Investigation of Bi2S3 nanorods photocatalytic activity under visible light for the degradation of dye from aquatic system

Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 was synthesized by employing solvothermal route. X-ray diffraction, UV-vis absorption, photoluminescence, Raman, scanning electron microscopic studies conrmed the structural, optical, morphological behaviors. The XRD pattern of Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 correlated well with JCPDS # 65-2435. The crystallite size was found to be 57, 49 and 40 nm. The photoluminescence spectra showed the semiconducting property of the prepared Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 . The absorption spectra of the Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 nanorods were well matched with the spectra of the previous report. The band gap of the Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 was calculated to be 1.56, 1.45 and 1.3 eV in reducing order. The morphology of the Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 samples showed the development of nanorods. 10 ml EG-Bi 2 S 3 sample showed better development of nanorods with the addition of ethylene glycol. The agglomeration was considerably reduced with the mixing of solvent. 10 ml EG- Bi 2 S 3 sample investigated showed 86% of eciency towards dye degradation. The narrow band gap, dened morphology of 10 ml EG- Bi 2 S 3 made a positive result towards ecient photocatalytic activity.


Introduction
Wastewater management has become an art nowadays. By better management skills, the heavy water scarcity created in this era in all developing and developed countries will be eliminated. Wastewater is drained out in tonnes of liters. Wastewater has been increased tremendously due to the high rate of developed industries which uses dyes, organic and inorganic salts, carcinogenic substances that are fairly combined with wastewater which pulled out form industries without pre-treatment. This causes a very adverse effect on aquatic, mankind and soil's fertility. These changes surely give upon all kinds of alternates in environmental cycle (Wang et al.2020). Not only dye industries, but from other skincare product industries and pharmaceuticals, there is also toxins and chemicals that are added to the water that is ushed out of industries (Recsetar et al.2021). Even the years move on, the chemical contaminants were added in wastewater that create more hardness to the water and it critically makes the researchers to reduce the contaminants from water and changes it to reuse (Hara yamamura et al.2020). By survey scan, over large increment in textile industries has been responsible for dye molecules that are mixed with the wastewater. Dyes has the high color, strong bonding and these molecules are very hard to remove (Alderete et al.2021). There is clear classi cation of dyes that includes acid, basic, azoic, disperse, reactive and direct dyes. Among these azoic, sulfur dyes will not dissolve in water and these molecules are not a competent that destroys the wastewater. Other group of dyes is soluble in water.
These dyes exhibit high stability and strong bonds that are not much easy one to separate from water (Hassan et al.2018). Twenty ve groups of dyes were identi ed now. Dyes are mostly used as color aerating agents. Among different dyes, methylene blue is commonly used in most of textile industries (Wang et al.2007). Methylene blue is 3, 7 -bis dimethylamine phenothlazin 5-ium chloride compound comes under synthetic basic dye family. This methylene blue dye is cationic which is highly stable and non-biodegradable. MB is mostly used as coloring agent and in medical it was used to nd the blocks in heart and it has wide applications in medical eld. However, over dosage of this dye will cause severe ailments in human beings and waste water mixed with water resources will make a adverse effects on aquatic animals (Soni et al.2020,Banerjee et al.2018.
To remove these types of organic pollutants various ways have been followed. Traditional, chemical and biological methods were followed. Traditional method has become out of fashion because these method is high cost and low e ciency. Advanced oxidation process (AOP) is the best method listed as compared with other process (Luo et al.2021). Photocatalysis is one among the AOP process which uses light sources as its basic element. The catalyst added dye solution is exposed to light illumination which excites the particles by giving photon energy and excites the particles from valence band to conduction band. The excited particles degrade the dye obtained in the solution. This process involves by creating four types of radicals like hydroxyl radicals, superoxide radicals, holes and electrons (Du et al.2021,Xu et al.2020. From all these aspects, photocatalysis is considered to be the best system for wastewater treatment. Nanoparticles are smaller in size which possess some individual properties that includes high surface to volume ratio, better optical properties and magnetic properties. These nanoparticles give good adsorption, better reaction rates and catalytic process. Hence, nanoparticles are widely used in all elds to attain the target (Singh et al.2012). The main theme of photocatalysis is that using visible light than UV light which will be considered to be better because of the negative impacts of UV light and the other reason is that, the sunlight has only 3% of the UV light spectra and most of visible light region. For practical application convenience sake, the visible light is preferred. Sul de based materials shows good comeback because of the narrow bandgap of metal sul de materials. This behavior is most positive one for the metal sul des which makes it more active in photocatalytic applications and many reports were present in metal sul des based photo degradation of methylene blue, rhodamine B and methyl orange dyes (Li et al.2012). Bismuth sul des possess Bi 2 S 3 formula is a semiconductor material with narrow bandgap of 1.4 eV. Bi 2 S 3 has been reported as an excellent performance on photocatalytic dye degradation applications (Zhang et al.2003). The physico-chemical properties which directly in uence the photocatalytic activity are morphology, crystal size and phase. Different synthesis methods for Bi 2 S 3 have been studied and reported so far (Ghanbari et al.2014, Sakthivel et al.2020, Ye et al.2020. Various morphologies like nanorods, nano owers, nanodots, nanowires are developed and investigated for photo based applications (Zhang et al.2001,Xia et al.2016, Wu et al.2020, Koh et al.2003). investigated the e ciency. Dasari Ayodhya and Gultene Veerabhadram synthesized GO-Bi 2 S 3 by sonochemical method and used to detect the ow of concentration of Hg 2+ . Xinshan Rong et.al., prepared Bi 2 S 3 / gC 3 N 4 and analyzed RhB degradation (Kumar et al.2016, Koutavarapu et al.2020, Huang et al.2013, Sharma et al.2013, Helal et al.2016, Lee et al.2017, Ayodhya et al.2018, Rong et al.2015. In this study, Bismuth Sul de (Bi 2 S 3 ) were prepared by employing solvothermal route by using ethylene glycol (EG) and deionised water as solvent. Bi 2 S 3 nanorods were investigated by standard characterization studies. Bismuth sul de nanorods optical properties was analyzed employing UV-vis spectroscopical studies and photoluminescence studies. The prepared nanorods morphology was studied with help of sscanning electron microscope. The development of nanorods has been thoroughly studied and its behavior towards photocatalytic dye degradation was investigated under visible light irradiation. The nanorods exhibited a good photocatalytic performance with high electron hole combination. Methylene blue dye was considerably degraded with almost 80% e ciency.

Materials And Methods
Bismuth nitrate (Bi(NO 3 ) 3 ), Thiourea, ethylene glycol, deionised water, ethanol, methanol, Whatmann lter paper were purchased form Sigma Aldrich with analytical grade. The chemicals were used without further puri cation. 0.01 M of Bismuth nitrate was suspended in 35 ml DI water and stirred using magnetic stirrer for complete dissolving. 0.03 M of thiourea is dissolved in 35 of DI water and stirred under magnetic stirrer. After completely dissolving, the thiourea solution is added drop wise to bismuth nitrate and completely stirred for about 1 hour to make it homogeneous. For the second solution, the same molarities ratio for Bi(NO 3 ) 3 and thiourea was taken as used before and the solvent comprised of 30 ml DI water and 5 ml ethylene glycol. Other procedure was followed as before. In the third solution, the solvent mixture ratio was changed to 25 ml DI water and 10 ml ethylene glycol and the other procedure was the same followed for the rst solution. All these three solutions were autoclaved at 160 0 c for 16 hours. After cooling down hydrothermal to room temperature, the black precipitate is collected and cleaned with demonized water, ethanol and methanol for about four times and it was kept in oven. The dried product is grinded and stored for further use. The structural, optical, vibrational properties were examined by X-ray diffraction studies, UV-vis absorption studies, photoluminescence studies, Raman studies. The grown nanorods morphology was studied by Scanning Electron Microscopic studies. Bi 2 S 3 nanorods photocatalytic action was performed by degrading Methylene blue dye.

Results And Discussion
Bi 2 S 3 structure was substantiated with standard JCPDS card is given in Fig. 1. The prepared pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 material structure was correlated with standard JCPDS. The obtained XRD pattern is in good agreement with JCPDS Card # 65-2435 with orthorhombic pattern with pbnm (62) space group. The 2θ values with hkl values at (020), (120), (130), (310), (230), (400), (240), (041), (510), (002), (431) and (132) were resembled with the JCPDS card (Jin et al.2013). The crystalline sizes were calculated by Scherer formula used in previous literature (Kuar et al.2013). The calculated crystallite sizes of pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 was found to be 57 nm, 49 nm and 40 nm. The crystalline size of the samples were considerably reduced for 10 ml EG -Bi 2 S 3 sample due to the addition of the solvent ethylene glycol (Kanade et al.2006). The higher angle peak shift occurred for 10 ml EG-Bi 2 S 3 is due to the maximum absorption of ethylene glycol and the growth of nanorods towards the corresponding plane (Zhao et al.2016).
The photoluminescence spectra of the pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 sample at excitation wavelength of 420 nm was given in Fig. 2. There are three clear peaks at 470, 485 and 487 nm. These peaks clearly illustrate the semiconductor properties of the material. The peak at 485 and 487 nm is created due to the presence of some structural defects on Bi 2 S 3 nanorods. The smaller nanorods with the effect of addition of ethylene glycol that are also a relevant option on the structural defect caused in the Bi 2 S 3 (Kawade et al.2014).
The optical property of the pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 was analyzed by UV-vis absorption spectra as depicted in Fig. 3. The broad absorption at 250 nm is due to the blue shift of the Bi 2 S 3 sample.
The bandgap energy was found with the Tauc's plot. The bandgap values calculated were 1.56, 1.45 and 1.3 eV for pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 . The bandgap values were decreased for 10 ml EG-Bi 2 S 3 sample. The bandgap was reduced to the reported value of Bi 2 S 3 . The reduce in bandgap will make the sample more e cient in visible light region. Reducing the bandgap value and deciding the narrow bandgap revealed to be more active in visible light region and it can have high possibility range in making real time applications (Salavati et al.2013;Xue et al.2014).
Vibrational properties of the pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 samples were characterized by Raman spectra as given in Fig. 4. A clear distinct peak at 962 cm − 1 is observed. This 962 cm − 1 peak explored Raman bands of Bi 2 S 3 nanorods. This band also con rms the sul de band of the Bi 2 S 3 . This result is also in good correlation as reported (Noordeen et al.2018;Panigrahi et al.2013).
The morphology of the pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 was studied by Scanning Electron Microscopy. Figure 5(a-f) illustrates the development of nanorods of pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 . Morphology of pure Bi 2 S 3 was de ned to be the initial stage of formation of unde ned nanorods which is clearly shown in 1µm scale (Fig. 5a). In Fig. 5d, smaller nanorods of Bi 2 S 3 is con rmed. Figure 5(b,e) revealed the morphology of nanosheets arranged in a clubbed manner. The agglomeration was decreased due to the addition of ethylene glycol. The thicker nanorods were started to decrease the size. The 10 ml EG-Bi 2 S 3 showed a distinct growth of nanorods in a well de ned manner. The addition of EG helps to grow the clear morphology of nanorods. There is a no agglomeration on the surface of nanorods. The solvent provided a better enhancement of nanorods development. The little agglomeration present on the surface of nanorods was clearly shown in Fig. 5(f) at 500 nm scale. The agglomeration was present on the other lesser ratio samples. The morphology plays a vital role in photocatalytic activity. The larger particles will decrease the photocatalytic performance by inhibiting active photoelectron charge to in Bi 2 S 3 . The clear and thin nanorods will better promote the charge transfer and makes the photocatalytic activity more fast and good. The decrease in size of nanorods corroborated with XRD results. The prepared nanorods will enhance the photocatalytic activity. 10 ml EG-Bi 2 S 3 sample will give an e cient output with good degradation percentage (Drmosh et al.2020;Zhu et al.2008;Miniach et al.2018).

Photocatalytic activity
The prepared pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 , 10 ml EG-Bi 2 S 3 samples were further investigated for its photocatalytic activity. The sample was get ready for 50 ml. The catalyst loaded was 0.1 g. The catalyst is added into 50 ml dye and stirred in dark about 30 min. All the solutions were kept in annular type photoreactor. Then, the solution was kept without disturbing for one hour to increase the absorption equilibrium of the dye with the catalyst. The visible light with 500 W was irradiated on the samples and the liquid sample was collected for every 20 minutes.
The degradation was examined by employing UV-vis absorption spectra as depicted in Fig. 6(a-d). The absorbance peak of Methylene blue at 609 nm and 668 nm was gradually decreased which was evident from the absorbance spectra. The degradation e ciency was calculated by the formula; Degradation % = [(A 0 -A t ]/A 0 )×100], where A 0 is initial absorbance at 0 minutes and A t is absorbance at t = 20, 40,… The degradation e ciency calculated was 55 %, 72 %, 73% and 86% for MB solution without catalyst, pure Bi 2 S 3 , 5 ml EG-Bi 2 S 3 and 10 ml EG-Bi 2 S 3 . The e ciency was con rmed better for 10 ml EG-Bi 2 S 3 sample (Kumar et al.2016). Figure 7 shows the relative concentration of the MB dye over time. The concentration of the Methylene Blue was decreased for the 10 ml EG-Bi 2 S 3 sample and correlated with UV-vis absorption spectra. When the light was passed on the solution, the electrons present in the catalyst absorbs the photon and gets excited. Valence band (VB) electrons will get su cient energy and transfers to conduction band (CB). The excited electrons will degrade Methylene blue molecules. When samples were subjected to light irradiation for certain hours, the catalyst completely degraded the dye molecules. From these three samples 10 ml EG-Bi 2 S 3 sample gives almost 86% of degradation. The morphology plays signi cant effect on photocatalytic process. The developed nanorods have a potential of degrading the dye molecules with complete e ciency. The undeveloped nanorods of the other two samples also showed good e ciency nevertheless the third sample with developed nanorods and less agglomeration gives higher output. Moreover, the band gap was also reduced which is more effective to work under visible light irradiation (Sharma et al.2017;Sarkar et al.2016). Addition of ethylene glycol does not directly involve in dye degradation but helps in better morphology growth. The reduce in thickness and agglomeration on nanorods e ciently attained the output which is desirable. Figure 8 shows the bar chart of e ciency of the samples. The superior photocatalytic activity of 10 ml EG-Bi 2 S 3 was optimized by adding 10 ml of ethylene glycol solvent, the uniformly developed nanorods and narrow bandgap. The main criteria for productive photocatalytic behavior are the narrow bandgap which is attained for 10 ml EG-Bi 2 S 3 that produces a better results under visible light irradiation (He et al.2017).

Conclusion
Wastewater management is the most important issue that was going on in this era. To make use of the wastewater for some other domestic purposes without harming the users have become the toughest job to complete. Pure and ethylene glycol assisted Bi 2 S 3 nanorods were synthesized by facile solvothermal route. Prepared samples explored orthorhombic structure. The crystalline size was also found and there were no extra peak identi ed and con rmed the purity of the prepared samples. Band gap energy of the samples was calculated to be 1.56, 1.44 and 1.3 eV. The band gap was decreased and becomes narrow which is the most positive side of 10 ml EG-Bi 2 S 3 . The undeveloped and developed nanorods were con rmed by SEM characterization. The growth of the nanorods was due to the addition of optimum amount of ethylene glycol. Ethylene glycol supported the growth of nanorods by reducing the agglomeration on the surface of Bi 2 S 3 . The nanorods provided the good photocatalytic degrading the synthetic dye under visible light for 2 hours. 86% of e ciency was obtained for 10 ml EG-Bi 2 S 3 . The narrow bandgap made the sample very e cient to work under visible light. The prepared nanorods are made of the uniform arrangement of the nanoparticles which absorbs light with most of its formation. 10 ml EG-Bi 2 S 3 samples possess most positive behaviors of narrow bandgap and uniform nanorods which helped the sample to develop high rate of photo generated electrons and recombination of electron-hole pairs that reduced the toxic dye within short duration of time. 10 ml EG-Bi 2 S 3 sample will be the most promising candidate for the future real time application on wastewater management. Declarations Zhang, W., Yang, Z., Huang, X., Zhang, S., Yu, W., Qian, Y., Jia, Y., Zhou, G. and Chen, L., 2001. Low temperature growth of bismuth sul de nanorods by a hydrothermal method. Solid state communications, 119(3), pp.143-146.