Facile synthesis of hollow Cu2O octahedral and spherical nanocrystals and their morphology-dependent photocatalytic properties
© Feng et al.; licensee Springer. 2012
Received: 9 April 2012
Accepted: 16 May 2012
Published: 30 May 2012
Herein, we report that octahedral and spherical Cu2O samples with hollow structures are synthesized in high yield by reducing Cu(EDA)22+ complex with hydrazine. A series of experiments are carried out to investigate the factors which impact on the morphology of the Cu2O samples. It is observed that ethylenediamine (EDA) serves as a molecular template in the formation of hollow structure. Octahedral Cu2O with solid structure is prepared without EDA. When EDA is added, Cu2O sample with hollow structure is formed. Different morphologies of Cu2O such as spherical and octahedral could be obtained by adjusting the concentration of EDA and NaOH. The temporal crystal growth mechanism is proposed. Furthermore, the photocatalytic activities of the as-prepared Cu2O nanoparticles are evaluated by monitoring two dyes (methyl orange and congo red) using UV-visible spectrophotometer. Results show that the order of photocatalytic activity of Cu2O with different morphologies is as follows: hollow octahedral morphology > hollow sphere morphology > solid octahedral morphology. The hollow octahedral Cu2O nanoparticles would be a promising material on applications for photocatalytic degradation of organic pollutants.
Hollow nanostructures have attracted considerable attention because of their unique physical and chemical properties that allow them to be widely used in catalysts, confined-space chemical reactors[1, 2], lithium-ion battery materials, controlled gene delivery[4, 5], and biomedical diagnosis and therapy[6, 7]. Up to date, the hollow structure of nanoparticles is highly desired and typically prepared by sacrificing templates, such as polystyrene, silica, or other inorganic crystals. For example, Yin et al. demonstrated the preparation of hollow CoO nanoparticles (NPs) by the oxidation of Co NPs. Since then, hollow nanocrystals of cobalt oxide and chalcogenides, MnO2, metal phosphide, etc. have been prepared through the Kirkendall effect using spherical Co, MnCO3, and metal particles as sacrificial templates, respectively[9–11]. Nevertheless, the conventional template approach has intrinsic disadvantages. For example, it is difficult to achieve high yield because of the complicated process. The shell structure may be destroyed in the template removal process if the structure is weak. The conventional template approach is time-consuming, expensive, and complicated in stringent control over a set of experimental variables. Thus, it remains a great challenge to develop feasible methods to prepare hollow nanocrystals with well-defined morphology.
Cuprous oxide (Cu2O) is a well-known p-type semiconductor and has a direct small bandgap of 2.2 eV, which endows it promising applications in solar energy conversion, as an electrode for lithium-ion batteries, gas sensors, and photocatalytic degradation of organic pollutants and decomposition of water into O2 and H2 under visible light[15–17]. So far, great efforts have been devoted to the synthesis of cuprous oxide with different shapes and sizes. Different morphologies such as nanocube, octahedral micro/nanocrystal, and hexapod-shaped microcrystal have been prepared[18–20]. Recently, some methods have been reported on the preparation of cuprous oxide with hollow structure[21–23]. However, these methods mainly focus on spherical morphology formation. Zeng and co-workers[21, 22] had prepared hollow Cu2O nanospheres and nanocubes through the Ostwald ripening effect using hydrothermal method. However, the preparation of hollow octahedral Cu2O nanocrystals usually requires hard templates. Limited reports are closely associated with template-free synthesis of hollow octahedral Cu2O nanocrystals. Wang and co-workers and other groups have found that the photocatalytic activity of the  surface is much higher than other surfaces, due to higher adsorption capacity on the  surface than others (e.g.,  surface). Cu2O with hollow structure and more  surface is extensively needed in photocatalysis. Thus, developing effective and facile methods for the synthesis of hollow Cu2O crystals especially with hollow octahedral morphology has become a key focus.
In this study, we mainly focused on a facile synthesis of hollow octahedral and spherical Cu2O nanocrystals by chemical reduction in which copper salt, sodium hydroxide, ethylenediamine (EDA), and the reducing agent hydrazine hydrate were involved. In this reaction, EDA served as a molecular template in the formation of hollow structure. The morphology of Cu2O nanoparticles can be easily tuned from spherical to octahedral with hollow structure by adjusting the concentration of EDA and NaOH. Moreover, the photocatalytic activities of the prepared Cu2O nanocrystals were investigated by methyl orange and congo red photodegradation.
Synthesis and characterization of Cu2O nanocrystals
All reagents purchased from the Shanghai Chemical Company (Shanghai, China) were of analytical grade and used without further purification. In each synthesis, 20 mL of NaOH (0.1 to 15 mol L−1) and varying amounts of EDA (0 to 500 μL; 99 wt.%) were added to a glass reactor (capacity 50 mL). Afterwards, 3 to 10 mL of Cu(NO3)2 (0.10 mol L−1) aqueous solution was added. Followed by a thorough mixing of all reagents, the reactor was then placed in a water bath with temperature controlled over 25°C to 100°C. Finally, hydrazine (50 μL; 35 wt.%) was added to reduce the Cu2+ ion to Cu2O. After 15 to 60 min, the cuprous oxide products with orange-red color were obtained. The resultant products were washed and harvested with centrifugation-redispersion cycles and dried at 60°C for 4 h in a vacuum oven. Further details on the synthesis can be found in supporting information 1 in Additional file1.
The crystallographic structure of the products were determined with X-Ray diffraction (XRD) (recorded on a Rigaku D/max-2200/PC (Rigaku Corporation, Tokyo, Japan); test conditions: Cu target at a scanning rate of 7°/min with 2θ ranging from 20° to 80°). The morphological investigations of scanning electron microscopy (SEM) images were taken on a field emission scanning electron microscope (FESEM, Zeiss Ultra; Carl Zeiss AG, Oberkochen, Germany). The transmission electron microscopy (TEM) images and electron diffraction patterns of the samples were captured on a JEOL/2100 F transmission electron microscope (JEOL Ltd., Akishima, Tokyo, Japan) at an accelerating voltage of 200 kV.
Photocatalytic properties of the prepared Cu2O nanocrystals
The evaluation of the morphology-related photocatalytic properties of these Cu2O samples was performed by constantly monitoring the photocatalytic decolorization of two dyes (methyl orange and congo red) in aqueous solution under visible light irradiation (ordinary household table lamp) by the changes in UV-visible (vis) absorption spectra. The typical procedure was as follows: 0.01 g of the prepared sample was dispersed into 15 mL of the corresponding dye aqueous solution (100 mg L−1 for methyl orange; 400 mg L−1 for congo red). Before illumination, the suspension was magnetically stirred in the dark for over 2 h to ensure adsorption equilibrium of the corresponding dye on the surface of the Cu2O crystal. Then, 300 μL of hydrogen peroxide was added to the solution, and the photocatalytic reaction was carried out with a 40-W daylight lamp (15 cm above the sample) used as a light source (ordinary household table lamp). The corresponding dye aqueous solution was then taken out in 20-, 40-, and 60-min intervals and centrifugated to exclude Cu2O. Finally, the corresponding dye aqueous solution was diluted fivefold and measured by a UV–vis spectrophotometer (Varian Cary 50, Varian Inc., Palo Alto, CA, USA).
Results and discussion
The morphology of the hollow spherical and octahedral Cu2O crystals
Tuning the morphology of the Cu2O crystals by varying the concentration of EDA
The exact growth mechanism of the hollow Cu2O nanocrystals is not very clear. We believe that EDA plays an important role as soft template for hollow structure formation. A classic rolling mechanism inspired by the natural phenomena of a piece of foliage or a piece of wet paper curling into scrolls during its drying process might be applicable for hollow Cu2O nanocrystal growth, which was used to explain the formation processes of metal chalcogenide nanotubes and nanorods[25, 26]. In that process, lamellar structured intermediates formed firstly. Then, in the reducing course, the lamellar intermediates subsequently rolled up from the edges because sufficient energy was provided to overcome the strain energy barrier. In this stage, nanotubes were usually formed. If the heating time was prolonged, the tubes broke down to afford rod bundles.
Other factors influenced the morphology of Cu2O crystals
The influence of NaOH concentration on morphological evolution was shown in supporting information 4 in Additional file1. When NaOH concentration was 0.1 mol L−1, the morphology was hollow octahedral. Accompanying with the increase of NaOH concentration, the morphology gradually converted to sphere. When NaOH concentration reached to 5 mol L−1, the Cu2O crystals were of sphere morphology. Therefore, the results showed that low NaOH concentration was favorable to the formation of hollow octahedral morphology and that high NaOH concentration was favorable to the formation of hollow sphere morphology. Thus, high temperature, low NaOH concentration, and low EDA usage are favorable to the formation of octahedral morphology.
Photocatalytic activity study
The photocatalytic activities of the Cu2O crystals with different morphologies and an TiO2 reagent (analytical grade, 100 nm, anatase phase) were evaluated by monitoring the decomposition of two dyes in aqueous solution (methyl orange and congo red) under visible light irradiation. Six samples with different morphologies were studied (shown in supporting information 5 in Additional file1). Our control experiment showed if only H2O2 was added in the system, no Cu2O, the decomposition of organic pollutants cannot be detected under visible light irradiation (supporting information 6 in Additional file1). To minimize the influence of adsorption, the photocatalytic experiment was carried out after stirring in the dark for 2 h.
After illumination (at 140 min, 160 min, 180 min), TiO2 did not show good catalytic activity under visible light irradiation (ordinary household table lamp). Samples d, e, and f with hollow morphology have the highest photocatalytic performance. The decomposition activity of sample a (solid octahedral) was the lowest. The order of photocatalytic activity of Cu2O with different morphologies was as follows: hollow octahedral morphology with rough surface (samples d, e, and f) > hollow octahedral morphology with smooth surface (sample c) > hollow sphere morphology (sample b) > solid octahedral morphology (sample a).
The photodecomposition results of congo red were shown in Figure7C and7D. The adsorption ability of the six samples was similar. The photocatalytic performance of the six samples in the experiment of decomposing congo red followed the similar trends in the experiment of decomposing methyl orange. TiO2 did not show good catalytic activity for decomposing congo red under visible light irradiation. Samples d, e, and f have the highest photocatalytic performance. The decomposition activity of samples a and b was the lowest. However, the photocatalytic performance of sample a in the experiment of decomposing congo red is better than that in the experiment of decomposing methyl orange. Morphology-related photocatalytic activities of Cu2O followed the similar trends in the experiment of decomposing methyl orange. The order is as follows: hollow octahedral morphology with rough surface > hollow octahedral morphology with smooth surface > hollow sphere morphology > solid octahedral morphology.
In summary, hollow octahedral Cu2O had the highest photocatalytic activity to degrade organic pollutants.
The mechanism of photocatalytic reaction
The first reaction usually caused photocorrosion of Cu2O, resulting in the loss of photocatalytic activity (reaction 1). In order to prevent the photocorrosion, hole consumption agents such as methanol were usually added to the reaction solution (reaction 4). In our experiment, the reactions were as follows: Firstly, the photogenerated electron and holes were formed by visible light excitation (Equation 1); Secondly, the electrons were scavenged by molecular oxygen O2 to yield · O2− (Equation 4). The · O2− reacted with H2O2 to produce · OH (Equation 7). Thirdly, the · OH was the key oxidizing agent to degrade most pollutants, so the degradation reaction occurred. The addition of H2O2 in our experiment was very important. It could accelerate the generation of ·OH. In our control experiments, if only H2O2 was added in the system, no Cu2O, the organic pollutants could not be degraded. If only Cu2O was added in the system, no H2O2, the organic pollutants could be degraded with a very slow speed. So, the addition of H2O2 was essential. TiO2 has a wide bandgap of 3.2 eV, so only the shorter wavelength solar energy can be utilized (λ < 387 nm). Although TiO2 had good performance in photocatalytic reaction under ultraviolet light irradiation[27–29], the photocatalytic property is weak under visible light irradiation. So, the TiO2 sample did not have photocatalytic activity in our experiment.
Total surface area (BET) result
Finally, different crystal surfaces have significant impact on the catalytic activity. It is reported that the photocatalytic activity of the  surface is much higher than other surfaces. Thus, octahedral nanoparticles (mainly coverd by  surfaces) have a higher photocatalytic activity than cubic-shaped nanoparticles and spherical nanoparticles (mainly coverd by  or  surfaces).
In summary, these results demonstrated that the photocatalytic activities of the microcrystals were related to their morphology. The highest photocatalytic activity of hollow octahedral Cu2O samples could ascribe to the larger surface area, high utilization efficiency of visible light, and many  surfaces.
In summary, octahedral and spherical Cu2O samples with hollow structures had been synthesized by a simple chemical reduction method. In this reaction, EDA played a key role in the formation of hollow structures. Different morphologies of Cu2O could be obtained by adjusting the concentration of EDA and NaOH. The photocatalytic activities of these as-prepared Cu2O microcrystals with different morphologies, such as solid octahedral, hollow sphere, and hollow octahedral, were investigated by photodegradation of two dyes (methyl orange and congo red). The results demonstrated that hollow octahedral Cu2O possessed the highest photocatalytic activity, which could ascribe to the larger surface area, high utilization efficiency of visible light, and many  surfaces. The hollow octahedral Cu2O would be a promising material on applications for photocatalytic degradation of organic pollutants.
This work is supported by the National Key Basic Research Program(973 Project) (2010CB933901), Important National Science & Technology Specific Project (2009ZX10004-311), National Natural Scientific Fund (No.20803040), Special Project For Nano-Technology From Shanghai (No.1052 nm04100), New Century Excellent Talent of Ministry of Education of China (NCET-08-0350), and Shanghai Science and Technology Fund (10XD1406100).
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