A simple additive-free approach for the synthesis of uniform manganese monoxide nanorods with large specific surface area

A simple additive-free approach is developed to synthesize uniform manganese monoxide (MnO) one-dimensional nanorods, in which only manganese acetate and ethanol were used as reactants. The as-synthesized MnO nanorods were characterized in detail by X-ray diffraction, scanning and transmission electron microscopy (TEM) including high-resolution TEM and selected-area electron diffraction, Fourier transform infrared spectrum, and nitrogen adsorption isotherm measurements. The results indicate that the as-synthesized MnO nanorods present a mesoporous characteristic with large specific surface area (153 m2 g−1), indicating promising applications in catalysis, energy storage, and biomedical image. On the basis of experimental results, the formation mechanism of MnO one-dimensional nanorods in the absence of polymer additives was also discussed.


Background
During the past decade, manganese oxides have attracted considerable research interest due to their distinctive physical and chemical properties and potential applications in catalysis, ion exchange, molecular adsorption, biosensor, and energy storage [1][2][3][4][5][6][7][8][9][10][11][12]. Particularly, nanometer-sized manganese oxides are of great significance in that their large specific surface areas and small sizes may bring some novel electrical, magnetic, and catalytic properties different from that of bulky materials. A wide variety of manganese oxides (e.g., MnO 2 , Mn 2 O 3 , and Mn 3 O 4 ) have been synthesized through various methods [13][14][15][16][17][18][19][20][21][22][23][24]. Among them, manganese monoxide (MnO) is a model system for theoretical study of the electronic and magnetic properties of rock salt oxides [25], and its nanoclusters interestingly exhibit ferromagnetic characteristics [26]. On the other hand, MnO is very interesting for its lower charge potential (1.0 V vs. Li/Li + ) compared to other transition metal oxides [27]. It has been reported that a relatively high voltage and energy density can be obtained when it was coupled with a certain cathode material to construct a full lithium ion cell [28].
In the present work, we report a simple, cost-effective, and additive-free method for the synthesis of uniform MnO nanorods with large specific surface area, in which cheap manganese acetate and ethanol were used as starting materials. The microstructures of the as-synthesized products were investigated using scanning electron microscopy (SEM) and transmission electron microscope (TEM). The as-synthesized MnO nanorods present a mesoporous characteristic and large specific surface area. More importantly, we have avoided the use of expensive polymer or surfactant additives during the synthesis process. The possible formation mechanism for MnO nanorods in the absence of polymer additives was also discussed.

Preparation of MnO nanorods
In a typical synthesis, 1.0 g of manganese acetate was put into 30 mL of anhydrous ethanol distilled freshly to form a homogeneous solution under stirring. The solution was transferred to a 40-mL Teflon-lined stainless steel autoclave. These manipulations were operated in a glove box under N 2 atmosphere. The autoclave was heated at 200°C for 24 h in an electric oven. After cooling to room temperature, the final products were washed with deionized water and ethanol several times and subsequently dried at 80°C for 6 h in vacuum.

Instruments and characterization
The phase purity of the obtained samples was examined by X-ray diffraction (XRD) using an MSAL-XD2 X-ray diffractometer with CuKα radiation (λ = 0.15406 nm) operating at 40 kV and 20 mA. Morphologies of the samples were characterized by field emission scanning electron microscopy (JSM6700F). The morphology and structure of the MnO nanorods were further investigated by TEM and high-resolution transmission electron microscopy (HRTEM; JEM-2010, 200 kV) with energydispersive X-ray spectroscopy (EDS; INCA X200). X-ray photoelectron spectroscopy (XPS) was carried out by means of a Shimadzu AXIS UTLTRADLD spectrometer (Shimadzu, Kyoto, Japan). Nitrogen adsorption-desorption measurements were performed using a Micromeritics Tristar 3000 gas adsorption analyzer (Micromeritics Instrument Co., Norcross, GA, USA). Fourier transform infrared (FTIR) spectrum was measured by an Equinox 55 (Bruker, Ettlingen, Germany) spectrometer ranging from 400 to 4,000 cm −1 . The morphology of the as-prepared sample was examined by SEM and TEM. Figure 2a shows a typical SEM image of MnO nanorods synthesized at 200°C for 24 h, revealing that the product displays a uniform nanorodlike morphology. It can be observed that the nanorod is composed of small NPs, and the coarse surface of the nanorod can also be seen, as shown in Figure 2b.  Figure 2e shows an enlarged TEM image, revealing the porous character of the nanorods. Figure 2f depicts an HRTEM image of one single nanorod, revealing that the obtained nanorod consists of small nanoparticle subunits. As shown in the inset of Figure 2f, the selected-area electron diffraction (SAED) pattern with polycrystalline-like diffraction also indicates that the nanorod is an ordered assembly of small nanocrystal subunits without crystallographic orientation, well consistent with the HRTEM results.

Results and discussion
The chemical composition of the as-prepared MnO nanorods was further confirmed by EDS analysis. The spectrum, taken from the center area of the nanorod, shows four strong signals of Mn, C, O, and Cu ( Figure 3). The atomic ratio of Mn and O is about 1.02, indicating that the as-prepared nanorods are consist of high-purity MnO rather than other manganese oxides (e.g., Mn 2 O 3 , Mn 3 O 4 , and MnO 2 ), in good agreement with the XRD results. The Cu and O may have resulted from the Cu gridding and C support membrane in the TEM observation.
The FTIR spectrum was further performed to substantiate the formation of MnO and the organic residue on the   [43], indicating the formation of MnO in the present work. In addition, strong absorptions at 3,442 cm −1 and weak absorptions around 2,800 to 3,000 cm −1 reveal the stretching vibrations of O-H and C-H, respectively. The absorption peak at 1,112 cm −1 corresponds to the C-OH stretching and OH bending vibrations, whereas the bands at 1,385, 1,580, and 1,636 cm −1 correspond to C-O (hydroxyl, ester, or ether) stretching and O-H bending vibrations [44,45]. These results indicate that some organic residues such as hydroxyl and carboxyl groups are present on the surface of the as-prepared MnO nanorods.
The presence of the residue functionalities on the surface of the as-synthesize MnO nanorods was further characterized by XPS measurements. As shown in Figure 5,  the survey spectrum shows the signals of Mn 2p, O 1s, and C 1s, indicating the presence of carbon element on the surface the nanorods. The presence of the organic groups was further confirmed by the C 1s spectrum. The inset in Figure 5 presents the C 1s core-level spectrum and the peak fitting of the C 1s envelope. Four signals at 284.8, 286.4, 287.6, and 288.8 eV were identified, which were attributed to carbon group (C = C/C-C, CH x ), hydroxyl groups or ethers (−C-OR), carbonyl or quinone groups (>C = O), and carboxylic groups, esters, or lactones (−COOR), respectively. These results also reveal the presence of organic functional groups on the surface of the nanorods, in good agreement with the FTIR results.
The porous characteristic of the as-synthesized MnO nanorods was examined by nitrogen adsorption isotherm measurements. The specific surface area and pore size distribution (PSD) of the MnO nanorods were obtained from an analysis of the desorption branch of the isotherms using the density function theory. As shown in Figure 6, an isotherm is typical for a mesoporous material with a hysteresis loop at high partial pressures. According to the Brunauer-Emmett-Teller analysis, the as-synthesized MnO nanorods exhibited large specific surface area of ca. 153 m 2 g −1 and pore volume of ca. 0.22 cm 3 g −1 . The inset in Figure 6 shows the Barrett-Joyner-Halenda PSD curve that was centered at ca. 3.9 nm, suggesting that the MnO nanorods possess uniform mesoporous structures.
To investigate the formation mechanism of the MnO nanorods, a series of time-dependent experiments were carried out. As shown in Figure 7a, numerous amorphous manganese precursor NPs with size of ca. 5 to 6 nm were observed when the reaction was executed for 1 h. Figure 7b shows that larger NPs with size of ca. 20 to 30 nm were formed when the reaction time was increased to 3 h. The inset in Figure 7b reveals that the lattice fringe is ca. 0.36 nm, consistent with the d 012 spacing for rhodochrosite MnCO 3 , indicating that the transformation from manganese precursor to MnCO 3 happened in the earlier stage. When the reaction time was increased to 6 h, many nanorod-like particles could be obtained besides dispersed NPs (Figure 7c). It can also be seen that the nanorod-like products were formed by the self-assembly of small NPs. Figure 7d shows an HRTEM image taken from two adjacent NPs. The lattice fringes were found to be ca. 0.36 and 0.26 nm, corresponding to the d 102 spacing for rhodochrosite MnCO 3 and the d 111 spacing for cubic MnO, respectively, suggesting that the transformation from MnCO 3 to cubic MnO was incomplete within a short time. When the reaction time was further increased to 12 h, a large number of nanorods were formed (Figure 7e). Figure 7f shows an HRTEM image of one nanorod aggregated by small nanocrystals, and the boundary can be observed among the NPs. The SAED pattern in the inset of Figure 7f presents a polycrystalline character of the nanorods,   indicating that the nanorod is of an ordered assembly of nanocrystals without crystallographic orientation.
On the basis of the above experimental results, the possible formation mechanism of the MnO one-dimensional nanorods in the present work was proposed, as schematically illustrated in Figure 8. Firstly, the reaction between manganese acetate and ethanol results in the formation of certain alcohol acetate complexes, e.g., CH 3 COOMnOC 2 H 5 , accompanied with the nucleation and growth of amorphous precursor NPs, which are then transformed into MnCO 3 nanocrystals (step 1). Secondly, with the increase of reaction time, the MnCO 3 precursor is decomposed into MnO nanocrystallites (step 2). Meanwhile, the generated MnO nanocrystallites are capped by  the short C-chain molecules forming oxide-organic hybrids, which act as build blocks to form novel MnO nanostructures. When two MnO building blocks come together, the capillary force between them facilitates the solvent removal and strengthens the agglomerate by van der Waals forces. Finally, with the increase of reaction time, directed self-assemblies of the oriented nanocrystallites and subsequent fusion lead to the formation of the MnO onedimensional nanorods (step 3).

Conclusions
In summary, uniform mesocrystalline MnO nanorods were prepared successfully by using manganese acetate and ethanol as starting materials. The as-synthesized MnO nanorods exhibited uniform morphology, large specific surface area, and narrow pore size distribution. The simple, cost-effective, and environmentally friendly synthesis can be scaled up to produce large quantities of porous MnO one-dimensional nanorods. Owing to their large specific surface area, the as-prepared MnO nanorods may have promising applications in energy storage, catalysis, and biomedical image. This method may also open a new avenue for the simple synthesis of porous functional materials with applications in the fields of energy and environment.