Engineering the Morphology and Crystal Phase of 3 D Hierarchical TiO2 with Excellent Photochemical and Photoelectrochemical Solar Water Splitting

A Carbon-Free and Free-Standing Cathode From Mixed-Phase TiO2 for Photo-Assisted Li-CO2 Battery

Li-CO₂ battery provides a new strategy to simultaneously solve the problems of energy storage and greenhouse effect. However, the severe polarization of CO₂ reduction and CO₂ evolution reaction impede the practical application. Herein, anodic TiO₂ nanotube arrays are first introduced as carbon-free and free-standing cathode for photo-assisted Li-CO₂ battery, and the photo-assisted charge and discharge mechanism is first clarified from the perspective of photocatalysis. Mixed-phase TiO₂ exhibits a long cycling life of 580 h (52 cycles) at 0.025 mA cm^-2 and delivers a high discharge specific capacity of 3001 µAh cm^-2 under UV illumination. The charge voltage dramatically reduces from 4.53 to 3.03 V under UV illumination. The improvement of photo-assisted Li-CO₂ battery performance relies on the synergistic effect of the hierarchical porous structure, strong UV absorption, efficient separation, and transfer of photo-generated electrons and holes at hetero-phase junction, and the facilitation of photo-generated electrons and holes on CO₂ reduction and CO₂ evolution reaction. This work can provide useful guidance for designing efficient photocathode for photo-assisted Li-CO₂ battery and other metal-air batteries.

Phase transition of individual anatase TiO2 microcrystals with large percentage of (001) facets: a Raman mapping and SEM study

TiO₂ has been extensively studied in many fields including photocatalysis, electrochemistry, optics, etc. Understanding the mechanism of the anatase-rutile phase transition (ART) process is critical for the design of TiO₂-based high-activity photocatalysts and tuning its properties for other applications. In this work, the ART process using individual anatase micro-particles with a large percentage of (001) facets was monitored and studied. Phase concentration evolution obtained via Raman microscopy was correlated with the morphological evolution observed in scanning electron microscope (SEM) images. The ART of anatase microcrystals is dominated by surface nucleation and growth, but the ART processes of individual anatase particles are distinctive and depend on the various rutile nucleation sites. Two types of transformation pathways are observed. In one type of ART pathway, the rutile phase nucleated at a corner of an anatase microcrystal and grew in one direction along the edge of the crystal firstly followed by propagation over the rest of the microcrystal in the orthogonal direction on the surface and to the bulk of the crystal. The kinetics of the ART follows the first-order model with two distinct rate constants. The fast reaction rate is from the surface nucleation and growth, and the slow rate is from the bulk nucleation and growth. In the other type of ART pathway, multiple rutile nucleation sites formed simultaneously on different edges and corners of the microcrystal. The rutile phase spread over the whole crystal from these nucleation sites with a small contribution of bulk nucleation. Our study on the ART of individual micro-sized crystals bridges the material gap between bulk crystals and nano-sized TiO₂ particles. The anatase/rutile co-existing particle will provide a perfect platform to study the synergistic effect between the anatase phase and the rutile phase in their catalytic performances.

Recent Advances in Solar Rechargeable Seawater Batteries Based on Semiconductor Photoelectrodes

With the ever-increasing demand for energy in the world, the tendency to use renewable energies has been growing rapidly. Sunlight, as an inexhaustible energy source, and the oceans, as one of the most valuable treasures on Earth, are available for free. Simultaneous exploitation of these two sources of energy and matter (sunlight and oceans) in one configuration can provide a sustainable solution for future energy supply. Among the various types of such energy storage and conversion systems, solar rechargeable seawater batteries (SRSBs) can meet this need by storing the chemical energy of seawater by receiving solar energy. SRSBs consist of two compartments: a closed compartment including a sodium metal anode in an organic liquid electrolyte, and an open compartment containing a semiconductor photoelectrode immersed in seawater, which are separated from each other by a ceramic solid electrolyte membrane. In this complex system, the photoelectrode is irradiated by sunlight, whereby electrons are excited and reach the Na metal anode after passing though the external circuit. The ceramic solid electrolyte harvests only sodium ions from seawater and transfers them to the anodic part, where the transferred ions are reduced to sodium metal atoms. At the same time, an oxygen evolution reaction takes place at the cathodic part. In this way, the battery is charged. The use of a photoelectrode in the charging process significantly increases the voltage efficiency of SRSBs to more than 90%, whereas a cell with only the seawater compartment (without a photoelectrode) will not deliver satisfactory performance. Therefore, to achieve very high efficiencies, designing an accurate system with the best components is absolutely necessary. This review focuses on the working principle of SRSBs, at the same time explaining the effect of key components on the performance and stability of SRSBs. The role of the semiconductor photoelectrode in improving the voltage efficiency of SRSBs is also described in detail, and finally strategies proposed to overcome obstacles to the commercialization of SRSBs are introduced.

Nanoengineering of Catalysts for Enhanced Hydrogen Production

Hydrogen (H2) has emerged as a sustainable energy carrier capable of replacing/complementing the global carbon-based energy matrix. Although studies in this area have often focused on the fundamental understanding of catalytic processes and the demonstration of their activities towards different strategies, much effort is still needed to develop high-performance technologies and advanced materials to accomplish widespread utilization. The main goal of this review is to discuss the recent contributions in the H2 production field by employing nanomaterials with well-defined and controllable physicochemical features. Nanoengineering approaches at the sub-nano or atomic scale are especially interesting, as they allow us to unravel how activity varies as a function of these parameters (shape, size, composition, structure, electronic, and support interaction) and obtain insights into structure–performance relationships in the field of H2 production, allowing not only the optimization of performances but also enabling the rational design of nanocatalysts with desired activities and selectivity for H2 production. Herein, we start with a brief description of preparing such materials, emphasizing the importance of accomplishing the physicochemical control of nanostructures. The review finally culminates in the leading technologies for H2 production, identifying the promising applications of controlled nanomaterials.

Boosting the Photocatalytic H2 Evolution and Benzylamine Oxidation using 2D/1D g-C3N4/TiO2 Nanoheterojunction

The present research aims at the elevation of solar-to-chemical energy conversion with extortionate performance and sustainability. The nanostructured materials are revolutionizing the water splitting technology into decoupled hydrogen with simultaneous value-added organic chemical production. Yet, the bottleneck in semiconductor photocatalysis is rapid charge recombination and sluggish reaction kinetics. Herein, we demonstrate an efficient and non-noble metal-based catalyst for successful redox reaction with a theoretical modeling through density functional theory (DFT) study. Implementing this robust approach on 2D/1D ultrathin g-C₃N₄ nanosheets and TiO₂ nanowires heterojunction, we achieved H₂ production of 5.1 mmol g^-1 h^-1 with apparent quantum efficiency of 7.8% under visible light illumination and 93% of benzylamine conversion to N-benzylidene benzylamine in situ. The interface of 2D g-C₃N₄ nanosheets and 1D nanowires provide ample active sites and extends the visible light absorption with requisite band edge position for the separation of photoinduced charge carriers with superior stability. The electronic properties, band structure, and stability of the heterojunction are further investigated via DFT calculations which corroborate the experimental results and in good agreement for the enhanced activity of the heterojunction.

Morphology-Governed Performance of Multi-Dimensional Photocatalysts for Hydrogen Generation

In the past few decades, extensive studies have been performed to utilize the solar energy for photocatalytic water splitting; however, up to the present, the overall efficiencies reported in the literature are still unsatisfactory for commercialization. The crucial element of this challenging concept is the proper selection and design of photocatalytic material to enable significant extension of practical application perspectives. One of the important features in describing photocatalysts, although underestimated, is particle morphology. Accordingly, this review presents the advances achieved in the design of photocatalysts that are dedicated to hydrogen generation, with an emphasis on the particle morphology and its potential correlation with the overall reaction performance. The novel concept of this work—with the content presented in a clear and logical way—is based on the division into five parts according to dimensional arrangement groups of 0D, 1D, 2D, 3D, and combined systems. In this regard, it has been shown that the consideration of the discussed aspects, focusing on different types of particle morphology and their correlation with the system’s efficiency, could be a promising route for accelerating the development of photocatalytic materials oriented for solar-driven hydrogen generation. Finally, concluding remarks (additionally including the problems connected with experiments) and potential future directions of particle morphology-based design of photocatalysts for hydrogen production systems have been presented.

TiO2 nanostructures with different crystal phases for sensitive acetone gas sensors

Gas sensors have become increasingly significant because of the rapid development in electronic devices that are applied in detecting noxious gases. Adjusting the crystal phase structure of sensing materials can optimize the band gap and oxygen-adsorptive capacity, which influences the gas sensing characteristics. Therefore, titanium dioxide (TiO₂) materials with different crystal phase structures including rutile TiO₂ nanorods (R-TiO₂ NRs), anatase TiO₂ nanoparticles (A-TiO₂ NRs) and brookite TiO₂ nanorods (B-TiO₂ NRs) were synthesized successfully via one-step hydrothermal process, respectively. The gas sensing characteristics were also investigated systematically. The sensors based on R-TiO₂ NRs displayed the higher response value (12.3) to 100 ppm acetone vapor at 320 °C compared to A-TiO₂ NRs (4.1) and B-TiO₂ NRs (2.3). Furthermore, gas sensors based on R-TiO₂ NRs exhibited excellent repeatability under six cycles and good selectivity to acetone. The outstanding sensing properties of gas sensors based on R-TiO₂ NRs can be ascribed to relatively narrow band gap and more oxygen vacancies of rutile phase, which showed a probable way for design gas sensors based on metal oxide semiconductors with remarkable gas sensing performances by the crystal phase adjustment engineering in the future.

High electron transfer of TiO2 nanorod@carbon layer supported flower-like WS2 nanosheets for triiodide electrocatalytic reduction

1D–2D multidimensional nanostructured TNRs@C@WS₂ has been prepared and introduced as an effective catalyst for the triiodide reduction reaction.