Synthesis of Graphene/Cu Nanoparticles by Photoreduction Method for Enhancing Thermoelectric Power Factor

Thermoelectric materials that can directly convert heat to electricity are attracting attention. Their thermoelectric efficiency is usually evaluated by thermoelectric power factor (<i>S</i>²<i>σ</i>), where S is Seebeck coefficient and σ is electrical conductivity. Graphene is a promising thermoelectric material due to its high electrical conductivity (<i>σ</i>) derived from its inherent high carrier mobility, good thermal stability and excellent mechanical strength. It has been reported that graphene composited with highly conductive precious metal nanoparticles (Pt, Au, Ag) forms conductive bridges and improves thermoelectric properties by increasing <i>σ</i> [1]. We have focused on copper as alternative metals to precious ones because copper is abundant in the earth and has as high electrical conductivity as precious metals. Therefore, the combination of graphene and copper nanoparticles (Cu NPs) is expected to improve the σ. In this study, we synthesized Graphene/Cu nanoparticles (Gr/Cu NPs) by the photoreduction method which does not require environmentally hazardous reductants [2], and evaluated their thermoelectric properties.<br/>Mixed solution of copper acetate and graphene was irradiated with UV light. The resulting precipitates were spin coated and annealed on a glass substrate to form thin films (Gr/Cu NP films). Structure of the thin films was analyzed by X-ray diffraction (XRD), and cross sections and surfaces of the thin films were observed by scanning electron microscopy (SEM). Their electrical conductivity (<i>σ</i>) was measured by van der Pauw method. Their Seebeck coefficient (<i>S</i>= Δ<i>V</i>/Δ<i>T</i>) was determined by measuring the potential difference (Δ<i>V</i>) and temperature difference (Δ<i>T</i>) by our hand-made system.<br/>SEM images of Gr/Cu NPs showed particles with several dozens of nanometers on the graphene sheet. Cross-sectional SEM images of the thin films showed the thickness of 50~70 nm on the glass substrate. From XRD pattern, the diffraction peak originating from Cu(111) appeared. These results indicate that the nanoparticles observed in the SEM images were Cu NPs. When the dependence on the amount of Cu NPs in the thin film was examined, the electrical conductivity <i>σ</i> showed the highest value at 10 wt%. This value is 1.5 times higher than the graphene thin film, and this indicates the formation of conductive bridges through Cu NPs on the graphene. On the other hand, <i>S</i> showed no significant dependence on the amount of Cu NPs in the thin film. As a result, the power factor (<i>S</i>²<i>σ</i>) of the optimized Gr/Cu NP thin films was 1.5 times higher than the graphene thin film.<br/><b>Acknowledgments</b><br/>This work was supported by Grant-in-Aid for Early-Career Scientists Grant Number 21K14479 and Grant-in-Aid for Scientific Research (C) Grant Number 19K05187 from JSPS KAKENHI, Japan.<br/><b>References</b><br/>[1] L. L. Shiau et al., Graphene-Metal nanoparticles for Enhancing Thermoelectronic Power Factor. IEEE Transactions on Nanotechnology. 18, 1114 (2019).<br/>[2] M. Miyagawa, et al., Aqueous synthesis of protectant-free copper nanocubes by a disproportionation reaction of Cu2O on synthetic saponite. Chem. Commun. 54, 8454 (2018).