Role of Associated Defects in Oxygen Ion Conduction and Surface Exchange Reaction for Epitaxial Samaria-Doped Ceria Thin Films as Catalytic Coatings

Effects of In-Air Post Deposition Annealing Process on the Oxygen Vacancy Content in Sputtered GDC Thin Films Probed via Operando XAS and Raman Spectroscopy

We investigate the ionic mobility in room-temperature RF-sputtered gadolinium doped ceria (GDC) thin films grown on industrial solid oxide fuel cell substrates as a function of the air-annealing at 800 and 1000 °C. The combination of X-ray diffraction, X-ray photoelectron spectroscopy, operando X-ray absorption spectroscopy, and Raman spectroscopy allows us to study the different Ce3+/ Ce4+ ratios induced by the post growth annealing procedure, together with the Ce valence changes induced by different gas atmosphere exposure. Our results give evidence of different kinetics as a function of the annealing temperature, with the sample annealed at 800 °C showing marked changes of the Ce oxidation state when exposed to both reducing and oxidizing gas atmospheres at moderate temperature (300 °C), while the Ce valence is weakly affected for the 1000 °C annealed sample. Raman spectra measurements allow us to trace the responses of the investigated samples to different gas atmospheres on the basis of the presence of different Gd-O bond strengths inside the lattice. These findings provide insight into the microscopic origin of the best performances already observed in SOFCs with a sputtered GDC barrier layer annealed at 800 °C and are fundamental to further improve sputtered GDC thin film performance in energy devices.

Isotope Exchange Raman Spectroscopy (IERS): A Novel Technique to Probe Physicochemical Processes In Situ

A novel in situ methodology for the direct study of mass-transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges, is developed. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, which is not accessible by conventional methods, enabling complementary insights for the study of ion-transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) is demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary-ion mass spectrometry (ToF-SIMS) characterization and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness, and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate the understanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research, and beyond.

Improved Structural Properties in Homogeneously Doped Sm0.4Ce0.6O2-δ Epitaxial Thin Films: High Doping Effect on the Electronic Bands

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality Sm_xCe_1-xO_2-δ films are here reported at a high doping level up to x = 0.4, thanks to the good lattice matching with the (110) oriented NdGaO₃ substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at the macroscopic and atomic level, respectively. Therefore, in epitaxial thin films, the homogeneous doping can be obtained even with the high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by X-ray photoemission and X-ray absorption show the O 2p band shift toward the Fermi level, which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x = 0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d e_g and t_2g states are well separated by the crystal field in the undistorted local structure even in the case of a high doping level up to x = 0.4.

The oxygen ion conductivity of Lu doped ceria

The oxygen ion conductivity of polycrystalline samples of Lu doped ceria is studied using impedance spectroscopy. Lutetium doped ceria is of particular interest as Lu has a similar ionic radius as the host cation Ce. The change of the ionic conductivity as a function of the Lu dopant fraction is investigated in detail revealing a similar behavior as Sm doped ceria that has one of the highest ionic conductivity in ternary cerium oxides. In comparison with simulations, the experimental dependence of the conductivity on the dopant fraction reveals that migration barriers for oxygen vacancy jumps around Lu ions are slightly higher than for jumps in pure ceria. The absolute conductivity is small due to the strong trapping of oxygen vacancies near Lu dopants.

Amorphous ZnO/PbS Quantum Dots Heterojunction for Efficient Responsivity Broadband Photodetectors

The integration of lead sulfide quantum dots (QDs) with a high-conductivity material that is compatible with a scalable fabrication is an important route for the applications of QD-based photodetectors. Herein, we first developed a broadband photodetector by combining amorphous ZnO and PbS QDs, forming a heterojunction structure. The photodetector showed detectivity up to 7.9 × 10¹² and 4.1 × 10¹¹ jones under 640 and 1310 nm illumination, respectively. The role of the oxygen background pressure in the electronic structure of ZnO films grown by pulsed laser deposition was systematically studied, and it was found to play an important role in the conductivity associated with the variation of the oxygen vacancy concentration. By increasing the oxygen vacancy concentration, the electron mobility of amorphous ZnO layers dramatically increased and the work function decreased, which were beneficial for the photocurrent enhancement of ZnO/PbS QD photodetectors. Our results provide a simple and highly scalable approach to develop broadband photodetectors with high performance.

A review of defect structure and chemistry in ceria and its solid solutions

Ceria and its solid solutions play a vital role in several industrial processes and devices. These include solar energy-to-fuel conversion, solid oxide fuel and electrolyzer cells, memristors, chemical looping combustion, automotive 3-way catalysts, catalytic surface coatings, supercapacitors and recently, electrostrictive devices. An attractive feature of ceria is the possibility of tuning defect-chemistry to increase the effectiveness of the materials in application areas. Years of study have revealed many features of the long-range, macroscopic characteristics of ceria and its derivatives. In this review we focus on an area of ceria defect chemistry which has received comparatively little attention - defect-induced local distortions and short-range associates. These features are non-periodic in nature and hence not readily detected by conventional X-ray powder diffraction. We compile the relevant literature data obtained by thermodynamic analysis, Raman spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Each of these techniques provides insight into material behavior without reliance on long-range periodic symmetry. From thermodynamic analyses, association of defects is inferred. From XAFS, an element-specific probe, local structure around selected atomic species is obtained, whereas from Raman spectroscopy, local symmetry breaking and vibrational changes in bonding patterns is detected. We note that, for undoped ceria and its solid solutions, the relationship between short range order and cation-oxygen-vacancy coordination remains a subject of active debate. Beyond collating the sometimes contradictory data in the literature, we strengthen this review by reporting new spectroscopy results and analysis. We contribute to this debate by introducing additional data and analysis, with the expectation that increasing our fundamental understanding of this relationship will lead to an ability to predict and tailor the defect-chemistry of ceria-based materials for practical applications.

Tuning of ionic mobility to improve the resistive switching behavior of Zn-doped CeO2

Correlation between the resistive switching characteristics of Au/Zn-doped CeO2/Au devices and ionic mobility of CeO2 altered by the dopant concentration were explored. It was found that the ionic mobility of CeO2 has a profound effect on the operating voltages of the devices. The magnitude of operating voltage was observed to decrease when the doping concentration of Zn was increased up to 14%. After further increasing the doping level to 24%, the device hardly exhibits any resistive switching. At a low doping concentration, only isolated Vo existed in the CeO2 lattice. At an intermediate doping concentration, the association between dopant and Vo formed (Zn, Vo)× defect clusters. Low number density of these defect clusters initially favored the formation of Vo filament and led to a reduction in operating voltage. As the size and number density of (Zn, Vo)× defect clusters increased at a higher doping concentration, the ionic conductivity was limited with the trapping of isolated Vo by these defect clusters, which resulted in the diminishing of resistive switching. This research work provides a strategy for tuning the mobility of Vo to modulate resistive switching characteristics for non-volatile memory applications.

Finely Tuned Structure and Catalytic Performance of Cerium Oxides by a Continuous Samarium Doping from 0 to 100

Cerium oxides are prevalent catalytic materials, and the lanthanide-doped ceria have attracted special interest since it is easy to tune the concentration of oxygen vacancies (V_O) by changing the doping content. The presence of V_O is generally believed to favor a catalytic reaction, but the formation of dopant-vacancy associations at a high doping concentration might produce an adverse effect. Herein, evolutions of the structural properties and catalytic performances in Sm-doped ceria (Sm_xCe_1-xO_2-δ, x = 0-1) are investigated to explore the doping effect of Sm³⁺ on the ceria-based nanoctrystals. The Sm_xCe_1-xO_2-δ films composed of nanoctrystals are elaborately prepared via electrodeposition under mild conditions to prevent phase separation. A combination of studies, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, photoluminescence (PL), and methanol electro-oxidation (MEO) reaction, reveals that variation trends for the V_O concentration and catalytic property of Sm_xCe_1-xO_2-δ are unsynchronized. The lattice structures of Sm_xCe_1-xO_2-δ nanoctrystals undergo a smooth and steady transition from F-type (fluorite CeO₂) to C-type (cubic Sm₂O₃) with the increase of Sm³⁺ contents. The structural transition occurs in the Sm³⁺ concentration range of 64-84%, within which the V_O concentration reaches the maximum as well. However, the optimal MEO performance is obtained at a relatively lower doping concentration of 24%. Above this concentration, significant dopant-vacancy associates are observed by XRD, Raman, and PL characterizations. It is inferred that, for these ceria-based nanocrystals, the dopant-vacancy association induced by high doping would impede the growth of catalytic performance despite all the benefits of V_O.

Preparative History vs Driving Force in Water Oxidation Catalysis: Parameter Space Studies of Cobalt Spinels

The development of efficient, stable, and economic water oxidation catalysts (WOCs) is a forefront topic of sustainable energy research. We newly present a comprehensive three-step approach to systematically investigate challenging relationships among preparative history, properties, and performance in heterogeneous WOCs. To this end, we studied (1) the influence of the preparative method on the material properties and (2) their correlation with the performance as (3) a function of the catalytic test method. Spinel-type Co3O4 was selected as a clear-cut model WOC and synthesized via nine different preparative routes. In search of the key material properties for high catalytic performance, these cobalt oxide samples were characterized with a wide range of analytical methods, including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, Raman spectroscopy, BET surface area analysis, and transmission electron microscopy. Next, the corresponding catalytic water oxidation activities were assessed with the three most widely applied protocols to date, namely, photocatalytic, electrocatalytic, and chemical oxidation. The activity of the Co3O4 samples was found to clearly depend on the applied test method. Increasing surface area and disorder as well as a decrease in oxidation states arising from low synthesis temperatures were identified as key parameters for high chemical oxidation activity. Surprisingly, no obvious property-performance correlations were found for photocatalytic water oxidation. In sharp contrast, all samples showed similar activity in electrochemical water oxidation. The substantial performance differences between the applied protocols demonstrate that control and comprehensive understanding of the preparative history are crucial for establishing reliable structure-performance relationships in WOC design.

Synthesis and Characterization of Samarium-Substituted Molybdenum Diselenide and Its Graphene Oxide Nanohybrid for Enhancing the Selective Sensing of Chloramphenicol in a Milk Sample

The electronic conductivity and electrocatalytic activity of metal chalcogenides are normally enhanced by following the ideal strategies such as substitution/doping of heterogeneous atoms and hybridization of highly conductive carbon supportive materials. Here, a rare earth element (samarium) was substituted with MoSe₂ using the simple hydrothermal method. The lattice distortion due to the substitution of Sm³⁺ with MoSe₂ was clearly observed by using high-resolution transmission electron microscopy analysis. As a consequence, the prepared SmMoSe₂ nanorod was encapsulated with graphene oxide (GO) sheets by using ultrasonication process. Furthermore, the GO-encapsulated SmMoSe₂ nanocomposite modified glassy carbon electrode (GO@SmMoSe₂/GCE) was used for the sensing of chloramphenicol. The results showed that the GO@SmMoSe₂/GCE revealed the superior electrocatalytic activity with low detection (5 nM) and sensitivity (20.6 μA μM^-1 cm^-2) to electrochemical detection of proposed analyte. It indicates that the substitution of Sm³⁺ and encapsulation of GO significantly increased both the electrical conductivity and electrocatalytic activity of MoSe₂.

Design of Oxygen Vacancy Configuration for Memristive Systems

Oxide-based valence-change memristors are promising nonvolatile memories for future electronics that operate on valence-change reactions to modulate their electrical resistance. The memristance is associated with the movement of oxygen ionic carriers through oxygen vacancies at high electric field strength via structural defect modifications that are still poorly understood. This study employs a Ce_1-xGd_xO_2-y solid solution model to probe the role of oxygen vacancies either set as "free" or as "immobile and clustered" for the resistive switching performance. The experiments, together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol % of GdO_1.5 doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration, the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry, leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. The research demonstrates guidelines for engineering of the oxide's solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the resistive switching for nonvolatile memory and logic applications.