Recent advances in rare-earth elements modification of inorganic semiconductor-based photocatalysts for efficient solar energy conversion: A review

Remediation of recalcitrant pollutants in water solution using visible light responsive cerium-doped tungsten trioxide nanoparticles

In order to attain a solar energy-driven photocatalyst for wastewater remediation, cerium-doped WO₃ (W_1-xCe_xO₃ with x = 0, 0.02, 0.04, 0.06, and 0.08) nanoparticles have been synthesized via a chemical co-precipitation technique. X-ray diffraction (XRD) analysis confirmed that W_1-xCe_xO₃ nanoparticles retained their monoclinic structure even after doping. The presence of the vast number of defects produced in the WO₃ lattice was corroborated by Raman spectroscopy. Scanning electron microscopy confirmed the spherical shape of the nanoparticles with particle size range 50-76 nm. The optical band gap of W_1-xCe_xO₃ nanoparticles decreases from 3.07 to 2.36 eV with an increase in x, as confirmed by UV-Vis spectroscopy. Photoluminescence (PL) spectroscopy confirmed that the minimum rate of recombination was observed for W_1-xCe_xO₃ with x = 0.04. Degradation efficiency was explored for methyl violet (MV) and rhodamine-B (Rh-B) with 0.01 g of photocatalyst in a photoreactor chamber having a 200-W xenon lamp as a visible source of light. The results showed that the maximum photo-decolorization towards MV (94%) and rhodamine-B (79.4%) was observed in x = 0.04 sample in just 90 min because of its least recombination rate, highest adsorption capacity, and optimum band edge positions. Intriguingly, it has been observed that the modification with cerium in WO₃ nanoparticles enhances the photocatalytic activity by narrowing the band gap and by efficaciously lowering the recombination rate due to electron entrapment by defects produced in the lattice.

Pollen Carbon-Based Rare-Earth Composite Material for Highly Efficient Photocatalytic Hydrogen Production from Ethanol-Water Mixtures

The unique electronic structure of rare-earth elements makes their modified semiconductor photocatalysts show great advantages in solar energy conversion. Herein, the pollen-like N, P self-doped biochar-based rare-earth composite catalyst (Er/LP-C) has been successfully synthesized, which combines the advantages of biochar and Er and is used for the first time in the field of photocatalytic hydrogen production from ethanol-water mixtures. Experimental results confirmed that the performance of photocatalytic hydrogen production under the full spectrum is up to 33.70 μmol/g in 6 h; this is due to the introduction of Er, which improves the carrier concentration, separation and transfer efficiency, and the driving force for the reduction reaction.

Abatement of NO/SO2/Hg0 from flue gas by advanced oxidation processes (AOPs): Tech-category, status quo and prospects

This review article provides a state-of-art insight into the removal of NO, SO₂ and elemental mercury (Hg⁰) from flue gas by using advanced oxidation processes (AOPs) method. Firstly, the main flue gas purification strategies based on AOPs would be classified as gas-gas, gas-liquid and gas-solid systems preliminarily, and the primary chemistry/mechanism of the above homogeneous/heterogeneous reaction systems were presented as the oxidation of NO, SO₂ and Hg⁰ by the oxidative free radicals (OH, O₂ and SO₄^-etc.). Secondly, the research progress and reaction pathways for separately or simultaneously removing NO, SO₂ and Hg⁰ from flue gas by AOPs has been reviewed elaborated and analyzed in more details. Notably, the wet/dry oxidation coupled with efficient absorption process would be a promising method of efficient removal of above gaseous pollutants. Subsequently, four types of assumed layout modes were described graphically. The application prospects of AOPs for the purification of flue gas from coal-fired boiler or industrial furnace were evaluated and found that the operation cost and utilization of oxidants must be reduced and improved respectively. Finally, the limitations in the current removal technologies based on AOPs are highlighted, meanwhile the future research directions are suggested, such as cut down the cost of oxidants and catalysts, improve the yield and valid utilization of highly reactive radicals and enhance the reactivity, resistance and stability of catalysts. Significantly, it is also envisaged that the review could enrich the knowledge repository to function as a scientific reference for the sustainable development of economical, effective and environment-friendly technologies for the abatement of a wide variety of emissions from flue gas, and further improve the feasibility and reliability of the strategies for moving from laboratory studies to large-scale development and industrial application.

Nanoscale design of 1D metal oxides derived from mixed Ni-MH battery/transition metal dust

Controllable recycling of End-of-life rechargeable nickel-metal hydride (Ni-MH) batteries and by-products of steelmaking to added-value functional nanostructures is desired but challenging. The present work introduces an innovative and high-yield microrecycling strategy to simultaneous synthesis of TM alloy (i.e., Ni-based superalloy) and RE oxide (REO) nanostructures from obsolete Ni-MH batteries mixed with zinc-rich electric arc furnace dust (EAFD). This strategy involves integration of high-temperature thermal isolation followed by thermal nanowiring techniques. The impure thermally-isolated REOs were purified and transformed into one dimensional (1D) nanorods of hybrid REOs. Besides, during high-temperature thermal isolation, defect-rich ZnO with tailored structures of nanorods and nanoribbons were fabricated using controllable vapour deposition. The electrochemical performance of ZnO nanoribbons for oxygen evolution reaction (OER) revealed a considerable overpotential reduction of 131 mV (18%) compared to pure commercial nano-ZnO. This approach is transformational in providing a scalable and cost-effective pathway to facilitate recycling of the challenging, yet critical, waste materials into functional nanostructures for energy and environmental applications.

Optically Active TiO2:Er Thin Films Deposited by Magnetron Sputtering

Titanium dioxide photoanodes for hydrogen generation suffer from a profound mismatch between the optical absorption of TiO₂ and the solar spectrum. To solve the problem of low solar-to-chemical efficiency, optically active materials are proposed. In this work, TiO₂ thin films containing erbium were deposited by radio frequency RF magnetron sputtering under ultrahigh vacuum conditions UHV. Morphology, structural, optical and electronic properties were studied. TiO₂:Er thin films are homogenous, with uniform distribution of Er ions and high transparency over the visible VIS range of the light spectrum. However, a profound 0.4 eV blue shift of the fundamental absorption edge with respect to undoped TiO₂ was observed, which can be attributed either to the size effect due to amorphization of TiO₂ host or to the onset of precipitation of Er₂Ti₂O₇ nanocrystals. Near-infrared NIR to VIS up-conversion is demonstrated upon excitation at 980 nm, while strong green photoluminescence at 525 and 550 nm occurs upon photon absorption at 488 nm.

M/TiO2 (M = Fe, Co, Ni, Cu, Zn) catalysts for photocatalytic hydrogen production under UV and visible light irradiation

Cu/TiO₂ photocatalysts can be considered a promising low-cost alternative to the well-known Pt/TiO₂ system for hydrogen production under UV-Vis irradiation.

A radially controlled ZnS interlayer on ultra-long ZnO-Gd2S3 core-shell nanorod arrays for promoting the visible photocatalytic degradation of antibiotics

Nanorod (NR) arrays offer commendable visible-light-driven photocatalytic performances. Herein, we describe the construction of a ternary ZnO-ZnS-Gd₂S₃ nanostructural array in which a sulfidation process is used to decorate a Gd₂S₃ shell layer with a ZnS interface over vapor-phase-grown vertically-aligned ZnO. With control over the shell-wall thickness, the shell layer of ∼25 nm wall thickness on the ultra-long ZnO NR arrays exhibited a higher catalytic efficiency close to 3.3, 2.0, 1.2, and 1.8 times those of the bare ZnO, the ZnO-ZnS, the Gd₂S₃-decorated (∼10 nm) and Gd₂S₃ shell-layered (∼40 nm) ZnO-ZnS core-shell structures, respectively. The core-shell geometry and the shell-wall thickness with maximized contact interface afforded increased light absorption in the visible region and effectively retarded the recombination rate of the photoinduced charge carriers by confining electrons and holes separately, thus providing advantages in terms of the degradation of the pharmaceutical residue tetracycline and the industrial pollutant 4-nitrophenol in wastewater.

Y2O3 decorated TiO2 nanoparticles: Enhanced UV attenuation and suppressed photocatalytic activity with promise for cosmetic and sunscreen applications

Nanoparticulate titanium dioxide (TiO₂) is widely used in cosmetic products and sunscreens. However, primarily due to their photocatalytic activity, some TiO₂ products have been shown to be cytotoxic. Thus, the aim of this study was to reduce the photoactivity and consequent cytotoxicity of TiO₂nanoparticles. As such, in this work, yttrium oxide (Y₂O₃) was deposited onto TiO₂, at 5% and 10% Y/Ti weight ratio, via a hydrothermal method. The nanocomposites produced, TiO₂@Y₂O₃ 5 and 10 wt%, were characterised to assess their physical, photochemical and toxicological properties. These materials exhibit a uniform yttria coating, enhanced UV attenuation in the 280-350 nm range and significantly reduced photoactivity compared with a pristine commercial TiO₂ sample (Degussa Aeroxide® P25). Furthermore, the comparative cytotoxicity and photo-cytotoxicity of these materials to a human keratinocyte cell line (HaCaT), was assessed using a colorimetric tetrazolium salt (MTS) assay. Following 24-hour incubation with cells, both Y₂O₃ loadings exhibited improved biocompatibility with HaCaT cells, compared to the pristine TiO₂ sample, under all subsequent test conditions. In conclusion, the results highlight the potential of these materials for use in products, applied topically, with sun protection in mind.

Photocatalytic removal of trichloroethylene from water with LaFeO3

Tricholorethylene (TCE) has been recognized as second common organic pollutant found in groundwater in Taiwan. Several advanced oxidation processes (AOPs) have been applied for TCE removal and photocatalytic is one of promising AOP techniques. In this study, LaFeO₃ is successfully synthesized via sol-gel method and investigated for its photocatalytic oxidation rate toward TCE in water. Experimental results indicate that 95% removal efficiency of TCE can be achieved in aqueous solution with LaFeO₃ (2 g/L) as photocatalyst within 1 h of Xenon lamp illumination. Additionally, the influences of initial TCE concentration, light intensity, photocatalyst loading, and pH value on the TCE removal efficiency are evaluated as well. The highest energy efficiency obtained in this study is 10.8 mg TCE/kWh and the value is higher than those reported in previous studies. Besides, removal mechanisms have been identified and the results indicate that the overall removal efficiency reaches 82%, with adsorption and photolysis accounting for 20% and 39%, respectively.

Development of Fe/Nb-based solar photocatalysts for water treatment: impact of different synthesis routes on materials properties

Semiconductors based on Fe/Nb oxides can present both solar sensitivity and high catalytic activity. However, there is still a lack regarding the comparison between different routes to produce Fe/Nb-based solar photocatalysts and the evaluation of the impact of the synthesis operating conditions on the material properties. In this work, Fe/Nb₂O₅ ratio, type of precipitating agent, presence/absence of washing stage, and temperature of calcination were verified to be the most relevant parameters in the synthesis by the co-precipitation method. These factors led to remarkable differences in the properties and performance of the photocatalysts produced by each distinct synthesis route. Composition, iron species present in the materials, crystallinity characteristics, and pH of the catalysts were affected, leading to different photocatalytic activities under UV-Vis light. Due to their characteristics, the synthesized materials are potential photocatalysts for application in solar processes. Graphical abstract ᅟ.