Surface engineering on CeO₂ nanorods by chemical redox etching and their enhanced catalytic activity for CO oxidation

Highly Dispersed Mn-Doped Ceria Supported on N-Doped Carbon Nanotubes for Enhanced Oxygen Reduction Reaction

The weak adsorption of oxygen on transition metal oxide catalysts limits the improvement of their electrocatalytic oxygen reduction reaction (ORR) performance. Herein, a dopamine-assisted method is developed to prepare Mn-doped ceria supported on nitrogen-doped carbon nanotubes (Mn-Ce-NCNTs). The morphology, dispersion of Mn-doped ceria, composition, and oxygen vacancies of the as-prepared catalysts were analyzed using various technologies. The results show that Mn-doped ceria was formed and highly dispersed on NCNTs, on which oxygen vacancies are abundant. The as-prepared Mn-Ce-NCNTs exhibit a high ORR performance, on which the average electron transfer number is 3.86 and the current density is 24.4% higher than that of commercial 20 wt % Pt/C. The peak power density of Mn-Ce-NCNTs is 68.1 mW cm-2 at the current density of 138.9 mA cm-2 for a Zn-air battery, which is close to that of 20 wt % Pt/C (69.4 mW cm-2 at 106.1 mA cm-2). Density functional theory (DFT) calculations show that the oxygen vacancy formation energies of Mn-doped CeO2(111) and pure CeO2(111) are -0.55 and 2.14 eV, respectively. Meanwhile, compared with undoped CeO2(111) (-0.02 eV), Mn-doped CeO2(111) easily adsorbs oxygen with the oxygen adsorption energy of only -0.68 eV. This work provides insights into the synergetic effect of Mn-doped ceria for facilitating oxygen adsorption and enhancing ORR performance.

Rare earth oxide based electrocatalysts: synthesis, properties and applications

As an important strategic resource, rare earths (REs) constitute 17 elements in the periodic table, namely 15 lanthanides (Ln) (La-Lu, atomic numbers from 57 to 71), scandium (Sc, atomic number 21) and yttrium (Y, atomic number 39). In the field of catalysis, the localization and incomplete filling of 4f electrons endow REs with unique physical and chemical properties, including rich electronic energy level structures, variable coordination numbers, etc., making them have great potential in electrocatalysis. Among various RE catalytic materials, rare earth oxide (REO)-based electrocatalysts exhibit excellent performances in electrocatalytic reactions due to their simple preparation process and strong structural variability. At the same time, the electronic orbital structure of REs exhibits excellent electron transfer ability, which can reduce the band gap and energy barrier values of rate-determining steps, further accelerating the electron transfer in the electrocatalytic reaction process; however, there is a lack of systematic review of recent advances in REO-based electrocatalysis. This review systematically summarizes the synthesis, properties and applications of REO-based nanocatalysts and discusses their applications in electrocatalysis in detail. It includes the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), methanol oxidation reaction (MOR), nitrogen reduction reaction (NRR) and other electrocatalytic reactions and further discusses the catalytic mechanism of REs in the above reactions. This review provides a timely and comprehensive summary of the current progress in the application of RE-based nanomaterials in electrocatalytic reactions and provides reasonable prospects for future electrocatalytic applications of REO-based materials.

Effect of Surface Treatment of Nanocrystalline CeO2 on Its Dephosphorylation Activity and Adsorption of Inorganic Phosphates

The surface of nanocrystalline cerium oxide (CeO2) was treated with various chemical agents by a simple postmodification method at 25 °C and atmospheric pressure. Hydrogen peroxide, ammonium persulfate, deionized water, ascorbic acid, and ortho-phosphoric acid were used in order to study and evaluate their effect on surface materials, such as surface area, crystallite size, number of surface hydroxyl groups, particle morphology, and Ce3+/Ce4+ ratio. Paraoxon-methyl (PO) decomposition and inorganic phosphate adsorption were used to evaluate the effect of surface treatment on catalytic and adsorption properties. CeO2 surface was studied by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and acid-base titration. While the treatment procedure affected the number of surface hydroxyl groups and the amount of bulk surface oxygen vacancies, only negligible changes were observed in the Ce3+/Ce4+ ratio. Interestingly, surface treatment affected the ability to decompose PO, but only a small effect on inorganic phosphate adsorption was observed, indicating the robustness of CeO2 for the latter. A mechanism for possible interaction of the used chemicals with the CeO2 surface was proposed.

Catalysts Prepared from Atomically Dispersed Ce(III) on MgO Rival Bulk Ceria for CO Oxidation

Atomically dispersed cerium catalysts on an inert, crystalline MgO powder support were prepared by using both Ce(III) and Ce(IV) precursors. The materials were used as catalysts for CO oxidation in a once-through flow reactor and characterized by atomic-resolution scanning transmission electron microscopy, X-ray absorption near-edge structure spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction, among other techniques, before and after catalysis. The most active catalysts, formed from the precursor incorporating Ce(III), displayed performance similar to that reported for bulk ceria under comparable conditions. The catalyst provided stable time-on-stream performance for as long as it was kept on-stream, 2 days, increasing slightly in activity as the atomically dispersed cerium ions were transformed into ceria nanodomains represented as CeOx and having increased reducibility on the MgO support. The results suggest how highly dispersed supported ceria catalysts with low cerium loadings can be prepared and may pave the way for improved efficiencies of cerium utilization in oxidation catalysis.

Co/CeO2/C composites derived from bimetallic metal-organic frameworks for efficient microwave absorption

CeO2, an n-type semiconductor material, has been widely used in microwave absorption (MA) due to its unique structural features such as oxygen vacancies and interstitial atoms. In this paper, Co/CeO2/C composites were prepared by a hydrothermal method followed by a pyrolysis process. The effect of different pyrolysis temperatures (650-950 °C) on the MA property of the composites was investigated. When the pyrolysis temperature was 850 °C, the Co/CeO2/C-850 composite exhibited outstanding MA behavior in the frequency range of 2-18 GHz, displaying a minimum reflection loss (RLmin) of -45.22 dB and an effective absorption bandwidth (EAB) of 4.61 GHz at a thin thickness of 1.75 mm. The MA performance of the Co/CeO2/C composites is mainly attributed to the dielectric loss due to interfacial polarization originating from different interfaces and dipole polarization caused by the oxygen vacancies in CeO2. In addition, the introduction of Co nanoparticles not only provides the magnetic loss but also modulates impendence matching for the current magnetoelectric coupling system.

Ultrasonically assisted surface modified CeO2 nanospindle catalysts for conversion of CO2 and methanol to DMC

This study developed a facile and effective approach to engineer the surface properties of cerium oxide (CeO2) nanospindle catalysts for the direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol. CeO2 nanospindles were first prepared by a simple precipitation method followed by wet chemical redox etching with sodium borohydride (NaBH4) under high intensity ultrasonication (ultrasonic horn, 20 kHz, 150 W/cm2). The ultrasonically assisted surface modification of the CeO2 nanospindles in NaBH4 led to particle collisions and surface reduction that resulted in an increase in the number of surface-active sites of exposed Ce3+ and oxygen vacancies. The surface modified CeO2 nanospindles showed an improvement of catalytic activity for DMC formation, yielding 17.90 mmol·gcat-1 with 100 % DMC selectivity. This study offers a simple and effective method to modify a CeO2 surface, and it can further be applied for other chemical activities.

Roles of Oxygen Species in Low-Temperature Catalytic o-Xylene Oxidation on MOF-Derived Bouquetlike CeO2

To realize efficient low-temperature catalytic o-xylene oxidation, MOF-derived CeO₂-X catalysts were prepared via the pyrolysis of MOF precursors with different ratios of cerium nitrate to trimesic acid. Among the synthesized catalysts, the bouquet like CeO₂-1 exhibited the best activity with T₅₀ and T₉₀ of 156 and 198 °C and the lowest activation energy of 60.67 kJ·mol^-1 (WHSV= 48 000 mL·g^-1·h^-1, o-xylene concentration = 500 ppm). o-Xylene was completely mineralized, and no change in conversion efficiency or CO₂ yield was found at 5 vol % H₂O for over 50 h. The rich active oxygen species (XPS: O_sur/O_latt = 0.69) and abundant oxygen vacancies (Raman: I_D/I_F2g = 0.036) of CeO₂-1 made crucial contribution to its superior catalytic activity. The O₂-TPD and H₂-TPR results confirmed that CeO₂-1 had more surface active oxygen and better mobility of bulk oxygen. Moreover, the reaction routes under different atmospheres were probed through in situ DRIFTS, in which oxygen vacancy played a key role in promoting the adsorption and activation of molecular oxygen and facilitating the migration of the bulk lattice oxygen.

Effects of chemical etching and reduction activation of CeO2 nanorods supported ruthenium catalysts on CO oxidation

In this work, pristine and NaBH₄ etched CeO₂ nanorods supported ruthenium (Ru) catalysts were synthesized and employed to investigate the effects of chemical etching and reduction activation treatment on CO oxidation. With 1 wt% Ru loading, the CeO₂ nanorods supported catalyst samples, after 6 wt% NaBH₄ etching treatment, showed significantly promoted H₂ consumption under 100 °C and low apparent activation energy (i.e., E_a ∼ 31.2 kJ/mol) for CO oxidation. In-situ CO-DRIFTS profiles revealed that, for the reduced sample, the observed CO adsorption at ∼ 2020 cm^-1 at 40 °C may be related to a strong RuO_x-CeO₂ interaction induced by the NaBH₄ etching treatment, which was supported by the oxygen vacancy analysis results of X-ray photoelectron spectroscopy and CO-temperature programmed desorption. The enriched surface defects on CeO₂ support due to the chemical etching and reduction treatments are believed to promote the interaction between RuO_x species and CeO₂, which is responsible for the enhanced activity of CO oxidation.

Recent advances in catalytic systems in the prism of physicochemical properties to remediate toxic CO pollutants: A state-of-the-art review

Carbon monoxide (CO) is the most harmful pollutant in the air, causing environmental issues and adversely affecting humans and the vegetation and then raises global warming indirectly. CO oxidation is one of the most effective methods of reducing CO by converting it into carbon dioxide (CO₂) using a suitable catalytic system, due to its simplicity and great value for pollution control. The CO oxidation reaction has been widely studied in various applications, including proton-exchange membrane fuel cell technology and catalytic converters. CO oxidation has also been of great academic interest over the last few decades as a model reaction. Many review studies have been produced on catalysts development for CO oxidation, emphasizing noble metal catalysts, the configuration of catalysts, process parameter influence, and the deactivation of catalysts. Nevertheless, there is still some gap in a state of the art knowledge devoted exclusively to synergistic interactions between catalytic activity and physicochemical properties. In an effort to fill this gap, this analysis updates and clarifies innovations for various latest developed catalytic CO oxidation systems with contemporary evaluation and the synergistic relationship between oxygen vacancies, strong metal-support interaction, particle size, metal dispersion, chemical composition acidity/basicity, reducibility, porosity, and surface area. This review study is useful for environmentalists, scientists, and experts working on mitigating the harmful effects of CO on both academic and commercial levels in the research and development sectors.

Shape effect of cerium oxide nanoparticles on mild traumatic brain injury

The catalytic performance and therapeutic effect of nanoparticles varies with shape. Here, we investigated and compared the therapeutic outcomes of ceria nanospheres (Ceria NSs) and ceria nanorods (Ceria NRs) in an in vivo study of mild traumatic brain injury (mTBI). In vivo TBI was induced in a mouse model of open head injury using a stereotaxic impactor. Outcomes including cytoprotective effects, cognitive function, and cerebral edema were investigated after retro-orbital injection of 11.6 mM of ceria nanoparticles. Ceria nanoparticles significantly reduced fluoro-jade B (FJB)-positive cells and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL)-positive cells, and restored mRNA levels of superoxide dismutase 1 (SOD1) and SOD2. They also decreased the cyclooxygenase-2 (COX-2) expression compared with the untreated control group. Comparing the two nanomaterials, Ceria NRs showed less stable and high-energy (100) and (110) planes, which increased the number of active sites. The Ce3+/Ce4+ molar ratio of Ceria NRs (0.40) was greater than that of Ceria NSs (0.27). Ceria NRs (0.059 ± 0.021) appeared to exhibit better anti-inflammatory effect than Ceria NSs (0.133 ± 0.024), but the effect was statistically insignificant (p = 0.190). Ceria nanoparticles also improved cognitive impairment following mTBI compared with the control group, but the effect did not differ significantly according to the nanoshape. However, Ceria NRs (70.1 ± 0.5%) significantly decreased brain water content compared with Ceria NSs (73.7 ± 0.4%; p = 0.0015), indicating a more effective reduction in brain edema (p = 0.0015). Compared with Ceria NSs, the Ceria NRs are more effective in alleviating cerebral edema following in vivo mTBI.

Highly Dispersed Pd Species Supported on CeO2 Catalyst for Lean Methane Combustion: The Effect of the Occurrence State of Surface Pd Species on the Catalytic Activity

The correlation between the occurrence state of surface Pd species of Pd/CeO2 for lean CH4 combustion is investigated. Herein, by using a reduction-deposition method, we have synthesized a highly active 0.5% PdO/CeO2-RE catalyst, in which the Pd nanoparticles are evenly dispersed on the CeO2 nanorods CeO2-R. Based on comprehensive characterization, we have revealed that the uniformly dispersed Pd nanoparticles with a particle size distribution of 2.3 ± 0.6 nm are responsible for the generation of PdO and PdxCe1−xO2−δ phase with –Pd2+–O2−–Ce4+– linkage, which can easily provide oxygen vacancies and facilitate the transfer of reactive oxygen species between the CeO2-R and Pd species. As a consequence, the remarkable catalytic activity of 0.5% Pd/CeO2-RE is related to the high concentration of PdO species on the surface of the catalyst and the synergistic interaction between the Pd species and the CeO2 nanorod.

Surface-Mounted Metal-Organic Frameworks: Past, Present, and Future Perspectives

Metal-organic frameworks (MOFs) are an emerging class of porous materials composed of organic linkers and metal centers/clusters. The integration of MOFs onto the solid surface as thin films/coatings has spurred great interest, thanks to leveraging control over their morphology (such as size- and shape-regulated crystals) and orientation, flexible processability, and easy recyclability. These aspects, in synergy, promise a wide range of applications, including but not limited to gas/liquid separations, chemical sensing, and electronics. Dozens of innovative methods have been developed to manipulate MOFs on various solid substrates for academic studies and potential industrial applications. Among the developed deposition methods, the liquid-phase epitaxial layer-by-layer (LPE-LbL) method has demonstrated its merits over precise control of the thickness, roughness, homogeneity, and orientations, among others. Herein, we discuss the major developments of surface-mounted MOFs (SURMOFs) in LbL process optimization, summarizing the SURMOFs' performance in different applications, and put forward our perspective on the future of SURMOFs in terms of advances in the formulation, applications, and challenges. Finally, future prospects and challenges with respect to SURMOFs growth will be discussed, keeping the focus on their widening applications.

Size-Controlled Synthesis of Pd Nanocatalysts on Defect-Engineered CeO2 for CO2 Hydrogenation

The size effects of metal catalysts have been widely investigated to optimize their catalytic activity and selectivity. However, the size-controllable synthesis of uniform supported metal nanoparticles without surfactants and/or additives remains a great challenge. Herein, we developed a green, surfactant-free, and universal strategy to tailor the sizes of uniform Pd nanoparticles on metal oxides by an electroless chemical deposition method via defect engineering of supports. The nucleation and growth mechanism suggest a strong electrostatic interaction between the Pd precursor and low-defective CeO₂ and a weak reducing capacity for low-defective CeO₂, resulting in small Pd nanoparticles. Conversely, large Pd nanoparticles were formed on a highly defective CeO₂ surface. Combined with various ex situ and in situ characterizations, a higher intrinsic activity of Pd for the CO₂-to-CO hydrogenation was found on large Pd nanoparticles with higher electron density owing to their stronger H₂ dissociation ability and H-spillover effects, as well as the larger number of oxygen vacancies generated in situ for CO₂ activation under hydrogenation conditions.

Tomographic Study of Mesopore Formation in Ceria Nanorods

Porosity in functional oxide nanorods is a recently discovered new type of microstructure, which is not yet fully understood and still under evaluation for its impact on applications in catalysis and gas/ion storage. Here we explore the shape and distribution of pores in ceria in three dimensions using a modified algorithm of geometric tomography as a reliable tool for reconstructing defective and strained nanoobjects. The pores are confirmed as "negative-particle" or "inverse-particle" cuboctahedral shapes located exclusively beneath the flat surface of the rods separated via a sub-5 nm thin ceria wall from the outside. New findings also comprise elongated "negative-rod" defects, seen as embryonic nanotubes, and pores in cube-shaped ceria. Furthermore, we report near-sintering secondary heat treatment of nanorods and cubes, confirming persistence of pores beyond external surface rounding. We support our experiments with molecular modeling and predict that the growth history of voids is via diffusion and aggregation of atomic point defects. In addition, we use density functional theory to show that the relative stability of pore (shape) increases in the order "cuboidal" < "hexagonal-prismatic" < "octahedral". The results indicate that by engineering voids into nanorods, via a high-temperature postsynthetic heat treatment, a potential future alternative route of tuning catalytic activities might become possible.

Insight into the Contributions of Surface Oxygen Vacancies on the Promoted Photocatalytic Property of Nanoceria

Oxygen vacancies (OVs) have critical effects on the photoelectric characterizations and photocatalytic activity of nanoceria, but the contributions of surface OVs on the promoted photocatalytic properties are not clear yet. In this work, we synthesized ceria nanopolyhedron (P-CeO₂), ceria nanocube (C-CeO₂) and ceria nanorod (R-CeO₂), respectively, and annealed them at 600 °C in air, 30%, 60% or pure H₂. After annealing, the surface OVs concentration of ceria elevates with the rising of H₂ concentration. Photocatalytic activity of annealed ceria is promoted with the increasing of surface OVs, the methylene blue photodegradation ratio with pure hydrogen annealed of P-CeO₂, C-CeO₂ or R-CeO₂ is 93.82%, 85.15% and 90.09%, respectively. Band gap of annealed ceria expands first and then tends to narrow slightly with the rising of surface OVs, while the valence band (VB) and conductive band (CB) of annealed ceria changed slightly. Both of photoluminescence spectra and photocurrent results indicate that the separation efficiency of photoinduced electron-hole pairs is significantly enhanced with the increasing of the surface OVs concentration. The notable weakened recombination of photogenerated carrier is suggested to attribute a momentous contribution on the enhanced photocatalytic activity of ceria which contains surface OVs.

Carbon cloth-based immunosensor for detection of 25-hydroxy vitamin D3

Vitamin D (VD) deficiency is a global health concern due to its serious health impacts, and at present, the monitoring of VD status is expensive. Here, a novel immunosensor for sensitive and label-free detection of 25-hydroxy vitamin D₃ (25VD₃) is reported. Nanostructured cerium(IV) oxide (nCeO₂) was anchored onto carbon cloth (CC) via electrophoretic deposition to fabricate a nanoplatform (nCeO₂/CC). Subsequently, bioactive molecules (anti-25VD₃ and BSA) were introduced to fabricate the nanobioplatform BSA/anti-25VD₃/nCeO₂/CC as an immunosensor. The analytical performance of the developed immunosensor was studied towards 25VD₃ detection. The immunosensor provides a broad linear range of 1-200 ng mL^-1, high sensitivity of 2.08 μA ng⁻¹ mL cm⁻², a detection limit of 4.63 ng mL⁻¹, and a response time of 15 min, which is better than that of previous reports. The biosensor exhibited high selectivity, good reproducibility, and excellent stability for about 45 days. The potential application of the proposed immunosensor was observed for real serum samples towards 25VD₃ detection that demonstrated a high correlation with the conventional enzyme-linked immunosorbent assay.

Formation of self-assembled Gd2O3 nanowire-like structures during epitaxial growth on Si(001)

The structural and morphological properties of gadolinium oxide (Gd₂O₃) grown at high temperatures with molecular beam epitaxy on Si(001) were investigated for different stages of growth. The Gd₂O₃ layers were grown at 850 °C with different oxygen partial pressures and substrate miscuts. RHEED and XRD investigations indicate an initial formation of silicide and a subsequent transformation into cubic Gd₂O₃ with (110) orientation. The surface exhibits nanowire-like structures oriented orthogonally along with the [110] directions of the substrate, as indicated by AFM. Since on 4° off-cut Si(001) substrates the nanowire-like structures are mainly oriented in only one [110] direction, the orientation of the formed Gd₂O₃ structures seems to be related to the dimer orientation of the (2 × 1) reconstructed Si(001) surface. The density and length of the nanowire-like structures can be tuned by a change in oxygen partial pressure. The results were discussed in terms of different physical effects, where a combination of desorption of silicon and the formation of a silicide layer in the initial stage of growth could be the reason for the growth behaviour, which is also supported through TEM investigations.

The formation of nanowire-like structures during epitaxial growth of Gd₂O₃ on Si(001) at high temperatures is investigated. The results are discussed by means of physical models.

Roles of Oxygen Vacancies in the Bulk and Surface of CeO2 for Toluene Catalytic Combustion

Catalytic combustion technology is one of the effective methods to remove VOCs such as toluene from industrial emissions. The decomposition of an aromatic ring via catalyst oxygen vacancies is usually the rate-determining step of toluene oxidation into CO₂. Series of CeO₂ probe models were synthesized with different ratios of surface-to-bulk oxygen vacancies. Besides the devotion of the surface vacancies, a part of the bulk vacancies promotes the redox property of CeO₂ in toluene catalytic combustion: surface vacancies tend to adsorb and activate gaseous O₂ to form adsorbed oxygen species, whereas bulk vacancies improve the mobility and activity of lattice oxygen species via their transmission effect. Adsorbed oxygen mainly participates in the chemical adsorption and partial oxidation of toluene (mostly to phenolate). With the elevated temperatures, lattice oxygen of the catalysts facilitates the decomposition of aromatic rings and further improves the oxidation of toluene to CO₂.

A new V2O5–MoO3–TiO2–SO42−nanostructured aerogel catalyst for diesel DeNOxtechnology

A new V₂O₅–MoO₃–TiO₂–SO₄²⁻nanostructured aerogel catalyst exhibits superior SCR activity compared to the V₂O₅–WO₃/TiO₂commercial catalyst (EUROCAT) at high temperatures (375–500 °C).

Engineering well-defined rare earth oxide-based nanostructures for catalyzing C1 chemical reactions

In this review, we summarize the nanostructural engineering and applications of rare earth oxide-based nanomaterials with well-defined compositions, crystal phases and shapes for efficiently catalyzing C1 chemical reactions.

Engineering CuOx–ZrO2–CeO2 nanocatalysts with abundant surface Cu species and oxygen vacancies toward high catalytic performance in CO oxidation and 4-nitrophenol reduction

CuO_x–ZrO₂–CeO₂ nanocrystalline catalysts were designed and synthesized by a solvent-free synthetic strategy, and exhibited excellent catalytic performance owing to the increased oxygen vacancies and better dispersed active metal species.

The solid-state in situ construction of Cu2O/CuO heterostructures with adjustable phase compositions to promote CO oxidation activity

Cu₂O/CuO heterojunctions were fabricated via in situ solid-state technology. Tuning the ratio of reactants enables optimization of the components of the Cu₂O/CuO heterostructures and their catalytic activities for CO oxidation.

CeO2/C nanowire derived from a cerium(III) based organic framework as a peroxidase mimic for colorimetric sensing of hydrogen peroxide and for enzymatic sensing of glucose

A metal organic framework obtained from cerium(III) and trimesic acid was pyrolyzed to obtain a novel nanostructure referred to as CeO₂/C nanowires. The experimental parameters temperature, precursor and gas atmosphere were optimized. The nanowires show good dispersion and a large number of oxygen vacancies, and this leads to excellent peroxidase-like activity. The nanowires are stable at pH values between 2 and 10, and in the 4-80 °C temperature range. The peroxidase-mimicking activity was exploited in a sensitive colorimetric method for determination of H₂O₂ by using 3,3',5,5'-tetramethylbenzidine as the chromogenic substrate. The absorbance at 652 nm increases linearly in the 0.5 to 100 μM H₂O₂ concentration range. If glucose oxidase is added to a solution containing glucose, H₂O₂ will be enzymatically produced. This was exploited to design a new method for determination of glucose. The optical response is linear in the 1-100 μM glucose concentration range, and the detection limit is 0.69 μM (at S/N = 3). The method was successfully applied to the determination of glucose in serum samples. Graphical abstractCeO₂/C nanowires prepared by Ce-MOF pyrolysis show peroxidase-like activity and are able to catalyze the oxidation of tetramethylbenzidine (TMB) by H₂O₂. This was applied to glucose oxidase-based colorimetruc determination of glucose in human sera.

Insights into the Pyrolysis Processes of Ce-MOFs for Preparing Highly Active Catalysts of Toluene Combustion

Metal organic frameworks (MOFs) have recently been used as precursors of the catalysts for the combustion of volatile organic compounds (VOCs). In the present work, three kinds of CeO2 catalysts were successfully synthesized from Ce-MOF-808, Ce-BTC, and Ce-UiO-66, with specific topological structures and coordinate environments. Catalysts with small particle size, stacking mode, and structural defects could be created by pyrolysis of Ce-MOFs, which affects the activity in the toluene combustion significantly. Raman spectra, XPS, and OSC studies were performed to reveal the formation of defect sites. The thermal redox properties were determined by H2-TPR. Catalytic activity tests were conducted on the toluene combustion, and CeO2-MOF-808 showed the best catalytic performance (T90 = 278 °C) due to its having the largest specific surface area, abundant active surface oxygen species, and low-temperature reducibility.

Synthesis and biomedical applications of nanoceria, a redox active nanoparticle

Background

Nanoceria has recently received much attention, because of its widespread biomedical applications, including antibacterial, antioxidant and anticancer activity, drug/gene delivery systems, anti-diabetic property, and tissue engineering.

Main body

Nanoceria exhibits excellent antibacterial activity against both Gram-positive and Gram-negative bacteria via the generation of reactive oxygen species (ROS). In healthy cells, it acts as an antioxidant by scavenging ROS (at physiological pH). Thus, it protects them, while in cancer cells (under low pH environment) it acts as pro-oxidant by generating ROS and kills them. Nanoceria has also been effectively used as a carrier for targeted drug and gene delivery in vitro and in vivo models. Besides, nanoceria can also act as an antidiabetic agent and confer protection towards diabetes-associated organ pathophysiology via decreasing the ROS level in diabetic subjects. Nanoceria also possesses excellent potential in the field of tissue engineering. In this review, firstly, we have discussed the different methods used for the synthesis of nanoceria as these are very important to control the size, shape and Ce³⁺/Ce⁴⁺ ratio of the particles upon which the physical, chemical, and biological properties depend. Secondly, we have extensively reviewed the different biomedical applications of nanoceria with probable mechanisms based on the literature reports.

Conclusion

The outcome of this review will improve the understanding about the different synthetic procedures and biomedical applications of nanoceria, which should, in turn, lead to the design of novel clinical interventions associated with various health disorders.

Graphical abstract

Structure-Activity Relationship of Defective Metal-Based Photocatalysts for Water Splitting: Experimental and Theoretical Perspectives

Photocatalytic water splitting is promising for hydrogen energy production using solar energy and developing highly efficient photocatalysts is challenging. Defect engineering is proved to be a very useful strategy to promote the photocatalytic performance of metal-based photocatalysts, however, the vital role of defects is still ambiguous. This work comprehensively reviews point defective metal-based photocatalysts for water splitting, focusing on understanding the defects' disorder effect on optical adsorption, charge separation and migration, and surface reaction. The controllable synthesis and tuning strategies of defective structure to improve the photocatalytic performance are summarized, then the characterization techniques and density functional theory calculations are discussed to unveil the defect structure, and analyze the defects induced electronic structure change of catalysts and its ultimate effect on the photocatalytic activity at the molecular level. Finally, the challenge in developing more efficient defective metal-based photocatalysts is outlined. This work may help further the understanding of the fundamental role of defect structure in the photocatalytic reaction process and guide the rational design and fabrication of highly efficient and low-cost photocatalysts.

Size Distribution Control of Copper Nanoparticles and Oxides: Effect of Wet-Chemical Redox Cycling

In this work, we studied the effect of liquid-phase redox cycling on the size of Cu nanoparticles and oxides. The mixed solution of sodium hydroxide and ammonium persulfate was applied as the oxidation system at room temperature, and ascorbic acid was used as reduction agent at 80 °C in the cycling process. It was found that pristine copper particles with average size of around 800 nm and wide distribution from 300 to 1300 nm could be turned into the resulting particles with the average size of around 162.3 nm with the distribution from 75 to 250 nm after 5 redox cycles. It was also observed that uniform copper oxide nanowires formed after 5 oxidation cycles could be easily reduced into fine copper nanoparticles. The critical tuning factors including the precursor size, morphology, defects, reaction time, and the way of adding oxidant were investigated. It was suggested that the synergetic driving effect of chemical reduction and nanostructure thermodynamic instability in solution accounted for the size reformation of the copper nanoparticles. This proposed method of size-shrinking could be developed as a general strategy for large-scale tuning the properties of copper nanoparticles for wide applications and extended to other metal particles as well.

Synthesis of Positively and Negatively Charged CeO2 Nanoparticles: Investigation of the Role of Surface Charge on Growth and Development of Drosophila melanogaster

Monodispersed cerium oxide nanoparticles (CeO₂ NPs) with positive and negative surface potential were synthesized by co-precipitation method using hexamethylenetetramine (HMT) and poly(vinylpyrrolidone) (PVP), respectively, as precipitating agents. Synthesized NPs were characterized with scanning electron microscopy (SEM), UV-Visible (UV-Vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and powder X-ray diffraction (XRD). Positively charged NPs of about 30 ± 10 nm in size formed within 5 h, aggregated in number, and resulted in larger-sized NPs as a function of time. The CeO₂ NPs were administered to Drosophila as a part of their diet to study the effects on the growth and development of Drosophila. While the positively charged NPs did not affect the growth of the third instar larvae, the negatively charged NPs delayed the growth of larvae by about 7 days. It required 7 more days to reach the stage of adult fly. TEM imaging of the larvae gut showed that positively charged NPs were found to be smaller, whereas the size of negatively charged NPs remained unchanged. This biodegradability could be the reason for the delayed larvae growth in the case of negatively charged particles. The distance covered by such second instar larvae fed with diet containing negatively charged CeO₂ NPs was significantly lower, and their size was significantly smaller when compared to the crawling activity and size of the third instar larvae of the control group. Such positively charged NPs have high potential for use as drug delivery carriers for the treatment of disease, and negatively charged NPs may play a rather detrimental role.

CeO2 Nanowire-BODIPY-Adenosine Triphosphate Fluorescent Sensing Platform for Highly Specific and Sensitive Detection of Arsenate

Effective and sensitive monitoring of arsenate in drinking water is significant for risk management of public health. Here, we demonstrated that a CeO₂ nanowire acted as an efficient quencher for small fluorescent molecules with a phosphate group, BODIPY-adenosine triphosphate (BODIPY-ATP) and riboflavin-5'-phosphate (Rf-P), and developed a CeO₂ nanowire-BODIPY-ATP platform for highly selective and sensitive detection of arsenate. The response strategy was based on the competitive coordination chemistry of CeO₂ nanowire between arsenate and phosphate group of BODIPY-ATP. Arsenate displaced adsorbed BODIPY-ATP to enhance fluorescence, allowing detection of arsenate down to 7.8 nM, which is lower than the WHO-defined limit of 130 nM. An excellent linear range of 20-150 and 150-1000 nM was obtained. Importantly, this system was simple in design and convenient in operation. Also, the platform exhibited excellent selectivity for arsenate without the interference of phosphate ions. Finally, the proposed method had been successfully employed for determination of arsenate in real water samples.

Pt-Embedded CuO x-CeO2 Multicore-Shell Composites: Interfacial Redox Reaction-Directed Synthesis and Composition-Dependent Performance for CO Oxidation

Exploring the state-of-the-art heterogeneous catalysts has been a general concern for sustainable and clean energy. Here, Pt-embedded CuO _x-CeO₂ multicore-shell (Pt/CuO _x-CeO₂ MS) composites are fabricated at room temperature via a one-pot and template-free procedure for catalyzing CO oxidation, a classical probe reaction, showing a volcano-shaped relationship between the composition and catalytic activity. We experimentally unravel that the Pt/CuO _x-CeO₂ MS composites are derived from an interfacial autoredox process, where Pt nanoparticles (NPs) are in situ encapsulated by self-assembled ceria nanospheres with CuO _x clusters adhered through deposition/precipitation-calcination process. Only Cu-O and Pt-Pt coordination structures are determined for CuO _x clusters and Pt NPs in Pt/CuO _x-CeO₂ MS, respectively. Importantly, the close vicinity between Pt and CeO₂ benefits to more oxygen vacancies in CeO₂ counterparts and results in thin oxide layers on Pt NPs. Meanwhile, the introduction of CuO _x clusters is crucial for triggering synergistic catalysis, which leads to high resistance to aggregation of Pt NPs and improvement of catalytic performance. In CO oxidation reaction, both Pt^δ+-CO and Cu⁺-CO can act as active sites during CO adsorption and activation. Nonetheless, redundant content of Pt or Cu will induce a strongly bound Pt-O-Ce or Cu-[O _x]-Ce structures in air-calcinated Pt/CuO _x-CeO₂ MS composites, respectively, which are both deleterious to catalytic reactivity. As a result, the composition-dependent catalytic activity and superior durability of Pt/CuO _x-CeO₂ MS composites toward CO oxidation reaction are achieved. This work should be instructive for fabricating desirable multicomponent catalysts composed of noble metal and bimetallic oxide composites for diverse heterogeneous catalysis.

Dual-responsive dithio-polydopamine coated porous CeO2 nanorods for targeted and synergistic drug delivery

Objective

The aim was to produce the first report of assembling degradable stimuli-responsive dithio-polydopamine coating with a cancer target unit for synergistic and targeted drug delivery.

Methods

A multifunctional drug delivery system was constructed by coating a dual-responsive dithio-polydopamine (PDS) on porous CeO₂ nanorods and subsequent conjugation of lactose derivative, where the PDS was formed by self-polymerization of dithio-dopamine (DOPASS).

Results

The multifunctional drug delivery system displayed excellent cancer targeted ability resulting from the conjugation of lactose derivative, which could specifically recognize the overexpressed asialoglycoprotein receptors on the surface of HepG2 cells. It also showed a dual-responsive property of glutathione and pH, achieving controllable drug release from the cleavage of disulfide bond and subsequent degradation of PDS in cancer cells. Moreover, the degradation of PDS led to the exposure of CeO₂ nanorods, which has a synergistic anticancer effect due to its cytotoxicity to cancer cells.

Conclusion

This work presents a good example of a rational design towards synergistic and targeted DDS for cancer chemotherapies.

A Novel Magnetically Recoverable Ni-CeO2-x/Pd Nanocatalyst with Superior Catalytic Performance for Hydrogenation of Styrene and 4-Nitrophenol

Metal/support nanocatalysts consisting of various metals and metal oxides not only retain the basic properties of each component but also exhibit higher catalytic activity due to their synergistic effects. Herein, we report the creation of a highly efficient, long-lasting, and magnetic recyclable catalyst, composed of magnetic nickel (Ni) nanoparticles (NPs), active Pd NPs, and oxygen-deficient CeO_2-x support. These hybrid nanostructures composed of oxygen deficient CeO_2-x and active metal nanoparticles could effectively facilitate diffusion of reactant molecules and active site exposure that can dramatically accelerate the reaction rate. Impressively, the rate constant k and k/m of 4-nitrophenol reduction over 61 wt % Ni-CeO_2-x/0.1 wt % Pd catalyst are 0.0479 s^-1 and 2.1 × 10⁴ min^-1 g^-1, respectively, and the reaction conversion shows negligible decline even after 20 cycles. Meanwhile, the optimal 61 wt % Ni-CeO_2-x/3 wt % Pd catalyst manifests remarkable catalytic activity toward styrene hydrogenation with a high TOF of 6827 mol_styrene mol_Pd^-1 h^-1 and a selective conversion of 100% to ethylbenzene even after eight cycles. The strong metal-support interaction (SMSI) between Ni NPs, Pd NPs, and oxygen-deficient CeO_2-x support is beneficial for superior catalytic efficiency and stability toward hydrogenation of styrene and 4-nitrophenol. Moreover, Ni species could boost the catalytic activity of Pd due to their synergistic effect and strengthen the interaction between reactant and catalyst, which seems responsible for the great enhancement of catalytic activity. Our findings provide a new perspective to develop other high-performance and magnetically recoverable nanocatalysts, which would be widely applied to a variety of catalytic reactions.

Co3O4@CeO2 hybrid flower-like microspheres: a strong synergistic peroxidase-mimicking artificial enzyme with high sensitivity for glucose detection

In recent years, the development of artificial nanostructured enzymes has received enormous interest in nanobiotechnology due to their advantages over natural enzymes. In the present work, different amounts (5, 10, and 20 wt%) of Co₃O₄ nanoparticle decorated CeO₂ hybrid flower-like microspheres (Co₃O₄@CeO₂) have been investigated for peroxidase-like activity and it was found that 10 wt% of Co₃O₄@CeO₂ exhibited excellent peroxidase-like activity for the catalytic oxidation of the 3,3',5,5'-tetramethylbenzidine (TMB) substrate in the presence of H₂O₂. The formation of more Ce³⁺ ions associated with the oxygen vacancies and a strong synergistic interaction between CeO₂ and Co₃O₄ may be responsible for the enhanced peroxidase-like activity. Based on their peroxidase activity, Co₃O₄@CeO₂ hybrid microspheres were used for the colourimetric detection of glucose. It was found that Co₃O₄@CeO₂ hybrid microspheres showed a substantial enhancement in the detection selectivity. The limit of detection (LOD) was also improved with a limit as low as 1.9 μM. Thus, we believe that Co₃O₄@CeO₂ hybrid flower-like microspheres with high peroxidase-like activity can be exploited for biosensing applications.

Rational design and controllable preparation of holey MnO2 nanosheets

A novel technique for the design and controllable preparation of holey MnO₂ nanosheets was first developed by an in situ redox reaction between the MnO₂ nanosheets and adsorbed Fe²⁺ ions at room temperature, in which Fe²⁺ ions originated from a redox reaction between Cu wire and Fe³⁺ ions.