Structural Stoichiometry and Phase Transitions of

Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502 Vol. 2(4), 1-8, April (2013) Res. J. Recent Sci.

Review Paper

Structural Stoichiometry and Phase Transitions of MoO3 Thin Films for Solid State Microbatteries Rao M.C.1*, Ravindranadh K.1, Kasturi A.2 and Shekhawat M.S.3 1

Department of Physics, Andhra Loyola College, Vijayawada - 520 008, INDIA 2 Department of Physics, Maris Stella College, Vijayawada - 520 008, INDIA 3 Department of Physics, Engineering College Bikaner, Bikaner – 334004, INDIA

Available online at: www.isca.in Received 2nd October 2012, revised 2012, accepted 2013

Abstract MoO3 is a potential material because of its wide range of stoichiometry with interesting behaviour, which includes chromogenic and catalytic properties. This leads to the applications in electrochromic display devices, optical memories, gas sensors and lithium batteries. The substochiometric films MoO 3-x with oxygen deficient contain excess metal atoms which act as doping centers; these centers control the electrical and optical film properties. The orthorhombic phase (α-MoO3) has a layered structure, which consists of double layers of MoO 6 octahedra held together by covalent forces in the (1 0 0) and (0 0 1) directions but by Van der Waals forces in the (0 1 0) direction. There are several approaches to prepare MoO 3 thin films including sputtering, chemical vapor deposition, electro-deposition and evaporation. In particular, chemical vapor deposition has been regarded as a suitable technique which could induce a homogeneous thin film using vaporized phases. Crystalline molybdenum oxide presents either an orthorhombic structure (α-phase) or a monoclinic structure (β-phase) of the perovskite-like type; both structures are essentially built up of corner-sharing MoO6 octahedra. A layered oxygen deficient orthorhombic MoO3 (α-phase) and monoclinic MoO3 (β-phase) are found to exhibit optical switching upon thermal, photo or electric excitations. This optical modulation (colouration/bleaching) is effectively used in many applications like smart windows, anti dazzling coatings and display devices. Thus the synthesis of large area Mo-oxide thin films by an economical route and the structure tailoring of deposited material for the end application seems to be of prime importance. This paper deals with the detailed technological aspects of properties, different structures of MoO3for the fabrication of all solid state lithium ion batteries with wide applications of MoO3films as electrochromic display devices and gas sensors. Keywords: MoO3, thin films, synthesis, structure, properties, nanostructured MoO 3, applications, display devices and gas sensors.

Introduction Transition metal oxides with diverse structures, properties, and phenomena have been the focus of much attention in recent years in view of their scientific and technological applications. MoO3, among the other transition metal oxides, exhibits interesting structural, chemical, electrical, and optical properties. MoO3 finds application as a cathode material in the development of high-energy density solid-state microbatteries. It is considered as a promising chromogenic/electrochromic material, as it exhibits electro, photo, and gaso chromic effects by virtue of which the material is of much interest for the development of electrochromic display devices, optical switching coatings, display devices, and smart window technology. Molybdenum oxide films and nano-crystals also find application in sensors and lubricants1. MoO3 is a potential material because of its wide range of stoichiometry with interesting behaviour, which includes chromogenic and catalytic properties. This leads to the applications in electrochromic display devices, optical International Science Congress Association

memories, gas sensors and lithium batteries. Different thin film deposition methods explored for the growth of molybdenum oxide films includes the thermal evaporation, electron beam evaporation, pulsed laser deposition, sputtering, electrodeposition, chemical vapour deposition and sol-gel process. Among these methods, dc magnetron sputtering received considerable attention because it is industrially practiced method for generation of films at low temperatures with required chemical composition on large area substrates 2. Redox-active transition metal oxides (i.e., V2O5, MoO3, and WO3) have shown considerable promise for application in the areas of electrochromics and batteries. The electrochromic effect observed in these materials has led to their use in the development of displays and smart windows. Smart window fabrication has seen the greater extent of advancement between the two with the major thrust arising from a need for more efficient control over heating and cooling conditions in office buildings. Development of new and improved electrochromic devices centers on increasing coloration efficiency and cycle life in addition to decreasing coloration response time. More 1

Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502 Vol. 2(4), 1-8, April (2013) Res. J. Recent Sci. recently, attempts at enhancing these properties have focused on fabrication of materials with nanoscale dimensions in hopes that structural modification on such small size scales will ultimately lead to improvement in switching times and amplification of the electrochromic response through reduction of diffusion lengths and increased surface area. Added interest in these materials for use in energy storage systems is fueled, in part, by the desire to maximize energy stored per unit mass, in addition to improving structural stability over numerous charge/discharge cycles. For cathode materials used in rechargeable batteries, it has been found that the component particle size and chemical structure greatly determine the resulting energy density and power performance 3. MoO3 have a great technical interest, due to their optical and electronic properties. Indeed these transition metal oxides can be switched between two different optical states prompted by photochromic, thermochromic or electrochromic effect. These materials can be used in other applications such as solid state microbatteries. The substochiometric films MoO3-x with oxygen deficient contain excess metal atoms which act as doping centers; these centers control the electrical and optical film properties. The orthorhombic phase (α-MoO3) has a layered structure, which consists of double layers of MoO6 octahedra held together by covalent forces in the (100) and (001) directions but by Van der Waals forces in the (010) direction 4. Figure-1 represents the idealized structure of α-MoO3.

Synthesis Molybdenum trioxide is a chemical compound with the formula MoO3. This compound is produced on the largest scale of any molybdenum compound. It occurs as the rare mineral molybdite. Its chief application is as an oxidation catalyst and as a raw material for the production of molybdenum metal. The oxidation state of molybdenum in this compound is +6. MoO3 is produced industrially by roasting molybdenum disulfide, the chief ore of molybdenum, 2 MoS2 + 7 O2 → 2 MoO3 + 4 SO2 The laboratory synthesis entails the acidification of aqueous solutions of sodium molybdate with perchloric acid, Na2MoO4 + H2O + 2 HClO4 → MoO3 (H2O)2 + 2 NaClO4 The dihydrate loses water readily to give the monohydrate. Both are bright yellow in color. Molybdenum trioxide dissolves slightly in water to give "molybdic acid." In base, it dissolves to afford the molybdate anion5. Figure-2 shows the unit cell of MoO3.

Figure-2 Unit cell of MoO3

Figure-1 Schematic representation of the idealized structure of αMoO3

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WO3, MoO3 and Nb2O5 are well-known electrochromic materials that show cathodic coloration with H+ or Li+ ion insertion. As-deposited amorphous MoO3 films are transparent, but when ions such as H+ or Li+ and electrons are electrochemically injected into these molybdenum oxide films, the color of the films changes to dark blue. However, despite three decades of intense studies, the physical mechanism of this color change is still not fully understood. The color change in the films is believed to be directly related to the double injection/ extraction of electrons and ions in the films, which can be written, in a simplified form, as xM+ + xe- + a-MoO3 a-MxMoO3

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Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502 Vol. 2(4), 1-8, April (2013) Res. J. Recent Sci. where M=H, Li, etc. The current models suggest that the optical absorption of the ion-intercalated a- MoO3 films is caused by electron exchange between adjacent Mo5+ and Mo6+ ions6. Amorphous molybdenum oxide films show pronounced electrochromism and have many, though not all, properties in common with tungsten oxide. There have been a number of studies on α-MoO3 films; however, very few adequately describe the structural properties of the colored films. Indeed, even the structure of a non-colored as deposited α-MoO3 film has not been clearly elucidated partially due to difficulties encountered with conventional diffraction techniques. Recently, Raman spectroscopy has been demonstrated to be successful in studying the microstructures of amorphous thin films 7. Currently, there are several approaches to prepare MoO 3 thin films including sputtering, chemical vapor deposition (CVD), electro-deposition and evaporation. Despite the development of various synthesis techniques, industrial scale deposition for applications such as smart windows still remains challenging. An improvement in the deposition technique for large area chromogenic films is demanded. In particular, chemical vapor deposition has been regarded as a suitable technique which could induce a homogeneous thin film using vaporized phases. Gesheva and Ivanova successfully deposited MoO3 thin films on silicon substrates by atmospheric pressure chemical vapor deposition (APCVD). The MoO3 thin films found by pyrolytic decomposition of molybdenum hex carbonyl (Mo(CO) 6) vapor used as a precursor where the chemical reaction was assigned to8, Mo(CO)6(g) + O2(g) → MoO3(s) + CO2(g) Transition metal oxides have been studied with respect to their electrochromic properties for application as smart windows and display devices. The most widely investigated oxide is tungsten oxide. Similar to tungsten oxide, molybdenum trioxide thin films, which also possess electrochromism, are easy to produce by different techniques including chemical vapor deposition (CVD). Although molybdenum trioxide has comparatively lower coloration efficiency for a given inserted electrical charge, the closer position of its optical absorption peak to the human eye sensitivity peak makes this material very attractive for many applications. With respect to this, mixed oxides of molybdenum and tungsten are also of great interest since they offer the additional possibility of tailoring the optical absorption spectra. This can be reached by changing the fraction of each of the two components. A detailed investigation of technology and properties of MoO3 thin films would be a basis for developing technological process for deposition of mixed structures of these two materials. The low-temperature CVD carbonyl process is suitable for deposition of these materials 9. MoO3 is an industrially important catalytically active oxide used for a number of oxidative reactions. There have been studies of the physical characteristics of molybdenum trioxide including single crystal and microcrystalline MoO3 as well as supported


molybdenum oxide catalysts. Particular studies have focused on methanol oxidation to formaldehyde on unsupported and supported molybdenum oxide catalysts. These studies show that for the partial oxidation of methanol to formaldehyde, CH3OH + 1/2O2 → CH2O + H2O the reaction proceeds via adsorption of the alcohol, O–H bond breaking and formation of a methoxy intermediate, C–H bond breaking (rate limiting) and desorption of the oxidized product formaldehyde and water from the surface. Since the catalyst is reduced in the process some sort of oxygen transport to the surface must occur to replace that lost at the reaction site 10. A layered oxygen deficient orthorhombic MoO3 (α-phase) and monoclinic MoO3 (β-phase) are found to exhibit optical switching upon thermal, photo or electric excitations. This optical modulation (colouration/bleaching) is effectively used in many applications like smart windows, anti dazzling coatings and display devices. Thus the synthesis of large area Mo-oxide thin films by an economical route and the structure tailoring of deposited material for the end application seems to be of prime importance. MoO2 exhibits monoclinic rutile structure built from infinite chains of edge sharing distorted MoO 6 octahedra held together by weak Van der Waal’s forces. It is quite difficult to prepare stable stoichiometric MoO2 in thin film form owing to the difficulties in maintaining oxygen partial pressure during reactive process. MoO3 on the other hand is a very promising and stable molybdenum oxide for the electrochromic applications. The reason for pronounced electrochromism in MoO3 is related to its oxygen deficient defect perovskite structure (α and β phases) providing suitable conduits for intercalation and deintercalation of ions, existence of polyvalent oxidation states and the electronic band structure providing near infrared absorption upon electric excitation, leading to bluish colouration11.

Structure In the gas phase, three oxygen atoms are double bonded to the central molybdenum atom. In the solid state, anhydrous MoO3 is composed of layers of distorted MoO6 octahedra in an orthorhombic crystal. The octahedra share edges and form chains which are cross-linked by oxygen atoms to form layers. The octahedra have one short molybdenum-oxygen bond to non-bridging oxygen. Figure-3 represents the crystal structure of MoO3. Molybdenum forms two simple binary oxides, namely, MoO 3 and MoO2. Formally MoO3 is a 4d0Mo (VI) compound, and the material is an insulator. A metastable β phase is known which adopts a ReO3-like structure similar to that of WO3. However, the thermodynamically favored α phase crystallizes in a unique orthorhombic crystal structure (space group Pbnm) with lattice constants a) 3.962 Å, b) 13.855 Å, and c) 3.699 Å. This structure is based on a series of bilayers that are oriented perpendicular to the [010] y axis as shown in figure- 4. Each

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Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502 Vol. 2(4), 1-8, April (2013) Res. J. Recent Sci. bilayer consists of two sub layers of distorted MoO 6 octahedra to give three crystallographically inequivalent oxygen sites. These involve singly coordinated (terminal) oxygen O(1), twocoordinate oxygen O(2) and three-coordinate oxygen O (3). Each O (1) oxygen is bonded to only one Mo atom by a very short Mo-O bond length of 1.67 Å. O (2) is 2-fold coordinated and located asymmetrically between two Mo centers with bond lengths of 1.73 and 2.25 Å. Lastly, the O (3) oxygen is symmetrically placed between two Mo centers in a sub layer with a bond distance of 1.94 Å and also connected to another Mo center in the other sub layer with a bond length of 2.33 Å12.

Molybdenum trioxide is the thermodynamically stable oxide of Mo under ambient conditions. Like most other transition metal oxides, when the partial pressure of oxygen is lowered, the average oxidation state of the Mo atoms decreases and the oxide is reduced. In MoO3, reduction is accommodated by the formation of two oxygen stoichiometry compensating defects: oxygen vacancies and crystallographic shear (CS) planes. Below oxygen to metal ratios of 2.90, seven stable and/or metastable sub-oxides have been identified between the compositions of MoO3 and MoO2: Mo18O52, Mo9O26, Mo8O23, Mo5O14, Mo17O47, η-Mo4O11 and γ-Mo4O11. The structural chemistry of these phases was described several decades ago in a series of papers by Kihlborg13. The layered structure of α-MoO3 is illustrated in Figure-514. Each layer is composed of two corner-sharing nets of MoO6 octahedra that link by sharing edges along [001]. The adjacent double-octahedral layers are then linked along [010] only by weak van der Waals forces. In projection along [010] one sees a corner-sharing net that is characteristic of the ReO3 structure. The α-MoO3 structure was refined by Kihlborg and may be described by the Pbnm space group with the following established lattice parameters: a = 3.9628 Å, b = 13.855 Å and c = 3.6964 Å.

Figure-3 Crystal structure of MoO3

Figure-5 Crystal structure of orthorhombic MoO3 showing the layered structure along the (010) direction

Properties

Figure-4 Idealized polyhedral representation of the α-MoO3 structure, The structure is composed of double octahedral layers stacked along [010]


MoO3 is an odorless, yellow or light blue, crystalline solid slightly soluble in water and soluble in acid. Its melting point is 1463 degrees Fahrenheit, or 795 degrees Celsius. At 2111 degrees Fahrenheit or 1155 degrees Celsius, MoO3 decomposes into other Molybdenum oxide compounds. The electronic structure and properties of the higher molybdenum oxides have not been systematically investigated. However, there is the general trend that with increasing metal to oxygen ratios, the electronic conductivity increases. Stoichiometric MoO3 is an insulator with a band gap of 3.1 eV. When the oxide is reduced,

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Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502 Vol. 2(4), 1-8, April (2013) Res. J. Recent Sci. by the introduction of impurity donor atoms or by oxygen deficiency, electronic donor levels are created near the bottom of the conduction band and, therefore, the reduced oxides behave as semiconductors. The variation in conductivity of the Mo oxides ranges from the insulating MoO3 to the semiconducting Mo18O52 (ρ = 78.1 Ω-cm) to the metallic ηMo4O11 (ρ = 1.66 x 10-4 Ω-cm). The anisotropy in these structures leads to speculation that the electronic properties will also be anisotropic; however, definitive data on these variations are not available15. Molybdenum oxide is a wide band-gap semiconductor. This material is usually available as a white or light-gray colored powder that turns blue after electron excitation. Crystalline molybdenum oxide presents either an orthorhombic structure (αphase) or a monoclinic structure (β-phase) of the perovskite-like type; both structures are essentially built up of corner-sharing MoO6 octahedra. Amorphous molybdenum oxide films, which are based on the monoclinic structure, exhibit better chromogenic properties than do crystalline films or single crystals16. Thin films of metal oxide insulator compounds are used in a wide range of applications, such as manufacturing semiconductor devices, e.g. transistors, and integrated circuits. In metal oxide compounds, like molybdenum oxide, thin films are difficult to be prepared by thermal evaporation process, because of decomposition and the preferential evaporation of the lower vapor pressure of the constituent atoms. In metal oxide compounds, the metallic constituent usually has the lower vapor pressure. Thus, these evaporated films may often contain an excess of metal atoms, which can act as donor or doping centers. It requires only one molecule per million to yield an impurity level of the order 1017 cm–3. It was suggested that, this dopant controls the electrical properties of the films, and so the energy diagram of the dopant films will be different from that of the undopant ones12.

Nanostructured MoO3 Nanostructured MoO3 in particular is an attractive material due to its unusual chemistry and electrochromism produced by multiple valence states. Some metal oxides can spontaneously spread over supports forming self-assembled monolayers, which have been applied to heterogeneous catalysis, but nanostructured MoO3 can exhibit greater activity, and the possibility of forming stable coordination environments could lead to improved selective oxidation and isomerisation of hydrocarbons, and suitable supported intercalation cathodes. Lamellar or layered α-MoO3 in its thermodynamically stable orthorhombic form, based on the bilayered arrangement of corner-shared MoO6 octahedra held together by covalent forces, is also used as an image rotation calibration standard for transmission electron microscopy, but currently only accessible in bulk form with a lamellar structure by tedious synthetic routes17.


In spite of its promising properties as a catalyst, only in the last years semiconducting MoO3 thin films have attracted interest because of their application as active elements in conductancetype gas sensors. MoO3 thin films have been found to be very sensitive to various gases such as NO, NO2, CO, H2 and NH3 in the temperature range of 300-6000C. However, the working temperature of the MoO3-based gas sensors needs to be reduced as a consequence of the low melting point of molybdenum trioxide and also for the possible grain growth of polycrystalline materials. The properties of the films formed are mainly dependent on deposition technique and deposition parameters. Therefore, extensive investigations of structural and physical properties to optimize the functional properties are essential. Quasi 1D structures, as reported in many recent publications, possess novel characteristics due to the large surface to volume ratio, that ensure a high percentage of surface atoms, the Debye length is comparable to their lateral dimensions, i.e., their conductivity is strongly influenced by surface reactions, their greater level of crystallinity reduce possible instabilities 18.

Applications MoO3 is used to manufacture molybdenum metal, which serves as an additive to steel and corrosion-resistant alloys. The relevant conversion entails treatment of MoO3 with hydrogen at elevated temperatures, MoO3 + 3 H2 → Mo + 3 H2O It is also a component of the co-catalyst used in the industrial production of acrylonitrile by the oxidation of propene and ammonia19. Because of its layered structure and the ease of the Mo(VI)/Mo(V) coupling, MoO3 is of interest in electrochemical devices and displays. Molybdenum trioxide has also been suggested as a potential anti-microbial agent, e.g., in polymers. In contact with water, it forms H+ ions that can kill bacteria effectively20. MoO3 has widespread industrial use. Its major use is as an additive to steel and other corrosion-resistant alloys. It is also used in the production of molybdenum products, as an industrial catalyst, a pigment, a crop nutrient, a component of glass, ceramics and enamels, a flame retardant for polyester and polyvinyl chloride resins, and as a chemical reagent. Occupational exposures are expected. Molybdenum trioxide passed the animal data screen, underwent a preliminary toxicological evaluation, and is being brought to the Carcinogen Identification Committee for consultation. This is a compilation of the relevant studies identified during the preliminary toxicological evaluation21. Since MoO3 thin films are n-type semiconducting oxide, its resistance is affected by the presence of gases. So, MoO3 thin films have been extensively used for detecting a variety of gases, like ammonia, CO, NO, and NO 222. MoO3 has found widespread use in heterogeneous catalysis both as a “stand alone” material or when activated by deposition onto a suitable oxide support. It has applications in hydro cracking and desulfurization processes, metathesis, and reactions

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Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502 Vol. 2(4), 1-8, April (2013) Res. J. Recent Sci. involving removal of nitrogen oxides and partial oxidation. MoO3 is also used in the production of synthetic MoO3 has also been proposed as a possible host for Li intercalation in Li-ion batteries23. MoO3 is a well-known catalyst for the oxidation of hydrocarbons and conversion of harmful NOx to nitrogen. Attempts have been made to investigate the gas sensing properties of MoO3. Efforts are also being made to improve the sensor characteristics using thick and thin film configuration/geometries. Significant improvement in the sensing pattern of MoO3 is observed in films obtained by spin coating technique. Pulsed laser deposition (PLD) has already been shown to be a versatile technique for the preparation of a variety of oxides in thin film form24.

studies of electrochromic a-MoO3 thin films, Solid State Ionics, 147 129–133 (2002) 4.

Diaz C., Lavayen V. and Dwyer C., Single-crystal micro and nanostructures and thin films of lamellar molybdenum oxide by solid-state pyrolysis of organometallic derivatives of a cyclotriphosphazene, J. Solid State Chem., 183, 1595– 1603 (2010)

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Todd McEvoy M. and Keith Stevenson J., Electrochemical Preparation of Molybdenum Trioxide Thin Films: Effect of Sintering on Electrochromic and Electro insertion Properties, Langmuir, 19, 4316-4326 (2003)

6.

Lee Y. J., Park C. W., Kim D. G., William Nichols T., Oh S. T. and Kim Y. D., MoO3 thin film synthesis by chemical vapor transport of volatile MoO3(OH)2, J. Cer. Processing Resea., 11(1), 52-55 (2010)

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Gesheva K., Szekeres A. and Ivanova T., Optical properties of chemical vapor deposited thin films of molybdenum and tungsten based metal oxides, Solar Energy Materials & Solar Cells, 76, 563-576 (2003)

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Bouzidi A., Benramdane N., Tabet-Derraz H., Mathieu C., Khelifa B. and R. Desfeux, Effect of substrate temperature on the structural and optical properties of MoO3 thin films prepared by spray pyrolysis technique, J. Matt. Sci. Eng. B, 97, 5-8 (2003)

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Street S. C. and Goodman D. W., Chemical and spectroscopic surface science investigation of MoO3 and MoO3/Al2O3 ultrathin films, J. Vac. Sci. Technol. A, 15(3), 1717-1723 (1997)

Conclusion Transition metal oxides with diverse structures, properties, and phenomena have been the focus of much attention in recent years in view of their scientific and technological applications. MoO3, among the other transition metal oxides, exhibits interesting structural, chemical, electrical, and optical properties. MoO3 have a great technical interest, due to their optical and electronic properties. The substochiometric films MoO3-x with oxygen deficient contain excess metal atoms which act as doping centers; these centers control the electrical and optical film properties. The orthorhombic phase (α-MoO3) has a layered structure, which consists of double layers of MoO6 octahedra held together by covalent forces in the (1 0 0) and (0 0 1) directions but by Van der Waals forces in the (0 1 0) direction. Crystalline molybdenum oxide presents either an orthorhombic structure (α-phase) or a monoclinic structure (βphase) of the perovskite-like type; both structures are essentially built up of corner-sharing MoO6 octahedra. A layered oxygen deficient orthorhombic MoO3 (α-phase) and monoclinic MoO3 (β-phase) are found to exhibit optical switching upon thermal, photo or electric excitations.

Acknowledgements The Corresponding author (M. C. Rao) is thankful to UGC for providing the financial assistance through Major Research Project (Link No. F. No. 40-24/2011(SR))

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