Electrochemical synthesis of ZnO branched submicrorods on carbon fibers and their feasibility for environmental applications
© Ko et al.; licensee Springer. 2013
Received: 15 April 2013
Accepted: 21 May 2013
Published: 3 June 2013
We investigated the structural and optical properties of the hierarchically integrated zinc oxide (ZnO) branched submicrorods on carbon fibers (ZOCF) by scanning/transmission electron microscopy, X-ray diffraction, and photoluminescence (PL) measurements. The ZnO submicrorods were facilely synthesized by an electrochemical deposition method on polyacrylonitrile-based carbon fiber sheets used as a substrate. After coating the ZnO seed layer on the surface of the carbon fibers, ZnO submicrorods were densely grown on the nuclei sites of the seed layer. The prepared ZOCF samples exhibited high crystallinity and good PL properties. A feasibility for environmental application in Pb(II) removal from aqueous solutions was also studied. The ZOCF adsorbent exhibited an excellent maximum adsorption capacity of 245.07 mg g−1, which could be practically used in Pb(II) removal from water. These fabricated ZOCFs are potentially useful for multifunctional and environmental devices.
KeywordsHierarchical submicrorods Zinc oxide Carbon fibers Heavy metal removal Electrochemical deposition
Over the past years, ZnO nano- or microstructures have attracted great interest in a wide range of application fields such as electronic, photonic, photovoltaic, piezoelectric, and chemical sensing devices due to their unique properties [1–5]. Recently, many efforts have been made to synthesize and integrate such ZnO nanostructures on specific substrates based on functional materials including graphene, paper fibers, and conductive fabric as well as flexible or foldable plastic substrates with less weight and cost-effective productivity because their physical and chemical properties can be improved [6–9]. Synthetic strategies, e.g., hydrothermal synthesis, sol–gel method, electrochemical deposition (ED), chemical vapor deposition, and laser ablation technique, have been developed to fabricate high-purity and high-crystallinity ZnO nanostructures on functional substrates. Among them, particularly, the ED method has many advantages in producing ZnO nanostructures [10–12]. For instance, ZnO nanostructures could be grown at low temperature (75°C to 85°C) for short preparation time utilizing the ED process. Furthermore, the shape and size of ZnO nanostructures were readily tuned by controlling the external cathodic voltage and concentration of growth solution. For this reason, it would be desirable to integrate ZnO submicron structures on carbon fibers by the ED method.
Meanwhile, carbon-based materials like carbon nanotubes and nanosized metal oxides (NMOs) including TiO2, Fe3O4, MnFe2O4, and ZnO have been promising in the removal of various toxic metals such as Cr(VI), Cd(II), Pb(II), and Hg(II) ions due to the larger surface area and higher porosity of nanostructured materials compared to bulk materials, allowing for efficient adsorption of heavy metal ions [13–16]. Although numerous methods were already practically used for heavy metal removal from aqueous solutions, adsorption techniques have come to the forefront and are effective and economical . However, NMOs are poor in mechanical strength and unfeasible in flow-through system. On the contrary, ZnO branched submicrorods on carbon fibers (ZOCF) can be employed as a complex adsorbent with the desired mechanical strength by using NMOs as host resources in permeable supports . Moreover, ZnO has been considered as a promising material because of its morphological variety with nontoxic property. It is very interesting to study the removal of Pb(II) by hierarchical ZnO structures. In this work, we prepared hierarchically integrated ZnO branched submicrorods on ZnO seed-coated carbon fibers by a simple ED method and investigated their structural and optical properties. An environmental feasibility of using ZOCF for the removal of Pb(II) metals was tested.
The removal of Pb(II) ions using ZOCF was carried out by the batch method, and the effects of various parameters such as the pH of the solution, contact time, and Pb(II) ion concentration were studied. The pH was adjusted to a desired level by adding HCl and NaOH into 50 mL of the metal solution. Then 2 × 3 cm2 of the ZOCF sample weighting 0.04 g was dipped into the metal solution. After that, the samples were agitated at room temperature using a shaker water bath (HB-205SW, Han Baek Scientific Company, Bucheon, Korea) at a constant rate of 180 rpm for a prescribed time to reach equilibrium. At the end of the predetermined time, the samples were taken out. The supernatant solution was carefully separated, and the concentration of Pb(II) ions was analyzed. The metal concentrations were determined by using an inductively coupled plasma spectrometer (ICP-7510, Shimadzu, Kyoto, Japan). Blank solutions (without adsorbent) were treated similarly, and the Pb(II) ion concentrations were recorded by the mass balance equation q e = V/m(C0 − C e ), where q e is the equilibrium adsorption capacity of Pb(II) ions (mg g−1) and C0 and C e are the initial and equilibrium concentrations of Pb(II) ions, respectively. Here, V is the volume of the solution (L), and m is the mass of the adsorbent (g).
Results and discussion
Comparison of some host-supported NMOs for heavy metal removal
0.030 mg g−1
0.029 mg g−1
0.35 mmol g−1
99.0 mg g−1
124 mg g−1
218.53 mg g−1
0.702 mg g−1
1.21 mg g−1
Municipal sewage sludge
42.4 mg g−1
176.33 mg g−1
245.07 mg g−1
We successfully fabricated hierarchically integrated ZnO branched submicrorods on carbon fibers, i.e., ZOCF, by the ED method using a simple two-electrode system. With an external cathodic voltage of −3 V for 40 min of growth time, the ZnO submicrorods could be densely self-assembled on the ZnO seed-coated carbon fibers, which exhibited a high crystallinity and a good optical property. Furthermore, the ZOCF adsorbent exhibited an excellent maximum adsorption capacity of 245.07 mg g−1 for Pb(II) metal from water. The experimental kinetic and adsorption data could be understood by theoretical equation and isotherm modeling. These well-integrated ZnO submicrorods on carbon fibers can be useful for various electronic and chemical applications with a great environmental property.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 2012–0007412).
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