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Electrocatalytic water splitting presents an exciting opportunity to produce environmentally benign hydrogen fuel to power human activities. Earth abundant Ni5P4 has emerged as an efficient catalyst for the hydrogen evolution reaction (HER) and its activity can be enhanced by admixing synergistic metals to modify the surface affinity and consequently kinetics of HER. Computational studies suggest that the HER activity of Ni5P4 can be improved by Zn doping, causing a chemical pressure-like effect on Ni3 hollow sites. Herein, we report a facile colloidal route to produce Ni5-xZnxP4 nanocrystals (NCs) with control over structure, morphology, and composition and investigate their composition-dependent HER activity in alkaline solutions. Ni5-xZnxP4 NCs retain the hexagonal structure and solid spherical morphology of binary Ni5P4 NCs with a notable size increase from 9.2-28.5 nm for x = 0.00-1.27 compositions. Elemental maps affirm the homogeneous ternary alloy formation with no evidence of Zn segregation. Surface analysis of Ni5-xZnxP4 NCs indicates significant modulation of the surface polarization upon Zn incorporation resulting in a decrease in Niδ+ and an increase in Pδ- charge. Although all compositions followed a Volmer-Heyrovsky HER mechanism, the modulated surface polarization enhances the reaction kinetics producing lower Tafel slopes for Ni5-xZnxP4 NCs (82.5-101.9 mV/dec for x = 0.10-0.84) compared to binary Ni5P4 NCs (109.9 mV/dec). Ni5-xZnxP4 NCs showed higher HER activity with overpotentials of 131.6-193.8 mV for x = 0.02-0.84 in comparison to Ni5P4 NCs (218.1 mV) at a current density of -10 mA/cm2. Alloying with Zn increases the material’s stability with only a ~10% increase in overpotential for Ni4.49Zn0.51P4 NCs at -50 mA/cm2, whereas a ~33% increase was observed for Ni5P4 NCs. At current densities above -40 mA/cm2, bimetallic NCs with x = 0.10, 0.29, and 0.51 compositions outperformed the benchmark Pt/C catalyst, suggesting that hexagonal alloyed Ni5-xZnxP4 NCs are excellent candidates for practical applications that necessitate lower HER overpotentials at higher current densities. 

Metal–semiconductor hybrid nanomaterials (HNMs) exhibit unique properties that are distinct from individual nanostructures, leading to promising applications in optical technologies. The interfacial linkage of semiconductor and metal nanoparticles (NPs) via cogelation is an effective strategy to produce HNMs that show strong plasmon‐exciton coupling and improved physical properties. However, optical properties of these hybrids show little to no tunability. Herein, CdSe/Ag hybrid aerogels that show tunable absorption and photoluminescence (PL) are produced by cogelation of CdSe nanorods (NRs) or NPs with Ag hollow NPs. Hybrid electronic states are created by overlapping the excitonic absorption of CdSe NRs or NPs with the plasmonic absorption of Ag NPs. Physical characterization of the hybrids reveals an interconnected network of hexagonal CdSe and cubic Ag NPs, linked by Ag⁺and Se²⁻surface species, without intervening ligands. PL spectra exhibit maxima at 640 and 720 nm for the CdSe NPs/Ag and CdSe NRs/Ag hybrids, respectively, corresponding to new radiative decay mechanisms. Time‐resolved PL data support the emergence of new radiative pathways, kinetically and energetically distinct from the excitonic and plasmonic properties of primary NPs. This new approach of metal–semiconductor hybrid formation through cogelation is intriguing for the design of high‐efficiency HNMs without detrimental PL quenching.