Composite aerogel membranes with well dispersed nano M-TiO₂@GO for efficient photocatalysis

Abstract

Photocatalytic technology plays a crucial role in addressing energy shortages and environmental pollution. TiO₂ has been widely applied in photocatalysis but encounters several issues, such as the easy recombination of photoexcited electrons and holes, the poor dispersion uniformity, the wide band gap, and the complex recovery process. In this work, a recyclable M-TiO₂@GO/modified chitosan composite aerogel membrane was designed and prepared for photocatalytic application, with TiO₂ nanoparticles generated by MXene oxidation (M-TiO₂) serving as the photocatalyst and modified chitosan acting as the matrix. The experimental results demonstrated that M-TiO₂ nanoparticles were in situ generated on the surface of graphene oxide (GO) nanosheets, forming M-TiO₂@GO nanoparticles with a 0D@2D structure. Due to the combined effects of M-TiO₂ and carbon (another product resulting from the oxidation of MXene), the band gap of M-TiO₂ nanoparticles was decreased from 3.10 eV to 2.06 eV, which significantly enhanced the photocatalytic efficiency of TiO₂. When the photocatalytic degradation performance of the aerogel membrane containing M-TiO₂@GO nanoparticles was evaluated, the sample C₁₅M₁₅G₅ (containing modified chitosan 15 mg/M-TiO₂ 15 mg/GO 5 mg) demonstrated nearly complete degradation of Rhodamine B and Acid Blue 1 within 120 min, exhibiting excellent photocatalytic performance. In addition, the antibacterial properties of C₁₅M₁₅G₅ were examined and found that the antibacterial rate reached 100% following ultraviolet irradiation. In summary, this study presents an innovative strategy to obtain well-dispersed nano TiO₂, which can be used to prepare composite photocatalytic aerogel membranes with excellent catalytic performance and complete recyclability.

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1. Introduction

Since TiO₂ was first discovered for water photocatalytic splitting in 1972,¹ it has drawn extensive attention due to its green, efficient, stable and reuseable features, and has been widely applied in numerous fields, such as photocatalytic hydrogen production,^2–4 CO₂ reduction,^5,6 and organic pollutant degradation,^7–9 to address the crisis of energy shortage and environmental pollution.¹⁰ The specific surface area, uniformity, and band gap of the photocatalyst will directly influence its photocatalytic performance,¹¹ therefore, enhancing the porosity or dispersion, doping and hybridization can effectively enhance the photocatalytic efficiency.^12,13 However, TiO₂ still presents some issues, such as a tendency to agglomerate, difficulty in recovery, and the easy recombination of photoexcited electrons and holes.

The high surface energy of TiO₂ will lead to obvious agglomeration during the catalytic process, thereby significantly reducing the photocatalytic performance. By grafting KH570 onto the surface of TiO₂, Qin et al. obtained modified TiO₂ that could be evenly dispersed on the surface of the fiber.¹⁴ However, as the photocatalytic process mainly takes place on the surface of TiO₂, the traditional surface modification technology will restrict the contact between TiO₂ and other substances, thereby influencing the photocatalytic performance of TiO₂. The in situ growth approach was frequently adopted to generate photocatalysts, thereby enhancing the distribution uniformity of photocatalysts. For example, Ćurković et al. fabricated the uniform nano-TiO₂ film through the sol–gel method, which exhibited effective photocatalytic performance (solution with 7.3 mg L⁻¹ Lissamine Green B was almost completely degraded within 120 min).¹⁵ This method leaves no substances on the surface of the catalyst and can mitigate the tendency of reduced catalytic performance. Nevertheless, the sol–gel method might sometimes yield Ti(OH)₄ and other precipitates, affecting the stability of the system. The in situ oxidation of titanium sources represented by Ti₃C₂T_x MXene (such as high-temperature calcination,¹⁶ hydrothermal method,¹⁷ addition of an oxidizing agent,¹⁸etc.) provides a solution to avoid the aforementioned problems, which is considered as an effective strategy to stably obtain nano TiO₂.

To enhance the photocatalytic activity of TiO₂, the structure of the photocatalyst is frequently regulated through the addition of a cocatalyst or doping.^19,20 The cocatalyst was often used to prevent the recombination of electrons and holes, thereby enhancing the photocatalytic efficiency of TiO₂.²¹ Among them, graphene oxide (GO) is an oxidized derivative of graphene, inheriting the outstanding properties of graphene, such as excellent optical properties, mechanical flexibility, good thermal stability, and chemical stability.^22,23 Moreover, it possesses excellent hydrophilicity, enabling GO nanosheets to form a stable dispersion in water or other solvents. Additionally, the specific surface area of GO nanosheets is large, and the surface is rich in active functional groups, thus it can effectively anchor the photocatalyst to form the composite photocatalytic structure, which is currently considered as one of the most popular strategies to enhance the catalytic performance.^24–27 For example, Yang et al. fabricated a composite material for the efficient photocatalytic degradation of decabromodiphenyl ether (BDE209) by uniformly anchoring TiO₂ nanoparticles onto GO nanosheets. The prepared TiO₂/GO photocatalyst exhibited excellent photocatalytic performance owing to the intense coupling between GO nanosheets and TiO₂ nanoparticles. Under the optimal photocatalytic degradation conditions, the degradation efficiency of BDE209 could exceed 90%, which offered an effective approach for designing highly active catalytic materials.²⁸ Alternatively, by doping C, N and other elements to form heterojunctions with TiO₂, the width of the band gap can be effectively reduced and the photocatalytic activity of TiO₂ can be improved.²⁹ For example, Wahyuni et al. employed a simple hydrothermal technique to prepare N-doped TiO₂ with urea serving as the N source, and successfully decreased the band gap width from 3.20 eV to 3.06–2.96 eV.³⁰

In addition, the recovery of TiO₂ is also an issue that needs to be addressed. Ding et al. successfully incorporated Fe₃O₄ into the photocatalyst system to facilitate the magnetic recovery of the material.³¹ They synthesized a CS/Fe₃O₄/TiO₂@MXene nanocomposite photocatalyst through a one-step hydrothermal method, which exhibited excellent bacteriostatic and degradation properties. Moreover, it can be readily recycled and reused through the external magnetic field by adding Fe₃O₄. Fernández-Ibáñez et al. employed flocculation agents to enhance the sedimentation of colloids in a pilot plant for catalyst recovery, resulting in an impressive 97% retrieval of TiO₂.³² However, the recycling process of TiO₂ described above is relatively complex. The preparation of composite materials of TiO₂ with polymers in the form of aerogels or membranes represents another important solution for this issue. This approach can simultaneously achieve relatively efficient photocatalysis and comprehensive recovery of TiO₂.^33–36 Chitosan (CHs), being a natural organic macromolecule, has been extensively employed as the matrix in catalytic systems for water pollutant treatment due to its environmental friendliness, easy availability, and degradability.^37–40

To achieve high photocatalytic performance, addressing the dispersion issue, the electron–hole recombination issue and the recycling issue of TiO₂, in this study, a M-TiO₂@GO/chitosan composite aerogel membrane with a 0D@2D in 3D structure (0D M-TiO₂ nanoparticles @ 2D GO sheet dispersed in a 3D chitosan aerogel membrane) was successfully designed and fabricated. M-TiO₂ was formed in situ through the oxidation of MXene, which was deposited on GO nanosheets to address the aggregation issue of TiO₂. Employing GO nanosheets as the cocatalyst, which can effectively enhance the photocatalytic performance by transferring photoexcited electrons and preventing the recombination of electrons and holes. In the meantime, M-TiO₂ could form the Schottky heterojunction with carbon (another product resulting from the oxidation of MXene), effectively narrowing the band gap. The complete recovery of M-TiO₂@GO was achieved by using modified chitosan as the matrix to form a composite aerogel membrane. The excellent photocatalytic performance of M-TiO₂@GO/modified chitosan composite aerogel membranes was demonstrated through the dye degradation experiments.

2. Experimental section

2.1. Materials

Chitosan (M_W = 100 kDa, deacetylation degree 85%), LiF (99.9%) and Acid Blue 1 were purchased from Shanghai Macklin Biochemical Co. Ltd. Hydrochloric acid (HCl) solution (36.0–38.0%) and anhydrous ethanol (AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide (EDC, 98%) and 3,4-dihydroxyhydrocinnamic acid (HCA, 98%) were bought from TCI (Shanghai) Chemical Industry Development Co., Ltd. Ti₃AlC₂ was acquired from Jilin 11 Technology Co., Ltd. TiO₂ (99.8%, 25 nm, anatase crystal type) (P25 TiO₂) was purchased from Meryer (Shanghai) Biochemical Technology Co., Ltd. Graphene oxide (GO) dispersion (0.5–5.0 μm, 5 mg mL⁻¹) was obtained from Nanjing XFNANO Materials Tech Co., Ltd. Hydrogen peroxide was bought from Shanghai Titan Scientific Co., Ltd. Rhodamine B was acquired from Shanghai Aladdin Biochemical Technology Co., Ltd. The qualitative filter paper was bought from Hangzhou Fuyang Beimu Jiangzhi Co., Ltd. All the reagents were used as received without further purification. Deionized (DI) water with a resistance of 18 MΩ cm was prepared via a lab water purification system and used in all experiments.

2.2. In situ generation of M-TiO₂ on GO nanosheets

2.2.1. Preparation of MXene nanosheets. LiF (1.6 g) was added to 20 mL of hydrochloric acid (9 M) and stirred for 30 min. Subsequently, Ti₃AlC₂ (1.0 g) was slowly introduced into the mixture. The resulting solution underwent a constant temperature reaction at 35 °C for 36 h. Following repeated centrifugation and washing, the pH of the mixture approached a range between 5 and 6. The supernatant after centrifugation was collected and ultrasonically treated in an ice bath for 60 min to ensure the uniform dispersion of single- or few-layer MXene nanosheets, which were then frozen and freeze-dried.

2.2.2. Generation of M-TiO₂ through MXene oxidation. The corresponding mass of MXene nanosheets was uniformly ultrasonically dispersed in 1 mL GO nanosheet dispersion solution. Then, 50 μL 30 wt% H₂O₂ solution was added, and ultrasonication was continued for 30 min to oxidize MXene into M-TiO₂ and obtain M-TiO₂@GO dispersed solution.

2.3. Preparation of aerogel membranes with TiO₂ and GO nanosheets

Modified chitosan, which is water-soluble and capable of being cross-linked, was prepared following a method modified from the literature-reported procedure.^41,42 Chitosan (0.5 g) was dissolved in dilute hydrochloric acid to obtain a 1% chitosan solution, and the pH value was adjusted to approximately 5.2. A mixture of deionized water and ethanol with a volume ratio of 1 : 1 was employed to dissolve EDC (0.36 g) and HCA (0.34 g), which were then added dropwise to the chitosan solution for the grafting reaction. The modified chitosan (CHs-HCA) was dialyzed using a dilute hydrochloric acid solution with a pH range of 4.6–4.8 as the dialysate, followed by freezing and freeze-drying.

A designed amount of CHs-HCA was dissolved in 1 mL deionized water, mixed with M-TiO₂@GO dispersion, and stirred at 60 °C. Under the influence of the residual H₂O₂ in the solution, CHs-HCA was oxidized and cross-linked. Eventually, H₂O₂ that did not take part in the oxidation reaction was decomposed under heating conditions. Subsequently, the resulting solution was cast onto a substrate membrane (qualitative filter paper), followed by freeze-drying to obtain the aerogel membrane consisting of CHs-HCA/M-TiO₂/GO (CMG), where the subscript of M represents the mass of TiO₂ generated after oxidation of MXene by theoretical calculation.

Similarly, P25 TiO₂ with a corresponding amount was uniformly dispersed along with GO nanosheets, and combined with CHs-HCA to produce the aerogel membrane consisting of CHs-HCA/TiO₂/GO (CTG) using the same method described above. The respective component masses for different samples are summarized in Table 1. And the preparation process of composite aerogel membranes is shown in Fig. 1.

Table 1 Formulation of composite aerogel membranes

Sample	CHs-HCA (mg)	MXene (mg)	Nano-TiO2 (mg)	GO (mg)
C20M10G5	20	7	—	5
C15M15G5	15	10.5	—	5
C10M20G5	10	14	—	5
C20T10G5	20	—	10	5
C15T15G5	15	—	15	5
C10T20G5	10	—	20	5

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Fig. 1 Preparation process of composite aerogel membranes.

2.4. Characterization

The Fourier transform infrared (FTIR) spectra of the chitosan before and after modification were recorded using a FTIR spectrophotometer (TENSOR II, BRUKER) with an ATR attachment. The grafting rate was calculated based on the integral of the corresponding peaks in the spectra collected from a Nuclear Magnetic Resonance (NMR) spectrometer (AVANCE NEO, BRUKER). The crystallographic structures of the MAX phase, MXene, and M-TiO₂ were determined using X-ray diffraction (XRD) (X-ray dynamic 500, Anton Paar). The samples were run at a scan rate of 5° min⁻¹ over an angular (2θ) range of 3°–90°. The morphology of GO nanosheets before and after loading M-TiO₂ was characterized using a Transmission Electron Microscope (TEM) (JEM-F200, JEOL, Japan). The changes in elemental valence states in MXene before and after H₂O₂ treatment were analyzed by X-ray Photoelectron Spectroscopy (XPS) (K-Alpha, Thermo Scientific, USA). Raman spectra were obtained using an inVia-Reflex (Renishaw) with a 532 nm excitation laser. The electrical properties of M-TiO₂ in water were tested using a zeta potential analyzer (Zetasizer Nano ZS90, Malvern, UK). The micromorphology of the composite aerogel membrane was observed using a Scanning Electron Microscope (SEM) (SU8010, Hitachi, Japan). The pore size distribution of the aerogel membrane was measured by mercury intrusion porosimetry (AutoPore V 9620, Micromeritics, USA). The absorbance of the dye solution after photocatalysis was measured using an ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer (UV-3600, SHIMADZU), and the change in concentration was calculated.

2.5. Photocatalytic performance

The composite aerogel membrane was immersed in a 100 mL, 5 ppm dye solution (Rhodamine B or Acid Blue 1) and irradiated with a 250 W ultraviolet high-pressure mercury lamp (with spectral energy centered at 365 nm) under constant temperature conditions maintained in a water bath. During this process, 5 mL aliquots of the solution were collected at 20 min intervals. UV-vis-NIR absorption spectra were recorded, and the change in dye concentration was calculated according to the principle of Lambert–Beer law, as shown in eqn (1).where C₀ is the initial concentration of the dye, A₀ is the initial absorbance, C is the concentration of the dye following photocatalysis, and A is the corresponding absorbance.

In addition, the obtained photocatalytic degradation data were analyzed, and the quasi-first-order reaction kinetics was used to describe the photocatalytic degradation process of Rhodamine B or Acid Blue 1 (eqn (2)),⁴³ and the rate constant of the degradation reaction, k_obs, was obtained.

where C₀ is the initial concentration of the dye, and C is the concentration of the dye at reaction time t.

During the cycling photocatalytic performance test, the aerogel membrane after initial photocatalysis (recorded as Cycle 1) was taken out, washed with deionized water and freeze-dried. The cleaned and dried samples were tested again for photocatalysis, and the tests were recorded as Cycle x (x = 2, 3, …).

3. Results and discussion

3.1. Preparation and characterization of M-TiO₂@GO

In this study, the dispersion of single-layer or few-layer MXene nanosheets was prepared by etching Ti₃AlC₂ in the LiF/HCl system and then freeze-dried for quantification. After the MXene and GO nanosheets were mixed and evenly dispersed, H₂O₂ was employed to oxidize the MXene attached to GO, thereby obtaining M-TiO₂@GO nanocomposite structures. Among them, the few-layer MXene was the key to achieving rapid oxidation and obtaining nano-sized TiO₂. Compared with the filtration-drying method for MXene preparation,¹⁸ the freeze-drying method adopted in this work could effectively prevent the restacking of the few-layer MXene, enabling the acquisition of M-TiO₂ nanoparticles with smaller particle size and uniform distribution within a short time (30 min). In addition, GO nanosheets were designed as a carrier, which not only enhanced the structural stability of M-TiO₂ but also provided an effective transfer path for photoexcited electrons. Since MXene was attached and oxidized on GO, the generated M-TiO₂ could be anchored on the GO nanosheet surface, effectively avoiding the agglomeration of M-TiO₂. In the meantime, M-TiO₂ formed Schottky heterojunctions with carbon, another product resulting from the oxidation of MXene, which can effectively narrow the band gap.

A series of characterization studies of MXene before and after oxidation were performed. Based on the XRD pattern (Fig. 2a), the characteristic peak of Ti₃AlC₂ at 38.76°⁴⁴ became unobservable, whereas the peak at 9.51° associated with Ti₃AlC₂ shifted to 6.91°, which was attributed to the transformation of Ti₃AlC₂ to MXene.⁴⁵ In conjunction with the morphological changes observed in Fig. 3b and S1,† these findings suggested that single- or few-layered MXene nanosheets have been successfully synthesized. In addition, the observed diffraction peaks at 25.2°, 38.0°, 47.5°, and 62.5° corresponded to the crystallographic planes (101), (004), (200), and (204), respectively, which are characteristic of anatase TiO₂ according to PDF #21-1272.^46,47 The peaks in the Raman spectrum of oxidized MXene powders (bottom spectrum in Fig. 2b) at approximately 150, 400, 520, and 676 cm⁻¹ were assigned to the E_g, B_1g, A_1g & B_1g, and E_g modes of anatase TiO₂,^48,49 respectively. The peaks observed at approximately 284 and 900 cm⁻¹ in the Raman spectrum and 900 cm⁻¹ in the FTIR spectrum (Fig. S2†) could be attributed to the oxidation of the sample induced by H₂O₂. This oxidation process resulted in the formation of oxygen-containing functional groups on the surface of the catalyst. The peak around 284 cm⁻¹ corresponded to the surface hydroxyl related mode.^50,51 The peak around 900 cm⁻¹ was attributed to the combination of Ti and the surrounding hydroxyl groups.⁵² Ti atoms will form –TiOO groups after treatment with H₂O₂, and the peak around 900 cm⁻¹ corresponded to the stretching vibration of the O_α–O_β bond.^53,54 Besides, the two broad peaks between 1200 and 1700 cm⁻¹ were characteristic of the D and G modes of graphitic carbon.⁵⁵

Fig. 2 Characterization of the structure and properties of M-TiO₂. (a) XRD patterns of Ti₃AlC₂, MXene and H₂O₂ treated MXene; (b) Raman spectrum; (c) high resolution XPS spectra of the C 1s region; (d) high resolution XPS spectra of the Ti 2p region of MXene before and after oxidation; (e) UV-vis DRS; (f) band-gap energy of M-TiO₂ and TiO₂ samples.

Fig. 3 Characterization of the morphology of M-TiO₂@GO. The TEM images of (a) GO, (b) MXene and (c) sample M₁₅G₅; EDS elemental distribution maps of (d) C, (e) O, and (f) Ti on sample M₁₅G₅.

XPS tests of the sample (Fig. 2c and d) further elucidated the evolution of the chemical composition and structure before and after H₂O₂ treatment. In Fig. 2c, the C 1s spectrum of MXene was deconvoluted into four peaks at binding energies of 289.2, 286.4, 284.8, and 281.7 eV, corresponding to C–F, C–O, C–C, and C–Ti bonds, respectively.^56,57 The disappearance of the C–Ti peak and the emergence of the C–O peak in MXene following H₂O₂ treatment suggested that MXene has undergone a complete reaction. As illustrated in Fig. 2d and S3† (high resolution XPS spectra of the O 1s region), MXene treated with H₂O₂ exhibited only TiO₂ peaks, confirming that MXene was oxidized by H₂O₂ to form TiO₂. The above results might be due to the single-layer or few-layer structure allowing MXene to fully contact with H₂O₂, resulting in complete oxidation. The above results collectively indicated that the oxidation of MXene by H₂O₂ resulted in the formation of TiO₂ and carbon.

To investigate the photocatalytic performance of the synthesized M-TiO₂ catalyst, the corresponding characterization was conducted. Fig. 2e presents the diffuse reflectance absorption spectra of both P25 TiO₂ and the prepared M-TiO₂ samples. The bandgap absorption edges of M-TiO₂ and P25 TiO₂ were approximately 500 nm and 400 nm, respectively, indicating that both materials primarily absorbed ultraviolet light. Notably, M-TiO₂ exhibited a broader absorption spectrum, attributed to the formed carbon during the oxidation process, which reduced the recombination of photogenerated electrons in TiO₂.⁵⁸ A plot of the transformed Kubelka–Munk function versus light energy⁵⁹ is presented in Fig. 2f, from which the approximately determined band gaps were 2.06 and 3.10 eV for M-TiO₂ and P25 TiO₂, respectively. The main reason for the reduction of the band gap in M-TiO₂ was that during the oxidation process of MXene, carbon (C), another oxidation product, formed a Schottky heterojunction with M-TiO₂, thereby resulting in C-doped M-TiO₂.⁶⁰ The introduction of carbon atoms would create an “intermediate band” between the valence band and the conduction band of TiO₂. The existence of these intermediate bands made it easier for electrons to transit from the valence band to the conduction band, thereby effectively reducing the width of the band gap. Owing to the better dispersion and wider light absorption range, M-TiO₂ was anticipated to utilize the solar energy more effectively, thereby exhibiting superior photocatalytic performance, as will be evidenced in the photocatalytic experiment section.

Upon comparing Fig. 3a and c, it was evident that nanoparticles had uniformly formed on the lamellar structure, and the nanoparticles were thought to be TiO₂. The EDS elemental distribution maps of M₁₅G₅ are shown in Fig. 3c–f. The particle size distribution of the synthesized M-TiO₂ nanoparticles was characterized using a Nano Measurer, with a minimum of 30 individual particles analyzed to ensure statistical reliability. As shown in Fig. S4,† the particle size of the generated M-TiO₂ was mainly distributed within the range of 1.0–5.5 nm. Similar functional groups on the surface of GO and MXene facilitated strong hydrogen bonding, leading to intimate interfacial interactions between them, consequently, MXene could effectively disperse on the surface of GO nanosheets.^61,62 TiO₂ nanoparticles were in situ generated on GO nanosheets by dispersing and adsorbing MXene onto GO, resulting in the formation of 0D@2D structured M-TiO₂@GO nanoparticles. As a carrier, GO nanosheets not only enhanced the structural stability of M-TiO₂ but also provided a transfer pathway for photoexcited electrons. As an intercalation agent, M-TiO₂ produced via MXene oxidation significantly reduced the propensity of GO to aggregate.

3.2. Structure and morphology of CMG aerogel membranes

In order to ensure the complete recovery of M-TiO₂@GO, a composite aerogel membrane was prepared by using non-toxic, green and biocompatible modified chitosan as the matrix. Chitosan is a polymer material soluble in dilute acid, but dilute acid solutions often react with fillers and would remain in the material to affect its properties. In this section, chitosan was grafted with HCA to render it water-soluble and enhance its processability. The infrared spectrum of chitosan after grafting was presented in Fig. 4a. Notably, the high abundance of O, N, and H within the system brought abundant hydrogen bonding sites to the system, thereby causing the redshift phenomenon of the characteristic peaks of the groups.⁶³ The peaks at 1665, 1590, 1375, 1151, and 1025 cm⁻¹ respectively corresponded to the absorption peaks of the carbonyl group (C=O), amino group (NH₂), methyl group (CH₃), C–O–C in the chitosan ring, and alcohol hydroxyl group (–OH). The peaks at 3342, 1668 and 820 cm⁻¹ respectively corresponded to the absorption peaks of the O–H stretching vibration of the phenol hydroxyl group, the carboxyl group (–COOH), and the out-of-plane bending vibration of C–H in the 1,2,4-trisubstituted benzene ring in the HCA molecule. Meanwhile, the two peaks at 1442 and 1525 cm⁻¹ respectively corresponded to the absorption peaks of the benzene ring skeleton. In the infrared spectrum of the prepared CHs-HCA, the peak at 1625 cm⁻¹, corresponding to the secondary amide group (–CONH-), emerged compared with chitosan, and the out-of-plane bending vibration of C–H in the 1,2,4-trisubstituted benzene ring and the absorption peak corresponding to the benzene ring skeleton appeared. Additionally, due to the substitution of the –NH₂ group in chitosan, the corresponding absorption peak transformed into the acromial peak in CHs-HCA.^64,65 These alterations demonstrated the success of the grafting reaction.

Fig. 4 Characterization of properties of CHs-HCA and the structure of the composite aerogel membrane. (a) Infrared spectrum and (b) ¹H-NMR of modified chitosan; (c) images of chitosan solution (left) and modified chitosan solution (right); (d) SEM image of the C₁₅M₁₅G₅ aerogel membrane cross-section.

The modified chitosan was analyzed using ¹H-NMR spectroscopy (Fig. 4b). The grafting degree of catechol was determined by comparing the relative peak areas of the catechol groups (3H, aromatic cyclic proton, δ 6.78, D₂O) and the acetyl groups (3H, –COCH₃, δ 2.06, D₂O) on the chitosan backbone. The calculated grafting degree was approximately 15%. And images of the chitosan and CHs-HCA aqueous solution (Fig. 4c) demonstrated the excellent water solubility of the modified chitosan. Furthermore, a composite aerogel membrane featuring a porous structure and high specific surface area was synthesized using modified chitosan as the matrix and incorporated with M₁₅G₅ (as illustrated in Fig. 4d). The pore size distribution of the composite aerogel membrane was characterized using mercury intrusion porosimetry. The results indicated that the average pore size of the prepared aerogel membrane was approximately 71.5 μm, with a porosity of 93.2%, thereby confirming its significant porous structure (as shown in Fig. S5†), which was consistent with the porous morphology observed in the SEM image.

In addition, to explore the distribution of M-TiO₂@GO within the modified chitosan matrix, cross-sections of the aerogel membranes with different components were observed by SEM. The results showed that the pore wall surface of the CHs-HCA aerogel membrane was smooth. As the relative content of CHs-HCA decreased, the surface of the pore wall became rougher, and no obvious agglomeration phenomenon emerged (as shown in Fig. 5b and c). Additionally, based on the EDS scanning results, it could be observed that M-TiO₂ was evenly distributed in sample C₁₅M₁₅G₅ (as shown in Fig. S6†). However, when the mass of CHs-HCA was decreased to 10 mg (sample C₁₀M₂₀G₅), the nano-fillers agglomerated, leading to the uneven dispersion of M-TiO₂@GO and forming the obvious granular structure (as shown in the red dashed line box in Fig. 5d). The uneven distribution of the catalyst resulting from this agglomeration structure might cause the decline of the photocatalytic performance of the composite system.

Fig. 5 Characterization of cross-section morphology of aerogel membranes with different M-TiO₂ contents. SEM images of (a) CHs-HCA, (b) C₂₀M₁₀G₅, (c) C₁₅M₁₅G₅ and (d) C₁₀M₂₀G₅.

3.3. Photocatalytic performance of composite aerogel membranes

The photocatalytic degradation of Rhodamine B and Acid Blue 1 was used as model reactions to evaluate the photocatalytic performance of M-TiO₂@GO/chitosan. The photocatalytic degradation performance of P25 TiO₂@GO and M-TiO₂@GO aerogel membranes was evaluated under identical conditions. Fig. 6a and b show the concentration–time curves of Rhodamine B and Acid Blue 1 degraded by different samples. UV-vis absorption spectra for the degradation of different dyes by C₁₅M₁₅G₅ are shown in Fig. S7.† It was observed that the composite aerogel membrane exhibited a little adsorption effect on these dyes under dark conditions, and the adsorption curve is shown in Fig. S8.† The M-TiO₂@GO composite aerogel membrane demonstrated superior photocatalytic activity compared to the P25 TiO₂@GO composite aerogel membrane. After 120 min of ultraviolet irradiation, C₁₅T₁₅G₅ degraded Rhodamine B by 86% and Acid Blue 1 by 72.36%, while C₁₅M₁₅G₅ degraded Rhodamine B by 96.03% and Acid Blue 1 by 98.86%. This could be attributed primarily to the narrower band gap of M-TiO₂ (2.06 eV), leading to enhanced catalytic degradation efficiency. Additionally, due to electrostatic interactions between MXene and GO, M-TiO₂ was synthesized in situ on the surface of GO during the oxidation process, ensuring intimate contact between the catalyst and co-catalyst. This configuration facilitated the transfer of excited electrons generated during catalysis and inhibited the recombination of electron–hole pairs, thereby improving the photocatalytic performance. Additionally, the dye degradation efficiency of C₁₅M₁₅G₅ samples prepared by heat drying in an oven (called C₁₅M₁₅G₅-H) was evaluated. It was observed that the dye degradation efficiency of these samples was significantly lower than that of the aerogel membrane with the same composition. After 120 min of ultraviolet irradiation, C₁₅M₁₅G₅-H degraded Rhodamine B by 77.94% and Acid Blue 1 by 93.62%, which was significantly lower compared to that of the C₁₅M₁₅G₅ sample. This discrepancy arose because the dye degradation process occurs on the surface of the catalyst TiO₂, and the porous structure (Fig. 4d) of the aerogel membrane provided a larger specific surface area, allowing for greater TiO₂ exposure and thus increasing the number of active sites and overall catalytic efficiency.

Fig. 6 Characterization of photocatalytic properties of aerogel membranes. Degradation of Rhodamine B: (a) plots of C/C₀ with time and (c) kinetic linear simulation curves using different samples, and (e) cycling performance of Rhodamine B degradation using C₁₅M₁₅G₅; degradation of Acid Blue 1: (b) plots of C/C₀ with time and (d) kinetic linear simulation curves using different samples, and (f) cyclic performance of Acid Blue 1 degradation using C₁₅M₁₅G₅.

Through the quasi-first-order kinetic simulation of the degradation curves for various materials, the slope of the fitted line represents the degradation rate constant. As illustrated in Fig. 6c and d, it was evident that the chitosan aerogel membrane exhibited a strong linear relationship. Furthermore, the data indicated that sample C₁₅M₁₅G₅ demonstrated the highest photocatalytic reaction rate constant for both dyes: the photocatalytic reaction rate constant of Rhodamine B was 0.02433 min⁻¹, and that of Acid Blue 1 was 0.03367 min⁻¹. This enhanced performance could be attributed to the formation of a Schottky heterojunction between M-TiO₂ and carbon generated from oxidized MXene. This significantly improved the separation efficiency of photogenerated carriers and narrowed the band gap of TiO₂,⁶⁶ thereby enhancing photocatalytic efficiency.

In the field of photocatalytic technology, it was imperative to develop synthetic materials that could be efficiently recovered and reused for wastewater treatment. Fig. 6e and f illustrat the cycling performance of a composite aerogel membrane in dye degradation. Specifically, five consecutive photodegradation experiments were conducted using a C₁₅M₁₅G₅ aerogel membrane. Upon initial application, the degradation efficiencies for Rhodamine B and Acid Blue 1 were 97.78% and 98.86%, respectively. After five successive uses, the degradation efficiencies decreased to 76.57% for Rhodamine B and 84.31% for Acid Blue 1. Despite this reduction, the C₁₅M₁₅G₅ aerogel membrane retained a high level of catalytic activity, demonstrating its excellent cycling performance.

3.4. Antibacterial properties of composite aerogel membranes

In addition to organic pollutants, pathogenic bacteria are also among the pollutant components in wastewater. Based on the efficient reaction mechanism of photocatalysis technology in degrading organic pollutants, it also offers a new idea for inactivating microorganisms in wastewater. We compared the antibacterial performance of sample C₁₅M₁₅G₅ under dark and light conditions, as shown in Fig. 7. It can be seen that the number of colonies in the Petri dish with C₁₅M₁₅G₅ added under dark conditions was slightly reduced compared with that in the blank sample. This is mainly due to the electrostatic interaction between the amino groups of CHs (positively charged) with the cell membranes (negatively charged) of bacteria.⁶⁷ This interaction induces significant alterations to the cell surface, resulting in changes in permeability. This subsequently triggers osmotic imbalance and the efflux of intracellular components, ultimately leading to cell death.⁶⁸

Fig. 7 Bacteriostatic properties of the samples against the colon bacillus under dark and UV-light conditions. (a) Petri dishes of the blank control group under dark conditions; (b) Petri dishes containing sample C₁₅M₁₅G₅ under dark conditions; (c) Petri dishes of the blank control group following UV exposure; (d) Petri dishes containing sample C₁₅M₁₅G₅ following UV exposure.

After ultraviolet irradiation, the number of colonies in the blank sample significantly decreased due to the bactericidal properties of ultraviolet light. However, in the culture dish containing C₁₅M₁₅G₅, no colonies were observed. This was primarily attributed to the fact that during irradiation, the photoelectron in the conduction band interacted with adsorbed O₂ on the surface, generating reactive oxygen species (ROS).⁶⁹ These ROS damaged bacterial cell membranes upon contact, penetrated the cells, and subsequently disrupted the protein structure or genetic material, ultimately leading to bacterial death.^67,69

3.5. Mechanism for photocatalytic degradation of dyes and antibacterial activity by M-TiO₂@GO

Based on the aforementioned findings, we built a schematic representation of the M-TiO₂@GO catalytic degradation of dyes and sterilization, as illustrated in Fig. 8. When M-TiO₂ nanoparticles absorb ultraviolet light or visible light containing a certain energy, electrons in the valence band (VB) would be excited to transition to the conduction band (CB), forming photogenerated electrons (e⁻) and leaving photogenerated holes (h⁺) in the valence band,^70,71 as shown in eqn (3).

M-TiO₂ + hν → M-TiO₂(e⁻) + M-TiO₂(h⁺) (3)

Fig. 8 Schematic illustration of the M-TiO₂@GO catalytic degradation of dyes and sterilization.

These holes could interact with water molecules on the surface of titanium dioxide to produce hydroxyl radicals (˙OH), as shown in eqn (4). However the photogenerated electrons migrated to the GO surface and reacted with dissolved oxygen in water to form superoxide radicals (˙O₂⁻), as shown in eqn (5) and (6).

H₂O + M-TiO₂(h⁺) → ˙OH + H (4)

M-TiO₂(e⁻) + GO → M-TiO₂ + GO(e⁻) (5)

O₂ + GO(e⁻) → ˙O₂⁻ (6)

Subsequently, the ROS such as ˙OH and ˙O₂⁻ exhibited strong oxidation/reduction activities. These ROS could degrade organic dyes in water into inorganic small molecules such as CO₂ and H₂O, or destroy the cell structure of bacteria, thus achieving the effect of efficient water purification.

4. Conclusion

In this study, we presented a green and rapid method for preparing well-dispersed M-TiO₂@GO photocatalysts and demonstrated that the composite aerogel membranes containing such photocatalysts exhibited excellent photocatalytic activity and recyclability. With MXene serving as the precursor of TiO₂, the in situ generation of TiO₂ was accomplished, effectively addressing the agglomeration issue of nano TiO₂. The two oxidation products of MXene, TiO₂ and carbon, successfully formed the Schottky heterojunction in the system, effectively reducing the band gap of TiO₂ from 3.10 eV to 2.06 eV, which resolved the relatively wide band gap problem of TiO₂ and thus significantly enhanced the photocatalytic efficiency. Additionally, GO was employed as the co-catalyst to alleviate the problem of photoexcited electron–hole recombination, and modified chitosan was prepared and used as a carrier to overcome the cumbersome issue of TiO₂ recovery. When the fabricated M-TiO₂@GO/chitosan composite aerogel membranes with a 0D@2D in 3D structure were applied to the dye degradation, after 120 min of ultraviolet light irradiation, the degradation rate of Rhodamine B reached 96.03%, and the degradation rate of Acid Blue 1 reached 98.86%. The composite membranes retained the ability to degrade 80% of the dye after five cycles. Moreover, after being exposed to ultraviolet light, the antibacterial rate of C₁₅M₁₅G₅ reached 100%. Overall, this work provided an effective strategy for green, efficient and recyclable photocatalytic degradation of organic pollutants.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

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

Acknowledgements

This study was financially supported by the National Key Research and Development Program of China (2022YFB3804905, 2022YFB3804900, and 2021YFA1201300) and National Natural Science Foundation of China (U23A20585).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5na00238a

‡ These authors contributed equally to this work.

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