Copper sulfide-macroporous polyacrylamide hydrogel for solar steam generation

Chemical Engineering Science 207 (2019) 516–526

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Copper sulfide-macroporous polyacrylamide hydrogel for solar steam generation Yu Sun, Jianping Gao ⇑, Yu Liu ⇑, Huiying Kang, Minhui Xie, Fuming Wu, Haixia Qiu ⇑ Department of Chemistry, School of Science, Tianjin University, Tianjin 300350, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


During solar evaporation, a heating

zone is formed at the air-water interface.
The solar steam generation device has been optimized comprehensively.
Solar photothermal conversion efficiency reached 92%.
The solar steam generation device shows excellent reusability and performance stability.

a r t i c l e

i n f o

Article history: Received 17 January 2019 Received in revised form 19 June 2019 Accepted 24 June 2019 Available online 25 June 2019 Keywords: Solar steam generation Photothermal conversion CuS nanoparticles Macroporous polyacrylamide hydrogel Nano composites

a b s t r a c t The use of abundant solar energy to produce clean water by a solar steam generation device is a promising strategy to solve the long-term water and energy shortage. The development and application of photothermal materials are an effective way to improve solar photothermal conversion efficiency of the device. Herein, an inorganic-organic light-absorbing material, copper sulfide-macroporous polyacrylamide hydrogel (CuS-m-PAM), is reported for the first time. The CuS-m-PAM was prepared by in-situ synthesis method via loading CuS nanoparticles into macroporous polyacrylamide hydrogel (m-PAM). CuS-m-PAM forms a heating zone at the air-water interface and CuS nanoparticles can efficiently convert the strongly absorbed light into localized heat, thus reducing the heat loss in the transfer process. The results demonstrate that the solar photothermal conversion efficiency of CuS-m-PAM-0.05 is the highest and can reach 92% under one sun illumination (1000 W m2). The rough surface of m-PAM and the plasma resonance effect of CuS nanoparticles improve the absorb ability to sunlight. The open macroporous structure of m-PAM also makes a great contribution to the water vapor escape of solar steam generation device. Moreover, the formation of N-Cu bond in CuS-m-PAM makes CuS nanoparticles more stabilization, and the solar photothermal conversion efficiency of CuS-m-PAM-0.05 remains 87.5% after 50 cycles of tests. CuS-m-PAM, as a novel light-absorbing material, can comprehensively optimize the solar steam generation device in light absorption, thermal insulation, water replenishment and steam exhaust, and thus provides a new practical method for maximum utilization of solar steam generation device. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Currently, the shortage of clean water caused by industrial development, climate change and population growth is a serious ⇑ Corresponding authors. E-mail address: [email protected] (H. Qiu). https://doi.org/10.1016/j.ces.2019.06.044 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

global problem that needs to be urgently solved (Elimelech and Phillip, 2011; Haddeland et al., 2014; Mekonnen and Hoekstra, 2016). In view of the abundant and inexhaustible sunlight, the use of solar energy to produce clean water by solar steam generation device seems to be a feasible way to solve the current global challenges of water as well as energy shortages (Chandrashekara and Yadav, 2017; Sharon and Reddy, 2015). Solar steam generation

Y. Sun et al. / Chemical Engineering Science 207 (2019) 516–526

device is the most common solar energy collection device and also clean water production device in the world (Liu et al., 2015a; Neumann et al., 2013; Zhang et al., 2014). The evaporation efficiency of water depends mainly on the light-absorbing material of solar steam generation device. At present, there are four main types of light-absorbing materials: noble metal plasma (gold nanoparticles, self-assembly of gold nanoparticles, and gold film), carbon-based materials (hollow carbon spheres, wood-graphene oxide, porous graphene, and graphene oxide aerogels), near infrared materials (TiOx, layered BiInSe3@nickel foam, SnSe@nickel foam, germanium nanocrystals, and Cu7.2S4), and organic photothermal conversion materials (indocyanine green (ICG) molecules, porphyrin, polyaniline, and polypyrrole) (Li et al., 2014; Liu et al., 2017a; Liu et al., 2017b; Sun et al., 2017; Yao et al., 2017; Yin et al., 2018; Zeng et al., 2014). For the noble metal plasma, the response of noble metalbased plasma to visible and infrared light has obvious overlap with the solar spectrum, but it has no plasma response in the ultraviolet light, and display low reuse rate and cannot be produced on a large scale (Liu et al., 2015a; Zhou et al., 2016). Carbon based materials stand out in the study of solar steam generation device, due to their wide band absorption characteristics and excellent chemical stability, but their application is limited by the complex preparation process (Ren et al., 2017). Organic photothermal conversion materials have the advantages of biodegradability and easy regulation of light absorption wavelength. However, these materials have poor photobleaching resistance (such as ICG) and are easy to decompose after illumination, making them difficult to be used for a long time. Semiconductor photothermal materials have unique absorption characteristics for near-infrared light. They have a wide range of species and are easy to adjust the absorption intensity of near-infrared light, so they have been widely concerned in many fields. According to different absorption principles, they _ the absorption of near infrared can be divided into two types. (I) is due to surface plasmon resonance effect, for example, chalcogenide copper-based semiconductor materials and transition metal _ the absorption oxide semiconductor materials (Li et al., 2014). (I_I) of near-infrared light is owing to intrinsic absorption band gaps, such as TiOx and BiInSe semiconductors that their adsorption is mainly depended on their intrinsic absorption band gaps (Wang et al., 2017a; Yao et al., 2017). CuS belongs to P-type semiconductor photothermal material. It has a defective structure that may cause surface carriers to migrate, thus forming a plasma resonance effect similar to noble metal nanoparticles. The plasma resonance effect of chalcogenide copper-based compound has no direct relationship with its morphology, but depends on the concentration of free carriers, so the stability of its solar photothermal conversion performance is high (Li et al., 2014; Tian et al., 2011). CuS is widely used in solar cells, gas sensors, thermal therapy and other fields. Up to now, the application of CuS nanoparticle or its composite in solar steam generation device is still rare. At present, solar steam generation device relies on lightabsorbing material to absorb sunlight and transfer the accumulated heat directly or through intermediate heat transfer fluid to bulk water (Gao et al., 2018; Liu et al., 2015b). Generally speaking, evaporation of water is a surface process, because preventing heat from spreading to bulk water can effectively improve the evaporation efficiency of water (Wang et al., 2014b). A material matrix with high absorption of sunlight and low thermal conductivity can help localize thermal energy (Kashyap et al., 2019). Therefore, it is necessary to find a kind of porous, hydrophilic and stable material to prevent the heat generated by photothermal conversion from spreading to bulk water. Hydrogel is a synthetic macromolecular network filled with a large amount of water. Hydrogels

517

with special functional groups provide a unique environment for in-situ synthesis of nanoparticles because they can limit the size of nanoparticles and enhance their stability. Polyacrylamide hydrogel (PAM) has porous structure, high thermal stability and hydrophilicity, so it is very suitable for solar steam generation device (Alam et al., 2017; Xie et al., 2016). In this article, a new type of inorganic-organic light-absorbing material, copper sulfide-macroporous polyacrylamide hydrogel (CuS-m-PAM), is reported. Hydrogel was synthesized in saturated salt solution, macroporous polyacrylamide hydrogel (m-PAM) was prepared by salt leaching process, and CuS nanoparticles were loaded in m-PAM by in-situ synthesis under the action of amino groups (Makaya et al., 2009; Wang et al., 2014a). The CuS-mPAM has the following advantages. First of all, CuS nanoparticles have good absorption in both visible light region and near infrared radiation region that can realize high-efficiency solar photothermal conversion. Secondly, m-PAM can prevent the synthesized CuS nanoparticles from aggregating because Cu2+ ions are more stable after complexation with ANH2 groups on m-PAM macromolecules. Thirdly, the rough porous structure of m-PAM has multiple scattering effects, which can improve the ability to absorb sunlight, and the open macroporous structure can improve the water replenishment and steam exhaust. Last but not least, the CuS-m-PAM forms a heating zone is separated from bulk water by the m-PAM, so water vapor is generated by thermal positioning on the evaporated surface. These advantages provide a new way for commercial application of the CuS-m-PAM in solar steam generation device. 2. Experimental 2.1. Materials Acrylamide (AM), N, N0 -methylene-bis-acrylamide (BIS), ammonium persulfate (APS), sodium sulfide (Na2S9H2O), sodium bisulfite (SBS), copper (II) chloride dihydrate (CuCl22H2O), sodium chloride (NaCl), magnesium sulfate (MgSO4), magnesium chloride (MgCl2), calcium chloride (CaCl2) and potassium chloride (KCl) were supplied by Shanghai Aladdin co., ltd. These reagents are analytical grade and need not be purified. 2.2. Preparation of PAM 1 g of monomer (AM) and 0.1 g of crosslinker (BIS) were added to 2 mL of deionized water and were stirred at normal temperature until they were completely dissolved. Then the initiator (APS) and the redox agent (SBS) were added to the above solution under stirring, and the mixture was poured into the polytetrafluoroethylene (PTFE) mold to polymerize and form hydrogel. 2.3. Preparation of m-PAM The m-PAM was prepared by salt leaching techniques using NaCl particles as porogen and saturated NaCl aqueous solution as solvent (Makaya et al., 2009). 1 g of AM, 0.1 g of BIS and 2 g of NaCl crystal (particle size distribution: 250–300 lm) were added to 2 mL of saturated NaCl aqueous solution, where AM and BIS dissolved but NaCl particles did not dissolve. Afterwards, the APS and SBS were added to the mixture and then the resultant solution was poured into the PTFE mold to carry out polymerization to obtain saline hydrogel. The saline hydrogel was rinsed with deionized water and soaked for 3 h. This salt leaching process repeated three times to obtain m-PAM. The hydrogels with AM and BIS mass ratio of 14:1, 12:1 and 10:1 were called m-PAM-14, m-PAM-12 and m-PAM-10.

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2.4. Preparation of CuS-m-PAM

2.7. Solar steam generation and desalination experiments

The as-prepared m-PAM-10 was placed in 10 mL of CuCl2 solution for 6 h and then soaked and rinsed with deionized water for 3 times. After that, it was soaked in 10 mL of Na2S solution for 3 h to induce formation of CuS, and then it was rinsed 5 times with deionized water. The CuS-m-PAMs prepared in CuCl2 solutions of 0.01, 0.03, 0.05, 0.1 and 0.2 mol L1 were called CuS-m-PAM-0.01, CuS-m-PAM0.03, CuS-m-PAM-0.05, CuS-m-PAM-0.1 and CuS-m-PAM-0.2, respectively.

The solar steam generation tests were carried out on different samples including pure water, m-PAM and CuS-m-PAM. In order to verify the water evaporation rate of the light-absorbing material, the solar steam generation device was established as shown in Fig. 1. The simulated solar illumination was provided by a Xe lamp (CEL-PE300L-3A, Education Au-light Co., Beijing, China) with AM 1.5G filter. The power density was kept at 1000 Wm2 (one sun). Weighing bottle of 25 mL (the bottle mouth diameter is 22 mm) loaded with sample and water were placed on an analytical balance (CITIZEN CX 301, accuracy: 0.1 mg), the weight loss of water was recorded in real time under one sun illumination, the temperature distribution was recorded by an infrared thermal imager (VHR 480, Infra Tec, Germany) and a digital thermometer (UT325, UNI-T, China). In order to demonstrate the application of the CuS-m-PAM in seawater desalination, pure water in the above process was replaced by simulated seawater (the simulated seawater was prepared as follows: 13.37 g of NaCl, 1.13 g of MgCl2, 1.63 g of MgSO4, 0.58 g of CaCl2 and 0.36 g of KCl were added into 500 mL of deionized water and dissolved completely to get a 3.4 wt% simulated seawater).

2.5. Characterization PAM, m-PAM-10 and CuS-m-PAM were freeze-dried and cut into thin slices. The surface morphology of the samples was observed by scanning electron microscopy (SEM, S-4800, Hitachi, Japan). The distribution and lattice spacing of the CuS nanoparticles were observed by transmission electron microscopy (TEM, Tecnai G2 F20, Philips, Holland). The AM, m-PAM-10 and CuS-mPAM-0.05 were also characterized by a Perkin-El MEP-1000 Fourier transform infrared spectrometer (FTIR). For the purpose of probing the surface elements of pure CuS, m-PAM-10 and CuS-m-PAM-0.05 and elucidating the properties of the formed copper complex, an Xray photoelectron spectroscope (XPS, PHI1600 ESCA System, PERKIN ELMER, US) with Mg Ka anode was used for XPS analysis. Pure CuS, m-PAM-10 and CuS-m-PAM-0.05 were ground into powder and characterized by an X-ray diffractometer (XRD, BRUKER AXS GMBH, D8-Focus). The analysis conditions are: reference target: Cu-Ka radiation (k = 1.54 Å), measuring current: 40 mA, and voltage: 40 kV. For analyzing the light absorption by light-absorbing material, the transmittance and reflectivity were measured with an ultraviolet–visible-near infrared spectrophotometer (UV–visNIR, Lambda 750, Perkin Elmer, US) in a wavelength region of 200–2500 nm. The absorption spectrum of CuS nanoparticle was measured by an ultraviolet–visible spectrophotometer (UV–Vis, TU-1901) with a detection limit of 10–6 mol L1. Contact angles were measured on a commercial contact angle system (OCA 20 of Data-Physics) using a 3 mL water droplet as the indicator at ambient temperature. The porosity of the CuS-m-PAM-0.05 was determined by mercury intrusion porosimetry (MIP, Auto Pore IV 9500, US). The thermal conductivity was measured through transient hot-wire method (Hot Disk TPS 2500S). Ion concentrations were measured using an inductively couple plasma optical emission spectrometry (ICP-OES, VISTA-MPX, Varian).

3. Results and discussion 3.1. Characterization of CuS-m-PAM Fig. 2a shows the schematic diagram of solar steam generation device that use the CuS-m-PAM as the light absorbing material and the polyethylene foam wrapped in cotton cloth as the thermal insulation and water supply module. CuS-m-PAM forms a heating zone at the air-water interface. When it is irradiated by sunlight, CuS nanoparticles absorb light to excite level transition of election and plasma resonance effect to release heat. The polymer network of m-PAM hydrogel that loads with CuS nanoparticles can capture some water. Therefore, CuS nanoparticles release heat after absorbing light and transfer it directly to this part of water. After absorbing heat, the water is heated and transforms into steam which rapidly escapes through the porous structure of CuS-mPAM. The first step of the water compensation process is to absorb water via the capillary force of cotton cloth. Then, through the capillary action and osmotic swelling of m-PAM, the water supplies on the evaporation surface sufficiently, and the water loss in the evaporation process is replenished, so that the solar steam generation device can maintain high-efficiency evaporation for a long time.

2.6. Water-absorbing test The PAM hydrogel is hydrophilic and has high water adsorption ability. The water-absorbing capacity was determined by recording the time-dependent movement of water with an infrared camera. The samples (PAM, m-PAM, CuS-m-PAM) each was cut into the small size (20 mm  10 mm  2 mm). To measure the swelling ratio (W%) of PAM, m-PAM and CuS-mPAM, they were soaked in 50 mL deionized water. These hydrogels were taken out every 1 h and the excess water on the surface was wiped off with filter paper, then the mass of these hydrogels were weighed. The above steps were repeated until the weights of these samples were no longer changed. W% was calculated according to the following equation:

W% ¼

m2  m1  100% m1

ð1Þ

where m1 is the weight of dry hydrogel and m2 is the weight of expanded hydrogel.

Fig. 1. Schematic setup of solar steam generation test.

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Fig. 2. Schematic diagram of solar steam generation device (a) and schematic process of CuS-m-PAM fabrication (b).

In addition, the excellent thermal insulation performance of polyethylene foam wrapped in cotton cloth further restricts heat transfer to bulk water, thus improving solar photothermal conversion efficiency. The solar steam generation device is highly dependent on the CuS-m-PAM absorbing material and is expected to achieve higher solar energy utilization. Fig. 2b shows the schematic fabrication process of CuS-m-PAM. Firstly, m-PAM was prepared by salt leaching process (Makaya et al., 2009). Secondly, m-PAM was soaked in CuCl2 solution to combine Cu2+ by ANH2 in the hydrogel, and then it was soaked in Na2S solution for a period of time to obtain CuS-m-PAM. The interaction force between Cu2+ and ANH2 ensures the successful loading of CuS nanoparticles (Wang et al., 2014a). When the CuSm-PAM was immersed in water for a long time, the water was still transparent and colorless, indicating that CuS nanoparticles are firmly fixed in hydrogel. This also ensures the good reusability of the CuS-m-PAM. The surface morphology of m-PAM-10, CuS-m-PAM-0.05 and PAM were observed by SEM, as shown in Fig. 3a–c. Pore size distribution of m-PAM-10, CuS-m-PAM-0.05 and PAM are shown in Fig. 3d–f. As can be seen from Fig. 3a, m-PAM-10 is interconnected macroporous structure with an average pore size of 265.5 um (Fig. 3d), which is consistent with the diameter of NaCl crystals. Fig. 3b and 2e show that the surface morphology and average pore size of CuS-m-PAM-0.05 are similar to those of m-PAM-10, indicating that the pore size of m-PAM is not affected by the loading of CuS nanoparticles. As can be seen from Fig. 3c, PAM has closed holes with an average pore size of 8.11 um (Fig. 3f), which seriously hinders water-absorbing capacity and is not conducive to efficient water supply of the solar steam generation device. This is consistent with the result of subsequent PAM water-absorbing test. As can be seen Fig. S1a–e, the loading amount of CuS nanoparticles in CuS-m-PAM-0.01 and CuS-m-PAM-0.03 is relatively low, and the distribution of CuS nanoparticles in CuS-m-PAM-0.05 hydrogels is dense and more uniform. However, CuS-m-PAM-0.1

and CuS-m-PAM-0.2 show a large aggregation of CuS nanoparticles, which hinders the transmission of light. The CuS-m-PAM0.05 with dense and uniform dispersion of CuS nanoparticles may be beneficial to improve the solar photothermal conversion efficiency. The microstructure information of CuS nanoparticles was further investigated by high resolution transmission electron microscope (HRTEM) images. Fig. 3g shows the HRTEM image of CuS-m-PAM-0.05 that has a lattice fringe of 0.28 nm corresponding to the (1 0 3) crystal plane of hexagonal phase CuS (pdf # 06-0464). To observe the effect of Cu2+ concentration on the particle size distribution of CuS nanoparticles, TEM images of CuS-m-PAM-0.01, CuS-m-PAM-0.03, CuS-m-PAM-0.05, CuS-m-PAM-0.1, and CuS-mPAM-0.2 are shown in Fig. S2a–e. It can be seen that when the Cu2+ concentration is higher, the CuS nanoparticles are more densely distributed and the particle size gradually increases. With the increase of Cu2+ concentration, the density of CuS nanoparticles increased gradually, which is consistent with SEM images. Fig. 4a-c shows the XRD patterns of CuS, m-PAM-10 and CuS-mPAM-0.05. XRD pattern of pure CuS nanoparticles shows four diffraction peaks at 27.7, 29.3, 31.7 and 47.9 °corresponding to (1 0 1), (1 0 2), (1 0 3) and (1 1 0) crystal planes of hexagonal phase (pdf # 06-0464), respectively (Tanveer et al., 2014). The m-PAM-10 has a wide diffraction peak and a weak diffraction peak at 21.7 and 36° as shown in Fig. 4b. In Fig. 4c, two slight diffraction peaks of CuS-m-PAM-0.05, whose 2h values are 31.7 and 47.9°, correspond to the (1 0 3) and (1 1 0) crystal planes of hexagonal phase (pdf # 06-0464). This proves successful loading of CuS nanoparticles in m-PAM-0.05. The FTIR spectra of AM, m-PAM-10 and CuS-m-PAM-0.05 in the range of 4000–500 cm1 were shown in Fig. 4d-f. For IR spectrum of AM in Fig. 4d, we can see two strong absorption peaks, stretching vibration of ANH2 at 3196 and 3354 cm1 respectively, and the stretching vibration absorbing peaks of 1673 and 1612 cm1 corresponding to C@O and C@C (Lin et al., 2007; Yang et al., 2015). Peaks at 1425 and 989 cm1 root in CAN stretching vibration and C@C

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a

c

b

e

200

250

Size (um)

300

Avg. size = 265.6 um

350

220

240

260

280

Size (um)

Avg. size = 8.11 um

Counts

Counts

Avg. size = 265.5 um

f

Counts

d

4

300

5

6

7

8

9

10 11 12 13 14 15

Size (um)

g

Fig. 3. SEM images of m-PAM-10 (a), CuS-m-PAM-0.05 (b), PAM (c); pore diameter distribution of m-PAM-10 (d), CuS-m-PAM-0.05 (e) and PAM (f); HRTEM image of CuS-mPAM-0.05 (g).

31.78(103) 47.94(110)

29.28(102) 27.68(101)

b

36 21.7 31.78(103)

c

47.94(110) 21.7

10

20

Transmittance (%)

Intensity (a.u.)

a

d

3354

e

2900

f 30

40

50

60

Ο

2θ ( )

70 4000

16731612 1425 989

3196

2928

3500

3000

2500

1673

1449

1669

1449

2000

1500 Wavenumber (cm-1 )

1000

500

Fig. 4. XRD patterns of CuS (a), m-PAM-10 (b) and CuS-m-PAM-0.05 (c); FTIR spectra of AM (d), m-PAM-10 (e) and CuS-m-PAM-0.05 (f).

out-of-plane bending (Li, 2018; Lin et al., 2007). As shown in Fig. 4e, two peaks at 1612 and 989 cm1 disappear, indicating that the double bonds do not exist any more because the AM monomer have polymerized into m-PAM-10. In Fig. 4e and f, the strong and wide peak in the range of 3196–3404 cm1 can put down to the overlap of ANH and AOH stretching vibration bands, and the peak at 1449 cm1 belongs to ACH2A deformation vibration (Song et al., 2014). Fig. 4f is the IR spectrum of CuS-m-PAM-0.05. The stretching vibration absorption peak of C@O varies slightly from 1673 to 1669 cm1 after loading CuS nanoparticles (Sharma and Lee, 2014). The absorption peak in the region of 3196–3404 cm1 is lower than that of m-PAM-10, possible owing to the interaction

between Cu2+ ions and polymer chains (Zhang and Li, 2017). The IR spectra of CuS-m-PAM-0.05 and m-PAM-10 are very similar, indicating that the loading of CuS nanoparticles has no effect on the structure of hydrogel. In order to determine the surface elements and the interaction between m-PAM-10 and CuS, the XPS spectra of CuS, m-PAM-10 and CuS-m-PAM-0.05 are given in Fig. 5 and Fig. S3. The XPS spectrum of m-PAM-10 (Fig. S3a) has three main peaks: C 1s, N 1s and O 1s. As shown in Fig. S3b, two peaks of CuS-m-PAM-0.05 appear at about 161 and 940 eV, which are assigned to S 2p and Cu 2p. Fig. 5a is the C1s XPS spectra of m-PAM-10. The three peaks correspond to C-C (284.5 eV), RCONH2 (287.7 eV) and C-OH (285.4 eV),

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284.5 eV

C 1s

399.5 eV

c

287.7 eV 285.4 eV 284.5 eV

b

287.7eV

285.1 eV 282

C 1s

284

286

288

290

Intensity (a.u.)

Intensity (a.u.)

a

400.3 eV 399.6 eV

d

396

398

Binding energy (eV)

Cu2p 1/2 952.6 eV oscillating line

f

Cu 2p

Cu 2p 3/2 932.6 eV Cu2p 1/2 952.7 eV

930

940

g

Cu 2p

Cu 2p 3/2 932.6 eV

950

400

N 1s

402

404

Binding energy (eV)

Intensity (a.u.)

Intensity (a.u.)

e

N 1s

S 2p

h 161.5 eV

960 156

Binding energy (eV)

S 2p

162.0 eV

158

160

162

164

166

168

Binding energy (eV)

Fig. 5. XPS spectrum of C 1s (a) and N 1s (c) spectrum of m-PAM-10, C 1s (b), N 1s (d), Cu 2p (e) and S 2p (g) spectra of CuS-m-PAM-0.05, Cu 2p (f) and S 2p (h) spectra of CuS.

respectively (Lima et al., 2016). The peak of C-OH is caused by the interaction of water molecules adsorbed on carbon atoms in m-PAM-10 (Zhang and Li, 2017). However, in Fig. 5b, the peak of C-OH in CuS-m-PAM-0.05 shifts to 285.1 eV. This may be caused by the adsorption of Cu2+. Fig. 5c is N 1s XPS spectra of m-PAM10. The two peaks at 399.5 and 400.3 eV correspond to nitrogen in cross-linking agent and a small amount of quaternary amide formed during hydrogel synthesis, respectively (Orozco-Guareño et al., 2010; Yang et al., 2009). It is worth noting that in Fig. 5d, there is only one peak at 399.6 eV, indicating that some N atoms on the CuS-m-PAM-0.05 surface have higher oxidized state. This phenomenon can be attributed to the formation of RCONH2Cu2+ complexes, in unshared pair electrons in N atom are contributed to the covalent bond between N and Cu2+. Therefore, electron cloud density of nitrogen atoms is significantly reduced, leading to higher binding energy (Orozco-Guareño et al., 2010). In the Fig. 5e, the two peaks at 932.6 and 952.7 eV correspond to Cu 2p3/2 and Cu 2p1/2, indicating the existence of Cu (II) in the CuS-m-PAM-0.05 (Ma et al., 2018; Nekouei et al., 2018). In addition, an oscillating line is probed between the peaks of Cu 2p3/2 and Cu 2p1/2, which proves that Cu2+ exhibits paramagnetism which is conducive to carrier migration (Nekouei et al., 2018; Wang et al., 2013). Notably, the binding energy (162.0 eV) of S 2p in CuS-m-PAM-0.05 is higher than that of S 2p in CuS (161.5 eV), indicating that CuS in CuS-m-PAM0.05 may be more stable (Wang et al., 2016). As can be seen from Fig. 6a, the CuS nanoparticles have broad absorption in both visible light and near infrared radiation regions, especially a strong adsorption centered at 980 nm. Researchers often used the visible light absorption of CuS for photocatalytic degradation of pollutants. Cai et al. found that the CuS hierarchical hollow microcubes exhibited excellent photocatalytic activity for degradation of organic dyes (methylene blue) under visible light (Cai et al., 2015). Meng et al. synthesized hierarchical flower-like CuS hollow nanospheres that exhibited outstanding visible-light

photocatalytic activity for the degradation of Rhodamine B (Rh B) and 2, 4-dichlorophenol in aqueous solution (Meng et al., 2013). The absorption property of CuS in near infrared region can be used for photothermal conversion. For instance, Li and Zhou et al. utilized CuS nanoparticles for photothermal ablation of tumor cells (Li et al., 2010; Zhou et al., 2010). In order to analyze the light absorptive capacity of PAM, m-PAM-10 and CuS-m-PAM-0.05, UV–vis-NIR spectrophotometer was used to collect their absorption spectra. As can be seen from Fig. 6b, by comparing the absorption spectra of PAM and m-PAM-10, we find that the latter has a higher absorbance (33%) mainly due to the multiple scattering of rough porous surface structure that may improve the optical absorption capacity. As shown in Fig. 6b inset, the transmission-r eflection-absorption spectra show that the CuS-m-PAM-0.05 has relatively high absorption (93%), low transmittance (3.1%) and reflectivity (4.0%), indicating that CuS-m-PAM-0.05 can absorb most of the sunlight. The detailed absorbances of the samples in ultraviolet, visible and near infrared regions are listed in Table S1. The light-absorbing material must have excellent hydrophilicity and water-absorbing capacity, which is essential for the efficient water supply of the solar steam generation device. The water contact angle test is a convenient method for determining the hydrophilicity or hydrophobicity of a material. Fig. S4 shows photographs of water droplets on PAM, m-PAM-10 and CuS-m-PAM0.05, demonstrating their excellent hydrophilicity. The contact angle of m-PAM-10 (27.0°) is smaller than that of PAM (41.3°), which seems to be due to the introduction of macropores that results in the decrease of the contact angle (The porosity of PAM and m-PAM-10 is 52.13% and 75.10% respectively). The contact angle of CuS-m-PAM-0.05 (25.4°) is similar to that of m-PAM-10, probably due to the low CuS content that has little effect on the contact angle as well as porosity (74.69%). Fig. 7 shows the water-absorbing capacity of PAM, m-PAM-10 and CuS-m-PAM0.05. In order to facilitate observation, Rh B solution (concentration

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1.0

the swelling ratio of hydrogel gradually decreased with the increase of BIS content. The m-PAM-10 has a lowest swelling ratio (4.98%), which is conducive to improving the evaporation rate of water. In order to obtain good water-absorbing capacity and low swelling ratio, m-PAM-10 was selected to load CuS nanoparticles. The loading of CuS nanoparticles basically has no effect on the swelling ratio of m-PAM-10.

a 980 nm

Absorbance

0.8 0.6 0.4

3.2. Photothermal conversion efficiency of CuS-m-PAM 0.2 0.0 300

400

500

600

700

800

900 1000 1100

Wavelength (nm) 100

b 100 80

T-R-A (%)

Absorption (%)

80

CuS-m-PAM-0.05

60

Absorption

60 40

Transmittance 20

40

m-PAM-10

Reflectivity

0 500

1000

1500

2000

2500

Wavelength (nm)

20

PAM

0

500

1000

1500

2000

2500

Wavelength (nm) Fig. 6. Absorption spectra of CuS (a), absorption spectra of the PAM, m-PAM-10 and CuS-m-PAM-0.05, respectively (inset: transmittance-reflectivity-absorption spectra of CuS-m-PAM-0.05) (b).

is 300 mg L1) was used instead of water in the experiment. The water absorption rate was measured by tracking the movement track of Rh B solution over time. As can be seen from Fig. 7, m-PAM-10 and CuS-m-PAM-0.05 absorb water immediately in contact with Rh B solution, and they take about 20 s to completely wet. In addition, the water absorption rate of the CuS-m-PAM-0.05 is not affected by the load of CuS nanoparticles. Fig. S5 shows the water-absorbing capacity of m-PAM-14, m-PAM-12, CuS-m-PAM0.01, CuS-m-PAM-0.03, CuS-m-PAM-0.1 and CuS-m-PAM-0.2. It can be found that changing BIS amount and CuS loading has little influence on the water absorption rate. The water-absorbing rate of CuS-m-PAM-0.05 is 197.6 kg m2 h1, which is much higher than the water yield under one sun illumination. Such high water absorbing rate ensure timely supply of moisture and prevents the light-absorbing material from drying during evaporation. In contrast, after PAM was contacted with Rh B solution for 20 s, a red watermark only appeared at the bottom, indicating that PAM absorbs water relatively slowly to porous hydrogel. This comparison shows that the driving force of macroporous hydrogel comes not only from hydrophilic property, but also from the capillary action of porous structures. As we all know, low water content of the heating zone may reduce the heat transfer in the light-absorbing material and help increase the evaporation rate of water (Kara and Pekcan, 2001). Table S2 shows the swelling ratio of PAM, m-PAM-14, m-PAM-12, m-PAM-10, CuS-m-PAM-0.01, CuS-m-PAM-0.03, CuS-m-PAM-0.05, CuS-m-PAM-0.1 and CuS-m-PAM-0.2. By comparing m-PAM-14, m-PAM-12 and m-PAM-10, we found that

Under one sun illumination, the change of surface temperature of the light-absorbing material (the area is 0.3799 cm2 and the thickness is 2 mm) was observed with the infrared thermal imager, and the mass of water evaporation was measured with the analytical balance (the experimental data of solar steam generation was calibrated with dark evaporation data). As shown in Fig. 8a, after 15 min of one sun illumination, the surface temperature of pure water is up to 39.4 °C, the temperature increases by 7 °C compared with the initial temperature. The surface temperature of m-PAM10 increases 16.5 °C, higher than that of pure water. The increase of temperature is mainly due to the reason that m-PAM-10 captures part of the water through the polymer network and effectively limits the heat dissipation to the underlying bulk water. Furthermore, under one sun illumination, the surface temperature of CuS-m-PAM-0.05 rapidly increases from 31.7 to nearly 55.3 °C within 15 min. The large temperature increase is due to CuS-mPAM-0.05 forming a heating zone at the air-water interface and capturing part of the water through the polymer network, where CuS nanoparticles release heat after absorbing light and transfer heat directly to water, thus reducing heat loss. To verify this, we tested the thermal conductivity of CuS-m-PAM-0.05 solar steam generation device (It consists of the CuS-m-PAM-0.05 and polyethylene foam wrapped in cotton cloth). The value is 0.048 W m1 K1, indicating better thermal insulation effect. In addition, we studied the temperature changes of pure water, m-PAM-10 and CuS-m-PAM-0.05 at different depths within 15 min under one sun illumination. As shown in Fig. S6a, temperatures at the selected points A, B, C and D of the solar steam generation device were for monitored. As shown in Fig. S6b–d, the temperature variation of pure water at these points is not large; but for m-PAM-10 and CuS-m-PAM-0.05, the temperatures at these points are obvious different. These evidences prove that pure water without solar steam generation device is heated by the traditional heating method. When there is a solar steam generation device floating on the water surface, the interface heating takes place. The variation curves of water quality with illumination time are shown in Fig. 8b. For CuS-m-PAM-0.05, the mass loss of water is the most (0.7339 kg m2), which is four times that of pure water (0.1598 kg m2). For evaluate the solar photothermal conversion efficiency of the light-absorbing material, the following formula was used to calculate:

g¼

qe qi

ð2Þ

Here, g represents the solar photothermal conversion efficiency, qi represents sun illumination at the power density of 1000 W m2, and qe represents the power of evaporation of the water, which can be represented as follow:

qe ¼ h  v ¼ h 

dm dt

ð3Þ

where h represents the heat of evaporation of water (2260 kJ kg1), dm represents the mass of evaporated water per square meter, dt represents time of change and v represents the water evaporation rate under one sun illumination minus the water

Y. Sun et al. / Chemical Engineering Science 207 (2019) 516–526

523

Fig. 7. Images that show the water-absorbing capacity of the PAM, m-PAM-10 and CuS-m-PAM-0.05 (the yellow box represents the wetted part). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

evaporation rate under dark reaction (Gao et al., 2018). Formula (4) can be derived from formula (2) and (3):

g¼

h dm  qi dt

ð4Þ

Since h and qi are constant, ƞ is determined by dm/dt that can be obtained from Fig. 8b. The water evaporation rate and solar photothermal conversion efficiency of various light-absorbing materials are shown in Fig. 8c. The water evaporation rate and solar photothermal conversion efficiency of m-PAM-10 is higher than that of m-PAM-14 and mPAM-12. It is ascribed to two reasons: (1) m-PAM-10 has a low swelling ratio, so it absorbs less water and reduces heat transfer in the heating zone. (2) Dense hydrogel dries much faster than the loose hydrogel, which was also reported in other literatures (Evingur et al., 2009; Kara and Pekcan, 2001). The water evaporation rate and solar photothermal conversion efficiency of CuS-mPAM-0.01, CuS-m-PAM-0.03 and CuS-m-PAM-0.05 increased with the increase of CuS nanoparticles. However, CuS-m-PAM-0.05, CuS-m-PAM-0.1 and CuS-m-PAM-0.2 gradually decreased, which may be due to the agglomeration of CuS nanoparticles and uneven distribution (interpreted in conjunction with SEM and TEM). It is worth noting that the solar photothermal conversion efficiency of CuS-m-PAM-0.05 under one sun illumination is higher than or similar to those of other materials reported in the literatures, as shown in Fig. 8d (Ghasemi et al., 2014; Hu et al., 2017; Li et al., 2016; Wang et al., 2017b; Xue et al., 2017; Zhang et al., 2017). Among them, the environment temperatures reported by Ghasemi et al. (2014), Wang et al. (2017b) and Xue et al. (2017) p (the signals with ) are very close to that of our work. The solar photothermal conversion efficiency of Ghasemi et al. and Xue et al. were 64% and 72% under one sun illumination, and that of

Wang et al. was not high (72.5%) under two sun illumination (Ghasemi et al., 2014; Wang et al., 2017b; Xue et al., 2017), all of which were lower than that of CuS-m-PAM-0.05 in the present work. The excellent solar photothermal conversion efficiency of CuS-m-PAM-0.05 is due to its outstanding absorbance, good solar photothermal conversion ability, preeminent hydrophilicity and capillary effect, which comprehensively optimizes the solar steam generation device from the aspects of light absorption, thermal insulation, water replenishment and steam exhaust. The repeatability and performance stability of solar steam generation device have great significance to practical application. Therefore, under the same conditions, we used the same CuS-m-PAM-0.05 to repeat solar steam generation test for 50 times. As shown in Fig. S7, the solar photothermal conversion efficiency remained at 87.5% after 50 times of tests, indicating excellent performance stability of the CuS-m-PAM-0.05. In order to study the application of the CuS-m-PAM-0.05 in seawater, we replaced the pure water with simulated seawater in the experiment, and the results indicate that the simulated seawater seemly did not affect the evaporation rate of the CuS-m-PAM0.05, but the results prove good characteristics of seawater desalination. In order to evaluate the desalination effect of the CuS-mPAM-0.05, the ion concentration of simulated seawater and desalinated water are measured (Fig. S8). Importantly, the concentrations of major ions (Na+, Mg2+, Ca2+ and K+) in desalinated water were reduced by four orders of magnitude and compared to the drinking water standards of the World Health Organization (WHO, the black dashed line) (Xu et al., 2019). It is clear that our desalinated water is fully compliant with drinking water standards. To further investigate the practicality of the CuS-m-PAM0.05, we carefully examined the concentration of copper ion in the remained water after the experiment. The detected copper

Y. Sun et al. / Chemical Engineering Science 207 (2019) 516–526

0.0

c 100

-0.1 -0.2 Pure water PAM-14 PAM-12 PAM-10 CuS-m-PAM-0.01 CuS-m-PAM-0.03 CuS-m-PAM-0.05 CuS-m-PAM-0.1 CuS-m-PAM-0.2

-0.3 -0.4 -0.5 -0.6 -0.7 0

5

10

15 min

Solar photothermal conversion efficiency Evaporation rates

80

1.6

-1 )

10 min

1.4

-2

5 min

Effciencies (%)

Mass change (kg m-2 )

b

0 min

1.2 1.0

60

0.8 40

0.6 0.4

20

0.2

15

20

25

0

30

0.0

Evaporation rates (kg m h

CuS-m-PAM-0.05

m-PAM-10

a

Water

524

Time (min)

d

100 95

Efficiency (%)

90 85 80 75

√ √ √

70 65 60

Ghasemi et al., 2014 Hu et al., 2017 Li et al., 2014 Li et al., 2016 Liu et al.,2015a Liu et al., 2017b Ren et al., 2017

55 50 0

1

2

3

4

5

6

Wang et al., 2017a Wang et al., 2017b Xue et al., 2017 Yin et al., 2018 Zhang et al., 2017 Zhou et al., 2016 Our work

7

8

9

10

Power density (kW m-2) Fig. 8. Infrared images of the pure water, m-PAM-10 and CuS-m-PAM-0.05 under one sun illumination for 15 min (a), evaporation mass loss of the light-absorbing material under one sun illumination for 30 min (b), solar photothermal conversion efficiency and corresponding water evaporation rate of the light-absorbing material under one sun illumination (c), comparing the solar thermal efficiency of the CuS-m-PAM-0.05 with other solar steam generation devices (d).

ion concentration is very low (0.01 mg L1), even much lower than the drinking water standards of the WHO (2 mg L1). In addition, the absorption peak of CuS was not found in the absorption spectra of the remained water after the experiment (as shown in Fig. S9). The above results eliminate the possibility of copper pollution caused by CuS.

4. Conclusions In conclusion, CuS-m-PAM was used as light-absorbing material for efficient solar steam generation device. CuS-m-PAM was prepared by in-situ synthesis method by loading CuS nanoparticles into m-PAM. The CuS-m-PAM forms a heating zone at the air-water

Y. Sun et al. / Chemical Engineering Science 207 (2019) 516–526

interface that limits heat to the molecular grid, and hence reduces the energy loss. Furthermore, rough surface, open porous structure and plasma resonance effect can adjust light absorption, balance water transportation and evaporation, thus achieving efficient solar photothermal conversion. Such the CuS-m-PAM-0.05 exhibits a solar photothermal conversion efficiency of 92% with a water evaporation rate of 1.46 kg m2 h1 under one sun illumination. The formation of NACu bond makes CuS nanoparticles more stabilization, and the solar photothermal conversion efficiency of CuS-m-PAM-0.05 remains at 87.5% after 50 cycles of tests. To sum up, the CuS-m-PAM will provide a new practical way for efficient utilization of solar steam generation device due to its simple preparation process, solar photothermal conversion efficiency and excellent reusability. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was supported by the National Science Foundation of China (51573126). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ces.2019.06.044. References Alam, A., Kuan, H., Zhao, Z., Xu, J., Ma, J., 2017. Novel polyacrylamide hydrogels by highly conductive, water-processable graphene. Compos. Part A Appl. Sci. Manuf. 93, 1–9. Cai, L., Sun, Y.G., Li, W.Y., Zhang, W.L., Liu, X.J., Ding, D.R., Xu, N.N., 2015. CuS hierarchical hollow microcubes with improved visible-light photocatalytic performance. RSC Adv. 5, 98136–98143. Chandrashekara, M., Yadav, A., 2017. Water desalination system using solar heat: a review. Renew. Sustain. Energy Rev. 67, 1308–1330. Elimelech, M., Phillip, W.A., 2011. The future of seawater desalination: energy, technology, and the environment. Science 333, 712–717. Evingur, G.A., Aktas, D.K., Pekcan, Ö., 2009. In situ steady state fluorescence (SSF) technique to study drying of PAAm hydrogels made of various cross-linker contents. Chem. Eng. Process. 48 (2), 600–605. Gao, M., Peh, C.K., Phan, H.T., Zhu, L., Ho, G.W., 2018. Solar absorber gel: localized macro-nano heat channeling for efficient plasmonic Au nanoflowers photothermic vaporization and triboelectric generation. Adv. Mater. 8 (25), 1800711. Ghasemi, H., Heinke, J., Biemans, H., Eisner, S., Florke, M., Hanasaki, N., Konzmann, M., Ludwig, F., Masaki, Y., Schewe, J., Stacke, T., Tessler, Z.D., Wada, Y., Wisser, D., 2014. Solar steam generation by heat localization. Nature Commun. 5 (1). Haddeland, I., Heinke, J., Biemans, H., Eisner, S., Florke, M., Hanasaki, N., Konzmann, M., Ludwig, F., Masaki, Y., Schewe, J., Stacke, T., Tessler, Z.D., Wada, Y., Wisser, D., 2014. Global water resources affected by human interventions and climate change. Proc. Natl. Acad. Sci. USA 111 (9), 3251–3256. Hu, X., Xu, W.C., Zhou, L., Tan, Y.L., Wang, Y., Zhu, S.N., Zhu, J., 2017. Tailoring graphene oxide-based aerogels for efficient solar steam generation under one sun. Adv. Mater. 29 (5), 1604031. Kara, S., Pekcan, Ö., 2001. Photon transmission technique for monitoring drying processes in acrylamide gels formed with various crosslinker contents. J. Appl. Polym. Sci. 80 (11), 1898–1906. Kashyap, V., Medhi, R., Irajizad, P., Jafari, P., Nazari, M., Masoudi, A., Marquez, M.D., Lee, T.R., Ghasemi, H., 2019. Capture and conversion of carbon dioxide by solar heat localization. Sustain. Energy Fuels 3 (1), 272–279. Li, Y., Lu, W., Huang, Q., Li, C., Chen, W., 2010. Copper sulfide nanoparticles for photothermal ablation of tumor cells. Nanomedicine 5 (8), 1161–1171. Li, B., Wang, Q., Zou, R.J., Liu, X.J., Xu, K.B., Li, W.Y., Hu, J.Q., 2014. Cu7.2S4 nanocrystals: a novel photothermal agent with a 56.7% photothermal conversion efficiency for photothermal therapy of cancer cells. Nanoscale 6 (6), 3274–3282. Li, X., Xu, W.C., Tang, M.Y., Zhou, L., Zhu, B., Zhu, S.N., Zhu, J., 2016. Graphene oxidebased efficient and scalable solar desalination under one sun with a confined 2D water path. Proc. Natl. Acad. Sci. USA 113 (49), 13953–13958. Li, M., 2018. Preparation and properties of novel modified humidity control composite materials. J. Macromol. Sci. B 57, 21–30.

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