(1) Flexible photothermographic nanogenerators based on MoS2/PU photothermal film and Te/PEDOT thermoelectric layer can be used to collect infrared light in the environment.
(2) Compared with traditional thermoelectric devices, photothermal electric nano-generators can achieve effective electrical output without space temperature gradient.
(3) This type of PTENG has the advantages of flexibility, shape adaptability, light weight, and simple manufacturing process.
ã€introduction】
With the rapid development of the economy, the consumption of fossil fuels has increased dramatically to meet energy demand, leading to global warming and environmental pollution. The development of clean and renewable energy technologies is critical to the sustainable development of human society. Energy harvesting technologies that convert environmental energy into electrical energy have received widespread attention. Nanogenerators originating from Maxwell's displacement current are widely regarded as a promising mechanical energy harvesting technology with enormous potential for application in blue energy, self-driven sensors and implant systems. In addition to mechanical energy, thermal energy is also very rich and ubiquitous in our living environment, but it is usually wasted, and pyroelectric effects and thermoelectric effects are now commonly used to develop thermal energy harvesting technology. The pyroelectric effect refers to a change in the polarization state in some anisotropic solids caused by temperature changes, causing the two polarized surfaces to generate an asymmetrical potential to generate electricity. In addition, when a thermoelectric effect occurs, electrical energy can also be generated by a temperature gradient in the material/device. Compared to pyroelectric energy harvesting devices, thermoelectric devices are widely considered to be more useful and effective in practical applications. However, when ambient temperatures are consistent in space, ie without any gradients, how to use thermoelectrics to collect thermal energy remains a key and problem to be overcome. The key challenge is to create a significant temperature difference (ΔT) in the unit to drive the thermoelectric generator. In our living environment, in addition to direct heat sources, light sources (such as infrared light) can also provide heat through photothermal effects. Based on the photothermal effect and the Seebeck effect, phototherm generators are rapidly being studied for converting light energy into electrical energy without a spatial temperature gradient in the environment. In order to produce the required temperature difference, the traditional method is to use various bulky components such as a vacuum cover, a concentrating lens, and a heat sink. These additional modules not only increase the weight and size of the thermoelectric generator, but also are detrimental to the entire device. Flexible, and flexible is very important for wearable electronic devices. Recently, Jung et al. reported a wearable portable phototherm generator that absorbs sunlight using a superlattice structure and produces a high temperature difference in the lateral direction. However, this solar absorber requires a complicated design process, and its bio-meltability may also be a big problem. Therefore, the development of new types of photothermal materials and device structures is the key to photothermal nanogenerators (PTENG).
Two-dimensional (2D) materials, such as graphene and transition metal sulfide (TMDC), have drawn great attention in the fields of electronics, catalysis, energy storage, and optics. Among them, molybdenum disulfide (MoS2) is a typical representative, which has excellent electronic properties and mechanical properties. In recent years, MoS2 has proven to be a photothermal material with higher absorbance than graphene oxide and gold nanorods. So far, most of the reports on MoS2 focus only on biomedical applications, such as cancer treatment or drug release, but there are few applications for energy harvesting techniques, especially phototherm generators.
[Introduction]
Recently, Dr. Xie Yannan, Ph.D., School of Energy, Xiamen University, and Dr. Lin Zonghong, Ph.D., Institute of Biomedical Engineering, Tsinghua University, Taiwan, reported a flexible photothermtoelectric nanogenerator (PTENG), which consists of a MoS2/PU photothermal layer. A thermoelectric device based on cerium (Te) nanowires is formed in combination. Due to the extremely large specific surface area of ​​the MoS2 nanoclusters, the MoS2/PU film has flexibility, transferability and good photothermal properties. Bismuth nanowires are used to make thermoelectric nanogenerators because of their good thermoelectric properties, such as low thermal conductivity and wide temperature range. By combining the photothermal layer with the thermoelectric device, PTENG can absorb infrared light, creating a temperature difference in the device, and the potential difference between the two electrodes can be established for power generation. Therefore, PTENG can generate electrical energy without a spatial temperature gradient. In addition, flexible and shape-adaptive PTENG can be well applied to photothermothermic energy conversion in wearable electronics and implantable electronic devices. Dr. Xie Yannan and Dr. Lin Zonghong are co-communication authors, and the first authors are the Ming Dynasty (Xiamen University) and Lin Yuxi (Tsinghua University, Taiwan).
[Full text analysis]
Figure 1: Schematic diagram and material characterization of photothermographic nanogenerators (PTENG) based on MoS2/PU photothermal film and Te/PEDOT thermoelectric layer.
(a) Photothermographic nanogenerator (PTENG) structure based on MoS2/PU photothermal film and Te/PEDOT thermoelectric layer;
(b) an SEM image of the MoS2 nanoclusters;
(c) an SEM image of the nanowires;
(d) Raman spectra of MoS2 nanoclusters;
(e) a Raman spectrum of the nanowires;
[Interpretation]
The researchers designed the structure of the flexible TENG as shown in Figure 1(a). As can be seen from the figure, the TENG consists of a flexible PET substrate, a layer of Te/PEDOT nanocomposite as a thermoelectric material, a pair of Ag electrodes spaced about 3 cm apart, and a layer as a photothermal medium. The composition of the MoS2/PU film above the electrode. MoS2 and Te nanomaterials were synthesized by one-step hydrothermal synthesis. The surface morphology and structural properties of the prepared samples were characterized by FESEM and Raman spectroscopy, respectively. As shown in Fig. 1(b), the molybdenum disulfide nanoclusters have a nanoflower structure with an average diameter of 2.5 μm. The nanoflower structure consists of a well-dispersed nanosheet structure, which has been reported. The three-dimensional molybdenum disulfide nanomaterials are structurally similar. Figure 1(d) reveals the Raman spectrum of the MoS2 nanoclusters, consisting of two characteristic Raman peaks with peaks at 376 cm -1 and 407 cm-1, respectively, and corresponding to the 2H-MoS2 and A1g modes, respectively. Here, it is produced by the vibration of the opposite plane between the Mo atom and the two S atoms. The A1g mode is due to non-planar vibration of S atoms in opposite directions. Fig. 1(c) is a FESEM image of a Te nanowire having an average width of 50 nm and an average length of 1.5 μm. One-dimensional nanostructures (such as nanowires) can be used to modify thermoelectric materials because the phonon scattering on the surface of the nanostructures is stronger and the thermal conductivity is lower. Figure 1(e) shows the Raman spectrum of a Te nanowire containing three active Raman bands. The strongest Raman peak is at 112 cm?1, which corresponds to the A1 mode, which is on the base plane of each atom. The chain expansion pattern of the movement is related. In addition, there are two smaller peaks beside the A1 peak, the E1 mode appears at 85 cm-1, and the E2 mode appears at 131 cm -1 due to the large number of bond bends and bond stretches in Te.
Figure 2: Thermal imaging of MoS2/PU film, optical photo and photothermal heating curve
(a) Thermal imaging of the MoS2/PU film;
(b) Optical microscopy images of MoS2/PU films at different MoS2 concentrations (0, 0.5, 1, 2, 3 wt %);
(c) Photothermal heating curves of Ag electrode (without MoS2/PU film) and MoS2/PU film at different MoS2 concentrations (0, 0.5, 1, 2, 3 wt %);
[Interpretation]
Figure 2 (a) shows the temperature profile and thermographic image obtained after 200 seconds of infrared laser irradiation of the MoS2/PU film. For the prepared film sample, the highest temperature appeared in the film area irradiated by the laser beam, and the heat was transmitted outward at the edge of the laser spot. This phenomenon indicates that the temperature rise is caused by infrared light. In addition, it can be clearly seen that the surface temperature of the sample having a large MoS2 content is high. The tendency of the temperature of the sample under illumination to change with time is plotted in Figure 2(c). All samples were rapidly warmed from the initial temperature (room temperature) within 30 seconds and finally maintained at equilibrium temperature. As the MoS2 content in the photothermal film increased (0.5, 1, 2, 3 wt%), the film equilibrium temperature increased significantly to 337, 339, 340 and 343 K. The results show that MoS2 can absorb infrared light and can effectively convert it into heat. In order to further study the photothermal effect of MoS2, two samples without MoS2 were prepared and compared under an infrared laser beam under the same conditions. As shown in Fig. 2(c), the equilibrium temperatures of PET and pure PU films were 310 K and 322 K, respectively, which were much lower than those containing MoS2. Therefore, it can be concluded that MoS2 is an excellent photothermal medium with a high infrared absorption rate. This increase in temperature is mainly due to the photothermal effect of the MoS2 nanoclusters, which has an extremely high specific surface area. As shown in Fig. 2(b), the surface morphology of the MoS2/PU film was studied by metallographic microscope. The results show that the density and uniformity of the distribution of MoS2 nanoclusters in the photothermal layer increase with the increase of the weight percentage of MoS2. The higher this corresponds to the enhanced photothermal performance. However, even if a higher heating temperature can be achieved (as shown in Fig. 2(c), when the concentration reaches 3 wt%, breakage and cracking occur, which is not conducive to film transfer. Therefore, based on the above discussion, this paper selects 2 The photothermal device is made of wt.% MoS2 photothermal film. The reason for the occurrence of cracks and cracks may be that when the weight percentage of MoS2 reaches 3 wt%, the MoS2 nanoclusters are more likely to aggregate, and the MoS2 nanoparticle is uniformly dispersed in the PU aqueous solution. Clustering is difficult. Therefore, when a MoS2/PU mixed solution is deposited on a PET substrate to prepare a film, the MoS2 content in some regions may be much higher than 3 wt%, while the MoS2 content in other regions may be much lower than 3 wt%. Even 0 wt%. Regions with different MoS2 concentrations may have different coefficients of thermal expansion. Therefore, during heating and drying of the wet film, the film may rupture due to thermal stress.
Figure 3: Schematic diagram of thermoelectric nanogenerator (TENG) and thermoelectric test
(a) a schematic diagram of a hot plate heating a thermoelectric nanogenerator based on a Te/PEDOT film;
(b) The open circuit voltage of TENG at a temperature difference of 45 K;
(c) Short-circuit current of TENG at a temperature difference of 45 K;
(d) Dependence of the open circuit voltage on the temperature difference caused by the hot plate.
[Interpretation]
The researchers placed one electrode (on the left side of Figure 3(a)) on the hot plate while the other electrode (on the right side in Figure 3(a)) was suspended in the air without contact with the hot plate. When the hot plate is heated and maintained at 323 K, the temperature of the left electrode rises rapidly, while the temperature of the right electrode remains at room temperature (about 303 K), thus establishing a temperature gradient between the two electrodes, making the two electrodes A voltage output is generated between them, as shown by the open circuit voltage (VOC) in FIG. 3(b). At a temperature difference of 45K, the peak output voltage can reach approximately 1.9 mV and the Seebeck coefficient is 42 μV/K, which can be compared to previously reported flexible TEGs. However, compared to some excellent work, the Seebeck coefficient is still very low and needs further improvement. The reason for the relatively low Seebeck coefficient may be the lower concentration of Te nanowires in the Te/PEDOT composite. However, as the concentration of Te increases, it is difficult to obtain a dispersed Te/PEDOT mixed solution, which is disadvantageous for the preparation of the Te/PEDOT thermoelectric film and is not conducive to the stable electrical output of the thermoelectric device. The VOC in Figure 3(b) did not show significant attenuation after reaching the maximum value, indicating that the Te/PEDOT thermoelectric layer has a low thermal conductivity. In addition, TENG has been subjected to multiple heating and cooling cycle tests, and VOC exhibits reliability and repeatability. Short-circuit current (ISC) has similar output characteristics to VOCs and has a peak value of approximately 1.5μA. It can be seen that as shown in Fig. 3(d), the peak output voltage (VOC) reaches 0.8, 1.0, 1 under different temperature differences of 20, 25, 32, 36, 40, 45 K, etc. .3, 1.5, 1.6 and 1.9 mV, there is a good linear relationship between the temperature difference and the output voltage, indicating that the TENG has great potential in self-powered temperature sensors.
Figure 4: Working diagram of PTENG and photothermal test
(a) a schematic diagram of irradiating PTENG with an infrared laser;
(b) a photo of a flexible PTENG;
(c) the output voltage of PTENG at different optical power densities;
(d) the output current of PTENG at different optical power densities;
(e) The output voltage of PTENG at different illumination times (optical power density is 2.625 W/cm2);
(f) PTENG output current at different illumination times (optical power density is 2.625 W/cm2)
[Interpretation]
As shown in Fig. 4(b), the assembled device has good flexibility and deformability, so that it has shape adaptability and is suitable for curved surfaces for a wide range of practical applications. In order to study the photothermographic properties of PTENG, an IrS2/PU film with electrodes was irradiated with an IR laser (λ = 808 nm). Once PTENG was exposed to an infrared laser, the photothermal layer could absorb infrared light and cause its own temperature to rise. High, therefore, the photothermal layer can be used as a heat source for the electrode to raise the temperature, while the temperature of the other electrode remains at the initial room temperature, that is, there is a temperature difference between the two electrodes, and the Seebeck effect causes a potential difference between the electrodes. The carrier is driven to flow through an external circuit to generate an output current. During the illumination process, there are two factors that affect the electrical output of the PTENG: one is the optical power and the other is the illumination time. Figures 4(c) and (d) show the relationship between the electrical output of PTENG and the laser power over 200 seconds of illumination time. When the infrared laser is turned on, the VOC increases rapidly. Under different optical power densities of 2.625, 2.3, and 2w/cm2, the VOC finally reaches saturation values ​​of 1.2, 0.7, and 0.5 mV, which indicates The higher the optical power, the larger the voltage output, and reveals the strong dependence of the photothermal effect of the MoS2/PU film on the optical power. The infrared laser is turned off after 200 seconds of illumination, and the VOC is gradually reduced to zero due to heat transfer between the device and the surrounding environment. ISC and VOC have similar trends. The maximum output values ​​of short-circuit currents at different optical power densities (2.625, 2.3, and 2 W/cm2) are 0.18, 0.06, and 0.03 μA, respectively. The above results show that PTENG can achieve adjustable output. Figures 4(e) and 4(f) show the effect of different illumination times (30, 50, 100, 150, and 200 s) on PTENG electrical output at the same laser power density of 2.625 W/cm2. When the time is increased from 30s to 100 seconds, the VOC and ISC rise sharply from 0.6 mV and 0.1 μA to 1.1 mV and 0.15 μA; when the illumination time increases to 200 s, the VOC and ISC reach the saturation maximum of 1.2. mV and 0.18 μA. The photothermographic characteristic curve is similar to the photothermal property of the material (Fig. 2(c)), which indicates that the temperature difference between the two electrodes is the driving force of the electric output. The main working mechanism of PTENG is formed by the coupling of photothermal effect and Seebeck effect. After 200 seconds of illumination, there was no significant attenuation in the electrical output.
Figure 5: PTENG related application diagram
(a) Schematic diagram of IR lamp illumination PTENG;
(b) PTENG is applied to windows to collect infrared light;
(c) the output voltage of PTENG under infrared illumination;
(d) Tandem PTENG is applied to irregular rocks to collect outdoor sunlight.
[Interpretation]
In the above experiment, one electrode of PTENG was illuminated with an infrared laser. However, selective illumination of concentrated light sources is rare in our living environment. As shown in Figures 5(a) and (b), the infrared lamp (Philips BR125) is used to illuminate the PTENG attached to the window. The MoS2/PU film and the Ag electrode have different photothermal characteristics, resulting in the MoS2/PU film. The temperature increase is higher than that of the Ag electrode, that is, a temperature difference is formed between the two electrodes, and a potential difference is generated based on the Seebeck effect, and the driving electrons flow through the external circuit between the electrodes. As shown in Fig. 5(c), when the infrared lamp is turned on and the illumination is continued for 50 seconds, the VOC of PTENG rises rapidly and stabilizes at 1.2 mV. To further demonstrate the shape-adaptive properties and practical applications of PTENG, 10 PTENGs are connected in series and attached to a rock with an irregular surface, as shown in Figure 5(d). Under outdoor sunlight (atmospheric temperature of 20 ° C), 10 PTENGs can achieve an effective electrical output, and the output voltage measured by a digital multimeter is 1.48 mV.
[summary and outlook]
The flexible PTENG reported herein has several unique advantages over previously reported opto-thermoelectric devices. First of all, PTENG can produce electrical output for a long time and stably. The device does not have any heavy cooling elements, such as vacuum housing and heat sink, making the device light and small, so it is suitable for wearable electronic devices. Secondly, PTENG The photothermal layer is based on a simple MoS2/PU film that does not require complex design processes such as superlattice structures. In addition, the MoS2/PU photothermal layer is non-toxic and biocompatible, which is critical for wearable electronics. Therefore, considering the high flexibility of the device, PTENG has great potential for application in optoelectronic energy harvesting of wearable electronic devices such as solar photovoltaics.
In summary, the flexible PTENG based on MoS2/PU photothermal film and Te/PEDOT thermoelectric layer has been systematically demonstrated and studied. The MoS2/PU layer has been carefully designed to provide excellent flexibility, portability and photothermal performance by adjusting the MoS2 content. By mixing the photothermal layer with the Te/PEDOT thermoelectric device, PTENG can utilize the coupling of the photothermal effect and the Seebeck effect to obtain ambient infrared light energy to generate electrical energy. In addition, the obtained PTENG has many advantages such as flexibility, shape adaptation, light weight, and simple manufacturing, and has great application potential in photoelectric energy harvesting of wearable electronic and implantable electronic products.
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