Construction of Structured Hydrotalcite Supported with Silver Halide and Its Enhanced Visible Light Photocatalytic Degradation of Methyl Orange

Abstract

To increase the interaction between a catalyst and large pollutant molecules in industrial wastewater, this study employed worm-like micelles created by surfactants as soft templates for synthesizing structured hydrotalcites with high specific surface areas and diverse pore sizes. Following this, the integration of these hydrotalcites with AgBr yielded supported, structured hydrotalcites that exhibited enhanced redox properties. Characterization techniques, including XRD, FT-IR, SEM, and EDS, validated the successful incorporation of AgBr into the structured hydrotalcites. Furthermore, UV–Vis DRS and electrochemical analyses revealed that the integration with AgBr narrowed the band gap of the hydrotalcites, thereby expanding their light absorption range. At 25 °C with an initial solution pH of 5 and an adsorbent dosage of 0.5 g/L, the efficiency of methyl orange removal by the composite material reached 97.69% after 60 min of dark adsorption. EPR and reactive species-trapping experiments revealed that the high-efficiency degradation of methyl orange was primarily attributed to the combined action of highly active h⁺, •O₂₋, and ¹O₂ species.

1. Introduction

Annually, approximately 2% of the total dyes produced globally are discharged into the environment as wastewater, amounting to roughly 100 tons of dye-laden wastewater being released into rivers every year. Azo dyes, such as methyl orange, comprise the bulk of this dye wastewater [1,2,3]. These dyes are highly stable in structure and are non-biodegradable, making it challenging for microorganisms to degrade them in natural settings. Their prolonged presence and accumulation in aquatic organisms pose a potential risk to water ecosystems [4,5]. Notably, the degradation of these dyes can produce various carcinogenic aromatic amines, making the treatment of dye-containing organic wastewater extremely difficult [6,7,8]. Consequently, there is an urgent requirement for efficient, scalable, portable, and cost-effective wastewater treatment and purification equipment, especially for textile factories located in remote areas. Among various methods for wastewater treatment and purification, photocatalytic degradation technology has garnered significant attention due to its high efficiency in degrading persistent dye pollutants. The current theoretical foundation of semiconductor photocatalysis is primarily based on the solid-state energy band theory. According to this theory, the energy band structure of a semiconductor is composed of multiple bands, including the conduction band (CB), the valence band (VB), and the band gap (Eg) located between them. When light irradiates the surface of a material, electrons in the valence band are excited to higher energy levels [9,10,11]. If the energy acquired by the excited electrons is equal to or greater than the band gap width of the photosensitive material, the valence band electrons will transition to the conduction band, resulting in positively charged photogenerated holes in the valence band. The photogenerated holes and electrons can oxidize water and reduce oxygen molecules, respectively, thereby generating highly reactive radicals (e.g., •O₂₋ and •OH), which directly react with the substrate or indirectly interact with photogenerated holes and electrons to achieve photocatalytic oxidation [12,13,14,15].

Notably, in the process of degrading organic pollutants, most photocatalytic materials exhibit light absorption primarily in the ultraviolet (UV) range. However, the poor visible light absorption and high recombination rate of photogenerated electron–hole pairs have resulted in suboptimal catalytic performance [16,17]. Due to their high specific surface areas and tunable structures, layered double hydroxides (LDHs) have been widely used in catalysis, pharmaceuticals, energy storage, adsorption, and flame retardants [18,19]. However, their large band gap restricts their light absorption to the UV region, and the high recombination rate of photogenerated carriers under UV irradiation significantly limits their application in photocatalysis [20,21,22]. Feng [23] utilized an exfoliated magnesium–aluminum–titanium layered double hydroxide (MgAlTi-LDH) and ultrathin graphene oxide (GO) to construct a composite material (LDH/GO) through electrostatic assembly technology and deeply investigated its performance in catalyzing the reduction of carbon dioxide under visible light. That study demonstrated that the incorporation of GO extended the light response range of the materials to the ultraviolet-visible region. Luo and other researchers [24] prepared a nickel–aluminum layered double hydroxide via the co-precipitation method and formed biochar nanomaterials (NiAl-LDH/BC) with S-shaped heterojunction structures. The results showed that the strong interaction between the LDH and biochar facilitated the efficient separation of photogenerated electrons and holes, improving the efficiency of the photocatalytic process. However, the modified structures were inactivated during recycling, and their photocatalytic efficiency was limited by rapid recombination. AgX (X = Cl, Br, and I), an excellent catalyst that strongly absorbs visible light through the surface plasmon resonance (SPR) effect and self-sensitization, has been used widely in photocatalytic processes [25]. The activity of AgBr in visible light was greatly improved by modifying zinc ferrite (ZnFe₂O₄), as suggested by Chnadel et al., which resulted in the efficient degradation of methyl orange dye [26]. The addition of AgBr can also inhibit the recombination of photogenerated electron–hole pairs, as illustrated by the results of Miao et al., who supported AgBr over terpolymer nanocomposites [27]. Furthermore, it was found that the pore distributions and properties of supports greatly affect the degradation efficiency and stability of photocatalysts by effectively inhibiting electron–hole recombination, as suggested by Joniana et al., when using structured Bi₂MoO₆ as a support [28].

The enhanced cross-linking density of micelles per unit area is facilitated by the interaction between the layered structure of hydrotalcite nanoflakes and worm-like micelles. This improves the spatial entanglement of the worm-like micelles and offers greater adsorption opportunities compared to conventional hydrotalcite [29]. Building on this theory, our team has made significant strides in recent years in developing high-specific-surface-area materials using worm-like micelles as soft templates. Tang et al. prepared a magnesium/aluminum layered double hydroxide (LDH) with mesopores and microstructure through a co-precipitation method, utilizing worm-like micelles as templates. Their results indicated a substantial increase in both the average pore size and specific surface area of the material, leading to improved adsorption and the removal of sulfonated lignite (SL) from oilfield wastewater [30]. Zhou et al. found that hierarchical double magnesium hydroxide (MgAl-LDH) nanoparticles with a flower-like morphology and large specific surface area could be synthesized using worm-like micelles as templates. This enhanced the exposure of adsorption active sites and improved the removal efficiency of pollutants [31]. As mentioned earlier, hydrotalcite carriers with a high specific surface area represent a promising option for minimizing the recombination rate of photogenerated electron–hole pairs in photocatalysis. To create more effective reagents, worm-like micelles composed of trimethyl bromide ammonium with varying alkyl chain lengths and salicylic acid were employed as soft templates. By combining this method with the co-precipitation technique, we induced the synthesis of hydrotalcite carriers characterized by a high specific surface area and large pores. Following this, a range of structured hydrotalcites with supported AgX were prepared through impregnation. This approach aims to tackle challenges such as the limited specific surface area of traditional hydrotalcites, the narrow range of visible light response, and the tendency of common AgX materials to agglomerate, ultimately achieving the efficient degradation of methyl orange in dye wastewater.

2. Materials and Methods

2.1. Materials

Hexahydrate magnesium nitrate (Mg(NO₃)₂·6H₂O), nonahydrate aluminum nitrate (Al(NO₃)₂·9H₂O), hexahydrate cerium(III) nitrate (Ce(NO₃)₃·6H₂O), silver nitrate (AgNO₃), potassium bromide (KBr), sodium chloride (NaCl), silver iodide (AgI), sodium iodide (NaI), and salicylic acid were purchased from Tianjin DaMao Chemical Reagent Factory (Tianjin, China). Sodium hydroxide (NaOH), anhydrous sodium carbonate (Na₂CO₃), dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), hexadecyltrimethylammonium bromide (CTAB), octadecyltrimethylammonium bromide (OTAB), and methyl orange (C₁₄H₁₄N₃NaO₃S) were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China). All chemicals were analytical grade and used as received without further treatment.

2.2. Synthesis of Catalysts

The structured hydrotalcite was prepared based on the co-precipitation method combined with the template technique. In brief, 0.05:2:1 molar ratios of Ce(NO₃)₃·6H₂O, Mg(NO₃)₂·6H₂O, and Al(NO₃)₂·9H₂O were sequentially added to 90 mL of distilled water, labeled as metal salt solution A. Solution B was prepared by dissolving 6.88 g of NaOH and 5.655 g of anhydrous Na₂CO₃ in water. At 60 °C, 0.69 g of salicylic acid was dissolved in 125 mL distilled water and ultrasonically dispersed for 1 h. Then, 0.5 mol/L of surfactant OTAB was dispersed into a 125 mL salicylic acid solution by ultrasonic stirring to obtain a stable and homogeneous micelle solution C. While stirring solution C at 40 °C on a heating magnetic stirrer, solutions A and B were slowly added dropwise at a rate of 1 mL/min using a separatory funnel, adjusting the pH to approximately 10. After the addition was complete, the mixture was aged for 12 h to form the hydrotalcite precursor. The resultant product was repeatedly washed with distilled water until the supernatant reached a neutral pH, dried at 70 °C for 12 h, ground, and then sieved to obtain the structured hydrotalcite, which was labeled as OTAB-CeMgAl-LDH. Similarly, DTAB-CeMgAl-LDH, TTAB-CeMgAl-LDH, and CTAB-CeMgAl-LDH were prepared using various surfactants and CeMgAl-LDH was synthesized under the same conditions without the addition of a surfactant.

Further, 1 g of the prepared structured hydrotalcite was dispersed in 50 mL of distilled water and stirred at 60 °C for 60 min. Subsequently, solid silver nitrate was added and the mixture was stirred at 60 °C in the dark for 60 min. Thereafter, a 10% excess volume of equimolar KBr solution was slowly added to promote the formation of AgBr nanoparticles. After 200 min, the final product was washed three to four times with anhydrous ethanol and then centrifuged with deionized water until the pH was neutral. The sample was dried at 75 °C overnight to obtain the supported structured hydrotalcite and was labeled as AgBr/OTAB-CeMgAl-LDH. Similarly, supported AgBr over various LDH mixtures were prepared and named AgBr/DTAB-CeMgAl-LDH, AgBr/TTAB-CeMgAl-LDH, AgBr/CTAB-CeMgAl-LDH, and AgBr/CeMgAl-LDH.

2.3. Characterization of Catalysts

The pore structure of the catalysts was characterized using an ASAP2010 physical adsorption instrument (Micromeritics, Norcross, GA, USA). The crystallinity and structure of the materials were analyzed by X-ray diffraction using a D8 ADVANCE diffractometer (Bruker, Bremen, German). Fourier transform infrared spectra were obtained with a NEXUS-670 spectrometer (Thermo Nicolet, Madison, WI, USA) within the wavelength range of 4000–400 cm⁻¹. The morphologies of the prepared photocatalysts were observed by using a JSM-6390A scanning electron microscope (JEOL, Tokyo, Japan). The band gap energy of the photocatalysts was determined using a Thermo Scientific K-Alpha UV-Vis spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). Mott–Schottky analysis was conducted on a CHI 760E electrochemical workstation (Chenhua Instrument Co., Ltd., Shanghai, China), employing a platinum electrode as the working electrode, a saturated calomel electrode as the reference electrode, and ITO as the counter electrode. Photoluminescence spectra were acquired using a steady-state/transient fluorescence spectrometer (Edinburgh Instruments Ltd., Livingston, Scotland, UK) at a 380 nm excitation wavelength. Additionally, transient free radicals in the solution were tested using a Bruker EMX Plus-6/1 electron paramagnetic resonance spectrometer (Bruker, Bremen, German).

2.4. Degradation Experiment of HPG

Photocatalytic activity was tested using a jacketed reaction vessel with a 300 W xenon lamp simulating sunlight as the light source. To eliminate errors caused by the potential adsorption of methyl orange, a 60 min dark adsorption period was conducted over the prepared catalyst to ensure adsorption equilibrium. Subsequently, the photocatalytic degradation experiment was performed under the conditions of 25 °C, pH 7, an adsorbent dosage of 0.5 g/L, an initial methyl orange concentration of 25 mg/L, a current intensity of 19 A, and a 50% AgBr support. The remaining concentration of methyl orange in the solution at any given time was determined by measuring its absorbance and referring to the standard curve of methyl orange. The concentration ratio (c_t/c₀) and degradation rate of the structured hydrotalcite supported with AgBr at any given time were calculated using Equation (1).

(1)$\mathit{Degradation}\mathit{rate}=\left(1-{\displaystyle \frac{c_t}{c_0}}\right)\times 100\%$

where (c₀) and (c_t) represent the initial and residual concentrations of methyl orange at any time, respectively, in mg/L. The degradation rate indicates the percentage degradation of methyl orange by the adsorbent.

3. Results and Discussion

3.1. Morphological and Microstructural Characterization

3.1.1. XRD Analysis

Figure 1 presents the XRD patterns of CeMgAl-LDH, OTAB-CeMgAl-LDH, AgBr/CeMgAl-LDH, and AgBr/OTAB-CeMgAl-LDH. The patterns reveal that all hydrotalcites exhibit characteristic reflections of Mg-Al hydrotalcite (PDF#35-0964) at 2θ values of 11.6°, 23.3°, 34.7°, 60.8°, and 62.0°, corresponding to the (003), (006), (012), (110), and (113) crystal planes, respectively. Additionally, AgBr/CeMgAl-LDH and AgBr/OTAB-CeMgAl-LDH show characteristic reflections of silver bromide (PDF#79-0149) at 2θ values of 26.8°, 31.0°, 44.3°, 55.1°, 64.7°, and 73.1°, corresponding to the (111), (200), (220), (222), (400), and (420) crystal planes, respectively. This indicates the successful support of AgBr particles onto the hydrotalcite surface with high crystallinity. Notably, the (012) and (110) reflections of hydrotalcite tend to disappear in AgBr/CeMgAl-LDH and AgBr/OTAB-CeMgAl-LDH, suggesting that the incorporation of silver ions into the hydrotalcite lattice cause the damage of layered structure to some extent [32].

3.1.2. BET Analysis

The pore structure of different hydrotalcites was analyzed through BET characterization with the results presented in Figure 2 and Table 1. As shown in Figure 2a, all hydrotalcites exhibit type IV isotherms with distinct H3 hysteresis loops, and the N₂ adsorption gradually increased and reached a noticeable inflection point at a P/P₀ of 0.8, indicating monolayer adsorption. Conversely, increasing P/P₀ causes a hysteresis loop due to macropore filling induced by the condensation of N₂ molecules within the hydrotalcite channels, leading to a rapid increase in adsorption volume. The pore size distribution curves in Figure 2b indicate that OTAB-CeMgAl-LDH and AgBr/OTAB-CeMgAl-LDH have a wider size distribution compared to CeMgAl-LDH and AgBr/CeMgAl-LDH. This can prove that the three-dimensional networks were constructed using worm-like micelles as templates. According to Table 1, the decrease in surface area over the supported hydrotalcite was found due to the covering or blocking of AgBr particles with a high dispersion of AgBr.

3.1.3. FT-IR Analysis

From the FT-IR spectra results as shown in Figure 3, it was found that all hydrotalcites exhibit metal–oxygen (M-O) vibration peaks within the 400–800 cm⁻¹ wavelength range corresponding to the hydrotalcite lattice. The absorption peaks observed at 800–1200 cm⁻¹ and 1356 cm⁻¹ can be attributed to the symmetric and asymmetric stretching vibrations of interlayer CO₃²⁻, respectively [33]. Additionally, the absorption peaks at 1600 cm⁻¹ and 3430 cm⁻¹ are caused by the bending and stretching vibrations of -OH in the interlayer water of the hydrotalcites. For OTAB-CeMgAl-LDH, the absorption peaks at 2850 cm⁻¹ and 2920 cm⁻¹ corresponded to the asymmetric and symmetric stretching vibrations of the C-H bonds in the alkyl side chains of the surfactant. In AgBr/OTAB-CeMgAl-LDH, the C-H bond stretching vibration absorption peak shifts to 2830 cm⁻¹ due to the attraction of AgBr, resulting in a longer C-H bond.

3.1.4. SEM Analysis

Figure 4 displays scanning electron microscope (SEM) images of hydrotalcites at various magnifications. As illustrated in Figure 4f–h, all the samples exhibit the characteristic hexagonal platelet structure, even after the introduction of AgBr [34]. When viewed under higher magnification, it becomes apparent that the layered structure of the hydrotalcite (seen in Figure 4a,e) undergoes a transformation towards three-dimensional growth. The presence of large pores, as shown in Figure 4c–g, is attributed to the worm-like micelle template. These observations align well with the results obtained from physical adsorption characterization.

3.1.5. EDS Analysis

Figure 5 displays the EDS spectra of AgBr/OTAB-CeMgAl-LDH. The uniform distribution of elements such as O, N, Ce, Mg, and Al on the material surface can be seen. Additionally, Figure 5d,h shows that the Ag and Br elements are also uniformly distributed across the hydrotalcite surface, suggesting that AgBr is highly dispersed on the structured hydrotalcite, which is advantageous for enhancing the photocatalytic activity by providing more active sites for producing radicals. Furthermore, the high correlation in the distribution of Ag and Br elements confirms their presence in the form of AgBr. The prominent color display in the EDS spectra might be due to the excess of Ag and Br elements, resulting in partial aggregation in the central region of the hydrotalcite.

3.2. Optical Properties Characterization

3.2.1. UV-Vis DRS Analysis

The UV-visible diffuse reflectance spectra of CeMgAl-LDH, OTAB-CeMgAl-LDH, AgBr/CeMgAl-LDH, and AgBr/OTAB-CeMgAl-LDH are illustrated in Figure 6a. For CeMgAl-LDH and OTAB-CeMgAl-LDH, a wide absorption range between 200 and 500 nm can be found, with strong absorption in the UV region (λ < 400 nm) but weaker absorption in the 400–500 nm range and no significant absorption at wavelengths λ > 400 nm. In contrast, AgBr/CeMgAl-LDH and AgBr/OTAB-CeMgAl-LDH show absorbance values in the of UV and visible light ranges, indicating an expanded light response range from 200 to 800 nm. Additionally, the structured hydrotalcites (OTAB-CeMgAl-LDH, AgBr/OTAB-CeMgAl-LDH) display a red shift to the visible region compared with the hydrotalcites (CeMgAl-LDH, AgBr/CeMgAl-LDH), which suggests that the structured hydrotalcites with multiscale pores show significantly enhanced visible light absorption [35,36].

The band gaps of the prepared samples determined based on the UV-visible diffuse reflectance spectra absorbance and the Tauc relation are shown in Equation (2):

(αhv)^1/n = K(hv − E_g)(2)

where α represents the absorption coefficient, which is proportional to the absorbance (A) in the UV-visible diffuse reflectance; hv denotes the photon energy in electron volts, eV; n is related to the semiconductor nature (n = 1/2 for direct band gap semiconductors and n = 2 for indirect band gap semiconductors [37,38]; hydrotalcite-based photocatalysts belong to the direct semiconductor category); K is a proportionality constant; and Eg is the band gap energy, eV. By plotting (αhv)² against hv and extrapolating the linear portion of the curve to the x-axis, the intercept gives the band gap value.

As shown in Figure 6b, the band gaps of CeMgAl-LDH, OTAB-CeMgAl-LDH, AgBr/CeMgAl-LDH, and AgBr/OTAB-CeMgAl-LDH are 2.99 eV, 2.98 eV, 2.78 eV, and 2.43 eV, respectively, which is consistent with the conclusions in Figure 6a. This phenomenon indicates that structured hydrotalcites with multiscale pores that use micelles as templates show a decrease in band gap energy, presenting a great opportunity to be excited by low-energy light sources. Consequently, the long wavelength light associated with low energy corresponds to an increased absorption range in the visible light spectrum.

3.2.2. Electrochemical Analysis

To further elucidate the band structure of AgBr/OTAB-CeMgAl-LDH, the flat band potential of the sample was determined using Mott–Schottky analysis (Figure 7a). The results indicate that the slope of the Mott–Schottky (M-S) curve is positive, confirming a typical n-type semiconductor of AgBr/OTAB-CeMgAl-LDH. Considering that 0.1–0.3 eV is above the conduction band to the Fermi level of n-type semiconductors, 0.241 V was selected in this study. The Mott–Schottky plot reveals that the flat band potential of the sample relative to the saturated calomel electrode was −0.57 V, yielding a conduction band potential of approximately −0.33 V [39,40]. Moreover, combining the band gap energy measured from UV-Vis DRS with Equation (3), the band structure of AgBr/OTAB-CeMgAl-LDH is depicted in Figure 7b.

E_g = E_VB − E_CB(3)

where Eg denotes the band gap energy, eV, while E_VB and E_CB represent the valence band and conduction band potentials, respectively, eV.

3.2.3. PL Analysis

The PL spectra of the catalysts at an excitation wavelength of 380 nm are shown in Figure 8. The results indicate that CeMgAl-LDH, OTAB-CeMgAl-LDH, AgBr/CeMgAl-LDH, and AgBr/OTAB-CeMgAl-LDH all exhibit strong fluorescence intensity peaks around 445 nm, while adding AgBr decreased the intensity, indicating the low recombination rate of photogenerated electron–hole pairs. Moreover, the fluorescence intensity of AgBr/OTAB-CeMgAl-LDH was significantly lower than that of AgBr/CeMgAl-LDH, suggesting that AgBr/OTAB-CeMgAl-LDH possesses a more favorable photogenerated electron–hole separation efficiency. This is attributed to the high specific surface area and hierarchical porous structure derived from the micelle template, as suggested above. More effective inhibition of the photogenerated electron–hole pair recombination can be obtained over high-dispersion AgBr [41,42,43].

3.3. Catalytic Properties Investigation

The effects of different catalysts (CeMgAl-LDH, OTAB-CeMgAl-LDH, and AgBr/OTAB-CeMgAl-LDH) on the photodegradation performance of methyl orange were investigated under 25 °C, pH 7, an initial methyl orange concentration of 25 mg/L, and a catalyst concentration of 0.5 g/L. The hydrotalcite without AgBr exhibited excellent adsorption performance for methyl orange, whereas the removal performance of AgBr/OTAB-CeMgAl-LDH was negligible due to pore blockage or a reduction in specific surface area, as shown in Figure 9a. However, for CeMgAl-LDH and OTAB-CeMgAl-LDH, almost no photocatalytic performance could be found after reaching the dark adsorption equilibrium. In contrast, AgBr/OTAB-CeMgAl-LDH exhibited an efficient degradation of methyl orange under simulated sunlight, indicating its high photoresponse capability, as suggested by the above result of the UV-Vis DRS. The degradation performance of the supported structure hydrotalcite with variations of halogenic Ag under optimized degradation conditions was investigated. As shown in Figure 9b, AgBr/OTAB-CeMgAl-LDH exhibited significant photodegradation capabilities for methyl orange, with 96.2% photodegradation rates after 60 min, whereas 63.7% photodegradation rates were reached for AgCl/OTAB-CeMgAl-LDH. Furthermore, almost no photodegradation activity for methyl orange was observed if AgI was used as the active species. This phenomenon is probably related to the band gap energy and the band edge positions of silver halide. In comparison with AgBr, AgCl possesses a larger band gap energy, resulting in a low light absorption capacity and weak photocatalytic activity. AgI not only has a large band gap energy but also has energy band edges that may be unfavorable for effective redox reactions, inhibiting the photodegradation of methyl orange [25].

The degradation rate of methyl orange over structured LDH supported with various AgBr formulations was measured under an initial methyl orange concentration of 25 mg/L and a catalyst dosage of 0.5 g/L, as shown in Figure 9c. AgBr/OTAB-CeMgAl-LDH exhibited a certain degree of adsorption capacity for methyl orange, achieving adsorption equilibrium after 60 min of dark adsorption. Under illumination, as the supporting AgBr increased from 25% to 50%, the degradation rate rose from 86.7% to 96.2% within 20 min, attributed to the increased number of active surface sites for absorbing light energy and photo-induced electron–hole pairs, thereby enhancing the efficiency of the photocatalytic reaction. However, excessive AgBr above 75% caused a decreasing trend of degradation rate due to the partial agglomeration of AgBr particles on the structured hydrotalcite surface, which narrowed the specific surface area of the composite material and decreased the utilization rate of light energy.

Figure 9d illustrates the structured hydrotalcites prepared from micelle surfactants with different carbon chain lengths (DTAB, TTAB, CTAB, OTAB). The results indicate that after reaching adsorption equilibrium, the adsorption removal rates of methyl orange by AgBr/DTAB-CeMgAl-LDH, AgBr/TTAB-CeMgAl-LDH, AgBr/CTAB-CeMgAl-LDH, and AgBr/OTAB-CeMgAl-LDH were 38.5%, 40.9%, 58.3%, and 74.7%, respectively. Similarly, the best photodegradation rate of 97% was obtained over AgBr/OTAB-CeMgAl-LDH after reaching absorbance equilibrium. This was likely due to the larger size of the long-chain surfactant molecules, which give more opportunity for an increase in the interlayer spacing and specific surface area of the hydrotalcites, resulting in a greater likelihood of the adsorption of pollutant molecules and potentially reducing the recombination of photogenerated electron–hole pairs, thereby improving the photocatalytic efficiency.

At 25 °C and an initial solution pH of 7, the photocatalytic performance of structured hydrotalcite (AgBr/OTAB-CeMgAl-LDH) was investigated by varying the catalyst dosage (0.1 g/L, 0.2 g/L, 0.3 g/L, 0.4 g/L, 0.5 g/L) for an initial methyl orange concentration of 25 mg/L. From Figure 9e, it can be seen that the removal efficiency of methyl orange increased with the catalyst dosage, and the best photodegradation rate of 96.7% was achieved at the dosage of 0.5 g/L due to the increasing active sites. Further increases in catalyst dosage led to a decrease in methyl orange degradation, presumably attributed to the excessive catalyst causing an incomplete reaction, which also reduces cost-effectiveness.

The pH level determines the charge state of the pollutant surface during the photocatalytic degradation of methyl orange, thereby influencing its interaction with the catalyst and its degradation efficiency. Thus, examining the initial solution pH is essential for understanding the photodegradation process. The effect of varying pH conditions on the photocatalytic performance of AgBr/OTAB-CeMgAl-LDH (0.5 g/L) was investigated under conditions of 25 °C and an initial methyl orange concentration of 25 mg/L, as illustrated in Figure 9f. The results revealed that AgBr/OTAB-CeMgAl-LDH achieved degradation rates exceeding 90% for methyl orange after 60 min of illumination across a broad pH range (1–13), indicating strong resistance to both acidic and basic environments during the photodegradation phase. However, a great effect of pH on the absorption performance was observed in the dark adsorption stage due to the protonation of hydroxyl groups on the surface of hydrotalcite nanosheets at low pH levels, which generated more positive charges that electrostatically attracted the sulfonic groups (SO₃⁻) of methyl orange, thereby increasing the adsorption efficiency. The present OH^- ions compete with SO₃⁻ for adsorption sites and this resulted in a decreased adsorption capacity at pH levels of 9, 11, and 13 [44].

The surface plasmon resonance (SPR) effect can be generated by trace amounts of Ag⁰ on the catalyst surface, which was controlled by adjusting factors such as the size and morphology of the metal particles. This control enhances the light absorption properties of Ag⁰ nanoparticles in the visible to near-infrared region, thereby extending their light response range. However, excessive Ag⁰ hinders the contact between the catalyst and light source, significantly impeding the practical application of AgBr/OTAB-CeMgAl-LDH in photocatalysis. To mitigate the loss of Ag during catalyst regeneration, H₂O₂ was used as a mild oxidizing agent to recover Ag⁰ from the photocorrosion and decomposition of AgBr/OTAB-CeMgAl-LDH, supplemented with KBr solution to replenish the Br source [45]. The degradation behavior of methyl orange solution (25 mg/L) using the recycled catalyst (0.5 g/L) was studied at 25 °C and pH 7. As shown in Figure 10, the favorable regeneration performance of AgBr/OTAB-CeMgAl-LDH caused it to maintain a removal rate of 83.28% for methyl orange after three cycles.

3.4. Photocatalytic Mechanism Analysis

Triethanolamine (TEOA) was utilized as a scavenger for photogenerated holes (h⁺), tert-butyl alcohol (TBA) for hydroxyl radicals (•OH), carbon tetrachloride (CCl₄) for photogenerated electrons (e⁻), and L-histidine (L-Arg) for singlet oxygen (¹O₂), in order to investigate the influence of various reactive species on the photodegradation of methyl orange by AgBr/OTAB-CeMgAl-LDH [46]. As shown in Figure 11a, the degradation rate of methyl orange was significantly decreased by 41.62% and 86.29% after introducing TEOA and L-Arg. This indicates that h⁺ and ¹O₂ are the active species involved in the reaction. Conversely, the degradation rate of methyl orange remained unchanged after the additions of CCl₄ and TBA, suggesting that the role of •OH and e⁻ in the reaction process is negligible. To further elucidate the formation of reactive species during the photocatalytic process, electron paramagnetic resonance (EPR) spectroscopy was employed with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as spin trapping agents for h⁺ and •O₂⁻, respectively. From the results shown in Figure 11b, it can be observed that the main peak intensity corresponding to •O₂⁻ was almost same with or without light irradiation. As suggested in the previous results, the weak •O₂⁻ signal in the EPR spectrum might be due to its partial oxidation to ¹O₂, and the TEMPO-h⁺ spectrum in the dark shows a main peak with an intensity ratio of approximately 1:1:1 attributed to TEMPO. Upon light irradiation, the peak intensity decreases due to the consumption of TEMPO by h⁺ generated from AgBr/OTAB-CeMgAl-LDH. The significant reduction in the spin trapping agent signal indicates the generation of a large amount of h⁺ over the catalyst, which is consistent with the results of the scavenging experiments [47]. Based on the results, it can be confirmed that the primary reactive species is h⁺, with trace amounts of •O₂⁻ and ¹O₂ also participating in the degradation reaction [48].

The proposed mechanism for the photocatalytic process is depicted in Figure 12. AgBr, as the primary photocatalytic active component, generates electron–hole pairs under light to initiate the photocatalytic reaction. The introduction of OTAB-CeMgAl-LDH significantly increases the specific surface area of the composites, which provides more active sites to enhance the opportunity for contacting methyl orange with catalysts. Firstly, electrons in the valence band of AgBr/OTAB-CeMgAl-LDH are excited to the conduction band and electrons transition from the valence band (VB) to the conduction band (CB), forming photogenerated electron–hole pairs (Equation (4)) with simulated sunlight irradiation. Due to the increase in the specific surface area of LDH when using worm-like micelle as templates, a great probability of the sample contacting pollutants is provided. Such structural modification also broadens the light response range and suppresses the recombination of electron–hole pairs. Therefore, O₂ adsorbed on the surface of the photocatalyst is reduced by photogenerated electrons (e⁻) in the conduction band (CB) to form superoxide radicals (•O₂⁻), or it is oxidized by photogenerated holes (h⁺) in the valence band (VB) to generate singlet oxygen (¹O₂) (Equations (5)–(7)). Both •O₂⁻ and ¹O₂ participate in the photocatalytic degradation of methyl orange. Furthermore, the highly oxidative holes (h⁺) in the VB degrade methyl orange into smaller molecular products such as H₂O and CO₂ (Equation (8)).

(4)$\mathit{AgBr}\mathit{/}\mathit{OTAB}-\mathit{CeMgAl}-\mathit{LDH} \stackrel{hv}{\to } e^{-}+h^{+}$

e⁻ + O₂ → •O₂⁻(5)

h⁺ + •O₂⁻ →¹O₂(6)

h⁺ + O₂ → ¹O₂(7)

MO + h⁺ +•O₂⁻ + ¹O₂ → H₂O + CO₂ + Products(8)

4. Conclusions

In summary, the supported structure hydrotalcite (AgBr/OTAB-CeMgAl-LDH) was successfully prepared via an impregnation method using structured hydrotalcite as a carrier for high-sensitivity photosensitive materials. The supported AgBr was uniformly dispersed with a narrower band gap and large specific surface area. The structural modification broadened the light response range and suppressed the recombination of electron–hole pairs, resulting in an excellent catalytic performance in the degradation of methyl orange. A high photocatalytic degradation rate of 97.70% can be obtained within 60 min and the well performance can be maintained even after three cycles driven by the combined action of h⁺, •O₂⁻, and ¹O₂. This work provides an innovative and rational insight into effective water treatment applications.

Author Contributions

Conceptualization, Y.T., G.C., and J.Y.; methodology, B.B.; validation, J.Y.; formal analysis, C.W.; investigation, Z.Y.; data curation, T.Y.; writing—original draft preparation, J.Y.; writing—review and editing, G.C. and Y.T.; supervision, Y.T.; project administration, G.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data that supports the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors are grateful for support of The Youth Innovation Team of Shaanxi Universities and the work of Modern Analysis and Testing Center of Xi’an Shiyou University.

Conflicts of Interest

Author Chunhui Wang was employed by Xi’an Changqing Tongxin Petroleum Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Table 1

Pore structure parameters of hydrotalcite.

Sample	Specific Area (m2/g)	Pore Volume (cm3/g)	Average Pore Width (nm)
CeMgAl-LDH	73.99	0.53	28.14
OTAB-CeMgAl-LDH	132.89	0.03	13.87
AgBr/CeMgAl-LDH	62.18	0.61	29.51
AgBr/OTAB-CeMgAl-LDH	101.26	0.38	15.00

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