Electrodeposition of Zn nanoparticles via nanoemulsion: a single-entity electrochemistry study

Steel continues to be a dominant construction component owing to its superior material and physicochemical properties. Corrosion/unwanted metal oxidation still plagues this sector leading to material failure. Zn remains a cornerstone coating material acting as a sacrificial anode; meanwhile, coating nanofunctionalization has been shown to enhance anticorrosion performance. Water-in-oil nanoemulsions were explored as confined nanoreactors for electrochemical functionalization using ultramicroelectrodes (UMEs) fabricated from carbon fiber, Pt, or steel wire, polished to form inlaid disc UMEs 7, 10, and 25 µm in diameter, respectively. Zn-containing nanoemulsions were stabilized by alkylphosphonium ionic liquids (ILs) with electrodeposition investigated via single-entity electrochemistry (SEE). Emulsions were prepared from ZnCl₂/LiCl-containing aqueous droplets dispersed in α,α,α-trifluorotoluene (TFT) along with IL salts as surfactants composed of trihexyltetradecylphosphonium (P₆₆₆₁₄ ⁺) paired with different anions, including Br^-, NTf₂ ^- (bis(trifluoromethylsulfonyl)imide), or TB^- (tetrakis(pentafluorophenyl)borate). CTAB (cetyltrimethylammonium bromide) was also used as a surfactant but induced poorer emulsion stability compared to the ILs. Dynamic light scattering (DLS) measurements showed noticeable changes in droplet size and stability when the IL type, water-to-oil ratio, salt concentration, and pH were changed. P₆₆₆₁₄Br performed better than P₆₆₆₁₄NTf₂ and P₆₆₆₁₄TB, becoming the focus. Zn electrodeposition signal is complicated by the H₂ evolution reaction (HER) occurring at similar potentials. The shape and frequency of SEE current transients recorded at -1.3 V with ZnCl₂-emulsions were compared to those with electro-inactive MgCl₂ to determine the relative contribution of HER to the current density. Integration of the i-t spikes allowed the charge transferred (Q) to be determined, from which the mean radius of water nanodroplets (r _drop) to be calculated. The r _drop at carbon fiber UMEs agreed well with DLS measurements, while Pt and steel UMEs demonstrated a 3× and 2× relative signal enhancement owing to their enhanced electrocatalytic activity towards HER. SEM analysis revealed the differences in these deposited structures, which are dependent on emulsion conditions and the substrate used. Overall, our studies show that nanoemulsion composition and droplet behaviour can strongly influence confined Zn electrodeposition. Mechanistic insights into the relationships among emulsion stability, stochastic electrochemical signals, and metal-deposition processes relevant to Zn-based corrosion protection systems are established.