**2. Colloidal synthesis of NCs**

*Colloids - Types, Preparation and Applications*

convenient way to stop carbon emissions is to move towards zero carbon-emitting resources. In view of this, hydrogen (H2) produced by photo/electrocatalytic water splitting has shown great potential to become the fuel of the future. The merits have been attributed to its high energy density and it produces only one by-product of water upon combustion [4]. Thus H2, which is a zero carbon-emitting fuel, can be a promising solution to the mitigation of climate change. The second method is to capture carbon from the atmosphere and then store it back into the geosphere. However, the geosphere sequestration of CO2 has no financial benefits. In contrast, chemical transformation of excess accumulated CO2 from air into valuable industrial products, such as fuels (methanol, ethanol), hydrocarbon (methane, ethylene) and chemicals (formic acid, acetic acid), is an effective way to solve both global warming and energy crises [5, 6]. Furthermore, it has economic significance from an industrial point of view. However, carbon-di-oxide reduction (CO2R) is a highly cumbersome and non-specific process. So far, several approaches, such as biochemical, chemical, thermal, photochemical and electrochemical catalysts have been explored to achieve aspirated activity and selectivity in this region [7–9]. Nonetheless, unlike other catalytic system electrocatalysts have gained tremendous attention due to its easy operation at ambient temperature and pressure. In addition, the selectivity of the product can be obtained by just adjusting reaction conditions, such as redox potential, electrode, and electrolyte, pH temperature and so on. The main advantage of using electrocatalysts is that they can be powered by renewable energy sources that emit zero carbon. For all these reasons, many research activities have shifted to the areas of EH2ER and ECO2RR (**Figure 1**) [10]. Over the years of time metal, metal oxide, metal sulphide have shown great promise in ECO2RR and EH2ER using electrocatalysis phenomenon. These electrocatalysts are being considered as a promising system that would be able to operate on a real scale without polluting environment. Whereas, there are still many limitations that are associated with electrocatalysts, such as high cost, poor product selectivity, high overpotential and low stability [11]. Colloidal Nanocrystal (C-NC) based electrocatalysts have become indispensable to overcome these limitations to certain extent owing to their larger surface to volume ratio, precise shape, long-term durability, and the plethora of configurations [12, 13]. These factors are important in influencing their efficiency, selectivity, and durability for EH2ER and ECO2RR. For example, variation in the size and/or shape causes alteration in reactivity at

*Electrochemical CO2 conversion into fuel and H2 production by using colloidal nanocrystal-based* 

**192**

**Figure 1.**

*electrocatalysts.*

The C-NC is an inorganic material with a size of 1–100 nm and surface covering of protecting capping agents like polymer and surfactants molecules. Generally, the inorganic part exhibits characteristic features, such as optical, electrical, magnetic, and catalytic, that can be tuned by changing their physicochemical parameters, while surface capping guarantees the stabilization of these structures and paves the way for synthesizing more complex structures [14, 15]. The physical parameters like morphology and chemical composition of C-NCs can be easily adapted by varying their reaction parameters like monomer concentration forming inorganic core of NCs and judicious choice of capping substances for surface covering. Over the last two-three decades, researchers have gained good control over synthesis of highquality and cost-effective NCs with uniform morphology and chemical constituents using colloidal synthesis [16–19]. The C-NCS approach has not only enhanced efficiency, selectivity of NCs, but also improved their service life. So far, researchers have found many commercial applications of C-NCs in various fields ranging from life sciences to the material world. One of the striking applications of C-NCs is in the field of biological imaging of cells, where quantum dots are used owing to their excellent fluorescent properties and also they do not photo-chemically bleach out like organic dyes [20]. Recently, quantum dots are being used commercially in LED displays also known as *QLED*-displays [21]. In addition, C-NCs-based photo/ electro-catalysts for ECO2RR and EH2ER are being developed to solve the energy crisis and global warming. However, their uses at economical scale in this area is still facing challenges. Deep insights of C-NCs synthesis and the effect of C-NCs physiochemical parameters on their electrocalytic properties need to be investigated for their successful applications at the economical level.

In general, C-NCs can be synthesized in both water and organic solvents. However, synthesizing a broad spectrum of NCs requires different reaction conditions that are much more feasible to achieve in organic solvents compared to water that is mainly used in the synthesis of noble metal C-NCs [14]. Therefore, in this section, authors will focus on organic phase C-NCs synthesis. Generally, C-NCs synthesis requires three major elements: 1) precursor molecules or building blocks forming inorganic core of NCs, 2) capping agents, and 3) organic solvents. Capping agents sometime act as solvent. The process of nanocrystal formation starts with transformation of precursor molecules into unstable and reactive species or monomers that usually occurs at quite high temperature. Thereafter, these monomers

**Figure 2.**

*The LaMer mechanism based formation of active monomers, burst nucleation, and subsequent slow growth of colloidal nanocrystals. Adapted from [23].*

lead to formation of C-NCs whose growth is mainly influenced by capping agents. The crystallization of C-NCs can be best understood using widely accepted LaMer mechanism as depicted in **Figure 2 [**22–25]. Based on this mechanism, NCs development considered to have three major steps. In the first phase, the precursor molecules are converted to the reactive species, or monomer, and then eventually reach to a supersaturated phase (I), where no particle or second phase is still visible. In the next step, reactive species concentrate to the critical limit of supersaturation (phase II), at which a thermodynamically feasible state (**Cmin**) is developed for nucleation, followed by monomers to form initial seed for nucleation. In the second phase, the supersaturation again drops at some point due to instant nucleation, reducing the monomer concentration below **Cmin** that triggers the third phase (III) of the mechanism, that is, the growth of NCs. In this phase, because of monomer concentrations remain below Cmin, therefore, NCs grow without forming further nuclei until they attain an equilibrium state. During the growth phase of C-NCs, capping agents also play an important role in determining the final morphology of NCs. First and foremost, the capping agent should have a tendency to adsorb on the surface of the growing NCs.

Secondly, capping agents are required to bind in such a way that it can desorb and adsorb on the surface of growing NCs during growth process, making growing NCs surface accessible for reactive monomers, yet surface covering of capping agents, overall, stabilize NCs [14, 26].

#### **2.1 Size control in C-NCs synthesis**

During NC synthesis, control over size uniformity is a prominent feature of contemporary synthetic methods. The LaMer mechanism discussed earlier not only explains the formation of NCs, but is also an approach to synthesize C-NCs with narrow size distributions. The essential element of LaMer approach to synthesize uniform-sized C-NCs is to divide crystallization into two disparate events; nucleation and growth. As discussed previously, the nucleation occurs for short periods of time, also known as burst nucleation, triggering a different growth phase where all nuclei then grow at the same rate without generating extra nuclei. The formation of extra nuclei during the growth phase can cause differences in the size of C-NCs because the newly formed nuclei will lag behind the previously growing NCs in growth kinetics. The

**195**

growing NCs [32].

**3. C-NCs-based catalysts for ECO2RR**

**3.1 Mechanistic insight of ECO2RR**

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems…*

method it is often used to synthesize C-NCs at large scale [28–30].

several methods utilize LaMer mechanism to grow uniform size C-NCs [24, 27]. The first example is seed-mediated growth method where reaction medium is introduced pre-developed nuclei at low monomer concentration to inhibit secondary nucleation. Therefore, these pre-developed nuclei further grow up to the desired size of C-NCs without creating extra nuclei. However, in order to obtain narrow size distribution of NCs, it is still required to control growth phase as well. The second example is hot injection method where the reaction medium at high temperature is rapidly supplemented with the precursor or reducing reagents to create a state of supersaturation that triggers subsequent burst nucleation. The next example is the heating up method where reaction medium pre-treated with precursor, and capping agents is heated at high temperature to induce the LaMer crystallization. Due to the simplicity of this

Unlike the bulk materials, the physiochemical properties of C-NCs are also strongly dependent on their shape. In this chapter, authors will further discuss how the shape of C-NCs can be adapted to improve the service life, selectivity and efficiency of electrocatalysts. Generally, C-NCs shapes can be tuned by means of both thermodynamic and kinetic controls. In C-NCs synthesis, capping agents are used, primarily, to obtain the desired shape using their specific binding nature on the surface of the growing NCs [31]. If the surface adsorption of capping agents causes the decrement in surface energy of any specific facet, then the obtain shape will be favored by thermodynamically. Whereas, if capping agents serves an obstruction between growing NCs and diffusing monomers then the resulting shape of C-NCs will be governed by kinetic factors [32, 33]. Thermodynamically, the growing NCs attain it most likely shape by reducing its total surface free energy. For example, during the formation of fcc NC, the capping agents first selectively adheres to the (100) plans, which in turn, decreases their surface free energy [32]. These selective adsorptions onto the (100) plans causes transformation of cuboctahedron into cubic structure due the subsequent growth of higher energy facets. Whereas, in the kinetic regime, capping agents selectively adhere to some specific facets to lower their growth rate compare to others, resulting in various NCs shapes. Simultaneously, capping agents inhibit the diffusion of pre-deposited atoms over the NCs surface. In the real situation, however, the final shape of C-NCs is governed by the comparable kinetics of diffusion and deposition of monomers to

Thermodynamically, CO2 is a quite stable molecule (bond dissociation enthalpy of C=O is ~750 KJ mol−1), so high energy is required for its activation. Moreover, the highest oxidation state of CO2, causes problems for its selective reduction [34]. A catalyst in this regard is an alternative that offers reactive sites for its selective and rapid transformations. In this chapter, the authors are focusing specially on NC-based electrocatalyst for ECO2RR. Generally, ECO2RR involves several proton/ electron transfers processes that take place at the cathode (catalyst). This process is considered to have three major stages. First, CO2 is absorbed on the catalytic surface and its binding strength depends on the structure and composition of the NC as well as the nature of the electrolyte. Once it is absorbed, an electron is transferred

*DOI: http://dx.doi.org/10.5772/intechopen.95338*

**2.2 Shape control in C-NCs synthesis**

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems… DOI: http://dx.doi.org/10.5772/intechopen.95338*

several methods utilize LaMer mechanism to grow uniform size C-NCs [24, 27]. The first example is seed-mediated growth method where reaction medium is introduced pre-developed nuclei at low monomer concentration to inhibit secondary nucleation. Therefore, these pre-developed nuclei further grow up to the desired size of C-NCs without creating extra nuclei. However, in order to obtain narrow size distribution of NCs, it is still required to control growth phase as well. The second example is hot injection method where the reaction medium at high temperature is rapidly supplemented with the precursor or reducing reagents to create a state of supersaturation that triggers subsequent burst nucleation. The next example is the heating up method where reaction medium pre-treated with precursor, and capping agents is heated at high temperature to induce the LaMer crystallization. Due to the simplicity of this method it is often used to synthesize C-NCs at large scale [28–30].

#### **2.2 Shape control in C-NCs synthesis**

*Colloids - Types, Preparation and Applications*

lead to formation of C-NCs whose growth is mainly influenced by capping agents. The crystallization of C-NCs can be best understood using widely accepted LaMer mechanism as depicted in **Figure 2 [**22–25]. Based on this mechanism, NCs development considered to have three major steps. In the first phase, the precursor molecules are converted to the reactive species, or monomer, and then eventually reach to a supersaturated phase (I), where no particle or second phase is still visible. In the next step, reactive species concentrate to the critical limit of supersaturation (phase II), at which a thermodynamically feasible state (**Cmin**) is developed for nucleation, followed by monomers to form initial seed for nucleation. In the second phase, the supersaturation again drops at some point due to instant nucleation, reducing the monomer concentration below **Cmin** that triggers the third phase (III) of the mechanism, that is, the growth of NCs. In this phase, because of monomer concentrations remain below Cmin, therefore, NCs grow without forming further nuclei until they attain an equilibrium state. During the growth phase of C-NCs, capping agents also play an important role in determining the final morphology of NCs. First and foremost, the capping agent should have a tendency to adsorb on the

*The LaMer mechanism based formation of active monomers, burst nucleation, and subsequent slow growth of* 

Secondly, capping agents are required to bind in such a way that it can desorb and adsorb on the surface of growing NCs during growth process, making growing NCs surface accessible for reactive monomers, yet surface covering of capping

During NC synthesis, control over size uniformity is a prominent feature of contemporary synthetic methods. The LaMer mechanism discussed earlier not only explains the formation of NCs, but is also an approach to synthesize C-NCs with narrow size distributions. The essential element of LaMer approach to synthesize uniform-sized C-NCs is to divide crystallization into two disparate events; nucleation and growth. As discussed previously, the nucleation occurs for short periods of time, also known as burst nucleation, triggering a different growth phase where all nuclei then grow at the same rate without generating extra nuclei. The formation of extra nuclei during the growth phase can cause differences in the size of C-NCs because the newly formed nuclei will lag behind the previously growing NCs in growth kinetics. The

**194**

surface of the growing NCs.

*colloidal nanocrystals. Adapted from [23].*

**Figure 2.**

agents, overall, stabilize NCs [14, 26].

**2.1 Size control in C-NCs synthesis**

Unlike the bulk materials, the physiochemical properties of C-NCs are also strongly dependent on their shape. In this chapter, authors will further discuss how the shape of C-NCs can be adapted to improve the service life, selectivity and efficiency of electrocatalysts. Generally, C-NCs shapes can be tuned by means of both thermodynamic and kinetic controls. In C-NCs synthesis, capping agents are used, primarily, to obtain the desired shape using their specific binding nature on the surface of the growing NCs [31]. If the surface adsorption of capping agents causes the decrement in surface energy of any specific facet, then the obtain shape will be favored by thermodynamically. Whereas, if capping agents serves an obstruction between growing NCs and diffusing monomers then the resulting shape of C-NCs will be governed by kinetic factors [32, 33]. Thermodynamically, the growing NCs attain it most likely shape by reducing its total surface free energy. For example, during the formation of fcc NC, the capping agents first selectively adheres to the (100) plans, which in turn, decreases their surface free energy [32]. These selective adsorptions onto the (100) plans causes transformation of cuboctahedron into cubic structure due the subsequent growth of higher energy facets. Whereas, in the kinetic regime, capping agents selectively adhere to some specific facets to lower their growth rate compare to others, resulting in various NCs shapes. Simultaneously, capping agents inhibit the diffusion of pre-deposited atoms over the NCs surface. In the real situation, however, the final shape of C-NCs is governed by the comparable kinetics of diffusion and deposition of monomers to growing NCs [32].
