Multifunctional CuO nanowire mesh for highly efficient solar

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Multifunctional CuO nanowire mesh for highly efficient solar evaporation and water purification Ying Xu, Jiaxiang Ma, Yu Han, Jingjing Zhang, Fuyi Cui, Ying Zhao, Xin Li, and Wei Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06679 • Publication Date (Web): 03 Feb 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Sustainable Chemistry & Engineering

Multifunctional CuO nanowire mesh for highly efficient solar evaporation and water purification Ying Xu a, Jiaxiang Ma a, Yu Han a, Jingjing Zhang a, Fuyi Cui a, b, Ying Zhao *a, Xin Li *a, Wei Wang*a a

State Key Laboratory of Urban Water Resource and Environment, School of

Environment, Harbin Institute of Technology, 73 Huanghe Road, Harbin 150090, China. b College

of Urban Construction and Environmental Engineering, Chongqing

University, 174 Shazheng street, Chongqing 400000, China.

* Corresponding Author Email address: [email protected] (W. Wang) [email protected] (Y. Zhao) [email protected] (X. Li)

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Abstract Pure water producing by solar distillation under no light concentration is attracting ever attention in the rural area with electricity limit due to its constant energy input. In the meanwhile, the polluted raw water in these areas is also lack of effective decontamination treatment. Rather than relying on external steps for decontamination process, photo-thermal materials with pollutant removal ability would have better water cleaning performance. Here, we designed a multifunctional photo-thermal material based on a copper mesh with abundant CuO nanowires. This CuO nanowires mesh exhibited a high solar absorption of 93 % and super-hydrophilicity for water transport, contributing to a high solar vapor efficiency of 84.4 % under one-sun illumination. Besides, the CuO nanowires possessed a great catalytic ability for the degradation of contaminants in raw water. Moreover, the diffusion inhibition test showed a clear antimicrobial effect of the CuO nanowire mesh on the bacteria. Hence, the as-prepared multifunctional CuO nanowire mesh allows for the incorporation of solar evaporation, pollutant degradation and antibacterial action, which holds great application potential in the pure water production in solar distillation. Keywords: Solar distillation; Evaporation; Antibacterial effect; Water treatment; Advanced oxidation processes.

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INTRODUCTION Pure water production by solar distillation under no light concentration is attracting ever-growing attention and holds immense promise in low development regions.1–6 Photo-thermal materials, which can efficiently harvest solar energy and convert it into heat, are introduced into the solar distillation system to improve the pure water production. 7–14 Conventionally, photo-thermal materials were placed at the bottom or dispersed in raw water to play a role in bulk heating, the efficiency of which were less than 50 %.15 Realizing that evaporation is an interface phenomenon, the photo-thermal materials were designed to float on water surface aiming at interface heating. 16–20 To further enhance the efficiency, multi-layer structured system consisting of photothermal materials and thermal insulation stuffs has been developed and achieved the high efficiency of above 70 % under one-sun, indicating the promising application.21– 27

In addition to the pure water production through solar evaporation, the raw water obtained from natural waterbody also needs to be managed due to the growing number of toxic and carcinogenic contaminants discharging.28–30 These organic pollutions appearing in raw water would be concentrated after evaporation process, resulting in more serious pollution and much heavier environmental risk. Hence, the raw water for evaporation also demands decontamination. Common approaches for contaminants treatment includes adsorption, photocatalytic degradation and advanced oxidation processes (AOP) et.al.

31–34As

the adsorption process only transfers pollutants, the

degradation method is the preferred method for water treatment. Rather than relying on 3



external steps for decontamination process, photo-thermal materials with pollutant removal ability would have better clean water generation performance. For example, Lou designed a paper-based photo-thermal material composite of rGO and TiO2 nanoparticles (NPs), which exhibited a photocatalytic performance in the meanwhile of solar-driven water evaporation.35 However, without thermal insulation layer, the solar vapor efficiency of this floating thin film was at a low level. This actually deviates from the original intention that we are aiming at increasing the production of distilled water. With the addition of foams and other auxiliary heat-collecting materials, the contact between photo-catalyst and pollutions in water will be reduced, which would lead to a decrease of decontamination chance.36 It is also worth noting that the equilibrium temperature of the most evaporators floating on the water surface is around 30 - 40 °C, which is suitable for the incubation of bacteria. The long-term accumulation of bacteria on the surface of the evaporator would cause biological corrosion in the meanwhile threaten the water quality of the distilled water. Hence, it is envisioned that if a solar evaporator system with highly efficient solar evaporation, excellent pollution decontamination capacity and antibacterial action is developed, it would be attractive to the remote area of water scarcity. Inspired by the purification mechanisms of recent water treatment, in this work, we introduced a multifunctional copper oxide (CuO) nanowire mesh combined with thermal isolated expandable polyethylene (EPE) foam for highly efficient solar evaporation and water purification. Through mimicking the transpiration of natural trees — water pumped by roots from the ground to top and finally released to the 4


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atmosphere through leaves, a CuO tree solar distillation system was conducted to evaluate its solar evaporation capacity (Figure 1). First, as a photo-thermal material, the morphology of abundant nanowires on the CuO mesh increased the light absorption to a high level (93.8 %).37 The hydrogen bond formed between CuO and water also endows the mesh with outstanding hydrophilicity, which can effectively facilitate the transportation and evaporation of water during solar driven evaporation.38 Under onesun illumination, the CuO tree system achieved a constant evaporation rate of ≈ 1.42 kg m-2 h-1 and a high solar vapor efficiency of ≈ 84.4 %. Second, as a catalyst, the upper layer as leaves of CuO tree system with a band gap of 1.2-1.9 ev performed excellent photocatalytic capacity for water purification. Meanwhile, the bottom part as roots of CuO tree can effectively activate the potassium monopersulfate (PMS, a commonly oxidant used in AOPs) and realize the oxidation process for pollution decontamination in underneath bulk with the absence of light.39 Moreover, the CuO nanowire mesh was proved processing excellent bacteriostatic capacity in the diffusion inhibition test of E. coli and S. aureus.40,41 Thus, this multifunctional CuO nanowire mesh, as a combination of photo-thermal material, catalyst for pollution decontamination and bacteriostatic material holds great application potential in the pure water production in solar distillation.

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Figure 1. Solar evaporation through the CuO nanowire tree system by mimicking water transportation and evaporation in natural trees. Combining with solar evaporation to produce distilled water, the CuO nanowire tree system can also degrade the pollutants bulk water and perform sterilization ability. EXPERIMENTAL SECTION Materials. Copper meshes were bought from Anpingxian mooring Lin metal wire mesh co., LTD. EPE foams for thermal insulation were obtained from Shenyang Rongfeng Packaging Material Co., Ltd. Anhydrous ethanol was obtained from China National Medicines Co., Ltd to clean the pristine mesh. NaOH (96 %), (NH4)2S2O8 (98 %), and PMS (KHSO5, ≥47%) were purchased from Sigma Aldrich to oxidize the copper mesh. Pyrrole monomer (Py, 99 %) and methyl orange (MO, 96 %) for the control sample fabrication was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. The nano carbon black was purchased from J. S. YA company. The cellulosic fiber (CF) papers used as control sample were purchased from the Kimberly Clark Kimwipes 6


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Corporation, USA. Chemical Reagents are of analytical grade and used without further purification. Fabrication of the CuO. To remove the dirt of the mesh before use, the commercial copper mesh (400 mesh) was first ultrasonically cleaned in ethanol and distilled water three times. Next, the cleaned copper mesh was immersed into a mixture of aqueous solution of 2.5 M NaOH and 0.13 M (NH4)2S2O8 for 30min. Then, the obtained Cu(OH)2 mesh was washed by distilled water and dried at room temperature for 24 h. Subsequently, Cu(OH)2 mesh was heat on the alcohol burner for a brief period of time to obtain the black CuO nanowire mesh. Characterizations. The morphologies of the samples were examined by a scanning electron microscope (SEM, Zeiss SUPRA 55 SAPPHIRE, V=2 kV). The wettability of each sample was investigated through the water contact angle (WCA), which was tested by a contact angle goniometer (Kino SL200B) with 2 μL water. The optical properties of the samples were measured by UV−Vis absorption spectra (SHIMADZU, UV 2550). The XRD diffraction patterns were recorded on a Bruker D8 ADVANCE X-ray diffractometer (Germany) using Cu Kα radiation (λ = 1.54060 nm) with the SampleSaver storage container under purified. The mechanical properties of relevant samples were performed on a tensile tester (SHIMADZU AG-1kN). Solar evaporation experiment. To imitate the transpiration ability of the natural tree, the CuO mesh (2 × 6 cm2) was folded and placed on a piece of foam (2 × 2 × 2 cm3) with CuO’s edges immersing in raw water. Then, the above configuration was transferred into a beaker (100 mL with an inner diameter of 5 cm), which was filled up 7



by 80 mL pure water. To eliminate the water evaporation from other open areas, the surrounding exposed water was covered by a cover-foam (1-cm-thick EPE foam) with a square hole (2 × 2 cm2). Besides, the mass change of foams subtracted from all subsequent measured evaporation rates as systematic error.42 To investigate the solar evaporation process, the water reservoir containing DI water was illuminated under a solar simulator (NBeT Solar-500). The evaporation rate was calculated as the mass-change record balanced by a 0.0001 accuracy electronic scale every 1 minute, which was connected to a computer for real-time monitoring. In the 3 days of recycle experiment, the above reservoir was illuminated under 1-sun for 8 h each day to test the stability of the CuO tree system in solar evaporation. The solar light intensity was calibrated using a powermeter (PM100D, Thorlabs Inc) equipped with a thermal sensor (S305C, Thorlabs Inc), which was for general broadband optical power (wavelength range 0.19 - 25 µm) measurement. IR thermal imager (FLIR T1040) was employed to measure the surface temperature of relevant samples. The solar evaporation experiment above was performed for 8 h, then the bulk water after evaporation and the collected distilled water was measured by ICP-OES (OPTIMA 5300 DV, PerkinElmer Instrument) to verify whether the Cu2+ could be evaporated to the purified water. Unless other specified, each solar evaporation experiment was performed under an intensity of 1000 W m-2, corresponding to 1-sun illumination. Degradation test of model dye solution and actual industrial sewage. MO was used as a model organic contaminant to evaluate the degradation capacity of the CuO nanowire mesh. The mesh (4 × 4 cm2) was placed in 50 mL of MO solution (20mg/L) 8


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in a beaker (100mL). For comparison, 1mM PMS was added into the above solution to verify the PMS activation capacity of CuO nanowire mesh under no light input. Then the light generated from a Xe-lamp with a power density of 1000 W/m2 was illuminated onto the CuO nanowire mesh composites or the MO solution. The removal rate of MO was monitored by the relative change of absorption peak intensity at the wavelength of 464 nm that is detected by UV-Vis absorption spectra (SHIMADZU, UV 2550). The recycle experiments of the degradation capacity of PMS and CuO mesh without light were performed 5 times to verify the stability of the catalytic capacity of the CuO roots. After finishing each cycle, the sample was washed with pure water. To verify the catalytic property of our solar evaporator in practical application, a kind of practical organic wastewater produced by local industrial process was conducted as the raw water for evaporation. The amount of organic pollutants in raw water was characterized with total organic carbon (TOC), which was analyzed using a total organic carbon (TOC) analyzer (TOC-VCPH, SHIMADZU, JAPAN) after filtering the sample by 0.45 µm membrane. Antibacterial assessment of the CuO mesh. The as-prepared CuO nanowire meshes were immersed in deionized water for 24 h before the assessment of their antimicrobial activity. A Gram-positive Staphylococcus aureus and a Gram-negative Escherichia coli were set as the model bacteria, the cell suspensions of which were incubated on a rotary shaker (150 rpm) at room temperature for 24 h, respectively. Diffusion inhibition zone (DIZ) tests were performed to assess the bactericidal effects of the as-prepared samples. Briefly, 100 μL of above bacterial culture was spread onto an LB agar plate. Mesh 9



samples (1 × 1 cm2) were then placed onto the plates with their surface in contact with the agar surface. After incubation at optimal temperature (37 °C) for 48 h, the colonies formed around the samples were examined. The bacterial rejection experiment was performed in the domestic sewage, which is rich in a variety of bacteria. As contrast, the polypyrrole (PPy) coated cellulose acetate membrane and carbon-spray paper were prepared as typical polymer photo-thermal material and paper-based photo-thermal material. The PPy coated membrane was fabricated through chemical oxidation in pyrrole monomer solution.43,44 The carbonspray paper was fabricated through spraying the carbon nanoparticles (dispersed in ethanol) onto the paper. Then, the CuO mesh, carbon-spray paper and PPy coated membrane were conducted as photo-thermal materials in the tree evaporation system. The raw water of the systems was replaced with domestic sewage of Harbin University of Technology. Subsequently, the above systems were tested for 5 days, 8 h each day under one-sun illumination. Afterwards, the growth of bacteria on the surfaces of the samples were observed by SEM. RESULTS AND DISCUSSION Fabrication of CuO nanowire mesh. The preparation process of the CuO nanowire mesh was illustrated in Figure 2a. First, the pristine copper mesh was oxidized into Cu(OH)2 mesh after a strong oxidation process in a mixed aqueous of NaOH and (NH4)2S2O8. Then, the Cu(OH)2 decomposes into dark CuO after a rapid heating process. As observed from the SEM image (Figure 2b), the pristine copper mesh exhibited regular pores with the average pore size of ≈ 35 μm. The surface of the copper 10


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fiber was smooth with the average diameter of ≈ 30 μm. After 30 min of oxidation, the surfaces of the copper fibers were covered by a bunch of Cu(OH)2 nano-needles as shown in Figure 2c. These uniform nano-needles with lengths in the range of 10 - 25 μm and diameters in the range of 200 - 500 nm were clearly observed, which grew vertically along the wire walls and filled up the original pores of the mesh. The formation of the Cu(OH)2 nano-needles follows a chemical oxidation process:45 Cu  2 NaOH  NH 4 2 S 2 O 8  Cu(OH) 2  NH 4 2 SO 4  Na 2 SO 4

(1)

In previous studies, the as-prepared Cu(OH)2 mesh could transform into traditional CuO mesh in above solution after a longer period of time (4 - 6 h).46 The structure of the obtained traditional CuO mesh was 2D nano-plates, which failed to satisfy the requirement of decreasing light transmittance (Figure S1). Hence, we firstly induced a facile and rapid heating method to prepare CuO nanowire mesh based on the decomposition reaction:47 Δ

Cu(OH)  CuO  H O 2

(2)

2

The dynamic decomposition process was recorded as shown in Figure 2e. A piece of blue Cu(OH)2 mesh (2 × 2 cm2) immediately turned into black mesh after heating up on the alcohol lamp for 1.2 s. This apparent change in color indicates that the reflection of the sample was significantly reduced. Correspondingly, the straight nano-needles bent into curved nanowires and intertwined with each other as observed in the SEM image (Figure 2d). The length and diameter of the nanowires are slightly smaller than that of the Cu(OH)2 due to the loss of water in the heating process. Fortunately, the pores of the mesh were still covered up by these nanowires, maintaining the 11



transmittance of the CuO nanowire mesh at a low level.

Figure 2. (a) Schematic representation of the fabrication of the CuO nanowire mesh. SEM images of (b) the pristine copper mesh, Cu(OH)2 mesh and (d) CuO nanowire mesh. Insets show the magnification views of the corresponding samples. (e) The optical images of the rapid heating process. The CuO nanowire mesh was obtained within 1.2 s by heating the Cu(OH)2 mesh with an alcohol lamp. Characterization of the samples. The chemical compositions of the as-prepared samples were confirmed by X-ray diffraction (XRD). As shown in Figure 3a, the two extremely strong peaks (marked with plus signs) corresponded to the (111) and (200) crystal planes of the Cu mesh. After oxidation, the diffraction peaks indicated the orthorhombic-phase Cu(OH)2 crystals (marked with asterisks) appeared, which is in agreement with the values in the standard card (JCPDS Card No. 03-0310). As proved by the appearance of the diffraction peaks marked with pounds of CuO (JCPDS Card 12


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No. 48-1548), the Cu(OH)2 was proved to be successfully decomposed into CuO and water after the heating process. It is worth noting that the peaks of the original copper mesh were still strong after oxidation and deposition process, indicating the main body of the copper mesh still existed. It is well accepted that the intense oxidation process on inorganic substance would probably cause a significant decline of the mechanical strength. Herein, the mechanical strength of the relevant samples was measured as shown in Figure 3b. The pristine copper mesh exhibited a high mechanical strength (64 MPa) and a high tensile strain (78 %). After oxidation, the mechanical strength of the Cu(OH)2 reduced by 12.5 %, indicating that the formation process of the Cu(OH)2 crystals on the copper mesh destroyed the pristine copper fiber to some extent. However, the main part of the Cu(OH)2 mesh hadn’t been oxidized and still has a strong mechanical strength (54 Mpa), which is also consistent with the XRD analyzation. Furthermore, the mechanical strength of CuO mesh had no significant decrease (52 Mpa), which is attributing to that the decomposition occurs only on the Cu(OH)2 crystal instead of the main body of the copper mesh. Interestingly, the tensile strain of CuO mesh slightly increases about 12 % in contrast with pristine CuO mesh, which is probably due to the intertwined CuO nanowires. To sum up, the mechanical behavior of the resultant CuO mesh was still maintained at a high level. The wettability of the as-prepared samples was also regarded as a crucial factor affecting water transport capacity during solar evaporation. As shown in Figure 3c, the pristine copper mesh with smooth surface exhibited a hydrophobic property, the water 13



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contact angle (WCA) of which maintained at 112° even after 60 s. In contrast, once the water droplets contacted the membrane surfaces of Cu(OH)2 mesh and CuO mesh, they spread out quickly and WCA reached 0 ° within 100 ms and 30 ms, respectively, indicating the superior hydrophilicity. As tested in Figure S2, the water transport process of the folded CuO mesh (2 × 4 cm2) from the bottom to the top surface could be finished within 20 s, suggesting an excellent water transport capacity. The water absorption (wt %) of the CuO mesh was determined by the weight increment after transportation, which is calculated to be 45.5 %. Above results proved the superior water absorption and transport capacity of the resultant CuO mesh, which would ensure sufficient water supply during the solar evaporation process. To analyze the absorption capacity changes between the as-prepared samples, the reflection rate (R %) and transmission rate (T %) of each mesh were measured by UVVis spectrophotometer. The absorption rate (A %) was calculated by the following expression:42 A=1-R-T

(3)

As shown in Figure 3b-d, after oxidation, the R of the Cu(OH)2 mesh was significantly increased in the blue light band compared with pristine copper mesh owing to the appearance of Cu(OH)2 nanoneedles, which could also be confirmed by the color change of the mesh (from amaranth to blue). Meanwhile, the T was significantly reduced to 0.2 % due to the overlapped Cu(OH)2 nano-needles array on the mesh. Therefore, in addition to the decrease of the absorption rate in the blue light band, the broadband optical absorption of the Cu(OH)2 mesh has been apparently enhanced from 14


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56.5 % to 75.8 %. After heating, the reflection in the blue band disappears due to the decomposition of Cu(OH)2, and the R decreases to merely 5.6 %. As expected, the T of the CuO nanowire mesh is still maintained at a low level of 0.6 %, indicating that the pores of the mesh were still locked by the CuO nanowires. The decrement of the reflection and transmission over CuO nanowire mesh leads to an increment of broadband light absorption of about 93.8 %, which is a high value among the photothermal materials, especially among inorganic metals.

Figure 3. (a) XRD pattern, (b) mechanical strength and (c) the dynamic WCAs of the as-prepared samples. (d) The reflectance spectra, (e) transmittance spectra and (f) absorption spectra of the as-prepared samples. Solar-driven evaporation test. As illustrated in Figure 4a, the as-prepared CuO nanowire mesh (2 × 6 cm2) was folded with its edges immersing in the bulk water as roots for water absorption and transportation. The foam with low thermal conductivity (0.02 W m-1 K-1) underneath the mesh was playing as the trunk for supporting and heat preservation. Once the illumination started, the light would be converted into heat by 15



the CuO nano-leaves. Then, the water at the upper surface, absorbed through the capillaries of CuO roots, would evaporate immediately into steam by utilizing the generated heat. To further substantiate the heating process during the solar evaporation process, the dynamic temperature changes of the CuO tree was recorded by IR thermal imager. As shown in Figure 4b, the surface temperature of the CuO nanowire mesh before illumination was 4 °C lower than the room temperature (20.5 °C) due to the natural evaporation. After illumination, the surface temperature of the CuO tree raised immediately to the equilibrium temperature (32.5 °C) in only about 40s and maintained at the equilibrium temperature even after 30 min (Figure 4c). In addition, the temperature of the surrounding vapor was measured to be 30.5 °C according to the temperature probe. The rapid start-up of the system was mainly attributed to the excellent heat conductivity of the copper (397 W m-1 K-1), which is a great advantage in practical application. Meanwhile, the temperature of the bulk water underneath the foam increased 1 °C after 30 min of illumination, indicating that the heat generated by light absorption was barely conducted to the bulk water owing to the great thermal localization of the foam. Subsequently, the mass change in every min of the systems under four different conditions over 30 min were recorded with electronic balance as shown in Figure 4d. The evaporation rate of the pure water under solar illumination was calculated to be 0.35 kg m-2 h-1, which is only 1.75 times that of the CuO tree system in dark (0.2 kg m2 h−1) because the light absorbance of the pure water was close to zero. After the induce of the CuO nanowire mesh (without tree system), the evaporation rate 16


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was increased to 0.7 kg m-2 h-1 due to the enhance solar absorption. However, a large portion of the heat produced by CuO nanowire mesh was transferred to the bulk water at the lower part, resulting in that evaporation rate of the which was only about half that of the CuO tree system (1.42 kg m2 h−1). Besides, the mass-change curve of the CuO tree system exhibited linearly increase in 30 min under one-sun solar illumination (1000W m-2) due to the rapidly reached equilibrium, which is consistent with the results obtained with the IR images. Above results proved that the water evaporation capacity could be significantly improved by the CuO tree system. To monitor the stability of the evaporator for pure water production, the recycle experiments were performed 3 days, and the mass changes and evaporation rates of each day were plotted in Figure S3. The evaporation rate of the system was demonstrated stable at 1.42 ±0.08 kg m2 h−1, indicating a great stability and repeatability of the CuO tree system. Regardless of the effect of different external conditions during evaporation, the solar vapor efficiency was regarded as a relatively accurate parameter to compare the evaporation capacity of different photo-thermal material. Here, the solar vapor efficiency of the CuO tree system η was defined as the following expression:48 η = mH/Aq

(4)

where m is the mass ﬂux, H is the total enthalpy of liquid-vapor phase change, A is the surface area of the absorber, and q is the normal direct solar irradiation (1000 W m−2). Without solar concentration, the efficiency of the CuO tree system was calculated to be 84.4 ± 2.4 %, which is superior to many previous reports due to its great thermal localization and efficient water transportation.49,50 The specific energy distribution over 17



the CuO tree system during solar evaporation was illustrated in Figure 4e, and the specific calculation process of each part was performed in the Supporting information. The energy loss of the reflection and transmission was calculated to be 6.2 % as described by the yellow part, which is an intrinsic property of the CuO mesh. The radiation loss (green, 1.5 %) and convection loss (purple, 2.5 %) were reduced to a low level, because the generated heat was first transferred to the ambient heated vapor (30 °C) instead of surrounding environment. The foam obstructed the heat radiation and conduction to the bulk water and effectively localized the energy. Therefore, the conduction loss (blue, 1.2 %) could only be transferred to bulk water from the roots of the CuO tree system, which significantly reduced the conduction loss. The sum of these five main energy consumption parts (84.4 % + 6.2 %+1.5 % + 2.5 % + 1.2 %) was ≈ 95.8 %, which means that the sum of the energy calculated did not exceed the initial input energy, and the incoming solar energy is approximately equal to the outcome. The advantages of the CuO tree system in the evaporation process were summed as that the system could start immediately under illumination, the energy loss could be significantly minimized.

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Figure 4. (a) Schematic illustration of the evaporation experiment. (b) Infrared images of the surface temperature over CuO tree, corresponding to t=0, 10, 20 and 40s after illumination. (c) Infrared image of surface temperature distribution top surface over the water reservoir after 900s illumination. Inner photograph shows the top surface over the water reservoir in evaporation experiment. (d) Comparison of the mass-changes over three different conditions: CuO tree in dark condition (red line), water under light (green line), CuO tree under light (black line). (e) Different energy distribution ratio of the solar evaporation process. Inset shows the Schematic illustration of energy distribution in evaporation process over CuO tree system. Catalytic property. The degradation performance of the CuO nanowire mesh was tested using MO as the typical contaminant model of organic matter. Because of the floating structure hindered the light through the underneath part, the photocatalytic function of CuO roots was limited in the bulk water. Hence, PMS as a kind of AOP is added to the raw water for better degradation effect. As a comparison, experiments were performed in the systems with MO solution alone, MO solution with PMS, MO solution 19



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with CuO nanowire mesh, and MO solution with both CuO/PMS, respectively. The removal rate of MO (R) in the solution can be calculated by the following express51,52: R= C/C0 × 100%

(5)

where C0 and C represent the initial concentration and the concentration of MO at the varying time, respectively. As shown in Figure 5a and b, the original MO aqueous solution showed a plateaued line in both dark and light, indicating that the MO has no self-degradable properties with or without light. The degradation of MO started (32 %) in the presence of CuO nanowire mesh under light due to its photocatalytic property. Since photo-catalysis mainly uses ultraviolet light region, which makes up only 7% of the solar spectrum, it's normal for the low rate of photocatalytic degradation under natural sunlight (1 sun).53 With the adding of PMS, the degradation of MO increased to 65 % and 85 % after 30 min in dark and light, respectively. Obviously, more than 90 % of MO degradation was obtained in the CuO/PMS system within 20 min and 10 min in dark and light, respectively. The accelerated removal of MO with CuO/PMS composites under solar illumination should be attributed to that the CuO nanowires can effectively activate the PMS. To monitor the stability of the evaporator for removal of contaminant in raw water, a recycle experiments (PMS + CuO mesh in dark) were performed 5 times in the dye solution. As shown in Figure S4, the removal rate of the dye solution was still at 82 % after continuously repeating test for 5 cycles, indicating that the stable degradation performance of the CuO nanowire mesh. To verify the catalytic property of the solar evaporator in practical application, a kind of practical organic wastewater produced by local industrial process was conducted as the raw 20


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water for evaporation. As shown in Figure S5, the TOC of the pristine raw water (1026.8 mg/L) reduced 58% after 8 h of reaction, revealing a great degradation capacity of our CuO nanowire mesh. The degradation level was consistent with that of other catalysts.54,55 Of course, compared to the model solution, the degradation effect of the CuO mesh in practical raw water was decreased, which is attributed to that the concentration of the pollutions is in actual industrial water is much larger and the composition is more complex. The whole clean water generation process of CuO nanowire tree system was schemed by Figure 5c. The contaminated water was simultaneously purified by three combined processes: (1) Activation of PMS by CuO nanowires root in bulk raw water; (2) Photodegradation of RhB with CuO nanowires in the upper surface; (3) Activation of PMS by Irradiated CuO nanowires in the upper surface.

Above results demonstrated a great catalytic performance of the CuO tree

system for the contaminant degradation.

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Figure 5 The MO removal in dark (a) and solar irradiation (b) under different conditions. (c) Schematic for multifunctional CuO nanowire mesh in contaminant degradation over solar vapor generation process. Antibacterial test. The bacterial problem was a new issue in the solar evaporation system due to the introduction of the floating photo-thermal materials. A persistently bacteriostatic system can prevent membrane damage by microbes and establish an effective barrier to isolate purified water and raw water to protect distilled water from biological contamination. As a widely accepted bactericidal method, the positive copper ion releasing from CuO mesh could be incorporated in the negative cell membrane, which causes leakage of intracellular substances and eventually causes cell death.54 To demonstrate the bacteriostatic effect of the as-prepared CuO mesh, E. coli (Gram negative) and S. aureus (Gram positive), as two kinds of representative bacteria 22


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recommended for antimicrobial assays, were spread evenly over the agar surface. Then the CuO meshes were placed on the agar medium and cultured for 48 hours. As shown in Figure 6a, bacteria grow evenly on the surface of blank agar after culture, in contrast, bacteria inhibition zones were clearly observed around the CuO mesh (1 × 1 cm2). The zone diameters of E. coli and S. aureus were 2.1 and 2.0 cm, respectively, indicating the excellent bacteriostatic effect on both Gram negative and positive bacteria. To find out the purifying behavior of the CuO tree system, the evaporation experiment was performed in pure water for 8 h under one-sun illumination. Subsequently, the concentrations of copper ion in original pure water in beaker and the distilled water collected from evaporation condensation were measured by ICP-OES. After 8 h of illumination, barely copper ion (0.001 mg/L) was detected from the distilled water. The results also confirmed that the copper ion was released sustainably (0.0027 mg cm-2 h1)

during the evaporation process, which could inhibit the growth of bacteria both on

material surface and in raw water. Moreover, the copper ion could not be evaporated with the steam, which confirmed the water safety for drinking. In addition, by means of facile preparation, a piece of magnified CuO nanowire mesh with a size of 0.3 × 0.4 m2 was prepared to prove its easy scale-up capacity, as shown in Figure 6c. To further confirm the barrier ability of CuO tree system for bacteria, the raw water was replaced by the domestic sewage obtained from Harbin University of Technology. The domestic sewage here was aiming at introducing multiple bacteria to the system. Then, the CuO mesh was tested in solar evaporation of the sewage for 5 days, 8 h each day under illumination. For comparison, PPy coated membrane and carbon-spray paper 23



were also conducted in the experiment as typical polymer-based and paper-based photothermal materials. As shown in Figure 6d, the carbon-spray paper was severely damaged after test by microbes due to the great biocompatibility of cellulose. The corrosion process of the fibers by bacteria (in the red circle) could be clearly observed in the inner magnified SEM image. Meanwhile, although the PPy coated membrane maintained its structure intact after the test because the polymers are hard to degrade, the surface of which was attached by a thick layer of bacteria as shown in Figure 6e. Besides, the chlorella (in the yellow circle) with green appearance was also observed on the surface of the PPy paper, which would probably affect the solar absorption of the photo-thermal material after a long period of operation. Notably, no bacteria were observed on the surface of the CuO mesh as shown in Figure 6f, indicating the persistent and excellent bacteriostatic capacity of the CuO tree system. It is to say that the CuO tree system acting as a barrier can effectively lock the bacteria in raw water. The nanowires of CuO mesh tended to gather after days of running in water, which is probably affected by the complex conditions in the domestic sewage. However, the cores of the mesh were still blocked by the nanocrystals of CuO and the solar absorption rate of the CuO mesh was still above 91.5 % after this long-term running (Figure S6). To sum up, solar sill system based on bacteriostatic CuO tree system would defend the bio-safety of water production in the long-term application.

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Figure 6. (a) Diffusion inhibition test E. coli (gram negative) and S. aureus (gram positive). (b) The copper ion concentration in the raw water, raw water after 8-hillumination in evaporation system and distilled water collected after evaporation. (c) Photograph of the scaled-up CuO nanowire mesh with a size of 0.3 × 0.4 m2. SEM images of the CF paper (d), PPy paper (e) and CuO mesh after evaporation in domestic sewage for 5 days. Insets show the magnification views of the corresponding samples. CONCLUSION In summary, to solve the water purity challenge in the rural area, we developed multifunctional photo-thermal material based on a copper mesh possessing abundant CuO nanowires. The abundant CuO nanowires effectively covered the surface of the copper mesh and increased the light absorption capacity to 93 %. By mimicking the transpiration of nature tree, the CuO tree system exhibited a high evaporation rate of 84.4 % and water yield of 1.42 kg m-2 h-1 even after several cycles. More importantly, the CuO nanowires possessed a great catalytic ability for the degradation of 25



contaminants in raw water in the meanwhile of solar evaporation, avoiding the extra process of the contaminated water disposal process. Moreover, the sustained release of copper ion could effectively inhibit the growth of bacteria, which is of great significance to the quality and boi-safety of the purified water in practical application. In addition, long-term stability and antibacterial activity of the solar distillation system will be assessed in our further study. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: Steady-state energy balance analysis; SEM image; photograph; cycle experiment of evaporation; cycle experiment of degradation; actual industrial water degradation; light absorption spectrum. AUTHOR INFORMATION Corresponding Authors *Email address: [email protected] (W. Wang) *Email address:[email protected] (X. Li) *Email address: [email protected] (Y. Zhao) ORCID Fuyi Cui: 0000-0002-4107-9398 Wei Wang: 0000-0002-0583-0682 Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51873047 and 51761145031), the State Key Laboratory of Urban Water Resource 26


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The multifunctional CuO nanowire mesh possesses excellent solar distillation capacity for pure water production, great catalytic ability for the contaminants’ degradation and bacteriostatic ability.

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Multifunctional CuO nanowire mesh for highly efficient solar

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