So far, we noticed the absence of light soaking and photo-shunt as potential advantages of n-type SnO2 NPs over ZnO-based ETMs. Next, we extended our investigations to the ATO NPs and clarified the nature of the polarity and the doping mechanism. We selected Sb3+for two reasons: firstly, Sb3+is expected to be a p-type dopant as compared to Sb5+; second, Sb3+(0.76Å) has an ion radius that is comparable to that of Sn4+(0.69Å), potentially facilitating substitutional doping rather than interstitial doping. Nevertheless, the nature of the doping mechanisms is usually more complex in oxides and especially in metal oxide NPs. The schematic diagram of the proposed doping mechanism is illustrated in Fig. 2A, i.e., the Sn4+sites in the host matrices are supposed to be partially substituted by the lower valence Sb3+cations, inducing shallow acceptor levels close to the
valence band. The Sb dopants introduced during spray pyrolysis are easily ionized [51–53]. Even in the case of oxide formation, Sb2O3 and Sb2O5, antimony would retain either the Sb3+ or Sb5+ state by sharing their 5s and 5p electrons. In the first case, one Sb3+cation could generate an additional shallow energy state in the SnO2 lattice; hence, the successful substitutional doping process can be described by the following equation, where the symbols ↑ ↓ denote the replacement of Sn4+cation by Sb3+cation in the lattice sites; Vis the shallow acceptor level; h+is the positively charged “free hole carrier.” To validate the feasibility of our concept, we first used a low Sb content (0.5 mol%) to dope SnO2 NPs during the synthesis of NPs. As depicted in Fig. 2B, the absorption edge of 0.5% ATO film shows a reduced absorption in the visible region compared to that of the pristine SnO2 film. X-ray powder diffraction (XRD) measurements were performed to characterize and compare the formation of pristine SnO2 and 0.5% ATO NPs. As described in Fig. 2C, the main diffraction peaks of 0.5% ATO NPs, oriented along the (110), (101), (200), and (211), are all well assigned to tetragonal rutile SnO2 (ICSD card: 154960), indicating the same rutile lattice structure. If you are looking for high quality, high purity, and cost-effective ATO, or if you require the latest price of ATO, please feel free to email contact mis-asia.