AU2021100320A4 - Development of solar driven photocatalyst and its application in degradation of organic pollutants - Google Patents

Abstract

Development of solar driven photocatalyst and its application in degradation of organic pollutants Abstract In this project, core-shell CdS@SnO2 particles have been prepared by Successive Ion Layer Adsorption and Reaction (SILAR) method. CdS is a well known low band gap semiconductor (of band gap ~ 2.4eV) and can thus harvest visible light of wavelength up to 520nm of the solar radiation. In our present work, we have implemented SILAR method with slight modification to coat thin CdS layer over fine SnO 2 particles to obtain core-shell CdS@SnO 2 particles. The SILAR method, which is usually used for the deposition of binary semiconducting thin films, has some advantage over other preparative methods, for example, this is a facile, less expensive and less time consuming technique, and it provides the provision to control the thickness of the film by adjusting the number of cycle of coating. In the present synthetic approach, fine SnO 2 powder has been prepared by hydrothermal method initially. The synthesized SnO2powder has been used as a substrate over which the CdS layer has been coated to obtain powder photocatalyst. SnO2 is a large band gap semiconductor and has no impact on visible light absorption. The synthesized core shell type CdS@SnO 2composite photocatalyst has been characterized with FTIR technique. The activity of the synthesized photocatalyst has been investigated under visible light towards the photooxidative degradation of Rhodamine B (RhB), which is a toxic organic contaminant. 1

Description

Development of solar driven photo catalyst and its application in degradation of organic pollutants

Abstract

In this project, core-shell CdS@SnO 2 particles have been prepared by Successive Ion Layer Adsorption and Reaction (SILAR) method. CdS is a well-known low band gap semiconductor (of band gap ~ 2.4eV) and can thus harvest visible light of wavelength up to 520nm of the solar radiation. In our present work, we have implemented SILAR method with slight modification to coat thin CdS layer over fine SnO2particles to obtain core-shell CdS@SnO 2 particles. The SILAR method, which is usually used for the deposition of binary semiconducting thin films, has some advantage over other preparative methods, for example, this is a facile, less expensive and less time consuming technique, and it provides the provision to control the thickness of the film by adjusting the number of cycle of coating. In the present synthetic approach, fine SnO 2 powder has been prepared by hydrothermal method initially. The synthesized SnO2powder has been used as a substrate over which the CdS layer has been coated to obtain powder photo catalyst. SnO2 is a large band gap semiconductor and has no impact on visible light absorption. The synthesized core shell type CdS@SnO 2 composite photo catalyst has been characterized with FTIR technique. The activity of the synthesized photo catalyst has been investigated under visible light towards the photo oxidative degradation of Rhodamine B (RhB), which is a toxic organic contaminant.

Introduction

The treatment and removal of pollutants from wastewater have become a crucial problem of environment and health. Development of semiconductor photo catalyst for the photo degradation of organics and dyes from wastewater has drawn immense attention in the past few years. A dye is a coloured compound that has an affinity to the substrate to which it is being applied. However, all coloured compounds are not dyes. To be as useful as a dye, a compound must show fastness to light, washing, heat and bleaching (1). Some common organic dyes are

Dyes are the main source of coloured organics generated as a waste from the textile dyeing process. Due to the high concentration of dyes in the effluents and the higher stability of modern synthetic dyes, the conventional biological treatment methods are not effective for removing the colour and degrading the dyes.

Filtration, coagulation, adsorption by activated carbon and treatment with ozone are some of the commonly used methods for dye degradation or removal. However, all these methods suffer from some drawbacks. For example, the use of charcoal is technically easy but is costly (2); in filtration, low molecular mass dyes can easily pass through the filter system; coagulation using alum, ferric salts or lime is a low cost process but the disposal of toxic sludge is a severe drawback; the ozone treatment does not require disposal but suffers from high cost. Therefore, an alternative and more reliable method is photo catalysis (3) in which, the photo catalyst can be reused after being used indicating the cost effectiveness of this method.

The main advantages of this method are:

1. Inherent destructive nature. 2. No mass transfer involved. 3. Can be carried out under ambient conditions and using atmospheric oxygen as the oxidant. 4. May lead to complete mineralisation of organic carbon into CO 2

. The use of semiconductor materials as photo catalysts has achieved great popularity over the past decade due to their ability to harness solar energy.

An efficient semiconductor must possess the following primary characteristics for photocatalytic reactions:

Low band gap to utilize most part of the solar light. Low recombination of photo generated electrons and holes to maximize reactivity of the catalyst. Correct band-edge positions for redox reactions. Stability to chemicals and light.

Photo catalysis is the catalysis of a chemical reaction under light irradiation. It is the acceleration of a photoreaction in the presence of a photo catalyst(4). As a photo catalyst, semiconductor materials can be used since they can harness light. For example, TiO 2 is a very common semiconductor which is used as a photo catalyst(5). It is a stable and a non-toxic semiconductor. The band gap of TiO 2 (Titanium dioxide) is 3.05eV andthus itabsorbs UV radiation because of which, it cannot harness much of the solar radiation as solar light comprises only 4% UV-radiation.

The mechanism of photo degradation of dyes using Ti 2semiconductor photocatalytic has been well documented in literature (6). The photo excited electron and hole pairs facilitate the oxidation and reduction reactions at the catalyst surface and generates hydroxyl free-radicals (OH•) and superoxide ions (02). These species behave as strong oxidizer to degrade toxic organic pollutants in wastewater.

When a photo catalyst is irradiated with radiation of energy equal to or greater than the bandgap energy of the semiconductor photo catalysts, it absorbs energy and creates a positively charged hole in the valance band (VB) and negatively charged electron in the conduction band (CB) by exciting the electrons in the valance band to the conduction band.

Objective and scope of the present work

In the present project work, we have developed low band gap core-shell structure of cadmium sulphide coated tin oxide (CdS@SnO 2) semiconductor based photo catalyst using a simple, inexpensive, solution based chemical method namely, successive ion layer and adsorption reaction (SILAR) and hydrothermal techniques. We have chosen cadmium sulphide (7) as it is a well-known low band gap semiconductor of band gap -2. 4eV.Therefore, it can absorb solar spectrum of wavelength-516 nm and thus can utilize a major portion of the sun's radiation. Solar light consists of 4% ultraviolet and 43% visible irradiations and thus the use of low bad gap semiconductor is realistic for harvesting the major portion of the solar light

In this project work we have used fine SnO 2 powder as a substrate, prepared by hydrothermal method, over which the CdS layer has been coated to obtain powder photo catalyst. SnO 2 is a large band gap semiconductor and plays no role in visible light absorption

Also, we have investigated the effectiveness of the developed photo catalyst towards the photo degradation of Rhodamine B under irradiation of visible light in the present project work.

Our Approach

SILAR stands for Successive Ion Layer Adsorption and Reaction. This method is usually used for film deposition. The SILAR method which is usually used for the deposition of binary semiconducting thin films has some advantages over other. The relative simplicity of the successive ion layer adsorption and reaction (SILAR) method, its potential application for layer area deposition, the requirement of relatively lower temperature, less expensive, less time consuming and the provision to control the thickness of the film by adjusting the number of cycle of coating makes this method very attractive. Most of the common methods for the preparation of CdS nanomaterial usually includes high temperature, inert atmosphere supported by argon, nitrogen, etc., the use of expensive chemicals like trioctyl phosphine oxide, trioctyl phosphine, etc., and the product formed usually suffers from unavoidable agglomeration(8). Therefore, based on the advantages of SILAR method described above, we have chosen this method for the preparation of [email protected], this method has been used for the first time to make CdS coated SnO2 for the photo degradation of RhB. In this project work, we have used tin oxide (SnO2) as a substrate (support) on the surface of which we have coated CdS. (Fig. 1)

Experimental

1. Hydrothermalsynthesis ofSnO 2 powder: In the present approach, fine SnO2 powder has been synthesized by hydrothermal method. Hydrothermal synthesis refers to the synthesis by chemical reactions of substances in a sealed heated solution above ambient pressure and boiling point. The hydrothermal synthesis of SnO 2

was done by following the below given procedure: Firstly, 1.72 g of sodium dodecyl sulphate [SDS:CH 3(CH 2)iiSO 4-Na+] was dissolved in 12 mL of distilled water. SDS is an anionic surfactant, frequently used as a template/capping agent to control the nucleation and subsequent growth of the particles in the synthesis of mesostructures of the material. This aqueous solution of SDS was acidified with 10 mL conc. HCl. Then 4.5 g stannous chloride (SnCl2 .2H 20.) was dissolved to this acidic solution and 6.2 mL 30% hydrogen peroxide (H 2 0 2 ) was added drop wise in that solution under continuous stirring condition. Next, the precipitation of metal (hydrous) oxide was carried out by adding ammonium hydroxide (NH 4 0H) drop wise to the resulting solution in stirring condition. The solution was then autoclaved at 120°C for 24 h. After the reaction, the sealed bottle was allowed to cool down naturally to room temperature. The resulting precipitate was centrifuged, washed with distilled water for a number of times to remove ions remained in the final product and dried in an oven at 100C for 2 h. Finally, as-synthesized product was calcined at 500°C for 2 h in air using a furnace to remove the organic surfactant remained in the product. (Scheme 1) (Fig. 2)

2. Synthesis of CdS coated SnO 2powder by SILAR method: 2 g of the prepared SnO2powder was taken in a centrifuge tube and 25 mL of100mM cadmium acetate (CdAc) solution in ethanol was added to it. It was then stirred to ensure SnO2 dispersion. This was then centrifuged to separate solid SnO2 . It was then washed with ethanol (EtOH) to remove excess cadmium acetate and centrifuged. After that, same volume of100mM sodium sulphide (Na 2 S) solution in ethanol was added to cadmium acetate coated Sn0 2 , stirred, centrifuged. This was again washed with ethanol to remove excess Na 2S. This whole process was repeated to complete 5 cycles. The resulting solid was then dried in an oven at 80°C for half-an-hour in a covered vial thus to get the product i.e., CdS coated Sn0 2. (Scheme 2) (Fig. 3).

The bands at 650cm-land 730cm-'correspond to Cd-S stretching band. The bands at 1620and 3550cm-lare attributed to bending vibration of water. The band at around 3401.5cm-1 may correspond to the stretching vibration of O-H bond. This band is due to the OH groups and the adsorbed water bound at the Sn02 surface. The peak at 447cm-1 might be due the stretching vibration of the terminal Sn-OH, while the peak at 613 cm-1 may correspond to the stretching modes of Sn-O-Sn. Although FTIR data does not actually confirm the formation of metal oxides, though we can obtain some data like: The bands at 650 cm-land 730 cm-'correspond to Cd-S stretching band. (Fig. 4)

Photodegradation of Rhodamine B (RhB) using the developed photocatalyst

Photocatalytic activity of the synthesized core-shell composite CdS@SnO 2was evaluated by investigating the degradation of Rhodamine B (RhB) dye in water. 200 mg of the developed CdS@Sn2 was taken and dispersed in 0.01 mM 50 mL solution of RhB. The suspension was exposed to visible light irradiation provided by three 200 W tungsten filament bulb (each produces light in the range of 420 nm to 800 nm) under constant stirring condition. 4.5 mL of the aqueous solution was taken out before the irradiation and after 2 h of irradiation and was centrifuged to remove any solid particles for analysing the degradation.

Results and discussions:

The absorbance spectra of an aqueous solution of RhB exposed to visible light irradiation and extracted from the reaction medium is displayed in fig.6. From fig.5 it can quite clearly be seen that the dark pink colour of RhB solution has become very faint pink after 2 h of irradiation indicating the degradation of RhB. This has further been proved by the data provided by the UV VIS spectrophotometer. (Fig. 5, 6)

The absorption peak at X=553 nm was found to diminish under the visible light irradiation, indicating the photocatalytic activity of the synthesized material in the degradation of RhB dye. The absorption peak at X=553 nm diminished after about 2 hours. The shift of the major absorption band of RhB dye to shorter wavelengths (hypsochromic shift) was, in accordance to literature (Zhao et al. J. Mater. Chem., 2007, 17, 2526 J. Phys. Chem. B, 2002, 106, 5022) due to the removal of ethyl groups, indicating the photo degradation of RhB dye. Plausible reaction mechanism is mentioned in Fig. 7.

Conclusion

The approach to use SILAR method has been quite successful to coat the particle i.e., CdS coated SnO2. From the experimental data, as shown in fig., 5, and the UV-VIS data, as shown in fig., 6, it can be concluded that the test for the application of the developed photo catalyst for the photo degradation of Rhodamine B has become quite successful. Further improvements can be made by comparing the results which we can get by using more than five or less than five cycles in the preparation of the photo catalyst CdS@SnO2by SILAR method. Also, the use of XRD data for the confirmation of the prepared photo catalyst instead of FTIR is more preferred. Nevertheless, the results have come to be very good with the photocatalystCdS@SnO 2prepared by using 5 cycles.

Therefore, SILAR method can be employed to develop an effective semiconductor photo catalyst.

References

1. Booth, Gorald. Dyes, General Survey, Wiley-VCH. a09-073. 2000. 2. J.M. Abdul, S. Vigneswaran, H.K. Shon, A. Nathapom and J. Kandasamy, Korean J. chem. Eng., 26, 724. 2009. 3. R.W. Matthews, Water Res., 20, 569. 1986. 4. Wu, Chang,; Decolourisation of reactive red 2 by advanced oxidation process: comparative studies of homogenous and heterogeneous systems. Journal of hazardous materials 128 (2-3): 265-75. 2005. 5. K Vinodgopal, PV Kamat; environmental science and technology; ACS publications. 1995. 6. Jaun Yang, Chuncheng Chen, Hongwei Ji, Wanhong Ma and Jincai Zhao; mechanism of TiO2 -assisted photocatalytic degradation of dyes under visible light irradiation: photo electrocatalytic study by TiO 2-film electrodes; ACS publications. 2005.

7. G.A. Martinez, M.G. Sanchez-Loredo, J.R. Martinez-Mendoza and Facundo Ruiz; Synthesis of CdS nanoparticles: a simple method in aqueous media; AZojomo (ISSN 1833-122X) Vol.1. 2005 8. M aAzad Malik; synthesis of TOPO-capped Mn-doped ZnS and CdS quantum dots; journal of materials chemistry. 2001. 9. Zhao etal. J. Mater. Chem., 2007, 17, 2526 J. Phys. Chem. B,106,5022. 2002. 10. Gayoung Lee, Huryul Lee, Myeong-Heon Um, and Misook Kang, Light scattering amplification on dye sensitized solar cells assembled by holly hock-shaped CdS-TiO2 composites, Bull Korean Chem Society. Vol. 33, No. 9. 11. Shang M, Wang W, Zhangg L, Sun S, Wang L, Zhou L. 3D Bi2WO 6/TiO 2heirarchicalheterostructure: controllable synthesis and enhanced visible photocatalytic degradation performances. J Phys Chem C 2009; 113:14727-31. 12. Kale BB, Baeg JO, Lee SM, Chang H, Moon SJ, Lee CW. CdIn 2 S 4 nanotubes and "marigold" nanaostructure: a visible-light photocatalyst. AdvFunct Mater B 2006;16:1349-54. 13. Chang Y, Teo JJ, Zeng HC. Formation of colloidal CuO nanocrystallites and their spherical aggregation and reductive transformation to hollow Cu 2 0 nanospheres. Langmuir 2005; 21:1074-9. 14. Growth and characterization of tin disulfide (SnS 2 ) thin film deposited by successive ionic layer adsorption and reaction (SILAR) technique. 2007, Journal of Alloys and Compounds.

Claims (2)

1. Development of solar driven photocatalyst and its application in degradation of organic pollutants

   Claim:

   • In this invention, core-shell CdS@SnO 2 particles have been prepared by Successive Ion Layer Adsorption and Reaction (SILAR) method. CdS is a well known low band gap semiconductor (of band gap ~ 2.4eV) and can thus harvest visible light of wavelength up to 520nm of the solar radiation. • In our present work, we have implemented SILAR method with slight modification to coat thin CdS layer over fine SnO2particles to obtain core-shell CdS@SnO 2 particles. • Most of the common methods for the preparation of CdS nanomaterial usually includes high temperature, inert atmosphere supported by argon, nitrogen, etc., the use of expensive chemicals like trioctyl phosphine oxide, trioctyl phosphine, etc., and the product formed usually suffers from unavoidable agglomeration( 8). Therefore, based on the advantages of SILAR method described above, we have chosen this method for the preparation of [email protected], this method has been used for the first time to make CdS coated SnO2 for the photodegradation of RhB. In this invention, we have used tin oxide (SnO2) as a substrate (support) on the surface of which we have coated CdS. • The application of the developed photocatalyst for the photodegradation of Rhodamine B has become quite successful

   Very thin CdS shell (few tens SnO2 nanometer core 2021100320

   thick layer) Fig.1 Core-shell structure of CdS coated SnO2(CdS@SnO2) photocatalyst by SILAR and hydrothermal method.

   10 mL HCl, 1.7 g SDS was 4.5 g, NH4OH was then dissolved in SnCl2.2H2O, added dropwise distilled water 6.2 mL H2O2 were added

   The ppt. was filtered, The ppt. was washed, dried hydrothermally Fine SnO2 powder and calcined treated at 120 ⁰C at 500 ⁰C for for 24 h. 2 h.

   Scheme 1 Flowchart synthesis of fine SnO2 powder.

   Fig.
2. 2 Fine SnO2 powder prepared via hydrothermal synthesis.

   Prepared 2 g SnO2 25 mL ethanolic was taken in a solution of CdAc centrifuge tube was added, stirred, centrifuged

   25 mL EtOH 25 mL Na2S 25 mL EtOH was was added, was added, added, stirred, stirred, stirred, centrifuged centrifuged centrifuged

   The resulting This process solid was dried at was repeated 80⁰C in an oven upto 5 cycles for half-an-hour..

   Scheme 2. Flowchart synthesis of CdS@SnO2.

   Fig: 3 Photograph of CdS@SnO2

   Fig. 4 FTIR data of the synthesized CdS@SnO2 photocaalyst

   Fig.5 Photograph of RhB before the irradiation and after 2 h.

   Fig. 6 UV-VIS data.

   Fig. 7 (Reaction mechanism)