Recently, the research on development of visible-light-active photocatalysts for photodegradation of organic pollutants has got much attention. Therefore, this study reports the synthesis of Co3O4-Bi2O3 heterojunction as visible-light responsive photocatalyst for photodegradation of rhodamine B dye. The Co3O4-Bi2O3 heterojunction was synthesized by coprecipitation method and characterized by XRD, EDS, SEM, TGA and FTIR. The as prepared Co3O4-Bi2O3 heterojunction was utilized as photocatalyst for photodegradation of rhodamine B dye using a 100 mg/L solution. It was observed that Co3O4-Bi2O3 showed best catalytic performance with ~ 92% degradation of rhodamine B dye than Co3O4 and Bi2O3 with 14 and 34% removal of rhodamine B dye, respectively. The rate constant for Co3O4-Bi2O3 catalyzed photodegradation of rhodamine B was 6 times and 3 times higher than rate constant for Co3O4 catalyzed and Bi2O3 catalyzed photodegradation of rhodamine B, respectively. The pH 8 was found as optimum pH for Co3O4-Bi2O3 catalyzed photodegradation of rhodamine B dye.
Research Article
Co3O4-Bi2O3 heterojunction; An effective photocatalyst for photodegradation of rhodamine B dye
https://doi.org/10.21203/rs.3.rs-463319/v1
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published 01 Jan, 2022
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Co3O4-Bi2O3
Heterojunction
Photodegradation
Rhodamine B
Kinetics analysis
The removal of organic pollutants from aqueous system by photocatalysis employing semiconductor metal oxides as photocatalysts plays a crucial role in treatment of wastewater due to its advantages of mild reaction conditions, complete mineralization of pollutants and low processing cost (Chen et al. 2018a) (Meng et al. 2019). An ideal photocatalyst for photodegradation of organic pollutants is the one which can effectively degrade the pollutants under irradiation of visible light. A narrow band gap semiconductor can be used as photocatalyst under irradiation of visible light, however the fast recombination of photo induced positive holes and electrons inhibits the photocatalytic activity (Huang et al. 2017) (Zhong et al. 2017). Therefore, attempts are made to inhibit the recombination of photo induced positive holes and electrons by developing the composite materials. In this respect, a number of studies have been reported for the synthesis of active visible-light photocatalysts for the treatment of wastewater contaminated with organic pollutants (Gan et al. 2018). The development of visible light responsive active photocatalyst for dyes contaminated wastewater is a hot topic among the researchers of photocatalysis.
The semiconductor bismuth oxide, Bi2O3, has recently attracted the interest of researchers due to suitable price, stable structure, and suitable band gap energies. The bismuth oxide, Bi2O3, exists in different crystal types: the α-Bi2O3, β-Bi2O3, γ-Bi2O3, δ-Bi2O3, ε- Bi2O3 and ω-Bi2O3 (Zhang et al. 2018) (Ho et al. 2013). Among different crystal types, the α-Bi2O3, β-Bi2O3 have been widely used in catalysis, chemical sensors, fuel cells, photovoltaic cells, and optical thin films. The α-Bi2O3 and β-Bi2O3 has band gap of 2.85 and 2.58 eV respectively, hence both can be activated under visible irradiation (Chen et al. 2018b) (Song et al. 2020). However, the fast recombination photoinduced electron-hole limits the practical application of Bi2O3 as visible light photocatalyst. Therefore, attempts have been made to reduce the rate of recombination of photoinduced electron-hole. The coupling of Bi2O3 with semiconductors is one of the effective ways to separate the photoinduced electron and hole. The coupling of Bi2O3 with a semiconductor metal oxide results in formation of a heterojunction interface with an electric field between two semiconductors. In this way, the electric field created at the heterojunction assists the transport of charges from one semiconductor to other resulting an effective separation between the photoinduced charges (Balachandran and Swaminathan 2012). Hence in Bi2O3-semiconductor heterojunctions, advantages such as separation of charges, increased lifetime of charges and enhanced transfer efficiency of the charges to adsorbed substrate can be achieved. In this study, the development of heterojunction formed by coupling of Bi2O3 with spinel cobalt tetroxide (Co3O4) is reported. The spinel cobalt oxide (Co3O4) which composed of Co (II) and Co (III) has been studied extensively due to its extra ordinary properties. The high catalytic activity of Co3O4 is thought to be due to adsorption of oxygen in different states, oxygen defects and variance in oxygen holes in Co3O4. Furthermore, the narrow band gap in the range of 1.2–2.1 eV makes Co3O4 attractive in photocatalysis for treatment of wastewater contaminated with organic pollutants (Malefane 2020) (Hu et al. 2019) (Luo et al. 2019) (Zhao et al. 2019) (Rao and Sunandana 2008). As Bi2O3 has oxygen vacancies in its crystals, and Co3O4 is rich with oxygen content, therefore Bi2O3 promote mobility and activity of lattice oxygen in Co3O4 which results in separation of photoinduced electron-hole pair and ultimately enhances the photo catalytic activity.
2.1 Synthesis of photocatalyst
The Co3O4-Bi2O3 hetero-structure was prepared by coprecipitation method. Typically, 12.5 mmol of cobalt nitrate hexahydrate and 12.5 mmol of bismuth nitrate pentahydrate were dissolved in 10 mL (1M) nitric acid solution under vigorous stirring. Then, 1M of sodium hydroxide solution was added dropwise to above mixed solution under continuous stirring at 60°C till pH 12 obtained. The solution was further stirred for 2 h at 60oC. As a result, a green precipitate was formed. The resultant precipitate was filtered, washed with ethanol, and distilled water till all unreacted ions were eliminated from prepared precipitate. Then, the washed precipitate was dried at 100oC overnight. Finally, the dried residue was calcined at 500oC for 3 hours which resulted black colored Co3O4-Bi2O3 hetero-structure.
For comparison, Co3O4 and Bi2O3 were also prepared. The Co3O4 was prepared as follows. A 100 mL solution of cobalt nitrate hexahydrate (0.03M) was mixed with a 100 mL solution of potassium carbonate (0.06M) under sitting. The resultant reaction mixture was stirred for 7 hours at 70°C. the obtained residue was filtered, washed, and dried at 100°C for 12 hours. Finally, the dried sample was calcined at 500°C for 3 hours to get Co3O4 particles.
The Bi2O3 was prepared by adding 1M sodium hydroxide solution to a solution containing 4.85g bismuth nitrate pentahydrate in 100 mL till pH 12 obtained. Concentrated nitic acid was used for dissolution of bismuth nitrate. The resultant precipitate was filtered, washed, and dried at 100°C for 12 hours. The dried product was calcined at 500°C to get light yellow Bi2O3 particles
2.2 Characterization
X-rays diffraction, energy dispersive spectroscopy, scanning electron microscopy, thermal gravimetric analysis and infrared spectroscopy were used for characterization of prepared material using JOEL-JDX-3532 Japan X-ray diffractometer, JEOL-JSM 5910 Japan Scanning electron microscope, JSM5910 UK Energy dispersive X-rays spectrophotometer, Perkin Elmer 6300 TGA analyzer and Bruker VRTEX70 Infrared spectrophotometer, respectively.
2.3 Catalytic activity
The photo-catalytic activities of synthesized particles were determined with photo degradation of rhodamine B dye. Typically, a solution of rhodamine B dye was charged with a predetermined catalyst dose and stirred under visible irradiation for 120 minutes. Progress of photo catalytic degradation was monitored by sampling and analyzing the reaction mixture at regular time interval. Blank experiments were performed by stirring the dye solution and dye solution with catalyst under irradiation and dark conditions, respectively. UV/Vis spectrophotometer (U-2800, HITACHI, Japan) was used for measurement of photocatalytic activity.
3.1 Characterization
The formation of Co3O4, Bi2O3 and Co3O4-Bi2O3 were confirmed by XRD analysis. The XRD of Co3O4 given in Fig. 1a consists of sharp diffraction peaks which show the crystalline nature of Co3O4. The diffraction peaks observed at 2θ ~ 18, 30, 37, 43 and 58° have been indexed to spinel structure of Co3O4 (Manickam et al. 2016) (Rao and Sunandana 2008) (R. Manigandan, K. Giribabu, R. Suresh, L. Vijayalakshmi, A. Stephen 2013). Similarly, the diffraction peaks at 2θ ~ 12, 26, 33, 36, 41, 46, 54 and 58 are observed in XRD of Bi2O3. All these diffraction peaks correspond to β-Bi2O3. The remaining weak diffraction peaks in XRD of Bi2O3 represent the γ-Bi2O3. Hence, it is concluded that Bi2O3 is composed of mixed phases of β-Bi2O3 and γ-Bi2O3 crystals, however the β-Bi2O3 is the main constituent in crystalline phase (Iyyapushpam et al. 2015) (Liang et al. 2014) (Huang et al. 2016).
The elemental composition of as-prepared material was investigated by energy dispersive spectroscopy using JSM5910, INCA200 UK. Figure 2 shows the energy dispersive spectra of the samples. The energy dispersive spectrum of Co3O4 given in Fig. 2a shows peaks for Co and O only which confirms the purity of the sample. The EDS analysis showed that prepared cobalt oxide is composed of 87.3 weight% Co and 12.68 weight% O. The energy dispersive spectrum of bismuth oxide (Fig. 2b) shows peaks for Bi, O and C. The EDS analysis showed that prepared bismuth oxide is composed 58.08, 9.97 and 4.95% Bi, O and C respectively. The existence of C may be due to impurities in precursor material. Similarly, the energy dispersive spectrum of cobalt oxide-bismuth oxide composite (Fig. 2c) shows that the composite is composed of 11.6, 70.83, 13.92 and 4.19% Co, Bi, O and C respectively.
The morphology of Co3O4, Bi2O3 and Bi2O3-Co3O4 was studied by scanning electron microscopy with JEOL-JSM-5910, Japan scanning electron microscope. JEOL-JSM-420, Japan coating machine was used for mounting and coating the samples with gold foil. The scanning electron micrographs given in Fig. 3 show that the particles of as-prepared samples are irregular in shape, non-agglomerated and dispersed. The non-agglomerated and dispersed particles have enhanced catalytic activity as the active centers are easily accessible to substrate molecules.
Thermal stability of as prepared samples was estimated by thermal gravimetric analysis using Perkin Elmer 6300 TGA analyzer. As given in Fig. 4, there was only about 5% loss in weight of the samples up to 700°C, which is attributed to loss of moisture content. The non-significant weight loss shows the stability of as prepared samples over a wide range of temperature.
The typical bonds and functional groups of as prepared samples were estimated by Fourier transform infrared spectroscopy (FTIR) using Bruker (VRTEX70 series). Figure 5 shows the FTIR spectra of as-prepared samples in which several peaks can be observed. The absorption peak at 1634 cm− 1 is attributed to -OH groups present at the surface of Co3O4-Bi2O3 heterojunction. The absorption bands at ~ 445 cm− 1 and 846 cm− 1 have been assigned to stretching vibrations of Bi-O and Bi-O-Bi bonds, respectively. The absorption bands at ~ 571 and 668 are representative peaks of Co3O4 due to Co-O stretching vibrations (Tang et al. 2018) (Hammad et al. 2016) (Li et al. 2017) (M. Ilyas 2010) (Saeed et al. 2012). The FTIR result show the successful formation of Co3O4-Bi2O3 heterojunction.
3.2 Catalytic activity
The photocatalytic activity of Co3O4, Bi2O3 and Co3O4-Bi2O3 was evaluated by performing photodegradation experiments of rhodamine B dye in the presence of 0.05g of afore mentioned catalysts under visible irradiation. A 50 mL rhodamine B (100 mg/L) was taken in Pyrex glass beaker and was stirred under irradiation for 30 minutes. Then, as blank reaction, 0.05g Co3O4/Bi2O3/Co3O4-Bi2O3 was added to dye solution and stirred for 30 minutes under dark to clarify the effect of adsorption. For evaluation of catalytic performance, the reaction mixture, was then irradiation under stirring. The change in concentration of rhodamine B due to catalytic degradation was monitored by measurement of absorbance at wavelength 554 nm using UV-visible spectrophotometer. As the normalized temporal changes in absorbance (At/Ao) of rhodamine B is during photocatalytic process are proportional to normalized concentrations (Ct/Co) of rhodamine B dye, therefore, the photocatalytic activity was expressed as At/Ao vs t as given in Fig. 6 (At: absorbance of rhodamine B at time t, Ao; Initial absorbance of rhodamine B, Ct: concentration of rhodamine B at time t, Co: initial concentration of rhodamine B). It was observed that Co3O4-Bi2O3 showed best catalytic performance with ~ 92% degradation of rhodamine B dye than Co3O4 and Bi2O3 with 14 and 34% removal of rhodamine B dye, respectively. The obtained results show that coupling of Co3O4 with Bi2O3 significantly promotes the catalytic activity.
Leaching experiment was also performed to confirm whether the photodegradation of rhodamine B is heterogeneous or homogeneous reaction. For this purpose, 0.1g Co3O4-Bi2O3 was suspended in 50 mL distilled water and stirred under irradiation for 120 minutes. Then, the Co3O4-Bi2O3 was filtered and a known solution of rhodamine B was added to filtrate to get a ~ 100 mg/L dye solution and analyzed with UV-visible spectrophotometer. Finally, the dye solution was again treated with irradiation and analyzed with UV-visible spectrophotometer after 120 minutes. The analysis showed that there was no change in concentration of the dye. Hence, it is concluded that Co3O4-Bi2O3 does not leach to aqueous medium in this study.
Co3O4-Bi2O3 is a second generation photocatalyst. The second generation photocatalysts, also called as heterojunctions were developed to overcome the drawback of first generation photocatalysts (Anwer et al. 2019). The single component metal oxides are classified as first generation photocatalysts. The fast recombination of electron-hole is a basic drawback of first generation photocatalysts. In second generation photocatalysts, the photoinduced electrons are confined in conduction band of one component of heterojunction while the holes are confined in valence band of the other component. This spatial separation of the photoinduced electrons and holes inhibits their recombination. As a result, active cites are generated at which degradation of organic pollutants takes place. The second-generation photocatalytic materials show light absorbance in the visible region (λ ≥ 420 nm) accompanied with lower band gap energies than first generation photocatalytic materials. Hence, the photodegradation of rhodamine B dye in present study can be described as follow.
The photoinduced electron, positive hole and OH radicals are the active species which contribute to photodegradation of rhodamine B dye. The role played by these species was confirmed by scavenging experiments. For this purpose, EDTA and BQ were separately used as scavengers each of which significantly suppressed the photodegradation activity. Since, the EDTA arrests the positive holes, therefore the activity decreased in the presence of EDTA. Similarly, the addition of BQ suppress the photodegradation because it reacts with super oxide anion radicals (Song et al. 2020) (Xu et al. 2019).
Based on above mechanism, the rate expression is written as
kobs, k1, k2, Ao and At is observed rate constant, 1st order rate constant, 2nd order rate constant, initial absorbance of rhodamine B and absorbance at time t respectively.
Figure 7 shows the kinetics treatment of the photodegradation degradation data of rhodamine B. The 1st order rate constant (k1) and 2nd order rate constant (k2) are given in Table 1. As the regression coefficient (R2) values are higher for 1st order kinetics treatment, therefore, it proposed that the degradation of rhodamine B dye in this study follows 1st order reaction kinetics. The rate constant for Co3O4-Bi2O3 catalyzed photodegradation of rhodamine B was 6 times and 3 times higher than rate constant for Co3O4 catalyzed and Bi2O3 catalyzed photodegradation of rhodamine B respectively. Hence, the formation of Co3O4-Bi2O3 heterostructure significantly boosts up the catalytic performance of Co3O4 and Bi2O3.
3.3 Effect of pH
The pH of solution significantly affects the catalytic activity; therefore, the pH was optimized as well in this study. The effect of pH on catalytic activity of Co3O4-Bi2O3 was evaluated by performing degradation experiment at pH 4, 6, 8 and 10. Figure 8 shows the effect of pH on photocatalytic degradation of rhodamine B dye (the numbers given at bar graphs represent the catalytic efficiency). Every experimental cycle was performed with a 0.05g Co3O4-Bi2O3 per 50 mL solution (100 mg/L). The reaction duration was 60 minutes. It was observed that increase in pH up to 8 favored the catalytic activity. The point of zero charge (PZC) for Co3O4-Bi2O3 has been reported as pH 8.4 (Ivanova-Kolcheva et al. 2020), hence the surface of Co3O4-Bi2O3 becomes negative at pH higher than PZC and positive at pH lower than PZC. At pH lower than PZC, both the rhodamine B and surface of the Co3O4-Bi2O3 are positive, therefore the catalytic activity is low at low pH due to electrostatic repulsion. Similarly, the negative surface of Co3O4-Bi2O3 at higher pH also opposes the adsorption of rhodamine B, hence the maximum catalytic activity exhibited at pH 8.
3.4 Effect of catalyst dose
The dependance of photocatalytic activity of Co3O4-Bi2O3 towards photodegradation of rhodamine B on photocatalyst dosage has been investigated by performing photodegradation experiments with different dosage of Co3O4-Bi2O3 under identical condition. In a model experiment, a 50 mL solution of rhodamine B dye (100 mg/L) was treated with a known amount of Co3O4-Bi2O3 for 60 minutes. Figure 9 shows the dependance of catalytic activity on catalyst dosage. It was observed that increase in catalyst dose from 0.01 to 0.05g increased the degradation from 18 to 36%, however further increase in catalyst dose caused a decreased in degradation efficiency. The enhancement in catalytic efficiency with increase in catalyst dose is due to two reasons: (1) The number of molecules of rhodamine B adsorbed increases with catalyst dosage, (2) the density of Co3O4-Bi2O3 particles under illumination increases with catalyst dosage. The decrease in catalytic efficiency at higher catalyst dosage is due to scattering of light. Hence, a catalyst dose of 0.05g/50 mL of dye solution is found to be optimum catalyst dosage in this study.
3.5 Effect of concentration
The initial concentration of dye also affects the catalytic activity, therefore the concentration dependance of catalytic activity has been also investigated. This investigation was carried out by performing separate photodegradation experiments of rhodamine B dye in the presence of 0.05g Co3O4-Bi2O3 using 100, 200 and 300 mg/L solutions of rhodamine B dye. Figure 10a shows the effect of concentration on catalytic activity Co3O4-Bi2O3 towards photodegradation of rhodamine B dye. It was found that catalytic activity decreased with increase in initial concentration of rhodamine B dye. The decrease in catalytic activity with increase in concentration of dye is due to three reasons (Balcha et al. 2016) (Saeed et al. 2017) (Nisar et al. 2021) (Adeel et al. 2021):
-
The pathlength of photon decreases with increase in concentration of dye
-
The rhodamine B dye absorbs photons significantly than catalyst at higher concentration of dye
-
The ration of OH radicals to rhodamine B molecules decreases with increase in concentration
The degradation data given in Fig. 10a was analyzed for kinetics studies using kinetics Eq. 7. Figure 10b shows the fitting of 1st order kinetics model (Eq. 7) to experimental data. The kinetics parameters given in Table 2 shows that rate constant decreases with increase in initial concentration of rhodamine B dye.
Conclusions
Herein, we prepared the Co3O4-Bi2O3 heterojunction as visible-light responsive photocatalyst by coprecipitation method for photodegradation of rhodamine B dye successfully. The Co3O4-Bi2O3 heterojunction was characterized by XRD, EDS, SEM, TGA and FTIR. The as prepared Co3O4-Bi2O3 heterojunction was utilized as photocatalyst for photodegradation of rhodamine B dye using a 100 mg/L solution. The photocatalytic activity of Co3O4-Bi2O3, Co3O4 and Bi2O3 towards photodegradation of rhodamine B dye was found as 92, 14 and 34%, respectively. The 1st order and 2nd order kinetics models were applied to the data of photocatalytic degradation of rhodamine b dye. the effect of pH, catalyst dose and initial concentration of rhodamine B dye on photocatalytic performance was investigated. The Co3O4-Bi2O3 heterojunction was found as an efficient visible light-driven photocatalyst for photodegradation of rhodamine B dye.
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Authors Contributions
Muhammad Saeed is main research supervisor, who has supervised the experimental work and drafted the manuscript.
Afifa Baig is research student who has carried out the experimental work.
Muhammad Asghar Jamal has helped in supervision of experimental work and drafting of manuscript.
Nadia Akram has helped in supervision of experimental work and drafting of manuscript.
Tanveer Hussain Bokhari has helped in drafting of paper.
Funding
Government College University Faisalabad Pakistan provided financial assistance under GCUF-RSP.
Availability of data and materials
Available on request
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