High Photocatalytic Performance of Modified Bismuth Oxychloride Semiconductor under Sunlight

Introduction

Water pollution has become a major environmental concern as it encourages the transmission of many waterborne diseases, especially in developing nations. The paper, textile, leather, printing and dyeing   industries which utilize a large amount of coloring materials such as dyes that are non-biodegradable and synthetically stable poisons are released into the nearby water bodies. This release of the effluent into the nearby water streams are considered to be a serious environmental hazard to both aquatic as well as human life¹. Different water treatment techniques such as Coagulation, flocculation, adsorption, membrane separation, UV/Ozone treatment have been employed for the removal of the organic pollutants from the wastewater^2-5. Among the various water treatment technologies, the development of advanced oxidation process in the past years have gained considerable interest by the researchers due to the low-cost, eco-friendliness and the potential to completely degrade the organic pollutants into harmless products such as carbon dioxide and water^6-10. Among many semi-conductor nanocatalysts bismuth-based semiconductors have gained tremendous consideration in the field of photocatalysis for its remarkable action against hazardous pollutants^11-13.Bismuth oxychloride is reported to show excellent photocatalytic activity in the degradation of variety of organic pollutants such as dyes, pesticides, phenols and microbes as it possesses a high chemical stability¹⁴. Many researchers have reported the excellent photocatalytic activity of Bismuth oxychloride photocatalyst which is attributed to the layered structure of[Bi₂O₂]²⁺ monolayer and dual ‘Cl’ layers¹⁵.However,its large band gap (3.17–3.54 eV) energy values have limited its activity as a photocatalyst towards many pollutants¹⁶. Hence, in order to overcomethese limitations many modifications of bismuth oxychloride photocatalysthave been researched by the materials scientists in the recent years.

Among which coupling and doping of the BiOCl photocatalyst with other suitable semiconductors are found to be effective in getting better photo-response and also facilitate the separationof photo induced electron–hole pairs which interact with the hydroxyl radicals to provide excellent photodegradation of the contaminants¹⁷.Hence, in our present work we have mainly focused on synthesizing a modified bismuth oxychloride photocatalyst via a facile low cost approach using water as the solvent, bismuth nitrate pentahydrate as the precursor, potassium chloride as the source of halogen and Aluminium fluoride as the dopant. The as-synthesized bismuth oxychloride photocatalyst have been employed for the degradation of the textile dyeAcid green 1. It is a nitroso dye with the molecular formula (C₃₀H₁₅FeN₃Na₃O₁₅S₃) and Molecular Weight(878.46). The dye shows excellent absorption and excellent light fastness. It is mainly used in solar salt industry and wool, silk and nylon fabric dyeing and printing can also be used for leather dyeing^{18, 19}. The chemical structure and UV-Visible absorption spectrum of the Acid green1dye is shown in figure 1.

Figure 1: shows the UV – Visible spectrum and Chemical structure of Acid green 1 dye. Click here to View figure

Experimental Section

Materials

All materials used in this experiment are of analytical grade. Bismuth nitrate pentahydrate (M.W. 485.07, purity 98 %) and Acid green 1 dye was purchased from S. D. Fine Chemicals., Potassium chloride from Merck and ethanol (99.9%) was purchased from Changshu Hongsheng Fine Chemical Co. Ltd. The solution pH was measured using Elico digital pH meter. The decolourization of dyes was monitored by UV-visible spectrophotometer (Hitachi- U2910).

Synthesis of AlF-BiOCl Photocatalyst

Bismuth nitrate pentahydrate is used as a precursor to prepare the bismuth oxychloride catalyst. In a typical synthesis, 0.1M bismuth nitrate pentahydrate is dissolved in 100mL double distilled water. About 3 wt % of Aluminium fluoride salt is added to the precursor solution and it is continuously stirred for about half an hour and this solution is labeled as A. Potassium chloride is used as a source of chlorine which is dissolved in a minimum quantity of double distilled water labeled as solution B. The solution B is added to A and the mixture is stirred continuously for 30 minutes. The pH of the solution is adjusted to 2 using aqueous ammonia solution. The stirring is continued for 7 hours and the obtained precipitate is repeatedly washed with double distilled water, dried in a hot air oven followed bycalcinations process. 0.5, 1, 2 and 4 wt % of AlF/BiOCl photocatalyst is prepared by adopting the same procedure and bare Bismuth oxychloride catalyst is prepared without the addition of Aluminium fluoride as the dopant.

Characterization

The instrument UV–spectrophotometer (Hitachi U2910) was used for measuring the decrease in absorbance of the dyes throughout the experiment for all the parameters optimized. The morphological characterizations of the synthesized AlF/BiOCl and BiOCl photocatalyst were investigated by HR-SEM with EDAX (FEI Quanta FEG 200-High Resolution Scanning Electron Microscope). The crystallinity and crystal phase of the as-synthesized photocatalysts were analyzed by X-ray diffraction (XRD, Rigaku) patterns with Cu-kα Radiation (λ = 1.54178 Å) in the range of 20–65 degree is scanned at 40 kV. The UV-DRS are carried out with the help of the instruments UV–spectrophotometer SHIMADZU/UV-2600, BET surface area analysis using Quantachrome instruments, Autosorb IQseries and Fourier transform infrared spectroscopy (Bruker Tensor – 27) for the confirmation of the metal-oxide bond in the synthesized photomaterial. 

Photocatalytic Degradation Experiments

About 1.13×10 ̄⁵mol/Lof Acid green 1 dye was prepared as the stock solution, from which required volume was withdrawn for each experiment. All the photocatalytic experiments were carried out on sunny days between 11 am to 2 pm. The suspensions were magnetically stirred in the dark for 15 minutes to attain adsorption-desorption equilibrium between dye and the catalyst. Before exposing the dye solution to sunlight irradiation, the initial absorption peak was recorded. To evaluate the effect of initial dye concentration, the concentration of the dye varied from 1.13×10ˉ⁵mol/L to 5.69×10ˉ⁵mol/L. The experiment was continued for 3 hours, at appropriate intervals small aliquots of the solution are withdrawn and the decrease in the absorbance is noted. The effect of pH was studied by adjusting the solution using NaOH and HCl. The blank experiments were performed in the dark at room temperature following the above procedure.The percentage decolourization was calculated using the formula

Where C₀ is the concentration of the dye solution at t=0 and C is the concentration of the dye solution at time t.

Results and Discussion

X-Ray Diffraction Analysis

The crystalline phase of the synthesized AlF-BiOCl and BiOClphotocatalyst is investigated by using powder X-ray diffraction analysis. The XRD pattern of the synthesized material is shown in Fig 2a and 2b. The sharp diffraction patterns observed for the synthesized AlF doped BiOCl and bare BiOCl photocatalyst suggests that these photocatalysts are highly crystalline in nature. The diffraction peaks at 2θ positions 12.1, 24.2, 26, 32.5, 33.6, 36.7, 41, 46.7, 49.8, 55.2, 58.7, 68.2, 75.1, 77.6 corresponds to the crystal planes (001) (002) (101) (101) (102) (003) (112) (200) (004) (104) (212) (114) (220) (214) and (006) respectively, according to JCPDS card No. 85-0861. As no additional diffraction peaks are observed in the doped BiOCl photocatalyst or no other peaks are observed which might arise from impurity, it is concluded that the synthesized Bismuth oxychloride photocatalyst samples are pure. In the XRD pattern, the peak (102) is found with maximum intensity indicating it to be the preferred growth plane²⁰.

Figure 2: a. shows the XRD pattern of the Synthesized Bismuth oxychloride photocatalyst. Click here to View figure

Figure 2: b. shows the XRD pattern of the synthesized AlF doped BiOCl photocatalyst. Click here to View figure

HR-SEM and EDAX Analysis

The morphology of the BiOCl and AlF doped BiOCl photocatalysts are studied from the scanning electron microscopic technique.It is found from the SEMmicrographs that the synthesized Bismuth oxychloride photocatalyst possess nanoflake like structure and the width of the nanoflakes ranges from 69.3 to 75.1nm and the AlF-BiOCl photocatalyst also possessed flake-like arrangement which is shown in the SEM images figures 3a and 3b.

Figure 3: a. shows the FE-SEM images of the synthesized Bismuth oxychloride photocatalyst. Click here to View figure

Figure 3: b. shows the FE-SEM images of the synthesized  AlF-BiOCl photocatalyst. Click here to View figure

Energy dispersive x-ray analysis is used to determine the major elemental composition in the synthesized materials²¹. Using this technique, the elementalcomposition of the synthesized materials is obtained withhigh resolution. The EDAX analysis data confirmed that the elements Bismuth, oxygen and chlorine are present in major composition without any other impurity in the BiOCl photocatalyst and the presence of aluminium, fluorine, Bismuth, oxygen and chlorine in the synthesized AlF-BiOCl photocatalyst is also confirmed from the EDAX data shown in figure 4a and 4b.

Figure 4: a. shows the EDAX images of the synthesized Bismuth oxychloride photocatalyst. Click here to View figure

Figure 4: b. shows the EDAX images of the synthesized AlF-BiOCl photocatalyst. Click here to View figure

UV-Diffuse reflectance spectrum

The band gap measurements of the synthesized  bismuth oxychloride and Aluminium fluoride doped bismuth oxychloride photocatalyst is done with the help of UV-Visible diffuse reflectance spectroscopy which paves way to understand whether there are changes in the band gap of the modified semiconducting photomaterial and whether the photomaterial absorbs the UV or visible radiation²². The band gap of the synthesized AlF-BiOCl and bare BiOCl photocatalyst is calculated using the following relation: E_g = 1240/λ, where λ is the cut off wavelength and E_g is the band gap energy in electron volts (eV). The band gap energy of the bismuth oxychloride photocatalyst is found to be 3.2evand the band gap value of the Aluminium fluoride doped bismuth oxychloride photocatalyst is calculated to be 2.86ev.

Figure 5: a. shows the UV-DRS plot of the synthesized Bismuth oxychloride photocatalysts. Click here to View figure

Figure 5: b. shows the UV-DRS plot of the synthesized Bismuth oxychloride and AlF-BiOCl photocatalysts. Click here to View figure

Fourier Transform Infra-Red spectroscopy

The metal-oxygen bond formation in the synthesized AlF-BiOCl and bare BiOCl photocatalyst is confirmed from the fourier transform infra-red spectroscopy shown in figure 6. The band observed at 3435cm^-1 in both AlF-BiOCl and bare BiOCl photocatalyst corresponds to the stretching vibrations of hydroxyl group. The peak at 1630 cm^-1 confirms the bending vibrations of the water molecules which are adsorbed on the surface of the photocatalyst. The broad bands between 412 and 850cm^-1 are due to the framework vibrations of Bi-O bonds. The peak at about 532 cm^-1 resulted from the symmetrical stretching vibration of the Bi-O band is a typical peak of BiOCl photocatalyst which is observed in both AlF-BiOCl and bare BiOCl photocatalysts^23-25.

Figure 6: shows the FTIR spectrum of the synthesized BiOCl and AlF-BiOCl photocatalyst Click here to View figure

BET Surface area measurements

Surface area is a crucial factor for determining the catalytic activity of the bare as well as surface modified photocatalysts^26,27. The specific surface area as well as the BJH pore size distribution of AlF-BiOCl and bare BiOCl photocatalysts are measured by Nitrogen adsorption- desorption isotherms and the plots are shown in Fig.7a and 7b. The N₂ adsorption-desorption plots for samples BiOCl and AlF/BiOCl (3 wt %) are categorized astype IV isotherms, which provesthe existence of pores in betweeneach inter-crossed linked nanoplates^28,29. From the BET surface area measurements, it is found that the specific surface area of the bismuth oxychloride photocatalyst is 18.726 m²/g with a pore volume of  0.036cc/g and the specific surface area of the AlF- Bismuth oxychloride photocatalyst is 14.837 m²/g with a pore volume of 0.030 cc/g.

Figure 7: a. shows the Nitrogen adsorption–desorption isotherms of the synthesized BiOCl photocatalyst. Click here to View figure

Figure 7: b. shows the Nitrogen adsorption–desorption isotherms of the synthesized AlF-BiOCl photocatalyst. Click here to View figure

Photocatalytic Degradation studies of Acid green 1 dye

Effect of pH

The influence of pH on the photocatalytic decolourization of acid green 1 dye (1.13×10^-5 mol/L) is studied by varying the pH range from 3 to 12 at a catalyst concentration of 15 mg per 50 ml of the dye solution. The maximum decolourization efficiency is observed at pH 5 which is shown in figure 8. The higher degradation of anionic acid green 1 dye at acidic pH=5 is due to its interaction with the H⁺ ions. On the other hand, in alkaline medium the negatively charged BiOCl catalyst  and the anionic acid green 1 dye competes for interaction with hydroxyl ions thereby resulting in low photocatalytic degradation process³⁰.

Figure 8: 1.13×10-5 mol/L of acid green 1 dye solution, 25mg of BiOCl catalyst, pH varied from 3 to 12, air flow rate = 2.5L/min under sunlight. Click here to View figure

Effect of Catalyst Concentration

For the effective decolourization of acid green 1 dye the bismuth oxychloride catalyst dosage is optimized by varying the catalyst concentration from 5mg to 55mg per 50mL of the dye solution at optimized pH level 5. The maximum decolourization is observed for 15mg of the catalyst dosage which is shown in figure 9. On increasing the BiOCl catalyst dosage a decrease in the photocatalytic dye decolourization process is observed. This may be due to the fact that on increasing the catalyst dosage the number of active surface area increases for the dye molecules to get adsorbed³¹ and by adding excess amount of catalyst dosage, turbidity in the dye solution gets increased which in turn decreases the sunlight penetration as a result of which there is a decrease in the dye decolourization process.

Figure 9: 1.13×10-5 mol/L of acid green 1 dye solution, BiOCl catalyst dosage varied from 5mg to 55mg, pH = 5, air flow rate = 2.5L/min under sunlight. Click here to View figure

Effect of initial dye concentration

The initial dye concentration of the acid green 1 dye is varied in the range 1.13×10^-5 mol/L to 5.69×10^-5 mol/L to find out the maximum dye concentration that can be decolourized with bismuth oxychloride photocatalyst (15mg/50mL of dye solution) at optimized pH level 5. The maximum decolourization of AG1 dye is observed at a concentration of 2.27×10^-5 mol/L which is shown in figure10. The experimental results shows that beyond the concentration level of 2.27×10^-5 mol/L there is a decrease in the dye decolourization process, this may be due to the fact that at higher dye concentration levels the number of active sites of the bismuth oxychloride photocatalyst available for the dye molecules is very less and hence the probability of the dye molecule to react with the hydroxyl ions is less which shows a retardation in the dye decolourization process³².

Figure 10: 15mg of BiOCl catalyst, pH = 5, initial acid green 1 dye concentration varied from 1.13×10-5 mol/L to 5.69×10-5 mol/L, air flow rate = 2.5L/min under sunlight. Click here to View figure

Effect of Different weight percent of AlF onto BiOCl photocatalyst

The photocatalytic response of both doped and undoped bismuth oxychloride photocatalyst is studied by doping different weight percent (0.5, 1, 2, 3 and 4 wt %) of aluminium fluoride which is shown in figure 11. The maximum dye decolourization is observed for 3wt% of aluminium fluoride doped bismuth oxychloride photocatalyst within 90 minutes whereas the bare BiOCl photocatalyst degraded AG1 dye only at 180 minutes under similar experimental conditions and under dark condition in the absence of the sunlight only 18% of the dye got decolourized this may be due to the adsorption of the AG1 dye onto the photocatalyst surface. The faster degradation of AG1 dye with AlF-BiOCl photocatalyst may be attributed to the decrease in size of the catalyst with the surface area. Greater the surface-active sites available, greater will be the photocatalytic activity. A second factor responsible for the increase in photocatalytic degradation is the decrease in the band gap energy values. Generally, in a semiconductor the decrease in the band gap values causes generation of electrons and holes more quickly which further reacts with the hydroxyl radicals to degrade the dye molecules³³.

Figure 11: shows the variation for different weight percent of AlF dopant concentration onto BiOCl catalyst for the photocatalytic decolourization of Acid green 1 dye. Click here to View figure

Kinetic studies

Langmuir-Hinshelwood (L-H) kinetic model is used here to study the kinetics of doped and undoped heterogeneous semiconductor photocatalyst. This (L-H) kinetic model has been widely used by several authors to find out the order of the reaction in heterogeneous semiconductor photocatalysis^34-37.The kinetic studies carried out for the photocatalytic degradation of AG1 dye follows pseudo first order kinetics according to the equation

ln [C₀]/[C] = kt

where k is the pseudo-first-order rate constant (min^-1), C₀  is the concentration of dyes (g L^-1 ) at t=0, C is the concentration of dyes at reaction time t (min). The linear fit between ln [C₀/C] and irradiation time‘t’ for bismuth oxychloride and Aluminium fluoride (0.5, 1, 2 and 3 wt %) doped bismuth oxychloride photocatalyst is shown in figure 12. The calculated average R² value for the bare bismuth oxychloride photocatalyst is 0.9514 and for different weight percent of AlF-BiOCl were found to be 0.9196, 0.9629 and 0.9095 respectively.

Figure 12: shows the pseudo-first order kinetic plot for the photocatalytic decolourization of Acid green 1 dye using different weight percent (0-3%) of Aluminium fluoride doped Bismuth oxychloride catalyst. Click here to View figure

Conclusions

Aluminium fluoride doped bismuth oxychloride and bare bismuth oxychloride photocatalyst is prepared via simple chemical precipitation technique. The optical and morphological characteristics of the synthesized BiOCl and AlF-BiOCl are studied and compared with the help of spectroscopy techniques such as XRD, FE-SEM, EDAX, FTIR, UV-DRS and BET surface area studies. The crystalline nature and hexagonal wurtzite structure of the synthesized BiOCl and AlF-BiOCl photomaterial is confirmed from the X-Ray diffraction studies. The SEM images showed that the synthesized photomaterial possessed a flake-like structure and the major elements such as bismuth, oxygen, aluminium and fluorine in the synthesized photomaterial is confirmed from the EDAX data. The FTIR studies confirmed the metal-oxide bond formation and a decrease in the band gap and surface area of the AlF-BiOCl catalyst compared to that of the bare BiOCl photocatalyst is studied from the UV-DRS and BET surface area analysis. Further the photocatalytic activity carried out for the acid green 1 dye showed that AlF doped bismuth oxychloride photocatalyst degraded the dye within 90 minutes under sunlight radiation at optimized conditions whereas the bare bismuth oxychloride photocatalyst took more under similar optimized conditions. The photocatalytic reaction followed pseudo-first order kinetics according to Langmuir-Hinshelwood kinetic model. Thus, on concluding, AlF-BiOCl photocatalyst coupled with air oxidation under sunlight has proved to be an efficient photocatalyst for the degradation of the acid green 1 dye in aqueous solution.

Acknowledgement

The authors sincerely thank Dr. S. Thennarasu, Sr. Principal Scientist, Department of Organic and Bio-Organic Chemistry, Central Leather Research Institute, Chennai, for his valuable suggestions given for the successful completion of the work. We also thank PG and Research Department of Chemistry, Holy Cross College (Autonomous), Tiruchirappalli, for providing thenecessary laboratory facilities.

Conflict of Interest

The authors declare that there is no conflict of interest.

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