Introduction

The hydrothermal system simulating the mineral formation is adapted to synthesize one dimensional (1D) nanostructured materials with high crystallinity. 1D nanostructures of Co3O4 have aroused great interest due to its mixed valence states [1–3]. The 1D nanostructures could be achieved by many preparative strategies [4–8]. Co3O4 exhibited potential application in a wide variety of fields, such as battery materials [9–11], catalysis [12, 13], and magnetic materials [4, 8, 14]. It is now commonly known that the properties of nanomaterials strongly depend on the shape of particles, which is a dominating factor to their ultimate performance and applications [8, 15–18]. In this paper, a hydrothermal pathway was carried out to investigate the controlled formation of 1D Co3O4. The hydrothermal procedure was monitored combinationally by the techniques of XRD, TEM, and FT-IR in order to deeply understand the function of anions in the formation of 1D nanostructures.

Experimental

The typical preparation of 1D Co3O4 is described as follows: 5 × 10−3 mol Co(NO3)2·6H2O and 1.25 × 10−3 mol hexamethylenetetramine (HMT) were added into the 10 mL 5 × 10−3 mol/L sodium dodecylsulfate aqueous solution under ultrasonication. The solution was then transferred into the Teflon-lined stainless steel autoclave for hydrothermal treatment at 160°C for 6 h. The hydrothermally obtained anion-intercalated cobalt hydroxides were rinsed with deionized water and anhydrous alcohol three times and suffered calcination in air at 500°C for 6 h to produce the final products.

Thermogravimetric (TG) analysis and differential thermal analysis (DTA) were carried out in air for the precursor of Co3O4 with a Rigaku Thermplus TG8120 (Rigaku, Shibuya-ku, Japan) with a heating rate of 5°C/min. Powder X-ray diffraction (XRD) patterns of the samples were measured using a Rigaku γA Mini diffractometer with Cu Kα radiation (λ = 0.154178 nm). The morphologies were observed by transmission electron microscopy (TEM) (obtained using a JEM-100CXII electron microscope (JEOL, Akishima-shi, Japan)) and scanning electron microscopy (SEM) (carried out by JSM-6700F). Fourier transform infrared spectra (FT-IR) of samples formed in KBr platelets were recorded with an Avatar 370 FT-IR spectrometer (ThermoNicolet, Waltham, MA, USA) in the range of 400 to 4000/cm to investigate the crystalline form of products.

Findings

The TGA-DTA analysis for precursors suggests two steps of losing weight: the weight loss below 100°C is mainly due to dehydration of physically absorbed water, while the decomposition of NO3− and CO32− ions, together with the dehydration of OH groups between brucite layers of Co(OH)2 during which occurs a simultaneous conversion into spinel Co3O4, contribute to the greater weight loss observed between 200°C and 500°C. Correspondingly, two endothermic peaks are shown in the DTA curve. The precursors have transformed completely into Co3O4 above 500°C (Figure 1).

Figure 1
figure 1

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves of precursors obtained hydrothermally (180°C, 4 h).

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The precondition to obtain the 1D nanostructures of Co3O4 is synthesizing the 1D precursor. The molar ratio of starting reactants was found as an important parameter; therefore, some experiments were carried out by varying both Co(NO3)2 and HMT amounts. Only within a very narrow range of the molar ratio of [Co2+] to [(CH2)6N4] (2:1 ~ 4:1), the wirelike precursors of cobalt basic nitrate carbonate could be obtained. The precursors and final products calcined at 500°C were further characterized by TEM and SEM techniques. A typical TEM image of Figure 2a reveals that the precursors display almost perfect 1D morphologies with widths about 100 nm. The lengths are estimated about 3 to 10 μm. Apparently, the aspect ratios of these 1D nanostructures are notably large. In addition, the precursors have proved the copiousness in quantity, and the uniformity in size distribution. The calcined products exhibit rough surfaces, as shown in Figure 2b, c, suggesting the polycrystalline feature. The energy dispersive spectrum (EDS) of the selected field, which is white framed in Figure 2c, was measured for the 500°C-calcined products, and the result displayed in Figure 2d indicates the molar ratio of Co/O equal to 0.72, in good agreement with the theoretic value of Co3O4.

Figure 2
figure 2

The morphology of the precursor (a), the calcined products (b and c), and EDS (d). SAED of the calcined products is inset in (b), and EDS results white framed in (c) are also exhibited in (d).

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As shown in Figure 3, XRD, TEM, and IR techniques were employed combinationally in order to gain some insights on the function of anions in the formation of 1D Co3O4.

Figure 3
figure 3

Temporal evolution of XRD patterns, TEM images, and IR spectrum. For precursors taken out of the autoclave at different stages of hydrothermal treatment: (a) 20, (b) 60, and (c) 120 min.

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Reaction time dependence of XRD patterns for precursors exhibited a phase transformation from initial metastable α-Co(OH)2 to the thermodynamically stable phase, which showed a similarity with Co(CO3)0.35Cl0.2(OH)1.1·0.74H2O, implying that NO3− ions occupied the same position as Cl− ions in the NO3− intercalated cobalt carbonate hydroxide.

The phase transformations were accompanied by the morphology alterations. As shown in Figure 3, nanosheets were obtained after 20 min of hydrothermal treatment. When the holding time was prolonged to 1 h, 1D nanostructures with the length of 2 μm coexisted with the nanosheets. After the holding time increased above 2 h, only nanowires were obtained.

FT-IR technique is an effective way to investigate the kind of anions intercalated between the Co(OH)2 layers. The precursor α-Co(OH)2 taken from solution at 20 min has a large interlayer space, designated literally as a pre-hydrotalcite-like phase as confirmed by IR and XRD. Water bending mode is also found at 1,601/cm, about 30/cm red shift due to strong interactions with the interlayer anions of NO3− and/or CO32−. In agreement with the structural assignment, absorption peaks at 1,384 and 839/cm (ν3 and ν2 vibration modes of NO3− with D3h symmetry) indicate that a large number of NO3− anions are included [19, 20]. With increasing the holding time, the intensity of the peak at 1,384/cm decreased obviously, and the ν2 vibrational modes appear at 830/cm, apparently lower than NO3− with D3h symmetry, indicating an increment in the perturbation of the intercalated NO3−. Simultaneously, two peaks appear at 1,480 and 1,312/cm, which can be assigned to symmetric vibrational mode (νs(ONO2)) and asymmetric vibrations (νas(ONO2))on monodentate nitrate, respectively. In addition, the appearance of a tiny peak at 1,312/cm suggests that the amount of NO3− is significantly reduced [19, 21]. The IR spectra for sample prepared after the holding time above 2 h shows two more absorptions at 1,506 and 1,035/cm, which belong to the ν3 and ν1 vibrational modes of carbonate anions with C2V symmetry. Compared with the data from Xu and Zeng [20], the two absorptions exhibited blue and red shifts, respectively. The perturbation can be attributed to stronger electrostatic interactions between the divalent CO32− anions and the brucite-like sheets.

In summary, the probable procedure for the formation of 1D Co3O4 could be described as follows: initially, a large amount of NO3− ions intercalated between the brucite layers, adopting D3h symmetry. The amount of CO32− gradually produced by hydrolysis and oxidation of HMT increased, leading to the substitution for some of NO3− ions and the alteration of symmetry from D3h to C2v. Consequently, the modification of interlayer forces resulted in the instability of intralayer interactions, and the two-dimensional nanosheets eventually transformed to 1D nanostructures. The morphology was preserved in calcination to produce 1D Co3O4.