Modification of BiOI Microplates with CdS QDs for Enhancing Stability

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Modification of BiOI Microplates With CdS QDs for Enhancing Stability, Optical Property, Electronic Behavior Towards Rhodamine B De-Colorization and Photocatalytic Hydrogen Evolution Debasmita Kandi, Satyabadi Martha, Arun Thirumurugan, and Kulamani M. Parida J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11938 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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The Journal of Physical Chemistry

Modification of BiOI Microplates with CdS QDs for Enhancing Stability, Optical Property, Electronic Behavior towards Rhodamine B de-colorization and Photocatalytic Hydrogen Evolution Debasmita Kandi,a Satyabadi Martha,a* Arun Thirumuruganb and K. M. Paridaa* aCentre

for Nano Science and Nano Technology, Institute of Technical Education and

Research, Siksha ‘O’ Anusandhan University, Bhubaneswar-751030 bInstitute

of Physics, Sachivalaya Marg, Bhubaneswar-751005

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Abstract Fast recombination of photoinduced charge carriers is a major problem in case of semiconductor based photo catalysts, which must be solved for their potential application in photocatalysis. In this work, photostable CdS QDs/BiOI composite have been successfully fabricated by two step precipitation-deposition method. The prepared samples were characterized by X-ray diffraction (XRD), UV-vis diffuse reflection spectroscopy (UV-vis DRS), photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Mott-Schottky and electrochemical impedance analysis. The potential application of CdS QDs/BiOI composite materials have been tested towards de-colorization of Rhodamine B (RhB) solution and hydrogen generation under solar light and visible light irradiation respectively. It has been observed that hydroxyl radicals, electrons and hole played a major role in de-colorization of RhB solution. Among all prepared photocatalysts, 4% CdS QDs/BiOI composite could able to decolorize 82 % of RhB solution in 1 h and 203 µmol/h of H2 under solar light and visible light irradiation, respectively. The highest activity has been ascribed to optimal loading of CdS QDs, well formation of composite and lowest recombination of charge carriers.

1. INTRODUCTION The production of fuel having highest specific enthalpy of combustion i.e. hydrogen (from water, renewable solar light) and de-colorization of organic pollutants by the process of photocatalysis could become one of the alternative techniques to overwhelm a bit of ever thriving energy dilemma and to live a healthy life in a pollutant free environment.1 A number of 2


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semiconductor photocatalysts have been developed since today to produce hydrogen and for deterioration of harmful organic pollutants. Among them Bismuth containing materials (Bi2O3,2, 3 Bi2S3, 4, 5 Bi2WO6, 6-8 BiVO4,9, 10 Bi2O2CO3, 11, 12 BiOIO3, 13, 14 CaBi2O4, 15 BaBiO3, 16 Bi4Ti3O12 17, 18

etc.) are attracting much attention to meet this energy and environment crisis. In particular

BiOI is of gigantic importance for its supreme optical and electrical properties and have demonstrated promising applications in photocatalysis, photo electrocatalysis, energy storage devices etc. These possess strongest photo response in the visible light region due to its narrow band gap energy (1.77–1.92 eV);

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unique anisotropic layered structure hence subsequently

improves the photocatalytic activity 3. However its photocatalytic activity is still not satisfactory due to some bottlenecks including its un-stability nature under strongly reducing agent, poor conductivity, high electron-hole recombination ability etc. This deficiency have been raised to some extent by further modification like forming composites with metal oxides (TiO2, ZnO, Bi2O3, SiO2, ZnTiO3), 24-28 metal salts (AgI), 29 noble metals (Pt, Ag), 30, 31 Bismuth oxyhalides (BiOCl, BiOBr) 32-34 and various carbon materials (graphene, MWCNT) 35, 36 etc. It has been reported that efficiency of Bismuth oxyhalides based materials have been greatly increased by modifying with different QDs (CdS, Cu2O, Cu2S) 37-39 etc. Song and co-workers have synthesized hydrothermally bulk CdS/BiOI Z-scheme type photocatalyst for Rhodamine B degradation.

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We have modified

BiOI microplates (MPs) with CdS QDs to improve its photocatalytic activity to a large extent. CdS QDs have been decorated uniformly on the surface of BiOI MPs to overcome the disadvantages of BiOI and to improve its catalytic activity as the electron-hole pairs are effectively separated after fabrication of the composite. Moreover, the multiple electron generation with single photon and band gap tunability of CdS QDs with respect to particle size maximizes the absorption range in visible light region. The limitation of CdS QDs is that it suffers from photo corrosion via self3



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oxidation which leads to un-stability. Here this major problem has been well resolved by compositing with BiOI MPs to enhance their individual stability, optical property, electronic behavior and photocatalytic activity. Thus, the composite of BiOI and CdS QDs is proved to be an ideal type-II heterostructure for better photocatalytic activity in the field of organic pollutant decomposition and hydrogen evolution. Encouraged by the above facts, we have investigated CdS QDs/BiOI composite via a generalized in situ method of preparation in which TGA are intelligently employed as capping agents for prevention of agglomeration of extreme small QDs. The prepared samples were characterized by XRD, UV-Vis DRS, PL, TEM, XPS, electrochemical study and the photocatalytic activity was evaluated by de-colorization of RhB as well as degradation of phenol and evolution of hydrogen gas. The result demonstrates the in situ modification of BiOI MPs surface by the CdS QDs which shows excellent degradation of the organic pollutant and enhanced production of hydrogen. Furthermore, the photocatalytic mechanism has expounded clearly on the basis of photoluminescence study and electrochemical study.

2. EXPERIMENTAL SECTION 2.1 Materials Bismuth nitrate pentahydrate, Potassium iodide, Sodium hydroxide, Cadmium nitrate tetrahydrate, Thioglycolic acid (TGA) were purchased from Merck and sodium sulfide from Himedia chemical

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company. All the reagents are of analytical grade and used in the reaction without any further purification. 2.2 Methods 2.2.1 Synthesis of BiOI MPs BiOI MPs was prepared by simple hydrolysis method. To prepare BiOI MPs, 0.0028 mole (1.385g) bismuth nitrate pentahydrate was dissolved in 80 mL (0.035mol/L) potassium iodide solution with stirring. At room temperature the solution was sonicated for 2 min and then stirred vigorously for 5h. A deep red color precipitate was obtained and collected by centrifugation. It was washed several times with double distilled water and dried at 600C for 12 h. 41 2.2.2 Synthesis of CdS QD sensitized BiOI MPs CdS QDs/BiOI composite with different percentage of QDs were synthesized by two step precipitation deposition method. Particular amount of BiOI MPs was taken in the reaction medium containing 75 mM of Cd (NO3)2.4H2O. To this solution requisite volume of TGA (mole ratio of TGA: CdS=1:2) was added and pH of the solution was maintained at 10.5 by drop wise addition of 1M NaOH solution. After adding 75 mM of Na2S the solution was stirred at 650C for 30 min and undergone aging for 90 min. Then deep red color composite was collected by centrifugation and dried in vacuum at 600C. 42 2.2.3 Photocatalytic de-colorization process The photocatalytic de-colorization process was carried out by taking 20 mL 20 ppm RhB solution containing 0.02 g of catalyst in 100 mL conical flask. The solutions are stirred at a speed of 300 5



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rpm for 60 min. Residual RhB concentrations were analyzed by a JASCO 750 UV-Vis spectrophotometer. The concentration of RhB was measured by spectrophotometric method at 554 nm. The average solar light intensity during the experiment is observed to be 102,000 lux. The active species involved in de-colorization process was detected by using different trapping agents like p-benzoquinone (p-BQ), dimethyl sulfoxide (DMSO), isopropyl alcohol (IPA), disodium ethylenediaminetetraacetate (2Na-EDTA) for superoxide, electron, hydroxyl radical and hole respectively. Similar reaction conditions were maintained for the de-colorization process in addition to that 5mM trapping agents were added.

The reusability of the concerned best

photocatalyst was tested as follows. After every de-colorization process the photocatalyst was washed with double distilled water and ethanol for several times and dried in oven. The dried sample was reused and this process was repeated under similar experimental conditions to inspect the reusability and durability of the photocatalyst. 2.2.4 Photocatalytic degradation of phenol The photocatalytic degradation of phenol was carried out by taking 20 mL 5 ppm phenol solution containing 0.02 g of catalyst in 100 mL conical flask. The experiment was performed for 1 h in the presence of solar light. The solutions are stirred at a speed of 300 rpm for 60 min. Residual phenol concentrations were analyzed by a JASCO 750 UV-Vis spectrophotometer. Before spectrophotometric analysis, the color was developed by adding 2 mL of 1M NH4Cl and the pH was maintained by adding conc. ammonium hydroxide solutions in the range of 9.8–10.2. After the pH adjustment, required amount of 4-aminoantipyrene (Merck, 98%) and potassium ferricyanide (Aldrich, 99.5%) were added to develop the color. The concentration of phenol was

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measured by the spectrophotometric method at 510 nm. The average solar light intensity during the experiment is observed to be 100,000 lux. 2.2.5 Photocatalytic Hydrogen generation The photocatalytic hydrogen generation was carried out in a reactor attached with a 125 W medium pressure Hg visible lamp with 1M NaNO2 as UV filter to irradiate light of wavelength equal to or greater than 400 nm. The average light energy density during the experiment is found to be 120 mW/cm2. 50 ml of 10 vol. % methanol solution containing 0.05g of powder catalyst was used to perform the reaction. The reactor containing the above solution with catalyst was stirred continuously in order to prevent the settling down of the catalyst. Nitrogen gas was purging inside the reactor before starting the experiment in order to evacuate the reactor and the process was repeated for several times. GC-17A using 5 Å molecular sieve column with a thermal conductivity detector (TCD) was used to detect the evolved hydrogen which was collected by the downward displacement of water. 2.2.6 Analytical characterization The phase purity and crystalinity of the synthesized catalysts were estimated by Rigaku Miniflex (set at 30 kV and 15 mA) using Cu Kα radiation (λ = 1.54 Å). The DRUV-Vis spectra of all the prepared samples were measured by JASCO 750 UV-Vis spectrophotometer in the range of 200800 nm, taking boric acid as the reference. The PL emission and excitation spectra were analyzed by a JASCO-FP-8300 fluorescence spectrometer in which Xenon lamp is used as excitation source. The emission and excitation spectra were recorded in the range of 520-640 nm and 420-540 nm respectively. The morphology, dispersion of CdS QDs on BiOI surface and microstructure of the 7



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catalysts were investigated by TEM-JEOL-2010- 200 kV instrument. X-ray photoelectron spectroscopy was performed with VG Microtech Multilab ESCA 3000 spectrometer using nonmonochromatised Mg Kα X-ray source. O 1s peak at 530.11 eV was used for binding energy correction. The electrochemical study of the prepared samples was carried out by multi-channel Ivium potentiostat. The photoelectrochemical (PEC) measurement was carried out in a Pyrex electrochemical cell by using working electrode, a platinum plate as a counter electrode and an Ag/AgCl as reference electrode. 0.1 M Na2SO4 of pH 6.5 was taken as electrolyte solution for PEC measurement. Light irradiation was carried out by using a 300 W Xe lamp with 400 nm cut off filter. Total organic carbon (TOC) measurement was carried out by using a TOC analyser, ANATOC series 2. 3. RESULTS AND DISCUSSION 3.1 XRD The crystal structure and phase purity of the prepared photocatalysts were determined by the Xray diffraction pattern. Figure 1 represents the X-ray diffraction (XRD) pattern of BiOI MPs and CdS QDs/BiOI composite with various wt. % of CdS QDs. All the diffraction peaks matches with the JCPDS no.10-0445 (BiOI) and JCPDS file no.75-0581(CdS QDs), clarifying the crystalinity as well as tetragonal and cubic structure of BiOI MPs and CdS QDs (a=5.810A0) respectively.

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Figure 1. XRD patterns of neat BiOI MPs and the CdS QDs/BiOI composites. The diffraction pattern indicates similarity between pure BiOI MPs and the composites which clearly concludes that BiOI crystal structure is not affected by the deposition of CdS QDs. The diffraction peaks are observed at 2 29.650, 31.620, 37.420, 43.650, 45.370, 51.370, 55.090 which corresponds to the (102), (110), (103), (113), (200), (114), (212) crystal planes of BiOI MPs and three signal are found at 2 values of 26.60, 44.10 and 51.80 corresponding the crystal planes (111), (220), (311) of CdS QDs (Figure S1). It is apparently visible from the above pattern that no impurity or secondary phases are present in the sample. It should be noted that no diffraction peaks for CdS QDs could be identified and it may be attributed to the ultra-small particle size, high dispersity and low loaded amount of CdS QDs. Moreover, no shifting of characteristic diffraction peaks are observed in the diffraction patterns of CdS QDs/BiOI composites which is supported by 9



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the fact that loaded CdS QDs were not introduced into the BiOI lattice it only modifies the surface of BiOI. The (102) peak of neat BiOI MPs becomes more and more intense with increasing the content of CdS QDs (up to 4 wt %) which concludes the increase of crystalinity of BiOI MPs with the loading amount of CdS QDs. It has been found that the pick intensity is decreased with further increase of CdS QDs content which confirms that the QDs gradually covers the surface of BiOI and may get agglomerated, as a consequence growth of the peaks are inhibited. It should be mentioned here that crystallite size of CdS QDs calculated by Scherrer equation from XRD peak broadening was found to be 2.48 nm which is smaller than its excitonic Bohr radius i.e. 5-6 nm. 43 Details about particle size are discussed in section 3.4. 3.2 UV-Vis DRS UV-Vis diffuse reflectance spectra (DRS) were utilized to determine the optical properties like photo absorption competence of BiOI MPs, CdS QDs and the series of the CdS QDs/BiOI samples. According to the spectrum (figure 2a), the BiOI sample shows broad absorption peaks below 690 nm which is attributed to the intrinsic oxygen vacancies present in it, while the CdS sample has exhibited strong absorption up to the range of   580 nm.

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Figure 2. (a) Absorbance spectra of (i) BiOI MPs (ii) CdS QDs (iii) 2% CdS QDs/BiOI (iv) 3% CdS QDs/BiOI (v) 4% CdS QDs/BiOI (vi) 5% CdS QDs/BiOI and (b, c, d) band gap energy of BiOI MPs, CdS QDs, 4 % CdS QDs/BiOI respectively. But after loading CdS QDs on BiOI, its absorption region has become broader and extended up to 750 nm and beyond this wavelength the absorbance gradient is small because of scattering effect. 44

Due to absorption by the QDs, the composite exhibits broad peak below 750 nm. The spectra

show the consequence of variation in content of CdS QDs on BiOI MPs. With increase the content of CdS QDs to 4 wt%, the broad absorption peak covers more area suggesting the contribution of absorption by the QDs. This indicates the composites show red shift and covers broad area in spectra as compared to the neat BiOI MPs which indicates the improvement of the optical property 11



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of both the constituents. The steep shape of the spectra is because of band gap alteration and was calculated by extrapolating the baseline and the steepest tangent of the UV–Vis spectra. Hence it is concluded that the composite could utilize almost the entire visible region owing to the potent coupling between these two components. It is only due to robust response towards visible light and broad size dispersal of CdS QDs

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and is evidenced by its TEM image. The band gap of

prepared samples has been calculated from the spectra with the help of Tauc plot using the following equation. 45 αhυ = A (hυ − Eg)n/2 where α, h, υ, Eg, and A are absorption coefficient, Planck constant, frequency of light, band gap, and proportionality constant respectively. The value of n is estimated depending on the type of optical transition whether direct or indirect. It is worthy to mention here that n=1, 3, 4, 6 for allowed direct transition, forbidden direct transition, allowed indirect transition, forbidden indirect transition. Most commonly the values of n=1 and 4 are used. The band gap energy of CdS QDs is found to be 2.24 eV (figure 2c) for direct optical transition and it is 1.78 eV in case of BiOI for indirect allowed transition. 28 The band gap energies are calculated as 1.77 eV, 1.74 eV, 1.70 eV and 1.71 eV for 2%CdS QDs/BiOI, 3%CdS QDs/BiOI, 4%CdS QDs/BiOI and 5%CdS QDs/BiOI respectively and all are directly allowed optical transitions. 28, 42 All the composites have smaller bandgap values than that of the neat materials because of change in optical property caused by effective interaction between the pure materials. 46 3.3 PL spectra

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Figure 3. PL spectra of CdS QDs/BiOI at (a) excitation wavelength, ex 460 nm (b) ex of 450, 455, 460 nm (c) PLE spectra at emission wavelength em of 560, 570, 580 nm. The optical properties like recombination and charge transfer process of photoinduced charge carriers in semiconductors can be determined effectively from the photoluminescence (PL) emission spectroscopy and photoluminescence excitation (PLE) spectroscopy. Here we have studied in detail the photoluminescence by using different excitation as well as different emission wavelengths. The above figure 3a shows the PL spectra of neat BiOI MPs and CdS QDs sentitized BiOI MPs (CdS QDs/BiOI). Broad and asymmetric peaks are found for the composite and this type of graph recommends the broad distribution of particle size. Hence it can be concluded that 13



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the asymmetry of PL peaks recommends that there is slow loss of energy followed by absorption, emission and energy transfer between the BiOI MPs and different sized QDs.

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The higher PL

intensity of BiOI MPs indicates faster recombination of charge carriers. The spectrum shows an intense peak at 582.3 nm (inset of figure 3a) and this is due to the band gap transition. The other two peaks formed might be due presence of defects. 44 From the figure 3a, it is also seen that the spectra of the CdS QDs/BiOI composite have lower intensity than the neat BiOI MPs (inset of figure 3a) which is attributed to effective quenching of electron-holes by means of their effective transfer between BiOI and CdS QDs. Among all the composites, 4% CdS QDs/BiOI shows lowest intensity which confirms the well formation and proper band edge alignment of the CdS QDs/BiOI composite and better photocatalytic activity. After using different excitation and emission wavelength it has been detected that CdS QDs also show excitation dependence PL-behavior like all other QDs. In addition to this the emission peaks are gradually blue shifted from ca. 550 nm to ca. 540 nm with increasing the excitation wavelength (ex) from 450 nm to 460 nm (figure 3b). This type of behavior is most commonly because of surface states which extensively affect the band gap of QDs. 48 Figure 3b shows the PL spectra of the composite at excitation wavelengths of 450, 455 and 460 nm. As shown in figure b (i) When (ex) is set at 450 nm PLE peak at ca. 550 nm is obtained with shoulder peaks at ca. 538 nm and 560 nm. But when (ex) is set at 455 nm [figure b (ii)] a broad PL peak is obtained at ca. 545 nm with a shoulder peak located at ca. 564 nm. This peak seems to be merely a single peak but it may be combination of two Gaussian peaks. The fact is that when (em) is set at 560, 570 and 580 nm, most intense PLE peaks at ca. 507, 510 and 537 nm are obtained respectively (figure 3c). From this, we can conclude that the convoluted peak of these absorption peaks give rise to a broad peak at ca. 540-550 nm as shown in figure 3b. Compared to figure 3b (i) and 3b (ii), two broad and intense peaks at ca. 540 nm and 580 nm with 14


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a shoulder peak at 564 nm are obtained when (ex) is set as 460 nm [figure 3b (iii)]. Hence the prepared composites are excited at 460 nm. It is also marked from figure 3c that with increase of (em) from 560 to 580 nm there is red shifting of peaks. These variations in PL peaks suggest that the set of various sizes of QDs (ca.3-4 nm) are uniformly dispersed on the surface of BiOI. 44 3.4 TEM analysis The detail morphology and structural information mainly the micro plate structure of BiOI, ultrasmall size of CdS QDs and its uniform distribution in 4 wt % CdS/BiOI composite were investigated by the TEM. Figure 4a represents the microplate like morphology of BiOI and figure 4b shows the SAED pattern of BiOI MPs in which it is clearly visible that the bright spots are arranged in a tetragonal shape without attaining a shape of concentric ring. That simply elucidates the single crystalline nature and tetragonal phase presenting (200), (110) plane of BiOI. Figure 4c and its inset figure show the HRTEM image of CdS QDs. The SAED pattern of CdS QDs on behalf of (311), (111), (220) planes is presented in figure 4d. The uniform distribution and attachment of CdS QDs (dark spots) on the surface of BiOI (light area) is clearly visible from the figure 4e. It was determined that the average particle size of CdS QDs is around 3.54 nm (0.13 nm) which is small enough to exhibit the quantum confinement effect. These two important properties i.e. crystalinity and ultra-small particle size tends to faster charge carrier diffusion towards BiOI surface which effectively separates the charge carriers and enhances its photocatalytic activity.

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Figure 4.HRTEM image and SAED pattern of BiOI MPs (a, b); HRTEM image and SAED of CdS QDs (c, d); TEM, HRTEM and SAED of 4% CdS QDs/BiOI (e, f, g). The BiOI microplates might be helping in preventing the aggregation and also uniform distribution of QDs. In figure 4f the lattice spacing were significantly observed with various orientations. The lattice spacing for BiOI was measured to be 0.30 nm and 0.28 nm nm which corresponds to the (102), (110) plane of tetragonal lattice respectively (upper inset of figure 4f). Lower inset of figure 4f shows lattice fringes arising from a CdS QDs, and the inter-planar spacing is deduced to be 0.354 nm and 0.205 nm which agrees well with (111), (220) reflection of cubic lattice of CdS. Figure 4g shows the SAED pattern of CdS QDs/BiOI composite in which co-occurrence of both tetragonal phase of BiOI and cubic phase of CdS QDs are remarkably detectable. The (220) plane of cubic CdS QDs is evident from the inner bright concentric ring and the ring pattern corresponds to the polycrystalline nature of CdS QDs. The (200), (110) plane of tetragonal BiOI is marked from the bright spots without concentric rings which refers to the single crystalline nature of BiOI MPs. As a whole the SAED pattern confirms the well crystalinity of the CdS QDs/BiOI composite and the little discontinuous pattern might be due to the irregularity in the coupling between the constituents. In summary, the CdS QDs were homogeneously loaded on the surface of the BiOI and the micro plates act as clapboards to separate CdS QDs, indicating the BiOI is the excellent supporting materials to improve the dispersion of the CdS QD. It increases surfaces to volume ratio and enhances reactive sites to the photocatalysts, which was favorable for higher photocatalytic activity. 3.5 XPS analysis 17



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Figure 5. XPS spectra for BiOI/CdS QDs in the regions of (a) Bi 4f, (b) O 1s, (c) I 3d, (d) Cd 3d and (e) S 2p.

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The surface compositions and the chemical states of the elements present in 4 % CdS QDs/BiOI are determined by X-ray photoelectron spectroscopy (XPS) which is shown in figure 5. XPS analysis showed that the composite contain the elements Bi, O, I, Cd and S. The photoelectron peaks are found to appear at binding energies of 159.2 eV and 164.4 eV for Bi (4f), 530.11 eV for O (1s), 619.2 eV, 630.5 eV for I (3d), 405.5 eV and 412.2 eV for Cd (3d) and at 159.9 eV, 165.1 eV for S (2p). The binding energy of oxygen at 530.11 eV is used to calibrate the shift of peak position. The symmetric peak obtained at 530.11 eV for O 1s in figure 5b is due to the presence of oxygen atom in the lattice form and hydroxyl oxygen on the surface. 49 Figure 5a represents two symmetric peaks for Bi (4f), which are positioned at 159.2 eV and 164.4 eV with peak difference of 5.2 eV and are ascribed to Bi 4f 5/2 and Bi 4f 7/2 respectively. This implies that the main elemental chemical state of Bi is trivalent (Bi3+). 28, 50 From the I 3d XPS spectra shown in figure 5c, two strong peaks centered at 619.3 eV and 630.5 eV are observed for I 3d 5/2 and I 3d 3/2 respectively which implies that iodine is present in iodide state, I- . 41 The Cd 3d5/2 and Cd 3d3/2 peaks are observed at 405.5eV and 412.2 eV (Figure 5d), which can be assigned to Cd2+ of CdS QDs.

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In figure 5e the peaks for S are centered at 159.9 eV and 165.1 eV for the transitions S

(2p1/2) and S (2p3/2) respectively confirming the presence of sulfur in -2 oxidation state. 40, 3.6 Mott-schottky analysis The energy band structure and charge transfer as well as recombination behavior is studied by the Mott-Schottky method and electrochemical impedance spectroscopy (EIS) under dark condition in three electrode systems using Ag/AgCl as the reference electrode, platinum as counter electrode dipped in 0.1 M Na2SO4.

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Figure 6. Mott-Schottky plot of (a) BiOI MPs, (b) CdS QDs The Mott-Schottky plot of BiOI and CdS QDs are represented in figure 6a and 6b. From the nature of the plot it is confirmed that both the constituents materials are n-type. The flat band potential was calculated by the extrapolation of the Mott-Schottky plot and the flat band potentials are found to be -0.99 V and -0.56 V for BiOI, CdS QDs respectively. The flat band potential is approximately equal to the conduction band potential for n-type semiconductors. Hence the positions of CB band for BiOI, CdS QDs are -0.99 eV and -0.56 eV respectively. From UV-DRS the band gap of BiOI, CdS QDs were found to be 1.78 eV and 2.24 eV. So, the valence band position of the BiOI and CdS QDs are calculated as 0.79 eV and 1.68 eV respectively. This band structure has greatly contributed towards the photocatalytic activities which are discussed in the next section. 3.7 EIS analysis Electrochemical impedance spectroscopy (EIS) study was performed to measure the charge transfer ability means the conductance, resistance of the photocatalysts which is shown in figure 7.

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Figure 7. Nyquist plot for the BiOI and 4% CdS QDs/BiOI.

Figure 7 shows the Nyquist impedance plot for BiOI and 4 % CdS QDs/BiOI composite which signifies more conductivity of the composite. The EIS data is fitted by the equivalence model provided in inset of figure 7. The electrolyte resistance is represented by Rs, charge transfer resistances and contact resistances at the counter electrode/electrolyte are also included in Rs. Electron-transfer resistance is Rct and CPE is the constant phase element that also signifies the double layer capacitance. Ws is the Warburg impedance. 52-54 The plot consists of semicircle part which contribute the charge transfer resistance (Rct) and constant phase element (CPE) at the composite-electrolyte interphase. 55 The high frequency part of the plot is semicircle in nature and low frequency part is almost straight line inclined at certain angle. The semicircle part describes about Rct and the straight line describes about the efficiency of material used as capacitor. 56 Larger the semicircle more is the resistance at interface and lesser is the conductance. Smaller semicircle means decrease of Rct which was recorded when CdS QDs were incorporated into BiOI MPs which 21



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suggests best separation efficiency of the composite in comparison to the constituent semiconductors. In other words the decreased value of Rct increases the photocatalytic activity of the as synthesized CdS QDs/BiOI composite. In addition, the straight line part is related to Warburg resistance and which results from the transfer or diffusion of ions in the electrolyte. In figure 7 it is clearly visible that Warburg region is small in case of the composite and larger in BiOI. The smaller Warburg region of composite indicates that the ion movement is more and consequently the conductance is also more for 4% CdS QDs/BiOI. Hence it can be concluded that the presence of CdS QDs have increased the electrical conductivity of BiOI MPs in the prepared composite and helps in enhancing the photocatalytic performance. 3.8. Photocurrent response The photocurrent measurement under dark and light illumination of BiOI MPs and 4% CdS QDs/BiOI composite was measured in 0.1 M Na2SO4 at a scan rate of 10mVs-1. Figure 8 shows the linear sweep voltammetry (LSV) plot of BiOIand 4% CdS QDs/BiOI composite under dark and light condition. From figure, it has been confirmed that under dark condition BiOI and 4% CdS QDs/BiOI composite could able to generate 2.6 A and 24.2 A of current respectively. The generation of current is significantly enhanced under light illumination. Under light illumination, BiOI and 4% CdS QDs/BiOI composite generated 4 A and 62A of current respectively. It is very clear that the composite generates more current than BiOI MPs. It can be concluded that modification of BiOI MPs with CdS QDs improves the charge separation process which enhanced the photocurrent generation under light irradiation. 57 The obtained result is also in good agreement with the PL behavior (figure 3a).

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Figure 8. Potential-Current density curves of BiOI MPs and 4% CdS QDs/BiOI composite under dark and light condition. 4. PHOTOCATALYTIC ACTIVITY 4.1 Photocatalytic activity towards de-colorization of RhB under solar light irradiation The photocatalytic activity of the prepared photocatalysts is tested towards the de-colorization of 20 ppm RhB solution under solar irradiation. The photocatalytic activity was tested by the following steps. In the first step, the RhB solution was de-colorized in absence of catalyst for 1 h. It was observed that only 2% of RhB got deteriorated itself. Second step involves 1 h adsorption mechanism of catalyst on active sites of the dye in dark condition. It was observed that the effect of adsorption is around 5% towards de-colorization of RhB solution. In the third step, 0.02 g of all the catalysts was used to de-colorize 20 mL of 20 ppm of RhB solution. The de-colorization rate of RhB solution with respect to time is presented in figure 9a. It is clear from the figure that BiOI MPs and CdS QDs could able to de-colorize 85 % and 90 % of RhB solution respectively. When the durability of the photocatalysts is tested by repeating the experiment with more than one cycle, 23



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it is seen that the stability of the parent material i.e. CdS QDs and BiOI is very low as their activity couldn’t retain after first cycle (Figure S2). In this investigation, we have tried to modify BiOI MPs with various amounts of CdS QDs in order to improve their photostability as well as to enhance their photocatalytic activity. It is observed that in case of the prepared composite materials, the percentage of de-colorization rate increases with increasing the content of CdS QDs up to 4 wt% of BiOI. From figure 9a, it is clear that 4% CdS QDs/BiOI shows 82 % de-colorization which is significantly greater than the other prepared composites. The order of de-colorization of the prepared composites is found to be 4% CdS QDs/BiOI > 3% CdS QDs/BiOI > 2%CdS QDs/BiOI > 5% CdS QDs/BiOI. Further loading of the QDs decreases the de-colorization rate by blocking the active sites of BiOI surface. This indicates that the content of CdS QDs plays vital role towards de-colorization of the dye. It is found that the parent materials have showed better result than the composites in the first one hour of the experiment. It is very attention-grabbing to observe that the activity of the 4 % CdS QDs/BiOI composite materials could able to retain even up to five cycles of experiments. So, it is concluded that the stability of the composite materials is significantly enhanced than the corresponding neat materials (figure 9b). Hence the formation of composite between BiOI and CdS QDs improves their individual stability tremendously which signifies the novelty of this work. Figure 9c signifies the spectral changes in concentration of the RhB solution by 4% CdS QDs/BiOI with increase of time period. During time period of 1 h, the absorption spectra of RhB was decreased significantly with increase in time interval of 20 min and the color became initial pink to light pink. So with increasing the time period the photocatalytic activity of 4% CdS QDs/BiOI composite increases significantly which helps in the de-colorization process. We have also tested the physically mixed CdS QDs and BiOI towards de-colorization of RhB. The result showed that the physically mixed sample could able to de-colorize only 45% 24


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which is significantly lower than all the prepared composite material. Moreover, the physically mixed material is very unstable than all other composite materials (Figure S3). The excellent photocatalytic activity of 4% CdS QDs/BiOI composite is well consistent with large absorption coefficient of CdS QDs, optimal amount of CdS QDs which prevents the coagulation on the BiOI surface, well formation of composite between the constituent semiconductors and low PL intensity. The large absorption coefficient of CdS QDs plays an important role in enhancing the photocatalytic activity. The large absorption coefficient of CdS QDs and optimal loading of CdS QDs results in harvesting more visible light by the composite. The harvestation of more visible light results in enhancing the de-colorization of the RhB solution. The well formation of composite materials as evidenced from TEM micrographs also played a great role for efficient photocatalytic activity of 4% CdS QDs/BiOI composite. The formation of composite materials results in interfacial charge transfer through the interface which results in inhibition of the recombination of the photo induced charge carriers (Scheme-1). The inhibition of the recombination of photo induced charge carriers played a vital role in enhancing photocatalytic activity of 4% CdS QDs/BiOI composite. In addition to the above discussed points, the lowest PL intensity of 4% CdS QDs/BiOI composite is one of the important criteria for enhanced photocatalytic activity. Due to low PL intensity, the recombination of charge carrier is less hence; it showed higher photocatalytic activity than all other composite materials. The less recombination of charge carriers and high conductivity is also confirmed from the electrochemical study. 4.1.1 Kinetics and mechanism of the de-colorization The rate of de-colorization is determined from figure 8a by plotting between C/C0 Vs time and using the following equation. 25



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Photo de-colorization rate = (C0-C/C0) x 100 Where C0 is the initial concentration at time t=0 min C is the concentration at time ‘t’ min Moreover, the photocatalytic de-colorization here follows pseudo-first order kinetic which is examined by Langmuir-Hinshelwood equation. ln (C0/C) = kt Where, k is the pseudo-first order rate constant or apparent rate constant. The kinetics is supported by the linear plot between ln (C/C0) and time of irradiation (figure 9d). The slope of this plot gives the value of k or kapp. The values of kapp for all the CdS QDs/BiOI composites are given in table 1. The kapp values follow pseudo-first order kinetics. From the kinetic plot it is also confirmed that 4% CdS QDs/BiOI displays better performance in the direction of photocataysis. Henceforth, it can be supposed that the composite is a better photocatalyst for the de-colorization of RhB. Table 1. Kapp values for the prepared samples in first half hour of the de-colorization of RhB. Catalyst

RhB (Kapp x 10-4 min-1)

Blank

128

BiOI MPs

2860

CdS QDs

3509

2 % CdS QDs/BiOI

878

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3 % CdS QDs/BiOI

1462

4 % CdS QDs/BiOI

2498

5 % CdS QDs/BiOI

485

4.1.2 Scavenger test involving the reactive species responsible for de-colorization The reactive species responsible for the deterioration of RhB can be detected by using the scavenging reagents such as p-benzoquinone (p-BQ), DMSO, IPA, EDTA for superoxide radical, electron, hydroxyl radical and hole respectively. 58 20 ppm RhB and 5mM scavenging solution are employed for this purpose. The figure 9e shows the species that are involved in de-colorization process of RhB solution. From the figure, it is clear that the species including hydroxyl radical, electrons and holes are responsible mainly for de-colorization process and superoxide radicals are the minor species in de-colorization of RhB. The data obtained after the scavenger test over 4% CdS QDs/BiOI for each species is presented in Table-2. When p-BQ is used as trapping agent for superoxide, the de-colorization rate is close to that of the 4 % CdS QDs/BiOI (without scavenger) which signifies superoxide radical played minor role towards de-colorization of RhB solution. In case of IPA and DMSO as scavenger for hydroxyl radical and electron, the de-colorization of RhB is minimum which signifies both the species have effective role in the de-colorization process. After introduction of EDTA, the de-colorization ability of the dye is more as compared to DMSO and IPA which signifies the role of hole is not as effective as hydroxyl radical and electron in decolorization process. So from the above experiment, it is concluded that hydroxyl radical, electrons

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and hole are responsible for de-colorization of RhB solution whereas super oxide radical played the minor role in the de-colorization process. Table 2. The values of C and C/C0 of RhB in presence of scavengers Scavenger

Species involved

Concentration,

C C/C0

(ppm) p-BQ

●

DMSO

O2─

4.45

0.22

e─

17.45

0.87

IPA

●

17.75

0.88

EDTA

h+

15.8

0.79

OH

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Figure 9. (a) Extent of de-colorization of RhB by the photocatalysts in every successive time interval for the first hour of the experiment, (b) photocatalytic stability and reusability of 4 % CdS 29



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QDs/BiOI composite (c) UV–Vis absorbance spectra of RhB as a function of time over 4 % CdS QDs/BiOI composite (d) kinetics of RhB de-colorization in first cycle and (e) effect of different scavengers on the de-colorization of RhB over 4 % CdS QDs/BiOI. 4.1.3. Mechanism of photocatalytic de-colorization of RhB Solution The mechanistic path way of the whole de-colorization process is discussed as follows. As discussed in UV-Vis DRS spectra, both the constituents of the composite absorb visible light as the band gap of BiOI MPs and CdS QDs is 1.78 and 2.24 eV respectively. As per the band gap alignment of BiOI (EVB = + 0.79 eV , ECB = −0.99 eV) and CdS QDs (EVB = +1.68 eV, ECB = − 0.56 eV), the photoexcited electrons get transferred from the CB of BiOI to that of CdS QDs and the direction of transfer of holes is opposite to it and is illustrated in scheme 1. The redox potential of O2 / ●O2─ (-0.33 eV) which is less negative than the CB of CdS QDs. Hence, the electrons accumulated at the surface of CdS QDs get trapped by the oxygen to form ●O2─ but it is confirmed from the scavenger test that it has very negligible role in de-colorization process. This suggests that ●O2─ formed reacts with proton produces H2O2 and subsequently ●OH is formed. The redox potential of OH─/●OH is 1.99 eV which is more positive than the potential of hole on VB of CdS QDs. So the formation of ●OH is not possible directly. But ●OH could be formed indirectly from ●

O2─ and it can be suggested that the amount of ●OH formed depends on the amount of ●O2─

formed. On the other hand, the holes at the surface of BiOI MPs are also involved in the photocatalytic activity. In summary, it can be concluded that the active species ●OH, h+ and e ─ plays very significant role in de-colorization of the dye.

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Scheme 1. The proposed mechanism for RhB de-colorization over the photocatalyst. In summary, the mechanism for de-colorization can be proposed as follows. CdS/BiOI + h → CdS (e ─) + BiOI (h+) CdS (e ─) + O2→ ●O2─ CdS (e ─) + ●O2─ +2H+→ H2O2 H2O2 + CdS (e ─) → ●OH + OH─ RhB + ●OH → De-colorization RhB + BiOI (h+) → De-colorization RhB + ●OH + CdS (e ─) + BiOI (h+) → De-colorization In order to exclude the possible interferences of photosensitization phenomena in case of RhB, we have also studied the photocatalytic activity of the prepared photocatalysts towards degradation of 31



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colorless organic compound i.e. phenol as a model organic pollutant. It has been observed that the prepared photocatalysts are also efficient towards degradation of phenol. The degradation procedure of phenol is similar to that of RhB and the degradation rate with respect to irradiation time is shown in figure 10a.

Figure 10. (a) Rate of degradation of phenol over the prepared photocatalysts in every successive time interval for the first hour of the experiment (b) kinetics of phenol degradation in first cycle (c) UV–Vis absorbance spectra of phenol over 4 % CdS QDs/BiOI as a function of irradiation time and (d) reusability graph of phenol over 4 % CdS QDs/BiOI composite. 32


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Figure 10b shows the kinetics of degradation of phenol in the first cycle of the experiment. The data represents the degradation process is of first order reaction. The degradation curve and spectral changes of decrease of concentration of phenol with respect to the irradiation time is shown in figure 10c. It is found from the figure that 4% CdS QDs/BiOI composite is also better in phenol degradation than the other prepared CdS QDs/BiOI composites and 63 % phenol were degraded in 1 h by this composite under solar light irradiation. The stability cum reusability of the composite towards degradation of phenol was tested in five repeated cycles under solar light and the result is shown in figure 10d. The material is also found to be stable for phenol degradation process as that of RhB. In order to sidestep tributary pollution in the real-world application of the photocatalyst, the mineralization of RhB and phenol was tested over 4% CdS QDs/BiOI at different irradiation times by TOC analysis. The degree of mineralization was estimated by using the following equation. 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑑𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑚𝑖𝑛𝑒𝑟𝑎𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 =

𝑇𝑂𝐶0 −𝑇𝑂𝐶𝑡 𝑇𝑂𝐶0

Where, TOC0 and TOCt are the of total organic carbon content of solution before and after irradiation respectively. The pictorial estimation of TOC removal efficiency with irradiation time is represented in figure 11. From the figure, the degree of mineralization was found to be 32 % in 60 min and it was reached to 60 % in 180 min for RhB and 32 % in 180 min for phenol.

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Figure 11. TOC removal efficiency in 10-4 M RhB and 5 ppm phenol over 4% CdS QDs/BiOI. 4.2 Photocatalytic activity towards hydrogen evolution under visible light To confirm the versatile photocatalytic activity of the prepared photocatalysts, these are also tested towards photocatalytic hydrogen gas production under visible light irradiation (>400nm). The experiment was first carried out without catalyst and then without light. It was observed that no hydrogen was evolved in both the cases which indicates that hydrogen gas was evolved by photocatalytic way. As per the stability is concerned the prepared composites show more H2 evolution than the constituent semiconductors. Photocatalytic hydrogen evolution of the synthesized composites were illustrated in figure 12 and from the figure it was found that 4% CdS QDs/BiOI composite has able to evolve maximum hydrogen of 610 µmol in 3h.

The

photocatalytic activity of the prepared samples towards H2 evolution follows the order: 4% > 3%> 2% >5% CdS QDs/BiOI. 5% CdS QDs/BiOI composite has showed minimum hydrogen evolution among the prepared samples. This is due to the reason mentioned earlier that overloading of CdS QDs blocks the active sites on BiOI surface which hinder the reaching of light on BiOI surface. 34


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This leads to subsequent decline of ejection of photoexcited electrons which are the main species to release hydrogen. The best photocatalytic activity of 4% CdS QDs/BiOI composite towards higher hydrogen evolution could be also described in view of large absorption coefficient of CdS QDs, optimal amount of CdS QDs loading, well formation of composite and low PL intensity. The effect of all these parameters have clearly described in our earlier section. The neat materials are also active towards hydrogen evolution but are very unstable than the composite materials (Figure S4). In order to know the stability of the 4% CdS QDs/BiOI composite photocatalyst, repeated experiments are carried out with evacuation of the photocatalytic system with an induction period of 3 h and are shown in Figure S5. From the repeated experiment graph, it was observed that the material could able to generate a steady rate of hydrogen production with 203µmol/h. The mechanism of hydrogen evolution over the composite material is presented in scheme-2. It was confirmed that H2 evolution reaction takes place on the surface of CdS QDs as per the band alignments of the constituents (scheme-2). Hydrogen evolution reaction takes place on the CB (−0.56 eV) of CdS QDs as per the band alignments of the constituents. CB of BiOI MPs (−0.99 eV) lies on more –ve potential than CdS QDs, as a result photoexcited electrons from BiOI transfers to CB of CdS QDs and the holes are transferred in opposite direction. As a result, more electrons are available on CB of CdS QDs which reacts with proton and produce H2 gas.

35



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Figure 12. Photocatalytic hydrogen evolution over the prepared composites.

Scheme 2. Proposed mechanism for hydrogen generation over the photocatalyst. 5. CONCLUSIONS

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CdS QDs/BiOI composite photocatalysts have been sucessfully synthesized by two step precipitation deposition method. In this investigation, we have significantly enhanced the photostability of CdS QDs and BiOI by making their composites. The well formation of 3.5 nm (average value) sized CdS QDs are observed from the TEM analysis and it is small enough than their Bohr’s exciton radius (5-6 nm). The optical activity of both the constituent semiconductors are tremendously increased which was evidenced from the UV-Vis spectroscopy. The composite absorbs maximum fraction of solar spectrum and the absorption range is extended up to 750 nm. The best photocatalytic activity of 4%CdS QDs/BiOI is in agreement with large absorption coefficient of CdS QDs, optimal amount of CdS QDs loading, well formation of composite and low PL intensity. The mechanistic path way and delay in recombination of charge carriers is verified from the photoluminescence study. Moreover, the excitation dependence PL behavior of CdS QDs in the composite has also been established. Further more, it has been established that hydroxyl radical, hole and electrons palyed major role in de-colorization of RhB solution. We believe this work will open up new tactic for fabricating efficient visible active photocatalysts and utilization of maximum fraction of solar light for energy and environmental applications. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected], [email protected], Tel. No. +91-6742351777, Fax. +91-674-2350642

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Supporting information XRD pattern of CdS QDs, photostability and reusability of CdS QDs and BiOI MPs towards decolorization of RhB, de-colorization study of RhB over physically mixed 4% CdS and BiOI, photostability and reusability of CdS QDs and BiOI MPs towards hydrogen generation and photostability and reusability of 4% CdS QDs/BiOI.

Acknowledgement The authors are very much thankful to the SOA University, management for their support and encouragement. We are obliged to Prof. P. V. Satyam, IOP, Bhubaneswar for the support of TEM analysis.

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Modification of BiOI Microplates with CdS QDs for Enhancing Stability

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