Zinc oxide-cellulose nanocomposite and preparation method thereof

Abstract

The present invention relates to a zinc oxide-cellulose nanocomposite made of cellulose and zinc oxide nanoparticles and to a preparation method thereof. The nanocomposite of the present invention is prepared by producing zinc oxide nanoparticles as seeds on the surface and/or inside of a cellulose film and growing the seeds into zinc oxide crystals to form a zinc oxide layer strongly attached to the cellulose film. The nanocomposite of the present invention has significantly improved piezoelectricity compared to simple cellulose piezoelectric paper and may be used for LEDs and photovoltaic power generation devices thanks to zinc oxide doped therein.

Claims

1. A method for preparing a cellulose-zinc oxide nanocomposite, the method comprising the steps of: mixing an organic solvent with a surfactant at a weight ratio of 1:3-1:10 and adding zinc oxide nanoparticles thereto to prepare a mixed solution; adding a cellulose solution to the mixed solution, followed by stirring; subjecting the stirred solution to a casting process and a washing process to prepare a cellulose film having zinc oxide seeds produced thereon and/or therein; and forming a zinc oxide layer on the cellulose film by growing zinc oxide crystals from the zinc oxide seeds wherein the cellulose film having the zinc oxide seeds is added to an aqueous solution containing zinc source and an amine-group containing compound, followed by stirring the aqueous solution at a temperature of 60-90 C. for 2-10 hours.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a scanning electron microscope (SEM) image showing the morphology of the surface of a cellulose film having zinc oxide seeds produced thereon by a pre-coating method according to an example of the present invention.

(2) FIG. 2 is a set of scanning electron microscope (SEM) images showing the surface (a) and cross section (b) of a zinc oxide-cellulose nanocomposite prepared by producing zinc oxide seeds using a pre-coating method.

(3) FIG. 3 is a set of scanning electron microscope (SEM) images showing the surface (a) and cross section (b) of a zinc oxide-cellulose nanocomposite prepared by producing zinc oxide seeds using a blending method.

(4) FIG. 4 shows the results of X-ray diffraction analysis of a zinc oxide-cellulose nanocomposite prepared by producing zinc oxide seeds using a pre-coating method according to an example of the present invention.

(5) FIG. 5 is an atomic force microscope (AFM) image showing the surface morphology, the uniformity of dispersion of seeds and the size of seeds on the surface of a cellulose film having zinc oxide seeds produced by a direct application method according to an example of the present invention.

(6) FIG. 6 shows the results of X-ray diffraction analysis of a zinc oxide-cellulose nanocomposite prepared by producing zinc oxide seeds using a pre-coating method.

BEST MODE

(7) The present invention relates to a zinc oxide-cellulose nanocomposite made of cellulose and zinc oxide nanoparticles and to a preparation method thereof. The nanocomposite according to the present invention has excellent piezoelectricity, biodegradability and biocompatibility, which is prepared by producing zinc oxide nanoparticles as seeds on the surface and/or inside of a cellulose film and further growing the zinc oxide nanoparticles to form a zinc oxide layer strongly attached to the surface of the cellulose film.

(8) If the zinc oxide layer is present in an amount of less than 5 wt % based on the total weight of the composite, an increase in the piezoelectric effect will be insignificant, and if the zinc oxide layer is present in an amount of more than 40 wt % based on the total weight of the composite, it will be highly brittle. For this reason, the zinc oxide nanoparticles are preferably present in an amount of 5-40 wt % based on the total weight of the composite. More preferably, the zinc oxide nanoparticles are contained in an amount of 10-20 wt %.

(9) Moreover, the zinc oxide nanoparticles preferably have a particle size of 50-400 nm, and more preferably 100-300 nm. If the particle size is less than 50 nm, an area that exhibits the piezoelectric effect will be small, and thus the composite will have an insufficient piezoelectric effect, and if the particle size is more than 400 nm, there will be a problem in that the nanoparticles aggregate. In addition, the zinc oxide nanoparticles grow into rod-like zinc oxide crystals that form a zinc oxide layer. The zinc oxide layer preferably has a thickness of 100-1000 nm.

(10) The zinc oxide-cellulose nanocomposite according to the present invention can overcome the disadvantage of highly brittle zinc oxide, because it comprises zinc oxide nanoparticles not only the surface, but also inside of the cellulose film.

(11) The cellulose-zinc oxide nanocomposite according to the present invention has a piezoelectric constant (d.sub.31) between 30 pC/N and 500 pC/N.

(12) The cellulose-zinc oxide nanocomposite according to the present invention has a Young's modulus of 2-6 GPa.

(13) The cellulose-zinc oxide nanocomposite according to the present invention is prepared by a method comprising the steps of: (S11) preparing a cellulose film; (S12) producing zinc oxide seeds on the surface and/or inside of the cellulose film; and (S13) growing zinc oxide crystals from the produced zinc oxide seeds to form a zinc oxide layer on the cellulose film.

(14) Step (S11) of preparing the cellulose film may be performed by any conventional method known in the art. For example, the cellulose film can be prepared by dissolving cellulose pulp to make a cellulose solution, and spin-coating the cellulose solution or casting the cellulose solution using a doctor blade. Specifically, sodium hydroxide, DMAc (N,N-dimethylacetamide) or NMMO (N-methylmorpholine-N-oxide) as a solvent is added to bulk cellulose to make a cellulose solution. The cellulose solution may be subjected to a spin-coating, extrusion or casting process to obtain a cellulose film.

(15) Next, zinc oxide seeds are produced on the surface and/or inside of the cellulose film (step S12). This step can be performed by a method selected from the group consisting of a pre-coating method, a blending method and a direct application method.

(16) In the pre-coating method, an aqueous solution containing a zinc source and an amine group-containing compound is prepared. Then, the cellulose film is added to the solution and stirred at a temperature of 40 C. to 90 C. for 1-12 hours to produce zinc oxide seeds on the surface and/or inside of the cellulose film. More preferably, the mixture of the cellulose film and the solution is stirred at a temperature between 50 C. and 80 C. for 3-10 hours to produce zinc oxide seeds.

(17) The zinc source that is used to produce the seeds is selected from the group consisting of zinc nitrate hexahydrate, zinc nitrate monohydrate, zinc citrate, zinc acetate, zinc acetate dihydrate, zinc bromide, anhydrous zinc bromide, zinc chloride, zinc fluoride, zinc iodide, and mixtures thereof. However, the zinc source is not limited thereto, and any compound may be used without limitation in the present invention, as long as it can provide zinc.

(18) Preferably, the zinc source is used in an amount of 0.01-5 wt %, and preferably 0.1-3 wt %, based on the total weight of the aqueous solution.

(19) The amine group-containing compound that is used in the present invention is preferably selected from the group consisting of triethanolamine, triethylamine, hexamethylenetetramine, ammonia, ammonium hydroxide, sodium hydroxide, potassium hydroxide, ethanolamine, and mixtures thereof. However, the amine group-containing compound is not limited thereto, and any compound containing an amine group may be used without limitation in the present invention.

(20) The amine group-containing compound is preferably used in an amount of 0.01-5 wt %, and more preferably 0.1-3 wt %, based on the total weight of the aqueous solution.

(21) FIG. 1 shows a SEM image showing the surface of a cellulose film having zinc oxide seeds produced thereon by the above-described pre-coating method.

(22) Another method is a method of producing zinc oxide seeds by blending. In this method, an organic solvent and a surfactant are mixed with each other, followed by ultrasonic stirring for 1-3 hours. The weight ratio of the organic solvent to the surfactant may preferably be 1:3 to 1:10, and more preferably 1:5 to 1:7. The organic solvent that is used herein may be dimethylacetamide (DMAc), acetone, cyclohexanone, 1-methyl-2-pyrrolidinone (NMP) or the like, and the surfactant may be sodium dodecyl sulfate (SDS), linear sodium alkyl benzene sulfonate (LAS), alcohol ethoxylate (AE) or the like. Next, 100 mg of zinc oxide nanoparticles are added to the mixed solution, followed by ultrasonic stirring for 1-3 hours. To the resulting zinc oxide/DMAc solution, 15 ml of a cellulose solution is added, and the mixture is stirred at a temperature of 50 C. to 80 C. for 1-5 hours to obtain a blend solution of cellulose/zinc oxide/DMAc. More preferably, the stirring is performed at a temperature of 60 C. to 80 C. for 2-4 hours. The blend solution is subjected to a casting process and a washing process to produce zinc oxide seeds on the surface and inside of the cellulose film.

(23) Still another method is a method of producing zinc oxide seeds by direct application. In this method, a zinc source and a solvent are mixed with each other to make a mixed solution, which is then applied to a dry cellulose film to produce seeds. Methods for applying the mixed solution to the cellulose film include a drop method of dropping the mixed solution using a spoid, a blade method of uniformly dispersing the dropped mixed solution using a glass or doctor blade having a sharp edge, and a spin coating method of applying the mixed solution using a spin coater. Application of the mixed solution is repeated 1-10 times, and the cellulose film having the solution applied thereto is maintained at a temperature of 70 C. to 150 C., thereby producing zinc oxide seeds on the surface of the cellulose film. Preferably, the temperature at which the cellulose film is maintained is 80 C. to 130 C. After the seeds are formed, the cellulose film is maintained at the same temperature for 10 minutes to 2 hours to volatilize the remaining solvent.

(24) The zinc source that is used to form seeds in this blending method may be the same as that used in the pre-coating method. Preferably, the zinc source may be selected from the group consisting of zinc nitrate hexahydrate, zinc nitrate monohydrate, zinc citrate, zinc acetate, zinc acetate dihydrate, zinc bromide, anhydrous zinc bromide, zinc chloride, zinc fluoride, zinc iodide, and mixtures thereof. However, the zinc source is not limited thereto, and any compound may be used without limitation in the present invention, as long as it can provide zinc.

(25) Preferably, the zinc source is used in an amount of 0.05-10 wt %, and preferably 0.5-7 wt %, based on the total weight of the solvent.

(26) The solvent is preferably selected from among ethanol, methanol and a solution having an alkyl group (C.sub.nH.sub.2n+1). However, the solvent is not limited thereto, and any solvent may be used without limitation, as long as it can react with the zinc source to produce a hydroxyl group (OH).

(27) FIG. 5 shows an AFM image of the surface of a cellulose film having zinc oxide seeds produced thereon by the above-described direct application method.

(28) Finally, step (S13) is a step of growing zinc oxide crystals from the zinc oxide seeds produced on the cellulose film, thereby forming a zinc oxide layer on the cellulose film.

(29) In this step, the cellulose film having the zinc oxide seeds produced therein and/or thereon is added to an aqueous solution containing a zinc source and an amine group-containing compound. Then, the aqueous solution is stirred at a temperature of 60 C. to 90 C. for 2-10 hours to grow zinc oxide crystals from the zinc oxide seeds on the surface of the cellulose film, thereby forming a zinc oxide layer on the cellulose film.

(30) Each of the zinc source and the amine group-containing compound is preferably used in an amount of 0.01-5 wt % based on the total weight of the aqueous solution.

(31) In the above-described method for preparing the cellulose-zinc oxide nanoparticle composite, the formation and growth of zinc oxide crystals occur as follows.

(32) The amine group-containing compound reacts with the zinc source to form a zinc-amine complex. Then, as the reaction temperature is increased, zinc hydroxide is formed. The formed zinc hydroxide is formed into very small zinc oxide particles on the surface or inside of the cellulose film, and the zinc oxide particles function as seeds. Such small particles aggregate to form nanosized zinc oxide. When a base such as sodium hydroxide or potassium hydroxide is used, it reacts with the zinc source to form zinc oxide, which is then formed into nanoparticles by hydrolysis as the reaction temperature is increased. The particle size of the formed zinc oxide is determined by the reaction temperature and time, the zinc concentration, pH acidity, etc. When the base is used, zinc hydroxide is directly formed by the reaction between the base and the zinc source, and then formed into nanoparticles as the reaction temperature is increased.

(33) The formation of zinc oxide nanorods from zinc oxide seeds by hydrothermal synthesis is influenced by additional chemicals such as organic ligands, metal ion impurities, pH acidity and the like (see Nature Materials, 10(9), p 596-601, August 2011). The Formation of zinc oxide nanorods is influenced predominantly by face-selective electrostatic interaction (see Crystal Growth and Design, 9, p 2570-2575, 2009). The growth of zinc oxide nanorods can be achieved by adding an amine group to the zinc oxide source to control the local reaction environment during growth. The amine group results in production of a complex, which has a charge but is not reactive, and the complex causes a portion having the opposite charge to be formed on the surface of zinc oxide so as to control the attachment of highly reactive zinc to the surface of crystals, thereby facilitating the growth of nanorod crystals.

(34) The zinc oxide nanoparticles have a particle size of 50-400 nm, and the zinc oxide layer composed of the grown zinc oxide nanorod crystals has a thickness of 100-1000 nm. Moreover, the zinc oxide layer is present in an amount of 10-70 wt % based on the total weight of the nanocomposite. In addition, the nanocomposite prepared according to the present invention has a piezoelectric constant (d.sub.31) between 30 pC/N and 500 pC/N.

(35) FIG. 4 shows the results of X-ray diffraction analysis of the cellulose-zinc oxide nanocomposite prepared by producing zinc oxide seeds using the pre-coating method. As can be seen in FIG. 4, the diffraction pattern of the zinc oxide is almost identical between the front side and the back side and shows peaks at 31.7, 34.4, 36.4, 47.5, 56.5, 62.8, 67.9, and 69.1. This suggests that the zinc oxide had the wurtzite crystal structure and was well formed on the cellulose film.

(36) FIG. 6 shows the results of X-ray diffraction analysis of the cellulose-zinc oxide nanocomposite prepared by producing zinc oxide seeds using the pre-coating method. As can be seen therein, the diffraction pattern of the zinc oxide shows peaks at 31.7, 34.4 and 36.4. This suggests that the zinc oxide had the wurtzite crystal structure and was well formed on the cellulose film. Particularly, the (002) crystal plane peak at 34.4 indicates that most of the zinc oxide grew in a direction perpendicular to the surface of the cellulose film.

(37) Hereinafter, the present invention will be described in further detail with reference to examples. It is to be understood, however, that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention as defined in the accompanying claims.

MODE FOR INVENTION

Examples

Preparation Example: Preparation of Cellulose Film

(38) The bulk cellulose, cotton pulp having a polymerization degree of 4,500 was cut into small pieces. Each of the cotton pulp and lithium chloride was dried in an oven at 100 C. to evaporate water. The cotton pulp was mixed with lithium chloride and anhydrous DMAc such that the weight ratio of pulp: lithium chloride: anhydrous DMAc was 1:8:90. The mixture was heated with stirring at 135 C. to dissolve the cellulose in the solvent. The wet cellulose film was cast onto a glass substrate using a doctor blade and treated with a 50:50 (v/v) mixture of isopropyl alcohol and deionized water, thereby obtaining a free-standing regenerated wet cellulose film.

Example 1: Production of Zinc Oxide Seeds by Pre-Coating, and Preparation of Zinc-Oxide Nanocomposite

(39) 0.005 mole of zinc nitrate hexahydrate and 0.005 mole of triethanolamine were dissolved in 500 ml of deionized water. The regenerated wet cellulose film (8080 mm) prepared in the Preparation Example above was added to and stirred in the solution at 60 C. for 6 hours to produce zinc oxide seeds on the surface and/or inside of the cellulose film.

(40) Next, the cellulose film having the produced seeds was washed with distilled water to remove unbound zinc oxide particles. The washed cellulose film was added to and reacted with an aqueous solution of 0.03 mole of zinc sulfate and 0.9 mole of ammonium chloride at 60 C. for 6 hours to grow rod-like zinc oxide crystals, thereby forming a zinc oxide layer having a thickness of about 5 m. The piezoelectric constant and Young's modulus of the zinc oxide-cellulose nanocomposite prepared as described above were measured to be 112 pC/N and 2.3 GPa, respectively.

Examples 2: Production of Zinc Oxide Seeds by Blending, and Preparation of Zinc-Oxide Nanocomposite

(41) 15 ml of DMAc was mixed with 100 mg of sodium dodecyl sulfate (SDS), followed by ultrasonic stirring for 2 hours. 100 mg of zinc oxide nanoparticles were added to the stirred solution to prepare a zinc oxide/DMAc solution. Then, 15 ml of a cellulose solution (cellulose pulp/DMAc weight ratio=1.5 wt %) was added to the zinc oxide/DMAc solution, and the mixture was blended by stirring at 60 C. for 2 hours. The resulting blend was subjected to a casting process to form a cellulose film having zinc oxide seeds produced thereon and/or therein.

(42) Then, the cellulose film having the produced seeds was washed with distilled water to remove unbound zinc oxide particles. The washed cellulose film was added to and reacted with an aqueous solution of 0.03 mole of zinc sulfate and 0.9 mole of ammonium chloride at 60 C. for 6 hours to grow rod-like zinc oxide crystals, thereby forming a zinc oxide layer having a thickness of about 5 m. The piezoelectric constant and Young's modulus of the zinc oxide-cellulose nanocomposite prepared as described above were measured to be 34 pC/N and 4.0 GPa, respectively.

Examples 3: Production of Zinc Oxide Seeds by Direct Application, and Preparation of Zinc-Oxide Nanocomposite

(43) 0.05 mole of zinc acetate was mixed with 400 ml of ethanol, and the mixture was applied to the regenerated wet cellulose film (8080 mm), prepared in the Preparation Example above, by a drop method using a spoid. The application process was repeated 10 times, and then the cellulose film was maintained at 100 C., thereby producing zinc oxide seeds on the cellulose film. After production of the seeds, the cellulose film was allowed to stand at the same temperature for 1 hour to volatilize the remaining solution. Then, the cellulose film having the seeds formed thereon was washed with distilled water to remove unbound zinc oxide particles. The washed cellulose film was added to and reacted with a solution of 0.05 mole of zinc nitrate and 0.05 mole of hexamethylenetetramine (HMTA) in deionized water at 90 C. for 2 hours to grow rod-like zinc oxide crystals, thereby forming a zinc oxide layer having a thickness of about 0.6 m. The piezoelectric constant and Young's modulus of the zinc oxide-cellulose nanocomposite prepared as described above were measured to be 420 pC/N and 4.1 GPa, respectively.