Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Microplotter printing of planar solid electrolytes in the CeO2-Y2O3 system

The formation process for planar solid electrolytes in the CeO₂-Y₂O₃ system has been studied using efficient, high-performance, high-resolution microplotter printing technology, using functional ink based on nanopowders (the average size of crystallites was 12-15 nm) of a similar composition obtained by programmed coprecipitation of metal hydroxides. The dependence of the microstructure of the oxide nanoparticles obtained and their crystal structure on yttrium concentration has been studied using a wide range of methods. According to X-ray diffraction (XRD), the nanopowders and coatings produced are single-phase, with a cubic crystal structure of the fluorite type, and the electronic state and content of cerium and yttrium in the printed coatings have been determined using X-ray photoelectron spectroscopy (XPS). The results of scanning electron (SEM) and atomic force microscopy (AFM) have shown that the coatings produced are homogeneous, they do not contain defects in the form of fractures and the height difference over an area of 1 µm² is 30-45 nm. The local electrophysical characteristics of the oxide coatings produced (the work function of the coating surface, capacitance values, maps of the surface potential and capacitive contrast distribution over the surface) have been studied using Kelvin-probe force microscopy (KPFM) and scanning capacitive microscopy (SCM). Using impedance spectroscopy, the dependence of the electrophysical characteristics of printed planar solid electrolytes in the CeO₂-Y₂O₃ system on yttrium content has been determined and the prospects of the technology developed for the manufacture of modern, intermediate-temperature, solid oxide fuel cells have been demonstrated.

Uniform Cu/chitosan beads as a green and reusable catalyst for facile synthesis of imines via oxidative coupling reaction

A nonprecious metal and biopolymer-based catalyst, Cu/chitosan beads, has been successfully prepared by using a software-controlled flow system. Uniform, spherical Cu/chitosan beads can be obtained with diameters in millimeter-scale and narrow size distribution (0.78 ± 0.04 mm). The size and morphology of the Cu/chitosan beads are reproducible due to high precision of the flow rate. In addition, the application of the Cu/chitosan beads as a green and reusable catalyst has been demonstrated using a convenient and efficient protocol for the direct synthesis of imines via the oxidative self- and cross-coupling of amines (24 examples) with moderate to excellent yields. Importantly, the beads are stable and could be reused more than ten times without loss of the catalytic performance. Furthermore, because of the bead morphology, the Cu/chitosan catalyst has greatly simplified recycling and workup procedures.

Uniform, spherical Cu/chitosan beads prepared using a software-controlled flow system as a green and conveniently recyclable catalyst for the efficient synthesis of various imines in short reaction time.

Cancer-Cell Imaging Using Copper-Doped Zeolite Imidazole Framework-8 Nanocrystals Exhibiting Oxidative Catalytic Activity

Copper-doped zeolite imidazole framework-8 (Cu/ZIF-8) was prepared and its peroxidase-like oxidative catalytic activity was examined with a demonstration of its applicability for cancer-cell imaging. Through simple solution chemistry at room temperature, Cu/ZIF-8 nanocrystals were produced that catalytically oxidized an organic substrate of o-phenylenediamine in the presence of H₂ O₂ . In a similar manner to peroxidase, the Cu/ZIF-8 nanocrystals oxidized the substrate through a ping-pong mechanism with an activation energy of 59.2 kJ mol^-1 . The doped Cu atoms functioned as active sites in which the active Cu intermediates were expected to be generated during the catalysis, whereas the undoped ZIF-8 did not show any oxidative activity. Cu/ZIF-8 nanocrystals exhibited low cell toxicity and displayed catalytic activity through interaction with H₂ O₂ among various reactive oxygen species in a cancer cell. This oxidative activity in vitro allowed cancer-cell imaging by exploiting the photoluminescence emitted from the oxidized product of o-phenylenediamine, which was insignificant in the absence of the Cu/ZIF-8 nanocrystals. The results of this study suggest that the Cu/ZIF-8 nanocrystal is a promising catalyst for the analysis of the microbiological systems.

Cultivation of a Cu/HMPC catalyst from a hyperaccumulating mustard plant for highly efficient and selective coupling reactions under mild conditions

Cu-containing activated carbon (eco-catalyst, Cu/HMPC, where ‘C’ defines ‘carbon’) was derived from a metal-hyperaccumulating mustard plant (HMP) by a simple chemical activation method. Transmission electron microscopy/selected area diffraction (HRTEM/SAED) results revealed that the Cu/HMPC has mainly three types of morphology [sheet-like morphology (2D), hollow-spheres (3D) and needle-like structures (1D)] which are interconnected. HRTEM-SAED, Raman and X-ray photoelectron spectroscopy (XPS) results confirmed the existence of Cu oxide species in Cu/HMPC. Content of Cu in Cu/HMPC was determined to be 1.03 wt%. The quality of graphitization in Cu/HMPC was discussed by using Raman and XRD results. The BET surface area of Cu/HMPC was determined to be 620.8 m² g⁻¹. The Cu/HMPC actively transformed a wide range of amines to imines under very mild reaction conditions. The catalyst Cu/HMPC gave products in excellent yields (98–61%) with very high TON/TOF values (1512/339–833/35 h⁻¹). To the best of our knowledge, this is the most efficient Cu-based heterogeneous eco-catalyst for the synthesis of imines among those reported to date. The Cu can be recovered from used Cu/HMPC by a simple HCl treatment. Versatility, heterogeneity and reusability of Cu/HMPC were tested. A possible mechanism has been proposed.

Cu/HMPC eco-catalyst with unique interconnected 1D/2D/3D morphology was derived from metal-hyperaccumulating mustard plant for highly efficient and selective coupling reactions.

Nanoscale Cobalt-Manganese Oxide Catalyst Supported on Shape-Controlled Cerium Oxide: Effect of Nanointerface Configuration on Structural, Redox, and Catalytic Properties

Understanding the role of nanointerface structures in supported bimetallic nanoparticles is vital for the rational design of novel high-performance catalysts. This study reports the synthesis, characterization, and the catalytic application of Co-Mn oxide nanoparticles supported on CeO₂ nanocubes with the specific aim of investigating the effect of nanointerfaces in tuning structure-activity properties. High-resolution transmission electron microscopy analysis reveals the formation of different types of Co-Mn nanoalloys with a range of 6 ± 0.5 to 14 ± 0.5 nm on the surface of CeO₂ nanocubes, which are in the range of 15 ± 1.5 to 25 ± 1.5 nm. High concentration of Ce³⁺ species are found in Co-Mn/CeO₂ (23.34%) compared with that in Mn/CeO₂ (21.41%), Co/CeO₂ (15.63%), and CeO₂ (11.06%), as evidenced by X-ray photoelectron spectroscopy (XPS) analysis. Nanoscale electron energy loss spectroscopy analysis in combination with XPS studies shows the transformation of Co²⁺ to Co³⁺ and simultaneously Mn^4+/3+ to Mn²⁺. The Co-Mn/CeO₂ catalyst exhibits the best performance in solvent-free oxidation of benzylamine (89.7% benzylamine conversion) compared with the Co/CeO₂ (29.2% benzylamine conversion) and Mn/CeO₂ (82.6% benzylamine conversion) catalysts for 3 h at 120 °C using air as the oxidant. Irrespective of the catalysts employed, a high selectivity toward the dibenzylimine product (97-98%) was found compared with the benzonitrile product (2-3%). The interplay of redox chemistry of Mn and Co at the nanointerface sites between Co-Mn nanoparticles and CeO₂ nanocubes as well as the abundant structural defects in cerium oxide plays a key role in the efficiency of the Co-Mn/CeO₂ catalyst for the aerobic oxidation of benzylamine.

Synthetically Tuned Atomic Ordering in PdCu Nanoparticles with Enhanced Catalytic Activity toward Solvent-Free Benzylamine Oxidation

Synthesis of ordered compounds with nano size is of particular interest for tuning the surface properties with enhanced activity and selectivity toward various important industrial catalytic processes. In this work, we synthesized ordered PdCu nanoparticles as highly efficient catalyst for the solvent-free aerobic oxidation of benzylamine. The Pd_xCu_1-x catalysts with different chemical compositions (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) were prepared by polyol method using NaBH₄ as a reducing agent and were well-characterized by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy (TEM) energy-dispersive analysis of X-rays, and X-ray absorption fine structure. The effect of different metal concentrations of Pd and Cu on the formation of Pd_xCu_1-x nanoparticles was investigated. The XRD and TEM confirmed the formation of ordered PdCu intermetallic phase with body-centered cubic (BCC) structure for the synthetic composition of Pd/Cu = 1:1. For compositions x = 0, 0.25, 0.75, and 1, Pd_xCu_1-x alloy with face-centered cubic (FCC) structure was observed, whereas mixed phase of BCC and FCC was observed for x = 0.4 and 0.6. The use of strong reducing agent (NaBH₄) was essential to synthesize PdCu ordered phase compared to weak reducing agents such as oleylamine and ascorbic acid. The PdCu nanocatalyst with ordered structure (BCC) showed excellent catalytic activity compared to Pd_xCu_1-x alloy nanoparticles with FCC structure. The atomic ordering in the PdCu intermetallic was the driving force for the enhancement in the catalytic activity with high benzylamine conversion of 94.0% and dibenzylimine selectivity of 92.2% compared to its monometallic and alloy counterparts. Moreover, ordered PdCu alloy showed good recyclability and activity toward the oxidation of different amines.

Highly active MnOx–CeO2catalyst for diesel soot combustion

Optimized Ce–Mn composite oxide, possessing abundant active Ce³⁺–Mn⁴⁺ions, exhibits excellent activity for catalytic soot combustion in NO_x/O₂.

Hemin-bound cysteinyl bolaamphiphile self-assembly as a horseradish peroxidase-mimetic catalyst

A horseradish peroxidase (HRP) mimetic catalyst was constructed by tethering hemin to the cysteinyl bolaamphiphile assembly through thiol–Fe bond. The prepared catalyst showed high catalytic activity comparable to HRP even at the high temperature.

Nanodiamond-Gold Nanocomposites with the Peroxidase-Like Oxidative Catalytic Activity

Novel nanodiamond-gold nanocomposites (NDAus) are prepared, and their oxidative catalytic activity is examined. Gold nanoparticles are deposited on carboxylated nanodiamonds (NDs) by in situ chemical reduction of gold precursor ions to produce NDAus, which exhibit catalytic activity for the oxidation of o-phenylenediamine in the presence of hydrogen peroxide similarly to a peroxidase. This remarkable catalytic activity is exhibited only by the gold nanoparticle-decorated NDs and is not observed for either Au nanoparticles or NDs separately. Kinetic oxidative catalysis studies show that NDAus exhibit a ping-pong mechanism with an activation energy of 93.3 kJ mol^-1, with the oxidation reaction rate being proportional to the substrate concentration. NDAus retain considerable activity even after several instances of reuse and are compatible with a natural enzyme, allowing the detection of xanthine using cascade catalysis. Association with gold nanoparticles makes NDs a good carbonic catalyst due to charge transfer at the metal-carbon interface and facilitated substrate adsorption. The results of this study suggest that diverse carbonic catalysts can be obtained by interfacial incorporation of various metal/inorganic substances.

Microwave synthesis of pure and doped cerium (IV) oxide (CeO2) nanoparticles for methylene blue degradation

Cerium (IV) oxide (CeO₂), samarium (Sm) and gadolinium (Gd) doped CeO₂ nanoparticles were prepared using microwave technique. The effect of microwave irradiation time, microwave power and pH of the starting solution on the structure and crystallite size were investigated. The prepared nanoparticles were characterized using X-ray diffraction, FT-Raman spectroscopy, and transmission electron microscope. The photocatalytic activity of the as-prepared CeO₂, Sm and Gd doped CeO₂ toward degradation of methylene blue (MB) dye was investigated under UV light irradiation. The effect of pH, the amount of catalyst and the dye concentration on the degradation extent were studied. The photocatalytic activity of CeO₂ was kinetically enhanced by trivalent cation (Gd and Sm) doping. The results revealed that Gd doped CeO₂ nanoparticles exhibit the best catalytic degradation activity on MB under UV irradiation. For clarifying the environmental safety of the by products produced from the degradation process, the pathways of MB degradation were followed using liquid chromatography/mass spectroscopy (LC/MS). The total organic carbon content measurements confirmed the results obtained by LC/MS. Compared to the same nanoparticles prepared by another method, it was found that Gd doped CeO₂ prepared by hydrothermal process was able to mineralize MB dye completely under UV light irradiation.

Designing CuOx Nanoparticle-Decorated CeO2 Nanocubes for Catalytic Soot Oxidation: Role of the Nanointerface in the Catalytic Performance of Heterostructured Nanomaterials

This work investigates the structure-activity properties of CuOx-decorated CeO2 nanocubes with a meticulous scrutiny on the role of the CuOx/CeO2 nanointerface in the catalytic oxidation of diesel soot, a critical environmental problem all over the world. For this, a systematic characterization of the materials has been undertaken using transmission electron microscopy (TEM), transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDS), high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM), scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS), X-ray diffraction (XRD), Raman, N2 adsorption-desorption, and X-ray photoelectron spectroscopy (XPS) techniques. The TEM images show the formation of nanosized CeO2 cubes (∼25 nm) and CuOx nanoparticles (∼8.5 nm). The TEM-EDS elemental mapping images reveal the uniform decoration of CuOx nanoparticles on CeO2 nanocubes. The XPS and Raman studies show that the decoration of CuOx on CeO2 nanocubes leads to improved structural defects, such as higher concentrations of Ce(3+) ions and abundant oxygen vacancies. It was found that CuOx-decorated CeO2 nanocubes efficiently catalyze soot oxidation at a much lower temperature (T50 = 646 K, temperature at which 50% soot conversion is achieved) compared to that of pristine CeO2 nanocubes (T50 = 725 K) under tight contact conditions. Similarly, a huge 91 K difference in the T50 values of CuOx/CeO2 (T50 = 744 K) and pristine CeO2 (T50 = 835 K) was found in the loose-contact soot oxidation studies. The superior catalytic performance of CuOx-decorated CeO2 nanocubes is mainly attributed to the improved redox efficiency of CeO2 at the nanointerface sites of CuOx-CeO2, as evidenced by Ce M5,4 EELS analysis, supported by XRD, Raman, and XPS studies, a clear proof for the role of nanointerfaces in the performance of heterostructured nanocatalysts.

Effect of water vapor on sulfur poisoning of MnOx–CeO2/Al2O3 catalyst for diesel soot oxidation

Water inhibits sulfate deposition and enhances the surface acidity during the sulfation of MnO_x–CeO₂/Al₂O₃, which promotes soot oxidation.

Highly efficient continuous-flow oxidative coupling of amines using promising nanoscale CeO2–M/SiO2 (M = MoO3 and WO3) solid acid catalysts

Nanoscale CeO₂–MoO₃/SiO₂ solid acid shows an outstanding catalytic performance in the oxidative coupling of amines under industrially-favourable conditions.

Sm doped mesoporous CeO2 nanocrystals: aqueous solution-based surfactant assisted low temperature synthesis, characterization and their improved autocatalytic activity

Mesoporous nanocrystalline high surface area CeO₂ synthesized at 100 °C exhibited improved autocatalytic activities by modifying defect states and reducibility upon Sm³⁺ doping.

β-Cyclodextrin supported MoO3–CeO2 nanocomposite material as an efficient heterogeneous catalyst for degradation of phenol

The β-cyclodextrin supported MoO₃–CeO₂ nanocomposite material is an efficient heterogeneous catalyst for the degradation of phenol to non-toxic pollutants at room temperature with continuous stirring and without light irradiation.

Graphene oxide as a metal-free catalyst for oxidation of primary amines to nitriles by hypochlorite

Graphene oxide catalyzes NaClO oxidation of primary benzyl and aliphatic amines to a product distribution comprising nitriles and imines.