Facile synthesis of mnwo4 bioi nanocomposites and their efficient photocatalytic and photoelectrochemical activities under the visible light irradiation

Journal of Science: Advanced Materials and Devices (2018) 331e341 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Facile synthesis of MnWO4/BiOI nanocomposites and their efficient photocatalytic and photoelectrochemical activities under the visiblelight irradiation V Ramasamy Raja a, b, A Karthika a, A Suganthi a, c, *, M Rajarajan d, ** a P.G & Research Department of Chemistry, Thiagarajar College, Madurai 625 009, Tamil Nadu, India K.P National College of Arts and Science, Batlagundu 624 202, Tamil Nadu, India Mother Teresa Women's University, Kodaikanal 624 102, Tamil Nadu, India d Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India b c a r t i c l e i n f o a b s t r a c t Article history: Received April 2018 Received in revised form 17 June 2018 Accepted July 2018 Available online 24 August 2018 The novel MnWO4/BiOI nanocomposite materials were successfully synthesized by a precipitation deposition method The as-prepared photocatalyst was characterized by XRD, SEM, EDAX, UV-DRS, FT-IR, TEM and BET techniques The as-prepared MnWO4/BiOI nanocomposites were further utilized to study the degradation of the Celestin blue aqueous solution under visible-light irradiation Absorption range and band gap energy, which are responsible for the observed photocatalyst behavior, were investigated by the DRS spectroscopy The photocatalytic test suggested that MnWO4/BiOI nanocomposites possess a higher activity for the degradation of these pollutants than the pure BiOI and MnWO4 under the visiblelight irradiation Among the as-prepared nanocomposites, the one of 3% MnWO4/BiOI displays the best photocatalytic activity of the degradation Factors, such as the effect of the catalyst dosage, the solution pH and the initial dye concentration affecting the photocatalytic activity were investigated The investigations of the adsorption kinetics and isotherm demonstrate that the adsorption process follows the pseudo-first-order kinetic model and the Langmuir adsorption isotherm, respectively © 2018 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: MnWO4-BiOI Nanocomposite Visible light Celestin blue Photocatalytic activity Introduction With the rapid development of economy and society, water contamination has been an increasing environmental problem for humans [1,2] To solve this trouble, abundance of efforts have been devoted to develop efficient methods for decomposing environmental pollutants, such as filtration, solvent extraction, chemical oxidation, adsorption, floatation and photocatalytic degradation Photocatalytic approaches have been devoted to extensive studies on the environmental purification [3e7] Transition metal tungstates belong to an important family of inorganic materials which have potential application in versatile * Corresponding author Mother Teresa Women's University, Kodaikanal 624 102, Tamil Nadu, India Fax: þ91 4542 241121 ** Corresponding author Fax: þ91 4542 241121 E-mail addresses: suganthiphd09@gmail.com (A Suganthi), rajarajan_1962@ yahoo.com (M Rajarajan) Peer review under responsibility of Vietnam National University, Hanoi fields and have been studied for many years [8] MnWO4 is a narrow bandgap semiconductor (Eg ~ 2.8 eV), which is expected to be a novel excellent photocatalyst [9] MnWO4 has nailed its extensive research interests due to its novel electrochromic, multiferroic, ionic properties and its significance in gas sensor, electrochemical and catalytic etc applications [10,11] However, the issues regarding the control of shape and size are considered as the negative effect on their applications As reported in literatures, semiconductor photocatalysts, such as MnWO4/Polyaniline [12], MnWO4/Reduced Graphene oxides [13], exhibit efficient photocatalytic degradation In addition to that MnWO4 when added to BiOI the degradation property seems to be enhancing Amongst the materials, BiOI exhibits as the most attractive semiconductor the most promising visible-light e harvesting photocatalytic properties, because of its narrow band gap and efficient sunlight-harvesting nature [14e16] BiOI has been recognized as one of the most promising photocatalytic materials also because of its strong oxidation ability and high stability, etc [17] Owing to its open and layered crystalline and electronic structure, https://doi.org/10.1016/j.jsamd.2018.07.003 2468-2179/© 2018 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) 332 V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 the BiOI shows outstanding photocatalytic performance [18,19] It is known that materials with an excellent photoabsorption in solar spectrum have been proved to be promising photocatalytic materials for the treatment of wastewater [20] Till now, numerous BiOI based heterostructured composite catalysts have been reported, including BiOI/Zn2SnO4 [21], BiOBr/BiOI [22], BiOI/ZnGe2O4 [23], BiOI/BiCOOH [24], p-BiOI/n-SnS2 [25], BiVO4/BiOI [26], BiOI/BiPO4 [27], Bi2O2CO3/BiOI [28], BiOI/BiOBr [29], and C3N4/BiOI [30] All these heterostructured composite catalysts show enhanced visible light activity as compared to pure BiOI The reason for their better photocatalytic ability is the formation of the interface junction which facilitates the effective separation and transfer of electronehole pairs In the present work, MnWO4/BiOI samples with different molar ratios were synthesized by the precipitation deposition method The photocatalytic activity of MnWO4/BiOI under visible-light was evaluated by the use of CB dye as the pollutant The applied synthesis methods have demonstrated a good reproducibility, and proved the possibility to obtain the uniform small particle sizes and the production of high purity products As-prepared samples were studied by different techniques The obtained MnWO4/BiOI nanocomposites exhibit significantly enhanced visible photocatalytic and photoelectrochemical (PEC) performances in comparison with the bare, MnWO4 and BiOI individually modified electrodes The visible light separation appeared in the merged hetrostructure presents the attributed enhanced photocatalytic and PEG activities This approach draws a potential application in synthesizing a broad range of hybrid nanostructures with prominent applications in photocatalysis and other relevant areas Experimental 2.1 Materials All chemicals and reagents such as manganese nitrate trihydrate (Mn(NO3)2$3H2O), glycine, sodium tungstates (Na2WO4$2H2O), bismuth nitrate pentahydrate (Bi(NO3)3$5H2O), ethylene glycol (EG) and potassium iodide (KI) were purchased from Merck Chemical India All reagents were of analytical grade and used without further purification Deionized (DI) water was used throughout the experiments 2.2 Synthesis of MnWO4 nanoparticles MnWO4 nanoparticles were prepared by the precipitation deposition method In a typical procedure, an aqueous solution of Na2WO4$2H2O (1 mmol) in the presence of different capping agents, such as valine, glycine and asparagine, was mixed with Mn(NO3)2$3H2O (1 mmol) aqueous solution and the solution was heated up to 80  C for 10 The brown precipitate was then centrifuged, washed out with distilled DI water and methanol for three times Finally, the final product obtained was dried at 80  C for 24 h 2.3 Synthesis of MnWO4-BiOI nanocomposites The MnWO4-BiOI nanocomposite was synthesized in two different BiOI concentrations (2% MnWO4-BiOI and 3% MnWO4-BiOI) according to the procedure previously reported in [31] Initially Bismuth nitrate (Bi (NO3)3$5H2O) and potassium iodide (KI) were dissolved in ethylene glycol This solution was added dropwise into the previously sonicated MnWO4 in double distilled water The mixture was stirred at room temperature for h, after that the product was washed with ethanol, double distilled water and dried at 80  C for 24 h BiOI was also synthesized by the same procedure without the addition of MnWO4 2.4 Characterization The optical properties were investigated using a UVevis-DRS The spectra were recorded in air at room temperature in the wave length range of 200e800 nm using the Shimadzu UV-2450 Spectrophotometer The crystal structure of the as-prepared photocatalysts was identified by the X-ray diffraction method using CuKa radiation (l ¼ 1.5418 Å) with a scanning angle (2q) of 10 e85 at a scan speed of ( )/min and a voltage of 40 kV and current of 300 mA The surface morphology of the sample was characterized with a JSM 6701F-6701 scanning electron microscope (SEM) operated at 25 kV attached with an energy dispersive electron microscopy (EDX) device Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were taken on a JEOL JEM-2100 electron microscope with an accelerating voltage of 200 kV The BET (Brunauer‒Emmett‒Teller) surface area was derived from the N2 adsorption e desorption isotherm by the Barrer-Joyner-Halenda (BJH) technique The FT-IR spectrum of the as-prepared catalysts was recorded using the FT-IR spectrometer JASCO-4200 Photodegradation experiments were performed in a HEBER immersion type photoreactor (HIPRMP125) 2.5 Photodegradation experiments The photocatalytic behavior of the as-synthesized samples were evaluated by degrading the Celestin blue under the visible light radiation A cylindrical quartz photoreactor was used to conduct the experiments, whereas a 300W Xenon arc lamp was used as a source of visible light The lamp was surrounded by a circulating water jacket for cooling Air flowing was kept bubbling continuously into the aliquot by an air pump in order to provide a constant source of dissolved oxygen 0.60 g/L of the 3% MnWO4-BiOI was dispersed in a 300 ml CB solution (6 mM) Then, the pH of the solution was adjusted using 0.1 M H2SO4 (or) 0.1 M NaOH, and the required amount of the photocatalyst was added into the vessel The CB solution was stirred for 30 in darkness before the visible light irradiation, to ensure the adsorptionedesorption equilibrium at regular time intervals ml aliquot was taken and centrifuged, filtered and the filtrate was analyzed in a UVeVis spectrometer to estimate the residual CB concentration, using the following defenition: Photodegradation (%) ¼ C0 À C/C0 Â 100, (1) where, C0 is the concentration of CB before the irradiation (t ¼ 0), and C is the concentration of CB after a certain irradiation time 2.6 Photoelectrochemical measurements The EIS was investigated with an electrochemical analyzer (CHI660B, Chen Hua Instruments, Shanghai, China) in a standard three-electrode system, which used a platinum wire as the counter electrode, a saturated calomel electrode (SCE) as the reference one and an ITO slice as the working electrode, respectively Typically, mg sample powder was dispersed ultrasonically in mL ethylene glycol, and 20 mL of the resulting colloidal dispersion was drop-cast onto a piece of the ITO slice with a fixed area of 0.5 cm2 and then dried at 80  C A 500 W Xenon arc lamp was utilized as the light source The electrochemical impedance spectra (EIS) were measured in the frequency range from 0.01 Hz to 100 kHz at 0.24 V, and the amplitude of the applied sine wave potential in each case was mV An 0.1 M Na2SO4 aqueous solution was used as the impedance liquid V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 Results and discussion 3.1 Optical absorbance analysis Two important factors, namely the optical absorption and the energy band feature of a semiconductor was employed for the determination of its photocatalytic activity Fig shows the DRS spectra of the as-prepared samples For the MnWO4 nanoparticles, strong absorption peaks at 400 nm and for the BiOI nanoparticles, an absorption edge at around 570 nm are observed, respectively For the MnWO4-BiOI composites, the absorption within the visible light range is apparently increased and a red shift appears when compared with pure BiOI The enhanced light absorption may lead to the formation of more electronehole pairs The above result indicates that dispersing MnWO4 on the BiOI surface leads to the enhanced absorption in the visible light range, electronehole pairs are formed due to the enhanced light absorption and this would be the promising mechanism for the photocatalysis application [32,33] Tauc relation was used to measure the optical band gap of the nanocomposites The UVevis absorption curves were observed and the absorption edges were calculated using the following formula: a,h,n¼ A (h,n ‒ Eg)n/2 Fig UVeVis-DRS of MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI 333 (2) where a, h, n, A, Eg are the absorption coefficient, the Planck's constant, the incident light frequency, a proportionality constant, and the band gap energy, respectively The obtained values of the band gap energy for MnWO4, BiOI, 2% MnWO4-BiOI and 3% MnWO4-BiOI are found as 2.65 eV, 1.80 eV, 2.15 eV and 2.05 eV, respectively and they are displayed in Fig Hence, 3% MnWO4BiOI absorbs more visible light than the pure materials MnWO4, BiOI Fig Tauc Plots of (a) MnWO4 (b) BiOI (c) 2% MnWO4-BiOI (d) 3% MnWO4-BiOI 334 V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 Fig FT-IR spectrum of MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI 3.2 FT-IR spectrum The infrared spectrum of the MnWO4-BiOI nanocomposite is shown in Fig The main characteristic peaks of MnWO4 and BiOI are in accordance with the reported respective results The absorption peak appears at 1084 cmÀ1 corresponds to the existence of MnWO4 The peak at 515 cmÀ1 is characteristic for the absorption of the MneO bond The vibration modes of MnWO4 are observed at 455, 592, 735, 792, and 885 cmÀ1, while the feature at 1587 cmÀ1 is due to the deformation vibration of physisorbed or chemisorbed water molecules [34] The peak at 489 cmÀ1 corresponds to the v(BieO) spectrum of the pure BiOI [35] The above IR characteristic results have revealed the formation and present of the MnWO4BiOI nanocomposite that is also confirmed by the XRD results as discussed below MnWO4, BiOI, 2% MnWO4-BiOI and 3% MnWO4-BiOI were determined as 20.15 nm, 26.45 nm, 23.09 nm and 26.54 nm, respectively, on the basis of Debye-Scherrer equation [36] 3.4 Morphological studies The detailed morphology and microstructure of the pure MnWO4, BiOI, and the 3% MnWO4-BiOI composite were investigated by SEM and EDAX As seen in Fig (a), (b) and (c), the morphology of the pure MnWO4 shows a sphere-shaped configuration in which the core part is not clear For BiOI, the picture shows 3.3 X-ray diffraction XRD technique was used to study the phase composition of synthesized samples of the pure MnWO4, the pure BiOI and the MnWO4-BiOI photocatalysts and results are shown in Fig The observed XRD peaks of the MnWO4 correspond to the cubic phase, matching well with standard pattern of the JCPDS card (file no: #14-0688) All the diffraction peaks of BiOI are in accordance with the orthorhombic phase, which is in agreement with the database pattern of the JCPDS card (file no: # 43-0543) as well As it is seen for the MnWO4-BiOI heterojunctions, the corresponding characteristic diffraction peaks of MnWO4 and BiOI coexist in the XRD patterns of the composite No other peaks are detected in the heterostructure spectra, indicating that no impurity species are formed between BiOI and MnWO4 This suggests that BiOI is well dispersed on the MnWO4 surface The average crystal sizes of Fig XRD Pattern of MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 the porous texture of the material aggregates The 3% MnWO4-BiOI composite appears to be of a small spherical structure and the average particle size is below 28 nm The EDAX data shown in Fig (d) also confirm the presence of the constituting elements, such as Mn, W, Bi, I and O in 3% MnWO4, BiOI composites The TEM measurement was performed in order to get more detailed information about the crystalline structure of the as-prepared photocatalysts In Fig 5(e), that the TEM image taken on the 3% MnWO4-BiOI nanocomposite sample shows the surface morphology of this sample From this, the standard grain diameter of the 3% MnWO4-BiOI composite varies between 23e47 nm The removal of intercalated and hydroxyl water adsorbed in the structure, release pore spaces and consequently create the configuration of mesoporous structures on the surface Fig (f) shows the SAED (selected area electron diffraction) pattern taken on the 3% MnWO4-BiOI nanocomposite The 3% MnWO4-BiOI composite is the composition of the cubic phase of MnWO4 and the 335 orthorhombic phase of BiOI which have been well identified with the XRD patterns The SAED patterns of the 3% MnWO4-BiOI nanocomposite confirm that the product is well of polycrystralline nature [37] 3.5 BET surface area analysis Generally, photocatalytic reaction takes place on the surface of the photocatalysts, and ultimately, it draws effects on its photocatalytic activity The specific surface area and the pore structure of the as-prepared MnWO4, BiOI and 3% MnWO4-BiOI nanocomposites were measured at À195.675  C and the nitrogen adsorptionedesorption isotherm was taken in order to explain the dissimilarity of the following photocatalytic presentation and results are depicted in Fig The three isotherms are identified according to the IUPAC classification as type IV by the mesoporous nature, indicated by the presence of the pores of 2e50 nm [38] The Fig SEM images of (a) MnWO4; (b) BiOI; (c) 3% MnWO4-BiOI; (d) EDAX spectrum of the 3% MnWO4-BiOI composite; (e) TEM image of 3% MnWO4-BiOI; and (f) SAED pattern of 3% MnWO4-BiOI 336 V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 Fig N2 adsorptionedesorption isotherms and pore size distribution plots for samples of (a) MnWO4; (b) BiOI; and (c) 3% MnWO4-BiOI hysteresis loops can be classified as type H3 The hysteresis loop appears for the relative pressure P/P0 between 0.6 and Table summarizes the BET surface area, the pore volume, and the average pore size of the as-synthesized samples as calculated on the basis of the recorded isotherms Both MnWO4, BiOI and the 3% MnWO4-BiOI nanocomposite possess all larger surface areas, and thus, they can provide more surface active sites for the adsorption of reactant molecules which is beneficial for achieving higher photocatalytic activity [39] 3.6 Electrochemical analysis Nyquist plots from electrochemical impedance spectra (EIS) are significant in elucidating the migration and transfer Table Parameters obtained from N2 adsorptionedesorption measurements Sample Specific surface area (m2/g) Average pore diameter (nm) Pore volume (cm3/g) MnWO4 BiOI 3% MnWO4-BiOI 45.36 34.75 75.45 29.32 20.77 46.50 0.2986 0.9161 1.5699 processes of photogenerated electronehole pairs in semiconductors [40] The prepared BiOI and 3% MnWO4/BiOI samples were used as electrode materials to measure their EIS and result is shown in Fig (a) The semicircle in the high frequency region is ascribed to the charge-transfer resistance (Rct), and it confirms the charge transfer through the electrode/electrolyte interface The smaller radius of 3% MnWO4/BiOI semicircle implies a higher efficiency of charge transfer in this composite than in the pure BiOI Hence, the coupling of MnWO4 with BiOI is considered beneficial in electron transport, and in the enhancement of the separation efficiency of the photogenerated charge carriers [41] The photo-induced holes or the formed hydroxyl radicals are the two main species responsible for the oxidization of the organic pollutants Disodium Ethylene Diamine Tetra Acetate (EDTA-2Na) was used as the hole scavenger and tert-butanol was used as an electron acceptor in investigating the main species involved in CB photodegradation over the 3% MnWO4/BiOI composite As is shown in Fig (b), the addition of EDTA-2Na greatly inhibited the degradation of CB, while the addition of tert-butanol could only slightly diminish the degradation rate of CB Therefore, it confirmed that both the holes and hydroxyl radicals play significant role in the CB photodegradation while the holes are the main reactive species V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 337 Fig Time dependent UVeVis spectral changes of CB (6 mM) in the presence of the 3% MnWO4-BiOI composite (0.60 g/L) activity towards the degradation of CB as compared to the bare MnWO4 and BiOI 3.7.2 Possible photocatalytic activity enhancement mechanism Based on the results mentioned above, a possible mechanism for the photocatalytic degradation of organic compounds by MnWO4/BiOI nanocomposite is proposed as displayed in Fig Further, the conduction band (CB) bottom and the valence band (VB) top of MnWO4 and BiOI can be calculated by the following equations: Fig Electrochemical impedance spectroscopy of BiOI (a) and 3% MnWO4-BiOI nanocomposite (b) Comparison of the photocatalytic activities of 3% MnWO4-BiOI nanocomposite for the degradation of CB under visible light irradiation with and without adding EDTA-2Na and t-butanol 3.7 Photocatalytic activity 3.7.1 Photodegradation of CB The photodegradation was monitored by examining the variations of the maximal absorption with respect to the irradiation time in the UVeVis spectra The photocatalytic activity of the 3% MnWO4-BiOI nanocomposite was evaluated from the degradation of mM of CB with an initial pH of The changes in the UVeVis spectra of CB as a function of irradiation time using 3% MnWO4BiOI (0.60 g/L) are presented in Fig It is important to notify that the contact time prior to the irradiation was 30 under the dark condition indicating the absorption of dyes on the active sites of the synthesized photocatalysts With the increase of the irradiation time, the absorption peak at lmax ¼ 645 nm decreases gradually and disappears after 180 of irradiation The photocatalytic degradation of CB in an aqueous solution in the presence of the 3% MnWO4-BiOI composite results in 90% degradation efficiency The degradation efficiency of the photocatalysts follows the order as: 3% MnWO4-BiOI > 2% MnWO4BiOI > BiOI > MnWO4 according to the results presented The 3% MnWO4-BiOI heterojunction exhibits the best photocatalytic EVB ¼ X e Ee ỵ 0.5 Eg (3) ECB ẳ EVB e Eg (4) where X is the electronegativity of the semiconductor, Ee is the energy of the free electrons on the hydrogen scale (À4.5 eV) and Eg is the band gap energy of the semiconductor As is it known, the estimated CB and VB of MnWO4 material are 2.07 and 4.72 eV, and of BiOI 0.8 and 2.6 eV, respectively As illustrated in Fig 9, under the visible-light irradiation, both the g-MnWO4 and BiOI are easily excited and engender the corresponding photogenerated electronehole pairs Once the electrons in the VB of MnWO4 and BiOI are excited to the CB of MnWO4 under visible-light irradiation, the photoinduced electrons on the CB of MnWO4 are transferred to the CB of BiOI, whereas the photogenerated holes on the VB of BiOI to the VB of MnWO4 [42e44] respectively These processes cause the gathering of electrons on the CB of BiOI and holes on the VB of the MnWO4 surface As an outcome, the separation of electrons and holes proceeds and completes causing the decrease of the recombination process leading to a highly efficient photocatalytic activity of the MnWO4/BiOI nano composites The mechanism of the degradation reaction can be summarized as the following: MnWO4/BiOI ỵ hn (Vis) / MnWO4/BiOI (hỵ ỵ e) (5) MnWO4/BiOI (e) þ O2 / MnWO4/BiOI þ OÀ (6) À 2OÀ ỵ 2H2O / 2OH ỵ H2O2 ỵ O2 (7) H2O2 ỵ MnWO4/BiOI (e) / OH ỵ OH ỵ MnWO4/BiOI (8) ỵ  CB ỵ O / OH/h / intermediates / degradation products (9) 338 V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 Fig Schematic diagram of the electron transfer in the 3% MnWO4-BiOI under visible-light irradiation The CB electrons (eÀ) are accumulated on the surface of MnWO4/BiOI Some of them are then scavenged by the oxygen on the surface of the photocatalyst to form the superoxide radicals (OÀ ), which can react with H2O to form H2O2 Some of them can be scavenged then by the H2O2 to form the hydroxyl radicals (OH) These are three active species being involved in the photocatalytic process 3.8 Optimization of the reaction parameters 3.8.1 Effect of pH Generally, the pH of the solution is an important parameter in the photocatalytic process The effect of pH on the photodegradation of the CB was studied in the pH range of 4e9 with an initial of the CB concentration of mM, and the catalyst dosage of 0.60 g/L The results are displayed in Fig 10(a) The photodegradation of the CB increases from pH 4e8 and then decreases as the pH increases to The alkaline or acidic conditions were adjusted by adding the appropriate amount of NaOH or H2SO4 to the solution This is due to the change in the electrostatic attraction or repulsion between the pollutant molecules and the catalyst The CB is a cationic dye and therefore, the electrostatic attraction between the dye molecules and the catalyst is greatly improved at pH ¼ [45] 3.8.2 Effect of the catalyst dosage In order to optimize the photocatalyst dosage on the degradation of the CB, experiments were carried out with varying the 3% MnWO4-BiOI dosage from 0.30 g/L to 0.70 g/L, at constant the CB concentration of mM and pH ¼ The results are shown in Fig 10(b) The photodegradation of the CB is negligible in the absence of the catalyst As the concentration of photocatalyst increases from 0.30 g/L to 0.60 g/L the degradation of the CB also increases In contrast, it causes the decrease in the photodegradation due to the presence of more active sites and also due to the increase in the total active surface area of the photoreaction [46] 3.8.3 Effect of the concentration The effect of the initial dye concentration on its photodegradation was investigated from mM to mM, at pH ¼ and at the 3% MnWO4-BiOI dosage of 0.60 g/L The results are shown in Fig 10(c) With the initial concentration increasing the CB photodegradation decreases At high concentrations, interruption by dye molecules of photons takes place- before reaching the catalyst surface, as a result of which the formation of reactive oxygen species on the photocatalytic surface is reduced The 3% MnWO4-BiOI concentration is the same for all initial concentrations of CB and therefore, the formation of OH remains constant [47] V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 339 Fig 10 (a) Effect of pH on the photodegradation of CB; (b) Effect of catalyst dosage on the photodegradation of the CB; (c) Effect of the concentration on the photodegradation of the CB; and (d) Kinetic regime on the photocatalytic degradation of the CB 3.8.4 Kinetics of the CB photodegradation The kinetics of the CB under the visible light over all the photocatalysts was investigated by applying the following lpseudo-first order equation: Àln (C0/C) ¼ k,t (10) where, k is the rate constant of pseudo-first order The plot of Àln (C0/C) versus irradiation time (where C0 is the initial concentration of the dyes and C is the dye concentration in the reaction time) was found to be linear as shown in Fig 10(d) This put forward that the photodegradation reactions follow the kinetics of the pseudo-first order The linear regression method was used to calculate the rate of the reaction constant (k) and the rate law for the photocatalytic degradation of the CB The apparent rate constants for MnWO4, BiOI, 2% MnWO4-BiOI, 3% MnWO4-BiOI were determined as 0.0115 Â 10À2 SÀ1, 0.0078 Â 10À2 SÀ1, 0.235 Â 10À1 SÀ1, 0.326 Â 10À1 SÀ1, respectively The observed results show that the 3% MnWO4-BiOI, nanocomposite has a higher rate constant than the other modified and their bare catalysts [48] 3.8.5 COD The COD was used as a measure of the oxygen equivalent to the organic content in a sample that was susceptible to oxidation to carbon-dioxide and water by a strong oxidant Ideal conditions were maintained for conducting the photocatalytic experiments Table COD removal (mg/L) of CB during the photodegradation using the 3% MnWO4-BiOI under visible light irradiation Time (min) COD removal efficiency (%) Celestin blue 30 60 90 120 150 180 15.68 28.37 43.67 60.45 75.34 90.00 Test samples were collected at the time intervals of every 30 The COD before and after the irradiation under visible light was estimated and results are shown in Table After the photodegradation at 180 under the optimum conditions the solutions have a significant decrease in COD up to 90.00% As an outcome smaller species were obtained by the degradation of organic matters (especially inorganic compounds) and hence the COD required decreases Conclusion In conclusion, a novel MnWO4/BiOI composite with visible light photocatalytic activity was successfully synthesized via a 340 V Ramasamy Raja et al / Journal of Science: Advanced Materials and Devices (2018) 331e341 precipitation deposition method and characterized by various techniques XRD results indicate that the as-prepared catalysts composed of the cubic phase MnWO4 and the orthorhombic phase BiOI The 3% MnWO4-BiOI nanocomposite was efficient for the degradation of the CB under visible light irradiation due to their unique morphology and optical property Evaluation and optimization of the effect of pH, catalyst dosage throughout the COD experiments indicated that under the studied conditions the parameters as pH ¼ 9, 0.60 mg/L MnWO4-BiOI and 180 irradiation time favor the COD efficiency Under the above condition approximately 90% of degradation was achieved within 180 of irradiation Correlation of photodegradation kinetics was considered with the model of the pseudo-first-order A plausible mechanism for the photodegradation was proposed To conclude this research work has proved that the synthesized nanocomposites can be utilized as a favorable photocatalyst for the degradation of organic dyes in the aqueous environment [18] [19] [20] [21] [22] [23] Acknowledgements [24] The authors thank the Management of Thiagarajar College for providing necessary laboratory facilities to carry out this work References [1] X.W Wang, H.W Tian, Y Yang, H Wang, S.M Wang, W.T Zheng, Y.C Liu, Reduced graphene oxide/CdS for efficiently photocatalystic degradation of methylene blue, J Alloys Compd 524 (2012) 5e12 [2] R.P Schwarzenbach, B.I Escher, K Fenner, T.B Hofstetter, C.A Johnson, U Gunten, B Wehrli, The challenge of micropollutants in aquatic systems, Science 313 (2006) 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and (f) SAED pattern of 3% MnWO4- BiOI 336... under the visible- light irradiation, both the g -MnWO4 and BiOI are easily excited and engender the corresponding photogenerated electronehole pairs Once the electrons in the VB of MnWO4 and BiOI