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Ever-growing demands for zinc–air batteries (ZABs) call for the development of advanced electrocatalysts. Single-atom catalysts (SACs), particularly those for isolating non-noble metals (NBMs), are attracting great interest due to their merits of low cost, high atom utilization efficiency, structural tunability, and extraordinary activity. Rational design of advanced NBM SACs relies heavily on an in-depth understanding of reaction mechanisms. To gain a better understanding of the reaction mechanisms of oxygen electrocatalysis in ZABs and guide the design and optimization of more efficient NBM SACs, we herein organize a comprehensive review by summarizing the fundamental concepts in the field of ZABs and the recent advances in the reported NBM SACs. Moreover, the selection of NBM elements and supports of SACs and some effective strategies for enhancing the electrochemical performance of ZABs are illustrated in detail. Finally, the challenges and future direction in this field of ZABs are also discussed.

Metal-linker bonds serve as the “glue” that binds metal ions to multitopic organic ligands in the porous materials known as metal–organic frameworks (MOFs). Despite ample evidence of bond lability in molecular and polymeric coordination compounds, the metal-linker bonds of MOFs were long assumed to be rigid and static. Given the importance of ligand fields in determining the behaviour of metal species, labile bonding in MOFs would help explain outstanding questions about MOF behaviour, while providing a design tool for controlling dynamic and stimuli-responsive optoelectronic, magnetic, catalytic, and mechanical phenomena. Here, we present emerging evidence that MOF metal-linker bonds exist in dynamic equilibria between weakly and tightly bond conformations, and that these equilibria respond to guest–host chemistry, drive phase change behavior, and exhibit size-dependence in MOF nanoparticles.

Strengthening the separation of photogenerated charge carriers is crucial for improving the efficiency of photocatalytic hydrogen evolution. Herein, t-Mn0.5Cd0.5S/Ov-WO3 (t-MCSW) triple-phase junctions with rich oxygen vacancies were developed using the calcination-hydrothermal method. The corresponding morphology and structure of the samples were examined by XRD, TEM and XPS. The formation of the S-scheme heterostructure in t-MCSW has also been confirmed with in situ XPS, work function analysis and free radical capture tests. The experimental results demonstrate that t-MCSW-7 exhibited optimal activity (194.2 mmol g−1 h−1), which was about 4 times higher than that of the individual Mn0.5Cd0.5S (t-MCS, 48.8 mmol g−1 h−1). The apparent quantum yield of t-MCSW-7 is 29.14% at 420 nm, and the material exhibits excellent stability after seven cycles of photocatalytic reaction. The excellent photocatalytic activity of t-MCSW-7 is attributed to more efficient separation of charge carriers by triple-phase junctions connected by homojunctions and heterojunctions. Moreover, the existence of oxygen vacancies broadens absorption spectra and accelerates surface charge transfer. The synergistic effect of phase junctions and oxygen vacancies leads to an enhancement of hydrogen evolution activity. This work provides a new idea for preparing efficient photocatalysts.

The magnetic and electronic structures of Fe4O5 have been investigated at ambient and high pressures via a combination of representation analysis, density functional theory (DFT+U) calculations, and Mössbauer spectroscopy. A few spin configurations corresponding to the different irreducible representations have been considered. The total-energy calculations reveal that the magnetic ground state of Fe4O5 corresponds to an orthogonal spin order. Depending on the magnetic propagation vector k, two spin-ordered phases with minimal energy differences are realized. The lowest energy magnetic phase is related to k = (0, 0, 0) and is characterized by ferromagnetic ordering of iron magnetic moments at prismatic sites along the b-axis and antiferromagnetic ordering of iron moments at octahedral sites along the c-axis. For the k = (1/2, 0, 0) phase, the moments in the prisms are antiferromagnetically ordered along the b-axis and the moments in the octahedra are still antiferromagnetically ordered along the c-axis. Under high pressure, Fe4O5 exhibits magnetic transitions with the corresponding electronic transitions of the metal–insulator type. At a critical pressure PC ∼ 60 GPa, the Fe ions at the octahedral sites undergo a high-spin to low-spin state crossover with a decrease in the unit-cell volume of ∼4%, while the Fe ions at the prismatic sites remain in the high-spin state up to 130 GPa. This site-dependent magnetic collapse is experimentally observed in the transformation of Mössbauer spectra measured at room temperature and high pressures.

This work is the first attempt to prepare Nd1−xCaxUxPO4 monazite–cheralite with 0 < x ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions (T = 110 °C for four days) of the Nd1−xCaxUxPO4·nH2O rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite–cheralite samples were obtained. However, α-UP2O7 was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite–cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of x = 0.08.

Benzene derivatives in wastewater have negative impacts on ecosystems and human health, making their removal prior to discharge imperative. In this study, Fe3O4@AC–NH2@Cu–opa (AC–NH2 = aminoclay, Cu–opa = [Cu(opa)(bipy)0.5(H2O)]n (H2opa = 3-(4-oxypyridinium-1-yl) phthalic acid)) nanoparticles (NPs) were synthesized as adsorbent and catalyst for phenolic compound removal from wastewater. Fe3O4@AC–NH2@Cu–opa NPs demonstrated outstanding performance in the adsorption of phenol, exhibiting a remarkable adsorption capacity of up to 166.39 mg g−1 according to the Langmuir model. The composite also exhibited higher Fenton activity toward the degradation of electron-rich organic phenolic pollutants, with a rate approximately 3.4 times higher than that of Fe3O4 alone. The high catalytic activity of the composite was attributed to the large surface area and abundant active sites of the 2D charge-separated Cu–MOF. Meanwhile, the superparamagnetism of the Fe3O4 core enabled magnetic recollection and reuse without any significant loss of activity. Therefore, use of Fe3O4@AC–NH2@Cu–opa/H2O2 shows potential in an efficient method for the removal of phenolic compounds from wastewater.

The construction of highly active and stable transition phosphide-based materials is widely regarded as an alternative approach to the use of Pt-based catalysts in the field of electrocatalytic hydrogen evolution. Herein, self-supported heterostructure Co–Ni phosphides (denoted as CoxNi1−x-P) were synthesized with different metal ratios by a low temperature electrodeposition strategy. Impressively, the optimized heterogeneous Co0.5Ni0.5-P nanocomposites displayed outstanding hydrogen evolution performance, with low overpotentials of 67 mV and 181 mV to deliver current densities of 10 mA cm−2 and 100 mA cm−2 in alkaline electrolyte. X-ray photoelectron spectroscopy revealed the optimized electronic structure of Co0.5Ni0.5-P, which led to an improvement in the conductivity. Density functional theory calculations demonstrated that the Co0.5Ni0.5-P heterostructure could provide a more optimal water-dissociation-related Volmer process for hydrogen evolution reaction (HER), in which water molecules could be easily activated on Co0.5Ni0.5-P with a low energy barrier. Moreover, the downshift of the d-band center confirmed the optimized H adsorption, further accelerating the HER kinetics.

The surface structure and composition of pre-catalysts play a critical role in the surface reconstruction process toward active species during the anodic oxygen evolution reaction (OER). Surface modified methods can accelerate the OER process of alloy ribbons, but the understanding of pre-catalysts and the structure/reactivity of the reconstruction (active) species is still insufficient. Herein, we report a two-step dealloyed Ni–Fe–P alloy ribbon as a highly efficient OER electrocatalyst. By adjusting the surface-derived component, we could regulate Ni/Fe hydroxide active species on the Ni–Fe–P alloy ribbon, enhancing the OER performance. The oxidation and release of P driven by dealloying plays a key role in constructing optimal β-NiOOH/FeOOH catalytic species on Ni–Fe–P. The optimal β-NiOOH/FeOOH active species enables Ni–Fe–P alloy to obtain a 104 mV of reduction in overpotential (at 10 mA cm−2) and a 78-fold increase in current density (at overpotential: 300 mV) compared to undealloyed Ni–Fe–P. Our work provides valuable insights into the relationship between the surface structure/composition of alloy bulk electrocatalysts and surface-reconstructed species and a rational design of a surface treatment process.

Based on the double activation models of epoxides, the design and synthesis of ionic porous organic polymers (iPOPs) is considered to be very attractive and promising but has remained a great challenge in recent decades owing to electrostatic interactions between charged groups. In this contribution, we developed a two-in-one strategy to fabricate metalloporphyrin-based iPOPs with unique nanostructures (named AlPor-QP@POP), which are composed of aluminum porphyrin units and three-dimensional quaternary phosphonium salts that work synergistically in the cycloaddition of CO2 with epoxides under mild conditions. The high symmetry of two monomers allows them to possess similar reactivity ratios and thus endows AlPor-QP@POP with densely located active sites, a large surface area and good CO2 capture capacity. More importantly, bifunctional AlPor-QP@POP has enormous potential to produce cyclic carbonates with simulated flue gas under ambient conditions. Moreover, AlPor-QP@POP can be readily recycled and efficiently reused more than ten times without an obvious decrease in catalytic activity. Finally, kinetic investigations and a comparative study have been conducted to understand the possible mechanism of CO2 catalytic cycloaddition.

Layered materials with a robust structure and reversible intercalation behavior are highly sought-after in applications involving energy conversion and storage systems, energy converting devices, supercapacitors, batteries, superconductors, photonic materials, and catalysis involving hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), solar cells and sensors. In the current study, quasi-2D rhombohedral Bi0.775Ln0.225O1.5 (Ln = La, Pr, Nd, Sm, and Eu) samples, synthesized by a solution combustion route, have been demonstrated to intercalate iodine reversibly. A solid–vapor reaction was employed to intercalate iodine at moderate temperatures, and deintercalation occurred on heating at higher temperatures. Expansion of the rhombohedral c-axis by ∼10 Å occurred, and the iodine between the interlayers existed as triiodide ions (I3−) in an unsymmetrical fashion. The amount of intercalated iodide has been determined from thermogravimetric analysis. Electron microscopic analysis confirmed these systems’ intercalation and subsequent lattice expansion. In the diffuse reflectance spectra, charge transfer from the triiodide ions to the host oxide was noticed, and it caused the absorption edge to fall beyond the visible region for the intercalated samples. XPS analysis of iodine intercalated Bi0.775Pr0.225O1.5 has shown the mixed valence states for Pr and the existence of I3− along with some IO3− species. The quasi-2D structure was stable during the thermal deintercalation process. The evaluation of iodine intercalated Bi0.775Ln0.225O1.5 (Ln = La, Pr, Nd, Sm, and Eu) samples as anode material in the lithium-ion battery system has given quite promising results, exhibiting fast Li+-ion diffusion, low charge transfer resistance, good reversible capacity, capacity retention (after cycling back to 10 mA g−1), and structural stability (after long cycles).

Electrochemical water splitting offers the potential for environmentally friendly hydrogen and oxygen gas generation. Here, we present the synthesis, characterization, and electrochemical analyses of four organic polymers where metalloporphyrins are the active center nodes. These materials were obtained from the polymerization reaction of poly(p-phenylene terephtalamide) (PPTA) with the respective amino-functionalized metalloporphyrins, where M = Fe, 1; Co, 2; Ni, 3; Cu, 4. Scanning and transmission electron microscopy images (SEM and TEM) show that these polymers exhibit a layer-type morphology, which is attributed to hydrogen bonding and π–π stacking between the metalloporphyrin nodes. The synthesized materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), UV-Vis spectroscopy, and Fourier-transform infrared spectroscopy (FT-IR). Among the materials studied, the cobalt-based polymer, 2, demonstrates a bifunctional electrocatalytic activity for oxygen (OER) and hydrogen (HER) evolution reactions with overpotentials (η10) of 337 mV and 435 mV, respectively. The Fe, 1, and Ni, 2, polymers are less active for HER with maximum current densities (jmax) of 12.6 and 19.1 mA cm−2 and η10 678 mV, 644 mV. Polymer 2 achieves a jmax of 37.7 mA cm−2 for HER and 133 mA cm−2 for OER. The copper-based material, 4, on the other hand, shows selectivity towards HER with an overpotential (η) of 436 mV and a maximum current density (j) of 45.5 mA cm−2. The bifunctional electrocatalytic performance was tested in the overall water-splitting setup, where polymer 2 requires a cell voltage of 1.64 V at 10 mA cm−2. This work presents a novel approach to heterogenized molecular systems, providing materials with exceptional structural characteristics and enhanced electrocatalytic capabilities.

Three new group IIIA metal phosphate–oxalate (MPO) compounds, namely [(CH3)2NH2]2[M2(HPO4)2(H2PO4)2(C2O4)] (M = Al (1), Ga (2)) and [(CH3)2NH2]2[In2(HPO4)2(H2PO4)2(C2O4)]·H2O (3), have been synthesized. Their crystal structures feature an anionic layer with the sql topology net. In particular, 1 displays a proton conductivity (σ) of 9.09 × 10−3 S cm−1 at 85 °C and under 98% relative humidity, which is the highest among MPOs. This study not only endows the main group metal-based MPO family with new members, but also contributes to further understanding of the structure-directing roles of amines and provides a feasible idea for improving the proton conductivity of MPOs.

Sulfur functionalized biocarbons were prepared from naturally abundant lignin alkali with sodium thiocyanate as an activation agent and a sulfur source. The resultant biocarbon sorbents showed a high mercury isolation ability from aqueous solutions, where high surface area and doping of sulfur significantly aid the uptake of mercury, i.e., 0.05 g of biocarbon sorbent removed 99% of mercury from 250 mL of simulated wastewater with an initial concentration of mercury of 10 mg L−1.

Zinc metal is widely considered as the primary option for constructing various aqueous batteries due to its cost-effectiveness, safety, and environmental friendliness. However, the Zn anode continues to be plagued by parasitic reactions and dendrite growth in aqueous electrolytes, limiting the practical implementation of zinc ion batteries (ZIBs) for large-scale energy storage. Herein, a foveolate ZnTe nanoarray is developed as a protective layer to enhance the chemical reversibility during Zn plating/stripping. The semi-conductive ZnTe with excellent ionic conductivity and hydrophobicity can effectually prevent the corrosion reactions, hydrogen generation and dendritic growth on the surface of the Zn anode. As a result, the Zn@ZnTe symmetrical cells achieve ultrahigh cycling stability (over 2800 h at 2 mA cm−2 and 1 mA h cm−2) and simultaneously deliver a low voltage hysteresis of 28 mV. Additionally, the durable Zn@ZnTe//V2O5 cells exhibit a remarkable capacity retention of 96.7% after 3000 cycles, surpassing that of the Zn//V2O5 cells. This work provides a straightforward and low-cost strategy to regulate the interface chemistry of the Zn anode, which may open a way for the development of practical ZIBs.

Optical thermometry is a non-contact temperature sensing technique with widespread applications. It offers precise measurements without physical contact, making it ideal for situations where contact-based methods are impractical. However, improving the accuracy of optical thermometry remains an ongoing challenge. Herein, enhancing the thermometric properties of luminescent thermometers through novel materials or strategies is crucial for developing more precise sensors. Hence, the present study focuses on the application of four-mode luminescence thermometric techniques in sol–gel synthesized Er3+/Yb3+ co-doped Ba2GdV3O11 phosphors for optical temperature sensing in the temperature range of 298–573 K. The upconversion (UC) luminescence is achieved under excitations of 980 nm or 1550 nm, resulting in bright yellow-green emission in the visible spectral range. Temperature sensing is realized by exploiting the UC emissions of 4S3/2, 2H11/2 and 4F7/2 bands, which represent intensity ratios of thermally coupled levels (TCELs) and non-thermally coupled levels (NTCELs) of Er3+/Yb3+, along with the emission lifetimes at 4S3/2. The relative sensitivity (Sr) values for TCELs exhibit a gradual decrease with rising temperature, reaching a maximum of 1.1% K−1 for 980 nm excitation and 0.86% K−1 for 1550 nm excitation at 298 K. Conversely, for NTCELs, the highest Sr value observed is 0.9% K−1 at 298 K for 1550 nm excitation. Moreover, the emission lifetimes at 4S3/2 yield notably high Sr values of up to 5.0% μs K−1 (at 425 K). Furthermore, the studied phosphors have a sub-degree thermal resolution, making them excellent materials for accurate temperature sensing. Overall, this study provides a promising new direction for the development of more precise and reliable optical thermometry techniques, which could have important implications for a range of scientific and industrial optical temperature sensing applications.

The inhibitory action of Schiff base complexes of 3d metals against the urease enzyme is well explored in the scientific community. However, the ability of such complexes in mimicking active metallobiosites of urease enzymes, possessing ureolytic behavior, still remains unexplored. With this aim firstly, two Zn(II)-complexes (PPR-HMB-Zn and PZ-HMB-Zn) have been developed from two different Schiff base ligands (HL1 = 2-((E)-(2-(piperidin-1-yl)ethylimino)methyl)-5-methylphenol and HL2 = 2-((E)-(2-(piperizin-1-yl)ethylimino)methyl)-5-methylphenol) and structurally characterized using single crystal XRD. The hydrolytic enzymatic activity of both complexes was demonstrated by the gradual increase in the absorption maxima at 425 nm for the formation of the p-nitrophenolate ion from catalytic hydrolysis mediated by the Zn(II) complexes with a disodium salt of p-nitrophenyl phosphate as a model substrate. Associated kinetic parameters, pH dependency and a relevant hydrolysis mechanism have also been explored. After confirming the hydrolytic ability, the complexes were exploited to mimic the hydrolytic activity of Jack bean urease that catalytically hydrolyses urea into ammonia and CO2. The change in the pH of the solution owing to the formation of ammonia under the complex catalysed hydrolytic action of urea has been monitored spectrophotometrically using the pH dependent structural change of phenol red. The amount of ammonia has been quantified using the Nessler's reagent spectrophotometric method. The ureolytic reaction mechanism has been investigated using density functional theory (DFT) calculations using the B3LYP and TPSSH methods for the systematic calculation of the interaction energy. In contrast to PZ-HMB-Zn, PPR-HMB-Zn functions more effectively as a catalyst due to the existence of a lattice-occluded water molecule in its crystal structure and the protonation of the non-terminal N to attract urea by H-bonding, which was further confirmed by AIM analysis.

A comprehensive density functional theory investigation has been presented towards the comparison of the C–H activation reactivity between high-valent iron-oxo and ruthenium-oxo complexes. A total of four compounds, e.g., [Ru(IV)O(tpy-dcbpy)] (1), [Fe(IV)O(tpy-dcbpy)] (1′), [Ru(IV)O(TMCS)] (2), and [Fe(IV)O(TMCS)] (2′), have been considered for this investigation. The macrocyclic ligand framework tpy(dcbpy) implies tpy = 2,2′:6′,2′′-terpyridine, dcbpy = 5,5′-dicarboxy-2,2′-bipyridine, and TMCS is TMC with an axially tethered –SCH2CH2 group. Compounds 1 and 2′ are experimentally synthesized standard complexes with Ru and Fe, whereas compounds 1′ and 2 were considered to keep the macrocycle intact when switching the central metal atom. Three reactants including benzyl alcohol, ethyl benzene, and dihydroanthracene were selected as substrates for C–H activation. It is noteworthy to mention that Fe(IV)O complexes exhibit higher reactivity than those of their Ru(IV)O counterparts. Furthermore, regardless of the central metal, the complex featuring a tpy-dcbpy macrocycle demonstrates higher reactivity than that of TMCS. Here, a thorough analysis of the reactivity-controlling characteristics—such as spin state, steric factor, distortion energy, energy of the electron acceptor orbital, and quantum mechanical tunneling—was conducted. Fe(IV)O exhibits the exchanged enhanced two-state-reactivity with the quintet reactive state, whereas Ru(IV)O has only a triplet reactive state. Both the distortion energy and acceptor orbital energy are low in the case of Fe(IV)O supporting its higher reactivity. All the investigated C–H activation processes involve a significant contribution from hydrogen tunneling, which is more pronounced in the case of Ru, although it cannot alter the reactivity pattern. Furthermore, it has also been found that, independent of the central metal, aliphatic hydroxylation is always preferable to aromatic hydroxylation. Overall, this work is successful in establishing and investigating the cause of enzymes’ natural preference for Fe over Ru as a cofactor for C–H activation enzymes.

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