Heterostructure TiO2 polymorphs design and structure adjustment for photocatalysis

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Phase transition behaviour and mechanism of 2D TiO2(B) nanosheets through water-mediated removal of surface ligands

Phase engineering is a central subject in materials research. The recent research interest in the phase transition behavior of atomically thin 2D materials reveals the important role of their surface chemistry. In this study, we investigated the phase transformation of ultrathin TiO2(B) nanosheets to anatase under different conditions. We found that the convenient transformation in water under ambient conditions is driven by the hydrolysis of surface 1,2-ethylenedioxy groups and departure of ethylene glycol. A transformation pathway through the formation of protonic titanate is proposed. The ultrathin structure and the metastable nature of the precursor facilitate the phase conversion to anatase. Our finding offers a new insight into the mechanism of TiO2(B) phase transition from the viewpoint of surface chemistry and may contribute to the potential application of ultrathin TiO2(B) nanosheets in aqueous environments.

Indium selenide/silver phosphate hollow microsphere S-scheme heterojunctions for photocatalytic hydrogen production with simultaneous degradation of tetracycline

Designing heterojunction photocatalysts with strong interfacial interactions is an effective way to reduce the recombination of photogenerated charge carriers. Here, silver phosphate (Ag₃PO₄) nanoparticles are coupled with hollow flower-like indium selenide (In₂Se₃) microspheres by a facile Ostwald ripening and in-situ growth method, resulting in the construction of In₂Se₃/Ag₃PO₄ hollow microsphere step-scheme (S-scheme) heterojunction with a large contact interface. The flower-like In₂Se₃ with hollow and porous structure provides a large specific surface area and numerous active sites for photocatalytic reactions to take place. The photocatalytic activity was tested by measuring the hydrogen evolution from antibiotic wastewater, and the H₂ evolution rate of In₂Se₃/Ag₃PO₄ reached 4206.4 µmol g^-1h^-1 under visible light, which is approximately 2.8 times greater than that of In₂Se₃. In addition, the amount of tetracycline (TC) degradation when it was used as a sacrificial agent is about 54.4% after 1 h. On the one hand, Se-P chemical bonds act as electron transfer channels in the S-scheme heterojunctions, which can facilitate the migration and separation of photogenerated charge carriers. On the other hand, the S-scheme heterojunctions can retain the useful holes and electrons with higher redox capacities, which is very favorable for the generation of more •OH radicals and the photocatalytic activity is greatly enhanced. This work provides an alternative design approach for photocatalysts toward hydrogen evolution in antibiotic wastewater.

Construction of TiO2(B)/Anatase Heterophase Junctions via a Water-Induced Phase Transformation Strategy for Enhanced Photocatalytic Hydrogen Production

The development of facile and green solution-phase routes toward the fabrication of TiO₂-based heterophase junctions with a delicate control of phase and structure is a challenging task. Herein, we report a simple and convenient method to controllably fabricate TiO₂(B)/anatase heterophase junctions, which was successfully realized by utilizing the ideal great solvent of water to treat the presynthesized TiO₂(B) nanosheet precursor at a low temperature of 80 °C. On the basis of phase structure transformation and morphology evolution data, the formation of these TiO₂(B)/anatase heterophase junctions was reasonably explained by a novel water-induced TiO₂(B) → anatase phase transformation mechanism. Benefiting from the desirable structural and photoelectronic advantages of more exposed active sites, enhanced light absorbance, and promoted separation of photogenerated electron-hole pairs, the thus-transformed TiO₂(B)/anatase heterophase junctions exhibit fascinating photocatalytic performance in water splitting. Specifically, with the help of Pt as a cocatalyst and methanol as a sacrificial agent, the H₂ production rate of optimized TiO₂(B)/anatase heterophase junction reaches 6.92 mmol·g^-1·h^-1, which is almost 7.1 and 2.1 times higher than those of the pristine TiO₂(B) nanosheets and the final anatase nanocrystals. More interestingly, the TiO₂(B)/anatase heterophase junction also delivers prominent activity toward pure water splitting to simultaneously produce H₂ and H₂O₂, with evolution rates of up to 1.10 and 0.55 mmol·g^-1·h^-1, respectively. Our work may advance the facile green solvent-mediated synthesis of metal oxide-based heterophase junctions for applications in energy- and environmental-related areas.

g-C3N4/TiO2-B{100} heterostructures used as promising photocatalysts for water splitting from a hybrid density functional study

Fabrication of heterostructures has been shown to be a good strategy to improve photocatalytic performance. By using first-principles calculation based on hybrid density functionals, the photocatalytic mechanism of g-C₃N₄/TiO₂-B{100} heterostructures is investigated to understand the process of water decomposition. We find that the reduction of the band gap of g-C₃N₄/TiO₂-B{100} heterostructures enhances the visible light response range. g-C₃N₄/TiO₂-B{100} heterostructures have direct band gaps, staggered band alignment, electron flow from g-C₃N₄ to TiO₂-B{100} surfaces and straddling water decomposition potential, and are potential Z-scheme photocatalysts. Photoinduced carriers can be effectively separated using the Z-scheme photocatalytic mechanism. Our results demonstrate that g-C₃N₄/TiO₂-B{100} heterostructures can enhance light absorption, prolong the life of photoinduced carriers, and further improve the photocatalytic activity. We believe that our findings can provide a reference for explaining the enhancement mechanism of the g-C₃N₄/TiO₂ photocatalyst as observed in the experiment.