Shape Control of Cobalt Carbonate Particles by a Hydrothermal Process in a Mixed Solvent: An Efficient Precursor to Nanoporous Cobalt Oxide Architectures and Their Sensing Property Huai-Ping Cong and Shu-Hong Yu*
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 210–217
DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Structural Research Laboratory of CAS, Department of Materials Science and Engineering, UniVersity of Science and Technology of China, Hefei 230026, P.R. China ReceiVed March 22, 2008; ReVised Manuscript ReceiVed September 26, 2008
ABSTRACT: Shape-controlled synthesis of CoCO3 crystals has been achieved by a hydrothermal process in a mixed solvent of water and ethanol (or glycerol). Uniform CoCO3 crystals with novel structures can be fabricated by tuning the volume ratio of water and ethanol (or glycerol). The results demonstrated that the as-prepared CoCO3 crystal can act as an efficient precursor for production of nanoporous Co3O4 particles with pore sizes of 2.5 and 60 nm by calcination at 500 °C. The nanoporous Co3O4 particles show sensitive response and reversibility in sensing performance.
1. Introduction Research on nanomaterials has been persistent due to their unique optical, electrical, magnetic, and catalytic properties compared with those of the bulk materials. Recently, more and more attention has been paid to the control of the morphologies and the microstructures of materials, because the materials with novel functionalities depend not only on the compositions but also on the shapes and sizes. Compounds with the same compositions but different morphologies and microstructures exhibit substantial differences in properties.1-5 Up to now, various methods have been used to synthesize different kinds of nanomaterials with unique morphologies and special structures, including nanowires,6 nanorods,7 naotubes,8 nanobelts,9 nanoplates,10 nanorings,11 nanoparticles,12 and other nanostructures.13 Metal carbonate is an important class of the inorganic materials and is used as the standard model system due to its abundance in nature and the wide applications in industry, such as paints, plastics, rubbers, and papers.14 Crystallization of the biomineral CaCO3 has been intensively studied in recent years as reviewed.15 Also, other metal carbonates such as BaCO3,16 CdCO3,17 MnCO3,17a,18 and PdCO317a have also been synthesized by mineralization approaches. It has been pointed out that mineralization of inorganic minerals usually has been carried out in aqueous solution. A few reports demonstrated that mineralization reactions in nonaqueous solution such as in alcohol, ethanol, 2-propanol, and diethylene glycol can vary the shape of CaCO3 crystals and phases.19 Polymer-controlled crystallization in a mixed solvent20 or in nonaqueous solution21 can result in entirely distinct results due to the changes in precursor solubility, precipitation kinetics, solution property, and aggregation forms of the polymer in a mixed solution. However, to the best of our knowledge, controlled synthesis of cobalt carbonate with novel structures and uniform shapes has been rarely studied.22 Spinel Co3O4 is an important p-type semiconductor due to its potential applications as a solid-state sensor,23 a heterogeneous catalyst,24 an electrochemical device,25 energy storage,26 and a rotatable magnet.27 Recently, more effort has been focused * To whom correspondence should be addressed. Fax: + 86 551 3603040. E-mail:
[email protected].
on the fabrication of cobalt oxide with novel nanostructures, such as nanocubes,28 nanofibers,29 nanotubes,30 nanorods,31 nanowalls,32 nanoboxes,33 hollow spheres,34 and porous nanostructures.35 Among the synthesis methods of this material is a two-step route to fabricate Co3O4 crystals with different shapes and novel structures; that is, the precursor compounds are synthesized in the first step, and then the precursor is calcinated to produce Co3O4. Cobalt hydroxide as the precursor has been used to prepare uniform Co3O4 nanoplates and flowers.36 A socalled polyol process has been developed for synthesizing Co3O4 with hierarchical structures by Cao et al.37 Co3O4 nanorod bundles have been synthesized from a single precursor [Co(CO3)0.35Cl0.20(OH)1.10].38 Recently, Han et al. reported a top-down approach to synthesize porous CdO by decomposition of CdCO3 crystals.39 In this paper, uniform cobalt carbonate crystals with different shapes have been synthesized in high yield by a hydrothermal approach through tuning the volume ratio of a mixed solvent composed of water and ethanol (or glycerol). Calciniation of the CoCO3 crystals results in the formation of nanoporous Co3O4 particles. The sensing property of the porous Co3O4 crystals has been examined.
2. Experimental Section 2.1. Materials and Preparation Procedures. All reagents were of analytical grade and were purchased and used as received without further purification. In a typical procedure, 1 mmol (0.2490 g) of cobalt acetate [Co(CH3COO)2 · 4H2O] and 3.95 × 10-2 mmol (0.0144 g) of cetyltrimethylammonium bromide (CTAB) were dissolved in a 24 mL mixture of distilled water and ethanol (or glycerol) with different volume ratios in a Teflon liner (capacity of 30 mL) with a magnetic stirrer at room temperature. Then, 5 mmol (0.3950 g) of NH4HCO3 was added to the solution described above and stirred for 10 min. Finally, the liner was sealed in a stainless steel autoclave. The autoclave was maintained at 160 °C for 24 h and then cooled to room temperature naturally. The obtained pink-colored products were centrifuged, washed several times with distilled water and anhydrous ethanol, and dried at 60 °C. The thermal decomposition of the pink-colored precursor to Co3O4 was performed at 500 °C for 3 h in air in the oven. 2.2. Characterization. X-ray powder diffraction (XRD) patterns of the products were obtained on a Japan Rigaku DMax-γA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). The SEM images were taken on a field emission scanning elecrton microscope (JEOL JSM-6700F, 15
10.1021/cg8003068 CCC: $40.75 2009 American Chemical Society Published on Web 11/20/2008
Shape Control of Cobalt Carbonate Particles
Crystal Growth & Design, Vol. 9, No. 1, 2009 211
Scheme 1. Illustration of the Working Principle of the Gas Sensing Measurement
kV). FT-IR spectra were recorded on a Bruker Vector-22 FT-IR spectrometer from 4000 to 400 cm-1 at room temperature. The composition of the samples was analyzed with an X-ray photoelectron spectroscopy (XPS) technique, which was conducted on an ESCALAB 250 (Thermo Electron Corp.). The thermogravimetric analysis (TGA) measurement was carried out on a Diamond TG/DTA thermal analyzer (Perkin-Elmer) with a heating rate of 10 K min-1 in air. BrunauerEmmett-Teller (BET) surface area measurement was performed on a Micromeritics ASAP-2000 nitrogen adsorption apparatus. 2.3. Experimental Procedure for Sensing Performance Measurement. Alcohol-sensing measurement was carried out on a homebuilt apparatus reported previously.40c The thick film sensors were made by depositing the thick films of Co3O4 particles on ceramic substrates with the connection through gold electrodes connected by four platinum wires. After calcination in air at 500 °C for 3 h, the platinum wires were connected with the leads of the gold electrodes to the instruments for gas sensing measurements. Then, the Ni-Cr-alloy coil used as the resistor was passed through the ceramic tubes and was attached to the electrode. Finally, the prepared sensors were fixed into the gas sensing apparatus (Scheme 1). During the measurement, the Vcircuit was applied to a certain value, and the heating voltage was used to heat the sensor. The detected output voltage of the load resistor connected with the sensor reflected the resistance change of the sensor, due to the various types and concentrations of the sensing gas. The higher value of the output voltage implied a lower resistance value and a better sensitivity of the sensor. The sensors were sealed into a bottle, and a given amount of ethanol was injected into the heating device part of apparatus with a microinjector and gasified quickly. The measurement data were collected through the computer.
3. Results and Discussion 3.1. Shape Control of Cobalt Carbonate Microcrystals in a Water/Ethanol Solvent. The phases of the as-prepared pink-colored samples obtained in pure water or ethanol solution were examined by the XRD patterns as shown in Figure 1. The intensities of the sharp diffraction peaks indicated that the product has good crystallinity. All of the diffraction peaks in the XRD patterns can be indexed as a pure hexagonal phase of CoCO3 with the following cell constants: a ) 4.661 Å, and c ) 14.96 Å (JCPDS card 78-0209). A mixture of the irregular and nearly spherical microspheres
