Template-free Synthesis and Crystal Transition of Ring-like VO2 (M

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Template-free Synthesis and Crystal Transition of Ring-like VO2 (M) Meng Wang, Zixiang Cui,* Yongqiang Xue,* and Rong Zhang Department of Applied Chemistry, Taiyuan University of Technology, Taiyuan, 030024, P. R. China

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S Supporting Information *

ABSTRACT: A new ring-like VO2 (M) was successfully synthesized for the first time by a template-free hydrothermal method, and the formation mechanism of the ring-like structure through aggregation of nanocrystals around the bubbles was proposed. Furthermore, the crystal transition temperature of ring-like VO2 (M) exhibits a more distinct size effect compared to nanoparticles. The results here point to the importance of carefully controlling the morphology of VO2 (M) to tune the crystal transition.

T

he great multifunctionality of materials arises largely from its structures. Among them, ring-like materials have always attracted considerable attention because of their unique geometrical configuration and hollow cavities as well as special applications.1−6 As the most widely studied inorganic functional material, vanadium dioxide is well-known for its reversible metal− insulator transition (MIT) between monoclinic phase VO2 (M) and rutile phase VO2 (R) at about 68 °C, which has extensive applications in various fields, especially in hydrogen storage,7,8 smart windows,9−11 photoelectric switches, and thermistor materials.12,13 Accordingly, various morphologies of VO2 (M) such as nanoparticles,14−17 nanowires,18,19 nanorods,20,21 nanosheets,22,23 nanobeams,24 and hollow structures25,26 have been synthesized to further enhance the material properties. However, there are no published studies reporting the synthesis of ring-like VO2 (M). In order to fabricate ring-like structures, a variety of significant synthetic strategies were developed including the use templates or not. Among them, the template method is the most common.27−33 However, the removal of the templates by either thermal or chemical etching is obviously uneconomical and complicated; consequently, these nanorings have poor stability. Another development to synthesize the ring-like structures is the template-free method,34−39 which not only has the advantages of template methods but also avoids the introduction of impurities. In this communication, we report a facile template-free hydrothermal method without any surfactants to synthesize the newly shaped hollow ring-like VO2 (M) (for detailed experimental procedures, see Supporting Information). Properly speaking, it can be considered to be the self-assembly of nanoparticles with smaller diameters into a hollow ring shape. The X-ray powder diffraction (XRD) pattern (Figure 1) of ring-like VO2 (M) exhibits peaks at 27.8°, 37.1°, 42.3°, and © XXXX American Chemical Society

Figure 1. XRD pattern of the as-synthesized ring-like VO2 (M).

55.5°, corresponding to (011), (−211), (−212), and (220) facets of monoclinic phase VO2 (JCPDS 76-0456), respectively. No impurity peaks were observed, confirming the high purity of the sample. Morphologies of the ring-like VO2 (M) were observed through the field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM). The FESEM image (Figure 2a) shows that the as-prepared hollow rings with the thickness of roughly 80 nm are uniform Received: January 26, 2018 Revised: June 4, 2018 Published: July 2, 2018 A

DOI: 10.1021/acs.cgd.8b00146 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 2. (a) High magnification FESEM image. (b) High resolution TEM image (c) gives its lattice fringes and (d) shows the corresponding SAED pattern of ring-like VO2 (M).

and well-proportioned. Figure 2b is the TEM image; it is apparent that the detailed TEM analysis of the ring-like VO2 (M) gives an average outer diameter of 350 nm and an inner diameter of about 100 nm. Lattice fringe d-spacing (Figure 2c) is 0.319 nm which is matched to the (011) lattice plane of VO2 (M). Taken together with the XRD data (Figure 1) and the HRTEM images (Figure 2c), it shows the lattice d-spacing values being consistent with monoclinic phase VO2, indicating that the samples with high crystallinity are very pure. Selected area electron diffraction (SAED) pattern (Figure 2d) shows that the ring-like VO2 (M) are polycrystalline and further confirms that the hollow rings are built from lots of smaller crystallites. The major polycrystalline diffraction rings with dspacings match well with the diffraction of (011), (−211), (−212), and (200) planes of monoclinic VO2 (M). Energydispersive X-ray spectroscopy (EDS) spectrum (Figure S1) of the ring-like VO2 (M) reveals that the sample only consists of O (66.67 at. %) and V (33.33 at. %) elements, consistent with the stoichiometry of VO2. X-ray photoelectron spectroscopy (XPS) survey spectrum (Figure 3a) illustrates that the ring-like VO2 (M) only contain V and O, and no other elements exist. High resolution XPS (Figure 3b) illustrates that the binding energies (BEs) for V 2p3/2 and V 2p1/2 are, respectively, 516.36 and 523.88 eV, which are in agreement with the reported values of V4+.40,41 The BE for O 1s is 530.02 eV that can be assigned to the O2− in the V−O.42 To further investigate the chemical bonding between vanadium and oxygen atoms, we performed Fourier-transform infrared spectroscopy (FTIR) measurement. Figure 4 displays characteristic absorption bands of the as-prepared VO2 (M). The absorption bands centered at 451 and 546 cm−1 can be attributed to the long-range stretching vibration of V−O−V bonds, while the intense peak at 750 cm−1 is characteristic of the first “‘rutile packing”’ of VO6 octahedra.43 The peak at 1005 cm−1 is characteristic of the stretching vibration of VO, while the absence of a peak at 1020 or 980 cm−1 indicates that the as-prepared samples do not contain the element of V(V) or V(III).44 In addition, the characteristic peaks at 3432 and 1592 cm−1 related to the stretching and bending mode of water molecules, respectively.45

Figure 3. (a) Survey XPS spectrum and (b) high resolution XPS for V 2p and O 1s of ring-like VO2 (M).

Figure 4. FTIR spectrum of the as-synthesized ring-like VO2 (M).

The reversible crystal transition from monoclinic VO2 (M) to rutile VO2 (R) of the ring-like sample can be clearly characterized by variable-temperature in situ XRD. The diffraction peak clearly shifted from (011)M to (110)R at 27° ≤ 2θ ≤ 29° (Figure 5a), and the peak corresponding to (310)M splits into two peaks of (130)R and (002)R at 61° ≤ 2θ ≤ 69° (Figure 5b) with the temperature increasing. B



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Scheme 1. Schematic Illustration of the Formation Mechanism of Ring-like VO2 (M)

concentration of 0.04 mol/L. When a concentration of 0.06 mol/L N2H4 was added into the reaction system, the morphologies of samples exhibit a poorly ring-like structure. More specifically, the ring-like morphology of samples prepared with N2H4 concentration of 0.08 mol/L has completely disappeared; that is to say, the larger amount of N2H4 accelerates the nucleation process and the large number of newly formed VO2 monomers tend to directly aggregate with each other, rather than aggregate on the surface of N2 bubbles.47 In addition, interestingly, the hydrothermal filling ratio is found to play another key role in the formation of the final VO2 (M) morphologies. Different filling ratios and the corresponding FESEM images are shown in Figure 6.

Figure 5. Variable-temperature magnified XRD patterns of ring-like VO2.

Since neither surfactants nor templates were used in our experiments, the amount of N2H4 is considered as the critical factor for the preparation of ring-like VO2 (M). We must pay attention to the chemical reaction in the hydrothermal process, and the chemical reaction formula could be described as the following: 4VO3− +N2H4 =N2 ↑ +4VO2 (precursor) + 4OH−

(1)

VO3−

In the hydrothermal process, the ions are reduced by hydrazine to VO2 monomers in the alkaline solution and lots of bubbles of N2 are produced. So, the newly formed smaller nanoparticles with high surface energy tend to aggregate around the gas−liquid interface,46 and the ring-like structure can be formed when not enough particles aggregate the bubbles, as schematically described in Scheme 1. To further understand the growth mechanism of the ringlike VO2 (M), we investigated the value of the molar ratio of VO 3− to N 2H4 in the reaction process and contrast experiments were performed (see Supporting Information). As shown in Figure S2, the final morphologies of VO2 (M) are sensitive to the amount of N2H4 and only an appropriate molar ratio of VO3− to N2H4 could keep the morphology unchanged. However, when the concentration of N2H4 is lower than 0.04 mol/L, there are only small particles in the final products, which may be related to the lack of sufficient N2 bubbles for the small crystals to aggregate around. Typically, the ring-like VO 2 (M) with uniform size are obtained with N 2 H 4

Figure 6. High magnification FESEM images of the VO2 (M) with different filling ratios (a) 40%, (b) 50%, (c) 70%, and (d) 80%.

It can be seen clearly from Figure 6a,b that the ring-like VO2 (M) morphologies can be obtained under the filling ratios of 40% and 50%. However, as shown in Figure 6c,d, when the filling ratios increased from 50% to 70% or 80%, the ring-like morphologies of VO2 (M) gradually disappeared. From eq 1, it is clear that the amount of N2 produced in the hydrothermal process led the real reaction system to deviate from that in pure water. That is to say, the pressure of the autoclave depends not only on temperature. With hydrothermal filling ratio increasing, the pressure also increases and N2 bubbles are harder to form in the solution, and the bubble numbers in unit volume solution will also correspondingly decrease. So, only a part of the newly formed small crystals can aggregate around the bubbles of N2; consequently, a small amount of ring-like structures were formed in the reaction process. C



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Furthermore, as shown in Figure 7 we can confirm that hydrothermal filling ratio has no effect on the VO2 (M) crystal

Figure 9. DSC curves of the ring-like VO2 (M) and spherical VO2 (M) with the diameter of 25 nm. Figure 7. XRD patterns for samples prepared with different filling ratios (a) 40%, (b) 50%, (c) 70% , and (d) 80%.

In summary, we developed a facile template-free hydrothermal method to fabricate ring-like VO2 (M) first. The asprepared ring-like VO2 (M) has much lower crystal transition temperature compared to the smaller nanoparticles, which can be attributed to its additional hollow cavity effect. This paper also provides a new strategy to reduce the crystal transition temperature. Furthermore, in the hydrothermal process, the gas bubbles were produced as aggregation centers to fabricate ring-like structures, and the formation mechanism of the ringlike VO2 (M) might be as important reference for the preparation of other ring-like nanomaterials.

structure, though the crystallinity of the ring-like VO2 (M) is slightly lower than that of the VO2 (M) prepared under higher filling ratios. Vanadium dioxide has attracted widespread attention because of their structural diversity, and among them VO2 (M) was widely studied for the crystal transition between VO2 (M) and VO2 (R). Figure 8 shows the crystal structures of the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00146. Detailed experimental procedures; Characterization methods; SEM images; EDS (PDF)

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Figure 8. Schematic of crystal transition between VO2 (M) and VO2 (R).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

rutile and monoclinic polymorphs of VO2. The rutile structure corresponds to a more isotropic arrangement and is based on adjacent [VO6] octahedra sharing edges along the c-axis. In contrast, the monoclinic phase corresponds to a small distortion of the [VO6] octahedra.48 The crystal transition behavior of the as-prepared ring-like VO2 (M) was determined by differential scanning calorimetry (DSC). It is widely known that particle size has great influence on crystal transition of VO2 (M), and smaller particle size leads to lower crystal transition temperature. By contrast, we also determined the crystal transition temperature of nanoparticles with the average diameter of 25 nm (Figure S3). As shown in Figure 9, the crystal transition temperature of ring-like VO2 (M) is 59.72 °C, which is much lower than 65.63 °C for nanoparticles (68 °C for the bulk) and also lower than that of nanosheets.23 It is obvious that the hollow cavities of ring-like structure have a more significant effect on the crystal transition temperature of VO2 (M) besides the size effect.

ORCID

Zixiang Cui: 0000-0001-8323-9612 Yongqiang Xue: 0000-0002-5707-2596 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (NSFC Nos. 21573157 and 21373147).

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Template-free Synthesis and Crystal Transition of Ring-like VO2 (M

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