Study on the preparation of ascorbic acid reduced ultrafine copper powders in the presence of different protectants and the properties of copper powders based on methionine protection

Abstract

High-purity, monodisperse, and low-oxygen submicron copper powder particles with particle sizes in the range of 100-600 nm were synthesized under alkaline conditions using ascorbic acid (C₆H₈O₆) as a reductant and copper chloride (CuCl₂·2H₂O) as a copper source. The redox potential of the Cu-Cl-H₂O system was obtained by calculations and plotted on pH-E diagrams, and a one-step secondary reduction process (Cu(ii) → CuCl(i) → Cu₂O(i) → Cu(0)) was proposed to slow down the reaction rate. The commonalities and differences in the nucleation and growth process of copper powders under methionine (Met), hexadecyl trimethyl ammonium bromide (CTAB), and sodium citrate dihydrate (SSC) as protectants and without the addition of protectants are compared, and the reaction mechanism is discussed. Among them, methionine (Met) showed excellent properties and the Cu₂O(i) → Cu(0) process was further observed by in situ XRD. The synthesized copper powder particles have higher particle size controllability, dispersibility, antioxidant properties, and stability, and can be decomposed at lower temperatures (<280 °C). The resistivity can reach 21.4 μΩ cm when sintered at a temperature of 325 °C for 30 min. This green and simple synthesis process facilitates industrialization and storage, and the performance meets the requirements of electronic pastes.

Fig. 1. pH–E plot of copper chloride at a concentration of 1.0 mol L⁻¹ in an aqueous ascorbic acid system at 25 °C.

Fig. 2. XRD patterns of the products at different stages of the reaction at pH = 12: (a) after the addition of ascorbic acid; (b) after the addition of NaOH; (c) at the end of the reaction.

Fig. 3. SEM images and particle size distribution of Cu powder prepared under different conditions: (a) Met-11.0; (b) Met-11.5; (c) Met-12.0; (d) CTAB-11.0; (e) CTAB-11.5; (f) Met-12.0; (g) SSC-11.0; (h) SSC-11.5; (i) SSC-12.0.

Fig. 4. (a) SEM image of directly reduced copper powder without protectant, (b) particle size distribution of directly reduced copper powder without protectant; (c) XRD patterns under different conditions, (d) localized enlargement.

Fig. 5. TEM images, SAED images, and EDS elemental distributions of the Cu powder prepared under the same conditions with the addition of different protectants: (a)–(c) without protectant; (d)–(f) with Met as the protectant; (g)–(i) with CTAB as the protectant; (j)–(l) with SSC as the protectant.

Fig. 6. XPS spectra and Auger spectra of ultrafine copper powders synthesized with different protectants at pH = 12.0: (a) C1s of none; (b) Cu LM2 of none; (c) O1s of none; (d) C1s of Met; (e) Cu LM2 of Met; (f) O1s of Met; (g) C1s of CTAB; (h) Cu LM2 of CTAB; (i) O1s of CTAB; (j) C1s of SSC; (k) Cu LM2 of SSC; (l) O1s of SSC.

Fig. 7. (a) In situ XRD plots of the Cu reduction process using Met as a protectant; (b) the corresponding contour map; (c) TG curves of Cu particles synthesized using Met as a protectant (in N₂); (d) the corresponding FT-IR profiles.

Fig. 8. Variation of resistivity of copper powder sintered samples with sintering temperature (225 °C, 250 °C, 275 °C, 300 °C, 325 °C).

Fig. 9. SEM images and object ratio of Cu powder particles prepared by reduction of externally purchased Cu₂O as a raw material; (a) SEM image of Cu₂O as a raw material; (b) SEM image after reduction by the same process; (c) XRD patterns before and after reduction.

Fig. 10. Mechanism of formation of the ultrafine Cu powder particles.