Assembled Porphyrin Nanofiber on the Surface of g-C₃N₄ Nanomaterials for Enhanced Photocatalytic Degradation of Organic Dyes

Abstract

In this work, a g-C₃N₄/porphyrin nanocomposite was fabricated through the self-assembling of monomeric Tetrakis (4-carboxyphenyl) porphyrin (TCPP) molecules with g-C₃N₄ nanomaterials. The characterizing results showed a good distribution of TCPP nanofibers with a diameter of < 100 nm and several micrometers in length on the g-C₃N₄ nanoflakes’ surfaces. The prepared g-C₃N₄/porphyrin nanocomposite had two bandgap energies of 2.38 and 2.7 eV, which could harvest a wide range of photon energy in the light spectrum, particularly in visible light. The obtained C₃N₄/TCPP nanocomposite revealed a remarkable photodegradation efficiency toward rhodamine B dyes, with a RhB removing rate of 3.3 × 10⁻² min⁻¹. The plausible mechanism for the photocatalytic performance of the g-C₃N₄/porphyrin photocatalyst for the RhB dye’s degradation was also studied and discussed.

1. Introduction

The development of global economics has caused many environmental issues d the discarding of pollutants in wastewater, posing serious threats to human and nature’s health [1,2,3]. Of the various pollutants disposed of in wastewater, organic dyes, originally derived from paper, food, mining, printing leather, and pharmaceutical industries, are highly toxic and contain carcinogenic agents and non-biodegradables. Most common organic dyes found in industrial wastewater can be listed as azo, phenothiazine, and triphenylmethane derivatives [4]. Among these, a commonly used fluorescent label or colorant in industries is rhodamine B (RhB). This organic dye is highly soluble in water and possesses extreme toxicity, being harmful to human health and the environment and posing risks such as teratogenicity, carcinogenicity, and mutagenicity [5,6,7,8,9,10,11,12]. Many strategies have been effectively utilized thus far for removing RhB dye from wastewater, such as redox degradation, photodegradation, coagulation, adsorption, and mixed approaches [13,14,15]. Each method has various advantages as well as disadvantages. Among them, the photodegradation process has been considered a promising pathway for RhB removal in aqueous media, using photon energy from the solar spectrum [16,17]. However, most photocatalysts for dye degradation can only harvest photon energy in ultraviolet light due to their large bandgap energy [18]. Therefore, there is an urgency to develop a photocatalyst that is able to absorb photon energy in the visible light region (approximately 42.3% in the solar spectrum).

Organic semiconductor nanostructures with low bandgap energy activated in the visible light region and high absorbing efficiency for photon energy have been extensively employed in photocatalysis. Among organic-based semiconductor photocatalysts, soft solid-state materials constructed from basic organic building blocks of $\pi$-conjugated porphyrin derivatives exhibit promising photocatalytic activity [19,20,21,22,23,24,25]. Many approaches have been effectively employed to prepare the $\pi$-conjugated organic semiconductors for photocatalysis applications [26,27]. Among these, the self-assembly of supramolecules is one of the most used methodologies to design and prepare soft nanostructured materials from monomeric organic molecules, including porphyrin derivatives. Porphyrin-based nanomaterials have been considered a new advanced nanomaterial for many applications, especially in photocatalysis. Porphyrin supramolecular nanostructures are commonly obtained via self-assembly from monomeric molecules and employed as photocatalysts. In order to enhance the photocatalytic activity, porphyrin nanostructures could be incorporated with other inorganic semiconductors to enable photocatalytic efficiency in a wide solar spectrum range and to improve the charge separation. Ionic self-assembly, reprecipitation, and surfactant-assisted self-assembly (SAS) are common self-assembly protocols used for the preparation of porphyrin nanostructures with controlled morphologies [28,29,30,31]. Based on each self-assembly technique, porphyrin monomers were designed and synthesized to obtain desired nanostructures. Self-assembled porphyrin nanostructures have been evidenced to be novel photocatalysts for application in environmental treatments [32]. For example, nanostructured porphyrin photocatalysts successfully obtained via self-assembly in the shape of microspheres, octahedra, and nanosheets exhibited remarkable performance for the photodegradation of several organic dyes [33]. However, free-standing nanostructured porphyrin photocatalysts have been hindered from practical application because of their low stability and quick recombining of electrons and holes. Thus, the incorporation of free-standing aggregates with other inorganic semiconductors could be a promising strategy to enhance charge separation, photon energy absorption, and stability. Our group also produced a range of porphyrin-based nanomaterials in regard to incorporate with several inorganic semiconductors, which revealed the enhanced photodegradation efficiency of organic dyes [30,34,35,36,37].

With low bandgap energy activated in the visible light region, high chemical and thermal stabilities, graphitic carbon nitride (g-C₃N₄) is an inorganic semiconductor, which has been extensively studied for photocatalytic applications such as water splitting, pollutant removal, photovoltaic solar cells, and chemical sensors [38,39,40]. Many approaches are available for the preparation of the g-C₃N₄ material, including, but not limited to, solution solvothermal methods, microwave heating-assisted approaches, mixing processes, hydrolysis, the sol-gel process, and mechanical grinding [41]. The g-C₃N₄-based nanocomposite showed high removal performance toward organic dyes in an aqueous solution [42]. However, a low surface area, fast recombining of generated electrons and holes, and a low photon energy-harvesting capability have limited the use of the g-C₃N₄ photocatalyst for pollutant degradation in practice [43]. To tackle these disadvantages, g-C₃N₄ could be doped (metal or nonmetal doping) or incorporated with other semiconductors to improve the photocatalytic efficiency. For example, the g-C₃N₄/NiSO₄ nanocomposite was facilely fabricated with a solvent evaporation method, which revealed a rapid removal of rhodamine B (RhB), Congo red (CR), methylene blue (MB), and indigo carmine (IC) under sunlight irradiation [44]. Durga et al. also incorporated g-C₃N₄ with Ca₂Fe₂O₅ to obtain the g-C₃N₄/Ca₂Fe₂O₅ nanocomposite [45]. The resultant nanocomposite exhibited an improved removal efficiency of up to approximately 96% toward methylene blue. However, most of the semiconductors incorporated with g-C₃N₄ were inorganic semiconductors; no organic semiconductors have been reported for incorporation with g-C₃N₄ for enhancing photocatalytic performance.

Herein, we propose a simple strategy to synthesize a g-C₃N₄/porphyrin hybrid material via the self-assembling of TCPP monomers with g-C₃N₄ nanomaterials. The resultant nanocomposite is well-characterized using scanning electron microscopy (SEM), FTIR spectroscopy, X-ray diffraction (XRD), and a UV-Vis spectrophotometer. The photodegradation evaluation of the synthesized nanomaterial was investigated for RhB removal. This work also proposes and discusses the plausible photodegradation mechanism of RhB using the g-C₃N₄/porphyrin photocatalyst.

2. Results and Discussion

The UV-Vis spectrophotometer is a reliable tool for investigating optical characteristics of materials. The optical properties of the TCPP monomers, free-standing assembled TCPP nanofibers, and g-C₃N₄/porphyrin materials were studied with UV–Vis spectra, and the result is shown in Figure 1. The UV-Vis spectra of the monomeric TCPP molecules revealed a sharp peak at approximately 412 nm, which was due to the transition of a_1u ($\pi$) to e^*_g($\pi$), which is the absorption of the Soret band. The a_2u ($\pi$) to e^*_g($\pi$) transition in porphyrin molecules was also presented, with the appearance of four weak Q-absorption bands ranging from 500 to 700 nm [46,47]. Interestingly, after the self-assembly, a strong peak at approximately 418 nm in the UV-Vis spectra of the TCPP aggregates was observed, which was assigned to the Soret absorption peaks, with a significant decrease in the peak’s intensity, which was primarily evident for the self-assembly of the TCPP monomer. The distinct bathochromic and hypsochromic shift in the Soret absorption band from 412 nm to 418 nm (blue shift) before and after the self-assembly indicated the supramolecular assembly of the monomeric porphyrin molecules following the J-type aggregates [31,48,49]. Furthermore, four Q-absorption bands in the monomeric porphyrin molecule transformed into one band at approximately 664 nm after aggregation, which was considered further evidence for the J-type aggregates. After the TCPP monomers self-assembled on the surface of g-C₃N₄ the UV-Vis spectrum of the hybrid material showed similar characteristic peaks, as evidenced in the TCPP aggregates’ UV-Vis spectra. The lower intensity of absorption peaks of the hybrid material’s UV-Vis spectrum in comparison to that of the TCPP aggregates demonstrated that the TCPP aggregates were well-integrated with the g-C₃N₄ nanomaterials.

The morphologies of the free-standing TCPP aggregates, g-C₃N₄, and g-C₃N₄/porphyrin hybrid material were studied using the SEM images (Figure 2). Figure 2a shows the fiber nanostructure of the free-standing TCPP aggregates of less than 100 nm in diameter and several micrometers in length. The g-C₃N₄ had a flake shape with a nanoscale thickness and a diameter of approximately 0.5 to 2 µm (Figure 2b). Figure 2c,d show the low and high SEM images of the g-C₃N₄/porphyrin. It could be observed that the assembled TCPP nanofibers were uniformly distributed on the surface of the g-C₃N₄ nanoflakes while retaining the morphologies of the free-standing porphyrin nanofibers (Figure 2a) and g-C₃N₄ (Figure 2b). The result was in good agreement with the UV-Vis results, and confirmed the successful synthesis of a well-defined g-C₃N₄/porphyrin hybrid material via self-assembly. The EDS analysis of g-C₃N₄ revealed the atomic percentages of C and N to be 36.62 and 63.38%, respectively, (Figure S1). The self-assembly of g-C₃N₄ with porphyrin witnessed an increase in the C atomic percentage (44.06%) and a decrease in the N atomic percentage (46.22%), as evidenced for the presence of porphyrin in the composite (Figure S2). The O atomic percentage of approximately 8.6% was further evidence of the presence of porphyrin, as it was the main element in the TCPP porphyrin molecules.

The bonding nature of the g-C₃N₄/porphyrin hybrid material was investigated with FTIR spectroscopy, and the result is shown in Figure 3. The FTIR spectrum exhibited a broad absorption band in the range of 3000 to 3500 cm⁻¹, which was attributed to the characteristic vibrations of the -OH and -COOH groups in porphyrin molecules and to the moisture. The intense vibration peaks at 1410 and 1640 cm⁻¹ were attributed to the C-O and C=O bonds in the carbonyl groups in the porphyrin molecules, respectively [29]. Firm peaks ranging from 1200 to 1650 cm⁻¹ were ascribed to the vibrations of the C-N heterocycles, as well as heptazine-derived repeating units [50]. The presence of vibration bands at 1321 and 1242 cm⁻¹ were attributed to the stretching vibration modes of the C-NH-C bridge or C-N(-C) trigonal units in g-C₃N₄ [51]. The characteristic vibration of the tri-s-triazine modes was also observed, with the appearance of a stretching band at 812 cm⁻¹. These results further demonstrated that the assembled TCPP porphyrin nanofibers were well-incorporated with the g-C₃N₄ nanoparticles to form a hybrid material. The FTIR spectrum of the monomeric TCPP porphyrin monomers showed similar characteristic vibration bands of porphyrin aggregates in the g-C₃N₄/porphyrin hybrid material, with a small shift, which demonstrated the successful self-assembly of porphyrin monomers into the porphyrin nanofibers.

The phase’s characteristics and structure of the hybrid material were studied using the X-ray diffraction (XRD) technique. Illustrated in Figure 4 are the XRD patterns of the prepared samples. The XRD of the monomeric porphyrin molecule revealed no diffraction peaks, conveying the amorphous nature of the porphyrin monomer. The XRD pattern of g-C₃N₄ (black line) depicted diffraction peaks at approximately 13 and 27.8°, which were in good agreement with the g-C₃N₄ material reported previously [52,53,54]. The strong peak at 27.8° attributed to the plane of (002) in g-C₃N₄ resulted from the layer stacking of the aromatic system. The weak peak at approximately 13^o was assigned to the (100) plane (JCPDS NO. 87-1526), which resulted from the tri-s-triazine unit stacking. The XRD pattern of the g-C₃N₄/porphyrin hybrid material also showed characteristic peaks of g-C₃N₄ at approximately 13 and 27.8°. The XRD pattern of the hybrid material showed a small shift in the diffraction peaks of g-C₃N₄, which might be attributed to the electronic interaction between porphyrin and g-C₃N₄. Intriguingly, several additional peaks with an asterisk at two theta values of 19 and 22° were also observed in the hybrid material’s XRD pattern, which belonged to the porphyrin aggregates. This result demonstrated the crystalline nature of the porphyrin aggregates [30,36], which was additional confirmation for the successful self-assembling of the TCPP monomer on the surface of g-C₃N₄.

The optical properties of the samples were further studied with the diffuse reflectance UV-Vis spectrophotometer. The result is depicted in Figure 5a. The spectrum of the free-standing g-C₃N₄ showed strong absorption in the ultraviolet region (300–400 nm). Interestingly, the diffuse reflectance UV-Vis spectrum of the g-C₃N₄/porphyrin hybrid material showed two broad peaks at approximately 450 nm and 650 nm, indicating that the hybrid material could favorably absorb light in the visible light and near-UV light region. This broad wavelength absorption in the light region was significantly favorable for photocatalytic applications. The difference between the photon energy and bandgaps was responsible for the optical absorption strength of the material, following the below equation:

$${\left(\mathrm{\alpha }\mathrm{h}\mathrm{\upsilon }\right)}^{1/\mathrm{n}}=\mathrm{A}\left(\mathrm{h}\mathrm{\upsilon }-{\mathrm{E}}_{\mathrm{g}}\right)$$

(1)

where E_g is the bandgap energy, α is the coefficient of the absorption band, h is Planck’s constant (6626.10⁻³⁴ Js), υ is the speed of light (3.108 ms⁻¹), and A is the absorption intensity. The Tauc plot was drawn based on (αhυ)² versus hυ, and, then, the bandgap energy of the material was determined using the Tauc plot. Figure 5b shows the Tauc plots of the free-standing g-C₃N₄ and g-C₃N₄/porphyrin hybrid material. The bandgap energies of g-C₃N₄ were calculated from the Tauc plot to be approximately 2.91 eV, only able to harvest the energy of photon energy from ultraviolet light. From the Tauc plot, two bandgap energies of the g-C₃N₄/porphyrin hybrid material were determined to be 2.38 and 2.7 eV, which were favorable for harvesting photon energy from visible and near-ultraviolet light. This result implied that the successful integration of the g-C₃N₄ and TCPP porphyrin aggregates could create a novel photocatalyst with bandgap energies in a wide range of light regions, particularly in visible light.

The photodegradation performance of the prepared g-C₃N₄/porphyrin hybrid material was investigated for RhB removal under simulated sunlight irradiation. Figure 6 reveals the effect of the g-C₃N₄ loading on the photodegradation behavior of the hybrid material. It was evident that the photodegradation performance of the g-C₃N₄/porphyrin hybrid material toward RhB was remarkably higher than that of the control, free-standing g-C₃N₄, and TCPP porphyrin aggregates. The RhB removal percentages of the free-standing g-C₃N₄ and TCPP aggregates were determined to be 38 and 42%, respectively, under photocatalytic experimental conditions. When g-C₃N₄ proportions in the g-C₃N₄/porphyrin increased, the photocatalytic degrading efficiency of RhB also increased. At a TCPP:g-C₃N₄ weight ratio of 1:1, the RhB removal percentage was approximately 71%, and enhanced significantly at a ratio of 1:2, with a degrading efficiency of 89%. The RhB degrading efficiency reached a maximum at a TCPP:g-C₃N₄ ratio of 1:3, with a removal percentage of 98%. When the g-C₃N₄ contents further increased, the removal efficiency of the RhB slightly decreased. Thus, the TCPP:g-C₃N₄ ratio of 1:3 was considered to be the optimal proportion of g-C₃N₄ in the hybrid material, and this ratio was selected for the preparation of a hybrid material for further photocatalytic studies.

The g-C₃N₄ with bandgap energies in the range of 2–3 eV has been extensively used as an effective photocatalyst under near-UV and visible light irradiation [55,56]. It has been evidenced that assembled porphyrin has a molecular structure similar to chlorophyll (photoactive molecules), which is a major component participating in many processes of biological energy transduction in algae and plants [57,58]. The bandgap energies of the hybrid material were determined to be 2.38 and 2.7 eV, indicating the effective harvesting of photon energy from a wide range of light regions, especially from the light in the visible light spectrum. Therefore, in this work, photocatalytic experiments were performed for the degradation of RhB using the prepared g-C₃N₄/porphyrin photocatalyst under simulated sunlight irradiation. The result is depicted in Figure 7. It can be seen from Figure 7a that the hybrid material exhibited remarkable photodegradation performance, with a RhB removal efficiency of almost 100% after irradiating with the simulated sunlight for 120 min. Illustrated in Figure 7b is the ln (C_t/C₀) vs. time plot for studying the photocatalytic reaction’s kinetics, in which C_t is the absorption peak’s intensity at time t, and C₀ is the absorption peak’s intensity at time zero. The photocatalytic performance was evaluated through the changes in the RhB absorption peak at a wavelength of 553 nm as a function of time. Under experimental conditions, the negligible self-sensitized degradation of RhB was observed with no decrease in the concentration of RhB. When free-standing g-C₃N₄ and TCPP porphyrin aggregates were used as photocatalysts, the RhB concentrations remarkably decreased, with a rate constant of cal. 3.75 × 10⁻³ and 5 × 10⁻³ min⁻¹, respectively. Intriguingly, when the g-C₃N₄/porphyrin hybrid material was employed as a photocatalyst, the RhB removal percentage remarkably increased, with a rate constant calculated to be approximately 3.3 × 10⁻² min⁻¹, which was approximately six- and ten-fold higher compared to the g-C₃N₄ and TCPP aggregates, respectively. In comparison to the other assembled porphyrin-based nanomaterials reported in the literature, this rate constant for the degradation of RhB using the g-C₃N₄/porphyrin hybrid material was higher, and much higher than the commercial photocatalyst of TiO₂ [30,34,36,59].

It is well-known that J-type porphyrin aggregates are of a photo-semiconductor property, because of their robust $\pi –\pi$ intermolecular interactions in the $\pi$- conjugate of the porphyrin molecules [20,31,37]. Due to the photo-semiconductor property, porphyrin aggregates via self-assembly can receive energy from photons to enable electrons to move to conduction bands from valence bands and form electron and hole pairs for the photocatalytic reaction [29,49,60]. Additionally, the photocatalytic performance of hybrid materials remarkably increased due to the improvement of charge separation induced through processes of an exciton-coupled charge transfer between the g-C₃N₄ and porphyrin aggregates [61,62]. The g-C₃N₄/porphyrin hybrid material had two bandgap energies of 2.38 and 2.7 eV, which enabled the photocatalyst to receive energy from photons in a wide range of the light spectrum, especially with visible light. Figure 8 proposes the possible photodegrading mechanism of the g-C₃N₄/porphyrin hybrid material for the removal of the RhB organic dye. When g-C₃N₄/porphyrin was irradiated with the sunlight region, the g-C₃N₄ and TCPP nanofibers harvested photon energy to enable the jumping of electrons from the valence band to the conduction band, generating electron/hole pairs [29]. Thanks to processes of the exciton-coupled charge transfer, newly generated electrons diffused from the conduction band of the g-C₃N₄ to the TCPP aggregates, and holes formed the valence band of the TCPP nanofibers to the g-C₃N₄ through the interface between the two materials. These transfer processes significantly avoided the electron and hole recombination; as a result, more electrons and holes could participate in the photocatalysis. Remarkably, the electrons reduced O₂ and H₂O to form free radicals, such as ^•O₂⁻, OH^•…; these free radicals oxidized the RhB dye to fewer toxic compounds. The holes also migrated and directly oxidized the RhB molecules to unharmed products.

3. Materials and Methods

3.1. Chemicals

Shanghai Macklin Biochemical Company provided monomeric tetrakis (4-carboxyphenyl) porphyrin (TCPP) molecules. Hydrochloric acid (HCl, 99%), ethanol (99%), sodium hydroxide (NaOH, 99%), and urea (CH₄N₂O, 99%) were purchased from Xilong Chemical Co., Ltd., Guangdong, China. All the chemicals without further purification and double-distilled water were employed for all experiments.

3.2. Preparation of g-C₃N₄ Nanomaterial

The g-C₃N₄ nanomaterial was prepared through the direct annealing of the urea compound. As in a typical process, a crucible with a cover containing 10 g of urea was heated at 550 °C for 3 h. The crucible was then naturally cooled to room temperature, after which a yellow powder was obtained as raw pristine. The raw g-C₃N₄ was washed with distilled water several times and ethanol to remove impurities. After that, the sample was completely dried at 70 °C for five hours.

3.3. Synthesis of g-C₃N₄/Porphyrin

An acid-based neutralization-induced self-assembly approach was employed to synthesize the g-C₃N₄@porphyrin hybrid material. As in a typical process, 8 mg TCPP was dissolved in 2 mL NaOH 0.1M to obtain solution A. Then, different proportions of the g-C₃N₄ nanomaterial were added into solution A, with various TCPP:g-C₃N₄ weight ratios of 0.5:1, 1:1, 1:2, 1:4, and 1:6. The resultant mixture was thoroughly mixed for 20 min through bath sonicating to obtain a homogeneous mixture of the g-C₃N₄ and porphyrin monomers. HCl 0.1M was gradually introduced to the mixture to induce the reprecipitation self-assembly of the porphyrin nanofibers on the surface of g-C₃N₄. The pH value was then adjusted to a pH of 6–7. The reprecipitates were collected with the use of centrifugation, thoroughly washed with double-distilled water, and dried at 60 °C for 12 h. The samples were stored at room temperature, in ambient humidity for other testing.

3.4. Material Characterization Techniques

The X′Pert PRO PANalytical instrument with a 0.15405 nm Cu-Kα radiation source was employed to obtain an X-ray diffraction (XRD) from the investigation of the structure and crystallinity of the C₃N₄@porphyrin hybrid material. Scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX, Hitachi S-4600, Fukuoka, Japan) were utilized to observe the morphology and the elemental composition of the sample. FTIR spectra were measured in a TENSOR II (Bruker) IR spectrometer by using KBr pellets. A UV-vis spectrophotometer (Jasco V730) in the range of 200–900 nm was used to obtain the optical absorption spectra of the materials. A UV-vis spectrophotometer was also used to study the photodegradation efficiency of the g-C₃N₄@porphyrin hybrid materials to remove RhB dye at an absorption wavelength of 553 nm.

3.5. Photocatalytic Experiments

The photodegradation efficiency of the g-C₃N₄@porphyrin hybrid materials for the removal of a simulated pollutant using RhB organic dye was studied under simulated sunlight irradiation generated from a 350 W xenon lamp (China, 350 W). Following the typical procedure, 0.4 mg of the g-C₃N₄@porphyrin hybrid material was introduced into 20 mL of a 10 ppm RhB solution. The adsorption equilibrium state of the hybrid material was established by placing the mixture in the dark for 2 h, before irradiating with light. The mixture was taken out after each time point and the UV–Vis spectra recorded for each sample. For comparison, the photodegradation performance of the free-standing g-C₃N₄ and TCPP nanofibers in the removal of RhB was obtained following the same photocatalytic protocol with the hybrid material.

4. Conclusions

A facile, self-assembling approach was successfully employed to fabricate the g-C₃N₄/porphyrin hybrid material through self-assembling TCPP porphyrin monomers on g-C₃N₄ nanoflake surfaces. The prepared porphyrin aggregates in a fiber shape of less than 100 nm in diameter and several micrometers in length were shown to be well-integrated in the g-C₃N₄ nanomaterials. The TCPP porphyrin aggregates were determined to form on the g-C₃N_4′s via J-type self-assembling pathway. The bandgap energies of the g-C₃N₄/porphyrin hybrid material were calculated to be 2.38 and 2.7 eV, which could make the hybrid material an efficient photocatalyst in a broad light spectrum range. Obviously, the hybrid material exhibited an enhanced photodegradation efficiency toward the rhodamine B dye with a degrading speed of up to 3.3 × 10⁻² min⁻¹, which was superior to other porphyrin-based photocatalysts reported previously. The enhanced photodegradation efficiency was due to the improved harvesting of photon energy, as well as the enhanced charge separation due to the exciton-coupled charge transfer processes between the g-C₃N₄ and TCPP aggregates. With a facile fabricating method and highly photocatalytic performance, the g-C₃N₄/porphyrin hybrid material is considered an advanced photocatalyst for many applications, such as environmental remediation and water splitting.