Electrochemical properties of ultrafine TiO₂-doped MoO₃ nanoplates prepared by one-pot flame spray pyrolysis

Abstract

Ultrafine TiO₂-doped α-MoO₃ nanoplates with a low thickness of 24 nm were prepared by one-pot flame spray pyrolysis, using water-soluble ammonium molybdate tetrahydrate as precursor. Pure MoO_x nanopowders had a mixed morphology of rod-like and cube-like crystals, and a complex crystal structure composed of α-MoO₃, β-MoO₃, and Mo₁₇O₄₇ phases. However, addition of TiO₂ influenced the morphology as well as the crystal structure of MoO_x. The TiO₂-doped MoO_x powders had a regular nanoplate-like morphology and single-crystalline α-MoO₃ structure. The initial discharge and charge capacities of the pure MoO_x powders were 1481 and 998 mA h g⁻¹ at a current density of 500 mA g⁻¹. In contrast, 5 wt% TiO₂-doped MoO₃ nanoplates had high initial discharge and charge capacities of 1728 and 1171 mA h g⁻¹, respectively. The discharge capacities of pure MoO_x, 5 wt% TiO₂-doped MoO₃, and 10 wt% TiO₂-doped MoO₃ nanopowders after 200 cycles were 676, 1022, and 831 mA h g⁻¹, respectively, and the corresponding capacity retentions measured from the second cycles were 68, 86, and 85%. The reversible discharge capacities of the TiO₂-doped MoO₃ nanoplates decreased from 1424 to 978 mA h g⁻¹ as the current density increased from 200 to 1000 mA g⁻¹, and the discharge capacity recovered to 1196 mA h g⁻¹ when the current density returned to 200 mA g⁻¹ after 50 cycles.

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Introduction

Molybdenum trioxide (MoO₃) exists in mainly three polymorphic forms, orthorhombic (α-MoO₃), monoclinic (β-MoO₃), and hexagonal (h-MoO₃). Nanostructured MoO₃ is attractive owing to its myriad potential applications in various fields such as energy storage, photochromic and electrochromic devices, gas sensors, and catalysis.^1–15 MoO₃ crystal preparation by conventional processes results in preferential growth along the 〈100〉 directions.¹⁶ Therefore, MoO₃ nanoplates of various sizes are mainly prepared by liquid solution methods.^14–18 However, the long processing times of these liquid solution methods result in micrometer-sized nanoplates with high thicknesses owing to the high vapor pressure of MoO_x and high mobility of MoO₃ tetrahedron along the diagonal directions.^19,20 The MoO₃ nanoplates prepared by hydrothermal and topochemical process had mean sizes of 1 and 1.5 μm, respectively.^14,15 Thus, the development of a process for synthesizing ultrafine MoO₃ nanoplates remains a conceptual challenge.

Combustion of an appropriate precursor solution by using flame spray pyrolysis has emerged as a promising and versatile technique for commercial production of nanosized ceramic, metal, and metal–ceramic composite powders in large-scale.^21–25 In particular, nanopowders of spherical or polyhedral shapes have been mainly prepared by flame spray pyrolysis, by maintaining an extremely short residence time of the order of few milliseconds inside the high-temperature diffusion flame. To the best of our knowledge, direct preparation of ultrafine nanoplates by high temperature flame spray pyrolysis above 2500 °C using inexpensive metal salts dissolved in distilled water, has not been reported in the literature.

Doping of other metal oxides was known as an effective way to improve the electrochemical performance of MoO₃ powders.^26,27 TiO₂ is one kind of good anode material for lithium ion battery. In this study, we report the synthesis of ultrafine α-MoO₃ nanoplates by one-pot flame spray pyrolysis of water-soluble ammonium molybdate tetrahydrate precursor solution. In addition, we have analyzed the effect of TiO₂ dopant on the preparation of α-MoO₃ nanoplates and their electrochemical properties for use as anode materials in LIBs.

Experimental

Pure MoO_x and TiO₂-doped MoO_x nanopowders were prepared directly by flame spray pyrolysis (Fig. S1†). The flame spray pyrolysis system used in this study was composed of a droplet generator, flame nozzle, powder collector, and blower.²⁵ An ultrasonic spray generator (1.7 MHz) with 6 resonators was used to generate droplets, which were subsequently carried into the high-temperature diffusion flame by the carrier gas, oxygen. Propane and oxygen were used as the fuel and oxidizer, respectively, in order to create the diffusion flame. The flame nozzle consisted of five concentric pipes. The carrier gas was used to supply the droplets generated from the precursor solution to the diffusion flame through the central pipe. The flow rates of the fuel, oxidizer, and carrier gases were maintained at 5, 40, and 10 L min⁻¹, respectively. The spray solution was prepared by dissolving a pre-determined quantity of ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) in distilled water to a concentration of 0.25 M. Titanium tetraisopropoxide (TTIP) was used as the precursor for TiO₂.

The morphologies of pure MoO_x and TiO₂-doped MoO_x nanopowders were investigated through scanning electron microscopy (SEM, JEOL JSM-6060), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800), and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F). The crystal structure of the nanopowders prepared in this study was investigated by X-ray diffraction (XRD, X'Pert PRO MPD) using CuKα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). The specific surface area of the samples was determined by Brunauer–Emmett–Teller (BET) method with N₂ as the adsorbate gas. Furthermore, the electrochemical properties of pure MoO_x and TiO₂-doped MoO_x nanopowders were analyzed by constructing a 2032-type coin cell. The anode was prepared from a mixing the MoO_x active material, carbon black, and sodium carboxymethyl cellulose (CMC) in the weight ratio of 7 : 2 : 1. The size of the electrode was 1 cm × 1 cm and the mass loading was about 1.6 mg cm⁻². Li metal and a microporous polypropylene film were used as the counter electrode and the separator, respectively. The electrolyte was composed of a solution of 1 M LiPF₆ in a mixture of ethylene carbonate/dimethyl carbonate (EC/DMC, 1 : 1 v/v) containing 2% vinylene carbonate. The discharge/charge characteristics of the samples were investigated by cycling potential in the range of 0.01–3 V at various current densities.

Results and discussion

The morphologies of pure MoO_x and TiO₂-doped MoO_x powders prepared by flame spray pyrolysis are shown in Fig. 1 and 2, respectively. The SEM and TEM images shown in Fig. 2 correspond to the MoO_x powder with TiO₂ doping concentration of 5 wt%. As can be seen from the figures, the MoO_x powders were in nanometer size, irrespective of TiO₂ additive. Furthermore, the SEM images indicated the absence of micrometer- or submicrometer-sized powders that are often formed due to the incomplete evaporation of the precursor. The drying and decomposition of the precursor droplet composed of Mo and Ti components resulted in the formation of micron-sized MoO_x–TiO₂ composite intermediate. The complete evaporation of MoO_x and TiO₂ occurred inside the high temperature diffusion flame, at above 2500 °C. It is believed that the formation of nanosized TiO₂-doped MoO_x powders would have proceeded via the nucleation and growth of the evaporated vapors of Mo and Ti components. As can be seen from the TEM images, the addition of the TiO₂ dopant influenced the morphology of the MoO_x nanopowders prepared by flame spray pyrolysis. The TEM images of pure MoO_x nanopowders shown in Fig. 1b–d indicate a mixed morphology, composed of rod-like and cube-like crystals. On the other hand, the TEM and SEM images of the TiO₂-doped MoO_x powders shown in Fig. 2 indicate the formation of regular plate-like morphology. The thickness of the TiO₂-doped MoO_x nanoplates, as measured from the high-resolution FE-SEM image shown in Fig. 2b, was estimated to be 24 nm. The lateral side of the TiO₂-doped MoO_x nanoplate as shown in Fig. 2d had clear lattice fringes separated by a lattice spacing of 0.63 nm. This lattice spacing value corresponds to the (020) plane of the α-MoO₃ crystal.²⁸ The spherical TiO₂ powders in the size range of few nanometers were observed in the high-resolution TEM image, as shown by white arrow in Fig. 2d. The spherical-shaped powder had clear lattice fringes separated by a lattice spacing of 0.32 nm. This lattice spacing value corresponds to the (110) plane of the rutile TiO₂ crystal.²⁹ Only small amount of Ti component was doped onto the MoO_x crystals. The corresponding dot-mapping image shown in Fig. 2e reveals the distribution of the Ti component over the MoO_x nanoplates. In addition, some segregated Ti components were also observed in the dot-mapping image, as shown by arrows in the figure.

Fig. 1 (a) SEM and (b)–(d) TEM images of the pure MoO_x powders prepared by flame spray pyrolysis.

Fig. 2 SEM, TEM, and dot-mapping images of the 5 wt% TiO₂-doped MoO₃ powders prepared by flame spray pyrolysis.

Fig. 3 shows the morphology of the 10 wt% TiO₂-doped MoO_x powders. As can be seen from the figure, these powders exhibit two different morphologies, namely rod and spherical shape. The spherical-shaped powder, as shown by a circle in Fig. 3d, had clear lattice fringes of the rutile TiO₂ crystal separated by a lattice spacing of 0.32 nm. The spherical-shaped powders observed in the FE-SEM and TEM images in Fig. 2 and 3 correspond to rutile TiO₂ crystals. In addition, the detailed structures of pure MoO_x and 5 wt% TiO₂-doped MoO_x powders were elucidated from the nitrogen adsorption and desorption isotherms (Fig. S2†). The pure MoO_x nanopowders generated type IV nitrogen adsorption and desorption isotherm with type H3 hysteresis loop, indicating the presence of slit-shaped pores. The pure MoO_x nanopowders had an average pore size of 18 nm. On the other hand, 5 wt% TiO₂-doped MoO_x powders were found to exhibit a dense structure without pores inside the plate-like crystals. The Brunauer–Emmett–Teller (BET) specific surface areas of pure MoO_x and 5 wt% TiO₂-doped MoO_x powders were found to be 20 and 5 m² g⁻¹, respectively. The formation of homogeneous plate-like crystals with dense structure decreased the BET surface area of the TiO₂-doped MoO₃ powders.

Fig. 3 TEM images of the 10 wt% TiO₂-doped MoO₃ powders prepared by flame spray pyrolysis.

Fig. 4 shows the XRD patterns of pure MoO_x and TiO₂-doped MoO_x powders prepared by flame spray pyrolysis. The diffraction pattern of pure MoO_x powders indicates a mixed crystal structure composed of α- and β-MoO₃ and oxygen deficient Mo₁₇O₄₇ phases.¹⁵ On the other hand, the diffraction pattern of TiO₂-doped MoO_x powders indicated the formation of phase-pure α-MoO₃. Diffraction peaks corresponding to TiO₂ were not observed in the XRD patterns because of fine crystallite size as well as the low doping concentration. The pure crystal structure resulted in the morphological homogeneity of the TiO₂-doped MoO₃ powders in Fig. 2 and 3.

Fig. 4 XRD patterns of the pure MoO_x and TiO₂-doped MoO₃ powders prepared by flame spray pyrolysis.

The mechanism underlying the formation of pure MoO_x and TiO₂-doped MoO₃ powders by the flame spray pyrolysis process is described in the Scheme 1. Here, pure MoO_x and TiO₂-doped MoO₃ powders were prepared from the completely evaporated vapors, analogous to the formation mechanism shown in Section 1. However, the TiO₂ dopant changed the morphology as well as the crystal structure of the MoO_x powders formed at a short residence time of the powders inside the high temperature diffusion flame. Typically, the growth mechanism involves the formation of MoO_x and TiO₂ nuclei, by random collisions of the MoO_x and TiO₂ molecules. Surface growth and coagulation resulted in the formation of MoO_x–TiO₂ composite as an intermediate product inside the high-temperature diffusion flame. Subsequently, quenching of the intermediate composite powders resulted in the formation of nanopowders with different morphologies. The metal oxides diffused inside the composite to form crystals of Mo and Ti components. Consequently, the phase separation of the composite occurred, resulting in the formation of MoO₃–TiO₂ Janus-structured composite nanopowder, in which TiO₂ nanopowder was attached in the plate or rod-like MoO₃ powders depending on the concentration of TiO₂.³⁰ Furthermore, plate or rod-like MoO₃ crystals were formed via subsequent crystal-oriented growth by particle annealing and rearrangement during the late flame process.²³ The addition of TiO₂ is believed to have improved the formation of the plate or rod-like MoO₃ crystals.

Scheme 1 Formation mechanisms of the pure MoO_x and TiO₂-doped MoO₃ powders in the flame spray pyrolysis.

The electrochemical properties of pure MoO_x and TiO₂-doped MoO₃ powders were investigated in the voltage range of 0.01–3 V vs. Li/Li⁺. Fig. 5a shows the initial discharge and charge voltage profiles at a constant current density of 500 mA g⁻¹. In this study, insertion of lithium into the anode is referred to as discharging and Li extraction from the electrodes is referred to as charging. The initial charge and discharge curves of the MoO_x powders had a similar shape, irrespective of the concentration of the TiO₂ additive. The initial discharge and charge capacities of the pure MoO_x powders were 1481 and 998 mA h g⁻¹, respectively, and the corresponding first coulombic efficiency was 67%. On the other hand, 5 wt% TiO₂-doped MoO₃ nanoplates had the initial discharge and charge capacities of 1728 and 1171 mA h g⁻¹, respectively, and the corresponding coulombic efficiency was 68%. Similarly, 10 wt% TiO₂-doped MoO₃ nanorods had the initial discharge and charge capacities of 1326 and 946 mA h g⁻¹, respectively, with the corresponding coulombic efficiency of 71%. The results indicate that the addition of small amount of TiO₂ improved the initial discharge and charge capacities of the MoO₃ nanoplates, while the addition of large amount of TiO₂ (10 wt%) decreased the initial discharge and charge capacities.

Fig. 5 Electrochemical properties of the pure MoO_x and TiO₂-doped MoO₃ powders; (a) initial discharge and charge voltage profiles at a current density of 500 mA g⁻¹, (b) cycling performances at a current density of 500 mA g⁻¹, (c) rate performances of the pure MoO_x and 5 wt% TiO₂-doped MoO₃ powders.

Fig. 5b shows the cycling performances of pure MoO_x and TiO₂-doped MoO₃ nanopowders at a constant current density of 500 mA g⁻¹. The discharge capacities of pure MoO_x, 5 wt% TiO₂-doped MoO₃, and 10 wt% TiO₂-doped MoO₃ nanopowders after 200 cycles were 676, 1022, and 831 mA h g⁻¹, respectively, and the corresponding capacity retentions measured from the second cycles were 68, 86, and 85%, respectively. Fig. 5c shows the rate performances of the pure MoO_x and 5 wt% TiO₂-doped MoO₃ nanopowders. As can be seen from the figure, the current density increased stepwise from 200 mA g⁻¹ to 1000 mA g⁻¹, and then returned to 200 mA g⁻¹. For each step, 10 cycles were measured to evaluate the rate performance. The reversible discharge capacities of the TiO₂-doped MoO₃ nanoplates decreased from 1424 to 978 mA h g⁻¹, with increase in current density from 200 to 1000 mA g⁻¹. The discharge capacity recovered to 1196 mA h g⁻¹ when the current density returned to 200 mA g⁻¹ after 50 cycles. On the other hand, the reversible discharge capacities of pure MoO_x nanopowders decreased from 1101 to 651 mA h g⁻¹, as the current density increased from 200 to 1000 mA g⁻¹. The addition of TiO₂ is considered to have improved cycling and rate performances as well as capacities of the MoO_x nanopowders by increasing the morphological homogeneity as well as the phase purity of α-MoO₃. The short diffusion length of lithium ions and electrons in nanoplate with low thickness improved the electrochemical properties of the TiO₂-doped MoO₃ powders.

Conclusions

In this study, we have investigated the effect of TiO₂ additive on the crystal structure and morphology of MoO_x nanopowders prepared by high-temperature flame spray pyrolysis. Results indicated that pure MoO_x nanopowders had a complex crystal structure and a mixed morphology. On the other hand, addition of TiO₂ influenced the crystal structure as well as the morphology of the MoO_x nanopowders. The 5 wt% TiO₂-doped MoO₃ nanoplates exhibited uniform morphology and single-crystalline α-MoO₃, formed by the nucleation, growth, and quenching of completely evaporated MoO_x and TiO₂ vapors. The ultrafine TiO₂-doped MoO₃ nanoplates had higher discharge and charge capacities and better cycling performances than those of the pure MoO_x nanopowders at a constant current density of 500 mA g⁻¹. Systematic studies performed in this study indicate that the addition of TiO₂ improved the electrochemical properties of the MoO₃ nanopowders by increasing the morphological homogeneity as well as the phase purity of MoO₃.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012R1A2A2A02046367). This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (201320200000420).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01780c

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