C,N and S-doped TiO2-characterization and photocatalytic performance for rose bengal dye degradation under day light

Accepted Manuscript Title: C, N and S- doped TiO2-Characterization and Photocatalytic performance for Rose Bengal dye degradation under day light Authors: B. Malini, G. Allen Gnana Raj PII: DOI: Reference:

S2213-3437(18)30528-1 https://doi.org/10.1016/j.jece.2018.09.002 JECE 2623

To appear in: Received date: Revised date: Accepted date:

4-7-2018 29-8-2018 1-9-2018

Please cite this article as: Malini B, Allen Gnana Raj G, C, N and Sdoped TiO2-Characterization and Photocatalytic performance for Rose Bengal dye degradation under day light, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

C, N and S- doped TiO2-Characterization and Photocatalytic performance for Rose Bengal dye degradation under day light

a.Department

of Chemistry,Government College of Engineering, Tirunelveli-,Tamil Nadu, India.

of Chemistry, Scott Christian College (Autonomous), Nagercoil-, Tamil Nadu, India.

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b.Department

Gnana Rajb

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*B.Malinia,b G.Allen ,

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(Affiliated to Manonmaniam Sundaranar University, Abishekapatti, Tirunelveli-627012, TamilNadu, India)

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Abstract

This article investigates the photocatalytic performance of carbon, nitrogen and sulphur-

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doped TiO2 (CNS-TiO2) nanoparticles for the degradation of rosebengal dye. CNS-TiO2 photocatalyst is found to be an efficient catalyst for the degradation of rosebengal dye and 100% degradation is achieved in 60 min. The effects of operational parameters like

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catalyst loading, concentration of dye and pH of dye solution on the rate of dye degradation are studied. Results show that the optimum conditions at which the

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maximum degradation of Rosebengal dye was achievable is at pH 6 at a catalyst loading of 0.1g/100mL and a dye concentration of 20ppm. The effect of dissolved oxygen and hydrogen peroxide on dye degradation is studied and the Chemical Oxygen Demand

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(COD) after irradiation is also found.

Keywords: Rosebengal; Dye degradation; Mineralization

1. Introduction

Contaminations of soil and water bodies by dyes that are released as effluents from textile industries and from other means create threatening ecological problems.

The

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constituents of these dyes are extremely toxic. These are dangerous, mutagenic or if nothing else at least destroys the aesthetic appeal of the receiving soil or water body [1]. The structural constituents of these dyes are metals and other chemicals which impart

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the above effects [2]. As per the reported investigations nearly 20% of those dyes used in textile industries are lost during the operational processes which pollute the aquatic ecosystems

and directly or indirectly affect the human health. There are different categories of dyes. One

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of the dyes that are extensively used belongs to xanthene class. Xanthene dyes are featured

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by the presence of xanthene nucleus where the chromophore is aromatic group. Rosebengal an anionic, organic, water soluble photosensitive dye being extensively used in

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textile and photochemical industries is an example of xanthene dye. It targets the corneal

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epithelium of humans. It also causes skin itchiness, irritation and blistering of skin. It can lead to redness and inflammation of eyes [3,4,5,6]. The attempts to reduce the toxicity of dye

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effluents have been carried out by different physical and chemical methods [7]. The hazardous materials have been degraded successfully by photocatalytic degradation via heterogeneous photocatalysis technique [8]. The salient advantages of it

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over the conventional traditional treatment techniques are that it removes even the low concentrations of pollutant and there are no further secondary treatment methods to be

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adopted. The above process breaks down the organic complex compounds into simpler fragments viz. CO2, H2O and simple mineral acid [9,10].The ambient operational temperature activity, low cost and resistivity to photocorrosion makes TiO2 based

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photocatalysts a novel one in the field of photocatalysis [11]. It is noteworthy to mention that despite several researches and thousands of

publications in the field of Titania based photocatalysis, this technique still suffers some technical barriers which impede its application for real time practice in large scale water treatment. The factors which hinder its commercialisation include: (a) The difficulty in removal of nanosized TiO2 powder from the slurry.

(b) Though this separation problem and problems related to agglomeration can be resolved by loading TiO2 on suitable support, this process reduces the active surface to volume ratios and decreases the mass transfer rates and the inactive support too plays its part in hindering the light harvesting process thereby decreasing the photocatalytic efficiency. (c) In real waters, the complexity of aqueous phase constituting different inorganic species are likely to induce potential interference and cause undesirable effects

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on photocatalysis.

(d) The sustainability, physico-chemical stability of doped and surface-modified

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TiO2 for repetitive use is little explored.

(e) The ways of using combined technologies (photocatalysis along with traditional technologies) for water treatment is still unexplored.

(f) The TiO2 based photocatalytic reactions are non-selective in nature.

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(g) Another inherent obstacle posed by the TiO2 based photocatalysis is its

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deactivation over time.

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Hence, it is pivotal that the above mentioned shortcoming be overcome so that TiO2 can be a viable option for photocatalytic applications on a large scale [12,13,14].

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Yet other two main factors that deprive the reliability on TiO2 based photocatalysis are the fast recombination of the generated e-/hole pair and its inability

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to trap visible light [15].

In order to enhance its activity suitable modifications in TiO2 have been

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suggested. Dye sensitization and doping with impurities (cation or anion) are the two main approaches to surpass these limitations. These cationic or anionic dopants decrease

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the Ea of TiO2 by band gap narrowing or intra band gap formation [16]. Metal doped titania suffers some drawbacks like thermal instability, enhanced

recombination of charge carriers, high cost of metal dopant, health hazard posed by leaching of metal ions and the possibility of photocorrosion. The alternate eco-friendly

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non-metal doping process has attracted much attention in the recent years as it provides more viability for visible light active photocatalysis [17,18]. Effective doping can be achieved in TiO2 at a low cost by modifying it with non-metals. The non-metal dopants influence the valence band through interaction with O2p orbitals. The localized states or p states of non-metal dopants generally form the impurity levels and lie above valence band which extends the optical absorption edge of TiO 2 [19]. For example,

extending the optical response of TiO2 to visible region is feasible by incorporation of nitrogen [20]. The incorporation of nitrogen via oxygen substitution results in bandgap narrowing due to the mixing of the nitrogen 2p and oxygen 2p states [21].On irradiation with visible light, electrons of the localized 2p states can be excited up to the individual conduction band, leaving holes on the localized states. The energy barrier between midgap states would suppress electron transfer, inhibiting recombination of photoelectrons and holes [13].

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Sulphur doping shows an antibacterial effect and has attained attention due to its thermal stability. The overlap of sulphur 3p states and oxygen 2p states facilitates the visible light catalytic activity of S-doped TiO2 [16, 22, 23]. Carbon doping increases the conductivity of the

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structure and enhances the separation of photo-generated carriers. Carbon in the doped

samples can also be the lattice defect of TiO2 to form interface states that effectively lower the band gap [24, 25].

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In general, the non-metal dopants effectively narrow the band gap of TiO2. The change

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of lattice parameters and the presence of trap states within the conduction band and valence bands from electronic perturbation give rise to band gap narrowing. This allows visible light

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photogenerated charge carriers [13].

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absorption and the presence of trap sites within the TiO2 bands increase the lifetime of Simultaneous doping with two elements onto TiO2 i.e co-doping of TiO2 can be

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considered as a much better process [26]. It is presumed that co-doped catalysts will show increased performance under visible light because of the possibility of sensitization

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by various mid gap energy sensitization energy levels introduced by the doping of different non-metals as well as better electron-hole separation due to the formation of

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heterojunctions. Co-doped TiO2 usually shows an enhanced photocatalytic activity in the visible range due to the merits benefited from each dopant [16]. The main focus of the present research work is to introduce carbon, nitrogen and

sulphur into the TiO2 lattice and to shift the absorption edge of TiO2 to 400-800nm range. The

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present results show enhanced photocatalytic activity for rosebengal dye degradation under day light illumination. 2. Experimental 2.1 Material The reagents used for the synthesis of CNS-TiO2 were titanium isopropoxide(Aldrich), thiourea (Aldrich) and isopropyl alcohol(Merck). Double distilled water was used for all experiments. Rosebengal dye was purchased from Himedia.

2.2 Sample preparation The simple hydrolysis process was used to synthesize CNS-TiO2 photocatalyst. The precursor for the synthesis of TiO2 was titanium isopropoxide. Thiourea was used as a source of carbon, nitrogen and sulphur. The synthesis procedure is as follows. Isopropyl alcohol (30mL) was taken in a beaker and to this titanium isopropoxide (10mL) was added and mixed well. This mixed solution was then added dropwise to desired amount of double distilled water in a beaker and subjected to continuous stirring for 4 h using a magnetic stirrer. thiourea (5wt%)

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dissolved in double distilled water was added to the above solution and stirring was continued

again for 6 h and then dried in a oven for 12h at 80°C.Solid product was formed. It was further

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calcined at 400°C in air to get the required CNS-TiO2 photocatalyst [27]. 2.3 Characterization

Powder X-Ray Diffraction (PXRD) of the obtained CNS-TiO2 powders was done by using X-ray Diffractometer (Bruker AXS D8 Advance) with CuKα radiations. Scanning

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Electron Microscope (SEM) measurements were carried out using SEM (Vega 3 Tescan)

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equipped with Energy Dispersive X-Ray Analysis (EDAX) (Bruker). Spectral changes of

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rosebengal during photocatalytic degradation were studied using Ultraviolet-Visible (UV-Vis) spectrophotometer (PG Instrument). Diffuse Reflectance Spectra (DRS) measurements of

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CNS-TiO2 and un-doped TiO2 photocatalysts were carried out using Ultraviolet-Visible-Near Infra Red (UV-Vis-NIR) Spectrophotometer (Varian, Cary 5000).

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2.4 Evaluation of photocatalytic activity The photocatalytic activity of the synthesised CNS-TiO2 was evaluated using aqueous

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solutions of rosebengal dye. To a known concentration of rosebengal dye solution definite amount of the synthesized CNS-TiO2 was added. Prior to solar irradiation, the suspension was stirred for 30 min in dark to facilitate the attainment of adsorption-desorption equilibrium of

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the dye on the catalyst surface. After this the dye solution was exposed to solar irradiation. The photocatalytic experiments were carried on a sunny day under clear sky conditions at Tirunelveli between 11a.m. to 2p.m. 5 mL of irradiated suspension was drawn out at regular

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time intervals and immediately centrifuged. The change in absorbance during the course of the reaction was monitored using a UV-Vis spectrophotometer. 3. Results and discussion 3.1 Characterization To analyse the crystal structure of the synthesized CNS-TiO2 photocatalyst, PXRD technique was used. It can be seen from Fig.1 that CNS-TiO2 exhibits the characteristic

peak of anatase (JCPDS No: 21-1272). The results of XRD are in good agreement with earlier reports [26]. Applying Debye Scherrer equation, on the diffraction peaks:

𝐷 = 𝑘𝜆/𝛽𝐶𝑜𝑠𝜃

(1) where D is the average crystallite size, k the constant which is taken as 0.89, λ the wavelength of the X-ray radiation (CuKα=0.154056nm), β the corrected band broadening is the full width at half maximum (FWHM) of the diffraction line, and θ is the diffraction angle. The calculated average crystallite size for the sample was 10nm.

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To identify the elements present in the synthesized CNS-TiO2 EDAX, a semiquantitative technique is employed. The EDAX spectrum of the synthesized CNS-TiO2 is

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presented in Fig.2(b) and that of un-doped TiO2 is presented in Fig.2(a). The EDAX spectrum reveals the incorporation of C, N, S, Ti and O in the synthesized photocatalyst.

SEM results of the prepared CNS-TiO2 are shown in Fig.3(c),(d) and that of undoped TiO2 in Fig.3(a),(b). It is obvious from the figure that there are irregularly shaped

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particles which can be considered as the aggregates of tiny crystals.

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The diffuse reflectance spectrum (DRS) of CNS-TiO2 and un-doped TiO2 was measured in the range of 250-800 nm to investigate the optical absorption properties of

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CNS-TiO2 photocatalyst and is shown in Fig.4. It can be noted from figure that CNS-TiO2

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showed significant absorption between 400 and 500 nm and the optical absorption in the UV region was also enhanced. The band gap narrowing of TiO2 by non-metal doping is the

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reason for this bathochromic shift. The band gap energy (Eg) of the doped samples are determined by the equation,

(2)

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𝐸𝑔 = 1239.8⁄𝜆

λ is the wavelength of the onset of the spectrum[24]. This lead to band gap

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narrowing to 2.91eV as compared to 3.2eV for un-doped TiO2 as obvious from Fig.4 and hence the doped photocatalyst is likely to be useful under visible light illumination. 3.2 Effect of catalyst amount The amount of photocatalyst being added to dye solution influences the rate of

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degradation. In case of dye solution with high amounts of catalyst, the aggregation of catalyst particles is also a factor which plays its role in altering the dye degradation rate [28]. The number of active sites and hence the number of OH radicals that take part in discolouration of dye increases with increase in catalyst amount. However, when the catalyst amount exceeds a certain limit the turbidity of the solution increases, blocking the penetration of radiation and thereby decreasing the degradation rate [29]. The optimal amount of CNS-TiO2

catalyst for the degradation of rosebengal was determined by varying the amount of catalyst from 0.02-0.16g/100mL at a constant dye concentration of 10 ppm. The percentage of dye degraded at different time intervals at various catalyst loadings is calculated and depicted in Fig.5. It is evident from figure that with the increase in the amount of catalyst from 0.020.16g/100mL there is an increase in percentage of dye degradation upto 0.1g and hence the optimal amount of catalyst for the present study was taken as 0.1g/100mL.

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3.3 Effect of dye concentration Only the quantity of dye adsorbed on photocatalyst surface is responsible for the photocatalytic process [30]. With a fixed catalyst amount and an increasing dye concentration

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a decrease in dye degradation is generally observed. The photocatalyst surface gets adsorbed

with more organic substances at high dye concentration. But only less photons and hence less OH˙ are available to reach the catalyst surface thereby diminishing the degradation rate [31].

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It is evident from the Fig.6 that as the initial concentration of the dye was varied from 5 ppm to 25 ppm in increments of 5 ppm at a constant catalyst loading of 0.1g/100mL it is seen that

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at 20 ppm there is maximum dye degradation. In this way, the optimum dye concentration was

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fixed as 20 ppm.

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3.4 Effect of pH The pH of the solution also affects the photodegradation efficiency. The surface charge

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of TiO2 particles is altered when there is a change in solution pH. As a result the adsorption of dye on the catalyst surface is altered which imparts a change in the degradation rate[32]. So, the effect of solution pH on dye degradation has been investigated in the range 4-8. It is evident

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from Fig.7 that as pH is increased from 4-8, there is an initial increase in the degradation values upto pH 6 followed by decrease in rate of degradation.

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3.5 Photocatalytic activity The degradation spectra of rosebengal dye (20 ppm) under sunlight in the presence of CNS-TiO2 photocatalyst (0.1g/100mL) at pH 6 is shown in Fig.8. It took 60 min to completely degrade rosebengal dye. As observed, a strong peak relating to

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absorbance of rosebengal at 545 nm turned out almost flat in 60 min term of time. When un-doped TiO2 was used 90% degradation occurred in 150 min. 3.6 Effect of dissolved oxygen In photocatalysis reactions dissolved oxygen acts as an electron acceptor. It acts as electron scavenger and traps the excited conduction band electron preventing recombination [31]. The degradation experiments were carried out in the presence of atmospheric O2 by taking

rosebengal (20ppm) CNS-TiO2 (0.1g) at pH (6) under sunlight illumination. Fig.9 shows that 100% percent dye degradation was achieved in 45 minutes whereas with un-doped TiO2 92.55% degradation occurred in 150 minutes. 3.7 Effect of hydrogen peroxide H2O2 was added to the dye solution (20ppm) to inhibit (e-/h+) recombination. H2O2 enhances the formation of hydroxyl radicals. These hydroxyl radicals act as good electron acceptors when compared to molecular oxygen, thereby proving to be an effective alternate for

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oxygen [32]. Thus, slightly enhanced degradation with H2O2 when compared to that in the presence of oxygen was observed as shown in Fig.10 and 100% degradation was achieved in

45 minutes whereas in the case of un-doped TiO2 it took 150 minutes to achieve 92.78%

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degradation.

3.8 Kinetics of the photocatalytic degradation of rosebengal The photodegradation of rosebengal dye at optimum conditions is shown in Fig.11 and

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the kinetic plots are shown in Fig.12. In order to determine the order of the reaction, the photo degradation reactions of rosebengal were carried out at optimum conditions, along with

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dissolved oxygen and along with hydrogen peroxide as depicted in Fig.12(a), 12(b), 12(c).

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The plot of lnC0/C vs. irradiation time shows a straight line behavior as observed in all

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case. The rate constant for the photo catalytic degradation of rosebengal was obtained from the first order rate equation

lnC0/C=Kt

(3)

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where C0 and C are the concentrations of substrate at time 0 and time t in minutes. K is the first order rate constant (min-1) determined from the slope of the straight line. The regression

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coefficient R2 of the experimental values was found to be 0.955, 0.961, 0.976 when dye degradation was carried out at optimum conditions, along with dissolved oxygen and along

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with hydrogen peroxide respectively. This confirms that the degradation of the dye molecules obeys the pseudo first order linear kinetics. optimum conditions optimum conditions along with dissolved oxygen optimum conditions along with hydrogen peroxide

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3.9 Mineralization With the goal of contaminant control in the degradation process, it is desirable to mineralize the organic pollutants into CO2 and H2O [33]. The COD varies during the photo catalytic degradation of rosebengal (20ppm, 100mL). Estimation of COD had been carried out by standard techniques. The proficiency of dye mineralization was estimated using the following expression [34] Mineralization %= {1-(COD/COD0)}*100

(4)

where COD and COD0 correspond to CODs at final and initial dye concentration. The suspension at optimum conditions was irradiated for 2 hours. The percentage mineralization was found to be 100 after 2 hours. 4. Mechanism The photo-catalytic degradation of the dye probably follows the following mechanism. Rosebengal molecule being photosensitive absorbs radiations and moves to its singlet excited state. It then undergoes inter system crossing and gives the triplet excited state. The CNS-

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TiO2 photocatalyst utilizes the incident light energy to excite the electron from its valence band

(VB) to the conduction band (CB). The hole left behind on the VB absorbs an electron from OH- to generate OH˙radical. The radicals oxidize the dye to its colourless leuco form that gets

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degraded to harmless compound.

Conclusion CNS-TiO2 photocatalyst was prepared by a simple hydrolysis process and its role in

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the photocatalytic degradation of hazardous rosebengal dye was investigated. The results conclude that CNS-TiO2 is an efficient catalyst for the photocatalytic degradation of rosebengal

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dye. The substrate concentration of 20ppm, catalyst amount of 0.1g/100mL and the solution

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pH 6 are found to be favourable for higher degradation rates. Photodegradation of the dye could

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be performed fast if the optimization of condition was accomplished.

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Acknowledgements

The authors would like to thank Sophisticated Test and Instrumentation Centre (STIC), Cochin and The Gandhigram Rural Institute (GRI), Dindigul for the sophisticated instrument

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Fig.1. PXRD of un-doped TiO2 and CNS-TiO2

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(b) Fig.2. EDAX spectra of (a) un-doped TiO2 (b) CNS-TiO2

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(c) (d) Fig.3. SEM images of (a),(b) un-doped TiO2 (c),(d) CNS-TiO2

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Fig.4. UV-Vis absorption spectra of un-doped TiO2 and CNS-TiO2

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Fig.5. Variation of percentage degradation after 60min irradiation as a function of catalyst loading

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Fig.6.Variation of percentage degradation after 120min irradiation as a function of dye concentration

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Fig.7. Variation of percentage degradation after 120min irradiation as a function of solution pH

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Fig.8. Time dependent photocatalytic degradation of rosebengal dye under sunlight irradiation using CNS-TiO2

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Fig.9. Effect of dissolved oxygen on dye degradation at optimum conditions

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Fig.10. Effect of H2O2 on dye degradation at optimum conditions

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Fig.11. Dye degradation at optimum conditions

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Fig.12(a). Kinetics plot for the photodegradation of rosebengal dye at

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Fig.12(b). Kinetics plot for the photodegradation of rosebengal dye at

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Fig.12(c). Kinetics plot for the photodegradation of rosebengal dye at