Synergistic adsorption–photocatalysis in α-Fe₂O₃/PDINH Z-scheme heterojunction for efficient azo dye wastewater treatment

Abstract

The integration of adsorption and photocatalysis in heterojunction composites offers a promising strategy for efficient azo dye degradation. Here, a novel α-Fe₂O₃/perylene-3,4,9,10-tetracarboxylic diimide (PDINH) Z-scheme heterojunction was synthesized via a facile solvent method, showcasing synergistic adsorption–photocatalysis for wastewater treatment. Zeta potential analysis (α-Fe₂O₃: +14.7 mV; PDINH: −24.3 mV at pH 5.0) and density functional theory (DFT) calculations (binding energy: −3.10 eV) revealed strong electrostatic interactions between α-Fe₂O₃ and PDINH, enabling uniform nanoparticle dispersion and forming a heterostructure with enhanced specific surface area. Electrochemical measurements confirmed that the Z-scheme heterojunction significantly accelerated charge carrier migration and suppressed electron–hole recombination, facilitated by an internal electric field from well-matched band alignment. Under visible light, the α-Fe₂O₃-15/PDINH composite achieved 93.4% removal of methyl orange (MO), outperforming PDINH alone (63.1%) due to its positive surface charge (+8.7 mV at pH 5.0) that enhanced selective adsorption of anionic dyes. Quenching experiments identified h⁺, ·O₂⁻, and ·OH as the primary reactive species, with the Z-scheme pathway retaining strong redox capabilities for efficient degradation. Notably, the composite exhibited an operational cost of $2.41 per ton, significantly lower than other reported processes, and maintained high efficiency (81.8% MO removal) over multiple cycles. This work demonstrates that the α-Fe₂O₃/PDINH composite integrates adsorption and photocatalysis synergistically, providing a low-cost, scalable solution for azo dye wastewater treatment with potential for industrial application.

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1. Introduction

Organic dyes, particularly azo dyes, pose significant environmental challenges due to their widespread use in textile industries and inherent toxicity, carcinogenicity, and persistence in water bodies.^1–4 With China's annual azo dye production exceeding 75 000 tons (50% of total dye output), efficient treatment of azo dye wastewater has become critical to mitigate ecological and human health risks.¹ Traditional chemical and biological methods, such as coagulation and anaerobic/aerobic processes, suffer from limitations including high operational costs, chemical consumption, and potential secondary pollution.^5–10 Photocatalytic technology offers a sustainable alternative, but its practical application is hindered by low charge carrier efficiency and limited adsorption capacity for refractory pollutants.¹¹

To address these challenges, integrating adsorption and photocatalysis in heterojunction composites has emerged as a promising strategy.^12,13 Porous heterostructures not only enhance specific surface area and the number of active reaction sites^14,15 but also serve as traps for photogenerated electrons, reducing electron–hole recombination by shortening charge carrier diffusion distances.^16–18 For example, rGH-AgBr@rGO nanosheets and MOF-Fe@Ti₃C₂ have demonstrated synergistic removal of organic contaminants through combined adsorption and Z-scheme charge transfer,^19,20 while mesoporous ErFeO₃/g-C₃N₄ heterojunctions enhance efficiency via pore structure-mediated pollutant enrichment.²¹ These designs highlight that pore size tunability and heterojunction types (e.g., Z-scheme,²² type-II²³) are critical for optimizing adsorption–photocatalysis synergy, bridging adsorption capacity and photocatalytic activity.

α-Fe₂O₃, a narrow-bandgap semiconductor, has gained attention for its visible-light responsiveness, low cost, and surface charge-mediated adsorption of anionic dyes.^24–29 However, its wide conduction band and low electron mobility lead to rapid charge carrier recombination and band structure constraints.³⁰ PDINH, an organic perylene diimide derivative, offers favorable photothermal stability, high electron affinity, and π–π stacking for efficient charge transport, with compatible band alignment for heterojunction formation.^31–33 Previous studies on PDINH-based composites such as Cu₂O/PDINH and PDINH/PCN, have demonstrated enhanced charge separation via internal electric fields derived from band alignment, effectively inhibiting electron–hole recombination.^34,35 This synergy makes PDINH an optimal partner for α-Fe₂O₃ in heterojunction architectures.

In this study, we report a novel α-Fe₂O₃/PDINH Z-scheme heterojunction synthesized through a simple solvent method. By leveraging strong electrostatic interactions, the composite achieves uniform dispersion of α-Fe₂O₃ on PDINH, forming a structure with enlarged specific surface area. The constructed Z-scheme pathway accelerates charge carrier migration while maintaining robust redox capabilities, enabling efficient degradation of anionic azo dyes. This work presents a low-cost, high-performance, and facile strategy for integrating adsorption and photocatalysis, offering new perspectives for azo dye wastewater treatment.

2. Materials and methods

2.1 Materials

PDINH (98%) were acquired from Innochem Reagents Co., LTD (Beijing, China). α-Fe₂O₃ (99%, average particle size: 100 nm), acid orange 7 (AO7), crystal violet (CV), methyl orange (MO), N,N-dimethylformamide (DMF, C₃H₇NO, 99.5%), p-benzoquinone (C₆H₄O₂, 98%), and tert-butanol (C₄H₁₀O, 99%) were obtained from Sinopharm Chemical Reagent Co., LTD (Shanghai, China). Potassium iodide (KI, 99%) was supplied by Aladdin Chemical Reagents Co., LTD (Shanghai, China). Ultrapure water (resistivity ≥18.2 MΩ cm at 25 °C, total organic carbon (TOC) <5 ppb) was prepared using a Milli-Q system (Merck Millipore, Darmstadt, Germany) and utilized for all experimental procedures. All chemicals were of analytical grade and used without further purification.

2.2 Synthesis of α-Fe₂O₃-x/PDINH photocatalyst

The α-Fe₂O₃-x/PDINH photocatalyst was synthesized via a simple solvent method, as outlined below and illustrated in Scheme 1. Specifically, 15 mg of α-Fe₂O₃ and 85 mg of PDINH were separately dispersed in 20 mL and 40 mL of DMF solution, respectively. The suspensions were subjected to sonication for 10 minutes to ensure uniform dispersion of the components. Subsequently, the two solutions were combined under magnetic stirring at 500 rpm for 30 minutes at room temperature (25 °C) to promote interfacial interaction between α-Fe₂O₃ and PDINH. To induce precipitation of the composite, 150 mL of ultrapure water was gradually added to the mixture while stirring was continued for an additional 30 minutes. The resulting suspension was then filtered through a membrane filter, and the collected solid was washed three times with ultrapure water to remove residual DMF and impurities. Finally, the wet solid was dried in a vacuum oven at 60 °C for 6 hours to obtain the α-Fe₂O₃-x/PDINH photocatalyst, where “x” denotes the weight percentage of α-Fe₂O₃ in the final composite (e.g., α-Fe₂O₃-15/PDINH indicates a 15 wt% α-Fe₂O₃ loading).

Scheme 1 Synthesis process of α-Fe₂O₃-x/PDINH photocatalyst.

2.3 Characterizations

The structural characteristics of the composites were systematically examined using X-ray diffraction (XRD, Rigaku D-MAX 2500, Japan) over a scanning range of 10° to 80° at a rate of 5° min⁻¹, enabling the identification of crystalline phases and lattice parameters. Fourier-transform infrared spectroscopy (FTIR, Thermo Fisher Nicolet iS50, USA) was employed to analyze functional groups, with spectra recorded in the range of 400–4000 cm⁻¹ using the KBr pellet method to confirm the successful integration of α-Fe₂O₃ and PDINH.

Surface morphology, particle size, and elemental composition were characterized by scanning electron microscopy (SEM, Hitachi SU8020, Japan) operated at 5 kV, complemented by energy-dispersive X-ray spectroscopy (EDS, Bruker XFlash 6130) with a spatial resolution of ≤1 μm to map the distribution of Fe, O, C, and N elements. The specific surface area, pore volume, and pore size were analyzed using a surface area and pore size analyzer (BET, Micromeritics ASAP 2460, USA). Full-spectrum X-ray photoelectron spectroscopy (XPS, Thermo Fisher EscaLab 250Xi, USA) was conducted to verify elemental composition and chemical states, while valence band X-ray photoelectron spectroscopy (VB-XPS) was used to determine valence band positions. The optical properties were assessed using UV-visible diffuse reflectance spectroscopy (UV-vis DRS, Shimadzu UV-3600, Japan), and the bandgap width³⁶ was calculated according to eqn (1):

where α is the absorption coefficient, h is Planck's constant, ν is the optical frequency, E_g is the bandgap width, and the index n depends on the semiconductor type (n = 0.5 for direct bandgap; n = 2 for indirect bandgap).

Optoelectronic behaviors were investigated through electrochemical impedance spectroscopy (EIS, CHI760E workstation) in a three-electrode system using a 5 mM [Fe(CN)₆]³⁻/⁴⁻ electrolyte. EIS measurements were performed over a frequency range of 10⁻² to 10⁵ Hz with an amplitude of 5 mV to evaluate charge transfer resistance. Photoluminescence (PL) spectroscopy (Horiba FluoroLog-3) was used to determine the fluorescence intensity of the photocatalyst. Photocurrent response measurements (I-T, CHI660E) were conducted under visible light (λ > 420 nm) using a Xe lamp (PL-X300D, Beijing Prinsep Scientific Co., Ltd., China; total luminous output: 50 W; luminous spectrum: 320–2500 nm; input power: 200 W) to assess charge carrier separation efficiency.

Additionally, zeta potential measurements were carried out using a Malvern Zetasizer Nano ZS90 to analyze surface charge properties. Samples were dispersed in deionized water and measured at pH 5 and 25 °C, with zeta potential values reflecting the electrostatic repulsion between particles. DFT calculations were conducted as described in our previous article.³⁷ The adsorption energies (E_a)^38,39 of α-Fe₂O₃-x/PDINH were calculated using eqn (2):

E_a = E_{(α-Fe2O3/PDINH)} − E_PDINH − E_(α-Fe2O3) (2)

where E_{(α-Fe2O3/PDINH)} is the energy of the α-Fe₂O₃-x/PDINH complex, E_PDINH and E_(α-Fe2O3) are the energies of PDINH and α-Fe₂O₃, respectively.

2.4 Photocatalytic performance evaluation

The photocatalytic activity was systematically evaluated using a photocatalytic reaction apparatus consisting of a 100 mL quartz reactor equipped with a circulating water system and a cooling fan to maintain isothermal conditions (25 °C). Briefly, a 20 mg L⁻¹ azo dye solution (50 mL, ultrapure water) of methyl orange (MO), crystal violet (CV), or acid orange 7 (AO7) was prepared, to which 25 mg of α-Fe₂O₃-x/PDINH was added. The mixture was adjusted to pH 5 and sonicated for 15 minutes to ensure homogeneous dispersion, followed by magnetic stirring in the dark for 1 h to achieve adsorption–desorption equilibrium prior to light irradiation.

Visible light irradiation was provided by a xenon lamp, and the photocatalytic performance of α-Fe₂O₃-x/PDINH was evaluated under visible light. During irradiation, 1 mL aliquots were withdrawn at set intervals, filtered through a 0.45 μm syringe filter (Jinteng, China), and analyzed using a UV-visible spectrophotometer (Shimadzu, UV-2600, Japan) to monitor dye concentration. Furthermore, the photocatalyst's recyclability was assessed over five consecutive cycles. After each run, α-Fe₂O₃-15/PDINH was recovered by vacuum filtration, then washed three times with ultrapure water, and dried at 60 °C for 8 h prior to reuse.

Quenching experiments were conducted to identify the primary reactive oxygen species involved in the catalytic process. Prior to light exposure, specific scavengers were introduced into separate reaction vessels: 0.05 mol L⁻¹ benzoquinone (BQ) to quench superoxide radicals (·O₂⁻), 0.5 mol L⁻¹tert-butanol (TBA) to quench hydroxyl radicals (·OH), and 0.1 mol L⁻¹ potassium iodide (KI) to quench photogenerated holes (h⁺). The concentrations of scavengers were selected based on preliminary titration experiments to ensure complete quenching without interfering with light absorption or catalyst aggregation. The degradation efficiency (η) of the azo dyes was calculated using eqn (3):

where C₀ and C_t denote the initial and t-time dye concentrations, respectively.

3. Results and discussion

3.1 Microstructure and chemical properties

The morphologies of PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH are visualized in Fig. 1. PDINH (Fig. 1a) presents as short rod-like fragments aggregated into irregular clusters, a result of sonication-induced fragmentation during synthesis, which aligns with our prior work.³⁷ In contrast, α-Fe₂O₃ (Fig. 1b) displays uniform spherical nanoparticles, while α-Fe₂O₃-15/PDINH (Fig. 1c) retains the rod-like matrix of PDINH with spherical α-Fe₂O₃ nanoparticles densely anchored on its surface, forming a heterostructured interface. EDS elemental mapping (Fig. 1d–h) further confirms homogeneous distribution of C, O, Fe (9.44 wt%), and N across the composite, verifying successful integration of α-Fe₂O₃ into the PDINH matrix.

Fig. 1 SEM images of (a) PDINH, (b) α-Fe₂O₃ and (c) α-Fe₂O₃-15/PDINH; (d–h) EDS elemental maps for C, O, Fe, and N in the composite; (i) zeta potential testing of PDINH and α-Fe₂O₃ (pH = 5.0); (j) DFT-optimized adsorption configuration of α-Fe₂O₃ on PDINH.

Zeta potential analysis (Fig. 1i) at pH 5.0 reveals complementary surface charges: PDINH exhibits a negative zeta potential (−24.3 mV), while α-Fe₂O₃ shows a positive charge (+14.7 mV). This charge disparity drives electrostatic attraction between the two components, a key factor in heterojunction formation. To quantify this interaction, DFT calculations (Fig. 1j) were performed to model the most stable adsorption configuration of α-Fe₂O₃ on PDINH. The optimized structure shows α-Fe₂O₃ nanoparticles adhering to the PDINH surface via robust electrostatic forces, with a calculated binding energy of −3.10 eV. This strong interfacial interaction not only stabilizes the composite structure but also facilitates efficient charge separation.⁴⁰

The specific surface area (SSA) and pore size distribution of PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH were evaluated through N₂ adsorption–desorption analysis. As shown in Fig. 2a, all samples exhibited type IV isotherms with H3 hysteresis loops, confirming their mesoporous nature.^41,42Fig. 2b presents the pore volume-pore size distribution curves. The pore size distribution of α-Fe₂O₃ is mainly concentrated in the range of 0–10 nm, while that of α-Fe₂O₃-15/PDINH is predominantly distributed between 10–40 nm, closely resembling the pore size distribution of PDINH. Specifically, the SSA values are 9.94 m² g⁻¹ for PDINH, 7.78 m² g⁻¹ for α-Fe₂O₃, and 11.9 m² g⁻¹ for α-Fe₂O₃-15/PDINH, with corresponding average pore volumes of 0.072 cm³ g⁻¹, 0.033 cm³ g⁻¹, and 0.100 cm³ g⁻¹, and average pore sizes of 28.95 nm, 16.85 nm, and 33.46 nm, respectively (Table S1). Compared to the individual components of PDINH and α-Fe₂O₃, the α-Fe₂O₃-15/PDINH composite exhibits a higher specific surface area and average pore volume. This enhancement is primarily attributed to the partial fragmentation of the rod-shaped PDINH morphology during the synthesis of the composite and the dense anchoring of α-Fe₂O₃ onto PDINH.

Fig. 2 (a) N₂ adsorption–desorption isotherms; (b) pore volume-pore size distribution curves; (c) FTIR spectra; (d) XRD spectra.

The surface chemical composition of the α-Fe₂O₃-15/PDINH adsorption photocatalyst was characterized by FTIR and XRD. In Fig. 2c, for α-Fe₂O₃-15/PDINH, the peak in the 500–700 cm⁻¹ range is attributed to the stretching vibration of Fe–O bonds in α-Fe₂O₃.⁴³ The peaks at 3159 cm⁻¹ and 3038 cm⁻¹ represent the stretching vibrations of N–H bonds in PDINH. The peaks in the range of 1500–1600 cm⁻¹ are assigned to the stretching vibrations of C–N bonds and the N–H bonds in PDINH. Additionally, the peak at 1680 cm⁻¹ originates from the asymmetric vibration of C=O bonds.^34,44,45 As depicted in Fig. 2d, the XRD pattern of α-Fe₂O₃-15/PDINH reveals characteristic peaks of PDINH at 11.95°, 24.96°, 27.1°, and 30.42°. The peaks at 24.96° and 27.1° are ascribed to the twisted arrangement distance and the π–π distance between PDINH frameworks, respectively.^46,47 The characteristic peaks of α-Fe₂O₃ at 24.14°, 33.15°, 35.61°, 40.85°, 49.48°, and 54.09°, correspond to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), and (1 1 6) crystal planes of α-Fe₂O₃ according to the standard card (PDF#33-0664).⁴⁸ The presence of these characteristic peaks of PDINH and α-Fe₂O₃ further validates the successful synthesis of the α-Fe₂O₃-15/PDINH adsorption photocatalyst.

XPS spectra were employed to analyze the surface chemical states of PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH, with results presented in Fig. 3. In the full-spectrum survey (Fig. 3a), PDINH shows characteristic signals of C 1s, N 1s, and O 1s, while α-Fe₂O₃ exhibits C 1s, O 1s, and Fe 2p peaks. For α-Fe₂O₃-15/PDINH, the combined elemental signals (C, N, O, Fe) confirm the successful integration of α-Fe₂O₃ into the PDINH matrix. With respect to the C 1s high-resolution spectra (Fig. 3b), α-Fe₂O₃-15/PDINH displays a dominant sp²-hybridized carbon peak at ∼284.8 eV, along with peaks at 286.5 eV (sp² C bound to –NH₂), 287.9 eV (O=C–N in PDINH), and 289.2 eV (C=O bond).⁴⁵ For the O 1s spectra (Fig. 3c), PDINH shows peaks at ∼531.2 eV (C=O bond) and 532.9 eV (carbonyl oxygen). α-Fe₂O₃ exhibits typical Fe–O bond signals, while α-Fe₂O₃-15/PDINH presents a composite O 1s profile.⁴⁹ In the Fe 2p spectra (Fig. 3d), α-Fe₂O₃ shows characteristic Fe³⁺ peaks at 711.0 eV (Fe 2p_3/2) and 724.5 eV (Fe 2p_1/2).^50,51 For α-Fe₂O₃-15/PDINH, these Fe³⁺ signals are preserved, confirming the successful loading of α-Fe₂O₃ onto PDINH. Moreover, the absence of additional Fe oxidation states (e.g., Fe²⁺) indicates the stability of α-Fe₂O₃ in the composite.

Fig. 3 XPS spectra of PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH: (a) full spectrum, (b) C 1s, (c) O 1s, (d) Fe 2p.

To clarify the band structure characteristics, the band positions of PDINH and α-Fe₂O₃ were determined by UV-vis DRS and VB-XPS. As depicted in Fig. 4a, PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH all exhibit remarkable light absorption capacities within the visible light range. By analyzing the plots of (αhν)²versus photon energy (Fig. 4b), the calculated bandgap widths show that PDINH has a bandgap of 2.01 eV, while α-Fe₂O₃ exhibits a narrower bandgap of 1.88 eV. Moreover, we ascertained the valence band (VB) positions from VB-XPS spectra (Fig. 4c and d), determining that PDINH has a VB position at 1.67 eV and α-Fe₂O₃ has a VB position at 2.45 eV. The appropriate bandgap widths and the well-matched band positions between PDINH and α-Fe₂O₃ are conducive to the construction of heterojunction structures. Such structures can effectively facilitate the migration of charge carriers, thereby further boosting their photocatalytic performance.

Fig. 4 (a) UV-vis DRS spectra of PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH; (b) bandgap widths calculations for PDINH and α-Fe₂O₃; VB-XPS spectra of (c) PDINH and (d) α-Fe₂O₃.

3.2 Photoelectric activity

The photoelectric properties of PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH were evaluated using an electrochemical workstation. In the photocurrent response experiment, the photocurrent magnitude reflects the migration rate of charge carriers. As shown in Fig. 5a, the photocurrent signal of α-Fe₂O₃-15/PDINH was notably stronger than those of single PDINH and α-Fe₂O₃, indicating that the incorporation of α-Fe₂O₃ onto PDINH enhances the migration rate of photogenerated charge carriers. The charge separation process of α-Fe₂O₃-15/PDINH under light conditions was investigated using EIS. A reduction in the curvature radius of the Nyquist plot indicates an increase in the rate of interfacial charge transfer. As depicted in Fig. 5b, the Nyquist curve radius for α-Fe₂O₃-15/PDINH was smaller than those of single PDINH and α-Fe₂O₃. This suggests that the presence of α-Fe₂O₃ in the composite accelerates the transfer efficiency of photogenerated charge carriers on the catalyst surface, thus facilitating the separation of electron–hole pairs. Photoluminescence spectroscopy was employed to assess the recombination rate of photogenerated charge carriers. As shown in Fig. 5c, when excited with 645 nm light, both α-Fe₂O₃-15/PDINH and PDINH exhibited prominent fluorescence emission peaks at 716 nm. Notably, compared with single PDINH, α-Fe₂O₃-15/PDINH exhibited a significant fluorescence quenching phenomenon. This indicates that the recombination rate of electron–hole pairs in α-Fe₂O₃-15/PDINH was significantly reduced compared to that in the individual photocatalysts,⁴⁵ further demonstrating the superior photocatalytic performance of α-Fe₂O₃-15/PDINH over single PDINH and α-Fe₂O₃.

Fig. 5 (a) photocurrent response spectra, (b) electrochemical impedance spectra, and (c) photoluminescence spectra of PDINH, α-Fe₂O₃, and α-Fe₂O₃-15/PDINH.

3.3 Photocatalytic degradation experiments

The impact of varying α-Fe₂O₃ loadings on the adsorption and degradation efficiency of MO was investigated using different proportions of α-Fe₂O₃-x/PDINH. As depicted in Fig. 6a, in the dark, the adsorption efficiency of MO by pure PDINH was the lowest. However, with increasing α-Fe₂O₃ loading, the adsorption performance of the α-Fe₂O₃-x/PDINH for MO gradually improved. Specifically, the adsorption capacities of pure PDINH, α-Fe₂O₃-15/PDINH, and α-Fe₂O₃-30/PDINH for MO were 2.5%, 38.1%, and 50.5%, respectively. In the subsequent photocatalytic stage, the MO degradation efficiency of α-Fe₂O₃-x/PDINH was significantly higher than that of PDINH alone. The MO degradation rate of PDINH was only 63.1%, while α-Fe₂O₃-15/PDINH achieved a rate of 93.4%, indicating that loading α-Fe₂O₃ onto the PDINH surface greatly enhances photocatalytic performance. Moreover, as the α-Fe₂O₃ loading increased, both the adsorption capacity and photocatalytic efficiency of α-Fe₂O₃-x/PDINH improved. However, an excessive α-Fe₂O₃ loading led to a decline in photocatalytic performance, probably due to the aggregation caused by the uneven distribution of α-Fe₂O₃ on the PDINH surface. In comparison to previously reported photocatalysts (e.g., Fe₂O₃/TiO₂ ⁵² and self-assembled PDINH⁴⁶), α-Fe₂O₃-15/PDINH exhibited superior MO removal efficiency. Detailed performance comparisons with other reported materials are provided in Table S2. This efficiency enhancement is attributed to the Z-type heterojunction, which accelerates charge carrier migration and suppresses electron–hole recombination, thereby boosting photocatalytic activity. Additionally, the incorporation of α-Fe₂O₃ significantly enhances the composite's selective adsorption of anionic azo dyes, further contributing to the improved removal of target pollutants.

Fig. 6 (a) The degradation of MO by different ratios of α-Fe₂O₃-x/PDINH; (b) cyclic degradation curves of α-Fe₂O₃-15/PDINH; (c) adsorption–photocatalysis for degradation of different azo dye; (d) effect of different scavengers on MO degradation in the α-Fe₂O₃-15/PDINH system. All experimental data and error estimates were obtained from three parallel experiments.

The reusability and structural stability of α-Fe₂O₃-15/PDINH was assessed through five consecutive photocatalytic cycles. As shown in Fig. 6b, the composite exhibited MO degradation rates of 95.0%, 86.4%, 84.1%, 82.4%, and 81.8%, respectively, demonstrating a high level of catalytic activity retention. To further corroborate the stability of the α-Fe₂O₃-15/PDINH composite, post-characterization of the recycled samples was performed after the five consecutive photocatalytic cycles. As revealed in Fig. S1, one can hardly see any difference between the recycled and original α-Fe₂O₃-15/PDINH sample both morphologically and chemically. These results intuitively demonstrate the excellent stability and recyclability of α-Fe₂O₃-15/PDINH.

3.4 Adsorption–photocatalysis mechanisms on α-Fe₂O₃-15/PDINH

At pH = 5.0, the zeta potential of α-Fe₂O₃-15/PDINH was determined to be +8.7 mV through zeta potential characterization, consistent with the electrostatic interaction trend of its components shown in Fig. 1i. This positive surface charge endows the composite with a strong affinity for anionic azo dyes, as electrostatic attraction drives the adsorption process. As depicted in Fig. 6c, adsorption–photocatalysis experiments using different azo dyes demonstrated that α-Fe₂O₃-15/PDINH had significantly higher adsorption efficiency for anionic dyes (e.g., AO7 and MO) than for cationic dyes such as CV. This result further confirms the composite's preferential adsorption capacity for anionic dyes, consistent with the mechanism of electrostatic interaction-enhanced adsorption proposed by Wang et al.⁵³ A comparison in Table S3 showcases that α-Fe₂O₃-15/PDINH outperforms most reported similar photocatalysts, prominently highlighting its superior adsorption–photocatalysis activity.

To clarify the photocatalytic degradation mechanism, quenching experiments were performed (Fig. 6d). KI, p-BQ, and TBA were used as scavengers for h⁺, ·O₂⁻, and ·OH, respectively. In the absence of quenchers, the removal efficiency of MO reached 94.1% after the reaction. However, upon introducing KI, p-BQ, and TBA, the removal efficiency of MO decreased to 89.6%, 61.0%, and 77.3%, respectively. These results indicate that h⁺, ·O₂⁻, and ·OH are involved in the degradation process, with ·O₂⁻ playing a dominant role.

To decipher the charge carrier behavior and photocatalytic mechanism, we determine the band positions of PDINH and α-Fe₂O₃ according to the formula E_g = E_VB − E_CB.⁵⁴ For PDINH, the valence band (VB) and conduction band (CB) positions are calculated as 1.67 eV and −0.34 eV, respectively. For α-Fe₂O₃, the VB and CB are positioned at 2.45 eV and 0.57 eV. Based on this analysis, a Z-scheme heterojunction mechanism is proposed to explain the separation and migration of photogenerated carriers.⁵⁵ As illustrated in Scheme 2, the well-matched band alignment between PDINH and α-Fe₂O₃ facilitates the formation of an internal electric field that is favorable for carrier separation. Under visible light irradiation, photogenerated electrons from the LUMO orbitals of α-Fe₂O₃ rapidly recombine with photogenerated holes from the HOMO of PDINH. Meanwhile, photogenerated electrons from the LUMO orbitals of PDINH can reduce O₂ to form ·O₂⁻, whereas photogenerated holes from the HOMO orbitals of α-Fe₂O₃ can oxidize H₂O to generate ·OH.

Scheme 2 Proposed photocatalytic mechanism of MO in the α-Fe₂O₃-x/PDINH system.

The formation of the Z-type heterojunction enables the more negative LUMO orbitals and more positive HOMO orbitals to participate in the redox reactions, substantially enhancing the redox capability of the photocatalyst. This ensures that electrons are retained in PDINH and holes in α-Fe₂O₃, facilitating effective charge carrier separation. This mechanism is consistent with the findings of Yang et al., who concluded that the introduction of Bi₂Sn₂O₇ and the formation of a heterojunction significantly improved the adsorption capacity and photocatalytic activity of UiO-66-NH₂ in their study of the adsorption and degradation performance of Bi₂Sn₂O₇/UiO-66-NH₂.⁵⁶ Given these insights, the photocatalytic degradation process of MO in the α-Fe₂O₃-15/PDINH system was summarized at eqn (4)–(8).

α-Fe₂O₃-15/PDINH + vis light → h⁺ + e⁻ (4)

h⁺ + H₂O/OH → ·OH (5)

e⁻ + O₂ → ·O₂⁻ (6)

·O₂⁻ + MO → CO₂ + H₂O (7)

·OH + MO → CO₂ + H₂O (8)

3.5 Adsorption–photocatalysis for mixed dye wastewater treatment

To assess the efficacy of α-Fe₂O₃-15/PDINH in treating complex dye contaminants, a mixed dyes wastewater containing MO (10 mg L⁻¹), AO7 (10 mg L⁻¹), and CV (10 mg L⁻¹) was prepared. As visualized in Fig. 7a, the UV-vis absorption spectra and inset photographs illustrate the entire adsorption–photocatalysis process. Specifically, the solution at 60 minutes before treatment represents the initial mixed dye solution. After 60 minutes of dark adsorption, the solution (corresponding to 0 minute in the spectra) shows reduced dye concentrations, and subsequent irradiation for 60–120 minutes achieves further degradation.

Fig. 7 (a) UV-vis absorption spectra of different pollutants; (b) COD removal of mixed dyeing wastewater by α-Fe₂O₃-15/PDINH. All experimental data were obtained from three parallel experiments.

Chemical oxygen demand (COD), a critical indicator of organic pollutant mineralization, was employed to further quantify degradation efficiency.⁵⁷ As depicted in Fig. 7b, under sequential dark-light irradiation, the COD removal efficiency reached 48.2% after the initial dark adsorption stage. When the reaction was extended to 120 minutes, a COD removal efficiency of 73.5% was achieved. These results indicate that α-Fe₂O₃-15/PDINH not only demonstrates strong adsorption-driven dye removal in the dark but also sustains efficient photocatalytic mineralization under illumination, showcasing its dual-function potential in tackling multi-dye wastewater systems.

3.6 Implications for industry

Beyond performance validation, this study further quantifies the economic feasibility of α-Fe₂O₃-x/PDINH for dyeing wastewater treatment. Based on the 2025 Chinese market prices, the cost components of some reported processes were estimated. As indicated in Table 1, the sequential chemical coagulation-electrooxidation (CC-EO) process incurs a total operational cost of ∼6.91 USD per t, covering electricity, chemical reagents, and process-specific fees.⁵⁸ For the sono-photocatalytic US/UV/ZnO/PS process, electricity and chemical costs are 7.1 USD per t and 147.5 USD per t, respectively.⁵⁹ In comparison, the total chemical consumption cost for the α-Fe₂O₃-x/PDINH system is estimated to be only ∼2.41 USD per t of treated wastewater. This value is strictly derived from actual reagent usage during synthesis and treatment processes, with α-Fe₂O₃ and PDINH contributing approximately 0.5 USD per t and 1.91 USD per t, respectively. This cost analysis, based on experimental consumption and prevailing market prices, clearly demonstrates the substantial economic advantage of our composite material over other reported photocatalytic systems.

Table 1 Cost comparison between the α-Fe₂O₃-x/PDINH adsorption–photocatalysis process and other dyeing wastewater treatment processes

Technics	CC-EO		US/UV/ZnO/PS		α-Fe2O3-x/PDINH
Item	Power & Chemicals	CC process	Power	Chemicals	Chemicals
Cost (USD per t)	6.5	0.41	7.1	147.5	2.41

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Crucially, while α-Fe₂O₃-x/PDINH utilizes simulated sunlight in laboratory tests, its photocatalytic mechanism remains compatible with natural sunlight. This inherent compatibility enables the potential replacement of artificial light sources with natural sunlight in industrial applications, helping to reduce electricity consumption. Furthermore, the material's excellent reusability (stable performance over multiple cycles) lowers material replacement costs. Combined, these features reduce both short-term operational expenditures and long-term capital investment, enhancing overall economic viability.

4. Conclusion

This study develops a synergistic adsorption–photocatalysis system based on α-Fe₂O₃-x/PDINH composites for efficient dye wastewater remediation. Through electrostatic anchoring of α-Fe₂O₃ nanoparticles onto PDINH, a Z-scheme heterojunction with optimized band alignment is constructed. The resulting modified surface properties and enhanced specific surface area facilitate selective adsorption of anionic dyes, achieving 93.4% removal efficiency for MO—significantly higher than pristine PDINH (63.1%). Quenching experiments identify h⁺, ·O₂⁻, and ·OH as the primary reactive species in the photocatalytic process, with ·O₂⁻ playing a dominant role. The Z-scheme mechanism accelerates charge separation, suppresses electron–hole recombination, and enhances redox capacity. Importantly, the composite exhibits excellent stability and reusability across multiple cycles, coupled with cost-effectiveness derived from low chemical consumption. This work demonstrates the feasibility of integrated adsorption–photocatalysis via interface engineering, providing a scalable advanced oxidation strategy. The mechanistic insights and material design principles offer new pathways for high-performance environmental catalysts.

Author contributions

Ying Hu: methodology, writing – original draft, writing – review & editing, funding acquisition. Rongkang Yan: conceptualization, investigation, formal analysis, writing – review & editing. Huiyan Pan: methodology, visualization, data curation. Ruyi Cai: visualization, data curation. Zhili Zeng: methodology, writing – review & editing. Jialun Jiang: validation, data curation. Meng Wang: methodology, formal analysis, data curation. Meng Shan: software, data curation. Shasha Liu: supervision, writing – review & editing. Hai Tang: resources, supervision, funding acquisition.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5cy00948k.

Acknowledgements

This work was financially supported by Natural Science Foundation of Anhui Province (2108085ME188), the Key Laboratory of Industrial Wastewater Treatment and Resource Utilization in Anhui Province (DHSZ2023-02), the Scientific Research Start-up Fund for Introduced Talents of Anhui Polytechnic University (2022YQQ083), and the Innovation and Entrepreneurship Training Program for College Students (202310363267).

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Footnote

† These authors contribute equally to this work.

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