Article pubs.acs.org/JPCC
Phase- and Size-Dependent Optical and Magnetic Properties of CoO Nanoparticles Xuemin He, Xueyin Song, Wen Qiao, Zhiwen Li, Xing Zhang, Shiming Yan, Wei Zhong,* and Youwei Du Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Jiangsu Provincial Laboratory for NanoTechnology and Department of Physics, Nanjing University, Nanjing 210093, P. R. China ABSTRACT: A simple pyrolysis method has been developed to synthesize microstructure-controlled CoO nanoparticles from cobalt acetylacetonate in oleylamine at or above 200 °C. XRD, SEM, and HRTEM analyses indicate that the cubic and hexagonal CoO nanoparticles with different morphologies, viz. spherical, quasi-cubic, and pyramidal, could be obtained via varying the precursor concentration, and the average size of hexagonal CoO nanoparticles increases with increasing reaction time or reaction temperature. XPS, TG-DTA, and FTIR analyses reveal that the as-synthesized nanoparticles are pure CoO with good thermal stability. Raman and UV−vis absorption spectra show that the optical properties of CoO nanoparticles are of obvious size effect, which revealed their characteristic feature. Whatever the crystal structure and particle shape are, the CoO nanoparticles with sizes of 33, 59, and 85 nm exhibit two band gaps, and the corresponding band gap differences are 1.84, 1.62, and 1.42 eV, respectively. The pure hexagonal CoO nanoparticles display complete room temperature paramagnetism, while the CoO nanoparticles that contain cubic phase show interesting magnetic behavior due to intrinsic antiferromagnetic structure and uncompensated surface spins, which were confirmed by VSM and ESR studies.
1. INTRODUCTION In the past decades, magnetic nanoparticles have been pursued intensively due to their importance in fundamental research and diverse technical applications. Compared with corresponding bulk materials, nanoparticles of transition metal oxide have a wide range of applications in ferrofluids for audio speakers, biosensors, magnetic storage media, battery materials, catalysts, powder compacts, targeted drug delivery, contrasting agents in magnetic resonance imaging, and alternatives to radioactivity because of their chemical stability and magnetic properties.1−4 Among them, cobalt monoxide (CoO) nanoparticles are of significant owing to their potential applications based on magnetic, catalytic, and gas-sensing properties.5−7 It is wellknown that the bulk CoO exhibits a rocksalt-type cubic structure, and it is antiferromagnetic (TN ≈ 298 K).8 When the particle size of CoO decreases to nanoscale, it has been gained special interest. So, as an example, it beats the superparamagnetic limit of small cobalt clusters by enclosing them with a shell of CoO,5 an effect due to the magnetic exchange coupling of ferromagnetic cobalt with antiferromagnetic CoO. Moreover, wurtzite-type hexagonal CoO has also been prepared and investigated as an “end member” of the solid solution Zn1−xCoxO,9 which is of particular interest as such transitionmetal-substituted semiconductors can exhibit ferromagnetism with up to high Curie temperatures.10,11 As can be seen from the above background, the particle size and crystal structure have important influences on the magnetic properties of CoO. Actually, in general, the electrical, optical, magnetic, thermal, mechanical, and catalytic properties of any particle depend decisively on its characteristics such as size, shape, and structure.12 Although there are a few reports on the © XXXX American Chemical Society
preparation of CoO particles, it is important to mention that pure CoO nanoparticles are even more difficult to be prepared because the surface of nanoscale CoO is oxidized easily. Therefore, there is still much work needing to be done in synthesizing pure CoO nanoparticles with controlled microstructures such as phase purity, average size, size distribution, crystallinity, and morphology. A number of techniques have been used for the production of CoO nanoparticles, such as the solvothermal method,13 organic method,14 and condensation method.15 To some extent, however, the CoO nanoparticles synthesized via these methods have unchanged crystal structure (rock-salt cubic structure) and poor thermodynamic stability. Recently, monodisperse MnO, FeO, and Ni nanoparticles have been prepared by the thermal decomposition of their corresponding metal acetylacetonates in oleylamine.16−18 In this process, oleylamine was used as both the medium and the stabilizing reagent. The organic media avoids the formation of oxide or hydroxide, and the use of organometallic precursors leads to particles with controlled size, shape, surface coordination, and crystallinity.19 Therefore, the above thermal decomposition method of organometallic salts in oleylamine is convenient and easy to handle. With respect to the research about magnetic properties of cubic CoO nanoparticles,20−26 they are conflicting. Flipse et al. have found that the large spin−orbit coupling of the electrons of the surface atoms can cause a change in magnetic ordering in the core of the CoO particles,23 while the ferromagnetic Received: December 23, 2014 Revised: March 6, 2015
A
DOI: 10.1021/jp5127909 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Table 1. Reaction Conditions and Structural Parameters of CoO Nanoparticles sample no.
precursor concn
reaction temp (°C)
crystal structure
lattice parameters (Å)
particle size (nm)
M1 M2 M3
1/15 1/30 1/60
reaction time (h) 1 1 1
220 220 220
fcc fcc/hcp hcp
a = 4.2586
59.20 33.48 85.05
M4
1/60
0.5
220
hcp
M5
1/60
2
220
hcp
M6
1/60
3
220
hcp
M7
1/60
1
200
hcp
M8
1/60
1
240
hcp
M9
1/60
1
260
hcp
a = 3.2490 c = 5.2092 a = 3.2481 c = 5.2103 a = 3.2492 c = 5.2112 a = 3.2511 c = 5.2081 a = 3.2506 c = 5.2085 a = 3.2489 c = 5.2119 a = 3.2505 c = 5.2094
52.42 102.25 127.05 49.24 110.20 148.96
15 min. During this time, an argon atmosphere was applied to the system to remove trace moisture and oxygen trapped in the reaction system, thereby giving a clear green solution. Under an Ar blanket, the solution was heated to 220 °C at a heating rate of 5 °C/min and kept at this temperature for 1 h. Then, the reaction mixture was cooled down to room temperature, and a green colloidal solution was obtained. The colloidal nanoparticles were separated upon the addition of ethanol and hexane, centrifuged, and washed using a mixture of ethanol and toluene solvent. At the end, the green products were dried under vacuum at 30 °C overnight. Herein, the obtained CoO nanoparticles sample is marked as M3. It is found that the shape and size of CoO nanoparticles can be governed by precursor concentration (that is, the molar ratio of Co(acac)3 and oleylamine). Therefore, the other two CoO samples (M2 and M1) could be produced by simply changing the concentration to 1/30 and 1/15, as shown in Table 1. What demand add is, the actual dissolved times (at 135 °C) of samples M1, M2, and M3 are 3 h, 30 min, and 15 min, respectively, and therefore resulting the nanopowders with colors of brown, hazel, and green, respectively. Sample Synthesis of Time and Temperature Sequences. Under similar reaction process, the precursor concentration remains unchanged at 1/60; the size of CoO nanoparticles can also be governed by reaction time and reaction temperature. As shown in Table 1, other three CoO samples (M4, M5, and M6) of time sequence were obtained at 220 °C for 0.5, 2, and 3 h, while other three CoO samples (M7, M8, and M9) of temperature sequence were obtained at 200, 240, and 260 °C for 1 h, respectively. It should be noted that the dissolved times (at 135 °C) of samples M4−M9 are the same 15 min and thereby produce all the green powders. 2.3. Characterization. The crystal structure was characterized by X-ray diffraction (XRD) using a Philips X’pert diffractometer with Cu Kα radiation. Quantitative analysis of the XRD data was undertaken with the MDI Jade 5.0 software. The particle shape was imaged with a FEI Sirion200 scanning electron microscope (SEM) operating at an accelerating voltage of 5 kV. According to the Nano Measurer software, the particle size D was obtained by calculating the number-average by manually measuring the equivalent diameter of >100 particles from SEM images. For transmission electron microscope (TEM) investigation, a drop of tested powder sample in
interaction in the small CoO nanoparticles (
