Introduction

The massive consumption of fossil fuels over the last several decades has led to the energy crisis and global warming [1, 2]. The photocatalytic conversion of CO2 into chemical fuels is widely believed to be a very promising approach to solve energy and environmental problems [3,4,5,6,7,8]. Although several photocatalysts, including CdS [9], TiO2 [10], ZnIn2S4 [11], and polymeric carbon nitride [12, 13], have been explored, the serious recombination of photogenerated charge carriers in these materials limits their photocatalytic efficiency [14, 15].

Bismuth tungstate (Bi2WO6), the simplest among the Aurivillius oxides, has attracted great attention on account of its layered structure, non-toxicity, and chemical stability [16, 17]. Despite these advantages, however, pristine Bi2WO6 exhibits only moderate photocatalytic CO2 activity because of extensive charge recombination. Therefore, various strategies, including morphology design [18], heteroatom doping [19, 20], heterojunction construction [21, 22], and coupling with cocatalysts [23], have been developed to enhance the photocatalytic CO2 reduction activity. Studies have confirmed that heteroatom doping is an effective approach to enhance the photocatalytic performance of Bi2WO6.

Herein, we propose a facile hydrothermal and calcination method to prepare ultra-thin carbon-doped Bi2WO6 nanosheets (Bi2WO6s) with improved photocatalytic CO2 reduction activity. The thickness of the C-doped Bi2WO6 nanosheets (C-Bi2WO6s) is within ~ 4.1 nm. Carbon doping not only successfully extended the light absorption range of the nanosheet but also greatly promoted charge separation, thereby inhibited recombination. The ultra-thin Bi2WO6 nanosheet exhibited relatively high CO2 photoreduction performance as a result of its increased active sites and excellent charge separation.

Experimental

Preparation of C-Bi2WO6s

C-Bi2WO6s were prepared by a facile hydrothermal and calcination method. In brief, 0.5 mmol of Na2WO4·2H2O and 0.025 g of hexadecyltrimethylammonium bromide (CTAB) were added to 40 mL of deionized water and mixed to obtain a solution. Then, 1 mmol of Bi(NO3)3·5H2O was added to the above solution, and the suspension was magnetically stirred for 60 min. The suspension was poured into a 50 mL Teflon-lined autoclave and hydrothermally treated at 120 °C for 24 h. The reactor was cooled to room temperature naturally, and the obtained sample was collected, washed three times with deionized water, and then dried at 60 °C in a vacuum oven for 12 h. The dry powder was placed in a tube furnace and calcined in a nitrogen gas atmosphere at 350 °C for 1 h with a heating rate of 3 °C/min. Finally, the obtained powder was denoted as C-Bi2WO6s. Carbon doping was realized by the carbonization of long carbon links in the residual CTAB.

Results and Discussion

Structure and Morphology

The X-ray diffraction (XRD) patterns of bulk Bi2WO6, pristine Bi2WO6s, and C-Bi2WO6s are shown in Fig. 1. The diffraction peaks of the samples could be indexed to those of orthorhombic Bi2WO6 (JCPDS Card No. 73–2020) [16]. No diffraction peaks assigned to carbon were observed in the XRD patterns, thus suggesting the high dispersibility of the element [24]. Compared with those of bulk Bi2WO6 and Bi2WO6s, the diffraction angles of C-Bi2WO6s showed no changes, thereby indicating that carbon doping does not change the crystalline phase of Bi2WO6 [25]. The XRD peaks of Bi2WO6s and C-Bi2WO6s were weaker and broader compared with those of bulk Bi2WO6, which could be attributed to the thinner layered structure of sheet-like Bi2WO6 [23]. According to the Scherrer equation, a smaller grain size results in broader XRD peaks.

Fig. 1
figure 1

XRD patterns of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s

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Field emission scanning electron microscopy (Fig. 2) and transmission electron microscopy (TEM; Fig. 3) were performed to investigate the microstructures of the samples. Interestingly, compared with bulk Bi2WO6, both Bi2WO6s and C-Bi2WO6s showed ultra-thin nanosheet structures, but the latter has small deformation, which could be attributed to the calcination treatment. Furthermore, this nanosheet-like structure of the samples was further confirmed by TEM and atomic force microscopy (AFM). The TEM micrographs in Fig. 3a and b reveal nearly transparent features, which suggests an ultra-thin nanosheet structure. The high-resolution TEM micrographs also demonstrated marked interplanar spacings of (200) and (020) planes, which correspond to the orientation of the Bi2WO6 nanosheet along the [001] projection [17]. The AFM image of C-Bi2WO6s shown in Fig. 3c reveals an average nanosheet thickness of ~ 4.1 nm, which corresponds to approximately three layers of Bi2WO6. These results demonstrate the intercalation of CTAB during the formation of the ultra-thin Bi2WO6 nanosheets. As a long carbon chain surfactant, CTAB could intercalate into the interlayers of Bi2WO6, thereby inducing the formation of ultra-thin Bi2WO6 nanosheets. Such an ultra-thin nanosheet structure could increase the availability of active sites and shorten the migration distance of charge carriers, thereby achieving effective charge separation [16, 26].

Fig. 2
figure 2

Field emission scanning electron micrographs of a bulk Bi2WO6, b Bi2WO6s, and c C-Bi2WO6s

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Fig. 3
figure 3

Transmission electron and high-resolution transmission electron micrographs of a Bi2WO6s and b C-Bi2WO6s. c Atomic force microscopic image of C-Bi2WO6s

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Energy-dispersive spectrometry (EDS) and elemental mapping were conducted to clarify the elemental composition of the C-Bi2WO6s catalyst. The EDS spectrum provided in Fig. 4 demonstrates that the elements Bi, W, C, and O are uniformly distributed in the C-Bi2WO6s catalyst. To further clarify the carbon content in the C-Bi2WO6s catalyst, C elemental analysis was carried out using an elemental analyzer. The elemental analysis result revealed that the atomic composition of C was about 0.36%.

Fig. 4
figure 4

a Field emission scanning electron micrograph and b corresponding EDS spectrum of C-Bi2WO6s. Elemental mappings of C-Bi2WO6s: c Bi, d W, e C, and f O

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X-Ray Photoelectron Spectroscopic Analysis

X-ray photoelectron spectroscopy (XPS) was performed on Bi2WO6s and C-Bi2WO6s, as shown in Fig. 5, to confirm the successful doping of carbon into Bi2WO6s. The survey XPS spectra confirmed the existence of Bi, W, O, and C in Bi2WO6s and C-Bi2WO6s. The high-resolution spectrum of Bi could be fitted to two peaks at 164.26 and 158.96 eV, which, respectively, correspond to Bi 4f5/2 and Bi 4f7/2 in Bi3+ [27]. After carbon doping, the binding energy of Bi shifted toward higher energy, thereby indicating the presence of Bi–C interactions. The binding energies of W at 37.36 and 35.26 eV in the XPS spectrum of Bi2WO6s correspond to the W6+ oxidation state. The binding energy of W also shifted toward higher binding energy, thus suggesting the successful doping of C. The binding energy of O in Bi2WO6s could be fitted to two peaks at 529.9 and 531.51 eV, which correspond to Bi–O and W–O, respectively. The binding energy of O 1s in C-Bi2WO6s slightly shifted, and the peak at 533.06 eV could be attributed to physically adsorbed water. The binding energy shifts of Bi, W, and O reflect strong interactions between Bi2WO6 and C [27, 28]. The binding energies of C 1s centered at 284.8, 286.3, and 288.8 eV could be assigned to C–C sp2-hybridized carbon on surface, C–O, and COO bonds, respectively, thus indicating that carbon was successfully doped into the Bi2WO6 nanosheets.

Fig. 5
figure 5

a X-ray photoelectron survey (XPS) spectra of Bi2WO6s and C-Bi2WO6s. High-resolution XPS spectra of b Bi 4f, c W 4f, d O 1 s, and e C1s of Bi2WO6s and C-Bi2WO6s

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N2 Adsorption–Desorption Isotherms

The curves of all Bi2WO6 samples were classified as type IV isotherms according to the Brunauer–Deming–Deming–Teller classification (Fig. 6) [29]. Moreover, the curves of all Bi2WO6 samples showed a type H3 hysteresis loop, suggests that slit-like pores formed due to the stacking and aggregation of plate-like particles, which is in agreement with the sheet-like morphology of Bi2WO6. The pore size distribution curves confirm the presence of mesopores and macropores in the Bi2WO6 samples. The Brunauer–Emmett–Teller (BET) surface areas of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s were measured to be 15.5, 43.8, and 42.7 m2/g, respectively. Compared with that of Bi2WO6s, the specific surface area of C-Bi2WO6s showed no obvious change, thus suggesting that the microstructure of Bi2WO6s is well preserved after carbon doping by thermal treatment. These results collectively confirm the formation of ultra-thin nanosheet structures, which increase the BET surface area of the material and provide more active sites for greater photocatalytic activity.

Fig. 6
figure 6

N2 adsorption–desorption isotherms of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s and the corresponding pore-size distribution curves

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CO2 Adsorption

The CO2 adsorption curves of the samples were measured and are shown in Fig. 7. Bi2WO6s and C-Bi2WO6s show similar CO2 adsorption performance, which is 4 times higher than that of the bulk Bi2WO6. This enhanced CO2 adsorption ability is mainly attributed to large specific surface area [16, 30]. The comparable CO2 adsorption capacity of the two nanosheet samples indicated that carbon doping does not significantly change the catalyst microstructure, which is consistent with the N2 adsorption–desorption results. Enhancements in CO2 adsorption capacity can reinforce the photocatalytic CO2 reduction activity of the prepared materials [31].

Fig. 7
figure 7

CO2 adsorption isotherms of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s

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UV–Visible Diffuse Reflectance Spectra

Figure 8 shows the UV–Vis absorption spectra of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s. The absorption edge of bulk Bi2WO6 was approximately 430 nm, which corresponds to a band gap of 2.88 eV. Moreover, the absorption edge of Bi2WO6s was blue-shifted compared with that of bulk Bi2WO6 because of the quantum size effect [32]. Compared with that of Bi2WO6s, the absorption edge of C-Bi2WO6s was red-shifted, thereby indicating uniform carbon doping. Extension of the absorption tail was also observed in C-Bi2WO6s, which suggests that doping-related mid-gap states are generated and that these states regulate the band structure of Bi2WO6s. C-Bi2WO6s showed enhanced light absorption in the full-range visible-light region, which implies that, besides the lattice, carbon is also doped on the surface of Bi2WO6. These results indicate that carbon doping could reduce the band gap of Bi2WO6 nanosheets and enhance their light absorption [33].

Fig. 8
figure 8

UV–Vis absorption spectra of the prepared samples

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Mott–Schottky Curves and Band Structure

Mott–Schottky plots were constructed, as shown in Fig. 9a–c, to confirm the band structures of the samples. All of the samples are typical n-type semiconductors, as evidenced by their positive slopes [34]. The flat-band potentials of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s were − 1.08, − 1.30, and − 1.11 V (vs. Ag/AgCl, pH = 7), respectively. A more negative conduction band (CB) compared with that of bulk Bi2WO6 implies that the reduction ability of electrons is enhanced [35]. The more positive CB potential of C-Bi2WO6s compared with that of Bi2WO6s may be attributed to carbon doping [36]. The corresponding band structures of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s are shown in Fig. 9d. The CB of the Bi2WO6 photocatalyst was more negative than the potential of the CO2/reduced-product redox couple and, thus, could provide sufficient driving force for CO2 reduction [33, 37, 38].

Fig. 9
figure 9

Mott–Schottky plots of a bulk Bi2WO6, b Bi2WO6s, and c C-Bi2WO6s. d Band structures of the samples

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Photocatalytic CO2 Reduction and In-Situ FTIR Analysis

The photocatalytic CO2 reduction performance of all samples was measured under full-spectrum irradiation. As shown in Fig. 10, the generation rates of CH3OH and C2H5OH for Bi2WO6s were remarkably enhanced by approximately 4.7 and 3.0 times, respectively, compared with those of bulk Bi2WO6. Compared with that of Bi2WO6s, the photocatalytic CO2 reduction activity of C-Bi2WO6s was further improved, and the corresponding generation rates of CH3OH and C2H5OH were 1.15 and 0.65 μmol/(g·h), respectively. This photocatalytic performance is comparable with those of other bismuth-based photocatalysts (Table S1).

Fig. 10
figure 10

Photocatalytic CO2 reduction performance of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s

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The photocatalytic reduction of CO2 over C-Bi2WO6s was investigated by in-situ Fourier transform infrared (FTIR) spectroscopy. As shown in Fig. 11, the peak at 1648 cm−1 can be attributed to H2O [39]. The peaks at 1684, 1670, 1662, 1575, 1559, and 1256 cm−1 can be attributed to carboxylate (CO −2 ) [39,40,41,42]. The peaks at 1696, 1636, 1623, 1617, 1458, 1449, 1436, 1406, 1397, and 1227 cm−1 are ascribed to bicarbonate (HCO −3 ) [39, 40, 42,43,44,45,46]. The peaks at 1654, 1361, 1340, and 1318 cm−1 refer to bidentate carbonate (b-CO 2−3 ) [40, 43, 46]. The peaks at 1569, 1541, 1532, 1522, 1508, 1498, 1488, 1474, 1466, and 1388 cm−1 are attributed to monodentate carbonate (m-CO 2−3 ) [39,40,41, 43, 47,48,49]. After light irradiation, a number of new peaks were observed. The peaks at 1375 and 1318 cm−1 are assigned to formate [50]. The peak at 1419 cm−1 is related to formaldehyde (HCHO) [35], and the peak at 1162 cm−1 is ascribed to methoxyl groups (CH3OH) [31, 51]. The presence of these peaks, which indicate intermediate products, reveals that CO2 conversion over C-Bi2WO6s is a multi-electron reduction process.

Fig. 11
figure 11

In-situ FTIR spectra of C-Bi2WO6s

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Mechanism of Photocatalytic Performance Enhancement

The charge transfer and separation dynamics of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s were studied by transient photocurrent response analysis and electrochemical impedance spectroscopy (EIS). The photocurrent intensity of C-Bi2WO6s was larger than those of bulk Bi2WO6 and Bi2WO6s (Fig. 12a), suggesting the best electron transfer efficiency and lowest charge carrier recombination rate of C-Bi2WO6s sample [52]. Anodic spikes were observed in all samples when the light was switched on because of the separation of photogenerated electrons and holes at the interface of Bi2WO6/electrolyte. A decrease in photocurrent was subsequently observed, because photogenerated holes tend to recombine with electrons instead of undergoing capture by reduced species in the electrolyte. The EIS Nyquist plots of all samples were also obtained, as shown in Fig. 12b. A smaller semicircular radius usually indicates lower charge-transfer resistance. Among the samples studied, C-Bi2WO6s showed the smallest arc radius, which suggests that charge transfer occurs fastest in this sample.

Fig. 12
figure 12

a Transient photocurrent responses of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s. b EIS Nyquist plots of bulk Bi2WO6, Bi2WO6s, and C-Bi2WO6s

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Conclusions

In summary, ultra-thin (~ 4.1 nm) carbon-doped Bi2WO6 nanosheets were prepared by a simple hydrothermal treatment and calcination method. The prepared Bi2WO6 nanosheets exhibited twofold enhanced photocatalytic activity for CO2 reduction compared with that of pristine ultra-thin Bi2WO6 nanosheets. This enhancement was attributed to the ultra-thin nanosheet structure and carbon doping. An ultra-thin nanosheet structure provides more active sites and shortens the diffusion distance of charge carriers, thereby suppressing charge recombination. Carbon doping extends the light absorption ability of the catalyst and promotes charge separation. This work highlights the importance of defect engineering in photocatalytic energy conversion and provides new insights for fabricating efficient two-dimensional photocatalysts.