Lagnamayee Mohapatra

A review on the recent progress, challenges and perspective of layered double hydroxides as promising photocatalysts

Considering the previous work on layered double hydroxides (LDHs) as novel photocatalysts, research on this group of materials has become one of the most exciting subjects of today. LDH has become an important class of layered materials having prospects in photocatalysis, wherein great attention has been paid to the exhaustive aerobic degradation of pollutants, photocatalytic water splitting, and CO2 photo-reduction. The unique structure, uniform distribution of different metal cations in the brucite layer, surface hydroxyl groups, flexible tunability, intercalated anions with interlayer spaces, swelling properties, oxo-bridged linkage, and high chemical stability are some of the important advantages of this group of materials. This article provides an up-to-date review on significant progress in the fabrication of LDH photocatalytic systems aiming at environmental clean-up and energy production, such as degradation of pollutants, photocatalytic H2 generation and photocatalytic CO2-reduction. This article, after discussing the recent significant progress in the synthesis of different photoactive LDH materials and photocatalytic applications through their structural and electronic properties, considers many typical examples. In particular, recent progress on the emerging strategies of LDH to improve their photocatalytic activity is also presented. Eventually, the future challenges and outlooks for this group of materials are also discussed.

Incorporation of Fe3+ into Mg/Al layered double hydroxide framework: effects on textural properties and photocatalytic activity for H2 generation

This present work highlights the successful preparation of the ternary series of (Mg/Al + Fe)-CO3 layered double hydroxides with a constant ratio of Mg/(Al + Fe) = 2 : 1 and their application for photocatalytic hydrogen generation from water. At Mg/(Al + Fe) = 2 : 1, the Al : Fe ratio was varied from 1 : 4 to lower the concentration of iron in the synthetic gel, in order to get samples with different amounts of iron in the brucite layers, and to find out the role of iron in photocatalytic activity. The presence of a hydrotalcite structure in the catalysts was clearly demonstrated from the powder X-ray diffraction (PXRD) pattern. The shifting of the diffraction plane d110 towards lower angles clearly indicated the amount of Fe3+ substitution in the brucite layer increases with increasing addendum (Fe3+) concentration up to a certain limit and thereafter shows a decreasing trend. Further characterizations like Fourier transform infrared (FTIR), thermogravimetry (TG) and differential thermal analysis (DTA) described the formation of amorphous Fe2O3 upon addition of a higher amount of Fe3+ than the optimum amount that can be accommodated in the brucite layer. Other characterizations like UV–Vis DRS, BET surface area, transmission electron microscopy (TEM), photoluminescence spectra (PL) and X-ray photoelectron spectral studies (XPS) were performed to detect the efficiency of the catalysts towards H2 evolution. Among all the prepared photocatalysts, LDH1(Mg/Al + Fe = 10 : 4 + 1), containing the highest amount of iron in the brucite layer, was found to be the most promising towards hydrogen evolution (301 μmol g−1 h−1) under visible light irradiation, which was attributed to the favourable surface structure and higher crystalline nature of hydrotalcites.

Design and development of a visible light harvesting Ni–Zn/Cr–CO32− LDH system for hydrogen evolution

In this work we have prepared a series of visible-light active Ni–Zn/Cr–CO32− LDHs with (Ni + Zn)/Cr ratio 2.0, while varying the Ni/Zn atomic ratio to 0 : 100, 25 : 75, 50 : 50, 75 : 25 and 100 : 0, and tested them for visible light photocatalytic hydrogen evolution. The photophysical and photocatalytic properties of the samples were evaluated thoroughly by PXRD, TEM and FESEM, UV-vis DRS, FTIR, PL and BET-Surface area. The PXRD measurement demonstrates a characteristic of hydrotalcite with long range order structure. The shifting of the d110 plane towards a lower angle clearly indicates that there is Ni2+ substitution in the brucite layer of Zn/Cr–CO32− LDH. Different diffuse reflectance spectra revealed that the absorption in the visible region is attributed to the metal-to-metal charge-transfer (MMCT) excitation of ZnII/NiII–O–CrIII to ZnI/NiI–O–CrIV. This oxo-bridged hetero-bimetallic assembly acts as a photo-induced centre for hydrogen evolution. The photocatalytic studies suggested that the Ni–Zn/Cr–CO32− LDH with Ni : Zn ratio of 75 : 25 exhibits the best photocatalytic activity under visible light radiation compared with the others. The detailed mechanism for the photocatalytic decomposition of water to hydrogen has also been discussed.

Characteristic Properties of Nanoclays and Characterization of Nanoparticulates and Nanocomposites

Clays have been one of the more important industrial minerals; and with the recent advent of nanotechnology, they have found multifarious applications and in each application, nanoclays help to improve the quality of product, economize on the cost and saves environment. The chapter describes key characteristics of nanoclays and their classification on the basis of the arrangement of “sheets” in their basic structural unit “layer”. Major groups include kaolin–serpentine, pyrophyllite-talc, smectite, vermiculite, mica and Chlorite. The structural, morphological and physicochemical properties of halloystite and montmorillonite nanoclays, representative of the 1:1 and 2:1 layer groups, respectively, are discussed as well. After briefly introducing the surface modification of clay minerals by modifying or functionalizing their surfaces and their incorporation into polymer matrices to develop polymer/clay nanocomposites, techniques that are being employed to characterize these nanoclays, in general, and the sample preparation for these techniques, in particular, are also reviewed in this chapter.

A review of solar and visible light active oxo-bridged materials for energy and environment

In efficient solar energy conversion for environmental and energy applications, the use of visible-light-active photocatalysis as a green and sustainable technology has become a promising technology. A visible-light-active photocatalyst (λ > 400 nm) for efficiently harvesting solar energy and addressing a recent primary concern has been designed and fabricated. An effective separation of photo-generated electron hole pairs and their rapid transport to the semiconductor surface is required for high-efficiency solar photocatalysis; therefore, significant efforts have been devoted to designing and developing hetero-binuclear oxo-bridged systems, owing to their unique physical and electronic properties, which make them promising candidates for photocatalysis. These materials exhibit excellent light-harvesting properties, due to metal-to-metal charge-transfer (MMCT) excitation, and possess band-gap energies that are more suitable for solar light absorption. Recently, hetero-binuclear oxo-bridged systems have facilitated the rapid progress in enhancing photocatalytic efficiency under visible light irradiation; therefore, this mini-review aims to provide a systematic study of heteronuclear oxo-bridged systems for environmental decontamination and artificial photosynthesis (water splitting and photocatalytic CO2 reduction). This review provides a few perspectives on the challenges hindering the further advancement of oxo-bridged materials for application in photocatalysis.