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. 2023 Jul 13;8(29):26055–26064. doi: 10.1021/acsomega.3c02153

Controllable Crystal Growth and Improved Photocatalytic Activity of Porous Bi2O3–Bi2S3 Composite Sheets

Yuan-Chang Liang 1,*, Yu-Hsun Chou 1, Bo-Yue Chen 1, Wei-Yang Sun 1
PMCID: PMC10373473  PMID: 37521655

Abstract

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Porous Bi2O3–Bi2S3 composite sheets were constructed through a combinational methodology of chemical bath deposition and hydrothermal reaction. The Na2S precursor concentration in the hydrothermal solution was varied to understand the correlation between the vulcanization degree and structure evolution of the porous Bi2O3–Bi2S3 composite sheets. The control of the etching rate of the Bi2O3 sheet template and the regrowth rate of Bi2S3 crystallites via suitable sulfide precursor concentration during the hydrothermal reaction utilizes the formation of porous Bi2O3–Bi2S3 sheets. Due to the presence of Bi2S3 crystallites and porous structure in the Bi2O3–Bi2S3 composites, the improved visible-light absorption ability and separation efficiency of photogenerated charge carriers are achieved. Furthermore, the as-synthesized Bi2O3–Bi2S3 composite sheets obtained from vulcanization with a 0.01M Na2S precursor display highly enhanced photocatalytic degradation toward methyl orange (MO) dyes compared with the pristine Bi2O3 and Bi2S3. The porous Bi2O3–Bi2S3 sheet system shows high surface active sites, fast transfer, high-efficiency separation of photoinduced charge carriers, and enhanced redox capacity concerning their constituent counterparts. This study affords a promising approach to constructing Bi2O3-based Z-scheme composites with a suitable microstructure and Bi2O3/Bi2S3 phase ratio for photoactive device applications.

Introduction

Satisfactorily, semiconductor-based photocatalysis is an eco-friendly and efficient technology to tackle the problem, which utilizes renewable solar energy to remove the organic dyes without any secondary pollutants. Bi2S3 and Bi2O3 are two important semiconductors extensively investigated for scientific applications. Bi2S3 is a narrow bandgap semiconductor with great promise in numerous photosensitive device applications.1−5 Due to unique shape- and size-dependent physical and chemical properties, Bi2S3 crystals with a large growth scale and different morphologies have been synthesized through various chemical routes.1−3 Moreover, Bi2S3 absorbs visible light and can be used as a visible-light photocatalyst.6,7 However, due to the facile recombination of photogenerated electrons and holes, the photocatalytic performance of Bi2S3 crystals is still highly desirable for substantial improvement. In addition, Bi2O3 has various phases and is also a visible light-sensitive semiconductor.8−10 It has been widely used in various photosensitive device applications due to its stable chemical nature, various morphologies, and diverse synthesis routes.11−14

Although Bi2S3 and Bi2O3 semiconductors have superior light harvesting abilities because of their narrow bandgap energies, their photoinduced charge separation efficiency under irradiation still needs marked improvement to make them suitable for photosensitive device applications with high performance. One promising strategy to improve the photoinduced charge separation efficiency in a single photosensitizer component is to couple it with another semiconductor to construct a suitable band alignment and enhance photoactive performance. It has been shown that Bi2S3 coupled with TiO2 or Bi2O2CO3 to form Bi2S3–TiO2 and Bi2S3–Bi2O2CO3 heterogeneous systems demonstrates enhanced photoactive performance for photosensitive device applications.15,16 Furthermore, Bi2O3-based heterogeneous systems such as Bi2O3–BiOCl, CuBi2O4–Bi2O3, and TiO2–Bi2O3 all demonstrate improved photoactive performance compared with their single constituent counterparts.11,12,17 Based on the above discussion, it might be interesting to integrate Bi2S3 with Bi2O3 to form a heterogeneous system and investigate the photoactive performance of this hybrid system. A well-defined Bi2O3–Bi2S3 composite has been synthesized using an ultrasonic-assisted synthetic method on a conductive substrate, and the resulting Bi2O3–Bi2S3 composite shows highly enhanced photoelectrochemical activity.18 Branched Bi2O3–Bi2S3 composites synthesized by a simple hydrothermal method without toxic substances demonstrate high photocatalytic activity under visible-light illumination.19 These examples present the feasibility of constructing a Bi2S3–Bi2O3 heterogeneous system for a photoactive device with improved efficiency.

Notably, looking into the synthesis routes developed for generating Bi2S3-based composites, shortcomings such as the use of harmful solvents and additives and the need for complex synthesis processes are generally involved.16 It has been shown that Bi2O3 is a material with a high tendency for sulfurization.20 This study will develop the vulcanization process to form Bi2O3–Bi2S3 sheet composites to avoid the generally involved shortcoming of using harmful solvents and additives during material synthesis. The hydrothermally induced vulcanization reaction on a chemical bath deposition (CBD)-derived Bi2O3 sheet template is adopted to prepare Bi2O3–Bi2S3 sheet composites. This synthesis methodology of Bi2O3–Bi2S3 sheet composites is environment-friendly because hydrothermally induced vulcanization uses no toxic solvents or surfactants. Meanwhile, the initially porous Bi2O3 sheet template for growing Bi2O3–Bi2S3 sheet composites has the advantage of large surface active sites and improves photoactive efficiency.8 The photosensitive property of the as-prepared Bi2O3–Bi2S3 composites was studied, and they displayed enhanced photosensitivity compared with the pristine Bi2O3 and Bi2S3. This excellent photoactive performance benefits from the increased light absorption, matched energy band alignment of Bi2O3/Bi2S3, and improved separation and transfer efficiency of photogenerated electron–hole pairs. The exact correlation between the vulcanization process-dependent microstructures and the photoactive performance of the Bi2S3–Bi2O3 sheet composites is proposed in this study.

Experiments

The Bi2O3–Bi2S3 composites were prepared by a two-step synthesis method. The first step is to prepare a porous Bi2O3 sheet template using chemical bath deposition (CBD). According to previous work, the porous Bi2O3 sheet films were grown on F-doped tin oxide (FTO) glasses by the CBD method.8 The 0.1 M bismuth nitrate was dissolved in deionized water to make a 50 mL stock solution. Furthermore, 3 mL of triethanolamine (TEA) and 0.2 M NaOH were added to the solution above to form a highly alkaline solution with a pH = 10. The CBD reaction was conducted at 65 °C for 2 h. After cooling to room temperature naturally, the FTO substrate with a layer of whitish film was washed with deionized water several times. Finally, the substrate was annealed at 300 °C for 1 h in air ambient to obtain porous Bi2O3 sheet films. Furthermore, the Bi2O3–Bi2S3 composites were prepared via a hydrothermal route. First, 0.01, 0.1, and 1 M Na2S precursor solutions with 12.5 mL were transferred into Teflon-lined stainless-steel autoclaves. The as-prepared porous Bi2O3 sheet films were placed into the autoclave with the film side facing down. The hydrothermal reaction was conducted at 160 °C for 3 h. The final products were collected and washed with deionized water several times and dried in air at 60 °C. The samples prepared at 0.01, 0.1, and 1 M Na2S precursor solutions are coded S1, S2, and S3 in this study.

The crystal structure of pristine Bi2O3, S1, S2, and S3 was determined by X-ray diffraction (XRD; Bruker D2 PHASER) with monochromatic Cu-Kα radiation in the two theta ranges of 20–60°. Field emission scanning electron microscopy (SEM; S-4800 Hitachi) investigates the surface morphology of prepared samples. High-resolution transmission electron microscopy (HRTEM; Philips Tecnai F20 G2) was operated at 200 kV to investigate the microstructure of samples. X-ray photoelectron spectroscopy (XPS ULVAC-PHI, PHI 5000 VersaProbe) recorded the elemental binding states of samples with argon ion sputtering to remove surface contamination. The etching depth herein is approximately 20 nm. The absorption spectra of samples were obtained using a UV–vis spectrophotometer (JASCO V750). Photoelectrochemical (PEC) and electrochemical impedance spectroscopy (EIS) properties of samples were investigated using the potentiostat (SP150, BioLogic). Herein, the as-prepared sample was used as the working electrode with an active area of 1 cm2. Meanwhile, a saturated Ag/AgCl (in saturated KCl) electrode and a Pt wire served as the reference and counter electrodes, respectively. An aqueous solution of 0.5 M Na2SO4 was used as the electrolyte. A 100 W Xe lamp was used as the illumination source, and the pure visible light was obtained using a 420 nm cutoff filter. The photodegradation experiment used the 10 mL methyl orange (MO) solution (5 × 10–5 M) as the target dye solution. The dark absorption–desorption equilibrium was conducted for 60 min. A 100 W Xe lamp with a 420 nm UV light cutoff filter was used as the visible light source. After the light irradiation, the MO solution with different photocatalytic reaction durations (0, 15, 30, 45, and 60 min) was taken out by the syringe for analyzing the residual concentration with a UV–vis spectrophotometer.

Results and Discussion

Figure 1a shows that the vertically aligned and uniform sheet-like Bi2O3 array was grown on the FTO substrate. The diameter of the sheets is approximately 2.5–3.5 μm. Moreover, great tiny pores were found in the Bi2O3 sheets. Figure 1b displays the SEM image of S1. After 0.01 M vulcanization of the Bi2O3 sheet template, S1 still exhibited a sheet-like morphology similar to the pristine Bi2O3 but with a substantially decreased size. Notably, the bamboo-like crystallites cross-linked with each other to form the sheet morphology, and pores existed between them. When the concentration of vulcanization was increased to 0.1 M, the size of the sheet structure of S2 increased, as revealed in Figure 1c. Notably, numerous tiny pores are exhibited on the S2 surface. The structure became looser in comparison with that of S1. Figure 1d presents the highest sulfurization concentration (1 M), making S3 display a significant sheet size change compared to S1 and S2. The sheet size of S3 was markedly large. Furthermore, no pores were observed on the S3 surface, which was relatively smooth. It has been shown that pristine Bi2O3 sheets will be etched, and regrowth of Bi2S3 occurs under enough sodium sulfide vulcanization reaction.19 Less S2– ions are expected at a low reaction concentration of 0.01 M; therefore, small and solid Bi2S3 crystallites will be formed after etching partial parent Bi2O3 sheets. The as-formed Bi2S3 oval crystallites are connected to form S1. Sulfur ions replaced the surface oxygen ions of the Bi2O3 template because of ion exchange during the vulcanization process. The etching rate of the Bi2O3 sheets and the regrowth rate of Bi2S3 crystallites competed during the hydrothermal reaction.20 As the Na2S concentration increased, both the etching rate of the Bi2O3 and the regrowth rate of the Bi2S3 increased. The Bi2S3 crystallites will produce more and form a larger sheet structure of S2 under the reaction condition. Finally, the parent Bi2O3 sheets will be fully etched at 1 M vulcanization conditions and wholly converted into Bi2S3 crystals. The sufficient Bi2S3 crystallites are densely contacted to form the solid, smooth, and sizeable Bi2S3 sheet structure of S3.

Figure 1.

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SEM micrographs of various sheets: (a) Bi2O3. (b) S1. (c) S2. (d) S3.

Figure 2a presents the XRD pattern of the Bi2O3 sheet template. Except for the diffraction peaks of the FTO substrate, the characteristic peaks of tetragonal-structured β-Bi2O3 are indexed (JCPDS 27-0050). The XRD result shows that CBD successfully synthesized the high-purity Bi2O3 phase, and the crystal growth plane of Bi2O3 was mainly (201) orientation. The XRD patterns of S1 and S2 are shown in Figure 2b,c, respectively. The sharp Bragg reflection peaks in the XRD patterns indicate that the as-synthesized products comprise the orthorhombic Bi2S3 phase (JCPDS 17-0320) and the tetragonal Bi2O3 phase. The XRD patterns reveal that the Bi2O3–Bi2S3 composite was successfully synthesized through the vulcanization method. Comparatively, in Figure 2b,c, it is found that when the Na2S concentration was increased, the Bragg reflection intensity of the Bi2O3 phase markedly decreased, and the Bragg reflection intensity of the Bi2S3 phase increased. This reveals an increased Bi2S3 phase content in the Bi2O3–Bi2S3 composite with an increased Na2S concentration during vulcanization. By contrast, after the vulcanization concentration is increased to 1 M, the XRD pattern of S3 in Figure 2d does not show the characteristic peak of the Bi2O3 phase, indicating the complete consumption of the Bi2O3 sheet template to form Bi2S3 crystals under this vulcanization condition.

Figure 2.

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XRD patterns: (a) Bi2O3. (b) S1. (c) S2. (d)S3. BO and BS denote Bi2O3 and Bi2S3, respectively.

Figure 3a,b shows morphological images of S1. The S1 sheets are formed by numerous oval crystallites linked to form a sheet structure, and abundant nanopores exist in the sheet structure. The diameter of the sheet is approximately 0.6–0.8 μm. The HRTEM images in Figure 3c,d show that the ordered lattice fringes with spacings of 0.25 nm and 0.32 nm are attributed to the interplanar distances of Bi2S3 (240) and Bi2O3 (201), respectively. The HRTEM images show powerful proof of the composite structure of Bi2O3 and Bi2S3 in S1. Figure 3e displays the selected area electron diffraction (SAED) of the randomly selected area of the sheet. The pattern exhibited sharp and bright spots arranged in several centric rings. The SAED pattern feature indicates the polycrystalline nature of the synthesized S1. According to the SAED pattern, the (201), (220), and (222) crystallographic planes were determined to correspond to the tetragonal Bi2O3 phase. The (220), (101), (130), (211), (240), (141), and (421) crystallographic planes were determined to correspond to the orthorhombic Bi2S3 phase. In addition, the line-scan energy dispersive X-ray spectroscopy (EDS) spectra profiles (Figure 3f) of the S1 sheet display that the Bi, O, and S elements are well distributed in the product. The O is concentrated in the center area, Bi is distributed equally on the whole sheet, and S is mainly distributed in the peripheral areas. The elemental distribution results confirm that both the etching and regrowth processes started from the periphery of the parent Bi2O3 and reveal that the Bi2O3–Bi2S3 composite was formed because of an insufficient vulcanization process that results in residual Bi2O3 crystals in S1.

Figure 3.

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TEM analysis of S1: (a,b) low-magnification images. (c,d) HRTEM images. (e) SAED pattern. (f) EDS line-scan profiles of the sample.

The representative low-magnification TEM images in Figure S1a,b also confirm that S2 has a sheet-like structure and great tiny pores on the sheet. The diameter of the sheet can reach approximately 1.3–1.6 μm. The TEM images herein are consistent with the SEM inspections. Figure S1c,d shows the local HRTEM micrographs of positions c and d marked in Figure S1b, respectively. Two different lattice fringes with spacings of approximately 0.25 and 0.32 nm could be resolved in Figure S1c,d, which agree well with the interplanar distances of Bi2S3 (240) and Bi2O3 (201), respectively. In addition, the heterogeneous Bi2O3–Bi2S3 sheet feature can be recognized by the grayscale contrast in the HRTEM images, in which Bi2O3 shows darker contrast while the Bi2S3 shows brighter contrast. Figure S1e shows the SAED pattern taken from Figure S1a; the pattern feature reveals the polycrystalline nature of S2 and the coexistence of the Bi2O3 and Bi2S3 phases. The EDS line scan profiles in Figure S1f display that Bi is evenly distributed on the entire sheet, while S is mainly distributed in the peripheral area. Notably, the O element is concentrated in the small central area. This result confirms the formation of a Bi2O3–Bi2S3 heterostructure of S2.

Figure 4a presents a morphological image of S3. Compared with S1 and S2, S3 has a denser and solid sheet structure. The diameter of the S3 sheet is approximately 2.0–2.4 μm and is markedly more prominent than S1 and S2. Figure 4b,c shows HRTEM images of different regions in Figure 4a. The lattice fringes with multiple orientations are observed in the HR images, which reveal the polycrystalline feature of the sheet structure. The lattice fringe distances of 0.25 nm and 0.35 nm corresponded to the interplanar spacings of Bi2S3 (240) and (130), respectively. Figure 4d shows the SAED pattern of the sheet structure in Figure 4a, and the pattern exhibits several diffraction rings consisting of sharp and bright spots that originated from various Bi2S3 crystallographic planes. Furthermore, the chemical composition of S3 was characterized by the EDS spectrum (Figure 4e). Figure 4e shows that only Bi and S are the primary constituent elements of S3, and the C and Cu signals come from the TEM grid. Further quantitative analysis demonstrates that the molar ratio of Bi and S is close to 2:3, in good agreement with the stoichiometric composition of Bi2S3. The TEM analysis reveals that S3 is in a pure Bi2S3 phase.

Figure 4.

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TEM analysis of S3: (a) low-magnification image. (b,c) HRTEM images. (d) SAED pattern. (e) EDS spectrum of the sample.

When the Bi2O3 sheet template was vulcanized with a 0.01 M Na2S precursor, the S 2p signal appeared between the Bi 4f5/2 and Bi4f7/2 peaks, as revealed in Figure S2a. The appearance of the S2p peak presents the formation of the sulfide phase through vulcanizing the Bi2O3 layer in the product, in which the S2p peak feature is not shown between the Bi 4f core-level peaks of the Bi2O3 sheet template.8 The binding energies of Bi 4f core-level peaks for S1 present the characteristic binding state of Bi3+ in the Bi2S3.21 Furthermore, the symmetric Bi 4f peaks reveal no metallic Bi appearing in the product after the vulcanization process. Figure S2b presents the O 1s spectrum of S1, and the appearance of the O 1s peak demonstrates that the product contains residual Bi2O3 phase, and the binding energies of the deconvoluted subpeaks in the O 1s spectrum show the bindings of lattice oxygen and adsorption oxygen species in the product.8Figure S2c displays the Bi4f XPS spectrum of S3; a more intense S 2p signal was observed than S1. Furthermore, no O 1s signal was detected, supporting the formation of pure Bi2S3 crystals, as observed in previous structural analysis results.

The optical absorption spectra of Bi2O3, S1, S2, and S3 are shown in Figure 5. The absorption edge of the pristine Bi2O3 sheet template is located at around 542 nm, which is consistent with the inherent band gap absorption of tetragonal β-Bi2O3.22 Furthermore, the light absorption range was appreciably widened and red-shifted for S1, S2, and S3. From the aforementioned structural analysis, S3 is a pure Bi2S3 phase, and its bandgap energy is evaluated to be approximately 1.37 eV in Figure S3 according to the plot of the modified Kubelka–Munk function versus the photon energy.8 This bandgap energy is close to the previously reported value of Bi2S3 nanorods.23 The light absorption result demonstrates that when the heterogeneous Bi2O3–Bi2S3 was formed in S1 and S2, a substantial red-shift of the absorption edge occurred concerning the pristine Bi2O3. The Bi2O3–Bi2S3 composites could utilize more light and produce photogenerated charge carriers than the initial Bi2O3 sheet template.

Figure 5.

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Absorbance spectra of various samples.

Figure 6a shows the transient photocurrent response of various samples under chopped illumination. Upon illumination, the photocurrent density of the samples increases rapidly, and the photocurrent density decreases instantaneously with the removal of illumination. The photocurrent density of each sample remains stable after cycling tests. The higher the photocurrent density is, the more efficient the separation efficiency of photoinduced charge carriers is, and then S1 is supposed to exhibit more efficient photoactive performance. When the light is turned on, Bi2O3, S1, S2, and S3 all show different degrees of spike-like transient response associated with recombining the photoinduced carriers at the surface states of semiconductors.24 The order of photocurrent intensity is as follows: S1 > S2 > Bi2O3 > S3. S1 shows the highest photocurrent density (0.14 mA/cm2) among all the samples. Furthermore, the Bi2O3–Bi2S3 composites (S1 and S2) show a higher photoresponse than the pristine Bi2O3 and Bi2S3, which is powerful evidence to verify the excellent photoresponse in the heterogeneous structure. S1 has a higher photoresponse performance than that of S2 herein can be attributed to several reasons. First, the photogenerated carriers are effectively separated due to Bi2O3 being maintained as the main body in S1. Although Bi2S3(S3) shows a wider absorption than Bi2O3, its photocurrent intensity is lower than that of Bi2O3. This might be due to the narrower band gap of Bi2S3(S3), which makes the photogenerated electrons and holes easy to recombine.19 Second, S1 and S2 are in different morphologies. The markedly smaller sheet size and abundant pores in S1 could lead it to exhibit more surface active sites than S2, which can be characterized by electrochemically active surface area (ECSA). The cyclic voltammetry curves of S1 and S2 at various scanning rates (0.1, 0.2, 0.3, 0.4, and 0.5 V/s) are recorded in Figure 6b,c, respectively. The electric double-layer capacitance value (Cdl) has been used to measure the ECSA, as it reflects the intrinsic activity of the catalyst. There is a linear relationship between the ECSA size and Cdl.25 The Cdl values from cyclic voltammetry curves at non-faradaic potential regions (−0.2 to 0.2 V) can be extracted by plotting the Δj = ja – jc at the middle potential against the scan rate, where ja is the anode current and jc is the cathode current. Cdl = Δj/2ν, and ν is the scan rate in mV s–1. The extracted slopes of Δj vs scan rate plot allow a comparison of the ECSA between S1 and S2.26 In Figure 6d, the Cdl of S1 is 0.041 mF cm–2, nearly 1.2 times larger than that of S2 (0.034 mF cm–2). The work of open-ended Fe2O3@CuO nanotubes with high porosity ensures sufficient electrolyte diffusion, revealing more active sites that could be exposed in the reaction environment.27 A larger ECSA of S1 than that of S2 was visibly displayed. Such a larger ECSA exposes the sample to more active sites, maintaining the sufficient electrochemical reaction between the S1 and electrolyte ions and thus delivering high photo-/electrocatalytic performance.28 The positive contribution of ECSA size to the PEC performance in other materials systems supports the observed results in this study.29

Figure 6.

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(a) Transient photocurrent density versus time curves of various samples under chopped irradiation at 1.5 V. CVs measured in the non-faradaic region of −0.20 to 0.20 V vs NHE with different scan rates: (b) S1. (c) S2. (d) Scan rate-dependent current density plots of S1 and S2.

EIS investigated the samples’ carrier transport and recombination dynamics under light illumination. Figure 7a shows the Nyquist plots of various samples. The arc radius size of Nyquist plots is proportional to the charge-transfer resistance of semiconductors.30 In Figure 7a, the pristine Bi2O3 and Bi2S3 (S3) have a larger semicircle radius compared to the heterogeneous samples (S1 and S2). Moreover, S1 shows the smallest radius, implying the minimum electron-transfer resistance and the fastest interface electron-transfer rate among various samples. Figure 7b exhibits the possible equivalent circuits for a quantitative analysis of the interfacial charge-transfer ability of the S1 and S2 composites. As the illustrations show, the intercept of the semicircle in the high-frequency region with a real axis symbolizes the solution resistance Rs. It depends on the electrolyte’s concentration and conductivity.31 C is a typical double-layer capacitance for the electrode surface.32 Rct is the electron-transfer resistance, and it can be estimated through the fitting of arc radii of the Nyquist curves. Moreover, R (Pt) and C (Pt) are the resistance and ordinary double-layer capacitor of the Pt counter electrode, respectively.33 The Rct values of S1 and S2 are estimated to be approximately 297 and 541 Ω, respectively. For comparison, the Rct values of the pristine Bi2O3 (1152 Ω) and S3 (1341 Ω) are also shown herein. The interfacial charge-transfer resistance was markedly reduced in the Bi2O3–Bi2S3 hybrid system. In conclusion, S1 exhibits fast interfacial charge transfer and practical carrier separation ability, which reduces the interfacial resistance, proving that the Bi2O3–Bi2S3 heterostructure with a suitable composition phase ratio helps to separate the photoexcited electron–hole pairs.

Figure 7.

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(a) Nyquist plots of various samples under light irradiation. (b) Possible equivalent circuit used for Rct evaluation of composite samples.

Figure 8 shows the Mott–Schottky (M–S) plots of S1, S2, and S3. Flat band potentials of all samples were calculated based on the intercept by extrapolating the linear part of the M–S plots to the x-axis. From Figure 8a–c, the flat band potential values of S1, S2, and S3 are approximately 0.29, 0.35, and −0.67 V, respectively (V vs NHE). The M–S curve benefits the determination of the position of the conduction band (CB). However, S1 and S2 are heterogeneous structures herein; their heterogeneous band alignment cannot be known directly from the M–S plots. By contrast, S3 is a pure Bi2S3 phase; the CB position can be evaluated from the flat band potential in Figure 8c. In general, the flat band potential of n-type semiconductors is about 0.1 V below the CB.34 Hence, the CB position (ECB) values of Bi2S3 herein are estimated to be approximately −0.57 V; this value agrees with the reported value.35 Considering the aforementioned calculated bandgap energy of S3, the valence band energy (EVB) of Bi2S3 is estimated to be approximately 0.80 V, based on Eg = EVB – ECB. For S1 and S2, the heterogeneous band alignment can be constructed from understanding the CB and VB positions of the Bi2O3 and Bi2S3 phases. The CB and VB positions of the Bi2O3 phase under similar synthesis conditions have been reported elsewhere.8 Furthermore, the density of charge carriers among various samples can be compared from the slope of the M-S plot. According to the relationship between 1/C2 and the applied potential (V), wherein C is interfacial capacitance.36 The tangent slope of the M–S plots is inversely proportional to the carrier density size of a semiconductor.24 According to the estimated slope size, the carrier density size of various vulcanization samples follows S3 < S2 < S1. The result indicates that Bi2O3–Bi2S3 heterostructures showed higher charge carrier separation efficiencies than their single constituent counterparts; the M–S analysis results herein supported the observations of the PEC above photoresponse results.

Figure 8.

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Mott–Schottky plots: (a) S1. (b) S2. (c) S3. The Mott–Schottky plots are obtained in a 0.5 M Na2SO4 solution at 1 kHz.

The photocatalytic activities of various samples were evaluated by photocatalytic degradation of MO aqueous solution under visible light irradiation (wavelength > 420 nm). The photodegradation efficiency of the MO solution with various photocatalysts is summarized in Figure 9a. The ratio of the residual concentration (C) to the initial concentration (Co) of the MO solution was used as the photodegradation degree.37 Notably, the dark balance tests are conducted at the equilibrium state of the adsorption–desorption process for 60 min. The C/Co decreased 13.5, 9.3, 3.4, and 5.5% for S1, S2, S3, and Bi2O3 under dark balance conditions, respectively. S1 has the best surface dye adsorption capacity. This is supported by the SEM observations that S1 has a small sheet size and is more porous than other vulcanization samples, which increases the surface area and surface adsorption. The worst surface dye adsorption capacity of S3 is mainly due to its large and solid sheet morphology engendered a smaller surface area among various samples. The sequential MO dye degradation rates are S3 < Bi2O3 < S2 < S1. Nearly complete photodegradation (approximately 95%) of the MO solution was achieved with S1 after 60 min of irradiation. S2, S3, and Bi2O3 photodegraded 75, 38, and 52% MO dyes after 60 min of irradiation, respectively. Compared to the single component S3 and Bi2O3, the Bi2O3–Bi2S3 composites (S1 and S2) exhibit enhanced photocatalytic properties. The photocatalytic activities of S1, S2, S3, and Bi2O3 are further compared with the pseudo-first-order rate constant k herein according to the pseudo-first-order model.38 The corresponding fitted kinetics curves of various samples are shown in Figure 9b. The k of S1, S2, S3, and Bi2O3 is 0.0527, 0.0231, 0.0079, and 0.0125 min–1, respectively. S1 exhibits the highest reaction rate constant k among various samples. The porous structure, high charge separation efficiency, and low interfacial charge-transfer resistance account for the superior photocatalytic activity of S1 in this study.

Figure 9.

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(a) C/Co vs irradiation time plot. (b) Plot of ln (Co/C) vs irradiation time for various samples.

Trapping experiments of reactive species during the photocatalytic process of S1 toward MO solution were conducted to understand the possible photodegradation mechanism. Three different quenchers, benzoquinone (BQ; a •O2– radical scavenger), isopropyl alcohol (IPA; a •OH radical scavenger), and ammonium oxalate (AO; an h+ scavenger), were used for scavenger tests.39,40 In Figure 10a, the presence of BQ reduced the removal rate of MO to 46.3%, while the presence of AO and IPA had a minor effect on the degradation of MO. The possible reason is that adding BQ to the MO aqueous system will cause BQ to capture •O2– and reduce the amount of active •O2– radicals used in the photocatalytic reaction, deactivating the S1 photocatalyst. The scavenger test results demonstrate that the •O2– is the main radical governing the photocatalytic process of S1 toward MO solution. Based on the previous results, a possible photodegradation mechanism of S1 toward MO dyes is proposed in Figure 10b. The photoinduced electrons would migrate to the CB of Bi2O3 from the CB of Bi2S3, and holes would migrate to the VB of Bi2S3 from the VB of Bi2O3 if the Bi2O3–Bi2S3 composite complies with the mechanism of type II heterojunction. However, the photoinduced holes on the VB of Bi2S3 are incapable of oxidizing OH– to produce •OH radicals because the VB edge potential of Bi2S3 (0.80 V vs NHE) is more negative than the potential of OH–/•OH (2.24 V) as presented in Figure 10b.38 Meanwhile, the potential of O2/•O2– is −0.33 V, which is more negative than that of the CB of Bi2O3, so the CB electrons could not quickly reduce the O2.38 It should be noted that this type II mechanism could simultaneously weaken the reducing ability of photoelectrons and the oxidizing ability of photoinduced holes during the photocatalytic process according to the relative potential positions among CB of Bi2O3, VB of Bi2S3, and redox potentials. Based on the discussion above, the type II photocatalytic mechanism might not be suitable to explain the photocatalytic mechanism of S1 toward MO dyes herein. Notably, according to the analysis results of the capture experiments, the S1 photocatalyst produces superoxide radicals (•O2–) with strong oxidizing ability during the photocatalytic process. Moreover, •OH and h+ radicals also have a certain degree of activity in the photocatalytic process. These results contradict the abovementioned analysis of the type II photocatalytic mechanism. Contrarily, the scavenger test results might evidence the construction of the direct Z-scheme heterojunction in the Bi2O3–Bi2S3 composite (Figure 10b). Upon light irradiation, both Bi2O3 and Bi2S3 components are excited to generate the charge carriers in their CBs and VBs, respectively. After constructing the Z-scheme heterojunction, the photoelectrons of Bi2O3 in the CB position could migrate to the VB position of Bi2S3 through the interfacial electronic field, resulting in the effective separation of charge carriers through the Bi2O3/Bi2S3 heterojunction. Meanwhile, via this Z-scheme migration of photoinduced charge carriers, the photoelectrons and holes are gathered on a more negative CB position of Bi2S3 to reduce oxygen molecules into •O2– radicals and a more favorable VB position of Bi2O3 to react with the H2O molecules to generate the ·OH radicals, respectively. Thus, the S1 photocatalyst simultaneously possesses a substantial reduction and oxidation ability compared to the single component during the photocatalytic process toward the MO solution. The possible action trails for active species were therefore proposed below

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Figure 10.

Figure 10

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(a) Degradation percentage of MO solution with S1 in the presence of various scavengers. (b) Schematic diagrams of the possible photodegradation process of S1 toward MO dyes.

Conclusions

CBD-derived porous Bi2O3 sheets were used as a template for vulcanization to form Bi2O3–Bi2S3 composite structures. At 0.01 M vulcanization, porous and small-sized Bi2O3–Bi2S3 composite sheets were produced. At 0.1 M vulcanization, the porous Bi2O3–Bi2S3 composite sheets with a larger size will be formed. When the vulcanization concentration is increased to 1 M, the porous Bi2O3 sheets transform into a solid and pure Bi2S3 sheet structure. S1 exhibits the best light absorption, the highest photogenerated charge separation efficiency, the smallest electron transfer impedance, and the highest carrier density among various samples. Furthermore, S1 shows good durability and the best photocatalytic decomposition toward MO dyes compared to the other control samples. The improved photodegradation performance can be ascribed to constructing the direct Z-scheme heterojunction, which promotes the practical separation of photoinduced charge carriers and the enhanced redox capacity. The findings herein might be of great value in developing porous oxide-sulfide composite materials for desirable photosensitive applications.

Acknowledgments

This research was funded by the National Science and Technology Council of Taiwan, grant number MOST 111-2221-E-019-062-MY3.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c02153.

  • TEM analysis of S2; low-magnification images; HRTEM images; SAED pattern; EDS line-scan profiles of the sample; Bi 4f XPS spectrum of S1; O 1s spectrum of S1; Bi 4f XPS spectrum of S3; absorbance spectrum of S3 (Bi2S3) sheets; and calculated band gap energy of S3 sheets (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c02153_si_001.pdf (260.4KB, pdf)

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Supplementary Materials

ao3c02153_si_001.pdf (260.4KB, pdf)

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