Highly salt-resistant and efficient dynamic Janus absorber based on thermo-responsive hydroxypropyl cellulose

Abstract

Recent advances in interfacial solar steam generation have made direct solar desalination a promising approach for providing cost-effective and environmentally friendly clean water solutions. However, developing highly effective, salt-resistant solar absorbers for long-term desalination at high efficiencies and evaporation rates remains a significant challenge. We present a Janus hydrogel-based absorber featuring a surface modified with thermo-responsive hydroxypropyl cellulose (HPC) and a hydrogel matrix containing photothermal conversion units, MXene, specifically designed for long-term seawater desalination. At the lower critical solution temperature, HPC undergoes phase separation, which results in the formation of a rough hydrophobic surface. This process creates a Janus evaporator structure that exhibits a high evaporation rate, excellent salt resistance, and long-term stability. Consequently, the hydrogel absorbers achieve an impressive evaporation rate (3.11 kg m⁻² h⁻¹) under one-sun irradiation. Salt residues are deposited only at the edges of the super-hydrophilic bottom. This process ensures long-term evaporator stability for continuous solar evaporation (>30 hours) in simulated seawater at an average evaporation rate of ∼2.58 kg m⁻² h⁻¹. With its unique structural design, achieved via a straightforward design process, the flexible Janus absorber serves as an efficient, salt-resistant, and stable solar steam generator for direct solar desalination.

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New concepts

This work presents an innovative concept of employing a Janus hydrogel-based absorber for solar desalination. By integrating thermo-responsive hydroxypropyl cellulose (HPC) and photothermal conversion units like MXene within the hydrogel matrix, a unique surface modification and structural transformation occur. The thermo-responsive nature of HPC enables it to undergo a phase transition at the lower critical solution temperature, leading to the formation of a hydrophobic upper surface and thus creating a Janus structure. This asymmetric structure plays a crucial role in enhancing multiple aspects of the absorber's performance. It not only facilitates high broadband light absorption and efficient light-to-heat conversion but also enables rapid water transportation while maintaining excellent salt resistance. The resulting Janus hydrogel absorber demonstrates remarkable evaporation rates both under standard one-sun irradiation and during long-term continuous solar desalination in simulated seawater. The strategic design showcases how carefully engineered materials with specific responsive properties can overcome the challenges in developing efficient and durable solar absorbers for seawater desalination and environmental protection, opening up new possibilities for the practical application of such materials in these crucial fields.

Introduction

Freshwater scarcity remains a critical global challenge, exacerbated by population growth and societal advancements. Solar-driven desalination has become a widely adopted solution to address this shortage.^1–3 In this approach, solar absorbers convert clean solar energy to heat for the production of freshwater from seawater. The process features low energy consumption and environmental sustainability.^4,5 Currently, various broadband light-absorbing materials such as semiconductors, polymers, metallic nanoparticles, and carbon-based materials are widely utilized in steam evaporation.^6–11 High evaporation rates and efficiencies have been achieved in solar-driven desalination through the meticulous selection of materials and structural design. However, the salt resistance of solar absorbers remains a critical factor in practical applications.^12,13 Seawater contains high salinity brine (10–20 wt%), along with various minerals, organic matter, and microorganisms. Prolonged exposure of solar absorbers to brine leads to salt crystal accumulation, which can (1) block water transport channels, (2) cover the surface and reduce sunlight absorption, and (3) compromise the mechanical stability of the solar absorber.¹⁴ Therefore, developing solar absorbers with high evaporation rates, excellent efficiencies, and outstanding salt resistance is crucial.

Currently, two main salt-resistance strategies are used for solar absorbers in direct contact with salt water: (1) to induce marginal crystallization, and (2) to prevent salt transport to the surface. The marginal crystallization strategy confines salt crystallization to the edges of the solar absorber. This strategy protects the central absorbent and evaporation surfaces.^12,13,15,16 However, periodic removal of salt crystals is necessary to ensure sustained performance. Furthermore, another common salt-resistance strategy involves using a Janus structure.¹⁷ This structure features a hydrophobic surface and a hydrophilic bottom. The hydrophobic surface absorbs light and generates water vapor, while the hydrophilic bottom ensures continuous water transport. Driven by the salinity gradient, salt ions diffuse back to the hydrophilic side and dissolve into the bulk water. This design enables the Janus structure to achieve both rapid steam evaporation and effective salt resistance. However, the Janus structure is typically constructed via physical stacking or hydrophobic modification, both of which introduce complexities in the preparation process. These methods often result in surface-to-bottom mismatches, lower evaporation rates, poor photochemical oxidation resistance, and limited mechanical stability.^15,18–20 Developing an integrated approach for fabricating solar absorbers with robust Janus structures remains a challenge.

Herein, we propose a Janus hydrogel-based solar absorber with a modified thermo-responsive hydroxypropyl cellulose (HPC) surface and an MXene-contained hydrogel matrix. HPC, a cellulose derivative with thermosensitive properties, is widely used in the design of thermochromic smart windows and artificial skin owing to its biocompatibility and non-toxicity.^21–23 Additionally, HPC exhibits good water solubility and expanded polymer chains when the temperature is below its lower critical solution temperature (LCST). This is because at a relatively low temperature, the thermal motion of water molecules is relatively weak, and the hydrogen bonding interaction between HPC molecules and water molecules dominates, enabling HPC to dissolve stably in water. When the temperature rises and approaches the LCST, the thermal motion of water molecules intensifies, and the hydrogen bonds between HPC molecules and water molecules begin to be gradually broken. This hydrophobic propensity prompts the HPC molecular chains to approach and aggregate, ultimately leading to a phase transition, known as the coil-globule transition.^21,22 The phase transition makes the hydrophobic upper surface of the Janus structure rough, irregular, and hydrophilic.²⁴ This condition creates an asymmetric Janus structure with enhanced light absorption capabilities and exceptional salt resistance. Furthermore, HPC exhibits rapid and stable thermo-induced phase transition characteristics, which facilitates the formation of efficient and stable Janus-structured hydrogel absorbers. The hydrophobic layer of the Janus structure, created via the phase transition of HPC, demonstrates high resistance to photochemical oxidation and enhanced light absorption. Furthermore, the layer offers effective water transport capabilities and long-term chemical stability, which makes it a promising candidate material for seawater desalination and environmental protection.

Result and discussion

Design and fabrication of a solar absorber with a Janus structure based on HPC@MPH hydrogel

Previous studies have demonstrated that hydrogels are excellent materials for solar water purification. Hydrogels can be engineered into solar absorbers to meet the requirements for efficient solar water purification. Through the incorporation of a solar absorber with high photothermal conversion efficiency into the polymer network, the hydrogel effectively collects solar energy for efficient water evaporation.^25,26 Among various photothermal additives, MXene, featuring a unique layered structure and metal-atom-terminated surface, is theoretically an excellent photothermal material with nearly 100% conversion efficiency.^10,27,28 MXene also represents an extensive solar spectrum absorption bandwidth and good thermal stability.^29,30 Also, the abundant hydrophilic terminal groups (i.e., Ti–OH, –O) on MXene nanosheets enhance wettability, enabling rapid water transport and improving the absorber's evaporation rate.^31–33 In this study, first, we prepared the MPH hydrogel by mixing MXene nanosheets (∼3.5 μm) (Fig. S1, ESI†) with the acrylamide (AM) monomer via free radical in situ polymerization. MXene nanosheets served as efficient photothermal agents to improve the photothermal conversion efficiency of the hydrogel. Additionally, the nanosheets established strong hydrogen bonding interactions with the poly-acrylamide chain to create a uniform and stable photothermal conversion system (Fig. S2, ESI†). Subsequently, HPC@MPH hydrogel was obtained by scraping and coating the HPC solution onto the upper surface of the MPH hydrogel. When utilized for solar evaporation, the HPC@MPH hydrogel served as a solar absorber, which stored thermal energy on exposure to light. When the temperature reached the LCST, the HPC layer on the upper surface of HPC@MPH gradually underwent a coil-globule transition (Fig. 1(a); details are in the experimental section), which made the hydrophilic upper surface of HPC@MPH hydrophobic.²⁴ However, the process was dynamically reversible. The transformation created an asymmetric hydrophilic–hydrophobic condition on the upper and lower surfaces of the HPC@MPH, which ultimately resulted in the formation of a Janus structure (Fig. 1(a)). The interface between the hydrophobic and hydrophilic layers of the Janus structure served as a thermal insulator, which reduced heat loss to the surrounding environment. The heat generated after the hydrophobic surface absorbed sunlight was mainly concentrated in the evaporation area. The hydrophilic layer effectively utilized this heat for water evaporation through efficient water transport, which in turn improved heat utilization efficiency. Additionally, the hydrophilic layer rapidly transported water to the evaporation area via capillary action to facilitate continuous evaporation.^12,20,34 Furthermore, the chain segments aggregated after the phase transition of HPC, which ultimately resulted in a significant transformation and roughening of the HPC surface.²² The increased roughness enhanced light absorption via strong scattering and multistage reflection. The irregular surface features scattered incoming light in multiple directions, which increased the likelihood of absorption by the HPC@MPH hydrogel. Moreover, multistage reflection due to the roughened surface further improved the absorption process. The combination of scattering and reflection resulted in a more efficient utilization of light energy (Fig. 1(b)).

Fig. 1 (a) Schematic diagram of HPC phase transition and the formation of Janus structure of HPC@MPH hydrogel. (b) Schematic illustration of the formation of multiple reflective channels and rough surfaces on HPC@MPH hydrogel with the transformation of HPC. (c) and (d) Mechanism of efficient salt resistance of the HPC@MPH hydrogel.

Fig. 1(c) illustrates the steam evaporation process and the structure of the Janus HPC@MPH absorber in brine. The Janus structure of the HPC@MPH hydrogel augments the desalination effect via two principal mechanisms. Firstly, it effectively curtails the deposition of salt on the hydrophobic surface. The hydrophobic upper surface exhibits a weak interaction with salt ions, thereby impeding the adhesion of salt. When exposed to illumination, the hydrophobic surface engenders steam that concomitantly sweeps away water, thereby further impeding the aggregation of salt (Fig. 1(d)). Secondly, it facilitates efficient salt ion transport and dissolution. The hydrophilic bottom of the Janus structure, by dint of the salinity gradient, draws in salt ions and transports them into the interior of the hydrogel or the main body of the solution through a porous architecture. Through this convection-propelled process, the HPC@MPH absorber restricts the deposition of salt on the upper hydrophobic layer and facilitates the expeditious dissolution of salt at the hydrophilic bottom (Fig. 1(d)). Thus, the absorber achieved rapid water evaporation and maintained excellent salt resistance. Moreover, in traditional Janus structures, the hydrophobic upper layer is typically created using special organic groups, which undergo photochemical oxidation when exposed to oxidative chemicals in water and high-energy light. This process results in the deterioration of surface hydrophobicity.^35,36 In contrast, the Janus hydrophobic layer constructed via the phase transition of HPC exhibited high resistance to photochemical oxidation and enhanced light absorption. These improvements ultimately led to efficient water transport and long-term chemical stability.

This observation indicated that HPC, owing to its polyhydroxy functional groups, could adequately cover the gel surface to create a homogeneous hydrogel at room temperature. The MPH hydrogel underwent significant volume expansion to form the HPC@MPH hydrogel after the scraping and drop-coating of the HPC solution (Fig. S3, ESI†). A large HPC@MPH hydrogel (∼13 cm × 13 cm) was also prepared to demonstrate the potential for large-scale practical applications (Fig. 2(a)). The internal microstructure of freeze-dried MPH and HPC@MPH hydrogels were characterized by scanning electron microscopy (SEM), as shown in Fig. 2(b) and Fig. S4, ESI.† The cross section of the MPH was composed of pores with a size ranging from 1–5 μm, demonstrating a typical structure of homogeneously gelated hydrogels.^29,37 Among them, holes of different sizes were interconnected with each other, forming interconnected pores and internal channels. Fig. 2(b) showed that after HPC scraping and infiltration, the internal pore structure of MPH remained unchanged and undamaged. A thin layer of HPC formed on its surface. Optical microscope images revealed that MPH exhibited a very smooth surface (Fig. 2(c)). After coating with HPC and undergoing a phase transition, the MPH surface became rough and irregular (Fig. 2(d)). To gain deeper insights, we further characterized the morphological variations of MPH and HPC@MPH using Atomic Force Microscopy (AFM). As can be seen from Fig. S5 (ESI†), the surface of HPC@MPH exhibits a significantly increased roughness, and its morphology becomes more irregular compared to MPH. The arithmetical mean (Ra) and root-mean-square (Rq) surface roughness values of MPH were 130 nm and 168 nm, respectively. For HPC@MPH, the Ra and Rq values expanded to 298 nm and 369 nm respectively, which represented an increase of 2.3 times and 2.2 times, respectively (Fig. S6, ESI†).

Fig. 2 Characterization of MPH and HPC@MPH. (a) Photograph of a large-sized HPC@MPH sheet. (b) Cross-sectional SEM images of HPC@MPH. (c) and (d) Optical microscope images of MPH and HPC@MPH at 40 °C. (e) High-resolution C 1s XPS spectra of MPH and HPC@MPH. (f) Contact angle changes in MPH and HPC@MPH at different temperatures (25 and 40 °C). (g) FTIR spectra of HPC, MPH, and HPC@MPH. (h) XRD patterns for HPC, MPH, and HPC@MPH 25, 38, 42, and 46 °C.

X-ray photoelectron spectroscopy (XPS) was performed to further validate the MPH structure (Fig. 2(e)). Compared with MPH, a new peak was observed at 286.4 eV in the C 1s spectrum of HPC@MPH. This peak was attributed to the C–O bond in HPC. Meanwhile, comparisons of the full XPS spectrum and binding energy changes of N 1s and O 1s between MPH and HPC@MPH indicated that the introduction of HPC did not alter the basic chemical structure or bonding state of MPH (Fig. S7 and S8, ESI†). Fourier transform infrared (FTIR) spectroscopy was used to further characterize the interactions between HPC and MPH (Fig. 2(g)). In the MPH spectrum, the C=O stretching vibration of the amide groups appeared at 1649.9 cm⁻¹, while the stretching vibration of –OH appeared at 3436.7 cm⁻¹. After the introduction of HPC, both the C=O and –OH characteristic peaks shifted to lower wavenumbers (1642.7 cm⁻¹ and 3428.2 cm⁻¹, respectively). This shift indicated strong hydrogen bonding interactions between HPC and MPH.^38,39 The presence of strong hydrogen bonding interactions ensured that HPC in the HPC@MPH structure would not detach or be damaged during the water evaporation process.

Subsequently, we characterized the changes in hydrophilicity and hydrophobicity of MPH and HPC@MPH during the heating process via contact angle measurements. At 25 °C, the contact angle of HPC@MPH was 47.9°, which was significantly lower than that of MPH (63.7°) (Fig. 2(f)). This observation indicated that the introduction of polyhydroxy HPC increased the hydrophilicity of MPH (Fig. S9, ESI†). At 40 °C, the contact angle of MPH increased slightly, while that of HPC@MPH increased significantly to 80.6°. This observation further confirmed that HPC underwent phase transition at around 40 °C and the surface structure changed from hydrophilic to hydrophobic to create a Janus evaporation structure.^21,23,24

X-ray diffraction (XRD) characterization and analysis were conducted to further investigate the phase transition behavior of HPC@MPH at different temperatures. The characteristic crystallization peak of HPC was 2θ = 20° (Fig. 2(h)).²¹ For HPC@MPH, no crystallization peak was observed at 25 °C. This finding indicated that when the temperature was below LCST, HPC was predominantly in the amorphous form on the surface of HPC@MPH. As the temperature approached LCST (38 °C), a weak crystallization peak appeared at 2θ = 21.6° for HPC@MPH, which indicated that HPC absorbed heat and underwent a partial phase transition. When the temperature reached or exceeded LCST (42 or 46 °C), the diffraction peak at 2θ = 21.9° for HPC@MPH significantly increased. This observation indicated that HPC molecules underwent local crystallization at this temperature to form a more ordered crystal structure.^21,24 The temperature-induced crystal structure change revealed the phase transition behavior of HPC molecules at LCST. The XRD results also confirmed the findings, regarding the changes in hydrophilicity and hydrophobicity, indicated by contact angle measurements.

To evaluate the optical absorption properties of the MPH and HPC@MPH solar absorbers, optical absorption, reflectance, and transmittance spectra were carefully measured using an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophoto-meter equipped with an integrating sphere (Fig. 3(a)–(c)). Both the MPH and HPC@MPH solar absorbers exhibited relatively low transmittance across the entire solar spectrum (Fig. 3(c)). However, the HPC@MPH solar absorber exhibited a significantly lower reflection (∼3%) for incident light (Fig. 3(b)). Consequently, the absorption spectrum of the HPC@MPH absorber exhibited high average absorption (>96%) across the broadband range from visible to near-infrared, which was higher than that of the MPH absorber (Fig. 3(a)). This observation was attributed to the phase transition of HPC at high temperatures after exposure to light, as well as the aggregation of chain segments. The increased surface roughness enhanced light absorption via strong scattering and multi-level reflection. The irregular surface features scattered the incident light in multiple directions, which increased the likelihood of absorption by HPC@MPH.^40,41

Fig. 3 (a) Absorption, (b) reflectance, and (c) transmittance spectra of MPH and HPC@MPH (thickness, 1.2 mm). (d) Measured surface temperature variation profiles of MPH and HPC@MPH under one-sun solar irradiation. (e) Self-made light-irradiation test devices at different incident angles. (f) Infrared thermal images of MPH and HPC@MPH at different incident angles (0°, 30°, 45°, and 60°) under one-sun solar irradiation. (g) and (h) Raman spectra showing the fitting peaks representing intermediate water (IW) and free water (FW) in MPH and HPC@MPH at 40 °C, respectively. (i) Changes in the IW/FW ratio in MPH and HPC@MPH with increasing temperature.

To further verify the effect of increased roughness resulting from the phase transition of HPC, we conducted tests under varying irradiation angles. When subjected to vertical irradiation at one solar intensity (1 kW m⁻²), the surface temperature of the HPC@MPH absorber rapidly increased to and stabilized at 41.7 °C, which was higher than that of the MPH absorber (40.5 °C) (Fig. 3(d)). This observation demonstrated the advantages of thermo-responsive HPC in light trapping and thermal energy management. Considering the changing positions of the sun throughout the day, maximizing solar energy utilization at various incident angles is crucial in high-efficiency solar absorbers. The surface temperatures of both MPH and HPC@MPH were measured when irradiated at one solar intensity at different incident angles (Fig. 3(e), (f) and Fig. S10, S11, ESI†). As the incidence angle increased from 0° to 60°, the surface temperatures of both MPH and HPC@MPH significantly decreased owing to the sharp decline in their projected area.^10,42 However, the surface temperature of HPC@MPH remained consistently higher than that of MPH. This observation confirmed that the excellent light absorption performance of MXene nanosheets, combined with the unique light-trapping structure of the surface, resulted in high light absorption and low directional dependence for HPC@MPH, which ensured good stability for photothermal conversion.

Subsequently, we further verified the impact of HPC on the internal water activation of the absorber, which was crucial for water evaporation. The Raman spectra in the O–H stretching region, with fitted peaks representing intermediate water (IW) and free water (FW) in both MPH and HPC@MPH at 25 °C and 40 °C are shown in Fig. 3(g), (h) and Fig. S12, S13 (ESI†), respectively. Calculations revealed that the IW/FW ratio in HPC@MPH was 0.97 at 25 °C, which was 1.23 times higher than that in MPH (Fig. 3(i)). This increase was attributed to the abundant hydroxyl hydrophilic groups in HPC, which interacted strongly with MPH through hydrogen bonding to induce a higher proportion of IW and further reduce the enthalpy of vaporization of water.^26,43,44 Furthermore, when the temperature increased to 40 °C, the IW/FW ratio in MPH slightly decreased. In contrast, the IW/FW ratio in HPC@MPH increased to 1.0, which was 1.28 times that of MPH (Fig. 3(i)). This enhancement could be attributed to the improved light absorption and heat storage capacity of HPC after phase transition, which strengthened the hydrogen bond interactions and facilitated the generation of IW. The HPC-modified MPH absorber exhibited excellent light absorption and photothermal conversion capabilities, which ultimately reduced the enthalpy of vaporization of water. These features demonstrated the significant potential of the HPC-modified absorber in water evaporation applications.

Fig. 4(a) shows the schematic of solar evaporation using MPH and HPC@MPH solar absorbers. The MPH absorber exhibited a symmetrical structure and a relatively smooth surface, which predominantly reflected the incident light and reduced light trapping ability. In contrast, the surface of the HPC@MPH absorber underwent a coil-globule transition owing to the presence of HPC as the temperature increased, which resulted in a shift from hydrophilic to hydrophobic characteristics. The transformation created an asymmetric hydrophilic–hydrophobic configuration on the upper and lower surfaces of the HPC@MPH absorber, which resulted in the formation of a Janus structure.^45,46 The transformation facilitated the transport of water molecules, which in turn enhanced the photothermal conversion efficiency for water evaporation in the HPC@MPH absorber.⁴⁷ Additionally, following the phase transition of HPC, its chain segments aggregated, which in turn resulted in a roughened surface on the HPC@MPH absorbers.²² The roughness further improved light absorption via strong light scattering and multistage reflection (Fig. 4(a)). Fig. 4(b) shows the COMSOL simulation results for the surface temperature variations of the MPH and HPC@MPH absorbers in the wet state under vertical irradiation at one solar intensity (1 kW m⁻²). The surface temperature of the HPC@MPH absorber rapidly increased to and stabilized at 30.1 °C within 1.3 min, which was higher than the 28.7 °C recorded for the MPH absorber. The inset of Fig. 4(b) shows the surface temperature distribution of both the HPC@MPH and MPH absorbers, obtained from the COMSOL simulation, after heat absorption under single sunlight irradiation.

Fig. 4 (a) Schematic of the solar-vapor generation of the MPH and HPC@MPH absorbers, respectively. (b) The surface temperature change curves of MPH and HPC@MPH absorbers obtained by COMSOL simulation under one solar irradiation. Inset: Schematic diagram of surface temperature change of MPH and HPC@MPH bulk. (c) Mass change of water over time for samples with MPH surface modified by different HPC contents under one sun solar illumination. (d) Mass change of water under one-sun illumination for HPC@MPH absorbers with different thicknesses. (e) Evaporation efficiency of the HPC@MPH absorbers with different thicknesses. (f) Variations in the evaporation mass of the HPC@MPH absorbers over time under different solar intensities. (g) Evaporation rate of the HPC@MPH absorbers after 100 hours test. (h) Evaporation rate of the HPC@MPH absorbers during five on–off light cycles.

The evaporation rate and efficiency were evaluated based on the mass loss of water over various illumination intervals. The conditions and detailed specifications of the customized solar evaporator systems are presented in the ESI.† The curves depicting the mass change of water over time for HPC@MPH absorbers with different HPC contents, as well as for MPH absorbers, were obtained under one-sun solar illumination (Fig. 4(c)). The corresponding water evaporation rates were calculated from the slopes of these curves. The evaporation rates of the HPC@MPH absorbers were consistently higher than those of the MPH absorbers. Fig. S14 (ESI†) shows the COMSOL simulated evaporation rate results. The HPC@MPH absorber exhibited a faster water evaporation rate compared with the MPH absorber. This observation could be attributed to the Janus structure of the HPC@MPH absorbers, along with the subsequent enhancement of surface light absorption and an increase in surface temperature, all of which synergistically contributed to the accelerated water evaporation rate. Additionally, as the content of HPC on the surface increased, the evaporation rate of the HPC@MPH absorbers initially increased before subsequently decreasing. The HPC@MPH-5.0 mg configuration exhibited the optimal evaporation rate (∼3.11 kg m⁻² h⁻¹). This observation was attributed to the enhancement of both water transport rate and surface light absorption in HPC@MPH absorbers as the HPC content increased, which ultimately improved the evaporation rate. However, when the HPC content was increased further, the HPC layer on the surface became too thick and formed an isolated layer or even detached from the surface (Fig. S15, ESI†). This condition hindered both water and heat transfer, which ultimately reduced the water evaporation rate. We further investigated the curves depicting the changes in the mass of water and solar-vapor conversion efficiencies for MPH and HPC@MPH absorbers with different thicknesses (Fig. 4(d)). When HPC@MPH absorbers with thicknesses of 0.6 mm, 1.2 mm, and 2.2 mm were used, the evaporation rates recorded were 3.01 kg m⁻² h⁻¹, 3.11 kg m⁻² h⁻¹, and 2.97 kg m⁻² h⁻¹, respectively. Their corresponding evaporation efficiencies were 89.5%, 91.5%, and 85.2%, respectively (Fig. 4(d) and (e)). The HPC@MPH absorbers consistently exhibited higher evaporation rates and solar-vapor conversion efficiencies compared with the MPH absorbers (2.55 kg m⁻² h⁻¹). The Janus-structured HPC@MPH absorber exhibits excellent performance in terms of both the evaporation rate (3.11 kg m⁻² h⁻¹) and the conversion efficiency (91.5%). This performance was better than that of most of the reported absorbers, such as carbon-based absorbers, plasmonic nanoparticles, and ceramic materials, etc. made of different materials, as well as absorbers with different structures such as foam, thin films, aerogels, and hydrogels, etc.^29,40,48,49 (Fig. S20 and Table S1, ESI†). This finding further validated the material and structural advantages of HPC in solar vapor generation. In subsequent experiments, we selected the HPC@MPH-5.0 mg sample with a thickness of 1.2 mm as the optimal configuration for further testing.

The changes in the mass of water for the HPC@MPH absorbers were measured under various light irradiation intensities ranging from 1 to 10 kW m⁻² (Fig. 4(f)). These measurements revealed that the evaporation rate increased with light intensity. Overall, the mass loss curves exhibited a consistent linear correlation, even at the high light irradiation intensity of 10 kW m⁻². This observation demonstrated the stable evaporation performance of the HPC@MPH absorbers. Furthermore, considering that practical applications typically involve harsh environments and prolonged irradiation periods, the stability and durability of solar absorbers are crucial.⁵⁰ To determine the reusability of the HPC@MPH absorbers, we conducted a 100-hours test using the same samples. In this test, after every 10 hours, the solar absorbers were removed and sealed overnight to simulate one day of use under real conditions. The following day, after replenishing some water, the solar absorbers were reassembled into the container and the cycle test continued.^10,12 The HPC@MPH absorbers maintained a stable average evaporation rate of ∼2.95 kg m⁻² h⁻¹, with only slight fluctuations in the data due to changes in weather conditions (Fig. 4(g)). This stability was attributed to the Janus structure formed by the phase transition of HPC, which exhibited high resistance to photochemical oxidation and demonstrated excellent durability and potential for practical application.

The cycling stability of HPC during heating and cooling was assessed to further confirm that the phase transition was a reversible process. We conducted five consecutive on–off light cycle experiments, where the HPC@MPH absorbers were continuously irradiated under one-sun solar illumination for 1 hour, followed by turning off the light source for 0.5 hours. Then, we recorded the changes in the evaporation rate of the HPC@MPH absorbers (Fig. 4(h)). During illumination, the water evaporation rate of the HPC@MPH absorbers was stable at ∼3.10 kg m⁻² h⁻¹, which further verified that the phase transition of HPC was both reversible and stable. Furthermore, the evaporation rate was stable at 1.69 kg m⁻² h⁻¹ during darkness. This observation was attributed to the short light-off duration and the presence of thermal residue. The brief light-off time ensured that the HPC did not dissolve into the bottom water, while the thermal residue contributed to a gradual evaporation process at a stable rate. Overall, the HPC@MPH absorbers exhibited excellent photothermal conversion capabilities and demonstrated stability and adaptability under long-term irradiation and day–night temperature variations. Thus, HPC@MPH is an efficient and stable material for seawater desalination. Furthermore, the fabrication process of the HPC@MPH absorber is remarkably uncomplicated, thereby promoting standardized and automated manufacturing. In combination with the favorable material compatibility and versatility exhibited by HPC, MXene, and the hydrogel matrix, it possesses a definite propensity for large-scale production.

To evaluate the stability and universality of the Janus HPC@MPH absorbers for efficient solar steam generation, we carefully investigated the time-dependent mass changes and evaporation rates of the evaporator in aqueous solutions of NaCl with four representative concentrations-0.8 wt% (the Baltic Sea), 3.5 wt% (the average salinity of the world ocean), 10 wt% (the Dead Sea) and 20 wt% (a hyper-saline condition) (Fig. 5(a) and (b)).²⁰ The results indicated that HPC@MPH absorbers could maintain a relatively stable evaporation rate in environments with varying salt concentrations. Specifically, under the simulated seawater concentration of 3.5 wt% NaCl, the HPC@MPH absorbers achieved an evaporation rate of 2.59 kg m⁻² h⁻¹. The evaporation rate of the HPC@MPH absorbers declined slightly as the concentration of NaCl increased. Nevertheless, in NaCl solutions with concentrations of 10 wt% and 20 wt%, the evaporation rates remained at 2.41 kg m⁻² h⁻¹ and 2.38 kg m⁻² h⁻¹, respectively, which demonstrated excellent salt-tolerance regulation ability of the HPC@MPH absorbers. In addition, we conducted water evaporation tests on the HPC@MPH absorbers in different salt solutions (3.5 wt% NaCl, KCl, MgCl₂, CaCl₂ and Na₂SO₄). As shown in Fig. S16 (ESI†), the HPC@MPH absorbers can also block these salt ions in their solutions, and keep a high and stable evaporation rate under one sun, demonstrating its excellent resistance capability to other varieties salts. The long-term stability of the HPC@MPH absorbers in simulated seawater (3.5 wt% NaCl) was further confirmed via continuous solar desalination for 30 hours under one-sun solar illumination at a mean evaporation rate of ∼2.58 kg m⁻² h⁻¹ (Fig. 5(c)). We further investigated the water evaporation capacity of HPC@MPH absorbers in simulated saltwater with high salt concentrations (20 wt% NaCl). The linear correlation between water mass loss and time over 10 hours demonstrated stable evaporation and desalination (Fig. 5(d)). After a 10-hours test, no salt crystals precipitated on the surface of the HPC@MPH absorbers (Fig. 5(d), inset). This observation demonstrated the excellent salt resistance of the evaporator. When the HPC@MPH absorber was tested for 10 hours in a nearly saturated NaCl solutions (25%), NaCl will precipitate under light, and salt deposition will occur at the edges of the evaporator. At this time, both the evaporation rate and efficiency have decreased. (Fig. S17, ESI†). However, there is no obvious salt deposition on the surface of the hydrogel evaporator. Only partial salt deposition occurs at the edge of the evaporation window (Fig. S18, ESI†). Owing to the formation of the Janus structure, the HPC@MPH absorber could limit salt deposition on the upper hydrophobic layer, which in turn facilitated rapid salt dissolution in the lower hydrophilic layer, driven by convection. This design resulted in outstanding salt tolerance and stability.^20,36,51,52

Fig. 5 (a) Changes in mass of the HPC@MPH evaporator tested in water with different salt contents for 1 hour. (b) Evaporation rates of the HPC@MPH absorber in 0.8, 3.5, 10, and 20 wt% NaCl simulated seawater. (c) Changes in mass and evaporation rates of the HPC@MPH absorber in 3.5 wt% NaCl simulated seawater. (d) Changes in mass of water and evaporation rate over 10 hours in a 20 wt% NaCl simulated seawater test. The inset images show no salt precipitated after the 10-hours test. (e) Concentrations of Na⁺, Mg²⁺, K⁺, and Ca²⁺ in actual seawater and collected desalinated water. (f) Changes in the mass of water after 10 hours for actual Bohai seawater. (g) Addition of methyl orange (MO) and methylene blue (MB) to simulate sewage treatment. (h) On September 17, 2024, in Tianjin, China, continuous measurement of solar flux and clean water generation was conducted using a large-scale HPC@MPH solar absorber (13 cm × 13 cm) assembled as a steam generator. Changes in the mass of water (red line), temperature (blue line), and solar flux (orange line) were recorded from 9:00 a.m. to 5:00 p.m.

To demonstrate the feasibility of the HPC@MPH solar absorber for actual seawater desalination, a seawater sample collected from the Bohai Sea in China was used as the saline solution. The HPC@MPH absorber was continuously irradiated for 10 hours, during which the change in evaporation rate was recorded. Additionally, the desalinated water was collected to measure the ion content via inductively coupled plasma mass spectroscopy (ICP-MS). The mean evaporation rate of the HPC@MPH absorber was 2.55 kg m⁻² h⁻¹, and a linear relationship was observed between water mass loss and time (Fig. 5(f)). The evaporation rates exhibited by the HPC@MPH absorber in both real seawater and simulated seawater display a remarkable similarity. This resemblance indicates that the employment of this simulated seawater in testing procedures is capable, to a certain degree, of mirroring the performance characteristics of the HPC@MPH absorber within the context of actual seawater conditions (Fig. S19, ESI†). Moreover, after desalination, the concentrations of the four major ions, Na⁺, K⁺, Ca²⁺, and Mg²⁺, were approximately four orders of magnitude lower than the standard levels set by the World Health Organization (WHO) and the US Environmental Protection Agency (EPA) (Fig. 5(e)). Methyl orange (MO) and methylene blue (MB) were intentionally added to the seawater as simulated pollutants to thoroughly assess the sewage treatment capability of the HPC@MPH absorber (Fig. 5(g)). The distinct UV-Vis absorption peaks corresponding to MO (∼465 nm) and MB (∼665 nm) were almost entirely removed after the desalination process. This observation indicated that barely any residual contamination was present in the resulting freshwater, which demonstrated the highly efficient nature of the sewage treatment provided by the HPC@MPH absorber.

To further investigate the actual performance under real factors such as varying sunlight intensity and incident angles, an outdoor solar desalination experiment was conducted under natural sunlight for a duration of 8 hours (from 9:00 a.m. to 5:00 p.m.). A large-scale HPC@MPH absorber (13 cm × 13 cm), assembled with polystyrene foam to serve as a steam generator, was utilized in this experiment. This setup mimicked real-world conditions and assessed the performance variations, durability, and stability of the HPC@MPH absorber under constantly changing environmental factors. Fig. 5(h) shows the variations in the mass of water, solar heat flux, and air temperature. From 10:00 a.m. to 2:30 p.m, the solar flux remained at a high value (∼550–800 W m⁻²). However, the sharp drop in solar heat flux between 1:00 p.m and 2:00 p.m. might have been due to dark clouds covering the sun. The changes in the mass of water continuously increased during this period. During the 8-hours test of the HPC@MPH absorber, the measured total evaporation was 9.59 kg m⁻², which demonstrated its continuous and highly efficient evaporation capabilities. Moreover, this observation further corroborated that the HPC@MPH absorber could maintain an efficient and stable evaporation rate even in real and complex environments. The excellent performance of the HPC@MPH absorber demonstrated its tremendous application potential in the field of seawater desalination.

Conclusions

In summary, we propose to use the thermo-responsive HPC to modify the surface of the hydrogel matrix containing the photothermal additive MXene to prepare an efficient hydrogel absorber. At the LCST, HPC will experience phase transition, resulting in the formation of a hydrophobic upper surface and thus an asymmetric Janus structure. The HPC@MPH absorber exhibited high broadband light absorption, efficient light to heat conversion, rapid water transportation, excellent salt resistance, and enhanced corrosion and oxidation. As a result, the HPC@MPH adsorber exhibits a rapid evaporation rate of 3.11 kg m⁻² h⁻¹ under one-sun irradiation intensity, and its evaporation efficiency has a negligible change even after 100-hours-long evaporation. Even during 30-hours continuous solar desalination in simulated seawater (with 3.5 wt% NaCl), the HPC@MPH evaporator can still maintain an evaporation rate of about 2.58 kg m⁻² h on average. The feasibility of the HPC@MPH absorber for practical seawater desalination and sewage treatment has also been successfully demonstrated. These characteristics render it a highly promising candidate material in the fields of seawater desalination and environmental protection.

Experimental section

Raw materials

Ti₃AlC₂ powders (particle size ∼400 mesh) were sourced from Laizhou Kai Kai Ceramic Materials Co., Ltd. Hydrochloric acid (HCl) was purchased from Tianjin Bohai Chemical Reagent Co., Ltd. Lithium fluoride (LiF) was purchased from Aladdin Reagent Co. Acrylamide (AM, 98%) and N,N′-methylene bis(acrylamide) (MBAA, 99%) and APS (ammonium persulfate) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd, China. Methyl orange (MO), methylene blue (MB) and hydroxypropyl cellulose (HPC) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd.

Synthesis of Ti₃C₂T_x MXene nanosheets

First, 2 g of LiF was dissolved in 40 mL of 9 M HCl solution under 30 min stirring. Then, 2 g of 400 mesh Ti₃AlC₂ powder was slowly added, and the mixture was stirred at 35 °C for 24 hours continuously. After the reaction, the mixture was cleansed by centrifuged at 5000 rpm for 3 min until the supernatant pH > 5. The precipitated MXene layer at the bottom was dispersed in ethanol by ultrasonication for 30 min. The dispersion was then centrifuged at 10 000 rpm for 30 min. Next, after distilled water was added, the precipitate was sonicated for 20 min and then centrifuged at 3500 rpm to get a uniform MXene suspension. Eventually, the resulting homogeneous MXene solution was freeze-dried to obtain MXene powder.

Preparation of MPH hydrogels

The MPH hydrogels were synthesized using MXene nanosheets and AM via free radical in situ polymerization. Firstly, 20 mg MXene nanosheets was added into 1.75 g distilled water and dissolved by ultrasound for 5 min. Then, 1.0 g AM and 10 mg MBAA were added into the MXene dispersion and stirred for 2 hours. After removing bubbles and adding 250 mg APS aqueous solution in the above dispersion. Finally, the resulting solution was injected into 0.5 mm, 1.0 mm and 2.0 mm thick molds, respectively for gelation at 60 °C for 15 hours.

Preparation of HPC@MPH hydrogels

Firstly, add 1 g of HPC to 199 g of distilled water and stir continuously at 25 °C for 12 hours until the HPC was fully dissolved in water, thus obtaining a 0.5 wt% HPC solution. After that, cut the MPH into pieces with a size of 2 × 3 cm. Then, drop-coat different amounts of the above 0.5 wt% HPC solution (2.5 mg, 5.0 mg and 10.0 mg) onto the upper surface of the MPH hydrogels and spread evenly. Finally, put the samples in a small quantity of water for 2 hours until the HPC was completely absorbed on the upper surface of the MPH and fully swollen, thus obtaining the HPC@MPH hydrogels.

SVG (solar-vapor generation) experiments

The experiments were typically conducted at an ambient temperature of about 25 °C and a humidity of about 50–60%. As a solar absorber and converter, the hydrogel was float-supported by a 3 mm thick polystyrene (PS) foam with a fixed square-dimensioned window in the middle for directly absorbing incident irradiation. Water evaporation experiments were carried out with a 1–10 kW m⁻² simulated solar flux output by a solar simulator (7ILX500P, SOFN Instruments Co., Ltd). The solar flux was measured by a solar power meter (CEL-NP 2000 Full-Spectrum Strong light power meter). The temperature was measured by an infrared thermal imager (Testo 869, Testo SE & Co. KGaA) and thermocouple. Once the light was on, the mass change was immediately tracked by a high-accuracy balance (OHAUS, AX224ZH, 0.1 mg in accuracy) and then real-time communicated to a desktop computer for calculating the evaporation rate and efficiency of solar steam generation.

Characterization

The morphology of the hydrogels was characterized using scanning electron microscopy (SEM, JSM-7800, Japan). X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250XI photoelectron spectrometer (Thermo Fisher Scientific, USA). X-ray diffraction (XRD) measurements were performed using a Rigaku Smart Lab 3 kW diffractometer (Rigaku, Japan). The FTIR spectra were obtained by a Bruker TENSOR27 FTIR spectrometer with a 4000–500 cm⁻¹ wavenumber range. The reflectance and transmittance were measured using a UV-vis-NIR spectrometer (Cray 5000) with an integrating sphere. The water contact angles were measured with a dynamic contact angle measuring instrument (JC2000D3M) at different temperatures. The concentration Na⁺, K⁺, Ca²⁺, and Mg²⁺ were tracked by inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7700) and inductively coupled plasma emission spectrometer (ICP-OES, Agilent 5110). Raman spectra were acquired utilizing a Thermo DXR2xi (Thermo Fisher Scientific, USA). The Shimadazu UV-2600 spectrophotometer was used to collect the ultraviolet-visible absorbance spectra for evaluating the simulated sewage and condensed water.

COMSOL simulation

We use COMSOL Multiphysics 4.4 to verify if the ability of localized heating and the evaporation rate of HPC@MPH absorber was better than MPH absorber. Here, we took the input solar energy of 1 sun, the ambient and water temperatures of 20 °C (293.15 K) and the evaporator surface temperature as boundary conditions. With these boundary and parameter matches, the temperature distributions in the COMSOL model were described by the following equations:

q = −k∇T (2)

where the E_in denotes the thermal energy input originating from solar irradiation; the x and t are the space vector and time, respectively; ρ, C_P and k are the mass density, specific thermal capacity and thermal conductivity of the matters respectively; T(x, t) signifies the local temperature, and u is the fluid flow speed of the aqueous medium. The environment temperature was set to 293.15 K, and the water-related parameters were taken directly from the COMSOL Multiphysics materials library.

Calculation of the energy conversion efficiency

The thermal conversion efficiency η was defined as the formula:
where m represents the evaporation rate (m = m_light − m_dark), m_light and m_dark are the evaporation rates with and without illumination respectively; h_fg is the liquid-vapor phase change enthalpy, A represents the light-adsorbing area of the surface, and q_solar represents the solar flux per area.

The equivalent water evaporation enthalpy in the MPH and HPC@MPH hydrogel were calculated by comparing the water evaporation of pure water and hydrogel during evaporation in the dark (ΔH_equ).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Mariñas and A. M. Mayes, Nature, 2008, 452, 301–310.
2. X. Zhou, F. Zhao, Y. Guo, B. Rosenberger and G. Yu, Sci. Adv., 2019, 5, eaaw5484.
3. M. A. Montgomery and M. Elimelech, Environ. Sci. Technol., 2007, 41, 17–24.
4. X. Zhou, Y. Guo, F. Zhao and G. Yu, Acc. Chem. Res., 2019, 52, 3244–3253.
5. P. Tao, G. Ni, C. Song, W. Shang, J. Wu, J. Zhu, G. Chen and T. Deng, Nat. Energy, 2018, 3, 1031–1041.
6. B. Yang, Z. Zhang, P. Liu, X. Fu, J. Wang, Y. Cao, R. Tang, X. Du, W. Chen, S. Li, H. Yan, Z. Li, X. Zhao, G. Qin, X. Chen and L. Zuo, Nature, 2023, 622, 499–506.
7. J. Wang, Y. Li, L. Deng, N. Wei, Y. Weng, S. Dong, D. Qi, J. Qiu, X. Chen and T. Wu, Adv. Mater., 2017, 29, 1603730.
8. X. Hu, W. Xu, L. Zhou, Y. Tan, Y. Wang, S. Zhu and J. Zhu, Adv. Mater., 2017, 29, 1604031.
9. Y. Ito, Y. Tanabe, J. Han, T. Fujita, K. Tanigaki and M. Chen, Adv. Mater., 2015, 27, 4302–4307.
10. X. Fan, Y. Yang, X. Shi, Y. Liu, H. Li, J. Liang and Y. Chen, Adv. Funct. Mater., 2020, 30, 2007110.
11. F. Zhao, X. Zhou, Y. Shi, X. Qian, M. Alexander, X. Zhao, S. Mendez, R. Yang, L. Qu and G. Yu, Nat. Nanotechnol., 2018, 13, 489–495.
12. L. Li, N. He, B. Jiang, K. Yu, Q. Zhang, H. Zhang, D. Tang and Y. Song, Adv. Funct. Mater., 2021, 31, 2104380.
13. X. Dong, Y. Si, C. Chen, B. Ding and H. Deng, ACS Nano, 2021, 15, 12256–12266.
14. Q. Zhang, G. Yi, Z. Fu, H. Yu, S. Chen and X. Quan, ACS Nano, 2019, 13, 13196–13207.
15. G. Ni, S. H. Zandavi, S. M. Javid, S. V. Boriskina, T. A. Cooper and G. Chen, Energy Environ. Sci., 2018, 11, 1510–1519.
16. Y. Tian, Y. Li, X. Zhang, J. Jia, X. Yang, S. Yang, J. Yu, D. Wu, X. Wang, T. Gao and F. Li, Adv. Funct. Mater., 2022, 32, 2113258.
17. H. Yao, P. Zhang, C. Yang, Q. Liao, X. Hao, Y. Huang, M. Zhang, X. Wang, T. Lin, H. Cheng, J. Yuan and L. Qu, Energy Environ. Sci., 2021, 14, 5330–5338.
18. C. Lei, Y. Guo, W. Guan and G. Yu, J. Polym. Sci., 2021, 59, 3084–3099.
19. Y. Kuang, C. Chen, S. He, E. M. Hitz, Y. Wang, W. Gan, R. Mi and L. Hu, Adv. Mater., 2019, 31, 1900498.
20. W. Xu, X. Hu, S. Zhuang, Y. Wang, X. Li, L. Zhou, S. Zhu and J. Zhu, Adv. Energy Mater., 2018, 8, 1702884.
21. Y. Guo, W. Guan, C. Lei, H. Lu, W. Shi and G. Yu, Nat. Commun., 2022, 13, 2761.
22. K. Yang, M. Chen, Q. Wang, S. Grebenchuk, S. Chen, X. Leng, K. S. Novoselov and D. Andreeva, Adv. Funct. Mater., 2022, 32, 2201904.
23. X. Lu, Z. Hu and J. Schwartz, Macromolecules, 2002, 35, 9164–9168.
24. Y. Jing and P. Wu, Cellulose, 2013, 20, 67–81.
25. Y. Guo, H. Lu, F. Zhao, X. Zhou, W. Shi and G. Yu, Adv. Mater., 2020, 32, 1907061.
26. X. Zhou, Y. Guo, F. Zhao, W. Shi and G. Yu, Adv. Mater., 2020, 32, 2007012.
27. X. Chen, P. Zhang, X. Chen, C. Luo, M. Chao and L. Yan, Desalination, 2024, 592, 118155.
28. Y. Yang, W. Fan, S. Yuan, J. Tian, G. Chao and T. Liu, J. Mater. Chem. A, 2021, 9, 23968–23976.
29. X. Ji, X. Fan, X. Liu, J. Gu, H. Lu, Z. Luan and J. Liang, Nano Lett., 2024, 24, 3498–3506.
30. Y. Lu, H. Zhang, Y. Wang, X. Zhu, W. Xiao, H. Xu, G. Li, Y. Li, D. Fan, H. Zeng, Z. Chen and X. Yang, Adv. Funct. Mater., 2023, 33, 2215061.
31. R. Li, L. Zhang, L. Shi and P. Wang, ACS Nano, 2017, 11, 3752–3759.
32. M. Mustakeem, J. K. El-Demellawi, M. Obaid, F. Ming, H. N. Alshareef and N. Ghaffour, ACS Appl. Mater. Interfaces, 2022, 14, 5265–5274.
33. M. Aizudin, M. K. Sudha, R. Goei, S. K. Lua, R. P. Pottammel, A. I. Y. Tok and E. H. Ang, Chem. – Eur. J., 2023, 29, e202203930.
34. P. Zhou, K. Yang, L. Liu, Q. Liu, N. Zhang and J. Xu, J. Colloid Interface Sci., 2024, 682, 795–803.
35. L. Zhang, B. Tang, J. Wu, R. Li and P. Wang, Adv. Mater., 2015, 27, 4889–4894.
36. J. Chen, J. L. Yin, B. Li, Z. Ye, D. Liu, D. Ding, F. Qian, N. V. Myung, Q. Zhang and Y. Yin, ACS Nano, 2020, 14, 17419–17427.
37. X. Guo, W. Qin, C. Gu, X. Li, M. Chen, H. Zhai, X. Zhao, H. Liu, B. Zhao, Y. Zhang, Y. Wang and S. Yin, Adv. Mater. Technol., 2024, 9, 2302072.
38. Y. Zhu, J. Liu, T. Guo, J. Wang, X. Tang and V. Nicolosi, ACS Nano, 2021, 15, 1465–1474.
39. S. Lin, Y. Zhong, X. Zhao, T. Sawada, X. Li, W. Lei, M. Wang, T. Serizawa and H. Zhu, Adv. Mater., 2018, 30, 1803004.
40. H. Ren, M. Tang, B. Guan, K. Wang, J. Yang, F. Wang, M. Wang, J. Shan, Z. Chen, D. Wei, H. Peng and Z. Liu, Adv. Mater., 2017, 29, 1702590.
41. T. Xue, F. Yang, X. Zhao, F. He, Z. Wang, Q. Wali, W. Fan and T. Liu, Chem. Eng. J., 2023, 461, 141909.
42. X. Wang, Q. Liu, S. Wu, B. Xu and H. Xu, Adv. Mater., 2019, 31, 1807716.
43. X. Zhou, F. Zhao, Y. Guo, B. Rosenberger and G. Yu, Sci. Adv., 2019, 5, eaaw5484.
44. L. Li, C. Xue, Q. Chang, X. Ren, N. Li, J. Yang, S. Hu and H. Xu, Adv. Mater., 2024, 36, 2401171.
45. L. Li and J. Zhang, Nano Energy, 2021, 81, 105682.
46. Y. Han, Y. Wang, M. Wang, H. Dong, Y. Nie, S. Zhang and H. He, Adv. Mater., 2024, 36, 2312209.
47. Z. Chen, J. Wang, H. Zhou, Z. Xie, L. Shao, A. Chen, S. Wang and N. Jiang, Adv. Funct. Mater., 2023, 33, 2303656.
48. Y. Wang, X. Liu, Q. Zhang, C. Wang, S. Huang, Y. Liu, T. Yu, R. Yang, G. Z. Chen, M. Chaker and D. Ma, Adv. Funct. Mater., 2023, 33, 2212301.
49. X. Zhou, M. Guo, M. Huang, Y. Xu and M. Liu, Chem. Eng. J., 2024, 501, 157659.
50. V. D. Dao, N. H. Vu and S. Yun, Nano Energy, 2020, 68, 104324.
51. S. Gao, X. Dong, J. Huang, J. Dong, F. Di Maggio, S. Wang, F. Guo, T. Zhu, Z. Chen and Y. K. Lai, Glob. Chall., 2019, 3, 1800117.
52. R. Hu, J. Zhang, Y. Kuang, K. Wang, X. Cai, Z. Fang, W. Huang, G. Chen and Z. Wang, J. Mater. Chem. A, 2019, 7, 15333–15340.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh01699h

‡ Authors with equal contributions.

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