Introduction

Control over the composition, shape, spatial location and/or geometrical configuration of functional nanostructures is of importance for nearly all practical applications of these materials [1–6]. One-dimensional (1D) nanostructures such as nanorods, nanowires, nanobelts and nanotubes with high aspect ratios have been attracting much attention lately due to their high potential applications in electronic device fabrications, sensors, etc. [7–15]. Recently, it has been demonstrated that several new geometrical configurations such as nanosprings [16], nanorings[16, 17], and nanohelices [18, 19] grown from 1D nanobelts or nanowires are of special interest owing to their unique periodic and elastic properties resulting in structural flexibility that provides additional opportunities for nanoengineering. The ability to construct artificial ringlike building units has implications in the rational design of complex nanostructures for precise nanofabrication. Although much progress has been made in the synthesis of nanorings by coiling of nanobelts, all these successes are based on materials with polar surfaces [16–19]. Here, we demonstrate a new mechanism of forming nanorings by coiling of nanobelts, i.e., cation-induced asymmetric strain on top and bottom surfaces of nanobelts. Different from previous work based on polar surfaces, our designed strategy can be applied to structures without anion- and cation-terminated surfaces.

As a well-known transition-metal oxide, vanadium pentoxide (V2O5·x H2O, normally, x falls in the range of 0–3) has been extensively studied due to its applications in the field of lithium–ion batteries [20], electric field-effect transistors [21], actuators [22], catalysis [23], and sensors [24]. Various 1D nanostructures of V2O5·x H2O such as nanorods [25], nanowires [25] and nanotubes [26, 27] have already been synthesized. However, it is a big challenge to rationally design a synthetic route for fabrication of vanadium pentoxide nanorings. In this work, V2O5·x H2O single-crystalline nanorings and microloops were successfully synthesized in the designed hydrothermal system.

Experimental Details

Synthesis

V2O5·x H2O nanorings and microloops were synthesized in a hydrothermal process. In a typical process, 0.2–0.4 g ammonium metavanadate (NH4VO3) was dissolved in distilled water (20–40 ml) to form a light yellow clear solution. Then 0.5–1 g magnesium nitrate was added to this solution under vigorous stirring. Afterward, nitric acid (5 mol/l) was added drop-wise under stirring until the final pH of the solution was 3–5. A clear orange solution was formed, which was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 160–190°C for 20–40 h. After the solution was cooled to room temperature, the obtained solid products were collected by centrifuging the mixture, which were then washed with absolute ethanol and distilled water several times and dried at 60–80°C for 4–8 h for further characterization. The anhydrous V2O5 nanostructure was obtained by annealing V2O5·x H2O nanorings and microloops in air under atmospheric pressure at 500°C for 2 h.

Characterization

The collected products were characterized by an X-ray diffraction (XRD) on a Rigaku-DMax 2400 diffractometer equipped with the graphite monochromatized Cu Ka radiation flux at a scanning rate of 0.02°s−1 in the 2θ range of 5–60°. Scanning electron microscopy (SEM) images were taken with a JEOL-5600LV scanning electron microscope, using an accelerating voltage of 20 kV. Energy-dispersive X-ray spectroscopy (EDS) microanalysis of the samples was performed during SEM measurements. The structure of V2O5·x H2O was investigated by transmission electron microscopy (TEM, Philips, TecnaiG2 20, operated at 200 kV). To analyze the water content in the sample, the sample was investigated using a thermogravimetric analyzer (Mettler Toledo TGA/SDTA851e), in flowing N2 and at a heating rate of 10°C/min. UV–Vis adsorption spectra were recorded on UV–Vis–NIR spectrophotometer (JASCO, V-570).

Results and Discussion

Typical XRD pattern of V2O5·x H2O nanorings and microloops displays a set of peaks characteristic of (00 l) reflections for the layered structure V2O5·x H2O (Fig. 1a), which is consistent with the reported data [28]. After calcining the products at 500°C for 2 h, XRD pattern (Fig. 1b) can be indexed as the orthorhombic V2O5 (space group Pmm a = 11.516, b = 3.566, c = 3.777 Å, JCPDS card no. 41-1426). EDS analysis was also used to determine the chemical composition of an individual V2O5·x H2O nanoring and microloop (Fig. 1c). The result shows that these nanorings and microloops contain only V and O elements. In order to analyze the water content in the current sample, TGA was carried out on them in N2. The TGA curve is shown in Fig. 2, which shows the as-obtained sample has two dehydration processes. The TGA plot shows a weight loss of 15% up to 600°C, which is related to the procedure of dehydration. The weight loss of 5.2% below 200°C can be attributed to the release of water absorbed on the sample, and the weight loss of 9.8% in the range of 160–600°C is believed to correspond to the release of water in the crystals [29]. Based on the XRD pattern, EDS spectrum and TGA analysis, the composition of the as-prepared products is V2O5·1·1H2O.

Figure 1
figure 1

XRD patterns of a V2O5·x H2O nanorings and microloops and b anhydrous V2O5 after annealing at 500°C for 2 h. c EDS pattern of the V2O5·x H2O nanorings and microloops

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Figure 2
figure 2

TGA curve of the as-synthesized V2O5·x H2O nanorings and microloops. XRD, EDS and TGA analyses indicate that the composition of the as-prepared products is V2O5·1·1H2O

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The morphology and structure of the products was observed by means of SEM and TEM. A low-magnification SEM image shows that dominant components of the as-synthesized products are nanobelts with the length ranging from several tens to several hundred micrometers, but a significant number of nanobelts have ring or loop shape (Fig. 3a). High-magnification SEM images show that there are three types of nanorings and microloops (1) circular rings and microloops (Fig. 3b, 3c, 3f, 3g), (2) trigonal rings and microloops (Fig. 3d, 3h), and (3) tetragonal rings and microloops (Fig. 3e, 3i). It can be seen that all these V2O5·x H2O nanorings have a perfect shape of complete rings with the smooth and flat surface (more nanorings and microloops are shown in Fig. 4). Detailed observation reveals that the circular nanorings and microloops have a diameter of 3–10 μm, coiling of nanobelts with the thickness of 20–30 nm and width of 300–500 nm.

Figure 3
figure 3

Low-magnification SEM image (a) and representative high-magnification SEM images (b–i) of V2O5·x H2O structures. The type (1) circular nanorings and microloops (b, c, f, g), the type (2) trigonal nanorings and microloops (d, h), and the type (3) tetragonal nanorings and microloops (e, i). Scale bars correspond to 20 μm (a) and 1 μm (b–i)

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Figure 4
figure 4

SEM images of the as-systhesized V2O5·x H2O nanorings and microloops. Scale bars correspond to 1 μm

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TEM and selected-area electron diffraction (SAED) provide further insight into microstructural details of these interesting structures. Figure 5a shows TEM image of one V2O5·x H2O single-crystalline nanoring with a diameter of about 3 μm. The corresponding SAED pattern shown in the inset shows the single-crystalline nature of the whole nanoring. Figure 5b is an enlargement of area marked in Fig. 5a, which clearly shows the complete nanoring is made by coiling of single-crystalline nanobelts. A high-resolution TEM image shows that the nanoring is single crystalline (Fig. 5c). The measured lattice shows that the resolved interplanar distance d = 2.1 Å can be determined as the [010] growth direction of these nanobelts [30].

Figure 5
figure 5

TEM characterizations of V2O5·x H2O nanorings. a TEM image of a single V2O5·x H2O nanoring. The inset is SAED pattern of a nanoring indicating the single-crystalline nature of the whole nanoring. b Enlargement of area marked in a showing the coiling of nanobelt to form the nanoring. c HRTEM image taken from a V2O5·x H2O nanoring. Scale bars correspond to 500 nm (a) and 100 nm (b)

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For the synthesis of inorganic nanorings by coiling of nanobelts, a polar surface–driven kinetic model has ever been proposed [16–19], and geometrically, the cation- and anion-terminated surfaces are the characteristic structure of these target compounds. To minimize the electrostatic interaction energy among the polar charges, the nanobelt dominated by polar surfaces tends to fold over, resulting in the formation of single-crystalline nanorings. Different from the previous work, the as-synthesized V2O5·x H2O is a layered structure material without a pair of positively and negatively charged polar surfaces in our process. The structure of V2O5·x H2O can be well described as stacking of well-defined bilayers of single V2O5 layer made of squarepyramidal VO5 units with water molecules residing between them along its c-axis (Fig. 6a) [28]. By projecting the structure model along the [100] direction, the ± (001) planes are terminated solely with negative O2− (Fig. 6b).

Figure 6
figure 6

Crystal structure and simulated morphology of V2O5·x H2O. a Monoclinic structure model of V2O5·x H2O. b The structure model of V2O5·x H2O projected along [100] direction or a-axis, displaying the structure of stacking bilayers of single V2O5 layer along c-axis. Water molecules are shown in green. c Simulated thermodynamic equilibrium morphology of V2O5·x H2O via the chemical bonding theory of single crystal growth

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The driven force for nanorings and microloops formation in our designed process is Mg2+-induced asymmetric strain on the top and bottom surfaces of V2O5·x H2O nanobelts. The ion-induced synthesis of inorganic nanostructures has been successfully carried out in our previous work [26]. From the viewpoint of both chemistry and crystallography, owing to the chain-based slab structure, V2O5·x H2O favors to form nanobelts along [010] direction or b-axis simulated by the chemical bonding theory of single crystal growth (Fig. 6c) [31]. While in the presence of Mg2+, both top and bottom surfaces of nanobelts tend to adsorb Mg2+ due to their terminated negative O2−. During the growth process of nanobelts, once the amount of adsorbed Mg2+ on the top or bottom surface is different, asymmetric strain emerges, and the so-called asymmetric strain [32] is realized by our designed cation-induced strategy. When Mg2+-induced asymmetric strain energy is larger than the elasticity energy, straight nanobelts tend to coil into a circle structure. This structure is further stabilized by hydrogen bonding between these negative polar surfaces through bridged water molecules. The formation mechanism of the current nanorings is shown in Fig. 7 in detail. During the coiling process, the chemical environment such as the ion concentration and pH value may vary the asymmetric strain, which thus leads to formation of some trigonal and tetragonal morphologies (Fig. 3d, 3e, 3h, 3i). In order to confirm the previous formation mechanism, we characterized the intermediate product at a shorter reaction time (10 h) by SEM (Fig. 8). Both ends and the screw coiling of V2O5·x H2O nanobelts can be clearly seen. When we performed the synthesis without Mg2+ in the reaction mixture while keeping other experimental conditions unchanged, only nanobelts can be obtained. Moreover, when Zn2+ was added instead of Mg2+, microloops were also obtained (Fig. 9), which confirmed our proposed cation-induced asymmetric strain formation mechanism.

Figure 7
figure 7

Schematic illustration of the formation process of V2O5·x H2O nanorings by Mg2+-induced asymmetric strain of nanobelts

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Figure 8
figure 8

SEM images showing the direct evidences of coiling of nanoblets into V2O5·x H2O nanorings and microloops. Scale bars correspond to 1 μm

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Figure 9
figure 9

SEM images of V2O5·x H2O microloops by coiling of nanobelts in the presence of Zn2+. a Low-magnification SEM images of the samples. b–e High-magnification SEM images of the microtube-like loops showing the hollow cavity and coiled nanobelts. Scale bars correspond to 20 μm (a), 1 μm (b–d), and 500 nm (e)

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To evaluate the optical properties of the obtained V2O5 nanorings and microloops, the UV–Vis spectrum of V2O5 are illustrated in Fig. 10. In comparison, we also characterized the UV–Vis spectrum of bulk V2O5 powders. As shown in the spectra, two major absorption bands for bulk V2O5 powders appear at about 330 and 470 nm, respectively. The absorption band above 450 nm corresponds to the band gap of V2O5. The absorption edge of V2O5 nanorings and microloops is blue-shifted compared to that of bulk V2O5 powders. The origin of the blue shift in the absorption edge is suggested to be the contribution of a quantum size effect in V2O5 nanorings and microloops [1, 7].

Figure 10
figure 10

UV–Vis spectra of as-obtained V2O5 nanorings and microloops, and bulk V2O5 powders

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Conclusions

In conclusion, single-crystalline V2O5·x H2O nanorings and microloops were synthesized in the solution-phase system, by the cation-induced asymmetric strain on layered structure V2O5·x H2O nanobelts. These as-synthesized V2O5·x H2O nanorings and microloops may extend the application of V2O5·x H2O in the area of gas sensors, actuators, and electric field-effect transistors. This work demonstrates that the novel nanoring structure, which has been observed previously only for polar surface dominated structure materials, is also available in other compounds without anion- and cation-terminated surfaces. Our proposed cation-induced strategy extends the existing formation mechanism of nanorings and can be applied to other materials.