Patent Application
20190217382
Generally, a nanowire is a nanostructure, with the diameter of the order of a nanometer (10−9 meters). Alternatively, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. At these scales, quantum mechanical effects are important, which coined the term, quantum wires. Many different types of nanowires exists, including superconducting, metallic for example, nickel, platinum, gold, etc., semiconducting for example, silicon nanowires, etc., and insulating for example, SiO2, TiO2, etc. Molecular nanowires are composed of repeating molecular units either organic or inorganic.
There are two basic approaches to synthesizing nanowires: top-down and bottom-up. A top-down approach reduces a large piece of material to small pieces, by various means such as lithography or electrophoresis. A bottom-up approach synthesizes the nanowire by combining constituent adatoms. Most synthesis techniques use a bottom-up approach. Initial synthesis via either method may often be followed by a nanowire thermal treatment step, often involving a form of self-limiting oxidation, to fine tune the size and aspect ratio of the structures.
There is a clear and present need for a simple, quick and economical method for synthesizing silver nanowires with uniform diameter. Further, there is a need for an electrochemical method for preparing silver nanowires with high density on an electrode surface, which does not require centrifugation and specific conditions to separate and store the silver nanowires.
The present invention relates to a method of production of nanowires, and more particularly relates to a method for synthesizing silver nanowires using a sol-gel template.
In an embodiment, a method of synthesizing silver nanowires is disclosed. The method involves the following steps: In the first step, the surface of an inert electrode is modified by a self-assembled monolayer of (3-mercaptopropyl) trimethoxysilane. The concentration of (3-mercaptopropyl) trimethoxysilane (3MPTS) solution ranges from 0.0001 to 0.1 M in ethanol or toluene as solvent. In another embodiment, the surface of the inert electrode modified by 3MPTS forms free hydroxyl groups in the presence of NaOH (0.01 to 1M) and HCl (0.001 to 0.5M). The inert electrode used in the present invention is a gold, platinum or palladium electrode. After the completion of the above step, a layer of sol-gel solution having a pH of 3.5 is deposited on the said surface modified inert electrode by chronoamperometry technique. In an embodiment, the pH of the sol-gel solution is 2 to 5 for 0.5 to 5 hours before the application of the sol-gel solution on the surface of the modified inert electrode. In another embodiment, the pH of the sol-gel solution ranges from 1 to 5. The sol-gel is used as template for the synthesis of the silver nanowires. The sol-gel solution comprises ethanol (as co-solvent), deionized water, alkoxysilane (as precursor), and potassium chloride or potassium nitrate (as electrolyte). In another embodiment, the sol-gel solution comprises ethanol, deionized water, potassium chloride, and M(OR)4, wherein M in M(OR)4 is Si or Ti, and R in M(OR)4 is —CH3, CH3CH2—, CH3CH2CH2—, or phenyl group. Further, one or more —OR— in M(OR)4 is substituted by R, wherein R is CH3, CH3CH2—, CH3CH2CH2—, or phenyl group. The potassium chloride, an electrolyte in the sol-gel solution could be substituted with KNO3.
During the condensation and electrodeposition of a sol-gel film on the surface of modified gold electrode, hydrogen bubbles are released from the said surface modified gold electrode to the sol-gel film resulting in the formation of nanochannels. The nanochannels formed in the aforementioned step are used to synthesize silver nanowires by immersion in a silver nitrate solution, wherein the concentration of silver nitrate solution is 0.0001 to 0.1 M. The silver nanowires are formed by electrochemical reduction of Ag+ (from AgNO3) to Ag through the nanochannels.
In an embodiment, the formation of the silver nanowires is analyzed by comparing the previously mentioned electrodes with another electrode without deposition of the sol-gel layer. The electrode without deposition of the sol-gel layer is placed in the 0.1 to 0.0001 M AgNO3 solution wherein a voltage of 1.0 to 1.5 V is applied for about 20 to 600 seconds in the three-electrode system previously mentioned. From the above comparison, it is concluded that, silver in the form of nanowires is deposited on the surface of the gold electrode.
In an embodiment, the sol-gel based template is dissolved in a hydrofluoric acid solution for 15 minutes, after the synthesis of the silver nanowires. The concentration of the hydrofluoric acid solution used for dissolving the sol-gel film is about 10 to 48% (w/v), and preferably 20% (w/v).
One aspect of the present disclosure is directed to a method of synthesizing silver nanowires, the method comprising: modifying surface of an inert electrode by a self-assembled monolayer of (3-mercaptopropyl) trimethoxysilane to form a modified gold electrode; depositing a layer of sol-gel film on the said modified gold electrode, wherein the sol-gel solution comprising: ethanol, deionized water, alkoxysilane(s) (e.g. tetraethyl orthosilicate (TEOS)), and a salt as electrolyte such as potassium chloride or potassium nitrate; forming nanochannels in the sol-gel film by releasing hydrogen bubbles from the said modified gold electrode to the sol-gel film, and immersing the nanochannels in a silver nitrate solution to form silver nanowires.
In one embodiment, the concentration of (3-mercaptopropyl) trimethoxysilane is 0.1-0.0001M. In another embodiment, the sol-gel solution is deposited on the gold electrode by chronoamperometry technique. In one embodiment, the pH of the sol-gel solution is 2 to 5 for 0.5 to 5 hours before of application of potential to the gold electrode. In another embodiment, the surface of the gold electrode modified by 3MPTS forms free hydroxyl groups in the presence of NaOH (0.01 to 1M) and HCl (0.001 to 0.5M). In one embodiment, the concentration of silver nitrate solution ranges from 0.0005M to 0.01M. In one embodiment of the method, the method further comprises dissolving the sol-gel film using hydrofluoric acid solution after the formation of the silver nanowire.
Another aspect of the present disclosure is directed to a method of synthesizing silver nanowires, the method comprising: modifying surface of a substrate; depositing a layer of sol-gel solution on the substrate; forming one or more nano channels on a sol-gel film; and immersing the nanochannels in a silver nitrate solution to form a silver nanowire. In one embodiment, the surface of the substrate is modified by a self-assembled monolayer of (3-mercaptopropyl) trimethoxysilane. In a related embodiment, the concentration of (3-mercaptopropyl) trimethoxysilane is 0.01M. In one embodiment, the substrate is a gold electrode. In another embodiment, the sol-gel solution comprises ethanol, deionized water, tetraethyl orthosilicate (TEOS), and potassium chloride. In a related embodiment, the sol-gel solution is deposited on the substrate by a chronoamperometry technique.
In another embodiment, the pH of the sol-gel solution is 3.5. In another embodiment, the pH of the sol-gel solution is between 1 and 5. In another embodiment, the surface modification of the substrates forms free hydroxyl groups on the surface of the substrate. In one embodiment, the nano channels are formed by release of hydrogen bubbles from the substrate to the sol-gel film. In another embodiment, the silver nanowires are deposited in the nano-channels by chronoamperometry technique. In one embodiment, the method further comprises dissolving the sol-gel film in hydrofluoric acid solution after preparation of the silver nanowires on the surface of the substrate. In one embodiment, the concentration of silver nitrate solution ranges from 0.0005M to 0.01 M.
Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
A description of embodiments of the present invention will now be given with reference to the figures. It is expected that the present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
Nanowire production can use several common laboratory techniques, including suspension, electrochemical deposition, vapor deposition, and VLS growth. Ion track technology enables growing homogeneous and segmented nanowires down to 8 nm diameter.
In suspension technique, a suspended nanowire is a wire produced in a high-vacuum chamber held at the longitudinal extremities. Suspended nanowires can be produced by, the chemical etching of a larger wire, the bombardment of a larger wire, typically with highly energetic ions, and indenting the tip of a STM in the surface of a metal near its melting point, and then retracting it.
VLS growth is a common technique for creating a nanowire. This process can produce high-quality crystalline nanowires of many semiconductor materials, for example, VLS—grown single crystalline silicon nanowires (SiNWs) with smooth surfaces could have excellent properties, such as ultra-large elasticity. This method uses a source material from either laser ablated particles or a feed gas such as silane.
VLS synthesis requires a catalyst. For nanowires, the best catalysts are liquid metal nanoclusters, which can either be self-assembled from a thin film by dewetting, or purchased in colloidal form and deposited on a substrate. The source enters these nanoclusters and begins to saturate them. On reaching supersaturation, the source solidifies and grows outward from the nanocluster. Simply turning off the source can adjust the final length of the nanowire. Switching sources while still in the growth phase can create compound nanowires with super-lattices of alternating materials. A single-step vapor phase reaction at elevated temperature synthesizes inorganic nanowires such as Mo6S9-xIx. From another point of view, such nanowires are cluster polymers.
Liquid/Solution-phase synthesis refers to techniques that grow nanowires in solution. They can produce nanowires of many types of materials. Solution-phase synthesis has the advantage that it can produce very large quantities, compared to other methods. In one technique, the polyol synthesis, ethylene glycol is both solvent and reducing agent.
The supercritical fluid-liquid-solid growth method can be used to synthesize semiconductor nanowires, e.g., Si and Ge. By using metal nanocrystals as seeds, Si and Ge organometallic precursors are fed into a reactor filled with a supercritical organic solvent, such as toluene. Thermolysis results in degradation of the precursor, allowing release of Si or Ge, and dissolution into the metal nanocrystals. As more of the semiconductor solute is added from the supercritical phase (due to a concentration gradient), a solid crystallite precipitates, and a nanowire grows uniaxial from the nanocrystal seed.
Liquid phase method has been widely used because of such advantages as using the nature of homogenous reaction, application of a wide range of solvents. Silver nanowire synthesis methods include polyol, intermediate grain growth, seedless and surfactant less wet chemical synthesis, seedless and surfactant assisted synthesis, template based synthesis and synthesis of silver coaxial nanowire and thermal solvent synthesis. Polyol and template based methods are used more frequently. Silver nanowires may also be prepared using alumina template by polyol method. The templates must be dissolved following nanowires preparation in the alumina template method. Different reactants and reagents are used in silver nanowire preparation by polyol method.
There is a clear and present need for a simple, quick and economical method for synthesizing silver nanowires with uniform diameter, and for methods for preparing silver nanowires with high density on an electrode surface, which does not require centrifugation and specific conditions to separate and store the silver nanowires.
The present invention generally relates to a method of production of nanowires, and more particularly relates to a method for synthesizing silver nanowires using a sol-gel template.
The present invention discloses a method of production of nanowires, and more particularly relates to a method for synthesizing silver nanowires using a sol-gel template.
The substrate disclosed in the present invention is not limited to gold (Au) electrode. The substrate could be any type of electrode that serves the purpose of the present invention. The concentration of the (3-mercaptopropyl) trimethoxysilane used in the present disclosure ranges from 0.0001 to 0.1 M in ethanol or toluene as solvent, and preferably 0.01 M. However, the concentration of the (3-mercaptopropyl) trimethoxysilane could vary accordingly and do not limit the concentration to the aforementioned range. In an embodiment, an intermediate layer of 3MPTS is formed between the gold electrode and sol-gel layer. The surface modification of the substrates forms free hydroxyl group on the surface of the substrate. The nanochannels formed by release of hydrogen bubbles from the substrate to the sol-gel film. The silver nanowires may be deposited in the nano-channels by chronoamperometry technique. The method may further comprise dissolving the sol-gel film in hydrofluoric acid solution after preparation of the silver nanowires on the surface of the substrate.
In an embodiment, provided herein is a method for modifying the surface of the gold electrode comprising the steps: i) Formation of SAM of 3MPTS: The gold electrode is immersed in the 0.01 M 3MPTS solution for 2 hours to form self-assembled monolayer (SAM) of 3MPTS. The time period could vary accordingly depending on the concentration of the 3MPTS used, ii) Formation of 2D network of SAM of 3MPTS and free hydroxyl groups: The modified gold electrode is washed with ethanol and deionized water. In the next step, the modified gold electrode is immersed in alkaline solutions and acidic solutions for about 30 minutes respectively to hydrolyze and condense the 3MPTS molecules. The aforementioned step results in the formation of a 2D network of SAM and some free hydroxyl (—OH) functional groups at the surface of the modified gold electrode, which is able to form a strong covalent bond with the sol-gel based template. In another embodiment, the surface of the gold electrode modified by 3MPTS forms free hydroxyl groups in the presence of NaOH (0.01 to 1M) and HCl (0.001M to 0.5M).
The second step involved in the synthesis of the silver nanowires is an electrochemical deposition of the sol-gel solution on the surface of the modified gold electrode. In an embodiment, the sol-gel solution disclosed in the present invention is prepared by mixing ethanol, deionized water, tetraethyl orthosilicate (TEOS), and potassium chloride (KCl). In another embodiment, the sol-gel solution comprises 10 mL ethanol, 10 mL deionized water, 500 μL tetraethyl orthosilicate (TEOS), and 0.3 g potassium chloride (KCl). The pH of the sol-gel solution is maintained to about 3.5 using an acidic solution, for example but not limited to 0.1M hydrochloric acid (HCl). The solution is then stirred slowly for about 3 hours to carry out the hydrolysis reaction.
In an embodiment, the sol-gel solution comprises ethanol, deionized water, alkoxysilane (precursor) such as tetraethyl orthosilicate (TEOS), and potassium chloride (electrolyte). In another embodiment, the sol-gel solution comprises ethanol, deionized water, potassium chloride, and M(OR)4, wherein M in M(OR)4 is Si or Ti, and R in M(OR)4 is —CH3, CH3CH2—, CH3CH2CH2—, or phenyl group. Further, one or more —OR— in M(OR)4 is substituted by R, wherein R is CH3, CH3CH2—, CH3CH2CH2—, or phenyl group. The potassium chloride, an electrolyte in the sol-gel solution could be substituted with KNO3. Several techniques are contemplated for depositing the sol-gel; in a particular example, the sol-gel solution was deposited on the substrate by a chronoamperometry technique. The pH of the sol-gel solution ranges from 1 to about 5. The pH of the sol-gel solution is preferably 3.5. In another embodiment, the pH of the sol-gel solution ranges from 2 to 5 for 0.5 to 5 hours before the application of the sol-gel solution to the gold electrode.
In an embodiment, the electrochemical deposition of the sol-gel layer on the surface of the modified gold electrode is carried out by chronoamperometry technique. The process for deposition of the sol-gel layer is described as follows: A three-electrode system comprising a counter electrode, reference electrode and a working electrode are placed in the sol-gel solution. The counter electrode, and the reference electrode used is a platinum (Pt) electrode, and Ag/AgCl, respectively, and the working electrode is the modified gold electrode.
The electrochemical deposition of the sol-gel layer is accomplished using Nova 1.6 program and AutoLab instrument, wherein a specific voltage is applied for a specific time period. The voltage applied in the above step is 900 V for a duration of about 300 seconds. Hydroxide ions are generated during the electrodeposition process, which act as a catalyst for the deposition of the sol-gel layer on the surface of the modified gold electrode. Hence, a base-catalyzed sol-gel process occurs at the surface of the modified gold electrode. Various reactions may be occur during the sol-gel deposition:
2H2O+2e−⇄2OH−+H2(g)
O2+2H2O+4e−⇄4OH−
O2+2H2O+2e−⇄H2O2+4OH−
In an embodiment, the sol-gel film is formed on the surface of the modified gold electrode. Hydrogen bubbles are released from the surface of the modified gold electrode to the sol-gel film during the condensation and formation of the sol-gel film. The generation of the hydrogen bubbles during the electrochemical synthesis of the sol-gel film causes the formation of one or more nanochannels in the sol-gel film. The nanochannels formed in the aforementioned step are used as template for the synthesis of the silver nano wires.
The final step in the synthesis of the silver nanowires is the electrodeposition of the silver nanowires on the nanochannels by chronoamperometry technique. In an embodiment, the nanochannels are immersed in a silver nitrate solution (AgNO3) to form the silver nanowires. The voltage applied for carrying out the electrodeposition process is in the range of 1.1 V to 1.5 V, and preferably 1.15V. Further, the concentration of the AgNO3 solution ranges from 0.0001M to 0.1M, and preferably 0.001M. The electrodeposition process is carried out for about 10 seconds to 600 seconds, and preferably 50 seconds. The silver nanowires are formed by electrochemical reduction of Ag+ (from AgNO3) to Ag through the nanochannels.
The formation of the silver nanowires is analyzed by comparing the previously mentioned electrodes (working electrode, counter electrode and the reference electrode) with another electrode. The electrode without deposition of the sol-gel layer is placed in the 0.001 M AgNO3 solution at a voltage of 1.15 V for about 50 seconds in the three-electrode system previously mentioned and compared by sol-gel coated electrode, placed in the same AgNO3 solution. From the above comparison, it is observed that, silver in the form of nanowires is deposited on the surface of the gold electrode modified by sol-gel film.
In an embodiment, the sol-gel based template is dissolved in a hydrofluoric acid solution for 15 minutes, after the synthesis of the silver nanowires. The concentration of the hydrofluoric acid (in water) solution used for dissolving the sol-gel film ranges from 10% to 48% (w/v), and preferably 20% (w/v). By decreasing the concentration of hydrofluoric acid, the time for dissolving the sol-gel film is increased. The sol-gel film could dissolved in any solution that serves the purpose of the present invention without limiting the scope of the invention.
In an embodiment, the synthesized silver nanowires are used in a variety of industries, such as the production of touch screens (laptop displays, cell phones, etc.), chemical sensors, optoelectronics, nanomachines (for the manufacture of nanorobots), and the production of solar cells. The advantage of the present invention include the synthesis of silver nanowires through electrochemical reduction of Ag ions on the surface of the gold electrode.
The concentration of (3-mercaptopropyl) trimethoxysilane may be 0.01M. In one example, the concentration of (3-mercaptopropyl) trimethoxysilane ranges from 0.005 to 0.05. The sol-gel solution is deposited on the gold electrode by an chronoamperometry technique, and the pH of the sol-gel solution ranges from 1 to 5, and preferably 3.5. The surface modification of the gold electrode forms free hydroxyl group on the surface of the gold electrode. Further, in one example, the concentration of silver nitrate solution ranges from 0.0005M to 0.01 M. The method could further comprise dissolving the sol-gel film using hydrofluoric acid solution after formation of the silver nanowire.
The method disclosed in the present invention is an economical method involving a fast preparation of the silver nanowires. The silver nanowires are produced with uniform diameter. The preparation of the template is easy and quick. Further, the amount of reactants used are very less. In the proposed method, the silver nanowires obtained have high density. In addition, the method requires no centrifugation and specific conditions for separation and storage of the nanowires.
The method of synthesizing silver nanowires may comprise modifying the surface of a gold electrode by a self-assembled monolayer of (3-mercaptopropyl) trimethoxysilane to form a modified gold electrode. The method then involves depositing a layer of sol-gel solution on the modified gold electrode to form a sol-gel film, wherein the sol-gel solution comprises ethanol, deionized water, alkoxysilane such as tetraethyl orthosilicate (TEOS), and potassium chloride or potassium nitrate. Subsequently, the method includes forming one or more nanochannels on the sol-gel film by releasing hydrogen bubbles from the said modified gold electrode to the sol-gel film, and immersing the nanochannels in a silver nitrate solution to form a silver nanowire
The foregoing description comprise illustrative embodiments of the present invention. Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only, and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Merely listing or numbering the steps of a method in a certain order does not constitute any limitation on the order of the steps of that method.
Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions. Although specific terms may be employed herein, they are used only in generic and descriptive sense and not for purposes of limitation. Accordingly, the present invention is not limited to the specific embodiments illustrated herein. While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description and the examples should not be taken as limiting the scope of the invention, which is defined by the appended claims.
