Search

Lead halide perovskites (LHPs) have emerged as promising candidates for a broad range of optoelectronic devices because of their unique physical properties. Methylammonium lead iodide (MAPI) perovskite has been the most commonly studied LHP due to its very promising optoelectronic properties. One of the main explored pathways for obtaining MAPI perovskites is the synthesis by ligand-assisted reprecipitation (LARP); however, this method is not totally understood from a phenomenological point of view. In this study, we took advantage of the development of a series of advanced in situtechniques to bring new insights into the pathway of this process that leads to MAPI perovskites from a precursor solution. First, we monitored the nucleation and growth processes of a solvated intermediate phase obtained viaLARP by correlating local information obtained by a direct visualization of the reaction medium using liquid-phase transmission electron microscopy (TEM) and more global information brought by synchrotron-based X-ray scattering measurements. This combined analysis, in real time and under representative conditions of the synthesis method, allowed us to decipher the structural evolution of the emerging phases, from amorphous and roughly spherical objects in the early stages of the process toward elongated ribbon-like morphology particles after a few seconds, through a crystallization process. Second, we followed in situthe transition of the as-obtained solvated intermediate phase toward the final phase of MAPI perovskite during a thermal treatment at 80 °C, using in this case a combination of gas-phase TEM, in image and diffraction modes, and temperature-resolved X-ray diffraction. We provided direct and unprecedented evidence of the fragmentation of the crystals, simultaneously to the occurrence of the structural transformation between the intermediate and final phases. Our approach, which involved also challenging development of new correlative methods, demonstrates the high interest of such an in situcorrelative study for a better understanding of the synthesis and properties of new emerging materials including LHPs.

The photocatalytic performance of Nb,N-codoped TiO2nanoparticles obtained via the sol–gel method was compared to that of N-doped TiO2. The study focused on investigating the effects of nitridation conditions on nitrogen insertion with a highlight on the nature of the doping sites in the photocatalyst depending on the initial presence of niobium in the TiO2. The photodegradation of methylene blue in solution under UV, visible, and simulated solar light was used to evaluate the photocatalytic activity of TiO2, Nb- or N-doped TiO2, and Nb,N-codoped TiO2nanoparticles. Codoped TiO2produced by mild thermal nitridation exhibits the best photocatalytic activity, with a strong contribution from visible light. On the contrary, the codoped TiO2produced by more intense thermal nitridation presents lower photocatalytic performances than TiO2despite a small improvement of activity in the visible range. In addition to material characterization (X-ray diffraction, UV–vis spectroscopy, and X-ray photoelectron spectroscopy), electron paramagnetic resonance and reversed double-beam photoacoustic spectroscopy measurements were used to identify the respective doping sites and ultimately propose the electronic band structure for each sample of Nb:TiO2, N:TiO2, and Nb,N:TiO2. Proper thermal nitridation conditions improve the charge compensation between Nb5+and N3–, thereby enhancing the photocatalytic activity. However, too intense nitridation conditions led to the generation of oxygen vacancies and a large amount of Ti3+acting as charge recombination centers, resulting in significant deterioration of the photocatalytic performances. This study highlights the importance of understanding the intricate charge compensation process in codoped (M,N) TiO2materials, as the photocatalytic performance cannot be elucidated solely by the cation/anion ratio but also by considering the nature of the doping sites generated during synthesis.