Plasma treated Bi2WO6 ultrathin nanosheets with oxygen vacancies for improved photocatalytic CO2 reduction

Qidi Lia, Xingwang Zhua, Jinman Yanga, Qing Yua, Xianglin Zhua, Jinyu Chu*a, Yansheng Du*a, Chongtai Wangb, Yingjie Huab, Huaming Lia and Hui Xu*a
aSchool of the Environment and Safety Engineering, Institute for Energy Research, Jiangsu University, Zhenjiang, 212013, China. E-mail:;;
bSchool of Chemistry and Chemical Engineering, the Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province, Hainan Normal University, Haikou, 571158, China

Received 22nd October 2019 , Accepted 29th November 2019

First published on 2nd December 2019

Oxygen vacancies on photocatalyst surfaces have a significant effect on the improvement of photocatalytic CO2 reduction performance. Plasma treatment can quickly and efficiently introduce oxygen vacancies on catalyst surfaces. In this work, we used plasma to treat Bi2WO6 ultrathin nanosheets to create more surface oxygen vacancies. Increasing oxygen vacancies on the surface of Bi2WO6 not only increases the catalyst's ability to absorb light, but also greatly promotes the separation of photogenerated electrons and holes. In addition, increasing the catalyst specific surface area provides more reaction sites due to plasma etching. In photocatalytic CO2 reduction, Bi2WO6 ultrathin nanosheets with more oxygen vacancies showed excellent activity, and the CO production rate was 40.6 μmol g−1 h−1. Our research provides a simple and fast pathway to improve the CO2 reduction performance of photocatalysts.


Global warming caused by excessive carbon dioxide emissions from the unrestricted combustion of fossil fuels has a serious impact on the global environment.1 Therefore, the use of renewable energy and carbon dioxide conversion or fixation is an effective way to address the above problems.2 Photocatalytic carbon dioxide reduction is an environmentally friendly and sustainable method that uses sunlight as an energy source to convert carbon dioxide into usable fuel or chemical feedstock with broad application prospects.3 Nowadays, a large number of semiconductor catalysts have been used in photocatalytic carbon dioxide reduction research. However, the problems of low visible light utilization efficiency and poor stability of the catalyst limit the development of photocatalytic CO2 reduction.4

Bismuth tungstate (Bi2WO6) is a semiconductor photocatalyst with visible light absorption and has a suitable band gap for photocatalytic CO2 reduction.5 In addition, Bi2WO6 has a special layered structure composed of perovskite-like (Bi2O2)2+ layers and fluorite-like (WO4)2− layers, which is beneficial to construct an ultrathin structure.6 And the ultrathin structure allows photogenerated carriers to be easily transferred from the inside of the material to the surface to facilitate the photocatalytic reaction.7 However, the problem with the ultrathin structure of Bi2WO6 is that photogenerated electrons and holes easily recombine, which results in a decrease in light utilization efficiency and limits the improvement of photocatalytic CO2 reduction activity.8 Therefore, many researchers use elemental impurities, structural heterojunctions and defect engineering to promote the separation of photogenerated hole charges in order to improve catalyst performance.9,10 Among them, it has been confirmed that the introduction of oxygen vacancies on the surface of the catalyst by defect engineering effectively prevents the recombination of photogenerated electrons and holes, which is attributed to the oxygen vacancies as traps for trapping photogenerated electrons and holes, thereby suppressing electron–hole recombination.11,12

Plasma treatment as a material surface modification method can produce defects on the surface of the catalyst by the etching of high-energy electrons contained in the plasma without affecting the original properties of the catalyst.13 Compared to chemical reagent treatment and high temperature heat treatment, the plasma treatment process is more environmentally friendly and the processing speed is faster.14,15 Non-thermal plasma can be produced at room temperature and atmospheric pressure by dielectric barrier discharge (DBD), which more suitable for plasma treatment under laboratory conditions.16

In this work, Bi2WO6 ultrathin nanosheets with a thickness of 0.8 nm were synthesized via a simple hydrothermal method and then treated with plasma to produce oxygen vacancies on the surface (Fig. 1). We tested the photocatalytic reduction of CO2 to CO before and after plasma treatment. It was found that the photocatalytic CO2 reduction performance was greatly improved. At the same time, the physical and chemical properties of Bi2WO6 before and after treatment were characterized. With the help of DFT calculations, we proposed that the mechanism of oxygen vacancies generated by plasma treatment affects the photocatalytic activity of Bi2WO6 ultrathin nanosheets.

image file: c9qi01370a-f1.tif
Fig. 1 Schematic illustration of the steps for the preparation of oxygen-vacancy Bi2WO6 ultrathin nanosheets.

Results and discussion

The crystal structure of the pristine Bi2WO6 and plasma treated Bi2WO6 was characterized by X-ray powder diffraction. As shown in Fig. S1, all of the diffraction peaks of the pristine Bi2WO6 can be well indexed to the standard Bi2WO6 (JCPDS 73-2020), showing the synthesis of Bi2WO6 by the hydrothermal method.17 After plasma treatment, all diffraction peaks of VO-Bi2WO6 were the same as those of the pristine Bi2WO6 without additional peaks, indicating that the crystal structure of Bi2WO6 was not affected by plasma treatment. In order to determine the morphology of the prepared catalyst, the pristine Bi2WO6 was characterized by FE-SEM and TEM. The SEM image (Fig. S2) clearly showed that the prepared samples had a sheet-like structure. At the same time, the TEM image (Fig. 2a) also displays a more transparent image, indicating that the material is thin. Then, the thickness of the prepared Bi2WO6 nanosheets was measured by AFM. As shown in Fig. 2b, the thickness of Bi2WO6 nanosheets is around 0.8 nm, which is close to the thickness of the monolayer Bi2WO6.17 To further observe the surface microstructure of the prepared catalyst, HRTEM was performed. The HRTEM image of Bi2WO6 is displayed in Fig. 2c, it confirmed that the lattice spacing of the pristine Bi2WO6 is 0.374 nm, corresponding to the (111) plane of Bi2WO6. Thus, all the above results prove that the prepared Bi2WO6 is an ultrathin nanosheet. In addition, changes in the microstructure were observed in the HRTEM images of the plasma treated Bi2WO6 ultrathin nanosheets. As shown in Fig. 2d, the lattice distance of 0.374 nm can be allotted to the (111) plane of Bi2WO6, and the crystal lattice of the VO-Bi2WO6 shows a certain lattice disorder compared with the untreated samples, which may be caused by the unsaturated coordination of metal atoms due to oxygen vacancies in the Bi2WO6.18 In the meantime, the N2 adsorption–desorption isotherm test of the catalyst revealed the effect of plasma treatment on the specific surface area of the catalyst. For the VO-Bi2WO6 (Fig. S3b), the specific surface area is 23.57 m2 g−1, which is better than that of the pristine Bi2WO6 (Fig. S3a) 18.97 m2 g−1. The increased specific surface area may be due to plasma etching of the surface of the Bi2WO6.19 Moreover, the larger specific surface area can also provide more reactive sites for photocatalytic CO2 reduction.
image file: c9qi01370a-f2.tif
Fig. 2 (a) TEM, (b) AFM and (c) HRTEM images of Bi2WO6; (d) HRTEM image and the corresponding structure model of VO-Bi2WO6.

To demonstrate that the catalyst has more oxygen vacancies due to plasma modification treatment, electron paramagnetic resonance (EPR) spectra were characterized. As shown in Fig. 3a, both VO-Bi2WO6 and Bi2WO6 have an EPR signal at g = 1.997, which is recognized as a single electron captured in the oxygen vacancy.20 Compared to the original Bi2WO6, the signal intensity of VO-Bi2WO6 is significantly enhanced, which proves that the plasma treatment can effectively increase the oxygen vacancy concentration of the catalyst. Furthermore, the surface chemical composition and chemical state of the catalyst were investigated by X-ray photoelectron spectroscopy (XPS). As shown in Fig. S4, the XPS survey spectra of both the pristine Bi2WO6 and VO-Bi2WO6 contain the peaks of Bi, O, W, and C and there were no other elemental peaks indicating that plasma treatment does not bring other impurities under an Ar atmosphere. In addition, the high-resolution XPS spectra of O 1s can be deconvolved into three peaks at 530.02, 531.82 and 532.78 eV (Fig. 3b): the peaks at 530.02 eV and 532.78 eV represent the lattice oxygen and the oxygen of surface adsorbing water molecules, respectively, and the peak at 531.82 eV is attributed to the oxygen atom signal near the oxygen vacancy.21 Obviously, the relative intensity of the peak at 531.82 eV in VO-Bi2WO6 was significantly higher than that for the original Bi2WO6, indicating that the oxygen vacancy concentration was enhanced after plasma modification treatment, which is also consistent with the ESR analysis. From Fig. 3c, compared with the pristine Bi2WO6, the binding energy of Bi 4f of VO-Bi2WO6 was shifted to a lower energy, which shows the decrease in the coordination number of Bi3+; however, the binding energy of W 4f did not show a significant shift (Fig. 3d). This indicates that the oxygen vacancies are located in the (Bi2O2)2+ layer and thus affect the binding energy of the Bi atoms.22 All results reveal that plasma modification can increase the oxygen vacancy concentration on the surface of Bi2WO6 ultrathin nanosheets.

image file: c9qi01370a-f3.tif
Fig. 3 (a) EPR spectra of Bi2WO6 and VO-Bi2WO6. XPS spectra of Bi2WO6 and VO-Bi2WO6 for (b) O 1s, (c) Bi 4f, and (d) W 4f.

To investigate the effect of increased oxygen vacancies on the separation of photogenerated electrons and holes, photoluminescence (PL) was first performed. The PL spectra of Bi2WO6 and VO-Bi2WO6 excited at a wavelength of 300 nm in the range of 500–600 nm are shown in Fig. 4a. The PL intensity of VO-Bi2WO6 is observed to be weaker than that of the pristine Bi2WO6, indicating that VO-Bi2WO6 has lower electron and hole recombination rates.23 Moreover, the fluorescence decay of VO-Bi2WO6 also represents that the carrier lifetime is prolonged during the photocatalysis process; as shown in Fig. 4b, the average fluorescence lifetime of VO-Bi2WO6 and Bi2WO6 is 1.06 and 0.89 ns, respectively.24 In addition, the photoelectric properties of the Bi2WO6 and VO-Bi2WO6 are further measured. The photocurrent curves of the pristine Bi2WO6 and VO-Bi2WO6 are shown in Fig. 4c. By simple calculations we can see that the photocurrent density of VO-Bi2WO6 was about 5 times that of VO-Bi2WO6 under photoexcitation, indicating that the separation efficiency of photogenerated electron–hole pairs of VO-Bi2WO6 was remarkably increased.25 It is further confirmed that the photogenerated electron and hole separation efficiency of the photocatalyst is improved. In the meantime, charge transfer in the photocatalyst was investigated by using the EIS Nyquist plot. As shown in Fig. 4d, the minimum arc radius of the Nyquist diagram of VO-Bi2WO6 is smaller than that for the original Bi2WO6, indicating that VO-Bi2WO6 has a lower charge transport resistance, which will make it easier for photogenerated electrons and holes to transfer from the bulk to the sample surface.26 All of the above characterization results demonstrate that the separation efficiency of photogenerated electron–hole pairs of samples with plasma treatment was significantly improved.

image file: c9qi01370a-f4.tif
Fig. 4 (a) Photoluminescence spectra and (b) time-resolved photoluminescence spectra of Bi2WO6 and VO-Bi2WO6; (c) transient photocurrent responses and (d) electrochemical impedance spectroscopy of Bi2WO6 and VO-Bi2WO6 under visible light irradiation.

To study the effect of increased oxygen vacancies on photocatalytic CO2 reduction, the comparative activities of CO2 photoreduction activities of Bi2WO6 and VO-Bi2WO6 catalysts were respectively measured. In the photocatalytic CO2 reduction system, acetonitrile was used to promote CO2 dissolution and TEOA played the role of a hole sacrificial agent.27 As shown in Fig. 5a, the untreated Bi2WO6 has a CO production rate of 16.8 μmol g−1 h−1. After increasing the surface oxygen vacancy concentration by Ar plasma treatment, the CO production rate of VO-Bi2WO6 was greatly increased to 40.6 μmol g−1 h−1, and compared with other photocatalysts (Table S1), VO-Bi2WO6 exhibits a good photocatalytic activity. At the same time, neither hydrogen (H2) nor methane (CH4) was detected in the reaction products of Bi2WO6 and VO-Bi2WO6 during the reaction, which indicates that the obtained catalyst has good selectivity for the photocatalytic reduction of CO2 to CO. In order to rule out the possible influence of some test factors, control experiments were further performed, and the results are shown in Fig. 5b. A very small amount of CO was detected (∼3 μmol g−1 h−1) without a catalyst or in the dark (Fig. 5b, columns 2 and 3), demonstrating that VO-Bi2WO6 has the characteristics of photocatalytic CO2 reduction. In addition, comparative experiments were carried out using carrier gas Ar instead of CO2. No CO generation was detected (Fig. 5b, column 4), further demonstrating that the CO produced by VO-Bi2WO6 photocatalysts is derived from the conversion of CO2. Meanwhile, the VO-Bi2WO6 has a peak external quantum efficiency of 0.125% at 405 nm (Fig. S5). Considering the important role of photocatalyst stability in photocatalytic reactions, VO-Bi2WO6 was subjected to photocatalytic experiments for 15 hours. As shown in Fig. 5c, the CO production increased linearly with time. The CO generation rate remained stable during the continuous reaction process for 15 hours, indicating the high stability of the VO-Bi2WO6 produced by the plasma treatment. At the same time, the XRD patterns of VO-Bi2WO6 before and after the 15 hours reaction also showed that the catalyst structure did not change (Fig. S6). To further clarify the conversion path of photocatalytic CO2 reduction to CO of VO-Bi2WO6, in situ Fourier-transform infrared spectroscopy (FT-IR) measurements were carried out. As shown in Fig. 5d, the peak at 1657 cm−1 corresponds to the adsorbed H2O, while those at 1619 and 1458 cm−1 correspond to the carboxylate (CO2).28,29 The other peaks belong to carbonate and bicarbonate (bidentate (b-CO32−) at 1355 cm−1, monodentate (m-CO32−) at 1298 cm−1, and bicarbonate (HCO3) at 1385, 1216 cm−1), respectively.30,31 Moreover, the concentrations of CO2 and HCO3 gradually increased with the reaction, indicating that CO2 and HCO3 are the main intermediates for the photocatalytic reduction of CO2 to CO by VO-Bi2WO6.

image file: c9qi01370a-f5.tif
Fig. 5 (a) CO generation of Bi2WO6 and VO-Bi2WO6; (b) comparison of the photocatalytic CO2 reduction performance under different conditions of VO-Bi2WO6; (c) time dependent CO generation process of VO-Bi2WO6 and (d) In situ FT-IR spectra of VO-Bi2WO6.

The band structure of the photocatalyst has an important influence on photocatalytic CO2 reduction, and the optical absorption of Bi2WO6 and VO-Bi2WO6 was firstly tested by UV-vis diffuse reflectance spectroscopy (DRS). As shown in Fig. 6a, the optical absorption of VO-Bi2WO6 in the 550–800 nm region is significantly enhanced compared with that for the pristine Bi2WO6, which can be attributed to oxygen vacancies enhancing metal oxide near-infrared optical absorption.32 At the same time, the band gap of the photocatalyst was estimated according to the curve of (ahv)2 and absorbed light energy. From Fig. 6a, the band gap of VO-Bi2WO6 is 3.00 eV which is smaller than 3.07 eV of the pristine Bi2WO6. Furthermore, based on the analysis of the XPS valence band (VB) spectrum, the VB maxima of Bi2WO6 and VO-Bi2WO6 were determined to be +1.74 eV and +1.62 eV, respectively (Fig. 6b). Therefore, it is easy to calculate the minimum conduction band (CB) of Bi2WO6 and VO-Bi2WO6, which is located at −1.33 eV and −1.38 eV, respectively (Fig. S7). Usually, electrons generated at a more negative CB position will have greater reducing power. VO-Bi2WO6 shows a more negative CB position than Bi2WO6, so better reduction ability should be obtained. In addition, density functional theory (DFT) calculations (Fig. 6c–f) can provide further theoretical support for the above. We performed theoretical calculations based on the models of perfect Bi2WO6 ultrathin nanosheets and VO-Bi2WO6 ultrathin nanosheets (Fig. 6d and f). As shown in Fig. 6e, the introduction of oxygen vacancies resulted in new defect levels in the forbidden band of VO-Bi2WO6 ultrathin nanosheets, compared to the perfect Bi2WO6 ultrathin nanosheets (Fig. 6c), and the corresponding band structure also provides further proof (Fig. S8). The new level of defects makes the electrons more susceptible to light excitation and transfers to the conduction band, which indicates a reduction in the band gap. Furthermore, the existence of oxygen vacancies also increases the density of states (DOS) at the edge of the VO-Bi2WO6 ultrathin nanosheet conduction band, which helps to enhance electrons transport.

image file: c9qi01370a-f6.tif
Fig. 6 (a) UV-vis diffuse reflectance spectra and (b) XPS valence spectra of Bi2WO6 and VO-Bi2WO6; (c)–(f) are DFT calculations for perfect Bi2WO6 ultrathin nanosheets and VO-Bi2WO6 ultrathin nanosheets.


In summary, we first fabricated Bi2WO6 ultrathin nanosheets via a hydrothermal method. Furthermore, based on this, oxygen vacancies were successfully introduced onto the catalyst surface by Ar plasma treatment, which greatly promoted the photocatalytic activity, converting CO2 to CO at an evolution rate of 40.6 μmol g−1 h−1. The increase of surface oxygen vacancies not only increases the light absorption, but also promotes the separation of photogenerated holes and electrons, so that more photogenerated electrons can effectively participate in the photocatalytic reaction. In addition, plasma etching enlarges the specific surface area of the material, which could provide more reactive sites. This work uses a fast and environmentally friendly method of plasma modification to treat Bi2WO6, improving its ability for the photocatalytic reduction of CO2 to CO.

Author contributions

Q. Li, J, Chu, Y, Du, and H. Xu designed experiments; Q. Li, X. Zhu, and J. Yang carried out experiments; Q. Yu, X. Zhu, C. Wang, Y. Hua, and H. Li analyzed experimental results; Q. Li wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.


This study was financially supported by the National Nature Science Foundation of China (21676128, 21776118, and 51902138), High-tech Research Key Laboratory of Zhenjiang (SS2018002), Jiangsu Funds for Distinguished Young Scientists (BK20190045), the Natural Science Foundation of Jiangsu Province (BK20190835), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province (KFKT2019002), Construction funding of High-level teachers, Jiangsu University (4111510008) and the High Performance Computing Platform of Jiangsu University. Prof. Ziran Chen at Sichuan Vocational and Technical College generously provided us with access to the Vienna ab initio simulation package.


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Electronic supplementary information (ESI) available: XRD pattern, XPS spectrum, SEM images, etc. See DOI: 10.1039/c9qi01370a

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