ARAOYINBO, Alaba Oladeji (USM, Pulau Pinang, 11800, MY)
AZIZAN, Aziz (USM, Pulau Pinang, 11800, MY)
SRIMALA, Sreekantan (Universiti Sains Malaysia, USM, Pulau Pinang, 11800, MY)
AZMI, Rahmat (USM, Pulau Pinang, 11800, MY)
AHMAD, Fauzi Mohd Noor (USM, Pulau Pinang, 11800, MY)
ARAOYINBO, Alaba Oladeji (USM, Pulau Pinang, 11800, MY)
AZIZAN, Aziz (USM, Pulau Pinang, 11800, MY)
SRIMALA, Sreekantan (Universiti Sains Malaysia, USM, Pulau Pinang, 11800, MY)
AZMI, Rahmat (USM, Pulau Pinang, 11800, MY)
| CLAIMS What is claimed is: 1. An electrochemical system for producing nanoporous structure, said electrochemical system comprising: a power supply with a negative end and a positive end; an electronic circuit board being disposed within a concealed box, wherein the electronic circuit board with an inlet and an outlet comprises one or more power resistors, wherein the inlet is electrically coupled to the positive end; a positive electrode containing the material for producing the nanoporous structure, wherein the positive electrode is electrically coupled to the outlet; a negative electrode; a monitor module electrically coupled between the positive and negative electrodes for monitoring the potential and current applied to the positive and negative electrodes; and a chamber for an electrolyte, wherein the positive and negative electrodes are situated in the electrolyte; wherein when the power supply is on, the one or more resistors of the electronic circuit board regulate the current with the chamber with different voltages to maintain the temperature of the electrolyte within the chamber, and the positive electrode is undergone a single step anodization, producing the nanoporous structure on the positive electrode. 2. The electrochemical system of claim 1, wherein the nanoporous structure is nanoporous alumina. 3. The electrochemical system of claim 1, wherein the power supply provides a dc supply. 4. The electrochemical system of claim 1, wherein the applied potential is 0-60V, preferably 20-60V. 5. The electrochemical system of claim 1, wherein the electrolyte is an aqueous solution of acid, neutral or base. 6. The electrochemical system of claim 5, wherein the aqueous acid solution is phosphoric acid. 7. The electrochemical system of claim 5, wherein the aqueous acid solution is titrated with a base to have a defined pH. 8. The electrochemical system of claim 7, wherein the base is sodium hydroxide. 9. The electrochemical system of claim 7, wherein when the aqueous acid solution is titrated to a pH of 1, 3, or 5, the pore size is 40-240nm. 10. The electrochemical system of claim 1, wherein the electrolyte is an aqueous neutral solution with a pH of 7, and the pore size is 10-70nm. 1 1. The electrochemical system of claim 10, wherein the aqueous neutral solution is made of phosphoric acid titrated by a base. 12. The electrochemical system of claim 1 , wherein the electrolyte is an aqueous basic solution of a pH of 13, and the pore size is 50-100nm. 13. A process for producing nanoporous structures, said process comprising: setting up the electrochemical system of claim 1 ; and subjecting the anode electrode to anodization; wherein when the anode electrode is aluminum electrode, the nanoporous alumina are produced. 14. The process of claim 13, wherein the process further comprises a post-anodization treatment of etching to increase the pore sizes of the nanoporous alumina. 15. The process of claim 14, wherein the etching is performed with acid and basic solutions but not neutral solution. 16. A nanoporous alumina produced by the process of any of claims 13-15. |
THE SAME
Field of the Invention
[0001] The present invention generally relates to nanotechnology, and more particularly to nanoporous alumina and further to a process and system for producing the nanoporous alumina.
Background of the Invention
[0002] There has been considerable advancement over the years in fabrication of nano scale materials. The reason for this advancement is due to the unique physical and chemical properties these materials possess. Nanoporous alumina is one of the most promising nanostructure materials, having contributed significantly to the advancement of nanotechnology because of its wide areas of applications in electronics, optical and magnetic fields.
[0003] Anodically generated nanoporous alumina is prepared electrochemically from aluminum metal. Aluminum is usually anodized in an acid electrolyte under controlled conditions; it is oxidized to form an alumina film. The conventional acid electrolytes normally used for anodizing aluminum are oxalic acid, sulphuric acid, phosphoric acid, and mixed acids. It was observed that pores emerged on the alumina surface and the pore densities as high as 10 11 pores per square centimeter of alumina have been achieved. The size of pores usually in the range of 5-250nm depending on the anodization conditions used has been reported.
[0004] The current methods of preparation of nanoporous alumina films have certain limitations. For example, the preparation of nanoporous alumina has been limited to strongly acidic electrolytes with pH less than 3; the use of a temperature controlled water bath is another limiting factor that is required to provide low or extremely low temperatures suitable for the growth of the pores; pore size increment that is attributed to high voltage and prolonged anodization time proves to be another challenge because it is energy and time consuming that makes it not cost effective. [0005] Therefore, there is an imperative need for a simple, fast and cost effective system and process for producing the nanoporous alumina.
Summary of the Invention
[0006] The present invention is to eliminate or attenuate the limitations associated with the current systems and processes for producing the nanoporous alumina.
[0007] One aspect of the present invention provides an electrochemical system for producing nanoporous structure. In one embodiment, the electrochemical system comprises a power supply with a negative end and a positive end, an electronic circuit board being disposed within a concealed box, wherein the electronic circuit board with an inlet and an outlet comprises one or more power resistors, wherein the inlet is electrically coupled to the positive end, a positive electrode containing the material for producing the nanoporous structure, wherein the positive electrode is electrically coupled to the outlet, a negative electrode, a monitor module electrically coupled between the positive and negative electrodes for monitoring the potential and current applied to the positive and negative electrodes, and a chamber for an electrolyte, wherein the positive and negative electrodes are situated in the electrolyte; wherein when the power supply is on, the one or more resistors of the electronic circuit board regulate the current with the chamber with different voltages to maintain the temperature of the electrolyte within the chamber, and the positive electrode is undergone a single step anodization, producing the nanoporous structure on the positive electrode.
[0008] Another aspect of the present invention provides a process for producing nanoporous structures. In one embodiment, the process comprises setting up the electrochemical system as described above; and subjecting the anode electrode to anodization; wherein when the anode electrode is aluminum electrode, the nanoporous alumina are produced.
[0009] Yet another aspect of the present invention provides a nanoporous alumina produced by the process described above.
[0010] The objectives and advantages of the invention will become apparent from the following detailed description of preferred embodiments thereof in connection with the accompanying drawings. Brief Description of the Drawings
[0011] Preferred embodiments according to the present invention will now be described with reference to the Figures, in which like reference numerals denote like elements.
[0012] FIG 1 shows a schematic diagram of the electrochemical system with power regulating module in accordance with one embodiment of the present invention.
[0013] FIG 2 shows an SEM micrograph of the top view of anodized aluminum at pHl and 60V potential for 1 and 3hrs.
[0014] FIG 3 shows an SEM micrograph of the top view of anodized aluminum at pH3 and 60V potential for 1 and 3hrs.
[0015] FIG 4 shows an SEM micrograph of the top view of anodized aluminum at pH5 and 60V potential for 1 and 3hrs.
[0016] FIG 5 shows an SEM micrograph of the top view of anodized aluminum at pH7 and 60V potential for 1 and 3hrs.
[0017] FIG 6 shows an SEM micrograph of the top view of anodized aluminum at pH13 and 20V potential for 2hrs.
[0018] FIG 7 shows an SEM micrograph of the top view of post anodization treatment of anodized aluminum at pHl .
[0019] FIG 8 shows an SEM micrograph of the top view of post anodization treatment of anodized aluminum at pH3.
[0020] FIG 9 shows an SEM micrograph of the top view of post anodization treatment of anodized aluminum at pH5.
[0021] FIG 10 shows an SEM micrograph of the top view of post anodization treatment of anodized aluminum at pH7.
Detailed Description of the Invention [0022] The present invention may be understood more readily by reference to the following detailed description of certain embodiments of the invention. [0023] Throughout this application, where publications are referenced, the disclosures of these publications are hereby incorporated by reference, in their entireties, into this application in order to more fully describe the state of art to which this invention pertains.
[0024] The inventors of the present invention have made several discoveries. First, the temperature during the anodization could be controlled with a power resistor, eliminating the requirement of a temperature-controlled water bath during the anodization. Second, when the temperature was maintained at the room temperature, the applied potential and current density could be lower, and the duration of anodization could be shorter, making the production process cost effective. Third, the electrolyte used during anodization could be acidic aqueous solutions, neutral aqueous solutions or basic aqueous solutions. Fourth, the pore sizes could be controlled by the utilization of different electrolytes. Finally, the post anodization treatment increases the pore sizes.
[0025] One aspect of the present invention provides an electrochemical system that is reliable, fast and inexpensive for producing nanoporous alumina with controlled pore sizes in strong acids, weak acids, and neutral and basic electrolytes.
[0026] Referring to FIG 1, there is provided a schematic diagram of the electrochemical system in accordance with one embodiment of the present invention. The electrochemical system 1 comprises a power supply 10, a switch 20, a power regulating module 30, a monitoring module 40, a positive electrode (anode) 50, a negative electrode (cathode) 60, a thermometer 70, and a chamber 80. Briefly, the power supply 10 is preferably a dc power supply for providing the electric power for the electrochemical system. The positive output of the power supply 10 is electrically coupled with the switch 20 that controls the supply of the electric power. The power regulating module 30 is electrically coupled with the switch, positive electrode 50 and monitoring module 40, where the power output from the power regulating module 30 is directed to the positive electrode and monitored by the monitoring module. The monitoring module 40 is electrically coupled with the negative electrode 60 and negative inlet of the power supply 10. The chamber 80 is used for containing an electrolyte in which the positive and negative electrodes and the thermometer are disposed; when the power is supplied, the anodization of the positive electrode is performed. The thermometer 70 monitors the temperature of the electrolyte in the chamber; it can be either electronically communicated with a controller (not shown) or the power regulating module to control the electric power supplied to the electrolyte in the chamber or manually read so that the power supply can be regulated accordingly. The term "anodization" as used herein refers to a process of subjecting a metal to electrolytic action at the anode of the electrochemical system.
[0027] In one embodiment of the present invention, the voltage from the power supply can be in the range of but not limited to 0 to 100V, preferably from 10-70V. In one preferred embodiment, the voltage ranges from 20 to 60V.
[0028] In one embodiment of the present invention, the direction of current flow is controlled by a start and stop device. The start and stop device can be any conventional electric switches.
[0029] In one embodiment of the present invention, the voltage and current are regulated by the power regulating module 30 that replaces the temperature controlled water bath used in the prior arts. The power regulating module 30 comprises an electronic circuit board that contains one or more power resistors. In one embodiment, the power resistors have wattage in the range of but not limited to 0 to 100 watts, preferably in the range of 5- 80 watts, and more preferably in the range of 10-50 watts. The power resistors are comprised of electric resistors that can be in the range of 0 to 1000 ohms, preferably in the range of 200 to 700 ohms, and more preferably in the range of 300 to 500 ohms. The power resistor can be Aluminum cased or White ceramic body power resistors. The one or more power resistors used in the power regulating module have dual functions: (1) limit the current flowing through the electrochemical cell with every increase in voltage; and (2) permit the supply of desired current/current density suitable for pore formation within the different electrolytes.
[0030] In one embodiment of the present invention, the positive electrode is aluminum. The aluminum electrode can be prepared by electrochemical polishing in 1 :4 volume mixture of f HC10 4 and C 2 H 5 OH at constant current density of 0 to 1000 mA/cm 2 for lmin at 10°C, preferably within 10 to 700mA/cm 2 , and more preferably within 300 to 500mA/cm 2 . The thickness of the aluminum electrode is, but not limited to, 0.1 to 1mm, preferably 0.1 to 0.5mm, and more preferably 0.3mm.
[0031] In one embodiment of the present invention, the cathode electrode is, but not limited to, a platinum electrode and the anode electrode is an aluminum template. [0032] In one embodiment, the monitoring module 40 across the electrodes records the voltage and current across the chamber. In one embodiment, the monitoring module is a multimeter to complement the effects of the power resistors and to confirm the exact amount of voltage and current supplied across the templates in the electrochemical chamber.
[0033] In one embodiment, the electrolyte is an aqueous acidic solution that is made of phosphoric acid. Its pH is titrated by a base, preferably sodium hydroxide. A wide range of acid electrolytes can be used in the present invention, such as sulfuric acid, oxalic acid, phosphoric acid etc.
[0034] In one embodiment, the electrolyte is an aqueous neutral solution that is made of a salt; the aqueous neutral solution can also be made by titrating an acidic solution with a base to reach a pH of 7. In one actual operation, nanoporous alumina films were prepared using the experimental setup shown in FIG 1. More specifically, the templates were electrochemically polished in 1 :4 volume mixtures of HC10 4 and C 2 H 5 OH at constant current density of 500mAcm ~2 for lmin at 10°C. The neutral electrolyte was prepared by titrating 10% phosphoric acid at pHl with 1M sodium hydroxide to pH7. A 60V dc power supply at room temperature was applied across the electrochemical cell for the process of developing the pores for 1 and 3 hrs respectively.
[0035] In one embodiment, the electrolyte is an aqueous basic solution that can be made of a base that is titrated by an acid to reach a desired pH. In one actual operation, nanoporous alumina films were prepared using the experimental set up shown in FIG 1. More specifically, the template was electrochemically polished in 1 :4 volume mixtures of HCIO 4 and C 2 H 5 OH at constant current density of 500mAcm "2 for lmin at 10°C. The basic electrolyte was prepared by titrating 20% phosphoric acid with 2.5M sodium hydroxide until it attains pH13. A 20V dc power supply at room temperature was applied across the electrochemical cell for the process of developing the pores for 2hrs.
[0036] The pH of the electrolytes controls the pore sizes; typically, the use of an acidic electrolyte results in the pore size in the range of 40-240nm; the use of an aqueous neutral solution with a pH of 7 with the pore sizes in the range of 40-70nm; and the use of an aqueous basic solution with the pore sizes in the range of 50-lOOnm.
[0037] The following are the examples of the nanoporous alumina produced by the electrochemical system of the present invention. [0038] FIG 2 shows an SEM micrograph of the top view of anodized aluminum at room temperature (30°C), pHl with electrolyte (phosphoric acid), 60V potential and current (70- 100mA) for 1 and 3hrs, wherein the pore sizes were 100-150nm.
[0039] FIG 3 shows an SEM micrograph of the top view of anodized aluminum at room temperature (30°C), pH3 with electrolyte (phosphoric acid), 60V potential and current (40-60mA) for 1 and 3hrs, wherein the pore sizes were 100-200nm.
[0040] FIG 4 depicts an SEM micrograph of the top view of anodized aluminum at room temperature (30°C), pH5 with electrolyte (phosphoric acid), 60V potential and current (10-30mA) for 1 and 3hrs, wherein the pore sizes were 50-100nm.
[0041] FIG 5 shows an SEM micrograph of the top view of anodized aluminum at room temperature (30°C), pH7 with electrolyte (neutral), 60V potential and current (0- 10mA) for 1 and 3hrs, wherein the pore sizes were 10-70nm.
[0042] FIG 6 shows an SEM micrograph of the top view of anodized aluminum at room temperature (30°C), pH13 with electrolyte (basic), 20V potential and current (0- 5mA) for 2hrs, wherein the pore sizes were 50-1 OOnm.
[0043] Another aspect of the present invention provides a process for producing the nanoporous structures using the electrochemical system as disclosed above. The process comprises setting up the electrochemical system and subjecting the anode electrode to anodization. When the aluminum electrode is used as the anode electrode, the nanoporous alumina are produced. The process can be performed at room temperature, low voltage and short duration. In one embodiment of the present invention, the process further comprises a post-anodization treatment to increase the pore sizes of the nanoporous alumina. The post-anodization treatment is an etching process using an acid solution as the etchant. The acid solution is phosphoric acid at pHl. Acid and basic solutions can be used but not the neutral solution. The process is performed by submerging or immersing the template into the prepared solution. In one embodiment of the present invention, the duration of the post-anodization treatment is, but not limited to, 5 to 60 minutes, preferably 45 minutes.
[0044] The following are the examples of the nanoporous alumina undergoing post- anodization treatment.
[0045] FIGs 7 to 10 show SEM micrographs of the top view of post anodization treatment of anodized aluminum at pHl, 3, 5 and 7. Post treatment with 5% phosphoric acid was prepared and the anodized aluminum templates were submerged into the solution for 45minutes. Before: pHl= 100-150nm; pH3=100-200nm; pH5=50-100nm; pH7=10- 70nm. After: pHl =200-3 OOnm; pH3=200-300nm; pH5=150-200nm; pH7= 40-150nm.
[0046] The electrochemical system and process of the present invention have several advantages over the prior arts. For example, the electrochemical system has a simple setup and is easy to use. The electrochemical system does not require the use of temperature controlled water bath; the production of nanoporous structures is cost effective. The electrochemical system provides and maintains the desired current density and voltage suitable for the growth of the pore sizes. The electrochemical system provides suitable and convenient temperature (usually room temperature) for the nucleation and growth of pore sizes. The electrochemical system and process are energy saving because they do not need to anodize at very high voltage and longer time. The electrochemical system and process are able to use acid free neutral or basic electrolytes for nucleation and growth of pore sizes. The electrochemical system and process provide production of nanoporous alumina structures at any desired and convenient pH.
[0047] While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited. Alternative embodiments of the present invention will become apparent to those having ordinary skill in the art to which the present invention pertains. Such alternate embodiments are considered to be encompassed within the scope of the present invention. Accordingly, the scope of the present invention is defined by the appended claims and is supported by the foregoing description.
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