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Article

Engineering Polyaniline Nanofibers/TiO2 for Enhanced Photocatalytic Degradation of Organic Contaminants: In-Depth Structural and Mechanistic Insights

by
Mohamed. A. Diab
1,*,
Heba A. El-Sabban
2,* and
Youngsoo Kim
1,*
1
Department of Chemistry, Yeungnam University, Gyeongsan 38541, Republic of Korea
2
School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea
*
Authors to whom correspondence should be addressed.
Catalysts 2026, 16(5), 464; https://doi.org/10.3390/catal16050464
Submission received: 2 April 2026 / Revised: 8 May 2026 / Accepted: 13 May 2026 / Published: 16 May 2026
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Abstract

This study presents the rational design of a visible-light-responsive TiO2/polyaniline (PANI) nanofiber heterostructure via in situ oxidative polymerization to overcome the limited visible-light absorption and rapid charge recombination of TiO2. Comprehensive characterization using XRD, FT-IR, XPS, SEM, UV–Vis DRS, and EIS confirmed the successful integration of TiO2 nanoparticles within a conductive polyaniline nanofiber network, enabling efficient interfacial charge transfer. The optimized TiO2/PANI-30 composite exhibited outstanding photocatalytic performance, achieving ~99% degradation of Basic Fuchsin dye within 40 min under visible light, significantly outperforming pristine TiO2. The enhanced activity is attributed to improved visible-light absorption, reduced bandgap energy, and suppressed electron–hole recombination, supported by optical and electrochemical analyses. Kinetic studies indicated pseudo-first-order behavior, with TiO2/PANI-30 showing the highest rate constant. Radical trapping experiments identified superoxide and hydroxyl radicals as the main active species, with •OH playing a dominant role. A direct Z-scheme charge transfer mechanism was suggested, preserving strong redox potentials and promoting reactive oxygen species generation. Additionally, the photocatalyst demonstrated excellent stability and reusability. These findings highlight the suggested potential of TiO2/PANI systems as efficient and sustainable photocatalysts for wastewater treatment.

1. Introduction

Hazardous pollutants are constantly being released into aquatic habitats as a result of the quick growth of industry. Among these pollutants, synthetic colours emitted by the paper, leather, textile, and pharmaceutical industries pose a serious threat to the environment due to their complex aromatic structures, high stability, and resistance to biodegradation [1,2,3]. It is estimated that large quantities of dye-containing wastewater are produced annually, posing severe ecological and health risks if discharged without adequate treatment. Conventional treatment techniques such as coagulation [4], adsorption [5], membrane filtration [6], and biological treatment [7] frequently have drawbacks such as high operating costs, secondary pollutants, and partial mineralization. Thus, advanced oxidation procedures (AOPs) [8], especially photocatalysis based on semiconductors [9], have shown promise as technology for the efficient degradation of persistent organic pollutants in water systems.
Among various photocatalysts, titanium dioxide has been widely considered due to its potent oxidative capacity, chemical stability, affordability, and low toxicity [10]. TiO2 exists mainly in anatase, rutile, and brookite phases and exhibits excellent photocatalytic activity under ultraviolet irradiation. Nevertheless, the application of TiO2 is insufficient due to several inherent drawbacks, including its wide bandgap (~3.0–3.2 eV). It primarily limits light absorption to the ultraviolet spectrum and the rapid recombination of photogenerated electron-hole pairs, which significantly reduces photocatalytic efficiency [11]. Therefore, extensive research efforts have focused on modifying TiO2 to enhance visible-light absorption and improve charge separation efficiency. To overcome these limitations, alternative approaches have been explored, including doping, defect engineering, surface modification, and heterojunction formation [12].
In recent years, constructing heterojunction photocatalysts has emerged as a highly effective approach to enhancing photocatalytic performance. In particular, Z-scheme heterojunction systems have attracted significant attention because they mimic the natural photosynthesis process, maintaining strong redox capacities [13]. In a typical Z-scheme system, the generated electrons in the conduction band (CB) of one semiconductor recombine with h+ in the valence band of another semiconductor, providing highly reductive electrons and strongly oxidative holes in the respective semiconductors. This unique charge transfer pathway enhances photocatalytic activity by suppressing charge recombination and preserving the high redox ability required for complex catalytic reactions [14]. Consequently, Z-scheme photocatalysts have demonstrated remarkable performance in pollutant degradation [15,16,17], CO2 reduction [18,19], and photocatalytic hydrogen evolution [20].
Conducting polymers have recently been introduced as efficient components in photocatalytic heterostructures due to their unique electronic structures and excellent visible-light absorption properties [21]. Among them, polyaniline (PANI) has attracted considerable interest because of its high electrical conductivity, environmental stability, tunable band structure, and facile synthesis [22]. PANI can act as an electron mediator and photosensitizer in semiconductor composites, effectively improving light harvesting and facilitating charge transfer [23]. Moreover, the formation of PANI nanofibers offers a large surface area. and interconnected conductive network. This enhances the quantity of the active catalytic sites and facilitates quick charge transmission. Recent studies have demonstrated that PANI-based nanocomposites exhibit significantly enhanced photocatalytic performance for the elimination of toxic dyes under visible light illumination [24].
Unlike conventional TiO2/PANI composites previously reported [25,26,27,28], this work specifically integrates TiO2 nanoparticles within a conductive polyaniline nanofiber network synthesized via in situ oxidative polymerisation. The nanofibrous morphology provides enhanced interfacial contact, directional charge transport pathways, and improved visible-light harvesting, which are not sufficiently explored in earlier TiO2/PANI systems.

2. Results and Discussion

2.1. The Photocatalyst’s Morphological and Optical Properties

The phase structure of the prepared TiO2 and TiO2/PANI was investigated using XRD analysis as presented in Figure 1a. According to TiO2 pattern, distinct peaks detected at 25.28°, 37.93°, 48.37°, 53.88°, 55°, and 68.99° with (101), (004), (200), (105), (211), (204) crystal facets are corresponding TiO2 (a, b = 3.77, and c = 9.49) as referenced by (ICDD:00-001-0562) [29]. The notation “(a, b = 3.77, and c = 9.49)” refers to the lattice parameters (unit cell dimensions) of the anatase TiO2 crystal structure. Specifically, a and b represent the unit cell dimensions along the horizontal crystallographic axes, while c corresponds to the vertical axis, with all values expressed in angstroms (Å). The equality of a and b (a = b = 3.77 Å) together with a larger c value (c = 9.49 Å) is characteristic of the tetragonal crystal system of anatase TiO2. The XRD pattern of pure PANI shows a quasi-crystalline nature at 15°, 20°, and 25° [30]. For the TiO2/PANI binary photocatalyst, the characteristic peak for anatase TiO2 appeared in the composite with no noticeable change in the intensity, indicating that the incorporation of PANI with TiO2 did not affect its crystallinity, which is a favorable feature for the photocatalytic degradation process [30].
The FT-IR spectra of TiO2 and TiO2/PANI binary photocatalyst are depicted in Figure 1b. Concerning TiO2 spectra, three distinct peaks can be detected at 693, 1620, and 3400 cm−1, corresponding to Ti-O stretching [31], -OH bending, and -OH bending vibrations [32,33], respectively. For pristine PANI, the peaks positioned at 1245, 1303, 1444, and 1554 cm−1 are credited to the C-N+● stretching vibration in heterocyclic secondary amines, C=N benzenoid stretching mode, and C=C quinoid rings [34]. Regarding the FT-IR spectra of the TiO2/PANI composite, the characteristic peaks for TiO2 and PANI appeared with slight shifting, indicating the successful interactions between them.
X-ray Photoelectron Spectroscopy (XPS) can provide valuable insights into the surface chemistry of TiO2/polyaniline composites, revealing key information about their elemental composition, chemical states, and interactions between the components. The survey spectrum showed the presence of Ti, O, N, and C, with specific peaks corresponding to each element (Figure 1c). The Ti 2p peaks (Figure 1d), around 458 eV and 464 eV, which are characteristic of Ti 2p3/2 and Ti 2p1/2, respectively, which are characteristic of Ti4+ in anatase TiO2. The absence of additional peaks related to Ti3+ suggests that the TiO2 maintains its chemical stability after composite formation. The O 1s peak near 530 eV presented in Figure 1e, represents the oxygen atoms in TiO2, which splits into a low-binding-energy component (~528.5 eV) and a high-binding-energy component (~531.6 eV), which correspond to Lattice oxygen (Ti–O) and surface hydroxyls or defects (Ti–OH) adsorbed species [35]. The relatively higher contribution of surface hydroxyl/defect oxygen is particularly important, as these sites act as active centers for the generation of reactive oxygen species (•OH radicals), thereby enhancing photocatalytic activity. In Figure 1f, the C 1s spectrum displays four distinct peaks. The C–C/C–H bond peak is located at 284.10 eV typically found in aromatic rings. The C-N bond is related to the peak at 284.35 eV [36]. Additionally, the peak at 286.08 eV is attributed to the C–O group, while the peak at 287.8 eV is ascribed to the C=O group [37]. These features indicate the presence of various carbon bonding environments within the composite, which can contribute to improved interfacial interaction and facilitate charge transfer between TiO2 and PANI. The N 1s peak, around 399–400 eV, corresponds to the nitrogen in PANI as shown in Figure 1g. Three distinct peaks are observed at binding energies of approximately 400.0, 399.4, and 398.3 eV. These peaks are attributed to protonated nitrogen species (–N+–), secondary amine groups (–NH–), and imine nitrogen (=N–), respectively [38,39]. Their presence confirms the successful polymerization of aniline monomers and the coexistence of different nitrogen bonding environments within the polyaniline structure. Additionally, Figure 1g presents the atomic percentages of the elements in the TiO2/PANI photocatalyst, highlighting its elemental composition.
The SEM images provide critical insight into the morphological features of TiO2 nanoparticles (NPs), polyaniline (PANI) nanofibers, and their combined structure in TiO2/PANI composite. TiO2 NPs (Figure 2a) appear as relatively uniform, quasi-spherical particles with diameters typically ranging from 20 to 50 nm. These particles tend to agglomerate slightly due to high surface energy, which is common in nanostructured oxides. The uniform distribution and nanoscale size are beneficial for photocatalytic applications, offering a high surface area and short diffusion paths for charge carriers. In Figure 2b, PANI NFs exhibit a distinct one-dimensional morphology, forming entangled, fiber-like networks. This fibrous architecture is characteristic of polyaniline synthesized under controlled polymerization conditions and is advantageous for improving electron transport pathways and interfacial contact with TiO2. In the SEM images of the TiO2/PANI composite (Figure 2c), both original components are clearly discernible. The TiO2 NPs are seen embedded within the fibrous network of PANI. This intimate contact suggests effective interfacial interaction, which, in photocatalytic applications, is necessary for effective charge separation. TiO2 nanoparticles provide an active site for photocatalytic processes, while the nanofiber matrix offers a conductive framework for electron transmission.
Figure 2d–i reflects the HAAFD of the composite along with the elemental mix of C, Ti, O, and N. The composite’s morphology reflects a well-integrated hybrid structure, retaining the characteristics of both components while indicating successful synthesis.
In Figure 3a, UV–Vis diffuse reflectance spectroscopy (UV–DRS) of TiO2, PANI NFs, TiO2/PANI-10, TiO2/PANI-20, TiO2/PANI-30, and TiO2/PANI-40 reveals a clear, systematic evolution of optical behavior that directly explains the improved visible-light photocatalytic performance when polyaniline nanofibers are added to TiO2. TiO2 shows the expected sharp absorption edge in the UV region (typically λ ≈ 344.5 nm) with a band gap at 3.2 eV when extracted from Tauc analysis of the Kubelka–Munk transformed reflectance (Figure 3b). In contrast, PANI NFs display broad, strong absorption across the visible region (polaron/bipolaron bands and π–π* transitions), so the PANI spectrum alone occupies much of the 400–800 nm window. When small amounts of PANI are combined with TiO2 (TiO2/PANI-10, 20, 30, and 40), the DRS shows the beginning of a visible tail and a minor shift in the apparent absorbance edge to the red. This tail represents two direct absorptions by PANI, along with the appearance of interfacial charge-transfer. As the PANI loading increases, the visible absorption becomes significantly stronger, and a broad band spanning 400–800 nm emerges, resulting in a pronounced red-shifted appearance of the composite spectrum. Tauc plots for TiO2/PANI then show an apparent band gap narrowing (2.55 eV), as presented in Figure 3b.
Electrochemical impedance spectroscopy (EIS) was employed to investigate the charge transfer behavior at the electrode–electrolyte interface, and the corresponding Nyquist plots are shown in Figure 3c. The Nyquist plots consist of a semicircle in the high-frequency region and a straight line in the low-frequency region. In this model, the semicircle diameter represents the charge transfer resistance, while the linear portion is associated with diffusion-controlled processes. A smaller semicircle diameter indicates lower interfacial resistance and more efficient charge transfer [40].
As observed, pristine TiO2 exhibits the largest semicircle, indicating high charge transfer resistance and rapid recombination of photogenerated charge carriers. In contrast, the TiO2/PANI composites show significantly reduced semicircle diameters, confirming enhanced charge separation and faster interfacial electron transfer. Among the samples, TiO2/PANI-30 displays the smallest semicircle, indicating the most efficient charge transport [41].
This enhancement can be attributed to the conductive nature of PANI, which facilitates electron mobility and suppresses charge recombination. Similar behavior has been reported in PANI-based composite systems, where the incorporation of PANI significantly reduces charge transfer resistance and improves electrochemical performance. These results confirm that the synergistic interaction between TiO2 and PANI plays a crucial role in improving charge transfer dynamics. As the PANI content increases, the radius of the semicircle decreases progressively up to TiO2/PANI-30, suggesting a continuous enhancement in interfacial charge separation and transfer efficiency. The TiO2/PANI-30 composite exhibits the smallest semicircle, which demonstrates its superior ability to transfer photogenerated electrons and suppress recombination. This efficient charge transport directly correlates with its highest photocatalytic activity among the studied samples. However, a further increase in PANI content beyond TiO2/PANI-30 (i.e., TiO2/PANI-40) leads to a slight enlargement of the semicircle, indicating increased resistance. This can be attributed to the excessive PANI loading, which may partially shield the TiO2 active surface or hinder light absorption by limiting photon penetration, thereby reducing the overall photocatalytic efficiency. Therefore, the EIS results clearly reveal that the optimal PANI incorporation in TiO2/PANI-30 effectively balances conductivity enhancement and light absorption, yielding the lowest charge transfer resistance and highest photocatalytic performance. The improved interfacial charge transfer in TiO2/PANI-30 is consistent with its superior photocatalytic degradation rate observed experimentally.
Figure 3d presents the XPS spectra of TiO2’s valence band (VB), where the VB edge is determined to be approximately 2.45 eV. This value reflects its strong oxidative potential. The accurate determination of the VB maximum is essential for constructing the band alignment and understanding the charge transfer mechanism in the composite system.
From the Tauc plots (Figure 3b), the optical band gap (Eg) of TiO2 is determined to be 3.2 eV. Using this value, the conduction band (CB) position can be estimated based on the well-established relationship:
EVB = ECB + Eg
ECB = EVB − Eg
Therefore, the CB value of TiO2 is estimated to be −0.75 eV. Figure 3e illustrates the proposed band alignment between TiO2 and PANI NFs. PANI is known to exhibit robust absorption in the visible region, enabling π–π* electronic transitions. Upon excitation, electrons are promoted from the HOMO level (approximately +0.8 eV) toward the LUMO level (around −1.9 eV), generating highly reactive charge carriers [42].

2.2. Experimental Investigation of Photocatalytic Degradation

As shown in Figure 4a, the impact of pH on the TiO2/PANI-30 photocatalyst’s zeta potential was investigated. Zeta potential was measured using a Zetasizer Nano ZS instrument (Malvern Instruments, Malvern, UK). The isoelectric point (IEP) is a key parameter used to evaluate the effectiveness of a surface in adsorbing charged contaminants. For the synthesized TiO2/PANI-30 catalyst, the IEP was identified at pH 6, showing that the surface is negatively charged above this pH and positively charged below it. Understanding the IEP is essential for interpreting the photocatalytic behavior of the material under varying pH conditions, as it provides valuable insight into surface charge interactions with pollutants.
Basic Fuchsin is a cationic dye, meaning that under most pH conditions, it carries a positive charge. When the solution pH is lower than the IEP of TiO2/PANI-30, both the dye and the surface of the catalyst are positively charged, resulting in strong electrostatic repulsion. As a result, less dye is adsorbed on the surface of the catalyst, which is essential for effective photocatalytic degradation. In such acidic conditions, not only is dye adsorption suppressed, but the availability of hydroxide ions (OH−) is also low, preventing hydroxyl radicals (•OH), important oxidizing species in the breakdown process, from forming. Additionally, excess protons (H+) can compete with dye molecules for surface sites and may scavenge photogenerated electrons, further hindering degradation efficiency.
As the pH approaches neutral conditions, the surface charge of the TiO2/PANI-30 composite becomes less positive and eventually negative, which enhances the electrostatic attraction between the negatively charged catalyst surface and the cationic BF dye molecules. This improved adsorption promotes more effective interactions between the dye and the reactive species generated upon light irradiation. This combination of improved dye adsorption and increased radical formation generally leads to higher degradation rates in neutral pH environments, in which the degradation efficiency of BF reaches 99% as presented in Figure 4a. At very high pH levels, while the catalyst surface remains strongly negative and dye adsorption continues to be favorable, other challenges may emerge. Excess OH− can sometimes lead to radical scavenging reactions or promote recombination of electron-hole pairs produced by photoexcitation. Furthermore, the TiO2/PANI-30’s stability may be compromised under strongly alkaline conditions, potentially leading to a decline in long-term photocatalytic performance. Figure 4b illustrates the effect of solution pH on the photocatalytic removal efficiency of BF dye.
The photocatalytic degradation of BF dye was systematically investigated using visible light irradiation using pristine TiO2 and a series of TiO2/PANI composite photocatalysts with varying PANI content (10, 20, 30, and 40 wt.%). The dye degradation performance was evaluated over a 40 min illumination period, and the results are summarized in Figure 5a,c. Pristine TiO2 exhibited limited photocatalytic activity under visible light, achieving only 30% degradation efficiency after 40 min (Figure 5c). This low performance is attributed to TiO2’s broad bandgap (3.2 eV), which limits its photoresponse predominantly to the UV region, limiting its activity under exposure to visible light. The incorporation of PANI NFs, a conductive polymer with strong visible-light absorption, greatly increased TiO2’s photocatalytic activity. The composites exhibited improved degradation efficiencies as the PANI content increased from 10 to 30 wt.%, indicating a more effective utilization of visible light and better charge separation. TiO2/PANI-10 and TiO2/PANI-20 achieved 80% and 91% degradation efficiency, respectively, showing progressive improvement with increasing PANI content. The optimal performance was observed with TiO2/PANI-30, which achieved 99% degradation of BF dye within 40 min of visible light exposure. This superior activity is ascribed to the synergistic effect between TiO2 and PANI, where light absorption is extended into the visible area by PANI and facilitates efficient photogenerated charge separation and transfer. Nevertheless, by increasing the content of PANI to 40 wt.% (TiO2/PANI-40), a minor decrease in degradation efficiency was detected (95.5%). This decrease is likely due to excessive PANI coverage on TiO2 surfaces, which may block active sites or hinder light penetration, thereby reducing overall photocatalytic activity. The degradation kinetics, analyzed using the pseudo-first-order model (Figure 5b), further confirmed that TiO2/PANI-30 exhibits the highest apparent rate constant among all samples, corroborating its superior photocatalytic efficiency. The apparent rate constant (k) derived from the linear fitting of Ln(C/C0) versus time clearly demonstrates the catalytic performance of the different systems. The TiO2/PANI-30 composite exhibits the highest k value (0.1096 min−1), indicating the fastest degradation kinetics and superior photocatalytic activity, which can be attributed to the optimal interfacial charge transfer between TiO2 and PANI that suppresses electron–hole recombination. In comparison, TiO2/PANI-40 (0.0749 min−1), TiO2/PANI-20 (0.0584 min−1), and TiO2/PANI-10 (0.0398 min−1) show progressively lower rate constants, suggesting that both insufficient and excessive PANI loading negatively affect the reaction kinetics. Pure TiO2 (0.0079 min−1) and the blank sample (0.0008 min−1) exhibit negligible activity, confirming that the enhanced k values in the composites originate from the synergistic effect between TiO2 and PANI. Overall, the variation in k values highlights that TiO2/PANI-30 achieves the most efficient photocatalytic degradation following pseudo-first-order kinetics. UV-Vis absorption spectra (Figure 5d) of the BF dye solution using TiO2/PANI-30 show a progressive decrease in the characteristic absorbance peak upon illumination, with a sharp reduction occurring only after the light was switched on for 40 min. This confirms the light-driven photocatalytic nature of the degradation process.
The influence of catalyst dosage on the photocatalytic degradation of BF dye was investigated using different amounts of the optimized TiO2/PANI-30 composite (0.1–0.7 g L−1), see Figure 6a,b. As shown by the degradation efficiencies, a clear dosage-dependent behavior was observed. At a low catalyst loading of 0.1 g L−1, the degradation efficiency reached only 65%, which can be attributed to the limited accessible active sites and insufficient capacity for light gathering. Raising the dose to 0.3 g L−1 significantly enhanced the degradation efficiency to 77%, reflecting improved photon absorption and more reactive surface areas, which encourage the production of reactive oxygen species. The maximum degradation efficiency of 99% was achieved at an optimized dosage of 0.5 g L−1 (Figure 6c). At this concentration, an optimal balance among light penetration, active surface area, and effective charge separation within the TiO2/PANI-30 composite was achieved, resulting in highly efficient photocatalytic performance. Though an additional increase in the dose of the catalyst to 0.7 g L−1 resulted in a reduction in degradation performance to 83%. This decrease is likely due to excessive catalyst loading causing light scattering and shielding effects, which limit photon penetration into the reaction medium. Furthermore, the effective surface area may be decreased by particle agglomeration at larger doses and hinder mass transfer between the dye molecules and active sites. These results demonstrate that catalyst dosage plays a critical role in the photocatalytic degradation of BF dye, with the ideal dose for TiO2/PANI-30, which was found to be 0.5 g L−1, ensuring maximum degradation efficiency while avoiding the adverse effects associated with excess catalyst loading [43,44,45].
The impact of the initial concentration of the dye is presented in Figure 6d. The degradation efficiency of the BF dye decreases noticeably with increasing initial dye concentration, as shown by the drop from about 99% at low concentration (20 ppm) to around 21% at higher concentration (50 ppm). This inverse relationship can be explained by several reasons. More dye molecules competed for the few active sites on the TiO2/PANI-30 catalyst surface at greater concentrations, reducing the availability of reactive sites per molecule. Furthermore, higher dye concentrations cause the dye to absorb more light, which restricts light penetration and lowers the amount of reactive species produced on the catalyst surface. The accumulation of intermediate products at higher concentrations may also hinder the reaction by blocking active sites. Overall, lower initial concentrations favour more efficient degradation due to better light utilization and more effective interaction between dye molecules and the catalyst [46,47,48].
Figure 6e illustrates the time-dependent photocatalytic performance of the TiO2/PANI-30 composite through simultaneous monitoring of TOC removal (%) and degradation efficiency (%). A steady and progressive increase is observed in both parameters with irradiation time, indicating continuous mineralization of organic pollutants. The degradation efficiency rises markedly from approximately 30% at the initial stage to nearly complete removal (~90%) after 40 min, reflecting rapid pollutant breakdown. In parallel, the TOC removal follows a similar upward trend, though at a relatively lower magnitude, reaching around 80–90% at the end of the reaction, which suggests effective but slightly slower mineralization compared to the primary degradation process. This distinction between degradation and TOC removal implies the formation of intermediate species during photocatalysis that are subsequently mineralized with prolonged irradiation. Overall, the results demonstrate the high activity and efficiency of the TiO2/PANI-30 system, highlighting its strong capability for both rapid degradation and substantial mineralization of contaminants under the applied conditions.

2.3. Evaluation of Stability and Identification of Reactive Species

Figure 7a illustrates the photocatalytic stability and the influence of reactive species scavengers on degradation efficiency. The catalyst exhibits high stability over five consecutive cycles, with only a slight decline in performance, which reached to 83 degradation efficiency, confirming its good reusability and durability. In the scavenger study (Figure 7a), degradation efficiency is somewhat reduced when EDTA is added, whereas benzoquinone (BQ) causes a more significant reduction, and isopropanol (IPA) leads to the most pronounced suppression. This behavior indicates that both superoxide radicals (•O2−) and hydroxyl radicals (•OH) are actively involved in the degradation process, with •OH identified as the dominant reactive species. Figure 7b shows the XRD patterns of the fresh and used catalysts, where the main diffraction peaks remain at similar positions, indicating that the crystalline structure is preserved after photocatalysis. However, a noticeable decrease in peak intensity accompanied by slight peak broadening is observed for the used catalyst, which can be explained by the dye molecules’ adsorption on the catalyst surface throughout the photocatalytic process. This observation further supports the surface interaction between the catalyst and dye without compromising the overall structural integrity.
As summarized in Table 1, the photocatalytic performance of the present TiO2/PANI nanofiber system is compared with previously reported TiO2/PANI-based photocatalysts for dye degradation, highlighting its improved efficiency and providing a clearer context for the advancements achieved in this study.

2.4. The Suggested Z-Scheme Pathway During the Photo-Catalytic Process

The band structure alignment and the scavenger findings show that the photocatalytic charge transfer route in this system follows a suggested Z-scheme transfer pathway instead of a typical type-II heterojunction [54,55]. TiO2 possesses a conduction band (CB) at approximately −0.75 eV and a valence band (VB) at +2.45 eV (vs. NHE), whereas PANI exhibits a LUMO level near −1.9 eV and a HOMO level around +0.8 eV. In a typical type-II heterojunction, electrons would transfer from the LUMO of PANI to the CB of TiO2, while holes would migrate from the VB of TiO2 to the HOMO of PANI. However, the holes accumulated in the HOMO of PANI (+0.8 eV) would not possess sufficient oxidation potential to generate •OH radicals, which contradicts the scavenger results showing the dominant role of •OH species.
In contrast, the observed photocatalytic behavior suggests a Z-scheme charge transfer route Scheme 1, where photogenerated electrons in the CB of TiO2 recombine with holes in the HOMO of PANI at the interface. This recombination pathway retains the highly reducing electrons in the LUMO of PANI (−1.9 eV) and the strongly oxidizing holes in the VB of TiO2 (+2.45 eV) [56]. As a result, electrons reduce dissolved oxygen to form •O2− radicals, while holes oxidize water or OH− ions to generate •OH radicals [57,58,59,60]. The significant suppression of degradation efficiency after the addition of IPA and BQ further confirms the major contribution of both •OH and •O2− reactive species. Consequently, the suggested Z-scheme mechanism promotes efficient charge separation while maintaining strong redox capability, leading to enhanced photocatalytic performance compared with a conventional type-II system [61].

3. Experimental

3.1. Materials and Chemicals

Aniline monomer, Titanium (IV) isopropoxide, HCl, ammonium persulfate, and isopropyl alcohol were obtained from Merck KGaA, Darmstadt, Germany.

3.2. Preparation of TiO2 NPs

The sol–gel technique was used to prepare TiO2 NPs. Initially, 100 mL of isopropyl alcohol and 12 mL of titanium (IV) isopropoxide were combined and continuously agitated for 2 h. The preceding mixture was then combined with 220 mL of deionized water and aged for four hours. The precipitate was frequently washed with DI H2O and kept dried, and calcined at 400 °C for 4 h to get TiO2 NPs [62].

3.3. Synthesis of TiO2/Polyaniline Nanofibers (PANI NFs)

TiO2/PANI NFs binary composite is prepared through a facile in situ oxidative polymerization technique. To find out how PANI loading affects TiO2/PANI photocatalytic performance, different weights of the synthesized TiO2 (0.3, 0.5, 0.7, 0.9 g) were mixed with 30 mL of HCl (1 M), followed by adding 0.5 mL aniline monomer with continuous stirring for 1 h. Finally, 0.5 g/20 mL of ammonium persulfate was added and stirred in an ice bath for 24 h. The prepared TiO2/PANI photocatalysts, designated as TiO2/PANI-10, TiO2/PANI-20, TiO2/PANI-30, and TiO2/PANI-40, respectively, were washed with water and ethanol multiple times and then vacuum-dried at 60 °C. Scheme 2 summarizes the preparation process of the TiO2/PANI binary photocatalyst.

3.4. Investigating the Photocatalytic Effectiveness Toward the Degradation of BF Dye

At room temperature, the photocatalyst degradation performance was evaluated under visible light irradiation by adding 0.5 g/L of photocatalyst to 50 mL of a BF solution (20 mg/L) for a degradation batch. After 30 min of adsorption in the dark, the cell was placed in a box equipped with a 300 W Xenon lamp with a cut-off filter set to 420 nm, ensuring that only visible light irradiation was applied by eliminating ultraviolet wavelengths below 420 nm. The light source was positioned at a fixed distance of ~15 cm from the reaction solution. The photocatalytic reactions were conducted in a cylindrical quartz reactor (100 mL) with continuous magnetic stirring to maintain suspension homogeneity. The effective irradiation area was approximately 20 cm2. All experiments were performed under identical conditions to ensure consistency and reproducibility of the results. A specific volume of sample was collected at predetermined intervals and filtered for analysis using a polyethersulfone membrane filter with a pore size of 0.22 μm. The UV–vis spectrophotometer was used to measure the BF concentrations after a particular period of light exposure.
The degradation efficiency (η) can be computed using the following equation:
η = (C0 − C)/C0) × 100%
where C0 shows the initial concentration, and C stands for the concentration following a particular period of time. The apparent rate of reaction constant (ka) is described by a linear fitting equation, depending upon Langmuir first-order kinetics, as follows:
ln (C0/Ct) = kat
Here, t refers to the reaction time, while k represents the degradation rate constant expressed in min−1.
Trapping tests show that •OH, •O2−, and h+ were quenched using isopropanol (IPA), P-benzoquinone (P-BQ), and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) during the BF degradation, respectively.
Total organic carbon (TOC) measurement was used to measure the extent of dye mineralization during the photodegradation process in order to assess the degradation route. TOC analysis enabled an assessment of the extent to which BF dye was mineralized. Before radiation, the starting TOC level (TOC0) was determined, while TOC values at designated time intervals (TOCt) were recorded during the reaction. The efficiency of mineralization was subsequently expressed as the percentage of TOC removal, calculated using the following equation [63]:
TOC removal (%) = (TOC0 − TOCt)/TOC0 × 100
where TOC0 is the initial total organic carbon value before irradiation, and TOCt is the TOC value at a given reaction time t.

4. Conclusions

In this study, a TiO2/PANI photocatalyst was successfully synthesized via an in situ polymerization method. The incorporation of conductive PANI nanofibers into TiO2 improved visible-light absorption and promoted efficient charge separation, resulting in enhanced photocatalytic activity. Among the prepared composites, TiO2/PANI-30 demonstrated the highest performance, achieving approximately 99% degradation of Basic Fuchsin dye within 40 min under visible-light irradiation.
Characterization results confirmed the formation of a well-integrated heterojunction with strong interfacial interaction, which facilitated charge transfer during photocatalysis. The degradation efficiency was influenced by factors such as pH, catalyst dosage, and dye concentration, with optimum performance obtained at neutral pH and a catalyst loading of 0.5 g L−1. Mechanistic analysis suggested a Z-scheme charge transfer pathway, which preserved highly reactive electrons and holes for the generation of •O2− and •OH radicals responsible for dye degradation. In addition, the photocatalyst exhibited good stability and reusability over repeated cycles.
Overall, the results demonstrate the potential of TiO2/PANI nanofibers as efficient visible-light-driven photocatalysts for wastewater treatment and environmental remediation applications.

Author Contributions

M.A.D., H.A.E.-S. and Y.K.: Conceptualization, Writing—original draft, Writing—review and editing, Validation; Methodology, Data analysis; Resources; Revision; Investigation; Visualization, and Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge financial support from the National Research Foundation of Korea (RS-2025-00573677).

Data Availability Statement

The whole data set presented in this article has been included in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Catalysts 16 00464 g001
Figure 1. (a) XRD patterns and (b) FTIR spectra of TiO2, PANI, and TiO2/PANI-30. (c) Survey spectrum of TiO2/PANI. High resolution CPS for (d) Ti2p, (e) O 1s, (f) C 1s, (g) N 1s, and (h) The elemental composition of TiO2/PANI-30.
Figure 1. (a) XRD patterns and (b) FTIR spectra of TiO2, PANI, and TiO2/PANI-30. (c) Survey spectrum of TiO2/PANI. High resolution CPS for (d) Ti2p, (e) O 1s, (f) C 1s, (g) N 1s, and (h) The elemental composition of TiO2/PANI-30.
Catalysts 16 00464 g001
Catalysts 16 00464 g002
Figure 2. FE-SEM images of (a) TiO2 NPs, (b) PANI NFs, and (c) TiO2/PANI-30 composite. (d) HAADF imaging, along with the related elemental mapping analysis of (e) elemental mix, (f) C, (g) Ti, (h) O, and (i) N.
Figure 2. FE-SEM images of (a) TiO2 NPs, (b) PANI NFs, and (c) TiO2/PANI-30 composite. (d) HAADF imaging, along with the related elemental mapping analysis of (e) elemental mix, (f) C, (g) Ti, (h) O, and (i) N.
Catalysts 16 00464 g002
Catalysts 16 00464 g003
Figure 3. (a) Spectra of UV-vis DRS for TiO2, PANI, and TiO2/PANI-10, 20, 30, and 40, (b) Tauc plots of TiO2 and TiO2/PANI-30, (c) Electrochemical impedance spectra of the photocatalysts, (d) VB-XPS of TiO2, and (e) Band edges alignment between TiO2 and PANI NFs.
Figure 3. (a) Spectra of UV-vis DRS for TiO2, PANI, and TiO2/PANI-10, 20, 30, and 40, (b) Tauc plots of TiO2 and TiO2/PANI-30, (c) Electrochemical impedance spectra of the photocatalysts, (d) VB-XPS of TiO2, and (e) Band edges alignment between TiO2 and PANI NFs.
Catalysts 16 00464 g003
Catalysts 16 00464 g004
Figure 4. (a) Zeta potential of TiO2/PANI-30 photocatalyst at different pH levels, and (b) The degradation efficiency of 0.5 g/L of BF (20 mg/L) under different pH ranges.
Figure 4. (a) Zeta potential of TiO2/PANI-30 photocatalyst at different pH levels, and (b) The degradation efficiency of 0.5 g/L of BF (20 mg/L) under different pH ranges.
Catalysts 16 00464 g004
Catalysts 16 00464 g005
Figure 5. (a) Photocatalytic degradation of 20 ppm BF using 0.5 g/L of different photocatalysts at pH 7, (b) pseudo-first-order kinetic plots, (c) degradation efficiencies of the photocatalysts, and (d) UV–Vis absorption spectra showing the decolorization of BF solution using TiO2/PANI-30 under visible light irradiation for 40 min.
Figure 5. (a) Photocatalytic degradation of 20 ppm BF using 0.5 g/L of different photocatalysts at pH 7, (b) pseudo-first-order kinetic plots, (c) degradation efficiencies of the photocatalysts, and (d) UV–Vis absorption spectra showing the decolorization of BF solution using TiO2/PANI-30 under visible light irradiation for 40 min.
Catalysts 16 00464 g005
Catalysts 16 00464 g006
Figure 6. (a) Photocatalytic degradation of 20 ppm BF using different dosages of TiO2/PANI-30 at pH 7, (b) pseudo-first-order kinetic plots, (c) degradation efficiency at various catalyst dosages, (d) effect of initial BF concentration on degradation efficiency, and (e) TOC analysis of BF degradation using TiO2/PANI-30.
Figure 6. (a) Photocatalytic degradation of 20 ppm BF using different dosages of TiO2/PANI-30 at pH 7, (b) pseudo-first-order kinetic plots, (c) degradation efficiency at various catalyst dosages, (d) effect of initial BF concentration on degradation efficiency, and (e) TOC analysis of BF degradation using TiO2/PANI-30.
Catalysts 16 00464 g006
Catalysts 16 00464 g007
Figure 7. (a) Stability cycles and scavenger effects on BF dye degradation using TiO2/PANI-30, and (b) XRD patterns of fresh and used TiO2/PANI-30 before and after photocatalytic experiments.
Figure 7. (a) Stability cycles and scavenger effects on BF dye degradation using TiO2/PANI-30, and (b) XRD patterns of fresh and used TiO2/PANI-30 before and after photocatalytic experiments.
Catalysts 16 00464 g007
Catalysts 16 00464 sch001
Scheme 1. The proposed Type-II vis Z-Scheme charge pathway transfer mechanism.
Scheme 1. The proposed Type-II vis Z-Scheme charge pathway transfer mechanism.
Catalysts 16 00464 sch001
Catalysts 16 00464 sch002
Scheme 2. Schematic illustration for the preparation procedures of the photocatalyst.
Scheme 2. Schematic illustration for the preparation procedures of the photocatalyst.
Catalysts 16 00464 sch002
Table 1. Comparison of TiO2/PANI-based photocatalysts for dye degradation.
Table 1. Comparison of TiO2/PANI-based photocatalysts for dye degradation.
PhotocatalystDyeLight SourceDegradation (%)Time (min)Stability/ReusabilityKey FeatureRef.
TiO2/PANI/GO- Thymol Blue
- Rose Bengal		UV-365 nm Hanovia lamp60
97		180Stable over 3 cycles
Stable over 3 cycles		Ternary composite[49]
PANI@Fe2O3@TiO2Methylene Bluesun-like radiation
Osram Ultra-Vitalux lamp (300 W)		96120Stable over 4 cyclesTernary composite[50]
PANI/CdS
PANI/CdS-ZnS
PANI/CdS-TiO2		Acid Blue-29Visible light A halogen liner lamp
(500 W, 9500 Lumens)		82.2
89.9
86.4		90Stable over 5 cyclesComparison between different composites[51]
TiO2/Bi2O3/PANIRhodamine BVisible LEDs99.650Stable over 4 cyclesTernary composite[52]
TiO2/polyaniline bilayerMethyl orange45 W fluorescent lamp 10060Stable over 10 cyclesBilayer composite[53]
TiO2/PANI nanofibersBasic FuchsinVisible light (420 nm cutoff)9940Stable over 5 cyclesZ-scheme heterojunctionPresent work
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Diab, M.A.; El-Sabban, H.A.; Kim, Y. Engineering Polyaniline Nanofibers/TiO2 for Enhanced Photocatalytic Degradation of Organic Contaminants: In-Depth Structural and Mechanistic Insights. Catalysts 2026, 16, 464. https://doi.org/10.3390/catal16050464

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Diab MA, El-Sabban HA, Kim Y. Engineering Polyaniline Nanofibers/TiO2 for Enhanced Photocatalytic Degradation of Organic Contaminants: In-Depth Structural and Mechanistic Insights. Catalysts. 2026; 16(5):464. https://doi.org/10.3390/catal16050464

Chicago/Turabian Style

Diab, Mohamed. A., Heba A. El-Sabban, and Youngsoo Kim. 2026. "Engineering Polyaniline Nanofibers/TiO2 for Enhanced Photocatalytic Degradation of Organic Contaminants: In-Depth Structural and Mechanistic Insights" Catalysts 16, no. 5: 464. https://doi.org/10.3390/catal16050464

APA Style

Diab, M. A., El-Sabban, H. A., & Kim, Y. (2026). Engineering Polyaniline Nanofibers/TiO2 for Enhanced Photocatalytic Degradation of Organic Contaminants: In-Depth Structural and Mechanistic Insights. Catalysts, 16(5), 464. https://doi.org/10.3390/catal16050464

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