Metal Oxysulfides: From Bulk Compounds to Nanomaterials

Controlled Synthesis of Terbium-Doped Colloidal Gd2O2S Nanoplatelets Enables High-Performance X-ray Scintillators

Terbium-doped gadolinium oxysulfide (Gd2O2S:Tb3+), commonly referred to as Gadox, is a widely used scintillator material due to its exceptional X-ray attenuation efficiency and high light yield. However, Gadox-based scintillators suffer from low X-ray spatial resolution due to their large particle size, which causes significant light scattering. To address this limitation, we report the synthesis of terbium-doped colloidal Gadox nanoplatelets (NPLs) with near-unity photoluminescence quantum yield (PLQY) and high radioluminescence light yield (LY). In particular, our investigation reveals a strong correlation between PLQY, LY, particle size, and Tb3+concentration. Our synthetic approach allows precise control over the lateral size and thickness of the Gadox NPLs, resulting in a LY of 50,000 photons/MeV. Flexible scintillating screens fabricated with the solution-processable Gadox NPLs exhibited a 20 lp/mm X-ray spatial resolution, surpassing commercial Gadox scintillators. These high-performance and flexible Gadox NPL-based scintillators enable enhanced X-ray imaging capabilities in medicine and security. Our work provides a framework for designing nanomaterial scintillators with superior spatial resolution and efficiency through precise control of dimensions and dopant concentration.

Tb- and Eu-doped yttrium oxyselenides as novel absorber layers for superstrate thin-film photovoltaics: improved spectral optical absorption and green-red phosphor activation

To generate and deliver alternative sustainable energy in the face of the current energy crisis, new materials that can capture solar energy and transform it into other useful energies are required. Rare-earth (RE) oxychalcogenides are now being used more frequently as up/down-conversion materials in established photovoltaic (PV) devices to boost their PV performance. Here, through an efficient microwave assisted synthesis procedure, novel nanoplate/sheet shaped nanomaterials of yttrium oxyselenide (YOSe) and its analogues doped with Tb and Eu (YOSe:Tb and YOSe:Eu) were successfully synthesized. Analyses of the structure, stability, morphology, light absorption, and electrochemistry were performed. This work showed that the parent YOSe exhibited green (543 nm) and red (615 nm) emission luminescence when doped with Tb and Eu with a luminescence quantum yield (LQY) of 0.56 and 0.53 for YOSe:Tb and YOSe:Eu nanomaterials, respectively. The surface and material conductivity of YOSe improved with the addition of the dopant elements, with the best outcome shown in YOSe:Eu, according to electrokinetic research evidenced by the enhanced current peaks, reduced charge-transfer resistance (Rct) and low impedance magnitude (Zmag) through electrochemical experiments. These improvements were induced by the distinctive properties of the dopant elements. PCEs of 0.25%, 0.67%, and 1.20% were obtained for YOSe, YOSe:Tb, and YOSe:Eu-based PV devices, respectively, using the nanomaterials as novel absorber layers in a superstrate device design. Our results can initiate further exploitation of the doped host structure for effective down-conversion NIR luminescence for applications in PV devices and to boost the PV performance of existing solar cells.

Topochemical Anionic Subunit Insertion Reaction for Constructing Nanoparticles of Layered Oxychalcogenide Intergrowth Structures

Intergrowth compounds contain alternating layers of chemically distinct subunits that yield composition-tunable synergistic properties. Synthesizing nanoparticles of intergrowth structures requires atomic-level intermixing of the subunits rather than segregation into stable constituent phases. Here we introduce an anionic subunit insertion reaction for nanoparticles that installs metal chalcogenide layers between metal oxide sheets. Anionic [CuS]- subunits from solution replace interlayer chloride anions from LaOCl to form LaOCuS topochemically with retention of crystal structure and morphology. Sodium acetylacetonate helps extract Cl- concomitant with the insertion of S2- and Cu+ and is generalized to other oxychalcogenides. This topochemical reaction produces nanoparticles of ordered mixed-anion intergrowth compounds and expands nanoparticle ion exchange chemistry to anionic subunits.

A new class of layered Bi2O2S nanopetals by one-pot supercritical-CO2 approach: A reliable electrocatalyst for analgesic bioflavonoid detection

Nanomaterials frequently draw a lot of interest in a variety of disciplines, including electrochemistry. Developing a reliable electrode modifier for the selective electrochemical detection of the analgesic bioflavonoid i.e., Rutinoside (RS) is a great challenge. Here in, we have explored the supercritical-CO₂ (SC-CO₂) mediated synthesis of bismuth oxysulfide (SC-BiOS) and reported it as a robust electrode modifier for the detection of RS. For a comparison study, the same preparation procedure was carried out in the conventional approach (C-BiS). The morphology, crystallography, optical, and elemental contribution analyses were characterized to understand the paradigm shift in the physicochemical properties between SC-BiOS and C-BiS. The results exposed the C-BiS had a nano-rod-like structure with a crystallite size of 11.57 nm; whereas the SC-BiOS had a nano-petal-like structure with a crystallite size of 9.03 nm. The B_2g mode in the optical analysis confirms the formation of bismuth oxysulfide by the SC-CO₂ method with the Pmnn space group. As an electrode modifier, the SC-BiOS achieved a higher effective surface area (0.074 cm²), higher electron transfer kinetics (0.13 cm s^-1), and lower charge transfer resistance (403 Ω) than C-BiS. Further, it provided a wide linear range of 0.1-610.5 μM L^-1 with a low detection and quantification limit of 9 and 30nM L^-1 and an appreciable sensitivity of 0.706 μA μM^-1 cm^-2. The selectivity, repeatability, and real-time application towards the environmental water sample with a recovery of 98.87% were anticipated for the SC-BiOS. This SC-BiOS unlocks a fresh avenue to construct a design for the family of electrode modifiers utilized in electrochemical applications.

Optical-Quality Thin Films with Tunable Thickness from Stable Colloidal Suspensions of Lanthanide Oxysulfide Nanoplates

In modern laser technologies, there is a need for coatings that would be compatible with flexible substrates while retaining the advantages of inorganic compounds in terms of robustness. As a first step in this direction, we developed here thin films of lanthanide oxysulfide, of optical quality, prepared by low-temperature dip coating. As a model compound in the family of oxysulfides, (Gd,Ce)₂O₂S anisotropic nanoplates were used. The films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and in situ UV and IR spectroscopic ellipsometry, showing that the band gap of the materials was preserved through the deposition process. The thickness of the films was tuned in a broad range, from a few nanometers to 150 nm, using different concentrations of the colloidal suspensions as well as single-layer and multilayer deposition. Lastly, thermal treatment of the thin films was optimized to remove the stabilizing organic ligands of the nanoparticles while preserving their integrity, as confirmed by SEM and XRD.

Synthesis, Characterization, and Electronic Properties of ZnO/ZnS Core/Shell Nanostructures Investigated Using a Multidisciplinary Approach

ZnO/ZnS core/shell nanostructures, which are studied for diverse possible applications, ranging from semiconductors, photovoltaics, and light-emitting diodes (LED), to solar cells, infrared detectors, and thermoelectrics, were synthesized and characterized by XRD, HR-(S)TEM, and analytical TEM (EDX and EELS). Moreover, band-gap measurements of the ZnO/ZnS core/shell nanostructures have been performed using UV/Vis DRS. The experimental results were combined with theoretical modeling of ZnO/ZnS (hetero)structures and band structure calculations for ZnO/ZnS systems, yielding more insights into the properties of the nanoparticles. The ab initio calculations were performed using hybrid PBE0 and HSE06 functionals. The synthesized and characterized ZnO/ZnS core/shell materials show a unique three-phase composition, where the ZnO phase is dominant in the core region and, interestingly, the auxiliary ZnS compound occurs in two phases as wurtzite and sphalerite in the shell region. Moreover, theoretical ab initio calculations show advanced semiconducting properties and possible band-gap tuning in such ZnO/ZnS structures.

Dual-mode vehicles with simultaneous thermometry and drug release properties based on hollow Y2O3:Er,Yb and Y2O2SO4:Er,Yb spheres

Employing luminescence thermometry in the biomedical field is undeniably appealing as many health conditions are accompanied by temperature changes. In this work, we show our ongoing efforts and results at designing novel vehicles for dual-mode thermometry and pH-dependent drug release based on hollow spheres. Hereby for that purpose, we exploit the hollow Y₂O₃ and Y₂O₂SO₄ host materials. These two inorganic hollow phosphors were investigated and showed to have excellent upconversion Er³⁺-Yb³⁺ luminescence properties and could be effectively used as optical temperature sensors in the physiological temperature range when induced by near-infrared CW light (975 nm). Further, doxorubicin was exploited as a model anti-cancer drug to monitor the pH-dependent drug release of these materials showing that they can be used for simultaneous thermometry and drug delivery applications.

Mixed-Anion-Oriented Design of LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) Nonlinear Optical Oxysulfides with Targeted Property Balance

Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap (E_g ≥ 3.0 eV) and large NLO effect (d_ij ≥ 0.5 × AgGaS₂) and simultaneously enlarge the birefringence (Δn) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa₃S₆O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa₃S₆O satisfy the property-balance demand (E_g: 3.21-3.27 eV and d_ij: 0.9-1.0 × AgGaS₂) as promising PM NLO crystals through gathering their property advantages between LaMGa₃O₇ and LaMGa₃S₇ by mixed-anion-oriented performance engineering. A study on the structure-property relationship indicates that heteroleptic (Ln/M)S₇O and GaS₃O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.

Heterogeneous Oxysulfide@Fluoride Core/Shell Nanocrystals for Upconversion-Based Nanothermometry

Lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) often suffer from weak luminescence, especially when their sizes are ultrasmall (less than 10 nm). Enhancing the upconversion luminescence (UCL) efficiency of ultrasmall UCNPs has remained a challenge that must be undertaken if any practical applications are to be envisaged. Herein, we present a Ln³⁺-doped oxysulfide@fluoride core/shell heterostructure which shows efficient UCL properties under 980 nm excitation and good stability in solution. Through epitaxial heterogeneous growth, a ∼4 nm optically inert β-NaYF₄ shell was coated onto ∼5 nm ultrasmall Gd₂O₂S:20%Yb,1%Tm. These Gd₂O₂S:20%Yb,1%Tm@NaYF₄ core/shell UCNPs exhibit a more than 800-fold increase in UCL intensity compared to the unprotected core, a 180-fold increase in luminescence decay time of the ³H₄ → ³H₆ Tm³⁺ transition from 5 to 900 μs, and an upconversion quantum yield (UCQY) of 0.76% at an excitation power density of 155 W/cm². Likewise, Gd₂O₂S:20%Yb,2%Er@NaYF₄ core/shell UCNPs show a nearly 5000-fold increase of their UCL intensity compared to the Gd₂O₂S:20%Yb,2%Er core and a maximum UCQY of 0.61%. In the Yb/Er core-shell UCNP system, the observed variation of luminescence intensity ratio seems to originate from a change in lattice strain as the temperature is elevated. For nanothermometry applications, the thermal sensitivities based on thermally coupled levels are estimated for both Yb/Tm and Yb/Er doped Gd₂O₂S@NaYF₄ core/shell UCNPs.

La4Ga2Se6O3: A Rare-Earth Oxyselenide Built from One-Dimensional Strips

The oxyselenide La₄Ga₂Se₆O₃ was obtained by reaction in NaCl flux. Its monoclinic crystal structure (space group C2/c, a = 21.2832(13) Å, b = 11.6272(7) Å, c = 6.0006(4) Å, β = 106.3430(10)°, Z = 4), which is a new type, consists of strips of edge-sharing OLa₄ tetrahedra and zigzag chains of corner-sharing GaSe₄ tetrahedra. The separation into distinct structural blocks with mostly ionic vs covalent bonding character was supported by analysis of the electron localization function and crystal orbital bond index. An experimental band gap of 1.8 eV was extracted from optical diffuse reflectance spectra. First-principles calculations suggest that the thermoelectric power factor of this compound would be enhanced by n-doping.

Liquid metals: an ideal platform for the synthesis of two-dimensional materials

The surfaces of liquid metals can serve as a platform to synthesise two-dimensional materials. By exploiting the self-limiting Cabrera-Mott oxidation reaction that takes place at the surface of liquid metals exposed to ambient air, an ultrathin oxide layer can be synthesised and isolated. Several synthesis approaches based on this phenomenon have been developed in recent years, resulting in a diverse family of functional 2D materials that covers a significant fraction of the periodic table. These straightforward and inherently scalable techniques may enable the fabrication of novel devices and thus harbour significant application potential. This review provides a brief introduction to liquid metals and their alloys, followed by detailed guidance on each developed synthesis technique, post-growth processing methods, integration processes, as well as potential applications of the developed materials.

Combining electrochemical and quantitative elemental analysis to investigate the sulfur poisoning process of ceria thin film fuel electrodes

This work deals with the effect of sulfur incorporation into model-type GDC thin films on their in-plane ionic conductivity. By means of impedance measurements, a strongly deteriorating effect on the grain boundary conductivity was confirmed, which additionally depends on the applied electrochemical polarisation. To quantify the total amount of sulfur incorporated into GDC thin films, online-laser ablation of solids in liquid (online-LASIL) was used as a novel solid sampling strategy. Online-LASIL combines several advantages of conventional sample introduction systems and enables the detection of S as a minor component in a very limited sample system (in the present case 35 μg total sample mass). To reach the requested sensitivity for S detection using an inductively coupled plasma-mass spectrometer (ICP-MS), the reaction cell of the quadrupole instrument was used and the parameters for the mass shift reaction with O₂ were optimised. The combination of electrical and quantitative analytical results allows the identification of a potential sulfur incorporation pathway, which very likely proceeds along GDC grain boundaries with oxysulfide formation as the main driver of ion transport degradation. Depending on the applied cathodic bias, the measured amount of sulfur would be equivalent to 1-4 lattice constants of GDC transformed into an oxysulfide phase at the material's grain boundaries.

[NiFe]-(Oxy)Sulfides Derived from NiFe2O4 for the Alkaline Hydrogen Evolution Reaction

The development of noble-metal-free electrocatalysts is regarded as a key factor for realizing industrial-scale hydrogen production powered by renewable energy sources. Inspired by nature, which uses Fe- and Ni-containing enzymes for efficient hydrogen generation, Fe/Ni-containing chalcogenides, such as oxides and sulfides, received increasing attention as promising electrocatalysts to produce hydrogen. We herein present a novel synthetic procedure for mixed Fe/Ni (oxy)sulfide materials by the controlled (partial) sulfidation of NiFe2O4 (NFO) nanoparticles in H2S-containing atmospheres. The variation in H2S concentration and the temperature allows for a precise control of stoichiometry and phase composition. The obtained sulfidized materials (NFS) catalyze the hydrogen evolution reaction (HER) with increased activity in comparison to NFO, up to −10 and −100 mA cm−2 at an overpotential of approx. 250 and 450 mV, respectively.

Synthesis, structure, and magnetic properties of the quaternary oxysulfides Ln 5V3O7S6 (Ln = La, Ce)

Two rare earth oxysulfides Ln 5V3O7S6 (Ln = La, Ce) have been synthesized and their structures determined. The two isostructural compounds crystallize in the orthorhombic space group Pmmn (no. 59). The structure features one-dimensional edge-sharing VS4O2 octahedron chains parallel to the b axis. The bonding between V and S/O is covalent, and between Ln 3+ and the rest of the matrix ionic. Magnetic susceptibility measurement revealed that V is in a mixed valence state of V3+ and V4+. Its magnetic behavior follows the Curie-Weiss law.

An Overview of Gadolinium-Based Oxide and Oxysulfide Particles: Synthesis, Properties, and Biomedical Applications

In the last decade, the publications presenting novel physical and chemical aspects of gadolinium-based oxide (Gd2O3) and oxysulfide (Gd2O2S) particles in the micro- or nano-scale have increased, mainly stimulated by the exciting applications of these materials in the biomedical field. Their optical properties, related to down and upconversion phenomena and the ability to functionalize their surface, make them attractive for developing new probes for selective targeting and emergent bioimaging techniques, either for biomolecule labeling or theranostics. Moreover, recent reports have shown interesting optical behavior of these systems influenced by the synthesis methods, dopant amount and type, particle shape and size, and surface functionality. Hence, this review presents a compilation of the latest works focused on evaluating the optical properties of Gd2O3 and Gd2O2S particles as a function of their physicochemical and morphological properties; and also on their novel applications as MRI contrast agents and drug delivery nanovehicles, discussed along with their administration routes, biodistribution, cytotoxicity, and clearance mechanisms. Perspectives for this field are also identified and discussed.

Design of metastable oxychalcogenide phases by topochemical (de)intercalation of sulfur in La2O2S2

Designing and synthesising new metastable compounds is a major challenge of today’s material science. While exploration of metastable oxides has seen decades-long advancement thanks to the topochemical deintercalation of oxygen as recently spotlighted with the discovery of nickelate superconductor, such unique synthetic pathway has not yet been found for chalcogenide compounds. Here we combine an original soft chemistry approach, structure prediction calculations and advanced electron microscopy techniques to demonstrate the topochemical deintercalation/reintercalation of sulfur in a layered oxychalcogenide leading to the design of novel metastable phases. We demonstrate that La₂O₂S₂ may react with monovalent metals to produce sulfur-deintercalated metastable phases La₂O₂S_1.5 and oA-La₂O₂S whose lamellar structures were predicted thanks to an evolutionary structure-prediction algorithm. This study paves the way to unexplored topochemistry of mobile chalcogen anions.

Great progress has been made in topochemistry of mobile oxygen anions, but metastable compounds have not yet been achieved by deintercalation of sulfur anions. Here, the authors prepare metastable oxychalcogenide phases by taking advantage of redox-reactive sulfur dimers embedded in a layered oxysulfide.

From Ta2S5 Wires to Ta2O5 and Ta2O5-x S x

Synthesis routes to forming novel materials are oftentimes complicated and indirect. For example, Ta₂S₅ has only been found as an unwanted byproduct of certain chemical reactions, and its properties were unknown. However, here we demonstrate the growth of Ta₂S₅ wires with steel-like tensile strength, which are also precursors for the first controlled synthesis of long, mesoscopic Ta₂O₅ wires and superconducting Ta₂O_5-x S _x wires. Single-crystal wires of tantalum pentasulfide, Ta₂S₅, were first grown using vapor transport from polycrystalline XTa₂S₅, sulfur, and TeCl₄ in fused-quartz tubes, where X = Ba or Sr. Crystals form as long wires with lengths on the order of a few centimeters and varying cross sections as small as 25 μm². They were found to have steel-like tensile strength, and their crystal structure was determined using X-ray diffraction to be monoclinic with space group P2/m and with lattice parameters a = 9.91(7) Å, b = 3.82(5) Å, and c = 20.92(2) Å. Electrical resistivity measurements reveal Ta₂S₅ to be a narrow band gap semiconductor with E _g = 110 meV, while a Debye temperature Θ_D = 97.0(5) K is observed in specific heat. Tantalum pentasulfide wires were then converted to insulating tantalum pentoxide (Ta₂O₅) wires after calcinating them for 30 min in air at 900 °C. Finally, tantalum pentoxide wires were converted to tantalum oxysulfide (Ta₂O_5-x S _x ) wires after annealing them in CS₂ vapor for 30 min at 900 °C. The oxysulfide crystal structure was determined using X-ray diffraction to be that of β-Ta₂O₅. Electrical and magnetic measurements reveal Ta₂O_5-x S _x to be metallic and superconducting with T _c = 3 K.