Xin
Ke
abc,
Bingqing
Xie
abc,
Jingguo
Zhang
*abd,
Jianwei
Wang
*ae,
Weiying
Li
b,
Liqing
Ban
ab,
Qiang
Hu
ab,
Huijun
He
ab,
Limin
Wang
ab and
Zhong
Wang
*ab
aMetal Powder Materials Industrial Technology Research Institute of China GRINM, Beijing 101407, China. E-mail: wangjianwei@grinm.com; wzwz99@126.com
bGRIPM Advanced Materials Co. Ltd, Beijing 101407, China. E-mail: zjg@gripm.com
cGeneral Research Institute for Nonferrous Metals, Beijing 100088, China
dChina Gricy Advanced Materials Co., Ltd, Chongqing 401431, China
eGRINM NEXUSX Advanced Materials (Beijing) Co. Ltd, Beijing 101407, China
First published on 15th January 2024
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.
Reductants and protectants are crucial in wet chemical synthesis. As the basis of the redox process, the reductant reduces divalent copper salt ions to the zero-valent state and further induces their nucleation and growth. The protectant is often used to functionalize the wet chemical synthesis of ultrafine copper powders, adsorbing on the surface of copper particles to reduce the surface energy to control growth, prevent aggregation, and hinder oxidation.13 In most cases, ascorbic acid is the most commonly used reductant in the synthesis of ultrafine copper powder particles, and the moderate reduction rate ensures strong controllability.14 Polyvinyl pyrrolidinium (PVP),15 cetyltrimethylammonium bromide (CTAB),16 alkyl amines (cetyl, octadecyl),17 and other macromolecular long carbon chain surfactants are chosen as protectants to improve dispersion and regulate the growth to avoid aggregation and oxidation. Additionally, complexing agents and small organic ligands may be used as protectants sparingly. Y. Shun et al.18 synthesized nanoscale (Cu NPs) and micrometer-sized Cu particles (Cu MPs) with narrow size distributions using ascorbic acid as a reductant and citric acid as a complexing agent and discussed their growth mechanisms; N. Kumar et al.19 synthesized copper nanoparticles in aqueous media using L-cysteine as a capping and functionalizing agent; H. J. Pereira et al.20 used L-alanine as a capping layer and stabilizer for the aqueous synthesis of submicron-sized copper metal particles under ambient conditions and found the nature of complex formation between L-alanine and the copper(II) precursor and demonstrated that L-alanine acted as an effective barrier on the surface of the copper particles conferring good thermal stability and delaying the onset of oxidation.
However, preparing size-controllable, homogeneous, and long-term stable copper nanomaterials in a simple reaction system is still challenging. The mechanism for its nucleation and growth process is not precise, and more importantly, the accessible oxidization properties of copper are the main problem that hinders its functional application.21 Therefore, to obtain a convenient, economical, and green aqueous phase synthesis method to realize monodisperse, easily detachable, and oxidation-resistant ultrafine copper powder particles, we focus on ascorbic acid and amino acids as the reductant and protectant, which, as a kind of naturally occurring vitamin, exists in large quantities in nature and has a very good reducing and antioxidant ability; and amino acids, which have NH3+ and COO−, which can have good coordination adsorption ability with copper, thus regulating the size and morphology of the formed copper powder particles and hindering the erosion of oxygen.22
In this study, to address the problems of difficulty to control, poor dispersion, and easy oxidation of ultrafine copper powder particles during the preparation process, the redox potential of the Cu–Cl–H2O system was obtained by calculation and the pH–E diagram was plotted to guide the synthesis process, and a one-step secondary reduction method was further proposed to regulate the reaction rate and to control the process of nucleation and growth of the ultrafine copper powder particles. High-purity, monodisperse, and low-oxygen ultrafine copper powder particles were synthesized with a 100–600 nm particle size. The commonalities and differences of the nucleation and growth processes under the effect of different protectants were compared by controlling the pH, and the reaction mechanism was discussed. It was found that the ultrafine copper powder particles prepared with methionine as the protectant were more controllable, with better dispersion and antioxidant ability.
The entire reaction was carried out in a three-neck flask, and a thermometer with a rubber stopper was inserted into the mixed solution under stirring to monitor the reaction temperature. A Mettler Toledo model FE28 pH meter was used to reflect the solution pH in real time. In each experiment, the pH values of NaOH, C6H8O6, and the protectant were monitored and recorded before and after the addition of the solution. The corresponding experimental conditions and the state and pH of the solution at different stages are shown in Table 1.
Sample | Protectant | pH | pH of Cu solution | pH and color after adding protectant | pH and color after the addition of ascorbic acid | pH and color after adding NaOH |
---|---|---|---|---|---|---|
Met-11.0 | Met | 11.0 | 1.60 | 0.57, brownish-black | −0.38, milky white | 11.0, orange |
CTAB-11.0 | CTAB | 11.0 | 1.55 | 1.61, brownish-yellow viscous | −0.41, milky white | 11.0, orange |
SSC-11.0 | SSC | 11.0 | 1.52 | 1.31, bright green | −0.29, milky white | 11.0, orange |
Met-11.5 | Met | 11.5 | 1.63 | 0.61, brownish-black | −0.37, milky white | 11.5, orange |
CTAB-11.5 | CTAB | 11.5 | 1.55 | 1.60, brownish-yellow viscous | −0.47, milky white | 11.5, orange |
SSC-11.5 | SSC | 11.5 | 1.52 | 1.29, bright green | −0.34, milky white | 11.5, orange |
None-12.0 | — | 12.0 | 1.53 | — | −0.47, milky white | 12.0, orange |
Met-12.0 | Met | 12.0 | 1.57 | 0.62, brownish-black | −0.38, milky white | 12.0, orange |
CTAB-12.0 | CTAB | 12.0 | 1.51 | 1.62, brownish-yellow viscous | −0.56, milky white | 12.0, orange |
SSC-12.0 | SSC | 12.0 | 1.53 | 1.39, bright green | −0.23, milky white | 12.0, orange |
The redox half-reaction equation for the reduction of Cu2+ with ascorbic acid as the reductant can be expressed by eqn (1) and (2):
C6H8O6 − 2e− = C6H6O6 + 2H+ | (1) |
Cu2+ + 2e− = Cu | (2) |
The standard electrode potential of Cu2+ as the oxidizing agent in this redox reaction is Eθ(Cu2+/Cu) = +0.342 V, and the standard electrode potential of ascorbic acid (C6H8O6) as the reducing agent is Eθ(C6H8O6/C6H6O6) = +0.08 V. The electrode potential of ascorbic acid (C6H8O6) is much lower than the electrode potential of Cu2+, causing a large potential difference, and theoretically it can be used as a reductant for Cu2+ to reduce it to Cu0.
The redox potential of ascorbic acid was calculated from the Nernst equation in eqn (3):
(3) |
In an aqueous system with CuCl2·2H2O as the copper source and C6H8O6 as the reductant, a Cu2+ concentration of 1.0 mol L−1 was used as a reference, the stability of copper and its compounds in different pH environments at room temperature of 25 °C was considered, and the pH–E diagram was calculated as shown in Fig. 1. Cu2+ can only be stabilized under acidic conditions at pH below 3.2, whereas with the addition of C6H8O6 the ligand reaction occurs between Cl− and Cu2+. That means Cu no longer exists in the free ionic state and a series of complexes are formed such as CuCl+, CuCl2·3Cu(OH)2, and CuCl. When copper is in the highly oxidized state of Cu(II), the thermodynamically more stable complexes formed with Cl− are more favorable for its oxidation, resulting in a lower and constant standard potential value within a certain acidic range, a decrease in the potential difference with C6H8O6, and a slowing down of both the redox reaction drive and reaction rate. When the pH in the solution increased to near 6, Cu(I) no longer existed in the form of the Cl− complex but began to transform into the more stable Cu2O, whose potential decreased linearly with the gradual increase of pH, but as the potential of ascorbic acid showed the same trend since the initial state, it made the potential difference between it and Cu2O larger than that with CuCl, and the reaction was easier to carry out, and finally complete reduction to Cu(0) occurred.
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. |
The XRD profiles of the products at different stages of the reaction in the presence of each protectant (None, Met, CTAB, SSC) at pH = 12.00 were determined by X-ray diffraction (XRD) as shown in Fig. 2. Fig. 2(a) shows the XRD pattern of the white precipitate formed after the addition of C6H8O6, and all the diffraction peaks correspond to the diffraction peaks of the standard CuCl (PDF#77-2383), which indicates that the white product during the reaction is CuCl. The samples are tested using the same amount, and among them, the diffraction peaks of the crystalline surface of (111) are all the strongest, and the peak intensity of different samples (None > SSC > CTAB > Met) is greatly different: the diffraction peak intensity of the (111) crystal plane without adding the protectant (sample None-12.0) is the highest, indicating that the crystallization degree of the CuCl formed is better, the grain is larger than other samples, and the growth of the crystal plane is more orderly. A trace amount of CuClO4 (PDF#38-0594) was also found in the product with CTAB as the protectant (sample CTAB-12.0). This is a result of the CTAB as a kind of cationic surfactant, which can act as a cross-linking agent in an aqueous solution to form complexes with a variety of compounds. Fig. 2(b) shows the XRD pattern of the orange-yellow precipitate formed when the pH was adjusted to 12.0 by the addition of NaOH, and the physical phase analysis showed that its diffraction peaks corresponded to those of the standard Cu2O (PDF#99-0041). The absence of the protectant (sample None-12.0) makes it difficult to fully react under these conditions due to the high degree of crystallization, ordered (111) crystal surface growth, and an untransformed CuCl phase in the product. The XRD pattern of the reddish-brown product after the final complete reduction is shown in Fig. 2(c), and the diffraction peaks correspond to those of the standard Cu (PDF#85-1326), indicating that all of them have been completely reduced to Cu(0) and that the number of (111) crystalline surfaces is greater. The amplification of the diffraction peaks of the (111) grain surface at 2θ = 43.316° reveals that the FWHM with Met as the protectant (sample Met-12.0) is significantly narrowed compared to the other conditions, which is analyzed in conjunction with the information on grain size and microscopic strain in Table 2 as the narrowing of the diffraction peaks due to the coarsening of the grains and weakening of the microscopic strain. It was further found that the diffraction peaks of Cu powder prepared with Met as the protectant (sample Met-12.0) were finer, higher, and sharper, indicating a higher degree of crystallinity. The results showed that the ultrafine Cu powders obtained by using C6H8O6 as the reductant and CuCl2·2H2O as the copper source underwent the reduction process of Cu2+(II) → CuCl(I) → Cu2O(I) → Cu(0) shown by the brown dashed line in Fig. 1. The accuracy of the pH–E diagram was demonstrated by combining the solution pH measured during the sampling phase (Table 1) with the stabilization zones of the substances in the corresponding solution environments in Fig. 1. Based on the above observations, the following secondary reduction reaction process is proposed to conform to the following reaction equation scheme at pH = 11.0–12.0:
Cu2+(II) + Cl− + C6H8O6 → CuCl(I) + C6H7O6 + H+ | (4) |
2CuCl(I) + OH− → Cu2O(I) + 2Cl− + H2O | (5) |
Cu2O(I) + 2C6H7O6 → 2Cu(0) + 2C6H6O6 + H2O | (6) |
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. |
Sample | Crystallinity/% | Grain size/Å | Microstrain/% | FWHM/rad |
---|---|---|---|---|
Met-11.0 | 27.74 | 420 | 0.067 | 0.239 |
CTAB-11.0 | 26.99 | 486 | 0.119 | 0.212 |
SSC-11.0 | 22.27 | 315 | 0.296 | 0.286 |
Met-11.5 | 27.39 | 538 | 0.104 | 0.193 |
CTAB-11.5 | 27.98 | 492 | 0.146 | 0.199 |
SSC-11.5 | 22.84 | 349 | 0.260 | 0.290 |
None-12.0 | 22.19 | 405 | 0.227 | 0.233 |
Met-12.0 | 26.20 | 619 | 0.128 | 0.179 |
CTAB-12.0 | 25.48 | 518 | 0.155 | 0.254 |
SSC-12.0 | 22.89 | 388 | 0.223 | 0.236 |
As a comparison, the SEM and particle size distribution of Cu powder obtained by direct reduction without adding any protectant under the same reaction conditions are shown in Fig. 4(a) and (b). We found that the particles of Cu powder without protectant are also separated from each other unbonded, which indicated that ascorbic acid itself has a good dispersing ability, in addition to the homogeneous morphology and size, and the average particle size (148.86 ± 18.59 nm) is much smaller than that of the Cu powder with the additive of protectant under the same conditions. This conclusion is also supported by the SEM plots and particle size changes of copper powder particles (Fig. S1–S15†) obtained by using different protectants prepared under different reaction conditions in the ESI.† The preparation of Cu powder under different conditions was analyzed by XRD as shown in Fig. 4(c), and X-ray diffraction peaks appeared at 2θ of 43.3°, 50.4°, 74.1°, and 89.9°, which corresponded to the (111), (200), (220), and (311) crystal planes of the standard Cu (PDF#85-1326), respectively, where the sharp diffraction peak of the (111) crystal plane reveals the high crystallinity of Cu powder particles. Local enlargement of the crystal surface of (111) at 2θ = 43.316° (Fig. 4(d)) revealed that the diffraction peak intensities and half-height widths are altered to varying degrees after the addition of the protectant, indicating that there are microscopic strains within the Cu powder particles, which in turn lead to distortion of the crystal surface and make the diffraction peaks deformed. The finer and higher (111) diffraction peaks of the Cu powders prepared with Met as a protectant indicated good crystallinity, while their microstrain was smaller and grain size was larger, indicating lower lattice defects.24
The grain size of the Cu powder particles calculated by the Scherrer formula (Table 2) also had a good correspondence with the SEM image. However, its grain size is much smaller than that of the particle size in SEM, indicating that the copper powder has undergone a secondary aggregate growth after a primary nucleation growth. In particular, the surface of the copper powder particles in Fig. 3(c) still has small particles that are not fully aggregated, which can be considered to be the second aggregation of the process of growth left behind, which is also consistent with Park et al. who proposed the “outbreak of nucleation–aggregation of the growth” of the two-stage growth model.25 The increase in pH promotes the secondary dissociation of ascorbic acid, while the addition of the protectant alters the surface free energy of Cu atoms, which in turn controls the rate of nucleation and the rate of reaction such that a higher critical free energy is required to reach the critical nucleation radius.
The TEM images, SAED images, and EDS elemental distributions are shown in Fig. 5, the particle size and dispersion properties of the Cu powder particles added with different protectants exhibit obvious differences, in which the Cu powder particles (about 500 nm) with Met as the protectant are much larger than the others, and at the same time the boundaries between the particles are clear and without agglomeration. The SAED results show that the copper powder particles prepared with Met as the protectant are more characterized by single crystals. In contrast, the copper powder particles prepared with other protectants are polycrystalline diffraction rings. This indicates that the growth behavior of using Met as a protectant is dominated by crystalline growth, while other protectants are dominated by aggregation growth. The corresponding element distribution diagrams confirm that the particles are Cu, with oxygen on the surface, which copper tends to adsorb. The EDS diagrams in Fig. 5(f) and (i) also confirm the characteristic elements of the corresponding protectant: N and S in Met and N in CTAB, indicating the participation of the protectants in the preparation process of the Cu powder and the source of differences between the copper powder particles. In addition, the brighter and more tightly packed Cu element with Met as the protectant indicated that it has higher Cu content and lower oxygen adsorption; subsequent XPS confirmed this conclusion, indicating that Met as a non-polar amino acid shows excellent properties different from conventional surfactants.26
The XPS data demonstrated the elemental content in the ultrafine copper particles synthesized with different protectants. The specific elements and their contents are shown in Table 3; the synthesized copper powders all have C, Cu, O, and uncleaned Cl and Na. Through the full spectrum analysis, it was found that there were differences in the purity and oxidation degree of the copper powders prepared with different protectants, among which the copper powders prepared with Met as the protectant had the highest purity of 92.81% and the lowest oxygen content of only 3.37%. The other main detection elements also corresponded to the pharmaceutical elements used in the experimental procedure and the elements analyzed by EDS energy spectroscopy.
Sample | Element | Peak position/eV | FWHM/eV | Area/eV | Weight/% |
---|---|---|---|---|---|
None-12.0 | Cu2p3 | 932.76 | 2.99 | 5153613.10 | 92.66 |
O1s | 531.20 | 3.33 | 1240929.57 | 4.13 | |
Na1s | 1071.69 | 2.89 | 245633.72 | 1.88 | |
C1s | 285.14 | 3.54 | 460643.40 | 1.02 | |
Cl2p | 199.93 | 5.32 | 49352.76 | 0.31 | |
Met-12.0 | Cu2p3 | 932.66 | 2.99 | 4762361.73 | 92.81 |
O1s | 531.02 | 3.21 | 9333932.86 | 3.37 | |
Na1s | 1071.22 | 2.97 | 227132.63 | 1.88 | |
C1s | 285.05 | 3.28 | 362220.37 | 0.87 | |
S2p | 163.11 | 3.07 | 93372.08 | 0.57 | |
Cl2p | 199.00 | 3.94 | 60165.73 | 0.41 | |
N1s | 399.98 | 3.31 | 31461.91 | 0.09 | |
CTAB-12.0 | Cu2p3 | 932.14 | 3.29 | 1990271.08 | 85.61 |
O1s | 530.74 | 3.36 | 911282.09 | 7.26 | |
C1s | 284.96 | 3.14 | 884416.06 | 4.67 | |
Na1s | 1070.99 | 3.17 | 63153.83 | 1.16 | |
Cl2p | 198.37 | 3.82 | 57024.77 | 0.86 | |
N1s | 402.38 | 3.49 | 69319.46 | 0.45 | |
SSC-12.0 | Cu2p3 | 932.63 | 3.03 | 4088104.21 | 90.36 |
O1s | 531.16 | 3.36 | 1025950.51 | 4.20 | |
Na1s | 1071.29 | 3.07 | 416265.68 | 3.91 | |
C1s | 285.12 | 3.65 | 412560.43 | 1.12 | |
Cl2p | 199.18 | 3.68 | 52932.52 | 0.41 |
In order to analyze the surface elemental valence and oxidation degree of the ultrafine copper powders in detail, Fig. 6 demonstrates the XPS spectra and Auger spectra of the micro- and nano-copper powders synthesized with different protectants at pH = 12.0. The binding energy of the C–C bond is used for charge correction based on 284.80 eV. Since the chemical composition and compositional content of Cu and Cu2O were differentiated by Cu LM2 spectra, the Cu and Cu2O peak positions in the Cu2p spectral line were very close to each other and indistinguishable. The C1s spectra of Fig. 6(a), (d), (h) and (k) revealed that the peaks of C all appeared at 284.80 eV, 286.00 ± 0.5 eV, and 288.40 ± 0.05 eV, which were attributed to C–C, C–O–C, and O–CO, respectively, and may originate from the fact that the o-dihydroxyl group on the five-membered ring structure of C6H8O6 undergoes dehydrogenation and oxidation to generate the neighboring diketone structure, and the C–O bond is converted to CO and adsorbed on the surface of the generated ultrafine copper powders.27 The Cu LM2 spectra of Fig. 6(b), (e), (i) and (l) revealed that the more pronounced peak at 916.80 ± 0.5 eV was caused by unreduced Cu2O, and the characteristic peaks of Cu appeared as the main peak at 918.50 ± 0.5 eV and the satellite peak at 921.50 ± 0.5 eV.28 It is noteworthy that the characteristic peak of residual CuCl also appears at 913.61 eV in Fig. 6(l), but none of the peaks of CuO (917.7 eV) appear. The O on the surface of copper powders was analyzed to determine the degree of oxidation. It is seen from Fig. 6(c), (f), (j) and (m) that O is mainly derived from incompletely reduced Cu2O (530.50 ± 0.5 eV) and CO in ascorbic acid (531.50 ± 0.5 eV). Among them, adsorbed water was also detected near 535.60 ± 0.20 eV in the copper powders without protectants and with SSC as a protectant, while it was absent from the surfaces of the copper powders with Met and CTAB as protectants, which was attributed to the methylthio group (CH3S–) in Met and the hydrophobic functional group (HO–) in CTAB as a hydrophobic functional group that hinders the adsorption of water, and further slows down the oxygen erosion.29 However, there is a weak peak attributed to C–O at 532.71 eV in Fig. 6(f), which is provided by a small amount of unoxidized hydroxyl oxygen in C6H8O6. A comprehensive evaluation of several protectants revealed that Met as a protectant resulted in the lowest level of oxidation.
The thermal stability of Met-coated copper powder particles was investigated using thermogravimetric analysis in an N2 atmosphere as shown in Fig. 7(c). With the gradual increase in temperature, the surface of the copper powder particles gradually decomposed leading to weight loss, and the weight loss can be divided into three stages: 35–190 °C, 190–280 °C and 280–600 °C. Among them, the mass loss of copper powder particles in the stage below 190 °C can be attributed to the evaporation of adsorbed and residual water on the powder surface, with a weight loss of only 0.42%. The weight loss was most pronounced at the 190–280 °C stage, reaching 1.99%, considering that the organic matter covered on the surface is mainly the rapid decomposition of Met. The mass loss at the 280–600 °C stage is also largely insignificant at only 0.52%, which may be related to the slow decomposition of the residual organic matter (Met and C6H8O6) and trace amounts of Cu2O. The organic matter in the surface layer was verified by the FT-IR pattern of Fig. 7(d), where more pronounced absorption peaks were found at 3750 cm−1, 2125 cm−1, and 1546 cm−1, which corresponded to the O–H, CCO, and CN stretching vibrations. It is indicated that both C6H8O6 as the reducing agent and Met as the protectant are adsorbed on the surface of the ultrafine copper powder particles to different degrees, which also corresponds to the EDS energy spectra and TG results. The variability in particle size, morphology, and degree of oxidation can only originate from Met as a protectant since the same reductant and copper source were used.
The variation of resistivity of copper powder particles with sintering temperature after low-temperature sintering is shown in Fig. 8. The results show that the resistivity tends to decrease with the increase of sintering temperature. The resistivity is basically unchanged at 250 °C, being 52.0 ± 0.6 μΩ cm. When the sintering temperature is increased to 275 °C and above, the resistivity decreases dramatically, and it is only 21.4 μΩ cm at 325 °C. This is due to the increase of sintering temperature, which leads to the gradual decomposition of the organic matter on the surface of the copper powder particles, and the bare copper atoms with higher activity are more likely to diffuse to form a sintered neck, which in turn forms a continuous conductive pathway to make the resistivity drop, and the SEM images (Fig. S16†) of the sintered tissues support this result very well.
Fig. 8 Variation of resistivity of copper powder sintered samples with sintering temperature (225 °C, 250 °C, 275 °C, 300 °C, 325 °C). |
In order to discuss the influence of Cu2O as an intermediate product on the reduction process of Cu, we prepared ultrafine Cu powder particles by reduction under the same process conditions using purchased Cu2O with essentially the same particle size as the intermediate Cu2O product as the raw material; the morphology and phase composition before and after the comparative reduction are shown in Fig. 9. Fig. 9(a) is the SEM image of the purchased Cu2O particles, which is indicated as pure Cu2O powder by the XRD pattern in Fig. 9(c), corresponding to the diffraction peaks of the standard Cu2O (PDF#99-0041), and no other impurity phases were found. The morphology is irregular and the particle size is extremely heterogeneous with particle sizes in the range of <1–10 μm. Fig. 9(b) shows the SEM image after reduction, which shows the similarity of the morphology with that of Cu2O as the raw material, indicating that the reduction reaction has a certain morphology inheritance, and the difference lies in the small spherical particles of about 200 nm bonded on the surface of the irregular large particles. The XRD pattern in Fig. 9(c) shows that the reduction is incomplete, and diffraction peaks of the Cu2O physical phase are still found at 2θ angles of 36.36°, 42.24°, 61.32°, and 77.32°, which correspond to the (111), (200), (220), and (222) crystallographic planes of standard Cu2O (PDF#99-0041), respectively. Diffraction peaks corresponding to the (111), (200), (220), and (311) crystal faces of the standard Cu (PDF#85-1326) were also found at 2θ angles of 43.24°, 50.44°, 73.40° and 89.90°. Using the same process to compare the purchase of Cu2O and CuCl2·2H2O as raw materials for the preparation of copper powder experiments (Fig. 2). Indeed, Cu2O has a key role in regulating the reduction rate during the nucleation growth process.30 However, incomplete reduction occurs with Cu2O as the raw material, which suggests CuCl, the reduction product of the first stage product, plays a relatively important role in the process. This indicates further that CuCl also has a facilitating role in regulating the reduction rate, ensuring a moderate reaction rate, which leads to a more controllable morphology and higher homogeneity of the morphology of the ultrafine Cu powder particles.
Based on the above results, the formation mechanism of the ultrafine Cu powder particles synthesized by the one-step secondary reduction method was hypothesized as shown in Fig. 10. More specifically, three key steps are involved, starting with CuCl2·H2O as the copper source and the addition of an organophilic ligand as a protectant to form a complex with Cu(II) and accompanying redox reactions throughout. When ascorbic acid (C6H8O6) with a conjugated structure is further added, one of the hydroxyl groups (–OH) in the o-dihydroxyl group on the ring structure ionizes, removing H+ and transferring a charge to the outer layer of the oxygen atom to form a ketone structure (CO) with the carbon atom, resulting in the formation of semi-dehydroascorbic acid (C6H7O6).31 The reduction drive from the transfer of charge initially reduces Cu(II) to form CuCl as a refractory white product, which provides an initial reduction driving force for the reduction of Cu(II) to Cu(I) as the first reduction of a one-step secondary reduction process, and in addition, CuCl also serves as a solid precursor for the subsequent redox reaction that can gradually release Cu(II) ions to measure the rate of the reaction. Meanwhile, the organic ligand protector adsorbed on the surface of CuCl acting on Cu(I) causes it to exhibit an electrostatic effect and limits agglomeration.32 Afterward, with the addition of large amounts of NaOH to make the environment of the reaction solution change from strongly acidic to strongly alkaline quickly, CuCl cannot exist and combine with OH− to undergo a proton reaction so that it is transformed into Cu2O. As the redox reaction proceeds, one of the remaining hydroxyl groups (–OH) in the semi-dehydroascorbic acid (C6H7O6) also ionizes prompting further reduction of the highly reactive Cu2O(I) to form Cu(0). The monomer concentration of Cu(0) atoms gradually increases to reach the threshold of supersaturated concentration and it will undergo nucleation to form Cu nano fine crystals, and the formed dehydroascorbic acid (C6H6O6) is also adsorbed on Cu(0) through ligand bonding to avoid oxidization, and after that, the reduced Cu(0) atoms will undergo ordered growth based on Cu nano fine crystals. Electrostatic interactions between carbon chains with different organic ligands as protectants and Cu(0) lead to differences in the surface state of Cu particles exhibiting different particle size morphology and dispersion, followed by secondary aggregation growth of residual Cu(0) atoms through Oswald ripening. The reaction continues until the Cu(0) atoms are completely consumed and no more Cu in any other valence state remains, wherein the formation of bidentate complexes with strong complexation between Met as a protectant and Cu(II) ions can significantly modulate the nucleation and growth process of Cu particles as well as the degree of antioxidant resistance.
Footnote |
† Electronic supplementary information (ESI) available: The SEM plots and particle size changes of copper powder particles obtained by using different protectants prepared under different reaction conditions (PDF). See DOI: https://doi.org/10.1039/d3na01146a |
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