Synthesis and optical property of one-dimensional spinel ZnMn2O4 nanorods
© Zhang et al; licensee Springer. 2011
Received: 23 December 2010
Accepted: 11 April 2011
Published: 11 April 2011
Spinel zinc manganese oxide (ZnMn2O4) nanorods were successfully prepared using the previously synthesized α-MnO2 nanorods by a hydrothermal method as template. The nanorods were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-Vis absorption, X-ray photoelectron spectroscopy, surface photovoltage spectroscopy, and Fourier transform infrared spectroscopy. The ZnMn2O4 nanorods in well-formed crystallinity and phase purity appeared with the width in 50-100 nm and the length in 1.5-2 μm. They exhibited strong absorption below 500 nm with the threshold edges around 700 nm. A significant photovoltage response in the region below 400 nm could be observed for the nanorods calcined at 650 and 800°C.
Spinel is an important class of mixed-metal oxides, which has the general chemical composition of AB2O4. In recent years, mixed transition-metal oxides with spinel structure have attracted much attention, owing to their various properties such as photocatalytic [1–4], electrochemical performance , magnetic [6, 7] properties, or being used as lithium ion batteries . Mn-doped ZnO has also aroused lots of interest because it had been predicted to be a room-temperature diluted magnetic semiconductor , which was later verified by experiments. Therefore, the Mn-Zn-O ternary systems belong to a class of interesting and useful materials in terms of their electrical and magnetic properties.
As one of the important mixed transition-metal oxides with spinel structure, ZnMn2O4 is a promising functional material and has become the focus of various researches owing to its potential applications. ZnMn2O4 could be used for the negative temperature coefficient thermistors on account of their unique electrical properties . Ferrari and the coworkers studied the catalytic activity of zinc manganite for the reduction of NO by several types of hydrocarbons [4, 11]. Those authors suggested that ZnMn2O4 was an efficient catalyst for the reduction of NO to N2, and, in all cases, its best selectivity to N2 and CO2 was at almost the maximum conversion temperature.
The physical and chemical properties of nanomaterials would be strongly affected by their particle sizes and morphologies . At present, a tremendous amount of comprehensive investigations are under way into the unique applications of one-dimensional (1D) nanostructures involving nanorods, nanotubes, nanowires, and nanobelts because they provide a great opportunity to investigate the dependence of optical/electrical properties, thermal transport, and mechanical performance on the nano-scaled dimensionality and size [13, 14]. 1D nanomaterials, especially nanorods have displayed enhanced performances in many fields such as catalysts , sensors , solar cells , and so on.
ZnMn2O4 particles could be prepared earlier by various methods, such as solid-state reaction [17, 18], sol-gel , co-precipitation method , and hydrothermal method [20, 21]. For instance, Bessekhouad and Trari  prepared spinel ZnMn2O4 powder by solid-state reaction under high temperature. Zhang et al.  fabricated ZnMn2O4 nanoparticles by a hydrothermal method expending 48 h. Fan et al.  successfully synthesized 1D single-crystalline spinel MFe2O4 nanotubes/nanorings by thermal transformation process. In this study, we prepare the ZnMn2O4 nanorods successfully using the α-MnO2 nanorods as templates. The ZnMn2O4 nanorods obtained thus are well characterized, demonstrating their morphology, optical, and photoelectric properties.
All chemicals used in this study were analytic-grade reagents and used without further purification. At first, α-MnO2 nanorods were prepared by a hydrothermal approach. In a typical synthesis procedure, 0.8686 g KMnO4 and 1.8 mL concentrated HCl (37 wt%) were added to 100 mL deionized water to form the precursor solution, which was then transferred into a Teflon-lined stainless steel autoclave with a capacity of 120 mL. The autoclave was sealed and hydrothermally treated at 140°C for 12 h. After the autoclave was cooled down to room temperature naturally, the black precipitates were collected by centrifugation and washed with deionized water and anhydrous ethanol several times to remove possible impurities or excess ions. The as-prepared sample was then dried in air overnight.
To obtain ZnMn2O4 nanorods, the prepared α-MnO2 nanorods (0.0174 g) were placed in 20 mL distilled water at room temperature to form a mixed solution by constant stirring. A Zn(NO3)2 solution (2.5 mL, 0.1 M) was then added to the solution, which was continually stirred for 5 min. Then NaOH solution (40 mL, 0.1 M) was directly added to the above solution dropwise, and the resulting mixture was maintained for 30 min under continuous stirring. The base-treated products were washed with distilled water and anhydrous ethanol several times. Next, the products were all calcined at 500, 550, 600, 650, and 800°C for 2 h in air.
The crystal structures and microstructures of the products were characterized by X-ray diffraction (XRD) (D/MAX-2400). The chemical composition of the samples was crosschecked by energy dispersive X-ray spectroscope (EDS, JSM-5600LV). The SEM (Quanta 200 FEG), transition electron microscopy (TEM)/high-resolution transition electron microscopy (HRTEM) (JEOL JEM-2000EX), and selected area electron diffraction (SAED, Tecnai G220) were employed to observe the samples morphology. UV-Vis spectrophotometer (JASCO, UV-550) and a home-built surface photovoltage (SPV) measurement system based on a lock-in amplifier (Stanford, SR830) were employed to test the optical and photoelectric properties. X-ray photoelectron spectroscopy (XPS, PHI 5600) was employed to reveal the surface chemical composition of the products. The chemical compositions of the different products were determined using Fourier transform infrared spectroscopy (FTIR, VERTEX 70).
Results and discussion
XRD patterns of the calcined samples
SEM images and elemental analysis of the nanorods
TEM and HRTEM images of the ZnMn2O4 nanorods
The UV-Vis diffuse reflectance spectra of the ZnMn2O4 nanorods
XPS spectra of the ZnMn2O4 nanorods
SPV spectra of the ZnMn2O4 nanorods
IR spectra of the ZnMn2O4 nanorods
In summary, one-dimensional spinel ZnMn2O4 nanorods were successfully fabricated using the α-MnO2 nanorods as template. The ZnMn2O4 nanorods mainly grew along the (211) crystalline plane with the width in 50-100 nm and the length in 1.5-2 μm. The optical band gap energies of the nanorods calcined at 500°C, 650°C, and 800°C were respectively estimated to be 1.2, 1.34, and 1.45 eV. As the calcination temperature increased, they presented with much improved crystallinity and photoelectric response. The simple method for preparing the ZnMn2O4 nanorods reported here could also be utilized to fabricate other manganates.
energy dispersive X-ray spectroscope
Fourier transform infrared spectroscopy
high-resolution transition electron microscopy
selected area electron diffraction
transition electron microscopy
X-ray photoelectron spectroscopy
This study was supported financially by the National Nature Science Foundation of China (No. 20877013, NSFC-RGC 21061160495), the National High Technology Research and Development Program of China (863 Program) (No. 2007AA061402), the Major State Basic Research Development Program of China (973 Program) (No. 2007CB613306), and the Excellent Talents Program of Liaoning Provincial University (LR2010090).
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