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Summer Research Fellowship Programme of India's Science Academies

Synthesis of BiOBr1-xClx photocatalysts: Compositional variation leading to improved activity under visible light irradiation

Bibek Samanta

Ramakrishna Mission Vidyamandira, Belur Math, Howrah 711202, India

Dr. C. Shivakumara

Indian Institute of Science, Bangalore 560012, India

Abstract

Visible light-driven water detoxification by semiconductor photocatalysts offers an attractive route to fight water pollution due to the simplicity of the process. A vast number of photocatalysts have been used to serve this purpose till date. Though Titania based photocatalysts are most widely explored in this field, they have shown significant absorption only in the UV region which limits its uses in visible light driven photocatalysis. Thus various modification of Titania along with the search for non-titania based photocatalysts is being practiced. In recent days, bismuth oxyhalides (BiOX, X=Cl. Br,I) based photocatalysts have attracted wide attention due to various suitable properties like photocatalytic efficiency and stability regarding water purification. These are V-VI-VII ternary compounds with a unique layered structure. The layered structure provides an internal electric field between positively charged Bi2O22+ layer and negatively charged X-(halide) layer which helps to prevent the recombination of photogenerated electron and holes leading to a improved photocatalysis phenomenon. Photocatalytic efficiency of these compounds are also dependent on exposure of some selected facets like {001} and the interlayer spacing in the [001] direction. The band gaps of the bismuth oxyhalides i.e BiOCl, BiOBr and BiOI are about 3.4, 2.8 and 1.8 eV respectively. So the absorption of these compounds can be tuned from UV region to visible region through compositional variation, which can be employed for improved photocatalytic action. Using this concept of band gap modulating, synthesis of a series of solid solutions of BiOCl and BiOBr expressed by the formula BiOBr1-xClxhas been attempted in this work. A facile solid state method has been adopted for the preparation of the target solid solutions. These solid solution semiconductor photocatalysts are expected to show an improved photocatalytic performance under visible light irradiation with increased absorption in the desired range.

Keywords: bismuth oxyhalides, semiconductor, internal electric field, solid solutions, band gap tuning, visible light irradiation

Abbreviations

Abbreviations
UV                                                                                 Ultra-violet
 RhBRhodamine B 
 MO Methyl orange 
 CRCongo red
 VBValence band 
CB  Conduction band
 XRDX-ray diffraction 
 SEMScanning electron microscopy 
 TGAThermogravimetric analysis
 FTIRFourier-transformed infra-red 

INTRODUCTION

Background/Rationale

Since the discovery of water splitting on TiO2 by Fujishima and Honda[1], photocatalysis has been a central method to address various environmental issues, specially the water pollution. TiO2 is the mostly used photocatalyst due to its stability, cost, availability etc[2]. But it poses a wide band gap of 3.0-3.2 eV which leads to absorption mainly in the UV region, which contributes to only around 4% of the total solar spectrum. Hence the use of pure TiO2 as efficient solar or visible light driven photocatalyst is limited[3]. Over the years, various modification of TiO2 in morphology, synthetic route, decoration with co-catalysts etc has been practiced to increase its performance under visible radiation. Simultaneous search of non-titania based photocatalysts were also of immense interest.

Therein, bismuth based photocalysts have attracted great deal of attraction due to their suitable band gap for visible light utilization[4]–[6]. Bismuth oxyhalides (BiOX, X=Cl, Br, I) are group V-VI-VII ternary compounds with significant photocatalytic activity. These oxyhalides have tetragonal matlockite structure with P4/nmm (no. 129) space group[7], [8]. They have unique layered structure where layers of [Bi2O2]2+ is interleaved between halide(X-) layers. There are covalent bonds between the atoms of [Bi2O2]2+ layer whereas the layers along c-axis show van der Waals force of attraction. This strong intralayer bonding and weak interlayer bonding makes this category of material promising for applications in the fields like photocatalysis, photoelectrochemical reactions, pharmacy etc. The internal electric field along c-axis and indirect nature of the band gap are helpful for separation of the photogenerated electron-hole pair which in term facilitate the overall process. Their photocatalytic activity also strongly depends on the amount of certain exposed facets[9]–[11]. So to optimize the materials property diverse field of synthesis procedures like solvothermal[3], [12]–[16], hydrothermal[11], [17], [18], co-precipitation [19][5], alcoholysis[10], microwave irradiation[18], sonochemical[20]have been employed. But all these processes suffer from various limitations like low yield; impurities coming from the solvents used, less crystallinity, rigorous chemical conditions etc. The solid state synthesis is much easier to perform with several advantages like greater yield, no solvent impurities, lesser duration and higher crystallinity[7], [8]. Recently, mixed oxyhalides solid solutions have gathered attraction due to their improved photocatalytic property obtained through band gap tuning towards visible region absorption[10], [14] [17][19][21].

Herein, a series of compounds with the chemical formula BiOBr1-xClxhas been synthesized via a facile solid state method. Structural studies were performed using XRD and SEM analysis. Thermal stability of these compounds was checked by TGA. Presence of Bi-O bonds was confirmed by FTIR. Band gaps of the prepared materials were determined using the UV-Vis diffuse reflectance data. Their photocatalytic activities were checked in terms of degradation of rhodamine B (RhB), methyl orange (MO) and congo red (CR).

Statement of the Problems

Following problems are generally encountered during visible light photocatalysis-

1. Preparing a photocatalyst of high purity and crystallinity.

2. Inefficient absorption of the provided radiation by the photocatalyst.

3. Difficulty in separation of the photogenerated electron-hole pair.

4. Significant yield of the target photocatalyst.

Objectives of the Research

Objectives of this project are the followings-

1. Facile synthesis of BiOBrxCl1-x photocatalysts via solid state method.

2. Controllable tuning of band gaps for utilization of visible light.

3. Examining the performances of the photocatalysts for both basic and acidic dyes.

4. Kinetic study of the photocatalysis.

5. Establishment of the reasons behind the behavior shown by the synthesized material.

LITERATURE REVIEW

Though bismuth oxyhalides are known in literature for a long time, their excellent photocatalytic activity came into observation quite recently. Among the bismuth oxyhalides BiOCl[15], [22]–[25]and BiOBr[9], [12], [26]–[29] are the mostly studied followed by BiOI[6], [17], [21], [30]. Various approaches are being practiced to modify the properties of these compounds in order to meet the requirements in the area of concerns. Some works focused on the facet dependency of photocatalytic property, they have synthesized compounds with greater amount of a particular exposed facet[9]–[11]. Variations in properties with change in morphology (e.g nanosheet[10], [11], [16], [24], [28], [31], nanoflower[29], microsphere[3], [4], [12], [13] , nanoplates[10], [14], [32] etc) have been studied in detail. Other methods like composite preparation[6], [15], [17], [33], heterojunction fabrication[6], [9], hybridization[15], [34], self assembly[35], solvent adjustment[31] etc also have been reported. However, there are fewer reports on modulating the band gaps through preparing solid solutions of two different bismuth oxyhalides[3], [10], [36], [37]. Besides, all of the reported solid solutions were prepared using solvent based methods like hydrothermal, solvothermal[3], [14], alcohololysis[10], co-precipitation[19], [38] etc, hence suffering from the limitations of impurities and less yield. Herein, based on this literature survey, a novel solvent free solid state synthesis of BiOBrxCl1-xsolid solutions is attempted in this work. Their photocatalytic activity is examined under visible light irradiation with both acidic and basic type dyes.

EXPERIMENTAL

Dye Preparation

Dye solutions of required concentration were prepared by dissolving exact amount of dye in distilled water taken in volumetric flask. The dyes were dissolved thoroughly and the water was filled up to the mark. The prepared stock solutions were kept in dark to ensure unwanted degradation.

To prepare 5 ppm dye solutions, 5 mg of the dye was transferred to a 1000 ml volumetric flask followed by proper dissolution.

f1.jpg
     Different dyes (5 ppm concentration) a) Rhodamine B b) Congo Red and c) Methyl Orange 

    Catalyst Synthesis

    Chemicals

    The required chemicals are –

    1.     Bismuth Oxide (Bi2O3)

    2.     Ammonium Chloride (NH4Cl)

    3.     Ammonium Bromide (NH4Br)

    Preparation Method

    3.2.2.1 Precursor measurement

    A simple solid state method was utilized to prepare the BiOBr1-xClx catalysts. The starting materials were taken according to their stoichiometry in the target catalyst. A total excess of 15% of the ammonium halides (distributed according their stoichiometric ratio) was maintained during preparation.

    t1.jpg
      Amounts of precursors taken for BiOBr1-xClxcatalysts preparation

      3.2.2.2 Grinding

      To prepare a catalyst, all the required precursors were taken in an agate mortar and were ground thoroughly with pestle to make fine powder. Grinding was done in open atmosphere and room temperature. The grinded material was collected in crucible of proper size.

      f2.jpg
        Grinded precursors for calcination

        3.2.2.3 Calcination

        The crucible was put into a furnace with lid. Then the following heating schedule was performed. Same heating schedule was followed for each catalyst.

        Heating schedule for catalysts preparation
         StepTemperature  Duration
        heating to 400 °C 2 hr 
        soaking at 400 °C   1 hr
         cooling to 50 °C 3 hr
        f3.jpg
          BiOBr1-xClxcompounds after calcination

          Photocatalytic Activity Measurements

          Chemical setup

          50 ml of the dye solution was taken in a clean and dry 100 ml beaker. A magnetic bead was kept inside the solution. An amount of 0.0500g catalyst was added to the solution.

          f4.jpg
            RhB dye with catalyst in beaker

            Dark period

            The beaker was kept inside the irradiation source box. It was stirred magnetically for 30 minutes in dark before starting the irradiation to ensure the establishment of adsorption-desorption equilibrium.

            Irradiation period

            After the dark period, the solution was irradiated with visible light (>420 nm) from a 500 W metal halide lamp. The source was 15 cm above the solution. The solution was continuously stirred magnetically throughout the whole degradation period.

            f5_1.jpg
              Metal halide lamp as illumination source

              Sample collection

              An amount of approximately 3 ml was being collected from the solution at proper intervals. These samples were collected in small glass vials for UV measurement.

              f6 1.jpg
                 RhB samples during illumination period for BiOBr1-xClx  a) x = 0.00,  b) x = 0.25,            c) x = 0.50, d) x =0.75    
                F6 2_2.jpg
                  RhB samples during illumination period for BiOBr1-xCl e) x = 1.00

                  f7.jpg
                    CR samples during illumination period for  a) BiOBr b) BiOCl
                    f8.jpg
                      MO samples during illumination period for  a) BiOBr   b) BiOCl

                      MATERIALS CHARACTERIZATION

                      XRD patterns were recorded using PANalytical X'Pert Pro Powder diffractometer with Cu Kα radiation (λ= 1.5418 Å) source and a nickel filter. Structural parameters were determined by Rietveld refinement using FullProf Suite-2000 software. Surface morphology was analysed using SEM (FEI Quanta 200). Thermogravimetric analysis was done using Mettler-Toledo system upto a temperature of 900 °C in presence of nitrogen as a carrier gas. UV-Visible diffuse reflectance spectra of the compounds were measured with Perkin Elmer Lambda 750 spectrophotometer. Absorbance of the dye solutions were recorded using Perkin Elmer UV-Visible spectrophotometer (Lambda 35).

                      RESULTS AND DISCUSSIONS

                      XRD Analysis

                      f9.jpg
                        a) Indexed powder XRD patterns of synthesized BiOBr1-xClx (x = 0.00, 0.25, 0.50, 0.75, 1.00) samples   b) Shift towards higher angles with increasing Cl content.

                        XRD patterns of the synthesized catalysts were analyzed for phase purity and confirmation of solid solutions formation. Both BiOBr and BiOCl has shown well matched pattern with their corresponding JCPDS data, card no. 09-0393 and 06-0249 respectively. The sharp and intense peaks referred to good crystallinity of the samples. A gradual right shift in the peaks of the catalysts with increasing amount of Cl is observed (fig. 11b) due to the smaller size of Cl atom with respect to Br atom, which is in well accordance with Bragg’s law-

                        nλ = 2dhklsin θ

                        where, n = order of reflection, λ=wavelength of the X-ray used, dhkl = interplanar spacing between (hkl) planes, 2θ = angle of diffraction of X-rays.

                        f10  a b.jpg
                          a) Observed, calculated and the difference XRD patterns of i) BiOBr, ii) BiOBr0.50Cl0.50 iii) BiOCl   b) Variation in cell parameters for BiOBr1-xClx

                          Rietveld refinement of the obtained XRD data shows that all samples belong to tetragonal phase of space group P4/nmm (no. 129). The structural data are summarized in fig. 14. A smooth linear decrease in structural parameters with increasing amount of Cl(fig. 12b) indicates the formation of solid solution. Based on the refined parameters schematic crystal structures of BiOCl was created using VESTA program (fig. 13).

                          f10 2_1.jpg
                            Crystal structure of BiOCl using VESTA
                            T3.jpg
                              Rietveld refined structural parameters for BiOBr1-xClx.
                              f11_1.jpg
                                Peak intensity ratios for the prepared compounds

                                A qualitative idea of the exposed facets present in the catalysts can be obtained from the intensity ratio of some selected peaks as shown above. The intensity ratio of peaks corresponding to (110) and (001) i.e. I(110)/(001) and the intensity ratio of peaks corresponding to (200) and (002) i.e. I(200)/(002) gives a qualitative idea about the amount of exposed (010) and (001) facets[16]. Greater amount of exposed (010) facets is characterized by I(110) < I(001) and I(200) < I(002) while the opposite i.e. I(110) > I(001) and I(200) > I(002) indicates the dominance of exposed (001) facets. As all samples have shown both I(110)/(001) and I(200)/(002) value greater than 1(fig.15), the samples posses greater amount exposed (010) faces than (001) faces.

                                SEM Analysis

                                f12_1.jpg
                                  SEM micrographs of a) BiOBr b) BiOCl

                                  Surface morphology of the compounds was investigated using SEM analysis. Plate-like structures were seen for both BiOCl and BiOBr. This is in well accordance with previous reports[4], [10], [14], [20], [32]. Average diameter of the plates was greater for BiOBr samples than BiOCl samples. BiOCl was more agglomerated in nature compared to BiOBr which affects the photocatalytic activity of the material.

                                  Thermogravimetric Analysis

                                  f13_1.jpg
                                    TGA curve of a) BiOBr  b) BiOCl0.50Br  c) BiOCl

                                    TGA was performed to check the thermal stability of the synthesized compounds. There was no noticeable change in weight up to 650°C, 580°C and 555°C for BiOBr, BiOBr0.50Cl0.50 and BiOCl respectively. This confirms the stability of the compounds under experimental temperature. The drastic change in weight after the temperatures mentioned is attributed to their decomposition to Bi2O3 and corresponding halogen gases[7], [8]. Weight losses of 27%, 21% and 12% were observed for BiOBr, BiOCl0.50Br0.50 and BiOCl respectively. Further decomposition of the Bi2O3 took place in the temperature range of 660°C to 750°C.

                                    FTIR Analysis

                                    f14_1.jpg
                                      FTIR spectra of BiOBrxCl1-x compounds

                                      FTIR spectra of the prepared compounds were recorded in the region of 2000-300 cm-1. Peaks around 500 cm-1 (lying within the marked area) are due to stretching of Bi-O bonds[25]. Shifting towards lower frequency with decreasing Cl/Br ratio is attributed to the increase of fraction of atoms with higher atomic weight leading to lowering of vibrational frequency[7].

                                      UV-Vis Absorption Spectroscopy

                                      f15_1.jpg
                                        a) UV-Vis absorption spectra of BiOBr1-xClx with corresponding plots of (F(R)hν)1/2 vs hν for a) x = 0.00 b) x = 0.25 c) x = 0.50 d) x = 0.75 e) x = 1.00

                                        Optical properties of the synthesized compounds were studied using UV-Vis diffuse reflection spectroscopy. Band gap of the compounds were measured using Kubelka-Munk method[25]. The values of band gaps found are listed in the table below-

                                        Band gaps of the synthesized photocatalysts
                                        Material Band gap(eV)
                                         BiOBr 2.89
                                         BiOCl0.25Br0.75 2.91
                                         BiOCl0.50Br0.50 3.02
                                         BiOCl0.75Br0.253.16 
                                         BiOCl 3.36

                                        The obtained values are in well accordance with previous reports[10], [14], [38], [39]. A gradual and desired decrease in the band gap values with increasing Br/Cl ratio is obtained. It is well known that the top of VB in BiOX (X = Cl, Br, I) is composed of 2p orbital of O atom and np orbital of halogen atom while the bottom of CB consists of 6p and 6s orbitals of Bi atom. Hence the VB top can be modulated through controlling the ratio of Cl 3p and Br 4p in BiOBrxCl1-x solid solutions[14]. CB bottomalso changes due to variation in Br/Cl ratio[27], [40]. Thus solid solution formation provides and effective pathway for band gap tuning in BiOBrxCl1-x compounds.

                                        f15 b.jpg
                                          Band gap structures of BiOBr1-xClx compounds

                                          Positions of the valence band top and conductance band bottom was determined using the formula-

                                          ECB = X - EC - 0.5Eg …. (1)

                                          Where ECB refers to CB bottom of the material at point of zero charge, X is absolute electronegativity of the material; EC is free electron’s energy on hydrogen scale and Eg is band gap of the material. From fig 20. it is seen that with incorporation of Br the VB top gradually decreases from 3.36 to 2.89 eV due to higher contribution of 4p electrons.

                                          Photocatalytic activity

                                          f16 1_1.jpg
                                            UV-Vis absorbance spectra of RhB/catalyst suspension as a function of time over BiOBr1-xClx   a) x = 0.00
                                            f16 2.jpg
                                              UV-Vis absorbance spectra of RhB/catalyst suspension as a function of time over BiOBr1-xClx   a) x = 0.00 b) x = 0.25 c) x = 0.50 d) x = 0.75 e) x = 1.00
                                              f17.jpg
                                                UV-Vis absorbance spectra of CR/catalyst
                                                suspension as a function of time over a) BiOBr b) BiOCl
                                                f18.jpg
                                                  : UV-Vis absorbance spectra of MO/catalyst suspension as a function of time over           a) BiOBr b ) BiOCl

                                                  Photocatalytic activity of the prepared photocatalysts was examined through monitoring the gradual change in the absorption maxima of the aqueous dye/catalyst suspension. Continuous decrement in the intensity of the absorption maxima indicates the degradation of the dye under visible light illumination. The absorption maximum for RhB shifts towards lower wavelength i.e. a blue shift is observed during degradation. This is attributed to the gradual de-ethylation of the RhB molecule over time by active oxygen species[7],[8]. The tetra-ethylated rhodamine molecule i.e., RhB dye shows an absorption maxima at 554 nm whereas the tri-, di-, monoethylated and completely de-ethylated form of rhodamine show absorptions centered at 539,525,505 and 497 nm respectively as depicted in fig. 21. Degradation of CR dye was below that of RhB dye. MO shows a little degradation with BiOCl while almost no significant degradation with BiOBr. This indicates the better applicability of this compound for basic dyes rather than acidic one.

                                                  f19.jpg
                                                    Percentage of dye degradation over BiOBr1-xClx
                                                    f20.jpg
                                                      Comparison between degradation of acidic and basic dyes on a) BiOBr b) BiOCl.   

                                                      Fig. 25 shows the degradation percentage of the RhB over time. During the dark period of 30 minutes for adsorption-desorption equilibrium, we observed 43.6%, 39.7%, 37.7%, 37.7%, 37.4% degradation of RhB on BiOBr1-xClx catalysts, where x = 1.00, 0.75, 0.50, 0.25 and 0.00 respectively. So the BiOBr1-xClx catalysts also posses reasonable adsorption capacity. 88.8%, 99.9%,97.4%,97.9% and 94.6% degradations were observed within 90,90,105,135,180 minutes for BiOBr1-xClx catalysts where x = 0.00, 0.25, 0.50, 0.75 and 1.00 respectively. MO shows only 21.53% degradation with BiOBr and insignificant degradation with BiOCl over 90 minutes.CR shows 85.92% degradation with BiOBr over 90 minutes and 73.28% degradation with BiOCl over 180 minutes. Comparison of the degradation percentage is presented in fig.26.

                                                      f21.jpg
                                                        Comparison between absorbance spectra of RhB/catalyst suspension over BiOBr1-xClx  catalysts for a) 60 min b) 90 min

                                                        Comparisons between the absorption curves of aqueous RhB/catalyst suspensions at 60 minute and 90 minute are presented in fig 27. This clearly shows that the compound BiOBr0.75Cl0.25 posses the best photocatalytic property followed by BiOBr0.25Cl0.75, BiOBr0.50Cl0.50, BiOBr and BiOCl. Amount of degradations for each compound within 60 and 90 minute are presented in the table below.

                                                        Comparison of RhB degradation percentage within 60 minute and 90 minute
                                                        Compound60 minute90 minute
                                                        BiOBr0.75 Cl0.25 85.25 99.89
                                                        BiOBr0.25 Cl0.75 82.0190.54 
                                                        BiOBr0.50 Cl0.50 79.0088.77 
                                                        BiOBr 75.6784.92 
                                                        BiOCl 69.6480.24 
                                                        f22.jpg
                                                          a) photodegradation of RhB over BiOBr1-xClx catalysts b) kinetics of RhB degradation over BiOBr1-xClx catalysts with corresponding rate constants (k).   

                                                          Photodegradation of RhB as a function of time is presented in fig 28a. The reaction kinetics of the photocatalysts was studied to evaluate their efficiency (fig 28b). All of the data are consistent with the general Langmuir-Hinshelwood equation-

                                                          R=-(dC/dt)=kKC/1+KC

                                                          Where R is degradation rate (mg L-1 min-1) of the dye, C is dye concentration (mg L-1), t is duration of irradiation, K is adsorption coefficient (L mg-1) and k is rate constant (min-1). When the value of C is very small, the above equation can be presented by-

                                                          ln(C0/Ct) =kKt =kappt…… (2)

                                                          Where C0 is initial concentration (at 0 min) of RhB dye solution and Ct is the concentration of the solution after a time t of visible light illumination.

                                                           From eq.(2) we can see that a plot of ln(C0/Ct) vs t will give a straight line whose slope will be equal to apparent first-order rate constant kapp which in this case are turned out to be 0.0057 min-1, 0.00915 min-1, 0.01099 min-1, 0.02486 min-1 and 0.00663 min-1 for BiOBr1-xClx catalysts where x = 1.00, 0.75, 0.50, 0.25 and 0.00 respectively. The rate for BiOCl0.25Br0.75 is 3.75 times higher than BiOBr and 4.36 times higher than BiOCl. This indicates that the compound BiOBr0.75Cl0.25 has the highest photocatalytic activity among all the prepared catalyst. All the three solid solutions have shown better rate constants than both pure BiOBr and BiOCl.

                                                           Dye degradation via semiconductor photocatalysis follows two main paths- photosensitization of dye and photo excitation of semiconductor. As the band gap values of BiOBr1-xClx with x = 1.0, 0.75 and 0.50 were above the required value for visible light absorption, major degradation in this case took place by dye photosensitization. The dye absorbs the incoming photons and the dye adsorbed on catalyst surface transferred their photo generated electrons to the CB of BiOBr1-xClx plates. These electrons are then utilized by the catalyst to generate active species which in turn helps to degrade the dyes.

                                                           For the rest, suitable band gap leads to effective utilization of the visible light and dye degradation took place by direct semiconductor photo excitation. Also the better plate-like morphology of the BiOBr may be assumed to facilitate the efficient separation of the photogenerated electron-hole pair, leading to improved photocatalysis.

                                                          CONCLUSION & RECOMMENDATION

                                                            BiOBr1-xClx compounds were prepared using a simple solid state method. Phase purity was confirmed using XRD. Refinement data showed the prepared compounds to be tetragonal. Band gaps of the compounds were controllably tuned through varying the Cl/Br ratio in the prepared compounds. Photocatalytic measurements showed BiOCl0.25Br0.75 as the best material whereas all the solid solutions were found to have better photocatalytic property than pure BiOBr and BiOCl. This was further confirmed by the first order apparent kinetic rate constants of the compounds which follows the order BiOBr0.75 Cl0.25 > BiOBr0.25Cl0.75 > BiOBr0.50Cl0.50 > BiOBr > BiOCl. Thus this work provides a convenient process to tune the band gap of the bismuth oxyhalides in a relatively easy manner leading to property enhancement which can be utilized in water detoxification process.

                                                           This work could be further extended to find the actual underlying mechanism of the photocatalysis process. Rate of electron-hole generation could be monitored through electrochemical analysis of photo-current for the compounds. The main active species for different compounds could be found through using proper scavengers (e.g. AgNO3, KI, benzoquinone and turt-butyl alcohol for electrons, holes, O2 and OH- respectively) during photocatalytic dye degradation process. Utilization of actual sunlight instead of halide lamp sources can also be checked for practical application purpose.

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                                                          ACKNOWLEDGEMENTS

                                                           I would like to express my deep sense of gratitude to Dr. C. Shivakumara for giving me the opportunity to work under his sincere guidance. His gentle and generous personality along with sharp observation and thinking power has been a true inspiration for me.

                                                          I am grateful to all my labmates – Sonali di, Lakshmi, sir, Pramod H, sir, Ms. Alisha and my fellow intern Ms. Anupama for their supports.

                                                          Very special thanks go to my coguide Abhilash G P who guided me in all possible ways. Learning the basics and all of the practical experiments from him was a real pleasure. His continuous and patient effort towards my project work was really appreciable. Thank you for the encouragement!

                                                          I am also thankful to the Indian Academy of Sciences for providing me such an opportunity to gain knowledge and experience.

                                                          I am grateful to revered Principal Maharaj of our institution Ramakrishna Mission Vidyamandira and Dr. Uttam Kumar Ghorai, HOD of our department, for giving me the permission to pursue this internship. Thanks to all my batchmates for their encouraging support.

                                                          Finally very special thanks to all my family members for their continuous support throughout the internship.

                                                          Thanks to everyone for your encouragement!!! May the Holy Trio – Sri Sri Thakur, Maa, and Swamiji bless you all.

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