Titanium dioxide (TiO2) has been the most intensively investigated binary transition metal oxide in the past four decades as indicated by Figure S1. Furthermore, the annual number of papers published on TiO2 has seen a continuous increase, particularly since the beginning of this century (Figure S2). This is understandable when one considers the wide range of applications of TiO2 from the conventional areas (i.e., pigment, cosmetic, toothpaste, and paint) to the later developed functional areas such as photoelectrochemical cell,(1-3) dye-sensitized solar cells (DSSCs),(4-11) photocatalysis,(12-24) catalysis,(25-31) photovoltaic cell,(32-34) lithium ion batteries,(35-41) sensors,(42-46) electron field emission,(47-51) microwave absorbing material, biomimetic growth, and biomedical treatments.(52-57) Nearly all these functional applications of TiO2 fall in the scope of energy, environment, and health, which are definitely the three most important and challenging themes facing the Human race that need to be addressed in this century. Besides the apparent merits including nontoxicity, elemental abundance, good chemical stability, and easy synthesis, TiO2 has attracted strong research interest worldwide due to its physicochemical properties for realizing various functions.(15, 58, 59) Especially, very encouraging progresses in photocatalysis and DSSCs with the involvement of TiO2 have greatly stimulated the rapid development of TiO2 crystals with controllable phase, size, shape, defect, and heteroatom.(58, 60-68)

Titanium dioxide crystals with tailored facets

In recent years, heterogeneous photocatalysis has received much research interest because of its powerful potential applications in tackling many important energy and environmental challenges at a global level in an economically sustainable manner. Due to their unique optical, electrical, and physicochemical properties, various 2D graphene nanosheets-supported semiconductor composite photocatalysts have been widely constructed and applied in different photocatalytic fields. In this review, fundamental mechanisms of heterogeneous photocatalysis, including thermodynamic and kinetics requirements, are first systematically summarized. Then, the photocatalysis-related properties of graphene and its derivatives, and design rules and synthesis methods of graphene-based composites are highlighted. Importantly, different design strategies, including doping and sensitization of semiconductors by graphene, improving electrical conductivity of graphene, increasing eloectrocatalytic active sites on graphene, strengthening interface coupling between semiconductors and graphene, fabricating micro/nano architectures, constructing multi-junction nanocomposites, enhancing photostability of semiconductors, and utilizing the synergistic effect of various modification strategies, are thoroughly summarized. The important applications including photocatalytic pollutant degradation, H2 production, and CO2 reduction are also addressed. Through reviewing the significant advances on this topic, it may provide new opportunities for designing highly efficient 2D graphene-based photocatalysts for various applications in photocatalysis and other fields, such as solar cells, thermal catalysis, separation, and purification.

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Graphene in Photocatalysis: A Review

Effectively harvesting light to generate long-lived charge carriers to suppress the recombination of electrons and holes is crucial for photocatalytic reactions. Exposing the highly active facets has been regarded as a powerful approach to high-performance photocatalysts. Herein, a hybrid comprised of {001} facets of TiO2 nanosheets and layered Ti3C2, an emerging 2D material, was synthesized by a facile hydrothermal partial oxidation of Ti3C2. The in situ growth of TiO2 nanosheets on Ti3C2 allows for the interface with minimized defects, which was demonstrated by high-resolution transmission electron microscopy and density functional theory calculations. The highly active {001} facets of TiO2 afford high-efficiency photogeneration of electron–hole pairs, meanwhile the carrier separation is substantially promoted by the hole trapping effect by the interfacial Schottky junction with 2D Ti3C2 acting as a reservoir of holes. The improved charge separation and exposed active facets dramatically boost the photo...

Hybrids of Two-Dimensional Ti3C2 and TiO2 Exposing {001} Facets toward Enhanced Photocatalytic Activity

Due to the increasingly polluted environment and the gradual depletion of fossil fuel reserves, the development of renewable technologies for environmental remediation and energy production is highly desirable. Over the past decades, oxide-based semiconductor photocatalysis has attracted much attention. On various frontiers for efficient photocatalysis, surface-tuning strategies for synthesis and modification of oxides on the nanometer scale have progressed at a fast pace. Hence, it is of significance to review recent advances in the development of surface tuning for oxide-based nanomaterials as activity-enhanced photocatalysts. In this review, special emphases, especially for recent advances in our group, are given to surface tuning of novel nanocrystallites for high thermal stability, hierarchical structure assembly, heterojunctional nanocomposites and high-energy-facet exposure, along with effective testing tools for photogenerated charge properties at the surfaces and/or interfaces. This is of great significance for fields related to energy and environment from scientific and engineering viewpoints.

Surface Tuning for Oxide-Based Nanomaterials as Efficient Photocatalysts

The g-C3N4/Ag3VO4 hybrid photocatalysts were prepared by Ag3VO4 anchoring on the surface of g-C3N4. The transmission electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photo-electron spectroscopy analyses demonstrated that Ag3VO4 nanoparticles well distributed on the surface of g-C3N4 and the g-C3N4/Ag3VO4 hetero-junctions were formed. Compared with pure g-C3N4 and Ag3VO4, the g-C3N4/Ag3VO4 hybrid materials displayed much higher photocatalytic activity for basic fuchsin degradation (20 mg/L, 50 mL) under visible-light irradiation. The 40 wt% g-C3N4/Ag3VO4 photocatalyst exhibited optimal removal rate constant of 0.92 h−1, which was 11.5 and 6.6 times higher than that of pure g-C3N4 and Ag3VO4, respectively. Density functional theory calculations indicated that complementary conduction and valence band-edge hybridization between g-C3N4 and Ag3VO4 could apparently increase separation efficiency of electron-hole pairs of g-C3N4/Ag3VO4 composites, which was confirmed by photoluminescence spectra. In addition, it was found that h+ and •O2−1generated in the photocatalytic process played a key role in basic fuchsin degradation.

Synthesis and characterization of g-C3N4/Ag3VO4 composites with significantly enhanced visible-light photocatalytic activity for triphenylmethane dye degradation

Tailored synthesis of well-defined anatase TiO2 nanocrystals with a high percentage of reactive facets has attracted widespread attention due to the scientific and technological importance. Here, high-quality nanosized anatase ultrathin TiO2 nanosheets, mainly dominated by {001} facets, were grown on graphene nanosheets by a simple one-pot solvothermal synthetic route. The obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of as-prepared TiO2/graphene composites for degradation of methylene blue (MB) under visible-light irradiation at λ ≥ 400 nm was investigated. The results show that TiO2/graphene nanocomposites have a higher photocatalytic activity than pure TiO2 and P25. This enhanced photocatalytic activity suggests that the photoinduced electrons in TiO2 pref...

Large Ultrathin Anatase TiO2 Nanosheets with Exposed {001} Facets on Graphene for Enhanced Visible Light Photocatalytic Activity

A novel ternary plasmonic Ag3VO4/AgBr/Ag hybrid photocatalyst was successfully fabricated via an in situ anion-exchange reaction between Ag3VO4 and KBr, followed by light reduction. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV–visible diffuse-reflectance spectroscopy (UV–vis DRS), and X-ray photoelectron spectroscopy. The photocatalytic activities of obtained photocatalysts were measured by the degradation of Rhodamine B and methylene blue under visible-light irradiation (λ ≥ 400 nm). As-prepared Ag3VO4/AgBr/Ag plasmonic photocatalysts exhibit wide absorption in the visible-light region and display superior visible-light-driven photocatalytic activities in degradation of organic contamination compared with pristine Ag3VO4, Ag3VO4/AgBr, and AgBr/Ag. This enhanced photocatalytic activity is attributed to the synergistic effects between Ag3VO4/AgBr-based heterostructured semiconductor photocatalysis and the surface plasmo...

Facile Synthesis of the Novel Ag3VO4/AgBr/Ag Plasmonic Photocatalyst with Enhanced Photocatalytic Activity and Stability

A heterostructured Ag3PO4/AgBr/Ag plasmonic photocatalyst was prepared by a rational in situ ion exchange reaction between Ag3PO4 micro-cubes and Br− in aqueous solution followed by photoreduction. The photocatalytic activities of obtained photocatalysts were measured by the degradation of methyl orange (MO) and methylene blue (MB) under visible light irradiation (λ ≥ 400 nm). Compared to AgBr/Ag, Ag3PO4/AgBr heterocrystals and pure Ag3PO4 crystals, the heterostructured Ag3PO4/AgBr/Ag plasmonic photocatalysts exhibit much higher photocatalytic activity and stability. This enhanced photocatalytic activity suggests that the synergetic effects of the heterostructured Ag3PO4/AgBr/Ag and the strong SPR of Ag NPs on the surface result in the high efficiencies of the photocatalytic activity and the improved stability. With the assistance of Ag3PO4/AgBr/Ag heterostructures, only 8 min and 12 min are taken to completely decompose MO and MB molecules under visible-light irradiation, respectively. Furthermore, the photodegradation rate does not show an obvious decrease during ten successive cycles, indicating that our heterostructured Ag3PO4/AgBr/Ag plasmonic photocatalysts are extremely stable under visible-light irradiation.

Heterostructured Ag3PO4/AgBr/Ag plasmonic photocatalyst with enhanced photocatalytic activity and stability under visible light

Porous Cd-doped ZnO nanorods have been successfully synthesized on a large scale by a precursor thermal decomposition route. Rod-like Zn1−xCdxC2O4 precursors with 1 μm in average length were firstly fabricated via a solvothermal process. After calcining the Zn1−xCdxC2O4 precursors, the porous Cd-doped ZnO nanorods were obtained. The as-prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the as-prepared Cd-doped ZnO nanorods for degradation of organic compounds and the printing and dyeing sewage under the 365 nm of UV light illumination were investigated. The results show that Cd-doped ZnO nanorods have a higher photocatalytic activity than pure ZnO and P25. This enhanced photocatalytic activity is attributed to the Cd doping, which increases the defects in the ZnO nanoparticles and reduces the bandgap width of ZnO. Furthermore, the photodegradation rate does not show an obvious decrease during ten successive cycles, indicating that our Cd-doped ZnO nanorods are very stable.

Fabrication of porous Cd-doped ZnO nanorods with enhanced photocatalytic activity and stability

BaTiO3 is a large band gap semiconductor, which only absorbs UV light in the solar spectrum. In this study, a series of BaTiO3–graphene nanocomposites with different weight addition ratios of graphene oxide (GO) have been synthesized by a facile one-pot hydrothermal approach, during which the reduced graphene oxide and the intimate interfacial contact between BaTiO3 nanoparticles and the graphene nanosheets were obtained. The as-prepared samples were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared (FTIR) spectroscopy. The photocatalytic activity of the obtained BaTiO3–graphene composites for the degradation of methylene blue (MB) under visible light irradiation at λ ≥ 400 nm was investigated, and a higher photocatalytic activity than that of pure BaTiO3 was observed. According to the experimental results, we suggest a new photocatalytic mechanism, in which the role of graphene in the BaTiO3–graphene nanocomposites is to act as an organic dye-like photosensitizer for large band gap BaTiO3. The photosensitization process of BaTiO3 by graphene transforms the wide band gap BaTiO3 semiconductor into visible light photoactivity in dye degradation. This reaction mechanism could widen the applications of BaTiO3–graphene nanocomposites in solar energy conversion, promoting our thinking on the microscopic charge carrier transfer pathway connected to the interface between the graphene and the semiconductor.

Wan-Sheng Wang

Papers

Large Ultrathin Anatase TiO2 Nanosheets with Exposed {001} Facets on Graphene for Enhanced Visible Light Photocatalytic Activity

Facile Synthesis of the Novel Ag3VO4/AgBr/Ag Plasmonic Photocatalyst with Enhanced Photocatalytic Activity and Stability

Heterostructured Ag3PO4/AgBr/Ag plasmonic photocatalyst with enhanced photocatalytic activity and stability under visible light

Fabrication of porous Cd-doped ZnO nanorods with enhanced photocatalytic activity and stability

BaTiO3–graphene nanocomposites: synthesis and visible light photocatalytic activity