Controlled Synthesis of Ag-SnO₂/α-Fe₂O₃ Nanocomposites for Improving Visible-Light Catalytic Activities of Pollutant Degradation and CO₂ Reduction

Abstract

The key to developing highly active α-Fe₂O₃-based photocatalysts is to improve the charge separation and efficiently utilize the electrons with sufficient thermodynamic energy. Herein, α-Fe₂O₃ nanosheets (FO) were synthesized using a metal-ion-intervened hydrothermal method and then coupled with SnO₂ nanosheets (SO) to obtain SO/FO nanocomposites. Subsequently, nanosized Ag was selectively loaded on SO using the photo-deposition method to result in the ternary Ag-SO/FO nanocomposites. The optimal nanocomposite could realize the efficient aerobic degradation of 2,4-dichlorophenol as a representative organic pollutant under visible-light irradiation (>420 nm), exhibiting nearly six-fold degradation rates of that for FO. Additionally, the Ag-SO/FO photocatalyst is also applicable to the visible-light degradation of other organic pollutants and even CO₂ reduction. By using steady-state surface photovoltage spectroscopy, fluorescence spectroscopy, and electrochemical methods, the photoactivity enhancement of Ag-SO/FO is principally attributed to the improved charge separation by introducing SO as an electron platform for the high-energy-level electrons of FO. Moreover, nanosized Ag on SO functions as a cocatalyst to further improve the charge separation and facilitate the catalytic reduction. This work provides a feasible design strategy for narrow-bandgap semiconductor-based photocatalysts by combining an electron platform and a cocatalyst.

1. Introduction

With the rapid development of industry, human beings are faced with increasingly serious environmental pollution. Among various forms of pollution, water pollution caused by discarding organic pollutants into water reservoirs is found throughout the world, which damages the natural ecosystem and harms the health of the living [1,2]. In particular, the chlorophenols and corresponding derivatives such as 2,4-dichlorophenol (2,4-DCP) are the representative organic pollutants for water bodies, which are commonly used in manufacturing herbicides, fungicides, antiseptics, and wood preservations [3,4,5]. The US Environmental Protection Agency has listed 2,4-DCP as one of the most toxic pollutants with potential carcinogenic and mutagenic risks to human health [6,7,8]. Therefore, it is highly desirable to develop green and efficient techniques to degrade organic pollutants, such as 2,4-DCP, in water.

Semiconductor photocatalysis, capable of degrading organic pollutants by the photocatalytic reaction using sustainable solar energy, is regarded as one of the most promising technologies. The key to realizing such a process is to design and fabricate efficient photocatalyst materials. For semiconductors such as photocatalysts, visible-light responsive ones are advantageous in terms of light absorption [9,10,11]. Taking the hematite α-Fe₂O₃ as an example, it is a narrow-bandgap metal oxide semiconductor (2.2 eV), which is stable, economic, and eco-friendly, and hence has been extensively investigated as a promising photocatalyst candidate [12,13,14]. Although the morphology control of two-dimensional (2D) α-Fe₂O₃ to obtain corresponding nanosheets could enlarge the specific surface area and improve the charge separation to a certain extent by shortening the charge transfer distance, its photocatalytic efficiency is still low due to poor charge separation, the unavailability of electrons with sufficient thermodynamic energy, and the lack of catalytic sites for target reactions [15,16,17].

For the photogenerated electrons of α-Fe₂O₃ nanosheets (denoted as FO for short) excited under visible-light irradiation, only some of them possess a high energy level above the conduction band (CB) of FO which could meet the thermodynamic requirement for the reduction of reactants such as O₂, etc. [18]. However, these high-energy-level electrons (HELEs) tend to quickly relax to the CB bottom and then recombine with the holes [19]. Therefore, it is rational to efficiently use the HELEs to realize the photoactivity enhancement of FO under visible-light irradiation. Currently, one of the most applied strategies is to introduce another semiconductor to couple with FO, forming a heterojunction. Traditional wide-bandgap semiconductors (WBS), such as ZnO, TiO_2, and SnO₂, with relatively high CB potentials (<~ 0 eV) become suitable choices [20,21,22,23]. Thus, the coupling of WBSs with FO would function as an electron platform for the HELEs, resulting in improved charge separation. Considering that SnO₂ has high electron mobility and suitable CB potential at around 0 eV, it is feasible to construct dimension-matched 2D/2D SnO₂/α-Fe₂O₃ heterojunctions (denoted as SO/FO) [20,24].

Moreover, catalytic efficiency is another crucial factor in determining photocatalytic performance [18]. If photocatalytic aerobic organic pollutant degradation is the target reaction, the catalytic process for the O₂ photoreduction significantly affects the overall degradation efficiency [25]. Therefore, it is necessary to introduce an effective cocatalyst on SO in SO/FO to further enhance photoactivity. Noble metals, such as Ag, Pt, and Au nanoparticles (NPs), have been reported as favorable cocatalysts in photocatalysis [26,27,28], performing the dual functions of electron acceptors and catalytic sites for activating reactants. Given that the economic cost of Ag is relatively low, selectively loaded Ag NPs on SO in the SO/FO heterojunctions would further facilitate electron transfer and catalytic reduction.

Based upon the above design scheme, herein, ultrathin FO was synthesized using the metal-ion-intervened hydrothermal method, which was coupled with SO obtained via the hydrothermal method to result in 2D/2D SO/FO heterojunctions. Subsequently, nanosized Ag was selectively loaded on SO to eventually obtain the ternary Ag-SO/FO nanocomposites. Under visible-light irradiation (>420 nm), the optimal Ag-SO/FO photocatalyst showed nearly six-fold photoactivity enhancement compared with FO for the aerobic degradation of 2,4-DCP. The steady-state surface photovoltage spectra, fluorescence spectra, and electrochemical results, etc., indicate that the enhanced photoactivity is due to improved charge separation by introducing SO as the electron platform and nanosized Ag as an electron acceptor. Additionally, nanosized Ag could facilitate catalytic reduction as a cocatalyst. This work improves the design scheme for efficient heterojunction photocatalysts.

2. Results

2.1. The Effect of Co-Modification by SO and Nanosized Ag on Photocatalytic Performance

As described in the experimental section, the FO-based nanocomposites were synthesized and then evaluated for photocatalytic aerobic degradation of organic pollutant 2,4-DCP under visible-light irradiation. As shown in Figure 1a, pure FO demonstrated a degradation rate of around 16%.

After coupling SO, the photoactivity of xSO/FO samples, where x% indicates the mass ratio of SO to FO, show obvious enhancement compared with FO (Figure S1a). In particular, 5SO/FO was the best binary sample and provided a degradation rate of 51%. Based on 5SO/FO, Ag NPs with different amounts were photo-deposited to obtain the ternary yAg-5SO/FO nanocomposites, where y% indicates the mass ratio of Ag relative to 5SO/FO. Among all Ag-modified samples, 2Ag-5SO/FO showed the largest degradation rate, which reached approximately six-fold that of FO (Figure S1b). According to the above results, the co-modification of SO and nanosized Ag with suitable amounts could effectively enhance the photocatalytic activities of FO. Considering that stability is another critical index used to evaluate photocatalysts, the recyclability of 2Ag-5SO/FO as the optimal photocatalyst was examined for five consecutive runs. As shown in Figure 1b, no obvious photoactivity decrease could be found after 5 runs for 10 h, indicating that the 2Ag-5SO/FO photocatalyst possesses favorable stability under reaction conditions.

2.2. Structural and Surface Characterization

The structural information of as-synthesized FO-based nanocomposites was collected by several characterization methods. The XRD patterns of FO, SO, 5SO/FO, and 2Ag-5SO/FO are demonstrated in Figure S2a. As shown in Figure S2a, the diffraction peaks of FO were 33.2, 35.62, 39.2, 40.8, 49.5, 54.0, 63.9, and 66.0°, which agree well with the ones of the standard α-Fe₂O₃ JCPDS No. 33-0664. For SO, the peaks at 26.6, 33.8, 37.9, 51.8, 54.7, 57.8, and 61.9° are ascribed to SnO₂ (JCPDS No. 41-1445) [14,17]. The peaks for 5SO/FO were consistent with those for FO. This indicates that the introduction of a tiny amount of SO does not change the crystalline structure of FO. Similarly, with the further introduction of nanosized Ag, for 2Ag-5SO/FO, no extra peaks appeared except those for FO [29]. Moreover, the light absorption of the FO-based samples was investigated by UV–vis DRS. Figure S2b depicts that neither SO nor nanosized Ag affected the light absorption of FO.

Furthermore, the morphology of typical samples was assured by TEM. As shown in Figure S3a, FO takes on 2D morphology, in agreement with [16,17]. Additionally, Figure S3b verifies that SO is also a 2D nanosheet, whose size is smaller than that of FO (Figure S3b). A heterojunction structure could be observed for 5SO/FO (Figure S3c), indicating the successful combination between FO and SO. For 2Ag-5SO/FO, in addition to the heterojunctions, such as 5SO/FO, the Ag NPs with an average diameter of around 5 nm were observed on SO (Figure 2a), evidencing the selective location of nanosized Ag on SO. Next, the chemical state of nanosized Ag was revealed by XPS. The XPS peaks at the binding energies of 373.9 and 367.8 eV were attributed to Ag 3d_3/2 and Ag 3d_5/2, respectively (Figure 2b) [30]. In summary, the 2Ag-5SO/FO nanocomposites were synthesized by co-modifying with SO and nanosized Ag, respectively.

2.3. Photogenerated Charge Separation and Catalytic Function

The charge separation is the key factor determining the photoactivity for semiconductor photocatalysis. Accordingly, the charge separation properties of typical samples were characterized to investigate the effect of SO and nanosized Ag as modifiers. When a photocatalyst capable of oxidizing H₂O is excited in the aqueous environment, it will produce hydroxyl radicals as captured by the coumarin to produce 7-hydroxycoumarin, whose fluorescence signal can be quantified to reflect the charge separation situation of the as-tested sample. The fluorescence spectra (FS) were demonstrated in Figure 3a and Figure S4.

5SO/FO presents the largest FS signal among xSO/FO samples, indicating that the loading of SO on FO improves the charge transfer by forming a heterojunction. After introducing nanosized Ag, for yAg-5SO/FO samples 2Ag-5SO/FO displays the largest signal, implying that an optimal amount of nanosized Ag could further improve the charge separation. Moreover, steady-state surface photovoltage spectroscopy (SS-SPS) is an advanced photophysical technique which can directly reveal the photogenerated charge separation and recombination by means of the surface potential difference of a semiconductor before and after illumination. Generally, the SPS intensity and charge separation are positively correlated. As shown in Figure 3b, the SS-SPS signal intensity sequence is FO < 5SO/FO < 2Ag-5SO/FO, consistent with the FS results [18].

Additionally, the electrochemical impendence spectra (EIS) and linear sweep voltammetry (LSV) curves were collected for typical samples under visible-light irradiation. As shown in Figure S5a and Figure 4a inset, 5SO/FO displayed the lowest semicircle radii among the SO-modified samples, suggesting that 5SO/FO has higher charge mobility/transport than FO. In Figure S5b, the introduction of nanosized Ag further reduces the capacitive arc radii of 5SO/FO, proving that Ag facilitates electron transfer. The LSV curves of FO, 5SO/FO, and 2Ag-5SO/FO are shown in Figure 4a. The photocurrent density signal intensities show the sequence of FO < 5SO/FO < 2Ag-5SO/FO, demonstrating the positive effects of SO and Ag on charge separation. Based upon the above results on the charge separation, it can be found that SO does improve the charge separation as the design scheme. When irradiated by visible light, only FO with a narrow bandgap could be excited. Normally, the photoelectrons of FO would be excited to different energy levels, some of which possess a high-energy level, denoted as HELEs, and hence capable of inducing photoreduction thermodynamically. However, the HELEs would relax to the valance band of FO quickly to recombine with the holes. Therefore, the improved charge separation of 5SO/FO indicates that the HELEs transfer from FO to SO, where SO functions as an electron platform [19,20]. In addition, the further improved charge separation for 2Ag-5SO/FO is caused by nanosized Ag on SO acting as an electron acceptor to facilitate the electron transfer. Accordingly, the introduction of both electron platform and electron acceptor effectively improves the charge separation to enhance the photoactivity.

Except for light absorption and charge separation, the catalytic efficiency for O₂ photooxidation is another essential factor affecting the photoactivity for the aerobic degradation of organic pollutants. Thus, the electrochemical reduction curves in the O₂ bubbled system were collected to investigate the catalytic function for O₂ reduction (Figure 4b). It was demonstrated that the onset potential gradually decreased in the sequence of 2Ag-5FO/SO < 5SO/FO < FO. This indicates that the introduction of SO benefits the catalytic oxidation of O₂. Noteworthily, 2Ag-5SO/FO showed the best catalytic function for O₂ reduction, which is attributed to the catalytic role of nanosized Ag.

2.4. Discussion

To further reveal the catalytic process mechanism for the visible-light aerobic degradation of 2,4-DCP, the trapping experiments were performed for typical samples by adding different scavengers (Figure S6). Specifically, ethylenediamine tetra-acetic acid disodium salt (EDTA-2Na), isopropyl alcohol (IPA), and benzoquinone (BQ) were used to trap the holes, hydroxyl radicals, and superoxo species, respectively. As shown in Figure S6, the existence of either EDTA-2Na and IPA significantly decreased the visible-light degradation rates of 2,4-DCP for FO, 5SO/FO and 2Ag-5SO/FO. This proves that holes and hydroxyl radicals are the main active species that participate the degradation of 2,4-DCP.

The applicability of fabricated samples was also checked for the representative industrial organic dyes BPA and MO (Figure 5a and Figures S7 and S8). Under identical reaction conditions with those for the photocatalytic aerobic 2,4-DCP degradation, the degradation rates of FO, 5SO/FO, and 2Ag-5SO/FO for BPA reached 13%, 35%, and 61%, respectively. On another aspect, the degradation rates of FO, 5SO/FO, and 2Ag-5SO/FO for MO reached 16%, 45%, and 88%, respectively. Compared with FO, 2Ag-5SO/FO realized 4.7-fold and 6-fold photoactivity enhancement for the BPA and MO degradation, respectively. The excellent photocatalytic performances of 2Ag-5SO/FO photocatalyst originate from the favorable charge separation and sufficient thermodynamic energy, and the catalytic function of Ag NPs [31]. Therefore, it can be inferred that 2Ag-5SO/FO might be capable of catalyzing visible-light CO₂ reduction. As expected, under visible light irradiation, the CO and CH₄ production rates for 2Ag-5SO/FO achieved 7.6 µmol g⁻¹ h⁻¹ and 1.5 µmol g⁻¹ h⁻¹, respectively, whose CO₂ conversion rate was nearly 8-fold of that for FO (Figure 5b). The above photocatalytic performances indicate that the design principle of Ag-SO/FO photocatalysts based on improving charge separation is applicable for both the aerobic degradation of organic pollutants and the CO₂ reduction.

Based on the above, the overall photocatalytic mechanism of the Ag-SO/FO photocatalyst is illustrated in Scheme 1. Under the visible-light irradiation, the photoelectrons of excited FO possessing sufficient thermodynamic energy transferred to the CB of as-coupled SO by crossing the interface, which further transferred to supported Ag NPs, resulting in improved charge separation. Moreover, nanosized Ag could function as the cocatalyst to activate the reactants. For the aerobic degradation of the organic pollutants, Ag could adsorb and activate O₂ to facilitate the catalytic reduction of O₂ with the consequence of speeding up electron consumption, which is beneficial to charge separation. Accordingly, the resultant holes and hydroxyl radicals would dominate the degradation of organic pollutants [32,33].

3. Materials and Methods

All the chemicals and reagents were of analytical grade and used as received.

3.1. Synthesis of FO

Ultrathin FO was prepared through the metal-ion-intervened hydrothermal method as previously reported [17]. In particular, Fe(NO₃)₃·9H₂O (0.40 g) and Al₂(SO₄)₃·18H₂O (0.16 g) were dispersed in the deionized (DI) water (10 mL), which was then magnetically stirred for 10 min to obtain a uniform liquid mixture. Subsequently, triethylamine (3 mL) was added dropwise under continuous stirring for 30 min. The resulting mixture was transferred to 50 mL Teflon-lined stainless-steel autoclaves and heated at 160 °C for 24 h, then cooled naturally. The product was collected by centrifugation, cleaned with absolute ethanol and DI water several times, and finally dried in the oven at 60 °C for 12 h. Then, FO was crushed to powder and calcined at 350 °C for 2 h.

3.2. Synthesis of SO

The hydrothermal method was utilized to synthesize SnO₂ nanosheets abbreviated as (SO). In the typical synthesis procedure, 4.5 mM of SnCl₂·2H₂O was added to 15 mL of DI H₂O and magnetically stirred for 15 min at room temperature to form the transparent solution. Then, 30 mL of 0.4 M NaOH aqueous solution was added drop-by-drop, maintained at pH = 10, and stirred for the next 2 h to achieve a translucent milky solution. The solution was then added to a 50 mL autoclave heated in an oven at 180 °C for 12 h. After natural cooling to room temperature, the precipitates were collected by centrifugations and alternatively washed 5 times with DI H₂O and ethanol. The precipitate was oven-dried at 80 °C and ground into powder to obtain SO.

3.3. Synthesis of SO/FO

For fabrication of 2D/2D (xSO/FO) nanocomposites, 1g of FO was dispersed in an equal volume ratio of aqueous ethanol mixture (50:50 mL) and alternatively sonicated and stirred for 30 min. The above-mentioned dispersion was then supplemented with SO in an optimized mass percent ratio (x = 1, 3, 5, and 7%) and the mixture was rapidly stirred for 2 h. The solution was dried in a water bath at 85 °C to completely evaporate all solvents and finally calcined at 350 °C for 2 h in a muffle furnace. The samples were marked as xSO/FO, where x% indicates the mass ratio of SO to FO.

3.4. Synthesis of Ag-SO/FO

For the photo-deposition of nanosized Ag on SO in 5SO/FO nanocomposite, 1 g of freshly prepared 5SO/FO was dispersed in an equal volume ratio of aqueous ethanol mixture (50: 50 mL), stirred, and sonicated for 30 min. Subsequently, the AgNO₃ solution was added into the above dispersions to prepare different mass ratios of Ag (0.5, 1, 2, and 3%) loaded on the surface of 5SO/FO and stirred for 30 min to achieve homogeneous dispersion. The beaker was then sealed with sealing film and bubbled with pure N₂ gas for 1 h to flush out adsorbed O₂ and facilitate an inert atmosphere for the photo-deposition reaction. The mixture solution was irradiated under a 300 W Xe lamp with a UV-cut-off filter (λ > 420 nm) for 3 h, with continuous stirring and water cooling to avoid evaporation. Finally, the mixture solution was centrifuged, and the resulting solid was washed with ethanol and DI water several times. Then, the solid was dried in the vacuum oven at 80 °C for 12 h. The resulting nanocomposites were denoted as yAg-5SO/FO, where y% indicates the mass ratio of nanosized Ag relative to 5FO/SO.

3.5. Photocatalytic Activity for Pollutants Degradation

The as-fabricated nanocomposites were evaluated for the degradation of 2,4-dichlorophenol (2,4-DCP) pollutants. For the photocatalytic degradation under visible-light irradiation, 0.1 g of the photocatalyst was dispersed in 50 mL 2,4-DCP solution with a concentration of 20 mg L⁻¹ and stirred in the dark for 30 min to achieve adsorption–desorption equilibrium. After that, the mixture was placed in an open glass reactor under continuous stirring and irradiation by a 150 W xenon lamp with a UV-cut-off filter (λ > 420 nm) for 1.5 h. Then, 5 mL of supernatant was centrifuged and analyzed for the remaining concentration of 2,4-DCP with a spectrophotometer. In addition, the same experimental protocols were also followed for the photocatalytic aerobic degradation of bisphenol A (BPA) and methyl orange (MO).

3.6. Photocatalytic Activity for CO₂ Conversion

In the following experimental procedure, 20 mg of photocatalyst and 5 mL of DI water were added to a 100 mL steel cylinder reactor with an exposed irradiation surface of 3.5 cm². For the next 20 min, pure CO₂ gas was bubbled into the mixed solution in the cylindrical reactor, then the CO₂ gas supply was switched off and the system was continuously stirred for the next 10 min to achieve equilibrium. The cylindrical photocatalytic reactor was irradiated with a 300 W xenon lamp (radiation source) for the next 4 h, with a UV-cut-off filter (λ > 420 nm) under a continuous flow of water to maintain photocatalytic reactor temperature and avoid solvent evaporation. Moreover, the incident light intensity was 380 mW/cm². After a certain period, 0.25 mL of the gaseous mixture was taken from the top head of the reactor using a syringe and tested for the production of carbon-containing products, such as CO and CH₄, using a gas chromatography model (GC-7920, Beijing).

3.7. Scavenger Experiments

Trapping experiments in the presence of three different types of scavengers, including isopropyl alcohol (IPA), ethylenediamine tetra-acetic acid disodium salt (EDTA-2Na), and benzoquinone (BQ), were performed to determine the reactive species involved in the degradation of aerobic organic pollutants 2,4-DCP under visible-light irradiation. In the typical experimental procedure, 0.1 g of the photocatalyst was dispersed in 50 mL of 20 mg/L solutions of 2,4-DCP and stirred in the dark for 10 min, then 0.8 mL of 1 mM of each of the following scavengers (i.e., IPA, EDTA-2Na, and BQ solution) were added separately to the above-mentioned solution and further stirred in the dark for 20 min to achieve adsorption–desorption equilibrium. After that, the mixture solution was exposed for 1.5 h in an open glass reactor with continuous stirring to a 150 W xenon lamp with a UV-cut-off filter (λ > 420 nm). Then, 5 mL of supernatant was centrifuged and analyzed for the remaining concentration of 2,4-DCP with a spectrophotometer and check the effect of added scavengers on the activity.

3.8. Recycle Experiment

For recycling testing, the photocatalyst was separated from the mixed solution by centrifugation, washed with acetone, followed by DI water, and then dried in the vacuum oven overnight at 70 °C, ready to be utilized for the following cycle.

3.9. Fluorescence Spectra Measurement

Hydroxyl radicals are essential in the photocatalytic process. To determine the number of hydroxyl radicals generated, a 50 mg photocatalyst was dispersed in 40 mL of 0.001 M coumarin solution and stirred in the dark for 30 min to attain adsorption–desorption equilibrium. The solution was then exposed for 1 h to a 150 W xenon lamp with a UV-cut-off filter (>420 nm). Finally, 5 mL of supernatant was centrifuged and analyzed for the production of the hydroxyl radicals using a spectrofluorometer F-7000 FL at excitation wavelengths of 332 nm.

3.10. Characterization

The as-prepared nanocomposite materials were analyzed by different characterization techniques. The crystalline structures were studied by Rigaku D/max-rB diffractometer (Rigaku, Japan) with Cu Kα radiation. The surface morphologies were checked by transmission electron microscopy (TEM) on an FEI Tecnai G2 S-Tw (USA) with an acceleration voltage of 200 kV. The optical bandgap was measured by realizing the UV–visible diffused reflectance spectra (UV–vis DRS) with a Model Shimadzu UV2550 spectrophotometer (Japan), with BaSO₄ as a reference. The surface composition and elemental chemical state of the samples were determined using an X-ray photoelectron spectroscopy (XPS) technique with a Model VG ESCALAB apparatus (Thermo Fisher Scientific, Waltham, MA, USA) and an Mg K X-ray source, and the binding energies were calibrated concerning the signal for adventitious carbon (BE = 284.6 eV). The charge separation property was tested using steady-state surface photovoltage spectroscopy (SS-SPS) using home-built equipment with a lock-in amplifier (SR830, USA) synchronized with a Xenon lamp (CHF XQ500 W) at room temperature. The photoelectrochemical and electrochemical investigation was carried out in a three-electrode configuration with a working electrode (catalyst pasted on FTO glass), a counter electrode (Pt foil), and a reference electrode (Ag/AgCl electrode) in an electrolyte of 0.2 M Na₂SO₄. To prepare the electrode, 20 mg of the catalyst was dispersed in a mixture of 20 μL (5 wt% Nafion ionomers) and 0.15 mL ethanol. This mixture was then sonicated for 30 min and then stirred continuously for 72 h to obtain a homogenous slurry. Cleaned and dried FTO glasses were cut into 3 × 4 cm. The samples were pasted by the doctor blade method and then calcined for 30 min at 300 °C in a muffle furnace. The FTO glass with coated samples was submerged in a quartz cell containing 60 mL of 0.2 M Na₂SO₄ solution, coupled with electrodes, and purged with highly pure O₂ gas for 20 min. The IVIUM V13806 electrochemical workstation (Netherland) was then used to measure electrochemical curves in the range of (0.2 to -2 V). Gamry’s electrochemical workstation was used to record electrochemical impedance spectra at an AC voltage magnitude of 0.05 V and a frequency range of 10³–10 Hz. A Gamry electrochemical workstation was also utilized to record the linear sweep voltammetry (LSV) under light irradiation in a quartz cell. A 500 W xenon lamp with a UV-cut-off filter (λ > 420 nm) was employed as the irradiation source. All electrochemical studies were conducted at standard room temperature.

4. Conclusions

In conclusion, a rationally designed ternary Ag-SO/FO heterojunction photocatalyst was synthesized and applied for efficient aerobic degradation of 2,4-DCP under visible-light irradiation. Compared with pristine FO, the optimal Ag-SO/FO photocatalyst exhibited nearly six-fold photoactivity enhancement, mainly due to the improved charge separation. Specifically, the HELEs of FO would transfer to the CB of SO and then to as-loaded Ag NPs. As the cocatalyst, Ag NPs could catalyze the O₂ reduction to effectively consume the electrons, which also benefits the charge separation. The photocatalyst is also applicable to the visible-light degradation of other organic pollutants (such as BPA and MO) and even CO₂ reduction. This work highlights the importance of modulating photoelectrons for the rational design of heterojunction photocatalysts.