Amphiphilic Janus patch-grafted hydrogels for salt-rejecting solar water desalination

Abstract

Interfacial solar seawater desalination is considered as one of the most promising sustainable techniques for producing fresh water. Janus hydrogel evaporators with a hydrophobic top layer and a hydrophilic bottom layer have been demonstrated as an effective way to accelerate water evaporation and reject salt ions. However, the existing strategies make it difficult to achieve precise control over surface wettability and confine a thin hydrophobic layer to the hydrogel's surface, both of which were considered to have a significant impact on solar seawater desalination. Herein, we fabricated amphiphilic Janus patch-grafted hydrogel evaporators, in which ultrathin hydrophobic polystyrene patches were uniformly and discretely distributed on the top of the hydrogel and hydrophilic quaternized poly(4-vinyl pyridine)s (QP4VPs) were entangled within the hydrogel network. By the rational design of the Janus patch size and surface coverage, the wettability of Janus hydrogels could be precisely regulated. The Janus hydrogel resulted in optimized solar water evaporation performance with an evaporation rate of 3.2 kg m⁻² h⁻¹ and Janus patch surface coverage of ∼60%. Moreover, the Janus hydrogel has a superior salt ion rejection ratio, which was attributed to the high ionic strength of the QP4VP-rich entangled layer. The amphiphilic Janus patch-grafted hydrogels outperformed all non-photothermal hydrogels and were comparable to photothermal hydrogels, in terms of water evaporation rate and salt ion rejection ratio, demonstrating their potential for solar seawater desalination.

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Xiayun Huang

Xiayun Huang is an associate professor in the Department of Macromolecular Science at Fudan University and the principal investigator in the State Key Laboratory of Molecular Engineering of Polymers. She received her PhD in 2014 from Texas A&M University, and did her postdoc in the Experimental Soft Matter Lab at Texas A&M University. She joined Fudan University as an assistant professor in 2016 and was promoted to an associate professor in 2021. She is an awardee of the Shanghai Pujiang Talent Award and her research focuses on the self-assembly of well-defined nanostructures and their functional application at surfaces and interfaces.

1. Introduction

Interfacial solar water evaporation has emerged as a reliable and sustainable seawater desalination technique to produce fresh water.^1–5 By managing photon management and localizing solar heat at the air/water interface, rather than heating the bulk water, solar harvesting is greatly enhanced and thermal loss is effectively suppressed.^6,7 Compared to other evaporator materials, such as covalent organic frameworks,^8,9 wood,^10,11 and textiles,^12–14 hydrogel evaporators have significantly higher evaporation rates due to the interaction between polymers and water molecules, which activates the water and reduces its evaporation enthalpy.^15,16 However, fast water evaporation causes a dramatic increase in local salt ion concentration at the interface, which eventually leads to the accumulation of salt crystals on the surface of the evaporator, reducing the evaporation rate and shortening the lifetime of the evaporator.^17–19

Several salt-rejection strategies have been explored to design hydrogel evaporators with high evaporation rate and excellent salt ion rejection performance, including diffusion and convection,^20,21 Janus structure,^22–25 salt ion repulsion effect,^26–28 and local crystallization.^29–31 Among these, the Janus hydrogel evaporator with a hydrophobic top layer and a hydrophilic bottom layer was promising, in which the hydrophobic top layer limits salt deposition and the hydrophilic bottom layer allows rapid salt dissolution so that dissolved salt diffuses back to seawater via convection.³² In particular, the wettability and thickness of the top layer have a significant impact on water evaporation rate. When the top layer is very hydrophobic or thick, the water supply for evaporation is severely restricted, thus dramatically reducing the evaporation rate.^33,34 However, fabricating Janus hydrogels with precise control over surface wettability and confining a thin hydrophobic layer to the hydrogel's surface is challenging. Yu et al. adjusted the surface hydrophobicity by creating island-like hydrophobic patches on polyvinyl alcohol (PVA)/Ti₂O₃ (13 wt% Ti₂O₃ particles in the PVA) hydrogel via a trichloro(octadecyl)silane (OTS) condensation reaction with dried hydrogel.³⁵ The water evaporation rate reached the maximum value (4.0 kg m⁻² h⁻¹ under one sun irradiation) when the hydrophobic patch surface coverage was 30%. On the hydrogel surface, the hydrophilic region leads to the rapid escape of water molecules while relatively long hydrophilic–hydrophobic contact lines promise considerable water evaporation. Nevertheless, the condensation reaction of OTS and the dried hydrogel inevitably modified the whole hydrogel rather than just the surface, resulting in a complex structure. On the other hand, Janus hydrogels made by stepwise hydrogel formation make it difficult to create thin hydrophobic top layers, resulting in a low evaporation rate.^36,37 Recent studies found that the high ionic strength of polyelectrolyte sufficiently repelled and entrapped the ions that were entering the polyelectrolyte hydrogel network, favouring better salt ion rejection.^38,39 Yet, the water evaporation rate of these hydrogels was relatively low (less than 1.7 kg m⁻² h⁻¹ under one sun irradiation),^40–43 and reached only 2.4 kg m⁻² h⁻¹ even when photothermal materials were incorporated.^44–46

In this work, we propose a precise surface modification strategy to produce Janus hydrogels, in which the wettability of the hydrogel's surface can be regulated by tuning the surface coverage of ultrathin Janus patches (Scheme 1). Driven by the electric field, charged polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) core–shell micelles were uniformly grafted onto the hydrogel with shell-forming block chains entangled within the hydrogel network. After solvent washing, the non-entangled polymer chains in the micelles were removed, leaving the hydrophobic PS on the top and the P4VP entangled within the hydrogel network. Except for the hydrophobic PS/P4VP Janus patches, amphiphilic Janus patches with a hydrophobic PS top layer and a hydrophilic quaternized P4VP (QP4VP) entangled layer were also developed. Despite the absence of photothermal material incorporation, the amphiphilic PS/QP4VP Janus patch-grafted hydrogel displayed a fast evaporation rate and excellent salt ion rejection ratio, due to its thinner hydrophobic top layer and the high ionic strength of the QP4VP entangled layer.

Scheme 1 Schematic illustration of hydrophobic Janus patch and amphiphilic Janus patch-grafted hydrogels for solar water evaporation and salt ion rejection.

2. Experimental section

2.1 Materials

PS-b-P4VP diblock copolymers (PS₄₈₀-b-P4VP₁₂₄, PDI = 1.05, SV-1; PS₁₁₆-b-P4VP₉₀, PDI = 1.09, SV-2; PS₂₈₈₀-b-P4VP₁₆₆₅, PDI = 1.10, SV-3) were purchased from Polymer Source Inc. Polyvinyl alcohol (PVA, M_n = 88 000 g mol⁻¹, alcoholysis degree = 99.8 mol%), methyl iodide (MeI, 98%), and simulated seawater were purchased from Adamas. All chemicals were used as received without further purification.

2.2 Synthesis of PVA hydrogel

The PVA hydrogel was prepared using the solvent-displacement method.^47,48 1.8 g of PVA was dissolved in 10 mL of DMSO and stirred at 90 °C for 30 min. The transparent solution was then poured into the mold and immersed in 1 L of water at room temperature to form the PVA hydrogel. The water was replaced by fresh water every 6 h till the DMSO was completely removed.

2.3 Preparation of SV and SVQ micelles

5 mg of PS-b-P4VP was dissolved in 5 mL of N,N-dimethylformamide (DMF) to obtain the diblock copolymer solution. Subsequently, 10 mL of 0.1 M acetic acid (HAc) was added dropwise to the polymer solution at the rate of 3 mL h⁻¹ to obtain the SV micelle suspension. After dialysis against 0.1 M HAc three times, SV micelles in 0.1 M HAc were obtained. Besides, 10 mL of methanol was added dropwise to the PS-b-P4VP solution at the rate of 4 mL h⁻¹, followed by the addition of 50 μL of MeI and reacted at 60 °C for 48 h. After the suspension was dialyzed against water three times, the SVQ micelles in water were obtained.

2.4 Formation of Janus patches on PVA hydrogel

The charged micelles were grafted onto the surface of the PVA hydrogel according to our previous work.⁴⁹ A direct current electric field of 20 V cm⁻¹ was applied using a source meter (Keithley 2400). After the electric field was removed, the micelle-grafted hydrogel was subsequently washed with water and chloroform. After evaporating the chloroform and rinsing with water, the Janus patches on the hydrogel were obtained.

2.5 Solar water evaporation experiment

The Janus patch-grafted hydrogel was installed in the self-made solar water evaporation device. The Newport Oriel Sol3A with an AM 1.5 G filter was used as the solar simulator. The solar flux was fixed at 1 kW m⁻² calibrated by Newport's 1916-R power meter before each experiment. In all experiments, sunlight was applied vertically to the upper surface of Janus patch-grafted hydrogel. The experiment was carried out at an ambient temperature of 20 °C and a humidity of 45%. The temperature of the evaporation surface was monitored using a Hikvision K20 thermal imaging infrared meter. The mass loss of the entire evaporation device was measured using a balance with 0.1 mg resolution (Mettler Toledo AL204), and the evaporation rate was the average of three tests.

2.6 Characterization

High-contrast transmission electron microscope (HT-TEM) images were obtained using a Hitachi HT7800. Scanning electron microscopy (SEM) images were obtained using a Zeiss Ultra55. Confocal laser scanning microscope (CLSM) images were recorded on a LEICA TCS SP8 confocal laser scanning microscope using a 40× immersion objective lens. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific K-Alpha spectrometer equipped with a monochromatic Al Kα X-ray source. The hydrodynamic size and zeta potential of micelles were determined using ZS90-2026 from Malvern. The water contact angle was measured using a Dataphysics OCA 20 contact angle analyzer. Differential scanning calorimetry (DSC) was performed using a TA Instruments DSC 250. For the IW/FW ratio and evaporation enthalpy measurement, the samples were equilibrated at −30 °C and 20 °C for 5 min before the test, respectively. And the heating rate was fixed at 2 °C min⁻¹ and 5 °C min⁻¹, respectively. The ion concentrations of the distilled water were measured using an Agilent 7500ce inductively coupled plasma-mass spectrometer (ICP-MS). The friction test was conducted at 25 °C using a ball-on-disk tribometer (Anton Paar TRB3) with a 6 mm diameter stainless steel ball (100Cr6). A 1 N load was applied at the 40 Hz frequency in a reciprocating mode over an 8 mm sliding distance. And the friction coefficient was calculated by the tribometer software based on the friction force and the load.

3. Results and discussion

3.1 Preparation of micelles

PS₄₈₀-b-P4VP₁₂₄ (SV-1) was first molecularly dissolved in DMF, which was a good solvent for both blocks (Fig. 1a). 0.1 M HAc, a good solvent for P4VP block chains but the non-solvent solvent for PS block chains, was added dropwise to the SV in DMF to form the SV-1 micelles with a PS core and a quaternized P4VP shell (30% quaternization in 0.1 M HAc, Note S1†). After dialysis against 0.1 M HAc, SV-1 micelles in 0.1 M HAc were obtained (Fig. 1a). In the meantime, the micellization was performed by dropwise adding methanol (MeOH) to the SV in DMF. Since MeOH is a good solvent for P4VP block chains but a non-solvent for PS block chains, the SV self-assembled into micelles with a PS core and a P4VP shell. The shell-forming P4VP block chains were then completely quaternized by reaction with methyl iodide (MeI) to form the positively charged P(Me4VP⁺)I⁻ shell-forming block. Upon complete quaternization of P4VP block chains with MeI (100% quaternization of P4VP), the nitrogen peak with a binding energy of 398 eV (attributed to the nitrogen in the pyridine group) entirely shifted to a higher binding energy of 401.5 eV (attributed to the quaternized pyridine group) (Fig. 1b).⁵⁰ After dialysis against water, SVQ-1 micelles in water were obtained (Fig. 1a). TEM observation revealed that both SV-1 and SVQ-1 micelles were monodispersed and spherical in shape. Since the shell of the micelles should be of a low contrast and thus invisible in the TEM image, the core diameters of SV-1 and SVQ-1 micelles were 42 and 43 nm, respectively (Fig. 1a). DLS traces showed that SV-1 micelles in 0.1 M HAc have an average hydrodynamic diameter of 61 nm with a polydispersity index (PDI) of 1.07. And the hydrodynamic diameter of SVQ-1 micelles in water was slightly larger with an average hydrodynamic diameter of 63 nm and PDI of 1.08 (Fig. 1c). Likewise, both micelles have high zeta potentials, in which the zeta potential of SVQ-1 micelles (39 mV) was a little bit higher than that of SV-1 micelles (38 mV) (Fig. 1d).

Fig. 1 Preparation of SV-1 and SVQ-1 micelles. (a) Schematic illustration of the preparation of SV-1 micelles in 0.1 M HAc and SVQ-1 micelles in water, and the corresponding TEM images. The inset images are the Tyndall effect of the micelle suspension. (b) N 1s XPS spectra of SV and SVQ-1 micelles. (c) DLS traces and (d) zeta potentials of SV-1 micelles in 0.1 M HAc and SVQ-1 micelles in water.

3.2 Formation of Janus patch-grafted PVA hydrogel

The charged SV-1 micelles in 0.1 M HAc and SVQ-1 micelles in water were grafted onto the surface of the PVA hydrogel according to our previous work.⁴⁹ Under the electric field, the micelles move towards the hydrogel and graft onto the hydrogel surface, with shell-forming block chains entangled within the hydrogel network (Fig. 2a). To realize micelles' grafting on the surface instead of penetrating into the PVA hydrogel, a small mesh size of PVA hydrogel (8 nm, Note S2†) was selected. Since the micelles' core diameter was much larger than the hydrogel's mesh size, the large PS core stayed above the hydrogel with QP4VP shell-forming block chains penetrated and entangled within the hydrogel network. After being sufficiently washed with water, the SV-1-n (n denotes the electric field application time in minutes) hydrogels were obtained when the SV-1 micelles were used, and the SVQ-1-n hydrogels were obtained when SVQ-1 micelles were used. As shown in Fig. 2a, the Nile red-dyed SVQ-1-30 micelle (red colour) was uniformly grafted onto the centre-circled area of the PVA hydrogel. To confirm that there are no SVQ-1 micelles penetrating into the PVA hydrogel network, a confocal microscope (Fig. 2b) was used to observe the spatial distribution of Nile red-dyed SVQ-1 micelles in PVA hydrogel; the hydrophobic Nile red was only located in the PS core of the micelle. In the 190 × 190 × 110 (x/y/z) μm³ region, the Nile red signal was only observed at the surface of the hydrogel with a depth of about 4 μm (the smallest depth observed, Note S3†), confirming that SVQ-1 micelles stayed only on the surface of the hydrogel. Moreover, the entanglement between quaternized P4VP and the hydrogel provides the SVQ-1 micelle-grafted hydrogel (SVQ-1-60) excellent stability under various durable operation conditions, including friction for 10 min, under onesun irradiation for 5 days, and immersion in simulated seawater for 5 days, indicated by no obvious changes in the average nearest-micelle distance (Fig. S1 and Note S4†).

Fig. 2 Formation of hydrophobic and amphiphilic Janus patch-grafted PVA hydrogels. (a) Schematic illustration of electric-field grafting of charged micelles onto the hydrogel and formation of Janus patches via solvent wash, obtaining hydrophobic and amphiphilic Janus patch-grafted hydrogels (SVJ-1 and SVQJ-1), respectively. Inset images are the Nile red-dyed SVQ-1 micelle-grafted PVA hydrogel (SVQ-1-30) and the corresponding SVQJ-1-30 after solvent wash. (b) Confocal microscope image and corresponding intensity profile along the z-direction of the SVQ-1-30 hydrogel. SEM images of (c–f) SV-1 and (g–j) SVQ-1 micelle-grafted PVA hydrogels, and the corresponding normalized distribution of the nearest-micelle distance. Inset images are the Janus patch-grafted hydrogels. (k) The micelle grafting density and Janus patch surface coverage of SV-1 and SVQ-1 hydrogels prepared by varying the electric-field application time. (l) Water contact angle of the PVA hydrogel, P(Me4VP⁺)I⁻ film, P4VP film, PS film, and (m) SV-1, SVJ-1, SVQ-1, and SVQJ-1 hydrogels prepared at different grafting time. (n) Evolution of friction coefficient signals over time, and (o) friction coefficients of the PVA hydrogel and SVQJ-1-60. (p) The SEM images of SVQJ-1-60 before and after friction for 10 min.

SEM images (Fig. 2c–e and g–i) showed that all the micelle-grafted hydrogels have the micelles uniformly and discretely distributed. When the electric field application time was fixed at 30 min, the SV-1-30 hydrogel has the average nearest-micelle distance of 88 nm, while the SVQ-1-30 hydrogel has a larger average nearest-micelle distance of 93 nm. The larger average nearest-micelle distance was due to stronger repulsion between SVQ-1 micelles than SV-1 micelles caused by a greater degree of quaternization in the shell-forming block chains. Upon extending the electric field application time from 30 min to 90 min, more and more micelles were grafted. The average nearest-micelle distances of SV-1 hydrogels were decreased from 88 nm to 60 nm (Fig. 2f). And the average nearest-micelle distances of SVQ-1 hydrogels were decreased from 93 nm to 63 nm (Fig. 2j). Accordingly, the micelle grafting densities increased from 1.38 × 10¹⁰ cm⁻² to 6.28 × 10¹⁰ cm⁻² for SV-1 hydrogels and increased from 1.30 × 10¹⁰ cm⁻² to 6.20 × 10¹⁰ cm⁻² for SVQ-1 hydrogels (Fig. 2k). Combined with the size and the grafting density of micelles, the surface coverage of micelles in SV-1 hydrogels increased from 20% to 92% while that in SVQ-1 hydrogels increased from 19% to 90% (Fig. 2k). Since P4VP shell-forming block chains of SV-1 micelles became hydrophobic in water, the water contact angles of all the SV-1 hydrogels were higher than that of the PVA hydrogel (29°) (Fig. 2l and m). In comparison, the hydrophilic QP4VP⁻ shell-forming block in the micelle led all the SVQ-1 hydrogels to have a smaller water contact angle. By increasing the SVQ-1 micelle grafting density, the hydrogel surface became more hydrophilic, from 11° (SVQ-1-30) to 7° (SVQ-1-90).

The micelle-grafted hydrogel was then washed with chloroform (CHCl₃). Since chloroform is a good solvent for the PS core and is immiscible with water, it only dissolved the PS core of micelles on the surface of the PVA hydrogel. In this way, the polymers in the micelles that were entangled within the hydrogel network remained, while the polymers that were not entangled within the hydrogel network were washed away. After evaporating the chloroform and rinsing with excess water, the PS block chains on the surface of the hydrogel were collapsed to the surface, forming Janus patches on the hydrogel. Solvent washing of the SV-1-n hydrogel resulted in hydrophobic Janus patches on the hydrogel (SVJ-1-n), in which PS block chains collapsed on the surface of the hydrogel while entangled P4VP block chains became non-quaternized and hydrophobic so as to be captured in the hydrogel network. In contrast, solvent washing of the SVQ-1-n hydrogel resulted in amphiphilic Janus patches on the hydrogel (SVQJ-1-n), in which PS block chains collapses on the hydrogel while QP4VP block chains were entangled within the hydrogel network.

After solvent washing, the centre-circled red colour disappears, indicating the complete dissolution of the PS cores; the Nile red that is located in the PS core could be washed off only when the PS core was dissolved. SEM images (inset images of Fig. 2c–e and g–i) showed that all the micelles on the hydrogel were removed, forming a smooth surface with ultrathin Janus patches grafted onto the hydrogel, which were difficult to distinguish. Since the entangled chains were securely entangled within the hydrogel network with no discernible displacement, the location and the size of Janus patches should be close to those of micelles. That is, the Janus patches' grafting density and surface coverage should be comparable to those of the micelle-grafted hydrogel. With the hydrophobic PS top layer exposed, the water contact angle of SVJ-1 and SVQJ-1 hydrogels was greatly increased, reaching that of the PS film as the Janus patch surface coverage approached ∼90% (Fig. 2l and m). The Janus patch surface also exhibited good abrasion resistance, maintaining a consistent friction coefficient over time (Fig. 2n). The exposed hydrophobic PS top layer significantly increased the friction coefficient to 0.16 (Fig. 2o), which was comparable to that of a PS film (0.19 (ref. 51)). In addition, the SEM image of SVQJ-1-60 after 10 minutes of friction showed no obvious difference from its original form (Fig. 2p), demonstrating the SVQJ-1-60's excellent abrasion resistance.

3.3 Solar water evaporation and ion rejection ratio under one-sun illumination

The solar water evaporation and ion rejection ratio of SVJ-1 and SVQJ-1 were evaluated under 1 kW m⁻² solar irradiation using the setup shown in Fig. 3a. Upon increasing the Janus patch surface coverage by prolonging the electric-field application time, the evaporation rate and efficiency of both SVJ-1 and SVQJ-1 hydrogels increased to their highest values at 60 min of grafting (SVJ-1-60 and SVQJ-1-60, respectively), and then dropped (Fig. 3b and S2, and Note S5†). All the hydrogel evaporators had low equilibrium surface temperatures ranging from 32 °C to 35 °C, which were close to the ambient temperature (20 °C) (Fig. 3c). In the hydrophobic Janus patch-grafted hydrogels, the SVJ-1-60 hydrogel had the highest evaporation rate of 2.7 kg m⁻² h⁻¹ and efficiency of 64%, which was greater than those of PVA (2.1 kg m⁻² h⁻¹, 58%) and pure water (0.37 kg m⁻² h⁻¹, 21%).⁵² Accordingly, the SVJ-1-60 hydrogel had the lowest evaporation enthalpy, suggesting that the states of the internal water were changed so as to require less energy to evaporate water from the hydrogel surface (Fig. 3d, S2 and S3†). Previous studies reported that the hydrogen bonding interaction between the hydroxyl groups of PVA and water molecules increased the content of intermediate water (IW) and decreased the content of free water (FW).^53,54 The introduction of hydrophobic Janus patches on the PVA hydrogel remarkably enhanced the IW/FW ratio (Fig. 3e, S4 and S5†). The IW/FW ratio further gradually rose with increasing the Janus patch surface coverage. Due to the hydrophobicity of both collapsed PS and the stretched P4VP block chains entangled in the hydrogel, the hydrophobic Janus patches effectively broke the hydrogen bonds in water, leading to an increase in the IW/FW ratio. As a result, the energy required for evaporation, evaporation enthalpy, was lowered. At ∼60% Janus patch surface coverage, the evaporation rate reached the highest value (Fig. 3f). When the Janus patch surface coverage further increased to ∼90%, even as the IW/FW ratio and the total length of the hydrophilic–hydrophobic contact line increased, the high coverage of the hydrophobic patches dramatically reduced the pathway for water evaporation, thus reducing the evaporation rate. Moreover, SVJ-1 hydrogels exhibited good ion rejection performance. With increasing Janus patch surface coverage, the ion rejection ratio gradually rose from 97.20% to 98.80% (Fig. 3f and S6†).

Fig. 3 Solar water evaporation and salt ion rejection performance under one sun irradiation. (a) Schematic illustration of the solar water evaporation setup. (b) The evaporation rate and efficiency of SVJ-1 and SVQJ-1 hydrogels prepared at different grafting time. The evaporation rate was the average of three tests. (c) The equilibrium temperature of SVJ-1 and SVQJ-1 hydrogels prepared at different grafting time. Inset images are the infrared thermal images of SVJ-1-60 and SVQJ-1-60 hydrogels. (d) The evaporation enthalpy and (e) IW/FW ratio of SVJ-1 and SVQJ-1 hydrogels prepared at different grafting time. (f) The evaporation rate and ion rejection ratio upon increasing the Janus patch surface coverage.

Compared with SVJ-1 hydrogels, SVQJ-1 hydrogels have amphiphilic Janus patches on the hydrogel with a hydrophobic PS top layer and a hydrophilic QP4VP entangled layer. The thinner hydrophobic top layer caused a dramatic rise in the IW/FW ratio (Fig. 3e). With increasing Janus patch surface coverage, the SVQJ-1 hydrogel achieved the highest evaporation rate and efficiency (3.2 kg m⁻² h⁻¹, 76%) at ∼60% surface coverage (Fig. 3f). Similarly, a significant drop in the evaporation rate was observed when the Janus patch surface coverage was increased to ∼90%. As shown in Fig. 3f, the ion rejection ratio of SVQJ-1 hydrogels was superior and improved steadily to 99.94% as surface coverage increased, because the high ionic strength of the QP4VP-rich entangled layer sufficiently repelled and entrapped the ions that penetrated the Janus patches, favouring better salt ion rejection.

The influence of Janus patch size on the evaporation rate and ion rejection ratio was also examined (Fig. 4a). PS₁₁₆-b-P4VP₉₀ (SV-2) and PS₂₈₈₀-b-P4VP₁₆₆₅ (SV-3) were selected to prepare SVQ-2 and SVQ-3 micelles, which were smaller and larger than the size of SVQ-1 micelles, respectively. After complete quaternization of P4VP block chains with MeI (Fig. S7†), the spherical SVQ-2 and SVQ-3 micelles were monodispersed with core diameters of 29 and 62 nm, respectively (Fig. S8†). DLS traces showed that SVQ-2 micelles in water have an average hydrodynamic diameter of 41 nm (PDI = 1.10), which was smaller than that of SVQ-1 micelles. And the hydrodynamic diameter of SVQ-3 micelles in water was 92 nm (PDI = 1.13), which was larger than that of SVQ-1 micelles. All the micelles displayed high zeta potentials of ∼40 mV (Fig. S5†), so that they were effectively grafted onto the PVA hydrogel under electric field and generated the amphiphilic PS/QP4VP Janus patches after solvent wash (Fig. 4b–e and S9†). Janus patch surface coverage increased as the electric field application time was extended (Fig. 4f). Similar to the SVQJ-1 hydrogel, SVQJ-2 and SVQJ-3 hydrogels obtained their highest evaporation rate when the Janus patch surface coverage reached ∼60% (SVQJ-2-60 and SVQJ-3-60), while the ion rejection ratio remained superior and continuously improved with increasing the surface coverage (Fig. 4g). At the Janus patch surface coverage of ∼60%, the Janus patch size caused slight variations of the evaporation rate (from 3 to 3.2 kg m⁻² h⁻¹) and ion rejection ratio (from 99.80% to 99.86%) (Fig. 4h). Therefore, the evaporation rate and ion rejection ratio were mostly determined by the types of Janus patch and Janus patch surface coverage, with a slight influence from the Janus patch size. Despite the fact that the amphiphilic Janus patch-grafted hydrogel was transparent and did not incorporate any photothermal materials, the SVQJ-1-60 hydrogel has a highest evaporation rate of 3.2 kg m⁻² h⁻¹, which indicates that it outperformed the majority of photothermal material encapsulated hydrogels. In the meantime, the SVQJ-1-90 hydrogel has a superior ion rejection ratio of 99.94% while maintaining an evaporation rate comparable to that of photothermal material encapsulated hydrogels (Fig. S10 and Table S1†). To the best of our knowledge, our work is the only example of Janus hydrogels with a fast water evaporation rate and excellent salt ion rejection ratio that did not incorporate any photothermal materials.

Fig. 4 Influence of Janus patch size. (a) Schematic representation of the influence of Janus patch size on solar water evaporation and salt ion rejection. SEM images of (b) SVQ-1-60, (c) SVQ-2-60, and (d) SVQ-3-60, and (e) the corresponding normalized distribution of the nearest-micelle distance. The inset images are the corresponding Janus patch-grafted hydrogels. (f) The Janus patch surface coverage of SVQJ-1, SVQJ-2, and SVQJ-3 prepared at different grafting time. (g) The evaporation rate and ion rejection ratio upon increasing the Janus patch surface coverage. (h) The evaporation rate and ion rejection ratio upon increasing the Janus patch size when the grafting time was fixed at 60 min.

The continuous solar water evaporation rate and ion rejection ratio were further studied by subjecting the SVQJ-1-60 hydrogel to simulated seawater for a 48-hour test under 1 kW m⁻² solar irradiation. As shown in Fig. 5a, the evaporation rate consistently maintained a high value of 3.2 kg m⁻² h⁻¹, with slight fluctuation over time. Both the condensate collected after 1 hour and 48 hours of evaporation had the ion concentrations falling well below the World Health Organization's (WHO) drinking water standards (Fig. 5b). Additionally, considering the day–night cycle of solar energy, we conducted a 5-day cyclability test. SVQJ-1-60 was exposed to solar irradiation of 1 kW m⁻² for 9 hours each day, with the remaining time spent stationary in the evaporator without any light exposure or other variables. The results indicated that SVQJ-1-60 exhibited excellent evaporation cyclability, with the evaporation rate stabilized at 3.2 kg m⁻² h⁻¹ (Fig. 5c).

Fig. 5 Durability of the SVQJ-1-60 hydrogel. (a) Mass change curves, evaporation rate and (b) salinity of desalination during the 48-hour evaporation test in simulated seawater. The inset images show the mass change curves after 1 hour and 48 hours of evaporation. (c) Mass change curves and evaporation rate during the 5-day cycling experiment, with 9 hours of exposure to 1 kW m⁻² solar irradiation every day.

4. Conclusions

In summary, we demonstrated a new wettability tailorable surface modification strategy to fabricate Janus hydrogels with an ultrathin hydrophobic PS patch uniformly and discretely distributed on the top of a PVA hydrogel and hydrophilic QP4VPs entangled within the hydrogel network. Through the rational design of the Janus patch size and surface coverage, the amphiphilic Janus patch-grafted hydrogels yielded an optimized solar water evaporation performance with an evaporation rate of 3.2 kg m⁻² h⁻¹ and efficiency of 76% at the Janus patch surface coverage of ∼60%. And the Janus patch size (from ∼30–60 nm) has little influence on the solar water evaporation performance. Due to the high ionic strength of the QP4VP-rich entangled layer, the prepared Janus hydrogel has a superior salt ion rejection ratio (i.e. 99.94% for the SVQJ-1-90 hydrogel). With fast water evaporation rates and excellent salt ion rejection ratios, the amphiphilic Janus patch-grafted hydrogels outperformed all the hydrogels that did not incorporate photothermal materials, and were comparable to photothermal material encapsulating hydrogels, highlighting their potential for solar seawater desalination.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial supports from the National Natural Science Foundation of China (No. 22071032, 51721002, and 52293473), and MOST (2022YFA1203001).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta02327g

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