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Article

Synthesis of Co3O4 Nanoparticles-Decorated Bi12O17Cl2 Hierarchical Microspheres for Enhanced Photocatalytic Degradation of RhB and BPA

by
Syed Taj Ud Din
1,
Wan-Feng Xie
1,2 and
Woochul Yang
1,*
1
Department of Physics, Dongguk University, Seoul 04620, Republic of Korea
2
School of Electronics and Information, University-Industry Joint Center for Ocean Observation and Broadband Communication, Qingdao University, Qingdao 266071, China
*
Author to whom correspondence should be addressed.
Int. J. Mol. Sci. 2022, 23(23), 15028; https://doi.org/10.3390/ijms232315028
Submission received: 21 October 2022 / Revised: 20 November 2022 / Accepted: 28 November 2022 / Published: 30 November 2022
(This article belongs to the Special Issue Molecular Aspects in Catalytic Materials for Pollution Elimination and Green Chemistry)
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Abstract

:
Three-dimensional (3D) hierarchical microspheres of Bi12O17Cl2 (BOC) were prepared via a facile solvothermal method using a binary solvent for the photocatalytic degradation of Rhodamine-B (RhB) and Bisphenol-A (BPA). Co3O4 nanoparticles (NPs)-decorated BOC (Co3O4/BOC) heterostructures were synthesized to further enhance their photocatalytic performance. The microstructural, morphological, and compositional characterization showed that the BOC microspheres are composed of thin (~20 nm thick) nanosheets with a 3D hierarchical morphology and a high surface area. Compared to the pure BOC photocatalyst, the 20-Co3O4/BOC heterostructure showed enhanced degradation efficiency of RhB (97.4%) and BPA (88.4%). The radical trapping experiments confirmed that superoxide (•O2−) radicals played a primary role in the photocatalytic degradation of RhB and BPA. The enhanced photocatalytic performances of the hierarchical Co3O4/BOC heterostructure are attributable to the synergetic effects of the highly specific surface area, the extension of light absorption to the more visible light region, and the suppression of photoexcited electron-hole recombination. Our developed nanocomposites are beneficial for the construction of other bismuth-based compounds and their heterostructure for use in high-performance photocatalytic applications.

1. Introduction

Rapid industrialization and population growth has led to a tremendous increase in environmental pollutions. These pollutants mostly consist of hazardous Azo dyes and phenolic compounds. Rhodamine-B (RhB) cationic Azo dye is an anthraquinone derivative. It is highly stable and non-biodegradable in nature and is classified as a carcinogenic and neurotoxic substance [1]. Aside from dyes, the other frequently used compound is the colorless Bisphenol-A. It is a diphenylmethane derivative and a raw material that is widely used in the fabrication of numerous polymeric materials [2]. Long-term exposure of BPA causes endocrine, neurological, and reproductive developmental disorders [3]. Therefore, it is crucial to eradicate RhB and BPA before waste is discharged into water reservoirs and landfills. Pollution-free environmental remediation technologies to degrade these organic pollutants have attracted substantial attention [4]. Among them, visible-light-driven photocatalytic technology has emerged as the most promising approach for wastewater cleaning and pollutant removal [5].
Recently, bismuth-based nanomaterials, such as BiPO4 [6], Bi2O2CO3 [7,8,9], Bi4Ti3O12 [10], Bi2MoO6 [11], Bi2WO6 [12], Bi2O3 [13], and BiOX (X = Cl, Br, I) [14,15], have attracted substantial attention for their use in photocatalytic applications, because O 2p and Bi 6s valence band hybridization not only narrows the bandgap but also enhances the mobility of photo-generated holes in the valence band. Similarly, a bismuth and oxygen-enriched bismuth-oxyhalide (Bi12O17Cl2 (BOC)) is a typical tetragonal phase compound composed of a layered structure with an alternate stacking of [Bi2O2]2+ sheets interleaved with [Cl]− groups, and it represents an important class of bismuth-based photocatalysts. The photocatalytic properties of nanobelts-like BOC were first reported by Xiao et al. in 2013 [16]. Since then, there have been many research efforts focused on the preparation of BOC with different morphologies, including nanobelts, nanosheets, and flower-like morphologies. For instance, Wang and colleagues [17] prepared BOC nanobelts through a solvothermal treatment using Bi(NO3)3.5H2O, NH4Cl, and NaOH as precursors in a solvent consisting of ethylene glycol (EG) and water for the photocatalytic degradation of BPA. Liu et al. [18] presented two-dimensional (2D)-BOC nanosheets oriented along the [002] direction which showed enhanced photocatalytic RhB degradation. Similarly, Fang et al. [19] prepared 3D BOC hierarchical nanostructures using a coprecipitation method followed by calcination, and these nanostructures demonstrated high photocatalytic efficiency for RhB degradation. Among the morphologies detailed above, the flower-shaped BOC has excellent characteristics, including high surface area, good adsorption capability, and maximum light absorption. Therefore, it is essential to develop a facile method to fabricate a 3D flower-like BOC. Despite these advantages, the 3D BOC still need to resolve the issues of a rapid electron-hole recombination rate and inappropriate redox potentials. Therefore, various research strategies have been developed to overcome these issues, including the fabrication of heterojunctions [20], element doping [21], noble metal deposition [22], and graphene decoration [23]. Among these methods, heterojunction preparation is the most effective approach because of the fast transfer rate of photo-generated electron and holes (e−-h+), which facilitates the separation of the photo-generated e−-h+ pairs in the photocatalysts, which play quite an important role in enhancing the photocatalytic activity of photocatalysts. For instance, He et al. [24] obtained a BOC/𝛽-Bi2O3 composite with flower-like micro/nano architectures that demonstrated good photocatalytic activity for the degradation of 4-tert-butyphenol under visible light. In another study, Huang et al. [25] prepared BiOI@BOC heterojunction photocatalysts with high exposure of the active BiOI (001) facet, which exhibited excellent photocatalytic performance for RhB and BPA degradation. Moreover, BOC heterostructures with non-bismuth-based compounds, such as CoAl-LDH/BOC [26] and Ag2O/BOC p-n junction catalysts [27], have also been reported; both exhibit significant photo-degradation efficiency under visible light irradiation.
Co3O4 is a traditional p-type semiconductor (band gap, Eg = 1.2~2.6 eV) with interesting electronic, magnetic, sensing, and catalytic properties [28]. In particular, Co3O4-based heterojunctions have yielded high photocatalytic activity; for example, the 0D/2D Co3O4/TiO2 heterojunction photocatalyst has exhibited enhanced photocatalytic activity under visible light irradiations [29]. In another study, Dai et al. [30] synthesized a Co3O4/BOC photocatalyst that showed effective visible-light-driven RhB photodegradation due to the more positive value of the valence band potential of Co3O4 relative to BOC. However, BOC is an n-type semiconductor, as indicated by its positive slope in the Mott-Schottky plot [31], and the more positive valence band (VB) potential than that of the Co3O4 counterpart. Therefore, the combination of Co3O4 with BOC is favorable for the formation of the p-n heterojunction. As a result, the photo-generated holes on the VB of BOC could be easily transferred to the VB of Co3O4 under light illumination, thus resulting in practical separation of photo-generated e−-h+ pairs of BOC and Co3O4, which would be beneficial for a photocatalyst in terms of photo-degradation efficiency. However, to our knowledge, there is a lack of research into using a Co3O4/BOC hierarchical microsphere photocatalyst for the degradation of RhB and BPA.
Herein, the synthesis of a BOC hierarchical microsphere and its decoration with Co3O4 nanoparticles (NPs) via a solvothermal method have been reported. The heterojunction formation of the Co3O4 NPs-decorated BOC (Co3O4/BOC) was evaluated through structural, morphological, spectroscopic, and electrochemical investigations. The photocatalytic degradation efficiency of the hierarchical microsphere Co3O4/BOC heterojunction was evaluated against RhB and BPA aqueous pollutants. The results showed that the 20-Co3O4/BOC heterostructure had an outstanding degradation efficiency of RhB (97.4%) and BPA (88.4%) after 140 min and 175 min of visible light irradiation, respectively, compared to pure BOC and other composite samples. The improved photocatalytic degradation performance could be ascribed to the synergetic effects of the larger active area of hierarchical microsphere, the extended light absorption to visible light range with Co3O4 NPs, and the suppression of e−-h+ recombination caused by the p-n junction formation of Co3O4/BOC.

2. Results and Discussion

2.1. Structural, Morphological, and Elemental Analyses

Figure 1 shows the morphology of the BOC fabricated with different volume ratios of ethylene glycol (EG) and ethyl alcohol (EtOH). BOC-1 (Figure 1a) synthesized in EG only and BOC-3 (Figure 1c) prepared in mixtures of EG and EtOH both have uniform 3D architectures, whereas BOC-2 (Figure 1b) prepared in pure EtOH solution shows a nanoparticle-like morphology with almost no agglomeration. Compared to BOC-1, the morphology of BOC-3 was more regular with a flower-like shape, and the size of the micro-flower was about 4 μm (Figure 1c). In addition, BOC-3 was also prepared with different solvothermal reaction times to understand the microspherical morphology growth and optimize the reaction time (discussed in Appendix A). BOC-3 synthesized following 6 h of solvothermal treatment was composed of many ultrathin nanosheets (inset of Figure 1), which is beneficial for photocatalytic degradation due to the increased specific surface area. Figure 1d shows the N2 adsorption-desorption isotherms of BOC-1 (dark), BOC-2 (red), and BOC-3 (blue). The isotherm plots showed type-IV isotherm and hysteresis loop curves [32]. The SBET values of BOC-1, BOC-2, and BOC-3 were 8.914, 8.160, and 15.720 m2/g, respectively, indicating that BOC-3 had the largest specific surface area.
Figure 2a shows a FESEM image of the 20-Co3O4/BOC synthesized with 20 mg of Co3O4 NPs and BOC-3. Its hierarchical morphology was almost identical to that of BOC-3, and it was not affected by the incorporation of Co3O4 NPs during the synthesis process. To investigate the existence of the nanosized Co3O4 in the BOC hierarchical morphology, HRTEM measurements of 20-Co3O4/BOC were performed, as shown in Figure 2b. The results confirmed that the Co3O4 NPs, which had an approximate diameter of 10 nm, were decorated on the surface of BOC. In the HRTEM, the fringe spacing of 0.23 nm belonged to the (222) crystal plane of Co3O4. By contrast, the fringe spacings of 0.272 nm and 0.31 nm, which respectively refer to the (200) and (117) crystal planes of BOC, have also been observed. Further, a high-angle annular dark-field (HAADF) image of 20-Co3O4/BOC was obtained (Figure 2c), in which the tiny black spots identified across the BOC surface indicated Co3O4 NPs; this finding was further confirmed by EDX analysis in Figure 2d,e. These results confirmed the 0D/3D morphology of the prepared photocatalyst might be conducive to the photocatalytic performance of the Co3O4/BOC heterostructure. Moreover, the obtained EDS mapping spectrum (Figure A2 in Appendix B) confirmed the presence of bismuth (Bi), oxygen (O), chlorine (Cl), and cobalt (Co) elements in the 20-Co3O4/BOC sample. The at.% and wt.% of the elements are also shown (inset table in Figure A2). Figure 2f presents the N2 adsorption–desorption isotherm plots for 20-Co3O4/BOC. The estimated SBET using N2 isotherms was found to be 14.873 m2/g, which was close to the SBET value of BOC-3. This result shows that the surface area of 20-Co3O4/BOC was slightly affected by the incorporation of Co3O4 NPs.
The XRD patterns of the pristine BOC-3, Co3O4, and 20-Co3O4/BOC are shown in Figure 2g. The observed peaks in the XRD patterns of BOC-3 and Co3O4 matched the BOC and Co3O4 crystallites (JCPDS cards #37-0702 and #42-1467), respectively. The 20-Co3O4/BOC samples showed all characteristic peaks of BOC-3. However, the characteristic peaks of Co3O4 were not observed in the prepared composite samples. The absence of Co3O4 characteristic peaks was attributed to the low amount of Co3O4 compared to BOC in 20-Co3O4/BOC composite samples. The structural properties of 20-Co3O4/BOC were further investigated using Raman spectroscopy, as shown in Figure 2h. All characteristic peaks of BOC have been observed in the Raman spectrum of pure BOC-3 [33]. The observed peak at 165.80 cm−1 belonged to the A1g internal stretching of the Bi-Cl bond [33,34]. Because of the oxygen-rich nature of BOC-3, the observed peaks in the range from 200 cm−1 to 500 cm−1 belong to the vibrational modes of Bi and O bonding. Among them, The peak at 470.51 cm−1 is the characteristic vibrational mode of BOC-3, which belongs to O-Bi-O bending modes. Further, the peak at 598.80 cm−1 represents Cl-Cl stretching modes [35]. Regarding Co3O4 NPs, all the characteristic peaks of Co3O4 appeared in the Raman spectra, indicating the successful formation of Co3O4 NPs, along with an extra peak at 481.15 cm−1 from the glass substrate. These observed peaks belonged to the F2g and Eg modes of the combined vibrations of the tetrahedral site and octahedral oxygen vibrations [36,37]. The Raman spectra of Co3O4 NPs-decorated BOC were also obtained. The Raman spectra of 20-Co3O4/BOC showed all the characteristic peaks of both Co3O4 and BOC-3 samples, along with an extra peak at 307.50 cm−1. Since both Co3O4 and BOC-3 are oxygen-rich compounds, the interconnection of Co3O4 and BOC through oxygen bonding led to a new peak formation in the 20-Co3O4/BOC sample. The observed intense peak belongs to the Bi-O(1) rocking and weak O(2) breathing modes in the 20-Co3O4/BOC heterostructures, thus confirming the successful heterojunction formation [38].
To analyze the chemical composition and chemical state of the elements, we performed X-ray photoelectron spectroscopy (XPS) of the 20-Co3O4/BOC heterostructure photocatalyst (Appendix C). The XPS survey spectrum clearly demonstrated that all peaks were attributable to Bi, O, Cl, and Co elements, revealing that the heterostructure consisted of Bi, O, Cl, and Co elements, as shown in Figure A3 (See Appendix C). The high-resolution XPS spectra of Bi 4f, C 1s, O 1s, and Co 2p for the heterostructure are respectively shown in Figure A3b–e. The two strong peaks at 159.63 and 164.93 eV were assigned to Bi 4f7/2 and Bi 4f5/2, respectively, which are the features of Bi3+ in BOC (Figure A3b). As depicted in Figure A3c, the O 1s profile could be deconvoluted into three peaks, thus indicating the existence of three different kinds of O species in the sample. The peaks observed at 529.674 and 530.568 eV were assigned to the lattice oxygen metal bonds and hydroxyl (•OH) functional groups in 20-Co3O4/BOC, respectively [39]; the peak at 531.592 eV corresponded to oxygen vacancies in Co3O4 in the 20-Co3O4/BOC composite sample [40]. Figure A3d shows the spectrum of Cl 2p, which contained diverse peaks at 198.58 and 200.139 eV, respectively. These can be attributed to Cl 2p3/2 and Cl 2p1/2 of the Cl− ions in the corresponding 20-Co3O4/BOC sample [41]. In Figure A3e, the Co 2p peak of 20-Co3O4/BOC showed Co 2p3/2 and Co 2p1/2 spin-orbit doublets. The peaks at 782.25 and 793.50 eV in 20-Co3O4/BOC corresponded to Co2+ ions, whereas the peaks observed at 779.50 and 792.50 eV were assigned to Co3+ ions, therefore indicating the coexistence of Co2+ and Co3+ in both samples [42].

2.2. Photocatalytic Performance

The photocatalytic performance of BOC-1, BOC-3, 10-Co3O4/BOC, 20-Co3O4/BOC, and 40-Co3O4/BOC was explored by degrading RhB dye in aqueous solution under visible light, as shown in Figure 3a,b. Figure 3a indicates that 20-Co3O4/BOC outperformed BOC-3, 10-Co3O4/BOC, and 40-Co3O4/BOC by decomposing RhB dye solution in 140 min. Further, the degradation rate of RhB in the presence of each photocatalyst could be determined by the pseudo-first-order kinetic model, as expressed in Equation (1).
ln C/C0 = kt,
where k, C0, and C represent the reaction rate constant, initial concentration, and remaining concentration at time t, respectively. Figure 3b shows the reaction rate and degradation efficiency of all photocatalysts used for RhB degradation after 140 min. 20-Co3O4/BOC had the highest degradation rate of 2.21 × 10−2/min and an efficiency of 97.4%. Moreover, we compared the RhB photocatalytic degradation performance of the single BOC and 20-Co3O4/BOC photocatalyst with previously reported, similarly structured semiconducting photocatalysts, as shown in Table A1 (In Appendix E). Both synthesized BOC and Co3O4/BOC hierarchical microspheres possessed relatively higher degradation performance under similar test conditions. This enhanced efficiency might be related to the high surface area of hierarchically structured BOC and the formation of a Co3O4/BOC heterojunction with Co3O4 NPs-decoration.
Moreover, the photocatalytic degradation of BPA was performed to evaluate the photocatalytic activity of BOC and Co3O4/BOC, as shown in Figure 3c,d. The BPA degradation results showed that 20-Co3O4/BOC efficiently decomposed BPA aqueous pollutant solution in 170 min. Moreover, the degradation of BPA in the presence of each photocatalyst followed the 1st-order reaction kinetic model. Figure 3d shows the reaction rate and degradation efficiency after 110 min for all the photocatalysts used for BPA degradation. 20-Co3O4/BOC had the highest degradation rate of 1.66 × 10−2/min and an efficiency of 88.4%, followed by BOC-3, BOC-1, and BOC-2.
We further conducted a reusability test for the 20-Co3O4/BOC in the presence of RhB and BPA pollutants, as shown in Figure A4 (in Appendix D). During RhB and BPA degradation, a consistent decrease in the degradation of RhB and BPA occurred after the 3rd cycle of degradation, and its degradation performance was slightly reduced. This slight reduction in photocatalytic degradation may be attributable to the adsorption of RhB and BPA molecules on the surface of the 20-Co3O4/BOC sample.

2.3. Analysis of Enhanced Photocatalytic Activity of Co3O4/BOC

Photocatalytic activity is mainly attributed to light absorption capacity and the separation and transfer efficiency of photoinduced charge carriers. Firstly, photoluminescence (PL) measurements were conducted to investigate the recombination rate of photo-induced e−-h+ pairs in BOC-3 and 20-Co3O4/BOC photocatalysts, as shown in Figure 4a. The PL emission intensity of 20-Co3O4/BOC was lower than that of BOC-3, thus indicating reduced recombination of e−-h+ pairs in the 20-Co3O4/BOC photocatalyst. The transient photocurrent response was also measured to provide further support to the efficient separation of photo-generated charges, as shown in Figure 4b. The 20-Co3O4/BOC had a higher photocurrent response than BOC-3. The higher photocurrent response of 20-Co3O4/BOC was attributed to the higher separation efficiency of the excitons and its longer lifetime. Moreover, the EIS Nyquist plot was obtained to examine the electrode/electrolyte interfacial charge transfer resistance, as shown in Figure 4c. 20-Co3O4/BOC showed a smaller arc radius than BOC-3. The inset in Figure 4c shows the circuit of the sample-solution in the EIS measurements. According to the model of the circuit, Rs is related to uncompensated solution resistance, Rp is related to the porosity of the electrode, and Rct represents the charge transfer resistance at the interface [43]. The Rct values for BOC and 20-Co3O4/BOC are 1.58 × 10−4 Ω and 82.93 × 10−4 Ω, respectively. The smaller Rct value of 20-Co3O4/BOC represents its lower charge transfer resistance, which is beneficial for high redox reactions during photocatalysis.
Further, the UV-vis DRS of Co3O4, BOC-3, and 20-Co3O4/BOC were measured to investigate the optical absorption ability, as shown in Figure 4d. Co3O4 showed absorption throughout the whole UV and visible range. The absorption edges for BOC-3 were located around 520 nm. In comparison, 20-Co3O4/BOC showed enhanced absorption in the visible range after loading Co3O4 NPs on BOC. The high absorption of 20-Co3O4/BOC was attributed to the strong contribution of Co3O4 in 20-Co3O4/BOC to the absorption of visible light. Ultimately, these results suggest that the formation of the heterojunction in 20-Co3O4/BOC heterostructure could effectively suppress the recombination of the photoexcited charge carriers and enhance the visible light absorption ability. Therefore, the photocatalytic performance could effectively be improved by the 20-Co3O4/BOC heterostructure.

2.4. Interfacial Charge Transfer Behavior and Photocatalytic Reaction Mechanism

The electronic structures of Co3O4, BOC, and Co3O4/BOC were analyzed by UV-Vis diffuse reflectance spectra (DRS), the Mott–Schottky (MS) plot, and valence band (VB) XPS measurements to elucidate the photocatalytic mechanism of the 20-Co3O4/BOC heterostructure during the photodegradation of RhB and BPA, as shown in Figure 5. First, the optical bandgap energies of BOC-3 and Co3O4 could be obtained through curve fitting of the Tauc plot of (αhν)n/2 versus hν (Figure 5a), where n = 4 for the Co3O4 direct band gap semiconductor and n = 1 for the indirect band gap semiconductor [44,45]. The obtained bandgap energies were 2.34 and 2.25 eV for BOC-3 and Co3O4, respectively.
Secondly, the Fermi energy (Ef) levels of the prepared Co3O4 and BOC-3 were obtained using MS analysis, as shown in Figure 5b,c, respectively. The MS plots of Co3O4 and BOC-3 showed negative and positive slopes, thus indicating p-type and n-type semiconducting behaviors, respectively [46,47]. Further, by extrapolating MS plots, the flatband potentials of Co3O4 and BOC-3 were found to be +0.037 and −0.51 V, respectively, vs. the standard calomel electrode (SCE). Note that the flatband potential (Efb) of the n-type and p-type semiconductors represents the Ef level. The Ef level vs. the normal hydrogen electrode (NHE) scale could be calculated using Equation (2). The resultant Ef of Co3O4 and BOC-3 were +0.277 and −0.266 eV vs. NHE, respectively
Efb (vs. NHE) = Efb (vs. SCE) + 0.244 (eV),
Valence band (VB) XPS measurement was conducted to determine the VB potentials of Co3O4 and BOC-3, as shown in Figure 5d. The VB XPS spectra revealed the VB maxima of 0.86 and 1.69 eV for Co3O4 and BOC-3, respectively. Thus, the VB potentials with respect to the Ef level were calculated to be 1.137 and 1.424 eV vs. NHE, respectively (Figure 6a). The CB minima of Co3O4 and BOC-3 were determined using the following Equation (3).
ECB (vs. NHE) = EVB (vs. NHE) − Eg,
where ECB, EVB, and Eg denote the sample’s CB potential, VB potential, and bandgap, respectively. As a result, the calculated CB potentials for Co3O4 and BOC-3 were −1.113 and −0.916 eV vs. NHE, respectively.
Therefore, based on the above analysis, we can obtain the band structures of Co3O4 and BOC in NHE scale before contact, as shown in Figure 6. When Co3O4 and BOC form the Co3O4/BOC heterojunction after they come into contact, the electrons will spontaneously migrate from BOC to Co3O4 through the Co3O4/BOC interface to align with the Fermi level because BOC has a higher Ef level than Co3O4. The migration of these electrons results in the band bending upward for BOC and downward for Co3O4, near the interface of BOC and Co3O4, respectively, as shown in Figure 6b. These band bendings lead to a depletion region at the Co3O4 and BOC-3 interface, thus resulting in the generation of an internal electric field (IEF) from BOC toward Co3O4 at the interface. After light is irradiated on the photocatalyst, the charge carriers simultaneously excite from VB to the CB in Co3O4 and BOC-3 to produce photo-generated e− and h+ pairs. Then, the photoexcited electrons in the CB of Co3O4 will quickly migrate to the CB of BOC-3, whereas the remaining holes in the VB of BOC-3 will migrate to the VB of Co3O4 because of the IEF directed from BOC to Co3O4 in the 20-Co3O4/BOC, which is a typical charge transport of a type-II heterostructure. As a result, the e−-h+ recombination is suppressed in the heterostructure system. Moreover, the unique 0D/3D morphology of 20-Co3O4/BOC will provide more active catalytic reaction centers and increase the active sites. Thus, the developed heterostructure could be suggested to be beneficial in photocatalytic degradation.
Finally, photocatalytic active radical detection experiments were conducted to investigate the photocatalytic reaction mechanism and validate our proposed heterojunction formation, as depicted in Figure 7. Here, BQ, IPA, and KI were used as scavengers of •O2− radicals, •OH radicals, and hole (h+), respectively, which are produced during the photocatalytic degradation of RhB [48]. Since 20-Co3O4/BOC composite decomposed the RhB dye efficiently compared to the other photocatalysts (shown in Figure 3a), a 20-Co3O4/BOC photocatalyst sample was chosen for active radical detection. As shown in Figure 7, in the presence of BQ and IPA, the degradation efficiency of RhB was significantly reduced from 98% (no scavenger) to 68.5% and 86.2%, respectively. However, no effect on the degradation efficiency was observed when using KI as an h+ scavenger. Therefore, •O2− and •OH radicals were found to be the reactive species with increased generation of •O2− during the photodegradation of RhB.
Based on the radical scavenger experiments, the possible reaction mechanism was proposed as shown in reaction (4)–(10); the excited electrons on BOC reduced the oxygen molecule (O2) into •O2− radicals, and then the •O2− radicals reacted with e- and hydrogen ions (H+), ultimately resulting in the generation of hydrogen peroxide (H2O2) radical, which was further reduced to •OH and hydroxyl ion (OH−). Then, the •O2−, •OH, and OH− finally decomposed RhB and BPA into small chain molecules.
Co3O4 + hν → Co3O4* (e− + h+)
BOC + hν → BOC* (h+ + e−)
Co3O4* (e− + h+) + BOC* (h− + e+) → Co3O4* (h+) + BOC* (e−)
O2− + e− → •O2−
O2− + e− + 2H+→ H2O2−
H2O2 + e− + H+ → •OH + OH−
RhB/BPA + •OH/OH−/O2− → decomposed products

3. Materials and Methods

3.1. Chemicals

Bismuth nitrate pentahydrate (Bi(NO3)3.5H2O, 99%), Cobalt acetate tetrahydrate (Co(CH3COO)2.4H2O), Potassium chloride (KCl, 99%), Ethylene glycol (EG) ((CH2OH)2, 99%), Dimethylformamide (DMF) (C3H7NO), Rhodamine-B (RhB) (C22H24N2O8, 99%), Bisphenol-A (BPA) (C15H16O2, 99%), 1,4-benzoquinone (BQ) (C6H4O2, 99%), and Fluorine doped tin oxide (FTO) glass were purchased from Sigma Aldrich Inc. (St. Louis, MO, USA). Ethanol (EtOH) (C2H5OH, 99%) and Isopropyl alcohol (IPA) (C3H8O, 99%) were purchased from DAEJUN Co., Ltd (Daejun, Korea). All reagents were used without any further purification.

3.2. Preparation of Co3O4 Nanoparticles

The Co3O4 NPs were prepared by the solvothermal method [29]. In a typical synthesis, 80 mg of Co(CH3COO)2.4H2O was dissolved in 60 mL of EtOH using a magnetic stirrer. The prepared solution was then transferred to a 100 mL Teflon-lined autoclave and heated at 150 °C for 4 h in a thermal oven to initiate the solvothermal reaction. After completion of the reaction, the autoclave was allowed to cool down to room temperature, at which point the raw Co3O4 was washed with EtOH and centrifuged at 15,000 rpm for 30 min. Following centrifugation, the collected sample was dried at 60 °C overnight to obtain the Co3O4 NPs.

3.3. Preparation of BOC and Co3O4/BOC

BOC was prepared by a solvothermal method, as shown in Scheme 1a. In a typical synthesis method, 2.186 g (4.5 mmol) of Bi(NO3)3.5H2O was dissolved in 17.5 mL of ethanol by ultrasonication, followed by stirring. The resulting solution was referred to as solution-A. At the same time, 1.5 mmol of KCl was dissolved in 17.5 mL of EG by ultrasonication, followed by stirring for 30 min. The resulting solution was referred to as solution-B. Next, solution-B was added dropwise to solution-A and then constantly stirred for 30 min. Then, the resulting combined solution was transferred to a 50 mL Teflon autoclave and heated for 6 h at 160 °C. The product obtained in this way was washed and dried overnight at 60 °C. Afterward, the gray powder was collected and calcinated in a muffle furnace at 450 °C for 1 h to obtain the targeted BOC hierarchical microspheres. The Co3O4/BOC samples were prepared using the identical synthesis procedure with the addition of Co3O4 NPs (10, 20, and 40 mg) into the combined solution (see Scheme 1b). Moreover, the BOC sample was prepared in pure EG and EtOH solutions to investigate the roles of EG, EtOH, and EG-EtOH mixed solvents during BOC synthesis. The samples prepared in EG, EtOH, and EG-ETOH mixed solvents were labeled as BOC-1, BOC-2, and BOC-3, and the sample prepared with the addition of 10, 20, and 40 mg of Co3O4 NPs were labeled as 10-Co3O4/BOC, 20-Co3O4/BOC, and 40-Co3O4/BOC, respectively.

3.4. Characterization of Samples

The crystal structure and phase purity were characterized by powder X-ray diffraction (XRD) in the 2θ range from 10~80° (2° min−1) using an X-ray diffractometer (Rigaku D/MAX-2500) with a Cu Kα irradiation source (λ = 1.54178 Å) and X-ray power of 40 kV/30 mA. Micro-Raman spectroscopy (XperRAM100, Nanobase Inc., Seoul, Korea) equipped with a monochromatic laser source (wavelength of 532 nm and power of 6 mW) was used to characterize the crystalline phase. The morphologies were examined using a field emission scanning electron microscope (SEM) (JSM-6700F, Jeol Ltd., Tokyo, Japan) and a transmission electron microscope (TEM) (“NEOARM “/JEM-ARM200F, Jeol Ltd.) equipped with an energy dispersive spectroscope (EDX). X-ray photoemission spectroscopy (XPS) (Veresprobe II, ULVAC-PHI Inc., Kanagawa, Japan) with a Monochromatic Al Kα X-ray source was used to examine the chemical compositions of the samples. The specific surface area was volumetrically assessed by measuring the nitrogen adsorption/desorption isotherms at 77 K using Microtrac, BELsorp-mini II. UV–vis diffuse reflectance spectra (DRS) were obtained using a spectrometer (V-750, Jasco Inc., Tokyo, Japan) equipped with a 60 mm integrating sphere while using BaSO4 as a reference. Photoluminescence spectra (PL) were collected using a spectrophotometer (FS5 fluorescence, Edinburg, United Kingdom) with an excitation wavelength of 375 nm. Electrochemical impedance spectroscopy (EIS) was performed using a three-electrode workstation (VSP Potentiostat, Biologic, Seyssinet-Pariset, France). A pt square plate (1 × 2 cm2) and a standard calomel were used as counter and reference electrodes, respectively. A clean fluorine-doped tin oxide (FTO) glass with an active surface area of 0.8 cm2 was used as a substrate for the working electrode, whereas an aqueous solution of 0.5 M Na2SO4 (80 mL) was used as the electrolyte. In the preparation of the working electrode, 0.1 mg of photocatalyst was added to 1 mL of DMF solution and sonicated for 1 h. Then, 50 μL of the dispersed solution was drop-casted on FTO and annealed at 160 °C for 1 h, which was further used in the electrochemical investigation.

3.5. Photocatalytic Activity Measurements

The photocatalytic characteristics of the samples were examined using a visible light source with a 300 W Xenon lamp (1000 W/m2) (CEL-HXF300, CEAULIGHT Co., Beijing, China) equipped with a UV-IR cutoff filter (420 nm > λ > 780 nm). A double wall jacket beaker with a surface area of 80 cm2 connected to a water chiller was used to perform the photocatalytic degradation measurement of the photocatalysts. The height from the surface of the pollutant solution to the light source was kept at 30 cm. In this study, RhB dye and BPA colorless pollutants were used to evaluate the degradation efficiency of the synthesized photocatalysts. Briefly, 20 and 40 mg of photocatalyst was used to degrade 40 mL of RhB dye (20 ppm) and BPA colorless pollutant (10 ppm), respectively. Before initiating the photocatalytic experiment, the RhB and BPA aqueous solutions were stirred for 60 min and 30 min, respectively, in dark conditions to attain an adsorption–desorption equilibrium of the photocatalysts. To investigate the photocatalytic degradation rate, 2 mL solution was taken from the RhB or BPA solution after a specific interval, and this solution was then centrifuged for 3 min at 5000 rpm to separate the photocatalyst, after which the absorbance spectrum of the supernatant using UV-visible spectrophotometer was measured. Further, radical trapping experiments were conducted to determine the dominant radical species involved in the photocatalytic decomposition of RhB and BPA. IPA, BQ, and KI of 2 mmol were used as trapping reagents to explore the active species, such as, •O2−, •OH radicals, and h+, respectively.

4. Conclusions

In this work, we successfully developed a 3D hierarchical BOC microsphere and a 0D/3D-Co3O4/BOC heterojunction photocatalyst composed of BOC decorated with Co3O4 NPs using a simple solvothermal synthesis method. The developed heterostructure showed a conventional type II charge transport phenomenon across Co3O4 and BOC-3 by forming a p-n heterojunction. The 0D/3D hierarchical morphology of the Co3O4/BOC could increase active sites because of its high surface area, suppressing e−-h+ recombination, and improving visible light absorption. The results of mechanistic studies have proven that the generation of an IEF from BOC-3 to Co3O4 led to a p-n junction and the formation of a type II heterojunction. Thus, benefiting from the above properties, the Co3O4/BOC sample demonstrated a higher reaction rate and a higher degradation efficiency than bare Co3O4 and BOC during RhB and BPA degradation. Conclusively, our developed photocatalyst should be considered a good candidate for pollutant degradation.

Author Contributions

S.T.U.D. contributed to the main idea and the writing of the original draft. W.-F.X. contributed to the writing—review and editing. W.Y. contributed to the writing—review and editing, supervision, project management, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022R1F1A1074441 and 2021H1D3A2A01100019).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

We also prepared BOC-3 in different solvothermal reaction times to explore the formation mechanism of the BOC microspherical morphology. Figure A1 represents the SEM images of the BOC samples prepared with solvothermal reaction times of 3, 6, 9, 12, and 18 h. The formation of a sphere-like morphology composed of ultrathin nanosheets could be observed after 3 h of solvothermal reaction (Figure A1a). When the solvothermal reaction time increased to 6 h, the nanosheets grew further, thus increasing the diameter of the microspheres with clear visibility of ultrathin nanosheets (Figure A1b). When the reaction time was increased to 9 h, the microspheres were covered with external BOC nanosheets, and the nanosheets did not grow further (Figure A1c). The BOC nanosheets were covered with BOC nanosheets after increasing the hydrothermal reaction time to 12 h and 18 h, respectively (Figure A1d,e). These results indicate that the formation of BOC microspheres quickly underwent nucleation, growth, and self-assembly.
Based on the above results, the formation of the BOC samples in the different solvent ratios proceeded as illustrated in Scheme 1. It could be concluded that Bi(NO3)3·5H2O will be hydrolyzed into [Bi2O2]2+ after dissolution in EtOH. Secondly, Cl− ions generated from KCl in EG solution could react with [Bi2O2]2+ to form Cl–Bi–O–Bi–Cl nuclei through the coulomb force during the solvothermal reaction [49], thus resulting in homogenous nucleation. As the reaction proceeded in the first 3 h of the solvothermal reaction time at 160 °C, the BOC nanosheets formed. These nanosheets formations could potentially have occurred due to the bonding of the OH functional group to Bi3+ ions in Cl–Bi–O–Bi–Cl complex, which grew vertically along the c axis due to its intrinsic crystal structure. Since BOC has a known silane-type structure with a space group of P4/nmm and lattice constants of a = 5.4 Å and c = 35.20 Å, which are related to √2as × √2bs × cs supercell, its layered structure was constructed through the combination of the 6-fold metal-oxygen (Bi-O) layer separated by the Cl layer [50]. Therefore, the formation of the nanosheets and their regulation with the OH functional group were considered to be favorable [51]. Thirdly, as the reaction progressed, the obtained nanosheets were assembled into microspheres, which could decrease the surface energy and achieve a stable structure. Finally, the hierarchical microspheres grew further and the size of the samples gradually increased.
Ijms 23 15028 g0a1 550
Figure A1. SEM images of BOC samples prepared after (a) 3, (b) 6, (c) 9, (d) 12, and (e) 18 h of solvothermal reaction time.
Figure A1. SEM images of BOC samples prepared after (a) 3, (b) 6, (c) 9, (d) 12, and (e) 18 h of solvothermal reaction time.
Ijms 23 15028 g0a1

Appendix B

Ijms 23 15028 g0a2 550
Figure A2. EDX spectrum of 20-Co3O4/BOC, and its elemental and atomic wt.% in the inset table.
Figure A2. EDX spectrum of 20-Co3O4/BOC, and its elemental and atomic wt.% in the inset table.
Ijms 23 15028 g0a2

Appendix C

Ijms 23 15028 g0a3 550
Figure A3. XPS survey spectra (a), core level spectra of (b) Bi 4f, (c) O 1s, (d) Cl 2p, and (e) Co 2p spectra for the prepared 20-Co3O4/BOC samples.
Figure A3. XPS survey spectra (a), core level spectra of (b) Bi 4f, (c) O 1s, (d) Cl 2p, and (e) Co 2p spectra for the prepared 20-Co3O4/BOC samples.
Ijms 23 15028 g0a3

Appendix D

Ijms 23 15028 g0a4 550
Figure A4. Cyclic photocatalytic degradation of (a) RhB and (b) BPA in the presence of 20-Co3O4/BOC sample.
Figure A4. Cyclic photocatalytic degradation of (a) RhB and (b) BPA in the presence of 20-Co3O4/BOC sample.
Ijms 23 15028 g0a4

Appendix E

Table
Table A1. Comparison of photocatalytic degradation of RhB in the presence of pure BOC and BOC composite sample with recently reported results.
Table A1. Comparison of photocatalytic degradation of RhB in the presence of pure BOC and BOC composite sample with recently reported results.
PhotocatalystRate Constant (min−1)Degradation Efficiency (%)Degradation Time (min)Lamp (C)Amount of Catalyst (mg) Solution Volume (mL)Solution Concentration (mg/L)Ref
Bi12O17Cl20.0115 min−183.5%140300 W Xenon lamp, (420–720 nm)204020This work
Co3O4/Bi12O17Cl20.021 min−197.4%
Bi12O17Cl21.93 × 10−3 min−140%240300 W Xenon lamp, (420–720 nm)408015[52]
Ag/Bi12O17Cl210.3 × 10−3 min−193%
Bi12O17Cl21.16 × 10−2 min−183%150300 W Xenon lamp, (400–780 nm)808020[53]
Bi-Bi12O17Cl22.19 × 10−2 min−199%
Bi12O17Cl20.074 min−1--20300 W Xenon lamp, (400 nm)105010[54]
Fe(III)-modified Bi12O17Cl20.157 min−1--
Bi12O17Cl20.0613 min−178%20300 W Xenon lamp, (400 nm)305010[23]
Graphene/Bi12O17Cl20.160 min−1--
Bi12O17Cl20.102 min−183%20300 W Xenon lamp, (400 nm)30505[55]
2D/2D g-C3N4/Bi12O17Cl20.353 min−199%
ZnO/NiO0.019 min−1--120320 mW/cm2. LED light505010[56]
g-C3N41.11 min−199%90 150 watt halogen lamp501005[57]
Carbon dot implanted g-C3N40.48 min−1--80300 W Xenon lamp, (320–780 nm) 103015[58]
BiVO4-Ni/AgVO30.133 min−1--30300 W Xenon lamp, (400 nm)305010[59]
Fe-BiOBr + H2O20.0646 min−198.23%60350 W xenon lamp (420 nm)305020[60]
SrSnO3/g-C3N4k1: 0.0083−1
k2: 0.0348−1		97.3250Sun Light1001005[61]

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Figure 1. FESEM images of (a) BOC-1, (b) BOC-2, and (c) BOC-3; insets show the respective SEM image with high resolution. (d) N2 adsorption–desorption isotherms of all samples.
Figure 1. FESEM images of (a) BOC-1, (b) BOC-2, and (c) BOC-3; insets show the respective SEM image with high resolution. (d) N2 adsorption–desorption isotherms of all samples.
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Figure 2. (a) FESEM image, (b) HRTEM image, (c) HAADF image, (d) EDX mapping, and (e) elemental EDX mapping of 20-Co3O4/BOC sample, (f) N2 adsorption-desorption isotherm of 20-Co3O4/BOC, (g) XRD patterns, and (h) Raman spectra of Co3O4, BOC-3, and 20-Co3O4/BOC micro flowers.
Figure 2. (a) FESEM image, (b) HRTEM image, (c) HAADF image, (d) EDX mapping, and (e) elemental EDX mapping of 20-Co3O4/BOC sample, (f) N2 adsorption-desorption isotherm of 20-Co3O4/BOC, (g) XRD patterns, and (h) Raman spectra of Co3O4, BOC-3, and 20-Co3O4/BOC micro flowers.
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Figure 3. (a) Photocatalytic degradation curves of RhB, (b) its degradation efficiency with reaction rate constant, in the presence of BOC-1, BOC-3, 10-Co3O4/BOC, 20-Co3O4/BOC, and 40-Co3O4/BOC, (c) photocatalytic degradation of BPA, and (d) its degradation efficiency with reaction rate constant in the presence of BOC-1, BOC-2, BOC-3, and 20-Co3O4/BOC.
Figure 3. (a) Photocatalytic degradation curves of RhB, (b) its degradation efficiency with reaction rate constant, in the presence of BOC-1, BOC-3, 10-Co3O4/BOC, 20-Co3O4/BOC, and 40-Co3O4/BOC, (c) photocatalytic degradation of BPA, and (d) its degradation efficiency with reaction rate constant in the presence of BOC-1, BOC-2, BOC-3, and 20-Co3O4/BOC.
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Figure 4. (a) Photoluminescence spectra, (b) Transient photocurrent response, (c) EIS Nyquist plots of BOC-3 and 20-Co3O4/BOC samples, and (d) UV–Vis diffuse reflectance spectra of Co3O4, BOC-3, 20-Co3O4/BOC, and 40-Co3O4/BOC.
Figure 4. (a) Photoluminescence spectra, (b) Transient photocurrent response, (c) EIS Nyquist plots of BOC-3 and 20-Co3O4/BOC samples, and (d) UV–Vis diffuse reflectance spectra of Co3O4, BOC-3, 20-Co3O4/BOC, and 40-Co3O4/BOC.
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Figure 5. (a) Tauc plots, (b,c) Mott–Schottky plots, and (d) valence band XPS of Co3O4 and BOC-3.
Figure 5. (a) Tauc plots, (b,c) Mott–Schottky plots, and (d) valence band XPS of Co3O4 and BOC-3.
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Figure 6. (a) Energy-level diagrams of Co3O4 and BOC and (b) interfacial charge transfer behavior and redox reaction process in 20-Co3O4/BOC heterostructure under visible-light irradiation.
Figure 6. (a) Energy-level diagrams of Co3O4 and BOC and (b) interfacial charge transfer behavior and redox reaction process in 20-Co3O4/BOC heterostructure under visible-light irradiation.
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Figure 7. Active radicals detection in the presence of various scavengers during RhB degradation.
Figure 7. Active radicals detection in the presence of various scavengers during RhB degradation.
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Scheme 1. Synthesis of (a) nanoflower-like BOC, and (b) 0D/3D Co3O4/BOC hierarchical micro flower.
Scheme 1. Synthesis of (a) nanoflower-like BOC, and (b) 0D/3D Co3O4/BOC hierarchical micro flower.
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Din, S.T.U.; Xie, W.-F.; Yang, W. Synthesis of Co3O4 Nanoparticles-Decorated Bi12O17Cl2 Hierarchical Microspheres for Enhanced Photocatalytic Degradation of RhB and BPA. Int. J. Mol. Sci. 2022, 23, 15028. https://doi.org/10.3390/ijms232315028

AMA Style

Din STU, Xie W-F, Yang W. Synthesis of Co3O4 Nanoparticles-Decorated Bi12O17Cl2 Hierarchical Microspheres for Enhanced Photocatalytic Degradation of RhB and BPA. International Journal of Molecular Sciences. 2022; 23(23):15028. https://doi.org/10.3390/ijms232315028

Chicago/Turabian Style

Din, Syed Taj Ud, Wan-Feng Xie, and Woochul Yang. 2022. "Synthesis of Co3O4 Nanoparticles-Decorated Bi12O17Cl2 Hierarchical Microspheres for Enhanced Photocatalytic Degradation of RhB and BPA" International Journal of Molecular Sciences 23, no. 23: 15028. https://doi.org/10.3390/ijms232315028

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