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.

Sustainable hydrogen production is an essential prerequisite of a future hydrogen economy. Water electrolysis driven by renewable resource-derived electricity and direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting are promising pathways for sustainable hydrogen production. All these techniques require, among many things, highly active noble metal-free hydrogen evolution catalysts to make the water splitting process more energy-efficient and economical. In this review, we highlight the recent research efforts toward the synthesis of noble metal-free electrocatalysts, especially at the nanoscale, and their catalytic properties for the hydrogen evolution reaction (HER). We review several important kinds of heterogeneous non-precious metal electrocatalysts, including metal sulfides, metal selenides, metal carbides, metal nitrides, metal phosphides, and heteroatom-doped nanocarbons. In the discussion, emphasis is given to the synthetic methods of these HER electrocatalysts, the strategies of performance improvement, and the structure/composition-catalytic activity relationship. We also summarize some important examples showing that non-Pt HER electrocatalysts could serve as efficient cocatalysts for promoting direct solar-to-hydrogen conversion in both photochemical and photoelectrochemical water splitting systems, when combined with suitable semiconductor photocatalysts.

Noble metal-free hydrogen evolution catalysts for water splitting

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

A review on g-C3N4-based photocatalysts

This paper presents a critical review of novel achievements in the modification of N–TiO2 photocatalytic systems aimed at enhancing TiO2 applications in the areas of energy conversion and environmental clean-up. Herein we studied the synthesis, physical properties, as well as synergism of modified N-doped TiO2. Based on the studies reported in the literature, metal, nonmetal and metal oxide modified N–TiO2 are very effective systems to extend the activating spectra to the visible range. Therefore, modified N–TiO2 play an important role in the development of efficient photocatalysts for future perspectives.

Development of modified N doped TiO2 photocatalyst with metals, nonmetals and metal oxides

Cerium dioxide was prepared by the precipitation method and found to be an efficient photocatalyst to degrade azodyes under visible light irradiation. Nonbiodegradable azodyes acid orange 7 (AO7) was selected as modal target to examine the photocatalytic activity of CeO2. AO7 could be efficiently degraded in aqueous suspension of CeO2 under visible light illumination. The catalyst was characterized by X-ray diffraction (XRD), N2 sorption, transmission electron microscopic image (TEM) and UV/vis absorption spectrum techniques. AO7 solution was quickly decolorized and partly mineralized under visible light irradiation with existing CeO2. The photodegradation rate of this azodye catalyzed by CeO2 is much faster than those occurring on commercial titania (Degussa P25) under otherwise identical conditions of visible light irradiation. Experiments were conducted to examine the adsorption mode of acid orange 7 on CeO2 and adsorption capacity at different pH values. The possible degradation pathway has been proposed for the photocatalytic degradations by using certain radical scavengers and gas chromatography–mass spectrometry (GC–MS) to determine intermediates. The enhanced photoactivity of the lanthanide oxide CeO2 was attributed to the superior adsorption capacity and special 4f electron configuration.

Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation

Self-doped Ti3+-enhanced TiO2 nanoparticles with a high-performance photocatalysis

Nitrogen-doped TiO2 nanocatalysts were successfully synthesized by adjusting a pH range using the ammonium nitrate and ammonia water as the nitrogen source. The samples were characterized by XRD, XPS and UV-DRS. When the total amount of ammonium nitrate and ammonia water was unchanged, different pH values were modified by changing the NH4NO3/NH3·H2O ratio to prepare nitrogen-doped TiO2. The prepared photocatalyst showed the highest photo-activity for the degradation of 2,4-dichlorophenol (2,4-DCP) under visible light when prepared at pH 5.87. XPS analysis showed the presence of nitrogen in two states doped in TiO2. The results indicated the photocatalytic activity of N-TiO2 is varied with the change of pH values, the amount of the nitrogen sources and water. The experimental results showed that the higher activity is due to the variation in the concentration and states of nitrogen-doped in TiO2. In the preparation methods, the photocatalyst was treated with the hydrogen peroxide before calcination, resulting in the decrease of nitrogen doped into the lattice and the photo-degradation rate of 2,4-DCP. The results suggested that the nitrogen source could be doped into the crystal lattice only in the form of reduction state as NH4+ ion during the calcination process.

New approaches to prepare nitrogen-doped TiO2 photocatalysts and study on their photocatalytic activities in visible light

Ordered nanocrystalline mesoporous cerium oxide was successfully synthesized using MCM-48 molecular sieves as a hard template. The long-range ordered mesostructure was characterized by low-angle and wide-angle X-ray diffraction (XRD), N2 adsorption−desorption technique, and transmission electron microscopy (TEM), and the obtained cerium oxide exhibited high similarity to the cubic Ia3d symmetry of the silica template. Due to smaller crystal size, the mesoporous materials have a blue shift in light absorption as compared with nonporous materials. X-ray photoelectron spectrum (XPS) and energy-dispersive X-ray spectrum (EDS) patterns confirm that mesoporous materials possess more surface vacancies than bulk CeO2. Photocatalytic degradation of azodye acid orange 7 under visible light illumination was used as a catalytic test reaction. The activity of mesoporous product in dye decolorization is substantially higher than those for a nonporous sample and TiO2 P25.

Jinlong Zhang

Papers

Development of modified N doped TiO2 photocatalyst with metals, nonmetals and metal oxides

Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation

Self-doped Ti3+-enhanced TiO2 nanoparticles with a high-performance photocatalysis

New approaches to prepare nitrogen-doped TiO2 photocatalysts and study on their photocatalytic activities in visible light

Ordered Mesoporous CeO2 Synthesized by Nanocasting from Cubic Ia3d Mesoporous MCM-48 Silica: Formation, Characterization and Photocatalytic Activity