The early stages of nanocrystal nucleation and growth are still an active field of research and remain unrevealed. In this work, by the combination of aberration-corrected transmission electron microscopy (TEM) and electrochemical characterization of the electrodeposition of different metals, we provide a complete reformulation of the Volmer-Weber 3D island growth mechanism, which has always been accepted to explain the early stages of metal electrodeposition and thin-film growth on low-energy substrates. We have developed a Generalized Electrochemical Aggregative Growth Mechanism which mimics the atomistic processes during the early stages of thin-film growth, by incorporating nanoclusters as building blocks. We discuss the influence of new processes such as nanocluster self-limiting growth, surface diffusion, aggregation, and coalescence on the growth mechanism and morphology of the resulting nanostructures. Self-limiting growth mechanisms hinder nanocluster growth and favor coalescence driven growth. The size of the primary nanoclusters is independent of the applied potential and deposition time. The balance between nucleation, nanocluster surface diffusion, and coalescence depends on the material and the overpotential, and influences strongly the morphology of the deposits. A small extent of coalescence leads to ultraporous dendritic structures, large surface coverage, and small particle size. Contrarily, full recrystallization leads to larger hemispherical monocrystalline islands and smaller particle density. The mechanism we propose represents a scientific breakthrough from the fundamental point of view and indicates that achieving the right balance between nucleation, self-limiting growth, cluster surface diffusion, and coalescence is essential and opens new, exciting possibilities to build up enhanced supported nanostructures using nanoclusters as building blocks.

Recently, electrochemical deposition is arising as a promising technique to synthesize supported nanoparticles which are important for many applications including electrocatalysis 1 and electroanalysis. 2 By means of electrochemical deposition, nanoparticles grow direclty from the substrate without the need of further sample preparation. Moreover, the technique is surfactant-free and cost-effective and allows to tune the nature of the nanoclusters by changing electrolyte composition and deposition parameters. 3 Several groups have electrodeposited metallic nanoparticles on various substrates such as glassy carbon, 4,5 highly oriented pyrolitic graphite, 6,7 indium-doped tin oxide, 8,9 carbon nanotubes, 10 and others. 11 However, in order to investigate the size-dependent properties of small nanoparticles and to use them as electrocatalysts or in sensing devices, distributions with low size dispersions need to be obtained. This still represents a challenge in the field of electrochemical deposition. In order to overcome this issue and improve nanoscale electrodeposition, it becomes extremely important to understand nanocluster nucleation and growth.Since many decades plenty of theoretical and experimental literature is available reviewing this topic, 3,12À14 with the most widely referred model being the one developed by Scharifker and Hills, 15 which has been slightly reformulated several times. 16À18 Conventionally, electrochemical nucleation and growth studies have been addressed indirectly by measuring the currentÀtime transients in potentiostatic experiments and correlating them to models that take into account the random nature of nucleation and the coupled growth of hemispherical nuclei under diffusion limitations. According to these models, an electrochemically formed nucleus grows by direct reduction of ions onto its surface if this action is thermodynamically favorable (i.e., if the Gibbs free energy is reduced by the addition of more atoms to the cluster), whereas it will dissolve if this condition is not fulfilled. Therefore, it has been normally considered that when a constant overpotential is applied, nuclei whose radii are bigger than a critical radius (r c ) are formed progressively in the surface and grow by direct attachment. The growth of each of them affects both the concentration of active species and the overpotential distribution in the cluster vicinity creating zones of reduced concentration and overpotential and thus reduced nucleation rate. Then, if multiple clusters are considered, their local zones of reduced nucleation rate spread and gradually overlap. This is a very complex problem which is frequently solved by approximating the areas of reduced nucleation rate by overlapping planar diffusion zones in which nucleation is fully arrested. 15À18 It is then foreseen that for progressive nucleation the number of growing nuclei can be expressed by an exponential law:where N 0 is the total number of active sites and A is the nucleation rate constant. Under these conditions, average particl...

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By using an optimized characterization approach that combines aberration-corrected transmission electron microscopy, electron tomography, and in situ ultrasmall angle X-ray scattering (USAXS), we show that the early stages of Pt electrochemical growth on carbon substrates may be affected by the aggregation, selfalignment, and partial coalescence of nanoclusters of d ≈ 2 nm. The morphology of the resulting nanostructures depends on the degree of coalescence and recrystallization of nanocluster aggregates, which in turn depends on the electrodeposition potential. At low overpotentials, a self-limiting growth mechanism may block the epitaxial growth of primary nanoclusters and results in loose dendritic aggregates. At more negative potentials, the extent of nanocluster coalescence and recrystallization is larger and further growth by atomic incorporation may be allowed. On one hand, this suggests a revision of the Volmer−Weber island growth mechanism. Whereas this theory has traditionally assumed direct attachment as the only growth mechanism, it is suggested that nanocluster self-limiting growth, aggregation, and coalescence should also be taken into account during the early stages of nanoscale electrodeposition. On the other hand, depending on the deposition potential, ultrahigh porosities can be achieved, turning electrodeposition in an ideal process for highly active electrocatalyst production without the need of using high surface area carbon supports.

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