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Insulating plasmonic photothermal heat of Ag nanoparticles by a thin carbon shell
Journal of Alloys and Compounds 791 (2019) 380e384
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Insulating plasmonic photothermal heat of Ag nanoparticles by a thin carbon shell Shengbin Cheng a, 1, Shiping Zhan a, 1, Xiaofeng Wu b, **, Guozheng Nie a, Shaobing Wu a, Junshan Hu c, Jin Li b, Shigang Hu b, Yanan Zhang b, Yunxin Liu a, * a b c
Department of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201 China Department of Information Science, Hunan University of Science and Technology, Xiangtan 411201 China School of Physics, University of Electronic Science and Technology of China, Chengdu 610054 China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2019 Received in revised form 18 March 2019 Accepted 24 March 2019 Available online 25 March 2019
Plasmonic photothermal effect of metal nanoparticles has been used for efficient cancer therapy and catalysis. Here, we show the insulation of Plasmonic photothermal heat generated in Ag nanoparticles by a thin carbon shell and their application for solar desalination of seawater. Ag, carbon and Ag with carbon shells (Ag@C) nanoparticles were synthesized by a hydrothermal method. The absorption of Ag@C core/ shell nanoparticles almost covers the whole visible light spectrum from red to violet, which can be tuned by changing the thickness of the carbon shells and the size ratio of Ag cores to carbon shells. Ag@C core/ shell nanoparticles with the size of 50 nm@18 nm exhibit a photothermal conversion efficiency 44% higher than carbon nanoparticles. Importantly, it is observed that carbon shells have excellent heat insulation property so that the temperature decay lifetime of Ag nanoparticles is enhanced by 49% after coating with a thin carbon shell of 18 nm. Ag@C nanoparticles supported by anodic aluminum oxide (AAO) templates exhibit a desalination rate 1000% higher than the direct evaporation of seawater under simulated solar irradiation and 220% higher than that of AAO templates supported carbon nanospheres. © 2019 Elsevier B.V. All rights reserved.
Keywords: Solar desalination Ag@C nanoparticles AAO
1. Introduction Noble metal nanoparticles usually exhibit a Surface Plasmon Resonance (SPR) phenomenon when they interact with light of a wavelength longer than their particle size [1e3]. Silver nanoparticles (Ag NPs) have been widely investigated for plasmonic photothermal therapy, photocatalytic reaction and modulation of upconversion luminescence since they display intense optical response arising from their localized surface plasmon resonance (LSPR) [4e8]. However, the naked Ag NPs are usually not stable in air which are usually coated with a layer of organic molecules for stabilizing them. Therefore, stabilizing Ag NPs with a suitable ligand is a key step for the subsequent applications. Among the reported ligands for Ag NPs, carbon shells have a large number of advantages, including good stability, strong absorption to visible and infrared light, high thermal insulation capability and good
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wu), [email protected] (Y. Liu). 1 These authors contrite equally. https://doi.org/10.1016/j.jallcom.2019.03.336 0925-8388/© 2019 Elsevier B.V. All rights reserved.
biocompatibility [8e14]. Here, we show the insulation of Plasmonic photothermal heat generated in Ag NPs by a thin carbon shell so that the temperature decay lifetime of Ag NPs is enhanced by 49% after coating with a thin carbon shell of 18 nm. Water scarcity is presently one of the most global challenges [14e19]. It is very urgent and desirable to develop advanced water purification technologies to improve the supplies of fresh water [20e25]. To enable efficient solar desalination of seawater, broadband and efficient light absorption is a critical first step. Carbon materials can absorb sunlight at a broadband from visible to infrared [25e29]. However, their photothermal conversion efficiency are usually not ideal for practical applications since the thermal vibration of amorphous carbon has much lower energy mode (called phonon in crystals) than crystal lattice vibration phonon of the crystals. In comparison to carbon materials, Plasmonic metal nanostructures exhibit much higher photothermal conversion efficiency due to their high energy of lattice vibration modes and have been extensively investigated for solar energy conversion due to their strong visible and infrared (IR) plasmonic responses [7,8,11]. Worth mentioning is that Plasmonic metal nanostructures have much narrower absorption bandwidths than
S. Cheng et al. / Journal of Alloys and Compounds 791 (2019) 380e384
carbon materials, so that a large portion of photons over the whole solar spectrum could not be utilized for photothermal conversion. A photothermal converter combining the high efficiency of Plasmonic metal nanostructures with the wide absorption bandwidth of carbon materials should be an ideal candidate for solar desalination of seawater [27e35]. Here, we show that Ag@C core/shell nanoparticles (Ag@C NPs) supported by AAO templates could achieve a desalination rate 1000% higher than the direct evaporation of seawater under simulated solar irradiation and 220% higher than that of AAO templates supported carbon nanospheres.
2. Results and discussion TEM images of carbon and Ag@C core/shell NPs are measured for demonstrating their particle structure, shape and size and shown in Fig. 1, It is clear (1a-1f) that Ag cores are coated with carbon shells to form heterostructured spheres. The particle size of Ag cores is tuned from 50 to 160 nm which corresponds to shell thickness change from 18 to 150 nm. The spherical morphology of pure carbon particles is clearly presented in (1g-1h). The real picture of mono-dispersed Ag@C core/shell NPs and carbon nanospheres after drying is shown in Fig. 2(a and b), from which black color is observed for both carbon and Ag@C NPs. The XRD pattern of Ag@C core/shell NPs is shown in Fig. 2(c) that confirms the existence of the face-centered cubic Ag (JCPDS No. 04-0783) crystal phase, while the amorphous carbon does not generate diffraction peak. The absorption properties of Ag@C core/shell NPs dispersed in ethanol are measured by a
381
UVevis spectrometer and shown in Fig. 2(d). The absorption peak of Ag@C core/shell NPs with a core diameter of 50 nm and a shell thickness of 18 nm centered at 485 nm, which can be modulated to 680 nm with increasing core diameter and shell thickness to 160 and 150 nm respectively. This indicates that the absorption of Ag@C core/shell NPs is highly depended on the particle size of cores and the shell thickness. Simultaneously increasing the size of cores and shells will lead to the red shift of the absorption peaks. In addition, it is noted that the absorption of all samples covers the whole visible spectrum, but the sample with a shell thickness of 150 nm exhibits much stronger absorption in the range of 600e850 nm than other samples which is mainly due to the enhanced absorption of carbon shell. The above absorption spectra indicate that there is a large range of overlapping between the absorption of Ag@C core/shell NPs and the solar spectrum. The photothermal conversion effect of Ag@C core/shell and pure carbon spheres is measured by an Infrared thermal imager (Infortech, Germany), with a measurement accuracy of ±1.5% (<0 resp. > 100) C and a temperature resolution better than 0.05 K, and then realtime communicated to a desktop computer for the evaluation of the evaporation rate and heat insulation efficiency. The temperature decay was repeatedly detected for three times for obtaining an average temperature decay with an error less than 4%. The carbon spheres or Ag@C core/shell NPs are first paved on a glass slide to form a rectangle film with the region of 10 30 mm and thickness of 0.5 mm, then transferred into an airtight chamber with simulated solar irradiation. Under the irradiation with power of 40 W for 10 min, the temperature of the carbon film is enhanced from room temperature (26.02 C) to 32.17 C, while those of Ag@C core/shell
Fig. 1. TEM images of Ag@C NPs prepared at 180 C for 4 h with the concentration of [Agþ]: (aeb) 0.1 M/160 nm @ 150 nm; (ced) 0.05 M/60 nm @ 130 nm; (eef) 0.025 M/50 nm @ 18 nm. (geh) TEM images of carbon spheres prepared at 180 C for 5 h. (ien) Statistical analysis of the diameter of Ag Core and Ag@C Core/Shell particles, (i,j) corresponding to the sample (aeb); (k, 1) corresponding to the sample (c,d); (m,n) corresponding to the sample (e,f); (o,p) the diameter distribution of Ag core and Ag@C core/shell particles: Sample 1 corresponds to (a,b), sample 2 corresponds to (c,d), and sample 3 corresponds to (e,f).
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Figure 2. (aeb) Photos for displaying the color of Ag@C core/shell nanoparticle and carbon nanosphere powders. (c) XRD pattern of Ag@C NPs. (d) Extinction spectra of Ag@C NPs with different [Agþ]: (Black curve) 0.1 M/160 nm @ 150 nm; (Blue curve) 0.05 M/60 nm @ 130 nm; (Red curve) 0.025 M/50 nm @ 18 nm; (Green curve) 0.015 M/35 nm @ 15 nm; (Pink curve) 0.01 M/30 nm @ 15 nm.
NPs films are enhanced from room temperature (26.02 C) to 32.81 C, 34.00 C and 34.88 C for particle sizes of 160 nm @ 150 nm, 60 nm @ 130 nm and 50 nm @ 18 nm, respectively (Fig. 3). The enhanced temperature for carbon spheres and Ag@C core/shell NPs is shown in Fig. 4 for intuitive comparison. The carbon spheres generated a temperature enhancement of △T ¼ 6.15 C while Ag@C core/shell NPs led to a temperature enhancement of 6.75 C, 8.00 C and 8.85 C for particle sizes of 160 nm @ 150 nm, 60 nm @ 130 nm and 50 nm @ 18 nm, respectively. It is clear that Ag@C NPs with particle size of 50 nm @ 18 nm exhibit a photothermal conversion efficiency 44% higher than pure carbon NPs. The temperature decay
curves of Ag and Ag@C rectangle films are clearly presented in Fig. 4 b), that Ag@C core-shell NPs release the absorbed heat obviously slower than Ag NPs due to the thermal insulation effect of carbon shells. Fitting the temperature decay data with the function y ¼ a ex=t þ b, the decay lifetime (t) of Ag NPs and Ag@C NPs with a shell thickness of 18 nm is determined to be 3.22 and 4.81 min, respectively. It means that the temperature decay lifetime of Ag NPs is enhanced by 49% after coating with an ultrathin carbon shell of 18 nm. For the application of solar desalination of seawater, black Ag@C NPs are successively supported by an AAO template and a porous
Fig. 3. Direct observation of plasmonic photothermal effect. The photothermal imaging photos of the rectangle films composed of carbon or Ag@C core/shell NPs under the simulated solar irradiation with the power of 40 W for 10 min. (a) Carbon, Temperature maximum is 32.17 C; (b) Ag@C NPs (160 nm @ 150 nm), Temperature maximum is 32.81 C; (c) Ag@C NPs (60 nm @ 130 nm), Temperature maximum is 34.00 C; (d) Ag@C NPs (50 nm @ 18 nm), Temperature maximum is 34.88 C).
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Fig. 4. a) Enhanced temperature (△T) of the carbon & Ag@C core/shell nanoparticle films under the simulated solar irradiation with the power of 40 W for 10 min; 1) carbon,△T ¼ 6.15 C; 2) Ag@C (160 nm @ 150 nm),△T ¼ 6.75 C; 3) Ag@C (60 nm @ 130 nm),△T ¼ 8.00 C; 4) Ag@C (50 nm @ 18 nm),△T ¼ 8.85 C; b) Normalized temperature decay of Ag and Ag@C nanoparticles after irradiation by a simulated solar light for 10 min for demonstrating the heat insulation efficiency of a thin carbon shell with thickness of 18 nm (Ag NPs have the same diameter as Ag cores in Ag@C NPs). The inset in a): Photos of four different samples dispersed in ethanol.
plastic film to form a Composite Membrane (CM). Ag@C core/shell NPs play a role of photothermal converters while AAO templates supply an energy-exchange platform for seawater and Ag@C NPs based on the capillary action of nanoscale holes and porous plastic films ensure the floating of the whole CM on water. AAO template have a thickness H ¼ 40 mm, a hole diameter D ¼ 250 nm and a gap between holes (hole center to center) L ¼ 450 nm. The selected Ag@C NPs have an average particle size of 450 nm, which can be well aligned on the surface of the AAO template for seawater desalination. The porous plastic film has a thickness of 1.0 mm, a hole diameter of 1.0 mm and a gap between holes of 0.5 mm. Fig. 5a and b shows clearly the model diagram of solar water desalination device based on Ag@C core/shell NPs, AAO and porous plastic film. The composite membrane with three functional layers has a broadband absorption to solar energy, localized photothermal converters (Ag@C NPs) and efficient transportation pathways for vapor that makes it particularly suitable for solar desalination of seawater. It can be clearly seen in Fig. 5 c) that the seawater in the beaker with a CM is completely desalinated while that in the beaker
without a CM is just desalinated by 15%, after irradiated by a simulated solar light source with the power of 40 W for 10 min. Ag@C NPs supported by AAO templates also exhibit a desalination rate 220% higher than that of AAO templates supported carbon nanospheres (see Fig. S1 and related discussion in Supporting Information). The higher desalination rate of Ag@C NPs is further confirmed by the crystallization of the salt on AAO templates (see Fig. S2 and related discussion in Supporting Information) which is in agreement with the comparison from the residual water.
3. Conclusions In summary, Black Ag@C core/shell nanoparticles were synthesized by a hydrothermal method. Their absorption almost covers the whole visible light spectrum from red to violet, which can be tuned by changing the thickness of the carbon shells and the size ratio of Ag nanoparticles to carbon shells. Plasmonic photothermal effect of Ag@C core/shell NPs is directly observed by a Thermographic system which is more efficient than pure carbon NPs. Ag@C core/shell nanoparticles with the size of 50 nm@18 nm exhibit a photothermal conversion efficiency 44% higher than carbon NPs. Meanwhile, carbon shells have excellent thermal insulation property so that the temperature of Ag nanoparticles with an ultrathin carbon shell of 18 nm day at a rate 25.4% slower than that of Ag nanoparticles without shells. Ag@C NPs supported by AAO templates exhibit a desalination rate 1000% higher than the direct evaporation of seawater under simulated solar irradiation and 220% higher than that of AAO templates supported Carbon nanospheres.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements Fig. 5. Model diagram of solar water desalination device based on Ag@C core/shell NPs. a) Experimental set-up for solar water desalination device. b) Enlarged model for demonstrating the three functional layers of the CM. c) Comparison of desalination efficiency of Seawater in a beaker with and without a CM.
This work was funded by the National Natural Science Foundation of China (Grant No. 61675067, 61575062 and 51675174) and the Scientific Research Fund of Hunan Province Education Department (Grant No. 16C0627, 17B090 and 16A072).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Insulating plasmonic photothermal heat of Ag nanoparticles by a thin carbon shell Shengbin Cheng a, 1, Shiping Zhan a, 1, Xiaofeng Wu b, **, Guozheng Nie a, Shaobing Wu a, Junshan Hu c, Jin Li b, Shigang Hu b, Yanan Zhang b, Yunxin Liu a, * a b c
Department of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201 China Department of Information Science, Hunan University of Science and Technology, Xiangtan 411201 China School of Physics, University of Electronic Science and Technology of China, Chengdu 610054 China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2019 Received in revised form 18 March 2019 Accepted 24 March 2019 Available online 25 March 2019
Plasmonic photothermal effect of metal nanoparticles has been used for efficient cancer therapy and catalysis. Here, we show the insulation of Plasmonic photothermal heat generated in Ag nanoparticles by a thin carbon shell and their application for solar desalination of seawater. Ag, carbon and Ag with carbon shells (Ag@C) nanoparticles were synthesized by a hydrothermal method. The absorption of Ag@C core/ shell nanoparticles almost covers the whole visible light spectrum from red to violet, which can be tuned by changing the thickness of the carbon shells and the size ratio of Ag cores to carbon shells. Ag@C core/ shell nanoparticles with the size of 50 nm@18 nm exhibit a photothermal conversion efficiency 44% higher than carbon nanoparticles. Importantly, it is observed that carbon shells have excellent heat insulation property so that the temperature decay lifetime of Ag nanoparticles is enhanced by 49% after coating with a thin carbon shell of 18 nm. Ag@C nanoparticles supported by anodic aluminum oxide (AAO) templates exhibit a desalination rate 1000% higher than the direct evaporation of seawater under simulated solar irradiation and 220% higher than that of AAO templates supported carbon nanospheres. © 2019 Elsevier B.V. All rights reserved.
Keywords: Solar desalination Ag@C nanoparticles AAO
1. Introduction Noble metal nanoparticles usually exhibit a Surface Plasmon Resonance (SPR) phenomenon when they interact with light of a wavelength longer than their particle size [1e3]. Silver nanoparticles (Ag NPs) have been widely investigated for plasmonic photothermal therapy, photocatalytic reaction and modulation of upconversion luminescence since they display intense optical response arising from their localized surface plasmon resonance (LSPR) [4e8]. However, the naked Ag NPs are usually not stable in air which are usually coated with a layer of organic molecules for stabilizing them. Therefore, stabilizing Ag NPs with a suitable ligand is a key step for the subsequent applications. Among the reported ligands for Ag NPs, carbon shells have a large number of advantages, including good stability, strong absorption to visible and infrared light, high thermal insulation capability and good
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wu), [email protected] (Y. Liu). 1 These authors contrite equally. https://doi.org/10.1016/j.jallcom.2019.03.336 0925-8388/© 2019 Elsevier B.V. All rights reserved.
biocompatibility [8e14]. Here, we show the insulation of Plasmonic photothermal heat generated in Ag NPs by a thin carbon shell so that the temperature decay lifetime of Ag NPs is enhanced by 49% after coating with a thin carbon shell of 18 nm. Water scarcity is presently one of the most global challenges [14e19]. It is very urgent and desirable to develop advanced water purification technologies to improve the supplies of fresh water [20e25]. To enable efficient solar desalination of seawater, broadband and efficient light absorption is a critical first step. Carbon materials can absorb sunlight at a broadband from visible to infrared [25e29]. However, their photothermal conversion efficiency are usually not ideal for practical applications since the thermal vibration of amorphous carbon has much lower energy mode (called phonon in crystals) than crystal lattice vibration phonon of the crystals. In comparison to carbon materials, Plasmonic metal nanostructures exhibit much higher photothermal conversion efficiency due to their high energy of lattice vibration modes and have been extensively investigated for solar energy conversion due to their strong visible and infrared (IR) plasmonic responses [7,8,11]. Worth mentioning is that Plasmonic metal nanostructures have much narrower absorption bandwidths than
S. Cheng et al. / Journal of Alloys and Compounds 791 (2019) 380e384
carbon materials, so that a large portion of photons over the whole solar spectrum could not be utilized for photothermal conversion. A photothermal converter combining the high efficiency of Plasmonic metal nanostructures with the wide absorption bandwidth of carbon materials should be an ideal candidate for solar desalination of seawater [27e35]. Here, we show that Ag@C core/shell nanoparticles (Ag@C NPs) supported by AAO templates could achieve a desalination rate 1000% higher than the direct evaporation of seawater under simulated solar irradiation and 220% higher than that of AAO templates supported carbon nanospheres.
2. Results and discussion TEM images of carbon and Ag@C core/shell NPs are measured for demonstrating their particle structure, shape and size and shown in Fig. 1, It is clear (1a-1f) that Ag cores are coated with carbon shells to form heterostructured spheres. The particle size of Ag cores is tuned from 50 to 160 nm which corresponds to shell thickness change from 18 to 150 nm. The spherical morphology of pure carbon particles is clearly presented in (1g-1h). The real picture of mono-dispersed Ag@C core/shell NPs and carbon nanospheres after drying is shown in Fig. 2(a and b), from which black color is observed for both carbon and Ag@C NPs. The XRD pattern of Ag@C core/shell NPs is shown in Fig. 2(c) that confirms the existence of the face-centered cubic Ag (JCPDS No. 04-0783) crystal phase, while the amorphous carbon does not generate diffraction peak. The absorption properties of Ag@C core/shell NPs dispersed in ethanol are measured by a
381
UVevis spectrometer and shown in Fig. 2(d). The absorption peak of Ag@C core/shell NPs with a core diameter of 50 nm and a shell thickness of 18 nm centered at 485 nm, which can be modulated to 680 nm with increasing core diameter and shell thickness to 160 and 150 nm respectively. This indicates that the absorption of Ag@C core/shell NPs is highly depended on the particle size of cores and the shell thickness. Simultaneously increasing the size of cores and shells will lead to the red shift of the absorption peaks. In addition, it is noted that the absorption of all samples covers the whole visible spectrum, but the sample with a shell thickness of 150 nm exhibits much stronger absorption in the range of 600e850 nm than other samples which is mainly due to the enhanced absorption of carbon shell. The above absorption spectra indicate that there is a large range of overlapping between the absorption of Ag@C core/shell NPs and the solar spectrum. The photothermal conversion effect of Ag@C core/shell and pure carbon spheres is measured by an Infrared thermal imager (Infortech, Germany), with a measurement accuracy of ±1.5% (<0 resp. > 100) C and a temperature resolution better than 0.05 K, and then realtime communicated to a desktop computer for the evaluation of the evaporation rate and heat insulation efficiency. The temperature decay was repeatedly detected for three times for obtaining an average temperature decay with an error less than 4%. The carbon spheres or Ag@C core/shell NPs are first paved on a glass slide to form a rectangle film with the region of 10 30 mm and thickness of 0.5 mm, then transferred into an airtight chamber with simulated solar irradiation. Under the irradiation with power of 40 W for 10 min, the temperature of the carbon film is enhanced from room temperature (26.02 C) to 32.17 C, while those of Ag@C core/shell
Fig. 1. TEM images of Ag@C NPs prepared at 180 C for 4 h with the concentration of [Agþ]: (aeb) 0.1 M/160 nm @ 150 nm; (ced) 0.05 M/60 nm @ 130 nm; (eef) 0.025 M/50 nm @ 18 nm. (geh) TEM images of carbon spheres prepared at 180 C for 5 h. (ien) Statistical analysis of the diameter of Ag Core and Ag@C Core/Shell particles, (i,j) corresponding to the sample (aeb); (k, 1) corresponding to the sample (c,d); (m,n) corresponding to the sample (e,f); (o,p) the diameter distribution of Ag core and Ag@C core/shell particles: Sample 1 corresponds to (a,b), sample 2 corresponds to (c,d), and sample 3 corresponds to (e,f).
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Figure 2. (aeb) Photos for displaying the color of Ag@C core/shell nanoparticle and carbon nanosphere powders. (c) XRD pattern of Ag@C NPs. (d) Extinction spectra of Ag@C NPs with different [Agþ]: (Black curve) 0.1 M/160 nm @ 150 nm; (Blue curve) 0.05 M/60 nm @ 130 nm; (Red curve) 0.025 M/50 nm @ 18 nm; (Green curve) 0.015 M/35 nm @ 15 nm; (Pink curve) 0.01 M/30 nm @ 15 nm.
NPs films are enhanced from room temperature (26.02 C) to 32.81 C, 34.00 C and 34.88 C for particle sizes of 160 nm @ 150 nm, 60 nm @ 130 nm and 50 nm @ 18 nm, respectively (Fig. 3). The enhanced temperature for carbon spheres and Ag@C core/shell NPs is shown in Fig. 4 for intuitive comparison. The carbon spheres generated a temperature enhancement of △T ¼ 6.15 C while Ag@C core/shell NPs led to a temperature enhancement of 6.75 C, 8.00 C and 8.85 C for particle sizes of 160 nm @ 150 nm, 60 nm @ 130 nm and 50 nm @ 18 nm, respectively. It is clear that Ag@C NPs with particle size of 50 nm @ 18 nm exhibit a photothermal conversion efficiency 44% higher than pure carbon NPs. The temperature decay
curves of Ag and Ag@C rectangle films are clearly presented in Fig. 4 b), that Ag@C core-shell NPs release the absorbed heat obviously slower than Ag NPs due to the thermal insulation effect of carbon shells. Fitting the temperature decay data with the function y ¼ a ex=t þ b, the decay lifetime (t) of Ag NPs and Ag@C NPs with a shell thickness of 18 nm is determined to be 3.22 and 4.81 min, respectively. It means that the temperature decay lifetime of Ag NPs is enhanced by 49% after coating with an ultrathin carbon shell of 18 nm. For the application of solar desalination of seawater, black Ag@C NPs are successively supported by an AAO template and a porous
Fig. 3. Direct observation of plasmonic photothermal effect. The photothermal imaging photos of the rectangle films composed of carbon or Ag@C core/shell NPs under the simulated solar irradiation with the power of 40 W for 10 min. (a) Carbon, Temperature maximum is 32.17 C; (b) Ag@C NPs (160 nm @ 150 nm), Temperature maximum is 32.81 C; (c) Ag@C NPs (60 nm @ 130 nm), Temperature maximum is 34.00 C; (d) Ag@C NPs (50 nm @ 18 nm), Temperature maximum is 34.88 C).
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Fig. 4. a) Enhanced temperature (△T) of the carbon & Ag@C core/shell nanoparticle films under the simulated solar irradiation with the power of 40 W for 10 min; 1) carbon,△T ¼ 6.15 C; 2) Ag@C (160 nm @ 150 nm),△T ¼ 6.75 C; 3) Ag@C (60 nm @ 130 nm),△T ¼ 8.00 C; 4) Ag@C (50 nm @ 18 nm),△T ¼ 8.85 C; b) Normalized temperature decay of Ag and Ag@C nanoparticles after irradiation by a simulated solar light for 10 min for demonstrating the heat insulation efficiency of a thin carbon shell with thickness of 18 nm (Ag NPs have the same diameter as Ag cores in Ag@C NPs). The inset in a): Photos of four different samples dispersed in ethanol.
plastic film to form a Composite Membrane (CM). Ag@C core/shell NPs play a role of photothermal converters while AAO templates supply an energy-exchange platform for seawater and Ag@C NPs based on the capillary action of nanoscale holes and porous plastic films ensure the floating of the whole CM on water. AAO template have a thickness H ¼ 40 mm, a hole diameter D ¼ 250 nm and a gap between holes (hole center to center) L ¼ 450 nm. The selected Ag@C NPs have an average particle size of 450 nm, which can be well aligned on the surface of the AAO template for seawater desalination. The porous plastic film has a thickness of 1.0 mm, a hole diameter of 1.0 mm and a gap between holes of 0.5 mm. Fig. 5a and b shows clearly the model diagram of solar water desalination device based on Ag@C core/shell NPs, AAO and porous plastic film. The composite membrane with three functional layers has a broadband absorption to solar energy, localized photothermal converters (Ag@C NPs) and efficient transportation pathways for vapor that makes it particularly suitable for solar desalination of seawater. It can be clearly seen in Fig. 5 c) that the seawater in the beaker with a CM is completely desalinated while that in the beaker
without a CM is just desalinated by 15%, after irradiated by a simulated solar light source with the power of 40 W for 10 min. Ag@C NPs supported by AAO templates also exhibit a desalination rate 220% higher than that of AAO templates supported carbon nanospheres (see Fig. S1 and related discussion in Supporting Information). The higher desalination rate of Ag@C NPs is further confirmed by the crystallization of the salt on AAO templates (see Fig. S2 and related discussion in Supporting Information) which is in agreement with the comparison from the residual water.
3. Conclusions In summary, Black Ag@C core/shell nanoparticles were synthesized by a hydrothermal method. Their absorption almost covers the whole visible light spectrum from red to violet, which can be tuned by changing the thickness of the carbon shells and the size ratio of Ag nanoparticles to carbon shells. Plasmonic photothermal effect of Ag@C core/shell NPs is directly observed by a Thermographic system which is more efficient than pure carbon NPs. Ag@C core/shell nanoparticles with the size of 50 nm@18 nm exhibit a photothermal conversion efficiency 44% higher than carbon NPs. Meanwhile, carbon shells have excellent thermal insulation property so that the temperature of Ag nanoparticles with an ultrathin carbon shell of 18 nm day at a rate 25.4% slower than that of Ag nanoparticles without shells. Ag@C NPs supported by AAO templates exhibit a desalination rate 1000% higher than the direct evaporation of seawater under simulated solar irradiation and 220% higher than that of AAO templates supported Carbon nanospheres.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements Fig. 5. Model diagram of solar water desalination device based on Ag@C core/shell NPs. a) Experimental set-up for solar water desalination device. b) Enlarged model for demonstrating the three functional layers of the CM. c) Comparison of desalination efficiency of Seawater in a beaker with and without a CM.
This work was funded by the National Natural Science Foundation of China (Grant No. 61675067, 61575062 and 51675174) and the Scientific Research Fund of Hunan Province Education Department (Grant No. 16C0627, 17B090 and 16A072).
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