J. Cent. South Univ. (2012) 19: 2125-2129

DOI: 10.1007/s11771-012-1254-4

Effect of OH^- on morphology of Cu₂O particles prepared through reduction of Cu(II) by glucose

WANG Yue-jun(王岳俊), ZHOU Kang-gen(周康根)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: Cu₂O particles with different shapes were prepared via reducing Cu(II) in alkaline system by glucose at 50 ℃. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the shape of Cu₂O particles changes with the change of concentration of NaOH. The different shapes of Cu₂O particles are due to the absorption of OH^- ions on Cu₂O particles, which arise the variety of growth mode of Cu₂O, and then influence the morphology of Cu₂O particles.

Key words: cuprous oxide; glucose reduction; morphology control; growth mechanism

1 Introduction

Cu₂O is widely used as additive of corrosion-proof coating, colorant of glass and bactericide. Cu₂O is a p-type semiconductor with a small band gap of about   2 eV [1], which makes it a promising material in the application of solar energy conversion, electronics, magnetic storage and catalysis [2-6]. Recently, some researches were carried out on the copper powder for electronic pastes prepared by H₂ reduction of Cu₂O as the precursor [7-8]. In this method, the shape and size of copper powder particles were determined by Cu₂O. Since the parameters, such as particle size and shape of copper particles are utmost important for conductive pastes, it is essential to investigate the morphology and size control of the Cu₂O particle. Many efforts have been devoted to the synthesis of Cu₂O with various morphologies such as octahedron [9-12], cube [13-17], sphere [10, 16, 18], star-like, flower-like [19] and tubular like [13].

However, most of the methods utilize organic additive as modifier [10, 13], irradiation of microwave [15, 18], or water-in-oil microemulsions [14]. Those processes are inconvenient and complex. In this work, Cu₂O particles were prepared via reducing Cu(II) by glucose in alkaline system. Different shapes and crystallinities of Cu₂O particles were obtained through changing the concentration of NaOH without any assistant equipment or reagents. The synthesis can be well repeated in high yield and satisfy the demand of industrialization. Furthermore, mechanism concerning morphology control was analyzed and discussed. The reaction condition of preparing spherical Cu₂O particles for producing copper powder used on electronic pastes was discovered.

2 Experimental

2.1 Materials

Copper sulphate (CuSO₄·5H₂O), sodium hydroxide (NaOH), glucose (C₆H₁₂O₆·H₂O) and solvents were of analytical grade and were used as received without further purification.

2.2 Preparation of Cu₂O particles

In different sequences of experiments, the volumes of CuSO₄, NaOH and glucose solutions with definite concentration were kept constant at 200 mL, respectively, and the concentrations of CuSO₄, NaOH and glucose were listed in Table 1.

Table 1 Reaction reagent concentration for preparation of Cu₂O

Firstly, the precursor was prepared by dropping

NaOH solution into CuSO₄ solution under stirring in flask at room temperature (25 ℃). Secondly, the precursor and glucose solutions were heated up to 50 ℃, respectively. Finally, the glucose solution was dumped into the reaction vessel rapidly to reduce the precursor at 50 ℃. The products were collected by centrifugation, washed for 5 times with distilled water and 2 times with ethanol, and then, dried at 65 ℃ for 24 h. Preparation processes of precursor and Cu₂O paste were both under stirring (500 r/min). The pH values were measured timely during the glucose reduction.

In the reaction, the Cu²⁺ ions are reduced first to Cu⁺ by glucose and Cu⁺ can react with OH^- to form Cu₂O, which is described as follows:

Cu(OH)₂=Cu²⁺+2OH^- (or CuO+H₂O=Cu²⁺+2OH^-) (1)

Cu²⁺+3OH^-+C₆H₁₂O₆=Cu⁺++2H₂O     (2)

Cu⁺+OH^-=CuOH                            (3)

2CuOH=Cu₂O+H₂O                          (4)

2.3 Measurement of quantities of OH^- absorbed on Cu₂O particles

Firstly, 4 g Cu₂O particles were put into 200 mL NaOH solution. Secondly, the suspension was stirred by ultrasonication for 30 min, and then the suspension was filtrated using 0.45 μm polyether sulfone membrane filter. Finally, the concentration of OH^- was measured in the filtrate by acid-base titration.

2.4 Characterization of Cu₂O particles

The XRD of the powder was carried out using a Rigaku D/Max 2550 X-ray diffractometer with Cu K_α radiation (λ=1.541 8 ?). The morphology and particle size of Cu₂O particles were investigated by SEM (JSEM-6360LV, Japan). TEM images were taken with a Hitachi Model-800 transmission electron microscope using an accelerating voltage of 200 kV.

3 Results and discussion

3.1 Characterization of Cu₂O particles

The XRD patterns of products with octahedron and spherical shapes are shown in Fig. 1. All the peaks can be indexed into the cubic symmetry of Cu₂O. No impurity peaks are observed, indicating that the products are phase-pure Cu₂O.

Cu₂O powders obtained from different experiments are characterized by SEM, as shown in Fig. 2. It can be deduced that spherical Cu₂O particles are synthesized when the concentration of NaOH is 2.50 mol/L. As the concentration of NaOH increases, quasi-spherical Cu₂O particles with angularity appear (Fig. 2(c)). When the NaOH concentration is 6.25 mol/L, the shape of Cu₂O particle is octahedron.

Fig. 1 XRD patterns of Cu₂O with different shapes:         (a) Octahedron; (b) Sphere

The TEM micrographs and the selected area electron diffraction (SAED) patterns of Cu₂O particles with different shapes are provided in Fig. 3. Figure 3(a) shows the TEM and SAED micrographs of the spherical Cu₂O particles when the NaOH concentration is     2.50 mol/L. The TEM images clearly indicate that the spherical particles are all polycrystals, as indicated by the ring-like SAED pattern taken on these spherical particles. Figure 3(b) shows the TEM and SAED micrographs of the octahedral Cu₂O particles when the NaOH concentration is 6.25 mol/L. The TEM image unambiguously reveals that it is a perfect Cu₂O single crystal, and the SAED pattern can be identified as the á111? zone axis projection of the Cu₂O reciprocal lattice. Therefore, both single crystal particles and polycrystalline powders can be prepared selectively in similar procedures by simply changing the concentration of NaOH.

3.2 Discussion

Two completely different mechanisms are usually used to explain the formation of single crystals and polycrystals [20]. A diffusion mechanism is generally accepted to explain the formation of monodispersed single crystals. According to this mechanism, the crystal nuclei will make a facet oriented growth via the constituent complexes diffusion to these primary particles, and the morphology of monocrystal is determined by the relatively growth rate of different crystal faces [12, 20-21]. For Cu₂O monocrystal, when (111) plane grows relative fast, the shape of Cu₂O crystal tends to be   cube. When (100) plane grows relatively fast, it tends  to be octahedron. On the other hand, an aggregation mechanism is also employed to explain the formation of monodispersed polycrystals [10, 20]. This mechanism considers that the aggregations of uniform single crystalline subunits lead to nuclei growing into the final larger polycrystalline assemblies.

Fig. 2 SEM images of Cu₂O particles prepared in presence of different concentrations of NaOH: (a) 2.50 mol/L; (b) 3.75 mol/L;    (c) 5.00 mol/L; (d) 6.25 mol/L

Fig. 3 TEM and SAED images of Cu₂O particles with different shapes: (a) sphere; (b) octahedron

It could be concluded that spherical Cu₂O polycrystals were obtained in this work because the growth mode of Cu₂O nuclei was aggregation, and octahedral Cu₂O single crystals were owing to the growth mode of (100) facet oriented growth. In this work, different shapes of Cu₂O crystal particles were obtained via changing the concentration of NaOH. The result indicates that the competition exists between aggregation growth and (100) facet oriented growth of crystal nucleus when the concentration of NaOH changes. It could be attributed to having a large number of OH^- on Cu₂O particles surface in alkaline systems.

In order to study the effect of OH^- on growth mode of Cu₂O crystal nucleus, the pH values were registered timely during glucose reduction of Cu(II), as shown in Fig. 4. And the absorption quantities of OH^- ions on Cu₂O particles at different pH values (NaOH solution with different concentrations) are listed in Table 2.

Fig. 4 Change of pH values during reaction in presence of different NaOH concentrations

Table 2 Absorption quantities of OH^- on Cu₂O in different alkaline systems

In this work, every reaction finished after 20 min. It can be seen in Fig. 4 that when NaOH concentration is 2.50, 3.75, 5.00 and 6.25 mol/L, the pH values change from 12.35 to 11.80, 12.65 to 12.18, 13.2 to 12.8 and 13.2 to 12.9 in reaction process, respectively. Table 2 shows that there is a strong adsorption between Cu₂O particles and OH^-, and the adsorption increases with the increase of NaOH concentration (pH value). It could be concluded that when NaOH concentrations are 2.50 mol/L and 3.75 mol/L, the absorbed quantity of OH^- ions on Cu₂O particles in the reaction process is less than   0.245 mmol/g, but when NaOH concentrations are   5.00 mol/L and 6.25 mol/L, it is more. Large number of OH^- adsorbed on surface of Cu₂O particles will arouse strong repulsion among them. The repulsion will become stronger while more OH^- ions are adsorbed. So, in this work, different shapes of Cu₂O crystal particles would be due to the following effects:

1) When the concentration level of NaOH is low, there are less OH^- ions adsorbed on surface of Cu₂O particles. Thus, repulsion between single-nuclei, molecule clusters, and primary particles is low, aggregation is the leading growth mode, and the crystal nucleus grow into spherical particles finally.

2) When the concentration level of NaOH is high, there are more OH^- ions adsorbed on the surface of Cu₂O particles. This brings about stronger repulsion among primary particles, which restrains the aggregation growth. Thus, diffusion mechanism is the growth mode of Cu₂O crystal nuclei. As to this work, it is accepted that the OH^- would be selectively adsorbed on (111) facet of Cu₂O crystals, then slow the growth rates of (111) facet [12]. The increase of NaOH concentration will augment the adsorption of OH^- on Cu₂O particle, in especial, on (111) crystal facet. So, the nuclei tend to preferentially grow along (100) directions and form octahedrons bounded by (111) facets.

4 Conclusions

1) When NaOH concentration is 6.25 mol/L, there are more OH^- ions adsorbed on surface of Cu₂O  particles, and the repulsion among primary particles restrains aggregation growth mode and high density of OH^- on (111) facet restrains the growth of (111) facet. As a result, relatively faster growing of (100) facet is eliminated, with morphology of Cu₂O forming into octahedron finally.

2) When NaOH concentrations are 2.50 mol/L and 3.75 mol/L, the repulsion is lower, aggregation is the leading growth mode, and Cu₂O nuclei grow into sphere finally.

3) When NaOH concentration is 5.00 mol/L, there is a balanced competition between aggregation growth and (100) facet growth. As a result, Cu₂O nuclei grow into quasi-spherical particles.

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(Edited by HE Yun-bin)

Foundation item: Project(50674100) supported by the National Nature Science Foundation of China

Received date: 2011-05-27; Accepted date: 2012-03-22

Corresponding author: ZHOU Kang-gen, Professor, PhD; Tel: +86-731-88836442; E-mail: zhoukg63@yahoo.com.cn