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Original article
11 2023
:16;
105286
doi:
10.1016/j.arabjc.2023.105286

Facile fabrication of BiOI/Bi2WO6 Z-scheme heterojunction composites with a three-dimensional structure for efficient degradation of pollutants

, , , , , , ,
a School of Mechanical Engineering, Chengdu University, Chengdu 610106, China
b Sichuan Institute of Industrial Technology, Deyang 618000, China
c Material Corrosion and Protection Key Laboratory of Sichuan Province, ZiGong 643002, China
d College of Resources and Environment, Xichang University, Xichang 615000, China

⁎Corresponding authors. fengwei233@126.com (Wei Feng), 13568666923@163.com (Wanming Zhang)

Received: , Accepted: ,
© The Author(s) 2023
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Three-dimensional BiOI/Bi2WO6 Z-scheme photocatalysts were synthesized by one-step hydrothermal method. Detailed characterization via SEM and TEM revealed that these photocatalysts exhibit a unique three-dimensional flower-like structure, composed of two-dimensional sheets. The results of photocatalysis mechanism analysis demonstrated that the BiOI/Bi2WO6 heterojunctions have formed a Z-scheme charge transfer mechanism. Specifically, electrons transfer from the Bi2WO6 conduction band to the BiOI valence band, facilitating both enhanced separation of photogenerated charges and the preservation of the strong reducibility of BiOI conduction band electrons and the strong oxidation of Bi2WO6 valence band holes. This dual effect leads to increased generation of free radicals and a consequent improvement in photocatalytic property. Remarkably, when the molar ratio of BiOI/Bi2WO6 is set at 40%, the first-order reaction rate constant reaches 0.089 min−1, which is nearly 5.9 times that of pure Bi2WO6.

Keywords

BiOI
Bi2WO6
Z-scheme heterojunctions
Photocatalytic property
1

1 Introduction

The issue of water pollution has gained substantial attention, leading to increasing concerns about the mitigation of organic pollutants in water in recent years (Jiang et al., 2022; Dou et al., 2020; Long et al., 2020). Photocatalysis, recognized as an environmentally friendly and sustainable technology, has emerged as a viable solution for the decomposition of organic pollutants. However, the conventional photocatalytic material, titanium dioxide (TiO2), possesses inherent limitations, primarily its exclusive absorption of ultraviolet (UV) light, which restricts its utilization of solar energy (Zhu et al., 2021; Guo et al., 2022; Sun et al., 2020; Zhu et al., 2022; Sun et al., 2019). Thus, the quest for photocatalysts responsive to visible light has become imperative. Bi2WO6, an innovative photocatalytic material capable of harnessing visible light, has gained significant research interest owing to its distinctive structural properties, narrow bandgap, and chemical stability (Fei et al., 2019; Ahsaine et al., 2016; Zhu et al., 2022; Qiao et al., 2018; Lai et al., 2019). Nonetheless, pure Bi2WO6 exhibits a drawback characterized by a high recombination of photogenerated charge carriers (Xiong et al., 2023; Xiong et al., 2022; Mohamed and Aazam, 2013; Liu et al., 2011). The introduction of semiconductor coupling represents an effective route to advance photocatalytic performance. Through precise microstructural control at heterojunction interfaces, this approach facilitates the separation of photogenerated electron-hole pairs, suppresses carrier recombination, and enhances quantum efficiency (Li et al., 2020; Chen et al., 2023; Qiang et al., 2021; Guo et al., 2019; Hu et al., 2019; Guo et al., 2022; Selvam et al., 2018; Xu et al., 2022; Vadivel et al., 2022).

Notably, three-dimensional (3D) structures offer an abundance of reactive sites for photocatalysis, enable multiple reflections of visible light, and enhance light source utilization efficiency, resulting in superior photocatalytic efficacy when compared to other morphologies (Chen et al., 2021; Shi et al., 2020; Xiong et al., 2022; Luo et al., 2019; Zhao et al., 2018). Zhao et al. (Zhao et al., 2018) synthesized various Bi2WO6 morphologies, including nano-flowers, nanoplates, and kont shapes, via modified hydrothermal processes, and evaluated their performance in the decomposition of ceftriaxone sodium under visible light irradiation. Among these, nano-flower Bi2WO6 exhibited the highest photocatalytic activity.

BiOI, characterized by a narrow bandgap of 1.7–1.9 eV and a broad visible light absorption range, possesses a unique layered structure that facilitates the efficient separation of photoinduced charges, rendering it a promising candidate for photocatalytic applications (Dou et al., 2021; Jiang et al., 2017; Tian et al., 2016; Zhao et al., 2013; Wang et al., 2011). Notably, BiOI readily forms Z-type heterojunctions with other semiconductors, enabling the retention of strong electron reduction and hole oxidation capabilities, thereby enhancing photocatalytic performance (Li et al., 2014; Chen et al., 2022).

Building upon our previous work, where we successfully synthesized 3D flower-like Bi2WO6 photocatalysts and identified the optimal hydrothermal temperature as 160 °C (Wang et al., 2022), the present study focuses on the development of BiOI/Bi2WO6 Z-scheme photocatalysts to mitigate photogenerated carrier recombination. These photocatalysts were prepared under hydrothermal conditions at 160 °C for 24 h. Various characterization techniques were employed to analyze their crystal structure, surface morphology, elemental distribution, chemical state, specific surface area, and optical properties. Methylene Blue (MB) aqueous solution was employed as the target pollutant to assess the photocatalytic performance of BiOI/Bi2WO6 photocatalysts. The study delves into the photocatalytic mechanism, electronic transfer processes within the synthesized BiOI/Bi2WO6 photocatalysts, and proposes a plausible Z-type transfer mechanism to elucidate the enhanced photocatalytic performance resulting from BiOI coupling.

2

2 Experimental

2.1

2.1 Synthesis of photocatalyst materials

The photocatalyst materials used in this study, namely Bi(NO3)3 (Analytical Reagent, AR), CH3COOH (AR), Na2WO4 (AR), C2H5OH (AR), and methylene blue (AR), were procured from Chengdu Kelong Chemical Co., Ltd, China. The BiOI/Bi2WO6 composite photocatalytic materials were synthesized via a one-step hydrothermal method. To prepare solution A, Bi(NO3)3 (2.94 g) and CH3COOH (5 mL) were dispersed in 20 mL of deionized water. Sodium iodide was then added to solution A to achieve atomic molar ratios of BiOI/Bi2WO6 at 20%, 30%, 40%, and 50%. In parallel, solution B was created by dispersing Na2WO4 (1 g) in 12 mL of deionized water. Solution B was slowly added to solution A while stirring continuously for 30 min. The resulting mixture was subsequently transferred to a reactor and heated at 160 °C for 24 h. The synthesized BiOI/Bi2WO6 composite photocatalysts were thoroughly washed alternately with deionized water and anhydrous ethanol until neutral pH was achieved. Finally, the samples were dried at 100 °C for 10 h. The BiOI/Bi2WO6 composite photocatalysts were denoted as x%BiOI/Bi2WO6, with “x” representing the respective atomic molar ratio.

2.2

2.2 Characterization

The phase composition of the samples was determined using a DX-2700X X-ray diffractometer (XRD) with a working voltage of 40 kV and a current of 30 mA. The scanning range was 20°-70°, and the scanning speed was set at 0.06°/s, utilizing Cu-Kα radiation as the source. Surface morphology was investigated using an FEI-inspect F50 scanning electron microscope (SEM) and EDS, operating at 5 kV, in addition to a JEM-F200 transmission electron microscope (TEM) and HRTEM, with an accelerated voltage of 200 kV. The specific surface area of the samples was determined by the V-sorb 2800S analyzer (BET). Elemental composition and valence states were analyzed using the XSAM800 X-ray photoelectron spectroscopy (XPS) with Mg-Kα radiation as the source. The recombination of photoinduced charges was assessed using a 4600 fluorescence spectrometer (PL) with xenon light as the source. Time-resolved photoluminescence (TRPL) spectra were acquired using the FLS1000 fluorescence spectrophotometer. Absorbance measurements were performed with a UV-3600 ultraviolet–visible spectrophotometer (DRS) in the wavelength range of 200–800 nm. Active free radicals generated under light exposure were quantified in situ using the CIQTEK EPR200-Plus electron paramagnetic resonance spectrometer (ESR).

2.3

2.3 Photocatalytic activity experiment

The photocatalytic performance of the samples was assessed by monitoring the degradation of a 10 mg/L methylene blue (MB) aqueous solution. In a 100 mL beaker, 100 mL of the solution and 25 mg of the sample powder were combined, and the mixture was stirred in darkness for 30 min. Subsequently, the solution was irradiated under xenon lamp light as the excitation source. Samples were extracted at 10-minute intervals, and their absorbances were measured.

3

3 Results and discussion

3.1

3.1 Phase composition

The phase composition of the synthesized materials was assessed using X-ray diffraction (XRD) analysis, as depicted in Fig. 1. In the XRD patterns, distinct diffraction peaks were observed for pure BiOI, pure Bi2WO6 and the BiOI/Bi2WO6 composites. The diffraction peaks at 28.3°, 32.9°, 47.2°, 55.3°, 58.6°, and 69.0° correspond to the (1 3 1), (2 0 0), (2 0 2), (3 3 1), (2 6 2), and (4 0 0) crystal planes of Bi2WO6, respectively. The peaks at 29.6°, 31.7°, and 39.4° can be attributed to the (1 0 2), (1 1 0), and (0 0 4) crystal planes of BiOI, respectively. Notably, these diffraction peak positions and intensities in the BiOI/Bi2WO6 composites closely resemble those of pure BiOI and pure Bi2WO6. These distinct peaks unequivocally indicate the successful formation of BiOI/Bi2WO6 composites.

XRD patterns of samples.
Fig. 1
XRD patterns of samples.

3.2

3.2 Morphology

SEM images of pure Bi2WO6, pure BiOI and 40%BiOI/Bi2WO6 are presented in Fig. 2. The micrographs reveal that Bi2WO6 exhibits a distinctive three-dimensional (3D) flower-like morphology comprised of two-dimensional (2D) nanosheets. These nanosheets possess an average diameter within the range of 2 to 4 μm, with lengths varying from tens to hundreds of nanometers, as illustrated in Fig. 2 (a, b). The SEM images of BiOI are presented in Fig. 2 (c, d), displaying a plate-like morphology. Intriguingly, Fig. 2 (e, f) demonstrates that the 40%BiOI/Bi2WO6 composite maintains a similar 3D flower-like morphology. The elemental distribution mappings of 40%BiOI/Bi2WO6 are depicted in Fig. 3, revealing uniform dispersion of Bi, I, O, and W elements throughout the matrix.

SEM images of samples: pure Bi2WO6 (a, b); pure BiOI (c, d); 40%BiOI/Bi2WO6 (e, f).
Fig. 2
SEM images of samples: pure Bi2WO6 (a, b); pure BiOI (c, d); 40%BiOI/Bi2WO6 (e, f).
EDS patterns of 40%BiOI/Bi2WO6.
Fig. 3
EDS patterns of 40%BiOI/Bi2WO6.

Further insight into the morphology is provided by the transmission electron microscopy (TEM) images shown in Fig. 4 (a, b). These images depict the 40%BiOI/Bi2WO6 composite at varying magnifications, showcasing the composition of two-dimensional sheets and the manifestation of a flower-like morphology with an approximate diameter of 3 μm. Fig. 4(d) offers a closer look at the crystal lattice fringes of 40%BiOI/Bi2WO6, with clear observation of distinct lattice spacings. These spacings measure approximately 0.321 nm and 0.302 nm, which can be ascribed to the (1 3 1) plane of Bi2WO6 (Cheng et al., 2018) and the (1 0 2) plane of BiOI (Park et al., 2014).

TEM and HRTEM images of 40%BiOI/Bi2WO6.
Fig. 4
TEM and HRTEM images of 40%BiOI/Bi2WO6.

3.3

3.3 Element composition

XPS spectra of 40%BiOI/Bi2WO6 composite is presented in Fig. 5, revealing the elemental composition of the material, predominantly consisting of Bi, W, I, and O. The high-resolution spectra of Bi 4f (Fig. 5(b)) exhibits two distinctive peaks at 158.8 and 164.1 eV, corresponding to Bi 4f7/2 and Bi 4f5/2, respectively. These peaks unequivocally signify the valence state of the Bi element as +3 (Dou et al., 2021; Wang et al., 2011). Furthermore, Fig. 5(c) displays the high-resolution spectra of I 3d, exhibiting peaks at 619.0 eV (I 3d5/2) and 630.4 eV (I 3d3/2). These peak positions suggest that the I element exists in the form of –1 (Dou et al., 2021; Wang et al., 2011). Fig. 5(d) presents the high-resolution spectra of W 4f, with characteristic peaks at 37.4 eV (W 4f5/2) and 35.2 eV (W 4f7/2). These peaks correspond to the valence state of W 6 + in the sample (Koutavarapu et al., 2020; Cheng et al., 2020).

XPS spectra of 40%BiOI/Bi2WO6.
Fig. 5
XPS spectra of 40%BiOI/Bi2WO6.

3.4

3.4 Optical properties

Fig. 6 presents the photoluminescence (PL) spectra of samples under investigation. It is evident that the BiOI/Bi2WO6 composites exhibit a significantly lower peak intensity compared to pure Bi2WO6. This observation suggests that the incorporation of BiOI effectively suppresses the recombination of photogenerated charge carriers, aligning with previous findings (Wang et al., 2022). As the I/W molar ratio increases, the PL peak intensity of BiOI/Bi2WO6 gradually decreases, with 40%BiOI/Bi2WO6 displaying the lowest PL peak intensity. However, it is noteworthy that the PL peak intensity of 50%BiOI/Bi2WO6 is slightly higher than that of 40%BiOI/Bi2WO6, suggesting that carrier separation decreases with excessive BiOI content (Lv et al., 2019). This behavior can be attributed to the adverse impact of excessive BiOI, leading to overlapping aggregation on the Bi2WO6 surface, which hinders the carrier transport path and reduces the separation of photogenerated charges (Zhu et al., 2016).

PL spectra of samples.
Fig. 6
PL spectra of samples.

To further analyze the fluorescence lifetimes, time-resolved fluorescence spectroscopy was employed (Fig. 7). The fluorescence decay curves were fitted using an exponential function (Equation 1) (Zhang et al., 2015; Yang et al., 2019);

（1）
I t = B 1 e x p ( - t / τ 1 ) + B 2 e x p ( - t / τ 2 ) where B and τ represent the amplitude and emission lifetime of each component. The average radiative lifetime of carriers (τave) was then calculated using Equation 2, with the results summarized in Table 1.
（2）
τ ave = B 1 τ 1 2 + B 2 τ 2 2 B 1 τ 1 + B 2 τ 2
Time-resolved transient PL decay of samples.
Fig. 7
Time-resolved transient PL decay of samples.
Table 1 Exponential decay-fitted parameters of fluorescence lifetime of samples.
Samples B1 τ1 (ns) B2 τ2 (ns) τave (ns)
Bi2WO6 3403.5930 0.22 10.5882 3.27 0.36
40%BiOI/Bi2WO6 3564.6953 0.22 10.4268 3.13 0.34

The relationship between fluorescence lifetime and photocatalytic performance remains a subject of debate. Some studies have suggested that longer fluorescence lifetimes are associated with better photocatalytic performance (Benlin et al., 2018). Conversely, others have proposed that shorter fluorescence lifetimes correlate with higher photocatalytic activity (Yang et al., 2018). In our study, the coupling of BiOI with Bi2WO6 resulted in a reduction in fluorescence lifetime from 0.36 ns to 0.34 ns. For multiphase composite photocatalysts, a shorter fluorescence lifetime implies faster migration of photogenerated carriers between heterogeneous interfaces, which is favorable for improving quantum efficiency and photocatalytic performance (Zhang et al., 2015; Yang et al., 2019).

Fig. 8(a) displays the UV–visible absorption spectra of the samples. Pure Bi2WO6 exhibits absorption primarily below approximately 450 nm. In contrast, the BiOI/Bi2WO6 samples exhibit a noticeable redshift in the absorption edge, extending the absorption range into the visible light region. The calculated band gaps for pure Bi2WO6, pure BiOI, and 40%BiOI/Bi2WO6 are 2.58 eV (Wang et al., 2022), 1.68 eV, and 2.22 eV, respectively (Fig. 8(b)). This shift in the absorption edge towards longer wavelengths, coupled with the narrower bandgap, demonstrates the enhanced visible light absorption capability of the BiOI/Bi2WO6 samples. These optical properties are crucial for facilitating efficient photocatalysis under visible light irradiation, as they allow for improved utilization of the solar spectrum and potentially higher photocatalytic performance.

UV–visible absorption spectra (a) and band gaps (b) of samples.
Fig. 8
UV–visible absorption spectra (a) and band gaps (b) of samples.

3.5

3.5 Specific surface area analysis

In previous study (Wang et al., 2022), the specific surface area of pure Bi2WO6 was determined to be 20.8 m2/g. To further elucidate the impact of BiOI coupling on the surface properties of Bi2WO6, we conducted a specific surface area analysis, focusing on the 40%BiOI/Bi2WO6 composite. Fig. 9 presents the pore size distribution curve and N2 adsorption–desorption isotherm of 40%BiOI/Bi2WO6. Remarkably, this composite exhibited a specific surface area of 27.9 m2/g. This result underscores that the introduction of BiOI into the Bi2WO6 matrix led to an enhancement in the specific surface area. The increased surface area provides a higher density of reactive sites and adsorption sites, which are instrumental for photocatalytic reactions (Wang et al., 2020).

The pore size distribution curve and N2 adsorption–desorption isotherm of 40%BiOI/Bi2WO6.
Fig. 9
The pore size distribution curve and N2 adsorption–desorption isotherm of 40%BiOI/Bi2WO6.

3.6

3.6 Electrochemical analysis

Fig. 10 presents the electrochemical impedance spectroscopy (EIS) curves and photocurrent curves for both pure Bi2WO6 and 40%BiOI/Bi2WO6, shedding light on their electrochemical characteristics. In Fig. 10(a), it is evident that the Nyquist diameter of 40%BiOI/Bi2WO6 is smaller compared to that of pure Bi2WO6. This observation indicates that the migration resistance of photogenerated charges within the 40%BiOI/Bi2WO6 composite is lower, signifying faster charge transfer abilities. This enhanced charge transfer capability is conducive to efficient charge separation in the photocatalyst (Huang et al., 2019; Yang et al., 2023; Feng et al., 2020). Fig. 10(b) illustrates that the photocurrent density of 40%BiOI/Bi2WO6 exceeds that of pure Bi2WO6. This outcome suggests that the 40%BiOI/Bi2WO6 composite exhibits a higher rate of photogenerated charge separation (Yang et al., 2023; Feng et al., 2020). The electrochemical tests collectively affirm that the coupling of BiOI has a positive impact on the generation, separation, and migration of photogenerated charges within the photocatalyst. These findings are in line with the results from the photoluminescence (PL) analysis, further reinforcing the enhanced photocatalytic activity of the BiOI/Bi2WO6 composite.

EIS (a) and photocurrent curves (b) of samples.
Fig. 10
EIS (a) and photocurrent curves (b) of samples.

3.7

3.7 Photocatalytic performance

The degradation curves of MB by the BiOI/Bi2WO6 photocatalysts are depicted in Fig. 11(a). Pure Bi2WO6 demonstrated a degradation degree of 70.6% (Wang et al., 2022); and pure BiOI demonstrated a degradation degree of 36.3%, while the BiOI/Bi2WO6 samples consistently exhibited higher photocatalytic activity compared to pure Bi2WO6 and pure BiOI. Specifically, the degradation degrees for 20%BiOI/Bi2WO6, 30%BiOI/Bi2WO6, 40%BiOI/Bi2WO6, and 50%BiOI/Bi2WO6 were recorded as 76.3%, 85.0%, 99.0%, and 86.3%. The observed enhancement in photocatalytic performance can be attributed to several factors. BET and UV–visible absorption spectra results reveal that the coupling of BiOI increases the specific surface area and light absorption capability of the Bi2WO6 photocatalyst. This expansion of active surface sites and enhanced light absorption contribute to the generation of more reactive sites and an increased production of charge carriers. Furthermore, the photoluminescence (PL) spectra reveal that the presence of BiOI accelerates the transfer of photogenerated charges, effectively suppresses recombination, and enhances quantum efficiency. These improvements are conducive to superior photocatalytic performance (Li et al., 2018; Zhang et al., 2019).

Photodegradation curves (a) and kinetics fitting curves (b) of samples.
Fig. 11
Photodegradation curves (a) and kinetics fitting curves (b) of samples.

To provide further insight, Fig. 11(b) presents the first-order kinetics curves of time and –ln(Ct/C0). The first-order reaction rate constants for pure Bi2WO6, pure BiOI, 20%BiOI/Bi2WO6, 30%BiOI/Bi2WO6, 40%BiOI/Bi2WO6, and 50%BiOI/Bi2WO6 were determined as 0.015 min−1, 0.004 min−1, 0.027 min−1, 0.035 min−1, 0.089 min−1, and 0.035 min−1. Notably, the first-order reaction rate constant for 40%BiOI/Bi2WO6 is approximately 5.9 times that of pure Bi2WO6.

Cyclic experiments were performed on 40%BiOI/Bi2WO6, the findings of which are depicted in Fig. 12. After 4 cycles, the degradation degree was 87.8%, indicating good reusability. The XRD pattern of 40%BiOI/Bi2WO6 before and after the cyclic experiment were shown in Fig. 13, revealed that the diffraction peak positions and intensities did not change significantly compared to the original sample. This suggests excellent structural stability after use.

The cyclic experiment of 40%BiOI/Bi2WO6 photocatalyst for MB degradation.
Fig. 12
The cyclic experiment of 40%BiOI/Bi2WO6 photocatalyst for MB degradation.
XRD pattern of 40%BiOI/Bi2WO6 before and after experiment.
Fig. 13
XRD pattern of 40%BiOI/Bi2WO6 before and after experiment.

The previous data on the degradation of MB by various photocatalytic materials were summarized, as shown in Table 2, the results show that the BiOI/Bi2WO6 photocatalytic composites prepared in this study have relatively high photocatalytic activity (Jiang et al., 2022; Cui et al., 2020; Zhang et al., n.d.; Chen et al., 2022; Guo et al., 2022; Cheng et al., 2022).

Table 2 MB degradation degrees by various photocatalysts reported in literatures.
Photocatalyst Ccatalyst g·L-1 CMB mg·L-1 Light Source Decolorization Degree Ref.
Bi2WO6/BiOCl 0.25 10 Xenon lamp (300 W) 88.0% in 75 min (Jiang et al., 2022)
Bi2WO6/Nb2CTx 0.5 15 Xenon lamp (500 W) 92.7% in 90 min (Cui et al., 2020)
Bi2WO6/ZIF-8 0.4 10 Mercury lamp (250 W) 95.5% in 120 min (Zhang et al., n.d.)
ZnO/Bi2WO6-CC 0.2 10 Mercury lamp (300 W) 96.9% in 100 min (Chen et al., 2022)
FeOOH/Bi2WO6 1 12 Metal halide lamp (300 W) 98.0% in 180 min (Guo et al., 2022)
Bi2WO6/Ni-MOF 0.5 20 Xenon lamp (300 W) 98.8% in 180 min (Cheng et al., 2022)
BiOI/Bi2WO6 0.25 10 Xenon lamp (250 W) 99.0% in 40 min Present work

3.8

3.8 Photodegradation mechanism

Fig. 14 illustrates the results of an active species experiment conducted on the 40%BiOI/Bi2WO6 photocatalyst. When scavengers such as AO (ammonium oxalate), BQ (benzoquinone), and IPA (isopropanol) were introduced, the degradation efficiencies of 40%BiOI/Bi2WO6 decreased from 99.0% to 42.4% (AO), 71.3% (BQ), and 73.2% (IPA). This substantial reduction in degradation degree strongly suggests that the primary active species responsible for the photocatalytic process is the photogenerated hole (h+), while •O2– and •OH free radicals also play a role in the degradation process.

The degradation degrees of 40%BiOI/Bi2WO6 in the presence of different scavengers.
Fig. 14
The degradation degrees of 40%BiOI/Bi2WO6 in the presence of different scavengers.

To further explore the generation of active free radicals during photocatalysis, we conducted electron spin resonance (ESR) measurements on the 40%BiOI/Bi2WO6 photocatalyst using DMPO as a free radical spin trapping agent. The results, as shown in Fig. 15, reveal the generation of •OH and •O2– radicals under illumination, indicated by the appearance of distinctive signal peaks.

ESR spectra of 40%BiOI/Bi2WO6: (a) DMPO-•OH; (b) DMPO-•O2–.
Fig. 15
ESR spectra of 40%BiOI/Bi2WO6: (a) DMPO-•OH; (b) DMPO-•O2–.

The band potential of a semiconductor is pivotal in determining photocatalytic performance, as it dictates the pathways of photogenerated charge transfer and active free radical formation. XPS valence band spectra were obtained for both pure BiOI and pure Bi2WO6 (Fig. 16). The valence band potentials of BiOI and Bi2WO6 were determined to be 1.34 eV and 3.58 eV, respectively. When converted to the standard hydrogen electrode potential, the valence band potential of BiOI is 1.20 eV (Wang et al., 2022; Wang et al., 2019). In conjunction with the UV–visible absorption spectra, the band gap of BiOI was found to be 1.68 eV, and its conduction potential was calculated to be –0.48 eV (1.20–1.68 eV). Similarly, the valence band potential of Bi2WO6 at the standard hydrogen electrode was determined to be 3.44 eV, with a band gap of 2.58 eV, resulting in a conduction potential of 0.86 eV (3.44–2.58 eV).

XPS valence band spectra of BiOI (a) and Bi2WO6 (b).
Fig. 16
XPS valence band spectra of BiOI (a) and Bi2WO6 (b).

Based on the findings from the active species experiment, ESR spectra, and XPS valence band spectra, we propose a schematic diagram of the photogenerated charge transfer and free radical generation pathways (Fig. 17). When excited by photons, both BiOI and Bi2WO6 generate photogenerated holes (h+) and electrons (e–) in their respective valence bands (VB) and conduction bands (CB). If the photoinduced charges transfer path follows the approach of type-II heterojunction, that is, electrons in BiOI CB will flow into Bi2WO6 CB, and holes in Bi2WO6 VB transfer to BiOI VB. Were these to happen, the Bi2WO6 CB electrons reduction is insufficient to react with O2 to generate •O2– radicals as the Bi2WO6 CB potential (0.86 eV) is positive than the electrode potential of O2/•O2– (–0.046 eV). Not only that, the BiOI VB holes oxidation is insufficient to react with OH– to generate •OH radicals as the BiOI VB potential (1.20 eV) is negative than the electrode potential of OH–/•OH (1.99 eV). As a result, •O2– and •OH radicals cannot be generated. However, ESR spectra confirm the generation of •O2– and •OH radicals under light irradiation. Therefore, it is reasonable to believe that photogenerated charges will not be transferred in the form of type-II heterojunction. A Z-type transfer mechanism for photogenerated charges is proposed based on the band potential of BiOI and Bi2WO6 and ESR results. Bi2WO6 CB electrons directly transfer to BiOI VB and recombine with the holes on its VB, retaining t Bi2WO6 VB holes with strong oxidability and BiOI CB electrons with strong reducibility. The Bi2WO6 VB potential is 3.44 eV, which is positive than the electrode potential of OH–/•OH (1.99 eV), and the BiOI CB potential is –0.48 eV, which is negative than the electrode potential of O2/•O2– (–0.046 eV).

Schematic diagram of photogenerated charge transfer and formation of free radicals in BiOI/Bi2WO6.
Fig. 17
Schematic diagram of photogenerated charge transfer and formation of free radicals in BiOI/Bi2WO6.

The observed potential differences suggest that the photogenerated holes and electrons possess sufficient oxidizing and reducing properties to facilitate the formation of •OH and •O2– radicals through reactions with OH– and O2, respectively. This Z-type transfer process not only mitigates the recombination of photogenerated electrons and holes in Bi2WO6 but also maintains the separation of holes with strong oxidizing properties and electrons with strong reducing properties (Wen et al., 2017; Dou et al., 2022; Chen et al., 2022). Ultimately, this mechanism is conducive to improving the photocatalytic performance of the 40%BiOI/Bi2WO6 composite.

4

4 Conclusions

In conclusion, this study focused on the modification of Bi2WO6 through the incorporation of BiOI, exploring the impact of varying BiOI concentrations (molar ratios of BiOI/Bi2WO6 at 20%, 30%, 40%, and 50%) on the structural and photocatalytic performance of Bi2WO6. The samples prepared using the hydrothermal method exhibited a distinctive three-dimensional flower-like morphology. By establishing Z-scheme heterojunctions through the integration of BiOI with Bi2WO6 and fine-tuning the microstructure of the heterogeneous interfaces, several significant improvements were observed. First, the visible light absorption was expanded, resulting in enhanced photocatalytic potential. Second, the specific surface area of the composite was increased, providing more active sites for catalysis. Most importantly, the Z-scheme transfer mechanism allowed for the preservation of electrons with strong reduction potential and holes with strong oxidation potential. This, in turn, effectively impeded the recombination of charge carriers and boosted quantum efficiency. Among the various compositions tested, 40%BiOI/Bi2WO6 demonstrated the most favorable characteristics. It exhibited the lowest charge carrier recombination rate and the highest photocatalytic activity, boasting a first-order reaction rate constant of 0.089 min−1, which was nearly 5.9 times that of pure Bi2WO6. The active species experiment results emphasized that photogenerated holes (h+) served as the primary active species, with the participation of •O2– and •OH radicals in the degradation process.

Funding

This study was supported by the Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan province (2023CL20, 2023CL01), China, the Higher Education Talent Quality and Teaching Reform Project of Sichuan Province (JG2021-1104), China, the Talent Training Quality and Teaching Reform Project of Chengdu University (cdjgb2022033), China.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. , , , . Electronic band structure and visible-light photocatalytic activity of Bi2WO6: elucidating the effect of lutetium doping. RSC Adv.. 2016;6(103):101105-101114.
    [Google Scholar]
  2. , , , . A novel three-dimensional heterojunction photocatalyst for the photocatalytic oxidation of crystal violet and reduction of Cr6+. Chemosphere. 2018;211:10-16.
    [Google Scholar]
  3. , , , . Photocatalytic degradation of MB by novel and environmental ZnO/Bi2WO6-CC hierarchical heterostructures. Mater. Charact.. 2022;189:111961
    [Google Scholar]
  4. , , , . Recent advances on Bi2WO6-based photocatalysts for environmental and energy applications. Chin. J. Catal.. 2021;42:1413-1438.
    [Google Scholar]
  5. , , , . Synthesis of novel muscovite loaded nano Ag/Cu2-FeO composites with excellent visible-light responsive photocatalysis. Opt. Mater.. 2022;124:112002
    [Google Scholar]
  6. , , , . Cu2−xSe/FeSe2 Z-type heterojunction demonstrate versatile boosting photoelectrochemical, electrocatalytic and photocatalytic properties. J. Alloy. Compd.. 2023;947:169496
    [Google Scholar]
  7. , , , . Plasmonic Bi promotes the construction of Z-scheme heterojunction for efficient oxygen molecule activation. Chemosphere. 2022;302:134527
    [Google Scholar]
  8. , , , . The simultaneous removal of heavy metals and organic contaminants over a Bi2WO6/mesoporous TiO2 nanotube composite photocatalyst. RSC Adv.. 2020;10(36):21228-21237.
    [Google Scholar]
  9. , , , . Flower-like Bi2WO6/ZnO composite with excellent photocatalytic capability under visible light irradiation. Chin. J. Catal.. 2018;39:810-820.
    [Google Scholar]
  10. , , , . Bi2WO6-wrapped 2D Ni-MOF sheets with significantly improved photocatalytic activity by a direct Z-scheme electron transfer. J. Alloy. Compd.. 2022;896:163055
    [Google Scholar]
  11. , , , . Bi2WO6/Nb2CTx MXene hybrid nanosheets with enhanced visible-light-driven photocatalytic activity for organic pollutants degradation. Appl. Surf. Sci.. 2020;505:144595
    [Google Scholar]
  12. , , , . One-pot solvothermal fabrication of S-scheme OVs-Bi2O3/Bi2SiO5 microsphere heterojunctions with enhanced photocatalytic performance toward decontamination of organic pollutants. Appl. Surf. Sci.. 2020;527:146775
    [Google Scholar]
  13. , , , . In-situ construction of 3D nanoflower-like BiOI/Bi2SiO5 heterojunctions with enhanced photocatalytic performance for removal of decontaminants originated from a step-scheme mechanism. Appl. Surf. Sci.. 2021;544:148883
    [Google Scholar]
  14. , , , . Fabrication of 3D flower-like OVs-Bi2SiO5 hierarchical microstructures for visible light-driven removal of tetracycline. Surf. Interfaces. 2022;29:101787
    [Google Scholar]
  15. , , , . Graphene quantum dots modified flower like Bi2WO6 for enhanced photocatalytic nitrogen fixation. J. Colloid Interface Sci.. 2019;557:498-505.
    [Google Scholar]
  16. , , , . Carbon quantum dots (CQDs) modified TiO2 nanorods photoelectrode for enhanced photocathodic protection of Q235 carbon steel. Corros. Sci.. 2020;176:108919
    [Google Scholar]
  17. , , , . 2D/2D Z-scheme heterojunction of CuInS2/g-C3N4 for enhanced visible-light-driven photocatalytic activity towards the degradation of tetracycline. Sep. Purif. Technol.. 2019;210:608-615.
    [Google Scholar]
  18. , , , . Ternary Ni2P/Bi2MoO6/g-C3N4 composite with Z-scheme electron transfer path for enhanced removal broad-spectrum antibiotics by the synergistic effect of adsorption and photocatalysis. Chinese J. Chem. Eng.. 2022;44:157-168.
    [Google Scholar]
  19. , , , . CdS nanoparticles decorated hexagonal Fe2O3 nanosheets with a Z-scheme photogenerated electron transfer path for improved visible-light photocatalytic hydrogen production. Chin. J. Chem. Eng.. 2022;43:266-274.
    [Google Scholar]
  20. , , , . Highly efficient visible-light-driven photo-Fenton catalytic performance over FeOOH/Bi2WO6 composite for organic pollutant degradation. J. Alloy. Compd.. 2022;816:152560
    [Google Scholar]
  21. , , , . Z-Scheme 2D/2D Heterojunction of black phosphorus/monolayer Bi2WO6 nanosheets with enhanced photocatalytic activities. Angew. Chem.-Int. Ed.. 2019;58:2073-2077.
    [Google Scholar]
  22. , , , . Facile construction of hierarchical flower-like Z-scheme AgBr/Bi2WO6 photocatalysts for effective removal of tetracycline: Degradation pathways and mechanism. Chem. Eng. J.. 2019;375:121991
    [Google Scholar]
  23. , , , . Construction of a hydrangea-like Bi2WO6/BiOCl composite as a high-performance photocatalyst. New J. Chem.. 2022;46:2627.
    [Google Scholar]
  24. , , , . Advances in Bi2WO6-based photocatalysts for degradation of organic pollutants. Molecules. 2022;27:8698.
    [Google Scholar]
  25. , , , . Enhancing visible light photocatalytic degradation performance and bactericidal activity of BiOI via ultrathin-layer structure. Appl. Catal. B: Environ.. 2017;211:252-257.
    [Google Scholar]
  26. , , , . ZnO nanosheets-decorated Bi2WO6 nanolayers as efficient photocatalysts for the removal of toxic environmental pollutants and photoelectrochemical solar water oxidation. J. Environ. Manage.. 2020;265:110504
    [Google Scholar]
  27. , , , . Facile one-pot solvothermal method to synthesize solar active Bi2WO6 for photocatalytic degradation of organic dye. J. Alloy. Compd.. 2019;801:502-510.
    [Google Scholar]
  28. , , , . Facile construction of novel Bi2WO6/Ta3N5 Z-scheme heterojunction nanofibers for efficient degradation of harmful pharmaceutical pollutants. Chem. Eng. J.. 2020;402:126165
    [Google Scholar]
  29. , , , . Facile hydrothermal synthesis of Z-scheme Bi2Fe4O9/Bi2WO6 heterojunction photocatalyst with enhanced visible light photocatalytic activity. ACS Appl. Mater. Interfaces. 2018;10(22):18824-18836.
    [Google Scholar]
  30. , , , . Bismuth oxyhalide nanomaterials: layered structures meet photocatalysis. Nanoscale. 2014;6:8473-8488.
    [Google Scholar]
  31. , , , . Nest-like structures of Sr doped Bi2WO6: Synthesis and enhanced photocatalytic properties. Mater. Sci. Eng. B. 2011;176:1264-1270.
    [Google Scholar]
  32. , , , . Historical development and prospects of photocatalysts for pollutant removal in water. J. Hazard. Mater.. 2020;395:122599
    [Google Scholar]
  33. , , , . Three-dimensional network structure assembled by g-C3N4 nanorods for improving visible-light photocatalytic performance. Appl. Catal. B: Environ.. 2019;255:117761
    [Google Scholar]
  34. , , , . Synthesis of GO/TiO2/Bi2WO6 nanocomposites with enhanced visible light photocatalytic degradation of ethylene. Appl. Catal. B: Environ.. 2019;246:303-311.
    [Google Scholar]
  35. , , . Enhancement of photocatalytic properties of Bi2WO6 nanoparticles by Pt deposition. Mater. Res. Bull.. 2013;48:3572-3578.
    [Google Scholar]
  36. , , , . Adsorption and UV/visible photocatalytic performance of BiOI for methyl orange, rhodamine b and methylene blue: Ag and Ti-loading effects. CrstEngComm. 2014;16:3155-3167.
    [Google Scholar]
  37. , , , . Iodine doped Z-scheme Bi2O2CO3/Bi2WO6 photocatalysts: Facile synthesis, efficient visible light photocatalysis, and photocatalytic mechanism. Chem. Eng. J.. 2021;403:126327
    [Google Scholar]
  38. , , , . Generation of hydrogen under visible light irradiation with enhanced photocatalytic activity of Bi2WO6/Cu1.8Se for organic pollutants under Vis-NIR light reign. J. Am. Ceram. Soc.. 2018;101(7):3015-3025.
    [Google Scholar]
  39. , , , . Reduced graphene oxide-mediated Z-scheme BiVO4/CdS nanocomposites for boosted photocatalytic decomposition of harmful organic pollutants. Sci. Total Environ.. 2018;635:741-749.
    [Google Scholar]
  40. , , , . Three-dimensional Z-Scheme Ag3PO4/Co3(PO4)2@Ag heterojunction for improved visible-light photocatalytic degradation activity of tetracycline. J. Alloy. Compd.. 2020;818:152883
    [Google Scholar]
  41. , , , . Influence of annealing temperature of TiO2 nanotubes via hydrothermal method on Ti foil for photocatalytic. Digest J. Nanomater. Biostruct.. 2019;14(2):463-470.
    [Google Scholar]
  42. , , , . Enhancing visible-light photocatalytic activity of Ag-TiO2 nanowire composites by one-step hydrothermal process. Mater. Lett.. 2020;279:128506
    [Google Scholar]
  43. , , , . Mediation of valence band maximum of BiOI by Cl incorporation for improved oxidation power in photocatalysis. Indus. Eng. Chem. Res.. 2016;55:4969-4978.
    [Google Scholar]
  44. , , , . Facile synthesis of broom stick like FeOCl/g-C3N5 nanocomposite as novel Z-scheme photocatalysts for rapid degradation of pollutants. Chemosphere. 2022;307:135716
    [Google Scholar]
  45. , , , . Visible light photocatalysis of BiOI and its photocatalytic activity enhancement by in situ ionic liquid modification. J. Phys. Chem. C. 2011;115:14300-14308.
    [Google Scholar]
  46. , , , . Few-layer BiVO4 nanosheets decorated with SrTiO3: Rh nanoparticles for highly efficient visible-light-driven overall water splitting. Appl. Catal. B: Environ.. 2020;279:119377
    [Google Scholar]
  47. , , , . Hydroxyl decorated g-C3N4 nanoparticles with narrowed bandgap for high efficient photocatalyst design. Appl. Catal. B: Environ.. 2019;244:262-271.
    [Google Scholar]
  48. , , , . Au/g-C3N4 heterostructure sensitized by black phosphorus for full solar spectrum waste-to-hydrogen conversion. Sci. China Mater.. 2022;65:974-984.
    [Google Scholar]
  49. , , , . Synthesis and photocatalytic performance of three-dimensional flower-like bismuth tungstate: Influence of hydrothermal temperature. Mater. Lett.. 2022;314:131892
    [Google Scholar]
  50. , , , . Fabrication of SnO2 nanopaticles/BiOI n-p heterostructure for wider spectrum visible-light photocatalytic degradation of antibiotic oxytetracycline hydrochloride. ACS Sustain. Chem. Eng.. 2017;5(6):5134-5147.
    [Google Scholar]
  51. , , , . Novel n-MoSSe/p-Co3O4 Z-scheme heterojunction photocatalyst for highly boosting photoelectrochemical and photocatalytic activity. J. Alloy. Compd.. 2022;926:166863
    [Google Scholar]
  52. , , , . Novel 0D/2D Bi2WO6/MoSSe Z-scheme heterojunction for enhanced photocatalytic degradation and photoelectrochemical activity. Ceram. Int.. 2022;48:31970-31983.
    [Google Scholar]
  53. , , , . Novel MoSSe/Bi2WO6 S-scheme heterojunction photocatalysts for significantly improved photoelectrochemical and photocatalytic performance. J. Alloy. Compd.. 2023;933:167784
    [Google Scholar]
  54. , , , . Fabrication of 2D/2D Z-scheme highly crystalline carbon nitride/δ-Bi2O3 heterojunction photocatalyst with enhanced photocatalytic degradation of tetracycline. J. Alloy. Compd.. 2022;895:162667
    [Google Scholar]
  55. , , , . Light-assisted synthesis MoSx as a noble metal free cocatalyst formed heterojunction CdS/Co3O4 photocatalyst for visible light harvesting and spatial charge separation. Dalton Trans.. 2018;47:6973-6985.
    [Google Scholar]
  56. , , , . Ternary compound heterojunction from isomerism h-CdS/c-CdSe exhibits boosting photoelectrochemical and hydrogen evolution reaction properties. Int. J. Hydrogen Energy. 2023;48:20324-20337.
    [Google Scholar]
  57. , , , . Boron nitride quantum dots decorated ultrathin porous g-C3N4: Intensified exciton dissociation and charge transfer for promoting visible-light-driven molecular oxygen activation. Appl. Catal. B: Environ.. 2019;245:87-99.
    [Google Scholar]
  58. Zhang X L, Yuan N, Chen T X, et al. Fabrication of hydrangea-shaped Bi2WO6/ZIF-8 visible-light responsive photocatalysts for degradation of methylene blue. Chemosphere, 307: 135949.
  59. , , , . Multichannel-Improved charge-carrier dynamics in well-designed hetero-nanostructural plasmonic photocatalysts toward highly efficient solar-to-fuels conversion. Adv. Mater.. 2015;27(39):5906-5914.
    [Google Scholar]
  60. , , , . Multichannel-improved charge-carrier dynamics in well-designed hetero-nanostructural plasmonic photocatalysts toward highly efficient solar-to-fuels conversion. Adv. Mater.. 2015;27:5906-5914.
    [Google Scholar]
  61. , , , . Synthesis of flower-like Bi2Sn2O7/Bi2WO6 hierarchical composites with enhanced visible light photocatalytic performance. J. Alloy. Compd.. 2019;788:1154-1161.
    [Google Scholar]
  62. , , , . Bi2WO6 nanoflowers: An efficient visible light photocatalytic activity for ceftriaxone sodium degradation. Appl. Surf. Sci.. 2018;436:854-864.
    [Google Scholar]
  63. , , , . Surface structure-dependent molecular oxygen activation of BiOCl single-crystalline nanosheets. J. Am. Chem. Soc.. 2013;135:15750-15753.
    [Google Scholar]
  64. , , , . Preparation and characterization of Cu-doped TiO2 nanomaterials with anatase/rutile/brookite triphasic structure and their photocatalytic activity. J. Mater. Sci. Mater. Electron.. 2021;32:21511-21524.
    [Google Scholar]
  65. , , , . Synthesis of Tin-doped three-dimensional flower-like bismuth tungstate with enhanced photocatalytic activity. Int. J. Mol. Sci.. 2022;23(15):8422.
    [Google Scholar]
  66. , , , . Three-phase mixed titania powder modified by silver and silver chloride with enhanced photocatalytic activity under UV-visible light. Nanomaterials. 2022;12:1599.
    [Google Scholar]
  67. , , , . One-step synthesis of flower-like WO3/Bi2WO6 heterojunction with enhanced visible light photocatalytic activity. J. Mater. Sci.. 2016;51(4):2112-2120.
    [Google Scholar]

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