Surfactant-free synthesis of Cu2O hollow spheres and their wavelength-dependent visible photocatalytic activities using LED lamps as cold light sources

A facile synthesis route of cuprous oxide (Cu2O) hollow spheres under different temperatures without the aid of a surfactant was introduced. Morphology and structure varied as functions of reaction temperature and duration. A bubble template-mediated formation mechanism was proposed, which explained the reason of morphology changing with reaction temperature. The obtained Cu2O hollow spheres were active photocatalyst for the degradation of methyl orange under visible light. A self-designed equipment of light emitting diode (LED) cold light sources with the wavelength of 450, 550, and 700 nm, respectively, was used for the first time in the photocatalysis experiment with no extra heat introduced. The most suitable wavelength for Cu2O to photocatalytic degradation is 550 nm, because the light energy (2.25 eV) is closest to the band gap of Cu2O (2.17 eV). These surfactant-free synthesized Cu2O hollow spheres would be highly attractive for practical applications in water pollutant removal and environmental remediation.


Background
Recently, semiconductor nanomaterials with different morphologies have attracted lots of interests because structure significantly influences their physical and chemical properties. Various morphologies, such as nanowires [1], nanocubes [2], nanocages [3], and octahedrons [4], have been synthesized for their interesting properties and applications. Among these nanostructures, hollow nanostructures are of particular interest because of their unique electrical, magnetic, thermal, and optical properties [5][6][7][8][9][10][11][12][13]. Hollow nanomaterials are widely used as nanoscale chemical reactors [14], high-performance catalysts [14][15][16], drug-delivery carriers [17,18], lithium-ion battery materials [19], and wavelength optical components for biomedical applications [20]. According to the reports related to the preparation of hollow materials, various methods have been developed which can be categorized into the following classes: template-mediated approaches [21], chemical etching [22], galvanic replacement [23], and the Kirkendall voiding [6]. Among the above methods mentioned, template-mediated approaches are the most usual and popular ones, which are based on selectively removing the cores in spherical core-shell particles by a solvent or calcination method.
Meanwhile, Cu 2 O photocatalyst can convert solar into chemical energy to degrade pollutants and can be used as a promising catalyst for environmental wastewater treatment in practical application [24]. Xe lamps and high-pressure mercury lamps with the power of 150 and 400 W, respectively, are usually used as light sources in photocatalytic experiment. They will introduce large amount of heat into the catalytic system, which makes it difficult to control the reaction temperature.
Herein, we investigate Cu 2 O hollow spheres via a facile aqueous solution method under different temperatures without the addition of a surfactant. In our research, hollow spheres with uniform diameter can be obtained through this surfactant-free method. Morphologies of Cu 2 O hollow spheres prepared under different temperatures are displayed and so does the supposed formation mechanism. In addition, photocatalytic activities of Cu 2 O hollow spheres are measured for the first time with a selfdesigned equipment using light emitting diode (LED) cold lamps with different characteristic wavelengths as photocatalysis light source. LED lamps with the power of 8 W, as typical cold light sources, are different from the highpower Xe lamps and mercury vapor lamps. There is no extra heat introduced into the catalytic system using LED cold light and the wavelength can be easily controlled. This one-pot method proceeds in aqueous medium with low temperatures and high reaction rates, which makes the as-produced Cu 2 O hollow spheres highly attractive for practical applications in water pollutant removal and environmental remediation.

Preparation of Cu 2 O hollow spheres
In a typical synthesis, 0.25 g of CuSO 4 · 5H 2 O were dissolved in 50 mL of deionized water with continuous stirring. Then, the transparent solution was kept in a 100-mL flask under different temperatures. We used N 2 H 4 · H 2 O (20%) to reduce Cu 2+ by fast injection of 1 mL N 2 H 4 · H 2 O into the solution and stirring at 750 rpm for 1 h. The color of the solution turned from dark blue to brick red with no extra alkali added. After that, the product was centrifuged at 3,250 × g for 10 min, washed with deionized water for several times, and finally dried in a vacuum at 60°C for 8 h.

Photocatalytic activities
Photocatalytic degradation of methyl orange (MO) was carried out in a self-designed equipment. Twenty milligrams of as-prepared Cu 2 O hollow spheres and 50 mL MO solution (10 mg/L) were kept in a 100-mL round-bottom flask with continuous stirring. Four 8-W LED lamps with the same characteristic wavelengths (450, 550, or 700 nm) were used for the first time as cold light sources which were mounted at 10 cm around the solution. Vigorous stirring was employed to ensure the adsorption equilibrium and eliminate any diffusion effect. The MO solution was kept in darkness for 15 min to get adsorption equilibrium and then under visible light.

Morphology and structure
Uniform Cu 2 O hollow spheres with rough surface were obtained by the simple one-step wet synthesis method. Figure 1 shows SEM images and diameter distributions of Cu 2 O spheres prepared under different temperatures. It can be clearly observed that the sizes and structures of Cu 2 O spheres changed under different conditions. When the reaction takes place in ice water bath keeping at 0°C, the obtained Cu 2 O spheres are well distributed in size with a diameter of 763 ± 83 nm (Figure 1a,d). Few spherical particles are broken into pieces (inset of Figure 1a), so we can clearly find the hollow structure of the big sphere. The big sphere is made up of small particles, leaving nanoscale holes on the surface. At 25°C, the spheres are bigger in size with a diameter of 1,521 ± 73 nm (Figure 1b,e). In addition, the hollow structure can be easily observed from the broken sphere (inset of Figure 1b). However, when the reaction temperature increase to 50°C, the sphere morphology change a lot, which could hardly be called hollow sphere. Cu 2 O spheres prepared at 50°C have rough surfaces with a diameter of 417 ± 51 nm (Figure 1c,f ). The reaction time should be strictly controlled within 1 h to prepare Cu 2 O spheres. Figure 2 shows the TEM, high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) images of Cu 2 O spheres prepared under different temperatures. Figure 2a shows the morphology of Cu 2 O spheres prepared at 0°C in which a hollow structure can be distinctly observed. The obtained Cu 2 O spheres are uniform in size with a wall thickness of 130 nm. The hollow structure shown in Figure 2b can also be observed when the reaction temperature increases to 25°C. Figure 2c shows the morphology of Cu 2 O spheres prepared at 50°C, in which nanoparticles aggregate together to form small sphere-like shape. A fringe spacing of 0.25 nm shown in the HRTEM images (Figure 2d,e,f) corresponds well to that of the lattice space of {111} of Cu 2 O crystals.
The composition and phase purity of the products prepared at different reaction temperatures were characterized by XRD as shown in   diffraction rings fit well with crystal planes of Cu 2 O. No other peak is observed in the XRD patterns, indicating that the products are phase-pure Cu 2 O crystals. There is no impurity such as cupric oxide or copper.

Formation mechanism
Hydrazine hydrate (N 2 H 4 · H 2 O) is used as the reductant to prepare Cu 2 O hollow spheres. As N 2 H 4 · H 2 O is an alkali reductant with strong reducing ability, after injecting N 2 H 4 · H 2 O, the solution turns into dark blue within several seconds and then changes to brick red gradually, which means that Cu(OH) 2 is generated and finally reduced to Cu 2 O. The formation of Cu 2 O hollow spheres in the reaction system can be represented by the following chemical reactions: The process of morphology changing under different temperatures can be explained as the following steps in Figure 4.
At a certain temperature, for example, at 25°C, after the addition of N 2 H 4 · H 2 O into CuSO 4 solution, Cu 2+ is reduced to Cu 2 O nanoparticles, and N 2 nanobubbles are generated at the same time. As there is no surfactant in the reaction system, Cu 2 O nanoparticles will tend to absorb on the surface of N 2 bubbles, so that Cu 2 O nanoparticles assemble into hollow spheres (Figure 1a), which can be referred to the Ostwald ripening process [47]. When the reaction takes place at 0°C, the reaction rate will slow down, resulting in smaller N 2 bubbles and spheres with smoother surface and tighter structure, which also agrees with the SEM results and diameter distribution ( Figure 1). The reaction rate increases along with the temperature rises. At 50°C, the reaction speed is too high for the nanoparticles to form uniform spheres. In Figure 1c, the hollow sphere structure could hardly be observed. N 2 nanobubbles escape faster so that the obtained Cu 2 O spheres are smaller.
On the other hand, Cu 2 O spheres are made up of nanoparticles. Crystallization rate increases with the rise of temperature to form bigger nanoparticles, so that the obtained Cu 2 O spheres would have rougher surface, which is also in agreement with the SEM results ( Figure 1).
Meanwhile, the morphology of Cu 2 O hollow spheres changes during the reaction time. The SEM images obtained at 0°C during different formation times are displayed in Figure 5a,b,c,d. Hollow structures have been formed at 30 min but with inconsistent diameters. With the reaction time increase to 6 h, Cu 2 O is oxidized into Cu 4 (OH) 6 SO 4 by oxygen dissolved in the solution, which can be confirmed by TEM and XRD results in Figure 5e,f.

Photocatalytic activities
A self-designed equipment was applied to carry out the photocatalytic degradation experiment as shown in Figure 6. Four 8-W LED lamps were used as cold light  To test the photocatalytic activities of obtained Cu 2 O hollow spheres, MO, a negatively charged molecule, was used in the photodegradation experiments. MO solution was kept in darkness for 15 min to get adsorption equilibrium. The adsorption curve in darkness is shown in Figure 7. It is found that adsorption have reached equilibrium after 15 min. Figure 8a,b,c,d,e,f shows the photocatalytic performance of Cu 2 O as photocatalysts for the degradation of MO. The experimental results disclose that Cu 2 O hollow spheres allow superior photocatalytic activity. Meanwhile, the time for concentration of MO solution to reach 1/e is summarized in Table 1, so we can have a clear look at the degradation of MO.
The results can be explained in two aspects, different wavelengths of visible lights and photocatalysts prepared     The structure with larger BET surface area could facilitate effective contacts between Cu 2 O spheres and organic contaminants, enhancing light harvesting and ultimately improving the photocatalytic activities. However, it shows almost the same effect under 700-nm wavelength among the three kinds of Cu 2 O spheres (Figure 8f). Maybe under 700-nm wavelength LED lamps, the structure of spheres is not the dominant factor of the photocatalytic activities.
An illustration of inter-particle electron transfer behavior is proposed as shown in Figure 8g. The uniform distributions of Cu 2 O hollow spheres have large active surface area, which enhances the effective adsorption of photons and provides a continuous pathway for the transportation of photoinduced electrons. The electrons in the valence band of Cu 2 O are excited to its conducting band, giving rise to the formation of electron and hole pairs. The obtained electrons and holes with high energy can combine with H 2 O and reduce MO into CO 2 and H 2 O.

Conclusions
We demonstrate a facile method to prepare Cu 2 O hollow spheres. Under the preparation at 0°C, 25°C, and 50°C, the obtained Cu 2 O hollow spheres have diameters of 763 ± 83, 1,521 ± 73, and 417 ± 51 nm, respectively. The corresponding surface area is 45.985, 31.961, and 20.944 m 2 /g, respectively. Cu 2 O hollow spheres are obtained by nanoparticles absorbing on the surface of N 2 bubbles and assemble together. A bubble template process is introduced to explain the formation mechanism. Importantly, Cu 2 O hollow spheres exhibit better photocatalytic activities for MO degradation under visible light. This is because the developed BET surface areas lead to more contact points, thus forming much more active sites between MO and the catalyst. So, Cu 2 O hollow spheres prepared at 0°C are the most effective for the degradation of MO. At the same time, 550 nm is the most suitable wavelength for Cu 2 O to photocatalytically degrade MO, because the light energy (2.25 eV) is closest to the band gap of Cu 2 O (2.17 eV).
The work not only provides insights into the Cu 2 O catalysis but is also useful for better catalyst design and water treatment industry. The LED lamps as cold light sources with no extra heat introduced into the reaction system are promoted in this work. In summary, we provide an efficient synthetic strategy for the fabrication of effective Cu 2 O visible photocatalyst in environmental treatment, and the self-designed catalytic equipment with single-wavelength LED cold light sources exhibits a novel model for the catalytic design.