Size influence on optical absorption property of ultra-fine nickel hollow spheres
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Nickel hollow spheres (NHSs) with different diameter have been synthesized by the autocatalytic reduction method. The morphology, particle size distribution and optical absorption property of NHSs were investigated. The optical absorption intensity obviously increases in ultraviolet—near infrared region with the decrease of particle size. While in infrared region, nickel hollow spheres have almost no absorption. After the heat treatment process, the grain sizes of samples become bigger and the absorptances decrease in UV–Vis–NIR region. For smaller particles, the absorption peak in ultraviolet range moves from 375 to 440 nm because of the increase of grain size after heat treatment.
KeywordsHollow Sphere Heat Treatment Process Nickel Sulfate Photon Correlation Spectroscopy Hollow Particle
Ultra-fine hollow spheres have been extensively studied in recent years. These hollow particles exhibit novel properties (for example, low density, large specific surface area, good permeability, and interesting optical properties), which make them attractive from both scientific and technological viewpoints. Potential applications for such hollow particles are artificial cells, catalysts, protection of biologically active agents, delivery of drugs, cosmetics, inks and dyes [1, 2, 3, 4, 5, 6, 7]. Among the methods used to fabricate hollow spheres, the template-synthetic route is often employed. To date, the templates used mainly include polystyrene sphere [8, 9, 10], resin sphere , micelles [11, 12, 13], microemulsion droplets [14, 15, 16, 17], silica colloid spheres , macromolecular micelles  and mixed micelles of macromolecules and surfactants .
Although various hollow spheres have been prepared, few reports about optical property of hollow sphere were found [21, 22, 23]. In this paper, nickel hollow spheres with different size were synthesized by the autocatalytic reduction method . The morphologies, size distribution and optical absorption property of the NHSs were characterized. The influence of particle size on optical absorption property of the NHSs was discussed.
Based on the previous work of our group , NHSs were prepared by means of autocatalytic reduction method. During the preparation process, analytically pure NiSO4 · 6H2O, NaH2PO2 · H2O, NaOH, acetic acid and citric acid as starting materials were used. Firstly, 20 g nickel sulfate, 0.1 g acetic acid and 0.15 g citric acid were dissolved in de-ionized water to get 200 mL Ni2+ mixture solution. About 3 g sodium hydroxide and 25 g sodium hypophosphite were dissolved in 200 mL de-ionized water in beakers, respectively. All solutions were preheated at 80 °C for 5 min. Then the Ni2+ solution and alkali solution were mixed by violent stirring, followed by producing a viridescent colloid. And then the sodium hypophosphite solution was added to the as-prepared colloid by evenly stirring. Finally, one dark-gray powder was obtained after the reaction had taken place for 5 min, and the powder was repeatedly washed with ammonia and de-ionized water. The final products were dried in vacuum furnace at 100 °C for 2 min. By increasing the quantity of sodium hydroxide from 3 g to 6 g with the step of 1 g, four samples (sample A–D) with different size were prepared.
The morphologies of hollow spheres were characterized through field emission scanning electron microscope (FE-SEM) with FEI SIRION 200 microscope, operated at acceleration voltage of 5.0 kV. The samples were coated gold by sputtering for 5 min at 1.5 mA. Transmission electron microscopy (TEM) images were recorded on a Philips CM200FEG microscope operating at 50 kV. X-ray powder diffraction (XRD) patterns of the samples were obtained on a D/max 2550V X-ray diffractometer using Cu Kα radiation at 40 kV and 100 mA. A Zetasizer Nano S photon correlation spectroscopy (PCS) nanoparticle size analyzer and a CIS-100 particle size & shape analyzer were used to estimate the particle size distributions of samples. Optical absorption properties of Ni hollow spheres were measured with a Cary 500 ultraviolet–visible–near infrared (UV-Vis-NIR) spectrophotometer at the wavelength range of 250–2500 nm and a EQUINOX 55 Fourier transform infrared-Raman (FTIR) spectroscope at the wavelength range of 2.5–25 μm.
Results and discussion
Morphology and structure
Optical absorption properties
In ultraviolet region, the absorptances of sample A and B decrease with the increase of wavelength, which are different from that of sample C and D. An absorption peak appears at 375 nm for sample C and D, which may be caused by small size effect of nano-particles. Some noble metallic nano-particles (Au, Ag) showed a plasma-resonance absorption in UV–Vis region [26, 27]. Although particle size of sample C and D are far bigger than that of nano-particles reported in literatures, the thickness of shell are nano-scale. So the peak at 375 nm may be due to the surface plasmon resonance absorption.
In visible region, the absorptance of samples decreases as wavelength increases. But sample A and B have different character from sample C and D. As wavelength increases in visible region, the absorptance of the former decreases more quickly than that of the latter. In near infrared region, absorptions of all samples are obviously weaker than that in UV-Vis region. Three peaks at 1385 nm, 2049 nm and 2213 nm may be caused by vibration of P–Ni bond.
Effect of heat treatment
NHSs with different diameter have been synthesized by the autocatalytic reduction method. The morphologies, size distribution and optical absorption property of NHSs were characterized. The results indicate that when the hollow Ni spheres become smaller, the optical absorption intensity obviously increases in ultraviolet—near infrared region. While in infrared region, NHSs have almost no absorption. After the heat treatment process, the grain size of samples became bigger and the absorptances decrease in UV-Vis-NIR region. For smaller particles, the absorption peak in ultraviolet range moves from 375 to 440 nm because of the increase of grain size after heat treatment.
This work is supported by National Natural Science Foundation of China (Grant no. 50474004), Shanghai Science and Technology committee Nano Special Fund (Grant no. 0552nm004), the “Dawn” Program of Shanghai Education Commission and the New-Century Training Program Foundation for Talents from the Ministry of Education of China.