NANOFIBERS AND PREPARATION METHODS THEREFOR

Abstract

The present invention discloses a method for preparing graphene nanofibers and non-woven fabrics using a fluid with a ultra-high draw ratio by means of a high-voltage electrospinning method. Compared with other methods for preparing graphene fibers (such as wet spinning, air-assisted spinning, etc.), the graphene fibers obtained by the present method have smaller diameters (about 100 nm to 500 nm) and a higher yield. The fibers themselves have better mechanical and electrical properties. The invention discloses a method for preparing ultra-fine graphene nanofibers and non-woven fabrics by electrospinning a mixed spinning liquid system of polymer and graphene oxide (the polymer is sodium polyacrylate). This method is highly efficient and environmentally friendly, and the resulted graphene nanofibers are the thinnest graphene fibers as currently known.

Claims

1-3. (canceled)

4. A preparation method for a nano-fiber, comprising steps of: (1): preparing a mixed spinning solution with an ultra-high draw ratio using sodium polyacrylate and graphene oxide, wherein the ultra-high draw ratio is a draw ratio of no less than 2000%, the graphene oxide (GO) in the spinning solution has sheets with sizes ranging from 20 μm to 30 μm and a concentration ranging from 0.5 wt % to 1.2 wt %, and a mass fraction ranging from 30% to 60% relative to a total mass of the sodium polyacrylate and the graphene oxide; and (2): electrospinning the mixed spinning solution prepared in step (1) to obtain a graphene oxide-sodium polyacrylate composite nanofiber, wherein the graphene oxide sheets in the composite nanofiber are overlapped and connected one after another along an axial direction of the fiber, and roll in a circumferential direction.

5. The preparation method of claim 4, further comprising step of: chemically reducing the composite nanofiber obtained in step (2) to obtain a reduced graphene oxide-sodium polyacrylate composite nanofiber.

6. The preparation method of claim 5, further comprising step of: subjecting the chemically reduced reduced graphene oxide-sodium polyacrylate composite nanofiber to a two-step thermal treatment to obtain a pure graphene fiber.

7. The preparation method of claim 4, wherein a mass ratio of the sodium polyacrylate (PAAS) to the graphene oxide in step (1) is 1:1.

8. The preparation method of claim 5, wherein the step of chemically reducing is fumigating at 95° C. for 12 hours using hydroiodic acid.

9. The preparation method of claim 6, wherein the thermal treatment is conducted in an inert atmosphere of 1000° C. and 2800° C. in turn for 1 hour.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 shows an electrospinning device of the present invention and a structure of a graphene nanofiber which is rolled by graphene sheets in a circumferential direction. The oriented graphene sheets are overlapped and connected continuously in an axial direction.

[0038] FIG. 2 shows measured drawing ratios of spinning solutions of graphene oxide and sodium polyacrylate at different ratios, and the sides are the corresponding polarizing optical micrographs and actual photos of the drawing process of the spinning solutions.

[0039] FIG. 3 shows the schematic of fracture process.

[0040] FIG. 4 shows the process for testing the drawing ratio of the solution.

[0041] FIG. 5 shows SEM images of the surface (a) and cross section (b) of the fiber according to embodiment 1.

[0042] FIG. 6 shows the tensile strength of the chemically reduced nanofibers obtained in embodiment 2. It can be seen from this figure that the nanofibers have good tensile strength, with tensile strength of 10.2 GPa and ultimate elongation of 0.93%.

[0043] FIG. 7 shows SEM images of the fiber fracture obtained in embodiment 1 after chemical reduction (a) and after thermal treatment (b).

[0044] FIG. 8 shows the photograph (a), SEM image (b) and cross section of the non-woven fabric (c), and cross section of a single fiber (d), wherein the cross section of non-woven fabric is cut with a blade.

[0045] FIG. 9 shows the distribution of fiber diameter, and it is shown that the diameters in the range of 100 to 500 nm occupy more than 90%.

[0046] FIG. 10 shows the tensile strength of the graphene nanofiber non-woven fabric obtained in embodiment 1. It can be seen that the tensile strength of the obtained graphene nanofiber non-woven fabric is 110 kPa, and the ultimate elongation is 6.4%.

[0047] FIG. 11 shows the nanofiber after chemical reduction (a) and the nanofiber after thermal treatment (b) in embodiment 3.

[0048] FIG. 12 shows the nanofiber after chemical reduction (a) and the nanofiber after thermal treatment (b) in embodiment 4.

[0049] FIG. 13 shows the nanofiber after chemical reduction (a) and the nanofiber after thermal treatment (b) in embodiment 5.

[0050] FIG. 14 shows dotted particles obtained by spinning in comparative example 2.

[0051] FIG. 15 shows the SEM image of the cross section of a micron-scale fiber obtained by wet spinning.

DESCRIPTION OF THE EMBODIMENTS

[0052] Hereinafter, that present invention will be specifically described with reference to the drawings and embodiments.

[0053] In the following embodiments and comparative examples, the polymer used includes:

[0054] sodium polyacrylate, Shanghai Yuanye Bio-Technology Co., Ltd., molecular weight M.sub.w=30 million.

[0055] In the following embodiments and comparative examples, the graphene oxides used include:

[0056] graphene oxide solution. HANGZHOU GAOXI TECHNOLOGY Co., Ltd., with transverse sizes distributed between 20 μm and 30 μm, single layer ratio of over 99%, and oxygen content of 30%-40%; and

[0057] graphene oxide solution, GAOCHAO Research Group of Polymer Department, ZHEJIANG UNIVERSITY, with transverse sizes distributed between 100 μm and 200 μm, single layer ratio of over 99%, and oxygen content of 30%-40%.

[0058] Those skilled in the art will know that: 1) chemical reduction only removes the oxygen-containing functional groups, without changing the morphology and size of the fibers; 2) the two-step thermal reduction of low-temperature and high-temperature after the chemical reduction removing the oxygen-containing functional groups can fix the defects on the surface of the fiber, and maintain the structure of the rolled, overlapped and connected graphene sheets.

[0059] In addition, in the following embodiments, unlike the fiber obtained after the thermal reduction which has a jagged section after breaking up, the fiber obtained after chemical reduction has no jagged section. This is because the polymer is still remained in the fiber which hasn't been subjected to the thermal treatment and a large amount of oxygen-containing functional groups are still remained on the surfaces of the sheets, and the sheets roll up at the breaking point to wrap the fracture surface under the high-energy electron beam of SEM. After thermal treatment, the sodium polyacrylate and the oxygen-containing functional groups are removed, so that the fiber is more excellent in conductivity. The jagged section results from the rigidity of graphene sheets after thermal treatment. It also indicates that the ultra-fine graphene nanofiber is broken up based on a ductile fracture mechanism caused by graphene interfacial slipping. In contrast, the cross section at the breaking point of the micron-sized graphene fiber which is thicker and obtained after thermal treatment at 1000° C. and 2800° C. for 1 hour in an inert atmosphere is flat (see FIG. 15), which indicates that this fiber is broken up based on a brittle fracture mechanism.

Embodiment 1

[0060] (1) An aqueous solution of graphene oxide at a concentration of 1 wt % and an aqueous solution of sodium polyacrylate at a concentration of 2.33 wt % are prepared, respectively, wherein the sizes of the graphene oxide (GO) sheets are distributed from 20 μm to 30 pam, and the average size is about 25 n.

[0061] (2) The two aqueous solutions are mixed in a mass ratio of 1:1 (that is, the graphene occupies 30 wt %) and homogenized by a homogenizer to obtain a mixed spinning solution of sodium polyacrylate and graphene oxide, and the draw ratio of the spinning solution at 25° C. is 3900%.

[0062] (3) The spinning solution is sucked into a 10 ml syringe, with the needle (21 #) of the syringe supplied with a positive high voltage of 15 kV, and extruded from the needle at a uniform speed of 0.08 mm/min. Two horizontally placed iron wires are used as the collecting device at 20 cm right below the needle and grounded (as shown in FIG. 1).

[0063] (4) The fibers are removed from the two iron wires, and fumigated at 95° C. for 12 hours with hydroiodic acid to obtain chemically reduced graphene nanofibers. FIG. 5 shows the SEM morphologies of the surface and cross section of the obtained chemically reduced graphene nanofiber, wherein the image of the cross section shows that the fiber has a structure in which the sheets are rolled in the circumferential direction.

[0064] (5) The tensile strength of the chemically reduced nanofiber is tested by means of uniaxial tension with a high-precision tensile testing machine. The tensile strength of the nanofiber is 10.2 GPa and the ultimate elongation is 0.93% (as shown in FIG. 6). The SEM image of the fiber after broken up is shown in FIG. 7a.

[0065] (6) The nanofibers which are chemically reduced by fumigating with hydroiodic acid at 95° C. for 12 hours are subjected to a thermal treatment of 1000° C. and 2800° C. at an inert atmosphere in turn for 1 hour. FIG. 7b shows a fracture surface of the obtained nanofiber. It can be seen that jagged graphene sheets present at the fracture of the broken fiber, and the fiber has graphene sheets that are rolled in the circumferential direction. The electrical conductivity of the thermal treated graphene nanofiber is measured through a four-point probe method, and the electrical conductivity is 1.1×10.sup.6 S m.sup.−1.

[0066] In the present embodiment, the average diameter of the fiber is 290 nm, the average size of the graphene sheets is 25 μm, the cross-sectional area of the fiber is 0.066 μm.sup.2, and the space between adjacent graphene sheets is 0.37 nm. Therefore, the number of the graphene sheets is 0.066/(25×0.37×10.sup.−3)≈7. That is, the graphene nanofiber is formed by 7 graphene sheets rolled in the circumferential direction as viewed from the cross-sectional direction.

Embodiment 2

[0067] (1) an aqueous solution of graphene oxide at a concentration of 2 wt % and an aqueous solution of sodium polyacrylate at a concentration of 2 wt % are prepared, respectively, wherein the sizes of the graphene oxide (GO) sheets are distributed from 20 μm to 30 μm, and the average size is about 25 μm.

[0068] (2) The two aqueous solutions are mixed in a mass ratio of 1:1 (that is, the graphene occupies 50 wt %) and homogenized by a homogenizer to obtain a mixed spinning solution of sodium polyacrylate and graphene oxide, and the draw ratio of the spinning solution at 25° C. is 2500%.

[0069] (3) The spinning solution is sucked into a 10 ml syringe, with the needle (21 #) of the syringe supplied with a positive high voltage of 15 kV, and extruded from the needle at a uniform speed of 0.08 mm min.sup.−1. A horizontally placed copper mesh is used as the collecting device at 20 cm right below the needle and grounded. After several hours, a non-woven fabric of graphene oxide nanofibers can be collected on the surface of the copper mesh.

[0070] (4) The above non-woven fabric of graphene oxide nanofibers is fumigated at 95° C. for 12 hours with hydroiodic acid, and then placed in a vacuum oven at 60° C. overnight. As shown in FIG. 8, the obtained non-woven fabric of graphene nanofibers is formed by randomly distributed nanofibers of the embodiment 1, and the fiber diameters are distributed from 100 nm to 500 nm (FIG. 0). As can be seen from FIG. 8d, the graphene oxide nanosheets are rolled in the circumferential direction.

[0071] (5) The obtained non-woven fabric is subjected to thermal treatment of 1000° C. and 2800° C. in turn for 1 hour in an inert atmosphere in order to obtain a non-woven fabric composed of pure graphene nanofibers. The tensile strength of graphene non-woven fabric is tested by uniaxial tension with a high-precision tensile testing machine. The tensile strength is 110 kPa and the ultimate elongation is 6.4% (as shown in FIG. 10). The electrical conductivity in-plane of graphene non-woven fabric is measured through a four-point probe method and reaches 3.18×10.sup.3S m.sup.−1. Its density is estimated to be 180 mg cm.sup.−3.

Embodiment 3

[0072] (1) An aqueous solution of graphene oxide at a concentration of 2.4 wt % and an aqueous solution of sodium polyacrylate at a concentration of 1.6 wt % are prepared, respectively, wherein the sizes of the graphene oxide (GO) sheets are distributed from 20 μm to 30 μm, and the average size is about 25 μm.

[0073] (2) Graphene oxide and sodium polyacrylate with the mass ratio of 1:1 are taken and dispersed in water to form a mixed solution, and the draw ratio of the spinning solution at 25° C. is 3150%.

[0074] (2) The spinning solution is sucked into a 10 ml syringe, with the needle (21 #) of the syringe supplied with a positive high voltage of 18 kV, and extruded from the needle at a uniform speed of 0.1 mm/min. A horizontally placed copper mesh is used as a collecting device at 20 cm right below the needle and grounded. After several hours, a non-woven fabric of graphene oxide nanofibers can be collected on the surface of the copper mesh.

[0075] (3) The above non-woven fabric of graphene oxide nanofibers is fumigated at 95° C. for 12 hours with hydroiodic acid, and then placed in a vacuum oven at 60° C. overnight. FIG. 11a shows a fracture surface of the separated reduced graphene oxide fiber. It can be seen that the graphene oxide nanosheets are rolled up circumferentially.

[0076] (4) The obtained non-woven fabric is subjected to thermal treatment of 1000° C. and 2800° C. in turn for 1 hour in an inert atmosphere in order to obtain a non-woven fabric composed of pure graphene nanofibers. FIG. 11b shows a fracture surface of the obtained reduced graphene oxide fiber. It can be seen that jagged graphene sheets present at the fracture of the broken fiber, and the fiber has sheets that are rolled in the circumferential direction.

[0077] It is also found by SEM that the non-woven fabric is formed by randomly overlapped graphene nanofibers, with diameters of fibers ranging from 100 nm to 440 nm and average diameter of 250 nm. The tensile strength of non-woven fabric is tested by uniaxial tension with a high-precision tensile testing machine. The tensile strength is 40 kPa and the ultimate elongation is 7.8%. The electrical conductivity in-plane of graphene non-woven fabric is measured through a four-point probe method and reaches 1.89×10.sup.3 S m.sup.−1. Its density is estimated to be 155 mg cm.sup.3.

Embodiment 4

[0078] (1) This step is similar to that of Embodiment 3, the difference therebetween is that two horizontally placed iron wires are used as the collecting device at 20 cm right below the needle and grounded (as shown in FIG. 1).

[0079] (2) The fibers between the two iron wires are removed and fumigated with hydroiodic acid at 95° C. for 12 hours to obtain chemically reduced graphene nanofibers having a diameter of about 255 nm. The fracture surface of the nanofiber is shown in FIG. 12a. The tensile strength of the chemically reduced fiber is tested by means of uniaxial tension with a high-precision tensile testing machine. The tensile strength of the nanofiber is 1.6 GPa and the ultimate elongation is 1.49%.

[0080] (3) The nanofibers which are chemically reduced by fumigating with hydroiodic acid at 95° C. for 12 hours are subjected to thermal treatment of 1000° C. and 2800° C. at an inert atmosphere in turn for 1 hour. FIG. 12b shows the obtained nanofiber. It can be seen that jagged graphene sheets present at the fracture of the broken fiber, and the fiber has sheets that are rolled in the circumferential direction. The electrical conductivity of the thermal treated graphene nanofibers is measured through a four-point probe method, and the electrical conductivity is 1.5×10.sup.5 S m.sup.−1.

[0081] In the present embodiment, the average diameter of the fibers is 255 nm, the average size of the graphene sheets is 25 μm, the cross-sectional area of the fiber is 0.049 μm.sup.2, and the space between adjacent graphene sheets is 0.37 nm. Therefore, the number of the graphene sheets is 0.051/(25×0.37×10.sup.−3)≈5. That is, the graphene nanofiber is formed by 5 graphene sheets rolled in the circumferential direction as viewed from the cross-sectional direction.

Embodiment 5

[0082] (1) An aqueous solution of graphene oxide at a concentration of 1 wt % and an aqueous solution of sodium polyacrylate at a concentration of 2.33 wt % are prepared, respectively, wherein the sizes of the graphene oxide (GO) sheets are distributed from 20 μm to 30 μm, and the average size is about 25 μm.

[0083] (2) The two aqueous solutions are mixed in a mas ratio of 1:1 (that is, the graphene occupies 30 wt %) and homogenized by a homogenizer to obtain a mixed spinning solution of sodium polyacrylate and graphene oxide, and the draw ratio of the spinning solution at 25° C. is 3900%.

[0084] (3) The spinning solution is sucked into a 10 ml syringe, with the needle (21 #) of the syringe supplied with a positive high voltage of 25 kV, and extruded from the needle at a uniform speed of 0.08 mm min.sup.−1. Two horizontally placed iron wires are used as a collecting device at 20 cm right below the needle and grounded (as shown in FIG. 1).

[0085] (4) The fibers are removed from the two iron wires, and fumigated at 95° C. for 12 hours with hydroiodic acid to obtain chemically reduced graphene nanofibers. FIG. 13a shows the cross section of the obtained chemically reduced graphene nanofiber. The tensile strength of the chemically reduced nanofiber is tested by means of uniaxial tension with a high-precision tensile testing machine. The tensile strength of the nanofiber is 0.7 GPa and the ultimate elongation was 0.55%.

[0086] (3) The nanofibers which are chemically reduced are subjected to thermal treatment of 1000° C. and 2800° C. at an inert atmosphere in turn for 1 hour. FIG. 13b shows the obtained graphene nanofiber. It can be seen that jagged graphene sheets present at the fracture of the broken fiber, the fiber has sheets that are rolled in the circumferential direction, and its size is substantially the same as that of the fiber before thermal treatment (FIG. 13a). The electrical conductivity of the thermal treated graphene nanofibers is measured through a four-point probe method, and the electrical conductivity is 2.7×10.sup.6 S m.sup.−1.

[0087] In the present embodiment, the average diameter of the fibers is 120 nm, the average size of the graphene sheets is 25 μm, the cross-sectional area of the fiber is 0.0113 μm.sup.2, and the space between adjacent graphene sheets is 0.37 nm. Therefore, the number of the graphene sheets is 0.0113/(25×0.37×10.sup.−3)≈1. That is, the graphene nanofiber is rolled by 1 graphene sheet in the circumferential direction as viewed from the cross-sectional direction.

Comparative Example 1

[0088] Comparative Example 1 is similar to Embodiment 1, and Comparative Example 1 differs from Embodiment 1 in that the graphene oxide having a size of 100 μm to 200 μm is used, the draw ratio of the spinning solution at 25° C. is determined to be 2200%. After the same spinning process as in Embodiment 1, it is found that the yield of the collected non-woven fabric is greatly reduced, which indicates that, although a sufficient draw ratio is provided, it is difficult for oversized graphene sheets (no less than 100 m, i.e., the ratio of the sheet size to the fiber diameter is no less than 200) to roll along the circumferential direction into nanofibers by drawing a jet in the process of electrospinning, and the phase separation between polymer and graphene occurs in the fiber.

Comparative Example 2

[0089] Comparative example 2 is similar to Embodiment 1, and Comparative Example 2 differs from Embodiment 1 in that the mass ratio of the graphene oxide aqueous solution to the sodium polyacrylate aqueous solution is 3:1 (that is, the proportion of sodium polyacrylate in the mixed solution is 25 wt %), and the drawing ratio of the spinning solution at 25° C. is determined to be 900%. After the same spinning process as in Embodiment 2, it is found that fibers cannot be collected on the copper mesh, but only dotted particles are formed (see FIG. 14). This result indicates that the graphene oxide spinning solution with a high solid content ratio has a low draw ratio, and the spinning jet cannot be drawn and refined under the action of electric field force. However, jet relaxation will happen.

Comparative Example 3

[0090] Comparative example 3 is similar to Embodiment 1, and Comparative Example 3 differs from Embodiment 1 in that the mass ratio of the graphene oxide aqueous solution to the sodium polyacrylate is 3:7 (that is, the sodium polyacrylate occupies 70 wt %), and the drawing ratio of the spinning solution at 25° C. is determined to be 4000%. After the same spinning and post-treatment as in Embodiment 1, it is found that the resulted non-woven fabric disappears after the thermal treatment, which indicates that, although an increased draw ratio can be provided by sodium polyacrylate with a higher content, graphene nanofiber cannot be obtained since the proportion of graphene oxide in the fiber is too small, so that the adjacent graphene sheets are not overlapped and connected after the thermal treatment.