18624
J. Phys. Chem. C 2007, 111, 18624-18628
Hierarchical Nanostructured Copper Oxide and Its Application in Arsenic Removal An-min Cao,† Jason D. Monnell,‡ Christopher Matranga,§ Jia-min Wu,† Liang-liang Cao,† and Di Gao*,† Departments of Chemical and Petroleum Engineering and CiVil and EnVironmental Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261 and Chemistry and Surface Science DiVision, National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, PennsylVania 15236 ReceiVed: September 12, 2007; In Final Form: October 4, 2007
A facile two-step process was developed to synthesize hierarchical nanostructured CuO. First, copper acetate reacts with ethylene glycol to produce organocopper precursors with tunable morphologies; second, the organocopper precursors are calcinated to produce CuO structures which preserve the morphology of the precursors. The product consists of nanometer-sized CuO crystallites self-organized into micrometer-sized monoliths with tunable complex structures, including microplates, doughnut-like structures, and multilayer microspheres. The doughnut-like CuO structure possesses high removal capacity for As(III) and can be easily separated and recycled during water treatment processes.
Introduction Hierarchical structured materials consisting of building blocks in multiple length scales at different levels, with the higher levels having a control or precedence over the lower levels, are of great interest for both fundamental research and technological applications.1-4 The property of such materials depends not only on the physicochemical nature of their organic and inorganic components but also on the synergy between these components.1,5,6 Thus, design and synthesis of hierarchical structures provide the capability to tailor material properties through tuning the accessibility of functional components, curvature of interfaces, and style and degree of the internal organizations. None of these parameters can be designed or controlled by traditional material engineering in a single length scale. As a result, there has been steady growth of research interest in developing a variety of approaches to construct hierarchical structures for the development of new functional materials.7-11 Copper oxide (CuO) is a versatile material with potential applications in sensors, heterogeneous catalysts, electron field emission sources, and solar cell devices.11-15 Different synthesis strategies have been developed for the fabrication of CuO nanostructures with various morphologies, including nanowire, nanosheet, nanowisker, and nanofiber.16-20 It is reported that the properties of the prepared CuO samples strongly depend on its morphologies and structures such as crystal sizes, orientations, stacking manners, aspect ratios, and crystalline densities.11-20 Most of these recent investigations, however, have been limited to the synthesis and application of CuO structures with simple 1- or 2-dimensional geometries. Reports on complex 3-dimensional (3D) CuO structures with well-defined morphologies are currently lacking as the control over the nucleation and growth of CuO remains a challenge. * To whom correspondence should be addressed. Phone: 412-624-8488. Fax: 412-624-9639. E-mail:
[email protected]. † Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh. ‡ Department of Civil and Environmental Engineering, University of Pittsburgh. § U.S. Department of Energy.
In this paper, a facile synthetic route is described to construct hierarchical copper oxide (CuO) structures. First, copper acetate reacts with ethylene glycol (EG) to produce organocopper precursors with tunable morphologies; second, the organocopper precursors are calcinated to produce CuO structures which preserve the morphology of the precursors. Without the use of templates, surfactants, or additives, this route is capable of synthesizing a variety of 3D complex CuO structures, including microplates, doughnut-like structures, and multilayer microspheres. These structures consist of nanometer-sized CuO crystals self-organized into micrometer-sized monoliths with a hierarchical architecture. While maintaining the high specific surface area characteristic of nanomaterials, the 3D hierarchical structures enhance mass transport in macroscales, promote the accessibility of the nanomaterials, and greatly facilitate dispersion, transportation, separation, and recycling of nanomaterials. As a demonstration, the synthesized doughnut-like CuO particles are used for removing As(III) from water with a high removal capacity. Experimental Section Material Synthesis. In a typical process, a certain amount of copper acetate [Cu(CH3COO)2] (Sigma-Aldrich) was added into 30 mL of ethylene glycol (J. T. Baker) to form a cloudy solution. The mixture was stirred with a magnetic stir bar and heated to 150 °C. The cloudy mixture turned clear in 10 min and became opaque again after another 20 min, indicating formation of an organocopper precursor. The product was collected after another 10 min by centrifugation-redispersion cycles with alcohol. This precursor was then calcinated at 400 °C for 2 h to obtain crystalline copper oxide. Characterization. The samples were characterized by powder X-ray diffraction (XRD, Philips X’pert diffractometer with Cu KR radiation, λ ) 0.1542 nm, 40 kV, 30 mA), transmission electron microscopy (TEM, JEOL 2000XF), scanning electron microscopy (SEM, Philips XL-30), BET surface area analyzer (Quantachrome Autosorb Automated Gas Sorption System), and X-ray photoelectron spectroscopy (XPS) (PHI 5600ci instrument).
10.1021/jp0773379 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/28/2007
Hierarchical Nanostructured Copper Oxide
Figure 1. SEM images of the organocopper precursors prepared with (a) 9, (b) 20, (c) 36, and (d) 110 mM Cu(CH3COO)2. The inset of a is a higher magnification SEM image. The circular openings on the outer shell of the particles are indicated by white arrows in d.
Arsenic Removal Experiments. A solution made by diluting As(III) standard solution (1000 mg/L, Sigma-Aldrich) in deionized water was used as the As(III) source. The acidity of the solution was adjusted using nitric acid. Different concentrations of As(III) solutions were prepared at a pH of 4.0. For each sample, 0.03 g of the synthesized CuO powder was added to 20 mL of As(III) solution. The mixture was agitated for 1 h and then placed for 5 h to establish equilibrium at room temperature. The CuO powder was then separated from the mixture by centrifugation. To determine the amount of As(III) removed by the CuO powder, As(III) concentration was measured before and after treatment using a Perkin-Elmer 1100B atomic absorption spectrophotometer. The adsorption isotherm was obtained by varying the initial As(III) concentrations. Commercial CuO nanopowder ( 50 nm), mesopores (diameter between 2 and 50 nm), and micropores (diameter < 2 nm). The surface of the synthesized CuO particles was examined by X-ray photoelectron spectroscopy (XPS). Spectra for the Cu 2p3/2 core level and shake up satellites are shown in Figure 5. Fits of the spectra show peaks (full-width-at-half-maximum given in parentheses) at 933.1 (2.6) and 934.5 (4.7) eV that are assigned to Cu+ and Cu2+ core features, respectively.31,32 Peaks at 940.8 (2.9) and 943.3 (2.1) eV are assigned to Cu2+ satellites.31,32 These spectral features are consistent with previous reports for CuO nanocrystals.32 Survey scans of the sample only find evidence for Cu, O, and adventitious C occurring at levels of 38.4, 45.5, and 16.1 at. %, respectively. The slightly higher than expected O/Cu ratio is believed to be caused by O in adventitious hydrocarbons adsorbed on the sample during handling. By using the relative areas of the three Cu2+ features
Hierarchical Nanostructured Copper Oxide
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18627 TABLE 1: BET Surface Areas and As(III) Removal Capacities of the Commercial, Doughnut-like, and Multilayer Spherical CuO Particlesa
samples commercial CuO with an average diameter of
