Nowadays, finding alternative sources of energy is an important global issue (dos Anjos et al.
2008; Li et al.
2013). The solution of this problem are ethanol fuel cells, which become more and more popular. This is related to the benefits of ethanol: it is less toxic compared to methanol, easier stored and transported than hydrogen, and available from renewable resources (Li et al.
2010; Du et al.
2011). However, the usage of ethanol as fuel generates various challenges, such as the difficulty to split the C–C bond and production of many by-products (Delpeuch et al.
2016). Therefore, the key challenge is to design and develop the appropriate type of catalysts. A good approach to obtain highly selective and active electrocatalysts for ethanol oxidation reaction (EOR), involves the preparation of multifunctional and multicomponent nanostructured particles (Antolini
2007; Antolini and Gonzalez
2011; Goel and Basu
2012; Zhang et al.
2017). These types of nanostructures are extensively synthesized and studied because they exhibit interesting properties, being key components in many catalytic applications. Very important is the possibility of precisely controlling the size, shape, and composition of the synthesized nanoparticles. Because catalysis is a surface effect, the catalyst needs to have the highest possible surface area, which is related to the size reduction (Sun et al.
2011). So far, the most promising group of nanocatalysts for ethanol oxidation are ternary catalysts based on mixtures of Pt, Rh, and SnO
2 designed by the Adzic group (Kowal et al.
2009b; Li et al.
2010,
2013). This type of ternary nanocatalysts is also studied by other groups (Higuchi et al.
2014; Delpeuch et al.
2016). This is related to the unique and individual role of each component in the ethanol oxidation pathway. The role of Rh is to cleave the C–C bond in ethanol, while SnO
2 provides OH species to oxidize intermediates and to free Pt and Rh sites for further ethanol oxidation (Li et al.
2013). Another important issue mentioned above, is the physical contact between the synthesized NPs. Kowal et al. (
2009a) studied ethanol oxidation reaction (EOR) on ternary NP system containing Pt, Rh, and SnO
2, showing a higher activity of such system, because the ethanol molecule has to be in contact with all the phases of the catalyst, which allows for a complete EOR. In their earlier work, they demonstrated the high efficiency of the catalyst of the same composition containing a Pt–Rh alloy (Kowal et al.
2009b). Another group (Higuchi et al.
2014) studied the Pt/Rh/SnO
2 catalyst, but they prepared a nanocatalyst only with partial contact between the respective nanoparticles. The authors confirmed that it is not clear, if for EOR it is better to have a nanoalloy between the metallic elements or is a physical contact between them sufficient. The investigation by Park et al. (
2008) shows that the control of alloy composition allows tuning the catalytic activity. On the other hand, Roth et al. (
2005) indicate that alloy formation does not seem to be a crucial requirement for a superior electrocatalytic performance, as long as a close contact between both phases is achieved. Therefore, understanding the synergistic effect occurring between all three components of the designed catalyst, preparing standard samples consisting of these three nanoparticles, control of their size, composition and contact between them, are key steps in further development of this field. One of the strategies, which can lead to successfully interconnecting the desired nanoparticles, is based on controlled assembly of separately synthesized nanoparticles.
One of the main factors determining the interactions of nanoparticles in the suspension is their charge. The presence of this charge causes that, according to the DLVO theory, in the low-salt conditions, the nanoparticles repel each other and their suspension remains kinetically stable. However, in a mixture of negatively and positively charged nanoparticles, electrostatic attraction between them occurs resulting in heteroaggregation (or agglomeration). The experimentally accessible measure of the state of nanoparticles charge is their zeta potential. A high zeta potential (negative or positive), usually |
ζ| > 30 mV, indicates the suspension stability. Generally, the charge of the metallic and metal oxide nanoparticles can be easily controlled by changing the pH of the suspension. The mechanism of surface charging is reported in detail in the monograph by Kosmulski (
2009). However, it is well known that electrostatic interactions, besides pH, are strongly influenced by ionic strength and/or use of ligands for nanoparticle stabilization (Mori et al.
2009).
Aziz et al. (
2013) analyzed the dependence of the zeta potential of aqueous suspension of SnO
2 nanoparticles on the pH, prior to their electrophoretic deposition. They have observed negatively charged nanoparticles at pH values from about 4.5 to 11. On the other hand, Kosmulski (
2009) measured the pH values of the isoelectric point for SnO
2 particle suspensions from various sources, as ranging between 3.8 and 5.6, depending on the method of their synthesis.
Controlling the zeta potential of individual nanoparticles is especially important, when designing and connecting different NPs to form multi-metal nanoparticle structures. Selecting an optimal value of the zeta potential allows to connect different types of nanoparticles such as metals and oxides. For example, heterointerfaces of TiO
2 and SnO
2 NPs were formed by Siedl et al. (
2012). By changing the pH of those nanoparticles, they have obtained oppositely charged suspensions, with zeta potential values of +24 and − 9 mV for TiO
2 and SnO
2 NPs, respectively, which allowed for the formation of a uniform network between them. The interaction of positively charged Pt-Ag alloy nanoislands with negatively charged graphene sheets resulted in the formation of hybrid composites for electro-oxidation of methanol (Feng et al.
2011).