- Nano Express
- Open Access
Surfactant-free Synthesis of CuO with Controllable Morphologies and Enhanced Photocatalytic Property
© Wang et al. 2016
Received: 5 December 2015
Accepted: 25 January 2016
Published: 3 March 2016
A green synthesis for nanoleave, nanosheet, spindle-like, rugby-like, dandelion-like and flower-like CuO nanostructures (from 2D to 3D) is successfully achieved through simply hydrothermal synthetic method without the assistance of surfactant. The morphology of CuO nanostructures can be easily tailored by adjusting the amount of ammonia and the source of copper. By designing a time varying experiment, it is verified that the flower- and dandelion-like CuO structures are synthesized by the self-assembly and Ostwald ripening mechanism. Structural and morphological evolutions are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-visible diffuse reflectance spectra. Additionally, the CuO nanostructures with different morphologies could serve as a potential photocatalyst on the photodecomposition of rhodamine B (RhB) aqueous solutions in the presence of H2O2 under visible light irradiation.
The photocatalytic performance, electrical and gas-sensing properties are strongly influenced by their morphology and size. Many investigations have been carried out to study the controlling of size, morphology and structure of materials during synthesis [1–4]. To achieve this, the studies of the crystal growth, morphology evolution processes and the corresponding mechanisms are significantly important. As an important p-type transition-metal oxide with a narrow band gap varying between 1.2 and 1.8 eV , CuO has been widely studied in thermal conductivity , optoelectronic device systems , CO oxidation , eradication of multi-drug resistant bacteria , Li ion batteries anodes , heterogeneous catalyst for olefin epoxidation , gas sensing [12, 13] and glucose sensor [14–18]. In the past decade, CuO with different morphologies such as nanoribbons , microworms , nanoplatelets , dandelions , sandwich , nanowires , nanotube arrays , nanourchins [2, 18] and nanorods  have been successfully synthesized through different methods with the assistance of surfactant such as CTAB, PVP, PEG and SDS. Since the surfactants invariably present residual surfactants or organic additives attached to the surfaces of products can block the active sites, it is a serious issue when considering applications in gas sensing or catalysis. Therefore, it is still a challenge to develop new green surfactant-free methods to synthesize well-defined CuO nanostructures . Zhang et al.  synthesized flower-like CuO microspheres by a hydrothermal route at 130 °C for 18 h without the assistance of surfactant. Sun et al.  synthesized two-dimensional CuO mesoplates and three-dimensinonal CuO mesospindles by an additive-free complex-precursor solution route.
Inspired by the green methods to synthesize various controllable CuO morphologies with surfactant-free building blocks, we present a simply low temperature hydrothermal synthetic method without the assistance of surfactant. And spindle-like, rugby-like, nanoleaves, nanosheets, microspheres and dandelions CuO nanostructures are synthesized through this green method. The synthesis is performed in an ethanol-water mixed solvent using copper source and ammonia as the variable. By designing a time varying experiment, it is verified that the flower- and dandelion-like CuO structures are synthesized by the self-assembly and Ostwald ripening mechanism. X-ray diffraction (XRD), scanning electron microscope (SEM) and UV-visible diffuse reflectance spectra are employed to characterize the obtained CuO nanostructures. Furthermore, these copper oxide nanostructures are found to be high qualified photocatalysts for the degradation of rhodamine B (RhB) under visible light irradiation in the presence of hydroxide water (H2O2).
Materials and synthesis
The morphologies and synthesis parameters of CuO nanostructures
The amount of NH3 · H2O (mL)
Cu(NO3)2 · 3H2O
Cu(COOH)2 · H2O
Cu(NO3)2 · 3H2O
Cu(COOH)2 · H2O
Cu(NO3)2 · 3H2O
Cu(COOH)2 · H2O
Powder XRD pattern is recorded on an X’Pert Philips diffractometer (Cu Kα radiation: λ = 1.5418 Å, 2θ range 20∼80°, accelerating voltage 40 kV, applied current 150 mA). The morphology of the products is investigated by field emission scanning electron microscopy (SEM, Hitachi S-4800).
The photocatalytic activity of the CuO nanostructures with different morphologies is evaluated by the degradation of a model pollutant RhB under the visible light irradiation with the assistance of hydrogen peroxide (H2O2) at ambient temperature. The original solution is prepared by mixing 5 mL H2O2 (30 wt%), 50 mL RhB solution (10−5 M) and 20 mg copper oxide powder together and then stirred in the dark for 60 min to ensure an adsorption-desorption equilibrium is established. Afterwards, the dispersion is irradiated by a 350-W xenon lamp equipped with a filter cutoff (λ ≥420 nm) under magnetic stirring. At given time intervals, the dispersion is sampled and centrifuged to separate the catalyst. The adsorption spectrum of the solution is then recorded with an UV-visible spectrophotometer (Shimadzu UV-3600).
Results and discussion
Crystal structures of the prepared CuO nanomaterials
Morphologies of CuO nanostructures synthesized by Cu(NO3)2 as a copper source
Morphologies of CuO nanostructures synthesized by Cu(COOH)2 as a copper source
Plausible mechanisms for the formation of CuO nanostructures
In the present case, CuO particles are synthesized directly by the decomposition of Cu(OH)2 or (Cu(NH3)4)2+ precursor under hydrothermal conditions without the presence of various surfactant. The crystal formation process can be divided into two stages: nucleation and crystal growth. When the amount of ammonia is less than 0.6 mL, Cu(OH)2 is formed in aqueous reaction medium, which transformed into CuO under hydrothermal conditions [Eqs. (1) and (2)]. When the amount of ammonia is over 1.5 mL, the soluble [Cu(NH3)4]2+ complex is formed, which transformed into CuO under hydrothermal conditions [Eqs. (3) and (4)]. The different growth unite might affect the competition between thermodynamics and kinetics during the reduction of precursors and nucleation and growth of CuO crystals . Comparing Fig. 2 with Fig. 3, we can conclude that the morphologies of CuO are not the same and the sizes of CuO become smaller when the copper source changes from Cu(NO3)2 to Cu(COOH)2. The effect of copper source on the structure of CuO may be that the NO3 − is inorganic strong acid root and the COOH− is organic weak acid root; in the synthesis of complex precipitation, the anions in the solution affect the nucleation and growth of the copper oxide precursor. From the above analysis, it is safe to say that the ammonia and the acid radical ion have an important effect on the formation of CuO morphology.
In summary, a green synthesis for spindle-like, rugby-like, nanoleave, nanosheet, dandelion-like and flower-like CuO nanostructures (from 2D to 3D) are successfully achieved through simply hydrothermal synthetic method without the assistance of surfactant. The formation of CuO nanostructures here is basically effected by the amount of ammonia and the copper source. We also found that the flower- and dandelion-like CuO structures are synthesized by the self-assembly and the Ostwald ripening mechanism. Additionally, the CuO nanostructures with different morphologies could serve as a potential photocatalyst on the photodecomposition of RhB aqueous solutions in the presence of H2O2 under visible light irradiation.
This work was financially supported by the NSFC (No. 51371093) and the MOE (No. IRT1251 & 20130211130003) of China.
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