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Constructing a direct Z-scheme system for photocatalysts is a promising strategy to enhance photocatalytic redox performance owing to its effective charge transfer; however, building a reasonable interfacial charge transfer bridge for charge selective recombination is still a challenge. Herein, a F-doped TiO2 quantum dot/g-C3N4 nanosheet Z-scheme photocatalyst is synthesized via a facile method of fluorine-containing solvothermal treatment. In this Z-scheme structure, F-TiO2 quantum dots (QDs, 3–7 nm) are bridged with g-C3N4 nanosheets through C–O covalent bonds. Thus, the oxidizability and reducibility of the composite photocatalyst and the interfacial charge transfer ability are greatly improved. Meanwhile, the TiO2 quantum dots provide more active sites and enhance the light harvesting, which are both beneficial for the photocatalytic reaction process. In this designed catalyst, the C–O covalent bond, as a chemical group bridge, provides an effective pathway for charge transfer and selective recombination, which greatly enhances the utilization of photogenerated electrons and holes. As a result, the FT/CNS-1-2 photocatalyst shows the highest photocatalytic activity for photocatalytic rhodamine B (RhB) decomposition, Cr(VI) reduction and phenol decomposition under visible light irradiation, which are about 17.1/4, 16/5.3 and 23.5/4.4 times higher than those of pure F-TiO2/g-C3N4, respectively. The excellent photocatalytic activity is mainly ascribed to the Z-scheme charge transfer model, which greatly enhanced the separation and transfer of charge carriers. In addition, the trapping experiments found that ˙O2− and ˙OH are the dominant active species in the photocatalytic process. This study of the chemically bonded Z-scheme F-TiO2/g-C3N4 photocatalyst provides ideas for the fabrication of such TiO2 QD based materials and a Z-scheme system for effective change transfer in photocatalysis processes.

Hard carbons have shown considerable promise as anodes for emerging sodium-ion battery technologies. Current understanding of sodium-storage behaviour in hard carbons attributes capacity to filling of graphitic interlayers and pores, and adsorption at defects, although there is still considerable debate regarding the voltages at which these mechanisms occur. Here, ex situ23Na solid-state NMR and total scattering studies on a systematically tuned series of hard carbons revealed the formation of increasingly metallic sodium clusters in direct correlation to the growing pore size, occurring only in samples which exhibited a low voltage plateau. Combining experimental results with DFT calculations, we propose a revised mechanistic model in which sodium ions store first simultaneously and continuously at defects, within interlayers and on pore surfaces. Once these higher energy binding sites are filled, pore filling occurs during the plateau region, where the densely confined sodium takes on a greater degree of metallicity.

Sodium ion batteries are a promising alternative to current lithium ion battery technology, providing relatively high capacity and good cycling stability at low cost. Hard carbons are today the anodes of choice but they suffer from poor rate performance and low initial coulombic efficiency. To improve the understanding of the kinetics of sodium mobility in these materials, muon spin rotation spectroscopy and density functional theory calculations were used to probe the intrinsic diffusion of sodium in a characteristic hard carbon sample. This revealed that atomic diffusion between sites is comparable to that observed in transition metal oxide cathode materials in sodium ion batteries, suggesting that the poor rate performance is not limited by site–site jump diffusion rates. In addition, diffusion was observed in the sodium that is irreversibly stored during the first cycle, suggesting that some of these sodium atoms are not immobilised in the solid electrolyte interface (SEI) layer but are still blocked from long range diffusion, thereby rendering the sodium electrochemically inactive.

In this paper, a computational study of Li, Na, and K adsorption and migration on pristine and defective graphene surfaces is conducted to gain insight into the metal storage and mobility in carbon-based anodes for alkali metal batteries. Atomic level studies of the metal adsorption and migration on the graphene surface can help address the challenges faced in the development of novel alkali metal battery technologies, as these systems act as convenient proxies of the crystalline carbon surface in carbon-based materials including graphite, hard carbons and graphene. The adsorption of Li and K ions on the pristine graphene surface is shown to be more energetically favourable than Na adsorption. A collection of defects expected to be found in carbonaceous materials are investigated in terms of metal storage and mobility, with N- and O-containing defects found to be the dominant defects on these carbon surfaces. Metal adsorption and migration at the defect sites show that defect sites tend to act as metal trapping sites, and metal diffusion around the defects is hindered when compared to the pristine surface. We identify a defect where two C sites are substituted with O and one C site with N as the dominant surface defect, and find that this defect is detrimental to metal migration and hence the battery cycling performance.

In this article fully atomistic Molecular Dynamics simulations were employed to study the behaviour of electrolyte salts (NaPF6, NaBF4, and NaTFSI) and different organic solvents (PC, EC, and EMC) in cylindrical carbon nanotubes, in order to reveal the storage mechanism. Organic solutions at 1 M concentrations were considered in bulk reservoir solutions, at the operational condition of sodium ion batteries. The effects of the solvents, nanotube diameter, and different anions (PF6 -, BF4-, and TFSI-) are quantified by calculating the number of ions inside the nanotubes, solvation number and radial distribution functions. The solvent, anion and cylindrical nanoconfinement can influence the organic electrolyte solution structure.

Correction for ‘A revised mechanistic model for sodium insertion in hard carbons’ by Heather Au et al., Energy Environ. Sci., 2020, 13, 3469–3479, DOI: 10.1039/D0EE01363C.

The effect of the pore wall model on the self-diffusion coefficient and transport diffusivity predicted for methane in graphitic slit pores by equilibrium molecular dynamics (EMD) and non-equilibrium MD (NEMD) is investigated. Three pore wall models are compared--a structured wall and a smooth (specular) wall, both with a thermostat applied to the fluid to maintain the desired temperature, and a structured wall combined with the diffuse thermalizing scattering algorithm of MacElroy and Boyle (Chem. Eng. J., 1999, 74, 85). Pore sizes ranging between 7 and 35 angstroms and five pressures in the range of 1-40 bar are considered. The diffuse thermalizing wall yields incorrect self-diffusion coefficients and transport diffusivities for the graphitic slit pore model and should not be used. Surprisingly, the smooth specular wall gives self-diffusion coefficients inline with those obtained using the structured wall, indicating that this computationally much faster wall can be used for studying this phenomenon provided the fluid-wall interactions are somewhat weaker than the fluid-fluid interactions. The structured wall is required, however, if the transport diffusivity is of interest.

The present work was designed to examine the interaction of collagen/calcium phosphate during the initial mineralization process. Circular dichroism analysis was performed to investigate the conformation change of collagen during the initial process. An ultraviolet photometer was used for turbidity measurements in situ to study the kinetic process of mineralization. It was revealed for the first time that during the initial mineralization process the collagen conformation changed markedly while amorphous/crystalline conversion and crystal ripening occurred. The mechanism for the conformation change was discussed based on the experimental results. The conformation change of the organic matrix may be a widespread phenomenon in biomineralization systems in the living body.