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Growth control of the oxidation state in vanadium oxide thin films
Precise control of the chemical valence or oxidation state of vanadium in vanadium oxide thin films is highly desirable for not only fundamental research, but also technological applications that utilize the subtle change in the physical properties originating from the metalinsulator transition (MIT) near room temperature. However, due to the multivalent nature of vanadium and the lack of a good understanding on growth control of the oxidation state, stabilization of phase pure vanadium oxides with a single oxidation state is extremely challenging. Here, we systematically varied the growth conditions to clearly map out the growth window for preparing phase pure epitaxial vanadium oxides by pulsed laser deposition for providing a guideline to grow high quality thin films with well-defined oxidation states of V₂⁺²O₃, V⁺⁴O₂, and V₂⁺⁵O₅. A well pronounced MIT was only observed in VO₂ films grown in a very narrow range of oxygen partial pressure P(O₂). The films grown either in lower (< 10 mTorr) or higher P(O₂) (> 25 mTorr) result in V₂O₃ and V₂O₅ phases, respectively, thereby suppressing the MIT for both cases. We have also found that the resistivity ratio before and after the MIT of VO₂ thin films can be further enhanced by one order of magnitude when the films are further oxidized by post-annealing at a well-controlled oxidizing ambient. This result indicates that stabilizing vanadium into a single valence state has to compromise with insufficient oxidation of an as grown thin film and, thereby, a subsequent oxidation is required for an 3 improved MIT behavior.
Mechanisms of Water-Stimulated Mg 2+ Intercalation in Vanadium Oxide: Toward the Development of Hydrated Vanadium Oxide Cathodes for Mg Batteries
As lithium-ion batteries approach their theoretical limits for energy density, magnesium-ion batteries are emerging as a promising next-generation energy storage technology. However, progress in magnesium-ion battery research has been stymied by a lack of available high capacity cathode materials that can reversibly insert magnesium ions. Vanadium Oxide (V 2 O 5 ) has emerged as one of the more promising candidate cathode materials, owing to its high theoretical capacity, facile synthesis methods, and relatively high operating voltage. This review focuses on the outlook of hydrated V 2 O 5 structures as a high capacity cathode material for magnesium-ion batteries. In general, V 2 O 5 structures exhibit poor experimental capacity for magnesium-ion insertion due to sluggish magnesium-ion insertion kinetics and poor electronic conductivity. However, several decades ago, it was discovered that the addition of water to organic electrolytes significantly improves magnesium-ion insertion into V 2 O 5 . This review clarifies the various mechanisms that have been used to explain this observation, from charge shielding to proton insertion, and offers an alternative explanation that examines the possible role of structural hydroxyl groups on the V 2 O 5 surface. While the mechanism still needs to be further studied, this discovery fueled new research into V 2 O 5 electrodes that incorporate water directly as a structural element. The most promising of these hydrated V 2 O 5 materials, many of which incorporate conductive additives, nanostructured architectures, and thin film morphologies, are discussed. Ultimately, however, these hydrated V 2 O 5 structures still face a significant barrier to potential applications in magnesium-ion batteries. During full cell electrochemical cycling, these hydrated structures tend to leach water into the electrolyte and passivate the surface of the magnesium anode, leading to poor cycle life and low capacity retention. Recently, some promising strides have been made to remedy this problem, including the use of artificial solid electrolyte interphase layers as an anode protection scheme, but a call to action for more anode protection strategies that are compatible with trace water and magnesium metal is required.
Investigation of Silica-Supported Vanadium Oxide Catalysts by High-Field 51 V Magic-Angle Spinning NMR
Supported V2O5/SiO2 catalysts were studied using solid state 51V MAS NMR at a sample spinning rate of 36 kHz and at a magnetic field of 19.975 T for a better understanding of the coordination of the vanadium oxide as a function of environmental conditions . Structural transformations of the supported vanadium oxide species between the catalyst in the dehydrated state and hydrated state under an ambient environment were revisited to examine the degree of oligomerization and the effect of water. The experimental results indicate the existence of a single dehydrated surface vanadium oxide species that resonates at -675 ppm and two vanadium oxide species under ambient conditions that resonate at -566 and -610 ppm, respectively. No detectable structural difference was found as a function of vanadium oxide loading on SiO2 (3% V2O5/SiO2 and 8% V2O5/SiO2). Quantum chemistry simulations of the 51V NMR chemical shifts on predicted surface structures were used as an aide in understanding potential surface vanadium oxide species on the silica support. The results suggest the formation of isolated surface VO4 units for the dehydrated catalysts with the possibility of dimer and cyclic trimer presence. The absence of bridging V-O-V vibrations (~200-300 cm-1) in the Raman spectra [Gao et al. J. Phys. Chem. B 1998, 102, 10842-10852], however, indicates that the isolated surface VO4 sites are the dominant dehydrated surface vanadia species on silica. Upon exposure to water, hydrolysis of the bridging V-O-Si bonds is most likely responsible for the decreased electron shielding experienced by vanadium. No indicators for the presence of hydrated decavanadate clusters or hydrated vanadia gels previously proposed in the literature were detected in this study.
Multiple Promotional Effects of Vanadium Oxide on Boron Nitride for Oxidative Dehydrogenation of Pro
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Atomic Layer Deposition of Vanadium Oxide to Reduce Parasitic Absorption and Improve Stability in n-i-p Perovskite Solar Cells for Tandems
Two critical issues associated with semi-transparent, n-i-p perovskite solar cells for 2-terminal tandem devices are parasitic absorption and long-term instability associated with the widely used spiro-OMeTAD and MoOx hole transport and buffer layers, respectively. In this work, we present an alternative hole contact bilayer that consists of a 30 nm undoped layer of spiro-TTB in conjunction with 9 nm of air-stable vanadium oxide (VOx) deposited via atomic layer deposition. The low absorption of UV and visible light in this bilayer results in the fabrication of a semi-transparent perovskite cell with 18.9 mA cm-2 of photocurrent, a 14% increase compared to the 16.6 mA cm-2 generated in a control device with 150 nm of doped spiro-OMeTAD. The ALD VOx buffer layer shows promise as a stable alternative to MoOx; an unencapsulated Cs0.17FA0.83Pb(Br0.17I0.83)3 device with ALD VOx and ITO as the top contact maintains its efficiency following 1000 hours at 85 degrees C in a N2 environment. Lastly, we use transfer matrix modeling of the optimized perovskite stack to predict its optical performance in a monolithic tandem cell with heterojunction silicon.
Co-sputtering of lithium vanadium oxide thin films with variable lithium content to enable advanced solid-state batteries
Advanced solid-state batteries most likely will entail aggressive structures or architectures with constraints that typically limit processing temperatures. Considering this, we have identified the importance of providing lithiated electrode materials at a modest processing temperature. Here we describe a pathway to meet this by the development of a co-sputtering process using lithium oxide and vanadium oxide targets which enables the growth of lithiated vanadium oxide (LVO) thin films for application in solid-state batteries. Analysis of the structure and film composition of samples deposited with different co-sputtering rate ratios and post-annealing shows that multiple phases of Li x V 2 O 5 likely coexist (i.e. α-, ε-, δ-, and γ-V 2 O 5 ), and that this is unchanged after electrochemical cycling. The co-sputtering process can tune the lithium content up to a highly lithiated state of at least Li 2 V 2 O 5 . Electrochemical half-cells showed a significant amount of lithium available on the first charge (delithiation of LVO). LVO samples post-annealed at 300 °C showed typical redox peaks for Li x V 2 O 5 for both one and two lithium insertion reactions, which were highly reversible in most cases. A thin-film solid-state battery prototype using LVO as a cathode had 20% of the expected capacity, although the coulombic efficiency is near 100% at a fast rate (22C). As a result, this co-sputtering technique represents an opportunity for low temperature synthesis of pre-lithiated cathodes for thin film batteries, and introduces a broader methodology of depositing metal oxides with different alkali metal contents.
Impact of Synthesis Method on Phase Transformations of Layered Lithium Vanadium Oxide upon Electrochemical (De)lithiation
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Lithium vanadium oxide (Li 1.1 V 3 O 8 ) thick porous electrodes with high rate capacity: utilization and evolution upon extended cycling elucidated via operando energy dispersive X-ray diffraction and continuum simulation
The phase distribution of lithiated LVO in thick (~500 μm) porous electrodes (TPEs) designed to facilitate both ion and electron transport was determined using synchrotron-based operando energy dispersive X-ray diffraction (EDXRD). Probing 3 positions in the TPE while cycling at a 1C rate revealed a homogeneous phase transition across the thickness of the electrode at the 1st and 95th cycles. Additionally, continuum modelling indicated uniform lithiation across the TPE in agreement with the EDXRD results and ascribed decreasing accessible active material to be the cause of loss in delivered capacity between the 1st and 95th cycles. The model was supported by the observation of significant particle fracture by SEM consistent with loss of electrical contact. Overall, the combination of operando EDXRD, continuum modeling, and ex situ measurements enabled a deeper understanding of lithium vanadium oxide transport properties under high rate extended cycling within a thick highly porous electrode architecture.
Formation of N2O greenhouse gas during SCR of NO with NH3 by supported vanadium oxide catalysts
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Effect of vanadium oxide addition on thermomechanical behaviors of borosilicate glasses: Toward development of high crack resistant glasses for nuclear waste disposal
The effect of of V2O5 additions (up to 8 mol%) on the physical properties (e.g., density, glass transition temperature and refractive indices) and mechanical behaviors (e.g., hardness, fracture toughness, elastic modulus) of a soda-lime borosilicate glass with a composition of 60.2SiO(2)-16.0B(2)O(3)-12.6Na(2)O-3.8Al(2)O(3)-5.7CaO-1.7ZrO(2) (mol%) have been investigated systematically. It was found that introducing V2O5 decreases the glass transition temperature and hardness while increases the refractive index and crack initiation load. Composition-property relationships, especially on mechanical properties, were examined in order to provide insights for the development of new glass materials with outstanding mechanical properties. The results show that V2O5 can be a glass component for engineering and insuring high crack resistant glasses and glasses with long-term stability. Additionally, elastic properties as a function of temperature were studied using Brillouin light scattering and temperature-independent elastic moduli behaviors were observed.
Low temperature performance of lithium/silver vanadium oxide cells
Lithium/silver vanadium oxide cells for low temperature applications have been developed. Prismatic and spirally wound AA cells were tested under constant load discharge of 0.3 to 1.8 amps or pulse discharge of 0.225 or 1.0 amps at temperatures from -40 to 25 C. At -40 C with current densities of 2.5 mA/cm2, 23 percent of theoretical capacity was achieved under constant load discharge and 40 percent of theoretical capacity was achieved under pulse test. Self-discharge estimates of 0.7 percent per year at 25 C were obtained from microcalorimetry. Preliminary safety testing of the cells revealed no violent performance under short circuit or crush tests.
Solvothermal synthesis and characterization of highly monodisperse organically functionalized vanadium oxide nanocrystals for thermochromic applications
Highlights: • Vanadium oxide nanocrystals is synthesized via an improved solvothermal method. • Vanadium oxide nanocrystals possess a structure of distorted paramontroseite VO{sub 2}(P). • Thermally annealed nanocrystal thin films exhibit good thermochromic properties. - Abstract: We report on the synthesis and characterization of highly monodisperse organically functionalized vanadium oxide (VO{sub x}) nanocrystals with a size of 7–10 nm. We show that how these nanocrystals can be prepared by an improved solvothermal reaction and confirm their composition and structure through various materials characterization techniques. Our results revealed that VO{sub x} nanocrystals have a crystalline structure almost identical to that of paramontroseite VO{sub 2}(P) and the majority of vanadium is in oxidation state of V(IV). VO{sub x} nanocrystal films can be readily prepared via a spin coating technique. Once thermally annealed at ∼350 °C under an air/N{sub 2} flow, the films exhibited good thermochromic properties (luminous transmittance (T{sub lum}) ≈49–66%, infrared switching efficiency (ΔT{sub 2000nm}) ≈28–43%, and solar modulating ability (ΔT{sub sol}) ≈7.4–10.7%), corresponding to a phase transition between monoclinic VO{sub 2}(M) and rutile VO{sub 2}(R).
Visualization of structural evolution and phase distribution of a lithium vanadium oxide (Li 1.1 V 3 O 8 ) electrode via an operando and in situ energy dispersive X-ray diffraction technique
We present Li 1+n V 3 O 8 (n = 0–0.2) has been extensively investigated as a cathode material for Li ion batteries because of its superior electrochemical properties including high specific energy and good rate capability. In this paper, a synchrotron based energy dispersive X-ray diffraction (EDXRD) technique was employed to profile the phase transitions and the spatial phase distribution of a Li 1.1 V 3 O 8 electrode during electrochemical (de)lithiation in situ and operando. As annealing temperature during the preparation of the Li 1.1 V 3 O 8 material has a strong influence on the morphology and crystallinity, and consequently influences the electrochemical outcomes of the material, Li 1.1 V 3 O 8 materials prepared at two different temperatures, 500 and 300°C (LVO500 and LVO300), were employed in this study. The EDXRD spectra of LVO500 and LVO300 cells pre-discharged at C/18, C/40 and C/150 were recorded in situ, and phase localization and relative intensity of the peaks were compared. For cells discharged at the C/18 rate, although α and β phases were distributed uniformly within the LVO500 electrode, they were localized on two sides of the LVO300 electrode. Discharging rates of C/40 and C/150 led to homogeneous β phase formation in both LVO500 and LVO300 electrodes. Furthermore, the phase distribution as a function of position and (de)lithiation extent was mapped operando as the LVO500 cell was (de)lithiated. In conclusion, the operando data indicate that (1) the lithiation reaction initiated from the side of the electrode facing the Li anode and proceeded towards the side facing the steel can, (2) during discharge the phase transformation from a Li-poor to a Li-rich α phase and the formation of a β phase can proceed simultaneously in the electrode after the first formation of a β phase, and (3) the structural evolution occurring during charging is not the reverse of that during discharge and takes place homogenously throughout the electrode.
Liquid Phase Exfoliation of Chemically Prelithiated Bilayered Vanadium Oxide in Aqueous Media for Li-Ion Batteries
Bilayered vanadium oxides are attractive for energy storage due to their high initial specific capacities, which could be stabilized by integrating the bilayers with conductive nanoflakes often produced in a form of aqueous dispersions. Therefore, exfoliation of the bilayered vanadium oxides in water with high yield is desirable. Here, this work introduces the first aqueous exfoliation of chemically prelithiated bilayered vanadium oxide (i.e., δ-Li x V 2 O 5 ·nH 2 O or LVO) followed by vacuum filtration to produce a free-standing film, exhibiting a lamellar stacking of the bilayered vanadium oxide nanoflakes as evidenced by scanning electron microscopy. Due to the hydrated nature of bilayered vanadium oxides, the relationship between interlayer water content and the vacuum drying temperature (105 °C vs 200 °C) was studied using X-ray diffraction, thermogravimetric analysis, and Raman spectroscopy. It was found that vacuum drying the LVO nanoflakes at 200 °C enabled more efficient removal of crystallographic water than drying at 105 °C, and did not induce a phase transformation. Scanning transmission electron microscopy confirmed the layered structure of the samples, which was more well-ordered in the 200 °C case and had no clear boundaries between flakes at the atomic scale. Furthermore, electrochemical testing in nonaqueous Li-ion cells revealed that vacuum drying at 200 °C led to improvements in ion storage capacity and electrochemical stability. Improvements in electrochemical charge storage properties of the electrodes obtained via LVO exfoliation and free-standing film assembly in water dried at 200 °C reveal that conventional battery electrode drying protocols need to be revised as new electrochemically active materials are synthesized, such as hydrated layered oxides with expanded interlayer regions. The remaining capacity fading can be attributed to the structural LVO degradation, dissolution of vanadium oxide in electrolyte, and parasitic effects of the remaining interlayer water molecules. Our results establish an environmentally friendly and safe approach to obtain two-dimensional (2D) bilayered vanadium oxide nanoflakes and create a pathway to constructing novel 2D heterostructures for improved performance in energy storage applications.
Comparison of distribution and toxicity following repeated oral dosing of different vanadium oxide nanoparticles in mice
Vanadium is an important ultra-trace element derived from fuel product combustion. With the development of nanotechnology, vanadium oxide nanoparticles (VO NPs) have been considered for application in various fields, thus the possibility of release into the environment and human exposure is also increasing. Considering that verification of bioaccumulation and relevant biological responses are essential for safe application of products, in this study, we aimed to identify the physicochemical properties that determine their health effects by comparing the biological effects and tissue distribution of different types of VO NPs in mice. For this, we prepared five types of VO NPs, commercial (C)-VO{sub 2} and -V{sub 2}O{sub 5} NPs and synthetic (S)-VO{sub 2}, -V{sub 2}O{sub 3,} and -V{sub 2}O{sub 5} NPs. While the hydrodynamic diameter of the two types of C-VO NPs was irregular and impossible to measure, those of the three types of S-VO NPs was in the range of 125–170 nm. The S- and C-V{sub 2}O{sub 5} NPs showed higher dissolution rates compared to other VO NPs. We orally dosed the five types of VO NPs (70 and 210 μg/mouse, approximately 2 and 6 mg/kg) to mice for 28 days and compared their biodistribution and toxic effects. We found that S-V{sub 2}O{sub 5} and S-V{sub 2}O{sub 3} NPs more accumulated in tissues compared to other three types of VO NPs, and the accumulated level was in order of heart>liver>kidney>spleen. Additionally, tissue levels of redox reaction-related elements and electrolytes (Na{sup +}, K{sup +}, and Ca{sup 2+}) were most clearly altered in the heart of treated mice. Notably, all S- and C-VO NPs decreased the number of WBCs at the higher dose, while total protein and albumin levels were reduced at the higher dose of S-V{sub 2}O{sub 5} and S-V{sub 2}O{sub 3} NPs. Taken together, we conclude that the biodistribution and toxic effects of VO NPs depend on their dissolution rates and size (surface area). Additionally, we suggest that further studies are needed to clarify effects of VO NPs on functions of the heart and the immune system. - Highlights: • We used commercial (C)-VO{sub 2} and -V{sub 2}O{sub 5} and synthetic (S)-VO{sub 2}, -V{sub 2}O{sub 3,} and -V{sub 2}O{sub 5} NPs. • The S- and C-V{sub 2}O{sub 5} NPs showed higher dissolution rates compared to other VO NPs. • S-V{sub 2}O{sub 5} and S-V{sub 2}O{sub 3} NPs more accumulated in tissues compared to other types of VO NPs. • Redox reaction-related elements and electrolytes were clearly altered in the heart. • With decreased body weight, immune and blood-related factors influence by all VO NPs. • Biodistribution and toxicity of VO NPs depend on their dissolution rates and size.
Polaron-induced metal-to-insulator transition in vanadium oxides from density functional theory calculations
Vanadium oxides have been extensively studied as phase-change memory units in artificial synapses for neuromorphic computing due to their metal-insulator transitions (MIT) at or near room temperature. Recently, injection of charge carriers into vanadium oxides, e.g., via optically via a heterostructure, has been proposed as an alternative switching mechanism and also potentially as a means to tune the MIT temperature. In this study, we explore the formation of small polarons in the low temperature (LT) insulating phases for V 3 O 5 ,VO 2 , and V 2 O 3 , and the barriers to their migration using density functional theory calculations. We find that V 3 O 5 exhibits very low hole and electron polaron migration barriers (<100 meV) compared to V 2 O 3 and VO 2 , leading to much higher estimated polaronic conductivity. We also link the relative migration barriers to the amount of distortion that has to travel when the polaron migrate from one site to another. Polarons in V 3 O 5 also have smaller binding energies to vanadium and oxygen vacancy defects. Furthermore, these results suggest that the triggering of the MIT via injection of charge carriers are due to the formation of small polarons that can migrate rapidly through the crystal.