Facile and Gram-scale Synthesis of Cu_2-xS Nanoplate for Photothermal Application by Sonochemical-assisted Reaction Based on High-concentration Copper Ion Complex Precursor

Recently, to solve the environmental issue caused by fossil fuels and the global energy crisis, numerous research on the conversion and utilization of solar energy are in progress. Humans mainly benefit from solar power by using solar cells, which mainly use a visible region of solar light. Because infrared ray occupies about half of the total sunlight, however, for efficient use of solar energy, it is important to absorb a wide region of light from solar radiation and convert it into as much energy as possible. Among others, photothermal materials attract a lot of attention and are developing because they provide a platform for low-cost and efficient light-to-heat conversion. This energy conversion occurs by localized surface plasmon resonance(LSPR), especially in nanoscale metallic materials. Generally, LSPR has been studied on noble metals that show LSPR in the visible region and have high carrier densities. However, they can show the photothermal effect only in a specific incident light range, which is determined by the size and shape of noble metal nanostructures. In addition, the application of noble metal photothermal materials is limited to wide applications due to their high cost.<br/>Copper-deficient p-type semiconductors, Copper sulfide(Cu_2-xS) attract attention as new photothermal conversion materials because of the low bandgap(~2.2eV) and strong LSPR in the NIR region. Furthermore, they have not only high photostability and low toxicity but are cost-effective due to their lower prices than other noble metals. Although many studies have been conducted on methods for synthesizing Cu_2-xS nanoplate like hydro-solvothermal, hot injection, and electrodeposition, they require harsh synthetic conditions such as high temperature and vacuum, many reactants, and complicated steps. Above all, since nanoplates are synthesized by suppressing the growth using a low concentration of precursor, the final synthesis yield is very poor. The limitations of existing synthetic methodologies with inefficient productive yields act as obstacles to the commercialization of photothermal conversion systems of Cu_2-xS nanoplates.<br/>According to the homogeneous nucleation and growth theory, the critical nuclear radius and critical nucleation free energy are inversely proportional to the supersaturation of the system. This means that a small diameter and many nuclei and initial nanoparticles can be synthesized even under low energy by using high-concentration precursors. However, as the supersaturation of the system increases, there is a limit to the control of nucleation reaction, hard aggregation, and growth rate due to thermodynamically high interfacial and surface energy. Therefore, a methodological breakthrough that can achieve mass production of uniform nanoplates is required to solve these problems.<br/>In this study, we suggest the sonochemical-assisted reaction method based on a high-concentration metal ion complex precursor solution at room temperature and atmospheric pressure. The ultrasound energy allows for burst homogeneous nucleation and subsequent growth is controlled by the diffusion of growth species because the concentration of the growth species reduces below the minimum concentration for nucleation by the burst nucleation. And the metal ion complex acts as a dispersant to prevent particle agglomeration and growth. Large-scale synthesis of Cu_2-xS nanoplates with controlled size and shape for a photothermal conversion is successfully demonstrated by the facile and versatile method. We systematically investigated the effect of the precursor concentration on the size and shape of the Cu_2-xS nanoplates by Field Emission Scanning Electron Microscopy(FE-SEM), Dynamic Light Scattering(DLS), X-ray Diffraction(XRD), X-ray photoelectron spectroscopy(XPS), and UV-vis spectroscopy. Furthermore, the photothermal conversion efficiency of the nanoplates is confirmed under the Xenon lamp and the 808nm laser.