Ce3+-ion, Surface Oxygen Vacancy, and Visible Light-induced Photocatalytic Dye Degradation and Photocapacitive Performance of CeO2-Graphene Nanostructures

Selective oxidation of methane to C2+ products over Au-CeO2 by photon-phonon co-driven catalysis

Direct methane conversion to high-value chemicals under mild conditions is attractive yet challenging due to the inertness of methane and the high reactivity of valuable products. This work presents an efficient and selective strategy to achieve direct methane conversion through the oxidative coupling of methane over a visible-responsive Au-loaded CeO2 by photon-phonon co-driven catalysis. A record-high ethane yield of 755 μmol h-1 (15,100 μmol g-1 h-1) and selectivity of 93% are achieved under optimised reaction conditions, corresponding to an apparent quantum efficiency of 12% at 365 nm. Moreover, the high activity of the photocatalyst can be maintained for at least 120 h without noticeable decay. The pre-treatment of the catalyst at relatively high temperatures introduces oxygen vacancies, which improves oxygen adsorption and activation. Furthermore, Au, serving as a hole acceptor, facilitates charge separation, inhibits overoxidation and promotes the C-C coupling reaction. All these enhance photon efficiency and product yield.

Advancing Chlorophyll Photostability: Dual Physicochemical Protection via Ce-Doped Hydrotalcite Organic-Inorganic Hybrid Pigments

In pursuit of enhancing the photostability of chlorophyll, a novel organic-inorganic hybrid pigment has been synthesized via a supramolecular intercalation assembly method, incorporating cerium-ion-doped hydrotalcite as the host matrix and chlorophyll as the intercalated guest molecule. This innovative pigment amalgamates the vivid coloration properties of organic dyes with the robust stability characteristic of inorganic substances. Determined from the detailed investigation of the structural evolution of chlorophyll during photodegradation, the dual physicochemical protection mechanism is critical to the advancement of chlorophyll photostability. It leverages the oxygen barrier attributes of the hydrotalcite's laminate structure and the ultraviolet light absorption and scattering capabilities of CeO2 nanoparticles formed in situ. Furthermore, Ce-doping introduces a redox cycle between Ce4+ and Ce3+ ions, which serves as a chemical defense by neutralizing reactive oxygen species that emerge during chlorophyll degradation. This multifaceted approach results in a substantial enhancement of photostability, with the hybrid pigment containing 0.3 Ce doped content, demonstrating a mere 5.90% alteration in reflectance at the 635 nm peak after 250 h of UV-accelerated aging. This breakthrough provides a dual physicochemical protective strategy that not only significantly prolongs the lifespan of chlorophyll pigments but also holds potential for broadening their application scope in various industries, including plastics and coatings, where color fastness and durability are paramount.

Theoretical Analysis of Superior Photodegradation of Methylene Blue by Cerium Oxide/Reduced Graphene Oxide vs. Graphene

Density functional theory and a semi-empirical quantum chemical approach were used to evaluate the photocatalytic efficiency of ceria (CeO2) combined with reduced graphene oxide (rGO) and graphene (GP) for degrading methylene blue (MB). Two main aspects were examined: the adsorption ability of rGO and GP for MB, and the separation of photogenerated electrons and holes in CeO2/rGO and CeO2/GP. Our results, based on calculations of the adsorption energy, population analysis, bond strength index, and reduced density gradient, show favorable energetics for MB adsorption on both rGO and GP surfaces. The process is driven by weak, non-covalent interactions, with rGO showing better MB adsorption. A detailed analysis involving parameters like fractional occupation density, the centroid distance between molecular orbitals, and the Lewis acid index of the catalysts highlights the effective charge separation in CeO2/rGO compared to CeO2/GP. These findings are crucial for understanding photocatalytic degradation mechanisms of organic dyes and developing efficient photocatalysts.

Tuning Pd species via electronic metal-support interaction for methane combustion

Utilizing catalytic combustion to convert methane (CH4) into CO2 and H2O stands as one of the most effective approaches for mitigating unburnt CH4 emissions from natural gas engines. Supported Pd catalysts have been extensively researched for their role in low-temperature CH4 combustion, with their catalytic activity greatly influenced by metal-support interactions. Surface interaction Pd phases, as a special type of Pd species originating from metal-support interactions on supported Pd catalysts, show controversial catalytic performance in CH4 combustion. Moreover, the impact of electronic metal-support interactions (EMSI, which refers to metal-support interactions associated with electron transfer) remains unclear. Hence, we opted for Ce-Zr solid solutions with different Ce:Zr molar ratios as supports and synthesized a range of supported Pd catalysts with varying EMSI intensities. Characterization revealed that as the oxygen vacancy concentration on the support increased, electron transfer weakened, leading to a higher Pd-O-Ce content, resulting in a lower CH4 activation barrier and better catalytic performance. This study offers a promising approach for regulating EMSI and active Pd species on supported catalysts in practical applications.

Cobalt-Doped Ceria Sensitizer Effects on Metal Oxide Nanofibers: Heightened Surface Reactivity for High-Performing Chemiresistive Sensors

Chemiresistive gas sensors based on semiconducting metal oxides typically rely on noble metal catalysts to enhance their sensitivity and selectivity. However, noble metal catalysts have several drawbacks for practical utilization, including their high cost, their propensity for spontaneous agglomeration, and poisoning effects with certain types of gases. As such, in the interest of commercializing the chemiresistive gas sensor technology, we propose an alternative design for a noble-metal-free sensing material through the case study of Co-doped ceria (Co-CeO2) catalysts embedded in a SnO2 matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co-CeO2 catalyst nanoparticles integrated with SnO2 nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co-CeO2@SnO2 NFs exhibited superior gas sensing characteristics toward isoprene (C5H8) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co-CeO2 catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO2 sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C5H8 (compared to pristine SnO2), exceptionally high selectivity, and a low detection limit of 100 ppb. The sensor also exhibited high stability for prolonged response-recovery cycles, attesting to the strong anchoring of Co-CeO2 catalysts in the SnO2 matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co-CeO2, demonstrate strong potential to completely replace noble metal catalysts, thereby advancing the development of the commercially viable chemiresistive gas sensors free from noble metals, capable of detecting target gases at sub-ppm levels.

Two decades of ceria nanoparticle research: structure, properties and emerging applications

Cerium oxide nanoparticles (CeNPs) are versatile materials with unique and unusual properties that vary depending on their surface chemistry, size, shape, coating, oxidation states, crystallinity, dopant, and structural and surface defects. This review encompasses advances made over the past twenty years in the development of CeNPs and ceria-based nanostructures, the structural determinants affecting their activity, and translation of these distinct features into applications. The two oxidation states of nanosized CeNPs (Ce3+/Ce4+) coexisting at the nanoscale level facilitate the formation of oxygen vacancies and defect states, which confer extremely high reactivity and oxygen buffering capacity and the ability to act as catalysts for oxidation and reduction reactions. However, the method of synthesis, surface functionalization, surface coating and defects are important factors in determining their properties. This review highlights key properties of CeNPs, their synthesis, interactions, and reaction pathways and provides examples of emerging applications. Due to their unique properties, CeNPs have become quintessential candidates for catalysis, chemical mechanical planarization (CMP), sensing, biomedical applications, and environmental remediation, with tremendous potential to create novel products and translational innovations in a wide range of industries. This review highlights the timely relevance and the transformative potential of these materials in addressing societal challenges and driving technological advancements across these fields.

Insight into mechanism of excellent visible-light photocatalytic activity of CuO/MgO/ZnO nanocomposite for advanced solution of environmental remediation

Environmental remediation has sought several innovative ways for the treatment of wastewater and captivated researchers around the globe towards it. Through this study, we aim to proceed with the efforts to foster sustainable and feasible ways for the treatment of wastewater. In this work, we report the sol-gel synthesis of CuO/MgO/ZnO nanocomposite and carry out their systematic characterization with the help of state-of-the-art analytical techniques, such as FTIR, SEM, TEM, PL, XRD, Raman, and AFM. The SEM along with TEM and AFM provided useful insights into the surface morphology of the synthesized nanocomposite on both 2D and 3D surfaces and concluded the well-dispersed behavior of the nanocomposite. The characteristic functional groups responsible for carrying out the reaction of Cu-O, Mg-O, and Zn-O were identified by FTIR spectroscopy. On the other hand, crystal size, dislocation density, and microstrain of the nanocomposite were calculated by XRD. For optical studies, photoluminescence spectroscopy was performed. Once the characterization of the nanocomposite was done, they were eventually treated against the toxic organic dye, methylene blue. The calculated rate constant values of k for CuO was 2.48 × 10-3 min-1, for CuO/MgO (2.04 × 10-3 min-1), for CuO/ZnO (1.82 × 10-3 min-1) and CuO/MgO/ZnO was found to be 2.00 × 10-3 min-1. It has become increasingly evident that nanotechnology can be used in various facets of modern life, and its implementation in wastewater treatment has recently received much attention.

A neoteric antibacterial ceria-silver nanozyme for abiotic surfaces

Community-associated and hospital-acquired infections caused by bacteria continue to yield major global challenges to human health. Bacterial contamination on abiotic surfaces is largely spread via high-touch surfaces and contemporary standard disinfection practices show limited efficacy, resulting in unsatisfactory therapeutic outcomes. New strategies that offer non-specific and broad protection are urgently needed. Herein, we report our novel ceria-silver nanozyme engineered at a molar ratio of 5:1 and with a higher trivalent (Ce3+) surface fraction. Our results reveal potent levels of surface catalytic activity on both wet and dry surfaces, with rapid, and complete eradication of Pseudomonas aeruginosa, Staphylococcus aureus, and methicillin resistant S. aureus, in both planktonic and biofilm form. Preferential electrostatic adherence of anionic bacteria to the cationic nanozyme surface leads to a catastrophic loss in both aerobic and anaerobic respiration, DNA damage, osmodysregulation, and finally, programmed bacterial lysis. Our data reveal several unique mechanistic avenues of synergistic ceria-Ag efficacy. Ag potentially increases the presence of Ce3+ sites at the ceria-Ag interface, thereby facilitating the formation of harmful H2O2, followed by likely permeation across the cell wall. Further, a weakened Ag-induced Ce-O bond may drive electron transfer from the Ec band to O2, thereby further facilitating the selective reduction of O2 toward H2O2 formation. Ag destabilizes the surface adsorption of molecular H2O2, potentially leading to higher concentrations of free H2O2 adjacent to bacteria. To this end, our results show that H2O2 and/or NO/NO2-/NO3- are the key liberators of antibacterial activity, with a limited immediate role being offered by nanozyme-induced ROS including O2•- and OH•, and likely other light-activated radicals. A mini-pilot proof-of-concept study performed in a pediatric dental clinic setting confirms residual, and continual nanozyme antibacterial efficacy over a 28-day period. These findings open a new approach to alleviate infections caused by bacteria for use on high-touch hard surfaces.

Humic acid-nanoceria composite as a sustainable adsorbent for simultaneous removal of uranium(VI), chromium(VI), and fluoride ions from aqueous solutions

In this article, the multifunctional behavior of novel, efficient, and cost-effective humic acid-coated nanoceria (HA@CeO2 NPs) was utilized for the sorptive removal of U(VI), Cr(VI), and F- ions at different conditions. The production cost of HA@CeO2 was $19.28/kg and was well characterized by DLS, FESEM, HRTEM, FTIR, XRD, XPS, and TGA. Batch adsorption study for U(VI) (at pH ~ 8), Cr(VI) (at pH ~ 1), and F- (at pH ~ 2) revealed that the maximum percentage of sorption was > 80% for all the cases. From the contact time experiment, it was concluded that pseudo-second-order kinetics followed, and hence, the process should be a chemisorption. The adsorption study revealed that U(VI) and Cr(VI) followed the Freundlich isotherm, whereas F- followed the Langmuir isotherm. Maximum adsorption capacity for F- was 96 mg g-1. Experiments in real water suggest that adsorption is decreased in Kaljani River water (~ 12% for Cr(VI) and ~ 11% for F-) and Kochbihar Lake water (25.04% for Cr(VI) and 20.5% for F-) because of competing ion effect. Mechanism was well established by the kinetic study as well as XPS analysis. Because of high adsorption efficiency, HA@CeO2 NPs can be used for the removal of other harmful water contaminants to make healthy aquatic life as well as purified drinking water.

α-Glucosidase Inhibitory Activity and Cytotoxicity of CeO2 Nanoparticles Fabricated Using a Mixture of Different Cerium Precursors

A mixture of three distinct cerium precursors (Ce(NO3)3·6H2O, CeCl3·7H2O, and Ce(CH3COO)3·H2O) was used to prepare cerium oxide nanoparticles (CeO2 NPs) in a polyol-mediated synthesis. Different ratios of diethylene glycol (DEG) and H2O were utilized in the synthesis. The properties of the synthesized CeO2 NPs, such as structural and morphological properties, were investigated to observe the effect of the mixed cerium precursors. Crystallite sizes of 7-8 nm were obtained for all samples, and all synthesized samples were confirmed to be in the cubic phase. The average particle sizes of the spherical CeO2 were between 9 and 13 nm. The successful synthesis of CeO2 can also be confirmed via the vibrational band of Ce-O from the FTIR. Antidiabetic properties of the synthesized CeO2 NPs were investigated using α-glucosidase enzyme inhibition assay, and the concentration of the synthesized CeO2 NPs was varied in the study. The biocompatibility properties of the synthesized CeO2 NPs were investigated via cytotoxicity tests, and it was found that all synthesized materials showed no cytotoxic properties at lower concentrations (62.5-125 μg/mL).

Polarized Active Pairs at Grain Boundary Boost CO2 Chemical Fixation

The chemical fixation of CO2 as a C1 feedstock is considered one of the most promising ways to obtain long-chain chemicals, but its efficiency was limited by the ineffective activation of CO2. Herein, we propose a grain boundary engineering strategy to construct polarized active pairs with electron poor-rich character for effective CO2 activation. By taking CeO2 as a model system, we illustrate that the polarized "Ce4+-Ce3+-Ce4+" pairs at the grain boundary can simultaneously accept and donate electrons to coordinate with O and C, respectively, in CO2. By the combination of synchrotron radiation in situ technique and density functional theory calculations, the mechanism of the catalytic reaction has been systematically investigated. As a result, the CeO2 nanosheets with a rich grain boundary show a high DMC yield of 60.3 mmol/gcat with 100% atomic economy. This study provides a practical way for the chemical fixation of CO2 to high-value-added chemicals via grain boundary engineering.

Effects of NO3-, Cl-, and CH3COO- anions and diethylene glycol on the morphological, structural, antidiabetic, and cell viability properties of CeO2 nanoparticles

Cerium oxide (CeO₂) nanoparticles (NPs) were synthesized using a modified conventional polyol method. The ratio of diethylene glycol (DEG) and water in the synthesis was varied, and three different cerium precursor salts (Ce(NO₃)₃, CeCl₃, and Ce(CH₃COO)₃) were used. The structure, size, and morphology of the synthesized CeO₂ NPs were studied. An average crystallite size of 13 to 33 nm was obtained from the XRD analysis. Spherical and elongated morphologies of the synthesized CeO₂ NPs were acquired. Average particle sizes in the range of 16-36 nm were obtained by varying different ratios of DEG and water. The presence of DEG molecules on the surface of CeO₂ NPs was confirmed using FTIR. Synthesized CeO₂ NPs were used to study the antidiabetic and cell viability (cell cytotoxicity) properties. Antidiabetic studies were carried out using α-glucosidase enzymes inhibition activity. CeO₂ synthesized using Ce(NO₃)₃ and CeCl₃ precursors showed approximately 40.0% α-glucosidase enzyme inhibition activity, while CeO₂ synthesized using Ce(CH₃COO)₃ showed the lowest α-glucosidase enzyme inhibition activity. Cell viability properties of CeO₂ NPs were investigated using an in vitro cytotoxicity test. CeO₂ NPs prepared using Ce(NO₃)₃ and CeCl₃ were non-toxic at lower concentrations, while CeO₂ NPs prepared using Ce(CH₃COO)₃ were non-toxic at all concentrations. Therefore, polyol-mediated synthesized CeO₂ NPs showed quite good α-glucosidase inhibition activity and biocompatibility.

La-Doped CeO2 Quantum Dots: Novel Dye Degrader, Antibacterial Activity, and In Silico Molecular Docking Analysis

The current work demonstrates a novel synthesis of different concentrations of La-doped (2, 4, and 6 wt %) CeO₂ quantum dots (QDs) using a hydrothermal approach. This research aimed to examine the dye degradation efficiency, antibacterial activity, and in silico molecular docking analysis of La-doped CeO₂ QDs. The structure, elemental composition, optical properties, d-spacing, and morphological features of QDs were examined using various methods. XRD spectra exhibited the cubic structure of CeO₂, and the crystallinity was suppressed upon La doping. TEM revealed the formation of cubic-shaped QDs of CeO₂, and the incorporation of La decreased agglomeration. UV-vis absorption spectra showed a red shift upon La doping, assigned to a decrease in band gap energy. 6% La-doped CeO₂ showed significant antibacterial activity against Escherichia coli at higher concentrations in comparison to ciprofloxacin. La-CeO₂ was proposed as a putative inhibitor of β-lactamase _E. coli and DNA gyrase _E. coli relying on the outcomes of a molecular docking analysis that was in improved accord with in vitro bactericidal activity. Moreover, the prepared QDs exhibited a remarkable photocatalytic degradation of methylene blue in a basic medium.

Effect of Pd-Doping Concentrations on the Photocatalytic, Photoelectrochemical, and Photoantibacterial Properties of CeO2

Cerium oxide (CeO2) can exhibit good photocatalytic and photoantibacterial activities. However, its light-harvesting property is rather limited due to its large band gap. In order to boost these properties, doping with metal ions can improve light absorption and charge mobility. In this report, CeO2 and palladium−doped CeO2 (Pd−CeO2) NPs were synthesized via the microwave-assisted synthesis method. The structural, optical, and morphological studies of CeO2 and Pd−CeO2 NPs were carried out using various techniques. Mixed phases of CeO2/Ce2O3 were observed in pure CeO2 (S−CeO2) and Pd−CeO2. However, the Ce2O3 phase gradually disappeared upon doping with a higher percentage of Pd. Almost spherical particles were observed with average sizes between 6 and 13 nm. It was found that the incorporation of Pd reduced the particle size. Moreover, band gap energies of S−CeO2 and Pd−CeO2 NPs were reduced from 2.56 to 2.27 eV, and the PL intensities were also quenched with more Pd doping. The shifts in the conduction band and valence band were found to cause the reduction in the band gap energies of S−CeO2 and Pd−CeO2 NPs. In the case of photocatalytic degradation of methylene blue, photoelectrochemical, and photoantibacterial activities, Pd−CeO2 NPs showed enhanced activities under visible light irradiation. Therefore, Pd−CeO2 NPs have been shown to be a visible-light active material.

Enhanced photocatalytic crystal-violet degradation performances of sonochemically-synthesized AC-CeO2 nanocomposites

Semiconductor-based photocatalysis is one of the favorable techniques for the wastewater treatment. Herein, we synthesized the activated carbon-decorated cerium dioxide (AC-CeO₂) nanocomposites via the facile ultrasonication method by using the biomass-derived AC nanoflakes and the sonochemically-synthesized CeO₂ nanoparticles. The AC-CeO₂ nanocomposites exhibited the aggregated morphology with the AC nanoflakes-anchored CeO₂ nanoparticles. Since the hybridization of conductive AC and semiconductive CeO₂ would lead to the increased photocarrier transport and the reduced photocarrier recombination, during the photocatalytic reaction, the AC-CeO₂ nanocomposites showed the enhanced crystal violet dye-degradation efficiency up to 97.9 % within 135 min. The results suggest that the AC-CeO₂ nanocomposites hold promise as a prominent photocatalyst for future green environmental technology.

Role of Anions in the Synthesis and Crystal Growth of Selected Semiconductors

The ideal methods for the preparation of semiconductors should be reproducible and possess the ability to control the morphology of the particles with monodispersity yields. Apart from that, it is also crucial to synthesize a large quantity of desired materials with good control of size, shape, morphology, crystallinity, composition, and surface chemistry at a reasonably low production cost. Metal oxides and chalcogenides with various morphologies and crystal structures have been obtained using different anion metal precursors (and/or different sulfur sources for chalcogenides in particular) through typical synthesis methods. Generally, spherical particles are obtained as it is thermodynamically favorable. However, by changing the anion precursor salts, the morphology of a semiconductor is influenced. Therefore, precursors having different anions show some effects on the final forms of a semiconductor. This review compiled and discussed the effects of anions (NO₃⁻, Cl⁻, SO₄^2-, CH₃COO⁻, CH(CH₃)O⁻, etc.) and different sources of S^2- on the morphology and crystal structure of selected metal oxides and chalcogenides respectively.

Exploring the Influence of Synthesis Parameters on the Optical Properties for Various CeO2 NPs

Cerium oxide (CeO₂) nanoparticles were synthesized with a chemical precipitation method in different experimental conditions using cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) as a precursor, modifying the solution pH, the reaction time, and Co atoms as dopants, in order to tune the band gap energy values of the prepared samples. The physical characteristics of the synthesized ceria nanoparticles were evaluated by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis analyses and photoluminescence measurements. XRD data revealed a pure cubic fluorite structure of CeO₂ NPs, the estimation of crystallite sizes by Scherrer’s formula indicates the formation of crystals with dimensions between 11.24 and 21.65 nm. All samples contain nearly spherical CeO₂ nanoparticles, as well as cubic, rhomboidal, triangular, or polyhedral nanoparticles that can be identified by TEM images. The optical investigation of CeO₂ samples revealed that the band gap energy values are between 3.18 eV and 2.85 eV, and, after doping with Co atoms, the E_g of samples decreased to about 2.0 eV. In this study, we managed to obtain CeO₂ NPs with E_g under 3.0 eV by only modifying the synthesis parameters. In addition, by doping with Co ions, the band gap energy value was lowered to 2.0 eV. This aspect leads to promising results that provide an encouraging approach for future photocatalytic investigations.

Pulicaria undulata Extract-Mediated Eco-Friendly Preparation of TiO2 Nanoparticles for Photocatalytic Degradation of Methylene Blue and Methyl Orange

Eco-friendly approaches for the preparation of nanomaterials have recently attracted considerable attention of scientific community due to rising environmental distresses. The aim of the current study is to prepare titanium dioxide (TiO₂) nanoparticles (NPs) using an eco-friendly approach and investigate their performance for the photocatalytic degradation of hazardous organic dyes. For this, TiO₂ NPs were prepared by using the aqueous extract of the Pulicaria undulata (L.) plant in a single step at room temperature. Energy-dispersive X-ray spectroscopy established the presence of both titanium and oxygen in the sample. X-ray diffraction revealed the formation of crystalline, anatase-phase TiO₂ NPs. On the other hand, transmission election microscopy confirmed the formation of spherical shaped NPs. The presence of residual phytomolecules as capping/stabilization agents is confirmed by UV-vis analysis and Fourier-transform Infrared spectroscopy. Indeed, in the presence of P. undulata, the anatase phase of TiO₂ is stabilized at a significantly lower temperature (100 °C) without using any external stabilizing agent. The green synthesized TiO₂ NPs were used to investigate their potential for the photocatalytic degradation of hazardous organic dyes including methylene blue and methyl orange under UV-visible light irradiation. Due to the small size and high dispersion of NPs, almost complete degradation (∼95%) was achieved in a short period of time (between 1 and 2 h). No significant difference in the photocatalytic activity of the TiO₂ NPs was observed even after repeated use (three times) of the photocatalyst. Overall, the green synthesized TiO₂ NPs exhibited considerable potential for fast and eco-friendly removal of harmful organic dyes.

Synergistic effect of MoO2/CeO2 S-scheme heterojunction on carbon rods for enhanced photocatalytic hydrogen evolution

The separation efficiency of photogenerated carriers is a key factor affecting photocatalytic hydrogen evolution activity. However, loading precious metals is a cost problem, so this work introduces cheap carbon rods...

Ce-Doped W18O49 Nanowires for Tuning N2 Activation toward Direct Nitrate Photosynthesis

Nitrate acts as a fundamental raw material in modern industrial and agricultural fields. Recently, photocatalytic nitrogen oxidation into nitrate has been expected to be an alternative method to replace the industrial nitrate synthesis process, which encounters many challenges, i.e., huge energy consumption and greenhouse gas emission. We synthesized Ce-doped W₁₈O₄₉ nanowires (Ce-W₁₈O₄₉) to realize photocatalytic nitrogen oxidation into nitrate under mild conditions. The defect state generated by coupling of Ce³⁺ introduction and surface plasma state acts as an "electron trap" to restrain photogenerated electrons, so as to facilitate the separation of photogenerated electron-hole pairs and prolong their lifetime. W₁₈O₄₉ doped with 5 mol % Ce exhibited the highest yield of nitrate (319.97 μg g^-1 h^-1) without any sacrificial agent, which is about 5 times higher than that of pristine W₁₈O₄₉. This work provides new insight into achieving high-efficiency photocatalytic nitrate evolution activity from direct N₂ oxidation by controlling the energy band structure of photocatalysts.

Defect-rich CeO2 in a hollow carbon matrix engineered from a microporous organic platform: a hydroxide-assisted high performance pseudocapacitive material

A microporous organic polymer (MOP) was utilized for the engineering of nanoparticulate CeO₂ in a hollow carbon matrix (H-C/CeO₂). After CeO₂ nanoparticles were incorporated into a hollow MOP platform (H-MOP) through the decomposition of cerium acetate, successive carbonization produced H-C/CeO₂. The redox feature of defective CeO₂ in a conductive carbon matrix induced promising pseudocapacitive behavior. In particular, the H-C/CeO₂ showed excellent electrochemical performance in an alkaline electrolyte (KOH), due to the hydroxide ion-assisted redox behavior of defective CeO₂. H-C/CeO₂-3 with an optimized amount of CeO₂ showed specific capacitances of up to 527 (@0.5 A g^-1) and 493 F g^-1 (@1 A g^-1). Even at high current densities of 10 and 20 A g^-1, the H-C/CeO₂-3 maintained high capacitances of 458 and 440 F g^-1, respectively. After 10 000 cycling tests, the H-C/CeO₂-3 retained the 94-95% capacitance of the first cycle.

Hollow Dodecahedra Graphene Oxide- Cuprous Oxide Nanocomposites With Effective Photocatalytic and Bactericidal Activity

In this study, a kind of graphene oxide-cuprous oxide (GO-Cu₂O) nanocomposites was fabricated with different morphologies to serve as a photocatalytic material for the degradation of organic/inorganic dyes under visible light and the bactericidal effect against pathogenic bacteria. The GO-Cu₂O was prepared with solid cube and hollow dodecahedra morphologies through in-situ synthesis, and characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Raman, Ultraviolet and visible spectrophotometry (UV/vis), and Fourier transform infrared spectroscopy. In comparison with cubic GO-Cu₂O, the absorption and degradation efficiency of the GO-Cu₂O dodecahedra (GCD) composite in Methyl orange (MO), Rhodamine B (RhB), and phenol was higher owning to the more active sites for the simultaneous dye and light absorption of hollow structure. The antibacterial effect of the GO-Cu2O dodecahedra was examined by the flat colony counting method with an excellent bactericidal effect against pathogenic bacteria. The possible mechanism for the preparation of GCD possessing the enhancement of the visible-light photocatalytic and antibacterial efficiencies were also investigated.

Photoenhanced Water Electrolysis in Separate O2 and H2 Cells Using Pseudocapacitive Electrodes

Water electrolysis has received much attention in recent years as a means of sustainable H₂ production. However, many challenges remain in obtaining high-purity H₂ and making large-scale production cost-effective. This study provides a strategy for integrating a two-cell water electrolysis system with solar energy storage. In our proposed system, CuO-Cu(OH)₂/Cu₂O was used as a redox mediator between oxygen and hydrogen evolution components. The system not only overcame the gas-mixing issue but also showed high gas generation performance. The redox reaction (charge/discharge) of CuO-Cu(OH)₂/Cu₂O led to a significant increase (51%) in the initial rate of H₂ production from 111.7 μmol h^-1 cm^-2 in the dark to 168.9 μmol h^-1 cm^-2 under solar irradiation. The effects of light on the redox reaction of CuO-Cu(OH)₂/Cu₂O during water electrolysis were investigated by in situ X-ray absorption and photoemission spectroscopy. These results suggest that surface oxygen vacancies are created under irradiation and play an important role in increased capacitance and gas generation. These findings provide a new path to direct storage of abundant solar energy and low-cost sustainable hydrogen production.

Insights into the Mechanism of Methanol Steam Reforming Tandem Reaction over CeO2 Supported Single-Site Catalysts

We demonstrated how the special synergy between a noble metal single site and neighboring oxygen vacancies provides an "ensemble reaction pool" for high hydrogen generation efficiency and carbon dioxide (CO₂) selectivity of a tandem reaction: methanol steam reforming. Specifically, the hydrogen generation rate over single site Ru₁/CeO₂ catalyst is up to 9360 mol H₂ per mol Ru per hour (579 mL_H2 g_Ru^-1 s^-1) with 99.5% CO₂ selectivity. Reaction mechanism study showed that the integration of metal single site and O vacancies facilitated the tandem reaction, which consisted of methanol dehydrogenation, water dissociation, and the subsequent water gas shift (WGS) reaction. In addition, the strength of CO adsorption and the reaction activation energy difference between methanol dehydrogenation and WGS reaction play an important role in determining the activity and CO₂ selectivity. Our study paves the way for the further rational design of single site catalysts at the atomic scale. Furthermore, the development of such highly efficient and selective hydrogen evolution systems promises to deliver highly desirable economic and ecological benefits.

Unraveling the Origin of Photocatalytic Deactivation in CeO2/Nb2O5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species

The study provides deep insight into the origin of photocatalytic deactivation of Nb₂O₅ after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO₂/Nb₂O₅ heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb₂O₅ and CeO₂) and as-prepared CeO₂/Nb₂O₅ heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO₂/Nb₂O₅ heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb₂O₅ to CeO₂. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.

Improved Oxidative Stability by Embedded Cerium into Graphene Oxide Nanosheets for Proton Exchange Membrane Fuel Cell Application

Investigation of the collaborative effect of cerium particles embedded in graphene oxide to enhance the chemical stability of a proton exchange membrane fuel cell (PEMFC) has been carried out. Synthesis of composite membranes (Nafion-GO/Ce-x) with Nafion solution as a polymer is synthesized by a solution casting method where (x = concentration of composite). The developed hybrid material was characterized by FT-IR and X-ray diffraction (XRD) for its phase identification while the chemical structure was characterized by XPS analysis. The enhancement in the chemical stability of the incorporated hybrid material is characterized by Fenton’s test showing a radical scavenging effect. It was found that the residual weight for Nafion ²¹² was 92.50% after 24 h and it was 94.32% for Nafion-GO/Ce-2 and 96.49% for Nafion-GO/Ce-4, proving the suitability of composite membranes for fuel cell applications.

Chlorine-Resistant Hollow Nanosphere-Like VOx/CeO2 Catalysts for Highly Selective and Stable Destruction of 1,2-Dichloroethane: Byproduct Inhibition and Reaction Mechanism

Developing economical and robust catalysts for the highly selective and stable destruction of chlorinated volatile organic compounds (CVOCs) is a great challenge. Here, hollow nanosphere-like VOx/CeO2 catalysts with different V/Ce molar ratios were fabricated and adopted for the destruction of1,2–dichloroethane (1,2–DCE). The V0.05Ce catalyst possessed superior catalytic activity, reaction selectivity, and chlorine resistance owing to a large number of oxygen vacancies, excellent low-temperature redox ability, and chemically adsorbed oxygen (O− and O2−) species mobility. Typical chlorinated byproducts (CHCl3, CCl4, C2HCl3, and C2H3Cl3) derived from the cleavage of C–Cl and C–C bonds of 1,2–DCE were detected, which could be effectively inhibited by the abundant acid sites and the strong interactions of VOx species with CeO2. The presence of water vapor benefited the activation and deep destruction of 1,2–DCE over V0.05Ce owing to the efficient removal of Cl species from the catalyst surface.

Metalloporphyrin immobilized CeO2: in situ generation of active sites and synergistic promotion of photocatalytic water oxidation

CoTCPP transfer photoexcited electrons to CeO₂ by d–f electron coupling. The in situ generation of catalytically active sites: reduction on CeO₂ accompanied with the creation of oxygen vacancies and oxidation on CoTCPP that transforms into CoOOH.

A CeO2 Semiconductor as a Photocatalytic and Photoelectrocatalytic Material for the Remediation of Pollutants in Industrial Wastewater: A Review

The direct discharge of industrial wastewater into the environment results in serious contamination. Photocatalytic treatment with the application of sunlight and its enhancement by coupling with electrocatalytic degradation offers an inexpensive and green technology enabling the total removal of refractory pollutants such as surfactants, pharmaceuticals, pesticides, textile dyes, and heavy metals, from industrial wastewater. Among metal oxide—semiconductors, cerium dioxide (CeO2) is one of the photocatalysts most commonly applied in pollutant degradation. CeO2 exhibits promising photocatalytic activity. Nonetheless, the position of conduction bands (CB) and valence bands (VB) in CeO2 limits its application as an efficient photocatalyst utilizing solar energy. Its photocatalytic activity in wastewater treatment can be improved by various modification techniques, including changes in morphology, doping with metal cation dopants and non-metal dopants, coupling with other semiconductors, and combining it with carbon supporting materials. This paper presents a general overview of CeO2 application as a single or composite photocatalyst in the treatment of various pollutants. The photocatalytic characteristics of CeO2 and its composites are described. The main photocatalytic reactions with the participation of CeO2 under UV and VIS irradiation are presented. This review summarizes the existing knowledge, with a particular focus on the main experimental conditions employed in the photocatalytic and photoelectrocatalytic degradation of various pollutants with the application of CeO2 as a single and composite photocatalyst.

Minimizing electron-hole pair recombination through band-gap engineering in novel ZnO-CeO2-rGO ternary nanocomposite for photoelectrochemical and photocatalytic applications

A novel ZnO-CeO₂-rGO (ZCG) ternary nanocomposite with varying ZnO/CeO₂ weight proportions was synthesized by a hydrothermal process for photoelectrochemical water splitting and photocatalytic application. XRD diffraction peaks of ZCG nanocomposites displayed the patterns of ZnO and CeO₂ nanoparticles, and SEM revealed irregular flake-like particles, which were uniformly decorated on the rGO matrix. Increase in the intensity ratio of D and G bands from Raman spectra revealed changes in oxygen bonding in the ZnO-rGO (ZG) and ZCG nanocomposites. The shift in the band edge positions and the decrease in the band gap with increase in the cerium oxide content in ZCG composites were observed from UV-Vis and Mott-Schottky plots. XPS results showed that Ce³⁺ fraction increased with an increase in the cerium oxide content in ZCG nanocomposites. The ZCG3 (85:15) nanocomposite exhibited decreased electron-hole recombination rate as evidenced from the photoluminescence and electrochemical impedance spectroscopy Nyquist plots. The characteristic frequency in Bode's plot shifted to a lower frequency for the ZCG3 electrode demonstrating low interfacial charge transfer resistance, and ZCG3 photoelectrode displayed a higher photocurrent density of 0.69 mA/cm² at 1.5 V compared with other photoelectrode. The optimized and highly efficient ZCG3 nanocomposite exhibited improved photocatalytic degradation of methylene blue (MB) with a reaction rate constant of 0.0201 min^-1. Combination of defects in the form of Ce³⁺ ion and surface oxygen vacancies coupled with rGO as the electron acceptor improved the charge carrier density and carrier transport in addition to the formation Schottky-type junction and the presence of an internal electric field.

Cerium doped magnetite nanoparticles for highly sensitive detection of metronidazole via chemiluminescence assay

Cerium doped magnetite nanoparticle (CDM) was synthesized via a co-precipitation method and used as the co-reactant of luminol-K₃Fe(CN)₆ chemiluminescent system. The physical-chemical features of CDM were studied by XPS, XRD, HRTEM, FESEM, VSM, BET, and FTIR analyses. This simple and highly sensitive nanoprobe enabled the determination of minor concentrations of metronidazole (MNZ). Owing to the quenching efficacy of MNZ in the studied chemiluminescence system, a linear range of 3.47 × 10^-6-9.37 × 10^-5 mol/L was obtained with a limit of detection of 3.91 × 10^-7 mol/L. This biosensor was used for MNZ detection in human serum samples, which was highly efficient. The outcomes of this study give credit to the proposed biosensor to be applied for detection of MNZ in biological samples.

Solar Degradation of Sulfamethazine Using rGO/Bi Composite Photocatalysts

Heterogeneous photocatalysts for water decontamination were obtained by the optimized synthesis of bismuth-functionalized reduced graphene oxide (rGO/Bi) using the Hummer method and microwave treatment. Sulfamethazine (SMZ) was used as model pollutant to evaluate the photocatalytic efficacy. Photocatalysts were characterized by VP-SEM, HRTEM, XDR, XPS, RAMAN, and FTIR analyses, which confirmed the effective reduction of GO to rGO and the presence of bismuth as a crystalline phase of Bi2O3 polydispersed on the surface. Their performance was influenced by the rGO/Bi ratio, microwave temperature, and treatment time. The as-obtained 5%rGO/Bi composite had the highest photocatalytic activity for SMZ degradation under visible light irradiation (λ > 400 nm), achieving 100% degradation after only 2 h of treatment. The degradation yield decreased with higher percentages of rGO. Accordingly, the rGO/Bi catalysts efficiently removed SMZ, showing a high photocatalytic activity, and remained unchanged after three treatment cycles; furthermore, cytotoxicity tests demonstrated the nontoxicity of the aqueous medium after SMZ degradation. These findings support the potential value of these novel composites as photocatalysts to selectively remove pollutants in water treatment plants.

Blue/red light-triggered reversible color switching based on CeO2-x nanodots for constructing rewritable smart fabrics

Photoreversible color switching systems (PCSSs) have attracted increasing attention in various applications, but in most PCSSs the discoloration process usually relies on harmful UV light as a stimulus and the recoloration requires high temperature. To solve these problems, we have designed and prepared CeO2-x nanodots as novel photocatalytic components in PCSSs that respond to two kinds of visible light. CeO2-x nanodots are prepared by a solvothermal reaction with l-ascorbic acid as the reducing agent. CeO2-x nanodots with a size of ∼2 nm have a high concentration of oxygen vacancies, which confers a broadened photoabsorption with an edge at 500 nm, as well as a weak photoabsorption tail in the visible region (500-800 nm). To realize the color switching, both the CeO2-x/Dye/H2O solution and CeO2-x/dye/hydroxyethyl cellulose (HEC)-coated fabrics have been prepared. Under blue (450 nm) light irradiation, both the solution and fabric show a rapid discoloration in 30 s and 150 s, respectively, due to the efficient photocatalytic reduction of the redox dye by CeO2-x. Conversely, red (630 nm) light irradiation with air confers a rapid recoloration in 35 s for the solution and 200 s for the fabric, resulting from CeO2-x-mediated self-catalyzed oxidation. In particular, the required images and letters can be remotely printed on CeO2-x/Dye/HEC-coated T-shirts with a 450 nm laser pen, and then erased with 630 nm light, with high reversibility and stability. Therefore, the present CeO2-x/Dye/HEC PCSSs have great potential to construct rewritable smart fabrics for various applications.