Surfactant-free synthesis of Cu2O hollow spheres and their wavelength-dependent visible photocatalytic activities using LED lamps as cold light sources
© Wang et al.; licensee Springer. 2014
Received: 11 October 2014
Accepted: 7 November 2014
Published: 22 November 2014
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.
KeywordsCuprous oxide Hollow spheres Surfactant-free Photocatalysis LED cold light sources
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, nanocubes, nanocages, and octahedrons, 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[5–13]. Hollow nanomaterials are widely used as nanoscale chemical reactors, high-performance catalysts[14–16], drug-delivery carriers[17, 18], lithium-ion battery materials, and wavelength optical components for biomedical applications. 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, chemical etching, galvanic replacement, and the Kirkendall voiding. 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.
Cuprous oxide (Cu2O), a typical p-type semiconductor with a direct band gap of 2.17 eV, has been broadly applied in photocatalysis, gas sensors[8, 25], solar cells[26, 27], photoelectrochemical cells[28, 29], and lithium-ion batteries. It is noticed that Cu2O with different shapes have attracted much attention. Many efforts have been made to obtain Cu2O nanomaterials. Wet chemical reduction[31–35], electrodeposition[11, 12, 36–38], solvothermal synthesis[39–41], and irradiation[42, 43] methods have been applied to prepare Cu2O nanocrystals. However, the reported synthetic routes are relatively complex and time consuming, typically involving expensive toxic solvents and surfactants, which make it difficult to purify as-produced Cu2O hollow nanostructure as well as produce it in large scale[25, 44–46]. Therefore, it is highly rewarding to facile synthesize functional Cu2O nanomaterials in a solution without a surfactant.
Meanwhile, Cu2O 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. 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 Cu2O 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 Cu2O hollow spheres prepared under different temperatures are displayed and so does the supposed formation mechanism. In addition, photocatalytic activities of Cu2O hollow spheres are measured for the first time with a self-designed 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 high-power 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 Cu2O hollow spheres highly attractive for practical applications in water pollutant removal and environmental remediation.
Copper sulfate pentahydrate (CuSO4 · 5H2O) and hydrazine hydrate (N2H4 · H2O) are purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), of analytical grade, and used without further purification.
Preparation of Cu2O hollow spheres
In a typical synthesis, 0.25 g of CuSO4 · 5H2O 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 N2H4 · H2O (20%) to reduce Cu2+ by fast injection of 1 mL N2H4 · H2O 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 degradation of methyl orange (MO) was carried out in a self-designed equipment. Twenty milligrams of as-prepared Cu2O 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.
The sample sizes and morphologies were investigated using scanning electron microscope (SEM) and transmission electron microscope (TEM). SEM images were performed with a Carl Zeiss Ultra 55 from Carl Zeiss AG, Oberkochen, Germany. TEM images were obtained with a JEOL JEM-2100 TEM operating at 200 kV from JEOL Ltd., Akishima, Tokyo, Japan. The crystal structures were examined by X-ray diffractometer (XRD; D8 Advance, Bruker, Ettlingen, Germany) with Cu Kα (λ =1.5418 Å) and 2θ from 20° to 80°. Ultraviolet–visible spectra (UV–vis, Lambda 500, PerkinElmer, Waltham, MA, USA) characterizations were carried out at the region from 350 to 600 nm. Nitrogen adsorption-desorption isotherms were collected on an autosorb-iQA3200-4 sorption analyzer (Quantatech Co., New York, NY, USA). The pore size distribution plots were obtained using the Barret-Joyner-Halenda (BJH) model.
Results and discussion
Morphology and structure
At a certain temperature, for example, at 25°C, after the addition of N2H4 · H2O into CuSO4 solution, Cu2+ is reduced to Cu2O nanoparticles, and N2 nanobubbles are generated at the same time. As there is no surfactant in the reaction system, Cu2O nanoparticles will tend to absorb on the surface of N2 bubbles, so that Cu2O nanoparticles assemble into hollow spheres (Figure 1a), which can be referred to the Ostwald ripening process. When the reaction takes place at 0°C, the reaction rate will slow down, resulting in smaller N2 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. N2 nanobubbles escape faster so that the obtained Cu2O spheres are smaller.
On the other hand, Cu2O spheres are made up of nanoparticles. Crystallization rate increases with the rise of temperature to form bigger nanoparticles, so that the obtained Cu2O spheres would have rougher surface, which is also in agreement with the SEM results (Figure 1).
To test the photocatalytic activities of obtained Cu2O hollow spheres, MO, a negatively charged molecule, was used in the photodegradation experiments.
Time for the intensity of MO to achieve 1/ e
Time to achieve 1/e(min)
The results can be explained in two aspects, different wavelengths of visible lights and photocatalysts prepared at various temperatures. As shown in Figure 8a,b,c, the photocatalytic results indicate that the obtained Cu2O spheres can photocatalyze MO degradation under visible light and the 550-nm wavelength light exhibits the most effective photocatalytic effect among all three spheres.
The structure with larger BET surface area could facilitate effective contacts between Cu2O 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 Cu2O 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 Cu2O 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 Cu2O 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 H2O and reduce MO into CO2 and H2O.
We demonstrate a facile method to prepare Cu2O hollow spheres. Under the preparation at 0°C, 25°C, and 50°C, the obtained Cu2O 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 m2/g, respectively. Cu2O hollow spheres are obtained by nanoparticles absorbing on the surface of N2 bubbles and assemble together. A bubble template process is introduced to explain the formation mechanism. Importantly, Cu2O 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, Cu2O 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 Cu2O to photocatalytically degrade MO, because the light energy (2.25 eV) is closest to the band gap of Cu2O (2.17 eV).
The work not only provides insights into the Cu2O 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 Cu2O 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.
The authors gratefully acknowledge the financial support by the National Basic Research Program of China (2013CB932500), the National High-Tech R&D Program of China (863 program, 2011AA050504), the National Natural Science Foundation of China (51102164), the Program for New Century Excellent Talents in University (NCET-12-0356), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. We also acknowledge the analysis support from the Instrumental Analysis Center of Shanghai Jiao Tong University.
- Wang W, Wang G, Wang X, Zhan Y, Liu Y, Zheng C: Synthesis and characterization of Cu2O nanowires by a novel reduction route. Adv Mater 2002, 14: 67–69. 10.1002/1521-4095(20020104)14:1<67::AID-ADMA67>3.0.CO;2-ZView ArticleGoogle Scholar
- Cao M, Hu C, Wang Y, Guo Y, Guo C, Wang E: A controllable synthetic route to Cu, Cu2O, and CuO nanotubes and nanorods. Chem Commun 2003, 15: 1884–1885.View ArticleGoogle Scholar
- Ho JY, Huang MH: Synthesis of submicrometer-sized Cu2O crystals with morphological evolution from cubic to hexapod structures and their comparative photocatalytic activity. J Phys Chem C 2009, 113: 14159–14164. 10.1021/jp903928pView ArticleGoogle Scholar
- Sui YM, Fu WY, Zeng Y, Yang HB, Zhang YY, Chen H, Li YX, Li MH, Zou GT: Synthesis of Cu2O nanoframes and nanocages by selective oxidative etching at room temperature. Angew Chem Int Ed 2010, 49: 4282–4285. 10.1002/anie.200907117View ArticleGoogle Scholar
- Li JT, Cushing SK, Bright J, Meng F, Senty TR, Zheng P, Bristow AD, Wu NQ: Ag@Cu2O core-shell nanoparticles as visible-light plasmonic photocatalysts. ACS Catal 2013, 3: 47–51. 10.1021/cs300672fView ArticleGoogle Scholar
- Liu H, Zhou Y, Kulinich SA, Li JJ, Han LL, Qiao SZ, Du XW: Scalable synthesis of hollow Cu2O nanocubes with unique optical properties via a simple hydrolysis-based approach. J Mater Chem A 2013, 1: 302–307.View ArticleGoogle Scholar
- Zhang L, Wang H: Cuprous oxide nanoshells with geometrically tunable optical properties. ACS Nano 2011, 5: 3257–3267. 10.1021/nn200386nView ArticleGoogle Scholar
- Zhang HG, Zhu QS, Zhang Y, Wang Y, Zhao L, Yu B: One-pot synthesis and hierarchical assembly of hollow Cu2O microspheres with nanocrystals-composed porous multishell and their gas-sensing properties. Adv Funct Mater 2007, 17: 2766–2771. 10.1002/adfm.200601146View ArticleGoogle Scholar
- Xu LS, Chen XH, Wu YR, Chen CS, Li WH, Pan WY, Wang YG: Solution-phase synthesis of single-crystal hollow Cu2O spheres with nanoholes. Nanotechnology 2006, 17: 1501–1505. 10.1088/0957-4484/17/5/056View ArticleGoogle Scholar
- Lu CH, Qi LM, Yang JH, Wang XY, Zhang DY, Xie JL, Ma JM: One-pot synthesis of octahedral Cu2O nanocages via a catalytic solution route. Adv Mater 2005, 17: 2562–2567. 10.1002/adma.200501128View ArticleGoogle Scholar
- Zhou ZH, Lin YL, Zhang PG, Ashalley E, Shafa M, Li HD, Wu J, Wang ZM: Hydrothermal fabrication of porous MoS2 and its visible light photocatalytic properties. Mater Lett 2014, 131: 122–124.View ArticleGoogle Scholar
- Hu P, Yu LJ, Zuo AH, Guo CY, Yuan FL: Fabrication of monodisperse magnetite hollow spheres. J Phys Chem C 2009, 113: 900–906. 10.1021/jp806406cView ArticleGoogle Scholar
- Chen ST, Zhang XL, Hou XM, Zhou Q, Tan WH: One-pot synthesis of hollow PbSe single-crystalline nanoboxes via gas bubble assisted Ostwald ripening. Cryst Growth Des 2010, 10: 1257–1262. 10.1021/cg901280aView ArticleGoogle Scholar
- Siegfried MJ, Choi KS: Elucidating the effect of additives on the growth and stability of Cu2O surfaces via shape transformation of pre-grown crystals. J Am Chem Soc 2006, 128: 10356–10357. 10.1021/ja063574yView ArticleGoogle Scholar
- Siegfried MJ, Choi KS: Electrochemical crystallization of cuprous oxide with systematic shape evolution. Adv Mater 2004, 16: 1473–1476.View ArticleGoogle Scholar
- Bao HZ, Zhang ZH, Hua Q, Huang WX: Compositions, structures, and catalytic activities of CeO2@Cu2O nanocomposites prepared by the template-assisted method. Langmuir 2013, 30: 6427–6436.View ArticleGoogle Scholar
- Zhu YF, Ikoma T, Hanagata N, Kaskel S: Rattle-type Fe3O4@SiO2 hollow mesoporous spheres as carriers for drug delivery. Small 2010, 6: 471–478. 10.1002/smll.200901403View ArticleGoogle Scholar
- Chen Y, Chen HR, Zeng DP, Tian YB, Chen F, Feng JW, Shi JL: Core/shell structured hollow mesoporous nanocapsules: a potential platform for simultaneous cell imaging and anticancer drug delivery. ACS Nano 2010, 4: 6001–6013. 10.1021/nn1015117View ArticleGoogle Scholar
- Yao Y, McDowell MT, Ryu I, Wu H, Liu N, Hu LB, Nix WD, Cui Y: Interconnected silicon hollow nanospheres for lithium-ion battery anodes with long cycle life. Nano Lett 2011, 11: 2949–2954. 10.1021/nl201470jView ArticleGoogle Scholar
- Zhang JZ: Biomedical applications of shape-controlled plasmonic nanostructures: a case study of hollow gold nanospheres for photothermal ablation therapy of cancer. J Phys Chem Lett 2010, 1: 686–695. 10.1021/jz900366cView ArticleGoogle Scholar
- Niu KY, Yang J, Kulinich SA, Sun J, Du XW: Hollow nanoparticles of metal oxides and sulfides: fast preparation via laser ablation in liquid. Langmuir 2010, 26: 16652–16657. 10.1021/la1033146View ArticleGoogle Scholar
- Niu KY, Yang J, Kulinich SA, Sun J, Li H, Du XW: Morphology control of nanostructures via surface reaction of metal nanodroplets. J Am Chem Soc 2010, 132: 9814–9819. 10.1021/ja102967aView ArticleGoogle Scholar
- An K, Hyeon T: Synthesis and biomedical applications of hollow nanostructures. Nano Today 2009, 4: 359–373. 10.1016/j.nantod.2009.06.013View ArticleGoogle Scholar
- Xu HL, Wang WZ, Zhu W: Shape evolution and size-controllable synthesis of Cu2O octahedra and their morphology-dependent photocatalytic properties. J Phys Chem B 2006, 110: 13829–13834. 10.1021/jp061934yView ArticleGoogle Scholar
- Zhang JT, Liu JF, Peng Q, Wang X, Li YD: Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors. Chem Mater 2006, 18: 867–871. 10.1021/cm052256fView ArticleGoogle Scholar
- Zhang N, Du YL, Zhang Y, Wang CM: A simple method for controlling the type of cuprous oxide semiconductors using different surfactants. J Mater Chem 2011, 21: 5408–5013. 10.1039/c0jm03535aView ArticleGoogle Scholar
- Diab M, Moshofsky B, Plante IJ, Mokari T: A facile one-step approach for the synthesis and assembly of copper and copper-oxide nanocrystals. J Mater Chem 2011, 21: 11626–11630. 10.1039/c1jm10638dView ArticleGoogle Scholar
- Liu YB, Zhou HB, Li JH, Chen HC, Li D, Zhou BX, Cai WM: Enhanced photoelectrochemical properties of Cu2O-loaded short TiO2 nanotube array electrode prepared by sonoelectrochemical deposition. Nano-Micro Lett 2010, 2: 277–284. 10.1007/BF03353855View ArticleGoogle Scholar
- Wang WZ, Huang XW, Wu S, Zhou YX, Wang LJ, Shi HL, Liang YJ, Zou B: Preparation of p-n junction Cu2O/BiVO4 heterogeneous nanostructures with enhanced visible-light photocatalytic activity. Appl Catal B Environ 2013, 134: 293–301.View ArticleGoogle Scholar
- Kuo CH, Huang MH: Morphologically controlled synthesis of Cu2O nanocrystals and their properties. Nano Today 2010, 5: 106–116. 10.1016/j.nantod.2010.02.001View ArticleGoogle Scholar
- Kim MH, Lim B, Lee EP, Xia YJ: Polyol synthesis of Cu2O nanoparticles: use of chloride to promote the formation of a cubic morphology. J Mater Chem 2008, 18: 4069–4073. 10.1039/b805913fView ArticleGoogle Scholar
- Zhang H, Ren X, Cui Z: Shape-controlled synthesis of Cu2O nanocrystals assisted by PVP and application as catalyst for synthesis of carbon nanofibers. J Cryst Growth 2007, 304: 206–210. 10.1016/j.jcrysgro.2007.01.043View ArticleGoogle Scholar
- Liang X, Gao L, Yang S, Sun J: Facile synthesis and shape evolution of single-crystal cuprous oxide. Adv Mater 2009, 21: 2068–2071. 10.1002/adma.200802783View ArticleGoogle Scholar
- Huang L, Peng F, Yu H, Wang H: Preparation of cuprous oxides with different sizes and their behaviors of adsorption, visible-light driven photocatalysis and photocorrosion. Solid State Sci 2009, 11: 129–138. 10.1016/j.solidstatesciences.2008.04.013View ArticleGoogle Scholar
- Kuo C-H, Huang MH: Fabrication of truncated rhombic dodecahedral Cu2O nanocages and nanoframes by particle aggregation and acidic etching. J Am Chem Soc 2008, 130: 12815–12820. 10.1021/ja804625sView ArticleGoogle Scholar
- Siegfried MJ, Choi K-S: Directing the architecture of cuprous oxide crystals during electrochemical growth. Angew Chem 2005, 117: 3282–3287. 10.1002/ange.200463018View ArticleGoogle Scholar
- Somasundaram S, Chenthamarakshan CRN, de Tacconi NR, Rajeshwar K: Photocatalytic production of hydrogen from electrodeposited p-Cu2O film and sacrificial electron donors. Int J Hydrogen Energy 2007, 32: 4661–4669. 10.1016/j.ijhydene.2007.06.028View ArticleGoogle Scholar
- Singh DP, Neti NR, Sinha ASK, Srivastava ON: Growth of different nanostructures of Cu2O (nanothreads, nanowires, and nanocubes) by simple electrolysis based oxidation of copper. J Phys Chem C 2007, 111: 1638–1645.View ArticleGoogle Scholar
- Xu Y, Jiao X, Chen D: PEG-assisted preparation of single-crystalline Cu2O hollow nanocubes. J Phys Chem C 2008, 112: 16769–16773. 10.1021/jp8058933View ArticleGoogle Scholar
- Teo JJ, Chang Y, Zeng HC: Fabrications of hollow nanocubes of Cu2O and Cu via reductive self-assembly of CuO nanocrystals. Langmuir 2006, 22: 7369–7377. 10.1021/la060439qView ArticleGoogle Scholar
- Zhang H, Zhang X, Li H, Qu Z, Fan S, Ji M: Hierarchical growth of Cu2O double tower-tip-like nanostructures in water/oil microemulsion. Cryst Growth Des 2007, 7: 820–824. 10.1021/cg0607351View ArticleGoogle Scholar
- He P, Shen X, Gao H: Size-controlled preparation of Cu2O octahedron nanocrystals and studies on their optical absorption. J Colloid Interface Sci 2005, 284: 510–515. 10.1016/j.jcis.2004.10.060View ArticleGoogle Scholar
- Chen Q, Shen X, Gao H: Formation of solid and hollow cuprous oxide nanocubes in water-in-oil microemulsions controlled by the yield of hydrated electrons. J Colloid Interface Sci 2007, 312: 272–278. 10.1016/j.jcis.2007.03.036View ArticleGoogle Scholar
- Meng XY, Tian GH, Chen YJ, Qu Y, Zhou J, Pan K, Zhou W, Zhang GL, Fu HG: Room temperature solution synthesis of hierarchical bow-like Cu2O with high visible light driven photocatalytic activity. RSC Adv 2011, 2: 2875–2881.View ArticleGoogle Scholar
- Su XD, Zhao JZ, Bala H, Zhu YC, Gao Y, Ma SS, Wang ZC: Fast synthesis of stable cubic copper nanocages in the aqueous phase. J Phys Chem C 2007, 111: 14689–14693. 10.1021/jp074550wView ArticleGoogle Scholar
- Wang WZ, Zhang PC, Peng L, Xie WJ, Zhang GL, Tu Y, Mai WJ: Template-free room temperature solution phase synthesis of Cu2O hollow spheres. Cryst Eng Comm 2010, 12: 700–701. 10.1039/b912514kView ArticleGoogle Scholar
- Jin L, Xu LP, Morein C, Chen CH, Lai M, Suib SL: Titanium containing γ-MnO2 (TM) hollow spheres: one-step synthesis and catalytic activities in Li/air batteries and oxidative chemical reactions. Adv Funct Mater 2010, 20: 3373–3382. 10.1002/adfm.201001080View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.