English

• 0.   Introduction

  As global environmental and energy issues become more and more prominent, photocatalytic treatment of pollutants is receiving more and more attention^[1-2]. Currently, photocatalyst studies have focused on ultraviolet (UV)‐driven TiO₂^[3-4] and ZnO^[5-6] as well as visible (Vis)‐responsive C₃N₄^[7-8] and Bi₂WO₆^[9-10]. However, sunlight reaching the Earth′s surface is 52%‐55% of near‐infrared (NIR) (> 790 nm), 42%‐43% of Vis (400‐700 nm), and 3%‐5% of UV (< 400 nm) in terms of energy^[11]. These photocatalysts are only a single response to limited wavelengths, such as UV and Vis light. The efficiency of solar energy utilization is greatly reduced. Researchers are trying to maximize the use of solar energy. In particular, many efforts have been made for the application of NIR light in photodegradation^[12-13], water decomposition^[14-15], CO₂ reduction^[16-17], and NO_x removal^[18-19].

  In recent years, some researchers have found that, unlike the single response of currently available photocatalysts, Cs_xWO₃ exhibits a good photocatalytic response in the UV, Vis, and NIR^[20-22]. For Cs_xWO₃, Cs⁺ can be inserted into the lattice. A mixed chemical valence of W⁶⁺ and W⁵⁺ is formed. It exhibits strong localized surface plasmon resonance (LSPR) absorption in the NIR range. Meanwhile, due to the abundant WO_6-x^[23] units in Cs_xWO₃, Cs_xWO₃ exhibits reversible photochromic activity. Because of the excellent properties of Cs_xWO₃, many researchers have investigated the preparation methods and photocatalytic properties of Cs_xWO₃‐based composites. Li et al.^[24] synthesized g‐C₃N₄@Cs_xWO₃ solvothermally by solvent‐thermal synthesis. Irradiation with full‐spectrum light was used for 240 min. Its degradation of toluene gas was about 413 g·m^-3. Li et al.^[25] used melamine resin as a template to prepare composite materials with an adsorption/autothermal photothermal catalytic synergistic effect of BiOBr/Cs_xWO₃@SiO₂ composite aerogel. The adsorption/autothermal photothermal catalyzed degradation of tetracycline was close to 100% after adsorption in the dark room for 30 min and light irradiation under a xenon lamp for 60 min. Ye et al.^[26] used hydrothermal synthesis of Cs_xWO₃/TiO₂ with Vis light irradiation for 120 min. The maximum degradation rate of rhodamine B (RhB) was 79.7%. Huang et al.^[27] experimented with photocatalytic nitrogen fixation by studying different atmospheres. They skillfully designed photocatalysts for dynamic structure repair, elucidated the aerobic photocatalytic nitrogen reduction reaction (pNRR) mechanism, and provided new ideas for photocatalytic experiments.

  In this work, Cs_xWO₃/TiO₂ composites with good morphology and uniform size were synthesized by the one‐step solvothermal method using self‐made Cs_xWO₃ and TiO₂ as raw materials. By adjusting the ratio between Cs_xWO₃ and TiO₂ in the composite, the adsorption and degradation effects of the composite on methylene blue (MB) solution were investigated. The cyclic stability test and the photocatalytic mechanism were also carried out. The Cs_xWO₃/TiO₂ composite synthesized in this paper has excellent full‑spectrum response properties. Its photocatalytic effect is greatly improved over that of the single raw material. Excellent photocatalytic stability was demonstrated in cyclic stability tests.

  1.   Experimental

  1.1   Materials

  All chemical reagents in this study were of analytical grade and could be employed without further purification. Tungsten hexachloride (WCl₆), cesium hydroxide monohydrate (CsOH·H₂O), glacial acetic acid, polyvinylpyrrolidone (PVP, M_w=50 000), tetrabutyl titanate (TBOT), and hydrochloric acid (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Anhydrous ethanol and isopropanol were obtained from Shanghai Titan Scientific Co., Ltd. Deionized water was prepared in‐house using laboratory purification systems.

  1.2   Synthesis of Cs_xWO₃/TiO₂

  First, 0.396 6 g (1 mmol) of WCl₆ was dissolved in 40 mL of anhydrous ethanol under magnetic stirring. Subsequently, 0.085 2 g (0.5 mmol) of CsOH·H₂O was introduced into the homogeneous mixture. After achieving a slightly yellow clarified solution through continuous stirring, 10 mL of glacial acetic acid and a predetermined quantity of PVP were sequentially added, followed by vigorous stirring for 30 min to ensure complete homogenization. The resultant solution was transferred into a PTFE‐lined stainless steel autoclave and subjected to hydrothermal treatment at 220 ℃ for 12 h. Upon completion of the reaction cycle, the system was passively cooled to ambient temperature. The precipitate underwent three sequential washing cycles using deionized water and anhydrous ethanol through centrifugation. Ultimately, the purified product was desiccated in a vacuum oven at 60 ℃ for 12 h, yielding dark blue nanoscale Cs_xWO₃ particles.

  Initially, 10 mL of TBOT was slowly added to 100 mL of anhydrous ethanol under continuous stirring for 30 min. Subsequently, 1 mL of deionized water was introduced dropwise to the mixture. After stirring for an additional 1 h, the solution was allowed to stand undisturbed for 1 h. The resulting suspension was centrifuged and sequentially washed with deionized water and ethanol. The collected precursor was dried at 80 ℃ for 12 h to yield a white intermediate powder. Finally, the precursor was calcined in a muffle furnace under static air atmosphere by heating to 550 ℃ at a rate of 5 ℃·min^-1 and maintained at this temperature for 5 h, resulting in TiO₂ powder.

  A predetermined quantity of TiO₂ was dispersed in 100 mL of isopropanol. Ultrasonication and magnetic stirring were alternately performed until a homogeneous suspension was formed. Subsequently, a controlled amount of Cs_xWO₃ combined with PVP was introduced into the mixture, and the pH was adjusted to be less than or equal to 2. After continuous magnetic stirring for 2 h, the homogeneous solution was transferred to a PTFE‑lined reactor and maintained at 200 ℃ for 6 h. Upon reaction completion, the product was cooled to ambient temperature, followed by three sequential washing cycles with deionized water and anhydrous ethanol. The final material was dried in a vacuum oven at 60 ℃ for 12 h. By systematically varying the mass fraction of Cs_xWO₃ in the Cs_xWO₃/TiO₂ composites (30%, 40%, 50%, 55%, 60%), a series of samples were synthesized and designated as 30% Cs_xWO₃/TiO₂, 40% Cs_xWO₃/TiO₂, 50% Cs_xWO₃/TiO₂, 55% Cs_xWO₃/TiO₂, 60% Cs_xWO₃/TiO₂, respectively.

  1.3   Characterization

  The sample morphology of the Cs_xWO₃/TiO₂ composites was observed using a VEGE 3 SBH‐EasyProbe type scanning electron microscope (SEM, operating voltage: 20 kV). The microstructure of Cs_xWO₃/TiO₂ composites was observed using a JEOL JEM‐F200 transmission electron microscope (TEM, operating voltage: 200 kV). The crystal structure of Cs_xWO₃/TiO₂ composites was analyzed using a Philips X′ Pert MPD Pro type rotary‐target X‐ray diffractometer (XRD, Cu Kα radiation, λ=0.154 nm, the working voltage was 40 kV, the current was 40 mA, 2θ=10°‐80°). Diffuse reflectance spectra (DRS) were obtained using a Japan Shimadzu UV‐3600 for light absorption and band gap calculations. UV‐Vis and UV‐Vis‐NIR tests were performed using the Japan Shimadzu UV‐3600, with different wavelengths for the two tests, 200‐800 nm for UV‐Vis and 200‐2 500 nm for UV‐Vis‐NIR. The surface elemental composition and valence states in Cs_xWO₃/TiO₂ composites were tested using a Thermo Scientific K‐Alpha X‐ray photoelectron spectrometer (XPS). Brunner‐Emmet‐Teller (BET) surface area analysis was performed using an automated surface area and pore size analyzer (AUTOSORBIQ, Quantachrome, USA).

  1.4   Photocatalytic activity

  The instrument used was an XPA‐7 photochemical reactor. Then, 10 mg of photocatalyst was added to 100 mL of MB solution (20 mg·L^-1). Stirring in a dark environment for 60 min, 5 mL of supernatant was taken every 10 min, centrifuged, and tested for its absorbance. Then the photocatalytic reaction lamp was turned on, and 5 mL of the supernatant was taken at certain intervals (UV: 10 min, Vis: 20 min, NIR: 1 h), centrifuged, and tested for its absorbance. The degradation rate (η) was calculated according to the following equation:

  $ \eta=\left(1-\rho / \rho_0\right) \times 100 \% $

  where ρ was the mass concentration of MB solution at time t and ρ₀ was the initial mass concentration (concentration before dark adsorption, 20 mg·L^-1) of MB solution.

  According to the Lambert‐Beer law, A was proportional to ρ. Therefore, the degradation rate can be calculated by the following equation:

  $ \eta=\left(1-A / A_0\right) \times 100 \% $

  where A was the absorbance of the MB solution at time t, and A₀ was the initial absorbance of the MB solution.

  1.5   Photocatalytic cycling performance experiment

  This experiment tested the photocatalytic cycling performance of the prepared 40% Cs_xWO₃/TiO₂ nanocomposite. At the end of each photocatalytic experiment, the remaining photocatalyst in the MB solution was filtered. The experiment was repeated five times to simulate the cycling process. Each photocatalytic cycling process was performed under the same conditions using 100 mL of MB (20 mg·L^-1) solution. Samples were taken every 20 min at the end of dark adsorption. The photocatalysts were filtered by centrifugation, and the photocatalytic performance of the photocatalysts was assessed spectrophotometrically by the change in methyl bromide concentration.

  2.   Results and discussion

  2.1   Analytical characterization of material crystallinity

  The crystal structure of the samples was tested and analyzed using XRD. The results are shown in Fig. 1.

  Figure 1

  

  Figure 1.  XRD patterns of (a) 40% Cs_xWO₃/TiO₂ and (b) a series of Cs_xWO₃/TiO₂ composites

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  Fig. 1a shows the XRD patterns of 40% Cs_xWO₃/TiO₂ composites compared with Cs_xWO₃ (PDF No.81‐1244) as well as standard cards of anatase TiO₂ (PDF No.71‐1166). As can be seen in Fig. 1a, the diffraction peaks at 13.66°, 23.40°, 27.68°, 33.86°, 36.54°, 44.48°, and 49.21° correspond to the standard characteristic peaks of the (100), (002), (200), (112), (202), (212), and (220) crystal planes of Cs_xWO₃, respectively^[28]. It is demonstrated that Cs_xWO₃ is successfully prepared using the solvothermal method, and the introduction of TiO₂ into the solvothermal reaction does not alter the structural features of Cs_xWO₃. The diffraction peaks at 25.28°, 37.79°, 48.04°, 53.88°, 55.07°, and 62.69° correspond to the standard characteristic peaks at (101), (004), (200), (105), (211), and (204) crystal planes of anatase TiO₂, respectively^[29]. It is shown that anatase TiO₂ is successfully prepared by hydrolysis. The solvothermal reaction does not change the structural features of TiO₂. For Cs_xWO₃/TiO₂ composites, all peaks can be attributed to Cs_xWO₃ or TiO₂ without other impurity peaks. The successful formation of Cs_xWO₃/TiO₂ heterojunction was demonstrated. It is particularly noted that, as shown in Fig. 1b, the intensity of the diffraction peaks of Cs_xWO₃ gradually increased with the increase of Cs_xWO₃ content.

  2.2   Analytical characterization of the chemical bonding state

  XPS was further applied to study the chemical states of atoms in TiO₂, Cs_xWO₃, and 40% Cs_xWO₃/TiO₂. The results are presented in Fig. 2.

  Figure 2

  

  Figure 2.  (a) Survey, (b, c) W4f, and (d, e) Ti2_p XPS spectra of different samples

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  As shown in Fig. 2a, the chemical binding energies of about 458.80 eV (Ti2p_3/2), 530.03 eV (O1s), 724.21 eV (Cs3d), and 35.45 eV (W4f_7/2) characterize the elements Ti, O, Cs, and W. The successful formation of Cs_xWO₃/TiO₂ heterojunction was further confirmed. The core energy level W4f XPS spectra of Cs_xWO₃ and 40% Cs_xWO₃/TiO₂ are shown in Fig. 2b and 2c. The higher energies, 35.79 and 37.95 eV, in W4f_7/2 and W4f_5/2 belong to W⁶⁺. The lower energies, 34.24 and 36.29 eV, in W4f_7/2 and W4f_5/2 are from the reduced W⁵⁺. Similarly, in 40% Cs_xWO₃/TiO₂ complexes, the higher energies, 35.45 and 37.43 eV, in W4f_7/2 and W4f_5/2 belong to W⁶⁺. The lower energies, 33.91 and 36.03 eV, in W4f_7/2 and W4f_5/2 come from reduced W⁵⁺. The interconversions between W⁶⁺ and W⁵⁺ provide the electron transfer and small polariton absorption. This favors carrier migration^[30]. For Cs_xWO₃ and its composites, it was observed that the W4f peak was shifted to the low‐energy region (35.79 eV negatively shifted by 0.34 eV), while the Ti2p peak was shifted to the high‐energy region (458.61 eV positively shifted by 0.19 eV) (Fig. 2d and 2e). The displacement of these peaks is due to the transfer of electrons from TiO₂ to Cs_xWO₃^[22].

  2.3   Morphology analysis

  To study the surface morphologies of the synthesized 40% Cs_xWO₃/TiO₂ nanoparticles, SEM analyses were carried out, and the results are shown in Fig. 3.

  Figure 3

  

  Figure 3.  (a‐c) SEM images, (d‐g) EDS element mappings, and (h) EDS spectrum of 40% Cs_xWO₃/TiO₂

  Inset: atomic fractions of the elements in 40% Cs_xWO₃/TiO₂.

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  As shown in Fig. 3a‐3c, Cs_xWO₃ showed nanoparticles with a polyhedral structure. TiO₂ was spherical, granular, and uniformly attached to the surface of Cs_xWO₃ nanoparticles. The successful formation of Cs_xWO₃/TiO₂ heterojunction was further confirmed. This structure facilitates the migration of photogenerated electrons between Cs_xWO₃ and TiO_2, reducing the recombination rate of photogenerated electron (e^-)‐hole (h⁺) pairs. Typical energy dispersive spectroscopy (EDS) element mappings and EDS spectrum of 40% Cs_xWO₃/TiO₂ nanoparticles are given in Fig. 3d‐3h. The inset in Fig. 3h shows the atomic fraction content of 40% Cs_xWO₃/TiO₂. The mass fraction content of Cs_xWO₃ in the composite product was calculated to be about 40% by using the atomic fraction content of elemental W and Ti to represent Cs_xWO₃ and TiO₂, respectively. This result was consistent with the content of the composite product in the previous experimental section. It can be seen from the EDS element mappings that the Cs_xWO₃ nanoparticles were uniformly bound to TiO₂ nanoparticles. EDS spectral analysis showed that the signals of each element appeared individually. The formation of Cs_xWO₃/TiO₂ heterojunction was further confirmed. SEM and EDS analysis results showed that the composites were solvent heat‐treated at 180 ℃. TiO₂ nanoparticles formed a stable crystalline shape while also firmly composited to the surface of Cs_xWO₃ nanoparticles.

  2.4   Microstructure analysis

  To further investigate the microstructure of the synthesized 40% Cs_xWO₃/TiO₂ nanoparticles. TEM analyses were carried out, the results are shown in Fig. 4.

  Figure 4

  

  Figure 4.  (a‐c) TEM images and (d) HRTEM images of 40% Cs_xWO₃/TiO₂

  Inset: corresponding lattice streaks.

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  As shown in Fig. 4a‐4c, the polyhedral structure of Cs_xWO₃ nanoparticles was combined with spherical TiO₂ nanoparticles on the surface. It was consistent with the performance of the SEM images. The successful formation of Cs_xWO₃/TiO₂ heterojunction was further confirmed. In Fig. 4d, the lattice streaks between the arrows above the nanoparticles correspond to the face‐to‐face spacing of the (002) and (200) crystal faces of Cs_xWO₃. The spacing was 0.38 and 0.32 nm, respectively. The lattice striations between the arrows below the nanoparticles are consistent with the facet spacing of the (101) crystal face of TiO₂. The spacing was 0.35 nm. It can be seen that the bonding between Cs_xWO₃ and TiO₂ was relatively uniform and tight.

  2.5   Analysis of optical properties of materials

  Fig. 5a and 5b show the UV‐Vis DRS and band gaps (E_g), respectively. As shown in Fig. 5a, the single Cs_xWO₃ had a wide absorption width. It is photo‑ responsive in the UV‐Vis as well as in the NIR. The single TiO₂ could only absorb UV light below 400 nm. The absorption edge was at 380 nm. when Cs_xWO₃ was compounded with TiO₂. The absorption peak was weakened in the 200‐400 nm UV range. But almost no effect occurred. The absorption peak was enhanced in the Vis range of 400‐790 nm. Its absorption spectrum was greatly enhanced in the Vis region. Fig. 5b shows the calculated band gap of the samples. The band gap of each sample was calculated based on the tangent. The E_g values of Cs_xWO₃, TiO₂, and 40% Cs_xWO₃/TiO₂ nanocomposites were 2.75, 3.34, and 2.65 eV, respectively. The band gap of the composites was reduced compared to the single Cs_xWO₃ with TiO₂. Fig. 5c and 5d show the valence band XPS (VB‐XPS) analysis of Cs_xWO₃ and 40% Cs_xWO₃/TiO₂. As can be seen from the figures, the E_VB (VB energy of valence band) of Cs_xWO₃ and 40% Cs_xWO₃/TiO₂ were 2.50 and 2.65 eV, respectively^[31]. Fig. 5e shows the UV‐Vis‐NIR DRS. The absorption peaks of the 30% to 50% binary complexes in the NIR optical range increased with the increase of the content of Cs_xWO₃. This is because the moderate introduction of TiO₂ enhances the localized surface plasmon resonance and small polariton jump effect of Cs_xWO₃. When the content of Cs_xWO₃ was 55% or above. The absorption peaks of the binary complexes in the NIR optical range started to decrease gradually. This is due to the agglomeration of Cs_xWO₃ nanoparticles as the content of Cs_xWO₃ increased. The Cs_xWO₃/TiO₂ heterojunction structure is disrupted. It can be seen that the moderate introduction of TiO₂ reduces the band gap and broadens the photo‐response range compared to a single material. It also improves the reduction of e^- and oxidation of h⁺.

  Figure 5

  

  Figure 5.  (a) UV‐Vis DRS and (b) band gaps of Cs_xWO₃, TiO₂, and 40% Cs_xWO₃/TiO₂; VB‐XPS spectra of (c) Cs_xWO₃ and (d) 40% Cs_xWO₃/TiO₂; (e) UV‐Vis‐NIR DRS of Cs_xWO₃ and Cs_xWO₃/TiO₂ composites

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  As shown in Fig. 6a (the test frequency was 10 000 Hz), the Mott‐Schottky curve of Cs_xWO₃ was positive, which shows that it is an n‐type semiconductor. The flat band potential of Cs_xWO₃ was 0.21 V. It is known that the VB energy (E_VB) and flat band energy of TiO₂ are 1.68 and -1.92 eV, respectively^[32]. As known from the above, the VB energy and conduction band energy (E_CB) of Cs_xWO₃ were 2.50 and -0.25 eV, respectively, and the VB and CB energies of TiO₂ were 1.68 and -1.92 eV, respectively. Compared to Cs_xWO₃, TiO₂ had a higher CB potential. As shown in Fig. 7, the electrons were transported from TiO₂ to Cs_xWO₃, which enhances the Cs_xWO₃ catalytic ability. This was consistent with the findings in Fig. 2.

  Figure 6

  

  Figure 6.  Mott‐Schottky curves of (a) Cs_xWO₃ and (b) 40% Cs_xWO₃/TiO₂

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  Figure 7

  

  Figure 7.  Transfer process diagram of photogenerated carrier

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  2.6   Pore size distribution analysis

  The specific surface areas and pore size distributions of the materials were analyzed, and the results are shown in Fig. 8.

  Figure 8

  

  Figure 8.  (a) N2 adsorption‐desorption isotherms, (b) pore size distribution curves, (c) specific surface areas in BJH, and (d) DFT modes of different samples

  Inset: corresponding enlarged isotherms (a) and curves (b).

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  Fig. 8a and 8b show the N₂ adsorption‐desorption isotherms and pore size distributions of some of the materials and their complexes tested at 77 K. It can be seen that the adsorption isotherms of Cs_xWO₃, TiO_2, and their complexes belong to the type Ⅳ line type in the IUPAC classification. For all the samples, when p/p₀ was about 1, the adsorption of N₂ was small relative to pressure at low pressure, the desorption curve lagged at medium pressure, and the adsorption was small at high pressure. And the pore size distribution in Fig. 8b was mainly in 0‐35 nm and 2‐35 nm. This indicates that a small portion of micropores (< 2 nm), as well as most mesopores (2‐50 nm), mainly exist in Cs_xWO₃, TiO₂, and their composites. The inset of Fig. 8b demonstrates the pore size distribution of each material in the range of 0‐7.5 nm. Among them, the microporous distribution of single materials was richer than that of composites. 40% Cs_xWO₃/TiO₂ had almost no micropores, which might be attributed to the fact that some small‐sized TiO₂ particles fill the micropores of the composites. Fig. 8c and 8d show the specific surface area analysis in Barret‐Joyner‐Halenda (BJH) and density function theory (DFT) modes, respectively. As can be seen from the figures, the Cs_xWO₃/TiO₂ composite differed from carbon materials such as graphene. It does not have a relatively large specific surface area and therefore does not have extremely strong dark adsorption properties. The specific surface area in the figure showed a trend of increasing, then decreasing. This was consistent with the performance of the dark adsorption trend in the subsequent photocatalytic process.

  2.7   Analysis of photocatalytic property

  The dark adsorption curves of Cs_xWO₃, TiO_2, and their composites on MB are shown in Fig. 9a. The dark adsorption of Cs_xWO₃ was 22.9%. The adsorption of TiO₂ on MB was almost 0 (0.4%). The adsorption effects of the composites were all between 20% and 30%. The enlarged portion of Fig. 9a shows dark adsorption plots of individual materials for 50 to 60 min. The adsorption effect showed a trend of increasing and then decreasing, and was consistent with the specific surface area of the composites. The highest adsorption effect of 27% was achieved when the loading of Cs_xWO₃ was 40%. After 30 min of dark adsorption, the concentration of the MB solution was almost unchanged. It can be inferred that the MB solution reached the dark reaction adsorption equilibrium state. The composites showed the highest photocatalytic degradation activity when the Cs_xWO₃ content was 40%. Fig. 9b shows the degradation profile of single and its composites for photocatalytic degradation of MB under mercury lamp (simulated UV). The degradation rate of MB reached 94.6% at 30 min. The solution degradation was complete at 40 min. Fig. 9c shows the degradation curve of MB photodegraded by a single and its composites under a xenon lamp (simulated Vis). The MB solution reached 91.4% degradation at 120 min. Fig. 9d shows the degradation curves of MB photodegraded by a single and its composites under the infrared (IR) lamp (simulated NIR). Since the time was on the long side, we added a set of blank controls (without any catalysts, other conditions were the same). From the blank control group, it can be seen that there was almost no degradation of the MB dye under the condition of no catalyst. The influence of factors such as the long time for this experiment was excluded. The degradation rate of the MB solution under NIR light reached 70.7% at 6 h. The results showed that the photocatalytic activities of the composites were all superior to those of single Cs_xWO₃ and TiO₂. Combined analytical (TEM, SEM, XRD, XPS) analyses show that the heterostructures were formed by solvothermal reaction between Cs_xWO₃ and TiO₂. Combined with Fig. 7, it could be seen that the electrons on TiO₂ were transferred to the surface of Cs_xWO₃. It enhances the carrier concentration on the surface of Cs_xWO₃ and reduces the probability of photocomposite e^-‐h⁺ generation. When the content of Cs_xWO₃ was low, the carrier concentration formed by it was insufficient. It is difficult to effectively avoid the rapid compounding of e^- and h⁺. When the content of Cs_xWO₃ was high, the particles were attracted to each other to form agglomerates during the solvent‐thermal reaction. The heterogeneous structure was destroyed, leading to a decrease in photocatalytic activity.

  Figure 9

  

  Figure 9.  (a) Dark adsorption curves and photocatalytic degradation under (b) UV, (c) Vis, and (d) NIR of MB by different samples

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  Fig. 10a and 10b show the pseudo‐primary kinetic models fitted for the photocatalytic degradation of MB dye by Cs_xWO₃, TiO₂, and their composites under simulated Vis and NIR light, respectively. The fitting process started at the photocatalytic reaction′s beginning (after the dark reaction′s end). The curves showed a linear relationship. The values of k under the Vis light (Fig. 10a) were 0.001 5 min^-1 (Cs_xWO₃), 0.001 2 min^-1 (TiO₂), 0.007 6 min^-1 (30% Cs_xWO₃/TiO₂), 0.018 min^-1 (40% Cs_xWO₃/TiO₂), 0.006 2 min^-1 (50% Cs_xWO₃/TiO₂), 0.005 8 min^-1 (55% Cs_xWO₃/TiO₂), and 0.005 2 min^-1 (60% Cs_xWO₃/TiO₂). The values of k under NIR light (Fig. 10b) were 0.021 h^-1 (Cs_xWO₃), 0.002 2 h^-1 (TiO₂), 0.13 h^-1 (30% Cs_xWO₃/TiO₂), 0.16 h^-1 (40% Cs_xWO₃/TiO₂), 0.12 h^-1 (50% Cs_xWO₃/TiO₂), 0.11 h^-1 (55% Cs_xWO₃/TiO₂), and 0.094 h^-1 (60% Cs_xWO₃/TiO₂). The trend of rate degradation constants was consistent with the results in Fig. 9.

  Figure 10

  

  Figure 10.  First‐order kinetic modeling of photocatalytic degradation of MB dye by different samples under (a) Vis and (b) NIR

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  Fig. 11 shows the photocatalytic analysis of 40% Cs_xWO₃/TiO₂ under ionic interference, which included photocatalytic degradation plots under UV, Vis, and NIR light. NaCl and Na₂CO₃, which were commonly used in the dyeing process, were used as interfering ions. As can be seen from the graphs, the addition of NaCl and Na₂CO₃ decreased the photocatalytic rate under UV light. However, it still reached 99.7%. The photocatalytic degradation rate under Vis and NIR light was decreased, but still had an excellent photocatalytic degradation rate (Vis: 91.4% for the blank group, 85.2% for NaCl, and 83.7% for Na₂CO₃. NIR: 70.7% for the blank group, 67.1% for NaCl, 65.8% for Na₂CO₃). It can be seen that the photocatalyst can still maintain a high catalytic activity under the interference of some ions.

  Figure 11

  

  Figure 11.  Photocatalysis of 40% Cs_xWO₃/TiO₂ under ionic interference under (a) UV, (b) Vis, and (c) NIR

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  Fig. 12 shows the photocatalytic analysis of 40% Cs_xWO₃/TiO₂ under the influence of different pH levels. Acidic conditions (cationic interference) were simulated by adjusting the pH to 4 with 0.5 mol·L^-1 HCl. Alkaline conditions (anionic interference) were simulated by adjusting the pH to 10 with 1 mol·L^-1 NaOH. Fig. 12a‐12c shows the photocatalytic effect in UV, Vis, and NIR, respectively. From the plots, it can be seen that the photocatalytic effect increased under acidic conditions. Under alkaline conditions, the photocatalytic effect decreased, but it still had high activity. It can be seen that the photocatalyst can still maintain high catalytic activity under the interference of anions and cations. It illustrates the wide application of photocatalysts.

  Figure 12

  

  Figure 12.  Photocatalytic test of 40% Cs_xWO₃/TiO₂ under the influence of different pH values under (a) UV, (b) Vis, and (c) NIR

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  The stability of the catalyst plays an important role in the photodegradation process. The 40% Cs_xWO₃/TiO₂ composite was selected for five repetitions of degradation of MB under Vis light. As shown in Fig. 13, the photocatalytic performance of the composite did not significantly decrease after five cycles (87.6%). It indicates that the prepared photocatalyst have excellent photocatalytic activity and stability.

  Figure 13

  

  Figure 13.  Cycling experiment of MB degradation by 40% Cs_xWO₃/TiO₂ composite

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  The SEM and TEM images, XRD patterns of 40% Cs_xWO₃/TiO₂ after the results of cyclic testing experiments are shown in Fig. 14. It can be seen from the figure that there was almost no change in the samples after cyclic testing experiments. It shows the high stability of the composites.

  Figure 14

  

  Figure 14.  (a, b) SEM images, (c, d) TEM images, and (e) XRD patterns of 40% Cs_xWO₃/TiO₂ after cyclic testing

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  2.8   Analysis of reactive radicals

  Many active species were produced during the photocatalytic degradation of dyes. These included e^-, h⁺, hydroxyl radicals (·OH), and superoxide radicals (·O₂^-). These species play different roles in different photocatalytic systems. The role of active species in the photocatalytic process is inferred to explore the photocatalytic mechanism by capturing experiments. In the capture experiments, isopropanol (IPA) acted as a trapping agent for ·OH. Benzoquinone (BQ) was used as a trapping agent for ·O₂^-. Ethylenediaminetetraacetic acid (EDTA) was used as a trapping agent for h⁺. The results of the trapping experiments on 40% Cs_xWO₃/TiO₂ samples under Vis and NIR light are shown in Fig. 15a and 15b. The results show that under Vis light, the degradation rate of MB decreased from 91.4% to 69.5% with the addition of BQ, 55.4% with the addition of IPA, and 34.8% with the addition of EDTA rapidly. Under NIR light, the degradation rate of MB decreased from 70.7% to 64.4% upon adding BQ, to 49.2% with IPA added, and to 66.9% with EDTA added. Based on these results, it is inferred that the main active species are h⁺ and ·OH, while ·O₂^- plays a lesser role. The effect of ·OH under NIR light was lower than that of Vis light. It is due to the lower energy of NIR light, which reduces the conversion of ·OH.

  Figure 15

  

  Figure 15.  Effect of different capture agents on the degradation rate of 40% Cs_xWO₃/TiO₂ composites against MB under (a) Vis and (b) NIR

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  2.9   Analysis of transient photocurrent, electrochemical impedance, and PL

  From Fig. 16a, it can be seen that the transient photocurrent of 40% Cs_xWO₃/TiO₂ was much larger than that of Cs_xWO₃ and TiO₂. This also indicates that the composites produce more carriers under the light. The heterojunction structure formed by Cs_xWO₃ and TiO₂ enhances the photogenerated carriers′ separation. The inset in Fig. 16b shows an enlarged view of the 0‐6 kΩ section. It is also evident from the electrochemical impedance distribution (Fig. 16b) that the arc radius (impedance) of the composites was much smaller than that of the single Cs_xWO₃ containing TiO₂. It can be seen that the composites had a much higher electron transfer efficiency in the photocatalytic process. This was consistent with the results of the transient photocurrent test. Fig. 16c shows the photoluminescence (PL) spectra of Cs_xWO₃ and 40% Cs_xWO₃/TiO₂ composites. As shown, the 40% Cs_xWO₃/TiO₂ composite had the lowest PL intensity under the same conditions of light excitation, indicating that it has the lowest e^-‐h⁺ complexation probability. This result was in complete agreement with the photoelectric efficiency analysis described above, further indicating that the 40% Cs_xWO₃/TiO₂ composite can accelerate the separation of photogenerated e^-‐h⁺ pairs.

  Figure 16

  

  Figure 16.  (a) Transient photocurrent response curves, (b) EIS, and (c) PL spectra of different samples

  Inset: 0‐6 kΩ impedance amplification curves.

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  2.10   Photocatalytic activity mechanism

  From the above analytical results, the photocatalytic mechanism of Cs_xWO₃/TiO₂ composites under UV and Vis light was proposed as shown below in Fig. 17. Cs_xWO₃ was excited under UV/Vis light irradiation. Photogenerated h⁺ were generated in the VB, and e^- were generated in the CB. e^- in the CB on TiO₂ could be transferred to the CB on Cs_xWO₃. e^- on Cs_xWO₃ combined with O₂ to form ·O₂^-. At the same time, h⁺ in the VB of Cs_xWO₃ could be transferred to the VB of TiO₂. H₂O would combine with the h⁺ in the VB on TiO₂ to form H₂O⁺. After that, H₂O⁺ decomposed to form ·OH and H⁺. These free radicals as well as h⁺ will degrade the dye (MB) to CO₂ and H₂O.

  Figure 17

  

  Figure 17.  Photocatalytic mechanism under Cs_xWO₃/TiO₂ composite under UV/Vis light

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  The photocatalytic mechanism of Cs_xWO₃/TiO₂ composites under NIR light is shown in Fig. 18 below. NIR light irradiates the Cs_xWO₃ surface and generates the LSPR effect. The large number of carriers in the Cs_xWO₃/TiO₂ composite generates a collective electromagnetic oscillatory behavior accompanied by the excitation of hot e^-‐h⁺ pairs. These hot e^- are excited to high‐energy LSPR states with corresponding h⁺ on OS1 (occupied state 1) or OS2 (occupied state 2). The vicinity of the bottom of the Cs_xWO₃ CB possesses an abundant impurity energy level for W⁵⁺. Therefore, the hot e^- in the LSPR state preferentially jumps to the CB of the lower energy surface state Cs_xWO₃ after relaxation. The h⁺ then remains in TiO₂. The effective separation of photogenerated e^- and h⁺ is realized. O₂ dissolves in the solution combined with e^- in the CB of Cs_xWO₃ to form ·O₂^-. At the same time, the cavities located in TiO₂ combine with H₂O to form H₂O⁺. After that, H₂O⁺ decomposes to form ·OH and H⁺. Eventually, these ·O₂^-, ·OH, and h⁺ catalytically produced by Cs_xWO₃/TiO₂ composites under NIR excitation are involved in the degradation reactions, such as MB. The dye (MB) is degraded into CO₂ and H₂O.

  Figure 18

  

  Figure 18.  Photocatalytic mechanism under Cs_xWO₃/TiO₂ composite under NIR light

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  Eq.1‐6 reveal the mechanism of photocatalytic degradation of dyes by Cs_xWO₃/TiO₂ composites.

  $ \mathrm{Cs}_x \mathrm{WO}_3 / \mathrm{TiO}_2+h \nu \rightarrow \mathrm{Cs}_x \mathrm{WO}_3\left(\mathrm{e}^{-}\right)+\mathrm{TiO}_2\left(\mathrm{~h}^{+}\right) $

  (1)

  $ \mathrm{Cs}_x \mathrm{WO}_3\left(\mathrm{e}^{-}\right)+\mathrm{O}_2 \rightarrow \mathrm{Cs}_x \mathrm{WO}_3+\cdot \mathrm{O}_2^{-} $

  (2)

  $ \begin{aligned} \mathrm{TiO}_2\left(\mathrm{~h}^{+}\right)+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{TiO}_2+\mathrm{H}_2 \mathrm{O}^{+} \rightarrow & \\ \cdot & \mathrm{OH}+\mathrm{H}^{+} \end{aligned} $

  (3)

  $ \begin{array}{r} \mathrm{h}^{+}+\mathrm{MB} \rightarrow \text { Degradation products } \rightarrow \\ \qquad \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \end{array} $

  (4)

  $ \begin{array}{r} \cdot \mathrm{OH}+\mathrm{MB} \rightarrow \text { Degradation products } \rightarrow \\ \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \end{array} $

  (5)

  $ \begin{array}{r} \mathrm{O}_2^{-}+\mathrm{MB} \rightarrow \text { Degradation products } \rightarrow \\ \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \end{array} $

  (6)

  3.   Conclusions

  In summary, full‐spectrum responsive Cs_xWO₃/TiO₂ composite photocatalysts were prepared by the solvothermal method. TEM, SEM, XRD, and XPS results show that the Cs_xWO₃ nanoparticles were tightly bonded with TiO₂ particles to form a heterogeneous structure. DRS results indicate that the optimal amount of Cs_xWO₃ in the Cs_xWO₃/TiO₂ composite photocatalysts was 40%. The composite photocatalyst had a small band gap (2.65 eV) and a low complexation efficiency of photoinduced carriers. Additionally, the transient photocurrent and electrochemical impedance analysed a higher electron transfer efficiency of 40% Cs_xWO₃/TiO₂. The photocatalytic effect of the composite photocatalysts was significantly improved compared with that of the single material under the irradiation of UV, Vis, or NIR light sources. After five cycles of treatment, the 40% Cs_xWO₃/TiO₂ nanocomposites exhibited high and stable photocatalytic activity. In addition, the method has the advantage of a simple process. It provides a reference value for the design of efficient and stable full‐spectrum photocatalyst materials as well as practical production.

  
  Acknowledgements: This work was financially supported by Shanghai Natural Science Foundation (Grant No.21ZR426200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Science Foundation of National Innovation Center of Advanced Dyeing and Finishing Technology (Grant No.2022GCJJ22) and National Natural Science Foundation of China (Grant No.51703123). Credit authorship contribution statement:   LIU Zhangyong: Writing‐original draft, supervision, visualization, conceptualization, methodology, validation; XU Lihui: Supervision, writing‐review & editing, funding acquisition, resources; YANG Yue: Methodology, validation; WANG Liming: Supervision, methodology; PAN Hong: Supervision, methodology; HUANG Xinzhe: Methodology, validation; FU Xueqiang: Methodology, validation; ZHANG Yingxiu: Methodology, validation; DOU Meiran: Methodology, validation; WANG Meng: Methodology, validation; TENG Yi: Methodology, validation.
  
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• 

  Figure 1  XRD patterns of (a) 40% Cs_xWO₃/TiO₂ and (b) a series of Cs_xWO₃/TiO₂ composites

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  Figure 2  (a) Survey, (b, c) W4f, and (d, e) Ti2_p XPS spectra of different samples

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  Figure 3  (a‐c) SEM images, (d‐g) EDS element mappings, and (h) EDS spectrum of 40% Cs_xWO₃/TiO₂

  Inset: atomic fractions of the elements in 40% Cs_xWO₃/TiO₂.

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  Figure 4  (a‐c) TEM images and (d) HRTEM images of 40% Cs_xWO₃/TiO₂

  Inset: corresponding lattice streaks.

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  Figure 5  (a) UV‐Vis DRS and (b) band gaps of Cs_xWO₃, TiO₂, and 40% Cs_xWO₃/TiO₂; VB‐XPS spectra of (c) Cs_xWO₃ and (d) 40% Cs_xWO₃/TiO₂; (e) UV‐Vis‐NIR DRS of Cs_xWO₃ and Cs_xWO₃/TiO₂ composites

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  Figure 6  Mott‐Schottky curves of (a) Cs_xWO₃ and (b) 40% Cs_xWO₃/TiO₂

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  Figure 7  Transfer process diagram of photogenerated carrier

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  Figure 8  (a) N2 adsorption‐desorption isotherms, (b) pore size distribution curves, (c) specific surface areas in BJH, and (d) DFT modes of different samples

  Inset: corresponding enlarged isotherms (a) and curves (b).

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  Figure 9  (a) Dark adsorption curves and photocatalytic degradation under (b) UV, (c) Vis, and (d) NIR of MB by different samples

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  Figure 10  First‐order kinetic modeling of photocatalytic degradation of MB dye by different samples under (a) Vis and (b) NIR

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  Figure 11  Photocatalysis of 40% Cs_xWO₃/TiO₂ under ionic interference under (a) UV, (b) Vis, and (c) NIR

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  Figure 12  Photocatalytic test of 40% Cs_xWO₃/TiO₂ under the influence of different pH values under (a) UV, (b) Vis, and (c) NIR

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  Figure 13  Cycling experiment of MB degradation by 40% Cs_xWO₃/TiO₂ composite

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  Figure 14  (a, b) SEM images, (c, d) TEM images, and (e) XRD patterns of 40% Cs_xWO₃/TiO₂ after cyclic testing

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  Figure 15  Effect of different capture agents on the degradation rate of 40% Cs_xWO₃/TiO₂ composites against MB under (a) Vis and (b) NIR

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  Figure 16  (a) Transient photocurrent response curves, (b) EIS, and (c) PL spectra of different samples

  Inset: 0‐6 kΩ impedance amplification curves.

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  Figure 17  Photocatalytic mechanism under Cs_xWO₃/TiO₂ composite under UV/Vis light

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  Figure 18  Photocatalytic mechanism under Cs_xWO₃/TiO₂ composite under NIR light

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•