First-Row Transition Metal Doping in Calcium Phosphate Bioceramics: A Detailed Crystallographic Study

Doped calcium phosphate bioceramics are promising materials for bone repair surgery because of their chemical resemblance to the mineral constituent of bone. Among these materials, BCP samples composed of hydroxyapatite (Ca10(PO₄)₆(OH)₂) and β-TCP (Ca₃(PO₄)₂) present a mineral analogy with the nano-multi-substituted hydroxyapatite bio-mineral part of bones. At the same time, doping can be used to tune the biological properties of these ceramics. This paper presents a general overview of the doping mechanisms of BCP samples using cations from the first-row transition metals (from manganese to zinc), with respect to the applied sintering temperature. The results enable the preparation of doped synthetic BCP that can be used to tailor biological properties, in particular by tuning the release amounts upon interaction with biological fluids. Intermediate sintering temperatures stabilize the doping elements in the more soluble β-TCP phase, which favors quick and easy release upon integration in the biological environment, whereas higher sintering temperatures locate the doping elements in the weakly soluble HAp phase, enabling a slow and continuous supply of the bio-inspired properties. An interstitial doping mechanism in the HAp hexagonal channel is observed for the six investigated cations (Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+) with specific characteristics involving a shift away from the center of the hexagonal channel (Fe3+, Co2+), cationic oxidation (Mn3+, Co3+), and also cationic reduction (Cu⁺). The complete crystallochemical study highlights a complex HAp doping mechanism, mainly realized by an interstitial process combined with calcium substitution for the larger cations of the series leading to potentially calcium deficient HAp.

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation

Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y₂Ru₂O₇ alternative through partial A-site substitution of Sr²⁺ for Y³⁺, efficiently improving its intrinsic OER activity. The optimized Y_1.7Sr_0.3Ru₂O₇ candidate observes a highly intrinsic mass activity of 1018 A g_Ru^-1 at an overpotential of 300 mV with excellent durability in 0.5 M H₂SO₄ electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr²⁺ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

Strontium-Doped Whitlockite Scaffolds for Enhanced Bone Regeneration

Bone graft substitutes to repair critical-sized bone fractures have experienced significant development over the last few decades. Among them, whitlockite (WH)-based bone grafts have proven to be effective in mediating bone healing. In the current study, a next generation, nature-inspired scaffold was developed with strontium-functionalized whitlockite nanoparticles (nSrWH) to enhance the intrinsic properties of WH. A series of nSrWH (with 2.5, 5, 7.5% Sr atomic substitution) were fabricated using a rapid-mixing wet precipitation route. Subsequently, the functionalized whitlockite was integrated into a gelatin-chondroitin sulfate scaffold and subjected to both in vitro and in vivo studies to investigate its osteogenic potential. Results indicated that nSrWH-containing scaffolds promoted osteogenic differentiation while inhibiting osteoclast activity. The positive impact of nSrWH was found to be dose-dependent, with the 7.5% Sr atomic substitution exhibiting the most significant results. Furthermore, the scaffold induced superior de novo bone regeneration compared to its undoped counterpart in the mouse calvarial critical-sized defect model. Collectively, these findings suggest that nSrWH nanoparticles inherit the beneficial properties of whitlockite, coupled by the therapeutic effects of Sr2+, operating in concert for an overall enhanced bone regeneration. As such, they constitute promising candidates to meet the biomedical requirements for bioactive bone graft substitutes.

Post-Substitution Modulated Robust Sodium Layered Oxides

Sodium layered oxides feature in high capacity and diverse composition, however, are plagued by various issues including limited kinetics and interfacial instability with residual alkali. Conventional substitution/doping and heterogeneous coating are promising to tackle the problems of bulk and surface, respectively, but normally insufficient to address both. Herein, a post-substitution strategy is proposed to modify primary sodium-layered-oxide particles that can simultaneously deal with bulk and surficial issues. As a typical example, post Ti-substitution for O3-NaNi1/3 Fe1/3 Mn1/3 O2 is successfully performed by adjusting thermodynamic driving force, resulting in depth-controllable Ti infusion from surface to bulk, as proved by energy dispersive spectroscopy maps collected at the cross-section. Residual alkali species are efficiently diminished and benefited from the surface-to-bulk osmotic reaction, significantly improving Coulombic efficiency. Moreover, remarkable enhancements in reversible capacity (135 mAh g-1 at C/10), rate capability (74% retention at 5 C), and long-term cycling stability (80% retention after 300 cycles at 2 C) are achieved by manipulating gradient-like Ti distribution in a primary particle that brings with increased kinetics and strengthened interfacial stability, surpassing those given by rough heterotic coating and homogeneous Ti-substitution. Such post-substitution is expected to provide a universal strategy to modify primary layered-oxide particles for developing advanced cathode materials of SIBs.

Li-Site Defects Induce Formation of Li-Rich Impurity Phases: Implications for Charge Distribution and Performance of LiNi0.5- xMxMn1.5O4 Cathodes (M = Fe and Mg; x = 0.05-0.2)

An understanding of the structural properties that allow for optimal cathode performance, and their origin, is necessary for devising advanced cathode design strategies and accelerating the commercialization of next-generation cathodes. High-voltage, Fe- and Mg-substituted LiNi0.5Mn1.5O4 cathodes offer a low-cost, cobalt-free, yet energy-dense alternative to commercial cathodes. In this work, the effect of substitution on several important structure properties is explored, including Ni/Mn ordering, charge distribution, and extrinsic defects. In the cation-disordered samples studied, a correlation is observed between increased Fe/Mg substitution, Li-site defects, and Li-rich impurity phase formation-the concentrations of which are greater for Mg-substituted samples. This is attributed to the lower formation energy of MgLi defects when compared to FeLi defects. Li-site defect-induced impurity phases consequently alter the charge distribution of the system, resulting in increased [Mn3+] with Fe/Mg substitution. In addition to impurity phases, other charge compensators are also investigated to explain the origin of Mn3+ (extrinsic defects, [Ni3+], oxygen vacancies and intrinsic off-stoichiometry), although their effects are found to be negligible.