Enhancing solar absorbance using a 2D graphene oxide/CuO composite film for efficient solar desalination

Abstract

Interfacial solar steam generation (ISSG) is a promising seawater desalination technique owing to the low cost, zero pollution and easy operation involved. In this work, we aimed to develop a novel 2D graphene oxide (GO)/CuO photothermal film via a facile ultrasonication method towards enhanced solar absorbance for highly efficient ISSG. The as-prepared GO/CuO film exhibited a sandwiched nanostructure, where the CuO nanoparticles were confined in the GO layers. This was beneficial to enlarge the spacing of the GO layers, leading to high light reflection in the GO/CuO film. Alternatively, the CuO nanoparticles could promote the incident light absorbance in the whole visible light range. Moreover, the water contact angle of the GO/CuO film was 57.4°, which is much lower than that of the pure GO film (74.8°), suggesting that a high-water transport rate can be expected. Consequently, the highest light absorption of the GO/CuO film was 83% in the entire sunlight range, whereas the corresponding value was 74% for the pure GO film. Under 1 kW m⁻² irradiation, the highest evaporation rate by the GO/CuO film reached 1.71 kg m⁻² h⁻¹ with an efficiency of 99.2%, which remained at 1.60 kg m⁻² h⁻¹ after 20 cycles of desalination. Furthermore, the GO/CuO film was applied to treat organic pollutants, which achieved a high removal efficiency of more than 90%.

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Water impact

Interfacial solar steam generation (ISSG) is considered a highly efficient and environmental-friendly desalination technique for producing portable water. Graphene oxide (GO) has been extensively explored in the ISSG research domain due to its excellent thermal conductivity, flexibility and multilayer structure. However, the unregulated GO has a high reflectivity of sunlight. Thus, to overcome this problem, we proposed the use of a novel GO/CuO composite film with nano-sandwiched structure. Owing to the increased GO layer spacing and the narrow band gap of CuO, the optical absorption of the composite film greatly improved. Besides, the increased layer spacing facilitates the transport of water molecules. The results show that the composite film demonstrated an enhanced ISSG performance. Moreover, it is verified that the composite film has excellent ability to remove salty ions and organic pollutants from water.

Introduction

With the rapid industrial development and increasing world population, the shortage of drinking water has become one of the most severe challenges for the survival of mankind at present. Despite the fact that most of the Earth is occupied by water, less than 3% is portable water.^1,2 In this case, various seawater desalination technologies have aroused attention. In the past few decades, reverse osmosis (RO), multi-stage flash (MSF) and electrodialysis (ED) have been developed and commercialized. However, these technologies suffer from the drawbacks of complex operation, high capital cost and secondary pollution.^3,4

The interfacial solar steam generation (ISSG) desalination technique utilizes solar energy to generate steam in a regulated area. Given that solar energy is abundant on the Earth, ISSG is deemed as efficient and low-cost seawater desalination technology.^5,6 To date, ISSG has been widely implemented in desalination, sewage treatment, photo-thermal power generation, solar sterilization and other related aspects.^7–9 Nevertheless, the improvement in the ISSG performance is limited by energy loss due to thermal radiation and heat transfer to the surrounding environment and water.¹⁰ Furthermore, the water path transporting bulk water to the evaporation surface is still unsatisfactory.¹¹ Besides, its low photothermal conversion efficiency has to some extent a result of the low optical absorption and low thermal conductivity of the evaporator.^12,13 Thus, great efforts have been devoted to overcoming these issues. For example, Ghasemi et al. prepared a double-layered film composed of carbon foam and expanded graphite for ISSG. It was demonstrated that the carbon foam effectively reduced the energy loss caused by thermal radiation to the surrounding environment. Under 10 kW m⁻² irradiation, the conversion efficiency reached 85%.¹⁴ Fang et al. prepared a layered microstructured activated carbon fiber cloth to improve the water transport. Benefiting from the capillary force provided by the cotton fiber non-woven fabric, the water transport rate was greatly enhanced. Consequently, the evaporation rate reached 1.59 kg m⁻² h⁻¹ under one sun irradiation.¹⁵

In terms of ISSG films, metal nanoparticles have attracted extensive attention due to their unique localized surface plasmon resonance (LSPR). When the local surface plasmon in metal nanoparticles is excited by the incident light, LSPR will be induced, which enhances the local electric field on the surface of the nanoparticles. In this case, a strong surface plasmon absorption band appears, causing high solar light absorption (>90%).^16,17 For example, Sheng et al. deposited Pd and Ag nanoparticles on natural bamboo, which realized the optical absorption of 99% in the wavelength range of 200 to 2500 nm. In contrast, the optical absorption was ∼60% for the raw bamboo. Therefore, the Pd/Ag/bamboo film enabled a conversion efficiency of 87% to be realized under 10 kW m⁻² irradiation.¹⁸ Unfortunately, the above-mentioned plasmonic ISSG films are associated with high cost, complex preparation and rare resources, and thus not applicable for large-scale desalination. Instead, metal oxides (MOs), as typical semiconductor materials, have been extensively developed as powerful photothermal candidates because of their highly tunable energy band and intrinsic thermalization process. When photons are adsorbed on the surface of MOs, the collective oscillation of surface electrons is triggered, and therefore the localized electric field is dramatically enhanced, which reinforces the adsorption of incident light. Afterwards, the surface plasmon attenuation is accelerated through electron–electron scattering, surface-electron scattering and radiation damping. Meanwhile, the high-energy electrons are excited. Due to the rapid diffusion of high-energy electrons, thermal energy is generated and dissipated in the MOs.^19,20 Significantly, MOs possessing a narrow band gap (<3.0 eV) can absorb sunlight efficiently in a wide range.²¹ For example, Wang et al. loaded corolla-like Co₃O₄ on nickel foam. The band gap of Co₃O₄ is about 1.5–2.5 eV, which favors the absorption of solar energy in the visible light to infrared range. The results demonstrated that the average optical absorption of Co₃O₄/nickel foam is over 90% in the wavelength range of 200–2500 nm, which is much higher than that of the pure nickel foam (∼50%).²² Liu et al. prepared moth-eye-like black TiO₂ nanorods with a narrow band gap of ∼3.0 eV on carbon cloth. In addition to the wide light absorption range due to TiO₂, the moth-eye-like layered nanostructure trapped the light in the nanorod gap, leading to multiple internal reflections until the light was completely absorbed. The results showed that the absorption of black TiO₂ nanorod/carbon cloth is 96% in the full spectrum range, which is nearly twice that of the pure carbon cloth.²³

As one of the typical 2D carbon materials, graphene oxide (GO) has been extensively explored in the ISSG research domain due to its excellent thermal conductivity, flexibility and multilayer structure, which is conducive to sunlight absorption and water transport. However, despite the advances achieved with GO, its high reflection limits the efficient absorption of sunlight.^24,25 It has been proven that the light reflection of pure GO films exceeds 25% in the range of 200–2500 nm, and remarkably it is more than 50% in the infrared region. Therefore, pure GO films cannot be employed as efficient solar absorbers.²⁶ Alternatively, owing to its narrow band gap of 1.2 eV, copper oxide (CuO) provides high solar absorption in the wide band of the visible and infrared regions.²⁷ For instance, Xu et al. prepared a CuO nanowire, which exhibited high solar absorption of over 93% in the whole solar spectrum. Significantly, the optical absorption of the CuO nanowire is very high in the infrared region.²⁸ Further, Zheng et al. prepared Cu/CuO foam via wet chemical oxidation reaction. The light absorption by the Cu/CuO foam reached more than 90% in the infrared light region, which was much higher than that of the original Cu foam (less than 40%).²⁹ Inspired by these reports, combining CuO with 2D GO is expected to deliver high photothermal conversion efficiency, benefiting from their high light absorption, unique 2D water transpiration path and excellent thermal conductivity.

In this work, an urchin-like CuO particle/GO film was prepared via a facile vacuum filtration method for high-performance ISSG. The morphology, structure and light absorption properties of the GO/CuO films with various CuO dosage were studied. The results indicated that the GO/CuO film has high solar absorption in the whole sunlight range. Specifically, the incorporation of CuO enlarged the GO layer, which favored an improvement in the rapid transport of water molecules. Consequently, the evaporation rate and efficiency of the GO/CuO films were enhanced effectively.

Experimental

Chemicals and materials

Cu(NO₃)₂·3H₂O (Macklin) and ammonia water (Sinopharm Chemical Reagent) were used as reagents. Polyvinylidene fluoride (PVDF) films with a pore size of 0.22 μm were purchased from Haiyan New Oriental Plasticizing Technology Co. Ltd. The PVDF films with abundant microchannels were used as the substrate, which facilitated the transport of water to the evaporation surface through the microchannels. All the chemicals were analytical grade reagents and used as received without further purification.

Preparation of urchin-like CuO

Initially, 0.42 g Cu(NO₃)₂·3H₂O was dissolved in 100 mL deionized water to obtain a Cu(NO₃)₂ solution. Subsequently, 19 mL 14.84 mol L⁻¹ ammonia water was quickly poured into the Cu(NO₃)₂ solution, stirred for 30 min, and then heated to 90 °C and stirred for 12 h to obtain a black precipitate. Finally, the black precipitate was washed with centrifugation several times, followed by drying at 60 °C for 12 h, resulting in the synthesis of urchin-like CuO particles.

Preparation of GO film

GO was prepared via the modified Hummers method and subjected to freeze-drying.²⁶ Subsequently, it was suspended in deionized water and ultrasonicated for 3 h to obtain a 5 mg mL⁻¹ GO suspension. Then, the GO suspension was vacuum filtered on a 0.22 μm hydrophilic polyvinylidene fluoride (PVDF) film. Finally, the GO film was dried under vacuum at 30 °C for 30 min.

Preparation of GO/CuO composite film

The urchin-like CuO particles with different masses were suspended in deionized water and ultrasonicated for 3 h to obtain certain concentrations of 0.67 mg mL⁻¹, 1.33 mg mL⁻¹, 2.66 mg mL⁻¹ and 5.32 mg mL⁻¹. As shown in Fig. 1a, 3 mL CuO suspensions with different concentrations were mixed with 5 mg mL⁻¹ GO suspension, and then ultrasonicated for 3 h to obtain a GO/CuO mixed suspension. The mixed suspension was vacuum filtered on hydrophilic PVDF films. After vacuum drying at 30 °C for 30 min, GO/CuO composite films with CuO concentrations of 0.33 mg mL⁻¹, 0.67 mg mL⁻¹, 1.33 mg mL⁻¹ and 2.66 mg mL⁻¹ were obtained, which were labeled as GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4, respectively.

Fig. 1 Schematic diagram of (a) preparation of GO/CuO composite film and (b) experimental setup for ISSG, and the inset is a digital image of the ISSG device.

Experimental set-up for solar steam generation

Fig. 1b displays a schematic diagram of the experimental setup for ISSG. Essentially, a xenon lamp (CEL-PE300L-3A, Ceaulight) with an optical filter for the standard AM 1.5G solar spectrum was employed to simulate the solar light source. The output optical power was regulated by adjusting the operating current of the xenon lamp, and the optical power density was calibrated using an optical power meter (PL-MW2000, Perfect Light, China). Regarding the ISSG test, the homemade ISSG device was placed perpendicularly on an electronic balance (CP114, Ohaus, USA) with an accuracy of 0.0001 g, and the reduction in the quantity of water was recorded using a computer under the irradiation of a xenon lamp. An infrared camera (FLIR-E6390, Estonia) was employed to measure the surface temperature of the film. It should be mentioned that the ambient temperature was maintained at 18 °C ± 1 °C and the humidity was maintained at 20% ± 3% during the desalination test. A digital image of the ISSG device is shown in the inset of Fig. 1b. The GO/CuO film was deposited on the top of polyurethane foam (PF), which was confined by a glass container. The PF acted as the supporting substrate. Besides, it can reduce the energy loss caused by heat conduction to the bulk water. Meanwhile, it is beneficial to concentrate the solar energy. The hydrophilic cotton slivers passed through the polyurethane foam, which can use the capillary force to pump the water from bottom to the GO/CuO film.

Results and discussion

The X-ray diffraction (XRD, SmartLab, Japan) patterns of CuO and GO/CuO are shown in Fig. 2a. Basically, the diffraction peaks of CuO match the standard XRD pattern very well (JCPDS: 48-1548). The two strong peaks located at 35.5° and 38.7° correspond to the (11-1) and (111) crystal planes of CuO, respectively.³⁰ In the XRD patterns of all the GO/CuO films, the typical diffraction peaks indexed to GO were observed at 10.7° and 26.0°, corresponding to the (001) and (002) crystal planes of GO, respectively.³¹ Upon increasing the concentration of CuO, the intensity of the peak related to CuO also increased. Fig. 2b shows the Raman spectra (DXR, USA) of CuO, GO and GO/CuO-2 film. In terms of CuO, there is an apparent characteristic peak at 287 cm⁻¹, which also appeared in the Raman spectrum of GO/CuO-2, suggesting the excellent coupling of GO with CuO.³² Different from CuO, the Raman spectra of GO and GO/CuO-2 feature the typical bands at 1350 cm⁻¹ and 1580 cm⁻¹, which correspond to the D and G bands of carbon materials, respectively. Further, the I_D/I_G of GO/CuO-2 is 0.97, which is higher than that of the GO film (0.93), indicating that GO/CuO-2 has a higher degree of defects.³³ The morphology of CuO, GO film and GO/CuO film were observed by scanning electron microscopy (SEM, Hitachi SU5000 Japan). Fig. 2c shows the SEM image of CuO, which exhibits a typical urchin-like structure with a diameter of ∼4 μm. This structure is beneficial to enlarge the layer spacing of GO and improve the incident light absorbance. Fig. 2d–h show the SEM images of the GO, GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4 films from the top view, respectively. It can be found that the surface of the GO film is quite smooth without presenting any other impurities. Unlike GO film, some obvious bulges emerged on the surface of GO/CuO due to the confinement of the CuO particles in the GO layers, which seems to be proportional to the concentration of CuO. Thus, the surface of GO/CuO became very rough, where the bulges were distributed over its entire surface. Significantly, these irregular bulges enable enhanced the refraction of incident sunlight. As shown in Fig. 2g, the enlarged SEM image from a selected area of GO/CuO-2 confirms the origin of the bulge, which is deemed as CuO particles. Fig. 2i and j exhibit the cross-sectional SEM image of GO and GO/CuO-2 film, respectively. They clearly highlight that the CuO particles were successfully confined in the GO layers, which is favorable for enlarging the layer spacing, and therefore strengthening the absorbance of solar energy. To confirm the structure of the GO/CuO film, Fig. 2k shows the cross-sectional SEM image of GO/CuO-2 and the corresponding mapping image of Cu, C and O, again demonstrating the novel structure described above.

Fig. 2 (a) XRD patterns of CuO, GO and GO/CuO film, (b) Raman spectra of CuO, GO and GO/CuO-2 film, (c–h) SEM images of CuO, GO, GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4 film, (i–j) cross-sectional SEM images of GO and GO/CuO-2 film, and (k) elemental mapping images of GO/CuO-2 film.

X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi) was employed to investigate the chemical composition of the GO/CuO-2 film. Fig. 3a shows the full survey spectrum, which exhibits C, O, and Cu signals. The XPS spectrum of Cu 2p in Fig. 3b exhibits two peaks at 934.4 and 953.8 eV, which correspond to Cu 2p_3/2 and Cu 2p_1/2, respectively. The peaks emerging at 940.1, 943.8, and 962.3 eV are ascribed to the satellite peaks of Cu²⁺. Fig. 3c shows the XPS spectrum of C 1s, which exhibits three peaks at 284.8, 286.7 and 288.6 eV, corresponding to the C–C bond, C–O–C bond and O–C=O bond, respectively. Fig. 3d shows the XPS spectrum of O 1s, in which the peaks located at 530.9, 532.2 and 533.5 eV are attributed to the lattice oxygen, adsorbed oxygen and oxygen vacancy, respectively.^34,35

Fig. 3 (a) Survey, (b) Cu 2p, (c) C 1s and (d) O 1s XPS spectra of GO/CuO-2.

The light absorbance performance of the GO/CuO films was measured using a UV-vis spectrophotometer (UV-vis, PE, Lambda1050). Fig. 4a and b show the light reflection and absorption of all the films in the broadband range of 200–2500 nm, respectively. The absorption can be calculated using eqn (1), as follows:

A = 1 − R − T (1)

where A (%), R (%), and T (%) represent the absorption, reflection and transmission, respectively.³⁶ In comparison with the pure GO film, the reflection of the GO/CuO films was significantly reduced, which was the reverse in the case of absorption. Upon increasing the concentration of CuO from GO/CuO-0.5 to GO/CuO-4, the absorption increased. Among the films, GO/CuO-4 exhibited the highest light absorption, which revealed the light absorption of more than 83% in the ultraviolet to visible light range. This is much higher than that of the pure GO film (74%). Remarkably, the GO/CuO-4 film maintained high light absorption in the whole wavelength range of sunlight.

Fig. 4 (a) Reflection and (b) absorption performance of GO and GO/CuO films, (c) surface temperature change, (d) mass change and (e) evaporation rate of pure water, GO film and GO/CuO film under one sun irradiation.

The photothermal conversion performance of the GO/CuO films was characterized by recording the variation in the surface temperature of all the films during evaporation under 1 kW m⁻² irradiation. Fig. 4c shows the temperature change of pure water and the surface of the GO film and GO/CuO films before and after solar irradiation. The initial surface temperature of all the films was 13.5 °C ± 0.3 °C, which increased rapidly when the light was turned on within 1 min. Specifically, the surface temperature of the GO film reached to 28.8 °C after 1 min irradiation, while that of GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4 was 29.8 °C, 32.7 °C, 33.2 °C and 35.1 °C, respectively. After 60 min, the surface temperature of the GO film stabilized at 43.2 °C, while that of GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4 remained at 44.1 °C, 45.7 °C, 46.4 °C and 47.5 °C, respectively. When the light was turned off, the surface temperature of the GO film, GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4 immediately dropped to 31.3 °C, 32.6 °C, 33.9 °C, 34.4 °C and 34.7 °C, respectively. Further, the surface temperature of the GO film, GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4 stabilized at 14.6 °C, 15.0 °C, 15.2 °C, 15.3 °C and 15.7 °C after 60 min, respectively. Thus, the surface temperature of all the GO/CuO films was higher than that of the GO film at any fixed duration, among which the GO/CuO-4 film was the highest. Fig. 4d shows the mass change in pure water caused by evaporation and all the films under 1 kW m⁻² irradiation within 2 h, where the mass change of pure water was employed as the reference. It can be seen that the mass change by all the ISSG films was much higher than that of pure water. The evaporation rates (kg m⁻² h⁻¹) were calculated using eqn (2), as follows:

where m, S and t are the mass change in pure water, effective surface area of the photo-thermal film and time, respectively.³⁷ As shown in Fig. 4e, the evaporation rate of pure water was 0.39 kg m⁻² h⁻¹, while that of the GO, GO/CuO-0.5, GO/CuO-1, GO/CuO-2 and GO/CuO-4 films was 1.51, 1.55, 1.60, 1.71 and 1.64 kg m⁻² h⁻¹, respectively. Importantly, the GO/CuO-2 film exhibited the highest evaporation rate among the films even after several tests. However, although the surface temperature of GO/CuO-4 is the highest among the films, its high CuO content may hinder the transport of water, which will cause a low evaporation rate.

The COMSOL Multiphysics 6.0 simulation software was used to clarify the mechanism of enhanced solar absorption enabled by the GO/CuO films. Based on the SEM analysis, a 3D physical model was established, shown in Fig. 5. In this model, the diameter of the CuO nanoparticles and the thickness of the GO layer were set to 3 μm and 0.2 μm, respectively. The colour variation suggests the change in the amount of light absorbed per unit volume (W m⁻³). Further, the wave equation with the refractive index form was employed, as follows:

∇ × (∇ × E) − k₀²n²E = 0 (3)

This is based on the assumption (μ_r = 1, σ = 0) and the equation ε_r = n², where μ_r, σ, ε_r, n, k₀ and E represent the relative permeability (H m⁻¹), electrical conductivity (S m⁻¹), relative dielectric constant, refractive index, free-space wavenumber (m⁻¹) and electric field (V m⁻¹), respectively. Fig. 5a shows the absorption of luminous power by the GO/CuO film under the irradiation of 3000 nm incident light. Obviously, the highest optical power reaches to 3.69 × 10¹⁷ W m⁻³ on the top of the CuO particles, while it was only 6.26 × 10¹³ W m⁻³ on the GO layer. Thus, this is firmly demonstrates that the incorporation of CuO particles in GO layer is beneficial to improve the incident light absorption. To further explore the absorption of solar energy by the CuO particles at different wavelengths, a 2D physical model was established, which follows the above-mentioned parameters. The only difference is that two adjacent CuO particles were inserted into the GO layer in this model. The incident light was set in the range of 400 nm to 800 nm, and the light intensity was uniformly set at 1 kW m⁻² at all wavelengths. Fig. 5b shows the cross-sectional image of luminous power absorbed by the GO/CuO film by varying the incident light wavelength in the range of 400 nm to 800 nm at an interval of 50 nm. Upon increasing the wavelength, the absorption of luminous power by the CuO particles gradually increased, while it remained almost unchanged in the GO layer. Particularly, the adsorption performance of the CuO particles was significantly enhanced in the long wavelength range, which is consistent with the above-mentioned UV-vis analysis of the GO/CuO film.

Fig. 5 COMSOL simulation of light absorption. (a) Luminous power distribution of the GO/CuO model from different views (3D, top and side). (b) Luminous power distribution of 2D GO/CuO model in the wavelength range of 400 to 800 nm.

Fig. 6a and b show the evaporation rate and evaporation efficiency of the GO/CuO-2 and GO films under different light irradiations, respectively. The evaporation efficiency (η) can be calculated using eqn (4), as follows:

where Q_s is the power density of illumination, m represents the mass change of water, t is the evaporation time, and H_LV is the liquid–vapor phase change enthalpy of water, including sensible heat and phase change enthalpy. C_p is the specific heat capacity of water and a constant of 4.18 J g⁻¹ K⁻¹, ΔT is the temperature increase of water, ΔH_vap is the enthalpy of vaporization (2257 kJ kg⁻¹ at 1 atm for water), and ṁ is the evaporation rate, which subtracts the evaporation rate at darkfield to eliminate the influence of intrinsic water evaporation (0.22 kg m⁻² h⁻¹).³⁸ Under one sun irradiation, the evaporation rate of pure water, GO and GO/CuO-2 film was 0.39 kg m⁻² h⁻¹, 1.51 kg m⁻² h⁻¹ and 1.71 kg m⁻² h⁻¹ with the evaporation efficiencies of 11.1%, 85.2% and 99.2%, respectively. Upon increasing the irradiation power, the evaporation rate and the corresponding efficiency increased. Remarkably, the evaporation rate and efficiency of the GO/CuO-2 film were the highest among the films under any light irradiation.

Fig. 6 (a) Evaporation rate and (b) efficiency of pure water, GO and GO/CuO-2 film, digital image of the surface temperature of (c) GO film and (d) GO/CuO-2 film with respect to time, and water contact angle of (e) GO film and (f) GO/CuO-2.

Basically, the majority of the photothermal energy enabled by the GO/CuO-2 film triggers evaporation, while the rest of the energy is lost through radiation, conduction and convection. Specifically, the radiation loss can be calculated using the Stefan–Boltzmann equation, as follows:

Φ = εAσ(T₁⁴ − T₂⁴) (5)

where Φ denotes the heat flux, ε is the emissivity of the GO/CuO-2 film (∼0.75), A is the evaporation area, σ is the Stefan–Boltzmann constant (5.67 × 10⁻⁸ W m⁻² K⁻⁴), and T₁ and T₂ are the surface temperature of the film under sunlight (319.55 K) and the ambient temperature (291.65 K), respectively. Thus, the radiation loss approaches 13.7%. Further, the convection loss can be calculated using eqn (6), as follows:

Φ = hAΔT (6)

where Φ is the heat flux, h is the convection heat transfer coefficient (5 W m⁻² K⁻¹), A is the area of evaporation, and ΔT is the temperature difference between the surface of the GO/CuO-2 film and the air. Consequently, the convection loss was ∼13.9%. Besides, the heat conduction was calculated using eqn (7), as follows:

Q = CmΔT (7)

where Q is the heat energy, C is the specific heat capacity of water (4.18 J g⁻¹ K⁻¹), m is the weight of the water at the bottom (15.62 g), and ΔT is the temperature difference of the water at the bottom before and after evaporation (6.1 K).³⁶ Given that the GO/CuO-2 film was separated from the bottom water by the polyurethane foam, the direct heat conduction from the GO/CuO-2 film to water can be ignored. Thus, the temperature increase in the bottom water mainly comes from the heat conduction by the environment. According to eqn (7), the energy gained by the ISSG device from the environment through heat conduction is 31.7%.

Fig. 6c and d show digital images of the surface temperature associated with GO and GO/CuO-2 during evaporation for different irradiation times, respectively. Obviously, a higher surface temperature was obtained using the GO/CuO-2 film compared to the GO film. Fig. 6e and f show the water contact angle of the GO and GO/CuO-2 films, respectively. Once water was dropped on the film, the contact angle of GO and GO/CuO-2 was 88.5° and 85.5°, respectively. Then, it immediately decreased to 74.6° for the GO/CuO-2 film, which is much smaller than that of the GO film (82.5°). As the contact time was prolonged to 60 s, the contact angle of GO/CuO-2 was 57.4°, whereas it was 74.8 for the GO film. Thus, GO/CuO-2 exhibited hydrophilic property compared to GO film, which enabled a fast water transport rate.

To explore the real desalination performance of the GO/CuO-2 film, simulated 35 000 ppm seawater was used as the target solution. Fig. 7a and b show the evaporation rate and evaporation efficiency of GO/CuO-2 under different light irradiations, respectively, where the GO film was employed as the control group. Under one sun irradiation, the evaporation rate of the seawater, GO and GO/CuO-2 film was 0.4 kg m⁻² h⁻¹, 1.48 kg m⁻² h⁻¹ and 1.65 kg m⁻² h⁻¹ with the corresponding efficiency of 11.1%, 83.4% and 95.2%, respectively. Likewise, both the evaporation rate and efficiency were proportional to the light irradiation for all the films. Most importantly, GO/CuO-2 possessed the highest evaporation rate and efficiency among the samples under any sun irradiation. Further, the durability of the GO/CuO-2 film was tested under one sun, where each test lasted for 2 h, as shown in Fig. 7c. After 20 tests, the evaporation rate of the GO/CuO-2 film remained above 1.60 kg m⁻² h⁻¹, while the evaporation rate of the GO film remained stable at ∼1.40 kg m⁻² h⁻¹.

Fig. 7 (a) Evaporation rate and (b) efficiency of the simulated seawater treated by the GO and GO/CuO-2 film under different light irradiations, (c) regeneration of the GO and GO/CuO-2 film in 20 cycles, and (d) concentration of cations in the raw and desalinated stream.

To evaluate the desalination capability of the GO/CuO-2 film, inductively coupled plasma (ICP, icap6300) spectroscopy was employed to measure the variation in the concentration of K⁺, Ca²⁺, Na⁺ and Mg²⁺ in the simulated seawater by ISSG. As shown in Fig. 7d, the concentration of K⁺ (481.39 mg L⁻¹), Ca²⁺ (392.06 mg L⁻¹), Na⁺ (9856.15 mg L⁻¹) and Mg²⁺ (1443.15 mg L⁻¹) was reduced to 1.9 mg L⁻¹, 0.75 mg L⁻¹, 8.67 mg L⁻¹ and 0.32 mg L⁻¹ after the ISSG by the GO/CuO-2 film, respectively. Obviously, the concentration of the target ions was reduced greatly, which is in accordance with the standard for drinking water defined by the World Health Organization (WHO, less than 200 mg L⁻¹).

To verify the feasibility of the GO/CuO-2 film in practical applications, natural water extracted from the Jinbo Lake of Ningxia University was taken as the target solution. The ion concentration of Jinbo Lake water was presented in a previous report.²⁶ As shown in Fig. 8a, the evaporation rate of the GO/CuO-2 film was always between 1.68–1.71 kg m⁻² h⁻¹ after 10 cycles. Further, the outdoor evaporation test of the GO/CuO-2 film was carried out in Ningxia University with Jinbo Lake water on Nov 20, 2022, as shown in Fig. 8b–d. It can be seen that the outdoor ambient temperature was very low. At 10:00, the ambient temperature was only 1 °C, while the surface temperature of the GO/CuO-2 film was 8.6 °C. At any time, the surface temperature of the GO/CuO-2 film was much higher than the ambient temperature. With a gradual increase in solar irradiance from 0.56 kW m⁻² to the maximum value of 0.79 kW m⁻², the evaporation rate of the GO/CuO-2 film rapidly increased from 0.29 to 1.62 kg m⁻² h⁻¹. After 12:00, the solar irradiance decreased rapidly, and finally dropped to 0.42 kW m⁻² at 16:00, while the evaporation rate of the GO/CuO-2 film decreased to 0.37 kg m⁻² h⁻¹.

Fig. 8 (a) Cycle performance of the GO/CuO-2 film using Jinbo Lake water, outdoor demonstration test of (b) atmosphere temperature and (c) surface temperature of the GO/CuO-2 film, and (c) solar irradiance and (d) evaporation rate of GO/CuO-2 film.

In terms of organic pollutant removal, 5 mg L⁻¹ methylene blue (MB) solution and methyl orange (MO) solution was used. Fig. 9a and b show the UV absorption spectra of MB solution and MO solution before and after removal, respectively. The removal rate can be calculated using eqn (8), as follows:

where C₀ is the absorbance of MB/MO solution before condensation and C_t is the absorbance of MB/MO solution after condensation.³⁹ According to the calculation, the removal efficiency was 99.3% and 93.5% for MB and MO, respectively, implying that the GO/CuO-2 film has excellent removal ability for organic pollutants. Fig. 9c summarizes the performance of various evaporators prepared using GO or CuO.^{28,31,40–49} Clearly, GO/CuO-2 demonstrated an exceptional evaporation rate and efficiency, making a promising candidate for high-performance ISSG.

Fig. 9 UV-vis absorption spectra of (a) MB and (b) MO solution before/after treatment, and (c) comparison of the ISSG performance of various evaporators.

Conclusions

A 2D GO/CuO composite film was developed for high-performance ISSG with enhanced solar absorbance. The CuO confined in the GO layers exhibited the typical urchin-like structure, which is beneficial to enlarge the layer spacing of GO, and thus improve the incident light absorbance. Consequently, the highest optical absorption of the GO/CuO film was 83%, which was much higher than that of the GO film (74%). The enhancement in light absorption induced by the CuO nanoparticles was further illustrated using the COMSOL simulation software. Towards desalination, the evaporation rate and efficiency of the optimized GO/CuO film under 1 kW m⁻² irradiation were 1.71 kg m⁻² h⁻¹ and 99.2%, while the corresponding values were 1.51 kg m⁻² h⁻¹ and 85.2% for the GO film, respectively. Further, the contact angle of the GO/CuO and GO film was 57.4° and 74.8°, respectively, highlighting the superior water transport rate of the GO/CuO film. In the case of synthetic seawater, the concentration of various cations could be effectively reduced by the GO/CuO ISSG device, and the treated water met the drinking water standards. Besides, the GO/CuO film also demonstrated strong ability to remove organic pollutants.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Natural Science Foundation of Ningxia (2022AAC05014).

Notes and references

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