Synthesis, characterization of Ag-doped CdS-WO 2 nanocomposite and effects of photocatalytic degradation in RhB under visible light irradiation

In this paper, the highly stable Ag/CdS-WO 2 nanocomposite was fabricated by a facile and capping agent-free hydrothermal technique. The fabricated Ag doped CdS-WO 2 nanocomposite were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and UV-vis diffuse reectance (DRS) spectroscopy. The photocatalytic performance of synthesized photocatalysts was evaluated for the photodegradation of rhodamine B (Rh B) under visible light irradiation (VLI). The parameters used for the optimization of the photocatalyst were pH, catalyst dose, oxidant dose, and irradiation time. Based on this, a possible reaction mechanism for the enhancement of photocatalytic activity of Ag/CdS-WO 2 has been proposed. Hence, we have a tendency to believe it might be a promising material that may be used for the photodegradation of organic pollutants present in wastewater.


Introduction
Water is extremely crucial in our environment for the regular weather and the co-evolution of life on Earth.
Almost all of the water on the planet is saline, making it un t for drinking and irrigation. Only 0.77% occurs as liquid fresh water and its distribution are very in homogeneous [1]. Drinking water should be free of pathogenic germs and contamination by toxic compounds like pesticides and industrial chemicals. Clean and fresh drinking water is essential to human and other life [2]. Water pollution is the contamination of water bodies, usually as a result of industrial wastes. For example, releasing inadequately treated wastewater into natural water bodies can lead to degradation of aquatic ecosystems. The waste bearing water, or e uent, and discharged into streams, lakes, or oceans, which in turn dispersed in the pollutants. Textile dyes and other industrial dyes are very easily mixed with fresh water from several industries [3]. Besides, these dyes can be considered as major water pollutants owing to their contained highly toxic organic compounds. In the current year it is estimated that some 30,000 million liters of pollutants are entering our river systems every day, 10,000 million liters from industrial units alone [4]. Synthetic azo dyes are among common environmental pollutants which are used in various 73 industries including textiles, papers, plastics, pharmaceuticals and cosmetic [5].
Advanced oxidation processes in a broad sense, are a set of chemical treatment procedures designed to remove organic and sometimes inorganic materials in water and waste water by oxidation through reactions with hydroxyl radicals (·OH) [6]. In real-world applications of wastewater treatment, however, this term usually refers more speci cally to a subset of such chemical processes that employ ozone (O 3 ), hydrogen peroxide (H 2 O 2 ) and/or UV light. Photocatalysis is the term can be generally used to describe a process in which light is used to activate a substance [7]. Photocatalysis promises a solution to challenges associated with the intermittent nature of sunlight which is considered as renewable and ultimate energy source to power activities on Earth [8]. Homogeneous photocatalysis refers to catalytic reactions in which both the reactants and the catalyst comprises only one phase and the photochemical process takes place in a homogeneous solution. There are 3 types of homogeneous photocatalysis are UV/H 2 O 2 , UV/O 3 and UV/O 3 /H 2 O 2 system. In Heterogeneous photocatalysis, UV/Semiconductor has most common are transition metal oxides and semiconductors, which have unique characteristics.
Semiconductor photocatalysis has been gained great attention as a green approach for complete removal of organic pollutants using freely obtainable solar energy source without discharge any secondary pollution [9].
Titanium dioxide (TiO 2 ) has been widely used as a photocatalyst in many environmental and energy applications due to its e cient photoactivity, high stability, low cost, and safety to the environment and humans [10]. When used in water treatment applications, TiOS 2 has a poor a nity toward organic pollutants, especially hydrophobic organic pollutants. Cadmium sul de, a visible-light responsive photocatalyst with a band gap of 2.4 eV, is one of the most prominent semiconductor photocatalysts [11]. In this study, the in uences of Ag/CdS-WO 2 Nano composite on the photodegradation of organic pollutant in aqueous solution were investigated. Hence, the objectives of this work are, (i) To synthesis Ag

Synthesis of CdS
In a typical procedure, 50 mL of 1 M cadmium nitrate, 25 mL of distilled water, 25 mL of ethanol and 0.34 g CTAB were mixed in a beaker. To this solution, 50 mL of 1M Na 2 S, 25 mL ethanol and 25 mL distilled water were added with vigorous stirring. Following this 20 mL of 2M NaOH was added with vigorous stirring which gives a light yellow precipitate. This mixture was transferred to a 250 mL Te on-lined stainless steel autoclave, followed by heating at 100°C for 2 h [12]. Following this, it was cooled to room temperature and the residue obtained was separated by centrifugation, washed several times with distilled water, ethanol and then dried in oven at 40°C, to get the resulting is the yellow coloured cadmium sulphide.

Synthesis of WO 2
2 g of tungstic acid and 0.895 g of CTAB were dissolved in 50 mL of distilled water. Then, the solution was transferred into a Te on-lined autoclave. The hydrothermal reaction was carried out in hot air oven at 120 ℃ for 12 h and then it allowed cooling at room temperature. The obtained yellow color precipitate was collected, washed with deionized water and ethanol several times to remove impurities, and dried in hot air oven at 60 ℃ for 6 h [13].

Synthesis of CdS-WO 2
For preparing CdS-WO 2 composite, 0.2 g of the above prepared WO 2 was dissolved in 80 ml ethanol to make a non-homogenous mixture and sonicated for half an hour. 1 g of CdS nanoparticles which was synthesized by the simple hydrothermal method was mixed in 100 ml of water and sonicated for half an hour. The above prepared WO 2 suspension was added into the CdS mixture and sonicated for 30 more minutes. The resulting mixture was lled into a Te on-lined stainless-steel autoclave at 200°C for 12 h. The obtained precipitates were ltered and rinsed many times with distilled water and ethanol. Finally, the formed product was dried at 60°C for 12 h [14].

Synthesis of Ag/CdS-WO 2 Nano composite
Ag/CdS-WO 2 (5 wt %) Nano composite was made by adding 0.5 g of CdS-WO 2 into 40 ml ethylene glycol solution with constant stirring for 20 min. afterward, the mixture was heated for 2 h at 180°C; then 0.025 g (2.7%) AgNO 3 was added with constant stirring for 30 min. The resulting mixture was cooled at room temperature, washed by ethanol three times, and nally dried at 60°C for 3 h [15]. 2.6. Characterization of photocatalysts X-ray diffraction (XRD) patterns of the synthesized photocatalysts were recorded using a X-ray diffractometer (Mini Flex II, Japan) with Cu Kα radiation (l ¼ 0.154 nm) at a scan speed of 3°/min. The phase purity was ascertained using X-ray diffraction. The Fourier transform infrared (FT-IR) spectra were recorded to study the interaction among CdS and WO 2 with a wavenumber ranging from 4000-400 cm −1 using a JASCO 460 plus FT-IR instrument. UV-vis diffuse re ectance spectra (DRS) were recorded using a Shimadzu 2100 spectrophotometer in the range of 200-800 nm.

Photocatalytic degradation studies
The photocatalytic activity of the as-synthesized photocatalyst was evaluated by the photo degradation of RhB under visible-light irradiation using a photocatalysis chamber. A 250W tungsten halogen lamp was employed as a source of visible-irradiation. A 75 mL of the RhB dye solution to achieve a catalyst concentration of 1.0 g/L. Prior to light irradiation, the suspension was magnetically stirred in the dark for 60 min to reach an adsorption-desorption equilibrium at room temperature [16]. During irradiation, 4 mL of aliquots was collected at regular time interval and then the photocatalyst was removed by centrifugation. The characteristic absorption wavelength of 554 nm for RhB was followed by using a UVvisible spectrometer (Jasco-630) Japan.  [19]. In the spectrum of WO 2 , the sharp peak at 542 cm −1 corresponds to the stretching vibration modes of O-W-O bonds. The absorption peaks at 1434 and 1654 cm −1 corresponds to the asymmetric and bending vibration of water on the surface of the WO 2 in the sample [20]. The characteristic broad peak at 3453 cm −1 may be due to the stretching frequency of the O-H stretching [21]. The sharp absorption peaks at 541 and 1038 cm −1 due to the Cd-S stretching modes and broad peak at 3527 cm −1 is for O-H stretching. The new distinct peak observed at 1026 cm −1 for the Ag doped CdS-WO 2 photocatalysts corresponds to asymmetric valence S=O vibration [22]. Thus, the FT-IR results clearly indicate the existence of Ag in the CdS-WO 2 photocatalyst.

Optical absorption studies
The light absorption properties of CdS, WO 2 , CdS-WO 2 and Ag/CdS-WO 2 photocatalysts were investigated by UV-vis diffuse re ectance spectroscopy and the spectral characteristics are displayed in Fig. 4

Effect of pH
The photocatalytic activities of prepared catalysts were investigated at different pH values (3)(4)(5)(6)(7)(8)(9). At increased pH (i.e., > 8) the degradation of rhodamine B decreased because of the formation of aromatic hydroxylated species that appears in the form of anions and competes with the adsorption of dye molecules which results in a decrease in degradation e ciency. Therefore the condition of optimum pH is mandatory for avoiding the wastage of excess quantity of catalyst; furthermore, it helps in maximum assimilation of visible light for enhanced photodegradation [26]. In Fig. 5 graph (a) shows the variation of pH trend of catalysts. CdS shows maximum degradation of 65% at neutral pH = 7. WO 2 shows better degradation because of the availability of the enhanced active sites on 75% at neutral pH (i.e., 7). Similarly, the CdS-WO 2 catalyst showed maximum degradation of 76% at neutral pH = 7. The ultimate degradation of 90% by Ag/CdS-WO 2 was even more than the other two catalysts. As a result of the loading of Ag on CdS-WO 2 surface, the possibilities of dye molecules adsorption on the surface of the catalyst enhance due to the increased availability of active sites.

Effect of catalyst concentration
For the optimization of catalyst dose in degrading rhodamine B dye, the amount of CdS, WO 2 , CdS-WO 2 , and Ag/CdS-WO 2 was varied from 10 to 90 mg/100 ml. Figure 6 shows that by increasing catalyst dose the degradation intensi es due to the availability of enhanced active sites. The trend remained up to a certain limit (i.e., Ag/CdS-WO 2 = 75, 150, 225, 300 mg/100 ml) above which the results showed reduced degradation of rhodamine B due to the formation of agglomerates [27]. Hence the maximum degradation is obtained at the above-mentioned catalyst loading of speci c catalysts.

Effect of oxidant concentration
In order to examine the role of S 2  Due to its high potential (2.6 eV) sulphate radicals which is a powerful oxidant that can able to participate in RhB degradation. This is the reason for decrease of rate [29].

Reaction Kinetics
For the quantitative study of RhB degradation, the pseudo rst-order kinetic model was employed to analyze the experimental data. For photocatalytic experiments, the equation is speci cally designed when we take the concentration of the pollutant in the mill molar range. and hydroxyl radical (·OH), which act as the active center and strong oxidizing agent for the photo catalytic activity [23,24,28,29]. The degradation of RhB under the irradiation of the stimulated visible light depends on two reactions like de-ethylation and decomposition of chromophore structure of the RhB. These processes can be characterized by the shift of the maximum absorption band (λ. max ) and change in the absorption maximum C max /C o max respectively [31]. Thus the process of producing photo excited e − can be used to generate more ·-O 2 from O 2 , subsequently accelerating reaction with organic dyes. At the same time, holes in the valence band (h + ) also have certain degrading ability to RhB.
Combined with two active species ·O 2 − and h + , photo catalyst can degrade organic dyes in a short time [32].
Thus, the promising reaction mechanism involving RhB dye photodegradation can be given as follows: showed the most pronounced effect on the decomposition of RhB. The addition of a minute amount of oxidant is an inexpensive way to greatly improve the degradation rate of a photocatalyst. The novel composite Ag/CdS-WO 2 has valuable applications in pollutant degradation.

Declarations
Competing interests: The authors declare no competing interests. Optimization of reaction parameter of pH using CdS, WO 2 , CdS-WO 2 and Ag/CdS-WO 2 Page 16/17 Optimization of reaction parameter of irradiation time using CdS, WO 2 , CdS-WO 2 and Ag/CdS-WO 2 Figure 10 Pseudo-rst-order kinetic tted curves of Rh B over Ag/CdS-WO 2 composite Figure 11 The schematic representation of Ag/CdS-WO 2 nanocomposite photocatalyst towards the degradation of RhB under VLI.