Extremely Black Vertically Aligned Carbon Nanotube Arrays for Solar

Aug 3, 2017 - It is reported that the optical absorptivity of an extremely black VACNT array is about 0.98–0.99 over a large spectral range of 200 n...
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Extremely Black Vertically Aligned Carbon Nanotube Arrays for Solar Steam Generation Zhe Yin,†,‡ Huimin Wang,†,‡ Muqiang Jian,†,‡ Yanshen Li,‡,§ Kailun Xia,†,‡ Mingchao Zhang,†,‡ Chunya Wang,†,‡ Qi Wang,†,‡ Ming Ma,‡,∥ Quan-shui Zheng,‡,§ and Yingying Zhang*,†,‡ †

Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Center for Nano and Micro Mechanics, §Department of Engineering Mechanics, and ∥Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China ‡

S Supporting Information *

ABSTRACT: The unique structure of a vertically aligned carbon nanotube (VACNT) array makes it behave most similarly to a blackbody. It is reported that the optical absorptivity of an extremely black VACNT array is about 0.98−0.99 over a large spectral range of 200 nm−200 μm, inspiring us to explore the performance of VACNT arrays in solar energy harvesting. In this work, we report the highly efficient steam generation simply by laminating a layer of VACNT array on the surface of water to harvest solar energy. It is found that under solar illumination the temperature of upper water can significantly increase with obvious water steam generated, indicating the efficient solar energy harvesting and local temperature rise by the thin layer of VACNTs. We found that the evaporation rate of water assisted by VACNT arrays is 10 times that of bare water, which is the highest ratio for solar-thermal-steam generation ever reported. Remarkably, the solar thermal conversion efficiency reached 90%. The excellent performance could be ascribed to the strong optical absorption and local temperature rise induced by the VACNT layer, as well as the ultrafast water transport through the VACNT layer due to the frictionless wall of CNTs. Based on the above, we further demonstrated the application of VACNT arrays in solar-driven desalination. KEYWORDS: vertically aligned carbon nanotube arrays, solar energy harvest, steam generation, desalination, blackbody absorber

1. INTRODUCTION Solar energy is a renewable and green energy which can be used in many ways. Nowadays, facing the problem of energy shortage, how to use solar energy effectively has become a hot topic. Solar energy can be used for solar photovoltaics, photocatalysis, and desalination.1,2 Particularly, desalination is a process that removes minerals from saline water. Furthermore, the pure water obtained through desalination can work as an even more versatile platform in the field of energy, such as aqueous batteries,3,4 aqueous solar cells,5−7 H2O splitting, etc. Main desalination techniques can be separated into two types, including membrane separation and thermal distillation.1,8 Solar-thermal-steam generation,9 which uses solar energy to enable the evaporation of water, is a typical approach of thermal distillation and can be used to provide water for drinking and agriculture, especially in remote areas where there is a lack of fresh water. In order to enhance solar-thermal-steam generation, pioneers10−13 have dispersed noble metal nanoparticles in water to harvest solar energy based on a surface plasmon heating mechanism, which led to a thermal conversion efficiency up to 24%. To further improve the efficiency, the following strategies have been considered: (i) Using a thin cap layer to localize the solar heat in the surface of water, and thus to decrease the thermal loss to bulk water.14−16 For example, an airlaid paper14 coated with metal nanoparticles was put on the © 2017 American Chemical Society

surface of water to localize the absorbed solar heat on the top layer of water, and a thermal conversion efficiency of 77.8% was obtained. Besides, a layer composed of graphite powder and a carbon foam was used, and a thermal conversion efficiency of 85% was achieved.15 (ii) On the basis of the above, designing the structure of the cap layer to increase the efficiency of solar energy harvesting.17−19 For example, an aluminum-based 3D porous membrane17 and a gold nanoparticle-based membrane18 fabricated by self-assembly metallic nanoparticles were recently reported for solar-thermal-steam generation, which showed a solar absorption of 96% and 99%, respectively. Besides metal nanoparticles, carbon materials which are more environmentally friendly and cost-effective compared to metals could also be used as solar thermal absorbers. Particularly, carbon materials mainly constructed with sp2-hybridized carbon atoms, such as graphite/graphene and carbon nanotubes (CNTs), are excellent optical absorbers due to the π-band’s optical transitions.20 Recently, a pioneering work reported a nitrogen-doped 3D porous graphene which acted as a solar thermal absorber, and an efficiency of 80% was achieved.16 The key factor to achieve higher efficiency is the design of carbon material structure. Actually, a vertically aligned CNT (VACNT) Received: June 15, 2017 Accepted: August 3, 2017 Published: August 3, 2017 28596

DOI: 10.1021/acsami.7b08619 ACS Appl. Mater. Interfaces 2017, 9, 28596−28603

Research Article

ACS Applied Materials & Interfaces

Figure 1. Experimental setup for solar-thermal-steam generation and the structure of the VACNT array. (a−c) Schematic illustration showing the experimental setup for solar steam generation using a VACNT array floating on the water to absorb solar energy and to localize the heat. (d) A tilted view SEM image of the VACNT array. (e) A magnified side-view SEM image of the VACNT array. (f) A transmission electron microscopy image of CNT bundles.

structure. It is worth to note that the ends of CNTs aggregate, forming a rough surface of array, and the alignment of the CNTs in the microscale is not perfect (Figure 1e), enabling the efficient trapping of light as discussed later. The average diameter of the CNTs is about 10 nm (Figure 1f), and the number of walls is around 6. Besides, the CNTs are clean with lack of obvious amorphous carbon, enabling good thermal transport along their axial direction with negligible thermal loss to the surroundings. As seen in Figure 1e and 1f, neighboring CNTs form bundles, endowing the whole CNT array with good integrity and acting as a bulk 3D porous structure. The areal density of VACNT arrays is calculated to be 8.5 × 1010 cm−2, indicating that the average interstitial space between neighboring CNTs is about 70 nm. Besides, according to highresolution SEM images, the interstitial spaces between neighboring CNTs are in the range of 40−190 nm. It is worth noting that although the number density of CNTs is very high a VACNT array has over 95% interstitial spaces between CNTs24,25 and is ultralight, enabling it to float on the surface of water and fast transportation of water through it. The solar-thermal-steam generation assisted by a VACNT array is found to be obviously prominent compared to that without VACNT arrays. First, the mass loss of water with and without the VACNT array under certain solar illuminations was tracked and recorded from 0 to 20 min. The used optical concentrations (Copt) are 1 and 15, which correspond to energy density of 1 kW m−2 and 15 kW m−2, respectively. As shown in Figure 2a, the mass loss with the assistance of a VACNT array under 1 kW m−2 is 1.9 times that without a VACNT array. Furthermore, under higher optical concentration, the difference in the mass change with and without the assistance of VACNT arrays became more distinct, which is consistent with previous reports.15 For example, the mass loss assisted by the VACNT array under Copt of 15 is 10 times that of the controlled sample. Figure 2b shows the evaporation rate of the water with and without the VACNT array under different optical concentrations. The evaporation rate of water assisted by VACNT arrays is obviously higher than the water without VACNT array assistance. Specifically, the evaporation rate of water assisted by VACNT arrays is 1.9 (Copt = 1), 5.2 (Copt = 5), 9.6 (Copt = 10),

array, where the CNTs are aligned to form a 3D forest-like structure, is well-known as the blackest material in the world.21,22 It has been reported that VACNT arrays possess a nearly constant optical absorptivity of 0.98−0.99 across a wide spectral range from ultraviolet (UV, 200 nm) to far-infrared (200 μm)21 and thus behave most similarly to a blackbody, promising its great potential for harvesting solar energy. In addition, the frictionless surface of CNT walls23 may lead to an ultrahigh fluid velocity of water outside the CNTs, which can further enhance the evaporation of water. Despite the high potential of VACNT arrays for enhancing optical absorption and steam generation, to the best of our knowledge the application of VACNT arrays in solar-thermal-steam generation has never been explored. Herein, we, for the first time, report the highly efficient solarthermal-steam generation by laminating a layer of VACNT array on the surface of water to harvest solar energy and to enhance the water evaporation. Our study showed that the VACNT arrays can absorb solar energy in the range of 280− 820 nm with an efficiency as high as 99%, resulting in a temperature increment localized at the top layer of water, while the bulk water remains at low temperature. Remarkably, the evaporation rate of water assisted by VACNT arrays is 10 times of that of bare water under solar illumination, and the thermal conversion efficiency can be as high as 90%. Based on the excellent performance of VACNT arrays, we further demonstrate their applications in desalination of seawater.

2. RESULTS AND DISCUSSION Figure 1a−c shows the experimental setup for the VACNT array-assisted solar-thermal-steam generation. Briefly, the VACNT array, which is floating on the surface of water contained in a baker under solar illumination, can absorb thermal energy and generate localized heat in the top layer of water, resulting in the generation of water steam. Figure 1d shows a typical scanning electron microscope (SEM) image of the VACNT arrays with a height around 280 μm. As seen in Figure 1d, the CNTs in the array are continuous from the bottom to the top and aligned vertically, forming a forest-like 28597

DOI: 10.1021/acsami.7b08619 ACS Appl. Mater. Interfaces 2017, 9, 28596−28603

Research Article

ACS Applied Materials & Interfaces

Figure 2. Enhanced steam generation induced by a thin layer of VACNTs. (a) Mass loss of water for systems with and without a VACNT layer under solar illumination of 1 kW m−2 (Copt = 1) and 15 kW m−2 (Copt = 15). (b) Evaporation rate of water for systems with and without a VACNT layer. (c) Solar thermal conversion efficiency with the assistance of a VACNT layer under different Copt. (d) Comparison of the steam generation performance of this work with typical references. “Ratio” indicates the ratio of water evaporation rate with and without the assistance of the layers floating on the water. The symbols in red correspond to the left axis (efficiency), and the symbols in blue correspond to the right axis (ratio). The number in the bracket refers to the number of references. (e−h) IR images taken at different time during illumination for water covered with a VACNT layer under solar illumination of 15 kW m−2. (i) Top view IR image taken after 20 min illumination and turning off the light from water covered with a VACNT layer. (j−m) IR images taken at different times during illumination from bare water under solar illumination of 15 kW m−2. (n) Top view IR image taken after 20 min illumination and turning off the light for bare water. All experiments are conducted at 22 °C with a relative humidity of 36%.

considered for the calculation of thermal efficiency due to the fact that the sensible heat is negligible in this system compared to the latent heat. Remarkably, the calculated thermal conversion efficiency in our experiment is 30% (Copt = 1), 60% (Copt = 5), 78% (Copt = 10), and 90% (Copt = 15). This result indicates that the percentage of lost energy decreased with the increase of optical concentration, which is consistent with previous work.15 A comparison of the steam generation performance between our work and recently reported ones27−30 is presented in Figure 2d. It is worth noting that due to the variety in experimental setup (such as the selection of water containers) and parameters (such as environmental relative humidity, temperature, and etc.) the comparison of efficiency between different works might not be straightforward. To exclude the influence of different experimental conditions, we used the ratio of evaporation rate with and without the VACNT array in the same experimental conditions

and 10.0 (Copt = 15) times that without the assistance of VACNT arrays. As a comparison, the reported evaporation rate of water with the assistance of an aluminum 3D porous membrane17 is 2.4 (Copt = 4) and with the assistance of a carbon foam15 is 3.4 (Copt = 10) times of that of pure water. It is noted that the enhanced water generation ratio by VACNT arrays of 9.6 at Copt = 10.0 and 10 at Copt = 15 are both the highest value compared to previously reported results,14−17 indicating the superiority of VACNT arrays for solar-thermalsteam generation. The solar thermal conversion efficiency under different solar illumination obtained with the assistance of VACNT arrays is shown in Figure 2c. The thermal efficiency (η) is defined as mhLV/qiCopt,14,15,17,19,26 where m is the mass flux, hLV the enthalpy of the liquid−vapor phase change (2.26 MJ kg−1), and qi the power density of nominal direct solar irradiation (1 kW m−2). It is worth noting that only the phase-change enthalpy is 28598

DOI: 10.1021/acsami.7b08619 ACS Appl. Mater. Interfaces 2017, 9, 28596−28603

Research Article

ACS Applied Materials & Interfaces

Figure 3. Optical properties of VACNT arrays. (a) UV−vis absorption spectra of pristine and wet VACNT arrays. (b) Reflectance and transmittance of a pristine VACNT array. (c) Reflectance and transmittance of a wet VACNT array. (d) Optical pictures showing the water steam generated by a thin VACNT layer (i) and the darkness of a VACNT array no matter in which direction to watch it (ii).

Figure 4. Preparation and treatment of VACNT arrays. (a) Schematic illustration showing the synthesis and treatment of VACNT arrays. (b) Schematic illustration showing the good contact between water and the VACNTs, enabling efficient heat transfer and water evaporation. (c) Topview SEM images of an as-grown VACNT array (left) and a treated VACNT array (right). The insets show the contact angle of water on the surface.

to show the steam generation performance of the VACNT array. The improved solar thermal conversion efficiency with the assistance of a thin layer of VACNT array could be partially ascribed to the locally temperature rise induced by the VACNT array. We monitored the temperature distribution in the water container (5−10 mL) under solar illustration for the systems with and without a VACNT array using an infrared (IR) camera. Figure 2e−n shows the IR images taken at different time. As shown in Figure 2e−h, the temperature of the top layer of water coated with a VACNT array locally rose, while the temperature of the whole water without the VACNT array rose uniformly (Figure 2j−m), indicating the obvious local temperature rise induced by the thin layer of VACNTs. The

local temperature rise reduced the heat loss through thermal radiation of the bulk water (as seen in Figure 2e−h), thus enabling effective utilization of the absorbed solar energy. The temperature of the water surface with the assistance of VACNTs was measured to be around 100 °C by a microthermometer, indicating predominant water evaporation. In contrast, the temperature of the bare water was uniform with a number of 40 °C, indicating obviously less water evaporation from the top surface of water. Figures 2i and 2n show the IR images taken from the top of the water after 20 min illumination and turning off the light, which further show the temperature difference of the top water surface between systems with and without the VACNTs. The existence of a VACNT layer on the water surface plays a role to localize the 28599

DOI: 10.1021/acsami.7b08619 ACS Appl. Mater. Interfaces 2017, 9, 28596−28603

Research Article

ACS Applied Materials & Interfaces

generation. The water transport through VACNTs has been studied both theoretically and experimentally,23,35−38 showing the fast water transport through CNT membranes owing to the almost zero friction. The perfect atomic structures of CNT walls39 will largely reduce the friction due to the incommensurability between CNTs and the water.35 Besides, the superhydrophobicity of the CNTs also led to weak interfacial forces between CNTs and water, contributing to the low friction and leading to a slip-flow mode.36 It should be noted that in this study the water mainly transferred through the interstitial spaces between CNTs rather than the inner channel of each CNT. As evidence, we used polydimethylsiloxane to fill the interstitial spaces of a VACNT array and open the ends of CNTs by reactive ion etching following a reported method24 and then used it for steam generation. No water evaporation was observed while keeping experimental conditions the same. Based on the superior water steam generation performance of VACNT arrays, we further demonstrated their application in desalination using natural seawater. Figure 5a illustrates the

thermal heat in the top layer of water, resulting in an elevated temperature of the surface and thus enhancing the water evaporation and leading to a high thermal conversion efficiency. The strong optical absorption ability of extremely black VACNT arrays plays a predominate role for the high solar thermal conversion efficiency observed in the above. As shown in Figure 3a, the optical absorbance (A) of our VACNT array is as high as 99% in the range of 280−820 nm, which is consistent with previous reports on VACNT blackbody absorbers,21,22 indicating the light is mostly absorbed by the array. Figure 3b shows the ultralow optical reflection and transmittance of VACNT arrays, which are all under 1.0%. The strong optical absorption of a VACNT array surface can be ascribed to its unique structure with plenty of voids and imperfect alignment.21 The coarse surface of the VACNT array traps the solar light into the array, which is continually deflected among the CNTs and eventually dissipated into heat. Besides, we compared the optical properties of pristine VACNT arrays and wet VACNT arrays after being used in water steam generation experiment. It is found that there is light difference between the optical properties of pristine VACNT arrays and wet VACNT arrays due to the slight change in the structure induced by wetting. As shown in Figure 3c, the reflection increases to about 1.25%, and the transmittance decreases to around 0.25%, resulting in a slightly decreased optical absorbance of wet VACNT arrays, which is still as high as 98%. It is worth noting that the optical absorptivity of our VACNT arrays is higher than that of other reported materials for solar steam generation,14,17 which is about 90% for a Au-coated airlaid paper and 96% for an aluminum 3D porous membrane. The excellent optical absorption ability of the VACNT arrays could also be directly seen by the naked eye (Figure 3d). As shown in the optical images taken by a camera, the VACNT array is very dark no matter from which direction to watch it. As seen in Figure 3d, the water steam generated by a thin layer of VACNTs under solar illumination could be obviously observed. The wetting properties of the VACNT layer will influence its performance of steam generation. A good contact between the water and VACNTs is required for fast heat transfer from CNTs to water. To promote the contact between the water and the VACNT layer, a hydrophilic surface is desired. In Figure 4a, the as-grown superhydrophobic31 VACNT array is transformed into a hydrophilic VACNT array by weak oxidation or acid corrosion32,33 (see details in Experimental Section). As illustrated in Figure 4b, the abundant functional groups at the end of VACNTs will lead to good contact of water with the VACNTs, benefiting the heat transfer from the CNTs to the surrounding water and thus leading to enhanced water stream generation.34 Figure 4c shows top view SEM images of the asgrown VACNT array and a treated VACNT array with the corresponding contact angle (CA) of water. After treatment, the morphology of the top surface of the VACNT array showed a slight change with the ends of neighboring CNTs aggregated more obviously, forming uniform hybrid structures which may also contribute to the change of wetting behaviors (Figure S2). The CA is 153° for the pristine VACNT array, indicating a superhydrophobic property. After plasma treatment, the surface of the VACNT layer became hydrophilic with a CA around 50°. Besides, the fast transport of water through VACNT membranes, which originates from the unique atomic structures of CNTs, also contributes to the enhanced water stream

Figure 5. Desalination of seawater using a VACNT array. (a) Experimental setup for desalination and its working mechanism. (b) Weight percentage of Na, K, Ca, Mg, and B in seawater samples (from the Huanghai Sea, China) before and after desalination, which are compared with the standard of drinking water quality. It is noted that there is currently no specific guideline value for K+ because the recommended daily dietary requirement for K+ is more than 3000 mg.

experimental setup of our desalination experiment and its working mechanism. The solar irradiation is absorbed by the VACNT array layer floating on the water in the single-basin still, and water steam is generated. The steam condenses at the ceiling surface and is collected in a water tank as clean water, which can be used for drinking, agriculture, and other purposes. Salinity is the saltiness or dissolved salt content of a body of water. The salinity of the seawater used in our experiment was around 26 wt ‰. We measured the salinity of the as-collected water, which was under the minimum detection limit of our salinity meter (