The encapsulation of inorganic nanoparticles into polymer particles opens the door to countless applications taking advantage of the properties of both phases. In this chapter the UV absorbing capacity of CeO2 nanoparticles and the film forming capacity of acrylic polymers are combined. A synthetic route to produce waterborne acrylic/CeO2 hybrid nanocomposites for UV absorbing coatings applications is presented. This strategy leads to encapsulated morphology of the CeO2 nanoparticles into the polymer particles and therefore to the lack of agglomeration during film formation. A mathematical model developed for inorganic/organic hybrid systems is able to explain the morphology evolution from the initial monomer droplet to the polymer particles. The films cast from these latexes are transparent and show excellent UV absorption that increases with the amount of cerium oxide nanoparticles in the hybrid latex. Finally, the photoactivity behavior that the CeO2 nanoparticles may have on the polymeric matrix is studied, discarding additional effects on the acrylic polymer matrix.
- waterborne polymer dispersions
- CeO2 nanoparticles
- hybrid nanocomposites
- UV absorption
The incorporation of metal oxide nanoparticles into polymer matrices opens the way to the production of novel nanocomposite materials due to the synergetic effect of each phase in the final properties, as well as the possibility to use them in many different applications. In this direction, in the last decade, many authors have studied the benefits in properties that could be obtained when combining CeO2 nanoparticles with polymers. For instance, catalytic [1, 2, 3], thermal , mechanical [5, 6], optical [7, 8, 9], anticorrosion [10, 11, 12], and barrier properties  of polymers have been considerably improved with the incorporation of CeO2 nanoparticles. Moreover, CeO2 nanocomposites can find application in many different fields such as chemi-sensors and photocatalyst for environmental applications [14, 15, 16], temperature and humidity sensors  or extractants for yttrium ions .
Taking advantage of the excellent UV absorption capacity of the CeO2 nanoparticles, it is really interesting to incorporate these nanoparticles into polymer matrices in the field of outdoor clear coatings. Waterborne acrylic polymers, synthesized mainly by emulsion polymerization process, are widely used as protective coatings for different surfaces due to their low toxicity and good quality film forming properties . However, the main drawback of these coatings is the photodegradation they suffer under UV light. Traditionally, organic UV absorbers and hindered amine light stabilizers (HALS) were used, but due to the increasing environmental pressure to reduce the volatile organic compounds (VOC) content in coatings, the use of metal oxides such as TiO2 [20, 21], ZnO [22, 23] and CeO2 [24, 25] have been considered as an attractive alternative. All of them absorb radiation around 400 nm  and posses a band gap energy of around 3 eV , which makes them good candidates for UV absorption purposes. There are some works in which TiO2 [28, 29, 30, 31] and ZnO [32, 33, 34, 35] nanoparticles have been incorporated into polymer matrixes to improve the UV absorbance capacity of the coating. Nevertheless, the photocatalytic activity of CeO2 nanoparticles is lower than that of TiO2 and ZnO , which might prevent a faster degradation of the acrylic coatings due to the presence of the metal oxide, making CeO2 nanoparticles ideal candidates for their incorporation into waterborne acrylic coatings.
In waterborne hybrid coatings, the final morphology of the hybrid system is governed by the different nature of the inorganic nanoparticles and polymer. Therefore, the control of the morphology of the hybrid system is challenging. The compatibility between both phases (thermodynamics) as well as the polymerization process (kinetics) will define the final morphology of the nanocomposite and thus, the final application . In the literature a variety of CeO2 nanoparticles (hydrophilic or hydrophobically modified) have been incorporated following different polymerization processes.
For instance, Fischer  and Mari  synthesized CeO2/polystyrene (PS) and CeO2/polymethylmethacrylate (PMMA) hybrids with raspberry like morphology, following the same procedure. They synthesized PS and PMMA latexes incorporating active groups (acrylic, methacrylic or phosphate groups) on the surface of either the PS or PMMA polymer particles, synthesized previously by minimemulsion polymerization. These active groups served as nucleating agents for the crystallization of the CeO2. As the CeO2 nanoparticles were generated in the surface of the polymer particles, this morphology was very favorable to take advantage of the catalytic behavior of the CeO2, giving for instance very efficient catalyst for the hydration reaction of 2-cyanopiridine to 2-picolinamide.
Another possibility to obtain pickering morphology is to use the inorganic nanoparticles to stabilize the polymer particles. CeO2 nanoparticles were used as pickering stabilizers in the miniemulsion polymerization of acrylates by Zgheib et al. . It was found that at least 35 wt% of CeO2 nanoparticles were necessary to obtain stable hybrid latexes at intermediate solids content (25 wt%). Therefore, the large amount of nanoparticles used and the solids content obtained, limited their application as coating. However, using other inorganic nanoparticles, such as SiO2 or TiO2, it has been possible to obtain high solids content latexes . The ability of these hybrid nanocomposites with Pickering morphology as protective coatings [40, 41] has been successfully proved.
Hawkett was the first one adsorbing amphiphilic macro-RAFT agents on the surface of inorganic nanoparticles and starting the polymerization from the macro-RAFT agents to obtain encapsulation of the inorganic nanoparticles [42, 43]. Garnier [44, 45], Warrant  and Zgheib  followed this method to encapsulate CeO2 nanoparticles. The hybrid acrylic/ CeO2 latexes were obtained by semibatch emulsion polymerization starting the polymerization from the macro-RAFT agent modified CeO2 nanoparticles. In general, good distribution of the CeO2 nanoparticles in the polymer particles was obtained and the CeO2 nanoparticles were located close to particle-aqueous phase interface and even encapsulated in some examples.
In this Chapter, a polymerization approach to produce waterborne hybrid (polymer/CeO2) dispersions with encapsulated CeO2 nanoparticles will be presented. The prediction of the evolution of the morphology during the polymerization will be illustrated by means of a mathematical model and finally the UV absorbing properties of the clear coatings produced from these hybrid latexes and the potential photochemical degradation of the coatings will be discussed.
2. Synthesis of acrylic/CeO2 hybrid nanocomposites by seeded semibatch (mini)emulsion polymerization
In the production of waterborne binders (for coatings), emulsion polymerization is the most popular process. However, when it comes into hybrid waterborne binders, miniemulsion polymerization emerged as an alternative process, to overcome the limitations of emulsion polymerization when the inorganic material must be incorporated into the polymer particles [48, 49, 50]. Moreover, equilibrium morphology simulations have demonstrated that if the nanoparticles present good wettability in the monomer phase, they can be encapsulated in monomer nanodroplets and hence hybrid latexes with encapsulated morphology can be produced .
The approach presented here to produce waterborne acrylic/CeO2 nanocomposite dispersions uses a two-step seeded semibatch (mini)emulsion polymerization process. The approach is well suited to produce hybrid latexes with CeO2 contents spanning between 0.5 and 5 wt% based on the polymer [52, 53]. In the first step, hybrid seed particles are synthesized by batch miniemulsion polymerization. In the second step the solids content and the final concentration of the CeO2 nanoparticles can be tuned by controlling the composition of the feed of the semibatch process. Two feeding strategies can be used:
Neat monomer preemulsion feeding. A preemulsion is fed containing monomers, emulsifier and water, to grow the already formed seed hybrid particles. In this case, all the CeO2 nanoparticles present in the final hybrid are only added in the seed prepared in the first step. Hybrid latexes with 40–50% solids content were synthesized [53, 54] with CeO2 contents up to 1 wt% in the final hybrid composite.
Hybrid monomer/CeO2 miniemulsion feeding. The same formulation of the miniemulsion used to synthesize the seed is used as feed allowing higher concentration of CeO2 in the final latex. Hybrid latexes with CeO2 contents up to 5 wt% were obtained at 40% of solids content .
Figure 1 presents the transmission electron microscopy (TEM) images of relevant latexes with low (1 wt%) and high (5 wt%) content of CeO2 nanoparticles produced using the two feeding strategies discussed above, respectively. As it can be seen, the polymer particle size distribution (PSD) obtained by both feeding strategies is different. Even if the PSD obtained for the hybrid seed is the same for both cases, with particles around 100 nm, the final PSD differs depending on the feeding strategy. When neat monomer preemulsion is fed (Figure 1a) the final PSD is narrow, suggesting lack of secondary nucleation during the semibatch process. Nevertheless, the PSD obtained when feeding the miniemulsion (second strategy) is broader, as particles between 25 and 600 nm can be found (Figure 1b). This is related to the miniemulsion stability and to the monomer droplet nucleation efficiency in the reactor. According to Rodriguez et al., the nucleation efficiency in a seeded semibatch miniemulsion polymerization is related to the stability of the miniemulsion fed (the higher the stability, the higher the nucleation of the entering droplets), and also to the ratio of the number of entering droplets with respect to the number of particles in the seed (the higher this ratio, the higher the number of fed droplets nucleate because their efficiency for capturing radical is higher) . In Figure 1b, very small polymer particles can be seen containing nanoceria, which is an indication that hybrid monomer droplets serve as monomer reservoirs when they enter into the reactor, but they do not lose their identity and finally they end up nucleating [55, 57].
In any case, the CeO2 nanoparticles (darker spots) are all present in the polymer particles in both cases (Figure 1a and b), and no one is present in the continuous water phase. It can be seen that the CeO2 nanoparticles aggregates are more centered in the polymer particles synthesized using the first strategy whereas they are more close to the border of the polymer particle in the hybrids synthesized using the second strategy. The difference comes from the feeding strategy used in each case. When the neat monomer preemulsion strategy is used, the monomer entering the reactor in the semicontinuous process covers de hybrid seed particles containing the CeO2 nanoparticles. However, in the case of the miniemulsion feeding, a large fraction of the entering hybrid droplets nucleates, and hence not all the fed monomer is used to grow the seed particles and as a consequence CeO2 nanoparticles aggregates are not fully encapsulated.
Nevertheless, the encapsulation of inorganic nanoparticles inside polymer particles cannot be proved just by TEM images, as sometimes the micrographs are not conclusive enough. Therefore, TEM Tomography studies were carried out to a representative area of the hybrid latexes prepared following the seeded semibatch (mini)emulsion strategy presented so far. The results demonstrated that the CeO2 nanoparticles were surrounded by polymer in all directions in both, the seed and the final polymer particles, demonstrating beyond any doubt the encapsulated morphology .
Furthermore, it is remarkable that every polymer particle contains one CeO2 nanoparticle aggregate in average. In Figure 1a and b it can be seen that the number of polymer particles with zero, two and three nanoparticles is very small. It is observed that the CeO2 aggregate size increases with the nanoceria content in the formulation of the hybrid nanocomposite; namely, the higher the CeO2 content, the larger the aggregates. Figure 1c and d presents the quantification of the CeO2 aggregate sizes in the hybrid latexes containing 1 and 5% of CeO2 nanoparticles. As it can be seen, aggregate sizes between 3 and 73 nm can be found for the hybrid latex containing 1% of CeO2 nanoparticles, whereas aggregate sizes between 3 and 123 nm can be found for the nanocomposite containing 5% of CeO2 nanoparticles. Volume average aggregate sizes are 26 and 50 nm, respectively. However, it should be mentioned that the initial average size of the CeO2 nanoparticles dispersed in the monomer mixture was 12 nm (measured by dynamic light scattering). Therefore, it seems that all the nanoparticles present in each monomer droplet aggregate during the first stages of the polymerization process to form a CeO2 aggregate per polymer particle. This effect will be discussed deeply in the following section.
One of the main advantages of having inorganic nanoparticles encapsulated in polymer particles is the lack of agglomeration during the film formation process, obtaining homogeneous distribution of the nanoparticles in the polymeric film and avoiding their leaching during the lifetime of the coating. Figure 1e and f show the hybrid films obtained after drying hybrid latexes with 1 and 5% of CeO2 nanoparticles. It can be seen that after film formation the nanoceria aggregates are homogeneously dispersed in the polymer matrix in both cases. The average CeO2 aggregate size was also analyzed and it was found that the average size in volume of the CeO2 nanoparticles in the film is 26 nm for the film containing 1% of CeO2 and 46 nm for the film with 5% of CeO2. Therefore, the average size of the CeO2 aggregates does not change during the film formation process in which the polymer particles coalesce between them, indicating that the encapsulation is an efficient method to avoid the agglomeration of the inorganic nanoparticles in the final film.
3. Evolution of particle morphology during the synthesis of acrylic/CeO2 hybrid nanocomposites synthesized by miniemulsion polymerization
The morphology obtained in a hybrid nanocomposite may affect directly the final application of the composite material as it has been shown in Section 1 of this chapter. The particle morphology will develop during the polymerization and the final particle morphology will be determined by the interplay of thermodynamics and kinetics. The equilibrium morphology is the one that minimizes the total interfacial energy () of the system, for an organic/inorganic system being the organic phase polymer (monomer), and it is given by:
where, Aij and 𝜸ij are the interfacial area and interfacial tensions respectively, between phase i and j, where P, I and W are polymer (monomer), inorganic material, and aqueous phase, respectively. In this particular case the CeO2 inorganic nanoparticles were previously modified in order to make them hydrophobic and more compatible with the monomers, so the interfacial tension 𝜸II should be very low because when the inorganic particles come into contact, the contact occurs between the same hydrophobic materials. Neglecting 𝜸II, Eq. (1) reduces to an equation that has the same mathematical form that the equation used to calculate equilibrium morphologies of two phase polymer-polymer systems [58, 59, 60]. Using the morphology map developed in these studies, Asua showed a similar one (see Figure 2) adapted to a polymer/inorganic system , where the gray phase represents the inorganic material and the white the polymer (monomer).
According to this morphology map presented in Figure 2, the possible equilibrium morphologies that can be obtained in a polymer/inorganic hybrid nanocomposite are core-shell (encapsulated), inverted core-shell, hemispherical or separated particles. During the miniemulsion polymerization, the system and thus, the composition of the monomer droplets, are changing as polymerization proceeds. The monomer becomes polymer, initiator or other compounds may incorporate into the polymer and grafting might occur between the polymer being formed and the inorganic material. All these factors will alter the interfacial tensions between the phases and hence, the final equilibrium morphology. In this way, it would be possible to shift from encapsulated morphologies in the initial miniemulsion to hemispherical or separated phases after polymerization. There are some examples in the literature in which the initiator type [62, 63], emulsifier amount  and monomer type  variations affected strongly the final particle morphology.
In order to analyze the effect that polymerization may have on the morphology of the system described in this chapter, the evolution of the acrylic/CeO2 nanocomposite is followed during the polymerization process (the hybrid seed preparation by batch miniemulsion polymerization) by cryo-TEM, analyzing samples withdrawn from the reactor at different monomer conversion, and the morphology map (Figure 2) is used as a reference to explain the different morphologies obtained, even if some of the morphologies presented are not at equilibrium. It can be observed that at the beginning in the miniemulsion, the CeO2 nanoparticles are well dispersed in the monomer droplets (Figure 2a). This means that the compatibility of the nanoceria with the monomer mixture is really good in the monomer droplets or in other words, that the interfacial tension between the acrylic monomers and the inorganic material, 𝜸IP, is low and the interfacial tension between the CeO2 nanoparticles and water, 𝜸IW, is high. This morphology is presented by the core-shell morphology on the left side where 𝜸IP/ 𝜸PW < 1 and | 𝜸PW- 𝜸IP|/ 𝜸IW < 1. It should be mentioned that the nanoparticles are sterically stabilized by the hydrophobic modification they bear in the monomer droplets.
In Figure 2b the acrylic/CeO2 nanocomposite system at 1% of conversion is shown. The morphology observed is completely different, as the nanoparticles tend to aggregate, which means that the incompatibility between the newly formed polymer and the surface of the CeO2 nanoparticles has increased or that 𝜸IP has become higher. This change in the morphology with the presence of polymer is observed too when the acrylic/CeO2 hybrid miniemulsion is prepared adding a polymer in order to increase the stability of the miniemulsion . At 8% of conversion, the difference becomes more evident, the nanoparticles are more aggregated and they tend to move towards the border of the polymer particles. The fraction of the polymer increases and thus, 𝜸IP increases. This way, the equilibrium morphology evolves following the red arrow crossing to the hemispherical region as shown in Figure 2. At 18 and 40% of conversion the CeO2 aggregates are more compact and most of the aggregates are situated in the border of the polymer particle (equilibrium position). It should be mentioned that all these morphologies are not at equilibrium, since more than one nanoceria aggregate can be found in the polymer particles. However, when full conversion is achieved, one single aggregate can be seen in each polymer particle, which corresponds to the hemispherical equilibrium morphology.
In the literature there are some mathematical models to predict equilibrium morphologies of hybrid systems [51, 59, 61]. However, these models are not enough to explain the evolution of the acrylic/CeO2 hybrid nanocomposites, since equilibrium morphology is not obtained until 40% of conversion is reached. Recently, Hamzelou et al.  developed a mathematical model for the dynamic evolution of this particular nanocomposite system. This approach provides the distribution of particle morphologies in the whole population of polymer particles. The distribution of particle morphologies is described by a distribution of clusters of CeO2 nanoparticles (aggregates) dispersed in the monomer phase (see Figure 3). According to their position in the particles, the clusters are divided into two different categories: those at equilibrium positions (red dashed line in Figure 3) and clusters at non-equilibrium positions (blue line in Figure 3). Thermodynamics are used to calculate the equilibrium morphology and all relevant kinetic events of the system including cluster nucleation, polymerization, polymer diffusion and cluster aggregation are taken into account. Figure 3 shows the simulated weight distributions for the CeO2 aggregates (clusters). It is shown that at 1% of conversion, most of the nanoceria aggregates are in nonequilibrium positions. At 18% of conversion, most of the aggregates are at equilibrium, however, in some of the polymer particles more than one aggregate can be found. At 100% conversion all the nanoceria aggregates are in equilibrium. TEM-like images are generated and they can be compared to the cryo-TEM images presented in Figure 2. It can be seen that CeO2 nanoparticles aggregates follow the same evolution in the experimental cryo-TEM images and in the TEM-like images generated from the model.
To summarize, the morphology evolution of the whole acrylic/CeO2 nanocomposite is as follows. During the first step, homogeneous distribution of the CeO2 nanoparticles in the monomer droplets is obtained in the hybrid miniemulsion. During the miniemulsion polymerization, the CeO2 nanoparticles aggregate and migrate to the surface of the polymer particles. Up to 40% of conversion, the concentration of monomer is high enough and the nanoparticles are able to move inside the monomer droplets towards equilibrium positions. Thus, the nanoceria aggregates are at the edge of the polymer particles, mostly surrounded by polymer, but not always encapsulated . During the second step (neat monomer feeding), the migration of the CeO2 aggregates is constrained due to the high internal viscosity of the particles. The monomer feeding is done under starved conditions and thus, the seed hybrid particles are covered by a shell of polymer leading to an encapsulated morphology. The proposed mechanism is graphically described in Figure 4.
4. UV absorption properties of acrylic/CeO2 hybrid nanocomposites
One of the main reasons to incorporate the CeO2 nanoparticles into waterborne clear coatings is their excellent UV absorption capacity. This can be assessed by measuring the UV absorbance of 50 μm thick hybrid films. It should be mentioned that all the hybrid films are transparent and yellowish (Figure 5). The color of the films increases with the CeO2 nanoparticle content from 1 to 5 wt% and hence, the transparency decreases. Even if the dispersion of the nanoparticles is good in all the hybrid films, the large sizes measured for the hybrid film containing 5% of CeO2 nanoparticle affect the transparency.
Figure 6 shows that the UV absorption of the hybrid films is higher in the presence of the nanoceria in the whole spectrum range (250–600 nm), but the absorption enhancement is most noticeable above 300 nm, where the pristine copolymer absorption is negligible. Furthermore, the higher the amount of CeO2 nanoparticles, the higher the absorption. However, scattering is observed for the film containing 5% of CeO2 nanoparticles due to the large size of aggregates obtained for this nanocomposite.
Photodegradation of the hybrid film is a major concern due to the photocatalytic activity of the CeO2 nanoparticles. In the literature, the photodegradation of hybrid acrylic coatings has been studied in different substrates such as glass, stone or wood [68, 69, 70, 71]. In these cases, the hybrid film was tested in a substrate and there might be two sources of radicals. One coming from the substrate and the other one from the nanoparticles present in the polymer matrix. To skip this problem the degradation behavior of the bare acrylic/CeO2 hybrid films was analyzed. Accelerated weathering tests were conducted in a solar box, for the nanocomposite film without nanoparticles and for the one containing 1% of CeO2. Different properties of the hybrid films exposed to UV light were measured . Thermal properties reveal one step thermal degradation (around 380°C) and negligible changes in the glass transition temperature (Tg) values for all the hybrid films before and after the exposure. Regarding the microstructure, molecular weight distributions (MWD) and the formation of cross-linked or gel structures were also analyzed. The results show that there is degradation of the polymeric film since the cross-linked fraction increases in the films, but there is no additional effect in the films containing metal oxide nanoparticles. On the other hand, neither the Fourier Transform Infrared Spectra (FTIR) nor the TEM micrographs show any significant difference in the films. It is therefore concluded that the possible photodegradation that CeO2 nanoparticles may produce in the bare hybrid films is negligible, owing to the similar properties obtained for the blank film and the hybrid film containing 1% CeO2 nanoparticles after the UV irradiation.
To study the effect of different metal oxides nanoparticles, a nanocomposite film with 1% ZnO nanoparticles was also synthesized following the same seeded semibatch polymerization approach as described in Section 2 . The morphology obtained in the final hybrid films was different to that obtained for the CeO2 hybrid films. The ZnO nanoparticles aggregate sizes were much bigger (~75 nm), preventing the homogeneous distribution of the nanoparticles in the film. However, the acrylic/ ZnO hybrid films presented higher UV absorption above 350 nm than the counterpart hybrids with CeO2. In the photodegradation studies carried out in the work mentioned above , even if it is known that the photocatalytic activity of the ZnO is larger than that of the CeO2, as it was mentioned in the introduction, the behavior of the hybrid films containing both types of nanoparticles did not differ significantly.
5. Conclusions and future perspectives
A polymerization strategy to synthesize waterborne hybrid acrylic/CeO2 nanocomposites for their application as UV blocking coatings has been discussed in this Chapter. The designed two-step polymerization approach is able to produce different loadings of CeO2 nanoparticles with industrially relevant solids content. Moreover, the strategy ensures the encapsulation of the nanoparticles in the polymer particles that avoids agglomeration during film formation process and provides good UV absorption properties, making these coatings good candidates as clear coats for outdoor applications. A mathematical model developed to predict the evolution of the particle morphology for polymer-polymer systems has been applied for the polymer-CeO2 hybrids and it is able to predict the evolution of the morphology of the two stage semicontinuous polymerization opening the door to the use of the model for optimization and control of waterborne polymer-inorganic particle morphology purposes.
The designed strategy opens the possibility to encapsulate other nanoparticles and extend the application region. The incorporation of hydrophobically modified ZnO nanoparticles has also been tested, providing film forming hybrid latexes with improved UV absorption capacity . Moreover, with the incorporation of a fluorinated monomer to the acrylic/ZnO hybrid system, anticorrosion properties have been improved. It was demonstrated that the incorporation of the ZnO nanoparticles by blending was not enough to improve the corrosion protection, whereas when the nanoparticles were encapsulated and hence, well distributed in the polymeric film, the benefits were substantial . Recently, many authors’ investigation has been directed to improve anticorrosion properties with the incorporation of CeO2 nanoparticles. For instance, polyurethane coatings containing CNT/CeO2 , polyacrylic acid/CeO2 coatings , CeO2/graphene-epoxy nanocomposite coatings  and water based polyurethane/ CeO2 coatings . None of these works obtained encapsulated morphology and hence, the possible aggregation of the nanoparticles during film formation and leaching could be a problem, even though the anticorrosion properties were improved in all the cases. This means that combining the strategy developed in this work, with the appropriate monomers and the anticorrosion properties that CeO2 nanoparticles exhibit in all the works mentioned above, synergetic effects could be obtained making these nanocomposites ideal candidates for corrosion protection.
Very recently, De San Luis et al.  incorporated quantum dots into core-shell particles made of polystyrene/divinyl benzene (DVB) as core and PMMA/DVB as shell. The cross-linked polymeric phases were synthesized in two stages following the strategy developed in this Chapter. Thanks to the encapsulated morphology obtained, the fluorescence emission of the QD containing core-shell particles was preserved for more time than any other work published so far. The same authors incorporated CeO2 nanoparticles obtaining PS/QD/CeO2/PMMA hybrid particles. Interestingly, the films casted from these hybrid particles exhibit increasing fluorescence under sunlight exposure . This opens the possibility to use CeO2 nanoparticles to enhance the optical properties of different technological devices.
Financial support from the Basque Government ELKARTEK KK-2016/00030 and KK-2017/00089 is greatly acknowledged. Miren Aguirre thanks the financial support given by the European Union (Woodlife project FP7-NMP-2009-SMALL-246434), the UPV/EHU (2984/2014) “Doktore berriak kontratatzeko eta horiek doktorego ondoko prestakuntza programetan sartzeko laguntza” and also the financial support received from Ministerio de Economía y Competitividad de España, Juan de la Cierva en Formación (FJCI-2014-22336). The SGIker UPV/EHU for the electron microscopy facilities of the Gipuzkoa unit is acknowledged. Programa de Grupos Consolidados from the Basque Government (IT999-16) is also gratefully acknowledged.