Synergistic enhancement of photocatalytic performance in flower-like CuS/RGO composite through integrated microwave absorption and photothermal effects

Abstract

In this study, flower-like copper sulfide/reduced graphene oxide (CuS/RGO) composite multifunctional photocatalysts were synthesized via a facile hydrothermal method, exhibiting outstanding microwave absorption and photocatalytic performance. The morphology, specific surface area, surface chemical state, microwave absorption properties, and photocatalytic activity of the composites were systematically characterized. The results revealed that the photodegradation performance of the flower-like CuS/RGO-20% composite was remarkably enhanced, showing a 3–4 fold improvement compared to pure RGO and pure CuS. Notable, the CuS/RGO composite demonstrated superior photocatalytic activity, attaining a methylene blue (MB) degradation efficiency of 76.8% under visible light irradiation within 120 min. Furthermore, the flower-like CuS/RGO-20% composite exhibited excellent photodegradation stability, retaining an MB removal rate exceeding 80% even after five consecutive cycles. In parallel, the multifunctional CuS/RGO composite displayed exceptional microwave absorption capabilities, achieving a minimum reflection loss (RL_min) of –28.4 dB at a thin thickness of 3.5 mm and a frequency of 6.4 GHz, along with a broad effective absorption bandwidth (EAB) of 3.52 GHz. This study offers valuable perspectives for the design of multifunctional photocatalysts aimed at the remediation of textile dyeing wastewater.

Introduction

With the development of industrialization and fast human population growth, the intensification of energy consumption and environmental pollution have led to great pressure on global energy and environment. The aquatic ecosystem and human health are seriously threatened by these organic pollutants released by the pharmaceutical, cosmetic, chemical, tanning, agricultural, and textile sectors.^1,2 Photocatalytic methods offer a promising strategy with widespread use to overcome the current energy and environmental crises. Encouragingly, most recently the photothermal effect that may also be a key factor in photocatalysis has been revealed, and the photothermal catalysis technology is considered to be a promising way to enhance the photocatalytic efficiency which gives a new vitality to the environment and energy production due to its environmental protection and cost-effectiveness. Until now, photothermal catalysis technology has been applied to photothermal-photocatalytic degradation, hydrogen production, sterilization, desalination, and other aspects.^3–5

As an important p-type semiconductor, CuS shows the potential candidate for photothermal photocatalysis with the integrated feature including photothermal and photocatalytic properties.^6–8 The photocatalytic activities of nanoparticles are largely dependent on their photo absorption and carrier-generation properties, which are usually affected by the materials and their structures. In particular, CuS can be prepared with morphologies such as nanoflowers, nanoplates, nanotubes, nanorods, nanoflakes, nanospheres, nanocubes, and complex hierarchical micro/nanostructures.^9–11 The unique morphology of these nanostructures is based on nanoscale aggregation and the nanoparticle units comprise the secondary structures, which are expected to improve and promote the photocatalytic properties of these materials.^12–14 Nevertheless, for simple nanostructures, the photogenerated electron-hole pairs from CuS are rapidly recombined, and their photoabsorption are low, which limits their photoelectric, photothermal, and photocatalytic applications. Moreover, the low surface-to-volume ration shows a low contact between the CuS photocatalytic materials and the reactants, which is obstacle faced in the practical application for photocatalysis. Additionally, under the influence of van der Waals force, the self-aggregation phenomenon of copper sulfide nanoparticles reduces the photocatalytic activity which limits its application in photocatalysis.

RGO has attracted considerable attention in the field of microwave absorption and photocatalytic due to its unique hierarchical structure, lightweight nature, and significant dielectric loss properties. ¹⁵ Photocatalytic materials with microwave absorption properties can, when absorbing microwaves, both utilize the thermal effect of microwaves to increase the temperature of the reaction system and promote the separation and migration of photo-generated charge carriers, thereby enhancing the reaction rate. ¹⁶ Additionally, when light is insufficient, catalytic activity can be maintained or enhanced solely by microwave drive, while under adequate light conditions, the “light + microwave” dual-drive mode can cover more complex scenarios, significantly expanding the application scope.^17–19

In this study, we synthesized a multifunctional CuS/RGO composite with both photocatalytic and microwave absorption properties through compositional regulation and structural design using a simple hydrothermal method. RGO effectively enhanced adsorption, increasing contact between the semiconductor and organic dyes, which improved photocatalytic efficiency. The multifunctional CuS/RGO composite also mitigated strong van der Waals forces between graphene sheets, preventing agglomerative stacking of nanosheets. Additionally, the composite’s large specific surface area and layered structure promoted multiple reflections and the absorption of electromagnetic waves. This work comprehensively examined the micromorphology, structure, chemical composition, microwave absorption, and photocatalytic performance of the CuS/RGO composite, along with the synergistic interactions among its components. Compared with the CuS and RGO, the synthesized CuS/RGO composite exhibits highly photocatalytic activity and can be applied to the treatment of dye-contaminated wastewater.

Experimental

Materials

Graphene oxide (GO) were commercially purchased from Yuechuang Science Co., Ltd. Ethanol absolute were commercially purchased from Shanghai Titan Scientific Co., Ltd. All other chemicals were of analytic grade and used as-received without any further purification.

Syntnesis of the CuS

1 mmol CuCl•H₂O was added in the 10 mL (BMIm)Cl-methanol mixed solvent (the molarity of (BMIm)Cl is 5.0 mol/L) and kept stirring at 65°C for 10 min to obtain green solution. Then 3 mmol thioacetamide (TAA) was mixed in the above mixture and kept stirring to obtain Yellow turbid solution. The DI water was then put into and maintained at 65°C for 12 h, and the solution gradually changed from brown to black. At the end of the reaction, the solution was cooled naturally, the resulted precipitates were collected via centrifuging, washing with DI water and ethanol absolute, and then dried at 60°C for 8 h to obtain the product that is denoted as CuS.

Syntnesis of the multifunctional CuS/RGO composite

Firstly, graphene oxide (GO) powder and flower-like CuS were separately dispersed in deionized water and sonicated for 1 h. The CuS dispersion was then added to the GO dispersion, and the mixture was stirred vigorously for approximately 30 min. Ascorbic acid was subsequently introduced, and the mixture was transferred to a Teflon-lined stainless-steel autoclave. A hydrothermal reaction was carried out at 160°C for 12 h. After cooling to room temperature, the solution was washed several times with ethanol. Finally, the multifunctional CuS/RGO composite was obtained by centrifugation at 5000 rpm and dried using a freeze-drying method for 24 h. An overview of the flowchart of the synthetic pathways for the CuS/RGO composite was shown in Figure 1. The content of RGO was varied simultaneously as 5%, 10%, 20%, 30%, and 50%, and the resulting composites were designated as CuS/RGO-X, where X represents the mass fraction of RGO.

Figure 1.

Schematic illustration of the synthesis pathway for CuS/RGO composite.

Characterization

The morphological characteristics of the samples were examined using an S-4800 field-emission scanning electron microscope (FE-SEM). The specific surface area was determined through Brunauer-Emmett-Teller (BET) analysis using a Micromeritics ASAP 2020 system. The surface chemical composition and oxidation states were analyzed via X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, USA). Raman spectroscopy was performed on a Renishaw in Via Raman system with a 532 nm laser excitation source. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer (Bruker, Germany) equipped with Cu-Kα radiation (λ = 0.154 nm), scanning over a 2θ range of 5–90° at a rate of 10°/min. PL spectra were collected by using a fluorescence spectrophotometer at room temperature with an excitation laser wavelength of 365 nm.

Photocatalytic performance test

The photocatalytic performance of the multifunctional CuS/RGO-X composite photocatalyst was evaluated through the degradation of MB in an aqueous solution under visible light irradiation (Xe lamp, 150 W, λ = 390–770 nm). In the photodegradation experiment, 30 mg of photocatalyst was added to 50 mL of MB solution (10 mg/L), and the suspension was stirred in the dark for 30 min to establish adsorption equilibrium. Subsequently, the suspension was exposed to visible light irradiation, and 4 mL aliquots were collected at 10-min intervals. The collected samples were then centrifuged to separate the catalyst powder from the solution. The MB concentration in the supernatant was determined by measuring its absorbance using a UV-Vis spectrophotometer (Shimadzu UV-2450), with the degradation efficiency assessed by monitoring the intensity of the characteristic absorption peak at 664 nm.

Microwave absorption performance test

The electromagnetic parameters were measured using a vector network analyzer (VNA, AV3672B-S, China) employing the coaxial-line method within the frequency range of 2–18 GHz. Prior to measurement, the as-prepared composite was thoroughly mixed with paraffin wax in a mortar for 5 min and subsequently molded into a cylindrical sample with an inner diameter of 3.04 mm, an outer diameter of 7.00 mm, and a thickness of 3 mm. The microwave absorption performance of the composites was assessed based on transmission line theory, ²⁰ by evaluating the reflection loss (RL).

Result and discussion

Morphological structure

The morphology and microstructure of the samples were characterized using scanning electron microscopy (SEM), as presented in Figure 2. The SEM images in Figure 2(a) clearly reveals the flower-like architecture of the synthesized CuS, which adopts a well-defined spherical morphology with an average diameter of approximately 200 nm. This hierarchical structure is composed of interconnected ultrathin nanosheets, each about 10 nm in thickness, forming an open microporous network that substantially increases the specific surface area of the CuS. Elemental mapping of the flower-like CuS (Figure 2(f)–(h)) demonstrates a homogeneous spatial distribution of Cu and S, confirming the uniform construction of the CuS microstructure. Figure 2(b) presents the SEM image of reduced graphene oxide (RGO), revealing a loose arranged architecture with characteristic curled, transparent, and folded nanoflakes. Figure 2(c) and (d) display the CuS/RGO-20% composite at different magnifications, illustrating the uniform dispersion of flower-like CuS nanoparticles across the RGO nanosheets. This close interfacial interaction between CuS and RGO not only promotes efficient charge transfer during photocatalytic processes but also contributes to enhanced microwave absorption by facilitating polarization and attenuation mechanisms.

Figure 2.

The typical SEM images of: (a) the flower-like CuS, (b) RGO, (c) the multifunctional CuS/RGO-20% composite, (d) high-magnification SEM images of the multifunctional CuS/RGO-20% composite, (e) EDX spectra of the flower-like CuS, (f) high-magnification SEM images of the flower-like CuS, and (g–h) elemental mapping of Cu and S in the flower-like CuS.

Specific surface area analysis

The specific surface area and pore size of materials are critical factors influencing their photocatalytic and microwave absorption performance. Generally, a larger specific surface area and smaller pore size provide more active sites for photocatalytic reactions, thereby significantly enhancing the photocatalytic efficiency. Moreover, nanostructured materials exhibit superior wave absorption capabilities over a broader frequency range. The specific surface area and pore size distribution of nano-CuS and the multifunctional CuS/RGO-20% composite was analyzed using BET measurements, with the results presented in Figure 3. Figure 3(a) and (b) display the N₂ adsorption-desorption isotherms and pore size distributions of nano-CuS and CuS/RGO-20% composite, respectively. Both materials exhibit increased adsorption at the high-pressure end of the isotherms and feature H3-type hysteresis loops, characteristic of slit-shaped pores formed by the stacking of lamellar structures. This observation is consistent with the microstructural analysis from SEM. The specific surface area of the multifunctional CuS/RGO-20% composite was determined to be 66.5 m²/g, significantly surpassing that of nano-CuS (10.8 m²/g), representing a 6.6-fold increase. This substantial enhancement in surface area facilitates the generation of additional active sites and promotes efficient charge carrier migration, thereby contributing to the improved photocatalytic performance of the composite.

Figure 3.

N₂ absorption-deposition isotherms of: (a) nano-CuS, (b) the multifunctional CuS/RGO-20% composite, (c) XRD pattern of nano-CuS and the multifunctional CuS/RGO-20% composite, (d) Raman pattern of flower-like CuS, RGO, and the multifunctional CuS/RGO-20% composite.

The crystalline phases of CuS and the multifunctional CuS/RGO-20% composite was examined using X-ray diffraction (XRD), and the results are presented in Figure 3(c). The XRD pattern of the synthesized nano-CuS exhibits characteristic diffraction peaks at 2θ values of 28.1°, 29.3°, 32.8°, 39.3°, 48.9°, 53.1°, and 59.5°, corresponding to the (101), (102), (103), (105), (110), (108), and (116) crystallographic planes of hexagonal-phase CuS, respectively. The absence of additional diffraction peaks confirms the high purity of the synthesized CuS, with no detectable impurities. The diffraction pattern of the multifunctional CuS/RGO-20% composite closely resembles that of hexagonal-phase CuS, indicating that the CuS structure remains intact within the composite. Additionally, as shown in the inset, a weak diffraction peak at 2θ = 11.2° is observed, corresponding to the multilayer structure of RGO. The absence of impurity peaks further confirms the high purity of the composite material.

Figure 3(d) presents the Raman spectra of CuS, reduced graphene oxide (RGO), and the multifunctional CuS/RGO-20% composite. Both RGO and the CuS/RGO-20% composite exhibits characteristic peaks at approximately 1354 cm⁻¹ and 1593 cm⁻¹, corresponding to the D and G bands of carbon materials, respectively. The D-band is associated with structural disorder and defects, while the G-band corresponds to the first-order scattering E_2g vibrational mode, which characterizes the sp²-bonded carbon structure. The relative intensities of the D and G bands, expressed as the intensity ratio (I_D/I_G), provide valuable insight into the local disorder and crystallinity of the material.^21,22 For RGO, the I_D/I_G ratio is 0.8, whereas for the multifunctional CuS/RGO-20% composite, this ratio increases to 1.36, indicating a significant increase in surface defects following the reduction of RGO. Additionally, a distinct absorption peak at 468 cm⁻¹ is observed for CuS, attributed to lattice vibrations. The presence of the same peak in the CuS/RGO-20% composite suggests that the introduction of RGO does not alter the crystal structure of CuS, consistent with the XRD results.

Chemical composition analysis

The surface elemental composition and electronic states of the multifunctional CuS/RGO-20% composite was analyzed using X-ray photoelectron spectroscopy (XPS), with the results presented in Figure 4. As shown in Figure 4(a), the overall XPS spectrum of the CuS/RGO-20% composite clearly reveals characteristic peaks corresponding to the elements Cu, S, C, N, and O, which are attributed to the Cu 2p, S 2p, C 1s, N 1s, and O 1s regions, respectively. Notably, the peaks corresponding to C and O are derived from the RGO component, and no additional elemental peaks were observed, indicating the purity of the sample. To investigate the detailed chemical states of copper and sulfur, high-resolution XPS spectra of the Cu 2p and S 2p regions were analyzed. As shown in Figure 4(b), the Cu 2p spectrum exhibits two peaks at 951.50 eV and 931.30 eV, corresponding to Cu 2p₁/₂ and Cu 2p₃/₂, respectively, which are typical for nano-CuS. Figure 4(c) presents the XPS spectra of S 2p, with electron binding energies at 161.33 eV and 162.48 eV, corresponding to S 2p₃/₂ and S 2p₁/₂ of S²⁻, respectively. Additionally, the C 1s spectrum revealed three peaks at 288.40 eV, 286.60 eV, and 284.80 eV, corresponding to C–C, C–O, and C=O bonds, respectively. The relatively weak intensities of the C–O and C=O peaks suggest that most oxygen-containing functional groups were effectively removed, indicating a high degree of reduction of graphene oxide (GO). These findings confirm the successful synthesis of the multifunctional CuS/RGO composite via the hydrothermal method.

Figure 4.

XPS spectra of: (a) the multifunctional CuS/RGO-20% composite, (b) C 1s spectra, (c) Cu 2p spectra, and (d) S 2p spectra XPS spectra of the multifunctional CuS/RGO-20% composite.

UV-visible diffuse reflectance spectroscopy (DRS) was employed to evaluate the light-harvesting capabilities of the as-synthesized CuS and CuS/RGO composite, as depicted in Figure 5(a). UV-visible diffuse reflectance spectroscopy (DRS) revealed that the CuS/RGO-20% composite exhibits significantly stronger absorption across both UV and visible regions compared to pure CuS, indicating improved photon harvesting efficiency. This enhancement can be attributed to the synergistic role of RGO, which not only increases the specific surface area and introduces a porous structure conducive to light scattering, but also extends the π-conjugation network, facilitating electron delocalization. Further analysis using the Kubelka–Munk method ²³ (Figure 5(b)) showed a narrowing of the optical bandgap from 2.03 eV for pure CuS to 1.91 eV for the composite, effectively extending the photoresponse into the visible region and promoting the generation of photogenerated charge carriers.

Figure 5.

(a) UV-Vis diffuse reflectance spectra (DRS) of pure CuS and CuS/RGO-20% composites, (b) Tauc plots for bandgap estimation based on the Kubelka–Munk method, (c) PL of the synthesized pure CuS and CuS/RGO-20%, (d) EIS spectra of pure CuS and CuS/RGO-20%.

As shown in Figure 5(c), pure CuS displays a strong emission peak around 454 nm, which is significantly quenched in the CuS/RGO composite. This decrease in PL intensity suggests efficient suppression of radiative recombination, likely due to facilitated electron transfer from CuS to the conductive RGO network. The Nyquist plot of the composite exhibits a smaller semicircle diameter in the high-frequency region compared to pure CuS, indicating lower charge-transfer resistance and more efficient interfacial charge separation.

Collectively, these results demonstrate that the incorporation of RGO not only enhances light absorption and narrows the bandgap but also promotes charge separation and transport, thereby synergistically boosting the photocatalytic activity of the CuS/RGO-20% composite.

Microwave absorption performance and mechanism

The complex permittivity components—real part (ε′), imaginary part (ε′′), and dielectric loss tangent (tanãδε)—of the CuS, RGO, and CuS/RGO-20% composite were systematically investigated over the 1–18 GHz frequency range, as illustrated in Figure 6(a) to (c). ²⁴ These parameters provide fundamental insights into the materials’ polarization behavior and energy dissipation mechanisms under alternating electromagnetic fields. It is generally known that μ′ values of electric loss absorbing materials should be around 1.0 and μ′′ values of electric loss absorbing materials should be around 0.0. ²⁵ In this work, μ′ values of CuS, RGO, and CuS/RGO-20% composite has negligible fluctuations around 1.0, and μ′′ values of CuS, RGO, and CuS/RGO-20% composites have negligible fluctuations around 0.0. The ε′ values, representing the dielectric polarization capability, are significantly enhanced in the CuS/RGO-20% composite due to the formation of abundant heterogeneous interfaces between flower-like CuS and conductive RGO nanosheets, which promote intensive interfacial polarization. Meanwhile, the elevated ε′′ values and corresponding tan δ_e indicate strengthened dielectric loss, attributable not only to interfacial polarization but also to dipole polarization arising from sulfur vacancies in CuS and residual functional groups in RGO.

Figure 6.

The (a–c) electromagnetic parameters of the nano-CuS, RGO, and the multifunctional CuS/RGO-20% composites, The RL values of: (d–f) nano-CuS, (g–i) RGO, and (j–l) the CuS/RGO-20% composites.

At the microstructural level, the three-dimensional hierarchical architecture of the composite facilitates multiple reflection and scattering of incident microwaves, effectively prolonging the propagation path and enhancing energy attenuation. Moreover, the conductive RGO network serves as an electron transfer platform, enabling the formation of micro-capacitor structures at the CuS/RGO-20% interfaces and generating additional interfacial polarization loss. The presence of defect-induced dipoles and the matched impedance characteristics further promote the penetration of electromagnetic waves into the absorber rather than reflection at the surface, thus favoring efficient microwave dissipation.

As evidenced in Figure 6(d) to (l), the reflection loss (RL) values were quantitatively evaluated based on transmission line theory. ²⁰ With increasing thickness, the RL_min peaks shifted toward lower frequencies—a typical quarter-wavelength attenuation behavior. Pure CuS attained an RL_min of –7.24 dB at 4.16 GHz with a thickness of 3.5 mm. In contrast, the CuS/RGO-20% composite achieved a markedly enhanced RL_min of –28.4 dB at 6.64 GHz under the same thickness, along with a broader effective absorption bandwidth. This superior microwave absorption performance stems from the synergistic integration of conductive loss from RGO, polarization loss from CuS, and optimized impedance matching, collectively enabling more effective incident wave capture and conversion into thermal energy.

Photocatalytic performance and mechanism

The photocatalytic performance of the CuS/RGO-X composite was systematically evaluated by monitoring the degradation of methylene blue (MB) under visible-light irradiation from a xenon lamp. ²⁶ Prior to photocatalytic testing, adsorption equilibrium was established in the dark using 30 mg of CuS, RGO, and CuS/RGO-5% samples. As shown in Figure 7(a), the adsorption capacities reached 4.18%, 2.56%, and 3.34%, respectively, with equilibrium attained within 20 min.

Figure 7.

(a) Degradation rates of MB by CuS, RGO, and CuS/RGO-5% composites in dark environment, (b) Degradation rates of MB by CuS, RGO, and the different concentration the multifunctional CuS/RGO-X composites, (c) Absorbance of MB at different times in the light of CuS/RGO-20%, (d) Cyclic photocatalytic degradation of MB by CuS/RGO-20% (30 mg).

Under visible-light irradiation, all CuS/RGO-X composite demonstrated significantly enhanced photocatalytic activity compared to the individual components (Figure 7(b)). The CuS/RGO-20% composite achieved the highest degradation efficiency of 76.8% within 120 min, representing a 3.6-fold and 2.7-fold improvement over pure CuS (21.3%) and RGO (28.4%), respectively. The optimal performance at 20% RGO content is attributed to the synergistic effects between CuS and RGO, which facilitate effective charge separation and broaden the visible-light response. The optimal performance at 20% RGO content is attributed to the synergistic effects between CuS and RGO, which facilitate effective charge separation and broaden the visible-light response. Beyond this optimal ratio, photocatalytic activity gradually declined: the CuS/RGO-30% and CuS/RGO-50% composite reached degradation efficiencies of 64.2% and 52.7%, respectively. This decrease is likely due to excessive RGO coverage that shields active CuS sites, reduces light penetration, and may also lead to increased agglomeration and reduced interfacial charge transfer. These results indicate that while RGO enhances charge separation and adsorption, its content must be carefully optimized to balance light accessibility, active site exposure, and charge transport.

This kinetic superiority confirms the crucial role of RGO in facilitating charge separation and extending the lifetime of photogenerated carriers. The time-dependent UV-Vis spectra (Figure 7(c)) further validate the efficient degradation, showing a systematic decrease in the characteristic MB absorption at 664 nm accompanied by a visible color lightening from blue to light blue. The practical applicability of the CuS/RGO-X composite was demonstrated through cycling tests (Figure 7(d)). After five consecutive cycles, CuS/RGO-20% maintained over 80% degradation efficiency, exhibiting exceptional stability under repeated operation. This remarkable recyclability is largely due to the protective role of RGO, which not only enhances dye adsorption but also mitigates direct contact between MB and CuS, thereby suppressing photocorrosion and promoting long-term catalytic activity.

The hierarchical porous structure of the composite provides abundant active sites for the adsorption and degradation of complex dye molecules, while the optimized band structure enables efficient solar energy utilization. In addition to the optimal CuS/RGO-20% sample, we further observed that while composites with higher RGO content (e.g. CuS/RGO-30% and CuS/RGO-50%) showed decreased photocatalytic efficiency due to potential active-site shielding and light penetration reduction, they still exhibited meaningful degradation activity and maintained the hierarchical porous architecture beneficial for pollutant adsorption. These characteristics, combined with the material’s robust stability across multiple cycles and its competitive performance when compared with recent photocatalysts, highlight the versatility of the CuS/RGO-X series. The obtained degradation results were compared with the literatures and presented in Table 1. It is noted that the current work achieves better photocatalytic degradation when exposed to the visible light source. This work thus marks a significant advancement in the design of multifunctional photocatalytic systems for sustainable wastewater remediation.

Table 1.

Comparison of the characteristic parameter for CuS/RGO photocatalyst and other photocatalysts related to this work.

Catalyst	Light source	Catalyst load	Dye concentration	Minimum reflection efficiency	Time	Reference
CuBTC/CuS	Xe (100 W)	—	20 mg/L	—	120 min	Souza et al. 27
g-C3N4/CuS	Xe (300 W)	50 mg	20 mg/L	—	120 min	Luo et al. 28
CdS	Osram (300 W)	50 mg	10 mg/L	—	60 min	Munyai et al. 29
Co nanoparticles	Sunlight	250 mg	50 ppm	—	40 min	Adekunle et al. 30
CuS/RGO composite	Xe (150 W)	30 mg	11 mg/L	−28.4 dB	120 min	This work

The multifunctional CuS/RGO-X composite exhibits a sophisticated dual-performance mechanism, as illustrated in Figure 8, enabling both efficient microwave absorption and photocatalytic degradation through complementary physical and chemical pathways.

Figure 8.

Schematic illustration for the microwave absorption mechanism of the multifunctional CuS/RGO-X composite.

The exceptional microwave absorption capability originates from multiple synergistic mechanisms. The heterogeneous interfaces between flower-like CuS and layered RGO create extensive polarization centers, leading to significant interfacial polarization. ³¹ The three-dimensional hierarchical architecture, comprising interconnected CuS nanoflowers and folded RGO nanosheets, establishes numerous micro-cavities and interfaces that promote multiple reflections and scattering of incident electromagnetic waves.^32,33 This structural design effectively prolongs the propagation path of microwaves within the material, enhancing energy dissipation through repeated absorption events.

Simultaneously, the conductive RGO network facilitates electron migration and tunneling effects, while the semiconductor nature of CuS contributes to dipole polarization, particularly through sulfur vacancy-induced dipoles. The presence of oxygen-containing functional groups and structural defects in RGO generates additional polarization relaxation sites and modifies Fermi energy levels, further impeding wave transmission. ³⁴ Quantum confinement effects in the nanostructured components cause energy level splitting, creating new absorption channels within the microwave energy range. These combined mechanisms—interfacial polarization, multiple scattering, dipole relaxation, and quantum effects—work in concert to convert electromagnetic energy into thermal energy, achieving an optimal balance between impedance matching and attenuation capability.

The photocatalysis mechanism of the multifunctional CuS/RGO-X composite in this experiment can be explained as follows (Figure 8) ³⁵ :

$$\begin{array}{c}\multicolumn{1}{c}{\mathrm{CuS}/\mathrm{RGO}+\mathrm{hv}\to \mathrm{CuS}(h^{+})+\mathrm{RGO}(e^{-}),}\\ \multicolumn{1}{c}{e^{-}+O_2\to •O_{2^{-}}}\\ \multicolumn{1}{c}{h^{+}+H_{2}O/\mathrm{OH}-\to •\mathrm{OH}}\\ \multicolumn{1}{c}{•\mathrm{OH}\mathrm{or}{O}_{2^{-}}\mathrm{or}{h}^{+}+\mathrm{Dye}(\mathrm{MB})\to C{O}_2\mathrm{and}{H}_{2}O(\mathrm{Product})}\end{array}$$

The RGO substrate serves as a superior two-dimensional platform that enhances dye molecule adsorption through π-π stacking interactions with the aromatic structure of methylene blue, effectively concentrating pollutants near active sites. Under visible light irradiation, the excited electrons in CuS rapidly transfer to the RGO matrix via the extended π-conjugation system, significantly suppressing electron-hole recombination. The holes remaining in the CuS valence band react with water molecules to generate hydroxyl radicals (·OH), while the electrons on RGO reduce adsorbed oxygen to superoxide anions (·O₂⁻). These highly reactive oxygen species subsequently initiate the oxidative degradation of MB molecules, ultimately mineralizing them into harmless products.

The integration of these two functional mechanisms within a single material system represents a significant advancement in multifunctional material design. The structural features that enhance microwave absorption—such as the hierarchical porosity and extensive interfaces—simultaneously contribute to improved photocatalytic performance by providing abundant active sites and facilitating charge carrier separation, demonstrating the ingenious synergy between the two apparently distinct functionalities.

Conclusion

The multifunctional CuS/RGO-X composite was successfully synthesized via a facile hydrothermal method, demonstrating a “kill two birds with one stone” capability by integrating efficient microwave absorption and visible-light photocatalytic performance. The synergistic interaction between flower-like CuS and RGO nanosheets confers the composite with a hierarchical porous architecture, low density, and abundant active interfaces, which collectively contribute to its dual functional characteristics. In terms of photocatalytic performance, the optimized CuS/RGO-20% composite achieved a methylene blue (MB) degradation rate of 76.8% within 120 min under visible-light irradiation, representing a 3–4 fold enhancement over pure CuS. Moreover, the composite retained over 80% degradation efficiency after five consecutive cycles, highlighting its excellent structural stability and recyclability, largely attributed to the protective role of RGO in suppressing CuS photocorrosion. Regarding microwave absorption, the composite exhibited a minimum reflection loss (RL_min) of –28.4 dB at 6.4 GHz and an effective absorption bandwidth (EAB) of 3.52 GHz (5.20–8.72 GHz) at a thin thickness of 3.5 mm. This superior performance stems from the synergistic effects of optimized impedance matching, multi-scale polarization, conductive loss, and multiple scattering within the three-dimensional porous network. By combining efficient dye degradation with strong microwave attenuation in a single material system, this work provides a strategic design paradigm for developing multifunctional composites suitable for complex environments such as integrated photocatalytic wastewater treatment and electromagnetic compatibility applications.