Improved Performance of Heat Source Free Water‐Floating Carbon Nanotube Thermoelectric Generators Controlling Wettability Using Atmospheric‐Pressure Plasma Jet and Waterproof Spray

Water‐floating carbon nanotube thermoelectric generators (CNT‐TEGs) can effectively power Internet of Things (IoT) applications. CNT‐TEGs produce electricity by floating on water to generate a temperature gradient in the CNT films, where water pumping via capillary action causes evaporation‐induced cooling in selected areas. However, the amount of electricity required for target applications needs to be increased. Therefore, the wettability of CNT films is controlled via various treatments to increase water evaporation and temperature difference in the films. The contact angle of the water droplet on the film surface related to the wettability decreases upon atmospheric‐pressure plasma jet irradiation and increases upon waterproof spray treatment. Intermediate wettability is obtained by combining the two aforementioned treatments. Under the environmental conditions of light irradiation (1 kW m−2) and wind (3 m s−1) at a water temperature of 20 °C, the CNT‐TEGs with the combined treatment exhibit a highest output voltage of 3.9 mV, which is 30% larger than that of the pristine CNT‐TEG. At a water temperature of 60 °C, a highest output voltage of 13.1 mV is achieved in the pristine CNT‐TEG. The performance enhancement of CNT‐TEGs by controlling the wettability of CNT films under various environmental conditions can facilitate advancements in IoT technology.


DOI: 10.1002/admi.202300171
effect. In particular, thin and flexible TEGs are becoming a promising power source for use in wearable electronics and sensing devices utilized in Internet of Things (IoT) technology. [1][2][3][4] Conventional TEGs comprise p-type and n-type semiconductors connected in series. [5][6][7] Electricity is generated by placing the TEGs on a heat source to create a temperature difference in the TEGs. To date, various attempts have been made to create a temperature difference in TEGs using heat sources. [8][9][10][11][12] When TEGs can generate electricity by creating a temperature difference without using heat sources, the use of TEGs is expected to expand significantly.
In a previous study, we used singlewalled carbon nanotubes (SWCNTs) as a thermoelectric material owing to their light weight, flexibility, and relatively high thermoelectric properties near room temperature. [13,14] Thus, we fabricated waterfloating TEGs using SWCNTs, namely CNT-TEGs, without a heat source. [15] In the fabricated water-floating CNT-TEGs, several pieces of SWCNT films were bonded to the polyimide substrate such that adjacent SWCNT films covered half a hole in the substrate and were connected in series using thin metal wires. The output voltage and power were generated by the temperature difference in the SWCNT films, in which water pumping via capillary action led to evaporation-induced cooling in selected areas. Furthermore, the output voltage and power increased when the films were exposed to sunlight and wind. Moreover, the direction of the temperature difference can be controlled depending on the position of holes in the substrate, indicating that the direction of heat flow is variable, and thus, TEGs with only p-type SWCNT film can be achieved. This finding is important for CNT-TEGs because it is challenging to maintain sustained n-type properties with SWCNTs. [16][17][18][19][20][21][22] Even though water-floating CNT-TEGs exhibit excellent characteristics, a further increase in performance is required to use the power source of wearable electronics and sensing devices. [23][24][25] Therefore, to increase the performance of the TEGs, one approach involves increasing the temperature difference in the TEGs, while maintaining the thermoelectric Water-floating carbon nanotube thermoelectric generators (CNT-TEGs) can effectively power Internet of Things (IoT) applications. CNT-TEGs produce electricity by floating on water to generate a temperature gradient in the CNT films, where water pumping via capillary action causes evaporation-induced cooling in selected areas. However, the amount of electricity required for target applications needs to be increased. Therefore, the wettability of CNT films is controlled via various treatments to increase water evaporation and temperature difference in the films. The contact angle of the water droplet on the film surface related to the wettability decreases upon atmospheric-pressure plasma jet irradiation and increases upon waterproof spray treatment. Intermediate wettability is obtained by combining the two aforementioned treatments. Under the environmental conditions of light irradiation (1 kW m −2 ) and wind (3 m s −1 ) at a water temperature of 20 °C, the CNT-TEGs with the combined treatment exhibit a highest output voltage of 3.9 mV, which is 30% larger than that of the pristine CNT-TEG. At a water temperature of 60 °C, a highest output voltage of 13.1 mV is achieved in the pristine CNT-TEG. The performance enhancement of CNT-TEGs by controlling the wettability of CNT films under various environmental conditions can facilitate advancements in IoT technology.

Introduction
Thermoelectric generators (TEGs) have a distinct ability of directly converting thermal energy into electricity via the Seebeck www.advmatinterfaces.de properties of the SWCNTs. When the amount of water evaporated from the SWCNT film surface increases, the temperature difference increases. Water evaporation via solar energy-toheat conversion using black nanomaterials as light absorber is widely investigated, [26] and the technique can be applied in our study. [27] Therefore, the control of water evaporation from the SWCNT film surface plays a crucial role in improving the performance of the CNT-TEGs. Therefore, the primary challenge is to control the wettability of SWCNT films.
Suitable approaches for the wettability control of SWCNT films include the use of an atmospheric-pressure plasma jet (APPJ) and waterproof spray. APPJ irradiation increases the wettability of SWCNT films, while waterproof spraying decreases the wettability of SWCNTs. APPJ is a nonequilibrium plasma that is stably generated without a vacuum process. [28][29][30] Since APPJ is extremely active at low temperatures, its treatment can be used for excellent surface modification without damage. The increase in wettability has been experimentally proven for many materials, such as metals, polymers, and fabrics. [31][32][33] Therefore, the wettability of SWCNTs can be increased using APPJ irradiation. [34][35][36][37] In contrast, waterproof sprays are commonly used for water-repellent treatment. Therefore, the wettability of SWCNTs can be decreased using a waterproof spray.
In this study, we report the fabrication and testing of waterfloating CNT-TEGs, where the wettability of SWCNT films was changed using APPJ irradiation and waterproof spray. SWCNTs with p-type properties were used to form films via vacuum filtering. The film characteristics, including the surface structure, wettability, and thermoelectric properties, are analyzed. The fabrication process and performance testing of the waterfloating CNT-TEGs were based on our previous study. [15] We demonstrate that wettability control is effective in improving the performance of CNT-TEGs.

Fabrication and Wettability Control of Water-Floating CNT-TEGs
The fabrication process, wettability control, and performance testing of the water-floating CNT-TEGs are shown in Figure 1. The fabrication process of the water-floating CNT-TEGs, except for the APPJ irradiation and waterproof spray, is described in our previous report. [15] In brief, for the film preparation in Figure 1a, SWCNTs in powder form were dispersed in ethanol to prepare a SWCNT dispersion solution with a concentration of 0.2 wt% using an ultrasonic homogenizer. The SWCNT films were prepared using a vacuum filtering. When the SWCNT films were dried for 24 h in air, they were removed from the membrane filter. The thickness of the SWCNT film was ≈50 µm.
As illustrated in Figure 1b, to assemble the CNT-TEGs, the SWCNT films were cut into 24 pieces, each measuring 10 mm in length and 10 mm in width. The substrate (80 mm × 60 mm, 125 µm thick) was a polyimide sheet (Kapton, DuPont) with 24 rectangular holes (5 mm × 4 mm) drilled into a staggered array. Twenty-four sections of the SWCNT films were bonded to the substrate with an adhesion bond such that the adjacent films were half-covered by a hole in the polyimide. The SWCNT films were connected in series using thin copper wires.
The wettability of the CNT-TEGs was controlled, as shown in Figure 1c. Four sample types were prepared. The CNT-TEG using the pristine SWCNT films is denoted as sample #1. To increase the wettability of the CNT-TEG, APPJ irradiation was applied (sample #2). The details of APPJ generation are described in our previous report. [31] In brief, an APPJ using argon (Ar) gas was generated at a frequency of 10 kHz, applied voltage of 4 kV, and Ar gas flow rate of 10 L min −1 . The APPJ was irradiated on the back surface of the SWCNT film in contact with water for 3 s, and the irradiation distance was maintained at 5 mm. To decrease the wettability of SWCNT films, a waterproof spray comprising fluoropolymer (Superhydrophobic, FK) was used. Waterproof spraying was performed on the back surface of each SWCNT film in contact with water for 3 s (sample #3). To create a medium wettability condition, water spraying was performed for 3 s. Thereafter, APPJ irradiation was performed for 50 s on the SWCNT films (sample #4). The performance testing system for CNT-TEGs is depicted in Figure 1d. The CNT-TEG floated on water and the output voltage was measured by connecting thin copper wires. To evaluate the performance of the CNT-TEGs under various environmental conditions, we varied the water temperature (20-70 °C), exposed artificial sunlight (1 kW m −2 ), and blew wind (3 m s −1 ).

Wettability Analyses of SWCNT Films
Side-view photographic images of the water droplets on the SWCNT films subjected to different treatments are illustrated in Figure 2. The contact angle of the water droplets on the surface of sample #1 is 88° (Figure 2a). Following APPJ irradiation, sample #2 exhibited a drastic decrease in the contact angle of the water droplet to 12°, indicating that the surface of the SWCNT film was almost completely hydrophilic ( Figure 2b). In Figure 2c, sample #3 shows a photographic image of the water droplet on the SWCNT film after spraying the fluoropolymer, highlighting the superhydrophobic property of the SWCNT film. When APPJ irradiation was performed after spraying the fluoropolymer of the SWCNT film (sample #4), the contact angle of the water droplets was 50°, which was an intermediate value between the contact angle of the film with APPJ irradiation and that of the pristine film ( Figure 2d). Therefore, the wettability of the SWCNT films can be controlled by the combined treatment of APPJ irradiation and fluoropolymer spraying. Figure 3 shows the surface morphologies of the SWCNT films subjected to different treatments. As shown in Figure 3a, sample #1 comprised CNT bundles with different diameters, and the aggregates of the bundles formed a nonuniform mesh structure. As shown in Figure 3b, sample #2 exhibited no noticeable change in the surface morphology of the SWCNT film after APPJ irradiation. As shown in Figure 3c, even when the fluoropolymer was sprayed on the surface of the SWCNT film, sample #3 exhibited no noticeable change www.advmatinterfaces.de in the surface morphology. This indicates that the fluoropolymer thinly coated the SWCNT surface, which was confirmed by X-ray photoelectron spectroscopy (XPS) analysis.

Structural Analyses of SWCNT Films
In Figure 3d, sample #4 shows the surface morphology of the SWCNT film treated by fluoropolymer spraying, followed by APPJ irradiation. The surface of the CNT bundles  www.advmatinterfaces.de appeared to be slightly rougher. However, Raman spectroscopy analyses showed that the crystallinity of the SWCNTs did not change significantly compared to that of the pristine SWCNTs, as provided in Figure S1 (Supporting Information). This indicates that APPJ irradiation caused little damage to the SWCNT structure. Therefore, the surface morphology did not significantly affect the hydrophilicity/hydrophobicity of the SWCNT films.
As shown in Figure 4, XPS analysis was performed to investigate the surface conditions of the SWCNT films subjected to various treatments. Figure 4a shows the peak intensities of the O1s spectrum of the SWCNT films. The peak of O1s was observed in sample #1 (pristine SWCNT film) because oxygen molecules adsorbed on the SWCNT surface were detected. When APPJ irradiation was performed on the SWCNT surface, sample #2 exhibited an increase in the peak intensity compared to sample #1, and the peak position was shifted to 532 eV. This phenomenon occurred because the water in the atmosphere was decomposed into OH radicals by APPJ and then attached to the SWCNT surface to form hydroxyl groups. [38,39] Therefore, the formation of hydroxyl groups on the SWCNT surface led to hydrophilicity of the SWCNT films, as shown in Figure 2b. The similar phenomena were observed when fabric was subjected to APPJ treatment. [33] When the fluoropolymer was sprayed on the surface of the SWCNT film, sample #3 exhibited an increase in peak intensity compared to samples #1 and #2. Oxygen, which is a constituent element of fluoropolymer, was detected. When consecutive fluoropolymer spraying and APPJ irradiation treatments were performed, sample #4 exhibited a significant increase in peak intensity and a shift in peak position to 532 eV. This indicates that the concentration of hydroxyl groups absorbed on the SWCNT surface increased because of the longer APPJ irradiation (50 s). Figure 4b illustrates the peak intensities of the F1s spectrum of the SWCNT films. Sample #3 exhibited a high peak intensity in the F1s spectrum. This analysis confirmed the presence of fluoropolymers on the SWCNT surface and led to hydrophobic properties in the SWCNT films. When APPJ irradiation was performed on sample #4, the peak intensity was significantly reduced; however, the peak was still detected. Therefore, hydroxyl groups and fluoropolymers were present on the surface of the SWCNTs in Sample #4. A similar trend was observed in the FT-IR analysis, as shown in Figure S2 (Supporting Information).

Thermoelectric Properties of SWCNT Films
The in-plane thermoelectric properties of the SWCNT films subjected to the different treatments are listed in Table 1. The measurements were performed on a surface that was not in contact with water. The pristine SWCNT film (sample #1) exhibited an electrical conductivity of 42 S cm −1 , Seebeck coefficient of 56 µV K −1 , and power factor of 12.7 µW m −1 K −2 . All the thermoelectric properties (electrical conductivity, Seebeck coefficient, and power factor) of the SWCNT films with different treatments exhibited almost the same values as those of the pristine SWCNT film. Therefore, the performance of the waterfloating CNT-TEGs was not significantly affected by the properties of the SWCNT films. Figure 5 illustrates the temperature distribution and performance of the water-floating CNT-TEGs with different treatments in response to various environmental conditions. The insets show thermographic images of the devices under the corresponding treatments and measurement conditions. As shown in Figure 5a-d, the output voltages of the four samples were measured under conditions of no light and no wind, while the water temperature was maintained at ≈20 °C. In the pristine CNT-TEG (sample #1), an output voltage of ≈0.55 mV was maintained for 3600 s (Figure 5a). Therefore, a stable output voltage was obtained when the CNT-TEG was floating on the water, demonstrating that thermoelectric power generation can be achieved in environments where no heat source is present. When APPJ irradiation was performed on the CNT-TEG (sample #2), the output voltage was stable for 3600 s;  Adv. Mater. Interfaces 2023, 10, 2300171 www.advmatinterfaces.de however, the value was reduced by 38% compared to that of sample #1 (Figure 5b). In the CNT-TEG sprayed with fluoropolymer (sample #3), the output voltage was lower than those of samples #1 and #2. Moreover, the measured output voltage in sample #3 fluctuated widely because the waterproofed SWCNT films allowed air to enter between the SWCNT films and water surface, resulting in an uneven temperature inside the SWCNT films (Figure 5c). When the CNT-TEG was subjected to waterproof spray following APPJ irradiation (sample #4), the approximate output voltage was stalely exhibited at 0.84 mV (Figure 5d), which was 1.5 times higher than that of the pristine CNT-TEG (sample #1). The thermographic image of the inset shows a slight temperature difference within the SWCNT film. Since the Seebeck coefficient of the SWCNT film was 55 µV K −1 , a temperature difference of ≈0.6 K was expected to be generated within the film. In Figure 5e-h, the output voltages of the four samples were measured at a condition of light (1 kW m −2 ) and wind (3 m s −1 ) while the water temperature was maintained at ≈20 °C. In all treatments, the output voltages increased compared to the corresponding values under environmental conditions with no light and no wind. In particular, sample #4 exhibited the highest value of 3.9 mV, which was 1.3 and 1.6 times higher than those of samples #1 and #2, respectively. A temperature difference was observed in the thermographic images. In sample #3, the measurement fluctuations were so severe that an accurate voltage could not be obtained. In our previous report, we measured the temperature distribution

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and performance of the water-floating CNT-TEGs with similar condition of sample #1 in an outdoor environment, [15] which is provided in Figure S3 (Supporting Information). The output voltage was stably obtained as sunlight intensity and wind velocity varied with time.
In Figure 5i-l, the output voltages of the four samples were measured at a condition of light (1 kW m −2 ) and wind (3 m s −1 ) while the water temperature was initially set at ≈70 °C following naturally cooling to room temperature. As the initial temperature of each sample could not be set exactly at 70 °C, we measured the output voltage when the water temperature reached 60 °C. Note that the output voltage of sample #3 could not be measured because of the severe fluctuations. In the higher-water-temperature region, sample #1 exhibited the highest output voltage among the three samples. The output voltage of sample #1 was 13.0 mV, while those of samples #2 and #4 were 10.8 and 12.4 mV, respectively. The insets of the thermographic images were measured at a water temperature of 60 °C. The temperature difference in the SWCNT films was clearly observed.

Discussion
We investigated the relationship between the contact angle of water droplet on the film surface and output voltage of the CNT-TEGs under various environmental conditions, as depicted in Figure 6. Under the condition of no light and no wind, and a water temperature of 20 °C (Figure 6a), the highest output voltage of 0.85 mV was exhibited at a contact angle of 50°. Therefore, the performance of the CNT-TEGs was enhanced by increasing the wettability compared with that of the pristine CNT-TEG. However, an excessive increase in wettability (contact angle of 12°) results in a decrease in performance. A similar trend was observed under light and wind conditions at a water temperature of 20 °C (Figure 6b). Note that the output voltage at a contact angle of 180° was not added to the figure because of the extremely large fluctuation of the measurement values. The highest output voltage of 3.9 mV was observed at a contact angle of 50°. Compared to the environmental conditions with and without light irradiation and wind while maintaining the water temperature at ≈20 °C, the output voltage was increased by a factor of 4.7 at a contact angle of 50°. Light irradiation contributed to an increase in the water evaporation rate on the SWCNT surface, while wind blowing contributed to a decrease in the density of water vapor near the SWCNT surface.
Under the conditions of light irradiation and wind at a water temperature of 60 °C (Figure 6c), the highest output voltage of 13.1 mV was observed at a contact angle of 88°. The contact angle exhibiting the peak output voltage shifted to a higher value than that at a water temperature of 20 °C. This phenomenon occurred because the temperature difference was maximized in the SWCNT film at a contact angle of 88°, owing to the increase in the evaporation rate of water on the SWCNT surface at higher water temperatures. Therefore, we demonstrated the enhancement of the performance of CNT-TEGs by controlling the wettability of the SWCNT films under various environmental conditions. These findings open a pathway for the use of CNT-TEGs as power supplies for sensors in IoT technology. However, the output voltage of the current CNT-TEGs is still low to be used as an IoT sensor. [40] Thus, it is challenging to increase the performance by changing the structure of CNT-TEGs, such as the film thickness and size.

Conclusion
To increase the performance of the water-floating CNT-TEGs, we controlled the wettability of the SWCNT films by applying various treatments, including APPJ irradiation, waterproof spray, and a combination of the above-mentioned two treatments. The wettability of the SWCNT films was increased by APPJ irradiation and decreased by the waterproof spray. The combination treatment resulted in intermediate wettability between the APPJ-irradiated and pristine films. The performance of CNT-TEGs was evaluated under various environmental conditions. Under the environmental conditions www.advmatinterfaces.de of light irradiation (1 kW m −2 ) and wind (3 m s −1 ) while maintaining a water temperature of ≈20 °C, the CNT-TEGs with the combined treatment exhibited a highest output voltage of 3.9 mV, which was 30% higher than that of the pristine CNT-TEG. When the water temperature increased to 60 °C while maintaining light and wind, the output voltage increased, and a highest output voltage of 13.1 mV was achieved in the pristine CNT-TEG. It was found that the optimal wettability of the SWCNT films varied with water temperature. Therefore, we demonstrated the enhancement of the performance of CNT-TEGs by controlling the wettability of the SWCNT films under various environmental conditions. To further increase the performance of CNT-TEGs, tuning their structures, such as film thickness and size, is challenging.

Experimental Section
SWCNTs synthesized using the super-growth method (ZEONANO SG101, purity > 99%, average diameter 3 nm, Zeon) were used as the starting material. [41] An ultrasonic homogenizer (Branson Sonifier SFX 250, Emerson Electric) was used to completely disperse the SWCNT powders in ethanol (purity > 99.5%, FUJIFILM Wako Pure Chemical). The primary dispersion conditions were that 100 mL of liquid was dispersed for 45 min using a tip with diameter of ≈12.7 mm at a maximum power of 20 W and an amplitude of 30% while the liquid was placed in a cold-water bath to prevent the increase in the temperature of dispersion liquid.
The vacuum filtering method was used to prepare the SWCNT films. A membrane filter (PTFE, 90 mm diameter: ADVANTEC) was placed in a filter holder in a suction bottle, and the dispersion solution was filtered by reducing the pressure in the suction bottle using a rotary pump to extract the material in the solution. A CNT-dispersed solution (40 mL) was dropped onto the filter and aspirated for 1 h to produce SWCNT films with diameters of ≈80 mm.
The wettability of the SWCNT films subjected to various treatments was investigated using contact angle measurements. A photograph of the water droplet was captured from the side using a charge-coupled device camera, and the contact angles of the water droplets were measured using a drop-shape analysis system (DSA100, Krüss GmbH) at ≈300 K. The volume of water droplets was set to 1 µL. The surface morphologies of the SWCNT films were analyzed by scanning electron microscopy (SEM, S-4800, Hitachi). The surface conditions of the SWCNT films were evaluated using XPS (JEOL JPS-9010MCY). The crystallinity of the SWCNT films was evaluated by Raman spectroscopy using a 532 nm diode-pumped solid-state laser (JUNO, Showa Optronics). The chemical structures of the SWCNT films were characterized by Fourier transform infrared spectroscopy (FTIR; JASCO FT/IR-4200).
The in-plane electrical conductivity σ of the SWCNT films was measured at 20 °C using a four-point probe method (Napson, RT-70 V). The in-plane Seebeck coefficient S was measured at 20 °C using a custom-built instrument. [42][43][44] One end of the thin film was connected to a heat sink, and the other end to a heater. The Seebeck coefficient was determined as the ratio of the potential difference across the membrane to the temperature difference measured using two 0.1 mm diameter K-type thermocouples pressed against the membrane. The in-plane power factor σS 2 was obtained from the measured electrical conductivity and the Seebeck coefficient.
In the performance measurement of the water-floating CNT-TEGs, a CNT-TEG was floated on 450 mL of water at initial temperatures of ≈20 and 70 °C. Wind was applied to the CNT-TEG using a compact circulator (PCF-HD15-W, IRIS OHYAMA Inc.) while the wind speed (3.0 m s −1 ) was measured using an anemometer (SP-82AT, Mother Tool Co.). The CNT-TEG was irradiated using an artificial solar illuminator (XC-100, SERIC Ltd.) to simulate direct sunlight (approximate light intensity: 1 kW m −2 ) and the intensity was measured using a solar power meter (DT-1307, CEM Instruments). The temperature distribution in the CNT-TEGs was measured using a thermographic camera (Type F30W, Japan Avionics). The output voltage was measured using a heat-flow logger (LR8432, Hioki Co.).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.