Perovskite-Like Strontium Bismuth Oxyhalides: Synthesis, Characterisation, Photocatalytic Activity and Degradation Mechanism

Abstract

Recent studies have demonstrated that bismuth oxyhalides with a 2D structure inhibit the recombination of electron–hole pairs. Further, perovskite-like strontium bismuth-based compounds with a special layered Sillen X1 structure have shown potential for use as effective visible-light photocatalysts. Here, a series of strontium bismuth oxyhalide composites were prepared under different calcination conditions. The sample compositions were controlled by modulating the calcination temperature and the secondary calcination time. The synthesised catalysts were characterised by various techniques to identify the product compositions. Under visible-light irradiation, the degradation efficiencies and photocatalytic activities of the different catalysts towards rhodamine B (RhB) and 2-hydroxybenzoic acid (2-HBA) were measured via UV–Vis PDA and electron paramagnetic resonance analyses. To explore the degradation mechanism, scavengers were utilised to detect the radicals produced in the photodegradation test. SrBiO₂Cl exhibited the best RhB degradation efficiency, of 0.0685 h⁻¹, and SrBiO₂Br exhibited a rate of 0.0984 h⁻¹. At 25 °C and 1 atm, the CO₂–CH₄ photocatalytic conversion efficiencies of the optimised SrBiO₂Cl and SrBiO₂Br samples increased to 0.037 and 0.053 μmol g⁻¹ h⁻¹, respectively. The findings confirm that the catalysts are highly recyclable and effective for environmental remediation, achieving the objectives of green chemistry.

1. Introduction

With the continuous advancements in science and technology, the requirements of human beings regarding quality of life are constantly increasing. Annually, more than 1.2 million tons of dyes are produced worldwide, and large quantities of pigments and synthetic dyes are released into the air, water, and soil environments. Compounds that cause environmental pollution, particularly triphenylmethane dyes, which are often used in the textile industry, are banned in many countries because of their high toxicity. However, since the goods produced from such compounds thrive in the market, they are still in use. Notably, approximately 10–20% of the wastewater produced in the dyeing and finishing industry is discharged yearly [1]. Therefore, dye treatment is a topic that requires immediate attention.

Many research methods have been developed to degrade organic pollutants, including adsorption, biodegradation, and photocatalysis. Among them, photocatalysis has attracted considerable attention because of its high efficiency and low energy consumption. Photocatalytic reactions conducted over semiconductor catalysts can convert solar energy into chemical energy and degrade pollutants under mild conditions. Therefore, photocatalysis is considered an effective and green technology [2].

Many semiconductor photocatalysts have been developed, such as TiO₂ and BiOX (X = Cl, Br, I). There are many studies regarding the application of TiO₂ in pollutant degradation and wastewater treatment, since it exhibits a high catalytic activity under light irradiation [3]. However, ultraviolet (UV) light only accounts for ~4% of sunlight, whereas visible light accounts for 30–40%. Conversely, the BiOX series photocatalysts have attracted significant research interest because of their special structures [1,4,5,6,7]. The BiOX crystal has a 2D layered structure ([Bi₂O₂]²⁺), which accounts for its highly asymmetric electrical, magnetic, and optical properties; thus, it is often used in industrial chemistry [4,5,8,9,10]. Further, owing to such properties, the BiOX crystal is conducive for use in separating electrons and holes, and it can achieve a high photocatalytic activity under visible-light irradiation [11]. Bismuth-based materials have a unique layered structure that facilitates the separation of electrons and holes, and they exhibit a high photocatalytic activity under visible-light irradiation [12]. In an ideal cubic symmetrical structure of ABO₃ perovskite oxides, the A-site cation is coordinated with twelve oxygen ions while the B-site cation forms a BO₆ octahedron in complexing with six oxygen ions [13]. In a perovskite-like A_n+1B_nO_3n+1 unit, n is the amount of BO₆ octahedra slabs in a block, n = 1–3 [14]. Notably, inorganic perovskite-like bismuth materials also have a modular structure [5,15]. Among them, bismuth strontium oxyhalides (SrBiO₂X) have been reported. The crystal structure of SrBiO₂X is composed of [BiSrO₂]⁺ layers separated by single Br^- layers, which can be regarded as a perovskite-like [16] and the Sillen X1 series structure. Thus, they are employed in organic syntheses and photocatalytic reactions under visible-light irradiation [16,17]; further, their application in dye degradation has been studied extensively [18].

Rhodamine B (RhB), also known as salt-based rosin, exists as a dark-green crystalline powder. In an aqueous solution, it exhibits green, yellow, and orange-red colours and it can emit fluorescence. In addition to being used as an industrial dyeing material, it is also widely used as a food additive. As a typical triphenylmethane dye with a benzene ring, it causes allergies in humans and is toxic to organisms. Furthermore, due to its detrimental effects on the oesophagus, it has been listed as a hazardous item. RhB induces physical discomfort, severe eye damage, irritation of the human mucous membranes, and cancer [19]. Recently, it was listed as an illegal food additive because of the related safety concerns. 2-Hydroxybenzoic acid (2-HBA) exists as a white powder at room temperature and is insoluble in water. It is a pollutant in industrial wastewater and can be found in rivers and wastewater-treatment systems. Further, due to its bacteriostatic effect, it is often used illegally as a food preservative. Recently, studies showed that at concentrations exceeding the stipulated standard, 2-HBA is detrimental to the environment and human beings. At high concentrations, it causes various disorders, with symptoms including gastric bleeding, blurred vision, and renal dysfunction [20].

In addition to studying the photodegradation of dyes, this study also aimed to achieve CO₂ reduction to generate organic compounds, such as CH₄, CH₃OH, and CO. In 2021, the world’s leaders participated in the United Nations climate change conference (COP26) to discuss strategies for reducing carbon emissions [21]. With the rapid development of industries, the quality of human life has gradually improved; however, carbon pollution has also gradually increased, causing problems such as global warming and climate change. Consequently, reducing carbon emissions has become a serious objective globally. The main source of pollution is the extensive use of fossil fuels by humans, which significantly increases the concentration of greenhouse gases, of which CO₂ exerts the most detrimental impact. CO₂ affects not only ecosystems and the environment but also the economy. Over the years, methods for reducing carbon emissions via carbon capture and storage technologies have been proposed, which are divided into ‘physical’ and ‘chemical’ methods. The physical methods include physical adsorption. For example, zeolite 13X, as an adsorbent, can remove more than 90% of the CO₂ in the flue, with outstanding recyclability. In addition, after the concentration of the desorbed CO₂ is adjusted, zeolite 13X can be used as a carbon source to facilitate the growth of microalgae, which have various valuable uses, including as an energy source or in biotechnology [22]. In this study, the chemical-absorption method was employed, wherein a sodium hydroxide solution was used as the absorbent to form solid sodium bicarbonate [23,24,25,26,27].

In this study, a series of strontium bismuth oxychloride and strontium bismuth oxobromide compounds were prepared by calcination. The calcination temperature and the secondary calcination time were controlled. The target pollutants, RhB and 2-HBA, in aqueous solutions were degraded to determine the optimal conditions and the photocatalyst with the highest degradation efficiencies. Additionally, the catalyst materials were analysed by various characterisation techniques. Further, we applied the catalysts for CO₂ reduction and CH₄ generation. This study provides insights into the visible-light-driven degradation of toxic organic compounds, with the goal of human-health protection and environmental remediation.

2. Results

2.1. As-Prepared Sample Characterisation

2.1.1. Powder XRD

In the solid-phase reaction method, the calcination temperature and the secondary calcination time exert a significant influence on the composition and morphology of the catalyst. In this experiment, the molar ratio of SrCO₃:BiOX (X = Cl, Br) was fixed at 1:1, and the calcination temperature was changed to 600 °C, 700 °C, 800 °C, and 900 °C, sequentially. Further, the secondary calcination time was varied between 24, 36, 48, and 60 h to synthesise a series of strontium bismuth oxyhalides.

Figure 1, Figures S1 and S2 the XRD patterns of the composite materials. The results indicate that the strontium bismuth oxyhalide series, including SrBiO₂Cl (JCPDS 00-049-0610), Bi₆O₇ (JCPDS 03-065-5490), Bi₃O₄Cl (JCPDS 00-036-0760), Bi₂₄O₃₁Cl₁₀ (JCPDS 01-075-0887), SrBiO₂Br (COD ID 37319), and Bi₃O₄Br (COD ID 33560) can be selectively prepared by a simple hydrothermal method. The XRD results are presented in Table 1.

At a molar ratio of 1:1 (SrCO₃:BiOCl) and a reaction temperature of 600 °C, the secondary calcination time was set to 24, 36, 48, and 60 h, sequentially. The XRD pattern is shown in Figure S1a. A comparison of the data reveals that the XRD pattern of the lattice consists of a combinatorial library of SrBiO₂Cl, Bi₃O₄Cl, and Bi₆O₇, without the starting reactant, SrCO₃. Figure S1b shows the XRD patterns of the S1B1C1 series calcined at 700 °C with secondary calcination times of 24, 36, 48, and 60 h. A comparison of the data reveals that the crystal phases produced by the samples synthesised at different calcination times are very similar. They are all mixtures of SrBiO₂Cl and Bi₂₄O₃₁Cl₁₀, and there is no incompletely reacted starting material remaining. The XRD patterns in Figure 1a and Figure S1c are similar, showing the XRD patterns of the S1B1C1 series calcined at 800 °C and 900 °C with secondary calcination times of 24, 36, 48, and 60 h. Comparative studies revealed that the pure-phase SrBiO₂Cl was synthesised.

Figure 1b and Figure S2 display the XRD patterns of the synthesised SrCO₃ and BiOBr at a 1:1 molar ratio, at various temperatures. Figure 1b shows the XRD pattern of the S1B1B1 series obtained at the calcination temperature of 600 °C, and the secondary calcination times of 24, 36, 48, and 60 h, sequentially. A database comparison revealed that the crystal phase is a mixture of SrBiO₂Br and Bi₃O₄Br and that there is residual unreacted starting material, SrCO₃. Figure S2a–c show the XRD pattern of the S1B1B1 series obtained at calcination temperatures of 700 °C, 800 °C, and 900 °C, with secondary calcination times of 24, 36, 48, and 60 h, sequentially. The samples synthesised at different calcination temperatures and secondary times are very similar; they are all pure-phase SrBiO₂Br and no incompletely reacted starting material remains.

According to the experimental results in Table 2, the photocatalytic activities of samples also depend on temperature [16]. We searched for the S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) photocatalysts with the better effect by changing the calcination temperature and time, its XRD is presented in Figure 1. The XRD pattern of the reaction at other temperatures can be seen in Figures S1 and S2.

2.1.2. High-Resolution Transmission Electron Microscopy–Energy Dispersive Spectroscopy

The TEM images of S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) are displayed in Figure 2. Figure 2a–d show that S1B1C1-800-(4 + 24) was synthesised from a mixture of SrCO₃ and BiOCl (molar ratio, 1:1); the calcination temperature was 800 °C, and the secondary calcination time was 24 h. Figure 2a shows a bright-field TEM image, in which the layered structure of the catalyst can be observed. Through the Bragg equation, the value of the d-spacing between the crystallographic planes can be calculated. At a beam angle of θ, the condition for constructive interference is given by nλ = 2dsinθ, where λ is the wavelength of the incident light, and θ is the angle formed by the incident light and the plane normal vector. Figure 2b is measured by comparison with the XRD patterns in the database, and the results show that the lattice fringe spacing (d-spacing) of the sample is d = 0.1445 nm, which corresponds to the (1 7 2) diffraction plane of SrBiO₂Cl. Figure 2c shows the SAED pattern of the electron diffraction on the selected area of the sample. After analysis, uniform and regular light spots are observed, which correspond to the monocrystalline phase. Figure 2d shows the EDS semi-quantitative analysis diagram of the sample, highlighting that the sample contains O, Cl, Sr and Bi. The photocatalyst contains O, Cl, Sr, and Bi at an atomic ratio of 15.61:20.62:29.56:34.21. Figure S3 displays the mapping analysis diagram of each element, highlighting that elements Sr, Bi, O, and Cl are all distributed on the catalyst.

Figure 2e–h reveal that S1B1B1-600-(4 + 24) was synthesised from a mixture of SrCO₃ and BiOBr (molar ratio, 1:1); the calcination temperature is 600 °C, and the secondary calcination time is 24 h. As can be observed in Figure 2e, the catalyst exists as overlapping flakes of different sizes.Figure 2f shows that the d (0.1999 nm) of the sample corresponds to the (0 6 1) diffraction surface of SrBiO₂Br. Figure 2g shows the SAED pattern and, based on the light spots, the sample is presumed to be polycrystalline. From Figure 2h, it can be determined that the sample contains C, O, Br, Sr, and Bi in the atomic ratio of 19.17:15.88:13.17:27.03:24.75. Figure S4 shows the mapping analysis of each element; Sr, Bi, O, and Br are all distributed on the catalyst.

2.1.3. Field Emission Scanning Electron Microscopy–EDS

We implemented FE-SEM–EDS to study the morphology and composition of the sample surface. Additionally, we identified Sr, Bi, O, and Cl or Br as the derivatised sample components, as shown in the EDS spectrum. Figure 3a–d display the SEM and EDS images of S1B1C1-800-(4 + 24). Figure 3a displays the sample with a magnification of 1000×, whereas Figure 3b has a magnification of 10,000×. The sample appears to have an irregular block-like structure, and it presents as a stacked, flat-plate structure. Figure 3c,d show the semi-quantitative analysis EDS diagram. The analysis results show that this catalyst contains four elements: Sr, Bi, O, and Cl. Figure 3e–h show the SEM and EDS images of S1B1B1-600-(4 + 24). Figure 3e shows an image of the sample with a magnification of 1000×, whereas Figure 3f has a magnification of 10,000×. The sample appears to have an irregular sheet-like structure, and it presents as a stack of plate-like structures. Figure 3g,h show the semi-quantitative EDS analysis diagrams, highlighting that the catalyst contains Sr, Bi, O, and Br elements.

2.1.4. X-ray Photoelectron Spectroscopy Spectra

The compositions of the SrBiO₂Cl and SrBiO₂Br/SrCO₃/Bi₃O₄Br samples were investigated through HR-XPS. The SrBiO₂Cl photocatalyst was composed of Sr 3d, Cl 2p, Bi 4f, and O 1s (Figure 4a–d). Figure S5 displays the survey XPS spectrum. The spectrum of the Sr 3d occupied state (Figure 4a) shows the formation of a doublet signal, with binding energies of 132.2 eV and 133.9 eV, associated with Sr 3d_5/2 and Sr 3d_3/2 orbitals, respectively. As shown in Figure 4b,c, Cl 2p has binding energies of 197.0 eV and 198.5 eV, associated with Cl 2p_3/2 and Cl 2p_1/2 orbitals, respectively, as expected of Cl with an oxidation number of −1 [5,28]. The binding energies of 158.0 eV and 163.3 eV correspond to the signals of Bi 4f_7/2 and Bi 4f_5/2, respectively, which are mainly the characteristic peaks of trivalent bismuth [29]. Figure 4d shows the spectrum of O 1s, in which there are two peaks. The signal peak at 528.8 eV belongs to the Bi–O bond, and the signal peak at 530.8 eV belongs to the Sr–O bond [30]. Figure 4e–h show the HR-XPS image of the S1B1B1-600-(4 + 24) sample; Figure S6 shows the full spectrum. In Figure 4e, there are two Sr 3d peaks at binding energies of 132.5 eV and 134.0 eV, corresponding to Sr 3d_5/2 and Sr 3d_3/2, respectively. Figure 4f shows the spectrum of Br 3d, with two peaks at binding energies of 67.4 eV and 68.0 eV, corresponding to Br 3d_5/2 and Br 3d_3/2, respectively, which belong to Br in its negative monovalent form [5]. Figure 4g shows the Bi 4f spectrum, with two peaks at 158.1 eV and 163.4 eV, corresponding to the signals of Bi 4f_7/2 and Bi 4f_5/2, respectively, the characteristic peaks of trivalent bismuth. Figure 4h shows the O 1s spectrum, displaying two signal peaks at 529.0 and 530.7 eV, corresponding to the Bi–O and Sr–O bonding signals, respectively.

The XPS spectra and photocatalytic properties of initial BiOX (X = Cl, Br) and SrCO₃ are important. However, the catalysts we used in our photocatalysis experiments underwent high-temperature calcination, which resulted in the transformation of BiOX (X = Cl, Br) and SrCO₃ into new catalytic species. Therefore, we have not discussed the XPS spectra and photocatalytic properties of initial BiOX (X = Cl, Br) and SrCO₃ in our study.

2.1.5. Ultraviolet–Visible Diffuse Reflectance Spectroscopy

Figure 5 shows the DRS diagram of the optimum photocatalyst at each temperature with the SrCO₃:BiOX synthesis ratio of 1:1. Figure 5a,b show that the sample’s colour is light yellow, and the absorption peaks of the catalyst samples at the four temperatures are all concentrated in the visible-light region (350–500 nm), indicating that the synthesised samples are all visible-light-driven photocatalysts.

To calculate the energy gap of the semiconductor, we implement the Tauc plot method using the formula, (αhν)^1/n = A (hν − E_g), where α is the absorption coefficient, h is Planck’s constant, ν is the light frequency, A is the proportionality, and E_g is the bandgap value. Since the synthesised samples are indirect transition photocatalysts [14], n needs to be substituted with 2 to carry out the calculation [31]. The calculated energy bandgaps of the samples in the S1B1C1 series fall between 2.3 and 3.0 eV. Since the samples of the S1B1B1 series are indirect transition photocatalysts [14], the calculation is performed by substituting n = 2 into the formula, yielding energy bandgap values between 2.8 and 3.2 eV.

2.1.6. FT-IR

The IR absorption peak of bismuth oxyhalide is mainly located below 1000 cm⁻¹. In Figure 6a,b, the absorption peak at 490 cm⁻¹ indicates the existence of the Bi–O functional group in the form of BiO₆, while the absorption peak at 551–557 cm⁻¹ indicates its existence in the form of (BiO)₂CO₃. This confirms the synthesis of a compound containing bismuth oxyhalide [32]. The absorption peaks at 698, 855, and 1400–1450 cm⁻¹ are judged to be the CO₃²⁻ characteristic peaks, which are influenced by the starting material, SrCO₃.

2.1.7. S_BET

Figure 7 presents the adsorption–desorption isotherms for S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) measured under N₂. According to the International Union of Pure and Applied Chemistry classification and the BET method, there are five type-IV adsorption isotherms with H3 hysteresis loops. The inset shows the corresponding S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) pore-size distributions. The S_BET values of S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) were calculated to be approximately 0.5912 and 1.3867 m²/g, respectively; their pore volumes and diameters were 0.0056 cm³ g⁻¹ and 57.09 nm and 0.01429 cm³ g⁻¹ and 48.52 nm, respectively. The larger S_BET and pore volumes are associated with the promotion of the transport of reactants and many surface active sites, which increase the photocatalytic activity. As a result, the larger S_BET and pore volume of S1B1B1-600-(4 + 24) significantly promote its photocatalytic activity.

2.2. Photocatalytic Reaction Activity

2.2.1. Photodegradation of RhB and 2-HBA

To confirm the self-degradation experiments of RhB under light conditions, we conducted UV–Vis analysis (Figure 8a) and found that no photolysis-based RhB degradation was observed within 60 min. Figure 8b shows the observed UV–Vis spectral changes after the photodegradation of RhB in aqueous SrBiO₂Cl. The absorption spectrum shows that the absorption band of the RhB solution drops rapidly, to approximately 550 nm, during irradiation.

For the experiment in this study, we employ an apparent pseudo-first-order model (ln(Co/C) = kt). A first-order linear fit, using the data presented in Table 2, shows that the maximum RhB degradation efficiencies (k) for S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) are 6.85 × 10⁻² h⁻¹ and 9.84 × 10⁻² h⁻¹, respectively. This indicates that the photocatalytic activity of SrBiO₂X samples also depends on temperature [16]. Therefore, the S1B1B1-600-(4 + 24) photocatalyst was substantially more effective than the other photocatalysts. The excellent photocatalytic activity may be attributed to its efficiency in harnessing visible light and its ability to separate electron–hole pairs owing to its layered structure. Of all the samples, the S1B1B1-600-(4 + 24) sample, with its larger S_BET, exhibited the best photocatalytic performance. Moreover, the S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) photocatalysts exhibited excellent recyclability and high chemical stabilities. After each usage cycle, S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) were extracted through centrifugal separation and subsequently reused. Over five cycles, the performance of the catalyst remained nearly stable, with no marked reduction in the photocatalytic activity (Figure 9a,b). The RhB degradation efficiencies remained at 95.74% and 98.37%, thus confirming the excellent recyclability. Based on the XRD patterns, the difference between the used and unused samples was not significant (Figure 9c,d). This indicates that the photocatalysts have high photostability.

An increase in the reaction times of S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) resulted in a decrease in the photodegradation efficiencies of 2-HBA (Figure 10). Specifically, after 108 h and 72 h of visible-light irradiation, the degradation efficiencies (k) of S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) for 2-HBA were 0.20 × 10⁻² h⁻¹ and 1.15 × 10⁻² h⁻¹, respectively.

2.2.2. CO₂ Photoreduction Performance

In studies with experiments involving reactors, information on the light intensity entering the reactor or irradiating the catalyst is often not provided; furthermore, the quantum yield is usually not calculated. Although most papers have reported the yield in μmol g⁻¹ h⁻¹, it is impossible to directly compare the yield in those studies with those obtained with other photocatalysts [33]. Increasing the light intensity may be beneficial for the photoreduction of CO₂. In this study, the CO₂–CH₄ conversion efficiencies of the S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) photocatalysts were determined to be 0.037 μmol g⁻¹ h⁻¹ and 0.053 μmol g⁻¹ h⁻¹, respectively, at 25 °C and 1 atm (Figure 11). CH₄ was the only product available from the gas-chromatographic data, confirming the excellent selectivity of these SrBiO₂X photocatalysts.

2.3. Mechanisms of RhB Photodegradation and CO₂ Photoreduction

The photocatalytic activity is related to the recombination rate of photogenerated electrons and holes, because the hydroxyl radicals and oxygen radicals required for photocatalysis originate from the redox of the photocatalyst electrons and holes. Therefore, the higher the emission intensity, the higher the recombination rate of electrons and holes and the lower the number of electrons and holes available for redox. This will affect the generation of hydroxyl radicals and oxygen radicals and consequently the photocatalysis efficiency. Therefore, PL analysis was conducted to learn about the photocatalytic efficiency of the catalysts. Figure 12a shows the PL diagram of the optimum photocatalyst at each temperature with the SrCO₃:BiOCl synthesis ratio of 1:1. The strong absorption region of this series of catalysts is between 500 and 580 nm, and the y-axis corresponds to the fluorescence emission intensity of the catalyst, which is low.

Figure 12b shows the PL diagram of the optimum photocatalyst at each temperature with the SrCO₃:BiOBr synthesis ratio of 1:1. The catalysts exhibit low intensities, particularly the best catalyst, S1B1B1-600-(4 + 24), which exhibits the lowest value. This result is also reflected in the photocatalytic efficiency obtained in the photocatalytic degradation experiment.

To understand the free radicals involved in the photocatalytic reaction mechanism, a free-radical scavenger was added to the reaction to measure the active species in the reaction. AO, SA, benzoquinone (BQ), and isopropanol (IPA) were employed as the scavengers for the holes (h⁺) [34], singlet oxygen (¹O₂) [35], oxygen radical (^•O₂⁻) [36], and hydroxyl radical (^•OH) [37], respectively.

In Figure 13a,d, η on the y-axis represents (C₀ − C)/C₀ × 100% (C: concentration of the RhB aqueous solution at a fixed time; C₀: initial concentration of the RhB aqueous solution). A low value indicates the degradation of the dye in the aqueous solution. The lower the change in the concentration, the higher the effect of the captured free radicals on the photocatalytic reaction. In the aqueous RhB solution added to S1B1C1-800-(4 + 24) or S1B1B1-600-(4 + 24), the smallest concentration change was observed when BQ was added. In this photocatalytic degradation reaction, the main active species is ^•O₂⁻, followed by h⁺.

In the photodegradation mechanism, the photogenerated electron holes generate free radicals with water and oxygen. The compound 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used as a free-radical scavenger [38]. DMPO was dissolved in an aqueous solution to detect whether the catalyst produces ^•OH during the photocatalytic reaction. If DMPO is dissolved in a methanol solution, it can be used to detect whether the catalyst produces ^•O₂⁻ during the photocatalytic reaction.

Figure 13b,e show the EPR spectra of S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) in the aqueous solution of DMPO. Only background noise was observed when the experiment was conducted in the dark (zero minutes after the light was turned on), and four main lines appeared after 5 min of visible-light irradiation. The intensity ratio of the characteristic peak is 1:2:2:1, and the intensity of the characteristic peak increases slightly as the irradiation time increases to 10 and 15 min. It is speculated that this catalyst will produce ^•OH free radicals in the photocatalytic reaction. Figure 13c,f show the EPR spectra of S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) added with DMPO in a methanol solution. Only background noise was observed when the experiment was conducted in the dark, and the characteristic peak of DMPO–^•O₂⁻ appeared after 5 min of visible-light irradiation. The characteristic peak became increasingly noticeable as the irradiation time increased to 10 and 15 min. It is speculated that this catalyst will generate ^•O₂⁻ radicals in the photocatalytic reaction, which is consistent with the experimental results of the active-species test.

After the valence band (VB) value of the photocatalyst sample was measured using an ultraviolet photoelectron spectrometer (UPS), the VB value and the energy gap value were substituted into the following formula: E_CB = E_VB − Eg, and the conduction band (CB) value of the sample was calculated and recorded in the detection data of active species. We can explore the action mechanism of the catalyst in the visible-light photocatalytic reaction. Figure 14a shows the photocatalytic reaction mechanism of the S1B1C1-800-(4 + 24) photocatalyst, and Figure 14b shows that of the S1B1B1-600-(4 + 24) photocatalyst. After the catalyst is irradiated with visible light, the electrons are excited from the VB to the CB, and they react with O₂ in the process of electron transfer to generate ^•O₂⁻. Further, the holes (h⁺) oxidise OH⁻ (or H₂O) during the transfer process to form ^•OH, separate the electrons and holes, and effectively reduce the electron–hole recombination rate. These free radicals facilitate the degradation of the RhB pollutant dye, achieving good photocatalytic efficiency.

From the CO₂-reduction experiments, we know that at 25 °C and 1 atm, SrBiO₂X facilitates the photocatalytic conversion of CO₂ to CH₄ (Figure 11). This highlights the decisive advantage of SrBiO₂X in CH₄ production and its great potential in applications involving CO₂-conversion catalysis. In this reaction, gaseous CO₂ is dissolved in, and reacts with, water to generate carbonic acid. Thereafter, hydrogen ions dissociate from the carbonic acid to further react, generating organic matter [39]. This indicates that the active substances for CO₂ reduction may be dissolved CO₂ and H₂CO₃. Figure 14 also shows the CO₂-photoreduction mechanism over the SrBiO₂X photocatalysts.

3. Experimental

3.1. Materials

The following analytical-grade reagents were employed without additional purification: Bi(NO₃)₃·5H₂O (Acros Organics, Geel, Belgium), SrCO₃ (Merck, Rahway, NJ, USA), KCl (PanReac Applichem, Darmstadt, Germany), KBr (SHOWA, Tokyo, Japan), melamine (Alfa Aesar, Ward Hill, MA, USA), RhB (TCI, Tokyo, Japan), ammonium oxalate (AO; Shimakyu, Fukushima, Japan), 2-HBA (Katayama, Osaka, Japan), p-benzoquinone (BQ; Alfa Aesar, Ward Hill, MA, USA), sodium azide (SA; Sigma-Aldrich, St. Louis, MO, USA), isopropanol (IPA; Merck, Rahway, NJ, USA), and NaOH (Shimakyu, Fukushima, Japan).

3.2. Instruments and Analytical Methods

X-ray diffraction (XRD) patterns were recorded using a diffractometer (MAC Science MXP18, Tokyo, Japan). Field emission transmission electron microscopy (FE-TEM), energy dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED) analyses were performed using a transmission electron microscope (JEOL JEM-2100F, Tokyo, Japan). Scanning electron microscopy with EDS (SEM–EDS) was performed using a scanning electron microscope (JEOL JSM-7401F, Tokyo, Japan) with an accelerating voltage of 15 kV. High-resolution X-ray photoelectron spectroscopy (HR-XPS) was performed using a spectrometer (ULVAC-PHI, Kanagawa, Japan) with an accelerating voltage of 15 kV. Fourier-transform infrared (FT-IR) spectroscopy was performed using a spectrometer (Thermo Nicolet 380, Massachusetts, USA) in transmission mode. Ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS, 300–800 nm) was performed with a spectrophotometer (Scinco SA-13.1, Seoul, Republic of Korea) at room temperature. The specific surface areas were computed by the Brunauer–Emmett–Teller (BET; Micromeritics Gemini 2370C, Micromeritics, USA) analysis, with liquid nitrogen employed as the adsorbate gas. For UV photoelectron spectroscopy (UPS), a spectrometer (Scinco SA-13.1, Seoul, Republic of Korea) was used. The electron paramagnetic resonance (EPR) spectra were characterised using a spectrometer (ELEXSYS E-580, Bruker. Corporation, Germany) equipped with a frequency counter (Agilent 5310A). To evaluate the CO₂ photoreduction, a gas chromatograph (Thermo Trace 1300, Waltham, MA, USA) with flame-ionisation and thermal-conductivity detectors was employed. The photoluminescence (PL) spectra were obtained using a fluorescence spectrophotometer (Hitachi F-7000, Tokyo, Japan).

3.3. Synthesis of BiOX (X = Cl, Br)

BiOX (X = Cl, Br) was synthesised starting with 5 mmol of Bi(NO₃)₃·5H₂O, to which ethanol (25 mL) was added, followed by continuous stirring for 30 min. Next, a 10 mL ethanol solution of 5 mmol KX (X = Cl, Br) was added, followed by continuous stirring for 4 h. The resulting solid precipitate was collected by filtration. Next, we washed the derivatised precipitate several times with deionised water and ethanol to remove all possible molecular or ionic species. Thereafter, we dried BiOX under a vacuum at 60 °C for 12 h.

3.4. Synthesis of SrBiO₂X (X = Cl, Br)

The SrBiO₂X (X = Cl, Br) samples were prepared by a solid-state reaction method. All the reagents used were of analytical grade and did not require further purification. SrCO₃ and the prepared BiOX were mixed (molar ratio, 1:1) using a pestle and mortar. The mixed powders were calcined in alumina crucibles in air at different temperatures for 4 h, after which they were ground and calcined again. The calcination was performed at 600 °C, 700 °C, 800 °C, and 900 °C, separately, in a muffle furnace, followed by cooling to room temperature. The synthesised samples were labelled according to the SrCO₃/BiOX molar ratio and the reaction temperature, as follows (see Table 3): S1B1C1-600-(4 + 24) to S1B1C1-900-(4 + 60), or S1B1B1-600-(4 + 24) to S1B1B1-900-(4 + 60).

3.5. Photocatalysis Experiments

3.5.1. Photocatalytic Degradation of RhB or 2-HBA

Before conducting the photocatalytic experiments, we prepared Pyrex glass vessels containing 100 mL of a 10 ppm RhB (or 2-HBA) solution. After that, we poured 50 mg of the photocatalyst into the reaction vessels. To achieve adsorption–desorption equilibrium between the catalyst surface and RhB (or 2-HBA), we stirred the suspension magnetically in the dark for 30 min before irradiation. The light source was kept at a distance of 30 cm from the reaction vessel, and the intensity of the visible irradiated light was 21 W·m⁻². Subsequently, we collected 5 mL of the resulting liquid and extracted the photocatalyst by centrifugation. UV–Vis spectroscopy was employed to measure the dye concentration after each reaction cycle.

3.5.2. Photocatalytic Reduction of CO₂

A quartz reactor containing 300 mL of 1 N aqueous NaOH was prepared, after which the aqueous NaOH was saturated with CO₂ by supplying high-purity (99.99%) CO₂ at a rate of 500 mL min⁻¹ using a mass-flow controller. Thereafter, 0.1 g of the photocatalyst was added, followed by irradiation. A syringe was employed to withdraw a 1 mL sample from the photoreactor for gas-chromatography analysis.

4. Conclusions

Through instrumental analyses, it was confirmed that a series of SrBiO₂X was synthesised by a simple calcination method in this study. XRD analysis highlighted the importance of temperature in the formation of products. The various SrBiO₂X catalysts synthesised were employed in the degradation of RhB dye, and the k values were 0.0685 h⁻¹ and 0.0984 h⁻¹. Further, after repeated use, the catalysts still maintained a degradation efficiency of more than 90%, indicating their excellent photodegradation efficiency and recyclability. The main active species in the RhB photodegradation was identified to be ^•O₂⁻. At 1 atm and 25 °C, the S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) photocatalysts afforded CO₂–CH₄ conversion efficiencies of 0.037 μmol g⁻¹ h⁻¹ and 0.053 μmol g⁻¹ h⁻¹, respectively. The proposed catalysts exhibit significant potential for application in environmental remediation through the visible-light-driven decomposition of toxic organic pollutants.

Author Contributions

Conceptualization, C.-C.C.; methodology, Y.-Y.L. and P.-H.L.; software, P.-H.L.; validation, F.-Y.L. and Y.-Y.L.; formal analysis, P.-H.L.; investigation, C.-S.L.; resources, C.-C.C.; data curation, Y.-Y.L. and P.-H.L.; writing—original draft preparation, Y.-Y.L.; writing—review and editing, C.-C.C. and Y.-Y.L.; visualization, F.-Y.L.; supervision, C.-C.C.; project administration, F.-Y.L.; funding acquisition, C.-C.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data is included in the article or Supplementary Materials.

Acknowledgments

We thank the National Taichung University of Education and the Precious Instrument Center for providing the instruments. We thank the Precious Instrument Utilization Center at National Central University, Taiwan (FA07B101X), the Instrumentation Center at Chung Hsing University, Taiwan (FA05B101X), and the Instrumentation Center at National Tsing Hua University, Taiwan (FA04B101X).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. XRD patterns of the as-prepared samples: (a) calcination conditions: SrCO3:BiOCl molar ratio = 1:1, time = (4 + 24)–(4 + 60) h, temp = 800 °C; (b) calcination conditions: SrCO3:BiOBr molar ratio = 1:1, time = (4 + 24)–(4 + 60) h, temp = 600 °C.

Figure 2. (a) FE-TEM image, (b) HR-TEM image, (c) SAED, and (d) EDS of the SrBiO2Cl (S1B1C1-800-(4 + 24)) sample obtained by calcination. (e) FE-SEM image, (f) HR-TEM image, (g) SAED, and (h) EDS of the SrBiO2Br/Bi3O4Br/SrCO3 (S1B1B1-600-(4 + 24)) sample obtained by calcination.

Figure 3. SEM images at (a) 1000× and (b) 10,000× magnifications. (c,d) EDS of the S1B1C1-800-(4 + 24) samples prepared through the calcination method. SEM images at (e) 1000× and (f) 10,000× magnifications. (g,h) EDS of the S1B1B1-600-(4 + 24) samples prepared through the calcination method.

Figure 3. SEM images at (a) 1000× and (b) 10,000× magnifications. (c,d) EDS of the S1B1C1-800-(4 + 24) samples prepared through the calcination method. SEM images at (e) 1000× and (f) 10,000× magnifications. (g,h) EDS of the S1B1B1-600-(4 + 24) samples prepared through the calcination method.

Figure 4. XPS spectra of the as-prepared SrBiO2Cl samples: high-resolution XPS spectra of (a) Sr 3d, (b) Cl 2p, (c) Bi 4f, and (d) O 1s. XPS spectra of the as-prepared S1B1B1-600-(4 + 24) (SrBiO2Br/Bi3O4Br/SrCO3) samples: high-resolution XPS spectra of (e) Sr 3d, (f) Br 3d, (g) Bi 4f, and (h) O 1s.

Figure 4. XPS spectra of the as-prepared SrBiO2Cl samples: high-resolution XPS spectra of (a) Sr 3d, (b) Cl 2p, (c) Bi 4f, and (d) O 1s. XPS spectra of the as-prepared S1B1B1-600-(4 + 24) (SrBiO2Br/Bi3O4Br/SrCO3) samples: high-resolution XPS spectra of (e) Sr 3d, (f) Br 3d, (g) Bi 4f, and (h) O 1s.

Figure 5. UV–Vis DRS of the as-prepared photocatalysts under different calcination conditions: (a) calcination conditions: SrCO3:BiOCl molar ratio = 1:1, temperature = 600–900°C; (b) calcination conditions: SrCO3:BiOBr molar ratio = 1:1, temperature = 600–900 °C.

Figure 6. FT-IR spectra of the as-prepared photocatalysts under different calcination conditions: (a) calcination conditions: SrCO3:BiOCl molar ratio = 1:1, temperature = 600–900 °C; (b) calcination conditions: SrCO3:BiOBr molar ratio = 1:1, temperature = 600–900 °C.

Figure 7. (a) Nitrogen adsorption–desorption isotherms, and (b) the corresponding pore-size distribution curve (inset) for S1B1C1-800-(4 + 24). (c) Nitrogen adsorption–desorption isotherms, and (d) the corresponding pore-size distribution curve (inset) for S1B1B1-600-(4 + 24).

Figure 8. (a) Self-degradation experiment of RhB under light irradiation. (b) Temporal UV–Vis adsorption spectral changes during the photocatalytic degradation of RhB. Photodegradation of RhB as a function of the irradiation time over different photocatalysts (c,d) under visible-light irradiation for S1B1C1-800-(4 + X, X = 24, 36, 48, 60) and (e,f) under visible-light irradiation for S1B1B1-600-(4 + X).

Figure 9. (a,b) Cycling runs and (c,d) XRD patterns acquired before and after the photocatalytic degradation of RhB over the S1B1C1-800-(4 + 24) and S1B1B1-600-(4 + 24) samples, respectively.

Figure 10. Photocatalytic degradation of 2-HBA over (a) S1B1C1-800-(4 + 24) and (b) S1B1B1-600-(4 + 24).

Figure 11. Photocatalytic activity of the as-prepared samples for CO2 photocatalytic reduction to methane over (a) S1B1C1-800-(4 + 24) and (b) S1B1B1-600-(4 + 24).

Figure 12. Photoluminescence spectra of the different strontium bismuth oxyhalides: (a) calcination conditions: SrCO3:BiOCl molar ratio = 1:1, temperature = 600–900 °C; (b) calcination conditions: SrCO3:BiOBr molar ratio = 1:1, temperature = 600–900 °C.

Figure 13. (a) RhB concentration, during the photodegradation as a function of the irradiation time, observed in S1B1C1-800-(4 + 24) with the addition of different scavengers: AO, SA, BQ, and IPA. EPR spectra of (b) DMPO–•OH and (c) DMPO–•O2− under visible-light irradiation. (d) RhB concentration, during the photodegradation as a function of the irradiation time, observed in S1B1B1-600-(4 + 24) with the addition of different scavengers: AO, SA, BQ, and IPA. EPR spectra of (e) DMPO–•OH and (f) DMPO–•O2− under visible-light irradiation.

Figure 13. (a) RhB concentration, during the photodegradation as a function of the irradiation time, observed in S1B1C1-800-(4 + 24) with the addition of different scavengers: AO, SA, BQ, and IPA. EPR spectra of (b) DMPO–•OH and (c) DMPO–•O2− under visible-light irradiation. (d) RhB concentration, during the photodegradation as a function of the irradiation time, observed in S1B1B1-600-(4 + 24) with the addition of different scavengers: AO, SA, BQ, and IPA. EPR spectra of (e) DMPO–•OH and (f) DMPO–•O2− under visible-light irradiation.

Figure 14. Schematic of the bandgap structures of (a) S1B1C1-800-(4 + 24) and (b) S1B1B1-600-(4 + 24).

Table 1

Crystalline-phase changes in the as-prepared samples prepared by a solid-state reaction method. Calcination conditions: SrCO₃:BiOX molar ratio = 1:1, temp = 600–900 °C, time = (4 + 24)–(4 + 60) h.

Time(h)	SrCO3:BiOCl = 1:1
	Temp. (°C)
	600	700	800	900
4 + 24	●▲◆■	●▼	●	●
4 + 36	●▲◆■	●▼	●	●
4 + 48	●▲◆■	●▼	●	●
4 + 60	●▲◆■	●▼	●	●
Time (h)	SrCO3:BiOBr = 1:1
	Temp. ( °C)
	600	700	800	900
4 + 24	● ▲ ■	●	●	●
4 + 36	● ▲ ■	●	●	●
4 + 48	● ▲ ■	●	●	●
4 + 60	● ▲ ■	●	●	●

● SrBiO₂Cl, ▲ SrCO₃, ◆ Bi₆O₇, ■ Bi₃O₄Cl, ▼ Bi₂₄O₃₁Cl₁₀, ● SrBiO₂Br, ■ Bi₃O₄Br.

Table 2

Pseudo-first-order rate constants for the degradation of RhB with photocatalysts under visible-light irradiation (calcination conditions: SrCO₃:BiOX molar ratio = 1:1, temp = 600–700 °C, time = (4 + 24)–(4 + 60) h).

Time(h)	SrCO3:BiOCl = 1:1
	Temp. (°C)
	600		700		800		900
	k (h−1)	R2	k (h−1)	R2	k (h−1)	R2	k (h−1)	R2
4 + 24	0.0662	0.9651	0.0578	0.9020	0.0685	0.9346	0.0128	0.9181
4 + 36	0.0130	0.9261	0.0405	0.9078	0.0133	0.9001	0.0455	0.9007
4 + 48	0.0240	0.9003	0.0466	0.9037	0.0142	0.8702	0.0194	0.9037
4 + 60	0.0257	0.9102	0.0622	0.9627	0.0118	0.8959	0.0171	0.9461
	SrCO3:BiOBr = 1:1
Time (h)	Temp. (°C)
	600		700		800		900
	k (h−1)	R2	k (h−1)	R2	k (h−1)	R2	k (h−1)	R2
4 + 24	0.0984	0.9419	0.0577	0.9000	0.0445	0.9489	0.0127	0.9016
4 + 36	0.0779	0.9055	0.0523	0.9025	0.0729	0.9549	0.0321	0.9454
4 + 48	0.0798	0.9029	0.0729	0.9049	0.0289	0.9031	0.0193	0.9016
4 + 60	0.0728	0.9348	0.0420	0.9010	0.0311	0.9412	0.0205	0.9307

Table 3

Codes of the as-prepared samples prepared by a solid-state reaction method. Calcination conditions: SrCO₃:BiOX molar ratio = 1:1, temp = 600 °C–900 °C, time = (4 + 24)–(4 + 60) h.

Time(h)	SrCO3:BiOCl = 1:1
	Temp. (°C)
	600	700	800	900
4 + 24	S1B1C1-600-(4 + 24)	S1B1C1-700-(4 + 24)	S1B1C1-800-(4 + 24)	S1B1C1-900-(4 + 24)
4 + 36	S1B1C1-600-(4 + 36)	S1B1C1-700-(4 + 36)	S1B1C1-800-(4 + 36)	S1B1C1-900-(4 + 36)
4 + 48	S1B1C1-600-(4 + 48)	S1B1C1-700-(4 + 48)	S1B1C1-800-(4 + 48)	S1B1C1-900-(4 + 48)
4 + 60	S1B1C1-600-(4 + 60)	S1B1C1-700-(4 + 60)	S1B1C1-800-(4 + 60)	S1B1C1-900-(4 + 60)
Time (h)	SrCO3:BiOBr = 1:1
	Temp. ( °C)
	600	700	800	900
4 + 24	S1B1B1-600-(4 + 24)	S1B1B1-700-(4 + 24)	S1B1B1-800-(4 + 24)	S1B1B1-900-(4 + 24)
4 + 36	S1B1B1-600-(4 + 36)	S1B1B1-700-(4 + 36)	S1B1B1-800-(4 + 36)	S1B1B1-900-(4 + 36)
4 + 48	S1B1B1-600-(4 + 48)	S1B1B1-700-(4 + 48)	S1B1B1-800-(4 + 48)	S1B1B1-900-(4 + 48)
4 + 60	S1B1B1-600-(4 + 60)	S1B1B1-700-(4 + 60)	S1B1B1-800-(4 + 60)	S1B1B1-900-(4 + 60)

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13050812/s1, Figure S1: (a)–(c) XRD patterns of as-prepared samples at 600–700 °C and 900 °C (calcination conditions: molar ratio SrCO₃:BiOCl = 1:1, time = (4 + 24)–(4 + 60) h); Figure S2: (a)–(c) XRD patterns of as-prepared samples at 700–900 °C. (calcination conditions: molar ratio SrCO₃:BiOBr = 1:1, time = (4 + 24)–(4 + 60) h); Figure S3: The mapping analysis diagram of each element, highlighting that elements Sr, Bi, O and Cl are all distributed on S1B1C1-800-(4 + 24); Figure S4: The mapping analysis diagram of each element, highlighting that elements Sr, Bi, O and Br are all distributed on S1B1B1-600-(4 + 24); Figure S5: The survey XPS spectrum of S1B1C1-800-(4 + 24); Figure S6: The survey XPS spectrum of S1B1B1-600-(4 + 24); Figure S7: Photocatalytic degradation of RhB as a function of irradiation time over different photocatalysts under different calcination method. (Calcination conditions: molar ratio SrCO3:BiOCl = 1:1, temperature = 600, 700, 900 °C); Figure S8: Photocatalytic degradation of RhB as a function of irradiation time over different photocatalysts under different calcination method. (Calcination conditions: molar ratio SrCO3:BiOBr = 1:1, temperature = 700, 800, 900 °C).

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