Introduction

The conversion and utilization of solar energy globally are widely recognized as pivotal solutions for mitigating the ongoing energy crisis and addressing pressing environmental concerns. Among the various pathways for energy conversion, the conversion of light into heat stands out as a conceptually straightforward and practically convenient approach. The heat generated through this process finds versatile applications in water purification and desalination [1,2,3,4,5], the development of personal heating textiles [6,7,8,9], bactericidal treatments [10, 11], catalytic conversions [12, Solar Heating Measurement

To assess sample temperatures, an IR camera (Testo 872) was utilized for capturing thermal images, while temperature changes were concurrently recorded using a K-type thermocouple (KAIPUSEN) in an air environment. Solar irradiation was administered via a solar simulator (MiniSol LED Solar Simulator, Newport). The simulated sunlight was directed perpendicular to the surface of the samples.

For the outdoor photothermal examination, the testing nanofibers and cotton cloth were positioned on the surface of the simulated skin (adorned with insulating tape possessing an emissivity similar to human skin). This configuration was sealed using heat-resistant tape, and a thermocouple was inserted between the simulated skin and the measured samples. The simulated skin was mounted on a PS foam to provide heat insulation and exposed to sunlight. These outdoor measurements took place in Stockholm, Sweden. Solar irradiance was quantified using a data-logging solar power meter (ISM 410, RS Components Ltd.), while wind speed and ambient temperature were continuously monitored using a flow anemometer (AVM-09, RS Components Ltd.).

Water Vapor Transmission Rate Test

A plastic cup with a diameter of 3.9 cm was filled with 30 mL of deionized water, and its opening was subsequently sealed using either nanofibers or cotton cloth. This prepared cup was then introduced into an environmental chamber set at 35 °C and a relative humidity of 30%. The mass of water that underwent evaporation was measured by periodically weighing the plastic cup on an electronic balance. Subsequently, the water vapor transmission rate was computed based on the recorded loss in the mass of water from the plastic cup.

Wicking Test

A pipette wetted a glass platform with 0.1 mL of deionized water. Subsequently, 5 × 5 cm2 samples were positioned on the moistened area, and the duration taken for the water to propagate to a circular region with a radius of 1.5 cm on the sample surface was noted. The wicking rate was determined by dividing the wicking area by the wicking time.

Mechanical Test

Tensile testing was conducted using an Instron 5960 universal testing machine (Instron, USA) equipped with a 100 N load cell. All nanofibrous samples were cut into 3 mm × 30 mm strips, and their thicknesses were measured using a micrometer. These strips were then tested at a constant speed of 5 mm min−1 with a gauge length of 10 mm. In this context, tensile stress was defined as the ratio of force to the initial cross-sectional area, while tensile strain represented the ratio of the change in length to the original gauge length. Young’s modulus was calculated from the slope of the linear region on the stress–strain curves obtained during testing. Additionally, toughness was determined by integrating the areas under the stress–strain curves. Each sample was tested five times to ensure statistical reliability, and the mean value was computed.

Results and Discussion

Preparation and Characterization of if-Cloth

In pursuit of scalability, a direct and budget-friendly fabrication technique known as electrospinning was selected for the mass production of a composite photothermal cloth (Fig. 1) [S1) and XPS surveys (Fig. 2c and Fig. S5). Analysis suggests that the C 1s spectra of MXene nanosheets could be fitted with four components corresponding to Ti–C–Ti (281.9 eV), C–C (284.1 eV), C–O (285.2 eV), and C=O (288.9 eV) [35, 36].

Cellulose acetate (CA), as one of the most crucial cellulose derivatives being biodegradable and biocompatible [2d) [39,40,41].

Comparing the FT-IR spectra of CA, RC, and the composite nanofibers in Fig. 2e, a pronounced band at 1733 cm−1, attributed to the C=O stretching vibration of the acetate group, was evident in the spectra of both CA and CA/MXene composite nanofibers [42]. However, this band nearly disappeared after the deacetylation process. Meanwhile, a hydroxyl band spanning 3030–3550 cm−1 emerged in RC and RC/MXene composite nanofibers, indicating the successful conversion of CA into RC [29]. This hydrolysis reaction endowed the RC-based nanofibers with improved mechanical strength, heightened surface hydrophilicity, and enhanced chemical resistance against organic and aqueous solutions across a broad pH range from 3 to 12 (Fig. S6).

By encapsulating varying quantities of MXene nanosheets, specifically at 1.3 wt%, 1.9 wt%, 3.3 wt%, 5.7 wt%, and 8.0 wt% (Supplementary Note S1, Fig. S7, and Table S2), the resulting RC/MXene composite nanofibers exhibited a consistent diameter of 0.25 ± 0.07 μm (Fig. 2f, Fig. S8 and Table S3). These nanofibers are labeled RC/MXene I/II/III/IV/V hereinafter. They were randomly stacked to build up nonwoven cloths of ~ 0.1 mm in thinness without detectable transmission of solar light through the nanofibers (Fig. S9).

Core Performances

Benefiting from the photothermal capabilities of MXene nanosheets, the absorptivity of RC/MXene composite nanofibers can be augmented by increasing the MXene content within the RC matrix across the entire spectrum of UV, visible, and NIR light (300–2500 nm) (Fig. 3a and Fig. S10). Accordingly, following a 2-min exposure to one-sun irradiation (1000 W m−2), the temperatures of the composite nanofibers rapidly escalate from an initial room temperature of 25.0 °C to their peak temperature (referred to as Tmax) of 45.9 °C, 50.5 °C, 52.2 °C, 55.4 °C, and 62.1 °C for RC/MXene I/II/III/IV/V, respectively (Fig. 3b, c). In contrast, the Tmax of the pristine RC cloth, with its white surface, only reaches 34.9 °C under identical conditions. MXene nanosheets into nanofibers leads to a darker surface giving a higher surface temperature (Fig. 3c). Notably, RC/MXene V exhibits the most desirable photothermal performance with a solar absorption (termed as Asolar) of 74.7% (Supplementary Note S2) and a Tmax of 62.1 °C, both of which are comparable to the polymer-free MXene film prepared via vacuum filtration of a MXene dispersion (Asolar ~ 68.6% and Tmax ~ 62.6 °C), as shown in Fig. S11 and Table S4. This suggests that the composite nanofibers, harboring embedded MXene nanosheets, can achieve a photothermal effect akin to that of the pure MXene film. Unlike the pure MXene film, these composite nanofibers exhibit enduring stability in harsh environments (Figs. S12 and S13).

Fig. 3
figure 3

Core performances of if-Cloth. a UV–Vis–NIR absorption spectra of RC and RC/MXene I/II/III/IV/V nanofibers. b Time course plots of their temperatures under one-sun irradiation (1000 W m−2). c Tmax of RC and RC/MXene I/II/III/IV/V nanofibers and digital images of RC, RC/MXene I, RC/MXene III and RC/MXene V nanofibers. d Plots of tensile stress–strain of RC and RC/MXene I/II/III/IV/V nanofibers. Water vapor transmission rate (e) and wicking rate (f) of RC/MXene IV, RC, CA and cotton cloths

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Furthermore, incorporating an appropriately balanced proportion of MXene nanosheets into RC nanofibers significantly enhances the mechanical performance of the bulk fibers, as depicted in Fig. 3d. Notably, the RC/MXene IV cloth exhibits a peak tensile stress of 8.7 ± 2.0 MPa, which is nearly six times that of the MXene-free RC cloth (1.5 ± 0.3 MPa) (Table S5). This enhancement in mechanical properties stems from the effective transmission of stress from the nanofillers to the composite matrix during the deformation process [43]. The alignment of MXene nanosheets can contribute to heightened tensile strength in individual fibers, thus bolstering the overall tensile strength of the fibrous membrane. Additionally, the polar terminal groups (–F, –O, and –OH) present on MXene can establish intermolecular hydrogen bonds with the abundant –OH groups on regenerated cellulose, potentially fortifying the entire composite structure [37]. However, excessive load of MXene nanosheets into RC nanofibers (as exemplified by RC/MXene V) could compromise the mechanical performance since excessive nanofillers can form large aggregations, introducing defects within the matrix that facilitate crack initiation and premature fracture.

Additionally, the adequately porous structure within the RC/MXene composite cloth (RC/MXene IV) facilitates the outward transport of water vapor, achieving a water vapor transmission rate (WVTR) of 13.3 mg cm−2 h−1. This figure closely parallels the WVTR of commercially available cotton cloth (13.0 mg cm−2 h−1) (Fig. 3e), underscoring the essential breathability of RC/MXene cloth for human comfort.

Concurrently, we conducted water-wicking rate tests on RC/MXene IV, RC, CA, and cotton cloth. For each sample, a 5 × 5 cm2 segment absorbed 0.1 mL of deionized water on the platform, and the wicking rate was calculated by dividing the wicking area by the wicking time for each specimen [44]. Notably, owing to the interconnected fibrous and hydrophilic structures inherent in the RC/MXene IV cloth, the established water transport channels allow for rapid capillary action to swiftly draw water beneath the cloth and disperse it. This results in the highest wicking rate (30.7 ± 5.1 mm2 s−1) among the various samples (Fig. 3f). This design feature ensures a continuous and rapid water uptake capability, facilitating prompt evaporation.

According to the above-mentioned measurements, the combined capabilities encompass photo-to-thermal conversion, breathability, rapid water uptake, and durability coalesce within a single RC/MXene composite cloth. This convergence opens avenues for various applications, including personalized heating management and solar-driven water evaporation. Henceforth, this multifunctional amalgamation is referred to as the integrated functional cloth or, simply, if-Cloth.

if-Cloth for Personal Heating Management

Since if-Cloth can be employed to collect solar energy into a heat source to stand cold climates, it could potentially suit outer cover of the autumn/winter coats for warming human body in daytime. To evaluate heating performance in real-life, experiments were conducted on the skin of a human wrist covered individually by pieces of if-Cloth and other conventional fabrics (cotton, fleece, lyocell, and cellulose) of 3.5 × 4.0 cm2 in size as reference, as shown in Fig. 4a. As a result, if-Cloth (RC/MXene IV) registers a surface temperature increase to 43.8 °C after a 20-min exposure to 0.6-sun irradiation. This temperature surpasses that of cotton (32.8 °C), fleece (31.1 °C), lyocell (32.5 °C), and cellulose (31.7 °C) fabrics under identical conditions. Consequently, the heat from solar energy effectively transfers to the skin underneath if-Cloth (RC/MXene IV), leading to a skin temperature of 39.1 °C. Notably, if-Cloth's warming effect is superior to the other reference fabrics, which range from 34.9 to 35.4 °C (Fig. 4b, c).

Fig. 4
figure 4

The if-Cloth for personal heating management. a The digital images of the arm skin covered by if-Cloth and other common fabrics as reference (cotton, fleece, lyocell and cellulose) of 3.5 × 4.0 cm2 in size and the corresponding thermal infrared images under 0.6-sun irradiation. b Time course plots of the temperature of human skin covered by if-Cloth and other common fabrics (cotton, fleece, lyocell and cellulose) under 0.6-sun irradiation and c their Tmax. d Detailed Isolar and Tambient in Stockholm area (59° 22′ 13″ N, 18° 3′ 35″ E, November 12, 2022). e The real-time temperature of the simulated skin was covered by if-Cloth, RC nanofibers and a white cotton cloth over 3 h under sunlight in Stockholm, Sweden

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Conventional textiles commonly retain body heat and inhibit its dissipation into the environment to keep the body warm [45, 46]. This approach involves insulating against thermal conduction and convection to maintain warmth. However, the efficacy of passive thermal radiation for heating is limited and struggles to adequately address complex outdoor conditions. In contrast, if-Cloth actively harnesses abundant outdoor solar radiation, converting it into heat to withstand colder climates. Consequently, garments constructed from if-Cloth can be envisioned as thinner and lighter (with a thickness of 0.1 mm and a weight of 30.1 g m−2) compared to conventional textiles, all while delivering superior warming capabilities for the human body.

To further showcase its remarkable solar heating prowess within cold climates, an outdoor assessment was conducted to gauge the solar heating capabilities of if-Cloth (RC/MXene IV), MXene-free RC nanofibers, and a white cotton cloth. This evaluation took place during autumn (on November 12, 2022) in Stockholm, characterized by an average ambient temperature (Tambient) of approximately 11.2 °C and an average solar intensity (Isolar) of roughly 588 W m−2. Throughout 3 h, the experimental setup (Fig. S14) was exposed to sunshine, with the temperatures of a simulated skin surface covered by the respective samples being closely monitored. Concurrently, real-time data encompassing solar irradiance, ambient temperature, wind speed, and simulated skin temperature were meticulously recorded (Fig. 4d, e and Fig. S15).

The simulated skin enveloped by if-Cloth (RC/MXene IV) attains an average temperature (Tave) of around 21.3 °C. This is notably warmer than the ambient temperature (11.2 °C) by 10.1 °C, as well as the RC nanofibers (14.1 °C) and the cotton cloth (15.7 °C) by 7.2 °C and 5.6 °C, respectively. During the peak solar intensity of approximately 654 W m−2 (at 13:37 pm), if-Cloth achieves a maximum temperature (Tmax) of 31.6 °C. If-Cloth effectively imparts energy-efficient heating to a surface, tolerating daytime application scenarios in chilly environments.

if-Cloth for Solar-Driven Water Evaporation

Impressively, when in a wet state, the if-Cloth displays a darker color compared to its dry state (Fig. 5a). Correspondingly, the wet if-Cloth (RC/MXene V) exhibits a significant increase in full light absorption (e.g., 87.7% of RC/MXene V in the 300–2500 nm range) compared to the dry state of if-Cloth (74.7%) (Fig. 5b, Fig. S16 and Table S6). In the wet if-Cloth, water acts as an intermediate medium between air and porous nanofibers, reducing light scattering at the water/cloth interface and thereby capturing more light (Supplementary Note S3) [27, 47]. Due to its excellent light harvesting properties, the wet if-Cloth accelerates the evaporation rate of water, leading to the rapid generation of water vapor in sunlight.

Fig. 5
figure 5

The if-Cloth for solar-driven water evaporation. a Optical images comparing the colors of if-Cloth in its dry and wet states. b UV–Vis-NIR absorption spectra of RC and RC/MXene I/II/III/IV/V nanofibers in their wet states. c Temperature profiles of if-Cloth and RC nanofibers wetted by pure water. Inset: IR images of if-Cloth (RC/MXene IV) and RC nanofibers under one-sun irradiation in equilibrium states. d Pure water evaporation kinetics of RC and RC/MXene I/II/III/IV/V nanofibers under one-sun irradiation. e Performance comparison of solar evaporators reported here and previously. f Synthetic seawater evaporation rates of if-Cloth (RC/MXene IV) in 10 repeated cycles under one-sun irradiation and rinsing by pure water

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For the corresponding experiment, a round-shaped if-Cloth with a diameter of 1.6 cm was employed as a steam generator. It was positioned atop a 2.0 cm-thick polystyrene (PS) foam heat barrier to minimize heat dissipation from the if-Cloth to the underlying bulk water. To ensure complete wetting of the hydrophilic if-Cloth, a cellulose cloth strip was utilized as a capillary water channel to transport water upward to the if-Cloth. Real-time monitoring of temperature fluctuations and water mass loss was achieved using an infrared (IR) camera and a digital scale, respectively, as shown in Fig. S17.

As depicted in Fig. 5c, a wet if-Cloth (RC/MXene V) reached a maximum temperature (Tmax) of 38.7 °C within 10 min under one-sun irradiation. It then maintained this equilibrium state to continually facilitate water evaporation. In contrast, the wet MXene-free RC cloth reached a Tmax of 25.6 °C under the same conditions, primarily due to its relatively lower photo-to-thermal conversion capability. By calculating the mass loss during pure water evaporation under one-sun irradiation, the water evaporation rates for RC and RC/MXene I/II/III/IV/V nanofibers were determined as 0.14 kg m−2 h−1, 0.70 kg m−2 h−1, 0.82 kg m−2 h−1, 1.02 kg m−2 h−1, 1.23 kg m−2 h−1, and 1.34 kg m−2 h−1, respectively (Fig. 5d). Notably, the incorporation of a higher concentration of MXene nanosheets into the nanofibers resulted in enhanced water evaporation rates. Consequently, the solar-to-water-evaporation efficiency was elevated, calculated at 9.5%, 47.5%, 55.7%, 68.2%, 82.3%, and 89.6%, respectively, for pure RC, RC/MXene I/II/II/IV/V nanofibers (Supplementary Note S4). Leveraging MXene nanosheets as photothermal additives [48,49,50,51], the solar-to-water-evaporation efficiency of if-Cloth (RC/MXene V) exceeded that of the MXene-free RC cloth by over 9 times. Compared with other artificial solar evaporators, if-Cloth demonstrated a superior capability for steam generation driven by sunlight (Fig. 5e and Table S7).

To further investigate the viability of if-Cloth for solar desalination, a synthetic seawater solution containing 3.5 wt% NaCl was prepared and subjected to continuous evaporation by if-Cloth under one-sun irradiation, utilizing the same measurement setup. As illustrated in Fig. S18, NaCl salts crystallized and accumulated on the surface of if-Cloth (RC/MXene IV) over a 24-h period of solar-driven synthetic seawater evaporation. Notably, the deposited salt could be easily cleaned by water (Movie S1). Consequently, if-Cloth maintained its performance, achieving a saltwater evaporation rate of 1.15–1.25 kg m−2 h−1 across sequential 10 cycles of testing (Fig. 5f and Fig. S19).

To provide a more comprehensive understanding of if-Cloth's real-life performance, a homemade solar desalination setup was employed to evaporate actual seawater (salinity: 3.6 g/kg). Within this configuration, a round-shaped if-Cloth (RC/MXene IV) measuring 1.6 cm in diameter was positioned within a plastic container and covered by a transparent plastic foil (Fig. S20). Exposed to typical solar irradiation (ranging from 350 to 850 W m−2) at Tambient of approximately 15–24 °C, the evaporated water was collected and weighed, yielding a total of 8.98 kg m−2 over a single day (July 11–12, 2023). In summation, the durability of if-Cloth aptly suits the demands of practical solar desalination and salt removal applications.

Conclusions

In summary, we have introduced a novel integrated functional cloth, if-Cloth, which is created by incorporating MXene nanosheets within regenerated cellulose nanofibers using a simple electrospinning technique. This if-Cloth (RC/MXene IV) exhibits strong photo-to-thermal conversion capability, rendering it effective as an outer layer for warming the human body in cold conditions. Equally significant, if-Cloth (RC/MXene V) is a valuable steam generator for clean water production, boasting an impressive water evaporation rate of 1.34 kg m−2 h−1 and a remarkable solar-to-water-evaporation efficiency of 89.6% under one-sun irradiation. Notably, if-Cloth employs a fossil-free energy-management approach, addressing the fundamental and critical human needs for water and warmth in an impressive synthesis of functionalities.