Cu2O particles re-precipitated at the bottom of the vessels. Through these processes we fabricated pure Cu2O particles without any precursors.
2.2.3. Materials Characterizations
A. High Resolution X-ray Diffractometer (XRD)
To determinate their crystal structures, the obtained Cu2O particles were characterized by X-ray diffraction (XRD) using a Bedi D1 diffractometer with Cu Kα radiation in a Bragg-Brentano geometry.
B. Scanning Electron Microscopy (SEM)
The SEM images for the Cu2O particles were taken by a Hitachi JSM 6700F. The samples were prepared by spreading the powders onto a carbon substrate on the sample holder followed by a conductivity improvement step by Pt deposition.
2.3. Results and Discussion
In this section, we discuss the influence of the experimental parameters such as copper(II) ion sources, surfactant concentration, base concentration, and synthetic method on the growth of Cu2O particles.
2.3.1. Characterization on the synthesized Cu2O particles
After altering the relevant experimental parameters we obtained the Cu2O powders with four different colors. The XRD was used to identify their respective phases and the results indicated pure Cu2O phases were present for all samples. The most intense XRD peak for the Cu2O powder was the (111) peak followed by the (200), (220), and (311).
The (111) peak was located at 2 theta of 36.4°, the (200) peak was positioned at 2 theta of 42.4°, the (220) peak was located at 2 theta of 61.4°, and (311) peak was located at 2 theta of 73.6°. The (110) peak, which was almost buried in the noises, was located at 2 theta of 29.6°. Figure 2.3 exhibits the XRD data.
We determine the colors of the Cu2O particles corresponding to various sizes and shapes by SEM images. The Cu2O particles with diameters larger than 1 µm exhibit red color. The Cu2O cubes with diameters from 300 to 1000 nm appeared in orange color. The Cu2O cubes with diameters from 100 to 300 nm revealed yellow color. The disordered Cu2O particles with diameter less than 300 nm were brown color.
2.3.2. The influence of surfactant concentration on the Cu2O growth
Surfactants are typically organic compounds containing hydrophobic and hydrophilic groups to reduce the interfacial tension between water and oil.
As a general rule, one expects that as the concentration of the surfactant (capping agent) increases, the resulting particle size would be decreased. It is because that the sites for further nucleation and growth are blocked by the
capping agent.
We adjusted the concentration of PEG from 0.002 to 2 M in order to explore the relationship between the particle sizes and surfactant concentrations.
The SEM images (figure 2.4) confirmed the results that the morphologies of the Cu2O particles revealed negligible change when we used CuCl2 as the copper(II) ion source. In contrast, when we used CuAc2 or Cu(NO3)2 as the copper(II) ion source, 2 M PEG led to the smallest Cu2O particles and the other concentrations of PEG produced Cu2O particles with identical sizes.
2.3.3. The influence of base concentration on the Cu2O growth
To attain the relation between base concentration and Cu2O particle sizes we controlled the concentration of the NaOH solution from 0.002 to 2 M.
Decreasing the concentration of base from 2 to 0.02 M we obtained the Cu2O particles with diameters varying from 1570 to 170 nm. However, when we reduced the concentration of NaOH further from 0.02 to 0.002 M, the Cu2O particles changed their shapes from uniform cubes to irregular spheres.
In thermodynamics, the Pourbaix diagram (figure 2.5) provides the equilibrium phase of a material at various pH and potentials. Through Pourbaix diagram we knew the states of copper ions at equilibrium and possibly their effects on subsequent Cu2O growth. There were other ions such as Cl-, SO42-, NO3-, and CH3COO- in the solution which may affect the states of copper ions at equilibrium, but the concentration for those ions were too small to influence the equilibrium states.
With addition of 2 M NaOH into the solution including copper(II) and PEG, the pH value of the solution was increased over 13 and the solution
became blue, which meant copper(II) existed as CuO22-. The CuO22- in the solution was likely to be surrounded by Na+. Thus the copper ions used to grow the Cu2O were released slowly and the Cu2O particles were found to grow gradually. From the SEM pictures at pH of 13 the Cu2O particles was approximately 1500 nm.
When 0.2 M PEG was added into the solution including copper(II) and PEG, the pH of the solution was increased to 12 to produce Cu(OH)2, which exhibited blue and small particles suspended in the solution (However, it is to be noted that the observation were inconsistent with the Pourbaix diagram.) The suspended blue particles disappeared instantly when the LAAS was added and the solution changed its color from light blue to orange. The SEM pictures presented that the particle size of Cu2O particles growth at pH of 12 is about 700 nm.
Adding 0.02 M NaOH into the solution including copper(II) and PEG, the pH value was 11 and the copper(II) in the solution also became Cu(OH)2. When the LAAS was added into the light blue solution containing suspended Cu(OH)2, the small Cu2O particles appeared with yellow color, changing the solution color from blue to yellow. The particle size of Cu2O growth at pH of 11 was smaller than that of pH of 12. It may result from the particle size of the Cu(OH)2. We found out that the Cu(OH)2 particles formed in the pH of 12 solution precipitated faster than the particles formed in the pH of 11 solution (shown in figure 2.6). We believe that the particle size of Cu2O at different pH changed significantly since the copper(II) exists in various forms at different pH values.
2.3.4. The influence of synthetic method on the Cu2O growth
We used two methods (method A and B) in preparing the Cu2O particles to study the relationship between the synthetic method and morphologies of Cu2O particles. In procedure A, the NaOH and the LAAS were added into the solution containing copper(II) sources and PEG. In this way we prepared the Cu2O particles with base concentrations from 0.002 to 2 M. On the other hand, when procedure B (NaOH and LAAS were mixed and injected into the solution containing copper(II) sources and PEG) was used to prepare the Cu2O particles, the base concentration was higher than 0.2 M. If the base concentration was lower than 0.2 M, the Cu2O particles could not be formed ever after aging for 6 hours.
Through SEM pictures we determined that procedure B was able to produce smaller Cu2O particles than procedure A under identical conditions.
We surmise that differences of copper(II) sources in the solutions were responsible. The copper ion sources for procedure A were CuO22- and Cu(OH)2 when the base concentrations were 2 and 0.2 M. In contrast, the copper ion sources for procedure B were copper(II) when the base concentrations were 2 and 0.2 M.
2.4. Conclusions
After careful analysis of our data, we come to several conclusions. First, changing the concentration of the surfactant (PEG) plays negligible influence over the particle sizes of the Cu2O. Second, the adding sequence of NaOH and LAAS influences the morphologies of the resulting Cu2O particles. When
the NaOH was added to the solution containing copper(II) before LAAS, the obtained Cu2O particles were bigger than the particles obtained from adding NaOH and LAAS simultaneously. Third, adjusting the pH value of the solution leads to distinct particle sizes, a fact from the differences of copper ion sources. Lastly, we have succeeded in synthesizing the Cu2O cubes with tunable edge length from 1570 to 170 nm through varying the NaOH concentrations of the solutions from 2 to 0.02 M.
Figure 2.1. Illustration of procedure A (above) and B (below), used to grow Cu2O particles.
Figure 2.2. Illustration of synthetic process of Cu2O particles.
red
orange
Figure 2.3. Four different sizes of Cu2O particles with four different colors.
yellow
brown
Method A and aging for 6 hours CuCl2 (0.005 M), PEG (200) (2 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 647 nm
Method A and aging for 6 hours CuCl2 (0.005 M), PEG (200) (0.2 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 642 nm
Method A and aging for 6 hours
CuCl2 (0.005 M), PEG (200) (0.02 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 661 nm
Method A and aging for 6 hours
CuCl2 (0.005 M), PEG (200) (0.002 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 655 nm
Method A and aging for 6 hours CuAc2 (0.005 M), PEG (200) (2 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 430 nm
Method A and aging for 6 hours
CuAc2 (0.005 M), PEG (200) (0.2 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 522 nm
Method A and aging for 6 hours
CuAc2 (0.005 M), PEG (200) (0.02 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 605 nm
Method A and aging for 6 hours
CuAc2 (0.005 M), PEG (200) (0.002 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 598 nm
Method A and aging for 6 hours
Cu(NO3)2 (0.005 M), PEG (200) (2 M), NaOH (0.2 M), and LAAS (0.05 M) Particle size: 927 nm
Method A and aging for 6 hours Cu(NO3)2 (0.005 M), PEG (200) (0.2
M), NaOH (0.2 M), and LAAS (0.05 M)
Particle size: 1109 nmMethod A and aging for 6 hours
Cu(NO3)2 (0.005 M), PEG (200) (0.02
M), NaOH (0.2 M), and LAAS (0.05 M)
Particle size: 1092 nmMethod A and aging for 6 hours
Cu(NO3)2 (0.005 M), PEG (200) (0.002
M), NaOH (0.2 M), and LAAS (0.05 M)
Particle size: 1121 nmMethod A and aging for 6 hours CuCl2 (0.005 M), PEG (200) (2 M), NaOH (0.2 M), and LAAS (0.05 M)
Method A and aging for 6 hours CuCl2 (0.005 M), PEG (200) (0.2 M), NaOH (0.2 M), and LAAS (0.05 M)
Method A and aging for 6 hours
CuCl2 (0.005 M), PEG (200) (0.02 M), NaOH (0.2 M), and LAAS (0.05 M)
Method A and aging for 6 hours
CuCl2 (0.005 M), PEG (200) (0.002 M), NaOH (0.2 M), and LAAS (0.05 M)
Figure 2.4. SEM images of Cu2O particles synthesized through different concentrations of PEG (200).
Figure 2.5. Part of the Pourbaix diagram for the Cu in different potentials and pH.
Figure 2.6. The color of the solution after adding 10 mL of various concentrations of NaOH into the solution containing copper(II) and PEG.
Method A and aging for 6 hours
Cu(NO3)2 (0.005 M), PEG (200) (0.2 M),
NaOH (2 M), and LAAS (0.05 M)
Particle size: 2513 nm
Method A and aging for 6 hours
Cu(NO3)2 (0.005 M), PEG (200) (0.2 M),
NaOH (0.2 M), and LAAS (0.05 M)
Particle size: 1109 nmMethod A and aging for 6 hours
Cu(NO3)2 (0.005 M), PEG (200) (0.2 M),
NaOH (0.02 M), and LAAS (0.05 M)
Particle size: 167 nmMethod A and aging for 6 hours
Cu(NO3)2 (0.005 M), PEG (200) (0.2 M),
NaOH (0.002 M), and LAAS (0.05 M)
Method A and aging for 6 hours
CuSO4 (0.005 M), PEG (200) (0.2 M),
NaOH (2 M), and LAAS (0.05 M)
Particle size: 1273 nmMethod A and aging for 6 hours
CuSO4 (0.005 M), PEG (200) (0.2 M),
NaOH (0.2 M), and LAAS (0.05 M)
Particle size: 582 nmMethod A and aging for 6 hours
CuSO4 (0.005 M), PEG (200) (0.2 M),
NaOH (0.02 M), and LAAS (0.05 M)
Particle size: 188 nmMethod A and aging for 6 hours
CuSO4 (0.005 M), PEG (200) (0.2 M),
NaOH (0.002 M), and LAAS (0.05 M)
Figure 2.7. SEM images of the Cu2O particles synthesized at different pH values of the solution.