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DOI: 10.1039/D4NR01930J (Paper) Nanoscale, 2024, Advance Article

De novo Cu-MOF@CNS nanocomposite coated on a cotton fibrils framework for sustainable solar-driven desalination†

Geetika Jainac, Sinu Sanghamitrab, Monalisa Mukherjeec, Mrinal Kanti Mandal*b, Rajib Ghosh Chaudhuri*b and Sandip Chakrabarti*c
aAmity Institute of Nanotechnology, Amity University Noida, UP 201313, India
bDepartment of Chemical Engineering, National Institute of Technology Durgapur, WB 713209, India. E-mail: mkmandal.che@nitdgp.ac.in; gcrajib@gmail.com
cAmity Institute of Click Chemistry Research & Studies, Amity University Noida, UP 201313, India. E-mail: schakrabarti@amtiy.edu

Received 4th May 2024 , Accepted 31st July 2024

First published on 22nd August 2024

Abstract

Environmental researchers are extremely concerned about addressing the declining availability of drinking water, which is a critical issue in many nations. Solar-driven desalination is an emerging and pioneering renewable approach to reduce potable water scarcity that is suitable for remote locations, developing countries, and disaster zones as it does not require additional energy supply. However, there are still issues with the scalable preparation of photothermal materials, such as achieving low cost and widening the assortment of useful applications. Conventional carbon- and metal-based absorbers are intricate and fragile, which make them difficult to install and transport in places with minimal infrastructure. Thus, a universal process for creating adaptable solar evaporators is sorely required. Herein, we have come up with a holistic approach using a solar absorber (GJ-01(Cal)) derived from a Cu-MOF (HKUST-1) and carbon nanosheets (CNSs) for generating potable water from saline water using solar radiation. The as-synthesized material provides high-performance photothermal water evaporation when illuminated under solar irradiation at the air–water interface. Moreover, its porous structure, high photothermal conversion efficiency, rapid water flow, and heat insulation make it appropriate for saline water desalination. CNS play a pivotal role in improving the photothermal features of the solar absorber (GJ-01(Cal)) in terms of conjugation to promote Cu(0) species and pyrrolic nitrogen (P-N) amplification and thereby enrich the p-type nature of the absorber's triphasic configuration. With these photothermal factors, the localised surface plasmon resonance (LSPR) of electrons increases and achieves high solar spectrum absorption. The GJ-01(Cal) was further coated on porous cotton fibrils (CF) that regulate photothermal interfacial evaporation (PTIE) by allowing water transportation via capillary action. This assemblage of the nanocomposite on CF efficiently evaporates water at a higher surface temperature of ∼47 °C under one solar illumination, achieving 4.23 kg m−2 h−1 of evaporation flux and 96.5% light-to-heat conversion efficiency. Interestingly, the GJ-01(Cal) coated over CF can be recycled at least 10 times. Additionally, it offers scalable production for higher photothermal efficiency with a flexible substrate as a solar evaporator and is beneficial for society paving new horizons towards a sustainable environment.

1. Introduction

Energy and water are vital for human life and economic development, and sustainable access to clean water is essential for global societal sustainability. In contrast, addressing the expanding demand for both resources poses substantial problems, especially considering the connection between the two, energy and water.1 Even though 71% of the Earth's surface is covered with water, 97.4% of it is in the form of brine that must be cleansed before usage with only 2.5% of it being freshwater, which includes groundwater, glaciers and surface water.2 Resources throughout the world will be under further stress in the ensuing decades if sustainable solutions are not found. The exploitation of groundwater by nations like the USA, India, Pakistan, Iran, and China during the last few decades has been mostly attributed to anthropogenic activity.3 This suggests that by 2025, global groundwater use will increase to 4600 km3 per year, putting the already fragile source of life in peril.4 However, massive efforts have been made to generate clean water from saline or contaminated water to address the freshwater scarcity.5 Scientists from all over the world are working tremendously hard to develop attainable solutions with respect to this concern to ensure the survival of humanity.

Researchers are exploring methods to desalinate seawater into potable freshwater, including vaporizing seawater using fossil fuels. However, this method produces harmful substances and increases costs. Solar energy may be considered an ideal candidate for desalination via photothermal conversion.6 Photothermal desalination is an efficient approach that combines the abundant resources of seawater and solar energy, generating heat, driving the evaporation of water, and leaving behind salt and impurities.7,8 This technique offers advantages such as advancing a green economy and high energy sustainability compared to conventional desalination technologies like ion exchange,9 ultraviolet filtration,10 reverse osmosis,11 and nanofiltration.12 High-efficiency photothermal materials for desalination are being developed to overcome the energy-intensive bulk heating used in conventional desalination technologies.13,14

Previous studies have shown that four characteristics are necessary for functional photothermal activity: broadband solar absorbability, thermal management, water conveyance, and water evaporation. A photothermal material with high photothermal conversion efficiency serves as the main component of photothermal desalination.15 Multiple photothermal desalination-based materials have been developed in recent years using a variety of materials, including metals and their oxides, carbon-based materials, and polymers.16 Smart nanomaterials are being developed to reduce heat loss from bulk water evaporation by targeted water heating.17 The advancement of this field has led to the engineering of metal–organic frameworks (MOFs) as novel solar nanoabsorbers. MOFs offer customized or reticular structures, tuneable functions, high porosity, and a large specific surface area.18,19 Furthermore, MOFs can be calcined at high temperature in the presence of an inert atmosphere, which makes them ideal templates for the formation of porous carbon, metal oxides, metals, or metal oxides/carbon composites due to their hierarchical porous structure.20 Nonetheless, materials like Cu-MOFs (HKUST-1) are ideal for hierarchical architectures with low thermal conductivity, maximizing solar energy absorption and water transportation. They effectively absorb optical radiation, making them suitable for photothermal water evaporation and plasmon-enhanced solar absorption due to their prevalence.21,22

In contrast, carbon-based materials, which stand out among photothermal materials for their characteristics of broad absorption range, high chemical stability, and high photothermal conversion efficiency, have attracted much interest in the field of solar energy application.23 Recently, pyrolytic mushrooms were applied as a light-absorbing layer by Xu et al.24 Their system could reach an evaporation efficiency of 78% due to the vertical holes in mushroom-derived carbon that can act as routes for the movement of water. Zhu et al. created a stable and effective evaporation layer by electrospinning carbon nanotubes inside hydrophilic polyacrylonitrile (PAN).25 The evaporation device demonstrated an evaporation efficiency of 90.8% because of the hydrophilic impact of PAN and the porosity modification. Irrespective of such advancements, there are certain pragmatic issues related to sustainability that need to be addressed. Carbon materials derived from biomass,26 graphene-based materials,27 carbon nanotubes,28 carbon aerogels,29 and carbon fibres30 have been extensively studied. Contrary to these other carbon-based materials, carbon nanosheets (CNS) are 2D carbon nanostructures with high surface area-to-volume ratios, enabling broadband absorption and excellent chemical stability. They can be synthesized in bulk using a facile, catalyst-free process, making them viable competitors for industrial applications.18

Additionally, MOF-derived porous carbon (MPC) materials are potential candidates for solar steam generators (SSG), as they have light absorption capability, and are non-toxic, environmentally friendly, and easy to manufacture.23 For instance, Ma et al. carbonized leaf-like MOF precursors after depositing them on a stainless-steel mesh using a basic solvent reaction to enhance the evaporation flux (1.22 kg m−2 h−1) with a higher evaporation efficiency of 84.3%.31 There are several reports depicting the improved stability of Cu nanoparticles on carbon substrates like graphene oxide for high photothermal efficiency.32 To date, a stable Cu-based black material has not yet been created that can efficiently absorb solar energy, enabling efficient photothermal conversion and combining freshwater production with high solar energy utilization. Herein, we have come up with a Cu-based black solar absorber (Cu-MOF@CNS) that enables all the properties needed for efficient photothermal conversion.

Usually, a substrate material is frequently needed for SSG systems onto which a variety of nanomaterials can be deposited to improve optical absorption. Wood,33 bamboo,34 polyurethane sponges,35 cotton, hydrogels,36 luffa sponges,37 aerogels,38 cloth,39 and others40 have been utilized as substrate materials to assemble the photothermal evaporator. Among them, cotton fabric is a cost-effective, versatile, and water-absorbent substrate material, offering flexibility, durability, and simplified design without the need for additional water uptake mechanisms.41 Systems based on cotton fabrics frequently employ a single interfacial evaporation layer, which may not maximize effectiveness. Efficiency may be increased and heat loss can be decreased using an indirect evaporation system including a double interface, such as that based on a hanging mode, jellyfish mode, arc mode, hollow cone mode, etc.42 Hence, we proposed a system that uses cotton fibrils (CF), which are readily accessible and placed in the hanging mode, suspended from the photothermal evaporator surface to facilitate indirect system evaporation.

In this research work, for the first time, we have focused on the development of an economically feasible photothermal material with high light-to-heat conversion efficiency that is coated over CF. Consequently, a phase-change-induced Cu-MOF@CNS (GJ-01(Cal)) material, made from HKUST-1, was investigated as a novel solar absorber when combined with CNSs. Furthermore, it was heat treated at high temperature under an inert atmosphere to form a MOF-based nanocomposite with a triphasic Cu/CuO/Cu2O configuration. The GJ-01(Cal) nanocomposite coated over porous CF induces the capillary rising of water through rejecting salts with efficient air–water interfacial heating. This provides consistent heat and controls photothermal interfacial evaporation (PTIE) at the water/air interface, much like that seen in mangrove trees. The CF-based photothermal desalination system allowed uniform distribution of localized heating by intercepting carried water for highly efficient photothermal desalination in the generation of potable water. Hence, GJ-01(Cal) is a cost-efficient material that exhibits 96.5% evaporation efficiency for sustainable photothermal desalination of saline water. This well-designed technology offers a promising solution for mitigating global freshwater shortages.

2. Experimental section

2.1 Materials

All chemicals were of analytical standard and used without any further purification. Cu(NO3)2·3H2O (99%) and benzene 1,3,5-tricarboxylic acid (95%) were obtained from Sigma Aldrich. Melamine (98%) was obtained from CDH Pvt. Ltd. Dimethylformamide (DMF), glycerol, and sulfuric acid were procured from Thermo Fisher Scientific India Pvt. Ltd.

2.2 Synthesis

2.2.1 Synthesis of MOFs. Cu(NO3)2·3H2O (0.5 mmol) and benzene 1,3,5-tricarboxylic acid (0.3 mmol) were dissolved in 32 mL of DMF. The reaction mixture was stirred for 30 min and transferred into an autoclave and kept for 20 h at 85 °C in a hot-air oven. A blue-coloured HKUST-1 solution was obtained. It was thoroughly washed with ethanol and deionized water, and subsequently dried at room temperature. Furthermore, it was heated in a tube furnace at 800 °C for 5 h under a nitrogen atmosphere and the product, HKUST-1(Cal), was cooled and collected.
2.2.2 Synthesis of CNSs. CNSs were synthesized based on the process outlined by Wang et al.,43 0.5 g of melamine was added to 10 mL of glycerol and forcefully agitated until the melamine was completely dissolved. The mixture was vigorously shaken while 10 mL of 98% sulfuric acid was added. The mixture was heated to 180 °C for 4 h in a 50 mL autoclave. The resulting black solid product was then washed several times with distilled water and ethanol to remove the impurities.
2.2.3 Synthesis of GJ-01(Cal). The two previously synthesized materials, HKUST-1 and CNSs, in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 w/w ratio, were ground well using a mortar and pestle. The resulting mixture was transferred to an alumina boat crucible and heated in a tube furnace at a high temperature of 800 °C for 5 h under a nitrogen atmosphere. The product, GJ-01(Cal), was cooled and collected, as shown in Fig. 1. The sample was subsequently used to characterize and test the properties of the resulting product.
image file: d4nr01930j-f1.tif
Fig. 1 Schematic diagram of GJ-01(Cal) nanocomposite synthesis.

2.3 Characterization of photothermal nanocomposite materials

XRD spectra were recorded using a Rigaku Smart Lab (9) diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range of 5°–90°. Absorption studies were performed using a UV-vis-NIR spectrometer (Shimadzu UV-3600 plus) operating at room temperature in the 200 nm–2500 nm range. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX) maps were obtained using an FEI NOVA NANO SEM-450 instrument. The study utilized high-resolution transmission electron microscopy (HRTEM), carried out using a Jeol JEM-2100F device, to thoroughly investigate the morphology and lattice spacing of the samples. An X-ray photoelectron spectroscopy study was performed using a PHI 5000 Versa Probe III instrument. Additionally, the micro-Raman spectra were obtained at ambient temperature using a Renishaw Raman spectrometric analyzer using an Ar+ laser with a laser intensity of ∼2.5 mW (λex = 514.5 nm) and a resolution of 0.5 cm−1. N2 adsorption was used to measure the pore size and surface area of the nanocomposite on a Quantachrome AUTOSORB-IQ-MP pore size, surface area and high-pressure surface analyzer at 77 K. The sample was degassed for two hours at 300 °C before conducting the BET measurements.

2.4 Photothermal desalination

Photothermal experiments were carried out using a calibrated 500 W xenon lamp. The light intensity was measured by using a lux meter. The length of the cotton fibrils (CFs) was around 4.5 cm and these were 30% coated with the above photothermal material dissolved in an ethanol–water solution using a dip-coating technique and then dried in an oven at 50 °C for 3 hours. The CFs were positioned in such a way that their coated part was above the level of the saline water while their uncoated surface was dipped into the saline water, whereupon the water rose by capillary action into the submerged CFs and rejected the salts in the bulk water solution. The coatings of photothermal material applied to CFs enhanced their light absorption capacity and this produced more heat at the air–water interface. The evaporator set-up generated water vapor as the saline water came into contact with the CFs in the presence of light. The vapor was then condensed back into liquid when it met the colder surface of the CFs. Simulated saline water was prepared with 890 mg L−1 of Ca2+, 3652 mg L−1 of K+, 1024 mg L−1 of Mg2+, and 7769 mg L−1 of Na+ salt compositions, which were placed in a glass beaker to dissolve them in deionized water. Furthermore, the beaker was placed over an electric balance (Precisa, with 0.1 mg accuracy) and the mass change at regular intervals was automatically recorded. The evaporation flux (E) was calculated from the collected data by using the following equation for normal low-saline water and simulated saline water:
 
image file: d4nr01930j-t1.tif(1)
where M is the mass loss of the saline water, A is the cross-sectional area available for evaporation and t is the time taken for the evaporation.44

The evaporation flux was calculated by subtracting the intrinsic water vaporization determined with and without any coating. The experiment was replicated three times, and average values were reported. The coated CFs were dried under ambient conditions and used for long-cycling testing, with the rejected salt being removed after each cycle. The rate of evaporation was contingent upon the surface temperature, given that the saturation vapor pressure rises with temperature. The temperature distribution in bulk water and at the absorber surface was recorded using an infrared thermometer. A comparison of the salinity content in the saline water after desalination was carried out using an inductively coupled plasma-optical emission spectrometer (ICP-OES) (OPTIMA 5300 DV, PerkinElmer Instruments, 0.1 mg L−1 accuracy).

3. Results and discussion

3.1 Characterization of the photothermal material, GJ-01(Cal)

3.1.1 XRD analysis. The X-ray diffraction pattern of the as-synthesized MOF composite, GJ-01(Cal), is represented in Fig. 2a. As shown in Fig. S1a,† all of the diffraction peaks of the as-synthesized HKUST-1 were indexed appropriately and synced with the simulated HKUST-1 data using CIF file no. 2300380.45 The diffraction peaks in the CNS (Fig. S1b†) appeared at approximately 24.2° (002) and 43.1° (100), and these two bands were broad, implying the presence of highly disordered but predominantly sp2-hybridized carbon.46 An apparent phase change was observed after calcining a mixture of HKUST-1 and CNS at 800 °C, which confirms the triphasic Cu/CuO/Cu2O configuration, as shown in Fig. 2a. The strong peaks at 28.10°, 32.75°, 35.51°, 38.15°, 48.92°, 61.72°, and 66.25°, respectively, were attributed to the (101), (110), (111), (411), (202), (022), and (220) planes, which demonstrate the formation of CuO crystals (JCPDS card no. 892531). The presence of Cu2O was confirmed by the prominent peaks at 29.84°, 36.19°, 43.70°, and 54.61°, which belong to the (110), (111), (101) and (203) planes, respectively (JCPDS card no. 770199).47 The diffraction peaks at 26.61°, 34.37°, 46.24°, and 50.95° correspond to the (110), (220), (111), and (200) planes, respectively, confirming the presence of Cu(0) (JCPDS card no. 40838).48
image file: d4nr01930j-f2.tif
Fig. 2 (a) XRD patterns of the GJ-01(Cal) nanocomposite and (b) UV-vis-NIR spectra of GJ-01(Cal) with AM 1.5.
3.1.2 UV-visible-NIR spectroscopy. The optical absorption spectrum demonstrates the broadband absorption properties of GJ-01(Cal) (Fig. 2b). The pristine samples of HKUST-1 and CNS, as shown in Fig. S2,† show absorbance from 300 to 800 nm, which is not broadband absorbance; however, after being mixed and calcined, the samples show a broadband absorbance (300–2000 nm), with GJ-01(Cal) displaying an enhanced absorbance percentage of approximately 95%. Therefore, GJ-01(Cal) was selected as the appropriate solar absorber to conduct further photothermal studies. The bandgap of GJ-01(Cal) was determined to be 2.22 eV, which acted as a semiconductor and absorbs light in the visible range, as illustrated by the Tauc plot in Fig. S2b.†
3.1.3 Morphological analysis. The morphological analysis of the as-synthesized GJ-01(Cal) was performed using FESEM (Fig. 3a and b). A crystalline octahedral-like structure with smooth surface edges can be seen in the FESEM image of pristine HKUST-1, as shown in Fig. S3.† High-temperature calcination may have caused a small amount of agglomeration in the nanostructured composite. The particles with quasi-spherical structure were composed of triphasic Cu/CuO/Cu2O and the sheet-like structure was the CNS. The triphasic Cu/CuO/Cu2O was bound to the CNS surface, as shown in Fig. 3a. Moreover, the formation of a nanocomposite with a highly porous structure was confirmed by analysis of the FESEM image (Fig. 3b). The EDX spectra and their mapping (Fig. S3†) reveal that the as-synthesized GJ-01(Cal) nanocomposite comprises copper, carbon, nitrogen, and oxygen in variable weight percentages and has consistent elemental distribution throughout its structure. The HRTEM image (Fig. 3c) indicates the encapsulation of triphasic Cu/CuO/Cu2O particles into the CNS sheet-like structure. The average particle size of the triphasic nanoparticles was also calculated using ImageJ software, and it was found to be 22–24 nm (Fig. 3d, inset). Likewise, the lattice spacing (Fig. 3d) of 0.24 nm corresponds to the (111) interplanar spacing of Cu2O, whereas the lattice spacing of 0.21 nm derived from the nanocomposite resembles the (111) interplanar spacing of Cu(0).49
image file: d4nr01930j-f3.tif
Fig. 3 Morphological analysis of GJ-01(Cal): (a and b) FESEM images and (c) HRTEM image of triphasic nanoparticles embedded in CNS; (d) lattice spacing with the size distribution histogram given in the inset.
3.1.4 XPS analysis. The chemical composition and elemental valence states were analyzed using the XPS technique, and it is also crucial to determine the efficiency of photothermal desalination. The survey spectra reveal CNS components, N 1s, C 1s, and O 1s, along with Cu 2p in the formation of HKUST-1(Cal) and GJ-01(Cal) nanocomposites (Fig. S4a–c†). In the C 1s spectra of HKUST-1(Cal), CNS, and GJ-01(Cal), a peak at 284.1 eV showed the presence of phenyl rings associated with the benzene ring from the organic linker (Fig. S4d–f†). A peak at 287.6 eV further indicates the presence of C[double bond, length as m-dash]O or C–N groups in both CNS and GJ-01(Cal) (Fig. S4e and f†).43 The carbonaceous framework contains three different types of nitrogen atoms: graphitic-N (G-N), pyrrolic-N (Py-N), and pyridinic-N (P-N), as shown in the deconvoluted N 1s spectra of HKUST-1(Cal), CNS, and GJ-01(Cal) (Fig. 4a–c). The peak of G-N at 401.3 eV reveals that the nitrogen atom was sandwiched in the graphene layer and takes the place of a carbon atom. The nitrogen atom that contributes two π electrons to the π system was referenced with respect to the peak present at 400.2 eV of Py-N. The initial P-N at 398.3 eV was identified from the N 1s peak, which was caused by the sp2 nitrogen atoms being coupled to two C atoms as a C[double bond, length as m-dash]N–C linkage.18 Additionally, it can be explained by the fact that the addition of CNS into the MOF profoundly impacted the increase of Py-N over the P-N peak in GJ-01(Cal), as shown in Fig. 4c, which plays a predominant role in the photothermal mechanism. The extensively deconvoluted O 1s spectra (Fig. 4d and e), which displays peak energies for OI, OII, and OIII, reveals three distinct components. The presence of surface lattice oxygen equates to the OI peak at 530.7 eV corresponding to the Cu+ while a peak at 529.8 eV reveals the lattice oxygen of Cu2+ ions.50 The vacant oxygen OII peak at 531.3 eV, shows increasing binding energy when the hydrogen atom eliminates valence charge.51 The OIII peak at 532.5 eV is a representation of the surface hydroxyl group or chemisorbed oxygen ions (O2−,O−, or O−2).52 The XPS spectra of Cu 2p of HKUST-1(Cal), and GJ-01(Cal) are shown in Fig. 4f and g. The two peaks corresponding to Cu 2p3/2 (934.4 eV) and Cu 2p1/2 (954.7 eV) were assigned to Cu2+.53,54 The Cu+ was indicated by the two peaks in the Cu 2p spectra, Cu 2p3/2 (933.7 eV) and Cu 2p1/2 (953.5 eV).54 The primary peak of Cu(0) in the Cu 2p XPS spectrum (Fig. 4g) was seen as a peak of Cu 2p3/2 at 932.6 eV and Cu 2p1/2 at 952.4 eV.55 In addition, the appearance of satellite peaks indicates electronic noise, which empirically supports the presence of Cu2+ in the triphasic system.32,56 Notably, the incorporation of CNS with HKUST-1(Cal), which resulted in the formation of GJ-01(Cal), showed an enhancement of the Cu(0) signal as compared with the Cu 2p spectrum of HKUST-1(Cal). This indicates that the presence of Cu(0) in the Cu nanoparticles plays a vital role in their photothermal activity.29,46,47
image file: d4nr01930j-f4.tif
Fig. 4 X-ray photoelectron spectroscopy (XPS) spectra: (a) N 1s spectra of HKUST-1(Cal), (b) N 1s spectra of CNS, (c) N 1s spectra of GJ-01(Cal), (d) O 1s spectra of HKUST-1(Cal), (e) O 1s spectra of GJ-01(Cal), (f) Cu 2p spectra of HKUST-1(Cal), and (g) Cu 2p spectra of GJ-01(Cal). (h) Micro-Raman spectrum of GJ-01(Cal).
3.1.5 Raman spectroscopy. The Raman spectrum of GJ-01(Cal) is displayed in Fig. 4h, with two major peaks at 1351 cm−1 and 1603 cm−1. The D band at 1351 cm−1 was formed by a breathing mode of the k-point photons of A1g symmetry, showing the presence of disorders as well as the borders and boundaries of amorphous carbon domains,57 while the G band at 1603 cm−1 was caused by first-order scattering of the E2g phonon of sp2 C atoms, suggesting graphitic carbon.58 The in-plane sp2 domain sizes have significantly decreased, and there has been an increase in disorder according to the ratio of D–G peak intensities, ID/IG.59 In the Raman spectrum of CNS (Fig. S3†), ID/IG was calculated to be 0.95.60 With the addition of HKUST-1 into CNS to form a MOF-based nanocomposite, GJ-01(Cal), the ID/IG was increased to 1.1, as shown in Fig. 4h.
3.1.6 BET analysis. BET analysis was used to measure the pore size and surface area of the GJ-01(Cal) nanocomposite. This indicated that N2 adsorption and desorption isotherms could be used to assess the surface area of the solar absorber material, GJ-01(Cal), as shown in Fig. 5. The nanocomposite has a pore volume of 8.88 × 10−2 cm3 g−1 and a specific surface area of 197.03 m2 g−1. Moreover, a mean pore diameter of 2.45 nm was determined using BET analysis.
image file: d4nr01930j-f5.tif
Fig. 5 (a) N2 adsorption/desorption isotherms, (b) pore volume distribution, and (c) pore size distribution as a graph of dV/dw vs. the pore width (where V is the pore volume and w is the effective pore width) of GJ-01(Cal).

3.2 Characterization of the pristine and coated cotton fibrils (CF)

The coating strategy of the CFs is presented in Fig. 6a, followed by an image of the CFs dipped into water where the CFs mimic a flower-like architecture from a top view (Fig. 6b), orchestrating PTIE at the water/air interface. The difference in both pristine and coated CFs can be observed from their FESEM images (Fig. S6†). The FESEM image of pristine CFs shown in Fig. 6c reveals the roughness and high porosity for water uptake via capillary action. Correspondingly, the CFs coated with the GJ-01(Cal) nanocomposite (Fig. 6d) shows increased porosity that enhances the surface-localised heating and heat insulation at the interface for efficient photothermal evaporation.
image file: d4nr01930j-f6.tif
Fig. 6 Optical images: (a) front view and (b) top view of coated CFs dipped into saline water. FESEM images of (c) pristine cotton fibrils (CFs) and (d) GJ-01(Cal) nanocomposite-coated CFs.

3.3 Photothermal desalination ability of the GJ-01(Cal) nanocomposite coated over CF

The efficacy of the GJ-01(Cal) material for generating steam through photothermal experiments conducted under one sun illumination was investigated. Fig. 7a shows a schematic illustration of the experimental set-up. The time-dependent mass change curves demonstrated that GJ-01(Cal) consistently displayed higher evaporation rate in the case of saline water when compared to HKUST-1(Cal) and CNS, as shown in Fig. 7b. Accordingly, the maximum evaporation fluxes of pristine CF and CF coated with GJ-01(Cal) for saline water were calculated to be 4.23 and 2.67 kg m−2 h−1, respectively (Fig. 7c), under one sun illumination. The highest surface temperature for GJ-01(Cal) was determined to be ∼47 °C (Fig. 7d). The photothermal conversion efficiencies of the constituent materials are presented in Fig. S7.† This revealed the high solar-to-vapor conversion efficiency of GJ-01(Cal), which was estimated to be 96.5% within 2 h, whereas other materials, such as HKUST-1(Cal) and CNS, have photothermal conversion efficiency values of 23.9% and 67.9%, respectively. In contrast, the mass changes of saline water and pure water observed over the time of 2 h for GJ-01(Cal) showed excellent evaporation flux rates for 10 cycles, as shown in Fig. S8.† Moreover, there was a gradual decrease in the evaporation flux due to salt accumulation over the CFs, which reduced the capillary rise of the water at the top of the CFs. Furthermore, the water vapor was condensed over a cold glass surface and collected. Subsequently, experiments using inductively coupled plasma-optical emission spectrometry (ICP-OES) were conducted to understand the photothermal desalination component of this research, as illustrated in Fig. 7e. The results showed that the salinity concentrations of frequently occurring salts (Ca2+, K+, Mg2+, and Na+) in saline water were reduced to below 10 mg L−1, which is significantly lower than WHO and EPA limits.
image file: d4nr01930j-f7.tif
Fig. 7 (a) Photothermal desalination experiment, (b) mass changes of saline water over time for HKUST-1(Cal), CNS, and GJ-01(Cal) under one sun illumination, and (c) comparison of evaporation fluxes with saline water, pristine cotton fibrils, CNS, HKUST-1(Cal), and GJ-01(Cal). (d) Surface temperatures of pristine cotton fibrils and GJ-01(Cal). (e) ICP-OES of different ions present in the saline water sample before and after desalination. Each error bar shows the standard deviation of the mean across three experiments.

4. Mechanistic analysis towards photothermal activity

The scientific scrutiny into the engine that drives the photothermal activity of the GJ-01(Cal) nanocomposite for its photothermal desalination performance has been divided. Thus, our understanding of the architecture of the photothermal set-up designed using CFs to achieve maximum evaporation, is as shown in Fig. 8. First, formation of the triphasic Cu/CuO/Cu2O configuration incorporated into CNS during calcining at 800 °C under an N2 atmosphere was significantly influential as this was necessary to induce a phase change in the Cu-MOF@CNS system. Nonetheless, the material absorbs light over the broadband solar spectrum and behaves as a visible light semiconductor with a bandgap of 2.22 eV. Correspondingly, the Raman spectrum of GJ-01 (Cal) in Fig. 4h shows an increase in the ID/IG ratio due to an enhancement in the D peak of GJ-01 (Cal) compared to that of the pristine MOF and CNS, which indicates an enhancement of defect density.61 Moreover, the defect investigation revealed the role of P-N and Py-N in enhancing the photothermal conversion efficiency via localized surface plasmon resonance (LSPR) in metallic nanoparticles. In general, N-doped graphene-like materials, such as CNS, possess increased G-N configuration due to n-type doping, whereas p-type doping was observed when P-N and Py-N are upregulated.62 The N 1s peaks in the XPS spectra of HKUST-1(Cal), CNS, and GJ-01(Cal ), shown in Fig. 4a–c reveal that p-type dopants promote the amplification of electronegative defects (P-N and Py-N), which is significant for their photothermal activity. The P-N is particularly important for widening the field of vision for the first NIR absorption to effect photothermal conversion.63 A localized heat zone at the air–water interface was necessary for higher evaporation efficiency and to limit heat transfer loss for improved steam generation. Comparing the Cu 2p data of HKUST-1(Cal) and GJ-01(Cal) revealed the formation of Cu(0) species as a result of the dominant growth of Cu nanoparticles, as can be seen in Fig. 4h. This displays an interesting enhancement of nucleation due to the intensified reduction of HKUST-1 to Cu(0) nanoparticles after the addition of CNS, followed by calcination at an elevated temperature. Cu(0) nanoparticles embedded in CNS are potential materials for solar-based desalination owing to their high solar spectrum absorption through LSPR and energy absorption for intended heating in narrow zones for photothermal activity.64 In summary, the amelioration of the p-type nature of the Cu/CuO/Cu2O triphasic configuration embedded in the CNS allows electrons to be generated as a result of (i) Cu reduction from the +2 to +1 oxidation state and also to Cu(0), (ii) P-N and Py-N, and (iii) vacant oxygen from the Cu/CuO/Cu2O triphasic configuration caged in the CNS (Fig. 4e and f) recombining effectively and thus emitting heat as a result of their being LSPR semiconductors,5 and (iv) CNS preventing Cu(0) from oxidizing in the presence of solar radiation, thus realizing a steady solar desalination performance via LSPR as seen in metals.64 In Table S1,† we compare the efficiency of the GJ-01(Cal) nanocomposite-coated CFs with that of other reported photothermal desalination devices.
image file: d4nr01930j-f8.tif
Fig. 8 Schematic mechanism of the photothermal desalination performance using GJ-01(Cal).

Subsequently, the solar absorber was coated over CFs to utilize its robust potential optimally. CFs have an extensive porous structure allowing liquids and vapors to enter them through capillary action and this enhances their wickability and inherent absorbing capacity.65,66,68 The low thermal conductivity of CFs effectively restrained heat loss from the evaporation interface into the bulk water. Furthermore, these CFs continuously supplied underlying bulk water while simultaneously rejecting salts through capillary action because of their porous structure, mimicking the natural world in a way akin to mangrove trees.39 Mangrove trees reject salts from saline water through negative pressure/tension in their mesophyll cell walls of the leaves and enhance water transportation through capillary action. This salt rejection eventually decreases the effective salt concentration at the air–water interface. Due to these colligative properties, the boiling point of the solution decreases and is nearly close to that of pure water. Afterwards, the transported water is intercepted by the CFs, which allow dispersion of the localized heating that has been provided by the solar absorber (GJ-01(Cal)), due to their low thermal conductivity and porous structure, as previously reported in the literature.67 This section was responsible for regulating PTIE at the water/air interface, which was attributed to providing the necessary water for evaporation.14 Salt accumulation hindered PTIE by causing solution saturation at the vaporization interface, causing congestion and contributing to vaporization issues. In summary, a simple and environmentally friendly desalination system was obtained using a strong solar absorber in conjugation with a framework made up of CFs. The precursors were made using affordable and eco-friendly materials. Consequently, it was thought that this unique combination of MOFs and CNS along with a novel architecture for photothermal desalination using CFs could produce a durable, low-cost desalination solution that would successfully address the global water shortage.

5. Conclusion

Herein, we report a promising material that has high solar-to-vapor conversion efficiency for the photothermal desalination performance of an emerging combination of Cu-MOFs incorporated into CNS, consisting of a phase-change-induced GJ-01(Cal) nanocomposite. The nanocomposite material absorbs broadband solar spectra and functions as a semiconductor with a bandgap of 2.22 eV. The amplification of pyrrolic nitrogen (Py-N) defects in the CNS, the enhancement of Cu(0) species, and the enrichment of the p-type nature of the triphasic Cu/CuO/Cu2O semiconducting configuration have all been suggested as contributing factors to the overall photothermal desalination activity. Additionally, Py-N assists in providing the photothermal effect with the aid of the LSPR mechanism from the triphasic configuration in the initial near-infrared absorption window. Cu(0) nanoparticles embedded in N-doped CNS achieve high solar spectrum absorption via LSPR. The solar absorber was coated over porous CFs that allowed water to be absorbed and vapors to pass through them. This allows localized dispersal of heat over the material, controlling PTIE at the water/air interface. On the other hand, solution saturation was a major issue with vaporization and a substantial hindrance to PTIE due to salt buildup at the vaporization interface. Thus, the system achieved a profitable photothermal conversion efficiency of 96.5% and maximum evaporation rates of 4.40 and 4.23 kg m−2 h−1 for pure water and saline water under one solar illumination, respectively. The MOF-based CNS nanocomposite with salt-rejecting activity also exhibits outstanding recyclability and stable water evaporation performance in the seawater desalination process. This material demonstrates resilient photothermal activity in a simple framework using inexpensive and environmentally friendly precursors for easily accessible potable water. We conclude that scalable production of the GJ-01(Cal) nanocomposite with its unique features such as high efficiency, salt-rejecting activity, and beneficial desalination performance holds great potential for sustainable desalination and water purification.

Author contributions

Geetika Jain was responsible for conceptualization, data curation, formal analysis, investigation, methodology, validation, and writing of the original draft. Sinu Sanghamitra performed the investigation and validation. Monalisa Mukherjee conducted the electron microscopy analysis of the materials, and reviewed and edited the manuscript. Mrinal Kanti Mandal, Rajib Ghosh Chaudhuri, and Sandip Chakrabarti were responsible for project administration, resources, and supervision, and writing, reviewing, and editing the manuscript.

Data availability

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

Conflicts of interest

There is no conflict of interest declared by the authors.

Acknowledgements

The authors are thankful to DST-SERD (grant No. DST/TMD/CERI/RES/2020/44) for the funding of the project. We thank Mr. Narender Kumar and Prof. Prodyut Ranjan Chakraborty from IIT Jodhpur for performing the thermal conductivity experiment.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr01930j
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