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The non-magnetic and magnetic polymorphs of iron oxide (Fe2O3) namely: alpha-Fe2O3 (hematite) and gamma-Fe2O3 (maghemite) respectively, were synthesized by a facile surfactant aided hydrogel route. The synthesized polymorphs were characterized by diffuse reflectance, photoluminescence and raman spectroscopy for optical properties whereas the morphological, structural, chemical and electronic state evaluation were performed by FESEM, HRTEM, XRD, and XPS. The charge transport and the stability of the materials were examined electrochemically. The photocatalytic activity of the synthesized polymorphs was evaluated for the degradation of 2-chlorophenol and 2-nitrophenol in the exposure of the visible region and complete spectrum natural sunlight. Both the polymorphs exhibited a significantly high activity for the degradation of the phenolic substrate in the exposure of the complete spectrum of sunlight, however, the activity in the visible region of the sunlight was relatively lower. A substantial increase in the activity in the visible region was noticed when the polymorphs were exposed to complete spectrum sunlight prior to the photocatalytic experiments. The comparison of the exposed and unexposed samples revealed the induction of defects that served as traps for the excited electrons and increased activity of the polymorphs. (C) 2018 Elsevier B.V. All rights reserved.

The broad bandgap tin (IV) oxide (SnO 2 ) is the least investigated semiconductor material for photocatalytic water decontamination in sunlight exposure. A detailed study covering the synthesis, characterization and the evaluation of photocatalytic activity of SnO 2 , in the natural sunlight exposure, is presented. The structural characterization by XRD revealed the formation of phase pure tetragonal SnO 2 with the average crystallite size of ∼41.5 nm whereas minor Sn 2+ states in the material were identified by XPS analysis. As explored by diffuse reflectance (DR) and photoluminescence (PL) spectroscopy, the material exhibited a distinct absorption edge at ∼3.4 eV. The morphological and microstructure analysis of the synthesized SnO 2 was carried out by FESEM and HRTEM. The electrochemical impedance spectroscopy (EIS) and chronopotentiometry (CP) predicted the better charge transport and retention ability of the material under illumination whereas the Mott-Schottky extrapolation prophesied the n-type behavior with the flat-band potential of −0.60 V. The photocatalytic activity of SnO 2 was assessed in the exposure of complete spectrum natural sunlight for the removal of 2,4,6-trichlorophenol. The HPLC and TOC analysis monitored the progress of degradation and mineralization whereas the released chloride ions were evaluated by ion chromatography. The effect of the transition metal ions (Fe 3+ , Cu 2+ , Ni 2+, and Zn 2+ ) as electron capture agents and H 2 O 2 as ROS generator was explored during the degradation process. The utility of the material for the simultaneous removal of chlorophenols in the mixture was also investigated. The SnO 2 exhibited sustained activity in the repeated use. Based on experimental evidence congregated, the mechanism of the removal process and the efficacy of SnO 2 for sunlight photocatalytic decontamination of water was established.

The MoO3 coated ZnO photocatalysts were synthesized for the optimum harvesting of the absorbed ultraviolet sunlight photons by initially permeating Mo6+ ions at the surface of pre-synthesized ZnO and finally transformed to MoO3 by thermal treatment in the air. The absorption spectra of the synthesized powders revealed the extension of the absorption edge in the visible region whereas, the photoluminescence spectroscopy established the supporting role of the MoO3 coating in gradually plummeting the excitons recombination. The growth of additional peaks in Raman as well as X-ray photoelectron spectra and the appearance of the corresponding low-intensity reflection substantiated the surface prevalence of MoO3. The absence of the individual particles of MoO3 in FESEM and the verification of coated layer by HRTEM images validated the authenticity of the adopted synthetic route. The electrochemical evaluation of the synthesized powders under illumination revealed the complete elimination of photocorrosion and the synergic role of the MoO3 layer for improved trap and transfer of charge carriers. The evaluation of the flat-band potentials of the coated powders by Mott-Schottky analysis revealed the suitability of the conduction band edges for the generation of superoxide anion radicals. The photocatalytic activity of the synthesized powders was assessed for the removal of chloro derivatives (mono-, di-, trichloroacetic acids) in comparison to pure acetic acid. A significant effect of the stability, polarity and stereochemical structure of the substrate on the photocatalytic removal process was observed and discussed. The experimental evidences from the time-scale chemical analysis were interpreted for the identification of the reactive oxygen species (ROS) involved in the degradation/mineralization process. The validation of the Langmuir-Hinshelwood kinetic model was also examined. Efforts were made to estimate the plausible route of the degradation/mineralization process.

Owing to the promising photocatalytic performance, the sheet-like WO

The issues of low photocatalytic activity and dissolution of ZnO in natural sunlight exposure were addressed by implanting p-type Mn3O4 on its surface. The optical analysis revealed the composite nature and suppression of exciton excitons recombination whereas the cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and chronopotentiometry substantiated the elimination of photocorrosion respectively, low charge transfer resistance and better charge retention ability for the as-synthesized composites. The Mott-Schottky analysis by staircase potential impedance spectroscopy (SPIES) verified the p-n nature while the measurement of flat band potential (Vfb) predicted the suitability of the band potentials for the enhanced generation of reactive oxygen species (ROS). The structure and morphology of the Mn3O4-ZnO composites were evaluated by x-ray diffraction (XRD) and field emission electron microscopy (FESEM) whereas the fine structure analysis was performed by high-resolution transmission electron microscopy (HRTEM) analysis that revealed the discrete fringe patterns of p-type Mn3O4 and ZnO in composites. The x-ray photoelectron spectroscopy (XPS) verified the simultaneous existence of Mn2+ and Mn3+ in Mn3O4 nanoparticles. Compared to pure ZnO, due to the existence of charge transfer synergy between the p-type Mn3O4 and n-type ZnO, the composites with moderate Mn3O4 loadings exhibited marked activity for the removal of stable pollutants like 4-bromo and 4-chlorophenol in natural sunlight exposure. The correlation of the findings from various analytic tools of chemical analysis such as high-performance liquid chromatography (HPLC), ion chromatography (IC), total organic carbon (TOC) measurements and GC–MS analysis revealed the persuasive role of Cl and Br groups in regulating the degradation process. The composite catalyst exhibited excellent stability and reproducible activity in the repeated scans.

In an effort to enhance the photocatalytic activity of cubic WO3 in sunlight exposure, its surface was modified by impregnating the Ce3+ ions ranging from 1% to 25% with a step of 5% with respect to the weight of WO3. Compared to pure WO3, the optical analysis by diffuse reflectance spectroscopy (DRS) revealed better absorption cross-section and red shift in the band edges for Ce loaded catalysts. The decreased intensity of photoluminescence (PL) emissions and the suppression of the Raman active bands of WO3 verified the recombination quenching ability of Ce surface states. The XRD analysis revealed the existence of Ce3+ states in the lower loadings (≤5%), whereas the majority of Ce4+ states were noticed at higher loadings. The FESEM analysis also verified the formation of individual particles of Ce(III,IV) oxides at the surface of WO3 at higher loadings. The XPS analysis of 10% Ce loaded samples also revealed the presence of mixed oxides of Ce at higher loading. Except for 1% Ce loaded WO3, the estimation of charge–discharge capacity, in comparison to pure WO3, revealed the enhancement in the charge retention ability with the increasing Ce loading. In comparison to pure WO3, the synthesized catalysts exhibited superior activity for the removal of 2-nitrophenol and 2-chlorophenol substrates in natural sunlight exposure. The analysis of the degradation data revealed that in the lower concentration the surface oxygen bonded Ce3+ states serve as electron trapping and transfer centers, whereas with the increasing surface density the synergic composite mechanism is the dominating mode. The exaggerated estimation in the EDX analysis of the samples loaded with 15% and 20% Ce also revealed the major surface coverage by the oxides. The salient feature of the study was the evaluation of the photocatalytic activity with the minimal catalyst loading of 350 mg L−1.

The photocatalytic activity of magnetically retrievable ZnO@γ-Fe 2 O 3 , synthesized by coating γ-Fe 2 O 3 , with ZnO in 5:1 and 10:1 ratio, was assessed for the mineralization of chlorophenol derivatives (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-tri chlorophenol) and compared with that of pure ZnO in natural sunlight exposure. The diffuse reflectance spectroscopy revealed the composite nature of the synthesized catalysts with the existence of dual band edges representing ZnO and γ-Fe 2 O 3 . The decreased emission intensity in the photoluminescence (PL) spectra rendered the contributory role of γ-Fe 2 O 3 in suppressing the charge carrier annihilation. The growth of Raman active bands of ZnO was observed with increasing density of coated layers. The morphology and structural changes were examined by XRD and FESEM. The cyclic voltammetric (CV) and electrochemical impedance spectroscopy (EIS) measurements, in the dark and under illumination, verified the elimination of photocorrosion and better trapping of the excited states in the synthesized coated catalysts as compared to pure ZnO. A noticeable increase in the degradation/mineralization ability, in comparison to pure ZnO, was observable for the ZnO coated on γ-Fe 2 O 3 support. The progress of the photocatalytic degradation process was monitored by HPLC while the mineralization ability of the synthesized catalysts was estimated by TOC and measuring the release ions by ion chromatography (IC). Based on repeated precise weight measurements, a 99% and 85% magnetic retrieval of 5:1 (ZnO@γ-Fe 2 O 3 ) and 10:1 (ZnO@γ-Fe 2 O 3 ) catalysts was established. The synthesized catalysts exhibited excellent stability and reproducibility in the photocatalytic performance.

Despite knowing the fact that vanadium pentoxide is slightly soluble in aqueous medium, its photocatalytic activity was evaluated for the degradation of phenol and its derivatives (2-hydroxyphenol, 2-chlorophenol, 2-aminophenol and 2-nitrophenol) in natural sunlight exposure. The prime objective of the study was to differentiate between the homogeneous and heterogeneous photocatalysis incurred by dissolved and undissolved V2O5 in natural sunlight exposure. V2O5 was synthesized by chemical precipitation procedure using Triton X-100 as morphology mediator and characterized by DRS, PLS, Raman, FESEM and XRD. A lower solubility of ∼ 5% per 100ml of water at 23 °C was observed after calcination at 600 °C. The study revealed no contribution of the dissolved V2O5 in the photocatalytic process. In sunlight exposure, V2O5 powder exhibited substantial activity for the degradation, however, a low mineralization of phenolic substrates was observed. The initial low activity of V2O5 followed by a sharp increase both in degradation and mineralization in complete spectrum sunlight exposure, was further investigated that revealed the decrease in the bandgap and the reduction in the particle size with the interaction of UV photons (420 nm) as this effect was not observable in the exposure of visible region of sunlight. The role of the chemically different substituents attached to an aromatic ring at 2-positions and the secondary interaction of released ions during the degradation process with the reactive oxygen species (ROS) was also explored.

CuO@ZnO core-shell catalysts, coated by varying the CuO layer density ranging from 0.5% to 10%, were synthesized with the aim to enhance the photocatalytic activity of ZnO in sunlight and control its photocorrosion. Initially, the Cu(2+) ions were impregnated on presynthesized ZnO by wet impregnation and finally converted to CuO layers by calcination. The optical and structural characterization of the synthesized powders was performed by DRS, PL, Raman spectroscopy, and XRD analysis, respectively. The homogeneity of the coated layers was explored by FESEM. The photocatalytic activity of CuO coated ZnO was investigated for the degradation of mononitrophenols (2-, 3-, and 4-nitrophenol) and dinitrophenols (2,4-, 2,5-, and 2,6-dinitrophenol) in the exposure of the complete spectrum and visible region (420-800 nm) of sunlight. The effect of the increasing density coated layers of CuO on photocatalytic activity was evaluated for the degradation of 4-NP. Compared to pristine ZnO, a substantial increase in the degradation/mineralization ability was observable for the catalysts coated with 0.5% and 1% CuO, whereas a detrimental effect was noticed for higher coating density. Prior to photocatalytic studies, as evaluated by cyclic voltammetry (CV), compared to pure ZnO, a significant suppression of photocorrosion was noticed, under illumination, for catalysts coated with lower CuO coating. The progress of the photocatalytic degradation process was monitored by HPLC while the mineralization ability of the synthesized catalysts was estimated by TOC. The estimation of the released ions and their further interaction with the excited states and the reactive oxygen was monitored by ion chromatography (IC).