EP3904561A1 - Method for producing micro and nanostructures - Google Patents

Abstract

A method for producing metallic micro- or nanostructures, preferably nanowires, comprising the steps of (a) providing an aluminum substrate; (b) Oxidation of the aluminum substrate by applying an electrical voltage in an electrolyte, a part of the aluminum substrate being oxidized to form an aluminum oxide layer which has a barrier layer which is adjacent to the aluminum substrate and which has an aluminum oxide layer with pores which is adjacent to the barrier layer; (c) breaking the barrier layer by gradually reducing the stress on the aluminum substrate; (d) if necessary, widening of the pores by adding a solvent; (e) if necessary, dissolving the barrier layer formed again in step (d); (f) electrochemical deposition of metallic micro- or nanostructures, preferably nanowires, in the pores of the aluminum oxide layer; (g) dissolving the aluminum oxide layer surrounding the metallic micro- or nanostructures, preferably nanowires, by adding a solvent for aluminum oxide and releasing the metallic micro- or nanostructures, preferably nanowires; (h) if necessary, cleaning of the nanowire suspension, if necessary drying (i) if necessary, regeneration of the aluminum substrate; and (j) optionally repeating steps (a) to (h), with rinsing steps optionally being provided between individual or several steps (a) to (i), the process being a continuous belt process in which the aluminum substrate forms the belt and whereby each of the steps (b), (d), (e), (f), (g), (i) and (j) takes place in an active basin and any rinsing steps take place in a sink.

Description

The present invention relates to a method for producing metallic microstructures or nanostructures, preferably nanowires, comprising the steps of providing an aluminum substrate; Oxidation of the aluminum substrate by applying an electrical voltage in an electrolyte; Breaking through the barrier layer by gradually reducing the voltage on the aluminum substrate; electrochemical deposition of micro- or nanostructures in the pores of the aluminum oxide layer; Dissolving the aluminum oxide layer surrounding the micro- or nanostructures by adding a solvent for aluminum oxide and releasing the micro- or nanostructures.

BACKGROUND TO THE INVENTION

Due to their special properties, micro- or nanostructures are used in numerous areas of technology. In the group of micro- or nanostructures, nanowires (or nanowires) are of particular industrial importance. One speaks of nanowires when it comes to structures that have a maximum diameter of a few 100 nm and that are much longer in the third dimension than in the other two. For the production of nanowires, a template, i.e. a template, is required in which the desired material is deposited. A suitable template is e.g. anodic, porous aluminum oxide, in which the corresponding material (e.g. galvanically) is built up and deposited. For example, an aluminum substrate can be anodized in a solution containing oxalic acid. The self-organizing layer (anodic aluminum oxide) forms orderly, parallel pores. A two-step procedure can improve the regular structure of the pores.

The pores in the substrate are insulating, chemically stable and they form an ideal template for the deposition of nanowires. The pore diameter can be adjusted using a suitable electrolyte (usually based on organic or inorganic acids) and the corresponding flow parameters. It has been observed that, under certain conditions, current oscillations occur during the anodizing process, which manifest themselves in a corrugated shape of the nanotubes.

The pore diameter in the template can, however, also be adjusted by chemical etching. The material is removed with diluted acids and the pore diameter is expanded. The distance between the pores can be adjusted by the electrolyte used.

The nucleation for pores usually takes place on surface defects. According to the prior art, a high-purity aluminum substrate with 99.99 to 99.999% by weight of aluminum is required for a template made of aluminum oxide in order to produce regular pores. Such substrates are extremely expensive and therefore not very suitable for the commercial production of nanowires. The use of less pure aluminum leads to irregular pores. In addition, the metallic structure and the surface structures are reflected in the pore growth (e.g. in the case of rolled foils).

The anodically produced porous aluminum oxide layers (PAA, porous anodic alumina) have a barrier layer made of aluminum oxide on the side oriented towards the substrate, which barrier layer insulates the substrate material. The barrier layer prevents the electrodeposition of metals in the pores, as it insulates the substrate material as a non-conductive insulator.

It could be shown on silicon wafers that this barrier layer can be removed by increasing the pH locally. However, this can also damage the PAA layer or the pores. Similar results could be achieved on ITO / Al substrates or on aluminum substrates by chemical etching with phosphoric acid. This allows the conductive substrate to serve as a cathode for the deposition. Alternatively, the entire PAA layer can be coated with gold, whereby the substrate is dissolved and the nanowires are built up using the upside-down method.

BRIEF DESCRIPTION OF THE INVENTION

The known processes for the production of metallic micro- or nanostructures are extremely complex, require very pure substrates and the production rate is low.

The object of the present invention is to provide a method for producing nanowires which is tolerant of impurities in the metal substrate material, which does not require expensive, high-purity aluminum or Si wafers, which would make the process uneconomical and nevertheless have high production rates.

1. (a) Bereitstellen eines Aluminiumsubstrats;
2. (b) Oxidation des Aluminiumsubstrats durch Anlegen einer elektrischen Spannung in einem Elektrolyten, wobei ein Teil des Aluminiumsubstrats zu einer Aluminiumoxidlage oxidiert wird, welche eine Barriereschicht aufweist, die dem Aluminiumsubstrat benachbart ist und welche eine Aluminiumoxidschicht mit Poren aufweist, die der Barriereschicht benachbart ist;
3. (c) Durchbrechen der Barriereschicht, indem die Spannung am Aluminiumsubstrat stufenweise verringert wird;
4. (d) gegebenenfalls Erweiterung der Poren durch Zugabe eines Lösungsmittels;
5. (e) gegebenenfalls anschließendes Auflösen der erneut in Schritt (d) gebildeten Barriereschicht;
6. (f) elektrochemische Abscheidung von metallischen Micro- oder Nanostrukturen, vorzugsweise Nanodrähten, in den Poren der Aluminiumoxidschicht;
7. (g) Auflösen der die metallischen Micro- oder Nanostrukturen, vorzugsweise Nanodrähte, umgebenden Aluminiumoxidschicht durch Zusatz eines Lösungsmittels für Aluminiumoxid und Freisetzen der metallischen Micro- oder Nanostrukturen, vorzugsweise Nanodrähten;
8. (h) gegebenenfalls Reinigung der Nanodrähte Suspension und gegebenenfalls Trocknung;
9. (i) gegebenenfalls Regenerieren des Aluminiumsubstrats; und
10. (j) gegebenenfalls Wiederholen der Schritte (a) bis (h),

wobei gegebenenfalls Spülschritte zwischen einzelnen oder mehreren Schritten (a) bis (i) vorgesehen sind,
wobei das erfindungsgemäße Verfahren ein kontinuierliches Bandverfahren ist, bei welchem das Aluminiumsubstrat das Band bildet und wobei jeder der Schritte (b), (d), (e), (f), (g), (i) in einem Aktivbecken erfolgt und allfällige Spülschritte in einem Spülbecken erfolgen.This object is achieved by a method for producing metallic microstructures or nanostructures, preferably nanowires, comprising the steps

1. (a) providing an aluminum substrate;
2. (b) Oxidation of the aluminum substrate by applying an electrical voltage in an electrolyte, part of the aluminum substrate being oxidized to form an aluminum oxide layer which has a barrier layer which is adjacent to the aluminum substrate and which has an alumina layer with pores adjacent to the barrier layer;
3. (c) breaking the barrier layer by gradually reducing the stress on the aluminum substrate;
4. (d) if necessary, widening of the pores by adding a solvent;
5. (e) optionally subsequent dissolving of the barrier layer formed again in step (d);
6. (f) electrochemical deposition of metallic micro- or nanostructures, preferably nanowires, in the pores of the aluminum oxide layer;
7. (g) dissolving the aluminum oxide layer surrounding the metallic micro- or nanostructures, preferably nanowires, by adding a solvent for aluminum oxide and releasing the metallic micro- or nanostructures, preferably nanowires;
8. (h) if necessary, cleaning of the nanowire suspension and if necessary drying;
9. (i) optionally regenerating the aluminum substrate; and
10. (j) if necessary, repeating steps (a) to (h),

where rinsing steps are optionally provided between individual or several steps (a) to (i),
wherein the process according to the invention is a continuous belt process in which the aluminum substrate forms the belt and wherein each of steps (b), (d), (e), (f), (g), (i) takes place in an active pool and any Rinsing steps take place in a sink.

Cleaning steps, preferably rinsing with a cleaning agent, can be provided between steps (a) to (i).

The production of the metallic microstructures or nanostructures, preferably nanowires, takes place according to the invention in several steps. The technical implementation takes place in the form of a conveyor system.

First, an aluminum substrate is provided in step (a). The aluminum substrate is a metal belt, for example an endless belt.

In step (b), the aluminum substrate is oxidized by applying an electrical voltage in an electrolyte. In the process, an aluminum oxide layer is formed which has a barrier layer which is adjacent to the aluminum substrate and is largely free of pores. In addition, the aluminum oxide layer has an aluminum oxide layer with pores on, which is adjacent to the barrier layer. Step (b) is preferably carried out by anodic polarization.

In step (c) the barrier layer is broken by gradually reducing the voltage on the aluminum substrate. This is preferably done in such a way that the voltage is reduced in step (c1) and the voltage is kept constant for a period of time in step (c2), steps (c1) and (c2) preferably being repeated at least once. As a result, the pores of the aluminum oxide layer extend through the barrier layer to the aluminum substrate.

In step (d) the pores can be enlarged by adding a solvent. The solvent is preferably a dilute mineral acid, for example dilute phosphoric acid. During the expansion of the pores in step (d) or in one of the rinsing steps, a barrier layer can again form. This can optionally be dissolved, for example with zincate (step e).

In step (f), metallic microstructures or nanostructures, preferably nanowires, are deposited by applying a voltage to the substrate and suitable electrolyte solutions. The electrochemically deposited nanowires are preferably an alloy containing at least one of the metals Ni, Co or Fe. The metals to be deposited or their alloys (in this case Ni, Co or Fe), as well as inert materials such as noble metals or carbon, are suitable as counter electrodes in this step.

Electrolytes can be used as electrolytes, including metal ions that correspond to the reduced metal in the metallic micro- or nanostructure. The corresponding metal ions in the electrolyte are required for the deposition of nanowires made of Ni, Co, Fe and their alloys. For the deposition of Ni nanowires electrolyte come with Ni ²⁺ ions, for nicotinamide nanowires electrolyte with Ni ²⁺ and Co ²⁺ ions in question and for CoNiFe- nanowires an electrolyte comprising Ni ^2+, Co ²⁺ and Fe ²⁺ Ions.

The following table 1 shows concentrations of electrolytes for the corresponding alloys. Table 1: Composition of individual electrolytes Concentration [mol / L] Metallic micro- or nanostructures, preferably nanowires made of metals: Ni NiCo CoNiFe NiSO ₄ 0.186 0.170 0.100 Ni (CH ₃ COO) ₂ 0.170 NiCl ₂ 1.262 1,250 0.310 CoCl ₂ 0.068 0.240 FeCl ₂ 0.160 H ₃ BO ₃ 0.650 0.650 0.650 NTS 0.010 0.010 0.010 5-sulfosalicylic acid 0.040

For these electrolytes, direct current or pulsed deposition has been described in the literature. NiCo was deposited under direct current (1.0-2.5 A / dm ² ). Pulse-reverse coating gave better results. The cathodic pulse is 60 ms followed by a reverse pulse of 20 ms. The current densities were set in such a way that the cathodic charge conversion was twice as high as the anodic one. The result was smoother layers compared to direct current deposition. The electrolytes described could also be used in the method according to the invention.

The metallic micro- or nanostructures, preferably nanowires, now deposited in the pores of the aluminum oxide layer are surrounded by the aluminum oxide layer of the pores. The aluminum oxide layer is dissolved by adding a solvent in step (g). For example, chromic acid or an alkaline solution, in particular NaOH, can be added as a solvent.

The aluminum substrate can be regenerated in step (i) (e.g. by electropolishing or mechanical processes such as grinding or polishing).

All process steps take place on a tape that is formed from the original aluminum substrate. The aluminum substrate is a metal strip, the individual steps (b), (c), (d), (e), (f), (g), (i), and any rinsing steps being carried out continuously on the aluminum substrate. Step (h) takes place separately from the aluminum strip and can also be carried out in a continuous operation. Thus, while the process is running, there are individual areas on the aluminum substrate in which the process steps run parallel to one another.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1

shows schematically an aluminum substrate (Al) with a barrier layer (BL) and a porous aluminum oxide layer (PAA).

Fig. 2a

shows a plan view of a porous aluminum oxide layer, so that the pores can be seen.

Figure 2b

shows a section through a template with an aluminum substrate layer (bottom), a barrier layer (middle) and a porous aluminum oxide layer (top).

Fig. 3

shows NiCo nanowires which, according to the invention, were deposited and released in an aluminum oxide layer.

Fig. 4

shows the cleaned NiCo nanowires Fig. 3 after cleaning.

Fig. 5

shows NiCo nanowires that were manufactured with pulse parameters according to the state of the art.

Fig. 6

shows schematically a conveyor system for carrying out the method according to the invention.

The method according to the invention for producing nanowires takes place on an aluminum substrate with a purity that can be less than 99.99% by weight. First, in step (b), the aluminum substrate is oxidized by applying an electrical voltage in an electrolyte, so that the surface of the aluminum substrate is oxidized. A barrier layer is formed which is adjacent to the aluminum substrate and an aluminum oxide layer with pores. This step (b) can be carried out by cathodic polarization. For example, an electrolyte with 3 mol / l oxalic acid in which the aluminum substrate is anodically polarized (60 V at 5-10 ° C) has proven suitable. In this step the complete aluminum oxide layer is built up. This consists of the structured porous aluminum oxide and the barrier layer (which interferes with the later process).

In Fig. 1 a schematic cross-section through the structure of a template is shown with the aluminum substrate Al and an aluminum oxide layer arranged thereon, which has a barrier layer BL and a subsequent porous aluminum oxide layer PAA.

In the Figures 2a and 2b Scanning electron microscope images of the porous aluminum oxide layer and a section through a template with aluminum substrate layer, barrier layer and porous aluminum oxide layer are shown.

The interfering barrier layer is then removed in step (c). A potential reduction of 2 V every 30 s has proven suitable. The barrier layer breaks through. This is mandatory for the later assembly of the nanowires.

In the optional step (d), the pores are widened by adding a solvent, a solution of 5% by weight phosphoric acid being found to be suitable and the pores being widened to the final diameter. The treatment time is e.g. 45 minutes and determines the final diameter (e.g. 80-120 nm).

• 120 g/l Natriumhydroxid
• 20 g/l Zinkoxid
• 50 g/l Kaliumnatriumtartrat
• 2 g/l Eisen(III)chlorid.

Since, after step (d), a thin oxide layer is formed again at the pore attachments (new barrier layer), this must be removed before the nanowires are built up. This can be dissolved with zincate according to the following conditions: Ultrasound-assisted treatment at 30 ° C for 40 s in the following solution:

• 120 g / l sodium hydroxide
• 20 g / l zinc oxide
• 50 g / l potassium sodium tartrate
• 2 g / l iron (III) chloride.

In step (e), nanowires are deposited by applying a voltage to the substrate and suitable electrolyte solutions.

• 0,23 A/dm² für 10 ms
• Off-Zeit für 25 ms

To build up the nanowires, an electrolyte for NiCo layers (see Table 1) is deposited and the pulse parameters are:

• 0.23 A / dm ² for 10 ms
• Off time for 25 ms

The nanowires deposited in this way, which are still in the aluminum oxide layer with pores, are in Fig. 3 shown.

1. a) Chromsäure: 0,5 M Phosphorsäure und 0,2 M Chrom(VI)oxid bei 60 °C für 30 min, anschließend 10 min ultraschallunterstützt. Das Aluminiumsubstrat wird in diesem Fall nicht angegriffen und kann für weitere Synthesen verwendet werden.
2. b) NaOH: die PAA Schicht wird in einer NaOH Lösung entfernt. Das Aluminiumsubtrat wird in diesem Fall ebenfalls angegriffen und muss für eine weitere Verwendung vorbehandelt werden.

The aluminum oxide layer surrounding the nanowires is dissolved, for example, by chromic acid or NaOH:

1. a) Chromic acid: 0.5 M phosphoric acid and 0.2 M chromium (VI) oxide at 60 ° C for 30 min, then ultrasonically assisted for 10 min. In this case, the aluminum substrate is not attacked and can be used for further syntheses.
2. b) NaOH: the PAA layer is removed in a NaOH solution. In this case, the aluminum substrate is also attacked and must be pretreated for further use.

The nanowires can now be released. This can be supported by ultrasound cleaning for e.g. 10 minutes in 2.5 M NaOH, 0.5 g / L SDS. The solution stays there overnight. The sample is further purified with water using decanting / centrifuging / dialysis or similar methods. Optionally, the nanowires can be dried in a drying cabinet or by means of lyophilization.

The aluminum substrate can be electropolished in step (i), for example in 40.8% by volume H ₃ PO ₄ , 38.8% by volume ethanol and 20.4% by volume water at 30 V and 42-45 ° C. for 5 minutes the aluminum substrate can be chemically polished or treated using a mechanical process.

The process according to the invention is a continuous process, each of steps (b) to (i) taking place in an active basin, it being possible for rinsing to take place in rinsing basins between the steps. The individual steps are in Fig. 6 shown schematically.

• Aktivbecken 1: Schritte (a) bis (c)
• Spülbecken (Reinigung)
• Aktivbecken 2: Schritt (d)
• Spülbecken S
• Aktivbecken 3: Schritt (e)
• Spülbecken S
• Aktivbecken 4: Schritt (f)
• Spülbecken S
• Aktivbecken 5: Schritt (g)
• Aktivbecken 6: (h)
• Spülbecken S
• Aktivbecken 7: Schritt (i)
• Spülbecken S

In a continuous belt process, aluminum as a substrate material is transferred as a belt over rollers into the individual process basins. The individual steps take place in the following basins:

• Active pool 1: steps (a) to (c)
• Sink (cleaning)
• Active pool 2: step (d)
• Sink S
• Active pool 3: step (e)
• Sink S
• Active pool 4: step (f)
• Sink S
• Active pool 5: step (g)
• Active pool 6: (h)
• Sink S
• Active pool 7: step (i)
• Sink S

The nanowires are isolated from basin 5 and released and cleaned according to step (i). In batch operation, the nanowires can be transferred from basin 5 to cleaning by means of settling / suctioning, centrifuging or by means of an electromagnet (in the case of magnetic nanowires). For continuous operation, the nanowires can be separated or concentrated from the process medium by means of an electromagnet or a continuous centrifuge.

In a comparative example, current parameters from the literature ( Tang, "Pulse reversal plating of nickel alloys," Transactions of the Institute of Metal Finishing 85, 2007 ) used ( Fig. 4 ). Due to the pulse parameters, which have been optimized for deposition on planar substrates, the nanowires grow together and form an agglomerate that can no longer be separated from one another.

Claims (13)

A method for the production of metallic micro- or nanostructures, preferably nanowires, comprising the steps (a) providing an aluminum substrate; (b) Oxidation of the aluminum substrate by applying an electrical voltage in an electrolyte, a part of the aluminum substrate being oxidized to form an aluminum oxide layer which has a barrier layer which is adjacent to the aluminum substrate and which has an aluminum oxide layer with pores which is adjacent to the barrier layer; (c) breaking the barrier layer by gradually reducing the stress on the aluminum substrate; (d) if necessary, widening of the pores by adding a solvent; (e) optionally dissolving the barrier layer formed again in step (d); (f) electrochemical deposition of metallic micro- or nanostructures, preferably nanowires, in the pores of the aluminum oxide layer; (g) dissolving the aluminum oxide layer surrounding the metallic micro- or nanostructures, preferably nanowires, by adding a solvent for aluminum oxide and releasing the metallic micro- or nanostructures, preferably nanowires; (h) if necessary, cleaning of the nanowire suspension and if necessary drying; (i) optionally regenerating the aluminum substrate; and (j) if necessary, repeating steps (a) to (h), where rinsing steps are optionally provided between individual or several steps (a) to (i),
wherein the process is a continuous belt process in which the aluminum substrate forms the belt and wherein each of steps (b), (d), (e), (f), (g), (i) and (j) in an active pool takes place and any rinsing steps are carried out in a sink. Method according to Claim 1, characterized in that step (b) takes place by anodic polarization. Method according to Claim 1 or Claim 2, characterized in that step (c) is carried out in such a way that the voltage is reduced in step (c1) and the voltage is kept constant for a period of time in step (c2), the steps (c1) and (c2) are preferably repeated at least once. Process according to one of Claims 1 to 3, characterized in that a dilute mineral acid is added as solvent in step (d). Method according to one of Claims 1 to 4, characterized in that in step (e) the barrier layer formed again in step (d) is dissolved - preferably with zincate. Method according to one of Claims 1 to 5, characterized in that electrochemically deposited micro- or nanostructures, preferably nanowires, are an alloy which comprises at least one of the metals Ni, Co or Fe. Method according to one of Claims 1 to 6, characterized in that the aluminum oxide layer surrounding the nanowires is dissolved in step (g) by adding chromic acid or an alkaline solution, in particular NaOH. Method according to one of Claims 1 to 7, characterized in that in step (i) the aluminum substrate is regenerated by electropolishing. Method according to one of Claims 1 to 8, characterized in that each step takes place in an active pool, with cleaning steps, preferably rinsing, being provided between the steps. Method according to one of Claims 1 to 9, characterized in that the electrochemical deposition in step (f) comprises the following steps:
Provision of the electrolyte, immersion of the substrate, application of a voltage or current-controlled signal against a counter electrode suitable material. Method according to one of Claims 1 to 10, characterized in that the counter-electrodes consist of the elements to be deposited, their alloys or inert materials during the electrochemical deposition. Method according to one of Claims 1 to 10, characterized in that the electrolyte contains Ni, Co or iron ions during the electrochemical deposition. Method according to one of Claims 1 to 10, characterized in that it takes place potentiostatically or preferably galvanostatically with direct current or pulsed.

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