A plasmonic interfacial evaporator for high-efficiency solar vapor generation

Abstract

The increasing energy and environmental concerns have spurred enormous research interest towards developing various renewable energy and sustainable environmental solutions. Photothermal conversion for interfacial solar vapor generation is a promising, green energy technology and efficient route for desalination and purification of seawater, i.e. for those parts where freshwater shortage is a severe concern and clean energy is not available. Eco-friendly, highly efficient and low-cost interfacial evaporators are highly desirable for the practical and widespread application of this technology. In this work, we have demonstrated a novel interfacial evaporator employing Cu₉S₅ nanonets with heterogeneous hexagonal holes as the photothermal conversion material and a microporous poly(vinylidene fluoride) membrane (PVDFM) as the supporting material. The Cu₉S₅/PVDFM evaporator displays a broadband (from 250 to 2000 nm) and large (∼91.7%) solar absorptance. The porous structures of Cu₉S₅ nanonets and PVDFM facilitate the water transportation, and the large optical absorption of Cu₉S₅/PVDFM converts most of the solar energy to thermal energy, producing water vapor with high efficiency. The Cu₉S₅/PVDFM evaporator exhibits solar vapor generation efficiencies of 80.2 ± 0.6% and 91.5 ± 1.1% under one-sun and four-sun irradiation, respectively, making it among the best copper sulphide-based solar evaporators reported so far. This Cu₉S₅/PVDFM evaporator is reusable, flexible, highly efficient, easy to prepare, easy to scale up, and controllable for tailoring, showing a promising future for interfacial solar vapor generation.

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Introduction

The ever-increasing clean energy and environmental needs have spurred enormous research interest around the world towards developing various renewable energy and environmental solutions. Efficient utilization of solar energy in various forms has provided good promise and also expanded our vision on the means of using solar energy, which is clean and free of the concerns of depletion. Among the various possible uses of solar energy,^1–6 photothermal conversion is one of the most promising and efficient approaches that can efficiently produce hot water and distilled clean water with high efficiency. Today, because of the increasing population growth, and extensive industrial and environmental contamination, many parts of the world are suffering from a serious freshwater shortage problem.^7,8 Interfacial solar vapor generation is becoming very attractive and efficient for seawater desalination, compared to the conventional route of heating up bulk water to generate steam/vapor. Interfacial solar vapor generation can localize the solar energy at the interface where vapor is generated and decrease the heat loss in the water evaporation process, enhancing the solar vapor generation efficiency and rate under low intensity of solar irradiation,⁹ because it does not need to heat the bulk water beneath the air–water interface that does not take part in generating vapor but also consumes the absorbed solar energy.

The structures of the interfacial evaporators are critical to achieve high efficiency.^10–14 Many types, such as integrated structure,^10,11 bi-layer structure^12,13 and multi-layer structure,¹⁴ have been reported. Photothermal conversion materials and supporting materials play key roles in these structures. Ideal photothermal conversion materials should have a large and wide absorption in the solar spectrum and be able to convert the absorbed solar energy into heat efficiently, and ideal supporting materials should have low density, low thermal conductivity, high porosity and excellent thermal stability. Common photothermal conversion materials include carbon-based materials (e.g., exfoliated graphite,¹⁵ carbon nanotube,¹⁶ graphite powder,¹⁷ graphene,¹⁸ graphene oxide,¹⁹ and reduced graphene oxide^12,20), plasmonic-metals (e.g., Au,^21–26 Ag,^27,28 Al,²⁹ and Pd³⁰), plasmonic-ceramics (e.g., TiN³¹ and Ti₃C₂ (ref. 32 and 33)), plasmonic-semiconductors (e.g., Cu_2−xS,^34–38 Cu_2−xSe,³⁹ Cu_2−xTe,⁴⁰ W₁₈O₄₉,^41,42 and WO_3−x (ref. 43)), and other light-harvesting materials (e.g., Ti₂O₃,⁴⁴ MoS₂,^45,46 and carbonized mushroom⁴⁷). And the supporting materials include paper-,^24,25,48 wood-,^30,49,50 foam-,¹⁵ membrane-,^10,12,17 gel-,^51–53 and gauze-based materials.⁵⁴ Although higher efficiencies are becoming feasible due to continuous development in this field, an ideal system needs not only high efficiency, but also an eco-friendly, efficient, cost-effective interfacial solar evaporator. The components inside should be inexpensive and non-toxic, and have long lifetime.

Gold nanoparticles have been investigated extensively in the solar-energy-harvesting field due to their good capability in absorbing sunlight effectively in a wide wavelength range from localized surface plasmon resonance (LSPR). For instance, Bae et al.²¹ reported a black gold membrane for solar vapor generation with a water evaporation efficiency of ∼57% under 20 sun. Naik et al.²² reported a plasmonic Au aerogel for water evaporation with a steam generation efficiency of ∼76.3% under 808 nm laser irradiation with a power density of 51 kW m⁻². Zhu et al.²³ reported an Au/nanoporous alumina template composite for solar vapor generation with a water evaporation efficiency of ∼65% under 4 sun. Deng et al.²⁴ reported an airlaid-paper-based Au nanoparticle film for solar vapor generation with a water evaporation efficiency of ∼77.8% under 4.5 sun. He et al.²⁵ reported an Au/filter paper composite for solar steam generation with an efficiency value of ∼85% under 10 sun. He et al.²⁶ reported an Au/poly(p-phenylene benzobisoxazole) nanofiber composite for solar steam generation with an efficiency value of ∼83% under 1 sun. Despite the gradual increase in the solar vapor generation efficiency of plasmonic-Au based evaporators, one major demerit is that the gold nanoparticles are easily aggregated and fused together at high temperature resulting from a long duration of solar irradiation, causing degradation of the water evaporation performance over time. The other major obstacle for gold-based evaporators is the high cost, which limits their widespread, large-scale production, even if the efficiency can be very high.

Eying to solve the high cost problem that gold-based evaporators have, other solar radiation absorbing materials are sought with LSPR as well as with some transition metal semiconducting materials.^31,32,55,56 For example, various solid phases of Cu_2−xS (0 ≤ X ≤ 1) have been reported, ranging from copper-rich Cu₂S to copper-poor CuS (e.g. chalcocite Cu₂S,⁵⁷ djurleite Cu_1.96S,⁵⁸ digenite Cu_1.8S,³⁶ anilite Cu_1.75S,³⁸ and covellite CuS³⁴) and with various nanostructures and morphologies (e.g., nanorods,³⁵ nanoplates,⁵⁹ nanonets,³⁶ nanowires,⁶⁰ nanoflowers,³⁴ nanotubes,⁶¹ and nanovesicles⁶²). Compared with other plasmonic materials (e.g., Au^21–26 and Ag^27,28), copper sulphide semiconductor materials display low cost, low cytotoxicity, and outstanding light-stability features, and have been regarded as some of the most promising green and sustainable energy materials for solar vapor generation,^13,34,35,63etc. For example, Wang et al.⁶³ reported an evaporator composed of CuS nanoparticles and a polyethylene hybrid membrane for solar vapor generation with a water evaporation efficiency of ∼63.5% under 1 sun. In our previous work,³⁴ an evaporator comprising CuS nanoflowers and a semipermeable collodion membrane (SCM) had a water evaporation efficiency of ∼68.6% under 1 sun. Wang et al.¹³ reported bi-layer structural evaporators composed of Cu_xS and mixed cellulose ester (MCE) film for solar vapor generation, e.g., an octahedral Cu₉S₅ nanocrystal/MCE composite with an evaporation efficiency of ∼60.1%, and granular CuS nanocrystal/MCE composite with an appreciable efficiency of ∼80% under 1 sun, respectively. The photothermal conversion material of Cu_xS nanoparticles was filtered on the surface of MCE film by a vacuum filtration method to form a bi-layer structural evaporation system.¹³ Although high water evaporation efficiency has been demonstrated, the Cu_xS nanoparticle located at the top layer could detach from the bottom layer (the MCE film) after a long duration of solar irradiation or/and high-usage levels due to the low adhesion between the top and bottom layers. Besides, the effect of natural water evaporation is very important and should be subtracted when calculating the water evaporation efficiency under solar irradiation, which was, however, neglected in some previous studies.^13,34,63 In view of this, the actual evaporation efficiencies of the interfacial evaporators in the above-mentioned studies are lower than what they reported. Therefore, it is essential to prepare an evaporator with high solar water evaporation efficiency and excellent reusability for solar vapor generation.

In this work, we report an integrative membraneous interfacial evaporator comprising Cu₉S₅ nanonets with heterogeneous hexagonal holes as a light-harvesting material and PVDFM as a supporting material for solar vapor generation. PVDFM has already been previously reported as an outstanding supporting material to prepare several desalination devices due to its major benefits of low density, microporous structure, and good hydrophilicity.^{10,27,32,64,65} Here, our Cu₉S₅/PVDFM device has several advantages: (1) broadband (250–2000 nm) and large solar absorptance (∼91.7%). (2) Unique porous structures. Both Cu₉S₅ and PVDFM are porous, i.e., the Cu₉S₅ nanonets have a unique heterogeneous hexagonal hole structure, and the PVDFM has a microporous structure. These unique porous structures can together facilitate the water transportation. (3) Excellent physical compatibility. Cu₉S₅ is distributed uniformly in the integrative membraneous Cu₉S₅/PVDFM composite, including the interior or/and the surface of PVDFM. (4) Easy fabrication process. The preparation procedure of the Cu₉S₅/PVDFM composite is easy and simple. (5) Low cost. The production cost of Cu₉S₅/PVDFM is low. (6) Excellent durability and reusability. This Cu₉S₅/PVDFM has been reused over 20 cycles without any decrease of water evaporation performance, showing outstanding reusability. Impressively, high water evaporation efficiencies of 80.2 ± 0.6% and 91.5 ± 1.1% were obtained under 1 sun and 4 sun irradiation, respectively. Therefore, it has shown great potential for solar vapor generation, and may be beneficial for other solar-heating applications such as sterilization, power generation, and solar distillation.

Results and discussion

The Cu₉S₅ nanonets showed a wide optical-absorption in the visible wavelength region of 300–700 nm due to the bandgap (∼1.5 eV) absorption and an increased light absorption in the NIR region due to the LSPR of the free carrier in Cu₉S₅ nanonets as shown in Fig. 1A. It is well known that an ideal solar-harvesting material should have an electromagnetic radiation absorption as high as possible over the full solar spectrum wavelength range (300–2500 nm), in which the visible wavelength region is from 400 to 700 nm, occupying ∼45% of the solar energy, and the IR wavelength region is from 700 to 2500 nm, accounting for ∼52% of solar energy.^44,53 The inset of Fig. 1B shows a photo of the Cu₉S₅/PVDFM disk with a diameter of 33.5 mm. To confirm its light-absorption capability, the UV-vis-NIR absorption spectrum was measured, calculated from its transmittance and diffuse reflectance spectra. The pure PVDFM was white and opaque with near-zero transmittance, high (∼95%) diffuse reflectance and very low (5%) absorption in the entire solar spectrum region, consistent with previous reports,^10,32,64 while the Cu₉S₅/PVDFM was black and had near-zero transmission, low reflection (<9%), and high (>91%) absorption, as shown in the transmission/reflection/absorption spectra in Fig. 1B. Specifically, it can absorb ∼91.4% of UV, ∼92.4% of visible, and ∼90.9% of infrared solar irradiation energy with a total solar absorptance of ∼91.7% in the 250–2000 nm wavelength region, as shown in Table 1.

Fig. 1 (A) UV-vis-NIR absorption spectrum of the as-obtained Cu₉S₅ nanonets. (B) UV-vis-NIR transmission/reflection/absorption spectra and digital photograph (inset of (B)) of 8 mg-Cu₉S₅/PVDFM.

Table 1 Solar absorption of Cu₉S₅/PVDFM in different solar spectrum regions

Sample	UV (250–400 nm)	Visible (400–760 nm)	Infrared (760–2000 nm)	Solar absorptance
AM 1.5	∼7%	∼50%	∼39.7%	∼91.7%
Cu9S5/PVDFM	∼6.4%	∼46.2%	∼36.1%

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Furthermore, the structure and morphology of Cu₉S₅/PVDFM were analyzed using a field-emission scanning electron microscope and a Leica microscope, respectively. A unique porous nanonet structure of Cu₉S₅ with heterogeneous hexagonal holes was seen in Fig. 2A and B, and it could effectively facilitate the transport of water from the bulk water to the water–air interface where water vapor generates. Meanwhile, the surface morphology of Cu₉S₅/PVDFM was a poly-porous structure with an average open size of 200 nm as shown in Fig. 2C and D. As shown in Fig. 2E, the initial surface contact angle was 75 ± 1° for the bottom side, showing a hydrophilicity which could facilitate the transport of the water from the bulk beneath the film to the water–air interface. The contact angle for the upper side was 125 ± 1° (see Fig. 2F), which demonstrated a good hydrophobicity that could benefit for localizing heat around the water drops on the film. The unique hydrophilic–hydrophobic characteristics of the film were easily recognized in that the bottom side (i.e., hydrophilic side) was smooth and the upper side (i.e., hydrophobic side) was rough, in accordance with a previous report.¹⁰

Fig. 2 (A) SEM image and (B) TEM image of Cu₉S₅ nanonets with heterogeneous hexagonal holes. SEM image (C) and optical microscope image (D) of 8 mg-Cu₉S₅/PVDFM. The surface contact angles of the bottom side (E) and upper side (F) of 8 mg-Cu₉S₅/PVDFM.

To study the solar vapor generation performance, water evaporation tests were performed on a series of Cu₉S₅/PVDFMs with different amounts of Cu₉S₅ nanonets added (i.e., 1, 2, 4, 8 and 12 mg per disk). Fig. 3A shows the weight changes of pure water covered with Cu₉S₅/PVDFMs under 1 sun irradiation. For comparison, the weight changes of pure water and pure water covered with pure PVDFM under 1 sun were also measured. Here, it is important to note that the weight change of pure water under a dark environment was also determined and subtracted from all the measured weight changes under solar irradiation to eliminate the effect of natural water evaporation. The average weight changes (i.e., the water evaporated weights) increased from 0.275 to 0.396, 0.526, 0.586 and 0.580 kg m⁻² when the amount of Cu₉S₅ nanonets increased from 1 mg to 12 mg. The increased content of Cu₉S₅ in Cu₉S₅/PVDFMs from 1 mg to 8 mg improved the amount of water evaporated by the film; however, adding excess amounts of Cu₉S₅ nanonets (e.g. 12 mg) may block the microporous channels of Cu₉S₅/PVDFM and inhibit the water transport and the release of water vapor, leading to a decrease of the amount of water evaporated. Therefore, the 8 mg-Cu₉S₅/PVDFM showed the maximum water evaporation weight, which was 2.8 and 3.6 times higher than that of pure PVDFM under 1 sun and pure water under 1 sun, respectively.

Fig. 3 The as-prepared samples for solar vapor generation. Weight changes (A) and evaporation rates (B) of pure water under a dark environment, pure water under 1 sun, pure water covered with pure PVDFM and pure water covered with Cu₉S₅/PVDFMs with different contents of Cu₉S₅ under 1 sun. The weight changes (C) and evaporation rates (D) of pure water covered with 8 mg-Cu₉S₅/PVDFM under 1, 2 and 4 sun, respectively. All above data are shown as the mean with error bars. (E) The solar vapor generation cycle performance of 8 mg-Cu₉S₅/PVDFM under 1, 2 and 4 sun, respectively.

To evaluate the water evaporation performance, the water evaporation rates (v) were calculated using the following equation (eqn (1))^17,34,35

where m_loss stands for the weight losses of the device due to water evaporation, D is the diameter of the as-prepared interfacial evaporator, and t represents the irradiation time of each water evaporation experiment. Likewise, to eliminate the effect of natural water evaporation, the evaporation rate of pure water under a dark environment was subtracted from all the solar-irradiation water evaporation rates. Fig. 3B shows the average water evaporation rates, which followed this order: 1 mg-Cu₉S₅/PVDFM < 2 mg-Cu₉S₅/PVDFM < 4 mg-Cu₉S₅/PVDFM < 12 mg-Cu₉S₅/PVDFM < 8 mg-Cu₉S₅/PVDFM, with the corresponding water evaporation rates of 0.550, 0.793, 1.053, 1.159, and 1.173 kg m⁻² h⁻¹, respectively. Obviously, the highest water evaporation rate was obtained with the 8 mg-Cu₉S₅/PVDFM.

To further investigate the solar vapor generation performance of Cu₉S₅/PVDFM, water evaporation tests were carried out under various solar intensities (i.e., from 1 sun to 4 sun). As shown in Fig. 3C, the average weights of the water evaporated by the 8 mg-Cu₉S₅/PVDFM were 0.586, 1.264 and 2.640 kg m⁻² under 1, 2 and 4 sun, respectively. Obviously, the water evaporation weights increased with the increased irradiation light intensity, so did the water evaporation rates (see Fig. 3D). As shown, the average water evaporation rates were 1.173, 2.527 and 5.280 kg m⁻² h⁻¹ under 1, 2, and 4 sun, respectively. Also, the solar evaporation rate of 5.280 kg m⁻² h⁻¹ under 4 sun was 12.5 and 16.1 times higher than that of pure PVDFM under 1 sun and pure water under 1 sun, respectively. This indicated an outstanding solar evaporation performance of the Cu₉S₅/PVDFM. To evaluate the reusability and durability of Cu₉S₅/PVDFM, the water evaporation tests of 8 mg-Cu₉S₅/PVDFM were repeated 20 times under different light irradiation densities. As showed in Fig. 2E, the corresponding water evaporation rates were close to 1.173, 2.527 and 5.280 kg m⁻² h⁻¹ under 1, 2 and 4 sun, respectively, indicating a stable performance of solar vapor generation of Cu₉S₅/PVDFM.

The IR photos of interfacial evaporators were further used to determine the surface temperatures of the evaporator under different intensities of solar irradiation. The initial temperature of pure water was about 25 °C for each solar vapor generation test. Fig. 4A–F show the IR photos of the temperatures after 30 min for pure water under a dark environment, pure water under 1 sun, pure water covered with pure PVDFM under 1 sun, and pure water covered with the as-prepared 8 mg-Cu₉S₅/PVDFM under 1, 2 and 4 sun irradiation, respectively. Accordingly, the surface temperatures were about 24.9, 30.7, 32.2, 36.1, 45.2 and 54.3 °C, after the corresponding solar vapor generation tests, as shown in the plot of temperatures change of Fig. 4G. As shown in Fig. 4H, the vapor generated was clearly observed under 4 sun.

Fig. 4 IR photos of pure water under a dark environment (A), pure water under 1 sun (B), pure water covered with pure PVDFM (C), and pure water covered with 8 mg-Cu₉S₅/PVDFM under 1, 2 and 4 sun (D–F), respectively; (G) the plot of their surface temperatures change; and (H) a photo of the vapor generated under 4 sun irradiation.

As is well known, another important parameter to evaluate the solar vapor generation performance is solar vapor generation efficiency (η), calculated based on the following equation (eqn (2))^46,48

where v denotes the evaporation rate, C represents the specific heat capacity of water (4.18 J g⁻¹ K⁻¹), ΔT is the temperature variation from the initial temperature (∼24.9 °C) of water to the final vapor temperature, Δ_vapH_m is the liquid–vapor phase change enthalpy, C_opt is the optical concentration and q_i is the incident solar irradiation density of 1 kW m⁻². Table 2 shows the details of the parameters of the solar vapor generation tests with different samples at different optical concentrations. As calculated using the above equation (eqn (2)), the water evaporation efficiencies of 8 mg-Cu₉S₅/PVDFM were about 80.2, 87.0 and 91.5% under 1, 2 and 4 sun, respectively. In comparison, the water evaporation efficiencies were only about 22.2 and 28.7% for pure water and pure water covered by pure PVDFM, respectively. As shown, the water evaporation efficiencies increased with the increased optical concentrations from 1 sun to 4 sun; further increase of solar intensity may lead to higher efficiency. However, considering that the practical application for solar vapor generation is outdoors under a solar intensity of ∼1 kW m⁻², further increasing the solar intensity is unnecessary. Interestingly, the water evaporation efficiencies of 91.5 ± 1.1% under 4 sun were very close to the value of solar absorptance of ∼91.7%. This might be attributed to the fact that the temperature of the bulk water beneath the air–water interface increased sharply under the higher simulated sunlight irradiation (e.g., 4 sun), as found in the IR photo of Fig. 4F. So the higher temperature of the bulk water beneath the air–water interface could accelerate the water evaporation. Moreover, the high bulk water temperature may cause some water bubbles to transpire into air directly without the phase transition from liquid to vapor.²⁶ So it is possible for the absorbers that the solar vapor efficiencies are close to or higher than the value of solar absorptance.^66,67

Table 2 Efficiency of the different samples at different optical concentrations

C opt	Sample	v (kg m^−2 h^−1)	ΔT (°C)	CΔT (kJ kg^−1)	ΔvapHm (kJ kg^−1)	η (%)
1	Pure water	0.328 ± 0.006	5.6 ± 0.5	23.41 ± 2.09	2427	22.2 ± 0.4
1	PVDFM	0.421 ± 0.007	7.3 ± 0.7	30.51 ± 2.93	2424	28.7 ± 0.5
1	8 mg-Cu9S5/PVDFM	1.173 ± 0.007	11.4 ± 0.9	47.65 ± 3.76	2415	80.2 ± 0.6
2	8 mg-Cu9S5/PVDFM	2.527 ± 0.010	20.6 ± 1.3	86.11 ± 5.43	2393	87.0 ± 0.7
4	8 mg-Cu9S5/PVDFM	5.280 ± 0.050	29.6 ± 2.1	123.73 ± 8.78	2371	91.5 ± 1.1

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In Table 3, we compared some of the water evaporation efficiencies of recent reports using different interfacial plasmonic evaporators (e.g., plasmonic-metal-based,^{21–26,29,30} plasmonic-ceramic-based,^31–33 and plasmonic-semiconductor-based^{13,34,35,38,41,63}). Here, it is noteworthy that the values of efficiency labelled by an asterisk are inaccurate due to neglecting the effect of natural water evaporation. To our knowledge, the effect of natural water evaporation is considerable and should be subtracted while calculating the water evaporation efficiency under solar irradiation. We estimate that the data could decrease ca. 10%. For instance, in this work, the efficiency of water evaporation of 8 mg-Cu₉S₅/PVDFM is ∼80.2% under 1 sun; however, the corresponding evaporation efficiency is ∼90% if the evaporation rate of pure water under a dark environment is not subtracted. In view of this, the water evaporation efficiency of our Cu₉S₅/PVDFM of ∼80.2% under 1 sun irradiation is top-ranking among the three plasmonic evaporators reported so far. Furthermore, with the advantages of the straightforward fabrication process, the inexpensive materials, and the excellent reusability, the Cu₉S₅/PVDFM demonstrated its strong competitiveness over other materials for solar vapor applications.

Table 3 Comparison of solar vapor generation performance of various interfacial plasmonic evaporators^a

Interfacial plasmonic evaporators	Classification of light-harvesting materials	Light sources	Power density (kW m^−2)	Efficiency	Ref.
a The symbol “*” refers to the data without subtracting natural water evaporation in their works.
Au/Al2O3 composite	Plasmonic-metal	Solar simulator	1	∼49%*	23
Al/Al2O3 composite	Plasmonic-metal	Solar simulator	1	∼57.5%	29
Au/filter paper composite	Plasmonic-metal	Solar simulator	1	∼62.5%*	25
Pd/wood composite	Plasmonic-metal	Solar simulator	1	∼69%	30
Au/PBO nanofiber composite	Plasmonic-metal	Solar simulator	1	83%	26
Paper-based AuNP film	Plasmonic-metal	Solar simulator	4.5	∼77.8%*	24
Black Au membrane	Plasmonic-metal	Solar simulator	20	∼57%	21
Au aerogel	Plasmonic-metal	808 nm laser	51	76.3%*	22
TiN/microfiber composite	Plasmonic-ceramic	Solar simulator	1	>80%*	31
Ti3C2/PVDFM	Plasmonic-ceramic	Solar simulator	1	84%*	32
Ti3C2/MCE membrane	Plasmonic-ceramic	Solar simulator	1	71%	33
W18O49/PTFE membrane	Plasmonic-semiconductor	Solar simulator	1	80.7%*	41
Cu7S4 nanocrystals film	Plasmonic-semiconductor	Infrared lamp	1	∼77.1%	38
CuS/PE membrane	Plasmonic-semiconductor	Solar simulator	1	∼63.9%*	63
CuS/SCM	Plasmonic-semiconductor	Solar simulator	1	∼68.6%*	34
Cu9S5/MCE membrane	Plasmonic-semiconductor	Solar simulator	1	∼60.1%*	13
CuS/MCE membrane	Plasmonic-semiconductor	Solar simulator	1	80 ± 2.5%*	13
CdS–Cu7S4 film	Plasmonic-semiconductor	Solar simulator	1.5	∼48.4%*	35
Cu9S5/PVDFM	Plasmonic-semiconductor	Solar simulator	1	80.2 ± 0.6%	This work

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Conclusions

In summary, we have demonstrated a Cu₉S₅/PVDFM with an integrative porous membrane structure, for solar vapor generation. The Cu₉S₅/PVDFM exhibits a large solar absorptance of ∼91.7% in the 250–2000 nm wavelength region, and enables the water evaporation efficiencies to reach 80.2 ± 0.6% and 91.5 ± 1.1% under the solar irradiation intensities of 1 sun and 4 sun, respectively. It has been reused over 20 times without performance degradation, exhibiting excellent reusability and durability. With its simple preparation technology, low-cost feature and high water evaporation efficiency, the Cu₉S₅/PVDFM composite shows great potential for solar vapor generation, and likely for other applications as well, such as water purification, sterilization, distillation, and desalination.

Experimental

Materials

N,N-Dimethylformamide (DMF) was purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. Poly (vinylidene fluoride) (PVDF) was purchased from Shandong Xiya Reagent Co. Ltd. Deionized water was purified using a Milli-Q system (Millipore).

Preparation of the Cu₉S₅/PVDF membrane

The preparation and characterization of Cu₉S₅ nanonets were based on the report in our previous work.³⁶ Specifically, the Cu₉S₅ obtained at a reaction time of 18 h was chosen as the photothermal conversion material. The Cu₉S₅/PVDFMs were prepared as follows.^10,27,65 Firstly, 2 g PVDF powder was added into 30 mL DMF solution and stirred to get a PVDF/DMF solution. Then, 116 mg Cu₉S₅ was added into a 10 mL beaker containing 1 mL PVDF/DMF solution to form a Cu₉S₅/PVDF/DMF solution mixture under sonication. The Cu₉S₅/PVDF/DMF solution was transferred with a 3 mL pipette to a flat glass surface, and cast with a 150 μm gap scraper to form the Cu₉S₅/PVDFM composite. The flat glass coated with Cu₉S₅/PVDFM was immersed in a deionized water bath for 0.5 h and the Cu₉S₅/PVDFM film was peeled off from the flat glass, washed with ethanol several times and dried in a vacuum oven at 45 °C for 1.5 h. Finally, the Cu₉S₅/PVDFM was trimmed into a round disk shape with a diameter of 33.5 mm. Hence, the Cu₉S₅/PVDFM with 8 mg Cu₉S₅ nanonets added (i.e., 8 mg-Cu₉S₅/PVDFM) was fabricated. The same method was used to prepare other samples with various amounts of Cu₉S₅ light absorber, e.g., 1 mg-Cu₉S₅/PVDFM, 2 mg-Cu₉S₅/PVDFM, 4 mg-Cu₉S₅/PVDFM, 12 mg-Cu₉S₅/PVDFM, and pure PVDFM (i.e., without any Cu₉S₅ nanonets added).

Characterization

The morphology and structure were analyzed with a field-emission scanning electron microscope (JSM-6700F, Japan) and Leica microscope (DM500, Germany). The contact angle was measured with a contact-angle analyzer (JC2000D1, China). The UV-vis-NIR transmittance and diffuse reflectance spectra were measured on a Lambda 950 UV-vis-NIR scanning spectrophotometer (PerkinElmer, America). The IR digital pictures were captured with an FLIR camera (E40, America).

Solar vapor generation

The solar vapor generation tests were conducted using a homemade chamber optical measurement system, with a PLS-SXE300/300UV solar simulator and other optical components (Perfect Light Technology Co. Ltd, Beijing). The optical irradiation intensities from 1 to 4 sun (kW m⁻²) were measured with an optical power meter (VLP-2000, China). The Cu₉S₅/PVDFM was placed in a 40 mm × 25 mm weighing bottle containing 20 mL deionized water as the solar vapor generation setup. Here, it is important to note that due to the difference of the hydrophilic–hydrophobic properties of the upper and bottom sides of the Cu₉S₅/PVDFM, the hydrophilic surface (called the bottom side) of the Cu₉S₅/PVDFM was placed face down so that it can establish contact with bulk water directly, so the hydrophobic surface (called the upper side) was placed upward. The weight change during solar vapor generation was recorded using a 4 decimal electronic precision balance (OHRUS FR224CN, accuracy: 0.1 mg). For cycling water evaporation tests, after each cycle, the wetted Cu₉S₅/PVDFM was dried under ambient conditions to simulate the actual usage.

Author contributions

F. Tao, K. Yin, and S. Cao prepared the samples. X. Chang, Y. Lei and D. Wang characterized the samples. F. Tao, K. Yin, S. Cao, and D. Wang performed the solar vapor generation tests. F. Tao wrote this manuscript. R. Fan, L. Dong, and Y. Yin discussed and critically read the manuscript. Y. Zhang and X. Chen designed this project and polished this manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

X. C. is thankful for the support from the U.S. National Science Foundation (DMR-1609061). Y. Z. and X. C. are thankful for the support from the National Key R & D Program of China (no. 2016YFB0300704). Y. Y. acknowledges the State Key Development Program for Basic Research of China (no. 2014CB643306). X. C. appreciates the NSF of Shanghai (no. 17ZR1440900). Y. L. acknowledges the National Nature Science Foundation of China (no. 51602195). F. T. is thankful for the support from the Doctoral Innovation Foundation of SMU (no. 2016ycx037), and the Doctoral Excellent Thesis Project Foundation of SMU (no. 2017BXLP005).

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