Semiconductor transition-metal oxides have been of much interest because of their unique properties and widely application. In particular, as a p-type semiconductor with a band gap of 2.17 ev, cuprous oxide (Cu₂O) is a promising material with potential applications in solar energy conversion, catalysis, and sensing [1] [2] [3], biosensor and magnetic storage devices [4] [5] [6] [7], and photocatalyst for degradation of organic pollutants and decomposition of water into O₂ and H₂ under visible light [8] [9].

Many efforts have been devoted to the synthesis of Cu₂O micro- and nanocrystals with various shapes [10] - [15] by different methods. Yongming Sui and his co-workers report a facile solution-phase route for the mass synthesis of Cu₂O crystals with different morphologies in the presence of poly (vinyl pyrrolidone) (PVP) [16]. Zhao et al. have prepared Cu₂O of various shapes by the reduction of copper nitrate with formic acid in hydrothermal condition [17]. Xu et al. have prepared a wide range of novel cuprous oxide microcrystals through an ethylene-diaminetetraacetic acid tetrasodium salt dihydrate (EDTA) reduction route by employing the EDTA molecule as both chelating reagent and reductant [18]. Wang et al. prepared Cu₂O cubes by reduction of copper sulphate with D-glucose in assist of sodium citrate and anhydrous sodium carbonate [19].

The methods mentioned in the literature require high temperature, special conditions, or tedious procedures. In this paper, we report a facile solution-phase method to synthesize uniform Tetragonal Cu₂O microcrystals with controlled monodispersity by PH value at low temperature. In particular, the synthesis does not require the assistance of a surfactant.

1) Materials

Cupric sulphate (90%) was obtained from Bodi chemical plant corporation, Tianjin. Sodium hydroxide was bought from Damao Chemical Reagent, Tianjin. D-glucose was received from Tianda Chemical Reagent, Tianjin. The above all reagents were analytically grade commercial materials. The powder was taken for characterization.

2) Preparations of Cu₂

In a typical procedure, an aqueous solution prepared by mixing 100 mL deionized water with 5 g copper sulfate, and stirred the mixture with a magnetic blender for about 20 min under room temperature. In the same way, 50 ml sodium hydroxide aqueous solution in certain concentration can be got. A dark blue precipitate was produced when the all above solutions were mixed in a four neck round-bottomed glass flask. The mixed solution was kept in a water bath at 80˚C. Then 50 mL of (3.6 g) glucose solution was slowly dropped into it with constant stirring for 30 min. Next, the dark blue precipitate gradually turned dark red, and then was allowed to cool to room temperature naturally. Afterward, the obtained particles were cleaned by deionized water, and dried at 60˚C for 20 h in a vacuum oven. Finally, the powder was taken for characterization.

3) Characterization:

The crystal phase of as-prepared products was characterized by an X-ray diffractometer (XRD) using Cu Kαradiation (λ = 1.54060 Å) in the range (20˚ - 80˚).

The morphology of the powders was investigated by field-emission scanning eletron microscopy (SEM) using S1500.

The infrared emissivity of the powders was tested by the Far infrared emissivity measurer S302, the test temperature is 34˚C.

The composition and purity of the products were first examined by XRD, and the results reveal that pure Cu₂O is obtained in all samples. Figure 1(a) displays representative XRD patterns of the Tetragonal (as show in Figure 1(b) SEM) as well as the standard card (JCPDS No. 65-3288), indicating that all the diffraction peaks are readily indexed to Tetragonal Cu₂O with no impurity, when the PH value of the solution is 12 or more. Figure 1(a) also indicates that the peak value of crystal plane (111) is relatively sharp and high. The strong and sharp peaks indicate that the (111) crystal surface grows optimally and the obtained Cu₂O crystals are highly crystalline. Figure 1(b) shows the particles size of products is uniform and have perfect monodispersity.

As showing in Figure 2 monodisperse of particles for various quality fractions of sodium hydroxide in the precursor solution. SEM observations indicate that, When the PH value is 9 and other experimental conditions are kept the same, Figure 2(a) shows obvious agglomeration in particles. When the PH value is 10 and 11, loose Cu₂O particles were obtained, as shown in Figure 2(b), Figure 2(c). Fully developed Cu₂O Tetragonal is observed when the sodium hydroxide concentration is increased to 0.6 M. The as-obtained Cu₂O crystals possess perfect monodispersity and Tetragonal morphology, as shown in Figure 2(d). So, increasing the reactant sodium hydroxide concentration enhances the reaction and increases the diffusion rate, or nucleation and growth rates. Hence, stable and dispersible particles are more easily formed. These results show that the initial solution PH value plays a key role in the formation of Cu₂O crystals.

The stabilities and monodispersities of Cu₂O are controlled by dispersants as a rule. the following growth mechanism of controlling dispersity with reactant sodium hydroxide can be proposed based upon our experimental results. When the appropriate contents of D-glucose, NaOH, and CuSO₄ are used at relatively high reaction temperatures, Cu₂O crystals can be synthesized from the following reactions.

Cu 2 + + 2OH − → Cu ( OH ) 2 ↓

Cu ( OH ) 2 + 2OH − → [ Cu ( OH ) 4 ] 2 −

2 [ Cu ( OH ) 4 ] 2 − + C 5 H 11 O 5 − CHO → Cu 2 O ↓ + C 5 H 11 O 5 − COOH + 4OH − + 2H 2 O

Sun et al. [20] have been demonstrated that Cu(II) can coordinate with excess OH⁻ ions to generate [Cu(OH)₄]²⁻ complexes. When the concentration of OH⁻ ions was higher enough, [Cu(OH)₄]²⁻ complexes would be formed. Therefore, it is proposed that the formation of Cu₂O with different dispersities may be related to the characteristics of the complex precursors synthesized in different reaction conditions (Equations (1)-(3)). The varied [Cu(OH)₄]²⁻ complexes formed in different conditions can modify the reduction process (Equation (3)), which might affect the competition between kinetics and thermodynamics during the reduction of precursors, nucleation, and growth of Cu₂O crystals. The similar rule has been reported [21], During the growth of Cu₂O crystal, the concentration of Cu²⁺ remains unchanged, the concentration of OH⁻ controls the PH value of the solution, and the concentration ratio of Cu²⁺ and OH⁻ affect the production rate of crystal orientation, that is to say, the concentration and activity of OH⁻ in the solution will affect the growth and crystal orientation of Cu₂O.

Schematic Illustration of the Process of Cu₂O Crystals as a Function of the PH value

The infrared emissivity test results of Tetragonal Cu₂O particles indicates that the infrared emissivity value is between 0.887 to 0.893, the average value is 0.89 as show in Figure 3, as the test temperature is 34˚C.

In summary, we have prepared uniform tetragonal Cu₂O in high yield by the reduction of cupric sulphate without surfactants. The concentration of source materials shows strong effects on the phase purity and morphology development of the products. The results indicate that the stable and dispersible Cu₂O particles could be prepared by adjusting the concentration of Sodium hydroxide concentration. When the PH value is 9, the obtained Cu₂O particles hold agglomeration. When the PH value is 10 and 11 leads to loose irregular Cu₂O. Fully developed monodisperse Cu₂O uniform tetragonal is observed when the sodium hydroxide concentration is increasing, the PH value is 12 or more. This method could be extended to prepare other similar inorganic oxides.

We thank Dr. Ling Zhang for helpful discussion. This work is supported by the Major special project of Shanxi Province Science and Technology (Grant No. 20181101020).

The authors declare no conflicts of interest regarding the publication of this paper.

Su, Q.L., Zhang, L., Liang, Y., Liu, Y., Liu, P. and Zhang, J.D. (2020) pH Controlled Synthesis of Tetragonal Cu₂O Particles. Journal of Materials Science and Chemical Engineering, 8, 46-52. https://doi.org/10.4236/msce.2020.88005