This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Heterogeneous photocatalyst materials for water splitting

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Jenny Schneider,*,† Masaya Matsuoka,‡ Masato Takeuchi,‡ Jinlong Zhang, Yu Horiuchi,‡ Masakazu Anpo,‡ and Detlef W. Bahnemann*,† †Institut fur Technische Chemie, Leibniz Universitaẗ Hannover, Callinstrasse 3, D-30167 Hannover, Germany ‡Faculty of Engineering, Osaka Prefecture University, 1 Gakuen-cho, Sakai Osaka 599-8531, Japan Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

Understanding TiO2 photocatalysis: mechanisms and materials.

Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental applications

Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

Nano‐photocatalytic Materials: Possibilities and Challenges

A fluoride mediated self-transformation method is proposed for the synthesis of hollow TiO2 microspheres (HTS) composed of anatase polyhedra with exposed ca. 20% {001} facets. Importantly, HTS exhibit tunable photocatalytic selectivity in decomposing azo dyes in water. The fluorinated HTS show preferential decomposition of methyl orange (MO) in comparison to methylene blue (MB). In contrast, the surface-modified HTS by either NaOH washing or calcinations at 600 °C favor decomposition of MB over MO. The surface chemistry and the surface structure at the atomic level are key factors in tuning the adsorption selectivity and, consequently, photocatalytic selectivity of HTS toward azo dyes.

Tunable photocatalytic selectivity of hollow TiO2 microspheres composed of anatase polyhedra with exposed {001} facets.

Control of the surface structure of inorganic materials, in particular titania (TiO2), by chemical processes under nonequilibrium conditions is of growing interest from scientific and utilitarian viewpoints. Titania is one of the most important materials because of its unique surface, electronic, and photocatalytic properties, which make this material applicable in many areas of science and technology ranging from adsorption, catalysis and photocatalysis to biomedicine, environmental monitoring and cleanup, energy conversion and storage, etc. Here we review the existing strategies for the synthesis of anatase TiO2 micro- and nanosheets with exposed high-energy {001} facets and for the assembly of these nanosheets into various hierarchical structures. The {001} facets are stabilized by specific capping agents (typically, fluoride), which are used to control the growth of titania crystals. The presence of high-energy facets in titania improves significantly its adsorption, electronic, and photocatalytic pro...

Anatase TiO2 with Dominant High-Energy {001} Facets: Synthesis, Properties, and Applications

Ag–TiO 2 multiphase nanocomposite thin films were prepared on quartz substrates by the liquid phase deposition (LPD) method from a mixed aqueous solution of ammonium hexafluouotitanate, silver nitrate and boric acid under ambient temperature and atmosphere followed by calcination at 500 °C for 1 h. The grain growth of anatase was depressed upon Ag + doping. However, silver ions not only promoted (or catalyzed) the formation of brookite phase but also reduced the phase transformation temperature of anatase to rutile. With increasing AgNO 3 concentration, the transmittance and band gap of the composite thin films decreased; however, the intensity of surface plasmon absorption (SPA) peaks increased and their peak position shifted to a longer wavelength range. When AgNO 3 concentration was higher than 0.03 M, the prepared samples consisted of anatase, brookite, rutile and metal silver nanocrystal particles, and their grain size ranges were 5–30 nm. The photocatalytic activity of the Ag–TiO 2 multiphase nanocrystal composite thin films prepared by this method exceeded that of pure TiO 2 thin films by a factor of more than 6.3 when AgNO 3 concentration was kept in the range of 0.03–0.05. This was attributed to the fact that there were many hetero-junctions, such as anatase/rutile, anatase/brookite, Ag/anatase, Ag/rutile and so on, existed in the Ag–TiO 2 multiphase nanocomposite films.

Fabrication and characterization of Ag-TiO2 multiphase nanocomposite thin films with enhanced photocatalytic activity

Mesoporous multiwalled carbon nanotubes/titanium dioxide (CNTs/TiO2) nanocomposites with low loading amounts (0–0.5 wt%) of CNTs embedded inside mesoporous TiO2 aggregates has been prepared by a simple one-pot hydrothermal method using titanium sulfate as titanium source. The as-prepared CNTs/TiO2 samples are carefully characterized, analyzed and discussed. In contrast to previous reports with high CNT loading, our results indicate that a low CNT loading slightly influences the textural properties (including crystallite size, degree of crystallinity, specific surface areas, and pore volume etc.) and UV-light absorption of the mesoporous TiO2 aggregates. The SEM and TEM results demonstrate that the CNTs are mostly embedded in the mesoporous TiO2 aggregates. Moreover, chemical bonds are formed at the interface between CNTs and TiO2, which is confirmed by the Raman, IR and XPS analyses. Significantly, we point out that PL analysis in terms of intensity of PL signals seems to not be a reliable way to monitor the recombination rate in the CNTs/TiO2 composite, due to the quenching effect of CNTs. Instead, the analysis of transient photocurrent responses is introduced, which definitely reflects CNTs as fast electron transfer channels in chemically-bonded CNTs/TiO2 composites with low CNT loading. Notably, the positive synergy effects of CNTs and TiO2 depend on both the CNT loading amount and the state of interfacial contacts. In our study, only these chemically bonded CNTs/TiO2 nanocomposites with appropriate loading amounts ( 0.1 wt%) results in a decrease in photocatalytic activity; a simple mechanical mixing of CNTs and TiO2 without forming chemical bonds at the interface also results in inferior photocatalytic performance.

Enhanced photocatalytic activity of mesoporous TiO2 aggregates by embedding carbon nanotubes as electron-transfer channel

CuO/Cu2O composite hollow microspheres with controlled diameter and composition were prepared without the addition of templates and additives by hydrothermal synthesis using Cu(CH3COO)2·H2O as a precursor. Increasing the precursor concentration from 0.02 to 0.2 M increased the diameter of the composite hollow microspheres from 500 nm to 5 μm. Moreover, the content of Cu2O in the composite hollow microspheres increased with increasing the reaction time or/and precursor concentration to produce a range of composite hollow microspheres with Cu2O contents from 20 to 80 wt %. A localized Ostwald ripening mechanism was proposed to account for the formation of CuO/Cu2O composite hollow microspheres. The photocatalytic activity experiment indicated that the prepared CuO/Cu2O composite hollow microspheres exhibited a higher photocatalytic activity for the photocatalytic decolorization of methyl orange aqueous solution under the visible-light illumination than the single phase CuO or Cu2O samples.

Shengwei Liu

Papers

Tunable photocatalytic selectivity of hollow TiO2 microspheres composed of anatase polyhedra with exposed {001} facets.

Anatase TiO2 with Dominant High-Energy {001} Facets: Synthesis, Properties, and Applications

Fabrication and characterization of Ag-TiO2 multiphase nanocomposite thin films with enhanced photocatalytic activity

Enhanced photocatalytic activity of mesoporous TiO2 aggregates by embedding carbon nanotubes as electron-transfer channel

Template-free hydrothermal synthesis of CuO/Cu2O composite hollow microspheres