Synthesis and Characterization of Pure Copper Nanostructures Using Wood Inherent Architecture as a Natural Template
© The Author(s). 2018
Received: 4 January 2018
Accepted: 17 April 2018
Published: 24 April 2018
The inherent sophisticated structure of wood inspires researchers to use it as a natural template for synthesizing functional nanoparticles. In this study, pure copper nanoparticles were synthesized using poplar wood as a natural inexpensive and renewable template. The crystal structure and morphologies of the copper nanoparticles were characterized by X-ray diffraction and field emission scanning electron microscopy. The optical properties, antibacterial properties, and stability of the hybrid wood materials were also tested. Due to the hierarchical and anisotropic structure and electron-rich components of wood, pure copper nanoparticles with high stability were synthesized with fcc structure and uniform sizes and then assembled into corncob-like copper deposits along the wood cell lumina. The products of nanoparticles depended strongly on the initial OH− concentration. With an increase in OH− concentration, Cu2O gradually decreased and Cu remained. Due to the restrictions inherent in wood structure, the derived Cu nanoparticles showed similar grain size in spite of increased Cu2+ concentration. This combination of Cu nanostructures and wood exhibited remarkable optical and antibacterial properties.
Metal nanoparticles have garnered wide attention in the scientific community thanks to their exceptional physical and chemical properties . Silver and gold have attracted particularly great interest given their unique plasmon resonance and high stability. However, the high cost of silver and gold limits their wide industrial application . Because copper is much cheaper and more abundant, copper nanoparticles (Cu NPs) may be considered a replacement for silver and gold NPs. Moreover, Cu-based NPs are gaining importance thanks to their catalytic, optical, antibacterial, and electrical conducting properties [3–5]. To fully utilize these properties, the size, purity, and shapes of copper must be well controlled. Therefore, various attempts have been proposed to synthesize NPs with a controlled shape and a specific size distribution, such as solution reduction, thermal decomposition, metal vapor synthesis, radiation methods, microemulsion techniques, mechanical attrition, and electrodeposition [6–10]. Among these, the solution reduction approach is a feasible and exceptionally versatile method for the preparation of Cu NPs. However, it is common to find nanoparticle molecules with spherical shapes; controlled NPs synthesis with other distinct surface morphologies can be accomplished using some unique organic/inorganic templates . Nevertheless, the template consumption in the preparation process is costly, and the procedure is tedious .
Another issue in utilizing these Cu NPs is their inherent propensity for surface oxidation in air and resultant aggregation . To avoid this problem, an inert environment (e.g., nitrogen or argon) is used . Other reports have presented various approaches that attempt to address the oxidation problem; such methods are generally based on minimizing exposure of the Cu NPs to oxygen through a protective layer at the particle surface. This layer may consist of polymers , organic ligands [16, 17], carbon and graphene , or inert metal ; however, these strategies require complex processes and/or special equipment.
In addition to the unique structure of wood, its lignocellulosic nature—composed of cellulose, lignin, and hemicelluloses—has a reducing and stabilizing effect on metal NPs given the electron-rich features of hydroxyl and phenolic groups in these components . Lin [25, 26] demonstrated that Pt NPs and Ag NPs with a controlled size and shape were successfully synthesized using wood nanomaterials in aqueous systems without employing any other reductants. They attributed the formation of NPs to the reducibility of hydroxyl and phenolic groups in wood components that reduce Pt ions and Ag ions. However, the sophisticated structure of wood has been underused such that the generated Cu NPs have been susceptible to oxidation in previous studies. Hence, wood components appear to be beneficial to NP stability if the NPs are synthesized using solid wood as a template.
In this study, we reported the success of a novel Cu architecture via chemical reduction within poplar wood as the natural template. The morphologies and crystal structure of the Cu NPs were characterized, and the stability, optical properties, and antibacterial properties of the hybrid wood materials were investigated.
From the sound sapwood portions of poplar (Populus tomentosa Carr.), samples with a dimension of 50 × 50 × 5 (longitudinal) mm3 were prepared and oven-dried at 103 °C to a constant weight.
Copper (II) chloride dehydrate (CuCl2·2H2O) and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Other analytical-grade chemical reactants were obtained from Beijing Chemical Reagents Co., Ltd. (Beijing, China).
Preparation of Wood/Cu Composites
Characterization of Cu Nanostructures
The X-ray diffraction (XRD) measurements of the NPs were carried out using a Bruker D8 advance diffractometer (Germany). The apparatus parameters were set as follows: Cu-Kα radiation with a graphite monochromator, voltage 40 kV, electric current 40 mA, and 2θ scan range from 5° to 90° with a scanning speed of 2°/min.
The morphologies of Cu nanostructures were examined using a field emission scanning electron microscope (FE-SEM, Hitachi SU8010, Japan) equipped with an energy dispersive X-ray spectroscope (EDS, EX-350, Horiba Scientific, Japan). The interior portions of longitudinal planes in the sample were mounted on conductive adhesives and were coated with gold sputter followed by observation using FE-SEM at a voltage of 5 kV.
Evaluation of Optical and Antibacterial Properties
The diffuse reflectance UV-VIS spectra were measured using a UV-VIS spectrophotometer (Cary-300) equipped with an integrating sphere. The scanning range was from 800 to 300 nm.
For bactericidal experiments, the hybrid wood materials were machined into round shape with diameter of 10 mm. The bacterial suspension (Escherichia coli) was applied uniformly on the surface of a nutrient agar plate before placing the samples on the plate (1 control and 2 treated samples per plate). The plates were incubated at 37 °C for 24 h, after which the average diameters of the inhibition zone surrounding the samples were measured with a ruler with up to 0.1 mm resolution.
Results and Discussion
X-ray Diffraction Analysis
The grain size of Cu NPs in group C, E, and F
Grain size (nm)
19.54 ± 1.87
18.34 ± 1.59
19.74 ± 2.41
18.93 ± 1.18
21.36 ± 3.09
20.08 ± 1.76
Optical and Antibacterial Properties
To leverage the inherent hierarchical, anisotropic architecture, and electron-rich components of wood, pure Cu NPs were derived with unique shapes and sizes through wood template methods. The Cu NPs exhibited a 3D structure along the wood cell lumina that consisted of corncob-like Cu deposits. The nanoparticle products depended strongly on the initial OH− concentration. With an increase in OH− concentration, Cu2O gradually decreased and Cu remained. As the Cu2+ concentration increased gradually, more Cu NPs were generated in the wood structure. The assembled structure of NPs invariably exhibited corncob-like Cu deposits in the wood templates. Due to the unique structure and components of wood, the oxidation and aggregation of Cu NPs could be circumvented. Additionally, this new hybrid wood material, combined with the advantages of wood and Cu nanostructures, exhibited remarkable optical and antibacterial properties.
The authors are very grateful for the support from the China Scholarship Council (CSC).
This work was supported by the Fundamental Research Funds for the Central Universities (no. 2016ZCQ01) and the National Natural Science Foundation of China (Project 51779005/E090301).
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.
YD, KW, YT, and QW conducted the experiments and gathered the data. SZ, HM, and JL conceived and supervised the work. YD analyzed the data and prepared the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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