Co3O4-Bi2O3 heterojunction; An effective photocatalyst for photodegradation of rhodamine B dye

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 Co 3 O 4 -Bi 2 O 3 heterojunction as visible-light responsive photocatalyst for photodegradation of rhodamine B dye. The Co 3 O 4 -Bi 2 O 3 heterojunction was synthesized by coprecipitation method and characterized by XRD, EDS, SEM, TGA and FTIR. The as prepared Co 3 O 4 -Bi 2 O 3 heterojunction was utilized as photocatalyst for photodegradation of rhodamine B dye using a 100 mg/L solution. It was observed that Co 3 O 4 -Bi 2 O 3 showed best catalytic performance with ~ 92% degradation of rhodamine B dye than Co 3 O 4 and Bi 2 O 3 with 14 and 34% removal of rhodamine B dye, respectively. The rate constant for Co 3 O 4 -Bi 2 O 3 catalyzed photodegradation of rhodamine B was 6 times and 3 times higher than rate constant for Co 3 O 4 catalyzed and Bi 2 O 3 catalyzed photodegradation of rhodamine B, respectively. The pH 8 was found as optimum pH for Co 3 O 4 -Bi 2 O 3 catalyzed photodegradation of rhodamine B dye.


Introduction
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) ). 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, Bi 2 O 3 , has recently attracted the interest of researchers due to suitable price, stable structure, and suitable band gap energies. The bismuth oxide, Bi 2 O 3 , exists in different crystal types: the α-Bi 2 O 3 , β-Bi 2 O 3 , γ-Bi 2 O 3 , δ-Bi 2 O 3, ε-Bi 2 O 3 and ω-Bi 2 O 3 (Zhang et al. 2018) (Ho et al. 2013). Among different crystal types, the α-Bi 2 O 3 , β-Bi 2 O 3 have been widely used in catalysis, chemical sensors, fuel cells, photovoltaic cells, and optical thin lms. The α-Bi 2 O 3 and β-Bi 2 O 3 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 Bi 2 O 3 as visible light photocatalyst. Therefore, attempts have been made to reduce the rate of recombination of photoinduced electron-hole. The coupling of Bi 2 O 3 with semiconductors is one of the effective ways to separate the photoinduced electron and hole. The coupling of Bi 2 O 3 with a semiconductor metal oxide results in formation of a heterojunction interface with an electric eld between two semiconductors. In this way, the electric eld 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 The Bi 2 O 3 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 ltered, washed, and dried at 100°C for 12 hours. The dried product was calcined at 500°C to get light yellow Bi 2 O 3 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.

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. 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 Co 3 O 4 given in Fig. 2a shows peaks for Co and O only which con rms 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 Co 3 O 4 , Bi 2 O 3 and Bi 2 O 3 -Co 3 O 4 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-signi cant 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- Leaching experiment was also performed to con rm whether the photodegradation of rhodamine B is heterogeneous or homogeneous reaction. For this purpose, 0.1g Co 3 O 4 -Bi 2 O 3 was suspended in 50 mL distilled water and stirred under irradiation for 120 minutes. Then, the Co 3 O 4 -Bi 2 O 3 was ltered and a known solution of rhodamine B was added to ltrate 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 Co 3 O 4 -Bi 2 O 3 does not leach to aqueous medium in this study.
Co 3 O 4 -Bi 2 O 3 is a second generation photocatalyst. The second generation photocatalysts, also called as heterojunctions were developed to overcome the drawback of rst generation photocatalysts (Anwer et al. 2019). The single component metal oxides are classi ed as rst generation photocatalysts. The fast recombination of electron-hole is a basic drawback of rst generation photocatalysts. In second generation photocatalysts, the photoinduced electrons are con ned in conduction band of one component of heterojunction while the holes are con ned 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 rst 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 con rmed by scavenging experiments. For this purpose, EDTA and BQ were separately used as scavengers each of which signi cantly 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 Page 7/19 k obs , k 1 , k 2 , A o and A t 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 (k 1 ) and 2nd order rate constant (k 2 ) are given in Table 1. As the regression coe cient (R 2 ) 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 Co 3 O 4 -Bi 2 O 3 catalyzed photodegradation of rhodamine B was 6 times and 3 times higher than rate constant for Co 3 O 4 catalyzed and Bi 2 O 3 catalyzed photodegradation of rhodamine B respectively. Hence, the formation of Co 3 O 4 -Bi 2 O 3 heterostructure signi cantly boosts up the catalytic performance of Co 3 O 4 and Bi 2 O 3 .

Effect of pH
The pH of solution signi cantly affects the catalytic activity; therefore, the pH was optimized as well in this study. The effect of pH on catalytic activity of Co 3 O 4 -Bi 2 O 3 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 e ciency). Every experimental cycle was performed with a 0.05g Co 3 O 4 -Bi 2 O 3 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 Co 3 O 4 -Bi 2 O 3 has been reported as pH 8.4 (Ivanova-Kolcheva et al. 2020), hence the surface of Co 3 O 4 -Bi 2 O 3 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 Co 3 O 4 -Bi 2 O 3 are positive, therefore the catalytic activity is low at low pH due to electrostatic repulsion. Similarly, the negative surface of Co 3 O 4 -Bi 2 O 3 at higher pH also opposes the adsorption of rhodamine B, hence the maximum catalytic activity exhibited at pH 8.

Effect of catalyst dose
The dependance of photocatalytic activity of Co 3 O 4 -Bi 2 O 3 towards photodegradation of rhodamine B on photocatalyst dosage has been investigated by performing photodegradation experiments with different dosage of Co 3 O 4 -Bi 2 O 3 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 Co 3 O 4 -Bi 2 O 3 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 e ciency. The enhancement in catalytic e ciency 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 Co 3 O 4 -Bi 2 O 3 particles under illumination increases with catalyst dosage. The decrease in catalytic e ciency 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.

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 Co 3 O 4 -Bi 2 O 3 using 100, 200 and 300 mg/L solutions of rhodamine B dye. Figure 10a shows the effect of concentration on catalytic activity Co 3 O 4 -Bi 2 O 3 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): 1. The pathlength of photon decreases with increase in concentration of dye 2. The rhodamine B dye absorbs photons signi cantly than catalyst at higher concentration of dye 3. 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 tting of 1st order kinetics model (Eq. 7) to experimental data. The kinetics parameters given Page 9/19 in Table 2 shows that rate constant decreases with increase in initial concentration of rhodamine B dye. A fa Baig is research student who has carried out the experimental work.

Conclusions
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 nancial assistance under GCUF-RSP.

Availability of data and materials
Available on request Zhao Z, Fan J, Deng X, Liu J (2019) One-step synthesis of phosphorus-doped g-C 3 N 4 /Co 3 O 4 quantum dots from vitamin B12 with enhanced visible-light photocatalytic activity for metronidazole  Effect of catalyst dose on catalytic activity of Co3O4-Bi2O3 towards photodegradation of rhodamine B Figure 10 Effect of concentration of dye on catalytic activity of Co3O4-Bi2O3 towards photodegradation of rhodamine B