Synthesis and optical property of one-dimensional spinel ZnMn2O4 nanorods

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.


Introduction
Spinel is an important class of mixed-metal oxides, which has the general chemical composition of AB 2 O 4 . In recent years, mixed transition-metal oxides with spinel structure have attracted much attention, owing to their various properties such as photocatalytic [1][2][3][4], electrochemical performance [5], magnetic [6,7] properties, or being used as lithium ion batteries [8]. Mndoped ZnO has also aroused lots of interest because it had been predicted to be a room-temperature diluted magnetic semiconductor [9], 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, ZnMn 2 O 4 is a promising functional material and has become the focus of various researches owing to its potential applications. ZnMn 2 O 4 could be used for the negative temperature coefficient thermistors on account of their unique electrical properties [10]. 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 ZnMn 2 O 4 was an efficient catalyst for the reduction of NO to N 2 , and, in all cases, its best selectivity to N 2 and CO 2 was at almost the maximum conversion temperature.
The physical and chemical properties of nanomaterials would be strongly affected by their particle sizes and morphologies [12]. 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 [15], sensors [9], solar cells [16], and so on. ZnMn 2 O 4 particles could be prepared earlier by various methods, such as solid-state reaction [17,18], solgel [19], co-precipitation method [18], and hydrothermal method [20,21]. For instance, Bessekhouad and Trari [18]

Synthesis
All chemicals used in this study were analytic-grade reagents and used without further purification. At first, α-MnO 2 nanorods were prepared by a hydrothermal approach. In a typical synthesis procedure, 0.8686 g KMnO 4 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 ZnMn 2 O 4 nanorods, the prepared α-MnO 2 nanorods (0.0174 g) were placed in 20 mL distilled water at room temperature to form a mixed solution by constant stirring. A Zn(NO 3 ) 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.

Characterizations
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).

XRD patterns of the calcined samples
The crystallinities and phase purities of the ZnMn 2 O 4 nanorods calcined were examined by powder XRD. Figure 1 shows the XRD patterns of the samples after calcination at different temperatures and as per the standard data card about the ZnMn 2 O 4 . After calcination at 650°C, all the XRD peaks of the calcined samples could be indexed to cubic ZnMn 2 O 4 with spinel structure (JCPDS file No. , where the diffraction peaks at 2θ values of 29.3°, 33.03°, 36.3°, 59.01°, 60.77°, and 65.1°are ascribed to the reflection of (112), (103), (211), (321), (224), and (400) planes of the spinel ZnMn 2 O 4 , respectively. No peaks from other phases are detected, indicating high purity of the products. However, in the products which were calcined at 500°C, the diffraction peaks of α-MnO 2 and ZnO could still be seen clearly, implying that the precursor was not converted into spinel ZnMn 2 O 4 completely even at such a calcination temperature. As for the sample treated at 800°C, two peaks of α-MnO 2 preferential growth along (110) and (220) crystal faces at the surface of ZnMn 2 O 4 nanorods were detected again because of partial decomposition of ZnMn 2 O 4 . Meanwhile, the peaks of α-MnO 2 and ZnO could also be seen unambiguously when the precursors were calcined at 550 and 600°C (see Figure S1 in Additional file 1). These phenomena indicate that the formation of ZnMn 2 O 4 began at the calcination temperature of about 650°C. On increasing the calcination temperature from 500 to 800°C, the peak widths of the ZnMn 2 O 4 nanorods become narrower, and the relative intensities of the characteristic peaks increase gradually, which implies that the spinel ZnMn 2 O 4 crystallites were growing and transforming from the disordered to a more ordered structure [23].

SEM images and elemental analysis of the nanorods
The general morphology of these nanorods was observed using scanning electron microscopy (SEM). Figure 2a displays an overview image of the α-MnO 2 nanorods. After coating Zn(OH) 2 onto the α-MnO 2 nanorods, the precursor of ZnMn 2 O 4 nanorods is obtained. The SEM images in Figure 2b show that the morphology does not change with the rod width in the range of 50-100 nm and the length in the range of 1.5-2 μm. The morphology of the ZnMn 2 O 4 obtained by calcination at 650°C of the precursor is shown in Figure 2c. It also displays a similar morphology to the MnO 2 nanorods. The EDS characterization at the same position was carried out. As shown in Figure 2d, it is clear that the nanorods consist of Zn, Mn, and O and the elemental ratio of Zn to Mn is determined to be about 1:2.

TEM and HRTEM images of the ZnMn 2 O 4 nanorods
The TEM and HRTEM were employed to further investigate the morphology of these nanocrystalline ZnMn 2 O 4 superstructures calcined at 650°C (Figure 3). TEM image confirms the nanocrystalline feature of the ZnMn 2 O 4 particles (Figure 3a,b), in which the individual rod width is in the range of 50-100 nm. Figure 3c shows the HRTEM image of ZnMn 2 O 4 nanorods. The inset in Figure 3c is the enlargement of the selected area, indicating the good crystallization of the nanoparticles. The fringes of d = 0.246 nm match that of the (211) crystallographic plane of ZnMn 2 O 4 , which is the strongest crystallographic plane, corresponding to the distance between (211) crystal planes of the spinel phase of zinc manganese oxide (JCPDS file No. 24-1133). Figure 3d is the SAED image of the ZnMn 2 O 4 nanorods, which shows the (211), (224), and (321) crystal planes clearly.

The UV-Vis diffuse reflectance spectra of the ZnMn 2 O 4 nanorods
The optical property of the ZnMn 2 O 4 nanorods can be observed by the UV-Vis diffuse reflectance spectroscopy. Figure 4a shows the UV-Vis diffuse reflectance spectra of the ZnMn 2 O 4 nanorods calcined at different temperatures. All the samples have strong absorption between 300 and 500 nm, and the absorption edges of these samples are all around 700 nm. The optical band gaps could be determined from the curves of (αhυ) n versus hυ, α being the optical absorption coefficient. The exponent n equals 1/2 for indirectly allowed and 2 for directly allowed transitions (Figure 4). Figure 4b is the (αhυ) 2 -hν curves for the ZnMn 2 O 4 nanorods calcined at different temperatures. The optical band gaps are calculated as 1.2, 1.34, and 1.45 eV for the samples calcined at 500, 650 and 800°C, respectively. No linear relation was found for n = 1/2, suggesting that the prepared ZnMn 2 O 4 may be a semiconductor allowing direct transitions at these energy levels [3].

XPS spectra of the ZnMn 2 O 4 nanorods
In ZnMn 2 O 4 , Mn and Zn atoms exist in the samples with more than one chemical state (A-sites or B-sites), bringing about several contributions with different binding energies in the XPS. Therefore, XPS was employed to reveal the surface chemical compositions of the nanocrystalline ZnMn 2 O 4 superstructures obtained from the calcinations of the precursor at 650°C (Figure 5). Figure 5a shows the survey spectra of the nanocrystalline ZnMn 2 O 4 . Elements of Zn, Mn, O, and adventitious C existed in the ZnMn 2 O 4 . The carbonate species adsorbed on the surface have appeared from the calibration for XPS instrument itself. As shown in the spectrum in Figure 5b, the peaks of 654.4 and 642.4 eV can be attributed to Mn 2p 1/2 and Mn 2p 3/2 , respectively. Figure 5c shows the Zn 2p peaks at binding energies of 1044.8 and 1021.6 eV. This reveals the oxidation state of Mn 3+ in the sample. Meanwhile, O 1s spectra of ZnMn 2 O 4 were also recorded (Figure 5d). The broad peak of O 1s can be fitted by two peaks at binding energies of 531.8 and 530.2 eV. The stronger peak at 530.2 eV is ascribed to the characteristics of oxygen in metal oxide, and the other peak at around 531.8 eV suggests the presence of other components, such as OH, H 2 O, and carbonate species adsorbed on the surface. In addition, XPS analysis confirms that the ratio of Zn to Mn in the sample is very close to 1:2, which is in good agreement with the formula of ZnMn 2 O 4 .
SPV spectra of the ZnMn 2 O 4 nanorods SPV method, a well-established technique for the characterization of photoelectric materials, can be used to investigate their photoelectric properties [24]. Figure 6 presents the obtained SPV amplitude spectra for the ZnMn 2 O 4 nanorods calcined at 500, 650, and 800°C and the schematic of the AC photovoltaic cells. As the calcined temperature goes on increasing, the SPV response intensities become stronger and stronger because the spinel ZnMn 2 O 4 nanorods become well ordered and the crystallinities enhance. For the ZnMn 2 O 4 nanorods calcined at 650 and 800°C, a significant SPV response band could be observed in the region 300-400 nm, which is directly related to the free charge carriers induced by the incident light. However, only a very small response can be observed about the product calcined at 500°C because of poor crystallinity of ZnMn 2 O 4 as well as the existence of α-MnO 2 , which almost cannot produce SPV response.

IR spectra of the ZnMn 2 O 4 nanorods
Infrared (IR) spectroscopy could provide plentiful information on the molecular structure and chemical bonding, which enables the characterization and identification of chemical species. Herein, we employed IR spectroscopy to comparatively analyze and identify the material compositions of different products. The IR spectra of the prepared samples treated at 500, 650, and 800°C are presented in Figure 7. The spectrum of the precursor calcined at 500°C shows a band at 531 cm -1 , which is attributed to the Mn-O vibrations of MnO 2 . The bands belonging to ZnMn 2 O 4 nanorods are considerably weak, probably because of the co-existence of α-MnO 2 and ZnO. When the temperature reached 650°C, a strong band at 516 cm -1 and a weak one at 623 cm -1 related to spinel ZnMn 2 O 4 appear. Upon increasing the treatment temperature to 800°C, these bands become more intense and shift to higher wavenumbers, from 516 to 547 cm -1 and from 623 to 642 cm -1 , which reveals the formed spinel nanorods have achieved an improved crystallinity [25].

Conclusions
In summary, one-dimensional spinel ZnMn 2 O 4 nanorods were successfully fabricated using the α-MnO 2 nanorods as template. The ZnMn 2 O 4 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 ZnMn 2 O 4 nanorods reported here could also be utilized to fabricate other manganates.