Semiconductor photocatalysis is recognized as a promising strategy to simultaneously address energy needs and environmental pollution. Titanium dioxide (TiO2 ) has been investigated for such applications due to its low cost, nontoxicity, and high chemical stability. However, pristine TiO2 still suffers from low utilization of visible light and high photogenerated-charge-carrier recombination rate. Recently, TiO2 photocatalysts modified by dual cocatalysts with different functions have attracted much attention due to the extended light absorption, enhanced reactant adsorption, and promoted charge-carrier-separation efficiency granted by various cocatalysts. Recent progress on the component and structural design of dual cocatalysts in TiO2 photocatalysts is summarized. Depending on their components, dual cocatalysts decorated on TiO2 photocatalysts can be divided into the following categories: bimetallic cocatalysts, metal-metal oxide/sulfide cocatalysts, metal-graphene cocatalysts, and metal oxide/sulfide-graphene cocatalysts. Depending on their architecture, they can be categorized into randomly deposited binary cocatalysts, facet-dependent selective-deposition binary cocatalysts, and core-shell structural binary cocatalysts. Concluding perspectives on the challenges and opportunities for the further exploration of dual cocatalyst-modified TiO2 photocatalysts are presented.

Dual Cocatalysts in TiO 2 Photocatalysis.

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Photocatalytic degradation has been unearthed as a promising strategy for environmental remediation, and the calling is endless for more efficient photocatalytic system. In this study, a novel oxygen vacancy-rich two-dimensional/two-dimensional (2D/2D) BiOCl-g-C3N4 ultrathin heterostructure nanosheet (CN-BC) is successfully prepared by a facile solvothermal method for degradation of non-dye organic contaminants. HRTEM observes the formation of heterojunction, while ESR and XPS unveil the distinct oxygen vacancy concentrations. Density functional calculations reveal that the introduction of oxygen vacancies (OVs) brings a new defect level, resulting in the increased photoabsorption. Under visible light irradiation, the OVs-rich optimum ratio of CN-BC (50CN-50BC) Exhibits 95% removal efficiency of 4-chlorophenol within 2 h, which is about 12.5, 5.3 and 3.4 times as that of pure BiOCl, g-C3N4 and OVs-poor heterostructure, respectively. The photocatalytic mechanism of OVs-rich 50CN-50BC is also revealed, suggesting that the synergistic effect between 2D/2D heterojunction and oxygen vacancies greatly promotes visible-light photoabsorption and photoinduced carrier separation efficiency with a prolonged lifetime, which is confirmed by multiple optical and electrochemical analyses, including DRS, steady-state photoluminescence spectra, electrochemical impedance spectroscopy, photocurrent response and time-resolved fluorescence spectra. This study could bring new opportunities for the rational design of highly efficient photocatalysts by combining 2D/2D heterojunctions with oxygen vacancies in environmental remediation.

Oxygen vacancy-rich 2D/2D BiOCl-g-C3N4 ultrathin heterostructure nanosheets for enhanced visible-light-driven photocatalytic activity in environmental remediation

Hydrogen production from water splitting by photo/photoelectron-catalytic process is a promising route to solve both fossil fuel depletion and environmental pollution at the same time. Titanium dioxide (TiO2) nanotubes have attracted much interest due to their large specific surface area and highly ordered structure, which has led to promising potential applications in photocatalytic degradation, photoreduction of CO2, water splitting, supercapacitors, dye-sensitized solar cells, lithium-ion batteries and biomedical devices. Nanotubes can be fabricated via facile hydrothermal method, solvothermal method, template technique and electrochemical anodic oxidation. In this report, we provide a comprehensive review on recent progress of the synthesis and modification of TiO2 nanotubes to be used for photo/photoelectro-catalytic water splitting. The future development of TiO2 nanotubes is also discussed.

One‐dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting

Exposing the highly active facets and hybridizing the photocatalyst with appropriate cocatalysts with right placement have been regarded as a powerful approach to high performance photocatalysts. Herein, TiO2 nanosheets (NSs) are in situ grown on highly conductive Ti3C2 MXene and then MoS2 NSs are deposited on the (101) facets of TiO2 NSs with mainly exposed high-active (001) facets through a two-step hydrothermal method. And a unique 2D-2D-2D structure of Ti3C2@TiO2@MoS2 composite is achieved. With an optimized MoS2 loading amounts (15 wt%), the Ti3C2@TiO2@MoS2 composite shows a remarkable enhancement in the photocatalytic H2 evolution reaction compared with Ti3C2@TiO2 composite and TiO2 NS. It also shows good stability under the reaction condition. This arises from: (i) the in situ growth of TiO2 NSs construct strong interfacial contact with excellent electronic conductivity of Ti3C2, which facilitates the separation of carriers; (ii) the coexposed (101) and (001) facets can form a surface heterojunction within single TiO2 NS, which is beneficial for the transfer and separation of charge carriers; and (iii) the MoS2 NSs are deposited on the electrons-rich (101) facets of TiO2 NSs, which not only effectively reduces the charge carriers recombination rate by capturing photoelectrons, but also makes TiO2 NSs expose more highly active (001) facets to afford high-efficiency photogeneration of electron-hole pairs.

2D/2D/2D heterojunction of Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity

One of the challenging issues in photocatalytic hydrogen (H2) production is to efficiently separate the photogenerated electron-hole pairs and require the enrichment of photogenerated electrons on the photocatalyst’s surface. Herein, a novel 2D-2D nanojunction of MoS2 nanosheets (NSs) selectively deposited on the (101) facets of TiO2 NSs with mainly exposed high-active (001) facets is prepared via a hydrothermal/annealing treatment combined with an photoreduction method using carbon ﬁber (CF) as templates. The obtained MoS2@TiO2 composite (selective deposition) photocatalyst exhibits a greatly enhanced photocatalytic H2 production activity at the optimal weight percentage of MoS2 (15 wt%), exceeding that of pure TiO2 NSs and MoS2@TiO2 composites (random deposition) by 32 and 3 times, respectively. The superior photoactivity of MoS2@TiO2 composites (selective deposition) is attributed to the synergistic promoting effects of the following factors: (i) the mainly exposed (001) facets of TiO2 NSs with higher surface energy in MoS2@TiO2 composites (selective deposition) facilitate the activation of water molecules and the photocatalytic reduction; (ii) the coexposed (101) and (001) facets can form a surface heterojunction within single TiO2 NS, which is beneﬁcial for the transfer and separation of charge carriers; (iii) the MoS2 NSs are selectively deposited on the electrons-rich (101) facets of TiO2 NSs, which can effectively reduce the charge carriers recombination rate by capturing photoelectrons. This study presents an inexpensive photocatalyst for energy conversion to achieve highly efficient H2 evolution without noble metals.

The selective deposition of MoS2 nanosheets onto (101) facets of TiO2 nanosheets with exposed (001) facets and their enhanced photocatalytic H2 production

Recently, near-infrared light (NIR) photocatalysts, such as up-converting materials, Bi2WO6, Cu2(OH)PO4, and carbon quantum dots, have been found and are attracting attention for environmental cleaning and energy conversion, because NIR light constitutes nearly half of the energy output. However, the photocatalytic efficiency is low for these new types of NIR photocatalysts, which has limited their widespread application owing to their insufficient NIR-light absorption. In this work, we use gold (Au) nanorods to enhance the NIR-light absorption of Bi2WO6, a typical visible and novel NIR-light photocatalyst, thus the enhancement of its NIR-light photocatalytic and photoelectrochemical properties are achieved. This can be attributed to the surface plasmon resonance (SPR) effects and wide-range NIR light harvesting of the Au nanorods. The present work provides key guidelines for the improvement of NIR-light photocatalysts and could also help design and prepare novel photocatalysts.

Bi2WO6 Nanosheets Decorated with Au Nanorods for Enhanced Near‐Infrared Photocatalytic Properties Based on Surface Plasmon Resonance Effects and Wide‐Range Near‐Infrared Light Harvesting

Highly efficient full solar spectrum (UV-vis-NIR) photocatalytic performance of Ag2S quantum dot/TiO2 nanobelt heterostructures

Synthesis of few-layer MoS 2 nanosheets-coated TiO 2 nanosheets on graphite fibers for enhanced photocatalytic properties

Towards full-spectrum (UV, visible, and near-infrared) photocatalysis: achieving an all-solid-state Z-scheme between Ag2O and TiO2 using reduced graphene oxide as the electron mediator

Xiaolin Hu

Papers

The selective deposition of MoS2 nanosheets onto (101) facets of TiO2 nanosheets with exposed (001) facets and their enhanced photocatalytic H2 production

Bi2WO6 Nanosheets Decorated with Au Nanorods for Enhanced Near‐Infrared Photocatalytic Properties Based on Surface Plasmon Resonance Effects and Wide‐Range Near‐Infrared Light Harvesting

Highly efficient full solar spectrum (UV-vis-NIR) photocatalytic performance of Ag2S quantum dot/TiO2 nanobelt heterostructures

Synthesis of few-layer MoS 2 nanosheets-coated TiO 2 nanosheets on graphite fibers for enhanced photocatalytic properties

Towards full-spectrum (UV, visible, and near-infrared) photocatalysis: achieving an all-solid-state Z-scheme between Ag2O and TiO2 using reduced graphene oxide as the electron mediator