Ceria Films Research Articles

Effect of annealing treatments on CeO2 grown on TiN and Si substrates by atomic layer deposition.

In this work, we investigate the effect of thermal treatment on CeO2 films fabricated by using atomic layer deposition (ALD) on titanium nitride (TiN) or on silicon (Si) substrates. In particular, we report on the structural, chemical and morphological properties of 25 nm thick ceria oxide with particular attention to the interface with the substrate. The annealing treatments have been performed in situ during the acquisition of X-Ray diffraction patterns to monitor the structural changes in the film. We find that ceria film is thermally stable up to annealing temperatures of 900 °C required for the complete crystallization. When ceria is deposited on TiN, the temperature has to be limited to 600 °C due to the thermal instability of the underlying TiN substrate with a broadening of the interface, while there are no changes detected inside the CeO2 films. As-deposited CeO2 films show a cubic fluorite polycrystalline structure with texturing. Further, after annealing at 900 °C an increase of grain dimensions and an enhanced preferential (200) orientation are evidenced. These findings are a strong indication that the texturing is an intrinsic property of the system more than a metastable condition due to the ALD deposition process. This result is interpreted in the light of the contributions of different energy components (surface energy and elastic modulus) which act dependently on the substrate properties, such as its nature and structure.

Structural, optical and electrical properties of CeO2 thin films simultaneously prepared by anodic and cathodic electrodeposition

CeO2 thin films were deposited on stainless steel (SS) and indium tin oxide (ITO)-coated glass by simultaneous anodic and cathodic electrodeposition, and the influence of negative potential on the formation of ceria films was studied with scanning electron microscopy, X-ray diffraction, Raman spectroscopy, van der Pauw measurements, UV-visible spectroscopy and X-ray photoelectron spectroscopy. The results show that CeO2 films on the anode are slightly affected by the potential, but the particle size, crystal orientation, strain, film thickness, resistivity and Ce(III) content of the films on the cathode increases with increasing potential on the SS substrate. Contradictory to the results of the SS cathode, redshift (Ed changed from 3.95 eV to 3.56 eV and Ei changed from 3.42 eV to 3.04 eV) occurring in the absorption spectrum of CeO2 deposited on the ITO-coated glass cathode indicates that the content of Ce3+ in the cathodic films is dependent on the adopted substrates and decreases as the applied potential is increased.

Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells

Gadolinia-doped ceria (GDC) films are synthesized by direct current magnetron sputtering under reactive conditions for hydrogen production by high temperature electrolysis (HTE). First, the sputtering process is investigated by varying the reactive gas flow rate. Working conditions in elemental sputtering mode (ESM) are adjusted to ensure a high incorporation of oxygen while keeping a high deposition rate. Heat treatments in air are found necessary to stabilize the sub-stoichiometric deposited oxide. The main characteristics of the films (crystallinity, chemical composition, porosity, etc.) are studied by Rutherford backscattering spectroscopy (RBS), optical transmission spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and scratch test and are compared to that of a conventional 2 μm thick GDC layer deposited by screen printing. The sputtered layer appears dense and adherent but kind of blisters are observed when deposited on half-cells. Performances of a thin GDC barrier layer (below 500 nm) sputter deposited on half cells are evaluated. Encouraging results are obtained since performances (i.e. about −1.70 A·cm−2 at 1.3 V and 800 °C in HTE mode) are slightly higher to that typically achieved with a classical manufactured cell (i.e. about −1.65 A·cm−2 at 1.3 V and 800 °C). A degradation rate of about 9% for 1000 h operation (at 800 °C with a current density of −0.5 A·cm−2, GDC layer thickness of 425 nm) is achieved.

Structure and chemistry of interfaces between ceria and yttria-stabilized zirconia studied by analytical STEM

Epitaxial undoped and Gd2O3-doped ceria films were grown by pulsed laser deposition on (1 1 1) faced Y2O3-stabilized zirconia (YSZ). Highly localized cerium reduction at the film-substrate interfaces is revealed by atomically resolved valence EELS mapping using Cs aberration-corrected scanning transmission electron microscopy. The chemical profiles reveal interdiffusion of Ce, (Gd), Y, Zr, forming an intermixing zone at the interface 7–9 (1 1 1) lattice planes wide. In its vicinity, the fraction of Ce3+ raises gradually over 6–8 lattice planes from zero in the bulk ceria to ≈100% in one single plane at the interface. Beyond this plane the Ce3+ fraction drops sharply within the YSZ substrate. In the vicinity of the interface systematic scan deflections are observed during EELS line scans. The advancing electron probe experiences a retarding force at the ceria side, and an accelerating force at the YSZ side, irrespective of the scan direction. This behavior is suggestive of coulombic interactions between the electron probe and a charged interface. This is interpreted as an indication of the presence of a space-charge situation at the YSZ/ceria interface, resulting from an excess negative charge at the ceria side (due to Ce3+cations) and an excess positive charge at the YSZ side (due to oxygen vacancies).

Influence of argon and oxygen pressure ratio on bipolar-resistive switching characteristics of CeO2−x thin films deposited at room temperature

Cerium oxide (CeO2−x) film was deposited on Pt/Ti/SiO2/Si substrate by rf magnetron sputtering at room temperature. Resistive switching characteristics of these ceria films have been improved by increasing oxygen content during deposition process. Endurance and statistical analyses indicate that the operating stability of CeO2−x-based memory is highly dependent on the oxygen content. Results indicate that CeO2−x film-based RRAM devices exhibit optimum performance when fabricated at an argon/oxygen ratio of 6:24. An increase in the oxygen content introduced during CeO2−x film deposition not only stabilizes the conventional bipolar RS but also improves excellent switching uniformity such as large ON/OFF ratio (102), excellent switching device-to-device uniformity and good sweep endurance over ~ 500 repeated RS cycles. Conduction in the low-resistance state (LRS) as well as in the low bias field region in the high-resistance state (HRS) is found to be Ohmic and thus supports the conductive filament (CF) theory. In the high voltage region of HRS, space charge limited conduction (SCLC) and Schottky emission are found to be the dominant conduction mechanisms. A feasible filamentary RS mechanism based on the movement of oxygen ions/vacancies under the bias voltage has been discussed.

Enhancing Full Water-Splitting Performance of Transition Metal Bifunctional Electrocatalysts in Alkaline Solutions by Tailoring CeO2–Transition Metal Oxides–Ni Nanointerfaces

Rational design of highly efficient bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is critical for sustainable energy conversion. Herein, motivated by the high activity of OER catalyst on water dissociation that is the rate-determining step of alkaline HER, a bifunctional catalyst of metallic nickel-decorated transition metal oxide nanosheets vertically grown on ceria film (ceria/Ni-TMO) is synthesized by composition controlling and surface engineering. Because of the idealized electronic structure of the active centers and the abundance of such sites, as well as a synergistic effect between the carbon cloth/ceria film and the in situ formed TMO/Ni nanoparticles, the as-synthesized ceria/Ni-TMO exhibited long-time stability and a low cell voltage of 1.58 V at 10 mA/cm2 when applied as both the cathode and anode in alkaline solutions. Moreover, it is the first time that pH-independent four-proton-coupled-electron-transfer processes and multiple adsor...

Room Temperature Polarization Phenomena in Nanocrystalline and Epitaxial Thin Films of Gd-Doped Ceria Studied by Kelvin Probe Force Microscopy

Thin films of the composition Ce0.9Gd0.1O2-δ and with a thickness of 200 nm were prepared on an Al2O3 (0001) substrate at 720°C or room temperature, to yield epitaxial or nanocrystalline materials. The effect of the microstructure on room temperature polarization were assessed by a combination of polarization with up to ±5 V using an AFM tip and a large silver back contact followed by surface potential mapping with Kelvin probe force microscopy. The nanocrystalline sample showed a low degree of polarization, which was reversible, and a fast relaxation process. The epitaxial thin film, in contrast, was already polarized at smaller biases and showed irreversible changes of the surface potential after polarization with +5 V. For lower biases, two distinct relaxation reactions were identified: a fast redistribution of charge over a large area in the first seconds after end of polarization and a slower process in the vicinity of the polarization contact, which took several minutes to relax. The slower polarization behavior is due to a bulk chemical diffusion process with a coefficient Dδ on the order of 10−17 m2·s−1, whereas the faster polarization can be assigned to a charge carrier redistribution within a space charge layer.

Infrared Spectroscopic Analysis of the Adsorption of Pyridine Carboxylic Acids on Colloidal Ceria

Surface adsorption of a homologous series of pyridine carboxylic acids on a hydrated colloidal cerium dioxide (ceria) film is characterized using the combination of experimental and computationally determined infrared (IR) spectra. Experimental analyses employ attenuated total reflectance (ATR) IR spectroscopy of deposited colloidal ceria thin films equilibrated with three pyridine carboxylic acids at pH 3.0, 5.5, and 8.5. The corresponding computational IR spectra for the energy-minimized intermediate and base forms of the pyridine carboxylic acids use density functional theory calculations at the B3LYP/6-311++G** level of theory. Solvent effects are modeled using both the COSMO implicit solvation model and the inclusion of explicit water molecules. Experimental IR spectra show that the adsorptive interactions between the pyridine carboxylic acids and ceria surface are due to the outer-sphere coordination of cerium ions in the films. Vibrational assignments based on combined experimental and computational results indicate that both pyridyl ring nitrogen and carboxylate functional groups account for the interaction of pyridine carboxylic acids at ceria surfaces. Experimentally determined Langmuir constants point to the intermediate form of picolinic acid (pyridine-2-carboxylic acid) as having the strongest adsorption to ceria compared to the other pyridine carboxylic acids investigated. The enhanced adsorption of picolinic acid is attributed to the adjacency of the protonated pyridyl nitrogen and the carboxylate group relative to nicotinic acid (pyridine-3-carboxylic acid) and isonicotinic acid (pyridine-4-carboxylic acid).

Novel gas sensor with dual response under CO(g) exposure: Optical and electrical stimuli

In this work, a lanthanum (La) doped ceria (CeO2) film, which depicted a dual gas sensing response (electric and optical) for CO(g) detection, was obtained by the microwave-assisted hydrothermal (HAM) synthesis and deposited by the screen-printing technique, in order to prevent deaths by intoxication with this life-threatening gas. An electric response under CO(g) exposure was obtained, along with an extremely fast optical response for a temperature of 380 °C, associated with Ce+4 reduction and vacancy generation. A direct optical gap was found to be around 2.31 eV from UV–Vis results, which corresponds to a transition from valence band to 4f states. Due to the anomalous electron configuration of cerium atoms with 4f electrons in its reduced state, they are likely to present an electric conduction based on the small polaron theory with a hopping mechanism responsible for its dual sensing response with a complete reversible behaviour.

Ion Charge Dynamics in Ceria-Based Metal Insulator Metal Structure

This paper reports on the room temperature ambipolar diffusion of oxygen vacancies and electrons in epitaxial films of pure CeO2 and 20% Sm-doped CeO2 a few tens of nanometers thick. To this aim, we fabricated heteroepitaxial structures consisting of an epitaxial ceria film sandwiched between a Pt upper electrode and a conducting substrate of Nb-doped SrTiO3 (lower electrode). The observed decay of the electrical conductivity was investigated. In particular, following this approach, we could measure room temperature diffusion coefficient in thin monocrystalline dielectric layers. While pure ceria is interesting for memory device applications, in doped ceria the resistive switching involves diffusion mechanisms that occur on much longer time scales. Under this respect, doped ceria is particularly suitable to be used in artificial neuronal devices.

Ab initio study of ceria films for resistive switching memory applications

The aim of this study is to investigate the charge distribution/relocation activities in relation to resistive switching (RS) memory behavior in the metal/insulator/metal (MIM) structure of Zr/CeO2/Pt hybrid layers. The Zr layer is truly expected to act not only as an oxygen ion extraction layer but also as an ion barrier by forming a ZrO2 interfacial layer. Such behavior of the Zr not only introduces a high concentration of oxygen vacancies to the active CeO2 layer but also enhances the resistance change capability. Such Zr contributions have been explored by determining the work function, charge distribution and electronic properties with the help of density functional theory (DFT) based on the generalized gradient approximation (GGA). In doped CeO2, the dopant (Zr) plays a significant role in the formation of defect states, such as oxygen vacancies, which are necessary for generating conducting filaments. The total density of state (DOS) analyses reveal that the existence of impurity states in the hybrid system considerably upgrade the performance of charge transfer/accumulation, consequently leading to enhanced RS behavior, as noticed in our earlier experimental results on Zr/CeO2/Pt devices. Hence it can be concluded that the present DFT studies can be implemented on CeO2-based RRAM devices, which have skyscraping potential for future nonvolatile memory (NVM) applications.

The Role of Oxygen Vacancy Mobility at the Oxide Bulk and Electrode Interfaces for Ceria-Based Memristive Devices

Resistive switching is a recently extensively studied phenomenon for use in non-volatile memories as promising building blocks of future electronics. In such devices transition metal oxides are operated at high electric field strengths to initiate non-linear and hysteretic current-voltage relations that can be addressed in their various resistance states by voltage pulse amplitude or polarity. Despite the known importance of oxygen vacancy defects in metal-oxide memristive devices, systematic strategies on tuning their concentration and mobility in the transition metal oxide are rare. In this work, we utilized the fluorite-type ceria-gadolinia binary system Ce1-xGdxO2-x/2 (GDC) to tune the oxygen vacancy concentration in order to correlate structure and defect chemistry to the memristive properties. The mixed conduction of GDC is well investigated at high temperature, and it is possible to extrinsically dope over the wide range of 3 to 30 mol% Gd; however, has only recently been reported for ambient and high electric fields1. Therefore, we can tune the oxygen vacancy concentration, migration and association energy by the acceptor dopant as a model system to directly investigate the influence on resistive switching. For this purpose Pt-GDC-Pt sandwich structures were fabricated as resistive switching devices with cross-bar electrodes. Based on hysteretic current-voltage profiles, a maximum in resistive switching (with ROFF/RON up to 100) is found for the best ion conducting2 20 mol% Gd-doped CeO2. In-operando Raman spectroscopy revealed a homogeneous reduction of the ceria film during electroforming, together with a strong increase in conductivity. Additional high energy XPS measurements we observe the electrostatic potential drop at the electrode-oxide interface. These experimental results directly prove that the electronic charge carriers generated by the formation and redistribution of oxygen vacancies at high electric fields is the origin of memristance in doped ceria films, and that the switching happens in a rather homogeneous manner. The importance of oxygen vacancy migration in our model is further verified by the doping study. Doping levels of 10-20 mol% resulted in good switching properties, whereas for low (<10 mol%) and high (>20 mol%) concentrations no resistive switching is observed. In both cases the number of mobile oxygen vacancies is not sufficient, either because of the low extrinsic doping level, or because of the formation of bixbyite-like domains with ordered or clustered "immobile" oxygen vacancies. This clustering is observed by the appearance of new vibrational modes in Raman spectroscopy due to the reduced symmetry in a defective crystal. Excitingly, one can study based on the findings the direct implication of "free" oxygen vacancies at low and high mobilites and also of "trapped" clustered vacancies on both, resistive switching and also local structural symmetry breaks3. In conclusion, we observed a clear correlation of oxygen vacancy mobility and switching characteristics. We suggest that a high concentration of mobile oxyen vacancies is of general importance when designing oxide materials for future computing applications.

Large-area release and transfer of ultrathin, freestanding nanocrystalline ceria films

Methods for fully releasing ultrathin ceria films onto an aqueous solution interface, and transferring the film onto arbitrary substrates are presented. The area of the released and transferred film is virtually unlimited, and the release procedure completely avoids etching steps which can be slow and contaminating. Transfer of the ceria film from one substrate to another substrate takes <10min. Furthermore, the procedure is most-likely applicable to a wide array of sample configurations. The possibility of applying the procedure to other materials and film dimensions is discussed. This method will be extremely useful for research and development of thin oxide films and membranes, and has potential for larger scale production.

Self-supported Gd-doped ceria films for electromechanical actuation: Fabrication and testing

In this study, we explored the feasibility of employing Gd-doped ceria (GDC) thin films (1–2μm) as functional, mechanically reliable material for microelectromechanical systems (MEMS). Self-supported structures, based on microscopic-scale GDC membranes, bridges, and cantilevers, were fabricated using Si-compatible processes and materials. With voltages of different amplitudes and frequencies and a variety of metal electrodes, we monitored structural stability and device response. The membrane-based structures displayed much higher stability under voltage and better mechanical robustness than those based on bridges or cantilevers. At low frequencies (a few Hz), the use of Ti contacts resulted in observable displacement of the membranes in the presence of moderately low voltage (≤10V/1.6μm), while Al, Cr, and Ni contacts did not provide such functionality. Although for all contact metals tested, formation of a blocking layer at room temperature is evident, for the case of Ti, the barrier height is much lower. In view of the fact that the crystallographic space group of weakly doped GDC is Fm-3m, the electromechanical response of the microfabricated GDC membranes is most likely electrostrictive, but a strict proof is not yet available. At high frequencies (>100kHz), the membranes produce lateral displacement as large as several microns due to Joule heating, i.e., a thermo-electromechanical response.

Electrical conductivity of RF-sputtered Gd-doped ceria film measured in across-plane mode

Gd-doped ceria (GDC, Ce0.86Gd0.14O2−δ) films are deposited on conductive substrates by RF-magnetron sputtering and their across-plane electrical conductivities (perpendicular to film plane) are measured as a function of temperature (300≤T≤400°C) and oxygen partial pressure (Po2) to assess the feasibility of their use as an electrolyte in miniaturized solid oxide fuel cells (micro-SOFCs). The films deposited at relatively high T=500°C, deposited on either dense or porous substrate, show columnar grains with diameter 200nm. The magnitude of conductivities in air is similar to those of pellet-form GDC. However, the electronic conductivities of GDC films in low Po2 are higher than that of pellet.

Properties of Gadolinium-doped Ceria (GDC) Films Deposited by Reactive Magnetron Sputtering Processes

Reactive magnetron sputtering deposition technique was used for formation of gadolinium doped ceria oxide (GDC) thin films 5- 10µm. Material characteristics and chemical compositions of GDC films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and 3D profilometry. In order to optimize the deposition of GDC to obtain high electrochemical performance of the cells, the influence of film thickness is studied. The GDC thin films were deposited on porous NiO–GDC (nickel oxide–Gadolinium stabilized ceria), the pre-sintered anode green tape were coated with a GDC electrolyte film by reactive magnetron sputtering using PEM (Plasma Emission Monitoring). An Alcatel SCM650 sputtering chamber was used for synthesizing the dense GDC layers. A Ce-Gd metallic target (90-10% at) was powered by a pinnacle + pulsed current generator from Advanced Energy. The structural, microstructural and morphological features of the half-cells were determined by XRD, SEM and 3D profilometry as deposited and after different annealing treatments. The obtained half-cell, 28 mm of diameter, anode supported solid oxide fuel cell, the porous and density of Ni-GDC10 composite anodes have been elaborated by tape casting method. The chosen ceramic powders in suspension were prepared with the mass ratio of 65:35 for NiO to GDC10. The microstructural morphological /porosity/surface features of the half-cell were analyzed by Scanning Electron Microscopy (SEM), XRD and 3D profilometry before and after different annealing treatment. Pycnometry and porosimetry techniques were primarily used to determine pore size and pore volume by equation Washburn and Ideal gas law-method. Finally, the EIS (Electrochemical Impedance Spectroscopy) measurements were done on Ni-GDC10/GDC10 half-cells under 60 sccm nitrogen by the means of a Solartron SI 1260 impedance / gain analyser from 20 MHz to 0.1 Hz with 11 points per decade.

Effect of template post-annealing on Y(Dy)BaCuO nucleation on CeO2 buffered metallic tapes

Substrate engineering is very significant in the synthesis of the high-temperature superconductor (HTS) coated conductor. Here we design and synthesize several distinct and stable Cerium oxide (CeO2) surface reconstructions which are used to grow epitaxial films of the HTS YBa2Cu3O7-δ (YBCO). To identify the influence of annealing and post-annealing surroundings on the nature of nucleation centers, including Ar/5%H2, humid Ar/5%H2 and O2 in high temperature annealing process, we study the well-controlled structure, surface morphology, crystal constants and surface redox processes of the ceria buffers by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and field-emission scanning electronic microscopy (FE-SEM), respectively. The ceria film post-annealed under humid Ar/5%H2 gas shows the best buffer layer properties. Furthermore, the film absorbs more oxygen ions, which appears to contribute to oxygenation of superconductor film. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.

Properties of Gadolinium-doped Ceria (GDC) Films Deposited by Reactive Magnetron Sputtering Processes

Reactive magnetron sputtering deposition technique was used for formation of gadolinium doped ceria oxide (GDC) thin films 5- 10µm. Material characteristics and chemical compositions of GDC films were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In order to optimize the deposition of GDC to obtain high electrochemical performance of the cells, the influence of film thickness is studied. The GDC thin films were deposited on porous NiO–GDC (nickel oxide–Gadolinium stabilized ceria), the pre-sintered anode green tape were coated with a GDC electrolyte film by reactive magnetron sputtering using PEM (Plasma Emission Monitoring). An Alcatel SCM650 sputtering chamber was used for synthesizing the dense GDC layers. A Ce-Gd metallic target (90-10% at) was powered by a pinnacle + pulsed current generator from Advanced Energy. The structural, microstructural and morphological features of the half-cells were determined by XRD and SEM as deposited and after different annealing treatments. The obtained half-cell, 28 mm of diameter, anode supported solid oxide fuel cell, the porous and density of Ni-GDC10 composite anodes have been elaborated by tape casting method. The chosen ceramic powders in suspension were prepared with the mass ratio of 65:35 for NiO to GDC10. Finally, Pycnometry and porosimetry techniques were primarily used to determine pore size and pore volume by equation Washburn and Ideal gas law-method.

Enhancing the oxygen ionic conductivity of (111) oriented Ce0.80Sm0.20O2-δ thin film through strain engineering

Several physicochemical properties can be tailored by tuning the lattice strain of the crystal. Here, we report the effect of lattice strain on oxygen ion conduction of oriented samarium doped ceria (SDC) films deposited over an amorphous quartz substrate. Lattice distortion was induced by varying the deposition rate in electron beam deposition technique (EB-PVD). Texture coefficient calculation and pole figure analysis confirms the (111) orientation in out‐off plane direction. Stress state was monitored using sin2Ψ method in tri-axial direction and the results indicate a reduction in compressive stress with an increase in deposition rate. A difference in the mean free path of adatom influences the surface morphology of SDC films with compressive stress leading to a significant impact on conductivity. Compared to higher compressive stress, thin film containing lower stress along (111) orientation exhibited a two order enhancement in conductivity. The faster ionic conductivity arises from the ease of oxygen ion hopping through the expanded lattice as compared to the highly-stressed crystal structure. Thus, the deposition rate in EB-PVD provides a valuable technique for engineering the strain to realize the potential enhancement in the efficiency of electrolyte for ITSOFC application.

Effects of Dopant Ionic Radius on Cerium Reduction in Epitaxial Cerium Oxide Thin Films

The role of trivalent rare-earth dopants on the cerium oxidation state has been systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. It was found that dopant rare-earths with smaller ionic radius foster the formation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter of the crystal lattice decreases without introducing macroscopic defects. The high crystal quality of our films allowed us to comparatively study both the ionic conductivity and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the Ce4+ → Ce3+ reduction as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tail...