Supporting Information for Sub-1 nm Nanowire Based Superlattice Showing High Strength and Low Modulus Huiling Liu,, Qihua Gong, Yonghai Yue,*,

Transcription

1 Supporting Information for Sub-1 nm Nanowire Based Superlattice Showing High Strength and Low Modulus Huiling Liu,, Qihua Gong, Yonghai Yue,*, Lin Guo*, and Xun Wang*, *To whom correspondence should be addressed. In situ Tensile Test. Preparation of Test Sample. To obtain single electrospun fibers, a homemade collector with two parallel metal wires was designed to construct parallel electric field. The parallel electric field could induce the charged fibers to arrange parallelly. In short time, several fibers were electrospun to parallel on the collector without acrossing with each other. Under the optimized electrospinning parameters, fibers with different diameters were prepared by adjusting the voltage and flow rate. Then a single fiber was transferred from the collector to a push-to-pull (PTP) device under optical microscope. Before tensile test, both ends of the fiber were fixed by electron beam deposited Pt in a FEI dual beam FIB/scanning electron microscope (SEM). Tensile Test Operation. In situ tensile tests were conducted on a Pico Indenter 85 nanoindenter coupled with a PTP device inside a Quanta 250 FEG scanning electron microscopy at 5 kv. Calibration of the conductive diamond flat punch (20 µm in diameter) was operated and confirmed before each test. After the punch was controlled to touch the semicircular end of the PTP accurately, the fiber was pulled under a displacement control mode at a displacement rate of 5 nm/s till the sample fractured. During the test, the force and displacement curve was recorded dynamically and real-time video was also taken. For loading-unloading cycle test, the fiber was loaded with gradually increasing strains under a constant displacement rate and full unloading in each cycle till the sample fractured. Quantitative Analysis. To calculate the stress and strain of the tested single fibers, their initial lengths, cross-sectional areas and the forces applied were determined. The initial length and the diameter of each sample were determined by SEM images before test (the cross-section of the fiber is assumed to be round). The force in the raw force-distance curve directly recorded by the punch includes the contribution of the sample and the PTP device. The PTP device is designed to be stiff along the tensile axis, resulting in a linear response of the force-distance curve of an empty PTP (as show in Figure S4). Thus the force applied on the fiber can be accurately determined by subtracting the contribution of the PTP device. For each test, the stiffness of the PTP device was confirmed when the fiber was totally fractured. For calculation on the elastic modulus of each fiber, the range of linear response between stress and strain at small strains was linear fitted first (the correlation coefficients were made sure to be above 0.99). Then the slope of the linear fitting was analyzed as the elastic modulus. Characterization. The morphologies and structures of the ultrathin nanowires and electrospun fibers were observed by using a Hitachi H-7700 transmission electron microscope operating at 100 kv and a field-emission scanning electron microscope (FESEM, Gemini LEO 1530). Fourier transform infrared spectra (FTIR) spectra were recorded with a Nicolet 205 FTIR spectrometer using the KBr pellet technique. Fluorescence spectra were recorded using the Edinburg FL920P transient spectrometer. Small-angle X-ray diffraction (SAXRD) characterization was carried on a Rigaku D/max-2500/PC X- ray diffractometer using Cu Kα radiation (λ = Å). Mechanical property tests on large scale fibers were conducted on an Instron 3342 universal testing machine (Instron, USA) at a loading rate of 0.5 mm/min with a gauge length of 20 mm. Water contact angles were measured on a Data-Physics 20 contact angle system at ambient temperature. Water droplets (2 μl) were located on the surface of the test mat, and five different measured points of a same sample were chosen to obtain an average value of the contact angle. The Adhesive force was recorded using a high-sensitivity microelectromechanical balance system (Data-Physics DCAT 11, Germany). A water droplet (5 μl) was carefully suspended with a copper ring, and the force was initialized to zero after the mat was placed on the balance table. A constant speed of 0.05 mms -1 was set to raise the mat to contact the water droplet. The balance table kept moving down after the contact until the water droplet completely broke away from the mat. S1

2 Figure S1. SEM images of the electrospun fibers from solutions with different nanowire concentrations at a) 15 wt%, b) 30 wt% and c) 45 wt%. Figure S2. SEM images of the electrospun fibers with the solvent of a) hexane and b) hendecane. S2

3 Figure S3. Electrospun fibers with controllable diameters of a) 1 μm, b) 540 nm and c) 370 nm. Through increasing the flow rate to mm/min, decreasing the voltage to 10 kv and keeping other conditions constant, the average diameter of the fibers was 1 μm (Figure a). When the voltage was increased to 20 kv, the average diameter of the fibers was 540 nm (Figure b). And the average diameter of the fibers was 370 nm by doubling the TBAB concentration to 0.14 wt% and keeping other conditions constant (Figure c). Figure S4. Representative force-displacement curve of an empty PTP device showing obvious linear responds. S3

4 Figure S5. a) SAXD patterns of the fiber under different strains. b-c) Photographs of the fiber fixed on a tensile stage under different strains. At first, the large scale fiber was fixed on a tensile stage as shown in Figure S4b. The initial length of the fiber between two fixed points was 24 mm. Quantitative strain on the fiber could be introduced through rolling the micrometer on the right side of the stage. At initial stage, i.e. 0% strain, a diffraction peak locating at 2θ=2.65 o was observed which indicated the existence of a periodic structure (the black line in Figure S4a). When a strain of 2.7% (rolling the micrometer by 0.65 mm) was introduced and fixed, the intensity of the diffraction peak obviously increased, indicating an enhancement of the alignment of the nanowires (the green line in Figure S4a). When the strain was further increased to 4.0% (rolling the micrometer by 0.95 mm, the moved distance could be clearly seen in the red frame in Figure S4c), the intensity of the diffraction peak simultaneously increased and moved to a higher position of 2.77 o. (The pink line in Figure S4a) The moving of the peak to a higher position indicated a closer contact between the nanowires under tensile effect. After the strain was recovered to 0% (the fiber was loose as shown in Figure S4d), the diffraction peak was almost kept and just slightly moved to a lower position. (The pink line in Figure S4a) The results demonstrated that the enhanced alignment of nanowires by stretch process could happen and still maintain under unloading, thus it would improve the tensile stress of stretched fiber. S4

5 Figure S6. Loading-unloading test on a large scale fiber with a diameter of 207 μm. Figure S7. Thermogravimetric analysis (TGA) curve of electrospun mat tested at 10 o C/min from R.T. to 800 o C in air. S5

6 Figure S8. a) TEM image of the Eu 3+ doped ultrathin nanowires. b) EDS result of the washed Eu 3+ doped nanowires. Figure S9. XRD pattern of the electrospun fibers consisting of Eu 3+ doped nanowires calcined at 500 o C for 1h. S6

7 Figure S10. SEM image of the electrospun fibers calcined at 500 o C for 1h. S7