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Article

Synthesis and Characterization of Visible-Light-Responsive TiO2/LDHs Heterostructures for Enhanced Photocatalytic Degradation Performance

by
Jing Wei
and
Liying Ren
*
Shandong Provincial Key Laboratory of Soil Conservation and Environmental Protection, College of Resources and Environment, Linyi University, Linyi 276000, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(17), 2582; https://doi.org/10.3390/w17172582
Submission received: 31 July 2025 / Revised: 27 August 2025 / Accepted: 30 August 2025 / Published: 1 September 2025
(This article belongs to the Special Issue Science and Technology for Water Purification, 2nd Edition)
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Abstract

A novel composite material comprising titanium dioxide and layered double hydroxides (TiO2/LDHs) was innovatively proposed and prepared using the co-precipitation method to overcome the shortcomings of titanium dioxide, such as low efficiency in separating electron–hole pairs induced by light and a low utilization rate of visible light. This material was used to study the visible-light-driven photocatalytic degradation of methylene blue. The experimental results show that by constructing efficient heterojunction structures through the alignment of interface band energies and regulating the interface charge transfer pathways, the recombination rate of photogenerated electron–hole pairs is significantly reduced, and the photocatalytic activity is greatly enhanced. Among the tested samples, the TiO2/LDHs composite material with an aluminum-to-titanium molar ratio of 1:1 (AT11) demonstrated the best photocatalytic performance. Within 70 min of simulated sunlight exposure, the degradation rate of methylene blue reached 98.2%, and the optimal concentration of the catalyst was 1 g/L. The photocatalytic process follows a first-order kinetic model. After four cycles of use, the degradation efficiency of methylene blue by the AT11 composite material was 78.93%, demonstrating good stability. The free radical capture experiments indicated that the main active substances for the photocatalytic degradation of methylene blue were h+ and ·OH. The constructed TiO2/LDHs heterostructure system significantly enhanced the photocatalytic performance of TiO2 materials, which was conducive to the efficient utilization of solar energy.

1. Introduction

Industrial advancement drives the widespread use of synthetic dyes in textiles, food processing, paper manufacturing, and pharmaceuticals. These compounds persist as major wastewater pollutants due to their molecular stability that resists standard treatments. Their polar groups enable water solubility while their chromophores impart color, collectively posing severe ecological threats to aquatic systems through persistent contamination [1,2,3,4]. These dye-derived organic pollutants exhibit carcinogenic, mutagenic, and toxic properties. When these compounds remain untreated, they jeopardize human health and ecological integrity. The effective elimination of such compounds from wastewater is thus critical for pollution control. Recent decades have seen advanced treatment technologies emerge, including adsorption [5,6], biological degradation [7,8], electrocatalysis [9], and photocatalysis [10,11]. Heterogeneous photocatalysis has emerged as a leading strategy for wastewater remediation, leveraging its potent oxidation power to mineralize diverse contaminants without secondary pollution. Its broad-spectrum efficacy against recalcitrant pollutants drives sustained research interest.
Conventional photocatalysts like TiO2 demonstrate robust photocatalytic activity. Under light excitation, they generate electron–hole pairs whose oxidation potential drives dye degradation, specifically oxidizing adsorbed molecules on the catalyst surface [12,13,14]. However, TiO2’s practical utility is constrained by its inherent limitations. Its wide bandgap (e.g., 3.2 eV in the anatase phase) confines its excitation at UV wavelengths, yielding negligible visible-light response and inefficient solar energy harvesting. Furthermore, TiO2 exhibits compromised photocatalytic efficiency due to synergistic limitations: weak surface adsorption capacity and rapid recombination of photogenerated charge carriers. These limitations reduce its performance under operational conditions [15,16,17,18]. To enhance the photocatalytic performance of TiO2, researchers employ three primary modification strategies: elemental doping [19,20], co-catalyst loading [21,22], and heterojunction engineering [23,24].
Doping modification can be classified into two types: metal doping and non-metal doping. Doping atoms are incorporated into the TiO2 crystal structure either by substitution (replacing Ti4+ atoms due to similar ionic radii) or by interstitial means (inserted or highly dispersed on the TiO2 surface in the form of single-nucleus complexes or clusters) [25]. The introduced ions affect the carrier recombination rate or bandgap width inside TiO2 by forming defects or altering the lattice type and ultimately influence the catalytic performance of the semiconductor [26]. Metal doping suppresses the recombination of photogenerated carriers on the surface of TiO2 and is currently the most widely used surface modification method. Metal ion doping includes transition metal ions, rare earth metal ions, and noble metal ions. Among them, the research on transition metal elements such as Pt [27], Ni [28], Zn [29], and Cr [30] is mainly focused on. Due to the multiple valence states of transition metals, the introduction of a small amount of transition metal ions can induce shallow trap energy levels, thereby enhancing electron capture and reducing carrier recombination. However, studies have shown that the doping of transition metal ions should be controlled within an appropriate range. When the amount of metal ions exceeds a certain limit, they become recombination centers, leading to an increase in the recombination rate of carriers [31,32]. The principle of non-metal doping is to mix the p orbitals of non-metals with the O 2p orbitals, thereby introducing impurity energy levels in the bandgap of TiO2 [25]. The valence band edge shifts upward, the bandgap narrows, and thus, the light response range of TiO2 redshifts, responding in the visible light range. Common non-metal doping elements include N [33], C [34], S [35], halogens [36]. They generally improve the photocatalytic performance of TiO2 by generating oxygen vacancies and narrowing the bandgap width.
The supported catalyst loads TiO2 onto various carriers (including carbon nanofibers, ceramics, etc.) [37,38] to solve problems such as the easy aggregation and difficult sedimentation of nanoparticles in a solution. Researchers have found that preparing magnetic composite photocatalysts by loading TiO2 onto magnetic carriers is expected to perfectly solve the problem of difficult separation and recovery [39]. Currently, the magnetic carriers that have been studied more extensively include Fe3O4, γ-Fe2O3, and spinel-type materials [40]. However, studies have found that direct contact between TiO2 and magnetic cores can reduce its photocatalytic activity. For instance, direct contact between Fe3O4 magnetic cores and TiO2 can trigger a “photo-dissolution phenomenon”, where Fe3O4 acts as an electron–hole recombination center, significantly reducing the photocatalytic activity and recyclability of the catalyst [39,40,41]. In addition, the performance of catalysts declines to varying degrees when loaded alone due to the significant reduction in the specific surface area, light absorption efficiency, and adsorption capacity of the composite materials after loading. Further research is still needed on the design, tunability, and photocatalytic activity of TiO2-based supported photocatalysts.
Constructing heterojunctions involves the combination of TiO2 with other semiconductors to achieve effective spatial separation and transfer of photogenerated carriers, thereby enhancing the overall photocatalytic activity of the catalyst. Park et al. [42] prepared In2S3/TiO2 composite materials. The formation of a heterojunction significantly enhanced the rapid separation of charges, resulting in the photocurrent density of this material being approximately 3.5 times that of pure TiO2. Yang et al. [43] constructed a heterojunction between TiO2 and Co3O4 and found that it had interface defects caused by a lattice mismatch. Through experiments and theoretical calculations, they discovered that the built-in electric field generated by electron transfer at the interface effectively promoted the migration of electron–hole pairs in opposite directions, thereby significantly enhancing the separation of photogenerated charges.
Among the aforementioned approaches, constructing heterojunctions with semiconductors, particularly 2D layered materials possessing tailored band alignments, proves highly effective for enhancing TiO2 photocatalysis. This strategy promotes charge separation while suppressing carrier recombination [44,45,46]. Layered double hydroxides (LDHs) are a class of two-dimensional lamellar crystalline compounds classified as anionic clays. Their structure features hexagonal or octahedral crystalline frameworks defining characteristic interlayer galleries [47]. The general chemical formula of layered double hydroxides (LDHs) is given by [M(1−x)2+Mx3+(OH)2]x+(An−)x/n·mH2O, where M2+ represents divalent cations (e.g., Ca2+, Mg2+, Fe2+, Co2+, Mn2+, Cu2+, Zn2+, Ni2+, etc.), M3+ denotes trivalent cations (e.g., Al3+, Cr3+, Fe3+, Mn3+, Co3+, etc.), An− signifies interlayer anions (e.g., CO32−, NO3−, SO42−, Cl−, etc.), and x represents the molar ratio of M2+ to M3+ [48,49]. LDHs feature expansive layered frameworks where bandgap engineering (2.0–3.4 eV) is achievable by varying M2+/M3+ cations. This tunability enables the effective construction of TiO2-based photocatalytic heterojunctions, enhancing visible-light absorption while offering abundant surface active sites. LDHs shorten migration pathways for photogenerated carriers, suppressing charge recombination. This collective effect markedly enhances overall photocatalytic efficiency [50,51,52,53].
We synthesized TiO2/LDHs composites with varied TiO2 loading via MgAl-LDHs layer reconstruction. The materials’ structure and morphology were characterized, while photocatalytic activity was assessed through methylene blue degradation under simulated sunlight. Reaction mechanisms were analyzed based on photocatalytic performance and radical trapping experiments.

2. Materials and Methods

2.1. Materials and Instruments

Butyl titanate (analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), absolute ethanol (analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), magnesium nitrate (analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and aluminum nitrate nonahydrate (analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were directly used for the preparation of composite materials. All aqueous solutions were prepared with deionized water.
The instruments used in this study are as follows: SX2-25-12 muffle furnace (Tianjin Zhonghuan Experimental Electric Furnace Co., Ltd., Tianjin, China), DS-20 vacuum drying oven (Tianjin Zhonghuan Experimental Electric Furnace Co., Ltd., Tianjin, China), SGY-1-type photochemical reactor (Nanjing Sidongke Electrical Appliance Co., Ltd., Nanjing, China), and CL19-1 Magnetic Stirrer (Shanghai SiLe Instrument Co., Ltd., Shanghai, China).

2.2. Synthesis of TiO2/LDHs

2.2.1. Preparation of TiO2 Sol

At room temperature, 4.4 mL of butyl titanate was dissolved in 26.4 mL of anhydrous ethanol (Beaker A) with stirring. Separately, Beaker B was prepared, containing 1.32 mL of triple-distilled water, 17.6 mL of anhydrous ethanol, and 0.62 mL of nitric acid. Using a peristaltic pump, Solution B was added to Beaker A at a rate of 30 drops/min with vigorous stirring. After complete addition, the mixture was continuously stirred for 30 min to yield a milky TiO2 sol.

2.2.2. Synthesis of LDHs Precursors

Under ambient temperature conditions, 12.80 g of magnesium nitrate hexahydrate (Mg(NO3)2 ·6H2O) and 9.38 g of aluminum nitrate nonahydrate (Al(NO3)3 ·9H2O) were weighed and dissolved in an appropriate amount of triple-distilled water in Beaker A. In this experiment, a mixed solution of NaOH and Na2CO3 was used to adjust the pH value of the system. According to the molar ratios of n(OH−)/[n(Mg2+) + n(Al3+)] = 2.2 and n(CO32−)/[n(Mg2+) + n(Al3+)] = 0.667, 6.75 g of sodium hydroxide (NaOH) and 5.29 g of sodium carbonate (Na2CO3) were weighed and dissolved in Beaker B. The solutions in Beakers A and B were simultaneously added dropwise into a beaker containing triple-distilled water at a flow rate of 30 drops per minute. At this point, the pH of the reaction system was stably maintained at 8–9. Using a low-temperature saturated co-precipitation method in a 60 °C water bath, a hydrotalcite with a Mg:Al molar ratio of 2:1 was synthesized. The reaction mixture was continuously stirred during the process to obtain the desired LDHs precursor solution. Upon completion of the reaction, the resulting precipitate was collected through filtration, washed until a neutral pH was achieved [54,55].

2.2.3. Synthesis of TiO2/LDHs Nanocomposites

The LDHs precursor dispersion was stirred for 20 min before dropwise infusion into the TiO2 sol. The continuous stirring (12 h) of this mixture initiated crystallization, followed by a 6 h static aging stage. After the mixture was centrifuged and washed to a neutral pH, the obtained precipitate was oven-dried at 80 °C. Subsequent grinding and calcination (500 °C, 2 h, air) [56] of the precipitate yielded the TiO2/LDHs nanomaterial with Al:Ti = 1:1, labeled AT11. Identical procedures produced the following Al:Ti variants: 1:2 (AT12), 1:3 (AT13), 2:1 (AT21), and 3:1 (AT31).

2.3. Characterization

The phase and structural composition of the materials were analyzed through X-ray diffraction (XRD, D/MAX2500, Rigaku Corporation, Tokyo, Japan) and Raman spectroscopy (Horiba LabRAM HR Evolution, HORIBA Ltd., Kyoto, Japan), Shimadzu Corporation, Kyoto, Japan). The microstructural morphology of the samples was characterized using scanning electron microscopy (SEM, ZEISS Sigma 360, Carl Zeiss AG, Oberkochen, Germany) and transmission electron microscopy (TEM, JEOL JEM-F200, JEOL Ltd., Tokyo, Japan). The optical absorption properties and bandgap structure of the materials were evaluated via ultraviolet–visible diffuse reflectance spectroscopy (Shimadzu UV-3600i Plus, Shimadzu Corporation, Kyoto, Japan).

2.4. Photocatalytic Experiments

The photocatalytic experiments were carried out in an SGY-1-type photochemical reactor, using a 300 W xenon lamp (full spectrum, 320–2500 nm) to simulate a visible light source. The photocatalytic degradation of the target pollutant was conducted using a 15 mg/L methylene blue solution (150 mL), and the catalyst concentration was 1 g/L. Prior to initiating the reaction, the mixture was stirred at 200 rpm in the dark for 20 min to achieve an adsorption equilibrium between the catalyst and the methylene blue solution. Throughout the photocatalytic experiment, magnetic stirring was maintained (200 rpm), and samples were collected every 10 min. After centrifugation, the absorbance of the supernatant was analyzed. A spectrophotometer was employed to measure the absorbance of methylene blue solutions at various concentrations at 665 nm and the maximum absorption wavelength of methylene blue. The absorbance–concentration standard curve was plotted, and through linear fitting, the standard curve equation was obtained as follows:
y = 0.0716 x + 0.0542 ,   R 2 = 0.9992

3. Results and Discussion

3.1. Material Characterization and Analysis

3.1.1. X-Ray Powder Diffraction

To determine the crystal structure and characteristics of the composite materials, XRD analysis was conducted on TiO2, LDHs, and AT11. The detection was carried out using Cu Kα radiation. The diffracted beam was filtered using a Ni monochromator with a wavelength (λ) of 0.15418 nm. The accelerating voltage and current were 40 kV and 200 mA, respectively. The scanning range of the diffraction angle (2θ) was from 10° to 80° with a scan step size of 0.02°. The characterization results are shown in Figure 1. The pure TiO2 material exhibited diffraction peaks at 2θ values of 25.30°, 37.79°, 48.04°, 53.88°, 55.07°, 62.69°, 68.30°, 70.03°, and 74.62°, corresponding to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO2 (ICSD #9852), respectively [57]. LDHs showed prominent diffraction peaks at 2θ values of 35.89°, 46.99°, and 62.13°, which align with the characteristic peaks of the standard card Mg0.67Al0.33(OH)2(CO3)0.165(H2O)0.48 (ICSD #6296), corresponding to the (012), (018) and (113) crystal planes of LDHs [57]. In the XRD pattern of the AT11 composite, the primary diffraction peaks observed for TiO2 were associated with the (101) and (200) planes, while those for LDHs were linked to the (012), (018), and (113) planes. Additionally, two new diffraction peaks appeared in the AT11 composite at 2θ values of 18.18°, 32.55°, and 49.19°, consistent with the characteristic peaks of MgTi2O5 (ICSD #37232), corresponding to its (200), (203), and (250) crystal planes. This indicates that the AT11 composite not only contains the TiO2 and LDHs phases but also forms a new crystalline phase structure of MgTi2O5. The crystallite sizes of the samples were calculated using the Scherrer formula. The particle size of TiO2(101) is 11.1 nm, the average particle size of LDHs(018) is 21.9 nm, and the average particle size of AT11(101) is 45.1 nm. The larger particle size observed for AT11 may be attributed to the formation of a composite structure, where TiO2 nanoparticles are embedded within the interlayers of LDHs during the synthesis process.

3.1.2. Morphological Analysis of TiO2/LDHs

The microstructure and morphology of the TiO2/LDHs composite were investigated using SEM and TEM, as illustrated in Figure 2. Figure 2a,b reveal the layered structure of LDHs and the lattice fringes of TiO2. The particle distribution is relatively uniform, although significant agglomeration is observed. Figure 2c demonstrates that the composite exhibits an irregular polygonal structure, with TiO2 nanoparticles dispersed within the LDHs lamellar structure [57]. This indicates that the layered architecture provides abundant loading sites for TiO2 particles [58]. Such structural modifications are hypothesized to enhance the photocatalytic activity of the composite, a supposition subsequently validated by the results of photocatalytic experiments.

3.1.3. Raman Spectroscopy

The Raman spectrum of the AT11 composite material is illustrated in Figure 3, where the absorption peak at 1065 cm−1 is attributed to the characteristic stretching vibration of CO32− within the interlayer of LDHs [19]. The absorption peak at 846 cm−1 corresponds to NO3−, which is a residue obtained from the decomposition of the precursor. The peak at 777 cm−1 is assigned to the Ti-O-Ti vibration, indicating the successful loading of TiO2 on the surface of LDHs [59]. The peak at 655 cm−1 reflects the interfacial coupling vibration of Ti-O-M (M = Mg/Al), demonstrating the bonding interaction within the heterostructure. In conjunction with the characteristic peaks at 18.18° and 32.55° observed in the XRD characterization of AT11, this peak is identified as the interfacial bonding vibration of Ti-O-Mg [60]. The peaks at 487, 428, and 385 cm−1 represent the v3/v4 vibrational characteristics of the tetrahedral [MO4] groups, indicating the presence of a defect structure with coexisting tetrahedral and octahedral configurations in the composite material. This structure, induced by anion intercalation or synthesis conditions, can optimize the material’s photocatalytic, adsorption, and ion exchange properties [44,61]. The peak at 256 cm−1 is attributed to the vibration of Ti-O-Al or Ti-O-Mg. The peak at 161 cm−1 is identified as the B1g vibrational mode of anatase TiO2, reflecting the symmetric stretching vibration of the Ti-O bond in the TiO2 crystal. Although the typical B1g peak of anatase TiO2 is located near 144 cm−1, the shift to 161 cm−1 may be due to the specific microstructural features of the sample, such as tensile stress introduced by heterointerfaces or surface defects. In summary, these findings demonstrate that TiO2 has been successfully loaded onto the surface of LDHs, forming a heterostructure that enhances the material’s photocatalytic, adsorption, and ion exchange properties.

3.1.4. UV–Vis Diffuse Reflectance Spectroscopy

Ultraviolet–visible diffuse reflectance spectroscopy was performed on TiO2 and AT11 to investigate the optical properties of composite materials, with the results presented in Figure 4. The data reveal that TiO2 exclusively exhibits a UV response, with an absorption edge at 400 nm. In contrast, AT11 demonstrates an extended absorption edge reaching 500 nm. This indicates that the composite formation between TiO2 and LDHs establishes an efficient heterojunction structure through interfacial band alignment. The LDHs layer consists of tunable metal elements, typically possessing a conduction band position higher than that of TiO2 and a valence band position significantly lower than that of TiO2. This band arrangement forms a type II heterojunction at the interface, facilitating the migration of photogenerated electrons from the LDHs conduction band to the TiO2 conduction band, while simultaneously transferring holes from the TiO2 valence band to the LDHs valence band. This process not only effectively suppresses carrier recombination but also extends the photoresponse range of the composite material into the visible light region through the narrow bandgap characteristics of LDHs [62].

3.1.5. Photoluminescence Spectroscopy

The photoluminescence behavior formed by the recombination of electron–hole pairs can reflect the separation, migration, and transfer of photogenerated carriers in semiconductors. Generally, the separation efficiency of electron–hole pairs is measured by photoluminescence spectroscopy (PL). The lower the PL spectrum intensity, the higher the separation efficiency of photogenerated carriers, and the better the photocatalytic activity of the material [63]. The PL spectra were used to study the electron–hole pair separation efficiency of the materials to explain the superior photocatalytic activity of TiO2/LDHs composites, as shown in Figure 5. It is evident from Figure 5 that both TiO2 and TiO2/LDHs are effectively excited at 320 nm. After TiO2 was combined with LDHs, the luminescence intensity decreased significantly. This change is attributed to the capture of photogenerated electrons on TiO2 by the positive charges on the surface of LDHs. This indicates that the TiO2/LDHs composites can effectively suppress the recombination of photogenerated carriers, reduce the recombination rate of photo-induced electron–hole pairs, and thereby enhance the overall photocatalytic activity of the materials.

3.2. Photocatalytic Performance of Photocatalysts

3.2.1. Impact of Diverse Composite Materials on Photocatalytic Performance

This study investigated the photocatalytic degradation efficiency of TiO2/LDHs composite materials (AT11, AT12, AT13, AT21, and AT31) with varying content ratios under simulated sunlight, using methylene blue as the target pollutant, as illustrated in Figure 6. The experimental procedure involved a 20 min dark reaction to achieve an adsorption–desorption equilibrium of the catalyst prior to photocatalytic testing. The results demonstrate that the pure TiO2 photocatalyst exhibits minimal adsorption capacity for methylene blue, while the composite photocatalysts show progressively enhanced adsorption with an increase in the LDHs content. However, the adsorption rate significantly decelerates when the Al:Ti molar ratio exceeds 1:1. Photocatalytic reaction analysis reveals that all TiO2/LDHs composites exhibit substantially higher photocatalytic activity compared with pure TiO2. The photocatalytic degradation efficiency initially increases and subsequently decreases with a rise in the LDHs content, reaching optimal performance at an Al:Ti molar ratio of 1:1 (AT11), thus achieving a remarkable 98.20% degradation rate of methylene blue within 70 min of reaction.
Based on the aforementioned results, the TiO2/LDHs composite material exhibits superior adsorption capabilities, which enhance the direct contact efficiency between the target degradation molecules, their photocatalytic oxidation intermediates, and the photogenerated holes on the composite surface. This synergistic effect of adsorption and photocatalysis significantly improves the overall photocatalytic efficiency. Furthermore, during the material compounding process, TiO2 is embedded into the LDHs layers and pores, ensuring its uniform dispersion on the LDHs carrier. This provides more active sites for photocatalytic reactions, reduces the agglomeration and shielding of photocatalytically active particles, and enhances the overall photocatalytic reaction efficiency. When the proportion of LDHs in the composite material is relatively low, TiO2 can be uniformly distributed on the surface and interlayer of LDHs. However, with the continuous increase in the LDHs content, the excessive LDHs lead to a relative reduction in active sites on the surface of the composite material, resulting in a decrease in its photocatalytic efficiency.

3.2.2. Kinetic Analysis of Photocatalytic Reactions

The photocatalytic degradation data were fitted using a pseudo-first-order kinetic model, with the results presented in Table 1 and Figure 7. The fitting results indicate that the linear correlation coefficients for all catalysts exceed 0.96, demonstrating that their photocatalytic degradation processes conform to the pseudo-first-order reaction kinetic model. Among them, AT11 exhibits the highest photocatalytic reaction rate constant of 0.0543 min−1, which is 7.5 times that of pure TiO2, thereby substantiating a significant enhancement in photocatalytic activity upon the combination of TiO2 with LDHs.
The comparison and analysis results of the catalytic degradation performance of the same type of photocatalytic materials on pollutants are shown in Table 2, where TiO2/LDHs not only show a high degradation rate for methylene blue but also a short period, demonstrating a promising application prospect for the photocatalytic degradation of pollutants.

3.2.3. Impact of Catalyst Concentration on Photocatalytic Performance

The experiment employed AT11 composite material as the catalyst, maintaining identical reaction conditions and procedures as the aforementioned photocatalytic process. The photocatalytic reaction was conducted for 70 min to investigate the impact of varying catalyst concentrations on the degradation of methylene blue. The results are illustrated in Figure 8.
The figure clearly illustrates that when the catalyst concentration is below 1 g/L, the degradation rate of methylene blue exhibits an increasing trend with the elevation of catalyst concentration. The augmentation of catalyst concentration facilitates the adsorption of more reactants on the catalyst surface, thereby enhancing light absorption and improving the generation rate of photogenerated holes, which consequently promotes the photocatalytic reaction. The maximum degradation rate of methylene blue is achieved at a catalyst concentration of 1 g/L. However, when the catalyst concentration exceeds 1 g/L, the degradation rate of methylene blue gradually decreases with the increase in catalyst dosage. This phenomenon can be attributed to the shielding effect of the excessive catalyst on incident light, which adversely affects the transmittance of the reaction solution and consequently reduces the generation rate of photogenerated holes. Therefore, under the experimental conditions of this study, the optimal catalyst concentration is determined to be 1 g/L.

3.2.4. Impact of Inorganic Anions on Photocatalytic Reactions

This study selected Cl−, NO3−, SO42−, and CO32− as representative inorganic anions to investigate their impact on the photocatalytic degradation of methylene blue. Using AT11 as the catalyst at a concentration of 1 g/L, the experiments were conducted with the addition of 5 mmol/L of NaCl, NaNO3, Na2SO4, and Na2CO3, respectively. The photocatalytic reactions were carried out for 70 min, and the results are presented in Figure 9, where Cl−, NO3−, and SO42− exhibit minimal impact on the degradation of methylene blue within the system, whereas CO32− demonstrates a significant inhibitory effect on the photodegradation of methylene blue. This phenomenon can be attributed to the weak acidity of CO32−, which, upon hydrolysis in an aqueous solution, readily combines with hydrogen ions to form HCO3− and OH−. Subsequently, HCO3− reacts with active species such as ·OH generated during the photocatalytic process, resulting in the formation of carbonate compounds. These compounds further cover the active sites, thereby reducing the catalytic activity.

3.2.5. Analysis of Photocatalytic Degradation Mechanism

The primary active species in photocatalytic oxidation reactions include photogenerated holes (h+), hydroxyl radicals (·OH), and superoxide radicals (·O2−). To explore the photocatalytic oxidation mechanism of methylene blue by TiO2/LDHs composites, this study conducted a radical trapping experiment. By introducing selective radical inhibitors (quenchers), the activity of specific radicals was blocked, thereby analyzing the contribution rate of different radicals to the degradation of the target pollutant. We individually added sodium bicarbonate [69] (NaHCO3, h+ scavenger), isopropanol [70] (IPA, ·OH scavenger), and p-benzoquinone [71] (BQ, ·O2− scavenger) to the reaction system, and conducted photocatalytic experiments using AT11 as the catalyst under the same reaction conditions. The results are shown in Figure 10.
As illustrated in Figure 10, the degradation efficiency of AT11 towards methylene blue decreased to 54.52%, 68.10%, and 87.63% upon the addition of NaHCO3, IPA, and BQ, respectively. This indicates that all three reactive species play significant roles in the photocatalytic degradation of methylene blue. Specifically, h+ and ·OH are identified as the primary reactive species responsible for the degradation, while ·O2− exhibits a limited influence on the photocatalytic reaction process.
Based on the above analysis, the reaction mechanism of the photocatalytic oxidation of methylene blue by TiO2/LDHs composite materials is speculated to be as follows. The LDHs component in the composite material exerts its adsorption performance, adsorbing organic pollutants on the adsorption sites, achieving the enrichment of organic pollutants and significantly increasing the adsorption capacity of organic pollutants on the catalyst surface. Under simulated sunlight irradiation, the TiO2/LDHs composite materials absorb light energy and become excited. The valence band electrons of O’s 2p orbitals transition to the hybridized Ti 3d orbitals and Al 3p orbitals in the conduction band, forming photogenerated electrons (e−), while photogenerated holes (h+) are formed in the valence band, generating electron–hole pairs. The type II heterojunction formed at the interface between TiO2 and LDHs drives the photogenerated electrons to migrate from the LDHs conduction band to the TiO2 conduction band, while the holes transfer from the TiO2 valence band to the LDHs valence band. This process promotes the transfer of charges, and the existence of intermediate energy levels in indirect bandgaps prolongs the recombination time of electron–hole pairs, resulting in a decrease in the recombination rate of photogenerated electrons and holes, and thereby enhancing the photocatalytic activity of the composite material. In the TiO2/LDHs composite material photocatalytic degradation of the methylene blue system, the main active groups involved in the reaction are photogenerated holes (h+) and hydroxyl radicals (·OH). h+ has strong oxidizing properties and can directly undergo redox reactions with methylene blue molecules. ·OH is generated by the combination of h+ with H2O and OH−, and it cooperates in oxidizing and degrading methylene blue.

3.2.6. Evaluation of Photocatalytic Stability in Composite Materials

The reusability of photocatalytic materials constitutes a critical factor in assessing the stability of catalyst performance. This study conducted four consecutive cycling experiments on AT11 to evaluate its photocatalytic degradation efficiency of methylene blue, with the results illustrated in Figure 11. The data demonstrate that, after four cycles of reuse, the degradation efficiency of AT11 for methylene blue decreased from 98.20% to 78.93%, indicating relatively favorable stability of the catalyst. The observed decline in activity may be attributed to two primary factors: minor sample loss during the recovery and washing processes, and the partial coverage of catalyst surface pores by reactants and products during the reaction, leading to the deactivation of active sites and subsequent reduction in degradation efficiency.

4. Conclusions

TiO2/LDHs composite materials with varying Al/Ti molar ratios were synthesized via the co-precipitation method. The formation of an efficient heterojunction structure through interfacial band alignment effectively suppressed carrier recombination and extended the photoresponse range of the composite materials into the visible light spectrum. The AT11 composite demonstrated optimal performance, achieving a 98.20% degradation rate of methylene blue (15 mg/L) under simulated sunlight irradiation for 70 min. The photocatalytic reaction followed first-order kinetics with a rate constant of 0.0543 min−1. After four consecutive cycles, the AT11 composite maintained a methylene blue degradation efficiency of 78.93%, exhibiting excellent stability.
The composite of TiO2 with LDHs demonstrates superior photocatalytic performance due to the synergistic effects of different catalyst components and modifications in the band structure. LDHs significantly enhance the adsorption capacity of the composite while mitigating the agglomeration of TiO2, thereby providing more effective active sites for the reaction and substantially improving the efficiency of photocatalytic degradation. Radical trapping experiments reveal that the primary active species responsible for the photocatalytic degradation of methylene blue by TiO2/LDHs are h+ and ·OH.

Author Contributions

Conceptualization, J.W. and L.R.; methodology, J.W. and L.R.; formal analysis, J.W.; data curation, J.W. and L.R.; writing—original draft preparation, J.W.; writing—review and editing, L.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by “Key Research and Development Program Project of Shandong Province”, grant number 2024TSGC0884.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors acknowledge the financial support through the Key Research and Development Program Project of Shandong Province, number (2024TSGC0884), Shandong, China.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 02582 g001
Figure 1. XRD patterns of TiO2, LDHs, and AT11.
Figure 1. XRD patterns of TiO2, LDHs, and AT11.
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Figure 2. Microscopic morphology of materials: (a,b) AT11 (TEM) and (c) AT11 (SEM).
Figure 2. Microscopic morphology of materials: (a,b) AT11 (TEM) and (c) AT11 (SEM).
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Figure 3. Raman spectroscopy spectrum of AT11.
Figure 3. Raman spectroscopy spectrum of AT11.
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Figure 4. UV–Vis spectra of materials: (a) TiO2 and (b) AT11.
Figure 4. UV–Vis spectra of materials: (a) TiO2 and (b) AT11.
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Figure 5. Photoluminescence spectra of materials.
Figure 5. Photoluminescence spectra of materials.
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Figure 6. Degradation effect of different photocatalytic materials on methylene blue: (a) Comparison of degradation effects of TiO2/LDHs with different Al:Ti molar ratios; (b) Comparison of degradation effects of TiO2 and LDHs.
Figure 6. Degradation effect of different photocatalytic materials on methylene blue: (a) Comparison of degradation effects of TiO2/LDHs with different Al:Ti molar ratios; (b) Comparison of degradation effects of TiO2 and LDHs.
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Figure 7. Pseudo-first-order kinetic fitting curve for photocatalytic degradation of methylene blue.
Figure 7. Pseudo-first-order kinetic fitting curve for photocatalytic degradation of methylene blue.
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Figure 8. Impact of catalyst concentration on methylene blue degradation.
Figure 8. Impact of catalyst concentration on methylene blue degradation.
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Figure 9. Effect of coexisting anions on degradation of methylene blue by AT11.
Figure 9. Effect of coexisting anions on degradation of methylene blue by AT11.
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Figure 10. Free-radical-trapping experiment.
Figure 10. Free-radical-trapping experiment.
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Figure 11. Recycling of AT11 composites.
Figure 11. Recycling of AT11 composites.
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Table 1. Kinetic parameters of photocatalytic degradation of methylene blue.
Table 1. Kinetic parameters of photocatalytic degradation of methylene blue.
SampleK/min−1R2
TiO20.007 20.968 7
AT130.009 10.976 6
AT120.010 20.987 5
AT110.054 30.980 5
AT210.016 80.965 4
AT310.027 00.969 6
Table 2. Comparison of photocatalytic properties between similar materials.
Table 2. Comparison of photocatalytic properties between similar materials.
CatalystPollutantLight SourceC0 (mg/L)Time (min)Efficiency (%)Ref.
g-C3N4/NiAl-LDHRhBVisible2024093.00[64]
CuMgAl-LDHsRhBVisible1024085.20[65]
g-C3N4/TiO2Methyl orangeVisible518073.55[66]
Fe-S/TiO2/GFCIbuprofenVisible1012080.04[67]
TiO2-SiO2RhBVisible5018097.80[68]
TiO2/LDHsRhBVisible157098.20This research
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Wei, J.; Ren, L. Synthesis and Characterization of Visible-Light-Responsive TiO2/LDHs Heterostructures for Enhanced Photocatalytic Degradation Performance. Water 2025, 17, 2582. https://doi.org/10.3390/w17172582

AMA Style

Wei J, Ren L. Synthesis and Characterization of Visible-Light-Responsive TiO2/LDHs Heterostructures for Enhanced Photocatalytic Degradation Performance. Water. 2025; 17(17):2582. https://doi.org/10.3390/w17172582

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Wei, Jing, and Liying Ren. 2025. "Synthesis and Characterization of Visible-Light-Responsive TiO2/LDHs Heterostructures for Enhanced Photocatalytic Degradation Performance" Water 17, no. 17: 2582. https://doi.org/10.3390/w17172582

APA Style

Wei, J., & Ren, L. (2025). Synthesis and Characterization of Visible-Light-Responsive TiO2/LDHs Heterostructures for Enhanced Photocatalytic Degradation Performance. Water, 17(17), 2582. https://doi.org/10.3390/w17172582

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