Electroless Deposition

Electroless deposition (ED) or electroless plating is defined as the autocatalytic process through which metals and metal alloys are deposited onto nonconductive surfaces. ^{[1] [2] [3]} These nonconductive surfaces include plastics, ceramics, and glass etc., which can then become decorative, anti-corrosive, and conductive depending on their final functions. Electroless deposition is a chemical processes that create metal coatings on various materials by autocatalytic chemical reduction of metal cations in a liquid bath. Electroless plating in contrast with electroplating processes, where the reduction is achieved by an externally generated electric current. ^{[4] [5]} Electroless deposition deposits metals onto 2D and 3D structures such as screws, nanofibers, and carbon nanotubes  ; unlike PVD, CVD, and electroplating which are limited to 2D surfaces. ^[6] Commonly the surface of the substrate is characterized via pXRD, SEM-EDS, and XPS. Key qualifying parameters of these methods are crucial for a researcher’ or company’s purpose for the conductive surfaces. Electroless deposition continues to rise in importance within the microelectronic industry, oil and gas, and aerospace industry.

Electroless nickel plating on metal parts.

History

Electroless deposition was serendipitously discovered by Charles Wurtz in 1846. ^[7] The nickel-phosphorus bath being used for his experiment spontaneously decomposed and formed nanoparticles. ^[7] Although Wurtz observed these nanoparticles he did not continue working on this discovery. ^{[2] [7]} 70 years François Auguste Roux rediscovered the electroless deposition process and patented it in United States as the ‘Process of producing metallic deposits. ^{[5] [7]} Roux deposited nickel on a substrate, but his invention did not seem to receive much commercial use. ^{[7] [5]} In 1946 the process was re-discovered by Abner Brenner and Grace E. Riddell while working at the National Bureau of Standards. ^{[5] [8] [9]} They presented their discovery at the 1946 Convention of the American Electroplaters' Society (AES); a year later, at the same conference they proposed the term "electroless" for the process and described optimized bath formulations, ^[10] that resulted in a patent. ^{[10] [11] [12]} However, Abner nor Riddell benefited financial from the filed patent. ^[13] The first commercial deposition was Leonhardt Plating Company in Cincinnati followed by the Kannigen Co. Ltd in Japan which revolutionized the industry. ^{[7] [2] [1]} The Leonhardt commercialization of electroless deposition was a catalyst for the design and patenting of several deposition baths including plating of metals such as Pt, Sn, Ag, and their alloys. ^{[1] [5] [12]}

The first mechanism for electroless deposition, atomic hydrogen mechanism, was proposed until Brenner and Riddell for a nickel deposition bath. ^{[4] [2]} This lead the way for other scientist to propose several other mechanisms. ^[7] The four examples of classical electroless deposition mechanism including: (1) Atomic hydrogen mechanism, (2) Hydride transfer mechanism, (3) Electrochemical mechanism, and (4) Metal hydroxide mechanism which will be discussed in the process section. ^[7] The four classical examples of electroless deposition are often linked to industrial processes. ^[7] Tollen's reaction deposits a uniform metallic silver layer via ED on glass which is referred to a silvering mirrors. ^{[14] [15]} This reaction is used to test for aldehydes as a basic solution of silver nitrate. ^[14] This reaction is often used as crude method used in chemistry demonstrations for the oxidation of an aldehyde to carboxylic acid, and the reduction of the silver cation into elemental silver. ^[14] Electroless deposition is an important process for industrial, and laboratory reactions in their quest to study the kinetics and test proposed mechanisms. ^{[1] [2] [4]}

Electroless deposition vs other plating methods

Electroless deposition is an important process in the electronic industry for metallization of substrates. Other metallization of substrates also include physical vapor deposition (PVD), chemical vapor deposition (CVD), and electroplating which produce thin metal films but require high temperature, vacuum, and a power source respectively. ^[16] This class is contrasted with electroplating processes, where the reduction is achieved by an externally generated electric current. ^{[5] [4]} Electroplating suffers from uneven current density due to the effect of substrate shape on the electrical resistance of the bath. ^{[1] [17]} These defects include loss of adhesion, contamination, and rigidity of the structure. ^[17]

Electroless deposition is advantageous in comparison to PVD, CVD, and electroplating deposition methods because it can be performed at ambient conditions. ^{[1] [4]} The plating method for Ni-P, Ni-Au, Ni-B, and Cu baths are distinct; however, the processes involve the same approach. The electroless deposition process is defined by four steps show in Fig.1 ^{[1] [2] [18]}

(1) Pretreatment or functionalization of the substrate cleans the surface of the substrate to remove any contaminants, determines the porosity of the elemental metal deposition, and the initiation site of the deposition.

(2) Sensitization is an activator ion that can reduce the active metal in the deposition bath.

(3) Activation accelerates the deposition of the active metal in the deposition bath.

(4) Electroless deposition is the process by which metal cation is reduced to elemental metal

The electroless deposition process is based on the principle of  redox reactions in which electrons are transferred during a chemical reaction. The chemical bath thus has a cathodic and anodic reaction in this one-pot reaction. ^[2] The design of the deposition process includes carefully chosen reducing and oxidizing agents based on their standard redox potentials values. ^[2] These redox principles are dictated by the  electroplating  principles where a metal cations are reduced by an electric current at the cathode  and a metal solid is oxidized at an anode. ^[19]

Figure 1. Steps in electroless deposition process. Adapted from Electroless copper deposition: A critical review.

Process

Fundamental process

Metallization of a substrate requires a uniform solid coating on the intended surface instead of precipitation of the metal salt requires a catalyst that is either the substrate itself or is applied to it beforehand. ^[2] In fact, the reaction must be autocatalytic ^[1] , so that it can continue after the substrate has been coated by the metal. ^[4] The electroless deposition process is based on redox chemistry in which electrons are released from a reducing agent and a metal cation is reduced to elemental metal. ^{[1] [2]} Equations (1) and (2) shows the simplified ED process where a reducing agent releases electrons, and the metal cation is reduced respectively. ^[4]

Reactions 1 and 2 describes the general process of the reduction of metals and the oxidation of a reducing agents. M is the metal cation (ex, Ni, Cu, Pt cations). M is elemental metal after reduction. Reductants (reducing agents) is a substance loses electrons and gets a higher oxidation state (ex. formaldehyde, hydrazine etc.). Oxidation products is the result of the reductants losing electrons (ex. formadedhyde transformation to formic acid. ze is the number of electrons transferred from the reductant to the metal cations.

Each component of the ED bath is complex because of the it needs a metal salt, reducing agent, stabilizers, complexing agents, pH variation, side product formation, bath lifetime, and plating rates. ^{[1] [4]} Stabilizers fine-tune the autocatalytic nature of the bath while controlling the heterogeneous deposition of nanoparticles. ^{[1] [2] [4]} Complexing agent s serve two purposes, prevention of  precipitation of the metal cation, and act as buffers for bath stability. ^{[2] [1]} pH stability works in combination with complexing agents to prevent rapid decomposition of the ED bath. ^{[7] [1]} Side products of the a deposition bath can negatively affect the bath by poisoning the catalytic site, and disrupt the morphology of the metal nanoparticle. ^{[1] [4] [7]} Bath lifetime and plating rates are the products of all the components of the bath depending of all the previously mentioned parameter. ^[6]

The ED bath has a final component which is the reducing agent. Once a reducing agent is added to the metal salt solution nanoparticles of elemental metal or metal alloys are deposited on the substrate (e.g., glass, textiles, plastics etc.). ^{[7] [20]} As the nanoparticles of metals are deposited on the surface of the substrate. ^[2] The layers of nanoparticles acts as a catalyst for continual deposition of the metal which is defined as autocatalysis. ^{[1] [4]}

Reaction of elemental zinc metal and copper(II) sulfate. Elemental zinc is dipped into a copper (II) sulphate solution. Red deposit is the reduction process in which Cu (II) is converted to elemental Cu. Elemental Zn is oxidized to Zn (II) and dissolves into solution.

The electroless deposition and electroplating bath actively performs cathodic and anodic reactions at the surface of the substrate. ^{[1] [2]} The standard electrode potential of the metal and reducing agent are important as a driving force for electron exchange. ^[2] The standard potential is defined as the power of reduction of compounds. Examples are shown in Table 1., in which Zn with a lower standard potential (-0.7618 V) act as a reducing agent to copper (0.3419 V). ^[1] The calculated potentials for the reaction of the copper salt and zinc metal is ~0.4169 V meaning the reaction is spontaneous.

Since electroless deposition also uses the principles of standard electrode potentials we are also able to calculate potential, E, of metal ions in a solution governed by the Nernst equation (3). ^[1]

${\displaystyle E=E^{0}-({0.592}|{2})log(Q)\quad \quad \quad (3)}$ ^[1]

E is the potential of the reaction, E⁰ is the standard reduction potential of the redox reactio, and Q is the concentration of the poducts divided by the concentration of the reactants ${\displaystyle [Products/Reactants]}$. ^[1] Metallizing

Standard Electrode Potentials for Zn and Cu table

Standard potentials governs the ability for elemental metal deposition on a substrate, however ED is a not as simplistic as described in a metal exchange process. Electrons for ED are produced by powerful reducing agents in the deposition bath for example formaldehyde, sodium borohydride, glucose, sodium hypophosphite, hydrogen peroxide, and ascorbic acid. ^{[1] [2]} These reducing agents have negative standard potentials that drive the deposition process

The standard potential of the reducing agent and metal salt is not the only determinant of the redox reaction for electroless deposition. Conventional deposition of the copper nanoparticles uses formaldehyde as a reducing agent. ^[21] But the E⁰ of formaldehyde is pH dependent. At pH 0 of the deposition bath is E⁰ of formaldehyde is 0.056 V, but at pH=14 the E⁰=-1.070. ^[22] The formaldehyde (pH 14) is a more suitable reducing agent than at pH=0 because of the lower negative standard potential which makes it a powerful reducing agent. ^[18] Formaldehyde (pH 14) allows the solution to be buffered without rapid degradation of the bath. ^{[18] [19]}

Four Classical Deposition Baths Mechanism

Although industrial companies patented and commercialized ED baths the mechanism remained elusive. ^{[1] [2] [4]} The electroless plating bath are influenced by several factors including pH, binding ligand, and the potential etc. of the solution. ^[7] The deposition is buffered to ensure there isn't a rapid drop or increase in the pH during the deposition process. ^{[1] [4]} Binding ligands/complexing agents are used to increase the solubility of the metal salts while maintaining bath stability for slower release of the metal cations. ^[1] The potential of the bath is determined by the standard potentials of a powerful reducing agent and the metal cations. ^[4] All these factors along with others determine the overall deposition kinetics. ^[4] Mechanisms for Ni-P, Cu, Pt, and Pd etc. have been proposed for decades however, proposed mechanisms for ED of elemental metal have remained elusive. ^[4] The first mechanisms proposed focused on the formation of a Ni-P code position nanoparticles onto a substrate. These mechanisms were proposed based on their redox reactions which is highlighted in each subsection. Electroless nickel plating uses nickel salts as the metal cation source and either hypophosphite (H₂PO₂^-) (or a borohydride-like compound) as the reducer. ^[4] A byproduct of the reaction is elemental phosphorus (or boron) which is incorporated in the coating. The classical deposition methods follows the following steps:

1) Diffusion of reactants (Ni²⁺ , H₂PO₂^-) to the surface

2) Adsorption of reactants at the surface

3) Chemical reaction at the surface

4) Desorption of products (H₂PO₂^-, H₂, H⁺, H^-) from the surface

5) Diffusion of the product from the surface or adhesion of the product onto the surface

Atomic Hydrogen Mechanism

Brenner and Riddle first proposed the atomic hydrogen mechanism for evolution of Ni and H₂ from a Ni salt, reducing agent, complexing agent, and stabilizers. ^{[1] [2] [4]} They used a nickel chloride (NiCl₂), sodium hypophosphite (NaH₂PO₂), commonly used complexing agents (e.g. citrate, EDTA, and tridentate), and stabilizers such as CTAB (cethyltrimethyl ammonium bromide). The redox reactions [4]-[6] proposes that adsorbed hydrogen (H_ad) reduces Ni²⁺ at the catalytic surface and has a secondary reaction where H₂ gas evolves. ^[4] In 1946 it was discovered that a Ni-P alloy and hydrogen gas was formed instead due to a secondary reaction of hypophosphite with atomic hydrogen to form elemental phosphorus. However, the atomic hydrogen mechanism did not account for the co-deposition of Ni-P. ^{[2] [5] [10] [4]} The hydride transfer mechanism was proposed by Hersh in 1955 which accounted for the deposition of elemental phosphorus. ^{[1] [4]}

Equation [3]-[5] describes the proposed 'Atomic Hydrogen Mechanism' by Brenner and Riddell

The Hydride Transfer Mechanism

Hersh proposed the hydride transfer mechanism which was expanded in 1964 by R.M. Lukes to explain the deposition of elemental P. ^{[2] [4]} Hydride transfer in a basic environment was purported [7] to form the hydride (H^-) which reduced the Ni²⁺ to Ni⁰[ 8], and combines with water to form H₂ gas [9]. ^[4] Lukes reasoned that the hydride ion came from the hypophosphite and thus accounts for the Ni-P codeposition through a secondary reaction. ^[4]

Equations [7]-[9] describe the proposed 'Hydride Transfer Mechanism' by Hersh

Electrochemical Mechanism

The electrochemical mechanism was also proposed by Brenner and Riddell but was later modified by others including scientist Machu and El-Gendi. ^[4] They proposed that a electrolytic reaction occurred at the surface of the substrate, and H₂ [11] and P [13] are by products of the Ni²⁺ ion reduction [10][11]. ^{[2] [7] [4]} The anodic reaction [10] has a reduction potential of 0.50 V. The cathodic reactions [11], [12], and [13] have reduction potentials of -0.25 , 0.00 V, and 0.50 V respectively. ^[4] The potential of the reaction is 1.25 V (spontaneous reaction). The electrochemical mechanism is the only classical mechanism that uses reduction potentials the other 3 classical mechanisms were tested in other ways including calorimetric studies. ^[4]

Equations [10]-[13] describe the proposed ' Electrochemical Mechanism' by Machu and El-Gendi

Metal Hydroxide Mechanism

Proposed in 1968, solvated Ni ions at the catalytic surface ionized water and forms a hydroxide coordinated Ni ion. ^[23] The hydrolyzed Ni²⁺ ion catalyzes the production of Ni, P, and H­₂. Water is ionized at the Ni surface [14], and Ni²⁺ ions coordinate with hydroxide ions [15]. ^[4] The coordinated Ni²⁺ is reduced [16] and NiOH⁺_ab is adsorbed on the substrate surface. ^[4] At the surface H₂PO₂^- reduces NiOH⁺_ab to elemental Ni⁰ [17]. ^[4] The released elemental H recombine to form hydrogen gas and [18] and elemental Ni catalyses the production of the P [19]. ^[4] The deposited Ni acts as a catatalyst due continued reduction by H₂PO₂^- [17]. ^[4] Cavallotti and Salvago also proposed that the NiOH⁺_ab [20] and water combination oxidises to Ni²⁺ and elemental H. ^[4] The NiOH⁺_ab participates in a competing reaction [21a] (refers to reaction [17] )and [21b] to for elemental Ni and hydrolized Ni respectively. ^[4] Finally H₂PO₂^- is oxidized [22] and elemental H [21a/21b] recombine to form and H₂ evolves for both reactions ^[4] .The overall reactions is shown in equation [23]. ^[4]

Equations [14]-[19] describes a step by step proposed reactions for 'Metal Hydroxide Mechanism' by Cavallotti and Salvago.

Equations [20]-[23] describes a step by step proposed reactions for 'Metal Hydroxide Mechanism' by Cavallotti and Salvago.

Industrial applications

Electroless deposition changes the mechanical, magnetic, internal stress, conductivity, and brightening of the substrate. ^{[1] [2] [4]}  The first industrial application of electroless deposition by the Leonhardt Plating Company electroless deposition has flourished into metallization of plastics. ^{[2] [24] [25]} , textiles ^[26] , prevention of corrosion ^[27] , and jewelry. ^[2] The microelectronics industry including the manufacturing of  circuit boards, semi-conductive devices, batteries, and sensors. ^{[1] [2]}

Metallization of plastics by electroless deposition

Typical metallization of plastics includes  nickel-phosphorus, nickel gold, nickel-boron, palladium, copper, and silver. ^[24] Metallized plastics are used to reduce the weight of metal product and reduce the cost associated with the use of precious metals. ^[28] Electroless nickel plating is used in  variety of industries including aviation, construction, textiles, and oil and gas industries. ^[23]

Electromagnetic interference shielding (EMI shielding)

EMI shielding refers to the process by which devices are protected from interference from the electromagnetic radiation. ^{[4] [29]} The interference negatively affects the function of the devices; EMI sources include radiowaves, cell phones, and tv receiver. ^{[4] [29]} The Federal Aviation Administration and the Federal Communication Commission prohibits the use of cellphones after the airplane is airborne to avoid interference with navigation. ^{[30] [31]} Elemental Ni, Cu, and Ni/Cu absorb noise signals in the 14 Hz to 1 GHz range. ^[4]

Oil and gas production

Elemental nickel coating prevents corrosion of the steal tubulars used for drilling. ^[4] At the core of this industry nickel coats pressure vessels, compressor blades, reactors, turbine blades, and valves. ^[4]

Schematic of oil rig setup. The steel tubulars are covered with elemental Ni which reduces corrosion rate. Sections 25, 26, and 27 are examples of where an elemental nickel coating would overlay the steel.

See Also

• Electroless copper deposition
• Electroless nickel-boron deposititon
• Nanoparticle
• Nanomaterial
• Nanotechnology

References

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