For controlling of the damage by CO₂ [1-5], photocatalytic CO₂ conversion technology is employed [6, 7]. The initial research [8] has studied solar light driving artificial photosynthesis widely [9, 10]. Especially, the heterojunction materials have caught the eyes [11, 12].

Layer graphite carbon nitride (g-C₃N₄) possesses a certain photocatalytic activity [13-17], which the narrow band gap (2.7 eV) responds to visible light [18], but suffer from the rapid recombination of photoinduced e⁻/h⁺ pairs. Fortunately, the formation of g-C₃N₄/metal oxides heterojunction contributes to the separation efficiency of carriers [19-25]. Among a large number of metal oxides that have been widely studied for photocatalytic conversion of CO₂, tungsten oxide (WO₃) is considered to be one of the promising materials due to its high oxidation potential and special catalytic properties [26]. The optical band gap of WO₃ is narrow (2.7 eV), and the holes generated on the valence band of WO₃ have strong oxidation ability. However, it is still a challenge to further modify WO₃ for improving the photocatalytic activity due to the low absorption and utilization efficiency of solar energy.

Three-dimensional ordered macroporous (3DOM) materials, billed as inverse opals, encompasses photonic crystals with regular and uniform porous structure. Photonic crystals have periodic modulations of the refractive index on the length scale of light wavelength [27]. The aperture of macropore is matched with a certain wavelength of light wave. So the light can propagate in it and be prevented from passing through Bragg diffraction, which is beneficial for the generation of energy band gap [28]. It has been found that 3DOM structure has the slow photon effect, which increases the length of light reaction path and enhances the light absorption efficiency of material during CO₂ photoreduction [29-33]. However, there are relatively few studies devoted to the design of WO₃ as 3DOM structure. Therefore, the Z-scheme photocatalyst with g-C₃N₄/3DOM-WO₃ heterojunction is fabricated for improving photocatalytic activity. The photogenerated media belonging to g-C₃N₄ and WO₃ will migrate because the different positions of valence band (VB) and conduction band (CB), which achieves effective separation of photogenerated electrons and holes. Consequently, the fabrication of Z-scheme heterojunction photocatalyst refers to a spectacular strategy to bring high-performance catalytic activity to CO₂ photo reduction.

Herein, the direct Z-scheme photocatalysts of g-C₃N₄ nanosheets decorated 3DOM-WO₃ (g-C₃N₄/3DOM-WO₃, abbreviated to 3DOM-CNW) were successfully fabricated by in-situ colloidal crystal template method. The Z-scheme heterojunction structure of 3DOM-CNW catalysts enhances the separation efficiency of photogenerated electron-hole pairs, which leads to the improvement of the catalytic activity for CO₂ photoreduction. The separation mechanism of photogenerated carriers in composite photocatalyst was proposed.

The 3DOM-WO₃ material was synthesized in virtue of colloidal crystal template (CCT) method, and 3DOM-CNW catalysts were fabricated by in-situ growing of g-C₃N₄ film on the pore wall of 3DOM-WO₃ as description in Supporting Information. During the in-situ synthesis processes of 3DOM-CNW catalysts, the mass ratios of g-C₃N₄ precursor (urea) and 3DOM-WO₃ are 2:1 and 4:1, which are denoted as 3DOM-CNW-2 and 3DOM-CNW-4, respectively. As shown in Fig. 1A, the two-dimensional silk-like ultrathin film of pure g-C₃N₄ can be observed, and their uniform ultrathin layers are potential to increase the specific surface area. The d-spacings of 0.372 nm (Fig. 1D) could be assigned to (020) crystal planes of WO₃ support. Figs. 1B and C clearly show the ordered and consistent macroporous structure. After introduction of g-C₃N₄ nanosheets, the macroporous structures of 3DOM-WO₃ and 3DOM-CNW-4 catalysts were perfect, indicating that the prepared procedures of 3DOM-CNW photocatalysts by in-situ growth method cannot destroy the 3DOM structure of WO₃ support.

To research the phase structure of 3DOM-WO₃ and g-C₃N₄ in 3DOM-CNW photocatalysts, the XRD patterns of g-C₃N₄, WO₃, 3DOM-CNW-2 and 3DOM-CNW-4 catalysts are shown in Fig. S1 (Supporting information). For the pure g-C₃N₄, the diffraction peak (2θ) of pure g-C₃N₄ nanocrystals at 27.1° can be indexed to the (002) crystal facets of g-C₃N₄ (JCPDS No. 87-1526), which is ascribed to the long-range interfacial stacking of the aromatic system. Another characteristic peak located at 13.0° is attributed to the in-plane structure stacking pattern [34]. According to the results of XRD patterns, the characteristic peaks of 3DOM-WO₃ support in the catalysts can be clearly observed, which are highly consistent with the standard monoclinic phase of WO₃ (JCPDS No. 20-1324). After introduction of a small volume of g-C₃N₄, the diffraction pattern of 3DOM-CNW-2 exhibits no obvious peaks indicative of crystalline phases. With increasing of g-C₃N₄ contents, the characteristic peak corresponding to (002) crystal plane of g-C₃N₄ can be found, and its diffraction peak intensity increases, while the intensity of diffraction peaks corresponding to 3DOM-WO₃ decreases. It suggests that the surface of 3DOM-WO₃ support is covered by g-C₃N₄ gradually. It is also noted that the diffraction peak of g-C₃N₄ (002) crystal plane shifts from 27.1° to 27.8°, indicating that the coupling between g-C₃N₄ and 3DOM-WO₃ may be related to the g-C₃N₄ (002) facets, which is ascribed to the interaction between 3DOM-WO₃ and g-C₃N₄ in the composite photocatalysts [35]. In addition, the shortened interlayer spacing of g-C₃N₄ results from the shift of the (002) diffraction peak, which is conducive to carrier transmission.

The functional groups and chemical bonds of photocatalysts were investigated by FT-IR (Fig. S2 in Supporting information). The absorption peaks of as-synthesized catalysts at 1248, 1325 and 1412 cm⁻¹ are assigned to the stretching vibrations occurring to the aromatic C-N bond, and the C-N stretching vibration peaks center at 1572 and 1639 cm⁻¹. For the g-C₃N₄, 3DOM-CNW-2 and 3DOM-CNW-4 catalysts, the characteristic peak at 808 cm⁻¹ is stemmed out of the out-of-plane ring vibration of triazine structure and attributed to the C-N heterocycle in the samples. In addition, the presence of a broad absorption peak at the wavenumber of 3137 cm⁻¹ is assigned to absorption conducted by water and N-H vibration in amine group. FT-IR can further prove that the 3DOM-CNW catalysts are encompassing g-C₃N₄ and 3DOM-WO₃, which is consistent with the results of TEM and XRD.

Fig. 2A exhibits the UV-vis diffuse reflectance spectroscopy of 3DOM-WO₃, g-C₃N₄, 3DOM-CNW-2 and 3DOM-CNW-4 catalysts. Compared with 3DOM-WO₃ and g-C₃N₄, the light absorption efficiency of 3DOM-CNW catalysts increases significantly, and the absorption edge of 3DOM-CNW displays red shift. It results in the enhanced light absorption capacity featured by the 3DOM-CNW with the integration of the g-C₃N₄ and 3DOM-WO₃. Thus, 3DOM-CNW catalysts with enhanced visible-light absorption efficiency make up the hopeful applications in photocatalytic reduction of CO₂. The photoluminescence (PL) spectrum is employed to evaluate the separation efficiency of photogenerated carriers and result is exhibited in Fig. 2B. The PL spectra of pure g-C₃N₄ and 3DOM-CNW catalysts have similar shapes and emission peaks located at 460 nm. The high intensity PL peak of pure g-C₃N₄ is assigned to the prompt reorganization of its own photogenerated carriers. The lower luminescence intensity of 3DOM-CNW catalyst indicates that the recombination of photogenerated electron-hole pairs encounters severe obstacle resulting from the coupling of g-C₃N₄ and 3DOM-WO₃ catalysts. The 3DOM-CNW-4 catalyst displays the lowest intensity compared to other catalysts at the same test conditions, which shows the huge reduction of reorganization rate for photogenerated electron-hole pairs. It demonstrates that 3DOM-CNW catalyst greatly improves the separation efficiency harbored by photogenerated carriers and expands the response range of visible light.

The promoted carrier transfer property of catalysts was evaluated by electrochemical impedance spectroscopy (EIS) in Na₂SO₄ solution (0.1 mol/L) under the condition of bias voltage of −1.3 V vs. RHE, and the frequency range is 10,000 Hz to 0.1 Hz. The Nyquist plots of WO₃, 3DOM-CNW-2 and 3DOM-CNW-4 catalysts are shown in Fig. S3 in Supporting information (light intensity marks 80 mW/cm²). The radius of semicircle in the spectrum is proportional to the charge transfer resistance (R_ct) of catalysts, which stands for the charge transfer capacity of catalysts and reactants. It is generally accepted that the lower the charge transfer resistance (R_ct), the faster the charge transfer process. 3DOM-CNW-4 catalyst has the smallest arc, indicating that photogenerated electron-hole pairs has effective separation in 3DOM-CNW-4 catalyst. On the basis of UV-vis DRS, PL and EIS obtained in the study, it can be confirmed that the 3DOM-CNW-4 catalyst has the best capability of visible-light capture as well as the separation efficiency of photogenerated carriers. The excited charges are rapidly transferred to the active surface of the 3DOM-CNW photocatalysts and interact with CO₂ to complete the reduction reaction.

To further investigate the chemical components and electronic sates of elements in the Z-scheme 3DOM-CNW catalysts, the X-ray photoelectron spectroscopy (XPS) spectra of W 4f regions for 3DOM-CNW catalysts was carried out as shown in Fig. S4. The existence of W(VI) in 3DOM-WO₃ catalyst is evidenced by the main peaks at 35.9 and 38.0 eV, which are assigned to the W 4f_7/2 together with W 4f_5/2 of W⁶⁺ species in WO₃ [36]. In contrast to 3DOM-WO₃ catalyst, the peaks of W element in 3DOM-CNW-4 catalyst are shifted to lower energy by 0.1-0.2 eV, revealing that the introduction of g-C₃N₄ can decrease the oxidation state of W in WO₃. It had been found that the binding energy of carbon element and nitrogen element has a positive shift [37]. Combined with the results of binding energy reduction of tungsten element, it indicates that the interaction between g-C₃N₄ and WO₃ causes the electron density to migrate from g-C₃N₄ to WO₃. After the generation of charge carriers, the binding effect of WO₃ on photoelectrons is weakened, the higher electron mobility in WO₃ directly injects the excited electron flow from its CB to the VB of g-C₃N₄.

Under the simulated irradiation of visible light (≥ 420 nm), the photocatalytic performance of 3DOM-CNW photocatalysts for CO₂ reduction was evaluated in a closed gas-circulation system in the presence of water vapor. And the product was analyzed by gas chromatography. Fig. 3 and Table S1 (Supporting information) exhibit the catalytic performances of 3DOM-CNW catalysts for CO₂ photoreduction with H₂O. As revealed in Figs. 3A and B, the pure g-C₃N₄ nanosheets show excellent performance in converting CO₂ to CO, and the CO formation rate is 25.2 μmol g⁻¹ h⁻¹, but the CH₄ product is undetected. During photocatalytic CO₂ reduction reaction, 3DOM-WO₃ catalyst has the low activity, which is attributed to the lower conduction band position of 3DOM-WO₃ catalyst. Compared with single-phase 3DOM-WO₃ and g-C₃N₄, the 3DOM-CNW binary catalyst has better photocatalytic performance for CO₂ reduction, demonstrating that the integration of 3DOM-WO₃ and g-C₃N₄ constitutes the direct Z-scheme heterojunction. The spatial separation of photogenerated electron-hole pairs is reliably formed. The 3DOM-CNW-4 catalyst exhibits the optimal formation rate of CO (48.7 μmol g⁻¹ h⁻¹), and its formation rate of CH₄ is 7.5 μmol g⁻¹ h⁻¹. Moreover, we further confirmed the formation rate of O₂ over the as-prepared catalysts. Among all the catalysts, 3DOM-CNW-4 catalyst shows the highest formation rate of O₂ (44.5 μmol g⁻¹ h⁻¹), which is higher than the stoichiometric rate according to the calculation of other production (39.45 μmol g⁻¹ h⁻¹). It may be attributed to the formation of other undetected hydrocarbons [38]. Therefore, it is concluded that 3DOM-CNW photocatalysts have the capability of upgrading the activity for CO₂ photo reduction because of its high photogenerated carrier separation and visible light capture efficiency.

The utilization efficiency of solar energy presents great significance for the photocatalytic reduction of CO₂ with H₂O as a reducing agent. The apparent quantum efficiency (AQE) is considered as a significant parameter in terms of the conversion of light energy and chemical energy. In Table S1,3DOM-CNW-4 photocatalyst shows the highest AQE value (2.21), which is 110-fold of that of 3DOM-WO₃ photocatalyst (0.02). It further confirms that the in-situ growing g-C₃N₄ in three-dimensional ordered macropores is an effective strategy for improving the application efficiency of solar energy. To investigate the stability of the formation rate of products (CH₄, CO and H₂) over 3DOM-CNW-4 photocatalyst, five activity test cycles were carried out under the exactly same conditions. As demonstrated in Fig. 3D, after the five test cycles, the formation rate of the products maintains the same high activity trend and exhibits no obvious decrease, indicating that the 3DOM-CNW photocatalysts have excellent cycle stability for CO₂ photoreduction.

According to the results of XPS, 3DOM-CNW photocatalysts have achieved the electron transmission path from 3DOM-WO₃ (CB) to g-C₃N₄ (VB) (Fig. 4A). On the basis of the above results, the mechanism of Z-scheme 3DOM-CNW photocatalysts for CO₂ reduction is proposed (Fig. 4A). In general, WO₃ has lower Fermi energy level and larger work function (6.23 eV), while g-C₃N₄ is a semiconductor with higher Fermi energy level and smaller work function (4.18 eV) (Fig. 4B). In situ growth of g-C₃N₄ film uniformly on the pore wall of 3DOM-WO₃ can promote the electrons transfer between g-C₃N₄ and 3DOM-WO₃ spontaneously until their Fermi levels are the same (Fig. 4C). Therefore, the internal electric field is obtained because of the opposite charges of 3DOM-WO₃ and g-C₃N₄. Derived out of the charge accumulation, the energy band edge of 3DOM-WO₃ is bent downward, while the energy band edge of g-C₃N₄ is bent upward resulting from the loss of electrons. Under light irradiation, the electrons are excited from VB to CB of both WO₃ and g-C₃N₄. The internal electric field, band edge bending and Coulomb interaction accelerate the recombination of some electrons (from CB of WO₃) and holes (from VB of g-C₃N₄). Hence, electrons with more negative potential provide powerful driving force oriented with the photocatalytic reduction of CO₂ to CH₄ and CO. In the half-reaction of water at the position of the VB of 3DOM-WO₃ with higher positive potential, the hydroxide radical is oxidized to generate O₂ and active hydrogen, which is an important active species that promotes carbon dioxide deoxygenation. Therefore, the oxygen generation reaction at the position of the VB plays an important role in the photocatalytic reduction of CO₂. In addition, the ultrathin layers structure from g-C₃N₄ as well as the macroporous structure of 3DOM-WO₃ cleverly increase the contact area. The promoted interaction provides a straight path for electrons transfer from 3DOM-WO₃ to g-C₃N₄. The photogenerated holes in g-C₃N₄ are quenched by this rapid electron injection, and the reorganization of photogenerated electron-hole pairs is effectively inhibited. Simultaneously, the low fluorescence intensity of 3DOM-CNW-4 also strongly proves that there is charge separation on the interface where 3DOM-WO₃ and g-C₃N₄ (Fig. 2B) are coexisting. In other words, the 3DOM-CNW direct Z-scheme photocatalyst not only realizes the rapid separation of photogenerated carriers, but also retains a better redox potential. The charge carrier transfer process (Fig. 4A) and the formation of internal electric field (Fig. 4C) are two important factors for the excellent redox capability of the 3DOM-CNW heterojunction photocatalyst, which is very beneficial for the photocatalytic reduction of CO₂.

Herein, the photonic crystal 3DOM-WO₃ was prepared by the colloidal crystal template method, and 3DOM-CNW catalysts were fabricated by in-situ growing of g-C₃N₄ film on the pore wall of 3DOM-WO₃. The CB position of 3DOM-WO₃ is close to the VB position of g-C₃N₄, which provides new sight for the construction of the all-solid-state Z-scheme photocatalyst for visible-light driving photocatalytic CO₂ reduction with H₂O. 3DOM-CNW catalyst overcomes the shortcomings of the mismatch between the CB position of traditional semiconductor materials and the redox potential required for the reaction. By constructing the direct Z-scheme system, the higher redox potential can be retained, which can realize the directional conduction of electrons between the double semiconductor materials, thereby separating the reduction and oxidation reactions. 3DOM-CNW catalysts have high the harvest efficiency of visible-light and the separation efficiency of photogenerated electron-hole pairs. 3DOM-CNW catalysts are representing outstanding performance for photocatalytic CO₂ reduction, and the formation rates of CO and CH₄ products over 3DOM-CNW-4 catalysts are 48.7 and 7.5 μmol g⁻¹ h⁻¹, respectively. The electron transfer mechanism was studied and verified. This research provides a new avenue for the construction of direct Z-scheme photocatalysts.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was supported by the National Natural Science Foundation of China (No. 21972166), Beijing Natural Science Foundation (No. 2202045) and National Key Research and Development Program of China (No. 2019YFC1907600).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.07.020