Enhanced light absorption and charge carrier’s separation in g-C₃N₄-based double Z-scheme heterostructure photocatalyst for efficient degradation of navy-blue dye

ABSTRACT

Effective wastewater treatment is essential due to the dye environmental threats. Photocatalysis offers an eco-friendly dye degradation solution. Herein, we report a double Z-scheme photocatalyst, i.e. LaFeO₃/Boron-doped g-C₃N₄/Fe₂O₃ (LFO/B-CN/FO), fabricated via a hydrothermal method. The formation of this ternary system is supported by structural, surface, and morphological analyses. Differential scanning calorimetry and thermal gravimetric analysis demonstrated the crystallization behavior of the composite. Because of synergetic effects in the ternary composite and boron doping, the optical band gap energy is reduced to 1.6 eV. Under visible light irradiation, the LFO/B-CN/FO nanocomposite degraded navy-blue dye by 98% in 130 min at neutral pH, surpassing CN, B-CN, and binary composites of B-CN with LFO and FO. The degradation process was rationalized using pseudo-first- and pseudo-second-order kinetic models, which confirmed high catalytic efficiency. The double Z-scheme heterojunction enhanced light absorption, charge separation, and carrier lifetime, hence improving the photocatalytic activity. Overall, the as-fabricated LFO/B-CN/FO ternary composite outperformed reference catalysts in terms of chemical reactivity, optical characteristics, and electrical performance. This work opens up a new route for the design and fabrication of double Z-scheme ternary composite photocatalysts for potential wastewater treatment and renewable energy applications.

GRAPHICAL ABSTRACT

KEYWORDS:

• Doping
• double Z-scheme
• photocatalysis
• pseudo-kinetics
• wastewater treatment

Introduction

Earth is currently facing major environmental problems as a result of the direct release of industrial toxins into the ecosystem (1). Rapid industrial growth and increasing anthropogenic pollution are major problems that are endangering the ecology, especially water bodies, and putting humans and other living things at risk (2). Water contains a range of contaminants, including heavy metal ions, pharmaceuticals, and chemical dyes. Over time, the need of clean water has been rising while its availability is on the decline. Therefore, in order to meet the demand for water, it is necessary to purify wastewater and make it suitable for consumption by humans and other organisms (3). Given the increasing environmental pollution caused by organic dyes, it is imperative to create photocatalytic technology that can efficiently transform hazardous dyes into water, carbon dioxide, and less toxic chemical compounds. Thus, in order to fulfill these requirements, it is necessary to develop a highly efficient visible light active photocatalyst that produces electron–hole (e⁻−h⁺) pairs for redox reactions. Furthermore, it has been noted that the utilization of conventional photocatalysts like CeO₂, TiO₂, and ZnO, among others, exhibits limited efficiency in harnessing visible light and experiences rapid recombination of photoinduced electron–hole pairs, hence reducing their practical applicability. Hence, our future strategy should prioritize the development of a novel photocatalyst that exhibits enhanced optical activity when exposed to visible light (4).

Researchers investigated that the design and fabrication of 2D g-C₃N₄-based heterostructure photocatalysts have the potential to act as highly efficient visible light active catalysts. They possess significant potential and provide benefits such as a large specific surface area, small charge diffusion distance, and excellent redox capability. However, rapid recombination of charges hindered the photocatalytic performance of individual 2D g-C₃N₄ nanosheets. Various methods, such as elemental doping, co-catalyst introduction, and heterojunction construction, have been extensively investigated to address this limitation. The synthesis of Z-scheme heterojunctions, composed of two or more semiconductors having a staggered band structure, has shown significant and promising steps forward. While compared to conventional type-II heterojunction photocatalysts such as g-C₃N₄/ZnO, TiO₂/g-C₃N₄, and g-C₃N₄/Ag₂CrO₄ etc., Z-scheme systems including WO₃/g-C₃N₄, SnS₂/g-C₃N₄, etc. demonstrate exceptional photocatalytic redox features upon stimulation by light, resulting in improved charge separation (5).

In order to improve the ability of Z-scheme system to absorb more light, efficiently separate charge carriers, and retain strong redox features, a new photocatalytic system based on g-C₃N₄, combined with metal oxides or nonmetal oxides, must be designed. The purpose of this system is to effectively utilize solar energy and improve the redox capabilities in Z-scheme heterojunctions based on g-C₃N₄ (6). A ternary Z-scheme photocatalyst, made up of BiFeO₃-g-C₃N₄-WO₃, was prepared using a wet chemical technique and subsequently utilized for water splitting to evolve H₂ and 2,4-dichlorophenol degradation, achieving rates of 90 μmol·h^–1·g^–1 and 63%, respectively. The improved performance, in comparison to pure g-C₃N₄, was attributed to the interfacial contacts between g-C₃N₄ and coupled BiFeO₃ and WO₃ constituents (7). Likewise, carbamazepine (CBZ) was degraded by Co₃O₄/CuBi₂O₄/SmVO₄, a dual Z-scheme heterojunction photocatalyst, under visible light, by 76.1% ± 3.81. The Z-scheme heterojunction's ability to promote effective electron–hole separation, better interfacial contact, and visible light absorption is responsible for this improvement (8). Reduced graphene oxide (rGO) was utilized as an electron mediator in the synthesis of the Z-scheme ternary photocatalyst ZnMoO₄/BiFeWO₆/rGO. Anthraquinonic acid blue 25 (AB 25) was degraded by ZM/BFW/rGO with remarkable photocatalytic performance; the degradation percentage was higher than that of the ZM (24%) and BFW (36%), when compared across 180 min. In addition, to confirm its promise for scalable wastewater treatment applications, the photocatalyst showed excellent stability with 89% efficiency after four cycles (9).

Likewise, a double Z-scheme heterojunction i.e. LaFeO₃/g-C₃N₄/BiFeO₃ was fabricated via a wet chemical technique and used for the degradation of ciprofloxacin. It was observed that the ternary nanocomposite exhibits superior performance compared to the individual counterparts i.e. pure g-C₃N₄, LaFeO₃, BiFeO₃ (10). Furthermore, a ternary composite i.e. g-C₃N₄/Ag₃PO₄/AgI was fabricated and utilized in the photocatalytic degradation of nitenpyram. The resultant photocatalyst revealed exceptional performance compared to the individual coupling counterparts. A double z-scheme WO₃/Bi₂O₃/g-C₃N₄ photocatalyst was synthesized and utilized in efficient visible light catalytic degradation of tetracycline pollutants. Similarly, α-Fe₂O₃/g-C₃N₄/TiO₂ ternary nanocomposite was fabricated via the two steps hydrothermal and calcination methods and utilized for the removal of Rhodamine B dye. The as-fabricated photocatalyst revealed 95.7% dye removal performance in only 50 min which was attributed to the synergetic interaction among the TiO₂, g-C₃N₄, and α-Fe₂O₃ counterparts (11).

The LaFeO₃ perovskite semiconductor has outstanding performance in advanced oxidation processes, especially in the elimination of organic pollutants, degradation of harmful gases, and producing hydrogen fuel through water splitting. Its small band gap of 2.1 eV, good stability, low cost, and nontoxic nature make it a desirable catalyst. Nevertheless, there are certain challenges to overcome, such as low surface area, high charge carrier’s recombination, and limited visible light response, which ultimately boosts the photocatalytic efficiency. Fabricating Z-scheme heterojunction between g-C₃N₄ and LaFeO₃ can remarkably improve the charge carrier’s separation, as a result, the photocatalytic performance of g-C₃N₄ can be enhanced. Similarly, the coupling of hematite (α-Fe₂O₃), a semiconductor photocatalyst with a band gap of 2.2 eV, can also improve the performance of g-C₃N₄ (12).

Thus, as expected, coupling of LaFeO₃ and Fe₂O₃ with boron-doped g-C₃N₄ can result in a ternary double Z-scheme heterojunction which can significantly promote the photocatalytic performance of g-C₃N₄. Since g-C₃N₄ has a highly negative conduction band potential (−1.05 V vs NHE) which can strongly reduce the adsorbed oxygen to ^•O₂⁻ (−0.33 V vs NHE). Furthermore, valence band potentials of LaFeO₃ (+2.21 V versus NHE) and Fe₂O₃ (+2.2 V vs NHE) are highly positive which can oxidize H₂O/OH⁻ to produce ^•OH radicals (+1.99 V vs NHE) (13). Herein, the as-fabricated LFO/B-CN/FO double Z-scheme heterojunction is utilized in photocatalysis for navy-blue dye degradation. The catalyst revealed exceptional performance due to the improved light absorption and charge carrier’s separation, as well as enhanced stability. This work will open up new routes for the design and fabrication of highly efficient Z-scheme photocatalysts for energy production and environmental remediation.

Materials and methods

Chemicals

Analytical grade chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. These include boric acid (H₃BO₃), melamine, citric acid monohydrate (C₆H₈O₇H₂O), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O), lanthanum(III) nitrate hexahydrate (La(NO₃)₃·6H₂O), HCl (35%), NaOH, ethanol, methanol, ethylene glycol, CH₃COONa, polyethylene glycol-600 (PEG-600), ethanol, benzoquinone (BQ), ethylenediaminetetraacetic acid (EDTA), and isopropyl alcohol (IPA) and navy-blue dye. Scheme 1 displays the structural formula of navy blue (C₃₂H₂₁N₅Na₂O₆S₂, molecular weight: 681.65).

Scheme 1. Chemical structure of navy-blue dye 5R (NB).

Synthesis of boron-doped g-C₃N₄ nanocomposites (B-CN)

Boric acid (0.25 g) was dissolved in 50 mL of deionized (D.I) water, which resulted in a colorless transparent solution. Subsequently, 5 g of melamine was introduced into the aqueous solution, followed by stirring at 500 rpm for 15–20 min. The mixture was then transferred to a Teflon-lined autoclave and maintained at 200°C for 12 h. Afterward, the resultant mixture was recovered from the autoclave and subjected to heating for 4 h at 550°C with a heating rate of 10°C per min. The preparation of pure g-C₃N₄ (CN) followed the same procedure as used for B-CN, but without the addition of boric acid (14).

Synthesis of LaFeO₃ (LFO)

The LFO nanoparticles were prepared via a facile gel-combustion process and then calcined at a temperature of 200°C for 2 hours. In a typical experiment, 5 mL of D.I water was taken in a 25 mL beaker and then 2.1014 g of C₆H₈O₇·H₂O, 2.0201 g of Fe(NO₃)₃·9H₂O, and 2.1650 g of La(NO₃)₃·6H₂O were added to it. To make a homogenous solution, the mixture was magnetically agitated for 20 min at 500 rpm. To create a gel, the resulting solution was heated at 80°C for 12 h in an oven. It was then placed in a 30 mL corundum crucible and calcined in a muffle furnace at 200°C for 2 h with a temperature rise of 5°C min⁻¹. Ethanol and DI water were used to wash the resulting powders after they naturally cooled to 25°C. The resulting material was then vacuum-dried for 4 hours at 100°C (15).

Synthesis of Fe₂O₃ (FO)

Typically, a solvothermal technique was used to synthesize FO nanoparticles. Initially, 80 mL of ethylene glycol was used to dissolve 8.08 g of Fe(NO₃)₃·9H₂O, which was then magnetically agitated at 500 rpm for 30 min at room temperature (25°C). About 2.0 g of polyethylene glycol-600 (PEG-600) and 7.20 g of CH₃COONa were then added to the mixture, and agitated at 500 rpm for 1.5 h. The resultant solution was then transferred to 100 mL volume autoclaves lined with Teflon and heated at 200°C for 22 h in an oven to undergo solvothermal treatment. When the autoclave cooled to ambient temperature (25°C), the precipitate was washed with pure water and ethanol several times and then dried in an oven at 60°C for 12 h. Finally, the precipitate was calcined in a muffle furnace at 470°C for 2 h in an air environment (16).

Synthesis of binary and ternary composites

A wet chemical method synthesis was used to synthesize the LaFeO_3–B-g-C₃N₄ (LFO/B-CN) composites, with B-g-C₃N₄ being chosen as the base material. In 100 mL of a water and ethanol (1:1) mixture solvent, a predetermined quantity (5 g) of previously synthesized B-CN was dissolved. The solution was then stirred (at 500 rpm) for 5 h while 5% by weight LFO was added. Last but not least, the solution was calcined in the air for 2 h at 500°C (temperature ramp = 5°C min^-1) after being dried for 12 h at 65°C. Fe₂O₃-B-g-C₃N₄ (FO/B-CN) and LaFeO_3–B-g-C₃N_4–Fe₂O₃ (LFO/B-CN/FO) composites were prepared using a similar process. 5% by weight of FO was utilized in place of LFO for the preparation of FO/B-CN composite, while 5% by weight of both LFO and FO were added to the B-CN dispersion in the case of LFO/B-CN/FO composites preparation (7). A pictorial representation of the procedure can be seen from Scheme 2.

Scheme 2. Step-wise synthesis process of LFO/B-CN–FO composite.

Characterization

The synthesized materials were examined for various characteristics, including phase changes, surface charge, electronic transitions, elemental composition, thermal stability, chemical structure, etc. UV–Visible spectrometer (UV1800ENG240 V, SOFT, Shimadzu Corporation, Japan, model UV-1800, 220–240V-50–60 Hz 140VA) was used to study the optical properties of the materials. Fourier-transform infrared spectrometry (FT-IR, with attenuated total reflection, ATR, accessory model IRSpirit, produced by Shimadzu, 220–240 V) was used for surface chemical analysis. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) (FEGSEM/EDS model number: 354900-7673-000-08 en, Germany, 30 kv), analysis was done to confirm the morphology and composition of samples. The transmission electron microscopy (TEM) technique was used for capturing the TEM images. X-ray diffractometer (XRD, Model no: AXRD-LPD System USA, 220 V, 20A) was used for measuring the XRD patterns of the samples. Furthermore, the photocatalyst's point of zero charge (pH_pzc) was assessed using the pH-drift method (pH meter, Model: HI 5521, powered by a 12 Vdc power supply). Similarly, UV–Visible spectrometer, in the wavelength range of 200–800 nm, was also used to assess the photocatalytic activities of these materials toward degradation of navy-blue dye.

Photocatalytic activity evaluation

The photocatalytic degradation tests of NB dye were conducted under visible light irradiation in order to assess the photocatalytic activity of the as-synthesized photocatalysts, as shown in Figure S1. The materials were also tested in the absence of light/catalyst. The least amount of NB dye was determined to have been degraded by using CN, B-CN, LFO/B-CN, FO/B-CN, and LFO/B-CN/FO photocatalysts in the dark. Thus, the degradation of NB was photocatalytic (light dependent) and was negligible in the dark. A 250 mL glass reactor was used for the NB dye degradation tests. To determine the adsorption equilibrium for each degradation test, 0.1 g of the catalyst was added to 100 mL of NB solution (50 mg/L) and stirred at 500 rpm in the dark for 30 min. The solution was then exposed to visible light for 130 min by using a 300 W Xenon lamp. After the photocatalytic reaction, the concentration of NB was measured using a UV–Visible spectrophotometer (UV1800ENG240 V, SOFT). The % removal was measured by using Equation (1) (1). Similarly, the pseudo-first-order and pseudo-second-order kinetics models were calculated using Equations (2–5) (17). (1) $\text{\%}\mathrm{Removal}={\displaystyle \frac{C_0-C_t}{C_0}\times 100}$(1) (2) $-\ln \left({\displaystyle \frac{C_t}{C_0}}\right)=\mathrm{kt}$(2) (3) ${\displaystyle \frac{t}{C_t}={\displaystyle \frac{1}{C_0}+{\displaystyle \frac{1}{K_2{C}_e^2}}}}$(3) (4) ${\displaystyle \frac{C_0}{C_t}={\displaystyle \frac{1}{k^{2}t}\left({\displaystyle \frac{1-C_0{k}^2}{C_0}}\right)}}$(4) (5) $-{\displaystyle \frac{C_t}{C_0}=\mathrm{kt}}$(5) where C₀ and C_t are the NB dye concentration (mg/L), before and after different time intervals in the presence of light, respectively. Where, k (min⁻¹) and k (M⁻¹min⁻¹) represents the degradation rate constant for the pseudo-first-order- and pseudo-second-order kinetics models respectively.

Effect of concentration on the λ_max of navy-blue dye

Double beam UV–visible spectrophotometer was used to measure the concentration of NB dye at its λ_max (i.e. 613 nm). At this wavelength, the broad absorption band is correlated to the chromophore of the azo group which is responsible for the appearance of blue color (18). The λ_max is not greatly changed with concentration; however, absorbance is continuously decreased. For this purpose, different concentration dye solutions were prepared. The absorbance versus wavelength plots, at various concentrations, are shown in Figure S1(a). Likewise, in this study, a calibration curve has been drawn for NB dye at the wave length (λ_max) of 613 nm for several solutions of different concentrations i.e. 5, 10, 20, 30, 40 and 50, 60, 70, 80, 90, and 100 mg/L. The calibration (absorbance vs concentration) curve obtained for NB is shown in Figure S1(b). The concentration range used was 5–100 mg/L with a regression factor (R²) = 0.998, slope = 2.312, and the intercept = 0.019.

Results and discussion

X-ray diffraction

The X-ray diffraction (XRD) patterns of CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO were measured as revealed in Figure 1(a). For CN, the in-plane structure of the trigonal nitrogen of tri-s-triazine layer units is represented by the less prominent peak at 2θ = 12.9° (100 plane). On the other hand, the more noticeable peak at 2θ = 27.4° (002) is linked to the conjugated aromatic segments interplane stacking, with an interlayer spacing of roughly 0.32 nm (the π–π stacking of heptazine rings (JCPDS Card No. 87-1526)). In B-CN, except for the peaks at 12.9° and 27.4°, the 2θ peaks at 20.4°, 41.7° and 50.23° are correlated to the crystallographic planes (103), (224), and (323), indicating the presence of Boron (JCPDS Card #: 01-070-0154) (19). In the case of FO/B-CN, the distinct diffraction peaks at 33.2°, 35.6°, 40.9°, 49.5°, 54.1°, 57.6°, 62.4°, and 64.0° corresponds to the crystal planes (104), (110), (113), (024), (116), (018), (214), and (300) of hexagonal α-Fe₂O₃ (JCPDS 33-0664) (20). Similarly, in LFO/B-CN sample, the peaks at 2θ values of 22.9°, 32.35°, 39.86°, 46.37°, 56.46°, 67.69°, and 76.98° correspond to the (101), (121), (220), (202), (240), (242), and (204) crystallographic planes of LaFeO₃ (JCPDS card no. 88-0641). Similarly, the corresponding peaks of CN, B-CN, LaFeO₃ and Fe₂O₃ can be seen in the XRD patterns of the LFO/B-CN/FO ternary composite (10).

Figure 1. (a) XRD, (b) UV-absorption spectra, (c) Tauc’s plots, and (d) FT-IR for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts.

UV–visible spectroscopy analysis

The powder samples were dispersed in a 1:1 ratio water/absolute ethanol mixture to conduct the UV–Visible absorption measurements and the corresponding spectra are revealed in Figure 1(b). As obvious, after boron doping, the intrinsic band gap of CN is slightly reduced and the B-CN revealed more absorption compared to the bare CN. Notably, after coupling LFO and FO, the optical absorption of binary (i.e. FO/B-CN and LFO/B-CN) and ternary (i.e. LFO/B-CN/FO) composites is significantly enhanced compared to the B-CN catalyst. This is due to the small band gaps of LFO and FO compared to the B-CN photocatalyst (14). The band gap energy of the samples was calculated via the Tauc plot equation (Equation 6) using the UV–visible absorption data. (6) $(\mathrm{\alpha h\upsilon }{)}^{\gamma }=A(\mathrm{h\upsilon }-E_{\mathrm{g}})$(6) In this equation, “E_g” represents the band gap energy, “A” is the proportionality constant, “υ” indicates photon frequency, “h” is the Planck constant, “α” is the absorption coefficient, and “γ” is the electronic transition, which might have the values of 2, 1/2, 2/3, or 1/3, depending on the kind of transition. Figure 1(c) shows the Tauc plots of CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO catalysts. When (αhυ)² was plotted against (hυ), an almost linear trend was seen, suggesting that direct permitted transitions are responsible for the absorption edge. The optical band gap (E_g) is represented by the value of the X-axis intersected by a straight line drawn. Clear and estimated optical energy band gap values of 2.7 and 2.5 eV were predicted for the CN and B-CN catalysts, respectively. These values are consistent with earlier research findings published in the literature (21). A red shift in the absorption edges was seen when FO/LFO was added to B-CN, suggesting that heterojunction has been formed between the counter parts. It appears that adding LFO and FO to B-CN increased its capacity to absorb visible light, which led to a slight red shift. This observation implies that the optical properties of the base material can be adjusted by coupling proper band gap semiconductor nanomaterials. The band gaps of CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO nanocomposites were determined to be 2.7, 2.5, 2.2, 1.8, and 1.6 eV, respectively, based on the Tauc's equation curves (22).

FT-IR spectroscopy analysis

Figure 1(d) reveals the FT-IR spectra of CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO catalysts measured in the wavenumber range of 500–4000 cm⁻¹. The FT-IR spectrum helps to assess the composition/surface functional groups of the catalysts. Notably, the peaks centered between 1200 and 1650 cm⁻¹ are responsible for the CN heterocycle stretching modes. The broad band observed in the range of 2800−3400 cm⁻¹ corresponds to the aromatic ring of the uncondensed terminal NH₂ group (23). Additionally, the peak at 2172 cm⁻¹ correlates to the asymmetric stretching vibration mode of cyano group (CN or NC). The binary and ternary composites exhibit all the major vibration peaks of CN, demonstrating that the structure of CN does not change during the coupling process. In the case of B-CN catalyst, the observed distinctive absorption peak at about 810 cm⁻¹ is associated with the bonds out-of-plane stretching vibrations (24). The distinctive absorption peak at 573 cm⁻¹ in the LFO/B-CN, FO/B-CN, and LFO/B-CN/FO composites could be attributed to the stretching vibration modes of Fe–O bonds. Peaks at 651, 756, and 873 cm^-1 are due to the presence of iron content confirming the successful fabrication of nanocomposites (25). Similarly, the peak at 530.2 cm⁻¹, corresponding to the lattice oxygen of La–O, showed the presence of LaFeO₃ in LFO/B-CN and LFO/B-CN/FO (10).

Scanning electron microscopy analysis

SEM analysis was done to investigate the morphology of the as-prepared samples as can be seen in Figure 2(a–e). Notably, the B-CN sample (Figure 2(b)) exhibits a smoother, more porous surface, and a more streamlined shape than the pristine CN (Figure 2(a)), which has a typical two-dimensional laminar structure (26). The SEM image of FO/B-CN (Figure 2(c)) demonstrates the aggregation of B-CN nanosheets along with ultra-small α-Fe₂O₃ nanoparticles anchored on its surface (27). Additionally, in LFO/B-CN, the LFO nanoparticles were dispersed across the B-CN nanosheets as revealed in Figure 2(d) (28). The SEM image of LFO/B-CN/FO ternary composite (Figure 2(e)) clearly demonstrates the presence of FO and LFO nanoparticles on the surface of B-CN catalyst. The potential photocatalytic reactions are possible due to their larger, rough and round surfaces (28). Figure 2(f) represents the SEM image of LFO/B-CN/FO ternary composite after usage of it, which reflects that no obvious changes were observed in its surface after its usage.

Figure 2. SEM images of (a) CN, (b) B-CN, (c) FO/B-CN, (d) LFO/B-CN, and (e) LFO/B-CN/FO photocatalysts.

Figure 3. TEM images of (a) CN, (b) B-CN, (c) FO/B-CN, (d) LFO/B-CN, and (e) LFO/B-CN/FO photocatalysts.

Transmission electron microscopy analysis

TEM was used to investigate the structural morphologies of the CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts as revealed in Figure 3(a–e). The 2D morphology and lamellar structure of CN and B-CN are confirmed as shown in Figure 3(a,b) (21). As obvious, the CN surface becomes rough after the introduction of boron. After coupling FO nanoparticles to the lamellar structure of B-CN, a heterojunction was formed as revealed in Figure 3(c) (14). Likewise, Figure 3(d) reveals the dispersion of LFO nanoparticles over the surface of B-CN in the binary LFO/B-CN composite. Notably, the TEM image of LFO/B-CN/FO reveals the presence of both LFO and FO nanoparticles over the surface of B-CN as displayed in Figure 3(e). This confirms the successful fabrication of ternary heterojunction (29).

Figure 4. Estimation of pH_pzc of CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts.

Point of zero charge (pH_pzc) of photocatalyst

One of the most important interfacial parameters is the pH_pzc, or pH at the point of zero charges. The quality and quantity of surface charge of photocatalyst is directly affected by changing the pH of solution, which then directly or indirectly affects the photocatalytic activity of the catalyst. In general, a number of factors, such as the photocatalyst type, structure, synthesis method, and nature of the medium have a substantial impact on the pH_pzc. The pH-drift method was used to estimate the pH_pzc values of CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts. The pH_pzc of the photocatalysts are the positions at which the initial and final pH of the aqueous solution containing the material/catalyst after 24 h are equivalent. By doing this, the point of zero charge pH values for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO were determined to be 6.4, 6, 6.51, 6.5, and 6.69, respectively, as shown in Figure 4 and Figure S2. The surface is positively charged when the pH of the solution is lower than the pH_pzc of the photocatalyst. On the other hand, the surface is negatively charged when the pH value is higher than the pH_pzc of the photocatalyst. Under neutral pH conditions, it is found that the surface charge of all photocatalysts is supposed to be negative as this pH is higher than the pH_pzc, while in acidic conditions, the surface charge is expected to be positive. Similar results were also repotted for pH_pzc of BSO-BCN/Y (10) (30).

Energy dispersive X-ray spectroscopy analysis

Figure S3(a–e) displays the EDS spectrum as well as the matching elemental composition peaks of CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO catalysts. As shown in Figure S3(a), the prominent peak in the CN EDS spectrum indicates the existence of C and N elements. Likewise, EDS spectrum peaks (Figure S3(b)) demonstrate the existence of C, N, and B. Using EDS spectra, the chemical makeup of the coupled FO with B-CN was assessed (Figure S3(c)). Similarly, Figure S3(d) displays the EDS spectrum of LFO/B-CN, whereas Figure S3(e) displays the EDS spectrum of LFO/B-CN/FO photocatalyst. The inset table shows that the ternary photocatalyst contains C, N, B, La, Fe, and O elements. Overall, these findings demonstrate the successful fabrication of the photocatalysts with high purity.

Thermal gravimetric analysis

TGA/DSC analyses for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts were carried out under an N₂ atmosphere in the range of 25−1000°C at 10°C/min to evaluate the thermal stability of the photocatalysts. The vaporization of the adsorbed H₂O or other volatile contaminants on the sample surface is the main cause of the initial mass loss, which occurs below 200°C, as illustrated in Figure 5(a,b) (31). Further heating to 530.27°C results in a 91.77% weight loss, which is explained by the chemical decomposition of CN. In the case of B-CN, the decomposition temperature was approximately 683.68°C, and the weight loss was 95.37% at 541.05°C. The weight loss for FO/B-CN was 91.73% at 531.15°C, indicating that the addition of FO enhanced the decomposition temperature and raised the peak temperature to 707.34°C. As obvious, the weight loss for LFO/B-CN photocatalyst was 87.25% at 520.88°C and the peak temperature was 565.61°C. Notably, in the case of LFO/B-CN/FO ternary nanocomposite, the decomposition temperature was raised. As a result, the percent weight loss was 43.82% at 642.89°C. This clearly indicates that LFO/B-CN/FO ternary composite is more stable at high temperatures. These findings reveal that the significantly enhanced thermal stability of B-CN is due to the strong interaction between LFO, B-CN and FO components (32).

Figure 5. (a) TGA and (b) DSC for CN, B-CN, FOB-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts.

Photocatalytic activities of the materials

Effect of catalysts dose

The as-fabricated photocatalysts were used to assess their photocatalytic activity for the degradation of NB dye. A 300-Watt Xenon lamp was used as a source of visible light irradiation. Furthermore, the degradation of NB dye was studied in the presence of photocatalysts under dark conditions, and in the presence of photocatalysts and light. As obvious, under dark conditions, very little amount of NB is removed by the LFO/B-CN/FO ternary composite. The as-synthesized photocatalysts revealed variable levels of photocatalytic activity, which is correlated to the generation, migration, and dissociation of photogenerated electron–hole pairs (${\mathrm{e}}^{-}/{\mathrm{h}}^{+}$). Figure 6(a) showed that after 130 min of visible light irradiation, CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts degraded ∼21.37%, 34.37%, 66.05%, 73.56%, and 91.13% of NB dye, respectively, under identical conditions (i.e. photocatalyst dosage of 0.1 g, 100 mL NB dye solution with 50 mg/L concentration, pH of 7, temperature of 25°C). These results reveal the potential significance of the as-synthesized photocatalysts in environmental remediation. The percentage degradation of NB dye removal was calculated using Equation (1) (33).

Figure 6. (a) Effect of different composites of CN, (b) effect of different NB dye concentrations, (c) effect of pH, and (d) effect of temperature.

The LFO/B-CN/FO ternary composite exhibits superior NB dye degradation performance compared to the CN, B-CN, FO/B-CN, and LFO/B-CN photocatalysts. The significant NB dye degradation performance of the LFO/B-CN/FO photocatalyst is attributed to the improved charge carrier’s separation and transfer and extended light absorption, as these are considered important factors during pollutant degradation reactions. Notably, this structural characteristic also strengthens the conductive structure and increases the photo-absorption of LFO/B-CN/FO ternary nanocomposite, which further encourages the separation of photo-excited (${\mathrm{e}}^{-}/{\mathrm{h}}^{+}$) pairs (34).

In addition, the photocatalytic degradation of NB dye was evaluated using different kinetics models such as first-order, second-order, third-order, zero-order, pseudo-first-order (PFO), and pseudo-second-order (PSO). For the degradation of NB dye, the PFO and PSO kinetics models revealed the highest regression factor (R²) values among others. Equations (2) and (5) were utilized to ascertain the kinetics of NB dye removal of PFO and PSO, respectively (33). As shown in Figure 7(a), the R² values of PFO kinetics model for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO were found to be 0.809, 0.935, 0.838, 0.938, and 0.869, respectively, at identical conditions (i.e. 0.1 g catalyst, 50 mg/L NB dye, 25°C temperature, and pH 7). These findings led to the conclusion that the photocatalytic degradation of NB dye follows the PFO kinetics model. Figure 7(a) illustrates the linear relationship between the inverse of $-\ln \left(C_t/C_o\right)$ and reaction time, which indicates the linear relationship for the removal of NB dye over the CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts. The plot of $-\ln \left(C_t/C_o\right)$ versus time “t” was used to calculate the corresponding rate constant “k” from the slope for PFO kinetics as shown in Equation (2). In addition, Equation (7) was used to calculate the half-life (t_1/2). (7) $t_{\frac{1}{2}}={\displaystyle \frac{0.693}{k}}$(7) Table S1 enlists the kinetic rate constants (k), half-lives (t_1/2), and regression coefficients (R²) for the removal of NB dye using CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts. These kinetic results showed similarity with previously reported literature (10). For CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts, the corresponding PFO rate constant (k) values were 0.0018, 0.0035, 0.0083, 0.0112, and 0.0179 per min. Notably, compared to other photocatalysts, LFO/B-CN/FO ternary composite has the highest rate constant value. Likewise, for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts, the half-life (t_1/2, min) values were found to be 378.68, 196.87, 82.79, 61.43, and 38.71 min, respectively. Again, it is obvious that the LFO/B-CN/FO photocatalyst has the shortest half-life (t_1/2). These results reveal that the LFO/B-CN/FO ternary nanocomposite exhibits the highest performance for NB dye degradation compared to the other photocatalysts under identical conditions.

Figure 7. PFO kinetics for (a) the effect of different photocatalysts, (b) the effect of different concentrations of NB dye, (c) the effect of pH, and (d) the effect of temperature.

The degradation of NB dye over the CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts followed the PSO kinetics model because they had regression factor values of 0.828, 0.940, 0.916, 0.955, and 0.977, respectively (see Table S2). When 0.1 g of each photocatalyst was added while maintaining other parameters constant, a linear relationship was found between the reaction time and the inverse of $-\left(C_t/C_{\mathrm{o}}\right)$ as shown in Figure 8(a). This indicates that the degradation process is adhered to the PSO kinetics model. The rate constant (k), regression factor (R²), and half-life (t_1/2, min) values for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts are shown in Table S2. The trend followed by these photocatalysts is similar to the previously published materials (35). After stirring in the dark for 30 min, the final concentration of NB dye was monitored at specific time intervals (i.e. 20 min) under visible light irradiation, to calculate the percentage degradation of NB dye using Equation (1). Additionally, Equations (5) and (8) were used to determine the kinetics of the NB dye degradation and their half-life (t_1/2 min). In these formulas, k stands for the rate constant (min⁻¹) for the PSO kinetics model, and C₀ is the initial concentration of NB dye, which was taken as 50 mg/L (36). (8) $t_{1/2}={\displaystyle \frac{1}{k^{\ast }\left[C_0\right]}}$(8) In addition to the PFO kinetics, as shown in Table S1, it was found that the degradation of NB dye by these photocatalysts followed PSO kinetics as shown in Table S2. It is found that the PSO rate constant (k) values for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalyst are predicted to be 0.0016, 0.0029, 0.0053, 0.0064, and 0.0073 (M⁻¹ min⁻¹), respectively. Further, LFO/B-CN/FO photocatalyst has the highest rate constant (k) value compared to the others as revealed in Table S2. Furthermore, the half-life (t_1/2, min) values for CN, B-CN, FO/B-CN, LFO/B-CN, and LFO/B-CN/FO photocatalysts are estimated to be 417.46, 238.14, 129.05, 107.94, and 94.93, respectively as shown in Table S2. This indicates that LFO/B-CN/FO has the shortest half-life (t_1/2 min).

Figure 8. PSO kinetics for (a) the effect of different composites of CN, (b) the effect of different NB dye concentrations, (c) the effect of pH, and (d) the effect of temperature.

Effect of initial concentration of navy-blue dye

We also investigated the effect of the initial concentration of NB dye on the photocatalytic activity of the photocatalysts. Target pollutants were selected while keeping all other parameters constant for different concentration of NB dye i.e. 25 to 100 mg/L in order to evaluate their impact on the photocatalytic performance of LFO/B-CN/FO photocatalyst. The catalyst dose, temperature and pH were maintained at 0.1 g, 25°C, and 7, respectively, under visible light irradiation and from the reaction mixture, 5 mL was taken at regular time intervals up to 130 min. The % degradation of NB dye after 130 min irradiation was measured to be 83.67, 91.13, 64.11, and 62.07% for 25, 50, 75, and 100 mg/L, respectively, as shown in Figure 6(b). The results concluded that at 50 mg/L NB solution, the maximum degradation happened and after that concentration decrease was noted in degradation as shown in Figure 6(b) and Tables S1 and S2. The interaction between NB dye molecules and the LFO/B-CN/FO photocatalyst surface-active sites is responsible for this behavior. The probability of NB dye molecules making contact with the active sites on the LFO/B-CN/FO photocatalyst surface increases with the increase in NB dye concentration (37). However, concentrations over this threshold might prevent NB dye molecules from efficiently making contact with the LFO/B-CN/FO photocatalyst (38). The interference may cause photocatalysis to proceed more slowly, which would reduce degradation efficiency. Moreover, the adsorption and degradation of NB dye molecules was facilitated by the catalyst surface, such as that of the LFO/B-CN/FO photocatalyst. At lower concentrations of NB dye (i.e. 50 mg/L), the probability of contact between NB molecules and the catalyst surface is maximum as can be seen in Figure 6(b). However, at concentrations above this threshold, the dye molecules start to take up a lot of the surface area that is accessible, thereby reducing the degradation performance of the photocatalyst (39).

After analyzing the plots, it was evident that the PFO (Figure 7(b)) and PSO (Figure 8(b)) reveal the highest regression factor (R²) values for NB dye, compared to the other kinetics models at identical factors. The regression factor values of PFO for 25, 50, 75, and 100 mg/L were found to be 0.972, 0.977, 0.988, and 0.955, respectively. It is clear from Table S1 data that 75 mg/L solution has the highest PFO reaction kinetics regression factor value. Similarly, the maximum rate constant value (k₁ = 1.027 E⁻⁴ min⁻¹) for PFO was observed at this concentration. This value is substantially higher than the k₁ value for other concentrations, which are 6.170 E⁻⁴, 3.286 E⁻⁴, and 7.673 E⁻⁵ for 25, 50, and 100 mg/L, respectively, as illustrated in Table S1. Additionally, Table S1 displays the half-life (t_1/2, min) values for PFO at different concentrations of NB dye. The half-life values were predicted to be 9031.0, 6743.9, 2108.8, and 1123.1 min for 100, 75, 50, and 25 mg/L, further demonstrating that in comparison to the other concentrations, the PFO half-life was lower for the 25 mg/L NB dye solution.

Similarly, the degradation data related to the influence of the initial concentration of NB dye were analyzed using the PSO kinetics model as shown in Figure 8(b). Table S2 reveals the regression factor values i.e. 0.972, 0.977, 0.988, and 0.955 for 25, 50, 75, and 100 mg/L, respectively. The highest regression factor value was observed for the 75 mg/L NB dye solution. Notably, Table S2 displays the PSO rate constant (k₂) values. As obvious, the highest PSO rate constant value was 0.0073 M^–1 min⁻¹ at 50 mg/L. This value is much greater than other values, like 0.0068 at 25, 0.0051 at 75, and 0.0051 M^−¹min⁻¹ at 100 mg/L. In addition, the half-lives (t_1/2, min) were determined for each dosage, and the results showed that their values at 25, 50, 75, and 100 mg/L are 100.58, 94.93, 134.0, and 135.35 min, respectively. This suggests that compared to other NB dye concentrations, the half-life at 50 mg/L is lower. The higher rate constant values indicate that the PFO and PSO appear to be favored over any potential competing reaction order, specifically at 50 mg/L concentration of NB dye solution (24). As a result, in relation to the NB dye concentration, the degradation process over the LFO/B-CN/FO photocatalyst tends to follow the PFO and PSO. This suggests that the concentration of NB dye slightly influences the degradation kinetics and the compatibility of PFO and PSO behavior due to the concentration of NB dye and is in agreement with the previous reports (40).

Effect of initial pH

The wastewater pH values fluctuate according to the dye content. Thus, we need to investigate how the pH of the medium influence the degradation of NB dye. In order to do so, the pH was adjusted while maintaining other parameters constant as shown in Table S1. This was done to evaluate the photocatalytic degradation performance of LFO/B-CN/FO photocatalyst toward NB dye. The degradation performance at pH 3, 5, 7, 9, and 11 were calculated to be 49.57, 70.75, 91.13, 61.19, and 15.54%, respectively, after irradiation for 130 min, as revealed in Figure 6(c). The maximum degradation performance of 91.13% was observed at pH 7. It is imperative to maintain a neutral pH during photocatalysis in order to keep the outstanding degradation performance of the photocatalyst. At extreme pH values, the catalyst surface may lose its active sites which may result in the formation of complexes (41). Conversely, when the pH drops below 7 and increases beyond 7, positive and negative charges can be visible on the catalyst surface. Under both basic and acidic conditions, the interaction of $\mathrm{O}{\mathrm{H}}^{-}$ with positive holes produces OH*. In a basic environment, $\mathrm{O}{\mathrm{H}}^{-}$ oxidizes, resulting in the surface of LFO/B-CN/FO containing OH* (42). On the other hand, the coulombic attraction between the positive active sites of the photocatalyst and $\mathrm{O}{\mathrm{H}}^{-}$ may limit the production of OH* in an acidic environment, thereby decreasing the effectiveness of the process. When it comes to anionic azo dyes, which are challenging to remove from negatively charged surfaces, the benefits of maintaining high pH levels are evident. Conversely, at lower pH values, electrons in the conduction band of the catalyst eliminate NB dye through the reductive breakage of azo bonds. Additionally, in acidic conditions, the surface area accessible for ${\mathrm{H}}^{+}$ absorption and dye degradation is decreased due to the heterojunction’s features of LFO/B-CN/FO photocatalyst. Consequently, the pH emerges as a critical factor influencing the characteristics of wastewater. It influences direct hole oxidation, direct electron reduction, and the activity of OH* in the conduction band. Protonation and deprotonation-induced pH variations also modify the dyes specifications, which have a fundamental impact on the dye’s redox activity and degradation features. The observed fluctuations in the photocatalytic degradation behavior of NB dye could perhaps be attributed to the substantial coulombic attraction on the LFO/B-CN/FO surface, particularly at the neutral pH (43).

The PFO and PSO kinetics models, which are depicted in Figures 7(c) and 8(c), respectively, were found to be followed during the photocatalytic degradation process after employing several kinetics models. The rate constant (k), regression factor (R²), and half-life (t_1/2) for PFO and PSO are shown in Tables S1 and S2, respectively. The observed rate constants (k₁) for PFO were determined to be 1.064 E⁻⁴, 1.830 E⁻⁴, 3.286 E⁻⁴, 1.476 E⁻⁴, and 2.123 E⁻⁵ min⁻¹ for pH values of 3, 5, 7, 9, and 11, respectively. At pH 7, a higher rate constant value was observed. Similar to this, the Regression factor (R²) values for PFO at pH 3, 5, 7, 9 and 11 were 0.978, 0.967, 0.981, 0.968, and 0.936, respectively, demonstrating quite excellent Regression coefficients. For pH values of 3, 5, 7, 9, and 11, the calculated half-life values were 6509.3, 3786.8, 2108.8, 4693.5, and 32636.3 min, respectively. In comparison to the other pH values determined and shown in Table S1, it was found that at pH 7, the rate constant value is higher and the half-life value is lesser; suggesting that neutral pH conditions are more fruitful in this case (44).

As can be seen in Figure 8(c), the degradation of NB dye over the LFO/B-CN/FO ternary nanocomposite also follows the PSO kinetics model. Table S2 contains the relevant data for the various parameters. At pH 3, 5, 7, 9, and 11, the rate constant values for PSO were determined to be 0.0040, 0.0057, 0.0073, 0.0052, and 0.0011 M⁻¹ min⁻¹, respectively. In a similar way, pH 7 has the highest rate constant. Additionally, the values of the regression coefficient (R²) were found to be 0.973, 0.961, 0.977, 0.962, and 0.923, at pH 3, 5, 7, 9, and 11, respectively. The rate constant and regression coefficients were found to be greater at pH 7. The half-life for PSO was also calculated as shown in Table S2. At pH 3, 5, 7, 9, and 11, the values were 169.4, 121.5, 94.93, 132.7, and 624.3 min, respectively. Again, a small value of the half-life was obtained at pH 7, compared to other pH values. Hence, it is confirmed that pH 7 is an appropriate value for photocatalytic degradation of NB dye over the LFO/B-CN/FO photocatalyst (36).

Effect of temperature

The removal of NB dye over time by LFO/B-CN/FO photocatalyst at various temperatures i.e. 25, 35, 45, and 55°C was also studied and the results are depicted in Figure 6(d). At the above-mentioned temperatures, the percentage decomposition of NB dye was 91.13, 98.10, 81.63, and 76.87%, respectively. It is clear that when the temperature rises from 25 to 35°C, the percentage degradation of NB dye increases. Nevertheless, the rate of NB dye removal decreased when the temperature increased to 55°C. This may be due to the fact that the active sites on the catalyst surface deactivate at high temperatures (37).

After putting temperature-dependent data through a number of kinetics models, it was determined that the data followed PFO as well as PSO kinetics models as presented in Tables S1 and S2 and Figures 7(d) and 8(d), respectively. Figure 8(d) displays the kinetic studies of NB dye degradation over LFO/B-CN/FO at various temperatures for PFO. The regression factor value for PFO was maximum at 25°C, reaching 0.987, as shown in Table S1. The PFO regression factor values at 35, 45, and 55°C were 0.961, 0.923, and 0.968, respectively. Additionally, it was found that the rate constant (k) values for PFO at 25, 35, 45, and 55°C were 3.325 E⁻⁴, 5.603 E⁻⁴, 3.165 E⁻⁴, and 2.059 E⁻⁴ min⁻¹, respectively. According to these findings, the rate constant value peaked around 35°C. Similarly, the half-lives (t_1/2) for PFO were 2083.9, 1236.6, 2188.9, and 3364.7 min at 25, 35, 45, and 55, respectively. In accordance with the previous findings, it was revealed that the half-life was highest at 55°C and lowest at 35°C (45).

Figure 9. Plots of lnK versus the inverse of temperature (in Kelvin) for the thermodynamic parameter of the photocatalytic removal of NB dye.

Likewise, the degradation of NB dye over LFO/B-CN/FO photocatalyst also followed the PSO kinetics model, as presented in Table S2 and Figure 8(d). The regression coefficient values for the PSO kinetic model were determined to be 0.987, 0.961, 0.923, and 0.968 at temperatures of 25, 35, 45, and 55°C, respectively. The rate constants (k₂) for 25, 35, 45, and 55°C were found to be 0.0074, 0.0083, 0.0071, and 0.0064 M⁻¹ min⁻¹, respectively. The PSO kinetics at these temperatures were found to have half-live periods of 93.14, 83.29, 96.51, and 107.10 min, respectively. At 35°C, the shortest half-life of 83.29 min was noted which implied that the half-life period increased with temperature after 25°C. These results are consistent with the findings reported in earlier studies (36).

Degradation thermodynamics studies

In order to investigate the thermodynamic basis of the photocatalytic degradation of NB dye, Equations (9–11) were used to compute the changes in enthalpy (ΔH°), entropy (ΔS°), and Gibbs free energy (ΔG°). In order to estimate the free energy changes, it is necessary to calculate the apparent equilibrium constant ($K_{\mathrm{app}}$) for the temperature range using the following equation. (9) $K_{\mathrm{app}}={\displaystyle \frac{C_{\mathrm{deg}}}{C_{\mathrm{undeg}}}}$(9) The photocatalytic degradation performance of NB dye is represented by $K_{\mathrm{app}}$. It is the ratio of NB dye concentration (degraded) on the surface of the photocatalyst to the NB dye concentration un-degraded at an equilibrium state in the bulk of the solution. The ΔG° values were determined by applying Equation (10) to study the thermodynamics process. This method determines the photocatalytic degradation performance of ternary composite for NB dye and improves the understanding of the required energy dynamics, providing a stronger basis for optimizing photocatalytic degradation mechanisms (30). In order to determine the changes in Gibbs free energy (ΔG°) and to get information regarding the photocatalytic degradation thermodynamics, Equation (10) was applied. (10) $\mathrm{\Delta }{G}^{\circ }=-\mathrm{RT}\ln K_{\mathrm{app}}$(10)

The photocatalyst degradation performance of the catalyst for NB dye is shown by the degradation equilibrium constant ($K_{\mathrm{app}}$), the R (universal gas constant) has a value of 8.314 J mol^–1K^–1. For further understanding of thermodynamics parameters, the following equation could be used. (11) $\ln K_{\mathrm{app}}=-{\displaystyle \frac{{\mathrm{\Delta H}}^{\circ }}{R}\left({\displaystyle \frac{1}{T}}\right)+{\displaystyle \frac{{\mathrm{\Delta S}}^{\circ }}{R}}}$(11) In Equation (11), T, ΔS°, and ΔH° present temperature (Kelvin), entropy change, and enthalpy change, respectively. The graph was obtained by plotting $\ln K_{\mathrm{app}}$ versus 1/T as shown in Figure 9. The intercept and slope were calculated from the values of ΔS° and ΔH°. Similarly, the ΔG° values were calculated as presented in Table S4. The ΔG° values were positive for CN, B-CN, and LFO/B-CN at all studied temperatures, which reveals that the degradation of NB dye over these photocatalysts is nonspontaneous. There is one exceptional case for LFO/B-CN photocatalyst because the ΔG° value is negative at 35°C which means that at this temperature, the degradation is spontaneous. Similarly, the ΔG° values are negative for FO/B-CN and LFO/B-CN/FO catalysts at all the applied temperatures. These negative values reveal that the degradation process of NB is spontaneous at all temperatures. These results provide a dynamic report for photocatalytic degradation of NB dye, which provides the significance of enthalpy and entropic contributions in various catalytic approaches, in thermodynamics complex frameworks. This complex thermodynamic study enhances our understanding of the photocatalytic process and capability, which makes a durable support to the area of environmentally remediation’s technology. The variations in Gibbs free energy (ΔG°) of NB dye degradation reveal that it is a spontaneous and feasible thermodynamics process. The ΔG° positive values for CN, B-CN, and LFO/B-CN photocatalysts reveal that the degradation pathway is nonspontaneous and needs external energy for the degradation of NB dye. The main cause is the barrier in high activation energy that restricts the active sites for surface adsorption. Conversely, the negative ΔG° values linked to FO/B-CN and LFO/B-CN/FO catalysts reveal the presence of spontaneous degradation mechanisms. Such a process indicates a favorable set of conditions for the reaction to occur, most likely as a result of improved charge carrier separation, improved adsorptive interactions, or increased availability of active sites properties that are specific to the binary (LFO/B-CN) and ternary (LFO/B-CN/FO) composites. These results emphasize the energy dynamics of NB dye degradation and show that how the composition of the photocatalyst affects the degradation pathways for thermodynamic viability.

Recyclability of photocatalyst

The success of photocatalytic processes is largely dependent on the life time and effectiveness of a photocatalyst. Using the same photocatalyst for NB dye degradation over several cycles, the studies were performed for recycling in order to evaluate the reusability of the LFO/B-CN/FO ternary nanocomposite. Eight consecutive cycles made up the recycling process, as shown in Figure 10(a,b). Centrifugation was done to recover the photocatalyst, which was then thoroughly cleaned with deionized water and absolute ethanol before being heated for a final time for 12 h at 65°C and then calcination was done at 650°C in order to remove the NB dye molecules from the catalyst materials for further use. After such treatments, the catalyst material was re-used for the degradation of NB dye. A slow but visible decline in photocatalytic activity was seen during the recycling tests. For each of the eight cycling runs, the degrading performance was found to be 98.10, 94.59, 90.80, 87.09, 85.92, 84.40, 82.21, and 79.96%, for first, second, third, fourth, fifth, sixth, seventh and eighth cycles, respectively. These outcomes highlight the catalyst's stability, reusability, and total recoverability from the reaction system. It was noted that the degradation performance slightly decreased with the re-use of these materials, which could have been caused by catalyst poisoning or mass loss during the NB dye degradation process. Notably, as shown in Figure 10(c,d), the reusability patterns observed for each run were consistent with the PFO and PSO kinetic models, similar to previously published reports (46).

Figure 10. (a) Reusability, (b) bar graph, (c) pseudo-first order, and (d) pseudo-second order of LFO/B-CN/FO.

Comparative study

Based on the literature cited in this work and elsewhere, it is observed that the catalytic and photocatalytic abilities of the materials are affected by several factors, including the chemical and physical nature and structure of the catalyst, and the arrangement of constituent moieties in the composite, the amount of catalyst, target pollutant and other experimental conditions. In this regard, Table S4 is added to highlight a comparison of the removal performance of the LFO/B-CN/FO photocatalyst with recently reported photocatalysts. Brief outlines for this comparative study are summarized here.

Polyaniline photocatalyst was used for the degradation of methyl orange and methylene blue dyes which showed 42 and 97% removal, respectively, in less than 3 h. The first-order kinetics was followed by the degradation process of both the dyes (47). Similarly, using a glucose-assisted hydrothermal process, LaFeO₃/g-C₃N₄/ZnO Z-scheme photocatalyst was synthesized. The 10%–LaFeO₃/g-C₃N₄/ZnO composite degraded 97.43% of 20 mg/L phenol in 150 min. Even after 5 catalytic cycles, the catalyst maintained 92.13% degradation performance, confirming its high stability (48). The g-C₃N₄/SnFe₂O₄/GO photocatalyst was used to decompose methylene blue dye that revealed 98% degradation in 1 h. This ternary nanocomposite was found reusable for five times (49). Similarly, a ternary composite of g-C₃N₄-WO₃/rGO was used for methylene blue dye degradation. The composite showed 76% dye degradation within 180 min. The degradation followed the PFO kinetics model (50). Likewise, a novel g-C₃N₄/ZnO/α-Fe₂O₃ ternary Z-scheme heterojunction photocatalyst showed 99.34% degradation performance for Tartrazine dye in 95 min. The degradation followed the PFO kinetics model and reusable for three times (29). Beyond these, another ternary composite of AgCl/Au/CN was used for rhodamine B dye degradation which showed 93.1% degradation within 85 min. The composite was reusable for four times (51). The ternary metal/g-C₃N₄/ZnO nanocomposite was used for EBT and MO dyes degradation which showed 23 and 24% degradation in 120 and 130 min, respectively (52). The Ag/FeWO₄/g-C₃N₄ composite was used for Rhodamine B dye degradation which showed 98% removal in 150 min (53). A ternary composite i.e. Ag/ZnO/S-g-C₃N₄ was used for MB dye degradation which showed 98% degradation within 90 min (40). The Trichosporon beigelii NCIM-3326 was used for NB dye degradation through microbial treatment. Under these conditions, 82% degradation was achieved within 48 h (54). Similarly, a novel g-C₃N₄/rGO/Bi₂Fe₄O₉ Z-scheme nanocomposite was used for Congo red dye degradation which showed 86.7% removal within 60 min (55). In the same manner, CoAl₂O₄ nanoparticles were used for NB dye degradation which showed 67% degradation in 180 min (56). LaNiO₃/g-C₃N₄/MoS₂ Z-scheme photocatalysts demonstrated pollutant removal capability with 96.6% Cr(VI) reduction and 91.6% TC degradation (57). Similarly, the Ag/g-C₃N₄/LaFeO₃ ternary nanocomposite showed efficiency of 98.97% (MB in 90 min) and 94.93% (TC in 120 min) (58). The g-C₃N₄ nanosheets/layered MoS₂/grapheme composite, which was synthesized by in situ adsorption, demonstrated better photocatalytic activity because of improved charge carrier separation via the interconnected surfaces and fast charge transfer pathways (59). The NiF nanoparticles were synthesized by thermally treating water to improve the adsorption of cationic crystal violet dye. Adsorption kinetics were evaluated using intra-particle diffusion, Elovich, pseudo-first-order, and pseudo-second-order models (60). The formation of ZnO-activated carbon nanocomposites was achieved through the co-precipitation. Congo red dye (CR) and malachite green (MG) were both readily adsorbed and photocatalyzed by the nanocomposites. The capacity of ZnO-AC was found to be greater than their other samples, while following the PSO adsorption kinetics mechanism. In the presence of hydroxyl radicals, photodegradation followed PFO (61).

It is clear from the table that in the current study, a new LaFeO₃/B-g-C₃N₄/Fe₂O₃ ternary composite which follows a double Z-scheme charge transfer mechanism was synthesized and used in the degradation of NB dye. The synthesized composite revealed 98% degradation within 130 mins under neutral pH and 35°C. This double Z-scheme demonstrates superiority over g-C₃N₄, B-g-C₃N₄, and their binary composites with LaFeO₃ and Fe₂O₃ metal oxides. In addition, as-fabricated ternary composite shows better dye degradation performance compared to most of the previously reported catalysts as mentioned in Table S4. The overall performance of the ternary photocatalyst could be attributed to the plentiful number of active sites, improved light absorption, and promoted charge carrier’s separation.

Photocatalytic process mechanism

In order to verify the active species involved in the dye degradation process, scavenger trapping experiments were carried out for NB dye degradation over the LFO/B-CN/FO ternary composite photocatalyst. The results are displayed in Figure S5(a, b). During the photocatalysis process, various radical scavengers, including benzoquinone (BQ), ethylenediaminetetraacetic acid disodium (EDTA-2Na), and isopropyl alcohol (IPA), were employed to identify the production of superoxide radicals, holes, and hydroxyl radicals, respectively. During each experiment, identical conditions i.e. photocatalyst dosage of 0.1 g, 100 mL NB dye solution with 50 mg/L concentration, pH of 7, the temperature of 35°C, and 5 drops of 1 mM related scavenger were employed. The scavenger trapping experiments over the LFO/B-CN/FO ternary photocatalyst revealed that the primary active species for the dye degradation process are the hydroxyl and superoxide radicals. The addition of EDTA-2Na resulted in a recorded degradation efficiency of approximately 91.2%, indicating that holes are not involved in the degradation process. The addition of BQ and IPA decreased the NB dye degradation performance to 54.48 and 44.81%, respectively. The scavengers trapping experiment suggests that both hydroxyl and superoxide radicals contribute to the dye degradation process (62).

To assess the photocatalytic performance of a catalyst, it is necessary to investigate its charge transfer mechanism. As shown in Scheme 3, under visible light irradiation, electron–hole pairs are produced in the LaFeO₃/B-g-C₃N₄/Fe₂O₃ (LFO/B-CN/FO) ternary composite. The electrons of B-CN excite to its conduction band while holes remain in its valence band. Due to the proper alignment between LFO and FO components and B-CN, the excited electrons of LFO and FO recombines with the holes of B-CN. Thus, the photogenerated electrons in the conduction band of B-CN have strong reducing power and reduce the adsorbed oxygen to superoxide radicals (${\mathrm{O}}_2^{-\bullet }$) (63). Meanwhile, the holes in the valence band of LFO and FO reveal strong enough oxidation potential and oxidize H₂O/OH⁻ to hydroxyl radicals ($\mathrm{O}{\mathrm{H}}^{\bullet }$). These reactive oxygenated species such as $\mathrm{O}{\mathrm{H}}^{\bullet }$ and ${\mathrm{O}}_2^{-\bullet }$ further proceed the NB dye degradation reaction and convert it into CO₂ and H₂O. Thus, the heterojunction follows a double Z-scheme charge transfer mechanism which has strong redox power due to the efficient charge carrier’s separation during the photocatalytic reaction (64). This suggests that the as-fabricated LFO/B-CN/FO photocatalyst can be considered an ideal catalyst for the degradation of organic pollutants (48). The electron–hole pair’s generation, separation and surface redox properties in the as-fabricated ternary composite can be expressed in terms of Equations (12–17). (12) $\begin{array}{rl}& \mathrm{LaFe}{\mathrm{O}}_3/\mathrm{B}-\mathrm{g}{\mathrm{C}}_3{\mathrm{N}}_4/\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{hv}\\ & \to \mathrm{LaFe}{\mathrm{O}}_3\left(e_{\mathrm{CB}}^{-}+h_{\mathrm{VB}}^{+}\right)/B-g\\ & -{\mathrm{C}}_3{\mathrm{N}}_4\left(e_{\mathrm{CB}}^{-}+h_{\mathrm{VB}}^{+}\right)+\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\left(e_{\mathrm{CB}}^{-}+h_{\mathrm{VB}}^{+}\right)\end{array}$(12) (13) $\mathrm{LaFe}{\mathrm{O}}_3\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)/\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)+\mathrm{B}-\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4\left({\mathrm{h}}_{\mathrm{VB}}^{+}\right)\to \mathrm{Recombination}$(13) (14) $\mathrm{LaFe}{\mathrm{O}}_{3_{\mathrm{VB}}}^{{\mathrm{h}}^{+}}+{\mathrm{H}}_2\mathrm{O}/\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{O}{\mathrm{H}}^{\bullet }$(14) (15) $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_{3_{\mathrm{VB}}}^{{\mathrm{h}}^{+}}+{\mathrm{H}}_2\mathrm{O}/\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{O}{\mathrm{H}}^{\bullet }$(15) (16) $\mathrm{B}-\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)+{\mathrm{O}}_2\to {\mathrm{O}}_2^{-\bullet }$(16) (17) $\mathrm{O}{\mathrm{H}}^{\bullet }+{\mathrm{O}}_2^{-\bullet }+\mathrm{LaFe}{\mathrm{O}}_3/\mathrm{B}-\mathrm{g}{\mathrm{C}}_3{\mathrm{N}}_4/\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{NB}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$(17)

Scheme 3. Schematic illustration of photocatalytic degradation of NB dye over LFO/B-CN/FO photocatalyst.

Enhanced charge carrier’s separation and transfer at the interface of LFO/B-CN/FO ternary composite is due to the proper band alignment between the counterparts. This probably reduces the likelihood of photogenerated holes and electrons recombining by fully utilizing them in redox reactions. As a result, the activity as well as stability of the ternary composite photocatalyst remarkably increases (65–68).

Future perspectives

Our future research will explore the application of the same composites for the degradation of a wider range of pollutants, aiming to validate their versatility and effectiveness. Additionally, these composites show potential for clean and sustainable energy solutions, such as CO₂ reduction and water splitting. We plan to investigate their conducting properties and solid-state conductivity. Furthermore, we will assess the applicability of these composites in energy storage devices, such as batteries and supercapacitors. The unique properties of the composites could lead to improved energy storage solutions, offering higher capacity, better stability, and longer lifespans. This broader scope will not only improve environmental remediation but also contribute to advancements in energy technology.

Limitations

The long-term durability of the photocatalysts under actual environmental conditions and the scalability of the hydrothermal synthesis process for industrial applications need to be assessed. The potential for efficiency fluctuations when using the photocatalysts on a larger scale is another drawback of our investigation. Furthermore, results acquired in a laboratory setting under controlled settings may not match the efficiency reported outside in the natural environment. The efficacy of the photocatalysts may be impacted by variables like temperature changes and sunshine intensity in natural settings that is the reason that our trials were carried out in artificial conditions. In order to guarantee consistent and dependable performance, these variables must be thoroughly evaluated and optimized for practical applications. To confirm the adaptability and efficacy of the ternary composite, further research should be carried out to explore its photocatalytic performance on a wider variety of contaminants and under natural environmental real samples.

Conclusion

A one-pot hydrothermal technique was used for the fabrication of CN, B-CN, binary composites (LFO/B-CN, FO/B-CN) and ternary composite (LFO/B-CN/FO). Various characterization techniques such as XRD, FT-IR, SEM, TEM, EDS, TGA/DSC, and UV–visible spectrophotometry were used to evaluate the structural, morphological, and chemical features of the as-fabricated photocatalyst. As obvious, boron doping remarkably reduced the band gap of CN photocatalyst and enhanced its light absorption. Coupling of LFO and FO significantly promoted charge carrier’s separation and transfer and also further improved the optical absorption of B-CN photocatalyst. The resultant LFO/B-CN/FO ternary composite degraded approximately 98% of navy-blue dye after visible light irradiation for 130 min at pH 7 and 35 °C for 50 ppm solution (100 ml). The photodegradation results followed pseudo-first and pseudo-second-order kinetics models. Under ambient conditions, all photocatalysts revealed pH values higher than their pH_pzc, indicating that they have negative charges. Thermodynamics studies revealed that the dye degradation over CN, B-CN, and LFO/B-CN is nonspontaneous, while spontaneous for FO/B-CN and LFO/B-CN/FO. Reusability and recycling tests revealed that the catalyst is highly stable, reusable, and completely recoverable from the reaction system over the course of eight consecutive cycles. This research highlights the potential of designing ternary heterojunctions as highly efficient photocatalysts for degrading environmental persistent pollutants.

Acknowledgments

The authors are grateful to Abdul Wali Khan University Mardan, Pakistan, and Prince Sultan University, Riyadh, Saudi Arabia for the overall support during this work. Authors would like to thank Prince Sultan University for paying the Article Publication Charges. All authors have contributed to this study at different stages. Wali Muhammad, Abbas Khan, and Sajjad Hussain: study design, method design, analytical protocol design, writing, reviewing, and editing. Hammad Khan and Rasha A. Abumousa: experimental assistant, discussion during writing, and reviewing. Iltaf Khan and Sikandar Iqbal: experimental assistant, activities, and data analysis. Rasha A. Abumousa, Mohamed Bououdina, and Muhammad Humayun: experimental assistant, discussion during writing, reviewing, and editing. All authors read and approved the final version of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

We declare that no special funds, grants, or other support were received during the preparation of this manuscript; however, the Department of Chemistry at Abdul Wali Khan University Mardan provided the basic chemicals and working space during this study.