Synergistic Integration of TiO₂ Nanorods with Carbon Cloth for Enhanced Photocatalytic Hydrogen Evolution and Wastewater Remediation

Abstract

The immobilization of titanium dioxide (TiO₂) nanostructures on conductive supports offers a promising strategy to overcome the intrinsic limitations of a wide band gap, poor visible-light absorption, and rapid charge recombination in photocatalysis. Herein, a rutile TiO₂ nanorods (TiO₂NRs) array was directly grown on carbon cloth (CC) via a hydrothermal method by using titanium tetrachloride (TiCl₄) seed solutions of 0.1, 0.3, and 0.5 M, designated as TiO₂NR0.1/CC, TiO₂NR0.3/CC, and TiO₂NR0.5/CC, respectively. Structural analysis confirmed that the TiO₂ NRs array is vertically aligned, and phase=pure rutile NRs strongly adhered to CC. The optical characterization revealed broadened absorption in the visible wavelength region and progressive band gap narrowing with the increasing seeding concentration. Photoluminescence (PL) spectra showed pronounced quenching in the fabricated TiO₂NRs/CC samples, especially with TiO₂NR0.3/CC exhibiting the lowest PL intensity, indicating suppressed charge recombination. Electrochemical impedance spectroscopy further demonstrated reduced charge transfer resistance, and TiO₂NR0.3/CC achieved the most efficient electron transport kinetics. Photocatalytic tests at λ ≥ 400 nm irradiation confirmed the enhanced hydrogen evolution performance of TiO₂NR0.3/CC. The hydrogen yield of 2.66 mmol h⁻¹ g⁻¹ of TiO₂NR0.3/CC was 4.03-fold higher than that of TiO₂NRs (0.66 mmol h⁻¹ g⁻¹), along with excellent cyclic stability across three runs. Additionally, TiO₂NR0.3/CC achieved 90.2% degradation of methylene blue within 60 min, with a kinetic constant of 0.0332 min⁻¹ and minimal activity loss after three cycles. These results highlight the synergistic integration of TiO₂ NRs with CC in achieving a durable, recyclable, and efficient photocatalytic platform for sustainable hydrogen generation and wastewater remediation.

1. Introduction

The continuous exhaustion of fossil fuel reserves and rising concerns over environmental pollution have accelerated the search for sustainable and renewable solutions for energy generation and wastewater remediation. Among emerging technologies, photocatalysis has attracted large attention as a green strategy that utilizes solar energy to drive both hydrogen (H₂) production through water splitting and organic pollutants degradation in wastewater [1,2]. Hydrogen, as a clean fuel with high energy density, represents an important element for future energy systems, while the removal of toxic dyes from textile and chemical industries is crucial to mitigating ecological risks. However, the effectiveness of photocatalytic treatment strongly depends on the design of efficient, durable, and recyclable photocatalysts, capable of overcoming intrinsic material limitations. In this context, TiO₂ has remained one of the most extensively studied semiconductor photocatalysts due to its chemical stability, low toxicity, abundance, and photocorrosion resistance. One-dimensional TiO₂ nanostructures, i.e., nanorods (NRs) and nanowires (NWRs), have gained particular interest because they provide oriented electron pathways, high aspect ratios, and improved light-harvesting ability compared to nanoparticulate TiO₂ [3]. Guo et al. reported the growth of rectangular bunched rutile TiO₂ NRs arrays on CC, enhancing the photocatalytic performance compared to conventional nanoparticle films [4]. Nevertheless, pristine TiO₂ often suffers from a wide band gap (~3.0–3.2 eV), restricting its activity primarily in the ultraviolet wavelength region that constitutes ~5% of the solar spectrum, and rapid recombination of photogenerated electron–hole pairs limits quantum efficiency [5]. Moreover, when employed as powder suspensions, TiO₂ catalysts pose difficulties in recovery and recycling, which hinders their large-scale deployment in wastewater treatment and H₂ evolution systems [5,6]. To address these challenges, some researchers explored the immobilization of photocatalysts on conductive and flexible supports. Carbon cloth (CC), a woven textile composed of carbon microfibers, has recently emerged as an attractive substrate for photocatalysis. Its high electrical conductivity allows the rapid extraction and transfer of photogenerated electrons from semiconductors, thereby suppressing electron–hole recombination and improving catalytic efficiency [7,8]. The mechanical flexibility and chemical stability of CC enable the fabrication of bendable, durable photocatalytic devices suitable for integration into flow systems or flexible reactors [9]. Furthermore, the three-dimensional porous network of CC facilitates the mass transfer of reactants and products, while its large surface area supports dense and uniform loading of nanostructured catalysts [10,11]. Importantly, the immobilization of photocatalysts on CC eliminates the need for centrifugation or filtration after reactions, enabling easy recyclability and preventing secondary pollution [12]. These features make CC a multifunctional scaffold that enhances photocatalytic activity, while ensuring practical applicability.

The advantages of CC have been demonstrated in numerous nanomaterials immobilized on CC for environmental and energy-related photocatalysis. Yi et al. synthesized ZnO@Ag₃PO₄ core–shell structures on CC, and the archived rhodamine B degradation efficiency was nearly 5–16 times higher than individual components due to efficient interfacial charge separation and fast electron transfer through CC [13]. Jian et al. reported that Co₃O₄/AgIO₄ heterojunctions on CC showed approximately 58-fold enhancement in dye degradation compared to bare Co₃O₄. This was attributed to synergistic S-scheme charge transfer mediated by conductive carbon fibers [14]. Chang et al. fabricated BiOBr/CuO composites on CC, which functioned as immobilized photocatalytic membranes with remarkable H₂ evolution activity under visible light [10]. Jiang et al. designed oxygen-deficient WO₃/TiO₂ NRs on CC, in which the conductive substrate collectively improved charge separation and visible-light absorption [12]. In another study, zinc indium sulfide (ZnIn₂S₄) nanosheets grown on carbon fibers delivered enhanced photocatalytic H₂ evolution, confirming that intimate coupling with CC facilitates directional electron flow [15,16]. These reports collectively highlight that CC not only serves as a passive support but actively contributes to improved photocatalytic charge dynamics.

Compared with powder catalysts, CC-supported systems exhibit multiple scientific advantages such as (i) electron transfer enhancement due to excellent conductivity of carbon fibers, which act as electron reservoirs, (ii) improved light utilization, as the woven 3D texture allows multidirectional photon penetration and scattering, (iii) structural stability and flexibility, enabling durable operation under mechanical stress and continuous operation of flow reactors, (iv) recyclability, as CC-supported photocatalysts can be easily retrieved and reused, and (v) interfacial synergy, where strong contact between nanomaterials and CC suppresses recombination and prolongs carrier lifetimes [16]. Despite these promising advantages of CC supporting various nanomaterials, relatively few studies have systematically investigated TiO₂ NR arrays grown directly on CC for dual photocatalytic functions of H₂ evolution and dye degradation. The aligned TiO₂ NRs’ morphology provides one-dimensional (1D) pathways for electron transport, while the CC substrate extracts electrons efficiently, thereby suppressing recombination and enhancing both reduction (H₂ generation) and oxidation (dye degradation) reactions. Moreover, the immobilization on CC overcomes the recovery issues, allowing for long-term operational stability and practical reusability. However, the detailed understanding of how nanorod growth orientation, interface structure, and defect states influence charge separation and photocatalytic efficiency in TiO₂NRs/CC systems remains limited.

Therefore, the present work focuses on the rational design and fabrication of TiO₂ NRs arrays directly grown on CC, and evaluates their photocatalytic performance for H₂ evolution and organic dye degradation under solar irradiation conditions. The present system uniquely achieves solar-to-hydrogen conversion without any costly noble metal co-catalysts, as well as exhibiting simultaneous organic dye degradation with good recyclability across multiple cycles.

By leveraging the synergistic integration of 1D TiO₂ with the conductive and flexible carbon substrate, this study aims to (i) enhance charge carrier separation and electron transport, (ii) maximize surface-active sites for photocatalytic reactions, (iii) ensure recyclability and structural durability, and (iv) provide insights into the mechanistic role of CC in promoting photocatalytic efficiency. This approach not only advances the application of TiO₂-based systems but also contributes to the new development of flexible, multifunctional photocatalytic platforms for addressing global energy and environmental challenges.

2. Results and Discussion

2.1. Surface Structure of TiO₂ NRs/CC

The XRD patterns of pristine CC and TiO₂ NRs grown on CC (i.e., TiO₂NRs0.3/CC) are shown in Figure 1. The pristine CC exhibits a broad hump at around 25°, corresponding to the amorphous graphitic carbon phase [17]. In contrast, the TiO₂NRs/CC exhibits sharp diffraction peaks located at 2θ = 27.88°, 36.60°, 39.62°, 41.76°, 44.52°, 54.76°, and 57.02°, which can be indexed to the (110), (101), (200), (111), (210), (211), and (220) planes of rutile TiO₂. The absence of characteristic anatase (101) reflection at 25.3° or brookite-related peaks confirms the formation of phase-pure rutile TiO₂ by hydrothermal treatment [18]. The dominant intensity of the (110) reflection indicates preferential orientation along this plane, which is a typical feature of rutile TiO₂ NRs grown in strongly acidic chloride-containing media. This result is consistent with earlier reports that chloride ions selectively adsorbed onto certain rutile crystal facets, thereby suppressing lateral growth and promoting anisotropic elongation along the c-axis to form rod-like structures [19]. The sharp and well-defined peaks further demonstrate the high crystallinity of the obtained TiO₂ NRs. This high crystallinity is beneficial for efficient charge transport in photocatalytic and photoelectrochemical applications [20]. The combination of seeding with TiCl₄ and subsequent hydrothermal growth ensured uniform nucleation and dense coverage of TiO₂ NRs on the CC substrate. This high crystallinity and phase purity might be helpful for enhancing electron transport and photocatalytic activity, making the TiO₂ NR/CC composites suitable for photoelectrochemical and energy-related applications [21].

2.2. Surface Morphology of TiO₂ NRs/CC

The morphological features of the pristine CC and TiO₂ NRs grown on CC were examined by employing scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. Figure 2a,b shows the surface morphologies of pristine CC, where the individual carbon fibers have exhibited a smooth surface, and they have a tightly woven textile structure. The absence of surface deposits confirms that the acid pretreatment effectively removed impurities, leaving behind a clean substrate suitable for TiO₂ nucleation. Upon hydrothermal growth, significant structural modifications were observed. The digital photograph in Figure 2c shows the overall whitish appearance of TiO₂NR0.3/CC, indicating uniform deposition of TiO₂ nanostructures. At lower SEM magnification (cf. Figure 2d), the carbon fibers are seen to be uniformly coated with vertically aligned TiO₂ NRs, forming a dense nanorod network across the substrate. With higher magnification (i.e., Figure 2e,f), it becomes clear that the nanorods are radially anchored onto the carbon fiber surface, creating a roughened texture compared to pristine CC. The nanorods are densely packed with diameters in the tens of nanometers and lengths extending into the micrometer range. Such dense vertical alignment can be attributed to the TiCl₄ seeding and chloride-assisted hydrothermal conditions, which promote anisotropic rutile TiO₂ growth through selective facet stabilization [20,22]. The TEM micrographs provide further insights into the structural features. Figure 2g shows a cluster of elongated TiO₂ NRs with uniform size distribution, while the high-resolution TEM image in Figure 2h displays individual nanorods with clear lattice fringes, confirming their single-crystalline nature. The SAED pattern in Figure 2i exhibits sharp diffraction spots characteristic of a well-ordered rutile crystal structure, in agreement with the XRD analysis. The formation of single-crystalline rutile nanorods directly on a conductive and flexible CC substrate ensures strong interfacial adhesion, enhanced electron transport, and high surface area, all of which are highly desirable for the practical applications of photocatalysis.

2.3. Surface Chemical Composition and States of TiO₂ NRs/CC

The chemical composition and oxidation states of TiO₂NR0.3/CC were examined by X-ray photoelectron spectroscopy (XPS). The survey spectrum (cf. Figure 3a) confirmed the presence of Ti, O, and C elements on the surface. The high-resolution Ti 2p spectrum (cf. Figure 3b) shows two peaks at ~458.6 eV (Ti 2p) and ~463.3 eV (Ti 2p₁/₂), with a spin–orbit splitting of ~5.7 eV. These values are characteristic of Ti⁴⁺ in stoichiometric TiO₂, confirming that titanium exists predominantly in the fully oxidized state without evidence of Ti³⁺ [23,24]. The O 1s spectrum (cf. Figure 3c) can be fitted into two components. The main peak at ~530.1 eV is attributed to lattice oxygen (Ti–O–Ti), while the higher binding energy shoulder at ~531.6 eV corresponds to surface hydroxyl groups or adsorbed oxygen species [25]. The presence of hydroxyl groups is advantageous, as they promote surface reactivity and improve photocatalytic performance. The C 1s spectrum exhibits a dominant peak at ~284.8 eV due to graphitic sp² carbon from the carbon fiber substrate, while the minor peaks at ~286.5–288.5 eV are associated with C–O and O–C=O groups (see Figure 3d). Such oxygen-containing functionalities are known to arise from oxidative pretreatment and serve as anchoring sites for TiO₂ nucleation. This analysis confirms the successful growth of rutile TiO₂ NRs with a Ti⁴⁺ oxidation state, strong bonding to the carbon fiber substrate, and surface hydroxylation, all of which are favorable for photocatalytic applications.

2.4. Optical and Photophysical Properties of TiO₂ NRs/CC

The optical and photophysical properties of TiO₂NRs/CC were systematically examined through UV–Vis absorbance (Figure 4a,b), Tauc band gap analysis (Figure 4c), and photoluminescence spectroscopy (Figure 4d–f). Figure 5a shows the diffuse reflectance UV–Vis spectra, revealing broadened absorption extending from the UV into the visible region, particularly for samples seeded with higher TiCl₄ concentrations. This red shift and enhanced visible-light absorption align with previous findings that carbonaceous supports effectively extend TiO₂’s light-harvesting capabilities through improved interfacial interactions and charge transfer dynamics. In Figure 4c, pristine TiO₂ NRs exhibited a band gap of 3.08 eV, corresponding to an absorption edge near 403 nm. When integrated with carbon cloth, a slight reduction in the band gap was observed, with values of 3.01 eV, 2.99 eV, and 2.99 eV for TiO₂ NRs grown using 0.1 M, 0.3 M, and 0.5 M TiCl₄, respectively. This red shift of about 0.07–0.09 eV indicates improved light absorption due to the interaction between TiO₂ and the conductive carbon substrate. The shift is likely attributed to interfacial states, defect levels, or C–Ti–O bonding, which extend the absorption edge marginally into the near-UV–visible region. Importantly, increasing the TiCl₄ concentration from 0.3 M to 0.5 M did not significantly alter the band gap, suggesting that the optical modification saturates around 0.3 M. This implies that further increases in the precursor concentration primarily influence the nanorod density and morphology rather than the electronic structure. From a practical perspective, 0.3 M TiCl₄ offers an optimal balance, as it achieves the desired band gap narrowing while minimizing excess precursor use. This trend is consistent with the literature reports where carbon doping or carbon support leads to band gap narrowing in TiO₂ composites [26]. The photoluminescence (PL) spectra of TiO₂ NRs deposited on CC are deconvoluted, as shown in Figure 4d–f. Two characteristic emission bands are observed for all samples, with the first peak appearing at 437 nm for TiO₂NRs0.1/CC and ~438 nm for TiO₂NRs0.3/CC and TiO₂NRs0.5/CC, corresponding to near band-edge excitonic recombination. The second peak occurs around 465–466 nm and is attributed to intrinsic defect states, mainly oxygen vacancies and Ti³⁺ centers. The maximum PL intensities were found to be 592 a.u. for TiO₂NRs0.1/CC, 462 a.u. TiO₂NRs0.3/CC, and 559 a.u. for TiO₂NRs0.5/CC. Especially, TiO₂NRs0.3/CC shows the strongest quenching effect. This reduction in PL intensity demonstrates improved charge separation and longer carrier lifetimes due to the conductive CC acting as an electron sink. Such enhanced charge dynamics are essential for maximizing H₂ generation efficiency and ensuring effective degradation of organic pollutants. Interestingly, at the 0.5 M concentration, the loading in PL intensity is slightly increased. This seems to be attributed to excess CC hindering light absorption or introducing recombination centers [27].

2.5. Charge Transport Resistance and Semiconductor Characteristics of TiO₂ NRs/CC

Electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S) analyses were further employed to probe the charge transport resistance and semiconductor characteristics of the TiO₂NRs/CC electrodes (Figure 5a–c). The Nyquist plots in Figure 6a represent that the bare TiO₂ NRs exhibit the largest semicircle diameter, corresponding to a higher charge transfer resistance (Rct), whereas the TiO₂NRs/CC composites display substantially reduced semicircles, confirming faster interfacial charge transport. Among them, TiO₂NRs0.3/CC demonstrates the smallest semicircle, indicating the most efficient electron transfer kinetics at the electrode–electrolyte interface. This improvement is attributed to the synergistic role of the CC, which enhances electrical conductivity and suppresses recombination by providing a direct pathway for electron collection. Mott–Schottky measurements (Figure 5b and c) further support these findings by elucidating the flat-band potential and carrier density. All potentials initially measured against the Ag/AgCl reference electrode were converted to the RHE scale by using the Nernst equation (E_RHE = EAg/AgCl + 0.197 + 0.059 × pH), where 0.197 V represents the standard potential of Ag/AgCl (saturated KCl) at 25 °C [2]. For pristine TiO₂ NRs (Figure 5b), the flat-band potential is observed at approximately –0.2 V vs. RHE, whereas for TiO₂NRs0.3/CC (Figure 5c), a more negative shift to –0.65 V is recorded. This cathodic shift suggests that CC incorporation not only alters the electronic environment but also enhances band bending, facilitating charge separation and a stronger driving force for reduction reactions [28]. Moreover, the slope of TiO₂NRs/CC electrodes implies a higher donor density compared to pure TiO₂, which is consistent with the literature reports on carbon-supported TiO₂ photoelectrodes. These electrochemical results collectively confirm that an optimal TiCl₄ concentration (0.3/CC) yields the most favorable charge transport and electronic properties. Excessive carbon loading (0.5/CC), however, slightly deteriorates performance. This might result from the partial blocking of active TiO₂ sites or the introduction of recombination centers, similar to previous observations in TiO₂/carbon hybrid systems [29]. Linear sweep voltammetry (LSV) measurements were further carried out to evaluate the photoelectrochemical (PEC) activity of the TiO₂NRs/CC electrodes (cf. Figure 5d). The experiments were performed in a standard three-electrode configuration using 0.5 M Na₂SO₄ aqueous solution (pH ~7) as the electrolyte under light illumination (AM 1.5G, 100 mW cm⁻²). The bare CC electrode exhibited a very low photocurrent density (~2 × 10⁻⁴ mA cm⁻² at 1.0 V vs. RHE), highlighting its limited contribution to photoactivity. Upon the integration of TiO₂ nanorods, the photocurrent density increased significantly, confirming the beneficial role of TiO₂ in light harvesting and charge generation. Notably, the TiO₂NRs0.3/CC electrode delivered the highest photocurrent density of ~2.1 × 10⁻³ mA cm⁻², outperforming both TiO₂NRs0.1/CC (~1.0 × 10⁻³ mA cm⁻²) and TiO₂NRs0.5/CC (~1.5 × 10⁻³ mA cm⁻²). This enhanced performance is attributed to the optimized nanorod morphology and interfacial contact achieved at the 0.3 M TiCl₄ precursor concentration, which synergistically promotes efficient charge separation and transport while maintaining a large active surface area. In contrast, an excessive Ti precursor concentration (0.5 M) likely leads to the overgrowth and partial aggregation of nanorods, which hinder charge transfer and increase recombination losses, thereby reducing the photocurrent output. These LSV results, in agreement with EIS and M–S analyses, demonstrate that 0.3 M TiCl₄ is the optimal condition for fabricating TiO₂NRs/CC electrodes with superior PEC performance.

2.6. Photocatalytic Performance of TiO₂NRs/CC

2.6.1. H₂ Evolution

The TiO₂ NRs-coated carbon fabric was placed on a polyurethane foam ring to float on an electrolyte surface in a splitable borosilicate glass reactor containing 20 mL of aqueous solution (90 vol% H₂O, 10 vol% triethanolamine (TEOA)). The reactor was purged with nitrogen gas for 30–60 min to remove oxygen. The sealed reactor was exposed to 100 mW/cm² light irradiation, and the reaction temperature was maintained at 25 °C using a cooling system. The photocatalytic H₂ evolution reaction was conducted for 1–4 h, with periodic headspace gas sampling to quantify H₂ production via GC analysis. Figure 6a and Figure 6b show the H₂ evolution and rate of H₂ evolution, respectively, using the as-prepared photocatalysts. Here, the bare TiO₂ NRs showed relatively low H₂ generation, producing only 41.8 µmol of H₂ after 4 h of illumination with a rate of 0.66 mmol h⁻¹ g⁻¹. When TiO₂ NRs were deposited on CC, the photocatalytic activity improved significantly due to enhanced electron transfer and reduced recombination. At 0.1 M TiCl₄, the composite produced 68.2 µmol of H₂ with a rate of 0.90 mmol h⁻¹ g⁻¹, while further increasing the concentration to 0.3 M resulted in a remarkable enhancement, reaching 120.5 µmol of H₂ after 4 h with the highest H₂ evolution rate of 2.66 mmol h⁻¹ g⁻¹. At 0.5 M TiCl₄, the H₂ evolution was slightly lower, with 114.3 µmol after 4 h and a rate of 2.24 mmol h⁻¹ g⁻¹, which suggests that an excessive precursor concentration may lead to denser growth and hinder charge mobility. Figure 6c shows the cyclic stability tests conducted for the optimized TiO₂NRs0.3M/CC sample showed excellent reproducibility across three consecutive cycles, with H₂ evolution values of 122.5, 120.5, and 121.5 µmol after 4 h, indicating negligible loss in activity and confirming the robustness of the photocatalyst. These results demonstrate that the combination of TiO₂ NRs with a conductive CC substrate, particularly when synthesized with 0.3 M TiCl₄, provides an efficient and durable system for photocatalytic H₂ production in TEOS aqueous solution.

2.6.2. Dye Degradation

The photocatalytic degradation of methyl blue (MB) dye using TiO₂NRs/CC is investigated under visible-light irradiation, as shown in Figure 7a–d. The UV–Vis absorption spectra (Figure 7a) clearly demonstrate the gradual reduction in MB intensity with the increasing irradiation time, confirming continuous dye degradation. Among the tested samples, TiO₂NRs0.3/CC achieved the most efficient photocatalytic activity, resulting in 90.2% degradation within 60 min. This superior performance highlights the crucial role of the TiO₂–carbon interface, which enhances light absorption, promotes charge separation, and accelerates interfacial electron transfer. The normalized C/C₀ plots in Figure 7b illustrate the degradation kinetics, with TiO₂NRs0.3/CC consistently outperforming other composites. To quantify the reaction kinetics, the pseudo-first-order model was applied, and the corresponding ln(C₀/C) curves are shown in Figure 7c. The calculated rate constants were 0.00216, 0.0122, 0.0177, 0.0332, and 0.0256 min⁻¹ for CC, TiO₂NRs, TiO₂NRs0.1/CC, TiO₂NRs0.3/CC, and TiO₂NRs0.5/CC, respectively. The TiO₂NRs0.3/CC sample exhibited the highest rate constant (0.0332 min⁻¹), which is nearly three times greater than pristine TiO₂NRs, demonstrating the strong influence of optimized CC loading on catalytic efficiency [30]. Such improvements have been consistently reported in TiO₂–carbon composites, where the conductive support suppresses electron–hole recombination and provides additional adsorption sites for pollutant molecules [31]. Recyclability of the TiO₂NRs0.3/CC photocatalyst is assessed across three consecutive cycles, as shown in Figure 7c. The MB degradation efficiency decreased slightly from 89.65% in the first run to 85.6% in the third cycle, indicating excellent structural stability and reusability of the composite electrode. The slight decline is attributed to surface fouling and partial active site deactivation during prolonged photocatalysis, in line with previous results [32]. For better context,

Table 1. TiO₂ NRs on CC or carbon fiber paper (CFP), for photocatalytic H₂ evolution and photocatalytic dye degradation.

Photocatalyst System	Light Source	Photocatalytic H2 Evolution	Photocatalytic Dye Degradation	Reference
TiO2@Sn3O4 nanorods	Visible-light PEC	5.23 µmol h−1 (vs. 1.13 µmol h−1 for TiO2 NRs)		[33]
Carbon fiber-reinforced polymers/TiO2 composite (powdered CFs mixed with TiO2)	UV–Vis, MeOH sacrificial	85.4 µmol g−1 h−1 (3 wt% CFs/TiO2), 2.87× vs. bare TiO2		[34]
Fe2O3/TiO2 immobilized on CFC	Visible light		97.54% color removal; ~84% COD removal (240 min)	[35]
UV-activated TiO2 coated felt (covalently anchored)	UV		95% Orange II in 180 min	[36]
TiO2 nanoparticles decorated on CFs	UV (λ ≈ 365 nm)		First-order kinetics	[37]
Defective TiO2/MoP heterojunction on CC	Solar light (panel; immobilized sheet)	14.7 µmol h−1 cm−2; ~19× higher than TiO2/CC		[38]
TiO2 nanorods (powder)	Vis (λ ≥ 400 nm), TEOA	0.66 mmol h−1 g−1		Present research
TiO2NR0.1/CC	Vis (λ ≥ 400 nm), TEOA	0.90 mmol h−1 g−1		Present research
TiO2NR0.3/CC	Vis (λ ≥ 400 nm), TEOA	2.66 mmol h−1 g−1	TiO2NR0.3/CC achieved 90.2% in 90 min	Present research

compares the TiO₂NRs/CC photocatalysts with previously reported TiO₂-based systems immobilized on carbon substrates. As seen in Table 1, the TiO₂NR₀.₃/CC developed in this work demonstrates significantly higher H₂ evolution rates and efficient dye degradation compared to earlier reports, underscoring the synergistic role of CC in enhancing photocatalytic performance.

2.7. Photocatalytic Mechanism

The enhanced photocatalytic activity of TiO₂ NRs grown on CC is attributed to the synergistic interplay between the 1D semiconductor nanostructures and the conductive carbon substrate. Under simulated solar irradiation, rutile TiO₂ NRs absorb photons with energies equal to or greater than their band gap, generating electron–hole (e⁻/h⁺) pairs according to the following process:

$$Ti{O}_2+hv\to e_{CB}^{-}+e_{VB}^{+}$$

(1)

The vertically aligned morphology of TiO₂ NRs facilitates the rapid transport of photogenerated electrons along the c-axis toward the CC substrate. Due to its excellent conductivity, CC acts as an electron reservoir and mediator, effectively extracting and shuttling electrons while simultaneously suppressing charge recombination. This efficient separation and migration of charge carriers is confirmed by the observed PL quenching and reduced charge transfer resistance in EIS measurements.

2.7.1. H₂ Evolution Pathway

The photogenerated electrons transferred from TiO₂ to the CC are rapidly delivered to protons (H⁺) in the aqueous medium, leading to H₂ evolution, as shown in the following equation.

$$2H^{+}+2e^{-}\to H_2\uparrow$$

(2)

Simultaneously, the photogenerated holes are consumed by TEOA, working as a sacrificial electron donor, as shown by the following equation.

$$h^{+}+TEA\to TEO{A*}^{+}\to Oxidation products$$

(3)

Through this synergistic charge separation mechanism, the H₂ generation efficiency is maximized, and TiO₂NR0.3/CC achieves the highest rate (2.66 mmol h⁻¹ g⁻¹).

2.7.2. Dye Degradation Pathway

Under visible-light irradiation, TiO₂ NRs generate charge carriers by following Equation 6. The electrons transferred to the CC reduce dissolved oxygen molecules to form reactive oxygen species (ROS), as shown in the following reactions,

$$O_2+e^{-}\to {•O}_2^{-}$$

(4)

$${•O}_2^{-}+H^{+}\to •H{O}_2$$

(5)

$$2•H{O}_2\to H_2{O}_2+O_2$$

(6)

$$H_2{O}_2+e^{-}\to •\mathrm{O}H+{OH}^{-}$$

(7)

Meanwhile, holes in TiO₂ oxidize water molecules or hydroxyl ions to produce hydroxyl radicals, as shown below,

$$h^{+}+H_{2}O/{OH}^{-}\to •\mathrm{O}H$$

(8)

The generated ROS ($•\mathrm{O}H$, ${•O}_2^{-}$, $H_2{O}_2$) and surface holes collectively degrade the methylene blue dye:

$$\mathrm{MB} + (•\mathrm{O}H/h^{+}/{•O}_2^{-}) \to {CO}_2+ H_{2}O+\mathrm{inorganic} \mathrm{ions}$$

(9)

This combined oxidative and reductive pathway results in rapid MB removal. Especially, TiO₂NR0.3/CC exhibits 90.2% degradation in 60 min.

3. Materials and Methodology

3.1. Materials

All chemicals were analytical grade from commercial suppliers. Tetrabutyl titanate (TBT, 98%, Alfa Aesar, Ward Hill, MA, USA) and titanium tetrachloride (TiCl₄) were titanium precursors for TiO₂ NRs synthesis. Carbon cloth from AvCarb, Lowell, MA, USA, was the substrate for TiO₂ coating. Hydrochloric acid (HCl, 35–38%) was a pH adjuster and catalyst; ethanol (99.9%) and acetone (99.5%) were solvents and cleaning agents. Triethanolamine (TEOA, 99%) served as an electrolyte and sacrificial agent for H₂ evolution and dye degradation. Methylene blue (MB, 95%) dye was used in degradation studies. All chemicals (i.e., TBT, TiCl₄, HCl, ethanol, TEOS, acetone, TEOA, MB, RhB) were purchased from S D Fine Chem Limited, Mumbai, India.

3.2. Synthesis of TiO₂NRs/CC

The TiO₂ NRs were grown on CC by a hydrothermal method. Initially, the CC was refluxed in concentrated HNO₃ acid at 120 °C for 4 h, during which the strong oxidizing conditions removed surface impurities and introduced oxygen-containing groups such as –COOH and –OH. These functional groups increased the hydrophilicity of the surface and provided active nucleation sites for TiO₂ growth [39]. The cleaned and dried carbon fabric was then immersed overnight in ethanol solutions containing TiCl₄ with concentrations of 0.1 M, 0.3 M, and 0.5 M. In ethanol, TiCl₄ was partially hydrolyzed to form amorphous Ti–O–Ti species, which uniformly coated the fiber and acted as seed layers [40]. The seeded fabric was annealed in a muffle furnace at 400 °C for 30 min, transforming the amorphous titanium species into crystalline TiO₂ nanocrystallites and improving adhesion to the carbon surface. In the next step, 4 mM of TBT, Ti(OC₄H₉)₄ was dissolved in a mixed solvent of 20 mL hydrochloric acid and 20 mL acetone, and stirred to obtain a clear solution. Here, acetone served as a cosolvent to slow down uncontrolled hydrolysis, while HCl provided both water and chloride ions: water molecules initiated the controlled hydrolysis of TBT, and Cl⁻ ions selectively adsorbed onto crystal facets to promote anisotropic growth [22]. This precursor solution and the seed-coated carbon fabric were transferred to a Teflon-lined autoclave and heated at 180 °C for 6 h. These conditions are consistent with the literature reports showing that rutile TiO₂ nanorods form preferentially in chloride-assisted acidic media under such conditions [41,42]. Under these circumstances, dissolved titanium species nucleated and preferentially grew into rod-like TiO₂ structures on the seeded surface, guided by the high acidity and chloride concentrations [43]. After slow cooling to room temperature, the white TiO₂ NRs were rinsed with DI water and ethanol to remove residual ions and organics, and then dried at 70 °C for 10 h to obtain a clean, uniform NRs array on the carbon fabric. The TiO₂ NRs grown on carbon fabric were designated according to the concentration of the TiCl₄ seed solution used during the seeding step. When 0.1 M TiCl₄ was employed, the sample was denoted as TiONR0.1/CC, while those prepared with 0.3 M and 0.5 M TiCl₄ seed solutions were labeled as TiONR0.3/CC and TiONR0.5/CC, respectively. This convention directly reflects the influence of the seeding concentration on the density and uniformity of TiO₂ NR growth on the CC substrate. The CC was weighed before and after the deposition of TiO₂ NRs. The bare CC was weighed before the growth of TiO₂ NRs. After hydrothermal deposition and subsequent treatment, the TiO₂ NRs NRs-decorated CC was dried and weighed again to determine the loading amount of TiO₂. Pristine TiO₂ NRs were synthesized without a CC substrate. The schematic illustration of the synthesis of TiO₂ NRs on CC is presented in Figure 8, with the key reactions outlined below.

• Hydrolysis of TiCl₄ in ethanol (during seeding),

$$Ti{Cl}_4+x{H}_{2}O\to Ti{O}_x{Cl}_{4-x}+xHCl$$

(10)

• Hydrolysis of TBT (in precursor solution),

$$Ti{{(OC}_4{H}_9)}_4+4H_{2}O\to Ti{\left(OH\right)}_4+4{(C}_4{H}_{9}OH)$$

(11)

• Condensation of hydroxide to TiO₂,

$$Ti{\left(OH\right)}_4\to Ti{O}_2+2H_{2}O$$

(12)

3.3. Characterization of TiO₂NRs/CC

The crystalline structure of the photocatalyst powders was characterized using X-ray diffraction (XRD) with a Bruker D8 Advance X-ray diffractometer. X-ray spectra were produced using a sealed tube with filtered Cu Kα radiation (wavelength 1.54 Å) operated at 40 kV and 40 mA. The surface morphology, including shape and size, of the heterostructure nanomaterials was examined using field emission gun scanning electron microscopy (FEG-SEM, JEOL JSM-7600F, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F30, Minneapolis, MN, USA) at an accelerating voltage of 300 kV. Optical absorption properties of the photocatalysts were measured using a PerkinElmer Lambda 950 UV-Vis-NIR optical spectrophotometer (New York, NY, USA). The photoluminescence (PL) properties were evaluated using a Jasco V-770 spectrofluorometer (Tokyo, Japan) at an excitation wavelength of 390 nm under ambient conditions. Additionally, X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Supra (Kratos Analytical, Stretford, UK) equipped with an Al Kα source to investigate the surface electronic states.

3.4. Photocatalytic Hydrogen (H₂) Evolution Using TiO₂NRs/CC

The photocatalytic H₂ evolution activity of the as-prepared photocatalysts was evaluated in a 30 mL splitable borosilicate glass reactor under visible-light irradiation. A TiO₂ NRs grown on a CC (3 cm diameter circle) was placed on a polyurethane foam ring to float on the surface of a 20 mL aqueous solution (90 vol% H₂O, 10 vol% triethanolamine (TEOA) used as the electrolyte. The light source was a 400 W lamp (HPL-N 400 W) with a cutoff filter ≥420 nm. Before light irradiation, the reactor was purged with nitrogen gas for 30 min to remove dissolved oxygen and exposed to light irradiation with an intensity of 100 mW/cm² measured by using an optical power meter (CEL-NP2000-2). The evolved H₂ gas was quantified hourly using gas chromatography (Agilent GC 7820A) equipped with a thermal conductivity detector, with nitrogen (N₂) as the carrier gas. No noble metal co-catalysts were added to enhance the photocatalytic H₂ evolution rate. Additionally, the photocatalytic degradation of organic pollutants in water was investigated to assess the dual functional properties of the photocatalytic substrate. The photodegradation of methylene blue (MB) dye was conducted under the same light source and irradiation intensity of the same light source, using the same TiO₂ NRs grown on CC for H₂ evolution. The photocatalytic substrate was floated on 100 mL of 0.01 gL⁻¹ MB dye solution. Before light exposure, the dye solution was stirred magnetically in dark conditions for 20 min to establish adsorption–desorption equilibrium. After specific intervals of light irradiation, the reaction solution was analyzed using a UV–Vis spectrophotometer (centered at a wavelength of 664 nm). The photocatalytic dye degradation efficiency (η) was calculated using the following equation:

$$\eta ={\displaystyle \frac{C_0-C_t}{C_0}}\times 100\%$$

(13)

where $C_0$ is the initial concentration of MB dye after adsorption–desorption equilibrium, and $C_t$ is the concentration of dye after a certain time under light irradiation.

3.5. Electrochemical Measurement

Electrochemical experiments were conducted using a PGSTAT 302N workstation configured with a standard three-electrode system, comprising an Ag/AgCl (3 M KCl) reference electrode for stable potential, a platinum wire counter electrode to complete the circuit, and a TiO₂NRs/CC working electrode. The TiO₂NRs/CC was submerged in a 0.5 M Na₂SO₄ aqueous electrolyte solution (pH 6.8) to maintain consistent experimental conditions. Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 100 kHz to 10 mHz to evaluate charge transfer and impedance characteristics. Mott–Schottky analysis was conducted to measure the flat-band potential and charge carrier density of the TiO₂NRs/CC electrode, critical for understanding its semiconductor properties. This analysis was executed at a fixed frequency of 1 kHz.

4. Conclusions

In this study, rutile TiO₂ NRs arrays were successfully synthesized for the first time on CC substrates via a TiCl₄-assisted hydrothermal route, with different seeding concentrations. The structural and spectroscopic analyses confirmed the formation of vertically aligned, single-crystalline rutile TiO₂ nanorods strongly anchored to the conductive carbon framework. Among the synthesized samples, TiO₂NR0.3/CC exhibited the most favorable balance between nanorod density, interfacial contact, and charge transport characteristics. Studies on optical studies revealed a red-shifted absorption edge and narrowed band gap relative to bare TiO₂ NRs; in addition, photoluminescence quenching and electrochemical impedance spectroscopy confirmed the efficient suppression of electron–hole recombination and reduced interfacial resistance. These enhancements give rise to superior photocatalytic activity, achieving a maximum H₂ evolution and rate with TiO₂NR0.3/CC photocatalysts, along with outstanding cyclic stability across multiple cyclic operations. Parallel degradation studies on TiO₂NR0.3/CC of methylene blue under visible light demonstrate 90.2% dye removal within 60 min, while maintaining good activity after three consecutive cycles. These results unequivocally demonstrate that CC serves as a passive support and as an active electron mediator, enhancing charge separation and promoting durable photocatalysis. The synergistic integration of TiO₂ NRs with CC effectively addresses the intrinsic drawbacks of pristine TiO₂ NRs, including limited solar absorption, poor charge transport, and difficulties in catalyst recovery. The integration of 1D TiO₂ NRs with flexible CC would provide a recyclable, structurally robust, and high-performance photocatalytic platform. Thus, the present work establishes a new promising pathway for the design of multifunctional photocatalysts for solar-driven H₂ production and wastewater remediation, with broad implications for energy conversion and environmental sustainability.