## 3 Result and Discussion
### Photothermal Conversion Performance Analysis  
To evaluate whether the PEG/GNPs composites are capable of absorbing and converting solar energy more effectively in comparison with pristine PEG when exposed to solar irradiation, we designed an experimental apparatus to conduct the photothermal conversion experiments on them, which was composed of two parts: photothermal conversion system and data acquisition system. The first system consisted of an insulated chamber, serial thermometer, temperature control device, temperature-sensing device, and a light source. The second system consisted of temperature logger with several thermocouples and a computer. Prior to testing, the sample melted at 85 \({}^{\circ}\)C was poured into a quartz beaker, then the thermocouple was put in the middle of the sample. During the process of cooling, it is imperative to keep the thermocouple in the correct position to ensure close contact with the sample. The variations in temperature of pristine PEG and PEG/GNPs-2% composite with the irradiation time are depicted in Figure 10. As can been observed, during the light irradiation period, the PEG/GNPs-2% composite can fully absorb and convert the light energy, as a result, the temperature exhibits a rapid rising trend. When the temperature achieves the melting value of the composite, a turning point appears in the temperature variation curve, indicating that the composite begins to process a phase change and store energy. After the illumination is removed, the temperature of the composite starts to decline, and the curve shows a constant temperature heat release platform, which indicates that the composite begins to crystallize. However, the temperature of pristine PEG  
Figure 9: Viscosity of the composites with different contents of GNPs.  
rises slowly and barely reaches its melting point, exhibiting a low efficiency in utilizing the light energy. Therefore, compared with pure PEG, the PEG/GNPs nanocomposites show better performance in photothermal energy conversion.  
To illustrate the reasons for the higher absorption efficiency of PEG/GNPs for solar irradiation compared with pure PEG, the Ultraviolet-visible-Near Infrared spectroscopy was used to measure the diffuse reflection spectra of PEG and PEG/GNPs composites. As shown in the Figure 11, the light absorption performance of PEG/GNPs composite is obviously much higher than that of pristine PEG in throughout at 200 to 2000 nm wavelength, suggesting that the PEG doped with GNPs make an outstanding increase in the solar light absorbance characteristic. Furthermore, the composite shows one total absorbance property in the different spectral region, which is essential to utilize the solar thermal energy to the greatest extent. This possibly can be attributed to a zero-bandgap structure of GNPs, which can theoretically absorb any wavelength, in addition, when the intensity of the incident light exceeds a certain threshold, the absorption of GNPs will reach saturation. As can be seen from photograph (inset in Figure 10), compared with PEG/GNPs composite, the pristine PEG has a white surface which can form a reflective layer that hinders the absorption of solar light. Accordingly, compared with pure PEG, the PEG/GNPs composites exhibit higher conductivity and better solar radiation absorption capacity, resulting in the more excellent performance in photothermal energy conversion. Therefore, the PEG/GNPs composites are expected to have great potential for solar energy conversion with respect to their better thermal conductivity and photothermal performance.  
Figure 11: The Ultraviolet-visible-Near Infrared diffuse reflection spectra of pure PEG and PEG/GNPs-2% composite.  
Figure 10: The variation in temperature with irradiation time of pure PEG and PEG/GNPs-2% composite.