Evaporation
Bio-Inspired Evaporation Through Plasmonic Film of Nanoparticles at the Air–Water Interface Zhenhui Wang, Yanming Liu, Peng Tao, Qingchen Shen, Nan Yi, Fangyu Zhang, Quanlong Liu, Chengyi Song, Di Zhang, Wen Shang*, and Tao Deng* Evaporation is critical to many fundamental processes[1–6] and important industry applications.[7–12] In power plants that utilize high pressure steam,[7] an efficient evaporation process can improve the overall system level power generation efficiency, and result in savings of billions of dollars for the world thermal power market. Efficient evaporation process can also enhance the performance of phase change based heat transfer systems,[8–10] such as heat pipes[8,9] and vapor chambers.[10] For the sterilization applications, the steam generation through evaporation helps minimize the potential contamination of medical devices by biological microbes.[11] The improvement of selective evaporation process can also significantly increase the separation efficiency of mixed solvents during distillation processes.[12] Evaporation is also vital to the proper functioning of biological systems.[13–17] For example, efficient evaporation during sweating helps the precise thermal regulation of human bodies.[13–15] In plants, transpiration, which involves the evaporation of water at the surfaces of leaves, not only cools the plants but also enables the mass transportation of mineral nutrients from roots to other parts of the plants.[16,17] In both sweating and transpiration, the control of local environment of evaporative surfaces, such as the control of temperature and the liquid flow within skin and leaf surfaces, is critical to achieve the efficient evaporation.[13–17] Inspired by the localized control of evaporative surfaces in biological systems, this work intended to achieve efficient evaporation in engineered systems through manipulation of the evaporative surfaces. In the common engineered systems, the evaporative surface is normally the liquid surface at the air–liquid interface. There is rarely any localized control of such evaporative surface as in biological systems. In this study, to mimic the porous biological evaporative surface, we utilized a self-assembly process to create films of gold nanoparticles at the air–water interface that could pump the liquid Z. H. Wang,[+] Y. M. Liu,[+] Dr. P. Tao, Q. C. Shen, N. Yi, F. Y. Zhang, Q. L. Liu, Dr. C. Y. Song, Prof. D. Zhang, Prof. W. Shang, Prof. T. Deng State Key Laboratory of Metal Matrix Composites School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200240, P. R. China E-mail: [email protected]; [email protected] [+]Z.H.W.
and Y.M.L. contributed equally to this work. DOI: 10.1002/smll.201401071 small 2014, DOI: 10.1002/smll.201401071
to the open surface during liquid evaporation through capillary flow (Figure 1a and Supporting Information Figure S1). Plasmonic heating was used to control the local temperature of the evaporative surface of the assembled film. Nobel metal nanoparticles possess extremely large absorption cross section under resonant light excitation.[18–24] The plasmonic photothermal conversion of gold nanoparticles within the assembled film is used to generate instant, intense and localized heat to drive the evaporation process.[25–31] Such controlled heating of the evaporative surface is similar to the heating of skins through the pumping of warm blood locally to the skin surface with the opening of blood veins (Figure S1, Supporting Information).[13–15] As the skin temperature increases, the evaporation rate also increases to help cool down the human body. The plasmonic thermal input in our system selectively heated up the evaporative surface of the assembled film. The thermal energy was transferred locally to the surface portion of water where evaporation happened. Such localized transfer minimizes the loss of thermal energy to the non-evaporative portion of water, and potentially enables much higher evaporation efficiency than processes that involve the heat transfer to bulk liquid.[11,12,31] As shown in Figure 1a, the plasmonic film converts the incident optical energy into thermal energy and generates a hot zone at the air–water interface. The vapor bubbles grow around the nanoparticles in the film due to the continuous heat generation from the plasmonic light-to-heat conversion process.[31–33] Once the bubbles reach the air–water interface, they burst and release the hot vapor inside. The film is located right at the air–water interface, so the vapor generated can be immediately released into air without losing energy to the unheated portion of liquid. In the evaporation process where the heat is not localized at the evaporative surface, such as the boiling of water in a kettle, the bubbles start the nucleation at the walls of the container, and then travel to the evaporative surface. As these bubbles travel within the bulk liquid to the evaporative surface, large portion of the thermal energy is lost due to the energy transfer to the non-evaporative portion of the liquid and the walls of the container. Such energy loss lowers the heat-to-evaporation conversion efficiency. In the system studied in this work, most of the thermal energy was utilized to heat up the water only at the evaporative surface. Such localized heaing minimized the heat loss and enhanced heat-to-evaporation conversion efficiency. The liquid water evaporated at the surface of film was quickly replenished through the capillary flow inside the porous film,
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nanoparticles. Consequently, the nanoparticles became less stable and were trapped at the air–water interface. The trapped gold particles then self-assembled into a free-floating film. Figure 1b shows a photograph of a uniform shinning film with a diameter > 3 cm at the air–water interface. The SEM image (Figure 1c), however, reveals a disordered network structure that should be ascribed to the random-walk nature of the self-assembly process.[36] Through analysis of the 2D projection of the SEM images, the selfassembled films have porosity of ≈ 40%. Figure 1d shows the absorption spectra of both the self-assembled film and the particle solution before the assembly process. The maximum absorption of the particle solution was around 520 nm, which agreed well with the theoretical prediction of the plasmonic absorption of gold nanoparticles with diameter ≈18 nm.[37] Due to the relatively delocalized plasmonic absorption in the assembled film,[38,39] the absorption spectrum of the film changed to a broadband absorption that extended across the full visible light range. To study the evaporation process, the cuvette containing the floating assembled film was placed on an analytic balance to measure the in situ weight loss (Figure 1e). A 532-nm laser with adjustable output power was used for the plasmonic heating of the film. The laser with a beam diameter of 5 mm was aligned perpendicularly to the surface of the film. The temperature distribution of the samples was visualized by an infrared (IR) camera. Figure 2 shows the side-view IR images of a cuvette with the plasmonic film at the top of solution. Before the Figure 1. a) Schematic illustration of efficient evaporation of liquid assisted by a floating laser was turned on, it showed a uniform plasmonic film of gold nanoparticles self-assembled at the air–water interface. b) A temperature distribution around 30.4 °C. photograph of a floating film with a diameter > 3 cm at the air–water interface. c) SEM images After illumination for 2 min (Figure 2b) at of self-assembled film of gold nanoparticles under low and high magnifications. d) Absorption 2 2 spectra of the film of gold nanoparticles and the aqueous solution of gold nanoparticles. power density of 5.09 W/cm (≈0.6 kJ/cm ), a localized hot zone was observed. For the e) Evaporation process monitored by a balance and an IR camera. convenience of discussion, the hot zone is similar to the liquid flow inside the skin or inside the xylem defined in this paper as the zone that has local temperature of the plants. As water evaporated from the surface, the free within two degrees of maximum temperature. The top porfloating film and the heating zone moved together with the tion of the water was heated by the film to 41.2 °C and generreceding air–water interface, and the same efficient evapora- ated a hot zone of ≈2.8 mm in depth after 2-min illumination. tion process continued. After 5-min illumination (≈1.5 kJ/cm2, Figure 2c), the surface The gold nanoparticles with a diameter of ≈18 nm were temperature reached 45.5 °C, and the hot zone was ≈2.5 mm synthesized using the citrate reduction approach.[34] The due to the increased temperature gradient. After 10-min assembled films were obtained by slow diffusion of vapor illumination (≈3 kJ/cm2, Figure 2d), the surface temperaof formic acid into the solution of the gold nanoparticles.[35] ture reached 47.3 °C, and the depth of the hot zone further As the formic acid vapor slowly dissolved into the solution, shrinked to ≈2.3 mm. The existence of the hot zone indicated formic acid was dissociated, and the dissociated hydrogen that most of the absorbed energy was confined locally in ions protonated the citrate group at the surface of the gold the evaporation zone. Figure 2e shows the plots of the
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small 2014, DOI: 10.1002/smll.201401071
Bio-Inspired Evaporation Through Plasmonic Film of Nanoparticles at the Air–Water Interface
film than for the cuvette with the solution of gold nanoparticles. Figure 3e shows the line profile of the temperature distribution from the top surface to 2 cm below the surface. The temperature of the film sample reached the maximum of 57.8 °C at 1.4 mm, and then quickly dropped to 40 °C at 5 mm. The hot zone was tightly confined at the air–water interface. By contrast, the temperature of the solution sample was 44.5 °C at 3.2 mm and slowly decreased to 40 °C at 2 cm. The broader hot zone of the particle solution indicated a bulk heating of the solution during the plasmonic heating process. It should be noted that the temperature recorded by the IR camera was actually lower than the real value, due to the infrared absorption and emission by the front walls of the cuvettes. The sideview images, however, provide the relative temperature profiles of the different portion of the solution inside the cuvette, and offer valuable information about the thermal energy distribution within the solution. As discussed in the above section, the depth of the hot zone for the cuvette with the assembled film depends on the laser power and the duration of laser illumination. For the cuvette with the solution of gold nanoparticles, besides the laser power and the duration of laser illumination, the absorption path length of the solution is another critical influencing factor for the depth of the hot zone. Hypothetically, if the concentration of the particles in solution further increases, the hot zone might be more localized and potentially matches that of the film due to the decrease of the Figure 2. Evaporation through the film of gold nanoparticles monitored by IR imaging and absorption path length. In such a situaweight loss analysis. Side-view IR images at different time under ≈1 W of laser illumination: tion, however, the concentration of the a) before laser was on; b) after 2 min of illumination; c) after 5 min of illumination; d) after 10 gold nanoparticles in the solution would min of illumination; e) evaporation weight loss under different laser power density: 0 (dotted be well above the percolation threshold. line), 5.09 W/cm2, and 10.18 W/cm2. The concentrated particles would tend to precipitate as agglomerates.[40–42] For evaporation weight loss as a function of illumination time the assembled films, there is no need to disperse particles under the laser power density of 5.09 W/cm2 and 10.18 W/cm2. into the evaporation liquid and the films have much better It was found that once the laser was turned on, the weight stability, even when most of the liquid evaporates away. loss increased almost linearly. After 10 min of illumination, Another advantage of using the film instead of the solution the stable evaporation rate reached about 0.2 mg/s and of nanoparticle is the maximized usage of the nanoparticles 0.4 mg/s for laser power density of 5.09 W/cm2 (≈3 kJ/cm2) in the evaporation process. Most of the particles used in the and 10.18 W/cm2 (≈6 kJ/cm2), respectively. Without laser film participated in the absorption and conversion of optical illumination, the evaporation rate was almost negligible energy into thermal energy. For the solution of gold nano(0.01 mg/s) as indicated by the dotted line in Figure 2e. particles, only particles within the absorption path length Figure 3 shows the optical and IR images of the participated in the absorption and conversion process, and plasmonic film and solution of gold nanoparticles particles outside the absorption path length were not utilized. (≈4.39 × 1013 particles/mL). As shown in the IR images Furthermore, the self-assembled films can be easily recycled (Figure 3b,d), under the same laser illumination the hot zone to enable the reuse of the evaporation medium (Figure S2, is much more localized for the cuvette with the self-assembled Supporting Information). With the potential use of approsmall 2014, DOI: 10.1002/smll.201401071
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Figure 3. Comparison between evaporation through the assembled plasmonic film and solution of gold nanoparticles. a,b) Optical image of an assembled film within a cuvette and IR side-view image of the same sample at 10.18 W/cm2 of laser illumination. c,d) Optical image of the solution of gold nanparticles within a cuvette and IR side-view image of the same sample at 10.18 W/cm2 of laser illumination. e) Temperature profiles of the film and solution samples after 10.18 W/cm2 of laser illumination for 2 min. f) Evaporation rate as a function of illumination time. g) Evaporation rate after 20 min illumination under different laser power.
priate flexible supporting structure for the assembled film, the approach can also be applied in large scale where large amount of vapor generation is required. Figure 3f shows the change of evaporation rate with the duration of laser (10.18 W/cm2) illumination for both the film and the solution of gold nanoparticles. For the film, the evaporation rate quickly jumped to a high value (≈0.4 mg/s) and reached a steady rate after 150 s. Wheras, for the solution, the evaporation rate gradually increased and it took ≈800 s before the rate became stable. The delay in reaching the steady state for the solution was ascribed to the travelling of the vapor bubbles to the surface of the solution, and also the related loss of heat to the bulk liquid. It is worth to mention that the amount of gold nanoparticles in the solution was more than 2 times of the particles in the assembled film, but at the steady evaporation stage, the evaporation rate of the solution was only half of that of the film. Figure 3g provides the plots of the evaporation rate for the film and the solution at different laser power. For both samples, as the laser power increased, the evaporation rate increased almost linearly. The rate of evaporation (v) and the heat power of evaporation (QE) can be defined in Equation 1 and 2, respectively, v = d m /d t QE =
(
(1)
)
dm × H E / d t = vH E / M M
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(2)
where m is the mass of the liquid (water), t is the time, M is the molar weight of water, HE is the molar heat of evaporation of water. If we assume the conversion of the input laser power (Qlaser) into the heat power of evaporation (QE) follows the below linear relationship, QE = α Qlaser
(3)
where α is a constant. Then v = QE M / H E = α Qlaser M / H E
(4)
Fitting the experimental results in Figure 3g with Equation 4 shows that α equals to 0.44 and 0.20 for the film and the solution of gold nanoparticles, respectively. The conversion from optical energy to the heat of evaporation for the solution of gold nanoparticles was about 20%. Most energy was lost through the heating of the bulk solution and the container as illustrated by the broad heating zone. For the floating plasmonic film, the conversion efficiency from optical energy to the heat of evaporation was about 44%, more than twice of the efficiency for the solution of gold nanoparticles. This analysis is also consistent with the IR imaging observation that the heat generated by the assembled film was strongly localized and efficiently utilized to evaporate the water from the top surface. In the experiment, there was about 10% of the incident laser light reflected away from
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small 2014, DOI: 10.1002/smll.201401071
Bio-Inspired Evaporation Through Plasmonic Film of Nanoparticles at the Air–Water Interface
the surface of the assembled film. Given that the 2D porosity of the film was ≈40%, the conversion efficiency from the absorbed light to the heat of evaporation was actually larger than 80% for the solid areas of the film. There are certainly some contribution to the energy loss through thermal diffusion, but the major cause of the energy loss in the film was from the laser light passing through the empty space of the film. It can be expected that films with denser and more closepacked structures than films used in this work will further improve the energy conversion efficiency. The photothermal conversion efficiency of the gold nanoparticles that are dispersed inside the solution greatly depends on the mismatch between the wavelength of the illumination and the plasmon resonance wavelength, and also the mismatch between these two wavelengths that is induced by the assembly states.[43] With the relatively broadband absorption of the self-assembled films of gold nanoparticles, the photothermal conversion efficiency actually is higher using the broadband illumination than using the narrow band illumination. The initial test using broadband illumination from a solar simulator confirmed the increase of the photothermal conversion efficiency and the evaporation rate, compared to the experiments using illumination from the 532-nm laser (Figure S3, Supporting Information). In summary, we have demonstrated a bio-inspired surface evaporation approach through the localized plasmonic heating by the free-floating film of gold nanoparticles at the air–water interface. Compared to other evaporation processes, the thermal energy is provided directly at the evaporative surface, leading to a fast and efficient evaporation. With this approach, the bulk liquid temperature is relatively undisturbed while the top surface reaches a large evaporation rate. The free-floating film can move downwards together with the air–water interface, thus the efficient evaporation can be sustained during the receding of the interface. This bio-inspired approach provides not only high efficiency in vapor generation, but also high efficiency in material usage of the plasmonic particles as almost all the nanoparticles in the film participated in the plasmonic heating process. With fine tuning of the micro and nanostructure of the films, the efficiency could be further improved. The facile evaporation design could be easily adapted and applied to other photothermal conversion systems, such as assembled films composed of other noble particles with different morphology and particle sizes, to provide fast and efficient evaporation. This approach will bring tremendous benefit for energy saving in solar drying, steam generation and distillation processes, and also open new paths in heat transfer and other related thermal applications.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
small 2014, DOI: 10.1002/smll.201401071
Acknowledgements This work was supported by Natural Science Foundation of China (Grant No: 91333115), Natural Science Foundation of Shanghai (Grant No: 13ZR1421500), and the Zhi-Yuan Endowed fund from Shanghai Jiao Tong University. The authors also want to thank Dr. Wang Zhang, Mr. Junlong Tian, and Mr. Wei Wang for their support and valuable discussions.
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[29] A. Sanchot, G. Baffou, R. Marty, A. Arbouet, R. Quidant, C. Girard, E. Dujardin, ACS Nano 2012, 6, 3434. [30] H. H. Richardson, M. T. Carlson, P. J. Tandler, P. Hernandez, A. O. Govorov, Nano Lett. 2009, 9, 1139. [31] Z. Fang, Y.-R. Zhen, O. Neumann, A. Polman, F. J. García de Abajo, P. Nordlander, N. J. Halas, Nano Lett. 2013, 13, 1736. [32] D. Lapotko, Opt. Express 2009, 17, 2538. [33] E. Lukianova-Hleb, Y. Hu, L. Latterini, L. Tarpani, S. Lee, R. A. Drezek, J. H. Hafner, D. O. Lapotko, ACS Nano 2010, 4, 2109. [34] G. Frens, Nature 1973, 241, 20. [35] Y. Zhang, Y. Xu, Y. Xia, W. Huang, J. Colloid Interface Sci. 2011, 359, 536. [36] T. A. Witten, L. M. Sander, Phy. Rev. Lett. 1981, 47, 1400. [37] S. Link, M. El-Sayed, J. Phys. Chem. B 1999, 103, 4212.
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Received: April 18, 2014 Published online:
small 2014, DOI: 10.1002/smll.201401071
Bio-Inspired Evaporation Through Plasmonic Film of Nanoparticles at the Air–Water Interface Zhenhui Wang, Yanming Liu, Peng Tao, Qingchen Shen, Nan Yi, Fangyu Zhang, Quanlong Liu, Chengyi Song, Di Zhang, Wen Shang*, and Tao Deng* Evaporation is critical to many fundamental processes[1–6] and important industry applications.[7–12] In power plants that utilize high pressure steam,[7] an efficient evaporation process can improve the overall system level power generation efficiency, and result in savings of billions of dollars for the world thermal power market. Efficient evaporation process can also enhance the performance of phase change based heat transfer systems,[8–10] such as heat pipes[8,9] and vapor chambers.[10] For the sterilization applications, the steam generation through evaporation helps minimize the potential contamination of medical devices by biological microbes.[11] The improvement of selective evaporation process can also significantly increase the separation efficiency of mixed solvents during distillation processes.[12] Evaporation is also vital to the proper functioning of biological systems.[13–17] For example, efficient evaporation during sweating helps the precise thermal regulation of human bodies.[13–15] In plants, transpiration, which involves the evaporation of water at the surfaces of leaves, not only cools the plants but also enables the mass transportation of mineral nutrients from roots to other parts of the plants.[16,17] In both sweating and transpiration, the control of local environment of evaporative surfaces, such as the control of temperature and the liquid flow within skin and leaf surfaces, is critical to achieve the efficient evaporation.[13–17] Inspired by the localized control of evaporative surfaces in biological systems, this work intended to achieve efficient evaporation in engineered systems through manipulation of the evaporative surfaces. In the common engineered systems, the evaporative surface is normally the liquid surface at the air–liquid interface. There is rarely any localized control of such evaporative surface as in biological systems. In this study, to mimic the porous biological evaporative surface, we utilized a self-assembly process to create films of gold nanoparticles at the air–water interface that could pump the liquid Z. H. Wang,[+] Y. M. Liu,[+] Dr. P. Tao, Q. C. Shen, N. Yi, F. Y. Zhang, Q. L. Liu, Dr. C. Y. Song, Prof. D. Zhang, Prof. W. Shang, Prof. T. Deng State Key Laboratory of Metal Matrix Composites School of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200240, P. R. China E-mail: [email protected]; [email protected] [+]Z.H.W.
and Y.M.L. contributed equally to this work. DOI: 10.1002/smll.201401071 small 2014, DOI: 10.1002/smll.201401071
to the open surface during liquid evaporation through capillary flow (Figure 1a and Supporting Information Figure S1). Plasmonic heating was used to control the local temperature of the evaporative surface of the assembled film. Nobel metal nanoparticles possess extremely large absorption cross section under resonant light excitation.[18–24] The plasmonic photothermal conversion of gold nanoparticles within the assembled film is used to generate instant, intense and localized heat to drive the evaporation process.[25–31] Such controlled heating of the evaporative surface is similar to the heating of skins through the pumping of warm blood locally to the skin surface with the opening of blood veins (Figure S1, Supporting Information).[13–15] As the skin temperature increases, the evaporation rate also increases to help cool down the human body. The plasmonic thermal input in our system selectively heated up the evaporative surface of the assembled film. The thermal energy was transferred locally to the surface portion of water where evaporation happened. Such localized transfer minimizes the loss of thermal energy to the non-evaporative portion of water, and potentially enables much higher evaporation efficiency than processes that involve the heat transfer to bulk liquid.[11,12,31] As shown in Figure 1a, the plasmonic film converts the incident optical energy into thermal energy and generates a hot zone at the air–water interface. The vapor bubbles grow around the nanoparticles in the film due to the continuous heat generation from the plasmonic light-to-heat conversion process.[31–33] Once the bubbles reach the air–water interface, they burst and release the hot vapor inside. The film is located right at the air–water interface, so the vapor generated can be immediately released into air without losing energy to the unheated portion of liquid. In the evaporation process where the heat is not localized at the evaporative surface, such as the boiling of water in a kettle, the bubbles start the nucleation at the walls of the container, and then travel to the evaporative surface. As these bubbles travel within the bulk liquid to the evaporative surface, large portion of the thermal energy is lost due to the energy transfer to the non-evaporative portion of the liquid and the walls of the container. Such energy loss lowers the heat-to-evaporation conversion efficiency. In the system studied in this work, most of the thermal energy was utilized to heat up the water only at the evaporative surface. Such localized heaing minimized the heat loss and enhanced heat-to-evaporation conversion efficiency. The liquid water evaporated at the surface of film was quickly replenished through the capillary flow inside the porous film,
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nanoparticles. Consequently, the nanoparticles became less stable and were trapped at the air–water interface. The trapped gold particles then self-assembled into a free-floating film. Figure 1b shows a photograph of a uniform shinning film with a diameter > 3 cm at the air–water interface. The SEM image (Figure 1c), however, reveals a disordered network structure that should be ascribed to the random-walk nature of the self-assembly process.[36] Through analysis of the 2D projection of the SEM images, the selfassembled films have porosity of ≈ 40%. Figure 1d shows the absorption spectra of both the self-assembled film and the particle solution before the assembly process. The maximum absorption of the particle solution was around 520 nm, which agreed well with the theoretical prediction of the plasmonic absorption of gold nanoparticles with diameter ≈18 nm.[37] Due to the relatively delocalized plasmonic absorption in the assembled film,[38,39] the absorption spectrum of the film changed to a broadband absorption that extended across the full visible light range. To study the evaporation process, the cuvette containing the floating assembled film was placed on an analytic balance to measure the in situ weight loss (Figure 1e). A 532-nm laser with adjustable output power was used for the plasmonic heating of the film. The laser with a beam diameter of 5 mm was aligned perpendicularly to the surface of the film. The temperature distribution of the samples was visualized by an infrared (IR) camera. Figure 2 shows the side-view IR images of a cuvette with the plasmonic film at the top of solution. Before the Figure 1. a) Schematic illustration of efficient evaporation of liquid assisted by a floating laser was turned on, it showed a uniform plasmonic film of gold nanoparticles self-assembled at the air–water interface. b) A temperature distribution around 30.4 °C. photograph of a floating film with a diameter > 3 cm at the air–water interface. c) SEM images After illumination for 2 min (Figure 2b) at of self-assembled film of gold nanoparticles under low and high magnifications. d) Absorption 2 2 spectra of the film of gold nanoparticles and the aqueous solution of gold nanoparticles. power density of 5.09 W/cm (≈0.6 kJ/cm ), a localized hot zone was observed. For the e) Evaporation process monitored by a balance and an IR camera. convenience of discussion, the hot zone is similar to the liquid flow inside the skin or inside the xylem defined in this paper as the zone that has local temperature of the plants. As water evaporated from the surface, the free within two degrees of maximum temperature. The top porfloating film and the heating zone moved together with the tion of the water was heated by the film to 41.2 °C and generreceding air–water interface, and the same efficient evapora- ated a hot zone of ≈2.8 mm in depth after 2-min illumination. tion process continued. After 5-min illumination (≈1.5 kJ/cm2, Figure 2c), the surface The gold nanoparticles with a diameter of ≈18 nm were temperature reached 45.5 °C, and the hot zone was ≈2.5 mm synthesized using the citrate reduction approach.[34] The due to the increased temperature gradient. After 10-min assembled films were obtained by slow diffusion of vapor illumination (≈3 kJ/cm2, Figure 2d), the surface temperaof formic acid into the solution of the gold nanoparticles.[35] ture reached 47.3 °C, and the depth of the hot zone further As the formic acid vapor slowly dissolved into the solution, shrinked to ≈2.3 mm. The existence of the hot zone indicated formic acid was dissociated, and the dissociated hydrogen that most of the absorbed energy was confined locally in ions protonated the citrate group at the surface of the gold the evaporation zone. Figure 2e shows the plots of the
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film than for the cuvette with the solution of gold nanoparticles. Figure 3e shows the line profile of the temperature distribution from the top surface to 2 cm below the surface. The temperature of the film sample reached the maximum of 57.8 °C at 1.4 mm, and then quickly dropped to 40 °C at 5 mm. The hot zone was tightly confined at the air–water interface. By contrast, the temperature of the solution sample was 44.5 °C at 3.2 mm and slowly decreased to 40 °C at 2 cm. The broader hot zone of the particle solution indicated a bulk heating of the solution during the plasmonic heating process. It should be noted that the temperature recorded by the IR camera was actually lower than the real value, due to the infrared absorption and emission by the front walls of the cuvettes. The sideview images, however, provide the relative temperature profiles of the different portion of the solution inside the cuvette, and offer valuable information about the thermal energy distribution within the solution. As discussed in the above section, the depth of the hot zone for the cuvette with the assembled film depends on the laser power and the duration of laser illumination. For the cuvette with the solution of gold nanoparticles, besides the laser power and the duration of laser illumination, the absorption path length of the solution is another critical influencing factor for the depth of the hot zone. Hypothetically, if the concentration of the particles in solution further increases, the hot zone might be more localized and potentially matches that of the film due to the decrease of the Figure 2. Evaporation through the film of gold nanoparticles monitored by IR imaging and absorption path length. In such a situaweight loss analysis. Side-view IR images at different time under ≈1 W of laser illumination: tion, however, the concentration of the a) before laser was on; b) after 2 min of illumination; c) after 5 min of illumination; d) after 10 gold nanoparticles in the solution would min of illumination; e) evaporation weight loss under different laser power density: 0 (dotted be well above the percolation threshold. line), 5.09 W/cm2, and 10.18 W/cm2. The concentrated particles would tend to precipitate as agglomerates.[40–42] For evaporation weight loss as a function of illumination time the assembled films, there is no need to disperse particles under the laser power density of 5.09 W/cm2 and 10.18 W/cm2. into the evaporation liquid and the films have much better It was found that once the laser was turned on, the weight stability, even when most of the liquid evaporates away. loss increased almost linearly. After 10 min of illumination, Another advantage of using the film instead of the solution the stable evaporation rate reached about 0.2 mg/s and of nanoparticle is the maximized usage of the nanoparticles 0.4 mg/s for laser power density of 5.09 W/cm2 (≈3 kJ/cm2) in the evaporation process. Most of the particles used in the and 10.18 W/cm2 (≈6 kJ/cm2), respectively. Without laser film participated in the absorption and conversion of optical illumination, the evaporation rate was almost negligible energy into thermal energy. For the solution of gold nano(0.01 mg/s) as indicated by the dotted line in Figure 2e. particles, only particles within the absorption path length Figure 3 shows the optical and IR images of the participated in the absorption and conversion process, and plasmonic film and solution of gold nanoparticles particles outside the absorption path length were not utilized. (≈4.39 × 1013 particles/mL). As shown in the IR images Furthermore, the self-assembled films can be easily recycled (Figure 3b,d), under the same laser illumination the hot zone to enable the reuse of the evaporation medium (Figure S2, is much more localized for the cuvette with the self-assembled Supporting Information). With the potential use of approsmall 2014, DOI: 10.1002/smll.201401071
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Figure 3. Comparison between evaporation through the assembled plasmonic film and solution of gold nanoparticles. a,b) Optical image of an assembled film within a cuvette and IR side-view image of the same sample at 10.18 W/cm2 of laser illumination. c,d) Optical image of the solution of gold nanparticles within a cuvette and IR side-view image of the same sample at 10.18 W/cm2 of laser illumination. e) Temperature profiles of the film and solution samples after 10.18 W/cm2 of laser illumination for 2 min. f) Evaporation rate as a function of illumination time. g) Evaporation rate after 20 min illumination under different laser power.
priate flexible supporting structure for the assembled film, the approach can also be applied in large scale where large amount of vapor generation is required. Figure 3f shows the change of evaporation rate with the duration of laser (10.18 W/cm2) illumination for both the film and the solution of gold nanoparticles. For the film, the evaporation rate quickly jumped to a high value (≈0.4 mg/s) and reached a steady rate after 150 s. Wheras, for the solution, the evaporation rate gradually increased and it took ≈800 s before the rate became stable. The delay in reaching the steady state for the solution was ascribed to the travelling of the vapor bubbles to the surface of the solution, and also the related loss of heat to the bulk liquid. It is worth to mention that the amount of gold nanoparticles in the solution was more than 2 times of the particles in the assembled film, but at the steady evaporation stage, the evaporation rate of the solution was only half of that of the film. Figure 3g provides the plots of the evaporation rate for the film and the solution at different laser power. For both samples, as the laser power increased, the evaporation rate increased almost linearly. The rate of evaporation (v) and the heat power of evaporation (QE) can be defined in Equation 1 and 2, respectively, v = d m /d t QE =
(
(1)
)
dm × H E / d t = vH E / M M
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(2)
where m is the mass of the liquid (water), t is the time, M is the molar weight of water, HE is the molar heat of evaporation of water. If we assume the conversion of the input laser power (Qlaser) into the heat power of evaporation (QE) follows the below linear relationship, QE = α Qlaser
(3)
where α is a constant. Then v = QE M / H E = α Qlaser M / H E
(4)
Fitting the experimental results in Figure 3g with Equation 4 shows that α equals to 0.44 and 0.20 for the film and the solution of gold nanoparticles, respectively. The conversion from optical energy to the heat of evaporation for the solution of gold nanoparticles was about 20%. Most energy was lost through the heating of the bulk solution and the container as illustrated by the broad heating zone. For the floating plasmonic film, the conversion efficiency from optical energy to the heat of evaporation was about 44%, more than twice of the efficiency for the solution of gold nanoparticles. This analysis is also consistent with the IR imaging observation that the heat generated by the assembled film was strongly localized and efficiently utilized to evaporate the water from the top surface. In the experiment, there was about 10% of the incident laser light reflected away from
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small 2014, DOI: 10.1002/smll.201401071
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the surface of the assembled film. Given that the 2D porosity of the film was ≈40%, the conversion efficiency from the absorbed light to the heat of evaporation was actually larger than 80% for the solid areas of the film. There are certainly some contribution to the energy loss through thermal diffusion, but the major cause of the energy loss in the film was from the laser light passing through the empty space of the film. It can be expected that films with denser and more closepacked structures than films used in this work will further improve the energy conversion efficiency. The photothermal conversion efficiency of the gold nanoparticles that are dispersed inside the solution greatly depends on the mismatch between the wavelength of the illumination and the plasmon resonance wavelength, and also the mismatch between these two wavelengths that is induced by the assembly states.[43] With the relatively broadband absorption of the self-assembled films of gold nanoparticles, the photothermal conversion efficiency actually is higher using the broadband illumination than using the narrow band illumination. The initial test using broadband illumination from a solar simulator confirmed the increase of the photothermal conversion efficiency and the evaporation rate, compared to the experiments using illumination from the 532-nm laser (Figure S3, Supporting Information). In summary, we have demonstrated a bio-inspired surface evaporation approach through the localized plasmonic heating by the free-floating film of gold nanoparticles at the air–water interface. Compared to other evaporation processes, the thermal energy is provided directly at the evaporative surface, leading to a fast and efficient evaporation. With this approach, the bulk liquid temperature is relatively undisturbed while the top surface reaches a large evaporation rate. The free-floating film can move downwards together with the air–water interface, thus the efficient evaporation can be sustained during the receding of the interface. This bio-inspired approach provides not only high efficiency in vapor generation, but also high efficiency in material usage of the plasmonic particles as almost all the nanoparticles in the film participated in the plasmonic heating process. With fine tuning of the micro and nanostructure of the films, the efficiency could be further improved. The facile evaporation design could be easily adapted and applied to other photothermal conversion systems, such as assembled films composed of other noble particles with different morphology and particle sizes, to provide fast and efficient evaporation. This approach will bring tremendous benefit for energy saving in solar drying, steam generation and distillation processes, and also open new paths in heat transfer and other related thermal applications.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
small 2014, DOI: 10.1002/smll.201401071
Acknowledgements This work was supported by Natural Science Foundation of China (Grant No: 91333115), Natural Science Foundation of Shanghai (Grant No: 13ZR1421500), and the Zhi-Yuan Endowed fund from Shanghai Jiao Tong University. The authors also want to thank Dr. Wang Zhang, Mr. Junlong Tian, and Mr. Wei Wang for their support and valuable discussions.
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Received: April 18, 2014 Published online:
small 2014, DOI: 10.1002/smll.201401071
