Mg-Fe-Al-O spinel: Preparation and application as a heterogeneous photo-Fenton catalyst for degrading Rhodamine B

It is an urgent need to develop new environmentally friendly spinel ferrites with high catalytic efficiency. In this work, a series of Mg-Fe-Al-O spinels with different ratios of Mg/Al were successfully synthesized by the reaction sintering method and were used as a heterogeneous photo-Fenton catalyst for degradation of Rhodamine B (RhB). The effect of different ratios of Mg/Al on the properties of Mg-Fe-Al-O spinel was characterized and analyzed through a range of advanced characterization techniques and DFT calculations. The influence factors on the photo-Fenton reaction catalyzed by Mg-Fe-Al-O spinels were systematically investigated. The results showed that the prepared Mg-Fe-Al-O spinels had larger lattice parameters, wider bandgap, and stronger magnetism, with the Mg content increased. Among them, Mg-9 (Mg_0.88Fe_1.88Al_0.23O₄) had the best catalytic performance in the photo-Fenton reaction. The degradation efficiency of RhB reached 98.45%, and the TOC removal efficiency was 83.47%. The elemental valence and PDOS of Mg-9 (Mg_0.88Fe_1.88Al_0.23O₄) spinels were closer to MgFe₂O₄. The photo-generated holes could directly oxidize water and hydroxyl to generate reactive oxygen species ·OH, improving the catalytic activity.

Synthesis of Template-Free Iron Oxyhydroxide Nanorods for Sunlight-Driven Photo-Fenton Catalysis

Designing a photocatalyst with high efficiency using semiconductor materials emerges as a promising approach for the treatment of wastewater. At the same time, it is very essential to develop nondestructive, green, and sustainable techniques for the degradation of refractory pollutants. Here, we have demonstrated a facile route to prepare iron oxyhydroxide nanorods (β-FeOOH) without employment of any templating agent via a light-driven solution chemistry pathway and explored the as-prepared nanorods as the photo-Fenton catalyst under solar light irradiation. The photocatalytic experiments were performed toward the degradation of the aqueous solution of two different pollutants, namely, methylene blue and rhodamine B dyes. We have illustrated the effect of pH of the solution together with the concentration of H₂O₂ during the degradation process and optimized the solution pH as well as the H₂O₂ concentration. The superb photocatalytic efficiency of β-FeOOH is attributed to the generation of reactive oxygen species in the presence of solar light, and these photo-produced reactive oxygen species assist the degradation process. The excellent photocatalytic efficacy and sustainability of β-FeOOH nanorods along with their effortless synthesis approach point to a cost-effective and environmentally benign pathway in fabricating a highly active photocatalyst for the degradation of organic dyes.

Photocatalytic degradation of organic dyes and phenol by iron-silicate glass prepared by the sol–gel method

A highly efficient photo-Fenton catalyst iron silicate glass was successfully prepared by a simple sol–gel method.

Toward Informed Design of Nanomaterials: A Mechanistic Analysis of Structure-Property-Function Relationships for Faceted Nanoscale Metal Oxides

Nanoscale metal oxides (NMOs) have found wide-scale applicability in a variety of environmental fields, particularly catalysis, gas sensing, and sorption. Facet engineering, or controlled exposure of a particular crystal plane, has been established as an advantageous approach to enabling enhanced functionality of NMOs. However, the underlying mechanisms that give rise to this improved performance are often not systematically examined, leading to an insufficient understanding of NMO facet reactivity. This critical review details the unique electronic and structural characteristics of commonly studied NMO facets and further correlates these characteristics to the principal mechanisms that govern performance in various catalytic, gas sensing, and contaminant removal applications. General trends of facet-dependent behavior are established for each of the NMO compositions, and selected case studies for extensions of facet-dependent behavior, such as mixed metals, mixed-metal oxides, and mixed facets, are discussed. Key conclusions about facet reactivity, confounding variables that tend to obfuscate them, and opportunities to deepen structure-property-function understanding are detailed to encourage rational, informed design of NMOs for the intended application.

Carbon Deposition on Hematite (α-Fe2O3) Nanocubes by Annealing in the Air: Morphology Study with Grazing Incidence Small Angle X-ray Scattering (GISAXS)

GISAXS has been used to study morphology change of α-Fe2O3 nanocubes after annealing processes. A submonolayer of the nanocubes was deposited on a Si(100) substrate. While an annealing at 400 °C in vacuum does not change a GISAXS pattern from as-prepared nanocubes submonolayer, subsequent annealing in air at the same temperature altered the GISAXS pattern significantly. SEM images showed that the air-annealed nanocubes were coated with thin layers which were identified as amorphous carbon layers based on Raman measurements. GISAXS simulations from morphologies of nanocube with 38 nm side-length and core-shell (nanocube-core and 7 nm thick carbon-shell) reproduced measured patterns from the vacuum- and the air-annealed nanocubes, respectively. The current study provides new approach for in-situ characterization of carbon deposition on a uniform shape nanoparticle through monitoring of deposited carbon thickness.

Identifying Iron-Bearing Nanoparticle Precursor for Thermal Transformation into the Highly Active Hematite Photo-Fenton Catalyst

The hematite photo-Fenton catalysis has attracted increasing attention because it offers strong oxidation of organic pollutants under visible light at neutral pH. In the present work, aqueous synthesis of hematite photo-Fenton catalysts with high activity is demonstrated. We compare photo-Fenton activity for hematite obtained by hydrolyzation at 60 °C or by a thermally induced transformation from iron-bearing nanoparticles, such as amorphous iron oxyhydroxide or goethite. A link between their structure and visible light photo-Fenton reactivity is established. The highest activity was observed for hematite obtained from goethite nanowires due to oblong platelet-like structure, high surface area and the presence of nanopores.

Fabrication and characterization of novel iodine doped hollow and mesoporous hematite (Fe2O3) particles derived from sol-gel method and their photocatalytic performances

In this work, iodine (I) doped hollow and mesoporous Fe₂O₃ photocatalyst particles were fabricated for the first time through sol-gel method. Phase structure, surface morphology, particle size, specific surface area and optical band gap of the synthesized Fe₂O₃ photocatalysts were analyzed by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS), BET surface analysis, particle size analyzer and UV-vis diffuse reflectance spectrum (UV-vis DRS), respectively. Also, electrochemical properties and photoluminescence spectra of Fe₂O₃ particles were measured. The results illustrated that high crystalline, hollow and mesoporous Fe₂O₃ particles were formed. The optical band gap values of the Fe₂O₃ photocatalysts changed between 2.104 and 1.93eV. Photocatalytic efficiency of Fe₂O₃ photocatalysts were assessed via MB solution. The photocatalytic activity results exhibited that I doping enhanced the photocatalytic efficiency. 1% mole iodine doped (I-2) Fe₂O₃ photocatalyst had 97.723% photodegradation rate and 8.638×10^-2min^-1 kinetic constant which showed the highest photocatalytic activity within 45min. Moreover, stability and reusability experiments of Fe₂O₃ photocatalysts were carried out. The Fe₂O₃ photocatalysts showed outstanding stability after four sequence tests. As a result, I doped Fe₂O₃ is a good candidate for photocatalysts.

Defect-induced enhanced photocatalytic activities of reduced α-Fe2O3 nanoblades

Bicrystalline α-Fe2O3 nanoblades (NBs) synthesized by thermal oxidation of iron foils were reduced in vacuum, to study the effect of reduction treatment on microstructural changes and photocatalytic properties. After the vacuum reduction, most bicrystalline α-Fe2O3 NBs transform into single-layered NBs, which contain more defects such as oxygen vacancies, perfect dislocations and dense pores. By comparing the photodegradation capability of non-reduced and reduced α-Fe2O3 NBs over model dye rhodamine B (RhB) in the presence of hydrogen peroxide, we find that vacuum-reduction induced microstructural defects can significantly enhance the photocatalytic efficiency. Even after 10 cycles, the reduced α-Fe2O3 NBs still show a very high photocatalytic activity. Our results demonstrate that defect engineering is a powerful tool to enhance the photocatalytic performance of nanomaterials.

Facet-Controlling Agents Free Synthesis of Hematite Crystals with High-Index Planes: Excellent Photodegradation Performance and Mechanism Insight

Hematite (α-Fe2O3) crystals with uniform size and structure are synthesized through very facile one-pot hydrothermal methods without any additive. The as-synthesized sub-micrometer-sized α-Fe2O3 crystals with small surface areas perform superb visible light photodegradation activities, even much better than most other α-Fe2O3 nanostructures with large surface areas. Profound mechanism analyses reveal that the microwave-assisted hydrothermal (Mic-H) synthesized α-Fe2O3 is enclosed by 12 high-index {2-15} facets. The structure and the low unoccupied molecular orbital (LUMO) of the high-index planes result in the excellent photocatalytic activity. This is the first report on the formation of {2-15} plane group of hematite, and the synthesis of the hematite particles with the {2-15} planes is very simple and no any facet-controlling agent is used. This study may pave the way to further performance enhancement and practical applications of the cheap hematite materials.

Morphology evolution of single-crystalline hematite nanocrystals: magnetically recoverable nanocatalysts for enhanced facet-driven photoredox activity

We have developed a new green chemical approach for the shape-controlled synthesis of single-crystalline hematite nanocrystals in aqueous medium. FESEM, HRTEM and SAED techniques were used to determine the morphology and crystallographic orientations of each nanocrystal and its exposed facets. PXRD and HRTEM techniques revealed that the nanocrystals are single crystalline in nature; twins and stacking faults were not detected in these nanocrystals. The structural, vibrational, and electronic spectra of these nanocrystals were highly dependent on their shape. Different shaped hematite nanocrystals with distinct crystallographic planes have been synthesized under similar reaction conditions, which can be desired as a model for the purpose of properties comparison with the nanocrystals prepared under different reaction conditions. Here we investigated the photocatalytic performance of these different shaped-nanocrystals for methyl orange degradation in the presence of white light (λ > 420 nm). In this study, we found that the density of surface Fe(3+) ions in particular facets was the key factor for the photocatalytic activity and was higher on the bitruncated-dodecahedron shape nanocrystals by coexposed {104}, {100} and {001} facets, attributing to higher catalytic activity. The catalytic activity of different exposed facet nanocrystals were as follows: {104} + {100} + {001} (bitruncated-dodecahedron) > {101} + {001} (bitruncated-octahedron) > {001} + {110} (nanorods) > {012} (nanocuboid) which provided the direct evidence of exposed facet-driven photocatalytic activity. The nanocrystals were easily recoverable using an external magnet and reused at least six times without significant loss of its catalytic activity.

Acetate-induced controlled-synthesis of hematite polyhedra enclosed by high-activity facets for enhanced photocatalytic performance

Uniform hematite polyhedra enclosed by high-activity facets could be selectively synthesized by acetates-induced synthetic strategy.

Enhanced visible light absorption and photocatalytic activity of [KNbO3]1−x[BaNi0.5Nb0.5O3−δ]x synthesized by sol–gel based Pechini method

[KNbO₃]_1−x[BaNi_0.5Nb_0.5O_3−δ]_x were synthesized by Pechini sol–gel method at low temperature. Visible light photocatalytic performance of this material was evaluated.

α-Fe2O3 nanopillar arrays fabricated by electron beam evaporation for the photoassisted degradation of dyes with H2O2

Vertically aligned α-Fe₂O₃ nanopillar arrays (NPAs) were fabricated by thermally oxidizing Fe NPAs on Si, quartz and F-doped SnO₂ (FTO) substrates prepared by glancing angle e-beam deposition (GLAD).

Etching treatment of vertical WO3 nanoplates as a photoanode for enhanced photoelectrochemical performance

An innovative etching method was developed to increase surface voids, active crystal facets and surface groups, which led to improved photocurrent performance.

Heterogeneous photo-Fenton reaction on hematite (α-Fe2O3){104}, {113} and {001} surface facets

The exposed surface facets play an important role in determining the catalytic performance of nanostructured materials. In this study, we report the synthesis of hematite nanoparticles with three varying morphologies with exposure of well-controlled {104}, {113} and {001} surfaces. The better shape control of hematite particles has provided a direct correlation between the surface facets and the photocatalytic performance. The catalytic photodegradation of MB using hematite nanoparticles reveals that the reaction follows the heterogeneous photo-Fenton process under visible light irradiation. The catalytic performance of hematite surface facets follows the order of {113} > {104} > {001}. Density functional theory (DFT) calculations were conducted to demonstrate the atomic surface structures and the corresponding charge distribution. The results indicate that the catalytic activity depends on surface atom arrangements as well as the number and the type of surface terminated hydroxyl groups bonding to underlying Fe atoms, where low valence states of Fe on {104} and {113} planes have the highest probability to be oxidized by H2O2 and the concurrently generated Fe((3+x)+) sites are more electronegative to accept electrons from activated dye molecules. The findings are of fundamental importance to understand the surface-dependence of photocatalytic properties, thus shedding new light on the catalytic application of hematite particles.

Band Gap Engineering of In2TiO5 for H2 Production under Near-infrared Light

In2TiO5, containing both early transition metal (d(0)) and p-block metal (d(10)), is a very promising candidate for possible application in H2 production because of its suitable edges of conduction and valence bands and the crystal structure, which is considered to favor mobility of charge carriers. Herein we report, for the first time, the synthesis of novel oxygen vacancies (OV), N-doped In2TiO5 (OV,N-In2TiO5) with controllable band gap. The resultant OV,N-In2TiO5 sample was prepared by a multistep sol-gel calcination process and studied as a near-infrared (NIR) light-driven photocatalyst for H2 production. OV and N-doping can effectively extend the photoresponse of In2TiO5 to the NIR region due to an interband springboard and the reduced band gap, thus leading to efficient NIR light photocatalytic H2 production activity with Pt as a cocatalyst.

Faceted metal and metal oxide nanoparticles: design, fabrication and catalysis

The review addresses new advances in metal, bimetallic, metal oxide, and composite particles in their nanoregime for facet-selective catalytic applications. The synthesis and growth mechanisms of the particles have been summarized in brief in this review with a view to develop critical examination of the faceted morphology of the particles for catalysis. The size, shape and composition of the particles have been found to be largely irrelevant in comparison to the nature of facets in catalysis. Thus selective high- and low-index facets have been found to selectively promote adsorption, which eventually leads to an effective catalytic reaction. As a consequence, a high density of atoms rest at the corners, steps, stages, kinks etc on the catalyst surface in order to host the adsorbate efficiently and catalyze the reaction. Again, surface atomic arrangement and bond length have been found to play a dominant role in adsorption, leading to effective catalysis.

Treatment of textile wastewater under visible LED lamps using CuO/ZnO nanoparticles immobilized on scoria rocks

ZnO and CuO thin films were synthesized by a sol–gel and direct crystallization method on granular porous natural scoria respectively and CuO/ZnO/scoria was used for water treatment under visible light irradiation.

Effect of oxidants on photoelectrocatalytic decolourization using α-Fe2O3/TiO2/activated charcoal plate nanocomposite under visible light

Effect of oxidants on the decolourization efficiency of Lanasol Yellow 4G in photoelectrocatalytic process using α-Fe₂O₃/TiO₂/ACP nanocomposite under visible light.