Non-Metal Sulfur Doping of Indium Hydroxide Nanocube for Selectively Photocatalytic Reduction of CO2 to CH4: A "One Stone Three Birds" Strategy

Photocatalytic CO2 reduction is considered as a promising strategy for CO2 utilization and producing renewable energy, however, it remains challenge in the improvement of photocatalytic performance for wide-band-gap photocatalyst with controllable product selectivity. Herein, the sulfur-doped In(OH)3 (In(OH)xSy-z) nanocubes are developed for selective photocatalytic reduction of CO2 to CH4 under simulated light irradiation. The CH4 yield of the optimal In(OH)xSy-1.0 can be enhanced up to 39 times and the CH4 selectivity can be regulated as high as 80.75% compared to that of pristine In(OH)3. The substitution of sulfur atoms for hydroxyl groups in In(OH)3 enhances the visible light absorption capability, and further improves the hydrophilicity behavior, which promotes the H2O dissociation into protons (H*) and accelerates the dynamic proton-feeding CO2 hydrogenation. In situ DRIFTs and DFT calculation confirm that the non-metal sulfur sites significantly weaken the over-potential of the H2O oxidation and prevent the formation of ·OH radicals, enabling the stabilization of *CHO intermediates and thus facilitating CH4 production. This work highlights the promotion effect of the non-metal doping engineering on wide-band-gap photocatalysts for tailoring the product selectivity in photocatalytic CO2 reduction.

Improved Charge Separation and CO2 Affinity of In2O3 by K Doping with Accompanying Oxygen Vacancies for Boosted CO2 Photoreduction

The CO2 photocatalytic conversion efficiency of the semiconductor photocatalyst is always inhibited by the sluggish charge transfer and undesirable CO2 affinity. In this work, we prepare a series of K-doped In2O3 catalysts with concomitant oxygen vacancies (OV) via a hydrothermal method, followed by a low-temperature sintering treatment. Owing to the synergistic effect of K doping and OV, the charge separation and CO2 affinity of In2O3 are synchronously promoted. Particularly, when P/P0 = 0.010, at room temperature, the CO2 adsorption capacity of the optimal K-doped In2O3 (KIO-3) is 2336 cm3·g-1, reaching about 6000 times higher than that of In2O3 (0.39 cm3·g-1). As a result, in the absence of a cocatalyst or sacrificial agent, KIO-3 exhibits a CO evolution rate of 3.97 μmol·g-1·h-1 in a gas-solid reaction system, which is 7.6 times that of pristine In2O3 (0.52 μmol·g-1·h-1). This study provides a novel approach to the design and development of efficient photocatalysts for CO2 conversion by element doping.

Transformation of 2D Flakes to 3D Hollow Bowls: Matthew Effect Enables Defects to Prevail in Electromagnetic Wave Absorption of Hollow rGO Bowls

High-efficiency electromagnetic (EM) wave (EMW)-absorbing materials have attracted extensive scientific and technical interest. Although identifying the dominant EM loss mechanism in dielectric-loss materials is indispensable, it is challenging due to a complex synergism between dipole/interfacial polarization and conduction loss. Modulation of defects and microstructures can be a possible approach to determine the dominant EM loss mechanism and realize high-efficiency absorption. Herein, 2D reduced graphene oxide (rGO) flakes are integrated into a 3D hollow bowl-like structure, which increases defect sites (i.e., oxygen vacancy and lattice defect) and reduces the stacked thickness of rGO. Despite their lower stacked thicknesses, the hollow rGO bowls with more defects exhibit lower conductivities but higher permittivities. Accompanied by the transformation from 2D flakes to 3D hollow bowls, the dominant EM loss mechanism of rGO transforms from conduction loss to defect-induced polarization. Furthermore, the defect engineering and structural design endow rGO with well-matched impedance and strong EMW-absorbing capacity. A minimum reflection loss of -41.6 dB (1.3 mm) and an effective absorption bandwidth of 4.8 GHz (1.5 mm) is achieved at a filler loading of 5 wt%. This study will provide meaningful insights into the development of materials with superior EMW-absorbing performances via defect engineering and structural design.

Zr, La-dual doped silver niobate for photocatalytic degradation of dyes under visible light irradiation

Sol-gel-assisted synthesis of silver niobate, 1%, 5%, and 10% Zr, La-dual doped silver niobates were carried out. Analysis done using XRD showed that increasing Zr and La dual doping caused the synthesized materials to adopt an AgNbO₃-like structure. This is also supported by FT-IR results. FESEM revealed that the silver niobate has a prism-like morphology while Zr, La-dual doped samples are irregular in shape. EDX mapping of the 10% Zr, La dual silver niobate confirmed the presence of Nb, Ag, Zr, and La metals. When compared with the silver niobate, the band gap energy of Zr, La-dual doped silver niobates are narrower, as shown by UV-Vis DRS measurements. It was revealed that dual doping of silver niobates with Zr and La has significantly improved the photocatalytic degradation of methylene blue (MB) and Rhodamine B (RhB) dyes. The 1% Zr, La-dual doped silver niobate showed the best photocatalytic results in terms of degrading MB while 10% Zr, La-dual doped silver niobate achieved the best performance when degrading RhB.

Superparamagnetic Iron Oxide Decorated Indium Hydroxide Nanocomposite: Synthesis, Characterization and Its Photocatalytic Activity

A simple and scalable liquid-based method was developed to produce a nanocomposite photocatalyst which was comprised of Fe3O4 nanoparticles (4-5 nm) decorated indium hydroxide nanorods (mean width 33 nm and average aspect ratio 2-3). The nanocomposite was produced at 25 ℃ in water via a hydroxide-induced co-precipitation ensued by a cathodic reduction during which the non-magnetic Fe(OH)3 intermediate was reduced to magnetic Fe3O4 at 20 V within 1 h. The incorporation of Fe3O4 nanoparticles served to bestow magnetic recoverability to the photocatalyst and helped enhance visible light absorption simultaneously. Interestingly, the addition of Fe3+ led to the formation of In(OH)3 nanorods rather than the commonly observed nanocubes. In comparison to the In(OH)3 system having a band gap of 4.60 eV), the band gap of the Fe3O4/In(OH)3 nanocomposite produced was determined to be 2.85 eV using the Tauc’s plot method. The effective reduction in band gap is expected to allow better absorption of visible light which in turns should help boost its photocatalytic performance. The Fe3O4/In(OH)3 nanocomposite was structurally characterized using a combination of PXRD, FESEM, EDS, and TEM and its paramagnetic property was proven with a positive mass susceptibility measured to be 1.30´10−5 cm3.g−1. Under visible light, a photocatalytic degradation efficiency of 83% was recorded within 1 hr for the nanocomposite using methylene blue as a dye. The photocatalytically-active Fe3O4/In(OH)3 should have good potential in visible-light driven waste water degradation once further optimized. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Thermally induced oxygen related defects in eco-friendly ZnFe2O4 nanoparticles for enhanced wastewater treatment efficiencies

Herein, a simple but highly effective strategy of thermal annealing to modulate oxygen vacancies related defects in ZnFe₂O₄ (ZFO) nanoparticles for obtaining enhanced wastewater treatment efficiencies is reported. The as-prepared nanoparticles were thermally annealed at three different temperatures (500 °C, 600 °C and 700 °C) and their phase purity was confirmed by X-ray diffraction (XRD). All samples were found to exhibit pure phases of ZFO with different crystallite sizes ranging from 10 nm to 25 nm. The transmission electron microscope (TEM) images showed well dispersed nanoparticles and a strong correlation of grain size growth with annealing temperature was established. The optical absorption and emission characteristics were estimated through UV-visible and Photoluminescence (PL) spectroscopy. Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) confirmed the variation of oxygen vacancies in the synthesized samples' lattice. The photocatalytic activities of all samples were investigated and the highest efficiencies were recorded for the ZFO samples annealed at 500 °C. Under high salinity condition, the organic dye degradation efficiency of the same sample remained the highest among all. The excellent dye degradation abilities in ZFO samples can be attributed to the abundance of oxygen vacancies in the crystal lattice that slow down the recombination rate during the photocatalysis process. Moreover, cytotoxicity tests revealed that all prepared ZFO samples showed insignificant cell structure effects on Picochlorum sp microalgae, as verified by Fourier-transform infrared (FTIR) spectroscopy. On the other hand, no significant changes were detected on the viable cell concentration and Chlorophyll a content. This work presents a systematic way to finely tune the crystal sizes and to modulate oxygen related defects in ZFO through a highly effective annealing approach to signify their potential in industrial wastewater and seawater treatment processes.

IrOx @In2 O3 Heterojunction from Individually Crystallized Oxides for Weak-Light-Promoted Electrocatalytic Water Oxidation

Multi-field coupling, especially photo-assisted electrocatalysis, has recently been studied to further improve the oxygen evolution reaction (OER). In this study, an n-type cubic In₂ O₃ semiconductor is employed for the first time to load IrO_x species (Ir-In₂ O₃ mass ratio: 17.6 %). Consequently, the IrO_x @In₂ O₃ heterojunction, which exhibits outstanding OER performance promoted by weak-light irradiation, is formed. Notably, IrO_x (approximately 1.7 nm in size) and In₂ O₃ are observed to crystallize independently during heterogeneous nucleation with no Ir atoms doped in the In₂ O₃ lattice. This avoids Ir loss and ensures the full exposure of all Ir-based sites. The IrO_x @In₂ O₃ heterojunction exhibits enhanced electrocatalytic water oxidation with overpotential values of 190 and 231 mV at current densities of 10 and 50 mA cm^-2 , surpassing all IrO_x -based catalyst results reported to date. Nano-sized IrO_x on the surface, irradiated by the weak-light beam of LED-365 (1.8 mW cm^-2 ), can be fully activated as an OER site. Moreover, the overpotential is further reduced to 176 and 210 mV to deliver the corresponding current. This work is anticipated to aid in the design of more efficient multi-field coupling OER systems.

Highly Enhanced OER Performance by Er-Doped Fe-MOF Nanoarray at Large Current Densities

Great expectations have been held for the electrochemical splitting of water for producing hydrogen as a significant carbon-neutral technology aimed at solving the global energy crisis and greenhouse gas issues. However, the oxygen evolution reaction (OER) process must be energetically catalyzed over a long period at high output, leading to challenges for efficient and stable processing of electrodes for practical purposes. Here, we first prepared Fe-MOF nanosheet arrays on nickel foam via rare-earth erbium doping (Er_0.4 Fe-MOF/NF) and applied them as OER electrocatalysts. The Er_0.4 Fe-MOF/NF exhibited wonderful OER performance and could yield a 100 mA cm⁻² current density at an overpotential of 248 mV with outstanding long-term electrochemical durability for at least 100 h. At large current densities of 500 and 1000 mA cm⁻², overpotentials of only 297 mV and 326 mV were achieved, respectively, revealing its potential in industrial applications. The enhancement was attributed to the synergistic effects of the Fe and Er sites, with Er playing a supporting role in the engineering of the electronic states of the Fe sites to endow them with enhanced OER activity. Such a strategy of engineering the OER activity of Fe-MOF via rare-earth ion doping paves a new avenue to design other MOF catalysts for industrial OER applications.

Defect Engineering of Photocatalysts towards Elevated CO2 Reduction Performance

Photocatalytic CO₂ reduction provides a promising solution to address the crises of massive CO₂ emissions and fossil energy shortages. As one of the most effective strategies to promote CO₂ photoconversion, defect engineering shows great potential in modulating the electronic structure and light absorption properties of photocatalysts while increasing surface active sites for CO₂ activation and conversion. This Review summarizes the recent progress in defect engineering of photocatalysts to promote CO₂ reduction performances from the following four aspects: 1) Approaches to defect (mainly vacancy and dopant) generation in photocatalysts; 2) defect structure characterization techniques; 3) physical and chemical properties of defect-engineered photocatalysts; 4) CO₂ reduction performance enhancements in activity, selectivity, and stability of photocatalysts by defect engineering. This Review is expected to present readers with a comprehensive view of progress in the field of photocatalytic CO₂ reduction through defect engineering for elevated CO₂ -to-fuels conversion efficiency.

Defect Induced Polarization Loss in Multi-Shelled Spinel Hollow Spheres for Electromagnetic Wave Absorption Application

Defect engineering is an effective approach to manipulate electromagnetic (EM) parameters and enhance absorption ability, but defect induced dielectric loss dominant mechanism has not been completely clarified. Here the defect induced dielectric loss dominant mechanism in virtue of multi-shelled spinel hollow sphere for the first time is demonstrated. The unique but identical morphology design as well as suitable composition modulation for serial spinels can exclude the disturbance of EM wave dissipation from dipolar/interfacial polarization and conduction loss. In temperature-regulated defect in NiCo2O4 serial materials, two kinds of defects, defect in spinel structure and oxygen vacancy are detected. Defect in spinel structure played more profound role on determining materials' EM wave dissipation than that of oxygen vacancy. When evaluated serial Co-based materials as absorbers, defect induced polarization loss is responsible for the superior absorption performance of NiCo2O4-based material due to its more defect sites in spinel structure. It is discovered that electron spin resonance test may be adopted as a novel approach to directly probe EM wave absorption capacities of materials. This work not only provides a strategy to prepare lightweight, efficient EM wave absorber but also illustrates the importance of defect engineering on regulation of materials' dielectric loss capacity.

Influence of sulfur amount in Ni-incorporated ZnIn2(O,S)4 on phase formation and the visible light photocatalytic hydrogen evolution reaction

In the present work, we propose Ni-doped ZnIn₂(O,S)₄/In(OH)₃ composite particles for visible-light photocatalytic HER. By adjusting the sulfur concentration while keeping the amounts of zinc, indium and nickel constant during the hydrothermal process.

Controlled Synthesis of Visible Light Active CuxS Photocatalyst: The Effect of Heat Treatment on Their Adsorption Capacity and Photoactivity

The effects of different precursor salts, stabilizing agents, and heat treatment parameters are already known to have an influence on the synthesis of nano-sized semiconductors in heterogenous photocatalysis. In the present work, CuxS materials were prepared by using different precursors (copper (II) chloride dihydrate or copper (II) acetate monohydrate) and shape tailoring/stabilizing agents, such as ethylenediaminetetraacetic acid/polyvinylpyrrolidone, and thiourea as the sulfur source. The polyvinylpyrrolidone (PVP) kinetically controlled the growth rate of the nanoplates, while ethylenediaminetetraacetic acid (EDTA) adjusted the nucleation process through the complexation of copper. A one-step hydrothermal method was used for the synthesis, and the materials were characterized by means of morphological and structural complementary investigation methods. Furthermore, the adsorption capacity and photocatalytic activity were also measured for these materials. It was found that the vacancy sites formed by changing the precursor salt, as confirmed by Raman measurements, affect the photocatalytic activity. The rise of the specific surface area was achieved by heat treatment, and concomitantly, the adsorption capacity of the treated samples was found to increase likewise.