**3.3 Effect of C-NCs size on ECO2RR**

The size is an important factor for C-NC-based electrocatalysis because different size of C-NCs show different activity/selectivity towards ECO2RR. Several studies have shown that alteration in size at nanoscale can affect both atomic distributions at various reaction sites (plans, edges, corners) and electronic structure of NCs, changing their catalytic characteristics. Therefore, the search for the optimal size C-NC showing the best efficiency, selectivity and durability requires contemporary research in this area. So far, to explore the effect of size towards ECO2RR, different sizes of electrocatalysts have been evaluated, however, our focus here is towards C-NC-based electrocatalysts. Colloidal synthesis of NCs is an excellent approach to study different sizes of NCs while keeping other factors such as shape and composition stable. Loiudice et al. synthesized spherical and cubical Cu C-NCs in the size range of 7.5–27 nm and 24–63 nm respectively for ECO2RR [46]. Their study has revealed unprecedented correlation between size of NCs and their electrochemical activity as well as selectivity for ECO2RR. The activity of Cu C-NCs increases as size of NCs within same morphology decreases, however, this does not hold while comparing cubical NCs with spherical. For example, the 44 nm cube has higher current density than 27 nm sphere. To understand this phenomenon, the propensity of Cu (100) facet towards ethylene production can provide a better understanding. Based on previous findings, it is widely accepted that the Cu (100) plane is selective for the electrochemical reduction of CO2 in ethylene. Additionally, edges in ethylene production are thought to be responsible for the absorption and stabilization of an important intermediate (i.e., COOH \*) as well as inhibiting EH2ER. In this study, the X-ray diffraction pattern has shown a higher contribution of the Cu (100) plane in the nanocube than in the nanosphere and Cu foil. Interestingly, among all Cu C-NCs, a size of 44 nm exhibited highest selectivity with 50% FE for ethylene and overall 80% for ECO2RR over EH2ER. This can be ascribed to changes in atomic configuration at different sites of C-NC due to differences in size. As nanocube move from smaller to larger sizes, the number of atoms at the edges and corners decreases, however, the number of atoms at the plane site increases. As a result, it came close to the morphology of a single crystal, where all the atoms of the surface are populated on the (100) plane. And, as previously stated edges also play an important role in electrochemical reduction of CO2 into C2 products. Therefore, an optimal balance between ratio of edge to plane site in 44 nm Cu C-NC, suggesting not only its unique selectivity for C2H4, but also an overall activity towards ECO2RR.

**199**

CO\*

**Figure 4.**

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

Similarly, Zhu et al. investigated the non-monotonic size-dependent selectivity of Au C-NCs for CO2 reduction into CO [47]. Among Au C-NCs of size 4, 6, 8, 10 nm synthesized, the 8 nm exhibited a highest selectivity for CO production with FE of ~90% at −0.67 V vs. RHE as shown in **Figure 4(c).** Based on the DFT calculations, the group concludes that dominance of edge sites at 8 nm C-NC facilitates selective CO2 reduction in hydrocarbons, whereas depletion of corner sites inhibits EH2ER. They have further reported the unique selectivity of Au13 C-NC towards EH2ER. Although Au13 facilitates the formation of COOH\* intermediate as compared to Au (211) and Au (111) facet, a stronger binding with CO\* intermediate

*TEM images of (a) the 8 nm Au NPs and (b) the C-Au NCs. (c) Potential-dependent FEs of the C-Au on electrocatalytic reduction of CO2 to CO. (d) Current densities for CO formation (mass activities) on the C-Au at various potentials. Free energy diagrams for electrochemical reduction of (e) CO2 to CO and (f) protons to hydrogen on Au(111) (yellow symbols), Au(211) (orange symbols), or a 13-atom Au cluster (red symbols) at* 

−*0.11 V. reprinted with the permission from [47]. Copyright © 2013, American Chemical Society.*

(ΔG*for*mation) of H\* intermediates on Au13 is lower than ΔG*for*mation of COOH\*, suggesting the generation of H2 at low overpotentials. Furthermore, this study is found to correspond to a size dependence study on the small size (2–15 nm) Cu C-NCs for ECO2RR. For particle sizes at 5–15 nm, significantly higher activity and selectivity were found for H2 and CO than for hydrocarbons. Moreover, particles smaller than 5 nm in size showed an exponential increase in the formation of H2 and CO relative to hydrocarbons. This unique property of small C-NCs is due to an increase in under coordinated catalytic sites, which, in turn, strongly stabilize and bind with H\*

 intermediates. Strong adsorption of H\* and CO\* intermediates is suggested to prevent subsequent hydrogenation of CO into hydrocarbons, so the increase in H2

The shape of the C-NC plays an important role in determining the selectivity/

Besides, Zhou et al. have illustrated the correlation between different shapes of C-NCs and their corresponding crystal planes using a stereographic triangle as depicted in **Figure 5(a) [**49]. The lower index surfaces, namely, the (100) (110) and (111) facets that lie on the three vertices of triangle, are present on the nanocrystal

activity of the electrocatalysts. Several findings have shown that the shapedependent C-NC activity/selectivity towards ECO2RR is typically associated with the presence of specific crystal plane. For example, Suen et al. have revealed that cubic Cu C-NC (C-Cu) with mainly (100) facet shows enhanced selectivity towards C2 products while octahedron Cu C-NC (O-Cu) with predominantly (111) facets

and

would lead to its lower tendency to produce CO2 reduction products. Additionally, as shown in **Figure 4(f )**, the free energy of formation

and CO production occurs at small (<5 nm) NCs.

shows selectivity for C1 products in ECO2RR [48].

**3.4 Effect of C-NCs shape on ECO2RR**

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

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

#### **Figure 4.**

*Colloids - Types, Preparation and Applications*

C-NCs-based electrocatalysts for ECO2RR.

**3.3 Effect of C-NCs size on ECO2RR**

(NP) surface and thus, alter the reactivity of NP-based electrocatalysts [41–43]. In addition, the geometric effect is an important factor that is crucial in stabilizing the specific intermediate during the reduction process, which leads to an increase in selectivity [44, 45]. When using C-NC-based electrocatalysts, electronic and geometric factors can be adapted to increase selectivity, stability and activity using several approaches, such as size adjustment, shape modification, compositional control, surface functionalization and reaction conditions. As discussed earlier, the nucleation and growth process can be optimized by changing the reaction conditions (thermodynamic and kinetic of the reaction) to achieve the desired C-NCs with well-defined composition, and morphology. Therefore, C-NCs-based catalytic systems have been found to be highly promising to lead the catalytic field that can achieve higher selectivity and efficiency than existing systems. Here, our focus will be mainly on to understand how properties like size, shape, morphology, composition and surface functionalization of nanocrystal affect efficiency and durability of

The size is an important factor for C-NC-based electrocatalysis because different size of C-NCs show different activity/selectivity towards ECO2RR. Several studies have shown that alteration in size at nanoscale can affect both atomic distributions at various reaction sites (plans, edges, corners) and electronic structure of NCs, changing their catalytic characteristics. Therefore, the search for the optimal size C-NC showing the best efficiency, selectivity and durability requires contemporary research in this area. So far, to explore the effect of size towards ECO2RR, different sizes of electrocatalysts have been evaluated, however, our focus here is towards C-NC-based electrocatalysts. Colloidal synthesis of NCs is an excellent approach to study different sizes of NCs while keeping other factors such as shape and composition stable. Loiudice et al. synthesized spherical and cubical Cu C-NCs in the size range of 7.5–27 nm and 24–63 nm respectively for ECO2RR [46]. Their study has revealed unprecedented correlation between size of NCs and their electrochemical activity as well as selectivity for ECO2RR. The activity of Cu C-NCs increases as size of NCs within same morphology decreases, however, this does not hold while comparing cubical NCs with spherical. For example, the 44 nm cube has higher current density than 27 nm sphere. To understand this phenomenon, the propensity of Cu (100) facet towards ethylene production can provide a better understanding. Based on previous findings, it is widely accepted that the Cu (100) plane is selective for the electrochemical reduction of CO2 in ethylene. Additionally, edges in ethylene production are thought to be responsible for the absorption and stabilization of an important intermediate (i.e., COOH \*) as well as inhibiting EH2ER. In this study, the X-ray diffraction pattern has shown a higher contribution of the Cu (100) plane in the nanocube than in the nanosphere and Cu foil. Interestingly, among all Cu C-NCs, a size of 44 nm exhibited highest selectivity with 50% FE for ethylene and overall 80% for ECO2RR over EH2ER. This can be ascribed to changes in atomic configuration at different sites of C-NC due to differences in size. As nanocube move from smaller to larger sizes, the number of atoms at the edges and corners decreases, however, the number of atoms at the plane site increases. As a result, it came close to the morphology of a single crystal, where all the atoms of the surface are populated on the (100) plane. And, as previously stated edges also play an important role in electrochemical reduction of CO2 into C2 products. Therefore, an optimal balance between ratio of edge to plane site in 44 nm Cu C-NC, suggesting not only its unique selectivity for C2H4, but also an

**198**

overall activity towards ECO2RR.

*TEM images of (a) the 8 nm Au NPs and (b) the C-Au NCs. (c) Potential-dependent FEs of the C-Au on electrocatalytic reduction of CO2 to CO. (d) Current densities for CO formation (mass activities) on the C-Au at various potentials. Free energy diagrams for electrochemical reduction of (e) CO2 to CO and (f) protons to hydrogen on Au(111) (yellow symbols), Au(211) (orange symbols), or a 13-atom Au cluster (red symbols) at*  −*0.11 V. reprinted with the permission from [47]. Copyright © 2013, American Chemical Society.*

Similarly, Zhu et al. investigated the non-monotonic size-dependent selectivity of Au C-NCs for CO2 reduction into CO [47]. Among Au C-NCs of size 4, 6, 8, 10 nm synthesized, the 8 nm exhibited a highest selectivity for CO production with FE of ~90% at −0.67 V vs. RHE as shown in **Figure 4(c).** Based on the DFT calculations, the group concludes that dominance of edge sites at 8 nm C-NC facilitates selective CO2 reduction in hydrocarbons, whereas depletion of corner sites inhibits EH2ER. They have further reported the unique selectivity of Au13 C-NC towards EH2ER. Although Au13 facilitates the formation of COOH\* intermediate as compared to Au (211) and Au (111) facet, a stronger binding with CO\* intermediate would lead to its lower tendency to produce CO2 reduction products.

Additionally, as shown in **Figure 4(f )**, the free energy of formation (ΔG*for*mation) of H\* intermediates on Au13 is lower than ΔG*for*mation of COOH\*, suggesting the generation of H2 at low overpotentials. Furthermore, this study is found to correspond to a size dependence study on the small size (2–15 nm) Cu C-NCs for ECO2RR. For particle sizes at 5–15 nm, significantly higher activity and selectivity were found for H2 and CO than for hydrocarbons. Moreover, particles smaller than 5 nm in size showed an exponential increase in the formation of H2 and CO relative to hydrocarbons. This unique property of small C-NCs is due to an increase in under coordinated catalytic sites, which, in turn, strongly stabilize and bind with H\* and CO\* intermediates. Strong adsorption of H\* and CO\* intermediates is suggested to prevent subsequent hydrogenation of CO into hydrocarbons, so the increase in H2 and CO production occurs at small (<5 nm) NCs.

#### **3.4 Effect of C-NCs shape on ECO2RR**

The shape of the C-NC plays an important role in determining the selectivity/ activity of the electrocatalysts. Several findings have shown that the shapedependent C-NC activity/selectivity towards ECO2RR is typically associated with the presence of specific crystal plane. For example, Suen et al. have revealed that cubic Cu C-NC (C-Cu) with mainly (100) facet shows enhanced selectivity towards C2 products while octahedron Cu C-NC (O-Cu) with predominantly (111) facets shows selectivity for C1 products in ECO2RR [48].

Besides, Zhou et al. have illustrated the correlation between different shapes of C-NCs and their corresponding crystal planes using a stereographic triangle as depicted in **Figure 5(a) [**49]. The lower index surfaces, namely, the (100) (110) and (111) facets that lie on the three vertices of triangle, are present on the nanocrystal

**Figure 5.**

*(a) Unit stereographic triangle of fcc single-crystal and models of surface atomic arrangement. (b) Unit stereographic triangle of polyhedral nanocrystals bounded by different crystal planes. Reproduced form [49] with permission from the Royal Society of Chemistry.*

cube, octahedral, and rhombic dodecahedral, respectively. These low index surfaces are said to be less energetic, that is, catalytically less active. However, the plains lying on three sidelines of triangle and one which is residing inside the triangle are known as high index surfaces, that is, catalytically more active and stable. The high index planes (310), (311), (331), and (321) are correlated with tetrahedron, trapezohedron, trisoctahedron, and hexoctahedron, respectively, as illustrated in **Figure 5(b).** The high energy of these polyhedron is attributed to predominance of atomic steps, islands and kinks on their surface. For example, a concave rhombic dodecahedron Au C-NC enclosed with multiple high index planes showed excellent activity and selectivity towards CO formation [50]. In addition, this particular structure of Au C-NC was found durable for longer period of time. Summing up, low coordinating high index planes with CN less than 7 exhibits high catalytic activity, selectivity, stability whereas low index planes with higher CN greater than 6 exhibits lower catalytic activity and stability.

Lattice strain has evolved as another factor that have a significant influence on electrocatalytic properties of C-NCs towards ECO2RR. It was studied that strain generates a distortion in lattice, which, in turn, alters d-band center of metallic C-NCs. The altered d-band center either facilitates or lowers the adsorption of intermediates, as a result of that changing catalytic properties. Typically, upshifting of the d-band center facilitates adsorption of reaction intermediates on the surface of C-NCs. This is because, as the d band center approaches the Fermi level, which is the highest occupied state, the antibonding orbitals move over it, causing them to empty. This, as a result, strengthens the binding strength of the reaction intermediate on the catalytic surface and, therefore, enhances the reaction kinetics [51]

In general, lattice strain in C-NCs can be induced by shaping C-NC in various morphologies, which leads to the inward displacement of atoms at high-energy locations, such as corners and edges, while the outward displacement of atoms on planes to gain overall crystal stability. These compression and expansion in the atomic arrangement in the NCS induce aeolotropic strain gradients that may improve the catalytic efficiency/selectivity of C-NCs [45]. Octahedral and Icosahedron C-NC of Pd with similar sizes were examined by huang et al. to investigate the effect of strain on ECO2RR. They observed that icosahedral/C C-NC shows higher FE (91% with −0.81 V vs. RHE) for CO production than octahedral/C C-NC in ECO2RR. The molecular simulations and DFT calculations showed that surface strain in icosahedral C-NC enhanced catalytic selectivity due to shifting in d-band center, which, in turn, facilitates absorption of a key intermediate (COOH\* )

**201**

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

in ECO2RR. Thus, surface strain in icosahedral C-NC boots catalytic efficiency and

The variation in composition of metal C-NCs is another intriguing factor that plays an important role in tuning electrocatalytic efficiency/selectivity towards ECO2RR. Among other effects of composition, the synergistic effect where the mutual synergy between both electronic and geometric effects determines the activity/selectivity of C-NCs towards ECO2RR is well known. The electronic effect can be understood by the concept of shifting in d-band center due to alteration in composition. In addition to the electronic effect, the geometrical effect also makes a significant contribution, where the particular atomic arrangement in the active center can modify the binding strength of the reaction intermediate, thereby improving the electrocatalytic efficiency/selectivity towards ECO2RR [44]. Therefore, to understand the synergistic chemistry between geometric and electronic effects in this section, the electrocatalytic efficiency, selectivity and durability of bimetallic C-NCs and doped C-NCs electrocatalysts towards ECO2RR

Until now, several bimetallic NCs have been investigated for ECO2RR. For example, Kortlever et al. have found an optimal composition of a novel Pd70Pt30/C electrocatalyst highly active and selective towards HCOOH production. Moreover, this has a remarkably lower onset potential close to 0 V vs. RHE, making it best catalyst till the date [53]. However, usage of noble high-cost metals discourages its application on economical scale. Incorporation of non-noble metals such as Cu, Ni, Fe, etc., in bimetallic system could serve a better alternative to address high cost and stability of electrocatalyst. Kim et al. synthesized different composition of NCs including Au, AuCu, AuCu3, Au3Cu, and Cu, which were assembled in a monolayer on glassy carbon while keeping precise control over morphology (Size, Shape etc.) [44]. When considering electronic effect solely, pristine Au NC should have shown higher activity due to its optimal binding with COOH and CO intermediates. Interestingly, they have observed higher activity for Au3Cu bimetallic C-NC than expected one. Therefore, electronic effect mere does not explain volcanic acti vity correlation for C-NCs. The geometric effect that works synergistically along with electronic effect ensure further stabilization of intermediates, thus, explaining optimal activity of Au3Cu bimetallic C-NC towards electrochemical CO2 reduction, among others. Previous studies have shown excellent properties of In based metal catalysts for selective CO2 conversion into HCOOH. However, these metal catalysts

Kown et al. synthesized In2O3-ZnO C-NCs that showed excellent selectivity towards formation of HCOOH [54]. The XRD pattern showed that pre reduction of these C-NCs during electrolysis leads to the formation of In-Zn bimetallic C-NCs. Among all Zn1-xInx NCs, In0.05Zn0.95 offered remarkable selectivity for HCOOH production at FE of 95% (−1.2 V vs. RHE) as well as with higher current density. The higher catalytic activity was seen in both Zn1-xInxO and Zn1-xInx with decreasing value of x. The XPS data has revealed predominance of O vacancies in bimetallic systems at lower x, which, in turn, decreases thickness of oxide layers in Zn1-xInx. Consequently, conductivity of Zn1-xInx C-NCs increases at lower x, therefore, facilitates rapid electron transfer processes in ECO2RR. Thus, highest catalytic activity of Zn0.95In0.05 C-NC is attributed to its remarkable conductivity. DFT calculations

suffer from limited current density and poor stability.

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

**3.5 Effect of C-NCs composition on ECO2RR**

selectivity for CO2R [52].

will be discussed.

*3.5.1 Bimetallic C-NCs*

in ECO2RR. Thus, surface strain in icosahedral C-NC boots catalytic efficiency and selectivity for CO2R [52].
