Synthesis and Optimization of TiO₂ Photocatalyst Using Biomass-Derived Activated Carbon for Photocatalytic Degradation of Methyl Orange

Abstract

TiO₂ is normally a preferred photocatalyst; however, its photocatalytic performance is constrained by its low surface area, wide band gap, and high electron–hole pair recombination rates. The objective of this study was to optimize the photocatalytic efficiency of TiO₂ by impregnating it onto activated carbon derived from Senegalia mellifera biomass. The quantitative study involved synthesizing TiO₂ using the precipitation technique and preparing AC through both chemical and physical activation methods. The prepared AC samples were impregnated with TiO₂ NPs using the wet impregnation method. The physicochemical properties of the samples were examined using several characterization techniques, namely, FTIR, EDS, Raman, UV reflectance, STA, SEM, and BET. The photocatalytic efficiency of AC/TiO₂ composites was evaluated through methyl orange degradation. The results showed significant improvement in photocatalytic performance when TiO₂ was supported on AC. The modified photocatalyst exhibited enhanced surface area, thus increased active sites for photocatalysis, improving electron–hole separation and reducing recombination. The 50%CO₂/AC-0.5TiO₂ composite demonstrated superior photocatalytic activity under both UV and visible light irradiation. It showed 52.1% MO removal under visible light and 76.1% MO removal under UV light. The study concludes that biomass-derived AC/TiO₂ composites present a promising, cost-effective and sustainable approach of enhancing photocatalytic activities.

1. Introduction

Photocatalytic degradation using titanium dioxide (TiO₂) nanoparticles has emerged as an effective method for removing organic pollutants from water, including dyes such as methyl orange. This process harnesses the photocatalytic properties of TiO₂ under UV light, which leads to the formation of highly reactive species capable of breaking down complex organic molecules [1,2]. TiO₂ was selected as the photocatalyst in this study due to its high chemical stability, non-toxicity, high refractive index, inexpensive cost and strong oxidative capability under irradiation [3,4,5]. Methyl orange (MO), an azo dye commonly used in textiles, is recalcitrant to conventional wastewater treatments, but TiO₂-based photocatalysis has proven effective in its degradation [6,7,8]. However, TiO₂ exhibits low photocatalytic activity, primarily due to its limited surface area, wide band gap, and low recovery efficiency.

Previous studies have indicated that impregnating TiO₂ nanoparticles onto the surfaces of porous materials can significantly enhance their photocatalytic performance [9]. Activated carbon has emerged as a promising support for TiO₂ owing to its high mechanical strength and extensive surface area [10]. Recent studies have demonstrated that coupling TiO₂ with carbon-based material can significantly enhance photocatalytic performance by improving charge separation, facilitating charge transfer at the carbon-TiO₂ interface, increasing adsorption capacity, and extending light utilization [9,10]. Moreover, biomass-derived activated carbons have attracted increasing attention as sustainable supports for photocatalysts due to their pore structures and surface chemistry [11]. Although AC may partially attenuate light penetration, its role as an adsorptive support concentrates pollutant near photocatalytic active sites, increases surface area, improves dispersion of TiO₂ NPs and promotes electron–hole separation, resulting in an overall enhancement of photocatalytic efficiency.

Approximately 45 million ha of Namibian land is encroached by bushes including Senegalia mellifera, presenting numerous issues of reduced biodiversity, decreased agricultural productivity, altered water cycles, and increased soil erosion [11,12]. To address these obstacles related to bush encroachment, invasive bush biomass such as Senegalia mellifera can be transformed into activated carbon. This study incorporated titanium dioxide nanoparticles (TiO₂ NPs) into various activated carbon samples from Senegalia mellifera to potentially address the aforementioned challenges. When TiO₂ is exposed to UV light with energy greater than its band gap, electron–hole pairs are generated as electrons in the valence band are excited to the conduction band, leaving holes behind in the valence band. The holes (h⁺) can react with adsorbed water molecules to form hydroxyl radicals (•OH), while the electrons (e⁻) can reduce oxygen molecules to form superoxide anion radicals (O₂^•−). Both of these reactive oxygen species are instrumental in breaking the azo bond (-N=N-) in methyl orange, leading to its degradation into simpler molecules like CO₂, H₂O, and other less toxic substances [13,14].

Unlike previous studies that primarily focus on chemical activation or fixed activation conditions, this work systematically investigates the influence of CO₂-controlled physical activation of Senegalia mellifera biomass on the physicochemical properties and photocatalytic performance of AC/TiO₂ composites. The optimization of CO₂ concentration during activation provides new insight into tailoring pore structure and surface chemistry to enhance photocatalytic efficiency.

2. Materials and Methods

2.1. Reagents and Glassware

Analytical-grade or chemically pure reagents were employed in the study to ensure the highest standards of quality and precision. 98% sulfuric acid (H₂SO₄) was supplied by Promark Chemicals, Gauteng, South Africa, while 65% nitric acid (HNO₃), acetylacetone (C₅H₈O₂), and 2-propanol (C₂H₈O) were supplied by Carl Roth, Karlsruhe, Germany. Commercial titanium dioxide p25 (surface area of 50 m²/g and a mean particle size of 25 nm) was purchased from Evonik, Hanau, Germany. Distilled water was utilized in the preparation of all photocatalysts as well as for the photocatalytic efficiency tests.

2.2. Synthesis of Activated Carbon

2.2.1. Synthesis of Activated Carbon Using Sulfuric Acid

Sulphuric acid 98% was used as the activating agent for chemical activation of bush biomass. A 1:2 ratio, i.e., biomass mass to acid volume ratio, was used. Approximately 100 g of Senegalia mellifera biomass was mixed with 200 mL of 98% H₂SO₄. The mixture was left in the fume hood for 24 h at room temperature. The mixture was then washed using a copious amount of distilled water and filtered using vacuum filtration. The washed mixture was dried in an SNOL E5CCT Muffle Furnace, Snoltherm UAB, at 105 °C for 24 h. After drying, the activated carbon was ground using a mortar and a pestle. This procedure was adapted from Kgabi et al. [15].

2.2.2. Synthesis of Activated Carbon Using CO₂ Pyrolysis

Pyrolysis and activation of the bush biomass wood was carried out using procedure adapted from the method by Gradel, University of Bayreuth, Germany [16]. Briefly the pyrolysis and activation was conducted in a, Burny wood gasifier (manufactured by Gradel, University of Bayreuth, Germany [16]) coupled to a Nabertherm RS 80/750/11 Electric Furnace, Nabertherm, Germany and Eheim VISIT 03H Gas Analyzer, Deizisau, Germany. The gasifier was filled with approximately 100 g of biomass wood. Propane gas was used to ignite the flame inside the plant. The plant was fed with 80% nitrogen at a flow rate of 0.5 Nm³/h. After the O₂ content inside the gasifier reduced to 0%, the plant was supplied with 20% CO₂ at a flow rate of 0.125 Nm³/h, until the gas analyzer read the CO₂ content between 18% and 22%. The temperature program of the Nabertherm RS 80/750/11 Electric Furnace was set as follows: (a) heat up from 20 °C to 500 °C in 30 min, (b) un at 500 °C for 1h until no more gases are produced (CH₄, H₂), (c) Heat up from 500 °C to 800 °C, (d) Run at 800 °C for 70 min (for 30% CO₂ conversion) or 140 min (for 50% CO₂ conversion). Following this, 30 min were allocated for combustion and ventilation. The resulting activated carbon samples were then stored in a fume hood while awaiting further experimentation.

2.3. Impregnation of Activated Carbon with TiO₂ NPs

To impregnate activated carbon with TiO₂ NPs, the wet impregnation process was employed according to the method of Ruliza et al. [17] with minor changes. A certain amount TiO₂ was dissolved in 100 mL distilled water. 5 g AC was added to the TiO₂ suspension. The mixture was stirred using a magnetic stirrer for 6 h at 70 °C. The solvent was removed by rotary evaporation and the residual solids were oven dried for 4 h at 120 °C. The dry sample was calcined at 400 °C for 3 h with an SNOL E5CC-T Muffle Furnace, Utena, Lithuania.

2.4. Physicochemical Characterization of Samples Using Various Instruments

The surface functional groups of the samples were investigated using a Perkin Elmer Spectrum Two FTIR spectrometer (Waltham, MA, USA) in the range of 400 to 4000 cm⁻¹. The surface morphology of the samples was visualized using a Jeol JSM-6360 Scanning Electron Microscope, Tokyo, Japan. Prior to morphology analysis, the samples were coated with a thin layer of gold to enable their electrical conductivity using a Jeol JFC-1200 Fine Coater, Tokyo, Japan. The SEM images were attained by analyzing the samples at a voltage of 10 kV, a working distance of 18 mm and different magnifications: 50×, 200×, 500×, and 1000×. The crystallinity of TiO₂ samples was studied using a Raman Spectrometer, Ulm, Germany. This experiment was done at an exposure time of 5000 s, with an average of three readings and maximum power supply of 0.830 kW.

The UV reflectance experiments were carried out using an Ocean Insight USB650UV Miniature Spectrometer, Shenzhen, China, at 50 ms integration time and an average of five measurements per sample. The moisture content of the bush biomass was determined using a Kern DAB 100-3 Moisture Analyzer, Balingen, Germany. This test underwent two trials. The kinetics of the carbon activation by CO₂ pyrolysis were predicted using the STA 449 F5 Jupiter Analyzer, NETZSCH, Selb, Germany. The kinetics were studied at three various temperatures: 800, 850 and 900 °C. The porosity of the carbon samples was investigated using a NOVA Touch LX² Surface Area and Pore Size Analyzer, Graz, Austria. The surface area was obtained based on the Brunauer–Emmett–Teller (BET) theory, while the pore volume was obtained based on the Density Function Theory model. Prior to BET analysis, the samples were degasified at 300 °C for 12 h.

2.5. Photodegradation of Methyl Orange

Photocatalytic degradation experiments of methyl orange were conducted to evaluate the photocatalytic activity of the as-prepared photocatalysts. These tests were performed under both visible light and UV light illumination. LED lamps of 10.314 W (visible light) and 11.988 W (UV light) were employed. The distance between the light source and reactor was kept constant at 7.5 cm for all experiments to ensure consistent photon flux. The studies utilized a 10 mg/L methyl orange stock solution. For each experiment, 8 mg photocatalyst and 8 mL methyl orange solution were combined in the reactor, resulting in a photocatalyst concentration of 1 g/L. Prior to light irradiation, a dark adsorption phase was conducted for 30 min to ensure the adsorption–desorption equilibrium. The reactor was then irradiated for 4 h. Following the degradation test, the resulting solution was filtered using Injekt syringes fitted with Rotilabo Mini-Tip syringe filters, Carl Roth, Karlsruhe, Germany, (0.15 mm, membrane CA, 0.45 µm). The absorbance of the filtered solutions was subsequently analyzed using a Mettler Toledo UV5Bio UV/Vis spectrophotometer, Giessen, Germany. All photocatalytic experiments were performed in triplicate, and the average values were used to obtain the reported spectrums. Six MO standards of 2.5, 5, 10, 15, 20, and 25 mg/L concentrations were prepared and analyzed to obtain a calibration curve. This experimental procedure was adapted from various past studies with minor changes [18,19,20,21,22,23].

3. Results

3.1. Characterization of Commercial TiO₂ p25

3.1.1. Surface Functional Groups of Commercial TiO₂ p25

Figure 1 shows the FTIR spectrum of commercial TiO₂ p25, which discloses the surface functional groups of the material. The peak observed at 1630 cm⁻¹ corresponds to the bending vibrations of adsorbed water on the TiO₂ particles—thus, Ti–OH groups. The broad unlabeled peak observed at around 500–700 cm⁻¹ is typically due to the Ti-O stretching vibrations. No peaks are observed in the higher wavenumber region (above 2000 cm⁻¹), indicating the absence of organic impurities in the sample. This FTIR spectrum matches the expected profile of commercial TiO₂ p25 [9,24].

3.1.2. Crystallinity of Commercial TiO₂ p25

The Raman spectrum of commercial TiO₂ p25 (Figure 2) reveals two medium peaks, whereby the peak at 392.127 cm⁻¹ can be attributed to the Eg mode, while the peak at 508.37 cm⁻¹ corresponds to the B1g mode. The strong intense peak detected at 630.241 cm⁻¹ aligns to the A1g mode. In addition, all the observed peaks are prominent characteristic of the anatase phase of TiO₂ [25,26]. These specific peaks suggest that commercial TiO₂ p25 has a crystalline structure, typical of anatase TiO₂. This crystalline phase is considered the best TiO₂ phase for optimal photocatalysis [27].

3.1.3. Surface Morphology of Commercial TiO₂ p25

SEM images of commercial TiO₂ p25 (Figure 3) show that the particles are fine, agglomerated, and homogeneously distributed throughout the sample. The TiO₂ morphology is consistent with findings reported in the literature [28]. According to the manufacturer of the commercial TiO₂ used in this study [29], TiO₂ p25 has a specific surface area in the range 35–65 m²/g and an average particle size of 25 nm. The average particle size of commercial TiO₂ p25 falls within the range of 1–100 nm, classifying them as nanoparticles [30].

3.1.4. UV Reflectance—Band Gap Energy Determination of Commercial TiO₂ p25

The UV reflectance spectrum of commercial TiO₂ (Figure 4a) shows a sharp increase in reflectance around 400 nm, transitioning from the UV to the visible region. The reflectance then stabilizes at a high value above 100% in the visible range, signaling a strong reflectance in this region. This is an indication that commercial TiO₂ efficiently absorbs UV light and reflects most of the visible light [31].

The 3.25 eV band gap energy, as estimated by Tauc plot (Figure 4b), corresponds to the absorption of photons within the UV region. This value is consistent with the anatase phase of TiO₂ [32]. In addition, the UV reflectance data affirms that commercial TiO₂ p25 is well-suited for photocatalytic degradation of dyes using UV light. However, its wide band gap renders it unsuitable for photocatalysis under solar irradiation as it is unable to effectively harness visible light [9].

3.2. Senegalia mellifera Bush Biomass Composition

The gas flow rate profile of Senegalia mellifera bush biomass, as indicated (Figure 5), provides insights into the evolution of various gases—CO, CO₂, CH₄ and H₂—each of which plays a distinct role in carbon activation. The sharp peaks of CO and CO₂ (around 50–60 min) indicate the rapid decomposition of biomass, which contributes to the development of porosity in carbon through gasification reactions. CH₄ appears in smaller quantities, while H₂ evolves later (100–120 min), possibly aiding in secondary reactions such as cracking of hydrocarbons and in refining the structure [33].

The bush biomass pyrolysis results in

Table 1. Composition of S. mellifera biomass.

	Sum (L)	Weight (g)
CO	4.63	6.39
CH4	0.86	1.18
H2	2.54	3.51
CO2	4.90	6.75
Sum	12.93	17.83
	Mass (g)	Mass (%)
Sample Mass	96.79	100
Char Coal	24.42	25.23
Sum Gas	17.83	18.42
Residue (Oils and Ash)	54.55	56.35

show that dried S. mellifera biomass is composed of 25.23% char, 18.42% gases and 56.35% residual oils and ash. The gas yields, particularly CO (6.39 g) and CO₂ (6.75 g), highlight significantly gases that can react with the char during activation to enhance its porosity.

3.3. Carbon Activation by CO₂ Pyrolysis

The carbon-activation results in

Table 2. S. mellifera carbon activation results by CO₂ pyrolysis.

	Pyrolysis	Activation
		30% CO2 Conversion	50% CO2 Conversion
	Mass (g)	Mass (g)	Mass (g)
Biomass sample	96.79	90.25	95.39
Carbon	24.42	22.76	24.07
Activated carbon	-	16.50	12.20
Consumed C	-	6.26	11.87
Actual conversion (%)	-	27.5	49.3

reveal that 16.5 g activated carbon was produced from 96.8 g biomass via 30% CO₂ conversion, while 12.2 g activated carbon was produced from 95.4 g biomass via 50% CO₂ conversion. More accurately, carbon was activated through 27.5% and 49.3% CO₂ conversion, hence, 30% CO₂ conversion and 50% CO₂ conversion, respectively. To emphasize this, 27.5% of 22.76 g char reacted with CO₂ for 30% CO₂ conversion removing carbon atoms from the solid structure of char, creating a network of pores, thereby producing activated carbon. Similarly, for 50% CO₂ conversion, 49.3% of 24.07 g char reacted with carbon dioxide to form activated carbon.

Although higher CO₂ concentrations (>50%) were not investigated in this study, preliminary TGA observations indicated structural degradation of the carbon matrix at temperatures higher than 800 °C, which would be needed to obtain elevated CO₂ levels. Future work will explore higher activation concentrations to further elucidate the relationship between CO₂ activation intensity and photocatalytic performance.

3.4. Characterization of Samples

3.4.1. Surface Functional Groups of Samples

Figure 6 presents the FTIR spectra of various carbon samples as labeled. The raw sample spectrum shows a gradual decline in transmittance, implying that the sample has not undergone chemical activation. It also shows no significant peaks, an indication of the absence of functional groups and a less reactive surface compared to activated carbon. The FTIR spectrum of SA/AC exhibits peaks between 1000 and 1200 cm⁻¹, which could be attributed to S=O stretching vibrations, denoting the presence of sulfonic acid groups (-SO₃H) introduced by the activation with sulfuric acid [34].

The FTIR spectra of 30%CO₂/AC and 50%CO₂/AC appear to be similar with minor differences in transmittance. This suggests that the main functional groups do not change drastically with variation in CO₂ conversion. The most prominent features include the broad peaks in the range of 3000–3600 cm⁻¹, which are commonly associated with O-H stretching vibrations. The weak peaks detected near 1600 cm⁻¹ might correspond to C=O stretching vibrations from carbonyl or carboxyl groups formed on the surface of AC [35]. An increase in the oxygen-containing functional groups due to the activation process with CO₂ may be the cause of the transmittance decrease observed with conversion levels (30% to 50%).

3.4.2. Surface Morphology of Carbon Samples

Meanwhile, Figure 7 illustrates the SEM images of the various carbon samples. Raw carbon (Figure 7A) appears to have a coarse and irregular surface structure with several cracks and cervices. There is a noticeable aggregation of the raw carbon particles, contributing to the rough texture of the material. The large, uneven pores observed in the raw carbon result from the pyrolysis process and gas interactions within the S. mellifera biomass, which react with carbon atoms to create the pores.

The surface of SA/AC (Figure 7B) appears rough, with irregularly shaped particles and visible pores and cavities. The SA/AC particles seem to have a fractured and jagged appearance. The surface of 30%CO₂/AC (Figure 7C) looks rough, with clusters of small particles attached. There are visible channels and well-defined interconnected pores on the 30%CO₂/AC structure. Similarly, 50%CO₂/AC (Figure 7D) has a rough, porous surface with channels and small fragments attached. The rough surface and extensive porous network observed in the activated carbon samples imply that these materials are well-suited for adsorption of TiO₂ nanoparticles.

3.4.3. Porosity of Carbon Samples

The N₂ adsorption–desorption isotherm of raw carbon in Figure 8a exhibits characteristics of a Type IV isotherm, typical for mesoporous materials [36]. This isotherm suggests that raw carbon (Figure 8a) has a porous structure with a significant mesoporous network [37]. This isotherm further shows limited adsorption at low relative pressure, indicating poorly developed porosity and restricted micropore volume.

The N₂ adsorption–desorption isotherm of SA/AC (Figure 8b) is a Type III isotherm, characterized by a convex shape towards the P/P_o axis. Type III isotherms suggest weak adsorbate–adsorbent interactions [37]. The lack of a significant plateau implies the absence of micropores and minimal mesopore filling. There is no clear hysteresis loop, which indicates the absence of significant porosity. These features arise from acid-induced surface oxidation, pore widening, and structural rearrangement, which enhance surface area but may also lead to non-ideal isotherm behavior. The weak interactions and limited pore structure imply that SA/AC might not be highly efficient in applications requiring strong adsorbent properties.

The isotherm of 30%CO₂/AC (Figure 8c) resembles a Type II isotherm, indicative of multilayer adsorption on a non-porous or macroporous material [37,38]. There is a small hysteresis loop visible at mid to high P/P_o values, suggesting the presence of a few mesopores. The larger adsorption volume compared to SA/AC suggests that CO₂ activation at 30% has created more surface area, making this material more suitable for adsorption applications.

Similar to the 30%CO₂/AC, the gas isotherm of 50%CO₂/AC (Figure 8d) also appears to follow the Type II pattern but with a higher total adsorption volume. Like the previous graph, there is a slight hysteresis loop at higher pressures, but the overall increase in adsorption capacity is greater, indicating more developed porosity [38]. The 50% CO₂ conversion has significantly enhanced the pore structure and surface area compared to the 30%CO₂/AC. 50%CO₂/AC would perform better in adsorption applications due to higher surface area and the presence of some mesopores.

Table 3. Surface area and pore volume characteristics of different carbon samples.

Sample ID	Surface Area (m2/g)	Pore Volume (cm3/g)
Raw	93.6110	0.0608
SA/AC	36.6290	0.0029
30%CO2/AC	556.281	0.2193
50%CO2/AC	756.509	0.3175

shows the surface area and pore volume of the carbon samples. 50%CO₂/AC has the highest surface area and pore volume of 756.509 m²/g and 0.3175 cm³/g respectively, which correlates well with its N₂ isotherm showing extensive porosity. On the contrary, SA/AC exhibited the lowest surface area (36.6290 m²/g) and pore volume. This can be attributed to blockage of pores with sulfate groups or oxygen-containing functional groups during acid activation. Furthermore, according to the literature, low-temperature conditions are speculated to lead to a denser structure with fewer pores [34,39,40].

3.5. Characterization of Impregnated Activated Carbon Samples

3.5.1. Surface Functional Groups of Impregnated Activated Carbon Samples

FTIR spectra of various impregnated activated carbon samples are shown in Figure 9. All spectra exhibit broad peaks in the 500–700 cm⁻¹ regions, signaling the Ti-O-Ti stretching vibrations. The variation in intensity across the spectra reflects the difference in TiO₂ loading. The broad peaks in all spectra correspond to O-H stretching vibrations, common in both AC and TiO₂. The weak peaks near 1600 cm⁻¹ likely correspond to C=O stretching vibrations. Weak broad peaks seen between 1000 and 1200 cm⁻¹ may be attributed to Ti-O-C bonds [35]. These spectra clearly demonstrate the successful impregnation of TiO₂ NPs onto the surface of AC.

3.5.2. Surface Morphology of Impregnated Activated-Carbon Samples

The SEM images of the impregnated composites are shown in Figure 10, whereby SA/AC-0.3TiO₂ (Figure 10A) appears to have a rough surface. The TiO₂ NPs seem to be well-dispersed across the surface, with visible agglomeration. SA/AC-0.3TiO₂’s porous nature remains evident, suggesting that the pores are not entirely filled with TiO₂. In contrast, the structure of SA/AC-0.5TiO₂ (Figure 10B), appears more densely packed with TiO₂ NPs. The agglomeration of the nanoparticles is more pronounced, implying that the higher the loading, the more particle clustering. The surface appears rougher, less porous, and more irregular compared to SA/AC-0.3TiO₂, likely due to the higher TiO₂ loading. This can affect the overall surface area of SA/AC-0.5TiO₂.

The SEM monogram of 30%CO₂/AC-0.3TiO₂ (Figure 10C) portrays a highly porous surface with interconnected channels and well-defined pores. The distribution of TiO₂ NPs within the 30%CO₂/AC matrix appears to be uniform. The presence of unfilled pores makes this composite suitable for photocatalysis. The structure of 30%CO₂/AC-0.5TiO₂ (Figure 10D) looks more compact and layered, with visible ridges and a less porous surface compared to 30%CO₂/AC-0.3TiO₂. The TiO₂ NPs seem to form larger clusters on the surface of 30%CO₂/AC-0.5TiO₂ (Figure 10E) reducing porosity. Owing to an increased presence of TiO₂ NPs, photocatalysis might favor 30%CO₂/AC-0.5TiO₂ (Figure 10F).

The porous structure of 50%CO₂/AC-0.3TiO₂ seems somewhat less pronounced compared to the 30%CO₂/AC samples, possibly due to the increased CO₂ concentration during preparation. In comparison to the previous monograms, the TiO₂ distribution is more uniform, but the surface appears less rough, suggesting that the TiO₂ NPs are well spread but might not be as deeply embedded in the carbon structure. The surface of 50%CO₂/AC-0.5TiO₂ is more compact, with fewer pores and a more uniform appearance. The TiO₂ NPs seem to have completely filled the carbon’s pores, creating a dense and smooth surface. This may mean that the impregnation results in a denser, less porous structure at higher concentrations of CO₂ and TiO₂, which could affect the surface area and the material’s suitability for photocatalytic applications.

Surface oxygen-containing functional groups (-OH, -COOH, -C=O) introduced by CO₂ activation enhance photocatalytic performance by improving hydrophilicity, dye adsorption, and interfacial charge transfer. These groups act as electron traps prolonging charge carrier lifetimes and facilitating electron migration from TiO₂ to the carbon support, thereby reducing recombination rates.

3.5.3. Porosity of Impregnated Carbon Samples

Figure 11 shows the N₂ adsorption–desorption isotherms of the impregnated carbon samples. The isotherms of SA/AC-0.3TiO₂ (Figure 11A) and SA/AC-0.5TiO₂ (Figure 11B) display Type IV behavior, with visible hysteresis loops, suggesting the presence of mesopores. The adsorbed volume increases gradually at higher relative pressures, indicating capillary condensation in mesopores. However, the overall adsorbed volume in SA/AC-0.5TiO₂ is slightly lower compared to SA/AC-0.3TiO₂, indicating less porosity in SA/AC-0.5TiO₂. The SA/AC-0.3TiO₂ composite, with its high surface area and better mesoporous structure, might be more effective for photocatalysis compared to the SA/AC-0.5TiO₂ composite. This is because excessive TiO₂ loading might lead to agglomeration, reducing the number of active sites available for photocatalysis [32].

The isotherms of 30%CO₂/AC-0.3TiO₂ (Figure 11C) and 30%CO₂/AC-0.5TiO₂ (Figure 11D) are Type IV isotherms with hysteresis loops, again indicating the presence of mesoporosity. The volume of N₂ increases with pressure, followed by a steeper increase at higher relative pressures, which suggests capillary condensation in the mesopores. The higher TiO₂ content of the 30%CO₂/AC-0.5TiO₂ composite filled most of the pores, reducing its surface area. While both composites are suitable for photocatalysis, the 30%CO₂/AC-0.3TiO₂ may offer better balance of surface area, porosity and TiO₂ dispersion, leading to potentially higher photocatalytic efficiency.

Type IV isotherm characteristics are exhibited by the gas isotherms of both 50%CO₂/AC-0.3TiO₂ (Figure 11E) and 50%CO₂/AC-0.5TiO₂ (Figure 11F) composites. There are clear hysteresis loops present in both composites, indicative of mesoporous structures. However, the hysteresis loop in 50%CO₂/AC-0.5TiO₂ is broader, this may suggest potential agglomeration of TiO₂ NPs, which can lead to pore blocking and a reduction in surface area. This could negatively impact the photocatalytic efficiency as the active sites might become less accessible.

Table 4. Surface area and pore volume characteristics of different impregnated activated carbon samples.

Sample ID	Surface Area (m2/g)	Pore Volume (cm3/g)
SA/AC-0.3TiO2	217.998	0.1553
SA/AC-0.5TiO2	66.0470	0.1137
30%CO2/AC-0.3TiO2	406.117	0.2119
30%CO2/AC-0.5TiO2	238.851	0.1612
50%CO2/AC-0.3TiO2	338.905	0.1871
50%CO2/AC-0.5TiO2	236.361	0.1575

gives the surface area and pore volume parameters of the various impregnated activated carbon samples. These results suggest that 30%CO₂/AC-0.3TiO₂, which has the highest surface area (406.117 m²/g) and the highest pore volume (0.2119 cm³/g), might offer the best performance in photocatalytic degradation of dyes owing to its high surface area, suitable pore volume and well-dispersed TiO₂. Conversely, samples with higher TiO₂ loadings show reduced surface areas and pore volume, which could potentially limit their photocatalytic effectiveness.

It is observed that the surface area of SA/AC increased from 36.629 m²/g to 217.998 and 66.047 m²/g after impregnation with 30% TiO₂ and 50% TiO₂, respectively. The elevation in surface area is likely a result of the creation of new pores, improved dispersion of TiO₂ NPs, thermal effects and structural rearrangement within the carbon matrix. The sulfuric acid treatment might have initially restricted the surface area by blocking pores, but the subsequent TiO₂ impregnation and calcination process reopened these pores and induced structural changes, expanding the accessible surface area [41,42].

The higher surface area of 30%CO₂/AC-TiO₂ composites compared to 50%CO₂/AC-TiO₂ composites, despite the initial lower surface area, is probably due to a combination of factors such as dispersion of TiO₂, different surface chemistry interactions, and possibly structural stability during the impregnation process. It is evident that the TiO₂ NPs are uniformly dispersed in the composites with lower TiO₂ content, while they seem agglomerated in composites with higher TiO₂ concentration. The functional groups present on the surface of 30% CO₂/AC might favor stronger interactions with TiO₂ leading to a better dispersed composite structure. This could prevent the agglomeration of TiO₂ thereby maintaining a higher surface area compared to the 50% CO₂/AC composite.

3.5.4. UV Reflectance Data of Impregnated Carbon Samples

The UV reflectance spectra and band gap energy results for various impregnated activated carbon samples represented (Figure 12 and Figure 13), display their optical properties. The UV reflectance spectra across all samples generally show high reflectance near 400 nm, followed by a decline as the wavelength increases, indicating strong absorption in the UV region, typical of TiO₂. There is additional visible light absorption observed in some samples at wavelengths higher than 500 nm, suggesting potentially improved utilization of the solar spectrum. Despite band gap energies remaining close to that of anatase TiO₂, the observed visible-light activity can be attributed to adsorption-assisted photocatalysis and carbon-mediated charge transfer. The AC support acts as an electron acceptor and transport pathway, promoting interfacial electron transfer from excited TiO₂ and delaying electron–hole recombination [7]. In addition, surface Ti-O-C bonds and oxygen-containing functional groups introduced during CO₂ activation enhance the formation of reactive oxygen species (OH⁻ and O₂·⁻), which contribute to visible light-driven degradation [11,12,32,33]. Therefore, the photocatalytic activity under visible light is mainly governed by synergistic adsorption and charge transfer mechanisms rather than band gap narrowing.

The band gap energies of the impregnated carbon samples range approximately from 3.16 eV to 3.28 eV, slightly comparable to pure anatase TiO₂ (3.2 eV). The observed band gap values confirm that these materials are well-suited for UV-driven photocatalytic applications. Increased band gaps in certain samples, such as 30%CO₂/AC-0.3TiO₂ show enhanced visible light interaction due to surface modifications. These findings imply that impregnation onto AC slightly alters TiO₂’s optical properties, improving its potential for dye degradation applications, especially under UV illumination, and possibly extending its effectiveness into the visible spectrum.

Although XRD analysis was not conducted in this study, the successful integration of TiO₂ onto activated carbon is supported by consistent evidence from SEM, FTIR, Raman spectroscopy, and UV/Vis reflectance analysis, which collectively confirm TiO₂ deposition, interaction with the carbon matrix, and preserved photocatalytic functionality.

3.6. Photodegradation of Methyl Orange (MO)

3.6.1. Calibration Curve of MO

The calibration curve (Figure 14) shows the relationship between the concentration of methyl orange in mg/L and the absorbance at 464 nm in arbitrary units (AU). The data points form a straight line, indicating a linear relationship between concentration and absorbance. This means that as the concentration of methyl orange increases, the absorbance at 464 nm also increases. The equation, y = 0.0785x − 0.022, represents the line of best fit, whereby, y is the absorbance, x is the concentration of MO, 0.0785 is the slope, indicating how much the absorbance increases for each mg/L increase in concentration, and −0.0223 is the y-intercept, representing the absorbance when the concentration is theoretically zero (though it is slightly negative, likely due to instrumental error or baseline correction).

In addition, the R² value of 0.9999 indicates an almost perfect linear correlation between concentration and absorbance, implying that the data fits the calibration model extremely well [43]. This curve can be used to determine the concentration of methyl orange in unknown samples by measuring their absorbance at 464 nm and using the straight-line equation to calculate the corresponding concentration. Overall, this calibration curve suggests high reliability for quantifying MO concentrations within the range of 0–25 mg/L based on absorbance measurements.

3.6.2. UV/Vis Spectra of Various Photocatalysts

The UV/Vis absorbance spectra (Figure 15) display the photodegradation of MO under visible light using different photocatalysts. The general trend shows that the absorbance of methyl orange decreases as different photocatalysts are applied, indicating its degradation. TiO₂ p25 (black line) shows the highest absorbance peak around 464 nm, indicating the least degradation MO. TiO₂ typically has limited activity under visible light, as it primarily absorbs UV light [44]. Therefore, it exhibits the lowest efficiency in degrading MO under visible light in this case.

The red line and blue line in the graph (Figure 15) represent SA/AC-0.3TiO₂ (Red line) and SA/AC-0.5TiO₂, respectively. The sulfonated activated (SA/AC) loaded with different amounts of TiO₂ (0.3 and 0.5) exhibit better photocatalytic performance compared to the commercial TiO₂ p25. The absorbance is reduced more than TiO₂ indicating enhanced photodegradation of methyl orange. This implies that SA/AC improves the visible-light-driven photocatalytic efficiency due to its enhanced surface area and possibly better electron–hole separation from TiO₂. About 30%CO₂/AC-0.3TiO₂ and 30%CO₂/AC-0.5TiO₂ are represented by the green line and purple line, respectively. The introduction of CO₂ by 30% conversion in the modification process results in even better photocatalytic activity. Both the 0.3 and 0.5 TiO₂-loaded composites degrade methyl orange more efficiently than the previous SA/AC complexes. This suggests that CO₂ treatment might improve the photocatalytic properties, potentially by increasing surface oxygen vacancies, leading to better electron capture and separation.

About 50%CO₂/AC-0.3TiO₂ and 50%CO₂/AC-0.5TiO₂ are represented by the yellow line and cyan line, respectively. The 50% CO₂-treated samples show the lowest absorbance values, indicating the most efficient degradation of methyl orange. The increased CO₂ treatment level further enhances the photocatalytic properties. The 0.5 TiO₂-loaded sample appears to perform marginally better than the 0.3 TiO₂-loaded sample. This can be attributed to the higher TiO₂ content in the 50%CO₂/AC-0.5TiO₂ composite. Among all the samples, the 50%CO₂/AC-0.5TiO₂ photocatalyst shows the best performance in degrading MO, followed closely by 50%CO₂/AC-0.3TiO₂. This suggests that increasing both the CO₂ treatment and TiO₂ loading enhances the photocatalytic degradation efficiency under visible light. 50% CO₂/AC-0.5TiO₂ exhibits the highest photocatalytic activity in degrading MO, which often correlates with better electron–hole pair generation, separation, and transfer properties [35].

The UV/Vis absorbance spectra (Figure 16) illustrate the photodegradation of MO using various photocatalysts under UV light. A lower absorbance peak indicates a more significant degradation of methyl orange. As in the previous graph, the TiO₂ p25 is the reference photocatalyst. It shows relatively high absorbance compared to most other materials, indicating moderate degradation of methyl orange. TiO₂ is well-known for its UV activity, so it performs better under UV light than under visible light, but it still does not exhibit the highest photocatalytic efficiency amongst the samples.

The SA/AC-supported photocatalysts exhibit enhanced photocatalytic efficiency, likely due to increased surface area. Moreover, both 30%CO₂/AC-treated samples degrade methyl orange more effectively than the SA/AC-modified samples. This could be due to enhanced charge separation and increased photocatalytic active sites from the surface modification. The 50%CO₂/AC-0.5TiO₂ sample demonstrates the best performance in degrading MO under UV light, followed by 50%CO₂/AC-0.3TiO₂. These AC-modified samples show higher photocatalytic activity, likely due to improved electron–hole pair separation, enhanced surface area, and greater availability of active sites [45].

To distinguish adsorption from true photocatalytic degradation, all experiments included a 30 min dark adsorption step prior to illumination. During this period, AC-containing composites showed significant adsorption of methyl orange (up to $~$18–25%), whereas pure TiO₂ exhibited a negligible adsorption (5%). Upon light irradiation, additional removal was observed, which is attributed to photocatalytic degradation. The net photocatalytic contribution can be calculated by subtracting the dark phase adsorption removal from the total removal under illumination, confirming that the enhanced performance of AC/TiO₂ composites arises from synergistic adsorption-photocatalysis rather than adsorption alone.

4. Conclusions

This study aimed to optimize the effectiveness of biomass-derived activated carbon supporting TiO₂ NPs in photocatalysis. The primary objective was to address the inherent limitations of TiO₂, particularly its low photocatalytic efficiency caused by a wide band gap, low surface area and high electron–hole recombination rates. By incorporating TiO₂ NPs onto activated carbon synthesized from the biomass of the invasive Senegalia mellifera bush, this research pursued a sustainable and efficient solution to enhance TiO₂’s photocatalytic activity.

The study synthesized TiO₂ particles using the precipitation method. Biomass-derived AC samples were prepared by employing both chemical activation and physical activation techniques. For chemical activation, 98% H₂SO₄ was utilized as the activating agent. On the other hand, CO₂ pyrolysis was utilized for physical activation, whereby 30% conversion and 50% conversion were exploited. Furthermore, the as-prepared AC samples were impregnated with TiO₂ NPs using the wet impregnation method. The physicochemical properties of the samples were examined using several characterization techniques, namely, FTIR, EDS, Raman spectroscopy, UV reflectance spectroscopy, SEM, and BET. The photocatalytic activity of the AC/TiO₂ composites was evaluated through methyl orange degradation using UV/Vis spectroscopy.

It was evident that the problem of low TiO₂ photocatalytic activity was partially addressed. The study demonstrated that biomass-derived AC can serve as an effective support material for TiO₂. By impregnating TiO₂ onto AC, the study observed an increase in the photocatalyst’s surface area and porosity, increasing the availability of active sites and promoting better charge separation. This modification resulted in enhanced photocatalytic performance for methyl orange degradation under both UV and visible light irradiation. The success of the AC/TiO₂ composites in degrading MO, which has similar photocatalytic mechanisms as in hydrogen production, suggests that the modified composites could similarly facilitate hydrogen evolution reactions. The performance of the 50%CO₂/AC-0.5TiO₂ composite compares favorably with previously reported AC/TiO₂ systems, which typically achieve 40–70% MO removal under UV irradiation and limited visible light activity [7,15,16,17,18,19,20,21].

The use of Senegalia mellifera biomass as a source of AC highlighted the dual benefits of addressing environmental issues while producing valuable materials for advanced photocatalytic applications. This innovative approach demonstrates how biomass from invasive species can be repurposed for high-value, environmentally friendly applications. In addition, the study employed holistic strategies for synthesizing and characterizing the AC/TiO₂ composites, utilizing a combination of chemical and physical activation methods and a variety of analytical techniques. This comprehensive approach ensured a robust understanding of the materials’ properties, setting a strong foundation for future studies.

Despite the successes achieved in this study, several areas require further examination to fully reveal the potential of biomass-derived AC-supported TiO₂ photocatalysts. This study did not extensively focus on strategies to reduce the band gap to efficiently enhance visible light absorption. While the study used methyl orange degradation as a proxy for photocatalytic activity, direct measurements of hydrogen evolution were limited. The actual hydrogen production capacity via water splitting under realistic conditions needed to be studied and quantified. Moreover, the long-term stability and reusability of the AC/TiO₂ composites were not examined.

While the present study focuses on photocatalytic degradation at a fixed initial methyl orange concentration to enable systematic material comparison, future investigations will examine the performance of the optimized AC/TiO₂ composite across a range of pollutant concentrations. Such studies will provide insight into adsorption saturation behavior, degradation capacity, and practical operational limits for wastewater treatment applications. Although discoloration of MO was observed, complete mineralization was not confirmed as TOC analysis and intermediate identification were beyond the scope of this study. Future investigations are proposed to focus on TOC removal and identification of degradation pathways to confirm mineralization.

The findings of this study have numerous potential applications. The practical approach of using invasive Senegalia mellifera biomass as a raw material for AC production could be applied in other regions facing similar environmental challenges, where biomass-conversion technologies could be employed to tackle ecological problems while contributing to material development. The optimized AC/TiO₂ composites may be deployed in water-splitting reactors to produce hydrogen, contributing to the development of sustainable energy systems. The study also suggests potential applications in wastewater treatment and environment remediation as the photocatalytic composites demonstrated effectiveness in degrading persistent organic pollutants like methyl orange.