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Structural, Optical and Photocatalytic Activity of Multi-heterojunction Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 Nanoflakes Synthesized via Submerged DC Electrical Discharge in Urea Solution

Abstract

In this research, a novel ternary multi-heterojunction Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst is fabricated via submerged DC electrical arc discharge in urea solution. FT-IR, XRD, EDS and PL results confirm the formation of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 multi-heterojunction. Formation of nanoflake morphology is revealed by FE-SEM and TEM images. The optical properties and intense absorption edge of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 reveal the proper visible light absorbing ability. The photocatalytic performance of the sample is investigated via the degradation of methylene orange (MeO) and rhodamine B (RB) under visible light irradiation. The photocatalytic activity of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 is compared with the synthesized sample in water, Bi2O3/Bi/Bi(OH)3, which exhibits much higher photocatalytic activity. Also, the stable photodegradation efficiency of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 after four cycles reveals the long-term stability and reusability of the synthesized photocatalyst. The PL intensity of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 shows an improved separation rate of electron–hole pairs and so enhanced photocatalytic performance. The improved photocatalytic activity can be ascribed to the formation of multi-heterojunctions, flake morphology and intrinsic internal electric field (IEF). Multi-heterojunction nanoflakes enhance the absorbance of visible light and facilitate the separation and transport of photogenerated electron holes through large IEF. Our work offers an effective method for the production of innovative bismuth-based photocatalyst with excellent prospects for the degradation of environmental pollutants and light harvesting for renewable energy generation under visible light.

Introduction

Energy crisis and environmental pollution are one of the major global problems in the present century [1]. Hence, semiconductor photocatalysts have attracted great attention in this matter. Photocatalysis with solar energy is one of the most advanced and encouraging techniques in reducing energy crisis and environmental pollution [1–, 2, 3, 4]. The light of the sun as an inexhaustible energy source is used for the removal of environmental pollutants as well as water splitting and CO2 photoreduction to generate hydrogen and carbon-containing fuels to overcome energy scarcity [5–, 6, 7, 8]. So, the synthesis of efficient photocatalysts with high performance in absorbing visible light and charge separation as well as photochemical stability is a key challenge [9]. For example, three-dimensional hierarchical CuCo2S4 microspheres with considerable photocatalytic activity in removing tetracycline hydrochloride. The high photocatalytic activity is related to the high charge separation efficiency as well as the large specific surface area [10]. Visible-light-driven photocatalysts with heterojunctions have found much attention in the degradation of a wide range of toxic compounds, especially hazardous dyes in wastewater [11, 12]. The photocatalytic activity depends not only on the material but also on the dimensions and structure as well as the composition of the photocatalysts [13, 14]. Bismuth-based complex oxide semiconductors are new promising materials for environmental remediation, water splitting and photocatalytic applications, which show a good photocatalytic performance under visible light [15].Among the diverse photocatalytic materials such as TiO2, ZnO, SnO2, WO3 and CeO2, bismuth-based photocatalysts are an important group with low toxicity compared to other photocatalysts which have photocatalytic performance for the degradation of organic pollutants in the environment as well as industrial hazardous wastes [16, 17]. In the electronic structure of bismuth oxide, the valence band consists of O 2p and Bi 6s hybrid orbitals. The Bi 6s orbital raises the mobility of photogenerated charge carriers and thus leads to a decrease in the band gap [15, 18]. Bi2O3 is a p-type semiconductor which has a direct band gap energy of less than 3.0 eV [19]. There are six main polymorphs for Bi2O3, which crystallize into different crystal structures [20]. The monoclinic α-Bi2O3 phase is the most stable phase at room temperature [21]. Thermodynamically, α-Bi2O3 is stable between 25 and 730 °C and then transformed to face-centered cubic δ-Bi2O3 phase at 730 °C and remains stable up to its melting point at 825 °C [20, 22]. The tetragonal β-Bi2O3 and body-centered cubic γ-Bi2O3 are metastable phases, which are formed at high temperatures and can stabilize down to room temperature or transform to α-Bi2O3 at high cooling rates [23, 24]. In addition, thermal transformation of α-Bi2O3 and β-Bi2O3 to Bi2O2CO3 and (BiO)4CO3(OH)2 was reported previously [25, 26]. Bismuth oxides, including α-Bi2O3, β-Bi2O3, γ-Bi2O3 and δ-Bi2O3, as well as Bi2O2CO3 and (BiO)4CO3(OH)2 show photocatalytic activity [20, 23–, 26, 27, 28, 29]. However, the small specific surface area, chemical instability and high recombination rate of photogenerated carriers of Bi2O3 demonstrate the main problem for applications of Bi2O3 as a photocatalyst merely [15]. Several approaches are developed to overcome the drawbacks of using a single-phase semiconductor as a photocatalyst, namely metal or ion doping, establishing composites or formation of multi-junctions [21–, 30, 31, 32]. Among them, the formation of multi-junctions has gained a lot of attention and has been used widely [32]. The construction of a heterojunction can efficiently improve photocatalytic activity by increasing the separation of photogenerated electrons and holes effectively, for example, 0D/2D S-scheme N-CDs/S-C3N4 heterojunction with excellent photocatalytic activity in degradation of organic pollutants and H2 evolution. The improved photocatalytic performance was attributed to the formation of S-scheme heterojunction with high redox potentials between nitrogen-doped carbon dots (N-CDs) and sulfur-doped carbon nitride (S-C3N4) semiconductor [33]. In addition, multi-heterojunctions can show better charge transport mechanism and thus higher photocatalytic activity. As an example of a heterogeneous photocatalytic reaction with efficient carrier separation and well-matched energy band structure, can refer to the organic/inorganic PDI-Urea/BiOBr S-scheme heterojunction with photocatalytic performance for the degradation of antibiotics such as ofloxacin, tetracycline as well as high–low junction OV-BiOBr/Cu2−xS with double defects namely, oxygen and copper vacancy for hydrogen evolution [34, 35]. In addition to these, we can refer to visible light photo-response AgI-loaded BiOI, Bi2S3/Bi2O3/Bi2O2CO3 ternary nanocomposites, Z-scheme Ag–AgI/BiOI–Bi2O3 photocatalysts and SnO2/Bi2O3/BiOI nanofibers with enhanced photocatalytic performance [36–, 37, 38, 39]. IEF is a factor in improving photocatalytic activity by means of simplifying transformation and separation of the photogenerated carriers. The alternative layer structure of (Bi2O2)2+ fluorite flake and (MO6)2− perovskite layers in bismuth-based binary metal oxides form IEF that enhanced the separation efficiency of photogenerated charge carriers [26, 40]. Bismuth subcarbonate with Bi2O2CO3 chemical formula is an n-type photocatalyst and has shown photocatalytic performance in the decomposition of pollutants in wastewater and polluted air [15, 41, 42]. The electronic structure of Bi2O2CO3 is composed of hybridized O 2p and Bi 6p orbitals in the conduction band and O 2p, Bi 6p and C 2p orbitals in the valence band [15]. The structure of Bi2O2CO3 is composed of (Bi2O2)2+ slabs that are intertwined by means of CO32− layers, which lead to a large IEF and improved photocatalytic performance [26, 43]. However, the photocatalytic application of Bi2O2CO3 under the solar spectrum is limited due to the wide band gap between 2.8 and 3.5 eV and low charge separation efficiency [26, 43, 44]. Hence, the formation of junctions between Bi2O2CO3 and other visible responsive photocatalytic semiconductors like α-Bi2O3 is an appropriate way to extend the photocatalytic activity even under visible light, such as α-Bi2O3/Bi2O2CO3 and Bi2O2CO3/Bi2S3, Bi2O2CO3/BiOCl [44, 45, 46]. (BiO)4CO3(OH)2 as an Aurivillius oxide with 2.17 eV band gap is a promising visible-light driven photocatalyst [26]. (BiO)4CO3(OH)2 has the same structure as Bi2O2CO3 and can be formed by replacing CO32− layers of Bi2O2CO3 with OH layers which are more polar and increase IEF of photocatalyst and thus lead to more photocatalytic performance [26, 47]. Therefore, Bi2O2CO3 and (BiO)4CO3(OH)2 are suitable candidates for construction of layered nanostructures with a large IEF and a high ability to separate photogenerated carriers [26, 43]. Consequently, the formation of junctions between (BiO)4CO3(OH)2 and Bi2O2CO3 can improve visible light photocatalysis activity [26, 48]. The interfaces between (BiO)4CO3(OH)2 and Bi2O2CO3, which are nearly contacted and have an efficient built-in electric field, improve the separation of photogenerated carriers and lead to better photocatalytic performance. (BiO)4CO3(OH)2/Bi2O2CO3 with internal polarized heterojunction was synthesized and shows enhanced photocatalytic activity due to the synergetic effect between IEF and carriers separation and indicated great potential in the application for wastewater treatment [26]. There are many growing numbers of methods for the preparation of single-phase or multi-heterojunctions of bismuth-oxide-based nanostructures. Hydrothermal synthesis of α-Bi2O3 photocatalysts with nanosheet structure, Bi/Bi2O3 nanoparticles’ synthesis by Nd:YAG laser ablation, solvothermal preparation of Bi/Bi2O2CO3 heterojunction photocatalyst, synthesis of photocatalytic dendritic α-Bi2O3/Bi2O2CO3 heterostructures with facile transformation, two-dimensional (BiO)4CO3(OH)2/Bi2O2CO3 heterostructure photocatalyst formation by in situ photosynthesis and flowerlike Au/Bi2O2CO3/Bi2O3 multi-heterojunction photocatalysts preparation by one-pot in situ growth, which are reported elsewhere [26, 43–, 49, 50, 51, 52]. Compared with several conventional techniques for nanostructures synthesis, DC electrical arc discharge in liquid provides a straightforward, flexible and cost-effective method for mass production of nanomaterials without environmental footprints [53, 54]. There are some experimental parameters including electrical current, composition of electrodes and the chemical nature of the liquid environment, which impress arc discharge process, as well as nanomaterials formation both during and after synthesis in liquid [55, 56]. The chemical nature of the liquid environment is a determining factor in the final composition of synthesized nanomaterials through electrical arc discharge, for instance, TiC nanoparticles synthesis via electrical arc discharge in organic liquids [57, 58]. Furthermore, Bi/Bi2O3 rodlike nanostructures were prepared via oriented aggregation of bismuth-based nanoparticles synthesized by DC arc discharge in water, where water acts as both a synthesis medium and oxidative environment [53].

In this research, novel Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 multi-heterojunction nanoflakes with considerable photocatalytic performance have been synthesized via DC electrical arc discharge in urea solution for the first time. By applying an appropriate DC current between bismuth electrodes, electrical arc discharge causes plasma generation in the solution and forms Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes. This compound exhibits enhanced photocatalytic performance towards degradations of MeO when compared with both single phases α-Bi2O3, Bi2O2CO3 and (BiO)4CO3(OH)2 compounds based on previous studies and synthesized Bi2O3/Bi/Bi(OH)3 by arc discharge in water. The photoactivity enhancement can be attributed to the formation of multi-heterojunction nanoflake morphology and improved IEF in Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanostructure. To the best of our knowledge, no research has been conducted around the investigation of photocatalytic performance of synthesized ternary Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanocomposite by electrical arc discharge in liquid with an efficient photocatalytic activity under visible light for energy and environmental application.

Experimental

Multi-heterojunction Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 and Bi2O3/Bi/Bi(OH)3 nanoflakes were synthesized by facile DC electrical arc discharge method in liquid [53, 54]. The synthesis part of nanostructures comprises of a heat-resistant glass container filled with urea in water solution with a concentration of 10 g/L for the synthesis of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 and deionized water for the synthesis of Bi2O3/Bi/Bi(OH)3. The urea was Merck with more than 99% purity. Two 99.9% purity bismuth electrodes were placed in front of each other in vertical mode: One electrode was held fixed, while the other one was movable to make electrodes close to each other in both experiments. The diameter and length of both electrodes were about 5 mm and 30 mm, respectively. The synthesis parts were connected to ARC 200 DC power supply, and a 40A DC current was applied to synthesis of nanostructures. The synthesis process begins by bringing electrodes close together. When electrodes were brought into contact, electrical arc discharge was generated and nanostructures formed. Each discharge lasted about 1–2 s and emitted a very shiny blue light. By establishing electrical discharge, the surface of electrodes was eroded through evaporation and sputtering, and then bismuth atoms and clusters were formed and dispersed in the solution at so lower temperature than the arc hot zone [54, 58, 59]. The suspension of dark nanostructures was  formed through synthesis, and the color of suspension became lighter as time passed due to the oxidation of nanostructures.

FTIR spectroscopy was used to determine the functional groups on the surface of the samples. FTIR spectra were conducted by a PerkinElmer spectrometer on samples using standard KBr pellets. To investigate the crystallographic and phase characteristics of the samples, the powder specimen was analyzed by X'Pert MPD system with Co Kα radiation at 1.78897 Å wavelength. To identify the microstructure, chemical composition, morphology and size distribution of nanostructures, a MIRA TESCAN field emission scanning electron microscope (FE-SEM) with energy-dispersive X-ray spectroscopy (EDS) and elemental mapping of nanostructures as well as a Zeiss EM900 transmission electron microscope (TEM) were used at room temperature. The optical properties of colloidal samples were determined by SCO-TECH SP UV-26 UV–visible spectrophotometer at wavelengths between 190 and 1100 nm at different times. To investigate the photocatalytic activity of synthesized samples, the powder specimen was carried out from solution by using a KTS 5000 rpm centrifuge. PL spectra were recorded by an AvaSpec-2048Tech spectrofluorophotometer, where a Nd:YAG laser with 3rd harmonic crystal was used for excitation of the samples at a wavelength of 355 nm.

The MeO and RB were used as the model pollutants to investigate the photocatalytic activity. The photocatalytic activity was done through dispersing of 45 mg photocatalyst powder in an 80 mL solution of dye (2.5 mg/L) in a glass reactor. It should be noticed that the solution was stirred in darkness for about 30 min to establish adsorption–desorption equilibrium between dye molecules and the surface of photocatalysts. Then, the solution was under irradiation for about 180 min while stirring constantly during irradiation to make a uniform mixture. The photocatalytic tests were applied at room temperature, and the setup was held in a chamber under a 15 W visible LED. During the photocatalytic experiment, each 15 min 3 mL of the solution was taken out to measure optical transmission spectra to reveal the concentration of dye by determining the maximum absorption wavelength of dyes. To evaluate the reusability of synthesized sample, cyclic photodegradation experiments of MeO on Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanocomposite were done under the same conditions. For doing cyclic photocatalysis tests, Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst was withdrawn from the solution by means of the centrifuge after each cycle. The solution sample is centrifuged at 4500 RPM for 10 min, then washed and cleaned via ultra-sonicating of the sample in deionized water several times and, after drying, reused again in a new solution of MeO with photocatalyst. The photodegradation procedure of MeO by the sample was redone in four cycles. Furthermore, various scavengers were used to investigate the role of reactive oxidative species in the MeO degradation process by Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2. To capture the generated holes, hydroxyl and superoxide radicals, the ethylenediaminetetraacetic acid (EDTA), isopropyl alcohol (IPA) and benzoquinone (BQ) were used in the photocatalytic process, respectively.

Results and discussion

The type of functional groups on the surface of nanostructures was identified by FTIR spectra, as illustrated in Fig. 1. The FTIR spectra of synthesized samples in urea solution and water are compared. The Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 spectrum reveals characteristic vibrating modes of the free carbonate ion with point group symmetry D3h. This is engrossed in several internal vibrations, including symmetric stretching mode ν1 at 1066 cm−1, out-of-plane bending mode ν2 at 848 cm−1, antisymmetric vibration ν3 at 1460 and 1384 cm−1, in-plane deformation ν4 at 695 and 673 cm−1 and symmetric stretching plus in-plane deformation modes, ν1 + ν4, at 1756 and 1730 cm−1[41, 42, 47]. The band groups of CO3−2 at ν1, ν2, ν3, ν4 and ν1 + ν4 reveal the formation of Bi2O2CO3. Moreover, the bands at 844 cm−1 and 300–800 cm−1 correspond to Bi-O-C and stretching modes of the Bi-O bonds in Bi2O2CO3, and Bi2O3, respectively [60]. The broad bands at 3600–3300 cm−1 and 1400 cm−1 are corresponding to O–H stretching vibration and O–H bending vibration, respectively, suggesting the presence of (BiO)4CO3(OH)2 in composition [61, 62].

Fig. 1
figure 1

FT-IR spectra of synthesized nanostructures by electrical arc discharge

The XRD patterns of synthesized samples are illustrated in Fig. 2. It is observed that there are different phases in the pattern of synthesized sample in water, which are related to the crystallographic characteristics of the Bi, Bi2O3 and Bi(OH)3 phases. The diffraction peaks assigned to Bi, α-Bi2O3, β-Bi2O3 and Bi(OH)3 are based on JCPDS NO. 00-005-0519, 01-071-2274, 00-027-0050 and 00-001-0898 standard cards. The crystallographic parameters of the bismuth phase are rhombohedral, a = b = 4.5460 Å, c = 11.86 Å, α = β = 90° and γ = 120°. Bismuth oxide formed in two crystal systems, monoclinic with a = 5.8486, b = 8.1661 and c = 7.5097 Å, α = γ = 90°, β = 113° and tetragonal with a = b = 7.7420, c = 5.6310 Å, α = γ = 90° and β = 113. In comparison with the pattern of synthesized sample in water, the pattern of the synthesized sample in urea solution reveals α-Bi2O3, (BiO)4CO3(OH)2 and Bi2O2CO3 phases. The α-Bi2O3 has the same reference code as that of synthesized in water [63]. The (BiO)4CO3(OH)2 phase is relevant to 00-038-0579 reference code with orthorhombic crystal system and a = 10.7716, b = 5.4898, c = 14.75740 Å and α = β = γ = 90° crystallographic parameters [64]. The crystal structure of Bi2O2CO3 is orthorhombic with a = 5.4680, b = 27.3200, c = 5.4680 Å, α = β = γ = 90° crystal parameters and is based on 01-084-1752 standard card [65]. Based on the calculation of semiquantitative analysis of multiphase systems, the α-Bi2O3, (BiO)4CO3(OH)2 and Bi2O2CO3 phases have 44%, 36% and 20% shares of structure, respectively. To describe the mechanism of multiphase formation of nanostructures, it should be noticed that establishing electrical arc discharge leads to the formation of a high-temperature plasma [59]. The plasma dissipates the solution molecules and produces neutral, ionic and molecular species of bismuth, water and urea. Formation of Bi, O, C and H ions or OH, H2, O2, CO and CO2 molecules in their ground or excited state in the plasma of electric arc discharge has been reported before [53, 57]. So, based on these species, the formation of oxide, oxide carbonate, hydroxide and carbonate oxide of bismuth could be explained.

Fig. 2
figure 2

XRD patterns of synthesized nanostructures by electrical arc discharge

Figure 3 shows the FE-SEM images of samples with different magnifications and size distribution histogram of the synthesized sample in urea solution. The FE-SEM images were obtained after keeping samples in their solutions for 15 days. The Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 has a nanoflake structure, and the approximate average thickness of nanoflakes was demonstrated by a size distribution histogram which was obtained by applying a Gaussian curve on the size histogram. The average thickness of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes is almost 22.0 ± 0.5 nm. The synthesized sample in water shows a nanoflake structure as well. Even though both samples indicate flake morphology, the Bi2O3/Bi/Bi(OH)3 is considerably more agglomerated than Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanocomposite. By increasing the agglomeration of nanostructures, the specific surface area will be reduced which will lead to less photocatalytic performance. Furthermore, FE-SEM elemental mapping and energy-dispersive spectrometry of synthesized nanoflakes in urea solution indicated the uniform distribution of Bi, O and C elements in the structure, as shown in Fig. 4.

Fig. 3
figure 3

FE-SEM images of synthesized nanostructures by electrical arc discharge in a, b urea solution, c, d water, and e size distribution histogram of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes

Fig. 4
figure 4

a FE-SEM image, b EDS spectrum, elemental mapping patterns of c Bi, d O, e C, f N, g combine and h combine with FE-SEM image of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes

Figure 5a and b shows the TEM image of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes with different magnifications, which confirms the SEM observation. With the formation of nanostructures with the more effective surface area, the interaction with dye molecules will increase. Therefore, it can be expected that by increasing the photocatalytic reaction sites, photocatalytic activity will improve [66, 67, 68]. Hence, the Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 with fine flake morphology will have more photocatalytic reaction sites and thus better photocatalytic performance than Bi2O3/Bi/Bi(OH)3.

Fig. 5
figure 5

TEM images of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes with different magnifications, a 200 nm and b 50 nm

Optical properties, especially the UV–visible spectrum of the samples, are an effective way to evaluate photocatalytic activity via measuring light harvesting efficiency over different ranges of visible spectrum [53]. The optical absorption spectra of colloidal samples over different times immediately after synthesis, 15 min, 30 min, 1 h, 4 h and 4 days are shown in Fig. 6. The spectra are shown at a wavelength between 190 and 900 nm. The intensity and wavelength of the optical absorption peak as well as the form of the spectrum depend on the morphology, size, material type of nanostructures and dielectric function of the surrounding medium [69]. The optical absorption spectra of the samples show an intense absorption at wavelengths between 190 and 400 nm, which is explained by electron transitions from the valance band to the conduction band of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 and Bi2O3/Bi/Bi(OH)3, which is the feature of semiconductor materials [26, 43, 60]. The intense absorption edge of both samples reveals their proper visible light absorbing ability. In addition, noble metallic nanoparticles reveal an absorption peak in the UV–visible spectrum which is explained by surface plasmon resonance (SPR) [70]. According to XRD results of synthesized sample in water the formation of bismuth phase in fresh samples are probable, hence the observable peak around 200 nm can be attributed to SPR of metallic bismuth phase in both samples [54]. The occurred changes in the absorption spectra can be explained by the disappearing of the bismuth plasmon absorption peak by oxidation of nanostructures. The optical properties and absorption edge of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 show that synthesized nanoflakes in urea solution can be a good candidate for visible light harvesting and possible photocatalytic activity than Bi2O3/Bi/Bi(OH)3.

Fig. 6
figure 6

UV–visible optical absorption spectra of synthesized colloidal samples in a urea solution, b water, over different times

The photocatalytic activity of the novel Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes with multi-heterojunctions and Bi2O3/Bi/Bi(OH)3 nanocomposite was evaluated by the degradation of MeO under visible light irradiation for 180 min. In addition, to reveal the capacity of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 for use as a proper photocatalyst in environmental applications, the photocatalytic performance of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 investigated for the degradation of RB as well. The optical transmission spectra of MeO solution during the degradation process by Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes exhibited considerable photocatalytic activity compared to that of Bi2O3/Bi/Bi(OH)3. The MeO and RB absorption peaks at 464 and 530 nm were considerably reduced by Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst. The concentration of the dye at different times during degradation can be obtained by Langmuir–Hinshelwood model [71]:

$$r = - \frac{{{\text{d}}C\left( t \right)}}{{{\text{d}}t}} = k_{abc} C\left( t \right)$$
(1)

where \(r\) is the degradation rate of the dye, \(C\left( t \right)\) reveals the concentration of dye at time t and \(k_{abc}\) is a constant for reaction rate. The degradation kinetics of the dye can be described by the following equation at low concentration of the dye:

$$\ln \left( {\frac{{C_{0} }}{C}} \right) = k_{abs} t$$
(2)

where C0 is the initial concentration of dye. The photocatalytic activity and dye degradation kinetics for the samples are presented in Fig. 7, which were obtained at different reaction times during the photodegradation process. Figure 7a illustrates the C/C0 plot versus irradiation time and, as obviously seen, Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 shows a highly efficient degradation rate. Figure 7b shows the degradation reaction kinetics, which were measured from the slopes of the − ln(C/C0) plots versus irradiation time [72]. The calculated \({k}_{abs}\) values for photodegradation of MeO and RB on Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 are 5.2 × 10–3 min−1 and 4.9 × 10–3 min−1, respectively, and for photodegradation of MeO on Bi2O3/Bi/Bi(OH)3 is 2.0 × 10–3 min−1. The promoted photocatalytic performance of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 can be attributed to its composition, absorption edge, flake morphology and multiphase structure. Based on the obtained results of this research, it can be concluded that the formation of multi-heterojunction can efficiently improve the photocatalytic activity of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst in comparison with Bi2O3/Bi/Bi(OH)3 photocatalyst. In addition, these results can be compared with other photocatalysts such as nanosheet Bi2O2CO3 and microrod BiOHC2O4/Bi2O2CO3 with photocatalytic performance for the removal of MeO in 50 mL aqueous solution of 10 ppm dye under 400 W xenon lamp irradiation. Hierarchical BiOHC2O4/Bi2O2CO3 heterostructure revealed the highest degradation kinetic with 4.7 × 10–3 min−1 values, which was considerably higher than single BiOHC2O4 and Bi2O2CO3 [73]. For silver-doped Bi2O3 photocatalysts with nanosheet morphology in photodegradation of MeO with 20 mg L−1 concentration under a 500 W Xenon arc lamp over 180-min irradiation time, the obtained degradation rate was less than 4 × 10–3 min−1 in the most optimal state [74]. Degussa P25 titania  was used for the removal of rhodamine B with 10 mg in 50 mL water solution under irradiation of 500 W Mercury-Xenon lamp with 6.4 × 10−4 min−1 degradation rate [8]. In all of these examples, the power of light sources was around 500 W while the photocatalytic results of this research obtained under only 15 W visible LED.

Fig. 7
figure 7

a Photocatalytic degradation of dyes and b Liner curves of ln(C/C0) versus the irradiation time for the photodegradation reaction kinetics of dyes by samples

The recyclability and reusability of photocatalysts are important factors in practical applications [75, 76]. The stability of the photocatalytic activity of a photocatalyst can be evaluated through the monitoring of photocatalytic degradation of MeO under light over several cycles. In fact, the stability of a photocatalyst can be proved over every cyclic photocatalytic degradation test [77]. Hence, efficiency should not decline noticeably with each cycle. The degradation efficiency of the recovered Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst after four cycles is shown in Fig. 8. As it is clear, after each cycle the photocatalytic activity of the sample was almost unchanged, which demonstrates high stability of photocatalyst after multiple reuse.

Fig. 8
figure 8

Photocatalytic stability test of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst

The possible photocatalytic degradation mechanism of MeO by Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 under visible light irradiation was investigated through trapping experiments of reactive species in photocatalytic tests by using of different radical scavengers, namely EDTA, IPA and BQ for scavenging the active h+, OH and O2 during the photocatalytic reaction [26, 43]. Based on Fig. 9, degradation values of the MeO decreased in all capturing experiments, which reveals that all the generated reactive species are effective in photodegradation of MeO. However, BQ as a superoxide radical scavenger plays more important role in the MeO degradation. As shown in Fig. 9, by capturing O2 the dye degradation will dwindle more than other h+, OH species.

Fig. 9
figure 9

Degradation curves of MeO with different scavengers under the visible light by Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 photocatalyst

The photocatalytic performance is deeply depend on the electronic and energy band structure of the semiconductor. Mott–Schottky measurements can be used to better study the energy band structure of a semiconductor. For instance, the band structure of S-scheme BP/BiOBr nano-heterojunction with photocatalytic performance in the degradation of tetracycline and evolution of oxygen, the flat-band potential of BPQDs and CN in CN/rGO@BPQDs high–low junctions system with photocatalytic degradation and H2O2 production, as well as study of semiconductor type and flat-band potential of S-C3N4 and PDI-Ala of S-Scheme heterojunction PDI-Ala/S-C3N4 organic photocatalyst in photodegradation of tetracycline and p-nitrophenol and production of H2O2 under visible-light irradiation, were measured by the Mott–Schottky method [78, 79, 80]. Although this method has a lot of advantages, it has been used for revealing the band structure of a single component in a multi-component system, while in the present work, the multi-heterojunction Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanocomposite has been synthesized during the in situ arc discharge process and there is no chance for the synthesis of single phases with this method. Therefore, the probable band structure of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 according to the calculated band gap energy level of it and the energy levels of literature are proposed in three possible degradation mechanisms. The energy band structure of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 multi-heterojunction before and after junction are proposed in Fig. 10 [26, 81, 82]. The Fermi level of α-Bi2O3 and (BiO)4CO3(OH)2 which are p-type photocatalysts are close to the valence band, and the Fermi level of n-type Bi2O2CO3 is close to the conduction band [26, 83]. By formation of multi-heterojunction, the energy band of α-Bi2O3, (BiO)4CO3(OH)2 and Bi2O2CO3 will move upward or downward to reach an equilibration state of Fermi level [26]. Under visible light irradiation, the photogenerated electrons in the conduction band of phases will transfer to downer CB, while the holes in the valence band are likely to transfer to the opposite position [82]. In this situation, the recombination of photogenerated carriers in photocatalyst will reduce and thus the formation of p-n-p or p-p-n Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 multi-heterojunction nanoflakes will improve the separation efficiency of the photogenerated electrons and holes which lead better photocatalytic performance.

Fig. 10
figure 10

Schematic diagram of three possible photogenerated charge separation for Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2, a before multi-heterojunctions formation, b–d after multi-heterojunctions formation

The PL spectrum of the Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 with the excitation wavelength at 355 nm was measured and compared with the PL spectrum of Bi2O3/Bi/Bi(OH)3 to display the photoactivity enhancement mechanism. Broad emission peaks in the range between 2 and 3 eV are found in the PL spectra of the samples, which is in agreement with the documents [24, 43, 51, 84]. In general, a superior PL intensity shows an upper recombination rate of photogenerated electrons and holes, and thus lower PL intensity expresses a lower recombination rate of photogenerated electrons and holes, which means an improved separation rate of electrons and holes and so better photocatalytic performance [43]. Based on PL spectra shown in Fig. 11, the PL intensities of the multi-heterojunction Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes are all lower than that of the Bi2O3/Bi/Bi(OH)3 at the same condition, which means the efficient separation rate of the carriers in Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2. The formation of the multi-heterojunction reduces the recombination rate of photogenerated electrons and holes and then is favorable to improving photocatalytic activity.

Fig. 11
figure 11

Photoluminescence spectra of synthesized nanostructures by electrical arc discharge in urea solution and water

Conclusion

In summary, new multi-heterojunction Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes were prepared by applying a 40 A DC current between two bismuth electrodes through DC electrical arc discharge in urea solution and investigated for using them as a visible light photocatalyst. The photocatalytic performance of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 is compared with that of Bi2O3/Bi/Bi(OH)3 which is synthesized in the same condition in water. FT-IR, XRD and EDS results revealed the formation of α-Bi2O3, Bi2O2CO3 and (BiO)4CO3(OH)2 phases. FE-SEM and TEM analysis revealed the fine flake morphology of Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 with a thickness of about 22.0 ± 0.5 nm. The optical absorption spectra of the samplesshow an absorption edge at a wavelength between 190 and 400 nm. The XRD and PL results confirm the formation of multi-heterojunction between α-Bi2O3, Bi2O2CO3 and (BiO)4CO3(OH)2 phases. In comparison with Bi2O3/Bi/Bi(OH)3, the Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes reveal lower agglomerated morphology, better absorption edge and lower recombination rate and thus much better photocatalytic activity. The results of this research introduce a novel multi-heterojunction compound with considerable multi-applicable photocatalytic performance in environmental applications via an appropriate and cost-effective synthesis method.

Availability of Data and Materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Abbreviations

DC:

Direct current

IEF:

Internal electric field

MeO:

Methylene orange

FT-IR:

Fourier-transform infrared spectroscopy

XRD:

X-ray diffraction

FE-SEM:

Field emission scanning electron microscopy

EDS:

Energy dispersive spectroscopy

TEM:

Transmission electron microscopy

PL:

Photoluminescence

EDTA:

Ethylenediaminetetraacetic acid

IPA:

Isopropyl alcohol

BQ:

Benzoquinone (BQ)

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Funding

This work was supported by Tarbiat Modares University, through grant #IG-39703.

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EH carried out the synthesis processes, analysis and photocatalytic processes of dye decomposition. RP and HD supervised synthesis, analyses of the materials and preparation of the manuscript. All authors discussed the results and contributed a revision of the manuscript. All authors read and approved the final manuscript.

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Correspondence to R. Poursalehi.

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Hashemi, E., Poursalehi, R. & Delavari, H. Structural, Optical and Photocatalytic Activity of Multi-heterojunction Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 Nanoflakes Synthesized via Submerged DC Electrical Discharge in Urea Solution. Nanoscale Res Lett 17, 75 (2022). https://doi.org/10.1186/s11671-022-03714-3

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Keywords

  • Bi2O3/Bi2O2CO3/(BiO)4CO3(OH)2 nanoflakes
  • Multi-heterojunction
  • Photocatalyst
  • Optical properties
  • Submerged DC electrical discharge
  • Urea solution