Fabrication of CdS/Ti3C2/g-C3N4NS Z-scheme composites with enhanced visible light-driven photocatalytic activity

The Ti3C2 and g-C3N4NS were obtained first, and the CdS/Ti3C2/g-C3N4NS Z-scheme composites were prepared via a facile hydrothermal synthesis, and their photocatalytic properties were investigated. The g-C3N4NS with a high surface area displayed higher adsorption and degradation capacity. Compared with Ti3C2/g-C3N4NS and CdS, the visible light photocatalytic activity of CdS/Ti3C2/g-C3N4NS composites was improved. The as-synthesized CTN-4:1 composite exhibited outstanding photocatalytic performance for degradation of orange II, approximately 3.2 and 10.7 times higher than that of Ti3C2/g-C3N4NS and CdS, respectively. The fabrication of CdS/Ti3C2/g-C3N4NS Z-scheme heterostructure using Ti3C2 as electron transfer medium improved the separation ability of the photoinduced e−-h+ pairs, thereby leading to the improvement of visible light-driven photocatalytic activity. This finding provides new insights into the construction of high efficiency Z-scheme heterostructure photocatalyst.


Introduction
With the development of industry, water pollution has become a serious problem. Photocatalysis has attracted much attention in resolving water pollution (Xu et al. 2019a(Xu et al. , 2019bSun et al. 2020). The two-dimensional (2D) layered materials have been widely studied due to their special physical and chemical properties Feng et al. 2018;Low et al. 2018). Ti 3 C 2 MXene is a promising 2D-layered photocatalyst material due to its fast electron transfer capability and rich surface properties .
Graphitic carbon nitride (g-C 3 N 4 ), as a nontoxic and easily prepared semiconductor material, was widely applied in the field of photocatalysis (Koyyada et al. 2021;Ibrahim and Gondal 2021;Yu et al. 2021;Phoon et al. 2021). Furthermore, g-C 3 N 4 nanosheets (g-C 3 N 4 NS) were obtained by hydrothermal treatment of g-C 3 N 4 , and their photocatalytic degradation properties were further improved (Dong et al. 2017;Wu et al. 2019). CdS is an excellent visible lightsensitive photocatalyst Zhang et al. 2016). However, the photocatalytic degradation effect of CdS was limited due to the problems of fast recombination of the photoexcited carriers and the photocorrosion (Peng et al. 2019;Zou et al. 2020). The CdS was combined with other semiconductor materials to form composites, which can effectively inhibit the recombination of the photoinduced e − -h + pairs and improve the photocatalytic degradation performance (Kang et al. 2017;Ran et al. 2017;Peng et al. 2019;Zou et al. 2020). Previous studies have demonstrated that the formation of CdS/g-C 3 N 4 heterojunction composites can effectively improve the photocatalytic degradation activity (Wang et al. 2019a, b, c;Liu et al. 2020a, b;Chen et al. 2019;Fiqar et al. 2021;Güy 2020).
In addition, the formation of Z-scheme heterojunction in the composite catalyst can also efficiently improve the photocatalytic activity (Zhao et al. 2019;Du et al. 2021;Li et al. 2019a, b, c;Jiang et al. 2018;Pourshirband and Nezamzadeh-Ejhieh 2021;Jo and Selvam 2017;Liu et al. 2021). The transport of photogenerated electron-hole pairs follows Z-shape pathway in Z-scheme composite, which is beneficial to improve the separation ability of e − -h + pairs and maintain strong oxidation-reduction potency . Successful ternary Z-scheme photocatalysts generally utilize noble metals as electron transfer components. To replace expensive metal particles, several promising MXene materials with small band gaps or metallic conductivity have been developed . Ti 3 C 2 MXene owns abundant hydrophilic functional groups (-OH, -O, -F) on its surface, which can construct a compact heterostructure with other semiconductors and provide many active sites. And its excellent conductivity ensures efficient carrier transfer capability (Gao et al. 2015;Zhou et al. 2017;Ai et al. 2019;Xiao et al. 2020;Fang et al. 2019). Recently, some studies found that the preparation of ternary Z-scheme heterostructure composites using Ti 3 C 2 as electron transfer medium can improve the photocatalytic degradation of pollutants (Ai et al. 2019;Li et al. 2019a, b, c;Ding et al. 2019;Sun et al. 2020;Yuan et al. 2021;Wu et al. 2020). To further improve visible lightdriven photocatalytic activity, it is still a great challenge to develop novel Z-scheme heterostructure photocatalysts with high efficiency and stability using Ti 3 C 2 as a charge transport medium via a simple method.
Herein, the g-C 3 N 4 NS and Ti 3 C 2 were obtained using urea and Ti 3 AlC 2 as raw materials, respectively, and then a 2D-layered CdS/Ti 3 C 2 /g-C 3 N 4 NS Z-scheme photocatalyst was prepared via a facile hydrothermal synthesis. The photocatalytic activity of as-prepared ternary composites was investigated under vis irradiation using orange II as an organic pollutant. This study may provide useful insights into utilizing Ti 3 C 2 as the mediator for controlling the transfer of photogenerated carriers in Z-scheme photocatalysis.

Preparation of Ti 3 C 2 and g-C 3 N 4 NS
Ti 3 C 2 was obtained by exfoliating the Al layers from Ti 3 AlC 2 with 40 wt% HF at 30 °C for 25 h. The precipitates were then filtered, washed with DI, and dried in the oven for 10 h. Synthesis of the g-C 3 N 4 NS: the urea was calcined at 500 °C for 6 h to obtain a g-C 3 N 4 bulk material, and 0.24 g of g-C 3 N 4 was added to 60 mL 6 M HCl, and treated with ultrasound for 30 min. The mixture was transferred to an autoclave (100 mL) and then heated at 110 °C for7 h. The precipitates were centrifuged, washed, and dried at 60 °C for 12 h in a vacuum oven. Preparation of Ti 3 C 2 /g-C 3 N 4 NS composite 0.5 g of Ti 3 C 2 and 0.6 g of g-C 3 N 4 NS samples were added to 50 mL DI under stirring, respectively. The two mixtures were then mixed and sonicated for 1 h. Subsequently, the suspension was heated at 80 °C in a water bath until the solvent was evaporated. The product was then dried at 60 °C for 12 h in a vacuum oven.

Fabrication of CdS/Ti 3 C 2 /g-C 3 N 4 NS and CdS/ g-C 3 N 4 NS composites
The obtained Ti 3 C 2 /g-C 3 N 4 NS (0.5 g) was dispersed in 60 mL of CdCl 2 solution and further stirred for 15 min, and certain amounts of Na 2 S were then added to the mixture solution under stirring. Subsequently, the mixture was transferred to an autoclave (100 mL) and then heated at 200 °C for 6 h. After the hydrothermal treatment, the precipitates were centrifuged, washed, and then dried at 85 °C for 12 h. The obtained CdS/Ti 3 C 2 /g-C 3 N 4 NS composites by controlling the mass ratio of CdS and Ti 3 C 2 /g-C 3 N 4 NS of 2:1, 3:1, 4:1, and 5:1 were denoted as CTN-2:1, CTN-3:1, CTN-4:1, and CTN-5:1, respectively. Schematic illustration of the synthesis procedure of CdS/Ti 3 C 2 /g-C 3 N 4 NS is depicted in Scheme 1. Meanwhile, the CdS/g-C 3 N 4 NS was obtained by the method of preparing Ti 3 C 2 /g-C 3 N 4 NS, in which the mass ratio of CdS and g-C 3 N 4 NS in CdS/g-C 3 N 4 NS was consistent with that in CTN-4:1.

Measurement of photocatalytic activity
The orange II solution (100 mL, 20 mg/L) was adjusted to pH 3 by HCl. The photocatalyst (0.04 g) was dispersed in an orange II solutions. The beaker was placed in a double-wall quartz cooling water jacket, and the temperature of the reaction mixture was maintained at 20 °C by circulating water. Mixed solution was stirred in a dark room for 1 h to reach the absorption-desorption equilibrium. The 300 W Xe lamp (PLS-SXE300c) with a UV cut-off filter (λ ≥ 420 nm) was selected as visible light source. All experimental runs were performed at 20 °C under continuous stirring. During the process of light irradiation, 5 mL suspension was taken out and centrifuged. The concentrations of orange II were determined using a UV-vis spectrometer (UV-8000) at 486 nm. Total organic carbon (TOC) was determined using TOC analyzer (Shimadzu TOX-5000). The degradation efficiency and TOC removal efficiency were calculated according to the equations in the literature (Kassahun et al. 2021).

Characterization
The characterization and performance of photocatalyst are shown in the Supporting Information (S1).

Structures and morphologies of samples
Fig. S1 shows the morphologies and the XRD patterns of g-C 3 N 4 and g-C 3 N 4 NS. The g-C 3 N 4 shows an irregularly wrinkled morphology, and the prepared g-C 3 N 4 NS presents a thin sheet shape. The XRD patterns show that two samples contain two peaks at about 13.0° and 27.4°, matching with the (100) and (002) crystal planes of g-C 3 N 4 , respectively. However, the peak of (002) plane of g-C 3 N 4 NS is stronger, indicating that the interlayer structure of g-C 3 N 4 NS is strengthened (Dong et al. 2017). Figure 1 displays the XRD patterns of the as-prepared samples. The peaks of Ti 3 C 2 at 2θ of 8.84°, 18.4°, 27.5°, and 60.7° correspond to (002), (004), (006), and (110) crystal planes of Ti 3 C 2 Cai et al. 2018), respectively. For Ti 3 C 2 /g-C 3 N 4 NS sample, the diffraction peaks of the Ti 3 C 2 and g-C 3 N 4 NS were presented (Xu et al. 2019a, b). For CdS/Ti 3 C 2 /g-C 3 N 4 NS composites, the CdS with cubic phase (JCPDS No 80-0019) is obtained in addition to Ti 3 C 2 and g-C 3 N 4 NS phases. No other peak is detected, thereby indicating that no other product is obtained. XRD results verify that CdS/Ti 3 C 2 /g-C 3 N 4 NS ternary composites are composed of Ti 3 C 2 , g-C 3 N 4 NS and CdS. With the increase of CdS content, the diffraction peaks of Ti 3 C 2 and g-C 3 N 4 NS decrease.
Scheme 1 Schematic illustration of the synthesis procedure of CdS/Ti 3 C 2 /g-C 3 N 4 NS  Figure 2 shows the Raman spectra of Ti 3 C 2 , CdS, g-C 3 N 4 NS, and d: CTN-4:1. The Raman peaks located at 326 cm −1 (A1g mode) corresponds to the out-of-plane stretching vibrations of Ti and C , and the peaks at 1456 and 1519 cm −1 can be attributed to the D-band and G-band of Ti 3 C 2 (Naguib et al. 2014). Additionally, the typical peaks at 301 and 600 cm −1 are assigned to the firstorder LO Raman peak and second-order LO phonon vibrational mode of cubic CdS (Rengaraj et al. 2011), respectively. And the peak at 474 cm −1 is referred as the Raman signal of g-C 3 N 4 NS (Sutar et al. 2020). For the CTN-4:1 composite, the Raman peaks of Ti 3 C 2 , g-C 3 N 4 NS, and CdS are shown, respectively. Raman spectra further confirm that the ternary composite is composed of Ti 3 C 2 , g-C 3 N 4 NS, and CdS. Raman spectra further confirmed the existence of Ti 3 C 2 , g-C 3 N 4 NS, and CdS in the ternary composite. Figure 3 presents the morphologies of the Ti 3 C 2 , Ti 3 C 2 /g-C 3 N 4 NS, and CdS/Ti 3 C 2 /g-C 3 N 4 NS samples. The prepared Ti 3 C 2 exhibits a multilayered morphology with accordion-like structure (Fig. 3a). In Fig. 3b, the g-C 3 N 4 NS is tightly coated around the Ti 3 C 2 , forming a rough surface. A rough surface is favorable because it enlarges the surface area and exposure of active sites, leading to improved charge transfer on the photocatalyst surface Xie et al. 2016;Tong et al. 2021). As shown in Fig. 3c-f, the SEM images of CdS/Ti 3 C 2 /g-C 3 N 4 NS composites show that the CdS particles are deposited and tightly attached on the interlayer and surface of Ti 3 C 2 /g-C 3 N 4 NS. With the further increase in CdS amount, the interlayer of the composite occupies a large amount of CdS particles, and the 2D-layered structure of the ternary composite is weakened (Fig. 3f). The EDX spectra and the corresponding elemental mapping were carried out for the CTC-4:1 sample (Fig. S2). Ti, C, N, S, and Cd elements are uniform distributed in the composite.
The HRTEM image of CTC-4:1 is shown in Fig. 4. The lattice spacings of 0.250 nm and 0.336 nm correspond to the (006) and (111) crystal facets in Ti 3 C 2 and CdS (Cao et al. 2013;Ding et al. 2019), respectively. The g-C 3 N 4 NS with low crystallinity do not show obvious lattice fringes (Wang et al. 2019a, b, c). A compact and continuous interface exists among Ti 3 C 2 , CdS, and g-C 3 N 4 NS, which can provide efficient separation and diffusion of photoinduced carriers (Kang et al. 2017). Figure 4 shows the XPS spectra of the CTN-4:1 and CdS. The XPS survey spectrum indicates that the CTN-4:1 is composed of Ti, C, N, S, and Cd elements (Fig. S3).

XPS analysis of samples
In the Ti2p XPS spectrum of CTN-4:1 (Fig. 5A), the peaks at around 454.7, 458.7, and 463.9 eV are attributed to the Ti-C, Ti2p3/2, and Ti2p1/2 in Ti 3 C 2 He et al. 2020). For the C1s spectrum (Fig. 5B), the peaks at approximately 286.26 eV can correspond to C-Ti on the CTN-4:1 surface (Peng et al. 2017), while the peaks at 284.6 and 288.36 eV are attributed to the C-C and C-N = C bonds in g-C 3 N 4 NS (Diao et al. 2020).
The S2p and Cd3d XPS spectra of CTN-4:1 and CdS were compared. In the S2p and Cd3d spectra of CTN-4:1, peaks at 161.9 and 163.2 eV can be corresponded to S2p 1/2 and S2p 3/2 of S 2+ (Ai et al. 2019) (Fig. 5C), and the peaks at 412.3 and 405.5 eV can be ascribed to Cd3d 3/2 and Cd3d 5/2 of Cd 2+ (Ai et al. 2019) (Fig. 5D). Compared with CdS, the binding energies of S2p and Cd3d in the CTN-4:1 show a shift to the right, suggesting the strong interactions between CdS and Ti 3 C 2 /g-C 3 N 4 NS, forming a heterojunction composite Wang et al. 2019a, b, c).

BET analysis of samples
The nitrogen adsorption-desorption isotherms of as-prepared samples are shown in Fig. S4. The specific surface area and pore volume values of all samples are summarized in Table 1. The surface area and pore volume values of obtained g-C 3 N 4 NS are evidently higher than that of g-C 3 N 4 . In contrast to the Ti 3 C 2 /g-C 3 N 4 NS, the CdS-loaded ternary composites increase the surface area and pore volume values. With the introduction of CdS, the CdS nanoparticles are dispersed on the surface of Ti 3 C 2 /g-C 3 N 4 NS, and the specific Fig. 2 Raman spectra of the as-prepared samples surface areas of the ternary composites are increased. However, with the further increase of CdS amount, the surface area of the sample decreases because a large number of CdS nanoparticles occupy the interlayer of the composite. Among the ternary composites, the CTN-4:1 sample presents the largest surface area, which may provide more reactive active sites for photocatalytic degradation (Elfiad et al. 2018).

Photocatalytic degradation of orange II
The effects of catalyst dosage and initial pH on orange II degradation using the CTN-4:1 as photocatalysts were determined by single factor experiments, and the results are shown in Fig. S5. The optimum conditions of photocatalytic degradation are 0.04 g catalyst and pH 3.0.
In Fig. S6, the adsorption and degradation curves of orange II exhibit that g-C 3 N 4 NS has higher adsorption and degradation performances than that of g-C 3 N 4 , which agrees well with the BET analysis. Figure 6 A presents the degradation efficiency (DE) of orange II using different samples as the photocatalysts. And the reaction rate constants in different catalyst systems were calculated by the pseudofirst-order model (Fig. 6B) and are listed in Table 2. Ti 3 C 2 and CdS show low degradation effect on orange II. Compared with Ti 3 C 2 /g-C 3 N 4 NS, the CdS-loaded ternary composites show enhanced photocatalytic degradation activity. From Fig. 6A and Table 2, it can be seen that the CTN-4:1 exhibits the highest degradation efficiency of orange II up to 91.5% within 60 min, and corresponding rate constant value (k = 4.2 × 10 −2 min −1 ) is about 3.2 times and 10.7 times higher than that of Ti 3 C 2 /g-C 3 N 4 NS and CdS, respectively. The UV-vis spectral variation of orange II solution with Fig. 3 SEM images of samples (a Ti 3 C 2 , b Ti 3 C 2 /g-C 3 N 4 NS, c CTC-2:1, d CTC-3:1, e CTC-4:1, and f CTC-5:1) different treatment times in the CTN-4:1 system is exhibited in Fig. 6C. It is clear that the typical adsorption peak of orange II at 486 nm decreased rapidly with the time increasing. Simultaneously, the color of the filtered solution changes from deep to colorless, indicating the degradation of orange II. The TOC removal efficiency of orange II in the CTN-4:1 system is about 76% at 60 min, indicating that orange II can be effectively mineralized (Fig. S7A). The reusability of the CTN-4:1 was examined by successive catalytic experiments under identical conditions. After each cycle, the catalyst was centrifuged, washed, and then dried at 85 °C for 12 h. As shown in Fig. S7B, the CTN-4:1 still maintains high catalytic performance after four cycles. In Fig. S6C, no evident difference is found for the structure of CTN-4:1 before and after the photocatalytic reaction, which indicates excellent reusability and stability. Figure 7A and Fig. S8A present the UV-vis absorption spectra of the samples. Ti 3 C 2 shows no obvious absorption edge. Compared with the CdS and Ti 3 C 2 /g-C 3 N 4 NS, all CdS-loaded ternary composites can cause an appropriate shift to long wavelengths. This indicated that the CdS/ Ti 3 C 2 /g-C 3 N 4 NS composite exhibits improved visible light response. The CdS and g-C 3 N 4 belong to direct transition semiconductor (Babar et al. 2019;Li et al. 2019a, b, c). The (αhν) 1/2 versus hv plots of samples (CdS, g-C 3 N 4 CN, and g-C 3 N 4 ) are analyzed ( Fig. 7B and Fig. S8B). And the band gap energy (Eg) data are shown in Fig. 7B and Fig. S8B. The E g of obtained g-C 3 N 4 NS (E g = 2.77 eV) is lower than that of g-C 3 N 4 (E g = 2.87 eV).

Analysis of photocatalytic mechanism
The PL spectra and transient photocurrent responses of the samples are shown in Fig. 8 and Fig. S9. From the Fig. 8 A and B, it can be seen that the CTN-4:1 shows lower PL intensity and higher photocurrent density compared with Ti 3 C 2 /g-C 3 N 4 NS and CdS. As shown in Fig. S9A and 9B, among the composites, CTN-4:1 exhibits the lowest PL intensity and maximum photocurrent intensity, indicating its higher separation efficiency of the photoinduced e − -h + pairs (Liu et al. 2020a, b;Yue et al. 2019;Allagui et al. 2019;Li et al. 2019a, b, c). As seen in Fig. 8C, by contrast, CTN-4:1 exhibits the smaller impedance radius, indicating the higher charge transfer and separation ability (He et al. 2019;Ren et al. 2020). The formation of heterojunction in ternary composites can promote the transfer and separation of photoinduced carries, which leads to the enhancement of photocatalytic activity.
Active species trapping experiments were conducted through orange II degradation in the CTN-4:1 system. In the test, BZQ (5 mM), EDTA (5 mM), and TBA (5 mM) were used as the scavengers of •O 2 − , h + , and •OH species, respectively. As shown in Fig. 9A, the DEs of orange II decrease after the addition of the scavengers, indicating that the •O 2 − , h + , and •OH species are formed, and •O 2 − and h + species are the main active species in the degradation system. Meanwhile, the ESR spectra of DMPO-•O 2 − in the CdS, Ti 3 C 2 /g-C 3 N 4 NS, and CTN-4:1 degradation systems were determined under vis irradiation. As shown in Fig. 9B, the six characteristic peaks corresponding to DMPO-•O 2 − are detected in the CTN-4:1 degradation system. The generation efficiency of •O 2 − in the CTN-4:1 system is slightly higher than those of CdS and Ti 3 C 2 /g-C 3 N 4 NS.
As a comparison, the photocatalytic activity and active species of CdS/g-C 3 N 4 NS system were also analyzed. As shown in Fig. S10A, compared with CTN-4:1, CdS/g-C 3 N 4 NS catalysis exhibits lower photocatalytic activity for degradation of orange II under the same conditions. The trapping experiments for the CdS/g-C 3 N 4 NS system show that both •O 2 − and h + are the major active species during the degradation process (Fig. S10B). As presented in Fig. S10C, the generation efficiency of •O 2 − in the CdS/g-C 3 N 4 NS system is lower than those of CTN-4:1. As can be seen from Fig. S10D, the characteristic signal peaks of DMPO-•OH signals can also be observed for the CTN-4:1 system, while the •OH radicals are basically not produced for the CdS/g-C 3 N 4 NS system. These results indicate that the formation of CdS/Ti 3 C 2 /g-C 3 N 4 NS Z-scheme heterojunction composites using Ti 3 C 2 , a charge transport bridge, can promote the production of the active species and effectively degrade orange II under vis irradiation. Fig. S11 shows the Mott-Schottky plots of the g-C 3 N 4 NS, Ti 3 C 2 , and CdS. The slopes of the lines are all positive, implying that the samples belong to n-type semiconductor. The flat potentials (E fb ) of g-C 3 N 4 NS, Ti 3 C 2 , and CdS are estimated to be − 1.11, − 0.22, and − 0.47 V vs. NHE by using Ag/AgCl (0.22 V vs. NHE) as the reference electrode, which are converted to − 0.89, 0, and − 0.25 V versus the normal hydrogen electrode (NHE), respectively. Normally, conduction band (E CB ) of an n-type semiconductor was about 0.1 eV more negative than its E fb (Sun et al. 2014;Ma et al. 2018). Thus, the E CB values of g-C 3 N 4 NS and CdS are estimated to be about − 0.99 and − 0.35 V vs. NHE, respectively. According to the band gap energies of samples, the valence band (E VB ) values of g-C 3 N 4 NS and CdS are calculated to be 1.78 and 1.92 V vs. NHE, respectively.
According to the above analysis, the plausible photocatalytic degradation mechanism of the CdS/Ti 3 C 2 /g-C 3 N 4 NS catalyst is depicted in Fig. 10. Under vis irradiation, e − -h + pairs can be generated on the CdS/Ti 3 C 2 /g-C 3 N 4 NS Z-scheme composite (Eq. 1). Ti 3 C 2 as a charge transport medium facilitates charge transfer. The photoinduced electrons in CdS can be transferred to Ti 3 C 2 . And the photogenerated holes in g-C 3 N 4 NS may migrate to the Ti 3 C 2 . As a result, the photogenerated carriers can be effectively separated by the Z-scheme heterostructure. Meanwhile, the E CB of g-C 3 N 4 NS (-0.99 V vs. NHE) is more negative than that of O 2 /•O 2 − (− 0.046 V) (Kandi et al. 2017), and the dissolved O 2 can combine with the electrons in the E CB of g-C 3 N 4 NS to produce •O 2 − in the photodegradation process (Eq. 2). In acidic degradation system, some of •O 2 − radicals  The photocatalytic materials that have been reported to degrade orange II are listed in Table S1. It is found that orange II can be effectively degraded using a small amount of CTN-4:1 as photocatalyst under visible light irradiation. In this study, prepared CdS/Ti 3 C 2 /g-C 3 N 4 NS composites have a high techno-economics in removing orange II.

Conclusions
In this work, a novel CdS/Ti 3 C 2 /g-C 3 N 4 NS Z-scheme composites with enhanced vis photocatalytic activity were prepared. In contrast to g-C 3 N 4 , obtained g-C 3 N 4 NS sample showed improved adsorption and photocatalytic degradation performances. It is found that the CdS/Ti 3 C 2 /g-C 3 N 4 NS Z-scheme heterojunction composites formed by using Ti 3 C 2 as a charge transport bridge could improve the vis absorption, enhance the separation ability of the photoexcited carriers, and promote the production of active species (•O 2 − , h + , and •OH), thus enhancing the photocatalytic degradation efficiency of orange II. Among them, the CTN-4:1 sample (4) O 2 − ∕h + + ∕OH + orangeII → CO 2 + H 2 O exhibited the highest vis photocatalytic activity. This study would provide useful insights into utilizing Ti 3 C 2 as the electron mediator in Z-scheme photocatalysis. Fig. 10 Proposed schematic diagram for the photocatalytic degradation of orange II with CdS/Ti 3 C 2 /g-C 3 N 4 NS catalyst