Monolith floatable dual-function solar photothermal evaporator: efficient clean water regeneration synergizing with pollutant degradation

Abstract

Meeting the growing demands of attaining clean water regeneration from wastewater and simultaneous pollutant degradation has been highly sought after. In this study, nanometric CuFe₂O₄ and plasmonic Cu were in situ confined into graphitic porous carbon nanofibers (CNF) through electrospinning and controlled graphitization, which were integrated onto a melamine sponge (S-FeCu/CNF) as a monolithic evaporator via a calcium ion-triggered network crosslinking method using sodium alginate (SA). This monolithic evaporator serves a dual purpose: harnessing solar-driven photothermal energy for water regeneration and facilitating synchronous contaminant mineralization through advanced oxidation processes (AOPs). The metal-modified FeCu/CNF graphitic porous carbon exhibited an enhanced light absorption property (≥95%) and was further securely anchored on the sponge by a calcium ion-triggered SA crosslinking technique, thereby efficiently restraining salt deposition. The FeCu/CNF evaporator demonstrated a solar-vapor conversion efficiency of 105.85% with an evaporation rate of 1.61 kg m⁻² h⁻¹ under one sun irradiation. The evaporation rate of the monolithic S-FeCu/CNF evaporator is close to 1.76 kg m⁻² h⁻¹, and an evaporation rate of 1.54 kg m⁻² h⁻¹ can be achieved even in 20% NaCl solution, with resistance to salt deposition and cycling stability. Synchronously, the monolithic D-S-FeCu/CNF evaporator also acts as a heterogeneous catalyst to activate peroxymonosulfate (PMS) and trigger rapid pollutant degradation, which also shows excellent catalytic cycling stability, producing clean water that satisfies the World Health Organization (WHO) standards. This work provides a potentially valuable solution for addressing desalination and wastewater treatment.

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New concepts

This study introduces a novel approach for clean water regeneration from wastewater while simultaneously degrading pollutants. By integrating nanometric CuFe₂O₄ and plasmonic Cu into graphitic porous carbon nanofibers (CNF) and embedding them into a melamine sponge, a monolithic evaporator (S-FeCu/CNF) is created. This evaporator utilizes solar-driven photothermal energy for water regeneration and facilitates contaminant mineralization through advanced oxidation processes (AOPs). Unlike conventional methods, the metal-modified FeCu/CNF structure enhances light absorption (>95%) and resists salt deposition through a calcium ion-triggered sodium alginate crosslinking technique. This innovation achieves a solar-vapor conversion efficiency of 105.85% and an evaporation rate of 1.61 kg m⁻² h⁻¹ under one sun irradiation. Even in 20% NaCl solution, the monolithic evaporator maintains an evaporation rate close to 1.54 kg m⁻² h⁻¹ with cycling stability. Additionally, it acts as a heterogeneous catalyst for peroxymonosulfate (PMS) activation, rapidly degrading pollutants to meet WHO standards. This work offers a promising solution for efficient desalination and wastewater treatment.

1. Introduction

Water pollution around the world is becoming increasingly serious, highly endangering human health and social sustainable development.^1–3 The acquisition of clean water from various polluted water sources has become important but is particularly challenging.^4–6 Solar-driven evaporation assisted by photothermal materials offers a promising solution for collecting clean water by isolating heavy metal ions and organic pollutants.^7,8 Solar distillation, an eco-friendly approach, is gaining traction for addressing freshwater scarcity.⁹ Recently, higher evaporation rates and photothermal conversion efficiencies were emphasized in only solar distillation;^10,11 however, wastewater containing residual pollutants still requires later comprehensive treatment when utilizing the polluted wastewater as evaporating object, which presents a further challenge for this application process.¹² Correspondingly, combining solar distillation with additional water treatment technologies not only can gain clean water resources but also can purify complex organic wastewater pollutants to alleviate the pressure on environmental management.¹³

Despite high water evaporation rates (>3 kg m⁻² h⁻¹) having been demonstrated in some special three-dimensional (3D) evaporators to ensure highly efficient water recovery,¹⁴ bifunctional evaporators encounter limitations and challenges when adding extra functionality (Table S1, ESI†). These proposed issues encompass: (1) challenges in mineralizing organic pollutants within individual carbon matrices, (2) inefficiencies in photothermal materials contacting organic molecule pollutants, and (3) integrating catalytically active substances into monolithic evaporators. Overcoming these hurdles promises highly efficient solar distillation coupled with simultaneous interfacial degradation to eliminate macromolecular pollutants through the use of advanced oxidation processes (AOPs).^15,16 In the pursuit of advancing photothermal evaporators, the focus should be on rationally designing photothermal materials. Carbon materials, with their broad solar energy absorption range, low production costs, and wide availability, are prime candidates.¹⁷ Traditional carbon materials often struggle to achieve excellent photothermal conversion properties, potentially hindering their sustainable use in photothermal applications.¹⁸ Incorporating metals or their oxides into carbon matrices efficiently improves photothermal conversion, endowing carbon matrices with additional metal catalytic reactive centers for catalytic degradation applications.¹⁹ AOPs, driven by diverse transition metals or oxides, have gained global attention for mineralizing large-molecule organic pollutants.²⁰ Their potent oxidative capabilities, rapid reaction rates, and minimal secondary pollution hinge on peroxomonosulfate (PMS) activation, generating various reactive oxygen species (ROS) like hydroxyl groups (˙OH), sulphates (˙SO₄⁻), superoxide radicals (˙O₂⁻), and singlet oxygen (¹O₂).²¹ Transition metals such as Fe, Co, and Cu and their oxides exhibit remarkable PMS activation efficiency.²² Composites combining carbon matrices with metal components enable efficient photothermal conversion and additional photothermal catalysis in the presence of light.²³ Carbon materials, with their broad spectral absorption, engineered structures, tunable properties, and environmental friendliness, show promise as metal supports in AOPs.²⁴ Optimizing metal–carbon combinations is crucial to enhance metal distribution and size states, thereby boosting catalytic activity.²⁵ Techniques like plasma excitation of transition metal nanoparticles or oxides, coupled with the broadband absorption of carbon materials, further enhance PMS photothermal activation and solar vapor production rates.²⁶ However, challenges persist in finely regulating the active metal state and carbon matrix structure, including morphology and porosity.²⁷

Noteworthy contributions in this realm include nanocomposites involving manganese carbide nanoparticles (Mn@NCNTs) in nitrogen-doped carbon nanotubes, which demonstrated enhanced catalytic degradation, reduced activation energy, and improved stability.¹⁰ Wu et al. loaded bimetallic MOF-derived carbon nanotubes (CoFe_0.8@NCNT@CA) onto cellulose aerogels, enhancing carbon nanotube adhesion through a semi-encapsulated porous structure and exposing numerous active sites while ensuring efficient mass transfer.²⁸ Effective encapsulation of ultrafine metal particles within porous carbon skeletons stabilizes active metal species, promotes electron transport, and facilitates reactant molecule proximity during catalytic oxidation through exposed reactive sites.²⁹ Recent research highlights the potential of electrospinning technology for accommodating metal catalytic species and mitigating active particle aggregation, thus enhancing catalytic properties and photothermal conversion efficiency.³⁰ While this advanced technique promises successful metal–carbon composite construction, a challenge arises.³¹ The two-dimensional (2D) interfacial vapor configuration, achieved by depositing photothermal materials on a polymer membrane, fails to establish effective aqueous contact with pollutant-laden wastewater.³² Consequently, the efficacy of AOPs is compromised. Thus, ensuring efficient contact between photothermal metal–carbon composites and wastewater is imperative.³³ The solution lies in the construction of 3D evaporators. These not only enhance contact with the underlying water but also expand the total evaporation surface area, harnessing additional energy to surpass the 2D theoretical limit.³⁴ Anchoring photothermal materials onto an evaporator has proved promising. Sodium alginate (SA), extracted from brown algae, emerges as a noteworthy candidate.³⁵ Its exceptional film-forming ability and high hydrophilicity enable the formation of SA membranes with an ion retention rate exceeding 99.4%.³⁶ This capability effectively addresses salt deposition issues.³⁷ In designing 3D evaporators, a hydrophilic melamine sponge serves as the substrate. SA acts as an adhesive, anchoring photothermal materials to the melamine skeleton. Therefore, this approach for anchoring photothermal materials onto an evaporator is promising, which can achieve efficient solar evaporation and catalytic degradation of organic pollutants at the same time by such a “two birds with one stone” strategy.

We developed a metal–carbon composite, FeCu/CNF, by incorporating CuFe₂O₄ and plasma Cu nanoparticles into porous carbon fibers using electrospinning and controlled graphitization. The iron precursors in synthetic fibers activate the porous carbon fibers, creating a network for pollutant diffusion. These carbon fibers have high surface area and porosity, synergizing metal and carbon matrix for PMS activation. We optimized the system for maximum photothermal efficiency. SA was used to firmly anchor photothermal FeCu/CNF to a superhydrophilic sponge skeleton (S-FeCu/CNF), enhancing solar evaporator performance. It achieved an outstanding evaporation rate of 1.76 kg m⁻² h⁻¹ under one sun irradiation and maintained a rate of 1.54 kg m⁻² h⁻¹ in a 20 wt% NaCl solution. The system effectively degraded methylene blue (MB) in 40 minutes, consistently achieving over 99% pollutant removal in various wastewater samples. This study introduces an integrated photothermal evaporator with promise for environmental management.

2. Results and discussion

The fabrication process of the S-FeCu/CNF composite material is presented in Fig. 1. Initially, precursor salts with varying mass ratios were mixed with PAN polymer to create a viscous spinning solution blend, which was then subjected to electrospinning to yield nanofibers. Subsequently, FeCu/CNF samples were generated by carbonizing FeCu plasma particles, both within and surrounding the calcined carbon nanofiber (CNF) framework, at a temperature of 700 °C. In this study, a unique tactic was employed in that iron precursor was introduced into the as-synthesized fiber polymer which can act as an activator for constructing the porous structure of activated carbon fibers benefiting the mass transfer and diffusion of pollutant and water. In the subsequent phase, the resulting FeCu/CNF was skillfully combined with sodium alginate to produce an FeCu/CNF-bearing sol, which was subsequently applied as a coating onto a sponge surface to produce a photothermal evaporator (S-FeCu/CNF). Following these stages, the structure and morphology of the FeCu/CNF samples were investigated using field-emission scanning electron microscopy (FESEM). The FESEM image indicates the successful encapsulation and uniform dispersion of FeCu nanoplasmas within the randomly entangled CNFs (Fig. 2a). After carbonization, the CNFs exhibited a minor reduction in diameter, measuring between 300 and 400 nm in diameter, without any noticeable structural collapse. Similarly, the FESEM images of FeCu/CNF@1, FeCu/CNF@1.5, and FeCu/CNF@2 nanocomposites portray a similar network of randomly interwoven CNFs, although the distribution of FeCu particles is less uniform as compared to FeCu/CNF@3 (compare Fig. S1–S3 (ESI†) to Fig. 2a). It is worth noting that plasma aggregation was observed in the FeCu/CNF@4 nanocomposite, indicating insufficient dispersion of excess FeCu, resulting in aggregation (Fig. S4, ESI†). Microstructural examinations were carried out using transmission electron microscopy (TEM). As demonstrated in Fig. 2b, numerous metal particles are uniformly dispersed within the CNFs creating abundant voids. Significantly, an abundance of minute metal particles occupying the interior spaces of CNFs can also be observed in the high-resolution TEM image (Fig. 2c).

Fig. 1 Schematic illustration of the synthesis steps of FeCu/CNF@3 nanocomposites and S-FeCu/CNF.

Fig. 2 (a) Low-magnification FESEM image of FeCu/CNF@3, with inset showing high-magnification FESEM image. (b) and (c) High magnification TEM images of FeCu/CNF@3. (d) XRD patterns of FeCu/CNF@1, FeCu/CNF@2 and FeCu/CNF@3. High-resolution XPS spectra of (e) Fe 2p, (f) Cu 2p, (g) C 1s, (h) O 1s of FeCu/CNF@3 sample. (i) Raman spectra of FeCu/CNF@1, FeCu/CNF@1.5, FeCu/CNF@2 and FeCu/CNF@3.

X-ray diffraction (XRD) analysis (Fig. 2d) was conducted to determine the crystal structure. For the FeCu/CNF@3 sample, XRD revealed peaks at 43.3° (111), 50.4° (200), and 74.1° (220), corresponding to elemental Cu (JCPDS card no. 04-0836). Additionally, a peak at 35.6° (220) indicated the presence of CuFe₂O₄ (JCPDS card no. 25-0283). Furthermore, distinct diffraction peaks at 26.1° and 45.0° on the (002) and (101) planes of hexagonal graphitic carbon (JCPDS card no. 56-0159) were observed, confirming the formation of graphitic carbon during carbonization at 700 °C.³⁸ Interestingly, for FeCu/CNF@1 and FeCu/CNF@2 samples, the XRD diffraction peak for elemental Cu diminished gradually, while the CuFe₂O₄ peak increased with decreasing iron acetyl acetone proportion, suggesting successful Cu incorporation into the carbon polymer matrix in FeCu/CNF@3. In addition, for Fe/CNF (Fig. S5, ESI†), XRD peaks at 30.2° (220), 35.6° (311), 43.3° (400), 53.7° (422), 57.3° (511), and 62.9° (440), along with clear Fe₂O₃ diffraction peaks (JCPDS card no. 39-1346), were observed. Similarly, for Cu/CNF, prominent XRD peaks at 43.3° (111), 50.4° (200), and 74.1° (220), representing elemental Cu (JCPDS card no. 04-0836), were noted. Notably, hexagonal graphitic carbon (JCPDS card no. 56-0159) also displayed significant diffraction peaks at 26.1° and 45.0° on the (002) and (101) planes, confirming the formation of graphitic carbon through carbonization at 700 °C. The proportional decrease in the XRD diffraction peak of elemental Cu and the increase in the CuFe₂O₄ peak with reduced iron acetyl acetone proportion in FeCu/CNF@1 and FeCu/CNF@2 samples suggest successful Cu incorporation into the carbon polymer matrix in FeCu/CNF@3. The chemical oxidation states and surface functional groups of FeCu/CNF@3 nanocomposites were assessed through X-ray photoelectron spectroscopy (XPS). The XPS analysis results (Fig. S6, ESI†) confirm the presence of Fe, Cu, C, N, and O elements within the FeCu/CNF@3 nanocomposite. The high-resolution Fe 2p XPS spectrum (Fig. 2e) reveals peak values at approximately 704.0–717.0 eV for Fe 2p_3/2 and 718.0–736.0 eV for Fe 2p_1/2. These peaks correspond to the surface oxidation state of Fe²⁺ at 710.0–711.0 eV (Fe 2p_3/2) and 724.0–725.0 eV (Fe 2p_1/2) (Gan et al., 2023). Similarly, the high-resolution Cu 2p XPS spectrum (Fig. 2f) exhibits two primary peaks spanning 928.0–944.0 eV and 945.0–962.0 eV, corresponding to Cu 2p_3/2 and Cu 2p_1/2, respectively. Each of these peaks can be further resolved into independent peaks corresponding to the surface oxidation state Cu²⁺, appearing at 932.6–933.0 eV (Cu 2p_3/2) and 952.0–953.0 eV (Cu 2p_1/2).³⁹ These XPS results validate the presence of Fe²⁺ and Cu²⁺, corroborating the earlier XRD findings. It is noteworthy that while XRD detected the diffraction peak of elemental Cu, XPS did not, potentially due to surface oxidation of elemental Cu exposed to air or the formation of a surface oxide layer during the pyrolysis synthesis process. The high-resolution C 1s XPS spectrum (Fig. 2g) exhibits peak binding energies at 284.4, 285.1, and 286.8 eV, corresponding to sp² graphite structure C=C, sp³ hybrid carbon C–C, and C–O bonds, respectively.⁴⁰ This sequence of carbon species indicates the presence of carboxyl groups and saturated carbon functional groups within the FeCu/CNF@3 nanocomposite. The high-resolution O 1s XPS spectrum (Fig. 2h) manifests binding energies at 530.0, 531.8, and 532.9 eV, respectively corresponding to metal oxides M–O, organic C–O, and C=O, signifying the presence of hydrophilic groups that facilitate water diffusion within the interfacial evaporation layer and the escape of water vapor. The high-resolution N 1s XPS spectrum (Fig. S7, ESI†) reveals two deconvolution peaks at 398.2 eV and 400.4 eV, attributed to pyridine–N and graphitic–N, respectively.⁴¹ This sp² hybrid nitrogen likely arises from the cross-linking of N-PAN-containing polymer during carbonization at elevated temperatures.

Raman spectra of FeCu/CNF@3, FeCu/CNF@2, FeCu/CNF@1.5, and FeCu/CNF@1 are presented in Fig. 2i. The D band at approximately 1345 cm⁻¹ is a result of sp² carbon atoms in CNF with a defective structure, while the G band at around 1594 cm⁻¹ originates from the sp³ graphene carbon structure.⁴² This observation aligns with the previous XRD and XPS results (see Fig. 2d–g), confirming the successful production of graphitic carbon materials. According to the Raman measurements, the relative intensity ratios of D-band and G-band (expressed as I_D/I_G) of FeCu/CNF@3, FeCu/CNF@2, FeCu/CNF@1.5, and FeCu/CNF@1 samples are 2.54, 2.2, 2.06, and 1.94, respectively. It is evident that the inclusion ratio of plasma significantly increased defect locations by generating holes and edges as it increased. Notably, the I_D/I_G values for CNF, Cu/CNF, and Fe/CNF were 2.72, 2.49, and 2.1, respectively (Fig. S8, ESI†), indicating substantial surface defects in pure carbon nanofibers.⁴³ With the successful inclusion of plasma, the I_D/I_G value decreased, indicating an increase in graphitization degree. Despite the relatively low degree of graphitization in FeCu/CNF@3, the surface defects of the sample were still preserved to a certain extent. These surface defects play a significant role in reducing the original thermal conductivity of the material.⁴³ Employing the Brunauer–Emmett–Teller (BET) model as illustrated in Fig. S9, S10 and Table S2 (ESI†), we have calculated the specific surface areas for various nanocomposite samples, namely FeCu/CNF@3, FeCu/CNF@2, FeCu/CNF@1.5, FeCu/CNF@1, Fe/CNF, and Cu/CNF, to be 291.29, 318.09, 205.02, 283.52, 335.52, and 172.42 m² g⁻¹, respectively. In addition, the non-local density functional theory (NLDFT) plots for these samples indicate the presence of a significant number of micropores.

The photothermal layer was constructed by depositing FeCu/CNF@3 onto a hydrophilic polytetrafluoroethylene (PTFE) film support via vacuum filtration. This configuration, as depicted in Fig. 3a, was designed to assess the intrinsic evaporation performance of the photothermal material itself, independently of any specialized evaporator structure. Specifically, the synthesized FeCu/CNF@3 photothermal layer, approximately 2.5 cm wide, was evenly distributed on the PTFE film, with an optimal mass load of 0.02 kg m⁻². The absorption spectrum of FeCu/CNF@3 (Fig. 3b) revealed an average solar absorption rate of 95.0% within the wavelength range of 200–2500 nm. Additionally, the corresponding thermal images (Fig. 3c) displayed a moistened FeCu/CNF@3 solar absorber reaching a maximum temperature of 45 °C under one sun solar irradiation. Comparing the thermal performance of FeCu/CNF@3 with that of other photothermal layers, as shown in Fig. S11 and S12 (ESI†), the FeCu/CNF@3 solar absorber achieved the highest maximum temperature of 45 °C under one sun solar irradiation, while FeCu/CNF@1, FeCu/CNF@1.5, FeCu/CNF@2, and FeCu/CNF@4 solar absorbers only reached a maximum temperature of 43 °C, 43 °C, 44.3 °C, and 39.2 °C, respectively. These results provide additional evidence of the superior photothermal performance of the FeCu/CNF@3 solar absorber, rendering it suitable for solar evaporation applications. Subsequently, a custom-made solar evaporator was employed to evaluate the water evaporation rate, enhancement factor, and solar evaporation efficiency of FeCu/CNF@3, along with control samples. This solar evaporator comprised polyethylene (PE) foam enveloped in hydrophilic dust-free air-laid paper, extending into a simulated bulk seawater environment. This arrangement facilitated the movement of water molecules through a network of capillaries to reach the surface of the solar absorber.

Fig. 3 (a) A photograph of FeCu/CNF@3 nanocomposite deposited on a PTFE membrane. (b) The UV-vis-NIR absorption spectrum of FeCu/CNF@3 nanocomposite, with reference to solar irradiation of AM 1.5 highlighted in orange. (c) Thermal images of FeCu/CNF@3 photothermal layer on water under one sun irradiation.

To assess the effectiveness of the solar evaporators featuring FeCu/CNF solar absorbers and control setups in simulated seawater, the water evaporation rate (m, kg m⁻² h⁻¹), enhancement factor (EF), and evaporation efficiency (%) were calculated using eqn (1)–(3). As a reference experiment, a control solar evaporator, composed of PTFE/air-laid paper/PE foam on simulated seawater, was employed without the inclusion of the solar absorber.

where Δm denotes the alteration in water mass (kg), S signifies the surface area of the photothermal layer (m²), and t corresponds to the duration of solar irradiation (h).where the variable m(SE) signifies the water evaporation rate of the FeCu/CNF or control solar evaporators, whereas m(blank) indicates the water evaporation rate of simulated seawater when exposed to one sun irradiation.where the variable m′ (expressed in kg m⁻² h⁻¹) denotes the disparity between the water evaporation rate of the FeCu/CNF or control solar evaporators and the water evaporation rate of simulated seawater without light exposure, h_LV signifies the total enthalpy of vaporization, while P_in represents the incident solar power during one sun irradiation.

The analysis of simulated seawater mass variation was conducted meticulously using a precision analytical balance, consistently applying one sun irradiation unless otherwise specified. The FeCu/CNF@3 solar evaporator demonstrated a remarkable water evaporation rate of 1.61 kg m⁻² h⁻¹ under one sun irradiation, substantially outperforming the control (0.69 kg m⁻² h⁻¹), FeCu/CNF@1 (1.48 kg m⁻² h⁻¹), FeCu/CNF@1.5 (1.50 kg m⁻² h⁻¹), FeCu/CNF@2 (1.57 kg m⁻² h⁻¹), FeCu/CNF@4 (1.40 kg m⁻² h⁻¹), Cu/CNF (1.69 kg m⁻² h⁻¹), and Fe/CNF (1.34 kg m⁻² h⁻¹) solar evaporators by factors of 2.33, 1.09, 1.07, 1.03, 1.15, 0.95, and 1.20, respectively (Fig. 4a, b and Fig. S13a, ESI†).

Fig. 4 (a) The cumulative mass change of water over time under distinct conditions: water in the absence of light (dark), water exposed to solar irradiation (blank), water in the presence of PTFE/air-laid paper/PE (control), CNF, and various FeCu/CNF solar evaporators subjected to one sun irradiation. (b) Rates of water evaporation. (c) Solar evaporation efficiency assessments with various FeCu/CNF solar evaporators against control. (d) The enhancement factor of FeCu/CNF@1, FeCu/CNF@1.5, FeCu/CNF@2 FeCu/CNF@3, and FeCu/CNF@4 solar evaporators against control under one sun irradiation.

To isolate the influence of solar irradiation on the evaporation rate, the measured evaporation rate in the presence of solar irradiation was subtracted from the total evaporation rate recorded in complete darkness, resulting in a value of 0.16 kg m⁻² h⁻¹. Furthermore, in comparison to the control, FeCu/CNF@1, FeCu/CNF@1.5, FeCu/CNF@2, FeCu/CNF@3, and FeCu/CNF@4 solar evaporators exhibited significantly enhanced solar evaporation efficiency percentages of 37.49, 91.8, 92.95, 97.84, 105.85, and 86.4%, respectively (Fig. 4c), along with solar evaporator EF values of 1.05, 2.24, 2.27, 2.38, 2.44, and 2.12, respectively (Fig. 4d).

Furthermore, the temporal evolution of the membrane surface temperature under one sun irradiation was scrutinized (Fig. S11, ESI†). The FeCu/CNF@3 solar absorber exhibited rapid temperature escalation to 42.7 °C within a mere 10 min, followed by a steady increase to 44.3 °C within 30 min, sustaining this elevated temperature. This performance surpassed that of FeCu/CNF@1, FeCu/CNF@1.5, FeCu/CNF@2, FeCu/CNF@4 and even the CNF solar absorber. Conversely, the control barely reached 30 °C, far below the temperature recorded by the FeCu/CNF@3 solar absorber. These outcomes underscore the substantial enhancements in water evaporation rate, EF, and solar evaporation efficiency achieved by the FeCu/CNF solar evaporator compared to the control, confirming the superior photothermal performance and pronounced thermal localization effects of the prepared FeCu/CNF solar absorber.

Upon comparing the cumulative mass change over time for Fe/CNF and Cu/CNF under one sun irradiation (Fig. S13, ESI†), it becomes evident that the water evaporation rate from the Cu/CNF solar evaporator, loaded with an identical mass, reaches an impressive 1.69 kg m⁻² h⁻¹, surpassing that of Fe/CNF (1.34 kg m⁻² h⁻¹). Additionally, the solar evaporation efficiency percentages for CNF, Fe/CNF, and Cu/CNF stand at 79%, 87.52%, and 115.48%, respectively (Fig. S13b, ESI†). The maximum temperature achieved by the Fe/CNF and Cu/CNF solar absorbers under one sun is 41.7 °C (Fig. S13c, ESI†). While it is apparent that Cu/CNF exhibits superior photothermal conversion performance, from the pollutant degradation perspective, FeCu/CNF@3 still remains the optimal choice due to both its enhanced pollutant degradation performance and its excellent photothermal properties.

The FeCu/CNF-based evaporator offers the flexibility to be tailored into either a bilayer evaporation structure or a monolithic evaporation unit, depending on specific application requirements. The S-FeCu/CNF sponge possesses dimensions of 3.4 cm in diameter and 0.5 cm in thickness. FESEM imaging reveals the skeletal structure of melamine sponge (Fig. S14a and b, ESI†). Upon sodium alginate (SA) coating, it is apparent that the melamine sponge transforms into a compact film-like material as SA fills the voids between the skeletal framework (Fig. S15a and b, ESI†). Furthermore, the SA-coated FeCu/CNF@3, chosen for its optimal photothermal properties, is thoroughly mixed with a predetermined mass of SA sol and encapsulated onto the melamine sponge to form S-FeCu/CNF (Fig. S16a and b, ESI†). This process is apparent in the microfiber-mixed SA sol uniformly filling the spaces between the framework of the sponge, resulting in a dense layer for sunlight absorption. The integration of SA endows the evaporator with salt-repelling properties, thereby enhancing salt resistance. It is imperative to note that the concentration of SA significantly influences the surface formation of the film. For instance, a high SA concentration (4 mg mL⁻¹) may result in the toughness of the material diminishing, making the evaporator susceptible to warping or even peeling (Fig. S17a and b, ESI†). Conversely, a low SA concentration (1 mg mL⁻¹) presents challenges due to the low viscosity and density of solute, rendering film formation problematic (Fig. S18a and b, ESI†). An optimum SA concentration (2 mg mL⁻¹) was ultimately achieved with stability and smoothness in polymer films (Fig. S19a and b, ESI†). Moreover, this optimal S-FeCu/CNF sponge evaporator exhibits remarkable flexibility (Fig. S20 and S21, ESI†), and both the bilayer and monolithic configurations can fully recover their form, even after several folding and unfolding cycles.

The interfacial solar vapor generation properties of S-FeCu/CNF were studied, with waste polyethylene foam (PE foam) utilized to isolate the S-FeCu/CNF evaporator from the water, effectively minimizing heat dissipation into the water. Depending on the amount of FeCu/CNF@3 photothermal material added (ranging from 10 to 50 mg), the samples were denoted as S-FeCu/CNF-10, S-FeCu/CNF-20, S-FeCu/CNF-30, S-FeCu/CNF-40, and S-FeCu/CNF-50, respectively. Among the samples, S-FeCu/CNF-40 recorded the highest evaporation rate of 1.76 kg m⁻² h⁻¹ under one sun irradiation (Fig. 5a and b). This rate significantly surpasses the evaporation rate of pure water, which stands at 0.66 kg m⁻² h⁻¹ (Fig. 4a), highlighting the exceptional solar vapor production capacity of the S-FeCu/CNF evaporator. This enhancement can be attributed to the thermal insulation layer design.

Fig. 5 (a) Assessment of solar evaporation performances; (b) water evaporation rates; (c) surface temperature fluctuations during the evaporation process on water under one sun; and (d) surface temperature fluctuations of S-FeCu/CNF sponges with various loadings of photothermal materials (PM) in air under one sun irradiation. (e) Infrared thermal image analysis depicting surface temperature variations in S-FeCu/CNF-40 samples on water under one sun irradiation.

Furthermore, water contact angle testing (Fig. S22, ESI†) demonstrates that the melamine sponge, SA-coated sponge, and S-FeCu/CNF-40 evaporator exhibit contact angles of 0°, indicating their super-hydrophilic nature. This attribute remains unchanged with the addition of PM and SA, facilitating rapid water movement to the evaporation interface. Superior hydrophilicity further promotes salt migration back to the salt solution, enhancing salt drainage performance. Additionally, the outstanding photothermal conversion capability is verified through temperature changes. The surface temperature of the S-FeCu/CNF-40 evaporator, for instance, surges from 18.5 °C to 38.5 °C within 20 min (Fig. 5c). Infrared imaging (Fig. 5e) illustrates that the evaporation temperature of the S-FeCu/CNF-40 sponge remains the lowest among comparison evaporators, because the high flux of vapor evaporating to dissipate a substantial amount of heat (Fig. S23, ESI†). In contrast, the surface temperatures of the other evaporators reach higher values than that of the 3D S-FeCu/CNF-40 (38.5 °C) sponge evaporator, such as the 2D FeCu/CNF@3 (44.3 °C) membrane evaporator. Dried surface temperature changes of the S-FeCu/CNF evaporator provide further insights into its photothermal conversion capabilities (Fig. 5d and Fig. S24, ESI†). As photothermal materials are loaded in greater quantities, the maximum surface temperature of the evaporator under one sun irradiation gradually increases. The S-FeCu/CNF-40 evaporator, for instance, witnesses a rapid temperature surge from 26 °C to 79.4 °C within 5 min, affirming its exceptional photothermal conversion performance.

The evaluation of evaporation performance in high-salinity solutions stands as another pivotal criterion. In Fig. 6a, the variation in the evaporation rate of the S-FeCu/CNF-40 evaporator is illustrated within brine solutions of varying salinities (0 wt%, 5 wt%, 10 wt%, and 20 wt% NaCl solutions) under one sun irradiation. This depiction reveals a gradual reduction in the evaporation rate, transitioning from 1.76 kg m⁻² h⁻¹ to 1.54 kg m⁻² h⁻¹ as the salinity of the NaCl solution increases (Fig. 6a and b). This decline can be attributed to the reduction in saturated vapor pressure resulting from the heightened salinity of the NaCl solution. Temperature variations of the S-FeCu/CNF-40 evaporator were recorded within brine solutions of varying salinities (0 wt%, 5 wt%, 10 wt%, and 20 wt% NaCl solutions) under 1 sun (Fig. S25 and S26, ESI†). The maximum surface temperature of the evaporator progressively increased with the concentration of the salt solution, increasing from 38.6 °C to 41.3 °C. This phenomenon can be attributed to the decrease in the required heat for evaporation as the evaporation rate decreases, consequently leading to heat accumulation on the evaporator surface and a corresponding slight elevation in temperature. Moreover, we assessed the evaporation performance of the S-FeCu/CNF-40 evaporator under diverse light conditions at a 20 wt% salinity level (Fig. 6c and d). Specifically, the evaporation rate of the S-FeCu/CNF-40 evaporator reached 2.26 kg m⁻² h⁻¹ under 1.5 sun and 3.24 kg m⁻² h⁻¹ under 2 sun, with accompanying gradual temperature increases (Fig. S27 and S28, ESI†). The peak surface temperature of the S-FeCu/CNF-40 evaporator reached 49.2 °C under 1.5 solar irradiation and 52.4 °C under 2 sun. Notably, the S-FeCu/CNF-40 evaporator displayed a noteworthy evaporation rate of 0.96 kg m⁻² h⁻¹ under 0.5 sun (representing natural environmental conditions), effectively showcasing its remarkable practical applicability. To further demonstrate the salt resistance of the S-FeCu/CNF-40 evaporator, it was submerged in a 20 wt% NaCl solution for a duration of 6 months (Fig. S29, ESI†). Remarkably, the photothermal material on the evaporator's surface remained unaltered, exhibiting no shedding or decomposition. Additionally, to simulate supersaturation within the evaporator, we positioned 1 g of NaCl crystals on the evaporator surface. As indicated in Fig. S30 (ESI†), the NaCl crystals on the evaporator's surface steadily dissolved, ultimately vanishing after 33 minutes. This outcome underscores the effective prevention of supersaturation within the evaporator. This phenomenon is attributed to the concentration disparity between the evaporator and the surrounding water body, whereby high-concentration salt solution readily migrates towards the low-concentration salt solution, facilitated by the superhydrophilic nature of the evaporator.

Fig. 6 (a) The accumulation of mass change over time and (b) rates of evaporation for varying NaCl solution concentrations: 0 wt%, 5 wt%, 10 wt%, 20 wt%. (c) The accumulation of mass over time under different solar irradiation: 0.5 sun, 1 sun, 1.5 sun, 2 sun (within a 20 wt% NaCl salt solution). (d) Evaporation rates within a 20% NaCl salt solution under varying sunlight intensities.

To assess the stability of the evaporator, cyclic evaporation performance was evaluated within a 20 wt% NaCl solution (Fig. S31, ESI†). After 15 cycles, the average evaporation rate remained stable at 1.50 kg m⁻² h⁻¹, with the surface of the S-FeCu/CNF-40 evaporator retaining its structural integrity and devoid of salt deposition. This substantiates the efficient stability of the S-FeCu/CNF-40 evaporator in high-salinity brine conditions.

Recognizing that many water sources are contaminated with multiple pollutants, the water obtained after evaporation through an evaporator typically lacks color but may still contain residual, undegraded pollutants. In particular, these residual highly concentrated pollutants become difficult to remove in the later treatment process, thereby causing worse environmental influence and treatment difficulty. Advanced oxidation techniques employing PMS have demonstrated notable capabilities in removing organic contaminants. Drawing inspiration from this, we further enhanced the D-S-FeCu/CNF evaporator system to concurrently facilitate PMS-mediated water purification and solar evaporation. In our investigation, a 20 mg L⁻¹ solution of MB served as the target pollutant. In the absence of PMS (Fig. 7a), the D-S-FeCu/CNF evaporator exhibited a mere 9% removal of MB which possibly correlated with adsorption behavior. However, the catalytic degradation activity can be demonstrated when introducing the PMS and corresponding monolithic evaporator as catalytic reactor. In the meantime, under varying intensities of solar irradiation (i.e., 0.5–2 kW m⁻²), the degradation efficiency substantially increased, reaching 80%, 96%, 96%, and 99% in the presence of PMS addition, respectively. Higher light intensity levels proved to be particularly advantageous, as they induced nanometallic plasma excitation and efficient light-to-heat effect, enhancing thermodynamics and kinetics behavior to promote overall catalytic activity. In addition, we explored the degradation rate of MB at different concentrations under one solar irradiation (Fig. 7b). Notably, the D-S-FeCu/CNF evaporator achieved a 98% degradation efficiency in just 25 min when the MB concentration was 5 mg L⁻¹. Even at higher concentrations such as 10 mg L⁻¹ and 20 mg L⁻¹, it still reached 98% and 96% degradation efficiency in 40 min and 50 min, respectively. Impressively, even when the MB concentration was as high as 50 mg L⁻¹, a substantial 65% degradation efficiency was achieved within 50 min. Such a result demonstrated the possibility of the dual-functional evaporator and reactor in treating wastewater and regeneration of clean water.

Fig. 7 (a) Depiction of the efficiencies in the degradation of MB (methylene blue). Reaction conditions: [MB] = 20 mg L⁻¹, [PMS] = 0.049 mM. (b) Efficiencies in the degradation of MB at various concentrations with the D-S-FeCu/CNF evaporator under 1 sun irradiation. Reaction conditions: [PMS] = 0.049 mM. (c) An optical image showcasing the simple degradation device for evaporation. Absorption spectra highlighting the initial solution and the condensed form of various organic pollutants present in the vapors generated by the D-S-FeCu/CNF evaporator: (d) MB, (e) CR, (f) RB, (g) MO, (h) TC, and (i) OTC.

The evaporation degradation device (Fig. 7c) effectively performed dual functions: evaporating water while concurrently degrading pollutants in the underlying water source and condensing the purified water. To assess the D-S-FeCu/CNF evaporator's ability to isolate pollutants, we measured the residual concentrations of various target contaminants in the regenerated water. MB, congo red (CR), rose bengal (RB), methyl orange (MO), tetracycline (TC), and oxytetracycline (OTC) were employed to simulate industrial dye wastewater samples. The results (Fig. 7d–i) unequivocally demonstrated a significant reduction in the concentrations of diverse pollutants in the condensate, indicating effective pollutant molecule interception. These findings underscore the distinctive and efficient nature of the S-FeCu/CNF, D-S-FeCu/CNF evaporator in removing pollutants at the solar evaporation interface, greatly expanding its applicability in treating water from various sources. It is noteworthy that the water evaporation performance of the S-FeCu/CNF, D-S-FeCu/CNF evaporator surpasses that of most dual-function solar absorbers (see Fig. S32 and Table S1, ESI†). In particular, the evaporation performance is easily limited when endowing the evaporator with an additional function. Therefore, the obtained dual-functional evaporator affords an appreciable evaporation property compared to most reported dual-functional evaporators. This exceptional performance encompasses a high evaporation rate of 1.76 kg m⁻² h⁻¹ and an outstanding evaporation efficiency of 105.85% in a 2D evaporation state. However, it is important to acknowledge that this apparent overestimation of evaporation efficiency may be attributed to factors such as prolonged evaporation time, elevated ambient temperatures, and the potential for the water in the beaker to absorb residual heat from the surroundings, consequently diminishing the enthalpy of water evaporation. Additionally, the increased height of the 3D evaporator naturally leads to a significant boost in the evaporation rate. To elucidate the degradation mechanism of pollutants (see Fig. S33a–c, ESI†) more comprehensively, we conducted additional free radical quenching experiments. Methanol (MeOH) and tert-butanol (TBA) were selected as quenching agents for ˙OH and ˙SO₄⁻, while p-benzoquinone (p-BQ) and L-histidine (L-his) served as quenching agents for ˙O₂⁻ and ¹O₂, respectively.^44–46 Several quenching agents exhibited inhibition of MB degradation. However, experimental results with calculated reaction rate constants indicated that ˙OH, ˙SO₄⁻, and ˙O₂⁻ were the primary reactive species, contributing to a 36% reduction in MB degradation efficiency. Furthermore, L-his demonstrated modest inhibition, reducing MB degradation efficiency by approximately 20%. These findings underscore the synergistic influence of these active species in the degradation process. The stability of the application plays a critical role in assessing its practical potential.^47,48 To assess the stability of S-FeCu/CNF, we conducted multiple cycles of methylene blue degradation experiments (Fig. S34, ESI†), demonstrating that S-FeCu/CNF maintains excellent degradation performance even after 5 cycles, consistently achieving over 85% degradation efficiency. These results highlight the promising practical application potential of the developed S-FeCu/CNF.

To further investigate the practical utility of the S-FeCu/CNF evaporator in water purification processes, an outdoor experiment was conducted to evaluate solar steam generation under natural sunlight conditions. To this end, a portable trapezoidal solar-driven interfacial evaporator was developed, constructed from a 3 mm transparent acrylic sheet with dimensions of 31 cm × 21.5 cm at the base, a front height of 7.5 cm, and a rear height of 24 cm (see Fig. 8a). This experiment was carried out on July 14, 2023, between 9:30 AM and 5:30 PM, aimed at outdoor water purification. Solar radiation flux and changes in evaporative mass were continuously monitored and recorded (as depicted in Fig. 8b) to assess the performance of solar steam generation. The seawater employed for this experiment was collected from the Bohai Sea in China. It was observed that the average natural outdoor solar irradiance was approximately 0.866 kW m⁻². Over the course of 9 h, the S-FeCu/CNF evaporator generated 16.63 kg m⁻² of fresh water, with an average evaporation rate of approximately 1.85 kg m⁻² h⁻¹ per unit, confirming the excellent practical evaporation capacity of the S-FeCu/CNF evaporator. Certain features of the developed evaporator make it appealing for multifunctional applications, placing it on a par with other high-performance photothermal materials.^49–51

Fig. 8 (a) Visual representation of the compact interfacial evaporation apparatus. (b) Time-evolution profile illustrating alterations in the evaporation rate of the S-FeCu/CNF-4 interfacial evaporation device in response to varying natural outdoor solar radiation levels. (c) Examination of alterations in metal ion concentrations before and after the solar-assisted seawater desalination process. (d) Photographic depiction of the assembly process for the large-scale evaporator.

To confirm the purification efficacy of the S-FeCu/CNF evaporator, inductively coupled plasma mass spectrometry (ICP-MS) was employed to analyze ion concentrations before and after solar desalination. As depicted in Fig. 8c, the concentrations of major ions (Na⁺, Ca²⁺, K⁺, Mg²⁺) in seawater were significantly reduced by several orders of magnitude. For instance, the initial concentration of Na⁺ in Bohai Sea water, which stood at 26 720 ppm, was reduced to a mere 1.43 ppm after desalination, signifying an ion removal efficiency exceeding 99.9%. Similarly, other ions such as Ca²⁺, K⁺, and Mg²⁺ were reduced to 0.7935 ppm, 0.516 ppm, and 0.4785 ppm, respectively, which comply with the drinking water quality standards outlined by the World Health Organization (WHO). Notably, the evaporator fabricated using the strategy developed in this study can be easily scaled up to a large size of 13 cm × 4 cm (Fig. 8d) and can be further expanded, which demonstrates the scalability of S-FeCu/CNF and its applicability in real-life situations.

3. Conclusions

In summary, we have developed versatile 3D dual-functional S-FeCu/CNF, D-S-FeCu/CNF evaporators, excelling in solar-driven evaporation and organic pollutant degradation for water purification. The FeCu/CNF@3 material, synthesized through electrospinning and carbonization, exhibits remarkable solar absorption across the entire spectrum (250–2500 nm), with absorption efficiencies of over 95%. The 2D FeCu/CNF membrane achieved an impressive 1.61 kg m⁻² h⁻¹ evaporation rate and 105.85% efficiency under one sun irradiation, which improved to 1.76 kg m⁻² h⁻¹ for the S-FeCu/CNF evaporator. Leveraging the superhydrophilicity and salt rejection of the composite (melamine sponge and sodium alginate), the S-FeCu/CNF evaporator maintains high efficiency, reaching 1.54 kg m⁻² h⁻¹ in 20 wt% ultrahigh-salinity brine, with remarkable stability over 15 cycles of continuous operation. Moreover, the D-S-FeCu/CNF evaporator efficiently removes organic pollutants at 99.9% from various wastewater samples. These results drive innovative photothermal evaporator designs for clean water production from diverse sources with organic contaminants. Future research is anticipated to delve deeper into real wastewater treatment, though there are likely to be significant challenges. In particular, the monolith evaporator, when processing a variety of materials in large quantities, may struggle to achieve rapid evaporation rates, thereby impacting production efficiency. Additionally, precise temperature control within the evaporator presents challenges: excessively high temperatures can cause material decomposition or degradation, while excessively low temperatures may hinder evaporation effectiveness.

Author contributions

Hongyao Zhao: investigation, writing – original draft. Danhong Shang: writing – original draft. Haodong Li: conceptualization, writing – original draft. Marliyana Aizudin: writing – review & editing. Xiu Zhong: visualization, formal analysis. Hongyang Zhu: formal analysis. Yang Liu: investigation. Zhenxiao Wang: formal analysis. Ruiting Ni: formal analysis. Yanyun Wang: formal analysis. Sheng Tang: formal analysis. Edison Huixiang Ang: writing – review & editing. Fu Yang: supervision, writing – review & editing, funding acquisition.

Data availability

All relevant data are within the paper and the ESI.† The data that support the findings of this study are available from the corresponding author, Fu Yang, upon reasonable request.

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.

Acknowledgements

Financial support for this research was graciously provided by several institutions, including the National Natural Science Foundation of China (Grant No. 21908085), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20190961), China Postdoctoral Science Foundation (2023M731422), and the Jiangsu Provincial Key Laboratory of Environmental Science and Engineering (Project No. JSHJZDSYS-202103). We also acknowledge funding support from the Ministry of Education, Singapore, through its Academic Research Fund Tier 1 (Grant No. RG10/22) and NIE-AcRF Grant (Grant No. RI 1/21 EAH). We extend our gratitude to the Instrumental Analysis Center at Jiangsu University of Science and Technology for their invaluable assistance with characterizations. Additionally, we would like to express our appreciation to shiyanjia lab (https://www.shiyanjia.com) for their generous support in providing access to the solid-state UV-visible spectrophotometer.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh00696h

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