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To treat impairments in hard tissues or overcome pathological calcification in soft tissues, a detailed understanding of mineralization pathways of calcium phosphate materials is needed. Here, we report a detailed mechanistic study of hydroxyapatite (HA) mineralization pathways in an artificial saliva solution via in situ liquid cell transmission electron microscopy (TEM). It is found that the mineralization of HA starts by forming ion-rich and ion-poor solutions in the saliva solution, followed by coexistence of the classical and nonclassical nucleation processes. For the nonclassical path, amorphous calcium phosphate (ACP) functions as the substrate for HA nucleation on the ACP surface, while the classical path features direct HA nucleation from the solution. The growth of HA crystals on the surface of ACP is accompanied by the ACP dissolution process. The discoveries reported in this work are important to understand the physiological and pathological formation of HA minerals, as well as to engineer the biomineralization process for bone healing and hard tissue repairs. 

Abstract Direct formation of ultra-small nanoparticles on carbon supports by rapid high temperature synthesis method offers new opportunities for scalable nanomanufacturing and the synthesis of stable multi-elemental nanoparticles. However, the underlying mechanisms affecting the dispersion and stability of nanoparticles on the supports during high temperature processing remain enigmatic. In this work, we report the observation of metallic nanoparticles formation and stabilization on carbon supports through in situ Joule heating method. We find that the formation of metallic nanoparticles is associated with the simultaneous phase transition of amorphous carbon to a highly defective turbostratic graphite (T-graphite). Molecular dynamic (MD) simulations suggest that the defective T-graphite provide numerous nucleation sites for the nanoparticles to form. Furthermore, the nanoparticles partially intercalate and take root on edge planes, leading to high binding energy on support. This interaction between nanoparticles and T-graphite substrate strengthens the anchoring and provides excellent thermal stability to the nanoparticles. These findings provide mechanistic understanding of rapid high temperature synthesis of metal nanoparticles on carbon supports and the origin of their stability. 

Supported bimetallic alloy nanoparticles are of great interest in various catalytic applications due to the synergistic effects between different metals for improved catalytic performance. However, it still remains a challenge to efficiently synthesize atomically mixed alloy nanoparticles with uniform dispersion onto a desired substrate. Here, in situ, rapid synthesis of atomically mixed bimetallic nanoparticles well‐dispersed on a conductive carbon network via a 1 s high‐temperature pulse (HTP, ≈1550 K, duration 1 s, the rate of 10⁴K s⁻¹) is reported. The high temperature facilitates the total (atomic) mixing of different metals, while the rapid quenching ensures the uniform dispersion of nanoparticles with fine features such as twin boundaries and stacking faults, which are potentially beneficial to their catalytic performance. By varying the ratio of the precursor salts and parameters in the HTP process, the composition, size, and morphology of the resultant nanoparticles can easily be tuned. Moreover, the synthesized bimetallic (PdNi) nanoparticles demonstrate excellent electrocatalytic performance for the hydrogen evolution reaction and hydrogen peroxide electrooxidation. This work provides a general strategy for a facile and rapid synthesis of bimetallic nanoparticles directly from their salts for a range of emerging applications.

 

For the first time, a fast heating–cooling process is reported for the synthesis of carbon‐coated nickel (Ni) nanoparticles on a reduced graphene oxide (RGO) matrix (nano‐Ni@C/RGO) as a high‐performance H₂O₂fuel catalyst. The Joule heating temperature can reach up to ≈2400 K and the heating time can be less than 0.1 s. Ni microparticles with an average diameter of 2 µm can be directly converted into nanoparticles with an average diameter of 75 nm. The Ni nanoparticles embedded in RGO are evaluated for electro‐oxidation performance as a H₂O₂fuel in a direct peroxide–peroxide fuel cell, which exhibits an electro‐oxidation current density of 602 mA cm⁻²at 0.2 V (vs Ag/AgCl), ≈150 times higher than the original Ni microparticles embedded in the RGO matrix (micro‐Ni/RGO). The high‐temperature, fast Joule heating process also leads to a 4–5 nm conformal carbon coating on the surface of the Ni nanoparticles, which anchors them to the RGO nanosheets and leads to an excellent catalytic stability. The newly developed nano‐Ni@C/RGO composites by Joule heating hold great promise for a range of emerging energy applications, including the advanced anode materials of fuel cells.