## 3 Results and Discussion  
### Results of Characterization of Materials  
#### 3.1.1 Platinum-Based Powder and Sintered Material  
It was found from the ICP-AES analysis that the content of platinum in the powder was 92.7 wt. %, and the dominant impurities were represented by tin (0.077 wt. %), copper (0.074 wt. %), silver (0.033 wt. %), palladium (0.022 wt. %), iron (0.014 wt. %), zinc (0.004 wt. %) and calcium (0.004 wt. %). In the sintered material, the content of platinum was 99.7 wt. %, and the dominant impurities were represented by the same elements: tin (0.086 wt. %), copper (0.072 wt. %), silver (0.037 wt. %), palladium (0.023 wt. %), iron (0.020 wt. %), zinc (0.014 wt. %) and calcium (0.005 wt. %). The observed difference in the content of platinum in these two materials was due to the fact that the particles in the powder were partially oxidized, since air was used as a carrier gas in the synthesis. Recently, platinum oxide nanoparticles were found to form in the course of spark ablation of platinum electrodes when employing air as a carrier gas [26]. It is known that platinum oxides are reduced to platinum (Pt\({}^{\delta}\)) at elevated temperatures [35], so the platinum oxide entering into the composition of the inorganic phase of the deposited ink was supposed to be reduced to a metallic state during the heat treatment and then get sintered, thus forming a polycrystalline material. The hypothesis of the occurrence of platinum oxide in the powder was supported by XPS data. The XPS spectrum of the powder in the Pt4f core level region (Figure 2, upper spectrum) was perfectly fitted by a single doublet comprising a Pt4f\({}_{7/2}\) line located at a binding energy of about 72.4 eV, which indicated the presence of PtO, according to [35; 36]. The XPS spectrum of the sintered material was also fitted by a single doublet (Figure 2, lower spectrum) with a Pt4f\({}_{7/2}\) line located at about 71 eV, suggesting the complete reduction of platinum oxide to a metallic state [35; 36]. It should be noted that the powder had a very limited solubility in the aqua regia used for the preparation of the solution for ICP-AES. Therefore, its dissolution was implemented in two steps. At the first step, the powder was processed in the concentrated hydrochloric acid that led to the dissolution of the platinum oxide. Then, the nitric acid was added to the obtained solution in the required amount in order to dissolve the remaining metallic platinum. At the same time, the sintered material was readily soluble in the aqua regia, as was expected for the metallic platinum. One may conclude from the analysis of the XRD spectra that both the powder and the sintered material contained only one crystalline phase, represented by platinum with the Fm3m crystal structure (Figure 3A,B). It follows from the XRD spectrum of the powder that there is an amorphous halo in the range of the scattering angles 20 = 29\({}^{\circ}\)-39\({}^{\circ}\), suggesting the presence of an amorphous phase, most likely represented by a platinum oxide since platinum atoms were found to be constituents of the PtO compound, as evidenced by the XPS data (Figure 2). The lines corresponding to platinum atoms involved in the metallic platinum were not observed in the XPS spectrum of the powder because the depth of the XPS analysis is few nanometers, so the photoelectrons emitted from the metallic platinum core of the oxidized particles did not escape from the particles. Figure 2: X-ray photoemission spectroscopy (XPS) spectra corresponding to the Pt4f core level region of the powder and sintered material. The spectra were acquired by using Al K\(\alpha\) radiation. Figure 3: _Cont._  
To estimate the content of the amorphous phase in the powder, we followed the internal standard method [37]. For this purpose, the platinum-based powder under study was mixed with ZnO powder (Sigma-Aldrich, Saint Louis, MO, USA, no. 544906), used as a crystalline internal standard, at a ratio of 1:1 by weight.