# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 2 Glass Strengthening: The Ion-Exchange Contribution
### Ion-Exchanged Noble Metal Nanoparticles in Glasses for Sers Applications
#### 3.2.3 Thermal Annealing in H\({}_{2}\) Controlled Atmosphere  
Another thermal post-processing strategy for a glass exchanged with noble metal ions involves the heating of the sample in a controlled hydrogen (H\({}_{2}\)) atmosphere. Although this technique requires larger precautions for the operator safety in comparison with that performed in air (O\({}_{2}\)), this thermal post-process favours the formation of both glass-metal nanocomposite (GMN) species within the glass and metal island film (MIF) on the glass surface due to the out-diffusion mechanism of the noble metal atoms as a result of the hydrogen penetration into the glass [161]. However, under particular conditions relating to hydrogen processing, it is possible to favour the formation of MIF species on the glass surface by minimizing that of GMN within the host material, as well reported in the works of [PERSON] and collaborators, to which we refer for more details [162; 163; 164]. In brief, the authors employed a thermal poling technique, through the use of a suitably shaped anodic electrode applied to the ion-exchanged silicate glass sample, in order to modify the distribution of noble metal ions (e.g., Ag\({}^{\star}\)) and thus control the formation of the metal nanoislands by means of a soft thermal annealing step in a hydrogen atmosphere. Figure 5 explains this approach together with an atomic force microscopy (AFM) image of the nanoislands so formed. Depending on the electrode periodicity, arrays of nanoislands can be obtained on the surface of the glass sample.  
During the poling step, the simultaneous presence of electrostatic repulsion effects generated by the applied DC electric field and those linked to the process temperature favours the migration of the silver ions towards deeper layers of the glass in correspondence of the poled region. Nevertheless, the low poling temperature (T\({}_{\text{p}}\)\(\sim\)250 \({}^{\circ}\)C) and the lack of impurities in the pristine silicate glass do not favour the formation of silver atoms and  
Figure 5: **(a)** A sketch of the mechanism of silver nanoislands growth by poling treatment following by a thermal annealing treatment in H\({}_{2}\) controlled atmosphere; **(b)** an AFM image of a nanoisland group in a not poled region after annealing treatment in H\({}_{2}\). Figure adapted from paper [164] under the terms of the Creative Commons Attribution 4.0 International License.  
metal nanoparticles in this area. Therefore, if the process parameters (i.e., duration and temperature) of the following thermal annealing in hydrogen are selected in such a way that the hydrogen penetration depth is not enough to reach the diffused silver ions, then any possibility of their reduction is precluded, and the poled zone remains almost empty of silver atoms and nanoparticles. On the contrary, in correspondence with the non-poled zone, the hydrogen in-diffusion is accompanied by an out-diffusion of the silver ions with their consequent reduction and formation of metal nanoislands on the glass surface [163]. A similar result was found by [PERSON] et al. in fluorophosphate glasses [99]. The possibility of drawing 2D-patterned nanoisland structures on the glass surface makes these substrates ready for their use in SERS applications.