Recently, great attention has been paid to fabricating direct Z-scheme photocatalysts for solar-energy conversion due to their effectiveness for spatially separating photogenerated electron–hole pairs and optimizing the reduction and oxidation ability of the photocatalytic system. Here, the historical development of the Z-scheme photocatalytic system is summarized, from its first generation (liquid-phase Z-scheme photocatalytic system) to its current third generation (direct Z-scheme photocatalyst). The advantages of direct Z-scheme photocatalysts are also discussed against their predecessors, including conventional heterojunction, liquid-phase Z-scheme, and all-solid-state (ASS) Z-scheme photocatalytic systems. Furthermore, characterization methods and applications of direct Z-scheme photocatalysts are also summarized. Finally, conclusions and perspectives on the challenges of this emerging research direction are presented. Insights and up-to-date information are provided to give the scientific community the ability to fully explore the potential of direct Z-scheme photocatalysts in renewable energy production and environmental remediation.

A Review of Direct Z-Scheme Photocatalysts

Review on the criteria anticipated for the fabrication of highly efficient ZnO-based visible-light-driven photocatalysts

Hydrogen (H2) production via photocatalytic water splitting is one of the most promising technologies for clean solar energy conversion to emerge in recent decades. The achievement of energy production from water splitting would mean that we could use water as a fuel for future energy need. Among the various photocatalytic materials, titanium dioxide (TiO2) is the dominant and most widely studied because of its exceptional physico-chemical characteristics. Surface decoration of metal/non-metal on TiO2 nanoparticles is an outstanding technique to revamp its electronic properties and enrich the H2 production efficiency. Metal dopants play a vital role in separation of electron-hole pairs on the TiO2 surface during UV/visible/simulated solar light irradiation. In this paper, the basic principles, photocatalytic-reactor design, kinetics, key findings, and the mechanism of metal-doped TiO2 are comprehensively reviewed. We found that Langmuir-Hinshelwood kinetic model is commonly employed by the researchers to demonstrate the rate of H2 production. Copper (Cu), gold (Au) and platinum (Pt) are the most widely studied dopants for TiO2, owing to their superior work function. The metal dopants can amplify the H2 production efficiency of TiO2 through Schottky barrier formation, surface plasmon resonance (SPR), generation of gap states by interaction with TiO2 VB states. The recent advances and important consequences of 2D materials, perovskites, and other novel photocatalysts for H2 generation have also been reviewed.

Photocatalytic hydrogen production using metal doped TiO2: A review of recent advances

Graphitic carbon nitride (g-C3N4) has gained remarkable acceptance as a visible-light-driven photocatalyst with a distinctive 2D structure and great stability. Owing to its superior features, g-C3N4 has been engaged in various scientific activities for environmental pollution abatement, production and storage of energy, and gas sensors. However, the visible-light efficiency of pure g-C3N4 is very poor and its separation from the phototreated systems is difficult. The most promising method to improve the photocatalytic activity and facilitate separation process is to introduce a magnetic compound over the g-C3N4 sheets. This review has mainly focused on the recent advancement in fabrication, characterization and application of magnetic g-C3N4-based nanocomposites. Accordingly, four primary g-C3N4-based nanocomposites are discussed based on the type of integrated magnetic material. The effects on the structure, physico-chemical properties, photocatalytic activity towards degradation of pollutants, hydrogen generation, solid phase extraction, lithium-ion batteries, gas sensors, and supercapacitors are also discussed in detail.

Review on magnetically separable graphitic carbon nitride-based nanocomposites as promising visible-light-driven photocatalysts

Applications of Phosphorene and Black Phosphorus in Energy Conversion and Storage Devices

Semiconductor photocatalysis is a promising approach to combat both environmental pollution and the global energy shortage. Advanced TiO 2 -based photocatalysts with novel photoelectronic properties are benchmark materials that have been pursued for their high solar energy conversion efficiency. Among the different morphological TiO 2 nanostructures, TiO 2 nanobelts (NBs) attract more attention due to their unique physical properties and ideal 1D ribbon-like morphology that is favorable for constructing heterostructures by assembling second-phase nanoparticles on the surface of the NBs. A large number of studies have proven that well-designed TiO 2 NB heterostructures can not only broaden the photocatalytically active light band of TiO 2 but also enhance the light absorption performance and the photo-induced carrier separation ability. The TiO 2 NB heterostructure has become a versatile and powerful tool for building high-performance TiO 2 -based photocatalysts, which has stimulated intense research activities focused on the growth, properties, and applications of the 1D TiO 2 NB and its heterostructures. This review attempts to cover all these aspects, including the underlying principles and key functional features of TiO 2 NBs and TiO 2 NB heterostructures in a comprehensive way and also discuss the prospects of this type of novel hybrid photocatalyst.

Heterostructures construction on TiO2 nanobelts: A powerful tool for building high-performance photocatalysts

Photocatalytic materials comprised of semiconductor nanostructures have attracted tremendous scientific and technological interest over the last 30 years. This is due to the fact that these photocatalytic materials have unique properties that allow for an effective direct energy transfer from light to highly reactive chemical species which are applicable in the remediation of environmental pollutants and photocatalytic hydrogen generation. Heterostructured photocatalysts are a promising type of photocatalyst which can combine the properties of different components to generate a synergic effect, resulting in a high photocatalytic activity. In this work, a heterostructured photocatalyst comprised of few-layered MoS2 nanosheets coated on a TiO2 nanobelts surface was synthesized through a simple hydrothermal treatment. The hybrid heterostructures with enhanced broad spectrum photocatalytic properties can harness UV and visible light energy to decompose organic contaminants in aqueous solutions as well as split water to hydrogen and oxygen. The mechanism of the enhancement is that the MoS2/TiO2 nanobelts heterostructure can enhance the separation of the photo-induced carriers, which results in a higher photocurrent due to the special electronic characteristics of the graphene-like layered MoS2 nanosheets. This methodology is potentially applicable to the synthesis of a range of hybrid nanostructures with promising applications in photocatalysis and other relevant areas.

Few-layered MoS2 nanosheets wrapped ultrafine TiO2 nanobelts with enhanced photocatalytic property.

A novel scaly Sn3O4/TiO2 nanobelt heterostructured photocatalyst was fabricated via a facile hydrothermal route. The scaly Sn3O4 nanoflakes can be synthesized in situ and assembled on surface coarsened TiO2 nanobelts through a hydrothermal process. The morphology and distribution of Sn3O4 nanoflakes can be well-controlled by simply tuning the Sn/Ti molar ratio of the reactants. Compared with single phase nanostructures of Sn3O4 and TiO2, the scaly hybrid nanobelts exhibited markedly enhanced photoelectrochemical (PEC) response, which caused higher photocatalytic hydrogen evolution even without the assistance of Pt as a co-catalyst, and enhanced the degradation ability of organic pollutants under both UV and visible light irradiation. In addition to the increased exposure of active facets and broad light absorption, the outstanding performance is ascribed to the matching energy band structure between Sn3O4 and TiO2 at the two sides of the heterostructure, which efficiently reduces the recombination of photo-excited electron–hole pairs and prolongs the lifetime of charge carriers. Both photocatalytic assessment and PEC tests revealed that Sn3O4/TiO2 heterostructures with a molar ratio of Sn/Ti of 2/1 exhibited the highest photocatalytic activity. This study provides a facile and low-cost method for the large scale production of Sn3O4 based materials in various applications.

Synthesis of scaly Sn3O4/TiO2 nanobelt heterostructures for enhanced UV-visible light photocatalytic activity

A SnO2 gas sensor was prepared by a two-step oxidation process whereby a Sn(II) precursor was partially oxidized by a hydrothermal process and the resulting Sn3O4 nanoplates were thermally oxidized to yield SnO2 nanoplates. The SnO2 sensor was selective and responsive toward ethanol at a temperature as low as 43 °C. This low sensing temperature stems from the rapid charge transport within SnO2 and from the presence of high-energy (001) facets available for oxygen chemisorption. SnO2/TiO2 nanobelt heterostructures were fabricated by a similar two-step process in which TiO2 nanobelts acted as support for the epitaxial growth of intermediate Sn3O4. At temperatures ranging from 43 to 276 °C, the response of these branched nanobelts is more than double the response of SnO2 for ethanol detection. Our observations demonstrate the potential of low-cost SnO2-based sensors with controlled morphology and reactive facets for detecting gases around room temperature.

High-Energy Faceted SnO2-Coated TiO2 Nanobelt Heterostructure for Near-Ambient Temperature-Responsive Ethanol Sensor

Synthesis of Y 3 Al 5 O 12 (YAG) powders respectively presents morphology control and chemical stoichiometry problems when employing the solid-state reaction or the wet-chemical route. YAG powder retaining the morphology of Al 2 O 3 powder was designed and synthesized via a partial wet-chemical process with yttrium ions precipitating on the Al 2 O 3 particles. The formation process of the Y-compound/Al 2 O 3 core–shell structure is discussed on the basis of zeta-potential measurements and HRTEM results. Two stages, including direct precipitation at the surface of the Al 2 O 3 particles and the assembly of the yttrium precipitate from explosive nucleation onto the yttrium compound-coated Al 2 O 3 particles, are proposed. A spherical surface reaction process is illustrated. A pure YAG phase can be realized at a temperature about 300 °C lower than that of the traditional solid-state reaction process.

Yana Wang

Papers

Heterostructures construction on TiO2 nanobelts: A powerful tool for building high-performance photocatalysts

Few-layered MoS2 nanosheets wrapped ultrafine TiO2 nanobelts with enhanced photocatalytic property.

Synthesis of scaly Sn3O4/TiO2 nanobelt heterostructures for enhanced UV-visible light photocatalytic activity

High-Energy Faceted SnO2-Coated TiO2 Nanobelt Heterostructure for Near-Ambient Temperature-Responsive Ethanol Sensor

Partial wet route for YAG powders synthesis leading to transparent ceramic: A core–shell solid-state reaction process