Hangdao Qinab,
Yingchang Yang
*a,
Wei Shia and
Yuanbin She
b
aCollege of Material and Chemical Engineering, Tongren University, Tongren 554300, China. E-mail: yangyc612@gmail.com
bCollege of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
First published on 15th April 2021
Few-layer two-dimensional (2D) Bi2O2CO3 nanosheets with a thickness of 4–5 nm were successfully fabricated via electrochemical exfoliation, followed by an exposure to ambient conditions. The formation process for these nanosheets was explored through ex situ X-ray diffractometer. The photocatalytic capacity of 2D Bi2O2CO3 nanosheets was investigated towards the degradation of ciprofloxacin. It was shown that 2D Bi2O2CO3 nanosheets exhibited better catalytic performance than Bi2O2CO3 nanoparticles synthesized by hydrothermal method under UV-Vis light irradiation. The enhanced photocatalytic activity is due to the larger specific surface area, as well as the lower band gap. Additionally, the radical trap experiments demonstrate that holes and hydroxyl radicals are of great importance in the degradation of ciprofloxacin. Finally, the 2D Bi2O2CO3 nanosheets show high stability in the photocatalytic degradation of ciprofloxacin, and could have a prospective application in the treatment of antibiotic wastewater.
Photocatalysis technology has proven to be an effective technology for the degradation of refractory organic compounds due to the highly reactive species generated in the interaction of the photocatalyst and UV light.4,5 The most widely used photocatalysts are TiO2 nanoparticles.6,7 However, due to their lower activity under visible light, many attempts have been made to look for alternatives. Recently, Bi-containing complex oxides have been found to be very effective in the degradation of organic contaminants under visible light irradiation.8,9 Among these oxides, Bi2O2CO3 has been studied by many researchers due to its layered structure, the ease of synthetic methods, the availability of precursors and nontoxic nature.10,11 Doping and coupling with other oxides can enhance the photocatalysis properties and efficiency of Bi2O2CO3.12,13 However, they will complicate the synthetics severely and increase the cost. Moreover, controlling the morphology of nanostructured Bi2O2CO3 seems to be a feasible way to improve the photocatalysis efficiency, as the morphology of nanomaterials has an impact on their catalytic performance. Bi2O2CO3 with different morphologies can be synthesized traditionally via solvothermal method by controlling the precursors, reactant ratios, and reaction temperature.14 However, to obtain ultrathin two-dimensional (2D) Bi2O2CO3 nanosheets with excellent photocatalytic activity is still a challenge. In 2013, Ag-doped Bi2O2CO3 microspheres comprising nanoplates (12 nm in thickness) were successfully fabricated through a hydrothermal method, and it showed improved photocatalytic activity towards methyl orange (MO) dye.15 Nevertheless, the synthesis process requires the addition of a polyvinylpyrrolidone template and KCl salt, and the loading of Ag nanoparticles. More recently, few-layer Bi2O2CO3 was fabricated through the electrochemical exfoliation of bulk Bi in 0.5 M Na2CO3 solution.16 This few-layer Bi2O2CO3 with a thickness of six atomic layers exhibits good activity and selectivity towards CO2 electro-reduction. In their results, Bi2O2CO3 was constructed through the reaction of oxidised exfoliated bismuth nanosheets with CO32− in the electrolyte. This method is facile and requires no surfactant. However, the Na+ intercalating agent used here is too small to exfoliate bismuth effectively, which will greatly reduce the synthesis efficiency.
In the current work, two-dimensional (2D) bismuthene nanosheets were fabricated through the electrochemical cathodic exfoliation of bulk Bi in a N,N-dimethylformamide solution of quaternary ammonium salt. These nanosheets were very unstable, even in ambient condition, as their ultrathin structures. When exposed to room air for a few days, they can be gradually transformed into ultrathin 2D Bi2O2CO3 nanosheets (4–5 nm in thickness) spontaneously. As a contrast, Bi2O2CO3 nanoparticles were synthesized by a traditionally hydrothermal method. The photocatalytic performances of these nanomaterials were evaluated in the photocatalytic degradation of ciprofloxacin in aqueous solution. To study the effect of the main active species in the degradation of ciprofloxacin, different scavengers were used to investigate the contribution of several active species. Furthermore, the stability of the as-prepared photocatalysts was evaluated.
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Scheme 1 Scheme illustration for the synthesis of 2D Bi2O2CO3 nanosheets through electrochemical exfoliation, followed by exposure to ambient condition. |
The TEM images of the exfoliated Bi products in TBA+, THA+, and TEA+ solutions are shown in Fig. 1a–c, respectively. It is clear that both of the exfoliated Bi products in the electrolytes of TBA+ and THA+ display the features of Bi nanoparticles. Fig. 1c and d exhibit the typical TEM and HRTEM images of the exfoliated materials in the electrolyte of TEA+. The TEM image (Fig. 1a) shows that this exfoliated Bi product comprises ultrathin nanosheets. As can be seen in the HRTEM image (Fig. 1b), obvious lattice fringes with a space of 0.33 nm can be observed, which correspond to the (012) plane of the layered Bi crystal, confirming the formation of the 2D bismuthene nanosheets. These results clearly reveal that the TEA+ cation with a diameter of 6.7 Å is the best intercalating agent for the exfoliation of 2D bismuthene nanosheets. Interestingly, the as-exfoliated ultrathin 2D bismuthene nanosheets in the electrolyte of TEA+ are prone to oxidation even exposed to room air (Scheme 1), which provides a facile and feasible way for the synthesis of 2D Bi2O2CO3 at room temperature. It was found that the bismuthene nanosheets obtained in the electrolyte of TEA+ could be completely transformed into greyish white Bi2O2CO3 when exposed to room air for seven days, while it would take twenty days and a month for products to be exfoliated in the electrolyte of TBA+ and THA+, respectively.
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Fig. 1 TEM images of the Bi product exfoliated in the electrolyte of TBA+ (a), THA+ (b), and TEA+ (c), and HRTEM image (d) of the exfoliated Bi product in the electrolyte of TEA+. |
Fig. 2a displays the XRD patterns of the as-exfoliated 2D bismuthene nanosheets and the samples that were exposed to room air for one day to seven days. The diffraction peaks for the as-exfoliated 2D bismuthene nanosheets at 22.3°, 27.1°, 37.8°, 39.5°, 48.7°, 55.9°, 64.5°, and 70.7° can be indexed to the planes of β-Bi (003), (012), (104), (110), (202), (024), (122), and (214), respectively, according to the XRD standard card for β-Bi (JCPDS card no. 44-1246).22 No diffraction peaks for other phases could be observed, illustrating that the pure material was obtained. When the 2D bismuthene sample was exposed to room air for one day to seven days, the typical characteristic diffraction peaks for bismuthene gradually disappeared, while the characteristic diffraction peaks for Bi2O2CO3 (JCPDS card no. 41-1488) increased steadily.11 Lastly, only the characteristic diffraction peaks for Bi2O2CO3 can be observed, indicating that the as-exfoliated 2D bismuthene was completely converted to Bi2O2CO3 after exposed to room air for seven days. It should be noted that increasing the temperature and air humidity could greatly reduce the reaction time.
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Fig. 2 (a) XRD patterns of the as-exfoliated 2D bismuthene nanosheets, and the samples that were exposed to room air for one day to seven days; (b) XRD patterns of E-Bi2O2CO3 and H-Bi2O2CO3. |
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Fig. 3 N2 adsorption–desorption isotherms (a), and the corresponding pore size distribution curves (b) of E-Bi2O2CO3 and H-Bi2O2CO3. |
Table 1 exhibits the textural parameters of the BET surface area (SBET) and pore volume for E-Bi2O2CO3 and H-Bi2O2CO3. It is clear that the SBET and pore volume of E-Bi2O2CO3 fabricated through electrochemical exfoliation followed by spontaneous oxidation here are much larger than those of H-Bi2O2CO3 synthesized via hydrothermal method, suggesting the advantage of this electrochemical method, which will greatly improve the photocatalytic performance of the photocatalyst.11 In comparison with the various structured Bi2O2CO3 materials prepared through hydrothermal method, the specific surface area of the few-layer E-Bi2O2CO3 nanosheets (97 m2 g−1) here is much larger than that of the flower-like Bi2O2CO3 (20.43 m2 g−1), sponge-like Bi2O2CO3 (50.60 m2 g−1) and plate-like Bi2O2CO3 (4.30 m2 g−1).26
Samples | SBET (m2 g−1) | Pore volume (cm3 g−1) | Thickness (nm) | Band gap (eV) |
---|---|---|---|---|
E-Bi2O2CO3 | 97 | 0.49 | 4–5 | 2.95 |
H-Bi2O2CO3 | 21 | 0.12 | — | 3.22 |
The SEM, TEM and AFM images of E-Bi2O2CO3 and H-Bi2O2CO3 are shown in Fig. 4. The SEM images in Fig. 4a and b indicate that E-Bi2O2CO3 consisted of well-defined nanosheets, and particles with other morphologies cannot be observed. In contrast, H-Bi2O2CO3 comprises irregular particles with different sizes. Moreover, it is seen from Fig. 4a and b that the size of E-Bi2O2CO3 is smaller than that of H-Bi2O2CO3, which is consistent with the result obtained by XRD. As shown in Fig. 4c–f, the morphology and microstructure of E-Bi2O2CO3 were further elucidated by TEM and AFM. The TEM images in Fig. 4c and d confirm that E-Bi2O2CO3 comprises ultrathin nanosheets. The HRTEM image (Fig. 4e) reveals a lattice spacing of 0.272 nm, agreeing well with the spacing of the (110) plane of the tetragonal Bi2O2CO3 crystal.27 In addition, the SAED pattern in the inset of Fig. 4e demonstrates an array of orderly diffraction spots indexed to the (110), (10) and (020) reflections, suggesting that the E-Bi2O2CO3 nanosheet is a well-defined single-crystal.27 Fig. 4f shows the AFM image of the E-Bi2O2CO3 nanosheet. As can be seen, the topographical profiles in Fig. 4f demonstrate that the thickness of the E-Bi2O2CO3 nanosheet is ca. 4.1–4.8 nm. According to the layer spacing of Bi2O2CO3 (∼0.68 nm), the layer number of E-Bi2O2CO3 can be assigned to be 6–7 layers.
The surface composition and the chemical state of the as-fabricated few-layer E-Bi2O2CO3 nanosheets were further investigated with XPS. The survey XPS spectrum of E-Bi2O2CO3 shown in Fig. 5a suggests the presence of Bi, C and O elements. The semi-quantitative results indicate that the atomic percentage of C, O and Bi are 26.4%, 54.05% and 19.56%, respectively. The high-resolution spectra of Bi 4f, C 1s and O 1s are depicted in Fig. 5b–d, respectively. It can be clearly seen from Fig. 5b that there are two individual peaks at 164.7 eV and 159.4 eV, which can be ascribed to the binding energy of Bi 4f5/2 and Bi 4f7/2 states, respectively. The C 1s spectra display two clear peaks at 284.9 eV and 289.0 eV, which correspond to the adventitious hydrocarbon from the environment and the CO32− in the E-Bi2O2CO3 sample.28 The O 1s spectra are shown in Fig. 5d, the well-defined peak centred at 530.7 eV can be ascribed to the carbonate species and adsorbed water on the Bi2O2CO3 surface.28
As shown in Fig. 6a, the UV-Vis DRS of E-Bi2O2CO3 and H-Bi2O2CO3 nanomaterials were obtained through a UV-Vis spectrometer. It can be seen that the photoabsorption for both samples ranging from the UV light region to visible light are shorter than 450 nm. Additionally, steep absorption curves can be observed in both samples, exhibiting that the visible light absorption is ascribed to the band gap transition rather than the transition from the impurity level.26 Furthermore, E-Bi2O2CO3 shows an absorption edge at about 450 nm, while the absorption edge of H-Bi2O2CO3 blue-shifted to 390 nm, which is due to the E-Bi2O2CO3 fabricated through electrochemical exfoliation followed by spontaneous oxidation here is much thinner than that of H-Bi2O2CO3 synthesized via hydrothermal method.11 To measure the band gaps of the Bi2O2CO3, the equation αhν = A(hν − Eg)n/2 is applied,26 and the plot of (αhν)1/2 vs. energy (hν) is displayed in Fig. 6b. It is clear that the band gaps estimated from the onset of the absorption edge are about 2.95 eV and 3.22 eV for E-Bi2O2CO3 and H-Bi2O2CO3, respectively. Moreover, it was found that the band gap of the as-fabricated E-Bi2O2CO3 here is lower than that of the Bi2O2CO3 nanomaterials synthesized through hydrothermal method or solvothermal method.11,24,26 Furthermore, PL measurements can provide important information on the charge separation. As can be seen in Fig. 6c, the PL intensity of E-Bi2O2CO3 is lower than that of H-Bi2O2CO3, which implies that the ultrathin E-Bi2O2CO3 can significantly restrain the recombination of the electron–hole pairs.
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Fig. 6 UV-Vis DRS (a), plots of the energy of absorbed light (b) and PL spectra (c) of E-Bi2O2CO3 and H-Bi2O2CO3. |
Compared to H-Bi2O2CO3, E-Bi2O2CO3 showed better catalytic performance. The enhanced catalytic activity of E-Bi2O2CO3 may be ascribed to the following aspects. On the one hand, the specific surface area of E-Bi2O2CO3 was much larger than that of H-Bi2O2CO3 (Table 1). Ultrathin E-Bi2O2CO3 nanosheets could increase the contact area between the catalyst and antibiotic molecules. The good adsorption capacity of E-Bi2O2CO3 was also in favor of the reaction between the organics and active species on the surface of the catalyst.11,29 On the other hand, the band gap determined the spontaneity of a photocatalytic reaction.30,31 The band gap of E-Bi2O2CO3 was less than that of H-Bi2O2CO3 (Table 1), and therefore the photocatalysis over E-Bi2O2CO3 was more reactive under UV-Vis light irradiation.
Mineralization of ciprofloxacin by E-Bi2O2CO3 photocatalysis was estimated by measuring the TOC removal of the reaction solutions. As shown in Fig. 7b, although the removal rate of ciprofloxacin was nearly 76.8% (Fig. 7a), the TOC removal was only 20.5%. The results indicated that the aliphatic intermediates generated from the ring-opening reaction might be more resistant towards further mineralization. The toxicity at different reaction times was also presented in Fig. 7b, where the toxicity was almost unchanged in the first 30 min, and subsequently it was gradually decreased. At the end of 60 min treatment, the toxicity decreased to 35.5%, suggesting the effective elimination of the toxicity of ciprofloxacin and the intermediates by E-Bi2O2CO3 photocatalysis.
The influence of the initial pH on ciprofloxacin degradation was investigated. It can be observed from Fig. 7c that the ciprofloxacin decay was strongly pH-dependent. E-Bi2O2CO3 showed the highest photocatalytic activity at pH 4.0, while it presented the lowest photocatalytic performance at pH 10.0. These observations could be explained by the surface charge of ciprofloxacin, and E-Bi2O2CO3 would be greatly affected by the solution pH.17
To identify the active species during the photocatalytic degradation of ciprofloxacin over E-Bi2O2CO3, the trapping experiments were carried out by adding suitable radical scavengers. Undoubtedly, electrons (e−) and holes (h+) are the principal contributors to the generation of active species, and may therefore restrain the degradation of organic pollutants.32,33 In the trapping experiments, TEA and anhydrous ethanol were used as the scavengers for holes and electrons, respectively. It can be seen from Fig. 7d that the removal efficiency of ciprofloxacin decreased from 76.8% to 18.0% in the presence of TEA. The addition of ethanol also reduced the ciprofloxacin removal efficiency to 28.9%. Moreover, t-BuOH and BQ were used as the capture agents for hydroxyl radicals (˙OH) and superoxide radicals (˙O2−), respectively. As presented in Fig. 7d, only a loss of 5.4% was found for the ciprofloxacin degradation efficiency in the presence of the BQ scavenger, while the ciprofloxacin removal efficiency decreased from 76.8% to 23.8% with the t-BuOH scavenger. These results demonstrated that the existence of hydroxyl radicals, as well as holes in the system, were of great significance in the photocatalytic degradation of ciprofloxacin. However, superoxide radicals contributed the least for the ciprofloxacin degradation, which was in good agreement with the previous studies,34 such as the photodegradation of ciprofloxacin over the magnetic 3D γ-Fe2O3@ZnO core–shell catalyst and the heterogeneous Fenton degradation of three kinds of antibiotics using the magnetic core–shell MnFe2O4@C-NH2 catalyst.35
The reusability of the photocatalyst is significant to evaluate the potential of its practical application. Therefore, five consecutive cycles were performed, and the results are presented in Fig. 8. At the end of each experiment, the solid photocatalyst was filtered, washed with 50% ethanol water, and then dried in an oven at 60 °C overnight. The photocatalysis experiment was repeated with a ciprofloxacin concentration of 10 mg L−1 in the presence of the recycled catalyst at pH 7.12. It can be observed that the E-Bi2O2CO3 photocatalyst maintained a high photocatalytic activity even after the fifth cycle. No distinct deactivation was observed since there was only 7.3% loss of the ciprofloxacin degradation rate after the last cycling. A slight loss of the used catalyst was inevitable during the recovery process, which might be one of the reasons for the reduction of the photocatalysis activity. These results indicated that the E-Bi2O2CO3 photocatalyst possessed good stability and reusability.
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