Grow Carbon Nanotube Using Nickel Electroplating

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Table of Contents

Introduction
Electroless Plating
The Watts Solution
Carbon Nanotubes
Methods of Preparation
Carbon Nanotubes Defects
Behavior in Electricity
Conclusion
Works Cited

Introduction

Nickel electroplating is used for electroforming, functional and decorative purposes. Nickel plating is divided into two categories, electroless plating and electrolytic plating. Electroplating involves covering metallic surfaces using electrically charged particles. During the electrolytic plating process, the voltage applied determines the quantity of metal deposited (Snyder par. 1). Electroless nickel plating is an important process due to the distinctive characteristics of the electroless nickel (EN) deposits. Nickel sulfate is the most commonly used nickel cations source while nickel acetate and nickel chloride are only used in a few applications. This is because nickel acetate is expensive while nickel chloride cannot be used to electroplate metals, such as aluminum. The most commonly used reducing agents during the nickel electroplating include dimethylamine borane ((CH₃)₂NHBH₃), sodium borohydride (NaBH₄), sodium hypophosphite (NaH₂PO₂.H₂O) and hydrazine (N₂H₄.H₂O). During the nickel-plating process, complexing agents (ligands) are used to decrease free nickel ions concentration, stop nickel salt precipitation, and buffer the solution to prevent rapid pH decrease. An electroless nickel (EN) plating solution can decompose spontaneously when stabilizers are not used in the plating solution. Stabilizers include unsaturated organic acids, heavy metal cations, such as lead and mercury, oxygenated compounds, and group VI elements. The rate of deposition and kinetics of the electroless nickel plating process are greatly influenced by temperature. This is because the reaction is catalytic and requires external energy to proceed. Generally, the electroless plating process involves selective reduction of metallic ions through catalytic action which involves oxidation and reduction (REDOX) reactions.

Electroless Plating

According to Mallory (2), electroless plating involves the covering of metallic surfaces with cobalt or nickel alloys without the application of external electricity sources. However, the term also involves any procedure which results in the precipitation of metals from aqueous media. Electroless nickel (EN) plating has several industrial and commercial applications due to the nature of electroless nickel deposits. The physical and chemical properties of the electroless nickel coatings depend on their composition. On the other hand, the composition of the electroless nickel coatings depends on the conditions in an electroless plating bath and the formulation of the bath. Often, the electroless nickel bath solution contains reducing agents, stabilizers, energy, source of nickel ions and complexing agents. Frequently, electroless nickel plating is used in coating aluminum and steel metals.

The concentration of stabilizers is crucial in the electroless plating baths because increasing the concentration of stabilizers beyond the critical limits results in inhibition of the electroless plating process. The stability of the electroless nickel plating bath is improved through air agitation of the electroless plating solution. However, agitation of the electroless plating solution using an inert gas, such as argon, does not affect the stability of the solution. Although pure oxygen improves the stability of the electroless nickel plating solution, the presence of oxy-anions, such as Br₃^– and NO₂^–, reduce the deposition rates. This is because the oxy-anions adsorb on the surface of the nickel substrate and prevent further deposition. Stabilizers used in the electroless nickel plating bath are either organic or inorganic. Organic stabilizers include acetylene dicarboxylic acid and propiolic acid. However, it is necessary to replenish continually the organic stabilizers since they are used up during the deposition reaction. This is crucial in maintaining the stability of the electroless nickel plating bath. The deposition reaction in the electroless nickel plating bath is the best at low pH, mainly between 3.8 and 4.5. Dilute sulfuric acid is frequently used to maintain the acidic condition of the plating bath to ensure that the reaction is uninhibited. During the electroless nickel plating process, it is important to ensure that the oxidation reaction is sustainable since this helps in preventing substrate dissolution. Sustainable oxidation reaction helps in producing continuously built thick deposits. At the beginning of the reaction, the deposition should exclusively and initially occur on the substrate before continuing to deposit on the initial substrate. This is important in producing nickel plating of uniform thickness.

The Watts Solution

The Watts solution is an electrolyte containing a mixture of nickel chloride (NiCl₂.6H₂O), boric acid (B(OH)₃) and nickel sulfate (NiSO₄.6H₂O), and is the most commonly used nickel plating bath. The Watts solution is used in decorative plating solutions and protection against corrosion. Nickel sulfate in the solution is used to provide the needed concentration of nickel ions, while boric acid is used to maintain the pH of the solution by acting as a weak buffer. Nickel chloride is used to increase conductivity and improve anode corrosion. The Watts solution results in bright nickel used in decorations and semi-bright nickel used for engineering purposes. The composition of the Watts bath varies depending on the purpose of the nickel electroplating. In nickel plating for decorative purposes, the solution contains 20 – 40 oz/gal of nickel sulfate, 5 – 7 oz/gal of boric acid, and 8 – 20 oz/gal of nickel chloride. In nickel plating for engineering purposes, the Watts bath contains 30 – 40 oz/gal of nickel sulfate, 5 – 7 oz/gal of boric acid, and 4 – 6 oz/gal of nickel chloride. The main advantages of the Watts bath include its simplicity and availability in high purity grade. Watts bath is also relatively cheap and deposits electroplated from the Watts bath solutions show little internal stress and are less brittle. Watts bath solution is also less corrosive to equipment as compared to nickel chloride solutions.

Carbon Nanotubes

Although carbon nanotubes were accidentally discovered in 1991, they have generated a lot of interest due to their many applications in industrial and domestic equipment. Some of the applications of carbon nanotubes include use in reinforced materials, semiconductor devices, catalysts, sensors and magnetic nanocomposites. Carbon nanotubes grow on negative electrodes in reaction chambers and are developed by rolling a sheet of graphene into hollow tubes. Carbon nanotubes have high mechanical strengths and good electrical properties, which makes them useful in many applications. Three major carbon nanotubes structures include the chiral, armchair and zigzag structures.

Carbon nanotubes are part of the Fullerenes family and are of two main types: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The single-walled carbon nanotubes exist as single cylinders while the multi-walled carbon nanotubes exist as concentric cylindrical tubes. Carbon nanotubes are produced using laser ablation, arc discharge, plasma-enhanced chemical vapor deposition, carbon vapor deposition as well as electrolytic methods. However, electrolytic methods are rarely used. Properties of carbon nanotubes vary depending on the method used to produce them. This is because different methods used in synthesizing carbon nanotubes result in different carbon nanotube structures and different levels of byproducts and defects. The four major deformities which arise during the synthesis of carbon nanotubes include topological, rehybridization, incomplete bonding and doping defects.

Methods of Preparation

Regardless of the method used in the synthesis of carbon nanotubes, the production methods are relatively similar. Carbon Nanotubes are usually produced using four methods, namely, laser ablation, arc-discharge, Plasma-Enhanced Chemical Vapor Deposition (PECVD) and Carbon Vapor deposition (CVD) methods. All these synthesis methods involve heat, metal catalysts and carbon feedstock. However, the carbon arc discharge method does not require metal catalysts during the synthesis of multi-walled carbon nanotubes.

In the laser ablation method, an intense laser pulse is used to ablate a carbon target in the presence of catalysts and an inert gas within a furnace. A constant supply of gas (either Helium or Argon) is allowed to flow through the tube, and this helps in transferring formed soot out of the reaction chamber. The formation of the carbon nanotubes occurs on cold substrates. The laser ablation method is mainly used in the synthesis of single-walled carbon nanotubes. However, the laser ablation method requires a high growth temperature to release carbon atoms from the carbon targets. Another disadvantage of the laser ablation method is that it cannot be used in high-volume processing.

The carbon arc-discharge method occurs in vacuum chambers containing an inert gas and two carbon electrodes. The inert gas increases the rate of carbon deposition while the carbon electrodes act as the source of carbon atoms (Wang and Yeow 2). A high direct current (DC) voltage applied across the two electrodes results in the generation of plasma from the inert gas. This evaporates carbon atoms which deposit on the cathode as carbon nanotubes while the anode is expended. The hard inner layer of the cathode contains a mixture of amorphous carbon, polyhedral particles and multi-walled carbon nanotubes. When a mixture of metal catalysts is put into the anode, single-walled carbon nanotubes may also be produced. The structure of the multi-walled carbon nanotubes depends on the gas used during the synthesis. For instance, insertion of boron results in the production of long zigzag multi-walled carbon nanotubes.

The PECVD method combines electrical energy and heating to produce carbon nanotubes. The electricity is supplied to the substrates through powdered electrodes. Combining electricity and heat helps in dissociating more carbon atoms. The PECVD method has the advantage of producing carbon nanotubes at low temperatures which makes it possible to grow carbon nanotubes on polymers (Gunderson 10). The method also makes it possible to produce vertically aligned carbon nanotubes.

In the carbon vapor displacement system, a hydrocarbon precursor (mainly, ethylene, acetylene, or methane) is broken down into different reactive species in the reaction chamber. The hydrocarbon is subjected to temperatures ranging between 550⁰C and 1000⁰C. When the right operating temperature is achieved, the gas source is closed and the carbon source valve is opened. The presence of catalysts on substrates and high temperatures in the reaction chamber helps in the formation of carbon nanotubes. The main catalysts used in the carbon vapor deposition method are the transition metals, such as nickel, iron and cobalt. Carbon nanotubes can be produced at low temperatures through carbon vapor deposition (CVD). The method is also the most efficient in the production of Single-Walled Carbon Nanotubes (SWCNTs) and Multi-Walled Carbon Nanotubes (MWCNTs). Single-walled carbon nanotubes are usually produced at higher temperatures while production of the multi-walled carbon nanotubes occurs at lower temperatures.

During the production of carbon nanotubes using the CVD method, metallic and carbonaceous impurities are produced. Metallic impurities arise from residual catalysts, such as nickel, while carbonaceous impurities are by-products that arise from the reaction. The carbonaceous impurities are removed through oxidation. This is achieved using gas phase and liquid phase purification. Gas-phase purification uses high temperatures to eliminate carbonaceous impurities, whereas liquid phase purification involves the use of acidic solutions to wash the carbon nanotubes. Metallic impurities are removed by evaporation. The carbon vapor deposition (CVD) method has several advantages, such as producing hollow and uniform carbon nanotubes, forming highly aligned ensembles and lack of macroscopic defects.

Electrolysis is the least commonly used method in the synthesis of carbon nanotubes. In this method, two graphite electrodes are immersed in molten ionic salts and current is passed through. After completion of the electrolysis process, the remaining carbonaceous material is dissolved in distilled water. This dissolves the ionic salt, and the dispersion is separated by filtration. The residues contain a mixture of carbon filaments, amorphous carbon, remnants of the salt ions, which cove some of the carbon, and multi-walled carbon nanotubes. However, the electrolysis method cannot be used in the production of single-walled carbon nanotubes.

Carbon Nanotubes Defects

Four major categories of carbon nanotubes exist and include topological, rehybridization, incomplete bonding and doping defects. Topological defects are localized and do not change the lengths of the carbon nanotubes, but only affect electronic structures on sidewalls in some locations along the nanotube. Graphite is mostly affected by the incomplete bonding defect where dislocations and vacancies arise. Incomplete bonding and doping defects have the ability to increase the chemical reactivity and electrical conductivity of carbon nanotubes.

Behavior in Electricity

Carbon nanotubes have unique electrical properties, and they are either semiconducting or metallic. These properties depend on the chirality and tube diameter of the Carbon nanotubes. The chirality of carbon nanotubes is represented by the integer pair (n and m). Carbon nanotubes with n – m = 3j (where j is a nonzero integer) are metallic. All the other carbon nanotubes are semiconducting. Carbon nanotubes are highly anisotropic in their dielectric properties, and this arises from their uni-dimensional structures. This increases the efficiency of carrying high current electricity in carbon nanotubes with minimum loss in terms of heating. The current density in most metallic conductors is limited by electromigration. Conducting carbon nanotubes have high current densities due to their small cross-sectional areas. Semiconducting SWCNTs have the advantage of operating at high frequencies and are used in integrated circuits to increase their number densities and speeds. SWCNTs have low electron scattering, which makes them useful for building transistors. Carbon nanotubes are also used for high voltage applications, such as providing reduced initiation voltage required for corona discharges (Chen 1).

Conclusion

Carbon nanotubes have gained widespread attention from scientists since their discovery in 1991. These materials have high tensile strength and good electrical properties which makes them applicable in a wide range of domestic and industrial equipment. Although several methods are used to produce carbon nanotubes, the CVD and PECVD methods are the most provide the best carbon nanotubes. The PECVD method has the advantage of producing carbon nanotubes at relatively low temperatures. Single-walled carbon nanotubes (SWCNTs) have excellent and unique mechanical, thermal and electronic properties, which makes them useful in various industrial and domestic applications.

Works Cited

Chen, Junhong. “Nanoscale corona discharge electrode US 7,504,628 B2”. United States Patent. 2009. Print.

Gunderson, Erick. Effect of cosputtered catalyst on growth and alignment of carbon nanotubes by plasma-enhanced chemical vapor deposition. Oregon, ORE: Oregon State University, 2010. Print.

Mallory, Glen. Electroless plating: fundamentals and applications. Electroless plating: fundamentals and applications. Ed. Glen Mallory and Juan Hajdu. Cambridge, CAM: Cambridge University Press, 1990. 1-55. Print.

Snyder, Donald. Nickel Electroplating. Products Finishing.2011. Web.

Wang, Yun and John Yeow. A Review of Carbon Nanotubes-Based Gas Sensors.Journal of Sensors.1.1 (2009): 1 – 24. Print.

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