S. Wageh

Graphene in Photocatalysis: A Review

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.

TiO2 nanosheets with exposed {001} facets for photocatalytic applications

TiO2 nanosheets with highly reactive {001} facets ({001}-TiO2) have attracted great attention in the fields of science and technology because of their unique properties. In recent years, many efforts have been made to synthesize {001}-TiO2 and to explore their applications in photocatalysis. In this review, we summarize the recent progress in preparing {001}-TiO2 using different techniques such as hydrothermal, solvothermal, alcohothermal, chemical vapor deposition (CVD), and sol gel-based techniques. Furthermore, the enhanced efficiency of {001}-TiO2 by modification of carbon materials, surface deposition of transition metals, and non-metal doping is reviewed. Then, the applications of {001}-TiO2-based photocatalysts in the degradation of organic dyes, hydrogen evolution, carbon dioxide (CO2) reduction, bacterial disinfection, and dye-sensitized solar cells are summarized. We believe this entire review on TiO2 nanosheets with {001} facets can further inspire researchers in associated fields.

Nitrogen-doped TiO2 microsheets with enhanced visible light photocatalytic activity for CO2 reduction

Abstract Nitrogen-doped anatase TiO2 microsheets with 65% (001) and 35% (101) exposed faces were fabricated by the hydrothermal method using TiN as precursor in the presence of HF and HCl. The samples were characterized by scanning electron microscopy, X-ray diffraction, N2 adsorption, X-ray photoelectron spectroscopy, UV-visible spectroscopy, and electrochemical impedance spectroscopy. Their photocatalytic activity was evaluated using the photocatalytic reduction of CO2. The N-doped TiO2 sample exhibited a much higher visible light photocatalytic activity for CO2 reduction than its precursor TiN and commercial TiO2 (P25). This was due to the synergistic effect of the formation of surface heterojunctions on the TiO2 microsheet surface, enhanced visible light absorption by nitrogen-doping, and surface fluorination.

Flexible Mg–Al layered double hydroxide supported Pt on Al foil for use in room-temperature catalytic decomposition of formaldehyde

Room-temperature catalytic decomposition of formaldehyde (HCHO) is regarded as one of the best methods for indoor HCHO purification and removal. Herein, for the first time, a flexible and bendable Mg–Al layered double hydroxide (LDH) on aluminum (Al) foil was in situ prepared and further decorated with platinum (Pt) nanoparticles (NPs) for catalytic oxidation decomposition of HCHO at room temperature. Such a Pt-loaded large-area flexible LDH catalyst can be directly used and easily regenerated because of its flexibility and relatively low production cost. Moreover, it exhibited excellent catalytic performance toward oxidation decomposition of HCHO at room temperature due to its hierarchical macro-/mesoporous structure that facilitates the diffusion of reactants and products. Furthermore, abundant hydroxyl groups on the catalyst surface are also beneficial to HCHO oxidation decomposition. This work may provide some new insight into the design and fabrication of advanced catalysts for indoor HCHO removal and air purification.

Effect of solvent and environmental conditions on the structural and optical properties of CdS nanoparticles

Changing the solvent and environmental conditions during the preparation of CdS nanoparticles have a marked effect on the crystal structure, size distribution, nanoparticle size, absorption and emission properties. X-ray investigations revealed that the CdS nanoparticles prepared with high surface tension solvent have clearly identified hexagonal structure. However, the nanoparticles prepared with high dipole moment solvent matched a cubic structure better than a hexagonal one. Optical absorption measurements proved that CdS nanoparticles prepared in aprotic solvent showed an increase in both size of nanoparticles and size polydispersity index (SPI) with a decrease of the dipole moment, viscosity and surface tension of the solvent, which are applicable to the nanoparticles prepared under argon and ambient atmospheres. Fluorescence spectroscopy showed that the CdS nanoparticles synthesized in aprotic solvent with high dipole moment revealed a high intensity band edge emission blue band with a small half width. Also, the increase of the dipole moment of the solvent leads to a decrease in the Stokes shift for CdS nanoparticles prepared under an argon atmosphere.

Effect of adding reducing agent on the structure and optical properties of one-pot preparation method of CdTe quantum dots

One-pot and facile preparation of highly luminescent and water-soluble CdTe nanoparticles was reported. The CdTe nanoparticles were prepared using cadmium acetate as a source of cadmium, 3-mercaptopropionic acid as capping and stabilizing agent, potassium tellurite as a source of Te, and sodium borohydride as reducing agent. The effect of adding different molar percentages of sodium borohydride was investigated. The prepared nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM) with energy-dispersive X-ray, X-ray diffraction, UV–Vis absorption spectroscopy, and photoluminescence. The results indicated that the concentration of sodium borohydride had a significant effect on the phase structure, particle size, composition and luminescence of CdTe nanoparticles. With increasing sodium borohydride concentration, the Te/Cd atomic ratios of the prepared CdTe sample increased and the phase structure gradually changed from cubic and hexagonal mixed phase to pure hexagonal phase. Also, the luminescence intensity was greatly enhanced by decreasing sodium borohydride concentration. The prepared nanoparticles with low percentages of sodium borohydride are promising for light emitting device applications and biological labels. However, the samples prepared with high percentage of sodium borohydride might be possible to use as an effective material for applications in solar cells and photocatalysis.

Structural and optical properties of CdZnTe quantum dots capped with a bifunctional Molecule

CdTe and ternary CdZnTe quantum dots capped with 3-mercaptopropionic acid in aqueous medium with different percentages of Zn have been prepared using K2TeO3 as a source of tellurium. The structure and morphology of these quantum dots were characterized by XRD, TEM, EDX, FTIR and Raman spectroscopy. The results revealed that the ternary CdZnTe quantum dots (QDs) have nearly spherical shape with hexagonal structure. Doping with Zn leads to a pronounced change in relative intensity of the X-ray diffraction peaks. The optical properties were investigated by using UV–Vis absorption and photoluminescence measurements. The UV–Vis showed a gradual increase in the optical band gap of ternary alloy compound of CdZnTe QDs plus a continuous shift of photoluminescence emission with increasing of Zn content. The color distribution analysis showed a gradual change in color with Zn percentage for red, green and blue components. The nanoparticle sizes were determined from optical absorption, XRD, Raman and HRTEM measurements indicating that the prepared binary CdTe and ternary CdZnTe capped with MPA are in a strong confinement regime as an indicative of quantum dots structure.