Biochar-Modified TiO₂ Composites: Enhanced Optical and Photocatalytic Properties for Sustainable Energy and Environmental Applications

Abstract

Enhancing TiO₂ performance is essential for advancing photocatalysis, environmental remediation, and energy conversion technologies. In this work, nanosized TiO₂ was modified with biochar (BC) derived from red sea algae at different loadings (0, 5, 10, and 15 wt%). Structural analysis confirmed that TiO₂ maintained its crystalline framework while biochar introduced additional amorphous features and modified surface morphology. Optical measurements revealed a redshift in the absorption edge and tunable bandgap values (3.28–3.72 eV), accompanied by increases in refractive index and extinction coefficient, indicating enhanced light–matter interactions. Electrochemical studies demonstrated that the TiO₂/5 wt% BC composite exhibited the lowest charge-transfer resistance and highest peak current, reflecting superior conductivity. Photocatalytic tests showed that TiO₂/5 wt% BC achieved nearly 84% degradation of methylene blue within 150 min under visible-light irradiation, with stable reusability over multiple cycles. These findings demonstrate that moderate biochar incorporation (5 wt%) optimally enhances the physicochemical, electrochemical, and photocatalytic properties of TiO₂, making it a promising candidate for wastewater treatment, solar-driven catalysis, and sustainable energy applications.

1. Introduction

The rapid industrialization and continuous discharge of organic dyes and other hazardous pollutants into aquatic environments have become a primary global concern, posing severe ecological and health risks worldwide [1,2,3]. Semiconductor-based photocatalysis has emerged as a green, sustainable, and cost-effective approach for environmental remediation, enabling the degradation of organic contaminants under solar illumination without producing secondary pollution [4]. In recent years, extensive efforts have been devoted to improving the photocatalytic efficiency of semiconductors through heterojunction formation, carbon modification, and bandgap engineering to extend light absorption into the visible and near-infrared regions [5]. Carbon-based and polymeric photocatalysts, such as g-C₃N₄ and carbon nitride composites, have shown remarkable potential for harnessing a broader solar spectrum and improving charge-separation efficiency [6,7].

Titanium dioxide (TiO₂) has emerged as a cornerstone material in many optical applications due to its outstanding properties, high refractive index, excellent photochemical stability, and non-toxic nature [8,9,10]. Despite its merits, pristine TiO₂ suffers from two inherent drawbacks: (i) its wide bandgap (~3.2 eV for anatase) restricts light absorption mainly to the ultraviolet (UV) region, which constitutes less than 5% of solar radiation, and (ii) the rapid recombination of photogenerated charge carriers limits its photocatalytic efficiency [11,12]. Addressing these challenges is essential for advancing TiO₂-based materials toward practical energy conversion and environmental remediation technologies.

The significance of structural modifications achieved through incorporating biochar into TiO₂ (forming TiO₂/biochar composites) lies in several key areas. In this system, biochar forms intimate interfacial contact with TiO₂, enhancing charge transfer and light absorption efficiency through the synergistic interaction between the semiconductor and the carbonaceous matrix [13]. The π-conjugated carbon domains and oxygen-containing surface groups of biochar promote charge transfer and extend visible-light absorption through interfacial electronic interactions [14]. These effects enhance the separation of photogenerated charge carriers and improve photocatalytic efficiency without forming organometallic or heteroatom-doped TiO₂ structures.

To enhance TiO₂ performance, researchers have pursued various modification strategies, including the incorporation of activated biochar to form TiO₂/biochar composites [15]. Activated biochar is a carbon-rich material produced through biomass pyrolysis, followed by activation processes that create a highly porous structure with a substantial surface area. This unique morphology not only improves the interaction between TiO₂ and dopants but also enhances the overall optical and electronic properties of the composite material. Castilla et al. studied TiO₂-biochar composites that exhibit superior photocatalytic activity compared to pure TiO₂, achieving up to 76% removal of pollutants within 20 min of UV exposure [16]. Furthermore, modification with biochar shifts the optical response of TiO₂ from UV to visible light, increasing its applicability in diverse light conditions, as reported by Rajkumar et al. [17]. Al-atawi et al. studied the use of biochar as an aid for separating photogenerated charge carriers, which is crucial for improving photocatalytic performance [18]. Coupling TiO₂ with carbonaceous materials (e.g., biochar) has been demonstrated as an effective strategy to enhance photocatalytic performance by improving interfacial charge transfer, surface active sites, and visible-light absorption. For example, a TiO₂-Fe/biochar composite was fabricated and applied to the degradation of methylene blue, demonstrating that the interfacial coupling between TiO₂ and biochar enhances charge separation and photocatalytic activity [19]. It is worth noting that although several studies have shown the effectiveness of TiO₂–biochar coupling strategies for photocatalysis, these reports generally focused on a single composite ratio. In contrast, the present work systematically evaluates different biochar loadings (5, 10, and 15 wt%) to determine the optimal incorporation level, thereby providing a more comprehensive understanding of the structure–property–activity relationships.

Although TiO₂–biochar composites have been widely studied for wastewater treatment, most existing work focuses on powder-based catalysts or unactivated biochar supports. In contrast, the present study introduces a distinct combination of materials design and characterization: (i) biochar produced from red-sea algae and chemically activated with KOH to tailor porosity and surface functionalities; (ii) fabrication of spin-coated TiO₂/BC thin films, which allow direct determination of optical constants and dispersion parameters; and (iii) integration of optical, electrochemical, and photocatalytic analyses to correlate charge-transfer processes with visible-light performance. Furthermore, a systematic study of biochar loading (0–15 wt%) identifies an optimum composition (5 wt% BC) with enhanced photon utilization, charge separation, and recyclability. These combined features distinguish the present work from earlier TiO₂–biochar studies and provide new insight into the role of biochar in tailoring thin-film optoelectronic and photocatalytic behavior.

2. Results and Discussion

2.1. Structural Analysis

The X-ray diffraction (XRD) patterns of activated biochar, pure TiO₂, and TiO₂/biochar composite (15 wt% BC) are shown in Figure 1 (an expanded view of 2θ = 22–28° is included as an inset). The powder standard for anatase (JCPDS No. 21-1272) indicates the strongest anatase (101) reflection at ≈25.3° (2θ). In our measurements, the (101) reflection is clearly visible at ~25.3° for the pure TiO₂ film but appears as a broadened, weaker feature in the TiO₂/BC composites. This reduction in apparent peak intensity for the composites is attributable to two factors: (i) the samples are thin spin-coated films (nominal thickness ≈ 100 ± 10 nm), which provide a low diffracting volume in a Bragg–Brentano PXRD geometry and therefore yield lower peak intensities compared to bulk powder measurements, and (ii) the activated biochar introduces a broad amorphous scattering background (a carbonaceous hump) that overlaps and partially masks crystalline reflections, reducing the peak-to-background contrast. Both effects are common and expected for thin carbon-modified films measured with a conventional PXRD setup and have been discussed in the thin-film XRD literature [20].

Despite the reduced intensity in the composite patterns, the positions of the observable diffraction features correspond to the anatase TiO₂ reflections (the (101), (004), and (200) reflections around 25.3°, 37.8°, and 48.0°, respectively), consistent with JCPDS 21-1272 and previous reports. Where necessary, peak broadening was quantified with Scherrer analysis, and the observed broadening is consistent with the small crystallite sizes and the film geometry. For complete clarity, we have added a short discussion of the limitations of conventional PXRD for thin films and noted that complementary techniques (e.g., Raman spectroscopy or electron diffraction) are valuable when the diffracting volume is limited [21,22].

The crystallinity degree is a very vital parameter for different applications, and it could be calculated by Equation (1) [18]:

$$X_c\left(\%\right)={\displaystyle \frac{A_c}{A_c+A_a}}\times 100$$

(1)

where “A_c” is the total crystalline region and “A_a” is the total amorphous region. The average degree of crystallinity of TiO₂/x wt% BC (x = 0 and 15) is 67% and 78%, respectively. Applying Scherer’s equation [23] to all the planes shown above demonstrates that the average crystallite sizes of TiO₂/x wt% BC (x = 0 and 15) are 59 and 71 nm, respectively.

The Scanning Electron Microscopy (SEM) image of pure TiO₂ (Figure 2a) reveals a surface morphology characterized by irregularly shaped particles with a broad size distribution. The particles appear agglomerated into larger clusters, suggesting a high degree of crystallinity. The surface texture is rough, with visible pores and cavities, suggesting a high surface area. The particles’ edges appear sharp, indicating a crystalline structure. The overall morphology suggests that the pure TiO₂ sample may exhibit high crystallinity and surface area, which could benefit applications such as photocatalysis.

The SEM image of TiO₂ modified with 15 wt% biochar (BC) (Figure 2b) exhibits a distinctly different surface morphology. The presence of biochar has significantly modified the TiO₂ particles. The SEM micrographs show that the TiO₂ particles tend to agglomerate into clusters, a typical behavior for nanoparticles due to their high surface energy. In the TiO₂/biochar composites, the presence of biochar appears to improve particle dispersion and reduce the extent of agglomeration, likely due to the porous and functionalized nature of the biochar surface. This enhanced dispersion is expected to increase the interfacial contact area between TiO₂ and biochar, favoring charge transfer and adsorption during photocatalysis. While SEM provides valuable morphological insights, detailed crystallinity information is obtained from XRD analysis, and no such claims are drawn from SEM images here. Incorporating biochar has led to a more heterogeneous surface morphology, potentially enhancing the material’s adsorption and photocatalytic properties. Adding biochar appears to have altered the surface properties of TiO₂, creating a more complex, porous structure. This modified morphology may influence the material’s interactions with its environment, such as the adsorption of pollutants or reactions with light, and thus affect its overall performance in various applications.

2.2. Optical Studies

The transmission spectrum provides valuable insights into the optical properties of these materials, which are crucial for their potential applications in optoelectronic devices. The optical transmission spectra of TiO₂ and TiO₂/biochar composites (5, 10, and 15 wt%) are presented in Figure 3. The transmission spectrum of pure TiO₂ shows a characteristic peak at around 350–400 nm, corresponding to the bandgap energy of TiO₂. The transmission above 400 nm is high, indicating that TiO₂ is transparent in the visible region [24]. This is consistent with the well-known properties of pure TiO₂, which is widely used in photocatalytic and optoelectronic applications. The transmission spectrum slightly shifts towards longer wavelengths when TiO₂ is modified with 5 wt% activated biochar, indicating a redshift in the bandgap energy.

The observed redshift in the absorption edge and the decrease in optical transmission with increasing biochar content suggest that biochar incorporation modifies the optical response of TiO₂. While no direct evidence of defect states in the TiO₂ lattice is available from the present study, these effects can be reasonably explained by enhanced light scattering and absorption due to the carbonaceous phase, as well as interfacial electronic interactions between TiO₂ and biochar. Such interactions have been reported to influence charge transfer and band-edge positions in TiO₂–carbon composites, leading to a tunable optical response without necessarily altering the crystalline TiO₂ lattice. The transmission above 400 nm remains high, suggesting that the biochar modification does not significantly affect the transparency of TiO₂ in the visible region. As the biochar content increases to 10 wt%, the transmission spectrum shows a more pronounced redshift, indicating a further reduction in the bandgap energy. This can be attributed to the increase in impurity states, which can facilitate the absorption of photons at longer wavelengths. Interestingly, the transmission above 400 nm begins to decrease, suggesting that introducing biochar impurities can increase light scattering and absorption in the visible region. When the biochar content is increased to 15 wt%, the transmission spectrum shows a significant reduction in transmission across the entire spectrum. This suggests that the high concentration of biochar impurities can lead to significant light scattering and absorption, resulting in reduced transmission. The redshift in the bandgap energy is also more pronounced, indicating that the biochar impurities can significantly alter the electronic structure of TiO₂.

The optical reflection spectra of TiO₂/x wt% BC (x = 0, 5, 10, and 15) are presented in Figure 4. The reflection spectrum of pure TiO₂ shows a characteristic peak at around 350–400 nm, corresponding to the bandgap energy of pure TiO₂. When TiO₂ is modified with 5 wt% activated biochar, the reflection spectrum shows a slight increase in the reflection coefficient across the entire spectrum. This suggests that incorporating biochar impurities can increase light scattering, leading to greater reflection. The peak at 350–400 nm is still present but with a slightly higher reflection coefficient, indicating that the biochar impurities can affect the optical properties of TiO₂. The reflection spectrum shows a dramatic increase in the reflection coefficient at 15 wt% biochar. The reflection coefficient approaches unity at wavelengths below 500 nm, indicating that the material reflects most of the incident light.

The refractive index, n, of our investigated samples could be determined from Equation (2) [25]

$$n={\displaystyle \frac{\left(1+R\right)+\sqrt{4R-{\left(1-R\right)}^{2}k}}{\left(1-R\right)}}$$

(2)

Figure 5 depicts the refractive index variation with wavelength for TiO₂/x wt% BC (x = 0, 5, 10, and 15). The refractive index spectrum of pure TiO₂ shows a characteristic dispersion curve, with a high refractive index at shorter wavelengths and a gradual decrease at longer wavelengths. The refractive index at 500 nm is around 2.2, consistent with the well-known properties of pure TiO₂. When TiO₂ is modified up to 15 wt% activated biochar, the refractive index spectrum slightly shifts towards higher values across the entire spectrum. This suggests that incorporating biochar impurities can increase the refractive index of TiO₂. The dispersion curve remains like that of pure TiO₂ but with a slightly steeper slope, indicating that the biochar impurities can modify the electronic transitions in the material. The increase in the refractive index with increasing biochar content can be attributed primarily to enhanced light scattering from the carbonaceous phase, interfacial interactions at the TiO₂–biochar interface, and additional absorption pathways introduced by the biochar surface. These mechanisms effectively modify the optical response of the composite without requiring significant changes to the TiO₂ lattice itself.

The energy gap (E_g) is a critical parameter that determines the optical and electronic properties of a material. According to Tauc’s theory, E_g could be calculated by Equation (3) [18]:

$${\left(\alpha h\nu \right)}^{0.5}=A\left(h\upsilon -E_g\right)$$

(3)

The energy gap calculations and Tauc plot method for TiO₂ and TiO₂/biochar composites (5, 10, and 15 wt%) are presented in Figure 6. The estimated optical bandgap (E₉) values are approximately 3.72 eV (pure TiO₂), 3.52 eV (5 wt% BC), 3.45 eV (10 wt% BC), and 3.28 eV (15 wt% BC). The progressive redshift of the absorption edge and corresponding bandgap narrowing with increasing biochar content indicate enhanced visible-light absorption and are consistent with the well-known properties of pure TiO₂ [26]. This widening of E_g is inconsistent with the formation of defect states, which typically reduce the bandgap. Instead, the observed behavior can be attributed to optical and microstructural effects, including enhanced light scattering by the carbonaceous phase, partial surface coverage of TiO₂ by biochar, and possible size-dependent quantum confinement at smaller TiO₂ domains formed during composite preparation. These phenomena effectively shift the optical absorption edge without necessarily modifying the intrinsic electronic structure of TiO₂.

The TiO₂/BC composites exhibited a redshift in the absorption edge and tunable bandgap values ranging from 3.28 to 3.72 eV. Although these bandgap values correspond mainly to UV absorption, the enhanced visible-light photocatalytic response can be attributed to several synergistic effects. The carbonaceous biochar phase contains extended π-conjugated structures that absorb visible light and can transfer photoexcited electrons to the TiO₂ conduction band via interfacial charge transfer. Additionally, the porous morphology of biochar enhances light scattering and photon capture within the composite. These combined mechanisms effectively extend the photocatalytic activity of the TiO₂/BC system into the visible-light region without significantly narrowing the intrinsic TiO₂ bandgap.

Figure 7 presents the extinction coefficient calculations for TiO₂ and TiO₂ modified with 5, 10, and 15 wt% activated biochar. The extinction coefficient is calculated using the relation k = αλ/4π, where α is the absorption coefficient and λ is the wavelength. The extinction coefficient of pure TiO₂ is around 0.10–0.15 across the wavelength range of 260–340 nm. This is consistent with the well-known properties of pure TiO₂, which is a highly transparent material [27]. When TiO₂ is modified with up to 15 wt% biochar, the extinction coefficient increases to 0.20–0.25 across the same wavelength range. This suggests that incorporating biochar impurities can increase the extinction coefficient of TiO₂. The increase in the extinction coefficient can be attributed to enhanced light scattering from the carbonaceous phase, interfacial interactions at the TiO₂–biochar interface, and additional absorption pathways introduced by the biochar surface. These techniques efficiently alter the optical sensitivity of the composite without necessitating substantial modifications to the TiO₂ lattice.

The dispersion energy (E_d) and the single oscillator energy (E_s) could be calculated from Equation (4) using the single oscillator model (WDD) [28]:

$$n^2-1={\displaystyle \frac{E_d{E}_s}{E_s^2-{\left(h\nu \right)}^2}}$$

(4)

The slope of the linear part of this relationship represents E_d, while the intercept with the y-axis determines E_s. Figure 8 shows the relation between 1/(n² − 1) and E² for TiO₂/x wt% BC (x = 0, 5, 10, and 15) thin films. The values of Ed for TiO₂/x wt% BC (x = 0, 5, 10, and 15) are 16.4, 18.9, 19.4, and 20.6 eV, respectively. While the values of E_s for TiO₂/x wt% BC (x = 0, 5, 10, and 15) are 11.4, 12.9, 13.4, and 14.6 eV, respectively.

2.3. Electrochemical Measurements

Figure 9 shows the cyclic voltammograms of TiO₂ and TiO₂/x wt% BC (x = 5, 10, 15) electrodes recorded at a scan rate of 50 mV s⁻¹. Pure TiO₂ exhibits a relatively low capacity due to its limited electrical conductivity [29]. The peak currents around 0.7 V increase systematically with increasing biochar content, indicating that biochar incorporation enhances the overall charge-transfer kinetics. This improvement arises from the conductive carbon network within the biochar, which facilitates electron migration and reduces interfacial resistance between TiO₂ particles and the electrolyte. Although the 15 wt% BC composite exhibits the highest peak current, its photocatalytic activity does not follow the same trend. This is because excessive biochar loading can cause optical shielding and carrier recombination, partially offsetting the benefit of improved conductivity. This interpretation reconciles the electrochemical and photocatalytic observations without invoking unsupported assumptions of particle agglomeration.

In contrast, the TiO₂/BC composites demonstrate enhanced charge–discharge capacities, indicating improved electrical conductivity upon BC modification. This agrees with the study by Zhang et al., who reported that incorporating BC into TiO₂ improved its electrochemical performance [30]. Furthermore, the charge–discharge profiles of TiO₂/BC composites exhibit a more rectangular shape compared to pure TiO₂, indicating improved cycling stability and rate capability upon BC modification. Compared to the study by Zhang et al., who reported a capacity of around 200 mA h g⁻¹ for their TiO₂/BC composite, our results show a higher capacity of around 250 mA h g⁻¹ for the 15 wt% BC sample [31]. This could be attributed to the differences in the preparation methods and conditions used in the two studies.

The magnitude of the current in cyclic voltammetry (CV) measurements, often referred to as the peak current, can be calculated using the Randles-Sevcik equation. This equation acts as a mathematical model in electrochemistry for determining the peak current (i_p) as shown in the Randles–Sevcik equation, as Equation (5) [32]:

$$i_p=2.69\times {10}^5{n}^{3/2}A{D}^{1/2}{v}^{1/2}c$$

(5)

In this equation, i_p represents the peak current, n denotes the number of electrons involved in the redox reaction, A is the area of the working electrode (in cm²), and D signifies the diffusion coefficient of the analyte (in cm²/s). The peak current is an essential parameter in electrochemical measurements, as it is directly related to the electrochemical reaction rate and surface area of the active material. The peak current of pure TiO₂ and TiO₂/biochar composites (5 wt%, 10 wt%, and 15 wt%) is shown in Figure 10. As shown in the figure, the peak current of pure TiO₂ is relatively low, indicating a limited electrochemical reaction rate and surface area. In contrast, the peak current of TiO2/BC composites is significantly higher than that of pure TiO₂, indicating an enhanced electrochemical reaction rate and surface area. Adding BC to TiO₂ increases its electrical conductivity, leading to a higher peak current. As the BC concentration increases up to 15 wt%, the peak current of the TiO₂/BC composites decreases. This could be due to agglomeration of BC particles at higher concentrations, leading to reduced electrical conductivity and increased resistance [23]. Compared to the study by Dong et al., who reported a peak current for their TiO₂/BC composite, our results show a higher peak current of around 3.2 mA for the 5 wt% BC sample [33].

The Nyquist plot is a visual representation utilized in electrochemical impedance spectroscopy (EIS) to examine and interpret the electrical characteristics of a system. The impedance spectra of pure TiO and TiO₂/biochar composites (5 wt%, 10 wt%, and 15 wt%) are shown in Figure 11. As shown in the figure, the impedance spectra of pure TiO₂ exhibit a large semicircle in the high-frequency region, indicating a high charge transfer resistance (R_ct) and a limited electrochemical reaction rate. As the BC concentration increases up to 15 wt%, the impedance spectra of the TiO₂/BC composites exhibit a larger semicircle in the high-frequency region, indicating a higher Rct and a slower electrochemical reaction rate. This could be due to the agglomeration of BC particles at higher concentrations, leading to reduced electrical conductivity and increased resistance.

2.4. Photocatalytic Activity

The photocatalytic performance of pristine TiO₂ and biochar-modified TiO₂ nanostructures was evaluated by monitoring the degradation of methylene blue (MB) under UV–visible irradiation (Figure 12a). The absorption peak of MB at ~656 nm gradually decreased with increasing irradiation time, confirming the progressive photodegradation of the dye. Among the studied samples, bare TiO₂ exhibited the lowest degradation efficiency, reflecting its limited light absorption and high recombination of photogenerated charge carriers. In contrast, the incorporation of biochar significantly enhanced photocatalytic activity. The TiO₂/5 wt% BC sample displayed the most pronounced reduction in MB intensity, indicating its superior ability to generate and sustain active species for dye degradation.

The enhanced visible-light photocatalytic performance of the TiO₂/BC composites can be attributed to the interfacial charge-transfer interactions between the biochar matrix and the TiO₂ nanoparticles rather than to intrinsic narrowing of the TiO₂ bandgap. The extended π-conjugated structure of biochar absorbs visible light and promotes electron transfer to the TiO₂ conduction band. At the same time, the porous carbon framework facilitates charge separation and limits electron–hole recombination. Consequently, photogenerated electrons reduce adsorbed O₂ to superoxide radicals (O₂•⁻), and holes oxidize surface hydroxyl groups or water to produce hydroxyl radicals (•OH), both of which contribute to dye degradation. Although no direct radical-trapping or EPR analyses were conducted in this work, the proposed mechanism is consistent with previous findings for TiO₂/carbon composites, where the carbon phase acts as an electron mediator to enhance ROS generation and photocatalytic efficiency [34,35].

The photocatalytic degradation efficiencies of all samples as a function of irradiation time are presented in Figure 12b. After 150 min of irradiation, the degradation efficiency reached ~53% for pristine TiO₂, whereas TiO₂/5 wt% BC, TiO₂/10 wt% BC, and TiO₂/15 wt% BC achieved efficiencies of approximately 84%, 69%, and 63%, respectively. The remarkable enhancement observed for the TiO₂/5 wt% BC nanostructure can be attributed to the synergistic role of biochar, which provides a high surface area, facilitates dye adsorption, and enhances charge separation by acting as an electron mediator. However, at a higher biochar loading (15 wt%), a slight reduction in activity was observed, likely due to excessive coverage of TiO₂ active sites and a light-shielding effect that reduced the availability of photons for photocatalysis. Overall, these results highlight the critical role of optimized biochar loading in modulating the photocatalytic efficiency of TiO₂ nanostructures. The superior activity of TiO₂/5 wt% BC underscores its potential for sustainable environmental remediation applications, particularly in wastewater treatment and dye degradation.

The photocatalytic degradation of Methylene Blue (MB) under visible-light irradiation by pristine TiO₂ and TiO₂/BC composites is shown in Figure 13a. Pure TiO₂ exhibited relatively low activity, with ~40% MB removal after 150 min. Upon introducing biochar, the degradation efficiency improved substantially, with the TiO₂/5 wt% BC composite achieving the highest performance, removing nearly 95% of MB within 150 min. Increasing the BC content beyond this optimum (10 and 15 wt%) led to a gradual decrease in photocatalytic efficiency, though both still outperformed pristine TiO₂. This suggests that 5 wt% BC represents the optimal loading for maximizing photocatalytic activity. The corresponding pseudo-first-order kinetics, plotted as ln(C_t/Co) versus irradiation time, are presented in Figure 13b. The apparent rate constants (k) were calculated as 0.0048, 0.0214, 0.0159, and 0.0127 min⁻¹ for TiO₂, TiO₂/5 wt% BC, TiO₂/10 wt% BC, and TiO₂/15 wt% BC, respectively. The 5 wt% BC composites thus demonstrated the fastest degradation kinetics, with a reaction rate about four times higher than that of pristine TiO₂.

The superior activity at 5 wt% BC can be attributed to several synergistic effects. Moderate biochar incorporation enhances visible-light absorption, provides abundant adsorption sites for MB molecules, and promotes efficient charge separation at the TiO₂–BC interface. At higher BC loadings (10–15 wt%), however, excess carbon may shield TiO₂ from light irradiation and cover active catalytic sites, reducing the overall efficiency. Therefore, controlling the biochar content is essential to achieving optimal photocatalytic activity. This optimization distinguishes our study from prior reports. For example, Rosa et al. [12] examined a TiO₂–Fe/biochar composite for MB degradation and achieved enhanced activity compared to bare TiO₂, but without investigating the effect of biochar content. Our systematic comparison across 5, 10, and 15 wt% biochar loadings revealed a clear optimum at 5 wt%, where excessive carbon (≥10 wt%) introduced light-shielding and active-site coverage effects that reduced efficiency. Such optimization studies are essential for guiding the rational design of TiO₂–biochar composites for real-world applications.

The long-term stability and reusability of photocatalysts are crucial for practical wastewater treatment applications. Figure 14 shows the recycling performance of pristine TiO₂ and TiO₂/BC composites during four successive cycles of MB degradation under visible-light irradiation. All samples retained photocatalytic activity after repeated use, although a gradual decline in efficiency was observed. Among the tested samples, TiO₂/5 wt% BC exhibited the highest stability, maintaining nearly 84% of its initial degradation efficiency after the fourth cycle. In contrast, pristine TiO₂ showed both the lowest overall activity and a more pronounced reduction upon reuse. The composites with higher BC contents (10 and 15 wt%) demonstrated good recyclability but experienced greater efficiency losses than the 5 wt% BC sample.

The superior reusability of TiO₂/5 wt% BC can be attributed to its optimal structure, in which biochar enhances charge transfer, prevents photocorrosion, and provides stable adsorption sites for MB molecules. The slight decrease in activity after multiple cycles is mainly due to partial catalyst loss during recovery and to possible surface fouling by dye intermediates. These results confirm that moderate biochar incorporation enhances photocatalytic activity and improves the durability of TiO₂ for sustainable environmental applications.

3. Materials and Methods

3.1. Fabrication of the Sample

The red sea algae were collected from the NEOM coast of Saudi Arabia. Chemicals such as titanium dioxide (TiO₂, anatase phase, nanopowder, ~25 nm average particle size, ≥99.7% purity, Sigma-Aldrich, St. Louis, MO, USA), ammonium hydroxide (NH₄OH, 40%), and potassium hydroxide (KOH, 85%) were purchased from Sigma-Aldrich and used as received without further purification. At room temperature, the algae were washed with distilled water and dried. The biochar was produced from dried algae powder through pyrolysis at 700 °C for 4 h under a nitrogen atmosphere. The resulting biochar was then chemically activated by mixing 10 g of the material with 100 mL of 5 M KOH and refluxing at 80 °C for 5 h with continuous stirring (150 rpm). After refluxing, the solid residue was thoroughly washed with deionized water to remove excess KOH, then dried at 60 °C. The activated biochar was then used for preparing TiO₂/BC composites. After pyrolyzing the algae powder at 700 °C, the resulting biochar was thoroughly washed with deionized water to remove residual chemicals. The biochar was then dried at 60 °C to ensure complete moisture removal before modification. Finally, TiO₂ was modified with biochar (BC) at varying concentrations of 5, 10, and 15 wt% to prepare TiO₂/biochar composites. To achieve uniform modification, TiO₂ and biochar were mixed using a ball-milling method for 2 h at 300 rpm, followed by calcination at 500 °C for 4 h under a nitrogen atmosphere. This step ensured optimal interaction between TiO₂ and the biochar matrix. The selection of 5%, 10%, and 15% biochar weight percentages was guided by previous studies [11] and our own optimization experiments. Lower percentages enhance charge transport, while higher percentages risk diminishing TiO₂ crystallinity due to excess carbon presence.

Thin films of TiO₂@BC were prepared by spin-coating (spin speed of 2000 rpm for 60 s) onto quartz substrates using colloidal TiO₂ nanoparticle solution, allowing for a 2 h adsorption period to ensure uniform adhesion of the nanoparticles onto the substrate surface. The process was repeated to achieve a uniform film with an average thickness of approximately 100 ± 10 nm, as measured using a profilometer. The films were annealed at 200 °C for 2 h to improve adhesion and eliminate solvent residues.

3.2. Experimental Techniques

X-ray diffraction (XRD) was performed using a LANScientific diffractometer (LANScientific Co., Ltd., Suzhou, Jiangsu Province, China) during the process. CuKα radiation, characterized by a wavelength of 0.15418 nm, was employed. The measurements were performed using a 2θ/θ scan configuration with a step size of 0.05°, covering a 2θ range from 10° to 80°. Furthermore, images acquired using scanning electron microscopy (SEM) were captured utilizing a JEOL-SEM JFC-1100E (JEOL, Ltd., Akishima, Tokyo, Japan) running at 25 kV. A double-beam spectrophotometer (JASCO model V-570 UV-VIS-NIR) (Japan Spectroscopy Co., Ltd., Hachioji City, Tokyo)was used to measure the optical transmittance (T) and reflectance (R) spectra of the produced thin films. The measurement was performed within the wavelength range of 200 to 2500 nm. The spectrophotometer resolution is ±0.1 nm.

To investigate the electrochemical properties of the TiO₂@BC composite, the Glassy Carbon Electrode (GCE, 3 mm diameter) was cleaned by polishing with alumina powder (0.05 µm particle size), rinsing with deionized water, and drying with nitrogen gas. Cyclic voltammetry (CV) analysis of the GCE was performed in a 6 M potassium hydroxide (KOH) solution as the electrolyte, within a potential range of −0.4 to 1.4 V and a scan rate of 50 mV/s at room temperature. Compared to other organic electrolytes, KOH electrolytes offer higher concentration and lower resistance, facilitating faster electrode kinetics. The TiO₂@BC/GCE was left to dry at room temperature or placed in an oven for a specific duration to evaporate the solvent and promote adhesion of the nanocomposite to the electrode surface. For electrochemical impedance spectroscopy (EIS) testing, an amplitude of 5 mV and a frequency range of 0–2000 Hz were used.

The photocatalytic activity of pristine TiO₂ and TiO₂/BC composites was evaluated using the degradation of methylene blue (MB) as a model pollutant. When organic dyes are employed as model pollutants, a possible photosensitization effect can influence the apparent photocatalytic activity. To minimize this effect, all TiO₂/BC samples were first kept in the dark for 30 min to reach adsorption–desorption equilibrium before illumination. Photocatalytic tests were conducted under a visible-light source using a cutoff filter. Control experiments without catalysts showed negligible dye degradation, confirming that the observed reaction rates were not dominated by dye self-sensitization. Moreover, the reproducibility of the degradation rates across multiple cycles and the absence of new absorption bands during the reaction indicate that the process is primarily driven by semiconductor photocatalysis, with interfacial charge transfer between TiO₂ and the biochar phase, rather than dye-sensitized photobleaching.

For each test, 50 mg of photocatalyst was dispersed in 100 mL of 10 mg L⁻¹ MB aqueous solution and stirred in the dark for 30 min to establish adsorption–desorption equilibrium. The suspension was then irradiated with a 300 W Xe lamp equipped with a UV–visible cutoff filter (λ > 420 nm for visible-light experiments), with continuous magnetic stirring to ensure homogeneous exposure. At regular time intervals (every 15 min), 4 mL aliquots were withdrawn, centrifuged to remove catalyst particles, and analyzed by UV–Vis spectrophotometry at 656 nm, the characteristic absorption peak of MB. The degradation efficiency was calculated using the relation:

$$\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\%={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(6)

where C₀ and C_t are the initial and instantaneous MB concentrations, respectively. The reaction kinetics were analyzed using the pseudo-first-order model ln(C_t/C₀) = −kt, where k represents the apparent rate constant.

The recyclability of the TiO₂/BC photocatalyst was tested for three consecutive cycles under visible light. The degradation efficiency remained nearly unchanged after the third cycle, demonstrating good structural and photocatalytic stability of the composite. Although only three cycles were examined in this work, the consistent activity suggests promising reusability. Future studies will extend the cycling tests to 5–10 runs to evaluate long-term durability and potential for large-scale or continuous applications. Reusability and stability tests were performed by recovering the catalyst after each run, washing it thoroughly with deionized water, and drying it at 60 °C under the same experimental conditions.

4. Conclusions

This study systematically investigated the structural, optical, electrochemical, and photocatalytic properties of biochar-modified TiO₂ nanostructures. The results reveal that moderate biochar incorporation plays a crucial role in enhancing performance: the TiO₂/5 wt% BC composite exhibited the most favorable characteristics, including reduced charge-transfer resistance, higher electrochemical activity, and superior photocatalytic degradation efficiency (~84% methylene blue removal within 150 min) with good recyclability. In contrast, higher biochar loadings (10–15 wt%) led to reduced efficiency due to light-shielding effects and excessive coverage of TiO₂ active sites. Overall, biochar derived from red sea algae provides an eco-friendly and sustainable route for engineering TiO₂ nanostructures with improved functionality. The improved visible-light activity and charge-transfer capability of the TiO₂/BC composites suggest that these materials are not only effective for environmental photocatalysis but also promising for sustainable energy technologies, such as photoelectrochemical water splitting and solar energy conversion, where efficient charge separation and photon utilization are essential.