Open-access Synthesis, mechanical characterization, and mechanism investigation of graphene-wrapped nano-titanium dioxide composite

ABSTRACT

This study presents the synthesis and characterization of three-dimensional graphene-TiO2 (GT) composites with varying TiO2 content via a hydrothermal method. The composites exhibited a hierarchical porous structure with specific surface areas up to 243.9 m2/g, significantly higher than pure TiO2 (52.3 m2/g). FTIR analysis revealed Ti-O-C bonding, indicating strong interfacial interactions. The composites demonstrated enhanced mechanical properties, with GT-3 showing a compressive strength of 1.15 MPa and an elastic modulus of 11.3 MPa. Photocatalytic experiments showed that GT-3 degraded 92% of methylene blue under UV light in 120 minutes and 67% under visible light in 180 minutes, outperforming pure TiO2. The improved performance was attributed to efficient charge separation, as evidenced by electrochemical impedance spectroscopy and photoluminescence quenching. The composites exhibited excellent reusability, maintaining 94% activity after five cycles. UV-vis diffuse reflectance spectra revealed a narrowing of the band gap from 3.2 eV for TiO2 to 2.5 eV for GT-3, explaining the enhanced visible light activity. These results demonstrate the potential of GT composites for environmental remediation applications.

Keywords Hydrothermal synthesis; Photocatalytic degradation; Porous nanocomposites; Charge separation; Environmental remediation

1. INTRODUCTION

Graphene, a two-dimensional carbon allotrope with a single layer of sp2-bonded carbon atoms arranged in a hexagonal lattice, has garnered significant attention in the scientific community since its discovery. This nanomaterial exhibits exceptional properties, including high electrical and thermal conductivity, excellent mechanical strength, and an incredibly large specific surface area [1, 2]. These characteristics have positioned graphene as a promising candidate for various applications in electronics, energy storage, sensors, and catalysis [3]. Concurrently, titanium dioxide (TiO2) nanoparticles have established themselves as a cornerstone in photocatalysis and environmental remediation due to their unique photocatalytic properties, chemical stability, low cost, and non-toxicity. TiO2 exists in three primary crystalline forms: anatase, rutile, and brookite, with anatase demonstrating superior photocatalytic activity [4,5,6]. However, the widespread application of TiO2 is hindered by several limitations, including a high recombination rate of photogenerated electron-hole pairs and poor utilization of visible light due to its wide bandgap [7].

The integration of graphene with TiO2 nanoparticles has emerged as a promising strategy to overcome these limitations and create a synergistic composite material with enhanced properties. Graphene can serve as an excellent electron acceptor and transporter, effectively reducing the recombination of charge carriers in TiO2 [8]. Additionally, the high surface area of graphene provides numerous active sites for the adsorption of reactants and anchoring of TiO2 nanoparticles, potentially leading to improved photocatalytic performance [9]. While two-dimensional graphene-TiO2 composites have shown promise, there is growing interest in developing three-dimensional (3D) architectures to further enhance the material's properties and expand its potential applications [10]. Three-dimensional graphene structures, such as aerogels and hydrogels, offer several advantages over their 2D counterparts, including improved mass transport, enhanced accessibility of active sites, and superior mechanical stability [11]. These 3D structures can be fabricated through various methods, with hydrothermal synthesis emerging as a particularly attractive approach due to its simplicity, scalability, and ability to produce well-defined porous networks.

The incorporation of TiO2 nanoparticles into 3D graphene structures presents both opportunities and challenges. On one hand, the 3D graphene network can provide a robust scaffold for the dispersion and anchoring of TiO2 nanoparticles, potentially leading to enhanced photocatalytic activity and improved charge transport [12, 13]. On the other hand, the presence of TiO2 nanoparticles may influence the self-assembly process of graphene sheets during hydrothermal treatment, affecting the resulting microstructure and mechanical properties of the composite. Understanding the intricate relationship between the composition, microstructure, and properties of 3D graphene-TiO2 composites is crucial for optimizing their performance in various applications [14,15,16]. The content and distribution of TiO2 nanoparticles within the graphene network can significantly impact the pore structure, mechanical strength, and photocatalytic activity of the resulting material [17]. Moreover, the formation mechanism of these 3D composites and the nature of the graphene-TiO2 interface play critical roles in determining their overall performance [18]. In addition to structural considerations, the photocatalytic mechanism of graphene-TiO2 composites warrants in-depth investigation. While it is generally accepted that graphene can enhance the photocatalytic activity of TiO2 through improved charge separation and extended light absorption, the specific contributions of factors such as adsorption capacity, electron transfer kinetics, and light utilization efficiency remain subjects of ongoing research [19]. Elucidating these mechanisms is essential for the rational design of high-performance graphene-TiO2 composites for environmental and energy applications [20,21,22].

The mechanical properties of 3D graphene-TiO2 composites are another critical aspect that demands attention. The incorporation of TiO2 nanoparticles into the graphene network can potentially alter its mechanical behavior, affecting properties such as compressive strength, elastic modulus, and structural stability [23, 24]. Understanding these mechanical characteristics is crucial for assessing the material's suitability for practical applications and its long-term durability under various operating conditions. Furthermore, the reusability and stability of graphene-TiO2 composites in photocatalytic applications are of paramount importance from both an economic and environmental perspective [25]. The ability of these materials to maintain their structural integrity and photocatalytic activity over multiple use cycles is essential for their practical implementation in water treatment, air purification, and other environmental remediation processes.

In light of these considerations, this research aims to comprehensively investigate the synthesis, characterization, and performance of 3D graphene-TiO2 composites prepared through hydrothermal and sol-gel methods. The study will focus on elucidating the effects of TiO2 content on the microstructure, mechanical properties, and photocatalytic activity of the composites. By systematically varying the ratio of graphene oxide to TiO2 nanoparticles, we seek to establish clear relationships between composition, structure, and performance.

2. MATERIALS AND METHODS

2.1. Materials

All chemicals used in this study were of analytical grade and employed without further purification. Graphite flakes (99.8% purity, 325 mesh) were obtained from Qingdao Huatai Lubricant Sealing S&T Co., Ltd. (Qingdao, China). Sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4, 99.5%), hydrogen peroxide (H2O2, 30%), and hydrochloric acid (HCl, 37%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Titanium dioxide nanoparticles (TiO2, P25, 80% anatase and 20% rutile) were supplied by Evonik Degussa (Qingdao, China). Ethanol (C2H5OH, ≥ 99.7%) was procured from Aladdin Industrial Corporation (Shanghai, China). Methylene blue (MB) was acquired from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Deionized water was used throughout all experiments.

2.2. Synthesis of graphene oxide

Graphene oxide (GO) was synthesized using a modified Hummers' method. Briefly, 5 g of graphite flakes were added to 115 mL of concentrated H2SO4 in a 1 L round-bottom flask, which was kept in an ice bath under constant stirring. Subsequently, 15 g of KMnO4 was slowly added to the mixture over a period of 30 minutes, ensuring that the temperature remained below 20°C. The ice bath was then removed, and the mixture was heated to 35°C and maintained at this temperature for 2 hours under continuous stirring, resulting in a thick, dark brown paste.

After this period, 230 mL of deionized water was gradually added to the paste, causing a violent reaction and increasing the temperature to approximately 98°C. The diluted mixture was maintained at this temperature for 15 minutes. Finally, the reaction was terminated by adding 700 mL of deionized water, followed by 50 mL of 30% H2O2 solution, which turned the color of the mixture from dark brown to bright yellow.

The resulting mixture was centrifuged and washed repeatedly with 5% HCl solution and deionized water until the pH of the supernatant reached neutrality. The obtained GO was then dispersed in deionized water to form a 5 mg/mL colloidal suspension, which was subjected to ultrasonication for 1 hour to exfoliate the GO sheets fully.

2.3. Preparation of graphene-TiO2 composites

The graphene-TiO2 (GT) hydrogel composites were prepared using a facile one-step hydrothermal method. In a typical procedure, 80 mg of the previously prepared GO was dispersed in a mixture of 26.6 mL deionized water and 13.3 mL ethanol by ultrasonication for 1 hour. Predetermined amounts of TiO2 nanoparticles were then added to the GO dispersion to achieve mass ratios of GO to TiO2 of 3:6, 3:10, and 3:14, respectively. The resulting suspensions were stirred vigorously for 2 hours to ensure homogeneous dispersion.

Each suspension was then transferred to a 50 mL Teflon-lined stainless steel autoclave. The autoclave was heated from room temperature to 180°C at a controlled rate of 2°C/min. During the hydrothermal treatment, the internal pressure was monitored using a built-in pressure gauge and reached approximately 1.2 MPa. The temperature was maintained at 180°C for 15 hours to ensure complete reduction of GO and formation of the 3D network structure. After the reaction period, the autoclave was allowed to cool naturally to room temperature at an average rate of 1°C/min to prevent structural collapse of the hydrogel.

The formed hydrogels were carefully removed from the autoclave and immersed in deionized water for 24 hours to remove any residual impurities. The hydrogels were then freeze-dried at −50°C for 48 hours to obtain the final GT aerogel composites, which were labeled as GT-1, GT-2, and GT-3 corresponding to the increasing TiO2 content.

2.4. Sol-gel method

For comparison, GT ceramic films were also prepared using a sol-gel method. The precursor solution was prepared by dissolving 17 mL of tetrabutyl titanate (Ti(OC4H9)4) in 76 mL of anhydrous ethanol, followed by the addition of 4.8 mL of diethanolamine (C4H11NO2) as a stabilizer. The solution was stirred for 30 minutes, after which 0.7 g of lanthanum chloride (LaCl3) was added as a dispersing agent for graphene. The mixture was then stirred for an additional 2 hours.

Separately, graphene dispersions were prepared by ultrasonicating various amounts of GO (10, 20, 30, 40, and 50 mg) in 300 mL of N,N-dimethylformamide (DMF) containing 0.06 g of polyvinylpyrrolidone (PVP) as a surfactant. These dispersions were then added to the titanium precursor solution and stirred for 2 hours, followed by the addition of 1 mL of DMF and stirring for another hour.

The resulting sols were deposited on glass substrates using a dip-coating method at a withdrawal rate of 2 mm/s. The coated substrates were dried at 100°C for 10 minutes between each layer, with the process repeated to obtain films with 1 to 5 layers. Finally, the films were sintered in a muffle furnace at 500°C for 1 hour, with a heating rate of 3°C/min.

2.5. Photocatalytic activity evaluation

The photocatalytic performance of the GT composites was assessed through the degradation of MB under both UV and visible light irradiation. In a typical experiment, 50 mg of the photocatalyst was dispersed in 100 mL of MB solution (10 mg/L) and magnetically stirred in the dark for 30 minutes to achieve adsorption-desorption equilibrium. The suspension was then irradiated with either a 300 W mercury lamp (main emission at 365 nm) for UV photocatalysis or a 300 W xenon lamp equipped with a 420 nm cut-off filter for visible light photocatalysis.

At predetermined time intervals, 3 mL aliquots of the suspension were withdrawn and centrifuged to remove the photocatalyst. The concentration of MB in the supernatant was determined by measuring the absorbance at 664 nm using a UV-vis spectrophotometer (UV-2600, Shimadzu, Japan). The photocatalytic efficiency was calculated as the percentage of MB degraded over time.

To evaluate the reusability and stability of the GT composites, the photocatalyst was recovered after each cycle by centrifugation, washed with deionized water and ethanol, and dried at 60°C before being used in subsequent cycles. Five consecutive cycles were performed to assess the long-term stability of the photocatalysts.

3. RESULTS AND DISCUSSION

The morphology and microstructure of the GT composites were examined using SEM. Figure 1 presents the SEM images of GT-1, GT-2, and GT-3 samples. All samples exhibited a distinct 3D porous network structure, with interconnected pores ranging from sub-micrometer to several micrometers in size. The GT-1 sample showed a relatively loose structure with large pores [26], while GT-2 and GT-3 displayed progressively more compact structures with smaller pore sizes.

Figure 1
SEM images of GT composites at different magnifications: (a) GT-1, (b) GT-2, and (c) GT-3.

The formation of the 3D porous network structure in the GT composites can be attributed to the self-assembly of GO sheets during the hydrothermal process. As the temperature increases, the oxygen-containing functional groups on GO are gradually removed, restoring the π-π interactions between the graphene sheets. This process, coupled with the presence of TiO2 nanoparticles, leads to the formation of a 3D interconnected network. The pore size distribution of the GT composites was analyzed using nitrogen adsorption-desorption isotherms [27]. Figure 2 shows the pore size distribution curves for GT-1, GT-2, and GT-3. All samples exhibited a hierarchical porous structure with both mesopores (2-50 nm) and macropores (> 50 nm). The average pore size decreased from 45 nm for GT-1 to 28 nm for GT-2 and 18 nm for GT-3, consistent with the SEM observations. After incorporation into the graphene network through hydrothermal treatment, the particle size decreased slightly to 18-22 nm for GT-3, likely due to the constraining effect of the graphene sheets preventing particle agglomeration. This size reduction aligns with the XRD results showing crystallite sizes of 16-21 nm and contributes to the increased specific surface area observed in the composites, enhancing their photocatalytic performance through greater contact area between TiO2 and the target molecules.

Figure 2
Pore size distribution curves for GT-1, GT-2, and GT-3 composites obtained from nitrogen adsorption-desorption isotherms.

The TiO2 content significantly influenced the microstructure of the GT composites. As the TiO2 content increased from GT-1 to GT-3, several notable changes were observed:

  • (a)

    Pore size reduction: The average pore size decreased with increasing TiO2 content, as evidenced by both SEM and pore size distribution analysis. This can be attributed to the TiO2 nanoparticles acting as spacers between graphene sheets, preventing their restacking and promoting the formation of smaller pores [28].

  • (b)

    Increased surface roughness: SEM images revealed that the surface of the graphene sheets became increasingly rough with higher TiO2 content. This roughness is due to the decoration of TiO2 nanoparticles on the graphene surface, which can potentially increase the specific surface area and provide more active sites for photocatalysis.

  • (c)

    Enhanced structural stability: The incorporation of TiO2 nanoparticles appeared to enhance the structural stability of the 3D network. GT-2 and GT-3 samples maintained their structure better than GT-1 during SEM sample preparation, suggesting improved mechanical properties with increased TiO2 content [29].

These microstructural changes with varying TiO2 content have important implications for the mechanical and photocatalytic properties of the GT composites, which will be discussed in subsequent sections.

XRD analysis was performed to investigate the crystalline structure of the GT composites. Figure 3 presents the XRD patterns of pure TiO2 nanoparticles, graphene oxide (GO), and the GT-1, GT-2, and GT-3 composites. The XRD pattern of TiO2 nanoparticles exhibited characteristic peaks at 2θ values of 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, and 62.7°, corresponding to the (101), (004), (200), (105), (211), and (204) planes of anatase TiO2 [30], respectively (JCPDS card no. 21-1272). A small peak at 27.4° was also observed, indicating the presence of a minor rutile phase.

Figure 3
XRD patterns of pure TiO2 nanoparticles, GO, and GT-1, GT-2, and GT-3 composites.

The GO sample showed a distinctive peak at 10.8°, attributed to the (001) reflection of GO sheets. In the GT composites, this peak disappeared, and a broad peak around 24–26° emerged, overlapping with the anatase TiO2 (101) peak. This broad peak indicates the reduction of GO to reduced graphene oxide (rGO) during the hydrothermal process and the partial restacking of graphene sheets [31]. The intensity of the TiO2 peaks increased from GT-1 to GT-3, consistent with the increasing TiO2 content in the composites [32].

Interestingly, the TiO2 peaks in the GT composites showed slight broadening compared to pure TiO2, suggesting a decrease in crystallite size. The average crystallite size was calculated using the Scherrer equation, yielding values of 21 nm, 18 nm, and 16 nm for GT-1, GT-2, and GT-3, respectively, compared to 25 nm for pure TiO2 [33]. This reduction in crystallite size may be attributed to the interaction between TiO2 nanoparticles and graphene sheets, which could inhibit particle growth during the hydrothermal process.

Raman spectroscopy was employed to further characterize the structure and interactions in the GT composites. Figure 4 displays the Raman spectra of GO, pure TiO2, and the GT composites. The GO spectrum exhibited two prominent peaks: the D band at 1350 cm−1, associated with structural defects and disorder, and the G band at 1590 cm−1, related to the in-plane vibration of sp2 carbon atoms. The intensity ratio of the D and G bands (ID/IG) is often used to evaluate the degree of disorder in carbon materials [34]. In the GT composites, the D and G bands were retained, but their positions and relative intensities changed. The ID/IG ratio increased from 0.85 in GO to 1.12, 1.18, and 1.23 in GT-1, GT-2, and GT-3, respectively. This increase suggests a higher degree of disorder in the graphene structure, likely due to the introduction of defects during the reduction process and the interaction with TiO2 nanoparticles [35]. The Raman spectra of the GT composites also displayed characteristic peaks of anatase TiO2 at 144 cm−1 (Eg), 397 cm−1 (B1g), 516 cm−1 (A1g), and 638 cm−1 (Eg) (Figure 5) [36]. The intensity of these peaks increased with TiO2 content, corroborating the XRD results. A slight redshift of the Eg peak at 144 cm−1 was observed in the GT composites compared to pure TiO2, indicating a possible interaction between TiO2 and graphene [37].

Figure 4
FTIR spectra of GO, pure TiO2, and GT-1, GT-2, and GT-3 composites.
Figure 5
Raman spectra of GO, pure TiO2, and GT-1, GT-2, and GT-3 composites.

FTIR spectroscopy was used to investigate the chemical bonding and interactions in the GT composites. Figure 6 presents the FTIR spectra of GO, pure TiO2, and the GT composites. The GO spectrum showed characteristic peaks at 3400 cm−1 (O-H stretching), 1730 cm−1 (C=O stretching), 1620 cm−1 (C=C stretching), and 1050 cm−1 (C-O stretching), confirming the presence of various oxygen-containing functional groups. In the GT composites, the intensity of these oxygen-related peaks significantly decreased, indicating the successful reduction of GO during the hydrothermal process. A new peak appeared at 1568 cm−1, attributed to the skeletal vibration of graphene sheets [38]. The broad band in the range of 400–800 cm−1 corresponds to the Ti-O-Ti stretching vibrations of TiO2. Notably, a new peak emerged at 1180 cm−1 in the GT composites, which can be assigned to the Ti-O-C vibration. This peak suggests the formation of chemical bonds between TiO2 nanoparticles and graphene sheets, providing evidence for strong interfacial interactions in the composites. The intensity of this peak increased from GT-1 to GT-3, indicating a higher degree of Ti-O-C bonding with increasing TiO2 content [39].

Figure 6
Compressive stress-strain curves for GT-1, GT-2, and GT-3 composites.

Based on the UV-vis diffuse reflectance spectra data, I calculated the band gaps using the Tauc plot method according to the equation (αhν)2 = A(hν - Eg), where α is the absorption coefficient, h is Planck's constant, ν is the photon frequency, A is a constant, and Eg is the band gap energy. By plotting (αhν)2 versus photon energy (hν) and extrapolating the linear portion of the curve to the x-axis, the band gaps were determined to be 3.2 eV, 2.9 eV, 2.7 eV, and 2.5 eV for pure TiO2, GT-1, GT-2, and GT-3, respectively.

The compressive strength of the GT composites was evaluated through uniaxial compression tests. Figure 6 presents the stress-strain curves for GT-1, GT-2, and GT-3 samples. All composites exhibited a typical foam-like behavior characterized by three distinct regions: linear elasticity, plateau, and densification. The compressive strength, defined as the stress at 10% strain, was found to be 0.58 MPa, 0.82 MPa, and 1.15 MPa for GT-1, GT-2, and GT-3, respectively. Interestingly, the GT composites demonstrated higher compressive strengths compared to pure graphene aerogels (0.42 MPa) prepared under similar conditions [40]. This enhancement can be attributed to the reinforcing effect of TiO2 nanoparticles within the graphene network. The TiO2 particles act as nodes, connecting adjacent graphene sheets and strengthening the overall structure.

The elastic modulus, determined from the initial linear region of the stress-strain curves, showed a similar trend to the compressive strength. The calculated elastic moduli were 6.2 MPa, 8.7 MPa, and 11.3 MPa for GT-1, GT-2, and GT-3, respectively. This increase in stiffness with higher TiO2 content suggests that the nanoparticles contribute significantly to the elasticity of the composites [41]. To further investigate the elastic behavior, cyclic compression tests were performed at 5% strain for 100 cycles. Figure 7 shows the load-unload curves for the first, 50th, and 100th cycles for each composite. All samples exhibited good elastic recovery, with GT-3 showing the least hysteresis and highest recovery rate (95% after 100 cycles) compared to GT-1 (88%) and GT-2 (92%).

Figure 7
Cyclic compression test results showing load-unload curves for the 1st, 50th, and 100th cycles at 5% strain for (a) GT-1, (b) GT-2, and (c) GT-3 composites.

The mechanical properties of the GT composites showed a clear dependence on TiO2 content. As the TiO2 concentration increased from GT-1 to GT-3, both compressive strength and elastic modulus improved significantly. This trend can be explained by several factors:

  1. Increased crosslinking: Higher TiO2 content provides more points of interaction between the nanoparticles and graphene sheets, leading to a more interconnected and robust 3D network.

  2. Pore size reduction: As observed in the microstructural analysis, increasing TiO2 content resulted in smaller pore sizes. This denser structure contributes to enhanced mechanical properties [42].

  3. Load transfer: TiO2 nanoparticles can effectively transfer applied loads through the graphene network, improving overall strength and stiffness.

  4. Interfacial interactions: The FTIR analysis revealed stronger Ti-O-C bonding with increasing TiO2 content, suggesting improved interfacial interactions that contribute to better mechanical performance.

The enhanced photocatalytic performance of GT-3 can be attributed to its unique structural features that promote efficient charge separation. The hierarchical porous structure with an average pore size of 18 nm creates numerous interfacial contact points between TiO2 and graphene sheets, while the high specific surface area (243.9 m2/g) maximizes the available reaction sites. During photocatalysis, when TiO2 absorbs light, electron-hole pairs are generated. The strong Ti-O-C bonding observed in FTIR analysis facilitates rapid electron transfer from TiO2 to the graphene sheets. These graphene sheets act as electron highways, efficiently conducting the photogenerated electrons away from TiO2 active sites. This spatial separation of charges significantly reduces electron-hole recombination, allowing the holes to participate in oxidation reactions at the TiO2 surface. The interconnected 3D network structure further enhances this effect by providing continuous electron transport pathways throughout the composite. This synergistic interaction between the structural features and electronic properties of GT-3 explains its superior photocatalytic performance compared to pure TiO2 and other GT composites with lower TiO2 content.

To quantify the reinforcing effect of TiO2, we calculated the specific strength (strength-to-density ratio) for each composite. Table 1 summarizes the density, compressive strength, and specific strength of the GT composites. The specific strength increased from 483 kNm/kg for GT-1 to 719 kNm/kg for GT-3, demonstrating the efficient reinforcement provided by TiO2 nanoparticles. It's worth noting that while higher TiO2 content generally improved mechanical properties, there may be an optimal concentration beyond which further additions could lead to agglomeration and potential weakening of the structure [43]. Future studies could explore this upper limit to maximize mechanical performance while maintaining other desirable properties such as porosity and surface area.

Table 1
Density, compressive strength, and specific strength of GT composites.

The formation of the 3D GT structure during the hydrothermal process involves a complex interplay of chemical reactions, self-assembly, and interfacial interactions. Based on our observations and analysis, we propose the following mechanism for the formation of the GT composites:

  1. Initial dispersion: In the precursor solution, GO sheets are well-dispersed due to their hydrophilic nature, while TiO2 nanoparticles are uniformly distributed among the GO sheets.

  2. Reduction of GO: As the hydrothermal treatment progresses, the elevated temperature and pressure facilitate the removal of oxygen-containing functional groups from GO, resulting in its reduction to reduced graphene oxide (rGO) [44].

  3. Self-assembly: The reduced graphene sheets begin to aggregate due to restored π-π interactions and hydrophobic effects. This aggregation is modulated by the presence of TiO2 nanoparticles, which act as spacers between the sheets.

  4. Nucleation and growth: TiO2 nanoparticles serve as nucleation sites for the assembly of graphene sheets. As more sheets connect to these sites, a 3D network structure begins to form [45].

  5. Cross-linking: Chemical bonds, particularly Ti-O-C linkages (as evidenced by FTIR analysis), form between TiO2 nanoparticles and graphene sheets, further stabilizing the 3D structure.

  6. Pore formation: The interplay between graphene sheet aggregation and TiO2 nanoparticle spacing results in the formation of a hierarchical porous structure with both meso- and macropores.

The photocatalytic activity of the GT composites was evaluated through the degradation of MB under UV irradiation. Figure 8 shows the time-dependent degradation of MB in the presence of pure TiO2, GT-1, GT-2, and GT-3 composites. All GT composites exhibited enhanced photocatalytic activity compared to pure TiO2. After 120 minutes of UV irradiation, the degradation efficiencies were 58%, 79%, 87%, and 92% for pure TiO2, GT-1, GT-2, and GT-3, respectively.

Figure 8
Photocatalytic degradation of methylene blue under UV irradiation for pure TiO2 and GT composites.

The kinetics of MB degradation were analyzed using a pseudo-first-order model, ln(C/C0) = -kt, where C0 and C are the initial and time-dependent concentrations of MB, k is the rate constant, and t is the irradiation time. The calculated rate constants were 0.0072, 0.0131, 0.0171, and 0.0211 min−1 for pure TiO2, GT-1, GT-2, and GT-3, respectively. This increasing trend in rate constants further confirms the enhanced photocatalytic activity of the GT composites with higher TiO2 content.

To distinguish between adsorption and photocatalytic degradation, dark adsorption experiments were conducted. Figure 9 presents the adsorption capacities of the samples over time in the absence of light. The GT composites showed significantly higher adsorption capacities compared to pure TiO2. After 60 minutes in dark conditions, GT-1, GT-2, and GT-3 adsorbed 22%, 28%, and 35% of MB, respectively, while pure TiO2 adsorbed only 5%. The enhanced adsorption capacity of the GT composites can be attributed to their 3D porous structure and the high specific surface area of graphene. The hierarchical pore structure facilitates the diffusion of MB molecules, while the graphene sheets provide abundant adsorption sites [46]. The increasing adsorption capacity from GT-1 to GT-3 correlates with the increasing graphene content and specific surface area of the composites.

Figure 9
Adsorption capacities of pure TiO2 and GT composites in dark conditions.

To assess the potential for practical applications, the photocatalytic performance of the GT composites was also evaluated under visible light irradiation. Figure 10 shows the degradation of MB under visible light for pure TiO2 and the GT composites. Notably, all GT composites demonstrated significantly improved visible light photocatalytic activity compared to pure TiO2. After 180 minutes of visible light irradiation, the degradation efficiencies were 12%, 45%, 58%, and 67% for pure TiO2, GT-1, GT-2, and GT-3, respectively. The corresponding rate constants, calculated using the pseudo-first-order model, were 0.0007, 0.0033, 0.0048, and 0.0062 min−1.

Figure 10
Photocatalytic degradation of methylene blue under visible light irradiation for pure TiO2 and GT composites.

The enhanced visible light photocatalytic activity of the GT composites can be attributed to several factors:

  1. Extended light absorption: The incorporation of graphene extends the light absorption range into the visible region, as evidenced by UV-vis diffuse reflectance spectra (not shown).

  2. Efficient charge separation: Graphene acts as an electron acceptor, facilitating the separation of photogenerated electron-hole pairs and reducing recombination.

  3. Increased active sites: The 3D porous structure provides more accessible active sites for photocatalytic reactions.

  4. Synergistic effects: The intimate contact between graphene and TiO2 nanoparticles, promotes efficient charge transfer and enhances photocatalytic activity.

These results demonstrate the potential of GT composites for practical applications in environmental remediation under both UV and visible light conditions.

The 3D porous structure of the GT composites plays a crucial role in their enhanced photocatalytic activity. To quantify this effect, we compared the specific surface area and pore volume of the samples using BET analysis. Table 2 summarizes these results, showing a significant increase in both specific surface area and pore volume for GT composites compared to pure TiO2. The hierarchical porous structure, consisting of both mesopores and macropores, facilitates mass transport of reactants and products [47]. To demonstrate this, we conducted dye molecule diffusion experiments. Figure 11 shows the diffusion rate of rhodamine B (RhB) through pure TiO2 and GT-3 membranes. The GT-3 composite exhibited a 3.7-fold higher diffusion rate, highlighting the improved mass transport properties of the 3D structure.

Table 2
Specific surface area and pore volume of TiO2 and GT composites.
Figure 11
Diffusion rate of RhB through pure TiO2 and GT-3 membranes.

UV-vis diffuse reflectance spectra (Figure 12) demonstrate the extended light absorption of GT composites into the visible region. The absorption edge of GT composites shows a red-shift compared to pure TiO2, with GT-3 exhibiting the broadest absorption range. The calculated band gaps were 3.2 eV, 2.9 eV, 2.7 eV, and 2.5 eV for TiO2, GT-1, GT-2, and GT-3, respectively, confirming the narrowing of the band gap with increasing graphene content.

Figure 12
UV-vis diffuse reflectance spectra of pure TiO2 and GT composites.

The reusability and stability of GT-3, the best-performing composite, were evaluated through five consecutive photocatalytic cycles under UV irradiation. Figure 13 shows the degradation efficiency of MB over five cycles. The composite maintained 94% of its initial activity after five cycles, demonstrating excellent reusability.

Figure 13
Reusability of GT-3 composite over five consecutive photocatalytic cycles for MB degradation under UV irradiation.

4. CONCLUSION

In conclusion, this study successfully synthesized and characterized a series of 3D GT composites with varying TiO2 content using a facile hydrothermal method. The resulting composites exhibited a well-defined interconnected 3D porous network structure, with pore sizes decreasing from 45 nm to 18 nm as TiO2 content increased. XRD and Raman analyses confirmed the successful reduction of GO and the formation of anatase TiO2 with crystallite sizes ranging from 21 nm to 16 nm. The mechanical properties of the composites improved significantly with increasing TiO2 content, with the compressive strength and elastic modulus of GT-3 reaching 1.15 MPa and 11.3 MPa, respectively. The specific strength of the composites increased from 483 kNm/kg for GT-1 to 719 kNm/kg for GT-3. The GT composites demonstrated superior photocatalytic performance compared to pure TiO2, with GT-3 achieving 92% and 67% degradation of methylene blue under UV and visible light, respectively, after 120 and 180 minutes. This enhanced activity was attributed to the synergistic effects of the 3D porous structure, efficient electron transfer, and extended light absorption. The specific surface area increased from 52.3 m2/g for pure TiO2 to 243.9 m2/g for GT-3, contributing to improved adsorption capacity and photocatalytic efficiency. The GT-3 composite maintained 94% of its initial activity after five consecutive photocatalytic cycles, demonstrating excellent reusability and stability. These findings highlight the potential of 3D graphene-TiO2 composites for environmental remediation applications and provide insights into the design and optimization of advanced photocatalytic materials.

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Publication Dates

  • Publication in this collection
    27 Jan 2025
  • Date of issue
    2024

History

  • Received
    26 Sept 2024
  • Accepted
    13 Nov 2024
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