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Lepidocrocite-type layered sodium titanate (NaxH2−xTi2O5) is widely used in environmental remediation because of its large specific surface area, formed by anisotropic crystal growth, and its ability to store and exchange cations between layers. Additionally, peroxo-titanate nanotubes (PTNTs), which are tubular titanates with peroxy groups, exhibit visible-light absorption capabilities, rendering them suitable for photocatalytic applications under visible light irradiation. However, because of cation exchange reactions, the Na+ concentration and pH of the solution can fluctuate under aqueous conditions, affecting the photocatalytic performance of the PTNTs. Herein, we evaluated the impact of cation exchange reactions on the photocatalytic degradation of Rhodamine B (Rh B) by PTNTs at controlled Na+ ratios. The observed pH of Rh B solutions increases due to the cation exchange reaction with Na+ and H3O+, leading to the formation of zwitter-ionic Rh B molecules, eventually weakening their adsorption and photodegradation performance. Moreover, the results indicate that inhibiting the pH increase of the Rh B solution can prevent the weakening of both the adsorption and photodegradation performance of PTNTs. This study highlights the significance of regulating the sodium ion content in layered titanate materials, emphasizing their importance in optimizing these materials' photocatalytic efficacy for environmental purification applications. [ABSTRACT FROM AUTHOR]

Using melt-derived LD glass powders and 5–20 M NaOH solutions, porous lithium disilicate (Li2Si2O5, LD) glass–ceramics were prepared by the cold sintering process (CSP) associated with the post-annealing technique. In this novel technique, H2O vapor originating from condensation reactions between residual Si–OH groups in cold-sintered LD glasses played the role of a foaming agent. With the increasing concentration of NaOH solutions, many more residual Si–OH groups appeared, and then rising trends in number as well as size were found for spherical pores formed in the resultant porous LD glass–ceramics. Correspondingly, the total porosities and average pore sizes varied from 25.6 ± 1.3% to 48.6 ± 1.9% and from 1.89 ± 0.68 μm to 13.40 ± 10.27 μm, respectively. Meanwhile, both the volume fractions and average aspect ratios of precipitated LD crystals within their pore walls presented progressively increasing tendencies, ranging from 55.75% to 76.85% and from 4.18 to 6.53, respectively. Young's modulus and the hardness of pore walls for resultant porous LD glass–ceramics presented remarkable enhancement from 56.9 ± 2.5 GPa to 79.1 ± 2.1 GPa and from 4.6 ± 0.9 GPa to 8.1 ± 0.8 GPa, whereas their biaxial flexural strengths dropped from 152.0 ± 6.8 MPa to 77.4 ± 5.4 MPa. Using H2O vapor as a foaming agent, this work reveals that CSP associated with the post-annealing technique is a feasible and eco-friendly methodology by which to prepare porous glass–ceramics. [ABSTRACT FROM AUTHOR]

Photocatalytic production of hydrogen peroxide (H2O2) from dioxygen (O2) and water (H2O) has shown promise for the artificial photosynthesis of liquid fuel. We previously demonstrated that an Al-based metal–organic framework (MOF) functions as a suitable platform for photocatalytic H2O2 production owing to the efficient suppression of H2O2 decomposition caused by the photocatalysts themselves, which increases the yield of H2O2. However, the photocatalytic efficiency of Al-based MOFs is often limited by their short-lived charge separation. The energy transfer process is a beneficial approach to promoting charge separation and thereby improving the photocatalytic activity; MOFs enable highly efficient energy transfer between organic linkers because they allow precise control of the arrangement of the building blocks. Herein, we demonstrate that an Al-based MOF composed of both porphyrin- and pyrene-based organic linkers (Al-TCPP(10-X)-TBAPyX) is a promising photocatalyst for producing H2O2 from O2 and H2O without additives under visible-light irradiation while simultaneously enabling efficient suppression of undesired H2O2 decomposition. Efficient energy transfer from 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy) to tetrakis(4-carboxyphenyl)porphyrin (TCPP) was driven within Al-TCPP(10-X)-TBAPyX, resulting in dramatically enhanced photocatalytic H2O2 production through optimization of the linker mixture ratio in the MOF structure. The present work not only proposes a new reaction pathway for H2O2 generation via1O2 intermediates, which is quite different from well-accepted mechanisms involving O2˙−, but also provides a promising strategy for designing catalysts to realize efficient photosynthetic H2O2 production. [ABSTRACT FROM AUTHOR]

Al-MIL-101-NH2, which was previously regarded as being inactive as a photocatalyst, produces hydrogen peroxide (H2O2) via O2 reduction under visible-light irradiation, accompanied by efficient suppression of undesired H2O2 decomposition. The low-coordination Lewis acid sites in trimetric Al-oxo clusters are crucial for the electron transfer to O2. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs) are promising materials for photocatalytic hydrogen peroxide (H2O2) production from water (H2O) and dioxygen (O2). However, H2O2 production from H2O and O2 using MOFs has remained challenging due to the fast recombination of photogenerated charges in the MOF matrix. Herein, a heterojunction structure was constructed by modification of an organic semiconductor, perylenetetracarboxylic diimide (PDI), upon a titanium-based MOF (MIL-125-NH2), forming MIL-125-PDI to promote the separation of photogenerated charges between PDI and MOFs. Photo/electrochemical characterizations confirmed that the constructed heterostructure indeed promoted the separation and transfer of photogenerated charges. As a result, the as-formed MIL-125-PDI displayed a H2O2 production rate of 4800 μM g−1 h−1, which was 4.3 times that of the pristine MIL-125-NH2 under visible-light irradiation (λ > 420 nm). Furthermore, MIL-125-PDI exhibited a better durability compared with MIL-125-NH2 with noble-metal Pt nanoparticles as co-catalysts. The results of trapping experiments and electron spin resonance (ESR) spectra of active species for H2O2 production indicated that a new one-step two-electron process of O2 reduction to H2O2 occurred in MIL-125-PDI, while the original one observed in MIL-125-NH2 was a two-step one-electron process. This work used MOFs as the main active component in photocatalysts, which opens up the new application of MOFs in photocatalysis and provides an effective strategy to design MOF-based photocatalysts for artificial photosynthesis of clean liquid fuels. [ABSTRACT FROM AUTHOR]

Porous materials have attracted considerable attention in materials science, chemistry and multidisciplinary areas of research due to their diverse structures, tailorable functionalities, large surface areas, remarkable optical transparency, confinement and shape selectivity effects. The precise architecture of porous photocatalytic materials has been extensively reported by the hybridization of semiconducting nanoparticles (NPs), light-responsive metal complexes and plasmonic metal NPs with silica-based micro-, meso- and macro-porous nanostructures. This review targets to cover state-of-the-art accomplishments in the surface engineering chemistry of zeolites, mesoporous silica, and metal-organic frameworks (MOFs) as excellent host materials for solar to chemical energy conversion and environmental remediation. The specific advantages and disadvantages of representative porous materials in photocatalysis have also been elaborated and summarized. Finally, key advances and prospects are presented to overcome the current challenges in photocatalysis and to inspire further studies in this rapidly evolving research field. [ABSTRACT FROM AUTHOR]

As an environmentally friendly and renewable energy source, hydrogen peroxide (H2O2) could be produced photocatalytically through selective two-electron reduction of O2 using effective photocatalysts. Metal organic frameworks (MOFs), as hybrid porous materials consisting of organic linkers and metal oxide clusters, have aroused great interest in the design of effective catalysts for photocatalysis under visible light irradiation due to their unique properties, such as large surface area, good chemical stability, and diverse and tunable chemical components. In this perspective, we highlight our recent progress in the application of various MOF-based nanomaterials for photocatalytic H2O2 production from the selective two-electron reduction of O2 in a single-phase system (acetonitrile) and two-phase system (water/benzyl alcohol). Photocatalytic H2O2 production in the single-phase system achieved a higher activity using NiO as a cocatalyst of the MOF rather than Pt. Photocatalytic H2O2 production in the two-phase system using various hydrophobic MOFs showed further improved activity compared to the single-phase system. It has been possible to design a hydrophobic MOF-based photocatalyst with high activity and stability under recycling conditions. These studies gathered in this perspective revealed the novel application of MOFs in the field of energy production. [ABSTRACT FROM AUTHOR]

Photocatalytic H2O2 production via two-electron reduction of O2 was realized by visible-light irradiation of a metal–organic framework, MIL-125-NH2, in the presence of TEOA and benzylalcohol. Deposition of NiO nanoparticles onto MIL-125-NH2 dramatically enhanced the catalytic activity. Further studies suggested that fast disproportionation of the O2Ṗ− intermediate to H2O2 resulted in the enhancement. [ABSTRACT FROM AUTHOR]

Layered alkali titanates of the lepidocrocite type are gaining enormous interest in various fields owing to their unique properties. These materials are mainly synthesized through a hydrothermal alkali treatment. However, this method uses a highly concentrated alkali solution, which has high environmental impacts and is therefore unsuitable for mass synthesis. Herein, we propose an efficient method for the large-scale synthesis of layered sodium titanate structures (Na 2- x H x Ti 2 O 5 ) using a recently reported bottom-up chemical process. The effects of the Na:Ti molar ratio in the peroxo-titanium complex ion precursor on the products are investigated through stoichiometric calculations for a molar ratio range of 10:1-1:1. The optimal ratio for the complete ionization of TiH 2 (which is the starting material) to form the peroxo-titanium complex ion is found to be 1.1:1. The amount of alkali raw material required is 99.6% lower than that required in the traditional hydrothermal method. The crystal structures and morphologies of the samples are almost identical regardless of the Na:Ti molar ratio. The precursor-derived peroxo bonds narrow the energy band gaps of the layered titanates even when the amount of titanium ions dissolved in the precursor increases. The proposed method is not only an efficient synthetic route for mass production but also has potential applications in the development of photofunctional materials., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Layered sodium titanate is a typical ion-exchanger for water purification aimed at removing cationic heavy metals and radionuclides. The material design of an ion-exchanger is effective for cation removal. For that purpose, understanding the basic impacts of crystallographic properties such as crystal size, morphology, and phase is critical for developing highly functional nanoscale ion-exchangers. In this study, we investigate the principal relationship between the crystallographic properties of seaweed-like sodium titanate mats (SSTs), which consist of a dititanate (H x Na 2- x Ti 2 O 5 ) phase of nanofibers synthesised by the alkaline hydrothermal method and their Sr 2+ sorption mechanism. A trititanate (H x Na 2- x Ti 3 O 7 ) phase, which has a micro-sized fibre morphology, was also synthesised using the same method by adjusting the NaOH concentration. The SST demonstrates a high ion-exchange selectivity of Sr 2+ against H + and a high maximum sorption capacity (2 mmol g -1 ), which was four times higher than that of the trititanate phase (0.49 mmol g -1 ). In contrast, the trititanate phase, which is the comparison target, had a low Sr 2+ ion-exchange selectivity and precipitated SrCO 3 . We conclude that these differences in Sr 2+ sorption mechanisms were derived from not only the unique morphology but also the crystal structure of sodium titanates. Although almost all of the Na + in dititanate with lamellar structure was consumed by the ion-exchange reaction, some Na + remained in the trititanate because there are two sites in the zigzag layered structure. These findings on the crystallographic properties of SST for Sr 2+ sorption may contribute to the functionalisation of a nanoscale ion-exchanger., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The development of new technologies for securing and recycling water resources are in high demand. A key focus of these technologies is the development of various ion exchangers or adsorbents that are used for the purification of aqueous solutions. Layered sodium titanate is one of the cation exchangers utilised in the removal of heavy metals and radionuclides from wastewater. To enhance the removal efficiency, the precise design of the crystal morphology, structure, and chemical composition is important. Herein, we synthesised a unique seaweed-like sodium titanate mat (SST) using a template-free alkaline hydrothermal process. The Co 2+ sorption capacity of SST was investigated by batch testing with cobalt(ii) nitrate. SST, which was synthesised from titanium sulphate in a 10 M NaOH solution at 200 °C, had a seaweed-like structure composed of randomly distributed nanofibres of layered sodium titanate that is approximately 9 nm in diameter. The crystal shape changed from roundish crystals to fibrous crystals as the hydrothermal reaction period increased. The Co 2+ sorption isotherm of SST was fitted with the Langmuir isotherm model and the maximum sorption density was 1.85 mmol g -1 . The selectivity of the Co 2+ sorption on SST was high in comparison to that of Ca 2+ and Mg 2+ . Herein, the Co 2+ sorption mechanisms of SST were studied in comparison with commercially available sodium titanate. Results show that controlling the crystal morphology, structure, and Na concentration of the layered titanate that can be ion-exchanged determines the cation sorption properties of sodium titanate., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)