methylene Blue

doi: 10.1063/1674-0068/cjcp2001009
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  • Photocatalytic degradation of organic pollutants has become a hot research topic because of its low energy consumption and environmental-friendly characteristics. Bismuth oxide (Bi$ _2 $O$ _3 $) nanocrystals with a bandgap ranging from 2.0 eV to 2.8 eV have attracted increasing attention due to high activity of photodegradation of organic pollutants by utilizing visible light. Though several methods have been developed to prepare Bi$ _2 $O$ _3 $-based semiconductor materials over recent years, it is still difficult to prepare highly active Bi$ _2 $O$ _3 $ catalysts in large scale with a simple method. Therefore, developing simple and feasible methods for the preparation of Bi$ _2 $O$ _3 $ nanocrystals in large scale is important for the potential applications in industrial wastewater treatment. In this work, we successfully prepared porous Bi$ _2 $O$ _3 $ in large scale via etching commercial BiSn powders, followed by thermal treatment with air. The acquired porous Bi$ _2 $O$ _3 $ exhibited excellent activity and stability in photocatalytic degradation of methylene blue. Further investigation of the mechanism witnessed that the suitable band structure of porous Bi$ _2 $O$ _3 $ allowed the generation of reactive oxygen species, such as O$ _2 $$ ^{-\cdot} $ and $ \cdot $OH, which effectively degraded MB.
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Large-Scale Synthesis of Porous Bi2O3 with Oxygen Vacancies for Efficient Photodegradation of Methylene Blue

doi: 10.1063/1674-0068/cjcp2001009

Abstract: Photocatalytic degradation of organic pollutants has become a hot research topic because of its low energy consumption and environmental-friendly characteristics. Bismuth oxide (Bi$ _2 $O$ _3 $) nanocrystals with a bandgap ranging from 2.0 eV to 2.8 eV have attracted increasing attention due to high activity of photodegradation of organic pollutants by utilizing visible light. Though several methods have been developed to prepare Bi$ _2 $O$ _3 $-based semiconductor materials over recent years, it is still difficult to prepare highly active Bi$ _2 $O$ _3 $ catalysts in large scale with a simple method. Therefore, developing simple and feasible methods for the preparation of Bi$ _2 $O$ _3 $ nanocrystals in large scale is important for the potential applications in industrial wastewater treatment. In this work, we successfully prepared porous Bi$ _2 $O$ _3 $ in large scale via etching commercial BiSn powders, followed by thermal treatment with air. The acquired porous Bi$ _2 $O$ _3 $ exhibited excellent activity and stability in photocatalytic degradation of methylene blue. Further investigation of the mechanism witnessed that the suitable band structure of porous Bi$ _2 $O$ _3 $ allowed the generation of reactive oxygen species, such as O$ _2 $$ ^{-\cdot} $ and $ \cdot $OH, which effectively degraded MB.

Li-rong Wang, Ting-ting Hou, Yue Xin, Wen-kun Zhu, Shu-yi Yu, Zi-cheng Xie, Shu-quan Liang, Liang-bing Wang. Large-Scale Synthesis of Porous Bi2O3 with Oxygen Vacancies for Efficient Photodegradation of Methylene Blue[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 500-506. doi: 10.1063/1674-0068/cjcp2001009
Citation: Li-rong Wang, Ting-ting Hou, Yue Xin, Wen-kun Zhu, Shu-yi Yu, Zi-cheng Xie, Shu-quan Liang, Liang-bing Wang. Large-Scale Synthesis of Porous Bi2O3 with Oxygen Vacancies for Efficient Photodegradation of Methylene Blue[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 500-506. doi: 10.1063/1674-0068/cjcp2001009
  • In recent years, handling water pollution in a simple way has become a hot research topic of practical concerns [1, 2]. Photocatalytic degradation of organic pollutants has attracted a great attention because of its low energy consumption and environmental-friendly characteristics [1-6]. Titanium dioxide (TiO$_2$) is considered as one of the most widely used photocatalysts due to its non-toxicity, good stability, and excellent photocatalytic activity. Unfortunately, TiO$_2$ is only responsive to ultraviolet light due to its wide band gap, which occupied no more than 4% of the solar spectrum [7-17]. In view of the better utilization of solar light, it is appealing to develop visible light-sensitive photocatalysts that were active enough for practical applications. Bismuth oxide (Bi$_2$O$_3$) nanocrystals with a bandgap ranging from 2.0 eV to 2.8 eV has attracted increasing attention during photocatalytic degradation of organic pollutants [18-22]. Over recent years, several methods [23-31] have been developed to prepare Bi$_2$O$_3$-based semiconductor materials, including sol-gel, Pechini, sono-chemical, solvothermal, hydrothermal, and microwave assisted methods. Among these methods, solvothermal and hydrothermal methods were widely used because of the possibility to obtain highly-crystalline powders with high purity and controlled morphology [24, 30, 32]. However, it is difficult to prepare efficient Bi$_2$O$_3$ catalysts in large scale by employing these methods, which significantly limits the possibility for industrial application of Bi$_2$O$_3$ catalysts in photocatalytic degradation of organic pollutants. Thus, developing a simple and feasible method for the preparation of Bi$_2$O$_3$ catalysts in a large scale is urgently desired.

    Herein, we successfully synthesized porous Bi$_2$O$_3$ in large scale, which also exhibited remarkable catalytic activity and stability in photocatalytic degradation of methylene blue (MB). Porous Bi$_2$O$_3$ was prepared by etching commercial BiSn powders at room temperature, followed by thermal treatment in air. Up to 25 g of porous Bi$_2$O$_3$ was produced in one pot. In the photocatalytic degradation of MB, porous Bi$_2$O$_3$ exhibited excellent efficiencies of $>$94% within 1 h. Further mechanistic studies revealed that the presence of oxygen vacancies in porous Bi$_2$O$_3$ promoted the chemsorption of O$_2$. Moreover, the suitable band structure of porous Bi$_2$O$_3$ allowed the generation of reactive oxygen species, such as O$_2$$^{-\cdot}$ and $\cdot$OH, directly contributing to the enhanced catalytic activity.

  • BiSn powder ($n_{\rm{Bi}}$:$n_{\rm{Sn}}$=44:56, $\geq$99.9%) was purchased from Chinese Institute of Metal Metallurgy. Hydrochloric acid (HCl, 36%-38%), iron(Ⅲ) chloride hexahydrate (FeCl$_3\cdot$6H$_2$O), commercial Bi$_2$O$_3$, and MB were obtained from Sinopharm Chemical Reagent Co., Ltd. All solvents and chemicals were in analytical grade and used as received without further purification. All aqueous solutions were prepared using deionized water with a resistivity of 18.2 M$\Omega$/cm.

  • To selectively etch Sn element from BiSn powder, 5 g of BiSn powder was added into a 500-mL flask containing 275 mL of H$_2$O, 7.6 g of FeCl$_3\cdot$6H$_2$O and 25 mL of hydrochloric acid (36%-38%), followed by stirring at 25 ℃ for 4 h. After that, the obtained powder was washed with water for several times until the pH of the supernatant was elevated to 7.0. Porous Bi was collected by centrifugation at 3500 r/min for 10 min, and dried at 70 ℃ under vacuum for 2 h. Porous Bi$_2$O$_3$ was obtained by calcining porous Bi in a muffle furnace at 600 ℃ for 0.5 h.

  • 50 g of BiSn powder was immersed in 2000 mL of solution containing 76 g of FeCl$_3\cdot$6H$_2$O and 250 mL of hydrochloric acid (36%-38%), followed by stirring at 25 ℃ for 6 h. Sn element in BiSn powder was selectively etched in this process. After that, the obtained powder was washed with water for several times until the pH of the supernatant was elevated to 7.0. 24.35 g of porous Bi was collected by centrifugation at 3500 r/min for 10 min, and dried at 70 ℃ under vacuum for 2 h. 25.25 g of porous Bi$_2$O$_3$ was obtained by calcining porous Bi in a muffle furnace at 600 ℃ for 1.0 h. This successful large-scale preparation of porous Bi$_2$O$_3$ provided the possibility for industrial application.

  • The morphology features were obtained by scanning electron microscopy (SEM, Quanta FEG 250). XRD patterns were recorded by using a Philips X'Pert Pro Super diffractometer with Cu-K$\alpha$ radiation ($\lambda$=1.54178 Å). Continuous scans were collected in the 2$\theta$ range of 10$^\circ$-80$^\circ$. X-ray photoemission spectroscopy was conducted at the Catalysis and Surface Science Endstation connected to the BL10B beamline in the National Synchrotron Radiation Laboratory in Hefei, China. UV-Vis tests were conducted on a TU-1901 at room temperature. ESR spectra were taken on JEOL JES-FA200 ESR spectrometer at 298 K. Inductively coupled plasma atomic emission spectrometry (ICP-AES, Atomscan Advantage, Thermo Jarrell Ash, USA) was used to determine the concentration of Sn. The Brunauer-Emmet-Teller (BET) surface area and pore sizes of sample were measured by N$_2$ adsorption and desorption on a Quadrasorb SI instrument (Quantachrome, USA) at liquid nitrogen temperature (77.3 K).

  • Photocatalytic degradation of MB was carried out in a 50-mL glass tube. In a typical reaction, 15 mg of photocatalyst and 30 mL of 10 ppm of MB solution were added into a 100-mL photoreactor. After ultrasonic treatment for 5 min, the obtained suspension was stirred under dark for 30 min to achieve adsorption equilibrium. Subsequently, the suspension was irradiated under vigorous stirring by a Xenon lamp (Perfectlight PLS-SXE300) under full-spectrum at the intensity of 250 mW/cm$^2$. The efficiency of the degradation of MB was monitored spectrophotometrically at intervals of 10 min by extracting 500 μL of the suspension from the reactor. The detailed process of the analysis was descripted as follows. 500 μL of the suspension was centrifuged at 8000 r/min for 10 min to remove the photocatalyst. The obtained supernatant fluid was diluted to 2 mL with deionized water for UV-Vis absorbance measurement at 663 nm. According to the light absorbance and standard curve, the degradation efficiency of MB was obtained.

  • To begin with, porous Bi was synthesized via etching commercial BiSn powders with the size of 5-30 μm using the mixed solution of hydrochloric acid and iron(Ⅲ) chloride at 25 ℃ for 4 h. The SEM images of BiSn and porous Bi are shown in FIG. 1 (a) and (b). After etching, the ratio of residual Sn element in porous Bi was determined to be $\sim$0.01wt% by ICP-AES, confirming the complete etching of Sn element. SEM images indicated the morphological change from pyknotic sphericities to porous structures during the process of etching (FIG. 1 (a) and (b)). After treating porous Bi with air in a muffle furnace at 600 ℃ for 0.5 h, porous Bi$_2$O$_3$ was obtained. X-ray diffraction (XRD) pattern of porous Bi was indexed to the metallic Bi (JCPDS No.85-1329) (FIG. 2).

    Figure 1.  (a) SEM image of BiSn powder, (b) SEM image of porous Bi, (c)-(d) SEM images of porous Bi$_2$O$_3$.

    Figure 2.  XRD patterns of the obtained samples.

    As for porous Bi$_2$O$_3$, the crystalline structures were attributed to $\alpha$-Bi$_2$O$_3$ (JCPDS No.71-2274) (FIG. 2). As shown in SEM images (FIG. 1(b)-(d)), both porous Bi and porous Bi$_2$O$_3$ exhibited a porous structure. In addition, the specific surface areas and pore structures of the obtained products were also characterized by N$_2$ adsorption-desorption isotherm. As shown in FIG. 3, the isotherms of porous Bi and porous Bi$_2$O$_3$ exhibited obvious hysteresis behavior, indicating that the products were mainly mesoporous [33]. Table I summarizes the surface areas and pore structures of porous Bi and porous Bi$_2$O$_3$. To investigate the electronic properties of porous Bi$_2$O$_3$, we conducted the X-ray photoelectron spectroscopy (XPS) measurements.

    Table Ⅰ.  The results of physical absorption.

    Figure 3.  (a) N$_2$ adsorption isotherm and pore size distribution of porous Bi at 77.3 K. (b) N$_2$ adsorption isotherm and pore size distribution of porous Bi$_2$O$_3$ at 77.3 K.

    The binding energies of Bi 4f$_{5/2}$ and Bi 4f$_{7/2}$ for porous Bi$_2$O$_3$ were 164.4 and 159.1 eV, respectively, which were similar to those of pure Bi$_2$O$_3$ (FIG. 4(a)) [34-36]. In the XPS spectra of O 1s, the peaks located at 529.6 and 531.0 eV were assigned to lattice oxygen and oxygen vacancies, respectively (FIG. 4(b)) [37, 38]. The observation of the peak located at 531.0 eV indicated the presence of the oxygen vacancies in porous Bi$_2$O$_3$. Moreover, electron spin resonance (ESR) spectroscopy was also performed to detect oxygen vacancies. As shown in FIG. 5, an obvious ESR signal with $g$=2.002 assigned to the trapped electrons by oxygen vacancy appeared for porous Bi$_2$O$_3$ [39]. As a result, plenty of oxygen vacancies were presented on the surface of porous Bi$_2$O$_3$. More importantly, porous Bi$_2$O$_3$ was able to be prepared in large scale, where $\sim$25 g of porous Bi$_2$O$_3$ was successfully acquired in one pot. Thus, porous Bi$_2$O$_3$ with oxygen vacancies was successfully acquired in large scale after thermal treatment of porous Bi at 600 ℃ with air.

    Figure 4.  (a) Bi 4f XPS spectra of porous Bi and porous Bi$_2$O$_3$. (b) O 1s XPS spectra of porous Bi$_2$O$_3$.

    Figure 5.  ESR signal of porous Bi$_2$O$_3$.

  • The photocatalytic performance of the as-obtained catalysts on photocatalytic degradation of MB was evaluated. The reaction was performed in 30 mL of dye solution with the concentration of 10 ppm under 1 atm of air at $\sim$25 ℃ with 15 mg of catalysts. FIG. 6(a) illustrates the activity of photocatalytic degradation of MB over different catalysts. In the absence of catalyst, 27% of MB was degraded after 1 h, which might be attributed to photodegradation. When using porous Bi$_2$O$_3$ as catalyst, MB was almost completely degraded under the same conditions (FIG. 6 (a) and (b)). As a comparison, 39% of MB was degraded over porous Bi. To further corroborate the superior activity of porous Bi$_2$O$_3$ in photocatalytic degradation of MB, commercial Bi$_2$O$_3$ was also employed to test the degradation efficiency. As shown in FIG. 6(a), only 40% MB was degraded over commercial Bi$_2$O$_3$ under the same reaction conditions. Besides, when increasing the concentration of MB solution to 20 ppm, 30 ppm, and 40 ppm from 10 ppm, porous Bi$_2$O$_3$ still exhibited excellent efficiencies of 94%, 92%, and 91% for the degradation of MB, respectively (FIG. 6(c)). Furthermore, the stability of porous Bi$_2$O$_3$ was studied by recycling the catalyst. As shown in FIG. 6(d), $\sim$96% of initial activity was retained in the degradation of MB after five rounds, indicating the remarkable catalytic stability of porous Bi$_2$O$_3$. As a result, porous Bi$_2$O$_3$ exhibited remarkable activity and endurance towards the photocatalytic degradation of MB.

    Figure 6.  (a) Photocatalytic activity of the obtained catalysts in the degradation of MB. (b) UV-Vis adsorption spectra of the MB solution with different degradation time. (c) Photocatalytic activity of porous Bi$_2$O$_3$ in degrading MB with different concentrations. (d) Photocatalytic activity of porous Bi$_2$O$_3$ over the course of five rounds of successive reaction.

  • In order to elucidate the role of oxygen vacancies, O$_2$ temperature programmed desorption (O$_2$-TPD) measurement of porous Bi$_2$O$_3$ was implemented. As shown in FIG. 7, a prominent peak of desorption appears at 328 ℃, which corresponds to the chemisorbed O$_2$. Thus, the presence of oxygen vacancies in porous Bi$_2$O$_3$ promoted the chemsorption of O$_2$. To further explore the origin of the distinction of porous Bi$_2$O$_3$ in the photocatalytic degradation of MB, we investigated its energy band structure. As shown in diffuse reflectance ultraviolet-visible (UV-Vis) spectra of these catalysts (FIG. 8(a)), porous Bi behaved a strong absorption across the full spectrum of 260-800 nm. As for porous Bi$_2$O$_3$, the adsorption edge was located at about 500 nm, in line with that of pure Bi$_2$O$_3$ nanocrystals. The calculated band gaps of porous Bi$_2$O$_3$ was 2.75 eV using a transformed Kubelka-Munk (K-M) plot (FIG. 8(b)). Synchrotron radiation photoemission spectroscopy (SRPES) was performed to further depict the energy band structures of porous Bi$_2$O$_3$ with a photon energy of 170.53 eV. As shown in FIG. 9(a), the value of valence band maximum for porous Bi$_2$O$_3$ was 6.65 eV below Fermi level ($E_{\rm{F}}$). Moreover, we also determined the work function ($\Phi$) of porous Bi$_2$O$_3$. The values of work function of porous Bi$_2$O$_3$ was determined to be 0.43 eV (FIG. 9(b)). Accordingly, the value of valence band maximum for porous Bi$_2$O$_3$ were 7.08 eV versus vacuum level. Considering the value of normal hydrogen electrode (NHE) was 4.5 eV vs. vacuum level, the value of valence band maximum for porous Bi$_2$O$_3$ was 2.58 eV vs. NHE.

    Figure 7.  O$_2$-TPD profile of porous Bi$_2$O$_3$.

    Figure 8.  (a) Diffuse reflectance UV-Vis spectra of the obtained samples of porous Bi and Bi$_2$O$_3$. (b) Transformed K-M function.

    Figure 9.  (a) Valence band spectrum, and (b) secondary electron cutoff of porous Bi$_2$O$_3$.

    In addition, combined with the determined band gap from transformed K-M function, the conduction bands (CB) of porous Bi$_2$O$_3$ was calculated to be -0.17 eV vs. NHE. Based on the above analysis, we illustrated the electronic band structure vs. NHE of porous Bi$_2$O$_3$ in FIG. 10. When the energy of photon was equal to or greater than the bandgap of porous Bi$_2$O$_3$, the electrons were excited from VB to CB, resulting in the formation of a hole (h$^+$) in the VB and an electron (e$^-$) in the CB. The electrons in CB were able to reduced O$_2$ adsorbed by oxygen vacancies to O$_2$$^{-\cdot}$. The highly oxidative species $\cdot$OH were produced as a consequence of the reduction of oxygen. The photo-generated h$^+$ in VB was also able to react with H$_2$O to form $\cdot$OH groups, leading to a constant stream of the surface $\cdot$OH groups [40-45]. Therefore, the efficient photocatalytic degradation of MB was smoothly proceeded (FIG. 10).

    Figure 10.  Schematic illustration of the photocatalytic degradation of MB.

  • In conclusion, we have successfully prepared porous Bi$_2$O$_3$ in a large scale via etching commercial BiSn powders, followed by thermal treatment with air. Porous Bi$_2$O$_3$ exhibited excellent activity and stability in photocatalytic degradation of MB. Further investigation of the mechanism witnessed that the suitable band structure of porous Bi$_2$O$_3$ allowed the generation of reactive oxygen species, such as O$_2$$^{-\cdot}$ and $\cdot$OH, which effectively degraded MB. This work not only provides a strategy for preparing Bi$_2$O$_3$-based photocatalysts in a large scale, but also paves a way to industrial applications for handling water pollution.

  • This work was supported by the National Natural Science Foundation of China (No.51801235, No.11875258, No.11505187, No.51374255, No.51802356, No.51572299, and No.41701359), the Innovation-Driven Project of Central South University (No.2018CX004), the Start-up Funding of Central South University (No.502045005), the Fundamental Research Funds for the Central Universities (No.WK2310000066, No.WK2060190081), Posdoctoral Science Foundation of China (No.2019M652797), Central South University Postdoctoral Research Opening Fund and the Fundamental Research Funds for the Central Universities of Central South University (No.2018zzts402).

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