With the rapid development of portable and wearable microelectronics, flexible thermoelectric materials have attracted much attention due to their ability to enable self-powered devices via utilizing the temperature difference between the skin and the environment. However, the scarcity of tellurium resources and its high toxicity pose some challenges to the commercial application of tellurium-based thermoelectric materials. It is thus critical to develop high-performance, low-cost, and non-toxic alternatives for flexible thermoelectric films. Copper(Ⅰ) selenide (i.e., Cu₂Se), as a representative liquid-like thermoelectric material, has attracted recent attention. Cu₂Se is an intrinsic p-type semiconductor composed of abundant, low-cost, and non-toxic elements, thus offering significant advantages in the field of sustainable energy. The existing studies report the fabrication of high-performance Cu₂Se thin films using conventional preparation techniques. However, these processes face significant limitations in pattern refinement and miniaturization, hindering their application in the fabrication of micro-thermoelectric devices. Inkjet printing technology, as a low-cost, efficient, and high-precision digital printing process, offers a promising solution to these challenges. This technique enables the preparation of complex patterns without masks or templates and is suitable for a variety of substrate materials. Therefore, this study was to prepare Cu₂Se nanopowders and explore their dispersion properties in different solvents to develop Cu₂Se inks suitable for inkjet printing technology, thereby offering insights into the preparation of flexible thermoelectric films and their application in next-generation wearable devices.

Cu₂Se nanoparticles were synthesized by the following methods. In method 1 (i.e., hydrothermal method), 5 mmol copper chloride, 2.5 mmol selenium powder, 0.05 mol sodium hydroxide were mixed with 50 mL deionized water, and continuously stirred at room temperature for 20 min, and then a certain amount of hydrazine hydrate solution was added, and the mixture was transferred to 100 mL PPL–stainless steel autoclave. Subsequently, the sealed autoclave reactor was heated at 180 ℃ for 20 h and then naturally cooled to room temperature. The precipitate at the bottom of the reactor was centrifuged at 10000 r/min and washed with ethanol and distilled water for several times to obtain Cu₂Se powder. In method 2 (i.e., hydrothermal method), 1 mmol of selenium dioxide and 2 mmol of copper acetate were mixed with 65 mL of deionized water, and then stirred at room temperature until there was no sediment. Afterwards, hydrazine hydrate was slowly added and stirred for 10 min. The mixture was transferred to 100 mL PPL–stainless steel autoclave, and then the sealed autoclave reactor was heated at 180 ℃ for 24 h. After cooling to room temperature, the resulting sediment was collected and centrifugally cleaned. The content of hydrazine hydrate was changed by the control variable method in the synthesis process to investigate its effect on the morphology. In method 3 (i.e., wet synthesis), 1.54 g polyvinylpyrrolidone (PVP) was dispersed in 77 mL deionized water, followed by 1.92 mmol selenium dioxide (9.6 mL) and 11.52 mmol ascorbic acid (28.8 mL) solutions and stirred for 15 min. Afterwards, 3.84 mmol cupric sulfate pentahydrate (9.6 mL) and 15.36 mmol -ascorbic acid (38.4 mL) solution were added successively. The solution was magnetically stirred at room temperature for 16 h. After washing with deionized water for several times, Cu₂Se core–shell nanoparticles were collected by centrifugation at 11000 r/min.

For Cu₂Se ink configuration, the prepared Cu₂Se powder was dispersed in deionized water and ethanol by an ultrasonic dispersion method. After ultrasonic treatment for 1 h, the dispersion effect was analyzed.

Cu₂Se nanoparticles are prepared by the three methods, and the crystal structure of the synthesized products is verified by the XRD patterns. The morphology and size of Cu₂Se particles prepared by different methods are different. In method 1, Cu₂Se powder prepared under alkaline conditions has a flake structure with a particle size of more than 10 μm, which does not meet the size requirements of inkjet printing. In method 2, under the influence of different contents of hydrazine hydrate, the synthesized powders show a large irregular flake structure (~500 nm) and small granular structure, and the particle size does not decrease significantly. The particle size of the powder is reduced when PVP is introduced into the subsequent synthesis process, indicating that the particles do not continue to grow into sheets during the synthesis process due to the coating of PVP, but remain small and irregular nanoparticles, and the particles are still agglomerated, which can cause a nozzle clogging in the subsequent printing process. In method 3, Cu₂Se particles synthesized by the wet method are spherical with a particle size of approximately 40 nm, and uniform in sizes without particle agglomeration, which can meet the requirements of material size for inkjet printing.

Cu₂Se particles synthesized by the wet method and dispersed under ultrasound in ethanol show a well-dispersion, and the particles in ink do not settle significantly after placing for a period of time, showing a good stability and suitable for inkjet printing.

Cu₂Se particles were prepared by the three methods for Cu₂Se ink suitable for inkjet printing, respectively, and nano-sized particles of Cu₂Se with uniform particle size and non-agglomeration were prepared by the wet method. The effective dispersion and stability of Cu₂Se particles were mainly attributed to the coating of PVP on the surface of the particles, improving the solubility of the particles in ethanol, effectively preventing the particle aggregation. This study could provide a foundation for the subsequent development of high-performance Cu₂Se thermoelectric thin films and their microdevices.