## 3 Results and Discussion
### Morphology and Surface Analysis  
SEM micrographs of Bi\({}_{2-x}\)Sb\({}_{x}\)Te\({}_{3}\) samples at different magnifications are presented in Figure 2. All samples show particles with clear hexagonal, or truncated-edge hexagonal, platelet morphology, in agreement with their rhombohedral layered crystal structure, with slight change of platelet size from one sample to another. In some cases, various platelets sharing a common axis are observed. Lateral size of hexagonal platelets varies in the range from 50 to 500 nm for different samples, where the thickness is typically around 50 nm. Sb\({}_{2}\)Te\({}_{3}\) sample display the largest lateral size of hexagonal platelets up to about 1 \(\upmu\)m, which aligns with its high crystallinity and narrower peak width observed in XRPD. A closer investigation of platelets performed by using TEM for the Bi\({}_{2}\)Te\({}_{3}\) sample, as an example, revealed crystalline domains (see Figure S5). There is a thin disordered layer on the nanoplatelets (see Figure S5b) which may be due to residual carbon materials or slightly different, disordered lattice. The synthetic method and the solvents used significantly influences surface chemistry of resultant materials. In the polyol synthesis, nanoparticles may have residual chemicals on the surface, which may influence their dispersion, sintering, or some other processing characteristics. Therefore, XPS analysis was performed on the as-made materials and the resultant spectra for Bi 4f, Sb 3d, and Te 3d regions are presented in Figure S6. Carbon residue is present on all samples with varying degrees. XPS peak fitting results are summarized in Table 1, disregarding the C and O content. For all samples, during the deconvolution process, oxide phases were identified. For Sb3d and Te3d regions, metallic and oxide phases were clearly visible. In the case of Bi4f region, high resolution spectra allowed determining metallic and oxide (Bi\({}_{2}\)O\({}_{3}\)) phases. The most remarkable outcome is the evidence on the presence of oxide phases on the surface of as-made nanoplatelets. Zeta potential (\(\xi\)-potential) analysis is a technique for determining the surface charge of nanoparticles in solution in colloidal form. Nanoparticles possess a surface charge that attracts a thin layer of ions of opposite charge to their surface, and when the surface charge is balanced by the attracted oppositely charged ions, the total surface charge becomes zero. This point is defined as the isoelectric point (_iep_) and is strongly dependent on the surface exposed species. \(\xi\)-potential is usually studied as a function of an additive, most commonly as a function of pH in the solution--where pH is adjusted by the step-wise addition of acid or base solution into the colloidal system. Study of _iep_ of the as-synthesized Bi\({}_{2-x}\)Sb\({}_{x}\)Te\({}_{3}\) materials as a function of pH is, therefore, expected to reveal the surface chemistry of as-made nanoplatelets. While all materials showed residual carbon, which should be the same type of organics due to the synthesis process used, the surface chemistry showed great variations in zeta potential analysis. Dependence of the pH at the _iep_ on the Sb content, \(\mathbf{x}\), for Bi\({}_{2-x}\)Sb\({}_{x}\)Te\({}_{3}\) samples is presented in Table 1. For the pure Bi\({}_{2}\)Te\({}_{3}\) (\(\mathbf{x}=0\)), the _iep_ was reached at pH 6.3, which shifts step-wise down to 3.3 with the increasing content of Sb (\(\mathbf{x}>0\)). A survey of _iep_ for various oxides led to the values  
Figure 2: SEM micrographs at different magnifications of as-synthesized: (**a**,**e**) Bi\({}_{2}\)Te\({}_{3}\); (**b**,**f**) Bi\({}_{1.8}\)Sb\({}_{0.2}\)Te\({}_{3}\); (c,g) Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\); and (**d**,**h**) Sb\({}_{2}\)Te\({}_{3}\). of 2.9 for Sb\({}_{2}\)O\({}_{3}\) and 8.2 for Bi\({}_{2}\)O\({}_{3}\), while no data were found for TeO\({}_{2}\)[29]. We performed zeta potential analysis on commercial TeO\({}_{2}\) powder and obtained an _iep_ value of 5.2.