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The symmetric confined growth to superstructured vanadium dioxide nanonet with regular geometrical pattern by a solution approach Deyu Guo, Zhengjing Zhao, JingBo Li, Jiasong Zhang, Ruibo Zhang, Zehao Wang, Pengwan Chen, Yongjie Zhao, Zhuo Chen, and Haibo Jin Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00897 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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Crystal Growth & Design

The symmetric confined growth to superstructured vanadium dioxide nanonet with regular geometrical pattern by a solution approach Deyu Guo, Zhengjing Zhao, Jingbo Li,* Jiasong Zhang, Ruibo Zhang, Zehao Wang, Pengwan Chen, Yongjie Zhao, Zhuo Chen, Haibo Jin* Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. Keywords patterned nanonet, vanadium dioxide, lattice match, self-assembly, thermochromic property, hydrothermal growth

ABSTRACT Controllable self-assembly of ordered and regularly-patterned semiconductor nanoarchitectures is of great interest in achieving fantastic functionalities and properties of nanomaterials in nanodevices. Here we demonstrate a symmetric confined growth methodology for fabricating geometrically-patterned and well-oriented two dimensional (2D) nanonet by a solution growth. A uniform orthogonal VO2 nanonet composed of single-crystalline nanowalls is selfassembled in a one-step process, and exhibits single-crystal-like crystallographic characteristics. It is revealed that the 4-fold symmetric structure of (001) TiO2 determines the orthogonal geometrical pattern of nanonet, in addition, the interfacial mismatch energy controls the horizontal growth

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direction and morphology of 1D nanocrystals competing with the surface energy. The unique VO2 nanonet exhibits excellent thermochromic performances due to its self-generated porosity and sluggish phase transition. The initial optical modulation temperature is near to room temperature. The solar modulating ability (∆Tsol) is up to 11.82% with the maximum visible light transmittance (Tvis-max) more than 70%. The proposed growth strategy could be adopted in more systems to perform self-assembly of regularly-patterned nanoarchitectures with well interconnectivity and preferred orientation, which offers promising opportunities for exploiting potential nanodevices in various applications.

INTRUDCTION The self-assembly of ordered two-dimensional (2D) nanostructures remains an aspiring pursuit for the integration of nanoscale electronic and photonic devices free from lithography technique.1-2 To achieve the fabrication of 2D nanostructures, tremendous efforts have been devoted to establishing effective methodologies, such as prepatterned template methods,3-6 postgrowth assembly methods7-10 and faceted nanostep methods.11-15 Epitaxial single-crystal nanoarrays were formed by using prepatterned block copolymer self-assemble-directed nanoporous templates on single-crystal Si substrate.5 Large-scale hierarchical organization of nanowire arrays was performed by the Langmuir-Blodgett (LB) technique.10 Tsivion et al. grew horizontal GaN nanowires with controlled crystallographic orientation on faceted surfaces by guided vapor-liquid-solid growth method.11 Liu et al. prepared hierarchical superstructures composed of ordered 2D bismuth compound nanostructures by using ordered sacrificial template.14-15 Those reports presented various successful techniques for fabricating controllable 2D nanostructures. However, researchers are still faced with great difficulties in achieving self-assembly of an ordered, highly-oriented and wellinterconnected 2D nanostructure, in which the 1D nanounits (single-crystal nanorods or nanowires)


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are orderly arrayed to form periodic geometrical units. The challenge lies in the simultaneous control of growth direction, crystallographic orientation, shape and large-range ordering of 1D nanounits. Vanadium dioxide (VO2) is a representative transition metal oxide, undergoing a metal-toinsulator phase transition (MIT) from the low-temperature monoclinic (M1) insulating phase to high-temperature rutile (R) metallic phase at 341 K. The transition is accompanied with dramatic changes in electric and optical properties,16-17 which provides great opportunities in various applications, such as the electric and optical switching devices,18 field-effect-transistors,19-20 thermal capacitor devices,21-22 memristors,23 microwave switch devices,24 etc. The ‘bottom-up’ fabrication approach of ordered 2D nanoarchitectures would promote its application in micro-optical and micro-electronic devices. In our previous work, a truss-structured VO2 nanonet composed of single-crystal nanorods was epitaxially grown on a sapphire (0001) substrate by the hydrothermal method.25 We speculated the formation of the triangular nanostructure was likely to be guided by the 3-fold symmetry structure of the (0001)-sapphire substrate. It means that the pattern of nanostructures could be controlled by specific symmetric structure of selected lattice-match substrates. The rutile TiO2 has a good latticematching relation with VO2. The lattice parameters of rutile VO2 are close to those of rutile TiO2 (Space group: P 42/mnm; rutile VO2, a=b=4.554 Å, c=2.8557 Å; TiO2, a=b=4.5937 Å, c=2.9587 Å).26 The monoclinic VO2is transformed from rutile VO2 through a little distortion. Therefore, it is expected that an orthogonal patterned VO2 network could be grown guided by the (001)-TiO2 substrate because the (001) plane has a 4-fold symmetric axis. In this work, we successfully fabricate a uniform orthogonal VO2 nanonet constructed by well-oriented single-crystal nanowalls on the rutile TiO2-(001) substrate by a facile one-step hydrothermal method. The effect of the lattice


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mismatch energy on the growth of 1D nanocrystals is discussed based on the experimental observations. The results demonstrate that the crystallographic orientation and geometric pattern of the 2D nanostructures can be controlled by selecting appropriate substrates, proving the universality of the proposed solution-based symmetric confined self-assembly strategy for patterned 2D nanostructures. EXPERIMENTAL SECTION Methods. The VO2 nanomaterials were grown by a hydrothermal method. All the reagents used in the experiments were analytically grade without further purification, and purchased from Sinopharm Chemical Reagent Co. LTD. In the typical procedure, the precursor of vanadyl oxalate solution was firstly prepared by dissolving 1 mmol V2O5 in oxalic acid aqueous solution with the molar ratio of V2O5 to oxalic acid of 1:16. The mixture was continuous stirring for 5 h at 70 °C to obtain blue transparent solution with the pH value of 1.65 adjusted by using the 1 M dilute sulfuric acid. The two-side polished rutile TiO2 and sapphire substrates were ultrasonically cleaned for 1 h in the solutions of acetone, deionized water, and isopropanol sequentially. After blow dried by nitrogen, the rutile substrates (size 10×10×0.5 mm) were obliquely placed in a 50 ml Teflon-lined autoclave filled with 40 ml precursor prepared above. For the 2 inches sapphire substrate, a 1 L Teflon-lined autoclave was used to grow the VO2 film. The sealed autoclaves were kept at 230 °C for 3 h for the hydrothermal growth. As a result, the substrates were covered with uniform films on the both sides. One side of film was eliminated from the substrate by hydrogen peroxide (30 wt %). Finally, the brown colored samples were acquired after annealed at 280 °C for 10 h in an air atmosphere with a pressure of 104 Pa.


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Characterization. The morphology of the synthesized film was examined by field emission scanning electron microscope (FESEM, Hitachi S-4800) and high-resolution transmission electron microscopy (TEM/HRTEM, FEI Tecnai G2 F20 S-TWIN). The pole figures were obtained by electron backscatter diffraction (EBSD) measurement to determine the crystal orientation of the VO2 nanonet. The scanning area of EBSD measurements is 5×5 µm2.The X-ray diffraction (XRD, D/max 2500, X’pert Pro PANalytical in 2θ-θ model, Cu-Kα radiation source) and Raman spectra (HR800, excitation wavelength 633 nm) were applied to characterize the growth orientation and crystal structure of the samples. X-ray photoelectron spectroscopy (Thermoescalab 250Xi, Al Kα radiation source) was employed to measure the chemical valence of vanadium ions. The electric conductivity of films was measured by a two-probe system from 30 °C to 100 °C using an Agilent U3606 multimeter equipped with a variable-temperature sample stage. The optical transmittance of samples was characterized by Shimadzu UV-3600 UV-vis-NIR spectrophotometer with a heat solid transmission accessory. RESULTS AND DISCUSSION Figure 1a shows the SEM image of the VO2 film grown on the rutile (001)-TiO2 substrate. A uniform orthogonal network nanostructure is obtained, which is composed of nanowalls with the average width of ~56 nm and length of ~475 nm. The nanowalls align themselves regularly with a right angle. Figure 1b shows the SEM image of the truss-structured 2D VO2 nanonet grown on the (0001)-sapphire substrate. Different from the nanonet on (001)-TiO2, the VO2 nanonet is composed of nanorods with the average width of ~50 nm and length of ~350 nm. The nanorods are regularly arranged with the angle of 120°between each other (details in our previous report25). Figure 1c shows the Raman spectrum of the VO2/TiO2 compared with those of TiO2 and VO2/Al2O3. Although it is difficult to distinguish the spectrum of VO2 in the VO2/TiO2 sample due to the heavy


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interference from the signal of TiO2 substrate,27 the characteristic peaks of the insulating M1 phase can be observed faintly at the position of 195 and 225 cm-1.25,28-29 The V 2p3/2 core-level peaks in high resolution XPS measurements confirm that the V3+ ions in the as-grown sample are converted into V4+ ions after the annealed treatment,30 as shown in Figure 1d (details in Figure S1, Supporting Information).

Figure 1 (a) SEM image of 2D VO2 nanonet on the (001)-TiO2 substrate grown for 180 min showing the preferable interconnectivity of 1D nanocrystals with right angle. (b) SEM image of 2D VO2 nanonet grown on c-Al2O3 exhibiting the truss structure. (c) Raman spectra of VO2 nanonet on (001)-TiO2 together with those of (001)-TiO2 substrate, VO2 nanonet on c-Al2O3 and c-Al2O3 for reference. (d) V 2p and O 1s core-level spectra of as-grown and annealed VO2 nanonets. Figure 2 shows the crystallographic orientation of as-grown and annealed VO2films on (001)TiO2. In the XRD patterns in Figure 2b, two XRD peaks at 62.7° and 64.8° are distinguished corresponding to TiO2-(002)T and VO2-(4ത 02)M [where subscripts “T”, “M” and “R” denote the rutile TiO2, the M1 VO2 (Space group: P 21/c; a=5.7517 Å, b=4.5378 Å, c=5.3825 Å, α=γ=90° ， β=122.64°) and the rutile VO2 respectively].31 No other peaks are detected, indicating the VO2


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nanonet is highly oriented. Figure 2a and 2c are the projections of crystal structures of TiO2 and VO2 towards the TiO2-(002)T and VO2-(4ത 02)M planes, respectively, showing the similar structure of these two compounds. The pole figures obtained from EBSD measurements reveal that the nanonet has a single-crystal-like structure with the (4ത 02)M preferred orientation, as shown in Figure 2(d-f). The (4ത 02)M pole figure, showing a single central peak in Figure 2d, confirms the perfect (4ത 02)M preferred orientation of the nanonet. The (020)M and (011)M pole figures in Figures 2e and 2f exhibit quasi-four-fold symmetric peaks, which are much like the diffractions of a (4ത 02)M oriented single-crystal VO2. The results indicate that the nanocrystals constructing the nanonet keep a strict orientation relation between each other, making the pole figures of the nanonet like those of a single crystal. An epitaxial growth of VO2 on the (002)T-TiO2 substrate is suggested behind this observation. In fact, there is a good lattice-matching relation between VO2-(4ത 02)M and TiO2-(002)T as illustrated in Figure 2a and 2c. Since the growth temperature (230 °C) is higher than the phase transition temperature of VO2, the VO2 nanonet would be grown in the high-temperature rutile structure during the hydrothermal process. The rutile VO2 and TiO2 have the same crystal structure and the closer unit-cell parameters as introduced above.26 The M1 VO2 is transformed from the rutile phase through a little distortion. The a- and b-axes of monoclinic VO2 corresponds to the cand a-axes of rutile VO2 respectively. The VO2-(4ത 02)M plane corresponds to the VO2-(002)R plane. The interfacial mismatch between VO2-(002)R and TiO2-(002)T is only ~0.87%. Therefore, the crystallographic relationship of VO2 and TiO2 supports the epitaxial growth of VO2 on TiO2 substrate.


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Figure 2 (a) The projection of rutile TiO2 structure with (002)T in view plane. (b) The θ-2θ scan XRD pattern of as-grown and annealed VO2 nanonets indicating the preferential orientation of (4ത 02)M//(002)T. (c) The projection of the monoclinic VO2 structure with (4ത 02)M in view plane. (d-f) (4ത 02)M, (020)M and (011)M pole figures of the VO2 nanonet on (001)T-TiO2. To clarify the detailed crystallographic relations between the VO2 nanonet and the TiO2 substrate, TEM investigations were carried out. Figure 3a shows a cross-section TEM image of the VO2/TiO2 interface. Three VO2 grains are presented in the image. The middle one is observed to vertically grow on the substrate with height of ~150 nm and width of ~56 nm, in consistence with the observation from SEM image in Figure 1a. No traces show the existence of grain boundaries across the grains, indicating the VO2 are grown in nanowalls to construct the nanonet. The other two grains to both sides of the middle one in Figure 3a should be the nanowalls with their wallplanes in the view-plane according to the orthogonal configuration of the nanonet in Figure 1a. The selected area electron diffraction (SAED) of the interface in Figure 3b shows clearly two groups of diffraction spots, one is from the VO2 nanonet, the other belongs to the TiO2 substrate according to the comparison of the SAED pattern in Figure 3b with that of the TiO2 substrate in Figure 3c. The SAED pattern of the TiO2 substrate belongs to the [11ത 0]T zone axis, The [110]T direction is indexed


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parallel to the (001)T interface in the view-plane in Figure 3a. The SAED pattern of VO2 belongs to the [12ത 2]M zone axis. It is determined that the (4ത 02)M plane is parallel to the interface and the ( 4ത 02 )M[122]M orientation of M1VO2 is parallel to (001)T[110]T of rutile TiO2. For an easy understanding of crystallographic relationship of the VO2/TiO2 interface, the SAED pattern of VO2 is also indexed with the tetragonal structure of rutile phase. The (4ത 02)M[122]M orientation in the monoclinic structure is equivalent to the (001)R[110]R in the rutile structure. Thus the crystallographic relation of (001)R[110]R//(001)T[110]T is obtained. This result definitely evidences that the VO2 nanonet is epitaxially grown on the TiO2 substrate. The high resolution TEM (HRTEM) image in Figure 3d displays a clean interface with clear lattice fringes on both VO2 and TiO2 without evidence of large mismatching strain due to the small mismatch of ~0.87% in the (001)R/(001)T interface. Figure 3e displays a piece of nanowall lying on the support. A group of SAED spots are obtained in the inset of Figure 3e by rotating the nanowall around its long axis. The long axis direction is determined along also the [122]M (or [110]R) direction. Figure 3f schematically illustrates the epitaxial orientation relation of the VO2 nanowalls and the substrate in the rutile structure, namely the nanowalls are grown along the (001)R[110]R (or (4ത 02)M[122]M) direction parallel to (001)T[110]T.


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Figure 3 (a) The cross-section TEM image of VO2/TiO2 interface. (b, c) The SAED patterns taken from the heterostructure interface and the TiO2 substrate respectively. (d) The high-resolution TEM image, showing continuous lattice fringes across the VO2/TiO2 interface. (e) TEM image of a piece of nanowall stripped from the substrate. The inset: corresponding SAED pattern, indicating the nanowall grown along the [122]M direction. (f) The schematic view of the orientation relation of VO2 nanowalls with TiO2 substrate. The growth process of the VO2 nanonet was investigated by interrupting the hydrothermal reaction. SEM images taken from the samples prepared at different reaction times are shown in Figure 4. The initial nucleation is observed after growing for ~60 min. The nuclei are mainly spindle-shaped nanorods in Figure 4a. With increasing the reaction time to 70 min, some nuclei stretch to several hundred nanometers. The nanorods are regularly arranged at right angle to each other and exhibit acuate ends as shown in Figure 4b and 4c. As the growth continues, more nuclei


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grow up, contact and join together to form the nanonet (Figure 4c and 4d). Figure 4e schematically illustrates the evolution of the nanonet from nucleation to growth. One question here arises, why it is nanowalls growing along {001}RR (or { 4ത 02 }MM)//{001}TT to form the orthogonal nanonet rather than a whole single-crystal film covering the TiO2 substrate or the nanowalls growing along other directions. The reason is discussed below in the light of thermodynamics and kinetics of crystal growth as well as crystallography.

Figure 4 SEM images of VO2 grown on TiO2 substrate for different reaction times and corresponding schematic view of the growth process. (a) 60 min (b) 70 min (c) 90 min (d) 120 min, the scale bar is 1 µm. (e) The schematic view of the growth process for VO2 nanonet. The M (or R) direction in the ( 4ത 02 )M (or (001)R) plane is the close-packed direction, as shown in Figure 2a and 2c. Thus, it is not surprise that the nuclei grow rapidly along


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these directions to form slender spindle-shaped nanocrystallines according to the Bravais–FriedelDonnay-Harker Law. Meanwhile the {011}M (or {110}R) planes were calculated to be the lowest surface energy plane in VO2.32-33 The growth of nanocrystals along the lowest surface energy plane is much favorable, while the growth vertical to the lowest energy plane is suppressed. Therefore, the nanowalls with the side surfaces of {011}M (or {110}R) planes are formed as illustrated in Figure 3f. The growth of VO2 on the (0001)-sapphire substrate is somewhat different from that on the rutile (001)-TiO2 substrate, where the nanorods are grown due to the low energy planes of VO2 parallel to the sapphire substrate.25 For VO2 nanonet grown on the (0001)-sapphire substrate, the formation of the truss network nanostructure was attributed to the 3-fold symmetry structure of sapphire (0001) which provided three-equivalent directions for the growth of nanorods. In this work, the rutile TiO2(001) substrate has a four-fold symmetric axis which provides two mutually perpendicular and equivalent T directions for the growth of VO2. Accordingly, the orthogonal nanonet is selfassembled. The results confirm the proposed growth mechanism for regularly-patterned nanoarchitectures: the geometrical pattern of nanonet is controlled by the multiple symmetry of selected lattice-match substrate, and the morphology of 1D nanounits is shaped by the low surface energy planes of nanocrystals. The (110)T-TiO2 substrate which possesses a two-fold symmetry axis was used to further verify the symmetric confined growth of VO2 nanocrystals. Intriguingly, the experimental results reveal that the lattice mismatch plays also a significant role in regulating the growth of nanocrystals, as well as the expected confirmation of the symmetric confined growth. Figure 5 shows the SEM image, XRD pattern and pole figures of VO2 film grown on the (110)T-TiO2 substrate. The VO2 film is composed of 1D nanocrystals compactly arranged in parallel. The XRD pattern indicates the preferred (011)M (or (110)R) orientation of the VO2 film. The preferential growth (011)M//(110)T is


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well understood in terms of the epitaxial growth due to the similar structure of VO2 and TiO2. The two-fold symmetry of (110)T substrate provides two equivalent directions opposite into a 180o angle, which results in the 1D nanocrystals aligning in parallel as shown in Figure 5a. According to the conclusion obtained above that the growth of nanocrystals is regulated by the low surface energy planes of nanocrystals, it is expected that the 1D nanocrystals would grow along the M (or R) direction because the M (or R) direction is the closest-packed direction in the (011)M (or (110)R) plane, and the lowest surface energy plane {011}M (or {110}R) will become the side planes of 1D nanocrystals, as shown in Figure 5c. However, the pole figures obtained from EBSD measurements indicate that the 1D nanocrystals are grown in fact along T/R (or M) with (002)R (or (4ത 02)M) as its side planes as shown in Figure 5d and 5e. In the firstprinciple calculations by Mellan and Grau-Crespo,33 the calculated surface energies are 0.29 J/m2 for (110)R and 0.96 J/m2 for (002)R. Appavooet et al. reported the similar results.32 The 1D nanocrystals take the high surface energy plane of (002)R as their surfaces. So there must be additional energy resource which overcomes the energy difference between the (110)R plane and the (002)R plane to drive the nanocrystals growing along T/R with the higher surface energy plane as their surfaces. The following discussion indicates that the additional energy comes from the lattice mismatch. In the growth mode along R, the 1D nanocrystals have the lowest energy surface of (110)R but the lattice mismatch along R//T reaches up to ~3.6%. In contrast, the growth along R//T has the smallest mismatch of ~0.87% but with higher surface energy plane of (002)R as side surfaces26,31. It suggests that the mismatch energy dominating over the surface energy controls the growth of nanocrystals on the (110)T-TiO2 substrate.


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Figure 5 (a) SEM image for the VO2 film grown on the (110)T-TiO2 substrate. (b) Corresponding XRD pattern. (c) The projection of rutile VO2 structure to (110)R. (d, e) The (11ത 0)R and (002)R pole figures corresponding to the orientation of VO2 film in (a). Apparently, the self-assembly of regularly-patterned nanostructures via the solution method is controlled by factors in crystallography, thermodynamic and growth kinetic aspects. In crystallography aspect, the lattice matching and multi-fold symmetry of substrates are indispensably necessary conditions. The low surface energy and anisotropic mismatch strain control the growth of 1D nanocrystals in thermodynamics aspect. Meanwhile, the growth rate of nanocrystals should not be too fast in order to obtain regularly-shaped crystals. For the chemistry of preparation of VO2 films, we found the pH dependent growth of VO2 films that the nanonet cannot be grown in vanadyl oxalate solution with high pH. Also we tried to use the V4+ ion precursor solution of vanadyl sulfate to grow the VO2 film, but it is failed. The phenomena suggest that the adsorption of oxalate groups plays a key role in the VO2 growth, which might work as a bridge to achieve the adsorption of V ions on substrate surface. The V5+ ions are


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reduced to V4+ and vanadyl oxalate is obtained when the molar ratio of vanadium to oxalic acid is less than 2:3 based on the following reactions:34 V2O5+H2C2O4→2VO2+H2O+2CO2 2VO2+2H2C2O4→2VOC2O4+2H2O In this work, the content of oxalic acid is far more than the reaction required, and the pH of precursor solution is 1.65. The excess oxalic acid will be ionized and coordinated to the vanadyl ion (VO2+) to form negatively charged vanadyl oxalate complexes at low pH values.35-38 The isoelectric points of the TiO2 is around 6.2.39 The adsorption of positive charge (i.e. protons) should dominate the TiO2 surface because the pH of the precursor solution is lower than 6.2. The vanadyl oxalate complexes tende to be adsorbed on the TiO2 substrate surface by the electrostatic force from adsorbed protons. Then the V ions would be bonded to the lattice oxygen in TiO2 to form nanonet by a dehydration reaction. The detailed process has been described in another our work.40 The electrical and optical properties are investigated as shown in Figure 6. The resistance of the prepared VO2 nanonet shows a change (∆R) of 4 orders of magnitude over the MIT transition, indicating the great conductivity modulation performance, as shown in Figure 6a. Different from the sharp transition of bulk VO2 and VO2 films deposited by vapor phase deposition,41-42 the transition of the prepared VO2 nanonet is sluggish, which implies the effect of the mismatch strain on the MIT transition. The onset temperature of the phase transition is ~43 °C and the endset temperature is ~72 °C in heating process. The peak temperatures of the derivation curves are ~60 °C on heating and ~50 °C on cooling respectively, lower than 68 °C of the MIT temperature of bulk VO2. The optical modulation properties of the samples are characterized by transmittance recorded from 30 °C to 100 °C, as shown in Figure 6b and 6c. The nanonet samples show desirable


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thermochromic performance. The solar modulating ability (∆Tsol) is up to ~11.82% (the calculation details in the Supporting Information), meanwhile the maximum visible light transmittance (Tvis-max) is kept to be more than 70% and the integrated luminous transmittance (Tlum, 380~780 nm) is more than 60%. The switching efficiency at 2500 nm (∆T2500) is ~57.95% with the high roomtemperature transmittance of more than 90%. The comprehensive thermochromic performance of nanonet is in the top level among the published results, which displays superior optical properties than multilayer and nanoporous films, even comparable to the composite films, as shown in Figure 6d.43-61 The high visible light transmittance, which is an essential property for smart windows,62 is benefited from the self-generated porosity of nanonet. The porosity of nanonet enables the samples to contain enough amount of VO2 for high infrared modulation but not significantly loss the visible light transmittance. Figure S2 shows the thermochromic properties of a compact VO2 film prepared from the nanonet via a re-crystallization annealing process. It is found that the Tvis-max of the compact film declines to ~60% and Tlum is down to ~52% although the ∆Tsol rises slightly up to ~12.43%, as listed in Table S1. Furthermore, the nanonet exhibits another advantage of low onset temperature of modulation, which makes the smart window start to work near room-temperature and gradually enhance its light modulation ability with the increase of environmental temperature. This advantage is ascribed to the sluggish transition resulting from the lattice mismatch. The lattice mismatch induces tensile strain in the (001)R (or ( 4ത 02 )M) plane which should result in the corresponding compressive strain along R (or M, i.e. the V-V pair chain). The compressive stress along the V-V pair chain does depress the MIT transition to low temperature due to the strain-induced realignment of d// or π* bands as suggested in refs.63-65 The continuous release of mismatch strain through the thickness of epitaxial VO2 films was verified by Fan et al.66 It is believed that the gradient strain along thickness induces the sluggish MIT transition of the epitaxial VO2 nanonet.


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Figure 6 The electrical and optical properties of annealed VO2 nanonet. (a) The resistance as the function of temperature for VO2 nanonet grown for 180 min and the corresponding differential curves. (b) Transmittance spectra of VO2 nanonet grown for 120 min at temperatures 30 °C and 100 °C. (c) Corresponding transmittance hysteresis loops of the VO2 nanonet at 2500 nm. (d) Comparison of thermochromic properties in this work with those of various reported VO2 films. The VO2 film grown on the (110)T-TiO2 substrate also shows a sluggish MIT transition but a rise of the MIT temperature (~79 °C) with respect to the bulk VO2 (68 °C) as shown in Figure S3. For the VO2 film grown on the (001)T-TiO2 substrate, the decrease of the MIT temperature is attributed to the compressive strain along R (or M, i.e. the V-V pair chain) resulting from the lattice mismatch. In the VO2 film on (110)T-TiO2, the raised MIT temperature is also ascribed to the lattice mismatch, but the VO2 undertakes the tensile strain along the V-V pair chain which definitely upraises the MIT temperature.26,63-65 CONCLUSION In summary, we have presented a general approach to self-assemble regularly-patterned 2D nanostructures by a facile hydrothermal treatment. The orthogonal VO2 nanonet composed of well-


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oriented single-crystalline nanowalls has been prepared on rutile TiO2-(001) substrate, which displays desirable electrical modulation and thermochromic performances. In this growth approach for self-assembly of patterned 2D nanonet, the geometrical pattern of nanonets is determined by the multi-fold symmetry of lattice-matching substrates, and the growth of 1D nanounits is regulated by the interfacial mismatch energy and the surface energy competitively. The proposed methodology which is free from pre-templates, surfactants, catalysts and other assisted treatments could be extended to other systems for self-assembling regularly-patterned 2D nanoarchitectures, paving a pathway for fabricating novel nanodevices in the bottom-up nanotechnology. ASSOCIATED CONTENT Supporting Information Available. Detailed XPS results of as-grown and annealed nanonets, the effect of high-temperature annealing on morphology and optical modulation of the nanonet, and the electric property of VO2 film grown on the (110)T TiO2 substrate. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: (J.L.) [email protected]; (H. J.)[email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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For Table of Contents Use Only

The symmetric confined growth to superstructured vanadium dioxide nanonet with regular geometrical pattern by a solution approach Deyu Guo, Zhengjing Zhao, Jingbo Li,* Jiasong Zhang, Ruibo Zhang, Zehao Wang, Pengwan Chen, Yongjie Zhao, Zhuo Chen, Haibo Jin*

Table of Contents Graphic and Synopsis

SEM and schematic image of orthogonal VO2 nanonet constructed by well oriented single-crystal nanowalls on the rutile TiO2-(001) substrate, which provides two mutually perpendicular and equivalent directions for the growth of VO2.


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Symmetric Confined Growth of Superstructured ... - ACS Publications

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