# Upcycled PET-Derived Carbon Foam Functionalized with Cu3SbS4–Sb2S3 Heterostructures for Efficient Interfacial Solar Desalination

Muzammil Hussain, Kassa Belay Ibrahim, Enrique Rodríguez-Castellón, Silvia Gross, Pawan Kumar, Stéphanie Bruyère, David Horwat, Elisa Moretti, Alberto Vomiero, Tofik Ahmed Shifa

► To cite this version:

Muzammil Hussain, Kassa Belay Ibrahim, Enrique Rodríguez-Castellón, Silvia Gross, Pawan Kumar, et al.. Upcycled PET-Derived Carbon Foam Functionalized with Cu3SbS4–Sb2S3 Heterostructures for Efficient Interfacial Solar Desalination. Small, 2025, 21 (39), 10.1002/smll.202506862. hal-05304087

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# Upcycled PET-Derived Carbon Foam Functionalized with Cu₃SbS₄–Sb₂S₃ Heterostructures for Efficient Interfacial Solar Desalination

Muzammil Hussain, Kassa Belay Ibrahim, Enrique Rodríguez-Castellón, Silvia Gross, Pawan Kumar, Stéphanie Bruyère, David Horwat, Elisa Moretti,* Alberto Vomiero,* and Tofik Ahmed Shifa*

Solar desalination is an emerging technique to produce fresh water utilizing renewable solar energy. However, the engineering of efficient photothermal material is a significant obstacle. In the present study, a carbon foam is synthesized from the upcycling of waste PET and hydrothermally functionalized with a heterostructure composed of Cu₃SbS₄ and Sb₂S₃. Material characterizations demonstrated the successful decoration of nanochannels on graphitic carbon foam (CF). The analysis of the optical properties in the UV/Vis-NIR spectral range demonstrated excellent absorption properties of 96% for Cu₃SbS₄–Sb₂S₃/CF compared to Sb₂S₃/CF (48%) and CF (68%) in near-IR. Photothermal desalination results reveal the evaporation rate of 2.82 kg m⁻² h⁻¹ for Cu₃SbS₄–Sb₂S₃/CF compared to 1.4 kg m⁻² h⁻¹ for Sb₂S₃/CF and 1.58 kg m⁻² h⁻¹ for CF, with 99% salt removal in condensed water. The formation of the composite leads to a high surface temperature and enhanced evaporation rate. The contact angle analysis confirmed the hydrophilic nature of the material that plays a crucial role in the solar desalination process. These findings elucidate the effective photothermal performance achieved through chalcogenide heterostructure engineering supported on upcycled carbon foam derived from waste PET, demonstrating a practical application aligned with circular economy principles in solar desalination.

## 1. Introduction

Fresh and clean water is a basic need of humans and is highly associated with human survival. However, due to the blooming in industrialization and urban development, the water resources are highly affected.[1–3] The utilization of seawater for drinking and energy production has consistently raised concerns, prompting the development of various strategies for desalinating seawater. A series of energy-intensive methods with complex technologies are used for water desalination, including electrodialysis (ED), forward osmosis (FO), multiple-effect distillation, reverse osmosis (RO), thermal desalination, solar desalination, and multi-stage flash distillation.[4] Among them, solar desalination is one of the emerging methods to treat seawater using renewable solar energy. Interfacial solar steam generation using solar energy is not only a cost-effective and straightforward technology, but it also

M. Hussain, K. B. Ibrahim, E. Moretti, A. Vomiero, T. A. Shifa
Department of Molecular Sciences and Nanosystems
Ca' Foscari
University of Venice
Via Torino 155, Venice 30172, Italy
E-mail: elisa.moretti@unive.it; alberto.vomiero@ltu.se;
tofikahmed.shifa@unive.it

M. Hussain
Department of Industrial Engineering
University of Padova
Via Venezia 1, Padova 35131, Italy

E. Rodríguez-Castellón
Department of Inorganic Chemistry
Crystallography and Mineralogy
Faculty of Science
Inter-university Institute of Research in Biorefinery I3B
University of Malaga
Málaga, Spain

S. Gross
Department of Chemical Sciences
University of Padova
Via Francesco Marzolo 1, Padova 35131, Italy

P. Kumar, A. Vomiero
Division of Materials Science
Department of Engineering Sciences and Mathematics
Luleå University of Technology
Luleå 97187, Sweden

S. Bruyère, D. Horwat
Université de Lorraine
CNRS
IJL, Nancy F-54000, France

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202506862

© 2025 The Author(s). Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/smll.202506862

<figure><img src="image_2.png" /></figure>
harnesses a free and clean energy source.[5] In photothermal ma-
terial for desalination, the solar energy can be transformed into
thermal energy that increases the water evaporation rate and
makes the process more efficient and greener under sunlight
irradiation.[4,6]

Material development for solar steam generators plays a signif-
icant role in enhancing the solar absorption and evaporation rate.
Considering these concerns, in recent years various carbon ma-
terials have been investigated, such as graphite, graphene oxide,
CNTs, biomass, and wood-derived carbonized carbon.[7–11] These
carbon-based materials have similar properties, such as good
light absorption and lightweight.[5] On the other side, carbon-
based systems are missing some key features that are required
for high solar desalination, they may have high synthesis cost
(graphene oxide and biochar), low water transport and light re-
flection due to the layered structure (graphene oxide), and hy-
drophobic nature (CNTs /graphene), which reduces the water
evaporation rate.[12,13] Furthermore, the high thermal conductiv-
ity of these carbon materials reduces the surface thermal prop-
erties, favoring heat dispersion in the water reservoir, reducing
an efficient interfacial solar desalination.[14] The development of
cost-effective carbon materials with low thermal conductivity, hy-
drophilicity, and high solar absorption, such as PET (polyethylene
terephthalate) upcycled carbon foam, could be an appealing solu-
tion. Waste PET upcycled carbon foam is an excellent carbon ma-
terial owing to the essential characteristics of solar evaporators,
such as high hydrophilicity and low thermal conductivity, which
prevent the surface heat loss and boost the evaporation rate.[15]

Semiconductors, plasmonic and hydrated transition metal chalcogenides, based nano-evaporators play a crucial role in the solar steam generation.[16–20] A bibliometric analysis shows that in recent years the nanoevaporators have attracted significant interest from the research community, as evidenced by a substantial increase in related publications.[6] Efficient solar steam generator design requires high solar absorption from UV/Vis to the Near Infrared region to guarantee absorbance of the full Sun spectrum, and a series of advanced materials is intensively investigated to obtain solar evaporation rates above 3 kg m⁻² h⁻¹.[21] In this context, sulfide-based materials attract strong interest in harvesting solar energy to produce thermal energy. Recently, copper-based sulfide nanostructures have proved their excellent potential as solar steam generators.[22–26] Sulfide-based nanostructures are characterized by their narrow band gaps (usually ranging from 1.0 to 2.4 eV), and this electronic property permits them to absorb a broad spectrum (UV/Vis-NIR) of solar radiation, which underlines the potential of sulfide-based nanostructures in advancing efficient and sustainable solar desalination processes.[27] Recently, ternary chalcogenides of copper-antimony-sulfide have attracted significant attention for solar cell applications due to their earth-abundance and low toxicity.[28–30] They have demonstrated promising achievements due to their high optical activity, catalytic surface properties, and tunable morphologies, with encouraging performances in areas such as dye degradation and CO₂ reduction.[31–33] However, despite their intriguing optical and thermal characteristics, their potential as photothermal materials for interfacial solar desalination remains largely unexplored. Interfacial solar desalination demands a highly efficient combination of materials that, at least, meet three essential criteria: high hydrophilicity coupled with low thermal conductiv-

ity, suitable wettability, and excellent solar light absorption[34].
Herein, we report a rationally designed heterostructure com-
posed of Cu₃SbS₄ and Sb₂S₃ (Stibnite) on carbon foam (CF).
The CF is derived from the upcycling of waste PET. The en-
gineered copper-antimony sulfide-based heterostructure grown
onto CF offers several key advantages for solar water desalination,
including enhanced light absorption across a broad spectrum,
improved photothermal conversion efficiency, and effective in-
terfacial heat localization (due to the low thermal conductivity,
porous and hydrophilic nature of the material which guarantee
effective interfacial heat exchange between the evaporator and
the absorbed water, resulting in a high evaporation rate). These
features collectively make it an excellent candidate for achiev-
ing a high water evaporation rate under solar illumination. The
obtained results prove that the heterojunction of Cu₃SbS₄ and
Sb₂S₃ with CF successfully engineers the morphology of the hy-
brid material and enhances the surface thermal property as com-
pared to Sb₂S₃/CF and the bare CF. In general, the combina-
tion of waste plastic-derived graphitic carbon with sulfide mate-
rial for the rational design of an efficient solar steam generator
promotes circular economy practices while enabling clean water
production.

**2. Results and Discussion**

Initially, the carbon foam (yield = 35.95%) was synthesized from
PET of waste plastic bottles using eutectic salt of NaCl and
ZnCl₂ and melamine by carbonization in an air environment.
To synthesize an efficient photothermal layer, the carbon foam
was further functionalized hydrothermally by copper-antimony
sulfide-based materials and investigated for interfacial solar de-
salination, see **Figure 1a**. The conversion mechanism of PET
to carbon foam is illustrated in Figure 1b, which starts with
the degradation of the PET chain by random scission at the
ester linkage (beta-hydrogen transfer), which creates two chain
fragments (1-carboxyl-terminated chain and 2-vinyl-terminated
chain). The vinyl-terminated fragment (2) decomposes into a
carboxyl-terminated fragment (1) by eliminating acetylene. Dur-
ing the first crosslinking, the amide reaction of the carboxyl
group of the carboxyl-terminated chain and the amino group of
melamine causes carboxyl-terminated PET degradation interme-
diate (5). The crosslinking of melamine with a carboxylic group
suppresses the formation of aromatic ring-terminated fragments
(4) from the carboxyl-terminated chain (2) and (3), and during the
carbonization process, the decarboxylation and dehydration re-
sult in the formation of a stable framework due to the catalytic
action of molten salt by owing to the Lewis acidity of ZnCl₂.
The compound (3) from vinyl-terminated chain fragments under-
goes intermolecular condensation to ease the self-crosslinking
structure (6). Zinc chloride not only promotes the decarboxyla-
tion but also removes the weak bonds of the crosslinked struc-
ture, which yields a more stable crosslinking structure (7). Dur-
ing the degradation and crosslinking process, the physical tem-
plate effect of NaCl and in situ formed small molecular com-
pounds such as CO₂ and H₂O causes a foaming effect. X-ray
diffraction analyses were performed to investigate the crystal
structure of the synthesized materials (namely: CF, SbS/CF, and
Cu, Sb₁.₅S/CF) in the angular range of 10°-80°. **Figure 2** shows
the broad Bragg peak at 2θ = 18°-32°, attributed to the (002)
<figure><img src="image_3.png" /><figcaption>**Figure 1.** a) Graphic presentation of CF production and decoration of Cu<sub>1</sub>Sb<sub>1.5</sub>S on CF for interfacial solar desalination, b) Carbonization mechanism for CF formation.</figcaption></figure>

diffraction by graphitic carbon of CF (JACPDS no. 41-1487). The Sb<sub>2</sub>S<sub>3</sub> grown on CF exhibits diffraction peaks at 2θ values of 11.1, 15.7, 17.5, 22.2, 24.8, 25.7, 28.4, 29.2, 35.5, 39.9, 40.3, 42.9, 46.7, 48.7, 52.9, 60.6 and 71.9° the crystal planes (110), (200), (120), (220), (130), (111), (230), (211), (221), (340), (141), (421), (501), (112), (351), (422), and (811) of stibnite (JACPDS no. 42-1393). The addition of Cu precursor during the hydrothermal synthesis induces the advent of another phase in addition to stibnite. The Cu/Sb ratio of 1:1.5 mm resulted in a heterostructure in which two different phases, stibnite (Sb<sub>2</sub>S<sub>3</sub>) and famatinite (Cu<sub>3</sub>SbS<sub>4</sub>) co-exist. As illustrated in Figure 2, the Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF sample exhibits the diffraction peaks at 2θ values of 18.4, 28.6, 29.9, 33.2, 47.7, 56.6, 59.4, 77.2 and 89.1° that can be indexed to Cu<sub>3</sub>SbS<sub>4</sub> (JACPDS no. 35-0581), with crystal planes of (101), (112), (103), (200), (204), (312), (224), (316), and (228), respectively. It is also evident from the Figure 2 that the peaks at 2θ value of 15.6, 17.5, 24.8, 32.3 and 48.7° can be indexed to Sb<sub>2</sub>S<sub>3</sub> (JACPDS no. 42-1393) with crystal plans of (020), (120), (130), (221), and (112) respectively. Moreover, the XRD patterns of samples with varied concentrations of Cu and Sb are pre-

sented in Figure S1a (Supporting Information). It is noteworthy that the CF did not induce any major crystal changes on the growth of the materials. This can be evidenced from Figure S1b (Supporting Information) where there is no significant alteration in the XRD pattern of Cu<sub>1</sub>Sb<sub>1.5</sub>S as compared to that of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF in Figure 2. Furthermore, the Rietveld Refinement was performed to determine the phase contents of the Cu<sub>3</sub>SbS<sub>4</sub>–Sb<sub>2</sub>S<sub>3</sub> heterostructure. The refinement was performed using (X'pert High Score Plus), and structural models were based on standard crystallographic data (Cu<sub>3</sub>SbS<sub>4</sub>: JACPDS no. 35-0581 and Sb<sub>2</sub>S<sub>3</sub>: JACPDS no. 42-1393). The refined pattern exhibited good agreement with the experimental data, with a χ<sup>2</sup> value of 5.28, indicating a reliable fit. Quantitative phase analysis revealed that the sample consists of ≈58 wt.% Cu<sub>3</sub>SbS<sub>4</sub> and 42 wt.% Sb<sub>2</sub>S<sub>3</sub> Figure S1c (Supporting Information). These results prove the structural integrity of the biphasic heterostructure (Cu<sub>3</sub>SbS<sub>4</sub>–Sb<sub>2</sub>S<sub>3</sub>).

X-ray Photoelectron Spectroscopy (XPS) analyses were conducted to check the elemental composition and chemical state of CF and nanostructure functionalized CF. The XPS survey
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** X-ray diffraction patterns of CF, SbS/CF, and Cu<sub>1</sub>Sb<sub>1.5</sub>S (a), and Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF and JCPDS cards for C, Sb<sub>2</sub>S<sub>3</sub>, and Cu<sub>3</sub>SbS<sub>4</sub> are also reported for direct comparison with the experiments.</figcaption></figure>

spectrum in Figure S2a (Supporting Information) shows sur-
face chemical composition (in atomic concentration%) was: C: 74.0%, N: 13.0%, O: 5.6% and Cl: 1.1%. The high resolution C 1s core level spectrum of CF (Figure S2b, Supporting Information) shows four contributions at 284.8 eV (69%), 286.1 eV (18%), 287.2 eV (7%) and 288.6 eV (7%), assigned to adventitious carbon/—C—C—/—C=C—, C—OH/—C=N, C=O, and —COOH, respectively. The high-resolution O 1s core level spectrum (Figure S2c, Supporting Information) shows three contributions at 530.8 eV (10%), 532.0 eV (51%), and 533.4 eV (39%), assigned to C=O/—COOH and C—OH, respectively. The N 1s spectrum has two contributions with similar relative intensities at 398.6 and 400.2 eV, assigned to pyridinic N and graphitic N. In addition, this sample presents a small amount of chlorine at the surface with a Cl 2p<sub>3/2</sub> signal at 197.3 eV, typical of chloride from final washing with HCl. Figure S3a (Supporting Information) presents the surface chemical composition of sample SbS/CF Figure S3b (Supporting Information) shows the C 1s core level spectra of the sample SbS/CF shows three contributions at 284.76, 286.2, and 288 eV assigned to /—C—C—/—C=C—, C—OH/—C=N, and

-COOH/-COO-. The N 1s in Figure S3c (Supporting Information) present similar information as observed in CF, with two deconvoluted peaks at 398.6 and 400.2 eV corresponding to pyridinic and graphitic carbon. The O 1s core level shows overlapping with Sb 3d<sub>5/2</sub>, which presents two contributions at 529.9 and 539.2 eV corresponding to Sb 3d<sub>5/2</sub> and the peak at 531.7 eV for C=O/-COOH (Figure S3d, Supporting Information). Figure S3e (Supporting Information) shows the deconvoluted peaks for the S 2p core level spectrum that presents two doublets S 2p<sub>3/2</sub>-S 2p<sub>1/2</sub>, with S 2p<sub>3/2</sub> binding energy values at 162.6 and 168.01 eV corresponding to sulfide and sulfate.

**Figure 3a** presents the surface chemical composition observed as: C: 23.9%, N: 5.1%, O: 5.6%, S: 15.1%, Cu: 17.1%, and Sb: 33.1%. The high resolution C 1s core level spectrum of sample Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF (Figure 3b) can be deconvoluted into three contributions at 284.8 eV (74%), 286.2 eV (19%), and 288.0 eV (7%), assigned to adventitious carbon/—C—C—/—C=C—, C—OH/—C=N, and —COOH/—COO—, respectively. The contribution due to C=O, observed in the case of sample CF, is not observed. The N 1s spectrum shows a broad peak centered at 400.1 eV (Figure 3c assigned to —C=N. The O 1s signal cannot be analyzed because it is fully overlapped with the Sb 3d<sub>5/2</sub> signal. For this reason, we used the Sb 3d<sub>3/2</sub> (Figure 3d) that appears at 539.5 eV, typical of Sb(III). This confirms the co-existence of two chemical states of Sb<sup>3+</sup> in Cu<sub>3</sub>SbS<sub>4</sub> and Sb<sup>3+</sup> in Sb<sub>2</sub>S<sub>3</sub>. The high-resolution Cu 2p core level spectrum (Figure 3e) presents a Cu 2p<sub>3/2</sub> at 932.0 eV without satellite. This binding energy value is typical of copper sulfide. Finally, Figure 3f shows the 2p core level spectrum presents two doublets S 2p<sub>3/2</sub>-S 2p<sub>1/2</sub> with S 2p<sub>3/2</sub> binding energy values at 161.8 and 168.3 eV, assigned to sulfide and sulfate, respectively, being the former the predominant. These findings demonstrate that the hydrothermal growth of Sb<sub>2</sub>S<sub>3</sub> and Cu<sub>3</sub>SbS<sub>4</sub>-Sb<sub>2</sub>S<sub>3</sub> on CF does not affect the elemental states/electronic structure of CF.

Scanning electron microscopy (SEM) analyses was performed to investigate the surface morphology of bare CF, SbS/CF, and Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF. In Figure S4a,b (Supporting Information), the as-synthesized CF surface presents the open cell pores developed due to the physical templating effect of molten salt and the foaming feature occurred as a result of carbon dioxide and water molecules released during degradation of PET.[15] In literature, some recent studies present a similar morphology of carbon foam with tiny open pores and flat sheets of CF, indicating its graphitic nature.[35] The surface morphology of the stibnite phase of Sb<sub>2</sub>S<sub>3</sub> on CF shows the elongated, densely packed nanorods that cover the CF surface, as shown in Figure S5a,b (Supporting Information). The nanorods present a good resemblance to literature-reported hydrothermally grown nanorods of stibnite Sb<sub>2</sub>S<sub>3</sub>.[36] Upon introducing Cu into the reaction system (representing the heterostructure of Cu<sub>3</sub>SbS<sub>4</sub>-Sb<sub>2</sub>S<sub>3</sub> on CF), a significant morphological transformation is observed. The nanorods evolve into vertically aligned nanosheets as shown in Figure 4a–c with different magnifications. Interestingly, it exhibits interconnected layers, forming abundant open channels that can facilitate mass transfer which is an advantageous feature for interfacial solar steam generation. Surface morphology plays a crucial role in the efficient performance of solar evaporators, and the aligned nanochannels in nanostructures facilitate directional water transport by reducing the vapor escape resistance and heat localization at the evaporation surface due to enhanced light
<figure><img src="image_6.png" /><figcaption>Figure 3. a) XPS survey spectrum of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF, high-resolution b) C 1s, c) N 1s, d) O 1s and Sb 3d, e) Cu 2p, f) S 2p.</figcaption></figure>

absorption. The elemental compositions of the samples were in-
vestigated through energy-dispersive X-ray spectroscopy (EDS),
and the results are presented in Figure S4 (Supporting Informa-
tion), Figure 4, and Figure S5 (Supporting Information), respec-
tively. The elemental mapping shows the uniform distribution of
elements in each respective sample. The TEM image in Figure 4d
further confirms the nanosheet morphology seen in SEM. The
thin, wrinkled edges in the TEM image indicate the presence of
ultrathin sheet-like domains, consistent with the SEM observa-
tion. The contrast variations suggest a heterogeneous structure,
possibly due to the coexistence of Cu<sub>3</sub>SbS<sub>4</sub> and Sb<sub>2</sub>S<sub>3</sub> phases
within the nanosheets. The existence of the two phases is fur-
ther corroborated by the HRTEM analysis (Figure 4e), which
reveals the distinct lattice fringes that are consistent with the
crystal planes of Cu<sub>3</sub>SbS<sub>4</sub> and Sb<sub>2</sub>S<sub>3</sub>. The obtained d-spacing
(SAED) values of 0.48, 0.32, and 0.16nm, were successfully
matched to the corresponding XRD crystal planes of Cu<sub>3</sub>SbS<sub>4</sub>
(101), (112), and (312); and the d-spacing 0.503nm corresponds to
Sb<sub>2</sub>S<sub>3</sub> crystal planes (020), (120). These observations confirmed
the successful synthesis of the heterostructure with retained
crystallinity.

Efficient photothermal conversion relies critically on the abil-
ity of materials to absorb a broad spectrum of solar radiation,
particularly spanning the ultraviolet (UV), visible (Vis), and near-
infrared (NIR) regions, where the majority of solar energy is
concentrated.[37,38] To understand the fundamental optical prop-

erties, the absorption spectra of the synthesized samples were
recorded across the UV/Vis to Near-IR range. Figure S6 (Support-
ing Information) shows the results of bare CF, SbS, and SbS/CF,
and various composites with different Cu/Sb ratio (1:1, 0.5:1,
1.5:1, 1:0.5, and 1:1.5). The selected sample demonstrate opti-
mum ratio Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF (i.e equivalent to Cu<sub>3</sub>SbS<sub>4</sub>-Sb<sub>2</sub>S<sub>3</sub>/CF)
that delivers the highest absorption is also depicted in **Figure 5a**.
The sample Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF exhibits an almost constant absorption
ranging between 94% and 96% in the full 300-2000 nm spectral
range. Bare CF and SbS/CF samples exhibit a significant drop in
absorbance in the NIR region. Specifically, the Sb<sub>2</sub>S<sub>3</sub> function-
alized CF shows a dramatic decrease in absorption from 90%
in UV/Vis region down to 48% (at 2000 nm) in the near-IR re-
gion, most likely due to the wide band gap of stibnite Sb<sub>2</sub>S<sub>3</sub>.
Although Sb<sub>2</sub>S<sub>3</sub> presents excellent absorption in UV/Vis region,
but declined in absorption activity in NIR is because of higher
wavelength (>800nm) and the photon with lower energy does
not absorb well due to integration of Sb<sub>2</sub>S<sub>3</sub> with CF, the Sb<sub>2</sub>S<sub>3</sub>
layer partially masks the highly NIR-absorptive carbon surface
with a material that performs poorly in NIR region. The uni-
form coating of Sb<sub>2</sub>S<sub>3</sub> nanocrystals can increase the surface re-
flectance of the material that interferes with the CF. Both factors,
the spectral mismatch and structural interference, explain why
SbS/CF shows lower overall light absorption than bare CF. The
enhanced absorption of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF is achieved due to the het-
erojunction of famatnite (Cu<sub>3</sub>SbS<sub>4</sub>) and stibnite (Sb<sub>2</sub>S<sub>3</sub>: Such a
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** a–c) SEM image of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF, d–f) HR-TEM of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF, g–i) elemental mapping and EDS mass sum spectrum of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 5.** a) UV-vis absorption analyses, b,c) Change in mass and evaporation rate of solar desalination, d,e) thermal images and graph of dry surface temperature of CF, SbS/CF, Cu₁Sb₁.₅S, and Cu₁Sb₁.₅S/CF, f,g) thermal images and graph of wet surface temperature of CF, SbS/CF, Cu₁Sb₁.₅S, and Cu₁Sb₁.₅S/CF.</figcaption></figure>

heterojunction of Cu₃SbS₄ with Sb₂S₃ extends the high absorption in the near-IR region. For instance, the study on optical activity for famatnite (Cu₃SbS₄) has been reported, illustrating the high optical absorption in near-IR due to its narrow bandgap of 0.9 eV.[³⁹] Another study reported the excellent optical absorption of Cu₃SbS₄ nanocrystals in the UV/Vis-NIR region,[⁴⁰] which demonstrates its potential as a photothermal material. Based on the comprehensive material structural characterization, especially optical absorption, the synthesized heterostructure could be an excellent material for the interfacial solar desalination process.

The interfacial solar evaporation experiment was performed under simulated solar light (AM 1.5G one sun irradiation 100 mW cm⁻²), the experimental detailed setup is illustrated in Figure S7a (Supporting Information). For a detailed comparison, the solar desalination test was conducted on different samples:CF, Cu₁Sb₁.₅S/CF, SbS/CF, and Cu₁Sb₁.₅S without CF. Figure 5b presents the mass change of the investigated samples, which elucidates the continuous reduction of water mass. The obtained results in Figure 5c reveal the excellent evaporation rate of 2.83 kg m⁻² h⁻¹ for Cu₁Sb₁.₅S/CF compared to the bare CF (1.58 kg m⁻² h⁻¹) and SbS/CF (1.4 kg m⁻² h⁻¹).
$h^{-1}$). The outperforming evaporation rate of $Cu_1Sb_{1.5}S/CF$ is attributed to the synergistic photothermal effect of $Cu_3SbS_4$ and $Sb_2S_3$. The improved results achieved due to the complementary absorption behavior of $Cu_1Sb_{1.5}S/C$ and the integration of $Cu_1SbS_4$ enable a broader spectral response compared to $Sb_2S_3$ functionalized CF, and their combined effect facilitates interfacial charge separation under solar irradiation within a heterostructure that boosts the energy to be dissipated nonradiatively as heat. The alignment of absorption activity (UV/Vis-NIR) with surface temperature ($Cu_3SbS_4-Sb_2S_3/CF > CF > Sb_2S_3/CF$) indicates the heterostructure facilitates more effective solar-to-thermal energy conversion. Figure 5d,e presents the thermal images and graphs of the investigated samples that were captured at the dry state (before desalination), Figure 5f,g presents the wet condition (during desalination). At the dry state Figure 5d,e, the $Cu_1Sb_{1.5}S/CF$ composite exhibits the highest surface temperature, reaching 76.4 °C, significantly surpassing that of pristine CF (68.3 °C) and SbS/CF (52.4 °C). Figure 5f,g shows the operational photothermal performance of the materials under wet conditions, which presents the relative trend consistent with the dry state of high photo-to-thermal conversion. Accordingly, the $Cu_1Sb_{1.5}S/CF$ composite shows the highest wet surface temperature of 41.4 °C followed by CF (37.6 °C) and SbS/CF (31.1 °C). During operational conditions at the wet surface, the lowering of temperature is obvious due to the cooling effect of saline water. The superior photo-to-thermal conversion indicates the effective radiation absorption (according to the optical absorption results) that maintained the heat localization at the wet-air interface. This thermal behavior under working conditions relates well to its highest evaporation rate. As revealed by scanning electron microscopy (SEM) of the investigated samples, it provides further insights to understand the crucial role of the surface morphology of solar evaporators. The vertically stacked nanosheets (nanochannels) increase internal multiple light reflections that help to trap solar photons. In results, it concentrates the photothermal energy at the interface that retains the produced heat locally, decreasing the thermal distribution into the bulk or surrounding water. The effect is proven with infrared imaging demonstrating explaining higher surface temperatures for the heterostructure. In comparison SbS/CF) rod-like morphology presents less surface interface for light interaction, fewer scattering due to fewer trapped photons, resulting in poorer photothermal and evaporation performance. The formation of the composite not only provides high absorption from UV/Vis-NIR but also facilitates the water transport due to nanochannels formation. The surface morphology plays a pivotal role in enhancing the performance of photothermal evaporators. Another interesting point is the comparison we made between $Cu_1Sb_{1.5}S$ ($1.9 \text{ kg m}^{-2} \text{ h}^{-1}$) vs $Cu_1Sb_{1.5}S/CF$ ($2.83 \text{ kg m}^{-2} \text{ h}^{-1}$). These findings indicate the significant contribution of the CF in addition to the optimum heterojunction between $Cu_3SbS_4$ and $Sb_2S_3$. In fact, the unique hydrophilic pores in the CF architecture provide high water transport toward the photothermal layer. In recent findings, Bai et al demonstrate similar outcomes for carbon foam/PVDF that show the evaporation rate of $1.27 \text{ kg m}^{-2} \text{ h}^{-1}$ due to its hydrophilicity.[15] Furthermore, the present study shows much improved results in comparison to our recently reported work on CuS functionalized CF showing an evaporation rate of $1.92 \text{ kg m}^{-2} \text{ h}^{-1}$.[41]

To provide experimental evidence on the hydrophilicity, the surface wettability using contact angle analysis was performed for the pristine CF and $Cu_1Sb_{1.5}S/CF$. Figure 6a shows the instantaneous water droplet absorption in less than a second for both samples. This evidence highlights the hydrophilic character of CF, which is attributed to the presence of oxygen and nitrogen in the graphitic skeleton of CF.[15,41] Hydrophilicity is a crucial factor in solar desalination to achieve a high evaporation rate, in the waste PET-derived carbon foam, the oxygen and nitrogen containing surface functional groups introduced during the carbonization of PET impart high wettability and surface energy to the CF, which allows water to be rapidly drawn into and foster the capillary action of vapor evaporation. As seen in the surface morphology of carbon foam in Figure S4a (Supporting Information) it presents tens of open-cell pores architecture formed due to the physical template effect of molten salts and foamy effect of in situ formed tiny molecular compounds such as $CO_2$ and $H_2O$ in the degradation and crosslinking process in the CF; these structural features act as continuous capillary channels for ensuring consistent water availability at the evaporation interface. The contact angle observation further solidifies these results, with rapid absorption of water droplets confirming the high wettability that drives the capillary movement of water from the bulk reservoir upward to the photothermal surface. Hu et al have demonstrated in a study on waste-derived carbon foam that wettability is a function of oxygen-containing groups and a proportional relation of the number of pores on the surface, which results in capillary movement of water molecules.[13] The $Cu_1Sb_{1.5}S/CF$ nanocrystal decoloration does not alter CF hydrophilic characters, is a requisite for efficient water transport and high evaporation at the interface due to constant water supply. The interfacial desalination process for the bare sample CF and the outperforming solar evaporator $Cu_1Sb_{1.5}S/CF$ was conducted thrice, the evaporation rate was measured every 10 min, and the mean evaporation rate is presented with the error bar in Figure 6b. The evaporation efficiency of $Cu_1Sb_{1.5}S/CF$ is calculated from the value obtained for water vaporization enthalpy from DSC (Differential Scanning Calorimetry) analysis. The $Cu_1Sb_{1.5}S/CF$ sample presents an excellent evaporation efficiency of 94.4%, supporting the performance of the synthesized solar evaporator. The evaporation efficiency is a key parameter (as well as the evaporation rate) to evaluate the potential of solar evaporators for desalination.[42] The present results outperform to recently reported carbon-derived solar evaporators, such as: MOF-porous carbon 64.41%, Co-N-C/CF 87%, and Carbon nanoparticle fabric 76%.[43-45]

The evaporation rate is a key parameter when evaluating the potential of solar evaporators. The stability test in this context ensures that this rate remains consistent, predictable over time, and reliable. Due to a high saline aqueous environment, reporting of the evaporation rate over a longer experimental time demonstrates how gradually salt accumulation affects the evaporation rate. The stability test was performed under one sun irradiation using a glass microfiber filter (Whatman GF/F, CAT No. 1825-047). The stability test presents a decline in evaporation from $2.86 to $2.51 \text{ kg m}^{-2} \text{ h}^{-1}$ in five hours Figure 6c. To confirm the salt crystallization on the surface of the solar evaporator, the real photographs were recorded before and after
<figure><img src="image_10.png" /><figcaption>**Figure 6.** a) contact angle analysis, b) evaporation rate with error bar c) stability graph of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF on 3.5% NaCl saline water under one sun irradiation, and d) Real photographs of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF before and after stability test.</figcaption></figure>

desalination. Figure 6d illustrates that there is no significant visual appearance of salt crystals on the surface, and the observed minor decline (~12.24%) in evaporation rate might have occurred due to saturation of salt concentration in the remaining water or in the cotton sponge that works as a water transport toward photothermal evaporators. The minor decline over the test interval signifies good operational stability of the photothermal system. Post-stability characterization was performed to assess the structural integrity and operational durability of the Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF solar evaporator after five hours of prolonged use. As shown in Figure S8 (Supporting Information), the XRD patterns remain essentially unchanged, with no detectable shifts in peak positions or emergence of new phases. This clearly confirms that the crystalline structure of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF remains stable under illumination and extended solar exposure. The Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF heterostructure

exhibits exceptional desalination performance. For instance, the sodium concentration before the desalination experiment (the saline feed water) was $1.2838 \times 10^4$ mg/L<sup>-1</sup>. Following the solar-driven evaporation experiment, we have condensed back the evaporated water and analyzed the sodium content by inductively coupled plasma optical emission spectroscopy (ICP-OES), which shows a significantly reduced sodium concentration of only $1.5 \times 10^1$ mg/L<sup>-1</sup>. This corresponds to a remarkable salt removal efficiency of ≈99% (Figure S7c, Supporting Information). To further solidify this finding, the ohmic resistance of condensed water and the feed saline water (before and after desalination) was also recorded. For both sample analyses, an equal amount of sample volume (5 mL) was taken in a beaker with an internal diameter of 3 cm, the ohmic value was recorded using a Digital Multimeter (DM01M), and the distance between the meter electrode was ~1.5 cm. Figure S7c (Supporting Information) shows that the ohmic resistance
<table><caption>**Table 1.** Comparison of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF with recently reported carbon-based nano solar steam generators under one sun irradiation.</caption><thead><tr><th>Solar Evaporators</th><th>Evap. Rate [kg m<sup>-2</sup> h<sup>-1</sup>]</th><th>Refs.</th></tr></thead><tbody><tr><td>N-doped carbon foam</td><td>1.39</td><td>[46]</td></tr><tr><td>MoS<sub>2</sub>-loaded carbon foam</td><td>1.48</td><td>[47]</td></tr><tr><td>3D Janus foam</td><td>1.78</td><td>[48]</td></tr><tr><td>Wood slice loaded Pd, Au, Ag nanoparticles</td><td>1.15</td><td>[49]</td></tr><tr><td>carbon-coated Fe<sub>3</sub>O<sub>4</sub> NPs</td><td>1.07</td><td>[50]</td></tr><tr><td>CuS/polyethylene membrane</td><td>1.02</td><td>[51]</td></tr><tr><td>GO/ Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> MXene</td><td>2.09</td><td>[52]</td></tr><tr><td>Co-N-C/CF</td><td>1.88</td><td>[44]</td></tr><tr><td>CuS-carbon foam</td><td>1.92</td><td>[41]</td></tr><tr><td>NiSe-hyd</td><td>1.85</td><td>[53]</td></tr><tr><td>CoS<sub>x</sub></td><td>1.40</td><td>[54]</td></tr><tr><td>HEAO/PDMS-coated wood</td><td>1.90</td><td>[55]</td></tr><tr><td>Cu<sub>3</sub>SbS<sub>4</sub>-Sb<sub>2</sub>S<sub>3</sub>/CF</td><td>2.82</td><td>Present study</td></tr></tbody></table>

of the condensed purified sample (5.02 MΩ) is significantly higher compared to saline water (0.327 MΩ). These observations indicated the dramatic decrease in salt concentration in purified water that ultimately reduced the conductivity of the water sample.

**Table 1** presents a detailed comparison of the evaporation rate of the recently reported solar steam generators with the present finding. Our work confirms the critical role of composite engineering for photothermal-derived desalination, the enhanced performance of Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF achieved due to the combined effect of CF presenting high hydrophilicity with low thermal conductivity (0.113 W m<sup>-1</sup> K<sup>-1</sup>)<sup>[37]</sup> and photothermal effect of Cu<sub>3</sub>SbS<sub>4</sub> and Sb<sub>2</sub>S<sub>3</sub> in enhancing both photothermal conversion and water evaporation performance, emphasizing the potential of composite microstructures for efficient solar desalination applications.

### 3. Conclusion

Interfacial solar desalination is an emerging technology for the production of fresh water utilizing renewable solar energy. The present study illustrates the upcycling of waste PET plastic into CF and further hydrothermal decoration of CF through Cu<sub>3</sub>SbS<sub>4</sub>-Sb<sub>2</sub>S<sub>3</sub> to prepare a solar steam generator. XRD and XPS analysis confirm the crystal structure and chemical state of the synthesized materials, and SEM-EDS present the nanochannels morphology. Optical analysis (UV/Vis-NIR) indicates excellent optical absorption (96%) that provokes the high photothermal output. Bridging the advantage of morphological transformation and the synergistic effect of heterostructuring, an enhanced evaporation rate of 2.82 kg m<sup>-2</sup> h<sup>-1</sup> was obtained, with 99% removal of salt concentration. These findings suggested the sustainable development of engineered nano-evaporators from chalcogenide-functionalized waste plastic-derived carbon material, which represents the rational design of a solar steam generator to enable circular economy practices.

### 4. Experimental Section

All the chemicals and reagents used in the present research work were purchased from Sigma Aldrich: CuCl<sub>2</sub>·2H<sub>2</sub>O, SbCl<sub>3</sub>·5H<sub>2</sub>O, NaCl, ZnCl<sub>2</sub>, Melamine, HCl, and thiourea.

**Synthesis of Cu<sub>3</sub>SbS<sub>4</sub>-Sb<sub>2</sub>S<sub>3</sub>/CF material:** Initially, the carbon foam was synthesized by using the procedure reported in,<sup>[15]</sup> In detail, 1.5g of waste PET bottle sheet was mixed with 0.75g of melamine and 3g of eutectic salt (ZnCl<sub>2</sub>/NaCl (58/42)) and carbonized at 340 °C for 30 min with a heating rate of 10 °C/min. After cooling to room temperature, the obtained product was washed with 0.5M HCl and distilled water repeatedly until neutral pH, and then dried at 60 °C (labelled as CF). The functionalization of CF with Cu<sub>3</sub>SbS<sub>4</sub>-Sb<sub>2</sub>S<sub>3</sub> was carried out using a hydrothermal synthesis approach. 1mM CuCl<sub>2</sub>·2H<sub>2</sub>O and 1.5mM SbCl<sub>3</sub>·5H<sub>2</sub>O were mixed in 100 mL of distilled water and sonicated for 20 min, the mixture was placed on a magnetic stirrer with the addition of 0.15g of CF and 2mM of thiourea and further stirred for 30 min. After stirring, the precursor mixture was transferred into a Teflon-lined stainless autoclave and placed in an oven for 24 h at 180 °C. After cooling to room temperature, the black powder was centrifuged with several washes of distilled water and ethanol. Finally, the obtained materials were dried at 60 °C in a drying oven and placed in a desiccator. The same procedure was followed to synthesize material with different ratios of Cu and Sb (1:1, 1.5:1, 0.5:1, 1:1.5 and 1:0.5) in order to optimize the best combination and labeled as Cu<sub>1</sub>Sb<sub>1</sub>S/CF, Cu<sub>1.5</sub>Sb<sub>1</sub>S/CF, Cu<sub>0.5</sub>Sb<sub>1</sub>S/CF, Cu<sub>1</sub>Sb<sub>1.5</sub>S/CF, and Cu<sub>1</sub>Sb<sub>0.5</sub>S/CF. The same reaction time, temperature, and CF concentration were followed to synthesize Sb<sub>2</sub>S<sub>3</sub> on CF with 1mM SbCl<sub>3</sub>·5H<sub>2</sub>O and 2mM thiourea, and the final product was labeled as SbS/CF.

**Characterization:** The UV/Vis absorption spectroscopy analysis (UV/Vis-NIR) was performed using an ultraviolet-visible spectrophotometer LAMBDA 1050<sup>+</sup>, Perkin Elmer with an integrating sphere. A Philips PW1050/37 diffractometer was used to carry out the X-ray diffraction (XRD) analysis. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 (Thermo Fisher Scientific USA) spectrometer with an x-ray source monochromated Al Kα 150W and energy pass 200 eV for the survey and 30 eV for high-resolution scans. The surface morphology and elemental mapping (SEM-EDS) analysis were carried out using an FEI Magellan 400 FEG-SEM with EDS detector, X-Max 80 mm<sup>2</sup> SDD (Oxford Instrument). The HR-TEM (High-resolution Transmission Electron Microscopy) micrographs were obtained via the JEOL ARM 200F microscope with 2 CS correctors of CC3M competence center of Institut Jean Lamour. The microscope was equipped with an MSC794 camera Gatan one view, JEOL HAADF detector, and a double-tilt sample holder. Hydrophilicity analysis was performed using the FTA 1000 Analyzer System for the contact angle measurements. Differential Scanning Calorimetry (DSC) analysis was carried out using a 1020 Series DSC 7 thermal analysis system, with the following temperature range 20 °C to 150 °C at 5 °C/min. The Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was performed with a Perkin Elmer Optima 5300SV spectrometer.

**Solar Desalination:** The interfacial solar desalination experiments were conducted by preparing photothermal evaporators through vacuum filtration. Specifically, 0.1 g of the material was first dispersed in 20 mL of distilled water and sonicated to ensure uniform suspension. The dispersion was then deposited onto a glass microfiber membrane (Whatman GF/F, CAT No. 1825-047) using vacuum filtration. After filtration, the membrane was dried at 60 °C to enhance film adhesion and mechanical stability. The resulting coated membrane served as the active photothermal layer in the solar evaporator. Then, 40 mL brine solution was taken in a glass beaker (0.6 M NaCl solution in the deionized water that was 3.5 wt.% average global seawater salinity) covered with polyethylene foam as a thermal insulator. A piece of membrane, 1x1 cm<sup>2</sup>, cut, was placed on the cotton sponge that facilitates water transport to the membrane and passes through the thermal insulator. The solar desalination experiment was performed under one sun radiation (air mass 1.5 global, AM1.5G, 100 Wcm<sup>-2</sup>), and the relative humidity was 35%. The surface temperature of the membrane was recorded in the dry state and during the desalination process using a thermal camera (FLIR C3-X).
The evaporation rate of the material was calculated using the following equation;

$$Evaporationrate = \frac{\Delta m}{S} \times \tau \qquad (1)$$

where Δm is the mass loss of water (kg), S is the area of the evaporation (m²), and τ is the evaporation time (h).

Evaporation efficiency (η) is calculated by using the following equation;

$$\eta = \frac{mh_{lv}}{C_{opt}q_{i}} \qquad (2)$$

where m is the mass flux kg m⁻² s⁻¹, hᵢₗ is the enthalpy of liquid, Copt is the optimal concentration, and qᵢ is solar radiation 1KW/m².

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work was supported by the MSCA-IF-2020, CoCaWS, 101027930, and PRIN 2022, CUP: H53D23003840001 projects. We greatly appreciate the efforts of contributors to present work for the successful accomplishment of results, especially Tiziano Finotte and Dr. Sabrina Fabris for the collection of XRD and DSC data. E.R.C. Thanks also goes to project TED2021-130756B-C31 funded by MCIN/AEI/10.13039/ 501100011033 and by “ERDF A way of making Europe” by the European Union NextGenerationEU/PRTR.

Open access publishing facilitated by Universita Ca' Foscari, as part of the Wiley - CRUI-CARE agreement.

## Conflict of interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

chalcogenides, circular economy, photothermal evaporators, plastic upcycling, solar desalination

Received: June 7, 2025

Revised: July 26, 2025

Published online: August 20, 2025

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