With a rapidly growing population, development of new materials, techniques and devices which can provide safe potable water continues to be one of the major research emphases of the scientific community. While the development of new metal oxide catalysts is progressing, albeit at a slower pace, the concurrent and rapid development of high surface area catalyst supports such as graphene and its functionalised derivatives has provided unprecedented promise in the development of multifunctional catalysts. Recent works have shown that metal oxide/graphene composites can perform multiple roles including (but not limited to): photocatalysts, adsorbents and antimicrobial agents making them an effective agent against all major water pollutants including organic molecules, heavy metal ions and water borne pathogens, respectively. This article presents a comprehensive review on the application of metal oxide/graphene composites in water treatment and their role as photocatalyst, adsorbent and disinfectant in water remediation. Through this review, we discuss the current state of the art in metal oxide/graphene composites for water purification and also provide a comprehensive analysis of the nature of interaction of these composites with various types of pollutants which dictates their photocatalytic, adsorptive and antimicrobial activities. The review concludes with a summary on the role of graphene based materials in removal of pollutants from water and some proposed strategies for designing of highly efficient multifunctional metal oxide/graphene composites for water remediation. A brief perspective on the challenges and new directions in the area is also provided for researchers interested in designing advanced water treatment strategies using graphene based advanced materials.

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Role of graphene/metal oxide composites as photocatalysts, adsorbents and disinfectants in water treatment: a review

In view of the situation that environmental issues become more serious day by day, recent studies on practical applications of TiO 2 photocatalysis for environmental purification are reviewed. Although the fundamental aspects and the mechanisms of TiO 2 photocatalysis have recently become quite well understood, effective photocatalytic environmental purifier, especially water purifier, could not be developed to the stage of real industrial technology. The removal rate of gaseous or aqueous contaminants is influenced by numerous parameters; UV light intensity, substrate concentration, O 2 partial pressure, humidity, substrate type, and so on. Moreover, TiO 2 photocatalyst essentially has a difficulty in decomposition of large amount of contaminants or refractory chemicals. As the solutions of these problems, combination with other processes such as advanced oxidation processes and improvement of the design of photocatalytic environmental purification systems are described. During the past several years, the strategies for effective design of the system are well discussed and evaluated. The reactor design for air- or water-purification can be classified into two main strategies: (1) enlargement of reactive surface area and (2) improvement of mass transfer. Based on these insights, very recent achievements for development of photocatalytic environmental purification system with our contribution in each aspect and future research directions are reviewed.

Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification

A series of cerium ion-doped titanium dioxide (Ce 3+ –TiO2) catalysts with special 4 f electron configuration was prepared by a sol–gel process and characterized by Brunauer-Emmett-Teller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Ce 3+ –TiO2 catalysts was evaluated in the 2-mercaptobenzothiazole (MBT) degradation in aqueous suspension under UV or visible light illumination. The experimental results demonstrated that the overall photocatalytic activity of Ce 3+ –TiO2 catalysts in MBT degradation was signigicantly enhanced due to higher adsorption capacity and better separation of electron-hole pairs. The experimental results verified that both the adsorption equilibrium constant (Ka) and the saturated adsorption amount (Gmax) increased with the increase of cerium ion content. The results of XPS analysis showed that the Ti 3+ ,C e 3+ , and Ce 4+ ions reside in the Ce 3+ –TiO2 catalysts. The results of DRS analysis indicated that the Ce 3+ –TiO2 catalysts had significant optical absorption in the visible region between 400 and 500 nm because electrons could be excited from the valence band of TiO2 or ground state of cerium oxides to Ce 4 f level. In the meantime, the dependence of the electron-hole pair separation on cerium ion content was investigated by the PL analysis. It was found that the separation efficiency of electron-hole pairs increased with the increase of cerium ion content at first and then decreased when the cerium ion content exceeded its optimal value. It is proposed that the formation of two sub-energy levels (defect level and Ce 4 f level) in Ce 3+ –TiO2 might be a critical reason to eliminate the recombination of

for 2-mercaptobenzothiazole degradation in aqueous suspension for odour control

The properties of pristine, free-standing graphene monolayers prepared by mechanical exfoliation of graphite are investigated. The graphene monolayers, suspended over open trenches, are examined by means of spatially resolved Raman spectroscopy of the G-, D-, and 2D-phonon modes. The G-mode phonons exhibit reduced energies (1580 cm-1) and increased widths (14 cm-1) compared to the response of graphene monolayers supported on the SiO2 covered substrate. From analysis of the G-mode Raman spectra, we deduce that the free-standing graphene monolayers are essentially undoped, with an upper bound of 2x10^11 cm-2 for the residual carrier concentration. On the supported regions, significantly higher and spatially inhomogeneous doping is observed. The free-standing graphene monolayers show little local disorder, based on the very weak Raman D-mode response. The two-phonon 2D mode of the free-standing graphene monolayers is downshifted in frequency compared to that of the supported region of the samples and exhibits a narrowed, positively skewed line shape.

Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers

Titanium dioxide (TiO2 ) is one of the most widely investigated metal oxides because of its extraordinary surface, electronic, and photocatalytic properties. However, the large band gap of TiO2 and the considerable recombination of photogenerated electron-hole pairs limit its photocatalytic efficiency. Therefore, research attention is being increasingly directed towards engineering the surface structure of TiO2 on the atomic level (namely morphological control of {001} facets on the micro- and nanoscale) to fine-tune its physicochemical properties; this could ultimately lead to the optimization of selectivity and reactivity. This Review encompasses the fundamental principles to enhance the photocatalytic activity by using highly reactive {001}-faceted TiO2 -based composites. The current progress of such composites, with particular emphasis on the photodegradation of pollutants and photocatalytic water splitting for hydrogen generation, is also discussed. The progresses made are thoroughly examined for achieving remarkable photocatalytic performances, with additional insights with regard to charge transfer. Finally, a summary and some perspectives on the challenges and new research directions for future exploitation in this emerging frontier are provided, which hopefully would allow for harnessing the outstanding structural and electronic properties of {001} facets for various energy- and environmental-related applications.

Facet-Dependent Photocatalytic Properties of TiO2-Based Composites for Energy Conversion and Environmental Remediation

The recent progress using Raman spectroscopy and imaging of graphene is reviewed. The intensity of the G band increases with increased graphene layers, and the shape of 2D band evolves into four peaks of bilayer graphene from a single peak of monolayer graphene. The G band will blue shift and become narrow with both electron and hole doping, whereas the 2D band will blue shift with hole doping and red shift with electron doping. Frequencies of the G and 2D band will downshift with increasing temperature. Under compressed strain, the upshift of the G and 2D bands can be found. Moreover, the strong Raman signal of monolayer graphene is explained by interference enhancement effect. As for epitaxial graphene, Raman spectroscopy can be used to identify the superior and inferior carrier mobility. The edge chirality of graphene can be determined by using polarized Raman spectroscopy. All results mentioned here are closely relevant to the basic theory of graphene and application in nanodevices.

Raman Spectroscopic Characterization of Graphene

An easy method for the preparation of core–shell structural Fe3C@graphite-like carbon hollow spheres

High photoactive and visible-light responsive graphene/titanate nanotubes photocatalysts: Preparation and characterization

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Raman Spectroscopic Characterization of Graphene

An easy method for the preparation of core–shell structural Fe3C@graphite-like carbon hollow spheres

High photoactive and visible-light responsive graphene/titanate nanotubes photocatalysts: Preparation and characterization