Mechanistic Insights and Kinetic Modeling of Photocatalytic Degradation of Organic Dyes Using TiO₂–Graphene Nanocomposites

636 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) Mechanistic Insights and Kinetic Modeling of Photocatalytic Degradation of Organic Dyes Using TiO₂–Graphene Nanocomposites https://currentsign journal.com/index. php/JCS/index Abu Bakar Hussain Zai Ishfaq Ahmed Kashaf Shehzadi Faisal Gulzar Nazeer Manzoor Imad Uddin Hadia Sharafat Ali Raza Vol. 3 No. 4 (2025) 637 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) Mechanistic Insights and Kinetic Modeling of Photocatalytic Degradation of Organic Dyes Using TiO₂–Graphene Nanocomposites Persistent organic dyes from industrial effluents contaminate water resources, requiring complex treatment methods. This study examines the enhanced photocatalytic degradation of methylene blue (MB) using a TiO₂–graphene nanocomposite that was created using a sol-gel assisted hydrothermal method. Complete characterization using XRD, SEM, BET, and DRS demonstrated the successful integration of graphene with TiO₂. This contributed to a narrower band gap, improved absorption of visible light, and a higher specific surface area. Superior photocatalytic performance has been demonstrated by the nanocomposite's degradation efficiency, which was significantly greater than that of pure TiO₂ when exposed to UV-visible light. Kinetic analysis revealed that the degradation process followed a pseudo-first-order model and that the nanocomposite had a substantially greater apparent rate constant. Mechanistic studies using radical scavenging experiments demonstrate that the primary reactive species that contribute to dye degradation are photogenerated holes (h⁺) and superoxide radicals (•O₂⁻). This illustrated how graphene may successfully accept particles and stop charge carrier recombination. Furthermore, the nanocomposite exhibited outstanding strength and reusability with minimal activity loss over a total of five consecutive cycles. With regard to these outcomes, the TiO₂–graphene nanocomposite is a highly effective and persistent photocatalyst for wastewater remediation, providing significant kinetic framework and mechanistic insights. Abu Bakar Hussain Zai* (Corresponding Author) Department of Chemistry, University of Baluchistan, Quetta, Pakistan Email: abubakarhussa[email protected] Ishfaq Ahmed Department of Chemistry, University of Science and Technology Bannu, kpk, Pakistan Email: [email protected] Kashaf Shehzadi Department of Chemistry, University of Agriculture Faisalabad, Pakistan Email: [email protected] Faisal Gulzar Department of Physics, University of Azad Jammu and Kashmir Muzaffarabad 13100, Pakistan Email: faisalg[email protected] Nazeer Manzoor Department of Chemistry, Riphah International University Faisalabad, Pakistan Email: [email protected] Imad Uddin Department of Chemistry University of Swabi, Pakistan. [email protected] Imad Uddin Department of Chemistry University of Swabi,Pakistan ([email protected]) Hadia Sharafat Department of Physics, Quaid-i-Azam University, Islamabad, Pakistan Email: [email protected] Ali Raza Department of Chemistry, University of Sahiwal, Pakistan Email: [email protected] Abstract 638 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) Keywords: TiO₂–Graphene Nanocomposite, Photocatalytic Degradation, Kinetic Modeling, Organic Dyes. Introduction The increasing incidence of water pollution, which is mainly caused by industrial effluents, is one of the most significant environmental problems of our time [1]. One of the most dangerous industrial pollutants is the manufacturing of synthetic organic dyes for clothing, textiles, printing, and cosmetics [2]. In spite of their ability to diminish in the presence of light and chemicals, these complex volatile substances show outstanding stability and persistence in aquatic environments [3]. When released unprotected, they severely damage the aesthetics of water bodies and drastically reduce light penetration, which challenges with photosynthesis and negatively impacts aquatic ecosystems [4]. In addition, numerous of these dyes and the waste products of their decomposition are hazardous, carcinogenic, and mutagenic, compromising human health and threatening biodiversity [5]. Conventional wastewater treatment methods like coagulation, adsorption, and biological degradation often fail to effectively transform these resistant organic substances [6].They usually transport the pollutants from one phase to another, generating secondary waste streams like sludge, rather than removing of them. As an outcome, there is a continuing and pressing demand for innovative, effective, and sustainable treatment methods that may dispose of these risky substances [7]. One highly promising Advanced Oxidation Process (AOP) at the moment is heterogeneous photocatalysis based on semiconductors [8]. Utilizing light energy, this approach effectively transforms organic contaminants into harmless byproducts like carbon dioxide and water [9]. The process starts when a photocatalyst absorbs photons with energy equivalent to or bigger than its bandgap, generating electron-hole pairs. These charge carriers can react with adsorbed oxygen and water molecules once they reach the catalyst's surface, releasing powerful ROS, primarily superoxide anions (•O₂⁻) and hydroxyl radicals (•OH) [10]. These ROS can oxidize and mineralize an array of organic shades due to their high oxidation potential and insufficient selective capacity. Titanium dioxide (TiO₂) has been extensively researched as a benchmark photocatalyst among other semiconductors due to its powerful photocatalytic activity, chemical stability, non-toxicity, and accessibility [11]. However, pure TiO₂'s large bandgap, which limits its light absorption to the ultraviolet (UV) region (a small portion of solar energy), and the rapid formation of photogenerated electron-hole pairs, which significantly decreases its quantum efficiency, are two major intrinsic limitations that prohibit it from being widely used in industries [12]. Combining TiO₂ with carbonaceous nanomaterials, especially graphene and its derivative graphene oxide (GO), is an increasingly common way to get around these restrictions [13]. The two-dimensional sheet of sp2 hybridized carbon atoms referred to as graphene has a special set of properties that make it a perfect part of a nanocomposite. Its remarkably high specific surface area offers dye molecules an excellent platform for adsorption, bringing them close to the catalytic sites[14]. Furthermore, its exceptional electrical conductivity makes it an excellent electron acceptor and transporter. When mixed with TiO₂, graphene may remove photogenerated electrons from the TiO₂ conduction band. This electron shuttle effect is essential because it effectively stops charge carrier recombination, which frees up more holes for the contribution of reactions of oxidation. This synergistic relationship may simultaneously enhance the separation efficiency of electron-hole pairs and increase the nanocomposite's light absorption range into the visible region because of graphene's photosensitizing effect [14], [15]. The corresponding TiO₂ graphene 639 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) nanocomposites consistently demonstrated enhanced photocatalytic capacity for dye degradation in contrast to pristine TiO₂. It is crucial for identifying the specific functions of graphene, the primary reactive species involved, the sequence of degradation intermediates, and the conclusion of degradation pathways [16]. Optimizing the composite material involves identifying which oxidants holes, hydroxyl radicals, or superoxide anions are predominant and how the presence of graphene influences their contribution[17]. Additionally, the development of accurate kinetic models must be done for a quantitative representation of the degradation process [18]. Researchers can move from qualitative observations to a predictive understanding of the reaction rate in relation to crucial operational parameters, which include light intensity, catalyst loading, and initial dye concentration, through kinetic analysis [19]. While models based on the Langmuir-Hinshelwood mechanism are frequently used to explain surface-mediated photocatalytic reactions, the complex connection between adsorption on graphene and reaction on TiO₂ sites in a nanocomposite requires the use of advanced modeling methods. A comprehensive kinetic model involving surface reaction rates, adsorption equilibria, and the effect of mass transfer can be a useful instrument for reactor design, scale-up, and process optimization in practical applications. Thus, in order to close the gap between material performance and fundamental process understanding, this work focuses on a comprehensive examination of the kinetic modeling and mechanistic aspects of the photocatalytic degradation of organic dyes using TiO₂–graphene nanocomposites. Materials and methods Sigma-Aldrich supplied titanium (IV) isopropoxide (TTIP, 97%), graphite powder, methylene blue (MB) dye, ethanol, nitric acid (HNO₃), and sodium hydroxide (NaOH). Each of the experiments used deionized (DI) water. Any analytical-grade reagent has been used without extra purification. Synthesis of Graphene Oxide (GO) A modified Hummers' method was employed for producing graphene oxide (GO). In the conventional process, 50 mL of concentrated H₂SO₄ was slowly incorporated with 2 g of graphite powder whereas constantly stirred in an ice bath. Then, maintaining the temperature below 10 °C, 6 g of KMnO₄ was added gradually. Following an hour of mixing at 35 °C, 100 mL of DI water was slowly added. The suspension became bright yellow if 10 mL of 30% hydrogen peroxide solution had been added in order to stop the reaction. After continually washing the product with HCl (5%) and DI water until the pH was neutral, it was dried at 60 °C to generate GO powder. 640 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) Figure 2.1: Synthesis of graphene oxide by hummer’s method. Synthesis of TiO₂–Graphene Nanocomposites Sol-gel assisted hydrothermal production was used to create TiO₂–graphene nanocomposites. In an average synthesis, 2 mL of TTIP was melted in 40 mL of ethanol for generating Solution A. 50 mg of GO were ultrasonically dispersed in 50 mL of a 1:1 v/v ethanol water mixture for 30 minutes to produce Solution B. Solution A was added dropwise to Solution B while stirring constantly to adjust the rate of hydrolysis. Following this, one milliliter of HNO₃ was added. Following two hours of mixing, the mixture was placed in a Teflon-covered stainless-steel autoclave and heated to 180 °C for twelve hours. Figure 2.2: Synthesis of TiO₂–Graphene Nanocomposites Characterization techniques 641 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) A wide range of techniques for analysis were used to fully analyze the synthesized samples in order to identify their vital features. Using Cu Kα radiation, X-ray diffraction (XRD) was employed to analyze the crystallite size and structural phase composition. Fourier Transform Infrared (FTIR) Spectroscopy was applied in the 400–4000 cm⁻¹ region to identify the functional groups that were present. The surface dimension and particle dispersion have been examined through scanning electron microscopy (SEM), and the surface area and pore size distribution were determined using Brunauer–Emmett–Teller (BET) analysis with N₂ adsorption–desorption, and Tauc plots were applied to optically calculate the bandgap from UV–Vis Diffuse Reflectance Spectroscopy (DRS) data. Photocatalytic Degradation Experiments The photocatalytic performance under UV–visible light has been evaluated using methylene blue (MB), a model organic dye. In the standard experiment, 100 mL of a 10 mg/L MB solution was mixed with 50 mg of photocatalyst. During illumination, the suspension had been stirred for 30 minutes in the dark to reach adsorption–desorption equilibrium. The solution was then exposed to ultraviolet irradiation using a 300 W Xe lamp coupled with a UV-visible cutoff filter. A UV-Vis spectrophotometer set at 664 nm was used for determining the percentage of dye retained after 5 ml aliquots were collected out at regular intervals and centrifuged. The equation that follows was employed to get the decrease in efficiency (η): η(%) = C0 −Ct C0 ×100 Here C0 and Ct are the initial and time-dependent dye concentrations, respectively. Kinetic and Mechanistic Studies Kinetic analysis was performed out by fitting experimental data to the pseudo-first-order and Langmuir–Hinshelwood (L–H) models. The apparent rate constant (k) is determined using the slope of the linear plot of ln(C₀/Cₜ) vs irradiation time. Reactive species trapping investigations were carried out to investigate the degradation mechanism using a variety of scavengers, comprising isopropanol (•OH scavenger), benzoquinone (•O₂⁻ scavenger), and EDTA (h⁺ scavenger). The behavior of electron transport and charge separation has been investigated employing photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). Reusability and Stability Tests The photocatalyst's stability has been evaluated over five further cycles of MB degradation under the exact same conditions. After every run, the catalyst was separated, washed with ethanol and DI water, dried at 60 °C, and recycled. The results of XRD tests revealed the structural stability of the TiO₂–graphene nanocomposite after cycling. Results and discussion X-ray Diffraction (XRD) 642 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) The TiO₂–graphene nanocomposite's successful formation and structural integrity are clearly shown by the X-ray diffraction (XRD) analysis. The crystalline structure of TiO₂ remains unmodified following its composite formation with graphene, as demonstrated by the distinct peaks in the diffraction pattern for the TiO₂/graphene sample that are exclusively indexed to the anatase phase of TiO₂ (JCPDS 21-1272). The pattern of the nanocomposite notably does not include the characteristic peak for graphene oxide (GO), suggesting that GO was effectively reduced to graphene during synthesis and then incorporated with the TiO₂ nanoparticles. The shortage of unusual crystalline phases and the continued existence of a sharp TiO₂ anatase structure indicate that graphene does not hinder the photocatalyst's crystallinity, which is important in encouraging efficient charge transfer and the kinetics of subsequent photocatalytic degradation demonstrated in the kinetic modeling research. Ourida et al. 2021 stated that to improve TiO2's photocatalytic reactivity for dye removal, graphene oxide (GO) is produced using the Hummers procedure and modified with TiO2 (xGO/TiO2 with x = 2, 5, 10, 15, and 30 weight percent). XRD, SEM/EDS, BET surface area, RDs, FTIR, and EIS were used for analyzing the synthetic materials. The photocatalysts exhibited significant photoreactivity when exposed to sunlight. After 10 minutes of direct sunlight, the dye removal reaction's efficiency improves from 14 to 81% when the percentage of GO is increased from 0 to 30%. In contrast with the other composites, the 30GO/TiO2 composites show superior photoactivity when placed under ultraviolet rays[20]. Therefore, the MO removal efficiency with the 30GO/TiO2 photocatalyst reaches 84% of its maximum value (96%) after 10 minutes of irradiation. On the other alternative, it takes 60 minutes to get the maximum yield of 81% by employing TiO2 alone. It was achievable to shorten the reaction time sufficient for attaining the steady-state by enhancing the catalyst with GO. Whenever applied to photodegradation data, the Weibull model gives the best fit (RMSE, ARE, R2, and t0.5) for both classical and fractal-like kinetic models. When compared to pure TiO2, the 30GO/TiO2 catalyst decreases the degradation time by half. The photodegradation by xGO/TiO2 demonstrates a decrease in mass transfer resistance inside a winding channel on the surface and in the fluid layer next to the catalyst particles. consequently, compared to pure TiO2, the mass transfer coefficient and intraparticle diffusivity on the 30GO/TiO2 catalyst grew by approximately 19 and 13 times, respectively [21]. 643 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) Figure 3.1: X-ray diffractograms of GO, pure TiO₂, and the TiO₂–graphene nanocomposite. The diffraction patterns confirm the successful synthesis of the composite, with all characteristic peaks corresponding to the anatase phase of TiO₂ (JCPDS 21-1272). The absence of a distinct GO peak in the nanocomposite pattern indicates the effective reduction of graphene oxide to graphene during the process. Scanning Electron Microscopy (SEM) Scanning electron microscopy (SEM), which demonstrates the morphological evolution of the photocatalyst, provides crucial details about the enhanced photocatalytic mechanism. Pure TiO₂ and the TiO₂–graphene nanocomposite are likely shown in images (A) and (B), respectively. The morphology of pure TiO₂ is densely aggregated nanoparticles, which minimizes surface area and promotes charge carrier recombination. On the other hand, the structure of the TiO₂–graphene nanocomposite (B) is substantially different, with TiO₂ nanoparticles equally distributed and attached to the wrinkled, sheet-like graphene matrix. Because it restricts TiO₂ aggregation, exposes further active sites for dye adsorption, and facilitates immediate interfacial electron transfer from TiO₂ to the graphene scaffold, this integrated, three-dimensional architecture serves as vital for the degradation kinetics. The mechanistic model of enhanced mass transfer and suppressed charge recombination, that together provides for the superior kinetic performance seen in the photocatalytic degradation process, is reinforced by this direct morphological evidence. 644 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) Mostafa et al.2016 explained that the sol-gel technique has been used to produce TiO2–graphene oxide nanocomposites for the degradation of a usual cationic dye solution. X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric-differential analyses, BrunauerEmmett-Teller surface area measurement, and scanning and transmission electron microscopy were used to characterize the synthesized photocatalysts. In addition, the degradation of a methylene blue aqueous solution under solar radiation was used to determine the photocatalytic activities of the samples. UV–vis spectroscopy was used to examine the color change of the solution. The greatest amount of photocatalytic decoloration (94%) was attained in 60 minutes, surpassing that of pure anatase under the same parameters. The findings indicate that the nanocomposite with 9.0 weight percent graphene oxide behaves significantly photo catalytically than either single-phase anatase or other composites containing different graphene oxide compositions. A modified kinetic model was employed for analyzing the experimental degradation data from the batch tests. Despite higher regression coefficients and fewer relative errors, this model had the ability to predict the performance. contrary research, the homogeneous dye degradation is significantly affected by the distribution of TiO2 nanoparticles (less than 20 nm) on graphene oxide sheets, which may additionally improve photocatalytic activity [22]. Figure 3.2: SEM images of the TiO₂–graphene nanocomposite (B–D) and aggregated pure TiO₂ (A) reveal improved TiO₂ dispersion on graphene sheets, which is crucial for enhanced photocatalytic activity. UV–Vis diffuse reflectance spectra (DRS) The UV-Vis diffuse reflectance spectra (DRS) offer crucial mechanistic data about the enhanced photocatalytic activity of the TiO₂–graphene nanocomposite. The spectra of the composite material generally shows an apparent redshift in the absorption edge when compared to pure TiO₂, 651 Journal for Current Sign Online ISSN (3006-1504) Print ISSN (3006-1490) compounds or producing ternary composites. 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