Synthesis of a novel one-dimensional Bi2O2CO3–BiOCl heterostructure and its enhanced photocatalytic activity

Yin Peng *, Qian Zhang and Peng-Fei Kan
The Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China. E-mail: kimipeng@mail.ahnu.edu.cn

Received 12th August 2020 , Accepted 23rd September 2020

First published on 23rd September 2020


Herein, a novel one-dimensional Bi2O2CO3–BiOCl heterostructure was synthesized via a hydrothermal method. For the first time, organic solvent CH2Cl2 was used as the Cl source and reaction solvent, and a new one-dimensional Bi2O2CO3 porous rod was used as the Bi source. BiOCl nanosheets uniformly and vertically grew onto the Bi2O2CO3 porous rod by crystallographic oriented epitaxial nucleation and growth, which provides the smallest penetration barrier for photo-generated charge carrier transfer at junctions due to the formation of high-quality interfaces between Bi2O2CO3 and BiOCl. Bi2O2CO3–BiOCl displays higher photocatalytic activity than Bi2O2CO3 porous rods and BiOCl nanosheets to degrade Rhodamine B (RhB), salicylic acid (SA) and phenol. This enhanced photocatalytic activity is attributed to the synergistic effect of the following factors: the formation of a high-quality interface between Bi2O2CO3 and BiOCl, suitable band alignment of Bi2O2CO3 and BiOCl, and one-dimensional ordered nanostructure. A novel route is offered in this work to synthesize a one-dimensional Bi2O2CO3–BiOCl heterostructure with high-quality interface.


Introduction

Organic pollutants, which are not easily degraded, are becoming an overwhelming problem because of the fast-growing industry and rising global population. Therefore, to develop a type of environmentally friendly technology for decomposing organic pollutants is very urgent. Fortunately, semiconductor photocatalysis has been widely studied as an effective way to solve environmental pollution.1–5 However, a photocatalyst with a junction-structure easily deactivates in practical applications due to its low-quality interface. Therefore, the interfacial quality plays a key role in the stability, photocatalytic activity, and reusability. At junction interfaces, the charge transfer rate is very fast and can be finished within the picosecond range, which indicates that the smallest energy barrier must be provided so as to improve the photocatalytic activity.6,7 A difference in lattice spacing likely causes a lattice mismatch at the junction interface when the two semiconductors form a heterostructure, resulting in a high penetration barrier at the junction interface, which is not responsible for the photocatalytic activity and photostability of the heterojunction photocatalyst.8–14 Recently, the synthesis of heterostructured photocatalysts with a high-quality interface gave rise to research interest all over the world.8,10,15–21 According to the reports, we believe that on both sides of the junction, a similar structure/composition will cause the smallest lattice mismatch, which can effectively facilitate the charge transfer across the interface.8–11,16–18

Bismuth subcarbonate (Bi2O2CO3), a typical layered structure interleaved with [Bi2O2]2+ slabs and [CO3]2− slabs along the c axial orientation, has greatly received attention for photodegrading organic/inorganic pollutants because of its low cost, non-toxic and chemical stability.22–36 BiOCl also possesses a layered structure that is the same as that of Bi2O2CO3. In recent years, bismuth oxychloride (BiOCl) has attracted considerable attention due to its excellent catalytic performance in the removal of pollutants, hydrogen production, CO2 reduction, water oxidation, organic synthesis, and nitrogen fixation.37–44 Due to the same layered structure, the heterojunction with a high quality interface can be formed between Bi2O2CO3 and BiOCl, which can effectively improve the photocatalytic performance.

In recent years, Bi2O2CO3–BiOCl heterostructures have been reported.45–49 For example, Xie et al. synthesized Bi2O2CO3–BiOCl hierarchical nanostructures using potassium chloride (KCl), solid bismuth nitrate pentahydrate, PEG 400, and potassium bicarbonate (KHCO3) as sources.50 Yu et al. synthesized a highly efficient photocatalyst of Bi2O2CO3–BiOCl flowers using Bi2O2CO3 flowers as the template and HCl as the Cl source.51 Zhang et al. obtained Bi2O2CO3–BiOCl nanosheets using HCl, bismuth nitrate pentahydrate, and NaOH as raw materials.52 However, the morphologies of these reported Bi2O2CO3–BiOCl heterojunctions are all nanosheets or the layered structure of nanosheet stacking and assembly. Other morphology such as one-dimensional structure has few reports.

In this study, a novel one-dimensional Bi2O2CO3–BiOCl heterojunction with a high quality interface was synthesized for the first time. Organic solvent CH2Cl2 was used as the Cl source and reaction solvent, and new one-dimensional Bi2O2CO3 nanorods were used as the template. BiOCl nanosheets uniformly and vertically grew onto the Bi2O2CO3 porous rod via an oriented epitaxial growth, providing the smallest penetration barrier for photogenerated charge carrier transfer at junctions due to the formation of high-quality interfaces between Bi2O2CO3 and BiOCl. The photocatalytic activity of the Bi2O2CO3–BiOCl heterojunction was evaluated by degrading Rhodamine B (RhB), salicylic acid (SA) and phenol. The results show that the Bi2O2CO3–BiOCl heterojunction exhibits outstanding photocatalytic activity. The enhanced photocatalytic activity is attributed to the formation of type II-junctions between Bi2O2CO3 and BiOCl with a high-quality interface. Moreover, the one dimensional structure is also favorable to directional transport and highly efficient separation of photogenerated charge carriers.

Experimental

Photocatalyst preparation

Bi(OHC2O4)·2H2O nanorods were obtained according to our earlier report.53
Bi2O2CO3 porous nanorods. Bi2O2CO3 porous rod was synthesized by calcining Bi(OHC2O4)·2H2O at 300 °C for 1 h in CO2.
Bi2O2CO3–BiOCl heterostructure. Bi2O2CO3 porous nanorods (0.051 g) were added into dichloromethane (CH2Cl2) and stirred for a while, and then the mixture was transferred to a Teflon-lined stainless-steel autoclave and reacted at 160 °C for 12 h. The product was collected after natural cooling to room temperature, washed 2–3 times, and dried at 60 °C in the air. When the volume of CH2Cl2 was 5 and 10 mL, the obtained samples were labelled S1 and S2, respectively.
BiOCl nanosheets. BiOCl nanosheets were obtained when the volume of CH2Cl2 was 20 mL and labelled S3.

Photocatalytic activity measurements

Photocatalytic activity of Bi2O2CO3–BiOCl heterostructures was evaluated by degrading dyes and organics under solar light irradiation. The light source used was a 300 W Xe lamp (CEL-HXF300F, Beijing China Education Au-light Co., Ltd). First, the obtained Bi2O2CO3–BiOCl powder (40 mg) was dispersed into RhB, SA or phenol (40 ml) at room temperature, and then stirred for 30 min in dark so as to ensure an adsorption/desorption equilibrium on the surface of the photocatalyst. Second, when the light was turned on, the photocatalytic degradation started. At given time intervals, about 4 mL of the suspension was withdrawn and centrifuged in order to remove Bi2O2CO3–BiOCl powder. Third, the concentration of RhB, SA or phenol vs. irradiation time was measured via UV-vis absorption spectroscopy.

Following the above experimental steps, the experiments of trapping active species were carried out. P-Benzoquinone (BQ) (0.001 mol L−1) acted as the scavenger of the superoxide radicals (˙O2), ammonium oxalate (AO) (0.01 mol L−1) as the scavenger of hole (h+), and t-butanol (0.01 mol L−1) as the scavenger of hydroxyl radicals (˙OH).

Electrochemical impedance spectroscopy (EIS) measurements

Electrochemical impedance spectroscopy (EIS) was performed on a CHI660D instrument (frequency: 0.1 Hz–100 kHz). The reference electrode used was a saturated calomel electrode, the counter electrode used was a platinum wire, and the working electrode used was a Bi2O2CO3 (or Bi2O2CO3–BiOCl, or BiOCl) film electrode on ITO. The detection electrolyte used was a [Fe(CN)6]3−/4− solution, containing K3Fe(CN)6, K4Fe(CN)6, and KCl.

Characterization

Field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800 microscope), X-ray powder diffraction (XRD) (Rigaku, D/max-γA X-ray diffractometer with Cu-Kα radiation, λ = 0.154178 nm), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) (JEOL-2010 microscope) were used to characterize the structure and morphology of the obtained samples. A UV-vis diffuse-reflectance spectroscopy (UV-2450 spectrophotometer), X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer RBD upgraded PHI-5000C ESCA system), room temperature photoluminescence (PL) spectroscopy (F-280 fluorescence spectrophotometer) were employed to characterize the optical properties of the obtained samples. A total organic carbon (TOC) analyzer (Multi N/C 2100, Jena) was used to analyze the removal efficiency of the RhB.

Results and discussion

In this study, Bi2O2CO3 porous rods were first synthesized, and then BiOCl–Bi2O2CO3 heterostructures with BiOCl nanosheets standing on the Bi2O2CO3 porous rods were obtained using Bi2O2CO3 porous rods as the support and CH2Cl2 as the Cl source and solvent. The as-obtained samples with different CH2Cl2 volume of 5, 10 and 20 mL were labelled as S1, S2 and S3, respectively. It can be seen from the XRD patterns of the as-made products (Fig. 1) that each diffraction peak is in agreement with Bi2O2CO3 with the tetragonal crystal phase (JCPDS No. 25-1464) when the volume of CH2Cl2 is 5 mL. However, two sets of XRD peaks that are indexed to tetragonal Bi2O2CO3 (JCPDS No. 25-1464) and tetragonal BiOCl (JCPDS No. 06-0249) can be discovered when the volume of CH2Cl2 changes from 5 to 10 mL, indicating the formation of the Bi2O2CO3–BiOCl heterostructure. Moreover, the intensity of the diffraction peaks of Bi2O2CO3 in S2 is obviously weaker than that in S1, which implies that Bi2O2CO3 nanorods gradually change to BiOCl with the volume of CH2Cl2 increases. When the volume of CH2Cl2 is 20 mL (S3 sample), only the peaks of the BiOCl can be found, and those of Bi2O2CO3 disappear. Therefore, it is confirmed that the obtained sample is BiOCl.
image file: d0ce01181a-f1.tif
Fig. 1 The XRD patterns of obtained samples Bi2O2CO3 and with different volumes of CH2Cl2 (0, 5, 10 and 20 mL) and standard card of Bi2O2CO3 and BiOCl.

Fig. 2 displays the SEM images of Bi2O2CO3, Bi2O2CO3–BiOCl heterojunctions and BiOCl with different volumes of CH2Cl2. Bi2O2CO3 porous nanorods having rough surfaces and porosity (Fig. 2A). The obtained Bi2O2CO3–BiOCl heterojunctions also show 1D rod-like structures. When the volume of CH2Cl2 is 5 mL (Fig. 2B), the thin BiOCl nanosheets grow vertically on the surface of Bi2O2CO3 nanorods along the long axial direction. With the volume of CH2Cl2 increases, the BiOCl nanosheets increase gradually, the gaps between BiOCl nanosheets become narrower and narrower (Fig. 2C). In our reaction system, Bi2O2CO3 nanorods act as the reactant to supply Bi ions. Therefore, the Bi2O2CO3 nanorods are gradually consumed, and finally Bi2O2CO3 completely disappears (Fig. 2D). The disappearance of the Bi2O2CO3 nanorod skeleton results in the collapse of the one-dimensional structure, which can explain the formation of BiOCl nanoplates when the volume of CH2Cl2 is 20 mL. The experiment, such as the reaction time and temperature has been studied, as shown in Fig. S1–S4. It can be found that raising the reaction temperature or prolonging the reaction time are beneficial to the formation of BiOCl nanoplates, and only under suitable reactive conditions, the Bi2O2CO3–BiOCl heterojunction can be obtained.


image file: d0ce01181a-f2.tif
Fig. 2 FE-SEM images of Bi2O2CO3 nanorods (A) and the obtained samples S1 (B), S2 (C) and S3 (D).

In order to deeply obtain the structural information, transmission electron microscopy (TEM) was used to characterize the Bi2O2CO3–BiOCl heterojunction (S2 sample). It can be clearly found from the TEM image of S2 (Fig. 3A) that the BiOCl nanosheets stand on the surfaces of the Bi2O2CO3 nanorods, in line with the SEM result. Fig. 3B shows the HRTEM image of the obtained S2 sample. The interplanar lattice spacing of 0.684 and 0.738 nm index to the (002) lattice plane of Bi2O2CO3 and the (001) lattice plane of BiOCl, respectively. The interfacial crystal lattice matches well between Bi2O2CO3 and BiOCl due to crystallographic oriented epitaxial nucleation and growth of BiOCl on the Bi2O2CO3 surface, which is favorable for the formation of the high-quality interface.8,10 Moreover, no obvious defects can be found at the interface from the HRTEM image. The above results prove the formation of the high-quality junction interfaces between Bi2O2CO3 and BiOCl. The HRTEM image and the corresponding selected-area electron diffraction (SAED) pattern of the BiOCl nanosheet recorded from the side are shown in Fig. 3C and D. The interplanar lattice spacing of 0.275 nm corresponds to the (110) planes of tetragonal BiOCl. The angle between the (110) and (200) planes in the SAED image (Fig. 3D) is 45°, consistent with the theoretical value of tetragonal BiOCl. Therefore, the BiOCl nanosheet grows along the [001] orientation and its side is the (001) crystal facet.


image file: d0ce01181a-f3.tif
Fig. 3 TEM (A), HRTEM (B and C) and SAED (D) images of the S2 heterostructure.

To investigate the chemical composition of S2, Bi2O2CO3 and S3, the XPS analysis was performed. Fig. 4A shows the high-resolution XPS spectra for the Bi 4f region. The peaks centered at 159.70 and 165.00 eV in S2 are indexed to Bi 4f7/2 and Bi 4f5/2, respectively. Compared with pure Bi2O2CO3, the peaks of Bi 4f in S2 remarkably shift to a higher binding energy, which indicates the formation of strong chemical interactions between Bi2O2CO3 and BiOCl. We also find the similar phenomenon in O 1s spectra (Fig. 4C). Fig. 4B shows the high-resolution XPS spectra of the C 1s, the peak at 289.4 eV is indexed to the carbonate ion in Bi2O2CO3,51 and the peak at 284.9 eV is ascribed to the adventitious hydrocarbon. Moreover, it can be found that the intensity of peak at 289.4 eV in S2 is obviously weaker than that in Bi2O2CO3, indicating that some Bi2O2CO3 is consumed. The peaks of Cl 2p can also be found in the S2 sample, proving the formation of BiOCl. The above-mentioned result proves that the heterostructure between Bi2O2CO3 and BiOCl with strong chemical interactions at the interface is formed, and is consistent with the HRTEM result.


image file: d0ce01181a-f4.tif
Fig. 4 High-resolution XPS spectra of Bi 4f (A), C 1s (B), O 1s (C) and Cl 2p (D) for Bi2O2CO3, S2 and S3.

The photocatalytic activity of the Bi2O2CO3–BiOCl heterostructure was tested using the RhB dye as a model pollutant under solar light irradiation. Fig. 5A depicts the correlation curves between the concentration and the irradiation time. It can be found that the Bi2O2CO3–BiOCl heterostructure displays the better photocatalytic performance than pure Bi2O2CO3 and BiOCl, and S2 exhibits the highest photocatalytic activity, RhB with the concentration of 20 mg L−1 can be decomposed completely in 8 min. A TOC analyzer was used to analyze the mineralization degree of RhB (Fig. 5B). The removal efficiency of RhB is 100% and 82% in 10 min using S1 and S2 as the photocatalysts. However, for Bi2O2CO3 and BiOCl, the removal yield of RhB only reaches 9% and 50% in 10 min. Compared with the nanosheet-structured Bi2O2CO3–BiOCl heterojunctions,50,51,54 our as-made samples exhibit outstanding photocatalytic performance to degrade organic pollutants. The possible reason could be the special one-dimensional structure, which owns remarkable transport characteristics of electrons and holes. There is no doubt that our Bi2O2CO3–BiOCl heterojunctions display the excellent photocatalytic activity compared with similar Bi-based heterostructures.55–58 The S2 sample is used to degrade RhB several times. It can be seen that S2 still maintains high photocatalytic activity after five cycles (Fig. 5C) and high photo-stability (Fig. 5D and S5). The UV-vis absorption spectra of RhB using S2 as the photocatalyst is shown in Fig. S6.


image file: d0ce01181a-f5.tif
Fig. 5 The photocatalytic degradation curve of RhB (20 mg L−1) (A) obtained by different photocatalysts, the TOC images of CO2 contents (B), cycle times of the photo-degradation of RhB using S2 as the photocatalyst (C) and XRD patterns of the S2 sample before and after cycling use (D).

SA and toxic phenol were also used as models to further test the photocatalytic performance of S2. Fig. S6A and C depicts the degradation curves of SA and phenol. It can be discovered that S2 displays the most outstanding photocatalytic activity among all the samples, and 100% SA and phenol are decomposed in 15 and 28 min under solar light irradiation. The UV-vis absorption spectra of SA and phenol are shown in Fig. S6C and D.

The optical absorption properties of S2 and pure Bi2O2CO3 and BiOCl samples were tested via UV-vis diffuse reflectance spectroscopy (DRS). The absorption range of S2 gradually shifts to the short wavelength with the increase in the loaded –BiOCl content (Fig. 6A). According to the plots of (αhν)1/2versus hν of Bi2O2CO3 and BiOCl (inset in Fig. 6A), the band gap (Eg) of Bi2O2CO3 and BiOCl is estimated to 2.96 and 3.33 eV, respectively.50 The VB-XPS for Bi2O2CO3 and BiOCl was characterized in order to draw out the positions of the valence band (VB) top and conduction band (CB) bottom (Fig. 6B). We can see that the EVB tops of BiOCl and Bi2O2CO3 are located at 2.80 and 1.78 eV, respectively. When combined with their band gaps derived from DRS and the CB bottom positions of Bi2O2CO3 and BiOCl, which can be obtained from the equation: ECB = EVBEg, ECB is estimated to be at −1.18 eV and −0.53 eV (Fig. 6C), respectively. Bi2O2CO3 and BiOCl can form a type II heterostructure (Fig. 6D). Therefore, the photo-generated electron will move to the CB of BiOCl, and photo-generated holes will move to the VB of Bi2O2CO3. From the Fig. 6C and D, it can be found that the potential of photo-generated electrons is −0.53 eV, but E0 (O2/˙O2) is only −0.046 eV vs. NHE, indicating that photo-generated electrons can effectively reduce O2 adsorbed on the surface of BiOCl nanosheets to ˙O2 through a one-electron reduction reaction.59 The potential of the photogenerated holes is 1.78 eV, but E0 (˙OH/H2O) is 2.7 eV vs. NHE. Therefore, water molecules/hydroxyl groups would not be directly oxidized by photogenerated holes to generate ˙OH radicals on the surface of Bi2O2CO3. Hence, the ˙O2 radicals and h+ are the main active species for efficient degrading RhB.


image file: d0ce01181a-f6.tif
Fig. 6 (A) UV-vis diffuse reflectance spectra of Bi2O2CO3, BiOCl and S2 heterostructures and the plots of (αhν)1/2vs. hν, (B) the VB-XPS spectra of Bi2O2CO3 and BiOCl, (C) schematic of the energy band of Bi2O2CO3 and BiOCl and (D) the possible charge separation.

In order to prove our result, trapping experiments were performed. Scavengers, such as 1,4-benzoquinone (BQ), ammonium oxalate (AO) and tert-butyl alcohol (TBA) were used to trap superoxide radicals (˙O2), holes (h+) and hydroxyl radicals (˙OH), respectively. As shown in Fig. 7, the photo-degradation efficiency of RhB significantly decreases when AO and BQ are added into the photocatalytic system. However, no obvious deactivation of the S2 photocatalyst can be found when the TBA was added into the photocatalytic system. The above results illustrate that the main active species are ˙O2 and h+ in the photocatalysis.


image file: d0ce01181a-f7.tif
Fig. 7 The photocatalytic degradation curves of RhB using the S2 sample as the photocatalyst, different scavengers were added into the RhB solution, respectively.

In order to demonstrate the separation efficiency of the photo-generated e–h+ pairs, the electrochemical impedance and room temperature photoluminescence (PL) of Bi2O2CO3, BiOCl and S2 were carried out, as shown in Fig. 8. Compared to Bi2O2CO3 and BiOCl (Fig. 8A), the PL intensity of S2 obviously decreases, which means that the recombination of photogenerated e–h+ pairs is suppressed, and then the photocatalytic performance is enhanced.60 As shown in Fig. 8B, the arc radius of S2 in the EIS Nyquist plots is significantly smaller than that of pure Bi2O2CO3 or BiOCl, suggesting a higher efficiency of charge transfer and a more efficient separation of photogenerated e–h+ pairs. N2 Nitrogen adsorption–desorption isotherms of the Bi2O2CO3, S2 and BiOCl samples are shown in Fig. S7. The BET surface areas of the Bi2O2CO3, S2 and BiOCl are calculated to 5.8912, 10.4274 and 5.7686 m2 g−1, respectively. The largest BET surface area of S2 is also a factor for its enhanced photocatalytic activity.


image file: d0ce01181a-f8.tif
Fig. 8 (A) The room temperature PL spectra (λex = 320 nm) and (B) the electrochemical impedance spectra of Bi2O2CO3, BiOCl and S2.

Conclusions

In summary, a novel Bi2O2CO3–BiOCl nanorod heterostructure with high quality interface has been synthesized for the first time via a hydrothermal method using CH2Cl2 as the Cl source and reaction solvent. BiOCl nanosheets uniformly and vertically grow onto the Bi2O2CO3 porous rod via crystallographic oriented epitaxial nucleation and growth, which provides the smallest penetration barrier for photogenerated charge carriers transfer at junctions due to the formation of high-quality interfaces between Bi2O2CO3 and BiOCl. Bi2O2CO3–BiOCl displays the higher photocatalytic activity than Bi2O2CO3 porous rods and BiOCl nanosheets to degrade RhB, SA and phenol. Trapping experiments prove that the main active species are ˙O2 radicals and h+ for efficient degrading RhB. The formation of the junction interface with high quality, suitable band alignment and one-dimensional ordered nanostructure can explain why the Bi2O2CO3–BiOCl exhibits outstanding photocatalytic activity. A novel route is offered in this work to synthesize one-dimensional Bi2O2CO3–BiOCl heterostructures with high-quality interface.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work is supported by the Natural Science Foundation of the Colleges and Universities in Anhui Province (KJ2017A309) and the Natural Science Foundation of Anhui Province (2008085MB33).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ce01181a

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