Photoreduction of CO₂ into energy-rich molecules (e.g., CO, CH₃OH, CH₄, C₂H₅OH, and C₂H₆) provides a sustainable way of CO₂ utilization^[1−4]. Numerous efforts have been devoted to design of different photocatalysts over the past decades. One of the most effective strategies is to rationally tailor bi- or multi-functional semiconductor homo/heterojunctions^[5−6].

WS₂ is a typical type of group-VI transition-metal di-chalcogenide, where the individual S-W-S atomic planes bonded through Van der Waals forces^[6]. WS₂ photocatalyst has received particular research interests due to its tunable bandgap, wide light absorption range and excellent charge mobility properies^[7]. Theoretically, it has been predicted that WS₂ layers have two distinct symmetries depending on the arrangement manner of S atoms, i.e., hexagonal close packing and trigonal prismatic coordination phase (2H) or the tetragonal symmetry and octahedral coordination phase (1T)^[8]. Notably, thermo dynamically stable 2H-WS₂ is suitable to form heterojunctions with other semiconductors for CO₂ photoreduction. And the 1T-WS₂ has suitable metal conductivity and plenty of active sites on the base plane and the edge areas, which could effectively transfer the electrons and thus acts as co-catalyst for CO₂ reduction. Interestingly, 1T/2H-WS₂ homojunctions can be formed by partial oxidation of metastable 1T-WS₂, enabling facile heterojunctions construction by integrating with other band structure-matching semiconductor materials^[9].

Herein, a novel heterojunction photocatalyst was prepared by integrating multiphase 1T/2H-WS₂ homojunction with WO₃ via one-pot hydrothermal treatment. Specifically, 1T/2H-WS₂/ WO₃ showed excellent catalytic performance for CO₂ photo- reduction using water vapor as electron donor, affording CO as the sole carbonaceous product with a production rate of 3.87 μmol∙g⁻¹∙h⁻¹ under visible light irradiation. Besides, under UV-visible light irradiation, the 1T/2H-WS₂/WO₃ photocatalyst showed higher rate of CO as 34.39 μmol∙g⁻¹∙h⁻¹. In addition, it exhibited excellent stability under photoreduction conditions. The formed heterostructure in 1T/2H-WS₂/WO₃ ensures a multi-step and cascade transfer pathway of photogenerated electrons, which facilitates accumulation of electrons on 1T-WS₂ for the photocatalytic CO₂ reduction and subsequently improves the selectivity of CO. This work provides new insight for designing and fabricating high-efficiency photo-active semiconductor-based photocatalysis for CO₂ utilization.

1 Experimental section

Synthesis of catalysts. One-pot hydrothermal method was adopted to prepare the 1T/2H-WS₂/WO₃ materials. 1.19 g (3 mmol) WCl₆ and 2.28 g (30 mmol) thioacetamide were dispersed in 40 mL DI water, and further stirred for 60 min. The obtained mixture was transferred into the 50 mL Teflon lined hydrothermal reactor and heated at 260 ℃ for 24 h. After cooling to room temperature, the product was collected by centrifugation at 8000 r∙min⁻¹, and washed thoroughly with ethanol and DI water. The product was vacuum dried overnight at 60 ℃.

Characterization. The Powder X-ray diffraction (XRD) patterns were obtained using a Rigaku SmartLab 3 kW diffractometer equipped with Cu Kα radiation. UV-vis diffuse reflectance spectroscopy (DRS) spectra were measured by Shimadzu UV-2550 spectrophotometer with an integrating sphere attachment. The morphologies were examined by field emission scanning electron microscopy (FE-SEM, Hotachi, Regulus-8100) at an acceleration voltage of 5 kV. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method. The pore size was obtained using the desorption isotherm through the Barrett-Joyner-Halenda (BJH) method. HRTEM images were taken on JEOL JEM-2100F field-emission high-resolution transmission electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was recorded on a thermos-scientific NEXSA system, using monochromatic Al Kα radiation (1486.6 eV).

Photocatalytic CO₂ reduction measurements. 15 mg of photocatalyst powder was carried out in a gas-closed system with a gas-circulated pump (EL-PAEM-D8 system, Beijing China Education Au-light Co., Ltd) equipped with a 300-W Xe lamp (PE300BUV, 200 ~ 780 nm). The 420 nm cutoff filter (420 nm < λ < 780 nm) was used to ensure that photocatalytic CO₂ reduction under visible light irradiation. During light irradiation, the gas products were analyzed by GC7920 with a flame ionization detector (FID) and a thermal conductive detector (TCD).

Photocatalytic CO₂ stability test. To evaluate the stability of prepared catalysts, the photocatalytic reactor was thoroughly dried at 100 ℃ for 2 h, then, 15 mg of catalysts were maintained in this reaction cell. Subsequently, high-purity CO₂ gas and water were introduced into the degassed system for further photoreduction performance evolution under visible light irradiation. The same measurement process was repeated for 4 times.

Photocatalytic activity calculation. The photocatalytic reaction rate (R) for product rate (μmol∙g⁻¹∙h⁻¹) was calculated as follows:

$ R=\frac{n}{mt} $

Where n refers to the number of moles of generated CO, m is the loading amount of catalyst (g), t is the irradiation time (h).

2 Results and discussion

The morphology of 1T/2H-WS₂/WO₃ heterostructures was measured using SEM technique. As shown in Fig.1(a), it displays typical flower-like microsphere morphology with the particle size of ~1 μm. The high-magnification SEM (Fig.1(b)) reveals that the flower-like microsphere was assembled by abundant nanoplates (~15 nm in thickness) as building blocks with highly branched structures. These congregated nanoplates create nanoscale 3D porous architecture on the exterior surface of WS₂, for faster CO₂ photoreduction. The microstructures of 1T/2H-WS₂/WO₃ were further investigated by the HRTEM (Fig. 1(c)). The lattice spacing of 0.27 nm corresponds to the plane of WO₃ (022). And the coexistence of the 1T-WS₂ and the 2H-WS₂ was confirmed. The compactly connected interface between 1T-WS₂, 2H-WS₂ and WO₃ is conducive to rapid charge transfer. The EDX elemental mapping images (Fig.1(d)) of the 1T/2H-WS₂/WO₃ heterostructure confirm the existence and W, S, and O distribution.

The XRD pattern of 1T/2H-WS₂/WO₃ is shown in Fig. 2(a). The diffraction peaks at 14.3°, 28.8°, 32.0°, 33.5°, 35.9°, 43.9°, 44.3° and 57.1° synchronized with the 2H-WS₂ (JCPDS No. 84-1398), assigning to (002), (004), (100), (101), (102), (006), (104), (008) planes, respectively. The peaks observed at 28.8°, 33.0°, 34.1° and 34.5° are associated with the (112), (022), (202),(220) planes of the triclinic WO₃ (JCPDS No. 32-1395)^[10].

The optical absorption characteristics of 1T/2H-WS₂/WO₃ were explored by UV-Vis DRS. As shown in Fig. 2(b), the 1T/2H-WS₂/WO₃ shows excellent visible light absorption, which can be attributed to the co-promotion of 1T-WS₂ and 2H-WS₂^[11]. As shown in Fig. 2(c), the BET surface area for 1T/2H-WS₂/WO₃ samples is 5 m²∙g⁻¹ and the pore volume is 0.03 cm³∙g⁻¹. The average pore size of the product based on the adsorption data is around 25.9 nm, which benefits the transports of CO₂, gaseous H₂O and CO products.

XPS measurements were adopted to give insight into the chemical environment and element composition of the 1T/2H-WS₂/WO₃ heterostructure. As shown in Fig. 2(d), both 2H-WS₂ phase and 1T-WS₂ phase can be evidenced. 1T-WS₂ form is characterized by W 4f signal whose W 4f_7/2 and 4f_5/2 component are located at 31.8 and 33.9 eV. Two peaks at 32.7 and 34.6 eV are attributed to the W 4f_7/2 and 4f_5/2 of 2H-WS₂^[12]. The peaks located at 35.7 (W 4f_7/2) and 37.7 eV (W 4f_5/2) correspond to the W—O bond of WO₃ species^[8]. The characteristic peaks of both WS₂ and WO₃ can be clearly observed, which further confirms the successful synthesis of the WS₂/WO₃ heterostructure. Two distinct peaks in the S 2p spectrum also confirm the existence of WS₂ (Fig. 2(e)), the peaks located at 161.3 and 162.3 eV belong to the S—W bond. These results demonstrate the successful formation of 1T/2H-WS₂/WO₃ heterostructures.

The photocatalytic activities of 1T/2H-WS₂/WO₃ for CO₂ reduction have been conducted and shown in Fig. 3(a) and 3(b). The dark reaction has been first explored before all tests, and only CO₂ signal can be observed in such a condition, which evidenced that there is no chemisorption of CO or O₂ on the photocatalyst surface. The 1T/2H-WS₂/WO₃ photocatalysts exhibited prominent and stable photocatalytic activities for CO₂ reduction into CO at a production rate of 3.87 μmol∙g⁻¹∙h⁻¹ during 64 h test under visible light irradiation. In addition, under the irradiation of UV-visible light, the 1T/2H-WS₂/WO₃ photocatalysts shows better catalytic activity, and the yield of CO was 34 μmol∙g⁻¹∙h⁻¹_. O₂ was detected by GC, indicating that the photoinduced holes were utilized for oxidizing water to generate oxygen and hydrogen ions via the half-reaction (2H₂O +4h⁺ → O₂ + 4H⁺). For stability measurements, the 1T/2H-WS₂/WO₃ photocatalysts almost keep its original activity even after four cycles (Fig. 3(c) and 3(d), indicating that it can act as a stable photocatalyst for CO₂ reduction.

Based on our study and literature survey, we propose a possible reaction mechanism for 1T/2H-WS₂/WO₃ involved photocatalysis. Firstly, the formation of 2H-WS₂/WO₃ hetero-junction contributes to build an efficient cascade charge transfer channel, which enhances the separation of photogenerated electron-hole pairs^[7,13]. Then, the charges are transferred to the surface of 1T-WS₂, which accelerates the separation of charges. Meanwhile, 1T-WS₂ plays a role as a co-catalyst, resulting in the high selectivity of CO generation^[9].

3 Conclusions

In conclusion, we successfully synthesized 1T/2H-WS₂/WO₃ heterostructures by one-pot hydrothermal method, which achieved photocatalytic reduction of CO₂ with H₂O, affording CO as the sole carbonaceous product with a production rate up to 3.87 μmol∙g⁻¹∙h⁻¹ under visible light irradiation, and 34.39 μmol∙g⁻¹∙h⁻¹ under UV-visible light irradiation. The formed heterostructures in the 1T/2H-WS₂/ WO₃ have highly qualified contact interface, contributed to the fast charge separation and transfer. We expect that this research provides a valuable reference for designing and fabricating photocatalytically active semiconductor heterojunction with high efficiency for solar-energy conversion.