Several approaches can be followed for the synthesis of nanomaterials. One of the most common synthetic routes to classifies them is in the “top-down” and “bottom-up” methodologies.

 “Top-down”

Through this strategy, the bulk material is reduced in size to produce nanoparticles. Mechanical milling, where the macroscopic material is converted into smaller particles by attrition; or laser ablation, where high-energy laser light is used to remove and vaporize the material from a solid surface, are some of the most applied techniques.

“Bottom-up”

In this approach, the nanoparticles are produced assembling atoms or molecules in a liquid or gas phase. In Flame Synthesis the starting material is evaporated, mixed with fuel and an oxidizing agent, and injected in a flame, forming the nanoparticles within the flame.

Reducing metal precursors in a liquid phase to form nanoparticles, often called wet colloidal synthesis, is also one of the most commons methodologies. It is completed upon mixed solutions of different ions or molecules, under controlled conditions to form the final nanomaterials. Let’s see the colloidal synthesis techniques in more detail.

Nucleation, growth and stability of the nanoparticles

In general, the synthesis of metallic nanoparticles can be divided into two main stages, namely nucleation (related to the formation of a new phase) and growth (which involves the growth of the nuclei in the final NPs). Besides, a stabilisation mechanism must be present; otherwise, the growing NPs inevitably result in the formation of bulk material through aggregation.

• Nucleation

Paying closer attention to the colloidal synthesis, we can distinguish between heterogeneous and homogenous nucleation.

Heterogeneous nucleation occurs at nucleation sites contacting the liquid or vapour. It has preferential sites as phase boundaries or impurities. This type of nucleation sees as the driving force in the seed-mediated synthesis.

Homogenous nucleation occurs spontaneously and randomly, but requires some factors, as a supersaturation state.

• Growth

Throughout the last decades, different theories have arisen to explain the phenomena that involve the growth of nuclei in final NPs. All of them can be subclassified into two large families. Growth mediated by atoms or growth mediated by nanoparticles. In the first family, atoms are used as building blocks through surface addition to influence growth. The second family comprises the formation of mesocrystalline nanoparticles in which the growth occur by initial nanoparticles addition rather than by atoms addition.

• Atom-mediated growth

Within this family, it is possible to classify a multitude of theories that explain, in a consolidated way, the observed phenomenology for the great majority of NPs. The most relevant growth theories are described below:

• LaMer mechanism, based on their research on aerosols and sulfur hydrosols, propose the separation between the nucleation and growth into three stages. The first stage is the formation of the metallic atoms. When they reach a certain level of saturation (Cmin), the energy barrier for self-nucleation can be overcome, and nucleation starts. Afterwards, there is a drop in the concentration of metallic atoms below the minimum supersaturation level and no more self-nucleation occurs, leading to the growth of the nanoparticles. The Lamer process only describes the nucleation and growth, not answering in the evolution of the NPs, shapes or size distribution.

• Ostwald Ripening. Taking into consideration that the solubility of nanoparticles is not independent of their size according to the Gibbs-Thomson relation, which states that the nanoparticles become unstable when the size is decreased. The Ostwald ripening theory proposes a mechanism of growing in which the nanoparticles of certain dimension go through a process of re-dissolution, leading to the growth of the big ones due to the new source of monomers. It states that small particles are more soluble than big ones and tend to lose to re-precipitate into larger particles.

• Digestive ripening, on the other side, can be considered as the opposite of Ostwald ripening. Small particles will grow at expenses of the biggest ones, which will re-dissolve and act as a source of monomers to the smallest nanoparticles. A more complex equation, derived from the Gibbs-Tomson relation, is necessary to explain the digestive ripening. The overall process can be summarised in three steps: 1) The addition of a ligand (e.g. dodecanethiol) that will help to break the larger nanoparticles and narrowing the sizes, 2) the purification of the nanoparticle to obtain an isolate ligand-nanoparticle system and 3) the heating, usually in high boiling point solvents, with the presence of the ligand to obtain monodisperse nanoparticles.

• The Finke-Watzky mechanism is a simultaneous process where the nucleation and growth happen at the same time, but still follow the condition of the critical size explained in the classical nucleation theory (CNT). The nucleation occurs with a constant rate K1, and the autocatalytic surface growth with a constant rate K2.

Nanoparticle-mediated growth

Thanks to the development of modern electron microscopy techniques (among other technologies) in recent decades an increasing number of reports have emerged that inform the synthesis of metal nanoparticles for which it is difficult to explain their growth according to atom mediated theories. The final products obtained by these processes occur through the fusion of nanoparticles formed in the early stages of the reaction, usually evolve into mesocrystalline nanostructures. Attending to the nanoparticles attachment method, nanoparticle-mediated growth can be classified in:

• Coalescence. During the formation of nanoparticles, they can present coalescence. Coalescence can be seen as the overall nanoparticle growth, where first occurs the aggregation of separate particles, followed by subsequent nanoparticle growth. For this process, there is no preference in the crystallographic planes.

• In the oriented attachment, the growth involves a self-organization of adjacent particles, sharing a common crystallographic orientation, and following by joining this particles at a planar interface

Stability

Once the nanoparticles forms, the colloidal stability in the dispersing medium plays an important role. Van der Waals, electrostatic or magnetic force can lead to nanoparticles agglomeration, aggregation or coalescence.

• Van de Waals interactions govern nanoparticles aggregation at the most basic level. Permanent or induced dipoles within the nanoparticles can result in net attractive forces between them and posterior aggregation.

• An electric double layer (EDL) formed by solvated ions or molecules can shield the surface charge, create repulsive interparticle forces and stabilise the system. We can distinguish two parts, the Stern layer, which consists of counter-ions adsorbed on the charged surface of the nanoparticle (NP), and the diffuse layer, an atmosphere of ions of opposite net charge surrounding the NP. The thickness of the EDL is called the Debye length.

• The DLVO theory (Derjaguin, Landau, Verwey and Overbeek, developers of the theory), for the colloidal stability, assumes that the total force between colloidal particles is composed for the van de Walls forces (attractive) and the EDL (repulsive).

• On the other hand, the steric stabilisation can be carried out absorbing large molecules at the surface of the particles, which creates a physical barrier that prevents aggregation. It can be used surfactants (e.g. hexadecyltrimethylammonium bromide, CTAB), polymers (e.g. Polyvinylpyrrolidone, PVP), proteins or other types of macromolecules. The stability is now determined by the solubility, average chain length, concentration or temperature, among others.

There are many other theories, but this summary focuses on the most common ones that are explained in the scientific literature.