WO2019139554A1

WO2019139554A1 - A production method for a metallic material with high surface area nanostructures

Info

Publication number: WO2019139554A1
Application number: PCT/TR2019/050017
Authority: WO
Inventors: Esin AKÇA; Çağla AKGÜN; Gökhan Demi̇rci̇
Original assignee: Aselsan Elektronik Sanayi Ve Ticaret Anonim Sirketi
Priority date: 2018-01-10
Filing date: 2019-01-09
Publication date: 2019-07-18

Abstract

The invention relates to a production method for high surface area materials in which three dimensional 5 metal oxide, metal or metal/metal oxide nanostructures with improved morphology and crystal structure are formed on a conductive substrate. Another purpose of the invention is to obtain a three-dimensional oxide nanostructures on conductive substrates by using the crystal growth method titled as "oriented attachment", which eliminates the complex process steps and the use of templates, binders or additive materials.

Description

A PRODUCTION METHOD FOR A METALLIC MATERIAL WITH HIGH SURFACE AREA

NANOSTRUCTURES

DESCRIPTION

Technical Field

The invention relates to a production method which enables the formation of three-dimensional metal oxide, metal or metal/metal oxide nanostructures having a high surface area at the surface of a substrate, which is inherently conductive or coated to make its surface conductive.

Prior Art

In a technical application, various templates (moulds), binders and/or additive materials are used for the production of metal or metal oxide powder materials and their coating on other substrates. There are many interfaces in these structures that are created by combining materials of different properties. The complex process steps used to assemble the various materials increase both the cost of the process and the probability of making an error. In addition, since different interfaces exhibit various characteristics in terms of material properties, such structures cannot provide sufficient efficiency in applications that require in material perfections.

In another technical application, the metal oxide nanostructures are formed directly on a conductive substrate, covering the surface, by a crystal growth method which is named "oriented attachment". In other studies, single crystal oxide nanostructures are directly grown on a substrate by creating larger surface area than that of the substrate. Copper oxide (CuO), zinc oxide (ZnO), manganese oxide (MnC ), nickel oxide (NiO), iron oxide (Fe2C>3), and tungsten oxide (WO3) are some of the materials used and/or synthesized in these studies. These metal oxide nanostructures, which are formed on certain substrates, are limited in terms of surface area, morphology and/or crystal structure due to the reasons caused by the crystal growth mechanism. It is also possible to obtain metal nanostructures by further reduction of these oxide nanostructures. However, when the previous studies are examined, it is observed that there are limited number of studies on the production of metallic nanostructures, which are obtained by the reduction of metal oxide nanostructures using various methods to metal form. Similar to the metal oxide growth, limited surface area and morphology problems are also encountered in obtaining metal nanostructures by reduction. The data on the metal/metal oxide nanostructures obtained through the use of these structures are also very limited.

The crystal growth method called “oriented attachment” is a method in which various structures can be directly obtained on conductive substrates without using the templates (moulds), binders and/or additive materials. In this method, nano-sized metal oxide/hydroxide particles dispersed in an aqueous media tend to aggregate in a certain crystal plane in a chemical or electrochemical system and form larger size crystals. As accepted in classical crystal growth theory, instead of atoms or molecules, larger particles form single crystal nanostructures by attaching to the end of crystals. These particles are attached to the substrate surface in a nonparallel orientation, thereby the total surface area is increased. The formation of metal oxide nanostructures by“oriented attachment” crystal growth method directly on a conductive substrate provides many advantages. First of all, unlike other techniques, this mechanism eliminates the complex steps required to work with nano powders. Secondly, the 3-dimensional nanostructures consisting of 1 - and 2-dimensional metal or metal oxide nano crystals in rod, leaf, pipe, or brush morphology can be grown on the metallic surface with a specific angle without requiring any template (mould), binder, additive material or complex equipment. Thus, a higher surface area, higher purity, lower cost and scalable oxide nanostructures are obtained compared to the initial state of the substrate. For example, nanostructured copper oxide crystals can be formed directly on a copper or copper coated substrate. However, this method also has some limitations in terms of surface area, morphology and crystal structure. The surface area, morphology and crystal structure of the nanostructure, which is obtained by the single axis crystal growth mechanism, which allows growth in only one directional, on a solid substrate are limited. The restrictions on these elements, which are very fundamental for nanotechnology applications, are detailed as follows.

Surface Area:

Metal oxide nanostructures should have a high surface area to highlight the advantages of the direct growth such as easier handling of nano particles, eliminating the need for binders and additive materials, and providing a better electrical contact. In a synthesis process where no substrate is used, the smallest crystal has the highest specific surface area (surface area / weight ratio). However, with the presence of a substrate, the small crystals nucleate and grow on the substrate while the nucleation sites are limited to the surface area of the substrate used. These nano crystals do not provide a large increase in the surface area compared to the initial surface area of the substrate because they coat the surface of the substrate as a layer. In other words, when only a single-layer of nano crystals is formed on the substrate with the“oriented attachment” crystal growth method, these nano crystals covering the substrate as a single thin layer do not significantly increase the surface area. In the“oriented attachment” method, in which the nucleation sites on the metallic substrate are limited to a single-layered crystal layer and which causes virtually no branching along the metal oxide crystal due to the uni-axial growth mechanism, the surface area can be expanded by increasing the height of only 1 - and 2-dimensional nano crystals. In order to obtain higher aspect ratio nanostructures, special process steps requiring high pressure and temperature are needed. In addition, it has also been observed that nano bars with high aspect ratio form bundles that limit the surface area. In this type of morphologies, the outermost nano bars were positioned almost parallel to the substrate surface and occupied the free surface area that could have been nucleated.

Morphology: In addition to the size and grain size distribution, the morphology of a nano material plays an important role in determining the performance of that material for a specific application. Since“oriented attachment” is a crystal growth mechanism that allows the crystal to grow in only one direction, the variety of morphologies that can be obtained through this method is limited. Therefore, when 3-dimensional tree like branched structures cannot be formed, only 1 - or 2-dimensional rod and plate-like crystal structures can be obtained.

Connection Characteristic and Crystal Structure: Another disadvantage of metal oxide nanostructures obtained by the“oriented attachment” method is the connection characteristics which are not suitable for semiconductor applications. The metal oxide-metal interfaces are formed by the direct growth of metal oxide nano crystals on the metal collector. Also, the charge carriers have to be carried along the semiconductor increasing the probability of recombination losses. As the distance that a charge must travel increase, the probability to meet crystal imperfections and consequently the noise level increases, leading to performance degradation. Furthermore, the single crystal nanostructures restrict the crystal planes and grain boundaries which might have potential use for some applications. The existing "oriented attachment" growth options are deficient in obtaining different crystal planes and grain boundaries on high surface area nanostructures.

1 - or 2-dimensional metal nanostructures obtained by the reduction of 1 -or 2-dimensional metal oxide nano crystals can be used as substrates with a high surface area, high electrical and/or thermal conductivity without requiring the use of complex and expensive equipments. These metallic substrates with high porosity have a great potential for various applications as a highly efficient catalyst, current and/or heat collector. A limited number of studies on metallic nanostructures obtained by the reduction of 1 - or 2- dimensional metal oxide nanostructures are available. Nonetheless, the issues regarding to the metal oxide growth and morphology are also valid for metal structures obtained by the reduction of metal oxides. Thus, the metal nanostructures obtained by the reduction of the 3-dimensional metal oxide nanostructures are also subject to similar restrictions.

The United States patent document no. US2013045328 refers to the electrodes synthesized from carbon nanostructures coated with a uniform and harmonious metal layer. In the preferred production process, first the carbon nanostructures are oxidized and then the surface preparation process is carried out by dipping the oxidized material into a solution of the desired pH value in order to form negative surface dipole. Following that, nanostructures are immersed in an alkaline solution containing an appropriate amount of non-noble metal ions to be absorbed in the surface reaction zone. Metal ions are then reduced either chemically or electrically. The nanostructures are exposed to a solution containing a salt of one or more noble metals which will be displaced by non-noble surface metal atoms absorbed by the galvanic displacement method. This process can be controlled and repeated to achieve the desired film coverage. The resulting nanostructures can be used as high-performance electrodes in the supercapacitors, batteries or other electrical storage devices.

The Russian patent document no. RU2493939 mentions nanostructures including the gate metals and sub oxides of gate metals, which are produced to be used as catalysis, membranes, filters and capacitor anodes, and their production method. In the production method proposed within the scope of invention, there are steps to subsequently oxidize and reduce the sub-oxides of the gate metals and then to quench the layered structures quickly to the point where they will be steady. As a result, nanostructures with high specific surface area are obtained.

Brief Description of the Invention

The purpose of the invention is to provide a production method which enables the production of metal oxide, metal or metal/metal oxide nanostructures with a high surface area, which are mechanically and electrically well connected to the conductive substrate, providing advanced morphology and crystal structure options. Another purpose of the invention is to provide a production method which enables to produce a high surface area by creating three-dimensional oxide nanostructures on conductive substrates by using "oriented attachment" the crystal growth method, which eliminates the complex process steps and the need for templates (mould), binders or additive materials.

Another purpose of the invention is to obtain metal nanostructures with a high surface area by reducing the metal oxides obtained through the 3-dimensional nanostructures.

Another purpose of the invention is to create advanced metal/metal oxide structures on 3-dimensional structures with high surface area by subsequent reduction and oxidation in a controlled manner.

Detailed Description of the Invention

In order to achieve the purpose of this invention, "A production method for a metallic material with high surface area nanostructures" is illustrated in the figures attached, that are briefly:

Figure 1 - (a) a graphical representation of the change in the electrochemical oxidation behavior of metal

(copper (Cu)) in multiple oxidation cycles at the current density of 0.5 mAcrrr² in 3 M KOH and (b) the electrochemical reduction of metal oxide to metal with a scan rate of 1 mVs^-1 in 3 M KOH. Anodic voltage steps resulting from the oxidation process are shown between A1 -12.

Figure 2- A graphical representation of the increase in the electrochemical active surface area after the first and second oxidation in 3 M KOH (For the second oxidation, samples that were oxidized and electrochemically reduced with 3 mAcrrr² in the first oxidation were used). The surface multiplier is the ratio of the electrochemically active surface area obtained after the process to the initial geometric area of the copper foil.

Figure 3- (a) A graphical representation of a combination of constant current and cyclic voltammetry data. The current density values of the alternating voltammetry chart, starting at -1 .6 V, are given with dots. The sample contains nanostructures obtained by the first reduction in the same solution of the first oxidation,. The oxidation peaks of A2-5 which are not visible in the graph are also shown in the insert. The time-voltage curve obtained during the third oxidation in the constant current scan is given by a straight line in the graph (b) Comparison of the amounts of oxides obtained in the oxidation up to the regions A10, A1 1 and A12.

Figure 4- SEM images of micron sized nanostructures obtained by electro-oxidation ((a) Cu(OH)2 after the first oxidation, (b) Nano Cu after the first reduction, (C) CuO after the second oxidation, and (d) Nano Cu after the second reduction)

Figure 5- SEM images of the reduced Cu nanostructures obtained after the anodizing of the Cu foil ((a) after the first reduction and (b) after the second reduction)

Figure 6- SEM images of micron sized nanostructures obtained by the second electrochemical oxidation ((a) Nano Cu surface after first reduction, (b) CU2O formation on Nano Cu 30 min after the A6 voltage step of the second oxidation, (C) CuO formation on Nano CU/CU2O at the end of step A10 of the second oxidation) The parts in the figures are numbered and their correspondences are given below.

A1-5. OH- adsorption in different copper crystal planes before the formation of CU2O.

A6. The formation of CU2O.

A7. The dissolution of CU2O.

A8. Formation of suspended solid 3-dimensional oxide growth precursors in the electrolyte.

A9. Increase of suspended solid 3-dimensional oxide growth precursors and initiation of 3- dimensional oxide nucleation.

A10. The growth of 3-dimensional oxide nanostructure.

A11-12. The use of 3-dimensional oxide growth precursors dispersed in electrolyte.

The present invention relates to a method of manufacturing a high surface area nanostructured metallic material, which has been developed in order to produce metal-based nanostructures having a high surface area with improved morphology and crystal structure, wherein; it comprises following process steps: placement of the metallic material to be developed in terms of surface area and morphology as an electrode in an electrolysis cell containing the basic aqueous electrolyte (KOFI, NaOH, etc.), formation of 3-dimensional metal oxide nanostructures (rods, sheets, and the like) that provide a high surface area on the metal surface and extend outwards from the metal surface, following the first electrochemical oxidation process (anodization),

first electrochemical reduction of metal oxide to metal form in the same electrolyte as a result of the removal of the oxygen atoms from the structure of the metal oxide material with the use of energy close to the lowest possible amount in practice (current density, voltage, temperature) sufficient for the complete realization of the reduction in order to protect the wide surface area of the 3-dimensional metal oxide nanostructure obtained by first electrochemical oxidation,

further oxidation of metal nanostructures which provides electrochemical re-oxidization of metal nanostructures by anodizing the metal nanostructures formed on the surface of the metallic material, that is obtained as a result of the first electrochemical reduction process, in the same basic electrolyte,

the application of electrochemical "further reduction" in the same electrolyte with the lowest energy input possible in practice aiming to remove the oxygen atoms present in the metal oxide nanostructures with a high surface area and advanced morphology as a result of the further oxidation process, while preserving the surface area and morphology of the structures as much as possible, multiple repetition of the further oxidation and reduction steps one after the other, if it becomes necessary in terms of morphology or surface area change,

obtaining metal oxide, metal or metal/metal oxide nanostructures with high surface area, advanced morphology and bonding characteristics on the surface of the final product metallic material, by completing further oxidation and reduction steps or by leaving them partially in a controlled manner.

In the preferred embodiment of the invention, the steps of further oxidation and further reduction are repeated in order to increase the surface area of the metallic material, prior to the step of obtaining the final product, which is a metallic material having high surface area metal oxide, metal or metal/metal oxide nanostructure on the surface. More than one repetition of the further oxidation and further reduction process steps ensures that materials with different morphology and surface areas are obtained. As shown in Figure 5, the increased number of further oxidation and reduction steps leads to a finer structure. In addition to increasing the total amount of oxide in further oxidation, it has a positive effect on the increase of surface area in the fine structures obtained.

In one embodiment of the invention, the metallic material for which surface area and morphology are desired to be developed can be directly used as a substrate. This metallic material is preferably chosen as copper.

In another embodiment of the invention, the metallic material for which surface area and morphology are desired to be developed is placed on another conductive, semi-conductive or insulating material in the form of a coating by electroplating, sputtering or similar methods.

In one embodiment of the invention, the oxidation (initial and/or further) of the metal surface is carried out by oxidizing the metal surface with oxygen at temperatures of 300-500 O with the method of "air oxida tion".

In one embodiment of the invention, the oxidation (initial and/or further) of the metal surface is carried out by oxidizing the metal in the autoclave under high pressure and temperature with the "chemical oxidation" method.

In one embodiment of the invention, the oxidation (initial and/or further) of the metal surface is carried out by oxidizing and anodizing the metal in a basic solution at the room temperature with the method of "electrochemical oxidation".

In another embodiment of the invention, the initial oxidation step is carried out by chemical or air oxidation, and the further oxidation step is carried out electrochemically. Since the surface area that can be obtained with further oxidation process steps depends on the surface area that is obtained in the initial oxidation process, the initial oxidation process parameters need to be optimized to give the highest surface area.

In one embodiment of the invention, the reduction of the metal oxide material to the metal form (first and/or further) is carried out in basic aqueous solution (NaOH, KOH, etc.) at the electrochemical reduction room temperature. The reduction is preferably carried out in the same electrolyte as the oxidation process, at low current densities in terms of reduction (<50 mAcrrr²) (preferably; -1 -10 mAcrrr²).

In one embodiment of the invention, the reduction of metal oxide material to a metal form (initial and/or further) is carried out under reducing gas. The process is carried out at £225 O, where the structure will be highly maintained by using a mixture of reducing gas containing hydrogen gas or formic acid vapor, preferably 3% and above as reducing gas.

In an embodiment of the invention, the reduction of metal oxide material to metallic form (initial and/or further) is carried out under the hydrogen plasma (50 seem, 300 W) for 20 min to 4 hours at temperatures below 75 O to decrease the required process temper ature less than it is required in hydrogen reduction.

In an embodiment of the invention, low current density is applied for electrochemical oxidation and a high concentration electrolyte solution is used in order to ensure that surface area increases and morphological changes occur on the metal surface are clearly observed. In an embodiment for the copper metal, it is preferred that the electrolyte solution is >0.5 M KOH and the geometric current density is <1 mAcrrr². In one embodiment of the invention, low current densities (<1 mAcrrr²) are used in the A6-A7-A8-A9 regions during the electrochemical oxidation to allow for increased CU2O content forming nano oxide growth precursors via dissolution.

In an embodiment of the invention, low current densities (<1 mAcrrr²) are used in steps A1 1 -A12 to allow the oxide growth precursors formed in the electrolyte to gather more on the nanostructures and increase the surface area due to the fact that the nano crystal growth rate is not too high.

In one embodiment of the invention, low current densities (<1 mAcrrr²) are used in the A9-A10 regions during electrochemical oxidation to allow the nano crystals of oxide growth precursors to increase the amount of nucleation and growth.

In one embodiment of the invention, the addition of the chemically obtained Cu(OH)2 solution increases the amount of oxide crystal growth precursor in the electrolyte, thereby resulting in the increase of the amount of oxide and surface area obtained in the A1 1 -A12.

In one embodiment of the invention, a certain amount of agglomeration of the structures with themselves is provided to increase the mechanical strength of the nanostructures by an energy input (temperature, voltage or current) which is higher than the minimum amount of energy practically applied to protect the structure during the reduction process.

In one embodiment of the invention, the surface area to be obtained by electrochemical oxidation/reduction is increased by the treatment of the substrate selected as the initial material to have a higher surface area prior to electrochemical oxidation and reduction processes by grinding, dendritic metallic coating, chemical etching and similar methods.

In one embodiment of the invention, CU2O is formed on the resulting copper nanostructures for copper metal to be terminated to the end of the first CU2O formation voltage step (A6), in a manner that is adjustable by the ratio of the amount of current required for the entire step to the current passed through this step.

In one embodiment of the invention, copper nanostructures are completely transformed into CU2O as a result of the continuation of the oxidation process until the end of the first CU2O formation voltage step (A6) for copper metal.

In one embodiment of the invention, metal nanostructures that are completely comprising of CU2O and contain more CU2O than the step A6 are obtained by removing the oxidative current after reaching the end of the voltage step indicated by A9 for copper metal.

In one embodiment of the invention, metal nanostructures containing different amounts of Cu(OH)2/CuO on CU2O are obtained for the copper metal when the voltage steps indicated by A10, A1 1 and A12 are reached.

In one embodiment of the invention, a method of production for a metallic material with high surface nanostructures according to Claim 1 , characterized by the metallic material to be developed in surface area and morphology is an alloy.

In one embodiment of the invention, there are high number of active metal crystal planes as shown in A1 - 5 voltage regions as a result of the formation of nano copper with high surface area. In one embodiment of the invention, increasing OH- in the A1 region, as a result of the high surface area copper nanostructure formation, is adsorbed.

Due to the method of the invention, it is possible to coat conductive, semi-conductive or non-conductive materials on metal, metal oxide, metal/metal oxide nanostructures.

In the context of the invention, the surface area and the morphology of the metallic material to be developed can be used directly as a substrate, or this metallic material is placed on another conductive, semi- conductive or non-conductive material by electroplating, sputtering or so on to obtain a conductive substrate. With the oxidation, 3-dimensional nano oxide structures are formed on the metal surface, providing a high surface area. Metal oxide structures cover the metal surface to form a layer. The scope of the present invention mainly involves anodizing and oxidization of the Cu surface in a basic solution at room temperature by the "electrochemical oxidation" method. However, other oxidation methods which produce 3-dimensional oxide may be used for the initial oxidation. For example; oxidation of the copper surface with ambient oxygen at 300-500 O by using the "air oxidation" method or oxidation of the Cu surface in the autoclave under high pressure and temperature by using the "chemical oxidation" method

The 3-dimensional metal oxide nanostructures obtained by the initial oxidation is reduced to the metal form of the material, while maintaining the provided high surface area. At this stage, in order to protect nanostructures on the surface of the material, the initial reduction process is carried out by removing oxygen atoms from the metal oxide structure with the lowest energy input possible in practice. In cases where more energy is being used than it is required, such as reduction at high temperatures, it is seen that nanostructures tend to agglomerate on themselves. Agglomeration is an undesirable condition for the reduction of high surface area provided by 3-dimensional nanostructures. This is prevented by using methods such as making the electrochemical reduction at low current densities, making the reduction with hydrogen gas at temperatures £ 225 O which will not disturb the structure, or ma king the reduction under hydrogen plasma in order to further reduce the process temperature.

The substrate, which is obtained as a result of the initial reduction and has metal nanostructures on its surface, is anodized in a basic solution. This process called "further oxidation" provides the electrochemical re-oxidation of the reduced metal nanostructures in the previous step and the formation of secondary oxide structures by "oriented attachment" on these structures. Anodization process at low current concentrations (preferably less than 1 mAcrrr² in Cu system) increases the surface area of the material due to the fact that it provides the time required for the dissolution/accumulation-based CU2O growth process, which is kinetically slow and 3-dimensional“oriented attachment” crystal growth process based on Cu(OH)2 addition to take place in a higher proportion. The high electrolyte concentration (preferably higher than 0.5 M KOH in the Cu system) and temperature (a temperature at which the evaporation rate does not cause a change in concentration) accelerate the oxidation process as it increases the reaction kinetics and the mass transfer rate in the electrolyte. The metal oxide nanostructures with a high surface area resulting from the repeated oxidation are subjected to "further reduction". When passing to the metal obtained by the reduction of CuO, about 20% of the mass loss is concerned. Furthermore, the increase in density between CuO having a density of 6.31 gem ³ and Cu having a density of 8.96 gem ³ indicates that the 3-dimensional oxide structure cannot remain exactly the same during reduction. With the reduction process, it is intended to remove the oxygen atoms within the structure without changing the morphology of the metal oxide nanostructures as much as possible (30% mass loss can occur as the transition from oxide to metal).

More than two repetition of the further oxidation and further reduction process steps ensure that materials with different morphologies and crystal structures with different surface ratios are obtained. For example, in a /copper oxide system, the metal surface obtained in the second reduction is more compact and thinner than the structure in the previous step, as shown in Figure 5. As another example, in the copper/copper oxide system, different properties of structures can be obtained if the reduction and oxidation steps are interrupted before completion. After the third reduction, in the case when the fourth oxidation is made only until the end of the A6 voltage step given in Figure 1 , a nanostructure consisting entirely of cuprous oxide (CU2O) is obtained (Figure 6b).

In one embodiment of the invention, the 3-dimensional metal oxide nanostructures obtained by oxidation in the initial and further reduction process steps are reduced to a metal by reduction treatments such as chemical, electrochemical, under reducing gas, under plasma and so on. Since electrochemical reduction can be performed at room temperature, it is good to maintain the structure. With electrochemical reduction process using low current densities, it is possible to achieve less degradation of the structure. For the reduction of metal oxide nanostructures, the use of electrochemical cell directly used in the production of metal oxide provides a great advantage. On the other hand, in the case of electrochemical reduction as a final treatment, some re-oxidation again on the copper is inevitable even after the process is done due to the strong oxidizing environment. A similar oxidation occurs during the rinsing process required to clean the electrolyte on the nanostructure. A similar situation is observed in the chemical reduction of metal oxides. Reduction is carried out under the gas so that the surface is completely free of oxides. Excessive energy input during reduction, especially in high mobility metals such as copper, leads to the reduction of surface areas due to the agglomeration of nanostructures on themselves. For example, copper requires a temperature of at least 1 50 under the hydrogen g as. Due to the long duration of reduction at this temperature and the mobility of the copper, there are some deteriorations in the structure. With the temperature rising above 225 , nanostructures are becoming extremely distorted. Due to the high activity of Fl⁺ ions formed by the use of hydrogen plasma instead of hydrogen reduction, it is possible to achieve lower temperatures and less degradation of the structure.

Within the scope of a production method for metallic material with high surface area nanostructures of the invention, the results obtained by the "oriented attachment" crystal growth method are improved by adding electrochemical oxidation step to the production process. 3-dimensional metal oxide nanostructures obtained by electrochemical oxidation are reduced to metal by chemical, electrochemical methods, or under reducing gas or plasma. The surface area, morphology and connection characteristics of the resulting metal, metal oxide or metal/metal oxide nanostructures are improved by the use of electrochemical oxidation steps introduced by the multiple oxidation-reduction process.

Within the scope of a production method for metallic material with high surface area nanostructures of the invention, is basically carried out in the form of repeated electrochemical oxidation and reduction processes by using the method of “oriented attachment”. As a result of these successive oxidation and reduction processes, 3-dimensional metal (Cu) high surface area nanostructures are obtained on the conductive substrate. During the first electrochemical oxidation, as shown in the first graph of Cu oxidation in Figure 1 (a), the voltage value rapidly rises to a wide voltage range of about -2.60 V (3M KOH). In this step, there is a voltage increase region indicated by A9, similar to that shown on the third oxidation graph in Figure 1 , and an A10 voltage region expressed by a short voltage decrease and then a re-increase after the maximum voltage value is reached.

In step A9 in the Figure 1 , the CU2O amount first increases and over time, the Cu(OH)2 based 3-dimensional structure starts to grow. Up to the peak point of the A9 step, CU2O grain sizes grow and form a porous CU2O layer on Cu. Cell voltage increases due to the growth of CU2O grains and oxide growth precursors in the electrolyte. At the same time, near the peak, solid Cu(OH)2 nucleation begins on CU2O. In the A1 0 region, CU2O is largely dissolved in electrolytes. The metal oxide growth precursors formed by dissolution in electrolytic are performing the growth of Cu(OH)2 and/or CuO single crystal rods. The voltage decreasing around 15 mV initially due to the CU2O dissolution during the formation of Cu(OH)2 and/or CuO after the peak of the main voltage step then begins to increase again. After the completion of the oxide crystal growth in the main voltage step, the voltage increases rapidly to +0.52 V, where the oxygen gas output starts.

3-dimensional nanostructures obtained by air oxidation and chemical methods show similar properties to those obtained by electrochemical oxidation. For this reason, air oxidation and chemical oxidation methods can be used instead of the initial electrochemical oxidation. Due to similar surface morphology and connection characteristics, the same restrictions apply to these structures and it can be improved by electrochemical initial oxidation and reduction processes.

There are time-dependent process steps such as CU2O formation and electrolytic dissolution during copper anodization, mass transfer of oxide growth precursors into the electrolyte, and orientation of nano particles containing copper. The effect of current density on surface areas obtained in the initial oxidation is shown in Figure 2. Keeping the current density low during oxidation (preferably less than 1 mAcrrr²) allows the time-dependent steps such as formation, dissolution, diffusion, directional bonding, etc. of CU2O in the electrolyte to be completed in a larger scale and increases the amount of oxide and hence the surface area obtained. Similarly, increasing the concentration and temperature of electrolyte have an increasing effect on the mass transfer rate and reaction kinetics of copper compounds. Low current density (preferably less than 1 mAcrrr² for copper), high chemical concentration (higher than 0.5M for copper) and anodization at a higher temperature than room temperature constitute optimum process conditions for all electrochemical oxidation steps. However, although there is a slight increase in surface area in the initial oxidation, there is no significant improvement in process control. The determination of intermediate reaction steps in seconds in the electrolyte other than the main voltage step is very difficult due to the reasons that the amount of the reaction products is low, the reaction times are short and the reactions are carried out in a liquid. Therefore, the mechanism of copper oxidation has not been fully understood to date. Therefore, due to the lack of process control, no significant improvement in surface area, morphology and crystal structure can be achieved.

After the initial electrochemical oxidation process, there comes initial reduction process. The initial reduction can be done by chemical or electrochemical methods as well as under reducing gas or plasma. However, as described previously, for metal oxide structures as well as for metal structures, there are common restrictions in terms of surface area and morphology, regardless of the initial oxidation and reduction methods used. The peak obtained from the initial reduction is shown in Figure 1 (b). The size of the reduction peaks is proportional to the amount of oxide in the structure and therefore to the increase in surface area obtained. The reduction graph of the initial copper foil and the reduction graph of the oxides obtained from the initial reduction provide information about the surface increase. The proportional increase achieved by the current density on the copper surface is also given in Figure 2.

Nano-copper material with a high surface area intended to be obtained within the scope of a production method for metallic material with high surface area nanostructures is obtained through the application of optimized process conditions such as low current density of the second electro-oxidation, high chemical concentration and high temperature. Voltage-time graphs with characteristics that vary with the continuation of the electrochemical oxidation cycles applied to the surface of the same copper sample are shown in Figure 1 (a). The main difference of the second and third oxidation cycles from the initial oxidation is the other potential steps that are expanded and became visible during the voltage rise. These potential steps, indicated by arrows and numbers in Figure 1 (a), correspond to intermediate reactions occurring during the oxidation. These voltage steps, which are not easily distinguished due to the fact that they are in seconds in the initial oxidation, are extending to hours due to the high amount of reaction products obtained because of the high amount of material present at the beginning of the second and third oxidation. Voltage steps that extend during the second and third oxidation also ensure that the oxidation mechanism can be understood and controlled. By using these voltage steps, it is possible to obtain sufficient liquid-solid samples for general characterization devices such as XRD, XRF and AES at desired stages and to examine the morphology and to understand the reaction mechanism by examining the morphology and the changes that occur in the surface area, morphology and connection characteristics as a result of these reactions. At the same time, it is possible to control surface area, morphology and connection characteristics by interrupting the process at the desired intermediate voltage step.

The surface area and morphology of the nano Cu structure, which is obtained by the reduction of the metal oxide structure by chemical or electrochemical methods or the reduction under gas or plasma for a second time, can be improved. The voltage steps shown in Figure 1 (a) are related to the increase in the crystal amount of metal oxide as a result of multiple electrochemical oxidations. The time required for more material oxidized by the reason of constant current density increases and the addition of more material to the growing crystal causes the oxidation step to extend in the next oxidation. The increase in the amount of oxide crystals can be understood from the increase in the area under the successive electrochemical reduction peaks in Figure 1 (b). Similarly, as shown in Figure 2, after the initial and second reduction, electrochemical active surface area increases proportionally with the amount of metal oxide crystals.

The increase in the electrochemical surface area at low current values may be related to the provision of sufficient time for time-dependent processes such as the formation of CU2O in the electrolyte during crystal growth, the dissolution, the diffusion of the oxide growth precursors to be performed at a higher rate. As more time is provided for these time-dependent processes to be performed at lower current densities, more oxide growth precursors are developed in electrolytes and 3-dimensional crystals can have a greater chance of growth. Since thinner crystalline structures are formed in the second and third oxidations, the increase in surface area is greater than the increase in oxide content.

The mechanism of oxidation of copper is not fully known. Flowever, the most widely accepted theory is that copper is dissolved as copper hydroxide ions (Cu[(OFI)4]^2·) in the electrolyte and then forms nano-size solid CU(OH)2 particles in the electrolyte. It is thought that Cu(OH)2 particles are then added to each other to form single crystals of Cu(OH)2 on the surface and following that Cu(OH)2 crystals lose water and cupric oxide (CuO) crystals are formed. Depending on the processing conditions of crystal growth, it is observed that the copper crystal growth precursors Cu[(OH)4]^2_ and Cu(OH)2 compounds cannot be used completely and the color change occurs in solution. Figure 3 shows the graph obtained by overlapping the graphs obtained from the measurements made in the form of constant current and cyclic voltammetry. Figure 3 gives information on both the crystal formation steps and the conditions under which the precursors remaining in the solution can be recovered in greater quantities. In case of optimized operation conditions and the use of a high surface area substrate the signals obtained in both current and cyclic voltammetry measurements are much stronger than normal. Since the presence of precursor compounds forming the copper oxide crystal in the electrolyte increases the surface area obtained, it is possible to carry out the initial oxidation and reduction by electrolysis and to use the remaining compounds from the first oxidization in the electrolyte in further oxidation. The optimum processing conditions for the A6-10 steps given in Figure 3, which increase the amount of oxide growth precursors, are also valid for A1 1 -12 steps where the remaining precursors in solution are used in crystal formation (preferably lower current density than 1 mAcrrr² and higher concentration than 0,5M KOH for copper). The extending voltage step by multiple oxidation in the A1 1 region and the high oxidation peak in the A12 region are related to the use of remaining precursors in solution in crystal growth process.

A1 -5: relates to the adsorption of OFF ions to the surface in different copper crystal planes and the crystal plane changes in copper surface prior to the actual oxidation reactions. Up to now, the oxidation behavior in this number has not been demonstrated on a single copper specimen before the CU2O formation voltage. These data show that the copper nanostructure obtained is very rich in terms of active crystal planes.

The fact that the reduction peak corresponding to the A1 zone is so distinctive is a special case provided by the copper nanostructure obtained. The high current obtained in this region associated with the reduction of adsorbed OFF ions in copper crystal plane shows that the copper structure has a high OFF adsorption capacity. Carbon monoxide and nitrate reduction reactions are carried out in the region, where hydrogen output is also started. It is possible to use the obtained copper nanostructure for these applications which are important in terms of environment and energy.

A6-10: A6 is related to the formation of CU2O and A7 is related to electrolyte dissociation of CU2O by dissolution. In the A8 peak, it is thought that suspended Cu(OH)2 crystals are formed in the electrolyte in which copper dissolved in Cu (I) form in the previous step. The amount of dissolved CU2O is increasing in A9 region. In a voltage region near the end of A9, the nucleation of Cu(OH)2 and CuO crystals, which increase the surface area, is also beginning. In the A10 region, which starts after the A9 voltage peak, CU(OH)2 and CuO crystal growth occurs due to the oxide growth precursors formed by CU2O dissolution.

A1 1 -12: Oxidation growth precursors formed in previous steps are used in the Cu(OH)2 based 3- dimensional crystal growth from A9 onwards. In cases such as the use of nanostructured copper, the amount of oxide growth precursors in the electrolyte at the end of rapid CU2O dissolution in the A10 region is increasing more rapidly than compared to the rate of crystal formation. The A1 1 -12 regions concern with increasing the surface area through the recovery of oxide growth precursors dispersed in solution. In addition to high surface area and high electrolyte concentration, low current density values for oxidation in constant current and low scan rates in 1 mVs^-1 applied in cyclic voltammetry allow the process steps to be clearly separated. The slope change, which forms the A1 1 voltage step of about 0.00 V and the A12 voltage step after +0.1 00 V, becomes more apparent during the second and third oxidations. Although there is no high current in the A1 1 oxidation peak given in cyclic voltammetry, there is a wide voltage step in the corresponding region in the constant current graph. A high oxidation current is observed in the cyclic voltammetry graph in response to the A12 voltage step, where the rate of increase in constant current oxidation is decreasing. It is observed that the reduction of the oxidizations made up to these steps increases the amount of oxide in the regions A1 1 and A12, Figure 3b. This information shows that after the main potential step, the precursors remaining in the solution come together and are combined to the crystal growth end. In other words, the scanning rate of 1 mAcrrr² and 1 mVs^-1 used in these experiments, using high chemical concentration (3 M KOH), gives the time required for crystal growth in terms of time and thus in the A1 1 -12 regions, the oxide growth precursor remaining in the electrolyte is recovered and the surface area is further increased. In other words, the surface area can be increased by ensuring that these precursors are incorporated into the crystal growth at a higher rate using process parameters. This surface area increase is also observed when copper oxide growth precursors are prepared outside and then added to the electrolysis cell.

Figure 4 shows the morphological changes obtained on metal and metal oxide nanostructures. During the second oxidation, new metal oxide branches are formed on the Cu nano bars obtained at the end of the initial reduction. In this way, instead of forming a dense layer on the Cu foil surface, the new metal oxide crystals form a suspended nanostructure. This means that due to the highly porous structure of the high surface area required for electron transfer, a different morphology is obtained, which provides very convenient conditions for mass transfer.

Cu nano rods with large grains after initial reduction turn into a thinner Cu nanostructure after the second reduction which increases the surface area, as shown in Figure 5.

The different crystal structures and connection characteristics obtained by the controlled and extended oxidation steps can be explained by the following examples. The 3-dimensional Cu nanostructures shown in Figure 6 (a) are transformed into the 3-dimensional CU2O nanostructure shown in Figure 1 (b) at the end of the A6 voltage step shown in Figure 1 (a). During further oxidation, structures are formed at different CU2O/CU ratios ranging from 0 to 1 on the initial Cu nano bars, depending on where the voltage step A6 is located. Since oxidation occurs at the outermost surface of Cu, a CU2O is obtained on the Cu core. For this reason, not only CU2O with a high surface area is obtained, but also the connection characteristics are improved by radial connection and the low crystal length helps to reduce recombination losses. Similarly, using the A10 voltage step, the amount of Cu(OFI)2/CuO accumulating on the CU2O can be adjusted. The CU2O CU(OFI)2 and CuO crystals start to grow in the A9 voltage step. In the A10 region, this growth is accelerated due to the oxidation growth precursors obtained from CU2O dissolution. The rates of CuO or CU(OH)2 nanostructures on CU2O can be adjusted by interrupting the process at different locations on A10- 12. Figure 6c shows the CuO leaves grown on fine CU2O grains which are prepared to provide new connection characteristics. It is possible to produce this structure obtained by proceeding to the end of step A12 with thinner CuO leaves on A10.

Claims

1. The present invention relates to a production method for a metallic material with high surface area nanostructures, which has been developed in order to produce metal-based nanostructures having a high surface area via improved morphology and crystal structure, wherein; the production method characterized by following process steps:

- placement of the desired metallic material to be developed as an electrode in an electrolysis cell containing the basic aqueous electrolyte (KOH, NaOH, etc.),

formation of 3-dimensional metal oxide nanostructures (rods, sheets, and the like) that provide high surface area on the metal surface and extend outward from the metal surface, following the initial electrochemical oxidation process (anodization),

first electrochemical reduction of material to metal form in the same electrolyte as a result of the removal of the oxygen atoms from the structure of the metal oxide material with the use of energy close to the lowest possible amount in practice (current density, voltage, temperature) sufficient for the complete realization of the reduction in order to protect the wide surface area of the 3-dimensional metal oxide nanostructure obtained by first electrochemical oxidation,

further oxidation of metal nanostructures which provides electrochemical re-oxidization of metal nanostructures by anodizing the metal nanostructures formed on the surface of the metallic material, that is obtained as a result of the first electrochemical reduction process, preferably in the same basic electrolyte,

the application of electrochemical "further reduction" in the same electrolyte with the lowest energy input possible in practice aiming to remove the oxygen atoms present in the metal oxide nanostructures with a high surface area and advanced morphology as a result of the further oxidation process, while preserving the surface area and morphology of the structures as much as possible,

multiple repetition of the further oxidation and reduction steps one after the other, if it becomes necessary in terms of morphology or surface area change,

2. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the repetition of the further oxidation and reduction steps in order to increase the surface area of the metallic material, respectively, prior to obtaining the final product, which is a metallic material having high surface area with metal oxide, metal or metal/metal oxide nanostructure.

3. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the selection of the metallic material to be developed in surface area and morphology as copper.

4. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the direct use of the metallic material to be developed in surface area and morphology as a substrate.

5. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the placement of the metallic material to be developed in surface area and morphology as a coating on a conductive, semi-conductive or other insulating material by using electroplating, sputtering or similar method.

6. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the initial oxidation of the metal surface is carried out by oxidizing the metal surface with oxygen at temperatures of 300-500 using the method of "air oxidation" instead of electrochemical oxidation.

7. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the initial oxidation of the metal surface is carried out by oxidizing the metal in the autoclave under high pressure and temperature using the "chemical oxidation" method, instead of electrochemical oxidation.

8. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the initial oxidation of the surface is carried out by oxidizing and anodizing the metal in a basic solution at the room temperature using the "electrochemical oxidation" method.

9. A method of production for a metallic material with high surface area nanostructures according to Claims 6,7,8, characterized by the initial oxidation step is carried out by chemical or air oxidation, and the further oxidation step is carried out electrochemically.

10. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the reduction of metal oxide material to metal form by using low current densities (~10 mAcrrr²) in the same electrolyte where electrochemical oxidation is preferred due to the ease of application of electrochemical reduction.

11. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the reduction of metal oxide material to metal form is carried out by using reduction with hydrogen gas, at temperatures of 175-225 , which will ensure the highest maintenance of the structure.

12. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the reduction of metal oxide material to a metal form is carried out under the form of hydrogen plasma (50 seem, 300 W) for 20 min to 4 hours at temperatures below 75 to decrease the required process temperature less than it is required in hydrogen reduction.

13. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the use of low current density (<1 mAcrrr²), and high concentration electrolyte solution (> 0.5 M KOH) during the electrochemical oxidation to increase the surface area and the amount of oxide obtained.

14. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the conversion of the morphology into a finer structure with high surface area by performing multiple repetition of the oxidation and reduction steps one after the other.

15. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the use of low current densities (<1 mAcrrr²) in the A6-A7-A8-A9 regions during the electrochemical oxidation to allow for increased CU2O content forming nano oxide growth precursors via dissolution.

16. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the use of low current densities (<1 mAcrrr²) in the A9-A10 regions during electrochemical oxidation to allow the nano crystals of oxide growth precursors in the electrolyte to increase the amount of nucleation and growth.

17. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the use of low current densities (<1 mAcrrr²) in steps A1 1 -A12 to allow the oxide growth precursors formed in the electrolyte to gather more on the nanostructures and increase the surface area due to the fact that the nano crystal growth rate is not too high.

18. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the addition of the chemically obtained Cu(OH)2 solution that increases the amount of oxide crystal growth precursor in the electrolyte, thereby resulting in the increase of the amount of oxide and surface area obtained in the A1 1 -A12.

19. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the step of obtaining a metallic material with a high surface area nanostructures in the form of conductive, semi-conductive or non-conductive materials on metal, metal oxide, metal/metal oxide nanostructures.

20. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by a certain amount of agglomeration of the structures with themselves is provided to increase the mechanical strength of the nanostructures by an energy input (temperature, voltage or current) which is higher than the minimum amount of energy practically applied to protect the structure during the reduction process.

21. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the increase in the surface area to be obtained via electrochemical oxidation/reduction by the treatment of the substrate selected as the initial material to have a higher surface area prior to electrochemical oxidation and reduction processes through sanding, dendritic metallic coating, chemical etching and similar methods.

22. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the formation of CU2O on the resulting copper nanostructures for copper metal to be terminated to the end of the first CU2O formation voltage step (A6), in a manner that is adjustable by the ratio of the amount of current required for the entire step to the current passed through this step.

23. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the transformation of copper nanostructures completely into CU2O as a result of the continuation of the oxidation process until the end of the first CU2O formation voltage step (A6) for copper metal.

24. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the metal nanostructures that are completely comprising of CU2O and contains more CU2O than the step A6 are obtained by removing the oxidative current after reaching the end of the voltage step indicated by A9 for copper metal.

25. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by obtaining metal nanostructures containing different amounts of CU(OH)2/CUO on CU2O, for the copper metal, when the voltage steps indicated by A10, A1 1 and A12 are reached.

26. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the metallic material to be developed in surface area and morphology is an alloy.

27. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the great number of active metal crystal planes as a result of the formation of nano copper with high surface area as shown in A1 -5 voltage regions.

28. A method of production for a metallic material with high surface area nanostructures according to Claim 1 , characterized by the adsorption of increasing OH- in the A1 region, as a result of the high surface area copper nanostructure formation.