Fabrication of Fe3O4/GO-based precious metal composite and research in comprehensive photocatalysis of organic pollutants with mechanism

Rui Wu; Xi Song; Junhong Wang

doi:10.25259/AJC_135_2025

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Original Article

2025

:18;

1352025

doi:

10.25259/AJC_135_2025

Fabrication of Fe₃O₄/GO-based precious metal composite and research in comprehensive photocatalysis of organic pollutants with mechanism

Rui Wu^,*, Xi Song, Junhong Wang

College of Chemical and Environment Science, Shaanxi University of Technology, Hanzhong, Shaanxi, 723001, China.

* Corresponding author: E-mail address: hftffc@163.com (R. Wu)

Received: 2025-02-12, Accepted: 2025-07-07, Epub ahead of print: 2025-09-29, Published: 2025-10-30

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Abstract

As a common antibacterial drug and organic dye, ciprofloxacin (CIP) and acid orange (AO) residues have posed a serious threat to ecosystems and human health. To solve this problem, Fe₃O₄/GO-based precious metal composites (Fe₃O₄/GO/M: Fe₃O₄/GO/Au, Fe₃O₄/GO/Ag) were prepared as catalysts to photocatalyze pollutants. The Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag) composites underwent comprehensive characterization, including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS)- mapping, and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and diffuse reflectance spectroscopy (DRS) were carried out. It was demonstrated that the Fe₃O₄/GO/M composite were successfully fabricated with excellent properties. The Fe₃O₄/GO and Fe₃O₄/GO/M composite performed adsorption and unique photocatalysis properties, respectively. The Fe₃O₄/GO/M composites were assessed through synergized adsorption-photocatalysis degradation of AO II and CIP. The pseudo-first-order and pseudo-second-order kinetic models, as well as Langmuir and Freundlich isotherms models were employed to evaluate the adsorption performance. The influence of various factors on photocatalysis were investigated in detail. The findings indicated that the composites demonstrated remarkable capabilities for pollutant removal, with the degradation rates of (84 ± 3.3) % for AO II by Fe₃O₄/GO/Au and (89 ± 3.9) % for CIP by Fe₃O₄/GO/Ag, respectively. The scavenger analysis and total organic carbon (TOC) was also conducted. The photocatalysis mechanisms was also highlighted in detail. It was primarily attributed to the surface plasmon resonance (SPR) of the precious metals and the charge transfer of graphene oxide (GO). Furthermore, the catalyst exhibited nice reusability in six consecutive cycle experiments. A preferable approach to fabrication of Fe₃O₄/GO/M composite and photocatalytic degradation of organic pollutants was established.

Keywords

Acid orange II

Ciprofloxacin

Fe3O4/GO/M

Photocatalytic degradation

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1. Introduction

Industrialization has significantly improved people’s quality of life. However, pollutants, such as organic dyes and pharmaceutical residues release to environment which pose considerable threat to ecosystems and endanger human’s health via the food chain [1]. As organic dye, acid orange (AO) not only contaminate environment but also cause irritation and damage to human’s skin and eyes. As common antibiotics in clinical practice, ciprofloxacin (CIP) can’t be completely metabolized by human body. Consequently, the AO and CIP persist and accumulate in water. Researchers devoted themselves to treatment of waste water and develop an efficient, cost-effective and dependable approach for eliminating pollution. To date, many technique, including membrane separation, adsorption, biodegradation and photocatalysis have been extensively employed for effective elimination of organic pollutants from wastewater. Among these techniques, photocatalysis play significant role in degradation of dyes [2-5]. Photocatalytic degradation technology, with the advantages of high efficiency, environmental friendliness, simple operation, low cost, no secondary pollution, shows great promise in environmental remediation. For photocatalysis, it is crucial to develop novel and highly efficient catalysts.

Photocatalysts, metal oxide (TiO₂, ZnO, P25 TiO₂, WO₃), precious metal nanoparticles (Au, Ag, Pt, and Pd NPs), metal sulfide (CdS, ZnS) and other material (graphene oxide- GO, BiVO₄, Bi₂Zr₂O₇, Ag₃PO₄, g-C₃N₄), are extensively utilized for photocatalytic degradation [6-13]. Heterostructured nano catalysts, such as DyVO₄/AgBr, TmVO₄, ZnS/ZnIn₂S₄/Fe₂O₃, SmMn₂O₅/Mn₂O₃/g-C₃N₄, CoFe₂O₄, g-C₃N₄/TiO₂/kaolinite, WO₃/GO, Bi/BiOF/Bi₂O₂CO₃, have attracted extensive attention from researchers. Hybrid or heterogeneous catalysts attracted strong attention for research [13-16]. Nevertheless, each material has its own advantages and disadvantages. P25 TiO₂ is a novel photocatalyst with unique catalytic performance, especially high efficiency. However, limited UV-light absorption and difficulty in separation are the major drawbacks. For most photocatalysts, the recovery is the greatest challenge. In addition, a single photocatalyst cannot completely meet the requirements of degradation. Composite materials with versatile properties and exceptional performance are necessary for photocatalytic applications. To address this, it is necessary to develop hybrid composite or heterogeneous catalysts, which combine recovery, ease of separation, and plasma resonance effect. Hybrid or heterogeneous catalysts with magnetic Fe₃O₄ can achieve rapid separation by an external magnetic field, with the significant advantage of recovery. Furthermore, graphene oxide (GO) is rich in hydroxyl, carboxyl, and epoxy functional groups, and these oxygen-containing groups endow GO with a significant advantage in removing pollutants from water [17,18]. In recent years, researchers have made significant advancements in the development of GO-based magnetic materials. By integrating Fe₃O₄ with GO, the individual strengths of the different materials are fully capitalized, and also synergistic effect is achieved, which enhances adsorption and degradation. Precious metals, such as Ag, Au, Pt, and Pd are traditional photocatalysts and serve as substrates for surface-enhanced Raman spectroscopy (SERS).

In the study, we fabricated Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag as photocatalysts. GO acted as a carrier and improved the dispersion and stability of the composite materials, which is beneficial for adsorbing pollutants [19]. As precious metal catalysts, Ag and Au NPs can enhance the ability to adsorption visible light. Ag and Au NPs are capable of capturing photogenerated electrons and effectively prevent their recombination with holes [20]. The integration of Ag and Au NPs significantly enhanced the activity of photocatalysts and promoted the transfer of photogenerated electrons. Consequently, Fe₃O₄/GO-based precious metal composite nanomaterials were designed and they were applied for photocatalytic degradation of antibacterial drug and organic dye. In addition, Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag as active SERS substrate were employed to detect pesticide residue in our previous work. Multifunctional three-layer Fe₃O₄/GO/M NP system not only act as SERS substrate, but also photocatalysts. Ultimately, the integration of pollutant monitoring and removal were achieved.

2. Materials and Methods

2.1. Chemicals and materials

Graphite powder (≥99.9%) was from Tianjin Damao Chemical Reagent Factory. Concentrated sulfuric acid (≥95%), potassium permanganate (≥99.5%), ferric chloride (≥99.5%), ethylene glycol (≥99.5%), silver nitrate (≥99.5%), isopropanol (≥99.7%), polyvinylimide (≥99%), ascorbic acid (AA) (≥99%), CIP (≥98%), and AO (≥95%) were purchased from Sinopharm Chemical Reagent Co. 3-aminopropyltriethoxysilane (APTES) (≥98%), tri-hydroxymethyl aminomethane (≥99.9%), dopamine hydrochloride (≥98%), peroxymonosulfate (PMS) (≥42%), trisodium citrate (≥99%) and chloroauric acid (≥99.9%) were produced from Aladdin. All chemicals were of analytical grade, and deionized water was from a Millipore water system.

2.2. Preparation of the materials

2.2.1. Synthesis of Fe₃O₄

Fe₃O₄ was synthesized by the solvothermal method as described in reference with little change [21]. Initially, 2.0010 g of FeCl₃ was dissolved in 60.0 mL of ethylene glycol. After complete dissolution, the pH of the solution was adjusted to alkaline with sodium hydroxide. The mixture was stirred for 30 min using a magnetic stirrer. Subsequently, the solution was transferred to a 100 mL polytetrafluoroethylene-lined reactor and kept at 180°C for 5 h. The Fe₃O₄ was separated with a magnet and dried in an oven at 60°C. The color of Fe₃O₄ is black.

2.2.2. Preparation of GO

The modified Hummer’s method was employed to synthesize GO [22]. Initially, 50.0 mL of sulfuric acid was introduced into a 500 mL beaker, and 1.002 g of graphite was added. The resulting mixture was placed in a water bath and maintained at 20°C for 1 h. Subsequently, 6.0020 g of potassium permanganate was introduced. The solution was under stirring for 3 h at 50°C, followed by continuous heating for 1 h at 90°C. 150.0 mL of deionized water was added to the mixture. Hydrogen peroxide was then introduced into the mixture until the bubbles disappeared. Subsequently, the mixture was centrifuged and washed the pH of sample to neutral pH, followed by ultrasonication for 2 h. The obtained GO was dried in an oven at 60°C. The GO is yellowish-brown.

2.2.3. Synthesis of Fe₃O₄/GO

To obtain GO with magnetism, 25.00 mL of isopropanol and 0.5000 g of ferric oxide were mixed and ultrasonicated for 40 min. Subsequently, 0.15 mL of APTES was added dropwise to the mixture, followed by an additional 10 min of ultrasonication. The mixture was transferred to a flask and subjected to reflux at 85°C for 3 h. After cooling to room temperature, the mixture was washed alternately three times with EtOH and deionized water. The free isopropanol and APTES were separated by magnet. The obtained product was then fully dispersed in 35.0 mL of deionized water. Meanwhile, 0.1500 g of GO was dissolved in 35.0 mL of deionized water. The two solutions were mixed and ultrasonicated for 30 min to achieve homogenous dispersion [23]. The mixed solution was stirred overnight at room temperature. Followed by separation with a magnet, Fe₃O₄/GO was obtained. The obtained Fe₃O₄/GO was dried in an oven at 60°C. Fe₃O₄/GO is grayish black.

2.2.4. Fabrication of Fe₃O₄/GO/Au nanocomposites

The Au NPs were obtained by redox reaction: 6 mL of ascorbic acid (AA) was diluted by 20 mL of deionized water. Then, 1 mL of sodium citrate solution was added, followed by 2.00 mL of 1% chloroauric acid. Subsequently, 0.5 g of Fe₃O₄/GO was dissolved in 100 mL of deionized water and sonicated for 30 min. After fully mixing, the pH of the solution was adjusted to 10 with sodium hydroxide solution, and sonication was repeated for 30 min. Then, 5.00 mL of PEI solution and the previously prepared Au sol were incorporated into the Fe₃O₄/GO solution, and maintained stirring for 2 h [24]. The mixture was then transferred to a water bath and kept at 80°C for 6 h. Finally, the Fe₃O₄/GO/Au was produced. The molar ratio of Fe₃O₄, GO, and Au was about 3.6 28 1. The Fe3O4/GO/Au was dried in an oven at 60°C.

2.2.5. Fabrication of Fe₃O₄/GO/Ag nanocomposites

Ag sol was obtained by chemical reduction method. The synthesis of Ag NPs and Fe₃O₄/GO/Ag was reported in our previous work with little change [24]. It involved sodium citrate, silver nitrate, and sodium borohydride. The Fe₃O₄/GO/Ag loading Ag NPs was in the presence of Tirs-HCl buffer and dopamine [25]. To obtain high load capacity of Ag, repeat the same procedure ten times. The molar ratio of Fe₃O₄, GO, and Ag was about 2:17:1. Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag were dark grey black.

2.3. Characterization

The crystal structure of Fe₃O₄/GO/M was collected by X-ray diffraction (XRD) (Bruker, German) and scanned within a 2θ range of 10° to 80°. The morphology of the samples was examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (Thermo Fisher, USA). The elemental nature of materials was assessed with energy-dispersive spectroscopy (EDS) (JEOL, Japan) and mapping. Furthermore, X-ray photoelectron spectroscopy (XPS) (Kratos, UK) measurements was conducted. The electrochemical impedance spectroscopy (EIS) (Gamry USA), cyclic voltammetry (CV) (Metrohm Autolab, Netherlands), and diffuse reflectance spectroscopy (DRS) (Shimadzu, Japan) were also recorded.

2.4. Adsorption and catalytic degradation of pollutants

2.4.1. Adsorption tests

A series of AO II and CIP standard solutions with different concentrations (4, 6, 8, 10, 20, 30, 40, 50, 60, 70, 80, and 90 mg/L), (2, 4, 6, 8, 10, 20, 30, 40 mg/L) were prepared respectively. The UV-visible absorption spectra of AO II and CIP were measured at wavelengths of 484 nm and 276 nm, respectively. The standard curves of AO II and CIP were established.

The adsorption kinetic model was established. For the same, 15 mg of Fe₃O₄/GO/Au was introduced into 50 mg/L of AO II solution. Similarly, 15 mg of Fe₃O₄/GO/Ag was incorporated into 30 mg/L of CIP solution. The Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag adsorbed AO and CIP in the dark, respectively. The AO II and CIP supernatants were collected at intervals of 30, 60, 90, 120, and 150 min post-reaction. The collected approximately 2.0 mL of supernatant was analyzed by UV-Vis spectrophotometer to evaluate the adsorption capacity and removal efficiency of the materials. Subsequently, a kinetic model was obtained through the experimental data, and kinetic parameters were calculated [26]. In addition, the comparison of the adsorption effects between Fe₃O₄/GO/Au and Fe₃O₄/GO, Fe₃O₄/GO/Ag and Fe₃O₄/GO were carried out.

To obtain the isotherm model, 15 mg of Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag were employed to adsorb different concentrations of AO II (50, 60, 70, 80, and 90 mg/L) and CIP (30, 35, 40, 45, and 50 mg/L). UV-Vis spectrophotometer was used to evaluate the adsorption capacity and removal efficiency. The experimental data were analyzed, and adsorption isotherm models were established [27].

2.4.2. Photocatalytic testing

Photocatalytic degradation experiments were carried out. The Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag photocatalyzed AO II and CIP, respectively [28,29]. The photocatalytic degradation behavior and the various factors, including the quantity of catalyst, initial concentration of pollutant, pH, and the amount of PMS were examined. Specifically, Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag were added to 10.0 mL of different concentrations of AO II and CIP, respectively. The mixed solutions were stirred, and adsorption equilibrium was achieved after approximately 180 min under dark conditions at room temperature. Subsequently, the mixtures were exposed to a xenon lamp with stirring. The irradiation intensity in photocatalytic tests is about 500 mW/cm². Samples were separated by magnet and collected 2.0 mL of supernatant at intervals of 0, 20, 40, 60, 80, 100, and 120 min. The supernatant was measured using UV-visible spectrophotometer.

3. Results and Discussion

3.1. Characterization analysis

3.1.1. Structural and morphological characterization

The XRD was employed to characterize the structure of Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag. The main diffraction peaks at 30.2°, 35.6°, 43.2°, 57.1°, and 62.7° correspond to Fe3O4 (joint committee on powder diffraction standards (JCPDS): No. 19-0629), as shown in Figure 1 (a). The diffraction peaks of GO are at 10.0°. For Fe₃O₄/GO/Au, additional diffraction peaks at 38.0° and 44.6°, 64.5°, and 77.5° were identified, corresponding to the (111, (200),(220), and (311) crystal planes of Au (JCPDS No. 04-0784) [30]. For Fe₃O₄/GO/Ag, besides Fe₃O₄ peaks, peaks at 38.1°, 44.2°, 64.3°, and 77.5° were attributed to the (111), (200), (220) and (311) crystalline planes of Ag NPs (JCPDS No. 04-0783), indicating the successful incorporation of Ag NPs [31]. The peaks at 62.7° for Fe₃O₄ were close to those at 64.5° and 64.3° for Au and Ag, leading to slight overlap. Similarly, the peak at 43.2° for Fe₃O₄ overlapped with those at 44.6° and 44.4° for Au and Ag, due to their close positions. The crystallite sizes of Fe₃O₄, Au, and Ag were 10 nm, 5 nm, and 5nm, according to the Scherrer model, respectively.

(a) XRD patterns and (b) FTIR spectra of the samples.

The GO and Fe₃O₄/GO composites were analyzed by infrared spectroscopy (IR) in the range of 500 cm^-1 to 4000 cm^-1, as illustrated in Figure 1(b). The main IR peaks of GO are located at 1721 cm-1, 1632 cm-1, 1395 cm-1, and 1042 cm-1, which correspond to hydrophilic functional groups, such as C=O and C-O-C, on the surface or edges of GO. The functional groups not only enhanced stability of the composites in aqueous solution, but also facilitated interaction with adsorbed molecular, thereby promoting the accumulation of adsorbed substances. For the IR of Fe₃O₄/GO composites, the peak of stretching vibration for -C=O in amide bond (-CONH) was identified at 1628 cm^-1, while the stretching vibration peak of the C-N was observed at 1403 cm^-1. Furthermore, the main peak at 596 cm^-1 was distinctly attributed to the stretching vibration of Fe-O. Evidently, the Fe₃O₄/GO composite was successfully synthesized according to IR analysis [32].

SEM, TEM image, and size histograms of Fe3O4/GO/M were analyzed, as shown in Figure 2, with insert lattice fringes. Figures 2a₁,a₃ represent the SEM, TEM, and size histograms of Fe₃O₄/GO/Au, and the lattice fringes of Au were inserted. Figure 2b₁,b₂ represent the SEM, TEM, and size histograms of Fe₃O₄/GO/Ag, and the lattice fringes of Ag were inserted. From the TEM image, the grey particles were Fe₃O₄ NPs, the black particles were Au and Ag NPs, and the film-like material in light color was GO. The image of GO is not very clear. The sizes of Fe3O4, Au, and Ag were less than 25 nm. TEM observation indicated that the NPs exhibit moderate agglomeration, with primary particle sizes less than 25 nm. As shown in Figure 2a₁ and Figure 2b₁, the SEM image of Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag exhibited distinct granular morphology. The successful synthesis of Fe₃O₄/GO/M can be primarily attributed to the GO with the advantage of abundant oxygen-containing functional groups and its high specific surface area, which facilitated effective deposition of precious metals.

(a1) SEM, (a2) TEM images and (a3) size histogramsof Fe3O4/GO/Au, (b1) SEM, (b2) TEM images and (b3) size histograms of Fe3O4/GO/Ag.

In order to confirm the compositions of Fe₃O₄/GO/Ag and Fe₃O₄/GO/Au, EDS-mapping analyses were conducted, as shown in Figure 3(a) and Figure 3(b), respectively. It was clearly identified that the peaks of Fe, C, O, and Ag in EDS and element mapping. Similarly, EDS analysis confirmed the presence of Fe, C, O, and Au in Fe₃O₄/GO/Au [33]. The Fe₃O₄/GO/M was comprehensively analyzed by XPS, and XPS analysis confirmed the presence of Fe, C, O, Ag, and Au in Fe₃O₄/GO/M, as shown in Figure 4. The binding energies of 711.1 eV and 724.2 eV correspond to Fe 2p_1/2 and Fe 2p_3/2, confirming the Fe²⁺ and Fe³⁺ in Fe₃O₄. Signals at 286.8 eV and 284.9 eV indicated the presence of different chemical states of C, specifically C-O-C and C-C or C=C bonds. Furthermore, the peaks at 532.3 eV and 530.5 eV witnessed the C-O and H-O for O 1s. Additionally, the peaks at 87.4 eV and 83.9 eV were attributed to Au 4f_5/2 and Au 4f_7/2, which were consistent with the binding energy of pure Au [34]. The double peaks at 368.2 eV and 374.2 eV witnessed the Ag 3d_5/2 and Ag 3d_3/2 [35]. These findings witnessed and confirmed the Fe, C, O, Ag, and Au in Fe₃O₄/GO/M, and they were in strong agreement with the results of characterization above. It was obvious that the Fe₃O₄/GO/M composites were successfully synthesized.

(a) EDX of Fe3O4/GO/Ag with element mapping and (b) EDX of Fe3O4/GO/Au with element mapping.

(a1) XPS of Fe3O4/GO/Au with high-resolution XPS of (a2) Fe, (a3) C, (a4) O and (a5) Au; XPS of (b1) Fe3O4/GO/Ag, and high-resolution XPS of (b2) Fe, (b3) C, (b4) O and (b5) Ag.

The Raman spectrum of GO and Brunauer-Emmett-Teller (BET) of Fe₃O₄/GO have been shown in Figures 5 (a and b). Raman spectroscopy revealed that the prepared GO exhibited two main absorption peaks, and they were located at 1583 cm^-1 (G peak) and close to 1350 cm^-1(D peaks). The peaks at 1583 cm^-1 and 1350 cm^-1 originated from the E₂g vibrational mode of sp² carbon and the vibration of sp3 carbon in graphite [36]. The average pore size of Fe₃O₄/GO was 18.7 nm.

(a) The Raman spectrum of GO, and (b) EBT of Fe3O4/GO.

3.1.2. Photoelectrochemical analysis of the photocatalysts

3.1.2.1. Optical characteristics analysis

The optical properties of the composites were assessed by DRS, as illustrated in Figure 6(a). When Fe₃O₄/GO matrix composites were stimulated by an external light, the electron (e^-) and hole (h⁺) pairs generated, due to the excitation of electrons from valence band (VB) to conduction band (CB). The band gap energies were determined using Tauc’s method [37]. The band gap energies of Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag were 2.58 eV and 2.46 eV, respectively. Generally, materials with narrow band gaps enable efficient visible-light absorption due to the requirement of low energy for electron excitation and wide band gap lead to strong oxidation. In our research, the band gap energy were less than 3, which facilitated absorption of visible light.

(a) The direct interband transition energies of samples. (b) Cyclic voltammogram and (c) electrochemical impedance spectra of samples.

3.1.2.2. Electrochemical characteristics analysis

For photocatalysts, the catalytic activity of the material, the photogenerated electron, hole separation and transfer are important for the photocatalytic efficiency. In order to investigate the electrical properties of the Fe₃O₄/GO matrix composites, CV and EIS were employed to examine electrochemical properties and charge transfer. The peak current densities of Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag were significantly greater than that of Fe₃O₄, GO and Fe₃O₄/GO, as shown in Figure 6(b). The results indicated that current and catalytic activity enhanced and great number of molecules participated in the reaction per unit time. The EIS of Fe₃O₄/GO/Au, Fe₃O₄/GO/Ag and Fe₃O₄/GO all exhibited semicircular shapes in Figure 6(c), and it indicated that charge transfer occurred. The radius of the semicircles of Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag were smaller than that of Fe₃O₄/GO. It was demonstrated that the reaction rate of the Fe₃O₄/GO/M surface was enhanced significantly. This phenomenon was attributed to the effective separation of photogenerated electron-hole pairs, which facilitated rapid process of charge transfer [38].

3.2. Adsorption experiments

In order to assess the performance of Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag, the adsorption behavior of the composites were examined via adsorption kinetics and adsorption isotherm models. A series solutions of AO II and CIP with different concentrations were measured by using UV-visible spectrophotometer to construct work curve. The work curves of AO II and CIP have been illustrated in supporting information (S1).

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3.2.1. Adsorption experiments

The relationship between adsorption time and adsorption efficiency for AO II and CIP have beem illustrated in Figures 7 (a, b). The adsorption process can be divided into three stages. In initial stage, AO II rapid diffused to the surface of the adsorbent, and AO II was effectively captured by the adsorption site of composite. The adsorption rate increased during the first 30 min. Then the adsorption rate continued to increase with slow rate, during 30 to 150 min. After 150 min, it exhibited negligible variation. The adsorbent was saturated, the adsorption process reached steady state ultimately. The adsorption behavior of the composites for CIP exhibited similar phenomena as AO, as illustrated in Figure 7(b). The adsorption equilibrium achieved between 150 and 180 min. The final adsorption rate of Fe₃O₄/GO and Fe₃O₄/GO/Au for AO II were 84.6% and 63.5%, respectively. The final adsorption rate of Fe₃O₄/GO and Fe₃O₄/GO/Ag for CIP were 78.2% and 52.4%, respectively. It was evident that adsorption rate of Fe₃O₄/GO is higher than Fe₃O₄/GO/MO. Because the surface of Fe₃O₄/GO was covered with Au and Ag NPs [39].

The effect of time on the sample of (a) AO and (b) CIP adsorption.

3.2.2. Adsorption kinetics

Pseudo-first-order and pseudo-second-order kinetic models were employed to evaluate the adsorption, as shown in Figure 8. Figures 8(a, b) illustrated the adsorption kinetic curves of Fe₃O₄/GO and Fe₃O₄/GO/Au for AO. While Figure 8(c) and Figure 8(d) depicted the adsorption kinetic curves of Fe₃O₄/GO and Fe₃O₄/GO/Ag for CIP. The corresponding kinetic parameters have been presented in Tables 1 and 2. It was indicated that the coefficients of determination (R²) of the pseudo-second-order kinetic model for both AO II and CIP were greater than the first-order kinetic model. Furthermore, the theoretical equilibrium adsorption capacities based on the pseudo-second-order model were closely with the practical values. It was demonstrated that the adsorption processes of Fe₃O₄/GO/Au for AO II and Fe₃O₄/GO/Ag for CIP followed pseudo-second-order kinetic model.

(a, c) Pseudo-first-order kinetic model, and (b, d) pseudo-second-order kinetic model of samples.

Table 1. The results of first-order kinetic model.

Samples	Qe. exp(mg/g)	Qe.cal(mg/g)	K₁(10^-2 l/min)	R²
Fe₃O₄/GO/Au	12	5.14	1.7	0.962
Fe₃O₄/GO/Ag	9.9	8.9	2.2	0.967

Table 2. The results of second-order kinetic model.

Samples	Qe.exp (mg/g)	Qe.cal (mg/g)	K₂ (10^-3 mg/(g·min))	R²
Fe₃O₄/GO/Au	12	12.58	5.9	0.998
Fe₃O₄/GO/Ag	9.9	11.54	2.9	0.991

3.2.3. Adsorption isotherm

Langmuir and Freundlich isothermal models involved in the adsorption. The influence of initial concentration of AO II and CIP was examined through adsorption isotherm experiments. The fitted curves have been presented in Figure 9; the results and the corresponding parameters have been detailed in Tables 3 and 4. Compared with correlation coefficients (R²) of the Freundlich model, the R² of Langmuir is close to 1. It suggested that the homogeneous monolayer adsorption occurred. The maximum adsorption capacities of Fe₃O₄/GO/Au for AO II and Fe₃O₄/GO/Ag for CIP were approximately 16.39 and 17.58 mg/g based on the Langmuir model.

(a, c) Langmuir isotherm and (b, d) Freundlich isotherm of samples.

Table 3. The fitting results of the Langmuir model.

Samples	Qm (mg/g)	KL (L/mg)	R²
Fe₃O₄/GO/Au	16.39	0.85	0.995
Fe₃O₄/GO/Ag	17.58	27.5	0.973

Table 4. The fitting results of the Freundlich model.

Samples	K_F (mg/g)/(mg/L)^1/n)	n	R²
Fe₃O₄/GO/Au	6.07	5.05	0.978
Fe₃O₄/GO/Ag	4.09	2.98	0.943

3.3. Catalytic experiment

3.3.1. Optimization of experimental parameter

3.3.1.1. The influence of catalyst

The quantity of catalyst plays a crucial role in the degradation of dyes. The dosage of Fe₃O₄/GO/Au (40, 50, and 60 mg) was examined. As depicted in Figure 10(a), the degradation efficiency of AO II increased from 31% to 84% with the increase of the amount of catalyst. It was attributed to the enlargement of contact area between the AO II and the composites, as a result of the generation of free radicals and enhancing degradation efficiency. However, the photodegradation activity of Fe₃O₄/GO/Au composites diminished when the dosage was escalated from 50 mg to 60 mg. This phenomenon can be ascribed to the fact that excessive catalyst obstructs light radiation. The optimum amount of catalyst can be considered as 50 mg. Similarly, the optimum amount of catalyst for CIP is also 50 mg in the presence of PMS, as in Figure 10(b). This phenomenon occurred because a low dosage of catalyst failed to provide sufficient active sites. While excessive catalyst blocked light radiation, which diminished light utilization and impaired catalytic performance [40].

The effect of catalyst dosage of (a) Fe3O4/GO/Au and (b) Fe3O4/GO/Ag and the effect of concentration on degradation of (c) AO and (d) CIP.

3.3.1.2. The influence of concentration of target pollutants

In order to examine the influence of the initial concentration of target pollutants, different concentrations of pollutants were examined. As illustrated in Figure 10(c), the degradation rate of AO II increased from 79% to 84% when the initial concentration of AO II was raised from 40 mg/L to 50 mg/L. However, subsequently, the degradation rate of AO II declined with further increase in the concentration of AO. This phenomenon can be attributed to the limited active site, which was provided by the fixed amount of Fe₃O₄/GO/Au. When the concentration of AO II increased, the number of AO II molecules surpassed the available catalyst, resulting in scarcity of active sites and consequent reduction in the degradation rate of AO. For CIP, the degradation efficiency was 89% when the initial concentration of CIP was 10 mg/L, as shown in Figure 10(d). The degradation efficiency of the Fe₃O₄/GO/Ag+PMS system diminished with increasing concentration of CIP. Because higher concentration of CIP adsorbs more photons, reducing light utilization efficiency. Furthermore, high concentrations of CIP can generate a great number of intermediate products or secondary products, which inhibit the degradation process by competing with CIP for active sites on the surface of photocatalyst [41].

3.3.1.3. The influence of pH

The pH of the solution affects the state of pollutants, the decomposition rate of PMS, and the charge properties of the catalyst surface, all of which are critical to degradation efficiency. The surface of Fe₃O₄/GO/Au exhibited positive charge in acidic environment, while it displayed negative potentials under neutral and alkaline conditions, as shown in Figure 11. When pH was 5, the maximum degradation efficiency was observed. Because the AO II dye with negative charge was prone to adsorb on Fe₃O₄/GO/Au with positive charge in acidic environment [42]. The adsorbed water molecules on the surface of the catalyst were replaced by hydroxyl groups, which hindered the formation of hydroxyl radicals and thus reduced the catalytic efficiency under neutral conditions. Additionally, the degradation efficiency of CIP was relatively low under acidic conditions and higher under alkaline conditions. When pH was below 5.9, CIP predominated as cations. When pH was above 8.9, anions were generated. When the pH is between 5.9 and 8.9, cations and anions coexist. Because both Fe3O4/GO/Ag and CIP carried positive charges in acidic pH, they repelled each other, leading to a decrease in catalytic efficiency. In contrast, they attracted each other and led to an increase in the degradation rate under alkaline and neutral conditions [23].

Zeta potentials of (a) Fe3O4/GO/Au and (b) Fe3O4/GO/Ag, effect of concentration on degradation of (c) AO and (d) CIP.

3.3.1.4. The influence of PMS

The impact of amounts of PMS on the degradation of CIP has been shown in Figure 12. As the concentration of PMS increased from 2 mg to 5 mg, the degradation rate of CIP rose. However, further increase in concentration of PMS from 5 mg to 7 mg resulted in a decline in degradation rate of CIP. The higher concentrations of PMS can react with ·SO₄^- to form·SO₅^-, and ·SO₅^- showed weak oxidizing ability.

The effect of PMS on degradation of CIP.

So, the optimal experiment parameters for photocatalytic degradation of AO II and CIP were observed. For AO, the dosage of catalyst and AO II was 50 mg and 50 mg/L, and the pH was 5. For CIP, the dosage of catalyst and PMS was 50 mg and 5 mg, the concentration of CIP was 10 mg/L, and the pH was 9.

3.3.2. Photocatalytic degradation performance

The degradation of AO II and CIP was systematically examined, as shown in Figure 13. Before illumination, the photocatalysts were subjected to 150 min in the dark in order to establish adsorption equilibrium. The photocatalytic degradation of the target pollutant is barely detectable in the absence of light in the control experiments. Compared with Fe₃O₄/GO, Fe₃O₄/GO/Au exhibited superior degradation of AO, and it achieved optimal catalytic performance with the degradation rate of 84%. Compared with Fe₃O₄/GO/Ag, Fe₃O₄/GO/Ag with PMS exhibited significant photocatalytic activity; the degradation rate was 89% for CIP after 140 min in the presence of light. The confidence intervals for photocatalytic degradation of AO II and CIP were (84 ± 3.3)%, (89 ± 3.9)%, at 95% confidence level.

Photocatalytic degradation of (a) AO and (b) CIP, and (c, d) corresponding kinetic simulation curves.

For Fe₃O₄/GO-based precious metal photocatalysts, precious metal NPs can capture visible light and exhibited localized surface plasmon resonance (LSPR). Fe3O4/GO served as a supporting matrix to facilitate light transmission and promote effective separation of charge. Additionally, PMS acted as a co-oxidant and improved the photocatalytic efficiency [43]. The time-dependent UV-profiles of AO II and CIP were provided as supporting information (S2). The XPS of Fe₃O₄/GO/M after degradation has been as supporting information (S3).

The kinetics of photodegradation for AO II and CIP were investigated using the Langmuir-Hinshelwood model [44]. For AO II, the degradation rate constant (k=8.8×10^-3 min^-1) of Fe₃O₄/GO/Au is greater than Fe₃O₄/GO(k=6.6×10^-3 min^-1). For CIP, the k of Fe₃O₄/GO/Ag+PMS system (k=1.3×10^-2 min^-1) was markedly higher than the PMS (k=4.2×10^-3 min^-1) and Fe₃O₄/GO/Ag (k=5.1×10^-3 min^-1) systems. The repeatability and stability of the Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag were shown in supporting information (S4). The comparison for photocatalytic degradation of AO II and CIP by other different catalysts has been displayed in supporting information (S5, S6). In order to assess the mineralization of CIP, TOC was analyzed. The TOC of AO II and CIP were 39% and 34% after degradation.

3.3.3. Reactive species analysis

In order to further investigate the photocatalytic mechanism, the capture of active species was conducted in the experiment [45]. As illustrated in Figure 14(a), the introduction of t-BuOH significantly diminished the photodegradation efficiency of AO, suggesting ·OH was the predominant reactive species for photocatalytic degradation of AO. When EDTA, MeOH, BQ, and t-BuOH were added, the removal rates of CIP were 70.3%, 49.8%, 60.9%, and 56.2%, respectively. Compared with the case without scavenger, the removal rates were reduced in the presence of different scavengers, as illustrated in Figure 14(b). The finding suggested that ·SO₄^-, ·OH and O2·- were the main reactive species and involved in the degradation of CIP in the Fe₃O₄/GO/Ag+PMS system.

Bursting experiment of (a) AO and (b) CIP.

3.3.4. Mechanism analysis

Based on photoelectrochemical analysis, potential photocatalytic mechanism were proposed. The catalytic degradation of AO II and CIP were illustrated in Figure 15(a, b). As is well known, Au and Ag NPs exhibit strong LSPR effects, which facilitated absorption of visible lights, and result of hot electrons. As an effective electron acceptor, GO loaded precious metal were beneficial to rapid absorption and transfer of electrons [46]. The energy of hot electrons was higher than the Fermi level of Au and Ag, electrons can directly transferred to the conduction band (CB) of GO and formed transport channel of interfacial electrons.

Schematic diagram of the potential photocatalysis mechanism of AO for (a) Fe3O4/GO/Au and (b) CIP for Fe3O4/GO/Ag.

The GO acted as an electron superhighway and accelerated the migration of electrons from Au and Ag to reaction sites and reduced energy loss. While the holes were kept on the surface of Au and Ag. This process can significantly reduce the probability of electron-hole recombination. In addition, because of GO and Au, and Ag, a Schottky barrier formed and promoted charge separation. The Schottky barrier prevented backflow recombination of electrons and holes, thereby extending the lifetime of charge carriers. The photoinduced electrons were then captured by O₂ and produced O₂^·-. Furthermore, the photogenerated holes reacted with water molecules and produced strong reactive ·OH. The generated free radicals effectively degraded dye molecules [47-49]. The generation of free radicals has been described by the following equations. In the equations, M represented Au and Ag. The photogenerated holes, •O₂^-, and ·OH cooperated to drive AO decomposition. The valence band potential (E_VB) and conduction band potential (E_CB) were calculated from the Eq. (1).

(1)

E_{VB} = χ - E_{e} + 0. {5E}_{g}, E_{CB} = E_{VB} - E_{g} .

χ represents electronegativity of photocatalyst [49], E_e is the free electron energy (4.5 eV), E_g is the band gap energy from DRS. According to previous literatures [50-51], the E_CB and E_VB values of GO for photocatalytic degradation of AO II were -0.85 ev and 1.61 ev.

(2)

M + h ν \to M^{*} \to M (e^{-}) + M (h^{+})

(3)

M (e^{-}) + GO \to GO (e^{-})

(4)

e^{-} + O_{2} \to \cdot O_{2}^{-}

(5)

h^{+} + {OH}^{-} \to \cdot OH

(6)

h^{+} + H_{2} O \to \cdot OH + H^{+}

The photocatalytic degradation of CIP and the aforementioned AO shared fundamental mechanistic similarity [52,53], but the role of MPS cannot be ignored. For photocatalytic degradation of CIP by Fe₃O₄/GO/Ag, photoinduced charge carriers appeared when visible light irradiated the photocatalyst, as shown in Figure 14(b). Initially, the photogenerated electrons were captured by O₂ and produced O₂^·-(Eqs. 2-6). Subsequently, O₂^·- and photogenerated electrons interacted with PMS to form ·SO₄^-. The ·SO₄^- can react with H₂O molecules to generate·OH [54,55]. The equations describing PMS-mediated free radical generation were as follows. Ultimately, the generated h⁺, ·OH, ·SO₄^- and O₂^·- radicals contribute to the decomposition of CIP Follow the same method, the E_CB and E_VB of GO for photocatalytic degradation of CIP were -0.91 ev and 1.67 ev (Eqs. 7-11), respectively.

(7)

e^{-} + O_{2} \to \cdot O_{2}^{-}

(8)

e^{-} + {HSO}_{5}^{-} \to \cdot {SO}_{4}^{-} + {OH}^{-}

(9)

\cdot O_{2}^{-} + {HSO}_{5}^{-} \to \cdot {SO}_{4}^{-} + H^{+}

(10)

\cdot {SO}_{4}^{-} + H_{2} O \to \cdot OH + {SO}_{4}^{2 -} + H^{+}

(11)

h^{+} + {HSO}_{5}^{-} \to \cdot {SO}_{4}^{-} + H^{+}

4. Conclusions

In summary, three-layered multifunctional Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag composite nanomaterials were successfully prepared and characterized by versatile techniques. The adsorption of AO II and CIP by Fe₃O₄/GO/Au and Fe₃O₄/GO/Ag was analyzed by Kinetic and isotherm models. The results indicated that the adsorption process followed pseudo-second-order kinetic model and monolayer adsorption. Additionally, Fe₃O₄/GO-based precious metal composite exhibited remarkable catalytic performance, with the degradation rates of 84% for AO II by Fe₃O₄/GO/Au and for CIP by Fe₃O₄/GO/Ag under optimal conditions. The degradation rate constants for AO II and CIP were 8.8×10^-3 min^-1 and 1.3×10^-2 min^-1, respectively. The photocatalytic mechanisms were simply discussed based on experimental results. The hybrid of GO and precious played significant role in photocatalytic degradation of pollutants. The integration of GO with precious metals facilitated electron transport and separation of charge carriers. It was demonstrated that the degradation of AO II by Fe₃O₄/GO/Au was primarily depended on ·OH and the degradation of CIP by Fe₃O₄/GO/Ag depend on ·SO₄^-, ·OH and ·O₂^-. The developed hybrid catalysts with three layer exhibited high stability, reusability, efficiency, especially the ease of separation compared with other work. In our previous work, Fe₃O₄/GO/M acted as SERS substrate and it was employed to detect pesticide residue. Combination with our previous research, this work offered novel perspective on the advancement of efficient SERS substrates and photocatalysts in water pollution monitoring and remediation. Fe₃O₄/GO/M composite materials enabled the integration of pollutant detection and simultaneous photocatalytic removal in water treatment. In addition, precious metal, especially Ag NPs, was a bacteriostatic agent with excellent performance, and they were also used as imaging probes in vitro and in vivo with nice biocompatibility. The fabricated high-performance, multifunctional, and hybrid Fe₃O₄/GO/M with three layers and magnetism is a promising photocatalyst, Raman substrate, adsorbent, bacteriostatic agent, and bioimaging probe. It is of great significance for them in wide application in environment, agriculture, industry, food, biomedicine, and public health field.

Acknowledgment

This work was supported by Key Research and Development Plan of Shaanxi Province (2024GX-YBXM-318), Qin Chuangyuan Project (2024PT-ZCK-38). Shaanxi Province Education Ministry Research Key Foundation (20JS015), Shaanxi provincial natural science basic research plan general project (2025JC-YBMS-146), The Brainstorm Project on Social Development by the Department of Science and Technology of Shaanxi Province (2025SF-YBXM-475). Education Research Project of Shaanxi University of Technology (JYYJ2023-16), National Natural Science Foundation of China (22177066).

CRediT authorship contribution statement

Rui Wu: Conceptualization, Funding acquisition, Investigation, Resources, Project administration. Xi Song: Writing original, Data curation, Validation, draft Methodology, Review, editing, Supervision. Junhong Wang: Formal analysis.

Declaration of competing 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.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of Artificial Intelligence (AI)-Assisted Technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_135_2025.

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1. Introduction

2. Materials and Methods

2.1. Chemicals and materials
2.2. Preparation of the materials
2.3. Characterization
2.4. Adsorption and catalytic degradation of pollutants

3. Results and Discussion

3.1. Characterization analysis
3.2. Adsorption experiments
3.3. Catalytic experiment

4. Conclusions

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