Article pubs.acs.org/JACS
Sub‑1 nm Nanowire Based Superlattice Showing High Strength and Low Modulus Huiling Liu,†,‡ Qihua Gong,§ Yonghai Yue,*,§ Lin Guo,*,§ and Xun Wang*,† †
Lab of Organic Optoelectronics and Molecular Engineering Department of Chemistry, Tsinghua University, Beijing, 100084, China Institute for New Energy Materials and Low-Carbon Technologies, Tianjin University of Technology, Tianjin, 300384, China § School of Chemistry, Beihang University, Beijing, 100191, China ‡
S Supporting Information *
ABSTRACT: Polymers possess special dimension-dependent processing flexibility which is always absent in inorganic materials. Traditional inorganic nanowires own similar dimensions to polymers, but usually lack near-molecular diameters and the related properties. Here we report that inorganic nanowires with sub1 nm diameter and microscale length can be electrospinningly processed into superstructures including smooth fibers and large-area mat by tuning the viscosity and surface tension of the colloidal nanowires solution. These superstructures have shown both flexible texture and excellent mechanical properties (712.5 MPa for tensile strength, 10.3 GPa for elastic modulus) while retaining properties arising from inorganic components.
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INTRODUCTION Inorganic materials possess numerous physical properties (such as optical,1 electrical,2 and magnetic3) that are of great importance for applications. The binding forces among the atoms of inorganic materials are mainly ionic bonds, covalent bonds, or a mixture of these two bonds which have high bond energy and polarity. As a result, compared with organic materials such as polymers, which have excellent processing flexibility,4 the processing of inorganic materials usually suffers from their brittle and stiff features. At the nanoscale, however, inorganic nanowires, as one-dimensional nanomaterials, are structurally similar to polymers from a viewpoint of dimensions. It is thus expected that polymer-analogue properties may appear when the diameter is further decreased to a size comparable with that of a polymer chain. It has been demonstrated that polymer-like growth kinetics and microscale conformations have been shown in 1.6 nm Bi2S3 nanowires.5 And recently, polymer-like macroscopic properties have been found in ultrathin nanowires with diameters approaching the critical value, sub-1 nm.6,7 However, the investigation on the applications of inorganic nanowires based on polymer-like properties is still limited. Electrospinning is a kind of powerful technique to process polymer materials, depending on the viscoelastic characteristic of polymer solutions.8 Although some nanowire-based superstructures have been achieved partly based on the recent developments on self-assembly of nanowires.9,10 For ultrathin nanowires, we wonder whether the polymer-like electrospinnability could emerge when they possess comparable diameters and backbone lengths as polymers and provide a new insight into the practical applications of inorganic nanowires. Recently, we have successfully synthesized ultrathin © 2017 American Chemical Society
GdOOH nanowires with diameters down to sub-1 nm and lengths up to the micrometer scale.6 When the ultrathin nanowires are dispersed in good solvent, the solution shows similar viscosity behavior as polymer solutions. And we find that the polymer-like property intensively depends on the morphology of these ultrathin nanowires, i.e., the sub1 nm diameter and the microscale length. Inspired by these results, the ultrathin GdOOH nanowires would be uniquely suitable to be processed into superstructures through electrospinning. In this work, we present the polymer-like electrospinnability of the sub-1 nm GdOOH nanowires. Well-structured electrospun fibers with controllable diameters are obtained, and the nanowires are perfectly oriented in every single fiber. The orientation makes the fiber possess excellent mechanical properties, demonstrated by in situ tensile test. For applications, the ultrathin nanowires are able to be processed into large-area free-standing mats with hydrophobic and high water-adhesive properties. Furthermore, luminescent properties can easily be introduced through doping Ln3+ ions. The research provides a viewpoint of exploring polymer-like properties of inorganic ultrathin nanowires. More importantly, it may demonstrate a new chance for inorganic ultrathin nanowires, paving the way to bring inorganic nanowires and polymers closer.
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EXPERIMENTAL SECTION
Synthesis of Ultrathin Nanowires. Ultrathin GdOOH nanowires were synthesized via a modified method as reported previously. Briefly, 1.2 g of GdCl3 was dissolved in 1.5 mL of deionized water under Received: March 31, 2017 Published: June 12, 2017 8579
DOI: 10.1021/jacs.7b03175 J. Am. Chem. Soc. 2017, 139, 8579−8585
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Journal of the American Chemical Society ultrasonication and mixed with 18 mL of ethanol to form a uniform solution. Then this solution was poured into the mixture of 3 mL of oleic acid and 6 mL of oleylamine in a 40 mL Teflon-lined autoclave with vigorous stirring and kept stirring for 15 min. The whole system was sealed and heated at 160 °C for 8 h, and then cooled down to room temperature. The product was transferred and dispersed in cyclohexane and then precipitated with ethanol and washed to remove excess surfactants. The cycle proceeded three times. To prepare the Eu3+ doped ultrathin nanowires, 1.164 g of GdCl3 and 0.432 g of Eu(NO3)3 were chosen as the precursors and other parameters were the same as that found above. Procedures of Electrospinning. Fibers with controllable diameter. For preparation for the electrospun solutions, ultrathin nanowires were dispersed in octane and mixed with tetrahydrofuran containing tetrabutyl ammonium bromide (TBAB) with a volume ratio of 9:1, reaching a nanowire concentration as high as 45 wt % and TBAB concentration of 0.07 or 0.14 wt %. The prepared electrospun solution was loaded in a 2 mL plastic syringe with a 9-gauge stainless steel needle which was fixed 13 cm higher than the plate collector vertically. Electrospinning was carried out with a voltage from 10 kV to 20 kV and a flow rate at 0.001 mm/min or 0.002 mm/min. Through tuning the voltage, flow rate, and the TBAB concentration, electrospun fibers with controllable diameter can be obtained. Large-Scale Free-Standing Macroassembly Mat. A macroassembly mat with a size of 150 × 200 mm2 was collected using a rotating drum with the height of 22 cm and radius of 5 cm at a rotational speed of 100 r/min. Here a coaxial spinneret consisting of an 8-gauge stainless steel inner needle and a rubber outer tube with a diameter of 3 mm was used. The flow rates of the standard electrospun solution (nanowires concentration 45 wt % and TBAB concentration 0.07 wt %) flowing through the inner needle and octane flowing through the outer needle were 0.001 mm/min and 0.000 15 mm/min, respectively. The voltage was fixed at 17 kV, and the distance between the spinneret and the rotating drum was 13 cm.
Figure 1. Unique viscosity of the ultrathin nanowire dispersion. (a) TEM image showing the sub-1 nm diameter of the ultrathin nanowires for electrospinning. The STEM image (inset bottom left, the scale bar is 10 nm) demonstrates the sub-1 nm diameter of the ultrathin nanowires. The photograph (inset upper right) displays the viscosity characteristic of the nanowires dispersion. The viscosity makes the dispersion trap air bubbles after gentle shaking. (b) Shear viscosity as a function of the rotation rate of solutions of nanowires in octane with different concentrations and the corresponding surface tension. With increasing concentration of the nanowires, the viscosity increases, and the surface tension decreases slightly. (c) Schematic diagram of the entangled network of ultrathin nanowires in solution. The nanowires with polymer-comparable diameter tend to form an entangled network and result in viscosity properties.
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RESULTS AND DISCUSSION Polymer-Like Electrospinnability of the sub-1 nm GdOOH Nanowires. The colloidal ultrathin nanowires for electrospinning were synthesized via a modified previously reported method.6 These nanowires have lengths up to the microscale and diameters less than 1 nm which is comparable to polymers, as shown in the transmission electron microscopy (TEM) image and scanning transition electron microscope (STEM) image (bottom left inset image in Figure 1a). Colloidal ultrathin nanostructures have exhibited many interesting shape-related behaviors11,12 in solutions mainly due to their inherent ultrathin dimension. In our case, the ultrathin nanowire solution presents an interesting viscous property (phenomenally, the inset in Figure 1a shows the viscosity through capturing air bubbles of the solution after shake). Although addition of nanoparticles into a solvent can generally increase the viscosity of the dispersion, the dependence of viscosity on the dimensions of the nanowires in our previous work6 should be given attention. Under the same low weight percentage (2 wt %) of the nanowires, the decreasing diameter and increasing length of the nanowires make an obvious positive contribution to the viscosity of the dispersion. At such low concentrations, the viscosity of the system with the thinnest and longest nanowires is increased by ∼70%. The variation trend is that the more similar the dimensions of the nanowires are to polymers, the more obvious the viscous properties the nanowire dispersion becomes. When the concentration is high, the viscosity is dramatically enhanced (Figure 1b). As the dimensions of the ultrathin nanowires are comparable to polymers, our case may be relatively similar to that of polymers. For long-chain polymer solutions, above a
critical concentration, the exponential increase of the viscosity as a function of the concentration is mainly attributed to the entangled polymer network.13 By analogy, the flexible dispersed ultrathin nanowires form bundles and construct a relative steady entangled nanowire network under high concentration, as shown in Figure 1c. The network would greatly limit the motion of the solvent molecules in the pores which in turn results in high viscosity. Although highly concentrated powder suspensions also have high viscosities, the special dimensions of the nanowires and their related entangled networks are absent. The highly viscous property of the polymer solutions commends them as an electrospinning technology with many related meaningful applications. We wonder if we can operate the sub-1 nm ultrathin nanowires on this aspect. We characterized three nanowire solutions with different concentrations using a digital rotation viscometer to quantify the shear viscosity. The result in Figure 1b shows that, at low shear rate, 2.5 rpm, the viscosities of these three solutions ranged from 1.20 pa·s to 1.55 pa·s. We found that the values belonged to the scope of the viscosity for electrospinning. Thus, electrospining might be suitable to process the ultrathin nanowires and build a bridge to combine the topological properties of polymers with inorganic nanowires. The nanowires can be directly electrospun without addition of polymers (polymers are always utilized to introduce the electrospinnability to nanostructures dispersions in most cases.14,15). The scanning electron microscopy (SEM) image in Figure 2a clearly shows the formation of fine-structured 8580
DOI: 10.1021/jacs.7b03175 J. Am. Chem. Soc. 2017, 139, 8579−8585
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Table 1. Surface Tension (mN/m) of Different Organic Solvents and the Corresponding Nanowires Dispersionsa pure solvent nanowires dispersion
toluene
cyclohexane
octane
28.622 31.410
25.171 24.903
21.642 22.380
a
The concentration of the nanowires is 45 wt %. The test is carried out at ambient temperature.
dispersion with the lowest surface tension. Meanwhile, when using octane as the solvent, the surface tension decreases with the increasing nanowire concentration as demonstrated in Figure 1b. Besides the surface tension, the boiling point of the solvent is also critical. If the boiling point is low (e.g., hexane, 69 °C), then nozzle-clogging would happen (Figure S2a); when the boiling point is too high (e.g., hendecane, 196 °C), the sprayed jets would adhere together (Figure S2b). Overall, when the action of surface tension is suppressed by the cooperated viscoelastic force and electrostatic repulsion, continuous fibers form and the beads can be effectively eliminated. Fibers with controllable diameters could also be fabricated via verifying the operating parameters of the electrospinning and the amount of TBAB as shown in Figure 2b−d (see more details in Figure S2). Compared to the fibers with diameters of 630 nm in Figure 2b, fibers with larger diameters of 1 μm were obtained by increasing the flow rate and decreasing the applied voltage; fibers with smaller diameters of 540 and 370 nm were fabricated through increasing the applied voltage and salt content, respectively. Increased flow rate can provide more sprayed materials, when the electric field strength and resulting elongation force on sprayed jets are reduced under lower applied voltage simultaneously, thicker fibers are always obtained. Inversely, higher voltage gives thinner fibers, while too high of a value is undesired for drawing out more nanowires. For more salt content, it can enhance the electrical conductivity of electrospun solution, resulting in stronger electrostatic stress. Nanowire-Based Superlattice. For electrospun polymer fibers, the high-voltage field that sprays high-speed charged jets can stretch them and then enhance the orientation of the molecule chains on the surface of the fibers.18 And we found that the ultrathin nanowires could perfectly align in every single fiber. To investigate the alignment, TEM characterization of single fiber and small-angle X-ray diffraction (SAXRD) of thick macroassembly mats were carried out. The TEM image in Figure 2e shows the perfect alignment (insert in Figure 2e shows the alignment mode) of the nanowires. A group of diffraction peaks locating at 2θ = 2.9° and 4.9° in Figure 2f further confirms the ordered assemblies of nanowires in the fibers. To give a quantitative characterization of the mean longrange order dimension of the alignment, Scherrer’s equation19 L = λ/(β cos θ) is used, where λ is the X-ray wavelength, β is the peak’s full-width at half-maximum, and θ is the peak’s angle of diffraction and we could calculate L = 23 nm. The alignment derives from two steps: the nanowires undergo a slight orientation when they first flow through a confined channel, the syringe; then the alignment of them is greatly enhanced by the stretch effect from the flowing acceleration and the Columbic force induced by the high electric field.20 In Situ Tensile Tests on Single Fibers in SEM. It has been widely reported that, for polymer and carbon nanotube fibers, a high orientation of their building blocks (i.e., molecule chains and carbon nanotubes, respectively) can greatly enhance
Figure 2. Uniform electrospun fibers with superaligned nanowires inside. SEM images of (a) large-scale area of electrospinning fibers and high magnification of the fibers with different diameters of (b) 630 nm, (c) 540 nm, and (d) 370 nm, the scale bar in c and d is 5 μm. The diameters of the fibers could be flexibly controlled by tuning the parameters including nanowires concentration, TBAB concentration, and applied voltage. (e) TEM image of a single fiber with oriented nanowires inside (insert is the modal of the aligned NWs). Under the stretch function of the high-voltage, the nanowires are arranged along the long axis of the electrospun fiber. (f) SAXRD spectrum of the fibers. A group of diffraction peaks indicate the highly ordered arrangement of the nanowires inside the fiber. The inset model shows the two-dimensional hexagonal arrangement of the nanowires.
fibers. The magnification in Figure 2b displays the average diameter of 630 nm. During the electrospinning procedure, the parameters of the electrospun solution greatly influence the morphology of the formed fibers. At low nanowires concentration (in pure octane), 15 wt %, only collapsed droplets and short fibers were observed (Figure S1a of the Supporting Information, SI); at 30 wt %, a numbers of beads formed within fibers (Figure S1b). When the concentration increased to 45 wt %, the number of beads in the fibers obviously decreased (Figure S1c). Finally, smooth fibers could be obtained through the addition of a trace amount of tetrabutyl ammonium bromide (TBAB). The evolution of the morphology of the fibers is mainly attributed to three kinds of forces loading on the electrospun jets.16 Viscoelastic force, as the most important force, can resist the fracture of the sprayed jets. The strength of the force can be obtained through increasing the concentration of the electrospun system.17 In our case, the viscosity indeed increases with the increasing nanowires concentration of the three solutions (Figure 1b). Second, the electrostatic repulsion on the surface of the sprayed jets tends to maximize the surface area and benefits the formation of smooth fibers. Here, we added TBAB to increase the net charge density of sprayed jets. However, the third force, surface tension, tends to convert liquid jets into droplets to minimize the surface area, thus leading to the formation of beads rather than fibers. The surface tension of nanostructure dispersion is greatly influenced by that of the dispersed solvent. As displayed in Table 1, among the three good solvents of the ultrathin nanowires, the surface tension of octane is the lowest. Therefore, we chose octane to endow the ultrathin nanowire 8581
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In situ tensile tests of fibers with gauge lengths of ∼2 μm and diameters ranging from ∼200 nm to ∼400 nm were conducted using a displacement control mode with a displacement rate of 5 nm·s−1 until a fracture of the sample was achieved. To calculate the stress of the fibers, their cross-sectional area was measured through SEM images before tensile testing. Figure 3d shows the corresponding true stress−strain curves (the effect from the PTP device has been removed as described in the SI). From Figure 3d, a strong size effect on the strength is revealed, the sample with the smallest diameter of 204 nm displays the highest tensile strength of 712.5 MPa and a calculated modulus (the modulus refers to the elastic modulus determined in the range of linear response between tensile stress and strain at small strains) of 10.3 GPa. Such desirable strength greatly depends on the well aligned ultrathin nanowires in the fiber. The orientation of the nanowires with small size brings more efficient van der Waals forces and friction between each other, leading to efficient load transfer to axial nanowires comparing with fibers with larger size. To further verify the advantage of the superstructure, the tensile strength and modulus of single polymer electrospun fibers are summarized in Figure 3h.21,31−42 The mechanical properties of the superstructure here stand in the upright area, indicating a desirable strength compared with polymer materials. As exhibited in Figure 3d,e, the strength and modulus of the fibers increased gradually with decreasing diameter. The inverse dependence was believed to rely on the increasing alignment level in fibers with decreasing diameter.43,44 For fibers with smaller diameters, the oriented arrangement of ultrathin nanowires by a high electric field becomes stronger. A similar trend has also been reported for electrospun polymer fibers with oriented molecule chains outside.21,24 Furthermore, with decreasing diameter, the defect intensity decreased dramatically which would greatly increase the strength. The SEM image of Figure 3f shows a typical fractured sample with irreversible elongation. When the fiber was loaded under uniaxial tensile, the nanowires overcame van der Waals force to start to slide against each other. As the initial failure appeared, it made the nanowires surrounding the zone slide more freely. Finally, fracture happened until the nanowires completely separated. The irreversible deformation is speculated to derive from the sliding of the nanowires. And the sliding mode is reasonable for the large fracture strain, leading to a relatively low modulus. The sliding is further confirmed by the observation of a fracture surface in Figure 3g (as for the ultrathin structure of nanowires, the drafted surface is more clear in the TEM image). On the basis of the alignment of the nanowires and the fracture mechanism, the fiber deformed uniformly before fracture happened as indicated by the in situ video during the tensile test (see Movie S1). The effect of nanowires orientation on the mechanical property of the fiber can also be confirmed by loading− unloading cycle experiments. At first, when the fiber experienced a strain to 16.8% (Figure 4a1−a3), the loading was fully removed. Upon unloading, the initial straight sample recovered to the state with obvious bending (Figure 4b1, Movie S2a), induced by the plastic deformation as nanowires sliding under uniaxial tensile tests. Under the same displacement rate in the first cycle, the sample was second loaded and finally fractured until the strain reached 21.4% (Figure 4b1−b3, Movie S2b, the yellow circle in Figure 4b3 again exhibited the plastic deformation of the fiber). The whole cycle test was recorded and given in Movie S2. Figure 4c gives the corresponding stress−strain curves of the two loading cycles. During the
their mechanical properties.21−24 In our case, the demonstrated alignment of the ultrathin nanowires is desired to make the electrospun fibers possess unique mechanical properties. The technology of an in situ tensile test of single nanowires has been demonstrated to be a powerful manner to investigate the mechanical properties of materials in nanoscale.25−30 Thus, in situ tensile tests on single fibers were conducted by a nanoindenter coupled with a push-to-pull (PTP) device inside a Quanta 250 FEG scanning electron microscopy. The basic configuration of the PTP device is shown in Figure 3a. After
Figure 3. In situ tensile test. SEM images of (a) the basic configuration of a PTP device and (b) enlarged SEM image taken from the yellow framed region in a. After the nanoindenter probe was positioned at the point marked by the arrow in a, the yellow shadow part in b was pushed to move at a constant speed under a displacement control mode. As the fiber was fixed across the gap, the loaded force on the fiber could be realized along the direction showed by the arrows. (c) A typical force-versus-displacement curve recorded by the indenter system. (d) True stress−strain curves of samples with different diameters and (e) the corresponding mechanical properties of the fibers. The breaking stress and modulus gradually increase with decreased fiber diameter. (f) SEM image of a fractured fiber. The irreversible elongation of the fractured fiber demonstrates the existence of plastic deformation. (g) TEM image of the fractured end of a single fiber, indicating a drafted fracture feature. The insert is the enlarged SEM image of the fractured end in f with a tilt angle of 30°. (h) Tensile strength and modulus of various single polymer electrospun fibers. The mechanical property of the fiber in this work stands in the area representing higher strength.
positioning the probe to touch the semicircular end of the PTP device accurately (as marked by the arrow in Figure 3a), the indentation load could be converted to the yellow shadow part in Figure 3b. Then a uniaxial tensile force on the tested fiber was loaded along the direction as marked by the two yellow arrows. At the same time, the force and displacement curve was recorded dynamically and real-time video was also taken. A typical corresponding force-versus-displacement curve is exhibited in Figure 3c. The true force-versus-displacement curve of a tested sample can be accurately extracted after removal of the contribution from the free PTP device (see more details in the SI). 8582
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macroassembly mat with an area of 150 × 200 mm2 (Figure 5a) as large as a piece of paper can be fabricated. The proportion of
Figure 4. Loading−unloading tests. (a) The first loading cycle with a1 the initial state and a3 ending strain of 16.8%. The three SEM images were captured at different tensile points, showing the uniform deformation of fiber during the tensile process. (b) The second loading cycle until the fracture of the fiber. SEM image b1 shows the state of the tested fiber after the force in first cycle was completely removed. The fiber finally fractured, and the fractured part was marked by the yellow frame. The scale bar in a and b is 2 μm. (c) Corresponding stress−strain curves of the two loading cycles. The blue circle represents the initial tightening process of the second cycle. (d) A 5-time loading−unloading cycle test with gradually increasing strain. The increased strain in every cycle was controlled to ∼4%.
Figure 5. Large-area macroassembly mat with hydrophobic and water adhesive surface and fluorescent property. Photographs of (a) a largescale free-standing macroassembly mat with the similar size of a piece of paper and (b) a drop of water held between the tips of a mat-coated tweezers and (the inset) the contact angle of the mat. (c) Force− distance curve recorded before and after the water droplet contacting with the mat. (d) PL emission spectra of Gd2O3:Eu3+ mat, the left insert is Gd2O3:Eu3+ mat and the right one is the Gd2O3:Eu3+ mat under UV light of 254 nm.
second tensile test, the sample was tightened from bending state first, resulting in a lower responding stress than that of first time. After the fiber was fully straightened, an inflection point appeared as marked by the blue circle in Figure 4c, followed by quickly increasing stress. Remarkably, after a cross point between the two curves, the second-time stress exceeded the corresponding value of the first loading cycle. It is thought that the improved mechanical properties of the second loading cycle relies on the increased orientation of ultrathin nanowires in the fiber after first loading cycle. When the initial sample was loaded under a uniaxial tensile, the nanowires inside were stretched and enhanced to further align. A series of SAXRD tests conducted on a rope consisting with electrospun fibers under different strains demonstrated the speculation (Figure S5). With increased strain, the enhancement of arrangement was indicated by the increased intensity of the diffraction peak. What is more important is that the enhancement could still maintain under unloading. A similar effect could also be observed in the large scale fiber (Figure S6). The improved orientation facilitates the load transfer between nanowires and reduces the defect density inside the fiber. Furthermore, the first tensile cycle also brought the sub-1 nm nanowires into closer contact to each other, and the van der Waals forces and friction were enhanced simultaneously, thus leading to higher strength and modulus.43,44 A similar principle has also been utilized to process carbon nanotube fibers with desirable mechanical property.22,23 The hardening response is also observed in a 5-time loading−unloading cycle test in Figure 4d. The hardening response can be clearly observed after every subsequent cycle. At the strain of 4.4%, the corresponding stress at this point is increased from 152 to 178, 198, 220, and 305 MPa, respectively. Fluorescent Property of the Macroassembly Mat. Under the optimized electrospinning condition, a free-standing
the inorganic component in the film is confirmed to be as high as 45 wt % by thermal gravimetric analysis (TGA) (Figure S7). The contact angle (CA) of the mat surface is 140.6 ± 2.3° (the inset in Figure 5b), indicating the high hydrophobic property of the mat. This high CA results from the hydrophobic long carbon chains covering the nanowires and the numerous air pockets constructed by the randomly stacked fibers in the mat. As the fibers interlace with each other and form a continuous network, they can provide van der Waals attraction between the mat and the water molecules in an “area-contact” way.45 Thus, the mat exhibits a strong adhesive behavior of water droplets. The photograph of Figure 5b shows that a drop of water can be held up by tweezers wrapped by the mat. Figure 5c shows the force−distance curve. The highest point confirms the water adhesive force to be at least 90.6 ± 3.4 μN.46 Numerous efforts have been made to dope Ln3+ into Gd-based compounds to introduce luminescent property.47,48 We chose Eu3+ to dope the GdOOH nanowires. Eu3+ is confirmed to be doped in through EDS mapping (Figure S8b) and the actual doping content is 3.1 mol % according to ICP data. The morphology of the doped nanowires is well maintained (Figure S8a), thus the corresponding electrospinnability is also retained. They were electrospun onto glass and calcined at 500 °C for 1 h to be transformed into an oxide (XRD pattern in Figure S9 demonstrates the cubic phase of Gd2O3). The calcined mat is shown in the left insert in Figure 5f and the SEM image in Figure S10 indicates that the morphology of the fibers is kept. The calcined film emitted red light under UV light at 254 nm, as shown in right insert. The emission peaks at 592, 653, and 706 nm correspond to Eu3+5D07F1,3,4 transitions, respectively, and the strongest 5D07F2 emission splits at 612 and 8583
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627 nm (Figure 5d). This fluorescent property arising from the inorganic component displays a primary chance to combine the characteristics of polymers and inorganics together to bring more insights into improving the development of ultrathin nanowires.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21431003, 21521091, and 51301011); China Ministry of Science and Technology under Contract of 2016YFA0202801; Research Fund for the Doctoral Program of Higher Education of China (20131102120053); and the Fundamental Research Funds for the Central Universities (YWF-17-BJ-Y-36).
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CONCLUSIONS The processing flexibility of organic materials is usually not suitable for inorganic materials due to their different properties. In our case, the polymer-comparable dimensions of the long, sub-1 nm inorganic nanowires endow them polymer-analogous properties which in turn introduce electrospinnability to this inorganic material. Macroassembly superstructures such as 1D smooth fibers with ordered inner structures are obtained through electrospinning. In situ tensile tests on a series of single fibers demonstrate high strength and low modulus, which can be attributed to the assembled superlattice. The van der Waals force between each nanowire in a fiber results in high strength. The alignment of every nanowire along the axis of a single fiber exhibits large fractured strain and a resulting low modulus. The electrospun two-dimensional structure with super hydrophobic and high adhesive properties can potentially be utilized as the surface that can transfer water droplets or act as an aqueous microreactor. And the luminescent property can endow the materials with the potential application for optical devices. This might provide an effective approach to organize nanowires which is of great importance for their application in nextgeneration devices. Furthermore, this work unveils a bit of the combination of the worlds of inorganic nanomaterials and polymers from an application perspective. We believe that this combination could bring new capabilities of scientific and technological significance.
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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/jacs.7b03175. Methods for in situ tensile tests; additional images of fibers under different operations; SAXRD of large scale fibers under different strains; TGA of the fibers; images, XRD and EDS of Eu3+ doped fibers (PDF) Video of in situ tensile test (AVI) Video of in situ tensile test (AVI) Video of in situ tensile test (AVI)
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Article
AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *
[email protected] *
[email protected] ORCID
Yonghai Yue: 0000-0002-8945-2032 Lin Guo: 0000-0002-6070-2384 Xun Wang: 0000-0002-8066-4450 Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest. 8584
DOI: 10.1021/jacs.7b03175 J. Am. Chem. Soc. 2017, 139, 8579−8585
Article
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DOI: 10.1021/jacs.7b03175 J. Am. Chem. Soc. 2017, 139, 8579−8585
