Sol-gel chemistry of transition metal oxides

Fluorine as a Key Element in Solid-State Chemistry of Mixed Anions 3d Transition Metal-Based Materials for Electronic Properties and Energy

Mixed anion compounds containing fluorine and based on 3d transition elements represent a class of materials with significant interest in solid-state chemistry. Indeed, their highly varied chemical composition, structural diversity, and the resulting electronic properties provide a rich playground for imagining new applications in the field of energy. The anions and the chemical bonds they form with the 3d transition elements are at the heart of this review. Key parameters such as electronegativity, hardness, and polarizability are introduced and discussed to better understand the charge capacity of the anion and the bonds formed in the solid. Oxyfluorides represent the most studied family due to the size similarity of the two anions, and part of the review is dedicated to the specific synthesis of these materials by systematically adjusting the fluorine content within various structures and analyzing the electronic and electrochemical properties of these compositions. The final sections focus on materials with structures often exhibiting a two-dimensional character, where ionic blocks coexist with covalent layers, such as fluorochalcogenides, fluoropnictides, and fluorotetrelides. The compositions and structures are systematically correlated with the electronic properties.

Protonolysis and Condensation Reactions of Alkoxido-Substituted Lindqvist {MW5} and Keggin {MPW11} Polyoxometalates: Comparative Experimental and Modeling Studies

An understanding of proton transfer and migration at the surfaces of solid metal oxides and related molecular polyoxometalates (POMs) and metal alkoxides is crucial for the development of reactivity involving protonation or the absorption/binding of water. In this work, the hydrolysis of alkoxido Ti- and Sn-substituted Lindqvist [(MeO)MW5O18]3- (M = Ti, 1; M = Sn, 2) and Keggin [(MeO)MPW11O39]4- (M = Ti, 3; M = Sn, 4) type polyoxometalates (POMs) to hydroxido derivatives and subsequent condensation to μ-oxido species has been investigated in detail to provide insight into proton transfer reactions in these molecular metal oxide systems. Solution NMR studies revealed the dependence of reactions not only on the nature of the heteroatom (Ti or Sn) but also on the type of lacunary (W5 or PW11) POM and also on the solvent (MeCN or DMSO). Tin-substituted anions 2 and 4 were much more susceptible to protonolysis than the Ti analogues 1 and 3 while reactions of {MW5} anions were generally faster than those of the {MPW11} anions. Subsequent condensation of the resulting hydroxido derivatives [(HO)MW5O18]3- (M = Ti, 5; M = Sn, 6) and [(HO)MPW11O39]4- (M = Ti, 7; M = Sn, 8) was significantly more facile for 5 and 7 and, in all cases, condensation was inhibited in DMSO. Quantitative comparisons of equilibria and reaction rates were provided by analysis of NMR kinetic experiments, while DFT calculations on these and the analogous {NbW5} reactions provided comparative energetics and reaction profiles that are consistent with experimental observations. These results add to the fundamental understanding of proton migration in metal alkoxide hydrolysis/condensation and related reactions at metal oxide surfaces.

Metal-Coordinated Polymer-Inorganic Hybrids: Synthesis, Properties, and Application

This review examines the recent advancements and unique properties of polymer-inorganic hybrid materials formed through coordination bonding (Class II hybrids), which enable enhanced functionality and stability across various applications. Here, we categorize these materials based on properties gained through complexation, focusing on electrical conductivity, thermal stability, photophysical characteristics, catalytic activity, and nanoscale self-assembly. Two major synthetic approaches to making these hybrids include homogeneous and heterogeneous methods, each with distinct tradeoffs: Homogeneous synthesis is straightforward but requires favorable mixing between inorganic and polymer species, which are predominantly water-soluble complexes. In contrast, heterogeneous methods are post-processing techniques that provide high area selectivity for inorganic precursors, allowing precise integration within polymer matrices. Finally, we highlight the role of hybrid linkers, namely metallosupramolecular polymers, in creating structural diversity. These can be organized into three main groups: metal-organic frameworks (MOFs), coordination polymers (CPs), and supramolecular coordination complexes (SCCs). Each of these groups introduces unique structural and functional properties that expand the potential applications of hybrid materials.

Lanthanum-Promoted Electrocatalyst for the Oxygen Evolution Reaction: Unique Catalyst or Oxide Deconstruction?

A conventional performance metric for electrocatalysts that promote the oxygen evolution reaction (OER) is the current density at a given overpotential. However, the assumption that increased current density at lower overpotentials indicates superior catalyst design is precarious for OER catalysts in the working environment, as the crystalline lattice is prone to deconstruction and amorphization, thus greatly increasing the concentration of catalytic active sites. We show this to be the case for La3+ incorporation into Co3O4. Powder X-ray diffraction (PXRD), Raman spectroscopy and extended X-ray absorption fine structure (EXAFS) reveal smaller domain sizes with decreased long-range order and increased amorphization for La-modified Co3O4. This lattice deconstruction is exacerbated under the conditions of OER as indicated by operando spectroscopies. The overpotential for OER decreases with increasing La3+ concentration, with maximum activity achieved at 17% La incorporation. HRTEM images and electron diffraction patterns clearly show the formation of an amorphous overlayer during OER catalysis that is accelerated with La3+ addition. O 1s XPS spectra after OER show the loss of lattice-oxide and an increase in peak intensities associated with hydroxylated or defective O-atom environments, consistent with Co(O)x(OH)y species in an amorphous overlayer. Our results suggest that improved catalytic activity of oxides incorporated with La3+ ions (and likely other metal ions) is due to an increase in the number of terminal octahedral Co(O)x(OH)y edge sites upon Co3O4 lattice deconstruction, rather than enhanced intrinsic catalysis.

Achieving Robust α-Alumina Nanofibers by Ligand Confinement Coupled with Local Disorder Tuning

As high-performance thermal protection and structure enhancement materials, oxide ceramic fibers have become indispensable in numerous areas, ranging from deep-sea exploration to supersonic aircraft. However, under extreme energy input, abnormal grain growth and inevitable vermiculate structure would break the fiber integrity, causing catastrophic structure failure. Nowadays, the design of nanoceramics brings potential answers for strengthening of mechanical properties, but with the diameter downsized to the nanoscale, the increasing structural susceptibility of ceramic fiber to phase transformation and grain growth becomes a huge barrier. Here, we propose a strong carboxylic ligand confinement strategy by the combination of formic and acetic acids to control the inorganic colloid growth for fabricating robust α-alumina nanofibers. The rapid hydrolysis and coordination of the carboxylate groups with aluminum together with subsequent concentration synergistically promote the formation of small and compact precursor colloids, laying a solid foundation for suppressing abnormal grain growth and achieving refined alumina grain structure. The local disorder induced by silica and boron oxide surrounding α-alumina grains imparts excellent mechanical properties and flexibility with no fractures observed even after 500 buckling cycles and a wide range of temperatures from -196 to 1100 °C, providing an enlightening paradigm for ceramic fiber strengthening.

Design of Silica-Cobalt Composite Microporous Structures with Dispersed Carbon Particles for Highly Permselective Gas Separation Membranes

Metal-doped silica membranes, fabricated via the sol-gel technique using metal nitrates, hold promise for high-temperature separation processes, such as H2 separation in steam reforming reactions. However, controlling the status of the doped metal is challenging and often leads to defect formation owing to the aggregation of metal oxides. In this study, we designed a uniform carbon-Co-SiO2 ceramic membrane using a one-pot sol-gel method with copolymerization, employing tetraethoxysilane and cobalt acetylacetone(III) (Co-(acac)3) as precursors. Organic chelate ligands within the amorphous silica network formed by the polymerization reaction were carbonized by calcination at 250-750 °C in an inert atmosphere. This approach suppressed defect formation and tailored the microporous structures to a wide range of separation systems. For example, the SiO2-Co-(acac)3 membrane calcined at 550 °C demonstrated a notable C3H6 permeance of 4.0 × 10-8 mol m-2 s-1 Pa-1 (GPU: 120), with a high C3H6/C3H8 selectivity of 46, attributed to the molecular sieving effect, whereas the membrane calcined at 650 °C exhibited a remarkable He permeance of 4.6 × 10-7 mol m-2 s-1 Pa-1 (GPU: 1400), with a high He/CH4 selectivity of 830. This study provides valuable insights into the development of defect-free carbon-cation-SiO2 ceramic membranes for a broad range of gas separation processes.

Evaluation of Admixture Silane Added into Cementitious Pastes

This manuscript evaluated the performance of silanes in cementitious matrices in the partial replacement of superplasticizers by silanes. For this, pastes with a water/cement ratio of 0.186 were produced and the superplasticizer admixture based on polycarboxylate esters was partially substituted by three types of silanes-vinyltriethoxysilane silanes (VTES), n-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTMS), and methacryloxypropyltrimethox-ysilane (MCPTMS)-in two substitutions levels (20% and 40%), and then tested in Portland cement pastes. Specific gravity, trapped air, mini-slump, and hydration kinetics (evaluated by isothermal calorimetry up to 48 h) of the pastes were determined in the fresh state. Thus, in the hardened state, the compressive and flexural strength tests (7 and 28 days), specific gravity, and absorption by immersion of the pastes were carried out. The results showed that the substitution of 20% and 40% of VTES and AEAPTMS considerably reduced the workability and increased the air content of the pastes in comparison to the reference sample. In contrast, the incorporation of 20% and 40% of MCPTMS did not significantly affect these properties. The presence of silane, for all analyzed samples, had a delaying effect on the hydration process: the maximum delay verified had a hydration peak in approximately 36 h for the 40% MCPTMS sample and the minimum delay verified had a hydration peak in approximately 11 h for the 20% VTES sample. The replacement of 20% and 40% by any of the silanes progressively reduced the flexural strength at both 7 and 28 days. In the compressive strength, as well as in the tensile strength in flexion, there was a decrease in the results when compared to the reference, except for the MCPTMS, at 7 and 28 days. In immersion absorption, all samples showed high percentages of absorption and void index when compared to the reference.

Designed functions of oxide/hydroxide nanosheets via elemental replacement/doping

Partial replacement of one structural element in a solid with another of a similar size was conducted to impart functionality to the solids and modify their properties. This phenomenon is found in nature in coloured gemstones and clay minerals and is used in materials chemistry and physics, endowing materials with useful properties that can be controlled by incorporated heteroelements and their amounts. Depending on the area of research (or expected functions), the replacement is referred to as "isomorphous substitution", "doping", etc. Herein, elemental replacement in two-dimensional (2D) oxides and hydroxides (nanosheets or layered materials) is summarised with emphasis on the uniqueness of their preparation, characterisation and application compared with those of the corresponding bulk materials. Among the 2D materials (graphene, metallenes, transition metal chalcogenides, metal phosphate/phosphonates, MXenes, etc.), 2D oxides and hydroxides are characterised by their presence in nature, facile synthesis and storage under ambient conditions, and possible structural variation from atomic-level nanosheets to thicker nanosheets composed of multilayered structures. The heteroelements to be doped were selected depending on the target application objectively; however, there are structural and synthetic limitations in the doping of heteroelements. In the case of layered double hydroxides (single layer) and layered alkali silicates (from single layer to multiple layers), including layered clay minerals (2 : 1 layer), the replacement (commonly called isomorphous substitution) is discussed to understand/design characteristics such as catalytic, adsorptive (including ion exchange), and swelling properties. Due to the variation in their main components, the design of layered transition metal oxide/hydroxide materials via isomorphous substitution is more versatile; in this case, tuning their band structure, doping both holes and electrons, and creating impurity levels are examined by the elemental replacement of the main components. As typical examples, material design for the photocatalytic function of an ion-exchangeable layered titanate (lepidocrocite-type titanate) and a perovskite niobate (KCa2Nb3O10) is discussed, where elemental replacement is effective in designing their multiple functions.

Review on the Polymeric and Chelate Gel Precursor for Li-Ion Battery Cathode Material Synthesis

The rapid design of advanced materials depends on synthesis parameters and design. A wide range of materials can be synthesized using precursor reactions based on chelated gel and organic polymeric gel pathways. The desire to develop high-performance lithium-ion rechargeable batteries has motivated decades of research on the synthesis of battery active material particles with precise control of composition, phase-purity, and morphology. Among the most common methods reported in the literature to prepare precursors for lithium-ion battery active materials, sol-gel is characterized by simplicity, homogeneous mixing, and tuning of the particle shape. The chelate gel and organic polymeric gel precursor-based sol-gel method is efficient to promote desirable reaction conditions. Both precursor routes are commonly used to synthesize lithium-ion battery cathode active materials from raw materials such as inorganic salts in aqueous solutions or organic solvents. The purpose of this review is to discuss synthesis procedure and summarize the progress that has been made in producing crystalline particles of tunable and complex morphologies by sol-gel synthesis that can be used as active materials for lithium-ion batteries.

A Comparative Study of Ni-Based Catalysts Prepared by Various Sol-Gel Routes

The use of heterogeneous catalysts to increase the development of green chemistry is a rapidly growing area of research to save industry money. In this paper, mesoporous SiO2-Al2O3 mixed oxide supports with various Si/Al ratios were prepared using two different sol-gel routes: hydrolytic sol-gel (HSG) and non-hydrolytic sol-gel (NHSG). The HSG route was investigated in both acidic and basic media, while the NHSG was explored in the presence of ethanol and diisopropyl ether as oxygen donors. The resulting SiO2-Al2O3 mixed oxide supports were characterized using EDX, N2 physisorption, powder XRD, 29Si, 27Al MAS-NMR and NH3-TPD. The mesoporous SiO2-Al2O3 supports prepared by NHSG seemed to be more regularly distributed and also more acidic. Consequently, a simple one-step NHSG (ether and alcohol routes) was selected to prepare mesoporous and acidic SiO2-Al2O3-NiO mixed oxide catalysts, which were then evaluated in ethylene oligomerization. The samples prepared by the NHSG ether route showed better activity than those prepared by the NHSG alcohol route in the oligomerization of ethylene at 150 °C.

Hydroxide-Derived Nanostructures: Scalable Synthesis, Characterization, Properties, and Potential Applications

Metal oxide nanostructures have received an increasing attention owing to their unique chemical and physical properties along with their widespread applications in various fields. This article provides an overview of the recent discovery - christened Hydroxides-Derived Nanostructures, or HDNs - in which hydroxide aqueous solutions (mostly tetramethylammonium hydroxide, TMAH) are reacted at temperatures < 100 °C and under atmospheric pressure with various metal-containing precursors to scalably prepare novel metal oxide nanostructures. In one case, a dozen commercial and earth abundant Ti-containing powders such as binary carbides, nitrides, borides, among others, are converted into new, 1D TiO2-based lepidocrocite (1DL) nanofilaments (NFs). Application-wise, the 1DLs show outstanding performance in a number of energy, environmental, and biomedical fields such as photo- and electrocatalysis, water splitting, lithium-sulfur and lithium-ion batteries, water purification, dye degradation, cancer therapy, and polymer composites. In addition to 1DL, the HDNs family encompasses other metal oxides nanostructures including magnetic Fe3O4 nanoparticles and MnO2 birnessite-based crystalline 2D flakes. The latter showed promise in electrochemical energy conversion and storage applications. The developed recipe provides a new vista in the molecular self-assembly synthesis of nanomaterials that can advance the field with a library of novel nanostructures with substantial implications in a multitude of fields.

From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

Nonaqueous sol-gel syntheses have been used to make many types of metal oxide nanocrystals. According to the current paradigm, nonaqueous syntheses have slow kinetics, thus favoring the thermodynamic (crystalline) product. Here we investigate the synthesis of hafnium (and zirconium) oxide nanocrystals from the metal chloride in benzyl alcohol. We follow the transition from precursor to nanocrystal through a combination of rheology, EXAFS, NMR, TEM, and X-ray total scattering (PDF analysis). Upon dissolving the metal chloride precursor, the exchange of chloride ligands for benzylalkoxide liberates HCl. The latter catalyzes the etherification of benzyl alcohol, eliminating water. During the temperature ramp to the reaction temperature (220 °C), sufficient water is produced to turn the reaction mixture into a macroscopic gel. Rheological analysis shows a network consisting of strong interactions with temperature-dependent restructuring. After a few minutes at the reaction temperature, crystalline particles emerge from the gel, and nucleation and growth are complete after 30 min. In contrast, 4 h are required to obtain the highest isolated yield, which we attribute to the slow in situ formation of water (the extraction solvent). We used our mechanistic insights to optimize the synthesis, achieving high isolated yields with a reduced reaction time. Our results oppose the idea that nonaqueous sol-gel syntheses necessarily form crystalline products in one step, without a transient, amorphous gel state.

Synthesis of pure and doped nano-calcium phosphates using different conventional methods for biomedical applications: a review

The applications of calcium phosphates (hydroxyapatite, tetracalcium phosphate, tricalcium phosphate (alpha and beta), fluorapatite, di-calcium phosphate anhydrous, and amorphous calcium-phosphate) are increasing day by day. Calcium hydroxyapatite, commonly known as hydroxyapatite (HAp), represents a mineral form of calcium apatite. Owing to its close molecular resemblance to the mineral constituents of bones, teeth, and hard tissues, HAp is often employed in the biomedical domain. In addition, it is extensively employed in various sectors such as the remediation of water, air, and soil pollution. The key advantage of HAp lies in its potential to accommodate a wide variety of anionic and cationic substitutions. Nevertheless, HAp and tricalcium phosphate (TCP) syntheses typically involve the use of chemical precursors containing calcium and phosphorus sources and employ diverse techniques, such as solid-state, wet, and thermal methods or a combination of these processes. Researchers are increasingly favoring natural sources such as bio-waste (eggshells, oyster shells, animal bones, fish scales, etc.) as viable options for synthesizing HAp. Interestingly, the synthesis route significantly influences the morphology, size, and crystalline phase of calcium phosphates. In this review paper, we highlight both dry and wet methods, which include six commonly used synthesis methods (i.e. solid-state, mechano-chemical, wet-chemical precipitation, hydrolysis, sol-gel, and hydrothermal methods) coupled with the variation in source materials and their influence in modifying the structural morphology from a bulky state to nanoscale to explore the applications of multifunctional calcium phosphates in different formats.

Refined structural studies on the fluorite-related polymorphs of sol-gel undoped and Eu3+-doped yttrium tantalates

Compounds with the general formula RE3MO7 (RE = rare earth ions; M = Ta, Nb, Sb, Ru, Ir, Os, Re, etc.), crystallize as a fluorite-related structure, forming polymorphs with different space groups. The space group strongly depends on the RE3+ and M5+ ionic radii and processing conditions. Structural characterization is well-established for the lanthanide series, but literature studies have divergent views about how to attribute yttrium tantalate (Y3TaO7) space groups-some authors have described the Y3TaO7 structure as orthorhombic and belonging to space group C2221 or Ccmm, whereas others have assigned a cubic Fm3̄m structure to it. Here, we have characterized the structure of undoped and Eu3+-doped Y3TaO7 (0.1 to 50 mol% of Eu3+) samples synthesized by the sol-gel method that crystallized as a cubic disordered fluorite-type structure, space group Fm3̄m. Their powder X-ray diffraction measurements, Rietveld analyzes and Raman spectra were used as a conclusive technique of the structural properties. We have also investigated whether a secondary phase (M'-YTaO4) emerged in the samples and compared the phase composition of each sample to their Raman spectra. Low-temperature photoluminescence measurements (∼15 K) using Eu3+ as a structural probe helped us analyze the inhomogeneous broadening observed in the emission spectra. These measurements can be used as an important tool to attribute the crystalline phases of rare earth tantalates and niobates.

Use of Ball Drop Casting and Surface Modification for the Development of Amine-Functionalized Silica Aerogel Globules for Dynamic and Efficient Direct Air Capture

Amine-functionalized silica aerogel globules (AFSAGs) were first synthesized via a simple ball drop casting method followed by amine grafting. The effect of grafting time on the structure and CO2 adsorption performance of the AFSAGs was investigated. The CO2 adsorption performance was comprehensively studied by breakthrough curves, adsorption capacity and rates, surface amine loading and density, amine efficiency, adsorption halftime, and cyclic stability. The results demonstrate that prolonging the grafting time does not lead to a significant increase in surface amine content owing to pore space blockage by superabundant amine groups. The CO2 adsorption performance shows obvious dependence on surface amine density, determined by both the surface amine content and specific surface area, and working temperature. AFSAGs with a grafting time of 24 h (AFSAG24) with a moderate surface amine density have optimal CO2 adsorption capacities, which are 1.78 and 2.14 mmol/g at 25 °C with dry and humid 400 ppm CO2, respectively. The amine efficiency of AFSAG24 with low CO2 concentrations, 0.38-0.63 with dry 400 ppm-1% CO2, is the highest among the reported amine-functionalized adsorbents. After estimation with different diffusion models, the CO2 adsorption process of AFSAG24 is governed by film diffusion and intraparticle diffusion. In the range of 1-4 mm, the ball size does not affect the CO2 adsorption capacity of AFSAG24 obviously. AFSAG24 offers significant advantages for practical direct air capture compared with its state-of-the-art counterparts, such as high dynamic adsorption capacity and amine efficiency, excellent stability, and outstanding adaptation to the environment.

Surface Chemistry of Biologically Active Reducible Oxide Nanozymes

Reducible metal oxide nanozymes (rNZs) are a subject of intense recent interest due to their catalytic nature, ease of synthesis, and complex surface character. Such materials contain surface sites which facilitate enzyme-mimetic reactions via substrate coordination and redox cycling. Further, these surface reactive sites are shown to be highly sensitive to stresses within the nanomaterial lattice, the physicochemical environment, and to processing conditions occurring as part of their syntheses. When administered in vivo, a complex protein corona binds to the surface, redefining its biological identity and subsequent interactions within the biological system. Catalytic activities of rNZs each deliver a differing impact on protein corona formation, its composition, and in turn, their recognition, and internalization by host cells. Improving the understanding of the precise principles that dominate rNZ surface-biomolecule adsorption raises the question of whether designer rNZs can be engineered to prevent corona formation, or indeed to produce "custom" protein coronas applied either in vitro, and preadministration, or formed immediately upon their exposure to body fluids. Here, fundamental surface chemistry processes and their implications in rNZ material performance are considered. In particular, material structures which inform component adsorption from the application environment, including substrates for enzyme-mimetic reactions are discussed.

Investigation of Structural and Antibacterial Properties of WS2-Doped ZnO Nanoparticles

ZnO nanoparticles, well-known for their structural, optical, and antibacterial properties, are widely applied in diverse fields. The doping of different materials to ZnO, such as metals or metal oxides, is known to ameliorate its properties. Here, nanofilms composed of ZnO doped with WS2 at 5, 15, and 25% ratios are synthesized, and their properties are investigated. Supported by molecular docking analyses, the enhancement of the bactericidal properties after the addition of WS2 at different ratios is highlighted and supported by the inhibitory interaction of residues playing a crucial role in the bacterial survival through the targeting of proteins of interest.

Hybrid Materials: A Metareview

The field of hybrid materials has grown so wildly in the last 30 years that writing a comprehensive review has turned into an impossible mission. Yet, the need for a general view of the field remains, and it would be certainly useful to draw a scientific and technological map connecting the dots of the very different subfields of hybrid materials, a map which could relate the essential common characteristics of these fascinating materials while providing an overview of the very different combinations, synthetic approaches, and final applications formulated in this field, which has become a whole world. That is why we decided to write this metareview, that is, a review of reviews that could provide an eagle's eye view of a complex and varied landscape of materials which nevertheless share a common driving force: the power of hybridization.

Mnemonic Rutile-Rutile Interfaces Triggering Spontaneous Dissociation of Water

Water interaction with mineral surfaces is a complex living system decisive for any photocatalytic process. Resolving the atomistic structure of mineral-water interfaces is thus crucial for understanding these processes. Fibrous rutile TiO2 , grown hydrothermally on twinned rutile seeds under acidic conditions, is studied in terms of interface translation, atomic structure, and surface chemistry in the presence of water, by means of advanced microscopy and spectroscopy methods combined with structure modeling and density functional theory calculations. It is shown that fibers while staying in stable separation during their growth, adopt a special crystallographic registry that is controlled by repulsion forces between fully hydroxylated and protonated (110) surfaces. During relaxation, a turbulent proton transfer and cracking of O─H bonds is observed, generating a strong acidic character via proton jump from bridge ─OHb to terminal ─OHt groups, and spontaneous dissociation of interfacial water via a transient protonation of the ─OHt groups. It is shown, that this specific interface structure can be implemented to induce acidic response in an initially neutral medium when re-immersed. This is thought to be the first demonstration of quantum-confined mineral-water interface, capable of memorizing its past and conveying its structurally encoded properties into a new environment.

Synthesis and Characterization of the Zinc-Oxide: Tin-Oxide Nanoparticle Composite and Assessment of Its Antibacterial Activity: An In Vitro Study

Introduction Nanoparticles (NPs) have been widely used for biomedical applications. Various methods of synthesis of NPs have been performed and the sol-gel technique is one of the most common and feasible methods. ZnO and SnO2 NPs are widely used due to their interesting properties and versatile medical applications. The present study aimed to synthesize a composite of ZnO- SnO2 NPs and evaluate its structural, morphological, and antibacterial properties. Materials and methods ZnO-SnO2 NPs were prepared via the sol-gel technique. The morphological study was performed by scanning electron microscopy (SEM) imaging, the structural study was performed by X-ray diffraction (XRD) analysis, and chemical studies were performed by Fourier transform infrared spectroscopy (FT-IR) and energy-dispersive X-ray spectroscopy (EDAX). Antibacterial properties of the NPs were assessed by the agar diffusion test and the area of bacterial growth that was inhibited was measured under high and low concentrations of the NPs. Results The SEM analysis confirmed the irregular shape and elemental composition of the synthesized NPs. The purity of the NPs was confirmed by the EDAX spectrum, which indicates the weight percentages of the elements in the NPs as follows: Sn-53.8%, Zn-12.5%, O-29.1%, and C-4.7%. The chemical bonds between the NPs were confirmed by Fourier transform infrared spectroscopy. XRD analysis confirmed the high degree of crystallinity of the NPs and orthorhombic structure of SnO2 and the hexagonal structure of ZnO. The zone of inhibition against S. aureus, S. mutans, and E. coli for low concentrations of the NPs was 24 mm, 26 mm, and 30 mm and for high concentrations of the NPs it was 26 mm, 28 mm, and 31mm and these values were similar to the control antibiotics.  Conclusion ZnO- SnO2 NPs were successfully prepared by the sol-gel method. The presence of NPs was confirmed and successfully characterized. The prepared NPs had a good antimicrobial effect against the tested pathogens.

La2O3-CeO2-Supported Bimetallic Cu-Ni DRM Catalysts

The present work is focused on nickel catalysts supported on La2O3-CeO2 binary oxides without and with the addition of Cu to the active component for the dry reforming of methane (DRM). The catalysts are characterized using XRD, XRF, TPD-CO2, TPR-H2, and low-temperature N2 adsorption-desorption methods. This work shows the effect of different La:Ce ratios (1:1 and 9:1) and the Cu addition on the structural, acid base, and catalytic properties of Ni-containing systems. The binary LaCeOx oxide at a ratio of La:Ce = 1:1 is characterized by the formation of a solid solution with a fluorite structure, which is preserved upon the introduction of mono- or bimetallic particles. At La:Ce = 9:1, La2O3 segregation from the solid solution structure is observed, and the La excess determines the nature of the precursor of the active component, i.e., lanthanum nickelate. The catalysts based on LaCeOx (1:1) are prone to carbonization during 6 h spent on-stream with the formation of carbon nanotubes. The Cu addition facilitates the reduction of the Cu-Ni catalyst carbonization and increases the number of structural defects in the carbon deposition products. The lanthanum-enriched LaCeOx (9:1) support prevents the accumulation of carbon deposition products on the surface of CuNi/La2O3-CeO2 9:1, providing high DRM activity and an H2/CO ratio of 0.9.

Tunable particle-agglomeration and magnetic coupling in bi-magnetic nanocomposites

A set of non-stoichiometric Zn-Co-ferrite nanoparticles (NPs) was prepared by thermal decomposition of metallic complexes, in the presence of oleic acid, and, after a ligand-exchange process, was coated by a hydrophilic surfactant: these NPs were used as seeds in a sol-gel self-combustion synthesis to prepare nanocomposites (NCs) with a fixed weight ratio. Our focus here is the development of an efficient synthetic approach to control the magnetic coupling between a hard-magnetic matrix (Sr-ferrite) and NPs. The physico-chemical synthetic conditions (temperature, pH, colloidal stability) were optimized in order to tune their effect on the final particles' agglomeration in the matrix. We demonstrate that our synthetic approach is a novel way to produce strongly magnetically coupled NCs, where the final extrinsic properties could be tuned by controlling (i) the agglomeration of seeds in the matrix and (ii) their elemental doping.

Gel transformation as a general strategy for fabrication of highly porous multiscale MOF architectures

The structure and chemistry of metal-organic frameworks or MOFs dictate their properties and functionalities. However, their architecture and form are essential for facilitating the transport of molecules, the flow of electrons, the conduction of heat, the transmission of light, and the propagation of force, which are vital in many applications. This work explores the transformation of inorganic gels into MOFs as a general strategy to construct complex porous MOF architectures at nano, micro, and millimeter length scales. MOFs can be induced to form along three different pathways governed by gel dissolution, MOF nucleation, and crystallization kinetics. Slow gel dissolution, rapid nucleation, and moderate crystal growth result in a pseudomorphic transformation (pathway 1) that preserves the original network structure and pores, while a comparably faster crystallization displays significant localized structural changes but still preserves network interconnectivity (pathway 2). MOF exfoliates from the gel surface during rapid dissolution, thus inducing nucleation in the pore liquid leading to a dense assembly of percolated MOF particles (pathway 3). Thus, the prepared MOF 3D objects and architectures can be fabricated with superb mechanical strength (>98.7 MPa), excellent permeability (>3.4 × 10^-10 m²), and large surface area (1100 m² g^-1) and mesopore volumes (1.1 cm³ g^-1).

Adsorption properties of silica aerogel-based materials

Silica aerogels have piqued the interest of both scientists and industry in recent decades due to their unusual properties such as low density, high porosity, low thermal and acoustic conductivity, high optical transparency, and strong sorption activity. Aerogels may be created via two-step sol-gel synthesis from different organosilicon compounds known as precursors. Various drying processes are employed to remove the solvent from the gel pores, the most common of which is the supracritical drying method. This paper highlights the potential of silica aerogels and their modifications as adsorbents for environmental cleanup based on recent researches. Following an introduction of the characteristics of aerogels, production techniques, and different categorization possibilities, the study is organized around their potential use as adsorbents.

Importance of Process Variables and Their Optimization for Oxidative Coupling of Methane (OCM)

Oxidative coupling of methane (OCM) is a promising process for converting natural gas into high-value chemicals such as ethane and ethylene. The process, however, requires important improvements for commercialization. The foremost is increasing the process selectivity to C₂ (C₂H₄ + C₂H₆) at moderate to high levels of methane conversion. These developments are often addressed at the catalyst level. However, optimization of process conditions can lead to very important improvements. In this study, a high-throughput screening (HTS) instrument was utilized for La₂O₃/CeO₂ (3.3 mol % Ce) to generate a parametric data set within the temperature range of 600-800 °C, CH₄/O₂ ratio between 3 and 13, pressure between 1 and 10 bar, and catalyst loading between 5 and 20 mg leading to space-time between 40 and 172 s. Statistical design of experiments (DoE) was applied to gain insights into the effect of operating parameters and to determine the optimal operating conditions for maximum production of ethane and ethylene. Rate-of-production analysis was used to shed light on the elementary reactions involved in different operating conditions. The data obtained from HTS experiments established quadratic equations relating the studied process variables and output responses. The quadratic equations can be used to predict and optimize the OCM process. The results demonstrated that the CH₄/O₂ ratio and operating temperatures are key for controlling the process performance. Operating at higher temperatures with high CH₄/O₂ ratios increased the selectivity to C₂ and minimized CO_x (CO + CO₂) at moderate conversion levels. In addition to process optimization, DoE results also allowed the flexibility of manipulating the performance of OCM reaction products. A C₂ selectivity of 61% and a methane conversion of 18% were found to be optimum at 800 °C, a CH₄/O₂ ratio of 7, and a pressure of 1 bar.

Highly customized porous TiO2-PANI nanoparticles with excellent photocatalytic efficiency for dye degradation

The present work encompasses a simple strategy to synthesize highly porous TiO₂ by incorporating PANI polymer into the sol-gel chemistry of Titanium dioxide/Titanium (IV)-iso-propoxide (TiO₂/TTIP). A series of TiO₂ samples by varying wt.% of PANI, have been synthesized. A probable growth mechanism has been presented for the formation of a porous ginger-like nanostructure of TiO₂-PANI (TP). HRTEM images reveal that the particle size range is 6-16 nm for pristine TiO₂ and 5-13 nm for TP samples. XPS analysis confirms the presence of the hydrogen bonds in-between surface hydroxyl groups (Ti-OH) of TiO₂ and the protonated nitrogen of PANI. UV-visible absorption study reveals a small shift towards longer wavelength for TP8 sample than that of pristine TiO₂ (λ_max = 314 nm) as well as reduction in E_g value from 3.02 eV to 2.89 eV. FTIR results confirm the successful interaction of PANI and TiO₂. BJH and BET analysis confirms an increase of porosity in TP8 sample with an average pore volume of 0.36 cm³ g^-1. High photocatalytic activity (98.77%) towards Methylene blue dye degradation is observed for TP8 sample having 8 wt% of PANI and it is explained through the combined effect of structural porosity of TiO₂ and synergic effect of PANI. The K_appa value at pH 11 (0.01372 min^-1) is found to be 7.84-folds higher than that of the photocatalytic reaction at pH 3 (K_appa = 0.00175 min^-1). While pristine TiO₂ exhibits the minimum removal efficiency (89.57%) with K_appa of 0.00756 min^-1. K_appa value of catalysis reaction for TP8 is found to be almost 2-fold higher than pristine TiO₂. Quantum Yield value for TP8 is found to be 3.59 × 10^-4 molecules photon^-1. This high Quantum Yield value of present photocatalytic system explicates the low energy consumption for the treatment of textile dye pollutant. Additionally, STY value (1.79 × 10^-5 molecules photon^-1 mg^-1) confirms the outstanding mineralization strength of TP8 by a unit mass for high amounts of MB dye per unit time. Thus, the present study offers an excellent photocatalyst i.e., TP8 having 8 wt% of PANI for the degradation of MB dye.

A Sol-Gel/Solvothermal Synthetic Approach to Titania Nanoparticles for Raman Thermometry

The accurate determination of the local temperature is one of the most important challenges in the field of nanotechnology and nanomedicine. For this purpose, different techniques and materials have been extensively studied in order to identify both the best-performing materials and the techniques with greatest sensitivity. In this study, the Raman technique was exploited for the determination of the local temperature as a non-contact technique and titania nanoparticles (NPs) were tested as nanothermometer Raman active material. Biocompatible titania NPs were synthesized following a combination of sol-gel and solvothermal green synthesis approaches, with the aim of obtaining pure anatase samples. In particular, the optimization of three different synthesis protocols allowed materials to be obtained with well-defined crystallite dimensions and good control over the final morphology and dispersibility. TiO₂ powders were characterized by X-ray diffraction (XRD) analyses and room-temperature Raman measurements, to confirm that the synthesized samples were single-phase anatase titania, and using SEM measurements, which clearly showed the nanometric dimension of the NPs. Stokes and anti-Stokes Raman measurements were collected, with the excitation laser at 514.5 nm (CW Ar/Kr ion laser), in the temperature range of 293-323 K, a range of interest for biological applications. The power of the laser was carefully chosen in order to avoid possible heating due to the laser irradiation. The data support the possibility of evaluating the local temperature and show that TiO₂ NPs possess high sensitivity and low uncertainty in the range of a few degrees as a Raman nanothermometer material.

Recent Progress in Solution Processed Aluminum and co-Doped ZnO for Transparent Conductive Oxide Applications

With the continuous growth in the optoelectronic industry, the demand for novel and highly efficient materials is also growing. Specifically, the demand for the key component of several optoelectronic devices, i.e., transparent conducting oxides (TCOs), is receiving significant attention. The major reason behind this is the dependence of the current technology on only one material-indium tin oxide (ITO). Even though ITO still remains a highly efficient material, its high cost and the worldwide scarcity of indium creates an urgency for finding an alternative. In this regard, doped zinc oxide (ZnO), in particular, solution-processed aluminum doped ZnO (AZO), is emerging as a leading candidate to replace ITO due to its high abundant and exceptional physical/chemical properties. In this mini review, recent progress in the development of solution-processed AZO is presented. Beside the systematic review of the literature, the solution processable approaches used to synthesize AZO and the effect of aluminum doping content on the functional properties of AZO are also discussed. Moreover, the co-doping strategy (doping of aluminum with other elements) used to further improve the properties of AZO is also discussed and reviewed in this article.

Chemical Preintercalation Synthesis of Versatile Electrode Materials for Electrochemical Energy Storage

ConspectusThe widespread use of electrical plants and grids to generate, transmit, and deliver power to consumers makes electricity the most convenient form of energy to transport, control, and use. Balancing electricity demand with electricity supply requires a mechanism for energy storage, which is enabled by electrical energy storage devices such as batteries and supercapacitors. In addition to the grid-level energy storage, we have all witnessed the quick growth of a number of applications that require autonomous power, illustrated by the Internet of Things, and electrification of transport. Batteries, when developed for targeted applications with specific requirements, require new materials with improved performance enabled by rational design on the atomic level. The material tunability knobs include chemical composition, structure, morphology, and heterointerfaces, among others. Synthesis methods that could enable control of these parameters while offering versatility and being facile are highly desired.In this Account, we describe a synthesis strategy for the creation of new intercalation host oxides, hybrid materials, and compounds with oxide/carbon heterointerfaces for use as electrodes in intercalation batteries. We begin by introducing a strategy called the chemical preintercalation synthesis approach and describing processing steps that can be used to tune the material's chemical composition, structure, and morphology. We then show how the chemical preintercalation of inorganic ions can be used to improve the ion diffusion and stability of the synthesized materials. We reveal how confined interlayer water can be controlled and how the degree of hydration affects the electrochemical performance. This is followed by a demonstration of the chemical preintercalation of organic molecules leading to unprecedented expansion of the interlayer region up to ∼30 Å and initial electrochemical characterization of the obtained hybrid materials. We then present evidence that the carbonization of the interlayer organic molecules is an efficient synthetic pathway for creating oxide/carbon heterointerfaces and improving the electronic conductivity of oxides, which leads to improved stability and rate capability during electrochemical cycling. The examples discussed in this Account show that the chemical preintercalation synthesis approach opens pathways for the preparation of materials that have not been synthesized previously, such as new phases, hybrid materials, and 2D heterostructures with advanced functionalities. We demonstrate that chemical preintercalation can be used to effectively tune the chemistry of the confined interlayer region in layered phases and form tight oxide/carbon heterointerfaces enabling control of the material properties at the atomic level.

Hydrothermal Stability of Hydrogen-Selective Carbon-Ceramic Membranes Derived from Polybenzoxazine-Modified Silica-Zirconia

This work investigated the long-term hydrothermal performance of composite carbon-SiO₂-ZrO₂ membranes. A carbon-SiO₂-ZrO₂ composite was formed from the inert pyrolysis of SiO₂-ZrO₂-polybenzoxazine resin. The carbon-SiO₂-ZrO₂ composites prepared at 550 and 750 °C had different surface and microstructural properties. A carbon-SiO₂-ZrO₂ membrane fabricated at 750 °C exhibited H₂ selectivity over CO₂, N₂, and CH₄ of 27, 139, and 1026, respectively, that were higher than those of a membrane fabricated at 550 °C (5, 12, and 11, respectively). In addition to maintaining high H₂ permeance and selectivity, the carbon-SiO₂-ZrO₂ membrane fabricated at 750 °C also showed better stability under hydrothermal conditions at steam partial pressures of 90 (30 mol%) and 150 kPa (50 mol%) compared with the membrane fabricated at 500 °C. This was attributed to the complete pyrolytic and ceramic transformation of the microstructure after pyrolysis at 750 °C. This work thus demonstrates the promise of carbon-SiO₂-ZrO₂ membranes for H₂ separation under severe hydrothermal conditions.

Structural, Optical, and Sensing Properties of Nb-Doped ITO Thin Films Deposited by the Sol–Gel Method

The aim of the present study was the development of Nb-doped ITO thin films for carbon monoxide (CO) sensing applications. The detection of CO is imperious because of its high toxicity, with long-term exposure having a negative impact on human health. Using a feasible sol–gel method, the doped ITO thin films were prepared at room temperature and deposited onto various substrates (Si, SiO2/glass, and glass). The structural, morphological, and optical characterization was performed by the following techniques: X-ray diffractometry (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV/Vis/NIR spectroscopic ellipsometry (SE). The analysis revealed a crystalline structure and a low surface roughness of the doped ITO-based thin films. XTEM analysis (cross-sectional transmission electron microscopy) showed that the film has crystallites of the order of 5–10 nm and relatively large pores (around 3–5 nm in diameter). A transmittance value of 80% in the visible region and an optical band-gap energy of around 3.7 eV were found for dip-coated ITO/Nb films on SiO2/glass and glass supports. The EDX measurements proved the presence of Nb in the ITO film in a molar ratio of 3.7%, close to the intended one (4%). Gas testing measurements were carried out on the ITO undoped and doped thin films deposited on glass substrate. The presence of Nb in the ITO matrix increases the electrical signal and the sensitivity to CO detection, leading to the highest response for 2000 ppm CO concentration at working temperature of 300 °C.

Tuning ceria catalysts in aqueous media at the nanoscale: how do surface charge and surface defects determine peroxidase- and haloperoxidase-like reactivity

Designing the shape and size of catalyst particles, and their interfacial charge, at the nanometer scale can radically change their performance. We demonstrate this with ceria nanoparticles. In aqueous media, nanoceria is a functional mimic of haloperoxidases, a group of enzymes that oxidize organic substrates, or of peroxidases that can degrade reactive oxygen species (ROS) such as H₂O₂ by oxidizing an organic substrate. We show that the chemical activity of CeO_2-x nanoparticles in haloperoxidase- and peroxidase-like reactions scales with their active surface area, their surface charge, given by the ζ-potential, and their surface defects (via the Ce³⁺/Ce⁴⁺ ratio). Haloperoxidase-like reactions are controlled through the ζ-potential as they involve the adsorption of charged halide anions to the CeO₂ surface, whereas peroxidase-like reactions without charged substrates are controlled through the specific surface area S_BET. Mesoporous CeO_2-x particles, with large surface areas, were prepared via template-free hydrothermal reactions and characterized by small-angle X-ray scattering. Surface area, ζ-potential and the Ce³⁺/Ce⁴⁺ ratio are controlled in a simple and predictable manner by the synthesis time of the hydrothermal reaction as demonstrated by X-ray photoelectron spectroscopy, sorption and ζ-potential measurements. The surface area increased with synthesis time, whilst the Ce³⁺/Ce⁴⁺ ratio scales inversely with decreasing ζ-potential. In this way the catalytic activity of mesoporous CeO_2-x particles could be tailored selectively for haloperoxidase- and peroxidase-like reactions. The ease of tuning the surface properties of mesoporous CeO_2x particles by varying the synthesis time makes the synthesis a powerful general tool for the preparation of nanocatalysts according to individual needs.

Solution combustion synthesis: the relevant metrics for producing advanced and nanostructured photocatalysts

The current developments and progress in energy and environment-related areas pay special attention to the fabrication of advanced nanomaterials via green and sustainable paths to accomplish chemical circularity. The design and preparation methods of photocatalysts play a prime role in determining the structural, surface characteristics and optoelectronic properties of the final products. The solution combustion synthesis (SCS) technique is a relatively novel, cost-effective, and efficient method for the bulk production of nanostructured materials. SCS-fabricated metal oxides are of great technological importance in photocatalytic, environmental and energy applications. To date, the SCS route has been employed to produce a large variety of solid materials such as metals, sulfides, carbides, nitrides and single or complex metal oxides. This review intends to provide a holistic perspective of the different steps involved in the chemistry of SCS of advanced photocatalysts, and pursues several SCS metrics that influence their photocatalytic performances to establish a feasible approach to design advanced photocatalysts. The study highlights the fundamentals of SCS and the importance of various combustion parameters in the characteristics of the fabricated photocatalysts. Consequently, this work deals with the design of a concise framework to link the fine adjustment of SCS parameters for the development of efficient metal oxide photocatalysts for energy and environmental applications.

Monitoring of CaCO3 Nanoscale Structuration through Real-Time Liquid Phase Transmission Electron Microscopy and Hyperpolarized NMR

Calcium carbonate (CaCO₃) is one of the most significant biominerals in nature. Living organisms are able to control its biomineralization by means of an organic matrix to tailor a myriad of hybrid functional materials. The soluble organic components are often proteins rich in acidic amino-acids such as l-aspartic acid. While several studies have demonstrated the influence of amino acids on the crystallization of calcium carbonate, nanoscopic insight of their impact on CaCO₃ mineralization, in particular at the early stages, is still lacking. Herein, we implement liquid phase-transmission electron microscopy (LP-TEM) in order to visualize in real-time and at the nanoscale the prenucleation stages of CaCO₃ formation. We observe that l-aspartic acid favors the formation of individual and aggregated prenucleation clusters which are found stable for several minutes before the transformation into amorphous nanoparticles. Combination with hyperpolarized solid state nuclear magnetic resonance (DNP NMR) and density functional theory (DFT) calculations allow shedding light on the underlying mechanism at the prenucleation stage. The promoting nature of l-aspartic acid with respect to prenucleation clusters is explained by specific interactions with both Ca²⁺ and carbonates and the stabilization of the Ca²⁺-CO₃^2-/HCO₃^- ion pairs favoring the formation and stabilization of the CaCO₃ transient precursors. The study of prenucleation stages of mineral formation by the combination of in situ LP-TEM, advanced analytical techniques (including hyperpolarized solid-state NMR), and numerical modeling allows the real-time monitoring of prenucleation species formation and evolution and the comprehension of their relative stability.

Hybrid Alginate-Brushite Beads Easily Catalyze the Knoevenagel Condensation On-Water

An innovative hybrid organic-inorganic material composed of alginate-brushite xerogel beads was successfully applied for the catalysis of the Knoevenagel condensation. The catalyst was derived from phosphated alginate xerogel microspheres formed from the ionotropic gelling effect of phosphated alginate. To this end, alginate was phosphated by the addition of diammonium hydrogen phosphate in a 1% w/w alginate gel. The phosphated alginate was subsequently precipitated by chelation of Ca²⁺ cations, generating a phosphated alginate hydrogel microsphere, which was washed and dried, forming hybrid organic-inorganic xerogel beads as a crystalline phosphate-rich mineral fraction covered by alginate. X-ray diffraction analysis revealed that the crystalline inorganic matrix of the material was composed predominantly of brushite. SEM analysis revealed plate-like, ribbon-like, or needle-like morphologies in the hybrid alginate-brushite beads. The hybrid material was tested as a catalyst for Knoevenagel condensation, which was performed ″on-water″ under mild conditions with aromatic aldehydes and activated methylene compounds, giving high yields (up to 97%). The reaction rate and product yield increased together with the reaction temperature for all reagents. The recyclable solid catalyst was effective for three runs, revealing the potential of the innovative hybrid catalyst as an eco-friendly heterogeneous catalyst.

Enhanced Antioxidant Ability of PEG-Coated Ce0.5Zr0.5O2-Based Nanofluids for Scavenging Hydroxyl Radicals

The antioxidant therapy to preserve residual hearing is relatively recent, and the search for effective antioxidants is still ongoing. Though nanoceria has shown promising radical-scavenging capability, improving its antioxidant ability and the dispersion stability of its nanofluid, which is critical to the desired site, i.e., cochlea, still remains a major challenge. The objective of the present work is to study the radical-scavenging capability of poly(ethylene glycol) (PEG)-coated CeO₂ and Ce_0.5Zr_0.5O₂ nanoparticles in water and the biologically relevant fluid (PBS buffer). Nanoparticles in the size range of 4.0-9.0 nm are synthesized using the coprecipitation method and characterized using suitable techniques. The scavenging and dispersion stability of the synthesized nanofluid are analyzed using a UV-vis spectrophotometer. It is found that the addition of PEG during the synthesis process promoted the generation of finer nanoparticles with a narrow size distribution and the doping of zirconium produced a large number of defects in the crystallite structure. The PEG coating over the nanoparticles improved the dispersion stability of nanofluids without affecting their surface reactivity, and it is found to be 94 and 80% in water and PBS, respectively, at 500 μM and 60 min, which is maintained till 90 min. The highest scavenging of hydroxyl radicals by PEG-coated Ce_0.5Zr_0.5O₂ is found to be 60%, which is significantly superior to that of CeO₂. The scavenging capability is found to be increased with the concentration of nanoparticles, showing the best scavenging activity at 190 and 150 μM for PEG-coated CeO₂ and Ce_0.5Zr_0.5O₂, respectively, and the scavenging in water is at par with that of PBS, indicating that these nanoparticles are suitable to be used in sites where a biologically relevant fluid is present, e.g., the cochlea. It is proposed that PEG-coated Ce_0.5Zr_0.5O₂ having an average size of ∼ 4 nm can be a potential antioxidant in relevant biomedical applications.

Solution-Based Synthesis Routes for the Preparation of Noncentrosymmetric 0-D Oxide Nanocrystals with Perovskite and Nonperovskite Structures

With the miniaturization of electronic-based devices, the foreseen potential of new optical nanoprobes and the assessment of eventual size and shape effects, elaboration of multifunctional noncentrosymmetric nanocrystals with ferroelectric, pyroelectric, piezoelectric, and nonlinear optical properties are the subject of an increasing research interest. Here, the recent achievements from the solution-based methods (coprecipitation in homogeneous and nanostructured media, sol-gel processes including various chemistries and hydro/solvothermal techniques) to prepare 0-D perovskite and nonperovskite oxides in the 5-500 nm size range are critically reviewed. To cover a representative list of covalent- and ionic-type materials, BaTiO₃ and its derivatives, niobate compounds (i.e., K/Na/LiNbO₃ ), multiferroic BiFeO_3, and crystals of lower symmetry including KTiOPO₄ and some iodate compounds such as Fe(IO₃ )₃ and La(IO₃ )₃ are systematically in focus. The resulting size, morphology, and aggregation state are discussed in light of the proposed formation mechanisms. Because of a higher complexity related to their chemical composition and crystalline structures, improving the rational design of these multifunctional oxides in terms of finely-tuned compositions, crystalline hosts and structure-property relationships still need in the future a special attention of the research community to the detailed understanding of the reaction pathways and crystallization mechanisms.

Sulfuric Acid Solutions of [Pt(OH)4(H2O)2]: A Platinum Speciation Survey and Hydrated Pt(IV) Oxide Formation for Practical Use

The systematic study of the platinum speciation in sulfuric acid solutions of platinum (IV) hydroxide {[Pt(OH)₄(H₂O)₂], HHPA} was performed with the use of a combination of methods. Depending on the prevailing Pt form, the three regions of H₂SO₄ concentration were marked: (1) up to 3 M H₂SO₄ forms unstable solutions gradually generating the PtO₂·xH₂O particles; (2) 4-12 M H₂SO₄, where the series of mononuclear aqua-sulfato complexes ([Pt(SO₄)_n(H₂O)_6-n]^4-2n, where n = 0···4) dominate; and (3) 12 M and above, where, along with [Pt(SO₄)_n(H₂O)_6-n]^4-2n species, the polynuclear Pt(IV) species and complexes with a bidentate coordination mode of the sulfato ligand are formed. For the first time, the salts of the aqua-hydroxo Pt(IV) cation [Pt(OH)₂(H₂O)₄]SO₄ (triclinic and monoclinic phases) were isolated and studied with a combination of methods, including the single-crystal X-ray diffraction. The formation of PtO₂·xH₂O particles in sulfuric acid solutions (1-3 M) of HHPA and their spectral characteristics and morphology were studied. The deposition of PtO₂·xH₂O was highlighted as a convenient method to prepare various Pt-containing heterogeneous catalysts. This possibility was illustrated by the preparation of Pt/g-C₃N₄ catalysts, which show an excellent performance in catalytic H₂ generation under visible light irradiation with a quantum efficiency up to 5% and a rate of H₂ evolution up to 6.2 mol·h^-1 per gram of loaded platinum.

Synthetic Routes to Crystalline Complex Metal Alkyl Carbonates and Hydroxycarbonates via Sol–Gel Chemistry—Perspectives for Advanced Materials in Catalysis

Metal alkoxides are easily available and versatile precursors for functional materials, such as solid catalysts. However, the poor solubility of metal alkoxides in organic solvents usually hinders their facile application in sol–gel processes and complicates access to complex carbonate or oxidic compounds after hydrolysis of the precursors. In our contribution we have therefore shown three different solubilization strategies for metal alkoxides, namely the derivatization, the hetero-metallization and CO2 insertion. The latter strategy leads to a stoichiometric insertion of CO2 into the metal–oxygen bond of the alkoxide and the subsequent formation of metal alkyl carbonates. These precursors can then be employed advantageously in sol–gel chemistry and, after controlled hydrolysis, result in chemically defined crystalline carbonates and hydroxycarbonates. Cu- and Zn-containing carbonates and hydroxycarbonates were used in an exemplary study for the synthesis of Cu/Zn-based bulk catalysts for methanol synthesis with a final comparable catalytic activity to commercial standard reference catalysts.

Hybrid Organic-Inorganic Materials and Interfaces With Mixed Ionic-Electronic Transport Properties: Advances in Experimental and Theoretical Approaches

The main goal of this mini-review is to provide an updated state-of-the-art of the hybrid organic-inorganic materials focusing mainly on interface phenomena involving ionic and electronic transport properties. First, we review the most relevant preparation techniques and the structural features of hybrid organic-inorganic materials prepared by solution-phase reaction of inorganic/organic precursor into organic/inorganic hosts and vapor-phase infiltration of the inorganic precursor into organic hosts and molecular layer deposition of organic precursor onto the inorganic surface. Particular emphasis is given to the advances in joint experimental and theoretical studies discussing diverse types of computational simulations for hybrid-organic materials and interfaces. We make a specific revision on the separately ionic, and electronic transport properties of these hybrid organic-inorganic materials focusing mostly on interface phenomena. Finally, we deepen into mixed ionic-electronic transport properties and provide our concluding remarks and give some perspectives about this growing field of research.

Development of Gold Nanoparticle-Based SERS Substrates on TiO2-Coating to Reduce the Coffee Ring Effect

Hydrophilic surface-enhanced Raman spectroscopy (SERS) substrates were prepared by a combination of TiO₂-coatings of aluminium plates through a direct titanium tetraisopropoxide (TTIP) coating and drop coated by synthesised gold nanoparticles (AuNPs). Differences between the wettability of the untreated substrates, the slowly dried Ti(OH)₄ substrates and calcinated as well as plasma treated TiO₂ substrates were analysed by water contact angle (WCA) measurements. The hydrophilic behaviour of the developed substrates helped to improve the distribution of the AuNPs, which reflects in overall higher lateral SERS enhancement. Surface enhancement of the substrates was tested with target molecule rhodamine 6G (R6G) and a fibre-coupled 638 nm Raman spectrometer. Additionally, the morphology of the substrates was characterised using scanning electron microscopy (SEM) and Raman microscopy. The studies showed a reduced influence of the coffee ring effect on the particle distribution, resulting in a more broadly distributed edge region, which increased the spatial reproducibility of the measured SERS signal in the surface-enhanced Raman mapping measurements on mm scale.

Cationic Vacancies in Anatase (TiO2): Synthesis, Defect Characterization, and Ion-Intercalation Properties

As one of the most studied materials, research on titanium dioxide (TiO₂) has flourished over the years owing to technological interest ranging from energy conversion and storage to medical implants and sensors, to name a few. Within this scope, the development of synthesis routes enabling the stabilization of reactive surface structure has been frequently investigated. Among these routes, solution-based synthesis has been utilized to tailor the material's properties spanning its atomic structural arrangement, or morphological aspects. One of the most investigated methods of stabilizing crystals with tailored facets relies on the use of fluoride-based precursors. Fluoride ions not only provide a driving force for the stabilization of metastable/reactive surface structures but also alter the reactivity of titanium molecular precursors and in turn the structural features of the stabilized crystals. Here, we review recent progress in the solution-based synthesis of anatase (one of the polymorphs of TiO₂) employing a fluoride precursor, with an emphasis on how cationic vacancies are stabilized by a charge-compensating mechanism and the resulting structural features associated with these defects. Finally, we will discuss the ion-intercalation properties of these sites with respect to lithium and polyvalent ions such as Mg²⁺ and Al³⁺. We will discuss in more detail the relevant parameters of the synthesis that allow controlling the phase composition with the coexistence of oxide, fluoride, and hydroxide ions within the anatase framework. The mechanism of formation of defective anatase nanocrystals has highlighted a solid-state transformation mostly implying an oxolation reaction (the condensation of hydroxide ions) that results in a decrease in the vacancy content, which can be synthetically controlled. The investigation of local fluorine environments probed by solid-state ¹⁹F NMR revealed up to three coordination modes with different numbers of coordinated Ti⁴⁺ and vacancies. It further revealed the occurrence of single and adjacent pairs of vacancies. These different host sites including native interstitial (and single/paired vacancies) display different ion-intercalation properties. We notably discussed the influence of the local anionic environments of vacancies on the thermodynamics of intercalation properties. The selective intercalation of polyvalent cations such as Mg²⁺ and Al³⁺ further supports the beneficial uses of defect chemistry for developing post-lithium-ion batteries. It is expected that the ability to characterize the local structure of defects is key to the design of unique, tailored-made materials.