Visible light-induced catalytic performance of composite photocatalyst synthesized with nanomaterials WO₃ and two-dimensional ultrathin g-C₃N₄

Most of the organic pollutants (especially colored dye pollutants) in industrial wastewater are difficult to degrade, and it is difficult to meet the standards for drinking water through conventional or enhanced treatment processes (Brillas 2020). Colored dyes are mutagenic, carcinogenic and allergic (Dassanayake et al. 2018). Studies have shown that even wastewater containing only small amounts of dyes could seriously affect the growth of aquatic organisms (Garcia et al. 2021). Among them, most dyeing industries prefer to use cationic dyes. However, cationic dyes are more toxic than anionic dyes because of their aromatic ring structure of delocalized electrons and special synthetic methods (Doltabadi et al. 2016). Therefore, it has become a challenge to find ways to efficiently degrade cationic dyes (such as rhodamine B (RhB), methylene violet (MV) and methylene blue (MB)) with low cost and high efficiency in industrial wastewater (Cui et al. 2015).

There are several methods for the treatment of dye wastewater, including ion exchange, flocculating sedimentation (Doltabadi et al. 2016), advanced oxidation technology (Dassanayake et al. 2018; Nidheesh et al. 2018), activated carbon adsorption (Doltabadi et al. 2016) and biodegradation (Dhall et al. 2012). Some of these physical technologies (such as activated carbon adsorption and flocculating sedimentation) have significant treatment efficacy, but they only transfer organic dyes from the liquid phase to the solid one for separation, which causes serious secondary pollution. The biological treatment method is not suitable for the treatment of dye wastewater which has certain toxicity and is difficult to degrade. Advanced oxidation technology has drawn wide attention because it is a way to use some strong oxidizing active groups (such as hydroxyl radicals, sulfate radicals and so on) to oxidize and decompose organic pollutants to generate small molecules such as water and carbon dioxide (Nidheesh et al. 2018). The most commonly used advanced oxidation methods for the removal of organic dyes are mainly ozonation and Fenton methods, electrochemical oxidation and photocatalytic oxidation. Among them, the ozonation and Fenton methods have significant effects on the degradation of dyeing pollutants. However, it requires continuously adding oxidants to the wastewater, which increases the treatment cost. At the same time, a large amount of sludge is produced in the Fenton oxidation process, while the initial investment of ozonation is large and more undegradable substances may be generated during the degradation process (Wang et al. 2019). The electrochemical oxidation method has demonstrated advantages for treating dyes wastewater, such as the space-saving electrocatalytic device, high degradation efficiency and stable treatment effect. However, a lot of electric energy would be consumed in the process of electrochemical reaction leading to a high processing cost. Furthermore, the electrode easily incurs side reactions such as hydrogen evolution, oxygen evolution and more.

The photocatalytic oxidation is one of the more advanced oxidation technologies. Its notable advantage is that photocatalysis is an efficient and environmentally friendly technology for the treatment of dyeing wastewater. However, ultraviolet photocatalytic energy consumption is huge which is hardly realistic in engineering applications (Dassanayake et al. 2018). Therefore, we grew an interest in the synthesis of catalysts responsive to visible light.

In recent years, the photocatalyst g-C₃N₄(CN) has drawn extensive attention from researchers because of its visible light-responsive performance, simple preparation methods, low price of raw materials and controllable electronic properties (Cui et al. 2020). CN is a type of two-dimensional (2D) polymer material mainly composed of carbon and nitrogen atoms. The band gap of CN prepared via the traditional thermal polycondensation method is around 2.6 eV. Compared with photocatalyst TiO₂, CN has a more active photocatalytic performance in the visible light range and a higher utilization rate of sunlight (Xu et al. 2016). However, CN still has some weaknesses, such as a small specific surface area, a severe recombination of photo-generated electron–hole pairs, poor dispersibility and a big band gap, which seriously confine its application in energy and environment (Pattnaik et al. 2019; Balakrishnan & Chinthala 2022).

In order to improve the visible light catalytic activity of CN, researchers have mainly reported two ways. One is to regulate the appearance of CN to synthesize ultrathin nanoplatelet g-C₃N₄ (UCN); the other is to construct heterojunctions with CN and metallic oxide. For the first way via regulating the appearance of CN to prepare UCN, many documents reported the improvement of its visible light catalytic activity. Lu et al. (2014) produced NH₃ by using ammonium chloride solid powder, and thermally exfoliated the bulk CN in an NH₃ atmosphere to prepare 2D nanoplatelet UCN with a specific surface area of 52.9 m²/g, which was 19 times of bulk CN (2.9 m²/g). Su et al. used the same method to prepare 2D nanoplatelet UCN. When the optimal dosage of 2D nanoplatelet UCN was 0.4 g/L, its degradation rate on enrofloxacin was 81.7％ within 1 h, and its reaction rate constant was 4.1 times higher than that of bulk CN under visible light irradiation conditions (Su et al. 2017). The visible light-induced catalytic activity of 2D nanoplatelet UCN was enhanced significantly. The structure of the 2D ultrathin nanosheet accelerated the electron transmission rate in the photocatalytic process and reduced the recombination rate of photo-generated electron–hole pairs (Aggarwal et al. 2021). However, 2D nanoplatelet UCN still suffers from insufficient visible light absorption and rapid recombination rate of photo-generated electron–hole pairs. For the second way, it is to establish a heterojunction structure with two photocatalytic materials. The key point of constructing heterojunctions is to find two photocatalysts with matching energy bands, which can effectively improve the separation efficiency of photo-generated electron–hole, leading to a desirable visible light-responsive catalytic activity. Therefore, to improve the photocatalytic performance of CN via constructing heterojunctions with metallic oxide, the selection of metallic oxide is a top priority. There are many metallic oxides with different energy bands, such as WO₃ (2.4–3.2 eV), ZnO (3.12 eV), MnO₂ (3.36 eV), Fe₂O₃ (2.06 eV) and so on (Jung et al. 2018; Xia et al. 2018; Geng et al. 2021; Wang et al. 2023). WO₃ is a common semiconductor catalyst with a band gap of 2.4–3.2 eV, which can match that of CN. It can capture the blue light part of the solar spectrum and be considered as a promising photocatalyst (Aravindraj & Roopan 2022). The heterojunction composite photocatalyst WO₃/CN is formed by the staggered band arrangement between CN and WO₃ (Lou & Xue 2016). The WO₃/CN can effectively reduce the band gap and the recombination rate of photo-generated electron–hole pairs, which can improve the photocatalytic properties of CN and photodegradation performance on refractory organic matter. Zhang et al. (2021) synthesized CN/WO₃ composite photocatalyst via low-temperature template method. The specific surface area of CN/WO₃ was 54.8 m²/g, 1.92 times that of CN. The photodegradation rate of CN/WO₃ was 4.27 times higher than that of CN. Wang et al. (2023) synthesized WO₃/CN photocatalyst via the hydrothermal method. Its band gap was 2.87 eV, and its photocatalytic activity was 6.4 times that of pure CN. Li & Du (2020) prepared composite photocatalyst WO₃/CN heterojunction by precipitation method and thermal polymerization. The degradation rate of WO₃/CN on Rhodamine B(RhB) reached 83% after 180 min of CN photocatalytic reaction. RhB was completely degraded when the WO₃/CN heterojunction kept reacting for 120 min. Nevertheless, the WO₃/CN heterojunction still has insufficient utilization of light energy and a high recombination rate of its photo-generated electron–hole pairs, which leads to a long photodegradation time, high energy gap or a low photodegradation rate.

According to the advantages and disadvantages of the prepared photocatalysts based on carbon nitride (CN) reported in the documents mentioned above, it was found that the photocatalytic performance of CN can be improved via regulating the appearance of CN to prepare UCN or synthesizing heterojunction with CN and WO_3. But even with this improvement, the enhancement of the active sites on its surface and visible light absorption capacity was still not sufficient. Hence, it is possible to further improve the photocatalytic performance of CN to prepare a composite photocatalyst by making full use of the advantages of 2D nanoplatelet UCN and WO₃ having a matching energy gap and adjust their appearances to be more favorable for light adsorption. The composite photocatalyst had a higher photo-generated electron transmission than CN, which led to a decrease in the recombination rate of photo-generated electron–hole pairs.

In this study, the foaming agent ammonium chloride was used to synthesize 2D nanoplatelet UCN via thermal polycondensation method which was previously introduced (Li et al. 2022). On this basis, the composite photocatalysts WO₃/2D-UCN (WU) were synthesized with WO₃ and UCN at different theoretical mass ratios via the precipitation method. SEM, X-ray diffraction (XRD), scanning probe microscopy (AFM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible near-infrared spectroscopy (UV-vis) and photoluminescence spectroscopy (PL) were used to characterize WU and explore its photocatalytic properties with different theoretical mass ratios of WO₃ to 2D-UCN. Under visible light irradiation (λ > 420 nm), the photocatalytic degradation mechanism of WU and its degradation efficacy on typical cationic dye-Rhodamine B (RhB) was investigated.

Urea (CH₄N₂O), melamine (C₃H₆N₆) and ammonium chloride (NH₄Cl) were all analytically pure, and purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Sodium tungstate (H₄Na₂O₆W) was offered by Shanghai Macklin Biochemical Technology Co., Ltd, concentrated hydrochloric acid (HCl, purity of 35% (mass percentage concentration)) was purchased from Guangzhou Biotechnology Co., Ltd, anhydrous ethanol (C₂H₆O) was bought from Tianjin Damao Chemical Reagent Factory. All reagents were analytical grade.

2D-UCN was synthesized via the thermal polycondensation method. First, weigh 3 g of melamine, 3 g of urea and 10 g of ammonium chloride and dissolve them with 50 mL of ultrapure water in a crucible. Next, place the crucible in the water bath at 80 °C with a shaking intensity of 300 r/min till the mixture solution dried out. Afterwards, transfer the remained substance in the crucible to a muffle furnace (XS5-5-1200, Shenzhen Zhongda Electric Furnace Factory, 2016) at a heating rate of 1 °C/min to 550 °C for 3 h to gain UCN. Then, cool the UCN to room temperature (25 °C), grind and bag it for use. For comparative purposes, the bulk g-C₃N₄ (BCN) was prepared by repeating the above steps without adding ammonium chloride.

The composite photocatalyst WO₃/UCN heterojunction (WU) was prepared via the precipitation method. First, weigh 200 mg of UCN and dissolve it in a beaker with 50 mL of ultrapure water and keep stirring at the speed of 300 r/min for 30 min to obtain UCN suspension. Next, disperse the suspension by ultrasonic apparatus (410HT, Shenzhen Jato Ultrasonic Cleaner Equipment. Co., Ltd, 2018) for 30 more minutes to obtain the UCN solution. Then, add 0.012 M of sodium tungstate solution dropwise to the UCN solution, disperse it uniformly by the ultrasonic apparatus for 1 h and put the beaker on a magnetic stirrer (89-2, Jintan Land Automation Instrument Factory, 2017) agitating for 9 h. Afterwards, add 6 mL of concentrated HCl into the beaker dropwise and keep stirring for 1.5 h. WU was collected by centrifugation of the mixture solution with sodium tungstate, UCN, concentrated HCl and ultrapure water mentioned above. Then washed WU several times with absolute ethanol and ultrapure water, respectively. The WU was dried at 90 °C overnight and ground into a fine powder for use. Finally, the composite photocatalysts were prepared under different conditions, changing the WO₃/UCN mass ratios, the stirring time, the amount of concentrated HCl, and the mixing time after adding concentrated HCl. The desirable heterojunction photocatalyst (WU_opt) was obtained at the optimum synthesis conditions which were determined via the orthogonal experimental design method.

The synthesis conditions had an important impact on the photocatalytic performance of the visible light-induced catalyst WU heterojunction. The orthogonal test design method was adopted to determine the optimum synthesis conditions. In the orthogonal test design, L_n (f^m) was used to represent the effects of various factors, where f and m denote the number of levels and factors, respectively and n denotes the total number of experiments at different test conditions. The factors of the synthesis experiments included the dosage of two-dimensional Ultrathin g-C₃N₄ (for short UCN), the theoretical mass ratio of WO₃ to UCN, the stirring time, the amount of concentrated HCl, the mixing time after adding concentrated HCl (denoted as M_UCN, M_WO3:M_UCN, t₁, V_HCl, t₂, respectively). Four levels were set for each factor. The evaluation index was set as the photodegradation rate (η) of RhB which was employed in the orthogonal test design to determine the optimal synthesis conditions of WO₃/UCN heterojunction.

The orthogonal test design consisted of several steps. First, a standard orthogonal test design table was selected. The ‘factor’ columns of the table were filled with their own levels. Next, experiments were conducted according to the orthogonal test design scheme. The test results were entered into the ‘index’ column in the L_n (f^m) format. Then, the sum (S) of each index value corresponding to the same level of each factor column was calculated. In addition, the average values of ‘K’ were gained through ‘S’ which were divided by the number of occurrences in each level. Finally, the extremum -"R’ value, the difference between the maximum and minimum values of ‘K’, was determined. ‘K’ was applied to denote the optimum combination of factors corresponding to the optimal experimental conditions, and ‘R’ was used to evaluate the degree of influence of each factor on the photocatalytic properties of the visible light-induced catalyst WO₃/UCN heterojunction in the synthesis process. Higher values of ‘R’ were associated with higher levels of influence on an index.

Hence, the influence degree of each factor on the photocatalytic performance of the WO₃/UCN heterojunction was evaluated based on the value of ‘R’, and the optimum synthesis conditions of WO₃/UCN were determined accordingly. Then the optimal heterojunction WO₃/UCN (WU_opt) was applied to do tests.

For different photocatalysts UCN, WO₃, and WU, their photocatalytic degradation efficiencies on RhB were tested under visible light irradiation conditions. The first step was to prepare the simulated dyeing wastewater by taking RhB as a target pollutant in accordance with the procedure as below, for instance, weigh 1 g of RhB, dissolve it in ultrapure water and obtain RhB solution (1 g/L). The simulated RhB wastewater (20 mg/L) was prepared by diluting a small amount of stock solution (1 g/L). The second step was to conduct photocatalytic degradation experiments. First, add 50 mg of photocatalyst into the beaker with 50 mL of RhB solution and keep stirring (200 r/min) for 60 min at 25 °C under dark conditions to reach adsorption–desorption equilibrium. Afterwards, turn on the cooling circulating water device and place the mixture solution with RhB and photocatalyst under the irradiation of visible light (10 cm away from the liquid surface of the beaker) for photocatalytic degradation reaction. During the experiments, the illuminant was a 500 W xenon lamp (HF-GHX-XE-300, Shanghai Hefan Instrument Co., Ltd, 2019) and the ultraviolet light was filtered with a 420 nm filter. Next, the mixture solution samples were taken at 10 min intervals and filtered with a 0.22 μm filter, and then the absorbance of the solution was measured by UV-vis spectrophotometer (λ = 554 nm, UV-5100, Shanghai Yuanxi Instrument Co., Ltd, 2016). The properties of the photocatalysts aforementioned were tested by repeating the above experimental steps.

To explore the stability of a photocatalyst, continuous cyclic degradation experiments were conducted. Once a photodegradation experiment of the photocatalyst (WU_opt) with optimum degradation efficiency at the visible light irradiation was complete, the used WU_opt was reclaimed by centrifugation and recovered for the second photodegradation experiment at the same conditions. This cycle was repeated five times for conducting photocatalytic degradation experiments of WU_opt on RhB. Its degradation rate was tested each time.

In order to explore the photodegradation mechanism of the optimum catalyst WU_opt, it is significant to disclose the possible oxidative active species produced in the photocatalytic process. The capture experiments were conducted with the steps below. For instance, to take RhB as a dyeing pollutant, the photodegradation experimental reactor was placed under the illuminance of a 500 W xenon lamp (HF-GHX-XE-300, Shanghai Hefan Instrument Co., Ltd, 2019) and the ultraviolet light was filtered with a 420 nm filter. Next, three scavengers, such as ethylenediaminetetraacetic acid (EDTA, 2 mmol/L), p-benzoquinone (BQ, 1 mmol/L) and isopropanol (IPA, 10 mmol/L) were added into the reactor one by one under the visible light irradiation condition. The scavengers reacted with free possible radicals, such as hole (h⁺), superoxide radical (⁠⁠) and hydroxyl radical (⁠⁠). The photocatalytic degradation rate of WU_opt on RhB was tested accordingly.

The orthogonal test design was conducive to determining the optimum synthesis conditions of the composite photocatalyst WO₃/UCN (WU). Five factors were selected below: UCN dosage (M_UCN), mass ratio of WO₃ to UCN (M_WO3:M_UCN), stirring time (t₁), concentrated hydrochloric acid volume (V_HCl) and mixing time after adding concentrated HCl (t₂). Four levels were set for each factor. According to the extremum R, the influence degree of each factor on photocatalytic performance was evaluated, which illustrated the optimal synthesis conditions of WU.

The orthogonal experimental results of the synthesis conditions on photocatalytic performance are shown in Table 1. The optimum synthesis conditions were determined based on the index-removal rate of photocatalysts on RhB (η). Higher values of R were associated with higher levels of influence on an index. For the five factors mentioned above, their corresponding R values were ranked in the sequence below (see Table 1), R-M_WO3:M_UCN > R-t1 > R-M_UCN > R-t₂ > R-V_HCl. It revealed that the mass ratio of WO₃ to UCN (M_WO3:M_UCN) has the strongest influence on the visible light-induced catalytic degradation performance of photocatalysts among the five factors, followed by t₁, M_UCN and t₂ while concentrated HCl volume (V_HCl) has the minimum impact.

Therefore, the influence degree of each factor on the removal rate of photocatalysts on RhB (η) was ranked as M_WO3:M_UCN > t₁ > M_UCN > t₂ > V_HCl. In addition, R-M_WO3:M_UCN, R-t₁ and R-M_UCN were significantly higher than R-t₂ and R-V_HCl, which disclosed that M_WO3: M_UCN, M_UCN and t₁ were the key factors influencing the photodegradation performance of visible light-responsive catalysts. ‘K’ was applied to represent the optimum combination of factors corresponding to the optimal synthesis conditions. A smaller value of K was connected with a lower level of impact on an index. As shown in Table 1, the optimal levels of the five factors (M_WO3: M_UCN, M_UCN, t₁, V_HCl and t₂) in the orthogonal test were M_WO3: M_UCN 1:1, M_UCN 100 mg, t₁ 9 h V_HCl 6 mL and t₂ 1.5 h corresponding to their maximum K-value, respectively. Therefore, the optimal synthesis conditions of the desirable composite photocatalyst were determined as M_WO3: M_UCN = 1:1, M_UCN = 100 mg, t₁ = 9 h, V_HCl = 6 mL and t₂ = 1.5 h. The corresponding prepared composite photocatalyst was denoted as WU_opt.

The elemental contents of W, O, N and C in WU_opt were analyzed with the help of Energy Dispersive Spectroscopy (EDS), which are shown in Figure 3(e). The weight percentage (wt%) of the four elements are 28.99, 9.49, 33.92 and 27.60%, respectively (Table 2). The distribution of elements W, O, N and C in the photocatalyst WU_opt was uniform and WO₃ nanoparticles were homogeneously loaded on the surface of UCN nanosheets, which further confirmed the successful preparation of WU_opt heterojunction.

The band gap can indicate the catalytic activity of the composite photocatalyst. A smaller band gap means a stronger light absorption capacity and a higher visible light catalytic activity. The band gaps of different composite photocatalysts were calculated with the Taucplot method. For UCN, WO₃ and WU_opt, their band gaps were 2.44, 2.38 and 2.13 eV, respectively (Figure 5(b)). Hence, the visible light absorption ability of composite photocatalyst WU_opt was much stronger than that of UCN. The synthesis of WU_opt with WO₃ and UCN led to a higher visible light catalytic activity.

The XPS analysis results mentioned above manifested that WU_opt had demonstrated the valence characteristics of elements contained in UCN and WO₃. Therefore, it strongly proved that the WO₃/UCN was successfully prepared.

The active free radicals play a main role in the photodegradation process. How to test the main types of active species is significant. They can be determined via free radical capture experiments. Effects of capture agents for different active species on the degradation rate of RhB are shown in Table 3. The photocatalytic degradation rate of WU_opt was only 2.1% after adding p-benzoquinone (BQ) which could capture the radicals (Table 3). In other words, the was the dominant active radical when the RhB was degraded. Likewise, after adding IPA and EDTA, the photocatalytic degradation rate of WU_opt on RhB was 49.3 and 76.2%, respectively, which revealed that was also the main active radical whereas the oxidation ability of h⁺ was negligible in the process of experiments.

X is the absolute potential of the semiconductor, EC is the electron-free energy (about 4.5 eV) relative to the hydrogen potential level and E_g is the band gap of the semiconductor.

A two-dimensional ultrathin carbon nitride g-C₃N₄ (UCN) was a new visible light-induced photocatalyst with a thickness of 4–7 nm. UCN was prepared by the foaming agent ammonium chloride via the thermal polycondensation method. By adding WO₃ to UCN, a noval promising composite catalyst WO₃/UCN(WU) with strong visible light-responsive photodegradation performance was gained via the precipitation method.

The optimal synthesis conditions were set as the theoretical mass ratio of WO₃ to UCN as 1:1, the stirring time of the mixture solution with UCN and sodium tungstate as 9 h, concentrated HCl as 6 mL which was poured into the mixture solution with an extra stirring time of 1.5 h. Under optimal conditions, the composite visible light-responsive catalyst WU has owned a satisfactory catalytic performance and multicycle photodegradation stability. WU_opt possessed a two-dimensional ultrathin structure with a thickness of 4–15 nm, which could accelerate the rate of electronic transmission and reduce the recombination rate of photo-generated electron–hole pairs. Besides, WU_opt had a wrinkled and porous appearance with a large number of active sites, conducive to incident light reflection and the utilization rate enhancement of light energy. WU_opt has presented broader infrared spectra than UCN. The light absorption edge of WU_opt was 524 nm while that of UCN was 465 nm. The band gap of WU_opt was 2.13 eV which was 0.3 eV less than that of UCN by characterization of UV-vis diffuse reflectance spectroscopy (DRS). Thus, the recombination rate of photo-generated electron–hole pairs of WU_opt decreased significantly. Taking advantage of the aforementioned characteristics, the optimal photocatalyst WU_opt exhibited about 2.1 times higher photocatalytic degradation efficiency on RhB (97.2%) within 50 min than that of UCN (only 53.4%). After being reused five cycles, WU_opt maintained a high RhB removal efficiency (91.8%) that was only 5.5% lower than that (97.3%) of the initial clean one.

The photodegradation mechanism of the novel composite photocatalyst WU_opt on RhB was that strong oxidation and degradation behaviors from the active free radicals ⁠, and h⁺ were generated in the process of photocatalysis at the visible light irradiation. The active free radicals and acted as a main role whereas h⁺ was a negligible active substance in the photodegradation process.

The authors would like to thank the National Natural Science Foundation of China (Nos. 51378129 and 51108094) and the Natural Science Foundation of Guangdong Province (No. 2020A030313321) for their financial support.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.