Preparation of hollow porous Cu2O microspheres and photocatalytic activity under visible light irradiation
© Yu et al.; licensee Springer. 2012
Received: 11 June 2012
Accepted: 18 June 2012
Published: 27 June 2012
Cu2O p-type semiconductor hollow porous microspheres have been prepared by using a simple soft-template method at room temperature. The morphology of as-synthesized samples is hollow spherical structures with the diameter ranging from 200 to 500 nm, and the surfaces of the spheres are rough, porous and with lots of channels and folds. The photocatalytic activity of degradation of methyl orange (MO) under visible light irradiation was investigated by UV-visible spectroscopy. The results show that the hollow porous Cu2O particles were uniform in diameters and have an excellent ability in visible light-induced degradation of MO. Meanwhile, the growth mechanism of the prepared Cu2O was also analyzed. We find that sodium dodecyl sulfate acted the role of soft templates in the synthesis process. The hollow porous structure was not only sensitive to the soft template but also to the amount of reagents.
Much attention has been focused on fabricating high-efficiency photocatalytic materials, which is one of the most potential routes to mitigating environmental pollution . Among them, metal oxide semiconductors, such as ZnO and TiO2, have attracted much attention owing to their high efficiency in the degradation of wide-ranged pollutants in which electron–hole pairs are generated under irradiation and degrade the pollutants absorbed on the surface of the photocatalytic materials [2–5]. Most of the metal oxide semiconductors have large band gaps, for example, 3.2 eV for ZnO  and 3.0 eV for TiO2, which are in the range of an UV spectrum. For this kind of materials, it is hard to generate electron–hole pairs under visible light irradiation because of the low photo energy, which therefore leads to lower photocatalytic efficiency and limits their large-scale applications. Therefore, in recent years, great efforts have been devoted to develop new photocatalytic materials with high efficiency under visible light irradiation.
As a typical p-type semiconducting material, cuprous oxide (Cu2O) possesses a stable, direct band gap of about 2.17 eV and a higher hall mobility up to 60 cm2/V·s , which has wide-scale applications in gas sensors, solar cells and lithium-ion batteries [7–9] owing to its unique optical and magnetic properties [9–15]. In 1998, Hara et al.  found that Cu2O can split water under visible light due to its low band gap. From then on, as a candidate of photocatalytic materials, Cu2O has attracted much research interests due to its important applications in degrading industrial dyeing wastewater, nitrogen-containing pesticides, etc. under visible light energy. Cu2O particles with different shapes, such as cube, octahedral, multipod, nanowire, hollow structure and porous spheres, have been synthesized [17–22]. It was found that the properties varied with the specific structure in different shapes . Especially, hollow-structured particles with large specific areas have widespread potential applications in photocatalysts , drug delivery carriers, lightweight fillers and gas sensors [25, 26]. Among lots of preparation approaches, the soft-template method is commonly used to synthesize Cu2O hollow-structured particles. So far, some polymer, such as EDTA-4Na , gelatin , PEG , PVP  and Oleic , were used as soft templates to fabricate hollow structures. Moreover, most reported photocatalytic studies related to Cu2O materials are mainly focused on UV-visible light irradiation [24, 32, 33]. In this paper, we used sodium dodecyl sulfate (SDS) as a soft template accompanied with gas bubble processes  to synthesize hollow porous Cu2O microspheres. The optical properties and photocatalytic activities under visible light irradiation were also investigated.
All of the chemical reagents used in the experiment were of analytical reagent grade and were directly used without further purification. To prepare the hollow porous Cu2O particles, CuSO4 was used as the Cu2+ source with SDS as the soft template. In a typical procedure, 180 mg SDS was dissolved into 45 ml deionized water under magnetic stirring for more than 20 min to form a stable micellar solution. Then, 1 ml (0.1 g/ml) CuSO4 solution, 0.04 ml (13 M) ammonia and 0.15 ml (5 M) NaOH solution were added in sequence to the above solution every 20 min. The color of the solution then turned to light turbid blue from clarification. After fully stirred, 0.18 ml (50 wt.%) N2H4·H2O solution was added dropwise as the reducing agent. As the reaction proceeded with constant stirring, the solution produced a lot of bubbles. The whole experiment process was under the condition of 20 °C for 40 min. When the reaction finished, the final products were separated by centrifugation and cleaned several times by filtration with plenty of deionized water and ethanol. Finally, the products were dried at 50 °C for 6 h in a vacuum oven.
The crystal structures of the as-prepared samples were identified by X-ray powder diffraction (XRD) using an advanced X-ray diffractometer (D8 ADVANCE, Bruker, Bremen, Germany) with a Cu-Kα rotating anode point source operating at 40 kV and 40 mA. The morphology and size were investigated by field emission scanning electron microscopy (SEM; Zeiss Ultra 55, Carl Zeiss Microscopy GmbH, Hamburg, Germany) at an accelerating voltage of 5 kV. The inner microstructure of the as-synthesized samples was studied by transmission electron microscopy (TEM; JEM-2100, JEOL, Tokyo, Japan). The optical absorption properties of the as-prepared Cu2O microspheres were characterized by UV-visible absorption spectroscopy with a He-Cd laser line of 325 nm as an excitation source (Lab-RAM HR 800 UV, HORIBA Jobin Yvon, Kyoto, Japan). The photocatalytic activity was analyzed by using methyl orange (MO) as a model pollutant molecule. The photocatalytic absorbance measurements were performed on a UNIC7000 spectrophotometer (Unic Company, USA) at 464 nm.
Results and discussion
In order to confirm the above explanations, experiment without SDS was also performed. As shown in Figure 5a, some well-crystallized particles in octahedral shapes were formed instead of hollow porous microspheres, which indicate the soft-template role of SDS in the reaction.
We have successfully synthesized hollow porous Cu2O microspheres with high purity by using a soft-template method at room temperature. This material has excellent photocatalytic activity under visible light irradiation in the degradation of MO owing to its unique optical properties and special morphology. The band gap was calculated to be 2.22 eV from its UV-visible absorbance spectrum. It was also found that the SDS acted as the soft template, and the amounts of ammonia and NaOH had important effects on the morphology of the products. These Cu2O microspheres with hollow porous structures may be a good candidate of photocatalytic materials under visible light irradiation.
This work is supported by the National High-Tech R & D Program of China (863, no. 2011AA050504), National Natural Science Foundation of China (no. 61006002), the U-M/SJTU Collaborative Research Program and the Analytical and Testing Center of SJTU, Shanghai Pujiang Program (no. 11PJD011), Natural Science Foundation of Shanghai (no. 09ZR1414800), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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