We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic–inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX2 (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A = Cs+, CH3NH3+, or CH(NH2)2+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Effective adsorption is of great importance to the photocatalytic degradation of volatile organic compounds. Herein, we succeeded in the preparation of anatase TiO2 with clean dominant {001} and {101} facets. By using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) equipped with a homemade reaction system and a coupling gas-dosing system, we found that TiO2 with dominant {001} facets exhibits higher toluene adsorption capacity than TiO2 with dominant {101} facets, which may be attributed to the different number of unsaturated 5c-Ti capable of forming the main active adsorption sites (terminal Ti-OH species). TiO2 with dominant {001} facets shows a significantly high photocatalytic degradation performance, with its degradation rate being 6 times higher than that of dominant {101} facets. Combined with simulation results, it is suggested that the synergetic effects of the formation of specific active adsorption sites, the low adsorption energy for toluene, and preservation of the free molecularly adsorbed water on the surface promote the degradation of gaseous toluene on the dominant {001} facets. This study exemplifies that the facet-dependent adsorption of volatile organic compounds is one of the most important factors to effectively engineer photocatalysts for air purification.

The electrocatalytic oxygen evolution reaction (OER) is an important half reaction in various technologies, such as metal air batteries and electrochemical water splitting. Transition metal chalcogenides, especially those based on Co and Ni, are emerging as promising OER catalysts, thanks to their high activity and low cost. However, it is still being debated whether they act as actual catalysts or as catalyst precursors, undergoing structural and morphological changes under OER conditions. To gain a better comprehension of this topic, we have developed a simple colloidal synthesis method for alloy nanocrystals (NCs) based on Ni, Co, S, and Se with a tunable composition, and we studied their structural and morphological evolution during OER. We found that binary CoSe, ternary Ni–Co–Se and quaternary Ni–Co–S–Se NCs, with the exception of NiSe, each undergo structural and morphological changes under OER conditions, forming the corresponding metal oxides/hydroxides, which act as the actual active catalysts. Interestingly, we discovered that the composition of the starting metal chalcogenide NCs plays a major role in dictating the crystallinity, conductivity and activity of such oxide/hydroxide materials. This compositional tuning, that is going from CoSe to Ni0.25Co0.65S0.4Se0.6 NCs, resulted in a ∼7 fold improvement in the OER activity in terms of turnover frequency. Our results suggest that the compositional engineering of metal chalcogenide materials could potentially be used to control their inevitable transformation into the corresponding oxide/hydroxide counterparts, eventually improving their OER activity.

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