Synthesis of Single Phase α-Cu₂Se Nanowires by Photoreduction Method for Thermoelectrical Application

Cu₂Se, a low-toxicity and low-cost p-type semiconductor, has attracted attention for various applications such as thermoelectric materials, photocatalysts, and solar cells due to its optical and electrical properties. Especially, there have been many reports that Cu₂Se exhibited high thermoelectric performance. For further enhancement of thermoelectric performance of Cu₂Se, nanostructuring is effective due to phonon scattering at nanostructure interfaces leading to thermal conductivity reduction. Recently, various Cu₂Se nanostructures have been synthesized. However, Cu₂Se usually shows a complex phase structure depending on temperature and Cu defects because Cu₂Se undergoes a reversible phase transition between the low-temperature phase <i>α</i>-Cu₂Se (monoclinic) and the high-temperature phase <i>β</i>-Cu₂Se (cubic) in the 350-410 K temperature range. Therefore, this makes it difficult to synthesize single-phase Cu₂Se nanostructures. We have focused on the photoreduction method as a synthesis of nanostructures. In the photoreduction method, strong reductant or toxic agent are not used, and the reaction speed is slow. In the synthesis of Cu₂Se nanostructures, the phase structure can be controlled by adjusting the light intensity and irradiation time due to the slow reaction. In this study, we aim to synthesize single-phase <i>α</i>-Cu₂Se nanowires (NWs) by photoreduction method and to evaluate their electronic structures.<br/>Cu₂Se NWs were synthesized by following method briefly. Selenium dioxide and β-cyclodextrin were mixed in distilled water and sonicated . This solution was slowly mixed with L(+)-ascorbic acid solution under continuous stirring. Obtained precipitates were centrifuged and washed. Se NWs were synthesized by redispersion of this precipitates in ethanol [1]. The synthesized Se NWs and copper acetate were mixed in ethanol solution and irradiated with UV light. The product was obtained by centrifugation and Cu₂Se NWs was synthesized. Structural characterization of the prepared samples was performed using scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), and diffuse reflection UV-visible spectroscopy (DR UV-vis).<br/>SEM images showed a nanowire structure with a diameter of about 500 nm and a length of several micrometers. It was found that the nanowire structure was maintained even after photoreduction. The EDX mapping results showed that the nanowires were composed of Cu and Se, and the atomic number ratio of Cu and Se was found to be about 2:1. The diffraction peaks in the XRD pattern of synthesized samples were attributed to <i>α</i>-Cu₂Se. These results indicate that <i>α</i>-Cu₂Se NWs were synthesized from Se NWs by photoreduction method.<br/>DR UV-vis spectra of the synthesized <i>α</i>-Cu₂Se NWs were measured. In the UV-vis absorption spectrum of <i>α</i>-Cu₂Se, the absorption due to the interband excitation from the valence band to the conduction band appears at &lt;800 nm, while the absorption due to the intraband electron excitation in the valence band appears at &gt;800 nm. For the region below 800 nm, the band gap of the sample was obtained by using the equation of Tauc et al [2]. The band gap of synthesized <i>α</i>-Cu₂Se NWs was found to be 1.17 eV. This band gap value was in good agreement with that of bulk <i>α</i>-Cu₂Se (1.20 eV [3]).<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] Qing, L <i>et al.</i> <i>Chem. Commun.</i> <b>1</b>, 1006 (2006).<br/>[2] Bin Liu <i>et al.</i> <i>Phys. Chem. Chem. Phys.</i> <b>17</b>, 13280 (2015).<br/>[3] Yizhou You <i>et al.</i> <i>Phys. Chem. Chem. Phys.</i> <b>23</b>, 9814 (2021).