Copolymerization And Solvothermal Synthesis of S-Scheme Cds/g-C3N4 to Improve the Photocatalytic Degradation Activity of Rhb Under UV-Visible Light

. The synthesis of photocatalysts with high charge separation and transfer efficiency are of immense significance in the process of using photocatalysis technology for wastewater treatment. In this study CdS/g-C 3 N 4 photocatalyst presented an improved morphology in its characterization using techniques such as SEM, DRS, PL, MS, EIS, and XRD, and enhanced photodegradation of oxcarbazepine. Different composites were obtained as confirmed by the various characterization techniques studied, including CdS/g-C 3 N 4 photocatalyst. The synthesized photocatalysts showed high visible light absorption efficiency within a range of ~655 to 420 nm. S-Scheme CdS/g-C 3 N 4 photocatalyst demonstrated high photocatalytic activity ascribed to high load separation and transition as shown in PL, Photocurrent reaction and EIS. It is understandable that CdS/g-C 3 N 4 photocatalyst have confirmed to be one of the ultimate promising entrants for ground-breaking photocatalyst scheming.


Introduction
Air and water pollution as result of intensive human activity is undoubtedly a major concern worldwide. This challenge urges the scientific community to look for sustainable and green solutions without imposing more strain on the environment. The use of materials which can be activated by renewable energy sources such as sunlight could be an ideal solution. Bismuth based photocatalysts are semiconductor materials which are capable of generating electron; hole pairs under UV light irradiation. These evolved charge carriers can then react with the surrounding water and oxygen molecules to generate reactive radicals which can thus induce photo-degradation of various pollutants, toxic substances and also, viruses and bacteria. Thus, Carbon based photocatalysts are suitable for the purification and detoxification of air and water, and in the purification of fuel.
Since the discovery of the photocatalytic properties of TiO 2 by Fujishima and Honda for the first time in 1972 (Fujishima and Honda, 1972), many researches have been conducted on improving the efficiency of semiconductor photocatalysts . A major challenge is to suppress the recombination of photogenerated electron-hole pairs, which are easily recombined due to the strong coulomb force Ye et al., 2017). In addition, for single-component photocatalysts, the dilemma of their application is the lack of good compatibility between strong redox capability and light response range (Bi et al., 2015;Fu et al., 2018). That is, in order to obtain a sufficient redox capability for a specific reaction to occur on a single photocatalyst, a larger band gap is required. However, the higher utilization of solar energy, the smaller band gap is required to increase the ability of absorbing light (Fu et al., 2017;Zhao et al., 2015). Therefore, it is of great significance to modify the photocatalyst against these contradictions Najafian et al., 2018;Mahdiani et al., 2018;Wu et al., 2019).
Nowadays, the researchers mainly focus on the development of visible-light-response photocatalysts for the decomposition of toxic dyes and organic pollutants, such as C-based (i.e., g-C 3 N 4, CdS/g-C 3 N 4, CdS) (Hu et al., 2013) and other semiconductors . Among those materials, CdS/g-C 3 N 4 has been reported to possess an excellent visible-light-driven photocatalytic activity, but the absorption capacity of solar energy is still limited .
The design of heterostructure photocatalysts has been considered to be an effective approach to extend the light responsive range and enhance the photo-generated carriers separation rate (Yu et al., 2009;Zhang et al., 2018). Carbon-based heterojunctions have been designed to improve the efficiency of the single phases. For instance, CdS/g-C 3 N 4 nanocomposite exhibited higher photocatalytic activities for degradation of dye and phenol than CdS/g-C 3  However, this study spotlights on the capacity and efficacy of reformation schemes to upsurge visible light responsive photocatalytic degradation of the Rhodamine B by CdS/g-C 3 N 4 photocatalyst nanocrystals for wastewater management and environmental treatment in directive to scheme potential operations for solving environmental problems. Consequently, this material is well-known as eye-catching candidates for environmental applications such as water purification, purified fuel production and treatment of waste water. In this systematic study, CdS/g-C 3 N 4 photocatalyst nanocrystals with their high photocatalytic activity are agreeably offered with their distinctive structure and innovative preparation procedures, and the enlightened concepts are then emphasized on extending CdS/g-C 3 N 4 photocatalyst preparation and synthesis.

Preparation of CdS nanorods
CdS nanorods were prepared through a solvothermal method (Jang et al., 2007. In general, 9.26 g of CdCl 2 ·2.5H 2 O and 9.26 g of NH 2 CSNH 2 were added into the Teflon-lined autoclave with a total capacity of 200 mL. Then, after 120 mL ethylenediamine was added, the autoclave was heated to 160 °C and maintained for 36 hours. The yellow products were collected and washed several times with deionized water and ethanol respectively.

Preparation of bulk g-C 3 N 4
The g-C 3 N 4 was prepared by copolymerization method (Zhang et

Preparation of CdS/g-C 3 N 4 Nanorods
The CdS/g-C 3 N 4 core/shell nanorods were fabricated through a chemisorption and self-assembly method . Accurately weighting 0.06 g g-C 3 N 4 was mixed with 20 mL methanol. Then, the above suspension was ultrasonicated for 1 h to reach a homogeneous suspension. 1.5 g CdS was then added in the suspension and continuous stirred for 24 h. Finally, the methanol was removed by rotary evaporator and dried in vacuum oven at 60°C. The g-C 3 N 4 first adsorbs on the surface of CdS nanorods and then undergoes a rolling mechanism and a regrowth process during the reaction. Hence, the g-C 3 N 4 could curl up and wrap around the CdS nanorods to from the CdS/g-C 3 N 4 composites (Pan, et al., 2012). Similar synthesis procedure was used to prepare the CdS/g-C 3 N 4 -0 mg, CdS/g-C 3 N 4 -87 mg and CdS/g-C 3 N 4 -173 mg composites, respectively. To a portion of the sample, there was calcination of 550 o C.

Characterization
X -ray diffractograms (XRD, Rigaku, SmartLab) were used in the range of 10-80• 2θ to analyze the phase purity and crystallite size of the as-synthesized photocatalysts. The morphologies and microstructures were carried out using electron microscopy and electron microscopy (TEM) with high resolution transmission. Use of high -resolution electron microscope (HR -TEM) JEOL JEM 2100 with an accelerating voltage of 200 KV, XRD, and TEM imagery was performed. XRD patterns for centrifuged and dried samples were also documented using X -ray Bruker D8 Advance X -ray diffractometer with source Cu Kα (π = 1,5406 A °). A spectrophotometer (Thermo fisher Evolution 220) has documented UV -Vis diffuse reflectance spectra (DRS). The carbon contaminant as reference contact was used to standardize all binding energies (C 1s= 284.6 eV).
Transient photocurrent response measurements were performed on an electrochemical system CHI-660E (Chenhua, China) with a standard three -Na2SO4 solution electrode system (0.2 mol / L) to evaluate the electrical properties of the sample. The platinum electrode and the Ag / AgCl saturated electrode were respectively used as counter electrode and reference electrode. 0.020 g of photocatalyst was spread in 3 mL of ethanol with ultrasonic treatment to prepare the working electrode, and the mixture solution was then dip-coated onto an ITO glass working electrode.
We measured all binding energies as their indication point using the carbon contaminant (C 1s= 284.6 eV). Electron spin resonance measurements (ESR) were analyzed on the X -band of a The Mott-Schottky curves were performed on an electrochemical workstation using a regular three -electrode device (CHI-660E, China). An aqueous solution containing 0.1 M of Na2SO4 was utilized as the electrolyte. The counter-and reference electrodes were platinum wire, and Ag / AgCl (saturated KCl), respectively. The working electrode was photocatalyst film electrodes mounted on fluoride-tin oxide (FTO) glass which was washed 1.5 cm x 1.0 cm.
In order to assess the electrical properties of the samples, electrochemical impedance spectroscopy (EIS) and transient photocurrent response measurements were performed on an electrochemical system CHI-660E (Chenhua, China), with a standard three -electrode system in solution Na2SO4 (0.2 mol / L). The platinum electrode and the Ag / AgCl saturated electrode were respectively used as counter -electrode and reference electrode.
To prepare the working electrode, 0.020 g of the photocatalyst was distributed with ultrasonic treatment in 3 mL of ethanol, and the mixture solution was then dip-coated onto an electrode working in ITO glass. The catalyst's photoluminescence spectroscopy (PL) was measured using the FLS1000 transient fluorescence spectrometer from Edinburgh-state/. A 450 W ozone-free xenon lamp with an excitation wavelength of 425 nm was the source of the excitement.

Photocatalytic activity evaluation
In short, 0.025 g of photocatalysts were suspended in aqueous solutions of Rhodamine B (RhB),   The XPS spectra agrees with the XRD spectra that confirms the successful phase transformation of cadmium sulphate from CdS/C3N4 photocatalyst up to CdS-d as the calcination temperature increases with increasing concentration of the schemed photocatalyst.     All of these scavengers decreased the activity of CdS/g-C 3 N 4 , but p-benzoquinone among them decreased the activity to the utmost degree, exposing the primary responsibility of O2 − radical in the photocatalytic reaction to decay.

Proposed mechanisms
A probable mechanism for photoactivity over the CdS/g-C 3 N 4 photocatalyst samples was proposed ( Fig. 9). In the subsequent heterojunction device CdS acts as a sensitizer between band edge locations, while g-C 3 N 4 acts as a substratum. The g-C 3 N 4 ECB is less negative than the CdS ECB; thus, CdS can act as a sink for the produced electrons. So, during photocatalytic removal of RB, we thought that the electrons provided in the g-C 3 N 4 CB could transfer to the CdS CB to form a are also capable of directly oxidizing RhB to the affected degradation products. As a result, the separation of carriers from charging is impressively increasing. Thus, the likelihood of recombination charging carriers is reduced, which results in significantly increased photoactivity.  Survey XPS spectra of C3N4/CdS, Cd3d, O1s. N1s C1s and S2s (a) and high resolution XPS spectra of (b) C1s (c) S2p (d) Cd3d Figure 3 EDX images of (a) 0mg CdS/g-C3N4 (b) 87mg CdS/g-C3N4 (c) 173mg CdS/g-C3N4 (d) g-C3N4

Figure 9
Schematic diagram of photocatalytic reaction over the CdS/g-C3N4 nanocomposite /PS system.