SnO2 heterostructured nanoparticle as visible light active photocatalyst for the removal methylene blue dye

Journal Pre-proof Impact of CdS/SnO2 heterostructured nanoparticle as visible light active photocatalyst for the removal methylene blue dye Emad E. El-Katori, M.A. Ahmed, A.A. El-Bindary, Aly M. Oraby

PII:

S1010-6030(19)30810-X

DOI:

https://doi.org/10.1016/j.jphotochem.2020.112403

Reference:

JPC 112403

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

15 May 2019

Revised Date:

13 January 2020

Accepted Date:

20 January 2020

Please cite this article as: El-Katori EE, Ahmed MA, El-Bindary AA, Oraby AM, Impact of CdS/SnO2 heterostructured nanoparticle as visible light active photocatalyst for the removal methylene blue dye, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112403

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Impact of CdS/SnO2 heterostructured nanoparticle as visible light active photocatalyst for the removal methylene blue dye Emad E. El-Katori1, M.A. Ahmed2*, A. A. El-Bindary3, Aly M. Oraby3 1Department

of Chemistry, Faculty of Science, Damietta University, Damietta-34517,

Egypt department, Faculty of Science, Ain Shams University, Cairo, Egypt

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2Chemistry

3Department

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of Chemistry, Faculty of Science, The New Valley University, El-Kharja72511, Egypt.

M.A. Ahmed

Graphical Abstract

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E-mail address: [email protected]

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Tel.: +20 103979568; fax: +20 224831836.

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Corresponding author

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A visible light driven CdS/SnO2 nanoparticles were synthesized for efficient degradation of methylene blue dye

Reflectance (%)

2 SnO2 CdS SnCdS1 SnCdS5 SnCdS10

1.5 1 0.5

400 600 Wave length (nm)

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200

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Highlights

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   

-Mesoporous CdS/SnO2 nanoparticles as visible light driven photocatalyst was prepared by facile route-CdS quantum dots reduce the band gap energy of SnO2 -Superoxide radicals are the main reactive species -The photocatalyst was recycled for five consecutive cycles

Abstract An efficient and recyclable CdS/SnO2 heterostructure containing various proportions of CdS (0-10) wt % was synthesized for photocatalytic degradation of methylene blue dye under UV and natural sunlight radiations. Mesoporous SnO2 nanoparticles of surface area 19.2 m2/g and particle size 34 nm with well definite slit pore structure were synthesized by sol-gel route using span as structure and pore directing agent. The physicochemical properties of the novel

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nanoparticles were investigated by X-ray diffraction [XRD], N2-adsorption-desorption isotherm,

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diffuse reflectance spectra [DRS], X-ray photoelectron spectroscopy [XPS], photoluminescence [PL] and high resolution transmission electron microscope [HRTEM]. CdS quantum dots of size

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9 nm, surface area 26.5 m2/g and wide mesoporous size (pore radius =119.2 Å) were

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incorporated homogeneously on the surface of SnO2 forming an efficient heterojunction that cause a remarkable reduction in the band gap energy of SnO2 from 3.52 to 2.53 eV. These novel

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heterostructures facilitate the electron transfer from CB of CdS of more negative potential (-0.52 eV) to that of SnO2 of less negative potential (-0.04 eV), thus enhancing the separation efficiency

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of the charge carrier and increasing the life time of the reactive radicals. It is emphasized that the photocatalytic reactivity of SnCd5 is more than twice that of SnO2 and CdS nanoparticles. Experiments with different quenchers indicate that the electrons conduction band and superoxide radicals are the predominant reactive species on SnCd5 hybrid nanoparticles, however, hydroxyl

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radical and positive holes are more efficient on pure SnO2 surface. The as-synthesized SnCd5 nanoparticles exhibits an excellent photocatalytic stability and the catalyst retains 83% of its reactivity after five consecutive cycles revealing that there is no deterioration in the catalyst structure.

Keywords: CdS quantum dots, CdS/SnO2 heterostructure; Sol-gel; Cd-O-Sn linkage; Superoxide radicals; Photocatalytic degradation of methylene blue dye. . 1. Introduction Synthetic dyes omitted from textile industries in wastewater causes negative effects to

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human health and aquatic life. The removal of these toxic pollutants from wastewater by

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traditional techniques as adsorption, coagulation, ion adsorption and reverse osmosis is

limited due to its high costs, difficulty of handling and possibility of transferring the primary

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pollutant into second one that require further treatment [1-4]. The production of large number of reactive radicals through a photocatalytic process is a green route for destruction of

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organic pollutants upon exposure of semiconductor to visible light [5-8]. TiO2, ZnO and

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SnO2 are promising semiconductors for photocatalytic removal of organic pollutants [9-19]. However, the rapid hydrolysis of titanium salts and the photocorrosion of ZnO limit the

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industrial applications of TiO2 and ZnO. SnO2 nanoparticles are effective in photocatalytic removal of organic pollutants due to its unique optical properties, chemically and thermal stability, non-toxicity and low cost metal precursors [20-23]. The wide energy band-gap (Ebg=3.5 eV) and the fast recombination of photoinduced charge carriers limits its

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photocatalytic reactivity to UV region that constitute only 5% of the solar energy. CdS nanoparticles with narrow band gap energy, tunable size, and excellent optical and electronic properties are involved in photocatalytic processes as dye degradation and hydrogen production. The incorporation of CdS on semiconductor surface improves the efficiency of charge carriers separation and shifts the photocatalytic response to visible region [24-31]. CdS exhibits more negative conduction band potential (-0.24 eV) than that of SnO2 (+0.01

eV) that facilitate the electron transfer from CdS to SnO2 conduction band which is accompanied by opposite positive hole transfer to valence band of SnO2 that enhances the oxidizing power of the photocatalyst [32-37]. CdS quantum dots have attracted various researches in the recent year due to their exceptional optical and electrical properties. They exhibits size dependent optical properties, high extinction coefficient and strong photostability make them a promising photocatalyst for efficient light harvesting materials in

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various photocatalytic process. Recently, more attention are paid to explore the influence of

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CdS quantum dots on the photoreactivity of TiO2 and ZnO surface for destruction and

removal of toxic pollutants. Up to our recent knowledge, there is no research work were

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conducted on synthesis of CdS quantum dots dispersed on the surface of SnO2 nanoparticles

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through chemical process. Reddy et al. prepare CdS/SnO2 by two step (coprecipitation/hydrothermal) method and they indicate that incorporation of CdS nanoparticles

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on SnO2 surface reduce the band gap energy from 3.3 to 2.9 eV and enhance the photocatalytic reactivity of photodegradation of methyl orange dye.. Zhang et al. prepare

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CdS/SnO2 by hydrothermal route for photocatalytic reduction of Cr(VI) [34]. Their results indicate that CdS nanoparticles are tightly combined with SnO2 which facilitate the formation of intimate contact interface between CdS and SnO2 which lead to production of large number of positive holes and superoxide radicals that is responsible for reduction process.

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Ghugal et al. [32] prepared CdS/SnO2 using incipient wetness route by mixing calculated amount of CdCl2 and SnO2 followed by evaporation of solvent. The photocatalytic reactivity of the prepared nanoparticles in degradation of acid violet dye was attributed to reduction in band gap energy and the production of hydroxyl and super oxide radicals. Our novel research is aimed to select the optimum direction in incorporating an appropriate amount of CdS on

SnO2 to reach a maximum photocatalytic degradation of MB dye as cationic pollutant model. Previous researches were conducted on synthesis of CdS/SnO2 by mechanical methods [3237]. However, these methods are usually associated with agglomeration of CdS nanoparticles on SnO2 surface that reduce the number of the active sites on the catalyst surface, prevent the light penetration and decrease the photocatalytic reactivity. In our, research, we use tritonX100 as structure and pore directing agent in manipulating mesoporous high surface area

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SnO2 nanoparticles. Various concentration of Cd(NO3)2 are dispersed on SnO2 surface

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followed by addition of an appropriate amount of Na2S to select the optimum catalyst

concentration which is found to be 5 wt% of CdS on the surface of SnO2 nanoparticles.

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Moreover, the chemical mode of preparation in our research increases the chemical

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interaction between SnO2 and CdS nanoparticles through formation of Cd-O-Sn bond that facilitate the heterojunction and improve the efficiency of separation of charge carriers. A

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plausible mechanism for destruction of MB dye using reactive radicals is postulated. The novel nanoparticles are suggested to be a promising candidate in a variety of photocatalysis

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applications in water purification.

2. Experimental 2.1. Materials

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SnCl4.5H2O, Cd(NO3)2.4H2O, Sodium sulphide (Na2S). Ammonia solution, Potassium iodide (KI), Silver nitrate (AgNO3), Benzoquinone, Isopropanol, Terephthalic acid, Methylene blue dye. All the chemicals were purchased from Sigma-Aldrich Company with analytical grades. 2.2. Synthesis of the photocatalyst 2.2.1. Synthesis of SnO2 nanoparticles

An appropriate amount of SnCl4.5H2O (Prolabo, 99.9%) dissolved in isopropanol are added to span-80 dissolved in isopropanol followed by constant stirring for one hour. Afterwhile, a few drops of ammonia solution [1M] were added slowly until the turbid sol was formed. The above solution was subjected to vigorous stirring for two hours followed by aging for 48 hours. Filtration and washing with de-ionized water was proceeded in order to remove all chloride ions. The above mixture is filtered and dried at 100ºC for 24 hours. Then, the dried sample is calcined

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at 500ºC for three hours.

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2.2.2. Synthesis of CdS

An equimolar concentration of Cd(NO3)2 and Na2S dissolved in bi-distilled water was added

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with vigorous stirring for three hours. Then, the solution is aged for two days followed by

2.2.3

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filtration, washing with bi-distilled water and dried at100⁰ C for 24 hours. Synthesis of CdS/SnO2 nanoparticles

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A fixed amount of Cd(NO3)2 dissolved in bi-distilled water was added with constant stirring for three hours to SnO2 nanoparticles at room temperature by a certain proportions in attempts to

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obtain ( 1, 3, 5,8 and 10) wt % CdS dispersed on SnO2 surface, then a solution of 1M Na2S dissolved in bi-distilled water was added drop by drops to the above solution until a complete precipitation of CdS on SnO2 was observed. The above solution was subjected for vigorous stirring for three hours. Then, the yellowish solid nanoparticles were filtered, washed with bi-

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distilled water and dried in an oven at 100°C for 24 h. The final samples were denoted as SnO2, CdS, SnCd1, SnCd3, SnCd5, SnCd8 and SnCd10 for pure SnO2, CdS and CdS/SnO2 containing (1, 3, 5, 8 and 10) wt% CdS, respectively. 2.3. Material characterization

The crystalline parameters and size of the prepared nanoparticles is estimated by P Analytical X’PERT MPD diffractometer using Cu (Kα1/Kα2) radiation. The XRD spectrum is investigated in a diffraction angle range from 10° to 90° with a step of 0.03° and integration time of 4 seconds per step. The surface area and the sample porosity was estimated by N2 Adsorption–desorption at 77 K

The point of zero charge was determined by mass titration method

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by a micrometrics apparatus with a residual pressure of 10-5 Torr.

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HRTEM JEOL 6340 electron microscope was involved for investigating the nanostructure of the prepared samples.

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the band-gaps values from Kubelka–Munk functions.

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JASCO spectrometer model V-570 is used for study the absorption spectrum and determine

The photodegradation of [MB] dye as cationic dye pollutant model was carried out under high

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pressure mercury lamp with intensity 16-watt and maximum absorption at 365 nm and under natural sunlight for comparing the photoreactivity of the prepared samples. The solution

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containing 0.1g of the photocatalyst suspended in 100 ml of MB dye of initial concentration 2x10-5 molL−1 was continuously stirred in dark for 30 min to determine the amount of dye removed via adsorption mechanism. Then, a mercury lamp is switch on for three hours and at each interval times, the solution samples were collected from the reactor at regular intervals,

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centrifuged and analyzed to determine the amounts of residual [MB] dye using UV-Vis spectrophotometer. The photocatalytic degradation process was carried out under natural sunlight irradiations using 150 ml quartz reactor. The sample tubes were placed on our laboratory platforms and the photocatalytic process starts at 2 p.m. for 3 h of sunlight exposure with average solar radiation

intensities varied between 1200 and 1450 W/m2 and the average temperatures varied between 30 and 35⁰ C during these photocatalytic degradation experiments. 3. Results and discussion 3.1. X-ray diffraction Fig. 1 depicts the diffraction pattern of the prepared nanoparticles. The diffraction pattern reflects the existence of various diffraction peaks at 2θ=26.6, 34.2, 37.6, 51.8, 54.8, 58.4, 61.9

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and 65.2˚ referred to (110), (101), (200) and (211) plane, which referred to the Cassiterite

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tetragonal (rutile type) structure of SnO2 (ICDD card No.41-1445) of space group P42/mnm. XRD diffraction pattern of CdS illustrates three broad prevailing peaks at 2θ=26.48, 44.125,

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52.225 that indexed to (111), (220) and (311) planes of cubic crystalline CdS (JCPDS No. 10-

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0454). The broadness in the diffraction peaks of cubic CdS suggests the existence of crystallites in quantum dot dimensions. The diffraction pattern of the nanocomposites resembles the same

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pattern of pure SnO2, however, the diffraction peaks referred to CdS cubic phase is completely missing. This situation reflects the quantum dots dimensions of CdS which cannot recognize as

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XRD cannot detect the crystallites with size below 50Å. It is interesting to notice the broading in the SnO2 diffraction peaks upon increasing CdS contents revealing the dispersion of CdS nanoparticles between SnO2 crystallites preventing them in size growing. The difference in ionic radii of Cd2+ and Sn4+ suggest that Cd2+ cannot occupy interstitial or substitution position in

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tetragonal SnO2 crystalline phase. The most proper situation is the distribution of CdS on the surface of tin oxide. The crystallite size calculated according to Debye-Scherrer equation to SnO2, CdS, SnCd1, SnCd5 and SnCd10 are 34, 9, 22, 18 and 13. 3.2. Surface characterization

Fig. 2 illustrates the adsorption-desorption isotherms, Va-t plots and pore size distribution curves. The adsorption isotherms are classified as type (II) with pronounced wide H3 hysteresis loop according to IUPAC classification closes at nearly P/Po=0.55 reflecting the existence of definite mesoporous structure. The existence of H3 hysteresis loop revealing the slit pore structure that arises from aggregates (assemblage of particles which are loosely coherent) of plate-like form giving rise to slit-shaped pores. The surface area is calculated by BET equation in

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its normal range of applicability with a value of 16.2 Å2 for the cross-section area of N2. Pore-

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size distributions calculated by BJH method applied to the desorption branch. The pore size

distribution of SnO2 and CdS exhibit a broad peak reflecting the pore widening in SnO2 and CdS

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nanoparticles. However, a sharp peaks were observed in the samples SnCd5 and SnCd10

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indicating the narrowing of the pore structure of SnO2 due to deposition of CdS inside the pore matrix. The surface area estimated by adopting BET is 19.4, 26.4, 22.8, 20.7 and 21.5 m2/g for

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SnO2, CdS, SnCd5 and SnCd10 [Table 1]. The surface parameters is a key factor in optimizing the photocatalytic reactivity of the prepared nanoparticles. It is should emphasized that

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mesoporous photocatalyst with high surface area is usually contains large number of active sites that responsible for destruction of organic pollutants. The porosity is another positive factor that can facilitate the diffusion of the organic pollutants. However, the strong adsorption of dye molecules affects the recycling behavior of the photocatalyst for many consecutive cycles as

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many active site can be blocked by the dye molecules. It is interesting to notice that the adsorption isotherms of SnO2 and SnO2 containing various proportions of CdS exhibit wide hysteresis loop revealing the existence of various mesoporous channels which can facilitate the trapping of MB dye molecules on the pore cavities to be easily destructed under the influence of

reactive radicals. CdS has no influence on the surface area and pore radius reflecting the dispersion of CdS as single layer molecules on SnO2 surface. 3.3. High resolution transmission electron microscope Fig. 3 depicts TEM images of SnO2, CdS, SnCd1, SnCd5 and SnCd10. Pure SnO2 contains particles of predominant hexagonal nanoparticles of dimension 34 nm. However, CdS contains nanoparticles with quantum dot dimensions which spreads as dots dispersed through the whole

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sample matrix with size vary between 6-9 nm. The particle dimensions of SnO2 are remarkably

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reduced upon incorporation of CdS as illustrated in table 1. It is clearly observed that CdS

nanoparticles are deposited strongly on the surface of SnO2 covering the active sites distributed

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on the oxide surface.

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Fig. 4 depicts the HRTEM image of the as-synthesized nanoparticles for estimating the lattice parameters. It is worthnoting the existence of SnO2 and CdS in perfect crystalline structure

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supporting the successful route of preparation in fabricating homogeneous nanocrystals without any distortion in crystal structure. It is clearly notice the existence of definite interplanar spacing

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of 0.34 nm corresponding to (111) plane of rutile tetragonal SnO2 crystal. However, the interplanar spacing observed at 0.334 nm refers to (111) plane of cubic CdS. On careful examining Fig. 4, one can notice the existence of two different line spaces for SnCd5, the first one is 0.34 and the second is 0.334 nm which attributed to SnO2 and CdS, respectively revealing

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the successful formation of CdS/SnO2 heterostructure. The SAED analysis [Fig. 4] shows various continuous diffraction rings at (110), (101), (200), (211) and (220) planes referred to tetragonal SnO2 nanoparticles. However, the existence of three prevailing diffraction spots at (111), (200) and (311) planes illustrates the formation of single crystal cubic CdS quantum dots. SAED analysis of SnCd5 illustrates various rings and spots at (110), (101), (200), (111), (200)

and (311) that belongs to SnO2 nanoparticles and CdS single crystal. It is should be emphasized that the results of HRTEM and SAED is considered an excellent proof for formation of CdS/SnO2 heterojunction. 3.4. XPS analysis XPS is carried out to investigate the chemical composition and the oxidation state of the photocatalyst. Fig. 5 represents XPS spectrum of SnCd5 that displays two sharp peaks at 486 and

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495.6 eV referred to Sn 3d3/2 and Sn 3d5/2, respectively. The peak observed for 3d5/2 is shifted to

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lower value than those reported in the literature revealing the strong interaction between SnO2 and CdS. The spectrum shows a broad peak at 529.6 eV ascribed to O1s binding energy. The

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existence of two distinct peaks at binding energies of 405 and 412 eV are referred to Cd3d3/2 and

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Cd3d5/2 respectively. Moreover a spin orbit separation of 6.7 between these two peaks suggests the presence of Cd2+ on the surface of SnO2. The doublet peak at 162.5 with a shoulder at 165.9

S 2p1/2 and S 2p3/2 state

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eV indicates the presence of S2−ions on the surface of the heterostructure which corresponded to

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3.5. FTIR analysis Fig. 6 represents the FTIR analysis of SnO2, CdS and SnCd5 nanoparticles to investigate the interaction between CdS and SnO2 nanoparticles. The spectrum displays a band at 620 and 506 cm-1 for SnO2 and SnCd5 that assigned to Sn-O-Sn bond The two bands observed at 3410 and

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1620 cm-1 referred to (O-H) of adsorbed water molecules. The spectrum of CdS and SnCd5 illustrate the existence of two bands at 1410 and 1100 cm-1 refereed to Cd-S bond. A new band at 1030 cm-1 can be assigned to Sn-O-Cd bond revealing the strong interaction between CdS and SnO2 nanoparticles [38, 39]. 3.4. Optical properties

The UV–Visible absorption spectrum of the as-synthesized nanoparticles is represented in Fig. 7. The spectrum illustrates a pronounced absorption band edge in the region 400-700 nm which originates from electronic transitions between valence and conduction band. CdS shows a strong absorption band in the visible region with band edge at 590 nm due to its narrow band gap energy (2.1 eV). A remarkable shift in the absorption spectrum of SnO2 upon incorporation of CdS nanoparticles due to formation of Cd-O-Sn linkage, this shift increases with increasing CdS

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content revealing a favorable shifting the photocatalytic response toward visible region.

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The energy gap (Eg) is calculated from the equation

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(αhv)2 = A (hv – Eg)n

where α is the absorption coefficient, A is a constant. n=2 for indirect transition and n=1/2 for

value of the optical energy gap (Eg).

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direct transition. An extrapolation of the linear region of a plot of (αhv)2 versus (hv) gives the

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The values of Eg, estimated for the samples from Fig. 7 by extrapolating the linear portion of the curve to (αhv)2 = 0, are 3.52, 2.2, 2.3.21, 2.88 and 2.53 eV for SnO2, CdS, SnCd1, SnCd5

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and SnCd10, respectively.

The valence and conduction band edge at the point of zero charge of semiconductor can be calculated according the following equation:

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EVB=X-Ee +0.5Eg ECB=EVB - Eg

Where [X] is the calculated electronegativity of the semiconductor which is geometrical means of the electronegativity of the constituent atoms, the value of [X] is 5.18 and 6.22 eV for CdS and SnO2, respectively. Ee is the free energy of free electron on the hydrogen scale (4.5 eV) and Eg is the estimated band gap energy of the semiconductor. Based on the above equation, the

calculated conduction band edge = - 0.04 eV and valence band edge = 3.48 eV for SnO2. However, the calculated conduction band edge = -0.52 eV and the valence band edge =+1.88 eV for CdS nanoparticles. This results reveal the possibility of electron transfer from CB of CdS to that of SnO2 which accompanied by narrowing band gap energy and improve the efficiency of separation of the charge carriers. 3.5. Photoluminescence

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Fig. 8 illustrates the PL spectrum of the as-synthesized samples as measuring the power of CdS

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in enhancing the quantum efficiency of the charge carriers separation and estimate the life time of the excited state of the charge carriers. The PL intensity is usually depends on the physical

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properties of the solid powder and the existence of any defects in the crystal structure. The PL

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spectrum of SnO2 illustrates the existence of two broad emission band in the visible region centered at 470 and 520 nm due to shallow defects level present in the nanoparticles [Fig. 8]. On

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careful examining Fig. 6, one can notice the remarkable reduction of emission intensity after incorporation of CdS nanoparticles on SnO2 surface indicating the successful capture of CdS

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electrons by SnO2 conduction band. The electron transfer from CB of CdS to SnO2 nanoparticles enhances the efficiency of charge carrier separation. This result indicates that the life time of the charge carriers in composite samples is higher than that in pure SnO2.

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3.6. Photocatalytic degradation of methylene blue dye 3.6.1 Kinetics of photocatalytic degradation of methylene blue dye The photocatalytic degradation of [MB] dye with definite concentration (2x10-5 mol/L) is

taken as cationic pollutant model to compare the reactivity of the prepared samples under UV light irradiation. Fig. 9 represents photocatalytic degradation of [MB] over the prepared sample indicating the removal of 90% of the dye during 180 minutes over SnCd5; however, only 48%

and 45% are removed over pure SnO2 and CdS nanoparticles. On examining the same figure, one can notice that the position of maximum wave length is not altered during the degradation process reveling that complete destruction of the dye without formation of secondary organic pollutant. Fig. 10 displays the variation of the photocatalytic activity of the samples determined at the maximal absorption wavelength of the dye (664 nm) with the time of irradiation. On careful examining Fig. 10, one can notice the removal of dye under dark conditions via

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adsorption mechanism does not exceed 15 % reflecting that the mineralization process occur

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predominantly through photocatalysis route.

Fig. 11 represents the first-order equation kinetics that describe the photocatalytic degradation of

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MB dye, the slope of the linear plot of first order graphical presentation estimate the apparent

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rate constants for the degradation process which found to be 0.0022, 0.0031, 0.0029, 0.0024, 0.0102, 0.0021 and 0.0022 min−1 for pure SnO2 CdS, SnCdS1, SnCd3, SnCd5, SnCd8 and

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SnCd10, respectively. 3.6.2 Effect of radical scavenger

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The role of positive hole, negative electron, hydroxyl and superoxide radicals were monitored using KI, isopropanol, benzoquinone and silver nitrate, respectively, on both pure SnO2 and SnCd5. The results demonstrated that the photodegradation percentage of MB dye was reduced

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from 48% into 16, 19, 41 and 42 % after addition of KI, isopropanol, benzoquinone and silver nitrate revealing that positive hole and hydroxyl radicals are the main reactive species for photocatalytic degradation of MB dye over SnO2 nanoparticles. However, photodegradation percentage of MB was reduced on SnCd5from 92% to 77, 79, 26 and 23 % after addition of KI, isopropanol, benzoquinone and silver nitrate [Fig. 12] reflecting that superoxide radicals and electron conduction band are the main reactive species in the degradation process This result

reveals that incorporation of CdS is associated with change in the mechanism of dye degradation that attributed to its photosensitization role in increase the concentration of electron conduction band which in turn increase in the concentration of superoxide radicals. 3.6.3. Detection of hydroxyl radicals The concentration of hydroxyl radicals can be investigated by measuring the

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photoluminescence of terephthalic acid. Fig. 13 illustrates the existence of prevailing peak at 423

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nm being increases in intensity with increasing the irradiation time for SnO2 and SnCd5

nanoparticles that referred to 2-hydroxy terephthalic acid which exhibits definite fluorescent

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peak. The existence of this prevailing peak indicates the production of large amount of OH

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radicals. It is interesting to notice a remarkable increase in the intensity of PL for the sample SnCd5 compared with pure SnO2 revealing the production of large number of hydroxyl radical

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upon incorporation of CdS on SnO2 surface,

3.6.4. Estimation of Total organic carbon (TOC)

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The estimation of TOC before and after dye removal reveal the degradation of dye into CO2 and H2O. TOC of MB dye solution was decreased from 78.4 mg/l to 10.1 mg/l revealing mineralized of 85 % of MB dye over SnCd5 nanoparticles. 3.6.5. Recycle of the photocatalyst

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The photocatalytic stability is measure the capability of the photocatalyst to re-use several times which reduce the environmental costs. The photocatalyst was subjected to five consecutive cycles for removal of MB dye. After each cycle, the nanoparticles were washed with distilled water and dried before the next cycle. It is clearly observed that the photocatalyst retains 83% of its reactivity after the five cycles revealing the high stability of the catalyst upon exposure to UV irradiations [Fig. 14]. On careful examining Fig. 15, one can notice that the crystalline pattern of

SnCd5 is not altered after the degradation process revealing the stability of the photocatalyst to be used for different cycles for degradation of organic pollutants. 3.6.6 Photodegradation of methylene blue dye under natural sunlight radiations For industrial and environmental purpose, the photocatalytic degradation of MB dye was carried out under natural sunlight radiations on pure SnO2, CdS and SnCd5 that is considered an optimal concentration of CdS. The results have pointed out the removal of 48% of MB dye over

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CdS through predominant photocatalytic route compared with 43% removal on pure SnO2.

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However, 83% RhB dye was removed on SnCd5 nanoparticles under natural sunlight radiations [Fig. 16 and 17]. This result reveal the successful role of CdS in shifting the photocatalytic

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response to visible region.

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3.6.7 Factors influence the photocatalytic degradation of [MB] dye

Mesoporous SnO2 is a wide band gap semiconductor that exhibit high surface area and definite

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pore structure compared with other semiconductors as ZnO and TiO2. However, the low quantum efficiency and the rapid electron-hole recombination rate reduce its photocatalytic

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applications. CdS is a notable narrow band gap energy that can photosensitize SnO2 and modulate the photocatalytic reactivity to visible region. The physicochemical investigation of the hybrid nanoparticles reveals the homogeneous distribution of CdS quantum dots of size 9 nm on SnO2 nanoparticles surface. The strong chemical interaction between SnO2 and CdS

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nanoparticles can be inferred from FTIR and PL analysis that illustrate the existence of Cd-O-Sn bond. This hybrid structure is a primary key for reducing the band gap energy to 2.53 eV and improves the quantum efficiency for charge carrier separation as proven by XRD, HRTEM, SAED, DRS and PL analysis. Moreover, the mesoporous structure of CdS and the appropriate surface area 26 m2/g enhances the surface properties of the nanocomposites that increases the

adsorption capacity which is key factor on optimizing the photocatalytic degradation rate of MB dye. The photocatalytic route is usually initiated by exposing CdS nanoparticles surface to UV or natural sunlight irradiation that produce large number of positive holes and negative electrons. The CB electrons of CdS nanoparticles with more negative potential can transfer across the interfacial boundary to CB of SnO2 with low negative potential. Likewise, the positive hole generated in VB of SnO2 at more positive potential will transfer to VB of CdS which is less

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positive potential. The electrons adsorbed on SnO2 surface can react with water to produce large

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number of superoxide radicals. The photocatalytic activity of CdS/SnO2 nanoparticles is largely influenced by the amount and size of CdS distributed on the surface of SnO2. In composites,

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there is a definite concentration of each constituent that exhibits maximum photocatalytic

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reactivity. The existence of CdS in quantum dot dimensions can improve the physical contact of large number of CdS with SnO2 nanoparticles which result in excellent photocatalytic efficiency.

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The optimal CdS concentration in our research is 5 wt % CdS dispersed on SnO2 surface which mineralize 90% of MB dye. This result reflects that this concentration is very suitable for

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production of large number of reactive radicals and exhibits a strong oxidizing power in the photocatalytic process. It seems that below this nominal value, the concentration of CdS is not sufficient to induce charge separation of the charge carriers. However, above this nominal concentration, there is a large possibility for particles aggregations and formation of excessive

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CdS layers that prevent light penetration to the photocatalyst surface and acts a new recombination centers. This recombination effect reduces the life time of the reactive and radicals and suppress the degradation rate of MB dye. Compared with pure SnO2 and CdS 5 wt% of CdS/SnO2 surface 94% of MB dye is completely removed under natural sunlight radiations. This remarkable reactivity can be attributed to the

influence of CdS in preventing the electron-hole recombination that achieved by electron transfer from CdS conduction band to SnO2 energy level. Then, the electrons can be trapped by the adsorbed oxygen to form superoxide radicals that consider a reactive species in degradation process as represented in Scheme [Fig. 18]. 4. Conclusions Herein, for the first time, CdS/SnO2 heterojunction with CdS in quantum dot dimensions were

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prepared for photocatalytic degradation of MB dye as cationic pollutant model. CdS quantum

ro

dots with size of 9 nm, surface area 26 m2/g, wide pore radius 119 Å and perfect cubic crystalline structure were homogeneously distributed on SnO2 surface without any distortion in crystalline

-p

parameters. HRTEM and PL analysis have pointed out the formation of heterojunction between

re

CdS and SnO2 nanoparticles that facilitate the electron transfer from CB of CdS of more negative potential to that of SnO2 that increase the quantum efficiency of the separation of the charge

lP

carriers. The existence of 5 wt% of CdS is considered the optimal concentration for maximize the photocatalytic reactivity. Photogenerated electrons and superoxide radicals play a

ur na

predominant role in dye degradation over SnCd5 nanoparticles; however, both positive hole and hydroxyl radicals are the main reactive species for dye degradation over pure SnO2. The novel nanocomposites are considered a promising photocatalyst for destruction of various organic

Jo

pollutants.

Author statement

I confirm that our manuscript titled [Synthesis of novel visible light driven photocatalyst CdS/SnO2 heterostructure for photocatalytic removal of methylene blue dye] is original

and not consider for publication in another journal. I confirm that all author agree with the submission.

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List of Figures Fig. 1 XRD pattern of SnO2, SnCd1, SnCd5 and SnCd10 Fig. 2-(a) N2-adsorption-desorption isotherm, (b) Vl-t plot and (c) BJH pore size distribution for of SnO2, SnCd1, SnCd5 and SnCd10 Fig. 3 TEM image of SnO2, SnCd1, SnCd5 and SnCd10

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Fig. 4 HRTEM and SAED image of SnO2, SnCd1, SnCd5 and SnCd10 Fig. 5 XPS of SnCd5 nanoparticles

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Fig. 6 FTIR of SnO2, CdS and SnCd5

Fig. 7 Diffuse Reflectance spectra and Tauc plot of SnO2, SnCd1, SnCd5 and SnCd10

-p

Fig. 8 Photoluminescence (PL) spectra of SnO2, SnCd1, SnCd5 and SnCd10

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Fig. 9 The absorption spectra of [MB] dye over SnO2, SnCd1, SnCd3, SnCd5, SnCd8 and SnCd10

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Fig. 10 The variation of photocatalytic degradation rate of [MB] dye over SnO2, SnCd1, SnCd3, SnCd5, SnCd8 and SnCd10 with time of irradiation

ur na

Fig. 11 The pseudo first order kinetics of degradation of [MB] dye over SnO2, SnCd1, SnCd3, SnCd5, SnCd8 and SnCd10

Fig. 12 The effect of various scavengers on photocatalytic degradation of [MB] dye over SnO2 and SnCd5

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Fig. 13 PL spectra of terephthalic over SnO2 and SnCd5 Fig. 14 Regeneration of SnCd5 for five consecutive cycles Fig. 15 XRD diffraction of SnCd5 before and after five consecutive cycles of dye degradation Fig. 16 The absorption spectra of [MB] dye over SnO2, CdS and SnCd5 under sunlight irradiation

Fig. 17 The variation of photocatalytic degradation rate of [MB] dye over SnO2, CdS and SnCd5 with time of irradiation under sunlight irradiation Fig. 18 Suggested scheme for electron transfer between SnO2 and SnCd5 under UV radiations

60

of

30

re

Intensity (a.u.)

-p

ro

SnCd10

lP

ur na Jo

SnCd1

CdS

SnO2

30

Fig. 1

SnCd5

60

2

Fig. 2

60

-p

ro

dv/dr

of

30

30

Jo

ur na

lP

re

Pore diameter

60

0.0 40

0.5

1.0

30

SnCd10

20 10 60

SnCd5 20 0 100

CdS

ro

50

60 0

20 0 0.5

re

0.0

-p

SnO2

40

lP

p/po

Jo

ur na

Fig. 2

of

V (cc/g))

0 40

1.0

Fig. 3

of

ro

-p

SnO2

re

lP

ur na

Jo CdS1

of

ro

-p

re

lP

ur na

Jo

SnCd5

Fig. 4

14000

Sn

3d3/2

35000 30000

12000

3d5/2

25000

O

10000

F

20000

D

1s

15000

8000

10000 6000

5000

478 480 482 484 486 488 490 492 494 496 498 500 502

C

540

545

550

1000

1000

402

404

406

408

410 412

414

416

ur na

Jo

418

420

422

900 156

lP

400

G

Fig. 5

re

1100 2000

535

-p

J

H

1200

3000

530

S

1300

4000

0

525

E

1400

3d5/2

5000

520

ro

Cd

6000

515

1500

3d3/2

7000

4000 510

of

0

158

160

162

164

166

I

168

170

172

174

176

2000

SnCd5

2000

-p

SnO2

-1

re

Wave number (cm )

Jo

ur na

lP

Fig. 6

ro

of

Transmittance (%)

CdS

0.010

0.008

0.006

0.002

of

SnO2 CdS SnCdS1 SnCdS5 SnCdS10

0.004

2

3

4

5

6

-p

Energy (eV)

re

1.8

lP

1.6 1.4 1.2 1.0

ur na

Reflectance (%)

ro

0.000

SnO2 CdS SnCdS1 SnCdS5 SnCdS10

0.8 0.6 0.4

Jo

0.2

0.0 200

Fig. 7

300

400

500

600

Wave length (nm)

700

800

4000

of

SnO2 SnCd1 SnCd5 SnCd10

ro

6000

-p

PL intensity (a.u.)

8000

400

Jo

ur na

Wave length (nm)

Fig. 8

600

lP

0 200

re

2000

SnCd1

450

550 650 750 Wavelength (nm)

of ro

850

1.2 1 0.8 0.6 0.4 0.2 0

-p

SnCd8

re

850

ur na

0

550 650 750 Wavelength (nm)

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Absorbance

0.5

Dye dark 10 min 20 min 30 min 40 min 50 min 60 min 90 min 120 min 150 min 180 min

SnCd5

450

1

650 750 Wavelength (nm)

Absorbance

1.2 1 0.8 0.6 0.4 0.2 0

550

lP

Absorbance

450

1.5

Dye dark 10 min 20 min 30 min 40 min 50 min 60 min 90 min 120 min 150 min 180 min

SnCd10

Dye dark 10 min 20 min 30 min 40 min 50 min 60 min 90 min 120 min 150 min

850

450

1.5 Absorbance

Absorbance

1.2 1 0.8 0.6 0.4 0.2 0

550 650 750 Wavelength (nm)

SnCd3

1 0.5 0 450

550 650 750 Wavelength (nm)

Dye dark 10 min 20 min 30 min 40 min 50 min 60 min 90 min 120 min 150 min 180 min

850

Dye dark 10 min 20 min 30 min 40 min 50 min 60 min 90 min 120 min 150 min

850

1 0.5

0 550 650 750 Wave length (nm)

850

1.2 1 0.8 0.6 0.4 0.2 0

zero dark 10-min 20-min 30-min 40-min 50-min 60-min 90-min 120-min 150-min 180-min

CdS

450

550 650 Wave length (nm)

ro

Fig. 9

-p

1.2 1

re

0.8 0.6 0.4

ur na

0.2

SnO2 CdS SnCd1 SnCd3 SnCd5

lP

Ct/Co

750

of

450

Absorbance

Absorbance

Dye dark 10 min 20 min 30 min 40 min 50 min 60 min 90 min 120 min 150 min

SnO2

SnCd8 SnCd10

0

-30

Jo

Fig. 10

20

70

Time (min)

120

170

220

of

2

SnO2

ro

CdS

1.5

-p

1 0.5 0 -0.5

100

ur na

Time (min)

Jo

Fig. 11

50

lP

0

re

lna/a-x

SnCd1

150

SnCd3 SnCd5 SnCd8

SnCd10

200

50

Without scavenger Benzoquinone

Silver nitrate

30

20

KI

Isoropanol

10

0

of

Removal of MB dye (%)

40

Without scavenger Isopropanol

re

KI

40

lP

60

Benzoquinone Silver nitrate

20

ur na

Removal of Rh B dye (%)

80

Jo

0

Fig. 12

-p

ro

Type of scavenger

Type of scavenger

zero

SnCd5

30000

30-min

25000

ro

60-min 90-min

20000

120-min

15000

-p

PL intensity (a.u.)

of

35000

10000

0 400

450

500

lP

350

180-min

re

5000

150-min

Wave length (nm)

ur na

25000

SnO2

zero 30-min 60-min 90-min 120-min 150-min 180-min

15000 10000

Jo

PL intensity (a.u)

20000

5000

0 350

400

450 Wave length (nm)

Fig. 13

500

2

3 Number of cycle

5

Jo

ur na

lP

re

-p

Fig. 14

4

of

1

ro

Removal o MB dye (%)

100 90 80 70 60 50 40 30 20 10 0

140

SnCd5 after 5-cycles for photocatalytic degradation 120

of

100

ro

80 60 40

-p

20 0

20

30

40

50

60

70

re

10

lP

(2)

700

ur na

SnCd5 before dye degradation

500 400 300 200

Jo

Intensity (a.u.)

600

100

0 10

Fig. 15

20

30

40

50

2

60

70

80

80

1 dye

SnCd5

dark 30-min

0.6

60-min 90-min

0.4

120-min 150-min

of

Absorbance

0.8

0.2 0 500

600 Wave length (nm)

1.2

re

CdS

0.8 0.6 0.4 0

ur na

0.2

Jo

400

800

Dye dark 30 min 60-min 90-min 120-min 150min 180-min

lP

Absorbance

1

700

-p

400

ro

180-min

500

600 Wave length (nm)

700

800

1.2

Dye

SnO2

dark

Absorbance

1

30-min

0.8

60-min

0.6

90-min

0.4

120-min 150-min

0.2

180-min

0 600 Wave length (nm)

700

800

of

500

ro

400

Fig. 16 1.2

-p

SnO2

1

CdS

re

0.6

SnCd5

lP

Ct/Co

0.8

0.4

ur na

0.2 0

-50

Jo

Fig. 17

0

50 100 Time (min)

150

200

of

ro

-p

re

lP

ur na

Jo

of

ro

-p

re

lP

ur na

Jo Fig. 18

List of Tables Table 1 Physicochemical properties of the prepared nanoparticles Table 2 Comparative study of photocatalytic degradation of MB dye over various photocatalyst

Surface area

Pore volume

Pore radius

(nm)

(m2/g)

(ml/g)

(Å)

SnO2

34

19.4

0.0908

89.5

CdS

9

26.5

0.157

SnCd1

22

22.8

0.089

86.5

75.9

SnCd5

18

20.7

84.5

52.3

SnCd10

13

0.0106

93.6

45.06

Table 2

Jo

Photocatalyst Our photocatalyst V2O5/ZnO ZnO/CuO CuInSe2 ZnO/ϒ-Mn2O3 ZnO CeO2 V2O5 CuO CeO2/V2O5 CeO2/CuO

ro 119.1

-p

0.087

lP 21.5

ur na

Sample

Removal of MB dye (%) 95% 95% 98% 80% 95% 95% 6.1 27.5 33.7 64.2 70.1

References [17] [18] [20] [23] [15] [40] [40] [40] [40] [40]

C constant

of

Particle size

re

Table 1

55.9 64.7

of

ro

-p

re

lP

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Jo