Phase evolution and thermochromism of vanadium oxide thin films grown at low substrate temperatures during magnetron sputtering

Vanadium oxides (VO_x) have been studied extensively for applications in thermochromic materials, electrochomics, and infrared detectors due to their unique phase transition characteristics. However, various vanadium oxide phases usually occur under different deposition conditions due to their particularly complex vanadium-oxygen system. In this research, V₃O₇, VO₂(B), VO₂(M), and V₂O₅ thin films were obtained as pure or mixed phases by controlling the substrate temperatures between 250 °C and 400 °C during magnetron sputtering. The microstructure and phase composition of vanadium oxide thin films were characterized and analyzed using X-ray diffraction and Raman spectroscopy. The phase evolution was dependent on the substrate temperature and could be clarified. Metastable V₃O₇ and VO₂(B) phases were obtained at substrate temperatures of 250–300 °C, while stable VO₂ and V₂O₅ phases were obtained at 350–400 °C. The surface morphology and optical properties of vanadium oxide thin films with different substrate temperatures were investigated in detail. Our results provide methods for transforming vanadium oxide phases under well controlled substrate temperatures.     

Porous silicon pillar and bilayer structure as a nucleation center for the formation of aligned vanadium pentoxide nanorods

Porous silicon single layer (PSM), bilayer (PSB) and pillar (PSP) structures have been evaluated as nucleation centers for vanadium pentoxide (V₂O₅) crystals. Deposition of vanadium precursor over different substrates (drop casting technique), followed by annealing treatment under Ar-H₂ (5% H₂) atmosphere, induced crystallization of vanadium oxide. With respect to c-Si/SiO₂ substrate, V₂O₅ nanorods with relatively large aspect ratio were formed over and within PSP structures. On the other hand, pores in PSM and PSB were found to be filled with relatively smaller crystals. Additionally, PSB provided a nucleation substrate capable to align the nanocrystals in a preferential orientation, while V₂O₅ crystals grown on PSP were found to be randomly aligned around the nanoporous pillar microstructure. Nanorods and nanocrystals were identified as V₂O₅ by temperature-controlled XRD measurements and evidence of their crystalline nature was observed via transmission electron microscopy. A careful analysis of electronic microscopy images allows the identification of the facets composing the ends of the crystals and its corresponding surface free energy has been evaluated employing the Wulff theorem. Such high surface area composite structures have potential applications as cathode material in Lithium-ion batteries.     

Phase cooperation of V<Subscript>2</Subscript>O<Subscript>5</Subscript> and Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> in the selective oxidation of H<Subscript>2</Subscript>S containing ammonia and water

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium oxide-based catalysts. The investigation was focused on the role of V₂O₅, and phase cooperation between V₂O₅ and Bi₂O₃ in this reaction. The conversion of H₂S continued to decrease since V₂O₅ was gradually reduced by treatment with H₂S. The activity of V₂O₅ was recovered by contact with oxygen. A strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V₂O₅ and Bi₂O₃. Temperature-programmed reduction (TPR) and oxidation (TPO) and two bed reaction tests were performed to explain this synergistic effect by the reoxidation ability of Bi₂O₃. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Photoconductivities in monocrystalline layered V2O5 nanowires grown by physical vapor deposition

Photoconductivities of monocrystalline vanadium pentoxide (V₂O₅) nanowires (NWs) with layered orthorhombic structure grown by physical vapor deposition (PVD) have been investigated from the points of view of device and material. Optimal responsivity and gain for single-NW photodetector are at 7,900 A W^-1 and 30,000, respectively. Intrinsic photoconduction (PC) efficiency (i.e., normalized gain) of the PVD-grown V₂O₅ NWs is two orders of magnitude higher than that of the V₂O₅ counterpart prepared by hydrothermal approach. In addition, bulk and surface-controlled PC mechanisms have been observed respectively by above- and below-bandgap excitations. The coexistence of hole trapping and oxygen sensitization effects in this layered V₂O₅ nanostructure is proposed, which is different from conventional metal oxide systems, such as ZnO, SnO₂, TiO₂, and WO₃.     

Oxygen deficient V2O3: A stable and efficient electrocatalyst for HER and high performance EDLCs

The demand of materials for energy conversion and storage is increasing to address the issues related to the depletion of fossil fuels. In the present study, oxygen deficient vanadium oxide (V₂O₃) has been synthesized in an autoclave at 500 °C for 4 h. The reaction temperature, time and atmosphere affect the pure phase formation of V₂O₃. The structural, morphological and surface characteristics of pure phase V₂O₃ has been analysed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The phase transition with respect to temperature has been confirmed by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The electrocatalytic hydrogen evolution reaction (HER) activity has been studied in an acidic medium. The synthesized sample shows higher cyclic stability for 1000 cyclic voltammetry (CV) cycles. It also exhibits a lower Tafel slope of 81.2 mVdec⁻¹. The electrochemical double layer capacitance (EDLC) measurements was done via CV measurements performed at various scan rates. The observed EDLC value of synthesized sample is 242.6 μFcm⁻². The study predicts potential applications of V₂O₃ nano structures for electrochemical applications like HER and supercapacitors.     

Reactivity of solid BaCe0.9Y0.1O3 towards liquid V2O5

In this work the reactivity in solid BaCe_0.9Y_0.1O_3-δ - liquid V₂O₅ system was investigated in order to evaluate the possibility of composite formation in this system. Impregnation of melted vanadium(V) oxide into solid Y-doped BaCeO₃ was performed in various temperature and time conditions. Due to the relatively high reactivity of components in this system an analysis of the synthesized materials was divided into discussion of the properties of the reactive layer and the inner part. As was shown, decomposition of Y-doped BaCeO₃ in contact with melted vanadium(V) oxide occurs already at temperature 700?°C and results in formation of CeVO₄ as one of the main phases. Moreover, the presence of Ba₈V₇O₂₂ as the intermediate phase was observed. The extension of reaction time and increase of temperature resulted in significant changes in the microstructure and chemical composition of the reactive layer. The incorporation of vanadium into yttrium doped BaCeO₃ structure for inner parts of the synthesized materials was postulated based on the modification of the materials electrical properties and chemical stability towards CO₂ and H₂O. Also, the presence of additional phase in the intergranular voids of doped-BaCeO₃ was suggested.     

New vanadium doped calcium titanate ceramic pigment

In this paper a new pink vanadium doped calcium titanate Ca(V_xTi_1−x)O₃ ceramic pigment in conventional ceramic glazes is obtained by ceramic route and characterized. The limit of solid solution is near by x = 0.2, higher amounts of vanadium crystallizes Ca₂V₂O₇ which dilute the real amount of saturated Ca(V_xTi_1−x)O₃ solid solution and diminish the intensity of colour. The unit cell parameter measurements of Ca(V_xTi_1−x)O₃ agrees with the substitution of Ti⁴⁺ by V⁵⁺ that is associated to a V⁵⁺-O charge transfer at 420 nm on UV-vis-NIR spectra of 5% glazed samples that explain the pink colour obtained. In order to avoid the limitation due to the suppressing of oxygen vacancies by high valence cation V⁵⁺ substitution in a Ti⁴⁺ site of CaTiO₃ perovskite for to preserve the charge neutrality of the lattice; Fe³⁺ and V⁵⁺ codoped samples Ca(Fe_xV_xTi_1−2x)O₃x = 0.1, 0.2 and 0.3 were prepared and show a brown colour fired 1000 °C, but 5% glazed do not produce colour indicating that iron codoping inhibits the pigmenting capacity of vanadium doped CaTiO₃ perovskite.     

Investigation of the conditions required for the formation of V(C,N) during carburization of vanadium or carbothermal reduction of V2O5 under nitrogen

Phase stability diagrams of V–C–N and V–O–C–N systems were constructed as a function of carbon activity, nitrogen partial pressure, oxygen partial pressure, and solution formation characteristics to determine the conditions for the formation of V(C,N) via the carburization of vanadium or carbothermal reduction of V₂O₅ under nitrogen. The diagram showed that only V, V₂C, and V(C,N) phases would be stable in the V–C–N system. From the diagram, it was also observed that only V(C,N) exists after the carburization of vanadium under nitrogen atmosphere more than 10^?5 atm. The diagram of V–O–C–N system suggests that V₂O₅ can be reduced to V(C,N) without forming VO, owing to the high stability of the V(C,N) phase. Using these stability diagrams, the conditions for preparing V(C,N) from vanadium or V₂O₅ were deduced and the validity of the diagrams was verified using the experimental results.     

XPS study of Li ion intercalation in V2O5 thin films prepared by thermal oxidation of vanadium metal

The intercalation and deintercalation mechanisms of lithium into V₂O₅ thin films prepared by thermal oxidation of vanadium metal have been studied by X-ray photoelectron spectroscopy (XPS) using a direct anaerobic and anhydrous transfer from the glove box (O₂ and H₂O < 1ppm), where the samples were electrochemically treated, to the XPS analysis chamber. Vanadium in the as-prepared oxide films is mostly (from 93 to 96% depending on samples) in a pentavalent state (V⁵⁺) with a stoichiometric O/V concentration ratio fitting that of V₂O₅. Four to seven percent of VO₂ is also observed. After the 1st and the 2nd intercalation steps at E = 3.3 and 2.8 V versus Li/Li⁺, respectively, the V2p core level spectra evidence a partial reduction to V⁴⁺ states with a remaining concentration of 73 and 56% of V⁵⁺, in agreement with the intercalation of about 1/2 mol of Li per V₂O₅ mol at each intercalation step. Intercalated lithium was observed at a binding energy of 56.1 eV for Li1s. Changes of the electronic structure of the V₂O₅ thin film after intercalation are evidenced by the observation, at a binding energy of 1.3 eV, of occupied V3d states (V⁴⁺) originally empty in the pristine film (V⁵⁺). The V2p and Li1s core level spectra show that the process of Li intercalation is partially irreversible. In the first cycle, 34 and 14% of the vanadium ions remain in the V⁴⁺ state after deintercalation at E = 3.4 and 3.8 V versus Li/Li⁺, respectively, indicating a partially irreversible process already after the 1st deintercalation. The analyses of C1s and O1s XP spectra show the formation of a solid-electrolyte interface (SEI). The analyzed surface layer includes lithium carbonate and Li-alkoxides.     

Synthesis, characterization and electrochemical properties of the V2O5·nH2O/AlO(OH)·nH2O xerogel composite

In this work, we report the synthesis, characterization and electrochemical properties of a new multicomponent material obtained from the polymerization of vanadium pentoxide in an inorganic matrix (alumina xerogel), forming a xerogel composite. The material has been characterized by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, electron microscopy, energy dispersive X-ray spectrometry, cyclic voltammetry and impedance spectroscopy. It was found that the V₂O₅ xerogel is dispersed in the alumina matrix, but its lamellar structure is not strongly affected, thus, its conductivity properties are maintained. Moreover, the electrochemical behaviour of the V₂O₅ xerogel dispersed in the alumina matrix is quite similar to that found for the V₂O₅ xerogel alone and the inorganic matrix leads to stabilization of V₂O₅ xerogel structure.     

Thermal oxidation of A<Superscript>III</Superscript>B<Superscript>V</Superscript> semiconductors with V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanolayers on the surface

The specific features of the interaction of vanadium(V) oxide nanofilms with the surface of gallium arsenide and indium phosphide semiconductors under thermal oxidation conditions have been considered. The kinetics and mechanism of thermal oxidation of GaAs and InP with deposited V₂O₅ layers 15 and 25 nm in thickness have been studied. It has been revealed that vanadium(V) oxide exerts a specific effect on the oxidation of gallium arsenide and indium phosphide as compared to other d-metal oxides. It has been established that the oxidation occurs with the formation of a phase predominantly consisting of indium phosphates or gallium arsenates and intermediate products based on vanadium compounds in different oxidation states. Schemes have been proposed for the development of the oxidation processes with due regard for the chemical nature of vanadium(V) oxide.     

Preparation and characterization of a model system for the study of monolayers and multilayers of vanadia supported on titania

A model system for the study of structural and chemical properties of monolayers and multilayers of vanadium oxide immobilized on titania is presented. Investigation of the planar oxide-oxide interface by XP, UV and IS spectroscopy indicated that vanadium immobilized by a single impregnation step exists as an incomplete heterogeneous layer containing well dispersed V⁴⁺ species. Increase of the vanadia loading by multiple impregnations led to vanadia agglomerates with higher apparent oxidation state of the vanadium. TD spectroscopy with O₂ and CO₂ as probe molecules revealed that the chemical reactivity of the vanadia surface species depends on their structure. The surface containing well-dispersed vanadia species exchanged oxygen more easily and showed pronounced interactions with CO₂.     

Infrared and raman characterization of V<Subscript>2</Subscript>O<Subscript>5</Subscript> on zirconia modified with WO<Subscript>3</Subscript> and activity for acid catalysis

Vanadium oxide supported on zirconia modified with WO₃ was prepared by adding Zr(OH)₄ powder into a mixed aqueous solution of ammonium metavanadate and ammonium metatungstate followed by drying and calcining at high temperatures. The characterization of prepared catalysts was performed by using FTIR, Raman, and XRD. In the case of calcination temperature at 773 K, for samples containing low loading V₂O₅ below 18 wt%, vanadium oxide was in a highly dispersed state, while for samples containing high loading V₂O₅ equal to or above 18 wt%, vanadium oxide was well crystallized due to the high V₂O₅ loading on the surface of ZrO₂. The ZrV₂O₇ compound was formed through the reaction of V₂O₅ and ZrO₂ at 873 K, and the compound decomposed into V₂O₅ and ZrO₂ at 1,073 K, these results were confirmed by FTIR and XRD. Catalytic tests for 2-propanol dehydration and cumene dealkylation have shown that the addition of WO₃ to V₂O₅/ZrO₂ enhanced both catalytic activity and acidity of V₂O₅-WO₃/ZrO₂ catalysts. The variations in catalytic activities for both reactions are roughly correlated with the changes of acidity.     

The distribution of lithium intercalated in V2O5 thin films studied by XPS and ToF-SIMS

The distribution of lithium in V₂O₅/V lower oxide duplex thin films prepared by thermal oxidation of V metal was analysed by XPS and ToF-SIMS after intercalation at 2.8 V versus Li/Li⁺ and de-intercalation at 3.8 V following cycling between 3.8 and 2.8 V in 1 M LiClO₄-PC. XPS analysis of the intercalated thin film evidenced a partial reduction (43 at.% V⁴⁺) of the V₂O₅ surface, the modification of its electronic structure and the presence of Li, consistent with the formation of the δ-Li_xV₂O₅ (0.9 ≤ x ≤ 1) phase. The Li in-depth distribution measured by ToF-SIMS shows a maximum in the outer layer of V₂O₅, but Li is also found at the oxide film/metal substrate interface indicating its diffusion across the inner layer of V lower oxides. The analyses performed after de-intercalation on the samples cycled 12, 120 and 300 times reveal the effect of aging on the trapping of lithium. A significant reduction (17-22 at.% V⁴⁺) of the V₂O₅ surface was measured after 300 cycles. The Li in-depth distribution shows a maximum at the interface between the outer layer of V₂O₅ and the inner layer of lower oxides. Aging favours the accumulation of lithium at this interface with a resulting enlarged distribution enriching the sub-surface of the outer layer of V₂O₅ and the inner layer of lower oxides after 300 cycles. Lithium is also found, but in smaller quantities, at the oxide film/metal substrate interface. Measurements performed in the non-electrochemically treated surface areas of the de-intercalated samples revealed the same type of modifications, evidencing the diffusion of lithium along the interfaces where it is trapped.     

In situ XAS investigation of the oxidation state and local structure of vanadium in discharged and charged V2O5 aerogel cathodes

We have examined the evolution of the oxidation state and atomic structure of vanadium(V) in discharged and charged nanophase vanadium pentoxide (V₂O₅) aerogel cathodes under in situ conditions using X-ray absorption spectroscopy (XAS). We show that the oxidation state of V in V₂O₅ aerogel cathode heated under vacuum (100 μTorr) at 220 °C for 20.5 h is similar to that of V in a commercially obtained sample of orthorhombic V₂O₅. In addition, lithium (Li) insertion during the first cycle of discharging leads to the reduction of V(V) to V(IV) and V(IV) to V(III) in a manner consistent with the stoichiometry of the sample (i.e. Li_xV₂O₅). Li extraction during charging leads to oxidation of V(III) to V(IV) and then V(IV) to V(V). Furthermore, the oxidation state of V in fully charged cathodes remains unchanged with cycling (upto at least the 16th cycle) from that of V in the control V₂O₅ aerogel cathode. However, the average oxidation state of V in discharged V₂O₅ cathodes increased with cycling. Moreover, the local structure of V in the discharged state has a higher degree of symmetry than that of the fully charged state. A significant change in the structure of the VV correlation of discharged cathodes is observed with cycling indicating the formation of electrochemically irreversible phases.     

Probing the Local Structure and Phase Transitions of Bi4V2O11‐Based Fast Ionic Conductors by Combined Raman and XRD Studies

In this article, we report the structural phase transitions in Bi₄V₂O₁₁ as observed from temperature‐dependent Raman scattering and X‐ray diffraction measurements. Four different types of highly disordered coordination polyhedra around the vanadium atoms with large dispersion of V–O bond lengths are observed in Bi₄V₂O₁₁ at ambient temperature. The observed V–O bond lengths could be grouped into two categories, viz. shorter <1.7 Å and longer >1.7 Å. The Raman modes of Bi₄V₂O₁₁ could be assigned to vibration of these bonds and V–O–V linkages. We could correlate the difference in degree of anharmonicity of the phonon modes with temperature to differences in V–O bond strength. The local structure of vanadium–oxygen network in Bi₄V_1.8Cu_0.2O_10.7 was also obtained by similar studies. The effect of highly disordered anion sublattice in the doped compound is reflected in the broadening of the Raman modes.     

Vanadia-molybdena catalysts for the oxidation of 2-picoline

A study has been made of the influence of catalyst composition on the gas phase oxidation of 2-picoline over mixed vanadium and molybdenum oxides supported on kieselguhr. It is shown that the formation of partial oxidation products is associated with the existence of a mixed oxide phase. The selectivity for the formation of pyridine-2-aldehyde reaches a maximum when one in six vanadium(V) ions in the V₂O₅ lattice is replaced by a molybdenum(VI) ion. It is suggested that the origin of the selectivity is the formation of isolated (V⁴⁺-O) species.     

Liquid Emulsion Membrane (LEM) Process for Vanadium (IV) Enrichment: Process Intensification

Abstract

A liquid emulsion membrane (LEM) system for vanadium (IV) transport has been designed using di‐2‐ethylhexyl phosphoric acid (D2EHPA), dissolved in n‐dodecane as carrier. The selection of extractant, D2EHPA, was made on the basis of conventional liquid‐liquid extraction studies. The work has been undertaken by first carrying out liquid‐liquid extraction studies for vanadium (IV) to get stoichiometric constant (n), and equilibrium constant (K_ex), which are important for process design.

Transport experiments were carried out at low vanadium (IV) concentration (ppm level). The studies on liquid emulsion membrane included i) the influence of process parameters i.e. feed phase pH, speed of agitation, treat ratio, residence time and ii) emulsion preparation study i.e., organic solvent, extractant concentration, surfactant concentration, internal strip phase concentration. When the strip phase concentration was 2 mol/dm³ (H₂SO₄) and feed phase pH 3 better extraction of vanadium was obtained. Higher V_m/V₁ gave higher extraction of vanadium (IV). A simplified, design engineer friendly model was developed.     

Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li_2xVO(H_2−xPO₄)₂ (0 < x < 2), have been undertaken. H⁺/Li⁺ ionic exchange from VO(H₂PO₄)₂ to Li₂VO(HPO₄)₂ leads to grain decrepitation. Further ionic exchange toward formation of Li₄VO(PO₄)₂ lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li₄VO(PO₄)₂ shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with V^V/V^IV and V^III/V^II redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of V^III to V^IV is involved.     

Lithium insertion property of Li2 2V2O5·nH2O

Lithium vanadium oxides have been prepared by the new solution processing in an aqueous hydrogen peroxide solution with lithium and vanadium alkoxides, LiO-n-C₃H₇ and VO(O-i-C₃H₇)₃, at low temperature, compared with conventional high temperature solid state reaction. Oxides having a layered structure isomorphic to that of γ-phase Li_xV₂O₅ were obtained. This “γ-like phase” oxide can be obtained at the nominal Li/V ratio of 1.5 almost as a single phase. However, formation of ω phase cannot be confirmed. The γ-like phase oxide contained water and organic compounds, and the water content n in Li_xV₂O₅·nH₂O was found to be about 2.4 for the γ-like phase oxide. Further as the result of the atomic absorption spectrometric method, the lithium content x in Li_xV₂O₅·nH₂O was estimated to be 2.2, and water molecules presumably exist in the interlayer space.Water content of the γ-like phase oxides, affects charge and discharge behaviours markedly. The lithium extraction-insertion capacity of the γ-like phase oxides were smaller, but the oxides had higher average potential compared with those of γ-phase oxide. As water content of γ-like phase oxides decreased, the lithium extraction-insertion capacity increased. Moreover, it should be noted that the average potential of γ-like phase oxides is at least 1 V higher than that of γ-LiV₂O₅.